Investigating the spatial distribution of plasma species using cavity ringdown spectroscopy and simulations

Advances in semiconductor etching depend on reliable plasma modelling and quantitative knowledge of near-surface behaviour of key reactive species. In this research, cavity ring down spectroscopy is utilised to determine number densities and gas temperatures of key species in technologically relevant low pressure ICPs of pure N₂, O₂ and Ar. Examples include N₂⁺(X) and N₂(A), O(³P) and O^-, and Ar⁺(⁴P) and Ar metastable and radiative states. Studies on all three gases are extended into the plasma-surface boundary layers to quantify absolute number densities as a function of height above a non-driven electrode. A clear decrease in absolute ion density is observed as the electrode is approached for N₂⁺ and O^-. A more gradual decrease is observed for the N₂(A) metastable species. In contrast, a modest increase is observed for O(³P). Lifetimes obtained for the neutral N₂(A) and O(³P) species in the plasma bulk are contrasted with near-surface lifetimes at a series of pressures. The rotational and vibrational temperatures of N₂(A) have also been investigated through a broadband cavity enhanced spectroscopic study at a series of pressures and powers, including within the capacitive regime. 2D-axisymmetric simulations are performed for N₂, O₂ and Ar, and compared with data in order to enhance reactive chemistry sets and refine simulation parameters.