Microstructure development of HfO_x thin films

Abstract

Morphological and crystallographic characteristics of HfO_x with different oxygen concentrations (0 ≤ x ≤ 2.5) and thicknesses (18 nm ≤ t ≤ 310 nm) were analyzed in this work before and after thermal annealing at 700^°C in N₂ atmosphere for 1h. The morphology of the as-deposited, low oxygen concentration films (t = 100 nm) is formed by a well-defined granular structure with grains around 20 nm in diameter. For higher oxygen concentration, the roughness increases, as a consequence of a very porous surface morphology. At the same time, the crystallographic structure changes from HCP with a {0002} preferred orientation to an amorphous structure as oxygen concentration increases. As the thickness of the HfO₂ films increases, we also observed the formation of a high surface porosity, with pore diameter ranging from 80 nm to 120 nm. The changes observed in morphology and crystallinity of the films, as we increase the concentration of contaminants, occurs because of the lowering of the adatoms surface diffusion, which prevents them from reaching lower energy points during the formation of the microstructure of the films. Furthermore, this lowered surface diffusion favors the process of grain renucleation, which leads to roughness increasing, and to an enhancement of the formation of porous surfaces. After annealing, all films exhibit the monoclinic crystallographic phase associated with HfO₂. Surface morphology of the films is consistent with a polycrystalline structure with grain diameters varying between 10 nm and 200 nm. As their size increases, the grains become very faceted, a finding consistent with the improvement in film crystallinity. Our results suggest that long-time annealing promotes the diffusion of oxygen from the SiO₂-Hf interface to the film, compensating any O₂ deficit in the film. Formation of the monoclinic phase is also favored by the improvement in the film stoichiometry promoted by thermal treatment. Also, the critical size of the nuclei, associated with a grain growth in a particular crystallographic orientation, decreases as a consequence of the high temperature during thermal annealing. Therefore, formation of faceted grains and increasing of surface roughness are favored in thicker films.