Influence of plasma radial nonuniformities on the properties of RF magnetron sputtered indium tin oxide (ITO) films

This study investigates the effects of radial nonuniformities in plasma parameters on the microstructural and optoelectronic properties of indium tin oxide (ITO) films deposited by radio-frequency (RF) magnetron sputtering. The diagnostic results showed that both the ion energy and ion flux incident on the substrate surface peak at the center of the substrate holder, with maximum values of approximately 32 eV and 4.1 A·m⁻², and decrease significantly toward the edges to 26 eV and 1.4 A·m⁻², respectively. Quantitative analysis reveals that the plasma is overwhelmingly dominated by Ar⁺ ions, while the ionization degree of sputtered indium atoms is extremely low (e.g., ~0.05% at the center). At such a low level, variations in the metal ionization fraction exert negligible influence on ITO film properties. Under an unbalanced magnetic field configuration, the electron temperature exhibits a radial gradient—lower in the center and accompanied by the emergence of both hot and cold electron populations near the periphery. These radial variations in plasma behavior directly affect the growth dynamics and properties of ITO films. Consequently, films deposited at central positions demonstrate superior performance, achieving a high optical transmittance exceeding 90% in the visible range and a low sheet resistance of 47.2 Ω/□, compared to those at the edge which exhibit a transmittance below 80% and a higher sheet resistance of 68.1 Ω/□. The findings conclusively identify the spatial distribution of Ar⁺ ion energy and flux as the primary factor governing surface activation and adatom mobility, thereby enabling the growth of high-performance ITO films.