Detailed studies of full-size ATLAS12 sensors

doi:10.1016/j.nima.2016.03.042

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

Volume 831, 21 September 2016, Pages 167-173

https://doi.org/10.1016/j.nima.2016.03.042 Get rights and content

Abstract

The “ATLAS ITk Strip Sensor Collaboration” R&D group has developed a second iteration of single-sided n⁺-in-p type micro-strip sensors for use in the tracker upgrade of the ATLAS experiment at the High-Luminosity (HL) LHC. The full size sensors measure approximately $97 \times 97 {mm}^{2}$ and are designed for tolerance against the $1.1 \times 10^{15} n_{eq} / {cm}^{2}$ fluence expected at the HL-LHC. Each sensor has 4 columns of 1280 individual $23.9 mm$ long channels, arranged at $74.5 μ m$ pitch. Four batches comprising 120 sensors produced by Hamamatsu Photonics were evaluated for their mechanical, and electrical bulk and strip characteristics. Optical microscopy measurements were performed to obtain the sensor surface profile. Leakage current and bulk capacitance properties were measured for each individual sensor. For sample strips across the sensor batches, the inter-strip capacitance and resistance as well as properties of the punch-through protection structure were measured. A multi-channel probecard was used to measure leakage current, coupling capacitance and bias resistance for each individual channel of 100 sensors in three batches. The compiled results for 120 unirradiated sensors are presented in this paper, including summary results for almost 500,000 strips probed. Results on the reverse bias voltage dependence of various parameters and frequency dependence of tested capacitances are included for validation of the experimental methods used. Comparing results with specified values, almost all sensors fall well within specification.

Introduction

The foreseen upgrade of the Large Hadron Collider (LHC) to the High-Luminosity LHC (HL-LHC) is scheduled to deliver collisions in 2022 [1]. To achieve a total cross-section of $3000 {fb}^{- 1}$ , the instantaneous luminosity of the HL-LHC is expected to reach $5 \times 10^{34} {cm}^{- 2} s^{- 1}$ at a centre of mass energy of 14 TeV. The increase in particle fluence necessitates an upgrade of the ATLAS inner detector: an all-silicon new inner tracker (ITk) [2] is proposed to replace the current SemiConductor Tracker (SCT) and Transition Radiation Tracker (TRT). The ITk layout as presented in the Letter of Intent [3] assumes silicon microstrip detectors to be used for 7 endcap disks, and 5 barrel layers. From simulations verified by experiments, the highest particle fluence in the barrel short strip layer is expected to be $5.3 \times 10^{14} n_{eq} / {cm}^{2}$ [4]. Including a safety factor of 2, candidate ITk sensors will have to be radiation hard up to levels of $1.1 \times 10^{15} n_{eq} / {cm}^{2}$ . The goal of the ITk Strip Sensor collaboration is to develop a silicon microstrip sensor that is suitable for use in the new ITk. Results of detailed studies of properties of full-size sensor prototypes are presented in this paper, whereas studies of radiation damage of $1 \times 1 {cm}^{2}$ miniature sensors are reported in [5], [6].

Section snippets

ATLAS12 large area sensors

The ATLAS12 sensors are the second iteration of sensors designed for the Upgrade ITk, superseding the ATLAS07 types [7]. To cope with the effects of radiation damage during the sensor lifetime, operation in partial depleted mode is foreseen towards the end of the detector lifetime. The sensor will need specially designed structures between the strips to guarantee strip isolation during its lifetime and mitigate radiation-induced surface damage whilst retaining a low inter-strip capacitance. A

ATLAS12A mechanical properties

The sensor mechanical specifications state the following:

•
nominal thickness: $310 \pm 20 μ m$ ;
•
thickness variation: $\pm 10 μ m$ across the sensor. This means some thickness variation is allowed between sensors, but not across a sensor;
•
sensor flatness when unstressed: $< 200 μ m$ ;
•
outer cut dimensions: $97540 \pm 25 μ m^{2}$ ;
•
inner cut dimensions: $95, 692 \pm 25 μ m^{2}$ ; and
•
no cracks or chips at the dicing line to extend further inwards than $50 μ m$ .

For visual inspection and evaluation of the above properties, a non-contact optical

Leakage current

Development of the leakage current against bias voltage $(V_{bias})$ for all 120 sensors is plotted in Fig. 4. The data was taken at $T = 21 ° C$ , and at a relative humidity (RH) of $40 \pm 10 %$ . Since the ATLAS12 sensors are very sensitive to the ambient humidity, the initial batch VPX12318 was tested in a dry $N_{2}$ environment. As can be observed from the graphs on VPX12318 however, a considerable number of sensors exhibit early, soft breakdown. The two subsequent batches were stored in dry atmosphere, and

Measurement techniques

To evaluate properties of individual strips, study the uniformity of electrical characteristics over the entire sensor surface, and compare against other sensors, full scans of individual sensor channels were made. A relatively simple Strip Test protocol, see Section 5.2, was used to check for strip shorts and pinholes, and measure the $C_{coupling}$ and $R_{bias}$ for each individual strip. A single probe needle was used for probing the initial 31 sensors in batch VPX12318, after that a custom 32

Summary and conclusion

Detailed studies on ATLAS12 sensors produced by Hamamatsu Photonics have been presented in this paper. Results from the evaluation are summarised in Table 1, and compared to the specifications set out in the ATLAS12 Technical Specification Document. 118 of 120 sensors tested satisfy the specifications for non-irradiated sensors for maximum bow, leakage current. Only 0.03% of probed strips were measured as defective; all other strips satisfied the requirements for $C_{coupling}$ and $R_{bias}$ . The values

Acknowledgements

The Irradiations were performed: with protons at the University of Birmingham MC40 cyclotron, supported by the H2020 project AIDA-2020, GA number 654168, and the UK׳s Science and Technology Facilities Council, at Cyclotron and Radioisotope Center (CYRIC), Tohoku University, with Y. Sakemi, M. Ito, and T. Wakui, at the Karlsruhe Institute of Technology (KIT) by A. Dierlamm, supported by the Initiative and Networking Fund of the Helmholtz Association, Contract HA-101 (Physics at the Terascale)

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Citation Excerpt :
The average bow and its RMSE are 51.18 μm and 9.30 μm respectively. These values are well within the specification and compare quite closely with a similarly measured sample of 100 ATLAS12A (51.72 μm and 12.36 μm) [14]. The thickness of the sensors is measured in a minimum of five locations on the sensor.
A novel microstrip sensor geometry, the stereo annulus, has been developed for use in the end-cap of the ATLAS experiment’s strip tracker upgrade at the HL-LHC. Its first implementation is in the ATLAS12EC sensors, a large-area, radiation-hard, single-sided, AC-coupled, n $^{+}$ -in-p design produced by the ITk Strip Sensor Collaboration and fabricated by Hamamatsu Photonics in early 2017. The results of the initial testing of two ATLAS12EC batches are presented here with a comparison to specification. The potential of the new sensor shape to reinvigorate endcap strip tracking is explained and its effects on sensor performance are isolated by comparing the bulk mechanical and electrical properties of the new sensor to the previous iteration of prototype, the conventional barrel ATLAS12A sensor. The surface properties of the new sensor are evaluated for full-size unirradiated sensors as well as for mini sensors unirradiated and irradiated with protons up to a fluence of $2.2 \times 1 0^{15}$ n_eq cm⁻². The results show that the new stereo annulus ATLAS12EC sensors exhibit excellent performance and the expected irradiation evolution.
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¹: Now at Syracuse University, USA.

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