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A selective sloped silicon dioxide etching method has been studied using a temperature controlled diode-type reactive ion etch system with CHCl₃, C₂F₆, H₂ and N₂ gases. The SiO₂ was covered by a polyimide mask with vertical sidewalls and window openings from 1.25-4 microns. Control of the sidewall slope in SiO₂ is possible through varying the ratios of CHCl₃ and C₂F₆. High percentages (95-100%) of CHCl₃ resulted in low etch rates of 420 angstroms/min. Large slopes of up to 65 degrees are possible. With C₂F₆ above 5%, larger etch rates close to 2000 angstroms/min. occur and etching becomes vertical with purely anisotropic profiles at 10% C₂F₆. In a simulation study, SiO₂ sidewall slope was found to be controlled by the isotropic deposition rate as well as the anisotropic etch rate. Optical Multichannel Analysis of the discharge showed an increase in silicon dioxide removal when the percentage of C₂F₆ was increased. (Abstract shortened with permission of author.)

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

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Optical diagnostic techniques have been developed and then used to investigate the chemistry of reactive species formed in CF₄ / O₂ rf parallel plate discharges, similar to those employed in semiconductor material processing. Oxygen atoms were detected by two photon laser induced fluorescence (LIF), and the technique was found to have a number of experimental caveats owing mainly to the high laser intensities required. In particular, amplified spontaneous emission (ASE), was observed from laser excitation of oxygen, and was found to influence the spontaneous fluorescence signal and thus question the use of LIF for ground state concentration measurements in these systems. The spin orbit states of the 3p ³P level were resolved for the first time, both in using high resolution excitation experiments and also as a consequence of detecting ASE. Spin orbit temperatures of less than 50° above ambient were observed. The absolute concentration of oxygen has been found to be 7.4 ± 1.4 x 10¹³ cm^-3 in a 50 mTorr, 100 W, 85% O₂ / CF₄ plasma. Optical emission was also used to study fluorine atoms and to examine the use of the actinometered emission technique as a measure of ground state concentrations. The latter was investigated directly by comparison with LIF measurements of O and CF₂, and in many cases shown to be a poor representation of the ground state concentration. To investigate the chemical and physical processes in the plasma, time resolved methods are required and a new technique, time resolved actinometry, has been developed, tested by comparison with LIF measurements and then used to study the kinetics of fluorine atoms. Results have shown the importance of wall reaction rates on the magnitude of the fluorine atom concentrations, and the sensitivity of these concentrations to the nature of the surface, particularly in the presence of oxygen and silicon. Oxygen has also been shown to be removed predominantly at the surface but the influence of gas phase reactions with CF_x radicals is apparent in discharges containing low percentages of O₂. Studies on an afterglow type, electron cyclotron resonance reactor have been carried out as a comparison to the parallel plate system, and high excitation and dissociation levels have been observed from differences in the emission intensities and from measured values of the absolute CF and CF₂ concentrations. The use of LIF as a diagnostic for CF has been investigated by probing the predissociation of the A²Σ⁺ state. Emission from the A²Σ⁺ (v = 2) level has been seen for the first time, and a J independent predissociation mechanism, with a rate of 3 x 10⁹ s^-1 has been observed.

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