The plasma state of a particle mixture is often described as the fourth state of matter. Under normal pressure, a plasma will only be created if a gas is heated to a temperature high enough to transform its molecules into ions and electrons. However, at very low pressures plasma can be formed without high temperatures.
Coating technology uses plasma to deposit thin layers of active material, for example to create a mirror or apply a non-stick coating to components.
Relatively delicate materials such as plastic, rubber and natural fibres are unsuitable for high-temperature processes. To apply coatings to such materials a low-pressure plasma is used, which operates at temperatures between 40°C and 120°C. The items are thoroughly cleaned and placed in a chamber, which is then evacuated. The vacuum system providing the medium and high levels required must be completely reliable, and its characteristics must be matched to the coating process.
A process gas – for example Argon – is then added to the chamber. High-frequency, microwave or electrical energy is applied to the gas, igniting a plasma flame. This splits the coating material into minute particles, which are then accelerated. The particles impact the items in the chamber with high energy, forming an even layer. Volatile by-products of the plasma are removed by the vacuum pump, and if necessary may be neutralized by an exhaust scrubber.
A variety of Busch products may be used to generate the medium and high vacuum levels required for plasma coating. Several variants of the COBRA range of screw vacuum pumps have proved to be particularly suitable, also in combination with a Panda vacuum-boosters. For smaller plasma coating plant with load-lock systems Fossa scroll vacuum pumps may be used, with combinations of Zebra rotary vane and Turbo molecular vacuum pumps for the process chamber.