V.N. Zhitomirsky

Recent progress in filtered vacuum arc deposition

Abstract During this decade significant advances have been made both in the understanding and implementation of filtered vacuum are deposition. Rigid rotor models have been analyzed statistically, and new models which treat the mutual influence of the electrons and ions on each other self-consistently, take into account the centrifugal force on the ions, and take into consideration collisions, have been formulated. It was shown that the plasma transport efficiency is limited by drifts caused by the centrifugal force and by the electric field generated by charge separation in the plasma. For a range of magnetic fields strengths for which the ions are not magnetized, i.e., confined to a Larmor radius less than the duct radius, the transport efficiency for Cu plasma is about 10%, and depends only weakly on the magnetic field strength. Increased transmission is found when the ions are magnetized, reaching about 50% for a 36–60 mT field in typical configurations. The plasma transport efficiency and spatial distribution has been measured over a large parameter range, and correlated with the various theories. The plasma beam may be approximated as a Gaussian distribution which is displaced in the B × G direction, where G is in the direction of the centrifugal force, while a displacement in the plane of symmetry is surprisingly found in the − G direction. The total convected ion current decreases exponentially with distance from the toroidal filter entrance. Macroparticle transport within the magnetic filter has been analyzed, and it has been shown that electrostatic reflection from the walls can occur if the magnetic field is weak. Filtered arc sources with improved throughput performance and novel geometries have been built, and are now available commercially. The range of coatings deposited with FVAD has been expanded to include metals, oxides, and nitrides, as well as diamond-like carbon. In several cases, coatings having the highest quality reported in the literature have been fabricated with the FVAD technique, and one commercial application has been reported.

Structure and mechanical properties of vacuum arc-deposited NbN coatings

Abstract Thin NbN coatings were deposited using a vacuum arc plasma gun connected to a straight plasma duct, with an imposed axial magnetic field. The substrates were cemented carbide bars having a composition of 90% WC, 1.8% TaC, 0.2% NbC, and 8% Co. The influence of the nitrogen pressure in the deposition system, which was in the range of 0.13–2 Pa, on the structure, phase composition, microhardness, and scratch critical load of the coatings was studied. It was shown that for nitrogen in the pressures range of 0.13–0.4 Pa the coating is composed of a mixture of two phases: hexagonal β-Nb2N and cubic δ-NbN, whereas at pressures of 0.67 Pa and above single-phase δ-NbN with a NaCl type structure was obtained. In most cases the coatings consisted of randomly oriented equiaxial grains. A maximum microhardness of 42 GPa was obtained for the two-phase coatings deposited at a nitrogen pressure of 0.4 Pa. However the maximal critical load of 95 N was obtained with the homogeneous δ-NbN coatings, deposited at a nitrogen pressure of 0.93 Pa, while the coatings with the hexagonal β-Nb2N phase had a much lower critical load (50 N). The NbN coatings with the highest critical load exhibited an average Vickers microhardness of 38 GPa.

Vacuum arc deposition devices

The vacuum arc is a high-current, low-voltage electrical discharge which produces a plasma consisting of vaporized and ionized electrode material. In the most common cathodic arc deposition systems, the arc concentrates at minute cathode spots on the cathode surface and the plasma is emitted as a hypersonic jet, with some degree of contamination by molten droplets [known as macroparticles (MPs)] of the cathode material. In vacuum arc deposition systems, the location and motion of the cathode spots are confined to desired surfaces by an applied magnetic field and shields around undesired surfaces. Substrates are mounted on a holder so that they intercept some portion of the plasma jet. The substrate often provides for negative bias to control the energy of depositing ions and heating or cooling to control the substrate temperature. In some systems, a magnetic field is used to guide the plasma around an obstacle which blocks the MPs. These elements are integrated with a deposition chamber, cooling, vacuum g...

Bias voltage and incidence angle effects on the structure and properties of vacuum arc deposited TiN coatings

TiN coatings were deposited on WC–Co bar substrates using a vacuum arc plasma gun connected to a cylindrical plasma duct in which an axial magnetic field was imposed. During deposition, the cathode arc current was 200 A, nitrogen pressure was 0.67 Pa, and the substrate temperature was 420°C. Substrate bias voltage (Vbias) was varied in the range of −40 to −600 V. The coating structure and properties were studied both on the sample face surface, i.e. normal to the plasma flux, and on its side surfaces. The structure and phase composition were studied using scanning electron microscopy (SEM) and X-ray diffraction (XRD). Microhardness and scratch critical load were studied using Vickers micro-indentation and scratch tests, respectively. The TiN coatings had a single-phase cubic δ-TiN structure and consisted of oriented columnar grains. No difference was observed in the preferred grain orientation and grain size at the substrate–coating interface for the coatings deposited on the face and the substrate side surface for every studied Vbias. The deposition rate decreased both on the face and the side surfaces, while the ratio between deposition rates on the face and the side surfaces increased from three to seven times when |Vbias| increased from 40 to 600 V. With increasing |Vbias|, the preferred orientation of the columnar grains changed from a mixture of (200) and (111) at −40 V to a strong (111) at −200 and −400 V. At −600 V, (111) remained dominant, while the (220) orientation also appeared. Increasing |Vbias| increased the grain size on the coating surface. A zone of equiaxed grains was observed near the substrate–coating interface, whose thickness increased with increasing |Vbias|. Possibly, the grain size growth was a thermal effect due to an increase in ion beam heating with increased |Vbias|. The grain size on the side surfaces was smaller than that on the face. The coating surface roughness and friction coefficient were smaller on the side surfaces than those on the face, while no differences in microhardness were observed.

Unstable arc operation and cathode spot motion in a magnetically filtered vacuum‐arc deposition system

Arc discharges were established on a Ti cathode in vacuum in an arc deposition system with a toroidal macroparticle filter. A magnetic field with a radial component of up to 3 mT on the cathode surface at the distance of 20 mm from the cathode center was applied in order to drive the cathode spots in an azimuthal motion on the front surface of the cathode, and an axial component of the field of up to 30 mT was applied parallel to the walls of the plasma ducts leading from the cathode region to the substrate in order to collimate the plasma beam. Cathode spot motion was observed by means of a television camera and VCR via a window installed at the substrate holder flange and a mirror located in the quarter torus. Ion current convected by the plasma beam was measured with a negatively biased probe. It was shown that the magnetic field of the coils located on the plasma duct has a strong influence on cathode spot behavior. If they produce a field stronger than 4 mT at the cathode surface, the spots move off ...

Vacuum arc deposition and microstructure of ZrN-based coatings

Abstract Thin coatings of ZrN, and bilayer coatings TiN/ZrN and ZrN/TiN of up to 3 μm thickness were deposited using a triple-cathode vacuum arc plasma gun connected to a straight plasma duct, where an axial magnetic field was imposed. The substrates were cemented carbide bars, having a composition of 90% WC, 1.8% TaC, 0.2% NbC, and 8% Co. The coatings were deposited with an arc current of 200 A, background nitrogen pressure of 0.4–2 Pa, substrate temperatures of 200 to 600 °C, and substrate bias voltages in the range of 0 to −200 V. The magnetic field in the duct was in the range of 1 to 10 mT. The structure and composition of the coating and interface morphology were studied by means of X-ray diffraction, Auger electron spectroscopy, transmission electron microscopy and scanning electron microscopy combined with energy dispersive spectroscopy analyses. It was shown that for nitrogen pressures higher than 0.4 Pa a single-phase ZrN coating with a NaCl-type structure was obtained. The microstructure of the ZrN coatings and ZrN and TiN layers of the bilayer coatings was found to be composed of (111) oriented columnar grains, although near the coating-substrate interface randomly oriented grains were also observed. In the bilayer coatings a sharp interface without intermixing between the TiN and ZrN layers was observed. The preferred grain orientation was independent of the substrate bias voltage and temperature. However, the coating grain size increased with the substrate temperature and decreased with the substrate bias voltage. It was shown that Co diffused from the cemented carbide substrate to the free surface of the coating, and its concentration there increased with the deposition temperature.

Ion current distribution in a filtered vacuum arc deposition system

Abstract The ion current distribution produced by a filtered vacuum arc deposition system consisting of a 90-mm diameter cathode, 122-mm internal diameter anode, and a 240-mm major radius / 80-mm minor radius quarter-torus duct macroparticle filter was measured with a nine-element multi-probe, and the total ion current was measured with a 130-mm diameter probe. Arcs were sustained on Ti and Sn cathodes, with currents of 300–340 and 160–175 A, respectively. Radial magnetic fields of up to 3 mT were imposed on the cathode surface for cathode spot rotation, and fields of up to 20 mT generally parallel to the duct were imposed for plasma guiding. In general, the plasma distribution function exiting from the duct was approximately Gaussian, but with an offset from the center of the duct. At the torus exit, the Ti plasma beam was offset 12 mm towards the outside of the torus at B = 4 mT, while with increasing toroidal field the beam center moved towards the center of the duct, and reached the center at B = 8 mT. The beam was offset towards the inside of the torus for B > 8 mT, reaching a displacement of up to 5 mm for B = 12 mT. The Sn beam, by contrast, was shifted towards the inside of the torus, and the displacement from the center was as much as 15 mm. In addition, for both cases an offset in the (B × g ) direction was observed, where B is the duct magnetic field vector and g is a vector pointing outward from the toroidal center of curvature. The displacements from the center were stronger for Sn plasma (up to 16 mm) than for Ti plasma (up to 7 mm). The displacement from the center in both directions grew with the distance from the exit of the torus. A beam steering coil, whose axis was normal to the axis of plasma duct, was used to shift the plasma beam center towards the chamber axis.

Filtered vacuum arc deposition of semiconductor thin films

The cathode spot vacuum arc produces a jet of highly ionized plasma plus a spray of liquid droplets, both consisting of cathode material. The droplets are filtered from the plasma by passing the plasma through a curved, magnetized duct. A radial magnetic field may be applied to the face of the cathode to rotate and distribute the cathode spots in order to obtain even erosion and avoid local overheating. The choice of axial magnetic field strength in the vicinity of the cathode is a compromise between a relatively high field desired to collimate a large fraction of the plasma flux, and the need to collect a substantial fraction of the plasma at the anode in order to reduce arc voltage and insure arc stability. The transmission of the filter duct increases with magnetic field strength until a saturation value is reached. Entrainment of the droplets in the plasma jet can decrease the effectiveness of the filter at high plasma flux. Semiconducting thin films of amorphous silicon were prepared using cathodes of heavily B-doped Si. Arcs of 35-A current produced a deposition rate of 10 /spl Aring//s. The electrical conductivity of the films was similar to conventional a-Si:H films deposited by conventional Silane based PACVD at high temperatures, but had a higher room-temperature conductivity. Transparent conducting films of Sn-O were deposited at rates of up to 100 /spl Aring//s using 160-A arcs on a Sn cathode while injecting O/sub 2/ gas in the vicinity of the substrate. Adjustment of the O content is critical for optimizing conductivity, and complicated by pumping effects of the arc. Optimal conductivity was achieved at an oxygen pressure of 6 mtorr. Conductivities equal to the best reported to date were achieved by subjecting the room-temperature deposited films to a 30-s rapid thermal annealing at 350/spl deg/C. Both the deposited and annealed films are amorphous. The deposition rates achieved by the filtered vacuum arc technique for these semiconductor films are an order of magnitude greater than achieved with conventional methods, while the conductivities are equivalent or better.

Magnetic control, in vacuum arc deposition - a review

The use of magnetic fields to control cathode spot location and motion and to collimate and direct the plasma flow in vacuum arc deposition apparatus is reviewed. Retrograde and acute angle motion are used to control the location and motion of cathode spots, in order to confine them to the cathode surface facing the substrates, to prevent local overheating, and to reduce macroparticle production. Axial fields are use to collimate cathode spot produced plasma jets, and to bend them around macoparticle occluding obstacles in filtered vacuum arc deposition. Advances have been made in understanding these phenomena, but theoretical models have not yet been formulated which can predict plasma behavior sufficiently well for apparatus design.

Influence of gas pressure on the ion current and its distribution in a filtered vacuum arc deposition system

Abstract A filtered vacuum arc deposition system consisted of a cathode, an annular anode, a quarter torus duct macroparticle filter, and a deposition chamber. Arcs were sustained on a Ti cathode in the presence of noble background gases helium and argon, and reactive gases nitrogen and oxygen. The gas pressure was continuously varied from 3 × 10 −5 Torr (4 mPa) to 0.1 Torr (13.3 Pa). A toroidal magnetic field of up to 20 mT, and a straight field in the deposition chamber of up to 10 mT were imposed for plasma guiding. The total saturation ion current was measured with a 130 mm diameter probe. The ion current density distribution was measured with a nine-segment multi-probe, and the individual probe element currents were fitted to a two-dimensional Gaussian distribution. It was shown that in the presence of a noble gas the total saturation ion current at first increases with increasing the background gas pressure, then achieves a maximum at a pressure of 10 mTorr (1.3 Pa) for He, and at 2 mTorr (0.26 Pa) for Ar, where its value is 1.4–2 times greater than in vacuum. With further increase in the pressure the ion current strongly decreases. In the presence of reactive gases this maximum is not observed, and the total ion current strongly decreases at pressures greater than 2–3 mTorr (0.26–0.4 Pa). In contrast to this, a maximum is observed in the ion current collected on small diameter individual probes of multi-probe, positioned towards the direction of the plasma beam displacement. With increasing gas pressure, the distribution width decreases, and a displacement of the beam center is observed both in the − g direction and in the ( B × g ) direction, where B and g are vectors of the toroidal field and centrifugal acceleration, respectively. The present results show that proper substrate positioning in the deposition chamber must take into account the beam displacement due to the background gas.