Dale E. Newbury
Objective: Evaluate the accuracy of the peak-to-background method for quantitative electron beam x-ray microanalysis of specimens with extreme topography.
Problem: Conventional quantitative electron probe microanalysis is restricted to surfaces that are flat to a relief of 100 nm amplitude or less. Otherwise, geometric effects on electron scattering and x-ray absorption violate the basic assumption of matrix correction calculations that composition is the only reason x-ray intensities differ between the unknown and the standard(s) (e.g., pure elements). Specimens examined in the scanning electron microscope (SEM) with semiconductor energy dispersive x-ray spectrometry (Si-EDS) are often characterized by extreme topography. Conventional quantitative x-ray microanalysis of rough specimens can be severely compromised. As shown in the table, the conventional analysis of IN100 alloy in the form of a sphere in which the analyzed location is on the backside of the sphere away from the Si-EDS results in large errors, especially for characteristic peaks with low photon energies below 4 keV.
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Table of Relative Errors for IN-100 Alloy Sphere (122
micrometer diameter; backside from EDS)
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Element
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C (wt. frac.)
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Conv. (raw) error
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Conv. (norm.) error
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P/B error
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Al-K
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0.0603
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-77%
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-71%
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-18%
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Mo-L
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0.0353 + 6
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-62
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-51
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+ 6
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Ti-K
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0.0519
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-16
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+7
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+19
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Cr-K
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0.0965
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-23
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- 3
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+12
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Co-K
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0.155
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-18
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+5
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- 0.3
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Ni-K
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0.601
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-14
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+9
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- 1
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Approach: To overcome the limitations of conventional analysis in such cases, an additional source of information must be supplied that can compensate for geometric factors. For point beam analysis, the peak-to-local background (P/B) method, which incorporates measures of both the characteristic and continuum (bremsstrahlung) x-rays at the same energy, can provide the necessary information for correction. While the P/B method has been available for many years, there exist no systematic studies of its efficacy for rough specimens. NIST-NIH Desktop Spectrum Analyzer (DTSA) provides an ideal x-ray spectrum-processing platform to study each stage of P/B corrections.
Results and Future Plans: The impact of the P/B corrections upon the accuracy of the analysis is shown in the table, where the relative errors are seen to be significantly reduced after P/B corrections. This on-going measurement program will be extended to other alloy systems as well as non-conducting targets to develop sufficient data to fully characterize the error distribution of the peak-to-background correction method.
Publications:
DE Newbury, Microbeam Analysis 2000, Inst. Physics Conf. Series #165, 427-428,
2000.
DE Newbury, Microscopy & Microanalysis, 6, suppl. 2, 928-929, 2000.
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Last Updated
March 5, 2002
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