Reconstruction of submicron size spatial distributions of implanted particles in solids by computer tomography

Abstract

By means of computer tomography, distributions of implanted ions in solids can be reconstructed in two or three dimensions. For this purpose, a set of depth profiles, measured under various implantation angles but otherwise identical conditions has to be provided. This approach is independent of the measuring technique by which the depth profiles are obtained, and therefore allows even the determination of submicron structures. For amorphous targets with rotationally symmetric implanted particle distributions, the evaluation is especially easy, as only two-dimensional concentration distributions need to be determined. Further, spatial distributions of radiation damage by transferred nuclear or electronic energy might be obtained, and the examination of implanted particle distributions after modification by diffusional processes is also possible. The mathematical problem to convert the set of angularly dependent depth profiles to a spatial particle distribution is treated by solving the corresponding matrix equation in an iterative way, as described in the literature on medical tomography. Special attention has to be given to the near-surface depletion in the depth profiles due to backscattering, which might lead to some distortion of the reconstructed distributions. To perform tomograpic measurements at an accelerator on a routine basis, it is advisable to modify the conventional experimental setup slightly, as described here. As a first example, the spatial distribution of 100 keV B in amorphized Si is derived from depth profiles which were measured previously by a nuclear reaction analysis technique with thermal neutrons. Those depth distributions showed slight deviations (about 20%) from the theoretically predicted ones (TRIM) in directions between about 30° and 60°. This results in a somewhat different distribution than predicted.