Quantitative analysis of the atomic nitrogen concentration in a remote plasma by means of mass spectrometry

Modulated beam mass spectroscopy is applied as a quantitative method for the measurement of the atomic nitrogen concentration in an expanding remote plasma. The plasma is created in a quartz tube by an electromagnetic surface wave (the upstream pressure ranges between 10 and 20 mbar). The recombined plasma with its N radicals is expanded into a reaction chamber where the pressure is about 10-3 mbar. In order to take into account the losses of atomic nitrogen by sticking and recombination at the inner walls of the mass spectrometer, a chopper has been inserted between the orifice of the mass spectrometer and the ionization chamber. Hence, the measured signal is separated into a beam and a background component, with different dependencies on the expansion properties. The beam-to-background ratio is found to depend on discharge parameters such as total pressure or gas mixture composition. A correction factor for the wall losses which is independent of these parameters allows the quantitative measurement of the N flux.