Electron Beam Evaporation (EBE) for multilayer / multimaterial optical coatings is applied in a heated (>250°C) high vacuum chamber by electron beam guns. Each gun emits a high-voltage (2-10kV) electron beam, that is vertically deflected and focussed into a water cooled rotating crucible, containing the coating material. The beam melts and evaporates metals, metaloxides or fluorides at temperatures of ca. 2000°C. Due to the high vacuum environment the material vapour moves as cloud to the substrates, mounted in a spherical calotte in the top of the chamber.
The vapour condenses on the surfaces and films grow mainly in a columnar structure. Rotation of the calotte provides to match surface geometries and an optimal coating thickness distribution. Optical thickness control is ensured by monochromatic quarterwave measurement in transmission or reflection on a witness sample, whereas the physical thickness is detected with a crystal oscillator.
In addition to electron guns, the thermal evaporation process can be generated from an evaporation boat as well, also called resistance heated source. It consists of high melting metals as Molybdenum, Tantalum or Tungsten, containing the coating material. The boats are heated by a strong electrical current to max. 1600°C.
Oxide materials are reactively deposited in an oxygen atmosphere of some 10-4 mbar in order to compensate the decomposition during evaporation.