Determination of fluorine atom density in reactive plasmas by vacuum ultraviolet absorption spectroscopy at 95.85 nm

Vacuum ultraviolet absorption spectroscopy was developed for the measurement of absolute fluorine (F) atom density in reactive plasmas. In order to minimize the influence of radiation trapping (self- absorption) in the light source, fluorescence at a wavelength of 95.85 nm from the F atoms in an electron-cyclotron resonance (ECR) CF4 plasma, which was operated with a low microwave power (0.1 kW) and a low gas pressure (1 mTorr), was employed as the probe emission. A windowless transmission system for the probe emission was constructed by connecting the ECR light source with the target plasma and the detection system using vacuum tubes having small slits. The connection tubes were differentially evacuated with turbomolecular pumps to prevent neutral particles from passing through between the ECR and target plasmas. The present method was applied to high-density CF4 and C4F8 plasmas produced by helicon-wave discharges. The accuracy of the measurement was examined carefully by evaluating various sources of error. In the present article, we have emphasized the evaluation of the radiation trapping effect in the light source plasma. (C) 1999 American Institute of Physics. [S0034-6748(99)04401-9].