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