Wednesday, July 17, 2013

1307.4338 (M. A. Mamun et al.)

The effect of heat treatments and coatings on the outgassing rate of
stainless steel chambers

M. A. Mamun, A. A. Elmustafa, M. L. Stutzman, P. A. Adderley, M. Poelker
The outgassing rates of four nominally identical 304L stainless steel vacuum chambers were measured to determine the effect of chamber coatings and heat treatments. One chamber was coated with titanium nitride (TiN) and one with amorphous silicon (a-Si) immediately following fabrication. One chamber remained uncoated throughout, and the last chamber was first tested without any coating, and then coated with a-Si following a series of heat treatments. The outgassing rate of each chamber was measured at room temperatures between 15 and 30 {\deg}C following bakes at temperatures between 90 and 400 {\deg}C. Measurements for bare steel showed a significant reduction in the outgassing rate by more than a factor of 20 after a 400 {\deg}C heat treatment (4x10-12 TorrLs-1cm-2 prior to heat treatment, reduced to 1.7x10-13 following heat treatment). The chambers that were coated with a-Si showed minimal change in outgassing rates with heat treatment, though an outgassing rate reduced by heat treatments prior to a-Si coating was successfully preserved throughout a series of bakes. The TiN coated chamber exhibited remarkably low outgassing rates, up to four orders of magnitude lower than the uncoated stainless steel. An evaluation of coating composition suggests the presence of elemental titanium which could provide pumping and lead to an artificially low outgassing rate. The outgassing results are discussed in terms of diffusion-limited versus recombination-limited processes.
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