- Email: [email protected]
The AMS-01 silicon tracker assembly and the industrial approach to detector construction
Nuclear Instruments and Methods in Physics Research A 473 (2001) 67–74
The AMS-01 silicon tracker assembly and the industrial approach to detector construction$ M. Pauluzzi* Dipartimento di Fisica and Sezione INFN, Universita" di Perugia, Via Pascoli, I-06125 Perugia, Italy
Abstract The Alpha Magnetic Spectrometer (AMS-01) has successfully performed in a precursor flight in June 1998. The experiment is scheduled for a 3-year operation on the International Space Station (ISS) in the year 2003. For the ISS flight, the completion of the tracker as well as a major upgrade of the detector (AMS-02) is in progress. In this contribution, we present results from the AMS-01 silicon tracker construction and describe the organization of the assembly for phase 2. Given the large scale of this silicon detector, the involvement of private industries in the construction is addressed. r 2001 Elsevier Science B.V. All rights reserved. PACS: 06.60.Mr; 29.40.Wk; 89.20+a; 95.55.Vj Keywords: Silicon detectors; Tracking and position-sensitive detectors; Cosmic ray detectors; Elementary particle detectors; Detector construction
1. Introduction The AMS-01 detector is a space-borne experiment aimed at precisely measuring the cosmic ray spectra to study anti-matter and the missing matter in the Universe [1]. It consists of a permanent magnet, a six plane silicon tracker, a time of flight system, a Cherenkov counter and anti-coincidence counters. We described elsewhere the AMS-01 detector [2] and the silicon tracker [3]. In June 1998, a successful precursor flight was made on shuttle STS-91, for a period of 10 days. During the test flight, both the experiment [4] and $
Invited talk at the ‘‘Vertex 2000, 9th International Workshop on Vertex Detectors’’, Sleeping Bear Dunes National Lakeshore, Michigan, USA, September 10-15, 2000 *Tel.: +39-075-5852765. E-mail address: [email protected] (M. Pauluzzi).
the silicon tracker [3,5,6] performed as expected. In addition, physics results have already been obtained [7–11]. The full version of the experiment, AMS-02 [12], will operate for 3 years on the International Space Station (ISS). The collaboration involved in the construction of the AMS silicon tracker is based on a core of institutions that started working together on the L3 Silicon Micro-vertex Detector at LEP (SMD) [13–15]. Table 1 provides a comparison between SMD, AMS-01 and AMS-02 silicon trackers. It is apparent from the table that the active surface covered with silicon detectors is for AMS-01 almost one order of magnitude larger than that of SMD. However, a careful design optimization has made it possible to have approximately the same number of readout channels. This was accomplished by increasing the readout pitch for
0168-9002/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 1 2 2 - 6
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M. Pauluzzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 67–74
Table 1 Assembly statistics of AMS-01 and AMS-02 compared to L3-SMD L3-SMD
AMS-01
AMS-02
No. of channels=ladder No. of bonds=ladder (average) No. of Si sensors=ladder (average)
1536 4608 2
1024 14,514 12.5
1024 13,856 11.9
No. of ladders Total No. of channels Total No. of bonds Total No. of Si sensors Active surface (m2 )
48 73,728 221,184 96 0.3
62 63,488 899,840 778 2.0
192 196,608 2,660,352 2286 7.0
Bonding pitch (mm)
50 (bending); 150–200 (non-bending) 4 (2 A.L.) 4.5–8.2
100 (bending) 150–200 (non bending; n-fold) 8 (2 A.L.) o5 mm
100 (bending)
Assembly period (months) Assembly precision (mm)
the bending plane coordinate, using an n-fold ambiguity in the non-bending plane readout [16] and by building much longer ladders (the ladder, see Fig. 1, is the main unit of the silicon tracker, built with 7–15 silicon sensors per ladder, the longest silicon units ever built). The area of silicon used for AMS-02 is about 3–4 times that of AMS01, and corresponds to more than 20 times in surface, with respect to a standard LEP silicon micro-vertex detector and 3–4 times in the number of readout channels. The construction of the AMS-01 silicon tracker took 8 months and 2 assembly lines, AMS-02 tracker construction is expected to take up to 16 months involving 3 assembly lines (not including spare ladders and contingency). All of this brings to an impressive assembly effort, a big step towards the LHC silicon detectors. We are, therefore, facing production on an industrial scale as already addressed in a previous contribution [16]. In this paper, we report on the experience gained during the AMS-01 tracker assembly and evaluate several implications arising from this scale of construction. In particular, the size of experiments such as AMS-02 or larger do require the organization, standardization and cost-effectiveness that only an industrial approach can guarantee. An industrial approach can be implemented in two different ways: either by private companies directly involved in the construction or
16 (2+1 A.L.) o5 mm
Fig. 1. AMS Silicon Tracker: exploded view of a ladder.
by public institutions such as universities and research laboratories adopting procedures commonly used in industries. Our personal feeling is that AMS-02 is at the limit at which either way is feasible and effective, while even larger experiments should preferably move towards construction by private companies. Statistics on AMS-01 assembly and organization for both AMS-01 and AMS-02 silicon tracker constructions are reported.
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M. Pauluzzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 67–74
2. Industrial approach Silicon detector technology is already reliable and established enough to always equip larger surfaces and volumes. What is large for physicists is usually equivalent to a small scale for industries; similarly, an established technology in physics is still challenging and semi-custom according to industrial standards. The AMS tracker is the largest silicon detector being built before the advent of LHC experiments [17]. It is large enough to become technically and economically suitable for assembly involving private companies devoted to small productions, R&D and semi-custom products. In any case, large assembly efforts do require a very efficient organization, precise documentation, standardization, compatibility, uniformity and cost-effectiveness, all of them typical of the industrial approach. Quality control (QC) and quality acceptance (QA) become important to ensure reliability, compatibility and consistency everywhere. The integration center, if any, is very useful as an independent QC and QA site to fully monitor and validate the assembly. Multiple assembly centers organization, integration, QC and QA are once more standard requirements in industries. Companies can effectively contribute during the early stages of an experiment as well, both in R&D and in the definition of assembly procedures: the assembly team can profit from their industrial experience and expertise in the optimization, standardization and automation of the assembly procedures. During AMS-01, companies had no direct involvement in assembly, only in parts procurement and manufacturing. However, during this phase, particular care has been devoted in developing assembly procedures as close as possible to industrial standards. This turned out to be very useful in the preparation for the second phase: in AMS-02, companies are involved in assembly as well, and documentation and assembly techniques were easily implemented by selected industries. The small size of candidate companies is important to ensure flexibility in their collaboration with the scientific experiments, both because
of uncertainties in the budget and schedules and because the state-of-the-art technologies often require improvements in progress. Involving industries in the construction results anyway in a more rigid schedule. In most cases, the collaboration is given a definite production window. Being (one of) the major customer(s) would increase the flexibility. The responsibility for delays is difficult to define when public research institutions are involved in procurement. To minimize delays, it is highly desirable that all parts are procured with a perfectly defined schedule and in all-good quantities. When the task of a private industry involves operations done at no cost in public institutions, losses and damages during these no-cost steps are also difficult to cope with from the company point of view since quantities purchased or to be produced by the company are defined by contract. Either allowances are foreseen or a buffer should be organized by the institutions performing or controlling the no-cost operations to replace the losses. Careful documentation of requirements, specifications, work, qualification and acceptance tests is in any case important.
3. AMS-01 and AMS-02 Silicon Tracker assembly organization Three assembly sites will contribute to the construction of the AMS-02 Silicon tracker. One will be a privately owned company [18]. This company will assemble half of the tracker modules for AMS-02. Table 2 gives an overview of the manpower working on the project on the different sites. Physicists and engineers will mostly be involved in coordination and testing.
Table 2 Personnel at AMS-02 Assembly sites Assembly centers
Technical staff
Physicists= engineers
Geneva & Zurich Universities Perugia University G&A company
4 4 3
2 2 2
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M. Pauluzzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 67–74
Four more physicists and three more technicians are working on integration and quality control at the integration center. Small companies have already participated in AMS construction during phase 1. Their contribution has been on the procurement of special custom components (high precision semi-custom silicon sensors cut [6] and ultrathin flexible upilex fanouts [19,21]) rather than a direct involvement in assembly as it will be the case for G&A in AMS-02. In all cases, the company profile was identical to the one characterized in a previous section. As an example, Fig. 2 shows the exceptional quality of the silicon sensors cut performed at Selmic [20]. This precision, with a standard deviation better than 5 mm not reachable on the market, has been obtained with standard equipment and a custom procedure developed together by the company and the AMS collaboration.
4. Assembly statistics from AMS-01 Table 3 reports the yield of AMS-01 production and Table 4 the details of the failures or problems on ladders during assembly. A ladder is considered marginal if it has defects that can, in principle, be repaired back to full functionality. In AMS-01, spare components were not enough, so defective ladders had to be used. We estimate that, were the repairs feasible, the final yield of good ladders would have been around 80%. Even so, the high failure rate could be evenly related to the atypical length of ladders and to the double-sided sensors choice. Repair is very time consuming and repair time must be taken into account for any schedule to be reliable. Vibration tests performed according to NASA specifications have demonstrated that encapsulation of ultrasonic wire bonds was not necessary.
Fig. 2. Distances between the edges of cut silicon wafer and the lithography reference crosses (t1, t2, l1 and l2) at the four corners (Design value 300 mm).
M. Pauluzzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 67–74 Table 3 Yield of silicon ladders in AMS-01 production
Required Produced
Used Yield
Quality
Number
Total Good Marginal Rejected
62 65 38 21 6 57
(Estimate)
Fraction (%)
59% 32% 9% 88% 80%
Table 4 Failure analysis of defective silicon ladders in AMS-01 production Defect
Marginal
Rejected
Handling=assembly Gluing Bonding Electronics High leakage current
7 1 3 9 11
7 1 2
This was later confirmed by both AMS-01 flight data analysis and visual inspection after AMS-01 dismounting. No bond failure was reported. Without encapsulation, ladders can be more easily repaired. It is useful to analyze in detail the different types of ladder defects arising during assembly. This study helps in optimizing procedures, defining the need of spare components for repair and establishing a reliable schedule for future assemblies. The summary is listed in Table 4. The total number of ladders listed in Table 4 does not correspond to the numbers reported in Table 3 since a given ladder could have more than one type of defect. Four of the electronics failures were due to mechanical defects of the front-end hybrid board that could not be replaced for lack of spares. Four of the ladders with higher than expected leakage current were due to a problem that was not completely understood in one assembly line. The increase in leakage current had no relation though with damages occurring during bonding. Due to redundancy, the assembly site has been shut down
71
and fully devoted to integration. The rest of the high leakage current failures occurred during integration on the plane: an unforeseen operation that proved necessary, was the accidental spreading of carbon fiber dust on the silicon. Given the complexity of the ladder, whenever assembly procedures were not carefully optimized, as in the above integration operation, failures were likely to occur. Viceversa, the gluing and bonding procedures were so successfully tuned that little defects were due to them. Again, because of complexity, the largest fraction of defective ladders were due to operator mistakes or mishandling. This particular failure demonstrates the need of developing, whenever possible, automated and operator-safe industrial procedures, not relying too much on operator (exceptional) expertise. To this purpose, standard though very precise and reliable equipment has been (will be) used for the construction of the AMS-01 (AMS-02) silicon tracker. A very good example of this is the gluing procedure: during SMD construction some gluing did require a lot of expertise to continuously change the amount of glue dispensed, depending on glue viscosity and environmental parameters. For similar gluing operations in AMS-01, we have successfully chosen a precise volumetric dispenser available in the market that, together with custom tools, automatically ensured the repeatability and uniformity of glue dispensing. Leakage current measurements at different steps during ladder assembly proved to be very useful to identify problems and to monitor the quality of the assembly procedures. The assembly sequence and tools were designed to allow more measurement than actually needed during the standard routine. They were useful during AMS-01 construction when the leakage current monitoring helped in discovering the need of a better cleaning after silicon sensors cutting and of better anti-static precautions during upilex gluing as well as the carbon fiber dust problem during integration. The statistics on the single ladder component yields is given for reference in Table 5. We have to emphasize that values in the table refer to the manufacturing process and have no relation with assembly. Usually, good components only are delivered by industries and the actual yield is
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M. Pauluzzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 67–74
Table 5 Yield statistics on ladder components manufacturing in AMS01 production (A single component can fail more than one test) Component
Action=test
Silicon production
Total 61% High leakage current p-strips 89% High leakage current n-strips 70% High leakage current sensors 82%
Silicon cut
Total Cut Shipment
94% 96% 98%
Long upilex fanout (p-side)
Total
58%
Electrical test Bonding test
66% 88%
Total
78%
Electrical test Bonding test
80% 93%
Short upilex fanout (n-side)
Yield
CFRP=airex mechanical Plates production structure Machining
94%
Decoupling capacitor chips
Electrical test (0 defects)
65%
VA preamplifiers
Electrical test (0 defects)
87%
FE hybrid packaging
Total
78%
90%
Summarizing, using the estimated final ladder yield of about 80% from AMS-01, for AMS-02 production we require 20% contingency in spare components plus 10% spare components to build 10% more fully functional spare ladders. Overall, 130% of the components required for the flight have been ordered. These orders have been placed on an all-good basis, in other words, to minimize fluctuations in quantity and delivery time.
5. Space qualification The AMS Silicon Tracker has already performed a successful 10 days flight on the Shuttle, and will have to operate on the ISS for a 3-year data-taking period. The tracker design and construction have been optimized for its use in space [16,22]. However, the tracker space qualification has not been fully performed by specialized firms. Instead, the collaboration has decided, in order to minimize costs, to: *
*
*
unknown to the customers. On the contrary, the AMS tracker collaboration participated in the testing procedures to minimize costs and monitor the production. Obviously, only good components were used in assembly. Silicon sensors and long upilex fanouts had a low yield, in both the cases, related to the length of the ladder. Having so many sensors in one ladder, single sensor specifications were much stricter than usual to eventually meet ladder specifications. For upilex fanouts, the length (and strip pitch) of the flex cable itself made the fabrication process quite difficult. In both the cases, producers and AMS-02 collaboration are working together to improve the fabrication process and, consequently, the yield. Acceptance tests ensure the reliability of components qualified as good.
*
*
understand the reasons behind each space qualification requirement; (a) identify critical materials, parts and processes with an impact on mission safety or other payloads; (b) identify possible space related failures which would only affect the experiment; upon agreement with NASA, only the requirements falling into category (a) have been tested and certified with standard qualification procedures, always in registered space industries; all other requirements have been addressed in the following ways, also depending on how critical the component is for AMS performances: * some requirements were simply not met, for materials, parts and processes not critical for mission safety nor for AMS; * whenever possible, materials already known to satisfy NASA specifications have been used; also ‘‘standard’’ space qualification procedures in specialized industries were adopted in some cases;
M. Pauluzzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 67–74 *
off-the-shelf standard components were used as often as possible with space qualification performed within the collaboration.
Non-standard space qualification procedures were carried out by the AMS collaboration. Participating institutions acquired space qualification expertise and developed the appropriate test facilities. In addition, facilities and tools already available to the High Energy Physics community were used, such as ion beams facilities. Procedures were executed according to standard NASA guidelines and specifications. As an example of requirements for space intentionally not met, the ultrasonic micro-bonds on the ladder have not been encapsulated for protection. The decision was taken after in-house vibration tests demonstrated that no failures would occur due to the high frequency random and acoustic excitations (10–2000 Hz) appropriate to the shuttle. In addition, a special test system was developed by the Perugia University to simulate Single Event Effects (SEL and SEU) on microelectronics components used in tracker readout and DAQ systems. Given the different durations of the precursor and ISS flight, more stringent space qualification criteria have, in general, been applied to AMS-02. Separate papers are in preparation on both the ladder and micro-electronics space qualification [23].
6. Choice of double-sided vs. single-sided silicon sensors The Silicon Tracker of the AMS experiment is no doubt a complex system, in particular its main unit, the ladder. The choice has been made to use double-sided silicon sensors instead of a simpler system based on single-sided sensors. For the detection of anti-matter, or for a conclusive new limit on its existence, the antihelium sensitivity of AMS during the ISS flight has to be of the order of 10 10 : Large angle nuclear scattering of ordinary matter nuclei occurring in the central planes of the silicon tracker can simulate the curvature of anti-helium nuclei
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[16,24]. To reach the required sensitivity, the non-Gaussian tails due to this particular background must be minimized by reducing the material inside the tracker. A detector built out of double-sided sensors has advantages: *
*
transparency of the detector (3.2% of a radiation length for AMS-01); less components: * lower costs, * less operations i.e. faster assembly and integration.
However, the usage of double-sided sensors has disadvantages: *
* *
components, tools and assembly procedures are usually more complex and lead to a lower yield both in components and ladders, higher costs, slower production for a fixed manpower.
As explained in a previous section concerning single components manufacturing, the low yield for silicon sensors in Table 5 is mostly related to ladder length rather than due to the double-sided sensor choice. In addition, the long upilex flex cable is required to bring out the signals from the second side of the ladder (transverse silicon strips) but, again, the low yield is mainly due to the length of the flex cable. Overall, the manufacturing yield of single components is moderately affected by the double-sided sensor choice. Considering instead the ladder assembly yield, failures related to the double-sided assembly can be estimated to be less than 10% (after repair), that is 50% of the total assembly failures. In general, complexity can be reduced by a careful design and optimization of assembly procedures, both being non-recursive efforts. The assembly time can be close to that of a single-sided detector with twice the number of sensors. The cost of a system based on double-sided silicon sensors has been evaluated to be sensibly less than 1.5 times that of an equivalent surface of singlesided sensors. Whenever transparency is mandatory for physics, as for AMS, the double-sided system has proved to be viable and therefore
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M. Pauluzzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 67–74
preferable. Otherwise, a single-sided sensors system is likely a safer approach. 7. Conclusions R&D, prototyping, or small scale assembly efforts would be very expensive if done in industries, because of scale and uniqueness. When technology becomes a standard and the scale of the experiment is large enough, small=medium sized companies can cost-effectively participate in mass production. In AMS-02, private companies will produce half of the silicon ladders. The companies can also contribute to R&D at earlier stages. The scientific community can profit from this early collaboration, by learning how to be more effective and tuning the procedures so that they will be easily adopted by any company involved in massive production. Silicon detectors are most definitely becoming a standard and reliable technology ready to make the leap to an industrial approach as it has happened in the past for scintillating counters. Some statistics from AMS-01 Silicon Tracker construction is also provided, together with solutions on procurement and assembly optimization that will be used for the AMS-02 assembly effort. The double-sided sensor approach has been proven effective and viable whenever detector transparency is important. Acknowledgements The successful flight of the AMS-01 Silicon Tracker has been made possible, among other contributions, by the valuable and untiring efforts made during the assembly by technicians, engineers and physicists from ETH Zurich, Geneva University and Perugia University-Sezione INFN. Looking forward to the next phase, my personal thanks to all of them.
References [1] R. Battiston, Nucl. Phys. B 65 (1998) 19. [2] G.M. Viertel, M. Capell, Nucl. Instr. and Meth. A 419 (1998) 295. [3] J. Alcaraz, et al., Il Nuovo Cimento 112A (11) (1999) 1325. [4] B. Alpat, Nucl. Phys. B 85 (2000) 15. [5] B. Alpat, et al., Nucl. Instr. and Meth. A 439 (2000) 53. [6] W.J. Burger, Nucl. Instr. and Meth. A 435 (1999) 202. [7] J. Alcaraz, et al., AMS Collaboration, Phys. Lett. B 461 (1999) 387. [8] J. Alcaraz, et al., AMS Collaboration, Phys. Lett. B 472 (2000) 215. [9] J. Alcaraz, et al., AMS Collaboration, Phys. Lett. B 484 (2000) 10. [10] J. Alcaraz, et al., AMS Collaboration, Phys. Lett. B 490 (2000) 27. [11] J. Alcaraz et al., AMS Collaboration, Helium in Near Earth Orbit, Phys. Lett. B, to be published. [12] B. Alpat, Nucl. Instr. and Meth. A 461 (2001) 272. [13] The SMD Study Group, CERN-LEPC 91-5, LEPC P4Add.1, April 1991. [14] G. Ambrosi et al., Nucl. Instr. and Meth. A 344 (1994) 133; M. Acciarri et al., Nucl. Instr. and Meth. A 351 (1994) 300. [15] M. Acciarri, et al., Nucl. Instr. and Meth. A 360 (1995) 103. [16] M. Pauluzzi, Nucl. Instr. and Meth. A 383 (1996) 35. [17] E. do Couto e Silva, Nucl. Instr. and Meth. A 473 (2001) 107, these proceedings. [18] G&A Engineering, Localit!a Miole 100, 67063 Oricola (Aq), Italy. [19] D. DiBitonto et al., Nucl. Instr. and Meth. A 338 (1994) 404; G. Ambrosi et al., Nucl. Instr. and Meth. A 361 (1995) 97. [20] Selmic Micromodules, Lumikontie 1, FIN-94600 Kemi, Finland. [21] CERN Surface and Material Group, Photomechanical and Printed Circuits Workshop (http:==cern.web.cern.ch= CERN=Divisions=EST=SM/welcome.htm). [22] R. Battiston, Nucl. Instr. and Meth. B 44 (1995) 274. [23] B. Alpat et al., Space Qualification Procedures, Tests and Results for Microstrip Silicon Detectors, in preparation; M. Menichelli, The Power Supply System of the Particle Tracker Detector for the AMS Experiment, Paper number N-18, contribution to National Aerospace & Electronics Conference, October 10-12, 2000. [24] S. Ahlen, Antimatter background in AMS from Single Nuclear Scatters or Fragmentations, AMS internal note, August 13, 1995; R. Battiston, INFN AE-96/38 (1996).
The AMS-01 silicon tracker assembly and the industrial approach to detector construction$ M. Pauluzzi* Dipartimento di Fisica and Sezione INFN, Universita" di Perugia, Via Pascoli, I-06125 Perugia, Italy
Abstract The Alpha Magnetic Spectrometer (AMS-01) has successfully performed in a precursor flight in June 1998. The experiment is scheduled for a 3-year operation on the International Space Station (ISS) in the year 2003. For the ISS flight, the completion of the tracker as well as a major upgrade of the detector (AMS-02) is in progress. In this contribution, we present results from the AMS-01 silicon tracker construction and describe the organization of the assembly for phase 2. Given the large scale of this silicon detector, the involvement of private industries in the construction is addressed. r 2001 Elsevier Science B.V. All rights reserved. PACS: 06.60.Mr; 29.40.Wk; 89.20+a; 95.55.Vj Keywords: Silicon detectors; Tracking and position-sensitive detectors; Cosmic ray detectors; Elementary particle detectors; Detector construction
1. Introduction The AMS-01 detector is a space-borne experiment aimed at precisely measuring the cosmic ray spectra to study anti-matter and the missing matter in the Universe [1]. It consists of a permanent magnet, a six plane silicon tracker, a time of flight system, a Cherenkov counter and anti-coincidence counters. We described elsewhere the AMS-01 detector [2] and the silicon tracker [3]. In June 1998, a successful precursor flight was made on shuttle STS-91, for a period of 10 days. During the test flight, both the experiment [4] and $
Invited talk at the ‘‘Vertex 2000, 9th International Workshop on Vertex Detectors’’, Sleeping Bear Dunes National Lakeshore, Michigan, USA, September 10-15, 2000 *Tel.: +39-075-5852765. E-mail address: [email protected] (M. Pauluzzi).
the silicon tracker [3,5,6] performed as expected. In addition, physics results have already been obtained [7–11]. The full version of the experiment, AMS-02 [12], will operate for 3 years on the International Space Station (ISS). The collaboration involved in the construction of the AMS silicon tracker is based on a core of institutions that started working together on the L3 Silicon Micro-vertex Detector at LEP (SMD) [13–15]. Table 1 provides a comparison between SMD, AMS-01 and AMS-02 silicon trackers. It is apparent from the table that the active surface covered with silicon detectors is for AMS-01 almost one order of magnitude larger than that of SMD. However, a careful design optimization has made it possible to have approximately the same number of readout channels. This was accomplished by increasing the readout pitch for
0168-9002/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 1 2 2 - 6
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M. Pauluzzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 67–74
Table 1 Assembly statistics of AMS-01 and AMS-02 compared to L3-SMD L3-SMD
AMS-01
AMS-02
No. of channels=ladder No. of bonds=ladder (average) No. of Si sensors=ladder (average)
1536 4608 2
1024 14,514 12.5
1024 13,856 11.9
No. of ladders Total No. of channels Total No. of bonds Total No. of Si sensors Active surface (m2 )
48 73,728 221,184 96 0.3
62 63,488 899,840 778 2.0
192 196,608 2,660,352 2286 7.0
Bonding pitch (mm)
50 (bending); 150–200 (non-bending) 4 (2 A.L.) 4.5–8.2
100 (bending) 150–200 (non bending; n-fold) 8 (2 A.L.) o5 mm
100 (bending)
Assembly period (months) Assembly precision (mm)
the bending plane coordinate, using an n-fold ambiguity in the non-bending plane readout [16] and by building much longer ladders (the ladder, see Fig. 1, is the main unit of the silicon tracker, built with 7–15 silicon sensors per ladder, the longest silicon units ever built). The area of silicon used for AMS-02 is about 3–4 times that of AMS01, and corresponds to more than 20 times in surface, with respect to a standard LEP silicon micro-vertex detector and 3–4 times in the number of readout channels. The construction of the AMS-01 silicon tracker took 8 months and 2 assembly lines, AMS-02 tracker construction is expected to take up to 16 months involving 3 assembly lines (not including spare ladders and contingency). All of this brings to an impressive assembly effort, a big step towards the LHC silicon detectors. We are, therefore, facing production on an industrial scale as already addressed in a previous contribution [16]. In this paper, we report on the experience gained during the AMS-01 tracker assembly and evaluate several implications arising from this scale of construction. In particular, the size of experiments such as AMS-02 or larger do require the organization, standardization and cost-effectiveness that only an industrial approach can guarantee. An industrial approach can be implemented in two different ways: either by private companies directly involved in the construction or
16 (2+1 A.L.) o5 mm
Fig. 1. AMS Silicon Tracker: exploded view of a ladder.
by public institutions such as universities and research laboratories adopting procedures commonly used in industries. Our personal feeling is that AMS-02 is at the limit at which either way is feasible and effective, while even larger experiments should preferably move towards construction by private companies. Statistics on AMS-01 assembly and organization for both AMS-01 and AMS-02 silicon tracker constructions are reported.
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M. Pauluzzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 67–74
2. Industrial approach Silicon detector technology is already reliable and established enough to always equip larger surfaces and volumes. What is large for physicists is usually equivalent to a small scale for industries; similarly, an established technology in physics is still challenging and semi-custom according to industrial standards. The AMS tracker is the largest silicon detector being built before the advent of LHC experiments [17]. It is large enough to become technically and economically suitable for assembly involving private companies devoted to small productions, R&D and semi-custom products. In any case, large assembly efforts do require a very efficient organization, precise documentation, standardization, compatibility, uniformity and cost-effectiveness, all of them typical of the industrial approach. Quality control (QC) and quality acceptance (QA) become important to ensure reliability, compatibility and consistency everywhere. The integration center, if any, is very useful as an independent QC and QA site to fully monitor and validate the assembly. Multiple assembly centers organization, integration, QC and QA are once more standard requirements in industries. Companies can effectively contribute during the early stages of an experiment as well, both in R&D and in the definition of assembly procedures: the assembly team can profit from their industrial experience and expertise in the optimization, standardization and automation of the assembly procedures. During AMS-01, companies had no direct involvement in assembly, only in parts procurement and manufacturing. However, during this phase, particular care has been devoted in developing assembly procedures as close as possible to industrial standards. This turned out to be very useful in the preparation for the second phase: in AMS-02, companies are involved in assembly as well, and documentation and assembly techniques were easily implemented by selected industries. The small size of candidate companies is important to ensure flexibility in their collaboration with the scientific experiments, both because
of uncertainties in the budget and schedules and because the state-of-the-art technologies often require improvements in progress. Involving industries in the construction results anyway in a more rigid schedule. In most cases, the collaboration is given a definite production window. Being (one of) the major customer(s) would increase the flexibility. The responsibility for delays is difficult to define when public research institutions are involved in procurement. To minimize delays, it is highly desirable that all parts are procured with a perfectly defined schedule and in all-good quantities. When the task of a private industry involves operations done at no cost in public institutions, losses and damages during these no-cost steps are also difficult to cope with from the company point of view since quantities purchased or to be produced by the company are defined by contract. Either allowances are foreseen or a buffer should be organized by the institutions performing or controlling the no-cost operations to replace the losses. Careful documentation of requirements, specifications, work, qualification and acceptance tests is in any case important.
3. AMS-01 and AMS-02 Silicon Tracker assembly organization Three assembly sites will contribute to the construction of the AMS-02 Silicon tracker. One will be a privately owned company [18]. This company will assemble half of the tracker modules for AMS-02. Table 2 gives an overview of the manpower working on the project on the different sites. Physicists and engineers will mostly be involved in coordination and testing.
Table 2 Personnel at AMS-02 Assembly sites Assembly centers
Technical staff
Physicists= engineers
Geneva & Zurich Universities Perugia University G&A company
4 4 3
2 2 2
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Four more physicists and three more technicians are working on integration and quality control at the integration center. Small companies have already participated in AMS construction during phase 1. Their contribution has been on the procurement of special custom components (high precision semi-custom silicon sensors cut [6] and ultrathin flexible upilex fanouts [19,21]) rather than a direct involvement in assembly as it will be the case for G&A in AMS-02. In all cases, the company profile was identical to the one characterized in a previous section. As an example, Fig. 2 shows the exceptional quality of the silicon sensors cut performed at Selmic [20]. This precision, with a standard deviation better than 5 mm not reachable on the market, has been obtained with standard equipment and a custom procedure developed together by the company and the AMS collaboration.
4. Assembly statistics from AMS-01 Table 3 reports the yield of AMS-01 production and Table 4 the details of the failures or problems on ladders during assembly. A ladder is considered marginal if it has defects that can, in principle, be repaired back to full functionality. In AMS-01, spare components were not enough, so defective ladders had to be used. We estimate that, were the repairs feasible, the final yield of good ladders would have been around 80%. Even so, the high failure rate could be evenly related to the atypical length of ladders and to the double-sided sensors choice. Repair is very time consuming and repair time must be taken into account for any schedule to be reliable. Vibration tests performed according to NASA specifications have demonstrated that encapsulation of ultrasonic wire bonds was not necessary.
Fig. 2. Distances between the edges of cut silicon wafer and the lithography reference crosses (t1, t2, l1 and l2) at the four corners (Design value 300 mm).
M. Pauluzzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 67–74 Table 3 Yield of silicon ladders in AMS-01 production
Required Produced
Used Yield
Quality
Number
Total Good Marginal Rejected
62 65 38 21 6 57
(Estimate)
Fraction (%)
59% 32% 9% 88% 80%
Table 4 Failure analysis of defective silicon ladders in AMS-01 production Defect
Marginal
Rejected
Handling=assembly Gluing Bonding Electronics High leakage current
7 1 3 9 11
7 1 2
This was later confirmed by both AMS-01 flight data analysis and visual inspection after AMS-01 dismounting. No bond failure was reported. Without encapsulation, ladders can be more easily repaired. It is useful to analyze in detail the different types of ladder defects arising during assembly. This study helps in optimizing procedures, defining the need of spare components for repair and establishing a reliable schedule for future assemblies. The summary is listed in Table 4. The total number of ladders listed in Table 4 does not correspond to the numbers reported in Table 3 since a given ladder could have more than one type of defect. Four of the electronics failures were due to mechanical defects of the front-end hybrid board that could not be replaced for lack of spares. Four of the ladders with higher than expected leakage current were due to a problem that was not completely understood in one assembly line. The increase in leakage current had no relation though with damages occurring during bonding. Due to redundancy, the assembly site has been shut down
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and fully devoted to integration. The rest of the high leakage current failures occurred during integration on the plane: an unforeseen operation that proved necessary, was the accidental spreading of carbon fiber dust on the silicon. Given the complexity of the ladder, whenever assembly procedures were not carefully optimized, as in the above integration operation, failures were likely to occur. Viceversa, the gluing and bonding procedures were so successfully tuned that little defects were due to them. Again, because of complexity, the largest fraction of defective ladders were due to operator mistakes or mishandling. This particular failure demonstrates the need of developing, whenever possible, automated and operator-safe industrial procedures, not relying too much on operator (exceptional) expertise. To this purpose, standard though very precise and reliable equipment has been (will be) used for the construction of the AMS-01 (AMS-02) silicon tracker. A very good example of this is the gluing procedure: during SMD construction some gluing did require a lot of expertise to continuously change the amount of glue dispensed, depending on glue viscosity and environmental parameters. For similar gluing operations in AMS-01, we have successfully chosen a precise volumetric dispenser available in the market that, together with custom tools, automatically ensured the repeatability and uniformity of glue dispensing. Leakage current measurements at different steps during ladder assembly proved to be very useful to identify problems and to monitor the quality of the assembly procedures. The assembly sequence and tools were designed to allow more measurement than actually needed during the standard routine. They were useful during AMS-01 construction when the leakage current monitoring helped in discovering the need of a better cleaning after silicon sensors cutting and of better anti-static precautions during upilex gluing as well as the carbon fiber dust problem during integration. The statistics on the single ladder component yields is given for reference in Table 5. We have to emphasize that values in the table refer to the manufacturing process and have no relation with assembly. Usually, good components only are delivered by industries and the actual yield is
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Table 5 Yield statistics on ladder components manufacturing in AMS01 production (A single component can fail more than one test) Component
Action=test
Silicon production
Total 61% High leakage current p-strips 89% High leakage current n-strips 70% High leakage current sensors 82%
Silicon cut
Total Cut Shipment
94% 96% 98%
Long upilex fanout (p-side)
Total
58%
Electrical test Bonding test
66% 88%
Total
78%
Electrical test Bonding test
80% 93%
Short upilex fanout (n-side)
Yield
CFRP=airex mechanical Plates production structure Machining
94%
Decoupling capacitor chips
Electrical test (0 defects)
65%
VA preamplifiers
Electrical test (0 defects)
87%
FE hybrid packaging
Total
78%
90%
Summarizing, using the estimated final ladder yield of about 80% from AMS-01, for AMS-02 production we require 20% contingency in spare components plus 10% spare components to build 10% more fully functional spare ladders. Overall, 130% of the components required for the flight have been ordered. These orders have been placed on an all-good basis, in other words, to minimize fluctuations in quantity and delivery time.
5. Space qualification The AMS Silicon Tracker has already performed a successful 10 days flight on the Shuttle, and will have to operate on the ISS for a 3-year data-taking period. The tracker design and construction have been optimized for its use in space [16,22]. However, the tracker space qualification has not been fully performed by specialized firms. Instead, the collaboration has decided, in order to minimize costs, to: *
*
*
unknown to the customers. On the contrary, the AMS tracker collaboration participated in the testing procedures to minimize costs and monitor the production. Obviously, only good components were used in assembly. Silicon sensors and long upilex fanouts had a low yield, in both the cases, related to the length of the ladder. Having so many sensors in one ladder, single sensor specifications were much stricter than usual to eventually meet ladder specifications. For upilex fanouts, the length (and strip pitch) of the flex cable itself made the fabrication process quite difficult. In both the cases, producers and AMS-02 collaboration are working together to improve the fabrication process and, consequently, the yield. Acceptance tests ensure the reliability of components qualified as good.
*
*
understand the reasons behind each space qualification requirement; (a) identify critical materials, parts and processes with an impact on mission safety or other payloads; (b) identify possible space related failures which would only affect the experiment; upon agreement with NASA, only the requirements falling into category (a) have been tested and certified with standard qualification procedures, always in registered space industries; all other requirements have been addressed in the following ways, also depending on how critical the component is for AMS performances: * some requirements were simply not met, for materials, parts and processes not critical for mission safety nor for AMS; * whenever possible, materials already known to satisfy NASA specifications have been used; also ‘‘standard’’ space qualification procedures in specialized industries were adopted in some cases;
M. Pauluzzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 67–74 *
off-the-shelf standard components were used as often as possible with space qualification performed within the collaboration.
Non-standard space qualification procedures were carried out by the AMS collaboration. Participating institutions acquired space qualification expertise and developed the appropriate test facilities. In addition, facilities and tools already available to the High Energy Physics community were used, such as ion beams facilities. Procedures were executed according to standard NASA guidelines and specifications. As an example of requirements for space intentionally not met, the ultrasonic micro-bonds on the ladder have not been encapsulated for protection. The decision was taken after in-house vibration tests demonstrated that no failures would occur due to the high frequency random and acoustic excitations (10–2000 Hz) appropriate to the shuttle. In addition, a special test system was developed by the Perugia University to simulate Single Event Effects (SEL and SEU) on microelectronics components used in tracker readout and DAQ systems. Given the different durations of the precursor and ISS flight, more stringent space qualification criteria have, in general, been applied to AMS-02. Separate papers are in preparation on both the ladder and micro-electronics space qualification [23].
6. Choice of double-sided vs. single-sided silicon sensors The Silicon Tracker of the AMS experiment is no doubt a complex system, in particular its main unit, the ladder. The choice has been made to use double-sided silicon sensors instead of a simpler system based on single-sided sensors. For the detection of anti-matter, or for a conclusive new limit on its existence, the antihelium sensitivity of AMS during the ISS flight has to be of the order of 10 10 : Large angle nuclear scattering of ordinary matter nuclei occurring in the central planes of the silicon tracker can simulate the curvature of anti-helium nuclei
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[16,24]. To reach the required sensitivity, the non-Gaussian tails due to this particular background must be minimized by reducing the material inside the tracker. A detector built out of double-sided sensors has advantages: *
*
transparency of the detector (3.2% of a radiation length for AMS-01); less components: * lower costs, * less operations i.e. faster assembly and integration.
However, the usage of double-sided sensors has disadvantages: *
* *
components, tools and assembly procedures are usually more complex and lead to a lower yield both in components and ladders, higher costs, slower production for a fixed manpower.
As explained in a previous section concerning single components manufacturing, the low yield for silicon sensors in Table 5 is mostly related to ladder length rather than due to the double-sided sensor choice. In addition, the long upilex flex cable is required to bring out the signals from the second side of the ladder (transverse silicon strips) but, again, the low yield is mainly due to the length of the flex cable. Overall, the manufacturing yield of single components is moderately affected by the double-sided sensor choice. Considering instead the ladder assembly yield, failures related to the double-sided assembly can be estimated to be less than 10% (after repair), that is 50% of the total assembly failures. In general, complexity can be reduced by a careful design and optimization of assembly procedures, both being non-recursive efforts. The assembly time can be close to that of a single-sided detector with twice the number of sensors. The cost of a system based on double-sided silicon sensors has been evaluated to be sensibly less than 1.5 times that of an equivalent surface of singlesided sensors. Whenever transparency is mandatory for physics, as for AMS, the double-sided system has proved to be viable and therefore
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preferable. Otherwise, a single-sided sensors system is likely a safer approach. 7. Conclusions R&D, prototyping, or small scale assembly efforts would be very expensive if done in industries, because of scale and uniqueness. When technology becomes a standard and the scale of the experiment is large enough, small=medium sized companies can cost-effectively participate in mass production. In AMS-02, private companies will produce half of the silicon ladders. The companies can also contribute to R&D at earlier stages. The scientific community can profit from this early collaboration, by learning how to be more effective and tuning the procedures so that they will be easily adopted by any company involved in massive production. Silicon detectors are most definitely becoming a standard and reliable technology ready to make the leap to an industrial approach as it has happened in the past for scintillating counters. Some statistics from AMS-01 Silicon Tracker construction is also provided, together with solutions on procurement and assembly optimization that will be used for the AMS-02 assembly effort. The double-sided sensor approach has been proven effective and viable whenever detector transparency is important. Acknowledgements The successful flight of the AMS-01 Silicon Tracker has been made possible, among other contributions, by the valuable and untiring efforts made during the assembly by technicians, engineers and physicists from ETH Zurich, Geneva University and Perugia University-Sezione INFN. Looking forward to the next phase, my personal thanks to all of them.
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