Performing elemental microanalysis with high accuracy and high precision by scanning electron microscopy/silicon drift detector energy-dispersive X-ray spectrometry (SEM/SDD-EDS)

Abstract

Electron-excited X-ray microanalysis performed in the scanning electron microscope with energy-dispersive X-ray spectrometry (EDS) is a core technique for characterization of the microstructure of materials. The recent advances in EDS performance with the silicon drift detector (SDD) enable accuracy and precision equivalent to that of the high spectral resolution wavelength-dispersive spectrometer employed on the electron probe microanalyzer platform. SDD-EDS throughput, resolution, and stability provide practical operating conditions for measurement of high-count spectra that form the basis for peak fitting procedures that recover the characteristic peak intensities even for elemental combination where severe peak overlaps occur, such PbS, MoS₂, BaTiO₃, SrWO₄, and WSi₂. Accurate analyses are also demonstrated for interferences involving large concentration ratios: a major constituent on a minor constituent (Ba at 0.4299 mass fraction on Ti at 0.0180) and a major constituent on a trace constituent (Ba at 0.2194 on Ce at 0.00407; Si at 0.1145 on Ta at 0.0041). Accurate analyses of low atomic number elements, C, N, O, and F, are demonstrated. Measurement of trace constituents with limits of detection below 0.001 mass fraction (1000 ppm) is possible within a practical measurement time of 500 s.

Fig. 1

Distribution of relative errors [(measured − true)/true × 100 %] using the k-ratio protocol with WDS measurements and matrix corrections with the NBS ZAF procedure FRAME (1975) [5]. Note that the histogram bins have a width of 1 % relative

Fig. 2

Distribution of relative errors observed for a commercial implementation of standardless analysis (2013). Note that the histogram bins have a width of 5 % relative

Fig. 3

a Analysis of NIST SRM 470 (K411 glass) as a flat, highly polished bulk sample (final polish with 100 nm alumina) using the k-ratio protocol with SDD-EDS measurements and NIST DTSA-II. Plot of Fe (normalized weight percent) vs. Mg (normalized weight percent) for 20 randomly selected analyses. Note the outlier (circled). b Analysis of NIST SRM 470 (K411 glass) as a flat, but slightly scratched bulk sample (scratches remaining after 1 µm diamond polish) using the k-ratio protocol with SDD-EDS measurements and NIST DTSA-II. Plot of Fe (normalized weight percent) vs. Mg (normalized weight percent) for 20 randomly selected analyses

Fig. 4

Analysis of NIST SRM 470 (K411 glass) in various geometric forms (flat, polished bulk; scratched surface after 600-grit grinding; shallow surface holes, chips, and shards) using the k-ratio protocol with SDD-EDS measurements and NIST DTSA-II. Plot of Fe (normalized weight percent) vs. Mg (normalized weight percent) [19]

Fig. 5

Analysis of NIST SRM 470 (K411 glass) in various geometric forms (flat, polished bulk; scratched surface after 600-grit grinding; shallow surface holes, chips, and shards) using the k-ratio protocol with SDD-EDS measurements and NIST DTSA-II: a Mg (normalized weight percent) vs. the raw analytical total (weight percent), including oxygen calculated by assumed stoichiometry [19], b Fe (normalized weight percent) vs. the raw analytical total (weight percent), including oxygen calculated by assumed stoichiometry [19]

Fig. 6

Automatic peak identification of potassium bromide, showing misidentification of BrLα,β as AlK; note also misidentification of minor BrLl peak as AsLα,β [–18]

Fig. 7

a Gold, beam energy 20 keV, showing correct identification of the Au L-family and M-family [–18]. b Gold, beam energy 10 keV, showing misidentification of the Au M-family as the Zr L-family and Nb L-family [–18]

Fig. 8

a Lead, beam energy 20 keV, showing correct identification of the Pb L-family and PbMα,β, but misidentification of PbMζ as WM and PbM₂N₄ as CdL [–18]. b NIST glass K230 with Pb as a major constituent, beam energy 20 keV; the Pb M-family is misidentified as SK and TcL, while the Pb L-family is ignored [–18]

Fig. 9

a SDD-EDS (four-detector array) spectra of manganese with a beam energy of E ₀ = 20 keV collected over a range of input count rates resulting in deadtime from 2 to 77 % showing the MnKα–MnKβ peak region. Note excellent superposition of the peaks after scaling to the MnKα peak integral. b MnL peak region. Note excellent superposition of the peaks after scaling to the MnLα peak integral

Fig. 10

a In-growth of coincidence (sum) peaks: NIST SRM 470 (K412 glass) over a sequence of detector deadtimes, b deadtimes from 1 to 48 %, with all spectra scaled to the full spectrum integral

Fig. 11

Comparison of the k-ratios measured simultaneously by SDD-EDS (red) and WDS (blue) in barium titantate, benitoite (Ba), and a series of Ba/Ti/Si glasses spanning a mass concentration range as high as Ba:Ti of 24:1 (Color figure online)

Fig. 12

SDD-EDS spectrum of PbS (galena) at a beam energy of E ₀ = 10 keV (red); residual spectrum (blue) after peak fitting with CdS and PbSe (Color figure online)

Fig. 13

SDD-EDS spectrum of the MoS₂ with E ₀ = 10 keV (red); residual spectrum after MLLSQ peak fitting (blue) (Color figure online)

Fig. 14

SDD-EDS spectrum of BaTiO₃ at a beam energy of E ₀ = 10 keV (red); residual spectrum (blue) after peak fitting with Ti and BaSi₂O₅ (sanbornite) (Color figure online)

Fig. 15

SDD-EDS spectrum of SrWO₄ at a beam energy of E ₀ = 10 keV (red); residual spectrum (blue) after peak fitting with SrF₂ and W (Color figure online)

Fig. 16

SDD-EDS spectrum of WSi₂ at a beam energy of E ₀ = 10 keV (red); residual spectrum (blue) after peak fitting with Si and W (Color figure online)

Fig. 17

a SDD-EDS spectrum of NIST microanalysis glass K2496 showing the Ba–Ti region of the spectrum around 4.5 keV (red), and the residual spectrum after fitting for Si, Ba, and Ti (blue). Beam energy 10 keV; dose 1000 nAs; spectrum integral 0.1–10 keV = 12.175 million counts; b The same measured spectrum but with fitting only for Si and Ba (red), revealing the low-level TiKα and TiKβ peaks in the residual spectrum (blue) (Color figure online)

Fig. 18

a SDD-EDS spectrum of NIST microanalysis glass K873 (red) and the residual spectrum after fitting for all constituents (blue). Beam energy 15 keV; dose 22500 nAs; spectrum integral 0.1–15 keV = 74.17 million counts. b The same measured spectrum but with fitting excluded for Ta and Ce (red), revealing the low-level Ce L-family and Ta M-family peaks in the residual spectrum (blue) (Color figure online)

Fig. 19

SDD-EDS spectrum of Cr₂N (red) and residual spectrum (blue) after MLLS fitting: a full spectrum, b expanded to show the low photon energy region (Color figure online)

Fig. 20

SDD-EDS spectrum of NIST microanalysis glasses K496 (blue) and K497 (red) (Color figure online)

Fig. 21

Analysis of S and Fe at several locations on a fragment of pyrite, FeS₂. The normalized mass concentrations, relative errors, and the raw unnormalized analytical total obtained following k-ratio protocol analyses with NIST DTSA-II (Fe and CuS standards) are indicated at selected locations (Everhart–Thornley detector SEM image)