[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

For soft x ray detection a suitable topology has been

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For soft X-ray detection a suitable topology has been designed with the radiation entrance window of the detector consisting of a continuous p+ implant without oxide gaps [35]. The schematic view of an SDD for X-ray spectroscopy based on this design is shown in Fig. 4.18 [36]. By using an equipotential electrode on the p-side, only the potential on the opposite side of the detector is varied to provide the drifting field, as shown in Fig. 4.19. As can be easily understood from the figure, also for this device, as in the case of the SDD shown previously, wherever the electrons are generated inside the detector volume, they are driven to the small collecting anode. An integrated voltage divider can be used to bias the p+ rings by just contacting externally the first ring next to the anode and the last one at the edge of the detector [37]. In order to reach a good response in the low energy range (a few hundred eVs) a very shallow implantation of the p+ back contact that acts as radiation entrance window has been obtained. A careful tailoring of this implantation in the fabrication technology of the Max Planck Institut (MPI) Halbleiterlabor has allowed to obtain devices with a pn-junction at the backside located at a depth of approximately 40 nm [38]. The curve of the quantum efficiency of a 300- µ m thick SDD is reported in Fig. 4.20 [39], showing a quantum efficiency higher than 60% above 200 eV. In Fig. 4.21 [35], the low energy spectrum of a Macrolon filter containing several light elements, irradiated with an X-ray beam and measured with an SDD, is shown. The energy resolution in corre- spondence of the oxygen peak (524.9 eV) is 92 eV FWHM.
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226 A. Longoni and C. Fiorini Last ring Ring #1 Anode Entrance window Path of Electrons n - Si p + Clear DGS n-JFET Fig. 4.18. Schematic diagram of the SDD for X-ray spectroscopy with integrated n-channel JFET. The transistor gate is connected to the anode ring by a metal strip - Potential [V] X [ m m] Y [ m m] Anode Entrance window 300 200 100 0 3000 2500 2000 1500 1000 500 0 0 50 100 150 200 250 Fig. 4.19. Energy potential for electrons inside an SDD with homogeneous entrance window. Possible electrons paths are shown in figure At high energy, the X-ray detection efficiency of the SDDs, limited by the total thickness of the wafer, typically 300 µ m, is about 90% at 10 keV and 50% at 15 keV.
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X-Ray Detectors and XRF Detection Channels 227 1000 Oxygen K edge Detector entrance window Silicon K edge 10000 100 0.0 0.2 0.4 0.6 Quantum efficiency 0.8 1.0 Energy [eV] Fig. 4.20. Quantum efficiency of a 300- µ m thick SDD. The curve has been cal- culated taking into account the absorption coefficients and the dead layer intro- duced by the backside p+ implantation. The points represent measured data (figure from [39]) The On-chip Electronics To fully exploit the benefit in terms of energy resolution and short shap- ing time arising from the low output capacitance typical of the SDD, both the preamplifier’s input capacitance and stray capacitances of the connection between detector and preamplifier have to be kept as small as possible. This
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  • Spring '14
  • MichaelDudley

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