Systematic characterization of key parameters of hermetic wafer-level Cu-Sn SLID bonding

Hermetic wafer-level encapsulation of atmosphere sensitive Micro-Electric-Mechanical Systems (MEMS) devices is vital to achieve a high yield, a high performance and a long operating lifetime. An interesting and gradually more employed packaging technique is flux-less wafer-level copper-tin (Cu-Sn) solid-liquid interdiffusion (SLID) bonding. The process is being employed for several next generation high performance infrared bolometers. The hermeticity and the reliability of the bond are studied through the key parameters: probability for void percolation, conversion into intermetallic compounds and residual stress using both characterization and modelling. Characterization using Scanning Acoustic Microscopy (SAM), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and optical microscopy showed different defects in the Cu-Sn bond in case of non-optimized process settings. Defects observed are incomplete conversion into the targeted intermetallic Cu₃Sn and voiding. Incomplete conversion influences the thermo-mechanical and environmental robustness of the bond negatively due to less favourable material properties of the intermediately formed Cu₆Sn₅ in comparison to the targeted Cu₃Sn. Voiding may result in lower thermo-mechanical robustness of the bond and loss of hermeticity. The probability for percolation through voiding is calculated. The reliability of the Cu-Sn bond is also studied using a Finite Element Model (FEM). The calculated residual stress in the Cu-Sn bond caused by volumetric shrinkage during formation of the IMCs is significant and should be taken into account during design to avoid a negative influence on the reliability of the bond.