Design and Fabrication of UHV Chambers
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What is the impact of material selection, surface finish, and design execution in ultra-high vacuum? What pumping power is actually needed for the application? And why can it be difficult to achieve good ultimate pressure?
Ultra-high vacuum (UHV) by definition begins at an absolute pressure of 10-7 mbar, which means that surface outgassing becomes critical to ultimate pressure at this pressure range. The flow in the UHV is molecular, the mean free path length is more than 1 km. If the pressure continues to drop to 10-12 mbar, the free path length grows to 10,000 km. The remaining particles left in the vacuum chamber now experience interactions only with the vessel walls, but not, or almost not, with each other. In this range, the materials and the surfaces of the chamber become massively more important. So what do you need to consider when designing, manufacturing and operating chambers and components for this pressure range?
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Criteria for material selection
As a start, you need high gas tightness for the chamber wall, as well as low intrinsic vapor pressure and low content of foreign gases. If this cannot be avoided, the material should at least outgas rapidly so that any troublesome residual gases can be pumped out quickly. In UHV, the chamber volume is not an important factor, at most it acts as a buffer during the pressure rise after the pump is switched off or pushed off. In this case, the residual gases come from the surfaces and volume of the vessel walls or installations.
Strength and corrosion resistance are further criteria. Since the sealing surfaces must not deform at a pressure difference of 1 bar, a sufficiently strong material is required.
Corrosion resistance must also be ensured under difficult conditions such as bakeout in atmosphere or with chemically active process gases. It is therefore important to test the materials for their resistance. Good stability during temperature changes and adapted expansion behavior are needed to ensure that the chamber is and remains tight. Materials for flanges and gaskets must be matched.
Stainless steel and copper have similar coefficients of thermal expansion and are therefore a good combination. Stainless steel and aluminum are only a limited match because after temperatures of over 150 °C, the flange connections are often no longer tight when they cool down.
Properties are followed by handling and availability of the materials, because they should be able to be processed with reasonable affordable effort, and must of course be available.
Due to the low demand in UHV technology, there is no own material development and one has to work with what is already available. Austenitic stainless steel is particularly suitable for UHV applications.
Effects in ultra-high vacuum
The following terms describe the effects that happen at surfaces in UHV:
1. Adsorption: Gas deposits on the surface of solids or liquids, such as particles sticking to the chamber wall.
2. Absorption: Gas trapped in solids or liquids. Absorption often follows adsorption. Particles previously attached only to the surface are now embedded in the chamber wall.
3. Desorption: Release of adsorbed gas into the environment. The particles retained by the first two effects detach from the chamber wall again.
4. Permeation: Transport of gas through a liquid or solid. Permeation = adsorption + diffusion + desorption.
Pressurization, adsorption and absorption are not problems because the particles are held and do not disturb the vacuum. Both effects happen on all surfaces that are in contact with atmosphere, as well with each ventilation. Desorption is the main opponent on the way to a good ultimate pressure. This is because particles attached to the outside of the chamber diffuse through the chamber wall during permeation, increasing desorption into the vacuum chamber.
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Interactions between the surfaces of a vacuum chamber and the surrounding gas.
Definition of working pressure
Some basics about the requirements for the materials have now been described. Next, the desired working pressure pWork must be determined in order to proceed with the construction of a vacuum chamber.
The backing pump, up to a maximum of 10-3 mbar pumps out the volume after which a high or ultra-high vacuum pump with a suitable pumping capacity provides the working pressure. For this purpose, it is necessary to calculate or at least estimate the gas loads resulting from desorption, permeation, leakage and process gases.
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Application report: Discovering new worlds - Vacuum chamber for “CHEOPS” space mission
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