Experimental electron energy distribution functions in argon, nitrogen and oxygen high-density and low-pressure reflex and microwave plasma sources

Abstract

The Langmuir probe method and Druyvesteyn procedure coupled with suitable software are used to measure the electron energy distribution functions (EEDFs) in atomic and molecular gases. Two complementary high-density and low-temperature plasma sources are used, i.e. a low-pressure direct current reflex discharge plasma source and a low-pressure electron cyclotron resonance (ECR) microwave discharge plasma source. The electron energy is restrained within the range 0–15 eV. The electron density and electron temperature are obtained directly from the EEDF data. The experiment is run within the pressure range 1.0–10−2 Pa with a gas flow rate of 1.0–3.0 sccm. The discharge mechanism and the kinetic and dynamic processes in reflex discharge and ECR plasmas play a key role in determining the shape of the EEDFs. The EEDF is bi-Maxwellian in the reflex discharge plasma and is Maxwellian in the ECR plasma, and is independent of the gas used. The distinctive feature of all the EEPDs in nitrogen and oxygen reflex plasmas and in the nitrogen ECR plasma is the presence of a complex structure independent of the plasma source, discharge mode, and/or radial position. The complex dip/shoulder structures obtained for the EEDFs in molecular plasmas are discussed by considering different inelastic interaction channels between electrons and heavy particles.