To increase the yield of the wafer-level Cu-Cu thermo-compression bonding method, certain surface pre-treatment methods for Cu are studied which can be exposed to the atmosphere before bonding. To inhibit re-oxidation under atmospheric conditions, the reduced pure Cu surface is treated by H₂/Ar plasma, NH₃ plasma and thiol solution, respectively, and is covered by Cu hydride, Cu nitride and a self-assembled monolayer (SAM) accordingly. A pair of the treated wafers is then bonded by the thermo-compression bonding method, and evaluated by the tensile test. Results show that the bond strengths of the wafers treated by NH₃ plasma and SAM are not sufficient due to the remaining surface protection layers such as Cu nitride and SAMs resulting from the pre-treatment. In contrast, the H₂/Ar plasma–treated wafer showed the same strength as the one with formic acid vapor treatment, even when exposed to the atmosphere for 30 min. In the thermal desorption spectroscopy (TDS) measurement of the H₂/Ar plasma–treated Cu sample, the total number of the detected H₂ was 3.1 times more than the citric acid–treated one. Results of the TDS measurement indicate that the modified Cu surface is terminated by chemisorbed hydrogen atoms, which leads to high bonding strength.

MEMS-compatible fabrication of nanoporous gold and the application for low temperature bonding are demonstrated. A cyanide-free electroplating solution is prepared for the Au–Sn alloy deposition. To investigate the influence of electroplating on Au–Sn alloy and nanoporous gold, different plating parameters and various sizes of patterns are designed and discussed. The optimized electroplating condition realizes 40 to 720 µm line width patterns fabricated on the same substrate. Low temperature substrate bonding at 200 °C is achieved with nanoporous gold and gold film, which has shear bond strength more than 60 MPa. The fracture inspection of the bonded area after shear tests verifies the bonding success. This study gives a study for fabricating on-chip nanostructure, and the results indicate the high feasibility of nanoporous gold for low temperature substrate bonding.