The problem is the square wafer, the edge effect becomes predominant and
the nice flow of resist happens only in a very limited (circular)
surface.

To avoid that, one of the way is to use a spinner with co-rotating
cover, the air flow responsible for most of the edge effect would
disappear. Either you buy a Karl-Suss one :-) or you simply use a half
Petri dish that you place on top of your sample to shield it from the
airflow. It works for standard photoresist (I developed that trick to
coat 10mmmX50mmm samples uniformly) but I've no experience with thick
PR.

Another way is to 'remove the edge' of your square wafer. You need to
use a round wafer the same thickness as your sample (doesn't really need
to be round anyway as soon as it is big enough, but the thickness need
to be the same) and you make a hole in the centre the same size as your
sample. Then you place them both on the spinner, your square sample
inserted in the middle of the big one: now you have something that looks
like a circular wafer. The discontinuity between the square sample and
the round wafer needs to be as small as possible, and you most probably
can fill it with PR - but I suspect that the more viscous the PR, the
larger the gap can be without affecting the spread. I you're afraid of
PR being sucked in the vacuum system, tape your wafer on the spinner
head instead of using vacuum to hold it - this is commonly done on
substrate with strongly corrugated backside. BTW I never tested that
trick tell us how it works if you do :-)
Franck

"-----Original Message-----
"From: Abang Annuar Ehsan [mailto:[email protected]]
"Sent: Wednesday, 30 April, 2003 18:31
"To: [email protected]
"Subject: [mems-talk] photoresist uniformity problem
"
"
"Dear sir,
"
"I'm spinning my photoresist (SU-8) on a 1" x 1" square silicon
"wafer sample
"using a lab spin-coater. I'm dispensing about 0.4 ml onto the
"surface using
"a syringe (static dispense). My spread spin is 500 rpm / 5 sec
"while spin
"speed is 4000 rpm/ 30 sec. However, I'm always getting an
"uneven thickness
"(in circular pattern) The centre looks different while sides are in
"different color. How do I improve uniformity? Your kind help is highly
"appreciated.
"
"Abang Annuar.
"
"
"
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