Production of metastable-state argon ions in an electron cyclotron resonance plasma investigated by laser-induced fluorescence spectroscopy

A laser-induced fluorescence spectroscopy (LIF) is a measurement technique of particle density and movement without disturbance to target fluids. For the LIF of ions generated in low-pressure argon (Ar) plasmas, Ar ions in a metastable excited state (Ar^+m) are generally target species, due to availability of excitation laser wavelengths. Therefore, careful consideration on production of Ar^+m in tested plasma source is necessary for appropriate plasma characterization. In this paper, we measured behaviors of Ar^+m in an electron cyclotron resonance (ECR) plasma source by the LIF method. Density and energy distribution of electrons are also investigated by a Langmuir probing method. The measured radial distribution of the plasma parameters indicated that the Ar^+m relative density (LIF signal intensity) has different distribution from the average-energy electrons and the ground state Ar ion densities. The Ar^+m relative density and the number of high-energy electrons over 15 eV showed similar pressure dependence and radial distribution. These LIF and probing results in ECR plasma suggests that presence of high-energy electrons is a key for the Ar^+m production.