High plasma-flux elevated temperature sputtering of Cu-Li alloys

Abstract

Copper-lithium alloys ranging in composition from 3–12 at.% Li have been exposed to sputtering by 3 × 10 16−6 × 10 17 100 eV He +/ cm 2 s at temperatures of 300–500°C at the UCLA PISCES plasma service. Weight loss and optical spectroscopy techniques were used to determine the sputtering-induced erosion of the binary alloys relative to pure copper. It was found that the weight loss of the alloy and the amount of copper in the plasma as measured by emission spectroscopy in some cases was reduced by a factor of five or more. Post-irradiation analysis by Auger electron spectroscopy and scanning electron microscopy show a correlation between lithium surface depletion, surface roughening, weight loss and partial erosion yields as measured by plasma emission spectroscopy. The results are explained in terms of formation of a lithium overlayer as a result of the elevated temperature and plasma ion bombardment. The formation of the overlayer is diffusion limited and depends on the sample conditioning, sample temperature, lithium concentration, method of preparation and plasma flux. The degree to which the copper erosion is reduced depends on the Li overlayer thickness, which is expected to depend on the sputtering yield of lithium on a copper substrate, the ion fraction of the sputtered lithium, the extent of subsurface lithium depletion and the duty cycle of the plasma device.