Cu-Cu Thermocompression Bonding with Cu-Nanowire Films for Power Semiconductor Die-Attach on DBC Substrates

Driven by the trend towards high power density and high reliability, new high-temperature die attach technologies are continuously being developed to take full advantage of the new generation of power devices. Conventional Cu-Cu thermocompression bonding has emerged as an attractive interconnection technique for 3D integration, enabling highstrength and low-resistance Cu interconnections under solidstate conditions. However, it typically requires copper bump structures as the bonding medium [1], which are complicated and costly to fabricate. Moreover, high process temperature and high external pressure combined with reducing atmosphere are typically required to achieve good Cu interconnects. In this study, a robust Cu-Cu bonding approach is proposed using prefabricated copper nanowire (NW) films as the bonding medium. The bonding process was carried out in air at a temperature of 250- 350 °C, an applied pressure of 15 - 25 MPa and a duration of 5 - 20 minutes. The individual effects of the process parameters and their interactions on the shear strength, the thickness of the bond layer and the porosity of the Cu joints were investigated by a two-level full factorial design. The results show that all three main process parameters have significant positive effects on the die shear strength in the investigated parameter range. In addition, temperature plays the most crucial role in reducing the porosity of the nanowire layer, followed by applied pressure and bonding time. Interactions between each of these parameters can be observed, implying that the densification of the sintered Cu nanowire layer was influenced by the interplay of different creep and diffusion mechanisms. Further experiments with nanowires of different diameters show that a reduction in diameter can lead to a reduction in the required surface melting or sintering temperature. However, thin nanowires with a diameter of 100 nm have shown low shear strength due to the low mechanical strength of the wire itself. Through the parameter studies, a dense and high strength bond can be achieved by an optimized parameter set and geometry of the nanowires.