Secondary Negative Cluster Ions for Structure Analysis

Among the recoil products generated by ion beam (>5 keV) bombardment of a surface are positive and negative molecular ions. Their means of formation is not well understood, likely involving both nearest-neighbor co-ejection and recombination events within the sputtered solid and in the short-lived plasma over the sputtered surface. The existence of the former mechanism is demonstrated in the dynamic secondary ion mass spectroscopy (SIMS) of As implants in Si following various anneal conditions in which As precipitation has been independently shown. Ion intensity of As sup (-) sub 2 relative to that of the monomer As sup (-) shows distinct and interpretable differences between homogenious and inhomogenious dopant distributions. This is of immediate use analytically and of interest in understanding the fundamentals of the sputter process. Other examples of secondary cluster ions from impurity level species are also discussed.