Publicaciones del IMSE

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Autor: Franco N. Bandi
Año: Desde 2002

Artículos de revistas


Architecture-Level Optimization on Digital Silicon Photomultipliers for Medical Imaging
F. Bandi, V. Ilisie, I. Vornicu, R. Carmona-Galan, J.M. Benlloch and A. Rodriguez-Vazquez
Journal Paper · Sensors, vol. 22, no. 1, article 122, 2022
resumen      doi      

Silicon photomultipliers (SiPMs) are arrays of single-photon avalanche diodes (SPADs) connected in parallel. Analog silicon photomultipliers are built in custom technologies optimized for detection efficiency. Digital silicon photomultipliers are built in CMOS technology. Although CMOS SPADs are less sensitive, they can incorporate additional functionality at the sensor plane, which is required in some applications for an accurate detection in terms of energy, timestamp, and spatial location. This additional circuitry comprises active quenching and recharge circuits, pulse combining and counting logic, and a time-to-digital converter. This, together with the disconnection of defective SPADs, results in a reduction of the light-sensitive area. In addition, the pile-up of pulses, in space and in time, translates into additional efficiency losses that are inherent to digital SiPMs. The design of digital SiPMs must include some sort of optimization of the pixel architecture in order to maximize sensitivity. In this paper, we identify the most relevant variables that determine the influence of SPAD yield, fill factor loss, and spatial and temporal pile-up in the photon detection efficiency. An optimum of 8% is found for different pixel sizes. The potential benefits of molecular imaging of these optimized and small-sized pixels with independent timestamping capabilities are also analyzed.

20-ps Resolution Clock Distribution Network for a Fast-Timing Single-Photon Detector
N. Egidos, R. Ballabriga, F. Bandi, M. Campbell, D. Gascon, S. Gomez, J.M. Fernandez-Tenllado, X. Llopart, R. Manera, J. Mauricio, D. Sanchez, A. Sanmukh and E. Santin
Journal Paper · IEEE Transactions on Nuclear Science, vol. 68, no. 4, pp. 434-446, 2021
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The time resolution of active pixel sensors whose timestamp mechanism is based on time-to-digital converters is critically linked to the accuracy in the distribution of the master clock signal that latches the timestamp values across the detector. The clock distribution network (CDN) that delivers the master clock signal must compensate process-voltage-temperature variations to reduce static time errors (skew) and minimize the power supply bounce to prevent dynamic time errors (jitter). To achieve sub-100-ps time resolution within pixel detectors and thus enable a step forward in multiple imaging applications, the network latencies must be adjusted in steps well below that value. Power consumption must be kept as low as possible. In this work, a self-regulated CDN that fulfills these requirements is presented for the FastICpix single-photon detector aiming at a 65-nm process. A 40-MHz master clock is distributed to 64 x 64 pixels over an area of 2.4 x 2.4 cm2 using digital delay-locked loops, achieving clock leaf skew below 20 ps with a power consumption of 26 mW. Guidelines are provided to adapt the system to arbitrary chip area and pixel pitch values, yielding a versatile design with very fine time resolution.

Photon counting detectors for X-ray imaging with emphasis on CT
R. Ballabriga, J. Alozy, F.N. Bandi, M. Campbell, N. Egidos, J.M. Fernandez-Tenllado, E.H.M. Heijne, I. Kremastiotis, X. Llopart, B.J. Madsen, D. Pennicard, V. Sriskaran and L.Tlustos
Journal Paper · IEEE Transactions on Radiation and Plasma Medical Sciences, vol. 5, no. 4, pp 422-440, 2021
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X-ray imaging is a widely used imaging modality in the medical diagnostic field due to its availability, low cost, high spatial resolution and fast image acquisition. X-ray photons in standard X-ray sources are polychromatic. Detectors that allow to extract the ‘colour ’ information of the individual X-rays can lead to contrast enhancement, improved material identification or reduction of beam hardening artifacts at the system level, if we compare them with the widely spread energy integrating detectors. Today, in the field of Computed Tomography (CT), prototypes of clinical grade systems based on Spectral Photon Counting detectors are currently available for clinical research from different companies. One of the key system components in that development is the X-ray photon detector. This paper reviews the photon detection hardware, from the conversion of X-rays into electrical signals to the pulse processing electronics. A review of available photon counting Application Specific Integrated Circuits (ASICs) for spectroscopic X-ray imaging is presented with emphasis on the CT medical imaging application.

Design of High-Efficiency SPADs for LiDAR Applications in 110nm CIS Technology
I. Vornicu, J.M. López-Martínez, F.N. Bandi, R. Carmona-Galán and A. Rodríguez-Vázquez
Journal Paper · IEEE Sensors Journal, vol. 21, no. 4, pp 4776-4785, 2021
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Single photon avalanche diodes (SPADs) featuring a high detection rate of near-IR photons are much desired for outdoor LiDAR based on direct time-of-flight (ToF). This article presents the complete design flow of a SPAD detector for LiDAR. First, the selection of the emitter wavelength is discussed, considering the maximum allowed power underlying eye safety regulations, solar irradiance, and reflected signal power. Then, the choice of the SPAD structure is discussed based on the TCAD simulation of quantum efficiency and crosstalk. Next, the proposed P-well/Deep N-well SPAD is explained. The electro-optical characterization of the detectors is presented as well. The performance of the time-of-flight image sensors is determined by the characteristics of the individual SPADs. To fully characterize this technology, devices with various sizes, shapes, and guard ring widths have been fabricated and tested. The measured mean breakdown voltage is 18 V. The proposed structure has a 0.4 Hz/µ m2 dark count rate and 0.5% afterpulsing. The FWHM (total) jitter and photon detection probability at 850nm wavelength are of 92 ps and 10%. All figures have been measured at 3 V excess voltage. Finally, the performance of the SPAD detector is analyzed by evaluating the signal-to-noise ratio at different acquisition times. Distance ranging measurements have been performed, achieving a depth resolution of 1 cm up to 6.3 m range.

Compact Macro-Cell with OR Pulse Combining for Low Power Digital-SiPM
I. Vornicu, F.N. Bandi, R. Carmona-Galan and A. Rodriguez-Vazquez
Journal Paper · IEEE Sensors Journal, vol. 20, no. 21, pp 12817 - 12826, 2020
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High-density digital-Silicon Photomultipliers call for high-performance Single Photon Avalanche Diodes (SPAD) front-ends. Power consumption and fill factor are significant concerns in this kind of sensors. This paper presents a compact and power-efficient macro-cell where several SPADs share the active recharge circuitry for increased fill factor. Integrated with 110nm technology for image sensors, the array of macro-cells has 30% fill factor. Also, following the first firing of any SPAD during the same macro-pixel dead-time, the other SPADs are disabled for power saving. Of course, subsequent triggers are lost. However, they would have been masked by the OR pulse combining scheme. Besides this event-driven disabling feature, the macro-cell includes circuitry to disable noisy devices - similar to other SiPM cells. Also, the macro-cell features control of the dead-time. This paper describes the macro-cell concept, its associated analysis and design equations. Key parameters of the design are discussed to optimize power consumption. Design scalability is contemplated as well. Experimental results proved that the power efficiency of the proposed scheme depends on the illumination power. Also, power efficiency is linked to the pulse overlapping probability. For example, power saving up to 30% is obtained with 4 sub-cells per macro-cell, when pulse overlapping is about 11% for correlated light or the pulse rate per sub-cell is about 100kHz for uncorrelated light.

A CMOS Digital SiPM with Focal-Plane Light-Spot Statistics for DOI Computation
I. Vornicu, F.N. Bandi, R. Carmona-Galán and A. Rodríguez-Vázquez
Journal Paper · IEEE Sensors Journal, vol. 17, no. 3, pp 632-643, 2017
resumen      doi      pdf

Silicon photomultipliers can be used to infer the depth-of-interaction (DOI) in scintillator crystals. DOI can help to improve the quality of the positron emission tomography images affected by the parallax error. This paper contemplates the computation of DOI based on the standard deviation of the light distribution. The simulations have been carried out by GAMOS. The design of the proposed digital silicon photomultiplier (d-SiPM) with focal plane detection of the center of mass position and dispersion of the scintillation light is presented. The d-SiPM shares the same off-chip time-to-digital converter such that each pixel can be individually connected to it. A miniature d-SiPM 8×8 single-photon avalanche-diode (SPAD) array has been fabricated as a proof of concept. The SPADs along each row and column are connected through an OR combination technique. It has 256×256μm2 without peripherals circuits and pads. The fill factor is about 11%. The average dark count rate of the mini d-SiPM is of 240 kHz. The average photon detection efficiency is 5% at 480 nm wavelength, room temperature, and 0.9 V excess voltage. The dynamic range is of 96 dB. The sensor array features a time resolution of 212 ps. The photon-timing SNR is 81 dB. The focal plane statistics of the light-spot has been proved as well by measurements.

Congresos


Low-Noise and High-Efficiency Near-IR SPADs in 110nm CIS Technology
I. Vornicu, F. Bandi, R. Carmona-Galán and A. Rodríguez-Vázquez
Conference · European Solid-State Device Research Conference ESSDERC 2019
resumen     

Photon detection at longer wavelengths is much desired for LiDAR applications. Silicon photodiodes with deeper junctions and larger multiplication regions are more sensitive to near-IR photons. This paper presents the complete electro-optical characterization of a P-well/Deep N-well single photon avalanche diode integrated in 110nm CIS technology. Devices with various sizes, shapes and guard ring widths have been fabricated and tested. The measured mean breakdown voltage is of 18V. The proposed structure has 0.4Hz/um2 dark count rate, 0.5% afterpulsing, 188ps FWHM (total) jitter and around 10% photon detection probability at 850nm wavelength. All figures have been measured at 3V excess voltage.

Design of a Compact and Low-Power TDC for an Array of SiPM´s in 110nm CIS Technology
F.N. Bandi, I. Vornicu, R. Carmona-Galán and A. Rodríguez-Vázquez
Conference · Conference on Ph.D Research in Microelectronics and Electronics PRIME 2017
resumen     

Silicon photomultipliers (SiPMs) are meant to substitute photomultiplier tubes in high-energy physics detectors and nuclear medicine. This is because of their -to name a few interesting Properties- compactness, lower bias voltage, tolerance to magnetic fields and finer spatial resolution. SiPMs can also be built in CMOS technology. This allows the incorporation of active quenching and recharge schemes at cell level and processing circuitry at pixel level. One of the elements that can lead to finer temporal resolutions is the time-to-digital converter (TDC). In this paper we describe the architecture of a compact TDC to be included at each pixel of an array of SiPMs. It is compact and consumes low power. It is based on a voltage controlled oscillator that generates multiple internal phases that are interpolated to provide time resolution below the time delay of a single gate. Simulation results of a 11b TDC based on a 4-stage VCRO in 110nm CIS technology yield a time resolution of 80.0ps, a DNL of ±0.28 LSB, a INL ±0.52 LSB, and a power consumption of 850μW.

VCRO-based TDCs in submicron CIS technology
F.N. Bandi, I. Vornicu, R. Carmona-Galán and A. Rodríguez-Vázquez
Conference · Workshop on the Architecture of Smart Cameras WASC 2017
resumen     

Time-to-Digital Converters (TDCs) based on Voltage Controlled Ring Oscillators (VCROs) provides a good trade off between area occupation, time resolution and power consumption. These specifications are determined by applications like nuclear medicine and high energy physics imaging, in which an accurate timestamp of the detected photons is needed and small area footprint to maximize fill factor is desired. This is specially true when the number of incident photons is low and oversampling is impossible. Other TDC architectures like pulse stretching, Vernier delay lines, time amplification or multi-path gated ring oscillator are able to provide finer time resolution at the price of higher area occupation and power consumption. If in sensor intregation is desired, these area and power increments are prohibitive. VCROs provides a large number of alternatives during the design phase, each one with their advantages and disadvantages. The first step is the selection of the stage topology, that is, single-ended, differential and pseudo-differential. In this application, pseudo-differential stages outperforms the other alternatives in terms of lower power consumption, lower jitter and better noise rejection. The second step consist in the selection of a pseudo-differential stage using a common metric. To this end, the two most used pseudo-differential stages were compared in terms of time resolution, by using the small signal model and the GBW product. Analytical expression points out that pseudo-differential stage with cross-coupled inverters have finer time resolution than pseudo-differential stage with cross-coupled PMOS. Pre-layout simulations support the analytical expression and shows a clear difference between the time resolution of each stage.

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