Effect of the Silicon Drift Detector on EDAX Standardless Quant Methods
Frank Eggert EDAX Inc. Materials Analysis Division, AMETEK, 91 McKee Drive, Mahwah, NJ 07430
frank.eggert@
ametek.com
Abstract: Two standardless quantitative methods for evaluating EDS X-ray spectra were investigated in regards their basic metrics. Both methods have similar total errors, but the error contributions are from different sources. In the P/B-based method, error is more related to counting statistics and therefore can benefit from high count rates achievable with modern silicon drift detectors. To reduce systematic uncertainties in the net-count-based standardless approach, mea- sured values need to be supported by data in a previously measured database. Using the P/B-based method, it is now possible to achieve standardless EDS quantification within ±10% relative deviation from true composition for 95% of results.
Keywords: X-ray spectra, standardless EDS quantification, silicon drift detector, P/B PeBaZAF Quant, eZAF Quant
Introduction When used with a scanning electron microscope (SEM),
energy-dispersive X-ray spectrometry (EDS) offers the micros- copist significant analytical capabilities. For example, the operator can obtain distribution maps that are color-coded according to elements or phases present in the analyzed mate- rial. If more analytical details are needed, it is necessary to ana- lyze the X-ray spectra from each distinguishable multi-element phase. From these spectra, quantitative values for the amounts of elements within each phase can be estimated. Sophisticated physical models, algorithms, and computer programs are needed to do this. Te resultant uncertainties are a combina- tion of the “precision” of the measured data and the “accuracy” available from the measurement setup, fundamental atomic databases, and the correctness of the matrix-correction mod- els employed to convert X-ray peak intensities into values for the amounts of elements in the specimen. While standardless quantitative analysis had been developed and accomplished for many years using conventional Si(Li) EDS detection systems, the advent of silicon driſt detectors (SDDs) has the potential to improve this type of analysis. Since the SDD can count over 100 times faster than the older Si(Li) detector, the SDD can quickly provide enough counts in X-ray peaks and background to significantly reduce the experimental scatter and improve the “precision” of the measurements. In this article two standardless methods used in EDAX
soſtware are compared without and with the high-count rate provided by an SDD. One of these methods, which depends more on the number of counts collected (counting statistics), benefits significantly from the SDD.
Materials and Methods Element identification is the first step toward getting
a proper view of the sample composition. No Quant model would be able to calculate the element concentrations properly if the assumption about which elements are present in the sam- ple is incorrect or wrongly evaluated. Tis is why EXpertID [1]
34 doi:10.1017/S1551929519001196
Figure 1: Two different quantification models, one based on evaluation of the measured net-counts of element peaks (green) and the other based on peak-to- background (P/B) ratios (green divided by purple).
www.microscopy-today.com • 2020 March
is used with all EDAX spectra evaluation soſtware to automati- cally identify the elements present. Tis procedure includes iterative evaluation of the residual spectrum, which is the mea- sured minus the reconstructed spectrum, to find small peaks which were not detected in other ways. Data from the spectrum. Figure 1 shows a simulated [2]
spectrum of GaP with indications of how raw data are obtained for the two different Quant methods used in EDAX standard- less analysis soſtware (following the definitions in [3]). One method is based on evaluation of the measured net-counts of element peaks above the background (P=measured counts minus the background, shown in “green”). Te other method is based on peak-to-background ratios (P/B=P divided by back- ground, shown as “green” divided by “purple”). In both cases, the measured P and P/B values are greater with higher element concentrations in the sample. But there is usually no linear rela- tionship between element concentration and measured element X-ray signal; moreover, other elements can cause large inter-ele- ment effects. For both methods, the measured input values for the quantification calculation (net intensities or P/B) require the X-ray background to be subtracted from the gross peak. In the case of the P/B method, the peak intensity must also be divided by the background; thus, accurate background values are par- ticularly important. If element lines overlap, a deconvolution of the peaks is required. Among the methods for determining the background,
EDAX uses the physics-based bremsstrahlung calculation [4,5] and a pure mathematical background approximation method,
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