Utilizing Co as a contact metallization for wafer-level Cu-Sn-In SLID bonding used in MEMS and MOEMS packaging

Abstract

Many MEMS and MOEMS devices require hermetic packaging with preferably no postprocessing after the MEMS device's releasing. Wafer-level Solid-Liquid Interdiffusion (SLID) bonding can provide simultaneous hermetic packaging and better electrical interconnects. Moreover, employing a physically deposited contact metallization on the device wafer instead of chemically deposited layers (such as electrochemical Cu) is of utmost importance as far as reducing the complexity of the MEMS/MOEMS packaging process integration is concerned. The current work studied the possibility of utilizing Co as a contact metallization layer for the low-temperature Cu-Sn-In-based SLID bonding. In order to guarantee the long-term reliability of the devices, a fundamental understanding of the formation and evolution of interconnection microstructures and mechanical characterization of the joint is of utmost importance. In this work, Cu-Sn-In electroplated Si chips were bonded to Co substrates at a temperature range 160-250°C. During the bonding process, a single intermetallic compound (IMC) (Cu,Co)6(Sn,In)5 formed at the bonding area, with no detectable Cu3Sn phase that causes voids formation. The Young's modulus and hardness of (Cu,Co)6(Sn,In)5 and Cu6Sn5, as a reference, were measured as 124.8±0.5 and 6.2±0.5, 114±1 and 6.7±0.5 MPa, respectively. Furthermore, the current study was able to produce a fully IMC joint of Cu-Sn-In/Co SLID system at 220°C for bonding time as short as 20 minutes.