| Themes > Science > Physics > Molecular Physics > Excitation Energy Transfer and Energy Migration > Theoretical Methods and Advances > Neutral Dissociation |
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, Cl2. 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 dissociation fragments. In the most recent variant of this technique, Sugai and collaborators 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. 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). |
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