- Email: [email protected]
Status of the L3 silicon microvertex detector
I| LI[I l f z l t l '.I: k'l,'l[Ik'l ;I
PROCEEDINGS SUPPLEMENTS
Nuclear Physics B (Proc. Suppl.) 32 (1993) 202-207 North-Holland
STATUS OF T H E L3 SILICON M I C R O V E R T E X D E T E C T O R O.Adriani/), S.Ahlend), G.Ambrosim), E.Babuccim), G.Barbagli/), A.Baschirotto0, R.Battistonm), A.Bayh), G.Bencze e) 1, P.Beneh), B.Bertuccim), M.Biasinim), G.Bileim), J.G.Boissevaink), M.Bosetti0, M.L.Brooks k) , ] .Busenitz c) , W.J .Burger h) , C.Camps b) , M.Caria m), G.Castellini a), R.Castello0, B.Checcucci m), A.Chen "), W.Y.Chenn),T.E.Coan k), V.Commichaub), D.DiBitonto c), S.Easo~), P.Extermann h), E.Fiandrini m), A.Gougas i), K.Hangarterb), C.Hauviller e), A.Herve 'e), H.Hoferq), 3.S.Kapustinski k), W.W.Kinnison k) , H.KirstV), V.R.Krastev m) 2, G.Landi/), M.Lebeau 3, P.Lecoqe), D.M.Leek), R.LeisteP), W.Lin,), W . L o h m a n n p), K.Liibelsmeyer a), M.MacDermottq), A.Marin d) F.Masciocchi h), G . M a t a y e) 4, G.B.Milierk), G.Millsk), H.Nowak p), G.Passaleva/,m), T.PenningtonC), T.Pauli), M.Pauluzzi'~), S.Pensotti t) , E.Perrin h) , P.G.Rancoita 0, M.Ratt aggi 0 , J.-P.Richeux h) , A.Santocchia m), M.SachwitzV), P.Schmitzb), B.SchSneichP), L.Servolim), R.Siedling a), K.SubhaniC), G.Terzi0, T . C . T h o m p s o n k), F.Tonisch v), G.Trowitzsch p), G.M.Viertel q) , H.Vogt p), S.Waldmeier q), S.Wang m) , R.Weill j), S.C.Yeh °), B.Zhou d) Presented by G.M. Bilei a) l. Physikalisches Istitut, RWTH, Aachen, FRG b)3. Physikalisches lstitut, RWTH, D-5100 Aachen, FRG C)The University o] Alabama, Tuscaloosa, USA a) Boston University, Boston, USA e) European Laboratory for Particle Physics, CERN, Geneva, Switzerland /)INFN Sezione di Firenze/University o] Firenze, Italy 9) INFN Sezione di Firenze and Istituto Ricerca Onde Elettromagnet~che, Italy h) University of Geneva, Geneva , Switerland i) Johns Hopkins University, Baltimore, USA J) Universite' de Lausanne, Lausanne, Switzerland k)Los AMmos National Laboratory, Los Alamos , USA t)INFN Sezione di Milano, Italy m)INFN Sezione di Perugia/University of Perugia, Italy n) National Central University, Chun#-li, Taiwan °)National Tsing Hua University, Hsinchu, Taiwan P)Deutsches Elektronen Synchrotron-Institut ]~r Hochenergiephysik, DESY-Ifh Zeuthen q) EidgenSssiche Teehnische Hochschule, ETH Z~rieh, Switzerland
A report on the status of the construction of the L3 Silicon Microvertex Detector is presented here. The detector will consist of two double sided AC coupled silicon layers equipped with re and z readout with an expected intrinsic resolution of ~ 6/~m and ~ 25/zm respectively. A description of the detector with its mechanical support, alignment system and readout electronics is presented. 0920-5632/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved.
O. A&'iani et al. /Status of the L3 silicon microvertex detector
1. I N T R O D U C T I O N A Silicon Microvertex Detector (SMD) is presently under construction for the L3 detector [1,2] at the CERN LEP storage ring. This detector will allow a two-dimensional measurement of charged particle tracks over the polar angle region 22 ° < 0 < 158 ° and over the full azimuthal region, improving the L3 impact parameter resolution, momentum and z coordinate measurements. For a detailed description of the SMD design parameters see Ref.[3]. In this paper we review the status of the project. In particular we report on silicon sensors tests, on the mechanical design and construction, and, finally, on the electronics system developement. The SMD installation is planned during the 1992/93 LEP shutdown, to allow commissioning and first data taking during the 1993 LEP run.
2. S I L I C O N
SENSORS
The silicon detectors, based on the INFN Pisa double sided design[4], are manufactured by CSEM[5]. The detectors are built on n-doped silicon wafers of 300 g m thickness and having a size of ~ 7 x 4 cm 2. The strip pitch is 25 pm on the p-side and 50 tim on the ohmic side; the readout pitch is 50 /~m on the p-side (re) and 150-200 1
3 4
Permanent address: Research Institute for Particle and Nuclear Physics,Budapest, Hungary Permanent address: Institute for Nuclear Researches and Nuclear Energy, Sofia, Bulgaria Permanent address: Laboratoires Physique des Particules, LAPP, Annecy, France Permanent address: Technical University of Budapest, Budapest, Hungary
203
pro, depending on the polar angle, on the ohmic side (z). With this choice we expect an intrinsic resolution of 6 and 25 pm in r¢ and z coordinate measurement, respectively. To AC-couple the strips to the preamplifier, we use a special integrated coupling capacitor on a quartz substrate[5], based on an improvement of the original Pisa design[11]; each dye consists now of 128 capacitors with 50 #m pitch. Each capacitor is protected by a set of diodes voltages spikes induced by beam losses on the detector. Each sensor is tested and characterized after delivery from CSEM. The basic electrical characteristics (depletion voltage, single strip and total leakage current, interstrip resistance, depletion voltage, voltage drop between guard-ring and strips, etc.) are measured at the University of Perugia and we define those detectors as good ones which, at the voltage of 1.5 times the depletion voltage, satisfy the following criteria: (i) total leakage current less than 2 pA; (ii) not more than two strips with a leakage current bigger than 50 hA, and none bigger than 100 nA (Fig. 1 shows a typical leakage current for each strip: the typical current is of the order of 100 pA. We note in the figure a clear pattern of higher current strips which is an artefact due to the finite size of the 64 channels probe card); (iii) interstrip resistance on the ohmic side between 2 and 15 MfL On the basis of tests performed on the first 40 sensors produced, we obtain, at the time of the writing, a fairly good production yield (..~ 70 %).
204
O. Adriani et aL / Status of the L3 silicon microvertex detector
3. M E C H A N I C A L
We have decided to use carbon fiber compo-
DESIGN
The reduced size of LEP vacuum pipes, gave the L3 detector the challenging capability to host a silicon vertex detector only supported on the end flanges of the L3 Time Expansion Chamber (TEC). This requires a rigid, accurate mechanical supporting structure of about 1 m long. Since the installation must be done with the beam pipe and the T E C in place, we adopted a scheme of having two semi-cylindrical halves joined together right before the installation (Fig.2).
4.0
5
f
~22.0
0
:.
-i . , .~ .
100
:
i .
,..
,
200
.
•
t
t
•
300
400
Fig. 1 Leakage current as functlon~of strip #
site to build the supporting shell. It has small CTE, high Young's modules, large radiation length and it is easy to manufacture. The support design has been completed at CERN: it is now under construction at NIKHEF and the delivery is expected by the end of october 1992. The SMD active area consists of two concentric ,~ 30 cm long silicon layers. Each layer consists of 12 modules (called ladders); given the space available, the outer layer consists of non overlapping ladders, having a 2 ° stereo angle rotation with respect beam axis to resolve tracking ambiguities in case of many particles, while the inner layer consists of 12 non rotated ladders, having about 10% of overlap. Each ladder (Fig. 2) is made with 2 half ladders, glued to a carbon fiber stiffener. Each halfladder is composed of two silicon sensors glued and bonded together: it is read-out at one end by two identical hybrids (one hybrid for r¢ and the other for z coordintate readout). In order to read the z strips with an identical hybrid as for the r¢ strips, as well as locate the heat load generated by these hybrids at the ladders end,
Fig. 2 View of the SMD Ladders
O. Adriani et al. ~Status of the L3 silicon microvertexdetector
a special 50 #m thick Kapton cable with Lshaped copper strips has been developed at the University of Alabama and manifactured at Max Levy Autograph[6]. This cable consists of dimensionally stable type E Kapton with a C H E 9 p p m / p p m / ° C that is plated with 2.5 p m of copper. Lines pattern of typically 15 prn width are chemically etched using ahigh precision ebeam litography technique and then plated on the input and output pad areas with 2 and 1 #m of nickel and gold, respectively, for final wire bonding. The overall length of these cables is 14 cn].
Special care has been dedicated to the SMD cooling system. Simulations done at Los Alamos have shown that positioning all readout electronics at the ladder ends we can desing an efficient cooling circuit to keep the thermal gradients to an acceptable level. The requirements of the cooling system were: (i) to remove a total power of ,,- 110 watts at each detector end; (ii) to maintain detectors and the mechanical structure as close as possible to 18.5 °C, the temperature of the TEC chamber; (iii) to minimize the thermal gradients so that the initial high-precision mechanical alignment can be maintained; (iv) to be a reliable, leakless, and low mass system. A solution matching these requirements has been found, based on a chilled water system operating below atmospherice pressure, already used for the L3 BGO calorimeter[7] and integrating the cooling circuit in the SMD mechanical support. For this purpose a water cooling channel is machined in the two aluminum end-flanges which support the readout hybrids. The readout hybrids have been built on a thick-film aluminum nitrate substrate, similar to the one used for the CDF vertex detector[14].
205
This substrate has a very good thermal conductivity (160 W/m °K) and it provides a good heat conduction from the VLSI readout chips to the cooling circuit. Furthermore, the coefficient of thermal expansion of aluminum nitrate is well matched to that of silicon. Care has also been taken to make sure the hybrid is thermally isolated from the nearest silicon detector in order to prevent conduction of heat from the VLSI chips to the sensors. An additional cooling circuit will be used to remove heat from the converter boards, which, for power regulation and signal driving functions, generate ,,, 2 watts/board (-,,50 W each end). In addition to water cooling, dry air is flushed from the center to the ends of the SMD in order to reduce the thermal gradients across the detector. The scale of mechanical tolerances required in the construction and assembly of the SMD must be comparable to the intrinsic position resolution of the strips detectors. In order to achieve an final SMD position resolution of ,,~ 10 pm, construction accuracy must be correspondingly high. High precision vertex reconstruction depends also on the accurate knowledge of the detector position in space and on its stability in time. To address these issues, great care is being devoted to:
(i) perform very precise laboratory measurements of the ladder components and their relative positions; (ii) perform an accurate measurement of the ladders position on the support; (iii) monitor in time the position of SMD with respect to TEC. Precise coordinate measurement machine (,,~ 3 pm precision) is being used to survey all the ladder components and to align them during the various phases of the assembly.
206
O. Adriani et aL / Status of the L3 silicon microvertex detector
To monitor SMD alignment stability with respect to TEC a capacitive readout system is used. This system is based on the simple idea of relating differences in capacitance to displacements in the relative positions of the two conductive plates forming the capacitor. This system has already been sucessfully used by other vertex detectors [8,13] and it is expected to provide an accuracy of few/~m's. It is also foreseen to illuminate the some strips on the silicon sensors with laser heads mounted on the TEC inner wall[13], to monitor the stability in time of the SMD support.
4. R E A D O U T
ELECTRONICS
The scheme of the SMD readout system is shown in Fig. 3. Data
DRP
Port
V.5 I Module # 1
Module # 2
Module # 3
/ Module # N
/
Sequencer
Digital Fanout for Control Siqnals Crate Master
Da•
& Cornmunication to Databox in P4
Fig. 3 SMD Readout Block Diagram
The readout of each half ladder requires two
hybrids (one for re and one for z side), each one equipeed with 6 SVX-H chips. The SVX-H is a full custom VLSI radiation hard chip developed at LBL for the CDF experiment, and fabricated by UTMC[9] using 1.2 pm feature size CMOS technology. The chip comprises an analog and a digital section. The analog section contains 128 channels of low noise high-gain charge amplifiers, each followed by a sample-and-hold stage, a threshold storage stage, a comparator and latch, and digital circuitry to control the serial multiplexed readout, which allows for various readout mode, like full or sparse (with zero suppression) modes. For a detailed description of the SVX chip see Ref. [15]. The VLSI chips production is completed and the hybrids fabrication is in progress[10]. At each SMD end forty-eight hybrids are needed to readout the complete detector. Each pair of adjacent hybrids are connected together and readout serially through an intermediate converter board placed inside the support. The converter contains low drop voltage regulators for the SVX analog and digital powers, an analog amplifier to drive the analog data out of the detector, a calibration circuit for the SVX charge injection and some bidirectional line drivers for the SVX control and digital-data lines. These boards have been developed using surface mounted technology at Perugia. Some prototypes have already been built and tested successfully, and the mass production is ready to start at CAEN[12]. Digital and analog data from the converters travel to optical boards placed at about 10 m from the detector. These optoboards digitize the analog data and also apply the voltage needed for the SVX calibration with a DAC circuit. Optodecoupling of the SMD from the DAQ electronics allows both a simple method to read the silicon n-side SVX-H, operating near the bias voltage, and an efficient separation of the DAQ ground
O. Adriani et al. ~Status of the L3 silicon microvertex detector
from the SMD ground, thus reducing ground loop problems[16]. After digitization, data are sent through optical fibers to a set of VME fast Data Reduction Processor (DRP) modules[17]. These modules are a modified version of the one used in the L3 TEC data acquisition system[i] and they are built around a Texas Instruments[18] processor chip (TMS 99105) with a 24 Mhz clock. The SMD readout system consists of three crates, each one houses 16 DRP's and 1 Crate Master for communication handling. The synchronization and the front-end electronics timing is realized by a special VME sequencer module. The heart of this module is based on a programmable micro sequencer chip (AM2910) producing the clock patterns required to operate the SVX-tt. Both sequencer and DRP prototype modules have been realized by Aachen II and DESY/Zeuthen groups.
5. C O N C L U S I O N S We presented the status of the construction of the L3 microvertex detector. We plan to install and to commission the detector during the 1993, to exploit its significant contribution to the L3 detector physics capabilities both at LEP phase I and II. ACKNOWLEDGMENTS We thanks the LSD-Pisa group for the many discussions concerning the use of double sided silicon detectors; in particular it is a pleasure to thank for its very valuable help L. Bosisio. We are deeply indebted to the LBL group for the help concerning the SVX readout chip. The contribution of NIKttEF to the SMD mechan-
207
ics construction is gratefully aknowledged. This work is partly supported by the Swiss National Foundation for Scientifich Research, by the Italian Institute for Nuclear Physics, by the German Bundesministerium ffir Forshung and Technologie, by the Taiwanese National Science Foundation and by the U.S. Department of Energy under grant DE-FG05-84ER40141.
References [1] B. Adeva et al. , Nucl. Instr. and Meth. A289 (1990) 35. [2] SMD Study Group, L3-/nternal Report ~924 and L3 Collaboration, CERN-LEPC 91-5, LEPC-4-ADD.1. [3] B. Alpat et al., Nucl. Instr. and Meth. A315 (1992) 197. [4] G. Bagliesi et al., INFN/PI AE 86/10. [5] C.S.E.M., Recherce et DEvelopment, Neuchatle, Switzerland. [6] Max Levy Anthograph, Inc., Philadelphia, PA, USA. [7] M. Bosteels, L E P / I M / M B / Y N (1985). [8] A. Breakstone et al., Nucl. Instr. and Meth. A281 (1989) 453; A. Breakstone et al., Nucl. Instr. and Meth. A305 (1991) 39. [9] UTMC,Colorado Springs, CO, U.S.A. [10] PROMEX, Santa Clara, CA, U.S.A. [11] G. Batignani et al., Nucl. Instr. and Meth. A277 (1989) 147. [12] C.A.E.N, Viareggio, Italy. [13] M. Caccia et al. Nucl. Instr. and Meth. A315 (1992) 143. [14] F. Bedeschi et al., Nucl. Inst. and Meth. A315 (1992) 188. [15] S. Klein/elder et al., Proc. IEEE NSS Syrup. October 1987. [16] O.Adriani et al., contribution to this vohtme. [17] V. Commichau, User's manual DRP V05, Aachen, April 1992. [18] Texas Instrument Inc., U.S.A.
PROCEEDINGS SUPPLEMENTS
Nuclear Physics B (Proc. Suppl.) 32 (1993) 202-207 North-Holland
STATUS OF T H E L3 SILICON M I C R O V E R T E X D E T E C T O R O.Adriani/), S.Ahlend), G.Ambrosim), E.Babuccim), G.Barbagli/), A.Baschirotto0, R.Battistonm), A.Bayh), G.Bencze e) 1, P.Beneh), B.Bertuccim), M.Biasinim), G.Bileim), J.G.Boissevaink), M.Bosetti0, M.L.Brooks k) , ] .Busenitz c) , W.J .Burger h) , C.Camps b) , M.Caria m), G.Castellini a), R.Castello0, B.Checcucci m), A.Chen "), W.Y.Chenn),T.E.Coan k), V.Commichaub), D.DiBitonto c), S.Easo~), P.Extermann h), E.Fiandrini m), A.Gougas i), K.Hangarterb), C.Hauviller e), A.Herve 'e), H.Hoferq), 3.S.Kapustinski k), W.W.Kinnison k) , H.KirstV), V.R.Krastev m) 2, G.Landi/), M.Lebeau 3, P.Lecoqe), D.M.Leek), R.LeisteP), W.Lin,), W . L o h m a n n p), K.Liibelsmeyer a), M.MacDermottq), A.Marin d) F.Masciocchi h), G . M a t a y e) 4, G.B.Milierk), G.Millsk), H.Nowak p), G.Passaleva/,m), T.PenningtonC), T.Pauli), M.Pauluzzi'~), S.Pensotti t) , E.Perrin h) , P.G.Rancoita 0, M.Ratt aggi 0 , J.-P.Richeux h) , A.Santocchia m), M.SachwitzV), P.Schmitzb), B.SchSneichP), L.Servolim), R.Siedling a), K.SubhaniC), G.Terzi0, T . C . T h o m p s o n k), F.Tonisch v), G.Trowitzsch p), G.M.Viertel q) , H.Vogt p), S.Waldmeier q), S.Wang m) , R.Weill j), S.C.Yeh °), B.Zhou d) Presented by G.M. Bilei a) l. Physikalisches Istitut, RWTH, Aachen, FRG b)3. Physikalisches lstitut, RWTH, D-5100 Aachen, FRG C)The University o] Alabama, Tuscaloosa, USA a) Boston University, Boston, USA e) European Laboratory for Particle Physics, CERN, Geneva, Switzerland /)INFN Sezione di Firenze/University o] Firenze, Italy 9) INFN Sezione di Firenze and Istituto Ricerca Onde Elettromagnet~che, Italy h) University of Geneva, Geneva , Switerland i) Johns Hopkins University, Baltimore, USA J) Universite' de Lausanne, Lausanne, Switzerland k)Los AMmos National Laboratory, Los Alamos , USA t)INFN Sezione di Milano, Italy m)INFN Sezione di Perugia/University of Perugia, Italy n) National Central University, Chun#-li, Taiwan °)National Tsing Hua University, Hsinchu, Taiwan P)Deutsches Elektronen Synchrotron-Institut ]~r Hochenergiephysik, DESY-Ifh Zeuthen q) EidgenSssiche Teehnische Hochschule, ETH Z~rieh, Switzerland
A report on the status of the construction of the L3 Silicon Microvertex Detector is presented here. The detector will consist of two double sided AC coupled silicon layers equipped with re and z readout with an expected intrinsic resolution of ~ 6/~m and ~ 25/zm respectively. A description of the detector with its mechanical support, alignment system and readout electronics is presented. 0920-5632/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved.
O. A&'iani et al. /Status of the L3 silicon microvertex detector
1. I N T R O D U C T I O N A Silicon Microvertex Detector (SMD) is presently under construction for the L3 detector [1,2] at the CERN LEP storage ring. This detector will allow a two-dimensional measurement of charged particle tracks over the polar angle region 22 ° < 0 < 158 ° and over the full azimuthal region, improving the L3 impact parameter resolution, momentum and z coordinate measurements. For a detailed description of the SMD design parameters see Ref.[3]. In this paper we review the status of the project. In particular we report on silicon sensors tests, on the mechanical design and construction, and, finally, on the electronics system developement. The SMD installation is planned during the 1992/93 LEP shutdown, to allow commissioning and first data taking during the 1993 LEP run.
2. S I L I C O N
SENSORS
The silicon detectors, based on the INFN Pisa double sided design[4], are manufactured by CSEM[5]. The detectors are built on n-doped silicon wafers of 300 g m thickness and having a size of ~ 7 x 4 cm 2. The strip pitch is 25 pm on the p-side and 50 tim on the ohmic side; the readout pitch is 50 /~m on the p-side (re) and 150-200 1
3 4
Permanent address: Research Institute for Particle and Nuclear Physics,Budapest, Hungary Permanent address: Institute for Nuclear Researches and Nuclear Energy, Sofia, Bulgaria Permanent address: Laboratoires Physique des Particules, LAPP, Annecy, France Permanent address: Technical University of Budapest, Budapest, Hungary
203
pro, depending on the polar angle, on the ohmic side (z). With this choice we expect an intrinsic resolution of 6 and 25 pm in r¢ and z coordinate measurement, respectively. To AC-couple the strips to the preamplifier, we use a special integrated coupling capacitor on a quartz substrate[5], based on an improvement of the original Pisa design[11]; each dye consists now of 128 capacitors with 50 #m pitch. Each capacitor is protected by a set of diodes voltages spikes induced by beam losses on the detector. Each sensor is tested and characterized after delivery from CSEM. The basic electrical characteristics (depletion voltage, single strip and total leakage current, interstrip resistance, depletion voltage, voltage drop between guard-ring and strips, etc.) are measured at the University of Perugia and we define those detectors as good ones which, at the voltage of 1.5 times the depletion voltage, satisfy the following criteria: (i) total leakage current less than 2 pA; (ii) not more than two strips with a leakage current bigger than 50 hA, and none bigger than 100 nA (Fig. 1 shows a typical leakage current for each strip: the typical current is of the order of 100 pA. We note in the figure a clear pattern of higher current strips which is an artefact due to the finite size of the 64 channels probe card); (iii) interstrip resistance on the ohmic side between 2 and 15 MfL On the basis of tests performed on the first 40 sensors produced, we obtain, at the time of the writing, a fairly good production yield (..~ 70 %).
204
O. Adriani et aL / Status of the L3 silicon microvertex detector
3. M E C H A N I C A L
We have decided to use carbon fiber compo-
DESIGN
The reduced size of LEP vacuum pipes, gave the L3 detector the challenging capability to host a silicon vertex detector only supported on the end flanges of the L3 Time Expansion Chamber (TEC). This requires a rigid, accurate mechanical supporting structure of about 1 m long. Since the installation must be done with the beam pipe and the T E C in place, we adopted a scheme of having two semi-cylindrical halves joined together right before the installation (Fig.2).
4.0
5
f
~22.0
0
:.
-i . , .~ .
100
:
i .
,..
,
200
.
•
t
t
•
300
400
Fig. 1 Leakage current as functlon~of strip #
site to build the supporting shell. It has small CTE, high Young's modules, large radiation length and it is easy to manufacture. The support design has been completed at CERN: it is now under construction at NIKHEF and the delivery is expected by the end of october 1992. The SMD active area consists of two concentric ,~ 30 cm long silicon layers. Each layer consists of 12 modules (called ladders); given the space available, the outer layer consists of non overlapping ladders, having a 2 ° stereo angle rotation with respect beam axis to resolve tracking ambiguities in case of many particles, while the inner layer consists of 12 non rotated ladders, having about 10% of overlap. Each ladder (Fig. 2) is made with 2 half ladders, glued to a carbon fiber stiffener. Each halfladder is composed of two silicon sensors glued and bonded together: it is read-out at one end by two identical hybrids (one hybrid for r¢ and the other for z coordintate readout). In order to read the z strips with an identical hybrid as for the r¢ strips, as well as locate the heat load generated by these hybrids at the ladders end,
Fig. 2 View of the SMD Ladders
O. Adriani et al. ~Status of the L3 silicon microvertexdetector
a special 50 #m thick Kapton cable with Lshaped copper strips has been developed at the University of Alabama and manifactured at Max Levy Autograph[6]. This cable consists of dimensionally stable type E Kapton with a C H E 9 p p m / p p m / ° C that is plated with 2.5 p m of copper. Lines pattern of typically 15 prn width are chemically etched using ahigh precision ebeam litography technique and then plated on the input and output pad areas with 2 and 1 #m of nickel and gold, respectively, for final wire bonding. The overall length of these cables is 14 cn].
Special care has been dedicated to the SMD cooling system. Simulations done at Los Alamos have shown that positioning all readout electronics at the ladder ends we can desing an efficient cooling circuit to keep the thermal gradients to an acceptable level. The requirements of the cooling system were: (i) to remove a total power of ,,- 110 watts at each detector end; (ii) to maintain detectors and the mechanical structure as close as possible to 18.5 °C, the temperature of the TEC chamber; (iii) to minimize the thermal gradients so that the initial high-precision mechanical alignment can be maintained; (iv) to be a reliable, leakless, and low mass system. A solution matching these requirements has been found, based on a chilled water system operating below atmospherice pressure, already used for the L3 BGO calorimeter[7] and integrating the cooling circuit in the SMD mechanical support. For this purpose a water cooling channel is machined in the two aluminum end-flanges which support the readout hybrids. The readout hybrids have been built on a thick-film aluminum nitrate substrate, similar to the one used for the CDF vertex detector[14].
205
This substrate has a very good thermal conductivity (160 W/m °K) and it provides a good heat conduction from the VLSI readout chips to the cooling circuit. Furthermore, the coefficient of thermal expansion of aluminum nitrate is well matched to that of silicon. Care has also been taken to make sure the hybrid is thermally isolated from the nearest silicon detector in order to prevent conduction of heat from the VLSI chips to the sensors. An additional cooling circuit will be used to remove heat from the converter boards, which, for power regulation and signal driving functions, generate ,,, 2 watts/board (-,,50 W each end). In addition to water cooling, dry air is flushed from the center to the ends of the SMD in order to reduce the thermal gradients across the detector. The scale of mechanical tolerances required in the construction and assembly of the SMD must be comparable to the intrinsic position resolution of the strips detectors. In order to achieve an final SMD position resolution of ,,~ 10 pm, construction accuracy must be correspondingly high. High precision vertex reconstruction depends also on the accurate knowledge of the detector position in space and on its stability in time. To address these issues, great care is being devoted to:
(i) perform very precise laboratory measurements of the ladder components and their relative positions; (ii) perform an accurate measurement of the ladders position on the support; (iii) monitor in time the position of SMD with respect to TEC. Precise coordinate measurement machine (,,~ 3 pm precision) is being used to survey all the ladder components and to align them during the various phases of the assembly.
206
O. Adriani et aL / Status of the L3 silicon microvertex detector
To monitor SMD alignment stability with respect to TEC a capacitive readout system is used. This system is based on the simple idea of relating differences in capacitance to displacements in the relative positions of the two conductive plates forming the capacitor. This system has already been sucessfully used by other vertex detectors [8,13] and it is expected to provide an accuracy of few/~m's. It is also foreseen to illuminate the some strips on the silicon sensors with laser heads mounted on the TEC inner wall[13], to monitor the stability in time of the SMD support.
4. R E A D O U T
ELECTRONICS
The scheme of the SMD readout system is shown in Fig. 3. Data
DRP
Port
V.5 I Module # 1
Module # 2
Module # 3
/ Module # N
/
Sequencer
Digital Fanout for Control Siqnals Crate Master
Da•
& Cornmunication to Databox in P4
Fig. 3 SMD Readout Block Diagram
The readout of each half ladder requires two
hybrids (one for re and one for z side), each one equipeed with 6 SVX-H chips. The SVX-H is a full custom VLSI radiation hard chip developed at LBL for the CDF experiment, and fabricated by UTMC[9] using 1.2 pm feature size CMOS technology. The chip comprises an analog and a digital section. The analog section contains 128 channels of low noise high-gain charge amplifiers, each followed by a sample-and-hold stage, a threshold storage stage, a comparator and latch, and digital circuitry to control the serial multiplexed readout, which allows for various readout mode, like full or sparse (with zero suppression) modes. For a detailed description of the SVX chip see Ref. [15]. The VLSI chips production is completed and the hybrids fabrication is in progress[10]. At each SMD end forty-eight hybrids are needed to readout the complete detector. Each pair of adjacent hybrids are connected together and readout serially through an intermediate converter board placed inside the support. The converter contains low drop voltage regulators for the SVX analog and digital powers, an analog amplifier to drive the analog data out of the detector, a calibration circuit for the SVX charge injection and some bidirectional line drivers for the SVX control and digital-data lines. These boards have been developed using surface mounted technology at Perugia. Some prototypes have already been built and tested successfully, and the mass production is ready to start at CAEN[12]. Digital and analog data from the converters travel to optical boards placed at about 10 m from the detector. These optoboards digitize the analog data and also apply the voltage needed for the SVX calibration with a DAC circuit. Optodecoupling of the SMD from the DAQ electronics allows both a simple method to read the silicon n-side SVX-H, operating near the bias voltage, and an efficient separation of the DAQ ground
O. Adriani et al. ~Status of the L3 silicon microvertex detector
from the SMD ground, thus reducing ground loop problems[16]. After digitization, data are sent through optical fibers to a set of VME fast Data Reduction Processor (DRP) modules[17]. These modules are a modified version of the one used in the L3 TEC data acquisition system[i] and they are built around a Texas Instruments[18] processor chip (TMS 99105) with a 24 Mhz clock. The SMD readout system consists of three crates, each one houses 16 DRP's and 1 Crate Master for communication handling. The synchronization and the front-end electronics timing is realized by a special VME sequencer module. The heart of this module is based on a programmable micro sequencer chip (AM2910) producing the clock patterns required to operate the SVX-tt. Both sequencer and DRP prototype modules have been realized by Aachen II and DESY/Zeuthen groups.
5. C O N C L U S I O N S We presented the status of the construction of the L3 microvertex detector. We plan to install and to commission the detector during the 1993, to exploit its significant contribution to the L3 detector physics capabilities both at LEP phase I and II. ACKNOWLEDGMENTS We thanks the LSD-Pisa group for the many discussions concerning the use of double sided silicon detectors; in particular it is a pleasure to thank for its very valuable help L. Bosisio. We are deeply indebted to the LBL group for the help concerning the SVX readout chip. The contribution of NIKttEF to the SMD mechan-
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ics construction is gratefully aknowledged. This work is partly supported by the Swiss National Foundation for Scientifich Research, by the Italian Institute for Nuclear Physics, by the German Bundesministerium ffir Forshung and Technologie, by the Taiwanese National Science Foundation and by the U.S. Department of Energy under grant DE-FG05-84ER40141.
References [1] B. Adeva et al. , Nucl. Instr. and Meth. A289 (1990) 35. [2] SMD Study Group, L3-/nternal Report ~924 and L3 Collaboration, CERN-LEPC 91-5, LEPC-4-ADD.1. [3] B. Alpat et al., Nucl. Instr. and Meth. A315 (1992) 197. [4] G. Bagliesi et al., INFN/PI AE 86/10. [5] C.S.E.M., Recherce et DEvelopment, Neuchatle, Switzerland. [6] Max Levy Anthograph, Inc., Philadelphia, PA, USA. [7] M. Bosteels, L E P / I M / M B / Y N (1985). [8] A. Breakstone et al., Nucl. Instr. and Meth. A281 (1989) 453; A. Breakstone et al., Nucl. Instr. and Meth. A305 (1991) 39. [9] UTMC,Colorado Springs, CO, U.S.A. [10] PROMEX, Santa Clara, CA, U.S.A. [11] G. Batignani et al., Nucl. Instr. and Meth. A277 (1989) 147. [12] C.A.E.N, Viareggio, Italy. [13] M. Caccia et al. Nucl. Instr. and Meth. A315 (1992) 143. [14] F. Bedeschi et al., Nucl. Inst. and Meth. A315 (1992) 188. [15] S. Klein/elder et al., Proc. IEEE NSS Syrup. October 1987. [16] O.Adriani et al., contribution to this vohtme. [17] V. Commichau, User's manual DRP V05, Aachen, April 1992. [18] Texas Instrument Inc., U.S.A.