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ADVERTIMENT. Lʼaccés als continguts dʼaquesta tesi queda condicionat a lʼacceptació de les condicions dʼús establertes per la següent llicència Creative Commons: ADVERTENCIA. El acceso a los contenidos de esta tesis queda condicionado a la aceptación de las condiciones de uso establecidas por la siguiente licencia Creative Commons: WARNING. The access to the contents of this doctoral thesis it is limited to the acceptance of the use conditions set by the following Creative Commons license: Universitat Autònoma de Barcelona Departament de Física Institut de Física d Altes Energies A novel Depleted Monolithic Active Pixel Sensor for future High Energy Physics Detectors Sonia Fernandez-Perez PhD Thesis April 2016 Supervised by: Dr. Cristobal Padilla Institut de Física d Altes Energies Edifici Cn E Bellaterra (Barcelona) Tutored by: Dr. Enrique Fernández Universitat Autònoma de Barcelona (UAB)/ IFAE Edifici Cn E Bellaterra (Barcelona) Universitat Autònoma de Barcelona Departament de Física Institut de Física d Altes Energies A novel Depleted Monolithic Active Pixel Sensor for future High Energy Physics Detectors Sonia Fernandez-Perez PhD Thesis April 2016 Supervised by: Dr. Cristobal Padilla Institut de Física d Altes Energies Edifici Cn E Bellaterra (Barcelona) Tutored by: Dr. Enrique Fernández Universitat Autònoma de Barcelona (UAB)/ IFAE Edifici Cn E Bellaterra (Barcelona) Contents Introduction vii 1 The Large Hadron Collider and the ATLAS experiment Physics environment The Large Hadron Collider The HL-LHC upgrade The ATLAS experiment Physics programme for HL-LHC Detector layout The ATLAS Inner Detector Pileup performance and future expectations ATLAS upgrades towards HL-LHC Requirements and challenges for the ATLAS Inner Detector upgrade Vertexing and tracking detectors in HEP experiments Interaction of particles with matter Detection of charged particles Multiple scattering Energy deposition of photons From a semiconductor to a silicon pixel detector Energy band structure: semiconductors pn-junction Silicon pixel sensors Signal generation and transport in a silicon sensor Detector readout Tracking and vertexing with tracking detectors Transverse momentum resolution Vertex and impact parameter resolution Requirements on detector design Radiation damage in silicon detectors Radiation induced damage in transistors Damage mechanism of ionizing radiation Changes in transistor properties Radiation damage in the silicon bulk Damage mechanism and defect generation iii 3.2.2 The NIEL hypotesis Changes in detector properties Pixel detectors: Developments and Trends Hybrid pixel detectors CMOS pixels for the ATLAS HL-LHC upgrade HV-CMOS pixel sensors HR-CMOS pixel sensors CMOS-on-SOI monolithic pixels Characterization of the radiation hardness to Total Ionizing Dose XTB01 transistors test structures Transistors characterization setup X-ray irradiation facility and setup Results Transistors response to Total Ionizing Dose Back Gate Effect in XTB Coupling between the buried oxide and the sensor diode Charge collection measurements Leakage current characterization T-Cell circuit operation Charge collection measurements with radioactive sources Measurements method MIP signal as a function of timing distribution Calibration and charge distribution Source bias voltage scan and depletion depth formation Position resolved charge collection behaviour edge Transient Current Technique setup Charge collection and depletion depth Acceptor Removal effect Test beam characterization Test beam instrumentation AIDA SBM FE-I4 telescope XTB01 analogue readout Test beam measurements and results Pixel search and Region-of-Interest definition Charge collection and charge sharing Selection criteria Spatial resolution Tracking efficiency Summary and Outlook 123 iv Bibliography 127 List of Figures 135 List of Tables 139 List of Acronyms 141 v Introduction The Standard Model of particle physics provides the best knowledge of the ultimate constituents of matter and their nature. However, the Standard Model provides no explanation for a number of fundamental observations. Examples of these are the gravitational interactions, the dark matter observed in galaxy studies or the matter and anti-matter asymmetry observed in the universe. Giant particle accelerators all over the world try to find answers to these fundamental questions. The LHC (Large Hadron Collider) located at CERN (Conseil Européen pour la Recherche Nucléaire) is the most powerful of them built to date. The LHC, operative since 2009, has four main experiments distributed around its ring. They are: CMS (Compact Muon Spectrometer), ATLAS (A Toroidal LHC ApparatuS), LHCb (Large Hadron Collider beauty), and ALICE (A Large Ion Collider Experiment). In 2012, after fifty years of searching, the existence of the Higgs Boson was experimentally confirmed by the ATLAS and CMS experiments. The upgrade of the LHC with the aim to extend its physics programme and to exploit all its possibilities is one of the highest priorities for the European Strategy for Particle Physics. The LHC expects to operate from 2026 to 2035 with an instantaneous luminosity of cm 2 s 1, which corresponds to a seven times increase with respect to the design value. This will allow precision measurements for the 125 GeV Higgs boson, the study of rare Standard Model processes, and searches for phenomena beyond the Standard Model. A major upgrade of the LHC machine and its detectors, called HL-LHC (High Luminosity LHC) is foreseen to achieve successful operation in such conditions. The HL-LHC upgrade opens a big technological challenge to the LHC machine itself as well as to the detectors, in particular to the systems closest to the interaction point. The ATLAS experiment is a proton-proton experiment at the LHC investigating a large variety of particle physics at the TeV energy scale, with the main focus on the electro-weak symmetry breaking mechanism, and physics beyond the Standard Model. The ATLAS Collaboration consists of more than 3000 scientists from 174 institutes in 38 countries. The ATLAS detector layout, composed of symmetrical and concentric sub-detectors, was designed to cover the maximum possible solid angle around the interaction point. The ATLAS detector is composed of a tracking detector immersed into a magnetic field to measure the particles position and momenta, two calorimeters to measure the particle and jet energies, and an spectrometer to detect muons. The current ATLAS tracking detector is composed of gas and solid state silicon detectors. It contains a dedicated vertex detector called Pixel Detector, which consists of four layers of segmented pixel silicon detectors. The Pixel Detector, located closest to the protonproton collision point, has the most stringent requirements of all sub-detector systems. Due vii to the high particle rate it must perform under high radiation levels and at the same time minimize the material budget. The present detector concepts constituting the vertex detector are hybrid modules developed at the cutting edge of the technology. The requirements imposed to the tracking detectors for the HL-LHC are at least one order of magnitude more stringent with respect to LHC in terms of radiation hardness, and number of particle traversing the detector per second and square centimetre. Thus, the HL-LHC ATLAS upgrade is a technological challenge, and it involves an extensive R&D effort. The ATLAS detector plans to install a new all silicon tracker for the HL-LHC upgrade. The final layout is under discussion, and the detector technology to be installed is not decided yet. This is definitely one of those very exciting periods in which technology development is being pushed by the needs in the High Energy Physics community. These periods have been happening since the construction of LEP (Large Electron Positron collider) and LHC leading to the discovery of new technology which afterwards was changing once and forever the world, as the creation of the WWW (World Wide Web), or the utilization of the developed accelerator and sensor technology for tumour treatments. The current hybrid pixel concepts used at the moment in ATLAS are unrivalled in terms of rate and radiation tolerance positioning them as a good candidate for HL-LHC. However, their material budget, production complexity, and their cost have boosted the development of the new detector concepts. The interest of CMOS-based pixel sensors have emerged due to their potential low cost in comparison with standard hybrid pixels and to the large area that must be covered in the outer layers. CMOS-based sensors use an industrial production process with a large throughput, a stringent quality assurance, and they are relatively cheap. They allow small pixel size fabrication, which improves the spatial resolution and the detection of two very near tracks. The possibility to produce smaller thickness of the sensor would also be beneficial for particle identification. Since several years an international community called the ATLAS CMOS Pixel Collaboration is seeking for new radiation-hard pixel sensor concepts, both hybrid and monolithic, based on industrial CMOS processes for HL-LHC. The work presented in this thesis is done within the framework of this collaboration. A novel and promising concept of a depleted monolithic active pixel sensor built on siliconon-insulator within a high voltage process has been fully characterized to evaluate its performance for the future ATLAS HL-LHC upgrade in this thesis. This promising sensor concept would reduce the material budget, pixel size, and cost with respect to hybrid approaches. The silicon dioxide layer used to separate the charge collecting diode from the electronics would reduce the coupling capacitance between charge collecting electrode and readout electronics with respect to other monolithic sensors. Additionally, the accomplishment of the HL-LHC requirements by a monolithic detector would lead to a new era of the high energy physics detectors, with a significant cost advantage and simpler detector assembly. An overview of the LHC, the HL-LHC and its particle physics environment is given in chapter 1. Subsequently, the ATLAS detector layout, and its upgrade towards HL-LHC is deviii scribed. The requirements and challenges of the tracking detectors are emphasized in chapter 1. The interaction of particles with matter is described in chapter 2 in order to introduce the operation of solid state detectors. The chapter proceeds with the building block of solid state detectors for particle tracking from a semiconductor to a pixel detector. The chapter closes with a description of the main features and requirements for the design of vertexing and tracking detectors. The effect of radiation damage on silicon detectors is extensively explained in chapter 3. This chapter covers the radiation effects suffered at the silicon surface, mainly at the electronics, and the radiation effects suffered in the silicon bulk. An overview on the current development and trend of pixel detectors, where hybrids, high voltage and high resistivity CMOS, and depleted monolithic active pixel sensors are covered, is given in chapter 4. This chapter also describes in detail the monolithic prototype under study on this thesis. A validation programme was defined and executed to evaluate the technology. The following chapters describe and discuss the performed measurements, and their results. Chapter 5 describes the radiation hardness characterization of the transistors to ionizing radiation. Total Ionizing Dose effects, Back Gate Effect, and the influence of the radiation induced charges in the silicon dioxide layer on the sensor are discussed. Chapter 6 describes the characterization of the charge collection properties at the diode. Different experimental techniques were used to extract the depletion depth and the electrical field shape on unirradiated and irradiated samples. The leakage current, the charge collected by diffusion and by drift, and hints to the Acceptor Removal effect are measured and discussed. The monolithic prototype under study in this thesis was also characterized in a pion beam test, which is described in chapter 7. The measured charge collection, charge sharing, spatial resolution, and tracking efficiency of the prototype are also explained in this chapter. This work concludes with an extensive summary, providing an outlook towards the future of depleted monolithic active pixel sensors on silicon-on-insulator technology for high energy physics. ix Chapter 1 The Large Hadron Collider and the ATLAS experiment The Large Hadron Collider (LHC) is the largest and most powerful particle accelerator in the world. Scientists at the LHC aim to study of the ultimate constituents of matter and the nature of their interactions. The LHC, which is taking data since 2009, has four main experiments distributed around its ring where the result of the proton-proton and ion-proton collisions are studied. The Standard Model of particle physics, despite its successes coming from confirmed predictions is known to be incomplete. The LHC plans to enlarge its physics programme in the next decades by carrying out a major machine and detectors upgrade called High Luminosity LHC (HL-LHC), scheduled for The upgrades to the detectors imply an enormous technological challenge, specially for the sub-detectors closest to the interaction point. All this is set into context in this chapter. The chapter starts with a description of the physics context nowadays. The LHC accelerator, the HL-LHC upgrade, and the physics programme for is introduced in section 1.2. Section 1.3 describes in detail the present layout of the A Toroidal LHC ApparatuS (ATLAS) detector as well as its scheduled upgrades towards HL-LHC. The chapter closes with an introspection on the requirements and technological challenges to optimize a tracking detector under such an environment as the HL-LHC. 1.1 Physics environment The Standard Model [1] of particle physics is a quantum field theory describing all elementary particles and their interactions. The elementary particles are divided into three main categories: 6 quarks, 6 leptons and 5 bosons (4 force carriers and the Higgs boson) as is shown in figure 1.1 [2]. Leptons are particles with spin 1/2 and -1,0,1 electrical charge, which appear as free particles. Quarks, with spin 1/2 and a fractional electrical charge, are confined in groups of particles called Hadrons. Additionally, quarks carry colour. Leptons and quarks are subdivided into three generations with a notable mass hierarchy. The origin and nature of the mass hierarchy is not explained by the Standard Model. The lightest and most stable particles make up the first generation. All the stable matter in our universe belong to the first generation. Particles from the second and third generation are produced in high energy processes, which 1 Chapter 1 The Large Hadron Collider and the ATLAS experiment shortly decay into first generation particles. The four force carriers mediate the interaction between the fundamental particles. Figure 1.1: Elementary particles in the Standard Model of particle physics [2]. The Standard Model describes three of the four fundamental forces in nature: electromagnetic, weak, and strong force. The electromagnetic force underpins all of chemistry. It acts between all charged particles, and it is mediated by a charge-less photon with spin 1. This mediator couples only to charged particles and will not interact with neutral particles such as itself. Since the photon is massless its interaction range is infinite. The weak force is responsible for the beta decays. It acts between all fermions and it is mediated by three bosons, the charged W ± bosons and the neutral Z boson. These bosons are massive, W ± = ( ± 0.029) GeV, Z = ( ± ) GeV [3, 4, 5, 6], and as a consequence the range of the weak interaction is short. The strong force is responsible for the confinement of quarks in hadrons and also for nuclear interactions. It is mediated by massless, coloured gluons (g) of spin 1. The electromagnetic and the weak force are unified into the electroweak force which builds the electroweak symmetry. However, the fact that the W ± and Z were experimentally measured to be so massive while the photon is massless means that the electroweak symmetry is broken. The mechanism to spontaneously break the electroweak symmetry, which generate the masses of the W ± and the Z boson, is called Higgs-mechanism [7, 8, 9, 10, 11], and it was proposed by Robert Brout, Francois Englert, Peter Higgs, Gerald Guralnik, C. R Hagen and Tom Kibble in The masses of the other particles are generated through the Yukawa interactions with the Higgs scale field [11]. The existence of the Higgs field results in at least one mass spin 0 boson referred to as the Higgs boson. Thus, the Higgs mechanism was postulated to explain the electroweak symmetry breaking, and its hunting has been the highlight of High Energy Physics (HEP) experiments all over the world. The Higgs Boson was discovered at the Conseil Européen pour la Recherche Nucléaire (CERN) in 2012 [12, 13]. On 2013 Francois Englert, 2 1.2 The Large Hadron Collider and Peter Higgs were awarded with the Nobel Prize for their work on the spontaneous symmetry breaking mechanism. The Standard Model is an extremely successful theory, whose predictions have been rigorously tested by a variety of experiments over many decades. Despite its successes, the Standard Model is known to be incomplete. It does not include gravitational interactions, lacks an explanation for dark matter and dark energy, has no mechanism to generate neutrino masses, and cannot describe the matter/anti-matter asymmetry observed in the universe. Thus, there are several theoretical models, not supported by any experimental evidence yet, that go beyond the Standard Model and try to answer some or all of these questions. These are the subject of searches at the LHC for the next decade. 1.2 The Large Hadron Collider The LHC [14] is a 27 km circular proton-proton collider located at CERN at around 100 m underground astride the Franco-Swiss border close to Geneva. The LHC was designed as proton-proton collider mainly due to two reasons. First, hadron collisions provide the possibility to probe a wide range of energies simultaneously and this makes hadron collisions a well suited tool to search for new particles with unpredicted masses. Second, in comparison to electrons, the loss of energy due to the synchrotron radiation is much smaller as is described in section 2.1. This allows to achieve higher collision energies. The LHC provides four interaction points where its four experiments are located as figure 1.2 shows. They are named: Large Hadron Collider beauty (LHCb), A Large Ion Collider Experiment (ALICE), ATLAS and Compact Muon Spectrometer (CMS). LHCb is primarily designed to investigate the b -quark physics and therefore provide an insight into the CP-violation phenomenon while ALICE is dedicated to research in heavy-ion physics and quark gluon plasma formation [15]. ATLAS and CMS are two general purpose experiments, mainly investigating the proton-proton collisions to study the electroweak symmetry breaking mechanism and new physics beyond the Standard Model (SM) like, for example, SUperSYmmetry (SUSY) [16] or extra dimensions. The luminosity (L) is the measure of the ability of a particle accelerator to produce a given number of interactions. The luminosity of a circular collider is given by the amount of particles traversing the interaction point in the detector per area and time: L = n b N 1 N 2 f (1.1) A where n b denotes the number of bunches per beam in the accelerator, N i the number of particles in the bunches of the two beams, f the collision frequency of the bunches and A
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