Tailoring Microstructure and Properties of Nitride Films: Manipulating Bombardment via Regulating Me⁺/Me²⁺ Ratios

Film optimization using high power impulse magnetron sputtering (HiPIMS) currently faces challenges in process control, primarily due to its reliance on empirical trial-and-error adjustment of the macroscopic parameters as well as the insufficient understanding of the underlying mechanisms. To address these issues, this study adopts concentration ratios of monovalent ions over divalent ions of the same metallic element (i.e., Me⁺/Me²⁺) in plasma as a function of key controlled discharge parameters. A mass spectrometer was employed for the in situ diagnostics of ionic species in HiPIMS discharges of Cr, Ti, and Al targets. The influence of discharge parameters on Me⁺/Me²⁺ ratios was systematically investigated. Combined with film characterization, the correlations of discharge parameters, ion concentrations, microstructure evolution, and mechanical properties were established. Results demonstrated that Me⁺/Me²⁺ ratios could be tuned significantly by varying discharge parameters. Decreasing the Me⁺/Me²⁺ ratio suppressed growth of columnar grains and promoted film densification due to enhanced high-energy bombardment. This study reveals the dominant role of the charge state distribution of metallic ions in HiPIMS on the microstructure and properties of nitride films, thereby providing a novel approach to deposition-process optimization, which can also be used as guidance for studies on ternary as well as high-entropy nitride films.