Silicon Drift Detector
from the horizontal surface of the specimen is well defined. A single detector in a tube, within the SEM specimen chamber, views a horizontal specimen at a well-defined take-off angle but does not provide the greatest count rate because of its lim- ited solid angle of photon collection. Te highest count rate is typically obtained with multiple detectors located close to the specimen. Figure 3a shows four separate 15 mm2
shaped SDDs arranged around a central hole for the electron beam. Kidney-shaped detectors allow the take-off angle to be better defined than detectors of round or tear-drop shape. Sig- nals from the four separate channels can be grouped together to create an effective sensitive area of 60 mm2
kidney-
. Tis configura-
Figure 8: Quantum efficiency of a 450 μm thick silicon detector as a function of X-ray energy from 50 eV to 25 keV. In this special case, the quantum effi- ciency of the internal X-ray entrance window composed of dielectric layers is shown along with two thicknesses of aluminum as a visible light absorber. The black curve shows the QE without aluminum. The blue curve is for an Al layer of 50 nm, which attenuates visible light by a factor of 103
. The red curve shows the effect of 150 nm of Al to block the visible light by more than a factor of 106. Note
that for the red curve the detector can collect some Li photons at 54 eV, but no Be (109 eV) or B (183 eV) photons.
signal shaping time of 500 ns [16]. The required operation temperatures for SDDs can all be accomplished with ther- moelectric coolers. Quantitative microanalysis with SDDs. Quantitative
analysis is more precise when more counts are collected in ele- ment peaks and more accurate when the X-ray take-off angle
tion also has the advantage that the detectors can be placed quite close to the specimen (Figure 3c). Reliable qualitative and quantitative analysis also demands a flat, well-polished speci- men oriented perpendicular to the electron beam. When these conditions are met, the SDD can deliver quantitative analyses that rival those from crystal spectrometers on an EPMA [11]. Compositional imaging. High count rates from the SDD
can produce high-quality element distribution maps in surpris- ingly short times (5–10 minutes) compared to the Si(Li) detector (hours). As mentioned above, this was the first use of commer- cial SDDs. Moreover, modern commercial X-ray spectrometer systems employ spectrum-imaging soſtware that can collect an entire spectrum at each image pixel, producing a data cube (2D image with 1D spectrum at each pixel). Distribution maps of individual elements and phases within the specimen can be extracted from this data cube aſter acquisition is complete. Easily interpreted elemental distribution maps are best
acquired from flat-polished specimens. In some cases, a flat- polished specimen is not available, but there is still a need to obtain meaningful X-ray maps showing the location of specific elements within a particu- lar field of view. Figure 9 shows four compositional images obtained with a 4-channel detector like that shown in Figure 3. Each of the four SDDs collected X-rays from a differ- ent angle. Note the strong shadow- ing in the individual X-ray maps, particularly for low-energy carbon X-rays (red), which are strongly absorbed along certain directions. However, by combining the signals from the four detectors, these shad- owing effects can be minimized; the combined X-ray map gives the impression that all the X-rays were collected from directly above the specimen (not shown). A particularly difficult compo-
Figure 9: X-ray maps of a rough surface composed of iron and kaolinite on a carbon substrate acquired in an SEM with four SDDs arranged around a center hole for the electron beam (as shown in Figure 3a). On a rough sur- face each detector has a different view of the specimen, and some X-rays will be blocked from reaching the detec- tor (the shadow effect). When the signals from the four channels are combined, the shadow effect would be largely mitigated (not shown). In these maps the blue, green, and red regions correspond to Fe, Al, and C, respectively. The maps (1024 × 768 pixels) were acquired in 5 minutes at 10 kV and 0.38 nA. Image width (field of view) = 368 μm.
52
sitional imaging task is to acquire X-ray maps from microstruc- tures with Li-rich phases. Figure 10a shows an X-ray spectrum of a lithium battery material. The Li-K peak at 54 eV is clearly separated from the noise at the far left, and the oxygen peak is at 525 eV. Figure
www.microscopy-today.com • 2020 September
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