The pixel detector is the innermost tracking device of the CMS experiment at
the LHC. It is built from two independent sub devices, the pixel barrel and the
end disks. The barrel consists of three concentric layers around the beam pipe
with mean radii of 4.4, 7.3 and 10.2 cm. There are two end disks on each side
of the interaction point at 34.5 cm and 46.5 cm. This article gives an overview
of the pixel barrel detector, its mechanical support structure, electronics
components, services and its expected performance.Comment: Proceedings of Vertex06, 15th International Workshop on Vertex
  Detector

B. Meier

Broennimann

Ch. Hörmann

D. Kotlinski

H.Chr. Kästli

K. Gabathuler

Kotlinski

Kästli

P. Robmann

R. Horisberger

S. König

S. Streuli

T. Rohe

W. Bertl

W. Erdmann

English

arXiv

The pixel detector is the innermost tracking device of the CMS experiment at the LHC. It is built from two independent sub devices, the pixel barrel and the end disks. The barrel consists of three concentric layers around the beam pipe with mean radii of 4.4, 7.3 and 10.2 cm. There are two end disks on each side of the interaction point at 34.5 cm and 46.5 cm. This article gives an overview of the pixel barrel detector, its mechanical support structure, electronics components, services and its expected performance

Kästli, H C

Bertl, W

Erdmann, W

Gabathuler, K

Hörmann, C

Horisberger, Roland Paul

König, S

Kotlinski, D

Meier, B

Robmann, P

Rohe, T

Streuli, S

CERN Document Server

arXiv:physics/0702182v1   21 Feb 2007 CMS Barrel Pixel Detector OverviewH. Chr. Ka¨stli a,∗, W. Bertl a, W. Erdmann a, K. Gabathuler a Ch. Ho¨rmann b,a,R. Horisberger a, S. Ko¨nig a, D. Kotlinski a, B. Meier c, P. Robmann b, T. Rohe a, S. Streuli caPaul Scherrer Institut, 5232 Villigen PSI, SwitzerlandbPhysik-Institut der Univerita¨t Zu¨rich, 8057 Zu¨rich, SwitzerlandcInstitut fu¨r Teilchenphysik, ETH Zu¨rich, 8093 Zu¨rich, SwitzerlandAbstractThe pixel detector is the innermost tracking device of the CMS experiment at the LHC. It is built from twoindependent sub devices, the pixel barrel and the end disks. The barrel consists of three concentric layers aroundthe beam pipe with mean radii of 4.4, 7.3 and 10.2 cm. There are two end disks on each side of the interactionpoint at ±34.5 cm and ±46.5 cmThis article gives an overview of the pixel barrel detector, its mechanical support structure, electronics compo-nents, services and its expected performance.Key words: CMS, Pixel Detector, LHCPACS: 29.40.Wk, 29.40.Gx1. IntroductionTracking and vertexing in a high multiplicityenvironment such as the LHC is a challenging task.In order to get a good secondary vertex resolution,one needs measurements as close to the interactionpoint as possible. To have acceptably low occupan-cies one needs a very fine segmented detector. TheCMS experiment uses a hybrid pixel detector with100× 150µm2 pixel size. The barrel part has threelayers at mean radii of 4.4, 7.3 and 10.2 cm. In theinnermost layer the occupancy reaches 6 · 10−4 atthe full LHC luminosity of 1034s−1cm−2.The main purpose of the pixel detector is the re-∗ Corresponding author.Email: hans-christian.kaestli@psi.chconstruction of secondary vertices (mainly for band τ physics) and the generation of track seedsfor the reconstruction in the full tracker. Alsotracks reconstructed with the pixel detector aloneis the only available tracking information at thefirst stage of the higher level trigger (HLT). Herespeed is more important than accuracy or effi-ciency. As pointed out in [2], pixel-only tracks canbe reconstructed in less than 20ms (110ms) perevent for regional (global) track finder. The pTresolution is σ(pT )pT= 0.055 + 0.017 · pT [GeV/c]and the impact parameter can be measured withσ(IP ) = 80µm for pT > 7GeV/c rising to 90µmfor pT = 4 GeV/c. The component of the ver-tex position along the beam line is measured atthe HLT. The resolution depends on the physicschannel (number and momentum of tracks) and isPreprint submitted to Elsevier Science 22 February 2007Fig. 1. Drawing of half of the barrel support structure (right) and detailed view of the geometric layout (left)typically / 50µm with an efficiency of > 94%.2. Mechanical structureThe smallest independent unit of the barrel sup-port structure is a half-shell. It is made out of53cm long and 0.25 mm thin carbon fiber laddersglued to aluminum cooling pipes with 0.3 mm wallthickness. To reach full spatial coverage ladders aremounted with overlap on alternating sides of thecooling tubes. This is shown in figure 1 on the left.Since the ladders are not tilted, charge sharing isenhanced between pixels due to Lorentz drift in the4T magnetic field and hence the spatial resolutionthrough analog signal interpolation is increased.Eight modules are screwed onto each ladder usingthermal grease to improve the heat transfer fromthe electronics components to the cooling tubes.This allows an easy removal of the mounted mod-ules.Three half-shells are mounted together at the endflange building up half of the barrel detector. Theleft and right half barrels are mechanically sep-arated. A six layer PCB is mounted on the endflange. It distributes power and signals to the indi-vidual modules. The PCB is equipped with LCDS(Low Current Differential Signal) driver chips andis otherwise passive. About 10cm long cables andcooling tubes connect the barrel support structureto the service half-cylinders. A sketch of the lateris shown in figure 2.Fig. 2. Drawing of a supply tube half-cylinder. The elec-tronics components are described in section 43. ModulesThe barrel layers are made out of two types ofsensor modules. Each layer has 32 half modules atthe edges of the half-shells and 128, 224 and 320 fullmodules for the three layers respectively. A sketchof a module is shown in figure 3. 16 (8) readoutchips (ROCs) are bump bonded to the full (half)sensors. A dedicated In bump bonding process has2Fig. 3. Drawing of a full barrel modulebeen developed at PSI [3] and the whole produc-tion is done in house. A three layer flexible highdensity interconnect (HDI) is glued on top of thesensor and wire bonded to the ROCs and to the to-ken bit manager chip (TBM) [7]. It is then glued toSi-Nitride base stripes which give mechanical sta-bility and reduces stress to the bump bonds due toan excellent CTE match to Si. Details about themodule assembly are described in these proceed-ings ([4]).3.1. SensorThe active area of a full (half) module sensoris 64.8×10.6 mm2 (64.8× 5.3mm2). It is a doublesided processed ”n+ on n” design. This is a pixe-lated high dose n implantation in a lightly n-dopedbulk material. The backside is p-doped forming thepn junction. The inter-pixel isolation is done witha moderated p-spray technique. There are mainlytwo reasons for this choice: (i) after irradiation,the bulk material will be type inverted. Depletionwill then begin at the structured n-side, allowingthe device to be operated in a partially depletedmode and thus at a lower bias voltage. (ii) A multi-ple guard ring structure on the p-side controls thevoltage drop from the large negative bias voltagetowards the sensor edge and allows to put all sen-sor edges to ground potential. This enables a safeoperation at very high bias voltages up to 600V.3.2. Readout chipThe readout chip (ROC) is fabricated in a com-mercial 0.25µm 5 metal layer CMOS process. It’smain purposes are– amplification and buffering of the charge signalfrom the sensor– zero suppression in the pixel unit cell. Only sig-nals above a certain threshold will be read out.This threshold can be adjusted individually foreach pixel by means of four trim bits. The trimbits have capacitive protection against singleevent upset (SEU), which have shown to reduceSEU by 2 orders of magnitude [5]. A typicalthreshold dispersion after trimming at T=-10◦Cis 90 electron equivalents with a noise of 170electrons.– level 1 trigger verification. Hit information with-out a corresponding L1 trigger are abandoned.– sending hit information and configuration dataout to the TBM chip– adjusting various voltage levels and offsets in or-der to compensate for chip-to-chip variations inthe CMOS device parametersThe ROC has 6 on chip voltage regulators. Thismakes it possible to operate the detector withoutvoltage regulator boards inside the CMS trackervolume, improves AC power rejection and stronglyreduces intermodule cross-talk. More details aboutthe ROC can be found in [5].3.3. Module readoutThe module is readout in a daisy chainedscheme. It is controlled by the token bit manager(TBM) chip. For each level 1 trigger the TBMgenerates a token bit which controls 8 or 16 ROCs.One ROC at a time sends its hit informationthrough a LVDS (low voltage differential signal)to the TBM chip where they are amplified andconverted into a LCDS (low current differentialsignal). The later has been chosen because of itslow voltage swing, in order to reduce cross-talk onthe kapton signal cable to the supply tube.There is one analog readout link running at 40MHzfor each half module and each full module in theoutermost layer and 2 such links for each full3Fig. 4. Schematic view of the signal flow for the barrel pixel detectormodule in the two inner layers. In order to reducereadout time, the pixel address is sent in a digi-tal coded analog signal, i.e. there exist 6 discreteaddress levels. The amplified sensor signal is sentpurely analog. Each hit pixel needs 6 clock cyclesor 150ns to be read out. The address level separa-tion has been measured with the entire system andwas found to be >30 times the RMS of the levelwidth. Thus, reliable address decoding is ensured.4. Detector control and readoutFigure 4 shows a simplified diagram of the sig-nal flow for the barrel pixel detector. The signalpath in the middle shows the fast detector controllink. The pixel front end controller (pFEC) sendsa clock signal with encoded fast control signals (L1trigger, reset, internal calibration and resynchro-nization signal) to the detector. The pFEC alsodownloads configuration data at run start and ifnecessary during runs via the industrial I2C pro-tocol, modified to run at a higher clock rate of40MHz. All connections between service tube andcounting room are optical links. The DOH (digitalopto hybrid) does the conversion between opticaland electrical signals. A PLL chip regenerates thefast signals and restores the clock. A delay chip canadjust the phase for each signal separately. TheGateKeeper chip blocks idle signals needed for au-tomatic gain adjustment of the optical links frombeing transmitted to the module. Finally there isa LVDS to LCDS level converter chip on the endflange print.The path on the right shows the readout part. TheTBM drives the analog signal all the way to theservice tube. A level translator chip (ALT) gen-erates signal levels acceptable to the laser driverchips of the analog optical link. Driver and opticaltransmitter are located on the analog opto hybrid(AOH). The signal is received in the counting roomby a VME front end driver unit (pxFED) [8]. Itconverts back to electrical signals, does a baselinecorrection, determines all pixel addresses and dig-itizes the analog pulse height. It then passes thisinformation to the CMS DAQ system.Some of these components need to be programmed.This happens through a dedicated digital link viaa standard I2C protocol. The boxes in gray showpixel specific full custom auxiliary ASICs devel-oped at PSI.4]2beam intensity [MHz/cm0 20 40 60 80 100 120inefficiency-410-310-210-110110 kHz triggertrackstracks (scint.)test pulsessimulation100 kHz triggertrackssimulationFig. 5. Measured efficiency in a pion beam at PSI as afunction of particle fluence for two different trigger rates.For comparison simulation results are shown (triangles).5. PerformanceBarrel modules have been tested in a high rate300 MeV/c pion beam at PSI. Figure 5 shows thedetection efficiency as a function of particle fluencefor trigger rates of 10 and 100kHz. Three differentmethods have been used. For low trigger rates ef-ficiencies are shown for tracks reconstructed witha beam telescope or using small scintillators only.The third method measures the efficiency for inter-nal calibration signals which are superimposed ontop of the beam induced data traffic. For compari-son, triangles show the efficiencies as expected fromsimulations. At high trigger rates the data lossesare dominated by readout related dead times. Theagreement between data and simulation is excel-lent here. For low trigger rates the measurementshows ≈ 0.5% higher inefficiency than expectedfrom simulations. This can be understood by radi-ation damage of the sensor under test.Resolution studies have been made in [9]. Usingpion beam test data together with simulations, oneexpects a resolution in rΦ between 10 and 20 µmfor sensors irradiated up to 6.7·1014neq/cm2, whichcorresponds to the first 4 years of operation in theinnermost layer. The z resolution depends on theazimuthal angle Θ and only weakly on the irradi-ation dose. For Θ < 60◦ the resolution is < 20µm,increasing to 40 µm at Θ = 90◦.6. Conclusion and outlookAn overview of the CMS barrel pixel detectorhas been given. The design is finalized and all sys-tem components have been tested extensively. Thedevice is still in the construction phase and willbe delivered to CMS in summer 2007. First protoncollisions are expected by the end of 2007 at theinjection energy of 450GeV per beam and at thefull energy of 7TeV in spring 2008.References[1] The CMS collaboration, CMS Tracker TechnicalDesign Report, CERN/LHCC 98-6[2] The CMS collaboration, CMS Physics TechnicalDesign Report, Vol I: Detector performance andSoftware, CERN/LHCC 06-001[3] Ch. Broennimann, Development of an Indium bumpbond process for silicon pixel detectors at PSI, Nucl.Instrum. Methods A 565 (2006) 303[4] S. Ko¨nig, Building CMS Pixel Barrel DetectorModules, These proceedings[5] H. C. Ka¨stli, Design and performance of the CMS pixeldetector readout chip, Nucl. Instrum. Methods A 565(2006) 188[6] D. Kotlinski, The Control and Readout System of theCMS Pixel Barrel Detector, Nucl. Instrum. MethodsA 565 (2006) 73[7] E. Bartz, The 0.25µm Token Bit Manager Chipfor the CMS Pixel Readout, Proceedings of the11th Workshop on Electronics for LHC and FutureExperiments, Heidelberg, Germany[8] Designed by M. Pernicka, et al., HEPHY Institute,Vienna, Austria5[9] A. Dorokhov, Performance of Radiation Hard PixelSensors for the CMS Experiment, Ph.D. thesisUniversity of Zu¨rich, 20066

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CMS Barrel Pixel Detector Overview

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