The problem of radiation damage for the electronics of the pixel detectors in the Inner Tracking System of the ALICE experiment is discussed. Simulations allowed to estimate the cumulated doses andparticle fluences during a ten year operational period. Several irradiation tests have been carried out on the various prototypes of the readout chips. The results obtained so far point out that the recent prototypes will retain their functionality up to doses and neutron fluences well above those expected in ALICE

Badalà, A

Barbera, R

Cavagnoli, A

Di Liberto, S

Meddi, F

Morando, M

Palmeri, A

Pappalardo, G S

Riggi, F

Scarlassara, F

Segato, G F

Soramel, F

Vannucci, Luigi

CERN Document Server

Irradiation tests of the pixel front-end readoutelectronics for the ALICE experiment at LHCA. Badala, R. Barbera, A. Palmeri, G. S. Pappalardo, F. RiggiINFN Catania and University of Catania, ItalyS. Di Liberto, F.Meddi, A.CavagnoliINFN Roma and University La Sapienza, Roma, ItalyM.Morando, F.Scarlassara, G.Segato, F.SoramelINFN Padova and University of Padova, ItalyL.VannucciINFN Legnaro, Italyfor the ALICE Collaboration(June 26, 2000)AbstractThe problem of radiation damage for the electronics of the pixel detectorsin the Inner-Tracking-System of the ALICE experiment is discussed. Simula-tions allowed to estimate cumulated doses and particle uences during a tenyears operational period. Several irradiation tests have been carried out onthe various prototypes of the readout chips. The results obtained so far pointout that the recent prototypes will retain their functionality up to doses andneutron uences well above those expected in ALICE.1Corresponding Author:Francesco Riggi, Department of Physics and Astronomy,University of Catania, I95129 Catania, ItalyFax ++39 095 383023, Tel. ++39 095 7195261, e-mail: Franco.Riggi@ct.infn.itNow at CERN-EP, Geneve, CHPACS number: 29.40.Gx, 07.50.-e, 07.89.+bKeywords: Pixel; Detector; Silicon; Radiation Damage; MicroelectronicsTypeset using REVTEX2I. INTRODUCTIONAn important part of the design of the Inner-Tracking-System (ITS) [1] of the ALICE ex-periment at the CERN LHC has been the implementation of radiation tolerant technologiesfor the electronic circuitry located close to the interaction region. Such problem is of specialconcern for the electronics of the pixel detectors, which constitute the innermost two layersin the ALICE ITS. These layers are essential for the reconstruction of the primary vertexand for the measurement of the impact parameter of the secondary tracks originating fromthe decay of strange, charm and beauty particles. Such requirements can only be fullled bydetectors which couple high precision and granularity, in order to operate near the collisionvertex, where the track density may exceed 50 tracks/cm2. The pixel detectors must surviveto an expected overall dose in the order of 200 krad in ten years [2,3]. This has demanded aneort in the electronic design which led to a sensible upgrading of the hardness with respectto the rst readout chips of the Omega family, still in use in experiments at SPS. This eortfrom the Microelectronics Group at CERN has been accompanied by a series of irradiationtests carried out in several laboratories with dierent probes.Sect.II discusses the radiation environment envisaged for the ITS during a realistic work-ing scenario of the ALICE experiment. The development of the pixel readout electronics isshortly presented in Sect.III, while some results concerning experimental tests of the radia-tion hardness for the readout electronics are reported in Sect.IV. Finally, some conclusionsare drawn in Sect.V.II. THE RADIATION ENVIRONMENT FOR THEPIXEL LAYERS IN THE ALICE ITSAt an energy around 6 A TeV, planned for the heavy-ion LHC program, a large particlemultiplicity is predicted by the current theoretical models, reaching values around 105fora central Pb-Pb collision. Full simulations of the interaction of the primary and secondary3particles generated by a heavy-ion collision with the overall structure of the ALICE detectorhave been carried out [3]. This allowed to evaluate the cumulated dose and particle uenceon the ITS detectors. Most of the simulations made use of the event generator programHIJING (Heavy Ion Jet INteraction Generator), version 1.35 [4]. This code was used togenerate central, peripheral and intermediate impact parameter collisions, with a propermixing of the events to simulate the real collisions. Primary and secondary particles weretracked using a GEANT package describing as close as possible the geometry and materialsplanned for the ALICE detectors.In order to scale from the expected dose per event, evaluated for individual collisionclasses, to the overall cumulated dose, a reasonable scenario during ten years data takingcan be found in the ALICE Technical Proposal (see Table I), which includes proper sharingof beam time between protons, Ca and Pb ions. A summary of the cumulated dose expectedin the individual ITS layers in an operational period of ten years is reported in Table II andFig. 1. The use of dierent event generators with the largest value of the pseudorapidity dis-tribution (dNch/d = 8000) could increase the overall dose up to 200 krad for the innermostpixel layer.Neutron uences have been estimated by FLUKA simulations, which give values around3  1011n/cm2(energy-integrated) for the pixel layers (see Table II). Recent simulations [5]also provide the contribution to the neutron uence due to the front absorber and the energydistribution of the secondary neutrons, with a maximum yield in the range 100 keV-1 MeV.III. DEVELOPMENT OF THE PIXEL READOUT ELECTRONICSThe rst series of the pixel front-end readout chips (Omega2 and Omega3) are employedin the WA97 and NA57 experiments at CERN SPS [6]. While satisfying many of the re-quirements for the ALICE experiment, they were rather sensitive to radiation. The Omega3front-end chip is a matrix of 16 x 127 active readout cells (50 x 500 m2), manifactured in1 m SACMOS process, giving the same component density of a 0.6 m standard CMOS4process. Each front-end cell includes a preamplier, a shaper lter, a discriminator, a delayline and the readout logic. A mask ip-op allows the cell to be disabled when it is toonoisy or defective (Fig. 2).Further requirements in terms of the electronic circuitry needed for the ALICE exper-iment demanded a process with both a high component density and a good tolerance toradiation.As a result of an extensive R&D work [7] done by the Microelectronics Group at CERN,two prototypes (ALICE1test and ALICE2test) have been developed. The rst is a small testchip (65 x 2 cells, each measuring 50 x 420 m2), developed in 0.5 mCMOS technology, withthe use of NMOS devices in enclosed geometry and guardrings. The use of such techniquesenhanced the radiation tolerance by a factor of 30-50 with respect to the Omega3 series(see following Section). Such performance is adequate for the requirement of the ALICEexperiment. However, the need for an even larger component density brought to the choiceof a deeper submicron technology (0.25 m) for the design of the ALICE2test chip. Similarto the previous chip, ALICE2test is a matrix of 65x2 cells, containing this time about 50000transistors. The design of this second test chip also enabled to incorporate some additionalfeatures required for the nal ALICE chip, such as an individual pixel threshold adjust anda delay line based on the use of a 8 bit counter.IV. EXPERIMENTAL TESTS OF THE RADIATIONDAMAGE FOR THE PIXEL ELECTRONICSSystematic studies concerning the radiation tolerance of front-end chips of the Omegaseries were carried out by X-rays [1,8], and using a60Co gamma source [9], 15 MeV elec-trons [10] and a proton beam [9]. These studies all demonstrated that these chips cannotsurvive to doses signicantly higher than 30 krad. As a result, a common protocol for theirradiation of the submicron technology prototypes was established. With the advent ofthe new prototypes, built in radiation-tolerant technology, additional irradiation tests were5performed with various sources [1]. First of all, X-ray irradiations were carried out at CERNunder the same conditions as for the individual transistors [8]. The result showed on a fullcircuit scale the eectiveness of enclosed NMOS devices and guardrings to prevent radiation-induced leakage. The ALICE1test chip was seen to degrade signicantly only after 600 krad.Irradiations by -rays from60Co at the Istituto Superiore di Sanita (Rome) were then carriedout at a dose rate of 610 rad/min, with a uniformity within 5 % on the chip. The evolutionof the supply currents (analog and digital), as well as the threshold and its dispersion werestudied as a function of the absorbed dose. The results pointed out a functionality of thechip up to doses around 1 Mrad, where a a degradation of the performance, including thenumber of responding cells, was seen. Partial recovery after a few days from irradiation wasobserved in the discriminator threshold, due to annealing eects. The tolerance to chargedhadronic particles was also investigated, by the use of 6.5 MeV protons at the CN acceleratorof the Legnaro National Laboratory, with a dose rate of 12 krad/min. Even in this case,serious degradation takes place only after 1 Mrad, with partial recover during the annealingphase. In another test [1] carried out at CERN with low energy electrons derived from theNA50 experiment, the chip was seen to survive up to a dose of 1.7 Mrad.Even though it is extremely dicult to compare results concerning the damage caused bydierent radiation sources, a qualitative agreement between all these results was observed,which allows to establish a radiation tolerance for such prototype, built in 0.5 m CMOStechnology, up to doses in the order of 1 Mrad (0.6-1.7 Mrad), which is a factor about 30-50with respect to the Omega3 chip.Similarly to the previous prototype, the rst measurements concerning the radiationtolerance of the ALICE2test chip were done by X-rays with a peak energy of 10 KeV [1,8].The chip was seen to be fully operational up to 30 Mrad, with a slight change of the pixelthreshold and noise after 10 Mrad. Even in such case, partial recovery due to annealing wasobserved. Further tests were carried out with60Co -rays up to 26 Mrad, observing a slightincrease of the analog current (Fig. 3, top), and with 6.5 MeV protons with doses of 9, 19and 48 Mrad (Fig. 3, bottom), where the same trend was observed. The chip ceased to work6at 48 Mrad. High energy protons were also used at the CERN SPS machine, with a uenceof about 9  1014protons/cm2over a 12 h period. A recent test [11] was also done with 14MeV neutrons at the Frascati neutron generator using the T(d,n) reaction. A uence upto 1.3  1012n/cm2was used, which corresponds roughly to 2.3  10121 MeV-equivalentneutrons/cm2. The chip was powered as usual during all the irradiations, monitoring boththe analog and digital currents (Fig. 4) which were seen to remain constant within 0.5 %.A detailed electrical characterization of the chip prior and after the irradiation was alsoundertaken, checking the shape of the signals from the preamplier and from the shaper,the linearity in the gain and the eciency response. No degradation eect was seen, up tothis neutron uence, which is approximately one order of magnitude higher than expectedin ten years ALICE scenario. For the sake of completeness, a summary of the irradiationtests which have been carried out so far on the two prototypes, is reported in Table III.V. SUMMARY AND CONCLUSIONAn extensive work has been carried out to implement radiation tolerant techniques in thepixel front-end electronics for the ALICE experiment in order to reach the required hardnessfor the full operational period of the experiment. Several prototypes of the readout chipshave been tested in various laboratories and employing very dierent probes, which includeX-rays,60Co -rays, low and high energy protons, electrons and neutrons. Even though itis not easy to standardize a comparison between dierent sources of radiation, a commonbehaviour has been obtained by dierent irradiations. For the rst prototype, built in 0.5m technology, a tolerance around 1 Mrad has been achieved, whereas the second may beconsidered as full operational up to doses of the order of 10 Mrad. This result is well inexcess for the requirements of the ALICE experiment, discussed in Sect.II, and allows toconclude that the R&D work, as far as the radiation tolerance is concerned, is now complete.We would like to stress that the conditions used in such irradiations are worser than theexpected scenario during the ALICE run, because only a few months of exposure per year7are planned, allowing annealing eects during the rest of each operational year.VI. ACKNOWLEDGMENTSThanks are due to theMicroelectronics Group at CERN, where the R&D work concerningthe radiation tolerance of the pixel electronics has been carried out, and to A.Morsch for hisFLUKA simulations of the neutron ux in the ITS. We are also indebted to F.Librizzi forhis help during the irradiation with neutrons.8REFERENCES[1] The ALICE Collaboration, Inner-Tracking-System Technical Design Report, ReportCERN/LHCC 99-12(1999).[2] F.Meddi and M.Morando, Nucl. Instr. and Methods A395(1997)416.[3] A.Badala et al., Internal Note ALICE/99-01.[4] X.N.Wang and M.Gyulassy, Phys. Rev. D44(1991)3501.[5] A.Morsch, private communication; see also information at the following Web address:http://Alisoft.cern.ch/oine/[6] F.Antinori et al., J.Physics G:Nucl.Part. Phys. 25(1999)473.[7] W.Snoyes et al., Nucl. Instruments and Methods A439(2000)282.[8] W.Snoyes, Proceedings of the Fourth Workshop on Electronics for LHC experiments,Report CERN/LHCC/98-36, p.114-118.[9] S.Di Liberto et al., Internal Note ALICE/98-43.[10] A.Badala et al., Internal Note ALICE/99-31.[11] A.Badala et al., Internal Note ALICE/2000, in preparation.9FIGURESFIG. 1. Cumulated dose in the ITS layers during a running period of ten years, together withthe contribution from pp, CaCa and PbPb collisions.10shaperpreampmaskFFtestFFlogicdataFFcomparatorCinCfbRfbthresholdsettingstrobepolarityanalog testinputleakage currentcompensationdelayClkFIG. 2. Block diagram of a cell in the ALICE2test pixel readout chip.11FIG. 3. Behaviour of the analog (dots) and of the minimum and maximum digital (triangles)supply currents for the ALICE2test chip during irradiation with -rays from60Co and 6.5 MeVprotons.12FIG. 4. Variation of the digital (open symbols) and analog (lled symbols) currents in theALICE2test chip as a function of the neutron uence. 14 MeV neutrons have been employed.Fluctuations are of the same order as the instrumental sensitivity.13TABLESTABLE I. ALICE scenario for an overall working period of ten years.Parameter pp Ca-Ca low L Ca-Ca high L Pb-PbL (cm 2s 1) 10303 102710291027Running time (s) 1082.5 1062.5 1065 106Rate (s 1) 1058 1033 1058 103No. of events 10132 10107.5 10114 101014TABLE II. Overall cumulated dose (krad) and neutron uence in ten years for the individualITS layers.Layer pp CaCa PbPb Total neutron uence (1011/cm2)1 26 77 27 130 3.22 7 23 8 39 3.13 2.3 7.7 2.8 13 3.54 0.8 2.9 1.0 4.7 3.35 0.4 1.2 0.4 1.9 3.76 0.3 0.9 0.3 1.5 3.315TABLE III. Summary of irradiation tests carried out on the prototypes of the ALICE front-endpixel readout chips.Source of radiation Prototype Max dose/particle uence Ref.10 KeV X-rays ALICE1test 800 krad [8]60Co -rays ALICE1test 1.5 Mrad [1]1 MeV electrons ALICE1test 2.6 Mrad [1]6.5 MeV protons ALICE1test 1.05 Mrad [1]10 KeV X-rays ALICE2test 30 Mrad [8]60Co -rays ALICE2test 26 Mrad [1]High energy protons ALICE2test 9  1014p/cm2[1]6.5 MeV protons ALICE2test 48 Mrad [1]14 MeV neutrons ALICE2test 1.3  1012n/cm2[11]16

Irradiation Tests of the Pixel Front-End Readout Electronics for the ALICE Experiment at LHC

Irradiation Tests of the Pixel Front-End Readout Electronics for the ALICE Experiment at LHC

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