AVS 53rd International Symposium
    Vacuum Technology Wednesday Sessions
       Session VT+AS-WeM

Invited Paper VT+AS-WeM9
Outgassing of Construction Materials for Vacuum Chambers and Coatings for Reduction of Outgassing

Wednesday, November 15, 2006, 10:40 am, Room 2000a

Session: Outgassing, Materials Coatings for Reduction of Outgassing
Presenter: J. Setina, Institute of Metals and Technology, Slovenia
Correspondent: Click to Email
In vacuum systems we have to deal with continuous gas desorption from the chamber walls and inflow from the technological process. Working pressure in a vacuum system is a counterbalance of total gas flow rate and available pumping speed. Vacuum technological processes are very diverse and span more than 15 decades of pressure: from coarse vacuum to extremely high vacuum (XHV). To properly design a vacuum chamber, the vacuum engineer needs, reliable data on outgassing of constructional materials for the working conditions. Extensive data can be found in the literature, but there is considerable scatter in the data due to differences in sample quality, preparation technique, cleaning procedure and measurement method. Different methods for measuring outgassing will be discussed. Vacuum gauges can also influence the result as they can outgass, pump or significantly change the gas composition by promoting chemical reactions on hot filaments. Therefore, the measurement setup has to be considered and evaluated carefully to determine the uncertainty of the results. In ultrahigh vacuum (UHV) systems the dominant gas is hydrogen that is dissolved in the material used in its construction and continuously diffuses towards the chamber walls and desorbs into the vacuum. To achieve UHV, outgassing has to be reduced to the lowest practical values. Established methods are pre-treatment of materials (vacuum firing, air bake) and in-situ baking of the vacuum chamber. For XHV systems, such treatments are often not sufficient and passive barrier coatings to reduce gas diffusion have been reported in the literature. Published results for coatings on stainless steel or aluminum such as TiN, BN and SiO2 are not consistent. Hydrogen suppression depends on film thickness and density and also on pre-treatment of the coating substrate. These conditions were not the same in the published works and important experimental details are often missing.