Silicon Drift Detector
stage. Te cylindrical shape of this type of SDD allows for an optimal signal charge col- lection speed. SDD sizes and arrays. Te low capaci-
tance of these detectors allowed large sensitive areas, as well as detector arrays where X-ray detection can be combined from several indi- vidual channels, to further increase the count- rate capability by increasing the field of view. A large number of SDD variations have been derived from this basic concept ( Figure 2): large-area oval-shaped detectors with sen- sitive areas from 5 mm2
up to 200 mm2 ,
hexagonally shaped arrays of SDDs, a drop- let-shaped configuration for capacitance minimization [12], and SDDs with integrated FETs and external FETs. A design particularly useful for the SEM is a circular array of four SDDs around a central hole, which brings the detectors as close as possible to the specimen, providing the maximum solid angle for X-ray collection (Figure 3). Energy resolution and P/B. Energy
resolution is defined as the FWHM intensity of the 5.898 keV Mn Kα line measured at a low counting rate of 1000 to 3000 cps. Tis specification is a leſtover from the old Si(Li) times where good energy resolution was only achieved at long signal shaping times τ and consequently low count rates. For SDDs this limitation in count rate is no longer required. For example, the energy resolution of an early cylindrical SDD in 1996 of 3.5 mm2
active
Figure 1: (a) Cross section of a cylindrical SDD with an integrated single-sided n-channel JFET. The green ring in the center represents the signal-electron collecting anode, and the red rings are the p+ drift rings (field strips), producing a graded electric field that forces the signal-electrons to the read node. The homogeneous radiation entrance window is at the back of the device where the p+ back con- tact can be seen. (b) The electron potential is shown with the field strips, the equipotential on the back contact, and the collecting node at the radial position 0.0. Once electrons are generated in the SDD volume, they experience at every point an electrical field component that drifts them toward the anode.
in microanalysis for detecting X-rays with energies from 30 eV up to 30 keV. Te traditional SDD for microanalysis has a cylindrical shape with an integrated n-channel junc- tion field-effect transistor (JFET) in the center to minimize read node capacitance. Field strips (driſt rings) produce a graded electric field that forces signal electrons to driſt toward the anode. Once the staircase of collected signal charges at the charge-sensitive amplifier output reaches its limit, a reset mechanism allows the process to repeat. Inte- gration of the transistor in the SDD reduced the total input capacitance dramatically, thus reducing the electronic noise and increasing the speed of signal processing. Tis con- figuration eliminates microphonic effects and delivers an on-chip-at-low-impedance signal to the next amplification
48
area, measured at room temperature with a shaping time of 500 ns, was 225 eV (FWHM) at 5000 cps. When the same SDD was cooled with a thermoelectric cooler (TEC) to approx- imately -20°C and measured with a processing time of 2 μs, it had an energy resolution of 152 eV (FWHM) at 5000 cps [6]. Te P/B ratio for the present work is
defined as the maximum number of counts in the signal peak of the 5898 eV Mn Kα line divided by the mean value of counts in the energy range from 900 eV to 1100 eV. For early
SDDs described above at -20°C, the P/B was 3000:1. While not fully satisfactory, this experimental detector was a good starting point. Today the P/B is typically between 15000:1 and 20000:1. It mainly depends on the total noise and the quality of the internal radiation entrance window. Te energy resolution of the above mentioned early SDDs
that were in use around 1996 was not fully satisfactory in com- parison with the conventional Si(Li). In a first development step the electronic noise was lowered from approximately 10 elec- trons (rms) [4] achieving an energy resolution of 152 eV at Mn Kα, with a 3.5 mm2
small SDD operated at -20°C, to less than
5 electrons (rms) in 2008 at 100,000 counts per second for a 10 mm2
detected in the late 90s.
www.microscopy-today.com • 2020 September active area. Te C Kα peak could not yet be properly
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80
