Abstract
Lowering energy consumption during thin film growth made by magnetron sputtering techniques becomes of particular importance in view of sustainable development goals. As a large fraction of the process energy is consumed for substrate heating with the purpose of providing sufficient adatom mobility to grow dense films, the most straightforward strategy towards more environment-friendly processing is to find alternatives to the thermally activated surface diffusion. A promising route is offered by high-mass metal ion irradiation of the growing film surface, which we show is very effective in densification of transition metal nitride layers grown with no external heating, such that Zone 2 microstructures of the structure-zone model are obtained in the substrate temperature Ts range otherwise typical for Zone 1 growth. The practical implementation of this technique relies on heavy metal targets operated in high power impulse magnetron sputtering (HiPIMS) mode to provide periodic metal-ion fluxes which are accelerated in the electric field of the substrate to irradiate layers deposited from direct current magnetron sputtering (DCMS) sources. A key feature of this hybrid HiPIMS/DCMS configuration is the substrate bias which is synchronized with the heavy metal ion fluxes for selective control of their energy and momentum. Time-resolved ion mass spectrometry analyses provide information about the time evolution of ion fluxes incident at the growing film surface which is a crucial input for film growth experiments. The major fraction of process energy is used at the sputtering sources and for film densification, rather than for heating of the entire vacuum vessel. For the model materials system of TiAlN the process energy consumption can be reduced by as much as 64% with respect to conventional methods, with no compromise on coating quality. Model materials systems include TiN and metastable NaCl-structure Ti1-xAlxN films, which are well-known for challenges in stoichiometry and phase stability control, respectively, and are of high relevance for industrial applications. A strong dependence of Ti0.50Al0.50N-based film properties on the mass of the HiPIMS-generated metal ions is found where layers deposited with Cr+ irradiation exhibit porous nanostructure, high oxygen content, and poor mechanical properties. In contrast, W+ irradiation gives films that are fully-dense even with the W fraction on the metal lattice as low as 0.09. Importantly, the precipitation of softer wurtzite AlN phase is avoided hence layers with high hardness and low residual stress can be obtained at low substrate temperatures. The high effectiveness of heavy ions in substituting for the thermally-activated mobility is explained by the large mass difference between the incident ion and the atoms constituting the film, which results in effective creation of low energy recoils, leading to film densification at low Ts. Due to their high mass, metal ions become incorporated at lattice sites beyond the near-surface region of intense recoil generation leading to further densification, while preventing the buildup of residual stress. The critical parameter that controls the growth is shown to be the average momentum transfer per deposited metal adatom. Studies of high-temperature properties of TiAlWN films grown by hybrid W-HiPIMS/TiAl-DCMS co-sputtering led to the discovery of a new age hardening mechanism, which is based on the formation of Guinier-Preston (GP) zones.
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