State-of-the-art and evolution of UFSD sensors design at FBK

doi:10.1016/j.nima.2020.164375

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Volume 978, 21 October 2020, 164375

https://doi.org/10.1016/j.nima.2020.164375 Get rights and content

Abstract

In the past few years, there has been growing interest in the development of silicon sensors able to simultaneously measure accurately the time of passage and the position of impinging charged particles. In this contribution, a review of the progresses in the design of UFSD (Ultra-Fast Silicon Detectors) sensors, manufactured at the FBK (Fondazione Bruno Kessler) Foundry, aiming at tracking charged particles in 4 dimensions, is presented. The state-of-the-art UFSD sensors, with excellent timing capability, are planned to be used in both ATLAS and CMS experiments detector upgrade, in order to reduce the background due to the presence of overlapping events in the same bunch crossing.

The latest results on sensors characterization including time resolution, radiation resistance and uniformity of the response are here summarized, pointing out the interplay between the design of the gain layer and the UFSD performances. The research is now focusing on the maximization of the sensor fill factor, to be able to reduce the pixel size, exploring the implementation of shallow trenches for the pixel isolation and the development of resistive AC-coupled UFSD sensors. In conclusion, a brief review on research paths tailored for detection of low energy X-rays or for low material budget applications is given.

Introduction

The UFSD development, started as an ERC¹ supported project in 2015, aims at the realization of silicon detectors for 4D tracking with excellent time and space resolution, able to achieve concurrently a time resolution of the order of tens of ps, and a space resolution $\approx$ tens of $μ m$ . The sensor technology used as baseline are Low Gain Avalanche Diodes (LGAD), an evolution of the n-on-p planar silicon sensor incorporating a low (10–30 range), controlled gain in the signal formation mechanism [1]. The charge multiplication conditions, where electrons and holes acquire sufficient kinetic energy to generate additional e/h pairs (electric field E $\sim$ 300 kV/cm), are obtained by implanting a layer of acceptors with appropriate charge density ( $ρ_{A} \sim 1 0^{16} {cm}^{3}$ ) below the n-p junction, the so-called gain layer. The key points of LGADs optimized for timing are: signals large and fast enough to assure excellent timing performance while maintaining almost unchanged levels of noise (low jitter term), reduced Landau fluctuations ( $\approx$ $50 μ m$ thin sensors), and a very uniform weighting field. A detailed description of the UFSD characteristics can be found in [2], [3].

Within the UFSD project, the FBK Foundry started developing LGADs in 2016. Since then, four productions of 50-micron thin UFSDs have been completed, covering several aspects of R&D work necessary to reach the project goal performances, including the radiation hardness of the sensors, which should match the requirements of the future High Luminosity Large Hadron Collider (HL-LHC) experiments.

Section snippets

UFSD Sensors: key performances

In this section, an overview of performances of the latest UFSD productions is reported, covering the topics on gain uniformity, radiation hardness an time resolution. To be noticed that the typical pad size of the state-of-the-art LGAD is of the order of 1–3 mm $^{2}$ . The roadmap towards fine-segmented LGADs, able to measure also the position of the traversing particle with high resolution, can be found in Section 3. It is worth mentioning that the latest UFSD production (UFSD3) was partially

Towards fine-segmented LGADs

In the current UFSD design, the area between read-out pads is hosting a Junction Termination Extension (JTE) on each side to contain the gain layers of the two adjacent pads, separated by a p-stop area for the electrical isolation of the pads (a p-type material implantation with a certain pattern). This design leads to a no-gain area for signal collection, due to the nominal distance between the two gain layers, and to an extra periphery of the gain implant where the charges are collected by

Future developments for low energy X-rays detection

A recently approved three-years R&D project aims at the reduction of the active and the physical thickness of the LGAD sensors, down to 20- $30 μ m$ , and at the implementation of a very thin rear entrance window to the active volume of the device. Such improvements open the way to different fields of application, such as low energy (keV) X-rays detection, and to very low material budget applications. In this project, the reduction of the size of the pad is not at the center of the development work.

Conclusion

The UFSD project started in 2015 with the goal of designing sensors suitable for 4D tracking in High Energy Physics experiments. Several productions implementing aggressive or optimized technological solutions have been completed and thoroughly studied. The state-of-the-art UFSD sensor has achieved: excellent time resolution ( $≃$ 30 ps); very good production uniformity and yield for sensors of $\sim$ 3 cm $^{2}$ ; optimization of the gain layer design to enhance operating parameters and radiation hardness

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We thank our collaborators within RD50, ATLAS and CMS who participated in the development of UFSD. Part of this work has been financed by the European Union Horizon 2020 Research and Innovation funding program, under Grant Agreement no. 654168 (AIDA-2020) and Grant Agreement no. 669529 (ERC UFSD669529), by the Italian Ministero degli Affari Esteri, by INFN, Italy Gruppo V and by the Dipartimento di Eccellenza, University of Torino, Italy (ex L. 232/2016, art. 1, cc. 314, 337). The work was

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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 n $_{e q}$ /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) $50 μ m$ LGAD sensors already deliver adequate charge collection >4 fC and time resolution <50 ps at voltages <400 V. The operation voltages of these $50 μ m$ devices are well below those at which single event burnout may occur.
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Citation Excerpt :
Therefore, those detectors are not suitable for counting and timing applications in proton therapy. It straightforward the benefits of using LGAD technology, which has improved in the last few years [19]. First, the signal is fast (order of few ns) and large enough, allowing single proton detection capabilities in the clinical range.
A fast 144-channel proton counter prototype, designed for monitoring the fluence rate of clinical proton beams, is based on a thin Low Gain Avalanche Detector (LGAD), segmented into 146 strips (114 $μ m$ width, 26214 $μ m$ length, 180 $μ m$ pitch). The layout of the sensor was designed in the framework of the Modeling and Verification for Ion beam Treatment planning (MoVe-IT) project in collaboration with Fondazione Bruno Kessler (FBK, Trento, Italy) and fourteen wafers were produced and delivered by FBK in 2020. In this paper, we present the laboratory characterization of the sensors performed on the entire wafer at FBK, right after production, and at the University of Turin after cutting the sensors using a probe station connected with a power device analyzer for static electrical tests and an infrared picosecond laser to study the dynamic properties. In addition, one sensor was tested with the clinical proton beam at National Center for Oncological Hadrontherapy (CNAO, Pavia, Italy).
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The Low Gain Avalanche Detector (LGAD) is a novel Silicon detector with precise timing and fast response. To study the transient response of LGADs, we need to obtain the detector signal using Transient Current Technique (TCT), for which a preamplifier with high bandwidth and low noise is required. Currently, the most widely used preamplifier board in the LGAD community is developed by University of California, Santa Cruz (UCSC). In this paper, we will introduce a newly designed LGAD preamplifier board. Compared with the UCSC board, the performance has been fully improved. The board contains a transimpedance amplifier and two-stage voltage amplifiers. The total charge gain of our preamplifier reaches 20.58 mV ns/fC, with good linearity in the input dynamic range of [0.7 fC, 66 fC]. The $-$ 3-dB bandwidth is about 870 MHz, meeting the demands of LGAD amplification. We conducted ⁹⁰Sr $β$ -scope tests using our preamplifiers and HPK 3.1 single sensors. The equivalent noise charge at 20 °C is 0.27 fC. The jitter contributed by our preamplifier is about 7.12 ps. The results show that our single-channel preamplifier has lower noise and better time resolution than the UCSC boards. Besides, a 9-channel preamplifier board is developed for multi-pad LGAD readout. All channels have good uniformity and low noise. Our preamplifier boards can be well applied in $β$ -scope or laser-scanning TCT tests of LGADs.
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The High Luminosity upgrade of the Large Hadron Collider highlighted the need for a time-tagging of tracks with a precision of tens of picoseconds. This requirement motivated the development of radiation hard silicon sensors dedicated to the time-of-interaction measurement of minimum ionizing particles. Low Gain Avalanche Diodes (LGADs) are silicon sensors with internal charge multiplication and are the baseline for the timing systems of the ATLAS and CMS experiments. These sensors use their gain to improve the signal to noise ratio (SNR) of detector systems and have been engineered to withstand the harsh radiation environment of the experiments. Fondazione Bruno Kessler (FBK) developed the LGAD technology through several production runs. The improved SNR and excellent time resolution made LGADs suitable also for medical, X-ray, and space applications. A feature of LGADs is the presence of a termination structure between regions with gain that results in areas without gain between the readout channels, reducing the fill factor of the devices. Different strategies to improve the fill factor of LGADs are being developed, such as double sided LGADs, resistive AC-coupled LGADs, and trench isolated LGADs. This paper summarizes the experience acquired at FBK with the realization of more than ten sensor batches. Selected results in radiation hardness, time resolution, fill factor, and different LGAD applications will be discussed.

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State-of-the-art and evolution of UFSD sensors design at FBK

Abstract

Introduction

Section snippets

UFSD Sensors: key performances

Towards fine-segmented LGADs

Future developments for low energy X-rays detection

Conclusion

Declaration of Competing Interest

Acknowledgments

Nucl. Instrum. Methods Phys. Res. A

Nucl. Instrum. Methods Phys. Res. A

Nucl. Instrum. Methods Phys. Res. A

4D tracking with ultra-fast silicon detectors

Rep. Progr. Phys.