Photographs of Cr arc discharge burning in vacuum and Ar atmosphere. Exposure = 1 ms.
Research on arc plasmas supports an ongoing programme on the coating-substrate interface preparation. Arc discharges are used for pretreatment of substrates prior to coating deposition.
By applying bias voltages up to -1200 V to substrates in an arc plasma environment, the metal ions produced in the cathode spot of the discharge can be accelerated and implanted in the material.
Arc plasmas are characterised with a high proportion of highly charged states - up to 5+ for Nb - which can be accelerated to enormous energies.
Nb arc plasmas contain high densities of multiply-charged metal ions. In contrast, the dominant metal species in magnetron sputtering plasmas are neutral atoms.
Influence of Process Gas on the Arc Discharge
The theory of arc discharges is well developed for vacuum environments, however industrially-relevant arc processing is often carried out in a gas environment. The presence of gas can affect significantly the charge state distribution of metal ions near the substrate. For example, highly charged states such as 5+ and 4+ vanish to negligible levels even at small pressures - 0.01 Pa. The average charge state can drop significantly at pressures of 0.1 Pa.
The presence of gas can lower the electron temperature of the plasma in the vicinity of the substrates and probably in the cathode spot itself due to ionisation and excitation collisions where electrons loose their energy. Additionally, collisions of metal ions with the neutral gas atoms can trigger a charge exchange reaction whereby an electron is transferred from the neutral particle to the ion.
Although researchers are forced to apply concepts of vacuum arcs to arcs burning in low pressures of gas, it is not always obvious that this link can be made.
The average charge states of metal ions in Nb arc plasmas decreases significantly as a result of the interaction with the process gas.
This work was performed in cooperation with Dr Andre Anders from the Lawrence Berkeley National Laboratory, USA.
Influence of the Magnetic Field Configuration
Magnetic fields are very often used in industrial processing with arcs because they increase the speed of the cathode spot motion on the cathode surface which results in diminished droplet size.
The geometry of the magnetic field influences the diffusion paths of electrons but also of ions via the ambipolar interaction. Measurements and finite element modelling of the magnetic field allow these paths to be determined.
The diffusion path and confinement of the plasma is determined by the configuration of the magnetic field.
The presence of a residual plasma has been detected when the magnetic field forms a large tunnel across the target area. Both the volume and lifetime of the residual plasma are diminished when the width of the tunnel is decreased.
Fast photographs illustrating different degrees of residual plasma confinement depending on the shape of the magnetic field. Exposure time = 100 microseconds.
The presence of a residual plasma has a strong effect on the chemical composition of the products of the arc spot. Time-resolved OES measurements of single spectral lines of Cr2+, Cr1+ and Cr0 illustrate a severe influence on the charge state distribution.
The influence of coil current on the CSD of ions is linked to the trapping of gas plasma in certain magnetic field configurations.
Experimentally recorded movies illustrating the motion of the cathode spot under different configurations of the magnetic field depending on the coil current Icoil = -1 A, 0 A and +1 A. Exposure = 100 us, Interval = 200us
The actual speed of motion of the cathode spot is 10 rounds per second.
Plasma Diagnostic Techniques used
- Optical Emission Spectroscopy (OES)
- Time resolved OEST
- Electrostatic probe diagnostics
- Magnetic field measurement and simulation