Cathodic Arc Deposition
Cathodic Arc Deposition is a widely used industrial-scale process for applying high quality thin film coatings. The process is based on low-voltage, high current cathodic arc physics that produce a dense, highly ionized plasma. Cathodic Arc Deposition is characterized by a nearly 100% ionized deposition plasma with relatively high-energy deposition ions.
Cathodic Arc Deposition works under vacuum conditions using specially designed deposition heads. Cathodic Arc Deposition can be operated in either DC or pulsed modes. In either case, a power supply applies a voltage which produces an arc discharge between an anode and cathode. The arc current is concentrated over a small surface area on the cathode which creates an extremely high current density (~ 1012 A/m2) at what are generally called “cathode spots”.
This high current density is associated with an extremely high power density (~ 1013 W/m2) that produces a localized phase transformation of the solid target (the cathode material) to an almost fully ionized deposition plasma. The plasma expands rapidly into the ambient vacuum towards the substrate.
At the time of deposition on the substrate, the plasma has ion velocities with kinetic energies of about 20 eV for light elements and 200 eV for heavy elements. This can be compared to sputtering, where the energy is a few eV at most.
There are a number of advantages to the higher ion energies associated with Cathodic Arc Deposition. For example, Cathodic Arc Deposition films tend to be denser and have better adhesion characteristics than films produced using other methods. The deposited atoms penetrate the surface, locking the coating to the surface with high adhesion.
The energetic ions created by Cathodic Arc Deposition also allow the use of lower substrate temperatures compared to other processes. This is because the Cathodic Arc Deposition ions carry sufficient energy to form dense, compact films without the need for additional thermal energy to be provided by the substrate.
Cathodic Arc Deposition’s high ionization fraction allows the deposition material to be controlled. For example by biasing the substrate, the impact energy of the ions on the substrate can be increased. The plasma stream can also be rastered using magnetic fields, which allows the deposition material to be moved about the surface, averaging the coating without moving the substrate.
For reactive deposition, Cathodic Arc Deposition allows chemically accurate films to be produced over a wide range of gas pressures. This eases the necessity for precise pressure control, which increases yield and reduces reworks, reducing the cost of the coating. By contrast, reactive sputtering commonly suffers from “target poisoning,” in which oxygen impinges on the surface of the target and forms oxides, influencing the sputter rate. This creates uniformity problems with the coating. Because of the energy associated with Cathodic Arc Deposition processes, target poisoning does not occur as easily, producing more uniform films with fewer problems.
The Cathodic Arc Deposition process produces so called “macro particles” (or droplets) along with the deposition plasma. Macro particles range in size from less than a micrometer to about 10 micrometers in diameter. For many coating applications (tool coatings for example) the macro particles are not detrimental and no measures are taken to eliminate them. However, for some applications (e.g. optical coatings) the macros degrade the coating sufficiently that they must be removed. This is generally accomplished using 90 degree magnetic filters that guide the deposition plasma away from the straight line trajectories of the macros. Using a filter, over 99% of macros are removed, producing high-quality, particle-free coatings.