Experimental and numerical investigations of electron characteristics in 2 MHz and 13.56 MHz inductively coupled hydrogen plasmas with an expansion region

Abstract

The electron characteristics are investigated in 2 MHz and 13.56 MHz inductively coupled hydrogen discharges with an expansion region. The influence of the gas pressure and radio-frequency of the power source on the electron energy probability functions (EEPFs), electron density, and electron temperature at high deposition power has been presented. The measured EEPFs in the driver region of the discharge evolve from a three-temperature Maxwellian distribution to a Maxwellian distribution as the pressure increases. Different characteristic frequencies calculated based on the measured plasma parameters show that stochastic heating of electrons dominates at pressures lower than 0.3 Pa and it has to be considered for pressures lower than 1.0 Pa, while Ohmic heating dominates at higher pressure. Furthermore, the EEPFs as a function of the total energy evolve from the identical shape to discrete shapes with axial position and pressure, indicating a transition of electron kinetics from nonlocal to local regimes. This can be explained by the calculated electron energy relaxation length. In order to verify the experimental results, COMSOL Multiphysics is used to calculate the electron density and electron temperature at different pressures and frequencies of power source. The simulated axial distributions of the plasma parameters agree well with the measured results at 5.0 Pa, while the calculated electron density is lower and the calculated electron temperature is higher at 1.0 Pa. In addition, there is no frequency dependence of axial resolved EEPFs, electron density, and electron temperature in high power deposition discharges (1.5 kW).