SDDs (Figure 1) are advanced detectors for high resolution, high count rate X-ray spectroscopy. Today’s SDDs have a unique design that provides them the ability to deliver a performance much higher than lithium drifted silicon or Si(Li) detectors.
Low X-ray energies, larger detector areas, and very less electronic noise that can be observed at short peaking times (high count rates) are the key features of SDDs, which make them an ideal replacement to Si(Li) detectors.
Figure 1. Silicon Drift Detector
SDDs are largely used for industrial scale applications such as X-ray fluorescence analysis (EDXRF) and electron microscopy (SEM/EDS). A typical SDD device is shown in Figure 2. There are different shapes of SDD devices available.
The device consists of a thermoelectric cooler for device cooling and on which the chip is installed. The droplet-like shape depicted in Figure 2 is widely used for smaller active areas of 10-20mm2. The radiation entrance window comprises a flat p-implanted region encased by a thin conductive layer to maintain the entrance window radiation hard.
Figure 2. Typical mounted and bonded SDD devices
X-ray Detector Fundamentals
Typical X-ray detection devices contain an active region consisting of fully depleted, high-resistivity silicon, a collection anode, and a front contact area. The bulk Si region absorbs the incident X-rays on the front contact area and subsequently electron-hole pairs are generated.
The energy of the incident X-rays decides the amount of charged carriers produced. These electrons and holes are drifted toward the anode (along the field lines) by a pre-established electric field between the anode and the front contact. A pre-amplifier converts the charge collected at the anode to a voltage.
It is possible to determine the incident X-ray energy by observing the magnitude of the voltage step after each pulse, i.e. after the absorption of each incident X-ray. An illustrative schematic of the electronics involved in an X-ray detector is shown in Figure 3. The fluctuations in the output waveform caused by noise limits the accuracy of the measurement of this voltage step.
Figure 3. Schematic of the electronics in an X-ray detector. The SDD dashed line illustrates the electrical impact of the integrated FET.
A Gaussian spread is created for a given energy as a result of the measurement imprecision. Therefore, the measured X-ray peaks are widened due to noise. The voltage steps and corresponding noise are illustrated in Figure 4.
Factors such as the pre-amplifier leakage current, input capacitance, and FET gain influence the noise. Averaging out the noise is done over longer shaping times to improve the resolution.
Figure 4. Illustration of voltage steps as a function of an absorbed X-ray. The noise fluctuations demonstrate the impact of noise and shaping time (top vs. bottom) on resolution.
Averaging of the noise is less over shorter shaping times used to drive higher count rates, causing more uncertainty in the voltage step and deteriorating resolution.
In addition, low energy X-rays must have low signal to noise ratio, revealing the significance of noise in the resolution of low energy X-rays.
Sources of Noise
Many different sources of electronic noise are available as characterized by the following equations:
Electronic noise ∝ shot noise + 1/f noise + thermal noise
shot noise ∝ Ileak
1/f noise ∝ C2in
Shot noise is the first factor and is caused by leakage current in the pre-amplifier. "1/f" noise is the next factor and is directly related to the capacitance squared. Thermal noise is the third factor and is related to the capacitance squared to the temperature and to the inverse peaking time.
Short noise becomes the predominant factor influencing the total resolution for an X-ray when the capacitance gets small enough, for instance as the size of the anode is reduced. Since temperature does not affect the shot noise, device cooling is not effective to achieve good resolution. Short noise is also now less dependent on a long shaping time.
Short noise eventually becomes small enough that resolution becomes almost fully limited by Fano broadening, which depends on statistical fluctuations in the radiation interaction with the Si crystal lattice and the charge production process. The theoretical best resolution is approximately 120eV while reaching this limit.
The resolution as quantified at the Mn Kα peak as a function of shaping time for a SDD and a diode detector is demonstrated in Figure 5. The lower capacitance of SDD means lower noise, which results in superior resolution with larger active areas and shorter shaping times. This means that superior resolution at superior count rates.
Figure 5. Energy resolution at Mn Kα as a function of peaking time for typical SDD and Si(Li) detectors
Modern X-ray Detectors – the Silicon Drift Detector (SDD)
Modern SDDs have extremely small sized anode with respect to their active area. The X-ray generated charged carriers (i.e., holes and electrons) are directed along these electric field lines to the dramatically smaller anode at the center of the detector. Since the capacitance of the device is proportional to the anode size, a very small anode leads to a drastically lower device capacitance. The anode capacitances typically observed are 25-150fF.
Having a very small anode helps achieve better resolution at shorter shaping times (higher count rates) owing to the fact that the electronic noise at short shaping times varies in proportion to capacitance squared, especially at low energies where the signal to noise is very less. If the noise is small enough, it is possible to operate the device at temperatures (~ -20 °C) that are readily achievable with a Peltier device. This avoids the use of LN2 cooling.
The front-end transistor of the amplifying electronics is directly integrated onto the detector chip and coupled to the collecting anode by a short metal strip to make use of the small output capacitance.
This avoids parasitic capacitance at bonding pads, lowering minimizing capacitance between the amplifier FET and the detector anode. Furthermore, noise due to cross-talk, electric pickup, and micro-phony effects are considered immaterial. Figure 3 schematically illustrates this impact.
Figure 7 shows a more modern SDD design involving an offset anode and FET. This is known as a "tear-drop" or "droplet" SDD. When the integrated FET is at the device center as shown in Figure 6, it is vulnerable to irradiation by incident X-rays. Moreover, the electrostatic fields surrounding the FET cause performance losses at low X-ray energies.
As shown in Figure 7, the FET is outside the active area when it is offset, and therefore it is not under the exposure of incoming radiation. This characteristic design can be seen widely in smaller area (10 mm2) detectors.
Figure 6. Example SDD detector design
Figure 7. Modern detector with offset anode and pre-amplifier FET
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.
For more information on this source, please visit Thermo Fisher Scientific – Materials & Structural Analysis.