Electron attachment mass spectrometry for the detection of electronegative species in a plasma

Electron attachment mass spectrometry (EAMS) has been implemented to detect electronegative species in a low pressure 13.56 MHz discharge. For this purpose a source of low energy electrons has been used in combination with a quadrupole mass spectrometer (QMS) and the signal of the negative ions, resulting from electron attachment to neutrals has been recorded as a function of the electron energy. Chemically active fluorocarbon gases like CF4 and CHF3 have been studied. EAMS provides much insight into chemistry in the plasma, especially into mechanisms of negative ion formation. It enables the detection of electronegative species, formed from the parent gas under plasma conditions, based on their different attachment cross sections. Moreover. the effective negative ion formation cross section in a plasma, taking into account the chemical conversion of the feed gas, can be determined. In fluorocarbon plasmas various species are formed, like C2F6 and C3F8 in a CF4 plasma and CF4 and C2F6 in a CHF3 plasma, which significantly influence the negative ion production mechanisms under discharge conditions. Because the active neutrals produced in the plasma have typically both a larger attachment cross section and a lower attachment energy threshold, negative ion formation is dominated by the plasma species and not by the parent gas.