A Bulk Driven Transimpedance CMOS Amplifier for SiPM Based Detection

The contribution of this work lies in the development of a bulk driven operationaltransconducctance amplifier which can be integrated with other analog circuits andphotodetectors in the same chip for compactness, miniaturization and reducing thepower. Silicon photomultipliers, also known as SiPMs, when coupled with scintillator materials are used in many imaging applications including nuclear detection. This thesis discuss the design of a bulk-driven transimpedance amplifier suitable for detectors where the front end is a SiPM. The amplifier was design and fabricated in a standard standard CMOS process and is suitable for integration with CMOS based SiPMs and commercially available SiPMs. Specifically, the amplifier was verified in simulations and experiment using circuit models for the SiPM. The bulk-driven amplifier’s performance, was compared to a commerciallyavailable amplifier with approximately the same open loop gain (70dB). Bothamplifiers were verified with two different light sources, a scintillator and a SiPM.The energy resolution using the bulk driven amplifier was 8.6% and was 14.2% forthe commercial amplifier indicating the suitability of the amplifier design for portable systems

Miniaturized Optical Probes for Near Infrared Spectroscopy

RÉSUMÉ L’étude de la propagation de la lumière dans des milieux hautement diffus tels que les tissus biologiques (imagerie optique diffuse) est très attrayante, car elle offre la possibilité d’explorer de manière non invasive le milieu se trouvant profondément sous la surface, et de retrouver des informations sur l’absorption (liée à la composition chimique) et sur la diffusion (liée à la microstructure). Dans la gamme spectrale 600-1000 nm, également appelée gamme proche infrarouge (NIR en anglais), l'atténuation de la lumière par le tissu biologique (eau, lipides et hémoglobine) est relativement faible, ce qui permet une pénétration de plusieurs centimètres dans le tissu. En spectroscopie proche infrarouge (NIRS en anglais), de photons sont injectés dans les tissus et le signal émis portant des informations sur les constituants tissulaires est mesuré. La mesure de très faibles signaux dans la plage de longueurs d'ondes visibles et proche infrarouge avec une résolution temporelle de l'ordre de la picoseconde s'est révélée une technique efficace pour étudier des tissus biologiques en imagerie cérébrale fonctionnelle, en mammographie optique et en imagerie moléculaire, sans parler de l'imagerie de la durée de vie de fluorescence, la spectroscopie de corrélation de fluorescence, informations quantiques et bien d’autres. NIRS dans le domaine temporel (TD en anglais) utilise une source de lumière pulsée, généralement un laser fournissant des impulsions lumineuses d'une durée de quelques dizaines de picosecondes, ainsi qu'un appareil de détection avec une résolution temporelle inférieure à la nanoseconde. Le point essentiel de ces mesures est la nécessité d’augmenter la sensibilité pour de plus grandes profondeurs d’investigation, en particulier pour l’imagerie cérébrale fonctionnelle, où la peau, le crâne et le liquide céphalo-rachidien (LCR) masquent fortement le signal cérébral. À ce jour, l'adoption plus large de ces techniques optique non invasives de surveillance est surtout entravée par les composants traditionnels volumineux, coûteux, complexes et fragiles qui ont un impact significatif sur le coût et la dimension de l’ensemble du système. Notre objectif est de développer une sonde NIRS compacte et miniaturisée, qui peut être directement mise en contact avec l'échantillon testé pour obtenir une haute efficacité de détection des photons diffusés, sans avoir recours à des fibres et des lentilles encombrantes pour l'injection et la collection de la lumière. Le système proposé est composé de deux parties: i) une unité d’émission de lumière pulsée et ii) un module de détection à photon unique qui peut être activé et désactivé rapidement. L'unité d'émission de lumière utilisera une source laser pulsée à plus de 80 MHz avec une largeur d'impulsion de picoseconde.----------ABSTRACT The study of light propagation into highly diffusive media like biological tissues (Diffuse Optical Imaging) is highly appealing due to the possibility to explore the medium non-invasively, deep beneath the surface and to recover information both on absorption (related to chemical composition) and on scattering (related to microstructure). In the 600–1000 nm spectral range also known as near-infrared (NIR) range, light attenuation by the biological tissue constituents (i.e. water, lipid, and hemoglobin) is relatively low and allows for penetration through several centimeters of tissue. In near-infrared spectroscopy (NIRS), a light signal is injected into the tissues and the emitted signal carrying information on tissue constituents is measured. The measurement of very faint light signals in the visible and near-infrared wavelength range with picosecond timing resolution has proven to be an effective technique to study biological tissues in functional brain imaging, optical mammography and molecular imaging, not to mention fluorescence lifetime imaging, fluorescence correlation spectroscopy, quantum information and many others. Time Domain (TD) NIRS employs a pulsed light source, typically a laser providing light pulses with duration of a few tens of picoseconds, and a detection circuit with temporal resolution in the sub-nanosecond scale. The key point of these measurements is the need to increase the sensitivity to higher penetration depths of investigation, in particular for functional brain imaging, where skin, skull, and cerebrospinal fluid (CSF) heavily mask the brain signal. To date, the widespread adoption of the non-invasive optical monitoring techniques is mainly hampered by the traditional bulky, expensive, complex and fragile components which significantly impact the overall cost and dimension of the system. Our goal is the development of a miniaturized compact NIRS probe, that can be directly put in contact with the sample under test to obtain high diffused photon harvesting efficiency without the need for cumbersome optical fibers and lenses for light injection and collection. The proposed system is composed of two parts namely; i) pulsed light emission unit and ii) gated single-photon detection module. The light emission unit will employ a laser source pulsed at over 80MHz with picosecond pulse width generator embedded into the probe along with the light detection unit which comprises single-photon detectors integrated with other peripheral control circuitry. Short distance source and detector pairing, most preferably on a single chip has the potential to greatly expedites the traditional method of portable brain imaging

Design and validation of key components for the readout electronics of future PET scanners

This thesis work discusses the design and validation of two circuit components used in the electronic readout of positron emission tomography (PET) scanners for biomedical applications: a constant fraction discriminator (CFD) and an integrated CMOS time to digital converter (TDC). The former is used in the read out of a double-head PET scanner already developed by the group of medical physics at INFN Pisa for non-invasive dose delivery monitoring in hadrontherapy. The goal of the work has been the optimization of the front-end PCB in terms of timing performances so as to reduce the dead time and resolution at system level. A new CFD board has been implemented and experimental results have shown a significant enhancement of the timing characteristics which have enabled performing in-beam PET data acquisition which is fundamental in hadrontherapy treatment. The design of an integrated CMOS TDC to be used for the time of flight measurement in a magnetic field-compatible PET block detector is the second topic of the thesis. The required time resolutions, linear behaviour as well as the communication with other readout elements have been taken into account in the definition of the circuit topology. Cadence and Verilog simulations have shown that a bin size of 100 ps can be obtained with the combination of a submicron technology (UMC 65 nm LLLVT) and a pipeline approach where a 10 bit systolic counter coupled to a 4 stage delay locked loop (DLL) are exploited. This translates into a nominal resolution of 29 ps. In addition, the use of a short DLL leads to a high linearity which is an issue in PET measurements. Despite lower resolutions are obtained in literature with different TDC topologies, achieving good performances in terms of both time resolution and linearity is not straightforward. The converter also features a real-time validation algorithm which is capable to reject noise inputs generated by the photodetector without impairing the acquisition capability of the system. A standard-cell unit has been also designed which is in charge of data buffering and serial communication with external readout boards. A 47 bit output word is provided by the semi-custom stage at a measurement rate which is selectable between 31.25 MHz and 62.5 MHz with a double hit resolution of 170 ns. An 8 channel prototype of 1.875 x 1.875 mm2 has been submitted in March 2013 in order to validate simulated data with experimental results

INTEGRATED SINGLE-PHOTON SENSING AND PROCESSING PLATFORM IN STANDARD CMOS

Practical implementation of large SPAD-based sensor arrays in the standard CMOS process has been fraught with challenges due to the many performance trade-offs existing at both the device and the system level [1]. At the device level the performance challenge stems from the suboptimal optical characteristics associated with the standard CMOS fabrication process. The challenge at the system level is the development of monolithic readout architecture capable of supporting the large volume of dynamic traffic, associated with multiple single-photon pixels, without limiting the dynamic range and throughput of the sensor. Due to trade-offs in both functionality and performance, no general solution currently exists for an integrated single-photon sensor in standard CMOS single photon sensing and multi-photon resolution. The research described herein is directed towards the development of a versatile high performance integrated SPAD sensor in the standard CMOS process. Towards this purpose a SPAD device with elongated junction geometry and a perimeter field gate that features a large detection area and a highly reduced dark noise has been presented and characterized. Additionally, a novel front-end system for optimizing the dynamic range and after-pulsing noise of the pixel has been developed. The pixel is also equipped with an output interface with an adjustable pulse width response. In order to further enhance the effective dynamic range of the pixel a theoretical model for accurate dead time related loss compensation has been developed and verified. This thesis also introduces a new paradigm for electrical generation and encoding of the SPAD array response that supports fully digital operation at the pixel level while enabling dynamic discrete time amplitude encoding of the array response. Thus offering a first ever system solution to simultaneously exploit both the dynamic nature and the digital profile of the SPAD response. The array interface, comprising of multiple digital inputs capacitively coupled onto a shared quasi-floating sense node, in conjunction with the integrated digital decoding and readout electronics represents the first ever solid state single-photon sensor capable of both photon counting and photon number resolution. The viability of the readout architecture is demonstrated through simulations and preliminary proof of concept measurements

A portable device for time-resolved fluorescence based on an array of CMOS SPADs with integrated microfluidics

[eng] Traditionally, molecular analysis is performed in laboratories equipped with desktop instruments operated by specialized technicians. This paradigm has been changing in recent decades, as biosensor technology has become as accurate as desktop instruments, providing results in much shorter periods and miniaturizing the instrumentation, moving the diagnostic tests gradually out of the central laboratory. However, despite the inherent advantages of time-resolved fluorescence spectroscopy applied to molecular diagnosis, it is only in the last decade that POC (Point Of Care) devices have begun to be developed based on the detection of fluorescence, due to the challenge of developing high-performance, portable and low-cost spectroscopic sensors. This thesis presents the development of a compact, robust and low-cost system for molecular diagnosis based on time-resolved fluorescence spectroscopy, which serves as a general-purpose platform for the optical detection of a variety of biomarkers, bridging the gap between the laboratory and the POC of the fluorescence lifetime based bioassays. In particular, two systems with different levels of integration have been developed that combine a one-dimensional array of SPAD (Single-Photon Avalanch Diode) pixels capable of detecting a single photon, with an interchangeable microfluidic cartridge used to insert the sample and a laser diode Pulsed low-cost UV as a source of excitation. The contact-oriented design of the binomial formed by the sensor and the microfluidic, together with the timed operation of the sensors, makes it possible to dispense with the use of lenses and filters. In turn, custom packaging of the sensor chip allows the microfluidic cartridge to be positioned directly on the sensor array without any alignment procedure. Both systems have been validated, determining the decomposition time of quantum dots in 20 nl of solution for different concentrations, emulating a molecular test in a POC device.[cat] Tradicionalment, l'anàlisi molecular es realitza en laboratoris equipats amb instruments de sobretaula operats per tècnics especialitzats. Aquest paradigma ha anat canviant en les últimes dècades, a mesura que la tecnologia de biosensor s'ha tornat tan precisa com els instruments de sobretaula, proporcionant resultats en períodes molt més curts de temps i miniaturitzant la instrumentació, permetent així, traslladar gradualment les proves de diagnòstic fora de laboratori central. No obstant això i malgrat els avantatges inherents de l'espectroscòpia de fluorescència resolta en el temps aplicada a la diagnosi molecular, no ha estat fins a l'última dècada que s'han començat a desenvolupar dispositius POC (Point Of Care) basats en la detecció de la fluorescència, degut al desafiament que suposa el desenvolupament de sensors espectroscòpics d'alt rendiment, portàtils i de baix cost. Aquesta tesi presenta el desenvolupament d'un sistema compacte, robust i de baix cost per al diagnòstic molecular basat en l'espectroscòpia de fluorescència resolta en el temps, que serveixi com a plataforma d'ús general per a la detecció òptica d'una varietat de biomarcadors, tancant la bretxa entre el laboratori i el POC dels bioassaigs basats en l'anàlisi de la pèrdua de la fluorescència. En particular, s'han desenvolupat dos sistemes amb diferents nivells d'integració que combinen una matriu unidimensional de píxels SPAD (Single-Photon Avalanch Diode) capaços de detectar un sol fotó, amb un cartutx microfluídic intercanviable emprat per inserir la mostra, així com un díode làser UV premut de baix cost com a font d'excitació. El disseny orientat a la detecció per contacte de l'binomi format pel sensor i la microfluídica, juntament amb l'operació temporitzada dels sensors, permet prescindir de l'ús de lents i filtres. Al seu torn, l'empaquetat a mida de l'xip sensor permet posicionar el cartutx microfluídic directament sobre la matriu de sensors sense cap procediment d'alineament. Tots dos sistemes han estat validats determinant el temps de descomposició de "quantum dots" en 20 nl de solució per a diferents concentracions, emulant així un assaig molecular en un dispositiu POC

Amplificador CMOS de baixo ruído para imagiologia médica

Mestrado em Engenharia Electrónica e TelecomunicaçõesA presente dissertação aborda o projecto de um frontend analógico integrado para sincronização e amplificação de sinais produzidos por um fotomultiplicador de silício. A solução proposta pretende possibilitar medidas de tempo com resoluções na ordem dos picosegundos, para implementação em equipamentos compactos dedicados à Tomografia por Emissão de Positrões, com capacidade para medida do tempo de voo de fotões (TOFPET). O canal de frontend completo foi implementado em tecnologia CMOS 130nm, e compreende blocos de préamplificação, integração de carga, equilíbrio dinâmico do ponto de operação, bem como circuitos geradores de correntes de referência, para uma área total em silício de 500x90 μm. A discussão de resultados é baseada em simulações póslayout, e as linhas de investigação futuras são propostas.An analogue CMOS frontend for triggering and amplification of signals produced by a silicon photomultiplier (SiPM) is proposed. The solution intends to achieve picosecond resolution timing measurements for compact timeofflight Positron Emission Tomography (TOFPET) medical imaging equipments. A 130nm technology was used to implement such frontend, and the design includes preamplification, shaping, baseline holder and biasing circuitry, for a total silicon area of 500x90 μm. Postlayout simulation results are discussed, and ways to optimize the design are proposed

CMOS Transducers and Programmable Interface Circuits for Resource-Efficient Sensing Applications

Modern sensors are complex systems comprising multiple sub-systems such as transducers, analog and mixed-signal interface circuits, digital processing circuits, and packaging. Over the last few decades, innovations in these sub-systems combined with their increased integration in complementary metal-oxide semiconductor (CMOS) processes have led to the rapid growth in sensors for the Internet-of-Things (IoT), wearable devices, and fundamental scientific instrumentation. This thesis introduces novel ideas for various parts of a sensor signal chain. First, CMOS-based transducers (i.e. sensing elements) are introduced. Single-photon avalanche diodes (SPADs) fabricated in 0.18µm and 0.13µm standard CMOS processes are demonstrated and characterized for various optical sensing techniques. A resistor fabricated using standard CMOS-BEOL layers in a 0.18µm process is used to demonstrate a compact, fully-integrated single-element flow sensor occupying less than 0.065mm2. Second, front-end interface circuits for single-photon optical detectors are introduced. A fully-integrated SPAD-based ambient light sensor using mostly digital circuits and fabricated in a 0.13µm CMOS process is highlighted. It consumes 125μW and achieves one of the lowest reported areas (0.046mm2) in the literature. A custom analog front-end (AFE) chip is fabricated in a 0.18µm CMOS process for interfacing with a commercial silicon photomultiplier (SiPM) for gamma spectroscopy. It incorporates tunability of dynamic range and integration time, thus making it suitable for different detectors (i.e. SiPM and scintillator crystal combinations). Third, non-linear analog-to-digital converters (NL-ADCs) are explored as a viable alternative to linear ADCs for information-aware, non-uniform quantization and a widely reconfigurable piecewise-linear analog-to-digital converter (PWL-ADC) prototype chip (0.18µm CMOS) is used to validate this. With a 7-bit output word, it achieves 5.6-bit to 9.5-bit resolution in user-defined regions of the input full-scale range (FSR), while consuming 105µW at a sampling frequency of 42kHz. Measurements with recorded ECG waveforms are used to highlight the application-specific advantages of the PWL-ADC. Finally, some of the aforementioned ideas are used at the system level in a gamma spectrometer realized on a printed circuit board (PCB). The PCB design includes the AFE and PWL-ADC IC chips, a commercial SiPM and scintillator crystal, and a FPGA-based digital back-end (DBE). Several linear and non-linear isotope spectra with variable energy bin-widths (dE/bin) are recorded and analyzed to demonstrate the utility of the proposed concepts for peak enhancement and improved peak discrimination in radiation spectroscopy

Photodetectors

In this book some recent advances in development of photodetectors and photodetection systems for specific applications are included. In the first section of the book nine different types of photodetectors and their characteristics are presented. Next, some theoretical aspects and simulations are discussed. The last eight chapters are devoted to the development of photodetection systems for imaging, particle size analysis, transfers of time, measurement of vibrations, magnetic field, polarization of light, and particle energy. The book is addressed to students, engineers, and researchers working in the field of photonics and advanced technologies

Optimized PET module for both pixelated and monolithic scintillator crystals

[eng] Time-of-Flight Positron Emission Tomography (TOF-PET) scanners demand fast and efficient photo-sensors and scintillators coupled to fast readout electronics. Nowadays, there are two main configurations regarding the scintillator crystal geometry: the segmented or pixelated and the monolithic approach. Depending on the cost, spatial resolution and time requirements of the PET module, one can choose between one or another. The pixelated crystal is the most extensive configuration on TOF-PET scanners as the coincidence time resolution is better compared to the monolithic. On the contrary, monolithic scintillator crystals for Time-of-Flight Positron Emission Tomography (ToF-PET) are increasing in popularity this last years due to their performance potential and price in front of the commonly used segmented crystals. On one hand, monolithic blocks allows to determine 3D information of the gamma-ray interaction inside the crystal, which enables the possibility to correct the parallax error (radial astigmatism) at off-center positions within a PET scanner, resulting in an improvement of the spatial resolution of the device. On the other hand, due to the simplicity during the crystal manufacturing process as well as for the detector design, the price is reduced compared to a regular pixelated detector. The thesis starts with the use of HRFlexToT, an ASIC developed in this group, as the readout electronics for measurements with single pixelated crystals coupled to different SiPMs. These measurements show an energy linearity error of 3% and an energy resolution below 10% of the 511 keV photopeak. Single Photon Time Resolution (SPTR) measurements performed using an FBK SiPM NUV-HD (4 mm x 4 mm pixel size) and a Hamamatsu SiPM S13360-3050CS gave a 141 ps and 167 ps FWHM respectively. Coincidence Time Resolution (CTR) measurements with small cross-section pixelated crystals (LFS crystal, 3 m x 3 mm x 20 mm ) coupled to a single Hamamatsu SiPM S13360-3050CS provides a CTR of 180 ps FWHM. Shorter crystals (LSO:Ce Ca 0.4%) coupled to a Hamamatsu S13360-3050CS SiPM or FBK-NUVHD yields a CTR of 117 ps and 119 ps respectively. Then, the results with different monolithic crystals and SiPM sensors using HRFlexToT ASIC will be presented. A Lutetium Fine Silicate (LFS) of 25 mm x 25 mm x 20 mm, a small LSO:Ce Ca 0.2% of 8 mm x 8 mm x 5 mm and a Lutetium-Yttrium Oxyorthosilicate (LYSO) of 25 mm x 25 mm x 10 mm has been experimentally tested. After subtracting the TDC contribution (82 ps FWHM), a coincidence time resolution of 244 ps FWHM for the small LFS crystal and 333 ps FWHM for the largest LFS one is reported. Additionally, a novel time calibration correction method for CTR improvement that involves a pico-second pulsed laser will be detailed. In the last part of the dissertation, a new developed simulation framework that will enable the cross-optimization of the whole PET system will be explained. It takes into consideration the photon physics interaction in the scintillator crystal, the sensor response (sensor size, pixel pitch, dead area, capacitance) and the readout electronics behavior (input impedance, noise, bandwidth, summation). This framework has allowed us to study a new promising approach that will help reducing the CTR parameter by segmenting a large area SiPM into "m" smaller SiPMs and then summing them to recover all the signal spread along these smaller sensors. A 15% improvement on time resolution is expected by segmenting a 4 mm x 4 mm single sensor into 9 sensors of 1.3 mm x 1.3 mm with respect to the case where no segmentation is applied.[cat] Aquesta tesi tenia com a objectiu la fabricació i avaluació d'un prototip per a detecció de fotons gamma en aplicació per imatge mèdica, més concretament en Tomografia per Emissió de Positrons amb mesura de temps de vol (TOF-PET). L'avaluació del mòdul va començar fent una caracterització completa del chip (ASIC) anomenat HRFlexToT, una versió nova i millorada de l'antic chip FlexToT, desenvolupat i fabricat pel grup de la Unitat Tecnològica del ICC de la Universitat de Barcelona. Aquesta avaluació inicial del chip compren des de la comprovació de les funcionalitats bàsiques fins a la generació d'un test automàtic per generar les gràfiques de linealitat corresponents durant el test elèctric. Un cop donat per bo, es va muntar en una placa demostrada, també ideada per l'equip d'enginyers del grup, i ja quedava llesta per realitzar les mesures pertinents. Tot seguit, es varen realitzar les mesures òptiques, que incloïa mesures de Singe Photon Time Resolution (SPTR) i de Coincidence Time Resolution (CTR). Aquest valors actuen com a figures de mèrit a l'hora de comparar les prestacions amb d'altres ASICs competidors del HRFlexToT. Es van obtenir valors de 60 ps de resposta pel que respecta al SPTR i de 115 ps de CTR en cristalls segmentats, una millora entorn del 20-30% respecte a la versió predecessora del chip. Aquests valors mostren ser el límit de l'estat de l'art actual i amb aquesta idea es van començar a fer altres mesures, en aquest cas amb cristall monolítics, blocs grans llegits per diversos fotosensors de les empreses Hamamatsu i FBK. Per altra banda, es va provar el funcionament del ASIC en configuració anomenada monolítica, on el cristall centellejador s'utilitza en blocs grans en coptes d’emprar cristalls segmentats, això abarateix el cost total del detector. Aquesta configuració degrada les propietats de CTR, un paràmetre crític a l'hora de tenir un producte bo i eficient. S’han obtingut mesures de 250 ps de CTR amb aquesta configuració, d’on es pot dir que l’HRFlexToT es trobar a l’estat de l’art de la tecnologia electrònica dedicada a TOF-PET amb cristalls segmentats i monolítics. Finalment, es va desenvolupar una nova eina simulació que consisteix en un sistema híbrid entre un simulador físic i un electrònic per tal de tenir una idea del comportament complet del mòdul detector. Una solució que ningú havia provat fins ara o que no es pot trobar en la literatura