Eden, Samuel Peter; (2003) Spectroscopic and electron impact studies of molecules relevant to plasma etching. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

SiO2 plasma etching is conventionally carried out using CF4, C2F6, C3F8, CHF3, and c- C4F8. These species have high global warming potentials (GWP) because they absorb strongly in the infrared and have very long residence times in the Earth's atmosphere. The current generation of plasma reactors release a large proportion of feed gas into the atmosphere. An effective way to meet the environmental targets imposed upon the industry is therefore to introduce new etch gases with minimal global warming effects. This thesis provides the spectroscopic analysis of a range of experimental results to assist in the evaluation of potential environmentally-friendly feed gases for the plasma etching of SiO2. CF3I is plausible alternative since, due to the weak C-I bond, it is possible to produce high yields of CF3(+) etching fragments in a plasma by direct electron impact dissociation and ionisation. The present results include the first electron scattering evidence for the dominantly singlet to triplet nature of the lowest energy electronic excitation; that from the HOMO localised on the iodine lone pair to the repulsive C-I [sigma]* LUMO. Comparisons with the photoabsorption spectra of CF3Br, CF3CI, CH3CI and CH3I suggest that CF3I maintains its ground state symmetry in the low-energy Rydberg excited states. Electron scattering results provide evidence for the formation of temporary negative ions at incident energies between 5 and 9 eV. The atmospheric lifetime of CF3I due to solar photolysis is calculated to be less than one day at low altitudes. Therefore, industrial emissions of the gas are expected to have negligible global warming and ozone depleting effects. C2F4 has attracted considerable interest as a feed gas for the plasma etching of SiO2 as it can be an excellent source of CF2(+) ions and radicals in an industrial plasma and has a short atmospheric lifetime due to reactions with OH radicals. High-resolution photoabsorption and electron energy loss spectra reveal new features including a proposed vibrational series coupled to the dissociative C=C [pi] [pi]* transition. New vibrational structure observed in the ionic ground state by photoelectron spectroscopy provides evidence to distinguish vibrational structure associated with valence and Rydberg states. Analogies with C2F4 have proved useful in the analysis of the previously unmeasured VUV photoabsorption spectrum of CF2CFCF3, another potential environmentally-friendly feed gas for SiO2 etching. The relatively high energy of the onset of pre-dissociative Rydberg excitations in the spectrum of CF2CFCF3 suggests that the molecule may be a less effective source of CF2(+) in an industrial plasma than C2F4. Precise experimental cross sections for electron and photon interactions with molecules are essential to understand the processes which can occur in future reactors and in the atmosphere. The present studies thus add to the body of research aimed to activate new etching chemistries which can reduce the environmentally damaging effects of the plasma processing industry.

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