Studies of electron ionization
This thesis presents the results of experimental investigations to determine the electron ionization cross-sections of gases used in plasma etching processes. The small molecules studied include boron trichloride (BCI3), nitrous oxide (N20), sulphur hexafluoride (SF6) and acetylene (C2H2). The knowledge of accurate electron ionization cross-sections is essential for the modelling and optimization of plasmas used in industry and for the purposes of atmospheric modelling. It is therefore surprising that despite the widespread industrial use of these small molecules there have been few investigations of their ionization. It is the lack of available electron ionization cross-section data that provides the motivation for the investigations detailed in this thesis. The formation and dissociation of singly charged (monocations), doubly charged (dications) and triply charged molecular ions (trications) are studied using pulsed electron ionization (30-200 eV) coupled with time-of-flight (TOF) mass spectrometry and a two-dimensional (2D) ion coincidence technique. The experimental technique and data analysis procedures adopted allow the contribution to the total ion yield from single and multiple ionization to be distinguished and quantified. Such information yields relative partial ionization cross-sections (PICSs) and also allows relative precursor specific PICSs to be determined, which quantify the contribution from single, double and triple ionization to the relative yields of the fragment ions formed following electron-molecule collisions. Relative precursor specific PICSs therefore provide a more 'in-depth' chemical picture of each PICS. The relative precursor specific PICSs determined, following electron interaction with N20, provide clear evidence of energetic ion loss in previous cross-section determinations detailed in the literature. In addition, the 2D ion coincidence technique provides information on the energetics involved in the dissociation of any multiply charged ions formed. In the case of BCI3, further insight on the possible dissociation dynamics of BC132+ is reported. Following the investigations of N2O, BCI3 and SF6, the apparatus was commissioned to produce absolute partial ionization cross-sections and absolute precursor specific partial ionization cross-sections from 30-200 eV. Experimentally, the commissioning involved the addition of a Spinning Rotor Gauge (SRG) and a new gas inlet line to the source region. These investigations have extended the data determined for the electron ionization of N20 and have also enabled the absolute cross-section data for the electron ionization of C2H2 to be determined.