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Woollam ellipsometer cryostat operation

1. In Figure 75a the same fitting analysis was performed except that the constraint g2 0 was enforced A constant adsorption rate is not the best model In Figure 75b the g2 from the original model was reset to zero and data generated assuming no change in adsorption rate occurred The last part of this temperature cycle 1290 1317 min was subjected to a similar analysis using a constant growth rate analysis The resulting Adsorb layer and data fits are shown in the following Figures Cryo 200 J A Woollam Co Inc Section 5 Ellipsometric Acquisition and Analysis e 49 Adsorption Layer x Comment adsorb layer rMat Name N ia t DataTime StartTime arama y g1 t g2 t t Replace Layer Start Time min Fit Delete Layer 10 fo Sid A l Fit i i gl Ajhr Fit Cancel g2 0s hh Fit Figure 76 Generated and Experimental Generated and Experimental 12 260 Taala Model Fit 137 10 _ M Fi 12 250 Exp E 2 9eV Mey i sea 290V 137 05 EE peli i V 12 240 a l i He d B ah i ai D c 137 00 c D O D 12 230 o D D ko O z TE 2 136 95 12 220 I i i ji i M j dovada n pil BAIN ine i 136 90 Han Marren a Vi vf 12 210 Rr e Co 136 85 l vi 1 12 200 0 136 80 0 1290 1295 1300 1305 1310 1315 1320 1290 1295 1300 1305 1310 1315 1320 Time in Minutes Time in Minutes Figure 77a b End of Temperature Cycle 1 Over the final 30 minutes of the
2. Cryo 200 VASE Cryostat Attachment May 10 1999 By J A Woollam Co Inc The CRYO 200 option adapts a Janis Research model ST 400 SuperTran UHV cryostat to a GB 700 VASE ellipsometer base This allows the user to control the sample temperature over an extremely wide range from 4 2 Kelvin to 700 Kelvin while making transmission measurements or VASE measurements The windows on the cryostat have a limited acceptance angle which in turn limits the VASE measurement angle of incidence to a range of 65 to 75 For a complete understanding of the functionality and operation of the cryostat and its support systems it 1s necessary for the user to read and understand all the manuals provided by Janis Research This application note is a VASE operation supplement and is not a substitute for the training and experience needed for the correct and safe operation of a cryostat PLEASE READ AND UNDERSTAND THE MANUALS Cryostat nomenclature can be found in the Introduction to Laboratory Cryogenics manual provided by Janis Research Schematic diagrams located at the back of the Operating Instructions for the Janis Research Supertran System are also useful This application note assumes the reader user understands the operation of the VASE instrument See VASE Hardware Manual and WVASE32 Software Manual This manual was produced using Doc To Help by WexTech Systems Inc Contents Section 1 Safety Concerns 3 UES ass rsa ess eas
3. O N O 10 Ellipsometric Data Acquisition Options m Optical Components Track Polarizer tracking tolerance Fixed Polarizer Setting V Zone Average Polarizer Auto Retarder Off H Retardance Min Delta j Revs Measurement Sample Type Dynamic Averaging Isotropic Sample M Dynamic Averaging Enabled Maximum of Revs r Monochromator Slit Width Max slit width um m Auto slit adjustment Threshold Intensity Min Usable Intensity m Principal Angle Scan Configuration I Enabled Delta Tracking Range Min Mar _ Transmission Measurement Cancel Reset to Defaults IV Split grating changes across angles Figure 64 In the above example the Zone Average Polarizer option has been activated Zone averaging the polarizer is strongly recommended as the standard measurement mode unless the fastest possible measurement rate 1s needed With the Save File During Scan option active the data will be saved to a prenamed file periodically during acquisition The file name and data comment are entered using the dialog boxes shown below Save As Save in I example E 3 test1 dat Enter a Comment for the Experimental Data File name ftesd dat Save as type Data Files dat bat x Cancel Generated and Experimental Model Fit Exp E time 0 559 min Exp E time
4. i i i i i i i i i i i i i Tools Needed Parts Needed in addition to the ellipsometer Rubber mallet 4mm key wrench 7 16 combination wrench 9 64 key wrench Small straight blade screwdriver Small phillips tip screwdriver e Table components 8 side rails 4 each 5 support rails 6 each 3 support rails 6 each 3 support brace 1 each Base each ee ee ee ee ee me ee ee ee ee ee ee ee ee ee ee ee y Cryo 200 J A Woollam Co Inc Cryo 200 J A Woollam Co Inc Tabletop 1 each e Cr yostat components CRYO 200 storage stand CRYO 200 cryostat assembly M5 by 16 mm socket head cap screws 4 each 1 45 ID copper gasket 1 each Flexible vacuum line 1 each 14 28 bolts and nuts 6 each 8 32 Socket head cap screws 6 each 8 32 nut plate 3 each 64 ID copper gasket 1 each Cryogen transfer line 1 each Cryogen dewar not included e Temperature controller components Temperature controller 1 each Power cord each Cold Junction Compensator each Thermocouple cable 1 each Heater cable 1 each e Exhaust port heater components Heater controller 1 each e Power cord 1 each e Heater cable each e Pumping station components e Assembled pumping station 1 each e Isobar power strip each Facility Needs e Well ventilated room e Computer Stand e 4 by 8 floor space plus room for computer stand with 9 c
5. 1 0 2 0 3 0 Photon Energy Cryo 200 J A Woollam Co Inc 4 0 5 0 6 0 1 0 2 0 3 0 4 0 5 0 6 0 eV Photon Energy eV Figure 87a b Native oxide silicon wafer from 4K to 650K Section 5 Ellipsometric Acquisition and Analysis e 55 This page left blank intentionally 56 e Section 5 Ellipsometric Acquisition and Analysis Cryo 200 J A Woollam Co Inc Index A Active Gauge Controller 5 7 15 16 Adsorb layer 48 51 Adsorption Rate 43 47 48 50 51 AllowAlignJogs 26 angle of incidence 4 17 20 24 43 45 AOI 4 11 12 attachment 26 AutoTune 30 B bayonet 17 34 39 42 46 53 C calibration 23 26 29 30 32 33 calibration curve 30 32 33 chamber contamination 21 35 Changing Samples 25 34 47 cold finger 15 17 25 30 32 34 36 38 41 cold junction compensator 9 14 cradle 35 cryogen 3 4 9 17 30 34 41 42 46 Cryogen Transfer 9 41 42 D DelOffsets 23 24 27 Dynamic Ellipsometric Acquisition 43 E Exchanging Cryostat And Standard Sample Stage 17 exhaust port heater 9 13 14 34 41 F fixedpolalways 26 flexible vacuum hose 4 11 16 18 21 Cryo 200 J A Woollam Co Inc G goniometer base 18 19 20 H hardware cnf 23 25 heater 9 13 15 30 33 34 41 42 46 heater tape 42 L LakeShore Model 330 5X Temperature Controller 29 leak 7 15 16 23 41 Leaks 6 15 38 41 multiple angle cryostat data 24 O o ring compression nut 17 34 39
6. A Woollam Co Inc Step 1 BE CAREFUL Cryogens and anything that have been in contact with cryogen are dangerously cold While step is universally applicable the exact procedures to use in transferring cryogen will need to be worked out for the particular setup of the Dewar and choice of cryogen There is no absolute procedure for initiating and maintaining cryogen transfer A few operational suggestions are presented below For additional cryostat operating instructions refer to Operating Instructions for the Janis Research SuperTran System from Janis Research Before using cryogen to cool the cold finger let the cryostat pump down to 10 Torr Once the cold finger assembly becomes cold most of the residual gas in the cryostat will stick to the cold surfaces including the cold surface of the sample If you start with less residual gas there will be less to stick to the sample and the other surfaces will cryo pump more effectively if they are not saturated with adsorbed gas during the initial cool down section 4 Cryostat System Operation 41 A cold bayonet will ice almost instantly in room air and obviously this ice will not melt away while immersed in liquid helium Be patient it may take awhile to cool the inside of the transfer line Try to prevent frost formation on the cryogen outflow port Proper precautions should be taken keep water and frost out of the ellipsometer Cycling the sample temperature
7. P PID 30 R radiation shield 35 37 47 S safety 3 17 34 35 sample mounting clamps 36 37 Sample Stage Exchange 17 sample z axis stage 19 20 storage pedestal 17 switchconfig 26 system calibration 24 25 T temperature controller 8 9 13 15 17 29 30 32 34 42 46 47 Temperature Cycling 42 45 51 TempReader 27 29 33 45 thermal cycling 45 thermocouple 8 9 14 29 30 32 33 34 46 54 transfer line 3 9 16 17 34 39 40 42 46 53 Turbo Pump 5 15 16 39 47 Type E thermocouple 29 30 type e_extended cnf 32 33 Index e 57 U Userl 27 32 43 46 V vacuum tee 6 7 11 16 W water vapor 16 32 41 47 Window Calibration 23 Window Strain Effects 23 WinkEffects 23 24 26 Work Table 7 8 WSCRIPT 25 WVASE3 2 19 23 27 31 32 37 4445 47 52 Z zoneavealways 26 zones 24 26 58 e Index Cryo 200 J A Woollam Co Inc
8. acaseseaaney enti mtet sos tetsatepaadcuadaaab E 3 Foni neS a 0y s8 sions 2 4 act as a at vant unidetee A iahes bantu tu hctiastecoias 3 Section 2 System Setup 5 Turpo Pump Se Paia E A 5 WOK Tabe SOON me e a a a a oa rire cneemiess 8 Turbo Pump Operation And Seal Checkout segsssctdidssccadsees sianlsoiaesiecogdededs pad eased a ogideskeets 15 Exchanging Cryostat And Standard Sample Stage ccccccccccsesccceseceeseeceeeeeeseseeeeeseeseneees 17 Cryostat To Sample Stace Exchane seeker e a E 17 Sample Stas to Cryostat Exehan Oe cir hie e ie ures ered antec eerie 19 Section 3 Configuring WVASE32 For Cryo 200 Attachment 23 Window Strain Effects And Window Calibration cccccccceesccccseeeceeeeeeeeeeceeeseeeeeneaeeeeens 23 Hardware cnt Entries For The Cryo 200 e ssesssseseseessrrssresrerssersreesasesaresssressresseesseesasesas 25 Section 4 Cryostat System Operation 29 LakeShore Temperature Controller Model 330 5X c cccccccsccessecessesescessecesssceesesvscesvsceasaees 29 TempReader exe Utility Program ccccccccccsssecceeecceneeeeeeeeseneeeeeeeeeeneeeaeeeeseneessaeeeseneeeaes 31 Removing Cold Finger Assembly From The Cryostat And Changing Samples 34 Inserting Cold Finger Assembly Into The Cryostat And Aligning The Sample 37 EVO CCN ram T cheb Sinead Pare atite a iterate hla Re Matunta a a tari iene s 4 Section 5 Ellipsometric Acquisition and Analysis 43 VASE Dynamic Ellipso
9. first temperature cycle a constant adsorption rate is a satisfactory model Note the gap in the data During this time a quick spectroscopic scan not shown was taken When the single wavelength monitoring resumed the VASE dynamic time reference was maintained so a continuous data set would be available for analysis Cycle 2 was subjected to a similar adsorption rate analysis with results shown below Generated and Experimental Generated and Experimental 12 10 180 140 8 180 Model Fit hy o 12 05 Exp P E 2 9eV 1 1150 140 6 Mi ed Fit aa 150 n 12 00 120 n 140 4 120 g D T A 11 95 90 6 140 2 90 amp gt 11 90 60 lt 140 0 60 11 85 30 139 8 30 11 80 0 139 6 0 1300 1350 1400 1450 1500 1550 1300 1350 1400 1450 1500 1550 Time in Minutes Time in Minutes Figure 7S8a b Temperature Cycle 2 For cycle 2 a constant adsorption rate was sufficient for the entire cycle time Also note the starting y and A values from cycle 2 were not the same as cycle 1 A reasonable explanation is that during the cool down of cycle 1 the sample adsorbed a non negligible amount of ice which was present at the 1194 min mark 50 e Section 5 Ellipsometric Acquisition and Analysis Cryo 200 J A Woollam Co Inc This implies that 1 the previous model assumption that the adsorb layer had zero thickness at 1194 min was probably wrong But remember this affects only the fitted for optical constants of the su
10. gently move the cryostat assembly to the goniometer base with the Z Axis micrometer knob on the input unit side Figure 40 pr r P y Figure 40 Figure 41 7 Install the four M5 socket head cap screws finger tight through the CRYO 200 base into the ellipsometer base 8 Using the 4mm key wrench tighten the four M5 screws Figure 41 Note Keeping the cryostat vacuum chamber under vacuum during storage is highly recommended This removes the possibility of chamber contamination minimizing the pump down time required before usage Cryo 200 J A Woollam Co Inc Section 2 System Setup 21 This page left blank intentionally 22 e Section 2 System Setup Cryo 200 J A Woollam Co Inc section 3 Configuring WVASE32 For Cryo 200 Attachment Window Strain Effects And Window Calibration Obviously windows are needed on a vacuum chamber to permit an optical probe to reach the sample without letting the vacuum leak out However with windows comes the possibility of strain effects which can perturb the polarization state of the probe beam These effects and the general WVASE32 scheme for handling them are discussed below The Cryo 200 comes with standard fused silica UHV windows and these windows generally have some strain Expensive fragile strain free windows which might be more appropriately named lower strain when shipped have been developed Regardless of the window type if there is d
11. of being calibrated before starting the multiple system calibrations that follow Launch the WSCRIPT exe script running application using the IGloballRun_WVASE_Tools menu See Figure 43 Load the ftdelwin wsc script file See Figure 44 below Run the script This should take about two hours The first time this procedure is completed without a J A Woollam Co representative in attendance the user should e mail the company the wvase32 current log file for validation of the procedure WY ASE Automated Script Program FIDELWIN WSC Eile Run Options List of WYVASE EPI Commands hardinit warm fi ick 2 hardcal type 1 det 0 el xxxxxxxxxxxxxxxi lel e l z dows 105 fitdeloffsets 105 winfitmode 0 wyll dows 1 05 fitdeloffsets 105 wintitmode 0 wvi1 dows 105 fitdeloffsets 1 05 winfitmode 0 wvll cal eal ftwindows 108 fitdeloffsets 108 winftmode D well Far of Dynamic Return Run Script Cancel Script Exit Figure 44 Hardware cnf Entries For The Cryo 200 The hardware cnf file contains the hardware configuration information that tells WVASE32 what kind of ellipsometer system is available and how to control that system Configuration entries specific to the Cryo 200 are described below Cryo 200 J A Woollam Co Inc Section 3 Configuring WVASE32 For Cryo 200 Attachment e 25 The Cryo 200 should be used at integer angles of incidence and is principal
12. of interest Finally perform the high temperature measurements of interest working from the lowest to the highest temperature The data at 3 1 eV is summarized in Figure 84 for an extended high temperature run The experiment started by ramping the sample up to 650 K As previously mentioned a better procedure would have been to start at 400 K and work up to 650 K An important feature in the data can be observed in first 200 minutes where the ellipsometric measurements show a continuous change even after the temperature was fully stabilized This was indicative of the surface morphology changing This is similar to the situation at low temperatures where the growing adsorption layer change the ellipsometric data However this high temperature change was irreversible This change was confirmed by noting the change in room temperature measurements from before and from after this experiment not shown Experimental Data Experimental Data 22 5 700 700 ExpP E 3 1eV ExpA E 3 1eV 22 0 600 600 V WY 21 5 500 500 D C C D D 6 O D O S 21 0 400 gt c 400 gt a lt 20 5 300 300 20 0 200 200 0 500 1000 1500 2000 2500 3000 0 500 1000 1500 2000 2500 3000 Time in Minutes Time in Minutes Figure 84a b Native oxide silicon wafer at elevated temperatures More detailed looks at the data are shown in Figure 5 The slope in y with constant temperature is easily seen for temperatures above 600K Also note the undershoot of the
13. room temperature and near 4K data is shown in the figures below Note how the critical point structures sharpen and shift for 4K as compared to 297K 52 e Section 5 Ellipsometric Acquisition and Analysis Cryo 200 J A Woollam Co Inc Experimental Data Experimental Data Exp E time 0 5 min 297k Exp E time 1590 min 4k Exp E time 0 5 min 297K Exp E time 1590 min 4k O O a lt 1 0 2 0 3 0 4 0 5 0 6 0 1 0 2 0 3 0 4 0 5 0 6 0 Photon Energy eV Photon Energy eV Figure 82a b Sulfur doped InP Comparison of 4K and 297K data Experimental Data Experimental Data Exp E time 0 5 min 297K Exp E time 1590 min 4K A D V Exp E time 0 5 min 297K Exp E time 1590 min 4K 1 0 2 0 3 0 4 0 5 0 6 0 1 0 2 0 3 0 4 0 5 0 6 0 Photon Energy eV Photon Energy eV Figure 83a b Sulfur doped InP Comparison of 4K and 297K data High Temperature Spectroscopic Measurement A few comments about high temperature operation of the cryostat are needed before some experimental results for a native oxide silicon sample are shown e Remove the transfer line bayonet from the cryostat before making high temperature measurements in the cryostat The transfer line bayonet may be damaged if exposed to temperatures above 475K e The cryostat should be under vacuum to prevent corrosion of the internal components which might occur if exposed to large oxygen concentrations at high temperatures Be c
14. temperature as the control point was changed Especially note the 2 3 minute lag of sample temperature as seen in the y with respect to the thermocouple in Figure 83b Experimental Data Experimental Data 680 660 660 ExpY E 3 1eV 640 V WY D 640 D z 7 620 F Oo Oo 620 7 g T a a ExpY E 3 1eV 600 600 580 580 200 250 300 350 400 450 500 400 410 420 430 440 450 460 Time in Minutes Time in Minutes Figure 85a b Native oxide silicon wafer at elevated temperatures 54 e Section 5 Ellipsometric Acquisition and Analysis Cryo 200 J A Woollam Co Inc After the high temperature measurements the same sample was measured at low temperatures A comparison of spectroscopic data for three different temperatures is shown in Figure 86 and Figure 87 The dominant features are due to the silicon substrate The surface overlayer although modified by the high temperature experiments is still thin 30A Thus although the data is sub optimal for the accurate determination of silicon optical constant the key features energy shifts and broadening changes of the temperature dependence are clearly present Experimental Data Experimental Data 40 180 35 30 160 ep id 25 D D 140 a a lt iy 120 10 5 100 1 0 2 0 3 0 4 0 5 0 6 0 1 0 2 0 3 0 4 0 5 0 6 0 Photon Energy eV Photon Energy eV Figure 86a b Native oxide silicon wafer from 4K to 650K Experimental Data Experimental Data lt E4 gt
15. the sample vacuum to the sample chuck Figure 34 Make sure goniometer dials 12 Check the angles in the Motor Settings and make sure they agree with agree with the WVASE32 the goniometer positions Zero the fine scales on the goniometers 1f settings necessary See VASE Hardware Manual Sample Stage to Cryostat Exchange Tools Needed e 3 16 key wrench e 4mm key wrench Procedures Cryo 200 J A Woollam Co Inc Section 2 System Setup 19 1 Set the system angle of incidence to 0 via the Move Angle command 2 Disconnect the vacuum from the sample chuck Figure 35 Connect the vacuum hose to the dummy connection on the sample goniometer Figure 35 Figure 36 3 Remove the four 1420 socket head cap screws that hold the sample chuck to the sample Z Axis stage Figure 36 4 Remove the two 20 socket head cap screws holding the sample z axis stage to the sample goniometer Figure 37 Figure 37 Figure 38 5 Store the removed Sample Stage components for reuse Figure 38 6 Using the 4mm key wrench remove the four M5 socket head cap screws holding the cryostat assembly to its storage stand Figure 39 These bolts will be used to fasten the cryostat assembly to the goniometer base 20 e Section 2 System Setup Cryo 200 J A Woollam Co Inc Figure 39 Be Careful The cryostat and 1 Using care not to strain the flexible vacuum hose if attached to the tilt stage are heavy cryostat chamber
16. 168 826 min Exp E time 337 309 min Exp E time 506 011 min Exp E time 674 627 min Exp E time 843 444 min 2 0 3 0 Photon Energy eV Figure 65 Figure 66 Sample data acquired near room temperature for a heavily sulfur doped InP sample 1s shown in Figure 67 Figure 68 Data from 6 different wavelength cycles are shown in the different Figures Note as discussed in Guide to Using WVASE32 the pseudo dielectric values lt gt and lt gt in Figure 68 are just numerically transformed from the y and A values Generated and Experimental Model Fit Exp E time 0 559 min Exp E time 168 826 mi Exp E time 337 309 min Exp E time 506 011 min Exp E time 674 627 mi Exp E time 843 444 min A in degrees 4 0 5 0 6 0 1 0 2 0 3 0 4 0 5 0 6 0 Photon Energy eV Figure 67a b Sulfur doped InP near 297K Cryo 200 J A Woollam Co Inc Generated and Experimental Generated and Experimental Model Fit Exp E time 0 559 min Exp E time 168 826 min Exp E time 337 309 min Exp E time 506 011 min Exp E time 674 627 min Exp E time 843 444 min 1 0 2 0 3 0 lt E5 gt Model Fit Exp E time 0 559 min Exp E time 168 826 min Exp E time 337 309 min Exp E time 506 011 min Exp E time 674 627 min Exp E time 843 444 min 4 0 5 0 6 0 1 0 2 0 3 0 4 0 5 0 6 0 Photon Energy eV Photo
17. 62 40 e Section 4 Cryostat System Operation Cryo 200 J A Woollam Co Inc Figure 62 23 Before using cryogen let the cryostat pump down to below 10 Torr 24 Assuming no leaks the amount of time it takes to pump down is related to the quantity of contaminants being released If proper UHV handling procedures were observed when changing the sample the dominant contaminant will be water vapor In general the amount of water vapor adsorbed on the cold finger and on the interior of the cryostat shell is directly related to the length of time those surfaces are exposed to atmosphere If the parts have been exposed to room air for several days it may require pumping overnight to achieve the desired pressure If the parts have been exposed for only the several minutes needed to change the sample pump down may take less than 30 minutes 25 If you suspect a leak try tightening the copper gasket clamping bolts in the manner previously described in the sealing section Sometimes there is a single bolt which can be tightened to cause a dramatic decrease in cryostat pressure If the main seal bolts are all sufficiently tightened then the copper gasket may have a defect If the gasket is defective it will need to be replaced Cryogen Transfer BE CAREFUL The exhaust port heater tape should be turned on before flowing croyogen Before using cryogen to cool the cold finger let the cryostat pump down to 10 Torr Cryo 200 J
18. 6951 0780 P O0 00646 0 00070 Winetteces 4 2 ledsos2 0 40560 0 00191 0200041 DelOffsets 2 0 45295 0 00178 0 00056 0 500 DelOffsets 2 00 0 5 deloffsets2 2 0 0 OQ delotisetss 2 0 06044 0 00563 0 00047 0 500 delorisers4 2 0 70477 0 00276 0 200054 0 500 deloriserss 2 lL 27227 0200052 0 00069 0 500 deLorisetso 2 leoeel2 02 00685 0200119 04500 ASlorrs ets7 2 Hz243092 0503636 0 00242 0 500 GelOrEsetso 2 23585260 0 04420 0 002777 6 0 3500 delortisets I 105519 0 007240 0 00055 10 500 d lotisetsl0 2 1 60354 0 00036 0 00069 0 500 delotisets l Is 23560509 0 00365 0200077 0 500 dSlorrsetslZ Z asto VOe Us00360 0 00066 0 500 delotrsers 3 2 Ae 23914 Os00646 0200070 02500 delorisere 4 2 S160073 02001L9L O 0000d1 30 500 The Userl value is the temperature passed to WVASE32 by TempReader exe The User subsection tells the program to look for an external User1 value to be used when acquiring dynamic data For the Cryo 200 the User1l value is the temperature passed to WVASE32 by the TempReader exe utility program User Userl l1 0 UserlCal 0 Cryo 200 J A Woollam Co Inc Section 3 Configuring WVASE32 For Cryo 200 Attachment e 27 This page left blank intentionally 28 e Section 3 Configuring WVASE32 For Cryo 200 Attachment Cryo 200 J A Woollam Co Inc Section 4 Cryostat System Operation LakeShore Temperature Controller Model 330 5
19. Co Inc Section 4 Cryostat System Operation 31 1 TempReader can poll the current sensor temperature and pass that value along to WVASE32 2 TempReader can download a thermocouple calibration curve and apply an optional user defined correction The utility can be used to poll the current sensor temperature from the controller using a serial communications port and then advise WVASE32 what the current temperature is WVASE32 can then store the current temperature USERI value with the measured ellipsometric data See Acquisition and Analysis Section The polling controls are in the upper left hand corner The update period can be selected between 0 5 and 60 seconds The same button is used to Start Polling and Stop Polling The Close Comm Port button may help clear the program if the program gets caught in a series of repeated errors Error information is presented in the text box on the right side Serial communications under Windows tends to produce some errors As a general matter TempReader deals with these errors by ignoring them and then restarting the polling process The goal is to send only correct temperatures to WVASE32 thus not all polling attempts are successful TempReader can also download a thermocouple calibration curve By default TempReader works with a file called type e_extended cnf and curve 12 The currently configured curve and file are displayed in the TempReader text box when the applicati
20. Once the valve is fully open the pressure should quickly drop to below 107 Torr If not there is probably a leak in one of the seals on the cryostat If there is a leak try tightening the clamping bolts You may want to close the cryostat vacuum valve and confirm that problem is really with the cryostat itself If the leak persists start the seal chck out procedure again to isolate which seals could possible be at fault Replace the copper gasket seal s and try pumping down again Exchanging Cryostat And Standard Sample Stage Cryostat To Sample Stage Exchange Tools Needed 4mm key wrench 3 16 key wrench Parts Needed J J J Procedures 1 2 The copper radiation shield 3 does not warm up or cool down as fast as the cold finger 4 Cryo 200 J A Woollam Co Inc Sample stage components Sample chuck assembly each Z axis stage each 1420 by 3 8 socket head cap screws 6 each The cryostat storage pedestal should be positioned behind the ellipsometer straddling the ellipsometer base connecting cables Set the system angle of incidence to 0 via the Move Angle command Allow the cold finger to come to room temperature If the cryostat has been cold the temperature controller may be set to 295 Kelvin to quickly warm up the sample If the cryostat has been hot cryogen may be used to cool the sample Completely unscrew the transfer line O ring compression nut and carefully withdraw the transfer li
21. X The LakeShore manual is The primary documentation for the LakeShore Model 330 5X Temperature the primary reference for the Controller is the LakeShore manual shipped with the system Only some of the temperature controller important features and operational considerations are discussed here The Cryo 200 is equipped with an extended Type E thermocouple to monitor temperatures from 4 to 700K The LakeShore controller does not have a calibration curve for this extended range thermocouple stored in permanent memory curves 1 10 See LakeShore page 2 8 The controller is shipped with extended Type E calibration values stored Curve 12 in non volatile RAM The curve will remain stored in memory during normal operations including turning the power off but the curve can be deleted by resetting to factory defaults See LakeShore page 3 13 If the curve becomes lost or corrupted a new curve must be downloaded before correct temperature measurements are possible There is a special format for downloading an extended Type E calibration which is found in the one page addendum to LakeShore manual The TempReader exe utility program can be used to download new calibration curves as discussed in the next section The extended Type E thermocouple used from 4 700K is not standard See the LakeShore manual addendum and the TempReader exe description Cryo 200 J A Woollam Co Inc Section 4 Cryostat System Operation e 29 It the temperature controll
22. ack to the quad detector Figure 56 10 Finger tighten the vacuum seal bolts making sure the sealing flanges are parallel 38 e Section 4 Cryostat System Operation Cryo 200 J A Woollam Co Inc Important Do not over 11 tighten the first bolts The goal is to produce an even pressure seal all around Using the 1 2 combination wrench gradually tighten the vacuum seal bolts Figure 57 to 15 lb ft Try not to tighten any one bolt more than 10 15 degrees of turn at one time The recommended tightening procedure is to cycle around the bolts working on every third skip 2 tighten 1 repeat The seal flanges should remain parallel throughout the entire bolt tightening sequence Figure 57 12 13 14 15 16 17 Cryo 200 J A Woollam Co Inc The pump down can then begin If the vacuum pump is currently off and has been brought up to atmospheric pressure then open both system vacuum valves and let the cryostat chamber rough out through the turbo pump when the pump is turned on If the vacuum pump is on but the cryostat chamber is at atmospheric pressure then the cryostat chamber will need to be slowly released through the turbo pump Monitor the turbo pump vacuum gauge while very slowly cracking the cryostat gate valve Try to keep the turbo pump pressure below 1 Torr About every thirty seconds or when the pressure is back below 0 1 Torr open the gate valve a little more but keep the press
23. amps should lie across the cold finger not along the cold finger See Figure 54 This will avoid blocking the measurement beam with the clamps Tighten the screws just enough to hold the sample securely Note Use the springs provided to help keep from straining or breaking the sample Inspect the radiation shield for frost or condensation If there is frost let the radiation shield warm up to room temperature Wipe condensation off with a clean soft cloth Reattach the radiation shield to the thermal anchor with the round view ports holes at O and 180 and the slotted view ports at 70 and 70 Inserting Cold Finger Assembly Into The Cryostat And Aligning The Sample Tools Needed Cryo 200 J A Woollam Co Inc Procedures 1 Insulated Gloves pair 1 2 combination wrench 1 each Clean room gloves 1 pair Small straight blade screwdriver 3 32 key wrench WVASE32 should be running with the hardware initialized and monochromator lamp turned on Use the HardwarelAcquirelAlign_Sample menu item to start the ellipsometer alignment There should be a white light ellipsometer probe beam and the Hardware window should show an alignment cross hair Use the left and right arrow keys in conjuction with optional accelerating modifier keys shift cntrl cntrl shift to zero the sample stage goniometer There is only one move per screen update If the detector stage goniometer is not zeroed use the H
24. an menu option This brings up the following dialog box C VASE Data wyl tf by e Min Cycle Period sec e Discrete Wavelengths 1 x Dynamic Fit amp 1 r Ellipsometric Data Acquisition Settings 7 2 pC Isotropic Sample Fixed Pol Setting 20 00 r 2 a Pol Zone Average ON Change Settings gt gt 5 fs Revs Meas co zo 9 fe Save File during scan every fis min Cancel Figure 63 HardwarelAcquirelDynamic_Scan If Discrete Wavelengths is selected then a predefined set of up to 9 wavelengths will be repeatedly measured Typically only one or two wavelengths would be used in this case to monitor temperature and or surface changes for example VASE Data should be selected for more detailed spectroscopic scans In either case each data point is assigned a measurement time and a grouping time The grouping time is just the measurement time of the first wavelength in a spectroscopic cycle Fora single wavelength scan the grouping and measurement times are the same The ellipsometric data is acquired using the current angle of incidence Additional acquisition settings can be accessed using the Change Settings gt gt button which brings up the following dialog box Section 5 Ellipsometric Acquisition and Analysis e 43 Zone averaging the polarizer is Strongly recommended Y in degrees 44 e Section 5 Ellipsometric Acquisition and Analysis 40 to
25. ardwarelSetup Motors dialog box zero that stage Using gloves to prevent contamination remove and dispose of the used copper seal gasket Section 4 Cryostat System Operation 37 5 Inspect the seal mating surfaces on the vacuum jacket and cold finger clean if any dirt or lint is found to avoid vacuum leaks 6 Install a NEW copper gasket Figure 55 Resist the temptation to re use gaskets it is an exercise in futility ee Coa E K N E ra lt Figure 55 7 Slowly insert the cold finger assembly into the cryostat shell with the sample surface facing and roughly normal to the white light ellipsometer alignment beam Try to center the cold finger assembly such that the gasket will be contacted near the final mounting position Moving the cold finger assembly across the copper gasket may scratch the gasket 8 Install the eight vacuum seal bolts back into their holes Figure 56 but do not tighten them not even finger tight 9 If necessary gently rotate the cold finger assembly in the shell to retroreflect the ellipsometer alignment beam back into the quad detector The quad detector intensity increase greatly when nearing alignment Do not try for perfect alignment at this time just get the cross hair on the screen with an intensity much above the background level For large misalignments dimming the room lights and using a piece of paper to detect the reflected beam may be useful in reflecting the light b
26. areful Some materials e Some materials are capable of out gassing toxic chemicals at high are capable of out gassing temperatures e g GaAs may out gas As Be careful and consider toxic chemicals at high what is going to happen to your sample at high temperatures under temperatures vacuum Coating windows and interior surfaces may affect future operation and the problems are complicated many times over if the contaminant is toxic e The surface morphology of some materials may be altered at high temperature Furthermore these changes are likely to be irreversible The example data presented below gives a good example of how this can happen Cryo 200 J A Woollam Co Inc Section 5 Ellipsometric Acquisition and Analysis e 53 It is best to start with lower temperatures and the move to higher temperatures At high temps the sample may be irreversibly changed The following experimental data was taken for a native oxide on silicon sample The single wavelength tracking data was acquired at 3 1 eV At this wavelength the silicon optical constants exhibit a strong dependence on temperature The data was not subjected to a detailed analysis because the sample surface was irreversible altered at temperatures above 600 K In fact this data provides an excellent example of how NOT to perform high temperature measurements As a general rule perform room temperature measurements first Next perform the low temperature measurements
27. ay produce better optical constants but these optical constants should be satisfactory for most experiments The thickness model for the Adsorb layer is summarized in the dialog box For times before Start Time the thickness is defined as tp For times after Start Time the thickness grows as a quadratic polynomial given by Thickness t g t g t t The rates g and gz are give in units of A hr and A hr hr respectively However note times are in minutes Thus the assumptions implicit in the Adsorb layer are that the optical constants are in fact constant for all times and that the layer grows in a manner no more complicated than a quadratic function A portion of the data shown in Figure 71 is shown analyzed using the model and adsorb layer from Figure 72 and Figure 73 The results are shown below 48 e Section 5 Ellipsometric Acquisition and Analysis Cryo 200 J A Woollam Co Inc Generated and Experimental Generated and Experimental 12 30 139 0 Model Fit 12 25 Exp E 2 9eV 138 5 w Model Fit ii ee Ny ExpA E 2 9eV 9 12 20 a D D 138 0 D 12 15 z O O c 187 5 5 12 10 lt 12 05 ie 12 00 0 136 5 0 1170 1200 1230 1260 1290 1320 1170 1200 1230 1260 1290 1320 Time in Minutes Time in Minutes Figure 74a b Temperature Cycle 1 fitting for both g and g gt For the above results the parameters g and g gt and the substrate optical constants were fit Leaving t
28. be powered and the displays active Toggle the vacuum gauge display to read the pump vacuum see the Edwards Active Gauge Controller manual Start the vacuum pump by pressing the Start button The pressure should quickly drop to below 10 Torr If not there is probably a leak in the seal between the turbomolecular pump and the vacuum tee If there is a leak try tightening th clamping bolts If the leak persists press the Stop Start button to stop the pump and let the system return to atmospheric pressure Replace the copper gasket seal and try pumping down again While monitoring the pump vacuum gauge very slowly open the main vacuum valve such that the pump gauge pressure never exceeds Torr Once the valve is fully open the pressure should quickly drop to below 10 Torr If not there is probably a leak in one of the seals at the ends of the flexible vacuum hose If there is a leak try tightening clamping bolts If the leak persists close the main vacuum valve but the turbo pump can be left running Replace the copper gasket seal s and try pumping down again Wait for the pump gauge pressure to reach 10 Torr Cryo 200 J A Woollam Co Inc Check cryostat seals 8 While monitoring the pump vacuum gauge very slowly open the cryostat vacuum valve such that the pump gauge pressure never exceeds 1 Torr The cryostat was shipped under vacuum so the pressure should not rise appreciably but go slowly for safety
29. bration sample inside If it is still present you can skip the steps relating to installation of a new sample Cryo 200 J A Woollam Co Inc Run WY ASE Tools x WYASE Automated Script Program LakeShore Communications Program TempReader Figure 43 Cancel 10 11 Remove the cold finger assembly referring to procedures described in the section on Removing Cold Finger Assembly From The Cryostat And Changing Samples Place the calibration sample on the cold finger This calibration should be performed near room temperature 297K and does not require a vacuum If this sample is only going to be used to calibrate the windows then it is probably unnecessary to replace the copper gasket when returning the cold finger assembly to the cryostat If the system is going to be pumped down then replace the seal Perform a proper coarse alignment as described in the section on Inserting Cold Finger Assembly In To The Cryostat And Aligning The Sample The seal clamping bolts should be tightened enough to prevent the sample from moving around Then a fine alignment should be performed Perform a fine calibration at 70 and select fitting for window effects when that dialog appears Figure 44 If the calibration is successful continue with the system calibration If the fine calibration was unsuccessful you may need to perform a coarse calibration Itis important to confirm that the system is capable
30. bstrate not the adsorption rate analysis And therfore 2 the fitted substrate optical constants from cycle 2 are closer to the actual 4 K optical constants of this sample For cycle 3 a constant adsorption rate was also sufficient to model the entire 4 K data region Note most of the time was spent acquiring spectroscopic data see next section However it was useful to take single wavelength data at the beginning and the end to determine the adsorption rate used in the next section and to confirm that a constant adsorption rate is the correct model Generated and Experimental Generated and Experimental 11 94 FELY Model Fit i i 11 92 Exp E 2 9eV 140 8 ta e 2 80 n 11 90 140 6 D O O D 11 88 9 1404 7 gt 11 86 lt 140 2 11 84 ae 11 82 0 139 8 0 1550 1600 1650 1700 1750 1800 1550 1600 1650 1700 1750 1800 Time in Minutes Time in Minutes Figure 79a b Temperature Cycle 3 The results from Cycles 2 and 3 are very similar The starting ellipsometric values are very similar suggesting that no further cleaning was achieved by the second warm up step and suggesting that the surface is probably free of most adsorbant when the 4 K temperature was reached There is a slight reduction improvement in the rate of adsorption for Cycle 3 The following table summarizes the adsorption rate for different sections of the temperature cycling experiment Table I Adsorption Rate Analysis R
31. can reduce the adsorption rate Details are in the data analysis section Monitoring Sample Cool Down With Temperature Cycling 42 e Section 4 Cryostat System Operation There will be variability in the time and manner that the storage Dewar builds up an over pressure and this will in turn affect the rate of cryogen transfer This variability will be between storage Dewars Even the same Dewar will change its characteristics as the cryogen fill level changes The transfer line bayonet should be warm and dry when inserted into the storage Dewar Admittedly this will boil off more cryogen than if the bayonet was pre cooled with liquid nitrogren for example However a cold bayonet will ice almost instantly in room air this ice can clog the transfer line and obviously this ice will not melt away while immersed in liquid helium A wet bayonet is virtually guaranteed to ice closed when inserted into cryogen In some situations it may be useful to use the over pressure created by inserting the bayonet to initiate cryogen flow After opening the bayonet valve on the transfer line two full turns to start is typical there may be a substantial wait before liquid cryogen begins to flow Be patient it may take awhile to cool the inside of the transfer line However if more than twenty minutes have elapsed then the possibility of icing should be considered To clear an iced transfer line remove the bayonet from the cryogen and raise the transf
32. current window effects are As shipped the ellipsometry windows were divided into 14 zones most of which are 1 wide and centered on an integer angle These values should not be directly modified by the user Any changes in the current window effect values will be entered by WVASE329 as the results of a calibration Cryostat nZones 14 angzone 0 69 5 70 5 angzone2 0 10 10 angzone3 71 70 5 71 5 angzone4 72 71 5 72 5 angzone5 73 72 5 73 5 angzone6o 74 73 5 74 5 angzone 7 5 74 5 75 5 angzone8 76 75 5 80 angzone9 69 68 5 69 5 angzonel0 68 67 5 68 angzonell 67 66 5 67 angzonel2 66 65 5 66 angzonel3 65 64 5 65 angzonel4 64 50 64 5 O O1 O1 O1 wineffects 2 1 34693 1 50901 0 00178 0 00056 winfitmode 2 26 e Section 3 Configuring WVASE32 For Cryo 200 Attachment Cryo 200 J A Woollam Co Inc wineffects2 2 0 00000 1 21400 0 winfitmode2 12 053869 0 00282 WINSTESCUSS 2 1le29 04 1 39070 0200563 0 00047 WINeGT tect s 2 1216626 L 25907 0 00273 0 00054 WIinetrectss Z2 O 265603 1 04749 0 00052 0 00069 WINCTESCESO 2 0 60369 0 TITS 0 00645 0 00119 WINEEESCEST 2 O 27 786 O 239115 O0403636 0 00242 WINeGEECCESO Z O08 332 0269950404420 0 002 78 WINMCTESCES9 2 HlhseS54 71 1 52092 0 00740 0 00055 WInetrectslO0 2 1L 66636 L 5S0376 0 00036 0 00069 Wanetrectslil 2Z Silio 9243 Slaa tS 0T C 00S63 02000 77 WiIneETectsi2 2 I 307A Alsat 0000 0 00066 vinei fects t3 s 122
33. d before downloading to the LakeShore controller See the LakeShore Model 330 5X Addendum for more information S9IJEXTENDED TYPE E ToS TLO P Shea 130 155 O Soe sor 1 50 E eS 3 24 16 180 O 6a 5 34 459 210 9 0AL 8 eee o 240 a Ea 10 Oecd 6 Zo SO OY I3 Lek 300 9 Ey 16 4 786 30 93 740 20 o EST 400 29 8642 30 TORES 450 aS AQ Loge 33 500 20 2 OZ 50 Zle 126 9 Sais ee ed aS 60 Ze Fe LOG 650 9 099 70 ae eae es Taa Ge GAZ 80 SEEE 800 0 302 90 aE S 850 Section 4 Cryostat System Operation 33 Removing Cold Finger Assembly From The Cryostat And Changing Samples Note Wear insulated gloves while handling cold or potentially cold items Wear latex or suitable clean room type gloves while handling parts internal to the cryostat to avoid contaminating them Tools Needed J J J J Procedures 1 pa Do not vent the cryostat 3 vacuum Shell until the sample and raditation shield have come to room temperature 4 Figure 46 5 34 e Section 4 Cryostat System Operation Insulated Gloves 1 pair 1 2 combination wrench each Clean room gloves pair Small straight blade screwdriver 3 32 key wrench Set the ellipsometer to the alignment mode and follow the instruction prompts given by the computer Jog the sample stage angle fine scale to zero using the right and left arrow keys on the computer keyboard Allow the cold finger to come to room temperature If the cryostat has been cold the tem
34. e Check seals at ends of the vacuum transfer line 16 e Section 2 System Setup e During pumpdown there can easily be two or more orders of magnitude difference in the pressure between the pump and cryostat chamber Even after pumpdown there can be an order of magnitude difference e If the system is well sealed water vapor will be the major contaminant in the chamber and it can take weeks to pump out Therefore it is recommended to keep the flexible vacuum line and the cryostat shell under vacuum to prevent water adsorption For extended idle times the valves can be closed and the turbo pump turned off e The start stop cycle of the pumping station is an automated sequence and may not immediately appear to be functioning The following procedures will check the integrity of the three copper gasket seals just installed The seal just above the turbo pump should have already been checked before assembling the table but check it again at this time l Make sure the main vacuum valve and the cryostat vacuum valve are completely closed Connect the pumping station Isobar to AC power separate circuit from the ellipsometer Make sure the power switch for each of the pumping station components is in the on position turbomolecular pump controller active gauge controller and roughing pump as well as the main power switch all switches are located on the back side of the pumping station The pumping station should
35. e parallel Using the two 1 2 combination wrenches Figure 4 gradually tighten the bolts to 15 lb ft in to turn increments using an alternating criss cross star pattern the seal flanges should remain parallel throughout the entire bolt tightening sequence Cryo 200 J A Woollam Co Inc Cryo 200 J A Woollam Co Inc 6 Figure 5 7 10 11 12 13 Connect Active Gauge Controller cable labeled Pump Vacuum to the gauge on the vacuum tee Figure 5 The other will be used later on the cryostat gauge The system is now ready to test the integrity of the copper gasket seal just installed If the seal needs to be replaced it is much more convenient to fix it before the full work table is assembled Start this check by making sure the main vacuum valve is completely closed Connect the pumping station Isobar power strip to AC power Make sure the power switch for each of the pumping station components is in the on position turbomolecular pump controller active gauge controller and roughing pump as well as the main power switch all switches are located on the back side of the pumping station The pumping station should be powered and the displays active Toggle the vacuum gauge display to read the pump vacuum see the Edwards Active Gauge Controller manual Start the vacuum pump by pressing the Start button The pressure should quickly drop to below 10 Torr If not the
36. e radiation shield may still be cold or hot Observe proper safety precautions Figure 50 10 Lay the cryostat in its cradle with the sample facing up Figure 51 Section 4 Cryostat System Operation e 35 Figure 51 11 Allow the radiation shield to come to room temperature 12 Using a small straight blade screwdriver remove the four flat head screws holding the radiation shield to the thermal anchor Figure 52 a o ep E Figure 52 Figure 53 13 Carefully slide the radiation shield off the cold finger Figure 53 14 Using the 3 32 key wrench remove the four screws holding the sample mounting clamps to the cold finger Figure 54 Figure 54 36 e Section 4 Cryostat System Operation Cryo 200 J A Woollam Co Inc 15 16 17 18 19 20 2i Remove the sample mounting clamps If necessary adjust the sample chuck height to compensate for sample thickness This is done via loosening the three screws that mount the sample chuck to the end of the cold finger sliding the sample chuck such that the face of the sample will be at the center of the cold finger and securely tightening the three mount screws Lay sample on the sample mount face up Note The back surface of the sample has to be flat and clean in order to make good thermal contact with the sample mount Lay the sample mounting clamps back over their screw holes and insert the hold down screws Note The mounting cl
37. ed for this step although it not depicted in Figure 10 8 Place the ellipsometer base on the tabletop near the main vacuum valve Figure 11 The front of the ellipsometer base should be 10 from the front of the tabletop The left side of the ellipsometer base should be 16 from the left side of the tabletop 10 e Section 2 System Setup Cryo 200 J A Woollam Co Inc Figure I 9 Carefully place the CRYO 200 assembly on the ellipsometer base Figure 12 oriented such that the z axis adjust knob faces toward the input unit when the AOI is at 0 Figure 12 Figure 13 10 Install four M5 socket head cap screws finger tight to mount the CRYO 200 assembly to the ellipsometer base 11 Use the 4mm key wrench to tighten the M5 socket head cap screws Figure 13 12 Install a new 1 45 ID copper gasket in the seal flange at the top of the main vacuum valve Figure 14 13 Mate the flexible vacuum hose 2 3 4 seal flange up to the main vacuum valve seal flange at the top of the vacuum tee 14 Install the six 28 bolts and nuts finger tight through the seal flanges making sure the seal flanges are parallel D y Bete a d PD tA ip 4 EY WZ gt a Ei a Joe TA 4 AX gt N y i RoN 5 ft K ve aa Figure 15 Figure 14 Using two 7 16 combination wrenches Figure 15 gradually tighten 15 the bolts to 12 lb ft in 1 4 to turn increments using an a
38. eiling e 15 amp 120 VAC outlet separate circuit from the ellipsometer Assembly Steps 1 Assemble the outer frame Figure 7 and Figure 8 A rubber mallet will be necessary to persuade the supports into place All four base supports two 3 two 5 should be in the 2 and 3 holes from the bottom Three of the top supports one 3 two 5 should be in the 2 and 3 holes from the top The last top support 3 should be in the 6 and 7 holes from the top this acts as a support for the cryogen transfer line and should be located on the left hand side of the table 2 Install the base shelf Section 2 System Setup e 9 Figure amp 3 Place the vacuum pumping station on the base located toward the back left hand corner as in Figure 11 The front of the pumping station should be 17 from the front of the base The left side of the pumping station should be 12 from the left side of the base 4 Connect the pumping station to the Isobar power strip There is a separate Isobar for the ellipsometer system 5 Install the four table support rails in the 17 and 18 holes from the bottom Figure 9 6 Install the 3 table center cross brace with the vertical brace to the right of the mounting rivets Figure 11 Figure 9 Figure 10 7 Making sure the main vacuum valve and pump vacuum gauge fits through the hole carefully install the tabletop Figure 10 Note the cross brace shown in Figure 11 should be install
39. er oscillates badly examine the P proportional control parameter It may need to be manually adjusted On the Cryo 200 the LakeShore controller regulates temperature solely by adjusting power delivered to a resistive heater embedded in the cold finger Thus to maintain temperature control heat dissipation either by evaporating cryogen below room temperature or by conductive convective losses above room temperature is required to pull against the heater Because the pull down heat dissipating forces in the system e g cryogen flow rate tend to be highly variable it is difficult to determine universally optimal PID control parameters The LakeShore controller is capable of sophisticated multi zone temperature PID control functions which are outside the coverage of this manual however the user is encouraged to try using the AutoTune feature of the controller for most control applications Occasionally the AutoTune feature will drive one of the PID control parameters too far in one direction requiring the user to make a manual adjustment see LakeShore page 3 10 For instance if temperature control oscillates very badly the P proportional value may be too large e g 650 and needs to be reset to a more stable value like 20 There are several factors which may affect accuracy and precision of the monitored thermocouple temperature The users should be aware of these potential limitation and should gauge their importance relative
40. er line to room temperature and make sure all water is off the end of the bayonet before reinsertion If the outside of the transfer line between the Dewar and the cryostat should start to frost then one can probably assume that the vacuum jacket on the transfer line has leaked In this case the transfer line will need to be removed warmed and dried and the vacuum jacket pumped back down before use If the transfer line jacket will not hold a vacuum the transfer line should not be used Once cryogen transfer is proceeding the transfer rate should be throttled back using the bayonet valve one full turn open is typical If an excessive amount of cryogen flows the heater tape on the cryogen outflow port will be unable to prevent the port from frosting over When this frost melts it will create water which could then drip into the mechanical and electrical components below Take precautions to keep this frost and the water when the frost melts off of the ellipsometer Paper towels and styrofoam cups work well for this task To hold at the desired operating temperature the bayonet valve typically only needs to be 14 turn open If the desired temperature is between room tempeature and the boiling point of the cryogen the temperature controller will need to be used Adjust the flow rate to keep the required compensating heater power in the center of the control range This way if cryogen flow rate changes the controller will be able to adj
41. esults Cycle 1 start 4 869 A hr Cycle 1 end 1 233 A hr Cycle 2 0 919 A hr Cycle 3 0 606 A hr Low Temperature Spectroscopic Measurement Cryo 200 J A Woollam Co Inc Low temperature spectroscopic measurements were taken during temperature cycle 3 for the sulfur doped InP sample The same adsorption rate analysis model as used for the data in Figure 79 was used for the results shown below For the spectroscopic data however the adsorption rate was held constant and the InP optical constants were fit over the full range of wavelengths measured instead of just the monitoring wavelength In particular a parametric semiconductor layer was used to model the substrate optical constants This model is based on Kramers Kronig consistent functions with Gaussian broadening Guassian broadening is needed to correctly model the sharp Section 5 Ellipsometric Acquisition and Analysis e 51 direct gap region for a material like InP especially at low temperatures Alternatively the optical constants could have been fit for on a wavelength by wavelength basis However in many cases the function based model is preferred because the physical K K consistency is built in The data and fit results for three wavelength cycles are shown below in Figure SO and Figure 81 Although not clearly visible due to the scale of these graphs the data for each wavelength cycle is slightly different because the adsorbing layer continues to grow throu
42. etectable window strain the usual situation then that strain needs to be accounted for not ignored and the accuracy of the strain model will be about the same whether a small or moderately large strain effect is present In general terms the effect of window strain can be divided into two parts Part 1 is a pure window effects which can be determined when the ellipsometer is calibrated for a very wide range of samples Part 2 is a window effect which merges completely with the measured A value for the sample Part 1 effects are referred to as WinEffects and Part 2 effects are referred to as DelOffsets See hardware cnf section below Cryo 200 J A Woollam Co Inc Section 3 Configuring WVASE32 For Cryo 200 Attachment e 23 With windows the measured w and A values must be interpreted as applying to the window sample window system as a whole Data files are tagged with the appropriate window effects information for use when analyzing the data To analyze multiple angle cryostat data use multiple models with data at one angle per model For normal operation it is not necessary to recalibrate window effects Previously stored values will be used A WinEffects system calibration requires a thermal oxide calibration sample 24 e Section 3 Configuring WVASE32 For Cryo 200 Attachment One tractable but unfortunate consequence of windows combined with a rotating analyzer ellipsometer is that the window effects can
43. fully less material will be adsorbed on to the sample 46 e Section 5 Ellipsometric Acquisition and Analysis Cryo 200 J A Woollam Co Inc 5 Close the cryostat vacuum valve By this time the cryostat will probably be cryo pumped to a pressure below the turbo pump and there is no need to cryo pump gas back streaming from the turbo pump 6 Turn the heater off and let the temperature fall to 4K 7 Let the cold finger assembly and radiation shield soak at 4K for 10 minutes 8 The initial cycle is complete The radiation shield and the whole of the cold finger assembly should now be cold and participating in cryo pumping the cryostat chamber Cycles 2 and 3 quickly ramp the sample temperature back above room temperature The goal is to clean the sample surface and transfer the adsorbed material to the still cold radiation shield The suggested schedule for cycles 2 and 3 are as follows 1 Stop the cryogen flow 2 Open the cryostat vacuum valve There will be a pulse of pressure as the heater turns on in the next step 3 Adjust the temperature controller set point to 305 K and turn on the heater 4 After the temperature passes 300 K wait one minute The material on the sample surface does not desorb instantaneously However do not wait too long or the radiation shield will start to warm up 5 Turn off the heater 6 Restart cryogen flow The transfer line should still be cold so this time cryogen flow should star
44. ghout the acquisition process That is why each measurement needs to have its own correct measurement time recorded The grouping time is only needed to ease graphing and experimental data selection Note as discussed in Guide to Using WVASE32 the pseudo dielectric values lt gt and lt gt in Figure 8 amp 1 are just numerically transformed from the y and A values Generated and Experimental Generated and Experimental 40 180 Model Fit f i Model Fit Exp E time 1590 316 min 160 Exp E time 1590 316 min 30 Exp E time 1635 131 min gt Yo Exp E time 1635 131 min n Exp E time 1680 148 min wo Exp E time 1680 148 min D 140 O D 2 20 2 c 120 gt lt 10 100 oc 80 1 0 2 0 3 0 4 0 5 0 6 0 1 0 2 0 3 0 4 0 5 0 6 0 Photon Energy eV Photon Energy eV Figure 80a b Sulfur doped InP near 4K Generated and Experimental Generated and Experimental Model Fit i i l Model Fit Exp E time 1590 316 min Exp E time 1590 316 min Exp E time 1635 131 min Exp E time 1635 131 min Exp E time 1680 148 min Exp E time 1680 148 min 1 0 2 0 3 0 4 0 5 0 6 0 1 0 2 0 3 0 4 0 5 0 6 0 Photon Energy eV Photon Energy eV Figure 8la b Sulfur doped InP near 4K The room temperature data displayed in Figure 67 and Figure 68 was subjected to a similar parametric semiconductor model analysis Of course there was no adsorbing layer to correct for A comparison of
45. he dynamic scan used a single wavelength 427 6nm 2 9 eV and a 72 angle of incidence A 50 revolution zone averaged Pol 20 acquisition was used The TempReader exe utility was put in a one second polling mode while the LakeShore controller units were set to K Section 5 Ellipsometric Acquisition and Analysis e 45 13 2 12 9 12 6 Y in degrees 12 3 12 0 11 7 1100 The data shown in Figure 71 encompass three cool down cycles and two intermediate warm ups The User1 values are thermocouple temperatures in K The User1 values will be the secondary Y axis values Double Y axis is set using GraphlDefaults Experimental Data Experimental Data 400 141 0 400 140 0 300 300 Q 139 0 ExpA E 2 9eV 200 2 8 200 E c 138 0 a lt 100 100 137 0 0 136 0 a a ae a 1600 1700 1800 1100 1200 1300 1400 1500 1600 1700 1800 Time in Minutes Time in Minutes Figure 7la b Thermal cycling of sulfur doped InP Userl is thermocouple temperature Cycle 1 1190 1310 min is qualitatively different from cycles 2 1310 1520 min and 3 1520 1800min Qualitatively comparing cycle 1 to 2 and 3 it is noted for cycle 1 that the starting y and A values are different that the initial rate of change of the data is greater and that there is more curvature These differences are due to the total amount of adsorbed material and the current rate of adsorption The data during the two warm up cycle
46. he oxide layer with room temperature thickness and optical constants is the most practical perhaps only method of dealing with the oxide overlayer Note that the assumed start time for adsorption was the 1194 minute mark and that 0 A of adsorbant was assumed present at that time Any adsorbant present at 1194 min was effectively subsumed into the substrate optical constants that were fitted This type of modeling is typical of the virtual substrate approximation commonly used for in situ monitoring of intentional deposition processes The key feature of a virtual substrate type model is that the growing layer can be very accurately characterized even when the exact underlying model is not perfectly defined If there is a good ellipsometric measurement a some time e g 1194 min and the underlying sample is of the correct type high index slightly absorbing is best the virtual substrate approach is possible To examine the impact of the g2 fit parameter on the analysis see the following figures Generated and Experimental Generated and Experimental 12 30 12 40 Model Fit Model Fit 12 25 Exp Y E 2 9eV Exp Y E 2 9eV 12 30 y 12 20 L C L M C P 12 15 3 12 20 gpp A D xe 5 xe ppp a me 5 i i gt 12 10 gt 12 10 12 05 12 00 0 12 00 0 1170 1200 1230 1260 1290 1320 1170 1200 1230 1260 1290 1320 Time in Minutes Time in Minutes Figure 75a b Temperature Cycle 1 with different constant rate model
47. hermocouple monitor temperature for the same reasons discussed in point 3 above 30 Section 4 Cryostat System Operation Cryo 200 J A Woollam Co Inc TempReader exe Utility Program The TempReader utility written by J A Woollam Co is a simple program that performs just the three basic tasks as described below It is not a general control program for the LakeShore If a more sophisticated WVASE32 LakeShore interfacing program is required for a particular experiment the user will need to contact the J A Woollam Co for more information on the WVASE32 external programming interface EPI A screen shot of the inactive TempReader window is shown below TempReader can be started from the GloballRun_WVASE_ Tools menu from inside WVASE32 or it can be launched directly from the operating system out of the wvase32 tools subdirectory TempReader assumes a baud rate of 1200 bit s using the computer s ComPort 1 If these values need to be changed contact J A Woollam Co for more information Make sure the LakeShore baud rate is set to 1200 See LakeShore page 3 13 TempHeader Curre 12 C W ASE324type e_extended cnf File Update Period zec fi Curme C WVASE Sey pe e exbended cnf i ae Curve 12 Not Folling Start Folling Close Comm Fort E Manual Send String Real Temp K Meas m m Down Load Curve 999 999 hs Figure 45 TempReader exe front screen Cryo 200 J A Woollam
48. ion 2 System Setup 13 24 Connect the exhaust port heater controller to the vacuum pumping station Isobar using the cord provided 25 Connect the cryostat thermocouple to the cold junction compensator using the cable provided Figure 21 and Figure 22 ETT e aw T ETI Signs TU ee 7 a Pie od ie i t t f t ttl Rae Figure 21 Figure 22 26 Using the small phillips tip screwdriver connect the cold junction compensator to the temperature controller Figure 23 Figure 24 Figure 25 14 e Section 2 SystemSetup Cryo 200 J A Woollam Co Inc 27 Connect the sample heater to the temperature controller with the cable provided Figure 24 and Figure 25 28 Connect the temperature controller to the vacuum pumping station Isobar using the cord provided 29 Connect Active Gauge Controller cable labeled Cryostat Vacuum to the gauge on the cryostat vacuum chamber Figure 26 aa go 7 Mgr y md a o aE S goaa 7 x lt Ju t f E gt gt 21 w h R z H m uo i ers i 7 k ba G Figure 26 This completes the installation of the CR YO 200 option on the ellipsometer base The ellipsometer installation may now be completed The cryostat was shipped with a thermal oxide 250A on silicon sample mounted on the cold finger Ellipsometer and data acquisition test can be performed Turbo Pump Operation And Seal Checkout Cryo 200 J A Wo
49. lternating criss cross star pattern the seal flanges should remain parallel throughout the entire bolt tightening sequence Install a new 64 ID copper gasket in the seal flange of the cryostat vacuum valve Figure 16 Figure 16 17 Mate the flexible vacuum hose 1 1 3 seal flange to the cryostat vacuum valve seal flange 18 Install the six 8 32 screws and three nut plates finger tight through the seal flanges making sure the seal flanges are parallel 19 Make sure the AOI can move the full 0 to 90 range without placing undo strain on the flexible vacuum hose refer to VASE Hardware manual for instructions on manually changing the AOI 20 Using the 9 64 key wrench Figure 17 gradually tighten the screws to 7 lb ft in 4 to 1 2 turn increments using an alternating crissross star pattern the seal flanges should remain parallel throughout the entire bolt tightening sequence 12 e Section 2 System Setup Cryo 200 J A Woollam Co Inc Cryo 200 J A Woollam Co Inc i Figure 17 21 Place the exhaust port heater controller on the tabletop next to the main vacuum valve Figure 18 22 Place the temperature controller on the tabletop next to the ellipsometer base Figure 18 i Figure 18 23 Connect the exhaust port heater to its controller using the cable provided Figure 19 and Figure 20 L D pone A 7 7 see ee ih es Figure 19 Figure 20 Sect
50. ly designed to work at 70 In the Hardware section of the hardware cnf file the following entry identifies attachmentl as cryostat In turn this causes WV ASE32 to look for the cryostat subsection for more configuration entries Hardware attachmentl 1 cryostat The following lines provide acquisition guidelines for WVASE32 to use when the presence of the Cryo 200 is detected Cryostat switchconfig 27 1 AllowAlignJogs 1 fixedpolalways 1 20 zoneavealways 1 The switchconfig entry tells the program that when sensor bit 27 is active the Cryo 200 is present There is a proximity switch in the sample stage base that detects the presence of the cryostat The AllowAlignJogs entry enables the left right arrow keys to be used for left right fine sample alignment but only with the Cryo 200 present The fixedpolalways entry tells the program that the polarizer must be fixed for a given scan needed because of possible window effects and that the fixed polarizer azimuth must be 20 or 20 if zone averaged The zoneavealways entry which is commented out not active in the above example would force zone averaged data acquisition While this is not an absolute requirement zone averaging the polarizer is highly recommended even without possible window strain The remaining Cryostat entries define the angle zones for the windows define how window effects will be calibrated and specify the
51. metric ACQUISITION ccccccseeecceeeeeeneceeeeeeceeeeseseeeeeeeseseeeeeeeaes 43 Monitoring Sample Cool Down With Temperature Cycling cecccceccccseseceseeeeeeeeeeeneeeaes 45 Adsorption Rate Data Analysis eseesesenseseressseessseessseresseresseressseessseresserosseresseeessserssseeo 47 Low Temperature Spectroscopic Measurement ccccccsescccesecceeseceeneeeaeseeeenseeseseeseneeeaes 51 High Temperature Spectroscopic Measurement ccccccseeeccseseeeeneecaeseeeenseeaeneeeeeseeeeneees 53 Index 57 Cryo 200 J A Woollam Co Inc Contents e i Section 1 User Safety Safety Concerns Operators and Observers MUST to be aware of these safety concerns 1 The evaporating gas from the cryogen Helium or Nitrogen is an asphyxiant While it is not toxic it will displace the oxygen in the room and will KILL you just the same The system MUST be set up to be n a WELL VENTILATED area 2 Use only INERT cryogens e g Helium or Nitrogen Do NOT USE liquid oxygen or hydrogen as the cryogen 3 The cryogen and anything it has been in contact with is unimaginably cold and will cause SEVERE frostbite cold burns The primary danger is from items that have been in contact with the cryogen They stay cold and must be allowed to warm up to room temperatures before being handled ALWAYS wear long sleeve shirts full length pants covered toe shoes safety glasses and insulated gloves when handling cry
52. n Energy eV Figure 68a b Sulfur doped InP near 297K Each measurement cycle appears in the legend denoted by its grouping time If dynamic data already exists in the experimental window when a new dynamic scan is started the new data can be appended to the old data thereby preserving the measurement time information YASE Dynamic Scan xi 2 Append to existing Experimental Data Figure 69 Alternatively even if the new data will not be appended to the old data the start time from a previous dynamic scan can still be used By default WVASE32 assumes the user will want to maintain a continuous measurement time reference Answering Yes in the following dialog box resets the dynamic time reference back to zero YASE Dynamic Scan x 2 Based on the start time from the last dynamic scan the current time is 0 62 min Set the new data time stamps to restart from 0 min i Cancel Yes Figure 70 In the examples described in the next sections many scans were started and saved as different files but the time reference from the first dynamic scan was retained Monitoring Sample Cool Down With Temperature Cycling Cryo 200 J A Woollam Co Inc This sections presents an acquisition and analysis example of thermal cycling to reduce adsorption by the sample surface at very cold temperatures The sample was a sulfur doped InP wafer Room temperature data were previously shown in Figure 67 Figure 68 T
53. ne bayonet from the cryostat by pulling the transfer line straight up Figure 27 and Figure 28 A step stool may be necessary to accomplish this step Caution The bayonet may be cold Observe proper safety precautions Section 2 System Setup 17 Figure 27 Figure 28 5 Using the 4mm key wrench remove the four M5 socket head cap screws that hold the cryostat assembly to the goniometer base Figure 29 These screws will be used to fasten the cryostat assembly to its storage stand AD Aa D Sa CR AFINA Figure 29 Be Careful The cryostat and 6 Using care not to strain the flexible vacuum hose if attached to the tilt stage are heavy cryostat chamber gently move the cryostat assembly to its storage stand Figure 30 PNM iy atii 7 Wie WE Whigs LLU P n Figure 30 Figure 31 18 e Section 2 System Setup Cryo 200 J A Woollam Co Inc 7 Install the four M5 socket head cap screws finger tight through the CRYO 200 base into the storage stand 8 Using the 4mm key wrench tighten the four M5 socket head cap screws Figure 31 9 Using the 3 16 key wrench bolt the sample z axis stage to the goniometer base with two 14 20 socket head cap screws Figure 32 Ze 5 I DaT N n a ol N Figure 32 10 Using the 3 16 key wrench bolt the Sample Chuck on the sample z axis stage with four 4 20 socket had cap screws Figure 33 ee ert genet pah Figure 33 Figure 34 11 Attach
54. not be fully disentangled from the measured y and A values at acquisition time Without windows a rotating analyzer system has an ambiguity with respect to sign of A which is not a serious problem because A can be mapped back to the 0 180 range for analysis However with windows present the measured A relates to the combined window sample window system and there is no general convenient mapping to apply With windows the measured y and A values are not intrinsic sample parameters The measured w and A values must be interpreted as applying to the complete window sample window system WVASE32 manages this complication by tagging the data files with the appropriate window effects information which is automatically loaded into the Model Options when the data file is loaded The spread of strain effects across the window also needs to be considered if data 1s to be taken at more than one angle As discussed in the next section the Cryo 200 attachment has its windows divided via software into different zones for different angle of incidence ranges As data is acquired at different angles window effects for that angle zone are tagged with the data file However for analysis any single model and corresponding experimental data can have only one set of window effects Therefore one can not simultaneously model cryostat data acquire at different angles of incidence using a single model To analyze multiple angle cryostat data use multiple model
55. ogen and items that have been in contact with cryogen Equipment Safety Cryo 200 J A Woollam Co Inc Operators NEED to be aware of these operational concerns 1 The transfer line MUST be removed from the cryostat for operation at temperatures above 475 Kelvin The transfer line will be damaged if it is overheated Section 1 Safety Concerns e 3 4 e Section 1 Safety Concerns 2 3 4 The cryostat vacuum shell MUST be under vacuum and continuously pumped during cryostat operation At elevated temperatures outgassing will occur and atmospheric contaminants oxygen will corrode the sample mount At low temperatures atmospheric gases will condense deposit on the sample altering the sample morphology on a continual basis making meaningful measurements of the sample difficult When the cryostat is mounted on the ellipsometer do not move the system angle of incidence AOI beyond the 0 to 90 range either manually or automatically Doing so will result in damage to the flexible vacuum hose The cryostat exhaust port tends to build up condensation or frost at high cryogen flows This condensation or frost should be wiped off as quickly at it is formed Water and electrical equipment do not mix Cryo 200 J A Woollam Co Inc section 2 System Setup Turbo Pump Setup Any foreign matter inside the Note Cleanliness is absolutely essential in this procedure Any foreign matter vacuum system will likely inside
56. ollam Co Inc Some general trouble shooting and operational comments are presented below UHV systems have a complex nature which is beyond the scope of this manual For an in depth understanding of UHV consult relevant books journals and experienced colleagues e Leaks have a large number of causes The most obvious are e Recycling used copper gaskets You might get lucky and get one to seal but how much time do you want to spend fixing a known source of trouble Always use fresh copper gaskets when assembling a UHV seal e Improper torque sequence Torquing of the seal bolts in an uneven manner will cause uneven compression of the gasket causing it to leak Loosen and re torque seal bolts in the prescribed fashion If the torquing is really bad the gasket will have to be replaced e Inadequate torque Seals rely on bolt tension to compress the gasket and hold it in intimate contact with the seal surface Torque the seal bolts to the prescribed value e Dirt on seal mating surfaces Dirt keeps the gasket from coming into intimate contact with the seal surfaces and may damage the seal surfaces Always check your seal surfaces for foreign material dirt lint dust hair etc before assemble a UHV seal Replace the gasket and clean the seal surfaces Section 2 System Setup e 15 It is recommended to keep the vacuum line and cryostat shell under vacuum to prevent water adsorption Check seal between pump and vacuum te
57. on is launched See Figure 45 The default curve and filename can be changed by using command line arguments which are accessible using a Windows shortcut to launch TempReader For example if the command line contains curve 13 curve type e_alternate cnf then TempReader will look for a file called type e_alternate cnf and download the values to curve 13 if directed to do so The format for a thermocouple cnf file can be seen the in the text of type e_extended cnf at the end of this section A linear user specified correction can be applied to the calibration curve before downloading using the Real Temp and Meas text boxes in the lower left corner By using known cryogenic control temperatures such as imersion in liq helium or liq nitrogren the actual measured thermocouple voltage can be measured by selecting the mV units display See LakeShore 3 3 A measurement with the cold finger assembly outside the cryostat and at equilibrium at room temperature can provide another known temperature assuming a secondary accurate temperature measurement is available It is not recommended to directly immerse the cold finger in ice water because water vapor is primary background gas at the vacuum pressures typical for the cryostat In all cases remember that even if the thermocouple were perfectly calibrated the temperature of interest is the temperature of the sample See end of previous LakeShore Tem
58. perature Controller section In Figure 45 the Meas boxes contain 999 which cause NO correction to be applied If exactly one of the Meas boxes contains a value which is not 999 then TempReader will adjust the downloaded calibration voltages by a constant offset to match the actual measured value for the specified temperature Of course the preferred method is to make the measured voltage match the thermocouple values using proper wiring techniques and the ice point buffer module If both Meas boxes contain values which are not 999 then a linear correction to the downloaded voltages is made such that the measured values match both specified temperatures A two point correction effectively shifts and tilts the calibration curve to match the known measured points 32 e Section 4 Cryostat System Operation Cryo 200 J A Woollam Co Inc TempReader does NOT perform a plausibility check on the user defined correction 3 TempReader can pass commands to the controller to test the serial link and to examine the controller NOVRAM Cryo 200 J A Woollam Co Inc Note TempReader does not perform a plausibility check on the user defined correction Therefore very inaccurate temperature read outs can occur A potential hazard exists if the LakeShore 1s instructed to control the temperature with an invalid calibration curve For example the heater might turn on and remain at full power indefinitely because the desired tem
59. perature can never be reached The same thing would also happen if an unreachable temperature was selected as the control point even with accurate calibration values TempReader can also pass single commands to the LakeShore and read the returned string The Manual Send String button sends the text from the box above the button and waits for a return if the string contained a characters See LakeShore 4 8 to 4 22 for a complete description of commands Do not send commands to the LakeShore while polling is active The primary purpose of this feature is to test the serial link to the controller The sensor units query command suni shown in Figure 45 will return a K C or mV depending on the display of the controller If this command succeeds one is assured that serial communication has been established There are a variety of commands used to control and query the LakeShore controller Tests with this feature may be a useful way for the user to develop a separate more sophisticated control package Also some of the NOVRAM values are more easily interrogated using the serial link Text can be clipped from the log box and pasted into different applications for printing File type e_extended cnf contains the following text The tabulated values are measured voltage in mV and corresponding temperature in degrees Kelvin Please note that for an extended type thermocouple both columns of numbers are modifie
60. perature controller may be set to 295 Kelvin to quickly warm up the sample If the cryostat has been hot cryogen may be used to cool the sample Note Do not vent the cryostat vacuum shell until the sample and raditation shield have come to room temperature Parts at elevated temperatures will corrode permanent or at reduced temperatures will frost instantly water contamination is time consuming to remove Completely unscrew the transfer line o ring compression nut and carefully withdraw the transfer line bayonet from the cryostat by pulling the transfer line straight up Figure 47 and Figure 47 Caution the bayonet may still be cold Observe proper safety precautions j t J J ya A Figure 47 Disconnect the cables from the sample heater sample thermocouple and the exhaust port heater Cryo 200 J A Woollam Co Inc Cryo 200 J A Woollam Co Inc 6 Note In order to reduce vacuum chamber contamination which will significantly increase the pump down time minimize the time the vacuum chamber is exposed to the atmosphere Be organized work quickly do not leave the system lay open unnecessarily 7 Close the cryostat vacuum chamber valve Figure 48 8 Using the 1 2 combination wrench remove the eight 5 1624 cap head bolts that seal the cryostat vacuum chamber Figure 49 Figure 48 Figure 49 9 Withdraw the cryostat by lifting it straight up out of the vacuum jacket Figure 50 Caution Th
61. re is probably a leak in the seal between the turbomolecular pump and the vacuum tee If there is a leak try tightening the clamping bolts If the leak persists press the Stop Start button to stop the pump and let the system return to atmospheric pressure Replace the copper gasket seal and try pumping down again After the pressure reaches 10 Torr press the Stop Start button to stop the pump and let the system return to atmospheric pressure The pumping station is now ready for installation in the system For additional information on the pumping station components refer to the appropriate manual included with the system There is no separate manual for the pumping station as a system Section 2 System Setup 7 Work Table Setup Note This procedure covers the setup of the CRYO 200 option as it relates to the ellipsometer and not the setup of the ellipsometer itself Please read Operating Instructions for the Janis Research SuperTran System from Janis Research Schematic diagrams at the end of the Janis manual contain a general cryostat layout and useful nomenclature Also read User s Manual Model 330 Autotuning Temperature Controller from LakeShore Section 2 6 2 2 contains useful thermocouple hook up information 8 e Section 2 System Setup OY i 6 i IF i d Figure 6 B e n n t ie q l i j i r i i i i i i L
62. s with data at one angle per model The Part1 WinEffects values can in principle be determined with each normal hardware calibration However the default operating procedure for the Cryo 200 is to not fit new window effects With the Cryo 200 present the following additional dialog box is displayed during the normal calibration procedure For normal operation the recommend action is to select No at this stage ASE Calibration o Recalibrating windows i optional Do pou want to recalibrate the windows Figure 42 If the WinEffects are to be recalibrated then it is recommended that a full window system calibration be performed A window system calibration determines appropriate WinEffects and DelOffsets A system calibration entails loading a piece of the thermal oxide calibration wafer provided with the system as the cryostat sample and then running a special script file Because the system calibration is a combined spectroscopic calibration and model fit the sample need only have an oxide thickness near the nominal 250A Other simple samples which can be fit to high accuracy without windows present could also be used The data acquisition and analysis takes about two hours If the windows are removed or substantial external heat is applied to bake out the chamber the windows will need to be recalibrated Otherwise the windows are typically stable Procedures 1 The CRYO 200 was shipped with the correct cali
63. s exhibit competing effects due to both temperature changes and desorption from the surface The key qualitative observation from Figure 71 is that the sample changes at low temperatures less rapidly for each subsequent temperature cycle A quantitative analysis of the adsorption rate is given in the next section The following procedures are suggested for thermally cycling the sample to reduce the adsorption rate It 1s suggested that data be acquired during these steps in the manner described above The following example assumes liquid helium is used Note the suggested dwell times in the cold phase for cycles 1 and 2 are much shorter than those shown in Figure 71 Those measurements were primarily for demonstration of the effect The suggested schedule for cycle 1 is as follows 1 Using the proper procedures described in earlier sections open the vacuum valves and pump the cyrostat down to 10 2 Adjust the temperature controller set point to 50 K and turn on the heater 3 Using the proper procedures described in earlier sections initiate cryogen flow 4 As temperature approaches 50 K throttle the cryogen flow back to about 1 2 turn open on the transfer linebayonet valve Let the heater try and hold the temperature around 50 K for about 2 minutes Often there is a large cryostat pressure drop between 50 and 4K and the goal of this pause at 50 K is to let surfaces other than the sample to also get cold In this way hope
64. t almost immediately 7 As the temperature approaches 50K close the cryostat vacuum valve 8 Hold at 4 K for 5 minutes 9 Ifon cycle 2 repeat steps for cycle 3 Additional cycles might have some benefit but they are expected to be minimal Cycle 3 may be omitted if desired But as shown in the next section there 1s in fact a measurable reduction in adsorption rate with the extra cycle in this experiment Adsorption Rate Data Analysis Cryo 200 J A Woollam Co Inc At very cold temperatures the sample under test will adsorb residual gas from the chamber In a UHV sealed chamber with base pressures below 10 most of the residual gas is water vapor which is slowly released from the inside surfaces of the cryostat Because water vapor is the primary contaminant it is useful to keep the cryostat sealed and under vacuum when not in use and it is useful to minimize exposure time of the cryostat parts to room air when changing samples The primary consequence of adsorbing material is that a thin overlayer will grow on the sample during the measurement procedure Ellipsometry has the wonderful capability of being sensitive to very thin overlayers when the overlayer is the subject of study However ellipsometry retains that sensitivity to overlayers even when the overlayers are just complications Overlayers themselves are a universal issue when dealing with ellipsometric data See Guide to using WVASE32 for a more in depth disc
65. the vacuum system will likely result in major damage to the pump Please result in major damage to the read the instruction manuals for the Edwards Active Gauge Controller and the pump Edwards Turbomolucular pump and controller before performing this procedure A schematic diagram located at the back of the Operating Instructions for the Janis Research Supertran System contains a general vacuum pump layout and useful nomenclature Figure 1 Tools Needed e Clean room gloves e 1 2 combination wrench 2 each Cryo 200 J A Woollam Co Inc Section 2 System Setup e 5 Incrementally tighten bolts in cross cross pattern to avoid leaks 6 e Section 2 System Setup Parts Needed Pumping Station 1 each e Pump Oil liter e 2 51 ID copper gasket 1 each e Vacuum tee with gauge and valve 1 each e 5 16 24 bolts with nuts and washers 8 each Assembly Steps 1 Fill the roughing pump with pump oil Figure 2 as per specification in 2 Figure 3 3 the pump manual Remove the caution notice when finished N J r 2 E ng i a z RE Install a new 2 51 ID copper gasket in the seal flange at the top of the turbomolecular pump Figure 3 Figure 4 Place the vacuum tee on the seal flange at the top of the turbomolecular pump with the gauge and valve facing away from the roughing pump Install the eight 5 16 24 bolts washers and nuts finger tight making sure the seal flanges ar
66. to the particular experiment under consideration 1 Thermocouple voltage measurements may have some offset induced by undesired junctions in the wiring These are effects are reduced by the cold junction compensating device in line with the thermocouple Additional offset correction capability is available using the TempReader exe utility to down load a modified calibration curve to exactly match one or two known temperatures to a measured voltage 2 The extended Type E thermocouple which covers a very wide temperature range 1s much less sensitive to temperature changes around 4K than it is around 300K 3 The sample temperature may not be exactly the same as the thermocouple The thermocouple is in close proximity with the heater and helium outlet in the cold finger assembly The thermocouple needs to be close to the hot cold source for control stability see Introduction to Laboratory Cryogenics page 65 However this permits a temperature offset to exist between the sample and the thermocouple Furthermore the offset between sample and thermocouple may depend not only on what the control temperature is but also on how much cooling heating is taking place to maintain that control temperature An experiment using a secondary temperature probe may be possible to characterize the sample thermocouple offset as a function of control temperature and controlling heater power 4 There is a time lag between the sample temperature and the t
67. ure below 1 Torr When the pressure no longer increases as the gate valve is opened a little more proceed to fully open the gate valve Because the cryostat has a small volume the cryostat will be mostly evacuated after a few minutes of the above procedure Inspect the transfer line bayonet for frost or condensation If there is frost let the bayonet warm up to room temperature Wipe condensation off with a clean soft cloth Insert the transfer line bayonet into the cryostat Figure 58 Lightly tighten the transfer line o ring compression nut Figure 59 Section 4 Cryostat System Operation e 39 aa os N at a lee eee was i lie a oe a eS a J Figure 58 Figure 59 18 With the transfer line installed the final fine alignment of the sample can be completed 19 To adjust the Y Axis alignment use the tilt stage adjustment knob behind the cryostat Figure 60 20 To adjust the X Axis alignment use the left and right arrow keys on the computer keyboard Figure 61 The sample stage will jog clockwise or counterclockwise one step every time the left arrow or right arrow key is depressed CTRL arrow makes 10 steps and CTRL SHFT arrow makes 100 steps Figure 60 Figure 61 21 When the sample is aligned press the escape key Follow the instruction prompts given by the computer 22 To adjust the Z Axis of the sample use the micrometer knob on the front of the cryostat tilt stage Figure
68. ussion of data modeling The principal added complication from adsorption is that the overlayer is continually changing However that continual change can in turn be used to help characterize Section 5 Ellipsometric Acquisition and Analysis e 47 the adsorption layer if certain assumptions about the adsorbant and the adsorption rate are valid The Adsorb layer has been added to the WVASE32 modeling capabilities to deal directly with the adsorption situation encountered with the cryostat A simple adsorption rate analysis model is shown below Layer 1 was derived from a room temperature analysis of the sample Layer 2 is just a place holder layer where the optical constants for the adsorbant material are stored accom ice OA inp Ox 23 985 A inp _s doped_tabulated_4k 1mm Figure 72 Layer 3 is the Adsorb layer which couples to optical constants from some other layer and defines how the adsorption should be modeled The Adsorb layer dialog box is shown below Layer Name Save rMat Name __ Material Gee t DataTime StartTime Thickness 00 10 grts gre M Start Time min mre _Delete Layer_ tp J Fit Ok gl Ajhr Fit Cancel g2 Afhrhr M Fit Figure 73 In this example the optical constants for Ice have been coupled in to the Mat Name box Ice optical constants were obtained from a very long adsorption experiment results not shown here Future experiments m
69. ust power up or down as needed For temperature controller operating instructions refer to chapter 3 of User s Manual Model 330 Autotuning Temperature Controller from Lakeshore When making low temperature measurements residual gas in the cryostat will continually deposit on the sample surface This deposition can be slowed but not eliminated In general a better vacuum room temperature base pressure will yield slower deposition rates Experiments have determined that cycling the sample temperature helps to reduce the adsorption rate Details of the temperature cycling procedure are given in the data analysis section Monitoring Sample Cool Down With Temperature Cycling Cryo 200 J A Woollam Co Inc Section 5 Ellipsometric Acquisition and Analysis VASE Dynamic Ellipsometric Acquisition Most data acquisition runs involving the cryostat will be made using the VASE dynamic measurement mode Each data point is assigned a measurement time and a cycle grouping time Set the angle before starting a dynamic scan Cryo 200 J A Woollam Co Inc Most data acquisition runs involving the cryostat will be made using the VASE dynamic measurement mode The VASE Dynamic mode records the acquisition time for each measurement needed for adsorption rate analysis at low temperatures and allows the Userl values temperatures to be recorded Dynamic data acquisition is started using the HardwarelAcquirel Dynamic_Sc

Woollam ellipsometer cryostat operation

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