Practical Electron Microscopy and Database

Table 4532a lists the properties of common X-ray detectors.

The X-ray detector crystal is behind the EDS detector window, and is commonly made up of a lithium (Li) impregnated silicon (Si) wafer that is maintained at cold temperatures by liquid nitrogen (N₂) stored in a Dewar located on the outside of the microscope.

The main requirements of EDS detectors for EMs are:
         i) Detect the entire energy range of x-rays, namely from 50 eV (Li-K) to incident beam energy of the microscope.
            i.a) For SEMs, the energy range of X-rays is up to 30 keV.
            i.b) For (S)TEMs with accelerating voltages up to 300 keV, the realistic energy range of X-rays is up to 50 keV.
         ii) High x-ray detection rates, resulting in reduced collection times.
         iii) Optimized physical geometry to maximize the x-ray collection by shortening the specimen-to-detector distance and leaving enough physical room for other detectors and accessories.

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.

Table 4532b lists the main reasons why EDS measurements in low-energy SEM or high-energy (S)TEM have not replaced each other.

Table 4532b. Comparison between the EDS measurements in low-energy SEM and high-energy (S)TEM*.

Note that the use of a large convergence angle, as occurs in the STEM mode with a focused probe minimizes the problem of channeling enhanced X-ray emission.

The schematics in Figure 4532 shows the electron optical column in a modern analytical electron microscope operated in STEM mode.

Figure 4532. Schematics of the electron optical column in a modern
analytical electron microscope operated in STEM mode.

[1] Ralf Terborg, Bruker.