Silicon Pixel Sensors Research Articles

Planar n-in-n quad module prototypes for the ATLAS ITk upgrade at HL-LHC

In order to meet the requirements of the High Luminosity LHC (HL-LHC), it will be necessary to replace the current tracker of the ATLAS experiment. Therefore, a new all-silicon tracking detector is being developed, the so-called Inner Tracker (ITk). The use of quad chip modules is intended in its pixel region. These modules consist of a silicon sensor that forms a unit along with four read-out chips. The current ATLAS pixel detector consists of planar n-in-n silicon pixel sensors. Similar sensors and four FE-I4 read-out chips were assembled to first prototypes of planar n-in-n quad modules. The main focus of the investigation of these modules was the region between the read-out chips, especially the central area between all four read-out chips. There are special pixel cells placed on the sensor which cover the gap between the read-out chips. This contribution focuses on the characterization of a non-irradiated device, including important sensor characteristics, charge collection determined with radioactive sources as well as hit efficiency measurements, performed in the laboratory and at testbeams. In addition, first laboratory results of an irradiated device are presented.

The Mu3e Experiment

The Mu3e experiment will search for lepton flavor violation in the neutrinoless muon decay μ+→e+e−e+ with a sensitivity down to 10−16 (90% C.L.) using the world most intense muon beam at the Paul Scherrer Institute. The μ+→e+e−e+ decay is strongly suppressed in the Standard Model whilst several Beyond the SM models predict observable effects accessible to the new generation of LFV experiments. The search for the μ+→e+e−e+ decay requires a large acceptance detector capable of measuring up to 2 × 109 decays of stopped muons per second with excellent momentum, space, and time resolution to suppress backgrounds below the 1016 level. The required Mu3e detector performance is possible thanks to tracking detectors based on thin monolithic active silicon pixel sensors (HV-MAPS) and very precise timing measurements using scintillating fibers and tiles both coupled to silicon photo-multipliers. The first phase of the Mu3e experiment has been recently approved. Following an intense R&D program the construction of the detector is about to start. First data taking is expected in 2019.

Track reconstruction for theMu3eexperiment based on a novel Multiple Scattering fit

The Mu3e experiment is designed to search for the lepton flavor violating decay μ + → e + e + e − . The aim of the experiment is to reach a branching ratio sensitivity of 10−16 . In a first phase the experiment will be performed at an existing beam line at the Paul-Scherrer Institute (Switzerland) providing 108 muons per second, which will allow to reach a sensitivity of 2 · 10−15 . The muons with a momentum of about 28 MeV/c are stopped and decay at rest on a target. The decay products (positrons and electrons) with energies below 53MeV are measured by a tracking detector consisting of two double layers of 50 μm thin silicon pixel sensors. The high granularity of the pixel detector with a pixel size of 80 μm × 80 μm allows for a precise track reconstruction in the high multiplicity environment of the Mu3e experiment, reaching 100 tracks per reconstruction frame of 50 ns in the final phase of the experiment. To deal with such high rates and combinatorics, the Mu3e track reconstruction uses a novel fit algorithm that in the simplest case takes into account only the multiple scattering, which allows for a fast online tracking on a GPU based filter farm. An implementation of the 3-dimensional multiple scattering fit based on hit triplets is described. The extension of the fit that takes into account energy losses and pixel size is used for offline track reconstruction. The algorithm and performance of the offline track reconstruction based on a full Geant4 simulation of the Mu3e detector are presented.

Photon counting phosphorescence lifetime imaging with TimepixCam.

TimepixCam is a novel fast optical imager based on an optimized silicon pixel sensor with a thin entrance window and read out by a Timepix Application Specific Integrated Circuit. The 256 × 256 pixel sensor has a time resolution of 15 ns at a sustained frame rate of 10 Hz. We used this sensor in combination with an image intensifier for wide-field time-correlated single photon counting imaging. We have characterised the photon detection capabilities of this detector system and employed it on a wide-field epifluorescence microscope to map phosphorescence decays of various iridium complexes with lifetimes of about 1 μs in 200 μm diameter polystyrene beads.

Module production of the one-arm AFP 3D pixel tracker

The ATLAS Forward Proton (AFP) detector is designed to identify events in which one or two protons emerge intact from the LHC collisions. AFP will consist of a tracking detector, to measure the momentum of the protons, and a time of flight system to reduce the background from multiple proton-proton interactions. Following an extensive qualification period, 3D silicon pixel sensors were selected for the AFP tracker. The sensors were produced at CNM (Barcelona) during 2014. The tracker module assembly and quality control was performed at IFAE during 2015. The assembly of the first AFP arm and the following installation in the LHC tunnel took place in February 2016. This paper reviews the fabrication process of the AFP tracker focusing on the pixel modules.

A beam monitor using silicon pixel sensors for hadron therapy

We report the design and test results of a beam monitor developed for online monitoring in hadron therapy. The beam monitor uses eight silicon pixel sensors, Topmetal-II-, as the anode array. Topmetal-II- is a charge sensor designed in a CMOS 0.35µm technology. Each Topmetal-II- sensor has 72×72pixels and the pixel size is 83×83µm2. In our design, the beam passes through the beam monitor without hitting the electrodes, making the beam monitor especially suitable for monitoring heavy ion beams. This design also reduces radiation damage to the beam monitor itself. The beam monitor is tested with a carbon ion beam at the Heavy Ion Research Facility in Lanzhou (HIRFL). Results indicate that the beam monitor can measure position, incidence angle and intensity of the beam with a position resolution better than 20µm, angular resolution about 0.5° and intensity statistical accuracy better than 2%.

Development of Edgeless Silicon Pixel Sensors on p-type substrate for the ATLAS High-Luminosity Upgrade

In view of the LHC upgrade phases towards the High Luminosity LHC (HL-LHC), the ATLAS experiment plans to upgrade the Inner Detector with an all-silicon system. The n-on-p silicon technology is a promising candidate to achieve a large area instrumented with pixel sensors, since it is radiation hard and cost effective. The presentation describes the performance of novel n-in-p edgeless planar pixel sensors produced by FBK-CMM, making use of the active trench for the reduction of the dead area at the periphery of the device. After discussing the sensor technology, some feedback from preliminary results of the first beam test will be discussed.

LHCb VELO upgrade

The upgrade of the LHCb experiment, scheduled for LHC Run-III, scheduled to start in 2021, will transform the experiment to a trigger-less system reading out the full detector at 40MHz event rate. All data reduction algorithms will be executed in a high-level software farm enabling the detector to run at luminosities of 2×1033cm−2s−1.The Vertex Locator (VELO) is the silicon vertex detector surrounding the interaction region. The current detector will be replaced with a hybrid pixel system equipped with electronics capable of reading out at 40MHz. The upgraded VELO will provide fast pattern recognition and track reconstruction to the software trigger. The silicon pixel sensors have 55×55μm2 pitch, and are read out by the VeloPix ASIC, from the Timepix/Medipix family. The hottest region will have pixel hit rates of 900Mhits/s yielding a total data rate of more than 3Tbit/s for the upgraded VELO. The detector modules are located in a separate vacuum, separated from the beam vacuum by a thin custom made foil. The foil will be manufactured through milling and possibly thinned further by chemical etching.The material budget will be minimised by the use of evaporative CO2 coolant circulating in microchannels within 400μm thick silicon substrates. The current status of the VELO upgrade is described and latest results from operation of irradiated sensor assemblies are presented.

Upgrade of the CMS Instrumentation for luminosity and machine induced background measurements

To optimise performance with the higher luminosity, higher beam energy and shorter bunch spacing of 25 ns at the LHC after 2014, an upgrade program is performed for the detectors to measure the luminosity and machine induced background. A new detector is the pixel luminosity telescope consisting of 8 telescopes, equipped with silicon pixel sensors, on both ends of the interactions point. The Fast Beam Conditions Monitoring system, using diamond sensors, is upgraded to 24 sensors, 12 on each end of the IP. In addition, dedicated fast ASICs produced in 130 nm commercial CMOS technology and dead-time free backend electronics using FPGAs for fast signal processing are being developed and built. Also, the part of the forward HCAL used for the luminosity measurement is instrumented with new readout electronics, in microTCA standards. The machine induced background measurement will be supported by a new system of direction sensitive quartz Cherenkov counters, with excellent time resolution. A data acquisition architecture is being developed that is common for all subsystems and allows for synchronization across different hardware. The design of the new system will be presented, and a report will be given on the performance of each subsystem measured in several test-beam campaigns and prototype operation in the last LHC run.

Charge collection and non-ionizing radiation tolerance of CMOS pixel sensors using a 0.18 μm CMOS process

The proposed Circular Electron Positron Collider (CEPC) will be primarily aimed for precision measurements of the discovered Higgs boson. Its innermost vertex detector, which will play a critical role in heavy-flavor tagging, must be constructed with fine-pitched silicon pixel sensors with low power consumption and fast readout. CMOS pixel sensor (CPS), as one of the most promising candidate technologies, has already demonstrated its excellent performance in several high energy physics experiments. Therefore it has been considered for R&D for the CEPC vertex detector. In this paper, we present the preliminary studies to improve the collected signal charge over the equivalent input capacitance ratio (Q/C), which will be crucial to reduce the analog power consumption. We have performed detailed 3D device simulation and evaluated potential impacts from diode geometry, epitaxial layer properties and non-ionizing radiation damage. We have proposed a new approach to improve the treatment of the boundary conditions in simulation. Along with the TCAD simulation, we have designed the exploratory prototype utilizing the TowerJazz 0.18μm CMOS imaging sensor process and we will verify the simulation results with future measurements.

Study of silicon pixel sensor for synchrotron radiation detection**Supported by Prefabrication Research of Beijing Advanced Photon Source (R&D for BAPS) and National Natural Science Foundation of China (11335010)

The silicon pixel sensor (SPS) is one of the key components of hybrid pixel single-photon-counting detectors for synchrotron radiation X-ray detection (SRD). In this paper, the design, fabrication, and characterization of SPSs for single beam X-ray photon detection is reported. The designed pixel sensor is a p+-in-n structure with guard-ring structures operated in full-depletion mode and is fabricated on 4-inch, N type, 320 μm thick, high-resistivity silicon wafers by a general Si planar process. To achieve high energy resolution of X-rays and obtain low dark current and high breakdown voltage as well as appropriate depletion voltage of the SPS, a series of technical optimizations of device structure and fabrication process are explored. With optimized device structure and fabrication process, excellent SPS characteristics with dark current of 2 nA/cm2, full depletion voltage < 50 V and breakdown voltage >150 V are achieved. The fabricated SPSs are wire bonded to ASIC circuits and tested for the performance of X-ray response to the 1W2B synchrotron beam line of the Beijing Synchrotron Radiation Facility. The measured S-curves for SRD demonstrate a high discrimination for different energy X-rays. The extracted energy resolution is high (<20% for X-ray photon energy >10 keV) and the linear properties between input photo energy and the equivalent generator amplitude are well established. It confirmed that the fabricated SPSs have a good energy linearity and high count rate with the optimized technologies. The technology is expected to have a promising application in the development of a large scale SRD system for the Beijing Advanced Photon Source.

Pixel front-end with synchronous discriminator and fast charge measurement for the upgrades of HL-LHC experiments

The upgrade of the silicon pixel sensors for the HL-LHC experiments requires the development of new readout integrated circuits due to unprecedented radiation levels, very high hit rates and increased pixel granularity. The design of a very compact, low power, low threshold analog very front-end in CMOS 65 nm technology is described. It contains a synchronous comparator which uses an offset compensation technique based on storing the offset in output. The latch can be turned into a local oscillator using an asynchronous logic feedback loop to implement a fast time-over-threshold counting. This design has been submitted and the measurement results are presented.

TimepixCam: a fast optical imager with time-stamping

We describe a novel fast optical imager, TimepixCam, based on an optimized silicon pixel sensor with a thin entrance window, read out by a Timepix ASIC. TimepixCam is able to record and time-stamp light flashes in excess of 1,000 photons with high quantum efficiency in the 400–1000nm wavelength range with 20ns timing resolution, corresponding to an effective rate of 50 Megaframes per second. The camera was used for imaging ions impinging on a microchannel plate followed by a phosphor screen. Possible applications include spatial and velocity map imaging of ions in time-of-flight mass spectroscopy; coincidence imaging of ions and electrons, and other time-resolved types of imaging spectroscopy.

Developments towards the LHCb VELO upgrade

The Vertex Locator (VELO) is a silicon strip detector surrounding the interaction region of the LHCb experiment. The upgrade of the VELO is planned to be installed in 2019–2020, and the current detector will be replaced by a hybrid pixel system equipped with electronics capable of reading out at a rate of 40MHz. The new detector is designed to withstand the radiation dose expected at an integrated luminosity of 50fb−1. The detector will be composed of silicon pixel sensors, read out by the VeloPix ASIC that is being developed based on the TimePix/MediPix family. The prototype sensors for the VELO upgrade are being irradiated in five different facilities and the post-irradiation performance is being measured with testbeams, and in the lab. These proceedings present the VELO upgrade and briefly discuss the results of the sensor testing campaign.

The LHCb VELO upgrade

The upgrade of the LHCb experiment, planned for 2019, will transform the experiment to a trigger-less system reading out the full detector at 40MHz event rate. All data reduction algorithms will be executed in a high-level software farm. The upgraded detector will run at luminosities of 2×1033cm−2s−1 and probe physics beyond the Standard Model in the heavy flavour sector with unprecedented precision. The Vertex Locator (VELO) is the silicon vertex detector surrounding the interaction region. The current detector will be replaced with a hybrid pixel system equipped with electronics capable of reading out at 40MHz. The detector comprises silicon pixel sensors with 55×55μm2 pitch, read out by the VeloPix ASIC, based on the TimePix/MediPix family. The hottest region will have pixel hit rates of 900Mhits/s yielding a total data rate more than 3Tbit/s for the upgraded VELO. The detector modules are located in a separate vacuum, separated from the beam vacuum by a thin custom made foil. The detector halves are retracted when the beams are injected and closed at stable beams, positioning the first sensitive pixel at 5.1mm from the beams. The material budget will be minimised by the use of evaporative CO2 coolant circulating in microchannels within 400μm thick silicon substrates.

Design optimization of pixel sensors using device simulations for the phase-II CMS tracker upgrade

In order to address the problems caused by the harsh radiation environment during the high luminosity phase of the LHC (HL-LHC), all silicon tracking detectors (pixels and strips) in the CMS experiment will undergo an upgrade. And so to develop radiation hard pixel sensors, simulations have been performed using the 2D TCAD device simulator, SILVACO, to obtain design parameters. The effect of various design parameters like pixel size, pixel depth, implant width, metal overhang, p-stop concentration, p-stop depth and bulk doping density on the leakage current and critical electric field are studied for both non-irradiated as well as irradiated pixel sensors. These 2D simulation results of planar pixels are useful for providing insight into the behaviour of non-irradiated and irradiated silicon pixel sensors and further work on 3D simulation is underway.

Development of Silicon Sensor Characterization System for Future High Energy Physics Experiments

The Compact Muon Solenoid (CMS) is one of the general purpose experiments at the Large Hadron Collider (LHC), CeRN and has its tracker built of all silicon strip and pixel sensors. Si sensors are expected to play extremely important role in the upgrades of the existing tracker for future high luminosity environment and will also be used in future lepton colliders. however, properties of the silicon sensors have to be carefully understood before they can be put in the extremely high luminosity condition. At Delhi university (DU), we have been working on the development of Si sensor characterization system, as part of the collaboration with the CMS experiment and RD50 collaboration. this works reports the installation of current-voltage (I-V) and capacitance-voltage (C-V) systems at DU.

The Mu3e Experiment at PSI

The Mu3e experiment will search for the lepton flavor violating neutrinoless muon decay μ+→e+e−e+ with a sensitivity down to 10−16 (90% C.L.) using the world most intense continuous muon beam at the Paul Scherrer Institute. This decay is strongly suppressed in the Standard Model (BR<10−56). Several Beyond the SM models predict observable effects accessible to the new generation of LFV experiments. The search for the μ+→e+e−e+ decay requires excellent momentum, spacial, and time resolution to suppress backgrounds, high rate capability (detect more than 109 stopped muons per second), and large acceptance. The required detector performance is possible thanks to tracking detectors based on thin monolithic active silicon pixel sensors with high granularity (HV-MAPS), which incorporate the complete read-out electronics on the sensor chip, and very precise timing measurements using scintillating fibers and tiles coupled to silicon photo-multipliers operating at very high particle rates.

The ToPiX v4 prototype for the triggerless readout of the PANDA silicon pixel detector

ToPiX v4 is the prototype for the readout of the silicon pixel sensors for the Micro Vertex Detector of the PANDA experiment. ToPiX provides position, time and energy measurement of the incoming particles and is designed for the triggerless environment foreseen in PANDA. The prototype includes 640 pixels with a size of 100 × 100 μm2, a 160 MHz time stamp distribution circuit to measure both particle arrival time and released energy (via ToT technique) and the full control logic. The ASIC is designed in a 0.13 μm CMOS technology with SEU protection techniques for the digital parts.

Optimization of radiation hardness and charge collection of edgeless silicon pixel sensors for photon science

Recent progress in active-edge technology of silicon sensors enables the development of large-area tiled silicon pixel detectors with small dead space between modules by utilizing edgeless sensors. Such technology has been proven in successful productions of ATLAS and Medipix-based silicon pixel sensors by a few foundries. However, the drawbacks of edgeless sensors are poor radiation hardness for ionizing radiation and non-uniform charge collection by edge pixels. In this work, the radiation hardness of edgeless sensors with different polarities has been investigated using Synopsys TCAD with X-ray radiation-damage parameters implemented. Results show that if no conventional guard ring is present, none of the current designs are able to achieve a high breakdown voltage (typically < 30 V) after irradiation to a dose of ∼ 10 MGy. In addition, a charge-collection model has been developed and was used to calculate the charges collected by the edge pixels of edgeless sensors when illuminated with X-rays. The model takes into account the electric field distribution inside the pixel sensor, the absorption of X-rays, drift and diffusion of electrons and holes, charge sharing effects, and threshold settings in ASICs. It is found that the non-uniform charge collection of edge pixels is caused by the strong bending of the electric field and the non-uniformity depends on bias voltage, sensor thickness and distance from active edge to the last pixel (``edge space"). In particular, the last few pixels close to the active edge of the sensor are not sensitive to low-energy X-rays ( < 10 keV), especially for sensors with thicker Si and smaller edge space. The results from the model calculation have been compared to measurements and good agreement was obtained. The model can be used to optimize the edge design. From the edge optimization, it is found that in order to guarantee the sensitivity of the last few pixels to low-energy X-rays, the edge space should be kept at least 50% of the sensor thickness.