Ultra-Low kV EDS
allows microscopy and elemental analysis to be done simultaneously in the same tool.
Lithium analysis . The lightest element that can be detected by EDS is lithium [ 10 – 12 ]. The lithium K X-ray from LI to K shells is not an allowed transition [ 13 ] because it fails the azimuthal quantum number (l) selection rule, with both energy levels in the transition having the same value of orbital angular momentum (l=0). T is means that likelihood of X-ray emission will be very low, unless bonding interaction causes the LI electron to take on more LII/III charac- teristics [ 14 ]. T erefore not all lithium compounds can generate lithium X-rays, for example, X-rays have never been detected from Li-metal oxides (LMO) used as cathodes in current- generation lithium-ion batteries. Hovington et al. [ 13 ] summarize the latest progress on detection of Li K in lithium-bearing compounds. The greatest role for Li analysis by EDS appears to be in the development of next-generation lithium batteries. Materials from this technology are showing significant levels of Li X-ray emission. For example, Figure 12 shows spectra collected from compounds being studied for potential roles in next-generation batteries: Li 2 S as a cathode material and lithium lanthanum zirconate (LLZ), a constituent in the solid electrolyte.
Discussion T e examples shown in this article demonstrate that with this technique useful elemental information is provided where the alternative would be to move the sample to a diff erent tool such as TEM, Auger, or SIMS for further analysis. T e majority of
Figure 9 : Investigation of the structures on the surface of a 20 nm SRAM semiconductor device. First column: image and maps collected at 3 kV. (a) SE image at 3 kV, (b) O K map, (c) Si K map, and (d) Ti L map. Second column: image and maps collected at 1 kV. (e) SE image at 1 kV, (f) O K map, (g) SiL map, and (h) Ti L map. Data at 1 kV shows enhanced image and maps of the surface structures.
elements have low-energy, characteristic X-ray lines that can be excited at accelerating voltages as low as 1.5 kV. T e lines oſt en have poor intensity and peak-to-background ratio and may be fully absorbed or have very low-emission probability in some compounds. Many of these lines are not useful for conventional quantitative microanalysis. However, the results here show useful qualitative analysis using X-ray spectra, X-ray maps showing elemental distributions, and the correction of overlaps in automatic peak identifi cation and deconvolution mapping.
26
Further work is clearly needed to maximize the potential of this technique. For example, there is interest to improve the characterization of these low-energy lines to provide composition determination in certain applications. The challenges for compositional analysis range from experimental to computational. Experimentally, one issue is the need to provide sample surfaces free of carbon-based contamination, or oxidation resulting from attempts to remove this contami- nation using plasma or similar cleaning methods. In terms of the development of quantitative analysis soſt ware, in addition
www.microscopy-today.com • 2017 March
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