Vacuum Chambers

Technical Notes

In vacuum systems, a chamber is a vessel that contains the vacuum at base pressures ranging over 10 orders of magnitude. The chambers’ physical characteristics are as varied as the applications for which vacuum is used. They can be as small as a few millimeters in diameter or as large as 100' long x 12' in diameter. They can be as simple as a regular four-way cross, or have hundreds of ports angled toward many internal focal points.

Geometries & Characteristics

While too numerous to list, and limited only by the customer’s imagination and intended application, the variety of chamber geometries can be classified simply as box, spherical, cylindrical, or D-shape. Each chamber geometry has its inherent pros and cons, and consequently, dictates the base pressure range and suited application(s).

Box Because metal boxes usually have at least one large o-ring seal, designers do not depend on them for UHV applications. Instead, boxes make excellent high vacuum coaters with large internal areas and easily opened doors. They work especially well to enable coating of large numbers of substrates during a single run. Another popular application involves outgassing and impregnation.

Box Chamber

Spherical The is a popular choice for UHV applications requiring radial component placement, such as pulse laser deposition and surface science. The numerous ports, all with axes oriented toward the center of the sphere, enable ample access to any central sample for analysis, measurement, and observation. Because spheres normally have knife-edged flanges (CF) and stainless steel bodies, they often allow evacuation to pressures in the 10-11 torr range.

Spherical Chamber

Cylindrical Cylindrical chambers can be subdivided into horizontal/vertical chambers or bell jars and their related components—feedthrough collars, service wells, and endplates. Horizontal & Vertical Cylindrical Chambers Vertical cylindrical chambers are typically longer and taller than feedthrough collars and have fewer ports. They are a good choice for thin film deposition by sputtering and evaporation. Large o-ring door seals restrict such chambers of non-UHV applications.

Cylindrical Chamber

Horizontal cylindrical chambers feature large internal areas and easily opened hinged doors. Their shape permits the use of instrumentation in a radial orientation. Due to the large o-ring seal between the hinged door and the chamber, they are not designed for UHV use.
Metal Bell Jars Metal bell jars are good multipurpose vacuum chambers used for various UHV applications. Their simple design lends itself to relatively easy manufacturing and low cost. Some designs include water cooling traces running on the exterior of the chamber. The typical metal bell jar has at least one side port for attaching a load lock or mouting a viewport to monitor the process. Glass Bell Jars We make our glass bell jars from Pyrex® to withstand thermal shock. Glass, inexpensive and transparent, has good high-vacuum properties; however, it is fragile. To protect personnel and the chamber, surround the glass chamber with a metal guard before applying a vacuum. An L-shaped rubber gasket seals the bottom edge of the glass against a metal base plate. Although the wall thickness is carefully controlled during manufacture, bell jars are individually blown into molds. No two are exactly alike. Endplates Endplates or baseplates (depending on mounting orientation) are flat metal flanges made to cap glass/metal bell jars and cylinders of standard diameters (12", 14", 18", and 24"). Endplates usually have one large flanged port where the HV pump is attached, plus a number of smaller ports for feedthroughs and components.	Metal Bell Jar Endplates
Service Wells Service wells, like endplates, act as transition stages between the pumping stack and the bell jar or other chamber. The physical distinction is that service wells are wide-bore tubes, closed at the bottom, with feedthrough flanges radially penetrating the tube walls. Because there is a much larger area for feedthrough ports and because the ports are not buried beneath the lowest surface, the service well’s primary advantages are number of feedthroughs and convenience.
Feedthrough Collars This name sometimes misleads, as a feedthrough collar has no feedthroughs. Like the service well, it has a number of radial ports for feedthroughs, but otherwise is simply a tube with flanges at each end, used as a transition piece between a bell jar and an endplate.	Feedthrough Collars
D-Shape The D-shape chamber combines the best features of box and cylindrical chambers. It has the box’s large area access door, the cylinder’s relatively thin wall material for the D-curve (reducing its weight), and the cylinder’s compatibility with the typical disk-shaped substrate holder/rotator. The large o-ring sealing the door prevents the D-shape chamber from reaching pressures in the UHV range.

Materials

The most commonly used material for high vacuum and UHV chambers is a 300-series stainless steel—most frequently 304L (carbon content >0.03%), which is available in sheet, tube, bar, plate, and forged forms. This steel, and others such as 316L, has desirable vacuum chamber properties: mechanically strong, machinable, and weldable; magnetic permeability close to 1; resistance to atmospheric corrosion; takes a high polish; and can be effectively outgassed by baking. For large space simulation chambers operating in the 10^-6 torr range, mild steel is a costeffective option. But often, it has unacceptable magnetic, corrosion, and outgassing properties.

For experiments disturbed by minuscule magnetic fields or radioactive backgrounds, or which require chamber walls with excellent thermal or electrical conductivity, weldable grades of aluminum (typically 6061) are used for high vacuum and UHV. Two problems prevent the regular use of aluminum for vacuum chambers:(a) it is more difficult to weld when vacuum integrity is required and (b) its low strength means it cannot be used for CF flange knife-edges without expensive modification.

Glass is a common chamber material in educational and some research laboratories. Many types of glass have low gas permeability (except for helium) and good vacuum characteristics. The background radioactivity dueto potassium 40 decays; however, it may be high enough to preclude it for some uses. Obviously, glass is more susceptible to damage and less easily modified by the average machine shop than metal, but glass bell jars offer the benefits of low cost, transparency, and simplicity.

When the earth’s magnetic field interferes with experiments, the complete chamber is sometimes made of mu-metal. However, anecdotal evidence suggest it has strange properties when used as a vacuum construction material (see Liners for an alternative solution).

Common materials such as brass are sometimes used, but not recommended. Less common, chemically inert materials such as Monel^® and Inconel^® have special applications. But zinc or cadmium-plated steels or screws should never be used inside a vacuum chamber. Their relatively high vapor pressure can cause operational adventures that are best avoided.

Water Cooling

For applications with high heat outputs, double chamber walls with channeling and baffles provide the most effective method of water cooling to provide a ‘‘cold wall.’’ For lower heat outputs, a water trace (a channel welded to the external chamber surface) is used. The trace covers a small fraction of the surface but, using data on heat input and thermal conductance, the trace pattern is designed for adequate cooling efficiency. Occasionally, copper tubing is brazed to the chamber’s exterior. Although less expensive, this is never as effective as the first two methods.

Water-cooled chambers should undergo rigorous testing procedures to ensure they are free of leaks. Testing methods include stress-testing all water cavities with a pressurized gas (typically dry nitrogen) and helium leak checking all internal and external welds.

Liners

Two common applications for chambers liners are: (a)reducing the effects of magnetic fields by installing multiple mu-metal liners inside the chamber (often a better solution than a mumetal chamber), and (b)preventing sputtered or evaporated materials reaching the chamber's surfaces using aluminum foil or thin (shaped) stainless liners.

NOTE: Personnel must be fully protected when changing liners used in toxic or harzardous processes.