It is normal to have resist thicker near the edge than the centre, that's the
physics of the spin coating. You can not completely avoid it. What I can
suggest, you can try to leave the sample on a nice flat surface for few minutes
once you done the spin coating. Bear in mind the photoresist is a kind of liquid
anyway, this will allow part of the liquid flow back from the edge to the
central part. Then you can do the baking afterwards. The other thing is your
sample is so small, why don't you try to spinning coating a larger area then
dice into smaller pieces?


Yingtao

-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of leiwangsdu
Sent: 11 February 2010 01:02
To: mems-talk
Subject: [mems-talk] Help for photoresist coating

Hello, everyone. I use the positive photoresist to do some lithography.
I have a big problem. My samples are rectangular which have dimensions 10mm*15mm
After the resist spinning, I found that the resist film around the edge of the
sapmle is thicker
than the resist in the center. This produces a gap when the contact exposure is
applied.
And the resist profile is not good enough after the development.
I  shall be highly obliged if anyone can help me out in this and provide some
advices.
Thanks in advance

Yours sincerely,

Lei Wang