Effect of asynchronous pulsing parameters on the structure and properties of CrAlN films deposited by pulsed closed field unbalanced magnetron sputtering (P-CFUBMS)

Controlled ion bombardment of growing thin films can be used to modify and improve the film structure and properties. Recently, higher energetic species (up to hundreds eV) were found in the plasma by pulsing the target(s) in magnetron sputtering. In this study, an electrostatic quadrupole plasma mass spectrometer (EQP) has been used in a pulsed closed unbalanced magnetron sputtering (P-CFUBMS) system to investigate the effect of different pulsing parameters (frequency and reverse time) on the ion energies and ion fluxes in the intrinsic plasma during Cr-Al-N film deposition. It is confirmed that pulsing both magnetrons in this P-CFUBMS configuration had a large effect on both the ion energies and ion fluxes generated within the plasma, which are shown to be strongly dependent on pulsing frequency and duty cycle. The effect of pulsing to provide a wide range of ion energies and ion fluxes on the film microstructure, mechanical and tribological properties was investigated using nanoindentation, microtribometry, X-ray diffraction (XRD), atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM) and scanning transmission electron microscopy (TEM). In the current study, by taking -50 V substrate bias into consideration, it was found that total ion energies with controlled pulsing parameters to achieve moderate values (70-120 eV) can effectively increase the density and decrease the grain size of Cr-Al-N films. On the other hand, pulsing regimes that produce excessive total ion energy (∼ 200 eV) result in an increase in the residual strain, and point and lattice defects in the film, which will significantly decrease the toughness and tribological properties of the film. Under optimum pulsing conditions (100 kHz and 5.0 μs), Cr-Al-N films with a dense nanostructure (column grain size of 10-40 nm) of super hardness and good wear resistance (41 GPa, 0.099 H/E ratio, 0.46 COF, and a wear rate of 3.4 × 10^{- 6} mm³N^{- 1} m^{- 1}) have been deposited using a controlled maximum ion energy bombardment of 122 eV at high ion flux.