Thank you very much for all your suggestions.

I used a homemade hotplate and power supply for the bonding process. I put
an alumina chunk on top of glass in order to apply some pressure and also to
fix the wire which is used as cathode.

 I was trying to remove the force after I shut off the electric field, But
the glass started to crack immediately after I remove the force, even before
I turned down the temperature. This may be caused by the influence of the
cooler air on top, since I don't have a chamber.

On Fri, Feb 27, 2009 at 1:52 PM, Brad Johnson  wrote:

> Hello Jingru,
>
> There are many things that can cause the cracking.  What do the cracks look
> like?  Are they round cracks or half moon cracks?  What type of bonder are
> you using, homemade hotplate and power supply or a commercial chamber
> bonder?  Are you bonding in vacuum or atmosphere?
>
> If you are seeing half moon cracks or round chunks of glass popping out
> then you have air (or gas) getting trapped between two bond fronts.  The gas
> will be compressed until it cracks the glass and vents.  If you have long
> cracks the follow your channels then the trapped gas in the channels may be
> cracking the glass, I've never seen this case but it could be an issue.
>
> Try this:
> 1.  If you are using a chamber bonder make sure it is setup properly and
> the upper and lower electrodes are clean and flat.  Metal electrodes tend to
> warp and pit over time.  If this is the case then it will cause poor contact
> with the bond stack and allow air or gas to become trapped.  SiC electrodes
> will become contaminated with Na after several bonds.  Give them a good
> clean in RCA over night and the conductivity should be back to new and
> uniform across the chuck.
>
> 2.  Try bonding in a vacuum or at a lower pressure.  If you bond at a low
> pressure make sure it is below 1e-2mbar or you risk the chance of generating
> a plasma in the chamber.  You'll know if you have a plasma because the
> current will stop dropping and the voltage will stop increasing and stay and
> a constant state.
>
> 3.  If you cannot bond in a vacuum and trapped gas is the issue the try
> bonding at a lower current.  This will slow the bond front and may help to
> allow the gas to vent before it gets trapped in the bond.
>
> 4.  If the cracking is general cracks across the wafer check your ramping
> and cooling times.  Let the wafer stack slowly heat up then let them set at
> the bonding temp to come to a nice uniform temp before bonding.  Any time I
> set up a first time bond I let the wafers ramp up to 350C over a 30 minute
> time.  Then I will heat to 400 and hold for 15 minutes to allow the tool and
> the wafer stack to stabilize.  After bonding I remove any force on the stack
> and cool the wafers slowly at about 100C over a 30 minute time.  If you have
> the ability to apply force do not heat or cool the stack with force, this
> can not only cause a shift in the wafer stack but also cause cracking..
>  This sounds like a very long recipe but once you have a working process you
> can optimize it for throughput.
>
> Good Luck
>
> Brad Johnson

--
Jingru