The Silicon Tracker is a large-surface silicon micro-strip detector that covers the full acceptance of the experiment in a single tracking station upstream of the spectrometer magnet and the inner-most part of the three tracking stations downstream of the magnet. Special emphasis has been put on module quality assurance at all stages of the production. Various tests are performed after each production step and each module goes through several burn-in cycles. The design of the LHCb silicon detectors is described and the main lessons learnt from the R&D phase are summarized. Focus will be on the experience from module production and the quality assurance program

Adeva, B

Agari, M

Bauer, J

Bay, A

Bernet, R

Bernhard, R P

Bettler, M O

Blouw, J

Borysova, M

Esperante-Pereira, D

Fauland, P

Frei, R

Fungueirino-Pazos, J L

Gallas, A

Gassner, J

Hofmann, W

Knecht, M

Lehner, F

Lois, C

Löchner, S

Maciuc, F

Nicolas, L

Ohrimenko, O

Pazos-Alvarez, A

Perrin, A

Pló-Casasus, M

Pugatch, V

Pérez-Trigo, E

Sakhelashvili, T

Schmelling, M

Schneider, O

Schwingenheuer, B

Smale, N

Steiner, S

Steinkamp, O

Straumann, U

Tran, M T

Van Hunen, J

Van Tilburg, J

Vervink, K

Vollhardt, A

Volyanskyy, D

Voss, H

Vázquez, P

Wenger, A

Yakovenko, V

CERN Document Server

The LHCb Silicon Tracker ∗November 20, 2007R. P. Bernhard1,‡, M. Agari2, C. Bauer2, J. Blouw2,†, W. Hofmann2,S. Lo¨chner2, F. Maciuc2, M. Schmelling2, N. Smale2, B. Schwingenheuer2,H. Voss2, M. Borysova3, O. Ohrimenko3, V. Pugatch3, V. Yakovenko3,A. Bay4, M.-O. Bettler4, P. Fauland4, R. Frei4, L. Nicolas4, M. Knecht4,A. Perrin4, O. Schneider4, M.T. Tran4, J. van Hunen4, K. Vervink4,B. Adeva5, D. Esperante5, A. Gallas5, J. L. Fungueirino Pazos5, C. Lois5,A. Pazos Alvarez5, E. Perez Trigo5, M. Plo Casasus5, P. Vazquez5,R. Bernet1, J. Gassner1, F. Lehner1, T. Sakhelashvili1, S. Steiner1,O. Steinkamp1, U. Straumann1, J. van Tilburg1, A. Vollhardt1,D. Volyanskyy1, A. Wenger11Physik-Institut der Universita¨t Zu¨rich, 8057 Zu¨rich, Switzerland;2Max Planck Institut fu¨r Kernphysik, 69117 Heidelberg, Germany;3National Academy of Sciences, Institute for Nuclear Research, 03680 Kiev, Ukraine;4LPHE, EPFL Lausanne, 1015 Lausanne, Switzerland;5Universidad de Santiago de Compostela, 15706 Santiago de Compostela, Spain;†now at Albert-Ludwigs Universita¨t Freiburg, 79104 Freiburg i. Br., Germany;‡now at Physics Institute, University of Heidelberg, 69120 Heidelberg, Germany.LHCb Note 2007-126AbstractThe Silicon Tracker is a large-surface silicon micro-strip detectorthat covers the full acceptance of the experiment in a single trackingstation upstream of the spectrometer magnet and the inner-most partof three tracking stations downstream of the magnet. Special em-phasis has been put on module quality assurance at all stages of theproduction. Various tests are performed after each production stepand each module goes through several burn-in cycles. The design ofthe LHCb silicon detectors is described and the main lessons learntfrom the R&D phase are summarized. Focus will be on the experiencefrom module production and the quality assurance program.∗Talk presented at the 8th International Conference on Large Scale Applications andRadiation Hardness of Semiconductor Detectors June 27-29, 2007 - Florence, Italy. Toappear in NIM.11 IntroductionThe Silicon Tracker is part of the tracking system of the LHCb experiment [2].It comprises two detectors, the Trigger Tracker (TT) and the Inner Tracker(IT). Silicon microstrip detectors with long readout strips and with strippitches of about 200 µm are used for both. The TT is a 150 cm wide and130 cm high four layer planar tracking station which is located upstream ofthe LHCb dipole magnet and covers the full acceptance of the experiment.The IT covers a roughly 120 cm wide and 40 cm high cross-shaped region inthe centre of three large planar tracking stations downstream of the magnetwith four detection layers each. In total, an active surface of about 12 m2is covered with about 272k readout channels. A common front-end readoutchip (the Beetle [3]) and readout link [4] are used by TT and IT. The designsof the detector modules are different. Including 15% spares, 148 TT modulesand 386 IT modules are needed. The module productions was split in twoproduction sites, one for TT and one for IT. A lot of effort and resourceswent into the quality assurance program at all steps of the module produc-tion. In particular, systematic “burn-in” programmes were set up throughwhich each module has to pass at least once during the production. A briefoverview of the designs of the modules, the main production steps and thequality assurance programmes for both sub-detectors will be given. For keyparameters the achieved module quality will be illustrated.2 Modul DesignAn important design goal for TT was to keep the dead material associatedwith front-end readout hybrids, mechanical supports, cooling, etc. outsideof the acceptance of the experiment. The resulting TT module design is il-lustrated in Fig. 1. Each TT module consists of seven silicon sensors1 with astack of either two or three front-end readout hybrids attached at one end.The seven sensors are organised in either two or three readout sectors. Forall modules, the four sensors closest to the readout hybrids form the firstreadout sector (“L sector”) and are directly connected to the lower-mostreadout hybrid. For “4-3” type modules (majority of the modules), the re-maining three sensors form a single readout sector (“M sector”), for “4-2-1”type modules (these are used in the central region of the detector) they are1The sensors used in the TT are 9.64 cm wide 9.44 cm long, with a thickness of 500µm and 512 readout strips with a strip pitch of 183 µm. The sensors are identical to theOB2 sensors used in the Outer Barrel of the CMS Tracker and were produced by HPK,Hamamatsu, Japan.2Figure 1: Detector module design for the Trigger Tracker.further split into an intermediate two-sensor sector (“M sector”) and a thirdsector consisting of the single sensor farthest from the hybrids (“K sector”).The sensors of the M sector and, where applicable, the K sector are connectedto the upper frontend hybrid(s) via Kapton interconnect cables of 38 cm, re-spectively 57 cm, in length. To give the modules mechanical stability, twothin fibre-glass/carbon-fibre rails are glued along the sides of the sensors andthe lower-most readout hybrid. Bias voltage is connected to the backplanesof the sensors via a thin Kapton cable that runs along the back of the mod-ule. Two detector modules, glued together end-to-end, form a 14-sensor longsuper-module that spans the full height of the LHCb acceptance. All (super-) modules are mounted inside one large detector box, in which an ambienttemperature of 5◦ C is maintained in order to suppress leakage currents andreverse annealing after irradiation. Mechanical supportframes, cooling pipes,and readout cables are all located outside of the LHCb acceptance. In thecase of IT, front-end hybrids, mechanical supports, cooling, etc. could notbe kept outside of the LHCb acceptance and a main goal in designing thedetector was to minimise this dead material. The resulting module design isillustrated in Fig. 2. The IT modules consist of one or two silicon sensors2that are connected via a short pitch adapter to a single readout hybrid. Sil-icon sensor(s), pitchadaptor and hybrid are glued onto a flat backplane thatconsists of a thin layer of polyetherimide foam sandwiched in between twosheets of carbon fibre of high thermal conductivity. The silicon sensors are2The sensors used in the IT are 7.6 cm wide 11 cm long, with a thickness of 320 µmin case of one-sensor modules and 410 µm for two-sensor modules. Each of them has 384readout strips with a strip pitch of 198 µm. The sensore were designed and produced byHPK, Hamamatsu, Japan.3Figure 2: Detector module design for the Inner Tracker.electrically insulated from the carbon-fibre sheets by a thin Kapton foil. Asmall aluminium insert (“cooling balcony”) is embedded into the backplaneat the location of the readout hybrid. It provides a direct heat path betweenthe front-end chips and a thin aluminium cooling rod onto which the mod-ules will be mounted. It also provides thermal contact between this coolingrod and the carbon-fibre sheets of the module backplane such that the latterform large cold surfaces and contribute to cooling the detector volume to thedesired ambient temperature of 5◦ C.3 Module ProductionThe TT module production is performed in two or three stages, dependingon the module type. Each production stage of the modules is followed bya two to three day long burn-in test (see Section 4). The production of amodule begins by placing all seven sensors and the lowermost hybrid on anassembly template, gluing the two carbon-fibre rails, measuring the sensorpositions, connecting the bias voltage cable on the back of the sensors, andbonding the ground connections and readout strips for the L sector of themodule. The L sector is fully operational afterwards and the module is testedin the first burn-in cycle. During the second production stage, the readouthybrid and Kapton interconnect cable for the M sector are mounted and thisreadout sector is bonded. The module is tested in a second burn-in cycleand is then finished in the case of a “4-3” type module. The “4-2-1” modulesenter the third production stage, in which the readout hybrid and Kapton4cable for the K sector are attached and bonded, followed by a third and finalburn-in test. With two assembly templates, an average production rate offive modules per week was achieved and maintained over the full duration ofthe production. Out of 150 modules produced, two were damaged beyondrepair due to handling errors in the production. Three more modules weresuccessfully repaired by replacing a damaged sensor or readout hybrid.The production of IT modules proceeds as follows: The pre-fabricatedsupport backplane is placed on an assembly template and the readout hybridand pitch adapter are positioned and glued onto the support frame. A firstreadout test is performed to assure the electrical functionality of the readouthybrid. One or two silicon sensors, depending on the module type, are po-sitioned on a second template, transfered to the assembly template using avacuum transfer jig, and glued onto the support frame using a silicone-basedglue. The bias voltage and ground connections are bonded and a first HVtest is performed. If the module has passed this test, the readout strips arebonded and with the finished module a comprehensive quality assurance pro-gram is performed, including the measurement of sensor positions and a 48 hburn-in test. Using five sets of assembly templates, the achieved productionrate was twelve modules per week.4 Quality AssuranceThe quality assurance played an important role and took up a significantfraction of the resources at all stages of the module production for bothTT and IT. The silicon sensors were qualified by the manufacturer but stillcomprehensive acceptance tests were performed upon reception. These testsincluded optical inspections, geometrical measurements of the dicing edges(which are used to accurately position the sensors during the module assem-bly), measurements of leakage currents, the measurement of the full depletionvoltage and the identification of bad strips. The quality of the sensors wasfound to be excellent and a very good agreement between our results andthe data provided by the manufacturer was found [5]. Readout hybrids wentthrough first quality assurance tests, including several temperature cyclesbetween -20◦ C and +60◦C, at the vendor3, where a dedicated test stand forthis purpose had been set up. Each hybrid was tested after the delivery ina further 96 h burn-in program, which included measurements of its powerconsumption and various readout tests [6]. As described in section 3, duringproduction each detector module passed through thorough burn-in tests atleast once. For the two productions sites, the burn-in setups and programmes3The readout hybrids were assembled by RHe Microsystems, Radeberger, Germany.5were different, but the essential tests were similar. Both include temperaturecycling (at least 36 h cycling between room temperature and 5◦C for TT,48 h cycling between +40◦C and -5◦C for IT) with detectors continuouslybiased at 500 V, as well as several measurements of IV curves up to 500V bias voltage. Extensive readout tests were performed, including pedestaland noise measurements and the measurement of signal pulse-shapes usingthe internal test pulse implemented in the Beetle readout chip. An arrayof infra-red laser diodes was used in the TT burn-in test stand to generatecharge on each readout sector and to measure signal pulse-shapes as well ascharge-collection efficiency curves using detector signals. More than 2 TByteof data were recorded in the TT burn-in tests alone. “Bad” channels (inter-rupts, shorts between two channels and pinholes) are identified by analysingthe noise on the strips and the pulse-shapes of the internal test pulses andlaser signals. Interrupts can be identified as channels with too little noise ortoo high test-pulse amplitude (due to a reduced load capacitance). Shortedstrips, as pairs of adjacent channels, have with too large noise or too smalltest-pulse amplitude (due to a larger load capacitance).For the IT modules, a second test is performed to identify shorts betweentwo adjacent channels: Internal test pulses with the same amplitude but al-ternating polarity are applied to consecutive Beetle readout channels. If twochannels are shorted, the two test pulses cancel out and the amplitude of theoutput signal for these two channels drops to zero. Pinholes are identifiedby reading out the modules with an open detector box and daylight shiningon to the silicon sensors. If a pinhole is present the leakage currents that arecreated in the silicon bulk flow through the front-end pre-amplifier. Withlight shining onto the sensors, these currents are so large and the amplifiersaturates even if no bias voltage is applied. The spread of the noise distribu-tion for the affected channels essentially drops to zero. The overall quality ofthe modules produced is very good. The fraction of bad channels is aroundor below 0.1 % for both the TT modules and the IT modules. Leakage cur-rents at 500 V and room temperature are typically around 0.5 µA or lessper silicon sensor. A few modules show higher than normal leakage currents,break down below 500 V or otherwise abnormal IV curves. The number ofthese modules is smaller than the number of spares. The measurement of thepositioning accuracy of the silicon sensors is important, because it is foreseento treat each detector module - resp. each readout sector in the case of TT- as one single unit in the software alignment of the detector. The relativemisalignment of sensors within a module therefore needs too be small com-pared to the expected spatial resolution of about 50 µm. As an example ofthe achieved precision, the distribution of relative sensor offsets and rotationon TT modules is shown in Fig. 3. One particular problem detected during6Figure 3: Relative offsets and rotations distributions of the TT silicon sensorsmodules.7the module production concerns the long-term behaviour of silver glue onaluminium surfaces. Silver glue was initially used on TT modules for thebias voltage connection to the aluminium-coated backplanes of the sensors.During production the ohmic resistance of these connections was measuredimmediately after the module assembly and then again after a few weeks tomonths. Initially, all connections showed a low resistivity, but later mea-surements showed that a significant fraction of them had a resistances of upto several hundred Ohms. The problem is attributed to a slowly continuingoxidation of the aluminium surface underneath the glue. In order to avoidpossible long-term problems, all bias voltage connections on the TT moduleswere subsequently modified with additional wire bonds. Silver glue is alsoused on IT modules to ensure the ground connection between the readoutchips and the aluminium cooling balcony as a common ground in the detectorbox. In order to avoid problems, also here additional thin ground wires havebeen implemented on all modules.5 Summary and OutlookThe production and testing of detector modules for the LHCb Trigger Trackeras well as the production of modules for the Inner Tracker have been com-pleted, the testing of Inner Tracker modules will be completed soon. Thetesting of modules dominated the overall production effort and determinedthe production rate. This was expected since special emphasis was put onthe quality assurance. The quality of the produced modules is very satis-factory with a small number of modules lost during the production. Theinitially foreseen module production rates were reached within the allocatedresources. To mentions is that the transition from prototyping to series pro-duction took much longer than anticipated. This was due to a wide rangeof reasons, including delivery problems at vendors (an example being pro-duction yield problems at the vendor of Kapton interconnect cables for theTT modules), less steep than expected learning curves (for example, trainingof bonding technicians), and some weak points in the module design thatbecame apparent only when the first final modules were produced (most ofthese concerned bias voltage and grounding) and required last-minute mod-ifications of the module design and/or assembly procedures. The focus ofattention now shifts to the assembly of detector modules into tracking sta-tions and their installation and integration in LHCb. The first TT moduleis already installed in the LHCb detector.8References[1] Ralf Bernhard et al., LHCb note 2007-126.[2] LHCb Reoptimized Detector Design and Performance - Technical DesignReport, CERN/LHCC-2003-030.[3] N. Bakel et al. LHCb note 2005-105.[4] A. Vollhardt et al. Proc. LECC 2005, CERN/LHCC-2005-038 (2005)187-191.[5] C. Lois, Nucl. Instr. and Methods. A568 (2006) 277-283.[6] F. Lehner, Proc. LECC 2005, CERN/LHCC-2005-038 (2005) 67-70.9

The LHCb Silicon Tracker

The LHCb Silicon Tracker

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