Room temperature wafer bonding of metal films using flattening by thermal imprint process

This paper reports on the surface flattening of metal films on wafers by the thermal imprint process and its application to wafer direct bonding. The ultraflat surface of a sapphire wafer was pressed onto the surface of a sputter-deposited Au film on a Si wafer for 600 s at 200 C with an applied pressure of 5 MPa by compressed air. As a result, the Au film surface was successfully flattened by thermal imprint process. The flattened Au films were bonded in a vacuum chamber after surface activation by Ar beam sputtering. In the case of bonding between two Au films on a Si wafer, the surface flattening of the Au films was determined to be effective when the bonding load is small and/or the Au films are relatively thick. In the case of bonding between a Si wafer and a Au film on a Si wafer, defects at the bonding interface were found to be dramatically reduced by the flattening of the Au film surface. Additionally, the Au-Si bonding using the flattened Au film was significantly stronger than that without flattening.