Hi Pavan,

The undercut can be easily measured by cleaving the substrate, clamping it
on its side (so that it is vertical) and looking at the side profile under
an ordinary optical microscope. I did this for measuring the undercut on ~3
microns of polyimide + 3 microns of resist, which was about the limit of
resolution for this technique for our optical microscope on 100x. 50 microns
of resist could be easily measured in this way.

It isn't safe to assume that the undercuts will be the same for the same
recipe on different substrates. The optimal bake temperatures and exposure
dose are substrate-dependent: the bake temp because of the different heat
transfer characteristics (our bake temps on glass are 10-15 degrees C higher
than on silicon, for the same resist); and the exposure dose because of the
different reflectance of the substrate-resist interface (and the
substrate-chuck interface if you have a UV-transparent substrate and a metal
chuck in your mask aligner). Bottom line is that you really need to develop
your recipe on the substrate that you intend to use.

Final note: you need to be careful not to damage the resist when scribing
the silicon. I found that the best way is to make a deep scribe line on the
top side that extends part of the way along the wafer, cleaving it by
flexing, and measuring the part that was not scribed. Next best is to scribe
the back side, but this can cause delamination if the resist is flexible.

Good luck!

Jason Milne
Microelectronics Research Group
The University of Western Australia


-----Original Message-----
From: mems-talk-bounces@memsnet.org [mailto:mems-talk-bounces@memsnet.org]
On Behalf Of Pavan Samudrala
Sent: Monday, 11 May 2009 10:41 PM
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