Through Si Vias Using Liquid Metal Conductors for Re-workable 3D Electronics

This paper describes the design and fabrication of liquid metal interconnects (vias) for 2.5D and 3D integration. The liquid metal is gallium indium eutectic with a melting temperature of approximately 15.7°C that is introduced into via openings of a silicon interposer. This liquid interconnect technology can be integrated with existing interposer technologies, such as capacitors and traditional (solid metal) through-silicon vias (TSVs). In addition, liquid metal interconnects can better accommodate thermal stresses and provide re-workability in case of chip failure. Our research efforts are focused on the integration of multi-chip modules using liquid metal interconnects. Our study encompasses Direct Current (D.C.) measurements and failure analysis using snake and comb structures at low temperature (10 degrees Kelvin) to slightly above room temperature (300 degrees Kelvin). The snake and comb structure allows us to measure electrical shorts and opens, as well as provide estimates of via yield and allows additional information for determination of possible failure mechanisms. In order to make electrical contact to the liquid metal interconnect interposer from both the top and bottom, test coupons have been fabricated with arrays of large numbers of vias. The interposer structure consists of a thin (200 um thick) silicon wafer with via holes filled with liquid metal. The test coupon consists of bottom and top silicon die with a thickness of 500 um. The bottom wafer incorporates a 2 um-thick daisy-chain metallization and 100 um copper tall vias, which are electrically isolated from each other and the underlying Si by patterned AL-X dielectric. The top wafer incorporates an array of 80 um tall, electroplated copper pillars and top daisy-chain metallization. Liquid metal containment mechanisms and structures have also been investigated. In our presentation we will describe the design, fabrication and characterization of this re-workable interposer with liquid metal interconnects. We will present D.C. resistance and X-ray imagery of the liquid metal filled via. In addition, we will provide failure analysis of via yield per chip.