the horizontal plane. The long sides of the pixels are also horizontal. The indicated positions refer to the center of the spectrometer, the target is at -95 cm. Fig.
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IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 42, NO. 4, AUGUST 1995
Construction and Characterization of a 117 cm2 Silicon Pixel Detector Erik H.M. Heijne CERN, CH-1211 Geneva 23, Switzerland representing collaborators from Detector R&D Collaboration RD 19 and Experiment WA97 E.H.M. Heijnel, F. Antinoril, D. Barberis3, H. Beker5, W. Beuschl, P. Burgerlo, M. Campbell1, E. Cantatore1*2, M.G. Catanesi2, E. Chesil, G. Darbo3, S. D'Auria6, C. Da Viav1, D. Di Bari2, S. Di Liberto5, D. Elia2, T. Gysl, H. Helstrupl, J.M. Heuser8, A. Jacholkowskil, P. Jarronl, W. Klemptl, I. Kralik8, F. Krummenacher12, J.C. Lasallel, R. Leitner7, F. Lemeilleurl, V. Lenti2, M. Lokajicek7, L. Lopez1, M. Luptak8, G. Maggi2, P. Martinengol, F. Meddi5, A. Menetreyl, P. M i d d e l k a m p l ~ ~M. , Morando4, A. Munnsl l , P. Musico3, F. Pellegrini4, S. Pospisi17, E. Quercighl, J. Ridky7, L. Rossi3, K. Safarikl, S. Saladino2, G. Segato4, S. Simone2, W. Snoeysl, G. Stefaninil and V. Vrba7
"and University of Bari2, Genova3, Padova4, Roma5, Glasgow University6, Universty Groups of Praha7, CERNl, I Inst. of Experimental Physics Kosice8, GHS Wupperta19, Canberra Semiconductor NVl0, GEC-Marconi (Caswe1l)l and Smart Silicon Systems ~ ~ 1 2
Abstract A silicon pixel detector, developed in RD19, and consisting of 4 planes, -30 cm2 each, is operating for the first time in the lead ion experiment WA97 at CERN. The 288 CMOS readout chips are bump-bonded to 48 Si detector matrices,
assembled in 8 identical arrays. The total number of pixel cells is nearly 300 000 and each cell, 75 pm x 500 pm, contains a complete signal processing chain. Overall dead area is less than 3%.
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Fig. 1 Scatter plot representing the positions of nearly 80000 particles in the 72k pixel detector in 10000 event triggers with interactions, taken during the proton run in the Omega spectrometer with the detector positioned perpendicular to the beam axis, downstream of the target. The dead area in this detector is less than 1%. 0018-9499/95$4.00 0 1995 IEEE
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I. INTRODUCTION For particle tracking and vertex detection in the inner region of detectors at the proposed Large Hadron Collider LHC the experiments will need robust devices with a precision of 10 p m on coordinate measurements and timetagging with jitter < 15 ns, in order to cope with the event 40 MHz. The true 2-dimensional repetition rate of semiconductor pixel detector with signal processsing in each cell is being developed for this application.
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An intermediate prototype pixel detector has been made in the framework of the CERN detector R&D collaboration RD19 [ l ] and this detector already now finds the first practical application in the high multiplicity, heavy ion experiment WA97. In fig.1 a scatter plot is shown of particle positions recorded with one of these detectors. A realistic evaluation of the basic concepts of the "smart" pixel detector can now be made and we can gain practical experience with reliability and manufacturability of a large microlectronic system in a physics experiment. Descriptions of the single detector assembly and the associated electronics have been published already in extenso [2-41 and only a few points will be repeated here. The emphasis in this paper is on the construction and operation of a system of practical size with a large number of components.
11. DETECTOR DESCRIPTION The experiment WA97 uses a telescope with several consecutive planes of high resolution silicon detectors, hi
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Fig. 2 Schematic diagram of the positions of the highresolution detector boxes in the WA97 silicon detector telescope. Pixel detector planes (Pix) are placed in boxes 1, 3, 5 and 6. Additional microstrip (y and z) and pad detectors (Pad) complete the setup. The telescope points to the target at a 48 mrad angle with the horizontal Pb beam axis (x). The magnetic field is in the z-direction and bends the particles i n the horizontal plane. The long sides of the pixels are also horizontal. The indicated positions refer to the center of the spectrometer, the target is at -95 cm.
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Schematic drawing in side-view of a fully hermetic. "logical" detector plane. The 1 m m thick ceramic is U-shaped and the detectors on their thin support are mounted on the open area in thc U.
positioned - 1 m downstream from the target and - 5 cm above the beam axis. In the November 1994 setup 4 of the telescope planes have been implemented using hybrid pixel detectors as indicated schematically in fig. 2. During the preceding proton calibration run in September already 3 pixel planes were installed but not in the same positions as in fig. 2.
A pixel detector plane is composed of 2 staggered arrays as shown in fig. 3. The basic unit on an array is called a 'ladder' and it is a hybrid assembly of 6 readout chips on a 5 4 mm x 5.8 mm high-resistivity silicon detector substrate. Each of the 1008 amplifiers on the readout chip called 'Omega2' is connected via a 38 pm diameter solder bump to its corresponding detector element. Figure 4 shows 2 ladders in top and rear view. One array has 6 such ladders mounted on a ceramic plate with a thin-film conductor pattern and in fig. 5 the 2 arrays which compose one plane
Fig. 4
Photograph of 2 ladders, w i t h the detector substrate upwards (top) and h i t h the 6 attached readout chips upwards. Each ladder contains 6048 sensitive sensor elements with associated electronics
Fig. 5
Photograph of one array mounted on the support frame and the mating array in front. Each array with its z-shape flat cable consists of 6 ladders on a ceramic substrate and one of the associated sets of local driver card and VME readout module is also shown. A single array has 32288 detecting elements and the second, identical array, staggered over the open "slots" completes a full detector plane. The smaller ceramic support with the 6 ladders IS mounted on a larger. thicker U-shaped ceramic on which the flexible circuit is glued.
are illustrated together with one of the sets of intermediate board and VME module. The pattern of detector segmcnts on a ladder is mirrored in the 6 readout chips, each with 16 x 63 active pixels and an extra row of pixels at the top for electrical testing. Each cell (75 pm x 500 pm, 0.037 m m 2 ) in the readout matrix contains a complete signal processing chain [3]. The timing sequence is sketched in fig. 6. The non-uniformities from chip to chip of the timing characteristics were found to determine the overall performance of the detector system. Therefore, some details Input Ii"
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Timing diagram for the signal processing in each pixel cell. The timing can be externally adjusted.
will be discussed. Following a signal of >6000 electrons from the Si detector cell the adjustable threshold comparator will produce a signal and the binary output is available after -120 ns. Awaiting an external strobe, this bit is stored in an individual delay line in the pixel itself. The spread in delay times between pixels and between chips imposes the rather long strobe duration of 900 ns. The event data strobed in the pixel memory by the first level trigger are reset if a higher level trigger rejects the event. If it is accepted, the binary data from the pixel rows are sequentially clocked out via the column shift registers, 16 columns i n parallel, as pairs of 16bit words, chip after chip, into an external FIFO memory. Full data readout at -2MHz takes - 600 ps for a 36-chip array, and all arrays are read in parallel. In the WA97 experiment the trigger rate is less than 1 kHz and overall data acquisition time is of the order of 1 ms and therefore the relatively slow pixel detector readout does not limit the data taking. In the future we expect to speed up the event-frame readout time by on-chip zero suppression and pixel masking. which are now implemented in the VME readout module.
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Approximately 100 ladders have been tested in this way. If one of the chips was found to be unuseable the complete ladder was rejected. Ladders with not more than 3 dead columns were accepted. The majority of the rejects is caused by failures in the electronics and this shows the need for improved testing facilities on the chip. The high detector current, which is the cause for 25% of the rejects, has not yet been understood. It is practically impossible to measure independently the currents of the pixel cells in the matrix, and in the assembled devices even the guard ring current can not easily be separated from the total reverse current.
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Fig. 7 Diagrams produced during the irradiation testing of one readout chip on a ladder. The top shows the distribution of hits by the electrons from the 90Sr source, the bottom gives the projections of the contents in rows and columns. The columns 1 and 16 are lo00 pm wide instead of 500 pm.
The arrays were constructed by mounting the accepted ladders onto the ceramic substrate. Once the array is completed it undergoes the final test in the laboratory, connected to the W E readout system. Using again a 90Sr source, the overall characteristics are studied in order to determine the actual settings for the programmable bias voltages. In fig. 8 we show examples of the timing properties for one ladder with compatible delays and another ladder which contains a faster chip.
111. COMPONENT TESTING
SCAN PIXEL DELAY WA97 CHIP2
On each 100 mm diameter silicon wafer* there are 68 Omega2 chips and these have been tested on a wafer probe for DC characteristics. With power applied it was found that non-functional circuits generally are characterized by voltage excursions in excess of 1 0 m V on one or several of the different bias terminals. The wafer maps indicating the bad circuits have been provided to the companyt who deposited the bumps on the readout wafers and diced these into chips. Before bonding the readout chips onto the detector substrates all components were visually inspected for flaws. The finished ladder assemblies as shown in fig. 4 were dispatched toCERNand Bari. A probe card was used to electrically test one chip after the other on the ladder. The test row was used to check overall functionality, including the 16 parallel shift registers and the delay timing response. Then the full matrix was tested with a long strobe duration, using a 90Sr radioactive source. An example of the result of such a test is shown in fig. 7. This example represents a perfect chip. The slight loss of counts at the middle left is caused by the shadow of a contacting needle. Typical flaws detected in this test concern a missing (part of) column, excessive detector current which prevents operation of the electronics and 'always-on' pixels, sometimes for practically the complete matrix. It is relatively easy to spot noisy, 'always-on' pixels, but finding dead pixels is more difficult as it requires sufficient statistics, i.e. -lo5 counts per chip. ~
* SACMOS3 technology of Faselec AG, Zurich, CH. t GEC-Marconi Materials Ltd, Caswell, UK.
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Response of the pixel cells (chip average) as a function of the timing of the applied strobe, for a 'good ladder (top), where all chips respond optimally for the same applied strobe, and for a 'bad ladder (bottom) where one chip is too fast in delay time.
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the 4 weeks of the Pb-ion run another plane was added and the geometry was as shown in fig. 2. The Pb beam particles have a charge of 82 and the energy was 450 GeV per 740
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IV.PERFORMANCE IN WA97 In September-October during the proton calibration run the telescope contained 3 pixel detector planes. Figs. 1 and 9 show a beam profile in the detector telescope for 10 000 events. A proton interaction ocumng in a detector plane, normally rejected in the experiment, is shown in fig. 10. In
Fig. 10 Display of a proton interaction in the first detector (top) with one track perpendicular to the beam direction, passing through several adjacent pixel cells. Reaction products are seen in the following plane (bottom).
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Fig. 11 Scatter plots of particle positions for 2000 events in all 4 pixel planes in operation in the WA97 experiment. Plane 3 here corresponds to plane 1 in the proton run (see figs. 1 and 9). The Pb beam passed centrally below each plane.
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nucleon. The beam with an intensity of lo6 per burst passed at a safe distance from the silicon detector telescope but each beam particle was accompanied by several delta-electrons visible in the detectors. I n fig. 11 one may notice the increased intensity at the bottom, close to the beam. A small fraction (1%) of the Pb ions interacts in the P b target and from the several thousands of tracks which emerge from such an interaction the number of hits seen in each detector telescope plane was 25 on average. Many of these hits are from delta electrons and shower photons, rather than from hadron tracks. The timing is common to all planes and the coincidence of delays and strobe has to be carefully tuned, in order to obtain optimal response with a minimum number of silent pixels. The hardware masking procedure was executed -once a week and consisted in turning off each pixel that produced an output signal during 1000 strobe cycles. For the complete system -500 pixels were affected, while an additional 5-15 cells need sofware masking as they have from time to time an excessive number of hits.
REFERENCES [I]
E.H.M. Heijne, F. Antinori, R. Amold, D. Barberis, H. Beker, W. Beusch, P. Burger, M. Campbell, E. Chesi, G. Darbo, C. DaVia', D. Di Bari, S . Di Liberto, D. Elia, C.C. Enz, M. Glaser, J.L. Guyonnet, T. Gys, H. Helstrup, J. Heuser, A. Jacholkowski, P. Jarron, S . Kersten, F. Krummenacher, R. Leitner, F. Lemeilleur, V. Lenti, M. Letheren, M. Lokajicek, L. Lopez, M. Lovetere, G. Maggi, P. Martinengo, G. Meddeler, F. Meddi, A. Menetrey, P. Middelkamp, M. Morando, A. Munns, P. Musico, C. Neyer, M. Pallavicini, F. Pellegrini, F. Pengg, S . Pospisil, E. Quercigh, J. Ridky, L. Rossi, K. Safarik, G. Segato, S. Simone, P. Tempesta, H. Verweij, G.M. Viertel and V. Vrba First operation of a 72k element hybrid silicon micropattem pixel detector array Nucl. Instr. Meth. A349 (1994) 138.
[2]
F. Anghinolfi, P. Aspell, K. Bass, W. Beusch, L. Bosisio, C. Boutonnet, P. Burger, M. Campbell, E. Chesi, C. Claeys, J.C. Clemens, M. Cohen Solal, I. Debusschere, P. Delpierre, D. Di Bari, B. Dierickx, C.C. Enz, E. Focardi, F. Forti, Y. Gally, M. Glaser, T. Gys, M.C. Habrard, E.H.M. Heijne, L. Hermans, R. Hurst, P. Inzani, J.J. Jaeger, P. Jarron, F. Krummenacher, F. Lemeilleur, V. Lenti, V. Manzari, G. Meddeler, M. Morando, A. Munns, F. Nava, F. Navach, C. Neyer, G. Ottaviani, F. Pellegrini, F. Pengg, R. Perego, M. Pindo, R. Potheau, E. Quercigh, N. Redaelli, L. Rossi, D. Sauvage, G. Segato, S . Simone, G. Stefanini, G. Tonelli, G. Vanstraelen, G. Vegni, H. Verweij, G.M. Viertel and A 1006 element J. Waisbard hybrid silicon pixel detector with strobed binary output IEEE Trans. Nucl. Sci. NS-39 (1992) 650 .
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M. Campbell, F. Antinori, H. Beker, W. Beusch, E. Chesi, E.H.M. Heijne, J. Heuser, P. Jarron, T. Karttaavi, L. Lopez, G. Meddeler, A. Menetrey, P. Middelkamp, C. Neyer, F. Pengg, M. Pindo, E. Quercigh, S . Simone and H. Venveij Development of a pixel readout chip compatible with large area coverage Nucl. Instr. Meth. A342 (1994) 52.
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H. Beker, W. Beusch, M. Campbell, M.G. Catanesi, E. Chesi, J.C. Clemens, P. Delpierre, D. Di Bari, E.H.M. Heijne, P. Jarron, V. Lenti, V. Manzari, M. Morando, F. Navach, C. Neyer, F. Pengg, R. Perego, M. Pindo, E. Quercigh, N. Redaelli, D. Sauvage, G. Segato and S . Simone
Although the components of this detector are known to be not radhard, no influence of the radiation environment has been noticed.
V. CONCLUSIONS The preliminary testing of the components is of primordial importance for a multichip detector. Close to 600 chips have been checked prior to array construction. A dead area of
