Effects of oblique-angle deposition on intrinsic stress evolution during polycrystalline film growth

Precise control of the residual stress in polycrystalline films, which remains a central but difficult task in a variety of emerging technologies, requires synergistic manipulation of multiple processing parameters. In addition to the substrate temperature and the rate of deposition, the angle of incidence of the deposition flux is known to affect structure evolution processes during film formation. However, its role in influencing the evolution of the intrinsic stress has not been determined. In this work, we investigate the effects of oblique-angle deposition on intrinsic stress evolution in polycrystalline gold and nickel films, using in situ stress measurements as well as ex situ surface and microstructure characterization. We find that as the angle between the surface normal and the direction of the deposition flux increases, the thickness at which film coalescence occurs increases and the post-coalescence stress shifts toward the tensile direction. We suggest that the first trend is associated with the effects of shadowing on the nucleation density in the pre-coalescence regime, and we attribute the second trend to an increase in surface roughness and shadowing effects associated with the dome-shaped surfaces of individual grains. According to this view, oblique-angle deposition on a mesoscopically rough surface eliminates or reduces condensation at and near grain boundaries, and therefore lowers the rate of adatom-grain boundary attachment, resulting in reduction in the compressive component of the intrinsic stress. The stress evolution map built from this work suggests routes to achieve specific stress levels in polycrystalline films.