ABSOLUTE TOTAL AND PARTIAL ELECTRON-IMPACT IONIZATION CROSS-SECTIONS OF HEXAMETHYLDISILOXANE

We studied the electron impact ionization of hexamethyldisiloxane (HMD SO), Si2O(CH3)(6), which is widely used in plasma-enhanced polymerizat ion applications. Appearance energies and absolute partial cross secti ons for the formation of fragment ions with relative intensities >1% o f the most abundant ion, the Si2O(CH3)(5)(+) fragment ion, were measur ed in a high resolution double focusing sector field mass spectrometer with a modified ion extraction stage far electron energies from thres hold to 100 eV. Dissociative ionization was found to be the dominant p rocess. The main ionization channel removes a complete methyl group to produce the fragment ion Si2O(CH3)(5)(+) with a cross section of 1.7 x 10(-15) cm(2) at 70 eV. The stoichiometric and isotope composition o f the various fragment ions was determined by using the high resolutio n (m/Delta m = 40,000) of the mass spectrometer used in these studies. The methyl ion is formed with considerable excess kinetic energy, whe reas all other fragment ions are formed with essentially no excess ene rgy. The experimental total single ionization cross section of HMDSO ( 2.2 x 10(-15) cm(2) at 70 eV impact energy) is in good agreement with the result of a semiempirical calculation (2.1 x 10(-15) cm(2) at the same energy). (C) 1998 Elsevier Science B.V.