Ultra-Low kV EDS
10 nm, and surface analysis becomes possible, the majority of elements still emit characteristic X-rays giving the possibility for some elemental characterization ( Table 2 ).
Results
The hardware solutions for low-energy X-ray analysis described above indicate that more low-energy X-ray lines are becoming accessible. T ese lines may not be as reliable for analysis as high-energy lines, but they do offer the potential for elemental analysis in applications and materials where previously no elemental information was attainable in the SEM. We now consider some applications that indicate the potential of a detector optimized for low-energy sensitivity: high spatial resolution nano-analysis, surface analysis, light element analysis, and lithium detection.
High spatial resolution analysis . Low accelerating voltage
imaging of nano-particles using within-lens detectors shows detailed topographical information and atomic-number contrast. Figure 7 shows the analysis of ceramic oxide nano-particles (20–80 nm) formed by the sparking of ferrocerium on a silicon substrate [ 6 ]. T e backscatter image ( Figure 7b ) shows three distinct gray levels suggesting the existence of three phases of diff ering mean atomic number within these particles. To achieve the necessary spatial resolution, X-ray maps were collected at an accelerating voltage of 2 kV. T e composite map ( Figure 7c ) confi rms that three distinct oxide phases are present: the darkest, a MgZn oxide; medium-gray-level Fe oxide; and the brightest, an LaCe oxide. T e elemental distribution was shown down to sub-10 nm scale with an acquisition time of 5 minutes. Additively manufactured Alloy 718 is a superalloy containing a range of precipitates of diff erent sizes [ 7 ]. It contains disk-shaped Ni 3 (Nb,Ti) γ ” precipitates on the order of 10–15 nm in thickness in a matrix mainly of Cr, Fe, and Ni ( Figure 8 ). Spectra collected at 3 and 1.5 kV from these precipitates and surrounding matrices demonstrate the improvement in spatial resolution achieved by reducing the accelerating voltage. In the 1.5 kV spectrum ( Figure 8c ), the Nb Mζ , and Ti L signals are clearer than the corresponding Nb L or Ti L signals in the 3.5 kV spectrum ( Figure 8b ). Note even at 1.5 kV that Fe and Cr are in the spectrum collected from the precipitate, showing that the X-ray generation region is not solely contained within the precipitate. A spectrum image dataset collected from an area containing these Ni 3 Nb γ ” precipitates ( Figure 8d ) demonstrates that it is still practical to study elemental distribution of features on this scale using X-ray maps collected at 1.5 kV. Surface analysis . As shown in Figure 1 , by reducing the accelerating voltage to very low values, the interaction volume of the electron beam is concentrated in the top few nm of a material. T is is used in electron imaging to identify surface features such as in complex semiconductor structures ( Figure 9 ). By reducing the accelerating voltage from 3 to 1 kV, the surface selectivity of the data improves, and imaging contrast is more clearly related to those structures on the sample surface. Collecting X-ray data under the same conditions makes the determination of what constitutes the surface of a complex structure, such as in this sample, much clearer [ 8 ]. For example the X-ray maps for O K and Si L reveal where these elements
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Figure 7 : Nanoparticles produced during sparking of ferrocerium [ 6 ]. (a) SE image and (b) BSE image that shows strong material contrast variation within the particles indicating that each particle is composed of multiple phases. Images were collected using a Zeiss GeminiSEM500 at 2 kV beam energy with InLens (SE) and EsB (BSE) electron detectors. (c) Composite EDS layered element map of ferrocerium particles showing iron (yellow), lanthanum and cerium (green), magnesium and zinc (magenta), and silicon (blue) superimposed on a BSE image.
are on the surface when collected at 1 kV, in contrast to the O K and Si K maps collected at 3 kV where elemental information from subsurface layers contributes signifi cantly to the signal.
www.microscopy-today.com • 2017 March
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