Hi Andreas,

It sounds like you have trapped air.  It is common to see round cracks or half
circle cracks due to air bubbles.   The bubbles are usually caused by two common
issues:
1.  Particles at the bond interface.  A particle will prevent direct contact
between the wafers.  As the bond front approaches the particle, air will get
trapped and can be compressed enough to crack the glass.  If this is the issue
you should see a small void in the center where the particle is.
2.  Multiple bond fronts colliding.  If your bond propagates at multiple points
air can get trapped when the two bond fronts meet.  It is common to see half
moon cracks if this is the case.  As the bond front moves the trapped air
compresses until it cracks the glass and vents.  Once the air is gone the glass
will bond leaving only the crack.  Try using a lower voltage to allow the air to
escape as the bond front moves across the wafer.  If you are using a "Home made"
bonder that requires you to move the electrode, try and place the electrode next
to the bond front every time you need to move the electrode to avoid trapped
air.

Anodic bonding is kind of the brute force of bonding because it has such a
strong bond energy.  Because of this particles and trapped air will result in
broken wafers.  If you are seeing circles then there is a good chance it is from
trapped air or particles.  If you see cracks or broken wafers then stress and
thermal shock need to be addressed by adjusting the bond temp cooling and ramp
time.

Brad

Brad Johnson
SUSS MicroTec
Bonder Product Specialist
Asia