Stress and microstructure of sputter-deposited thin films: Molecular dynamics investigations

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Abstract
In magnetron sputter deposition the intrinsic mechanical stress of films of refractory metals passes a tensile stress maximum, decreases and then becomes compressive with decreasing working gas pressure. To elucidate the dependence of internal tensile stress on film growth and microstructure, a two-dimensional molecular dynamics simulation is employed. The evolving structure is determined at an atomic level and the stress is calculated as a function of the kinetic energy of adatoms and fast neutralized inert gas atoms which arrive at the film surface. These energies are dependent on the deposition geometry and the working gas pressure. The theoretical results are in qualitative agreement with experiment and reveal that the initial increase in tensile stress with increasing adatom and neutral incident kinetic energy is caused by a microstructural change from microcolumnar growth to a more densely packed atomic network with closed micropores. Interatomic attractive forces producing tensile stress can act most effectively in such a structure. The subsequent decrease in stress is attributed to a less defect-rich, better ordered absorbate structure formed at higher kinetic energies of arriving species. Stress is found to vary in a similar fashion as a function of incident kinetic energy for both sputtered atoms and fast neutrals. The compressive stress regime has not been investigated.

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