Absolute measurement of vacuum ultraviolet fluxes from inductively coupled plasmas via metal surface photoemission currents

Abstract

Quantifying vacuum-ultraviolet (VUV) fluxes typically requires vacuum compatible spectrometers and is often associated with significant cost and effort. A simple technique for the absolute measurement of local VUV fluxes from plasmas using the photoemission from a set of coated metal plates, is described. The radiant power from a 13.56 MHz hydrogen plasma operating at 40-100 mTorr and with an RF input power from 100 to 120 W was investigated by irradiating a set of 2cm diameter Au, Ag and Cu plates. The variation in photoemission currents was compared with the photoelectric yield curves to estimate the absolute flux incident on the surfaces in the 113-190 nm range. The measured fluxes were found to have an uncertainty of 5 – 30% when compared with the VUV spectrometer measurements. The VUV output power was found to have a maximum at a pressure of 70-80 mTorr and to increase with RF power. In all cases, the VUV output power was measured to be approximately 12 - 16% of the RF input power to the matching network, in good agreement with spectroscopy results.