Characterization of Interfacial Adhesion of Cu-Cu Bonding Fabricated by Thermo-Compression Bonding Process

Four-point bending tests were performed to investigate the interfacial adhesion of Cu-Cu bonding fabricated by thermo-compression process for three dimensional packaging. A pair of Cu-coated Si wafers was bonded under a pressure of 15 kN at 350 degrees C for 1 h, followed by post annealing at 350 degrees C for 1 h. The bonded wafers were diced into 30 mm x 3 mm pieces for the test. Each specimen had a 400-mu m-deep notch along the center. An optical inspection module was installed in the testing apparatus to observe crack initiation at the notch and crack propagation over the weak interface. The tests were performed under a fixed loading speed, and the corresponding load was measured. The measured interfacial adhesion energy of the Cu-to-Cu bonding was 9.75 J/m(2), and the delaminated interfaces were analyzed after the test. The surface analysis shows that the delamination occurred in the interface between SiO2 and Ti.