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. (Demis D. John, 2017)