Introduction to Vacuum Coating by Thermal Evaporation
January 7, 2021
The material must be heated to a temperature that results in substantial high vapor pressure, which limits the use of high-melting material and the choice of the container. Evaporation can be achieved by heating wires electrically or depositing them in electrically conductive crucibles made of a material with a significantly higher melting point. Oxides can be evaporated out of boat-shaped evaporators. Image 2 shows a selection of various thermal evaporators.
Larger coating machines typically create web coating for packaging foils. Films like aluminium are applied to plastic foils in “roll-to-roll” web coaters. These thin films create a protective barrier against air and moisture which prolongs the freshness and shelf life of consumer goods.
In these production machines with high foil throughput (several meters per second!) the gas flow into the vacuum system is immense. The large foil surfaces to be coated generate large amounts of degassing. Generally, the vacuum systems consist of large oil diffusion pumps for pumping the air supported by cold panels to condense the water vapour. These cold panels are cooled by cryogenic refrigerators, or cryochillers, sometimes called “Polycold machines”.
Special cryochillers are designed to cool to temperatures of 110 K. This provides up to 200,000 l/s of pumping speed for water vapor inside the chamber. Fore-vacuum is generated by a roots blower systems.
An other application would be the production of costume jewelry. The optical effects are generated by special coatings adhered via thermal evaporation.
As you can imagine, the list of such applications can go on. This blog post series is just an introduction to get you started on this engaging field and vacuum systems application. In our next blog post we’ll continue our journey and introduce sputter technology and some of its typical applications.