Arc evaporation – What is an arc?
An arc can be defined as a discharge of electricity between two electrodes. Everyday examples include static electricity or lightning.
The arc evaporation process
A high current, low voltage arc is struck on a cathode’s surface to start the arc evaporation process, which creates a cathode spot—a small, extremely energetic emitting region that is typically a few microns across. A crater is left on the cathode surface as a result of a high velocity jet of vapourized cathode material travelling at a speed of 10 km/s due to the extremely high localised temperature at the cathode spot (about 15000 °C). The cathode spot only lasts a little time before it self-extinguishes and re-ignites in a new location near to the original crater. The arc appears to be moving as a result of this activity.
Arc evaporation – High ionization
The strength of the plasma jet is most perpendicular to the cathode surface and is highly ionised (30-100%) with neutral particles, clusters, and macro-particles (droplets).
Arc evaporation – A self-sustaining discharge
The arc is a self-sustaining discharge that can theoretically support huge currents through the emission of electrons from the cathode surface and the subsequent re-bombardment of the surface by positive ions in a high vacuum. The fundamental steps of arc evaporation are depicted in the animation below (secondary electron emission and re-bombardment are not included).
Arc evaporation – Reactive deposition
A compound film will be deposited if a reactive gas is added during the evaporation process dissociation, ionisation, and excitation can happen during interaction with the ion flux.
Arc evaporation – Advisable to control the cathode spots
The cathode spot wanders around randomly without the help of an applied magnetic field, evaporating microscopic asperities and forming craters. However, if the cathode spot remains at one of these evaporative locations for an extended period of time, as illustrated above, it may release a significant number of macro-particles or droplets. Due to their weak adhesion and potential to penetrate the coating, these droplets are harmful to the coating’s performance. Even worse, if the cathode target material—such as aluminum—has a low melting point, the cathode spot may evaporate through the target, evaporating the target backing plate material or allowing cooling water to enter the chamber. As a result, magnetic fields are utilised to regulate the motion of the arc, which is essentially a conductor that can carry electricity. When using cylindrical cathodes, the cathodes can revolve during the deposition process. Aluminium targets can be employed, and the amount of droplets is decreased, by avoiding letting the cathode spot stay in one place for an extended period of time. Some businesses additionally employ filtered arcs that separate the droplets from the coating flux using magnetic fields.