Effect of negative substrate bias voltage on the structure and properties of CrN films deposited by modulated pulsed power (MPP) magnetron sputtering

Abstract

As a variation of high power pulsed magnetron sputtering technique, modulated pulsed power (MPP) magnetron sputtering has shown the capability of maintaining a good deposition rate while achieving a high degree of ionization of the sputtered material with low ion energies. It is critical to usefully utilize the negative substrate bias voltage (Vb) to attract these ions towards the substrate to enhance the ion bombardment on growing films by controlling the kinetic energy and the behaviours of ions and electrons arriving on growing films. In this study, CrN thin films have been deposited by MPP in a closed field unbalanced magnetron sputtering system at different Vb varied from 0 to −150 V. The peak and mean substrate ion current densities were measured during the depositions as a function of Vb. The films were annealed at 450 °C in Ar for 1 hr in an effort to release the defects and residual stress in the as-deposited films. The structure and properties of as-deposited and annealed films were characterized by electron probe micro-analysis, x-ray diffraction, scanning electron microscopy, transmission electron microscopy, nanoindentation, and ball-on-disc wear test. An increase in the Cr/N ratio of the film was observed as the Vb was increased negatively to above −70 V, which resulted in the formation of the hexagonal Cr2N film at Vb = −150 V. A preferred (3 1 1) texture was observed in the CrN films deposited as Vb increased from −50 V to −100 V. The residual stress of the films increased as the Vb was increased from 0 to −100 V and then decreased with further increasing the Vb. The increase in the Vb led to grain refinement and an increase in the hardness of the films, but the wear resistance of the films decreased rapidly as the Vb was increased to −150 V.