We are currently fusion bonding two silicon wafers together. In wafer 1 there
is a circular cavity, of diameter 7mm, etched into its polished surface to a
depth of about 10 microns. The thickness of wafer 2 is reduced to about 20-40
microns by lapping and polishing to produce a diaphragm of this thickness above
the cavity. The cavity is vented to air by a channel etched into wafer 1.

We are observing bowing of the wafer into the cavity. Does anybody have
experience of this process? Comments on the this effect and the likelyhood of
the process generating significant inplane stresses in the diaphragm would be
helpful.