抄録
The effect of neon glow discharge cleaning (GDC) for the reduction of water outgassing using a 304 stainless steel chamber was studied. A pumping line through an orifice of limited pumping speed to 10 l/s and a normal pumping system of pumping speed of 180 l/s were prepared for the pump-down of the chamber during and after GDC. When the pump-down of the chamber after exposure to air was carried out, the pressure decreased in inverse proportion to time in the pumping period up to 72 h and reached a steady constant value in the pumping period over 72 h. The attained pressure after the trial of GDC with the ion dose of 0.5 Coulomb/cm2 in the time dependent range of pressure was nearly the same as that by only the normal pumping system without help of GDC. This result was interpreted as that the desorbed water from the chamber wall by GDC is not so effectively pumped out of the chamber, because of the dependence of removal rate of water on pumping speed. Therefore, the pumping speed of the normal pumping system was increased further to 270 l/s using a cryopump and the chamber was pumped until a steady constant pressure of the order of 1×10-6 Pa was achieved. Then the trial of GDC for further pressure improvement was made 3 times every other day from the pumping time of near 25 h. As a result, the pressure was improved by 2 orders of magnitude from 1.3×10-6 to 1.3×10-8 Pa. It was presumed that this result is caused by the dissociation of adsorbed water on the chamber surface during GDC.
