There are a few potential causes for this:

1: The thermal expansion of the epoxy is probably significantly higher
than either of the other substrates.  If the epoxy is thick enough, then
its CTE, plus the difference in yield strength and modulus between the
silicon and glass could account for the bow.

2: Uneven heating - if you are performing this bonding on a hotplate or
other one-sided heating apparatus, then the two substrates may actually
be at different temperatures during the bond.

3: Cooling parameters vs. nature of the epoxy. If this epoxy does not
cure completely, and is instead a thermoplastic material (meaning, its
viscosity changes with response to temperature), then its possible that
the material is not "set" at the bonding temperature  This means that
the cool down process will cause the adhesive to drop below the glass
transition temperature after the bond is completed.  Even though the
glass and silicon have the same STE, they have different thermal
conductivity, so the rate at which they cool down is different.  By the
time the adhesive "freezes", the substrates are two different
temperatures (the glass will be hotter), and will shrink differently
when returning to room temperature.

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 ? ??
Sent: Friday, June 26, 2009 5:30 PM
To: mems-talk@memsnet.org
Subject: [mems-talk] wafer bonding warpage

Hi all:
i am bonding one glass wafer and one silicon wafer with epoxy and their
TEC is the same. But why does it have near 500um warpage after bonding?

Thank you!

Andrew Du