Structural modifications of hafnium oxide films prepared by ion beam assisted deposition under high energy oxygen irradiation

This work deals with high-energy ion beam assisted deposition (IBAD) of HfO2 on Si(100) substrates. Hafnium vapor was generated from a metallic hafnium target with simultaneous bombardment with oxygen ions accelerated at energies of 1–20 keV, a much higher energy regime than in other IBAD works. The transport ratio (TR), defined as the ratio between the hafnium arrival rate and the oxygen ion dose, was in the range of 0.5–10. The substrate was not heated during deposition. The films’ structure and properties were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron probe micro-analysis (EPMA), and Knoop microhardness. Significant structural modifications were observed with parameter variation. Films consisting of tetragonal and cubic structure or mixtures of these with the monoclinic phase were obtained. To the authors’ knowledge this is the first report of non-monoclinic phases observed in IBAD films. Measurements of film stoichiometry show that the films are oxygen deficient; the Hf/O ratio appears to be approximately 1:1.5 despite variations in TR. At 20 keV acceleration energy the microhardness increased linearly with TR, reaching a maximum of 25 GPa at a TR value of 10. The high hardness is associated with the new tetragonal phase.