H. S. Maciel

Basic Characteristics of Gliding-Arc Discharges in Air and Natural Gas

Gliding-arc discharges have been utilized in plasma-assisted combustion processes, among various other applications, due to their properties of high electron density and chemical selectivity in a transitional regime. However, basic characteristics relative to the relations between the fundamental parameters of discharge, like mass flow rate, breakdown voltage, and frequency of repetition (number of discharge breakdowns per half cycle), have not been completely studied. In this paper, an ac-powered gliding-arc discharge having a reverse vortex flow configuration is built to carry on a basic investigation on discharges in air, natural gas, and mixture of both. Electrical measurements, optical emission spectroscopy, and mass spectrometry are the techniques used for these investigations. The results presented in this paper describe the dependence of the breakdown voltage, frequency of discharges, and conversion rates of methane and molecular oxygen with respect to the variation of the mass flow rate (directly related to the residence time) and discharge current.

Longitudinal magnetic field effect on the electrical breakdown in low pressure gases

The electrical breakdown has been investigated for low-pressure argon and nitrogen discharges under the influence of an external longitudinal magnetic field. Plane-parallel aluminum electrodes (5 cm diameter) separated by a variable distance d (4.0 cm < d < 11.0 cm) were sustained with a dc voltage (0 < V < 1 kV). A Helmholtz coil was used to produce an uniform magnetic field(B) parallel to the discharge axis. Paschen curves were obtained and the secondary electron emission coefficient (g), the first Townsend ionization coefficient (a) and the ionization efficiency(h), were plotted with respect to the variation of the reduced field (E/P). To observe the effect of the magnetic field these curves were plotted for fixed values of B=0 and B=350 Gauss. As consequence of the longitudinal magnetic field, the free paths of the electrons in the Townsend discharge are lengthened and their lateral diffusion is reduced, thus reducing electron losses to the walls. The data presented in this paper give a quantitative description of the B-field effect on the Townsends coefficients and overall it is concluded that the DC electrical breakdown of the gases is facilitated if a longitudinal magnetic field is applied along the discharge axis.

Nanoindentation study of Ti6Al4V alloy nitrided by low intensity plasma jet process

Abstract The present research has been conducted aiming the study of mechanical characteristic improvement of Ti6Al4V alloy surface. The proposed process is based on an ion nitriding method performed in a low intensity plasma jet reactor. An objective of the study is to enlarge the applicability of this alloy in several industrial areas, mainly foccusing the biomedical applications. The reactor of low intensity nitrogen plasma jet is a system governed by principle of gas expansion through a constriction orifice. The whole device is constituted by a plasma source chamber maintained at relatively high pressure and a vacuum chamber where the nitriding is processed, both chambers being separated by a wall having one central orifice. The D.C. electric discharge is run between a cathode located in the source chamber and the wall of the processing chamber playing the role of anode. The reactive plasma jet of nitrogen emerges from the constriction and expands into the vacuum chamber, where the sample is placed to intercept the plasma jet. The nitrided surfaces of Ti6Al4V samples are characterized by means of structural and physical analyses to formulate the correlations between surface characteristics and process parameters.

Anisotropic reactive ion etching in silicon, using a graphite electrode

Abstract Monocrystalline silicon is etched in a simple reactive ion etching system with mixtures of SF6, Ar and H2 to obtain deep trenches. A graphite electrode is used to increase the anisotropy of the etching processes. The effect of varying flow, pressure and power levels on etch rate and anisotropy has been studied. Addition of Ar to SF6 results in an increase of ion bombardment of the graphite electrode. This will increase the carbon content in the plasma. Using Ar additions, wall slopes of approximately 60° are obtained; without Ar the etching is almost perfectly isotropic. Addition of H2 to the SF6-Ar mixtures will decrease the free fluorine content in the plasma. This will decrease the etch rate and increase the anisotropy of the process. Anisotropic etching has been achieved and 30 μm deep vertical trenches have been etched to obtain micromechanical structures.

Dielectric characteristics of AlN films grown by d.c.-magnetron sputtering discharge

Abstract Aluminium nitride(AlN) films have attracted much interest as a promising opto-electronic material. In this work, AlN films were deposited on p-type silicon by d.c.-magnetron sputtering using different nitrogen concentrations in a mixture with argon and at different substrate temperatures. Structural analysis of the AlN films was performed by X-ray diffraction. The AlN capacitors (MIS structures) were obtained by evaporating aluminium on the back of the silicon wafer, and on circular areas on the film. These capacitors were used to obtain quasi-static current–voltage characteristics at room temperature. The effect of substrate heating on produced AlN films and MIS capacitors is shown. Capacitance–voltage measurements were obtained and indicated that the substrate heating plays an important hole on the MIS capacitor dielectric properties.

End-point detection of polymer etching using Langmuir probes

The adequate determination of the end point of a plasma-etching process is very important for integrated circuit fabrication. In this paper, the authors propose a new method, making use of the floating potential, as determined by a single Langmuir probe with a radio frequency (RF) choke. For the etching of a polymer film with an oxygen plasma using a reactive ion-etching system, this method yields a reproducible and reliable signal, which was successfully used to detect the end point for several wafers, it is better than the method using the DC self-bias voltage as the end-point detection signal, and approximately as good as when using emission spectrometry-at least when the resist area is larger than 4.4 cm/sup 2/-whereas it uses a much cheaper equipment set. Langmuir probe measurements indicate that the floating potential changes are caused by several mechanisms: the average mass change, the plasma density, the average electron temperature, and the electron energy distribution all change after the end point of the etching.

Deep trench etching in silicon with fluorine containing plasmas

Abstract Single crystal silicon was etched with mixtures of SF6, CBrF3, Ar and O2, using different electrode materials to obtain deep trenches. The etch rates, both vertically and horizontally increase when the relative flow of SF6 increases. When using aluminium or stainless steel electrodes, the amount of SF6 has to be limited to 10% of the total flow of fluorine containing gases to obtain wall profiles with an angle of over 80°. However, in all these cases considerable surface roughness is observed. A solution to this problem is the use of a graphite electrode, which permits the use of SF6 as the sole halogen containing gas to obtain vertical walls. Depending on the Ar addition, processes with good anisotropy and without surface roughness can be obtained.

Study of SF6 and SF6/O2 plasmas in a hollow cathode reactive ion etching reactor using Langmuir probe and optical emission spectroscopy techniques

In this work, electrical and optical studies of SF6 and SF6/O2 plasmas generated in a hollow cathode reactive ion etching reactor were performed using the Langmuir probe and optical emission spectroscopy techniques, respectively. We carried out an investigation aimed at understanding the influence of radio-frequency power, gas pressure and O2 gas mixing ratio on plasma parameters, namely electron temperature, electron density and electronegativity, and also atomic fluorine density. The results indicate an increase of up to one order of magnitude in electron density and atomic fluorine in the overall gas volume when compared with a conventional reactive ion etching plasma generated under the same operation conditions.

Surface improvement of EPDM rubber by plasma treatment

The surface of ethylene-propylene-diene monomer (EPDM) rubber was treated in N2/Ar and N2/H2/Ar RF plasmas in order to achieve similar or better adhesion properties than NBR (acrylonitrile-butadiene) rubber, nowadays used as thermal protection of rocket chambers. The surface properties were studied by contact angle measurements and by x-ray photoelectron spectroscopy (XPS). The treated surfaces of the EPDM samples show a significant reduction in the contact angle measurement, indicating an increase in the surface energy. XPS analyses show the incorporation of polar nitrogen- and oxygen-containing groups on the rubber surface. After plasma treatment the presence of oxygen is observed due to surface oxidation which occurs when the samples are exposed to the air. Atomic force microscopy and scanning electron microscopy analyses indicate a decrease in the EPDM rubber surface roughness, promoted by surface etching during the plasma treatment. Strength tests indicate improvement of about 30% and 110% in the adhesion strength for the plasma treated EPDM/polyurethane liner interface and for the EPDM/epoxy adhesive interface, respectively. The adhesion strength of the EPDM/liner is similar to that obtained for the NBR/liner, which indicates that EPDM rubber can safely be used as thermal protection of the solid propellant rocket chamber.

Formation of electrostatic double-layers and electron-holes in a low pressure mercury plasma column

Experimental studies of the formation of electrostatic double layers (DLs) and electron-holes (e-holes) are reported. The measurements were performed in the positive column of a mercury arc discharge operating in the low-pressure range of (2.0–14.0) × 10−2 Pa with current density in the range of (3.0–8.0) × 103 A m−2. Stable and unstable modes of the discharge were identified as the current was gradually increased, keeping constant the vapour pressure. The discharge remains stable until a critical current from which a slight increase of the current leads to an unstable regime characterized by high discharge impedance and strong oscillations. This mode ceased after a DL was formed in the plasma column. To induce the DL formation and to transport it smoothly along the discharge column, a low intensity B-field (7–10) × 10−3 T produced by a movable single coil was used. The B-field locally increases the electron current density and makes the DL form at the centre of the magnetic constriction where it remained at rest. Electrostatic potential structures compatible with ordinary DLs and multiple-layers could be formed in the plasma column by dealing with the combined effects of the operational parameters of the discharge. It is noticeable that a pure e-hole, which is a symmetric triple-layer having a bell shape potential profile, could easily be formed by means of this experimental technique. A partial kinetic description, based on the space charge structure derived from an experimental e-hole, is presented in order to infer the charged particle populations that could contribute to the space charge of the e-hole. Evidence is shown that strong e-hole formation might be driven by an ion beam, therefore it could not be formed in isolation since its formation requires a nearby ion accelerating potential structure. Probe measurements of the plasma properties, at various radial positions of the stable positive column, are also presented. In the stable mode, prior to current limitation, the probe data reveal a substantial radial decrease of the electron drift velocity. This result calls for a review of the free fall theories of low pressure plasma columns to take into account this non-uniformity of the electron drift velocity.