Abstract
Optical emission spectroscopy has been widely used in low-temperature argon plasma diagnostics. A coronal model is usually used to analyze the measured line ratios for diagnostics with a single temperature and density. However, many plasma processing conditions deviate from single temperature and density, optically thin conditions, or even coronal plasma conditions due to cascades from high-lying states. In this paper, we present a collisional-radiative model to investigate the validity of coronal approximations over a range of plasma conditions of Te = 1–4 eV and Ne = 108–1013 cm−3. The commonly used line ratios are found to change from a coronal limit where they are independent of Ne to a collisional-radiative regime where they are not. The effects of multiple-temperature plasma, radiation trapping, wall neutralization, and quenching on the line ratios are investigated to identify the plasma conditions under which these effects are significant. This study demonstrates the importance of the completeness of atomic datasets in applying a collisional-radiative model to low-temperature plasma diagnostics.
Highlights
Published: 24 November 2021Argon plasmas are widely used for plasma processing applications, and the information on the thermodynamic properties of plasma, such as electron temperature, density, and electron energy distribution function, plays an important role in the control and performance of plasma applications [1,2,3].Plasma spectroscopy is a non-intrusive diagnostic technique that provides information on the thermodynamic properties of plasma and the atomic-level population distributions and radiative properties
multiconfiguration Dirac–Hartree–Fock (MCDHF)-GF/Los Alamos National Laboratory (LANL)-GF/flexible atomic code (FAC)-GF refers to the MCDHF/LANL/FAC oscillator strengths to be used in addition to the NIST oscillator strengths
A CR model employing a complete set of atomic data is constructed to verify the coronal approximation for the optical emission spectroscopy (OES) analysis in plasma processing
Summary
Argon plasmas are widely used for plasma processing applications, and the information on the thermodynamic properties of plasma, such as electron temperature, density, and electron energy distribution function, plays an important role in the control and performance of plasma applications [1,2,3]. Plasma spectroscopy is a non-intrusive diagnostic technique that provides information on the thermodynamic properties of plasma and the atomic-level population distributions and radiative properties. Argon optical emission spectroscopy (OES) has been used to obtain electron temperature and density information from the measured line ratios, and more recently, the shape of the electron energy distribution functions [4,5,6,7]. A population kinetics model should be built to couple the atomic-level population distributions and radiative properties with the plasma thermodynamic properties. For low-electron-density plasmas, a coronal model has been generally adopted to interpret the spectral line intensity distribution [1,2,3], where an excited atomic state emitting a line of interest is assumed to be populated from the ground state or a metastable state by collisional excitation and depopulated by spontaneous emission to the lower states.
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