A new bonding technology using gold and tin multilayer composite structures

A bonding technology which utilizes chromium, gold, and tin and gold deposited directly on the backside of a device die to form a multilayer composite is reported. The substrate accepting the die is coated with chromium and gold layers. The die and the substrate are brought into contact and heated to 310-320 °C. Due to the unique feature of the gold-tin alloy system, the tin layer melts first and dissolves the gold layers of the composite to produce a solution mixed with solid, which in turn would dissolve a portion of the gold layer on the substrate to develop a near eutectic bonding. In the composite, since the tin layer is protected by an outer gold layer in the same vacuum cycle, tin oxidation, which is a major cause of difficulty in achieving quality bondings, is reduced. This technology thus eliminates the requirement of preforms, prevents tin oxidation, and provides precise control of the bonding thickness. Results of bonding 4-mm by 4-mm GaAs dice on alumina substrates show that high-quality bondings are obtained as determined by a scanning acoustic microscope (SAM)