Wafer-Level Low-Temperature Solid-Liquid Inter-Diffusion Bonding With Thin Au-Sn Layers for MEMS Encapsulation

Abstract

A novel solid-liquid inter-diffusion (SLID) bonding process is developed allowing to use thin layers of the Au-Sn material in wafer-level microelectromechanical systems (MEMS) packaging while providing a good bonding strength. The bond material layers are designed to have a robust bond material configuration and a metallic bond with a high re-melting temperature, which is an important advantage of SLID bonding or with its alternative name, transient liquid phase (TLP) bonding. The liquid phase in SLID bonding is the gold-rich eutectic liquid of the Au-Sn material system, where the bonding temperature is selected to be 320 °C for a reliable bonding. The average shear strength of the bonds is measured to be 38±1.8 MPa. The hermeticity of the package is tested with the He-Leak test according to MIL-STD 883, which yields a leak value lower than 0.1×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> atm.cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> /s. The vacuum inside the package without a getter is calculated as 2.5 mbar after cap wafer thinning. The vacuum level is well preserved after post-processes such as annealing at 400 °C and the dicing process. These results verify that thin layers of Au-Sn materials can be used reliably with the SLID or TLP bonding technique using the new approach proposed in this study.