Diffusional hillock growth in Ag stress migration bonding for power device interconnections

We propose an interconnection method, called silver (Ag) stress migration bonding, in which perfect bonding can be achieved by migration of Ag atoms driven by the stress gradient in an Ag film at about 250 °C. In such low-temperature bonding at atmospheric pressure, some stress is generated at the bond interface during the die attach process. On heating, numerous hillocks grow on the Ag films deposited on substrates to release residual stress in the thin films. A bonding interface is formed between the substrates as the hillock growth turns into abnormal grain growth. When the sputtering process is properly optimized to maximize the stress migration with suitable bonding temperature and time, perfect bonding can be obtained. These results pave the way for the use of the stress migration bonding method in various applications such as flip-chip packaging, and future electronic devices.