Silicon Pixel Sensors Research Articles

Cooling and noise performance of the ATLAS ITk strip system tests

The current tracking system of the ATLAS detector will be replaced by the new Inner Tracker (ITk) to cope with the challenging conditions expected at the High Luminosity Large Hadron Collider. This new tracking system will be an all-silicon detector consisting of silicon pixel sensors in the inner most layers and silicon micro-strips sensors in the outer layers. A central barrel section surrounds the interaction point and two end-cap sections cover the forward regions. This contribution focuses on the results of the system tests for the ITk strips detector in which several close-to-final detector components are evaluated before production. The barrel system tests structure at CERN consists of up to eight staves, while for the end-caps a structure at DESY is loaded with up to twelve petals. Staves (petals) consist of core structures loaded with square (trapezoid) shaped sensors of various lengths and strip pitches, and include readout and power electronics mounted on top of the sensors. Objects are mechanically held in place in a support structure and connected to the electrical, optical and cooling services as true as possible to the final system. With these setups, it is possible to validate the detector design, including the verification of the detector data acquisition, powering and cooling. This article gives an overview of the system tests and summarizes their current status by showing a selection of test results.

Performance of 3D trench silicon pixel sensors irradiated up to [formula omitted]

Tracking particles at extreme fluences requires the accurate measurement of the charged particle timing at the pixel level in order to cope with the increased occupancy of HL-LHC experiments. Maintaining this precision throughout the operational life of the detector is crucial. This work demonstrates that the 55×55μm2 wide 150μm thick 3D trench-type pixels, developed by the TimeSPOT Collaboration, maintain the performance of non-irradiated sensors even after exposure to fluences as high as 1⋅10171MeVneqcm−2. The preliminary results of a beam test characterization using minimum ionizing particles at the SPS North area beam facility reveal that the charge collection and the time resolution of the irradiated sensors are comparable to those of non-irradiated ones, by increasing the operational bias voltage. A minor reduction in detection efficiency, approximately 4%, is observed post-irradiation. Currently, 3D trench-type pixels are among the fastest pixel detectors available for tracking charged particles and hold significant promise for future tracking system upgrades. This preliminary evaluation suggests their suitability for use in even harsher radiation environments than High Luminosity Large Hadron Collider (HL-LHC) such as future experiments at the Future Circular Hadron Collider (FCC-hh).

Beam test performance studies of CMS Phase-2 Outer Tracker module prototypes

A new tracking detector will be installed as part of the Phase-2 upgrade of the CMS detector for the high-luminosity LHC era. This tracking detector includes the Inner Tracker, equipped with silicon pixel sensor modules, and the Outer Tracker, consisting of modules with two parallel stacked silicon sensors. The Outer Tracker front-end ASICs will be able to correlate hits from charged particles in these two sensors to perform on-module discrimination of transverse momenta (p T). The p T information is generated at a frequency of 40 MHz and will be used in the Level-1 trigger decision of CMS. Prototypes of the so-called 2S modules were tested at the Test Beam Facility at DESY Hamburg between 2019 and 2020. These modules use the final front-end ASIC, the CMS Binary Chip (CBC), and for the first time the Concentrator Integrated Circuit (CIC), optical readout and on-module power conversion. In total, seven modules were tested, one of which was assembled with sensors irradiated with protons. An important aspect was to show that it is possible to read out modules synchronously. A cluster hit efficiency of about 99.75 % was achieved for all modules. The CBC p T discrimination mechanism has been verified to work together with the CIC and optical readout. The measured module performance meets the requirements for operation in the upgraded CMS tracking detector.

3D silicon pixel sensors for the CMS experiment tracker upgrade

The upgrade program of the CMS experiment (CMS Collaboration, 2024) in view of the LHC High Luminosity phase (HL–LHC) includes a replacement of the silicon pixel tracker. This is necessary to guarantee the same tracking performance of the current detector under harsher operating conditions, including higher radiation fluences and hit rates. The first layer of the central (barrel) detector will be located at radial distance of 3 cm from the interaction point where the radiation fluence after 7 years of operation will be 1.9×1016neq/cm2. Following an extensive R&D program, comprising both laboratory and test beam measurements, 3D sensors will be employed in this layer due to their higher radiation tolerance and lower power consumption after irradiation. Test beam measurements after irradiation up to 1.6×1016neq/cm2 showed a hit detection efficiency larger than 96% at normal incidence with less than 2% masked pixels, for applied bias voltages between 100 V and 130 V.

Embedded FPGA developments in 130 nm and 28 nm CMOS for machine learning in particle detector readout

Embedded field programmable gate array (eFPGA) technology allows the implementation of reconfigurable logic within the design of an application-specific integrated circuit (ASIC). This approach offers the low power and efficiency of an ASIC along with the ease of FPGA configuration, particularly beneficial for the use case of machine learning in the data pipeline of next-generation collider experiments. An open-source framework called “FABulous” was used to design eFPGAs using 130 nm and 28 nm CMOS technology nodes, which were subsequently fabricated and verified through testing. The capability of an eFPGA to act as a front-end readout chip was assessed using simulation of high energy particles passing through a silicon pixel sensor. A machine learning-based classifier, designed for reduction of sensor data at the source, was synthesized and configured onto the eFPGA. A successful proof-of-concept was demonstrated through reproduction of the expected algorithm result on the eFPGA with perfect accuracy. Further development of the eFPGA technology and its application to collider detector readout is discussed.

Measurement of Si pixel sensor alignment for the ALICE ITS detector

Abstract A Large Ion Collider Experiment (ALICE) experiment is one of the four experiments at the Large Hadron Collider (LHC) designed to investigate the status of matter under very high energy densities produced during heavy-ion collisions. The ALICE inner tracking system (ITS) consists of seven concentric cylindrical layers of monolithic silicon pixel sensors known as ALICE pixel detector (ALPIDE). The sensors are used to reconstruct the paths of charged particles generated in the collisions. The sensor alignment of the detector must be adjusted to a high precision standard. The adjustment objective is to obtain a detector that can undertake high-resolution measurements. This paper introduces a method for measuring the reference markers utilized in sensor alignment determination. Markers engraved at the chip corners have been detected using the Hough transform, Canny edge detection, and template matching techniques. The distances between two markers are measured to determine the accuracy of the pixel sensor alignment before and after assembly. The proposed methods exhibit an accuracy exceeding 99% and demonstrate high speed analysis. The average processing times for detecting the circle and cross markers are 105.9 ms/image and 113.8 ms/image, respectively. The sensor alignment of the detector must be adjusted to a high precision standard. However, recent studies have shown deviations of up to 5 μ m above the desired value in the measured sensor position. Such deviations do not represent a major issue, nevertheless it is important to measure them in order to speed-up and make more accurate the recursive track-based alignment procedure used to reconstruct the position of each pixel sensor in the tracking detector. The proposed method offers a promising solution to deliver precise and rapid measurements for a large number of examined objects.

Characterisation of highly irradiated 3D trench silicon pixel sensors for 4D tracking with 10 ps timing accuracy

3D trench silicon pixel sensors, recently developed by the TimeSPOT collaboration, have shown excellent performance in terms of spatial resolution, timing precision and detection efficiency. The combination of these three features make them one of the best candidate for inner tracking detectors operating in high luminosity hadron colliders experiments. This article presents systematic characterisations of these devices made with minimum ionising particles on irradiated sensors with neutrons up to 2.5 ⋅ 1016 1 MeV neq cm−2. The results show that 3D trench pixels have extremely high resistance to radiation. The measured time resolution and the detection efficiency of irradiated sensors match those of non-irradiated ones if a slightly higher bias voltage, few tens of Volts, is applied to the pixels. As of today, 3D trench pixels are the only sensors capable of achieving 10 ps time resolution after being irradiated at extremely high fluences, extending by far the capabilities of future tracking systems of HEP experiments operating under extreme conditions.

Ultra-large scale array silicon pixel sensors with uniform and low leakage current for advanced X-ray light sources

The development of silicon pixel sensors (SPS) with high operating voltage, low leakage currents, and large arrays can contribute to improving the energy and spatial resolution of advanced X-ray light source detection systems. The Future Detection System comprises a hybrid-pixel detector with a collective resolution of 2048 × 2048 pixels, each measuring 100 μm× 100 μm. It consists of 16 p-i-n SPSs, where each sensor has an array size of 1024× 256 pixels. In this paper, the design of the pixel and guard rings is optimized to achieve uniform and ultra-low pixels leakage currents under high operating voltage. The high leakage current uniformity of the designed sensor is demonstrated through several tests conducted on small scale array SPS. The leakage current of the tested pixels is in the range of 0.50–0.55 pA at room temperature with less than 5% leakage deviation on the whole array. It is accompanied by breakdown voltages greater than 1000 V. The optimized 256× 128 pixel SPS showcases uniform leakage currents below 0.6 pA per pixel at room temperature, as evidence in both the edge and central pixels. The 1024× 256 pixels SPS is then manufactured based on the optimized design results. The obtained results show that the breakdown voltage is greater than 1000 V and the leakage current of the pixel is less than 2.5 pA. In addition, the interpixel capacitance of the sensor also reach an ultra-low level of 16 fF. This study paves the way for the development of a robust semiconductor device solution for applications where ultra-fast and large panel-pixel detectors in advanced X-ray light source detection systems are required.

A novel silicon pixel sensor for beam monitoring applications at heavy-ion accelerators

This paper describes a silicon pixel sensor for non-interceptive real-time beam monitoring at heavy-ion accelerators. The total size of the sensor is 4 mm × 5 mm. It has 64 (row) × 120 (column) square pixels, each single of which is in the size of 40 μm × 40 μm. With the exposed sensing pad, this sensor can directly collect the charge in the media over the pixels. The in-pixel circuit mainly consists of a low-noise Charge Sensitive Amplifier (CSA) to establish the signal for the energy reconstruction and a discriminator with a Time-to-Amplitude Converter (TAC) for the Time of Arrival (TOA) measurement. The analog signal from each pixel is accessible through time-shared multiplexing over the entire pixel array. This paper will discuss the design of this IMPix-S1 sensor.

Expected performance of the High Energy Particle Detector (HEPD-02) tracking system on board of the second China Seismo-Electromagnetic Satellite

The China Seismo-Electromagnetic Satellite (CSES) is a scientific space program that aims to deepen the comprehension of the time correlation between the main earthquake shocks and an increase in the electron flux in the inner Van Allen belt. For this purpose, the suite of payloads on board the second CSES satellite (CSES-02) consists of several detectors. The launch of CSES-02 is foreseen in 2024. The present work is focused on the second generation High-Energy Particle Detector (HEPD-02), developed by Limadou Italian collaboration, and in particular on its tracking system (Direction Detector — DD) to reconstruct the incident particle angle. The DD is made of five standalone tracking modules, consisting, as first and unique case, of monolithic silicon pixel sensors (MAPS), namely ALTAI. This chip was developed in the framework of the ALICE (A Large Ion Collider Experiment) ITS sensor studies. The results on the DD performance are discussed based on the demanding requirements of space applications.

Breakdown performance of guard ring designs for pixel detectors in 150 nm CMOS technology

Silicon pixel sensors manufactured using commercial CMOS processes are promising instruments for high-energy particle physics experiments due to their high yield and proven radiation hardness. As one of the essential factors for the operation of detectors, the breakdown performance of pixel sensors constitutes the upper limit of the operating voltage. Six types of passive CMOS test structures were fabricated on high-resistivity wafers. Each of them features a combination of different inter-pixel designs and sets of floating guard rings, which differ from each other in the geometrical layout, implantation type, and overhang structure. A comparative study based on leakage current measurements in the sensor substrate of unirradiated samples was carried out to identify correlations between guard ring designs and breakdown voltages. TCAD simulations using the design parameters of the test structures were performed to discuss the observations and, together with the measurements, ultimately provide design features targeting higher breakdown voltages.

ATLAS ITk strip detector for the Phase-II Upgrade

The inner detector of the present ATLAS experiment has been designed and developed to function in the environment of the present Large Hadron Collider (LHC). At the ATLAS Phase-II Upgrade, the particle densities and radiation levels will exceed current levels by a factor of ten. The instantaneous luminosity is expected to reach unprecedented values, resulting in up to 200 proton-proton interactions in a typical bunch crossing. The new detectors must be faster and they need to be more highly segmented. The sensors used also need to be far more resistant to radiation, and they require much greater power delivery to the front-end systems. At the same time, they cannot introduce excess material which could undermine tracking performance. For those reasons, the inner tracker of the ATLAS detector was redesigned and will be rebuilt completely. The ATLAS Upgrade Inner Tracker (ITk) consists of several layers of silicon particle detectors. The innermost layers will be composed of silicon pixel sensors, and the outer layers will consist of silicon microstrip sensors. This contribution focuses on the strip region of the ITk. The central part of the strip tracker (barrel) will be composed of rectangular short (∼2.5 cm) and long (∼5 cm) strip sensors. The forward regions of the strip tracker (end-caps) consist of six disks per side, with trapezoidal shaped sensors of various lengths and strip pitches. After the completion of final design reviews in key areas, such as Sensors, Modules, Front-End electronics, and ASICs, a large scale prototyping program has been completed in all areas successfully. We present an overview of the Strip System and highlight the final design choices of sensors, module designs and ASICs. We will summarise results achieved during prototyping and the current status of pre-production and production on various detector components, with an emphasis on QA and QC procedures.

The IMPix-S, a Silicon Pixel Sensor for Gaseous Detector at HIRFL and HIAF

The IMPix-S, a Silicon Pixel Sensor for Gaseous Detector at HIRFL and HIAF

Evaluation of planar silicon pixel sensors with the RD53A readout chip for the Phase-2 Upgrade of the CMS Inner Tracker

The Large Hadron Collider at CERN will undergo an upgrade in order to increase its luminosity to 7.5 × 1034 cm-2s-1. The increased luminosity during this High-Luminosity running phase, starting around 2029, means a higher rate of proton-proton interactions, hence a larger ionizing dose and particle fluence for the detectors. The current tracking system of the CMS experiment will be fully replaced in order to cope with the new operating conditions. Prototype planar pixel sensors for the CMS Inner Tracker with square 50 μm × 50 μm and rectangular 100 μm × 25 μm pixels read out by the RD53A chip were characterized in the lab and at the DESY-II testbeam facility in order to identify designs that meet the requirements of CMS during the High-Luminosity running phase. A spatial resolution of approximately 3.4 μm (2 μm) is obtained using the modules with 50 μm × 50 μm (100 μm × 25 μm) pixels at the optimal angle of incidence before irradiation. After irradiation to a 1 MeV neutron equivalent fluence of Φeq = 5.3 × 1015 cm-2, a resolution of 9.4 μm is achieved at a bias voltage of 800 V using a module with 50 μm × 50 μm pixel size. All modules retain a hit efficiency in excess of 99% after irradiation to fluences up to 2.1 × 1016 cm-2. Further studies of the electrical properties of the modules, especially crosstalk, are also presented in this paper.

The first test of a monolithic silicon pixel detector ASIC for the detection of electromagnetic showers of TeV energy in the FASER experiment at the LHC

The FASER experiment at the LHC is designed to look for new, long-lived fundamental particles. A W-Si preshower detector is currently under construction, with the objective of enabling the discrimination of photon pairs with O(TeV) energies and separation down to 200 μm, extending the discovery potential of the experiment. The detector will be based on a new monolithic silicon pixel sensor in 130 nm SiGe BiCMOS. The first test of the ASIC from a pre-production run shows that the building blocks of the monolithic silicon pixel detector are functional.

The quality assessment of the MPD new inner tracker based on thin silicon pixel sensors with large area

The quality assessment of the MPD new inner tracker based on thin silicon pixel sensors with large area

The Pixel Luminosity Telescope: a detector for luminosity measurement at CMS using silicon pixel sensors

The Pixel Luminosity Telescope is a silicon pixel detector dedicated to luminosity measurement at the CMS experiment at the LHC. It is located approximately 1.75 m from the interaction point and arranged into 16 “telescopes”, with eight telescopes installed around the beam pipe at either end of the detector and each telescope composed of three individual silicon sensor planes. The per-bunch instantaneous luminosity is measured by counting events where all three planes in the telescope register a hit, using a special readout at the full LHC bunch-crossing rate of 40 MHz. The full pixel information is read out at a lower rate and can be used to determine calibrations, corrections, and systematic uncertainties for the online and offline measurements. This paper details the commissioning, operational history, and performance of the detector during Run 2 (2015–18) of the LHC, as well as preparations for Run 3, which will begin in 2022.

Ultra-high voltage silicon pixel sensor with less soft-breakdown for X-ray free electron laser

X-ray free electron laser (XFEL) requires operation of an advanced silicon pixel sensor (SPS) under ultra-high voltage for detection of fast electron–hole pairs. In this study, a high-voltage tolerant SPS is designed and fabricated by considering suppressing the soft-breakdown of the current collection ring (CCR) and increasing the breakdown voltage of the pixel array after dicing the sensor. It is found that a soft-breakdown behavior of CCR in one diced SPS occurred before a hard breakdown happening in the pixel and it may cause the degradation of maximum hard breakdown voltage in pixel region, which, in particular, turns worse as the increasing of the number of guard rings. An optimized method with the expansion of the non-sensitive area is used to effectively prevent the occurrence of soft-breakdown and increase the operation voltage of the SPS. Ultimately, a high performance SPS with a maximum breakdown voltage exceeding 1000 V and reversed leakage of ∼1.5 pA per pixel at bias 500 V is obtained. Together with the optimized multi-guard ring structure, a record ultra-high avalanche breakdown voltage of over 2100 V is obtained, which provides a promising approach to develop ultra-fast detector in an XFEL.

Cluster-Locating Algorithm Based on Deep Learning for Silicon Pixel Sensors.

The application of silicon pixel sensors provides an excellent signal-to-noise ratio, spatial resolution, and readout speed in particle physics experiments. Therefore, high-performance cluster-locating technology is highly required in CMOS-sensor-based systems to compress the data volume and improve the accuracy and speed of particle detection. Object detection techniques using deep learning technology demonstrate significant potential for achieving high-performance particle cluster location. In this study, we constructed and compared the performance of one-stage detection algorithms with the representative YOLO (You Only Look Once) framework and two-stage detection algorithms with an RCNN (region-based convolutional neural network). In addition, we also compared transformer-based backbones and CNN-based backbones. The dataset was obtained from a heavy-ion test on a Topmetal-M silicon pixel sensor at HIRFL. Heavy-ion tests were performed on the Topmetal-M silicon pixel sensor to establish the dataset for training and validation. In general, we achieved state-of-the-art results: 68.0% AP (average precision) at a speed of 10.04 FPS (Frames Per Second) on Tesla V100. In addition, the detection efficiency is on the same level as that of the traditional Selective Search approach, but the speed is higher.

Development of novel low-mass module concepts based on MALTA monolithic pixel sensors

The MALTA CMOS monolithic silicon pixel sensors has been developed in the Tower 180 nm CMOS imaging process. It includes an asynchronous readout scheme and complies with the ATLAS inner tracker requirements for the HL-LHC. Several 4-chip MALTA modules have been built using Al wedge wire bonding to demonstrate the direct transfer of data from chip-to-chip and to read out the data of the entire module via one chip only. Novel technologies such as Anisotropic Conductive Films (ACF) and nanowires have been investigated to build a compact module. A lightweight flex with 17 μm trace spacing has been designed, allowing compact packaging with a direct attachment of the chip connection pads to the flex using these interconnection technologies. This contribution shows the current state of our work towards a flexible, low material, dense and reliable packaging and modularization of pixel detectors.