Thermomechanical reliability challenges induced by high density Cu TSVs and metal micro-joining for 3-D ICs

The thermo-mechanical reliability challenges induced by high-density Cu TSVs and metal micro-joining are discussed. Cu TSV with the diameter of 20-μm induced the maximum compressive stress of ∼1 GPa at the Si substrate adjacent to them after annealed at 300°C. Depart from Cu TSV, the stress/strain in Si substrate changed to tensile stress and finally going to zero, where the TSV pitch is larger than twice of TSV size. However, in high density Cu TSV array with small TSV pitch, the Si substrate within small TSV spacing keep large compressive stress, which will seriously affect the mobility in active Si area, and thus device characteristics. Also, these large compressive stress leads to not only extrusion and peeling of Cu TSV, but also die cracking. The thermo-mechanical stress was produced during the bonding using high-density metal bumps. CuSn bump of 20-μm size has induced compressive stress of 140MPa beneath Si wafer surface, and it penetrates deeper area with large stress value after the bonding. The drain current and electron mobility of n-MOSFET which was located 15μm distance from microbump are changed by ∼10 % due to the local tensile stress of 500 MPa induced by microbump. Electron mobility changed varying with the distance from microbump. Influences of mechanical stress induced by Cu TSVs and microbump-underill joining on device characteristics were also evaluated.