Silver Joints Between Silicon Chips and Copper Substrates Made by Direct Bonding at Low-Temperature

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In this paper, pure silver (Ag) joints between silicon (Si) chips and copper (Cu) substrates are produced successfully at a temperature much lower than its melting point. Silver is ductile and has low-yield strength. It can deform to release the shear stress caused by the large mismatch in coefficient of thermal expansion between Si and Cu. Silver also has the highest electrical conductivity and thermal conductivity among metals. As a bonding medium and interconnect material, it can provide the best electrical and thermal performances. In experiments, Ag in the form of foil is chosen as the bonding medium. Prior to bonding, the Si chips are coated with thin Cr and Au layers. The Si chip, Ag foil, and Cu substrate are bonded together in one step. The bonding process is conducted at 250°C in 50-mtorr vacuum environment. There is no molten phase involved during the bonding process. The resulting joints exhibit nearly perfect quality. No voids are observed at the Si/Ag and Ag/Cu bonding interfaces. The bonding strengths at these two bonding interfaces pass MIL-STD-883G standards. We believe that the bonds at the Au/Ag and Ag/Cu interfaces are formed by short-range interdiffusion. Since the melting point of Ag is 961°C , the Ag joints are expected to sustain high-temperature. The 250°C bonding temperature is the typical reflow temperature of Sn3.5Ag solders used in electronic industries. This novel bonding process can be applied to various electronic devices that require high-thermal performance or high-operation temperature. It is particularly valuable to packaging high-temperature devices.