Solidification and interfacial interactions in gold–tin system during eutectic or thermo-compression bonding for 200 mm MEMS wafer level hermetic packaging

In this work, Au–Sn eutectic bonding and Au–Sn thermo-compression bonding are studied for applications in hermetic packaging at wafer level. Eutectic bonding experiments were performed under vacuum or pure nitrogen at temperatures between 300 and 350 °C while thermo-compression bonding experiments were performed under vacuum at 270 °C. During these experiments, the solidification of electrodeposited Au–Sn alloy as well as the interaction of this alloy with W₂N layers are studied. Some supplementary specific brazing experiments were performed using commercial sheets of eutectic Au–Sn alloy in order to understand the mechanisms of interactions between the Au–Sn alloy and the W₂N layer and of solidification of the Au–Sn eutectic alloy. The melting and solidification process of eutectic Au–Sn alloy were studied by differential scanning calorimetry under different geometrical configurations such as commercial eutectic Au–Sn sheets alone, brazing joints performed by commercial eutectic Au–Sn alloy and samples made by thermo-compression bonding. Bonded wafers with good mechanical properties were characterized by cross-section scanning electron microscopy using energy dispersive X-ray mode. Some samples were characterized by transmission electron microscopy. The mechanical strength of the seal was checked by shear tests.