Thanks all the advises for the variation problem of SU-8.

The variation of the Su-8 layer now is much smaller (within 1 µm). The main
reason of the large variation I had before is inappropriate cleaning and
dehydrating. After cleaning by Piranha wet etch and dehydrating bake, the
surface is much more smooth. Also, when dropping the SU-8 on Si before
spinning, try to make it occupy a large area instead of a high-hill. This
also helps a lot.

Meifang

-----Original Message-----
From: mems-talk-bounces@memsnet.org [mailto:mems-talk-bounces@memsnet.org]
On Behalf Of Meifang Lai
Sent: Thursday, 12 June 2008 11:55 AM
To: mems-talk@memsnet.org
Subject: [mems-talk] thickness variation of SU-8 layer

Hi all,

I am just starting to pattern a 50-µm-thick SU-8 layer on Si wafer by
photolithography. However, I found the SU-8 layer is not uniform. The center
of the SU-8 layer is about 8 µm higher than the part near the edge (except
the edge beads). My spin speed is: 500 rpm for 5 s with acceleration of 100
rpm/second, then spin at 3000 rpm for 40 s with acceleration of 1000
rpm/second. If any one has any solution for this problem, please tell me.