Influence of driving frequency on the metastable atoms and electron energy distribution function in a capacitively coupled argon discharge

TL;DR

This study uses particle-in-cell simulations to explore how varying driving frequency affects metastable atoms and electron energy distribution in capacitively coupled argon plasmas, revealing frequency-dependent plasma behaviors and EEDF shapes.

Contribution

It introduces a detailed simulation analysis of metastable levels and their impact on plasma properties across a wide frequency range, highlighting new insights into plasma dynamics.

Findings

Plasma density decreases with metastable inclusion, more at higher frequencies.

Multistep ionization contribution increases with excitation frequency.

Electron temperature rises with metastables and drops as frequency increases.

Abstract

One-dimensional particle-in-cell simulation is used to simulate the capacitively coupled argon plasma for a range of driving frequency from 13.56 MHz to 100 MHz. The argon chemistry set can, selectively, include two metastable levels enabling multi-step ionization and metastable pooling. The results show that the plasma density decreases when metastable atoms are included with higher discrepancy at higher excitation frequency. The contribution of multistep ionization to overall density increases with excitation frequency. The electron temperature increases with the inclusion of metastable atoms and decreases with excitation frequency. At lower excitation frequency, the density of Ar** (3p5 4p, 13.1 eV) is higher than Ar* (3p5 4s, 11.6 eV), whereas, at higher excitation frequencies the Ar* (3p5 4s, 11.6 eV) is the dominant metastable atom. The metastable and electron temperature profile evolve from a parabolic profile at lower excitation frequency to a saddle type profile at higher excitation frequency. With metastable, the electron energy distribution function (EEDF) changes its shape from Druyvesteyn type, at low excitation frequency, to bi-Maxwellian, at high frequency plasma excitation, however a three-temperature EEDF is observed without metastable atoms.