Diode laser-induced fluorescence measurements of metastable argon ions in a magnetized inductively coupled plasma

S Jun,H Y Chang,R Mcwilliams

doi:10.1063/1.2201894

Abstract

Velocity distribution functions of metastable argon ions (3d′4F7∕2) have been measured to obtain metastable ion density and temperature by the diode laser-induced fluorescence (LIF) technique in magnetized inductively coupled plasma as a function of pressure, rf power, and magnetic field strength. Calculated density from a rate equation agrees with the trends observed in the experimental data. From the calculation, the metastable ion density should be over 107cm−3 to obtain a LIF signal. From a dc bias experiment, it is suggested that the spatial potential can be the dominant ion heating source, and a simple global model for ion temperature is constructed. In this model, approximately 0.01% and 10% of total spatial potential energy can contribute to ion and neutral temperatures, respectively. The measured ion temperature agrees with the calculation.

Highlights

Laser-induced fluorescenceLIFwas first applied to plasma in the 1970s.1,2 Since it has been one of the most powerful diagnostics for the detection of ions, neutrals and radicals in the plasma
A metastable argon ion3dЈ4F7/2͒ density and temperature are measured as a function of pressure, rf power, and magnetic field by a diode LIF method in magnetized inductively coupled plasma
Metastable argon ions are generated from direct electron-impact excitation of ground state neutrals, and they are dominantly lost by lifetime decay

Summary

INTRODUCTION

Laser-induced fluorescenceLIFwas first applied to plasma in the 1970s.1,2 Since it has been one of the most powerful diagnostics for the detection of ions, neutrals and radicals in the plasma. LIF has such good spectral resolutionϳ0.0025 eV ion or neutral temperature that many experiments on ion or neutral heating have been conducted in low-temperature processing plasmas. These efforts explained ion heating qualitatively from electron-ion collisions, potential in the presheath, and wave interaction, but these were not enough to explain why ion temperature is much higher than room temperature in the center of the plasma. We have used the diode LIF scheme to measure metastable argon ion velocity distribution functions in magnetized inductively coupled plasma to obtain metastable ion density and temperature. We suggest ion and neutral heating by spatial potential, and construct a model to explain why the ion temperature in the center of the plasma can be much higher than room temperature qualitatively and quantitatively

EXPERIMENTAL SETUP

Theoretical model for ion temperature

Findings

CONCLUSION