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 1853a 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 1853a. 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. One must take great care in determining whether measured carbon characteristic peak is valid or not since the detection efficiency are normally limited by absorption band edges for various materials in the detector window at low energies (normally below 600 eV). Table 1853 lists C-examples of thicknesses at which the thin-film approximation is no longer valid due to X-ray absorption effects in specific materials.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%.


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

However, in many cases, carbon is one of the most difficult contaminants, incorporated from fab-environment, to be detected by analytical TEM such as TEM-EDS and TEM-EELS techniques because of two reasons:
        i) Hydrocarbon (HC) contamination, from the chamber surfaces, vacuum pumps and sample surface migration and then its reaction with the electron beam, often induces background carbon signal. Therefore, it is difficult to distinguish between HC and real carbon contaminants from in-line process.
        ii) The TEM-EDS and TEM-EELS sensitivities are not high enough to detect carbon contents at interfaces or in structures which is able to affect electrical properties of devices.

Figure 1853b shows the percentage of x-ray transmitted through an H2O-ice contamination layer depending on the thickness of the H2O-ice layer up to 1 μm. As expected, the absorption effect of ice layer is greatest for the low-energy boron x-rays and the least for the silicon signal. Such H2O-ice layer is normally formed in cryo-TEM measurements.

Calculated percentage absorption of characteristic x- rays in an H2O-ice contamination layer

Figure 1853b. Calculated percentage absorption of characteristic x- rays in an H2O-ice contamination layer. [2]. X-ray energies: B-K = 183 eV, C-K = 277 eV, O-K = 525 eV, N-K = 392 eV and Si-K = 1.739 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).
[2] M. Malac and R.F. Egerton, Calibration Specimens for Determining Energy-Dispersive X-ray k-Factors of Boron, Nitrogen, Oxygen, and Fluorine, Microsc. Microanal. 5, 29–38, (1999).