For what it's worth, here is an optical description of the effect Bill and
Garrett describe.

Proximity printing is highly  subject to the effects of Fresnel diffraction and,
more specifically, Huygen's wavelet's.  Huygen showed that as light passes
through a hole or a slit the uniform wavefront becomes "segmented" into a series
of wavelets of similar phase.  At the edge of the slit the wavelet has the
effect of actually "bending" the light around the edge thus creating an exposure
gradiant under the dark area of the mask.  The larger the print gap the longer
the lateral propagation of the wavelet, thus the larger the change in dimension.

For more info, see "Intro to Microlithography" by Wilson, Thompson and Bowden or
"Optics" by Mueller

----- Original Message -----
From: mems-talk-bounces@memsnet.org 
To: General MEMS discussion 
Sent: Fri May 16 17:46:58 2008
Subject: Re: [mems-talk] sidewall angle of photoresist

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