Hi there,

High topographies are a challenge both in terms of resist
coating as well as exposure (depth-of-focus limitations).
Small pits are comparatively easier than mesas or bigger
trenches, but whatever you do, you will end up with pits
filled and a much higher effective resist thickness there
as compared to non-pit areas. This has to be taken into
consideration when optimising the exposure energy - and
in extreme cases it might be impossible to find a workable
"window" between overexposure of non-pit areas and under-
exposure in the pits.
There are many tricks to optimise spin-coating for high
topographies. The biggest challenge is getting uniform
coverage (or sometimes coverage at all) in the "shadow"
behind topographic features, which divert the spinning
resist. Slower spin speed, ramping, high dispense volumes
(drenching the wafer in resist) helps.
Better, because non-directional, is spray-coating (EVG
has machines for this). Even better electrophoretic resist
application, a completely isotropic plating process - but
the resist chemistries are completely different (and very
limited choice), which poses other challenges.
We have used both these techniques to deposit 7 - 10um
conformal resist on anisotropically etched trenches of up
to 350um depth (you definitely can't spin here any more!)
and patterned metal traces of 75um width crossing those
trenches.
Please feel free to contact me (or post another question
to MEMS-Talk) if you need further information.

Greetings,
        Frank Berauer
        Senior R&D Engineer
        Hewlett-Packard Singapore


-----Original Message-----
From: Bill Moffat [mailto:BMoffat@yieldengineering.com]
Sent: Tuesday, April 23, 2002 10:42 PM
To: mems-talk@memsnet.org
Subject: RE: [mems-talk] pit coverage - with resist


Hi,
   Sounds like a job for SU8 from Microchem which I have seen 300 micron
thick
or a photosensitive Polymide which can be up to 100 micron thick.  Bill
Moffat

-----Original Message-----
From: xiaohong [mailto:xiaohong@ee.ualberta.ca]
Sent: Monday, April 22, 2002 3:05 PM
To: mems-talk@memsnet.org
Subject: [mems-talk] pit coverage - with resist


 Hello all - I recently switched from Semiconductor to MEMS and have
never had to worry about the kind of topography I am seeing now - Anyone
have suggestions on a resist and program that could be used for 70um
deep pit coverage No small geometrics to resolve......


 Thanks Mike O' in Alberta
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