I'd say that the odds are good that the Si membrane may be drawn towards the
glass - the net negative charge at the surface of the glass wafer will probably
draw it.  Whether it will be enough for contact is a different question

However, for the second part of the question, I'll have to say "that depends".
At a bonding temperature of 350ºC, probably not, since the interdiffusion of
gold into gold is still fairly low.

However, if the bond takes place at 400ºC, there may be more of an issue.  The
electrostatic forces that occur during the anodic bond my be sufficient to bring
the two into intimate contact, which may allow for interdiffusion between the
gold layers (this is the same mechanism that occurs when performing a gold-to-
gold thermocompression bond, which is also typically performed in the
neighborhood of 400ºC, and is sufficient to form a hermetic seal).

Bonding in air vs. vacuum can definitely change the balance of things.  In the
case of bonding with air, I can see two significant effects.  If we assume that
the substrates are already in very intimate contact when the voltage is turned
on, then the air inside the cavity will have no where to go.  So, for the first
effect, if the electrode starts to flex towards the glass, then the air on the
glass side will increase in pressure, creating some resistance to further
flexing.  Once the bonding is done, this will promote the movement of the
electrode back to its original position (although from then on, external
variations in air pressure will cause flexure to the electrode - this is how
pressure sensors work).

If the substrates are not in completely intimate contact prior to voltage (due
to wafer warp or TTV issues, etc.), then at least some of the air in the cavity
will escape.  There will be less resistance to flexing of the electrode towards
the glass.  Plus, upon completion of the bond, air can no longer re-enter the
cavity (since the electrostatic forces of bond will generaly overcome all but
severe warpage and TTV), and so the cavity will effectively contain a partial
vacuum.  So , when the exterior is re-pressurized to atmospheric, the eletrode
will remain partially deflected towards the glass, since the pressure
differential will push on it.

Wnen bonding in vacuum, you basically guanrantee that the second case is true -
the only difference is, instead of having a partial vacuum in the cavity, you
will have a nearly complete vacuum in the cavity- you will pretty much always
have flexure of the electrode towards the glass.

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 Reza Rashidi
Sent: Wednesday, February 04, 2009 12:32 AM
To: mems-talk@memsnet.org
Subject: [mems-talk] Pull-in during anodic bonding process

Hi everyone,

I am going to anodically bond si to glass. There is a 53 um thick square
membrane with side 1.6 mm in the middle of si chip. Also, there is a 10 um
cavity and a gold electrode on the back of si membrane that forms a capacitor
when bonding to a glass (Pyrex) with similar gold electrode.
I was wondering if electrostatic force during bonding process pulls in membrane
toward glass (as there is just a small 10 um gap between them). If so, does gold
electorde on membrane still stick on the electrode of glass after the process.
If not, does the membrane return to the original position after the process. How
is the difference of effect between bonding in vacuum or air.

Any help will be highly appreciated.

Thanks,
Reza Rashidi