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
The arc evaporation process begins with the striking of a high current, low voltage arc on the surface of a cathode that gives rise to a small (usually a few microns wide) highly energetic emitting area known as a cathode spot. The localised temperature at the cathode spot is extremely high (around 15000 °C), which results in a high velocity (10 km/s) jet of vapourised cathode material, leaving a crater behind on the cathode surface. The cathode spot is only active for a short period of time, then it self-extinguishes and re-ignites in a new area close to the previous crater. This behaviour causes the apparent motion of the arc.
Arc evaporation – High ionization
The plasma jet intensity is greatest normal to the surface of the cathode and contains a high level of ionization (30-100%) multiply charged ions, neutral particles, clusters and macro-particles (droplets).
Arc evaporation – A self-sustaining discharge
Theoretically the arc is a self-sustaining discharge capable of sustaining large currents through electron emission from the cathode surface and the re-bombardment of the surface by positive ions under high vacuum conditions. The animation below shows the basic arc evaporation process (secondary electron emission and re-bombardment are not shown).
Arc evaporation – Reactive deposition
If a reactive gas is introduced during the evaporation process dissociation, ionization and excitation can occur during interaction with the ion flux and a compound film will be deposited.
Arc evaporation – Advisable to control the cathode spots
Without the influence of an applied magnetic field the cathode spot moves around randomly evaporating micoscopic asperities and creating craters. However if the cathode spot stays at one of these evaporative points for too long it can eject a large amount of macro-particles or droplets as seen above. These droplets are detrimental to the performance of the coating as they are poorly adhered and can extend through the coating. Worse still if the cathode target material has a low melting point such as aluminium the cathode spot can evaporate through the target resulting in either the target backing plate material being evaporated or cooling water entering the chamber. Therefore magnetic fields are used to control the motion of the arc, which is basically a current carrying conductor. If cylindrical cathodes are used the cathodes can also be rotated during deposition. By not allowing the cathode spot to remain in one position too long aluminium targets can be used and the number of droplets is reduced. Some companies also use filtered arcs that use magnetic fields to separate the droplets from the coating flux.