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

2. 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 International Workshop on Radiation Imaging Detectors

3. 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 International Workshop on Radiation Imaging Detectors

4. 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 International Workshop on Radiation Imaging Detectors

5. 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 International Workshop on Radiation Imaging Detectors

6. 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 International Workshop on Radiation Imaging Detectors

7. 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 International Workshop on Radiation Imaging Detectors

8. 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 International Workshop on Radiation Imaging Detectors

9. 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 International Workshop on Radiation Imaging Detectors

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

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

12. Electric field simulations of an avalanche diode
V/cm
2011 International Workshop on Radiation Imaging Detectors

13. A p-n junction at steady state
2011 International Workshop on Radiation Imaging Detectors

14. A p-n junction at steady state
2011 International Workshop on Radiation Imaging Detectors

15. A p-n junction in Geiger-mode
2011 International Workshop on Radiation Imaging Detectors

16. 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 International Workshop on Radiation Imaging Detectors

17. 2011 International Workshop on Radiation Imaging Detectors
Avalanche phases
Idle
Build-up
Quench
Recharge
2011 International Workshop on Radiation Imaging Detectors

18. 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 International Workshop on Radiation Imaging Detectors

19. Electric field simulations of an avalanche diode
2011 International Workshop on Radiation Imaging Detectors

20. 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 International Workshop on Radiation Imaging Detectors

21. 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 International Workshop on Radiation Imaging Detectors

22. 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 International Workshop on Radiation Imaging Detectors

23. 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 International Workshop on Radiation Imaging Detectors

24. 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 International Workshop on Radiation Imaging Detectors

25. 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 International Workshop on Radiation Imaging Detectors

26. 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 International Workshop on Radiation Imaging Detectors

27. 2011 International Workshop on Radiation Imaging Detectors
Spread/quench phase
Electron
Hole
p+ (2 V) } x } 1
n-well (20 V)
2011 International Workshop on Radiation Imaging Detectors

28. 2011 International Workshop on Radiation Imaging Detectors
Spread/quench phase
Electron
Hole
p+ (3.3 V) } x } 1
n-well (20 V)
2011 International Workshop on Radiation Imaging Detectors

29. 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: ns
p+ (0 V) } x } 1
n-well (20 V)
2011 International Workshop on Radiation Imaging Detectors

30. 2011 International Workshop on Radiation Imaging Detectors
Animation
2011 International Workshop on Radiation Imaging Detectors

31. 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 International Workshop on Radiation Imaging Detectors

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

33. 2011 International Workshop on Radiation Imaging Detectors
Jitter
PMT: 28ps
CMOS SPAD: 47ps
[Becker & Hickl]
2011 International Workshop on Radiation Imaging Detectors

34. 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 International Workshop on Radiation Imaging Detectors

35. 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 International Workshop on Radiation Imaging Detectors

36. 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 International Workshop on Radiation Imaging Detectors

37. 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 International Workshop on Radiation Imaging Detectors

38. 2011 International Workshop on Radiation Imaging Detectors
SPAD Camera: 32x32 Array
Created in collaboration with ESA
Earthglow sensor
Backup satellite navigation
2011 International Workshop on Radiation Imaging Detectors

39. 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

40. 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 International Workshop on Radiation Imaging Detectors

41. Alpha particle exposure (cont’d)
Dark count rate (Hz)
Total dose (Gy)

42. Alpha particle exposure (cont’d)
Before
DCR
Row
Column
2011 International Workshop on Radiation Imaging Detectors

43. Alpha particle exposure (cont’d)
After (400 11MeV)
DCR
Row
Column
2011 International Workshop on Radiation Imaging Detectors

44. Random telegraph signal noise
Dark count rate (Hz)
Time (s)
2011 International Workshop on Radiation Imaging Detectors

45. 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 International Workshop on Radiation Imaging Detectors

46. 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 International Workshop on Radiation Imaging Detectors

47. 2011 International Workshop on Radiation Imaging Detectors
Gamma-ray exposure
2011 International Workshop on Radiation Imaging Detectors

48. 2011 International Workshop on Radiation Imaging Detectors
Gamma-ray exposure
Readout failure
(recovered after
annealing)
Insignificant change
Jump from gamma-rays
2011 International Workshop on Radiation Imaging Detectors

49. 2011 International Workshop on Radiation Imaging Detectors
Gamma-ray exposure
Before
DCR
Row
Column
2011 International Workshop on Radiation Imaging Detectors

50. 2011 International Workshop on Radiation Imaging Detectors
Gamma-ray exposure
After 10kGy and annealing
DCR
Row
Column
2011 International Workshop on Radiation Imaging Detectors

51. 2011 International Workshop on Radiation Imaging Detectors
Gamma-ray exposure
2011 International Workshop on Radiation Imaging Detectors

52. 2011 International Workshop on Radiation Imaging Detectors
MRI Compatibility
Ability to withstand strong magnetic fields is crucial in hybrid PET/MRI system
2011 International Workshop on Radiation Imaging Detectors

53. Noise in magnetic fields
<2% shift
2011 International Workshop on Radiation Imaging Detectors

54. 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 International Workshop on Radiation Imaging Detectors 54

55. 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 International Workshop on Radiation Imaging Detectors

56. 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 International Workshop on Radiation Imaging Detectors

57. 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 International Workshop on Radiation Imaging Detectors

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

59. 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 International Workshop on Radiation Imaging Detectors

60. 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 International Workshop on Radiation Imaging Detectors

61. 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:
2011 International Workshop on Radiation Imaging Detectors