Lei,

I have experienced this issue with similar size substrates.

Try covering the substrate with a smaller piece of silicon and using flood
exposure to expose the edgebead.  Then, develop it away.

Another approach is to use a photolithography mask with a rectangular pattern
slightly smaller than your substrate, align the mask to the substrate, and use
proximity contact or soft contact mode during exposure of the edgebead. Then,
develop it away.

Sagi Mathai
Information and Quantum Systems Lab
Hewlett-Packard Laboratories

-----Original Message-----
From: mems-talk-bounces@memsnet.org [mailto:mems-talk-bounces@memsnet.org] On
Behalf Of leiwangsdu
Sent: Wednesday, February 10, 2010 5:02 PM
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