. UHV Chambers – A Design Engineer’s Rules of Thumb and Good Practices

Case Study
UHV Chambers – A Design Engineer’s Rules of Thumb and Good Practices

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UHV Chambers

Vacuum chambers can be designed and manufactured in a range of shapes and sizes to suit a variety of applications and to facilitate different industrial and scientific processes. However, the design of most vacuum chambers can be broken down to a few key areas common to all chambers There are also some aspects that can be easily overlooked but that take a vacuum chamber from good to great.

Geometry and Structure

The major load acting on any vacuum chamber will be that of atmospheric pressure which acts equally and evenly in all directions. For this reason, the most common shape of a vacuum chamber regardless of size is a cylinder. The structural integrity of the chamber is always a good starting point for design.

Consider the reason for your chamber and how it will be used, are you going to need ports or passthroughs for electrical signals, power, fluid, optics? All of these features are likely to reduce the strength of the chamber, so sizing and placing these features is critical.

The next challenge to be encountered will be how to seal any ports, or the ends of the cylinder, such that there’s an enclosed volume in which to generate the vacuum. For small chambers up to 0.5m diameter it can be possible to use a flat sheet to seal the end of the chamber. Chambers up to 1m diameter will require either thicker material to resist deformation or reinforcement ribs, both of which promote survivability to the increasing loads exerted by atmospheric pressure. For larger chambers it is common to use dished (or torispherical) ends as the geometry is better suited to distribute the load exerted on them.

It is worth reminding that when looking at the loads exerted on a chamber, the level of vacuum for which the chamber is being designed is largely irrelevant as the difference between achieving a rough vacuum (0.1 mbar) and achieving an ultra-high vacuum UHV (1×10-9 mbar), the load only increases by less than 0.01%.

 

thermal vacuum chamber

 

Materials

The consideration to material choice is inherent to designing the chamber structure and ports; to designers with limited experience in UHV, material selection plays a key role in chamber performance, notably the minimum achievable pressure and the speed at which you can get to that pressure (pump-down).

The surface finish of vacuum-facing hardware will affect the quantity of retained moisture in the substrate and its cleanliness level. Water and contaminants can be desorbed from the substrate under UHV and lower the vacuum quality, furthermore these substances can then be reabsorbed onto other surfaces (e.g., the items being subject to vacuum!). Normally, the smoother the internal surface, the better the vacuum performance and by choosing materials with high resistance to corrosion, such as stainless steel, ensures this surface continues to perform run after run.

For long vacuum runs, strain due to “creep” may need to be considered, this affects steels to a lesser extent than aluminium, although this is down to operational frequency, pump-down duration, base-pressure and how the loads and stresses are realised in the chamber. Unfortunately, there a few rules of thumb for these cases.

Vapour pressure and material diffusivity are often overlooked when designing either chambers, or hardware subject to UHV. Vapour pressure (usually baselined at a set temperature) describes the conditions at which a material will experience a phase-change, changing from solid to liquid, or liquid to gas. Materials such as Zinc alloys may sublimate (change directly from a solid to a gas) under heat and UHV conditions. It goes without saying, avoid using materials with these sorts of properties! The diffusion rate, when expressing in reference to vacuum, refers to the rate at which small gas molecules permeate through the material, this mainly effects base-pressure performance.

 

Temperature Controlled Crystal Microbalance

Seals

Where the distinction between a rough vacuum and an ultra-high vacuum is particularly important is at sealing faces between two parts, poorly designed joints have the potential to undermine the ability of the vacuum chamber to create and sustain UHV. Fortunately, there are a few options commonly available that if used will take away some of the complexity.

These standard seals fall into two categories:

The second option uses the permanent deformation of a gasket material (typically copper) to achieve a strong UHV seal (ISO-CF).

When selecting a sealing technology, consideration should be given to:

  • The level of vacuum required (Rough, Medium, High, Ultra High)
  • The need to assemble/re-assemble the joint
  • What other equipment may be required to be fitted to the chamber in both the short and long term

Usually, the extendibility of the chamber is largely unknown. How could anyone know what might be needed in 10- or 20-years’ time? But this is nevertheless, an important consideration as it allows for future expansion of the chambers capabilities. This will play a part in selecting which arrangement of ports and seals is required to get the best performance from the vacuum chamber in both the immediate and long-term timescales.

Common attachments to chambers include:

  • Pumps
  • Sensors (pressure, temperature, film-thickness sensors, TQCMs, RGAs)
  • Electronics and Electrical Power (sensor heads, emission sources, active electronic equipment, solenoids and motion control systems)
  • Fluidic systems (temperature control, emission sources, cold traps)
  • Mechanical (wiggle sticks)
  • Optics (viewports, optical windows, fibre-glass feedthroughs)

 

Vacuum Chamber

Other Considerations

A chamber can be a significant asset to any organisation, but it does require certain infrastructure. It’s important to consider exactly where the chamber will reside, how it will be installed and operated on a day-to-day basis. Furthermore, it is important to consider your enabling and ancillary infrastructure, which may include:

There are many other factors that impact the design of the chamber, these extend far beyond the basic design principals of the chamber itself, with every element of the wider vacuum system playing its part. In some cases, the location of supporting systems is critical to successful operation, as they can contribute to a noisy RF environments, mechanical vibration, and magnetic fields (this is not a comprehensive list). This is often overlooked!

Importantly, for the successful operation of the chamber, it’s not just about generating a vacuum. At some point it will be necessary to bring the chamber back up to atmospheric pressure, this needs to be done in a safe a controlled manner, particularly if re-pressurisation must be done with a gas other than air. Which may bring about its own safety consideration when opening the chamber as the environment inside the chamber may have an oxygen content below a safe breathing level.

Safety is paramount and should never be taken lightly when operating a vacuum chamber as there is in effect a large stored potential energy as well as other sources of risk from chemicals, cryogenics, hot surfaces and electrocution to name a few, all these and more need to be considered to design and safely operate a vacuum chamber.

 

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