Absolute concentrations of reactive species in etching plasmas by high-sensitivity UV absorption spectroscopy

Many of the reactive species of interest in etching plasmas (for example, CF, radicals and SIF, etch products in fluorocarbon plasmas) absorb light in the UV spectral region (200 - 300 nm). Measurement of these weak absorbances (10(-2) - 10(-4) for a single pass) allows their absolute concentration to be determined. Low-resolution spectra can be obtained by broad-band absorption spectroscopy, using a Xe are lamp as the light source and a small monochromator equipped with a CCD or diode array detector [1]. With relatively long integration times excellent sensitivity can be achieved. The recently developed Cavity Ring-Down Spectroscopy (CRDS) technique has much higher spectral resolution, and (potentially) higher sensitivity. In this technique, the pulsed tunable output of an excimer pumped doubled dye laser is injected into a high-Q optical cavity in which the plasma is included. The absorbance as a function of wavelength is then deduced from the lifetime of the light pulse in the cavity. This technique offers the possibility of real-time (1 second) absolute concentration measurements. Results have been obtained for the detection of CF, CF2, AIF and SiF2 radicals in capacitively-coupled radio-frequency plasmas in fluorocarbon gases.