X-Ray Microanalysis in the Variable Pressure (Environmental) Scanning Electron Microscope

Abstract

Electron-excited x-ray microanalysis performed in the variable pressure and environmental scanning electron microscopes is subject to additional artifacts beyond those encountered in the conventional scanning electron microscope. Gas scattering leads to direct contributions to the spectrum from the environmental gas, as well as remote generation of x rays by electrons scattered out of the focussed beam. The analyst can exert some degree of control over these artifacts, but depending on the exact situation, spurious elements can appear at the trace (< 0.01 mass fraction), minor (0.01 mass fraction to 0.1 mass fraction), or even major (> 0.1 mass fraction) levels. Dispersed particle samples give the least compromised results, while fine scale microstructures are the most severely compromised. Procedures to optimize the situation based upon specimen preparation as well as spectral processing are described.

Keywords: energy dispersive x-ray spectrometry; environmental scanning electron microscopy (ESEM); variable pressure scanning electron microscopy (VP-SEM); x-ray mapping; x-ray microanalysis; x-ray spectrometry.

Fig 1

Schematic diagram illustrating formation of electron scattering “skirt” around the unscattered beam in a VPSEM-ESEM. Elastic scattering leads to transfer of electrons from the focused beam to the skirt. Inelastic scattering leads to inner shell ionization and subsequent emission of characteristic x rays from the gas, which will be collected by the EDS if emitted into the solid angle defined by the collimator.

Fig 2

(a) Detection of x-ray emission from the environmental gas (H₂O) at various pressures for a gas path length of 6 mm, an incident beam energy of 20 keV, normal incidence, and a 2.5 cm diameter carbon target: base: 53 Pa (0.4 torr); 266 Pa (2 torr); 1600 Pa (12 torr); 2800 Pa (21 torr); (b) plot of the x-ray intensity ratio O/C as a function of pressure.

Fig. 3

Development of scattering skirt as calculated with Eq. (1) (Fig. 1) at a beam energy of 20 keV for various gases and a gas path length of (a) 5 mm; (b) 15 mm.

Fig. 4

Remote scattering of beam electrons. Target: 500 µm diameter wire of 40Cu-60Au (selected from NIST Standard Reference Material (SRM) 482 Copper-Gold Alloys) embedded in a large (2.5 cm diameter) aluminum disk; beam energy 20 keV; normal incidence. Pressure: 50 Pa (0.4 torr); 200 Pa (1.5 torr); 600 (4.5 torr); 1000 Pa (7.5 torr); 1600 Pa (12 torr); (b) Plot of x-ray intensity ratio Al-K/Cu-K vs pressure.

Fig. 5

Spectra of a particle of NIST Glass K230 (approximately 50 µm in dimension) on a carbon disk at pressures of 266 Pa (2 torr) and 1330 Pa (10 torr) showing increase in the skirt contribution from the carbon as well as direct excitation of the environmental gas (water vapor) by beam and backscattered electrons.

Fig. 6

Charging effects observed with NIST glass 1070 (Mg = 0.075 mass fraction; Si = 0.0187 mass fraction; Ca = 0.0893 mass fraction; Zn = 0.01 mass fraction; Ba = 0.0112 mass fraction; Pb = 0.0928 mass fraction; O = 0.0343 mass fraction) under various environmental gas conditions. (a) 266 Pa (2 torr) showing the correct Duane-Hunt limit at 15 keV; (b) 67 Pa (0.5 torr) showing reduced intensity near the Duane Hunt limit and a limit of 13 keV; (c) 53 Pa (0.4 torr) showing a Duane Hunt limit of 12 keV; (d) Comparison of spectra recorded at 266 Pa (2 torr) and 53 Pa (0.4 torr) with a linear intensity scale, showing loss of PbLα due to charging, and severe reduction in intensity for ZnKα and ZnKβ.

Fig. 7

Application of the pressure shift method [8]. Correction of skirt contributions to the x-ray spectrum using data from Fig. 4 experiment: (a) 40Cu-60Au in Al mount, E₀ = 20 keV, 200 Pa (1.5 torr) H₂O; (b) 400 Pa (3 torr); (c) difference spectrum, 400 Pa to 200 Pa; (d) “no scattering” spectrum calculated using equation (2) with difference spectrum and a linear multiplier of 2.

Fig. 7

Application of the pressure shift method [8]. Correction of skirt contributions to the x-ray spectrum using data from Fig. 4 experiment: (a) 40Cu-60Au in Al mount, E₀ = 20 keV, 200 Pa (1.5 torr) H₂O; (b) 400 Pa (3 torr); (c) difference spectrum, 400 Pa to 200 Pa; (d) “no scattering” spectrum calculated using equation (2) with difference spectrum and a linear multiplier of 2.

Fig. 8

Application of the pressure shift method [8]. Correction of skirt contributions to the x-ray spectrum to a complex microstructure (Raney nickel). (a) Backscattered electron SEM image showing phase distribution; three distinct phases can be recognized. (b) EDS spectra observed on the three phases with a pressure of 50 Pa at a beam energy of 20 keV; (c) EDS spectra observed on the three phases with a pressure of 665 Pa at a beam energy of 20 keV; (d) Pressure corrected spectrum for the high—Ni phase showing the NiL and AlK peaks; (e) Pressure corrected spectrum for the high—Ni phase showing the NiKα and NiKβ peaks.

Fig. 8

Application of the pressure shift method [8]. Correction of skirt contributions to the x-ray spectrum to a complex microstructure (Raney nickel). (a) Backscattered electron SEM image showing phase distribution; three distinct phases can be recognized. (b) EDS spectra observed on the three phases with a pressure of 50 Pa at a beam energy of 20 keV; (c) EDS spectra observed on the three phases with a pressure of 665 Pa at a beam energy of 20 keV; (d) Pressure corrected spectrum for the high—Ni phase showing the NiL and AlK peaks; (e) Pressure corrected spectrum for the high—Ni phase showing the NiKα and NiKβ peaks.

Fig. 8

Application of the pressure shift method [8]. Correction of skirt contributions to the x-ray spectrum to a complex microstructure (Raney nickel). (a) Backscattered electron SEM image showing phase distribution; three distinct phases can be recognized. (b) EDS spectra observed on the three phases with a pressure of 50 Pa at a beam energy of 20 keV; (c) EDS spectra observed on the three phases with a pressure of 665 Pa at a beam energy of 20 keV; (d) Pressure corrected spectrum for the high—Ni phase showing the NiL and AlK peaks; (e) Pressure corrected spectrum for the high—Ni phase showing the NiKα and NiKβ peaks.

Fig. 9

Use of a polycapillary x-ray optic (shown schematically) to serve as a spatial filter to restrict the acceptance area for x-ray collection. The plot shows the fall in intensity as a function of source position [14].

Fig. 10

Analytical blank: (a) High purity carbon substrate; 20 keV beam; 200 Pa (1.5 torr) H₂O; (b) Double-sided adhesive carbon tape on high purity carbon substrate; 20 keV beam; 200 Pa (1.5 torr) H₂O.

Fig. 11

Particles of NIST SRM 1633 (Trace metals in fly ash): (a) micrograph of particles dispersed on double-sized adhesive attached to carbon planchet; (b) operational blank measured at location “BL1” in Fig. 5 (a); (c) operational blank measured at location “BL2” in Fig. 5 (a); (d) operational blank measured at location “BL3” in Fig. 5 (a); (e) particle spectrum measured with beam centered on particle “A” compared to operational blank; (f) particle spectrum measured with beam centered on particle “C” compared to operational blank; (g) particle spectrum measured with beam centered on particle “D” compared to operational blank.

Fig. 11

Particles of NIST SRM 1633 (Trace metals in fly ash): (a) micrograph of particles dispersed on double-sized adhesive attached to carbon planchet; (b) operational blank measured at location “BL1” in Fig. 5 (a); (c) operational blank measured at location “BL2” in Fig. 5 (a); (d) operational blank measured at location “BL3” in Fig. 5 (a); (e) particle spectrum measured with beam centered on particle “A” compared to operational blank; (f) particle spectrum measured with beam centered on particle “C” compared to operational blank; (g) particle spectrum measured with beam centered on particle “D” compared to operational blank.

Fig. 11

Particles of NIST SRM 1633 (Trace metals in fly ash): (a) micrograph of particles dispersed on double-sized adhesive attached to carbon planchet; (b) operational blank measured at location “BL1” in Fig. 5 (a); (c) operational blank measured at location “BL2” in Fig. 5 (a); (d) operational blank measured at location “BL3” in Fig. 5 (a); (e) particle spectrum measured with beam centered on particle “A” compared to operational blank; (f) particle spectrum measured with beam centered on particle “C” compared to operational blank; (g) particle spectrum measured with beam centered on particle “D” compared to operational blank.

Fig. 11

Particles of NIST SRM 1633 (Trace metals in fly ash): (a) micrograph of particles dispersed on double-sized adhesive attached to carbon planchet; (b) operational blank measured at location “BL1” in Fig. 5 (a); (c) operational blank measured at location “BL2” in Fig. 5 (a); (d) operational blank measured at location “BL3” in Fig. 5 (a); (e) particle spectrum measured with beam centered on particle “A” compared to operational blank; (f) particle spectrum measured with beam centered on particle “C” compared to operational blank; (g) particle spectrum measured with beam centered on particle “D” compared to operational blank.

Fig. 12

Particles of NIST glass K309 deposited on thin (~20 nm) carbon film supported on a copper grid: (a) low magnification view of particle dispersion showing 80 µm (on edge) windows. Spectra of individual, micrometer-sized particles of NIST glass K309, beam energy 20 keV; pressure: (b) on bulk carbon adhesive tape, note the large carbon peak; (c) on carbon thin film (≈ 20 nm) carried on copper grid. Note artifact peaks of Cu-L and CuKα, CuKβ peaks from scattering onto grid, but near absence of carbon peak.

Fig. 12

Particles of NIST glass K309 deposited on thin (~20 nm) carbon film supported on a copper grid: (a) low magnification view of particle dispersion showing 80 µm (on edge) windows. Spectra of individual, micrometer-sized particles of NIST glass K309, beam energy 20 keV; pressure: (b) on bulk carbon adhesive tape, note the large carbon peak; (c) on carbon thin film (≈ 20 nm) carried on copper grid. Note artifact peaks of Cu-L and CuKα, CuKβ peaks from scattering onto grid, but near absence of carbon peak.

Fig. 13

VP-ESEM image of a fiber of NBS glass 230 suspended over a hole in a carbon block: (a) position shown where the fiber passes over the edge of the hole; (b) the x-ray spectrum of a 16 µm diameter fiber obtained at 266 Pa (2 torr, H₂O) with the 20 keV beam placed in the center of the hole; (c) spectrum obtained where the same fiber is attached to the carbon tape on the bulk carbon.

Fig. 13

VP-ESEM image of a fiber of NBS glass 230 suspended over a hole in a carbon block: (a) position shown where the fiber passes over the edge of the hole; (b) the x-ray spectrum of a 16 µm diameter fiber obtained at 266 Pa (2 torr, H₂O) with the 20 keV beam placed in the center of the hole; (c) spectrum obtained where the same fiber is attached to the carbon tape on the bulk carbon.

Fig. 14

Use of a fiber of lithium tetraborate glass as a specimen support: (a) EDS spectrum with direct beam on fiber; 400 Pa (3 torr); 20 keV; (b) EDS spectrum contributed by skirt, a factor of 3 lower.

Fig. 15

(a) ESEM image of an unknown particle attached to lithium tetraborate glass fiber; (b) EDS spectrum with direct beam on particle; 400 Pa (3 torr); 20 keV.

Fig. 16

(a) VP-ESEM image of a polished block of NBS glass K411 (SRM) mounted in silver-loaded epoxy and held in a titanium block (b) EDS spectrum at low pressure (< 50 Pa, 0.4 torr) with no gas scattering showing the peaks expected (K-lines of O, Mg, Si, Ca, and Fe); (c) EDS spectrum at 665 Pa (5 torr) of water vapor showing additional peaks for silver (Ag-L) and titanium (Ti-K).

Fig. 16

(a) VP-ESEM image of a polished block of NBS glass K411 (SRM) mounted in silver-loaded epoxy and held in a titanium block (b) EDS spectrum at low pressure (< 50 Pa, 0.4 torr) with no gas scattering showing the peaks expected (K-lines of O, Mg, Si, Ca, and Fe); (c) EDS spectrum at 665 Pa (5 torr) of water vapor showing additional peaks for silver (Ag-L) and titanium (Ti-K).

Fig. 17

X-ray mapping in the VPSEM/ESEM: Raney nickel, E₀ = 20 keV; all maps at 4 mm gas path; H₂O at pressures of 133 Pa (1 Torr), 665 Pa (5 Torr), 1330 Pa (10 Torr), 2000 Pa (15 Torr); E₀ = 20 keV; (a) Al x-ray maps; (b) Ni x-ray maps.