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Development of the Silicon Drift Detector for Electron Microscopy Applications


Lothar Strüder,1 * Adrian Niculae,2 *lothar.strueder@pnsensor.de


Abstract: In the 50 years since the first mating of semiconductor- based energy-dispersive X-ray spectrometry (EDS) with the scan- ning electron microscope (SEM), this hybrid instrument has become an indispensable microanalytical tool. In the last two decades a new detector, the silicon drift detector (SDD), has overtaken earlier Si(Li) technology and has made EDS in the SEM and TEM faster and bet- ter. This article tells the story of the SDD development and describes improvements in count rate capability, energy resolution, and detector geometry that bring to SEM microanalysis exceptional precision and stability. Quality maps of element distributions can now be obtained in minutes instead of hours.


Keywords: silicon drift detector (SDD), sideward depletion, SEM, energy-dispersive X-ray spectrometry (EDS), elemental mapping


Introduction Te silicon-based energy-dispersive spectrometer (EDS),


which can produce an entire X-ray spectrum without moving parts, has been serving microanalysis for over 50 years. But in 1984, based on the work of Gatti and Rehak [1], development of a new type of EDS detector that could collect X-ray photons 100 times faster than previous semiconductor devices began. In fact, over the last decade, the silicon driſt detector (SDD) has completely superseded the previous EDS technology in every important performance parameter. A semiconductor-based EDS was first applied to X-ray


microanalysis in 1968 by attaching a 3 mm-thick Si(Li) detecting crystal to the column of an electron probe micro- analyzer (EPMA) [2]. At that time, the reigning method for measuring X-rays in electron-beam instruments was the crystal spectrometer that produced peaks about 10 eV wide compared to 600 eV wide for early Si(Li) detectors. Even so, the EDS became popular because most elements (Z = 11 to 92) could be detected in 5 minutes, compared to scanning four separate crystals for 30 minutes to detect the elements present in the specimen. Also, since there was considerable freedom in locating the EDS detector within the specimen chamber, the Si(Li) device was quickly adapted to the scanning electron microscope (SEM). Te mechanism for detecting X-rays in a Si-based detec-


tor is straightforward in principle. An X-ray entering the active or sensitive region of the detector creates a number of electron-hole pairs proportional to the energy of the incoming photon. In the case of the SDDs the electrons are collected at the readout node while the holes are absorbed in the rectifying p+ contact. Te pair creation energy w in silicon is about 3.68 eV at room temperature. Tis means an X-ray photon of 1 keV produces on average 272 signal electrons. By measuring this small amount of charge, the detector determines the energy of each individual photon as it contributes to the X-ray spectrum. Te initial mating of the old Si(Li) detector with the SEM and


46 doi:10.1017/S1551929520001327


the transmission electron microscope (TEM) was a success- ful marriage for over three decades. In that time the energy resolution (peak width) of this device improved from 600 eV to 130 eV, but there were significant drawbacks. X-ray photon counting was rather slow at good energy resolutions, typically only 3000 counts per second (cps), which forced long collection times to obtain a precise quantitative analysis and hours-long acquisition times for quality compositional images (X-ray ele- ment maps). Tere was also the inconvenience of cooling the Si(Li) crystal and its field effect transistor (FET) amplifier to near liquid nitrogen temperatures to reduce noise in the signal. About two decades ago an EDS detector system contain-


ing a new sensor based on the novel SDD technology became available [3]. Tese SDD sensors were fabricated by a team in Munich [4], which was dedicated to development of detectors for space research. Tey adopted the lithographic techniques and planar technology of the microelectronics industry for detector fabrication. However, to realize the concept of side- ward depletion, an important modification was introduced: the double-sided processing of silicon wafers. Te SDD over- came the difficulties present with Si(Li) detectors. Recent SDDs with thermoelectric cooling to -20°C collect X-rays at least two orders of magnitude faster, at slightly better energy resolution, and without the need for liquid nitrogen cooling. Today all high-resolution commercial X-ray spectrometers


employ some variation of the SDD. SDDs are used in more than 300,000 instruments for science and industry to detect parti- cles, X-rays, and scintillation light. Some of the many applica- tions of SDDs are as tracking detectors in high-energy physics experiments, as electron spectrometers in neutrino experi- ments, as X-ray spectrometers for astronomy, as light detec- tors coupled to scintillators, as high-speed X-ray spectrometers in X-ray fluorescence units, and in microanalysis systems for electron microscopy [5]. Tis overview will concentrate on the development of SDD spectrometers for X-ray microanalysis in electron microscopes, including the latest achievements in terms of energy resolution and count rate capability.


The Physical Concept of SDDs Early work and the concept of sideward depletion. Since


their invention in 1983 [1], SDDs have leſt deep traces in the field of radiation detection. Emilio Gatti (1926–2016), a fre- quent visitor at Brookhaven National Laboratory (BNL), and Pavel Rehak (1945–2009), a scientist at BNL, discussed the topic of how to move the signal electrons generated in a fully depleted silicon crystal to a point-like, low-capacitance read- out node. In 1969, Boyle and Smith invented the charge coupled device (CCD) [6] for imaging and signal processing purposes.


www.microscopy-today.com • 2020 September Peter Holl,1 and Heike Soltau2


1PNSensor GmbH, Otto-Hahn-Ring 6, 81739 Munich, Germany and University of Siegen, Walter-Flex Str.1, 51228 Siegen, Germany 2PNDetector GmbH, Otto-Hahn-Ring 6, 81739 Munich, Germany


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