The DarkSide-20k collaboration is preparing to equip 20 m^2 of SiPMs working
in liquid argon at 86 K for the direct search of WIMPs. The collaboration had
to solve many technological aspects, such as the development of SiPM optimized
for operation in liquid argon, the readout of large SiPM-based detectors, the
reliable packaging of more than 200000 SiPMs using radiopure materials. The
packaging solutions available for cryogenic applications and the performances
of the newest cryogenic extended gain SiPMs from FBK will be discussed

Kochanek, I.

English

arXiv

The DarkSide-20k collaboration is preparing to equip 20 m2 of SiPMs working in liquid argon at 86 K for the direct search of WIMPs. The collaboration had to solve many technological aspects, such as the development of SiPM optimized for operation in liquid argon, the readout of large SiPM-based detectors, the reliable packaging of more than 200000 SiPMs using radiopure materials. The packaging solutions available for cryogenic applications and the performances of the newest cryogenic extended gain SiPMs from FBK will be discussed

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DOI 10.1393/ncc/i2019-19062-5Colloquia: EuNPC 2018IL NUOVO CIMENTO 42 C (2019) 62SiPMs for cryogenic temperatureI. Kochanek for the DarkSide CollaborationLaboratori Nazionali del Gran Sasso - L’Aquila, Italyreceived 5 February 2019Summary. — The DarkSide-20k collaboration is preparing to equip 20 m2 of SiPMsworking in liquid argon at 86 K for the direct search of WIMPs. The collaborationhad to solve many technological aspects, such as the development of SiPM optimizedfor operation in liquid argon, the readout of large SiPM-based detectors, the reliablepackaging of more than 200000 SiPMs using radiopure materials. The packagingsolutions available for cryogenic applications and the performances of the newestcryogenic extended gain SiPMs from FBK will be discussed.1. – DarkSide-20kDarkSide is a stepped program for the realization of increasing size dark matterexperiments toward the direct detection of WIMP interactions on a liquid argon target.DarkSide-50 is based on a two-phase Time Projection Chamber (LAr TPC) with a fiducialmass of 47 kg of Underground Liquid Argon (UAr). The underground origin of argon,extracted from a CO2 well in Colorado, shows a depletion of 39Ar, which represent thehighest background for argon-based detectors, in excess of 1400. DarkSide-50 is insertedin an active Liquid Scintillator Veto (LSV) inside a cylindrical Water Cherenkov Veto(WCV) to further reduce the background. The experiment is installed at LaboratoriNazionali del Gran Sasso, under 1400 m of overburden rock providing shield againstcosmic rays. DarkSide-50 demonstrated exceptional results in the WIMP search, thanksto the extremely low background achieved [1], [2].Particle interactions in the active volume produce a scintillation light (S1) and ion-ization electrons. Electrons are drifted by means of an electric field to the liquid-gasinterface, extracted in the gas phase. There the accelerated electrons produce a secondlight pulse (electroluminescence), called S2. Photons from S1 and S2 are detected by 383” photo-multipliers (PMTs) installed on top and bottom of the TPC. The presence ofthe S1 and S2 allows to reconstruct the position of the original interaction with centime-ter resolution. The discrimination of nuclear recoils from electromagnetic interaction isperformed by a Pulse Shape Discrimination (PSD) of the S1 scintillation with a rejectionbetter than 108 [2].DarkSide-20 is the next installation and will be based on a scaled design with anactive (fiducial) mass of 23 t (20 t) [3].Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 I. KOCHANEK for the DARKSIDE COLLABORATION2. – Light detection in DarkSide-20kAccurate light detection is of primary importance for the DarkSide program. Theexperience from DarkSide-50, where the collaboration was forced to run the PMTs atvery low bias and to adopt cryogenic low-noise preamplifier, pushed to start an intenseR&D program for the development of innovative silicon-photomultipliers (SiPM) basedphotodetectors.A SiPM is constituted by a large collection of parallel SPADs (Single Photon AvalancheDiode) of typical size 20 − 50 μm. Each SPAD operates independently in Geiger mode.A signal is generated when few SPADs are triggered by incoming photons or by dark-rate. SiPMs typically exhibit an extraordinary charge resolution O(1%), at least anorder of magnitude better than PMTs. Furthermore, the photon detection efficiency,which includes both the quantum efficiency and the SPAD fill factor, can read 60% onthe full detector surface. On the other hand, the dark-rate from SiPMs lays around104 − 105 cps/mm2 at 300 K, with the addition of correlated noises of about 15-20%.SiPMs are small size devices (tens to hundreds of mm2), while for DarkSide-20k a3” PMT replacement is required. In DarkSide this is obtained by grouping SiPMs inarrays of 25 cm2, called tiles, with a cryogenic front-end board that pre-amplifies andaggregates all the signal to a single analog output. The challenge is to reliably assemblea Photo Detector Module (PDM), see Figure 1 left, capable of operating at 87 K with asignal to noise ratio (defined as gain divided the baseline noise) larger than 8 and a darkrate smaller than 80 mcps/mm2. Up to now, SiPMs are produced by FBK (FondazioneBruno Kessler) developed in collaboration with DarkSide for low after pulsing and lowdark rate operations in liquid argon.For low background detectors, a high radio-purity of all the components building thedetector is required. SiPMs are made of high purity silicon and are intrinsically radio-pure, however a PDM includes electronic components and connectors. A second challengefor the collaboration is to identify components and procedures that will maintain thecontaminations of U/Th at the level of mBq/PDM.Finally in Darkside-20K, about 10000 PDMs are foreseen. This requires the use ofhighly scalable packaging techniques and the deployment of a industrial-grade packagingfacility capable of supporting our requirements in term of reliability and radio-cleanliness.The PDMs are installed in a 25 units structure called motherboard (MB) providingFig. 1. – A Photo Detector Module, which includes a tile with 24 mounted SiPMs and a front-endboard in an acrylic cage (left). A motherboard with mounted 25 PDMs (right).SiPMs FOR CRYOGENIC TEMPERATURE 3Table I. – CTE values and radio-cleaness in U/Th/K chains for common substrate materials.Material CTE [ppm/K] U/Th [mBq/kg] 40K [mBq/kg]Silicon 3-5 very low very lowFR4 12-14 very high very highPolyimide/Kapton 20 1-10 >10Arlon NT 5-7 100 1000Selected Fused Silica 0.5 0.05 -interface to the TPC. The final goal for radio-purity is to reach around 10 mBq/MB.In October 2018 we produced the first full motherboard. The next step is to movethe production of SiPM to a commercial foundry that can provide higher productionthroughput and better repeatability. INFN selected LFoundry. However, before startingthe large scale production we need to finalize the design parameters of the SiPMs. Thisis performed in a dedicated setup at LNGS capable of characterizing the dark-rate andthe correlated noises as function of temperature in the range 40 to 300 K [4]. In the samesetup it is possible to further optimize the design of the front-end electronics to matchat best the parameters of the light detectors.3. – Packaging solutionsFor packaging the tiles of DarkSide-20k several aspects have to be taken under con-siderations:• The Thermal Expansion Coefficient (CTE) of the silicon is quite low, a compatiblesubstrate is required to avoid mechanical stress on the silicon die. From Table I,it is clear that the best solution for the substrate would be silicon itself. SiliconPCBs are not trivial to make, considering the requirement of insulation, stray ca-pacitance (between traces and substrate) and the requested low impedance groundplane (to limit the noise pickup). Arlon 55NT is the baseline solution. An R&Dis ongoing on multilayer hybrid fused silica-polimide PCB. This alternative wouldsignificantly reduce the neutron background from U/Th α-n production. The sec-ond contribution in background is from connectors, where glass-fiber reinforcednylon exhibits significant contaminations. Electronic components are at the levelof 60-100 μBq/kg.• The current bonding procedure uses conductive cryogenic epoxy EPO-TEK EJ2189.The epoxy is applied by a robot dispenser, Nordson EFD, with nine dots patternper SiPM. Then the SiPMs are positioned with a manual die bonder, Westbond7200CR. Finally the tiles are cured for one hour at 100◦C. The conductive epoxyis required because the backside contact of the SiPM is TTiN based and no directsolder is possible on this finish. For the first motherboard we bonded more than600 dies and none showed any problem in liquid nitrogen, leading to a lower limiton the packaging reliability of >99.8%. However, the silver loaded epoxy is notradio-clean and we have no data on long term stability. A new procedure that weare working on, involves indium/solder (eutectic) bump bonding or gold studs ballbonding. Both solutions were tested and provided very strong bonds. LFoundryis currently producing dies with a copper-based backside contact pads that will beused to fine tune the bonding procedure.4 I. KOCHANEK for the DARKSIDE COLLABORATION05000100000 25000 50000 75000 100000filtered amplitude [au]count1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 2778910111213tile #SNR VoV:457Fig. 2. – Single photoelectron peaks (left) and signal to noise distribution for tested tiles (right).• The anode contact is on the top side of the SiPM and, up to now, it is connected viaa wire bonding. We use 25 μm aluminium wire, 24 bonds per PDM. This makes themanipulation of the tiles very complicated. Since we want to optimise the detectorfill factor there is no dead space around the tile and the silicon surface can notbe touched to avoid damaging the wire bondings. At the same time identifyingbroken wires requires a characterization of the tile on a test bench. This resultsin a very complicated detector installation. A better solution would be providedby the use of Through Silicon Vias (TSV). We are in contact with LFoundry thatis developing TSVs for SiPMs according to our specifications. The bump bondingtechnique that we plan to use is already compatible with TSVs.4. – Tiles characterizationFor the production of the first motherboard we produced more than 30 PDMs. Weestablished a rigorous test procedure to qualify them. The procedure includes tests atroom temperature and in liquid nitrogen. The tests are targeted to verify that all 24SiPMs of each tile are working according to our specifications and that the front-endboard is operating as expected. Finally we acquired the waveform of the photoelectronsproduced by a laser. The analysis (based on advanced digital signal processing algo-rithms) showed that for all the PDMs the photoelectron peaks are well distinguishablewith a signal to noise ratio above 8, Figure 2.5. – ConclusionsDarkSide-20k succeeded in an ambitious program to deploy radio-pure cryo-graded,SiPM-based, large photodetectors. The development included R&D in SiPM at FBK, inextremely low noise preamplifier board at LNGS, in the most avanced silicon packagingtechniques at the Princeton University. The next step is the realization of a cutting edgesilicon packaging facility at LNGS, called Nuova Officina Assergi (NOA) where DarkSideand other experiments can build their future photo-detectors.REFERENCES[1] DarkSide Collaboration, Phys. Rev. Lett., 121 (2018) 081307;[2] DarkSide Collaboration, Phys. Rev. D, 98 (2018) 102006;[3] DarkSide Collaboration, The European Physical Journal Plus, 133 (2018) 131;[4] F. Acerbi, et. al, IEEE Transactions on Electron Devices, 64, 2 (2017) 16618176;

SiPMs for cryogenic temperature

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