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Ultra-Low kV EDS


Figure 5 : Spectra collected at 3 kV from CaSO 4 using a windowless 100 mm 2 detector (X-Max Extreme - red) compared to a 150 mm 2 detector (X-MaxN150 - yellow) with polymer window at an identical take-off angle on the same SEM. The windowless design shows signifi cant improvement in intensity. For example, the Ca L-series is indistinct using the conventional detector because of strong absorption in the polymer window.


windowless operation and solid angle improvements, increases in count rate of around 75–100× have been achieved versus a conventional 10 mm 2 SDD detector, and between 5–10× versus a 150 mm 2 SDD detector for the N K line. Figure 5 compares spectra for CaSO 4 , collected at 3 kV under identical conditions, from a 100 mm 2 windowless detector and a conven- tional 150 mm 2 windowed detector. T e increase in sensitivity is clear; the Ca L lines, heavily absorbed by a polymer window, are easily detected with the windowless detector. One fi nal, critical design issue for low kV EDS on the SEM is analytical working distance, that is, the microscope working distance at which EDS is practical. Conventional EDS-detector working distances have been relatively short for FE-SEM, in the range of 5–15 mm. However, when operating at sub-3 kV accelerating voltage, working distance must be reduced further for two reasons. Firstly, the beam size increases with working distance [ 3 ], and many microscopes are specifi ed at about 1 mm for best resolution at 1 kV. Secondly, the contrast of within- lens secondary, and particularly backscatter, signals decreases rapidly with increasing working distance; optimal is normally 5 mm or less. T erefore, a further barrier to low-energy EDS is the need to work at short working distance to achieve suffi cient image resolution and contrast. A further benefit of using an oval-shaped sensor is that the detector can oſt en be designed to work at working distances in the range of 3–5 mm, about half that of a conventional detector. Again, this can be seen in the schematic representation of this design versus a conventional circular EDS detector shown in Figure 4 .


Figure 6 : Spectra collected with a 100 m 2 windowless detector (X-Max Extreme) at 3 kV for metals in the sixth row of the periodic table. Note the N lines detected for Bi (135 eV), Hf (200 eV), Ta (205 eV), W (21 eV), Ir (235 eV), Pt (245 eV), and Au (250 eV). Samples were plasma-cleaned for 30 minutes but show variable intensities of the C Kα peak, which makes particular contributions to the Ir N, Pt N, and Au N peaks.


X-ray lines for low-energy analysis . Conventional microanalysis at 20 kV or higher uses K lines for most common elements in metals, oxide ceramics, and minerals up to and including the fi rst-row transition elements. For heavier elements L or M lines are typically used. Low-energy X-ray analysis requires the use of a very diff erent collection of X-ray lines, using K lines for only the lightest elements and making use of low-energy L, M, and even N lines for other elements. Low-energy lines (< 1 keV) present a number of challenges for EDS including the following: low intensity, poor peak-to-background, poorly understood X-ray absorption (including self-absorption), and transitions from valence electrons, meaning line positions and intensities may change depending on the bonding environment of the constituent elements. Table 2 shows the highest-energy X-ray line series available for EDS analysis at diff erent accelerating voltages in the range 1–5 kV. Only at 5 kV can all naturally occurring elements be detected using K, L, or M lines, and this requires using L lines for fi rst-row transition elements. At 3 kV, N lines are required for Th and U, and elements such as Sn require low-energy M lines. At 2 kV a line for every element may be detectable, although some like Si Ll at 92eV require the most sensitive detectors, and further effort is required to achieve accurate spectrum processing of these lines [ 4 ]. Increasing interest in N lines ( Figure 6 ) [ 5 ] is being driven by their potential use for detecting some important heavy elements (for example, Ta and W) in semiconductor devices and superalloys. Even at 1 kV, where EDS spatial resolution is often significantly less than


Table 2 : Highest-energy X-ray lines available for elements at different accelerating voltages from 5 kV–1 kV. As accelerating voltage is decreased, the highest-energy available lines will be lower in energy with reduced intensity and reduced peak-to- background. Even at 1 kV characteristic lines may be excited for most elements.


V acc 5 kV


3.5 kV 2 kV


1.5 kV 1 kV


K line


3 Li to 20 Ca 3 Li to 17 Cl 3 Li to 13 Al 3 Li to 11 Na 3 Li to 9 F


L line


21 Sc to 49 ln 18 Ar to 45 Rh 14 Si to 37 Rb 12 Mg to 33 As 12 Mg to 27 Co


2017 March • www.microscopy-today.com M line


50 Sn to 92 U 46 Pd to 83 Bi 38 Sr to 73 Ta 38 Sr to 66 Dy 38 Sr to 60 Nd


N Line


90 Th to 92 U 74 W to 92 U 67 Ho to 92 U 62 Sm to 92 U


No available line


34 Se to 37 Rb 10 Ne to 11 Na, 28 Ni to 37 Rb 23


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