Hello Robert,

AML wafer bonders are specified down to 2*10-6 mBar, and importantly, the
wafers are held in separation of ~8mm during the pumpdown & heating phase.
This enables fast pumpdown and good outgassing of the bonding surfaces
prior to bringing the wafers into contact using the in-situ alignment
capability, and performing the bond. With this large separation between
the wafers you can be reasonably sure that the pressure level that you
measure is actually the pressure level between the wafers.

Regards
Jan Kowal