Investigation of Au-AuSn Bonding Below Eutectic Temperature for Gigahertz Bulk Acoustic Wave Transmission

This work reports a novel approach to form chip-level Au to AuSn bonding below eutectic temperature, and the techniques to investigate the bonding interface quality for gigahertz bulk acoustic wave (BAW) transmission. The primary study involved bonding a $6\times 6$ mm MEMS chip with BAW transducer arrays to an $8\times 8$ mm silicon substrate by an intermediate Au-AuSn layer deposited by e-beam evaporation. The bonding was achieved by using a custom-made chip bonder where the compression was done at room temperature and later heat treated in an N2 ambient oven at 200°C to form the bond. The bonding quality was evaluated by pulsating the MEMS transducers with 80 ns pulsed-RF signal at 1.465 GHz to generate a packet of bulk acoustic waves and then assess the reflected echo signals. With good interface bonding, it is hypothesized that acoustic energy can propagate through the bonded layer and back to the transducers. By analyzing the received echo signals on the transducer array, we predicted whether acoustic transmission through the bonding interface occurred. Electro-acoustic measurements were done pre/post bonding to compare the pulse-echo signature differences. There are a total of 32 transducer arrays, which cover half of the MEMS chip area and were used to correlate with the CSAM inspection to determine the uniformity of the bond.