The best solution would be to dispense a solvent (not acetone, but I don't
have the exact name with me right now) on the back side of the wafer as it
is spun. The solvent wraps around and removes a certain amount of resist
at the edges of the top side. Time and speed determine how much resist is
removed. However, your coater must be equipped to do this.
In fact, ours is not, so we have several ways to remove the bead. My
favorite is a cleanroom swab with a VERY small amount of acetone on it.
After the last "usual" spin, I spin the wafer again at about 400-500 rpm.
I simply bring the swab into contact with the wafer's edge, keeping the
swab as parallel to the wafer as possible. Also repeat with the swab held
perpendicular to the edge to remove the resist hair that grows there. You
must not use too much acetone, as this will cause the resist to retract and
form another bead further in from the edge.
It takes some practice, but we've used many methods (even these
contraptions that hold a bit of dental floss), I believe the above to be
the most repeatable method.
-Steven
At 11:46 PM 1/30/00 , Patrick Leech wrote:
>Dear Colleagues,
>I would like to know of a simple and effective way of removing the edge
>bead after spinning of AZ4620 resist on a 3" silicon wafer. At present,
>after spinning at 4,000 rpm, a 2-3 microns lip of resist develops around
>the rim of the wafer.
>
>Thank You,
>
>(Dr.) Patrick Leech,
>CSIRO Division of Manufacturing Science and Technology,
>Private Bag 33, Clayton South MDC,
>Victoria, Australia, 3169.
>
>
>
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Steven F. Nagle
Ph.D. Candidate http://www-mtl.mit.edu/~middle/
Room 10-007 middle@mtl.mit.edu
Massachusetts Institute of Technology Tel: 617-253-2011
77 Massachusetts Avenue Fax: 707-215-2906
Cambridge, MA 02139
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"Stanley looked quite bored and somewhat detached,
but then penguins often do."
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