The key task of a spectroscopist is discovering and identifying the elemental peaks in the acquired spectrum. Isolated peaks can be located and identified easily, however overlapped peaks and low amplitude peaks may not be so easily discovered.
EDS detectors have a specified resolution which is defined as the FWHM of the spectral peak at the Mn-Ka X-ray energy, with a typical value being 130eV for SEM microanalysis systems.
The energy resolution however is not constant across the complete spectral energy range but varies as the square-root of the energy from approximately 50eV for C-K to 160eV for Cu-Ka.
Another challenge is that resolution is only valid for the slowest electronics processing setting of the detection system. When a faster electronics processing setting is used for higher data collection rates, the resolution necessarily degrades.
At every X-ray energy, WDS has the benefit of superior spectral resolution. This offers the benefit of easily resolving elemental X-rays which are closely spaced to one another. Since there is no change in the electronics settings for the system, spectral resolution does not change with acquisition conditions.
Furthermore, the intrinsic spectral background is not affected by the Bremsstrahlung intensity and is considerably lower than EDS which aids in the discovery of very small amplitude peaks.
The spatial resolution of electron microanalysis is based primarily on the incoming electron beam energy and sample density, and to a lesser extent on the elemental X-ray energy.
The following examples compare the results of WDS and EDS to illustrate the advantages of WDS in the discovery and identification of the elemental peaks in a range of materials.
It is not possible to avoid certain elemental peak overlaps of the discovered elements even at high beam energies. This example of Barium Titanate (BaTiO3) is a perfect example. Except the L-lines at 4.4keV, there are no other X-ray lines for Ba, which can be easily used for discovery.
Those lines have a very similar energy as the Ti-K lines and appear overlapped in an EDS spectrum. The WDS has enough spectral resolution to separate the lines and confirm the existence of both elements in the sample.
Steel alloys have a broad range of transition metal additions to optimize the properties as needed. Furthermore, the composition levels of the different alloying elements can range from major (>10%) to minor (>1%), and even to trace (<1%). In order to identify the sample, the right discovery and identification of all of the elements is critical.
It is expected that using high beam energy, high energy X-rays would be produced for all of the elements such that they would be isolated in the EDS spectrum. Manganese is a principal minor or trace alloy addition whose Ka X-ray is heavily overlapped with the Cr-Kb and whose Kb overlaps heavily the Fe-Ka X-rays. By using WDS alone, the analyst can determine the Mn contribution with high confidence.
Cinnabar (HgS) and Galena (PbS) are sulfide compounds having elemental peaks that directly overlap with the S-K X-ray peak. Only a broad peak with possible distortion from symmetric is seen by the EDS, but the WDS spectrum clearly isolates the individual contributions of the X-rays of each element to the analysis.
Complicated semiconducting compounds are being studied for unique electrical properties. They normally have thin layers and solid-solution mixtures of elements that have similar atomic numbers for optimum properties.
Identification, discovery and quantification are required by the producers to correlate with the properties to optimize their application. As intermediate to low beam energies are utilized for spatial resolution analyses, most of the elemental X-ray peaks overlap in an EDS spectrum, especially for X-ray line families having many different energies. Only the use of WDS can determine the unique contributions of all of the elements in the sample.
A common glass basis is aluminum-silicon-oxide, however in order to fine-tune the final properties, other additions are very important. The composition levels are trace or minor and X-ray peaks may be difficult to discover on a relatively high background intensity even when they are isolated.
There are extreme challenges when two small amplitude peaks overlap, as in the case for Zn-L and Na-K. The EDS spectrum shows only a single low amplitude peak on a background (P/B ~ 5) but the WDS shows 2 clear peaks for the trace elements on a low background (P/B ~ 10).
Silicon Overlaps in Semiconductor Samples
Refractory materials are present in many defects and structures in semiconductors. Identification would be simple if analyses were performed at high beam energies, but spatial resolution preferences restrict analyses to low beam energies.
Only M-line X-rays are generated at these energies and many of these overlap with the substrate Si-K X-rays, like Ta-M and W-M. The EDS spectra peaks are slightly broader and mask the presence of the impurity. By using the WDS alone, the elements are uniquely discovered and identified.
High temperature alloys have a number of refractory elements for enhanced properties. Many of these elements will be in low concentrations so the X-ray peak contribution may be quite small. Low beam energy analyses cause additional difficulties by requiring the use of higher order X-ray lines containing many peaks, all of which can overlap with other elemental peaks.
The EDS spectrum of a Zr alloy shows only the expected asymmetry of the primary Zr-La peak but no distinct contribution of other elemental X-rays. The WDS spectrum, in contrast, shows a distinct contribution of a trace amount of Nb masked in the tail of the primary Zr-L X-rays.
WDS is complementary to the EDS technique and offers the needed confidence in discovering and rightly identifying all of the elemental x-ray peaks in a spectrum for further processing.
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This information has been sourced, reviewed and adapted from materials provided by EDAX.
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