We have demonstrated a technique of using implanted ¹⁸O to quantify in situ the amount of oxygen in the near-surface layers of sputtered silicon during dynamic secondary ion mass spectrometry, SIMS. This technique is proving useful in better understanding the phenomenon of chemical segregation at the surface during sputtering when oxygen is introduced to the sputter region.

The use of oxygen-containing primary ions or oxygen gas flooding of the sample surface is a common practice used to increase ionization efficiency and thus detection limits of trace elements in SIMS analysis. However, the use of oxygen during dynamic SIMS can in some cases create artifacts in the measured depth profile. Specifically the measured decay length (the sputtered depth required for the ion intensity to drop an order of magnitude in intensity) of impurity elements present as a delta function, step function or implant can change when oxygen is delivered to the surface, setting up an oxygen concentration gradient from the surface into the bulk.

A form of chemical segregation due to thermodynamic effects, specifically the relative affinities for oxygen of the impurity element and silicon resulting in chemical segregation to, or away from, the oxygen-rich sputtered surface, is a proposed mechanism for the observed changes in decay length for several trace elements in silicon. Whether the impurity element must segregate completely to the backside of a surface oxide layer, or just simply away from the outermost layers of the sputtered surface, to produce such changes in decay length has been unclear and debated in the SIMS community.

The decay length of several impurity elements in silicon has been measured over a range of surface oxygen conditions, quantified using ¹⁸O implant internal standards. The results indicate that the segregation at the surface is both element specific and oxygen dependent.