3.4.2 Processing – Cleaning
Clean surfaces are a prerequisite in vacuum technology. All impurities must be removed from the surfaces, so they do not desorb under vacuum conditions and produce gas loads or deposit on components.
Initially a pre-treatment is required, for example, with a high pressure cleaner, to remove coarse dirt. Subsequently, the components will be cleaned in a multi-chamber ultrasonic bath. The first cleaning is under ultrasonic conditions with the addition of special cleaners, which clean and degrease the surfaces. Contaminations are coated with surfactants, lifted from the surface and bound in the cleaning bath. The pH of the cleaning bath must be adjusted to the chamber material. In other baths, the detergent is completely removed, by pre- rinsing followed by thorough rinsing with hot, deionized water. And quickly after that, drying must be done in hot, dust-free and hydrocarbon-free air. Large chambers are cleaned with steam or high-pressure cleaners with the addition of special cleaning agents. After that, it must be washed again several times with hot deionized water, quickly followed by a drying in hot air.
After cleaning, the vacuum-side surfaces must only be touched with clean, lint-free gloves. The packaging used is PE plastic films, and sealing surfaces and knife edge profiles are protected with PE caps.
The surface of cleaned components still represents an outgassing source. Specifically adsorbed water molecules and traces of hydrocarbons from storage in air are the biggest sources of residual gas sources in UHV. In order to effectively remove these from the surfaces, UHV chambers are heated. Under continuous evacuation at a pressure less than 1 · 10-6 hPa, the components are typically heated to temperatures of 150°C to 300°C for about 48 hours.
Foreign atoms bonded to the surfaces through physisorption or chemisorption obtain thermal energy through these processes, with which they can dissolve the bond and become released from the surface. The molecules released from the surface must be removed from the system by the vacuum pump. After cooling, a final pressure is obtained that has been reduced by several orders of magnitude. If the chamber is vented, the surfaces are once more become with molecules. Using dry nitrogen as the flood gas and brief exposure times to a dry atmosphere cannot completely prevent water buildup on the surfaces, only reduce it. In order to attain the final pressure within a tolerable pump-down time, smaller than 1 · 10-8 hPa, another bakeout is inevitable.