Abstract
Low-temperature epitaxial growth of refractory transition-metal nitride thin films by means of physical vapor deposition has been a recurring theme in advanced thin-film technology for several years. In the present study, 150-nm-thick epitaxial HfN layers are grown on MgO(001) by reactive high-power impulse magnetron sputtering (HiPIMS) with no external substrate heating. Maximum film-growth temperatures Ts due to plasma heating range from 70 to 150 °C, corresponding to Ts/Tm = 0.10–0.12 (in which Tm is the HfN melting point in K). During HiPIMS, gas and sputtered metal-ion fluxes incident at the growing film surface are separated in time due to strong gas rarefaction and the transition to a metal-ion-dominated plasma. In the present experiments, a negative bias of 100 V is applied to the substrate, either continuously during the entire deposition or synchronized with the metal-rich portion of the ion flux. Two different sputtering-gas mixtures, Ar/N2 and Kr/N2, are employed in order to probe effects associated with the noble-gas mass and ionization potential. The combination of x-ray diffraction, high-resolution reciprocal-lattice maps, and high-resolution cross-sectional transmission electron microscopy analyses establishes that all HfN films have a cube-on-cube orientational relationship with the substrate, i.e., [001]HfN||[001]MgO and (100)HfN||(100)MgO. Layers grown with a continuous substrate bias, in either Ar/N2 or Kr/N2, exhibit a relatively high mosaicity and a high concentration of trapped inert gas. In distinct contrast, layers grown in Kr/N2 with the substrate bias synchronized to the metal-ion-rich portion of HiPIMS pulses have much lower mosaicity, no measurable inert-gas incorporation, and a hardness of 25.7 GPa, in good agreement with the results for epitaxial HfN(001) layers grown at Ts = 650 °C (Ts/Tm = 0.26). The room-temperature film resistivity is 70 μΩ cm, which is 3.2–10 times lower than reported values for polycrystalline-HfN layers grown at Ts = 400 °C.
Highlights
Refractory transition-metal (TM) nitride thin films are employed in a wide variety of applications due to their unique combination of properties including high hardness,1–5 scratch and abrasion resistance,6 low coefficient of friction,7 high-temperature oxidation resistance,8–10 and tunable optical,11,12 electrical,12–14 and thermal15 properties
The results demonstrated that synchronizing the substrate bias with the metal-rich portion of high-power impulse magnetron sputtering (HiPIMS) pulses provides film densification, microstructure enhancement, surface smoothening, and decreased film stress with no measurable Ar incorporation
Prior to initiating film-growth experiments, reactive HiPIMS discharges in mixed Ar/N2 and Kr/N2 environments are characterized in order to design substrate-bias strategies as described in Subsections III B and III C
Summary
Refractory transition-metal (TM) nitride thin films are employed in a wide variety of applications due to their unique combination of properties including high hardness, scratch and abrasion resistance, low coefficient of friction, high-temperature oxidation resistance, and tunable optical, electrical, and thermal properties. TM nitrides have gained considerable interest over the past few decades and become technologically important for use as hard wear-resistant coatings, decorative coatings, and diffusion barriers; the latter because of their high thermal stability and low electrical resistivity. a)Current address: Division of Solid-State Electronics, The Ångström Laboratory, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden. Epitaxial TM and rare-earth nitride [TiN, CrN, HfN, CeN, ZrN, and VN (Ref. 11)] layers have been grown on MgO(001) by reactive dc magnetically unbalanced magnetron sputtering (DCMS) in either Ar/N2 or pure N2 environments at elevated temperatures Ts, typically between 600 and 850 °C, using low-energy, high-flux ion irradiation of the film surface during growth. In many TM-nitride applications, including diffusion barriers, there is a strong drive to grow dense high-crystalline quality films at much lower temperatures in order to minimize deposition cycling times and allow the use of thermally sensitive substrates.
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