Tissue Oxygen Sensor

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Recent progress of the ATLAS Planar Pixel Sensor R&D

M. Bombena,1,∗

a Laboratoire de Physique Nucleaire et de Hautes ´Energies (LPNHE) Tour 12-22, 4, place Jussieu, FR-75252 Paris Cedex 05

Abstract

The foreseen luminosity upgrade for the LHC (a factor of 5-10 more in peak luminosity by 2021) poses serious con- straints on the technology for the ATLAS tracker in this High Luminosity era (HL-LHC). In fact, such luminosity increase leads to increased occupancy and radiation damage of the tracking detectors.

Figure: System Model & Simulation Flow for Tissue Oxygen Sensor

To investigate the suitability of pixel sensors using the proven planar technology for the upgraded tracker, the ATLAS Planar Pixel Sensor R&D Project was established comprising 17 institutes and more than 80 scientists. Main areas of research are the performance of planar pixel sensors at highest fluences, the exploration of possibilities for cost reduction to enable the instrumentation of large areas, the achievement of slim or active edge designs to provide low geometric inefficiencies without the need for shingling of modules and the investigation of the operation of highly irradiated sensors at low thresholds to increase the efficiency.

Figure: System Model & Simulation Flow for Tissue Oxygen Sensor

In the following I will present results from the group, concerning mainly irradiated-devices performance, together with studies for new sensors, including detailed simulations. c⃝2011 Published by Elsevier Ltd.

Figure: System Model & Simulation Flow for Tissue Oxygen Sensor

Keywords:

Silicon pixel detectors, planar sensors, radiation damage to detector materials (solid state), detector simulations,

1. Introduction

ATLAS is a general purpose detector for the study of primarily proton-proton collisions at the

Lhc .

The ATLAS Inner Detector [3, 4] provides charged-particle tracking with high efficiency over the pseu- dorapidity range |η| < 2.5. The pixel detector system is the innermost element of the Inner Detector. The pixel detector contains approximately 80 million channels and provides pattern recognition capabil- ity in order to meet the track reconstruction requirements of ATLAS at the full luminosity of the LHC of L = 1034cm−2s−1.

Figure: System Model & Simulation Flow for Tissue Oxygen Sensor

∗Corresponding Author

1on behalf of the ATLAS Planar Pixel Sensor R&D Project https://twiki.cern.ch/twiki/bin/view/Atlas/PlanarPixelUpgrade

/ Physics Procedia 00 (2021) 1–9

Consisting of three barrel layers (at radii between 50.5mm and 122.5mm), and six discs, the pixel detec- tor counts a total of 1744 pixel modules, which are mounted allowing for a three-hit track-reconstruction of charged secondary-particles. Each module contains a 250 µm thick n-in-n pixel sensor of 62.6 × 18.6mm2 with pixel cells of 50×400 µm2. Connected to each sensor are 16 7.4×11.0mm2 ATLAS FE-I3 readout chips with a total of 46080 channels. Both the sensors as well as the electronics of the present ATLAS pixel modules were specified to work up to a fluence of 1015neq/cm2 and an ionising dose of 50 MRad.

Figure: System Model & Simulation Flow for Tissue Oxygen Sensor

While the nominal luminosity of the present LHC accelerator is 1034cm−2s−1, an upgrade to increase the luminosity by a factor of ten (five with luminosity leveling) is planned to be carried out in a two phase process . After a first shutdown, foreseen for 2017, the Phase 1 of LHC will start, with the goal luminosity of (2 −3) × 1034cm2s−1. A second shutdown will take place after 2020; then the Phase 2 will start, and the expected luminosity is 5 × 1034cm−2s−1 with luminosity levelling. By 2030 a total integrated luminosity of O(3000fb−1) is envisaged. Based on this scenario the innermost layer of the ATLAS pixel system will have to sustain fluences above 1 × 1016 neq/cm2 ; see also figure 1.

Figure: System Model & Simulation Flow for Tissue Oxygen Sensor

Fig. 1. Fluences in neq/cm2 expected for a phase II integrated luminosity of 3000 fb−1. A safety factor of 2 is applied. The main effects on the pixel sensors from these large fluences are an increase of the leakage current and a reduction in the charge collection efficiency. The final goal is to retain a good hit-efficiency up to the final integrated luminosity.

Another high-luminosity related effect is the increase of the occupancy of the detector. A natural option is to reduce the size of the elementary pixel cell; at the same time the spatial resolution will improve. The reduced size of the elementary cell is just one of the features of the new read-out chip for the pixel system, the FE-I4 chip; the total surface of the FE-I4 chip is 20.0 × 18.6 mm2. Moreover, replacing the strips of the Inner Detector with pixel detectors might be an option to handle the occupancies foreseen at the

Hl-Lhc.

Hence, in view of a possible pixel system replacement in 2017; and then, maybe, for a whole new tracking system after 2020, a new Pixel System is unders study. The new pixel sensors will have not only to sustain the harsher environment, but also to show high geometrical acceptance: for the future the material budget restrictions and the geometrical limitations ask for geometry inefficiency to be below 2.5%. Hence the inactive areas of the future pixel sensor should be less than 450 µm wide .

Different sensor options are being developed in parallel to address the challenges imposed by the forseen luminosity upgrades. They include diamond sensors and 3D-sensors, with implants going through the silicon bulk . Optimizing the well-known technology of planar silicon pixel sensors for the ATLAS

3

detector at an upgraded LHC accelerator is carried out within the Planar Pixel Sensor project .

2. The Project

Planar pixel sensors are the current technology for the ATLAS pixel system and a standard for tracking detectors in High Energy Physics. A lot of experience in designing, optimizing and producing planar sensors has been accumulated in the last 30 years; during these three decades planar sensors proved to be a reliable technology. There is still a lot of research on planar sensors and a lot of suppliers are on the market, assuring high quality and relatively low cost productions. Hence planar sensor are one good option for future tracking detectors. The ATLAS Upgrade Planar Pixel Sensor (PPS) R&D Project is a collaboration of 17 groups and more than 80 scientists aiming to explore the suitability of planar pixel sensors for the ATLAS upgrade.

Primary goal of the project is to demonstrate that planar sensors have the requested radiation hardness for the HL-LHC. To investigate this, several planar sensors technologies are under evaluation, including p-bulk (Section 3) and n-bulk (Section 4) option; regardless of the bulk, the group is looking only at sensors collecting electrons, based on their smaller trapping time compared to holes .

A reduction in sensor thickness (with respect to the 250 µm of the current ATLAS pixels) is investigated (Section 5), to reduce the effects of trapping after irradiation and the material budget. In case of a complete replacement of the Inner Detector a solution with more than 4 layers of pixels is possible. In this way an area of roughly 10 m2 should be equipped with pixel modules. Hence it is crucial to minimize the costs of the future pixel system. PPS is evaluating several cost-reduction scenarios, including new bump-bonding technologies, given the fact that the bump-bonding cost dominated the cost of the current ATLAS pixel detector. The possibility to use p-bulk sensor is favorable in the view of a cost reduction, having only one side patterned contrary to two-side patterned n-bulk sensors.

The new sensors will have to assure a close to 100% geometrical acceptance, so different options toward a reduction of the inactive area are pursued in the collaboration, such as slim edge geometries(Section 6) and active edge detectors (Section 7).

Many of the new ideas for sensors are tested in advance by means of sensor simulations (Section 8) thanks to a TCAD software. In Section 9 the plans of the PPS R&D collaboration will be outlined, together with some general remarks on the work done so far.

3. P-Bulk Overview

In this Section a quick overview of the PPS activities on p-bulk sensors will be given. Standard n-in-n sensors need guard-rings opposite to pixel side: the production requires to pattern both sides of the wafer. On the contrary, p-bulk does not need such a double processing, having only an homo- geneous implantation on the back side. This should allow for a significant cost reduction. Another major advantage of p-bulk over n-bulk is that there is no type inversion for irradiated material .

A problem under investigation for the n-in-p detectors is the drop between the high voltage applied to the back side and the readout electronics potential ( 0V) which occurs solely on the sensor top side facing the readout electronics. In figure 2 a sketch of the p-bulk sensor geometry is displayed, highlighting the issue with the voltage drop on the sensor in a region close to the electronics.

Covering the sensor with a thin layer of BCB (Benzo Cyclo Butene) proved to prevent sparks up to 1000

V.

Several contributors are working with p-bulk sensors in the PPS R&D group, namely CiS Forschungsin- stitut f¨ur Mikrosensorik und Photovoltaik GmbH, Micron Semiconductor Ltd., Hamamatsu Photonics K.K. (HPK) and Max Planck Institut f¨ur Physik - Max Planck Institut Halbleiterlabor (MPI-HLL). A variety of sensor thicknesses is currently investigated by the different contributors. In table 1 a summary is given.

More details are given in .

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Fig. 2. P-bulk sensor sketch; potential of the different parts are indicated, while detectoro is biased.

150/75

Table 1. P-bulk sensor suppliers and relative wafer thicknesses

4. N-Bulk Sensors: Results For Irradiated Devices

To assess the radiation hardness of n-bulk sensors, irradiation campaigns have been carried out and crucial observables, such as the charge colleced per impinging particle and hit efficiency, were studied as a function of the delivered dose. In the following details on the studies performed on irradiated n-in-n sensors will be given.

The starting point for the PPS R&D project are the ATLAS single chip pixel modules . Planar n-in-n ATLAS sensors, already assembled with a FE-I3 chip in a single chip module (SCM) were irradiated with neutrons at the TRIGA nuclear reactor of the Jozef Stefan Institute in Ljubljana. Modules were irradi- ated with fluences up to 2 × 1016neq/cm2 and then characterized with radioactive sources and beams . In figure 3 the two experimental setups are shown.

Fig. 3. left: Schematics of the 90Sr setup, with source, collimator, module and scintillator detector; right: the CERN SPS testbeam setup. Pions were incoming from the right, traversing the first arm of the telescope, then the black box with the DUTs, As displayed on the left, the 90Sr source was enclosed by a collimator. The module was cooled with dry ice and operated at temperatures below -50◦C. A trigger scintillator was mounted below the SCM.

The SCMs were also operated in testbeams at the CERN SPS beamline (see figure 3, right) in July and October 2010. Thanks to the testbeam, it was possible to compare the energy depositions of high-energy pions with those of beta-electrons and to be able to observe the effect of different incidence angles after high fluences. The testbeam was conducted with the EUDET beam telescope which uses six MIMOSA26 chips as telescope planes. The six telescope planes were defining the tracks and up to six devices under

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