Mehmet,

The problem you are having is probably due to thermal stress.  Based on
your description, I am guessing that you are performing your bonding on
a hot plate - thus, only using single-side heating.  Because the thermal
conductivity of glass is very poor, you probably have a significant
temperature gradient between the glass at the bond interface, and the
glass at the top surface.

Upon cool-down of the stack, this will create a large amount of thermal
stress in the glass.  Under normal circumstances, with blank glass, this
will lead to a noticeable bow in the bonded stack.  In your case you
have stress concentrating points (the holes)  to make matters worse, by
using a diamond drill, the holes themselves will also have some built-in
stress, and most likely an uneven (non-smooth) wall for the holes.
These are ideal propagation points for stress fractures.

Your best bet would be to perform double sided heating.  If this cannot
be accomplished, you may be able to improve things by placing a large
conductive mass (aluminum or steel chuck), and allow the whole stack to
be come hot prior to applying the potential.  Then, after the bonding is
done, turn off the hot plate and allow the whole stack to cool to room
temperature or near it gradually - this will at least reduce thermal
shock.

Best Regards,
Chad Brubaker

EV Group
invent * innovate * implement
Senior Process Technology Engineer - Direct: +1 (480) 305 2414, Main: +1
(480) 305 2400 Fax: +1 (480) 305 2401
Cell: +1 (602) 321 6071
E-Mail: C.Brubaker@EVGroup.com, Web: www.EVGroup.com

-----Original Message-----
From: mems-talk-bounces@memsnet.org
[mailto:mems-talk-bounces@memsnet.org] On Behalf Of Mehmet Aykol
Sent: Tuesday, March 17, 2009 7:56 AM
To: MEMS discussion
Subject: [mems-talk] Glass cracking after anodic bonding

Hi all,Few weeks ago someone else asked about glass cracking after anodic
bonding. I am having the same problem but form a different reasons. My
cracks seem to nucleate form the holes I drilled using a diamond drill bit
(3/4mm) attached to a Dremmel. The cracks start after I start cooling down
the device. I ramp down the temperature as well. My bonding conditions are
500C and 700V. I am using a spring loaded gold pin as the top electrode.
How can I avoid these cracks?

--
Mehmet Aykol
University of Southern California
EE - Electrophysics
Phone: (213) 821-4090