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

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)
$((\mathbf{Cu},\mathbf{Co})_{6}(\mathbf{Sn}_{2}\mathbf{In})_{5}$
formed at the bonding area, with no detectable Cu3Sn phase that causes voids formation. The Young's modulus and hardness of
$(\mathbf{Cu}, \mathbf{Co})_{6}(\mathbf{Sn},\mathbf{In})_{5}$
and
$\mathbf{Cu}_{6}\mathbf{Sn}_{5}$
. as a reference, were measured as
$124.8\pm 0.5$
and
$6.2\pm 0.5,114\pm 1$
and
$6.7\pm 0.5$
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.