Electron microscopy
 
EDS Measurement of Carbon
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The background counts in TEM-EDS are much lower than those in SEM-EDS spectrum. Due to the high background counts in SEM-EDS, an artificial carbon (C) peak is always visible and thus a value of more than 2% carbon is normally measured even though there is no carbon in the specimen. This artefact is due to the window in the detector. The EDS windows are normally SATW windows and their material has a specific transmission profile with a strong absorption edge just above but very close to the C X-ray energy, resulting in an artificial peak at the C energy position. Therefore, it is the strong absorption of the background (continuum) X-rays that produces the artefact peak. Note that SATW detector windows are AP* ultrathin polymer windows manufactured by Moxtek and are almost supplied by all EDS detector companies. However, a TEM-EDS spectrum taken from the same specimen materials does not show such a artefact peak at the carbon energy because the spectrum consists mostly of characteristic X-rays.

Figure 1853 shows the deconvolution of an EDX spectrum with peak overlaps, taken from a material that contains O, Ti, N, and C elements.

Deconvolution of an EDX spectrum with peak overlaps

Figure 1853. Deconvolution of an EDX spectrum with peak overlaps. [1]

As discussed on page4650, X-ray absorption is a function of the energy of X-rays. Low energy peaks will be more strongly absorbed than high energies ones. For thick TEM samples, k-factor correction due to X-ray absorption is needed in order to accurately quantify EDS measurements. Table 1853 lists C-examples of thicknesses at which the thin-film approximation is no longer valid due to X-ray absorption effects in specific materials.

Table 1853. Examples of limits to the thin-film approximation caused by X-ray absorption: Maximum thicknesses of thin specimens for which the absorption correction (or error) is less than ±10% and ±3%.

Material

10% error in kAB
3% error in kAB
Absorbed X-ray lines
Primary X-ray lines
Thickness (nm)
SiC
13 3 Si Kα and C Kα Si Kα (1.739 keV) and C Kα (0.277 keV)

 

[1] J. Berlin, T. Salge, M. Falke, and D. Goran, Recent Advances in EDS and EBSD Technology: Revolutionizing the Chemical Analysis of Chondritic Meteorites at the Micro- and Nanometer Scale, 42nd Lunar and Planetary Science Conference, 2723, (2011).

 

 

 

 

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