Optical Characteristics and Mass-charge Composition of Plasma of a Pulsed Vacuum Arc Discharge with a Magnesium Cathode

Measurements of discharge voltage, current, and cathode erosion rates in a vacuum-arc centrifuge are described. Such a centrifuge consists of a high-current vacuum-arc source imbedded in a uniform axial magnetic field. Plasma produced by the vacuum-arc discharge is collimated and rotated by the magnetic field. This rotation leads to radial centrifugal separation across the plasma column. Voltage-current characteristics reveal that the plasma resistance is independent of the axial magnetic field. A minimum turn-on voltage is required by the discharge, independent of the conducted current. This turn-on voltage is probably associated with sheath voltages at the electrodes. An appropriate electrical analog for the discharge is that of a diode in series with a pure resistance. Impedance matching of the electrolytic pulse-forming network to such a load is examined. Typically, 90 percent of the stored electrical energy is found to be coupled to the plasma load, making this a very efficient source for the centrifuge. Cathode erosion rates for carbon and magnesium cathodes are presented. These measurements are compared with those of other investigators at lower currents.

An experimental technique for the determination of electric conductivity and temperature of plasma is presented. The technique is based on comparing the signals that are produced by a pulsed magnetic field in the circuits of two probes located within the studied plasma and outside of it. The proposed technique for the measurement of plasma parameters was tested experimentally in the context of measuring the electric conductivity and temperature of plasma flux formed in cathode spots of a high-current pulsed vacuum arc with a magnesium cathode.

The azimuthal component of the force, which establishes rotation in vacuum arc centrifuges, is investigated. It is found that the design of the anode grid is one important factor influencing rotation. A range of tungsten-wire grids have been studied experimentally in a vacuum centrifuge operating with a magnesium cathode, and angular velocities have been determined by cross correlation of voltage probe signals. It has been verified that angular velocity increases when grids with higher effective electrical resistivity are used, as predicted theoretically. Grid heating during the 14 ms operating pulse increases resistivity and should also increase angular velocity; this effect has been observed experimentally and agrees with predictions.

The report presents experimental research results on a pulsed vacuum arc discharge with a cathode made of commercially available boron. The unique properties of boron make it attractive as a coating material, and many studies are now pursued to investigate different discharge systems with sputtering of boron-containing targets [1–3]. Boron is a wideband-gap semiconductor having a high specific resistance $( \sim 1.8$ MOhm $\times $ cm) under normal conditions. Because of the so high resistance, the initiation of an arc discharge with a pure boron cathode requires preliminary heating of the cathode to temperatures above $600^{\circ}\mathrm {C}$. The discharge is initiated by a flashover of an alumina ceramic insulator separating the cathode and trigger electrode.

Arc dynamics and discharge mode transitions of vacuum arcs on Cu/Cr50/50 electrodes were investigated using high-speed camera technique in a wide parameter range. Current pulses with variable AC frequency of 25 _ 100 Hz as well as pulsed DC currents with duration from 5 to 20 ms have been studied. The total charge transferred by the arc was kept constant through the variation of current amplitude. In addition, the influence of mean contact separation speed in the range of 0.3 up to 1.3 m/s was considered. The characteristics of the voltage recordings and optical imaging of the observed arcs indicate the conditions for the change between high-current anode modes. Optical and electrical characteristics of the arc as well as the existence diagrams of various modes are presented and discussed.

We have designed, fabricated and characterized an ion source based on a vacuum magnetron discharge. The magnetron discharge is initiated by a vacuum arc discharge, the plasma of which flows onto the magnetron sputtering target working surface. The vacuum arc material is usually the same as that of the magnetron target. The discharges operate at a residual pressure of 3 × 10−6 Torr without working gas feed. Pulses of vacuum arc (30 μs) and magnetron discharge (up to 300 μs) are applied simultaneously. After ignition by the vacuum arc, the magnetron discharge runs in a self-sustained mode. Cu–Cu, Ag–Ag, Zn–Zn, and Pb–Pb pairs of magnetron target material and vacuum arc cathode material were tested, as well as mixed pairs; for example, Cu vacuum arc cathode and Pb magnetron target. An ion beam was extracted from the discharge plasma by applying an accelerating voltage of up to 20 kV between the plasma expander and grounded electrodes. The ion beam collector current reached 80 mA. The ion beam composition, analyzed by a time-of-flight spectrometer, shows that the beam consists mainly of singly-charged (about 90%) and doubly-charged (about 10% current fraction) magnetron target material ions. The ion beam radial current density non-uniformity was as low as ±5% over a diameter of 6.6 cm, which is the diameter of the source output aperture.

The characteristics of plasmas in a titanium hydride vacuum arc ion source were experimentally investigated by a temporally- and spatially-integrated optical emission spectroscopy method. A plasma emission spectral fitting model was developed to calculate the plasmas temperature and relative density of each particle component, assuming plasmas were in local thermodynamic equilibrium state and optical thin in this study. The good agreement was founded between the predicted and measured spectra in the interesting regions of 330–340 nm and 498–503 nm for Ti+ ion and Ti atom respectively, while varying the plasma temperature and density. Compared with conventional Boltzmann plot method, this method, therefore, made a significant improvement on the plasma diagnosis in dealing with the spectral profile with many lines overlapped. At the same time, to understand the mechanism of the occluded-gas vacuum arc discharge plasmas, the plasmas emission spectra, ion relative density, and temperature with different discharge conditions were studied. The results indicated that the rate of Ti metal evaporation and H desorption from the electrode would be enhanced with arc current, and the ionization temperature increased with the feed-in power of arc discharge, leading more H+ and Ti+ ions, but reducing the H+ proportion in arc discharged plasmas.

Low-pressure high-current pulsed magnetron discharge with electron injection from a vacuum arc plasma emitter

The behavior of multicharged ions in the cathode spot of pulsed copper vacuum arc is studied by the 2D3V electrostatic Particle-In-Cell Direct Simulation Monte Carlo method. This method tracks the position and velocity of electrons, neutrals, and copper ions charged from +1 to +4 simultaneously, which couples with external circuit physics as well as heat conduction at the cathode. The general thermofield electron emission developed in recent years is used in the vacuum arc simulation. The simulation starts from complete vacuum until the arc current reaches the steady state at about 3 A limited by the external circuit, and the arc voltage is between 20 and 30 V. During the discharge, the cathode temperature increases from room temperature to around 8000 K. The breakdown process is visualized by the distribution of ion density at different stages of arc discharge: from a small volume of cathode spot to a conductive current path between electrode gaps. The vacuum arc plasma is found to be highly ionized, with an average charge state slightly above two and electron density on the order of 1020 cm−3. The positively charged ions move in the direction from the cathode to anode, which is opposite to the direction of the applied external field. Ion energies at the steady state increase from 20 to 200 eV when charge states increase from +1 to +4. This indicates that the electrostatic acceleration of ions is caused by a dynamic space-charge field in the breakdown process of pulsed vacuum arc discharge.

Structural and morphological characterization WCxNy thin films grown by pulsed vacuum arc discharge in an argon–nitrogen atmosphere

Structural and chemical composition analysis of WCN produced by pulsed vacuum arc discharge

A pulsed vacuum arc discharge emits a plasma as well as macroparticles (MPs) in the form of micrometer-sized molten droplets of cathode material. Due to their direction of flight and submicrometer to 100-mum diameter, these MPs often pose a contamination threat for both spacecraft-based thrusters and thin-film deposition systems. The velocity, mass, and charge of copper MPs emitted by a 100-A arc was experimentally measured and compared to a model based on thermionic electron emission. The MP velocity was determined by using a time-of-flight velocity filter. The charge was calculated by measuring particle deflection in a transverse electric field. The model predicts, and the experimental results verify, that the charge on the MPs becomes positive once the plasma is extinguished, and the MP travels in a vacuum, as would occur in a pulsed vacuum arc, versus a dc arc. Experimental results show a roughly quadratic dependence of particle charge on the particle diameter (q ~ D2), with a 1-mum particle having a positive charge of ~1000 electronic charges (1.6 times 10-16 C), and a 5-mum particle having a charge of ~25 000 electronic charges. The model is particle temperature dependent, and gives q ~ D2 at 1750 K and q ~ D1.7 at 2200 K. Arguments are also made for limitations on particle temperature due to radiative and evaporative cooling.

We describe recent results of our investigations of a repetitively pulsed vacuum arc plasma source with pure boron cathode. Boron is a semiconducting material with high specific resistance (about 1.8 MOhm.cm) under normal conditions. The high resistance and strong temperature dependence of the boron cathode material determine the features of the arc discharge ignition and cathode spot behavior. For arc ignition with pure boron, it is necessary to preheat the cathode up to 1000 °C. Here we describe a novel arc triggering technique that allows cathodic arc operation with pure boron while the cathode temperature is only about 600 °C. Our vacuum arc ion source produces a 450 mA pulsed ( $100-300 \mu{\mathrm{m}}$ ) space-charge compensated beam of boron ions of singly- and doubly ionized charge states with no gas and few impurity atoms. The arc discharge current was up to 100 A. We have investigated the discharge and basic characteristics of the boron plasma source using two cathode samples made by casting and hot pressing.

The fast development of microelectronics means for metallization 1) smaller width of trenches and vias, 2) higher aspect ratio, 3) transition to copper. Conventional PVD methods and even adapted versions (e.g. directed sputtering) have large problems to meet the future demands, especially to realize complete filling of the deeper structures. A very promising alternative is represented by special versions of vacuum arc deposition. The plasma beam generated in the vacuum arc discharge is distinguished by its complete ionization and by its high kinetic energy. Up to now this technique seems to be not acceptable for microelectronic technologies due to the inherent problem of droplet emission. By the development of the pulsed High Current Arc (HCA) the melt splashing effect is markedly reduced. For completely droplet-free deposition an additional magnetic filtering was developed. Due to its specially designed high efficiency and due to the high rate of the HCA it allows the integration in the time cycle of a cluster tool. Complete copper filling of 1 μm trenches with aspect ratios up to 3 has been achieved. Further optimization includes the effects of enhanced subatrate temperature and of scattering on a gas atmosphere. Based on the thorough analysis of the dependence of the growth rate on orientation and on the geometrical conditions in the neighborhood a theoretical model has been developed. According to this concept, the deposition must be considered as a dynamic process including sputtering and redeposition on the highly activated surface.

Summary form only given. Intense pulsed heavy ion beams have a wide area of applications including nuclear fusion, materials processing, and plasma production. Pulsed ion beams are usually generated by a pulsed power ion diode with surface flashover ion source. However, in the diode the producible ion species and the purity of the beam are limited. To generate variety of ion species we have developed a new type of ion diode. In the diode a gas puff plasma gun and a vacuum arc plasma gun are used as ion source. A pulsed power generator of output voltage 200 kV, current 20 kA, pulse duration 100 ns is used to apply the acceleration voltage to the magnetically insulated acceleration gap of d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A-K</sub> = 10 mm. The plasma gun is placed 100 mm upstream from the acceleration gap. With gas puff plasma gun, nitrogen ion beam is successfully accelerated and ion current density more than 50 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> has been obtained. The purity of the ion beam is evaluated by a Thomson parabola ion analyzer and found that > 85% of accelerated ions are nitrogen. To generate aluminum ion beam vacuum arc ion source has been developed. By the pulsed vacuum arc discharge between the coaxial electrodes, ion current density > 100 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> has been observed at 100 mm downstream from the gun.