The emission intensities in the range of 2000–8000 Å of CF, CF2, O, F, CO, CO+, and CO2 produced in CF4/O2 radio-frequency discharges, operated at 1 Torr of pressure and 50 W of power input, have been used to follow the etching process of Si and SiO2, as a function of the CF4/O2 feed composition. The addition of small amounts of nitrogen or argon to the plasma mixtures has permitted the determination of the effect of the oxygen addition to the gas feed on the electron densities of the plasma for a wide range of electron energies. The relative concentration profiles of F and O, as well as of CO and CO2, have been determined with this technique, as a function of the oxygen content in the feed. The important role played by atomic F as active etchant for both Si and SiO2 substrates has been confirmed.
This paper deals with the measurement of N 2 (A 3 + u ) metastable state density in a dielectric barrier discharge in nitrogen and nitrogen with small admixtures of oxygen, operating in a Townsend-like discharge regime. The measurement is made by optical-optical double resonance-LIF, calibrated by a method based on the measurement of the ratio of nitrogen second positive system and NO-γ emissions, and of NO density by LIF. A metastable density of the order of 10 13 cm −3 was found in a nitrogen diffuse discharge. Addition of small oxygen concentrations to the discharge drives a transition to the filamentary regime that appears to be caused not by a marked decrease of the metastable density in the discharge but rather by a considerable increase of its quenching rate. Such an increase, due to collision quenching by O 2 and O, strongly reduces the survival of the metastable between two discharge pulses. These observations are consistent with the idea that the diffuse regime can be due to a space charge memory effect due to the nitrogen triplet metastable, which is cancelled by the introduction of oxygen in the gas feed.
Rate constants for N2(A,v) quenching by O, for levels v=2–7, by O2 for levels v=3–7, and by NO for levels v=2–4, have been measured in this work. This is the first data set for the quenching by O of vibrational levels v>3. The results of this work are based on the measurement by laser induced fluorescence (LIF) of N2(A,v) decay in a rf pulsed postdischarge, supported by LIF measurements of NO density. O atom density is deduced by N2(A,v=0,1) decay using the known rate constants of N2(A,v=0,1) quenching by O, O2, and NO. Finally, from appropriate scaling of LIF results for the various v levels, N2(A,v) vibrational distributions are deduced, showing a quite low vibrational excitation of the triplet metastable, characterized by an average Boltzmann vibrational temperature of the order of 2000–2500 K with some superimposed structures.
Abstract.This paper offers an outline of Laser Induced Fluorescence (LIF) diagnostics and practical recommendations for its use in atmospheric pressure discharges. LIF principles, technical requirements and rationalization of experimental outcomes by modelling are addressed. Important issues that are particularly relevant to small scale, spatially inhomogeneous discharges, like plasma-jets, are emphasized. For the first time, all collision processes and the spatial non-homogeneity of the laser beam are together accounted for in the LIF model. Saturation characteristics are discussed and used for the assessment of model parameters. A calibration procedure is discussed and implemented. Gas temperature measurements by LIF are also addressed. The whole description of the technique is given, without loss of generality, through the example of its application to the OH radical. Notes on other diatomic radicals, CH, NO and CN, are given along the paper. Some results in a RF plasma-jet are presented as an example of application in a discharge system where all the concepts developed in the paper are applied.
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