Low Gain Avalanche Detectors Research Articles

Low Gain Avalanche Detectors (LGAD) for particle physics and synchrotron applications

A new avalanche silicon detector concept is introduced with a low gain in the region of ten, known as a Low Gain Avalanche Detector, LGAD. The detector's characteristics are simulated via a full process simulation to obtain the required doping profiles which demonstrate the desired operational characteristics of high breakdown voltage (500 V) and a gain of 10 at 200 V reverse bias for X-ray detection. The first low gain avalanche detectors fabricated by Micron Semiconductor Ltd are presented. The doping profiles of the multiplication junctions were measured with SIMS and reproduced by simulating the full fabrication process which enabled further development of the manufacturing process. The detectors are 300 μm thick p-type silicon with a resistivity of 8.5 kΩcm, which fully depletes at 116 V. The current characteristics are presented and demonstrate breakdown voltages in excess of 500 V and a current density of 40 to 100 nAcm−2 before breakdown measured at 20̂C. The gain of the LGAD has been measured with a red laser (660 nm) and shown to be between 9 and 12 for an external bias voltage range from 150 V to 300 V.

Radiation hardness of thin Low Gain Avalanche Detectors

Low Gain Avalanche Detectors (LGAD) are based on a n++-p+-p-p++ structure where an appropriate doping of the multiplication layer (p+) leads to high enough electric fields for impact ionization. Gain factors of few tens in charge significantly improve the resolution of timing measurements, particularly for thin detectors, where the timing performance was shown to be limited by Landau fluctuations. The main obstacle for their operation is the decrease of gain with irradiation, attributed to effective acceptor removal in the gain layer. Sets of thin sensors were produced by two different producers on different substrates, with different gain layer doping profiles and thicknesses (45, 50 and 80 μm). Their performance in terms of gain/collected charge and leakage current was compared before and after irradiation with neutrons and pions up to the equivalent fluences of 5⋅1015 cm−2. Transient Current Technique and charge collection measurements with LHC speed electronics were employed to characterize the detectors. The thin LGAD sensors were shown to perform much better than sensors of standard thickness (∼300 μm) and offer larger charge collection with respect to detectors without gain layer for fluences <2⋅1015 cm−2. Larger initial gain prolongs the beneficial performance of LGADs. Pions were found to be more damaging than neutrons at the same equivalent fluence, while no significant difference was found between different producers. At very high fluences and bias voltages the gain appears due to deep acceptors in the bulk, hence also in thin standard detectors.

A High-Granularity Timing Detector for the Phase-II upgrade of the ATLAS calorimeter system: detector concept description and first beam test results

The expected increase of the particle flux at the high luminosity phase of the LHC (HL-LHC) with instantaneous luminosities up to 7.5⋅1034 cm−2s−1 will have a severe impact on the ATLAS detector performance. The pile-up is expected to increase on average to 200 interactions per bunch crossing. The reconstruction performance for electrons, photons as well as jets and transverse missing energy will be severely degraded in the end-cap and forward region. A High Granularity Timing Detector (HGTD) is proposed in front of the liquid Argon end-cap and forward calorimeters for pile-up mitigation. This device should cover the pseudo-rapidity range of 2.4 to about 4.0. Low Gain Avalanche Detectors (LGAD) technology has been chosen as it provides an internal gain good enough to reach large signal over noise ratio needed for excellent time resolution. The requirements and overall specifications of the High Granular Timing Detector at the HL-LHC will be presented as well as the conceptual design of its mechanics and electronics. Beam test results and measurements of irradiated LGAD silicon sensors, such as gain and timing resolution, will be shown.

Development of Ultra-Fast Silicon Detectors for 4D tracking

In this contribution we review the progress towards the development of a novel type of silicon detectors suited for tracking with a picosecond timing resolution, the so called Ultra-Fast Silicon Detectors. The goal is to create a new family of particle detectors merging excellent position and timing resolution with GHz counting capabilities, very low material budget, radiation resistance, fine granularity, low power, insensitivity to magnetic field, and affordability. We aim to achieve concurrent precisions of ∼ 10 ps and ∼ 10 μm with a 50 μm thick sensor. Ultra-Fast Silicon Detectors are based on the concept of Low-Gain Avalanche Detectors, which are silicon detectors with an internal multiplication mechanism so that they generate a signal which is factor ∼10 larger than standard silicon detectors. The basic design of UFSD consists of a thin silicon sensor with moderate internal gain and pixelated electrodes coupled to full custom VLSI chip. An overview of test beam data on time resolution and the impact on this measurement of radiation doses at the level of those expected at HL-LHC is presented. First I-V and C-V measurements on a new FBK sensor production of UFSD, 50 μm thick, with B and Ga, activated at two diffusion temperatures, with and without C co-implantation (in Low and High concentrations), and with different effective doping concentrations in the Gain layer, are shown. Perspectives on current use of UFSD in HEP experiments (UFSD detectors have been installed in the CMS-TOTEM Precision Protons Spectrometer for the forward physics tracking, and are currently taking data) and proposed applications for a MIP timing layer in the HL-LHC upgrade are briefly discussed.

Study of gain homogeneity and radiation effects of Low Gain Avalanche Pad Detectors

Silicon detectors with intrinsic charge amplification implementing a n++-p+-p structure are considered as a sensor technology for future tracking and timing applications in high energy physics experiments. The performance of the intrinsic gain in Low Gain Avalanche Detectors (LGAD) after irradiation is crucial for the characterization of radiation hardness and timing properties in this technology. LGAD devices irradiated with reactor neutrons or 800 MeV protons reaching fluences of 2.3×1016neq/cm2 were characterized using Transient Current Technique (TCT) measurements with red and infra-red laser pulses. Leakage current variations observed in different production lots and within wafers were investigated using Thermally Stimulated Current (TSC). Results showed that the intrinsic charge amplification is reduced with increasing fluence up to 1015neq/cm2 which is related to an effective acceptor removal. Further relevant issues were charge collection homogeneity across the detector surface and leakage current performance before and after irradiation.

Gain and time resolution of 45 μm thin Low Gain Avalanche Detectors before and after irradiation up to a fluence of 1015 neq/cm2

Low Gain Avalanche Detectors (LGADs) are silicon sensors with a built-in charge multiplication layer providing a gain of typically 10 to 50. Due to the combination of high signal-to-noise ratio and short rise time, thin LGADs provide good time resolutions. LGADs with an active thickness of about 45 μm were produced at CNM Barcelona. Their gains and time resolutions were studied in beam tests for two different multiplication layer implantation doses, as well as before and after irradiation with neutrons up to 1015 neq/cm2. The gain showed the expected decrease at a fixed voltage for a lower initial implantation dose, as well as for a higher fluence due to effective acceptor removal in the multiplication layer. Time resolutions below 30 ps were obtained at the highest applied voltages for both implantation doses before irradiation. Also after an intermediate fluence of 3×1014 neq/cm2, similar values were measured since a higher applicable reverse bias voltage could recover most of the pre-irradiation gain. At 1015 neq/cm2, the time resolution at the maximum applicable voltage of 620 V during the beam test was measured to be 57 ps since the voltage stability was not good enough to compensate for the gain layer loss. The time resolutions were found to follow approximately a universal function of gain for all implantation doses and fluences.

Dependence of charge multiplication on different design parameters of LGAD devices

The Low Gain Avalanche Detectors (LGADs) have the unique characteristic to provide amplification to the particle signal by using a controlled local avalanche mechanism. These devices are being explored by the HEP community because, while it is known that in standard silicon detectors the signal-to-noise ratio falls drastically with fluence, in LGAD devices this can be made sufficiently high to potentially overcome this problem. However, the existing irradiation studies show that at high fluences, the LGAD internal gain disappears and is no more significant. In this work, we have studied the effect of various design parameters of the LGAD device on its gain characteristic, at different irradiation levels. With the acquired understanding, we have also tried to tune the LGAD gain so to be suited for operation in a high radiation environment.

Developments and first measurements of Ultra-Fast Silicon Detectors produced at FBK

Segmented silicon sensors with internal gain, the so called Ultra-FAST Silicon Detectors (UFSD), have been produced at FBK for the first time. UFSD are based on the concept of Low-Gain Avalanche Detectors (LGAD), which are silicon detectors with an internal, low multiplication mechanism (gain ∼ 10). This production houses two main type of devices: one type where the gain layer is on the same side of the read-out electrodes, the other type where the gain layer is on the side opposite to the pixellated electrodes (reverse-LGAD). Several technological splits have been included in the first production run, with the aim to tune the implantation dose of the multiplication layer, which controls the gain value of the detector. An extended testing on the wafers has been performed and the results are in line with simulations: the fabricated detectors show good performances, with breakdown voltages above 1000 Volts, and gain values in the range of 5–60 depending on the technological split. The detectors timing resolution has been measured by means of a laboratory setup based on an IR picosecond laser. The sample with higher gain shows time resolution of 55 ps at high reverse bias voltage, indicating very promising performance for future particle tracking applications.

Ultra-Fast Silicon Detectors for 4D tracking

We review the progress toward the development of a novel type of silicon detectors suited for tracking with a picosecond timing resolution, the so called Ultra-Fast Silicon Detectors. The goal is to create a new family of particle detectors merging excellent position and timing resolution with GHz counting capabilities, very low material budget, radiation resistance, fine granularity, low power, insensitivity to magnetic field, and affordability. We aim to achieve concurrent precisions of ∼ 10 ps and ∼ 10 μm with a 50 μm thick sensor. Ultra-Fast Silicon Detectors are based on the concept of Low-Gain Avalanche Detectors, which are silicon detectors with an internal multiplication mechanism so that they generate a signal which is factor ∼ 10 larger than standard silicon detectors.

Beam test results of a 16 ps timing system based on ultra-fast silicon detectors

In this paper we report on the timing resolution obtained in a beam test with pions of 180GeV/c momentum at CERN for the first production of 45µm thick Ultra-Fast Silicon Detectors (UFSD). UFSD are based on the Low-Gain Avalanche Detector (LGAD) design, employing n-on-p silicon sensors with internal charge multiplication due to the presence of a thin, low-resistivity diffusion layer below the junction. The UFSD used in this test had a pad area of 1.7mm2. The gain was measured to vary between 5 and 70 depending on the sensor bias voltage. The experimental setup included three UFSD and a fast trigger consisting of a quartz bar readout by a SiPM. The timing resolution was determined by doing Gaussian fits to the time-of-flight of the particles between one or more UFSD and the trigger counter. For a single UFSD the resolution was measured to be 34ps for a bias voltage of 200V, and 27ps for a bias voltage of 230V. For the combination of 3 UFSD the timing resolution was 20ps for a bias voltage of 200V, and 16ps for a bias voltage of 230V.

Simulation of Low Gain Avalanche Detector characteristics based on the concept of negative feedback in irradiated silicon detectors with carrier impact ionization

In this study the main characteristics of silicon Low Gain Avalanche Detectors (LGAD), the dependencies of the collected charge versus bias voltage and fluence, are calculated to fit experimental data. The calculations are based on two previously developed Ioffe Institute models of radiation degradation in Si detectors: 1) a model of two effective energy levels of radiation-induced defects, and 2) a mechanism of internal negative feedback responsible for the gain degradation in irradiated Si detectors originating from the avalanche multiplication at the detector junction. The combination of these models describes well the properties of irradiated p-i-n detectors in a wide range of fluences. For simulating the LGAD characteristics the models are adapted to its n+-pbi-p-p+ structure, where the built-in boron-doped layer pbi produces high electric field sufficient for carrier impact ionization. It is shown that the developed models give adequate quantitative description of the experimental results for the LGADs up to the fluence of 2×1015 n/cm2 including the detector pulse response; however, additional boron removal from the pbi layer is required to have the best correlation with the experimental data. Similar to the physical model developed for silicon strip detectors operated at high voltage, the results are interpreted in terms of the internal negative feedback mechanism. It is shown that in irradiated LGADs this feedback leads to the transfer of a significant fraction of the potential drop from the built-in layer toward the p+ contact. It initiates two negative effects, which both cause the gain degradation with irradiation: the lowering of the electric field in the n+-pbi region that reduces the multiplication probability, and the increase of the collection time and trapping-related charge losses.

Technology developments and first measurements on inverse Low Gain Avalanche Detector (iLGAD) for high energy physics applications

The first Inverse Low Gain Avalanche Detector (iLGAD) have been fabricated at IMB-CNM (CSIC). The iLGAD structure includes the multiplication diffusions at the ohmic contact side while the segmentation is implemented at the front side with multiple p+ diffusions. Therefore, iLGAD is p on p position-sensitive detector with a uniform electric field all along the device area that guarantees the same signal amplification wherever a particle passes through the sensitive bulk solving the main draw of the LGAD microstrip detector. However, the detection current is dominated by holes flowing back from the multiplication junction with the subsequent increase of the transient current pulse duration in comparison with conventional LGAD counterparts. Applications of iLGAD range from tracking and timing applications, like determination of primary interaction vertex, to medical imaging. The paper addresses the optimization of the iLGAD structure with the aid of TCAD simulations, focusing on the electric field profiles of iLGAD and LGAD microstrip structures and the corresponding gain. The electrical performance of the first fabricated samples is also provided. For the first time, we have the experimental demonstration of the signal amplification of these novel iLGAD detectors.

TCAD simulation of Low Gain Avalanche Detectors

In the present work, detailed simulation using Technology Computer Aided Design (TCAD) tool, Silvaco for non-irradiated and irradiated LGAD (Low Gain Avalanche Detector) devices has been carried out. The effects of different design parameters and proton irradiation on LGAD operation are discussed in detail. An already published effective two trap bulk damage model is used to simulate the radiation damage without implementing any acceptor removal term. The TCAD simulation for irradiated LGAD devices produce decreasing gain with increasing fluence, similar to the measurement results. The space charge density and electric field distribution are used to illustrate the possible reasons for the degradation of gain of the irradiated LGAD devices.

Recent technological developments on LGAD and iLGAD detectors for tracking and timing applications

This paper reports the latest technological development on the Low Gain Avalanche Detector (LGAD) and introduces a new architecture of these detectors called inverse-LGAD (iLGAD). Both approaches are based on the standard Avalanche Photo Diodes (APD) concept, commonly used in optical and X-ray detection applications, including an internal multiplication of the charge generated by radiation. The multiplication is inherent to the basic n++–p+–p structure, where the doping profile of the p+ layer is optimized to achieve high field and high impact ionization at the junction.The LGAD structures are optimized for applications such as tracking or timing detectors for high energy physics experiments or medical applications where time resolution lower than 30ps is required. Detailed TCAD device simulations together with the electrical and charge collection measurements are presented through this work.

Ultra-fast silicon detectors (UFSD)

We report on measurements on Ultra-Fast Silicon Detectors (UFSD) which are based on Low-Gain Avalanche Detectors (LGAD). They are n-on-p sensors with internal charge multiplication due to the presence of a thin, low-resistivity diffusion layer below the junction, obtained with a highly doped implant. We have performed several beam tests with LGAD of different gain and report the measured timing resolution, comparing it with laser injection and simulations. For the 300μm thick LGAD, the timing resolution measured at test beams is 120ps while it is 57ps for IR laser, in agreement with simulations using Weightfield2. For the development of thin sensors and their readout electronics, we focused on the understanding of the pulse shapes and point out the pivotal role the sensor capacitance plays.

Design and fabrication of an optimum peripheral region for low gain avalanche detectors

Low Gain Avalanche Detectors (LGAD) represent a remarkable advance in high energy particle detection, since they provide a moderate increase (gain ~10) of the collected charge, thus leading to a notable improvement of the signal-to-noise ratio, which largely extends the possible application of Silicon detectors beyond their present working field. The optimum detection performance requires a careful implementation of the multiplication junction, in order to obtain the desired gain on the read out signal, but also a proper design of the edge termination and the peripheral region, which prevents the LGAD detectors from premature breakdown and large leakage current.This work deals with the critical technological aspects required to optimize the LGAD structure. The impact of several design strategies for the device periphery is evaluated with the aid of TCAD simulations, and compared with the experimental results obtained from the first LGAD prototypes fabricated at the IMB-CNM clean room. Solutions for the peripheral region improvement are also provided.

RD50 Collaboration overview: Development of new radiation hard detectors

Silicon sensors are widely used as tracking detectors in high energy physics experiments. This results in several specific requirements like radiation hardness and granularity. Therefore research for highly performing silicon detectors is required. The RD50 Collaboration is a CERN R&D collaboration dedicated to the development of radiation hard silicon devices for application in high luminosity collider experiments. Extensive research is ongoing in different fields since 2001. The collaboration investigates both defect and material characterization, detector characterization, the development of new structures and full detector systems. The report gives selected results of the collaboration and places an emphasis on the development of new structures, namely 3D devices, CMOS sensors in HV technology and low gain avalanche detectors.

Radiation effects in Low Gain Avalanche Detectors after hadron irradiations

Novel silicon detectors with charge gain were designed (Low Gain Avalanche Detectors - LGAD) to be used in particle physics experiments, medical and timing applications. They are based on a n++-p+-p structure where appropriate doping of multiplication layer (p^+) is needed to achieve high fields and impact ionization. Several wafers were processed with different junction parameters resulting in gains of up to 16 at high voltages. In order to study radiation hardness of LGAD, which is one of key requirements for future high energy experiments, several sets of diodes were irradiated with reactor neutrons, 192 MeV pions and 800 MeV protons to the equivalent fluences of up to Φeq=1016 cm−2. Transient Current Technique and charge collection measurements with LHC speed electronics were employed to characterize the detectors. It was found that the gain decreases with irradiation, which was attributed to effective acceptor removal in the multiplication layer. Other important aspects of operation of irradiated detectors such as leakage current and noise in the presence of charge multiplication were also investigated.

MO‐F‐CAMPUS‐T‐05: Design of An Innovative Beam Monitor for Particle Therapy for the Simultaneous Measurement of Beam Fluence and Energy

Purpose:Monitoring the prescribed dose in particle therapy is typically carried out by using parallel plate ionization chambers working in transmission mode. The use of gas detectors has several drawbacks: they need to be calibrated daily against standard dosimeters and their dependence on beam quality factors need to be fully characterized and controlled with high accuracy. A detector capable of single particle counting is proposed which would overcome all these limitations. Combined with a gas ionization chamber, it will allow determining the average particle stopping power, thus providing an effective method for the online verification of the selected particle energy and range.Methods:Low‐Gain Avalanche Detectors (LGADs) are innovative n‐in‐p silicon sensors with moderate internal charge multiplication occurring in the strong field generated by an additional p+ doping layer implanted at a depth of a few µm in the bulk of the sensor. The increased signal‐to‐noise ratio allows designing very thin, few tens of microns, segmented LGADs, called Ultra Fast Silicon Detectors (UFSD), optimized for very fast signal, which would be suitable for charged particle counting at high rates. A prototype UFSD is being designed for this purpose.Results:Different LGAD diodes have been characterized both in laboratory and beam tests, and the results compared both with those obtained with similar diodes without the gain layer and with a program simulating the signal in the sensors. The signal is found to be enhanced in LGADs, while the leakage current and the noise is not affected by the gain. Possible alternative designs and implementations are also presented and discussed.Conclusion:Thanks to their excellent counting capabilities, UFSD detectors are a promising technology for future beam monitor devices in hadron‐therapy applications. Studies are ongoing to better understand their properties and optimize the design in view of this application.

First measurements of segmented silicon tracking detectors with built-in multiplication layer

Silicon sensors with a built-in multiplication layer, also known as Low Gain Avalanche Detectors (LGAD), are a new technology of potentially radiation hard silicon sensors developed at CNM (Centro Nacional de Microelectrónica) in the framework of the CERN-RD50 collaboration. The concept of the LGAD technology is to produce a tracking sensor with an intrinsic low gain due to charge multiplication in order to enhance the signal-to-noise ratio. This makes LGAD detectors appealing for the high energy physics community which foresees a possible application in harsh radiation environments such as the inner detectors of the experiments at the High Luminosity LHC, where a fluence of 2×1016 neq/cm2 is expected in the innermost pixel layer of the tracker.LGAD pad diodes with a multiplication factor of 10 at 200V before irradiation have been produced and studied but it still has to be proven that this technology can be applied to segmented devices. The electric field profile at the p–n junction plays a crucial role for the multiplication mechanism that could be affected by implantation edge effects in highly segmented devices.In this paper the results of the first measurements of segmented LGAD devices from 285μm thick float zone wafers will be discussed. Time resolved Transient-Current-Technique analysis has been performed on strip devices and charge collection measurements have been carried out on pixel detectors to test the LGAD technology on highly segmented devices.