Copper thermocompression for MEMS encapsulation:Master’s thesis

Copper thermocompression is a promising wafer-level packaging technique, as it allows the bonding of electric contacts simultaneously to hermetic encapsulation. In thermocompression bonding the bond is formed by diffusion of atoms from one bond interface to another. The diffusion is inhibited by barrier forming surface oxide, high surface roughness and low temperature. Aim of this study was to establish a wafer-level packaging process for MEMS (Mi-croElectroMechanical System) mirror and MEMS gyroscope. The cap wafer of the MEMS mirror has an antireflective coating that limits the thermal budget of the bonding process to 250°C. This temperature is below the eutectic temperature of most common eutectic bonding materials, such as Au-Sn (278°C), Au-Ge (361°C) and Au-Si (370°C). Thus a thermocompression bonding method needed to be developed. Copper was used as a bonding material due to its low cost, high self-diffusivity and resistance to oxidation in ambient air. The bond structures were fabricated using three different methods and the bonding was further enhanced by annealing. The bonded structures were characterized with scanning acoustic microscopy, scanning electron microscope and the bond strength was determined by shear testing. Exposing the bond structures to etchant during Cu seed layer removal was found to drastically increase the surface roughness of bond structures. This increase proved detrimental to bond strength and dicing yield and thus covering the bond surface during wet etching is recommended. The native oxidation on copper surfaces was completely removed with combination of ex situ acetic acid wet etch and in situ forming gas anneal. Successful thermocompression bonding process using sputtered copper films was established at a low temperature of 200°C, well below the thermal limitation set by the antireflective coating. The established wafer bonding process had high yield of 97% after dicing. The bond strength was evaluated by maximum shear strength and recorded at 75 MPa, which is well above the MIL-STD-883E standard (METHOD 2019.5) rejection limit of 6.08 MPa