Lift-Off with DUV Imaging + PMGI Underlayer
This process is intended for Deep-UV Exposure on the ASML DUV Stepper. PMGI is used as the underlayer. This process uses PMGI SF-5, 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).
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 left so the PR doesn't fall over/collapse.
- Also make sure to use a very vertical evaporation - EBeam4 or EBeam1. EBeam3 is not vertical.
Suggested process for Liftoff
De-H2O Bake (eg. 220°C, 1m+). Optionally Oxygen ash instead.
HMDS soak 10s+
Spin PMGI SF-5 @ 4krpm (rcp 7)
PMGI-Bake: 220°C, 3min (lift-pins Brewer hotplate). Recipe "00 220deg, 3min Vac". Enable Vacuum with overhead valve, only for wafer landing and then turn off.
Spin UV-6-0.8 @ rcp 6 (3.5krpm)
--> POLOS underside clean: 2000rpm, ACE/ISO/N2
Pre-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 dish prepared
--> not stirring, instead very slow/gentle lift/drop at various angles or gentle swish
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 openings.
Metal Evaporation - EBeam#4 or EBeam#1 vertical/4-inch holder
Lift-off in NMP, facing down or vertical.