Spatial structures of different particles in helicon plasma

Abstract

The spatial density structures of different particles (high-energy electron excited ionic and low-energy electron excited neutral particles) in both discharge and plume plasmas of a helicon source were characterized by an optical emission spectroscope (OES) and a Langmuir probe. Filters of 480 nm band pass and 600 nm high pass were used to distinguish the ionic and the excited neutral particles, respectively. The ion energy distributions at the outlet of the discharge tube with different magnetic field were obtained by a four-grid retarding field energy analyzer (RFEA). Results show that as RF input power increased, the helicon discharge modes change from a capacitive (E mode) to an inductive (H mode) to a wave coupling or a helicon discharge (W mode). After reaching the W mode, neutral particles are basically saturated, but ions will experience another growth as the power increases. Moreover, the reversed applied magnetic field can change the axial distribution of ion density (ionization region). The IEDF test results show that the maximum (most probable) ion energy increases with increasing input power. Meanwhile, the reversed magnetic field (+ 50 A) can increase the maximum ion energy by about 15 eV, which is believed to be the ionization/acceleration zone is close to the ion energy test point. Therefore, the directed ion energy is more correlated with the ion density distribution excited by high-energy electrons.