Hi William,

Can you consider using a metallic backside coating prior to your photo
process?  I would suggest sputtering 300 Ang TiW on the backside of the
wafer, the reflectivity of TiW is relatively poor, and it can be removed
very easily after photo using a wet etch in peroxide.

Justin C. Borski
MEMS Program Manager
Advanced MicroSensors Inc.
[email protected]



-----Original Message-----
From: William Lanford-Crick [mailto:[email protected]]
Sent: Friday, May 14, 2004 11:28 PM
To: General MEMS discussion
Subject: [mems-talk] Photlith on transparent materials


Hi Group,
I am having difficulty patterning wide-bandgap semiconductors which are
transparent to the 436 nm in our g-line stepper.  The incident radiation
seems to be reflecting off the wafer chuck and back up into the photoresist.
The causes the entire die to be somewhat exposed.  How can I prevent this?

I have found some improvement by additionally spinning photoresist onto the
backside of the wafer (AZ5214E at 2k rpm--I have not measured the thickness
at that speed).  I suspect that using a very thick PR layer would solve the
problem (by absorbing the radiation) but I would need to remove it
immediately after developing the patterns, and I don't see an easy way to do
that.

Any suggestions?  Are there any spin-on materials that have high absorption
at 436 nm and are easy to remove (without disturbing PR)?

Thanks for any suggestions,
William B Lanford-Crick
Micro and Nanotechnology Lab, UIUC


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