Sputtering and migration of trace quantities of transition metal atoms on silicon

We have investigated the behavior of low levels of transition metal atoms on silicon surfaces subject to nitrogen bombardment. Submonolayer coverages of gold, iron, copper, molybdenum and tungsten were deposited on [100] silicon surfaces. Samples were analyzed using 270 keV He+ time-of-flight backscattering before and after irradiation with 6 mC of 270 keV N+ at current levels in the hundreds of nanoamps. The yield of sputtered metallic atoms ranged from 1.0 X 10(-3) per incident nitrogen ion to 3.3 X 10(-3) per incident ion. Lower yields were correlated with migration of the metallic species into the silicon. The implications for ultra-high sensitivity measurement of contamination on silicon wafers by time-of-flight heavy-ion backscattering spectrometry are discussed.