AR PC 5040 Protective coating for KOH etching - NanoLabStaff/nanolab GitHub Wiki

by Marthe Linnerud ([email protected])

AR-PC 5040 Protective coating for KOH etching

AR-PC 5040 is a PMMA-based coating that effectively protects the wafer during KOH etches for concentrations up to 40 wt% KOH, etching temperatures up to 85C, and durations up to 8 hours.

[!IMPORTANT] You should only use the EBL spinner when spin coating both the adhesion promoter and the protective coating.

1. Cleaning

  • Solvent clean with acetone, 3 min, and IPA, 1min
  • Plasma clean, 100/100 O2 for 1 min
  • Dehydration bake, 180C for 5 min

2. Priming

  • Spin coating adhesion promoter AR 300-80 (EBL spinner)
    • Spin up: 1000 rpm for 4 sec. Acceleration 250 rpm/sec
    • Spinning: 4000 rpm for 68 sec. Acceleration 500 rpm/sec
    • Spin down: 0 rpm, 8 sec. Acceleration 500 rpm/sec
  • 180C for 2 min (makes the adhesion layer uniform, 15 nm)

3. Spinning AR-PC 5040

  • Spin up: 500 rpm, 3 sec. Acceleration 250 rpm/sec
  • Spinning: 1500 rpm, 60 sec. Acceleration 500 rpm/sec
  • Spin down: 500 rpm/sec to 0 rpm for 4 sec

I have not performed a systematic test of the best spin recipe. But including a spin-down made the film more uniform, and it works allright.

4. Tempering Use the programmable hotplates

  • 140 C for 1.5 min
  • Ramp to 190C, 10C/min
  • 190C for 15 min
  • Ramp down to 30C, 2C/min

5. Removal of AR-PC 5040 after KOH etch

  • AR 600-71 overnight
  • Acetone bath for 5 min
  • IPA bath for 3 min
  • Dry with N2 gun

I use the AR-PC 5040 to protect the front side of my silicon wafer while I etch through the whole wafer from the back side. In these etches, I etch windows through a 200 um (100) Si wafer that is coated with Si3N4 on both sides (etch stop on the front side and etch mask on the back side defining the windows). I use 30wt% KOH and etch at 80C. I often overetch the sample to make sure that all of the silicon has been etched away. My experience is that the protective coating effectively protects the back side for at least 3.5 hours, which is the longest I have left my samples in the KOH solution.

The adhesion promoter is key for good protection. I have also tested HMDS as an alternative adhesion promoter, but it does not yield the same adhesion quality as AR 300-80.