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ELECTRON COLLISION PROCESSES 43 dissociation fragments. In the most recent variant of this technique, Sugai and collaborators5 measured absolute and relative dissociation cross sections for various molecules relevant to plasma technology (SiH4, CF4, CH4) using threshold ionization mass spectrometry. With this technique, absolute cross sections can be obtained only for those radical species whose ionization cross sections are known in the near-threshold region. Another method, the ''chemical getter'' method, which traps the dissociation products, has been used to determine the total dissociation cross sections for CF4 and CH4. This technique has very limited applicability to other targets. A method with high potential for detection of neutral dissociation fragments in the ground state is laser- induced fluorescence. This method has not, so far, been applied to molecules of interest to plasma processing. Although many fragments can be detected by this method, a limitation is that tunable lasers are available over a limited wavelength range (roughly 250 to 800 nm) that does not include absorption wavelengths of some prominent dissociation fragments in processing plasmas. The dissociation cross section has been calculated theoretically for a few molecules of interest to plasma processing, most notably Cl2 and NF3, for which the calculated cross section agrees reasonably well with experimental data.6 This calculation is based on the application of the complex Kohn variation method, which, in principle, could be used to calculate cross sections for other targets. Calculations have also been performed for BCl3 and SiC2 (V. McKoy, California Institute of Technology, private communication, 1995). ELECTRON-IMPACT EXCITATION For atoms, an extensive database of electron-impact excitation cross sections exists. However, due to their corrosive and reactive nature, some atoms relevant to plasma processing (e.g. F, C1, Br) have not been studied. In principle, there is no reason that these atoms could not be studied. Vibrational excitation data are available for many molecules, but only a few that are used in plasma processing, again due to the corrosive nature of many plasma processing molecules. Exceptions are CF4, SF6, and some freons. There is also a shortage of measured data for electronic excitation (particularly to stable states) of complex molecules. Theoretical methods for calculating the electronic excitation of molecules are maturing and, when properly benchmarked by measurements, can provide cross sections with adequate accuracy. ATTACHMENT Cross sections for dissociative attachment range from values (for electronegative gases) that are several orders of magnitude larger than cross sections for positive ion formation, to values (for SiF4 and CF4) that are several orders of magnitude smaller than cross sections for positive ion formation. Even when the dissociative attachment cross sections are small, these processes are important for full characterization of the plasma. The negative ions formed may influence the free-electron density and may also participate in nucleation of parasitic dust. Although there have been few measurements, recent exceptions used Fourier transform mass spectrometry techniques and beam techniques to determine dissociative attachment cross sections for SiF4 and CF47 and SiH4.8 Some experimental effort has been devoted to the study of attachment to vibrationally excited molecules.9 There exist virtually no data on electron attachment to radicals, although such species are produced in large numbers in plasmas. Only a few research groups are engaged in these types of measurements. MOMENTUM TRANSFER, SWARM, AND DISCHARGE MEASUREMENTS For purposes of modeling electrical discharges in gases, it may not be possible or even necessary to use complete, detailed information about the dynamics of electron-molecule collision processes that produce dissociation. It may be sufficient to consider only total or averaged "effective" cross sections that apply to
