Gain Guided GaSb laser Flow

AKA Old Process Flow for GaSb based laser Adopted from Process Flow for Tunnel Junctions and TLM Written by Adam Crook July 1, 2010. Modified by Thien-An Nguyen August 10, 2010. Modified by Rodolfo Salas 9/15/11. Modified by NTS 08/07/13. Litho steps have been updated in Index Guided Process Flow – NTS 3/2014. Updated 150928, NTS.

[Note: Sample must be cleaved and in sample holders to begin.]

Silicon Nitride Deposition

• Refer to PECVD procedure

  • O2 clean chamber for 30 minutes (Full Recipe on Wiki)

  • Prime chamber: Recipe AMC_SIN for 3 minutes (Full Recipe on Wiki)

• Deposit ~200 nm of SiNx onto sample: run AMC_SIN for 10 minutes

Laser Stripe Lithography

Prep

• Setup hotplate and thermometers

• Take photoresist out of the refrigerator because it needs to be at room temp

  • Crack PR fridge to let fumes get vacuumed up, then open.

  • AZ 5214E-IR

• Check MJB4 lamp power

  • Check the power unit is working and on CH1/365

  • Insert empty mask holder, raise the stage, and expose for 10-20 seconds

  • Look at the output power listed on the power unit, it should be ~7.5 mW/cm^2

  • If it is not, you need to email Mertech to calibrate the lamp.

  • Power less than 6.5 can ruin your litho. 6.5-7.5 is not ideal but OK for process dummies.

Solvent Clean

• Solvent Clean (ACE, IPA, DI: 2 min)

  • Create a clean workstation before beginning.

    • Spray bench with IPA if needed, then wipe clean

    • Do this by getting clean wipes from the dispenser and lay them down anywhere you plan to work.

    • For Regrowth, layer wipes from back to front, and right to left, and go up to/over lip of bench

  • Setup solvent beakers and dipper basket

  • Note: The dipper baskets are labeled. Only use those that are appropriate for your immediate task. Otherwise, keep them in the bin and close the bin to prevent any contamination.

  • Always fill each subsequent beaker higher than the previous

  • Note: Gentle agitation during cleaning process is recommended

  • Acetone: 2 min

  • 2-propanol: 2 min

    • Go get UHV foil for boat

    • Scrap UHV foil is ideal, never trust another person’s boat

  • DI water cascade rinse: 2 min

  • Make foil boat

  • N2 blow dry

  • Put samples in boat

• Dehydration bake

  • 150C oven: 5 min

  • Let Samples cool in boat

• Setup hotplate for prebake with thermometers

  • Set hotplate to prebake temperature

  • Thermometers are setup up in a particular way. Short silver is at NE position, tall silver is NW, tall black is SW, short black is SE. Thermometers should be on flat foil and positioned so that the edges of the hotplate are tangential to the thermometer. All thermometers should be rotated so that the dial is in normal, easy reading position.

• Clean up acetone and IPA beakers

Spin PR

• Spinner Prep

  • Make sure spinner chuck is clean

    • Our own 0.5” chuck should be clean

    • Spray wipe with acetone and wipe chuck off, swap gloves, repeat

      • Don’t get solvent on inside

    • Spray wipe with IPA and wipe chuck off

    • N2 dry chuck

    • If chuck is not at spinner you want it at, assume it’s dirty

  • Make sure you have pipettes for PR

  • Note: Set spinner to almost max time, always use your timer!

  • Check the spinner by placing a junk wafer piece or non-critical sample on it. Make sure it spins correctly and at the correct RPM (4k) before placing on a critical sample or PR.

• Spin on resist (AZ 5214E-IR, 4k rpm, 30 sec)

  • Check Sample 1 and N2 blow

  • Put Sample 1 on chuck and center

  • Check centering and RPM

  • Note: When putting PR onto the wafer, try not to put on so much that it leaks off the sides because this can get into the vacuum and mess things up. Also, make sure that there is an adequate amount, as to completely coat the wafer with an even amount. If the integer number of drops you must apply is too little (too much) for the sample, wait for it to spread a little before spinning (spin immediately after application)

  • Put PR (AZ 5214E-IR) in pipette carefully and drop a few drops into the PR container (to eliminate air)

  • Drop on PR, no bubbles, and Spin

  • 4k rpm

  • 30 sec

  • Check wafer when done. No streaks.

  • Repeat as needed

  • When done, empty pipette into PR container and return to fridge

  • Clean chuck as above

  • Clean spinner bench

• Pre-Bake (evaporate solvents before lithography)

  • 100C hotplate: 1 min

  • Let samples cool in boat while you prep aligner

  • Check that mask is clean

  • Set hotplate to IR bake temperature

Exposures

• First exposure (Laser 2013 Mask 1, 11 sec)

  • Choose stage for your sample

  • Prepare height of aligner stage to be close to (but lower than) what you will want with the sample.

  • Put Laser 2013 Mask 1 (laser lines) onto holder, ~1 mm from edge, turn on vacuum, slide into aligner, and tighten screws. Only touch the mask corners and edges.

  • Note: When placing mask onto the holder, place it so that the chrome side of the mask faces away from the holder. (Brown side faces sample after placement.) Keep your hand under the mask when moving the loaded mask holder.

  • Load sample onto stage and lower stage a few turns

  • While looking from the front of the aligner at the sample (between the mask and stage), rotate contact lever, and then raise the stage til it looks like the sample and its reflection on the mask are about to touch.

  • Align features, then raise stage til contact:

    • The best way to tell contact is by looking (through the microscope) for rings to appear at a corner of the sample or an edge

    • Can also look for a brightening of the mask metal, meaning there’s pressure on the mask

    • Whichever corner or edge displays contact first on your first sample will probably be the corner/edge to display contact first for the other samples

    • Note: Will feel a tiny increase in resistance of the knob

  • Parameters

    • Align + expose

    • Soft contact (we do this now to avoid sample movement)

    • 11 sec on MJB4 contact aligner

Note: Do not face the aligner when the wafer is being exposed. Also, make sure to move your other samples away from the aligner, as not to ruin them from scattered UV light. Usually this means putting a toolbox or bin between them and the aligner.

• Lower the stage using the front knob about four numbers (knob number increases)

  • Center the stage rotation knob then Rotate contact lever

  • Unload sample and Repeat as needed

• IR bake (Image Reversal Bake)

  • 115 C hotplate: 115 sec (1 min 55 sec)

  • Note: Very important step, do not over/under bake

  • Remove mask from aligner

• Second exposure (Flood exposure = w/o mask)

  • 60 sec flood exposure on MJB4 aligner

  • Do not use hard contact (can’t select for flood exposure)

  • Note: Only put one wafer on at a time so as to not ruin wafers with scattered UV light.

Develop and Inspect

• Develop with MF-26A

  • 30 sec in MF-26A followed immediately by DI cascade rinse: 2min

  • Note: Gentle agitation the whole time.

  • Note: Does not stop developing after you take the wafer dipper out of the developer solution, make sure to immediately dunk into DI water and rinse

  • N2 blow dry

  • Put stuff away and clean up bench

  • Inspect Pattern

The key parameter for this process is the IR bake. If the pattern looks like features are way too big (overdeveloped), then the sample was under baked. If the features look too small (underdeveloped), then it was over baked. Also, too long of an Exposure 1 can lead to underdev, and too short can lead to overdev.

SiNx etch

• Don PPE

• Clean workspace and set out beakers

• Mix acid solution (Sol 17): BOE(6:1):H2O 1:10 (0.1) = 9 mL BOE, 90 mL H2O

  • Measure DI water (90 mL) in PP grad cylinder and pour into Etch beaker

  • Pour BOE (9 mL) into PP grad cylinder using PP funnel (one hand on acid, one hand on funnel)

    • Pour excess acid from PP grad cylinder into the excess BOE beaker

  • Pour BOE acid into Etch beaker (acid to water)

  • Triple rinse graduated cylinder and funnel

  • Aspirate excess BOE beaker and triple rinse

• Etch samples for 80 seconds

• DI rinse 2 min

• Blow dry with N2

• Repeat etch as needed

• Clean beakers and workspace

• Remove PPE

Dividing Line Lithography

• Same as laser line litho except use Mask 2: Metal.

Top Contact Metallization

• Refer to CHA1 Procedure

• It is likely you will need two or three 2-3 hour slots to do the stack.

• Start vent of CHA 1 (takes about 12 min)

• Go to acid hood, prepare and use surface cleaning solution

  • HCL:DI (~1:10, 40:120 mL)

  • Dip samples for 15 sec in solution (precision is not needed here)

  • Rinse for 30 sec in DI

  • Blow dry with N2

  • This part takes about 10 minutes, so start it once you vent CHA #1

Note: This takes 20-45 minutes depending on number of samples.

• Take samples immediately to CHA #1

  • Vacuum and clean hearth and sealing surface

  • Load samples

  • Load sources (Ti, Pt, Au)

  • Check Mirrors

  • Pump down

• Deposit contact metal (for top contact near epilayer we contact p+ GaSb)

  • p GaSb: Ti/Pt/Au (100/100/2000 A)

  • Ti takes about 17 easy minutes in total

  • Pt is a pain to evaporate, goes very slow, needs high power, and will spit easily, so you can’t try to go fast and you MUST watch it. Also, the evaporator chamber will get very hot, so watch that too (if it goes above 90 C, immediately ramp down). The dep rate will likely be under 0.1 A/s. 1 A/min is normal once it starts evaporating. It is likely you will need two slots to finish the Pt; it is important that you get at least 10 A of Pt (100A Ti / 10+ A Pt) down during your first slot to protect the Ti from oxidation when venting, if your slots are not back-to-back. Also factor in chamber cool down time during Pt—you might have to ramp down and wait for over 10 minutes for the temp to drop below 60 C (45 C is better) to resume evaporation.

  • Au takes ~40 minutes of evaporation time at 1 A/sec, and the chamber will heat up, so watch that. If Au is going slower, it could take 2-3 hours for just the gold stack (Vent to Vent).

• Lift off metal in Acetone or Kwik Strip

  • 40-60 min 90C Kwik Strip, or in acetone

    • 6 hours or more total is OK. Do not force metal off.

  • Collect all waste, as it is metal contaminated

  • Perform a Spray check

    • Remove mesh basket from KS and hold over metals waste beaker (it's an over and in position) so that all waste is caught in beaker

    • Spray samples gently with acetone along the laser lines, for each sample

    • Repeat perpendicularly

    • Repeat parallel

    • Use your judgment to see if more acetone spraying is needed

    • Do a quick, gentle N2 dry on each sample

    • Do a gentle DI sprayer rinse of each sample

    • Inspect to see if sample needs more KS/acetone time

  • If sample/s need more, return to KS

  • If sample is done, remove from basket and N2 dry

  • Empty metals waste beaker as needed into larger properly labeled waste container

  • If you want, put samples into the Kwikstrip (KS) for another 20 minutes to ensure they are good.

  • DI rinse and N2 dry as appropriate

  • When done with liftoff for the day:

    • Empty petri dish of KS into metal waste beaker. Put KS into petri dish, carefully swirl around, empty into beaker. Repeat as needed to clean petri dish.

  • When done with metals waste beaker for the day:

    • Pour out beaker into larger waste container. Fill half to two-thirds with DI water, swirl around, pour into larger waste container. Due to KS, there will be bubbles, Repeat til bubbles go away and beaker seems clean-ish (triple rinse should be about right).

Lap Substrate

• Prep topside of samples for lapping

  • Take PR out of the fridge (AZ 5214E-IR)

  • Set up solvent beakers

    • Fill subsequent beaker higher than previous

  • Acetone: 2 minutes

  • 2-Propanol: 2 minutes

  • DI rinse: 2 minutes

  • Make foil boat

  • N2 blow dry

  • Dehydro bake in boat – 150 C for 5 minutes

• Set spinner to 4 krpm

  • Spin on PR 4 krpm / 30 seconds

  • Prebake 100 C for 1 minute

  • Flood expose 11 seconds

  • IR bake 115 C for 1:55

  • Clean spinner chuck and workspace

• See Lapping Procedure

• Clean Sample for Bottom Contact Metalization

• Bond piece to glass slide

Bottom Contact

• Similar to top contact, but for bottom contact we contact n GaSb

• Deposit contact metal

  • n GaSb: Au/Ge/Ni/Au (100/100/100/1000)

  • n GaSb: Au/Sn/Ni/Au (50/50/?/1000 A) or Au/Sn/Au (50/50/1000 A)? No.

• Lift off metal in Acetone or Kwik Strip and clean

Clean and Anneal

• Release piece from glass slide

  • Bring samples to PL lab fume hood and put into metal wire basket

    • Can only fit two one-third glass slides w/ lasers on them in basket

  • Fill Laser Liftoff acetone petri dish with acetone, to about 3/4 full

  • Place basket in acetone petri dish and confirm samples are submerged

  • Top off petri dish with acetone spray bottle (maybe 9/10 full)

  • Cover with larger beaker

  • Check periodically to top off petri dish and see if samples lifted off

    • A top off every 4-18 hours is warranted until liftoff

    • To check, lift up basket an inch and gently spray a sample with acetone from the side to see if it moves

    • Be careful of it sliding too much, so never hold it high or at a large tilt. Right over the petri dish almost flat is best

    • Liftoff seems to take over 24 hours

• Clean off PR

[In progress]

• Furnace Anneal?

• Release from glass slide

Final Packaging

• Cleave into individual lasers

• Indium mount bottom (top?) contact (SiNx side) to copper piece (gets bonded upside down)