Nitrogen is the decomposition byproduct of exposing the photosensitive
ingredients (the basis of "vesicular" microfilm-forming vesicules or
microscopic bubbles in a polymer containing diazonium salts-applying heat
and causing the bubbles to expand and create an optical image). Because of
the long exposure times needed for thick resists, the resist will soften
from the energy and start forming N2 faster than they can reabsorbed or
diffused . The simplest fix is shorter, multiple exposures. Cooling the
substrate may help also, but that's not easily done on an aligner. If
you've ever opened a very old bottle of resist, you may have noticed that
it's pressurized. Again, N2 evolution as the decomposition byproduct.
Mac McReynolds
R&D Manager
Caliper Technologies Corp.
605 Fairchild Drive
Mountain View, CA 94043-2234
Tel: 650-623-0710
Fax: 650-623-0521
> ----------
> From: Jennifer Scalf
> Reply To: [email protected]
> Sent: Wednesday, January 16, 2002 2:29 PM
> To: [email protected]
> Subject: [mems-talk] More problems w/ AZ4620.
>
> There has been a lot of discussion about 4620 recently, which prompts me
> to ask a related question:
>
> I have unsuccessfully attempted to use 4620 recently. The problem that
> I have had is a "bubbling" of the resist immediately after exposure.
> The "bubbles" nucleate in all open areas. The resist sheds in circular
> flakes from the open areas - sometimes even before developing. If there
> are features surrounded by lots of open area, these features flake off
> as well. Has anyone else seen anything like this?
>
> The recipe that I am using is based on recipes from Clariant, recipes I
> found on various websites, and exposure tests that I personally
> performed:
>
> Hotplate bake at 150degC for 30min
> Vapor phase application of HMDS
> Apply AZ4620 and spin at 4krpm for 30.0sec
> Softbake on hotplate for 200sec at 110degC
> Resulting resist thickness = 7.5um
> Exposure for 17sec (intensity = 4.5 mW/cm^2) in hard contact with mask
> Develop in diluted AZ400K (1:4 AZ400K:water) for 2-3min
> Blow dry
>
> I suspect this may be an adhesion-related problem, but would be
> surprised considering that some of the affected feature sizes are >50um
> and the resist thickness is >5um. I also have tried increasing the
> softbake time to get rid of excess solvents, but this has not alleviated
> the problem.
>
> One other comment: the exposure dose that I determined to be optimum was
> much less than that suggested in the recommended 10um process on the AZ
> P400 data sheet. My exposure dose was 76.5mJ while the recommended dose
> is 400mJ.
>
> Thanks in advance for any information or suggestions you might be able
> to offer,
> Jennifer
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