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The Silicon Tracker Readout Electronics of the Gamma-ray Large Area Space Telescope Luca Baldini, Alessandro Brez, Thomas Himel, Masaharu Hirayama, R. P. Johnson, Wilko Kroeger, Luca Latronico, Massimo Minuti, David Nelson, Riccardo Rando, H. F.-W. Sadrozinski, Senior Member, IEEE, Carmelo Sgro’, Gloria Spandre, E. N. Spencer, Mutsumi Sugizaki, Hiro Tajima, Johann Cohen-Tanugi, Marcus Ziegler one of 16 layers of tungsten foils. The charged particles pass Abstract—A unique electronics system has been built and through up to 36 layers of position-sensitive detectors tested for reading signals from the silicon-strip detectors of the interleaved with the tungsten, the “tracker,” leaving behind Gamma-ray Large Area S pace Telescope mission. The system tracks pointing back toward the origin of the gamma ray . amplifies and processes signals from 884,736 36-cm strips using In the LAT the tracker and calorimeter are segmented into 16 only 160 W of power, and it achieves close to 100% detection “towers,” as illustrated in Fig. 1. efficiency with noise occupancy sufficiently low to allow it to Each of the 16 tracker modules is composed of a stack of 19 self trigger. The design of the readout system is described, and “trays,” as can be seen Fig. 3. A tray is a stiff, lightweight results are presented from ground-based testing of the carbon-composite panel with silicon-strip detectors (SSDs) completed detector system. bonded on both sides, with the strips on top parallel to those on the bottom. Also bonded to the bottom surface of all but Index Terms— Application specific integrated circuits, Data acquisition, Gamma-ray astronomy, Multichip modules, S ilicon the 3 lowest trays, between the panel and the detectors, is an radiation detectors array of tungsten foils. Each tray is rotated 90 with respect to I. INTRODUCTION T he Large Area Telescope (LAT) of the Gamma-ray Large- Area Space Telescope (GLAST) mission – is a pair- conversion gamma-ray detector similar in concept to the previous NASA high-energy gamma-ray mission EGRET on the Compton Gamma-Ray Observatory . High energy (>20 MeV) gamma rays convert into electron-positron pairs in Manuscript received October 9, 2005. T his work was supported in part by the U.S. Depart ment of Energy under Grant 22428-443410 and in part by the Agenzia Spaziale Italiana (ASI). Luca Baldini, Alessandro Brez, Luca Latronico, Massimo Minuti, Carmelo Sgro’, and Gloria Spandre are with the Istituto Nazionale di Fisica Nucleare and the Dipartimento di Fisica, Università di Pisa (INFN Pisa), Largo B. Pontecorvo, 3 PISA, Italy. T homas Himel, Wilko Kroeger, David Nelson, Mutsumi Sugizaki, and Hiro T ajima are with the Stanford Linear Accelerator Center (SLAC), Fig. 1. Cutaway view of the LAT instrument. Each tower in the 4×4 Menlo Park, CA 94025. array includes a tracker module and a calorimeter module. Yohann Cohen T anugi is currently with SLAC but worked at INFN the one above or below. The detectors on the bottom of a tray Pisa during much of the time that the LAT T racker was in development. combine with those on the top of the tray below to form a 90 R. P. Johnson (phone: 831-459-2125, fax: 831-459-5777, email: stereo x,y pair with a 2 mm gap between them. email@example.com), H. F.-W. Sadrozinski, E. N. Spencer, and Marcus Ziegler are with the Santa Cruz Institute for Particle Physics (SCIPP), University of California, Santa Cruz, CA 95064 . II. REQUIREMENT S Masaharu Hirayama is currently with the Joint Center for The tracker electronics were designed with a goal of Astrophysics, University of Maryland, Baltimore County, 1000 Hilltop operating with under 200 microwatts of conditioned power per Circle, Baltimore, MD 21250. He worked on the LAT T racker electronics while a member of SCIPP. channel, in order to allow us to launch a detector with close to Riccardo Rando is with the Istit uto Nazionale di Fisica Nucleare and a million readout channels. Of course, low power has to be the Dipartimento di Fisica, Università di Padova, I-35131 Padova, Italy. balanced against noise and efficiency requirements. buffering such that the dead time is negligible at trigger rates as high as 10 kHz. The system should be designed to minimize the impact of single point failures. The 16 independent tracker modules already go a long ways toward achieving that goal. However, even within a single tracker module we have built in enough redundancy that in nearly all cases failure of a single component will cause a loss of no more than 64 channels out of 55,296. III. A RCHIT ECT URE Fig. 4 partially illustrates the architecture of the tracker readout system. The figure represents one of the four sides of each of the 16 tracker modules. Each module side has 9 readout boards and each board supports 24 GTFE chips, for a total of 1536 amplifier channels, and 2 GTRC chips. Each channel has a preamplifier, shaping amplifier, and discriminator similar, although not identical, to the prototype circuits described in . The amplified detector signals are discriminated by a single threshold per GTFE chip; no other measurement of the signal size is made within the GTFE. Each GTFE can be programmed at any time by either GTRC to send data and trigger signals to either the left or the right and to receive commands from only either the left or right GTRC (except that the command to set the direction can be Fig. 3. Inverted view of one tracker module, with a sidewall removed. Nine MCMs and 2 flex-circuit cables are visible. received at any time from either GTRC). This architecture establishes a redundancy in the control and readout that Achieving optimal angular resolution requires highly allows the rest of the system to continue to function even in efficient detector layers placed as close as possible to the the event of loss of any single chip or readout cable converter foils. Our goal was to minimize dead regions Trigger information is formed within each GTFE chip from a between the SSDs (and between tracker modules) and to have logical OR of the 64 channels, of which any arbitrary set can be an efficiency for detecting a minimum-ionizing particle of >98% masked. The OR signal is passed to the left or right, within the active region of each SSD. depending on the setting of the chip, and combined with the The LAT tracker must also provide the principal trigger. A OR of the neighbor, and so on down the line, until the GTRC practical trigger can only be formed if the noise rate from a receives a logical OR of all non-masked channels. This “layer- single layer is not too high. Furthermore, the noise occupancy OR” trigger is sent down as a “trigger request” to the TEM for for a given trigger should not be too high (<5×10–5), or else the trigger processing. In addition a counter in the GTRC data volume will be prohibitive. measures the length of the layer-OR signal (time-over- The readout system should have sufficient speed and threshold) and buffers the result for inclusion in the event data stream. The usual tracker trigger is formed from a coincidence of trigger requests from 3 consecutive x,y pairs of tracker layers. Upon receipt of a trigger acknowledge, each GTFE chip latches the status of all 64 channels into one of 4 internal event buffers. A 64-bit mask, can be used to mask any subset of channels from contributing data, as may be necessary in case of noisy channels. When a read-event command is sent to the GTRC chips, and relayed to the GTFE chips, the event data read from the event buffer are written into the output register. From there the data flows to one or the other of the GTRC chips and gets passed down to the TEM. Fig. 2. View of approximately ¼ of an MCM, mounted on a tray in a tracker module, prior to cable installation. Fig. 4. Architecture of the tracker readout system, depicting one side of one tracker module. For brevity, only 3 of 9 layers are shown, and only 6 of 24 GT FE chips are shown within each layer . T he arrows from GT RC to GT RC indicate the flow of data packets. T he opposing flow of the readout token is not shown. amplifier output, and as a result, the system has never had any IV. M ECHANICAL INT EGRAT ION problems with coherent noise causing the pedestal (or The MCMs are mounted on the edges of the trays to effective threshold) to wander. Since the threshold can only minimize dead space between tracker modules, which requires a be adjusted per set of 64 channels, using one of the two 7-bit method to carry 1536 detector strip signals plus 16 bias DACs in the GTFE, it is important to minimize the threshold connections around the 90º corner to the SSDs. That is variation from channel to channel. That was accomplished for accomplished by a 1-layer Kapton flexible circuit that is the most part by the feedback system on the shaper, in which a bonded over a 1-mm radius machined into the edge of the differential amplifier stabilizes the DC output level , and by polyimide-glass PWB. See the x-ray image in careful design of the comparator. Fig. 5 . The GTFE chip has a built-in charge injection system Minimizing the dead space between tracker modules also controlled by a 64-bit calibration mask and the second DAC. calls for very low profile connectors on the MCM. We chose Each DAC has two 6-bit linear ranges, and the 7th bit is used to 37-pin, single-row, surface mount nano-connectors with 25-mil select the high or low range. The mask is used to select any pin spacing, manufactured by Omnetics. Two cables connect a subset of the 64 channels for injection of charge. The set of 9 MCMs to the TEM. Each cable is a 4-layer Kapton calibration command causes a step voltage, set by the DAC, to flexible circuit. be applied to each of the selected channels for a duration of 512 clock cycles. V. FRONT -END READOUT ASIC Two other 64-bit masks control which channels contribute to The GTFE design achieves low power in large measure by the trigger and the data flow, as described in Section III. All of keeping the amplifiers and digitization schemes very simple. a chip’s masks and control registers can be read back The first stage is a folded cascode, with the input transistor nondestructively by commands addressed to the chip. bias current supplied at 1.5 V, and an output source follower. The tracker’s pipelined, buffered readout system allows the It is AC coupled to the second stage (shaping amplifier), which detector trigger to be active while data are being read from the has only a single integration plus a source follower that is DC tracker. For this to work properly, it is crucial for the digital coupled to the discriminator. The main supply voltage is readout system to operate quietly enough not to disrupt the nominally 2.65 V. Good noise performance is achieved using a sensitive amplifiers. That was achieved by careful attention to 1490 m by 1.2 m input transistor, biased at 38 A, and a several design details, including the following: shaper output peaking time of about 1.5 s. For the 36-cm All digital communication between chips takes place by strips (about 41 pF load) the equivalent noise charge (ENC) is low-voltage differential signaling (LVDS), with the about 1500 electrons, compared with a most probable signal of exception of the hard reset line and the bus used to read 32,000 electrons for a minimum-ionizing particle (MIP) passing register contents from the GTFE chips back to the GTRC perpendicular through the 400 m thick silicon. chips. The discriminator, a simple comparator, sits very close to the The 20 MHz digital clock runs continuously throughout the system. Furthermore, all shift registers in the T able I. Summary of tracker performance metrics, based on the 2 nd command decoders and the event readout system shift through 17 th tracker modules manufactured. T he noise occupancy and efficiency are quoted for the same operating threshold. continuously, whether in use or not. Through prototype Metric Measurement studies we found this to be crucial. If the power load in Power consumption per channel 180 W the digital part of the system changes significantly, the Layer hit efficiency within active area >99.4% resulting change in the ground potential appears at the Active area fraction within a tracker 95.5% input of the amplifiers and can cause the system to trigger module erroneously. Overall tracker active area fraction 89.4% T racker noise occupancy <5×10 –7 The digital activity on the MCM is kept well separated T hreshold variation within a chip (rms) <9% (typically 5.2%) from the analog supplies, ground, and bias points. The T ime-over-T hreshold resolution for a 43% FWHM analog bias and filter connections never form loops single hit Fig. 2 for a photograph of one end of an MCM mounted on a around the digital busses, which are restricted to the top Number of dead channels 0.20% tray. Number of noisy channels (occupancy 0.06% two layers of the 8-layer board. >10 4 ) In addition to the left-right redundancy in control and The analog and digital parts of the GTFE chips operate on readout, some other fault protection features are designed into separate 2.5V supplies. Furthermore, the analog portion the MCM. All low-voltage power flowing into the MCM has its ground bus locally tied to the chip substrate passes through resettable poly-switches, which heat up and throughout, while the digital return current flows on metal open the circuit in case of a short on the MCM. that ties to ground off of the chip. This scheme did not cause any problems with latch-up susceptibility. Analog VIII. SYST EM PERFORMANCE and digital sections of the chip are separated by a barrier Based on measurements made on 16 flight tracker modules, a consisting of two wells biased to the corresponding tracker module consumes on average 10.0 W of power while supply voltage, with a series of ground contacts in taking data at a nominal level of activity. This corresponds to between. only 180 W of power per channel Both ASICs were implemented in the Agilent 0.5-m 3-metal The equivalent noise charge (ENC) of the SSD/amplifier CMOS process (AMOS14). All ASICs were probe tested on system has been measured channel by channel by fitting the wafers to ensure that only good chips were used in MCM threshold curves accumulated by using the internal calibration assembly . system to inject charge while scanning the threshold. The VI. READOUT CONT ROLLER ASIC fitted ENC varies channel by channel roughly in the range from 1200 to 1800 electrons, with a mean of around 1500 electrons. The GTRC buffers all command, clock, data, trigger, and The overall normalization of the ENC (and the amplifier gain) reset signals between the GTFE chips and the TEM. It has two has some uncertainties arising from the calibration of the event buffers for the data, each capable of holding the DACs and our knowledge of the capacitance of the charge addresses of up to 64 hit strips. It also has a configuration injection capacitors. register, in which several options may be set. The register can The noise occupancy was directly measured by generating be read back nondestructively. random triggers and reading out the resulting data. The typical The GTRC also includes special logic for handling the layer- occupancy measured at the level of a single tray is less than OR trigger primitive generated by the GTFE chips. The GTRC 10–6 . calculates and stores the length of the layer-OR for each event. High layer-by-layer detection efficiency is critical to The trigger acknowledge starts the counter. Hence the count optimization of the angular resolution, and hence the gamma- corresponds to the time-over-threshold of the largest signal in the layer, minus the round-trip time from layer-OR to trigger acknowledge. The time-over-threshold (TOT) provides information on the energy deposition in the SSDs that is useful for background rejection. VII. M ULT I -CHIP M ODULE (MCM) After the flexible circuit has been bonded to the PWB and trimmed, the small surface-mount components are reflow soldered, and then the connectors are attached by screws and hand soldered. The 26 chips are glued directly to the PWB and wedge wire bonded. After functional testing the wires and chips are potted with epoxy (Hysol FP4450/4451 dam and fill), and then the remainder of the board is conformal coated. See Fig. 5. X-ray cross section of the edge of the MCM with the right -angle interconnect. T he pads on the flexible circuit at the left -hand edge of the photograph are for the wire bonds that go to the plane of SSDs. ray-source point-spread function, or PSF. Within a plane of 16 full report on these measurements is found in . SSDs, the fraction of area that is active is 95.5%, taking into The effects of ionizing radiation were measured up to a dose account the small gaps (0.2 mm) between SSD wafers and the of 10 kRad, more than 10 times the expected dose over a 5-year dead region around the perimeter of an SSD. Including the mission. That level of radiation was found not to have a dead area between tracker modules, the active fraction of the significant effect on any aspect of the performance of the overall tracker (16 tower modules) is 89.4%. However, the ASICs. The main effect of the radiation on the detector system effects of the dead fraction are greatly reduced by the fact that will be increasing leakage current in the SSDs. The integration each tungsten converter plane is divided into 16 squares that time of the amplifiers is short enough that this expected fit directly over the SSD active areas. Furthermore, the tracking increase will have only a minor effect on the noise budget at code can reconstruct the photon vertex to determine whether it end of life, even at the upper limit of the operating temperature lies within a dead region, in which case at least the first range (35ºC). measurement is expected to be missing and the resolution correspondingly reduced. Therefore, there is real benefit to IX. CONCLUSION keep the efficiency within the live area as high as possible. With over half of the tracker modules built and fully tested, Inefficiency comes from two sources: dead channels and low the GLAST LAT tracker readout electronics have been fluctuations in ionization, but in practice it is dominated by the demonstrated to meet all of the design goals. In particular, the former. Dead channels due to broken detector strips and to detector system has been demonstrated to detect minimum broken amplifiers number a few per ten thousand. Dead ionizing particles with hit efficiencies >99% and with noise low channels due to broken connections between the detector enough such that the tracker can provide the primary trigger strips and the amplifiers are more common, although their for the LAT instrument. Furthermore, that is accomplished number diminished greatly following experience with building with power consumption low enough (160 W) to allow the the first tracker module. 880,000 channel instrument to operate continuously in space. The overall efficiency was measured for each layer of each tracker module using cosmic-ray tracks that pass through the A CKNOWLEDGMENT active regions of the SSDs . For example, the first tracker We would like to thank Thomas Borden, Richard Gobin, module built had mechanical interconnect problems, resulting Albert Nguyen, David Rich, Jeff Tice, Roger Williams, and in an efficiency, averaged over its 36 layers, of 98.6%, while the Charles Young of SLAC, Tim Graves of Sonoma State following 16 flight tracker modules were all more than 99.4% University, and Kamal Prasad for their dedicated work efficient. supporting the assembly and testing of the LAT tracker Measurement of the time-over-threshold (TOT) of the signal readout electronics boards. We thank Dieter Freytag, Gunther is not strictly required for operation of the detector system, but Haller, and Jeff Olsen of SLAC and Sergei Kashiguine of SCIPP it does provide information on the energy deposition in the for their contributions to the electronics design. R.P. Johnson SSDs that is useful for background rejection. For example, it thanks William Atwood of SCIPP for many useful discussions can readily identify charged particles emerging from the and analysis support during the electronics development. calorimeter and ranging out in the tracker. It can also help distinguish a single background electron track from a high - REFERENCES energy photon conversion that results in electron and positron  W.B. Atwood, “GLAST : applying silicon strip detector technology tracks nearly on top of each other. For simplicity and low to the detection of gamma rays in space,” Nucl. Instrum. Meth..A, power consumption, the tracker electronics meas ure the TOT vol. 342, p. 302, 1994. only on the layer-OR trigger primitives, but that is sufficient in  N. Gehrels and P. Michelson, “GLAST : the next -generation high the low-occupancy environment of a GLAST gamma-ray event. energy gamma-ray astronomy mission,” Astropart. Phys., vol 11, p. 277, 1999. The digital readout of the tracker system works as designed.  D.J. 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