UV-LED exposure system for low
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1. The sample was exposed at 100 mJ cm 20 mW cm for 5 s and developed for 15 s in diluted 1 4 AZAOOK developer Figure 6 shows various microstructures that were successfully patterned on bare silicon wafers with the UV LED system The photomask was partitioned into different sections to accommodate both dark field and light field areas to verify the patterning capability of the system under both types of mask design scenarios With the dark field mask region photoresist molds were created for electroplating of various MEMS structures like ring inductors Fig 6a and channel shaped arrayed electrodes Fig 6b The inverse photoresist patterns were generated similarly with the light field region Fig 6c and feature sizes in the range of 50 100 um were achieved which are typical for MEMS microfluidics applications The images were acquired with a USB microscope which proved to be a low cost supplementary module lending additional imaging capability to the UV LED exposure system which could be useful when patterning relatively large geometries Ors photoresist mold 7 _ oe ESD um i ranean i ge Silicon ll Figure 6 Images of patterned microstructures suitable for various MEMS and microsystem applications with relatively large feature sizes Proc of SPIE Vol 9052 90521T 5 Downloaded From http proceedings spiedigitallibrary org on 06 11 2014 Terms of Use http spiedl org te2. equipped with a digital user interface liquid crystal display LCD and keypad which provides user defined automatic electronic control on exposure time and power allowing accurate tuning of exposure dose within a large process window Our prototype lithography system costs less than 250 and employs only off the shelf components including UV LEDs 380 nm microcontroller digital to analog D A converter and electronic elements for the LED control circuitry The functionality and patterning capability of the system is verified with a minimum achievable feature size of 5 um 2 SYSTEM LEVEL DESIGN OPERATION AND TEST Designing a fully functional photolithography tool requires a suitable light source and proper control of its emission Commonly used high pressure mercury Hg UV lamps are usually powered from mains supply 110 220V AC possibly requiring transformers and AC DC convertors depending on the exact lamp model For instance service manual for a common mask aligner Karl Suss MJB3 states ignition voltages of 30kV at start up open circuit DC voltages reaching 180V and currents of 50A which are operating conditions with potentially life threatening risks Mercury lamps also require a cooling mechanism to operate within the specified pressure limits and to prevent lamp explosions which may result in possible harm to the instrument and to the operator by accidental mercury poisoning Furthermore Hg lamps require a warm up time to operate
3. operation Figure 2 Photographs showing some of the screens in the user interface menu The interactive screens allow the user to set up the exposure parameters modify entered settings repeat previous exposure settings or cancel operation 2 3 Control Unit The UV light intensity of the LEDs is digitally tuned with the designed control unit which involves a microcontroller D A converter and LED driver circuitry Fig 3 The control unit communicates between different system components to sequentially manage all operations User defined decimal values for power and duration are represented with binary values in the microcontroller Arduino Uno R3 board To control the UV LED radiant power a D A converter is used to convert the digital values into analog voltages Vpac which in return adjust the LED driver circuitry To achieve high accuracy a 12 bit D A converter MCP4725 was selected which provided 4096 discrete voltage levels Figure 3 UV LED driver circuitry Proc of SPIE Vol 9052 90521T 3 Downloaded From http proceedings spiedigitallibrary org on 06 11 2014 Terms of Use http spiedl org terms The LED driver circuitry is based on a type of linear current regulator consisting of an operational amplifier LM741 OPAMP to adjust the biasing of a transistor BD139 NPN BJT The collector current Ic is controlled by Vpac which tunes the intensity of the LEDs Through changes in Vpac current Ic flowing into the LED ar
4. Ds either to ON or OFF states User Interface Control Unit Microcontroller D A converter __ Meroconaler Ramet LED driver circuitry Peripherals Exposure Unit USB microscope w YU JE U J F E R AE A E E A E E R AEA E E EE ne S e a EE A AA EN N E ENEA R E A Figure 1 Block diagram of the UV LED exposure system Proc of SPIE Vol 9052 90521T 2 Downloaded From http proceedings spiedigitallibrary org on 06 11 2014 Terms of Use http spiedl org terms 2 2 User Interface The UV LED system hides the complexity of circuit operation behind a simple interactive user interface Fig 2 An LCD screen and keypad interfaced with the microcontroller allow users to input instructions view operational messages depending on system state and properly adjust exposure parameters depending on process requirements The interactive display also allows scrolling through the user defined settings and a help menu When the system boots up a password is requested to prevent accidental UV exposures The LCD main menu has three main options 1 New 2 Repeat and 3 Help New is used to set up a new exposure configuration where exposure duration up to 99 9 s with 0 1 s increments and radiant power up to 20 mW cm with 1 mW increments would be entered and confirmed The sub menu appearing after has the options to start the exposure modify the entered information or cancel the
5. UV LED exposure system for low cost photolithography Murat Kaya Yapici and Ilyas Farhat Department of Electrical and Computer Engineering Khalifa University Abu Dhabi UAE 127788 ABSTRACT This paper reports the development of a low cost portable light emitting diode LED based UV exposure system for photolithography The major system components include UV LEDs microcontroller digital to analog D A converter and LED control circuitry The UV LED lithography system is also equipped with a digital user interface LCD and keypad and permits accurate electronic control on the exposure time and power Hence the exposure dose can be varied depending on process requirements Compared to traditional contact lithography the UV LED lithography system is significantly cheaper simple to construct using off the shelf components and does not require complex infrastructure to operate Such reduction in system cost and complexity renders UV LED lithography as a perfect candidate for micro lithography with large process windows typically suitable for MEMS microfluidics applications Keywords UV LED lithography low cost lithography light emitting diode maskless lithography UV LED exposure dose control MEMS microfluidics microfabrication 1 INTRODUCTION Since the early 1960s lithographic techniques have played a central role in the advancement of semiconductor process technologies which in turn has fueled the development of integrated circuits IC
6. and optics to filter out the desired wavelength from multiple emission peaks In comparison apart from their low cost UV LEDs are inherently advantageous to operate and control compared to Hg lamps UV LEDs require only a few volts 3 4 V DC to generate the needed monochromatic UV emission without resorting to additional optics The low operating voltages and currents of LEDs create naturally safe operating conditions and alleviate the extra measures required for Hg lamp operation These unique features allow the realization of a compact portable exposure system with user controlled emission behavior 2 1 UV LED Exposure System The prototype UV LED exposure system has four major functional blocks Fig 1 which are 1 the user interface 2 control unit 3 exposure unit and 4 peripherals The operator is prompted to enter the exposure settings through the user interface and based on the user specifications the control unit automatically tunes the exposure unit to output UV light with the desired intensity and duration Upon beginning of each new run the system can accept new settings for exposure power and duration without requiring a restart as well as storing the previous run s configurations should the user decide to repeat the exposure with the same parameters To perform such higher level sequential operations a microcontroller based topology was selected to drive the UV LEDs rather than using simple switches that would place the LE
7. critical technology only to state of the art laboratories As a result widespread dissemination of lithography technology beyond microelectronics towards other engineering disciplines and fundamental sciences and possible applications therein are inherently limited due to the cost factor To address this problem we have developed a low cost portable light emitting diode LED based UV exposure system which makes photolithography possible even in a basic laboratory Recently LEDs with monochromatic emission and spectral peak in the UV range have become commercially available at low cost 0 67 LED rendering UV LEDs as a possible light source for photolithography thereby offering significant reduction in tool cost and infrastructure needs Some demonstrations of UV LED lithography include maskless lithography with micro pixel LED arrays and fabrication of high aspect ratio curved structures microchannels and micropatterns using packaged UV LEDs murat yapici kustar ac ae muratyapici gmail com phone 971 2 5018344 fax 971 2 4472442 Optical Microlithography XXVII edited by Kafai Lai Andreas Erdmann Proc of SPIE Vol 9052 90521T 2014 SPIE CCC code 0277 786X 14 18 doi 10 1117 12 2046123 Proc of SPIE Vol 9052 90521T 1 Downloaded From http proceedings spiedigitallibrary org on 06 11 2014 Terms of Use http spiedl org terms In this work we report a complete prototype lithography system
8. ers Appl Phys Lett 67 21 3114 3116 1995 6 Guo L J Nanoimprint lithography methods and material requirements Adv Mater 19 4 495 513 2007 7 Quate C F Scanning probes as a lithography tool for nanostructures Surf Sci 386 1 3 259 264 1997 8 Yapici M K Zou J A Novel Scanning Probe Array with Multiple Tip Sharpness for Variable Resolution Scanning Probe Lithography Applications Proc of the 8 IEEE NANO 18 21 2008 Proc of SPIE Vol 9052 90521T 6 Downloaded From http proceedings spiedigitallibrary org on 06 11 2014 Terms of Use http spiedl org terms 9 Yapici M K Zou J A Novel Micromachining Technique for the Batch Fabrication of Scanning Probe Arrays with Precisely defined Tip Contact Areas J Micromech Microeng 18 8 085015 2008 10 Melchels F P W Feijen J Grijpma D W A review on stereolithography and its applications in biomedical engineering Biomaterials 31 24 6121 6130 2010 11 Jorge P Stereolithography Materials Processes and Applications Springer New York 2011 12 Harriot L R Limits of lithography Proc IEEE 89 3 366 374 2001 13 Holmes S J Mitchell P H and Hakey M C Manufacturing with DUV lithography IBM J Res amp Dev 41 7 1997 14 Helbert J N Handbook of VLSI Microlithography Principles Tools Technology and Applications 2 edn Noyes William A
9. he LEDs was found to be approximately 3 5 cm o o Mai e Teon M a oe eee Sao othe Mee y a a Mt er A PI i n ewe O a a Me eh teh Bh Re ee eo Se Pe Pee Perse a 05856 r teks 85 02656 8 0000858685 20 Ba 65 s 0 8 odpo 9900 gt ORERE A ee fe BE Besetes 086 ae fas F k A A A A al aeh a t Sl S Figure 4 Schematic and photographs showing a the zigzag arrangement of LEDs b the assembled UV LED array on protoboard covering more than 3x3 in rectangular area c UV LED array during operation 2 5 Final Packaged System The complete packaged prototype of the UV LED exposure system is shown in Figure 5 For the system enclosure black acrylic material was chosen to minimize unwanted sidewall reflections Figure 5a shows the assembly of the system where the user interface components were placed on the front wall for easy access and others components were secured inside the enclosure Sufficient space was allocated for the inclusion of a fan to avoid possible heating of the LEDs especially during longer exposure durations where temperature differences could cause drift in LED emission characteristics Figure 5b shows the picture of the system during operation where UV reflections can be observed A manual slider and sample stage were also designed to allow proper alignment of samples under the exposure unit Fig 5c An optional USB microscope is attached to the sy
10. ndrew Publishing Norwich New York 2001 15 http www superbrightleds com moreinfo component leds Smm uv led 15 degree viewing angle 380 nm 30mw 632 Accessed 26 January 2014 16 Jeon C W Gu E and Dawson M D Mask free photolithographic exposure using a matrix addressable micropixellated AlInGaN ultraviolet light emitting diode Appl Phys Lett 86 22 1105 2005 17 D Elfstr m B Guilhabert J McKendry S Poland Z Gong D Massoubre E Richardson B R Rae G Valentine G Blanco Gomez E Gu J M Cooper R K Henderson and M D Dawson Mask less ultraviolet photolithography based on CMOS driven micro pixel light emitting diodes Opt Express 17 26 23522 23529 2009 18 Suzuki S Matsumoto Y Lithography with UV LED array for curved surface structure Microsyst Technol 14 9 11 1291 1297 2008 19 Kim J K Paik S J Herrault F and Allen M G UV LED lithography for 3 D high aspect ratio microstructure patterning 14th Solid State Sensors Actuators and Microsystems Workshop 481 484 2012 20 Guijt R M and Breadmore M C Maskless photolithography using UV LEDs Lab Chip 8 8 1402 1404 2008 21 Huntington M D and Odom T W A portable benchtop photolithography system based on a solid state light source Small 7 22 3144 3147 2011 22 Karl Suss MJB3 Mask Aligner Operator s Reference Manual P N 080AA261 1289 Section 3 2 1 23 P
11. ray is controlled and therefore the UV LED emission characteristics are easily modulated Table 1 tabulates various Vpac values for selected radiant powers ranging from 5 20 mW cm With the selected 12 bit D A converter Ic could be varied with 0 5 mA resolution Table 1 List of LED radiant power values and associated current and voltage values in the LED control circuitry D A input D A output Vpac 438 6 mA 2 903 V 0880 1 0744 V 2 4 Exposure Unit For the construction of the exposure unit InGaN UV LEDs RL5 UV0315 380 with peak wavelength of 380 nm and long durability 2000 h were used The needed voltage to operate is in the range of 2 4 to 3 5 V which is considered to be low voltage The designed exposure unit consisted of an array of 172 LEDs placed uniformly on approximately 3x3 in area Due to the preset spacing of holes on the protoboard and 5 mm LED tube diameter placing the LEDs side by side with minimum tilt in the vertical axis was difficult to achieve Therefore a zigzag arrangement was used Fig 4a which eliminated possible vertical tilt and resulted in higher density of LEDs providing nearly equal light illumination over the entire area With the zigzag arrangement the number of wires was reduced as each two adjacent columns of LEDs shared a common ground Figure 4b and 4c show the actual assembly of the LEDs and the array under operation respectively The optimum distance between the sample stage and t
12. rms The UV LED system was also tested on smaller geometries to evaluate resolution capabilities The minimum features on the photomask were straight lines with 5um width which were successfully replicated onto glass substrates under the same lithography conditions described above Figure 7 shows optical microscopic images of the patterned array of stripes having 5 um width and pitch with uniform coverage Although lithography on oxide surfaces is a non trivial process due to well known resist adhesion problems successful results were achieved without using surface treatment methods such as HMDS hexamethyldisilazane priming or conducting extensive process optimizations Further reductions in pattern size would be possible by optimizing process conditions and using higher resolution masks a N b pass 10 um c Figure 7 Optical microscope images of parallel array of 5 um stripes obtained with Carl Zeiss inverted microscope a image at 60x magnification b zoom in image of the outlined region and c perspective view rendered with Carl Zeiss Zen image post processing software 4 CONCLUSION A complete prototype of a tunable UV LED based photolithography system was developed and its functionality was verified through successful patterning of features down to 5 um Compared to traditional contact lithography tools the UV LED lithography system reported in this work is significantly cheaper less than 250 simpler to const
13. roduct data sheet AZ5214E image reversal photoresist Clariant Corporation Somerville NJ Proc of SPIE Vol 9052 90521T 7 Downloaded From http proceedings spiedigitallibrary org on 06 11 2014 Terms of Use http spiedl org terms
14. ruct with readily available components does not require complex infrastructure to operate and offers full flexibility on exposure parameters The system is portable compact offers low voltage DC operation and can be set up in any basic laboratory The reduced system cost and complexity renders UV LED lithography as a perfect tool for microfabrication applications allowing patterning capability within a wide range varying from hundreds to only few microns The prototype UV LED exposure system overcomes the access limitations financial and operational hurdles of conventional tools and can facilitate broader dissemination of lithography technology which is fundamental for micro and nanosciences REFERENCES 1 Madou M J Fundamentals of Microfabrication The Science of Miniaturization CRC Press Boca Raton Florida 2011 2 Levinson H J McCord M A Cerrina F Allen R D Skinner J G Neureuther A R Peckerar M C Perkins F K Rooks M J Rai Choudhury P editor Handbook of Microlithography Micromachining and Microfabrication SPIE Press London 1 1997 3 Maldonado J R X Ray Lithography Where it is Now and Where it is Going J Electron Mater 19 7 699 709 1990 4 Pease R F Lithography and Other Patterning Techniques for Future Electronics Proc of the IEEE 96 2 248 270 2008 5 Chou S Y Krauss P R Renstrom P J Imprint of sub 25 nm vias and trenches in polym
15. s and more recently micro nano electro mechanical systems M NEMS Today for various applications many different types of lithographic techniques are employed including electron beam lithography X ray lithography nanoimprint lithography scanning probe lithography stereolithography and photolithography Among these photolithography is the most widely used technique in microfabrication In photolithography a light source is used to generate radiation usually in the ultraviolet UV region The selective passage of UV light through a photomask falling onto a photosensitive polymer photoresist allows transfer of the geometrical patterns on the mask to the resist layer The light source therefore is the key enabler of photolithography and also defines the resolution limits of the technology For the light source modern lithography systems in high volume production environments use excimer lasers like krypton fluoride KrF 248 nm and argon fluoride ArF 193nm to generate deep ultraviolet DUV emission allowing patterning of features below 32 nm On the other hand academic or small volume R amp D centers mostly use steppers or mask aligners with gas discharge lamps mercury lamps having typical UV emission peaks at 436 nm g line 405 nm h line 365 nm i line and achieve sub micron feature sizes However the high cost of photolithography tools and associated infrastructure requirements limits the access to this
16. stem which could be interfaced with a personal computer to view large features on the mask or patterned sample Proc of SPIE Vol 9052 90521T 4 Downloaded From http proceedings spiedigitallibrary org on 06 11 2014 Terms of Use http spiedl org terms ef pe tpt Cat atte e UV LED dssembly BoP Micmac yang and control circuitry aS Manual slider r Silicon f Hele wafer f ff war User interface N i Sample stage b Reflection of LEDs c i Figure 5 Photographs of the constructed UV LED exposure system a inside of the casing showing the UV LED assembly and control circuitry b system during typical exposure and c overall view of the complete system 3 UV LED LITHOGRAPHY PROCESS AND PATTERNING RESULTS To demonstrate the functionality of the prototype UV LED exposure system we performed photolithography using AZ5214E i line photoresist Clarient Somerville NJ whose compositional and spin characteristics are well established For substrate materials silicon wafers and pyrex glass slides were chosen due their widespread use and readily availability To conduct micropatterning experiments first the substrates were cleaned and dehydrated at 110 C for 3 min on a hot plate After 5 min cool down under ambient conditions photoresist was uniformly applied and spin coated at 5000 rpm for 30 s This was followed by an immediate soft bake at 100 C for 1 min yielding 1 25 um film thickness
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