Ultraviolet cavity ring-down spectroscopy of free radicals in etching plasmas

Many reactive species of interest in technological plasmas absorb light in the UV spectral region (200-300 nm). Measurement of these weak absorbances (typically 10^-2-10^-4 for a single pass) allows us to determine their absolute concentration. Low-resolution absorption spectra of these systems have previously been obtained by broad-band absorption spectroscopy. Here we present spectra obtained using laser cavity ring-down spectroscopy, which has much higher spectral resolution, and potentially higher sensitivity. Spectra were obtained for CF, CF₂, AlF and SiF₂ radicals in capacitively-coupled radio-frequency plasmas in fluorocarbon gases. This technique offers the possibility of real-time (1 s) absolute concentration measurements during wafer processing.