I am working on an e-beam process that defines metal gates with extremely high sensitivity to the repeatability/resolution of the defined geometry. I am looking to improve our recipe since we currently use acetone at room temperature as our lift-off solvent.
We currently use a PMMA bilayer for the resist stack. The first layer is PMMA 450k-A2 with a bake time/temp of 90 secs/180 C, and the second layer is PMMA 950k-A2 with the same bake time/temp.
As I said, our current lift-off recipe is soak in acetone for 1 hour at room temperature. I often see black residue after lift-off under the SEM (coined "the black veil of death" in literature), and also have seen what looks like metal particulates in places they should not be. I am aware of many other solvents that can be used instead of acetone to potentially improve our yield.
From talks with others, 3 different solvents have been recommended to me (I'm sure there are plenty of others), and I'm trying to decide which one to use for the best resolution/repeatability.
1. Microposit PG-Remover (NMP-based)
2. Microposit 1165 (NMP-based)
3. Technistrip D350 (DMSO-based)
Is anybody familiar with the pros/cons of these different solvents for lift-off? Any other recommendations? Anything to watch out for (etching of metals/oxides that could occur over time)? I've read literature on DMSO vs. NMP, but practically are there big differences for lift-off?
Thanks for the help in advance.
Dear John, I guess that NMP will be good for PMMA removal only heated up to 60 C.
Toluene/acetone mixture can help. Also acetic acid treatment reported in https://arxiv.org/pdf/1301.4106.pdf for good cleaness (applicability depend on the certain material system).
Good luck!
Dear John,
While using acetone for lift off, you may need to heat it to 40-50 C while maintaining a surplus of solvent available and the beaker being sealed with foil.
NMP based solvents are known for cleaner lift off and can be heated to 60-80 C due to lower vapor pressure. However, it may etch Al and Cr and thus, acetone should be a safer option there in my opinion.
These solvents should all do the job provided proper temperatures as suggested and proper agitation. A key for a successful liftoff is letting your PMMA patterns have upside-down T shape, so that the metal film can be shake off easily.
@Nikhil Tiwale
What makes you think NMP based solvent will etch aluminum? I haven't found any evidence that they do.
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