Relationships between grain structure and stress in thin Volmer-Weber metallic films

Abstract

In Volmer-Weber growth, islands that nucleate on the substrate surface impinge and coalesce into grains of a continuous film. During deposition of these polycrystalline films, the intrinsic stress for materials of sufficiently high mobility often evolves through three distinct stages, switching from compressive to tensile and back to compressive. Many studies of stress evolution during this process have indicated that a tensile stress develops as the islands coalescence, with the peak stress occurring when the film become continuous. The magnitude of this tensile stress is strongly dependent on the grain structure. The grain structure is in turn strongly dependent on atomic processes at the substrate surface at the onset of film growth. In this study, Kinetic Monte Carlo simulations were used to investigate the relationship between characteristics of the amorphous surface, nucleation and growth of islands, and the tensile stresses observed as films form. It is demonstrated that island nucleation on amorphous substrates can be dominated by the spatial characteristics of the amorphous surface. The simulation parameters providing the best fit to experimental data from gold deposited on silicon nitride included a trapping energy of ET = 0.69 eV. The compressive stresses that develop have also been shown to reversibly change during interruptions of growth. One proposed model for this reversibility is that the compressive stress is related to adatom trapping and de-trapping at grain boundaries, while others attribute the stress to surface changes. In the current study, intrinsic stresses monitored in-situ using a capacitive curvature measurement system are studied with respect to the film grain structure, deposition rate, and substrate temperature.

(cont.) The kinetics of the post-deposition tensile rise receive special attention. The "reversible" compressive stress exhibited by polycrystalline, low melting point fcc metal films is found to be absent in epitaxial cases. The stress magnitude in polycrystalline films is shown to be inversely related to grain size and very weakly dependent on temperature. Densification stresses from abnormal grain growth are found to account for the post-deposition tensile rise. Finally, the compressive stresses observed during the deposition of polycrystalline, high mobility gold films is explained using a simple model of trapping of adatoms as grain boundary interstitials.