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Production and Quality Assurance of Detector Modules for theLCHb Silicon Tracker

LHCb Silicon Tracker group, including the following authors; Volyanskyy, D;Steinkamp, O; Bernhard, R P; Bernet, R; Gassner, J; Koestner, S; Lehner, F; Needham,M; Sakhelashvili, T; Steiner, S; Straumann, U; van Tilburg, J; Vollhardt, A; Wenger,

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LHCb Silicon Tracker group, including the following authors; Volyanskyy, D; Steinkamp, O; Bernhard, R P;Bernet, R; Gassner, J; Koestner, S; Lehner, F; Needham, M; Sakhelashvili, T; Steiner, S; Straumann, U; vanTilburg, J; Vollhardt, A; Wenger, A (2008). Production and Quality Assurance of Detector Modules for the LCHbSilicon Tracker. In: Barone, M; Gaddi, A; Leroy, C; Price, L; Rancoita, P G; Ruchti, R. Astroparticle, Particle andSpace Physics, Detectors and Medical Applications, Proceedings of the10th Conference, Villa Olmo, Como, Italy,8-12 October 2007. Singapore, 1019-1023.Postprint available at:http://www.zora.uzh.ch

Posted at the Zurich Open Repository and Archive, University of Zurich.http://www.zora.uzh.ch

Originally published at:Barone, M; Gaddi, A; Leroy, C; Price, L; Rancoita, P G; Ruchti, R 2008. Astroparticle, Particle and Space Physics,Detectors and Medical Applications, Proceedings of the10th Conference, Villa Olmo, Como, Italy, 8-12 October2007. Singapore, 1019-1023.

LHCb Silicon Tracker group, including the following authors; Volyanskyy, D; Steinkamp, O; Bernhard, R P;Bernet, R; Gassner, J; Koestner, S; Lehner, F; Needham, M; Sakhelashvili, T; Steiner, S; Straumann, U; vanTilburg, J; Vollhardt, A; Wenger, A (2008). Production and Quality Assurance of Detector Modules for the LCHbSilicon Tracker. In: Barone, M; Gaddi, A; Leroy, C; Price, L; Rancoita, P G; Ruchti, R. Astroparticle, Particle andSpace Physics, Detectors and Medical Applications, Proceedings of the10th Conference, Villa Olmo, Como, Italy,8-12 October 2007. Singapore, 1019-1023.Postprint available at:http://www.zora.uzh.ch

Posted at the Zurich Open Repository and Archive, University of Zurich.http://www.zora.uzh.ch

Originally published at:Barone, M; Gaddi, A; Leroy, C; Price, L; Rancoita, P G; Ruchti, R 2008. Astroparticle, Particle and Space Physics,Detectors and Medical Applications, Proceedings of the10th Conference, Villa Olmo, Como, Italy, 8-12 October2007. Singapore, 1019-1023.

Production and Quality Assurance of Detector Modules for theLCHb Silicon Tracker

Abstract

The LHCb experiment, which is currently under construction at the Large Hadron Collider~(CERN,Geneva), is designed to study CP violation and find rare decays in the B meson system. To achieve thephysics goals the LHCb detector must have excellent tracking performance. An important element of theLHCb tracking system is the Silicon Tracker, which covers a sensitive surface of about 12~m2 withsilicon microstrip detectors and includes about 272k readout channels. It uses up to 132~cm longdetector modules with readout strips of up to 38~cm in length and up to 57~cm long Kaptoninterconnects in between sensors and readout hybrids. The production of detector modules has beencompleted recently and the detector is currently under installation. A rigorous quality assuranceprogramme has been performed to ensure that the detector modules meet the mechanical and electricalrequirements and study their various characteristics. In this paper, the detector design, the moduleproduction steps, and the module quality assurance programme are briefly described.

LHCb note 2007-131

Production and Quality Assurance of DetectorModules for the LHCb Silicon Tracker∗

October 12, 2007

D. Volyanskyy5, O. Steinkamp5, M. Agari1, C. Bauer1, J. Blouw1,†, W. Hofmann1,S. Lochner1, F. Maciuc1, M. Schmelling1, N. Smale1, B. Schwingenheuer1, H. Voss1,M. Borysova2, O. Ohrimenko2, V. Pugatch2, V. Yakovenko2, A. Bay3, M.-O. Bettler3,P. Fauland3, R. Frei3, L. Nicolas3, M. Knecht3, A. Perrin3, O. Schneider3, M.T. Tran3,J. van Hunen3, K. Vervink3, B. Adeva4, D. Esperante4, A. Gallas4, J. L. Fungueirino Pazos4,C. Lois4, A. Pazos Alvarez4, E. Perez Trigo4, M. Plo Casasus4, P. Vazquez4, R.P. Bernhard5,R. Bernet5, J. Gassner5, S. Kostner5,‡, F. Lehner5, M. Needham5,‡, T. Sakhelashvili5,S. Steiner5, U. Straumann5, J. van Tilburg5, A. Vollhardt5, A. Wenger5

1Max Planck Institut fur Kernphysik, 69117 Heidelberg, Germany;2National Academy of Sciences, Institute for Nuclear Research, 03680 Kiev, Ukraine;

3LPHE, EPFL Lausanne, 1015 Lausanne, Switzerland;4Universidad de Santiago de Compostela, 15706 Santiago de Compostela, Spain;

5Physik-Institut der Universitat Zurich, 8057 Zurich, Switzerland;†now at Physics Institute, University of Heidelberg, 69120 Heidelberg,Germany;

‡now at CERN, 1211 Geneve, Switzerland.

Abstract

The LHCb experiment, which is currently under construction at the Large HadronCollider (CERN, Geneva), is designed to study CP violation and find rare decays in theB meson system. To achieve the physics goals the LHCb detector must have excellenttracking performance. An important element of the LHCb tracking system is the SiliconTracker, which covers a sensitive surface of about 12 m2 with silicon microstrip detectorsand includes about 272k readout channels. It uses up to 132 cm long detector moduleswith readout strips of up to 38 cm in length and up to 57 cm long Kapton interconnectsin between sensors and readout hybrids. The production of detector modules has beencompleted recently and the detector is currently under installation. A rigorous qualityassurance programme has been performed to ensure that the detector modules meetthe mechanical and electrical requirements and study their various characteristics. Inthis paper, the detector design, the module production steps, and the module qualityassurance programme are briefly described.

∗Presented at the 10th ICATPP Conference on Astroparticle, Particle, Space Physics, Detectorsand Medical Physics Applications, Villa Olmo, Como, Italy, October 8-12, 2007. Proceedings will bepublished by World Scientific.

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1 Introduction

Since bb hadrons at the LHC will be predominately produced at low polar anglesin the same forward cone, the LHCb spectrometer [1] is designed as a single-arm,forward spectrometer with an angular coverage from 10 mrad to 300(250) mrad in thehorizontal (vertical) plane. The Silicon Tracker is part of the LHCb tracking system.It comprises two sub-detectors: the Trigger Tracker (TT) and the Inner Tracker (IT).Both use single-sided silicon sensors and a common readout1 and infrastructure suchas high voltage, low voltage and cooling. Around 30 physicists from 5 universitiesand laboratories from 4 countries are currently involved in the LHCb Silicon Trackerproject.

The TT is a 150 cm wide and 130 cm high planar tracking station, which is located justin front of the entrance of the LHCb dipole magnet. Charge particle fluxes of about5×105cm−2s−1 are expected in the innermost region of the TT. The detector consists offour planar detection layers of silicon microstrip sensors that cover the complete LHCbacceptance. The detection layers are arranged according to the x–u–v–x configuration:the first and the last layer have vertical strips, while the second and third have stripswhich are rotated by an angle of +5o and −5o, respectively. The layers are enclosed ina common thermally-insulating box that also provides electrical and optical shieldingand mechanical support. A detailed description of the TT detector layout can be foundin Ref. [4].

The IT covers a roughly 120 cm wide and 40 cm high cross-shaped area around thebeam pipe in the centre of three planar tracking stations downstream of the mag-net. Although the IT covers only 1.3% of the sensitive surface of the tracking sta-tions, approximately 25% of all charged particles from the pp interaction point thatreach the tracking stations will pass through the IT. Charge particle fluxes of about1.5×105cm−2s−1 are expected in the innermost region of the IT. There are four sep-arate IT boxes in each of the tracking stations: above, below, to the left and to theright of the beam pipe. They provide electrical and thermal shielding for the siliconsensors and readout electronics. Each IT box contains four detection layers of siliconmicrostrip detectors with a x–u–v–x topology, as in the TT. A detailed description ofthe Inner Tracker detector layout can be found in Ref. [5].

2 Module design

The basic detector unit of the TT is the half-module. It consists of seven 500 µm thicksilicon sensors2 and a stack of readout hybrids located at one end. To provide mechan-ical stability two carbon fibre rails are glued along the sensor edges. By bonding thestrips on adjacent sensors, the half-module is segmented into two or three readout sec-

1Both detectors use the same readout chip (the Beetle [2]) and readout link [3].2TT sensors are 9.64 cm wide and 9.44 cm long, and carry 512 readout strips with a strip pitch of

183 µm.

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tors. This allows to limit the hit occupancy and to decrease the total load capacitanceat the input of a readout amplifier. There are two different types of half-modules: LMKand LM half-modules. The LMK half-modules are placed in the innermost part of thedetector around the beam pipe, where the occupancy is highest. In these half-modules,the seven silicon sensors are electronically divided into three independent readout sec-tors: an outer four-sensor long readout sector, an intermediate two-sensor long readoutsector and an inner readout sector consisting of a single silicon sensor. The LM half-modules are placed outside the beam pipe region. These half-modules are electronicallydivided into two readout sectors: an outer four-sensor long readout sector and an innerthree-sensor long readout sector. Each readout sector is independently read out bya front-end hybrid located at the end of the half-module. This leads to two (three)stacked readout hybrids for the LM (LMK) half-module. The four-sensor long read-out sectors are directly connected to their front-end readout hybrids via a short pitchadapter. In order to connect the three-sensor and two-sensor readout sectors of the LMand LMK half-modules to the corresponding readout hybrids, an interconnect Kaptoncable3 with a length of 39.1 cm is used. A similar cable with a length of 58 cm isemployed to connect the one-sensor readout sectors of the LMK half-modules to theirreadout hybrids.

IT detector modules consist of one 320 µm thick or two 410 µm thick silicon sensors4

that are connected via a short pitch adapter to a single readout hybrid. Silicon sen-sor(s), pitch adapter and readout hybrid are glued onto a flat backplane that consistsof a thin layer of polyetherimide foam sandwiched in between two sheets of carbon fiberof high thermal conductivity. A thin Kapton foil is used to insulate the silicon sensorsfrom the carbon-fibre sheets. To provide a direct heat path between the readout hybridand a thin aluminium cooling rod onto which the modules will be mounted, a smallaluminium insert (“cooling balcony”) is embedded into the backplane at the locationof the readout hybrid.

3 Module production

The series production of TT modules was launched in September 2005 and was com-pleted in December 2006. Including 15% spares, 149 half-modules have been produced:119 LM half-modules and 30 LMK half-modules. The production of TT modules pro-ceeded in two or three stages, depending on the module type. During the first pro-duction step the seven silicon sensors and the lowest readout hybrid were placed onan assembly template, the two carbon fibre rails were glued along the sensor edges,the bias voltage cable was connected to the sensors, and the readout strips and theground connections of the four-sensor readout sector were bonded. After this stage, thefour-sensor sector was fully operational. At the next production step, the readout hy-

3The interconnect Kapton cable has a thickness of 0.010 cm and a width of 6 cm and carries 512signal lines with a fan-in section which reduces the sensor pitch of 183 µm to the 112 µm pitch of thecable.

4IT sensors are 7.8 cm wide and 11 cm long, and carry 384 readout strips with a strip pitch of198 µm.

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brid and a 39.1 cm long interconnect Kapton cable for the three-sensor and two-sensorreadout sectors of the LM and LMK half-modules were mounted and the sensors withinthese sectors were bonded together. After this step, the assembly of a LM half-modulewas completed. For the LMK half-modules, a third production stage was performedto attach and bond the readout hybrid and a 58 cm long interconnect Kapton cable ofthe one-sensor readout sector.

The series production of IT modules was launched in January 2006 and was completedin September 2007. In total, about 415 modules have been produced. The productionof IT modules proceeded in several steps. At first, the pre-fabricated support backplanewas placed on an assembly template and the readout hybrid and pitch adapter werepositioned and glued onto the support. Then, a first readout test was performed toassure the functionality of the readout hybrid. Depending on the module type, oneor two silicon sensors were further positioned on a second template, transfered to theassembly template using a vacuum transfer jig, and glued onto the support using asilicone-based glue. At the next step, bias voltage and ground connections were bondedand a first HV test was performed. Finally, the readout strips were bonded.

4 Quality assurance

A crucial role in the quality assurance programme for the detector modules was playedby the comprehensive “burn-in” test, through which each detector module passed atleast once during its assembly. This test allowed to investigate the long-term behaviourof the detector modules at different thermal conditions, to study their electrical char-acteristics and to search for defective channels. The burn-in setups and programmesfor IT and TT differed in details, however their main features were similar. The typicalburn-in programme took around 36 hours for TT and 48 hours for IT and includedtemperature cycling between room temperature and +5◦C in case of TT and between+40◦C and -5◦C in case of IT. During the cycling, the modules were continuously bi-ased at 500V and leakage currents were monitored as a function of temperature andtime. Furthermore, the leakage currents were measured as a function of the applied biasvoltage at cold and warm temperature. These measurements provided a search for thebreakdown voltages of the silicon sensors. In order to investigate the noise performanceof the detectors, pedestal runs were repeatedly taken at cold and warm temperatures.Measurements of the signal pulse shapes were performed at both temperatures usingan internal test pulse implemented in the Beetle readout chip and, in case of the TTsetup, using an array of infra-red laser beams that permitted to generate charges atpredefined locations on each readout sector. This allowed to verify the optimal timingsettings of the readout electronics and to study pulse shape characteristics as a functionof different parameters. In particular, the dependence of the signal amplitude on theapplied bias voltage was investigated and the charge-collection efficiency curves wereobtained. In case of TT, the burn-in programme was running fully automatically. Thiswas achieved by using the LabView programming language that allowed to control allcomponents of the burn-in test-stand, including readout electronics and data taking.An extensive and uninterrupted measurement programme was running even overnight

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and over the weekends. The need for operator intervention was limited to searching ofdefective channels at the beginning and at the end of the test and to exchanging thehalf-modules.

The overall quality of the modules is very good. The fraction of defective channels (in-terrupts, shorts, pinholes) is only 0.13% for TT and around 0.1% for IT. Leakagecurrents per sensor at 500V and at room temperature are typically below 0.4 µA. Asmall number of modules were graded as spares due to higher or unstable leakage cur-rents at 500V. Around 2 TByte of data have been accumulated in the TT burn-in testsalone.

5 Outlook

The production of detector modules for the LHCb Silicon Tracker has been completed.The results of the quality assurance programme demonstrate that the detectors meetthe mechanical and electrical requirements for the LHCb Silicon Tracker. The detectorinstallation has been started and will be completed by the end of 2007.

References

[1] LHCb Reoptimized Detector Design and Performance - Technical Design Report,CERN/LHCC-2003-030

[2] N. Bakel et al., LHCb note 2005-105

[3] A. Vollhardt et al., Proc. LECC 2005, CERN/LHCC-2005-038 (2005) 187-191

[4] D.Volyanskyy, The Trigger Tracker and a Monte Carlo Study of the B0s → J/ψη′

Decay in the LHCb Experiment, CERN-THESIS-2007-065

[5] LHCb Inner Tracker - Technical Design Report, CERN-LHCC/2002-029

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