Appearance mass spectrometry of neutral radicals in radio frequency plasmas

Abstract

High-sensitivity space-resolved measurements of the number density of neutral free radicals in reactive plasmas have been accomplished by appearance mass spectrometry. This technique is based on several electron volts difference among the appearance potentials for electron-impact ionization. First, some difficulties accompanied with the appearance mass spectrometry are considered, especially on the influence of excited parent molecules and radical sticking. Second, spatial distributions of neutral radical CH3 and CH2 in a parallel plate rf discharge of methane at a pressure of 0.01–0.3 Torr have been measured. The results are well described by a one-dimensional numerical modeling whose input parameters are provided by comprehensive measurements of electron energy distribution, ionic composition, and radical sticking coefficients. Third, the density distributions of CF3 and CF2 radicals in a fluorocarbon etching plasma at a CF4 pressure of 0.01–0.2 Torr have been measured. Finally, several applications of the appearance mass spectrometry to investigations of radical collision processes in gas phase and on solid surfaces are reported, e.g., cross section measurements for electron-impact dissociation of methane into CH3 and CH2 radicals.

Detection of free radicals in low-temperature gas-grain reactions of astrophysical interest

“…On the other hand, this increases the probabilities of the C 2 H 6 dissociation in reaction (4) due to the higher electron concentration, and also of the three-particle recombination CH 3 + H + (CH 4 ) → CH 4 + (CH 4 ) which competes with reaction (3). The most significant difference, though, between our methane discharge and those of , Kojima et al (1989), Sugai et al (1990), Sugai & Toyoda (1992), Dagel et al (1996) is that the walls of the discharge tube in the present study were cooled with liquid nitrogen vapor of about 120 K, while in the cited papers the walls were at temperatures of 300-400 K. At 120 K, the saturated C 2 H 6 vapor pressure is about a factor of 1.25 × 10 4 lower than the pressure at 300 K. This implies that the sticking probability on the 120 K walls of a gas discharge tube is far above that on the 300-400 K walls. An estimate shows that the C 2 H 6 density would decrease with the increase of the sticking coefficient.…”

“…It seems that most of the products formed in a methane gas discharge, which enter the cold substrate together with CH 4 molecules, is composed of hydrogen atoms, CH 3 radicals produced by dissociation of CH 4 molecules by electron impact, reaction (2), and C 2 H 6 molecules formed in the discharge. The results of the mass-spectroscopy analysis of the products formed in a pure CH 4 gas discharge have been presented by , Kojima et al (1989), Sugai et al (1990), Sugai & Toyoda (1992), Dagel et al (1996). For a 5-25 W radio-frequency discharge with a plasma temperature of about 400 K and a low CH 4 pressure of ∼0.005-0.02 Torr, the concentrations of the discharge products were found to be [CH 3 ] = (2.5-4.7) × 10 11 cm −3 , [C 2 H 6 ] = 10 11 -10 12 cm −3 , [CH 2 ] ∼ 10 9 cm −3 , while, the [H] value was estimated not to exceed 10 13 cm −3 .…”

“…In the present study, the CH 4 pressure in discharge was set to a value of the order of several Torrs, i.e. several times as great as in the cited papers by , Kojima et al (1989), Sugai et al (1990), Sugai & Toyoda (1992), and Dagel et al (1996). It is hard to predict how such an increase in the CH 4 pressure would alter the C 2 H 6 density in the gas discharge, since the rising CH 4 base pressure is expected to increase the probability of the three-particle reaction, (3), with a greater yield of C 2 H 6 .…”

Detection of nitrogen metastable molecules by using the threshold ionization mass spectrometry

“…The method is utilized for the detection of neutral activated species in a semiconductor process, and many fruitful results have already been obtained [3]. There are numerous reports of the detection of excited nitrogen [4,5].…”

Detection of Nitrogen Metastable Molecules by using the Threshold Ionization Mass Spectrometry.

“…By detecting N 2 (A 3 Σu + ) by threshold ionization mass spectrometry making use of the difference between the ionization voltage from the N 2 (A 3 Σu + ) state and that from the ground state nitrogen molecule, the number density of the state is determined. The method is utilized for the detection of neutral activated species in a semiconductor process, and many fruitful results have already been obtained [3]. There are numerous reports of the detection of excited nitrogen [4,5].…”