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ELECTRON COLLISION PROCESSES 41 5 Electron Collision Processes INTRODUCTION In modeling and simulation of plasma processing applications, quantitative data are needed on the many reactions involving neutral species and positive and negative ions under practical industrial conditions, especially for those substances that are used directly in plasma deposition and etching. For each chemistry to be considered, ideally, electron collision processes involving all possible reactants, products, and intermediates must be investigated. Cross sections or probabilities for the various reaction channels (ionization, excitation, dissociation, attachment, and recombination) will depend on plasma conditions of temperature and on the energy state of the target species. In this chapter, the general availability of electron-impact cross section data is discussed for each reaction channel. Potential sources of new data, both experimental and theoretical, are described. A set of typical plasma processing substances serve as examples in the discussion of electron-collision cross section availability. These include deposition compounds SiH4 and SiO2; etching gases C1, Br, C12, HCl, F2, HBr, BCl3, and CF4, and decomposition and etching products SiClx and SiBrx. IONIZATION Atoms For constituent atoms present in plasma etching and deposition (F, C1, Br, C, N, S, H, O), ionization cross sections have been measured to within ±20%.1 The situation is similar, if not better, for rare gases used as buffers and dilutants. Single ionization cross sections for all rare gases are known to better than ±8%, for instance. Little ionization data is available for atoms in excited or metastable states,with the exception of laser-excited alkali atoms and metastable rare gases, H, N, and O. Molecules For plasmas with complex molecules present in the feed gas, detailed understanding requires identification of the specific ions formed by electron collisions. Thus, cross sections are needed for production of parent molecular ions and dissociative ionization products. These are distinct from total ionization cross sections (the sum for all channels), which are measured in some cases but do not provide adequate information on the specific ionic species produced. Channel-specific ionization cross sections are available for a variety of plasma processing compounds, including SiH4, CF4, SF6, CCl2F2, and O2, as well as the common purge gas N2. Total ionization cross sections have been reported for C12 and F2. Changes in these cross sections may occur when the target molecules are vibrationally or electronically excited. Experimental techniques are not well developed to study these effects quantitatively. Experimental and theoretical results for the ionization of vibrationally excited molecules show qualitatively a shift of the ionization threshold to lower energies and a significant enhancement of the cross section in the lower energy region near threshold,2 two effects that can drastically affect the ionization balance in a low-temperature plasma. Many dissociative-ionization cross section measurements made prior to 1990 are suspect because many experiments did not properly account for the fact that fragment ions can be produced with excess kinetic energies that are far greater than thermal energies. Ion losses and other discrimination effects involving energetic fragment ions have seriously compromised many dissociative ionization cross section measurements prior to 1990. For the complex molecules used in plasma processing there are only a few
