Silicon Drift Detector


Te transfer of signal charges in a CCD was performed by rap- idly changing voltages of isolated shiſt electrodes on the sur- face of the CCD to create time-variant potential wells inside the semiconductor. In contrast to this approach, Rehak and Gatti were exploring a constant driſt field such that the detec- tor could be operated in DC mode without changing voltages. Terefore, Rehak called the SDD a phase-less CCD or a pnCCD, a discretely transferring SDD, to highlight the similarities and differences of the two devices. A double-sided polished high-resistivity n-type silicon


wafer can be entirely depleted of mobile charge carriers if both wafer surfaces are covered with rectifying p+ (Figure 1a). Only a very small ohmic n+


junctions contact is needed to


apply reverse bias voltages and drain off all mobile charge car- riers. With proper bias voltages the depletion occurs from both wafer surfaces until the detector volume is completely sensi- tive. Te small ohmic n+


node is then the most positive point


in the detector structure where all the signal electrons will be collected (Figure 1b). As this node can be made geometrically very small, its capacitance with respect to all other contact becomes small as well. Te collected signal electrons generate a voltage swing ΔV at the collecting node that is proportional to the number of collected electrons according to ΔV = ΔQ/C (ΔV is the voltage swing, ΔQ the amount of charge, and C the total node capacitance). For example, a 40 fF node capacitance yields a voltage change of 4 μV for every collected electron. As can be seen from equation (1) the noise, or more accu-


rately the equivalent noise charge ENC, expresses the influence of various physical noise sources on the total noise of a detector readout as a function of detector and transistor parameters. It can be understood as the number of fluctuating electrons that must be injected into the input of the amplifier to generate the measured total noise at the output.


ENC=+ + m


α21 2


g CA a C A qI A tot


kT 2


1 τ πτ (1) f tot 23


2 L


ature; α is a parameter describing the input noise source; gm is the transconductance of the first transistor; A1


In equation (1) k is the Boltzmann constant; T is the temper- , A2


mentary charge; and IL 2 and 1 is a DC electron current, for example,


the thermally generated leakage current or dark current. Te first term under the square root is called the series noise and scales withCtot


is large, the series noise increases. For a given capacitance C, the noise contribution can be reduced only by extending the sig- nal processing time τ, with the consequence of reading out the signals at a low rate, or by increasing gm


τ. Tat means if the total input capacitance of the first transistor.


Te second term is independent of the signal processing time τ, but again this is proportional to the total input capacitance Ctot


2 . Tis contribution is called low frequency noise. Both com-


ponents, the series noise and the low frequency noise, heavily profit from a low input capacitance. Te last term is indepen- dent of the total input capacitance but proportional to the sig- nal processing time τ. Tis depends on the thermally generated leakage current or dark current and can be controlled by an appropriate operating temperature and an optimized sensor


2020 September • www.microscopy-today.com , and A3


are constants describing the frequency-depending filtering; af parameterizes the so-called “low frequency noise”; q is the ele-


fabrication technology. To summarize: if you want to readout fast at a low noise, the total input capacitance must be mini- mized. To obtain the system energy resolution, the noise con- tribution from equation (1) has to be quadratically added to the Fano noise (equation 2) caused by statistical fluctuations of the ionizing process:


ENC q FE w


fano =⋅ ⋅ x where F is the Fano factor in silicon (F = 0.115), Ex (2) is the X-ray


energy, and w is the pair creation energy. Te Fano contribu- tion to the full width at half maximum (FWHM) describes the theoretically achievable lower limit of the energy resolu- tion. More details about the physics of SDDs can be found in [4,7]. First commercial SDDs. Te very first mention of SDDs


as new and compact Peltier-cooled X-ray spectrometers was in 1995 [8]. Te SDD chips were developed and fabricated at the semiconductor laboratory of the Max Planck Institute for Extraterrestrial Physics in Munich. Te first commer- cially available SDD detector systems were offered by Röntec, now Bruker Nano, in 1997 for element mapping applications [9,10] with SDD chips delivered from the Munich team. Te XFLASH system from Röntec produced count rates that were 10×–100× higher than Si(Li) systems, and the thermoelec- tric cooler kept the detector at -30°C without liquid nitrogen [3,4]. Tis allowed detailed element distribution maps to be acquired in a few minutes versus several hours. But at this time, for serious X-ray spectrometry requiring good energy resolution, excellent low-energy X-ray detection, good peak- to-background (P/B) performance, and reliable background measurements, a Si(Li) detector was still needed in addition to the SDD employed for X-ray mapping. Several years of contin- uous improvements in device fabrication and pulse-handling electronics followed, resulting in the production of an SDD system that counts remarkably fast, while at the same time providing excellent energy resolution on peaks. Tis allowed overlapping lines to be separated better than previously pos- sible with the Si(Li) detector. Te following sections tell the story of these improvements.


Performance Improvements of SDDs Figures of merit. Typical figures of merit for the qualifi-


cation of SDDs are the same as for Si(Li) detectors: (a) energy resolution at the 5.898 keV manganese Kα line from an 55


Fe


radioactive source, (b) the P/B ratio at Mn Kα, (c) energy res- olution at the 277 eV carbon Kα line, (d) count rate capabil- ity and energy resolution at various processing times, and (e) stability and reproducibility. Tese parameters are important for electron beam microanalysis (in SEMs and TEMs) and for X-ray fluorescence analysis [11]. For these analytical tech- niques, beyond a high count rate capability, an X-ray spectrom- eter should produce narrow peaks (good energy resolution) to separate peaks from different elements and a high P/B to detect small amounts of elements. For use in electron microscopy the solid angle value, low X-ray background, and cost are also important issues. Detector shape with FET integration. Figure 1 shows what came to be the SDD configuration commonly employed


47


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