SIMS PROFILE QUANTIFICATION BY MAXIMUM-ENTROPY DECONVOLUTION

The action Of the Probe ions in SIMS depth profiling causes a number of mass transport phenomena. Consequently the data obtained are both broadened and shifted in a manner dependent on the species of the matrix, impurity and probe, and the experimental conditions. Quantification methods based on implanted standards and crater depth measurement are hampered by inaccuracies in the depth measurement, and take no account of the blurring processes. This paper reports the development of an alternative method. The blurring processes are not yet well enough understood for complete modelling. An empirical model, valid in the dilute limit, is that the true depth distribution is convoluted with an instrumental response to give the SIMS signal. If the response function is carefully defined, inversion of the convolution equation should give a fully quantified depth profile, correcting for blurring effects and any differential shift, mapping primary ion dose density (or time) back to depth and mapping signal intensity back to concentration in one mathematical operation. The sample from which the response function is measured becomes a complete concentration and depth standard. Deconvolution is non-trivial as there is no unique solution in the real case, where the profile data and measured response consist of a finite number of data points. The method used here is based on the use of maximum entropy and returns the least biased (least structured) of the possible solutions. The use of this method to fully quantify two samples will be demonstrated and the results compared with ordinary quantification.