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Silicon pad detectors for LCCAL: characterisation and first results

Silicon pad detectors for LCCAL: characterisation and first results. Antonio Bulgheroni University of Milan – Italy on behalf of LCCAL: Official INFN R&D project, official DESY R&D project PRC R&D 00/02

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Silicon pad detectors for LCCAL: characterisation and first results

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  1. Silicon pad detectors for LCCAL: characterisation and first results Antonio Bulgheroni University of Milan – Italy on behalf of LCCAL: Official INFN R&D project, official DESY R&D project PRC R&D 00/02 Contributors (Como, LNF, Padova, Trieste): M. Alemi, M.Bettini, S. Bertolucci, E. Borsato, M. Caccia, P.Checchia, C. Fanin, G. Fedel, J. Marczewski, S. Miscetti , M. Nicoletto, M. Prest, R. Peghin, L. Ramina, E. Vallazza.

  2. Scintillator Calorimeter Layout Total of 50 layers 27 X0 Absorber 25 x 25 x 0.3 cm3 3 layers 725 Pads ~ 1 x 1 cm2 2, 6 and 12 X0 Silicon pad detectors 25 Cells 5 x 5 cm2

  3. Sensor details 6 cm • Main characteristics: • Sensor thickness: 300mm • Resistivity: 4-6k • AC coupling • Silicon dioxide thickness: 265 nm • Bias grid and guard ring • 3M bias resistors • Symmetric structure ~0.9 cm ~0.9 cm 7 cm Bias pad Guard ring pad

  4. Hybridisation details • Hybridisation through conductive glue • Analogue Readout Chip: VA-HDR9c (IdeAs) • VA  Viking family • HDR  High dynamic range • 9c  Four selectable gains *Measured value

  5. Motherboard design • 6 sensors per motherboard with serial readout. • Status of production: • 16 sensors available • 2 motherboards fully and 1 partiallyequipped • Signal routing through Erni connectors

  6. Test beam: preliminary results* Two electrons with energy 750 MeV X silicon chambers Y silicon chambers Cluster recognition First layer Second layer Third layer *Plots filled only with pads with SNR>10s

  7. How we get there: details of the sensor characterisation 2 technological runs First batch of 11 sensors (spring ’02) Second batch of 9 sensors (summer ’02) Next batch available in summer ‘03

  8. KI 590 & 595 HF & QS CV meters Manualprober SCS 4200 With 2 SMU Measurement setup GP - IB Triax & Bnc

  9. A SMU2 SMU1 A I – V Measurement: the method Guard ring • Total and bias current measured directly by an SMU • Guard ring current calculated from the others two Bias Total

  10. I – V measurements: the results High guard-ring current (> 50mA) Dicing edge too close to the guard ring structure, but it works and the sensor performances are NOT spoiled! Cutting edge Guard ring p+ Depletion region n Solution: keep the cutting edge far away from the guard ring! n+ metal

  11. Soft breakdown • Bias current reasonable (few mA) • Strange shape with a “soft” breakdown • n+ or metal shallow impurities on the backplane Depletion region n n+ Impurities Metal Solution: replace the implanted backside contact with a diffused one, but …

  12. I – V Characteristics

  13. C – V measurements • Useful technique to calculate the depletion voltage • Quite uniform behaviour of the depletion voltage

  14. Leaky capacitor Good AC coupling “Leaky” pads: the discovery • In batch 12: the great part (90%) of the pads seems to be DC coupled • In batch 13: only few percents

  15. “Leaky” pads: the explanation • No pin holes in SiO2 • Surface leakage  residua of polysilicon after the etching of the polysilicon layer • Equivalent circuit with two opposite diodes. Readout metal Resistor Al bridge Polysilicon residua Bias grid

  16. “Leaky” pads: the solution • External AC coupling  SMD capacitors on the PCB • Next batch will be DC coupled with external capacitors • It will improve also the quality of the p-n junction reducing the thermal budget SMD capacitor

  17. Summary • Three motherboards equipped with 16 sensors • First test beam on going • Yield satisfactory • New batch with simpler technology has been started yet promising a better yield Thanks

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