Lift-Off with DUV Imaging + PMGI Underlayer

Developed by Demis D. John, ~2017-2019

This process is intended for Deep-UV Exposure on the ASML DUV Stepper. PMGI is used as the underlayer, which is exposed at the same time as the imaging resist.

This process uses PMGI SF-5 @ 4krpm, which spins to about 130nm (maximum metal thickness = 130nm/3 ≈ 45nm) and undercuts laterally about 130-140nm (minimum gap between metals = (2 x 140nm) + 30nm ≈ 350nm).

Adjust spin speed or switch to a different PMGI formulation to tailor the underlayer thickness to your desired metal thickness.

Tips

Please see our [Bi-Layer Lift-Off Tutorial] to understand the limitations and requirements for good lift-off.

• Remember that gaps in between adjacent metal traces can lift-off during the develop!

• Need underlayer thickness 2x to 3x the desired metal thickness.

• The underlayer will develop laterally from both sides which can lift-off the imaging resist, so:
  • Minimum gap between adjacent metals should then be: less than 0.5 * underlayer thickness
  • Want at least 30-50nm of underlayer width left to support the imaging resist, so the PR doesn't fall over/collapse.

• Make sure to use a vertical evaporation - EBeam4 or EBeam1. EBeam3 is not vertical.

Suggested Process for Liftoff

• De-H2O Bake (eg. 220°C, 1m+). Optionally Oxygen ash instead (faster and more effective).
• Spin PMGI SF-5 @ 4krpm (rcp 7)
  • --> POLOS underside clean: 2000rpm, ACE/ISO/N2
• PMGI-Bake: 220°C, 3min (BrewerSci lift-pin hotplate). Can just place wafer directly onto hotplate surface. Optionally can use lift-pins & Recipe "00 220deg, 3min Vac"; Enable Vacuum with overhead valve, only for wafer landing and then turn off to prevent sliding.
• Spin UV-6-0.8 @ rcp 6 (3.5krpm)
  • --> POLOS underside clean: 2000rpm, ACE/ISO/N2
• Soft-Bake = 135°C, 1min (builtin hotplate)
• (check underside for particulates)
• ASML Exposure
  • Default: Exp = 37.5mJ // foc = –0.10
  • Try this exposure dose - might need to do a FocArray (smaller Image Distribution eg. 5x5) to find proper exposure.
• PEB = 135°C, 1.5min (built-in hotplate)
• Dev (300MiF) = 50sec (CRITICAL time)
  • --> with cassette & H2O rinse dish prepared
  • --> not stirring, instead very slow/gentle lift/drop at various angles or gentle swish just to mix developer slightly.
• DI rinse : very Gently! Dunk into prepared DI dish, and dump/fill gently 2x. Don’t allow direct hard water to hit PR surface.
• PEii Technics O2: 30sec. May increase feature size openings.
• Metal Evaporation - EBeam#4 or EBeam#1 vertical/4-inch holder
• Lift-off in NMP, facing down or vertical.

Data

• For PMGI Baked at 220°C for 3min, the PMGI dissolves in AZ 300MIF at a rate of approximately ~600nm/min.

Images / Examples

SEM of holes in PR with above process. Lateral measurement of undercut is only approximate because circle may not have been cut through the exact diameter.