EDS Measurement of Oxygen
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Light elements such as nitrogen (N K) and oxygen (O K) are detectable with different modern EDS detectors, e.g. ultra-thin window X-ray detectors (see page4589). However, an absorption correction will be needed even for the thinnest TEM specimens. Note that absorption does not only happen in the specimen itself but also in surface layers, e.g. contamination, and intentionally coated carbon and metal conductive layers, and in the detector window.

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 Table 1739 lists O-examples of thicknesses at which the thin-film approximation is no longer valid due to X-ray absorption effects in specific materials.

Table 1739. 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)
Al2O3
113 14 Al Kα and O Kα Al Kα (1.486 keV) and O Kα (0.525 keV)
MgO
   25 Mg Kα and O Kα Mg Kα (1.253 keV) and O Kα (0.525 keV)
SiO2
 167 14 Si Kα and O Kα Si Kα (1.739 keV) and O Kα (0.525 keV)

Figure 1739 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 1739. Calculated percentage absorption of characteristic x- rays in an H2O-ice contamination layer. [1]. 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] 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).

 

 

 
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