Time-dependent excitation in high- and low-frequency chlorine plasmas

Abstract

Time-resolved voltage, current and optical emission from Cl, Cl+, Cl+2, and small amounts of added Ar were studied in 0.3 Torr chlorine discharges at 13.2 MHz and 220 kHz, above and below the ion transit frequency (ITF). Emissions, measured as a function of position, were deconvoluted to correct for the finite fluorescence lifetimes and extract the electron impact excitation rates. At 220 kHz, emission falls to zero at the voltage zero point crossings due to electron energy relaxation and attachment. Emission and excitation in a sheath peak sharply about 70 ns before electrode voltage reaches its positive maximum and are stimulated by electron current. Emission then falls to zero and reaches a second maximum ∼300 ns after the electrode voltage reaches its negative peak. This emission is excited by a secondary electron avalanche from ions crossing the sheath. Excitation maxima in the center of the discharge lag the voltage peaks by ∼25°. Analyses of time and spatially resolved ratios of emission from Cl and Ar show that actinometric techniques can give an erroneous measure of atom concentrations unless the viewing area and phase are taken into account. At 13.2 MHz, ions cannot respond to the time-varying fields, and excitation in the sheath peaks in phase with the anodic part of the cycle with minimal excitation during the cathodic phase. Emission from Cl+ was not observable. The lack of high-energy ion bombardment drastically reduces secondary emission and excitation in the cathodic sheath. In the center of the discharge, excitation peaks twice per cycle, but unlike the low-frequency case, it does not fall to zero. The electron energy relaxation frequency (νu) is estimated to be 64 MHz.