Interfacial reactions between Cu and Sn, Sn-Ag, Sn-Bi, Sn-Zn solder under space confinement for 3D IC micro joint applications

Recently, three-dimensional integrated circuit (3D IC) integration technology consisting of TSV and micro joints has been viewed as one of the very promising solutions to go beyond Moore´s law. Typical solder joints for 3D IC stacking technique under development today has a solder volume about 1000 μm³, which is roughly 1/500 that of a conventional flip-chip solder joint with usually an 100 μm in diameter. One well-perceived effect of such a small solder volume is the solder joint would have very high possibility to end in a relatively large fraction of intermetallic compounds (IMCs) after common reflowing or thermal compression bonding process. Under this circumstance, instead of solder, the mechanical properties of IMCs and other reaction-induced microstructural changes and characteristics will undoubtedly play dominant roles on the reliability of the micro joints. In this study, solid-state interfacial reactions under a very confined space between Cu and Sn, Sn-Ag, Sn-Bi, Sn-Zn solders are systematically investigated. The objective of this paper is not merely to propose and identify some critical reliability concerns arising from interfacial reactions but to provide essential data for microelectronic packaging industry to the design of sturdy enough micro solder joints for multichip stacking applications. Sandwich structures of Cu/Solder/Cu are fabricated through chip-to-chip thermal compression bonding process. The thickness of the solder layer in this study is well-controlled at 10 μm. High temperature storage tests are conducted by isothermal aging at 120 °C, 150 °C, 180 °C, and 200 °C for different time periods, respectively. Issues to be discussed in this paper include (a) Impingement behavior of IMC grains after solid-state isothermal aging, (b) Rise of concentrations of minor inert alloying constituents in low solder volume joint, (c) Promising approach to reduce the growth rate of Cu-Sn - eactants within limited solder volume joint. These critical issues will be proposed and discussed through experimental evidence, and the implications based on these finding will be discussed as well.