Air turbulence can be the answer.  When engineers were  experimenting with super
thick resist on photo masks they used to put a second blank mask a short
distance above the mask being coated.  This protected the resist surface from
air turbulence.  You may want to use a variation of this.  Bill Moffat

-----Original Message-----
From: [email protected] [mailto:mems-
[email protected]] On Behalf Of Yingtao Tian
Sent: Wednesday, June 27, 2012 9:36 AM
To: [email protected]
Subject: [mems-talk] Ultrathick SU8 photoresist processing

Dear All,

I am trying to make very thick SU8 photoresist on silicon wafer, however, I
found that, even the resist was spun coated, the surface will not be flat. I
tried to put over 200 um thick resist but the thickness can vary between 180 -
220 across the whole wafer. I have tried to adjust the levelness of the hotplate
by using an inclinometer and it was almost perfectly levelled. Somebody
suggested that the air turbulence in the cleanroom does matter for such very
thick resist. But, I have no evidence to doubt it.

Anybody has ideas?

Thanks,
Yingtao
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