Whoops,

I missed the mention that the SU-8 is old.  As Michi mentioned, once past the
expiration date, there is a loss of solvent and perhaps a slight amound of
premature cross-linking.

Just solvent evaporation can be a major contributor.  If I remember correctly
(no guarantees), the SU-8 2100 is ~77% solids, and the SU-8 2050 is ~ 72-73%
solids.  So a swing of 4-5% solids is enough to triple the viscosity (from ~150
P - 15,000 cP - to ~500 P).

Best Regards,
Chad Brubaker

EV Group       invent * innovate * implement
Technology - Tel:  480.727.9635, Fax:  480.727.9700  e-mail:
c.brubaker@EVGroup.com, www.EVGroup.com

-----Original Message-----
From: Brubaker Chad
Sent: Wednesday, November 29, 2006 11:00 AM
To: General MEMS discussion
Cc: Brubaker Chad
Subject: RE: [mems-talk] SU-8 2100 layer thicker than expected

Gareth,

SU-8 2000 materials have a bit of temperature sensitivity when it comes to the
material viscosity - this is more pronounced with the higher solids content
materials such as SU-8 2100 (which is, I think, somewhere around 77% solids).

If your clean room is a bit on the cold side (20ºC vs. 24ºC, for instance) this
could have an impact. I wouldn't expect just a temperature variation to cause
the film thickness difference you are seeing, but it may contribute.

Other contributors may be things such as exhaust, use/lack of use of a covered
coat module, completeness of soft-bake...