A New Failure Mechanism in thin Gold Films at Elevated Temperatures

High temperature (320°C-360°C) life tests of integrated circuits with TiW/Au metalization have resulted in failures due to void formation in the metal films which result in loss of continuity. Hillocks are also observed. The failures occur with and without power applied to the circuits, so electromigration can be discounted as a cause. Failures occur in the vicinity of oxide steps after several hundred hours at 340°C. Hillock growth is more extensive for films deposited on room temperature substrates than on heated substrates indicating that the effect may vary with grain size. Doping of the gold with tantalum decreases the hillock growth under some conditions. Coating of the metal with silicon nitride prevents failures from occurring and voids and hillocks from forming for at least 500 hrs at 360°C. Silox and sputtered SiO2 coatings were not as successful. When a second TiW layer is deposited over the gold, hillock and void formations are prevented, but gold whiskers are observed to grow laterally from the exposed edge of the gold metalization. Hillock growth and void formation is believed to be due to a self-diffusion process which occurs in the presence of strains within the gold film. These strains may be due to a mismatch in the thermal expansion coefficients of the gold and the TiW, SiO2 and Si on which it is deposited.