Low temperature (Ts/Tm < 0.1) epitaxial growth of HfN/MgO(001) via reactive HiPIMS with metal-ion synchronized substrate bias

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 T s due to plasma heating range from 70 to 150 °C, corresponding to T s /T m = 0.10-0.12 (in which T m 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/N 2 and Kr/N 2 , 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/N 2 or Kr/N 2 , exhibit a relatively high mosaicity and a high concentration of trapped inert gas. In distinct contrast, layers grown in Kr/N 2 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 T s = 650 °C (T s /T m = 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 T s = 400 °C.

Texture and Stress Evolution in HfN Films Sputter-Deposited at Oblique Angles

“…Increasing film porosity at higher substrate inclination angles α could be the main contribution to the loss of electrical conductivity in OAD HfN films due electron scattering at the column boundary [37]. At α = 5 • , ρ = 81 µΩ•cm, a value which is consistent with that of 70 µΩ•cm reported for polycrystalline HfN films on glass at 300 • C [38] or epitaxial HfN films on MgO(001) obtained by pulsed-laser deposition (44-75 µΩ•cm) [28] or by high-power impulse magnetron sputtering (70 µΩ•cm) [39]. Note that the resistivity of stoichiometric epitaxial HfN/MgO(001) layer grown at 650 • C is 14.2 µΩ•cm [40].…”

Plasma-Assisted ALD of Highly Conductive HfNx: On the Effect of Energetic Ions on Film Microstructure

“…This major decrease in the ρ e value is primarily attributed to the improvement in the HfN x film microstructure, enabled by the impingement of ions with a larger mass and higher energy. Relevantly, Villamayor et al also recently showed that an increase in the mass of impinging ions by adopting Kr-N 2 plasma, instead of Ar-N 2 plasma, led to an improvement in the crystalline quality of the sputtered HfN x films [20]. The above results can be understood from the fact that the nature of ion-surface interaction depends on the extent of energy and momentum transfer.…”

“…Altogether, these excellent material properties lead to the very low ρ e of the HfN x films grown using Ar-H 2 plasma with energetic ion bombardment. Impinging ions during plasma processes are known to initiate several interactions with the growing films, for example, enhanced ad atom surface diffusion leading to decrease in the defect density; bulk lattice atom displacements resulting in a collision cascade; sputtering and ion-induced damage; ion implantation at higher ion energies [20,[43][44][45][46][47][48][49][50][51]. In regards to the material properties, Hultman et al and Petrov et al showed that energetic ions in a N 2 and Ar-N 2 plasma discharges respectively with |V bias | value in the range of 150 V-250 V can lead to annihilation of defects and reduction in nano-porosity in sputtered TiN films [52,53], and attributed the decrease in defect density to the near-surface and sub-surface diffusion processes [44,45].…”

“…These results demonstrated that the impingement of energetic ions during the film growth can significantly improve the chemical and associated electrical properties of HfN x thin films prepared by ALD. In parallel, Villamayor et al recently showed that an increase in the mass of impinging ions positively affects also the crystallinity of HfN x films grown by PVD, which contributes to the decrease in electrical resistivity [20].…”

“…Typically, the polycrystalline HfN x films prepared by PVD exhibit a resistivity of ca. (1-2)•10 -4 Ωcm for a typical film thickness of ~ 200 nm [17,19,20]. On the other hand, the growth of low resistivity HfN x films by techniques employing a metal-organic precursor, such as chemical vapor deposition (CVD) or atomic layer deposition (ALD), is very challenging [13,14,[21][22][23].…”