Unstable kinetic roughening during the island coalescence stage of sputtered tantalum films

Kinetic roughening of tantalum films during the initial growth stages has been studied by atomic force microscopy, scanning electron microscopy, and dynamic scaling theory. Different from the time-independent scaling behavior for continuous film growth, an intriguing unstable kinetic roughening occurs during island coalescence. In such case, roughness exponent α increases with growth time, accompanied by lower growth exponent β and higher coarsening exponent η. Detailed analysis of film surface morphology and simple phenomenological models suggests that this unstable behavior is related to the pronounced lateral growth of surface islands, which arises from the combined effect of the formation of grain boundary and the covering of heterogeneous substrate surface.