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ELECTRON COLLISION PROCESSES 42 examples of reliable data for dissociative ionization, such as CH4, SiH4, CF4, and SF6; generally the data are lacking. As noted in Chapter 3, "Radiative Processes and Diagnostics," this lack of data for dissociative ionization, especially near threshold, can significantly limit the ability to quantitatively interpret mass spectrometric measurements. This is the case because a common tactic for measurement of radical concentrations with mass spectrometry is to reduce the electron impact energy to near threshold. Generally, this approach allows discrimination between a signal from a parent molecule ionizing dissociatively and the radical of interest. However, if the parent molecule is vibrationally excited (for example), its dissociative ionization cross section may increase near threshold (see discussion above). This results in a significant drop in confidence in the technique, and may invalidate it completely. Very recently, there have been efforts to measure ionization and dissociative ionization cross sections for complex metal-organic and silicon-organic molecules used in deposition plasmas.3 When considering ion formation by dissociation in discharges, polar dissociation must also be taken into account. Although this is usually a minor process for positive-ion formation, it can make a significant contribution to negative-ion formation. Radicals are a class of highly reactive molecular species that are frequent products in plasma processing. Electron-impact ionization cross sections have been measured to ±20% for SiFx and CFx (x = 1-3), NF2, NF, and SO. These studies apply to ground state targets only. Only a few research groups are actively studying electron-impact ionization, and in even fewer cases have the research programs focused on substances pertinent to plasma processing. Theoretical Methods and Advances For molecules and radicals, the state of the art has until recently consisted of empirical and semiempirical methods and simplistic additivity rules. Recently there have been several new developments: (1) two modified additivity rules that attempt to account for molecular bonding; (2) the Deutsch-Mark formalism, which combines a Gryzinsky-type energy dependence with quantum mechanically calculated molecular structure information (also applicable to atoms); and (3) a new binary-encounter dipole theory. Neutral Dissociation In cases where electron-impact dissociation produces electronically excited fragments that decay radiatively, studies use the optical excitation function technique. Emissions have been measured over a wide spectral range. Several molecules relevant to plasma processing have been studied, providing dissociation cross sections for CF4, SF6, NF3, BCl3,4 and several freons and halogenated methane compounds. Using traditional beam techniques to measure cross sections for electron-impact dissociation of molecules into neutral products is extremely difficult, especially when the dissociation fragments are in the ground state or do not radiate. This difficulty is caused by the lack of sensitive methods for detecting neutral fragments. One beam technique that has been used successfully for measurement of dissociation cross sections involves fast neutral beams formed by charge transfer in conjunction with coincident product detection techniques. In these configurations there can be significant uncertainty in the excited state distribution of the neutral target molecules formed in the charge-transfer process. This method has been applied to the relatively simple molecules N2, O2, CO, and, most relevant to plasma processing, C12. The technique can be extended to other simple molecules that are dominated by two-fragment break-up channels. A serious limitation will arise when the method is applied to more complex polyatomic molecules for which many break-up channels with different numbers of fragments compete. This is at present the only suitable method for studying the dissociation of free radicals into neutral ground-state fragments. Another beam technique is the so-called two-electron-beam technique, in which the first electron beam is used to dissociate the target molecules and a second electron beam "downstream" is used to probe the
