One possibility that comes to mind.  Very early technical paper by Mary Long in
the 80's on bubble formation in thick resist at exposure.  17 microns is pretty
thick.  During exposure the heat of the exposure lamp causes bubble formation
inside the resist.  If there is a bubble  partially on the expose section it
will give a half circle in the resist line that is developed away.  Mary was at
Motorola or Arizona University in Tucson when she gave the paper.  My colleague
and I would have thought the problem would have been reversed, mouse bites in
the aluminum because of the reflectivity of the aluminum.  Try longer gentler
exposure.  Also when we got mouse bites in aluminum we increase the prime time
from 5 minutes to 20 minutes.  The lower number of hydroxyl ions on metal
require longer HMDS exposure for the same contact angle. Bill Moffat

-----Original Message-----
From: mems-talk-bounces@memsnet.org [mailto:mems-talk-bounces@memsnet.org] On
Behalf Of Lihua Li
Sent: Tuesday, January 12, 2010 10:42 PM
To: mems-talk@memsnet.org
Subject: [mems-talk] Lithography on LTO

I have a recipe setup for 17um resist with AZ4620. The process worked fine on
Aluminum coated wafers. When I used same recipe on LTO coated wafers, I can see
mouse bites right after exposure.

Here is my current recipe setup:

1, Vapor prime
2, 1st resist spin @ 2000rpm
3, Soft bake at hot plate, 90 sec at 105C 4, 2nd resist spin  @ 2000rpm 5, Soft
bake on tho plate, 120 sec at 110C 6, Wait for 2hr to rehydrate the resist 7,
Expose with MA150, total exposure time is 30sec (5 sec exposure and wait for
60sec before next exposure) 8, Develop

I changed soft bake temperature (up and down), tried pre-bake with convection
oven, tried dehydration bake before vapor prime, but the mouse bites is still on
the wafers. Is this related to the thermal conduction of LTO films? How can I
improve my process? Unfortunately my MA150 does not have the option of purging
N2 during exposure.

Thanks!