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Microlithography Chapter 2 E Beam Lithography b

1. a pe S 100s O MONTE CARLO SIMULATION di a DOUBLE GAUSSIAN CURVE FIT a as lt 10 0 a a 1 0 ra 0 1 D im D 0 01 Ww O 0 001 5 0 0001 z 0 02 0 05 0 1 0 2 0 5 1 0 2 0 5 0 10 R um FIGURE 2 11 Simulated profile of the energy absorbed from an electron beam exposure Although it is generally best to avoid forward scattering effects when possible in some instances they may be used to advantage For example it may be possible to tailor the resist sidewall angle in thick resist by adjusting the development time 20 As the time increases the resist sidewall profile will go from a positive slope to vertical and eventually to a negative or retrograde profile which is especially desirable for pattern transfer by liftoff 2 3 2 Backscattering As the electrons continue to penetrate through the resist into the substrate many of them will experience large angle scattering events These electrons may return back through the resist at a significant distance from the incident beam causing additional resist exposure This is called the electron beam proximity effect The range of the electrons defined here as the distance a typical electron travels in the bulk material before losing all its energy depends on both the energy of the primary electrons and the type of substrate Fig 2 12 shows a plot of electron range as a function of energy for three common materials 21 The fraction of electrons that are backscat
2. Deflection of the electron beam is used to scan the beam across the surface of the sample As with lenses it can be done either magnetically or electrostatically The coils or plates are arranged so that the fields are perpendicular to the optical axis as shown in Fig 2 8 a Deflecting the beam off axis introduces additional aberrations that cause the beam diameter to deteriorate and deviations from linearity in X and Y increase as the amount of deflection increases These effects limit the maximum field or deflection size that can be used As with lenses magnetic deflection introduces fewer distortions than electrostatic deflection Double magnetic deflection using a pair of matched coils is sometimes used to further reduce deflection aberrations However electrostatic deflection can achieve much higher speeds since the inductance of the magnetic deflection coils limits their frequency response and eddy currents introduced by the magnetic fields may further limit the speed of magnetic deflection Since deflection systems are frequently placed inside the final lens care must be taken to prevent the fields from interacting with conducting metal parts Usually the final lens will be shielded with ferrite to minimize eddy currents Some tools use multiple deflection systems where high speed short range deflection is done electrostatically while long range deflection is magnetic In either case the field size of the tool is limited by aberrations o
3. If the focal plane aperture includes an annular ring then some of the dark field electrons pass through to expose the resist The unfocused dark field image of the mask can thereby be used to provide a background dose correction to compensate for proximity effect using a technique similar to GHOST 40 see Sec 2 4 3 3 Although this compensation scheme is still in the design stage it holds the promise of proximity effect correction without any loss of throughput 90 As in cell projection the mask is sequentially scanned and the image shifted and reduced onto the wafer However because the scattering features can be much thinner than the holes of cell projection patterns can be fabricated at smaller dimensions and the demagnification of the mask can be decreased to 5 A much larger chip can then be fabricated with up to 210 pixels 91 Massive support struts between the cells are not imaged onto the wafer since the patterns are shifted into place as they are illuminated While the mask structure is similar to those used for x ray lithography the support struts provide greater dimensional stability 84 and use of reduction optics makes mask fabrication simpler The throughput of a fully developed SCALPEL tools which to date has only been modeled is expected to be comparable to that of an optical stepper while delivering resolution on the scale of 0 1 um However several questions remain concerning its practical use At energies in
4. Optional descum in a barrel etcher 150W 0 6 Torr Oo Mount in evaporator and pump down to 210 Torr Evaporate 10 nm Cr then 100 nm Au Remove from evaporator soak sample in methelyne chloride for 10 min Ran f Agitate substrate and methylene chloride with an ultrasonic cleaner for 1 min to complete the liftoff Rinse in IPA Blow dry 141 2 7 2 2 EBR 9 EBR 9 is an acrylate based resist poly 2 2 2 trifluoroethyl chloroacrylate 142 sold by Toray Inc 143 This resist is 10 times faster than PMMA 10 C cm at 20 KV Its resolution is unfortunately more than 10 times worse than that of PMMA 0 2 m EBR 9 excels for mask writing applications not because of its speed PBS is faster but because of its long shelf life lack of swelling in developer and large process latitude EXAMPLE PROCESS EBR 9 POSITIVE MASK PLATE 1 Starting with plate purchased with a coating of EBR 9 skip to step 5 Starting with a mask plate purchased with a coating of photoresist soak mask plate in acetone gt 10 min to remove the photoresist Rinse in isopropanol blow dry 2 Clean the plate with RIE in oxygen Do not use a barrel etcher RIE conditions 30 sccm O 30 mTorr total pressure 90 W 0 25 W cm 2 min Immediately spin EBR 9 4 krpom 1 min 400 nm Bake at 170 to 180 C oven for 1h Expose with e beam 50 kV 25 C cm Make sure the plate is well grounded Other accelerating voltages may be used The dose scales roughly wit
5. Table 2 1 Characteristics of SEM based lithography systems In all cases the resolution is high depending for Nabity and Raith on the chosen SEM All of these systems have relatively small stage motion 2 in The Nabity and Raith devices are add on products while the Leica Nanowriter is an integrated system Leica Lithography Systems Ltd JC Nabity Lithography Systems Raith GmbH Model NPGS Elphy Plus EBL Nanowriter Alignment Automated or manual Automated or manual Automated SOE Automated accuracy limited by Automated 0 1 um accuracy Automated with laser Stitching stage with laser stage stage 0 40 kV for typical SEM but 0 40 kVfor typical SEM but Energy depends on target instrument dependson target instrument TOTO OOK Mid range gt 0 4 us per DAC speed Low gt 10 us per exposure point exposure point 2 6 MHz but Mid range gt 1us per p 100 kHz may be limited by SEM exposure point 1 MHz deflectors Throughput Settling time of scan coils Settling time of scan Settling time of scan coils limited by transmission rate of ISA bus coils Support for any automated optional laser Stage stage optional laser controlled controlled Control PC compatible ISA bus PC compatible computer DOS Windows DOS Windows Po Gnmpanoie Fee A out i Mid range gt 100k for Mid to high range Cost p pattern generator only SEM gt 1000k for a complete generator only SEM purchased separately purchased separately lithography sys
6. 0 6 Torr Os 0 5 min 7 Bake to harden resist 30 min 120 C Heat and cool slowly 8 Etch chrome in wet etch from Transene or Cyantek acetic acid and ceric ammonium nitrate 1 min Rinse in water Blow or spin dry 9 Strip PBS with RIE in O or by soaking in acetone rinse in IPA blow dry 2 7 2 4 ZEP A relative newcomer to e beam lithography is ZEP 520 from Nippon Zeon Co 148 ZEP consists of a copolymer of chloromethacrylate and methylstyrene Sensitivity at 25 kV is between 15 and 30 C cm an order of magnitude faster than PMMA and comparable to the speed of EBR 9 Unlike EBR 9 the resolution of ZEP is very high close to that of PMMA ZEP has about the same contrast as PMMA Lines of width 10 nm with pitch 50 nm have been fabricated with this resist 149 150 The etch resistance of ZEP in CF RIE is around 2 5 times better than that of PMMA but is still less than that of novolac based photoresists ZEP is reported to have a long shelf life 150 One disadvantage in using this resist is that like PMMA its sensitivity to electrons makes it difficult to inspect with a SEM Resist lines shift and swell under high magnification SEM viewing so it is necessary to judge the resolution of the resist by inspecting the etched patterns EXAMPLE PROCESS ZEP PATTERNING OF SiO HOLES 1 Prepare oxidized Si wafer Spin ZEP 520 at 5 krpm for thickness 300 nm 2 Bake at 170 C 2 min 3 Expose at 25 kV 15 to 30 C cm Other acceler
7. 131 M Angelopoulos J M Shaw K Lee W Huang M Lecorre M Tissier Lithographic applications of conducting polymers J Vac Sci Technol B9 6 3428 1991 132 M Angelopoulos N Patel J M Shaw N C Labianca S A Rishton Water soluble conducting polyanilines Applications in lithography J Vac Sci Technol B11 6 2794 1993 133 I Haller M Hatzakis R Srinivasan High resolution positive resists for electron beam exposure IBM J Res Develop 12 251 1968 134 M Hatzakis Electron resists for microcircuit and mask production J Electrochem Soc 116 1033 1969 135 PMMA vendors include OCG Microelectronic Materials Inc 5 Garret Mountain Plaza West Paterson NJ 07424 800 222 4868 Microlithography Chemical Corp 1254 Chestnut St Newton MA 02164 617 965 5511 617 965 5818 fax Mead Chemical Co 10750 County Rd 2000 PO Box 748 Rolla MO 65401 314 364 8844 136 G H Bernstein D A Hill On the attainment of optimum developer parameters for PMMA resist Superlattices and Microstructures 11 2 237 1992 137 B P Van der Gaag A Sherer Microfabrication below 10nm Appl Phys Lett 56 481 1990 138 D W Keith R J Soave M J Rooks Free standing gratings and lenses for atom optics J Vac Sci Technol B9 6 2846 1991 139 W C B Peatman P A D Wood D Porterfield T W Crowe M J Rooks Quarter micrometer GaAs Schottky barrier diode with high video re
8. 1994 63 M G R Thomson R Liu R J Collier H T Carroll E T Doherty R G Murray The EBES4 electron beam column J Vac Sci Technol B5 1 53 1987 64 D W Peters D C Fowlis A von Neida C M Rose H A Waggener W P Wilson EBES4 Performance of a new e beam reticle generator SPIE vol 1924 193 1993 65 H C Pfeiffer D E Davis W A Enichen M S Gordon T R Groves J G Hartley R J Quickle J D Rockrohr W Stickel E V Weber EL 4 a new generation electron beam lithography system J Vac Sci Technol B11 6 2332 1993 66 P F Petric M S Gordon J Senesi D F Haire EL 4 column and control J Vac Sci Technol B11 6 2309 1993 67 J D Rockrohr R Butsch W Enichen M S Gordon T R Groves J G Hartley H C Pfeiffer Performance of IBM s EL 4 e beam lithography system Proc SPIE 2437 160 1995 68 R Kendall S Doran E Weissmann A servo guided X Y theta stage for electron beam lithography J Vac Sci Technol B9 3019 1991 69 R Innes Yaw compensation for an electron beam lithography system J Vac Sci Technol B12 3580 1994 70 H Elsner P Hahmann G Dahm H W P Koops Multiple beam shaping diaphragm for efficient exposure of gratings J Vac Sci Technol B11 6 2373 1993 71 K Nakamura T Okino S Nakanoda I Kawamura N Goto Y Nakagawa W Thompson M Hassel Shearer An advanced electron beam litho
9. 1995 155 OCG Microelectronic Materials Inc 5 Garret Mountain Plaza West Paterson NJ 07424 800 222 4868 156 Shipley Inc 455 Forest St Marlboro MA 01752 800 343 3013 157 D Macintyre S Thoms High resolution electron beam lithography studies on Shipley chemically amplified DUV resists presented at the MNE Conference September 1996 to appear in Micro and Nano engineering 96 Procedings of the International Conference on Micro and Nano engineering S P Beaumont ed vol 29 158 E Reichmanis L F Thompson Polymer materials for microlithography in Annual Review of Materials Science v 17 R A Huggins J A Giordmaine J B Wachtman Jr eds Annual Reviews Palo Alto 1987 p 238 159 T Yoshimura Y Nakayama S Okazaki Acid diffusion effect on nanofabrication in chemical amplification resist J Vac Sci Technol B10 6 2615 1992 160 E A Dobisz C R K Marrian Sub 30nm lithography in a negative electron beam resist with a vacuum scanning tunneling microscope Appl Phys Lett 58 22 2526 1991 161 A ClaBen S Kuhn J Straka A Forchel High voltage electron beam lithography of the resolution limits of SAL601 negative resist Microelectronic Engineering 17 21 1992 162 D A Mixon A E Novembre W W Tai C W Jurgensen J Frackoviak L E Trimble R R Kola G K Celler Patterning of x ray masks using the negative acting resist P SI CMS J Vac Sci Technol
10. B11 6 2834 1993 163 A E Novembre D A Mixon C Pierrat C Knurek M Stohl Dry etch patterning of chrome on glass optical masks using P SI CMS resist Proc SPIE 2087 50 1993 164 C W Lo W K Lo M J Rooks M Isaacson H G Craighead A E Novembre Studies of 1 and 2 keV electron beam lithography using silicon containing P SI CMS resist J Vac Sci Technol B13 2980 1995 165 K J Stewart M Hatzakis J M Shaw D E Seeger E Neumann Simple negative resist for deep ultiraviolet electron beam and x ray lithography J Vac Sci Technol B7 1734 1989 166 K G Chiong S Wind D Seeger Exposure characteristics of high resolution negative resists J Vac Sci Technol B8 1447 1990 167 K G Chiong F J Hohn Resist patterning for sub quarter micron device fabrications Proc SPIE 1465 221 1991 168 N LaBianca J D Gelorme High aspect ratio resist for thick film applications Proc SPIE 2438 846 1995 169 W Moreau C H Ting High sensitivity positive electron resisit US Patent 3934057 1976 170 S Mackie S P Beaumont Solid State Technology 28 117 1985 171 M J Rooks C C Eugster J A del Alamo G L Snider E L Hu Split gate electron waveguide fabrication using multilayer poly methy methacrylate J Vac Sci Technol B9 6 2856 1991 172 Microlithography Chemical Corp 249 Pleasant St Watertown MA 02172 617 926 3322 2919 fa
11. D Berger J M Gibson R M Camarda R C Farrow H A Huggins J S Kraus Projection electron beam lithography A new approach J Vac Sci Technol B9 6 2996 1991 88 J A Liddle S D Berger C J Biddick M I Blankey K J Bolan S W Bowler K Brady R M Camarda W F Connely A Crorken J Custy R C Farrow J A Felker L A Fetter B Freeman L R Harriott L Hopkins H A Huggins C S Knurek J S Kraus D A Mixon M M Mkrtchyan A E Novembre M L Peabody W M Simpson R G Tarascon H H Wade W K Waskiewicz G P Watson J K Williams D L Windt The Scattering with Angular Limitation in Projection Electron Beam Lithography SCALPEL System Jpn J Appl Phys 34 6663 1995 89 J A Liddle H A Huggins S D Berger J M Gibson G Weber R Kola C W Jurgensen Mask fabrication for projection electron beam lithography incorporating the SCALPEL technique J Vac Sci Technol B9 6 3000 1991 90 G P Watson S D Berger J A Liddle W K Waskiewicz A background dose proximity effect correction technique for scattering with angular limitation projection electron lithography implemented in hardware J Vac Sci Technol B13 2504 1995 91 H W P Koops Microcircuit Engineering 88 North Holland New York 1989 p 217 92 G E Shedd and P E Russel The scanning tunneling microscope as a tool for nanofabrication Nanotechnology 1 67 1990
12. Values are 0 3 4 5 and 600 With release 6 0 the bersion number changes to three digits Two byte signed integer contains last modification time of library two bytes each for the year month day hour minute and second as well as time of last access same format and marks beginning of library word 1 0x1C hex bytes in record word 2 0x0102 the token for bgnlib word 3 year of last modification words 4 8 month day hour minute second word 9 year of last access time words 10 14 month day hour minute second ASCII string contains a string which is the library name The string must adhere to CDOS file name conventions for length and valid characters and may contain file extensions such as db Eight byte real contains 2 8 byte real numbers The first is the size of a database unit in user units The second is the size of a database unit in meters For example if your library was created with the default units user unit 1 m and 1000 database units per user unit then the first number would be 0 001 and the second number would be 10 Typically the first number is less than 1 since you use more than 1 database unit per user unit To calculate the size of a user unit in meters divide the second number by the first No data is present This marks the end of a library Two byte signed integer contains creation time and last modification time of a structure in the same format as that of BGNLIB and marks the b
13. by a liftoff process using a bilayer of PMMA and P MMA MAA see Sect 2 7 4 2 In all cases the designer must consider the thickness roughness and process compatibility of the material used for e beam alignment marks as well as the mark shape required for specific e beam tools 2 6 4 CAD Programs CAD programs range from the very expensive schematic capture tools for VLSI to simple and inexpensive polygon editors At the high end are widely used circuit capture simulation and layout tool sets from Cadence 101 and Mentor Graphics 102 Other high end packages are sold by Silvar Lisco 103 Integrated Silicon Systems 104 and a number of other vendors 105 These tools run almost exclusively on UNIX workstations and generate the standard intermediate format GDSII also known as Calma Stream format as well as the machine specific MEBES format Software tools in these sets include analog and digital simulators silicon compilers schematic capture wire routers design rule checkers and extensive cell libraries for CMOS BiCMOS and bipolar technologies In the mid range of expense are the programs from Design Workshop 106 DW2000 and Tanner Research 107 L Edit Design Workshop implements a fully functional graphical editor with the unusual feature of providing not only GDSII format but also output in machine specific formats for MEBES JEOL and Leica systems DW2000 includes an integrated command language for algorithmic patte
14. filled with a raster pattern and the beam jumps from one shape to the next via a direct vector By taking over the scan coils and beam blanking a SEM can be used as a simple but high resolution lithography tool SEM conversions have evolved greatly in the past twenty years primarily due to improvements in small computers and commercially available DAC boards Early designs used relatively slow computers that sent primitive shapes rectangles trapezoids and lines to custom hardware The custom pattern generator filled in the shapes by calculating coordinates inside the shapes and feeding these numbers to the DACs While this approach is still the best way to avoid data transmission bottlenecks and is used in commercial systems inexpensive SEM conversions can now rely on the CPU to generate the shape filling data A typical configuration uses an Intel CPU based PC with a DAC card plugged into an ISA bus In this case the CPU can generate data much faster than it can be transmitted over an ISA bus FIGURE 2 15 The vector scan writing strategy a Patterns are split into fields The stage moves from field to field as shown by the arrows Full patterns are stitched together from multiple fields b In many vector scan systems the fields are further tiled into subfields A major DAC 16 bits deflects the beam a small Gaussian spot to a subfield boundary and a faster DAC 12 bits deflects the beam within a subfield SEM conversion kits ty
15. pneumatic table ideally it should also be located on the ground floor Finally the temperature should be well controlled ideally to within a tenth of a degree This is particularly important if good placement accuracy is required This section begins with a description of the smallest e beam systems namely SEM conversions and proceeds to the largest commercial mask production tools We conclude the section with a listing of e beam fabrication services 2 5 2 SEM and STEM Conversions Any tool for microscopy optical electron or scanning probe may be adapted to work in reverse that is for writing instead of reading Converted electron microscopes suffer the same limitations as light microscopes used for photolithography namely a small field of view and low throughput Nevertheless for a subset of research and R amp D applications converted SEMs offer a relatively inexpensive solution Of the many custom designed SEM conversions most use a single set of digital to analog converters DACs from 12 to 16 bits wide to drive the scan coils of the microscope The beam is modulated with an electrostatic or magnetic beam blanker which is usually located near a crossover of the beam Alternatively the beam can be blanked magnetically by biasing the gun alignment coils or not blanked at all In the later case the beam must be dumped to unused sections of the pattern Figure 2 15 illustrates the vector scan method in which shapes are
16. 90 W 0 25 W cm 5 min Immediately spin AZ5206 3 krpm Bake at 80 C for 30 min Expose with e beam 10 kV 6 C cm Make sure the plate is well grounded Other accelerating voltages may be used but the dose will be different 6 Develop for 60 s in KLK PPD 401 developer Rinse in water 7 Descum important Same as step 2 above for only 5 seconds Or use a barrel etcher 0 6 Torr oxygen 150W 1 min Ge Oe 8 If this is a Cr plate etch with Transene Cr etchant 1 5 min If this is a MoSi plate then RIE etch 0 05 Torr total pressure 0 05 W cm 16 sccm SFe 4 2 sccm CF4 1 min 9 Plasma clean to remove resist same as step 2 above for 3 min Other UV sensitive resists used for e beam include EBR900 153 from Toray 143 8 uC cm at 20 kV the chemically amplified resist ARCH 154 from OCG 155 8 16 uC cm at 50 kV and the deep UV resists UVIII and UVN from Shipley 156 157 The latest offerings from Shipley have been optimized for DUV 248 nm exposure and have higher resolution than that of AZ5206 The use of DUV resists allows exposure by both photons and electrons in the same film thereby reducing e beam exposure time 2 7 3 Negative Resists Negative resists work by cross linking the polymer chains together rendering them less soluble in the developer Negative resists tend to have less bias often zero than positive resists However they tend to have problems with scum insoluble residue in ex
17. 93 N C MacDonald W Hofmann L Y Chen J H Das Micro machined electron gun arrays MEGA Proc SPIE 2522 220 1995 94 W Hofmann L Y Chen N C MacDonald Fabrication of integrated micromachined electron guns J Vac Sci Technol B13 2701 1995 95 N Shimazu K Saito M Fujinami An approach to a high throughput e beam writing with a single gun multiple path system Jpn J Appl Phys 34 6689 1995 96 T H P Chang D P Kern L P Murray Arrayed miniature electron beam columns for high throughput sub 100 nm lithography J Vac Sci Technol B10 2743 1992 97 D A Crewe D C Perng S E Shoaf A D Feinerman Micromachined electrostatic electron source J Vac Sci Technol B10 2754 1992 98 G W Jones S K Jones M D Walters B W Dudley Microstructures for control of multiple ion or electron beams IEEE Trans Electr Dev 36 2686 1989 99 E Kratschmer H S Kim M G R Thomson K Y Lee S A Rishton M L Yu T H P Chang Sub 49nm resolution 1 keV scanning tunneling microscope field emission microcolumn J Vac Sci Technol B12 3503 1994 100 E Kratschmer H S Kim M G R Thomson K Y Lee S A Rishton M L Yu T H P Chang An electron beam microcolumn with improved resolution beam current and stability J Vac Sci Technol B13 2498 1995 101 Cadence Design Systems 555 River Oaks Parkway San Jose CA USA 408 943 1234 Se
18. Chemical Society Washington DC 1984 122 C G Willson Organic Resist Materials and L F Thompson Resist Processing in Introduction to Microlithography Second Edition L F Thompson C G Willson M J Bowden eds American Chemical Society Washington DC 1994 123 A Weill The spin coating process mechanism in The Physics and Fabricaton of Microstructures and Microdevices M J Kelly C Weisbuch eds Springer Verlag Berlin 1986 p 51 124 T Tanaka M Morigami and N Atoda Mechanism of resist pattern collapse during development process Jpn J Appl Phys 32 6059 1993 125 The program SELID is available from Sigma C GmbH Rosenheimer Landstr 74 D 85521 Ottobrunn Germany 49 89 609 60 51 126 T E Everhart in Materials in Microlithography L F Thompson et al eds American Chemical Society Washington DC 1984 127 Gold etch solution type TFA from Transene Co Rowley MA 128 Chrome etch type CR 14 from Cyantek Corp 3055 Osgood Ct Fremont CA 94538 129 M Kurihara M Arai H Fujita H Moro oka Y Takahashi H Sano Primary processes in e beam and laser lithographies for phase shift mask manufacturing II SPIE vol 1809 72th Annual BACUS Symposium 50 1992 130 C A Kondek L C Poli A submicron e beam lithography process using an overcoating conducting polymer for the reduction of beam charging effects on lithium niobate and quartz Proc SPIE vol 2194 p 366 1994
19. Design Systems Inc 555 River Oaks Parkway San Jose CA 95134 See also the web site http www cadence com The following is a description of the GDSII Stream data format Release 6 0 the most commonly used format for electron beam lithography and photomask production 197 This appendix omits the description of tape formatting since disk files and disk file images on tape and other media are now the norm 198 The pattern data is considered to be contained in a library of cells Cells may contain geometrical objects such as polygons boundaries paths and other cells Objects in the cell are assigned to layers of the design Different layers typically represent different processing steps for exposure on separate mask plates Geometrical objects may also be tagged with datatypes which can be used for any purpose but are most commonly used to group together similarly sized objects for compensation of the proximity effect There is no explicitly stated limit to the level of hierarchy the degree of cell nesting however most CAD programs impose a limit of around 32 levels GDSII interpreters will either impose such a limit explicitly or will impose an implicit limit by running out of memory during recursive operations 2 9 1 Order of records A GDSII Stream file has a great deal of flexibility but must contain at least the following 1 A header record 2 One or more Stream records 3 Library name record 4 End of libr
20. Lett 60 848 1988 8 M J Rooks C C Eugster J A del Alamo G Snider E Hu Split gate electron waveguide fabrication using multilayer PMMA J Vac Sci Technol B 9 2856 1991 9 P H Woerlee G A M Hurkx W J M J Josquin and J F C M Verhoeven Novel method of producing ultrasmall platinum silicide gate electrodes Appl Phys Lett 47 7 700 702 1985 10 E Anderson V Boegli M Schattenburg D Kern and H Smith Metrology of electron beam lithography systems using holographically produced reference samples J Vac Sci Technol B9 6 3606 3611 1991 11 R Viswanathan D Seeger A Bright T Bucelot A Pomerene K Petrillo P Blauner P Agnello J Warlaumont J Conway and D Patel Fabrication of high performance 512K static random access memories in 0 25 um complementary metal oxide semiconductor technology using x ray lithography J Vac Sci Technol B11 6 2910 2919 1993 12 S Y Chou H Smith and D A Antoniadis Sub 100 nm channel length transistors fabricated using x ray lithography J Vac Sci Technol B4 1 253 255 1986 13 P W Hawkes and E Kasper Principles of Electron Optics Academic Press London 1989 14 P Grivet Electron Optics Elsevier Oxford Pergamon imprint 1965 15 E Munro Numerical modelling of electron and ion optics on personal computers J Vac Sci Technol B8 6 1657 1665 1990 16 H Boersch Experimentell
21. Thus it is important to consider the bandwidth of the deflection system when purchasing a SEM for beamwriting The other major limitation of a SEM is its stage Being designed for flexible imaging applications SEM stages are not flat and even when equipped with stepper motor control are no more accurate than 1 to 5 um Periodic alignment marks can be used to stitch fields accurately but this requires extra processing as well as the use of photolithography for printing alignment marks The mark mask would presumably be fabricated on a commercial system with a laser controlled stage Fortunately alignment with a converted SEM can be quite accurate especially when using Moir patterns for manual alignment Automated alignment in the center of a SEM writing field is at least as good as in large commercial systems Alignment at the edges of a SEM field will be compromised by distortions which are typically much larger than in dedicated e beam systems Laser controlled stages can be purchased for SEMs but these are usually beyond the budgets of small research groups Electron beam lithography requires a flat sample close to the objective lens making secondary electron imaging difficult with an ordinary Everhart Thornley detector a scintillator photomultiplier in the chamber A few high end SEMs are equipped with a detector above the objective lens or can be equipped with a microchannel plate on the pole piece These types of detectors are a great ad
22. advantage of ion beam lithography is the lack of a proximity effect which causes problems with linewidth control in EBL Another advantage is the possibility of in situ doping if the proper ion species are available and in situ material removal by ion beam assisted etching The main reason that ion beam lithography is not currently widely practiced is simply that the tools have not reached the same advanced stage of development as those of EBL Finally it should also be noted that modern computer simulation tools together with a detailed understanding of the underlying physics in many cases allows one to accurately predict exploratory device characteristics without ever having to build actual hardware This is especially true for silicon transistors 2 2 Elements of electron optics 2 2 1 Introduction The part of the EBL system that forms the electron beam is normally referred to as the column An EBL column Fig 2 4 typically consists of an electron source two or more lenses a mechanism for deflecting the beam a blanker for turning the beam on and off a stigmator for correcting any astigmatism in the beam apertures for helping to define the beam alignment systems for centering the beam in the column and finally an electron detector for assisting with focusing and locating marks on the sample The optical axis Z is parallel to the electron beam while X and Y are parallel to the plane of the sample Electron optics are a very close
23. associated with any one element the total length of all the strings plus twice the number of attribute value pairs must not exceed 128 or 512 of the element is an sref aref or node For example if a boundary element used a property attribute 2 with property value metal and property attribute 10 with property value property then the total amount of property data would be 18 bytes This is 6 bytes for metal odd length strings are padded with a null plus 8 for property plus 2 times the 2 attributes 4 equals 18 The following records are not supported by Stream Release 3 0 BOX No data is present This marks the beginning of a box element 2D00 BOXTYPE Two byte signed integer contains the box type The value of the boxtype must be in the 2E02 range of 0 to 63 PLEX Four byte signed integer a unique positive number which is common to all elements of 2F03 the plex to which this element belongs The head of the plex is flagged by setting the BGNEXTN 3003 EXDEXTN 3103 MASK 3706 seventh bit therefore plex numbers should be small enough to occupy only the rightmost 24 bits If this record is omitted then the element is not a plex member Plex numbers are not commonly used Four byte signed integer applies to pathtype 4 Contains four bytes which specify in database units the extension of a path outline beyond the first point of the path The value can be negative Four byte signed integer Applie
24. be fractured into fields subfields and even sub subfields For shaped beam machines or if the data is to be proximity corrected medium and large sized shapes should be sleeved so that the edges of shapes are exposed separately from the interiors For shaped beam machines this allows the edges to be exposed with a small shaped size that has better resolution for proximity corrected patterns this allows finer control over the dose delivered to the shapes Frequently a bias also known as sizing may be applied to the pattern shapes to account for resist characteristics or process steps that affect the final device linewidth For Gaussian beam machines a reasonable pixel size must be selected A good compromise is usually to use a pixel size of about half the beam diameter Larger pixel sizes may speed up throughput while smaller pixel sizes will reduce line edge roughness and improve feature size control The machine field size is usually a fixed multiple of the pixel size Field sizes may range from less than 100 um for high resolution high accuracy work to more than 1 mm for high speed low resolution lithography When designing a device such as a transistor you would organize the fabrication in a set of steps e g mesa ohmics gate etc Each step is assigned to a layer in the CAD tool and multiple layers are displayed as overlapping patterns usually in different colors Much later on the layers will be split apart into separate
25. by scanning electron beams J Vac Sci Technol B13 2757 1995 189 D R Allee X D Pan A N Broers C P Umbach ultra high resolution electron beam patterning of SiOz A review in Science and Technology of Mesoscopic Structures S Namba C Hanmaguchi T Ando eds Springer Verlag Tokyo 1991 p 362 190 M J Lercel G F Redinbo F D Pardo M Rooks R C Tiberio P Simpson H G Craighead C W Sheen A N Parikh D L Allara Electron beam lithography with monolayers of alkylthiols and alkylsiloxanes J Vac Sci Technol B12 6 3663 1994 191 R C Tiberio H G Craighead M Lercel T Lau C W Sheen D L Allara Self assembled monolayer electron beam resist on GaAs Appl Phys Lett 62 476 1993 192 S W J Kuan C W Frank Y H Y Lee T Eimori D R Allee R F W Pease R Browning Ultrathin Poly MMA resist films for microlithography J Vac Sci Technol B7 1745 1989 193 M B ttcher L Bauch Surface imaging by silylation for low voltage electron beam lithography J Vac Sci Technol B12 3473 1994 194 C Pierrat S Tedesco F Vinet T Mourier M Lerme B Dal Zotto J C Guibert PRIME process for deep UV and E beam lithography Microelectronic Engineering 11 507 1990 195 C Pierrat New model of polymer silylation application to lithography J Vac Sci Technol B10 2581 1992 196 M Irmscher B H fflinger R Springer Compara
26. not more than 99 If the GENERATIONS record is not present a value of 3 is assumed ASCII string contains the name of the attribute definition file This record is present only if there is an attribute definition file bound to the library The attribute definition file name may include directory specifiers and an extension See FONTS Maximum size is 44 bytes Bit array contains 2 bytes of bit flags Bit 15 rightmost specifies template data Bit 14 specifies external data also referred to as exterior data All other bits are currently unused and must be cleared to 0 If this record is omitted then all bits are assumed to be 0 Further information about template data can be found in the GDSII Reference Manual Information about external data can be found in the CustomPlus User s Manual Two byte signed integer contains the node type The value of the node type must be in the range of 0 to 63 Two byte signed integer contains the attribute number The attribute number is an integer from 1 to 127 Attribute numbers 126 and 127 are reserved for the user integer and user string properties which existed prior to Release 3 0 ASCII string contains the string value associated with the attribute named in the preceding PROPATTR record Maximum length is 126 characters The attribute value pairs associated with any one element must all have distinct attribute numbers Also there is a limit on the total amount of property data that may be
27. pattern files Some of these layers may be patterned with photolithography some with e beam For example you may design the geometry of each layer and place all of this information in the transistor cell Now you can put this cell at a number of other locations to create say a NAND logic gate If you have not simply copied the transistor but rather have created instances of the cell somewhat like a function called in a program then any modifications in the transistor cell will be instantiated all over the NAND gate The NAND gate is now a higher level cell which can be used as part of say a half adder The hierarchy of an entire circuit is continued in this way Of course when building circuits from a standard technology such as CMOS all of the basic component cells are usually purchased as part of the CAD program a library of cells and may even be placed and connected automatically as part of a symbolic CAD package 2 6 2 Avoiding Trouble Spots An e beam lithographer would be unlikely to use any high level design tools Rather the lithographer must deal with data at the lower geometrical level If the scale of critical dimensions is far larger than the e beam tool s placement errors then the designer is free to place features anywhere For instance a set of 5 reticles with 5 um design rules and 0 5 um overlay error budget will demand little except stability of a commercial e beam system However when the design requires a direct wri
28. plot showing resolution as a function of beam convergence angle for an electron beam column at 30 kV The plot assumes an energy spread of 1 5 eV a source diameter of 20 nm anda fixed demagnification of 5 The diagram in Fig 2 9 shows how these sources contribute in a typical column In systems with thermionic sources spherical aberrations tend to be the limiting factor for beam diameter while chromatic aberrations dominate in field emission systems For a given beam current there will be an optimum combination of convergence angle and system demagnification Resolution can generally be improved in most systems by using a smaller beam limiting aperture at the expense of reduced beam current and throughput In systems where the demagnification can be varied increasing the demagnification will also improve resolution at the expense of reduced beam current 2 3 Electron solid interactions Although electron beam lithography tools are capable of forming extremely fine probes things become more complex when the electrons hit the workpiece As the electrons penetrate the resist they experience many small angle scattering events forward scattering which tend to broaden the initial beam diameter As the electrons penetrate through the resist into the substrate they occasionally undergo large angle scattering events backscattering The backscattered electrons cause the proximity effect 18 where the dose that a pattern feature receives is affec
29. the 100 kV range resists are proportionally less sensitive and the energy delivered to the substrate will be larger than in conventional e beam systems The effect this may have on transistor thresholds and mobility is still unknown 2 5 8 Other E Beam System Research 2 5 8 1 STM writing The scanning tunneling microscope STM has been used to write nanometer sized patterns in research experiments It simply consists of a sharp tip used as a field emission cathode that is scanned a few nanometers above the surface of the sample Resolution is obtained not by lenses but rather by keeping the tip so close to the surface that the electrons do not have a chance to diverge However the technique is severely limited in writing speed and the resist thickness it can expose and has seen only a few very limited applications STM lithography is discussed in Sect 8 8 3 and in the review article by Shedd and Russel 92 2 5 8 2 Parallel beam architectures microcolumns In addition to the projection systems described above several other new architectures have been proposed for increasing the parallelism of e beam lithography One proposal is to build an integrated matrix of electron sources producing an array of parallel beams within one column 93 94 In contrast researchers at NTT have proposed the use of an array of micromachined beam blankers and objective lenses illuminated by a single high current electron gun 95 Other researchers are de
30. the SCALPEL technique 87 Electrons 1 that hit the scatterer the patterns on the mask are scattered and most are filtered out by the aperture Electrons traveling through the membrane 2 3 are demagnified through the aperture and form a high contrast image on the substrate The mask is a pattern of tungsten supported on a low stress silicon nitride membrane The membrane is supported on a silicon wafer with periodic silicon support struts lace tae not shown Courtesy of Lucent Technologies Inc reduced image Instead of using an absorbing mask Koops and Grob 85 proposed and researchers at AT amp T Bell Laboratories 86 88 now known as Lucent Techologies later implemented the idea of using a scattering mask to produce a high contrast image with a technique commonly used in transmission electron microscopy Figure 2 26 illustrates the technique scattering with angular limitation in projection electron beam lithography or SCALPEL Electrons traveling through a thin typically 150 nm silicon nitride membrane are focused by a lens and pass through an aperture the back focal plane filter Electrons scattered by the adsorber typically 50 nm of Au or W are most likely not to pass through the aperture By choosing an optimal accelerating voltage 95 kV for the membrane thickness 100 nm of low stress silicon nitride and adsorber 50 nm W the contrast at the substrate can be as high as 95 with a transmission of 55 89
31. the beam is steered in a circular pattern defining the width of a line while the stage is moved over the length of the line or curve This is a relatively slow way of writing a long line but avoids spatially localized stitching errors Instead the placement and drift errors are averaged over the length of the feature The Raith Elphy Plus is not only available for SEM conversions but is also used as the pattern generator for Leica s LION LV1 e beam system see below Even the most expensive SEM conversion kit will be limited by the SEM s slow magnetic deflection large distortion and small stage Next we look at fully integrated commercial systems 2 5 3 3 Leica EBL Nanowriter FIGURE 2 16 Leica EBL 100 shown here with a 100 kV LaB6 electron source and a conventional SEM stage The system is also available with a TFE source and laser controlled stage Courtesy of Leica Lithography Systems Ltd Somewhere between a converted SEM and a full featured e beam system is the Leica EBL Nanowriter Fig 2 16 This system takes its electron gun and upper column from the Leica EBPG e beam system its deflection and imaging systems from the Leica 400 SEM series 51 and adds custom pattern generation hardware The pattern generator uses 16 bit DACs and has a deflection rate up to 1 MHz for vector scan operation With an optional laser stage 5 3 nm resolution this system costs substantially less than large e beam systems and competes more directl
32. thermal field emission cathode with a 100 um objective aperture Data is from JEOL Gaussian spot e beam systems using 50 kV acceleration and a short working distance objective 5 lens courtesy of JEOL Ltd Leica e beam tools are also distinguished from those of JEOL by their use of a single objective lens one working distance and scaleable writing fields with 2 32768 or 2 65536 pixels across the field In the case of the EBML 300 field sizes up to 3 2 mm may be used although the benefit of using such a large field is debatable Emitter C1 dectrostatic Supress amp y guniens Extractor A Focus ANODE m Gun k gimert Tat K cols Shift K N C2 magreic lens RK Blanking cell Blanking aperture Spray aperture Upper main deflector A Sublid d deflector A Lower main def ector A ie i iA B Fastfocus amp stigmation jk a8 ae C3 magniaic ters Substrate FIGURE 2 19 Schematic of the Leica VectorBeam 100 kV column with a thermally assisted field emission electron source courtesy of Leica Lithography Systems Ltd The largest systems from Leica are also equipped with 100 kV TFE emitters and have stages with up to 8 in travel Additional features include a glancing angle laser height sensor for dynamic field size corrections and dynamic focus astigmatism corrections features more commonly found on high speed maskmaking tools Systems using large writing fields with defl
33. tone reversal rotation sizing and overlap removal Sigma C also offers a hierarchical proximity effect correction program CATS can be combined with the proximity effect corrector PROXECCO from AISS GmbH This software is an important alternative to the converters sold by e beam manufacturers JEBCAD 116 is a less extensive and less expensive tool for viewing and manipulating JEOL and Leica formats JEBCAD will read in GDSII J01 SPD and several low level fractured formats it will output machine formats for JEOL and Leica systems Operations in JEBCAD include adding and deleting polygons moving copying and adding arrays of objects Design Workshop 117 provides one of the most economical ways of producing machine specific formats for JEOL Leica and MEBES tools DW2000 s low level fracturing modules are quite slow compared to alternative software but are available at a small fraction of the cost 2 7 Resists Electron beam resists are the recording and transfer media for e beam lithography This section is not intended as a review of research in resists or as a guide to resist chemistry for this the reader is referred to Chap 4 and to several review papers 118 122 Instead we present here a few standard resist systems and some useful recipes for processing and pattern transfer The commercially available resists described here are summarized in Table 2 5 The usual resists are polymers dissolved in a liquid solvent Liquid resi
34. 4 such as telecentric deflection and the sliding chuck wayless stage with yaw compensation Unlike EL 4 the Excaliber will keep the field size below 1 mm thereby decreasing beam settling times while the stage moves continuously i ii M i a KA at it i Ei PRS ir dt p ii ks Hi X Re N I m j 5 f it Thi gt lt xa il i 5 i i G z0 11 FIGURE 2 25 Schematic of the IBM EL 4 column for shaped beam lithography On the right the dashed ray trace corresponds to the source and the solid trace to the shaped spot 66 Courtesy of IBM Corp 2 5 6 3 JEOL shaped spot systems JEOL s JBX 8600DV 71 provides 0 1 m resolution at 30 A cm for direct write applications The system uses two stage electrostatic deflection and handles 6 in wafers The JBX 7000MVII 72 has been developed as a 4 reticle making system for 256 Mbyte DRAM class devices As with most shaped spot systems the JEOL machines can create a map of distortion values for the deflection so that patterns can be mapped more precisely onto optically generated features The JBX 7000MVII handles up to 7 in plates with a laser stage measurement unit of 0 6 nm 1024 Overlay accuracy is 30 nm 8 and placement accuracy is 40 nm The attention to absolute pattern placement accuracy is always much more extensive in dedicated maskmaking tools than in direct write machines To control thermal expans
35. 988 p 252 146 Pre spun mask plates are sold by Hoya Electronics Co Ft Lee NJ Balzers Optical Co Marlborough MA see the Semiconductor International Buyer s Guide for other vendors 147 Mead Chemical Co 10750 County Rd 2000 PO Box 748 Rolla MO 65401 314 364 8844 148 Nippon Zeon is represented in the US by Nagase California Corp 710 Lakeway Suite 135 Sunnyvale CA 94086 408 773 0700 149 K Kurihara K lwadate H Namatsu M Nagase H Takenaka K Murase An electron beam nanolithography system and its application to Si nanofabrication Jpn J Appl Phys 34 6940 1995 150 T Nishida M Notomi R Iga T Tamamura Quantum wire fabrication by e beam lithographyusing high resolution and high sensitivity e beam resist ZEP 520 Jpn J Appl Phys 31 Pt 1 no 12B 4508 1992 151 J Pacansky R J Waltman Solid state electron beam chemistry of mixtures of diazoketones in phenolic resins AZ resists J Phys Chem 92 4558 1988 152 Hoechst Celanese Corp AZ Photoresist Products 70 Meister Ave Somerville NJ 08876 908 429 3500 153 M Kurihara M Komada H Moro oka N Hayashi H Sano EBR900 processes in e beam and laser beam lithographies for photomask production Proc SPIE 2437 240 1995 154 A E Novembre R G Tarascon O Nalamasu L Fetter K J Bolan C S Knurek Electron beam and x ray lithographic characteristics of the optical resist ARCH Proc SPIE 2437 104
36. Etec Systems Excaliber and Leica Lithographie Systeme Jena ZBA 31 32 2 5 6 3 JEOL shaped spot systems 2 5 6 4 Cell projection 2 5 7 SCALPEL 2 5 8 Other e beam system research 2 5 8 1 STM writing 2 5 8 2 Parallel beam architectures microcolumns 2 5 9 Electron beam fabrication services 2 6 Data preparation 2 6 1 Pattern structure 2 6 2 Avoiding trouble spots 2 6 3 Alignment marks 2 6 4 CAD Programs 2 6 5 Intermediate formats 2 6 5 1 GDSII Stream 2 6 5 2 CIF 2 6 5 3 DXF 2 6 5 4 PG3600 2 6 6 Low level formats 2 7 Resists 2 7 1 Charge dissipation 2 7 2 Positive resists 2 7 2 1 PMMA 2 7 2 2 EBR 9 2 7 2 3 PBS 2 7 2 4 ZEP 2 7 2 5 Photoresists as e beam resists 2 7 3 Negative resists 2 7 3 1 COP 2 7 3 2 Shipley SAL 2 7 3 3 Noncommercial negative resists P SI CMS and EPTR 2 7 4 Multilayer systems 2 7 4 1 Low high molecular weight PMMA 2 7 4 2 PMMA copolymer 2 7 4 3 Trilayer systems 2 7 5 Inorganic and contamination resists 2 7 6 Other research scanning probes and thin imaging layers 2 8 Acknowledgements 2 9 Appendix GDSIl Stream Format 2 10 References 2 1 Introduction 2 1 1 Definition and historical perspective Electron beam lithography EBL is a specialized technique for creating the extremely fine patterns much smaller than can be seen by the naked eye required by the modern electronics industry for integrated circuits Derived from the early scanning electron microscopes the technique in brief consists of sca
37. It is the standard positive e beam resist and remains one of the highest resolution resists available PMMA is usually purchased 135 in two high molecular weight forms 496 K or 950 K ina casting solvent such as chlorobenzene or anisole PMMA is spun onto the substrate and baked at 170C to 200C for 1 to 2 hours Electron beam exposure breaks the polymer into fragments that are dissolved preferentially by a developer such as MIBK MIBK alone is too strong a developer and removes some of the unexposed resist Therefore the developer is usually diluted by mixing in a weaker developer such as IPA A mixture of 1 part MIBK to 3 parts IPA produces very high contrast 136 but low sensitivity By making the developer stronger say 1 1 MIBK IPA the sensitivity is improved significantly with only a small loss of contrast The sensitivity of PMMA also scales roughly with electron acceleration voltage with the critical dose at 50 kV being roughly twice that of exposures at 25 kV Fortunately electron guns are proportionally brighter at higher energies providing twice the current in the same spot size at 50 kV When using 50 kV electrons and 1 3 MIBK IPA developer the critical dose is around 350 uC cm Most positive resists will show a bias of 20 to 150 nm i e a hole in the resist will be larger than the electron beam size depending on the resist type thickness and contrast and development conditions and beam voltage When exposed to more than 10 time
38. R Vi A u RR FIBSIPATE STAGE y FIGURE 2 17 Schematic of the JEOL JBX 5D1I system with LaB emitter The system features two objective lenses for two different working distances courtesy of JEOL Ltd JEOL s JBX 6000 implements a number of improvements on the 5DII The LaBs emitter is replaced with a thermal field emitter eliminating the need for one of the condenser lenses The pattern generator speed is increased to 12 Mhz and the PDP 11 controller is replaced with a VAX The system uses the same set of two objective lenses and for a given objective lens the magnification is fixed that is the DAC s deflection is not scaled with the field size As can be seen in the graph of figure 2 18 the ultimate spot size is somewhat improved over that of the LaBs machine but more importantly the current density at smaller spot sizes is greatly improved The JBX 6000 runs at 25 kV or 50 kV With higher current density comes the property that the probe size is sometimes smaller than a pixel For example consider a pixel grid of spacing 0 0025 um If the rastering beam skips every n grid points then the pixel area is n 0 0025 um With a current of 10 nA and a dose of 200 uC cm we must have n 0 0025 um 200 uC cm 10 nA exposure time for one pixel and since the minimum exposure time is 1 12 MHz 0 08 us the smallest value of nis 9 In this case the pixel spacing is 22 5 nm and the spot size according to Fig 2 18 is 12 n
39. SPIE Handbook of Microlithography Micromachining and Microfabrication Volume 1 Microlithography Chapter 2 E Beam Lithography by Mark A McCord Stanford University Michael J Rooks Cornell University Table of Contents 2 1 Introduction 2 1 1 Definition and historical perspective 2 1 2 Applications 2 1 3 Alternative techniques 2 2 Elements of electron optics 2 2 1 Introduction 2 2 2 Electron sources 2 2 3 Electron lenses 2 2 4 Other electron optical elements 2 2 4 1 Apertures 2 2 4 2 Electron beam deflection 2 2 4 3 Beam blanking 2 2 4 4 Stigmators 2 2 5 Other column components 2 2 6 Resolution 2 3 Electron solid interactions 2 3 1 Forward scattering 2 3 2 Backscattering 2 3 3 Secondary electrons 2 3 4 Modeling 2 4 Proximity effect 2 4 1 Introduction 2 4 2 Proximity effect avoidance 2 4 3 Proximity effect correction 2 4 3 1 Dose modulation 2 4 3 2 Pattern biasing 2 4 3 3 GHOST 2 4 3 4 Software 2 5 Systems 2 5 1 Environment 2 5 2 SEM and STEM conversions 2 5 3 Commercial SEM conversion systems 2 5 3 1 Nanometer Pattern Generation System NPGS 2 5 3 2 Raith pattern generators 2 5 3 3 Leica EBL Nanowriter 2 5 4 Gaussian vector scan systems 2 5 4 1 JEOL systems 2 5 4 2 Leica Lithography Systems 2 5 4 3 Leica Lithographie Systeme Jena Jenoptik LION 2 5 5 Gaussian spot mask writers 2 5 5 1 Etec MEBES systems 2 5 5 2 Lepton EBES4 2 5 6 Shaped Spot and Cell Projection Systems 2 5 6 1 IBM EL 4 2 5 6 2
40. W cm 2 min 3 Immediately spin COP 3 krpm 4 Expose 10 kV 0 3 C cm Other accelerating voltages may be used The dose will be different 5 Spray develop MEK methyl ethyl ketone ethanol 7 3 for 30 s 6 Rinse in MIBK methyl isobutyl ketone isopropanol 1 3 for 30 s using spray or spinner 7 Rinse in isopropanol for 30 s spray or spinner Blow dry with nitrogen 8 Inspect pattern repeat steps 5 7 as necessary 9 Descum in a barrel etcher 150 W 0 6 Torr Os 0 5 min 10 Etch chrome in wet etch from Transene or Cyantek acetic acid and ceric ammonium nitrate 1 min Rinse in water Blow or spin dry 11 Strip with RIE in Oz or by soaking in acetone rinse in IPA blow dry 2 7 3 2 Shipley SAL Shipley Inc 156 produces the popular SAL resist which comes in a variety of versions and viscocities SAL has three components a base polymer an acid generator and a crosslinking agent After exposure a baking cycle enhances reaction and diffusion of the acid catalyst leading to resist hardening by cross linking Common alkaline photoresist developers will dissolve the unexposed regions The acid reaction and diffusion processes are important factors in determining the resolution 159 and a tightly controlled postexposure baking process is required The postexposure bake is usually on a feedback controlled hotplate with a suction holder to ensure good thermal contact The extent of the cross linking reaction is
41. YPE 2A02 PROPATTR 2B02 PROPVALUE 2C06 present if there are any reference libraries bound to the current library The name for the first reference library starts at byte 0 and the name of the second library starts at byte 45 decimal The reference library names may include directory specifiers separated with and an extension separated with If either library is not named its place is filled with nulls ASCII string contains names of textfont definition files This record must be present if any of the 4 fonts have a corresponding textfont definition file This record must not be present if none of the fonts have a textfont file The name of font 0 starts the record followed by the remaining 3 fonts Each name is 44 bytes long and is null if there is no corresponding textfont definition Each name is padded with nulls if it is shorter than 44 bytes The textfont definition file names may include directory specifiers separated with and an extension separated with Two byte signed integer contains a value of 0 for square ended paths that end flush with their endpoints 1 for round ended paths and 2 for square ended paths that extend a half width beyond their endpoints Pathtype 4 signifies a path with variable square end extensions see BGNEXTN and ENDEXTN Two byte signed integer contains a positve count of the number of copies of deleted or backed up structures to retain This number must be at least 2 and
42. a current density at the sample of 1600 A cm The EBES4 column uses a TFE electron gun operating at 20 kV and a single beam crossover at the center of a high speed beam blanker 63 The pattern generator operates at up to 500 MHz and the high overall throughput allows production of a 16 Mbit DRAM mask in 30 min 64 A robot arm is used to load mask plates from a magazine module to the alignment and temperature equilibration chambers and later to the exposure chamber The internal mask carrier is made from the glass ceramic Zerodur which minimizes substrate temperature variations during exposure The EBES4 automatically loads each mask plate into the carrier establishes electrical contact to the substrate and verifies the contact resistance The EBES4 mask writer has a spot size of 0 12 m uniformity to 50 nm 3 stitching error of 40 nm and repeatability overlay accuracy of 30 nm over a 6 in reticle Table 2 3 Comparison of Gaussian spot raster scan mask making systems Lepton Inc Etec Systems Inc Model EBES4 MEBES 4500 Resolution 0 125 um spot 0 25 um features automated optional direct write on Alignment wafers automated mask writing only Field 256 um x 32 um stripes continuous 1 1 mm maximum stripe length continuous motion motion Energy 20 kV 10 kV current 1600 A cm 400 A cm Density Speed 500 MHz 160 MHz Samples 6 inch plates 8 inch plates Stage laser controlled 5 nm resolution laser controlled 6 6 nm r
43. a polygon or an actual line Translation programs support different subsets of DXF and translate the structures into GDSII using various sets of rules Users of DXF are advised to submit sample patterns for conversion before investing a lot of time in CAD work and to bear in mind that the DXF file used for one vendor may not work at all for a different vendor Therefore the cost of data conversion should be considered when choosing an apparently inexpensive CAD tool 2 6 5 4 PG3600 PG3600 and its predecessor PG3000 are used primarily by optical pattern generators built by GCA These reticle printers use a high brightness lamp and a variable rectangular shutter to print patterns onto mask plates The rectangle can be rotated to create angled features and rectangular flashes are often overlapped to create curves circles and other shapes Because of its popularity in reticle generation many e beam systems support the use of PG3600 even though the format would normally be considered low level and machine specific There are a number of disadvantages over GDSII e Overlaps must be removed by the conversion software This can be very time consuming e The format is formally a specification for 9 track tape using the EBCDIC character set Some conversion programs require the disk format to use EBCDIC and some allow a mapping into ASCII e Polygons in the CAD program are translated into overlapping rotated rectangles This process is prone to err
44. adowing 9 The problem with these tricks is that they may not be capable of exposing arbitrary patterns although they may be useful for making isolated transistor gates or other simple sparse patterns Another specialized optical technique can be used to fabricate gratings with periods as small as 0 2 um by interfering two laser beams at the surface of the sample 10 Again the pattern choice is very restricted although imaginative use of blockout and trim masks may allow for the fabrication of simple devices X ray proximity printing may be a useful lithographic technique for sub 0 25 um features 11 Again it requires a mask made by EBL and since the mask is 1 this can be a formidable challenge However if the throughput required exceeds the limited capabilities of EBL this may be an attractive option The disadvantage is that x ray lithography is currently an extremely expensive proposition and the availability of good masks is limited It also requires either a custom built x ray source and stepper or access to a synchrotron storage ring to do the exposures With care x ray lithography can also be extended to the sub 0 1 um regime 12 The final technique to be discussed is ion beam lithography The resolution throughput cost and complexity of ion beam systems is on par with EBL There are a couple of disadvantages namely limits on the thickness of resist that can be exposed and possible damage to the sample from ion bombardment One
45. an with a range intermediate between the forward scattering distribution and the backscattering distribution 100 E 10 S keid F ic 1 0 0 1 0 10 20 30 40 50 60 Energy keV FIGURE 2 12 Electron range as a function of beam energy for PMMA resist silicon and gallium arsenide From Brewer 1980 2 3 4 Modeling Electron scattering in resists and substrates can be modeled with reasonable accuracy by assuming that the electrons continuously slow down as described by the Bethe equation 22 while undergoing elastic scattering as described by the screened Rutherford formula 23 Since the different materials and geometries make analytic solutions difficult Monte Carlo techniques where a large number of random electrons are simulated are commonly used The input to the program contains such parameters as the electron energy beam diameter and film thicknesses and densities while the output is a plot of energy deposited in the resist as a function of the distance from the center of the beam Curve fitting with Gaussians and other functions to the simulated energy distribution may also be employed In order to get good statistics the energy deposition for a large number 10 000 to 100 000 of electrons must be simulated which can take a few minutes to an hour or so on a personal computer Software for Monte Carlo simulation of electron irradiation is available from several sources 24 27 Such simulations are often used
46. analog of light optics and most of the principles of an electron beam column except for the rotation of the image can be understood by thinking of the electrons as rays of light and the electron optical components as simply their optical counterparts In order to operate an EBL machine generally it is not necessary to understand the underlying math andphysics so they will not be discussed here although several excellent texts are available should the reader desire more information 13 14 In addition computer programs are available that allow easy and accurate design and simulation of optical components and columns 15 2 2 2 Electron Sources Electrons may be emitted from a conducting material either by heating it to the point where the electrons have sufficient energy to overcome the work function barrier of the conductor thermionic sources or by applying an electric field sufficiently strong that they tunnel through the barrier field emission sources Three key parameters of the source are the virtual source size its brightness expressed in amperes per square centimeter per steradian and the energy spread of the emitted electrons measured in electron volts The size of the source is important since this determines the amount of demagnification the lenses must provide in order to form a small spot at the target Brightness can be compared to intensity in light optics so the brighter the electron source the higher the current in the e
47. and controlling the electron beam Underneath the column is a chamber containing a stage for moving the sample around and facilities for loading and unloading it Associated with the chamber is a vacuum system needed to maintain an appropriate vacuum level throughout the machine and also during the load and unload cycles A set of control electronics supplies power and signals to the various parts of the machine Finally the system is controlled by a computer which may be anything from a personal computer to a mainframe The computer handles such diverse functions as setting up an exposure job loading and unloading the sample aligning and focusing the electron beam and sending pattern data to the pattern generator The part of the computer and electronics used to handle pattern data is sometimes referred to as the datapath Fig 2 2 shows a picture of a typical commercial EBL system including the column chamber and control electronics electron gun D A converters deflection amplifiers registration unit laser interferometer stage controller final lens electron detector computer vacuum system pattern data storage FIGURE 2 1 Block diagram showing the major components of a typical electron beam lithography system vibration isolation table 2 1 2 Applications Currently electron beam lithography is used principally in support of the integrated circuit industry where it has three niche markets T
48. arks 2 are used to correct for the placement of individual patterns Chip marks can be used to correct for individual chip rotation and gain to better match a badly adjusted optical stepper The marks are not shown to scale Typical wafer marks 1 are 200 um long and typical chip marks 2 are 10 um long rrrrrr The size of a chip may be on the order of centimeters and in photolithography the chips or entire wafers are aligned at once While e beam systems can align to global marks alone the best tolerance lt 0 1 m will be achieved when the alignment marks are within several hundred micrometers of the critical region The designer may therefore wish to split the e beam layer into smaller sections so that critical regions can be aligned individually If these critical regions e g gates are arranged in a regular pattern then arranging the sequence of e beam writing will be simple If the critical regions are placed randomly in the chip the designer will have a time consuming job of arranging the e beam sequence and avoiding field boundaries Alignment marks must be patterned in previous steps of the device fabrication A zero level is sometimes used for the sole purpose of placing robust alignment marks on the sample before any actual device data are written Typically the designer includes a photolithography step simply for patterning alignment marks as trenches to be etched into the substrate The best alignment of lay
49. ary beam of electrons and are insensitive to the much larger spread of secondary electrons The highest resolution patterns were formed in NaCl crystals where 50 keV electrons were used to drill holes of 1 5 nm diameter 186 but the patterns could not be transferred to any useful material While negatively exposed AIF makes an excellent etch mask 185 for fluorine based RIE the process has not been applied to any useful devices Recent research in metallic compound resists 187 188 has concentrated on mixing AIF3 and LiF to reduce the dose needed for dissociation to provide more uniform films and to expose these films with the lower current density and lower voltage 20 to 50 kV available in common e beam exposure tools Slots in these films of width 5 nm have been made with 30 keV electrons 188 At doses similar to those of the metal fluorides silicon dioxide 189 has also been used for nanometer scale patterning 2 7 6 Other Research Scanning Probes and Thin Imaging Layers A great deal of research in electron beam exposure of nanometer scale patterns is in the field of scanning probe microscopy SPM which is covered in Sect 8 3 3 For an excellent review of SPM lithography see also the review article by Shedd and Russel 92 At low voltage 1 kV and at higher energies self assembled monolayer 190 191 films have demonstrated high resolution but suffer from a very high defect density and difficulty in pattern transfer Very thi
50. ary token An example of a common record order see below for record descriptions follows HEADER version number BGNLIB last modification date LIBNAME library name GENERATIONS see below UNITS data units BGNSTR begin structure STRNAME structure name BOUNDARY begin boundary polygon LAYER layer number DATATYPE a label associated with this item XY coordinates ENDEL end of element etc ENDSTR end of structure cell ENDLIB end of library 2 9 2 Record description The GDSII Stream file format is composed of variable length records The minimum record length is four bytes Records can be infinitely long The first four bytes of a record are the header The first two bytes of the header contain a count in eight bit bytes of the total record length The count tells you where one record ends and another begins The next record begins immediately after the last byte included in the count The third byte of the header is the record type also known as a token The fourth byte of the header describes the type of data contained within the record see table below The fifth through last bytes of a record are data 2 9 3 Data type description The data type value is found in the fourth byte of the record Possible types and values are Data Type Value oO No data present Bit array Two byet signed integer Four byte signed integer Four byte real not used Eight byte real ASCII string O oa R Ww hM Two and four by
51. ating voltages may be used The dose will be different Develop in xylene p dioxane 20 1 for 2 min Blow dry Descum in barrel etcher 0 6 Torr of oxygen 150W 1 min Etch oxide in 4 min intervals to avoid resist flow 15 mTorr total pressure 42 sccm CF 5 sccm Hp 0 03 W cm oxide etches at 15 nm min 7 Remove residual resist with oxygen RIE 30 sccm Os 30 mTorr total pressure 0 25 W cm 5 min OV S 2 7 2 5 Photoresists as e beam resists Most photoresists can be exposed by e beam although the chemistry is quite different from that of UV exposure 151 Because electrons cause both positive exposure and cross linking at the same time a photoresist film exposed with electrons must be developed with a strong developer for positive behavior or the same film can be blanket exposed with UV light and then developed in a weak developer for negative behavior One of the best photoresists for positive e beam exposure is AZ5206 152 145 This resist has sensitivity around 6 uC cm contrast 4 and good etch resistance With resolution around 0 25 um and very simple processing AZ5206 is one of the best alternatives for high speed mask production EXAMPLE PROCESS AZ5206 POSITIVE MASK PLATE 1 Soak mask plate in acetone gt 10 min to remove the original photoresist Rinse in isopropanol blow dry 2 Clean the plate with RIE in oxygen Do not use a barrel etcher RIE conditions 30 sccm O gt 30 mTorr total pressure
52. atos CA 95030 408 866 1851 fax 408 866 4839 115 SIGMA C GmbH Rosenheimer Landstr 74 D 85521 Munich Germany phone 49 89 609 6051 fax 49 89 609 8112 caprox sigma c de U S distributor Raith Co 6 Beech Rd Islip NY 11751 516 224 1764 516 224 2620 fax 73164 1330 compuserve com 116 JEBCAD is sold by JEOL USA 111 Dearborn Rd Peabody MA 01960 508 535 5900 In Japan JEOL Ltd 1 2 Musashino 3 chome Akishima Tokyo 196 0425 42 2187 117 Design Workshop 4226 St John s Suite 400 D D O Quebec H9G 1X5 514 696 4753 fax 514 696 5351 118 E Reichmanis L F Thompson Polymer materials for microlithography in Annual Review of Materials Science vol 17 R A Huggins J A Giordmaine J B Wachtman Jr eds Annual Reviews Inc Palo Alto CA 1987 p 235 119 E Reichmanis A E Novembre Lithographic resist materials chemistry in Annual Review of Materials Science vol 23 R A Laudise E Snitzer R A Huggins J A Giordmaine J B Wachtman Jr eds Annual Reviews Inc Palo Alto CA 1993 p 11 120 C Grant Willson Organic resist materials theory and chemistry in Introduction to Microlithography L F Thompson C G Willson M J Bowden eds ACS Symposium Series 219 American Chemical Society Washington DC 1983 p 87 121 Materials for Microlithography Radiation Sensitive Polymers L F Thompson D G Willson J M J Fr chet eds ACS Symposium Series 266 American
53. auert 18 D 44227 Dortmund Germany 0231 97 50 000 or Raith USA 6 Beech Rad Islip NY 11751 516 224 1764 516 224 2620 fax 73164 1330 compuserve com 45 PROXECCO distributed byTranscription Enterprises Limited 101 Albright Way Los Gatos CA 95030 408 866 1851 fax 408 866 4839 46 Y Pati A Teolis D Park R Bass K Rhee B Bradie and M Peckerar An error measure for dose correction in e beam nanolithography J Vac Sci Technol B8 6 1882 1888 1990 47 Raith GmbH Hauert 18 D 44227 Dortmund Germany 0231 97 50 000 or Raith USA 6 Beech Rd Islip NY 11751 516 224 1764 516 224 2620 fax 73164 1330 compuserve com 48 J C Nabity Lithography Systems PO Box 5354 Bozeman MT 59717 USA 406 587 0848 jcnabity aol com 49 Data Translation Inc 800 525 8528 50 J C Nabity M N Wybourne A versatile pattern generator for high resolution electron beam lithography Rev Sci Instrum 60 1 1989 51 The Leica SEM division and the Zeiss SEM TEM division have merged to form a new separate comany Leo Electron Optics US Address One Zeiss Drive Thornwood NY 10594 800 356 1090 52 R Kendall S Doran E Weissmann A servo guided X Y theta stage for electron beam lithography J Vac Sci Technol B9 3019 1991 53 R Innes Yaw compensation for an electron beam lithography system J Vac Sci Technol B12 3580 1994 54 H Ohta T Matsuzaka N Saitou New electron optica
54. ave up to 200 pairs of coordinates A path must have at least 2 and a boundary at least 4 pairs of coordinates The first and last point of a boundary must coincide A text or SREF element must have only one pair of coordinates An AREF has exactly three pairs of coordinates which specify the orthogonal array lattice In an AREF the first point locates a position which is displaced from the reference point by the inter column spacing times the number of columns The third point locates a position which is displaced from the reference point by the inter row spacing times the number of rows A node may have from 1 to 50 pairs of coordinates A box must have five pairs of coordinates with the first and last points coinciding No data is present This marks the end of an element ASCII string contains the name of a referenced structure Two byte signed integers the first 2 bytes contain the number of columns in the array The third and fourth bytes contain the nunber of rows Neither the number of columns nor the number of rows may exceed 32 767 decimal and both are positive No data is present This marks the beginning of a node Two byte signed integer contains the text type The value of the text type must ge in the range of 0 to 63 Bit array contains 2 bytes of bit flags for text presentation Bits 10 and 11 taken together as a binary number specify the font Bits 12 and 13 specify the vertical presentaton 00 means top 01 mea
55. bake for 1 min on a large hotplate 115 C Cool for gt 6 min Develop for 6 min in Shipley MF312 water 1 1 Be sure to check for underdevelopment Descum 30 s with oxygen RIE same as step 2 10 s 10 Etch with Transene or Cyantek Cr etchant 1 5 min 11 Plasma clean to remove resist Same as step 2 5 min 9 STON eE O 2 7 3 3 Noncommercial negative resists P SI CMS and EPTR Although not yet commercialized a very promising negative resist is P SI CMS which combines the high speed of CMS chloromethylstyrene with the etch resistance of SI Sheet u methacrylate This resist offers at least 10 times the plasma etch resistance of SAL 162 164 Its silicon component gives excellent resistance to etching in an oxygen plasma by forming a surface layer of silicon oxide The sensitivity is similar to that of SAL 10 uC cm at 40 kV but the resolution is around 0 2 um P SI CMS will be a good choice when etch resistance is more important than resolution The epoxy type resist 165 167 developed at IBM is a combination of a novolac epoxy resin o cresol novolac glycidyl ether and an onium salt triphenylsulfonium hexafluoroantimonate photoinitiator EPTR is a high speed resist 6 uC cm at 50 kV with relatively high contrast 6 4 and high resolution 50 nm While the resolution of EPTR is comparable to that of Shipley SAL the epoxy formulation allows EPTR to be extended to layer thicknesses exceeding 200 um 168 The
56. ch the beam passes on its way down the column There are several types of apertures A spray aperture may be used to stop any stray electrons without materially affecting the beam itself A blanking aperture is used to turn the beam on and off by deflecting the beam away from the aperture hole the aperture intercepts the beam when not writing A beam limiting aperture has two effects it sets the beam convergence angle alpha measured as the half angle of the beam at the target through which electrons can pass through the system controlling the effect of lens aberrations and thus resolution and also sets the beam current A beam limiting aperture is normally set in an X Y stage to allow it to be centered or aligned with respect to the optical axis It is best to have a beam limiting aperture as close to the gun as possible to limit the effects of space charge caused by electron electron repulsion Apertures may be heated to help prevent the formation of contamination deposits which can degrade the resolution of the system If not heated the apertures typically need to be cleaned or replaced every few months With platinum apertures cleaning is easily accomplished by heating the aperture orange hot in a clean burning flame Shaped beam systems also have one or more shaping apertures which can be square or have more complicated shapes to allow the formation of a variety of beam shapes such as triangles etc 2 2 4 2 Electron beam deflection
57. combined distribution FIGURE 2 14 Schematic showing how the GHOST technique can be used to correct for the proximity effect The top curves show the energy distribution in the resist for a group of seven lines from the primary exposure and from the GHOST exposure The bottom curve is the resulting final energy distribution showing the dose equalization for all the lines Section 2 5 Systems 2 5 1 Environment For best results systems should be installed in a clean quiet environment 60 Hz noise is pervasive in most systems To minimize this careful consideration must be paid to the grounding of the system components to prevent ground loops Also analog and digital grounds should be kept separate as much as possible to minimize high frequency noise components One useful method for tracking noise problems is to place the beam on the edge of a mark and monitor the electron detector output with a spectrum analyzer while disconnecting various suspect noise sources Acoustical noise can be a significant problem especially in systems with field emission electron sources In such systems the demagnification of the field emission source and thus the demagnification of vibrations is much less than that of LaBs systems Stray magnetic fields are also a common problem Mechanical pumps transformers and fluorescent lights should be moved at least 10 ft from the column if possible The system should be well isolated from mechanical vibrations with a
58. data processing system for electron beam lithography J Vac Sci Technol B10 1 133 142 1992 37 K Cummings R Frye E Rietman Using a neural network to proximity correct patterns written with a Cambridge electron beam microfabricator 10 5 lithography system Appl Phys Lett 57 1431 1433 1990 38 J Jacob S Lee J McMillan and N MacDonald Fast proximity effect correction An extension of PYRAMID for circuit patterns of arbitrary size J Vac Sci Technol B10 6 3077 3082 1992 39 B D Cook S Y Lee Fast proximity effect correction An extension of PYRAMID for thicker resists J Vac Sci Technol B11 2762 1993 40 G Owen and P Rissman Proximity effect correction for electron beam lithography by equalization of background dose J Appl Phys 54 6 3573 3581 1983 41 M Gesley and M A McCord 100 kV GHOST electron beam proximity correction on tungsten x ray masks J Vac Sci Technol B12 6 3478 3482 1994 42 Y Kuriyama S Moriya S Uchiyama and N Shimazu Proximity effect correction for x ray mask fabrication Jpn J Appl Phys 33 6983 6988 1994 43 T Abe S Yamasaki T Yamaguchi R Yoshikawa and T Takigawa Representative Figure Method for Proximity Effect Correction ll Jon J Appl Phys 30 11 2965 2969 1991 44 CAPROX trademark of Sigma C GmbH Rosenheimer Landstr 74 D 85521 Ottobrunn Germany 49 89 609 60 51 Distributed by Raith GmbH H
59. ding magnetic lens converges the beam The complementary lenses reduce chromatic aberration just as in a compound optical lens A high resolution automated stage substrate cassette loader and substrate height measuring system complete the LION LV1 as a full featured system Low voltage operation avoids substrate damage and proximity effects and offers the capability of three dimensional patterning by tailoring the electron penetration depth However the disadvantage is in greatly complicated resist processing If the beam does not penetrate the resist there will be significant effects from resist charging 56 and placement errors due to charging may be dependent on the writing order and on the shape of the pattern itself Charging may be avoided by using a resist trilayer with a conducting center e g PMMA on Ti on polyimide or by using a conducting overlayer see sect 2 7 1 Increased processing is required also for removing the resist layer over alignment marks In a production environment this complexity adds significantly to the cost of ownership Gun Chamber Blanker Condenser lens Alignment and stigmator system Valve SE detector a mpera RTS Compound objective lene N J FIGURE 2 20 Low voltage column developed by ICT GmbH used in the LION e beam system from Leica The beam blanker is directly above the anode The objective lens combines electrostatic and magnetic elements to reduce the net chromat
60. drift compensation and substrate temperature control Real time correction of focus gain and rotation provide stitching errors 3 of 50nm 59 The MEBES 4500 can be used as a metrology tool to characterize its own stitching and linearity However when errors appear in both the writing and the reading process as would be caused by interferometer mirror defects then a machine cannot measure its own distortions In this case two or more MEBES machines can be used to check for consistency lon pumps Stigmator 1 muaf Centering coil 1 Aupiliary blanker Column timing operture T First vonsfer lens Second tranderlens JUUL ins 160 MHz blanker driver at 502 Stignator 2 Cemering ool Objective lens Deflection system Mask FIGURE 2 22 Schematic of the MEBES IV TFE column Etec Systems Inc The source optics include the extractor V focus V1 and suppressor V The high speed beam blanker assembly is a U shaped transmission line designed to deflect the beam twice with one blanking pulse Courtesy of Etec Systems Inc As with any Gaussian beam system throughput decreases as resolution density of the pixel writing grid increases One way to increase the resolution without sacrificing speed is to implement a graybeam strategy where the pixels on edges of features have dwell times and placements modulated on a per pixel basis This allows the bulk of a pattern to be written on a
61. e also http Awww cadence com 102 Mentor Graphics Corp Gateway Marketing Center P O Box 5050 Wilsonville OR 97070 800 547 3000 fax 503 685 8001 E mail gen del gateway mentorg com 103 Silvar Lisco 703 E Evelyn Av Sunnyvale CA 94086 800 624 9978 408 991 6000 fax 408 737 9979 104 Integrated Silicon Systems P O Box 13665 Research Triangle Park NC 27709 800 422 3585 105 Refer to the Semiconductor International Buyer s Guide issue for a list of other CAD vendors 106 Design Workshop 4226 St John s Suite 400 D D O Quebec H9G 1X5 514 696 4753 fax 514 696 5351 107 Tanner Research 180 North Vinedo Av Pasadena CA 91107 818 792 3000 fax 818 792 0300 108 DXF to GDSII conversion software is available from Artwork Conversion Software 1320 Mission St 5 Santa Cruz CA 95060 408 426 6163 109 For information on ordering these programs and on the Berkeley Industrial Liaison Program see http www eecs berkeley edu ILP Catalog index html 110 R W Hon C H Sequin A Guide to LSI Implementation Second Edition p 79 XEROX Palo Alto Research Center 3333 Coyote Rd Palo Alto CA 94304 1980 111 C Mead L Conway Introduction to VLSI Systems Addison Wesley Reading MA 1980 112 See http info broker isi edu 1 mosis 113 These rules provided by S Reynolds ISI MOSIS 4676 Admiralty Way Marina del Rey CA 90292 114 Transcription Enterprises Limited 101 Albright Way Los G
62. e convergence angle of the system so that electrons are confined to the center of the lenses at the cost of greatly reduced beam current A magnetic lens is formed from two circularly symmetric iron or some other high permeability material polepieces with a copper winding in between Fig 2 6 shows a cross section through a typical magnetic lens along with some magnetic flux lines copper windings i ey 4 l i iron shell a ted electron polepieces optic axis FIGURE 2 6 Cross section through a magnetic lens with lines showing the magnetic field distribution The divergence of the magnetic flux along the optical axis imparts a force on electrons back towards the optical Z axis resulting in focusing action The magnetic field also causes a rotation of the electrons and the image about the Z axis in a corkscrew fashion Although this does not affect the performance of the lens it does impact the design alignment and operation of the system For instance the deflection system must be rotated physically with respect to the stage coordinates Also when aligning a column X and Y displacement in the upper regions of the column will not correspond to the same X and Y displacement at the target Finally changes in focus or changes in the height of the sample can cause a slight rotation in the deflection coordinates This must be properly corrected or stitching and overlay errors will result Magnetic lenses particularly th
63. e Bestimmung der Energieverteilung in Thermisch Ausgelosten Elektronenstrahlen Z Phys 139 115 146 1954 17 M Gesley Thermal field emission optics for nanolithography J Appl Phys 65 3 914 926 1989 18 T H P Chang Proximity effect in electron beam lithography J Vac Sci Technol 12 1271 1275 1975 19 D F Kyser and N S Viswanathan Monte Carlo simulation of spatially distributed beams in electron beam lithography J Vac Sci Technol 12 6 1305 1308 1975 20 M Hatzakis Recent developments in electron resist evaluation techniques J Vac Sci Technol 12 6 1276 1279 1975 21 G Brewer ed Electron Beam Technology in Microelectronic Fabrication Academic Press 1980 22 R Birkhoff in Handbuck der Physik E Fluegge ed Springer Berlin and New York 53 1958 23 K Murata D Kyser and C Ting Monte Carlo simulations of fast secondary electron production in electron beam resists J Appl Phys 52 4396 4405 1981 24 University of Califronia Berkeley Department of Electrical Engineering Berkeley CA USA 25 Leica Ltd Cambridge UK USA 708 405 0213 0147 fax UK 44 223 41 1 411 211 310 fax 26 Sigma C GmbH Rosenheimer Landstr 74 D 85521 Ottobrunn Germany 49 89 609 60 51 27 AISS GmbH represented byTranscription Enterprises Limited 101 Albright Way Los Gatos CA 95030 408 866 1851 fax 408 866 4839 28 S A Rishton and D P Kern Point exposure d
64. e aspect ratio exceeds roughly 5 1 most resists undergo mechanical failure features will fall over during development due primarily to surface tension in the rinse portion of the development sequence 124 Recently commercial software for simulating electron beam exposure of polymer resists has become available 125 a 100 x Coron a i h a Fimreaamon o Ds 0 e Logi Glecton Dose FIGURE 2 29 Film thickness versus exposure dose for a positive b and b negative resist Contrast is defined as the slope of the linear portion of the falling or rising section of the curve 3 3 Fimretention Q 5 D D Log Glecton Dose The primary goals of e beam lithography are high resolution and high speed high sensitivity Unfortunately the highest resolution resists are usually the least sensitive We can see a reason for this trend when we consider the limit of resist sensitivity If a very sensitive resist has a critical dose of 0 1 uC cm and a pixel is 0 1 um ona side then only 62 electrons are needed to expose the pixel 126 At this sensitivity even small changes in the number of electrons will cause variations in the dose delivered to each pixel If the sensitivity is increased further then the number of electrons in each pixel becomes too small to allow an even exposure of the pattern To look at it another way if we wish to decrease the pixel size then the resist will have to be made e
65. e final lens may be liquid cooled to maintain a controlled temperature which is critical for stable operation of a system Electrostatic lenses have worse aberrations than magnetic lenses so they are not as commonly used They are most often found in the gun region as a condenser lens since they can be combined with the extractor or anode used to pull electrons out of the cathode and they are easily made for ultrahigh vacuum use and are bakeout compatible Also aberrations in the condenser lens tend to be less important system performance is usually dominated by the aberrations of the final lens A simple electrostatic lens as shown in Fig 2 7 consists of three consecutive elements like apertures the outer two being at ground potential and the inner at some other variable potential that controls the lens strength The electric potentials set up by such a lens tend to pull an electron that is traveling away from the optical axis back towards the axis resulting in the focusing action optical axis NA Pcs Vf K ro VLLLLLLL LLLLLLLL I V7ZZZ222 FIGURE 2 7 Cross section through an electrostatic Einzel lens The focus of the lens is controlled by the voltage applied to the center electrode 2 2 4 Other electron optical elements Other optical elements include apertures deflection systems alignment coils blanking plates and stigmators 2 2 4 1 Apertures Apertures are small holes through whi
66. e structures Proc SPIE 1089 124 1989 181 A N Broers Micromachining by sputtering through a mask of contamination laid down by an electron beam in Proceedings of the First International Conference on Electron and lon Beam Science and Technology R Bakish ed Wiley New York 1964 p 191 182 R Voss R B Laibowitz A N Broers Niobium nanobridge DC SQUID Appl Phys Lett 37 656 1980 183 C P Umbach S Washburn R A Webb R Koch M Bucci A N Broers R B Laibowitz Observation of the h e Aharonov Bohm interference effects in sub micron diameter normal metal rings J Vac Sci Technol B4 383 1986 184 P Mankiewich H G Craighead T R Harrison A Dayen High resolution electron beam lithography on CaF Appl Phys Lett 44 468 1984 185 E Kratschmer M Isaacson Nanostructure fabrication in metals insulators and semiconductors using self developing metal inorganic resist J Vac Sci Technol B4 1 361 1986 186 M Isaacson A Muray In situ vaporization of very low molecular weight resists using 1 2 nm diameter electron beams J Vac Sci Technol 19 1117 1981 187 W Langhenrich A Vescan B Spangenberg H Beneking Microelectronics Engineering 17 287 1992 W Langhenrich H Beneking Jpn J Appl Phys 32 6248 1993 188 J Fujita H Watanabe Y Ochiai S Manako J S Tsai S Matsui Sub 10 nm lithography and development properties of inorganic resist
67. ection angles exceeding 5 to 10 milliradians make use of a number of higher order corrections including deflection linearization maps field rotation maps dynamic focus and stigmation tables and even shift corrections for the dynamic focus coil 2 5 4 3 Leica Lithographie Systeme Jena Jenoptik LION One of the most unique Gaussian vector scan systems is the LION LV1 from Leica Lithographie Systeme Jena GmbH 55 a company better known for its large mask making machines previously sold only in Eastern Bloc countries The LION LV1 combines a column designed by ICT GmbH Heimstetten Germany with the pattern generator from Raith GmbH This pattern generator has the unusual feature that it allows continuous path control of curves In this mode the beam is held close to the center of the field while stage motion defines the shape of a Bezier curve The ICT column is very similar to that used in the Leo 982 SEM 51 except for the use of a beam blanker and higher bandwidth deflection coils see Fig 2 20 In this system proximity effects are avoided by using beam energies as low as 1 to 2 keV Although the voltage may be set as high as 20 kV the system s selling point is low voltage avoiding both damage to the substrate and complications due to the proximity effect The column provides a spot size as small as 5 nm at 1 kV through the use of an unusual compound objective lens An electrostatic lens produces a diverging field while the surroun
68. ection coil calibrations inan electron beam block exposure system J Vac Sci Technol B12 6 3404 1994 79 M Kawano K Mizuno H Yoda Y Sakitani K Andou N Saitou Continuous writing method for high speed electron beam direct writing system HL 800D J Vac Sci Technol B11 6 2323 1993 80 G H Jansen Coulomb Interactions in Particle Beams Academic Boston 1990 81 S Berger D J Eaglesham R C Farrow R R Freeman J S Kraus J A Liddle Particle particle interaction effects in image projection lithography systems J Vac Sci Technol B11 6 2294 1993 82 Y Someda H Satoh Y Sohda Y Nakayama N Saitou H Itoh M Sasaki Electron beam cell projection lithography Its accuracy and its throughput J Vac Sci Technol B12 6 3399 1994 83 Y Nakayama S Okazaki N Saitou H Wakabayashi Eelctron beam cell projection lithography A new high throughput electron beam direct writing technology using a specially tailored Si aperture J Vac Sci Technol B8 1836 1990 84 J A Liddle C A Volkert Stress induced pattern placement errors in thin membrane masks J Vac Sci Technol B12 6 3528 1994 85 H P W Koops J Grob Springer Series in Optical Sciences X ray Microscopy Springer Berlin 1984 vol 43 86 S D Berger J M Gibson New approach to projection electron lithography with demonstrated 0 1um linewidth Appl Phys Lett 57 2 153 1990 87 S
69. eginning of a structure ASCII string contains a string which is the structure name A structure name may be up to 32 characters long Legal characters are A through Z a through z 0 through 9 underscore question mark and the dollar sign No data is present This marks the end of a structure No data is present This marks the beginning of a bounary element polygon No data is present This marks the beginning of a path element No data is present This marks the beginning of a structure reference element a reference or call to another cell in the library No data is present This marks the beginning of an array reference element an array of cells No data is present This marks the beginning of a text element Two byte signed integer contains the layer number The value must be from 0 to WIDTH OF03 XY 1003 ENDEL 1100 SNAME 1206 COLROW 1302 NODE 1500 TEXTTYPE 1602 PRESENTATION 1701 STRING 1906 STRANS 1A01 MAG 1B05 ANGLE 1005 REFLIBS Four byte integer contains the width of a path or text lines in database units A negative value for width means that the width is absolute i e is not affected by the magnification factor of any parent reference If omitted zero is assumed Four byte signed integer contains an array of XY coordinates in database units Each X or Y coordinate is four bytes long Path and boundary elements may h
70. egins with the bytes 00 06 00 02 telling us that the first record contains 6 bytes that the first record is type 00 the header and that record contains data of type 02 two byte signed integer The corresponding ASCII representation on the right is read from left to right 02000200 60000201 1C000300 02000600 000000 01000E00 02000200 60002500 01000E00 000010 42494045 4C504D41 58450602 12002500 EXAMPLELIB 000020 413E0503 14000300 02220600 59524152 RARY gt A 000030 60000000 01000E00 02000200 60000205 cae 000050 58450606 0C001 100 01000E00 02000200 EX 000060 0100020D 06000008 04000045 4C504D41 AMPLE 000070 0000F0D8 FFFF0310 2C000000 020E0600 Perry 000080 FFFF204E 00001027 0000204E 00001027 N N 000090 0000F0D8 FFFFFOD8 FFFFFOD8 FFFFFOD8 0000A0 00000004 04000007 04000011 04001027 0000B0 00000000 00000000 00000000 00000000 0000C0 The following is an ASCII representation of this file created by the program SDUMP 198 which translates the token numbers into the names listed in the previous section HEADER 3 BGNLIB 96 37 LIBNAME EXAMPLELIBRARY GENERATIONS 3 UNITS 1 0000000000000E 03 1 0000000000000E 09 BGNSTR 96 14 1 17 STRNAME EXAMPLE BOUNDARY LAYER 1 DATATYPE 0 XY 10000 10000 20000 10000 20000 10000 10000 10000 10000 10000 ENDEL ENDSTR ENDLIB 2 10 Re
71. er their relatively low throughput compared to photolithography systems has kept them firmly rooted in the maskmaking market e FIGURE 2 21 Gaussian spot raster scan writing strategy The stage is moved continuously while the beam is rastered perpendicular to the stage motion This technique used by the Etec MEBES tools is one of the most common for mask generation 2 5 5 1 Etec MEBES systems The most popular and well established mask writing tool is the MEBES from Etec Systems Inc 57 The MEBES uses a focused Gaussian spot writing a pattern in stripes while moving the stage continuously The beam deflection is primarily in one direction perpendicular to the motion of the stage Fig 2 21 Of course some small deflections are needed in the direction of stage travel to compensate for stage placement errors These correction values are provided by the feedback system of the laser controlled stage The 10 kV TFE electron gun provides a current density 58 at the mask plate of 400 A cm The MEBES is designed for high throughput mask making with minimum feature size 0 25m Figure 2 22 shows the MEBES IV TFE column design with three beam crossovers compared to one crossover in the Lepton column A 160 MHz transmission line beam blanker is located at the third crossover Since Etec Systems has implemented a full range of error compensation techniques including a glancing angle height sensor dynamic focus corrections periodic
72. er 2 to layer 1 will be achieved when layer 1 contains the marks used for aligning layer 2 and when the marks are as close as possible to critical areas If the material of layer 1 is unacceptable for alignment e g a 20 nm thick metal layer then both layers will have to be aligned to a third reference pattern the zero level Alignment to a third layer adds a factor of 1 4 to the overlay error Well designed marks are commonly destroyed by processing For example ohmic metalizations become very rough when annealed The rough marks are fine for optical alignment but the lumps may cause the e beam alignment hardware to trigger at the wrong locations A good solution to this problem is to fabricate alignment marks as deep etched trenches deeper than 1 m Plasma etched or wet etched trenches may be used Such pits will not change after high temperature processing unless material is deposited in them and unlike Au are compatible with MOS processing Other examples of effective alignment marks are W on Ti Pt on Ti and Au on Cr Au is compatible with GaAs processing but to maintain a smooth film the alignment marks must be patterned after the annealing steps In each of these cases the Ti or Cr provides improved adhesion to the substrate A 200 nm thick layer of Pt or Au provides a good alignment signal and 10 to 20 nm of Ti or Cr under the high Z material provides improved substrate adhesion Metal films can be patterned with very smooth edges
73. es An alternative is to define a grid and compute the interaction of the pattern shapes with the grid and vice versa 35 however the accuracy and flexibility of this technique may be limited An optimal solution may also be arrived at by an iterative approach 36 Finally neural network techniques have been applied to the problem of proximity correction 37 while not an attractive technique when implemented on a digital computer it might be advantageous if specialized neural network processors become a commercial reality Many of the algorithms in use assume that the energy distribution has a double Gaussian distribution as discussed in Sec 2 3 2 4 3 2 Pattern biasing A computationally similar approach to dose modulation is pattern biasing 38 39 In this approach the extra dose that dense patterns receive is compensated for by slightly reducing their size This technique has the advantage that it can be implemented on EBL systems that are not capable of dose modulation However the technique does not have the dynamic range that dose modulation has patterns that contain both very isolated features and very dense features will have reduced process latitude compared to when dose modulation is used since the isolated features will be under dosed while the dense features will be overdosed Pattern biasing cannot be applied to features with dimensions close to the scale of the pixel spacing of the e beam system 2 4 3 3 GHOST A third techn
74. esolution controller 6 g controller 146 mm travel inch travel USA 510 783 9210 France 33 42 58 68 94 Contact USA 908 771 9490 Japan 81 425 27 8381 2 5 6 Shaped Spot and Cell Projection Systems All of the e beam tools described above focus the beam into a small spot and shapes are formed by rastering the beam This spot is the reduced image of the source often referred to as the gun crossover which has a current intensity profile resembling a Gaussian distribution The time needed to paint a shape can be eliminated by forming the electron beam into primitive shapes rectangles and triangles and then exposing large areas with single shots of the beam The optics of these shaped spot systems is shown schematically in Fig 2 24 The upper aperture typically uses a square to form two sides of a rectangle and the overlap of the lower aperture defines its length and width More complex shapes are fractured into rectangles and triangles before exposure electron a source square aperture 13 x condenser lens iI deflector FIGURE 2 24 Shaped spot optics a The first square aperture defines two sides of a rectangle and the second square aperture defines the other two sides Deflectors second determine the overlap and thus the length and width of the aperture rectangle as shown in b By deflecting over a corner oe triangles can be formed The shaped spot image would be shaped spot further demagnified by magnetic lense
75. f the deflection system some tools introduce dynamic corrections to the deflection focus and stigmators in order to increase the maximum field size at the cost of additional complexity b a N S po Yee Ge ea A FIGURE 2 8 Schematic showing the magnetic electrostatic field distribution for a a simple beam deflector or alignment device energized for diagonal deflection and b a stigmator The optical axis is perpendicular to the plane of the page 2 2 4 3 Beam blanking Blanking or turning the beam on and off is usually accomplished with a pair of plates set up as a simple electrostatic deflector One or both of the plates are connected to a blanking amplifier with a fast response time To turn the beam off a voltage is applied across the plates which sweeps the beam off axis until it is intercepted by a downstream aperture If possible the blanking is arranged to be conjugate so that to first order the beam at the target does not move while the blanking plates are activated Otherwise the beam would leave streaks in the resist as it was blanked The simplest way to ensure conjugate blanking is to arrange the column so that the blanking plates are centered at an intermediate focal point or crossover In very high speed systems more elaborate blanking systems involving multiple sets of plates and delay lines may be required to prevent beam motion during the blanking and unblanking processes 14 2 2 4 4 Stigmators A sti
76. fast coarse grid while edges are written with a finer resolution 60 2 5 5 2 Lepton EBES4 The EBES4 mask writer from Lepton Inc 61 also uses a Gaussian spot with a patterning strategy similar to that of the high resolution machines In this system the coarse fine DAC beam placement is augmented with an extra third deflection stage and the mask plate is moved continuously using the laser stage controller to provide continuous correction to the stage position Unlike the high resolution JEOL machines each stage of deflection has a separate telecentric deflector instead of simply a separate set of DACs for high speed operation Patterns are separated into stripes similar to writing fields 256 m wide see Fig 2 23 These stripes are separated into 32 m subfields cells which are further subdivided into 2 um sub subfields microfigures A spot of 0 125 m diameter fills in the microfigure with a raster pattern STRIPE MA ROFIGURE ag CELL MICAOFIGURE Ouhd X 2 hd MAX PIXEL 0 125 pM FIGURE 2 23 Writing strategy of the Lepton EBES4 mask writing tool pattern data is cut into stripes 256 um wide The stripes are fractured into smaller cells containing macrofigures The macrofigures are split into even smaller microfigures which are finally written as a set of pixels 62 Courtesy of Lepton Inc The entire EBES tool has been designed for high speed with a current of 250 nA delivered in a 0 125 m spot for
77. ferences 1 M Hatzakis Electron resists for microcircuit and mask production J Electrochem Soc 116 1033 1037 1969 2 M G Rosenfield M G R Thomson P J Coane K T Kwietniak J Keller D P Klaus R P Volant C R Blair K S Tremaine T H Newman and F J Hohn Electron beam lithography for advanced device prototyping Process tool metrology J Vac Sci Technol B11 6 2615 2620 1993 3 S A Rishton H Schmid D P Kern H E Luhn T H P Chang G A Sai Halasz M R Wordeman E Ganin and M Polcari Lithography for ultrashort channel silicon field effect transistor circuits J Vac Sci Technol B6 1 140 145 1988 4 C P Umbach C Van Haesendonck R B Laibowitz S Washburn R A Webb Direct observation of ensemble averaging of the Aharonov Bohm effect in normal metal loops Phys Rev Lett 56 386 1986 5 V Chandrasekhar M J Rooks S Wind and D E Prober Observation of Aharonov Bohm Electron Interference Effects with Periods h e and h 2e in Individual Micron Size Normal Metal Rings Phys Rev Lett 55 1610 1613 1985 6 S Washburn R A Webb Aharonov Bohm effect in normal metal quantum coherence and transport Advances in Physics 35 375 1986 7 B J van Wees H van Houten C W J Beenakker J G Williamson L P Kouwenhoven D van der Marel C T Foxon Quantized conductance of point contacts in a two dimensional electron gas Phys Rev
78. gh gt 3M USA 510 783 USA 518 USA 415 244 9210 France 33 535 5900 USA 518 535 7594 415 244 Contact n a 42 58 68 94 Jab n 0425 5900 Japan 81 7612 fax or in Japan 81 425 27 reer 87 425 43 1111 Japan 81 3 8381 5294 2061 2 5 7 SCALPEL Cell projection uses small reticle areas to avoid spherical aberration and to minimize space charge effects A natural extension of the idea would be to separate a large pattern into many small sections etch each section into its own area of the aperture wafer and then select and stitch the patterns together using a set of two deflectors There are a number of limitations to this extension of cell projection 1 20 um of silicon is needed to stop 50 kV electrons 83 so the pattern must include deep holes Because the aspect ratio of these holes is limited lines can be no wider than 2 um therefore the electron optics must demagnify the pattern by a factor of at least 20 to produce linewidths of 0 1 um This limits the area available for cell patterns 2 Multiply connected e g doughnut shaped patterns require complementary stencil masks so the throughput and available pattern area is further reduced 3 Residual stress in the stencil mask will distort the mask in a pattern dependent way and since stencil masks absorb most of the electron energy the changing temperature will also cause similar pattern dependent distortions 84 scatterer mem brane lens FIGURE 2 26 Schematic of
79. gmator is a special type of lens used to compensate for imperfections in the construction and alignment of the EBL column These imperfections can result in astigmatism where the beam focuses in different directions at different lens settings the shape of a nominally round beam becomes oblong with the direction of the principal axis dependent on the focus setting resulting in smeared images in the resist The stigmator cancels out the effect of astigmatism forcing the beam back into its optimum shape Stigmators may be either electrostatic or magnetic and consist of four or more poles eight is typical arranged around the optical axis They can be made by changing the connections to a deflector as shown in Fig 2 8 b With proper mixing of the electrical signals a single deflector may sometimes perform multiple functions including beam deflection stigmation alignment and blanking 2 2 5 Other column components A number of other components may be found in the column which although not important to the electron optics are nonetheless critical to the operation of the system A Faraday cage located below the final beam limiting aperture is used to measure the beam current in order to ensure the correct dose for resist exposure It can be either incorporated directly on the stage or a separate movable assembly in the column The column will also typically have an isolation valve that allows the chamber to be vented for maintenance while the gun is
80. graphy system for sub half micron ultra large scale production the distortion corrector technology J Vac Sci Technol B8 6 1903 1990 72 T Komagata H Takemura N Gotoh K Tanaka Development of EB lithography system for next generation photomasks Proc SPIE 2512 190 1995 73 H C Pfeiffer Projection exposure with variable axis immersion lenses a high throughput electron beam approach to suboptical lithography Jpn J Appl Phys 34 6658 1995 74 Y Someda H Satoh Y Sohda Y Nakayama N Saitou H Itoh M Sasaki Electron beam cell projection lithography Its accuracy and its throughput J Vac Sci Technol B12 6 3399 1994 75 G H Jansen Coulomb interactions in particle beams J Vac Sci Technol B6 1977 1988 76 K Hattori R Yoshikawa H Wada H Kusakabe T Yamaguchi S Magoshi A Miyagaki S Yamasaki T Takigawa M Kanoh S Nishimura H Housai S Hashimoto Electron beam direct writing system EX 8D employing character projection exposure method J Vac Sci Technol B11 6 2346 1993 77 K Sakamoto S Fueki S Yamazaki T Abe K Kobayashi H Nishino T Satoh A Takemoto A Ookura M Oono S Sago Y Oae A Yamada H Yasuda Electron beam block exposure system for a 256 M dynamic random access memory J Vac Sci Technol B11 6 2357 1993 78 A Yamada K Sakamoto S Yamazaki K Kobayashi S Sago M Oono H Watanabe H Yasuda Deflector and corr
81. h the voltage Develop for 4 min in 3 1 MIBK IPA rinse in IPA blow dry in nitrogen Descum important Same as step 2 above for only 5 s If this is a Cr plate etch with Transene Cr etchant 1 5 min If this is a MoSi plate then RIE etch 0 05 Torr total pressure 0 05 W cm 16 sccm SF 4 2 sccm CFa 1 min 10 Plasma clean to remove resist same as step 2 above for 3 min SiO O aaa a 2 7 2 3 PBS Poly butene 1 sulfone is a common high speed positive resist used widely for mask plate patterning For high volume mask plate production the sensitivity of 1 to 2 C cm is a significant advantage over other positive resists However the processing of PBS is difficult and the only advantage is the speed of exposure Plates must be spray developed at a tightly controlled temperature and humidity 144 Contrast is poor with 2 For small to medium scale mask production the time required for plate processing can make PBS slower than some photoresists 145 See Sect 2 7 2 5 EXAMPLE PROCESS PBS POSITIVE MASK PLATE Start with plates spun with PBS 146 2 Expose 25 kV 1 0 to 1 6 C cm Other accelerating voltages may be used The dose will be different 3 Spray develop 101 C humidity 301 in MIAK 5 methyl 2 hexanone 2 pentanone 3 1 147 30 s Rinse in MIAK 2 propanol 3 2 10 C Spin dry under nitrogen Inspect pattern repeat steps 3 and 4 as necessary ae 6 Descum in a barrel etcher 150 W
82. he first is in maskmaking typically the chrome on glass masks used by optical lithography tools It is the preferred technique for masks because of its flexibility in providing rapid turnaround of a finished part described only by a computer CAD file The ability to meet stringent linewidth control and pattern placement specifications on the order of 50 nm each is a remarkable achievement Because optical steppers usually reduce the mask dimensions by 4 or 5 resolution is not critical with minimum mask dimensions currently in the one to two um range The masks that are produced are used mainly for the fabrication of integrated circuits although other applications such as disk drive heads and flat panel displays also make use of such masks An emerging market in the mask industry is 1 masks for x ray lithography These masks typically have features ranging from 0 25 um to less than 0 1 um and will require placement accuracy and linewidth control of 20 nm or better Should x ray technology ever become a mainstream manufacturing technique it will have an explosive effect on EBL tool development since the combination of resolution throughput and accuracy required while technologically achievable are far beyond what any single tool today is capable of providing The second application is direct write for advanced prototyping of integrated circuits 2 and manufacture of small volume specialty products such as gallium arsenide integrated circuits a
83. high aspect ratio and thicknesses accessible with EPTR make it uniquely suited for micromechanical applications Table 2 5 Comparison of commercially available electron beam resists Tone Resolution Sensitivity at 20 kV Developer Contact nm uC cm reference PMMA positive 10 100 MIBK IPA 135 EBR 9 positive 200 10 MIBK IPA 143 PBS positive 250 1 ea ln 147 ZEP positive 10 30 xylene p dioxane 148 AZ5206 positive 250 6 KLK PPD 401 152 COP negative 1000 0 3 MEK ethanol 7 3 147 PA negative 100 8 4 MF312 water 156 2 7 4 Multilayer Systems 2 7 4 1 Low high molecular weight PMMA Multilayer resist systems are useful for several purposes when an enhanced undercut is needed for lifting off metal when rough surface structure requires planarization and when a thin imaging top layer is needed for high resolution Figure 2 28 showed the simplest bilayer technique where a high molecular weight PMMA is spun on top of a low molecular weight PMMA The low weight PMMA is more sensitive than the top layer so the resist develops with an enhanced undercut At high energies gt 20 kV thin PMMA lt 0 5 m will not normally develop an undercut profile the best resist profile will be perpendicular to the substrate The moderate undercut from this technique is useful when liftoff is required from densely packed features The two layer PMMA technique was patented in 1976 by Moreau and Ting 169 and was later improved by Mackie and Bea
84. highly refined In fact simple capstan motors and push rods have been used at IBM 52 53 The stage controller receives a target location from a computer drives the motors to a point close to this location then sends an interrupt back to the computer and corrects the field position by applying an electronic shift This shift is applied continuously in real time to compensate also for stage drift and low frequency vibration In comparison the laser stage built by Raith for SEM conversions applies corrections to relatively slow piezoelectric translators on the stage itself By moving and measuring an alignment mark at various locations in the writing field laser stages are used to calibrate the deflection gain deflection linearity and field distortion that is the stage is used as an absolute reference and the deflection amplifiers are calibrated using the stage controller Other common features of commercial systems include a flat stage a fixed working distance contrasting with a SEM and automated substrate handling A flat stage keeps the sample in focus but requires the use of a detector either on or above the objective pole piece Most commonly a microchannel plate or a set of silicon diodes is mounted on the pole piece The market niche for commercial Gaussian spot high resolution e beam tools has been primarily in research and to a lesser extent for small scale production of MMICs high speed T gate transistors and integrated optics
85. ibsecur libname reflibs fonts i attrtable generations lt FormatType gt UNITS lt structure gt ENDLIB lt FormatType gt FORMAT FORMAT MASK ENDMASKS lt structure gt BNGSTR STRNAME STRCLASS lt element gt ENDSTR lt element gt lt boundary gt lt path gt lt SREF gt lt AREF gt lt text gt lt node gt lt box gt lt property gt ENDEL lt boundary gt BOUNDARY EFLAGS PLEX LAYER DATATYPE XY lt path gt PATH EFLAGS PLEX LAYER DATATYPE PATHTYPE WIDTH BGNEXTN ENDEXTN XY lt SREF gt SREF EFLAGS PLEX SNAME lt strans gt XY lt AREF gt AREF EFLAGS PLEX SNAME lt strans gt COLROW XY lt text gt t TEXT EFLAGS PLEX LAYER lt textbody gt lt node gt NODE EFLAGS PLEX LAYER NODETYPE XY lt box gt BOX EFLAGS PLEX LAYER BOXTYPE XY lt textbody gt TEXTTYPE PRESENTATION PATHTYPE WIDTH lt strans gt XY STRING lt strans gt STRANS MAG ANGLE lt property gt PROPATTR PROPVALUE 2 9 6 Example GDSII Stream file The following is a dump of a minimal GDSII Stream file consisting of just one polygon boundary The GDSII file was created with the program DW2000 from Design Workshop The binary dump was created on a VAX with the VMS command DUMP The hex numbers are read backwards from right to left with each pair of digits representing a byte Reading the first line below we see that the file b
86. ic aberration Beam diameter at 1kV is approximately 5nm Courtesy of LLS Jena GmbH Table 2 2 Comparison of Gaussian spot vector scan systems All of these systems are equipped with thermally assisted Schottky field emission electron sources Leica Lithography Leica Lithographie Jeni Ine Systems Ltd Systeme Jena GmbH Model JBX 6000FS Vectorbeam LION LV1 Resolution minimum spot 5 nm 8 nm 5 nm size Alignment automated automated Automated Stitching automated automated Automated scaleable 16 bits in up to Field size maximum 80 or 800umatS0 800 umat50kVor400 scaleable 16 bits um field at 100 kV Energy 25 50 100 kV 10 to 100 kV 1 to 20 kV Speed of highest of class gt 0 04 us mid range gt 0 4 us per pattern a E pEr EXpOSUrS per exposure point 25 exposure point 2 6 generation p MHz MHz Stage laser controlled 0 6 nm 6 inch laser controlled 0 6 nm 6 laser controlled 2 5 nm travel inch travel 162 mm travel Control computer VAX VMS VAX VMS PC compatible Cost Expensive gt 3M Expensive gt 3M Expensive gt 1M USA 518 535 5900 Japan USA 708 405 0213 USA 708 405 0213 Contact 0425 42 2187 1 2 Musashino 0147 fax UK 44 1223 0147 fax UK 44 1223 3 chome Akishima Tokyo 196 411 123 211 fax 411 123 211 fax 2 5 5 Gaussian Spot Mask Writers While both of these systems are promoted for mask making their basic technologies could be adapted for direct writing on wafers Howev
87. inum or germanium 175 177 forming a so called trilayer resist After the top layer is exposed and developed the pattern is transferred to the interlayer by RIE in CF or by Cl in the case of aluminum The interlayer serves as an excellent mask for RIE in oxygen The straight etch profile available from oxygen RIE allows the fabrication of densely packed high aspect ratio resist profiles Such resist profiles can then be used for liftoff or for further etching into the substrate FIGURE 2 30 a Resist cross section PMMA on P MMA MAA on PMMA for the lift off of a T shaped gate b Metal gate lifted off on GaAs Courtesy of R C Tiberio et al 180 If we start with Hatzakis s bilayer scheme PMMA on the bottom and copolymer on the top and then add another top layer of PMMA we have a structure that can be developed into a mushroom shape 180 as shown in Fig 2 30 In this technique a heavy dose is given to the central line and a lighter dose to the sides Mutually exclusive developers are used to form the T gate shape and a thick layer of metal is lifted off This technique is widely used to form MESFET gates with low capacitance and low leakage from the small contact area and low resistance from the large metal cross section 2 7 5 Inorganic and Contamination Resists Some of the first high resolution e beam exposures were made with contamination lithography by simply using the electron beam to crack contaminants so
88. ion of the plates temperature monitoring and stabilization is far more elaborate Like other manufacturers JEOL creates a map of stage nonlinearity by measuring a set of marks turning the plate in 90 increments and measuring the set again The resulting stage distortion map is used to reduce the runout due to imperfections in the stage mirror In fact each individual plate holder has its own specific distortion table which is identified automatically by reading a bar code on the cassette 2 5 6 4 Cell Projection The throughput of shaped beam tools is primarily limited by the average beam current in the spot and by the pattern density The average beam current for cell projection is modestly larger than for variable shaped beams Both are limited by Coulomb interaction to a few microamperes However by replacing the simple beam shaping aperture with a more complex pattern a cell projection system can greatly increase the pattern density without sacrificing throughput In cell projection systems the upper deflector steers the beam into one of a number of hole patterns or cells The shaped beam is deflected back to the center of the column and is demagnified by another lens forming an image on the substrate The shaping aperture is made of a silicon membrane around 20 m thick patterned with holes and coated with gold or platinum To maintain small aberrations and high resolution the cell is demagnified by a factor of 20 to 100 and the fi
89. ions of the fast and slow electronics but sacrifice some speed The largest distinction of these commercial Gaussian spot systems and in fact all commercial e beam systems is the use of high precision laser controlled stages Stage controllers from Hewlett Packard or Zygo use the Zeeman effect to split the line of a He Ne laser The split frequency laser beam is reflected off a mirror attached to the stage and the beat frequency from the two lines is measured by high speed electronics When the stage moves the beat frequency shifts according to the Doppler effect and the stage position is calculated by integrating the beat counts While often referred to as interferometers these stages actually have more in common with radar speed guns Analysis of multiple points on the stage mirror allows the measurement of X Y and rotation about Z yaw Stage precision is often given in terms of a fraction of the laser s wavelength a precision of 128 5 nm is commonly used in commercial systems and the best stages now use 1024 0 6 nm Even though the controller reports the stage location to this precision the accuracy of the stage is limited by unmeasured rotations about the X and Y axes and by bow in the mirrors These nonlinearities called runout limit the absolute placement accuracy to the order of 0 1 um over 5 cm of stage travel The high precision in reading the stage position means that the stage motors and drive do not have to be
90. ique for proximity correction GHOST 40 has the advantage of not requiring any computation at all The inverse tone of the pattern is written with a defocused beam designed to mimic the shape of the backscatter distribution Fig 2 14 The dose of the GHOST pattern ee 1 ee is also set to match the large area backscatter dose After the defocussed inverse image is written the pattern will have a roughly uniform background dose GHOST is perhaps an underutilized technique under ideal conditions it can give superb linewidth control 41 Its disadvantages are the extra data preparation and writing time a slight to moderate loss of contrast in the resist image and a slight loss in minimum resolution compared to dose modulation due to the fact that GHOST does not properly correct for forward scattering 2 4 3 4 Software A number of companies for some time have had proprietary software for proximity correction 25 42 43 Just recently commercial proximity packages have become available or are about to become available 44 45 At present these are limited in their accuracy soeed and data volume capability while excellent for correcting small research patterns they may have difficulties with complex chips Finally several packages have been developed at university and government laboratories some of which might be available to an adventurous user with excessive amounts of free time 38 46 primary pattern GHOST pattern
91. is technologically ahead of optical mask writers this may not continue in the future However EBL will continue to provide a resolution advantage over the optical mask writers which may be important for advanced masks using phase shift or optical proximity correction For 1 mask fabrication i e x ray EBL will continue to be the most attractive option 3 0 07 Microns Jf Spacer d FIGURE 2 3 Micrograph of a portion of an integrated circuit fabricated by electron beam lithography The minimum dimensions are less than 0 1 um Courtesy of S Rishton and E Ganin IBM Optical lithography using lenses that reduce a mask image onto a target much like an enlarger in photography is the technique used almost exclusively for all semiconductor integrated circuit manufacturing Currently the minimum feature sizes that are printed in production are a few tenths of a micrometer For volume production optical lithography is much cheaper than EBL primarily because of the high throughput of the optical tools However if just a few samples are being made the mask cost a few thousand dollars becomes excessive and the use of EBL is justified Today optical tools can print 0 25 um features in development laboratories and 0 18 um should be possible within a few years By using tricks optical lithography can be extended to 0 1 um or even smaller Some possible tricks include overexposing overdeveloping phase shift and phase edge masks and edge sh
92. is used to generate the X and Y beam deflections and to program a second board which provides the signals for blanking control The beam is deflected from shape to shape in a writing field vector scan mode with the unique feature that the raster for filling arbitrary polygons can be defined by the user Arbitrary polygons can be designed with up to 200 vertices and the user can specify the raster to be parallel to any side of the polygon A unique feature of the NPGS is that the user has control over the exposure spot spacing in X and Y allowing the critical dimension e g perpendicular to grating lines to be filled with greater accuracy see Sect 2 5 2 Circles and circular arcs are swept using a polar coordinate approach with user control of the exposure spot spacing in rand As with any ISA system the data throughput is limited to around 100 kHz and like most pattern generators exposure points filling the features can be spaced by multiples of the DAC resolution 2 while still allowing full resolution for feature placement To provide for lower doses at reasonable currents the Nabity system strobes the blanker at each exposure point 50 For systems without a beam blanker the Nabity Pattern Generation System NPGS can be programmed to dump the beam at user defined locations within the writing field however this imposes significant limitations on the exposure spot spacing or on the lowest deliverable dose for a given beam curren
93. istribution measurements for proximity correction in electron beam lithography on a sub 100 nm scale J Vac Sci Technol B5 1 135 141 1987 29 M Rosenfield S Rishton D Kern and D Seeger A study of proximity effects at high electron beam voltages for x ray mask fabrication 1 Additive mask processes J Vac Sci Technol B8 6 1763 1770 1990 30 E Kratschmer Verification of a proximity effect correction program in electron beam lithography J Vac Sci Technol 19 4 1264 1268 1981 31 K K Christenson R G Viswanathan and F J Hohn X ray mask fogging by electrons backscattered beneath the membrane J Vac Sci Technol B8 6 1618 1623 1990 32 Y Yau R F W Pease A lranmanesh and K Polasko Generation and applications of finely focused beams of low energy electrons J Vac Sci Technol 19 4 1048 1981 33 M A McCord and T H Newman Low voltage high resolution studies of electron beam resist exposure and proximity effect J Vac Sci Technol B10 6 3083 3087 1992 34 M Parikh Self consistent proximity effect correction technique for resist exposure GPECTRE J Vac Sci Technol 15 3 931 933 1978 35 H Eisenmann T Waas and H Hartmann PROXECCO Proximity effect correction by convolution J Vac Sci Technol B11 6 2741 2745 1993 36 K Harafuji A Misaka K Kawakita N Nomura H Hamaguchi and M Kawamoto Proximity effect correction
94. l column with large field for nanometer e beam lithography system Proc SPIE 2437 185 1995 55 Jenoptik Technologie GmbH Microfabrication Division D 07739 Jena Germany 49 3641 653181 voice 49 3641 653654 fax The electron beam lithography division of Jenoptik has recently been acquired by Leica Lid Cambridge UK to form Leica Lithographie Systeme Jena GmbH USA 708 405 0213 0147 fax UK 44 223 411 411 211 310 fax 56 J Ingino G Owen C N Berglund R Browning R F W Pease Workpiece charging in electron beam lithography J Vac Sci Technol B12 3 1367 1994 57 M Gesley F Abboud D Colby F Raymond S Watson Electron beam column developments for submicron and nanolithography Jpn J Appl Phys 32 5993 1993 58 M Gesley MEBES IV thermal field emission tandem optics for electron beam lithography J Vac Sci Technol B9 6 2949 1991 59 H Pearce Percy R Prior F Abboud A Benveniste L Gasiorek M Lubin F Raymond Dynamic corrections in MEBES 4500 J Vac Sci Technol B12 6 3393 1994 60 A Murray F Abboud F Raymond C N Berglund Feasibility study of new graybeam writing strategies for raster scan mask generation J Vac Sci Technol B11 6 2390 1993 61 Lepton Inc Murray Hill NJ 07974 908 771 9490 62 D M Walker D C Fowlis S M Kugelmass K A Murray C M Rose Advanced mask and reticle generation using EBES4 Proc SPIE 2322 56
95. lectron beam A beam with a wide energy spread which is undesirable as will be shown in the section on lenses is similar to white light while a beam with a narrow energy spread would be comparable to monochromatic light Although the energy spread of the source is important space charge interactions between electrons further increase the energy spread of the beam as it moves down the column Boersch effect 16 An electron source is usually combined with two or more electrodes to control the emission properties as shown in Fig 2 5 17 tt Si H gt lt TE 4 wie aie oes HInDi b a ZZA i ONERE BETS Se eR aj A SR DAA D FIGURE 2 4 Cross section drawing of a typical electron beam column along with a raytrace of the electrons as they pass through the various electron optical components Courtesy of Leica Lithography Systems Ltd Table 2 1 summarizes the properties of common sources For many years the standard thermionic electron source for lithography optics was a loop of tungsten wire heated white hot by passing a current it Tungsten was chosen for its ability to withstand high temperatures without melting or evaporating Unfortunately this source was not very bright and also had a large energy spread caused by the very high operating temperature 2700 K More recently lanthanum hexaboride has become the cathode of choice due to a very low work function a high brightness is obtained at an opera
96. lity The unusual feature of this simple system is its support for exposure simulation and semiautomatic proximity effect correction Pattern data can be generated with the simple CAD program included or imported from a DXF AutoCAD file The higher end Raith system known as Elphy Plus supports the full range of e beam operations including control of a laser controlled stage and corrections for workpiece rotation gain and orthogonality The laser stage also manufactured by Raith allows field stitching to better than 0 1 m While the primary control is still a PC compatible computer the limitations of the ISA bus are circumvented by using a Separate computer and integrated DAC as the pattern generator In this way the PC transmits only the coordinates of the corners of a shape and the patterning hardware generates all of the internal points for exposure Data throughput is thereby increased to 2 6 MHz 0 4 s point minimum however many SEM deflection systems will be limited to less than 1 MHz due to the inductance of the coils and low pass filters in the imaging system The Elphy Plus system supports fully automated mark detection and field stitching All standard e beam data formats are supported Useful features of the Raith Elphy Plus system include support of data representation in polar coordinates greatly reducing the data required to represent circles bit mapped pattern exposure and a path writing mode In the path writing mode
97. lso work with the OCT database but are distributed as is and without support While these programs are distributed for only a shipping fee 109 the real cost is the time and expertise required for installation and for working around bugs Magic and VEM generate patterns in CIF format which is supported by some mask vendors or may be translated to GDSIl 2 6 5 Intermediate Formats 2 6 5 1 GDSII GDSIl also known as Calma Stream was originally developed by the Calma division of General Electric Rights to the Calma products have changed hands several times and are now owned by Cadence Design Systems GDSII is by far the most stable comprehensive and widely used format for lithography GDSII is a binary format that supports a hierarchical library of structures called cells Cells may contain a number of objects including Boundary which may be used to represent polygons or rectangles Box which may be used to represent rotated rectangles Path which may be used to represent wires Text for annotation either on the CAD screen or the device Sref to include an instance of one structure cell inside another and Aref similar to Sref but providing an array instance of a cell There are 64 available Layers numbered 0 to 63 Each primitive object Boundary etc lies on one of these layers Each layer number typically represents one mask or electron beam exposure step in a process A specification of GDSII format appears i
98. m In this example the pixel spacing is arger than the spot size and the exposed features may develop as a lumpy set of connected dots The problem will be even more pronounced when using high speed resists large field sizes and larger currents One solution would be to implement a faster pattern generator however JEOL s approach is to retain the superior noise immunity of the 12 MHz deflector and instead to use less current when necessary or to increase the spot size by using a larger aperture Alternatively one can purposely defocus the beam The NPGS system see Sect 2 5 3 1 attacks the problem by allowing different pixel spacings in X and Y or in r and It is interesting to note that future high resolution systems under development at Hitachi 54 are likely to resemble the JEOL Gaussian spot tools with field sizes gt 500 um and a single stage electrostatic deflector Small fields avoid the complexities of dynamic focus and astigmatism corrections and allow the short working distance needed to reduce the spot size Single stage deflectors limit the bandwidth speed of the system but improve intrafield stitching between deflections of coarse and fine DACs The design tradeoff is clearly between high speed and high accuracy 2 5 4 2 Leica Lithography Systems Electron beam systems from Leica Lithography Systems Ltd LLS are a combination of products previously manufactured by Cambridge Instruments the electron beam lithography divi
99. magnified cell size is kept below 10 m to reduce aberrations 76 Space charge effects also reduce the feature edge sharpness but these can be compensated by using a current dependent dynamic refocusing of the image 75 80 82 Cell projection has not yet achieved the throughput of optical steppers but as a transitional technology may provide the resolution needed for near term 256 Mbyte DRAM production Table 2 4 Comparison of shaped spot systems Leica IBM Corp Etec Systems Inc JEOL Inc Lithographie Hitachi Inc Systeme GmbH Model EL 4 Excaliber under JBX ZBA 31 32 HL 800D Cell development 7000MVII WePrint 200 Projection _ 0 15 um features 0 2 30nm CD 0 25 um 50 nm pesoluuan 50 nm CD control oleum as oes control CD control Alignment automated automated automated automated automated Field 10 mm maximum 1 mm 1 5mm 1 3 mm Energy 75 kV 100 kV 20 kV 20 kV 50 kV Speed 2 3 wafers hour 100 kV 20 kV 20 kV 50 kV up to 7 inch Samples 8 inch 8 inch plate 8 inch 8 inch wayless stage wayless stage electrostatic electrostatic sel controlled oer a laser conventional clamping sliding clamping sliding laser controlled controlled stage cassette Stage chuck servo chuck servo conventional conventional _ to cassette powered laser powered laser stage stage automated control with yaw control with yaw loadi oading compensation compensation system under Cost not for sale development high gt 3M high gt 3M hi
100. n films with lower defect density have been fabricated with Langmuir Blodgett techniques 192 Such thin imaging layers are important for low voltage 193 exposures and in situ processing However the imaging layer must be transferred into an intermediate film which is subsequently used as the etch or liftoff mask This process adds substantially to the cost and complexity of processing An alternative approach to generating a thin imaging layer on top of a thick resist is the use of surface silylation In the PRIME silylation process 193 196 electron beam exposure prevents the subsequent silylation of attachment of silicon containing molecules to the resist surface The silylated regions act as a mask for oxygen plasma etching of the resist film 2 8 Acknowledgements The authors would like to thank the many people who helped in the editing proofreading and checking of this chapter primarily the SPIE reviewer and copy editor and including also Sylvia Chanak Cadence Dennis Costello Cornell Mark Gesley Etec George Lanzarotta Raith Alex Liddle Lucent Francois Marquis Design Workshop Beth Moseley Hitachi Joseph Nabity Nabity Lithography Systems Yasutoshi Nakagawa JEOL Hans Pfeiffer IBM Rainer Plontke Leica Jena John Poreda Lepton and Bernard Wallman Leica 2 9 Appendix GDSII Stream Format Portions of the GDSII Stream Format Manual Documentation No B97E060 Feb 1987 reprinted with permission of Cadence
101. n the appendix to this chapter portions of which are reprinted by permission of Cadence Design Systems 2 6 5 2 CIF The Caltech Intermediate Format or CIF 2 0 is specified officially in A Guide to LSI Implementation Second Edition by R W Hon and C H Sequin 110 and a nearly identical description appears in Introduction to VLSI Systems by C Mead and L Conway 111 This format is far simpler than GDSII and has the advantage that it is readable using only ASCII characters While providing nearly all of the functionality of GDSII there are a few differences e Names of cells are not supported Instead cells are numbered e Datatypes are not supported These are commonly used to assign different doses within a pattern Therefore proximity effect correction requires patterns to be split into multiple layers e There is no limit on the number of vertices in a polygon therefore CIF interpreters either set arbitrary values or simply run out of memory e The array structure a square array of cells of n m elements is not supported and so the users of CIF have invented extensions to the format These extensions have not been added to the CIF standard CIF is widely used by universities using the Berkeley CAD tools to design circuits for the MOSIS integrated circuit foundry service 112 MOSIS requires a number of sensible restrictions on CIF data 113 Polygons P must have at least three points other than this arbitrary polygo
102. nal cell size on the wafer is only 2 to 10 m The wafer containing these patterns also contains a simple rectangular aperture for general purpose pattern generation in a standard shaped spot mode While a number of cell patterns may be placed on the beam shaping wafer it is clear that the cell projection technique is advantageous and economical only for highly repetitive designs with small unit cells namely memory chips Patterns for cell projection will require proximity correction by shape modification 38 39 or through a variation of the GHOST technique 40 see Sect 2 4 3 To achieve throughput comparable to that of optical steppers cell projection tools must reduce the shot count by a factor of around 100 Current machines have achieved shot reductions on the order of a factor of 10 and have throughputs of less than 10 wafer levels hour for a 6 in wafer populated with 256 Mbyte DRAMs 10 shots chip IBM 73 Hitachi 74 75 Toshiba 76 Fujitsu 77 78 and Leica have developed cell projection tools targeted for 256 Mbyte DRAM manufacture Leica s WePrint 200 instrument is a modified version of the ZBA 32 Hitachi also offers a cell projection shaped spot system for sale the HL 800D Common features of cell projection systems include continuous stage motion 79 and resolution around 0 2 um Hitachi s HL 800D reduces the cell reticle by a factor of 25 while Fujitsu uses a factor of 100 and Toshiba uses a factor of 40 The final de
103. nd optical waveguides Here both the flexibility and the resolution of electron beam lithography are used to make devices that are perhaps one or two generations ahead of mainstream optical lithography techniques Finally EBL is used for research into the scaling limits of integrated circuits Fig 2 3 3 and studies of quantum effects and other novel physics phenomena at very small dimensions Here the resolution of EBL makes it the tool of choice A typical application is the study of the Aharanov Bohm effect 4 6 where electrons traveling along two different paths about a micrometer in length can interfere constructively or destructively depending on the strength of an applied magnetic field Other applications include devices to study ballistic electron effects quantization of electron energy levels in very small structures 7 8 and single electron transistors To see these effects typically requires minimum feature sizes of 100 nm or less as well as operation at cryogenic temperatures FIGURE 2 2 A commercial electron beam lithography tool courtesy of JEOL Ltd 2 1 3 Alternative Techniques It is prudent to consider possible alternatives before committing to EBL technology For chrome on glass optical mask fabrication there are optical mask writers available that are based either on optical reduction of rectangular shapes formed by framing blades or by multiple individually controlled round laser beams Although at present EBL
104. nning a beam of electrons across a surface covered with a resist film sensitive to those electrons thus depositing energy in the desired pattern in the resist film The process of forming the beam of electrons and scanning it across a surface is very similar to what happens inside the everyday television or CRT display but EBL typically has three orders of magnitude better resolution The main attributes of the technology are 1 it is capable of very high resolution almost to the atomic level 2 it is a flexible technique that can work with a variety of materials and an almost infinite number of patterns 3 it is slow being one or more orders of magnitude slower than optical lithography and 4 it is expensive and complicated electron beam lithography tools can cost many millions of dollars and require frequent service to stay properly maintained The first electron beam lithography machines based on the scanning electron microscope SEM were developed in the late 1960s Shortly thereafter came the discovery that the common polymer PMMA polymethyl methacrylate made an excellent electron beam resist 1 It is remarkable that even today despite sweeping technological advances extensive development of commercial EBL and a myriad of positive and negative tone resists much work continues to be done with PMMA resist on converted SEMs Fig 2 1 shows a block diagram of a typical electron beam lithography tool The column is responsible for forming
105. ns are accepted Wires W must have at least one point Round Flashes R must have a non zero diameter The delete definition DD command is not allowed Symbols cells may not be redefined Lines are limited to 509 characters of text The following ASCII characters should not be used as blanks square brackets single quotes and periods e User extensions are allowed but ignored Wires are extended beyond the two extreme endpoints by half the wire width e The comment layer has a name ending with the letter X All geometry on this layer is read by MOSIS but is totally ignored however any syntax error in this layer may cause the CIF file to be rejected e The bonding pad layer is named XP in all technologies 2 6 5 3 DXF DXF format is produced by the program AutoCAD as well as by a number of other inexpensive CAD programs for Windows DOS and the Macintosh These programs were not designed for lithography and so contain structures e g three dimensional figures that have no meaning in this area Also the common jargon e g cell has been replaced with less familiar terminology e g block Like CIF this format does not support datatype numbers DXF is useful only after it has been translated into GDSII by a program such as that sold by Artwork Conversion Software 108 or those of various mask vendors In DXF there can be considerable confusion over such issues as whether an enclosed line represents
106. ns middle and 10 means bottom Bits 0 through 9 are reserved for future use and must be cleared If this record is omitted then top left justification and font 0 are assumed ASCII String contains a character string for text presentation up to 512 characters long Bit array contains two bytes of bit flags for SREF AFREF and text transformation Bit 0 leftmost specifies reflecton If it is set then reflection about the X axis is applied before angular rotation For AREFs the entire array lattice is reflected with the individual array elements riidly attached Bit 13 flags absolute magnification Bit 14 flags absolute angle Bit 15 rightmost and all remaining bits are reserved for future use and must be cleared If this record is omitted then the element is assumed to have no reflection and its magnification and angle are assumed to be non absolute Eight byte real contains a magnification factor If omitted a magnification of 1 is assumed Eight byte real contains the angular rotation factor measured in degrees counterclockwise For an AREF the angle rotates the entire array lattice with the individual array elements regidly attached about the array reference point If this record is omitted and algle of zero degrees is assumed ASCII string contains the names of the reference libraries This record must be 1F06 FONTS 2006 PATHTYPE 2102 GENERATIONS 2202 ATTRTABLE 2306 EFLAGS 2601 NODET
107. nses are far from perfect Spherical aberrations result from the tendency of the outer zones of the lenses to focus more strongly than the center of the lens The resultant diameter is ds 1 2C a where C is the spherical aberration coefficient of the final lens and a is the convergence half angle of the beam at the target Using an aperture to limit the convergence angle thus reduces this effect at the expense of reduced beam current Chromatic aberrations result from lower energy electrons being focused more strongly than higher energy electrons For a chromatically limited beam the diameter is d C aDV V where C is the chromatic aberration coefficient DV is the energy spread of the electrons and V is the beam voltage Finally quantum mechanics gives the electron a wavelength L 1 2 Veer nm although much smaller than the wavelength of light 0 008 nm at 25 kV this wavelength can still limit the beam diameter by classical diffraction effects in very high resolution systems For a diffraction limited beam the diameter is given by dy 0 6 L a To determine the theoretical beam size ofa L the contributions from various sources can be added in quadrature d da df df df 100 sphencal aberration Vb 30 kV ie ahar chromate aberration AV 1 5 eV source sine limsat C 60 mm diffraction beam diameter Co 40mm dy 20 nm uo beam diameter nm 1 10 beam convergence angle milliradians FIGURE 2 9 A
108. oluble in nonpolar solvents such as chlorobenzene A developer such as ethoxyethanol iso propanol is used on the top imaging layer stopping at the PMMA Next a strong solvent such as chlorobenzene or toluene is used on the bottom layer This technique has been used to fabricate 1 um memory arrays with thick gate metalizations A more common use of P MMA MAA is as the bottom layer with PMMA on top In this case the higher speed of the copolymer is traded for the higher resolution of PMMA 175 For simplicity a single developer is used the nonpolar solvent working on the PMMA and the polar solvent developing the copolymer Effective developer combinations include ethylene glycol monoethyl ether methanol 3 7 and MIBK IPA 1 1 The undercut of this process is so large that it can be used to form free standing bridges of PMMA a technique developed by Dolan 176 and used extensively for the fabrication of very small superconducting tunnel junctions Other shadowing and step edge techniques for fabricating small lines and junctions are covered in the chapter by Howard and Prober 177 The polymer PMGI polydimethylglutarimide is used for the same purpose as P MAA MAA 178 179 2 7 4 3 Trilayer systems Bilayer techniques using P MMA MAA or PMGI work well because the polar nonpolar combination avoids intermixing of the layers Almost any two polymers can be combined in a multilayer if they are separated by a barrier such as Ti SiOz alum
109. on services is provided in the U S by the National Nanofabrication Users Network NNUN based primarily at Cornell and Stanford universities Services provided through this network and the list of other sites changes so often that it is more appropriate to refer the reader to the World Wide Web page http www cnf cornell edu which provides information about the services of the NNUN and other nonaffiliated U S fabrication centers A list of mask vendors can be found in the Semiconductor International Buyer s Guide 2005 Note The National Nanofabrication Users Network is now the National Nanotechnology Infrastructure Network NNIN More information on this thirteen member network can be found at Www nnin org 2 6 Data Preparation 2 6 1 Pattern Structure Preparation of pattern data for electron beam lithography may begin with a high level symbolic or mathematical description of a circuit with the algorithmic description of a pattern e g a Fresnel lens or with a simple geometric layout A computer aided design CAD program is usually used to lay out or at least inspect the pattern and to generate output in a standard exchange format A separate program is then used to convert the intermediate format to machine specific form This last step can be quite involved since in most cases all hierarchy must by removed flattened polygons must be reduced to primitive shapes e g trapezoids or triangles and rectangles and the pattern must
110. one Since the mantssa is shifted four bits at a time it is possible for the left three bits of a normalized mantissa to be zero A zero value is represented by a number with all bits zero The representation of negative numbers is the same as that of positive numbers except that the highest order bit is 1 not 0 2 9 4 Record types Records are always an even number of bytes long If a character string is an odd number of bytes long it is padded with a null character The following is a list of record types The first two numbers in brackets are the record type and the last two numbers in brackets are the data type see the table above Note that the data type e g two byte signed integer refers to the type of data to follow in the record not to the number of bytes in the record The first two bytes of the record header contain a count in eight bit bytes of the total record length The third byte of the header is the record type also known as a token shown below and the fourth byte is the data type All record numbers are shown in hexidecimal For example in the HEADER record 00 is the token and 02 is the data type HEADER 0002 BGNLIB 0102 LIBNAME 0206 UNITS 0305 ENDLIB 0400 BGNSTR 0502 STRNAME 0606 ENDSTR 0700 BOUNDARY 0800 PATH 0900 SREF 0A00 AREF 0B00 TEXT 0C00 LAYER 0D02 Two byte signed integer contains data representing the GDSII version number
111. or e Like CIF datatypes are not supported and so features with different doses must appear in different layers e A hierarchy of cells is not supported The pattern must be flat and so may use a great deal of disk space 2 6 6 Low Level Formats Conversion from one of the above formats to a machine specific format usually involves flattening the hierarchy of cells fracturing polygons into primitive shapes and splitting the pattern into fields and subfields The resulting machine specific formats e g MEBES JEOL51 and BPD usually use far more disk space than the hierarchical forms These files must be carefully checked for software errors and may require manipulation for sizing tone reversal mirroring and so on One way of verifying a conversion is simply to convert the low level format back to GDSII so that it can be displayed with the original CAD tool Unfortunately the pattern would have lost its cell structure so the data set may be too large for the graphical editor A special class of display and manipulation software is required that can handle very large flat data sets The CATS program from Transcription Enterprises 114 and CAPROX from Sigma C 115 offer not only viewing and manipulation of machine formats but also will fracture GDSII directly into these formats These conversion programs support machine formats from Etec Systems MEBES AEBLE Hitachi JEOL Leica GCA and others Operations include Boolean functions
112. pically do not include the faster 12 bit DAC c The primitive shape is filled in by rastering the spot Between shapes the beam is turned off blanked and is deflected in a direct vector to the next shape An alternative deflection strategy not shown is to use the major DAC to deflect the beam to the origin of each primitive shape The bus limits the deflection speed to around 100 kHz that is to a dwell time per point of 10 us What dwell time is required With a 16 bit DAC and a SEM viewing field of 100 um the size of a pixel the smallest logically addressable element of an exposure field is 100 um 2 1 5 nm and its area A is the square of this The charge delivered to this pixel in a time t is It where J is the beam current This must equal the dose times the pixel area Given a beam current I on the order of 50 pA and a required dose D around 200 uC cm typical for PMMA we have a pixel dwell time t DA I 910 s 2 1 or a deflection speed of 11 MHz This being impossible with an ISA bus we must either space out the exposure points apply a short strobe to the beam blanker or use a combination of the two When the exposure points are spaced every n pixels that is when the 2 available exposure points are reduced by a factor of n then the pixel area and thus the dwell time is increased by a factor of n Note that the placement of features can still be specified to a precision of 2 within the writing field while
113. piece can use marks which are separate and larger than those used for chip alignment Large global alignment marks are useful for the exposure of full wafers since the machine can be programmed to search for the first mark Typical marks used for global alignment are large crosses of width 2 to 6 um and length 100 to 200 um placed at the top bottom left and right sides of the wafer as illustrated in Fig 2 27 Alternatively a few of the marks used for chip alignment could also be used for global alignment this would allow global alignment on small pieces of a wafer Alignment to chip marks is especially useful as a diagnostic of the maskmaking tool allowing the measurement of displacements as a function of chip location For large patterns that take a long time to write it may improve registration and placement accuracy if the machine stops periodically every 5 to 10 minutes is typical to reregister to the alignment marks This corrects for thermal or other drifts that can occur during the writing process For single level processes or maskmaking reregistering to a single mark is sufficient to correct for drift rrrrrr rrrrrr rrerererre oe rPreeeeree FIGURE 2 27 Alignment marks used for electron beam lithography Marks are typically etched pits in the wafer or high Z metal such as gold platinum or tungsten Global marks 1 are used to correct for the overall shift rotation and gain of the wafer and chip m
114. posed areas swelling during development and bridging between features A reasonable starting point for developing a negative resist process is to choose a development time twice as long as the time needed to clear the unexposed resist and an exposure dose just sufficient to ensure acceptable resist thickness loss on all features e g no more than 10 From there fine tuning of development time dose and postexposure bake conditions may be needed to optimize feature sizes improve critical dimension control and minimize resist scum 2 7 3 1 COP COP is an epoxy copolymer of glycidyl methacrylate and ethyl acrylate P GMA co EA commonly used for negative exposure of mask plates 147 122 This is a very high speed resist 0 3 C cm at 10 kV with relatively poor resolution 1 um 158 COP also has relatively poor plasma etch resistance and requires spray development to avoid swelling Because cross linking occurs by cationic initiation and chain reaction the cross linking continues after exposure Therefore the size of features depends on the time between exposure and development Unless speed is very critical COP is probably not a good choice for a negative resist EXAMPLE PROCESS COP NEGATIVE MASK PLATE 1 Soak mask plate in acetone gt 10 min to remove photoresist Rinse in isopropanol blow dry 2 Clean the plate with RIE in oxygen Do not use a barrel etcher RIE conditions 30 sccm O 30 mTorr total pressure 90 W 0 25
115. rbed onto the substrate These carbonaceous and silicaceous contaminants are produced from oil in the vacuum pumps or from organic residue on the sample surface By using the contamination as a mask for ion milling wires as narrow as 50 nm were made in the 1960s 181 Later the technique was used for fabricating nanometer scale superconducting devices 182 and metal lines for the study of electron transport in mesoscopic devices 183 The dose required for the deposition of contamination depends on how much oil and other contaminants are in the vacuum system an untrapped diffusion pump provides an ample supply but the dose is very high typically in the range of 0 1 to 1 C cm The high dose limits its application to very sparse patterns Cracked hydrocarbons provide poor selectivity for etching or milling so the choice of metals is also limited for instance it is not practical to pattern aluminum this way The contamination can be easily cleaned by heating the substrate to 100 C Another technique for producing nanometer scale patterns again using doses on the order of 1 C cm is the use of metal fluorides A high current density of electrons causes the dissociation of materials such as AIF3 MgF2 NaCl LiF KCI and CaF 184 at doses around 10 to 20 C cm At lower doses 1 to 3 C cm AIF acts as a negative resist developed in water 185 One reason for the very high resolution is that these materials are modified by the prim
116. rce size energy spread vacuum A cm sr eV requirement Torr tungsten 108 25 um 2 3 10 thermionic LaBg 10 10 um 2 3 10 thermal Schottky 108 20 nm 0 9 10 field emitter cold Sane 10 field emitter A anm a 10 A technology that is now available to EBL as well as in many electron microscopes is the thermal field emission source It combines the sharp tungsten needle of the field emission source and the heating of the thermal source Because the tip operates at a temperature of about 1800 K it is less sensitive to gases in the environment and can achieve stable operation for months at a time Although thermal field emitter is the common name it is more properly called a Schottky emitter since the electrons escape over the work function barrier by thermal excitation It features a brightness almost as high as the cold field emission sources a very small virtual source size and a moderate energy spread The tungsten is usually coated with a layer of zirconium oxide to reduce the work function barrier A heated reservoir of zirconium oxide in the electron gun continuously replenishes material evaporated from the tip It requires a vacuum in the range of 10 Torr which although much better than required for the thermionic sources is readily achievable with modern vacuum technology A light bakeout might be required to remove water vapor after the system has been vented LaB sources are still preferred for shaped beam sys
117. rn definition Design Workshop runs under the Macintosh OS UNIX and Windows NT The Tanner Research tools run on PC compatibles Macintoshes and several UNIX workstations output is in CIF or GDSII Both Design Workshop and Tanner Research have implemented a less extensive set of companion tools rule checkers routers simulators etc and concentrate on the core graphical editors Inexpensive graphical editors include AutoCAD and other general purpose CAD tools for PC compatibles and the Macintosh AutoCAD and other similar programs generate DXF format which must be converted to GDSII with a separate program 108 AutoCAD has the disadvantage that it was not designed for lithography and so can generate patterns such as 3D structures that cannot be rendered by e beam systems Also DXF format does not support datatype tags which are used to specify individual dose values for geometrical shapes Datatype tags are important when compensating manually or automatically for the proximity effect see Sect 2 4 At the very low end are the free programs from UC Berkeley Magic and OCT VEM which run on UNIX workstations Magic is a widely used program geared for MOSIS compatible CMOS processing Magic is restricted to rectangles at right angles Manhattan geometry and has no support for polygons The VEM polygon editor in conjunction with the OCT database manager provides support for polygons A number of companion simulation and routing tools a
118. rtant not to use an electron gun evaporator since x rays and electrons in the evaporator will expose the resist Another approach to charge dissipation is the use of a conducting polymer either as a planarizing layer under the resist or as a coating over the resist The commercial polymers TQV Nitto Chemical Industry and ESPACER100 Showa Denko have been used for this purpose 129 130 Both are coated at a thickness of about 55 nm and have a sheet resistance around 20 M TQV uses cyclohexanone as the casting solvent which swells and dissolves novolac resins present in most photoresists and SAL and so a water soluble PVA polyvinyl alcohol layer is needed to separate the resist from the TQV ESPACER100 has the advantage that it is soluble in water and so can be coated directly onto many resists TQV is removed with methyl isobutyl ketone isopropanol MIBK IPA the developer used for PMMA ESPACER is removed in water Other water soluble conducting polymers can be prepared from polyaniline doped with onium or triflate salts 131 132 2 7 2 Positive Resists In the simplest positive resists electron irradiation breaks polymer backbone bonds leaving fragments of lower molecular weight A solvent developer selectively washes away the lower molecular weight fragments thus forming a positive tone pattern in the resist film 2 7 2 1 PMMA Polymethyl methacrylate PMMA was one of the first materials developed for e beam lithography 133 134
119. s informationis used only when reading the data into a new library If this record is present it should occur between the BGNLIB and LIBNAME records Two byte signed integer contains an array of Access Control List ACL data There may be from 1 to 32 ACL entries each consisting of a group number a user number and access rights This information is used only when reading the data into a new library If this record is present it should occur between the BGNLIB and LIBNAME records The following record types are either not used not released or are related to tape formatting TEXTNODE 1400 SPACING 18 UINTEGER 1D USTRING 1E STYPTABLE 2406 STRTYPE 2502 ELKEY 2703 LINKTYPE 28 LINKKEYS 29 TAPENUM 3202 TAPECODE 3302 STRCLASS 3401 RESERVED 3503 2 9 5 Stream syntax in Bachus Naur representation An element shown below in CAPITALS is the name of an actual record type An element shown in lower case means that name can be further broken down in to a set of record types The following table summarizes the Bachus Naur symbols SymbolMeaning Is composed of An element which can occur zero or one time Choose one of the elements within the braces The elements within the braces can occur Q zero or more times ae The elements within braces must occur one or more times These elements are further defined in the Stream syntax list Or lt stream format gt HEADER BGNLIB LIBDIRSIZE SRFNAME l
120. s lower in the ready for column dem agnification b Because of their increased parallelism over Gaussian raster scan tools shaped spot systems are much faster However throughput is still limited by the remaining serialism by stage movements and in a few cases by data transfer times Shaped spot systems can readily be extended to 0 15 m resolution compared to the 0 25 m resolution of the Gaussian raster beam systems While there is no well defined standard for the comparison of throughput we can say that the throughput of shaped spot machines remains under 10 wafers hour making them superior to Gaussian systems but not competitive with optical steppers which produce typically 40 to 80 wafers hour The market for high speed shaped spot systems remains in maskmaking direct write prototyping and low volume production of 0 15 m scale features 2 5 6 1 IBM EL 4 Shaped spot systems have been pioneered but never sold by IBM The latest version EL 4 combines an extraordinarily large number of lenses 65 67 Fig 2 25 with a unique three stage deflection for optimum speed The final lens termed a variable axis immersion lens VAIL provides minimized off axis aberrations or maximum field coverage as well as telecentric beam positioning with the beam landing normal to the substrate thereby reducing stitching errors due to substrate height variance The system runs at 75 kV with a LaBg emitter providing up to 50 A cm at the subs
121. s the optimal positive dose PMMA will crosslink forming a negative resist It is simple to see this effect after having exposed one spot for an extended time for instance when focusing on a mark The center of the spot will be crosslinked leaving resist on the substrate while the surrounding area is exposed positively and is washed away In its positive mode PMMA has an intrinsic resolution of less than 10 nm 137 In negative mode the resolution is at least 50 nm By exposing PMMA or any resist on a thin membrane the exposure due to secondary electrons can be greatly reduced and the process latitude thereby increased PMMA has poor resistance to plasma etching compared to novolac based photoresists Nevertheless it has been used successfully as a mask for the etching of silicon nitride 138 and silicon dioxide 139 with 1 1 etch selectivity PMMA also makes a very effective mask for chemically assisted ion beam etching of GaAs and AlGaAs 140 EXAMPLE PROCESS PMMA POSITIVE EXPOSURE AND LIFTOFF 1 Start with 496K PMMA 4 solids in chlorobenzene Pour resist onto a Si wafer and spin at 2500 rpm for 40 to 60 seconds 2 Bake in an oven or on a hotplate at 180 C for 1 h Thickness after baking 300 nm 3 Expose in e beam system at 50 kV with doses between 300 and 500 uC cm Other accelerating voltages may be used The dose scales roughly with the voltage Develop for 1 min in 1 3 MIBK IPA Rinse in IPA Blow dry with nitrogen
122. s to pathtype 4 Contains four bytes which specify in database units the extension of a path outline beyond the last point of the path The value can be negative ASCII string Required for Filtered format and present only in Filtered Stream files Contains the list of layers and data types included in the data file usually as specified by the user when generating the Stream file At least one MASK record must follow the FORMAT record More than one MASK record may follow the FORMAT record The last MASK record is followed by the ENDMASKS record In the MASK list data types are separated from the layers witha semicolon Individual layers or data types are separated with a space a range of layers or data types is specified with a dash An example MASK list looks like this 1 5 7 10 0 63 ENDMASKS No data is present This is required for Filtered format and is present only in a Filtered 3800 LIBDIRSIZE 3902 SRENAME 3A06 LIBSECUR 3B02 Stream file This terminates the MASK records The ENDMASKS record must follow the last MASK record ENDMASKS is immediately followed by the UNITS record Two byte signed integer contains the number of pages in the Library directory This information is used only when reading the data into a new library If this record is present it should occur between the BGNLIB record and the LIBNAME record ASCII string contains the name of the Sticks Rules File if one is bound to the library Thi
123. sensitive to electrons and the pattern developed in it is transferred by dry etching into a thicker underlying layer reduce the forward scattering effect at the cost of an increase in process complexity Higher beam voltages from 50 kV to 100 kV or more also minimize forward scattering although in some cases this can increase the backscattering When writing on very thin membranes such as used for x ray masks higher voltages reduce the backscatter contribution as well since the majority of electrons pass completely through the membrane 31 Conversely by going to very low beam energies where the electron range is smaller than the minimum feature size the proximity effect can be eliminated 32 The penalty is that the thickness of a single layer resist must also be less than the minimum feature size so that the electrons can expose the entire film thickness The electron optical design is much harder for low voltage systems since the electrons are more difficult to focus into a small spot and are more sensitive to stray electrostatic and magnetic fields However this is the current approach in optical maskmaking where a 10 kV beam is used to expose 0 3 um thick resist with 1 um minimum features on a 5 mask In more advanced studies a 1 5 kV beam has been used to expose 70 nm thick resist with 0 15 um minimum features 33 A technique that can be used in conjunction with this approach in order to increase the usable range of electron energy is
124. sion of Philips and most recently products from the former Jenoptik Microlit Division Leica sells eight different models of Gaussian spot vector scan machines the EBL Nanowriter has been described above Systems in the mid range of resolution include the EBML 300 a LaBg tool directly evolved from the Cambridge line and the EBPG 5 a LaBg machine evolved from the Philips line The EBPG 5 is comparable to the JEOL JBX 5DII in resolution but has accelerating voltage up to 100 kV The EBMLand EBPG are both known for their versatile control software On Leica s high end is the VectorBeam with optics evolved from the Philips EBPG line and control electronics and software evolved from the Cambridge EBML line The VectorBeam Fig 2 19 has a thermal field emission electron source running at 100 kV and a 6 in stage motion with up to 1024 0 6 nm precision The 25 MHz pattern generator has the useful feature that it is able to hold a small pattern in a buffer so that repeated patterns do not have to be retransmitted to the pattern generator This can significantly decrease the transmission overhead time when writing a large array of simple figures 10 o vw Beam Diameter nm z gt _ So 10 0 001 0 01 O 1 1 10 100 1000 Beam Cunent n FIGURE 2 18 Probe beam diameter versus current for a a LaBs cathode with a 120 um objective aperture b a thermal field emission TFE cathode with a 40 um objective aperture and c a
125. sponsivity at 118 m Appl Phys Lett 61 294 1992 140 R C Tiberio G A Porkolab M J Rooks E D Wolf R J Lang A D G Hall Facetless Bragg reflector surface emitting AlGaAs GaAs lasers fabricated by electron beam lithography and chemically assisted ion beam etching J Vac Sci Technol B9 2842 1991 141 Note that this liftoff process allows the use of ultrasonic agitation because chrome sticks very well to silicon The ultrasonic process causes lines of aluminum to peal off the surface A common belief is that once the substrate is dry the metal cannot be made to separate from the surface This is not necessarily true If the metal pattern adheres well to the substrate e g Cr or Ti then further ultrasonic agitation in the solvent may well continue the liftoff process and improve the yield of devices 142 T Tada Highly sensitive positive electron resists consisting of halogenated alkyl chloroacrylate series polymer materials J Electrochem Soc 130 912 1983 143 Toray Marketing and Sales 1875 S Grant St Suite 720 San Mateo CA 94402 415 341 7152 Toray Industries 1 8 1 Mihama Urayasu Inc Chiba Japan 144 K Nakamura S L Shy C C Tuo C C Huang Critical dimension control of poly butene sulfone resist in electron beam lithography Jpn J Appl Phys 33 6989 1994 145 M Widat alla A Wong D Dameron C Fu Submicron e beam process control Semiconductor International May 1
126. ss sensitive to avoid statistical variations in the exposure Although there is room for improving the sensitivity of both high and low resolution resists the statistics of resist exposure will eventually limit the resist sensitivity and exposure rate In the following we describe some common resists categorized as either positive removed where exposed or negative retained where exposed single layer or multilayer and organic or inorganic 2 7 1 Charge Dissipation A common problem is the exposure of resist on insulating substrates Substrate charging causes considerable distortion when patterning insulators and may contribute significantly to overlay errors even on semiconductors 56 A simple solution for exposure at higher energies gt 10 kV is to evaporate a thin 10nm layer of gold gold palladium alloy chrome or aluminum on top of the resist Electrons travel through the metal with minimal scatter exposing the resist The film is removed before developing the resist When using Au or Au Pd the metal film is removed from the top of the resist with an aqueous KI I solution 127 A chrome overlayer would be removed with chrome etch 128 Aluminum can be removed from the resist with an aqueous base photoresist developer Acid mixtures or photoresist developer for removing aluminum will sometimes react with exposed e beam resist therefore aluminum is not the best choice for charge dissipation When evaporating any metal it is impo
127. st is dropped onto the substrate which is then spun at 1000 to 6000 rpm to form a coating 123 Further details on resist application can be found in Chapter 4 After baking out the casting solvent electron exposure modifies the resist leaving it either more soluble positive or less soluble negative in developer This pattern is transferred to the substrate either through an etching process plasma or wet chemical or by liftoff of material In the liftoff process a material is evaporated from a small source onto the substrate and resist as shown in Fig 2 28 The resist is washed away in a solvent such as acetone or NMP photoresist stripper An undercut resist profile as shown aids in the liftoff process by providing a clean separation of the material FIGURE 2 28 Two bilayer e beam resist structures a A ge gm high molecular weight PMMA High My PMs PMA is spun on top of a slightly io a more sensitive bottom layer ae of low molecular weight w q J N PMMA The resist is developed in methyl isobutyl Low Mp PMs P MMA co MAA ketone isopropanol MIBK IPA typically 1 3 a b giving a slight undercut b PMMA is spun on top of the copolymer P MMA co MAA The structure is typically developed in MIBK IPA 1 1 giving a large undercut In this case MIBK develops PMMA and IPA develops the P MMA co MAA In the liftoff c d process metal is evaporated as shown in c The resist is then removed in a liquid sol
128. still under vacuum and operational All parts of an electron beam column exposed to the beam must be conductive or charging will cause unwanted displacements of the beam Often a conductive liner tube will be placed in parts of the column to shield the beam from insulating components Finally the system needs a method of detecting the electrons for focusing deflection calibration and alignment mark detection Usually this is a silicon solid state detector similar to a solar cell mounted on the end of the objective lens just above the sample Channel plate detectors and scintillators with photomultiplier tubes may also be used Unlike scanning electron microscopes which image with low voltage secondary electrons EBL systems normally detect high energy backscattered electrons since these electrons can more easily penetrate the resist film The signal from low energy secondary electrons may be obscured by the resist 2 2 6 Resolution There are several factors that determine the resolution of an electron beam system First is the virtual source size a divided by the demagnification of the column M resulting in a beam diameter of dj d M In systems with a zoom condenser lens arrangement the demagnification of the source can be varied but increasing the demagnification also reduces the available beam current If the optics of the column were otherwise ideal this simple geometry would determine the beam diameter Unfortunately le
129. t refer to discussion above Mark alignment on the NPGS is performed by calculating the correlation between the measured mark image and the user defined mark pattern Signal processing such as averaging and edge enhancement can be executed before the alignment correlation allowing the use of low contrast or rough marks If the user supplies precisely defined marks usually printed with a mask made on a commercial maskmaking tool then NPGS can be used to correct for global rotation scaling and nonorthogonality NPGS can control motorized stages providing fully automated sample movement and pattern alignment However SEM stages are typically orders of magnitude slower than those of dedicated e beam tools and do not provide feedback to the deflection system see Sect 2 5 4 Angled lines polygons and arbitrarily shaped features are all supported and data can be imported in common e beam formats GDSII Stream CIF and a subset of DXF AutoCAD 2 5 3 2 Raith pattern generators The Proxy Writer SEM conversion kit is Raith s low end PC based pattern generator Like the Nabity system the Proxy Writer is a vector scan system Unlike the Nabity NPGS the Proxy Writer has only manual alignment and patterns are limited to single writing fields Corrections for rotation shift and orthogonality are applied to single fields with single patterns these corrections are not applied globally to correct the workpiece rotation and stage nonorthogona
130. te e beam layer with 0 05 um alignment the placement of alignment marks becomes critical and e beam stitching errors can significantly affect device performance and yield It is important for the designer to consider the limitations of the e beam system before laying out any pattern Consider the case of a pattern targeted for a high resolution Gaussian beam system such as the Leica EBPG or the JEOL JBX series For high resolution work the writing field may be as small as 80 um Larger patterns are formed by moving the sample and stitching fields together Field stitching errors will be around 20 nm so any fine lines in the pattern e g a narrow gate should not be placed at a field boundary 2 6 3 Alignment Marks Electron beam lithography may be used to pattern optical masks and their corresponding alignment marks steppers and contact aligners have specific design requirements for these marks However we will discuss here only the marks used for direct write e beam layers There are two phases of alignment 1 correction for the placement and rotation of the wafer or piece and 2 correction for the placement of individual chips on the wafer The e beam tool aligns each pattern file in its final fractured form to a mark before writing the pattern If your alignment tolerance is greater than 0 5 um then the individual chip alignment will not be necessary Global alignment that is correction for the placement and rotation of the work
131. te signed integers use the usual twos complement format for negative values The more significant bytes appear first in the file so that by default no byte swapping is required when reading the integers with a big endian CPU e g Intel processors Byte swapping is required when reading or writing integers with a little endian machine such as a VAX Real numbers are not represented in IEEE format A floating point number is made up of three parts the sign the exponent and the mantissa The value of the number is defined to be mantissa 16 exponen If S is the sign bit E are exponent bits and M are mantissa bits then an 8 byte real number has the format SEEEEEEE MMMMMMMM MMMMMMMM MMMMMMMM MMMMMMMM MMMMMMMM MMMMMMMM MMMMMMMM The exponent is in excess 64 notation that is the 7 bit field shows a number that is 64 greater than the actual exponent The mantissa is always a positive fraction greater than or equal to 1 16 and less than 1 For an 8 byte real the mantissa is in bits 8 to 63 The decimal point of the binary mantissa is just to the left of bit 8 Bit 8 represents the value 1 2 bit 9 represents 1 4 and so on In order to keep the mantissa in the range of 1 16 to 1 the results of floating point arithmetic are normalized Normalization is a process whereby the mantissa is shifted left one hex digit at a time until its left four bits represent a non zero quantity For every hex digit shifted the exponent is decreased by
132. ted by electrons scattering from other features nearby During this process the electrons are continuously slowing down producing a cascade of low voltage electrons called secondary electrons Zum a Zum 9 FIGURE 2 10 Monte Carlo simulation of electron scattering in resist on a silicon substrate at a 10 kV and b 20 kV From Kyser and Viswanathan 19 1975 Figure 2 10 shows some computer simulations of electron scattering in typical samples 19 The combination of forward and backscattered electrons results in an energy deposition profile in the resist that is typically modeled as a sum of two Gaussian distributions where a is the width of the forward scattering distribution b is the width of the backscattering distribution and e is the intensity of the backscattered energy relative to the forward scattered energy Fig 2 11 shows an example of a simulated energy profile 2 3 1 Forward Scattering As the electrons penetrate the resist some fraction of them will undergo small angle scattering events which can result in a significantly broader beam profile at the bottom of the resist than at the fxtop The increase in effective beam diameter in nanometers due to forward scattering is given empirically by the formula a 0 9 R V where R is the resist thickness in nanometers and V is the beam voltage in kilovolts Forward scattering is minimized by using the thinnest possible resist and the highest available accelerating voltage
133. tem USA 708 405 0213 Contact 406 587 0848 406 586 9514 QErTany 89 028 0720000 708 405 0147 fax UK jcnabity aol com fak i i 44 1223 411 123 211 fax 2 5 4 1 JEOL systems JEOL s popular JBX 5DII Gaussian vector scan system uses a LaBg emitter running at either 25 or 50 kV Figure 2 17 shows the 5DII with two condenser lenses and two objective lenses Only one of the objectives is used at a time the operator has the choice of using the long working distance lens for a field size of 800 um or the short working distance lens for an 80 um field at 50 kV The fields are twice as large at 25 kV The pattern generator runs at 6 MHz gt 0 167 us per exposure point and the stage has a precision of 1024 0 6nm As with all commercial systems alignment field stitching and sample handling are fully automated In fact one drawback for research purposes is that there is no manual mode of operation The system is capable of aligning to within 40 nm 2 and writing 30 nm wide features over an entire 5 in wafer or mask plate JEOL systems are known for their simple high quality sample holders The 5DII is one of the highest resolution though not one of the fastest e beam tools in the LaBg class LaB CATHODE ALIGNMENT COIL APERTURE BEAM BLANKER 1ST LENS BEAM STOP ALIGNMENT COIL 2D LENS SAD LENS STIGMATOR ANAL APERTURE OCTOPOLE DEFLECTOR AND 47H LENS OCTOPOLE DEFLECTOR AND STH LENS BACKSCATTERED BL ECTRON DETECTO
134. tems since the total current provided by the thermal field emission source is inadequate for this application B mm TIP RADIUS RU L a Lo Be REAL SOURCE CROSSOV R WEHNELT sii ANODE TIP rous Fi re OSum b ZrO W e VIRTUAL J SOURCE N 200 SUPPRESSOR EXTRACTOR m 750 um FIGURE 2 5 Electrode structure and relevant dimensions for a LaBg gun and b thermal field emission gun The electrodes are circularly symmetric about the optical axis The Wehnelt and suppressor are biased negative with respect to the cathode while the anode and extractor are positively biased From Gesley 17 1989 2 2 3 Electron Lenses Electrons can be focused either by electrostatic forces or magnetic forces Although electron lenses in principle behave the same as optical lenses there are differences Except in some special cases electron lenses can be made only to converge not diverge Also the quality of electron lenses is not nearly as good as optical lenses in terms of aberrations The relatively poor quality of electron lenses restricts the field size and convergence angle or numerical aperture that can be used The two types of aberrations critical to EBL are spherical aberrations where the outer zones of the lens focus more strongly than the inner zones and chromatic aberrations where electrons of slightly different energies get focused at different image planes Both types of aberrations can be minimized by reducing th
135. tered e is roughly independent of beam energy although it does depend on the substrate material with low atomic number materials giving less backscatter Typical values of e range from 0 17 for silicon to 0 50 for tungsten and gold Experimentally e is only loosely related to ee the backscatter energy deposited in the resist as modeled by a double Gaussian Values for e tend to be about twice e 2 3 3 Secondary Electrons As the primary electrons slow down much of their energy is dissipated in the form of secondary electrons with energies from 2 to 50 eV They are responsible for the bulk of the actual resist exposure process Since their range in resist is only a few nanometers they contribute little to the proximity effect Instead the net result can be considered to be an effective widening of the beam diameter by roughly 10 nm This largely accounts for the minimum practical resolution of 20 nm observed in the highest resolution electron beam systems and contributes along with forward scattering to the bias that is seen in positive resist systems where the exposed features develop larger than the size they were nominally written A small fraction of secondary electrons may have significant energies on the order of 1 keV These so called fast secondaries can contribute to the proximity effect in the range of a few tenths of a micron Experimentally and theoretically the distribution of these electrons can be fit well by a third Gaussi
136. the shapes are filled in with a more coarse grid In the above example we can set n to 11 so that the dwell time is increased to 1 1 10 s 91 kHz increasing the pitch of exposure points to 16 5 nm This spacing is a good match to the resolution of PMMA and allows fine lines to be defined without any bumps due to pixelization However when we require 100 times the current 5000 pA in this example the exposure point spacing must be increased by a factor of 10 possibly leading to rough edges Some pattern generators see Sect 2 5 3 1 avoid this problem by allowing different exposure point spacings in the X and Y or in the r and theta directions thereby allowing a larger exposure point spacing in the less critical dimension To use a SEM without a beam blanker one must consider the large exposure point spacing required for common resists Lack of a beam blanker leads to the additional problem of artifacts from the settling of scan coils and exposure at beam dump sites Many SEM manufacturers offer factory installed beam blankers Retrofitted blankers are also sold by Raith GmbH 47 The scan coils of a SEM are designed for imaging in a raster pattern and so are not commonly optimized for the random placements of a vector scan pattern generator Settling times are typically around 10 us for a JEOL 840 to as long as 1 ms for the Hitachi S800 where the bandwidth of the scan coils has been purposely limited to reduce noise in the imaging system
137. therefore affected by the thermal conductivity of the sample and by the cooling rate after the bake Resolution of 30 nm has been demonstrated at very low voltage 160 and 50 nm wide lines have been fabricated using high voltage 161 SAL 606 has 0 1 um resolution in 0 4 um thick films exposed with 40 keV electrons at 8 4 uC cm The novolac base polymer has etching properties similar to those of positive photoresists Unlike photoresist the shelf life of SAL is on the order of six months at room temperature Refrigeration extends the shelf life to several years but care is required to avoid condensation when the resist is dispensed to smaller containers SAL is a sensitive resist 7 to 9 uC cm at either 20 or 40 kV and so is suitable for mask writing It is interesting to note that unlike PMMA the critical dose of SAL does not scale proportionately with accelerating voltage Although it is not as sensitive as other negative resists COP CMS or GMC SAL has far better process latitude and resolution EXAMPLE PROCESS SAL NEGATIVE MASK PLATE Soak mask plate in acetone gt 10 min to remove photoresist Clean the plate with RIE in oxygen Do not use a barrel etcher RIE conditions 30 sccm O gt 30 mTorr total pressure 90 W 0 25 W cm 5 min Immediately spin SAL 601 4 krpm 1 min Bake in 90 C oven for 10 min This resist is not sensitive to room light Expose at 50 kV 11 C cm Be sure the plate is grounded Post
138. ting temperature of around 1800 K The beam current delivered by thermionic sources depends on the temperature of the cathode Higher temperatures can deliver greater beam current but the tradeoff is an exponentially decreasing lifetime due to thermal evaporation of the cathode material Field emission sources typically consist of a tungsten needle sharpened to a point with a radius less than 1 um The sharp tip helps provide the extremely high electric fields needed to pull electrons out of the metal Although cold field emission sources have become common in electron microscopes they have seen little use in EBL due to their instability with regard to short term noise as well as long term drift which is a much more serious problem for lithography than microscopy The noise is caused by atoms that adsorb onto the surface of the tip affecting its work function and thus causing large changes in the emission current Heating the tip momentarily flashing can clean it but new atoms and molecules quickly readsorb even in the best of vacuums In addition atoms may be ionized by the electron beam and subsequently accelerated back into the tip causing physical sputtering of the tip itself To minimize the current fluctuations the electron source must be operated in an extreme ultra high vacuum environment 10 Torr or better TABLE 2 1 Properties of the electron sources commonly used in electron beam lithography tools source type brightness sou
139. tive evaluation of chemically amplified resists for electron beam top surface imaging use J Vac Sci Technol B12 3925 1994 197 Portions of the GDSII Stream Format Manual Documentation No B97E060 Feb 1987 reprinted with permission of Cadence Design Systems Inc 555 River Oaks Parkway San Jose CA 95134 408 943 1234 See also the web site http Awww cadence com 198 A useful set of GDSII utilities is available for the VMS operating system This set includes programs for syntax checking dumping to ASCII building from ASCII rotating and scaling cells printing cell hierarchies printing data extents and displaying layer occupation For purchase information contact the Cornell NanoScale Facility at 607 255 2329 or information cnf cornell edu
140. to place a layer with a high atomic number such as tungsten underneath the resist This has the effect of further limiting the range of the backscattered electrons FIGURE 2 13 SEM micrograph of a positive resist pattern on silicon exposed with a 20 kV electron beam demonstrates the proximity effect where small isolated exposed areas receive less dose relative to larger or more densely exposed areas From Kratschmer 30 1981 2 4 3 Proximity Effect Correction 2 4 3 1 Dose modulation The most common technique of proximity correction is dose modulation where each individual shape in the pattern is assigned a dose such that in theory the shape prints at its correct size The calculations needed to solve the shape to shape interactions are computationally very time consuming Although the actual effect of electron scattering is to increase the dose received by large areas for practical reasons proximity correction is normally thought of in terms of the large areas receiving a base dose of unity with the smaller and or isolated features receiving a larger dose to compensate Several different algorithms have been used In the self consistent technique the effect of each shape on all other shapes within the scattering range of the electrons is calculated The solution can be found by solving a large number of simultaneous equations 34 unfortunately this approach becomes unwieldy as the number of shapes increases and their size decreas
141. to generate input parameters for proximity effect correction programs see next section Alternatively experimental data can be obtained by measuring the diameter of exposed resist from a point exposure of the beam at various doses 28 or by measuring the linewidths of various types of test patterns such as the tower pattern 29 2 4 Proximity effect 2 4 1 Introduction The net result of the electron scattering discussed in the previous section is that the dose delivered by the electron beam tool is not confined to the shapes that the tool writes resulting in pattern specific linewidth variations known as the proximity effect For example a narrow line between two large exposed areas may receive so many scattered electrons that it can actually develop away in positive resist while a small isolated feature may lose so much of its dose due to scattering that it develops incompletely Fig 2 13 shows an example of what happens to a test pattern when proximity effects are not corrected 30 2 4 2 Proximity Effect Avoidance Many different schemes have been devised to minimize the proximity effect If a pattern has fairly uniform density and linewidth all that may be required is to adjust the overall dose until the patterns come out the proper size This method typically works well for isolated transistor gate structures Using higher contrast resists can help minimize the linewidth variations Multilevel resists in which a thin top layer is
142. trate Wafers are held on the stage by electrostatic clamping which is claimed to provide improved flatness superior thermal stability and lower contamination than conventional front surface reference wafer chucks An advanced feature of the EL 4 is its use of redundant data registers anda cyclic redundancy code for checking the validity of the many gigabytes of data flowing into the system Another unique feature is the use of a servo guided planar stage which slides on a base plate without guide rails moved by push rods coupled with friction drives to servo motors outside the vacuum chamber The stage is positioned entirely through feedback from a multi axis laser controller 68 69 2 5 6 2 Etec Systems Excaliber and Leica Lithographie Systeme Jena ZBA 31 32 The Leica Jena ZBA 31 32 70 handles plates up to 7 in and wafers up to 8 in The 31 is a maskmaking tool and the 32 is a direct write instrument Like Etec s AEBLE and Excaliber systems the ZBA writes while the stage is moving The ZBA delivers 20 A cm Its continuous stage motion and cassette to cassette wafer loader give it relatively high throughput when using high speed resist The latest generation of commercial shaped spot systems will offer resolution to 0 1 m Under development at Etec is the Excaliber with a field emission source larger stage and higher resolution than its predecessor the AEBLE 150 The Excaliber system incorporates a number of features from IBM s EL
143. umont 170 by the use of a weak solvent xylene for the top layer of PMMA Use of a weak solvent prevents intermixing of the two layers A further refinement of the technique 171 substituted MIBK a solvent of intermediate strength for the xylene PMMA of various molecular weights dissolved in MIBK can now be purchased commercially 172 EXAMPLE PROCESS LIFTOFF OF THIN METAL WITH PMMA BILAYER Clean wafer on the spinner by spraying with acetone then isopropanol Spin dry Spin 495 K MW PMMA 2 in any solvent 4 krpm for 30 s for a thickness 50 nm Bake at 170 180C for 1 h Spin 950 K MW PMMA 2 in MIBK 4krpm for 30 s for a thickness 50 nm Bake at 170 180C for 1 h Expose at 50 kV 350 to 450 C cm Develop in MIBK IPA 1 3 for 1 min Rinse in IPA blow dry Optionally remove surface oxide of GaAs with 10 s dip in NH OH H20 1 15 Blow dry Evaporate 15 nm of Au Pd 3 2 alloy 210 Torr base pressure 0 5 nm s O Lift off by soaking in methylene chloride Optionally finish with mild ultrasonic agitation Ol ON Pe 2 7 4 2 PMMA copolymer A larger undercut resist profile is often needed for lifting off thicker metal layers One of the first bilayer systems was developed by Hatzakis 173 In this technique a high sensitivity copolymer of methyl methacrylate and methacrylic acid P MMA MAA 174 is spun on top of PMMA The exposed copolymer is soluble in polar solvents such as alcohols and ethers but ins
144. vantage for lithography since they allow the operator to decrease the working distance and thus the spot size while keeping the sample flat and in focus With patterning speed limited by beam settling and bus speed it is clear that inexpensive SEM conversions cannot match the high speed writing of dedicated e beam systems However a SEM based lithography system can provide adequate results for a wide variety of applications at a small fraction of the cost of a dedicated system The number of applications is limited by stitching alignment and automation Practical applications include small numbers of quantum devices metal lines junctions SQUIDs split gates small numbers of transistors small area gratings small masks tests of resists and direct deposition The main limitations with SEM lithography are observed with writing over large areas or when deflection speed and throughput are critical Specifically difficulties with stitching and or distortions due to the electron optics of the microscope can become significant SEMs are not practical for most mask making integration of many devices over many fields large area gratings multifield optical devices or any application requiring a large substrate 2 5 3 1 Nanometer Pattern Generation System NPGS The SEM conversion kit sold by J C Nabity Lithography Systems 48 is built around a Windows based PC compatible with an ISA bus A 16 bit multifunction board from Data Translation 49
145. veloping discrete components for miniaturized single beam electron sources and columns 96 98 In an ongoing effort at IBM researchers are seeking to shrink the lenses and other optical components to micrometer sizes using micromachining techniques thereby building a high performance low voltage electron beam column 96 99 100 Low voltage has both advantages and disadvantages over high voltage lithography see Sect 2 5 4 3 but is required here simply because of the small size of the components In this design an entire e beam column is only several millimeters high assembled from micromachined silicon membranes supported on anodically bonded silicon and pyrex wafers This concept is still in the early development stages Microcolumn research seeks to provide exposure parallelism by building an array of small columns If they can be produced cheaply enough maintenance would be simplified by the use of disposable electron optics Although the optics may be inexpensive the control system for a large array of columns may be very expensive While many technical hurdles have already been overcome the ultimate success of beam arrays may be decided solely by economics 2 5 9 Electron Beam Fabrication Services In addition to commercial mask vendors many institutions offer services on large high resolution e beam tools Payment for services varies widely from purely collaborative work to hourly fees or contracts Public access to many fabricati
146. vent leaving the pattern d Solvents such as acetone and methylene chloride are used to dissolve the resist If we expose a positive resist to a range of doses and then develop the pattern and plot the average film thickness versus dose we have a graph as shown in Fig 2 29 The sensitivity of the resist is defined as the point at which all of the film is removed Ideally the film thickness would drop abruptly to zero at the critical dose In practice the thickness line drops with a finite slope If D is the largest dose at which no film is lost actually the extrapolation of the linear portion of Fig 2 29 a to 100 and if Dz is the dose at which all of the film is lost again actually the extrapolation seen in Fig 2 29 a then we define the contrast of the resist by log1o D2 D T 2 2 The same expression defines the contrast of a negative resist the film is retained where irradiated when D and D are the points shown in Fig 2 29 b A higher contrast resist will usually have a wider process latitude as well as more vertical sidewall profiles In order to help minimize bias and proximity effects positive resists should usually be exposed and or developed as lightly as possible while still adequately clearing the resist down to the substrate for all features In electron beam lithography especially at beam voltages of 50 kV or more it is possible to make resist structures with very high aspect ratios Unfortunately when th
147. x 173 M Hatzakis PMMA copolymers as high sensitivity electron resists J Vac Sci Technol 16 6 1984 1979 M Hatzakis High sensitivity resist system for lift off metallization U S Patent No 4024293 1977 174 P MMA MAA and PMMA may be purchased from OCG Microelectronic Materials Inc 5 Garret Mountain Plaza West Paterson NJ 07424 800 222 4868 or from the Microlithography Chemical Corp 249 Pleasant St Watertown MA 02172 617 926 3322 2919 fax 175 R E Howard E L Hu L D Jackel Multilevel resist for lithography below 100nm IEEE Trans Electron Dev ED 28 11 1378 1981 176 G J Dolan Offset masks for lift off photoprocessing Appl Phys Lett 31 337 1977 177 R E Howard D E Prober Nanometer scale fabrication techniques in VLSI Electronics Microstructure Science vol 5 Academic Press New York 1982 178 H Takenaka Y Todokoro A PMMA PMGI two layer resist system for stable lift off processing Proc SPIE 1089 132 1989 179 M P de Grandpre D A Vidusek M W Legenza A totally aqueous developable bilayer resist system Proc SPIE 539 103 1985 M W Legenza D A Vidusek M P Grandpre A new class of bilevel and mono level positive resist systems based on a chemically stable imide polymer Proc SPIE 539 250 1985 180 R C Tiberio J M Limber G J Galvin E D Wolf Electron beam lithography and resist processing for the fabrication of T gat
148. y with the high end Raith Elphy Plus Without a laser stage the EBL will suffer from the same limitations as SEM conversions namely lack of stage flatness and the need for alignment marks for calibration The system is available with a LaBg or Schottky thermal field emitter TFE and acceleration up to 100 kV The system is unusual in offering such high voltage and a TFE emitter in a low cost system 2 5 4 Gaussian vector scan systems Like the converted SEMs Gaussian vector scan systems use the writing strategy of stopping in each field deflecting the beam from shape to shape and filling in the shapes with a raster pattern Large commercial systems however break the deflection into two or more sections usually making use of a 16 bit DAC for subfield placement and a faster 12 bit DAC for deflection inside the subfield see Fig 2 15 This is the scheme used in systems from JEOL and some of the systems from Leica Leica s EBPG series and the Vector Scan VS tools built by IBM use an alternative technique the slower DACs are used for placing the origin of each primitive shape and the faster DACs are used for filling in the shape In addition to deflecting the beam with separate DACs systems from Hitachi and Leica use these separate DACs to drive physically separate deflectors magnetic or electrostatic JEOL systems in contrast use a single stage electrostatic deflector Single stage deflectors have fewer problems with matching deflect

Microlithography Chapter 2 E Beam Lithography b

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