Reliability of die to wafer bonding using copper-tin interconnections

摘要

Reliability of copper-tin interconnections using transient liquid phase bonding was investigated. CuSn to Cu die to wafer bonding was achieved and characterized before and after aging. Thermal cycle tests (TCT) consisting of 500 cycles between -40°C and 125°C was compared with high temperature storage (HTS) at 125°C for 84h, which roughly accounts for the same thermal budget. Electrical and mechanical tests show that TCT induces larger damage than HTS does: Kelvin resistance increase after TCT is twice larger than after HTS; ultimate shear force has a 70% decrease after TCT, whereas 50% after HTS. This result means that TCT failure mechanisms not only include chemical aging by Cu/Sn interdiffusion but also thermomechanical stress build-up during cycling. In light of these findings, capillary underfill (CUF) material was introduced to fill the gap between the dies and the wafer in order to assess its ability to limit thermomechanical stress during cycling. After TCT, electrical yield has a 70% decrease without CUF, whereas only 20% with CUF. Regarding Kelvin resistance, 64% increase was measured without CUF, whereas 37% with CUF. Therefore, CUF has a significant positive impact enabling to limit electrical performance degradations. Interconnections cross section analyses reveal Kirkendall voids located at the Cu/Cu_3Sn interface. After aging, some cracks develop mainly through these voids which therefore seem to be the most detrimental defects to overcome.