Novel 3D Pixel Sensors for the Upgrade of the ATLAS Inner Tracker

The ATLAS experiment will undergo a full replacement of its inner detector to face the challenges posed by the High Luminosity upgrade of the Large Hadron Collider (HL-LHC). The new Inner Tracker (ITk) will have to deal with extreme particle fluences. Due to its superior radiation hardness the 3D silicon sensor technology has been chosen to instrument the innermost pixel layer of ITk, which is the most exposed to radiation damage. Three foundries (CNM, FBK, and SINTEF), have developed and fabricated novel 3D pixel sensors to meet the specifications of the new ITk pixel detector. These are produced in a single-side technology on either Silicon On Insulator (SOI) or Silicon on Silicon (Si-on-Si) bonded wafers by etching both n- and p-type columns from the same side. With respect to previous generations of 3D sensors they feature thinner active substrates and smaller pixel cells of 50 × 50 and 25 × 100 µm2. This paper reviews the main design and technological issues of these novel 3D sensors, and presents their characterization before and after exposure to large radiation doses close to the one expected for the innermost layer of ITk. The performance of pixel modules, where the sensors are interconnected to the recently developed RD53A chip prototype for HL-LHC, has been investigated in the laboratory and at beam tests. The results of these measurements demonstrate the excellent radiation hardness of this new generation of 3D pixel sensors that enabled the project to proceed with the pre-production for the ITk tracker.publishedVersio

ATLAS ITk Pixel Detector - Overview

In the high-luminosity era of the Large Hadron Collider, the instantaneous luminosity is expected to reach unprecedented values, resulting in up to 200 proton-proton interactions in a typical bunch crossing. To cope with the resulting increase in occupancy, bandwidth and radiation damage, the ATLAS Inner Detector will be replaced by an all-silicon system, the Inner Tracker (ITk). The innermost part of the ITk will consist of a pixel detector, with an active area of about 13 m2. To deal with the changing requirements in terms of radiation hardness, power dissipation and production yield, several silicon sensor technologies will be employed in the five barrel and endcap layers. Prototype modules assembled with RD53A readout chips have been built to evaluate their production rate. Irradiation campaigns were done to evaluate their thermal and electrical performance before and after irradiation. A new powering scheme – serial – will be employed in the ITk pixel detector, helping to reduce the material budget of the detector as well as power dissipation. This contribution presents the status of the ITk-pixel project focusing on the lessons learned and the biggest challenges towards production, from mechanics structures to sensors, and it will summarize the latest results on closest-to-real demonstrators built using module, electric and cooling services prototypes

Towards the construction of a new tracker for the ATLAS detector at the HL-LHC

Particle tracking is one of the most fundamental aspects of high-energy physics research. By tracking the path of charged particles produced in high-energy collisions, physicists can study the properties of the particles and gain insight into the fundamental forces and interactions that govern the behavior of matter at the smallest scales. One of the most essential components of particle tracking is the interaction of charged particles with matter. When a charged particle passes through matter, it ionizes the atoms and molecules in its path, producing a trail of electrons and ions that can be detected and analyzed. Especially when dealing with high particle rates, semiconductor detectors are commonly used to detect the passage of ionizing particles, generating electrical signals that can be localized with high precision. By analyzing these signals, physicists can reconstruct the path of the particle and determine its properties. The need for high-precision tracking detectors arose in the 1970s with the development of the first colliders and the discovery of short-lived particles. These colliders produced particles with extremely high energies, making it difficult to detect them with traditional detectors. To detect these particles, compact detectors had to be developed and placed as close as possible to the point of interaction. These requirements led to the development of semiconductor detectors. Among semiconductor detectors, silicon detectors have become the most widely used tracking system in high-energy physics experiments. Silicons are ideal for tracking thanks to their low material cost and increasing industrial development in this material processing making them an attractive option for large-scale experiments. In particular, modern techniques have allowed the fabrication of pixel detectors which allow 3D spatial resolutions of the order of 10 μm. For these reasons, silicon pixel detectors are a standard choice in tracking systems in high-energy physics experiments. Pixel detectors have played a crucial role in the advancement of high-energy physics research, and the ATLAS and CMS experiments at the Large Hadron Collider (LHC) are excellent examples of this application. The LHC is the world’s largest and most powerful particle accelerator. Located at the European Organization for Nuclear Research (CERN) in Switzerland, it is designed to accelerate protons and heavy ions to nearly the speed of light and collide them at four experiment sites. The LHC is the culmination of decades of research and development, and its construction and operation represent an enormous technological and scientific achievement. The discovery of the Higgs boson in 2012, a missing piece in the Standard Model particle puzzle, was made possible also thanks to the high-precision tracking capabilities of pixel detectors. However, with an upgrade of the LHC planned, the High-Luminosity LHC, the ATLAS detector needs to be updated to cope with the increase in luminosity, improve the sensitivity of analyses already carried out, and widen the possibility of discoveries. As part of this upgrade, a new all-silicon detector, ITk, will replace the current ATLAS tracking system during the Long Shutdown 3 (2026-2028). The ITk detector will consist of a Pixel detector at a small radius and a large area Strip detector surrounding it. The main challenge facing the ITk detector will be the harder radiation environment, where both the level of radiation in the detector and the number of particles per square centimeter will increase by about a factor of seven. To address the radiation constraints, innovative high radiation-hard 3D pixel detectors will be used for the innermost layer of the ITk Pixel detector. The 3D sensor technology was used for the first time in HEP detectors in 2014 for the upgrade of the current ATLAS Pixel detector: a single layer (IBL) was partially equipped with 3D sensors for a surface of just 375 cm2. However, this first application paved the way for using 3D technology in future LHC detector upgrades. After several years of R&D, in 2020 ATLAS decided to use the 3D technology as baseline for the innermost pixel layer, with a cell size of 25x100 μm2 in the barrel and 50x50 μm2 in the forward region. Despite the decision to use 3D technology in the ITk detector based on first proto- type samples, there was the recommendation to continue with more prototypes and in pre-production the validation of the radiation hardness up to the ultimate fluence of 2 · 1016neq/cm2 and ionizing dose of 10 MGy. This Ph.D. thesis has been developed primarily inside the ITk Pixel project, focus- ing on two main specific aspects: the full qualification to the ultimate fluence of the 3D devices and the qualification of the local supports for the detector forward region. More specifically: Chapter 1 provides an overview of the LHC accelerator, including the machine performance in Run-2. Additionally, this chapter discusses the High Luminosity LHC pro gram, which aims to upgrade the LHC to increase by almost one order of magnitude its collision rate luminosity. The ATLAS experiment is described in detail in Chapter 2, including the detector configuration for Run-3, and the various subsystems that make up the detector. This chapter also discusses the physics goals of the ATLAS experiment, including the Higgs physics, W and Z physics, and SuperSymmetry searches. Chapter 3 provides an introduction to silicon detectors for high-energy physics, including a discussion of particles’ interactions with matter and the pn-junction. This chapter also explores the use of silicon detectors in particle physics experiments. Chapter 4 focuses on the ATLAS Inner Tracker for Phase II, which is part of the ATLAS upgrade program. This chapter also provides an overview of the Calorimeter, Muon Spectrometer, and Trigger and Data Acquisition systems upgrades. Additionally, this chapter discusses the ITk layout and its expected tracking performance. Chapter 5 explores the ITk 3D module qualification, both in Genova Laboratories and during the beam test campaigns carried out at Deutsches Elektronen-Synchrotron (DESY) and CERN. In the Genova laboratory standard measurements such as sensor IV and electronics tuning and characterization are performed, while the main results from the test beam campaigns are the detector efficiencies measurements. In particular, prototype modules, so-called RD53A, were tested in 2020 with an electron beam at DESY, obtaining an efficiency higher than 97% with both unirradiated and irradiated modules. In 2022 pre-production ITk 3D modules were tested with a protons beam at Proton Synchrotron (PS) at CERN, and nearly 99% efficiency was achieved with unirradiated modules, while at Super Proton Synchrotron (SPS) always at CERN, after irradiation the pre-production modules had shown the efficiency of 98,5% (99,9%) with modules placed perpendicularly (titled of 15 degrees) with respect to the beam. Therefore it is shown how the sensors meet the ITk requirement to have a mean efficiency higher than 96% (97%) in the perpendicular (tilted) configuration after irradiation. Chapter 6 focuses on various aspects of the ITk Outer Endcap Local Support structures. The section includes an overview of the Outer Endcap, details on the local support structures, and their manufacturing and assembly process. It also covers studies on foam density, metrology, and thermal properties, as well as information on the ITk Production Database, used to store all the results of the test done during the ITk detector construction. Finally, this section discusses the impact of detector active components misalignment on tracking performances, including the alignment procedure and misalignment studies with the ITk layout

Towards the construction of a new tracker for the ATLAS detector at the HL-LHC

This thesis work focuses on the upgrade of the ATLAS tracking system, known as ITk, for the High Luminosity period of the LHC. The thesis primarily focuses on two main aspects: the characterization of ITk Pixel End-Cap local supports and the characterization of ITk Pixel 3D modules. For the local supports, which provide mechanical stability and cooling to the Pixel detector in the Outer Endcap region, density distribution measurements of carbon foam blocks used in Half-Rings revealed homogeneity with a maximum dispersion of 13 percent. Thermo-mechanical property measurements confirmed compliance with deformation specifications, with absolute deformation variations less than 2 μm per degree. An effective detection study of anomalies in heat propagation within the local supports was conducted, ensuring reliability in identifying dysfunctions. Regarding the ITk 3D pixel modules, the characterization included the qualification of RD53A modules and pre-production modules. The RD53A modules demonstrated high hit efficiency both before and after irradiation, with hit efficiencies exceeding 96% and 98% after irradiation for different pixel pitches. The pre-production modules utilizing 3D sensors and ITkPixV1.1 readout chips showed excellent performance, achieving nearly 99% efficiency in the unirradiated configuration and 98.5% efficiency (99.9% for tilted modules) after irradiation

Test of ITk 3D sensor pre-production modules with ITkPixv1.1 chip

ITk detector, the new ATLAS tracking system at High Luminosity LHC, will be equipped with 3D pixel sensor modules in the innermost layer (L0). The pixel cell dimensions will be either 25x100 µm2 (barrel) or 50x50 µm2 (endcap), with one read-out electrode at the centre of a pixel and four bias electrodes at the corners. Sensors from pre-production wafers (50x50 µm2) produced by FBK have been bump bonded to ITkPixv1.1 chip at IZM. Bare modules have been assembled in Genoa on Single Chip Cards and characterized in laboratory and at test beam

Test of ITk 3D sensor pre-production modules with ITkPixv1.1 chip

ITk detector, the new ATLAS tracking system at High Luminosity LHC, will be equipped with 3D pixel sensor modules in the innermost layer (L0). The pixel cell dimensions will be either 25x100 µm2 (barrel) or 50x50 µm2 (endcap), with one read-out electrode at the centre of a pixel and four bias electrodes at the corners. Sensors from pre-production wafers (50x50 µm2) produced by FBK have been bump bonded to ITkPixv1.1 chip at IZM. Bare modules have been assembled in Genoa on Single Chip Cards and characterized in laboratory and at test beam

Qualification of the first preproduction 3D FBK sensors with ITkPixV1\

The ITk detector, the new ATLAS silicon tracking system for High Luminosity LHC, will be equipped with 3D pixel sensor modules in the innermost layer (L0). The pixel cell dimensions will be 25x100 μm² in the barrel and 50x50 μm² in the end-caps, with one read-out electrode at the centre of each pixel and four bias electrodes at the corners. Sensors from pre-production wafers (50x50 μm²) produced by FBK have been bump bonded to ITkPixV1.1 chips at IZM. Bare modules have been assembled in Genoa on Single Chip Cards and characterized in laboratory and at test beams. Few of these modules have been irradiated in Bonn and at the CERN IRRAD facility. Preliminary results of their characterization after irradiation will be shown, including measurements performed during SPS test beam campaigns in Summer 2022

Qualification of the first pre-production 3D FBK sensors with ITkPixV1 readout chip

The ITk detector, the new ATLAS silicon tracking system for the High Luminosity LHC (HL-LHC), will be equipped with 3D pixel sensor modules in the innermost layer (L0). The pixel cell dimensions will be 25×100 μm$^{2}$ in the barrel and 50×50 μm$^{2}$ in the end-caps, with one readout electrode at the centre of each pixel and four bias electrodes at the corners. Sensors from pre-production wafers (50×50 μm$^{2}$) produced by FBK have been bump-bonded to ITkPixV1.1 chips at IZM. Bare modules have been assembled in Genoa on Single Chip Cards (SCCs) and characterized in laboratory measurements and in test beam campaigns. Some of these modules have been irradiated in Bonn and at the CERN IRRAD facility. Preliminary results of their characterization after irradiation are shown, including measurements performed during test beam campaigns at CERN SPS in Summer 2022

Novel 3D Pixel Sensors for the Upgrade of the ATLAS Inner Tracker

The ATLAS experiment will undergo a full replacement of its inner detector to face the challenges posed by the High Luminosity upgrade of the Large Hadron Collider (HL-LHC). The new Inner Tracker (ITk) will have to deal with extreme particle fluences. Due to its superior radiation hardness the 3D silicon sensor technology has been chosen to instrument the innermost pixel layer of ITk, which is the most exposed to radiation damage. Three foundries (CNM, FBK, and SINTEF), have developed and fabricated novel 3D pixel sensors to meet the specifications of the new ITk pixel detector. These are produced in a single-side technology on either Silicon On Insulator (SOI) or Silicon on Silicon (Si-on-Si) bonded wafers by etching both n- and p-type columns from the same side. With respect to previous generations of 3D sensors they feature thinner active substrates and smaller pixel cells of 50 × 50 and 25 × 100 µm2. This paper reviews the main design and technological issues of these novel 3D sensors, and presents their characterization before and after exposure to large radiation doses close to the one expected for the innermost layer of ITk. The performance of pixel modules, where the sensors are interconnected to the recently developed RD53A chip prototype for HL-LHC, has been investigated in the laboratory and at beam tests. The results of these measurements demonstrate the excellent radiation hardness of this new generation of 3D pixel sensors that enabled the project to proceed with the pre-production for the ITk tracker

Novel 3D Pixel Sensors for the Upgrade of the ATLAS Inner Tracker

The ATLAS experiment will undergo a full replacement of its inner detector to face the challenges posed by the High Luminosity upgrade of the Large Hadron Collider (HL-LHC). The new Inner Tracker (ITk) will have to deal with extreme particle fluences. Due to its superior radiation hardness the 3D silicon sensor technology has been chosen to instrument the innermost pixel layer of ITk, which is the most exposed to radiation damage. Three foundries (CNM, FBK, and SINTEF), have developed and fabricated novel 3D pixel sensors to meet the specifications of the new ITk pixel detector. These are produced in a single-side technology on either Silicon On Insulator (SOI) or Silicon on Silicon (Si-on-Si) bonded wafers by etching both n- and p-type columns from the same side. With respect to previous generations of 3D sensors they feature thinner active substrates and smaller pixel cells of 50 × 50 and 25 × 100 µm2. This paper reviews the main design and technological issues of these novel 3D sensors, and presents their characterization before and after exposure to large radiation doses close to the one expected for the innermost layer of ITk. The performance of pixel modules, where the sensors are interconnected to the recently developed RD53A chip prototype for HL-LHC, has been investigated in the laboratory and at beam tests. The results of these measurements demonstrate the excellent radiation hardness of this new generation of 3D pixel sensors that enabled the project to proceed with the pre-production for the ITk tracker