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WIOPEDIA - The WIOTEC® electroplating encyclopaedia


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E

Evaporation in a high vacuum

In this process, the material to be evaporated, from which the layer is to be created, is heated in a vessel made of refractory metal or ceramic until it melts and then evaporates. However, it can also be converted directly from a solid to a gaseous state. The vaporisation temperatures are in the range between 1500 and 2000 °C. The particle energy at these evaporation temperatures is about 0.2 to 0.3 eV. Thus, it is considerably lower than in the plasma-assisted PVD process. Vapour-deposited coatings have a considerably lower adhesive strength than sputtered* coatings. However, this is countered by layer growth rates: they are two orders of magnitude higher in evaporation processes than in sputtering processes.

Three main processes are used in the production of vapour-deposited coatings:


Resistance-heated evaporation:
The material to be evaporated is filled into a boat made of refractory metal (e.g. tungsten, tantalum, molybdenum). The boat is clamped between two electrodes and heated by an electric current flowing through it. This process is used to vaporise pure materials. Alloys cannot be evaporated without segregation phenomena due to different melting and evaporation temperatures of the individual components, so that the alloy partners build up separately from each other in the resulting layer.

Flash Evaporation:
A process for the evaporation of chemical compounds and alloys.
It is also a resistance-heated process, but - in contrast to the previous procedure - the evaporation material is not filled into the boat in advance. First, the temperatures of the boat must be raised to temperatures above the evaporation temperature of the higher evaporating alloy partner.
Then the powdered evaporation material is allowed to trickle into the glowing shuttle via a so-called oscillating conveyor. When the individual alloy particles hit the surface of the shuttle, evaporation takes place immediately, so that segregation phenomena are excluded.

Electron beam evaporation:
Electron beams that can be steered by magnetic fields melt/evaporate the material in a cooled copper crucible. This process is suitable for the production of alloy layers from the individual components. This is achieved by oscillating the electron beam between two crucibles containing the individual components. The exact composition of the layer can be adjusted by the dwell time of the electron beam on the respective crucibles.

* = Sputtering, also called cathode sputtering, is a physical process in which atoms are released from a solid by bombardment with high-energy ions and pass into the gas phase. (Source: WIKIPEDIA)

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