Dependence of film structure on the film structure-independent equivalent film thickness in magnetron sputtering deposition of Ag thin films

In this work, I have investigated the structures and properties of Ag thin films deposited by magnetron sputtering onto glass substrates with temperatures of 150 and 600 °C for film structure-independent equivalent film thicknesses in the range of 20–400 nm. The Ag thin film morphologies observed using scanning electron microscopy and atomic force microscopy showed the following distinguishable changes: an Ag thin film with an equivalent film thickness of 20 nm deposited at a substrate temperature of 150 °C displayed a film microstructure of oblate grains separated by voids, while those with equivalent film thicknesses of 50 nm or more displayed microstructures consisting of flat-topped grains without any obvious voids between them. In comparison, an Ag thin film with an equivalent film thickness of 20 nm deposited at a substrate temperature of 600 °C displayed a microstructure consisting of isolated spherically shaped grains with a uniform diameter of approximately 40 nm and spaced at uniform intervals; an Ag thin film with an equivalent film thickness of 50 nm displayed a microstructure of more oblate grains; Ag thin films with equivalent film thicknesses of 100 and 200 nm displayed microstructures of highly isolated, flat-topped, mound-shaped grains; and an Ag thin film with an equivalent film thickness of 400 nm displayed a microstructure of continuous flat-topped, mound-shaped grains. In addition, the Ag thin films with equivalent film thicknesses of 20 and 50 nm deposited at 600 °C exhibited higher compressive stresses. The quantitative results of optical-transmittance and electrical resistivity measurements were consistent with the changes in thin film morphology. The morphological structures of the Ag thin films deposited at 600 °C result from the high surface diffusivity of the Ag atoms, which do not wet the glass substrate, whereas the morphologies of the Ag thin films deposited at 150 °C result from in-place grain growth following the formation of multiple nuclei because of the low surface diffusivity of the Ag atoms at this temperature. The observed thin film microstructures are unexplained by the classical structure model for sputter-deposited metal thin films, which does not consider either the high surface diffusivity of adatoms that do not wet the substrate or the increase in surface area required to dissipate the energy accumulating in grains during film deposition. The results obtained in this study provide a fundamental description and explanation of the grain structure of metal thin films with thicknesses of a few tens of nanometers or less.