Surface cleaning on aluminum for ultrahigh vacuum using supercritical fluid CO2 with H2O and NaCl as additives

Ozone treatments have been shown to reduce the outgassing rates in ultrahig h vacuum for several metals. We have further studied surface cleaning emplo ying supercritical fluid CO2 which has high solubility for hydrocarbons. Th e sample measured was a half section of an anodic oxidized Swagelok fitting (A2024) 10.8 mm diameter and 8.5 mm in length saw sectioned along the axis . The cut surface was used as the reference surface. The treated and untrea ted surfaces were analyzed by x-ray photoelectron spectroscopy, evaluating the surface carbon density obtained from the ratio of the convoluted area o f Al((2)p), O((1)s), and C((1)s) peaks. Carbon density of the untreated sur face is 87%. Following the treatment with supercritical fluid CO2 at 50 deg rees C and 80 atm for 2 h, the carbon was reduced to 65%. Treating at 2 h a t 70 degrees C and 94 atm showed the same 65% carbon. Adding water and incr easing the supercritical fluid CO2 pressure to 100 atm showed carbon at 57% . However, adding some salt (NaCl) to the mixture produced a dramatic carbo n reduction to as low as 13%. No pressure dependence was observed. (C) 1999 American Vacuum Society. [S0734-2101(99)22204-X].