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

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 40-87 mTorr and with an radio frequency (RF) input power from 100 to 120 W was investigated by irradiating a set of 2 cm 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.