Silicon Drift Detector
line was already reduced to 124 eV for a 10 mm2
In 2008 [13] the FWHM of the Mn Kα large detector with a P/B above 15000:1
[13]. Te best guaranteed values today are 121 eV at shaping times τ around 1 μs allowing for very high count rates and a P/B beyond 15000:1. Te latter values are already very close to the theoretically achievable energy resolution given by the Fano ionization sta- tistics, which is 119 eV at 5.9 keV. Te total electronic noise is 2 electrons (rms) due to the reduction of
input capacitance to less
than 50 fF—the key to obtaining excellent spectral performance and count rate capa- bility [14]. Low-energy X-ray detection. Starting
in 2007 the proper detection of B (at 180 eV) became achievable [11]. At
this time C Kα
Figure 2: Examples of SDD geometries. Beyond the classical round or square-shaped SDDs, oval and droplet shapes have been realized, as well as a variety of arrays built to increase the effective sen- sitive area and the field of view. (a) Large-area large area oval-shaped detectors with sensitive areas from 5 mm2
up to 200 mm2 more than 3 cm2, (c) the tear-drop detector shaped array designed to reduce capacitance has an inter-
nal FET located outside the irradiated area, (d) tear-drop detector mounted on a ceramic substrate, which is coupled to a Peltier cooler underneath. The 16 metal pins surrounding the ceramic and Peltier element serve as electrical connections to the outside of the TO-8 housing, the bottom part of which is shown in gold color.. The cover with the external entrance window is not shown.
, (b) hexagonal array comprised of a 61-channel SDD with a total area of
could be measured with a resolution of 42 eV (FWHM). A precondition to enter that energy domain was the very low noise of the detec- tor system, but that was not sufficient by far. Te X-ray attenuation length is about 100 nm at 300 eV. As there are typically several tens of nm of layers covering the rectifying p+
implant for passivation reasons, a frac-
tion of the incoming radiation is absorbed in these layers before entering the highly doped reverse-biased p+
diode. Tis highly doped
region has undergone radiation damage dur- ing the implantation process. Te subsequent annealing repairs and activates the disturbed silicon lattice as much as possible. Te interac- tion of the low-energy X-rays in these layers results in partial signal collection leading to a decreased energy resolution. At carbon K (277 eV) the theoretical
Figure 3: A flat four-channel detector for an SEM. (a) Plan-view of counts per second. four kidney-shaped sensors
around a center hole, (b) perspective view, (c) four-channel detector mounted between the SEM final lens and the sample. This configuration provides a high solid angle (up to 1.5 sr) for a detector with a total count rate capability of 6 × 106
2020 September •
www.microscopy-today.com
limit of Fano noise originating from ioniza- tion statistics is 26 eV (FWHM). The mea- sured width is the result of the Fano and electronic noise on one side, but a certain fraction is also due to partial signal elec- tron loss in the internal radiation entrance window. Thus, the problem of not reach- ing the Fano-limited energy resolution at X-ray energies below 500 eV is related to the quality of the radiation entrance win- dow. Because the physics and the technol- ogy of the X-ray entrance window became better understood and then implemented in the fabrication process, the energy resolu- tion at the 277 eV carbon K peak improved from 85 eV at the beginning of development in 2004 to 35 eV by 2019 [14], while at the same time the P/B ratio at the Mn Kα line went up from 3000:1 to more than 15000:1 (Figure 4). At that point, X-ray lines for light elements down to Li (54 eV) could be clearly detected (Figure 5).
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