Silicon drift detector

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A silicon drift detector (SDD) is a relatively new radiation detector for measuring ionizing radiation . Among other things, they are used in X-ray spectrometers for the detection of X-rays . The semiconductor detector was presented in 1984 by E. Gatti and P. Rehak and has since been further developed for different areas of high-energy physics and X-ray spectroscopy .

construction

The basic principle of a silicon drift detector essentially corresponds to a pn diode or a photodiode made of silicon with a highly doped p-conductive (p + ) and a moderately doped n-conductive area. An externally applied electrical voltage (mainly) depletes the n-doped area at the transition to the p + -doped area, i.e. the field of the applied voltage leads to a displacement of the majority charge carriers (in the n-doped area the electrons ). As a result, the space charge zone , which is already present without voltage, widens . Incident X-rays are absorbed and generate electron-hole pairs , which can be separated in this low-charge region due to the applied voltage without recombining . So the electrons drift to the n-doped area and reach the anode . The holes (defect electrons), on the other hand, drift to the p-doped area and reach the cathode of the detector crystal. This basic structure can easily be implemented vertically in a thin n-doped silicon substrate (for example a wafer ), the anode and the cathode each being attached flat on one side of the substrate. However, this structure has disadvantages, because on the one hand the flat anode has a large electrical capacitance, which leads to a long signal forming time , and on the other hand only a relatively small space charge zone results.

A clear improvement was achieved with the drift chamber presented by E. Gatti and P. Rehak in 1984. In contrast to the above-described structure, a thin n-doped silicon substrate ( the a wafer) on both sides with a p + provided doped region and contacting (the cathode). The n-doped bulk silicon was only contacted via a relatively small contact on one of the sides. Despite the small dimensions of the anode, it is possible to deplete the entire wafer by means of an externally applied electrical voltage. The space charge zones (both sides of the substrate) which are already present without voltage initially grow with the magnitude of the voltage until both space charge zones touch, so that a depleted area is formed between the two p + -doped areas. If the voltage is increased further, the space charge zone spreads further laterally outside the p + -doped areas in the direction of the anode.

Electron-hole pairs generated by incident X-rays that are absorbed in this low-charge region are separated due to the applied voltage. The holes (holes) drift to the p + contacts and the electrons in the opposite direction into the middle of the substrate between the two p + contacts. By superimposing a second voltage parallel to the wafer surface on the space charge zone, the electrons can drift in a controlled manner to the anode, where they are fed to an amplifier circuit or evaluation electronics. In this way the basic structure of the semiconductor drift chamber presented by E. Gatti and P. Rehak is obtained.

Nowadays, the typical structure deviates more or less from this basic concept. Modern SDDs are usually manufactured in a cylindrical shape on a high-purity silicon wafer . To increase efficiency, several p-doped areas are arranged in a ring around a cylindrical n-doped anode in the middle of the wafer. Standard methods of semiconductor technology are used, for example photolithographic structuring , ion implantation for doping or deposition of silicon dioxide and aluminum . In addition to the originally two-sided (structured) version, alternative variants with only one-sided structuring were presented in the literature, in which transistors were additionally integrated as preamplifiers on the detector crystal, cf. u. a. Scholze et al., Pieolli et al. as well as Friedbacher and Bubert.

Advantages and disadvantages

Due to the small thickness and thus the smaller detector volume compared to Si (Li) detectors , SDD already have a lower efficiency above approx. 10 keV. However, this is in the RFA hardly disturbing, since the radiation intensity is usually high enough. The (volume-dependent) leakage currents are also significantly lower, which reduces the noise of the output signal. It is therefore sufficient to cool them down to around −20 ° C with small Peltier coolers . Because of this (and because of the more efficient production on wafers) they are smaller and cheaper than Si (Li) s. Since the electrical signals are collected in the middle of the silicon drift detector on a small anode , their electrical capacitance of the anode is lower than that of Si (Li) detectors, which allows a measurement time that is ten times faster. Furthermore, the production with standard semiconductor technology processes on typically 4-inch or 6-inch wafers allows the simple integration of one or more transistors that can be used as preamplifiers. This means that the preamplifier is even closer to the detector material than with the Si (Li) detectors, which in turn enables better electronic evaluation. For these reasons, they are increasingly replacing the Si (Li) detectors.

literature

  • Emilio Gatti, Pavel Rehak: Semiconductor drift chamber - An application of a novel charge transport scheme . In: Nuclear Instruments and Methods in Physics Research . tape 225 , no. 3 , September 1984, pp. 608-614 , doi : 10.1016 / 0167-5087 (84) 90113-3 .
  • Gerhard Lutz : Semiconductor Radiation Detectors . Springer-Verlag Berlin Heidelberg, 1999, ISBN 978-3-540-71678-5 , p. 125-136 .
  • Peter Holl: Construction and testing of a silicon drift chamber - diploma thesis . MPI-PAE-EXP-EL-150, 1985 ( cern.ch ).
  • Lothar Strüder , G. Lutz, P. Lechner, H. Soltau, P. Holl: Semiconductor detectors for (imaging) X-ray spectroscopy . In: Kouichi Tsuji, Jasna Injuk, René Van Grieken (eds.): X-Ray Spectrometry: Recent Technological Advances . John Wiley & Sons, 2004, ISBN 0-471-48640-X , pp. 133-193 .

Individual evidence

  1. a b c Frank Scholze u. a .: X-Ray Detectors and XRF Detection Channels . In: Burkhard Beckhoff u. a. (Ed.): Handbook of Practical X-Ray Fluorescence Analysis . Springer, Berlin / Heidelberg 2006, ISBN 978-3-540-28603-5 , pp. 199-308 .
  2. ^ Emilio Gatti, Pavel Rehak: Semiconductor drift chamber - An application of a novel charge transport scheme . In: Nuclear Instruments and Methods in Physics Research . tape 225 , no. 3 , September 1984, pp. 608-614 , doi : 10.1016 / 0167-5087 (84) 90113-3 .
  3. a b c L. Strüder, G. Lutz, P. Lechner, H. Soltau, P. Holl: Semiconductor detectors for (imaging) X-ray spectroscopy . In: Kouichi Tsuji, Jasna Injuk, René Van Grieken (eds.): X-Ray Spectrometry: Recent Technological Advances . John Wiley & Sons, 2004, ISBN 0-471-48640-X , pp. 133-193 .
  4. ^ A b David Bernard Williams, C. Barry Carter: Transmission Electron Microscopy: A Textbook for Materials Science . Springer, 2009, ISBN 978-0-387-76500-6 , pp. 588 .
  5. L. Pieolli, M. Grassi, M. Ferri and P. Malcovati: A Low Noise 32-Channel CMOS Read-Out Circuit for X-ray Silicon Drift Chamber Detectors . In: Giovanni Neri (Ed.): Sensors and Microsystems: AISEM 2010 Proceedings . Springer, 2011, ISBN 978-94-007-1324-6 , pp. 259–264 ( limited preview in Google Book search).
  6. Gernot Friedbacher, Henning Bubert (ed.): Surface and Thin Film Analysis . 2nd Edition. John Wiley & Sons, 2011, ISBN 978-3-527-63694-5 , pp. 273 ( limited preview in Google Book search).
  7. ^ Frank Eggert: Standard-free electron beam microanalysis . BoD - Books on Demand, 2005, ISBN 978-3-8334-2599-8 , pp. 12-13 .