The properties of aluminum oxide and nitride films prepared by d.c. sputter-deposition from metallic targets

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Thin films of aluminum nitride and aluminum oxide were prepared at 540 K substrate temperature by d.c. magnetron sputtering from a metallic Al-target in pure nitrogen and oxygen atmosphere, respectively. Characterization of the films was performed by X-ray diffraction, X-ray reflection and by optical transmission spectroscopy. The stoichiometric composition of the films was ascertained by X-ray photoelectron spectroscopy. As a function of reactive gas pressure, the deposition rate showed, in general, a logarithmic decrease with increasing pressure. Oxide films were deposited at about a fourth the rate of nitride films as consequence of the lower sputtering yield. For the oxide films, specific gravity and static refractive index showed weak changes with pressure and amounted to 2.7 g/cm3, which is only approximately 68% of the bulk value, and 1.65, respectively. In contrast, low-pressure sputter-deposition of nitride films resulted in high-density films of approximately 97% the bulk value and a refractive index close to 2.1. Nitride films show a strong (002) texture (c-axis orientation) with large nanograins up to 90-nm length. Oxide films are in general amorphous. The very different structure of nitride and oxide films is related to the more complicated crystal lattice of the oxide and to higher-atomic Al–O complexes ejected from the target. It is concluded that stronger thermalization of these species compared to those ejected by nitride sputtering is responsible for insufficient enhancement of the mobility of the ad-atoms.