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Radiation Tolerance of DEPFET Active Pixel Sensors

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Radiation Tolerance of DEPFET Active Pixel Sensors. 8th International Conference on Large Scale Applications and Radiation Hardness of Semiconductor Detectors June 27-29, 2007 – Florence - PowerPoint PPT Presentation

Text of Radiation Tolerance of DEPFET Active Pixel Sensors

  • Radiation Tolerance of DEPFET Active Pixel Sensors 8th International Conference on Large Scale Applications and Radiation Hardness of Semiconductor Detectors June 27-29, 2007 Florence

    S. Rummel, L. Andricek, M. Bataglia*, D. Contarato*, P. Giubilato*, M. Porro, G. Ferrari**, G. Lutz, H.-G. Moser, S. Pozzi**, R.H. Richter*LBNL Berkely; **Politecnico Milano

  • OverviewDEPFET principle

    DEPFET as vertex detector for ILC

    Requirements with respect to radiation hardness

    Investigations of irradiated structures

  • DEPFET Depleted Field Effect Transistor Combination of detector grade silicon with first p-FET amplification stage in each pixelPotential minimum for electrons is created under the channel by sideward depletion and an additional n-dopingElectrons in the internal gate modulate the transistor currentSignal charge is removed via a clear contactLarge sensitive volume due to fully depleted bulkLow noise caused by a small input capacitance and internal amplificationTransistor can be switched off by external gate charge collection is then still active!

  • DEPFET Matrix operationColumn parallel architecture for fast readoutRow wise readout operation: Sample-Clear-Sample no charge transferLow power dissipation only one row active while readout

  • DEPFET as vertex detector for ILC) IP resolution: [Tesla TDR]Good point resolution Small material budget 0.1 % X0 per layerFrame rate ~ 20kHz line rate 20MHzSufficient radiation hardness

  • ILC conditionse from Beamstrahlung

    n backscattered form the calorimeter

    Total dose around 360krad in 10 years [LDC DOD]

    Moderate cooling with cold gas stream

  • Motivation Space charge sign inversion is not expected to be a problemBUT: Several remaining questions:Noise contribution from leakage current? (50s integration time)What is the extend of the threshold voltage shift? Does the performance of the FET degrade under irradiation?

    Leakage vs. Dose:101010101010DI / V [A/cm3]ILC10111012101310141015Feq [cm-2]-6-5-4-3-2-1n-type FZ - 7 to 25 KWcmn-type FZ - 7 KWcmn-type FZ - 4 KWcmn-type FZ - 3 KWcmn-type FZ - 780 Wcmn-type FZ - 410 Wcmn-type FZ - 130 Wcmn-type FZ - 110 Wcmn-type CZ - 140 Wcmp-type EPI - 2 and 4 KWcmp-type EPI - 380 Wcm[M.Moll PhD Thesis][M.Moll PhD Thesis]

  • OverviewSingle pixel structures with 6m gate lengthCurrent based readoutCharacterization with respect to:Electric characteristics (Vth, gm, gq)Leakage current (NIEL)Spectroscopic performanceNoise power density (1/f noise)

  • pirradiated - Electric characteristicsThreshold voltage shift small compared to the expectations for a dielectric exceeding 200nm Increase of sub threshold slope indicate a increase of 1/f-noisegm is noticeably reduced by 15%all terminals grounded during irradiation, 30 MeV p

  • p-irradiated - Leakage - Temperature dependenceDependence on operation voltages -> additional contribution which is not bulk generatedThick detector, bulk generated current will decrease with thicknessAt 0C around 20e/s! -> L=50s -> 31e contribution to noiseIrradiation far beyond 10 years of ILC operation

  • Spectroscopy setup


  • p-irradiated sample Spectroscopic performanceD1D2SG1G2

  • p-irradiated; Fe55 spectrumGood spectroscopic performance!Separated Ka Kb peak!=1s T=-10CENCnoi = 13.5e-

  • p - Irradiation - SummaryDEPFET fully operable after 3*1012 neq/cm

    Threshold voltage shift acceptable

    Gm slightly decreased by 15%

    Noise contribution due to leakage current is tolerable

  • n irradiated - Electrical characteristicNo thresholdvoltage shift observed!No significant increase in subthreshold slope!

  • n irradiated - Spectroscopic performanceLeakage around 8.9e/s @ 23CAt low temperatures the performance is similar to unirradiated structures!

  • n irradiated Fe55 spectrum=6s T=6CENCnoi =3.1e-

  • n Irradiation - SummaryTransistor performance unchanged

    No interface damage observed

    Good spectroscopic performance

  • Noise power densityCorner frequency increased Expectation from sub threshold slope are confirmed

  • Conclusion

    Irradiated structures are operable after irradiation Threshold voltage shift is in an acceptable regionBoth structures show good spectral resolutionNoise power density behaves as expected from sub threshold slope

    The DEPFET double pixel structure could be considered as radiation hard with respect to the ILC requirements

  • Leakage current Leakage current into the internal gate Contributions from bulk and surfaceAdditional shot noise


  • NIEL Bulk damage ILC

  • Shaper measurement vs. CDSENC depends on: Shaping timeResponse function

    Frame timeBandwidthCDS timeBoth variants are sensitive to the same noise sources, but in different extend

  • Influence of the radiation NIEL (Non Ionizing Energy Loss)Mainly damage to the bulk Ionizing Energy Loss ( charged particles) Damages interface and bulk

    Macroscopic effects:Shift in threshold voltageIncrease of leakage current bulk and interfaceIncrease of 1/f noiseChange in effective doping

    Skip?Improved spectrum!Add unirradiated points