A vacuum coating system for multilayer optical coatings
We report on the design and development of a vacuum coating system intended for fabricating multilayer optical coatings. Basically the system consists of a) vacuum pumps and valves, b) vacuum chamber, c) pressure gauges, and d) power supplies. The dielectric materials are evaporated using molybdenum boats. A jig was fabricated for fabricating the boats from molybdenum sheets. In this system it is possible to use at least three evaporation sources; three evaporations can be made succesively without breaking the vacuum. A glow discharge ring, connected to a high voltage power supply, was installed for in-situ cleaning of the substrates. A schematic diagram of the system is shown. At present it operates at a base pressure of 1.2 x 10-3 mmHg. However it was observed after a coating process, using magnesium fluoride, that the pressure dropped to 4.6 x 10-6. Using liquid nitrogen in the cold trap may further reduce the base pressure one order of magnitude. Currently being developed is a computer program for designing multilayer dielectric coalings and a rotating substrate holder. The rotating holder will allow the coating of several substrates without necessarily opening the vacuum chamber. An optical thickness monitor still has to be installed in the present system.