1. INAF - Osservatorio Astrofisico Catania II PRIN 2006 Meeting NEWS FROM SINGLE PHOTONS Sergio Billotta

2. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 2 Summary  Single Photon Avalanche Diode (SPAD)‏ What it is How it works Dark After pulse Photon Detection Efficiency (PDE)‏  Silicon PhotoMultiplier (SiPM)‏ What it is Dark After pulse Linearity Charge spectrum Time jitter Photon Detection Efficiency (PDE)‏  Conclusions

3. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 3 SPAD what it is OXIDE METAL N+N+ N + Gettering P+ Sinker SPAD = Single Photon Avalanche Diode STMicroelectronics SPAD  Planar device  Thin junction depletion layer (~ 1  m)‏  Low breakdown voltage (15 – 30 V)‏  Photodetector active area: defined by the metal ring used to contact the N + thin polysilicon layer doped with arsenic (Diameter: 10 ÷ 100  m)‏  High-electric field active region: defined by an P + enrichment diffusion  Local gettering sites: provided by an external ring doped by a heavy POCl 3 diffusion (to reduce the defectivity in the device active area)‏

4. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 4 SPAD how it works OXIDE METAL N+N+ N + Gettering P+ Sinker Semiconductor junction diodes reverse-biased few Volt above the breakdown voltage. The electric field within SPAD depletion layer is so high (higher than 3 x 10 5 V/cm) that a single carrier (photo / thermal electron) injected in this region can trigger a self-sustaining avalanche multiplication process. A sharp current pulse of few milliamps and with sub-nanosecond rise time is produced.( If the first carrier is photogenerated, the current rising edge marks the photon arrival time). Once the breakdown current has been detected, it is quenched by a large series resistor (passive quenching) or by a suitable quenching circuit (active quenching). The diode is thus turned off for a suitable hold-off time that allows the charge stored within the depletion layer to dissipate. The voltage is restored to the bias value and the device is ready to detect another photon.

5. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 5 SPAD dark STM SPAD array It is manufactured by the integration of 25 pixels with a square geometry of 5 x 5. For these devices, STMicroelectronics has designed three different pixel diameters: 20, 40 and 60  m. Separation distances between adjacent pixels are in the range of 160 and 240  m according to different diameters. Anode contacts are in common for each row, while each cathode is separately contacted and available from outside by different pads. The typical breakdown voltage is about 30 V. We have measured the dark counts rate of each pixel of several array of SPADs, and we have found a fairly good uniformity of it. Median room temperature dark count rate at 4V overbias as a function of SensL SPAD device area. Applications of Silicon Photon Counting Detectors, Stewart et al., JMO in press

6. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 6 SPAD a fter pulse It depends on:  Trap concentration in the junction depletion layer  number of carriers generated during a Geiger pulse. It could strongly enhance the total dark count rate. During an avalanche some carriers are trapped by deep levels in the multiplication region released after a statistically fluctuating delay they can re-trigger a Geiger event correlated with the previous avalanche pulse Dark count rate at several hold-off time estimation of the afterpulsing effects

7. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 7 PDE = QE x P t x A e QE=Quantum Efficiency P t =Avalanche Probability A e =Geometrical Efficiency Probability for a photon to generate an e–h pair in the active thickness of the device QE = (Dielectric layer transmittance) x QE internal Probability for a photon that has crossed the dielectric layer to generate an e–h pair in the active thickness. wavelength dependent. Can be maximized, implementing an anti-reflective coating (ARC)‏ ARC SPAD photon detection efficiency (1)‏

8. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 8 PDE = QE x P t x A e QE=Quantum Efficiency P t =Avalanche Probability A e =Geometrical Efficiency There is a finite probability for a carrier to initiate an avalanche when passing through a high-field region. In case of a photogeneration event, 2 carriers are created travelling in opposite directions P t = P e + P h - P e P h Electron and hole breakdown initiation probabilities In case of photogeneration on the right side, the situation is symmetrical and only electrons contribute to the triggering probability, thus, P t = P eM. In the central region, both carriers contribute to a different extent as a function of the interaction position and the Pt value is between P eM and P hM When a pair is generated in the left side of the high-field region, the electron is directly collected at the n+ terminal; thus, it does not contribute to the triggering probability. The hole is forced to cross the whole high-field region and so its triggering probability is maximized and P t = P hM. SPAD photon detection efficiency (2)‏

9. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 9 PDE = QE x P t x A e QE=Quantum Efficiency P t =Avalanche Probability A e =Geometrical Efficiency A active / A total for SPAD = 1 SPAD photon detection efficiency (3)‏ OXIDE METAL N+N+ N + Gettering P+ Sinker All the exposed area is active Photon Detection Probability (PDP)‏

10. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 10 Photon Detection Efficiency % PDE of a 40  m STM SPAD SPAD photon detection efficiency Photon Detection efficiency STM  Pixel size: 40  m  V Brk : 26V  V BIAS : 10% -> 15%  H.O.: 6  s STM  Pixel size: 32  m  V Brk : 29V  V BIAS : 10% -> 30%  H.O.: 40/45  s SensL  Pixel size: 50  m  V Brk : 28V  V overvoltage : 1.5V -> 4V MPD  Pixel size: 50  m  Operating conditions set by the electronics inside the module Applications of Silicon Photon Counting Detectors, Stewart et al., JMO in press

11. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 11  Single Photon Avalanche Diode (SPAD)‏ What it is How it works Dark After pulse Photon Detection Efficiency (PDE)‏  Silicon PhotoMultiplier (SiPM)‏ What it is Dark After pulse Linearity Charge spectrum Time jitter Photon Detection Efficiency (PDE)‏  Conclusions

12. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 12 SiPM what it is Silicon PhotoMultiplier  Matrix of n pixels in parallel  Each pixel: SPAD + R quenching Analog Device => the output signal is the sum of the signals from all fired pixel Q out = C x (V R – V BR ) x N fired Schematic cross-section of a half single cell of the SiPM fabricated at STMicroelectronics Catania R&D clean room facility a) SEM top view of a SiPM prototype fabricated at STMicroelectronics Catania R&D clean room facility; b) detail of optical trenches between adjacent pixels

13. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 13 SiPM dark Dark count pulses:  s => single pixel pulse  d => two simultaneously pixels pulse  a = > Characteristics of the single-pixel dark pulse (equal of single photon pulse)‏  rise time  ~ hundreds of ps  recovery time  τ = R quenching · C micro-cell ~ 20-30 ns Measured noise rate as a function of V − V bd afterpulses Dark rate as a function of overbias for a SensL SiPM at room temperature and at -20°C Applications of Silicon Photon Counting Detectors, Stewart et al., JMO in press

14. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 14 SiPM after pulse Sketch of the electronics for the self-correlated timing, employed for afterpulse measurements. For uncorrelated events (SiPM without cross-talk and afterpulses): The random noise follows the Poisson law The distribution of the arrival time between two events is exponential We measured the distribution of time intervals between two consecutive dark pulses at 20°C for several bias voltages, and built the corresponding histograms. The lower time threshold was around 15-20ns, therefore preventing us from attaining a direct measurement of cross-talk. The afterpulse effect shows up in such a distribution as a pronounced deviation from the perfect exponential distribution of the uncorrelated dark noise, namely a prominent peak around 200ns.

15. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 15 If N photon x PDE << N total Output signal  N fired When 50% of the cells fire the deviation from linearity is 20% Best working condition => N photons < N cells If  t >  R the SiPM dynamic range is larger.   t = duration of the light signal   R = single pixel recovery time SiPM dynamic range - linearity

16. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 16 SiPM charge spectrum By making use of a dedicated data acquisition system one can characterize the SiPM on an event-by-event basis. Using this method we built the charge distribution histogram under several different light intensity values. We employed a red laser diode (650nm) pulsed at 1kHz, whose light was conveyed onto the sensor by means of an optical fiber. A typical charge spectrum under very low light level for a 10x10 device biased at 6% OV. The multipeak structure reflects the detection of 1-18 photons per event. For this sensor we measured a 3  resolving power around 20 and a 2   resolving power around 45. Sketch of the electronics for the charge and time measurements, employed for SiPM response characterization.

17. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 17 SiPM time jitter STM SiPM:  SiPM area: 0,5 x 0,5 mm 2  Pixel active area: 32 x 32  m 2  Fill factor: 36%  N° pixels: 10 x 10  6% overvoltage  Apparatus:  Pulsed laser diode ( = 650 nm, 1kHz, pulse: 40ps)‏  Optical fiber A typical timing spectrum under very low light level for a 10x10 device biased at 6% OV. The average number of detected photons was around 6. The time calibration of the TDC was 50ps/channel, therefore the time resolution (sigma) is 135ps. Sketch of the electronics for the charge and time measurements, employed for SiPM response characterization. The width (FWHM) of the statistical distribution of the delay between the true arrival time of the photon at the sensor and the measured time marked by the output pulse current leading edge.

18. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 18 PDE = QE x P t x A e QE=Quantum Efficiency P t =Avalanche Probability A e =Geometrical Efficiency dead region:  determined by the guard ring  structure preventing optical cross-talk  space between the cells for the individual resistors Considering that the area of a cell can be very small (in the order of 30x30  m 2 ) even few microns of dead region around the cell have a very detrimental effect on the geometrical efficiency. Best filling can be achieved with a small number of big cells => SATURATION !!! A active / A total SiPM photon detection efficiency

19. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 19 STM  1 x 1 mm 2  20 x 20 pixels  Pixel size: 32  m  Fill factor: 36%  V BIAS : 32V  Gain = 8 x 10 5  Temp: 24°C SiPM photon detection efficiency SensL  1 x 1 mm 2  1144 pixels  Pixel size: 20  m  Fill factor: 43%  V BIAS : 29.5V, 30V, 31V and 32V  Temp: 20°C STM  0.5 x 0.5 mm 2  10 x 10 pixels  Pixel size: 32  m  Fill factor: 36%  Temp: 20°C  V Brk : 29.5V Hamamatsu  1 x 1 mm 2  20 x 20 pixels  Fill factor: 61%  V BIAS : 70V  Gain = 7 x 10 5  Temp: 25°C 10% 12% 5% 7% Gate 500 ns 1 st method Gain overestimated: after pulse 2 nd method underestimated: no amplitude discrimination Applications of Silicon Photon Counting Detectors, Stewart et al., JMO in press

20. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 20 Conclusions SiPMAdvantages:  robust and compact  sensitivity to extremely low photon fluxes providing proportional information with excellent resolution and high photon detection efficiency  extremely fast response with low fluctuation (sub-ns rise time and <100ps jitter)‏  low bias voltage (<100V)‏  low power consumption (<50μW/mm2)‏  long term stability  insensitive to magnetic fields (up to 15T) and EM pickup  low cost (in the future! now ~140$/mm 2 ) + low peripheral costsDisadvantages:  silicon quality (dark rate, after-pulse)‏  effective area of the cells (gain, fill factor, dynamic range, recovery time  optical cell insulation (optical cross-talk)‏  quenching resistor (recovery time, dynamic range)‏SPADAdvantages:  solid state technology: robust, compact, mechanically rugged and less expensive  Geiger mode  high internal gain of 10 5 - 10 6  faint sources  high quantum efficiency  large standardized output signal  no Read Out Noise  high sensitivity for single photons  excellent timing event for single photo electrons (<< 1ns)‏  good temperature stability  devices operate in general < 100V  no nuclear counter effect (due to the standardized output)‏Disadvantages:  BINARY DEVICE – one knows there was at least one electron/hole initiating the breakdown but not how many of them !!!!!  Max diameter 100  m.

21. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 21 INAF - Osservatorio Astrofisico Catania II Meeting PRIN 2006 NEWS FROM SINGLE PHOTONS Sergio Billotta Grazie

22. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 22 SPAD time jitter TIMING JITTER SPAD TIMING JITTER (TJ) or TIMING RESOLUTION: the width (FWHM) of the statistical distribution of the delay between the true arrival time of the photon at the sensor and the measured time marked by the output pulse current leading edge. STM SPAD:  40  m  20% overvoltage  Two regimes:  Few photons on the photosensor  Many photons on the photosensor  We need :  A pulsed laser system ( = 408 nm, FWHM ~ 300 ps)‏  A multiple grey filter (to reduce the intensity)‏  A reference PMT (illuminated by the non-attenuated laser light)‏ (a)In single photon regime: FWHM ~ 300ps => the measured time reflects the time structure of the laser pulse (b)In the multi photon regime: FWHM ~ 160 ps (which also included the electronics as well as the PMT contribution) => Strong reduction of the statistical fluctuations in the arrival time of photons PMT START SPAD STOP Time to Amplitude Converter (TAC)‏

23. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 23 SPAD photon detection efficiency  Xenon Lamp  Pre-filtering system  Monochromator  Integrating Sphere  Reference photodiode  SPAD (+ Camera)‏  One ammeter  (AQC + Counter)‏  One PC PDE = # Detected photons / # Incident photons Camera conceived to be anchored to the integrating sphere Distance (SPAD – sphere centre) = Distance (photodiode – sphere centre)‏ BK7 window with MgF 2 anti reflection coating AQC (OACt)‏ vacuum Peltier Stabilized T SPAD down to -20°C SPAD biasing SPAD (~ 10 ns) quenching Hold – off time variation (400 ns ÷ 6  s)‏ TTL output

24. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 24 SiPM what it is The cell is square shaped, with a 50μm/30μm side over active area ratio, and a resulting 36% fill factor. We produced a SiPM made of a 10x10 array with common anode. Each cell has a breakdown voltage around 29.5V at room temperature, with a variation coefficient of 35mV/°C. For our tests the SiPM was biased at voltages between 31.5V and 33V.

25. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 25 SiPM OV SiPM breakdown voltage as a function of the temperature. The breakdown voltage increases linearly with a temperature coefficient of 35.5 mV/°C. 400 channels SiPM output dark noise pulses. The device was biased at 10% OV; three types of signals can be seen: (i) single pulses coming from the activation of a single pixel; (ii) double pulses corresponding to events affected by cross- talk effects and (iii) small amplitude pulses due to the afterpulsing

26. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 26 SiPM after pulses Probability distribution of the time interval between two consecutive signals (solid line). The open circles represent the contribution due to the dark counts, the open squares the afterpulses. Example of measured noise rate as a function of the discriminator threshold. The threshold is normalized to the 1- photon signal amplitude. Operation at 0.5 photon threshold is safely in the plateau region

27. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 27 PDE = QE x P t x A e SiPM Camera Reference PhotoDiode Integrating Sphere Catania PDE measurament system  Xenon Lamp  Pre-filtering system  Monochromator  Integrating Sphere  Reference photodiode  SiPM  Two ammeter  One PC Hamamatsu MPPC Development of Multi-Pixel Photon Counter (MPPC), K. Yamamoto et al. 2006 Higher geometric factor value ! These PDE values are including the cross-talk and after-pulse = > more estimation to separate this contribution! SiPM photon detection efficiency (4)‏

28. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 28 SiPM photon detection efficiency (4)‏ The PDE at 700nm, measured at 32.5V bias voltage, as reconstructed with our method using logic signal durations of 50ns as a function of the number of photons impinging on the SiPM. Apart from the first point, measured at very low flux, the behaviour is constant as expected.

29. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 29 SiPM applications Positron Emission Tomography (PET)‏ The two 511 keV photons form the annihilation radiation are absorbed by a scintillator material in the scanning device, creating a burst of light which is detected by a photodetector (usually PMT or SiPM). This technique depends on simultaneous or coincident detection of the pair of photons; in fact photons which do not arrive in pairs (i.e., within a few nanoseconds) are ignored. Hence it is possible to localize the source along a straight line of coincidence. A statistics from tens- of-thousands of coincidence events is then collected. The scanner records these signals and transforms them into images. The resulting map shows the tissues in which the molecular probe has become concentrated, and can be interpreted by a nuclear medicine physician or radiologist for the patient's diagnosis and treatment plan. Positron emission tomography (PET) is a new nuclear medicine imaging technique which produces three-dimensional images or maps of functional processes in the body. It is heavily used in clinical oncology (medical imaging of tumors and search for metastases) and for clinical diagnosis of certain diffuse brain diseases. In order to conduct the scan, a short-lived radioactive tracer isotope which has been chemically incorporated into a metabolically active molecule, is injected into the living subject. There is a waiting period while the metabolically active molecule becomes concentrated in tissues of interest; then the research subject or patient is placed in the imaging scanner. The radioisotope emits a positron which travels few millimeters before it combines with an electron.

30. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 30 COLD laboratory  Detectors characterization:  Traditional detectors: CCDs  Innovative ones:  Diamond detectors  SiC detectors  Single Photon Avalanche Diode (SPAD)‏  Silicon Photon Multiplier (SiPM)  Designing and testing of control electronics, cryogenics and mechanical equipments for detectors  Software development for management of astronomical instruments  Collaboration in spatial and terrestrial telescope instrumentation

31. II PRIN 2006 meeting - Bled 26-28 Mar '08Sergio Billotta - NEWS FROM SINGLE PHOTONS 31 Our apparatus  < 1nm - = 1100 ÷ 150 nm Electro-optical characteristics to demonstrate the scientific program applicability Reference photodiode SPAD camera Integrating sphere