ADVANCED FEG SEM-EDS ANALYSIS USING AN ANNULAR FOUR-CHANNEL SILICON DRIFT DETECTOR
Analysis of fine-scale sub-micron structures requires low accelerating voltages. Consequently, only low to intermediate energy X-ray lines can be evaluated, and these have many peak overlaps that requires deconvolution. Examination of nano-scale structures also requires low probe currents, which would give low X-ray count rates with conventional EDS detectors. The additional time required to acquire sufficient data for deconvolution risks altering the specimen as a result of beam-sample interaction or sample contamination. The annular BRUKER XFlash® FlatQUAD silicon drift detector (SDD) has allowed us to overcome these limitations and offers additional benefits as we demonstrate here.
The annular SDD is inserted between the pole piece and sample and has a large solid angle (1.1 sr). This geometry is minimizing shadow effects and is therefore ideally suited for the analysis of topographically complex, three-dimensional samples. Furthermore, X-rays are collected from four separate detector segments and signals are processed in parallel by four detection units allowing high count rates at low dead-time. Even at the lowest beam current (<10 pA), uncoated, beam sensitive and non-conductive samples can be investigated under high vacuum in natural state [1,2]. We further highlight the possibilities and limitations of quantitative analysis of low to intermediate X-ray lines with a spatial resolution of ~60 nm. This will be demonstrated on polished samples of extraterrestrial platinum group element alloys (<1 μm) hosted by Ni-Cr spinels of the Barberton Greenstone Belt (2, 3).
We conclude that SEM-EDS at low accelerating voltages and low beam current with an annular SDD provides high spatial resolution and high detection sensitivity without the necessity of applying a conductive coating or working in low vacuum. Compared to low vacuum analysis, this approach avoids beam skirting effects. In addition, hydrocarbon contamination is reduced. The possibility to analyse beam sensitive or precious specimens in a close to natural state with little preparation and to study fine scale structures and surface layers will stimulate new approaches for a wide range of applications.
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