Silicon Drift Detector


is detected. Tis corresponds to a continuous operation for 10 years with 350,000 cps. By studying the spectra as a function of the signal processing time, the P/B ratio, and the response to low-energy X-rays, the damaging mechanisms can be ana- lyzed. Count rate capability. Te next performance parameter


Figure 4: Energy spectrum of emissions from a 55 Fe source. The measured


energy resolution is 122 eV (FWHM) at the Mn Kα line (5898 eV), and the P/B ratio is 15900:1. The silicon escape peaks from the Kα and Kβ lines at 4.15 keV and 4.75 keV are clearly visible. The other features at 1.49 keV and 2.0 keV are fluo- rescence lines from Al-K and Zr-L lines, materials used in the experimental setup. The Y-axis shows the number of counts per analog-to-digital units (adu), that is, the number of counts per ADC bin on a logarithmic (left) and a linear scale (right).


Long-term stability and reproducibility. Under heavy


irradiation of X-rays and electrons, the energy resolution can deteriorate. Two major contributions are well known: (1) deg- radation of the internal radiation entrance window and (2) the increase of dark current. One sensitive way of quantifying these effects is the onset of increased FWHM in the Mn K line at 5.9 keV. By irradiating the SDD with photons from a Mo X-ray tube with its prominent 17.35 keV Kα line and strong bremsstrahlung continuum or from a radioactive 55 degradation effects can be studied. Typically, 1014


Fe source, X-rays can


impinge on the detector before the widening of the Mn Kα line


that improved in the past 10 years was the readout speed—the number of X-ray photons per second the readout node can pro- cess and convert into digital units (X-ray photon counts as a function of energy). From the interaction point of the incom- ing X-ray with the silicon lattice, the generated signal charges (electrons) must be forced to driſt to the collecting read node. During the driſt phase they experience diffusion and elec- trostatic repulsion in the driſt direction at the bottom of the potential parabola (Figure 1b). Te upper limit of the count rate is governed by two fac-


tors: the speed of the signal electrons along the driſt direc- tion and the speed of the processing electronics. Te former is mainly determined by the effective electric field, the saturation velocity, the driſt distance, and the temperature; while the lat- ter depends on the total input capacitance, the transconduc- tance of the first FET, and the device temperature. With driſt fields of the order of 500 V/cm, driſt velocities of


10 μm/ns are attainable at -20°C. Tis translates in a time dis- tribution spread of signal charge arrival at the collecting node of approximately 30 ns (rms) aſter a driſt length of 3 mm. Tis leads to a rise time of the first amplifier of approximately 50 ns, taking into account the capacitance of the read node and the properties of the first amplifying FET. If the signal processing time of the amplifier is shorter


than the spread of the arrival times of the signal charges, not all electrons reach the read node during the shaping time τ of the amplifying system. In this case a ballistic deficit occurs. As a consequence, the amplitude measurement is degraded, and the energy resolution deteriorates. Terefore, special attention is dedicated to fabricating the p+ driſt rings that generate the electric field such that the driſt of the signal charges gets as close as possible to the electron saturation velocity in silicon. In addition, the ballistic deficit problem can be partially mitigated by using modern digital pulse processors. Both a short collection time of the signal electrons and a


low total capacitance of the input node lead to further improve- ments of the energy resolution at very short signal processing times. Figure 6 shows that even at an analog shaping time of 125 ns, the energy resolution for the most recent generation of SDDs was below 127 eV with a signal rise time shorter than 50 ns. Te count rate capability of current SDDs recorded with a standard digital pulse processor (DPP) can be nearly 600,000 output counts per second for an incoming count rate of 1.45 × 106


counts/second (Figure 7). More sophisticated DPPs


Figure 5: Composite low-energy spectra measured with a droplet SDD. The fully blue spectrum was taken from a pure Li sample partially oxidized; the shaded spectrum is from a commercially available radiation entrance window made of beryllium and coated with boron hydrate. The carbon peak in both spectra originates from the glue and carbon contamination in the SEM. The 4σ event threshold was set to 25 eV, corresponding to a system noise of less than 1.8 electrons (rms). The Li-K line at 54 eV (14 electron hole pairs created) is clearly separated from the noise. The measured FWHM of B-K at 183 eV is 32 eV, and the width of C-K is 36 eV.


50


may increase the above count rate by another factor of two, still maintaining an energy resolution of 145 eV (FWHM). To achieve good energy resolution at even higher count rates, the detector active area can be increased by combining the signals from several medium-sized multichannel SDDs (Figure 2) to reduce the ballistic deficit effect described above. Quantum efficiency. A high quantum efficiency (QE)


over the full energy range from 50 eV to 30 keV is required to satisfy the need for a high number of recorded X-ray photons


www.microscopy-today.com • 2020 September


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