CMOS Monolithic Active Pixel Sensors Research Articles

Response of the MIMOSIS-1 CMOS Monolithic Active Pixel Sensor to particle beams with different dE/dx

The ultra-thin and highly granular CMOS Monolithic Active Pixel Sensors (MAPS) are typically optimized for high rate high precision tracking, which implies the use of a very thin active medium and digital readout. Both features hamper using the devices for identifying low momentum particles by means of dE/dx. Still, MAPS feature charge sharing and typically clusters of more than one fired pixel per impinging particles are formed. It was previously shown that the number of fired pixels per cluster scales with the dE/dx, which allowed identifying highly ionizing nuclear fragments [1]. Assuming a sufficiently strong response to different dE/dx, this approach could also be considered for distinguishing minimum ionizing particles (MIP) from light fragments like alpha particles in tracking detectors. In this work, we study this response with particle beams with a dE/dx of up to four times the ones of MIPs, for non-irradiated and irradiated chips, with different sensing nodes as implemented in the MIMOSIS-1 prototype used for the vertex detector of the CBM experiment.

First results of the upgraded ALICE Inner Tracking System in LHC Run 3

Major upgrades of the ALICE experiment at CERN were completed during the LHC Long Shutdown 2 (2019–2021). The ALICE detector is currently taking data and has been doing so from the start of the third period of operation of the LHC (Run 3) on July 5th, 2022. One key part of these upgrades is the new Inner Tracking System (ITS2), a full silicon-pixel vertexing and tracking detector constructed entirely with CMOS monolithic active pixel sensors (ALPIDE). The ITS2 consists of three inner layers (50 μm thick sensors) and four outer layers (100 μm thick sensors) covering 10 m2 and containing 12.5 billion pixels with a pixel pitch of 27 μ m × 29 μm. It offers a significant improvement in impact-parameter resolution and tracking efficiency, thanks to the increased granularity, the very low material budget (0.35% X0 /layer in the inner barrel) as well as a smaller beam pipe radius.The ITS2 was successfully installed in the ALICE experiment in May 2021, followed by a period of comprehensive on-site commissioning, before starting data taking in July 2022. In this paper, the detector construction and commissioning will be introduced briefly. The performance results from the first phase of proton-proton collisions recorded to date in LHC Run 3 will be discussed in detail, which include detector calibration, long-term evolution of the ALPIDE sensor threshold and noise, and the first measurement of the impact parameter.

Tolerance of the MIMOSIS-1 CMOS Monolithic Active Pixel Sensor to ionizing radiation

MIMOSIS is a CMOS Monolithic Active Pixel Sensor (CPS) developed to equip the Micro Vertex Detector (MVD) of the Compressed Baryonic Matter (CBM) experiment at FAIR/GSI. The sensor will combine a spatial resolution of -5 µm with a time resolution of 5 µs and provide a peak hit rate capability of ∼80 MHz/cm2. To fulfil its task, MIMOSIS will have to withstand ionising radiation doses of ∼5 MRad and fluences of ∼7 × 1013 neq/cm2 per year of operation.The paper summarises major requirements of the CBM-MVD and compares them to the detection performances of the first full scale prototype, called MIMOSIS-1, recently evaluated in the laboratory and with particle beams. The tolerance of the sensor to the expected ionising radiation load was evaluated; the paper describes the measurements performed and their outcome.

Results from single event effect tests with MIMOSIS-1

Being installed as close as 5.5 mm to the beam axis, the Micro Vertex Detector (mvd) of the cbm experiment will be exposed to a sizable flow of heavy beam ions and nuclear fragments. The cmos Monolithic Active Pixel Sensor for the mvd, mimosis, must resist the related heavy ion impacts without permanent damage or frequent interrupt of operation as caused by single event effects (see). We motivate the requirements on the sensor and introduce our concept for protecting the device against sees. Moreover, we report the results of a related test campaign carried out with the first full size sensor prototype, mimosis-1, and different heavy ion beams at gsi.

Performance of the ALICE upgraded inner tracking system

Major upgrades of the ALICE (A Large Ion Collider Experiment) detector are underway and will be completed during the LHC long shutdown 2 in order to start operation in 2022 for LHC run 3. One key part of this upgrade is the new Inner Tracking System (ITS2), a full silicon-pixel detector constructed entirely with CMOS monolithic active pixel sensors. The upgraded ITS2 detector consists of three inner layers (50 μm thick sensors) and four outer layers (100 μm thick sensors) covering 10 m2 and containing 12.5 billion pixels with a pixel pitch of 27 μm × 29 μm. Compared with the silicon tracking system used during the LHC run 1 and run 2, the increased granularity, the very low material budget (0.35% X 0/layer in the inner barrel) as well as a smaller beam pipe radius, will result in a significant improvement of impact-parameter resolution and tracking efficiency. The assembly of the detector and services finished in December 2019, and the detector was fully installed in the ALICE experiment in May 2021. A comprehensive commissioning phase (on the surface) was completed in December 2020 to validate the detector performance. In this paper, an overview of the design and construction, as well as the performance of the ITS2 studied from the on-surface commissioning will be discussed in detail.

Impact of X-ray induced radiation damage on FD-MAPS of the ARCADIA project

Recent advancements in Monolithic Active Pixel Sensors (MAPS) demonstrated the ability to operate in high radiation environments of up to multiple kGy’s, which increased their appeal as sensors for high-energy physics detectors. The most recent example in such application is the new ALICE inner tracking system, entirely instrumented with CMOS MAPS, that covers an area of about 10 m2. However, the full potential of such devices has not yet been fully exploited, especially in respect of the size of the active area, power consumption, and timing capabilities.The ARCADIA project is developing Fully Depleted (FD) MAPS with an innovative sensor design, that uses a proprietary processing of the backside to improve the charge collection efficiency and timing over a wide range of operational and environmental conditions. The innovative sensor design targets very low power consumption, of the order of 20 mW cm−2 at 100 MHz cm−2 hit flux, to enable air-cooled operations of the sensors. Another key design parameter is the ability to further reduce the power regime of the sensor, down to 5 mW cm−2 or better, for low hit rates like e.g. expected in space experiments. In this contribution, we present a comparison between the detector characteristics predicted with Technology Computer Aided Design (TCAD) simulations and the ones measured experimentally. The comparison focuses on the current-voltage (IV) and capacitance-voltage (CV) characteristics, as well as noise estimated from in-pixel capacitances of passive/active pixel matrices. In view of the targeted applications of this technology, an emphasis is set on the modeling of X-ray induced radiation damage at the Si-SiO2 interface and the impact on the in-pixel sensor capacitance. The so-called new Perugia model has been used in the simulations to predict the sensor performance after total ionizing doses of up to 10 Mrad.

Deep Learning Signal Discrimination for Improved Sensitivity at High Specificity for CMOS Intraoperative Probes.

The challenge in delineating the boundary between cancerous and healthy tissue during cancer resection surgeries can be addressed with the use of intraoperative probes to detect cancer cells labeled with radiotracers to facilitate excision. In this study, deep learning algorithms for background gamma ray signal rejection were explored for an intraoperative probe utilizing CMOS monolithic active pixel sensors optimized toward the detection of internal conversion electrons from n}{}^{99m}nTc. Two methods utilizing convolutional neural networks (CNNs) were explored for beta-gamma discrimination: 1) classification of event clusters isolated from the sensor array outputs (SAOs) from the probe and 2) semantic segmentation of event clusters within an acquisition frame of an SAO which provides spatial information on the classification. The feasibility of the methods in this study was explored with several radionuclides including 14C, 57Co, and n}{}^{99m}nTc. Overall, the classification deep network is able to achieve an improved area under the curve (AUC) of the receiver operating characteristic (ROC), giving 0.93 for 14C beta and n}{}^{99m}nTc gamma clusters, compared to 0.88 for a more conventional feature-based discriminator. Further optimization of the lower left region of the ROC by using a customized AUC loss function during training led to an improvement of 31% in sensitivity at low false positive rates compared to the conventional method. The segmentation deep network is able to achieve a mean dice score of 0.93. Through the direct comparison of all methods, the classification method was found to have a better performance in terms of the AUC.

Helium radiography with a digital tracking calorimeter—a Monte Carlo study for secondary track rejection

Radiation therapy using protons and heavier ions is a fast-growing therapeutic option for cancer patients. A clinical system for particle imaging in particle therapy would enable online patient position verification, estimation of the dose deposition through range monitoring and a reduction of uncertainties in the calculation of the relative stopping power of the patient.Several prototype imaging modalities offer radiography and computed tomography using protons and heavy ions. A Digital Tracking Calorimeter (DTC), currently under development, has been proposed as one such detector. In the DTC 43 longitudinal layers of laterally stacked ALPIDE CMOS monolithic active pixel sensor chips are able to reconstruct a large number of simultaneously recorded proton tracks.In this study, we explored the capability of the DTC for helium imaging which offers favorable spatial resolution over proton imaging. Helium ions exhibit a larger cross section for inelastic nuclear interactions, increasing the number of produced secondaries in the imaged object and in the detector itself. To that end, a filtering process able to remove a large fraction of the secondaries was identified, and the track reconstruction process was adapted for helium ions.By filtering on the energy loss along the tracks, on the incoming angle and on the particle ranges, 97.5% of the secondaries were removed. After passing through 16 cm water, 50.0% of the primary helium ions survived; after the proposed filtering 42.4% of the primaries remained; finally after subsequent image reconstruction 31% of the primaries remained. Helium track reconstruction leads to more track matching errors compared to protons due to the increased available focus strength of the helium beam. In a head phantom radiograph, the Water Equivalent Path Length error envelope was 1.0 mm for helium and 1.1 mm for protons. This accuracy is expected to be sufficient for helium imaging for pre-treatment verification purposes.

Upgrading the Inner Tracking System and the Time Projection Chamber of ALICE

The ALICE experiment at CERN is undergoing a major upgrade during the Long Shutdown 2 (LS2) of the LHC during 2019-2020. The key elements regarding the central barrel are the installation of a new Inner Tracking System (ITS) and the upgrade of the large Time Projection Chamber (TPC). The TPC is currently being upgraded with a new readout system, including new GEM-based Readout Chambers and new front-end electronics enabling us to operate it in continuous mode. The new ITS based on CMOS Monolithic Active Pixel Sensors will significantly improve the impact parameter resolution and the tracking efficiency, especially for particles with low transverse momenta, as well as the readout-rate capability. In the following, the upgrades and their assembly and commissioning status will be outlined.Furthermore, an outlook will be given on plans for the upgrade of the ITS foreseen for the third long shutdown of the LHC in 2025-2026. For this upgrade, the three innermost layers could be replaced by cylindrical detector layers, made of curved wafer-scale-sized CMOS sensors, to lower even further the material budget and to improve the pointing resolution.

Heavy-flavour Measurements in Pb–Pb Collisions with the Upgraded ALICE Inner Tracking System

During the second LHC long shutdown (LS2) the Inner Tracking System (ITS) of ALICE (A Large Ion Collider Experiment) will be replaced by seven layers of CMOS Monolithic Active Pixel Sensors (MAPS). The latest innovations in silicon imaging technology allow for the construction of large, ultra-thin silicon wafers which can further improve the capabilities of the ALICE tracker. The research and development studies towards the construction of a novel vertex detector have started. The detector installation has been proposed for the third LHC long shutdown (LS3) during which the three innermost layers shall be replaced by three cylindrical layers of large curved CMOS wafers. This upgrade (ITS3) will further improve the impact parameter resolution and the tracking efficiency of low momentum particles. The innermost layer will be positioned closer to the interaction point and the material budget will be reduced down to 0.05$\%X_0$ per layer. Monte Carlo simulations of a simplified ITS3 geometry within the ITS2 design indicate an improvement in the impact parameter resolution and the tracking efficiency, which are of crucial importance for measurements of heavy-flavour hadrons. This contribution shows the improved performance for the example of the $\Lambda_{\mathrm{b}}$, for which the significance of its measurement is extracted based on these MC simulations. A significant improvement by almost a factor of three in the low momentum region compared to the ITS2 is observed.

Upgrade of the ALICE inner tracking system: Construction and commissioning

ALICE (A Large Ion Collider Experiment) is the CERN LHC experiment optimized for the study of the strongly interacting matter produced in heavy-ion collisions, in particular the characterization of the quark-gluon plasma. After the successful operation of the experiment during the first two runs of the LHC, the ALICE collaboration is currently working on a major upgrade of its detectors, to be installed during the 2019–2020 Long Shutdown (LS2). The main goal is to increase the readout capabilities and allow recording of Pb–Pb minimum bias events at rates in excess of 50 kHz, which is the expected Pb–Pb interaction rate at the LHC after LS2. One key part of the upgrade is the construction of a new Inner Tracking System (called ITS2) that will significantly improve the impact parameter resolution of the tracks, tracking efficiency, and readout capacity which enables precise measurements of low momentum charged particles. The ITS2 consists of seven approximately-cylindrical detector layers of CMOS Monolithic Active Pixel Sensors (MAPS) with the sensor matrix and readout integrated on a single chip, named ALPIDE (ALice PIxel DEtector), covering an area of 10 m2 and containing about 12.5 billion pixels. All layers are mounted on ultra-lightweight carbon support structures with an embedded cooling system. This allows a reduction of the material budgets down to about 0.3% X0 for the inner layers and 1% X0 for the outer layers, with respect to the previous ITS system. After a brief overview of the upgrade motivation, details of the overall layout will be given as well as the construction and commissioning status and plans.

Progress on the radiation tolerance of CMOS Monolithic Active Pixel Sensors

CMOS Monolithic Active Pixel Sensors (CPS) are ultra-light and highly granular silicon pixel detectors suited for highly sensitive charged particle tracking. Being manufactured with cost efficient standard CMOS processes, CPS may integrate sensing elements together with analogue and digital data processing circuits into one monolithic chip. This turns into 50 μm thin sensors, which provide an outstanding typical spatial resolution of fewμm and a detection efficiency for minimum ionizing particles above 99.9%. The radiation tolerance of CPS was initially constrained by the limits of the CMOS processes used for their production but has been improved by orders of magnitudes during the last years. This work reviews the related R&D on the radiation tolerance of traditional CPS with partially depleted active medium as pioneered by the MIMOSA-series developed by the IPHC Strasbourg. Procedures for assessing radiation damage in those non-standard pixels are discussed and the major mechanisms of radiation damage are introduced. Techniques for radiation hardening are shown. Moreover, recent results on next generation CPS featuring fully depleted sensors based on improved CMOS processes are summarized.

Observations on MIMOSIS-0, the first dedicated CPS prototype for the CBM MVD

The Micro Vertex Detector (MVD) of the future Compressed Baryonic Matter (CBM) experiment at FAIR will have to provide a spatial precision of ∼5μm in combination with a material budget of 0.3%–0.5%X0 for a full detector station. Simultaneously, it will have to handle the rate and radiation load of operating the fixed target experiment at an average collision rate of 100kHz (4–10AGeV Au+Au collisions) or 10MHz (up to 28GeV p-A collisions). The harsh requirements call for a dedicated detector technology, which is the next generation CMOS Monolithic Active Pixel Sensor MIMOSIS. We report about the requirements for the sensor, introduce the design approach being followed to cope with it and show first test results from a first sensor prototype called MIMOSIS-0 .

A Prototype Readout System for the ALPIDE Pixel Sensor

The Alice pixel detector (ALPIDE) chip is a CMOS monolithic active pixel sensor developed by the A Large Ion Collider Experiment collaboration. It measures 15 mm $\times30$ mm and contains a matrix of $512 \times 1024$ pixels, each one measuring $26.88\,\,\mu \text{m}\,\,\times 29.24\,\,\mu \text{m}$ . The sensor can achieve a spatial resolution of $5~\mu \text{m}$ , and an event time resolution of 2– $4~\mu \text{s}$ . The ALPIDE chip is suitable to be used as a sensor plane for the construction of a high-precision beam telescope, considering its excellent tracking performance. As a prototype of a single plane of the future ALPIDE-based beam telescope, we present a readout system for the ALPIDE chip in this paper. Chip threshold calibration was carried out with this system. The capability to read out the hit rate of 6 MHz/cm2 was achieved in an X-ray radiation experiment. The spatial resolution of $\sim 5~\mu \text{m}$ was measured in a laser beam test.

The upgrade of the ALICE Inner Tracking System

The Inner Tracking System (ITS) of the ALICE experiment will be upgraded during the second long LHC shutdown in 2019–2020. The main goal of the ALICE ITS upgrade is to enable high precision measurements of low-momentum particles (<1 GeV/c) by acquiring a large sample of events, benefiting from the increase of the LHC instantaneous luminosity of Pb-Pb collisions to ℒ = 6 ⋅ 1027 cm−2 s−1 during Run 3. Working in this direction the ITS upgrade project is focusing on the increase of the readout rate, on the improvement of the impact parameter resolution, as well as on the improvement of the tracking efficiency and the position resolution. The major setup modification is the substitution of the current ITS with seven layers of silicon pixel detectors. The ALPIDE chip, a CMOS Monolithic Active Pixel Sensor (MAPS), was developed for this purpose and offers a spatial resolution of 5 μm. The use of MAPS together with a stringent mechanical design allows for the reduction of the material budget down to 0.35% X0 for the innermost layers and 1% X0 for the outer layers. The detector design was validated during the research and development period through a variety of tests ensuring the proper operation for the full lifetime inside ALICE. The production phase is close to completion with all the new assembled components undergoing different tests that aim to characterize the modules and staves and determine their qualification level. This contribution describes the detector design, the measurements performed during the research and development phase, as well as the production status.

FoCal: A highly granular digital calorimeter

This contribution discusses the recent progress in the development of a highly granular digital electromagnetic calorimeter proposed as an upgrade to the ALICE detector. This forward electromagnetic calorimeter (FoCal) must be able to discriminate decay photons from direct photons at very high energy, which requires extremely high granularity and a small Molière radius. A dedicated R&D program is ongoing to develop the technology needed for such a high-granularity device. Within this program we have constructed a prototype of a digital electromagnetic calorimeter based on CMOS monolithic active pixel sensors (MAPS). This prototype has demonstrated the unique capabilities of such a highly granular digital calorimeter, providing unprecedented shower profile measurements and good linearity and energy resolution. The prototype was based on the MIMOSA chip, which is however not fast enough for application in a full detector at the LHC. As a next step, the ALPIDE chip developed for the ALICE Inner Tracker Upgrade is being investigated for performance with high occupancy. This contribution presents results from the current prototype, the performance of the ALPIDE and plans for the next prototype.

TCAD simulation of a single Monolithic Active Pixel Sensors based on High Voltage CMOS technology

ABSTRACTA model of a High Voltage CMOS (HV-CMOS) Monolithic Active Pixel Sensor (MAPS) has been modelled using Technology Computer Aided Design (TCAD). The model has incorporated both the active region and the on-pixel readout circuits which were comprised of a source follower amplifier and an integrated charge amplifier. The simulation has examined the electrical characteristics and response output of a HV-CMOS MAPS sensor using typical dimensions, levels of doping in the structural layers and bias conditions for this sensor. The performance of two alternate designs of amplifier have been examined as a function of the operating parameters. The response of the sensor to the incidence of Minimum Ionizing Particles (MIPs) at different energies has been included in the model.

A process modification for CMOS monolithic active pixel sensors for enhanced depletion, timing performance and radiation tolerance

For the upgrade of its Inner Tracking System, the ALICE experiment plans to install a new tracker fully constructed with monolithic active pixel sensors implemented in a standard 180 nm CMOS imaging sensor process, with a deep pwell allowing full CMOS within the pixel. Reverse substrate bias increases the tolerance to non-ionizing energy loss (NIEL) well beyond 10131MeVneq∕cm2, but does not allow full depletion of the sensitive layer and hence full charge collection by drift, mandatory for more extreme radiation tolerance. This paper describes a process modification to fully deplete the epitaxial layer even with a small charge collection electrode. It uses a low dose blanket deep high energy n-type implant in the pixel array and does not require significant circuit or layout changes so that the same design can be fabricated both in the standard and modified process. When exposed to a 55Fe source at a reverse substrate bias of −6 V, pixels implemented in the standard and the modified process in a low and high dose variant for the deep n-type implant respectively yield a signal of about 115 mV, 110 mV and 90 mV at the output of a follower circuit. Signal rise times heavily affected by the speed of this circuit are 27.8+∕−5 ns, 23.2+∕−4.2 ns, and 22.2+∕−3.7 ns rms, respectively. In a different setup, the single pixel signal from a 90Sr source only degrades by less than 20% for the modified process after a 10151MeVneq∕cm2 irradiation, while the signal rise time only degrades by about 16+∕−2 ns to 19+∕−2.8 ns rms. From sensors implemented in the standard process no useful signal could be extracted after the same exposure. These first results indicate the process modification maintains low sensor capacitance, improves timing performance and increases NIEL tolerance by at least an order of magnitude.

Radiation damage caused by cold neutrons in boron doped CMOS active pixel sensors

CMOS Monolithic Active Pixel Sensors (MAPS) are considered as an emerging technology in the field of charged particle tracking. They will be used in the vertex detectors of experiments like STAR, CBM and ALICE and are considered for the ILC and the tracker of ATLAS. In those applications, the sensors are exposed to sizeable radiation doses. While the tolerance of MAPS to ionizing radiation and fast hadrons is well known, the damage caused by low energy neutrons was not studied so far. Those slow neutrons may initiate nuclear fission of 10B dopants found in the B-doped silicon active medium of MAPS. This effect was expected to create an unknown amount of radiation damage beyond the predictions of the NIEL (Non Ionizing Energy Loss) model for pure silicon. We estimate the impact of this effect by calculating the additional NIEL created by this fission. Moreover, we show first measured data for CMOS sensors which were irradiated with cold neutrons. The empirical results contradict the prediction of the updated NIEL model both, qualitatively and quantitatively: the sensors irradiated with slow neutrons show an unexpected and strong acceptor removal, which is not observed in sensors irradiated with MeV neutrons.

A Design of a New Column-Parallel Analog-to-Digital Converter Flash for Monolithic Active Pixel Sensor.

The CMOS Monolithic Active Pixel Sensor (MAPS) for the International Linear Collider (ILC) vertex detector (VXD) expresses stringent requirements on their analog readout electronics, specifically on the analog-to-digital converter (ADC). This paper concerns designing and optimizing a new architecture of a low power, high speed, and small-area 4-bit column-parallel ADC Flash. Later in this study, we propose to interpose an S/H block in the converter. This integration of S/H block increases the sensitiveness of the converter to the very small amplitude of the input signal from the sensor and provides a sufficient time to the converter to be able to code the input signal. This ADC is developed in 0.18 μm CMOS process with a pixel pitch of 35 μm. The proposed ADC responds to the constraints of power dissipation, size, and speed for the MAPS composed of a matrix of 64 rows and 48 columns where each column ADC covers a small area of 35 × 336.76 μm2. The proposed ADC consumes low power at a 1.8 V supply and 100 MS/s sampling rate with dynamic range of 125 mV. Its DNL and INL are 0.0812/−0.0787 LSB and 0.0811/−0.0787 LSB, respectively. Furthermore, this ADC achieves a high speed more than 5 GHz.