Characterization of indium-tin oxide sputtering targets showing various densities of nodule formation

Eight Sn-doped In₂O₃ (ITO) target materials for direct-current (DC) magnetron sputtering, showing various rates of nodule formation, were subjected to analyses of lattice parameter, relative density, electrical properties, optical absorption spectra in 0.5-5 eV and oxygen deficiency, δ, in (In_0.905Sn_0.095)₂O _3+x-δ. The δ value was evaluated by heating the sample powders in a closed-system oxygen analyzer involving a solid electrolyte and analyzing their oxygen uptake rate from the Ar + O₂ circulating gas. The oxygen uptake by the powders started at approximately 623 K, and the successive oxygen release took place at higher temperatures than 823 K. For each sample, the lattice parameter monotonically decreased with the amount of oxygen absorbed, much like energy absorption maximum at approximately 1 eV. Among all the samples, energy absorption maximum at approximately 1 eV decreased linearly with decreasing conduction electron density, n. The correlations of δ with conduction electron density and relative density were not good, compared with that with lattice parameter. These results have implied that, for the samples with relative density higher than 99.5%, locally inhomogeneous conduction electron density, due to local deviation from the uniform distributions of tin ion and oxygen vacancy, increased the rate of nodule formation. A convenient manner for detecting the target quality has been suggested based on the experimental results.