I agree. Ensuring even evaporation of solvent during the bake is essential.
Covering the wafer with a glass dish can help.
You will also see big variations due to edge bead (i.e. thicker on the
outside). A resting step after spinning but before the bake is highly
recommended.

On 27 June 2012 12:54, Bill Moffat  wrote:

> 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: mems-talk-bounces+bmoffat=yieldengineering.com@memsnet.org [mailto:
> mems-talk-bounces+bmoffat=yieldengineering.com@memsnet.org] On Behalf Of
> Yingtao Tian
> Sent: Wednesday, June 27, 2012 9:36 AM
> To: mems-talk@memsnet.org
> 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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