Stress, strain, and microstructure in thin tungsten films deposited by dc magnetron sputtering

Abstract

Tungsten thin films were deposited on glass substrates by direct-current planar magnetron sputtering. The induced thickness-averaged film stress within the plane of the film was determined with the bending-beam technique and changed from compressive to tensile on increasing working-gas pressure. The microstructure of these films was investigated by cross-sectional transmission electron microscopy. Compressively stressed films consisted of tightly packed columnar grains, whereas in films with a maximum value for the tensile stress the onset of a void network surrounding the columnar grains was observed. High-pressure conditions resulted in dendritic-like film growth, which brought about complete relaxation of internal stresses. The α phase was predominantly found in films under compression, while an increasing amount of β-W coincided with the transition to the tensile stress regime. Special attention was focused on stress-depth dependence and the development of two overlapping line profiles in x-ray diffraction (XRD) diagrams with film thickness as observed in compressively stressed films. Both findings constitute a remarkable result in respect of stress-depth distributions in thin films: the presence of two sublayers in a monophase film, one experiencing tensile and the other compressive stress. The occurrence of a modest tensile stress maximum present in the substrate-adjacent part of the film was explained by an elastically accommodated volume reduction, associated with a phase transformation (β into α) of initially formed β-W. Furthermore, a comparison of bending-beam stresses and XRD lattice strains (utilizing the sin2 ψ method) provided a consistent view of the mechanical behavior of the differently strained sublayers in this monophase (α-W) film.