APEX-SZ Bolometer Array Fabrication Steps v2

Jared Mehl

6-6-2005

 

  1. Obtain Si wafers, either 500 um thick or 150 um thick (bonding to handle wafer still a research project in progress)
  2. Clean wafers in sink6 piranha bath and DI water rinse, then spin dry in srd.
  3.  Deposit ~ 1 um silicon nitride (SiN) onto wafers in tystar17.  Use LSNSTDxxx recipe and calculate the deposition based on current process monitoring data posted on Microlab website.  Time should be roughly 4 hours.
  4. Aluminum and titanium deposition in cpa.  Load wafers and wait for system to pump down to base pressure P < 2e-7 Torr.  Turn throttle on and adjust Ar gas flow obtain pressure of 6 mTorr.   Deposit 400 Å Al using 1kW, 40.0 cm/min and 1 pass.   Deposit 800 Å Ti using 1.5 kW, 31.3 cm/min and 2 passes.  Be sure to coat a few dummy wafers for alignment corrections in later steps, focus/exposure tests, and T_c checks.
  5. If using 150 um wafers and bonding to 350 um handle wafer, you would also coat backside of thin wafer with >1 um Al and do the bonding step.  Still a work in progress!
  6. Ti lithography step:  primeoven setting “0” (1 min HMDS), svgcoatx “1”  “1” (1.1 um I-line PR, 60 sec bake).  Shrink gcaws2 aperture to 37.5 + 5 small divisions in x direction and 37.5 + 15 small divisions in y direction (7 mm x 6 mm aperture).   Load APEX ALIGN mask and run ALIGN;1 program to print alignment marks.  Develop alignment marks in svgdev “1” “7” (no PEB yet).  Back to gcaws2, load ARRAY TITANIUM mask and run program HEXPIX,1,2 to print array, then load blank mask and run HEXPIX;3 to clear contact pads.   Be sure to print at least one dummy wafer here for later alignment corrections.  Then svgdev “8” “1” for Post Exposure Bake and svgdev “1” “7” to develop wafers. 
  7. Etch titanium in ptherm.  Run oxygen scourge on chamber and spacer wafers (15 min, 100 sccm O2, and 200 W) and SF6 scourge (2-3 min, 60 sccm SF6, 125 W) to clean chamber.  Run wafers in ptherm, with base pressure < 15 mTorr on left side gauge ( < 0.040 Torr on MKS gauge) and cooling water temp low (< 20 C), for roughly 3:30 minutes at 60 sccm and 125 W.  Then clean off PR with acetone bath, isopropanol rinse, DI water rinse, and N2 gun dry.
  8. Al lithography step: same as Ti (#6) except do a 90s bake on PR in svgcoatx, and only shrink gcaws2 aperture 10 small divisions per side.  Load ARRAY ALUMINUM mask, run a focus/exposure test, then run program HEXPI2,5-8 with appropriate shiftx and shifty settings on your dummy to correct for any alignment shifts.  Incorporate any alignment shifts into HEXPI2,1,2 program and print your real wafers with this program.   Do not develop real wafers yet.  Then, contact print APEX ARRAY mask with ksaligner, using vacuum contact, 9.0 s exposure time, and 30 um alignment gap.  You have to align the wafer alignment mark to the bottom of the mask alignment mark to account for gcaws2 offsets.  Run your dummy wafer first to check alignment and exposure time.  Run PEB and develop in svgdev.
  9. Etch Al in lam3.  Run 3 dummies first.  Then run standard recipe with following changes:  Reactor #2 COMPL:TIME and WAIT:00:25, Reactor #3 OVERETCH:0%, and AIRLOCK #1 WAIT:00:30.  Clean off PR, either with acetone or ptherm O2 plasma if acetone doesn’t work (technics-c has been giving us trouble…).
  10. Gold web lithography step: same as Al (#8) but no ksaligner, except on need 60s PR bake.  Use ARRAY GOLDWEB mask and run HEXPI2,1,2 (with appropriate alignment shifts).
  11. Liftoff gold web:  Clean off gold wire bundles and boats in acetone/isopropanol with sonicator. Deposit 20 Å Ti on crystal monitor and 300 Å Au in v401.  Deposit gold with a rate of ~10 Å/s.  Leave in acetone bath overnight (cover in aluminum foil) for liftoff to occur.  Rinse wafer with isopropanol and DI water afterwards.
  12. Gold lead lithography step: same as goldweb (#10).  Load mask ARRAY GOLDLEAD and run HEXPI2,1,2 in gcaws2.
  13. Liftoff gold lead: same as goldweb (#11) except deposit 20 Å Ti and ~400 Å Au (this still needs to be calibrated).
  14. Gold blob lithography step: same as goldweb (#10), except use 2.8 um PR and bake for 120s.  Load mask ARRAY GOLDBLOB and run HEXPI2,1,2 in gcaws2.
  15. Liftoff gold blob: same as goldweb (#11) except now deposit 20 Å Ti and  ~1 um of gold.  Will need to do multiple v401 runs to reload boats with gold.  Only depostion ~2000 Å at a time and wait ~ 10 minutes to prevent wafer overheating.
  16. Nitride web lithography:  same as goldweb (#10) except use 2.8 um PR and bake for 120s.  Load mask ARRAY NITRIDE and run HEXPI2,1,2 in gcaws2.
  17. Etch SiN in ptherm.  You have to be very careful here, if you overetch the nitride at all you will weaken the spiderwebs to the point that they disintegrate on release step.   Do same scourge step as #7.   Etch with SF6 at 60 sccm and 125 W for roughly 14-16 minutes (???).  The etch time seems to vary a bit.  One should run a dummy nitride wafer first to calibrate this.
  18. Dice wedge out in disco.  Dice to the inside of aluminum cutting lines except for line near contact pads, where you should dice to outside to limit undercutting of contact pads.
  19. Deposit gold backshort in v401.  You can cut a wedge-sized triangle in aluminum foil and put kapton tape in the corners to hold the wedge onto the top of the chimney.  Use 20 Å Ti sticking layer and ~ 2500 Å Au backshort.
  20. Release wedge in xetch.   Use the program parameters 1.5 Torr XeF2, 0.0 Tor N2, 20 seconds/cycle, and roughly 150 cycles.  The number of cycles will vary and you must verify total undercutting in microscope (you should be able to see this on fake TES island in web opposite the real TES).
  21. Remove PR in ptherm with O2 plasma at 100 sccm, 200 W for ~7 minutes.  Check for total PR removal.

 

*** Note we may soon be reversing the order of Ti and Al lithography and etching steps.  Stay tuned! ***