Cathodic vacuum arcs produce highly ionized plasmas from virtually all solid (or even liquid) metallic elements. As long as a material is sufficiently conducting to serve as an arc cathode, it can be used in a cathodic arc discharge; therefore, not only metals but only semi-metals (graphite), highly doped semiconductors (Si, Ge), and hot semiconductors (B) have been used to produce plasma from which ions can be extracted . Cathodic arc plasma are characterized by the presence of multiply charged ions. Ions are formed in the plasma can be extracted and accelerated by the strong electric field between extraction electrodes.
The original development of metal ion sources of the "Mevva" type dates back to the early 1980s when the (now retired) group leader Ian Brown and coworkers were tasked to provide uranium ions for the "Bevalac" accelerator. Their work was honored with an R&D 100 Award. The term "Mevva" is derived from Metal Vapor Vacuum Arc, which is by now an often used but historical term. The highly successful "Mevva" development showed that that the arc produces fully ionized plasma that -- in most cases -- contains multiply charged ions. Therefore, the term vacuum arc plasma is better suited than vapor.
The final ion energies are typically from a few keV to a few hundred keV depending on the charge state as well as on the extraction voltage according to E=eQVextract, where Q is the charge state number (Q= 1,2,3,...) and Vextract is the extraction voltage ( = potential difference between first and last grid of the extraction system).
Schematic of the plasma generator (cathode - anode arc discharge, trigger for starting the arc, and three-grid extraction system based on the acceleration-deceleration principle.
Over the years, the group under the leadership of Ian Brown has developed a "family" of Mevva ion sources, characterized by multiple cathodes numbers (up to 18 in Mevva V), and large beam diameter (up to 50 cm). In more detail, here are the steps of vacuum arc development and "members of the Mevva family"
1. MicroMEVVA
A miniature embodiment, 6 cm long, 1.5 cm in diameter. Beam currents
of 15 mA at 15 kV have been made.
2. MEVVA II
Single-cathode design in which the cathode is a central rod surrounded
coaxially by an alumina insulator and a trigger electrode. Pulse length
typically 250 µs, arc current 50-500 A. This version was the basis
of the first R&D 100 Award.
Mevva II
Publication highlight:
I. G.
Brown, J. E. Galvin, and R. A. MacGill, "High current ion source," Appl. Phys.
Lett., vol. 47, pp. 358-360, 1985.
3. MEVVA III
Embodiment residing completely in vacuum, about 5 cm long and 5 cm
in diameter. Ion beam pulses similar to MEVVA II, hundreds of milliamperes,
but limited duty cycle due to thermal load issues.
4. MEVVA IV
16 cathode version, cathodes mounted on revolver (Gatling-gun-like
fashion) allowing changing the cathode material without breaking vacuum.
Operational extractor voltage up to 100 kV.
5. MEVVA V
Similar to MEVVA IV, with 18 cathodes and some improved features to
enhance ease of operation and maintenance. Our "workhorse"
for broad beam ion implantation.
Mevva V is still occasionally used today (2005).
General Publication highlights:
Mevva V is also frequently modified for basic vacuum arc research, such in studies of the effect of magnetic fields. One important area of research was to enhance and fine-tune the ion charge state distributions. An axial magnetic field turned out to be a good practical tool. The main result of this research was summarized in:
Publication highlight:
6. MEVVA VI
Very broad beam version (about 1000 beamlets, total ion beam diameter 50 cm)
with single cathode. This version and DC operation earned a second
R&D 100 Award.
7. MEVVA VII
Metal ion source with filtered vacuum
arc plasma for macroparticle-free ion implantation. This version was
developed for research and demonstration purposes only.
Publication highlight:
8. LEVA
LEVA is an acronym for "Low Energy Vacuum Arc" ion source. 16 cm long,
6 cm in diameter, with 19 beamlet with a total ion beam diameter
of 1.5 cm. Low ion energy refers to the range of order 1 keV, achieved
with three-grid-extractor system of almost symmetric potential distribution
(acceleration-deceleration).
9. MAGIS
MAGIS is an acronym for "MArx-Generator-based Ion Source". This is a short-pulse ion source in which the discharge is a high-current vacuum spark, rather than an arc (i.e., the voltage between anode and cathode is much higher than the arc-typical 20 Volts). High charge states have been obtained (up to +8 for tungsten and +6 for copper), however, due to limited quility of hig vacuum and very low duty cycle of pulses, significant amounts of hydrogen and other non-metallic contaminiatios were found.
Publication highlights:
See also Publications
The flexibility achieved by using vacuum arc ion sources for non-semiconductor implantation is unmatched by other implanters. Most metallic elements or combinations of metallic elements of the periodic table can be implanted. Recent developments in our implanter technology allow simultaneous implantation of gas and metal ions. Recent investigations using vacuum arc ion implantation include:
1. Implantation of refractory metals such as W, Ta and Ti into steels and aluminum alloys to improve corrosion resistance in acidic and caustic environment.
2. Implantation of noble metals into ceramic materials to tailor the surface electrical resistivity of high voltage ceramic insulators. A direct correlation between the retained dose and the high voltage resistivity has been established when Pt ions were implanted in high purity alumina. The desired resistivity can be achieved by controlling the total implanted dose. The picture below shows Fenghua Liu (PAG) setting up the implantation of an actual "accelerator column" for Jefferson Lab (formerly CEBAF).
3. Co-implantation of gas and reactive metal ions have been used to increase the surface hardness of structural materials such as steel or aluminum alloys. Zirconia and alumina, for instance, have been produced by co-implantation of Zr+O and Al+O respectively. The increase in hardness occurs because the precipitates restrict the motion of dislocations near the surface.
References:
· "Hybrid Gas-Metal Co-Implantation with a Modified Vacuum Arc Ion Source", E.M. Oks, G.Yu. Yushkov, P.J. Evans, A. Oztarhan, I.G. Brown, M.R. Dickinson, F. Liu, R.A. MacGill, O.R. Monteiro and Z. Wang, Nucl. Instr. Meth. B 127/128, 782-786 (1997).
· "Surface Resistivity Tailoring of Ceramics by Metal Ion Implantation", F. Liu, M.R. Dickinson, R.A. MacGill, A. Anders, O.R. Monteiro, I.G. Brown, L. Phillips, G. Biallis and T. Siggins, 10th Int. Conf. on Surface Modification of Metals by Ion Beams, Gatlinburg, TN, September 21-26, 1997
· "Synthesis of Sub-Surface Oxide Layers by Hybrid Metal-Gas Co-Implantation into Metals", I.G. Brown, F. Liu, O.R. Monteiro, K.M. Yu, P.J. Evans, N. Dytlewski, A. Oztarhan, S.G. Corcoran and D. Crowson, 10th Int. Conf. on Surface Modification of Metals by Ion Beams, Gatlinburg, TN, September 21-26, 1997
See also Publications
The vacuum arc ion source provided an excellent opportunity to study fundamental properties of vacuum arcs: the ion extraction system forms an ion beam that can be analyzed with conventional techniques such as Faraday cups. The most fruitful approach was when out self-made time-of-flight (TOF) spectrometer was used. A short 150 ns "slice" of the beam was selected by a pulsed gate electrode system, and the "slice" drifted to a detector in about 1 m distance. The vacuum is typically 10-6 Torr or better, so the mean free path is much longer than the TOF instrument. Because the ion energy is proportional to the charge state according to E=eQVextract, as mentioned before, higher charged ions arrive at the detector earlier. The detector receives species-dependent signals. The amplitudes can be calibrated and quantitative measurements of charge state distributions are possible. By changing the discharge conditions (e.g., applying a magnetic field), the vacuum arc plasma changes, and this is reflected in the charge state composition of the beam. We have measured extensively ion charge state distributions for over 50 elements.
Publication highlights:
See also Publications
For more information on contact André Anders, or Ian Brown.
Plasma Applications Group Home Page