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Diploma Thesis

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1. operator s capabilities time of the switching the modes and subjective image processing sample s features defects usually reveal in topography or surface potential mapping softness stiffness of the surface cause restrictions for the impact force and demonstrate both scrapping effect and rip offs driving convolution and extra capacity It should be mentioned that all these listed items have been noticed in our study The experiment and data processing should be considered and planned on the basis of the literature concerned with issue and all the mentioned restrictions Therefore the work resulted in this Master s Thesis is organized as follows n chapter Semiconductors background the specific semiconductor properties of high k materials are described and compared with the LaLuO n chapter Methodical Section the classification and features of different SPM modes are given The applicability of the equipment used for the measurements is described in details from the structure of the piezo scanner to the abilities of certain modes The KPFM is discussed both with details of gradient mode KPFGM The Nanolithography of charge is overlooked since it is itself the technique of charge injection in this work Finally the software and future prospects of the study from positions of SPM and samples behavior are monitored n chapter Experimental part information about the samples used in our research with methods of growth which
2. 0 08 0 02 0 500 1000 1500 2000 2500 3000 3500 Time sec Figure 46 Experimentally obtained dependence of L t of charge for Sample 6 3 V 1 sec 58 Nonetheless it is possible to evaluate the order of diffusion coefficient D from the Diffusion law L D t by considering L FWHM Diffusion coefficient D is the slope of L t seen in Figure 46 It is measured to be nearly 8 5 107 cm sec From the Einstein Smoluchowski relation for charges D u ks T g Mobility u can be estimated as 4 10 cm V sec The values are in accordance by order for similar semiconductor LaScO found in literature 7 10 Still it wasn t possible to precisely define the size of the charge spots for different temperature conditions in Section 5 2 4 which is necessary for defining the activation energy E from the prescribed Arrhenius law D Do exp Ea keT 5 3 Nanolithography observations Considering the material s property to store the injected charge for rather long period of time Nanolithography was tested on practical raster image The raster sample logo of the Lappeenranta University of Technology similar to the symbol amp was drawn in the 8 um variant on the surface of Sample 6 See Figure 47 The applied Voltage range was 5 V to O V thus injected charge was negative to the background Auf 24 0 a 40 45 5 0 5 6 0 65 um a A O 55 09 Cn n a b C Figure 47 Charge nanolithography in Sample 6 a S
3. 6 with artifact on the right Mag z curve to define the Driving distance value in Semicontact Mode Electrical chargeability of Samples 6 and 7 KPFM results for Sample 6 after charging 3 V 10 sec FWHM vs Voltage dependence for Sample 6 KPFM and Sample 7 KPFGM Surface Potential for ellipse shaped defects in Sample 6 Mapping and Profile Surface Potential for defects after charging 3V 10 sec Mapping and Profile 3D reconstruction of the surface defect found in KPFM Surface Potential Surface Potential image of a charged series for Sample 6 charging duration 10 sec Surface Potential profile for the series of charges Surface Potential image of a charged series by duration Charging voltage 43V Potential height Pot t dependence for different charging duration for Sample 6 KPFM Surface Potential image of area after 3 hours on atmosphere Sample 6 Potential height Pot t dependence for Samples 6 7 and 6 2 cleavage KPFM Surface Potential image of Sample 6 2 after charging 6 V 10 sec 30 C Potential height Pot t dependence with temperature for Sample 6 2 6 V 10 sec Potential distribution image of Sample 6 2 2 min after charging 6 V 10 sec 30 C Surface Potential profiles for Sample 6 2 2 min after charging 6V 50 C 10 30 sec Surface Potential profiles and images for charges 7 V 7 V and reversed position Potential height Pot t dependence for different polarity signs for Sample 7 Pot
4. check that MAG SIN is crossing the horizontal axis then set the phase for the maximum absolute value of MAG SIN The sign of MAG SIN should be taken such that if Bias Voltage is positive then MAG SIN is decreasing Concurrently it is required to monitor the noise level Finally check that SetPoint 0 FB Gain is nearly 0 5 1 d Close the feedback loop simultaneously check that MAG SIN becomes zero e Check the parameters of Il pass set SURFACE POTENTIAL to be measured 8 Scanning in KPFM a Check that feedback is closed in I pass b Measure the amplitude of oscillations in I pass to obtain the range for dZ c Use the option CURVES and set the range to be 20 200 nm then set the position of a measured point by cursor and measure the amplitude See Figure 26 in Results d On the basis of this measurement set up the dZ parameter If the amplitude of oscillation is very small then leave it zero 08801505170 19155122 0812 5183 088 3 52 40 0415850 um Figure 23 Image revealing artifacts caused by the excess value of lift height 41 e scan the surface by SCAN RUN For too high dZ the white damaged area will be seen Figure 23 It is required to decrease the dZ parameter due to these artifacts 9 Charging amp Lithography measurements In the LITHO option choose Mode Vector and Method Bias Voltage then create the new sample It can be a point or a line Then it is required to set
5. d Still it was clearly seen that the positive peak is higher than the negative The charging of the material is easier with the positive applied voltage than negative It can be concerned with difference in charge mobility which should be further determined 56 40 8 8 8 Potential mV m e 0 200 400 600 800 1000 1200 Time sec Figure 44 Potential height Pot t dependence for different polarity signs for Sample 7 The results for experiments with Sample 47 are presented in the Figure 44 which shows the appropriate conformity of both measurements Charging is easier in the case of positive applied potential difference The Nova software allows also calculating the total injected charge integral of the profile multiplied by the square surface is resulting in the volume of potential and it has shown 4 times more total value for the positive charging than negative 5 2 6 Force gradient measurements The Kelvin Probe Force Gradient Microscopy KPFM gradient mode was applied to define the spot size with maximum lateral resolution As it was told before KPFGM provides measuring the force gradient by phase angle shift in the degrees According to our results the maximum resolution is on the range of 94 nm in experiments of charging the Sample 7 by 3 V 10 sec regime See Figure 29 A better locality was achieved both by KPFGM and by using thin 20 nm Platinum tips Resolution better than 250 nm was obtained
6. mo n e 2 J WQZosc where B is frequency bandwidth and Z is mean square amplitude of the cantilever vibration More specifically when contact of the probe with roughness causes the cantilever to bend the position of laser beam on the photo detector changes Misbalance in the photocurrent Alz is measured as difference in height Z because DFL I 16 24 Al Al AD Als Al Shift in horizontal axis is measured as LF Al Al AL Al M3 Measured difference DFL LF is used by a computer system which responds by compensating voltage to the scanner to minimize the DFL LF variation Here should be noted that nominal force does not matter it is only important to support the permanent force values Accuracy of the scanner positioning is almost 107 m and laser causes small inaccuracy Therefore main scan artifacts appear due to the feedback delay of the scanner To eliminate artifacts it is necessary to reduce the speed of scanning Nevertheless system performs part of the transformations of constant slope and offset curves As a result the measurement appears as checking the value of the measured parameter at a given point x y on the scanned area Parameter x y averaged over the value for surrounding 8 points Figure 15 Bini RII b a C Figure 15 Algorithm of processing the relative measurement by nearest 8 points 16 a Measured values b Distribution by values c Selection of app
7. G M McClelland S Chiang Atomic scale images of lateral frictional forces Electrostatic force microscope 1987 Y Martin D W Abraham H K Wickramasinghe Detection of charge as small as single electron Inelastic tunneling spectroscopy STM 1987 D P E Smith D Kirk C F Quate Phonon spectra of molecules in STM Laser driven STM 1987 L Arnold W Krieger H Walther Imaging by non linear mixing of optical waves in STM Ballistic electron emission microscope 1988 W J Kaiser 1988 Probing of Schottky barriers on a nanometer scale Inverse photoemission microscope 1988 J H Coombs J K Gimzewski B Reihl J K Sass R R Schlittler Luminescence spectra on nanometer scale Near field acoustic microscope 1989 K Takata T Hasegawa S Hosaka S Hosoki T Komoda Low frequency acoustic measurements on 10 nm scale Scanning noise microscope 1989 R Moller A Esslinger B Koslowski Tunneling microscopy with zero tip sample bias Scanning spin precession microscope 1989 Y Manassen R Hamers J Demuth A Castellano nm lateral resolution images of paramagnetic spins Scanning ion conductance microscope 1989 P Hansma B Drake O Marti S Gould C Prater 500 nm lateral resolution images in electrolyte Scanning electrochemical microscope 1989 O E Husser D H Craston A J Bard Absorption microscope spectroscop
8. and qualitative criterions of future prospects for LaLuO dielectric films In purposes of methodical interest of this study the sequence of experiments was presented step by step with system functions described and the certain parametrical values for settings were given The definite numerical parameters can be further used for investigations of surface properties and for verification of the results 63 Few important weaknesses must be called for this research Some shortcomings are associated with the mentioned restrictions and could be evaded by inclusion of more functional measuring systems In particular the numerical values for surface potential measurements should be perceived with discretion because the applied Thermal Module was causing high level of interference noise Mainly it was eliminated by using the Nova software for image processing and the Fourier transforms Presumably the noise was caused by grounding and lowered the potential values Nevertheless the trend of potential drop is consistent with the results of all the other experiments thus it was justified to be correct The model describing charge behavior in view of Tunneling into the interface layer should be overlooked with strong attention In some models the charge is leaking into the oxide layer between semiconductor and silicon wafer and only after that it is spreading by the diffusion mechanism Obtained results have shown constancy of spot size in time f
9. be justified to be only due the Thermal module noise however numerical values for charging limiting potential and relaxation time for Sample 46 presented here were justified to be correctly described It can be concluded that the amount of injected charge depends on the applied voltage time length of charging and the material properties The relaxation time is 2 h for Sample 6 and nearly 20 min for Sample 7 but it was not be precisely measured due to the low value of Pot t for Sample 7 and high noise After that it is possible to recall at least three other factors affecting the charges It is needed to describe the tip form and height not examined in this work temperature pressure conditions which also cause the additional layer of water molecules 19 and the polarity of applied Potential difference 5 2 4 Temperature dependence The NTegra Aura Thermal Mode was applied to investigate the temperature dependence of the charge behavior It allows heating up to 150 C in the vacuum conditions The Sample 6 was prepared for the investigation See Section 4 1 The average potential of the Sample 6 2 was 0 8 V which must be taken into account while evaluate the time dependence below The charging regime 6 V 10 sec was used for temperatures 30 C 50 C 70 C 90 C 110 C Charging at 30 C is presented on Figure 39 a Measured Pot t is smaller than expected It is seen in the Figure 40 that potential is decreasing slight
10. both the perspective material and method of study Second interest was in finding capabilities of certain SPM modes e g AFM KPFM KPFGM in such investigation For this purposes the NT MDT NTegra Aura system was used This device allowed combining the Contact Semicontact AFM topography measurements with KPM modes namely force mode and force gradient mode The chosen technique and device permitted the accurate study of surface properties however the application of such system put definite restrictions to our experimental conditions Limitations and inaccuracies can be distinguished to six main categories device features creep of piezo ceramics system background noise and time of scanning software used mainly feedback delay and methods for data processing pumping system limitations only medium vacuum of 2 10 bar is possible to reach which causes limitation of the quality factor Q for cantilever s tip oscillation and water film of few nm thickness existing on the sample s surface cantilever and tip properties large size of the cantilevers surface lead to an additional electrical interaction with the surface the tip form is not clearly defined at the same time with the tip radius which can lead to convolution effects and restrictions of the lateral resolution found at best on device used as one nm in AFM tens in KPFGM and about hundred in KPFM tips have certain range of softness applied voltage was limited by 10 V
11. by charging 7 V 10 sec for Sample 7 which is thought to be nearly 570 nm for standard KPFM mode under the same mentioned charging conditions Thus it is possible to track the linear approximation for the red line on Figure 29 which will give the result of nearly 25 nm for maximum lateral resolution for KPFGM This value is in agreement with the level of resolution of KPFGM discussed in literature 27 47 The results for Sample 7 charging series of three points 7 V 10 sec are presented on the Figure 45 which was preliminary processed by uniform transformation in Nova image Analysis 57 O A Q S e E 2 zm 240 nm nm gt gi e f J I BN A ce 0 i a Taa Plane um E 20 co 70 5 71 0 TAs 72 0 TAZ um Figure 45 Potential profile and KPFGM image of charged 7 V 10 sec series for Sample 7 Still the better lateral resolution doesn t allow to speculate the certain values for potential of the spots and mainly KPFM AM was used in our study However it s possible to think of the charge values in degrees and track down the Potential height time dependence similar to the presented above for KPFM Pot t 1 t Special programs for these transforms can be used The large noise level didn t allow to find the actual size of the spots L required for calculation of mobility diffusion coefficient and activation energy 0 12 N EC sample 6 Tos 3V 10 sec Linear Fit of L
12. device it is possible to protect the sample and measuring system from acoustic noise and to obtain the depressurized atmosphere this feature is missing in BRUKER Multimode 8 basic configuration In enhanced devices the hood can decrease the affect of electromagnetic noises and undesirable optical radiation After the pump is switched ON the safety hood is plugged into the vacuum pipe Vacuum is used for many reasons e g it creates the reproducible atmosphere and it minimizes the affection of dust particles and gas molecules to the cantilever and tip This increase the Quality factor Q However the most important effect of pumping is the drying effect The fact is that atmosphere air contains water molecules as moisture Therefore in reality all the surfaces are covered by a water layer of few nanometers thickness It can affect the measuring regime of the AFM topography and also be the reason of charge dissipation In reduced atmosphere the surface tension of water is lowered down dramatically and thickness of the aqueous water layer decreases It is required to apply the additional heating up to 350 C to dry the surface completely but it is apparent that sample and device are not suitable for such heating Residual water layer in our measurements is chemically adsorbed on the dielectric surface and its thickness has a value of nearly 3 nm It must be noted that the resonant frequency parameter for cantilever fo is changing in time becau
13. dielectric constant k 32 However experimental data related to high k applications is not available and its systematical study is required according to authors of 8 2 3 Properties and features of LaLuO LaLuO oxide thin films made of a stoichiometric ceramic target by Pulsed Laser Deposition PLD has proven 8 its appropriate morphology of nearly 2 nm roughness AFM stoichiometry by X Ray reflectometry XRR La Lu 1 1 1 and dependence of dielectric constant to the growth conditions 8 11 For instance the thermal method PLD has shown higher results for k 2 32 in nearly two times in comparison to the value for the film grown in room temperature k 17 Both internal photoemission IPE and photoconductivity PC measurements have shown the value of energy barrier 5 3 eV The Capacitance Voltage C V measurements have shown low leakage current density depending on the film thickness which resulted in calculating the Capacitance Equivalent Thickness CET giving the k 32 Further study of electrical properties of the LaLuO dielectric thin films was concluded as significant 14 3 Methodical Section 3 1 Scanning Probe Microscopy SPM fundamental and classification Scanning probe Microscopy is the type of microscopy techniques in which a physical probe is scanning the sample s surface The start of this microscopy was established by foundation of the Scanning Tunneling Microscope STM in 1981 In STM surface
14. first time Lateral spreading of the charged spots indicate the diffusive mechanism to be predominant in charge dissipation This allowed to estimate the diffusion coefficient and mobility Using simple electrostatic model it was found that charge is partly leaking into the interface oxide layer TIIVISTELM Lappeenrannan teknillinen yliopisto Teknillinen tiedekunta Matematiikan ja fysiikan laitos Pavel Geydt Kelvin Probe Force Microscopy KPFM characterization of lanthanum lutetium oxide high k dielectric thin films Pro gradu tutkielma 2013 69 sivua 48 kuvaa 4 taulukkoa ja 2 liitett Tarkastajat Professori Erkki L hderanta C Sc Mikhail Dunaevskiy Hakusanat high k eriste LaLuOs s hk varaus Atomivoimamikroskooppi KPFM Lantaani lutetium oksidiohutkalvoja LaLuOs tutkittiin erityisesti niiden k ytett vyyden kannalta teollisessa mikroelektroniikassa Pyyhk isymikroskopian SPM avulla voitiin kuvantaa pinnan topografiaa ja tutkia sen s hk isi ominaisuuksia Ty ss vertailtiin materiaalin ominaisuuksia varausk ytt ytymist 6 nm ja 25 nm leveill n ytteill sek my s k ytettyj SPM n eri toimintatiloja Erityisesti k yt ss oli kelvin probe force mikroskopia KPFM jolla tutkittiin paikallisia potentiaalieroja tarkalla sivuttaistarkkuudella Mittauksissa havaittiin eroja morfologiassa varautuvuudessa ja varauksien haihtumisessa molemmissa n ytteiss Polaarisuusilmi havaittiin ensimm ist
15. in bending of the cantilever Then bending angle is detected on the photo detector by shift of laser beam and recorded by system at each point Finally tip s trajectory profile is displayed as the scanned line Probe surface interaction is described by attraction repulsion model When the tip is close to the surface then it is engaged in complex power interaction due to the elastic properties of atomic shell 15 It is possible to distinguish three areas of elastic impact depending on value of applied force as described in Figure 5 16 n contact LENNARD JONES potential repulsive interaction W mode AFM ionic or Pauli repulsion 2 for distances d 2 js Non contact a g fo amp Ved mode AFM V d 4 c K a dj L 0 repulsion attraction short range long range tip sample distance d attractive interaction Van der Waals forces for distances d gt 2 Ve d Figure 5 Lennard Jones potential equation and curve 15 mage courtesy of Soft Matter Physics Division University of Leipzig Germany 0 const This Figure represents graph of the Lennard Jones potential In the left part of the curve can be seen a sector of Contact Mode The probe is in direct contact with the sample it pushes the surface The strength of applied pressure is given by the system as the SetPoint parameter in such way that tip do not create destructive impact to the material it also depends
16. or contamination area then will be monitored exactly the properties of these imperfections instead of the sample material properties When formulating a task operator of SPM device should consider the existing information about the sample and abilities of microscope i e the Modes The definite impact to the sample gives reaction in real time and examiner relies on personal experimental sense However critical mistakes can be prevented by an approximate first probe experiment General basics presented below including system parameters MAG DFL amplification lift height dZ Voltage etc and sequence of handling the images can be assumed as universal for future investigations 4 1 LaLuO thin films Two samples of high k dielectric lanthanum lutetium oxide were objects of our investigation Samples were obtained from the Laboratory of Research Center Julich Germany and they looked like dark squares nearly 1 cm each The first sample it is further called Sample 6 due to its catalogue name consisted of Si wafer covered with 6 nm thick LaLuO3 coating made by MBE technique in room temperature Due to the observed features defects of its morphology little attention might be paid on the basics of this method Molecular beam epitaxy is a technology based on the evaporation of material to the crystal substrate wafer applied in extra high vacuum conditions It can be used for growing the heterostructures and thin films however
17. ordinary influence of structural defects on the properties of the sample as well as fragility and specific imperfections of the probe tips 64 Finally on the basis of the results along with review of literature related to the issue of Kelvin probe microscopy few possible ideas for further research can be named 1 All the previously mentioned shortcomings should be eliminated Especially errors in the measurement of time 2 Limitations of the measuring system can be overcome by using a more advanced device Thus ranges of exposure might be expanded vacuum heating cooling moisture content voltage etc Also it can be possible to measure the mechanical characteristics e g adhesion and stiffness which is possible in advanced systems 3 Study the impact of the surface water layer and its properties e g Newtonian properties also in case of ionic and viscous fluids The forces occurring might be valuable in description of SPM operation in different modes and Nanolithography of charge 4 Study the impact of electromagnetic radiation on the LaLuO3 surface potential properties and lithography both in cases of dark light conditions 5 Using the developed methodology seems meaningful to study various classes of structures ODs photovoltaics in light dark fiber materials penometals polymers even biological structures such as living cells 6 To develop the computational methods of data analysis for different SPM mode
18. parameter is the resulting value of light intensity for photo detector Due to the peculiarities of the reflection from the cantilever surface and the photo detector s positioning in space the final setting of the probe must be conducted by the system MAG parameter For this purpose in the APPROACH option with indicated DFL on the left two 38 additional windows can be switched on to be indicated 1 the AIMING spot and 2 a plot for MAG in time domain See Figure 21 With the help of photo detector s screws a maximum value of MAG 15 is needed to be obtained 3 Setting the scanner At first the electronic calibrations can be installed in the system by pressing the Settings Load calibrations Except from this step the scanner should be mechanically calibrated In Nova scheme active window the CLOSE LOOP XY NL buttons are used for these purposes followed by pressing the RUN button On video screen or ocular of the optical microscope there can be noticed movement of sample in respect to the cantilever and its shadow These calibrations are overwhelmingly important due to the nonlinear mechanical properties of the piezo ceramic tube The tube can stagnate with time and thus creep effect will result in the image s artifacts see below Finally the buttons NL XY CLOSE LOOP should be pressed again in reversed order 4 Creating the vacuum conditions The safety hood should be put upside measuring Head and sample In NTegra Aura
19. sessi n nennen nana 31 3 4 1 NT MDT NTegra AURA device features sssssessseesseeeeeennn nennen 31 3 4 2 BRUKER Multimode 8 device features sssssseeeeeeeeeeeennnn 32 3 5 Advances in SPM equipment and techniques essere nnne nnne 32 3 6 Software for data and image processing ccccccccccsssesseeeceeeecaeeenseeececeeeseaaeeeeeeeeeeeaaa 34 4 Experimental Parte es oe eee eee ea RUNE E EE ERE SEY 35 4 T LaLuOsthin fitis sccdsavecesdsevenenecovencscvevenescvacenesesacenevecesesesesaseveveseseventargevexeats caeecicaetevectets 35 4 2 Sequence of the measurement esee sn nn aan eaa aa ana aaaaana 37 EI cc M 44 5 1t Topogtaphy eir te eee eter there teh etu eire ed 44 5 2 Electrical charge behavior ssssessseeesssseseeeen nennen nennen nennen nnne nsns nana 45 52 15 Electrical chargeability neret etri 45 5 2 2 Limiting potential of charging ree EEEE EE e a Katk 49 5 2 3 Induced charge relaxation timMe ccccccccccccccssssssesecceeeeseeesseeeeeeeesseaaeeeeeeeeesaeaagess 50 5 2 4 Temperature dependence cccccccssssssssecccceccaeaeseeeeceeeesaaeesseeeeeeessaeaaseeeeeeeesaaaggass 53 5 25 The etfect of polarity Of charge du pP 56 5 2 6 Force gradient measurements eooo E e A EE AE ETER 57 5 3 Nanolithography ObDSErvatiOns ccccccccccssssssseeccececsaeeeeseeeeeeeeseeaeeseeeeeeeesssaeaseseeeeeeeeaaa 59 Conclusi
20. the methodology described above 43 5 Results 5 1 Topography The topography measurements demonstrated smooth surface Figure 25 with 2 nm noise 8 nm 14 10 um 56 2 u 48 46 44 48 42 40 Figure 25 The 3D recovery of surface topography for sample 6 with artifact on the right Few found artifacts did not affect the results of our experiments at least for the future study only the homogenously smooth surfaces were taken into account To improve the resolution of scans it was needed to find proper value of height i e the Driving distance between the tip and the sample in Semicontact mode which was carried out by applying the proper MAG system parameter nearly 2 0 3 0 nA E NEM P i 7 n Pa N yd H ff f Jj 2 tf N i J rf n if j i f 2 f e E f j n i 3 p E 100 50 0 50 100 150 Height nm Figure 26 Mag z curve to define the Driving distance value in Semicontact Mode For correct measurements the graph for Mag z Figure 26 should be obtained straight irregularity due to the Young s modulus is seen and its slope multiplied by the Mag provide nearly 80 nm amplitude for Semicontact 44 5 2 Electrical charge behavior Charging experiment includes two stages 1 Charging of the point by Nanolithography LITHO in Nova software and 2 Measurement of the Surface Potential As a result the topography scanning is operated in AFM Semic
21. transistor technology Two possible regimes of movement can be distinguished in nanolithography They are separated by the type of used sample image and tip movement consequence The first algorithm is called Vector lithography It uses the specified commands of tip trajectories as simple geometrical objects squares points lines circles etc High operating speed is the main advantage of Vector lithography In our work the charging experiments were performed with this type of Nanolithography by using points to inject the charges The second algorithm is called Raster lithography because it uses the raster images to obtain information of the required impact The tip is moving by the whole image line by line as pixels are measured to obtain value of color intensity While AFM scanning measurements are providing mapping by desired Parameter x y in Raster lithography the system uses information of every pixel to result its color intensity on the specimen This type of lithography was tested in our work See Section 5 3 to obtain microscopic image of LUT logo 30 3 4 State of the art systems for SPM The equipment for surface investigations has been developed since it has shown prospects for high technology applications The first topografiner was invented in 1972 which gave the basis for construction of STM 29 A large variety of other Microscopes was launched by scientific groups however first prototypes are usually intended to be sin
22. 10 54 50 ce ei 35 J Normal t distribution ei N j e STA M N J XL 0 0 5 1 0 1 5 2 0 2 5 3 0 um Figure 41 Potential distribution image of Sample 6 2 measured 2 min after charging 6 V 10 sec 30 C Double Gaussian shape can be explained by existence of the interface oxide layer IL between LaLuO and Si wafer Tunneling into the IL is the reason of second acclivous curve which spreads faster than the first narrow curve Time for this divergence was nearly 10 min In the Figure 41 is shown the potential distribution profile for Sample 6 2 after two minutes from charging 6 V 10 sec 30 C with standard Gaussian shape calculated as Normal distribution The observed difference can be admitted to the acclivous second Gaussian This leakage effect should be examined in more details mV Sedo eo e eO S con eco dO 0 5 0 05 1 0 15 2 0 25 30 Plane um Figure 42 Surface Potential profiles for Sample 6 2 measured 2 min after charging 6 V 50 C with different lithography duration 10 sec and 30 sec Comparing the results for Sample 6 2 with charging by different durations widening can be found on the charge profile with increasing the charging length for Sample 6 2 See Figure 42 55 5 2 5 The effect of polarity of charge While measuring the limiting potential for Sample 7 the difference in heights on the potential profiles was o
23. 1500 2000 2500 3000 3500 Time sec Figure 36 Potential height Pot t dependence for different charging duration for Sample 6 The results for three charging times are shown in Figure 36 It is seen that charging time reflects on the height of potential curve Also it affects its shape Gaussian becomes thicker FWHM and spot lateral size L become wider with increasing the duration of charging The relaxation time is more than one hour not measured exactly in this experiment It should be mentioned here that after the experiments with charging on Sample 46 the measuring system NTegra Aura was devacuumized After 3 hours of reaction with ambient atmospheric air moisture and water film should be remembered the charge spots were still visible See Figure 37 According to the potential height at start 198 mV the result after 3 h was 58 mV i e fallen by 3 4 times while FWHM increased only by nearly 30 nm 51 Thus Sample 6 shows high sustainability to the ambient conditions and remarkable relaxation time of nearly 2 hours e wo in st in N in O in co T O r lt i N r O N S limiting potential of charging series e 1V 3V 2 E 3V 1V g O OS 0 18 20 7078 IE 8 ONS 2 08 886 um Figure 37 KPFM Surface Potential image of area after 3 hours on atmosphere marked blue For the Sample 87 was carried out similar experiment with longer charging lengths 10 sec 30 sec 90 sec Relaxation ti
24. 38 40 f v N v Help Apply py Width 128 Height 12 CAUsers Acer Desktop DIPLOMA Pavel redact Pavel old o6pN26 2 LUT mdt 25 Surface Pc Ready Figure 24 The Nova Image Analysis main window with settings on the right 42 In the DATA option of Nova press Analysis button on the right Few operations as Subtract plane are usually needed To remove the feedback delay effects artifacts Select Region option can be used Select region Flatten correction 1D Fit lines by area Fit lines by X Exclude the area selected To reduce the noise level Fourier Analysis can be operated In the FTT filtering option it is needed to choose peaks except the central one As final result the fitted and organized image is obtained Therefore it becomes possible to measure potential lateral size and other characteristics of the injected charge Due to the varying names of the settings and wide opportunities of the image processing software they seem to be out of range of this Master s Thesis Few artifacts can be named to be avoided in measurements They appear due to 1 Incorrect cantilever selection For example for living cells only those who have spring constant 0 01 N m should be selected Some cantilevers have much larger stiffness 5 10 N m By using such cantilevers to the soft objects their surface would be destroyed in Contact Mode The information
25. HfSiO4 11 6 5 1 8 La205 30 6 2 3 Y203 15 6 2 3 a LaAIOs 30 5 6 1 8 LaLuOs 32 5 2 2 1 Besides the mentioned parameters the locality of charge can be considered to be necessary for storing the data in SONOS and NROM technologies The locality can be measured by the 12 charge dissipation in the thin films by the measurements of surface potential However precise device is required to track charge behavior position migration and dissipation Three main mechanisms are discussed to explain the charge dissipation 6 7 1 Charge leakage into the conductive silicon wafer This mechanism is driven by Tunneling effect The total injected charge Q is exponentially decreasing in time domain and observed by decrease of local potential Q is also called integral charge since it is calculated as the integral of surface profile curve multiplied by surface area of the local charge 2 Charge drift It is described as the Coulomb repulsion of charges of same sign This causes lateral drift current jg i t P W E The total charge is not changing but the same time local potential is falling down concurrently with lateral widening of charged spot 3 Diffusion mechanism This mechanism of charge dissipation can be described as random walk of charges via trapping centers Total charge Q do not change diffusion current is given by formula jairr D Vp where D is diffusion coefficient and p is the density of charges Observed lateral widen
26. LAPPEENRANTA UNIVERSITY OF TECHNOLOGY Faculty of Technology Degree Programme in Technomathematics and Technical Physics Pavel Geydt KELVIN PROBE FORCE MICROSCOPY KPFM CHARACTERIZATION OF LANTHANUM LUTETIUM OXIDE HIGH K DIELECTRIC THIN FILMS Examiners Professor Erkki Lahderanta C Sc Mikhail Dunaevskiy ABSTRACT Lappeenranta University of Technology Faculty of Technology Degree Programme in Technomathematics and Technical Physics Pavel Geydt Kelvin Probe Force Microscopy KPFM characterization of lanthanum lutetium oxide high k dielectric thin films Master s thesis 2013 69 pages 48 figures 4 tables and 2 appendices Examiners Professor Erkki L hderanta C Sc Mikhail Dunaevskiy Keywords high k dielectric LaLuO3 local charge AFM KPFM Lanthanum lutetium oxide LaLuOs thin films were investigated considering their perspective application for industrial microelectronics Scanning probe microscopy SPM techniques permitted to visualize the surface topography and study the electric properties This work compared both the material properties charge behavior for samples of 6 nm and 25 nm width and the applied SPM modes Particularly Kelvin probe force microscopy KPFM was applied to characterize local potential difference with high lateral resolution Measurements showed the difference in morphology chargeability and charge dissipation time for both samples The polarity effect was detected for this material for the
27. MBE is exigent and rather slow method with growth 35 rate nearly 1000 nm hour Vacuum required for this technique is 10 Pa the cleanness of the materials must be at least 99 999999 Material is evaporated in heated tigel and then transferred by the molecular source to the heated wafer 45 The basic scheme of MBE operation is presented in Figure 20 SILICON WAFER REGULATORS 2 DOPANTS AP eA 9 A Lu La O S S a SOURCE HEATERS Figure 20 Experimental facility scheme a and device b used for MBE Adopted from Image courtesy of Gusev A I Academic Russia To prevent confusion in the further presented Results must be separately noted that Sample 6 was divided in two parts before the temperature measurements to avoid overheat The exact part of the sample used for further investigations was called Sample 6 2 Results for its comparing investigations are presented on the Figure 38 and in Section 5 2 4 The own potential of this sample was measured to be nearly 0 8 V 0 6 V and partly it was attributed to the noise of working Thermal Module Nevertheless the potential difference between own and applied potential values was considered in further calculations Note that Sample 6 2 was originally a part of Sample 6 thereby they were expected to exhibit same properties however the measured properties were different It is assumed to be caused by structural nonuniformity of
28. PFM 19 KPFM Energy resolution Spatial resolution mode meV FM Possibly sub nanometer resolution depending on tip apex 10 20 Typically 25 nm sub nanometer resolution also possible AM 5 depending on sample 29 3 3 Nanolithography of charge Atomic Force Microscope is not only the instrument to study surface but also a device providing modification of the surface condition in nanometer scale Firstly the sharp probe tip can be used in manipulating atoms 28 however for large solid samples it is possible to call another valuable option of AFM the lithography Programmably controlled tip movement can be combined in this technique with applying the impact force i e strong pressure or electrical voltage to obtain the modified atomic state on the surface during the tip s trajectory drawing Since the scanning regime is operated in non destructive impact when system uses the mentioned SetPoint parameter to influence the surface atoms only with elastic force the lithography is different by the enlarged value of SetPoint for mechanical modifying Second possibility occurs when the external voltage is applied to the tip in accordance to the sample With charge nanolithography it becomes possible to inject the required amount of electrical charges into the sample usually dielectric which could be used to deposit information in bits Another option is oxidizing the small area of semiconductors in
29. Sample 46 Only two coatings were investigated in this work The second sample it is called Sample 7 consisted of Si wafer with 25 nm film of LaLuOs made by the Pulsed Laser Deposition PLD technique with additional heating 450 C PLD is a preparation of coatings by condensation to the substrate surface of the products of reaction between the target and with impulse laser beam with power nearly 10 W cm These methods are widely used in production of thin layers See Table 4 each of them have advantages and weaknesses For our study only the quality of the surface is meaningful and it should be pointed out that MBE is considered to show excellent surface quality 36 Table 4 Thin film deposition technique comparison chart 46 ALD CVD Sputtering PLD MBE Thickness uniformity Good Good Good Fair Fair Film epitaxy Fair Poor Poor Good Good Stoichometric uniformity Good Good Fair Good Good Number of materials Fair Fair Good Good Fair Low temp deposition Good Poor Good Fair Fair Production yield Good Good Fair Fair Poor 4 2 Sequence of the measurement In this sequence is presented basic operational principles of Scanning Probe Microscope NT MDT NTegra Aura The device allows measurement and operating of the data with Nova software package Presetting Preliminary all the facilities should be turned ON and warm up for few minutes 1 The probe installation Operatin
30. Tip radius is critical factor for limiting the resolution of AFM scanning e g for 10 nm radius the lateral resolution of topography is limited to few nm Usually tip radius have rather large value from R 30 nm for Tungsten coated to R 20 nm for thin Platinum coated and R 2 nm for Si tips without additional coatings Coatings increase R Figure 10 but they provide special features e g ability to measure electrical or magnetic properties A tip coating seems to be fragile and limits the possible voltage range for electrical measurements by 10 V If the metal layer will be broken it can cause convolution effects seen in the measured topography Figure 10 SEM image of NN T190 HARS tips radius 50 nm angle 12 Image courtesy of K Tek Nanotechnology NT MDT Russia 20 Elasticity coefficient of cantilever ky is in interval 0 001 N m 10 N m 17 k is related to the magnitude of displacement of the tip height AZ and force F by equation dk SA The smaller kr the more suitable probe is for measuring delicate specimen such as living cells typically 0 01 0 03 N m Large k values are used in tapping mode since magnitude of the forces is less to increase the scanning speed For the correct working conditions AFM tips should provide the resonant oscillation properties The resonant freguencies of the cantilever oscillation have bandwidth 10 1000 kHz labeled by manufacturers Bending freguency is determined by the
31. about softness is always mentioned by manufacturer 2 Exceeded value of scan rate When there is a small disturbance in the analyzed topography i e a hill then tip position will be higher than the surface for some moment of time however cantilever is still moving This can lead to the stretched lines after the roughness found in Semicontact topography At the same time tip is always returning to the start position on the scan image originally to the left side thus the artifacts would be seen from the left side The light area on the Figure 23 does not mean that there is some object on the left of the image it is surely an artifact Compare Figure 22 with the handled image on Figure 4b 3 Incorrect lift height dZ In KPFM when applying high values of dZ drive amplitude would be larger than possible for measurement of Surface potential It is seen how tip slips upwards from the sample because of the imperfections in the scanned surface Figure 23 The mentioned artifacts can be reduced by proper operation conditions Thus the system parameters procedures and settings shown above seem to be valuable for practice However some operations are called in different way in other systems and few items calibration and landing can be done automatically in more advanced devices Nonetheless the experience and skills are required for proper operation of the SPM The results of measurements of LaLuO presented below are obtained with the help of
32. alculate the derivative of force F 1 dC Fpc 3 77 Uac U x y 28 dF 1d C E m 2 gzz Uae U x y which corresponds to the phase shift 2 Ag x y zo Uk U x y Thus by measuring the phase angle dependence of U x y and finding its minimum it becomes possible to define U x y with significant accuracy 21 27 The accuracy is better because dF dz substantially decreases the electrostatic interaction of the sample with tip cone and cantilever which both seem to be considerable but independent from z i e dConst 0 Table 2 Comparison of methods of measuring the surface potential 19 Spatial Method Description Energy Resolution resolution Better than 10 KPFM Measuring local CPD of the sample surface 5 20 meV nm KP Measuring CPD of the whole sample surface 1 meV 50 nm 26 Measuring energy spectroscopy of the Better than PES 20 meV whole sample surface 100 nm Measuring electron beam induced current Not a quantitative Better than 70 SEM to map the surface potential method nm When comparing AM and FM methods of KPFM one should mention that regardless the lateral resolution of the KPFM FM is higher data is usually recorded in degrees of phase shift This is because KPFGM mapping is based on distortion of the phase fluctuations In order to get values in mV special conversion is required Table 3 Typical spatial and energy resolution of FM and AM mode K
33. as constant in time domain as in the previous experiment 49 159 100 150 Figure 34 Surface Potential profile for the series of charges For sample 6 there are no observed limiting potential according to Figure 33 and potential profiles shown in the Figure 34 The Sample 7 does not demonstrate the existence of critical potential difference such as the Sample 6 at even wider range of 7 V 7 V We preferred to use the 3 V 10 sec charging regime for ability to compare effects of temperature and time Critical Voltage for tips was nearly 10 V thereby it was necessary not to go over this limit 5 2 3 Induced charge relaxation time Charge cannot be located within the dielectric layer for the unlimited period of time after injection This is observed when potential value U decreases Relaxation time is the time length when the peak value of potential drops in e 2 718 times It is supposed to be one of the main parameters of charge behavior thus it is needed to define precisely the certain potential values and real times of the measurements by algorithm described in Section 5 2 The series of 3 points was injected in the Sample 6 to find the dependence of potential with time Pot t for different charging duration 1 sec 10 sec 30 sec See Figure 35 50 um 690 70 0 71 0 m 1 sec 240 e 10 sec 4 30 sec 220 200 180 160 S t 140 Q amp 120 0 500 1000
34. bserved See Figure 43 a c The existing own average potential of Sample 47 was negligibly small about 0 02 V thus it couldn t affect the results 130m 3018 ce se Voss mV cato ec mV 10 oi oi o 3 S a 0 05 10 15 20 25 30 35 40 45 50 b 0 05 10 1S 20 25 30 35 40 45 50 Figure 43 Surface Potential profile for a Charges 7 V 7 V b Charges 7 V 7 V reversed Surface Potential images of c Charges 7 V 7 V d Charges 7 V 7 V reversed It can be noted here that for Sample 6 such effect wasn t apparent See Figure 33 because of the experimental sequence for the Sample 6 the mentioned series of charged points was prepared for some operating time assumed as 10 seconds for charging of every next point and additional 5 seconds for manual switching of modes According to Figure 27 the dependence of voltage is not absolutely straight two points are higher i e have larger slope than the others See green line on Figure 27 These points are concerned with the positive 1 V and 3 V charging while blue points are associated with negative voltages 5 V 3V 1 V Thus green and blue lines on Figure 27 for Sample 6 reveal dependence of polarity sign The polarity effect can be anticipated for both Samples 6 and 7 To prove the effect we interchanged the applied potential differences from 7 and 7 to 7 and 7 to exclude the factors of the measuring time See Figure 43 b
35. d 10 Influence of ambient pressure and moisture conditions was established Depressed atmosphere is causing the decrease of water layer thickness and contributes to the charge retention and retardation of the potential decrease which was demonstrated in the temperature measurements for 6 nm thick sample 11 At the same time LaLuO has shown high sustainability of injected charges in common room conditions It was demonstrated that after the device was devacuumized for 3 hours such room air and moisture exposure was followed by detection of charge still remaining in the surface of dielectric layer for 6 nm sample 12 Morphological difference between the two high k coatings obtained by MBE and PLD techniques was determined 25 nm film made by PLD possesses better structural characteristics of surface uniformity which is compulsory for industrial applications Concurrently the 6 nm sample obtained by MBE method had structural defects and the increased capabilities for injection and reservation of charge for Nanolithography 62 Summary In this work LaLuOs was investigated for the surface potential mapping by Kelvin probe microscopy for the first time Results for high resolution AFM measurements of surface morphology are also demonstrated For this purposes the methodology of Scanning Probe Microscopy measurements was developed on practice Measurements were conducted using multifunctional NT MDT NTegra Aura system providing opport
36. e 1989 J Weaver H K Wickramasinghe nm lateral resolution absorption images spectroscopy Scanning chemical potential microscope 1990 C C Williams H K Wickramasinghe Atomic scale images of chemical potential variation Photovoltage STM 1990 R J Hamers K Markert Photovoltage images on nanometer scale Kelvin probe force microscope 1991 M Nonnenmacher M P O Boyle H K Wickramasinghe Contact potential measurements on 10 nm scale Apertureless near field optical microscope 1994 F Zenhausern M P O Boyle H K Wickramasinghe Optical microscopy at nm resolution Appendice II continued Future technologies taken from 40 S Morita Roadmap for Scanning Probe Microscopy 2006 with STM in the 1990s STM STS using a well controlled tip with a defined electronic orbital at its apex after 2015 i Inelastic Tunnel Spec 5 STM observations of 10 layers below surface in 2015 koomaan amma in 1986 STM measurements with phase of tunnel electrons in 2013 STM with Standardization of STS with in situ controlled tips in 2010 some mages STM STS with hybrid functionalized tips in 2009 Combined multi STMs with atomic resolution in 2008 STM STS combined with nc AFM in 2006 2000 2005 2010 2015 Fig 2 3 Roadmap of STM 3 Atomic Force Microscopy 21 Atomic resolution imaging of hydrophilic and hydrophobic interactions A
37. e of both 6 nm and 25 nm thick samples The surface inhomogenities have range of 2 nm 2 Structural artifacts were found in sample of thickness 6 nm obtained by MBE technique by the Surface Potential studies in KPFM mode The objects were up to 2 um in diameter had an ellipse shape and consisted of negatively charged core with potential of 30 mV and positively charged elongated cloud area with 140 mV The background noise in KPFM was nearly 4 mV 3 Samples of LaLuO are capable for the charge lithography though their susceptibility to the applied potential differs in more than three times At room temperature and medium vacuum conditions value of the measured potential growth for 3 V was at level of 200 mV for 6 nm thick film while for 25 nm thick film the growth was nearly 50 mV This can be corresponded to the 4 fold distinction in sample width due to the formula E U d 4 Lateral resolution for samples in KPFM with tungsten tips had range of 350 nm the application of thin platinum tips with radius of 20 nm increased resolution only up to 250 nm 5 Lateral resolution of nearly 100 nm was achieved in KPFM gradient mode with thin platinum tips while maximum achievable resolution was estimated to be nearly 25 nm Therefore KPFGM is more preferable for dimensional studies of area with injected charge 6 Lateral size of the charged area was broadening with time as t i e in accordance with the Diffusive model of charge sp
38. e technologies are using mainly Si3N4 as a gate dielectric however the search for suitable materials still continues The material should provide significant density of charges per nano size local volume 2 2 High k dielectrics Models of charge dissipation Hafnium compounds are used for processors of 22 nm technology by Intel Corporation in 2013 4 Hafnium oxide HfO satisfies the essential criterions for prominent high k semiconductor oxide it is the most used and studied high k The requirements of a new oxide are 5 1 k value must be high enough to be used economically for a reasonable number of years 2 The oxide is in very close contact with the Si channel thus it must be thermodynamically stable with Si 3 The oxide must be kinetically stable and able to be processed at 1000 C at least for 5 seconds in present process flows 4 The oxide must act as an insulator by having band offsets with Si of over 1 eV to minimize carrier injection into its bands 5 The oxide must form a good electrical interface with Si 6 The oxide must have few bulk electrically active defects New candidate for gate oxide is required since 2009 despite the high k of Hf and HfO Table 1 Comparison between semiconductors for probable replacing of SiO 5 k Gap eV CB offset eV Si Fal SiO 3 9 9 3 2 Si4N4 7 5 3 2 4 Al Os 9 8 8 2 8 not ALD Ta Os 22 4 4 0 35 TiOo 80 3 5 0 SrTiO3 2000 3 2 0 ZrO 25 5 8 1 5 HfOs 25 5 8 1 4
39. e value of surface response potential U in mV to the voltage V applied to the tip dU dV m sample 6 t sample 7 B negative positive S E S cC 2 o n Voltage V Figure 27 Electrical chargeability of Samples 46 and 47 Note the green squares for positive and blue squares for negative charging for Sample 6 Black curve is for their absolute values 45 The chargeability of samples 6 and 7 as linear slopes for red and black curves differs nearly in three times which can be seen in the Figure 27 It is easier to inject the charge into the Sample 6 Data is suggested to be given for same moment however blue dots show slightly lowered values due to the time difference But the main reason for observed difference for these charge injections will be discussed later Here are given maximum values obtained in experiments with no loses occurring due to the surface defects or electrical system noises 46 47 48 49 50 um 45 42 43 44 41 61 62 63 64 65 um 66 67 68 69 70 200 150 artifact charge 100 50 m defect o 61 62 963564 65 068 67 650 1698 70 a b m Figure 28 KPFM results for Sample 6 after charging 3 V 10 sec a AFM surface scan 10x10 um is smooth b Surface Potential Mapping reveals charge in the center FWHM 365 nm Time for the observed drop potential is more than 1 h for Sample 6 and 0 3 h for Sample 7 Peak is decreasi
40. ed It has optional modes to develop the system parameters and possibilities However the most commonly used modes are included in basic construction AFM STM Phaselmaging MFM KPFM PeakForce Torsional Resonance mode etc 32 This device is operated with the ScanAssyst technology to simplify the operational algorithms for researcher Some of the modes are proprietary ScanAssyst PeakForce KPFM PeakForce Tuna mode etc Device provides a larger variety of operating conditions and scanners from 400x400x400 nm to 125x125x5 um and capable to investigate the mechanical properties of fragile objects polymers and living cells 33 3 5 Advances in SPM equipment and techniques In works 19 21 23 28 are presented calculations of the advantages of the vacuum for AFM scanning It is explained in view of the increasing of Quality factor of the cantilever s oscillation because less amount of gas molecules are hitting the tip Vibrations become easier and their magnitude MAG increases Thus it is possible to decrease the system multiplying parameters which cause additional noise At the same time vacuum has a drying effect and water layer covering the sample disappears It can lead to better interpretation and accuracy of the results because water layer conceals the adhesion and accelerate the charge leakage Few research groups are still considering the properties of the liquid 34 36 in their works and they describe the mechanics of the wa
41. ential profile and KPFGM image of charged 7 V 10 sec series for Sample 7 Experimentally obtained dependence of L t of charge for Sample 6 3 V 1 sec Charge nanolithography in Sample 46 raster sample litho image 3D reconstruction Charge nanolithography in Sample 86 Negative sample and Surface Potential image Appendice II History of SPM 10 12 13 14 15 16 17 18 19 20 21 22 23 Table 1 SPM techniques and capabilities SPM techniques and capabilities Scanning tunneling microscope 1981 G Binnig H Rohrer Atomic resolution images of conducting surfaces Scanning near field optical microscope 1982 D W Pohl 50 nm lateral resolution optical images Scanning capacitance microscope 1984 J R Matey J Blanc 500 nm lateral resolution images of capacitance variation Scanning thermal microscope 1985 C C Williams H K Wickramasinghe 50 nm lateral resolution thermal images Atomic force microscope 1986 C Binnig C F Quate Ch Gerber Atomic resolution on conducting non conducting surfaces Scanning attractive force microscope 1987 Y Martin C C Williams H K Wickramasinghe 5 nm lateral resolution non contact images of surfaces Magnetic force microscope 1987 Y Martin H K Wickramasinghe 100 nm lateral resolution images of magnetic bits heads Frictional force microscope 1987 C M Mate
42. esis is oriented on special variety of technigues and modes i e AFM KPFM and KPFGM another name is KPFM Frequency Modulation 15 3 2 Atomic Force Microscopy AFM main components and principle of operation Atomic Force Microscopy AFM is an experimental method to study local properties of the surface based on Van der Waals interaction between a solid probe tip and the sample surface The first Atomic Force Microscope was invented in 1986 by G Binnig K Quate and K Gerber Due to nanometer sharpness of the tip probe the AFM has nanometer and even sub nanometer atomic resolution 13 14 Depending on the type of tip sample interaction it becomes possible to measure the local parameters of topography surface potential mechanical properties stiffness adhesion friction magnetic properties etc Photodetector Laser beam Cantilever Figure 4 Operational principle of AFM Image courtesy of Connexions Rice University USA The operational principle of the AFM is based on mechanical force between the probe and the surface and the measured system parameters are describing the relief as opposed to the STM MSM and other techniques A special detecting console is used to register roughness It is called cantilever and include sharp tip at the end Figure 4 Van der Waals interaction defines the certain force acting the tip corresponding to the SetPoint however surface roughness creates additional force which results
43. fine the desirable materials KPFM seems to be appropriate technique for such investigations It allows to study the local potential with both accuracy of potential and high lateral resolution Due to the two pass technique the surface topography and surface potential mapping are obtained simultaneously Growing number of papers concerned with fundamentals of KPFM and its application for research of electrical properties of semiconductors proves its significance Despite the fact that dielectric constant of LaLuO is record high which is believed to be essential for gate oxide experimental data revealing its surface electrical properties is missing One can find only literature of LaLuO growth conditions crystal structure and morphology but no available data of chargeability surface potential and charge carriers mobility which are necessary for industrial applications Due to the prospective properties of LaLuO the desired study was carried out Thin films of LaLuO 1 6 nm obtained by MBE and 2 25 nm obtained by PLD at 450 C were investigated in idea of possible semiconductor application It was presumed to measure surface morphology and electrical properties compare the methods of growth of such films and to determine possibility of nanolithography for LaLuO3 Therefore the motivation of this work was to investigate the properties of high k dielectric thin films of LaLuO3 by means of Kelvin Probe Force Microscopy i e merging
44. force applied to the tip resonant frequency dielectric constant cantilever s stiffness lateral size of the charged spot quality factor of the cantilever oscillations tip radius charging duration relaxation time potential difference bending frequency loftiness mobility work function of the material numerical coefficient for different vibrational modes phase shift 1 Introduction Silicon Integrated Circuit IC technology has rapidly developed driven by the continuous increase in device functionalities Facing the growing demand in computational performance of microchips the more effective semiconductor devices are required While crystal size has been decreasing in last four decades at the same time number of transistors per crystal is growing intensively Thereby the transistors performance is satisfying the Moor s Law Nowadays the size of transistor nanoelements is on industrial range of recently designed and fabricated 22 nm devices produced from 2012 But it is known that size decreasing results in undesirable heating Furthermore a size less than 5 nm for transistors is unachievable due to quantum restrictions and emerging exponential losses of electrical current The idea of decreasing voltage seems not applicable because voltage has a predicted minimum of 0 2 V Second solution is in the increasing of the dielectric width to prevent the formation of undesirable capacity on the gate That s wh
45. formula 16 A JEJ E pS where is the cantilever s length E is Young modulus J is a cantilever s moment of inertia o is material density S is the cross surface area and X is numerical coefficient for different vibrational modes Figure 11 L A 3 52 A 22 04 A 617 Figure 11 Major mechanical modes of tip s bending vibrations 16 Quality factor Q is related with resonant frequency fo and width df of Mag f resonance curve For vibrating cantilever Q is a measure of energy loss of oscillation fo 300 kHz Q in air is nearly 100 18 19 Q Af In UHV conditions Q grows by factor of few hundred nearly 500 In addition Q can lead to the explanation of the increasing resolution of gradient mode mentioned earlier Considering time scale of amplitude change in force mode 18 it is 2Q T fo However in phase modulation gradient method 1 Tt fo Thus time scale t is nearly 500 times smaller for UHV which is reason for rise in spatial resolution 19 21 The Scanner Scanner is a device that moves the sample relatively to the AFM probe to perform raster scanning in AFM Piezo scanner consists of a radially polarized piezo ceramic tube made usually of PZT material with metal electrodes coating on the four sides Figure 12 Scanners with constructions of plates and bimorph elements are also possible Two types of mounting the scanner are used First is scanning by sample when piezo
46. g with the Scanning probe microscope is done not only by the computer and SPM device but also by the hands Since probe installation is a delicate procedure and it is performed manually it is needed to follow the regular algorithm Firstly the probe is taken from its box with adhesive coating on the bottom It should be lifted by the short side probe is rectangular with the help of tweezers At this time the small dots on the long sides can be found by eyes though with difficulty It is the cantilevers itself with length of nearly 150 uim and width 35 um Sizes can be noted descendingly probe 5 mm cantilever 35 um tip 5 um tip s apex 20 nm The cantilever is recognizable only with optical microscope and tip s apex is touching the surface to observe its shape an electronic microscope is needed The sample of investigation is placed on the polymer plate made of policor protective compound and fixed This plate is put on the scanner carefully without applying too much force on the fragile piezotube Then the sample surface is electrically grounded to the Earth The measuring Head with probe holder should be placed above the sample in distance of 3 mm with the help of Head s screws Otherwise tip can touch the sample and become rendered unusable 37 2 Setting the probe Using the Nova software in AIMING option the maximum value for laser intensity on the photo detector should be obtained Thus it is needed to turn the sc
47. gle option devices 30 Nowadays multifunctional devices have appeared which provide opportunities for comprehensive and precise investigations Few prominent SPM platforms can be briefly mentioned Each of them offers appropriate features 3 4 1 NT MDT NTegra AURA device features NTegra Aura device is the SPM for studies in the controlled conditions of low vacuum gases liquids and external magnetic fields with more than 40 measuring modes included 31 STM AFM contact semi contact non contact MFM EFM SCM Kelvin Probe Microscopy Lithography etc This allows investigating physical and chemical properties of the specimen with accuracy almost 2 nm The system permits high frequency regime of operation which is essential for vibrating oscillations in Semicontact mode At the same time sensitivity of the synchronous detector is up to 0 01 degree Scanning system realizes the scanning by sample scanning by the probe and double scanning modes of operation Maximal scanning field is limited by 0 2 mm x 0 2 mm x 20 um with scanning step nearly 0 001 nm Device was used in this work Its main internal components are presented on Figure 18 Figure 18 NTegra Aura device without vacuum hood mage courtesy of PortalNano ru Ministry of Education and Science of Russia 31 3 4 2 BRUKER Multimode 8 device features Multimode 8 device provides opportunity to use a variety of SPM methods with highest resolution and operational spe
48. graphy roughness Van der Waals interaction is denied while tip is used as a reference electrode KPFM differs from EFM because in II pass an additional feedback loop to the voltage Upc is applied so that F vanishes It is achieved when voltage applied to the probe Upc begins to change and adjusts to the feedback as long as F not equals to zero at each scanned point Z x y This occurs if Upc U x y then values for certain points is recorded by system as local value of U x y Therefore map of the surface potential is obtained KPFM provides the highest lateral resolution of local potential measurements in comparison to all other techniques KP PES SEM See Table 2 KPFM was first presented by Nonenmacher in 1991 24 and method is recommended as unique tool to characterize the electric properties of semiconductor metal surfaces and semiconductor devices at nanoscale Figure 16 Demonstration of a AFM tip used for KPFM 25 and b Kelvin Probe 26 27 It should be noted that measured local potential difference is equal to the work function of the 9tip 9 sample e surface electrons U x y Vopp Where Psampie and Prip are work fucntions of the sample and tip and e is electron charge 19 With direct contact and applied electrical potential Fermi levels of both materials are aligned thus potential of the sample will shift to the level of tip The external electrical bias nullifies the current simultaneously the vo
49. he distance However this mode is widely used as a part of two pass technique In this regime of AFM operation in first pass first scanning of the line the Semicontact topography is measured and then on the base of the topography the tip goes above the same line with constant specified uplift height Zig It seems like the tip returns back to the left side of the image line and tries to show zero topography In second pass the strong long range forces can be measured e g electrostatic force in KPFM The measurement in second pass is more sensitive due to the absence of Van der Waals forces and also it is more precise due to the z vertical gradient of measured forces That is because of the simple assumption that only tip s apex is interacting with point on the surface but not the whole tip cone and rather big cantilever plate In second pass such forces can be negotiated and force influencing the tip is connected only to the apex which gives correction to the position of cantilever found from equation k is cantilever s spring constant 16 dF ak Simultaneously phase angle is shifted Q is Quality factor i e measure of energy losses Q dF a The phase shift of the cantilever Ap is measured by the block unit in accordance to shift in resonant change of DFL regarding the exciting electrical signal Since Ouality factor and stiffness are known for cantilever thereby measuring the phase shift it is possible to calculate the derivati
50. ich is pointing the beam onto a cantilever and b 4 sectional photodiode measuring the intensity of laser light reflected from the cantilever to each of its four sections See Figure 14 b In order to improve the reflection a special coating is applied on the back side of the cantilever e g a thin metal film 23 Photo detector Figure 14 Simplified scheme of the feedback working principle a and photo detector b 16 Before measurements the system is adjusted in such a way that laser beam hit the cantilever and fall into the exact center of 4 cell photo detector The intensity of light falling on each section should be the same When additional force F for example caused by the interaction of the tip with the surface topography appears in scanning this leads to a bending of the cantilever Cantilever bending causes changing in the angle of the reflected laser beam thus observed shift of the laser spot at the photo detector appears The presence of four sections in photodiode permits measuring these small shifts by the difference in photocurrent from different sections Measurement of the angle of the cantilever deflection DFL allows measuring the tip surface interaction force In Figure 14 it is also shown the feedback system FB FB performs a regulation function to maintain a constant influence on the probe in a constant force regime it is F Minimum resolution of forces in the AFM can be calculated by 19 2kk TB
51. idn t affect the results um Figure 32 3D reconstruction of the surface defect found in KPFM Surface Potential One can conclude that samples are uniformly smooth and chargeable nearly 200 mV and 50 mV for samples 6 and 7 respectively in 3 V 10 sec regime For more precise description of charge behavior we conducted further research The artifacts found in Sample H6 See Figure 32 should be avoided 48 5 2 2 Limiting potential of charging It was necessary to find the range for non destructible voltage that can be applied to the material in our research as well as the limiting potential The series of charges was injected to LaLuO film at the distance of 1 um The charging time of 10 sec was identical for all five charging voltages 5V 3V 1V 41V 43V Then we used the KPFM for mapping of the charges Figure 33 The difference in time of the measurement for certain spot must be accounted at the same time as local potential of the surface itself nearly 0 02 V However one can suggest the difference in the charge properties depending on the charge sign further discussion of the effect of polarity is given later Figure 33 Surface Potential image of a charged series for Sample 6 charging duration 10 sec Comparable results for peak height of the potential profile for 3 V 10 sec charging 180 mV nearly the same as in our previous experiment was found Potential curves were decreasing slightly and FWHM w
52. ing for charged spot is proportional to t Results for ternary rare oxides e g DyScO3 and GdScO3 have shown its promising conformity for abovementioned six requirements with observed k 20 35 The studied Sc based oxides has shown even better values of k 28 33 with more appropriate morphology and energy band structure 8 11 Measurements of LaScO has shown its considerable locality in range of hundred nm though it is less than lateral size observed for SiO with embedded Si nanocrystals material SiO nc Si which was measured at best to be nearly 25 nm 11 high k oxide Figure 2 Experimental scheme of charge measurements by AFM Injection scanning 7 13 Further results for LascO3 have shown the predominant mechanism of charge leakage when local charge observations were performed by Atomic Force Microscope The scheme of experiment is shown on Figure 2 A sharp tip of a Microscope is injecting charge by applying bias voltage Then AFM tip is scanning the surface to study the map of surface potential thus obtaining the values of potential decrease and lateral spreading with high accuracy It must be noted that tunneling was claimed as the main mechanism for LaScO3 however the lateral widening was found It was explained by diffusion inside the interface layer IL The role of this oxide layer remains unclear In literature was discussed one more La based oxide LaLuO3 and it has shown even higher value of
53. is attached to a sample holder used in NTegra Aura device Sample surface is moving and pattern is measured more accurately because optical detection system is not moving Second assembly is scanning performed by probe tip when sample has a fixed position and piezo scanner is attached to the moving probe z electrode Az y electrodes gt x electrodes Figure 12 Operational principle of piezo scanner s tube movement The piezoelectric effect is used for precise movements of scanner Piezo ceramic resizes under an applied voltage The equation of the inverse piezoelectric effect 16 uj dijk Ex where uj is strain tensor E is electric field component dix are the coefficients of the piezo coefficient s tensor Tensor of piezoelectric coefficients depends on the properties of piezoelectric ceramics When voltage applied to the x electrodes have different signs tube is deflected in the x direction See Figure 12 central image same situation for y electrodes Thus probe can be laterally moved along the surface in the x y dimension Upon application to the z electrode 22 voltage with respect to both x y electrodes See Figure 12 right image either elongation Az or shortening of piezo occurs depending on the sign of the voltage It enables to displace the probe in z direction normal to the surface Thus movement of the probe in three dimensions x y z is possible for scanning Scan areas range from few
54. ita Roadmap of Scanning Probe Microscopy NanoScience amp Technology 2006 41 P Vettiger et al The Millipede More than one thousand tips for future AFM data storage IBM J Res Develop 44 323 2000 42 Leo Gross Recent advances in submolecular resolution with scanning probe microscopy Nature chemistry 3 2011 68 43 Gwyddion software website Accessed 17 May 2013 http gwyddion net 44 Yo Shen et al Simulating and interpreting Kelvin probe force microscopy images on dielectrics with boundary integral equations Rev of Sci Instrum 79 023711 2008 45 John R Arthur Molecular beam epitaxy Surface Science Vol 500 2002 46 Joon Hyung Shim Ph D Thesis Nanoscale Thin Film Ceramic Fuel Cells Stanford 2009 47 M S Dunaevskiy et al Nanolocal charge injection in thin SiO layers with an embedded nc Si by the AFM tip JTP Letters 33 20 2007 in Russian 69 Appendice l List of Figures Figure 1 MOSFET and flash memory constructions Figure 2 Experimental scheme of charge measurements by AFM Figure 3 Types of Scanning Probe Microscopy Family of KPM methods Figure 4 Operational principle of AFM Figure 5 Lennard Jones potential equation and curve Figure 6 Scheme of scanning process in SPM Figure 7 AFM Constant force and Constant distance modes with topography Figure 8 Distance in Semicontact mode Three principles of AFM modes Figure 9 Scheme of the cantilever
55. kertaa t m nkaltaisissa n ytteiss J nnitepisteiden sivuttainen levi minen viittaa hallitsevien mekanismien olevan diffuusiivisia Yksinkertaisen elektrostaattisen mallin avulla huomattiin varauksien osittain vuotavan rajapintakerrokseen Acknowledgements am pleased to thank people who influenced on my interest in the subject of this Master s Thesis Since was always keenly interested in the natural sciences the work in this area has been for me a truly exciting and meaningful experience I tried with all diligence to understand the problems of Scanning Probe Microscopy and found the prospects for further fruitful research in the field of physical science First of all want to thank my supervising Professor Erkki Lahderanta for his help in choosing a topic support at all stages of the Diploma Thesis and for the possibility to study at Lappeenranta University of Technology Without him this work would have been impossible would also like to express my admiration and deepest gratitude to the staff of Laboratory of Optics of Surface loffe Physical Technical Institute RAS Individually Professor Alexander Titkov for his professional help and support of my interest in Probe Microscopy material support and responsive leadership Also my second supervisor Mikhail Dunaevskiy for his mentorship and intensive help in writing the final version of the Master s Thesis Then of course Prochor Alekseev and Peter Dementyev for things wha
56. ltage value is defined by system as the local contact potential difference Therefore this method permits to calculate the sample work function if the tip s i is known Concurrently the information from second harmonic can be further processed by system to get information of the local dielectric constant local capacity and its high frequency dispersion 3 2 3 Force gradient mode in Kelvin Probe Microscopy KPFGM KPFGM is the development of KPFM mode by using the information of the force gradient dF dz instead of force F for processing data 19 21 23 In second pass of KPFGM the phase shift A is measured instead of cantilever oscillation amplitude change This is why it is also called the KPFM FM Frequency Modulation mode while KPFM is a common Amplitude Modulation AM mode KPFM When measuring the phase shift of the resonance cantilever oscillation the resolution is considerably higher Table 3 than that for amplitude measurement Figure 17 Amplitude change Frequency change H V N JA Fixed freguency V uU VA p 3 JA VAN v V Fixed amplitude W Da A a nm gt Z position of tip 1 Z position of tip x 8 W it Sample Surface 3 i a i Sample Surface Figure 17 Comparison between Amplitude Modulation a and Freguency Modulation b 19 Mathematical description of KPFGM is discussed in many works as it seems to be perspective technigue The essence of theory becomes clear if we c
57. ly as in the previous measurements and showed Pot t 1 t dependence The expected decreasing with temperature is connected to the rise in conductivity and mobility of charges within the dielectric layer However there is another conflicting effect that should be accounted When temperature is growing the layer of the water molecules is getting thinner The charge leakage 53 by this chemically adsorbed water film is terminated thus potential should decrease more slowly 590 59 5 60 0 60 5 61 0 61 5 a b 9 595 60 0 60 5 610 61 5 Figure 39 KPFM Surface Potential image of Sample 6 2 after charging 6 V 10 sec 30 C a t 3 min Pot 31 mV b t 17 min Pot 15 mV DN NPN U Q o o O N N N N A 20 Potential mV 0 200 400 600 800 1000 1200 1400 Time sec Figure 40 Potential height Pot t dependence for different temperature conditions for Sample 6 2 6 V 10 sec In the case of the temperature measurements one more probable effect was found in the surface properties One can see the complex structure of the potential profile for 30 C the first measurement See Figure 41 which is cross section of Figure 39 a The potential curve which had the bell shape earlier here demonstrate the complex shape of two hills which is not concerned with curvature of the surface it is the double Gaussian form Such effect is in accordance with the results for dielectric film with similar composition LaScO 7
58. me values were smaller nearly 0 3 h However size of the spots was nearly constant FWHM 360 nm for all three injected charged spots and didn t depend on the charging length Further investigation should be carried out on this issue 180 170 160 150 140 130 120 V o 100 90 80 70 60 50 Potential m 30 20 sample 6 e sample 7 4 sample 6 2 e 3 o T T T ja T y r T T 1 0 200 400 600 800 1000 1200 Time sec Figure 38 Potential height Pot t dependence for Samples 6 7 and 6 2 cleavage 52 Figure 38 shows comparison of the potential decreasing for different samples under similar conditions 3 V 10 sec room temperature It is seen that both surfaces of 6 and 7 have same Pot t 1 t dependence Results for Sample 6 2 See Experimental consequence in Section 4 1 means the results of non defect surface while the Thermal Module was turned ON Applied Voltage was even larger 6 V 10 sec however the measured potential for the scanned area is 6 times smaller One must note that such Module gave additional noise and affected the resulting values Still the tendencies Pot t 1 t are similar for all three presented results Second probability to describe the lowered values is by the impact of the defects on Sample 6 Earlier it was considered to be negligible in our experiments but defects could partly result into positive outcome on charging Still it can
59. n multitip platforms Millipede 40 41 This construction allows measuring the surface with increased speed and can be used in Nanolithography for production of precise marks on the coatings to save data The multiprobe scanning probe microscope SPM in which several tips or cantilevers are moved independently is supposed to be a versatile tool for electrical characterization at nanometer scales 28 A big amount of works is discussing the developing of chemical bond study due to the opportunities to detect electrical forces at sub nanometer range 40 42 Here can be noted that some details of chemical interaction and chemical reactions can be studied using the Scanning Probe Microscope platforms preciseness OF i o Om 1 i EFM with sub Uhra high resolution Li difficulty of technology i picosecond EFM using point dipole 2 L H s au development of high i spatial resolution and i i JB long durability probe for nano device 2015 atomically resolved EFM of microwave circuit 1999 eal cim semi sed charge distribution in 2007 QU local Fermi level 5D charge distribution liquid 1999 mz electric potential inbio moleculeor A measurement 2012 iui atomically resolved dl 212 _living cell 2015 local work function semicond uctor device analysis by AFMP 2008 measurement in vacuum 1999 nm
60. nanometers to several tens of microns depending on scanner and the voltage applied Piezo scanner in AFM can move probe relative to the sample in all three directions x y Zand scan with accuracy nearly 10 m 20 AZ reaction Figure 13 Piezo ceramic disadvantages a nonlinearity b creep c hysteresis 16 Piezo ceramics have deficiencies 16 which should be considered when measuring and storing the scanner First of all nonlinearity of piezoelectric ceramics exists Figure 13 a This reveals in deviation from the linear dependence of the change in piezo length with high unit voltage over 100 V mm Second effect is creep Figure 13 b which is the delay in response to the controlling field V This is usually seen in the first scanning point as appearance of a white strip in left side of the frame That s why first point is usually cropped by imaging software and not visualized Third some inaccuracy always exists because of hysteresis properties of piezo ceramic tube to change the length in direction of the electric field Figure 13 c This is the reason why measurement is carried at one direction which is mainly forward see Figure 6 Photo detector Photo detector is the device to measure the deflection caused by the force in the AFM tip in real time of scanning the surface Figure 14 a For this purpose the optical detection system is used It is measuring the bends of cantilever and consists of a 1 mW laser source wh
61. ndependent of freguency w from the first and second harmonics by w 19 1dC 1 Foo 2 5 Uac UG y Vac ac E dz Uac U x y Uac sin wt Foy a do Uae cos 2ot It can be seen that the first harmonic of the electrostatic force F depends on the local value of the potential U x y for the AFM probe Amplitude of the forced oscillation frequency measured in II pass for the cantilever at w is proportional to the magnitude of the first harmonic of the electrostatic force F Since the values of dC dz Upc and Uac are recorded in Il pass the resulting mapping of F x y will contain information only about the distribution of the surface potential U x y Force accuracy in this method is piconewtons It should be noted 26 that the measured difference AV includes not only the capacity value of the probe and the sample but also local potential value CPD 19 This value characterizes the local properties of the surface heterogeneity influencing the magnitude of the electron work function and the embedded charge which will be described for the case of KPFM 3 2 2 Kelvin Probe Force Microscopy KPFM KPFM is a two pass microscopic study of surface potential 22 23 KPFM is similar to the principle of EFM Topography is measured in I pass Semicontact mode After that probe is uplifted and in Il pass the magnitude of electrostatic interaction of sample with an oscillating probe is studied Thus topo
62. ng slightly and it has the bell shape or so called Gaussian form See Fig 41 FWHM nm 400 300 100 m sample 6 KPFM e sample 7 KPFGM Voltage V Figure 29 FWHM vs Voltage dependence for Sample 6 KPFM and Sample 7 KPFGM Coordinate of expected intersection of the FWHM axis denotes operational limit for lateral size in KPFM modes 46 The full width at half maximum FWHM parameter is not changing in time in our experiment FWHM for Sample 6 is 365 nm 3 V 10 sec Figure 28b while for Sample 7 it is estimated as 360 nm Thus the FWHM do not depend on the sample Using the 20 nm thin Platinum tips FWHM 300 nm was achieved for Sample 7 not presented here however by using KPFGM method lateral resolution is better than 100 nm in same conditions Figure 29 19 27 Though the FWHM parameter strongly depends on the applied voltage According to the Figure 29 it is possible to track the linear approximation to the potential axis to find the value for the maximum lateral resolution about 250 nm for KPFM and 25 nm for KPFGM Thus the KPFGM mode is preferable for the measurements because of better resolution however it cannot provide the numerical values of the Surface potential because it is measured in degrees of phase shift That s why for our further experiments KPFM mode was used tracking the values in mV results for KPFGM see later One must admit that strange shape
63. ng with cantilever s resonant frequency with distance almost 100 nm above the sample without touching it When vibrating tip is getting closer to the surface repulsive force is growing and amplitude of oscillations is decreasing thus feedback system is regulating the specified SetPoint value Feedback commands the scanner to shrink thus sample is again moving from the tip until amplitude becomes corresponding to SetPoint One should note that while in Contact mode SetPoint gt DFL in Semicontact mode SetPoint lt amplitude MAG In such way the middle line of the cantilever trajectory is kept constant this distance from surface dZ is used as relief Ideally dZ must be equal to half of Amplitude of oscillations In this case probe bites the surface in its slowest position and impact is more gentle even applicable for living cells Contact mode i etPoint gt DFL n S Z 7 lift distance n cili am s Figure 8 a Distance in Semicontact mode 16 b Principles of three AFM modes 17 Non contact mode See Figure 8 b corresponds to the case when the tip is oscillating with its own resonant frequency fo and it is not touching surface at all The half amplitude of 18 oscillations is less than distance between surface and cantilever s middle line Zur lift height i e 10 100 nm Usually this mode is not applied at room temperature due to the weak dependence of tip sample interaction from t
64. nology NIST 2001 30 Russell Young John Ward and Fredric Scire The Topografiner An Instrument for Measuring Surface Microtopography Rev Sci Instrum 43 999 1972 31 NTegra Aura device Technical details from NT MDT website Accessed 20 May 2013 http www ntmdt com device aura ii 32 Bruker Multimode 8 technical details from BRUKER website Accessed 23 May 2013 http bruker com products surface analysis atomic force microscopy multimode 8 technical details html 33 Barbara Foster New Atomic Force Microscopy AFM Approaches Life Sciences Gently Qualitatively and Correlatively American Laboratory 24 2012 34 T Fukuma and S P Jarvis Development of liquid environment frequency modulation atomic force microscope with low noise deflection sensor for cantilevers of various dimensions Rev Sci Instrum 77 043701 2006 35 B Reischl et al Free Energy Approaches for Modeling Atomic Force Microscopy in Liquids J Chem Theory Comput 9 2013 36 Aleksander Labuda and Peter Grutter Atomic Force Microscopy in Viscous lonic Liquids Langmuir 28 2012 37 G Binnig H Rohrer Nobel Lecture Scanning tunneling microscopy from birth to adolescence Rev Mod Phys 59 1987 38 H O Jacobs et al Resolution and contrast in Kelvin probe force microscopy J Appl Phys 84 1168 1998 39 J Hutter J Bechhoefer Calibration of atomic force microscope tips Rev Sci Instr 64 1993 40 Seizo Mor
65. o thin dielectric layers J Appl Phys 110 084304 2011 8 J M J Lopes et al Amorphous lanthanum lutetium oxide thin films as an alternative high k gate dielectric App Phys Lett 89 222902 2006 9 J M J Lopes et al La based ternary rare earth oxides as alternative high k dielectrics Microelectronic Engineering 84 2007 10 P A Aleksev et al Locally injected charge behavior in nanothin films of LaScO3 high k dielectric on Silicon wafer JTP Letters 39 9 2013 11 Y Y Gomeniuk et al Electrical Properties of LaLuO3 Si 100 Structures Prepared by Molecular Beam Deposition ECS Transactions 33 3 2010 12 G Binnig H Rohrer Scanning tunneling microscopy Helv Phys Acta vol 55 6 1982 13 G Binnig C F Quate and C Gerber Atomic Force Microscopy Phys Rev Lett 56 1986 66 14 Bharat Bhushan Editor Scanning Probe Microscopy in Nanoscience and Nanotechnology Vol 2 NanoScience and Technology Springer 2010 15 Holger Sch nherr and G Julius Vancso Atomic Force Microscopy in Practice Scanning Force Microscopy of Polymers Springer Laboratory 2010 16 V Mironov Fundamentals of Scanning Probe Microscopy N Novgorod 2004 in Russian 17 NanoWizard Handbook JPK Instruments Version 1 3 p 8 2005 18 T R Albrecht et al Frequency modulation detection using high Q cantilevers for enhanced force microscope sensitivity J Appl Phys 69 668 1991 19 W Meli
66. objects were found in the first measurements of Surface Potential See Figure 28b The object can be described as ellipse with length nearly 2 um highly negative core and positive cloud area shifted to the core See Figure 30 The concentration of these objects seemed to be nearly three on the 20 x 20 um scan image They were preliminary distinguished as the surface defects caused by mistakes in MBE method of obtaining thus the area with ellipses were not further examined 525910 0 5 m m 15 10 30 25 20 15 10 20 53 5 54 0 54 5 55 0 55 5 56 0 56 5 a b 0 5 10 5 KL 2 5 30 35 Figure 30 Surface Potential for ellipse shaped defects in Sample 6 a Mapping b Profile 47 140 100 120 140 120 2 g eye e e i R O N o o a b 05 10 15 20 25 30 35 Plane um Figure 31 Surface Potential for defects after charging 3 V 10 sec a Mapping b Profile Such artifacts should be distinguished in order to prevent affecting on the final measurements of charge behavior Although the ellipses have been perceived for the oddities reaction to the voltage applied was tested it was found that average 28 mV core potential Figure 30 is increased up to the 140 mV Figure 31 thus the potential difference is nearly 170 mV which is in good agreement with the surface area without defects compare with Figure 27 Still they were not found in further KPFM scans and d
67. on the probe s stiffness Feedback system maintains the constant value of SetPoint DFL Deflection parameter DFL corresponds to the measured force Measurement results in the two dimensional map of measured surface Parameter x y e g if the parameter is height Z then image shows the Z x y which is dimensional topography in every pixel of image Figure 6 Figure 6 Scheme of scanning process red is straightway blue is forward 16 Data recording is performed in straightway j is number of pixel line is number of position i j 256 1024 Relief in AFM can be measured in two possible regimes Constant force and Constant distance Figure 7 depending on number of included feedback loops It should be noted that the Contact Mode is not applicable for soft and living objects due to the significant forces used Perhaps it is the basis for precise measurements of solid specimen in metrology In the middle of graph of Lennard Jones Potential Figures 5 8 a it is possible to mark the area of Semicontact Mode measurements In this mode the probe performs harmonic oscillations and it rattles sample s surface The impact is less than in Contact mode 17 F const Constant force AZ F f 0000800 0000000 0000000 X a Scanning b Relief c Scanning d Relief Figure 7 AFM Constant Force a and Constant distance c modes with topography b d 16 Initially the probe is vibrati
68. ons xr RA RR OPE AR 61 Summary meteo ced dd dI dd dde d EEUU EU 63 ReTGrenCOSitsmusaataatrasta ae aeaa aaaea ean aeea a Tea eea Te aeaa Te Tea eae Tea eal eee HUN UKKKIKEKIE NNN 66 Appendices List of Abbreviations AFM ALD CET CPD DFL EEPROM EFM FWHM KPFGM KPFM LF MAG MBE MOSFET NROM PLD QDs SHINOS SONOS SP SPM STM UHV Atomic Force Microscopy Atomic Force Microscope device Atomic Layer Deposition Capacitance Equivalent Thickness Contact Potential Difference Deflection signal difference between top and bottom halves of the photodiode Electrically Erasable Programmable Read Only Memory Electric Force Microscopy Full Width at the Half Maximum of signal Interface oxide Layer Kelvin Probe Force Gradient Microscopy KPFM FM Kelvin Probe Force Microscopy Amplitude Modulation Difference signal between left and right halves of the photodiode Magnitude of AFM probe oscillations in Semicontact mode Molecular Beam Epitaxy Metal Oxide Semiconductor Field Effect Transistor Nitride Read Only Memory Pulsed Laser Deposition Quantum dots Silicon Hi k Nitride Oxide Silicon Silicon Oxide Nitride Oxide Silicon Surface Potential do not confuse with SetPoint which is system parameter Scanning Probe Microscopy Scanning Tunneling Microscopy Ultra High Vacuum List of Symbols D Trel AQ diffusion coefficient width of the dielectric layer electric field
69. ontact mode the Surface Potential mapping is made in second pass by KPFM To check the ability of the samples for charging the probe measurement is needed Thus the probe potential 1 V providing the potential difference of nearly 1 Volt to the surface local potential was applied for 10 seconds by the small sized tip s apex approximately R 20 nm for probes with Pt coating To process the charge behavior measurements it is necessary to define the real time on the scanned position on the line of the image If the scan time length is 3 min 10 sec but the desired spot is placed in the center of the scan then one can take the time for the charged point 5 sec 310 2 sec 160 sec rather than zero or 310 sec 5 seconds is the operational time for manual setting of the Surface Potential mode from the Nanolithography contact mode used for charging After defining the existence of charged spot 3 V 10 sec charging was used for control experiment See Figure 28b The AFM results show clean smooth surface while there is a charged spot on KPFM By automated measurement technique for the same position on the sample it is possible to preliminary track the peak potential drop and evaluate lateral size and relaxation time with non destructive impact to a surface To find the possible destructive potential difference the limiting potential experiment was arranged afterwards 5 2 1 Electrical chargeability The chargeability can be described as th
70. or some conditions The used tips did not allow the voltage more than 10 V In future studies seems reasonable to use blunt tips with lower resistivity for charge limit investigations The water layer affected the losses of total charge because medium vacuum is not enough to dry the surface properly Inert gases atmosphere high vacuum and heating at least 350 deg are required to thoroughly dry the surface This can be implemented in more enhanced devices However it was found that even medium vacuum conditions increase the Quality factor of the probes for the experiment in more than 50 times In terms of the experiment it would be worth to implement the line charging rather than point charging which is more relative to the actual industrial memory devices Nevertheless the objective factors of measured potential decrease and charge spreading do not depend on such form Also the experiments aimed to determine the engineering parameter of capacitance equivalent thickness CET associated with the relative value of dielectric constant supposed to be carried out It is worth noting that at the time of using the device few remarkable details were found on practice while attention to those is not clearly emphasized in the literature For example the sample drift caused by impact of the needle probe and thermal expansion had maxima as 5 micron per hour Also noted the existence of the wave front of charge divergence on the first scans non
71. reading In the first 10 min the injected charge is leaking from the film to the substrate which is revealed as decrease of the total charge Q and the existence of double Gaussian shape on surface potential profile in first measurements which is in accordance to the data for the similar materials in literature Tunneling into the interface layer was established as the reason of partial decrease in total charge in LaLuO3 dielectric thin films 7 Relaxation time for 6 nm thick sample is nearly 2 hours while for the 25 nm sample it is nearly 20 minutes The potential level is decreasing proportionally to 1 t 8 Charging time affects the size and the form of the surface profile Increasing of the charging duration lead to wider spot size bigger value of potential and thicker Gaussian shape Temperature exposure decreases the potential which is caused by rise in mobility of charge carriers The influence of charging time and temperature effect can also be attributed to the Tunneling of charge and Diffusive model of charge dissipation 61 9 A polarity effect was found for LaLuO which is in contradiction with the literature data for similar dielectric materials It was determined that for LaLuO charge is more than in two times easily injected by application of positive potential difference This fact may be explained by distinction in mobilities of charge carriers but in any case the results for impact of polarity should be further verifie
72. resolved local Fermi level electric potential measurement 2005 semiconductor device analysis by EFM almost established partly developed nm resolved local work function measurement 2000 2005 2010 2015 present 5 years later 10 years later Figure 19 Roadmap of EFM family by 2006 40 33 According to 28 atomic manipulation will become a common procedure in nearest future The Scanning Probe Microscopy Roadmap 2006 also calls Nanolithography as one of the most probably enhanced techniques in nearest decade 40 Finally it is worth saying that development of the data analysis could become as one of probable advantages in SPM for example in study of the KPFM FM Due to the improved resolution of this method it can be used to obtain the map of electrical properties However special treatment should be performed to obtain values of potential Partly this problem had been solved by AFM research group in loffe Institute 3 6 Software for data and image processing Software is used in Scanning Probe Microscopy at two stages to process the data feedback and to handle the scanned images Since all algorithms of processing the data have the same mathematical basis though with details certain image processing programs are strongly valuable Many purchasers of SPM platforms have their own appropriatory packages e g in this work is used NT MDT Nova Image Analysis FemtoScan can be supposed al
73. rews of probe holder and the resulting red spot cursor should be situated nearly at the center of AIMING window See Figure 21 here DFL is below zero LF is above zero Close to zero values for DFL and LF parameters would be desired Changes of these parameters will be used further by feedback system After that the laser spot should be placed right to the center of screen by manually rotating the photo detector s screws Values of system intensity LASER for platinum tips fpN11Pt and the Nova package are nearly 32 36 Nova 10261508 Debug Version File View Settings Tools SemiContact v T FB Gain 1 000 Mag 0 poems Scheme Aiming Video Landing Backward Moving 0 35 mm WE E Auto SetPoint Forward Moving 0 35 mm CEEA OLIN Points Period s Pause Color W Aver 1000 9 00 200 Figure 21 Working window of the Nova program Set regime is Semicontact used option is APPROACH chosen parameter SetPoint is 10 Further mentioned options are seen at the left up DATA AIMING RESONANCE APPROACH SCAN CURVES LITHO The system performs measurements of MAG parameter The AIMING window is seen at the right Finally in the RESONANCE option it is required to find the resonant frequency for the cantilever which has the value of 100 200 kHz indicated on the factory box It depends on the cantilever material stiffness its length temperature and individual features As it was told before LASER
74. ropriate value by exclusion 3 2 1 Electric Force Microscopy EFM Electric Force Microscopy is a two pass technique which enables to obtain not only the topography but also the surface potential U resulting in map U x y 21 Each line of the AFM frame is scanned twice Semicontact mode is called the I pass and it measures surface topography In the II pass non contact AFM is performed probe moves over the surface at a distance of Zi and repeats the trajectory measured in the I pass Additional voltage 25 U Uae UgcSin ot is applied between the probe and the surface Thereby AFM probe must be conductive e g it must be coated with a metal layer usually Pt or Au The electrostatic interaction energy of the probe with the sample is where C is the capacitance between probe and surface This capacity depends on the z distance between the probe tip and the surface Z component of the electrostatic force acting on the probe is dE dCU 57 dz an 2 In this case the derivative is negative for electrostatic attractive force Since the applied voltage is changing periodically the interaction force between the probe and the surface will also change periodically 1dc j F z t 2 dz Uac U x y Vac sin wt where U x y is the local value of surface potential at the certain position x y below the AFM probe The equation for the force can be divided into three terms distinguishing the part Foc which is i
75. s and conditions for more precise and automatic application For example to use different ranges of tip sample interaction force which is still developed in devices nowadays 7 Use more functional programs for image and data analysis and develop the processing software to compare the materials suitable for high k application The listed studies and development of experimental system may provide fundamental value and can be used to improve the existing technologies 65 References 1 S Sze Physics of Semiconductor Devices 3rd Ed Wiley Interscience 2007 2 Dov Frohman Bentchkowsky Floating gate transistor and method for charging and discharging same Intel Corporation Patent US3660819 1972 3 Marvin H White et al Advancements in nanoelectronic SONOS nonvolatile semiconductor memory NVSM devices and technology International Journal of High Speed Electronics and Systems 16 2 2006 4 S Borkar Design perspectives on 22 nm CMOS and beyond Proc 46th IEEE Design Autom Conf pp 93 94 2009 5 J Robertson New High K Materials for CMOS Applications Comprehensive Semiconductor Science and Technology 2011 6 G H Buh et al Real time evolution of trapped charge in a SiO layer An electrostatic force microscopy study Appl Phys Lett 79 2010 1999 7 M S Dunaevskiy P A Alekseev P Girard E Lahderanta A Lashkul and A N Titkov Kelvin probe force microscopy of charge dissipation in nan
76. se of pumping Frequency peak becomes narrow Quality factor Q grows up and shifts to the left by few hundred Hertz Simultaneously measured MAG parameter is growing up to nearly 50 and it should be decreased by system amplification settings Pumping lasts nearly 39 one hour until 10 bar and during this time it seems reasonable to set up the mentioned parameters of scanner and probe and then check feedback for further measurements 5 Setting the feedback In the Semicontact Mode it is used to protect surface from scrapping for the first measurement because it affects the surface less than Contact the SetPoint 0 6 MAG should be specified In the APPROACH option the LANDING button should be pressed By this action the oscillating probe tip will come close to the surface and in few seconds the defined system parameter MAG will become equal to SetPoint This is the demonstration of negative feedback when system reaction is keeping the MAG parameter constant in time Thus the tip surface influence is the same in all the measured areas of the sample Scanning in Semicontact mode is performed as the tip is oscillating with frequency nearly 200 kHz while cantilever is tracking the surface under the set distance if lower SetPoint then the position under the sample is lower One should keep in mind that distance between cantilever and sample is not measured as numerical values as well as the absolute value of MAG is not necessary to know I
77. seem valuable in case of found undesirable surface defects is given As this work has the methodical value the SPM and particularly KPM measurements are described step by step n chapter Results chapter the essence of the research is presented by the discussion of the measurements n Conclusions the obtained results of parameters and theories are combined by the statements and proposals followed by the Summary part where the entire work for purposes of the Master s Thesis is surveyed with justification of the obtained results and ideas for future studies 10 2 Semiconductors background Storing and processing the data can be claimed as main backbones for 21 Century of information Memory devices are required to perform these basic operations and they are recently based on the transistor s technology This semiconductor device is operating with gate drain and source as main constituents 1 Since charges are stored in nano small volume precise methods of their investigation are needed e g local potential measurements 2 1 Semiconductor materials and memory devices For instance one of the most widely used types of semiconductor memory is flash memory EEPROM data is retained for long period of time by transistors which include the data saving material under the gate Figure 1 The operational principle is based on injection of electrons by Tunneling mechanism into the floating gate 2 Since electrical charge is re
78. so as a multifunctional instrument of analysis while among the freeware programs can be mentioned Gwyddion 43 which relies on processing the images for a variety of file formats it is working in shade tones Here should be clarified that all images obtained in scanning are made with imitational colors since SPM is not an optical method 44 All images in the Results Chapter are presented in the red black tones 34 4 Experimental Part In this Chapter the methodology of our study is presented with experimental sequence One can confidently suppose that the experiment is largely dependent on the available information about the samples Without denying the chemical law of definite proportions method of growing the sample regime and conditions with preliminary visual information about the sample roughness reflection contaminants and fractures seems highly significant Frequently the required information is missing When measurements are carried out for other researchers often happens that they do not provide important information about their materials for the background It is a difficult question for Scanning probe microscopy specialists to examine the surfaces properly If any artifacts contaminants etc are even macroscopic apparent to the naked eye then results of the experiment can be distorted This is associated with high locality of SPM measurements now as a drawback of SPM if the measurements are performed on the defect
79. t is enough that these parameters are constant 6 Scanning in Semicontact Mode AFM In the SCAN option Frequency parameter should be set to 0 7 1 and chosen scan size to 10 micron then press RUN button It will take approximately 5 min to finish one scanning image of that size Further it is necessary to process the topography results See Section 5 1 and make decision about new scanning zone in case of any found defects or asperities One should remember that higher scanning rates varied by number of points and Frequency parameter can lead to linear artifacts See Figure 22 compare with results on Figure 28 b At the same time charges in our study are supposed to be dynamic systems thus it is needed to find optimum speed of scanning 8 3 g g F eo ES t Y g S g T 61 62 63 64 65 66 67 68 69 70 um Figure 22 The raw image obtained for Surface Potential It needs to be fitted 40 7 Setting the Kelvin probe Mode At first the Kelvin probe Mode should be turned on from Semicontact Then operate the procedure of checking a Open the II pass regime by the button of the same name b Watch the MAG SIN curve this signal is proportional to electrostatic force Fy on the oscillograph in the right and turn off the feedback c If MAG SIN is on its maximum then decrease the Lock in Gain then use Amplitude of electrical stimulation nearly 0 2 0 5 By changing the phase of the generator
80. t these people have taught me in practical research for their thorough answers to many of my questions and weighty moral support during my stay in St Petersburg Finally thank my dear friends for the exciting time of our studies and my beloved girlfriend Maria for encouragement and patience during the time of writing this paper Lappeenranta May 2013 Pavel Geydt Table of Contents Te Tvidgers Wesen ERE 8 2 Semiconductors background eeeesssessssssseseeee nennen eene ness sna naar nenne sanas 11 2 1 Semiconductor materials and memory devices sess 11 2 2 High k dielectrics Models of charge dissipation ccccccsssssscecccccecseseseeeeeceeeeseaaeseeseeess 12 2 3 Properties and features of LALUO3 sssssssseesee enne naa nnn nennen 14 3 Methodical Section cuu URSUS STN t t lt 15 3 1 Scanning Probe Microscopy SPM fundamental and classification 15 3 2 Atomic Force Microscopy AFM main components and principle of operation 16 3 2 1 Electric Force Microscopy EFM eeeeeeee nnne nnns 25 3 2 2 Kelvin Probe Force Microscopy KPFM seeeeeee nnns 27 3 2 3 Force gradient mode in Kelvin Probe Microscopy KPFGM eeeeee 28 3 3 Nanolithography Of Charge tmt tte eee ettet 30 3 4 State of the art systems for SPM sseesssesssssssseeen nennen enne nennen
81. tained inside the gate it switches the transistor into nonconductive state corresponding to the logical 0 When reverse Voltage is applied to the control electrode of such transistor the electrons are migrating back to the silicon and create the conductive channel corresponding to logical 1 Oxide Le Control Gate Oxide Layer 20V OV xide Layer b Floating Gate M Y 5i0 m high k MOS FET Flash Memory ov 20V a b C d Figure 1 a MOSFET and b flash memory construction c Write operation voltage applied to the control gate causes a tunnel current to flow through the oxide layer thereby injecting electrons into the floating gate d Erase operation voltage applied to the silicon substrate releases the electrons accumulated at the floating gate mage courtesy of TDK Corp Reducing the size of the elements as another trend of high technology leads to losses of current through the thin gate dielectric layer According to International Technology Roadmap for Semiconductors reports and recent manufacturing technologies the silicon dioxide is relic since 2008 and enhancement of materials with larger value of relative permittivity is mentioned as one of the way to overcome the size limits It is worth mentioning that even in transistors with another operating principles e g NROM SHINOS and SONOS structures electrons are stored in localized position preserving a bit of 11 information 3 Thes
82. tarting raster sample image b Image of the logo 2 5 x 2 5 um Light area is O V dark area is 5 V c 3D reconstruction image of the LUT logo The color gradation was not apparent for four supposed voltages To obtain better resolution and recognition it was needed to consider the effect of polarity While the positive charge is injected easier than the negative the positive potential difference should be associated with the symbol but not with the background Thus using the inversed negative lithography sample and larger lateral scale the symbol become recognizable See Figure 48 59 a T g 8 a m oo oe e nr N A 89 E g ey a 3 e U ie aki L ot eb ek 253 0052 40022 51002 6 a b um Figure 48 Charge nanolithography in Sample 6 a Negative Sample b Surface Potential image of the logo 8 x 8 um Light area is O V dark area is 5 V The charge was keeping its shape for a period of few minutes It can be noted that special techniques of sample correction should be used in lithography of charge as well as photomask correction is performed in Photolithography Demonstration of lithography by charging the points in LaLuOs thin films is a qualitative and visible proof of the possibilities of NTegra Aura system KPFM mode and the remarkable properties of the investigated dielectric for Nanolithography 60 Conclusions 1 AFM studies of surface topography indicated smooth and uniform surfac
83. ter layer tip interaction as well as water sample They are also studying the properties of ionic liquids 36 on the specimen and the affection of viscous liquids to the results of scanning These investigations seem necessary to be implemented in living cells investigation due to the fragile structure of the cell membranes which are also covered by liquid layer Progress in this study is expected in 2015 Figure 19 Distinguishing of the mechanical properties can result in the additional information about the surface adhesion stiffness and phase 32 33 The development of the mathematical basis of such systems has already been used by Multimode 8 device PeakForce QNM mode The main claim of the research groups is that tip structure seems to be predominant factor in scanning resolution Some researchers adhere to the idea of sharp nanometer thin tips even consisting of one carbon nanotube However in Binnig and Rohrer works in the 80s authors 32 already stated as fact though monatomic tips are necessary for STM the shape of AFM tips should be cone like 37 The fundamental work on the increasing of the resolution and contrast in KPFM had resulted in the idea of not sharp but blunt elongated tips 38 Tips should be accurately calibrated 39 and long durable probes providing high spatial resolution for SPM is predicted until 2015 40 Remarkable attention in enhancement of AFM is put to the 2 pass technologies of AFM or eve
84. the Voltage enough for lithography on the basis of chargeability See Chapters 5 2 1 and 5 2 4 After that set the Contact mode for scanning remembering the SetPoint unlock the feedback and set the Feedback to DFL SetPoint should be changed to be larger than DFL signal and after that close the loop Finally the charging is started by pressing the RUN button To scan the result it is needed to set the Semicontact mode set up the Feedback to MAG signal instead of DFL Type in SetPoint which was used before in contact mode then close the loop Finally the scanning is operated by pressing the SCAN RUN buttons 10 Handle the obtained images Software Nova Image analysis can be used for this purpose Figure 24 Second variant is Gwyddion package Nova seems to be more functional for appropriatory NT MDT file formats VT Image Analysis 1 0 26 1508 Surface Potenti jm File Image Analysis Curve Analysis Transform Filters View Options Help s HI EX Xxa0800Ko ZA e ofa LA A lati BOSSA PQQ VY 40238 Tfi M 1 FitLines misa Data Transformations Deconvolution Editing Equilization 4 Flatten Correction 1D Fit Average Fit by Average Poly Fit Lines Fit Lines by Area Fit Lines by Histogram Flatten Correction 2D Fourier Analysis O Statistics Section Analysis Grain Analysis 20 23 Fit Lines by Area H m Polynomial Order 4 Fit line 18 20 22 24 26 5 um Offset 35 40 um 32 34 36
85. tomic resolution imaging in a liquid Atomic resolution imaging of single spin Detection of molecule vibration identification of molecule 2000 2005 2010 2015 present 5 years later 10 years later Fig 3 6 Future predictions for novel AFM techniques
86. topography map of the heights with roughness is measured by the tunneling current in vacuum between probe and conductor 12 Since many opportunities were presumed for technology of semiconducting materials and insulators structure of the microscope was developed to the construction with small reflective cantilever plate which reflected the laser light onto the photo detector Working principle became independent from conductivity instead of it the Van der Waals attractive repulsive interaction revealed topography for solid materials and even liquids This construction including the photo detector and cantilever is called Atomic Force Microscope AFM 1986 13 Since these two devices provided significant enhancement in studying of surface properties their developers Binnig and Rohrer received Nobel Prize in 1986 _SPM_ STM AFM EFM NSOM Tunneling Wan der Vaals Work function SCM current 1 Contact SThM 2 Semicontact AKPFM PTMS KPFGM 3 Non contact E 0000 Figure 3 Types of Scanning Probe Microscopy Family of KPM methods For the last three decades at least 30 other types of Scanning Probe devices have appeared They distinguish the information source light radiation noise capacity etc Each of them permit measurement of specific forces e g electrical forces magnetic interaction The basic classification for SPM methods is given in Figure 3 It is impractical to discuss all the possibilities of SPM thereby this Th
87. tz J Shen A C Kummel and S Lee Kelvin probe force microscopy and its application Surface Science Reports 66 1 2011 20 NT MDT Ntegra Aura User Manual 21 M S Dunaevskiy CSc Thesis Atomic Force Microscopy of Si Ge nanosmall insulas Methods of diagnostics and nanolithography of charge S Pet 2007 loffe PTI RAS in Russian 22 S Sadewasser Th Glatzel Kelvin Probe Force Microscopy Measuring and Compensating Electrostatic Forces Springer Series in Surface Sciences volume 48 2011 23 A N Titkov P Girard Electrostatic Force and Force Gradient Microscopy Principles Points of Interest and Application to Characterisation of Semiconductor Materials and Devices Applied Scanning Probe Methods II NanoScience and Technology 2006 24 M Nonnenmacher M P O Boyle and H K Wickramasinghe Kelvin probe force microscopy Appl Phys Lett 58 25 2921 1991 25 NT MDT Sover Next User s guide 26 Kelvin Probe information site Accessed 17 May 2013 http www kelvinprobe info introduction surfaceanalysis htm 27 M Dunaevskiy et al Analysis of the lateral resolution of electrostatic force gradient microscopy J Appl Phys 112 064112 2012 28 Franz J Giessibl Advances in atomic force microscopy Review of Modern Physics 75 2003 67 29 J S Villarrubia The topografiner An instrument for measuring surface microtopography in A Century of Excellence in Standards Measurements and Tech
88. unity to study samples in vacuum The main highlights of this work can be established 1 By comparing the KPFM and enhanced KPFGM techniques lateral resolution 300 nm and 100 nm respectively was obtained for LaLuO films 2 The potential profile height is falling down proportionally to 1 t Concurrently the charged spots were widening proportional to t The total charge Q is leaking into the interface layer during first 10 min after that Q do not change At the same time with the complex shape of potential profile the dominant mechanism of diffusion was established for charge dissipation 3 The order for values of diffusion coefficient is 10 cm sec the order for mobility is 10 cm V sec These values are significant for comprehensive studies and comparing of high k dielectric materials they can be used to develop the technology of IC The establishing of the polarity effect should be emphasized By applying the positive voltage the total charge is nearly 4 times more stored and three times easily injected In this Master s Thesis the chargeability parameter was introduced for convenience few original pictures were created to demonstrate the occurring phenomena For visibility issues the data was presented in graphs instead of numerical tables The consequence of the study was declared in advance and then followed Thereby the general methodology integrated into this study resulted in the conclusions of quantitative
89. ve of the force influencing the tip It is worth noting that the derivative shows sharper change in the force parameters it can be tracked more accurately e g in Chapter 5 will be compared results for KPFM and KPFGM The constituent elements of the AFM For further detailed discussion of AFM capabilities it is necessary to describe its basic components One can recall 4 main elements of AFM scheme 16 1 probe attached to a flexible cantilever 2 piezo scanner used to move the sample relative to the tip 3 optical detection system laser and photo detector providing information of the bending angle of cantilever 4 feedback system In addition it is possible to name few separate additional components measurement electronic unit personal computer vacuum pump vibration isolation table etc 19 The probe Probe is the starting element of the AFM setup It is usually a pointed pyramidal needle with tip angle 10 20 degrees fixed on a flexible cantilever unit Figure 9 Most often tips have slightly elongated shape but it can be considered as a perfect cone for simplicity Probes are made of Polysilicon or Si3N4 Dopants cause undesirable increase of apex radius R Si az Figure 9 Scheme of the cantilever with tip in forced movement 16 Three main parameters characterize the tips 1 tip s apex radius usually called as tip radius R 2 cantilever elastic coefficient kz and 3 cantilever resonant frequency w
90. with tip in forced movement Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 Figure 18 Figure 19 Figure 20 Figure 21 Figure 22 Figure 23 Figure 24 SEM image of NN T190 HARS tips radius 50 nm angle 12 Major mechanical modes of tip s bending vibrations Operational principle of piezo scanner s tube movement Piezo ceramic disadvantages a nonlinearity b creep c hysteresis Simplified scheme of the feedback working principle and photo detector Algorithm of processing the relative measurement by closest 8 points Demonstration of AFM tip used for KPFM and Kelvin Probe Comparison between Amplitude Modulation and Frequency Modulation modes NTegra Aura device without the vacuum hood Roadmap of EFM family by 2006 Experimental facility scheme and device used for MBE Working window of the Nova program The raw image obtained for Surface Potential Image revealing artifacts caused by the excess value of lift height The Nova Image Analysis main window Figure 25 Figure 26 Figure 27 Figure 28 Figure 29 Figure 30 Figure 31 Figure 32 Figure 33 Figure 34 Figure 35 Figure 36 Figure 37 Figure 38 Figure 39 Figure 40 Figure 41 Figure 42 Figure 43 Figure 44 Figure 45 Figure 46 Figure 47 Figure 48 The 3D recovery of surface topography for sample
91. y huge interest has turned to materials with high values of dielectric permittivity k High k is the only viable solution according to Semiconductors Roadmap Reports and these materials will be viable in few years outlook While recent processors technology maintained by the Intel Corporation inclined to application of hafnium compounds HfO k 25 according to S Sze the needs for computing processors and memory RAMs applications might be distinguished A possible solution for rapid memory applications can be found in transistors with floating gate In these structures the high k dielectric is used to make the gate thicker The search for materials with high dielectric permittivity still continues there are tens of materials with giant k values up to 10 however such materials are not suitable for ICs from the viewpoints of energy band structure and technological interaction with Si wafer Thus the record values are held by the Sc and La oxides LaLuO3 k 32 These leading high k semiconductors are produced mainly by methods of ALD PLD and MBE To characterize the material as a prospective dielectric for industrial nano small transistors one should take into account such properties as parameters of its interaction with Si wafer surface adhesion ability to be introduced to the surface thermal and chemical stability and the material should have fine morphology without defects Hence comprehensive studies are needed to de

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