Influence of helium metastable states on the E-H mode transition in an inductively coupled plasma

Abstract

A helium (He) inductively coupled plasma (ICP) source combined with two diode laser systems with the wavelengths of 396.5 nm (21S–41P) and 1082.9 nm (23S–23P) was configured to investigate the influence of He metastable states on the E-H mode transition. The population densities of 21S and 23S states were measured by laser absorption spectroscopy as the functions of RF power, He pressure, and the distance from RF antenna. Correlations between the E-H mode transition and the population density ratio of the 23S metastable state to the 21S metastable state as well as the line intensity ratio (LIR) of the 33D-23S transition to the 31P–21S transition were investigated. The E-H mode transition of He ICP was analyzed by the electron impact ionization rate and it was confirmed that the 23S metastable state plays a key role in the E-H mode transition.