Ultimate strength and morphological structure of eutectic bonds

Abstract

Eutectic bonding is a technique commonly used in the electronics industry to fasten silicon chips to substrates coated with gold. The reverse process of bonding small gold tabs to large semiconductor substrates is also possible. Gold plated Kovar tabs have been bonded to a hot pressed silicon-germanium alloy (80 wt % Si) and to single crystals of silicon and germanium. The joining alloys used were gold-silicon (2 wt % Si) and gold-germanium (22 wt % Ge) which melt at 370‡C and 356‡C, respectively. Ultimate tensile loads have been measured and found to range from approximately 7.5 to l8 kilograms. Low tensile loads were associated with semiconductor surfaces which showed little evidence of dissolution, apparently protected by a surface oxide. A preliminary etch in 10% HF increased failure load considerably. Fracture then took place within the semiconductor material. Low strength bonds exhibit a lamellar eutectic structure, while high strength bonds exhibit lace-like grain boundary penetration of gold. Bonds on single crystals have considerable microstructural detail. Some show evidence of dendritic solidification, while others show evidence of eutectic solidification.