Amani,
STR 1075 is nominally intended for ~10 µm features, and as such, is not the best
possible choice for a 100 µm thick coat (although 30 µm is not too bad). With a
similar resist, AZP4620 (which has a slightly lower solids content, at ~41%
rather than ~44%), the thickest I was able to achieve in a single coat was 65
µm. In order to achieve a 100 µm coat with the STR 1075, you would need to
perform a double coat at least, and perhaps a triple coat or more (I had a few
advantages in the resist processing system I used - an EVG150, which is designed
for thick resist applications).
In general, for all thick resists, there are a few general guidelines to keep in
mind:
1. For the softbake, make sure not to expose the wafer too rapidly to too high a
temperature. A hot plate with ramped proximity pins, which can allow a gradual
approach to the hot plate, is ideal. If you expose the wafer too quickly, the
solvent in the resist closest to the wafer will flash to vapor without being
able to escape, and will cause bubbles in the coat.
2. Edge bead is going to be a major issue as well. A good edge bead removal
process will help improve later exposure characteristics.
3. Thick resists have one unifying characteristic - they are all high solids
(and thus low solvent) content. This provides its own challenges. Basically,
the resist is so thick, and the solvent content so low, that, during the initial
stages of the spin, the solvent of the top layer of resist evaporates
sufficiently to significantly increase the viscosity, while the resist in the
under-layer remains at the same, lower viscosity. This causes irregular flow
velocities between top and bottom, which can lead to "wrinkling" of the resist.
The EVG150 integrates an automated cover for the coating module, which allows
for coating in a solvent rich environment, preventing the drying of the resist,
and allowing improved resist uniformity.
For the thicker regions of the range you are looking at, you may also want to
look at another material - AZ50XT from Clariant. This material is nominally for
50 - 75 µm films, and can be stretched to 100 µm without too much trouble.
Best Regards,
Chad Brubaker
EV Group invent * innovate * implement
Technology - Tel: (602) 437-9492, Fax: (602)437-9435 e-mail:
[email protected], www.EVGroup.com
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-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of Amani Salim
Sent: Friday, August 06, 2004 7:48 PM
To: [email protected]; [email protected]
Subject: [mems-talk] tips for making thick 1075 photoresist
Dear Colleagues,
I am trying to make a very thick photoresist (using 1075 positive thick PR)
and eventually will exposed it using a clear field mask,
the thickness am looking so far is from 30-100 microns, if anyone has any
tips for me to make this process a success (exposure time, soft bake and
post bake temp and time, ect..), that'll be great, thanks
Amani Salim
******************************************
Research Assistant
Department of Electrical and Computer Eng.
U of Minnesota,T.C.
UMBM Lab
Office phone: 612 626-0590
Lab phone: 612-626-7188
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