The presence of gaseous molecules, whether slow or fast moving, is what gives rise to pressure. A vacuum is created by reducing the number of molecules that exist within, for example, a chamber or a flask. However, by reducing the number of molecules that exert a pressure on the internal surface of such a chamber, one reduces the pressure. Unfortunately, this causes “additional” molecules to enter into play.
These additional molecules have two main sources: near-surface molecules that would otherwise be snagged or entangled in the microscopic roughness of the surface; and, molecules that are embedded within the internal surface of the chamber itself and that “permeate” out of the bulk to the surface as the vacuum increases and the restraining pressure is removed.
As if this was not complicated enough for the vacuum engineer to have to deal with, there are other sources of gas contaminants which need to be considered. These include external contaminants such as fingerprints and the human grease that they contain; reaction products; backstreaming of oil from the pumps they are in into a vacuum atmosphere; and the vapour pressure of vacuum components themselves, for example seals.
These various (and combined) sources of “additional” molecules prevent vacuum pumps from easily obtaining and maintaining the level of vacuums that they can-and should—be able to obtain in a timely and predictable manner. The solutions are numerous: to ensure that the material from which these items are made are as low in contaminants as possible; ensure that latex dust-free gloves are used when assembling components; that sources of oil are (absolutely) isolated; that wherever possible, rubber and plastics are replaced by other “non-molecular producing” materials; and finally, that the components are “baked”.
What is a vacuum system bake out?
Baking involves heating items (ideally up to 300 - 400 ͦ C, if ultra-high vacuum (UHV) is to be achieved). However, pump manufacturers usually specify a maximum bake-out temperature in the high vacuum pump flange to 120 ͦ C. However, if heat sources (such as radiation heating) are used in the vacuum equipment, then the admissible radiated power must not be exceeded.
Limitations of a vacuum bake-out
Baking procedures for dealing with molecules emanating from the surface and sub-surface (of, for example, a chamber) will depend open the vacuum levels required, its end use and the system itself. Baking out of the chamber to volatise water and hydrocarbon residues would be an obvious exception. If the chamber is a bakeable, metal-gasketed UHV system, then baking the system (especially into a rougher pump) should be standard practice.
O-ring seals present their own genre of issues. The vacuum bake out temperature and its effectiveness will be limited by the temperature tolerance of the O-ring material. For example, Viton shouldn’t be baked above 160 ͦ C since it will slowly begin to break down. More importantly, all bakeable materials (wherever they are located in the system) must be fairly thermally homogeneous, otherwise cold-spots will collect the very contaminants that need to be removed.
Flushing the chamber with hot gas, such as nitrogen, can be effective in removing surface contaminants. If the nitrogen is passed through a clean gas heater, and allowed to slowly flow through the chamber for up to 30 minutes, contaminants will be caught up in the flow and exited through the exhaust portal. However, time and temperatures need to be worked out for each system. Whilst fairly easy to undertake, this nitrogen-purging method is only a partial solution for high vacuum systems.
Timing is everything
Finally, in a 1961 scientific paper regarding baking using Turbomolecular Pumps (TMPs) for the CERN project, it is stated that “pump-down by means of TMPs takes between two and three weeks, mainly driven by the outgassing of the many layers of super-isolation contained in the cryostat insulation vacuum”. However, this is an exceptional case, and pump-downs taking days rather than weeks are considered to be more acceptable in most applications.
Learn more about how contaminants enter vacuum systems, the implication this has on vacuum processes as well as how to mitigate the risk of contamination by downloading our eBook.