Transient evolution of electron energy distribution function during microwave plasma breakdown for material processing

摘要

The dynamics of plasma-surface interaction, often utilized for various material processing and nanoparticle synthesis, depends on electron energy distribution function (EEDF) of plasma. While most of the applications employ non-thermal plasmas at low pressure employing magnetic field, present work explores dynamics of plasma EEDF at atmospheric pressure (760 torr) where magnetic field is no longer required. Time evolution of EEDF during initial stage of microwave (2.45 GHz) plasma formation along a small capillary tube (8 mm diameter and 5 cm length) filled with a noble gas (Ar, He or Ne) is investigated using Monte Carlo simulation. The result shows that the plasma initially evolves with transient phase where electron density and its energy keeps on increasing with time, followed by the equilibrium phase at t ～ 200 ns where EEDF relaxation is observed. The EEDF of Ar and Ne plasma indicates higher ionization and higher energy electrons than He plasma and hence can be relied for use in high-temperature treatment of surfaces. There are some applications where weakly ionized plasmas are proved to show better results and hence He plasmas should be preferred. Due to lower ion mobility of ions, Ar and Ne plasmas are also preferable for smaller size nanoparticle formation.