Cross sections of electron-impact dissociative ionization and neutral radical production from fluorocarbon molecules or neutral radicals are reviewed. In addition, positive and negative ion production on solid surfaces bombarded by fluorocarbon ion beams are reviewed.

View full abstract

To investigate the mechanism of C₄F₈ dissociation, we made extensive measurements of electrons and radicals, as well as a theoretical study. These showed that the amount of light fluorocarbon radicals increased with increasing electron density. The dissociation of C₄F₈ was analyzed by using rate equations. The total dissociation rate coefficient of C₄F₈ was 1×10^-8cm³/s, and CF₂ radicals were mainly generated from products of C₄F₈ dissociation. F was mainly generated from CF₂ by electron-impact dissociation and lost by pumping. We estimate that the C₂F₄ density was roughly comparable to the densities of CF and CF₃, and that the surface loss probability of C₂F₄ increased with increasing electron density.

View full abstract

Negative ions strongly affect the transport and chemical reactions in fluorocarbon plasmas. In this article, a review is given of elementary processes concerning negative ions in fluorocarbon plasmas. Typical behaviors of negative fluorine ions in low-density and high-density plasmas are briefly described.

View full abstract

Polymerization processes in low-pressure plasmas used for industrial surface processing are surveyed. Special attention is given to fluorocarbon plasmas, because of their importance in reactive ion etching of semiconductor elements. Diagnostic methods for the detection of large molecules are described, including mass spectrometry, infrared absorption and laser techniques. By means of a new mass spectrometric method, large amounts of polymeric species have been detected in radio-frequency fluorocarbon plasmas. Mechanisms of macromolecule formation under low-pressure conditions are discussed, and energetically favorable negative ion-assisted polymerization channels are proposed. Several fluorocarbon gases are studied: CF₄, C₂F₆, C₄F₈ and C₅F₈. Polymerization efficiency increases with increasing size of the parent gas molecule, and with its decreasing fluorine to carbon ratio. Gas phase polymerization has important consequences for plasma chemistry and surface processing. It is strongly related to fluorocarbon film deposition on the surface. The presence of the film determines the etching performance of the plasma, introduces new surface reactions for radicals and positive ions, and can result in dust particle formation.

View full abstract

Surface reaction process of fluorocarbon radicals (CF_x) in the contact hole etching for ultra-large-integrated circuits (ULSIs) is presented. When H₂ is added to fluorocarbon gases, the surface reactions of CF and CF₂ radicals are intensively reported. CF₂ radicals contribute considerably to the fluorocarbon film deposition under plasma irradiations. The behaviours of CF radical densities in the plasma over the film composition on the chamber walls are investigated.

View full abstract

This article reviews recent work concerning the surface production and loss mechanisms of CF_x radicals and their role in polymer deposition processes occurring in radio-frequency plasmas in fluorocarbon gases used for the selective etching of SiO₂ layers in microelectronic device fabrication. In capacitively-coupled plasmas, CF_x radicals are often produced predominantly at the powered electrode surface. In fluorine-rich plasmas the instantaneous back-scattering of neutralised, fragmented incident CF_x⁺ ions is the dominant mechanism. Simultaneously, the radicals are destroyed by recombination at the various surfaces of the reactor. This process is most efficient when the fluorine atom concentration is high, and probably leads to the re-creation of volatile CF₄. Therefore, the different reactor surfaces can behave either as net sources or sinks for the radicals. When the fluorine concentration is low, another surface production mechanism dominates the production of CF₂, and involves a long-lived surface intermediate. This mechanism is linked to the formation of a polymer layer at the surface via heavy neutrals and ions formed in the gas phase by concatenation reactions of CF₂ radicals. Finally, the results obtained in higher density (inductively-coupled, electron cyclotron resonance and helicon) sources is compared to the results in capacitively-coupled sources. In this case, similar surface production and loss mechanisms occur, but the relative importance is changed due to the higher degree of fragmentation, the higher ion fluxes and the lower gas pressure.

View full abstract

This article describes the following basic mechanisms concerning the radial plasma transport in the open magnetic configuration: 1)derivation of standard mapping for the coupled dynamics of magnetic moment and gyration angle, 2)radial transport due to drift and bounce frequency resonances, 3)relationship between the intrinsic ambipolarity of the radial fluxes of each species and symmetry property, and 4)selective control of the radial fluxes of each species using rotating RF field, and its application to the profile control of electrostatic potential.

View full abstract

Ion and electron flux profiles could be affected by the geometric of the target. In this study, a groove target with the groove width close to the ion gyro-radius was used to investigate this effect to the sheath formation. Several other types of target were also studied for comparison. It was found that the unbalance of ion and electron fluxes induced potential differences in the targets. Due to the potential difference electric currents are generated in the targets to compensated for the differences. When insulators are inserted in the targets, the sheath potentials change to balance ion and electron fluxes at the each position in the targets.

View full abstract

Ion Cyclotron Range of Frequency (ICRF) heating on Large Helical Device (LHD) is characterized by its high power (up to 12MW) and by steady state operation (30 minutes). The LHD is a helical device (with a major radius of 3.9 m and a minor radius of 0.6m) with super-conducting coil windings (l=2, m=10). The main physical purpose is to investigate currentless and disruption-free steady state plasmas. Research and development for ICRF heating have been carried out in recent years. A high RF power transmission system has been developed, which consists of stub tuners, ceramic feed-through and ICRF heating loop antenna. The RF transmission system was tested and withstood 58kV for 10 seconds and 40kV for 30 minutes. The RF voltage corresponds in the case of a plasma loading resistance, 5Ω to a transmitted RF power capability of 3.4MW and 1.6MW. In addition, a pre-matching stub tuner was very effective in reducing the RF voltage. The reduction rate was confirmed to be one third, which leads to a higher ICRF injection efficiency because of reduction of RF power loss in the transmission system. Furthermore a proper procedure to effectively carry out aging of the RF antenna was found in terms of selecting the RF pulse length, repetition rate and RF voltage.

View full abstract

The behavior of ionized carbon during the simulated plasma disruption is investigated with the Magneto-Plasma-Dynamic (MPD) Arc Jet. The temporal and spatial distributions of the ionized carbon were measured by emission spectroscopy. Distributions of CII and CIII were obtained. For the isotropic graphite target, the emission intensity increased as the target is exposed by the heat flux from plasma. Two consecutive peaks of intensity were observed at the point near the target surface. A simple model of redeposition and surface roughness could explain these phenomena.

View full abstract