3.6.4. WDS vs. EDS Detection

The energy dispersive (EDS) and wavelength dispersive (WDS) systems both have benefits and disadvantages. The main differences between the systems are in detector efficiency and resolution. Schematic diagrams showing the components of the WDS and EDS systems are shown in Figure 3.6.4a and 3.6.4b.

Figure 3.6.4a. Schematic representation of a wavelength-dispersive spectrometer (Goldstein et al. 1981).	Figure 3.6.4b. Schematic representation of an energy-dispersive spectrometer (Goldstein et al. 1981).

The overall efficiency of collecting the X-rays produced is poor for both WDS (<0.2%) and EDS (><2%) with the EDS better because the detector is closer to the sample. Most X-rays produced from the sample do not make it to the detection systems. In a WDS system about 30% of the X-rays entering the detector are actually counted; whereas, in an EDS system about 100% of the incident X-rays are counted. As a consequence, the minimum useful probe spot size is larger for WDS (about 2 µm). The greater efficiency of EDS permits the use of beam diameters as small as 50 Å (0.05 µm). ><0.2%) and EDS (<2%). The EDS system is better because the detector is located closer to the sample. However, the resolution of the WDS system is far superior (Figure 3.6.4c).

Figure 3.6.4c. Comparison of the resolution of the proportional counter, SI(LI) semiconductor detector, and several analyzing crystals. Note that the axes are logarithmic (after Maurice et al. 1979).

Most X-rays produced from the sample do not make it to the detection systems. In a WDS system, about 30% of the X-rays entering the detector are actually counted; in an EDS system, almost 100% are counted. As a consequence, the minimum useful probe spot size for WDS is about 2 mm;  the greater efficiency of an EDS permits the use of beam diameters as small as 50 Å (0.05 mm).

The instantaneous spectral acceptance range differs greatly between the two systems. WDS spectrometers can only examine the portion of the spectrum for which they are positioned, whereas in EDS entire useful energy range can be examined simultaneously. Finally, spectral artifacts produced by stray X-rays and electrons are more rare with WDS. The EDS detector, located close to the sample, receives many stray X-rays and electrons and suffers peak distortion, peak broadening, escape peaks, absorption, and internal Si fluorescence.

Copyright 1997-2003, James H. Wittke

Last update: 01/18/2006 01:47 PM.