The reason for this distortion is that the glass-transition temperature
for the resin is in the ~105 - 110C range.  Hard baking at 120C is
significantly above this temperature.

The reason why a hard-bake temperature is designated at a temperature
above the glass transition temperature is that, especially for thick
films, the diffusion rate for the solvent in a solid film is extremely
low, so that removal of the last few percentage points of solvent
becomes virtually impossible.

By heating above the glass transition temperature, the resist becomes
semi-liquid, which greatly improves the diffusion rate of the solvent.
This provides the ultimate purpose for the hard bake - full removal of
solvent from the polymer.

Best Regards,
Chad Brubaker


-----Original Message-----
From:  Rupesh Sawant
Sent: Tuesday, March 14, 2006 10:01 AM
To: General MEMS discussion
Subject: [mems-talk] Post bake for AZ P4620 photoresist

In my experiments, I tried a post bake procedure for a 20 u thick AZ
P4620 photoresist spun on a glass slide. i observed that the features
were destroyed after the post bake. the post bake was done for 5 min
at 120 deg. C. According to the literature, post bake is an optional
step for thick photoresist layer and the features may distort due to
"Reflow" if the hard bake is done. Can someone tell me a clear  reason
for this distortion for thick photoresist.