Amani,
The primary issue here is the large degree of thermal mismatch between silicon
and SU-8 (silicon is ~3 ppm/C, while SU-8 is closer to 50). Compounding it, you
have a layer that is a significant fraction of the thickness of your wafer
(~1/5). A third extenuating factor may be the design of your patterns - sharp
cornered (90 degress resolving to a point) chrome features in your mask are
usually a bad idea with SU-8, as the point of that feature will become a stress
focus, more likely leading to a crack.
That being said, I can make three suggestions to help reduce the cracking:
1. Try to ramp down your soft bake, just like you ramped it up. SU-8 (un-
crosslinked) has a glass transition temperature of 55C. So, if you bake at 65C,
then 95C, and then a final extended step a 60C-65C,it should help to reduce the
stress somewhat, since it will allow the film to reach a "relaxed" state at 60 -
65C, rather than at 95C, to the expansion differential at room temp is (47ppm/C
* 35C = 1645 ppm, or 164.5 µm) vs. (47ppm/C * 70 C = 329 um).
2. If possible, round the corners of the chrome (and the "hole") features in
your mask. This will reduce the possibility of these points serving as
nucleation sites.
3. After develop, perform a final bake up to 65C (this is a hint from MCC). I've
not tried it myself, but I have heard from other users that they have had
positive results.
Best Regards,
Chad Brubaker
EV Group invent * innovate * implement
Technology - Tel: (602) 437-9492, Fax: (602)437-9435 e-mail:
[email protected], www.EVGroup.com
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-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of Amani Salim
Sent: Tuesday, July 13, 2004 8:39 AM
To: [email protected]
Subject: [mems-talk] Su-8 craking problem
Dear colleagues,
Any suggestions for avoiding cracking in Su8?
am using the 2100 Su8, and am making 100 microns features and follow
exactly the recipe suggested by microchem, seems like theres still small
cracks at the border of my 10 microns structures,
any suggestions are very helpful,
regards,
Amani Salim
******************************************
Research Assistant
Department of Electrical and Computer Eng.
U of Minnesota,T.C.
UMBM Lab
Office phone: 612 626-0590
Lab phone: 612-626-7188
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