Jiang,

The issue may be that the bubbles are in the su-8 prior to dispense on
the wafer.  The high viscosity of the thicker formulations can be so
great that bubbles will not float out of the solution, (especially
smaller bubbles), and may deposit onto the substrate.

One way to eliminate these bubbles is to placed a sealed container of
the SU-8 (sealed so it doesn't dry out) into an oven a moderate
temperature (60 - 80 C).  This will reduce the viscosity of the
material, allowing the bubbles to rise out.

Best Regards,
Chad Brubaker

 -----Original Message-----
From: Jiang Ziling
Subject: [mems-talk] su-8 air bubble problem

  I am using Su-8 and frequently there are air bubbles left in the
film after spin coating. It is more serious with thicker films. I
tried to get rid of them by putting the pre-spinning resist-coated
wafer in vacuum dessicator, but the bubbles won't go. Anyone have
experience in this?