Elemental thin film depth profiles by ion beam analysis using simulated annealing - a new tool

Abstract

Rutherford backscattering spectrometry (RBS) and related techniques have long been used to determine the elemental depth profiles in films a few nanometres to a few microns thick. However, although obtaining spectra is very easy, solving the inverse problem of extracting the depth profiles from the spectra is not possible analytically except for special cases. It is because these special cases include important classes of samples, and because skilled analysts are adept at extracting useful qualitative information from the data, that ion beam analysis is still an important technique.We have recently solved this inverse problem using the simulated annealing algorithm. We have implemented the solution in the 'IBA DataFurnace' code, which has been developed into a very versatile and general new software tool that analysts can now use to rapidly extract quantitative accurate depth profiles from real samples on an industrial scale. We review the features, applicability and validation of this new code together with other approaches to handling IBA (ion beam analysis) data, with particular attention being given to determining both the absolute accuracy of the depth profiles and statistically accurate error estimates.We include examples of analyses using RBS, non-Rutherford elastic scattering, elastic recoil detection and non-resonant nuclear reactions. High depth resolution and the use of multiple techniques simultaneously are both discussed. There is usually systematic ambiguity in IBA data and Butler's example of ambiguity (1990 Nucl. Instrum. Methods B 45 160-5) is reanalysed. Analyses are shown: of evaporated, sputtered, oxidized, ion implanted, ion beam mixed and annealed materials; of semiconductors, optical and magnetic multilayers, superconductors, tribological films and metals; and of oxides on Si, mixed metal silicides, boron nitride, GaN, SiC, mixed metal oxides, YBCO and polymers. OverviewWe will review our recent software developments in thin film depth profiling with ion beam analysis (IBA, see the 6 Centre de Física Nuclear da Universidade de Lisboa, Avenida Prog. Gama Pinto 2, 1699 Lisboa Codex, Portugal Glossary in section 2 for an explanation of the acronyms) in the context of the work of the community; discussing the scientific implications of these developments. We hope to show that these developments improve the usability of IBA to such an extent as to effectively establish a new depth profiling tool.We start (section 2) with a simplified overview of IBA since we believe that our new software tool (the IBA

Statistically sound evaluation of trace element depth profiles by ion beam analysis

“…This issue of ambiguity during manual deconvolution of IBA data is of course not new and has been discussed previously (e.g. [4] in 2003). However since then still a large number (e.g.…”

“…From this probabilistic comparison between the ideal forward calculated data and the experimental data the most probable choice of the trace element depth profile can be determined. The forward calculation of IBA data is described in much detail for example in [4,18,7]. Here only the general idea behind the forward calculation and how it can be linearized for trace level concentrations will be described.…”

“…This approach has been applied to the deconvolution of IBA data [1,2,3] and is summarized in the review by Jeynes [4]. There have been many approaches to determining depth profiles by fitting experimental scattering data starting with the early work by Doolittle [5] evolving into complete code packages like DataFurnace [6] or SimNRA [7].…”

“…In comparison only 3000 MCMC steps were performed in [9] to infer the confidence intervals. This shows why the maximum likelihood approach has been generally considered too computationally expensive due to the large number of forward calculations required [4]. But due to the linearization of the forward calculation in NRADC, such a large number of forward calculations become manageable and a full deconvolution including confidence intervals both based on the maximum likelihood approach takes less than ≈ 10 minutes on a normal office PC.…”

Accurate Determination of Quantity of Material in Thin Films by Rutherford Backscattering Spectrometry

“…The actual algorithm for the calculation of the RBS spectrum expected for any given sample (the "forward model") has been given in detail in [50]. Computer codes are used for these calculations [51], and the best of these codes have been demonstrated accurate for RBS to at least 0.2% [46].…”