Monolithic Active Pixel Sensors Research Articles

Towards 2D dosimetry using monolithic active pixel sensors and a copper grating

Higher energy and intensity X-ray radiotherapy treatments are coming into wider use, having the benefit of requiring fewer treatment fractions and fewer hospital visits per patient. However, small percentage errors in multileaf collimator positioning and dose become bigger problems with higher doses per fraction. Hence, real-time treatment verification becomes essential. Where devices downstream from the patient suffer from scattering in the patient, upstream devices can disturb the therapeutic beam. Here, a method is proposed for performing dosimetry upstream using monolithic active pixel sensors, which can be made thin enough to disturb the beam by <1%. In order to calculate the dose to the tumour, a verification device needs to make a measurement of the photon field. Some photons will Compton scatter an electron in the silicon and generate a signal. However, this signal is obscured by energy deposits from contamination electrons, originating from Compton scattering in the accelerator head and air. Often extensive build-up material is added to verification devices to reduce the electron contamination and enhance the photon signal. However, this leads to degradation of the beam intensity to the patient. Instead we propose using thin strips of 50 µm thick copper in a grating pattern and measuring the difference in the signal with and without it. The contamination electrons are relatively undisturbed by the presence of the thin copper strips and the photon signal generated via Compton scattering is enhanced under the strips. Hence the difference in the two signals mostly consists of energy deposits originating from the therapeutic photons. From this the dose to the patient can be derived. The 50 μm of copper is thin enough to keep the beam attenuation below 1%, but in itself would give a total signal response which consists of 38% contamination electrons. Using the grating technique, we show that the electron contamination can be reduced to 2.6% of the total signal. This allows the photon signal only to be extracted from the data and thus the dose to patient with a very thin upstream detector can be calculated using a Monte Carlo model to extrapolate the photon flux into the tumour.

Pixel Chamber: A solid-state active-target for 3D imaging of charm and beauty

The Pixel Chamber project purpose is the production of the first solid state active target capable of performing continuous, high-resolution (O(μm)) 3D tracking. The aim is to create a bubble chamber-like high-granularity stack of hundreds of very thin monolithic active pixel sensors (MAPS) glued together. In this paper, the R&D to develop the first prototypes with ALPIDE sensors, designed for the ALICE experiment at the CERN LHC, will be presented. Tracking and vertexing algorithms were developed to reconstruct tracks and vertices inside Pixel Chamber. They were tested on Monte Carlo simulations which show that it is possible to obtain a high efficiency for the reconstruction of hadronic tracks, and for the primary and secondary vertices inside the detector. The tracking algorithm has been tested on test beam data to reconstruct long tracks produced in a single ALPIDE sensor parallel to a beam. Results show that it is possible to obtain very good performances in long track reconstruction on a single ALPIDE. Among the possible applications, Pixel Chamber used as an active target, has the potential to allow precision measurements of charm and beauty production. Even more interesting are possible medical and astrophysical applications. The usage of the Pixel Chamber as scatterer detector in Compton Cameras would reduce by orders of magnitude the numbers of gammas required for performing a precise source imaging, both for in vivo monitoring during hadron therapy and for astrophysics measurements.

The novel, truly cylindrical, ultra-thin silicon detector for the ALICE Inner Tracker System

The ALICE collaboration is planning to replace its innermost tracking layers during the Large Hadron Collider (LHC) Long Shutdown 3 with a novel detector that will be as close as 18 mm to the interaction point and as thin as <0.05%X 0 per layer. To achieve these numbers, a wafer-scale Monolithic Active Pixel Sensor (MAPS) in 65 nm technology is being developed. This sensor, fabricated on 300 mm wafers, will reach dimensions of up to 280 mm by 94 mm. Sensors are subsequently thinned down to values between 20 μm to 40 μm, where they become flexible and are bent into truly cylindrical half-barrels.Following the publication of a Letter of Intent by ALICE and the endorsement by the Large Hadron Collider Committee (LHCC) end of 2019, a very active research and development programme on bent silicon detectors has started.This contribution reviews the detector concept, the physics motivations, and lays out the R&D path. Mechanical integration tests with ultra-thin silicon wafers as well as electrical test of bent MAPS, including beam test results, will be shown and demonstrate the feasibility of this new class of tracking detectors.

Latest developments and characterisation results of DMAPS in TowerJazz 180nm for High Luminosity LHC

The last couple of years have seen the development of Depleted Monolithic Active Pixel Sensors (DMAPS) fabricated in TowerJazz 180nm with a process modification to increase the radiation tolerance. While many of MAPS developments focus on low radiation environment, we have taken the development to high radiation environment like pp-experiments at High Luminosity LHC. DMAPS are a cost effective and lightweight alternative to state-of-the-art hybrid detectors if they can fulfil the given requirements for radiation hardness, signal response time and hit rate capability. The MALTA and Mini-MALTA sensors have shown excellent detection efficiency after irradiation to the life time dose expected at the outer layers of the ATLAS pixel tracker Upgrade. Our development focuses on providing large pixel matrixes with excellent time resolution (<2ns) and tracking. This publication will discuss characterisation results of the DMAPS devices with special focus on the new MALTA2 sensor and will show the path of future developments

Performance Studies of the ATLASpix HV-MAPS Prototype for Different Substrate Resistivities

The ATLASpix high-voltage monolithic active pixel sensor (HV-MAPS) was designed as a technology demonstrator for the ATLAS ITk Upgrade and the CLIC tracking detector. In this contribution new results from laboratory-based energy calibration measurements using fluorescence X-rays are presented for the ATLASpix_Simple matrix. These findings are complemented by new results from test-beam studies with inclined tracks, in which the active charge collection depth is determined.

The MAPS foil

We present a method of embedding a Monolithic Active Pixel Sensor (MAPS) into a flexible printed circuit board (FPC) and its interconnection by means of through-hole copper plating. The resulting assembly, baptised “MAPS foil”, is a flexible, light, protected, and fully integrated detector module. By using widely available printed circuit board manufacturing techniques, the production of these devices can be scaled easily in size and volume, making it a compelling candidate for future large-scale applications.A first series of prototypes that embed the ALPIDE chip has been produced, functionally tested, and shown to be working.

Simulation of an all-layer monolithic pixel vertex detector for the Belle II upgrade

The Belle II experiment is evaluating an upgrade program aiming at the time frame 2026–2027 for the installation of the upgraded detectors, to improve the detector performance and robustness against beam-induced backgrounds. The VTX concept, a fully-pixelated system based on depleted monolithic active pixel sensors, has been developed as a replacement of the current pixel-and-strip vertex detector. A simulation framework for the VTX has been developed and integrated with the standard Belle II simulation, facilitating the comparison among different layouts. Simulation results (such as tracking efficiency and vertex resolution) on benchmark physics channels are presented, showing a significant improvement with respect to the current detector.

Towards MightyPix, an HV-MAPS for the LHCb Mighty Tracker upgrade

The Mighty Tracker is a proposed upgrade to the downstream tracking system of LHCb for operations at luminosities of up to 1.5 × 1034 cm−2 s−1 starting with the LHC Run 5 data taking period. It foresees the replacement of the most central area of the current scintillating fibre tracker with High Voltage CMOS (HV-CMOS) pixel sensors. Due to the increased luminosity, occupancy will be too high for track reconstruction in the fibre tracker and the fibres would no longer withstand the radiation damage. HV-CMOS sensors have demonstrated a significant radiation tolerance and good performance making them an ideal choice of technology for the LHCb experiment. Monolithic Active Pixel Sensors (MAPS) fabricated in HV-CMOS processes provide fast charge collection via drift and allow the implementation of the readout on the same die as the sensitive volume. Due to the use of commercial processes, these sensors can be fabricated at low cost as no hybridisation with bump bonds is required. Since they are not fully depleted, the inactive volume can be reduced by thinning to achieve a total die thickness of 50 to 100 microns. A dedicated sensor called the MightyPix is developed for this programme. It is based on the HV-MAPS families MuPix and ATLASPix and tailored to the requirements of LHCb. To demonstrate the feasibility of this technology for the LHCb environment, prototypes have been irradiated. These sensors are tested in terms of efficiency, time resolution and power dissipation in temperature controlled environments.

The SiD Digital ECal Based on Monolithic Active Pixel Sensors

The SiD detector concept capitalizes on high granularity in its tracker and calorimeter to achieve the momentum resolution and particle flow calorimetry physics goals in a compact design. The collaboration has had a long interest in the potential for improved granularity in both the tracker and ECal with an application of monolithic active pixel sensors (MAPS) and a study of MAPS in the SiD ECal was described in the ILC TDR. Work is progressing on the MAPS application in an upgraded SiD design with a prototyping design effort for a common SiD tracker/ECal design based on stitched reticules to achieve 10 × 10 cm2 sensors with 25 × 100 micron2 pixels. Application of large area MAPS in these systems would limit delicate and expensive bump-bonding, provide possibilities for better timing, and should be significantly cheaper than the TDR concept due to being a more conventional CMOS foundry process. The small pixels significantly improve shower separation. Recent simulation studies confirm previous performance projections, indicating electromagnetic energy resolution based on digital hit cluster counting provides better performance than the SiD TDR analog design based on 13 mm2 pixels. Furthermore, the two shower separation is excellent down to the millimeter scale. Geant4 simulation results demonstrate these expectations.

DECAL: A Reconfigurable Monolithic Active Pixel Sensor for Tracking and Calorimetry in a 180 nm Image Sensor Process.

In this paper, we describe DECAL, a prototype Monolithic Active Pixel Sensor (MAPS) device designed to demonstrate the feasibility of both digital calorimetry and reconfigurability in ASICs for particle physics. The goal of this architecture is to help reduce the development and manufacturing costs of detectors for future colliders by developing a chip that can operate both as a digital silicon calorimeter and a tracking chip. The prototype sensor consists of a matrix of 64 × 64 55 m pixels, and provides a readout at 40 MHz of the number of particles which have struck the matrix in the preceding 25 ns. It can be configured to report this as a total sum across the sensor (equivalent to the pad of an analogue calorimeter) or the sum per column (equivalent to a traditional strip detector). The design and operation of the sensor are described, and the results of chip characterisation are reported and compared to simulations.

CMOS pixel sensors optimized for large ionizing dynamic

Monolithic active pixel sensors (MAPS) are now well established as a technology for tracking charged particles, especially when low material budget is desirable. For such applications, sensors focus on spatial resolution and pixels with digital output or modest charge measurement ability are well suited. Within the European Union STRONG-2020 project, which focuses on experiments using hadrons, the TIIMM (Tracking and Ions Identifications with Minimal Material budget) joint research activity intends to expand granular MAPS capacity to energy-loss (ΔE) measurement for ion species identification. The TIIMM prototypes are developed in the Tower Jazz 180 nm CMOS image sensor (CIS) process. The Time-Over-Threshold (ToT) method is applied to the sensor for the energy-loss measurement. The main design details and the preliminary test results from laboratory measurements of the initial TIIMM prototype are presented in this work.

Testbeam performance results of bent ALPIDE monolithic active pixel sensors in view of the ALICE Inner Tracking System 3

The ALICE Inner Tracking System has been recently upgraded to a full silicon detector consisting entirely of monolithic active pixel sensors, arranged in seven concentric layers around the LHC beam pipe. Further ahead, during the LHC Long Shutdown 3, the ALICE collaboration is planning to replace the three innermost layers of this new ITS with a novel vertex detector. The proposed design features wafer-scale, ultra-thin, truly cylindrical MAPS. The new sensors will be thinned down to 20–40 µm, featuring a material budget of less than 0.05% x/X 0 per layer, unprecedented low, and will be arranged concentrically around the beam pipe, as close as 18 mm from the interaction point.Anticipating the first prototypes in the new 65 nm CMOS technology node, an active R&D programme is underway to test the response to bending of existing 50 µm thick ALPIDE sensors. A number of such chips were successfully bent, even below the targeted innermost radius, without signs of mechanical damage, while retaining their full electrical functionality in laboratory tests. The curved detectors were subsequently tested during particle beam campaigns, where their particle detection performance was assessed.In this contribution, testbeam highlights from the data analysis of bent ALPIDE sensors, will be presented. It was proved that the current ALPIDE produced in the 180 nm CMOS technology retains its properties after bending. The results show an inefficiency that is generally below 10−4, independent of the inclination and position of the impinging beam with respect to the sensor surface. This encouraging outcome proves that the use of curved MAPS is an exciting possibility for future silicon detector designs, as not only the sensor can survive the bending exercise mechanically, but the enticing attributes that make it attractive for use in the inner tracking layers are comparable to the flat state.

Recent results with radiation-tolerant TowerJazz 180 nm MALTA sensors

To achieve the physics goals of future colliders, it is necessary to develop novel, radiation-hard silicon sensors for their tracking detectors. We target the replacement of hybrid pixel detectors with Depleted Monolithic Active Pixel Sensors (DMAPS) that are radiation-hard, monolithic CMOS sensors. We have designed, manufactured and tested the MALTA series of sensors, which are DMAPS in the 180 nm TowerJazz CMOS imaging technology. MALTA have a pixel pitch well below current hybrid pixel detectors, high time resolution (<2ns) and excellent charge collection efficiency across pixel geometries. These sensors have a total silicon thickness of between 50–300 μm, implying reduced material budgets and multiple scattering rates for future detectors which utilise such technology. Furthermore, their monolithic design bypasses the costly stage of bump-bonding in hybrid sensors and can substantially reduce detector costs. This contribution presents the latest results from characterisation studies of the MALTA2 sensors, including results demonstrating the radiation tolerance of these sensors.

ALICE - ITS3 — A bent, wafer-scale CMOS detector

ALICE is developing the ITS3 (inner tracker system) as upgrade of the inner layers of the presently installed ITS with the aim to improve the pointing resolution by factor of two and to lower the material budget to an unprecedented value of 0.05% X0 per layer.Its three layers are based on Monolithic Active Pixel Sensors (MAPS) thinned down to 20–40 μm. The configuration employs half-cylinders, which are bent around the beam pipe with bending radii of 18, 24 and 30 mm, respectively. The sensors are produced using stitching techniques allowing to reach a length of 28 cm. As consequence, the detector consists of six MAPS sensors only. The material budget is kept to a minimum using carbon foam elements to hold the stitched sensors in place, as well as by employing airflow cooling.This contribution summarizes the achieved R&D results on bending and thinning, the mechanical prototype, beam tests and the 65 nm prototype ASIC test, as well as the path towards the last remaining milestone, the implementation of a wafer-scale, stitched sensor design.

Development and characterization of a DMAPS chip in TowerJazz [formula omitted] [formula omitted] technology for high radiation environments

The increasing availability of commercial CMOS processes with high-resistivity wafers has fueled the R&D of depleted monolithic active pixel sensors (DMAPS) for use in high energy physics experiments. One of these developments is a series of monolithic pixel detectors with column-drain readout architecture and small collection electrode allowing for low-power designs (TJ-Monopix). It is designed in a 180nm TowerJazz CMOS process and features a pixel size of 33μm×33μm. The efforts and improvements on the front-end electronics and sensor design of the current iteration TJ-Monopix2 increase the radiation hardness and efficiency while lowering the threshold and noise.

Development and testing of a radiation-hard large-electrode DMAPS design in a 150 nm CMOS process

The LF-Monopix chips are depleted monolithic active pixel sensors that follow the large-electrode design approach and implement a fast synchronous read-out architecture. They are designed in a 150 nm CMOS process and make use of large voltages (>250V) and highly resistive substrates (>2kΩ⋅cm) to collect charge through drift and enhance their radiation hardness.Samples of the first prototype (“LF-Monopix1”) with a thickness of 100μm were irradiated to assess the tolerance of the chip’s substrate and front-end circuitry to the surface and bulk damage doses expected at modern collider experiments. The device remained fully operational, with only a very small gain degradation and an increase in noise by less than 25% after a total ionizing dose of 100Mrad. Efficiency measurements in a sample exposed to a neutron fluence of 1×1015neq/cm2 showed that at least 96% of all minimum ionizing particles going through a fully depleted detector are recorded in less than 25ns. In the latest design (“LF-Monopix2”) the column length was tripled and the pixel pitch reduced by 40% with respect to its predecessor. The chip was successfully thinned down while keeping its breakdown voltage above 400V and achieving a front-end threshold dispersion of ∼100e− after tuning.

Charge Collection Dynamics of the ARCADIA Passive Pixel Arrays: Laser Characterization and TCAD Modeling

Monolithic Active Pixel Sensors (MAPS) represent one of the most promising technologies for the next generation of radiation detectors. The ARCADIA project aims at the development of Fully Depleted (FD) MAPS employing a production process compatible with a 110 nm commercial CMOS technology. The first engineering run of the project included matrices of active pixels with embedded analog and digital frontend electronics and passive test structures such as passive pixel arrays, MOS capacitors and backside diodes. Although the produced samples were already characterized from the electrical point of view, a thorough study of the charge collection dynamics of the passive pixel arrays was still missing. In this paper we show the results of the dynamic characterization of a group of passive pixel arrays with different pixel pitches (50, 25 and 10 μm) and different pixel layouts. The tested samples have been illuminated from the backside with an infrared and a red laser with wavelengths equal to 1,060 nm and 660 nm, respectively. The pixel arrays have been mounted on a custom readout PCB connected to an external amplifier with 1 GHz bandwidth and the signals have been acquired through a fast digital oscilloscope. We employed both focused and unfocused laser spots to evaluate the change in the measured signal as a function of the laser spot position and the average response of the pixel arrays. An excellent agreement has been demonstrated by comparing the measured signals with the results of transient TCAD simulations and a time for 50% charge collection of 7.8, 4.2 and 2.6 ns has been predicted and experimentally validated in pixels with 50, 25 and 10 μm pitch, respectively.

Heavy-ion beam test of a monolithic silicon pixel sensor with a new 130 nm High-Resistivity CMOS process

The Topmetal-M is a newly designed monolithic silicon pixel sensor for charged particle tracking based on a novel detector structure and implemented in a new 130 nm High-Resistivity (>1 kΩ cm) CMOS process. This sensor integrates the functionality of the Monolithic Active Pixel Sensor (MAPS) and the Topmetal sensor. It has a matrix of 512 × 400 pixels with the pitch of 40μm×40μm. Heavy-ion campaigns with a 181Ta35+ beam have been performed to study the performance of this Topmetal-M sensor. The test results demonstrate it has an excellent response to particle energy deposition. Furthermore,this sensor is still fully functional after exposure to ∼14.1 krad, and no Single Event Latch-up (SEL) occurred. In addition, the baseline, the Equivalent Noise Charge (ENC), and the number of bad pixels show relatively good performance.

Research on Calculation Method of Radiation Response Eigenvalue of a Single-Chip Active Pixel Sensor.

In this paper, we present a calculation method for the radiation response eigenvalue based on a monolithic active pixel sensor. By comparing the statistical eigenvalues of different regions of a pixel array in bright and dark environments, the linear relationship between the statistical eigenvalues obtained by different algorithms and the radiation dose rate was studied. Additionally, a dose rate characterization method based on the analysis of the eigenvalues of the MAPS response signal was proposed. The experimental results show that in the dark background environment, the eigenvalues had a good linear response in the region of any gray value in the range of 10–30. In the color images, due to the difference in the background gray values in adjacent color regions, the radiation response signal in dark regions was confused with the image information in bright regions, resulting in the loss of response signal and affecting the analysis results of the radiation response signal. For the low dose rate radiation field, as the radiation response signal was too weak and there was background dark noise, it was necessary to accumulate frame images to obtain a sufficient response signal. For the intense radiation field, the number of response events in a single image was very high, and only two consecutive frames of image data needed to be accumulated to meet the statistical requirements. The binarization method had a good characterization effect for the radiation at a low dose rate, and the binarization processing and the total gray value statistics of the response data at a high dose rate could better characterize the radiation dose rate. The calibration experiment results show that the binarization processing method can meet the requirements of using a MAPS for wide-range detection.

The Tangerine project: Development of high-resolution 65 nm silicon MAPS

The Tangerine project aims to develop new state-of-the-art high-precision silicon detectors. Part of the project has the goal of developing a monolithic active pixel sensor using a novel 65 nm CMOS imaging process, with a small collection electrode. This is the first application of this process in particle physics, and it is of great interest as it allows for an increased logic density and reduced power consumption and material budget compared to other processes. The process is envisioned to be used in for example the next ALICE inner tracker upgrade, and in experiments at the electron-ion collider.The initial goal of the three-year Tangerine project is to develop and test a sensor in a 65 nm CMOS imaging process that can be used in test beam telescopes at DESY, providing excellent spatial resolution and high time resolution, and thus demonstrating the capabilities of the process. The project covers all aspects of sensor R&D, from electronics and sensor design using simulations, to prototype test chip characterisation in labs and at test beams. The sensor design simulations are performed by using a powerful combination of detailed electric field simulations using technology computer-aided design and high-statistics Monte Carlo simulations using the Allpix2 framework. A first prototype test chip in the process has been designed and produced, and successfully operated and tested both in labs and at test beams.