What is Outgassing?
Several factors contribute to a system's gas load. Outgassing is frequently the dominant process at pressures below 0.1 mbar. It occurs as a result of previously adsorbed molecules desorbing, bulk diffusion, penetration, and vaporization. Adsorption is characterized by five (or six) classification isotherms and involves the two basic processes of physisorption and chemisorption. You can lessen outgassing in your vacuum system with the following techniques.
1. Maintenance and Handling
These procedures include basic processes that take little time and are mainly conducted ex-situ on specific pieces. They are efficient against coarse and fine surface pollution and can lower outgassing rates by up to five orders of magnitude. Proper material preparation is essential for achieving low outgassing rates and UHV.
To decrease outgassing, a bakeout should follow cleanup. Items must be handled with care once material preparation has begun. It avoids contamination, as a set of fingerprints, for example, might take many days to desorb.
Limit exposure to moisture as much as possible.
2. Surface Treatment
Surface treatments, such as mechanical polishing and electropolishing, lower the net surface area by decreasing roughness.
Mechanical polishing is frequently employed to remove gross impurities, whereas electropolishing replaces an amorphous surface layer with an ordered oxide layer. Electropolishing works very well on hydrogen/hydrocarbons.
3. Passivation
Passivation by coatings generates a barrier layer against pollutant adsorption and penetration. Coatings are often applied at elevated temperatures (200-500°C) via CVD, PVD, or sputter coating and can be:
Passive — a simple impediment
Active — extracting and trapping gasses (H2, CO, H2O, O2, and N2) from the chamber. These coatings need to be heated up regularly to keep surface sites clean.
4. Backfilling and Purging
A continual passage of a dry gas through the chamber can eliminate contaminants and lower the quantity of water vapor. Even a little purge might help to reduce outgassing. When a purge flow is interrupted, humidity can quickly climb to over 30%.
Backfilling or venting with N2 can also help minimize water vapor in systems frequently vented to the atmosphere. A very innovative bakeout/purge approach employs inert gas pumping/purging cycles during the bakeout to achieve a quicker process.
Conclusion
Outgassing, which is frequently the most important gas source in HV and UHV, might restrict the possible vacuum in a system.
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