A novel source–drain follower for monolithic active pixel sensors Gao, C.; Aglieri, G.; Hillemanns, H. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
09/2016, Volume:
831
Journal Article
Peer reviewed
Monolithic active pixel sensors (MAPS) receive interest in tracking applications in high energy physics as they integrate sensor and readout electronics in one silicon die with potential for lower ...material budget and cost, and better performance. Source followers (SFs) are widely used for MAPS readout: they increase charge conversion gain 1/Ceff or decrease the effective sensing node capacitance Ceff because the follower action compensates part of the input capacitance. Charge conversion gain is critical for analog power consumption and therefore for material budget in tracking applications, and also has direct system impact. This paper presents a novel source–drain follower (SDF), where both source and drain follow the gate potential improving charge conversion gain. For the inner tracking system (ITS) upgrade of the ALICE experiment at CERN, low material budget is a primary requirement. The SDF circuit was studied as part of the effort to optimize the effective capacitance of the sensing node. The collection electrode, input transistor and routing metal all contribute to Ceff. Reverse sensor bias reduces the collection electrode capacitance. The novel SDF circuit eliminates the contribution of the input transistor to Ceff, reduces the routing contribution if additional shielding is introduced, provides a way to estimate the capacitance of the sensor itself, and has a voltage gain closer to unity than the standard SF. The SDF circuit has a somewhat larger area with a somewhat smaller bandwidth, but this is acceptable in most cases.
A test chip, manufactured in a 180nm CMOS image sensor process, implements small prototype pixel matrices in different flavors to compare the standard SF to the novel SF and to the novel SF with additional shielding. The effective sensing node capacitance was measured using a 55Fe source. Increasing reverse substrate bias from −1V to −6V reduces Ceff by 38% and the equivalent noise charge (ENC) by 22% for the standard SF. The SDF provides a further 9% improvement for Ceff and 25% for ENC. The SDF circuit with additional shielding provides 18% improvement for Ceff, and combined with −6V reverse bias yields almost a factor 2.
•A source–drain follower (SDF) for monolithic active pixel sensors is proposed.•The SDF reduces the input capacitance increasing the charge conversion gain.•The SDF can provide improved noise-power performance.•The SDF with shielding provides further improvement.•The SDF with shielding provides a way to measure the sensor capacitance.
We report on a systematic study of time resolution made with three different commercial silicon photomultipliers (SiPMs) (Hamamatsu MPPC S10931-025P, S10931-050P, and S10931-100P) and two LSO ...scintillating crystals. This study aimed to determine the optimum detector conditions for highest time resolution in a prospective time-of-flight positron emission tomography (TOF-PET) system. Measurements were based on the time over threshold method in a coincidence setup using the ultrafast amplifier-discriminator NINO and a fast oscilloscope. Our tests with the three SiPMs of the same area but of different SPAD sizes and fill factors led to best results with the Hamamatsu type of 50×50×μm 2 single-pixel size. For this type of SiPM and under realistic geometrical PET scanner conditions, i.e., with 2×2×10×mm 3 LSO crystals, a coincidence time resolution of 220 ±4 ps FWHM could be achieved. The results are interpreted in terms of SiPM photon detection efficiency (PDE), dark noise, and photon yield.
Within the R&D activities for the upgrade of the ALICE Inner Tracking System (ITS), Monolithic Active Pixel Sensors (MAPS) are being developed and studied, due to their lower material budget (~0.3%X0 ...in total for each inner layer) and higher granularity (~20μm×20μm pixels) with respect to the present pixel detector. This paper presents the design and characterization results of the Explorer0 chip, manufactured in the TowerJazz 180nm CMOS Imaging Sensor process, based on a wafer with high-resistivity (ρ>1kΩcm) and 18μm thick epitaxial layer. The chip is organized in two sub-matrices with different pixel pitches (20μm and 30μm), each of them containing several pixel designs. The collection electrode size and shape, as well as the distance between the electrode and the surrounding electronics, are varied; the chip also offers the possibility to decouple the charge integration time from the readout time, and to change the sensor bias. The charge collection properties of the different pixel variants implemented in Explorer0 have been studied using a 55Fe X-ray source and 1–5GeV/c electrons and positrons. The sensor capacitance has been estimated, and the effect of the sensor bias has also been examined in detail. A second version of the Explorer0 chip (called Explorer1) has been submitted for production in March 2013, together with a novel circuit with in-pixel discrimination and a sparsified readout. Results from these submissions are also presented.
This paper summarizes CERN R&D work done in the framework of the European Commission's FP6 BioCare Project. The objective was to develop a novel "time-based" signal processing technique to read out ...LSO-APD photodetectors for medical imaging. An important aspect was to employ the technique in a combined scenario for both computer tomography (CT) and positron emission tomography (PET) with effectively no tradeoffs in efficiency and resolution compared to traditional single mode machines. This made the use of low noise and yet very high-speed monolithic front-end electronics essential so as to assure the required timing characteristics together with a high signal-to-noise ratio. Using APDs for photon detection, two chips, traditionally employed for particle physics, could be identified to meet the above criteria. Although both were not optimized for their intended new medical application, excellent performance in conjunction with LSO-APD sensors could be derived. Whereas a measured energy resolution of 16% (FWHM) at the 511 keV photo peak competes favorably with that of 'classical' PMTs, the coincidence time resolution of 1.6 ns FWHM with dual APD readout is typically lower. This is attributed to the stochastic photon production mechanism in LSO and the photon conversion characteristic of the photo diode, as well as to the fluctuations in photon conversion, albeit the APD's superior quantum efficiency. Also in terms of CT counting speed, the chosen readout principle is limited by the intrinsic light decay in LSO (40 ns) for each impinging X-ray.
Over the last years interest in using time-of-flight-based Positron Emission Tomography (TOF-PET) systems has significantly increased. High time resolution in such PET systems is a powerful tool to ...improve signal to noise ratio and therefore to allow smaller exposure rates for patients as well as faster image reconstruction. Improvement in coincidence time resolution (CTR) in PET systems to the level of 200ps FWHM requires the optimization of all parameters in the photon detection chain influencing the time resolution: crystal, photodetector and readout electronics. After reviewing the factors influencing the time resolution of scintillators, we will present in this paper the light yield and CTR obtained for different scintillator types (LSO:Ce, LYSO:Ce, LGSO:Ce, LSO:Ce:0.4Ca, LuAG:Ce, LuAG:Pr) with different cross-sections, lengths and reflectors. Whereas light yield measurements were made with a classical PMT, all CTR tests were performed with Hamamatsu-MPPCs or SiPMs S10931-050P. The CTR measurements were based on the time-over-threshold method in a coincidence setup using the ultra fast amplifier-discriminator chip NINO and a fast oscilloscope. Strong correlations between light yield and CTR were found. Excellent results have been obtained for LYSO crystals of 2×2×10mm 3 and LYSO pixels of 0.75×0.75×10mm 3 with a CTR of 175ps and 188ps FWHM, respectively.
A time driven readout scheme for PET and CT using APDs and SiPMs Powolny, F.; Auffray, E.; Brunner, S. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
05/2010, Volume:
617, Issue:
1
Journal Article
Peer reviewed
In this paper we present the time-driven readout scheme for photodetectors in the domain of PET/CT applications. In our first test period as partner of the BioCare Consortium supported by the ...European Commission's FP6 framework program, we put emphasis on a scheme to be used with a LSO-APD detector suitable for both CT and PET, using exclusively electronics circuits developed for the LHC program at CERN.
Compared to standard PMT-based systems, the proposed time-based readout method together with CERN ASICs produced comparable performance in terms of energy resolution, i.e. 15% FWHM, but had a markedly inferior timing response of 1.6
ns FWHM in a dual APD system. This is not attributed to the readout scheme itself but to crystal-inherent photon statistics and insufficient photodetector gain of the APDs.
However, in our new follow-up test program employing the same electronics and readout scheme, silicon photomultipliers (SiPMs) replacing the previously used APDs showed excellent timing behavior of 420 and 235
ps FWHM for 1 and 3 photoelectrons, respectively. Timing resolution with 511
keV gammas on LSO reached 400
ps FWHM without time walk corrections.
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