How to reduce outgassing in vacuum systems
January 18, 2022
4 min read
There are four main ways you can reduce outgassing in your vacuum system. These are: cleaning and handling, surface treatment, passivation, and purging and backfilling. In this blog, we will take a closer look at each of these methods.
This blog is based on the article in Applied Science and Convergence Technology 26 (5): 95-109 (2017) R Grinham and A Chew, courtesy of www.vacuumscienceworld.com
Four ways to reduce outgassing in vacuum systems
There are four main ways you can reduce outgassing in your vacuum system. These are: cleaning and handling, surface treatment, passivation, and purging and backfilling. In this blog, we will take a closer look at each of these methods.
1. Cleaning and handling
These 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.
Generally, the cleaning process will involve the following steps:
Remove gross contaminants such as rust, grease or paint
Remove fine contaminants such as oils, cutting lubricants and adsorbed species such as water
Remove hydrogen from the materials bulk (most metals)
This cleaning should be followed by bakeout for reduced outgassing rates. This effectively reduces outgassing by creating a smooth surface.
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.
Diagram 3: An example of cleaning, Glow Discharge Cleaning-1
Learn about the basics of outgassing such as the contributing mechanisms, typical outgassing vales and more in our blog post “Introduction to outgassing”.
2. Surface treatment
Surface treatments reduce the 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 four.
For example, for stainless steel with a typical outgassing rate of ~2e-7 mbar/s/cm2 electropolishing reduces outgassing by a factor of 30.Meanwhile, mechanical polishing reduces outgassing by a factor of 50 and a 30-hour bakeout at 250°C by a factor of over 70,000.
Diagram of effect of surface roughness on outgassing
3. Passivation
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 (NEG) coatings require periodic activation by heat to keep surface sites free.
As mentioned, bakeout is one of the most commonly used and reliable passivation techniques.
From 100–500°C is required to remove water vapour; higher temperatures up to 1000°C are required for hydrogen removal from the material’s bulk.
Longer and repeat baking leads to lower outgassing rates, as shown in the graph below.
Diagram 5: Bakeout cycling
Purging and backfilling
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 below.
Diagram 6: vent/purge cycling effect on outgassing
Backfilling, or venting, with N2 can also reduce water vapour for systems regularly let up to the atmosphere — as shown in the graph above. A relatively new technique of bakeout/purge uses inert gas pumping/purging cycles during bakeout and gives a faster bakeout as shown in the graph below
Diagram 7 Bake with purge gas and pressure cycling3
Summary
Outgassing can limit the achievable vacuum in a system and is often the most important gas source in HV and UHV. There are several techniques to reduce outgassing as much as possible. These include actions to be taken ex-situ such as surface cleaning and treatments and vent-purge cycling and system bake-out.
No matter your application and vacuum system requirement, we can provide expert analysis and recommendations to ensure your vacuum system is reliable and built-for-purpose.
Reference
1 M. Li, H. F. Dylla, Reduction of Outgassing Rate by Glow Discharge Cleaning, Journal of Vacuum Science & Technology A 13.3 (1995): 571-575
2 H. F. Dylla, D. M. Manos, P. H. LaMarche, Correlation of outgassing of Stainless Steel and Aluminium with Various Surface Treatments, Journal of Vacuum Science & Technology A 11.5 (1993): 2623-2636, M. Suemitsu et al., Ultrahigh-Vacuum Compatible Mirror-Polished Aluminium-Alloy Surface: Observation of Surface-Roughness-Correlated Outgassing Rates, Journal of Vacuum Science & Technology A 10.3 (1992): 570-572
3 A Harpham, F. Tapp, A. Keen, N. Condon, Rapid Attainment of Vacuum System
Base Pressure by Combination of Pumping & Purging, BOC Edwards, Third EUVL Symposium (2004), J. Zhou, S. D. Dasso, Cycle Purging a Vacuum Chamber During Bakeout Process, U.S. Patent No. 5,879,467 9 Mar. 1999
