I have worked with AZ50XT and experienced the same problem (although in my case,
the layer was a bit thicker, at 105µm).  The ultimate cause for the bubbling was
an insufficient bake - I increased the temperature a bit, for the same amount of
time.  I also have vacuum hotplates on my coater (an EVG150), which allowed me
to increase solvent removal a bit more.

Another possibility for the bubbles is the extremely long exposure you are
using.  6 minutes is very extensive, especially depending on intensity of lamp
(which, on a broadband lamp house, has a typical intensity of 10 - 15
mW/cm2 measured at i-line).  This mans you would be delivering a dose of ~4
J/cm2, which is very excessive.  For a 30µm film, your dose should be in the
~1000 mJ/cm2 range (more likely less).

Additionally, you can try an "interval exposure" (this is a common feature on
more advanced mask aligners such as an EVG620).  This allows the exposure to be
pulsed in intervals - for example, a 5 - 20 second exposures, each with a wait
time of 15 seconds in between.  This allows some cooling of the resist t occur,
as well as dispersal of any generated nitrogen.

Best Regards,
Chad Brubaker


-----Original Message-----
From: Yu, Zeta (Tak For)
Sent: Sunday, January 29, 2006 10:43 AM
To: General MEMS discussion
Subject: [mems-talk] Bubbling and burnt phenomena of AZ50 after exposure

HI Fellows,

I am using AZ50, a positive PR. After exposing the PR under UV of 6min
for thickness about 30um, the surface is like burnt, with some pattern
on top, the surface is like burnt. Moreover, towards the end of
developing, it is obvious to see gas bubble on the wafer surface, which
severely damages the pattern. Can anyone share the way to remove these
problems? Thanks.