Pavan,
A little bit more complex than you imagine. 1) The exposure
light is not 100% collimated. Lets say 1% out of collimation. As soon
as you have light that is not vertical you get refraction when the light
hits the resist. An increase that can be as much as 20 degrees
dependent upon the angle of the light and the refractive index of the
resist. This produces a triangle of partially exposed resist at the
edges of the exposure area. The unexposed resist will etch a specific
rate lets say 5% of the rate of etch of the exposed resist. So you will
always get undercut. It depends upon the resist chemistry and the
developer chemistry and temperature. Also as you said the exposure has
some effect. Controlling developer temperature is the strongest factor
in controlling the amount of undercut. For more information contact me
directly. Bill Moffat
________________________________
From: mems-talk-bounces@memsnet.org on behalf of Pavan Samudrala
Sent: Mon 5/11/2009 7:41 AM
To: General MEMS discussion
Subject: [mems-talk] Lithography on glass wafers
Hello all,
I need to find undercut when a photoresist is developed using a certain
developer. I was planning to do lithography on a glass wafer and check =
for
the undercut from backside. I was wondering how significantly the =
exposure
times would vary. My actual product is on silicon and am not sure if its
safe to assume that the undercut on glass wafer be treated as undercut =
on my
silicon wafer?? I am positive that the undercut has to do mainly with =
the
developer and photoresist chemistry but am not sure to what extent =
exposure
affects it.
Does anyone has experience determining the undercut easily when you have =
the
patterns close to 50um thick and is difficult to focus the bottom using
optical microscope.
Regards,
Pavan Samudrala