A charge-state resolved optical emission spectroscopy method applicable to vacuum-sputtered and ablated metal-containing plasma to determine the time-resolved monovalent and divalant titanium ion number densities

Metal plasma has been widely applied in hard coatings or metallization of vias and trenches in semiconductor device fabrication. Ion irradiation plays a vital role in the film properties. Previous methods have focused on the Ti atom and Ti⁺ion number densities; however, there is a lack of a practical optical emission spectroscopy method for measuring the Ti²⁺ion number density. High-charge-state ions lead to high compressive stress, especially in high-power impulse magnetron sputtering (HiPIMS) and vacuum arc plasma devices with a high ionization fraction. In this work, we present a novel charge-state resolved OES method to obtain the time-resolved Ti⁺and Ti²⁺ion number densities. This method is based on the excited-state cycle mechanisms of Ti⁺(4p) and Ti²⁺(4p) emitting states, as determined by kinetic investigations using a collisional-radiative model. In the excited-state cycle mechanisms, the Ti⁺/Ti²⁺line-ratio is found to be sensitive to the ion ratio, and the Ti²⁺line-ratio is sensitive to the electron density. The latter can decouple the influence of the electron density on the Ti⁺/Ti²⁺line ratio, allowing the Ti⁺and Ti²⁺ion number densities to be determined by combining the above line ratios. This method is verified in a vacuum arc titanium metal plasma source.