These techniques include relatively simple methods which take a short amount of time and are mostly performed on individual parts ex-situ. They’re effective against gross and fine surface contamination and can reduce outgassing rates by anything from 50% to five orders of magnitude. Proper material preparation is vital to achieve low outgassing rates and reach UHV.
Cleaning should be followed by bakeout for reduced outgassing rates. It’s important that items are handled carefully once material preparation has begun. This prevents contamination, as a set of fingerprints (for example) can take several days to desorb. The length of time in which there’s exposure to moisture should be limited wherever possible.
Surface treatments reduce the net surface area by reducing roughness; the most common techniques are mechanical polishing and electropolishing.
Mechanical polishing is often one of the first material treatments used to remove gross contaminants, while electropolishing replaces an amorphous surface layer with an ordered oxide layer. Electropolishing is particularly effective against hydrogen/hydrocarbons. The net effect of reducing surface roughness is shown below in diagram two.
Passivation via coatings creates a barrier layer against contaminant adsorption and permeation. Coatings are usually applied via CVD, PVD or sputter coating at raised temperature (200-500°C) and can be:
- Passive — a simple barrier
- Active — pumping gases (H2, CO, H2O, O2 and N2) from the chamber and trapping them. These (Non-Evaporable Getter) coatings require periodic activation by heat to keep surface sites free.
A constant flow of a dry gas through the chamber can remove contamination and reduce water vapour concentration. Even a short purge is effective at reducing outgassing. After a purge flow stops, humidity can rise to over 30% within a few hours. You can see these effects represented in the graph.
Backfilling, or venting, with N2 can also reduce water vapour for systems regularly let up to the atmosphere — as shown in diagram 4. A relatively new technique of bakeout/purge uses inert gas pumping/purging cycles during bakeout and gives a faster bakeout as shown in diagram 5.
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