Double junction photodiode for X-ray CMOS sensor IC
TL;DR: In this paper, a CMOS compatible P+/Nwell/Psub double junction photodiode pixel was proposed, which can efficiently detect fluorescence from CsI(Tl) scintillation in an X-ray sensor.
Abstract: A CMOS compatible P+/Nwell/Psub double junction photodiode pixel was proposed, which can efficiently detect fluorescence from CsI(Tl) scintillation in an X-ray sensor. Photoelectric and spectral responses of P+/Nwell, Nwell/Psub and P+/Nwell/Psub photodiodes were analyzed and modeled. Simulation results show P+/Nwell/Psub photodiode has larger photocurrent than P+/Nwell photodiode and Nwell/Psub photodiode, and its spectral response is more in accordance with CsI(Tl) fluorescence spectrum. Improved P+/Nwell/Psub photodiode detecting CsI(Tl) fluorescence was designed in CSMC 0.5 μm CMOS process, CTIA (capacitive transimpedance amplifier) architecture was used to readout photocurrent signal. CMOS X-ray sensor IC prototype contains 8 × 8 pixel array and pixel pitch is 100 × 100 μm2. Testing results show the dark current of the improved P+/Nwell/Psub photodiode (6.5 pA) is less than that of P+/Nwell and P+/Nwell/Psub photodiodes (13 pA and 11 pA respectively). The sensitivity of P+/Nwell/Psub photodiode is about 20 pA/lux under white LED. The spectrum response of P+/Nwell/Psub photodiode ranges from 400 nm to 800 nm with a peak at 532 nm, which is in accordance with the fluorescence spectrum of CsI(Tl) in an indirect X-ray sensor. Preliminary testing results show the sensitivity of X-ray sensor IC under Cu target X-ray is about 0.21 Vm2/W or 5097e−/pixel @ 8.05 keV considering the pixel size, integration time and average energy of X-ray photons.
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01 Nov 2015
TL;DR: The design and measurement of low noise 32-channel snapshot X-ray readout integrated circuit (ROIC) is proposed, which adapts low noise preamplifier and S/H circuit and shows a dynamic range larger than 11000:1 with 3.6pF integration capacitor.
Abstract: The design and measurement of low noise 32-channel snapshot X-ray readout integrated circuit (ROIC) are proposed in this paper. This ROIC has 32 analog channels collecting weak current from detectors, then transfers them to voltage and outputs in differential mode. This ROIC adapts low noise preamplifier and S/H circuit. Measure results show a dynamic range larger than 11000:1 with 3.6pF integration capacitor. The dynamic range is higher than the previous design's and very close with a famous commercial IC.
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TL;DR: In this article, the intrinsic absorption spectra of high-purity single-crystal germanium and silicon have been measured at 77\ifmmode^\circ\else\text degree\fi{}K and 300\ifmode^''circ\decrease\textdegree\fi {}K, respectively.
Abstract: The intrinsic absorption spectra of high-purity single-crystal germanium and silicon have been measured at 77\ifmmode^\circ\else\textdegree\fi{}K and 300\ifmmode^\circ\else\textdegree\fi{}K. The spectral regions studied encompassed a range of absorption coefficient from 0.1 ${\mathrm{cm}}^{\ensuremath{-}1}$ to ${10}^{5}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ for each material. The germanium data may be interpreted as indicating a threshold for direct transitions at 0.81 ev at 300\ifmmode^\circ\else\textdegree\fi{}K and at 0.88 ev at 77\ifmmode^\circ\else\textdegree\fi{}K. The threshold for indirect transitions was placed at 0.62 ev and 0.72 ev for 300\ifmmode^\circ\else\textdegree\fi{}K and 77\ifmmode^\circ\else\textdegree\fi{}K, respectively. For silicon the data were not as readily interpreted However, there is an indication that the threshold for direct transitions should be placed at about 2.5 ev and the threshold for indirect transitions at 1.06 ev and 1.16 ev at 300\ifmmode^\circ\else\textdegree\fi{}K and 77\ifmmode^\circ\else\textdegree\fi{}K, respectively.
874 citations
11 Feb 2001-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: In this article, a novel active pixel sensor (MAPS) for charged particle tracking made in a standard CMOS technology is proposed, which has a special structure, which allows the high detection efficiency required for tracking applications.
Abstract: A novel Monolithic Active Pixel Sensor (MAPS) for charged particle tracking made in a standard CMOS technology is proposed. The sensor is a photodiode, which is readily available in a CMOS technology. The diode has a special structure, which allows the high detection efficiency required for tracking applications. The partially depleted thin epitaxial silicon layer is used as a sensitive detector volume. Semiconductor device simulation, using either ToSCA based or 3-D ISE-TCAD software packages shows that the charge collection is efficient, reasonably fast (order of 100 ns), and the charge spreading limited to a few pixels only. A first prototype has been designed, fabricated and tested. It is made of four arrays each containing 64×64 pixels, with a readout pitch of 20 μm in both directions. The device is fabricated using standard submicron 0.6 μm CMOS process, which features twin-tub implanted in a p-type epitaxial layer, a characteristic common to many modern CMOS VLSI processes. Extensive tests made with soft X-ray source ( 55 Fe) and minimum ionising particles (15 GeV/ c pions) fully demonstrate the predicted performances, with the individual pixel noise (ENC) below 20 electrons and the Signal-to-Noise ratio for both 5.9 keV X-rays and Minimum Ionising Particles (MIP) of the order of 30. This novel device opens new perspectives in high-precision vertex detectors in Particle Physics experiments, as well as in other application, like low-energy beta particle imaging, visible light single photon imaging (using the Hybrid Photon Detector approach) and high-precision slow neutron imaging.
395 citations
TL;DR: This paper presents a comparison of three photodiode structures in terms of spectral sensitivity, noise and dark current, and presents the n-well /p-sub diode as the best performing structure.
Abstract: While great advances have been made in optimizing fabrication process technologies for solid state image sensors, the need remains to be able to fabricate high quality photosensors in standard CMOS processes. The quality metrics depend on both the pixel architecture and the photosensitive structure. This paper presents a comparison of three photodiode structures in terms of spectral sensitivity, noise and dark current. The three structures are n+/p-sub, n-well/p-sub and p+/n-well/p-sub. All structures were fabricated in a 0.5 mum 3-metal, 2-poly, n-well process and shared the same pixel and readout architectures. Two pixel structures were fabricated-the standard three transistor active pixel sensor, where the output depends on the photodiode capacitance, and one incorporating an in-pixel capacitive transimpedance amplifier where the output is dependent only on a designed feedback capacitor. The n-well /p-sub diode performed best in terms of sensitivity (an improvement of 3.5 times and 1.6 times over the n+/p-sub and p+/n-well/p-sub diodes, respectively) and signal-to-noise ratio (1. 5times and 1.2 times improvement over the n+/p-sub and p+/n-well/p-sub diodes, respectively) while the p+/n-well/p-sub diode had the minimum (33% compared to other two structures) dark current for a given sensitivity.
78 citations
TL;DR: In this paper, a new CMOS image sensor cell using n/sup +/-ring-reset structure is presented, which can isolate the photon-sensing area from the defective field oxide edge.
Abstract: We present in this letter for the first time a new CMOS image sensor cell using n/sup +/-ring-reset structure, which can isolate the photon-sensing area from the defective field oxide edge. The experimental results demonstrate that the severe dark current degradation of the conventional CMOS active pixel image sensor fabricated by a standard CMOS logic process is significantly alleviated. Through optimizing the layout arrangement, as high as 45% fill factor can be obtained. The dynamic range of this new cell can thus be improved by more than 10/spl times/ compared to a conventional cell.
32 citations
TL;DR: Columnar CsI(Tl) screens are now routinely used in indirect digital x ray imaging detectors as discussed by the authors and provide high density, high atomic number, and high scintillation efficiency.
Abstract: Columnar CsI(Tl) screens are now routinely used in indirect digital x ray imaging detectors. The CsI(Tl) scintillator provides high density, high atomic number, and high scintillation efficiency. These properties, coupled with the fact that CsI(Tl) can be grown in columnar form, provide excellent spatial resolution, high x-ray absorption, and low noise resulting in detectors with high overall detective quantum efficiency (DQE(f)). While such screens are now commercially available, developments leading to further improvements in scintillator performance are ongoing at RMD. Here we report on the recent progress in developing very thin (10 μm) to very thick (~3 mm) columnar screens and discuss their application potential in digital radiology and nuclear medicine.
30 citations