Silicon pixel detectors produced according to the ATLAS Pixel Detector design
were tested in a beam at CERN in the framework of the ATLAS collaboration. The
detectors used n+/n sensors with oxygenated silicon substrates. The
experimental behaviour of the detectors after irradiation to 1.1 10**15
n_eq/cm**2 and 600 kGy is discussed. At the sensor bias voltage of 600 V the
depleted depth is measured to be 229 um, the mean collected charge is 20000
electrons, the detection efficiency is 98.2% and the spatial resolution is 9.6
umComment: 8 pages, 6 figures, Pixel2002 conferenc

Lari, T.

English

arXiv

Silicon pixel detectors produced according to the ATLAS Pixel Detector design were tested in a beam at CERN in the framework of the ATLAS collaboration. The detectors used n+/n sensors with oxygenated silicon substrates. The experimental behaviour of the detectors after irradiation to 1.1 10**15 n_eq/cm**2 and 600 kGy is discussed. At the sensor bias voltage of 600 V the depleted depth is measured to be 229 um, the mean collected charge is 20000 electrons, the detection efficiency is 98.2% and the spatial resolution is 9.6 u

Lari, Tommaso

CERN Document Server

arXiv:hep-ex/0210045v1  18 Oct 2002Test Beam Results of ATLAS Pixel SensorsTommaso LariINFN, Sezione di Milano, Via Celoria 16, I-20133 Milano, ITALYOn behalf of the ATLAS Pixel CollaborationAbstractSilicon pixel detectors produced according to the ATLAS Pixel Detector designwere tested in a beam at CERN in the framework of the ATLAS collaboration. Thedetectors used n+/n sensors with oxygenated silicon substrates. The experimentalbehaviour of the detectors after irradiation to 1.1 · 1015 neq cm−2and 600 kGy isdiscussed. At the sensor bias voltage of 600 V the depleted depth is measured tobe 229 µm, the mean collected charge is 20000 electrons, the detection efficiency is98.2% and the spatial resolution is 9.6 µm.1 IntroductionAt the LHC silicon microstrip and pixel detectors will be exposed to unprecedentedlevels of ionizing and hadronic radiation. As a consequence of displacement damagearising from hadronic radiation, the silicon detectors will change their characteristicsduring their lifetime [1, 2]. Potential radiation-induced performance degradation is aserious issue, which has been studied in detail.The ATLAS Pixel Detector [3, 4] is required to be tolerant to an hadronic fluence 1of 1015 neq cm−2and an ionizing dose of 550 kGy. In order to verify their ability tomeet these requirements, some pixel detectors produced according to the ATLASdesign have been exposed to 110% of the target fluence and dose and have beenoperated in a test-beam experiment at CERN. Measurements of the thickness of thedepleted region, of the efficiency and uniformity of charge collection, of hit detectionefficiency and spatial resolution have been performed and are reported here.1In this paper the fluences are converted in neq cm−2where neq is the number of particles withthe non-ionizing energy loss of a 1 MeV neutron.12 Tested devices and experimental setupThe ATLAS Pixel sensors [5] consist of n+ implants on a high resistivity n-bulksubstrate. This choice allows the detector to be operated in partial depletion modeafter type inversion due to irradiation, as the n-p junction and the depleted zoneare located near the pixels. The substrate is oxygenated to improve its radiationhardness.The pixel dimensions are 50× 400 µm2 and the sensor thickness is 250 µm. Theisolation between pixels is achieved with the p-spray technique [5, 6]. A networkof bus lines, one every other column pair of pixels, is connected to an n+ implantdot near each pixel. This bias grid allows to bias the detector during testing, beforeattachment of the front-end electronics.The irradiated devices had a 8×8 mm2 sensor connected to a front-end electronicschip [7, 8, 9] of the same size. In ATLAS, pixel sensors will have a size of 16×64 mm2and will be connected to 16 front-end chips.Every pixel cell in the sensor is connected via bump-bonding[3] to a matchingcell in the front-end chip, which includes a charge preamplifier and a discriminator.The pulse height is obtained from the time the signal stays over the discriminatorthreshold. The Time-Over-Threshold (ToT) is calibrated [9] for each pixel by injectinga known charge, with an accuracy of about 10%. During test-beam operation the pixelthresholds had a mean value of 3000 electrons with a dispersion of 300 electrons rms.The noise was 400 electrons.Irradiation was performed after bump-bonding at the CERN PS with a beamof 20 GeV protons [10]. The detectors received the total ionizing dose of 600 kGy,corresponding to a fluence of 1.1× 1015 neq cm−2.The test beam was performed at the CERN SPS accelerator with a pion beam of180 GeV/c momentum. A beam telescope [11] consisting of 4 pairs of double-sidedsilicon microstrip modules was used to track the particles. The position resolution oftracks projected onto the devices under test was 6 µm.During irradiation, between irradiation and the test beam operation and duringthe test beam operation the devices were kept below 0 0C, suppressing the timedependence of defect concentration in the silicon. At the LHC, the cumulative ef-fect of defect annealing after 10 years of operation is predicted to reduce the initialconcentration of defects, so that the results presented here are conservative.3 Thickness of depletion zoneThe measurement of the depletion zone depth is performed with data taken withparticle beam incident on the sensor at an angle of 300 respect to the normal to thepixel plane (Fig. 1), as described in [12, 13]. A particle crossing the detector produces2a cluster of hits consisting of the pixel cells that collect a significant fraction of thecharge released in the subtended segment of the track. To measure the depth of thedepleted region the maximum depth of track segment was used (Fig. 1); it was definedas the distance of the center of the track-segment subtended by a given pixel from thepixel itself. Since the entrance points of the tracks are uniformly distributed, all ofthe depths vary continuously and the maximum observed depth D is a measurementof the depletion depth within the detector.D                                                                                                                                                                                                                                                                                                                                                                                 td∆xFigure 1: Schematic view of an irradiated sensor crossed by a track. The hatchedzone corresponds to the not-depleted zone.In Fig. 2 the depletion depth measurements of the irradiated sensors are plottedas a function of the applied bias voltage. The three different symbols refer to threesensors irradiated to the same fluence. The continuous line is the prediction basedon the model [1] for the depletion depth of oxygenated sensors irradiated to 1.1 ×1015 neq cm−2before annealing, assuming a uniform space charge, so that the depletiondepth scales as the square root of the bias voltage. The measured depletion depthis in good agreement with this assumption. At 600 V, which is the maximum biasvoltage foreseen in ATLAS, the sensors are almost fully depleted.4 Charge collection efficiency and uniformityThe sum of the charges collected by the pixels in a cluster (cluster charge) providesa measurement of the charge collection efficiency of the sensor.In Fig. 3 the average cluster charge at normal incidence is plotted against thedepletion depth. As expected, the two quantities are proportional. At 600 V anaverage signal of 20000 electron is seen, to be compared with 25000 electrons collectedby not irradiated and fully depleted sensors. Both numbers are affected by the 10%3Bias (V)200 300 400 500 600 m)µDepletion (050100150200250Depletion vs Bias2 n/cm15Oxy1, 1.1 102 n/cm15Oxy2, 1.1 102 n/cm15Oxy3, 1.1 10Figure 2: Measured depletion depth as a function of bias voltage, for three differentirradiated assemblies. The solid curve is the model prediction.uncertainty on the absolute scale provided by charge calibrations. The ratio betweencharge and depletion depth, coming from the fit to the data is Q/d = 87± 9 e−/µm. m)µDepletion (0 50 100 150 200 250Charge (ke)0510152025Q/d      = 0.08725 2 n/cm15Oxy1, 1.1 102 n/cm15Oxy2, 1.1 102 n/cm15Oxy3, 1.1 10Charge vs DepletionFigure 3: Average charge collected at normal incidence, as a function of depletiondepth.The charge collection uniformity have been studied by considering the mean clus-ter charge as a function of the impact point of traversing particles relative to thecenter of a pair of pixel cells (Fig. 4) as done in [14] The coordinate axes x, y areparallel to the short (50 µm) and long (400 µm) edges of the pixels, respectively, andare centered in x at the centre of a single pixel and in y between two adjacent pixelsbelonging to a column pair.The uniformity is pretty good, except in the region between two columns where thebias grid is located (y = 0). Here a significantly reduced signal is observed. No loss4is observed between pixel columns where the bias grid is not present (y = ±0.4 mm)or between pixel rows.-0.4-0.3-0.2-0.1 0 0.1 0.2 0.3 0.4 -0.02-0.0100.010.0202.557.51012.51517.520charge vs x-yFigure 4: Average charge collected (in thousands of electrons) as a function of theposition inside a pair of pixel cells.5 Detection efficiencyAt the LHC the pixel detectors will be required to identify the bunch crossing whichhas produced a particle with an efficiency in excess of 97%. As the LHC bunchcrossing rate is 40 MHz, the charge collected by pixel must be detected in a timewindow of 25 ns.Since the extracted beams used at CERN did not have an RF structure and thereadout electronics was operated with a 40 MHz asynchronous clock (with respect tothe particles), the phase between the arrival time of every particle and the clock wasmeasured. During operation at the LHC this phase can be set for each individualmodule. Further, for an accepted trigger of beam particle the pixel hits belonging to16 contiguous “bunch crossings” (clock periods) were read out. This allowed to studythe timing behaviour of the detectors [15, 16].In Fig. 5 the detection efficiency is plotted against the difference between thebeam trigger and the beginning of the clock period in which hits are looked for. Themaximum efficiency is 98.2% which is well within the Pixel Detector requirements.6 Spatial resolutionSpatial resolution is a key feature of the Pixel Detector, which strongly affects thevertex-finding performances of ATLAS. If there is no charge sharing between pixels,the hit position is reconstructed as the centre of the fired pixel, and the resolution σ500.20.40.60.810 10 20 30 40 50 60 70nsefficiencyFigure 5: Detection efficiency versus time.is related to the pixel pitch p by σ = p/√12. If the charge released by the particleis shared by adjacent pixels, charge interpolation is possible, leading to improvedresolution. The amount of charge sharing depends on the particle incidence angle,the thickness of depleted zone and the charge collection performances.In what follows, resolution along the short (50 µm) pixel direction will be consid-ered.In Fig. 6 the distribution of the residuals between the hit position measured bythe pixel detector and the track position provided by the beam telescope is shownfor normal incidence and an incidence angle of 100. The latter was chosen to berepresentative of ATLAS tracking conditions [17]. The lighter histograms representthe one-pixel cluster residuals, the darker ones include all clusters. The resolutionis worse at normal incidence because of the larger fraction of one-pixel clusters, forwhich no charge interpolation is possible. A gaussian fit to the residuals distributionyields σ = 13.4 µm at normal incidence and σ = 9.6 µm at 100 incidence angle.7 ConclusionsATLAS Pixel detectors with oxygenated silicon sensors and rad-hard electronics havebeen irradiated to 1.1× 1015 neq cm−2fluence and 600 kGy dose, 10% more than thedesign values. They have been tested in a beam and were found to meet all ATLASspecifications. The thickness of the depletion zone was found to be 229 ± 8 µm at600 V of reverse bias voltage. The mean signal height was 20000±10% electrons, to becompared with 25000± 10% electrons before irradiation. The hit detection efficiencyin a time window of 25 ns was 98.2% and the spatial resolution at 100 incidence angle6Figure 6: Residuals between the position measured by the pixel detector and by thebeam telescope at normal incidence (left) and 10 degrees incidence angle (right). Thelighter histogram is for one-pixel clusters, the darker one includes all clusters.was 9.6 µm.AcknowledgementsThe work reported here represents the joint effort of many individuals in the AT-LAS Pixel collaboration. I would like to thank all of them, in particular John Richard-son and Kevin Einsweiler from LBNL, Berkeley, and Attilio Andreazza, FrancescoRagusa and Clara Troncon from the Milan ATLAS group.References[1] G. Lindstro¨m et al. [ROSE Collaboration], Nucl. Instrum. Meth. A 466, 308(2001).[2] Z. Li, presentation given at Pixel 2002 conference (2002), in these proceedings.[3] The ATLAS Pixel collaboration, ATLAS Pixel Detector Technical Design report,CERN/LHCC-98-13 (CERN 1998).[4] M. Garcia-Sciveres, presentation given at Pixel 2002 conference (2002), in theseproceedings.[5] M. S. Alam et al., Nucl. Instrum. Meth. A 456, 217 (2001).I. Gorelov et al., Electrical characteristics of silicon pixel sensors, ATL-INDET-2001-011 (2001), to be published in Nucl. Instr. and Meth., sec. A.7[6] R. H. Richter et al., Nucl. Instrum. Meth. A 377, 412 (1996).[7] K. Einsweiler, presentation given at Pixel 2002 conference (2002), in these pro-ceedings.[8] R. Beccherle, presentation given at Pixel 2002 conference (2002), in these pro-ceedings.[9] J. Richardson, presentation given at Pixel 2002 conference (2002), in these pro-ceedings.[10] A. Saavedra, presentation given at Pixel 2002 conference (2002), in these pro-ceedings.[11] J. Treis, P. Fischer, L. Klingbeil, H. Krueger, N. Wermes and T. Lari, Nucl.Instrum. Meth. A 490, 112 (2002) [arXiv:physics/0210004].[12] I. Gorelov et al., Nucl. Instrum. Meth. A 481, 204 (2002).[13] T. Lari and F. d. Ragusa, CERN-THESIS-2001-028.[14] C. Troncon, IEEE Trans. Nucl. Sci. 47, 737 (2000).[15] F. Ragusa, Nucl. Instrum. Meth. A 447, 184 (2000).[16] A. Andreazza, Performance of the Atlas Pixel Detector Prototype Modules, tobe published in the proceedings of the 6th international conference on positionsensitive detectors (Leicester, 2002).[17] T. Lari, CERN/ATL-INDET-2001-004 (2001).8

Test beam results of ATLAS Pixel Sensors

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