Interfacial reactions and diffusion path in gold–tin–nickel system during eutectic or thermo-compression bonding for 200 mm MEMS wafer level hermetic packaging

Abstract

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 310 and 400 °C while thermo-compression bonding experiments were performed under vacuum at 270 °C. Nickel layers 350 nm thick are used as wetting layers on both cap and device sides. During these experiments, the interaction of electrodeposited Au–Sn alloy with nickel layers is studied either in solid or liquid state. The diffusion paths at 270 °C as well as during isothermal solidification of the joint at 310, 350 and 400 °C are determined in relation with the thicknesses of the reaction layers formed at the interfaces and the microstructure of the joint. The effective interdiffusion coefficient in the ζ-Au₅Sn phase is deduced at 270 and 310 °C. Moreover, the melting and solidification process of electrodeposited Au/Sn layers with an overall eutectic composition were studied by Differential Scanning Calorimetry in a specific configuration that does not involve the interaction of Au–Sn alloy with the nickel layer. Bonded wafers with good mechanical properties were characterized by cross-section Scanning Electron Microscopy using Energy Dispersive X-ray mode. The mechanical strength of the seal was checked by blade plate insertion technique.