Stress, strain, and microstructure of sputter-deposited Mo thin films

Abstract

Mo thin films were deposited on glass substrates using direct-current (dc) planar magnetron sputtering. Mechanical determination of the internal stresses, using the bending-beam technique, yielded typical compressive-to-tensile stress transition curves with increasing working-gas pressure. The microstructure of the compressively stressed films consists of tightly packed columns, whereas in the tensily stressed films the development of a void network structure surrounding the columnar grains is observed. At elevated working-gas pressures the onset of microcolumns is observed in the initial stage of film growth. Determination of lattice strains by x-ray diffraction (XRD), utilizing the sin2 ψ method, encounters more difficulties than the more straightforward stress determination by the bending-beam method. Here special attention is focused on deviations from linear dependence of dψ with sin2 ψ along with asymmetry of XRD line profiles that results from stress-depth profiles as well as lateral stress distributions in the tensily stressed films. These anomalies and the discrepancy between bending-beam stresses and XRD lattice strains, observed for high working-gas pressures, can be interpreted in terms of microstructural features revealed by cross-sectional transmission electron microscopy.