Interface formation in the US-wedge/wedge-bond process of AlSi1/CuNiAu contacts

Interface formation between 25-m AlSi1 wire and flash-Au substrate metallization during a bonding time of 50 ms has been investigated. Only a few milliseconds after the ultrasonic power is switched on, intermetallic phase growth starts, continuing until the end of the wire-bonding process. During the entire bonding time, the fraction of the interface covered with Au 8Al3 increases, and at the end of the bonding time, the interface is nearly completely covered with that phase. Finite-element modeling (FEM) of the temperature in the interface region indicates maximum temperatures well below 100°C, thus making solely thermally activated intermetallic phase growth impossible. However, it is demonstrated that the phase growth observed during the ultrasonic wire-bonding process could result from an accelerated diffusion process caused by a higher vacancy concentration. The accelerated diffusion process would have an activation energy Q of 0.36 eV.