Total sputtering yield of Ag/Cu alloys for low energy argon ions

Abstract

Measurement of the total sputtering yields of Ag/Cu two-phase alloy targets for normally incident 200 and 100 eV Ar + ions have been performed. The dose was approximately 10 19 ions and the target temperature was held at about 20°C. A graph of total sputtering yield versus Ag/Cu (at.%) composition is “V-shaped” with the yield of all compositions being lower than either pure Cu or pure Ag. The total sputtering yield is defined as the number of sputtered atoms of any type divided by the number of incident ions. The total yield was calculated assuming stoichiometric component ejection during the entire bombardment time. The validity of this assumption and its effect on the results is discussed. The surfaces of the sputtered targets were covered with a variety of pronounced cones, ridges, and pebble-like features. It is hypothesized that a redeposition “shadow effect” is responsible for the lowered yield. When the developing surface features or projections become tall enough, material ejected from one projection will be redeposited onto neighboring projections resulting in a reduced sputtering yield. Interestingly, the “V-shaped” yield versus composition curve is very similar in structure to the Ag/Cu solidification curve in the Ag/Cu phase diagram. A plausible explanation for this correlation follows from the fact that the average crystallite sizes of the alloy compositions scale with the solidification curve. Compositions with the lowest melting temperature have the smallest average crystallite sizes. The smaller the crystallites the faster they will be covered by the yield lowering surface projections. Selective sputtering and surface diffusion fed “seed” cones appear to be the two most important mechanisms contributing to the formation of the roughened surrace topography.