Spectroscopic Ellipsometry Characterization of ITO Films Prepared by Reactive MF Dual Magnetron Sputtering: Effect of Substrate Temperature

The reactive middle frequency (MF) dual magnetron sputtering is a favorable process for industrial applications. Production of low resistivity (~10^-4 Ohm*cm) ITO films by this method requires their heat treatment. The ITO film formation and evolution at elevated substrate temperatures is not properly addressed because the phase diagram of this material is not known. Thus, in situ spectroscopic ellipsometry (SE) was applied to characterize the growth of ITO films at heated substrates (Ts=RT-510 °C). The results of in situ and ex situ SE were compared with the film resistivity, stoichiometry (elastic recoil detection analysis) and structure (X-ray diffraction).
The in situ SE indicates formation of the rough film with graded optical properties. This grading is stronger for the films deposited without substrate heating. The free electron parameters were determined from parameterization of the film optical constants in Drude-Lorentz approach. Their behavior with increasing Ts agrees with concomitant resistivity dependence, measured by the four point probe method. Thus, in situ SE monitoring of the free electron parameters provides a tool for real-time contactless characterization of the growing film resistivity. Quantitative characterization of the resistivity by SE requires further improvement of the optical model. The existence of the resistivity grading through the film thickness was indicated by this method for the growth without heating. At heated substrates this grading vanishes after deposition of ~50 nm layer and homogeneous film start growing. The films grown at the Ts=400 C have resistivity of 1.2*10^-4 Ohm*cm that is comparable with the best results achieved with ceramic targets. The SE data analysis demonstrated that improvement of the film resistivity with increasing Ts is mainly due to enhancement of the free electron mobility. It is accompanied by improvement of the film stoichiometry, changes of texture and crystallinity.