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Outgassing

The pressure in a vacuum chamber would fall very quickly to the pump’s ultimate pressure, if all of the gas were located inside the empty space inside the chamber. However, there are several sources of gas present in every chamber and these will limit the system’s ultimate pressure.
To a first approximation, the chamber pressure is given by the following equation.

P(t)=Qtot(t) / Seff

Stacks Image 3477571

Gas sources in a vacuum chamber

P(t): Chamber pressure, as a function of time
Qtot(t): Total gas load, as a function of time
Seff: Effective pumping speed inside the vacuum chamber
The gas load in a vacuum chamber is made up of leaks, both real and internal, gases deliberately introduced by the user, backstreaming from the pump and, most importantly, gas released from the surfaces inside the chamber. This release of gas from material surfaces is known as outgassing.
A properly designed and manufactured chamber won’t have significant leaks. Providing an appropriate pump technology has been chosen, backstreaming will be negligible. The gas load is then dominated by the outgassing rate. As can be seen from the above formula, the chamber pressure can be improved by either increasing the pumping speed or decreasing the outgassing rate. Pumping speeds for commercially available vacuum pumps vary over roughly 3 orders of magnitude (from 5 l.s-1 for “appendage” ion pumps to 5,000 l.s-1 for large turbomolecular pumps). In contrast, outgassing rates for materials used in vacuum systems vary over roughly 10 orders of magnitude (from Neoprene - 4 x 10-5 mbar.l.(cm2.s)-1 after 1 hours pumping, to baked oxygen free copper - 5 x 10-15 mbar.l.(cm2.s)-1). Clearly the choice of materials, and any procedures to reduce outgassing rates, are very important parts of the design of the system.
Definition
Definition of outgassing
The pressure in a vacuum chamber would fall very quickly to the pump’s ultimate pressure, if all of the gas were located inside the empty space inside the chamber. However, there are several sources of gas present in every chamber and these will limit the system’s ultimate pressure.
To a first approximation, the chamber pressure is given by the following equation.

P(t)=Qtot(t) / Seff

Stacks Image 2472

Gas sources in a vacuum chamber

P(t): Chamber pressure, as a function of time
Qtot(t): Total gas load, as a function of time
Seff: Effective pumping speed inside the vacuum chamber
The gas load in a vacuum chamber is made up of leaks, both real and internal, gases deliberately introduced by the user, backstreaming from the pump and, most importantly, gas released from the surfaces inside the chamber. This release of gas from material surfaces is known as outgassing.
A properly designed and manufactured chamber won’t have significant leaks. Providing an appropriate pump technology has been chosen, backstreaming will be negligible. The gas load is then dominated by the outgassing rate. As can be seen from the above formula, the chamber pressure can be improved by either increasing the pumping speed or decreasing the outgassing rate. Pumping speeds for commercially available vacuum pumps vary over roughly 3 orders of magnitude (from 5 l.s-1 for “appendage” ion pumps to 5,000 l.s-1 for large turbomolecular pumps). In contrast, outgassing rates for materials used in vacuum systems vary over roughly 10 orders of magnitude (from Neoprene - 4 x 10-5 mbar.l.(cm2.s)-1 after 1 hours pumping, to baked oxygen free copper - 5 x 10-15 mbar.l.(cm2.s)-1). Clearly the choice of materials, and any procedures to reduce outgassing rates, are very important parts of the design of the system.
Mechanisms
Outgassing mechanisms
There are several different mechanisms involved in the release of gas that has been dissolved in or adsorbed on solid materials. A simplified view of these mechanisms is shown in the accompanying image.
Stacks Image 2469

Outgassing mechanisms

Desorption
Desorption is the release of gas from a surface. The atoms or molecules that later make up the gas can reach the surface by a variety of routes. The most common source is adsorbed material from when the surface has been exposed to atmospheric conditions. Desorption is also the final stage of the diffusion and permeation processes.
The bonding mechanism between the adsorbed material and the surface can be classified as either physisorbtion or chemisorbtion. Particles that are physisorbed to a surface are easily released at room temperatures; they are normally desorbed during the initial pump-down period. Particles that are chemisorbed to a surface are more strongly bonded to the surface than physisorbed particles, they are desorbed at a lower rate.
Desorption is a thermally-stimulated process, that is to say: desorption rate increases with the temperature of the surface. It is also a function of the coverage / number of layers of adsorbed material. Desorption can be stimulated by irradiating the surface with photons, electrons or ions.
Vaporisation
Vaporisation is the phase transition of material to the gas phase. The vapour-pressure of a solid material, which increases with temperature, is the partial pressure of the vapour in thermodynamic equilibrium with its solid phase. Materials which have a high vapour pressure at the operating temperature of the chamber should not be used inside a vacuum chamber - these include: alloys containing zinc, lead, cadmium, selenium, sulphur and phosphorus.
Diffusion
Gas molecules are dissolved into the bulk of a material during it’s production and while it is exposed to air. If one surface is then exposed to a lower pressure (e.g. it is inside a vacuum chamber), the dissolved gases will diffuse to this surface, where they can be desorbed.
For metals used for vacuum chambers, only hydrogen has a high enough mobility to make a significant contribution to the outgassing rate. In contrast, polymers contain more dissolved gases than metals and these gases have much higher mobilities than they would have in a metal. This means that outgassing due to diffusion for polymers is high for a wide range of light molecules.
Permeation
Permeation is the mechanism by which gas passes through a solid wall. It is a three-step process consisting of: adsorption onto the outer wall; diffusion through the bulk material; desorption from the inner wall.
The permeation rate is the product of the gas solubility and the diffusion constant. It varies considerably with temperature and gas type. For the materials used for high vacuum and UHV chambers, the only gas with a significant permeation rate is hydrogen.
Time dependance
With certain approximations, the time-dependant pressure equation is:

P(t)=P0.exp(-Seff.t/V) + (Qdes(t) + Qdif(t) + Qper(t))/Seff

P0.exp(-Seff.t/V) is the contribution to the gas load from gas in the volume of the chamber.
Qdes(t) is the gas load from desorption. It typically varies with t-1.
Qdif(t) is the gas load from diffusion. It initially varies with t-0.5.
Qper(t) is the gas load from permeation. It does not vary with time.
The accompanying graph shows an approximation of the pump-down of a vacuum chamber kept at constant temperature. It can be seen that the dominant source of gas varies during the pump-down process.
It should be emphasised that the x-axis in the graph is log(t). While it takes just a few minutes until the gas from the chamber is removed, it will take approximately 10 thousand years before permeation is the dominant process.
Stacks Image 3278

Pump-down of a vacuum chamber

Conversion factors
Conversion factors
As with all vacuum technology, a wider variety of units are used. The SI unit is W.m-2, which is equivalent to Pa.m3.(m2.s)-1. However, the most commonly used units are Torr.l.(cm2.s)-1 and mbar.l.(cm2.s)-1. The conversion factors are shown below.
mbar.l.(cm2.s)-1 Pa.m3.(m2.s)-1 Torr.l.(cm2.s)-1 W.m-2
1 mbar.l.(cm2.s)-1 1 1.000 x 103 7.501 x 10-1 1.000 x 103
1 Pa.m3.(m2.s)-1 1.000 x 10-3 1 7.501 x 10-4 1.000 x 100
1 Torr.l.(cm2.s)-1 1.333 x 100 1.333 x 103 1 1.333 x 103
1 W.m-2 1.000 x 10-3 1.000 x 100 7.501 x 10-4 1
Materials for UHV
Materials for Ultra High Vacuum
Over the last 50 years there have been many exhaustive studies of outgassing rates for different materials using a wide variety of preparation techniques. An internet search will soon reveal outgassing rates for almost every material used in a vacuum system.
To summarise the vast literature that exists on this subject, the following materials are the ones most commonly used in the fabrication of UHV chambers. These materials combine mechanical strength with low outgassing rates:
- Stainless steel (austenitic, typically 304, 316L or 316LN)
- Aluminium (not anodised)
- Copper (oxygen free)
- Glass (borosilicate)
- Ceramics (porcelain or alumina, fully vitrified)
Materials such as Kapton® (polyimide), PTFE (fluoropolymer) and PEEK (thermoplastic) have low enough outgassing rates that they can be used, in moderation, inside UHV chambers for electrical insulation; however, they do not have the strength for structural applications.
Reducing outgassing
Methods to reduce outgassing
Many techniques to reduce outgassing rates have been developed during the long history of studies into materials and techniques for ultra high vacuum (UHV). At first sight the different materials and techniques can seem overwhelming. The good news is that our suppliers: Allectra, Mantis Deposition and UHV Design all have long histories of developing components and systems for the UHV environment. Their products are manufactured to the exacting standards required to minimise outgassing, so material preparation has already been optimised. However, the user needs to be aware that UHV systems, and components in these systems, need careful cleaning, handling, baking and venting in order to minimise the outgassing rate.
Material preparation
Heating components in a vacuum furnace will remove a large amount of dissolved gas. The maximum temperature varies with material; it is around 1,000º C for stainless steel and 500º C for copper.
Surface treatments, such as electropolishing, are known to reduce the outgassing rate.
Cleaning and handling
All forms of grease, including finger grease, have a high vapour pressure in a vacuum system. It is important for UHV applications to degrease anything that will be inside the chamber and to only handle cleaned items when wearing suitable gloves.
Baking
The desorption process can be accelerated if the vacuum chamber is heated, while under vacuum conditions. The maximum temperature, and temperature ramp, are limited by the materials used, but temperatures between 150º C and 200º C are commonly used. A bakeout controller will also monitor the chamber pressure (typically 10-6 mbar during the bakeout cycle.
In a UHV chamber, a properly designed bakeout cycle will remove most of the gas that has been adsorbed on the exposed surfaces inside the chamber. This will reduce the outgassing rate from these surfaces by 4 orders of magnitude, compared to the outgassing rate after 1 hours pumping.
Venting
Dry nitrogen, rather than air, is often used to vent UHV chambers when they need to be brought to atmospheric pressure. This minimises the amount of water adsorbed inside the vacuum chamber.
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