Sampo,
The probable reason for the increase in adhesion on the diffusion
oxidation is the diffusion oxidation is a tighter molecular construction and
will not admit moisture as readily as the deposited oxidation. This is a more
open molecular construction and moisture can be absorbed more easily. The first
level of non adhesion is caused by absorbed moisture. The second level is the
need to match the surface tension of the material to the surface tension of the
liquid you are depositing ,in this case PMMA. Inexpensive possible fix is to
bake the wafer dry and very quickly coat with HMDS to change the surface tension
to match PMMA. Best fix is to use a vacuum vapor primer to totally dehydrate
under vacuum and while the parts are under vacuum HMDS vapor is pulled in to
react with the material. We have no direct knowledge of PMMA surface tension
but we have worked with ebeam resists that are probably similar. If you know
the molecular weight of the PMMA it will help. Rule of thumb guide lines :- 1)
Standard positive resist molecular weight 80,000 needs a prime time of 5 minutes
on Si02. 2) Obsolete resists older AZ resists molecular weight 13,000 prime time
1 minute on Si02. 3) Obsolete Kodak resist 809 molecular weight 5,000 prime time
10 seconds on Si02. 4) Experimental high resolution resist molecular weight 1700
prime time 2 seconds on Si02. As the molecular weight of the resist drops the
amount of time exposed to HMDS vapor drops. I think the PMMA will be a low
molecular weight. Let me know if I can help running tests. Bill Moffat
-----Original Message-----
From: Sampo Tuukkanen [mailto:[email protected]]
Sent: Tuesday, December 17, 2002 8:59 AM
To: mems
Subject: [mems-talk] PMMA resist adhesion in wet etching.
Hi,
I'm trying to work with conductor-insulator layer-structures and now I'm trying
to etch SiO2 with BHF and gold with KI/I2. I use PMMA as etching mask, but it
seems like PMMA is stripping of on the SiO2-surface. How can I get better
adhesion between PMMA and SiO2?
One thing I noticed was, that adhesion is better to the oxidiced silicon than to
the SiO2 that is evaporater to the surface. But that is maybe because evaporated
oxide is of very bad quality compared to oxidiced one.
- MSc Sampo Tuukkanen
____________________________________________
Sampo Tuukkanen, [email protected]
Room K215, Department of Physics
University of Jyväskylä, Finland
Tel. +35814 260 2392
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