Wafer bonding process with reduced overlay distortion

A method comprising:

• forming a first plurality of bonded wafers using a plurality of process recipes, the forming comprising, for each one of the first plurality of bonded wafers, striking a backside of a first wafer with a striker to bond the first wafer to a second wafer, each of the first plurality of bonded wafers being formed using a different one of the plurality of process recipes, the first wafer being held by an upper wafer holder by applying vacuum to a first vacuum surface area at a first vacuum level, the first vacuum surface area being between 0.1% and 5% of a total area of the backside of the first wafer, the first vacuum level being between 0 kPa and −100 kPa, the second wafer being held by a lower wafer holder by applying vacuum to a second vacuum surface area at a second vacuum level, the second vacuum surface area being between 80% and 100% of a total area of a backside of the second wafer, the second vacuum level being between 0 kPa and −100 kPa;
• measuring overlay distortions for each of the first plurality of bonded wafers, and based thereon determining residual overlay distortions for each of the first plurality of bonded wafers;
• selecting one of the first plurality of bonded wafers with a lower value of the residual overlay distortions;
• determining a target process recipe associated with forming the selected one of the first plurality of bonded wafers;
• and
• forming a second plurality of bonded wafers using the target process recipe.

A method comprising:

• forming a plurality of bonded wafers by performing a cycle of bonding, each cycle comprising:
• receiving a wafer set to be bonded, the wafer set comprising a first wafer and a second wafer, the first wafer being held by a first wafer holder by applying vacuum to a first vacuum surface area at a first vacuum level, the first vacuum surface area being formed by activating a subset of chuck vacuum zones of the first wafer holder disposed around an outer edge of the first wafer, the second wafer being held by a second wafer holder by applying vacuum to a second vacuum surface area at a second vacuum level,
• measuring an overlay distortion of a previous bonded wafer, and based thereon determining a residual overlay distortion of the previous bonded wafer,
• determining a process recipe based on the previous residual overlay distortion,
• positioning the first wafer to align with the second wafer and being separated by a bond gap distance, and
• striking the backside of the first wafer with a striker to bond the first wafer and the second wafer together to form a bonded wafer.