Electron impact ionization of tetramethylsilane (TMS)

We studied the electron impact ionization of tetramethylsilane (TMS), Si(CH₃)₄, which is utilized in plasma polymerization applications, using various mass spectrometric techniques. Absolute partial cross-sections for the formation of the parent ion and 15 fragment ions were measured in a high resolution double-focusing sector field mass spectrometer from threshold to 90 eV. Complementary measurements of mass spectral cracking patterns of TMS were also carried out using two quadrupole mass spectrometers. Dissociative ionization of TMS was found to be the dominant process. The Si(CH₃)₃⁺ ion is by far the most abundant fragment ion with a maximum cross-section of about 1 × 10^-15 cm². In contrast, the maximum parent ionization cross-section is less than 2% of that value. On the basis of the measured partial ionization cross-sections, mass spectral cracking patterns and appearance energies of the various ionic products, we propose a collision-induced decomposition scheme for the TMS molecule. Three major decomposition channels were identified which all involve the total or partial removal of a methyl group. The agreement of our measured absolute partial ionization cross-sections with earlier data obtained by a different technique is generally poor.