Low-Temperature Pressure-Less Silver Direct Bonding

A light-emitting diode (LED) die and a glass substrate, both of which were coated with a few-micrometers-thick silver layer, was successfully direct-bonded above 200°C in air without any pressure. In this silver direct bonding, the following three factors are essential: 1) keeping surfaces of silver layers clean; 2) bonding with a suitable solvent that can be reduced and has a boiling point near the sintering temperature; and 3) avoiding excessive heating. Oxygen concentration in a sintering atmosphere has a great impact on bonding. The surface of a silver layer shows roughening by abnormal grain growth as increasing oxygen resulting in increase of bonding strength. The abnormal grain growth on the surface of silver effectively contributes to the increase of junction area. Sintering at too high temperature, however, causes the formation of large voids, which migrated towards silver/glass interfaces resulting in the reduction of bonding strength. An LED die bonded at 240°C for 2 h can maintain high bonding strength through a heat run test at 350°C for 2 h. Furthermore, transient thermal resistance of an LED die bonded with the silver direct bonding is lower than that of an LED die bonded by traditional eutectic bonding with Au-Sn alloy. This bonding method can realize high thermal conductivity with low-temperature pressure-less process and can contribute to high-power semiconductor devices.