High Granularity Timing Detector Research Articles

Overview of the ATLAS High-Granularity Timing Detector: project status and results

The increase of the particle flux (pile-up) at the high-luminosity phase of the Large Hadron Collider (LHC) with an instantaneous luminosity up to L ≈ 7.5 × 1034 cm-2 s-1 will have a severe impact on the ATLAS detector reconstruction and trigger performance. A High Granularity Timing Detector (HGTD) will be installed in the forward region for pile-up mitigation and luminosity measurement. This detector, based on Low Gain Avalanche Detectors and custom ASICs, will provide a time resolution of 30 ps per track at the beginning of HL-LHC and 50 ps at the end. This proceeding paper will summarise the overall specifications of the HGTD as well as the project status.

A high granularity timing detector for the ATLAS Phase-II upgrade: Project overview and status

A High Granularity Timing Detector (HGTD) has been proposed for the ATLAS Phase-II upgrade to mitigate the pile-up effects at the High-Luminosity Large Hadron Collider (HL-LHC). The HGTD is based on silicon devices, Low Gain Avalanche Detectors (LGAD), aiming to provide 30–50 ps time resolution per track. Significant progress has been made in the development of various detector components, particularly the radiation-resistant sensor and readout chip. This proceeding concisely summarizes the project overview and current status of the HGTD project.

Low Gain Avalanche Detectors for the ATLAS High Granularity Timing Detector: Laboratory and test beam campaigns

The High Granularity Timing Detector (HGTD) is designed for the mitigation of pile-up effects in the ATLAS forward region and for bunch per bunch luminosity measurements. HGTD, based on Low Gain Avalanche Detector (LGAD) technology and covering the pseudorapidity region between 2.4 and 4.0, will provide high precision timing information to distinguish between collisions occurring close in space but well-separated in time. Apart from being radiation resistant, LGAD sensors should deliver 30 ps time resolution per track for a minimum-ionizing particle (35 ps per hit) at the start of lifetime, increasing to 50 ps per track (70 ps per hit) at the end of HL-LHC operation. Each readout cell has a transverse size of 1.3×1.3 mm2 leading to a highly granular detector with about 3 millions of readout electronics channels. A dedicated ASIC for the HGTD detector, ALTIROC, is being developed in several phases producing prototype versions of 2×2, 5×5 and 15×15 channels. HGTD modules are hybrids of the LGAD and ALTIROC connected through flip-chip bump bonding process. Several test beam campaigns have been conducted at DESY and CERN SPS between 2021 and 2022. A summary of the results from LGAD-alone and hybrids will be presented.

An FPGA-based front-end module emulator for the High Granularity Timing Detector

This paper introduces an FPGA-based front-end module emulator developed for the High Granularity Timing Detector (HGTD) within the ATLAS experiment at LHC. The emulator serves as a debugger for the HGTD readout system during the stage when the front-end module is not available. Using a Xilinx-Spartan 7 FPGA, the emulator mimics the behavior of the ASIC utilized in the front-end module. In addition, it shares the same dimensions and connectors as its successor, the real front-end module. This emulator has been effectively employed in the design and testing of the HGTD.

Radiation tolerance of the MUX64 for the High Granularity Timing Detector of ATLAS

The MUX64 ASIC is a 64-to-1 analog multiplexer to accommodate 64 inputs, with one addressed to output for ADC readout. It is developed for monitoring of the Low-Gain Avalanche Detectors (LGAD) detector modules in the High Granularity Timing Detector (HGTD) of the ATLAS Phase-II upgrade. The MUX64 chips will be used in the radiation field of high-luminosity pp collisions at LHC to an integrated luminosity of 4000 fb-1. This work presents the radiation tolerance study for the MUX64 being tested with 80 MeV protons and X-ray exposures for damages caused by Non-Ionizing Energy Loss (NIEL) and Total Ionizing Dose (TID), respectively. The irradiated samples demonstrated tolerance to the NIEL to a fluence of 3.21 × 1015 (Si, 1 MeV) neq/cm2, and the TID of 7.46 × 105 Gy (Si).

HGTD DC/DC converter in low temperature and magnetic field operation

The BPOL12V is a DC/DC converter designed to supply power to the High Granularity Timing Detector (HGTD) as part of the ATLAS Phase II upgrade project. The HGTD operates in an environment characterized by low temperatures and a magnetic field. Ensuring the reliable functionality of the BPOL12V under such conditions is of utmost importance. This paper outlines a series of functionality tests for the BPOL12V, including efficiency, ripple, and rise/fall edge assessments across various operational scenarios. The performance of the BPOL12V consistently meets the requirements of the HGTD, whether it operates in low temperatures down to -30 °C or in the presence of a 0.4 T magnetic field.

Performance of a front-end prototype ASIC for the ATLAS High Granularity timing detector

This paper presents the design and characterisation of a front-end prototype ASIC for the ATLAS High Granularity Timing Detector, which is planned for the High-Luminosity phase of the LHC. This prototype, called ALTIROC1, consists of a 5 × 5-pad matrix and contains the analog part of the single-channel readout (preamplifier, discriminator, two TDCs and SRAM). Two preamplifier architectures (transimpedance and voltage) were implemented and tested. The ASIC was characterised both alone and as a module when connected to a 5 × 5-pad array of LGAD sensors. In calibration measurements, the ASIC operating alone was found to satisfy the technical requirements for the project, with similar performances for both preamplifier types. In particular, the jitter was found to be 15 ± 1 ps (35 ± 1 ps) for an injected charge of 10 fC (4 fC). A degradation in performance was observed when the ASIC was connected to the LGAD array. This is attributed to digital couplings at the entrance of the preamplifiers. When the ASIC is connected to the LGAD array, the lowest detectable charge increased from 1.5 fC to 3.4 fC. As a consequence, the jitter increased for an injected charge of 4 fC. Despite this increase, ALTIROC1 still satisfies the maximum jitter specification (below 65 ps) for the HGTD project. This coupling issue also affects the time over threshold measurements and the time-walk correction can only be performed with transimpedance preamplifiers. Beam test measurements with a pion beam at CERN were also undertaken to evaluate the performance of the module. The best time resolution obtained using only ALTIROC TDC data was 46.3 ± 0.7 ps for a restricted time of arrival range where the coupling issue is minimized. The residual time-walk contribution is equal to 23 ps and is the dominant electronic noise contribution to the time resolution at 15 fC.

Destructive breakdown studies of irradiated LGADs at beam tests for the ATLAS HGTD

In the past years, it has been observed at several beam test campaigns thatirradiated LGAD sensors break with a typical star shaped burn mark whenoperated at voltages much lower than those at which they were safely operatedduring laboratory tests. The study presented in this paper was designed todetermine the safe operating voltage that these sensors can withstand. Manyirradiated sensors from various producers were tested in two test beamfacilities, DESY (Hamburg) and CERN-SPS (Geneva), as part of ATLAS HighGranularity Timing Detector (HGTD) beam tests. The samples were placed in thebeam and kept under bias over a long period of time in order to reach a highnumber of particles crossing each sensor. Both beam tests lead to a similarconclusion, that these destructive events begin to occur when the averageelectric field in the sensor becomes larger than12 V/μm.

Characterization of the response of IHEP-IME LGAD with shallow carbon to Gamma Irradiation

Low Gain Avalanche Detectors (LGAD) for the High-Granularity Timing Detector (HGTD) are crucial in reducing pileups in the High-Luminosity Large Hadron Collider. Numerous studies have been conducted on the bulk irradiation damage of LGADs. However, few studies have been carried out on the surface irradiation damage of LGAD sensors with shallow carbon implantation. In this paper, the IHEP-IME LGADs with shallow carbon implantation were irradiated up to 2 MGy using gamma irradiation to investigate surface damage. Important characteristic parameters, including leakage currents, breakdown voltage (BV), inter-pad resistances, and capacitances, were tested before and after irradiation. The results showed that the leakage current and BV increased after irradiation, whereas overall inter-pad resistance exhibited minimal change and remained above 109 Ω before and after irradiation. Capacitance was found to be less than 4.5 pF with a slight decrease in the gain layer depletion voltage (V gl ) after irradiation. No parameter affected by the inter-pad separation was observed before and after irradiation. All characteristic parameters meet the requirements of HGTD, and this design can be used to further optimization.

Performance in beam tests of carbon-enriched irradiated Low Gain Avalanche Detectors for the ATLAS High Granularity Timing Detector

The High Granularity Timing Detector (HGTD) will be installed in the ATLAS experiment to mitigate pile-up effects during the High Luminosity (HL) phase of the Large Hadron Collider (LHC) at CERN. Low Gain Avalanche Detectors (LGADs) will provide high-precision measurements of the time of arrival of particles at the HGTD, improving the particle-vertex assignment. To cope with the high-radiation environment, LGADs have been optimized by adding carbon in the gain layer, thus reducing the acceptor removal rate after irradiation. Performances of several carbon-enriched LGAD sensors from different vendors, and irradiated with high fluences of 1.5 and 2.5 × 1015 neq/cm2, have been measured in beam test campaigns during the years 2021 and 2022 at CERN SPS and DESY. This paper presents the results obtained with data recorded by an oscilloscope synchronized with a beam telescope which provides particle position information within a resolution of a few μm. Collected charge, time resolution and hit efficiency measurements are presented. In addition, the efficiency uniformity is also studied as a function of the position of the incident particle inside the sensor pad.

Inverse LGAD (iLGAD) Periphery Optimization for Surface Damage Irradiation

Pixelated LGADs have been established as the baseline technology for timing detectors for the High Granularity Timing Detector (HGTD) and the Endcap Timing Layer (ETL) of the ATLAS and CMS experiments, respectively. The drawback of segmenting an LGAD is the non-gain area present between pixels and the consequent reduction in the fill factor. To overcome this issue, the inverse LGAD (iLGAD) technology has been proposed by IMB-CNM to enhance the fill factor and provide excellent tracking capabilities. In this work, we explore the use of iLGAD sensors for surface damage irradiation by developing a new generation of iLGADs, the periphery of which is optimized to improve the performance of irradiated sensors. The fabricated iLGAD sensors exhibit good electrical performances before and after X-ray irradiation.

MUX64, an analogue 64-to-1 multiplexer ASIC for the ATLAS high granularity timing detector

We present the design and the performance of MUX64, a 64-to-1 analogue multiplexer ASIC for the ATLAS High Granularity Timing Detector (HGTD). The MUX64 transmits one of its 64 inputs selected by six address lines for the voltages or temperatures being monitored to an lpGBT ADC channel. The prototype ASICs fabricated in TSMC 130 nm CMOS technology were prepared in wire-bonding and QFN88 packaging format. A total of 280 chips was examined for functionality and quality assurance. The accelerated aging test conducted at 85 °C shows negligible degradation over 16 days.

Design and testing of long flexible printed circuits for the ATLAS High Granularity Timing Detector demonstrator

The High Granularity Timing Detector for the ATLAS upgrade is under construction to meet the challenges of the HL-LHC. In order to connect a module, the basic detector element, to the surrounding peripheral electronic board, a flexible printed circuit (FPC) is used as an interconnection for data transmission and power distribution. An identical design for all FPCs is required except for their length, depending on the module position on the detector active area. The design and qualification of a preliminary FPC version, manufactured in 13 different lengths (from 28.5 to 73.2 cm), are presented.

Performance of ALTIROC2 readout ASIC with LGADs for ATLAS HGTD picosecond MIP timing detector

ALTIROC2 is a 225-channel ASIC designed in CMOS 130 nm to read out a 15 × 15 matrix of 1.3 mm × 1.3 mm Low Gain Avalanche Diodes (LGAD) for the ATLAS HGTD (High Granularity Timing Detector). The targeted combined time resolution of the sensor and its readout electronics range from 35 ps/hit (initial) to 65 ps/hit (end of operational lifetime). Each ASIC channel integrates a high-speed preamplifier followed by a high speed discriminator and two TDCs for Time-of-Arrival and Time-Over-Threshold measurements as well as a local memory. This front-end must exhibit a small jitter while keeping a challenging power consumption of less than 4.5 mW per channel. This conference proceeding summarizes the ASIC architecture, its measured performances compared to simulation, along with the requirements for the ATLAS HGTD experiments.

A High-Granularity Timing Detector for the ATLAS Phase-II upgrade

A High-Granularity Timing Detector for the ATLAS Phase-II upgrade

Design and testing of LGAD sensor with shallow carbon implantation

The low gain avalanche detectors (LGADs) are thin sensors with fast charge collection which in combination with internal gain deliver an outstanding time resolution of about 30 ps for Minimum Ionizing Particles (MIP). High collision rates and consequent large particle rates crossing the detectors at the upgraded Large Hadron Collider (LHC) in 2028 will lead to radiation damage and deteriorated performance of the LGADs. The main consequence of radiation damage is loss of gain layer doping (acceptor removal) which requires an increase of bias voltage to compensate for the loss of charge collection efficiency and consequently time resolution. The Institute of High Energy Physics (IHEP), Chinese Academy of Sciences (CAS) has developed a process based on the Institute of Microelectronics (IME), CAS capability to enrich the gain layer with carbon to reduce the acceptor removal effect by radiation. After 1 MeV neutron equivalent fluence of 2.5 × 1015 neq/cm2, which is the maximum fluence to which sensors will be exposed at ATLAS High Granularity Timing Detector (HGTD), the IHEP-IME second version (IHEP-IMEv2) 50μm LGAD sensors already deliver adequate charge collection >4 fC and time resolution <50 ps at voltages <400 V. The operation voltages of these 50μm devices are well below those at which single event burnout may occur.

Performance in beam tests of irradiated Low Gain Avalanche Detectors for the ATLAS High Granularity Timing Detector

The High Granularity Timing Detector (HGTD) will be installed in the ATLAS detector to mitigate pile-up effects during the High Luminosity (HL) upgrade of the Large Hadron Collider (LHC) at CERN. The design of the HGTD is based on the use of Low Gain Avalanche Detectors (LGADs), with an active thickness of 50 μm, that allow to measure with high-precision the time of arrival of particles. The HGTD will improve the particle-vertex assignment by measuring the track time with a resolution ranging from approximately 30 ps at the beginning of the HL-LHC operations to 50 ps at the end. Performances of several unirradiated, as well as neutron- and proton-irradiated, LGAD sensors from different vendors have been measured in beam test campaigns during the years 2018 and 2019 at CERN SPS and DESY. This paper presents the results obtained with data recorded by an oscilloscope synchronized with a beam telescope which provides particle position information within a resolution of a few μm. Collected charge, time resolution and hit efficiency are presented. In addition to these properties, the charge uniformity is also studied as a function of the position of the incident particle inside the sensor pad.

Overview of CNM LGAD results: boron Si-on-Si and epitaxial wafers

Low Gain Avalanche Detectors (LGADs) are n-on-p silicon sensors with an extra p-layer below the collection electrode which provides signal amplification. When the primary electrons reach the amplification region new electron-hole pairs are created that enhance the generated signal. The moderate gain of these sensors, together with the relatively thin active region, provide precise time information for minimum ionizing particles. To mitigate the effect of pile-up at the HL-LHC the ATLAS and CMS experiments have chosen the LGAD technology for the High Granularity Timing Detector (HGTD) and for the End-Cap Timing Layer (ETL), respectively. A full characterization of recent productions of LGAD sensors fabricated at CNM has been carried out before and after neutron irradiation up to 2.5 × 1015 neq/cm2. Boron-doped sensors produced in epitaxial and Si-on-Si wafers have been studied. The results include their electrically characterization (IV and bias voltage stability) and performance studies (charge and time resolution) for single pad devices with a Sr-90 radioactive source set-up. The behaviour of the Inter-Pad region for irradiated 2 × 2 LGAD arrays, using the Transient Current Technique (TCT), is shown. The results indicate that the Si-on-Si devices with higher resistivity perform better than the epitaxial ones.

Design of AC-coupled low gain avalanche diodes (AC-LGADs): a 2D TCAD simulation study

AC-Coupled Low Gain Avalanche Diodes (AC-LGADs) are a new type of silicon sensors which conceived for experiments at future colliders. As an evolution of the standard Low Gain Avalanche Diodes (LGADs), AC-LGADs have similar fast timing performance. The innovation is that AC-LGADs are available to provide fine temporal and spatial resolution simultaneously, thanks to two key features: AC-coupled readout through the dielectric layer and a continuous resistive n+ implant. The Institute of High Energy Physics (IHEP) High-Granularity Timing Detector group is developing its first version of AC-LGAD sensors. This paper present the impact of following parameters on AC-LGAD sensor performance: n+ dose, dielectric material and thickness, pad/pitch ratio and the distance between pad and cathode. All data are obtained from 2D Technology Computer-Aided Design (TCAD) simulations. The simulations will benefit future design and optimization.

Performance of LGAD sensors with carbon enriched gain layer produced by USTC

The Low Gain Avalanche Detector (LGAD) technology is proposed for the ATLAS High Granularity Timing Detector (HGTD) and the CMS Endcap Timing Layer (ETL) towards the High-Luminosity Large Hadron Collider (HL-LHC), for which radiation hardness is one of the key requirements. In this paper we report results from the measurements on LGAD sensors designed by the University of Science and Technology of China (USTC) and fabricated by the Institute of Microelectronics of the Chinese Academy of Science (IME, CAS). These sensors have been irradiated with irradiation fluences of 8×1014, 1.5×1015, 2.5×1015 and 6×1015 cm−2 1 MeV neutron equivalent. Both un-irradiated and irradiated sensors were measured with a 90Sr-based beta scope at −30∘C to extract the collected charge and time resolution. The results show that sensors with carbon implantation can provide collected charge of more than 4 fC and time resolution better than 70 ps at appropriate bias voltage even for the equivalent fluence of 2.5×1015 cm−2. Both the collected charge and the time resolution meet the design specification of HGTD.