Hi Andreas,

    Cracking is likely caused by trapped air bubbles, assuming that you bond
under non-vacuum environment. Ways to avoid this include bonding under
vacuum, and/or initiating bonding from the center of the wafer then letting
the bonding front propagate slowly towards the edges, so as not to trap any
bubbles. Cleanliness might also be an issue as a particle as small as 1um
can cause mm-scale voids in the bonding interface.

Masa