3.4 Vacuum chambers
The heart of a vacuum system is the vacuum chamber, which is tailored to specific application. It encloses the application and reliably separates it from the outside or protects the surrounding from the processes inside. Irrespective of whether a fine vacuum is required for drying processes, a medium or high vacuum for plasma processes or ultra-high vacuum for surface studies: – The vacuum chamber must always mechanically bear the pressure difference from the atmosphere.
Vacuum vessel within the European Union are not subject to any specific guidelines upon which a design and calculation has be based on. They are not pressure equipment (Pressure Equipment Directive 2014/68/EU applies to components with an internal gauge pressure greater than 500 hPa) and they are not classified as machines according to the Machinery Directive 2006/42/EC. Nevertheless, they must be designed, calculated, manufactured in a safe and reliable way and tested prior to commissioning.
The calculation of wall thickness for cylindrical tubes, spherical bodies, flat floors or mold parts, such as dished ends can be done using the AD-2000 leaflets. The AD 2000 regulation was actually designed by the “pressure vessel working committee“ for the calculation of pressure vessels, but which also describes the load condition “external overpressure”. Here you will find, for example, equations for calculating the required wall thicknesses that include the “elastic buckling” or “plastic deformation” of cylindrical tubes.
With rectangular chambers or similar designs, the deflection of the surfaces and the emerging tensions must be checked. If they are too high, the wall thickness must be increased or the areas must be reinforced, for example by additional welded ribs. For that, helpful programs that perform mechanical calculations using the finite element method (FEM) can be used to optimize the chamber design. In addition to the permissible mechanical stress, it is also necessary to check, whether under the load conditions “outside atmosphere, vacuum inside” the sealing surfaces remain flush with each other. If the sealing surfaces warp, leaks can occur that prevent the use of the chamber.
Figure 3.17: EUV source chamber with cooling profiles and water-cooled flanges
The basic shape of the chamber is often derived from the application. For the chamber body a cylindrical tube should be selected if possible, since it is the ideal for material input and stability. For smaller nominal diameters, a flat bottom can seal a tube side, larger diameters should be sealed by dished ends to limit the material input and the mass of the chamber. Example: Chamber diameter of 600 mm, requires a flat bottom, about three times the wall thickness as of a dished end. A main flange with fitting lid allows access to the chamber, a door hinge increases ease of use. Chamber feet on the outside ensure stability, eye bolts or hoisting shackles allow safe transport.
If the chamber is to be tempered or if internal heat sources lead to excessive heating of the chamber cover, a chamber cooling system must be provided. This can be achieved by welded cooling profiles or a pillow plate cooling for large areas (Figure 3.18) or even as a double-walled container.
Figure 3.18: Space simulation chamber with pillow plate cooling
Often, a chamber is designed in a dialog between user and designer according to an experiment or a process. An alternative to individually tailored chambers are standard vacuum chambers. These are preconfigured base bodies, complemented by freely selectable ports. They offer a faster and cheaper alternative to a completely customized vacuum chamber.