Radiation-Hardened, Single-Photon Imagers Edoardo Charbon12 Matthew W. Fishburn1 1Delft University of Technology, Netherlands 2EPFL, Switzerland

2011 International Workshop on Radiation Imaging Detectors Outline Motivation and applications of single-photon avalanche diodes (SPADs) Physics of SPADs Radiation damage in SPADs Outlook 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Outline Motivation and applications of single-photon avalanche diodes (SPADs) Physics of SPADs Radiation damage in SPADs Outlook 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

Applications using time-correlated, single-photon detection Positron emission tomography (PET) Quantum key distribution Fluorescence imaging Fluorescence correlation spectroscopy Fluorescence lifetime imaging microscopy Spectrally-resolved lifetime imaging 3D imaging with structured light Random number generators 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Why SPADs? Time-correlated, single-photon imagers Identical operation in magnetic fields >9T Important for dual PET/MRI systems in medical imaging Resilient detectors Operational over wide temperature range Insensitive to strong magnetic fields Easy to use Lower bias voltages than PMTs CMOS friendly Higher integration possibilities Cheap Ready for a space-based future 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

Applications using time-correlated, single-photon detection Positron emission tomography (PET) Quantum key distribution Fluorescence imaging Fluorescence correlation spectroscopy Fluorescence lifetime imaging microscopy Spectrally-resolved lifetime imaging 3D imaging with structured light Random number generators 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Outline Motivation and applications of single-photon avalanche diodes (SPADs) Physics of SPADs Radiation damage in SPADs Outlook 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors What is a SPAD A single-photon avalanche diode (SPAD) is a p-n junction constructed to operate “above” the breakdown voltage Avalanches occur when single carriers injected into diode, such as when a single photon impinges on diode Avalanches can be electrically detected, and are synchronous with carrier injection time 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Avalanche breakdown When a free carrier is injected into the diode Carrier rapidly accelerated (enormous electric field) Carrier ionizes silicon atoms, generates more carriers In linear-mode, ionization significant for electrons but not holes In Geiger-mode, ionization significant for holes and electrons Diode is in Geiger-mode when expected number of generated carriers for a carrier exceeds one 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

Views of an avalanche diode Cross-section Top Active area STI STI Guard ring Well 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

Views of an avalanche diode Cross-section Top Active area STI STI Guard ring Well 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

Electric field simulations of an avalanche diode V/cm 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

A p-n junction at steady state 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

A p-n junction at steady state 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

A p-n junction in Geiger-mode 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Operating point Breakdown voltage Temperature dependent, around +20mV per deg C for many CMOS diodes Excess bias Voltage “above” breakdown voltage Overvoltage Coupled resistance Usually a weakly turned on transistor, with tunable “resistance” 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Avalanche phases Idle Build-up Quench Recharge 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Idle Phase Idle No free carriers in the depletion region High electric field Sufficient to cause carrier ionization p+ (0V) } n-well (~20V) 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

Electric field simulations of an avalanche diode 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Idle Phase Idle No free carriers in the depletion region High electric field Sufficient to cause carrier ionization p+ (0V) n-well (~20V) 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Idle Phase Idle No free carriers in the depletion region High electric field Sufficient to cause carrier ionization p+ (0V) } Multiplication } Drift n-well (~20V) 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Idle Phase Idle No free carriers in the depletion region High electric field Sufficient to cause carrier ionization Majority carriers exist p+ (0V) } Multiplication } Drift Electron Hole n-well (~20V) 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Idle Phase Idle No free carriers in the depletion region High electric field Sufficient to cause carrier ionization Majority carriers exist Resistor: ~100 kOhm Capacitor: ~50 fF p+ (0V) } Multiplication } Drift Electron Hole n-well (~20V) 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors e-/h+ pair is injected Electron Hole Injection of a free carrier into the depletion region Light Tunneling Traps An electron-hole pair in this case Majority carriers not shown anymore to simplify picture p+ (0 V) } x } 1 n-well (20 V) 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Build-up phase Electron Hole Carrier ionizes other carriers, and eventually positive feedback helps form an avalanche Occurs in a small (<1 um radius) region of diode Ionization process is statistical Time scale: ~30 ps p+ (0 V) } x } 1 n-well (20 V) 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Spread/quench phase Electron Hole Carriers moving across drift region cause a voltage drop Avalanche spreads in the planar directions External circuitry eventually quenches avalanche Voltage drops “below” Vbd Aided by inductance component from drift region Time scale: ~1 ns p+ (0 V) } x } 1 n-well (20 V) 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Spread/quench phase Electron Hole p+ (2 V) } x } 1 n-well (20 V) 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Spread/quench phase Electron Hole p+ (3.3 V) } x } 1 n-well (20 V) 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Restoration Electron Hole External circuitry restores applied voltage to desired level “above” breakdown voltage RC process Majority carriers flow, so charge doesn’t cause another avalanche Time scale: 5-100 ns p+ (0 V) } x } 1 n-well (20 V) 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Animation 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Figures of merit Photon detection probability Uncorrelated noise Correlated noise Jitter Note: FOMs very sensitive to operating conditions Temperature Excess bias (overvoltage) 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

Photon detection probability Excess bias 4V Breakdown at 18V CMOS diode PDP Wavelength (nm) 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Jitter PMT: 28ps CMOS SPAD: 47ps [Becker & Hickl] 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Noise Uncorrelated noise: dark count rate (DCR) Avalanches do occur while diodes are under no light Measured in counts per second (cps) or Hz Correlated noise: after-pulsing, cross-talk Hot-carrier luminescence causes cross-talk Defects in silicon can hold, release single carriers on nanosecond time scales 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Noise sources Thermal Trap-assisted thermal After-pulsing Trap-assisted tunneling Tunneling For most diodes, noise rates are generally in the 1-10 Hz/sq. micron range A fraction of diodes contain most of the noise Tunneling-limited diodes have worse noise 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

Expectations of radiation effects Gamma-ray exposure Increase in lattice defects should cause noise increase No effect on PDP X-ray exposure No effect Alpha particle exposure 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Outline Motivation and applications of single-photon avalanche diodes (SPADs) Physics of SPADs Radiation damage in SPADs Outlook 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors SPAD Camera: 32x32 Array Created in collaboration with ESA Earthglow sensor Backup satellite navigation 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

Chip details 32x32 pixel array GAA transistors in pixel Pixel contains 6 um diameter SPAD 1-bit memory (event/no event) Direct access circuitry Two modes Direct access to 1 row Rolling shutter mode

Alpha particle exposure Exposure to alpha particles at energies of 11 MeV and 60 MeV up to 400 Gy 21 day annealing time after irradiation 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

Alpha particle exposure (cont’d) Dark count rate (Hz) Total dose (Gy) 2011-07-03

Alpha particle exposure (cont’d) Before DCR Row Column 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

Alpha particle exposure (cont’d) After (400 Gy @ 11MeV) DCR Row Column 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

Random telegraph signal noise Dark count rate (Hz) Time (s) 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors X-ray exposure X-ray energy: ~40keV Total dose: 0.25 – 0.5 mGy No observed change 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Gamma-ray exposure Co60 exposure (~1.25 MeV gamma-rays) Two campaigns Total dose of 10 kGy [1 MRad] Total dose of 300 kGy [30 MRad] Annealing for one week at room temperature after both exposures 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Gamma-ray exposure 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Gamma-ray exposure Readout failure (recovered after annealing) Insignificant change Jump from gamma-rays 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Gamma-ray exposure Before DCR Row Column 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Gamma-ray exposure After 10kGy and annealing DCR Row Column 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Gamma-ray exposure 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors MRI Compatibility Ability to withstand strong magnetic fields is crucial in hybrid PET/MRI system 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

Noise in magnetic fields <2% shift 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors MRI Compatibility Time resolution in 9.4T Delta FWHM <10ps: Test conditions: External laser source Commercial TDC 2011-07-03 2011 International Workshop on Radiation Imaging Detectors 54

2011 International Workshop on Radiation Imaging Detectors Outline Motivation and applications of single-photon avalanche diodes (SPADs) Physics of SPADs Radiation damage in SPADs Outlook 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

Application-specific outlook: PET Growing promise for PET/MRI systems with avalanche diodes Working prototype systems (UCDavis) Commercial interest Evidence implies SPADs can handle 1 kGy dose with no impact on performance Exposures above 1 kGy will cause increases in noise 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

Application-specific outlook: 3D imaging SPADs can be used in 3D imaging systems in hostile environments Open question: will SPAD-based systems be competitive with CMOS systems? Greater interest in other techniques for 3D imaging 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

Application-specific outlook No problem to perform “short-running” work with SPADs in space Fluorescence imaging Quantum key distribution 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Summary of results Equivalent doses of 11MeV alpha particles have order of magnitude larger effect on noise than 1MeV gamma-rays No measurable problems from x-rays No distortions in 9.4T magnetic field in timing, noise rates Still unknowns Correlated noise Very long term (>5 year) performance 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

2011 International Workshop on Radiation Imaging Detectors Unknowns Radiation’s effect on STI-bound SPADs Radiation’s effects on correlated noise Predicting device damage from radiation Long term concerns SPADs don’t rely on silicon dioxide, tend to age more gracefully than transistors 2011-07-03 2011 International Workshop on Radiation Imaging Detectors

References Our website: http://ens.ewi.tudelft.nl Fundamentals of single-photon avalanche diodes based on chapter 7 of “Single-Photon Imaging” edited by Seitz et. al. to be published July 2011 by Springer RTS data from “RTS Noise Characterization in Single-Photon Avalanche Diodes” by Karami et. al. published in Electron Device Letters Volume 31 Issue 7 Radiation data from “A gamma, x-ray and high energy proton radiation-tolerant CIS for space applications” by Carrara et. al. published in 2009 ISSCC proceedings Magnetic field data from “Environmental effects on avalanche propagation in silicon” by Fishburn et. al. to appear in 2011 NSS proceedings Our website: http://ens.ewi.tudelft.nl 2011-07-03 2011 International Workshop on Radiation Imaging Detectors