US Patent 7897431 Stacked semiconductor device and method

A method of stacking wafers includes: providing a first wafer including a first metal connection layer; forming a first passivation layer over the first metal connection layer; forming a first bondpad in the first passivation layer to form a first bondpad layer; providing a second wafer including second metal connection layer; forming a second passivation layer over the second metal connection layer; forming a second bondpad in the second passivation layer to form a second bondpad layer; forming at least one of a first conductive adhesive layer over the first bondpad layer and a second conductive adhesive layer over the second bondpad layer; and stacking the second wafer on the first wafer by bonding respective faces of the second bondpad layer with the first bondpad layer via the at least one of the first conductive adhesive layer and the second conductive adhesive layer.