Axial diagnosis of radio-frequency capacitively coupled Ar/O2 plasma

Abstract

The capacitively coupled Ar plasma containing oxygen, driven by a radio frequency of 27.12 MHz, is investigated by laser-induced photo-detachment technique assisted with a Langmuir probe. The plasmas with different amounts of oxygen are obtained by changing the flow of Ar and oxygen, each of which is controlled by a mass flow controller. The axial distribution of plasma characteristic can be measured by changing the relative axial position of the Langmuir probe between the parallel electrodes. The electron density and electron temperature are calculated from the current-voltage curve measured by the scanning Langmuir probe, and the electronegativity is obtained from the current curves of the probe with the laser-induced photo-detachment technique. The negative ion density can be calculated from the electron density and the electronegativity. It is shown that with oxygen flow rate increasing, the dissociative attachment of oxygen molecules with electrons will consume the electrons with the middle energy in the electron energy probability function (EEPF) measured by Langmuir probe. The EEPF evolves from Druyvesteyn to Maxwellian distribution due to the thermalization by the e-e interaction with applied power increasing. It is worth mentioning that a depression in the EEPF curve will appear when discharging high-pressure Ar gas containing oxygen. This depression can also be caused by the dissociative attachment of oxygen molecules with electrons where the threshold energy is around 4.5 eV. The axial profile of the electron density is calculated from the EEPF changing from a linear rise in pure Ar plasma to a flater phase of the distribution due to the negative ions such as oxygen introduced into the plasma. The electron temperature changes a little at different axial positions. The rise of negative ion density nearby the sheath of powered electrode is due to the dissociative attachment caused by the collision of oxygen molecules with energetic electrons. In addition, the axial distribution of electronegativity takes on a shape of spoon, which results from the consequence of generation and loss of negative ions in the process of the ambipolar-electric-field-driven diffusion to the plasma center.