Optical characteristics of sputtered tantalum oxynitride Ta(N,O) films

Tantalum oxynitride films, Ta(N,O), were prepared by reactive RF magnetron sputtering. Investigation was made on optical properties of films deposited at various sputtering parameters such as RF power, substrate temperature and sputtering gas ratios: N2/O2 flow ratio and Ar fraction (Ar/(Ar+N2+O2)). The deposition rates vary from 1.9 to 5.8 nm min−1 and 5.5 to 11.8 nm min−1, respectively, as the Ar fraction increases from 0.5 to 0.91 and N2/O2 flow ratio from 0.25 to 4, respectively. The deposition rates are proportional to the substrate temperature and the RF power from 1.2 to 5.5 nm min−1, too. By using the SEM and SPM analyses, the Ta(N,O) film not only shows a columnar structure but also has a smooth surface with a RMS roughness 0.22 nm for a film as-deposited at 300 °C. The roughness increases to 7.0 nm after annealing at 800 °C for 1 h. The microstructure of Ta(N,O) films closely relates to the introduced N2/O2 flow ratios, and shows significant effects on optical properties. By TEM microstructure analysis, a fine structure, in which nano-particles embedded in an amorphous matrix, was shown. The refractive index (n) decreases from 2.29 to 1.95 in the visible wavelength region when the N2/O2 flow ratio changes from 4 to 1. Furthermore, the refractive index is inversely proportional to the substrate temperature and proportional to the RF power. The reflectance (0.01–0.25) and the transmittance (0.73–0.9) change with sputtering parameters.