I agree with Mr. Moffat's assessment.  Light collimation from a proximity
aligner typically deviates a few degrees from the normal angle of the mask
plane.  Sidewall angles in the developed resist pattern will highlight this
deviation.  One should expect that this angle is constant for a given exposure
and development recipe with varying exposure distances.  The most profound
effect of the exposure gap will be on the feature dimension.  The print bias
will change as the exposure gap changes.  In other words, a via feature printed
at a 25 µm exposure gap will have a smaller print bias than the same via feature
printed with a 100 µm exposure gap.


Best Regards,
Garrett Oakes

-----Original Message-----
From: Bill Moffat [mailto:BMoffat@yieldengineering.com]
Sent: Wednesday, May 14, 2008 2:59 PM
To: General MEMS discussion
Subject: Re: [mems-talk] sidewall angle of photoresist

The side wall angle depends primarily on 2 factors.  1) The angle of
collimation, no your light source is not 100% collimated.  2) The light
is now unfortunately bent as it enters the resist.  This is a function
of the refractive index of the resist.  Experience with hundreds of
image reversal tests for lift off, leads us to expect angles from +22
degrees with normal exposure and develop.  Reversal and flood exposure
gives any angle from +22 to -22 dependent upon the level of flood
exposure.