Evident thickness effect on structure and mechanical properties of molybdenum films

Abstract

Magnetron sputtering was used to prepare nanocrystalline Molybdenum (Mo) films with a thickness (h) ranging from 200 to 1000 nm. Transmission Electron Microscope, Scanning Electron Microscope, X-Ray Diffraction, and Atomic Force Microscope were used to examine the micro-structure of as-deposited films. Using nanoindentation, the mechanical properties of Mo films were investigated. The micro-structure and mechanical properties were found to be dependent on h. With increasing h, the tensile residual stress increased from 88 to 435 MPa; the average roughness increased from 1.87 to 2.93 nm; the critical shear stress (τc) to initiate plastic deformation ranged between 10.8 and 12.3 GPa; and the hardness decreased from 6.70 to 4.72 GPa, as described by the Hall-Petch equation. The mechanism for plastic deformation was hypothesized to be screw dislocation nucleation and motion. Using Hertz contact theory, the theoretical τc matched the experimental results quite well. With decreasing h, the blocking effect of the grain boundary on dislocation motion was deemed to be the strengthening mechanism.