X-ray spectra suffer significantly degraded spatial resolution when measured in the variable-pressure scanning electron microscope (VPSEM, chamber pressure 1 Pa to 2500 Pa) as compared to highvacuum SEM (operating pressure < 10 mPa). Depending on the gas path length, electrons that are scattered hundreds of micrometers outside the focused beam can contribute 90% or more of the measured spectrum. Monte Carlo electron trajectory simulation, available in NIST DTSA-II, models the gas scattering and simulates mixed composition targets, e.g., particle on substrate. The impact of gas scattering at the major (C > 0.1 mass fraction), minor (0.01 ≤ C ≤ 0.1), and trace (C < 0.01) constituent levels can be estimated. NIST DTSA-II for Java-platforms is available free at: http://www.cstl.nist.gov/div837/837.02/epq/dtsa2/index.html).
Quantitative electron-excited x-ray microanalysis by scanning electron microscopy/silicon drift detector
energy dispersive x-ray spectrometry (SEM/SDD-EDS) is capable of achieving high accuracy and high
precision equivalent to that of the high spectral resolution wavelength dispersive x-ray spectrometer even
when severe peak interference occurs. The throughput of the SDD-EDS enables high count spectra to be
measured that are stable in calibration and resolution (peak shape) across the full deadtime range. With this
high spectral stability, multiple linear least squares peak fitting is successful for separating overlapping peaks
and spectral background. Careful specimen preparation is necessary to remove topography on unknowns and
standards. The standards-based matrix correction procedure embedded in the NIST DTSA-II software engine
returns quantitative results supported by a complete error budget, including estimates of the uncertainties
from measurement statistics and from the physical basis of the matrix corrections. NIST DTSA-II is
available free for Java-platforms at: http://www.cstl.nist.gov/div837/837.02/epq/dtsa2/index.html).
The typical strategy for analysis of a microscopic particle by scanning electron microscopy/energy dispersive spectrometry x-ray microanalysis (SEM/EDS) is to use a fixed beam placed at the particle center or to continuously overscan to gather an “averaged” x-ray spectrum. While useful, such strategies inevitably concede any possibility of recognizing microstructure within the particle, and such fine scale structure is often critical for understanding the origins, behavior, and fate of particles. Elemental imaging by x-ray mapping has been a mainstay of SEM/EDS analytical practice for many years, but the time penalty associated with mapping with older EDS technology has discouraged its general use and reserved it more for detailed studies that justified the time investment. The emergence of the high throughput, high peak stability silicon drift detector (SDD-EDS) has enabled a more effective particle mapping strategy: “flash” x-ray spectrum image maps can now be recorded in seconds that capture the spatial distribution of major (concentration, C > 0.1 mass fraction) and minor (0.01 ≤ C ≤ 0.1) constituents. New SEM/SDD-EDS instrument configurations feature multiple SDDs that view the specimen from widely spaced azimuthal angles. Multiple, simultaneous measurements from different angles enable x-ray spectrometry and mapping that can minimize the strong geometric effects of particles. The NIST DTSA-II software engine is a powerful aid for quantitatively analyzing EDS spectra measured individually as well as for mapping information (available free for Java platforms at: http://www.cstl.nist.gov/div837/837.02/epq/dtsa2/index.html).
Scanning electron microscopy with energy dispersive x-ray spectrometry (SEM/EDS) is a powerful and flexible
elemental analysis method that can identify and quantify elements with atomic numbers > 4 (Be) present as major
constituents (where the concentration C > 0.1 mass fraction, or 10 weight percent), minor (0.01≤ C ≤ 0.1) and trace (C <
0.01, with a minimum detectable limit of ≈± 0.0005 - 0.001 under routine measurement conditions, a level which is
analyte and matrix dependent ). SEM/EDS can select specimen volumes with linear dimensions from ≈ 500 nm to 5 μm
depending on composition (masses ranging from ≈ 10 pg to 100 pg) and can provide compositional maps that depict
lateral elemental distributions. Despite the maturity of SEM/EDS, which has a history of more than 40 years, and the
sophistication of modern analytical software, the method is vulnerable to serious shortcomings that can lead to incorrect
elemental identifications and quantification errors that significantly exceed reasonable expectations. This paper will
describe shortcomings in peak identification procedures, limitations on the accuracy of quantitative analysis due to
specimen topography or failures in physical models for matrix corrections, and quantitative artifacts encountered in xray
elemental mapping. Effective solutions to these problems are based on understanding the causes and then
establishing appropriate measurement science protocols. NIST DTSA II and Lispix are open source analytical software
available free at www.nist.gov that can aid the analyst in overcoming significant limitations to SEM/EDS.
The extraordinary throughput of the silicon drift detector energy dispersive x-ray spectrometer (SDD-EDS) enables
collection of EDS spectra with much higher integrated counts within practical time periods, e.g., 100 s or less, compared
to past experience with the Si(Li)-EDS. Such high count SDD spectra, containing one million to ten million counts,
yield characteristic peak intensities with relative standard deviation below 0.25%, a precision similar to that achieved
with wavelength dispersive spectrometry (WDS), the "gold standard" of microprobe analysis, but at lower dose because
of the greater solid angle of the SDD-EDS. Such high count SDD-EDS spectra also enable more accurate quantification,
nearly indistinguishable from WDS for major and minor constituents when the WDS unknown-to-standard intensity
ratio ("k-value") protocol is followed. A critical requirement to satisfy this measurement protocol is that the specimen
must be a highly polished bulk target. The geometric character of specimens examined in the scanning electron
microscope (SEM) often deviates greatly from the ideal flat bulk target but EDS spectra can still be readily obtained and
analyzed. The influence of geometric factors such as local inclination and surface topography on the accuracy of
quantitative EDS analysis is examined. Normalized concentration values are subject to very large errors, as high as a
factor of 10, as a result of deviation of the specimen geometry from the ideal flat bulk target.
Electron-excited x-ray spectrum image (XSI) elemental mapping can now be performed in remarkably short time, 30
seconds or less, with the silicon drift detector energy dispersive x-ray spectrometer (SDD-EDS). Major constituents
(concentration, C > 0.1 mass fraction) and minor constituents (0.01 < C < 0.1) can be mapped with such short duration
scans, and trace constituents (C < 0.01) can often be mapped in 300 second scans. Constraints imposed by the older
Si(Li)-EDS are greatly reduced with the new SDD-EDS technology, enlarging the range of application of elemental
mapping. While high speed mapping has numerous applications, mapping times up to 30 minutes duration are useful for
higher pixel density that can reveal unexpected fine spatial details, finer than the x-ray interaction volume appears to
permit. Longer duration SDD-EDS mapping enables recording a deeper x-ray gray scale, permitting compositional
contrast to be observed at much lower values for major, minor, and trace constituents.
Milliprobe x-ray fluorescence (mXRF) with x-ray spectrum imaging (XSI) enables elemental mapping over centimeter
lateral distances with a resolution of 40-150 μm. While highly complementary to classic elemental mapping scanning
electron microscopy/energy dispersive x-ray spectrometry (SEM/EDS), mXRF has several advantages: (1) The lack of
electron bremsstrahlung in the XRF spectrum, except for the elastic scattering of the primary continuum, means that the
inherent detection sensitivity of mXRF is better. (2) The broad continuum excitation of mXRF enables sensitive access
to secondary x-rays with photon energies in the range 10 keV to 40 keV, which are either not efficiently excited or are
completely inaccessible with SEM. (3) The range of penetration of x-rays (with minimal sideways scatter) is typically 10
to 100 times the range of an electron beam, enabling deeper probing into the specimen or even viewing the specimen
through protective covering such as glass or plastic. (4) The vacuum requirements of mXRF are much less than even
environmental SEM, and primary excitation and secondary detection can occur through an atmospheric gas path if
required. The scale of mXRF-XSI mapping is particularly useful for attacking problems that require following
compositional structures over a wide spatial scale, the classic "micro-to-macro" spatial scale challenge.
Compositional measurements of microscopic particles by scanning electron microscopy/energy dispersive x-ray
spectrometry (SEM/EDS) typically assume particle homogeneity so that a representative EDS spectrum can be collected
by continuously bracket scanning the particle (overscanning). Particles are often found to be complex structures
comprised of smaller entities that are elementally different, knowledge of which is inevitably lost with particle
overscanning. Heterogeneity can be directly visualized with x-ray spectrum image mapping performed in a high
resolution thermal field emission gun SEM combined with the silicon drift detector (SDD)-EDS. SDD-EDS is capable
of x-ray collection with output count rates in excess of 1 MHz, enabling spectrum image mapping with useful pixel
density (128x128 or more), intensity range (8 - 16 bits), and compositional sensitivity (detection to approximately
1 weight percent) with a total time of 3 - 30 s when a high beam current (e.g., 50 nA) is utilized. Spectrum image
datacubes can range from 100 Mbyte to several gigabytes. NIST Lispix contains extensive image processing tools to
extract spectral and image information from such large datacubes. In addition to particle chemical heterogeneity,
spectrum image mapping can directly reveal the effects of geometric factors (size, shape, curvature) that modify x-ray
generation and emission from particles and which must be considered in particle quantification calculations.
Natural and synthetic microstructures with micrometer- to nanometer-scale features present a significant challenge to chemical analysis techniques. As the dimensions of features are reduced, the number of atoms and molecules to be analyzed becomes so small that useful analytical signals can only be obtained through optimization of the entire measurement process: e.g., the use of high brightness radiation sources, high efficiency spectrometers, and long counting times. Techniques based upon beams of electrons, photons, ions, and neutrons and generally incorporating some form of microscopy are available. The suite of characterization techniques can provide a wide variety of information on elemental and molecular composition, morphology, and crystal structure on a scale ranging from micrometers to nanometers.
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