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The Compact Muon Solenoid Silicon Tracker: Testing of Hybrids, Modules and Substructures at Operating Temperature M. Pöttgens, III. Physikalisches Institut B, RWTH Aachen, on behalf of the CMS Tracker Collaboration Abstract--The Compact Muon Solenoid (CMS) is one of two general purpose detectors which are foreseen to operate at the Large Hadron Collider (LHC), which is presently being built at the European laboratory for particle physics (CERN) in Switzerland. The Central Tracker of CMS consists of a Pixel System, which is located close to the interaction point and a Silicon Strip Tracker (SST) which instruments the intermediate and outer region. The SST is composed of 15148 Silicon Microstrip Detector Modules which contain the read-out electronics (hybrids) and sensors. These modules will be assembled into substructures with control electronics and optics for transmitting data. The substructures will be integrated into the subsystems of the SST. The SST will be operated for up to ten years in the harsh radiation environment of the LHC. The lifetime of the SST will be extended by operating the detector at lowered temperature. The sensors, which are very delicate parts in respect to radiation damage, will be operated at a maximum temperature of –10°C. Since the assembly of the modules as well as the mounting on Substructures is done at room temperature, tests in a CMS-like environment are necessary to prove the mechanical and electrical stability. I. T INTRODUCTION Large Hadron Collider (LHC) is a future accelerator which will be located at the European laboratory for particle physics (CERN) in Switzerland, where the construction of the machine has already begun. The principle aim of the accelerator is to provide an environment where evidences of the Higgs boson and supersymmetric particles can be studied. When the machine is ready in 2007 protons will be accelerated in two counter-rotating beams to a center of mass energy of 14 TeV. The desired luminosity is 1033 cm-2 s-1 at the beginning, later during the high luminosity phase it will be increased to 1034 cm-2 s-1. At a later stage also heavy ions will be accelerated to 5.5 TeV per nuclei. The collisions of these vast objects compared to protons are expected to lead to a region where the density of energy is high enough to create a quark-gluon plasma. One of the two general purpose experiments at the LHC is the Compact Muon Solenoid (CMS). The central tracking system of CMS is formed by a pixel detector and a silicon strip tracker (SST). The size of the active HE detection area of the SST is 198 m2, which makes it the largest all silicon tracker ever built. II. OPERATION CONSTRAINTS The SST is planned to be operated in the harsh radiation environment of the LHC for at least ten years. Some irradiated parts of the SST will suffer an average fluence of 1.6x1014 1MeV-equivalent neutrons per cm2 during this period. For this reason radiation hardness is a mayor issue for the constituents of the SST. To reduce the radiation induced loss of performance, the SST is cooled down to avoid reverse annealing effects that deteriorate the properties of the sensors. The temperature needed to guarantee this is –10°C. Therefore the cooling system has to act as a heat sink in a way that –10°C is the maximum temperature of the silicon of all sensors. The ASICS used in the tracker were specially designed to tolerate high radiation doses and have shown to be still performing well after being irradiated with doses expected during operation in CMS. III. HYBRIDS, MODULES AND SUBSTRUCTURES The SST of CMS is divided into four sub-detectors. The inner barrel (TIB) and the outer barrel (TOB) instrument the barrel region of the SST while the forward region is instrumented by the inner disks (TID) and the tracker end caps (TEC). Some information on the basic electronics is given below. A. The Hybrids The hybrids house the read-out electronics for the silicon sensors. They are composed of a four layer polyimide circuit and bring power and control lines to the ASICS which are mounted on them. Each hybrid houses four or six analogue pipeline voltage (APV) chips as well as one phase locked loop (PLL), one 2:1 multiplexer (MUX) and one detector control unit (DCU) chip. The APVs shape, amplify, buffer and multiplex the signals coming from it’s 128 analogue input channels. The PLL is used for timing adjustment and trigger and clock decoding. The differential output of two APVs is multiplexed onto one differential line by the MUX chip. Monitoring of some interesting physical values is provided by the DCU. Such values are the temperatures of the silicon 0-7803-8701-5/04/$20.00 (C) 2004 IEEE Ceramic Support Piece APV25S1: Four-Layer Kapton Circuit MUX: The mass production of modules was started in 2003. Presently about 2100 good modules have been produced. The production suffered from bad vias and weak bonds on hybrids as well as shortance of qualified thick sensors. amplifier, shaper, buffer multiplexer PLL: clock distribution, trigger reconstruction and distribution DCU: measurement of hybrid voltage supply, temperatures, detector leakage current Figure 1: A hybrid with read-out and support chips. sensors, the hybrid and the DCU chip itself, the two hybrid supply voltages and the current on the high voltage line used to deplete the sensors. Before being mounted on modules a pitch adapter is glued to the hybrid and bonded to the inputs of the APVs. This is necessary to adjust the pitch on the APVs to the pitch of the sensors. A picture of a hybrid is shown in Fig. 1. The first larger amount of hybrids was delivered for the “milestone 200” at the beginning of 2002. At that time a ceramic substrate was used for the hybrid circuits which caused severe problems mainly because of the high density of vias and difficulties soldering the read-out cable to the substrate. An R&D program had to be started to identify a more suitable technology for the substrate. At the beginning of 2003 a four layer kapton flex circuit laminated onto a ceramic substrate for mechanical stability was found to be the best solution. In mid 2004 the sequence of glue and kapton layers was modified to reduce problems during the metallization step for the vias. B. The Modules The modules are the smallest detector units. They consist of a hybrid, one or two sensors, a capton circuit which brings the high voltage to the sensors and a carbon fiber and/or graphite frame which serves as mechanical support. A typical module is depicted in Fig. 2. High mechanical precision and good reproducibility are achieved by assembling the modules on gantry robots. HV Pitch Connector Adapter Sensors Kapton Circuit Hybrid Wire Bonds C. The Substructures A substructure for the TEC can be seen in fig. 3. The substructures have to provide the modules with cooling, clock, trigger, control signals and power. The cooling is done via titanium pipes through which a coolant is running. Small aluminium blocks connect these pipes to the modules. Since all control and read-out signals are transmitted to and from the SST via optical links, the substructures have to be equipped with optical to electrical converters. The digital opto-hybrid module (DOHM) is used to bring digital information like clock, trigger and slow control from the outside of the tracker to the communication and control units (CCU). The CCUs are organised in a token ring like architecture which implies that a number of CCUs share one DOHM. The analogue data is just transmitted unidirectional. Opto-hybrids convert the analogue output of the modules and couple them into optical fibers for off detector digitization. Digital Opto-Hybrid Module Support Cables Cooling Pipes The construction of a module is finished when the electrical connections between the strips of the sensors and the inputs of the read-out chips are done. Carbon Fiber Support Plate Inter-Connect Board Analogue Opto-Hybrid Figure 3: Picture of a fully equipped back petal. The digital opto-hybrid module is specific for this type of petal. The back side is also equipped with modules so that just the small area at the edges of the petal is insensitive to ionizing particles. The basic built up of the four substructures for the subdetectors is identical but their shapes and thus the number of modules per substructure varies a lot. The inner barrel uses shells on which the modules are organised in strings, the outer barrel uses rods which will be inserted into a carbon fiber structure, the inner disks are arranged in three rings per disk and the end caps will be equipped with petals. The assembly of the substructures has just started. Very promising results concerning the performance were obtained. Carbon Fiber Frame Figure 2: A wedge shaped CMS module which will be used for ring 6 in the end caps. A total number of 15148 modules will instrument the SST. Optical Fibers IV. TEST PROCEDURES The experience gained during the pre-production series of hybrids and modules was used to specify an optimum set of tests which is suitable to discover potential problems. To ensure a good quality of the test objects and to identify critical steps in the chain of production it is crucial to record results in a centralized database. Visual inspections are as important as alignment checks and electrical and functional tests. The 0-7803-8701-5/04/$20.00 (C) 2004 IEEE careful handling of the fragile objects is obligatory. In this article just functional tests can be mentioned. A. Tests on Hybrids Functional tests on hybrids and modules are done with a hardand software that was developed especially for testing purposes. The APV read-out controller (ARC) system provides full hybrid support like power, trigger, clock and slow control and is distributed among all institutes involved in module production. The first test at low temperature is done after the hybrids are equipped with and bonded to a pitch adapter. A standard test includes nine cycles between 20°C and –20°C and one between –25°C and 35°C. The tests are done to prove the mechanical and electrical stability of the objects in a CMS-like environment. The tests are done in a special teststation where four hybrids can be thermal cycled and read-out at a time. A few of these test stations are operable at CERN, UCSB and Mexico. The set of tests performed in this test station is a pedestal and noise run, a calibration run using the internal calibration circuit of the APV and a test where signals are injected into the pitch adapter lines. These tests allow to spot open connections (missing bonds or open pitch adapter lines), short circuited and bad APV channels. Short circuited channels have to be removed because they can reduce the gain of the APV, opens can be rebonded although this time consuming operation is just done if many opens are found and bad APV channels cannot be repaired. A hybrid with four APV is of grade A if less than three channels are open or noisy. It fails if channel are shorted or more then four are open or noisy. In any other case it is graded B. For six APV hybrids less than four noisy of open strips are tolerated for grade A, grade B is applied if less than six channels are problematic and any number of shorted or more than six noisy or open channels lead to grade C. The tests at low temperature revealed no mechanical problems but one functional problem was observed on the hybrids. When the hybrid power is switched on the PLL starts automatically an auto calibration cycle. When the cycle has finished successfully clock and trigger signals are output. It was found out that at low temperature the chip does not seem to finish the auto calibration cycle which causes no clock and trigger to be present on the hybrid. Since one can restart the PLL by assessing the control registers this problem is not considered to be severe. At present thermal tests on hybrids are done in a much wider range of temperature. They are extended to even destructive tests to check the robustness of the interlayer connections since the vias of some batches of hybrids were found to be badly metallised. This caused a stop in hybrid production which will be restarted when the quality of the vias has improved. B. Tests on Modules During the pre-series of module production a lot of experience was gained on the behaviour of faults on modules. It was shown that the various kinds of defects can be identified by certain signatures in the data. The official read-out used for single module qualification is the ARC system. Compared to hybrid tests a high voltage power supply for sensor depletion and a LED pulser for signal injection is added to identify typical failures on modules. Tests on modules are done completely automated with the ARC software (ARCS) which also interfaces an especially designed high voltage power supply and LED pulser. The most frequent problems are strips that are not connected to the read-out because of missing bonds (opens), short circuited channels (shorts) and short circuited read-out capacitors (pinholes). A module fails the qualification test if more than 2% of the channel are considered to be bad or the leakage current is higher than 10 µA per sensor. The grade is B if the number of bad channel is less than 2% but higher than 1%. If the leakage current measured on the sensor level is exceeded by a factor of five on a module, the grade A and B changes into AF or BF respectively. When module assembly and bonding is completed each module is intensively tested in an rf-shielded box to exclude pick up of noise from external sources which could falsify the data. These tests are done in the bonding centers which can repair all bonding related failures. The test of the modules at low temperature includes typically ten cooling cycles and lasts up to three days. Typically up to ten modules are mounted at a time and read out electrically using a multiplexer that feeds the date into the CMS-like system which is more similar to the final read-out. No LED pulser is placed inside the test-box for space and feed through reasons. Because of their duration these tests are called long-term tests. The decision on a standardized procedure is obliged to the single sub-detector groups. The aim of this long-term test is to assure the mechanical and electrical stability of the modules. For the TEC community for example this test on single modules is done as long as there are not enough fully equipped petals. C. Test on Substructures The substructures will be installed into the final mechanics of the tracker. Thus they are very advanced in respect to the final system where the read-out is also done via optical fibers. Thus they do not only allow to check the performance of the electronics but also the cooling capability of the system as it will be in CMS. Since the slow control is organized in a token ring architecture a certain amount of substructures is grouped together. The test of a single substructure is therefore done without a digital optical link. This will only be done in system tests, beam tests and during the final integration. Up to now most experience on the read-out of substructures was gained during test beams and in the system test. Procedures for the qualification of all types of substructures are almost completely defined. The preparation of test stands for mass production has been finished in all centers as for example in UCSB (Fig. 4). 0-7803-8701-5/04/$20.00 (C) 2004 IEEE the gain also increases so that the overall performance is not reduced. The same investigation was done on modules to check the influence of the sensors. These noise distributions are shown in Fig. 6. Figure 4: The most advanced test tool for substructures is the multi rod burn in test stand at the University of California, Santa Barbara, that can house up to eight rods at a time. Entries per bin Noise Histogram @ 20°C V. RESULTS 10 3 10 2 Noise Histo Entries 24064 Mean 1.411 RMS 0.09274 10 1 From the great amount of data just some plots can be shown here. The focus is put on the performance of the test objects for operation at cold. 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Noise in ADC Counts Noise Histogram @ -20°C 10 3 10 2 Entries per bin Entries per bin Noise Histogram @ 20°C Noise Histo Entries Mean RMS 8192 0.871 0.0548 10 3 10 2 Noise Histo Entries 24064 Mean 1.531 RMS 0.1085 10 10 1 0 1 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Noise in ADC Counts 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Noise in ADC Counts Entries per bin Noise Histogram @ -20°C 10 3 10 2 Noise Histo Entries Mean RMS 8192 0.9261 0.08438 10 1 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Noise in ADC Counts Figure 5: Noise distributions of hybrid channels at cold vs. room temperature taken in deconvolution mode. The mean noise expressed in ADC counts rises by 0.05 while the distribution broadens at the same time. No additional failures of hybrids were observed at cold compared to room temperature apart from the PLL behaviour which turned out to be harmless. The power dissipation of the hybrid rised by about 10% at cold when operated with identical register settings of the APVs. It was shown that the power dissipation does not change if the registers are tuned. The noise distribution of the hybrids at cold and at room temperature is shown in Fig. 5. The APVs were operated in deconvolution mode which will be used during the high luminosity phase of CMS. The mean noise expressed in ADC counts is slightly higher. At the same time it was shown that Figure 6: Noise distributions of module channels at cold vs. room temperature taken in deconvolution mode. The mean of the noise expressed in ADC counts rises by 0.12. A slight increase of noise can also be seen on modules. The channels with higher or lower noise compared to the central distribution can be identified as bad for certain reasons. Problems at low temperature would result in a higher number of bad channels at cold compared to room temperature. The change of noise versus temperature is rather small compared to the change of signal height which is shown in Fig. 7. The change of signal height is not to be seen as a precision measurement since the change of temperature on the APV internal calibration circuit is not exactly known. A precise measurement of the signal to noise ratio can only be derived from the signals of minimal ionizing particles on the sensors. The change of signal height with temperature is about 12 ADC counts which corresponds to an increase in the gain of about 16% if changes in the injection circuit is neglected. High voltage stability of the modules is of great importance for the operation of the tracker. Bad electrical contacts to the sensors would result in the loss of a full module. Even worse are modules that draw too much current because in the design of the tracker it is not foreseen to switch off single but groups of modules. A typical measurement of the detector leakage current versus temperature is shown in Fig. 8. As expected the current increases exponentially with increasing temperature. This property of the sensors shows the importance of a sufficient heat sink so that thermal runaway is improbable. 0-7803-8701-5/04/$20.00 (C) 2004 IEEE Pulse Height vs. Temperature 89 85 81 Peak Mode 73 -20 -15 -10 -5 0 5 10 15 Temperature [°C] Figure 7: Maximum pulse height of test pulses, which were generated by the APV internal signal injection circuit, versus temperature. The ICAL register of the APVs was set to 58 which corresponds to a injection charge of 36250 electrons. A lot of data was taken in beam tests in 2004. A partly equipped shell of the tracker inner barrel was tested, as well as fully equipped petals and rods. The data shown here is derived from two fully equipped petals. A front petal and a back petal were cooled down and tested in May and June 2004. This configuration corresponds to a complete TEC control ring. In total 124 read-out channels of 51 modules were fed into two final front end digitizers (VME FED9U) which provide 96 input channels each, final multi service cables were used and the cooling was done in the same way as it will be done in the experiment. The control ring was also implemented optically with a prototype of the digital optohybrid module (DOHM). The control and read-out system was therefore very similar to the final one. As an example the signal to noise ratio of modules of each TEC geometry is shown in Fig. 9. Signal / Noise 77 45 40 35 30 25 20 15 10 5 0 1 320 µm sensors 2 500 µm sensors 3 4 3 4 5 6 7 5 6 7 Ring number Deconvolution Mode 30 Signal / Noise Pulse Height in ADC Counts 93 all modules in deconvolution mode and higher than 25 for peak mode. These preliminary results were gained using a cluster finding algorithm asking for a signal to noise ratio higher than five for the seed strip. Neighbouring strips are added to the cluster if their ratio is higher than three. The noise of the cluster is identified by the seed noise. A final analysis with the official analysis tool ORCA is foreseen to be done in future. 25 20 15 10 5 0 1 2 Ring number Figure 9: Signal to noise ratio of TEC modules with different geometry. The modules of rings five, six and seven are assembled with sensors of 500 µm silicon while the other geometries are equipped with sensors of 320 µm thickness. The modules with thicker silicon sensors show a higher signal to noise ratio although two sensors are mounted in series. Ring one to four modules are equipped with only one sensor of 320 µm thickness which has a positive effect on the occupancy, the radiation damage and the noise. The disadvantage of smaller signal is partly compensated by the reduction of noise. The length of the silicon sensors increases from ring 1 to ring 4 as well as for ring 5 to ring 7. The decrease of the signal to noise ratio from ring 5 to ring 7 shows this expected behaviour. The thinner sensors do not show the expected behaviour which can be explained by not optimum timing settings during this run. VI. CONCLUSIONS Figure 8: A typical measurement showing the influence of the temperature on the leakage current. The signal to noise ratio was determined in a 120 GeV pion beam. The temperature range of the silicon lay between -6.5°C and -16.5°C. The performance of the petal from the noise point of view is good since the signal to noise is higher than 15 for The tests done on hybrids, modules and substructures are very convincing that the performance of the SST at operating temperature will be good. No severe problems of any components were observed due to low temperature. A reasonable fraction of modules has been produced up to now. The problems observed on the vias of the hybrid circuit result in a delay of production which tightens the production schedule. Latest beam test results have shown that the SST is also well advanced concerning the read-out. 0-7803-8701-5/04/$20.00 (C) 2004 IEEE