Electron Beam Evaporation
Electron Beam Evaporation (EBE) is the conventional coating technique. Due to its high evaporation rate the coating time is relatively short. In addition to the large capacity it has a good cost-performance ratio.
Coatings are done in a heated ( > 250 °C) high vacuum chamber using electron beam guns. Each gun emits a high-voltage electron beam that is focused into a water cooled rotating crucible, containing the coating material. The beam melts and evaporates the material at temperatures of about 2000 °C.
Rather than generated with an electron gun Thermal Evaporation (TE) can also be achieved with an evaporation boat which is heated by a strong electrical current.
Due to the high vacuum environment the material vapour moves as cloud to the substrates, mounted in a spherical or plano calotte in the top of the chamber.
The vapour condenses on the surface of the substrate forming a film with mainly columnar structure. To match surface geometries and to achieve an optimal coating thickness distribution the calotte is being rotated. Optical thickness control is ensured by monochromatic quarter-wave measurement on a witness sample, whereas the physical thickness is detected with a crystal oscillator.
Oxide materials are reactively deposited in an oxygen atmosphere of some 10-4 mbar in order to compensate the decomposition during evaporation.
Many different coating materials (metals, metal-oxides or fluorides and sulfides) can be used, so the application range is from DUV to IR. Due to the porous columnar structure EBE coatings show some thermal drift.