Contrib. Plasma Phys. zyxwvu 35 (1995) 6, zyxwvu 395-404 Ionization Cross Sections of Silane and Disilane by Electron Impact E. KRISHNAKUMAR') and S. K. SRIVASTAVA Jet Propulsion Laboratory, California Institute of Technology 4800 zyxwvu Oak Grove Drive, Pasadena, CA 91 109, U.S.A. Abstract Partial ionization cross sections for silane and disilane by electron impact have been measured for electron energies in the range zyxwvut of threshold to 1000 eV using a crossed beam geometry and employing a pulsed electron beam and a pulsed ion extraction technique. The cross sections were normalized by using the relative zyxwvutsr flow technique. For ease of use in various plasma modeling we have fitted the cross sections to an empirical formula. 1 Introduction Ionization of molecules produces various fragment ions with yields depending on the energy of the ionizing agent and the nature of the molecule. This process, called dissociative ionization, is a major contributing factor to the gross ionization cross section in the case of molecules. Most of these fragment ions generally have substantial kinetic energies and, therefore, special care is needed for making accurate measurements of the cross sections. This is particularly true for electron impact dissociative ionization of molecules where special efforts have to be made to extract ions formed in the interaction region without affecting the electron beam. As a result, relatively few measurements have been reported on these molecules. Recently, we have used a pulsed electron beam in combination with a pulsed ion technique to ensure complete collection of all ions irrespective of their initial kinetic energies without affecting the electron beam [ 11. Here, we report our measurements on silane and disilane using the same technique. Normalized values of cross sections have been obtained by using the relative flow technique [l, 21. Silane and disilane are very important molecules for the semiconductor industry. Cross sections for the formation of various ions by electron impact ionization of these molecules are essential for modeling the plasmas containing them. Even though several reports have been published in the past on the appearance energies of various positive fragment ions from silanes [3 - 71, there is only one on the measurement of absolute cross sections as a function of incident electron energy for these molecules. This is by CHATHAM et al. [8] in the energy range from threshold to zyxwvuts 200 eV for both silane and disilane. TURBAN et al. zyx [9] have reported the relative values of cross sections in the range of 10 to 90 eV for silane. This measurement is more of historical value as the apparatus and the operating conditions used by TURBAN et al. [9] were not really designed for studying the variation of cross sections as a function of electron impact energy. CHATHAM et al. made three different experiments to obtain the absolute values of cross sections as a function of electron impact energy. In their first set up the total ionization cross section for each gas was measured relative to that of an inert gas whose ionization cross section was well known. This measurement was free of ion mass and kinetic energy discrimination effects. In the second set up they obtained cracking patterns (i.e., spectra of fragment ions) by minimizing the discrimination effects, but at ') Present address: Tata Institute of Fundamental Research, Bombay 400005, India.