Sputtering and reactive effects in a stainless steel tube during glow discharge treatment

Abstract

Glow discharge treatment of stainless steel vacuum components is of current interest for removing surface contaminants and reducing the gas desorption rate. A study has been made of the effects of sputtering the inside of a stainless steel (EN58B) tube of 28 mm dia. × 135 mm length in Ar, Ar + O 2 and Ar + H 2 glow discharges at 10 −1 torr, 0.23 mA/cm 2 and 350 V. The tubular cathode was mounted in a glass vessel and the gas admitted to one end of the tube. The Al-electrode of an hf crystal microbalance, mounted in a window cut mid-way in the tube wall, was exposed to both ion bombardment and film deposition. A nett film growth rate of one atom monolayer/min or less was measured for all gases over sputtering periods up to 6 h. Failure to obtain an equilibrium film thickness was attributed to a non-uniform current density distribution arising from disturbance by the probe as use of a tube with a disc cathode at both ends still gave a nett film growth rate at the centre. Although the gases were predried by passage through a LN 2-trap and a LN 2 cold finger was used in vessel (ultimate pressure 10 −5 torr) films grown in Ar (99.999%) were always oxidized. At the film growth rate observed a partial pressure of an oxygen active gas of 10 −8 torr would have been sufficient to ensure metal oxidation. The metal oxide deposit was only effectively reduced in Ar + H 2 when the vessel LN 2-finger was not used so that desorbed water-vapour was present. Film growth at 0.5 Å/min in Ar + H 2 with the cold finger gave a resistivity of 2.84 ωcm compared with 7 × 10 −3 ωcm for 1 Å/min in Ar + H 2 without the cold finger. X-ray microprobe spectra are given for deposits formed on aluminium and silica substrates mounted in the tube window and for silica coated in a diode system. The results obtained show that glow discharge treatment of hollow cathode bodies can result in the formation of surface deposits whose preferred growth and metal and gas composition can critically depend on the uniformity of ion bombardment and the pressure of gaseous impurities in the discharge gas.