Spatial distribution of the absolute densities of CFx radicals in fluorocarbon plasmas determined from single-path infrared laser absorption and laser-induced fluorescence

Abstract

The spatial distribution of the absolute density of CFx (x=1–3) radicals and their translational temperatures in an electron cyclotron resonance (ECR) plasma generated from a tetrafluorocarbon (C4F8) gas were examined using infrared diode-laser absorption spectroscopy (IRLAS) without a multiple reflection cell, namely, single-path IRLAS. Furthermore, we have developed a method of measuring CF and CF2 radical densities using single-path IRLAS combined with laser-induced fluorescence (LIF) spectroscopy. This method enables us to measure the spatial distribution of absolute radical densities with high accuracy, because of the IRLAS infrared laser beam and the LIF ultraviolet laser beam having identical paths. Under all the conditions studied, a spatially hollow distribution of the CF2 radical density is formed; the CF2 radical density in the vicinity of the chamber wall is much higher than that in the plasma. However, the spatial distribution of the CF radical density differs greatly from that of the CF2 radical density. The translational temperatures of CF and CF2 radicals are evaluated to be ∼700 K. On the basis of the measured results, we clarify the mechanisms of the formation of the spatial distribution, and conclude that the hollow distribution of the CF2 radical density is not caused by radical generation from the chamber wall, rather, the dominant mechanism for the formation of this distribution is the electron-impact dissociation of C4F8 gas in the ECR region and diffusion from the upper part of the plasma chamber under the present plasma conditions where the flux of ions incident to the chamber wall is low.