This page is mostly a rewrite of Leland's "Ultra Thin Fab Flow" document to have it easily-accessible on the wiki for new group members and as a quick reference for current members. If you plan on doing fab, do not depend solely on this wiki page.

Senior group members: please add tips and images to this page based on your past experiences (and mistakes)! They help everyone who comes after you!

Table of Contents

Pre-Photolithography:

1. Take photoresist (AZ5214 is what we normally use) out of the fridge to start warming up. Takes about 10 minutes.
2. Turn hotplate to 110°C. We normally use the hot plate in hood J23.
3. Triple Rinse the following
   1. Acetone dish
   2. IPA dish
   3. Developer beaker/petri dish
   4. Developer rinse beakers (should be 2 of them)

Preparing Cover Slip

We use a cover slip (a glass slide with the sample attached) to improve consistency and prevent the sample from flipping/breaking as often, unless you are really good at fab.

1. Rinse cover slip with acetone, then IPA. Place on cleanroom wipe.
2. Blow dry cover slip and tweezers, pick it up and blowdry more. Make sure it's dry!
3. Spin 6 drops photoresist (AZ5214) at 1k RPM onto the cover slip
4. Remove from spinner and place on cleanroom wipe

Prepping Sample for Cover Slip

1. Blow dry epi-side surface of sample.
2. Rinse sample in acetone, then IPA, then blow dry (both sides, but be careful)
3. Place dry sample epi-up on the cover slip (on the PR side)
4. Using wide tweezers, gently press opposite corners of the sample on the cover slip to promote good adhesion
5. Inspect back of cover slip (don't place sample epi-side down on the cleanroom wipe!) to see rainbows created by thin PR. This means you have good adhesion. TODO: Add photo of rainbows on back of coverslip

Photolithography

See MJB4 tips and Photoresist Recipes for more information on using MJB4 and tips on getting good-looking PR.

If you need sub-micron gratings, use some tips from Abhilasha's sub-micron grating lithography or consider using Raith (EBL).

1. Rinse sample/cover slip with acetone. Acetone shouldn't remove the sample from the coverslip (has a hard time diffusing into PR)
2. Rinse with IPA
3. Spin 2 drops of AZ5214 onto the sample at 5k RPM for 1 minute.
   1. NOTE: Some group members spin at different speeds. Different speeds will result in different PR thicknesses. The faster the RPM, the thinner the PR, but they shouldn't be orders of magnitudes different.
      For reference, AZ5214 1 min @ 5k RPM should be around 1.7 um. 1 min @ 4k PRM is usually about 2 um.
4. Prebake sample with PR on the 110°C hotplate for 50 seconds to 1 minute. The hot plate temperature is more consistent in the center. Let cool on a metal surface for 10-30 seconds (this shouldn't take long at all)
5. Edge-bead removal. Use an acetone-soaked q-tip to remove the outer 1-2mm PR from the sample.
   1. Soak q-tip in acetone
   2. Dab on CR wipe to remove excess
   3. Run along sample edge (using cleaved surface as a guide)
6. Expose with MJB4 for 12-13 seconds (lamp power should be ~7.5mW).
   1. NOTE from fab flow document (thanks Leland!): The MJB4 can be cantankerous for high resolution photolithography and it is absolutely critical to see rainbows if you want to achieve ~1um critical dimensions! See example image:
7. Develop for 30-45 seconds (again, talk to senior group members!) in AD-10 developer, holding with tweezers by cover slip. If you drop the cover slip it can be tricky to grab it again.
8. Do a 45-60 second postbake on the newly developed sample on a hotplate at 110°C. This is recommended, especially so if you are doing a long etch (>2 minutes). You should see the PR change color during this postbake.
   1. Note that if you postbake too long it will be hard to remove the PR later.
9. If your sample is underdeveloped, use either Plasmatherm 2 or Oxford 80 RIE for an O2 descum.
   1. Recipe is PCL O2 Etch. Run for 2 minutes, 20 sccm, 100 RF, 80 strike pressure, 50 set pressure

Metallaization

1. (Optional) Remove surface oxides by dipping sample in a mix of 5:1 HCl: H20 (so-called HCl dip) for 60 seconds before loading into CHA. See the following procedure (This is kind of hard to read?):
   
   1. Triple rinse acid beaker+teflon dipper+rinse beaker
   2. Use rinse beaker to measure 200mL H2O and add to acid beaker (using beaker graduations is fine b/c its a ratio)
   3. Use rinse beaker to measure 50mL of HCl and add to acid beaker
   4. Triple rinse the rinse beaker and fill with water for rinsing
   5. Place samples into teflon dipper and put dipper in etch bath for 60 seconds
   6. Move samples to rinse beaker
   7. Dispose of etchant and triple rinse acid beaker
   8. Fill acid beaker with H2O and move dipper to this beaker
   9. Dump rinse beaker and triple rinse, then fill with H2O again
   10. Move dipper back to rinse beaker (you have effectively triple rinsed the sample+dipper)
   11. Remove sample from dipper and blow dry
   12. Clean up your stuff
2. Metal deposition with CHA. See the CHA1 Operating Procedures page and the CHA1 Logbook page for recipes. Some tips:
   1. Pt can be hard to evaporate. During the first soak you can open the shutter and make sure the e-beam is hitting the Pt.
   2. The e-beam can heat the sample and cause PR to reflow, though Leland says in the original fab flow document that he accidentally hit 84°C (don't go past 60!) and liftoff still worked fine.
3. Perform PR liftoff/cleaning with the Clean Process outlined below

Wet Etch

Refer to Wet Etch Recipes for the recipes we normally use. We normally etch III-Vs in hood E02 because the other hoods are not III-V compatible.

The procedure is generally the same between etches.

1. Triple rinse everything you will need (acid beakers, rinse beakers, stir bars, dipper baskets, graduated cylinders, etc.)
2. Prep the acid solution. Not gonna describe that here so if you are a new group member, shadow a senior member to see how it works at least once. Here are some tips:
   1. Always add acid to water, never water to acid. This is especially true for strong acids.
   2. Measure liquids with graduated cylinders and solids with analytical balances. Being off on your measurements leads to inconsistent etch times.
   3. Always check the expiration date of your H2O2 before use. We have run into etching problems with this before.
   4. If doing a citric acid etch, after adding H2O2 wait 15 minutes before starting your etch. The reaction between the H2O2 and the acid solution is endothermic, and you want the solution to come back to room temperature. Not sure about other acid solutions.
   5. Make sure you triple rinse everything you use when you start and when you finish.
3. Remove PR using the Clean Process outlined below.

Clean Process

Mesa sidewalls can be coated in PR during the photolith process (especially if you mesa etch before metallization) and this can contribute to higher dark currents. This process seeks to clean mesa sidewalls by hot PR remover and sonication. Coincidentally, this is also an excellent way to perform metal liftoff. All beakers should be kept incredibly clean! Don’t put you hands in them, don’t put mystery tweezers in them, only objects you know are clean! If you think you “dirtied” a beaker then “clean” it by 20 minutes of high sonication with the beaker full of acetone, then 20 minutes of sonication with the beaker full of methanol, then 20 minutes of sonication with the beaker full of IPA (blow dry).

Other prep: fill 600 mL plastic beakers each with 150 mL of H2O. These beakers and water act as a buffer to the sonication, and we are still waiting on the 250 mL beaker holder from VWR. The sonicator bath can be too full given the larger volume of these beakers so it is best to fill the sonication to the operating level and the use one of the beakers to scoop out around 600 mL of water (if you are only using 1 beaker). This puts the water level about perfect for sonication.

(Leland was working on a 100C version of this process, and was also looking at measuring solution temperatures using digital thermometers.)

Rinse every beaker with acetone, then IPA before use. This process looks daunting but it is mostly just beaker juggling.

Kwikstrip

1. Rinse hot kwikstrip with acetone/IPA, blow dry. Pour 200mL of kwikstrip into beaker
2. Put beaker on hot plate and set to 60 C and spin at 600 RPM. Wait about 20 minute for solution to heat up.
3. Place sample into teflon dipper basket and dip into hot kwikstrip for 20 minutes.
   1. The cover slip usually comes out at this phase and can shadow the sample during cleaning so feel free to pull the dipper and grab the cover slip after the first minute or two
4. Rinse kwikstrip beaker with acetone/IPA, blowdry
5. Fill kwikstrip beaker with 200mL of kwikstrip. This kwikstrip will not go on the hot plate.
6. Transfer teflon dipper from hot kwikstrip beaker into 400mL sonication kwikstrip beaker. Place sonication kwikstrip beaker into 600mL beaker. Sonicate at 100% power for 10 minutes.
   1. The majority of the liftoff typically happens in this beaker
7. Remove 400 mL sonication kwikstrip beaker from 600mL beaker.

Acetone

1. Rinse acetone beaker with acetone/IPA, blow dry, then fill with 200mL of acetone.
2. Transfer teflon dipper from sonication kwikstrip beaker into acetone beaker
3. Remove teflon dipper from acetone beaker (hold it in the air so it doesn’t touch anything dirty)
4. Fill acetone beaker with 200mL of acetone (first 200mL was to rinse off the kwikstrip)
5. Place 400mL sonication acetone beaker into 600mL beaker and sonicate at 100% power for 5 minutes
6. Remove 400 mL acetone beaker from 600 mL beaker

Methanol

1. Rinse methanol beaker with acetone/IPA, blow dry
2. Fill methanol beaker with 200 mL of methanol
3. Transfer teflon dipper from acetone beaker into methanol beaker
4. Remove teflon dipper from methanol beaker (hold it in the air so it doesn’t touch anything dirty)
5. Fill methanol beaker with 200mL of methanol (first 200mL was to rinse off the acetone)
6. Place 400mL sonication methanol beaker into 600mL beaker and sonicate at 100% power for 5 minutes
7. Remove 400mL methanol beaker from 600mL beaker

IPA

1. Rinse IPA beaker with acetone/IPA, blow dry
2. Fill IPA beaker with 200mL of IPA
3. Transfer teflon dipper from methanol beaker into IPA beaker
4. Remove teflon dipper from IPA beaker (hold it in the air so it doesn’t touch anything dirty)
5. Fill IPA beaker with 200mL of IPA (first 200mL was to rinse off the methanol)
6. Place 400mL sonication IPA beaker into 600mL beaker and sonicate at 100% power for 5 minutes
7. Remove dipper from sonication IPA beaker on cleanroom wipes. Before taking the sample out, try to blow it off to ensure that nothing from your tweezers dissolves onto the mesa sidewalls by excess IPA

You are done! Be careful of any future solutions that may touch the mesa sidewalls because this was a lot of work to clean them