Alpha- vs. beta-W nanocrystalline thin films: A comprehensive study of sputter parameters and resulting materials' properties

Tungsten thin films have exceptional thermal and mechanical properties, compared to other pure metal coatings, making them indispensable in many contemporary high-tech applications. Their superior properties, however, are strongly dependent on film microstructure, specifically including comprising phases, and hence, the employed deposition parameters, making a thorough process control essential. This study therefore investigates the different material properties of the bcc α and the A15 β phase of tungsten with respect to process parameters. The formation of β-W is mainly dependent on process gas pressure. While all films deposited at elevated pressure consist of β-W, the onset of β formation can already be observed for low pressure deposition, if magnetron power is reduced sufficiently. However, β-W formation for all deposition pressures can be suppressed by applying a substrate bias during deposition, leading to an α-W configuration. This indicates that the alpha-to-beta transition during film growth predominantly depends on adatom energy, rather than solely on the inclusion of residual impurities. Simultaneously, mechanical and electrical performance could be improved, while maintaining a nanocrystalline microstructure.