H. F. Dylla

Jefferson Lab

Newport News VA 23606

In ambient temperature XHV-UHV systems the only residual gases that are evident after proper surface conditioning of both the vacuum system under study and the residual gas analyzer used to characterize the system are: hydrogen, methane and carbon monoxide/carbon dioxide. Understanding the source of these residual gases can lead to higher performance systems at potentially less cost for the materials, surface preparation and pumping hardware. The source for residual hydrogen is unquestioned: hydrogen is the most ubiquitous gas dissolved within the bulk of the materials used for the construction of XHV-UHV systems (stainless steel, aluminum, copper, etc). The source for CH₄ and CO_x is less obvious but is presumably related to small concentrations of accessible carbon and oxygen impurities located within the near surface layer of the vacuum vessel wall. It is interesting to note that small surface concentrations (a few at.%) of carbon and oxygen are always observed on metal surfaces even after extensive cleaning techniques have been followed in UHV conditions. For example, quasi static surface concentrations of residual C and O have been observed on stainless steel after high fluence atomic hydrogen bombardment [1]. The author has shown that the residual surface concentrations can be maintained at quasi static levels from re-supply of oxygen from the passivation oxide layer with relatively modest oxygen diffusion coefficients [2]. Similar mechanisms may be responsible for maintaining small residual surface concentrations of carbon and oxygen and the resulting residual gases in XHV-UHV systems. Open questions include: (1) the role of hydrogen diffusion from the bulk which may assist the transport of carbon and oxygen species in the passivation oxide layer, and (2) the surface reactions that are responsible for the formation of the volatile species (CH₄ and CO_x) that are observed in the residual gas spectra.

[1.] K. G. Tschersich and J. von Seggern, in Proc. 5th International Symposium on Plasma Chemistry, Edinburgh, 1981, B. Waldie, Ed. (Edinburgh, 1981)

[2.] H. F. Dylla and W. R. Blanchard, J.Vac.Sci.Technol. A1, 1297-1301 (1983)