Recently, the customer demanded on the higher densities, performance, and smaller sizes on packaging technologies in electronic industry by development of three dimensional integrated circuits (3D ICs). Among all interconnection technologies, especially through-silicon-via (TSV) provides the shortest length and the highest density which lead to significantly reduced signal delay and power consumption. TSV integration is stacked using micro bumps and through Si via, moreover, the diameter of micro bumps will be about 10um to 20um, being one order of magnitude smaller than that of a flip chip solder joint. With the reduction of micro bump size, the current density in micro bump will increase at least by a factor of 100 under the same power demand. Furthermore, when different Si chips are stacked together, joule heating would be serious and the actual operation temperature of 3D IC packaging will significantly increase. As a result, this newly 3D IC technology is expected to confront electrical reliability problems associated with electromigration(EM). In addition, the excessive intermetallic compound(IMC) growth in micro bump can degrade the mechanical and electrical reliability during long-term environments. Therefore, it is essential to understand the fundamental mechanisms of IMC growth kinetics.
In this paper, the effects of solder thickness and non conductive adhesive(NCA) on the IMC growth kinetics and EM performance of Cu/Sn-2.5Ag microbump were systematically investigated. Quantitative in-situ analyses on the IMC growth kinetics during in-situ annealing and EM lifetime test were performed in a scanning electron microscope chamber under current stressing conditions with current density of 1.5 x 10^5 A/cm^2 at 130 ~ 150℃.
In case of solder thickness effect on Cu/Sn-2.5Ag microbump, Cu/Thick Sn-2.5Ag(6 μm) microbump with spreading solder structure showed Cu6Sn5 and Cu3Sn phase growths and then IMC phase transition stages with increasing annealing time. By the way, Cu/Thin Sn-2.5Ag(4 μm) microbump without solder spreading, remaining solder was transformed to Cu6Sn5 right after bonding and had only a phase transition of Cu6Sn5 to Cu3Sn during annealing. IMC growth followed a linear relationship with the square root of the annealing time due to a diffusion-controlled mechanism. Measured activation energies for the growth of the Cu3Sn phase during the annealing were 0.80 and 0.71 eV for Cu/Thick Sn-2.5Ag and Cu/Thin Sn-2.5Ag, respectively.
In case of NCA effect on Cu/Ni/Sn-2.5Ag microbump, EM lifetime test was conducted with convection oven on Cu/Ni/Sn-2.5Ag microbumps of three structures. Also, the characteristics of IMC were investigated by in-situ SEM. Ni3Sn4, (Ni,Cu)3Sn4, and NCA trap were observed in the Ni/solder interface after reflow. As a result, the EM lifetime of Cu/Ni/Sn-2.5Ag microbump with NCA trap is shorter than other microbumps due to reduction of cross sectional areas by the NCA trap. And high Joule heating occurred at this area. EM lifetime of Cu/Ni/Sn-2.5Ag microbump without NCA trap has longest than other microbumps. Thus, It is suggested that Cu/Ni/Sn-2.5Ag microbump without NCA trap has better electrical reliability than other microbumps.