Polycrystalline films grown by PVD techniques exhibit a variety of microstructures with respect to the size, the morphology, and the relative orientation of the crystallites.
These features have, for instance, implications for the mechanical strength and the electrical conductivity of the film. The microstructure is determined primarily by surface and bulk diffusion processes, which are controlled by the deposition temperature, the bombardment by energetic species, and the incorporation of impurities that act as inhibitors for the crystal and grain growth. Deposition at relatively low temperatures (typically lower than 0.4Tm, where Tm is the melting temperature of the deposited material) using state-of-the-art sputtering techniques allows only for surface diffusion to be activated leading to the formation of films with a columnar microstructure and intercolumnar porosity [see Fig. 10(a) for CrN films].
Growth of films using HiPIMS is characterized by high ionic fluxes to the substrate (up to several hundreds of milliamperes per square centimeter) of relatively low energies (several tens of electronvolts), as was discussed in Section II B. These growth conditions trigger and/or enhance surface diffusion leading to film densification
as shown in Fig. 10(b). Upon increasing the flux of ions available at the substrate (achieved, e.g., by increasing the peak target current), repeated nucleation occurs
resulting in suppression of the columnar structure and transition from a dense polycrystalline to a globular nanocrystalline microstructure,as shown in Figs. 10(c)–10(d). When high energetic fluxes are not available during growth,globular microstructures are consequence of segregation of impurity phases, which hinder the crystal and grain growth. It is, therefore, evident that the low-energy high-
flux ion irradiation during HiPIMS can be used to overcome the characteristically underdense and rough microstructures and obtain morphologies unique for
low-temperature sputter deposition. This, in turn,allows growth of films with higher hardness,lower friction coefficient,and improved scratch and wear as well as corrosion resistance,as compared to films deposited by DCMS.
The highly ionized fluxes available in HiPIMS in combination with the use of a substrate bias result in ion energies in the order of several hundreds to thousands of electronvolts, which can be used to engineer the film–substrate interface and enhance the film adhesion. Typical example is a CrN/NbN film deposited on steel substrates etched using HiPIMS (pretreatment by Nb+ ions at a substrate bias voltage of 1000 V) exhibited a scratch test critical load (Lc) of 56 N, which was higher than the values of 25 N obtained for films deposited on substrates etched by a DCMS plasma (substrate bias voltage of 1000 V at an Ar pressure of 0.8 Pa),which exhibit a significantly lower ionization degree for both Ar+ and metal ions. It should also be pointed out here that the Lc values achieved on HiPIMS etched substrates are comparable to those for films grown on substrates cleaned using high fluxes of Ar+ ions generated by an external ionization source. This fact indicates that HiPIMS can be used as an alternative process to improve the quality of the filmsubstrate interface and enhance the film adhesion, when
no external source for increasing the Ar ionization is available.