Problems Encountered in Depth Profiling of Nitrogen and Oxygen in Silicon by Means of Secondary Ion Mass Spectrometry

Abstract

Secondary Ion Mass Spectrometry (SIMS) has become the technique most frequently used for compositional in-depth analysis in the 1 nm to 10 urn regime. It has been noticed, however, that one may encounter implications and problems in SIMS depth profiling studies which are not due to insufficient signal heights. For example, the damage introduced in the sample by primary ion impact can alter the original composition profile significantly. Effects due to collisional displacement [1,2] and radiation-enhanced diffusion [3,4] have been observed. In order to keep bombardment-induced atomic mixing as small as possible, the primary ion energy should not exceed ∼1 keV [5]. Radiation-enhanced diffusion can occur not only in the bulk but also at the surface of the ion-bombarded sample. Accordingly, adsorbed gases may diffuse into the sample [6]. These findings are important in SIMS experiments because (i) saturation of positive secondary ion yields requires incorporation of oxygen rather than mere adsorption [7] and (ii) uptake of residual gases will result in unwanted background levels of the respective atomic ions (e.g. H−, C−, 0−) [8] and, equally important, of molecular ions (e.g. 30Si1H+ = 31p+) [g]. in this contribution we report on a systematic study of background generation in depth profiling of nitrogen and oxygen in silicon.