|
Theory of Operation
Electron beam heating is an efficient way of achieving temperatures in excess of 3,500°C (6,300°F) for uniform thin film, optical coating and vacuum metallurgic processes. The all-metal-sealed models are bakeable to 230°C (446°F). The e-Gun source and companion control power supply have been designed to be part of any high or ultra-high vacuum system.
The e-Gun source is a self-accelerated electron beam device. A beam of electrons, held at a high negative potential, is produced by the hot tungsten filament. The beam of electrons leaving the filament is magnetically focused and then deflected 270° by the integral permanent magnet. The electron beam is accelerated into the evaporant material contained within the water-cooled, grounded crucible. The beam strikes the evaporant material with a spot of approximately 1/8" diameter and an intensity of approximately 25 kW/cm2. Sweeping the electron beam across the evaporant material contained in the crucible is performed by an electro-magnetic system, with either manual or automatic controls.
The rate at which source materials may be evaporated is dependent upon power input, charge size, charge shape, and the characteristics of the material to be evaporated. The highest evaporation rates are obtained with materials that have low evaporation temperatures and low thermal conductivity.
The evaporant va cannot become contaminated because the focused electron beam strikes only the evaporant source material in the crucible. The beam deflection and effective shielding keeps the filament hidden from the evaporant va. The crucible is cooled so efficiently that its surface never gets warm enough to react with the evaporant material.
The e-Gun source will operate in a system base pressure as low as 5 x 10-11 Torr. The magnitude of pressure, which normally increases during evaporation, depends on the pumping capacity of the system and the cleanliness of the evaporant material. This pressure increase is minimized by the high thermal efficiency of the source. There is minimal outgassing of surrounding surfaces because only the evaporant is heated.
All of the Thermionics single, linear motion, three and five position e-Guns are 100% metal-sealed, thus eliminating “O” ring outgassing. Outgassing could require additional pumping capacity and sometimes result in sample contamination.
|
e-Guns
