One inexpensive solution you might try on your spinner:
   1) Put enough PMMA on to cover the wafer surface
   2) Set spread speed to ~500 RPM  for ~1 sec and set spin speed to the
same (~500 RPM) and the time to 30 sec.
   3) When the spinner begins the spin step, manually increase the speed
until you reach your target speed near the end of the 30 seconds.

You might have to play with the spinner's exhaust velocity to get it to
work just right. I've used this technique to coat wet etched features
that were 20 microns deep with resist. Since they were wet etched they
did have tilted sides which helps.

Good luck,
  Mike


>>> [email protected] 08/26/03 11:40 AM >>>
Does anyone have a recommendation for a conformal layer of PMMA? I have
8 micron steps that have been difficult to cover. I have succeeded with
a  bilayer of PMMA, each layer being 1.6 microns, for a total of 3.2
microns. At that thickness, though, ebeam lithography becomes very
difficult.

Thanks,

Randy Grow
Ph.D. Candidate
Dept. of Applied Physics
Stanford University


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