Characterization of O2/Ar inductively coupled plasma studied by using a Langmuir probe and global model

Abstract

An O2/Ar inductively coupled plasma is investigated by a Langmuir probe and a global model (volume averaged model). The electron density, electron temperature and electron energy distribution function (EEDF) are measured at different O2 contents, gas pressures and applied powers. At fixed pressure and power, the electron density first drops quickly with the O2 ratio and then tends to saturate in the high O2 ratio range. The effective electron temperature exhibits completely opposite behaviors at low and high pressures. This is caused by the different evolving behaviors of low and high energy electrons of the EEDFs with the O2 ratio. Both the Langmuir probe and the global model predict that the electron density of O2/Ar mixed plasma first increases, peaks and then drops constantly, upon increasing the pressure. An analysis based on the simulation reveals that the non-monotonic variation of electron density with the pressure is due to the non-monotonic variation of the ionizations from both ground state O and metastable O*. Due to the strong ionizations, the electron density increases linearly with the power. The effective electron temperature is unchanged because the EEDF shape that determines the electron temperature is not varied upon increasing the power. The calculated electron density and temperature when varying the power agree better with the experiments at high pressure, i.e. 45 mTorr. The quantitative deviation between the model and the experiment when varying the pressure and the O2 ratio can be explained by two aspects. (1) The electron energy probability function is assumed to have a Maxwellian distribution in the global model while the realistic EEDFs vary significantly with the pressure and/or the O2 ratio, as revealed by the experiment. (2) The power transfer efficiency (i.e. the fraction of the power coupled into plasma) increases with the pressure.