Melvin,

  Flatness, bow, and warp will all contribute to poor uniformity on your bond.
The TTV required, depends on the wafers and process itself.  Typically 25um of
bow or warp, and 1-3um TTV are recommended for wafer direct bonding.  Please
feel free to contact me directly for some process help with your bonder.
Applications support is a free service we offer to all customer for the life of
our tools.

Best Regards,

Brian Douglas
Suss Microtec Inc.
Applications Engineer - Bonders
Phone: (802) 244 - 5181 x202
Fax:  (802) 560 - 0001