The primary reason you are seeing that issue has to do with the path that the
resist takes to spin off of the wafer. Typically, it is pictured that the
pathway of the resist is radial from the center of the wafer, and, at the
initial point of the spin, this is true.
But, as the layer gets thinner, the resist flow path begins to take on more of a
spiral profile. On a round substrate, this is no problem, since the shape still
follows the general profile of a spiral (although the effect can still be seen
on wafers flats, with a discolored area that can be seen at the portion of the
wafer behind the flat).
Since the profile is a spiral, what is happening is that the spiral is broken at
the flats of the square, and so less resist flow is occurring to the corners of
the substrate.
We've used specialized chucks designed for square substrate coating, which
allows the resist to continue flowing to reach the corners of the substrate.
Best Regards,
Chad Brubaker
EV Group-Technology, Tel: (602) 437 9492 x 119, Fax: (602) 437 9435
E-mail: [email protected], Web: www.EVGroup.com,
-----Original Message-----
From: Abang Annuar Ehsan [mailto:[email protected]]
Sent: Wednesday, April 30, 2003 3:31 AM
To: [email protected]
Subject: [mems-talk] photoresist uniformity problem
Dear sir,
I'm spinning my photoresist (SU-8) on a 1" x 1" square silicon wafer sample
using a lab spin-coater. I'm dispensing about 0.4 ml onto the surface using
a syringe (static dispense). My spread spin is 500 rpm / 5 sec while spin
speed is 4000 rpm/ 30 sec. However, I'm always getting an uneven thickness
(in circular pattern) The centre looks different while sides are in
different color. How do I improve uniformity? Your kind help is highly
appreciated.
Abang Annuar.
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