A phase map for sputter deposited niobium oxides

Abstract

A Nb target was sputtered in radio frequency (rf) Ar/O2 and Ne/O2 discharges, and films were grown on unheated fused silica, 〈111〉-cut Si, and carbon ribbon. The transition from Nb metal to niobia film growth was studied as a function of three independent process parameters: cathode voltage, gas O2 content, and rare gas type. On the basis of x-ray diffraction results, resistivity, optical transmission, refractive index, Nb:O ratio, and infrared absorption characteristics, the following phases were identified in the films: bcc Nb, NbO, NbO2, x-niobia, amorphous niobia, microcrystalline niobia, and crystalline niobia (where niobia=Nb2O5−x with 0≤x≤0.2). In situ optical spectrometry of the discharge was used to monitor the emission intensity I(λ) of four radiative electronic transitions of the neutral excited Nb atom to ground state (λ=5344, 5079, 4101, and 4059 Å). Changes in I(λ) and the growth rate was used to (1) determine the set of process parameters at which target surface oxidation occurred, and (2) estimate the fractional flux of atomic Nb and Nb bonded to O in an unspecified molecular form (Nb-oxide) incident on the substrate. Film structure was found to depend to some extent upon this fractional flux. A phase map was constructed in which film structure and the Nb and Nb-oxide fractional flux were graphed onto process parameter space, demonstrating the equivalent effect of different sets of process parameters.