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ATLAS Tracker Upgrade: Silicon Strip Detectors for the sLHC
Nuclear Physics B (Proc. Suppl.) 215 (2011) 154–156 www.elsevier.com/locate/npbps
ATLAS Tracker Upgrade: Silicon Strip Detectors for the sLHC Liv Wiik a on behalf of the ATLAS Collaboration a
Physikalisches Institut, Albert-Ludwigs-Universit¨ at Freiburg, Herrman-Herder-Str. 3, 79104 Freiburg, Germany After the successful start of the Large Hadron Collider (LHC) in November 2009 plans for a luminosity upgrade, called super-LHC (sLHC), are already advancing well. With a planned luminosity of 5 × 1034 cm−2 s−1 the ATLAS detector has to cope with both a high instantaneous and integrated luminosity, providing a challenging environment for the tracking and vertexing detectors. Hence a new tracker is foreseen for the sLHC operation. As the radiation dose will increase according to the integrated luminosity, novel radiation hard detectors are required. A large R&D program is underway to develop silicon sensors with sufficient radiation hardness. In this article measurements of silicon strip sensors designed by the ATLAS Silicon Strip Sensor Upgrade Collaboration are presented. Both measurements of sensors irradiated to the expected sLHC fluences for different detector regions as well as comparisons between measurements of full size sensor properties and their technical specifications are shown. Furthermore the design challenges of the inner detector layout and support structures are presented.
1. Introduction A luminosity upgrade by a factor of five of the Large Hadron Collider (LHC), to the superluminous Large Hadron Collider (sLHC), is being considered as an extension of the LHC physics program. In order to fully exploit the physics potential given by the upgrade the performance of the existing ATLAS detector must be preserved if not improved. The increase of the luminosity will have a significant effect on the detectors by the large increase in radiation damage or the number of pile-up events per bunch crossing. The most significant upgrade of the ATLAS detector will be the replacement of the Inner Detector. 2. ATLAS SCT upgrade The existing ATLAS Inner Detector (ID) consists of a Silicon Pixel Detector (Pixel), a Silicon Strip Detector (SCT) and a Transition Radiation Tracker (TRT). It is expected that the TRT will not be able to cope with the expected sLHC occupancy. This leads to an all silicon ID for the upgrade consisting of a 4 layer pixel system, and five silicon microstrip layers. The inner three layers are designed to replace the SCT, and will be based on silicon detectors with 24 mm long strips, 0920-5632/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2011.03.163
the so called short strip layers. The outer two layers will be equipped with 96 mm long strip detectors, and are aimed to substitute the ATLAS TRT [1]. The upgrade of the ID will be completed with a set of end-cap disks at each side. Since the radiation damage scales with luminosity novel radiation hard silicon detectors are required to withstand the expected fluences of up to 1.2 × 1015 neq cm−2 in the short strip region. 3. Silicon Strip Detectors Silicon strip sensors, referred to as ATLAS07, were designed by the ATLAS Silicon Strip Sensor Upgrade Collaboration and produced by Hamamatsu Photonics [2] on n-in-p float zone material using 6” wafers [3]. Each wafer comprises a large area strip sensor of 9.75 × 9.75 cm2 and 24 1 × 1 cm2 miniature strip sensors. The sensors have a thickness of 320 μm and a pitch of 74.5 μm. These senors were produced to study strip isolation schemes, high-voltage performance, punchthrough protection, and charge collection as a function of the irradiated fluence. The miniature sensors were irradiated at the Reactor Center at the Josef Stefan Institute in Ljubljana using neutrons, and both at the Cyclotron and Radioisotope Center (CYRIC) at Tohoku University and
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L. Wiik / Nuclear Physics B (Proc. Suppl.) 215 (2011) 154–156
the Karlsruhe Institute of Technology using protons. 3.1. Mini Strip Detectors The charge collection of the irradiated sensors were measured at Freiburg University, Liverpool University, Jozef Stefan Institute in Ljubljana, UC-Santa Cruz, Tsukuba University/KEK and at IFIC-Valencia. The measurements were performed using either a 1064 nm laser or a 90 Sr source. There is good overall agreement between the measurements taken at the different sites, using various measurement and readout systems.
(a)
(a)
(b)
Figure 2. Charge collection as a function of the bias voltage measured at different sites. The sensors were irradiated with proton to fluences of 6 × 1014 neq cm−2 in (a) and 1.3 × 1015 neq cm−2 in (b).
(b)
Figure 1. Charge collection as a function of the bias voltage for sensors irradiated with neutrons to fluences of 5 × 1014 neq cm−2 in (a) and 1 × 1015 neq cm−2 in (b). Measurements taken at various different sites
Fig. 1 shows the collected charge as a function of the bias voltage for neutron irradiation from [4]. The fluences are 5 × 1014 neq cm−2 in Fig. 1(a) and 1 × 1015 neq cm−2 in Fig. 1(b). The neutron irradiated samples from Ljubljana and Tsukuba/KEK were annealed for 80 minutes at 60 ◦ C, which is supposed to lead to a roughly 15% larger signal. Hence the data taken at the remaining sites was scaled to an annealed state. A summary of the charge collection of the pro-
156
L. Wiik / Nuclear Physics B (Proc. Suppl.) 215 (2011) 154–156
ton irradiated samples prior to annealing is shown in Fig. 2 [4]. Again the measurements taken at the different sites are in good agreement. The fluences are 6 × 1014 neq cm−2 for Fig. 2(a) and 1.3 × 1015 neq cm−2 in Fig. 2(b). The ATLAS07 sensors collect the expected charge. Assuming a voltage limit of 500 V 11 − 14 Ke− are generated with proton irradiated sensors and 8 − 10 Ke− for the neutron irradiated samples, showing that these sensors are sufficiently radiation hard for the upgrade of LHC. Furthermore annealing studies were conducted with these sensors, showing that controlled annealing at 20 ◦ C is a useful tool to reduce power dissipation and to recover a fraction of the signal in heavily irradiated silicon sensors, assuming annealing times of 100-300 days [5]. 3.1.1. Full Size Sensors The large area sensors are segmented into four strip regions, two of which are axially aligned, i.e. with the strips parallel to the edge of the sensor, and two which have stereo strips, inclined at an angle of 40 mrad. There are 1280 strips with a length of 2.38 cm in each segment [6]. Out of the 30 sensors produced, 19 were tested at the Academy of Science of CR and the Charles University in Prague, Cambridge University, Geneva University and Stony Brook University. The main results of the measurements and the comparison to the ATLAS07 specifications are summarized in Table 1. Measurements of the leakage current, the full depletion voltage, the coupling capacitance, the bias resistance, the inter-strip capacitance and resistance, showed that all tested sensors satisfied the ATLAS07 specifications. In addition to these measurements strip scans were performed on six sensors. The measurements on the 23040 tested strips exhibited no defects such as pinholes, punch-through defects, shorts, or openings of metal strips [7]. 4. Module Integration Concepts A new detector design is needed for the ATLAS tracker upgrade to meet the high demands set by the sLHC environment. A highly integrated system is essential due to the high sensor modularity.
Table 1 Comparison of measured parameter values and with technical specification [7].
Leakage Current Full Depletion Voltage Coupling Capacitance at 1 kHz Silicon Bias Resistance Current through dielectric Strip Current Inter-strip Capacitance Inter-strip Resistance per cm
ATLAS07 Specification < 200 μ A < 500 V
Measurement
> 20 pF/cm
28 pF/cm
1.5±0.5 MΩ
1.3 - 1.6 MΩ
Idiel < 10 nA
< 10 nA
200 - 370 nA 190-245 V
No explicit < 2 nA limit 1.1 pF 0.7 - 1.8 pF > 10x Rbias ≈ > 150 GΩ 15 MΩ
A straight stave concept was proposed by the ATLAS collaboration for the Inner Detector upgrade. This stave will supply the services for cooling, powering, a readout, and mechanical stability for several sensors, which will form a unit. It is beyond the scope of this report to discuss the stave concept in details, however further information can be found in [1]. REFERENCES 1. J.Kierstad, et al., Nucl. Instr. And Meth. A 579(2007)801 2. Hamamamatsu Photonics. Available: http://www.hamamatsu.com 3. Y. Unno, et al., Nucl. Instr. and Meth. A (2010), doi:10.1016/j.nima.2010.04.080 4. A.Affolder, communication in the ATLAS Upgrade Week, Feb. 2009 5. G.Casse et al. PoS(VERTEX 2009)008 6. G.Casse et al., Nucl. Instr. And Meth. A 487 (2002) 465-470 7. J.Bohm et al., Nucl. Instr. and Meth. A (2010), doi:10.1016/j.nima.2010.04.094
ATLAS Tracker Upgrade: Silicon Strip Detectors for the sLHC Liv Wiik a on behalf of the ATLAS Collaboration a
Physikalisches Institut, Albert-Ludwigs-Universit¨ at Freiburg, Herrman-Herder-Str. 3, 79104 Freiburg, Germany After the successful start of the Large Hadron Collider (LHC) in November 2009 plans for a luminosity upgrade, called super-LHC (sLHC), are already advancing well. With a planned luminosity of 5 × 1034 cm−2 s−1 the ATLAS detector has to cope with both a high instantaneous and integrated luminosity, providing a challenging environment for the tracking and vertexing detectors. Hence a new tracker is foreseen for the sLHC operation. As the radiation dose will increase according to the integrated luminosity, novel radiation hard detectors are required. A large R&D program is underway to develop silicon sensors with sufficient radiation hardness. In this article measurements of silicon strip sensors designed by the ATLAS Silicon Strip Sensor Upgrade Collaboration are presented. Both measurements of sensors irradiated to the expected sLHC fluences for different detector regions as well as comparisons between measurements of full size sensor properties and their technical specifications are shown. Furthermore the design challenges of the inner detector layout and support structures are presented.
1. Introduction A luminosity upgrade by a factor of five of the Large Hadron Collider (LHC), to the superluminous Large Hadron Collider (sLHC), is being considered as an extension of the LHC physics program. In order to fully exploit the physics potential given by the upgrade the performance of the existing ATLAS detector must be preserved if not improved. The increase of the luminosity will have a significant effect on the detectors by the large increase in radiation damage or the number of pile-up events per bunch crossing. The most significant upgrade of the ATLAS detector will be the replacement of the Inner Detector. 2. ATLAS SCT upgrade The existing ATLAS Inner Detector (ID) consists of a Silicon Pixel Detector (Pixel), a Silicon Strip Detector (SCT) and a Transition Radiation Tracker (TRT). It is expected that the TRT will not be able to cope with the expected sLHC occupancy. This leads to an all silicon ID for the upgrade consisting of a 4 layer pixel system, and five silicon microstrip layers. The inner three layers are designed to replace the SCT, and will be based on silicon detectors with 24 mm long strips, 0920-5632/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2011.03.163
the so called short strip layers. The outer two layers will be equipped with 96 mm long strip detectors, and are aimed to substitute the ATLAS TRT [1]. The upgrade of the ID will be completed with a set of end-cap disks at each side. Since the radiation damage scales with luminosity novel radiation hard silicon detectors are required to withstand the expected fluences of up to 1.2 × 1015 neq cm−2 in the short strip region. 3. Silicon Strip Detectors Silicon strip sensors, referred to as ATLAS07, were designed by the ATLAS Silicon Strip Sensor Upgrade Collaboration and produced by Hamamatsu Photonics [2] on n-in-p float zone material using 6” wafers [3]. Each wafer comprises a large area strip sensor of 9.75 × 9.75 cm2 and 24 1 × 1 cm2 miniature strip sensors. The sensors have a thickness of 320 μm and a pitch of 74.5 μm. These senors were produced to study strip isolation schemes, high-voltage performance, punchthrough protection, and charge collection as a function of the irradiated fluence. The miniature sensors were irradiated at the Reactor Center at the Josef Stefan Institute in Ljubljana using neutrons, and both at the Cyclotron and Radioisotope Center (CYRIC) at Tohoku University and
155
L. Wiik / Nuclear Physics B (Proc. Suppl.) 215 (2011) 154–156
the Karlsruhe Institute of Technology using protons. 3.1. Mini Strip Detectors The charge collection of the irradiated sensors were measured at Freiburg University, Liverpool University, Jozef Stefan Institute in Ljubljana, UC-Santa Cruz, Tsukuba University/KEK and at IFIC-Valencia. The measurements were performed using either a 1064 nm laser or a 90 Sr source. There is good overall agreement between the measurements taken at the different sites, using various measurement and readout systems.
(a)
(a)
(b)
Figure 2. Charge collection as a function of the bias voltage measured at different sites. The sensors were irradiated with proton to fluences of 6 × 1014 neq cm−2 in (a) and 1.3 × 1015 neq cm−2 in (b).
(b)
Figure 1. Charge collection as a function of the bias voltage for sensors irradiated with neutrons to fluences of 5 × 1014 neq cm−2 in (a) and 1 × 1015 neq cm−2 in (b). Measurements taken at various different sites
Fig. 1 shows the collected charge as a function of the bias voltage for neutron irradiation from [4]. The fluences are 5 × 1014 neq cm−2 in Fig. 1(a) and 1 × 1015 neq cm−2 in Fig. 1(b). The neutron irradiated samples from Ljubljana and Tsukuba/KEK were annealed for 80 minutes at 60 ◦ C, which is supposed to lead to a roughly 15% larger signal. Hence the data taken at the remaining sites was scaled to an annealed state. A summary of the charge collection of the pro-
156
L. Wiik / Nuclear Physics B (Proc. Suppl.) 215 (2011) 154–156
ton irradiated samples prior to annealing is shown in Fig. 2 [4]. Again the measurements taken at the different sites are in good agreement. The fluences are 6 × 1014 neq cm−2 for Fig. 2(a) and 1.3 × 1015 neq cm−2 in Fig. 2(b). The ATLAS07 sensors collect the expected charge. Assuming a voltage limit of 500 V 11 − 14 Ke− are generated with proton irradiated sensors and 8 − 10 Ke− for the neutron irradiated samples, showing that these sensors are sufficiently radiation hard for the upgrade of LHC. Furthermore annealing studies were conducted with these sensors, showing that controlled annealing at 20 ◦ C is a useful tool to reduce power dissipation and to recover a fraction of the signal in heavily irradiated silicon sensors, assuming annealing times of 100-300 days [5]. 3.1.1. Full Size Sensors The large area sensors are segmented into four strip regions, two of which are axially aligned, i.e. with the strips parallel to the edge of the sensor, and two which have stereo strips, inclined at an angle of 40 mrad. There are 1280 strips with a length of 2.38 cm in each segment [6]. Out of the 30 sensors produced, 19 were tested at the Academy of Science of CR and the Charles University in Prague, Cambridge University, Geneva University and Stony Brook University. The main results of the measurements and the comparison to the ATLAS07 specifications are summarized in Table 1. Measurements of the leakage current, the full depletion voltage, the coupling capacitance, the bias resistance, the inter-strip capacitance and resistance, showed that all tested sensors satisfied the ATLAS07 specifications. In addition to these measurements strip scans were performed on six sensors. The measurements on the 23040 tested strips exhibited no defects such as pinholes, punch-through defects, shorts, or openings of metal strips [7]. 4. Module Integration Concepts A new detector design is needed for the ATLAS tracker upgrade to meet the high demands set by the sLHC environment. A highly integrated system is essential due to the high sensor modularity.
Table 1 Comparison of measured parameter values and with technical specification [7].
Leakage Current Full Depletion Voltage Coupling Capacitance at 1 kHz Silicon Bias Resistance Current through dielectric Strip Current Inter-strip Capacitance Inter-strip Resistance per cm
ATLAS07 Specification < 200 μ A < 500 V
Measurement
> 20 pF/cm
28 pF/cm
1.5±0.5 MΩ
1.3 - 1.6 MΩ
Idiel < 10 nA
< 10 nA
200 - 370 nA 190-245 V
No explicit < 2 nA limit 1.1 pF 0.7 - 1.8 pF > 10x Rbias ≈ > 150 GΩ 15 MΩ
A straight stave concept was proposed by the ATLAS collaboration for the Inner Detector upgrade. This stave will supply the services for cooling, powering, a readout, and mechanical stability for several sensors, which will form a unit. It is beyond the scope of this report to discuss the stave concept in details, however further information can be found in [1]. REFERENCES 1. J.Kierstad, et al., Nucl. Instr. And Meth. A 579(2007)801 2. Hamamamatsu Photonics. Available: http://www.hamamatsu.com 3. Y. Unno, et al., Nucl. Instr. and Meth. A (2010), doi:10.1016/j.nima.2010.04.080 4. A.Affolder, communication in the ATLAS Upgrade Week, Feb. 2009 5. G.Casse et al. PoS(VERTEX 2009)008 6. G.Casse et al., Nucl. Instr. And Meth. A 487 (2002) 465-470 7. J.Bohm et al., Nucl. Instr. and Meth. A (2010), doi:10.1016/j.nima.2010.04.094