EBE

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.