Our lab has had this problem with AZ4620 and some other resists.  Our
solution is to perform a "pirahna etch" cleaning with HF dip of the wafers
before the photo resist step.  This yields a pure silicon surface ready for
lithography.  It is good processing practice to do a pirahna etch or some
other type of robust wafer clean (RCA clean) before subsequent process
steps, as one nevers really knows what is on the wafer surface "as
delivered" from the supplier.  This is especially recommended for lapped
wafers.

Good luck!

--
Patrick Roman, M.S.
Microsystems Engineer
Code 553 - Detector Systems Branch
Room E042 Building 11
NASA - Goddard Space Flight Center
Greenbelt, MD  20771

-----Original Message-----
From: kris
Sent: Friday, November 04, 2005 5:51 PM
To: mems-talk@memsnet.org
Subject: [mems-talk] Bubbles in Photoresist


Hello All,

I am following the following steps for AZP4903 recipe.
I have bubbles problems

[1]. Dehyrate Sio2 wafer at 150C.
[2] Spin HMDS for 4000 r.p.m for 40 sec.
[3] spin AZP4903 to shoot for the thickness of 15
microns. Spin is usually done for longer periods of
time to evaporate all the solvent in the photoresist.
[4} Softbake the wafer from room temperature to 110C
on contact hot plate. This takes almost 10 mins.

when i expose the photoresist to the light(25mW/cm2).
I have seen bubbles in the PR which were not observed
during the softbaking step.

Can anyone suggest me on this.