Development and Characterization of Microscale Interconnects and Packaging for 3-D Microsystems

A soldering technology, solid-liquid-inter-diffusion (SLID), using Cu and Sn is chosen as the connection approach. SLID Cu/Sn is a lead free, low temperature, low pressure process. Moreover, a special characteristic for SLID process is that, intermetallic compound (Cu3Sn) can be formed at a lower temperature (250°C~325°C[3]) than its melting point (675°C[4]). Thus once the solder joint is made, it can stand fairly high temperature during the succeeding process, such as molding, 2-level packaging, etc.

Due to the special nature of the SLID process, it can also stand the process itself repeatedly. The intermetallic Cu3Sn is observed to be quite inert while exposed to moisture, so it is a good candidate material for both interconnects and sealing rings. The process is relatively simple to perform, and no complicated equipment is needed. As an alternative, eutectic Au/Sn solder technique will also be investigated.

Effort will be made to develop a 3D assembly method for heterogeneous microsystem based on SLID Cu/Sn or/and eutectic Au/Sn process. Firstly, 1-level interconnect, i.e., electrical and mechanical contact between MEMS die and ASIC, should be developed. Secondly, sealing process should be developed, preferably using the same connection approach, which is another Ph.D project running parallel.

Specific tasks related to this project will be:
• Develop simulation model of uniformity of electroplated micro-bumps as a function of mask layout and geometrical pattern density;
• Electrical and mechanical characterization of interconnects;
• Effect of Cu to IMC ratio on stress and yield of joints
• Understanding and optimization of IMC formation