Development of n+-in-p large-area silicon microstrip sensors for very high radiation environments – ATLAS12 design and initial results

doi:10.1016/j.nima.2014.06.086

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

Volume 765, 21 November 2014, Pages 80-90

https://doi.org/10.1016/j.nima.2014.06.086 Get rights and content

Abstract

We have been developing a novel radiation-tolerant n⁺-in-p silicon microstrip sensor for very high radiation environments, aiming for application in the high luminosity large hadron collider. The sensors are fabricated in 6 in., p-type, float-zone wafers, where large-area strip sensor designs are laid out together with a number of miniature sensors. Radiation tolerance has been studied with ATLAS07 sensors and with independent structures. The ATLAS07 design was developed into new ATLAS12 designs. The ATLAS12A large-area sensor is made towards an axial strip sensor and the ATLAS12M towards a stereo strip sensor. New features to the ATLAS12 sensors are two dicing lines: standard edge space of 910 μm and slim edge space of 450 μm, a gated punch-through protection structure, and connection of orphan strips in a triangular corner of stereo strips. We report the design of the ATLAS12 layouts and initial measurements of the leakage current after dicing and the resistivity of the wafers.

Introduction

We have been developing n⁺-in-p silicon microstrip sensors with p-type silicon wafers aiming for a cost-effective and radiation-tolerant solution for covering an area over 100 m² in very high radiation environments. A specific application of such a silicon microstrip sensor is a replacement of the outer part of the inner tracker in the ATLAS detector [1] for the high luminosity large hadron collider (HL-LHC) [2].

The current inner tracker of the ATLAS detector consists of a pixel tracker (Pixels) at radii of 5–12 cm, a silicon microstrip tracker (SCT/Strips) at 30–51 cm (Barrel cylinders) and at 28–56 cm (Endcap discs) and a transition radiation tracker (TRT) at 56–107 cm. The silicon sensor area is 2.7 m² and 62 m² in Pixels and Strips, respectively. For the HL-LHC, not only the entire inner tracker is to be replaced but also the TRT is to be replaced with Strips [3]. The latest layout of the inner tracker is Pixels at 4–25 cm and Strips at 40–100 cm. The silicon area is 8.2 m² and 193 m² (122 Barrel and 71 Endcap per m²), respectively. In both Pixels and Strips, the areas are increased to approximately three times that of the original ATLAS detector.

With the integrated luminosity of 3000 fb⁻¹ and a safety factor 2, a number of particles passing through an unit area in the tracker volume has been simulated [4], as shown in Fig. 1. The typical fluences are approximately: 2.2×10¹⁶ 1 MeV-neutrons equivalent ( $n_{eq}$ )/cm² at 3.7 cm for the Pixels, and $1 \times 10^{15} n_{eq / {cm}^{2}}$ at 31 cm and $5 \times 10^{14} n_{eq / {cm}^{2}}$ at 60 cm for the Strips. Charged particles dominate inside and neutral particles, mainly neutrons, dominate outside a radius of 25 cm. Our goal of the radiation-tolerant silicon microstrip sensor is to cope with a fluence of $\geq 1 \times 10^{15} n_{eq / {cm}^{2}}$ and to be robust against both charged and neutral particles.

Section snippets

Radiation-tolerant n⁺-in-p silicon strip sensors

The sensor of n⁺-implant readout in p-type silicon substrate (n⁺-in-p) is intrinsically more radiation-tolerant. This is because the p–n junction develops from the readout implants before and after radiation damage as the radiation-induced levels in the silicon band-gap are primarily acceptor states (p-type) [5]. The fact leads to a number of virtues: (1) the sensors can be operated under partially depleted conditions when the full depletion voltage is extremely high after radiation damage; (2)

ATLAS12 wafer layouts

Developing from the ATLAS07 sensors, we have designed a new set of strip sensors, the ATLAS12A and the ATLAS12M large-area main sensors and miniature sensors in 6-in. wafer. The wafer layouts of the ATLAS12A and the ATLAS12M sensors are shown in Fig. 3. Both ATLAS12A and ATLAS12M large-area main sensors are made to have four “blocks of short strips” (segment). The ATLAS12A main sensor is prototyping an “axial” strip sensor where the strips are running parallel to the sensor edges. The ATLAS12M

Leakage current

The wafer processing of 120 ATLAS12A and 45 ATLAS12M wafers was completed using wafer lots of 6 in., p-type, FZ, and 320 μm thick. Out of these wafers, 30 ATLAS12A and 26 ATLAS12M wafers were diced using stealth dicing technology [21]. For the ATLAS12A dicing, 25 wafers were given standard and 5 slim dicing. For the ATLAS12M, 3 wafers were given standard and 23 slim dicing; thus, in total, 28 standard and 28 slim dicing samples were obtained. The leakage currents as a function of the bias voltage

Summary

We have been developing novel radiation-tolerant n⁺-in-p silicon microstrip sensors for very high radiation environments, aiming for an application in the inner tracker of the ATLAS detector upgrade for the HL-LHC. Radiation tolerance has been studied with ATLAS07 sensors and with independent structures. Incorporating the results obtained for the minimum edge space for holding 1 kV bias voltage and the novel concept of “gated” PTP structure, the ATLAS07 design is developed into new ATLAS12

Acknowledgments

The research was partly supported by the Ministry of Education, Youth and Sports of the Czech Republic (Grant number LG13009), the German Federal Ministry of Education and Research, and the Helmholtz Association, the Japan Society for Promoting Science KAKENHI A (Grant number 20244038) and KAKENHI C (Grant number 20540291), the Japan MEXT KAHENHI for Research on Priority Area (Grant number 20025007) and for Scientific Research on Innovative Areas (Grant number 23104002), the Slovenian Research

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Measuring the border of the active area on silicon strip sensors
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Silicon strip sensors for the ATLAS Inner Tracker (ITk) have been designed to provide reliable particle detection in the high-radiation environment of the High-Luminosity Large Hadron Collider. One important design criterion for their development is the minimization of inactive sensor areas, which affect the hermiticity of particle detection inside the detector. In previous measurements of ATLAS silicon strip sensors, the charge-collecting area of individual strip implants has been mapped and found to agree with the sensor strip pitch and strip length. For strip implants next to the sensor bias ring, the extent of their charge-collecting area towards the inactive sensor area was previously unknown, which limited the accuracy of both overall detector hermiticity estimates and the position resolution for particle detection at the sensor edge. Therefore, measurements were conducted to map the area of charge collection for sensor strips at the edge of the active sensor area using a micro-focused X-ray beam. This publication presents measurements showing the extent of charge collection in the edge strips of silicon strip sensors for two generations of ATLAS ITk strip sensor modules. The measurements confirmed that charge deposited in a strip implant that is neither connected nor grounded leads to capacitive coupling to the adjacent strip, where it is indistinguishable from a hit in that strip.
Electrical characterization of surface properties of the ATLAS17LS sensors after neutron, proton and gamma irradiation
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Surface properties of the long-strip barrel, full-sized and miniature sensors have been studied before and after proton, neutron and gamma irradiation up to the maximal fluences and radiation doses specified for the ITk Strip tracker. Sensors have been irradiated by protons at CYRIC, Tohoku University (Japan), the Proton Irradiation Facility at CERN, Karlsruhe Inst. Tech. (Germany) and at the University of Birmingham (UK), by neutrons from the Ljubljana TRIGA reactor (Slovenia) and by gamma rays from the $^{60}$ Co source in UJP Praha (Czech Republic).
It has been verified that the surface radiation damage does not influence the sensor functionality. The breakdown voltage is well above the maximum operational voltage. All the tested surface parameters, such as the inter-strip resistance and capacitance, coupling capacitance and bias resistance satisfy the ATLAS ITk specifications for strip sensors.
Testbeam studies of barrel and end-cap modules for the ATLAS ITk strip detector before and after irradiation
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In order to cope with the occupancy and radiation doses expected at the High-Luminosity LHC, the ATLAS experiment will replace its Inner Detector with an all-silicon Inner Tracker (ITk), consisting of pixel and strip subsystems.
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Development of n+-in-p large-area silicon microstrip sensors for very high radiation environments – ATLAS12 design and initial results

Abstract

Introduction

Section snippets

Radiation-tolerant n+-in-p silicon strip sensors

ATLAS12 wafer layouts

Leakage current

Summary

Acknowledgments

Nuclear Instruments and Methods in Physics Research Section A

Nuclear Instruments and Methods in Physics Research Section A

Nuclear Instruments and Methods in Physics Research Section A

Nuclear Instruments and Methods in Physics Research Section A

Nuclear Instruments and Methods in Physics Research Section A

Nuclear Instruments and Methods in Physics Research Section A

Nuclear Instruments and Methods in Physics Research Section A

Nuclear Instruments and Methods in Physics Research Section A

Nuclear Instruments and Methods in Physics Research Section A

Nuclear Instruments and Methods in Physics Research Section A

Nuclear Instruments and Methods in Physics Research Section A

Development of n⁺-in-p large-area silicon microstrip sensors for very high radiation environments – ATLAS12 design and initial results

Radiation-tolerant n⁺-in-p silicon strip sensors