UV bake  - bake at ramping temp under DUV exposure will help. Stripping the
resist afterwards is tough.

Few points:

1. make sure the softbake is long at around 95 deg.
2. Let the wafers sit for at least 30 min at room temperature before
exposure to allow moisture to diffuse into the resist. If there is no
moisture, the photo chemical reaction - this might cause your low TG.

-----Original Message-----
From: mems-talk-bounces+shay=mizur.com@memsnet.org
[mailto:mems-talk-bounces+shay=mizur.com@memsnet.org] On Behalf Of hamed
khoojinian
Sent: Friday, July 19, 2013 9:58 PM
To: mems-talk@memsnet.org
Subject: [mems-talk] Undesirable reflowing of positive photoresist on hard
bake.

Hello all,

I am new to lithography so I apologize if questions are excessively
ignorant.

I am having trouble hard baking thick film of AZ4620. I am spinning on ~20um
twice. My 40um of AZ4620 exposes and develops well, but on hard baking on
hotplate the pattern is lost immediately as the PR reflows into a dome. This
PR will be dry etched (so it will see high heat eventually).

Hard baking:
I am hardbaking 40-60um of AZ4620 at 90C for 30min (reflows in under 10min).
I tried ramping 20-50C, holding overnight. Little rounding at edges, but a
two days later it had reflowed even more.

Other resists have had some problems, but I am still trying out new resists
(right now i'm on 4903, and two negative resists).

So can someone give me some guidelines on how to hard bake 4620 so I won't
have to change the process much? I'm very confused because I thought the Tg
for 4620 was not this low, and because I do single spins of 4620 (~10-15um)
and hardbake at 90C without reflow! Why would thicker application drop the
Tg so much?

Should I try exposing post development to UV to crosslink the top before
hard baking? Would that inhibit solvent evaporation too much? "Low and Slow"
bake over 3 days? Or just give up on 4620... (any AZ class PR that would
work here?)

Thanks in advance,
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