Electron energy distributions in inductively coupled plasma: Effect of Coulomb collisions

Abstract

The effects of gas pressure, electron density and coil current on the electron energy distribution functions (EEDFs) in inductively coupled discharges of argon are studied numerically. The EEDF for low gas pressure is close to the Maxwellian, whereas the EEDF for high gas pressure is lower than the Maxwellian in the high energy tail due to inelastic collisions. Since the energy loss due to inelastic collisions is compensated by the energy deposition due to the induced electric field, the EEDF near the quartz wall becomes close to the Maxwellian. However, this EEDF is not the real Maxwellian because the velocity distribution of electrons near the wall is anisotropic in velocity space and hence in nonequilibrium. The factor xi was proposed which represents the magnitude of the effect of Coulomb collisions. The factor is the ratio of electron-electron collision frequency to electron-atom inelastic collision frequency. The effect of Coulomb collisions on the EEDF is negligibly small for xi less than 0.01. The distribution function for the azimuthal component of electron velocity varies significantly as the radial position changes, whereas the distribution functions for the other components do not vary so much.