Axial diagnosis of electron and negative ion behaviors in capacitively coupled O2-containing Ar plasma driven by 27.12 MHz

Abstract

Capacitively coupled O2-containing Ar plasma driven by a radio frequency (RF) of 27.12 MHz has been investigated. The electron energy probability function (EEPF) was measured with a Langmuir probe. The electronegativity was measured with a laser-induced photodetachment (LIPD) technique in combination with a Langmuir probe. The probe measurement results show a transition of the EEPF from bi-Maxwellian to single-Maxwellian and finally to a Druyvesteyn distribution as RF input power or discharge pressure was increased. This transition indicates the evolution of the heating mode in the Ar plasma by changing the discharge conditions. Adding electronegative O2 gas into Ar plasma leads to the deviation of the EEPF from the pure Ar plasma case. This deviation becomes more serious at high pressure due to the inelastic collisions of electrons with oxygen molecules. Additionally, the addition of O2 not only lowers the electron density in the axial direction but also smoothens the electron density distribution close to the powered electrode in comparison to the linear electron density with the axis in the Ar plasma case. LIPD measurement results show that electronegativity in 5% O2-containing Ar plasma tends to be high as close to the powered electrode and to be a V-shaped distribution along the axis direction with the increase in the pressure. This behavior of the negative ion distribution may be caused by the combined effects of recombination of negative and positive ions and the pseudo-γ mode of negative ions with oxygen neutrals.