Epitaxial and textured TiN thin films grown on MgO(100) by reactive HiPIMS: the impact of charging on epitaxial to textured growth crossover

Ultra-thin TiN films were grown on MgO(100) substrates by high power impulse magnetron sputtering at growth temperatures of 35 degrees C-400 degrees C. Epitaxial TiN films were obtained when a metallic substrate holder is used, while only textured films were achieved for the films grown by HiPIMS using a substrate holder with a ceramic shadow mask. For the entire range of growth temperatures, the epitaxial films are denser and exhibit a higher growth rate compared to the textured films. Also, chemical composition analysis reveals that all the epitaxial TiN films are N-deficient sub-stoichiometric. As the growth temperature increases, the surface roughness of both epitaxial and textured films decreases, however the epitaxial films generally show less roughness than the textured films. In comparison to epitaxial films previously grown by dc magnetron sputtering, the HiPIMS grown epitaxial films are sub-stoichiometric and according to x-ray measurements, they are in a higher strain state whilst presenting higher film density and atomically smoother surfaces. A HiPIMS-induced negative charging of the TiN islands formed on top of the insulating mask is concluded to be the main reason for the growth mode transition from two dimensional Stranski-Krastanov to three dimensional Volmer-Weber when the shadow mask is used. Acceleration of the ionized species of the HiPIMS plasma including Ti+, N+, and Ti2+ toward the substrate when they approach the substrate holder is the reason for the observed changes in the film densities, growth rates, and surface roughness.