THE EFFECT OF WATER-VAPOR ON ATOM PRODUCTION BY GLOW-DISCHARGE SPUTTERING

The effect of traces of water vapour on the production of free atoms by sputtering in a glow discharge has been investigated using argon containing up to 140 ppm of water vapour. Atomic absorption measurements were performed on various elements in steel, aluminium, brass and zinc alloys and in all cases the water vapour reduced the number of free atoms. The extent of the reduction depended on the type of sample being sputtered and the sputtering current. At the chosen operating conditions (40-50 mA sputtering current, 4-6 Torr Ar), 140 ppm water vapour produced a reduction in absorbance which varied from 12% for Ni in steel to about 77% for Cr in aluminium. At lower sputtering currents the reduction was much greater (> 95% for Cu and Al in brass at 20 mA). The results indicate that the reduction in the number of free atoms is due both to interference with the sputtering process, which results in lower sample erosion rates, and to some process which occurs after the atoms are sputtered, probably gas phase reactions between the atoms and water molecules or fragments. These results underline the need for care in the design and operation of sputtering systems to ensure that contamination by water vapour is kept to a minimum.