Development of ASIC for SiPM sensor readout

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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

Thermal Fluctuation Noise in Mo/Au Superconducting Transition-Edge Sensor Microcalorimeters

In many superconducting transition-edge sensor (TES) microcalorimeters, the measured electrical noise exceeds theoretical estimates based on a thermal model of a single body thermally connected to a heat bath. Here, we report on noise and complex impedance measurements of a range of designs of TESs made with a Mo/Au bilayer. We have fitted the measured data using a two-body model, where the x-ray absorber and the TES are connected by an internal thermal conductance Gae. We find that the so-called excess noise measured in these devices is consistent with the noise generated from the internal thermal fluctuations between the x-ray absorber and the TES. Our fitted parameters are consistent with the origin of Gae being from the finite thermal conductance of the TES itself. These results suggest that even in these relatively low resistance Mo/Au TESs, the internal thermal conductance of the TES may add significant additional noise and could account for all the measured excess noise. Furthermore, we find that around regions of the superconducting transition with rapidly changing derivative of resistance with respect to temperature, an additional noise mechanism may dominate. These observations may lead to a greater understanding of TES devices and allow the design of TES microcalorimeters with improved performance

Novel Designs for Application Specific MEMS Pressure Sensors

In the framework of developing innovative microfabricated pressure sensors, we present here three designs based on different readout principles, each one tailored for a specific application. A touch mode capacitive pressure sensor with high sensitivity (14 pF/bar), low temperature dependence and high capacitive output signal (more than 100 pF) is depicted. An optical pressure sensor intrinsically immune to electromagnetic interference, with large pressure range (0–350 bar) and a sensitivity of 1 pm/bar is presented. Finally, a resonating wireless pressure sensor power source free with a sensitivity of 650 KHz/mmHg is described. These sensors will be related with their applications in  harsh environment, distributed systems and medical environment, respectively. For many aspects, commercially available sensors, which in vast majority are piezoresistive, are not suited for the applications proposed

Development of an ultra-low field magnetic resonance imaging scanner and DC SQUID based current sensors for the investigation of hyperpolarization techniques

Kernspinresonanzspektroskopie und Kernspinresonanztomographie sind etablierte Verfahren in der Strukturanalyse und der medizinischen Bildgebung. Aufgrund der hohen Kosten bei der Anschaffung und dem Betrieb der Spektrometer und Tomographen, welche haupts{\"a}chlich aus den ben{\"o}tigten supraleitenden Elektromagneten resultieren, gibt es ein wachsendes Interesse an kosteng{\"u}nstigen Ger{\"a}ten. Spektrometer und Tomographen auf Basis von normalleitenden Elektromagneten erlauben kosteng{\"u}nstige Systeme, was jedoch aufgrund der niedrigeren Magnetfeldst{\"a}rke und einer damit einhergehenden niedrigeren Probenpolarisierung zu Lasten des Messsignals geht. Um Signalverlust teilweise zu kompensieren werden in Niederfeldsystemen Detektoren auf Basis von gleichstrombetriebenen (DC) supraleitenden Quanteninterferometern (SQUIDs) verwendet, welche eine deutlich h{\"o}here Empfindlichkeit als konventionelle Detektionsspulen aus Kupfer besitzen. Zus{\"a}tzlich bieten neuartige Hyperpolarisierungsmethoden auf Basis von Parawasserstoff, welche bei niedrigen Magnetfeldst{\"a}rken im Bereich weniger mT anwendbar sind, die M{\"o}glichkeit, die Probenpolarisierung durch die {\"U}bertragung der Spinordnung von Parawasserstoff-Kernen auf Wasserstoff-Kerne der Probe um mehrere Gr{\"o}ssenordnungen zu erh{\"o}hen. Zur erfolgreichen Hyperpolarisierung der zu untersuchenden Proben werden Polarisierungstransfer-Katalysatoren ben{\"o}tigt. In dieser Arbeit wird zum Einen die Konzeption und der Aufbau eines Ultra-Niederfeld Kernspinresonanzspektrometers/-tomographen mit einem DC SQUID basierten Magnetfeldsensor zur kontrollierten Charakterisierung von neu entwickelten Polarisierungstransfer-Katalysatoren f{\"u}r Hyperpolarisierungsanwendungen vorgestellt. Der gesamte Aufbau wurde durch weitestgehende Vermeidung von metallischen Komponenten auf m{\"o}glichst niedrige Magnetfeldrauschwerte $S_B^{1/2}$ und homogene Magnetfelder hin optimiert, was sich in einem Magnetfeldrauschen $S_B^{1/2}=1.15\,\textrm{fT/Hz}^{1/2}$ im Bereich wei{\ss}en Rauschens und Linienbreiten der Kernspinresonanz $<1\,$Hz zeigt. Zum Anderen wurden im Rahmen der Arbeit DC SQUID basierte Stromsensoren zur Erfassung der Kernspinresonanz-Signale entworfen, welche auf dem Niedertemperatur-Supraleiter Niob basieren. Dabei konnte sowohl ein Supraleiter/Normalleiter/Supraleiter (SNS) als auch auf einen Supraleiter/Isolator/Supraleiter (SIS) Herstellungsprozess zur{\"u}ckgegriffen werden. Der Stromsensorentwurf wurde an die kritische Stromdichte $j_c$ des jeweiligen Herstellungsprozesses angepasst, was in unterschiedlichen SQUID-Induktivit{\"a}ten und dadurch in verschiedenen Ankoppelschemata der Signalaufnehmer-Spulen an das SQUID resultiert. Transport- und Rauscheigenschaften wurden bei einer Temperatur von $T = 4.2\,$K bestimmt. F{\"u}r die SNS basierten Stromsensoren konnte eine Eingansempfindlichkeit $1/M_{in}=37.8\,\mu$A gefunden werden, was in Kombination mit einem Flussrauschen $S_{\Phi}^{1/2} = 590\,\textrm{n}\Phi_0\textrm{/Hz}^{1/2}$ im Bereich wei{\ss}en Rauschens zu einer Stromempfindlichkeit $S_{i}^{1/2} = 21.9\,\textrm{pA/Hz}^{1/2}$ f{\"u}hrt. Mit SIS basierten Stromsensoren konnte eine Eingansempfindlichkeit $1/M_{in}<1\,\mu$A erreicht werden. Die tats{\"a}chliche Stromempfindlichkeit konnte jedoch nicht bestimmt werden, da aufgrund von herstellungsbedingten Schichtisolationsproblemen sehr hohe Flussrauschwerte resultierten. Anhand von hyperpolarisiertem Pyridin konnte ein Signalverstärkungsfaktor von $\leq200$ gemessen werden. Daran anschlie{\ss}end wurden drei weitere Probensubstanzen untersucht, welche sowohl $^1$H- $^{19}$F-Kerne enthalten und in Hochfeldmessungen vielversprechende Resultate zeigten. Dabei zeigten zwei Probensubstanzen ein Verhalten des Polarisationstransfers, wie er gem{\"a}{\ss} der etablierten Theorie auf Basis von J-Kopplung. Die dritte Probe hingegen zeigt ein Verhalten, was nicht mit J-Kopplung der Kerne erkl{\"a}rt werden kann und auf alternative Hyperpolarisierungsmechanismen schlie{\ss}en l{\"a}sst

DESIGN OF SMART SENSORS FOR DETECTION OF PHYSICAL QUANTITIES

Microsystems and integrated smart sensors represent a flourishing business thanks to the manifold benefits of these devices with respect to their respective macroscopic counterparts. Miniaturization to micrometric scale is a turning point to obtain high sensitive and reliable devices with enhanced spatial and temporal resolution. Power consumption compatible with battery operated systems, and reduced cost per device are also pivotal for their success. All these characteristics make investigation on this filed very active nowadays. This thesis work is focused on two main themes: (i) design and development of a single chip smart flow-meter; (ii) design and development of readout interfaces for capacitive micro-electro-mechanical-systems (MEMS) based on capacitance to pulse width modulation conversion. High sensitivity integrated smart sensors for detecting very small flow rates of both gases and liquids aiming to fulfil emerging demands for this kind of devices in the industrial to environmental and medical applications. On the other hand, the prototyping of such sensor is a multidisciplinary activity involving the study of thermal and fluid dynamic phenomenon that have to be considered to obtain a correct design. Design, assisted by finite elements CAD tools, and fabrication of the sensing structures using features of a standard CMOS process is discussed in the first chapter. The packaging of fluidic sensors issue is also illustrated as it has a great importance on the overall sensor performances. The package is charged to allow optimal interaction between fluids and the sensors and protecting the latter from the external environment. As miniaturized structures allows a great spatial resolution, it is extremely challenging to fabricate low cost packages for multiple flow rate measurements on the same chip. As a final point, a compact anemometer prototype, usable for wireless sensor network nodes, is described. The design of the full custom circuitry for signal extraction and conditioning is coped in the second chapter, where insights into the design methods are given for analog basic building blocks such as amplifiers, transconductors, filters, multipliers, current drivers. A big effort has been put to find reusable design guidelines and trade-offs applicable to different design cases. This kind of rational design enabled the implementation of complex and flexible functionalities making the interface circuits able to interact both with on chip sensors and external sensors. In the third chapter, the chip floor-plan designed in the STMicroelectronics BCD6s process of the entire smart flow sensor formed by the sensing structures and the readout electronics is presented. Some preliminary tests are also covered here. Finally design and implementation of very low power interfaces for typical MEMS capacitive sensors (accelerometers, gyroscopes, pressure sensors, angular displacement and chemical species sensors) is discussed. Very original circuital topologies, based on chopper modulation technique, will be illustrated. A prototype, designed within a joint research activity is presented. Measured performances spurred the investigation of new techniques to enhance precision and accuracy capabilities of the interface. A brief introduction to the design of active pixel sensors interface for hybrid CMOS imagers is sketched in the appendix as a preliminary study done during an internship in the CNM-IMB institute of Barcelona

Application of Silicon Photomultipliers to Positron Emission Tomography

Historically, positron emission tomography (PET) systems have been based on scintillation crystals coupled to photomultipliers tubes (PMTs). However, the limited quantum efficiency, bulkiness, and relatively high cost per unit surface area of PMTs, along with the growth of new applications for PET, offers opportunities for other photodetectors. Among these, small-animal scanners, hybrid PET/MRI systems, and incorporation of time-of-flight information are of particular interest and require low-cost, compact, fast, and magnetic field compatible photodetectors. With high quantum efficiency and compact structure, avalanche photodiodes (APDs) overcome several of the drawbacks of PMTs, but this is offset by degraded signal-to-noise and timing properties. Silicon photomultipliers (SiPMs) offer an alternative solution, combining many of the advantages of PMTs and APDs. They have high gain, excellent timing properties and are insensitive to magnetic fields. At the present time, SiPM technology is rapidly developing and therefore an investigation into optimal design and operating conditions is underway together with detailed characterization of SiPM-based PET detectors. Published data are extremely promising and show good energy and timing resolution, as well as the ability to decode small scintillator arrays. SiPMs clearly have the potential to be the photodetector of choice for some, or even perhaps most, PET systems