Glen,
  A simple box inside of another box is what we use for close alignment.
The bottom square is patterned.  Then a clear square of slightly larger
dimensions is used on the mask on the next layer.
  We then use multiple size boxes with different gaps, down to about 1
micron.  This allows us to get pretty good alignment.
  We have also found that we need to come in and out of contact to get very
high accuracy.  If the vacuum holding your wafer to the chuck is not good,
this can cause the wafer to shift.  We solved this problem by upgrading our
vacuum pump.

David Nemeth
Senior Engineer
Sophia Wireless, Inc.
14225-C Sullyfield Circle
Chantilly, VA

-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Glen Landry
Sent: Wednesday, May 28, 2003 9:10 AM
To: [email protected]
Subject: [mems-talk] Photolithography alignment marks


I am looking for other people's ideas for alignment marks.  I am using a
Karl Suss MJB3 aligner and I want at least 0.5 micron accuracy.  The marks
I am using consist of a ~100micron feature for rough alignment, and then a
vernier.  The vernier consists of five rectangles, 2 microns wide, and
separated by 5 microns.  The rectangles on the second mask are separated by
5.5 microns in such a way that when alignment is correct, the center
rectangles align.  When alignment is 0.5micron off, the rectangle one from
center is aligned.  It looks like this (excuse the crude drawing):

   O     O     O     O     O       base vernier
O      O      O      O       O    top vernier (aligned)

These marks can get me to better than 0.5micron, but I have to almost be in
vacuum contact before I can see my alignment clearly.  I have thought of
better alignment marks, but at $600/mask I want to get it right the first
time.  I am wondering if it would work better if the top marks were
larger/smaller than the bottom marks (I am using a dark field mask) or if I
would still be able to get as good of an alignment if my marks were say
5microns wide so I could see through them without being in very close
contact.  If anyone has a CAD/PDF design of marks that work for them,
please e-mail me.  Thanks.

Glen Landry


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