ELECTRON-ATTACHMENT MASS-SPECTROMETRY FOR THE DETECTION OF ELECTRONEGATIVE SPECIES IN A PLASMA

Electron attachment mass spectrometry (EAMS) has been implemented to d etect electronegative species in a low pressure 13.56 MHz discharge. F or this purpose a source of low energy electrons has been used in comb ination with a quadrupole mass spectrometer (QMS) and the signal of th e negative ions, resulting from electron attachment to neutrals has be en recorded as a function of the electron energy. Chemically active fl uorocarbon gases like CF4 and CHF3 have been studied. EAMS provides mu ch insight into chemistry in the plasma, especially into mechanisms of negative ion formation. It enables the detection of electronegative s pecies, formed from the parent gas under plasma conditions, based on t heir different attachment cross sections. Moreover. the effective nega tive ion formation cross section in a plasma, taking into account the chemical conversion of the feed gas, can be determined. In fluorocarbo n plasmas various species are formed, like C2F6 and C3F8 in a CF4 plas ma and CF4 and C2F6 in a CHF3 plasma, which significantly influence th e negative ion production mechanisms under discharge conditions. Becau se the active neutrals produced in the plasma have typically both a la rger attachment cross section and a lower attachment energy threshold, negative ion formation is dominated by the plasma species and not by the parent gas.