hi, Ruilin:

thx for the input. Since the wrinkle will also happen even if I dont do any
exposure and just heat the soft-baked sample to around 45C. So I dont think this
has too much to do with stress. My theory is: for the wrinkle to happen, two
conditions are necessary, 1. excessive solvent left in the SU-8 layer, 2.
surface layer has been completely dried before the PEB.

One thing can back up my theory is that someone here has reported that wrinkle
happened during PEB when they have a thin metal layer on the top of SU-8 layer.
In that case, top layer is a metal layer which is of course dried, so when the
remained solvent try to evaporate, wrinkle happens (this will possibly happen
even for under 100um SU-8 layer with metal on the top).

I end up solved the problem by doing the exposure right after the softbake, that
way, my top layer wont completely dried yet. And during PEB, when the solvent
try to escape, it wont have too much problem to do so..

However, I still can not find a way to bake out almost all of my solvent, for
SU-8 50 which contains 31% solvent. I can only bake out about 24%. But for the
first 500um layer, since I can bake out about 28% solvent, wrinkle did not
happen during PEB. The second layer is my biggest challenge. thx

Guocheng Shao

--- On Wed, 12/3/08, 郑瑞麟(Ruilin Zheng)  wrote:

From: 郑瑞麟(Ruilin Zheng) 
Subject: Re: [mems-talk] very thick layer of SU-8
To: [email protected], "General MEMS discussion" 
Date: Wednesday, December 3, 2008, 9:48 AM


Hello, Guocheng,

I experienced what you call wrinkles during post exposure bake, and my
structures were also distorted.
I think this are caused by the severe stress during soft-bake and exposure,
especially for ultra thick layer.
pls try to lower your baking temperature for both soft and post bake, and
greatly extend your baking time at lower temperature. That may help you solve
the wrinkles.
Good luck!


--
Best Regards,

郑瑞麟

Ruilin Zheng

Address:
Pen-Tung Sah MEMS Research Center,
Xiamen University, Xiamen,361005,
P.R.China