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

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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.