Mass spectrometric detection of reactive neutral species: Beam-to-background ratio

Mass spectrometry and appearance potential mass spectrometry (APMS) have re cently gained importance for detection and quantitative measurements of rea ctive radical species in plasmas using line-of-sight sampling of reactive s pecies. In this work, we have characterized the contributions to the mass s pectrometer signal from the line-of-sight "beam" component and the backgrou nd component of the species in the ionizer of the mass spectrometer. The be am signal is proportional to the number density of the species in the plasm a, while the background component of the signal depends on Various factors like the vacuum system design and pump speeds. Single differential pumping of the mass spectrometer is found to be inadequate as the background signal dominates the beam signal for radical and stable neutral species. The beam to background ratio for CFx (x = 1-3) radicals is smaller than 0.25 and th e large background signals of the species of interest necessitates implemen tation of modulated beam mass spectrometry using a mechanical chopper in th e beam path. The uncertainty in the beam component measurement is found to be as large as +/- 180%. High beam-to-background signal ratio is achieved u sing three stages of differential pumping, and this vastly reduces the unce rtainty in the beam component measurement to less than +/- 10%. (C) 1999 Am erican Vacuum Society. [S0734-2101(99)10505-0].