Barriers to Quantitative Electron Probe X-Ray Microanalysis for Low Voltage Scanning Electron Microscopy

Abstract

Low voltage x-ray microanalysis, defined as being performed with an incident beam energy ≤5 keV, can achieve spatial resolution, laterally and in depth, of 100 nm or less, depending on the exact selection of beam energy and the composition of the target. The shallow depth of beam penetration, with the consequent short path length for x-ray absorption, and the low overvoltage, the ratio of beam energy to the critical ionization energy, both contribute to minimizing the matrix effects in quantitative x-ray microanalysis when the unknown is compared to pure element standards. The low beam energy restricts the energy of the atomic shells that can be excited, forcing the analyst to choose unfamiliar shells/characteristic peaks. The low photon energy shells are subject to low fluorescence yield, so that the peak-to-continuum background is reduced, severely limiting detectability. The limited resolution of semiconductor energy dispersive spectrometry results in frequent peak interference situations and further exacerbates detection limits. Future improvements to the x-ray spectrometry limitations are possible with x-ray optics-augmented wavelength dispersive spectrometry and microcalorimeter energy dispersive spectrometry.

Keywords: electron probe x-ray microanalyzer; energy dispersive spectrometry; low voltage microanalysis; scanning electron microscope; wavelength dispersive spectrometry; x-ray spectrometry.

Fig. 1

Si-EDS (FWHM = 129 eV at MnKα, 61 eV at C K) spectrum of WSi₂ showing interferences (SiKα at 1.740 keV with WMα at 1.775 keV) revealed by W-peak fitting with multiple linear least squares. The thin-line trace shows the residuals after removal of the W-peak structure.

Fig. 2

Si-EDS (FWHM = 129 eV at MnKα, 61 eV at C K) spectrum of strontium orthosilicate, SrSiO₄ (SiKα at 1.740 keV with SrLα at 1.806 keV),

Fig. 3

Si-EDS (FWHM = 129 eV at MnKα, 61 eV at C K) spectrum of brass CuZn (CuLα at 0.928 and ZnLα at 1.009 keV)

Fig. 4

Si-EDS (FWHM = 129 eV at MnKα, 61 eV at C K) spectrum of stainless steel, CrFeNi (CrLα at 0.571 keV; FeLα at 0.711 keV; and NiLα at 0.849 keV).

Fig. 5

(a) Si-EDS (FWHM = 129 eV at MnKα, 61 eV at C K) spectrum of BaTiO₃ excited in the low voltage analysis region with E₀ = 3 keV; (b) spectrum of BaTiO₃ as measured with the microcalorimeter EDS.

Fig. 6

The C K and O K region of the x-ray spectrum showing interferences with L- and M-family peaks of heavy elements.

Fig. 7

Periodic Table showing typical choices of atomic shells for operation in the conventional beam energy range, E₀ = 10 keV to 30 keV.

Fig. 8

Periodic Table showing choice of atomic shells available for operation in the low beam energy range, E₀ = 5 keV and U = 1.1.

Fig. 9

Periodic Table showing choice of atomic shells available for operation in the low beam energy range, E₀ = 2.5 keV and U = 1.1. Note significant loss of elements that can be effectively measured.

Fig. 10

Behavior of the peak (P) and peak-background (P/B) ratio as a function of overvoltage

Fig. 11

Comparison of resolution vs photon energy for various spectrometers.

Fig. 12

Microcalorimeter energy dispersive x-ray spectrum of WSi₂, showing separation of the SiK peak from the W-M family x-ray peaks.

Fig. 13

Microcalorimeter energy dispersive x-ray spectrum of SrSiO₃, showing separation of the SiK peak from the Sr-L family x-ray peaks.

Fig. 14

Microcalorimeter energy dispersive x-ray spectrum of brass, showing separation of the CuL family peaks from the ZnL family peaks.

Fig. 15

Microcalorimeter energy dispersive x-ray spectrum of a complex alloy, showing separation of the L-family peaks for Mn, Fe, Co, Ni and Cu.