| CPC H01L 24/94 (2013.01) [H01L 22/12 (2013.01); H01L 24/741 (2013.01)] | 20 Claims |

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1. A method comprising:
placing a first wafer on a first wafer stage;
placing a second wafer on a second wafer stage in opposition to the first wafer stage;
pushing a center portion of the first wafer to contact the second wafer, wherein a bonding wave propagates from the center portion to edge portions of the first wafer and the second wafer; and
when the bonding wave propagates from the center portion to the edge portions of the first wafer and the second wafer, reducing a stage gap between the first wafer stage and the second wafer stage.
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