Environmental effects on the adhesion of gold microcircuit films

Adhesion is an important factor in controlling properties and performance of thin film devices. It is a critical factor in hybrid microcircuits with multilayer films and dissimilar metal interconnects where diffusion of copper from leads during processing and environmental effects during service can modify the adhesion strength of the gold conductive films. Previous work using gold and gold-copper alloy films to simulate different stages of processing and service showed that copper in solution improved film adhesion. More importantly, it took a combination of stressed overlayers and nanoindentation to trigger interfacial fracture of the gold-copper alloy films. The improvement in performance scaled directly with an increase in film strength. However, during two years air exposure telephone cord buckles formed at the gold-copper alloy film edges, grew slowing across the film surface, and eventually covered the sample. Formation of these buckles shows that a significant degradation in interfacial fracture strength had occurred in these films. We characterized the size and shape of the blisters that formed during nanoindentation of the as-deposited films and in the films following aging. These measurements were then combined with mechanics-based models to determine residual stresses and interfacial fracture energies. This analysis shows that air aging decreased the mode I interfacial fracture energy for the gold-copper alloy film from 3.2 J/m² to 1.5 J/m². A similar decrease in fracture energy has been observed for many systems exposed to hydrogen from processing and environmental exposure, including copper films, beryllium films, steels and iron-and nickel-based superalloys. This paper describes the effect of environment on resistance of gold-copper alloy film systems to premature interfacial failure, and by comparison with previous studies shows it can be attributed to hydrogen embrittlement.