SINGLE PHOTON AVALANCHE DIODE (SPAD): FROM SINGLE ELEMENT TO ARRAY (SPADA) (SWORD) G. Bonanno, M. Belluso, F. Zappa, S. Tisa, S. Cova, P. Maccagnani, D. Bonaccini Calia, R. Saletti, R. Roncella, S. Billota INVASION OF DETECTORS INAF – Catania Astrophysical Observatory Dept. of Electronics, Politecnico di Milano ESO European Southern Observatory Dept. of Information Engineering, Pisa University SPADA: an Array of SPAD Detectors for Astrophysical Applications Electro-Optical Characteristics of a SPAD M. Belluso, G. Bonanno, S. Billotta, A. Calì, S. Scuderi, M. Mazzillo, P.G. Fallica, D. Sanfilippo, E. Sciacca, S. Lombardo Characterization of SPAD Arrays MANUFACTURERS: ST Microelectronics R&D of Catania IMM-CNR R&D of Catania LAMEL, IMM-CNR R&D of Bologna

Taormina, 2005Scientific Detectors Workshop2 Detectors for Fast Transient Phenomena and Adaptive Optics CCD can really be used for all kind of stellar observations? CCD are integrating devices, with relatively slow readout (even with multi-port architectures) and show a read noise (!!!!/??). Techniques of signal intensifying “on-chip” (L3 CCD) or off-chip (MCP coupling) are good only for the observation of faint sources. How to observe faint and fast photon sources with the highest QE? How can obtain time resolution and fast gating for Adaptive Optics without complex controllers? Suitable detectors ? Silicon detectors operating in photon counting regime

Taormina, 2005Scientific Detectors Workshop3 Basically, a SPAD detector is a semiconductor junction diode that can sustain an avalanche multiplication process when reverse biased. Instead of a standard Avalanche Photodiode (APD), which is operated below the breakdown voltage in the analog multiplication regime, a SPAD is biased above its breakdown voltage. Applied voltage levels are such that the electric field is sufficiently tall to generate ionization from impact. APD – SPAD comparison APDSPAD BIAS : MODE : GAIN : Below BD linear e-/ph Above BD Geiger infinite AMPLIFIERTRIGGER

Taormina, 2005Scientific Detectors Workshop4 SPADA System (Curvature Wavefront Sensor) Current Status The SPADA chip is assembled on a ceramic holder and is cooled by a Peltier module. The cooling system has been designed in collaboration with ESO and finalized via industrial contracts. SPADA chip SPADA element Each SPAD is biased and driven by an appropriate detection and control electronics. Core of the Detection electronics are the integrated Active-Quenching Circuits (iAQC), one for each SPAD. MORE DETAILS ON THE POSTER CURRENT STATUS LENSLETS Array ESO SPADA chip realized DETECTION Boardrealized DATA-PROCESSINGrealized MECHANICAL HOUSING & CRYOSTAT under constr. DATA ACQUISITION SOFTWARE realized THE COMPLETE SYSTEM IS EXPECTED AT THE END OF THIS YEAR.

Taormina, 2005Scientific Detectors Workshop5 DETECTION AND CONTROL ELECTRONICS The entire Detection Board provides not only the 12 chips containing a total of 60 iAQCs, but also 15 quad output drivers for a total of 60 differential lines (requested for compatibility with the existing MACAO systems). Core of the Detection electronics are the integrated Active-Quenching Circuits (iAQC), one for each SPAD. Each iAQC is able to bias the correspondent SPAD above the breakdown voltage, generate a digital pulse each time a photon is detected and quickly quench the SPAD (by lowering its bias below breakdown). At the end of the cycle, the SPAD is ready to detect a new photon.

Taormina, 2005Scientific Detectors Workshop6 Adaptive Optics: The system acts as a Curvature WaveFront System (CWFS). By means of a moving membrane, the conjugated plane is moved with sinusoidal oscillations “before” (detected counts A) and “after” (detected counts B) the SPADA focal plane. The curvature signal is then evaluated as: (A-B)/(A+B) by an “on board FPGA circuit”. The acquisition is repeated in free running. Fast Transient Phenomena: In this operational mode, data corresponding to the 60-pixel SPADA detector are acquired every integration time window (tw settled in the range 100µs-100ms), allowing time-tagging of incoming photons, with quite enough resolution for the various astronomical applications, as for example fast events due to gamma rays burst, or low luminous flux with high temporal variation, like pulsar, polar and asteroseismology observations. Astrophysical Applications

Taormina, 2005Scientific Detectors Workshop7 Electro-Optical Characteristics of a SPAD To reduce the dark counts at few counts per second, is mandatory to operate the SPAD at temperatures below 0 °C. A mechanical housing working under vacuum conditions hosts the packaged detector and can be easily interfaced with the measurement apparatus. Dark counts and after-pulsing Quantum Efficiency Response Uniformity MASK OF DIFFERENT DIAMETER SINGLE SPAD Manufactured by STM The response uniformity has been measured by using a recently available facility constituted by a three axes translator that mount on top a reflective objective illuminated by a 10 µm pin-hole through a fiber optic that pick-up the monochromatic light from the Char. Sys. Optical Fiber In a 20 µm 25°C, dark is constant vs the hold-off (switched-off) time  no after-pulsing. Dark depends on the overvoltage and ranges from 300 to 1500 counts/s. The dark rate may be affected by after-pulsing  traps in Si (releasing of charge at different time). QE measurements have been carried-out on 10, 20, 50 µm SPADs at overvoltages of: 10, 20 and 30 %. For all the SPADs we find a QE typical of Si devices that peaks at nm. We found a QE dependence from the SPAD diameter and the overvoltage. At each movement the corresponding count rate is acquired and stored in a file. Simultaneously an IDL procedure read the file and produces a 3D plot of the currently acquired data. The 3D plot shows a 50 x 50 µm scan with 5 µm resolution. The 5 X 5 µm central area presents a flat response (5000 counts/s), confirming the response uniformity in that area. MORE DETAILS ON THE POSTER

Taormina, 2005Scientific Detectors Workshop8 Characterization of SPAD Arrays For SPAD Array test, another mechanical housing working under vacuum conditions and hosting the packaged detector as well as the AQC boards have been deigned and realized. Dark counts MapQuantum Efficiency 5 X 5 40 µm SPAD ARRAY MASK Manufactured by STM The cryogenic system is able to operate the array detector at a temperature adjustable in the -30 ÷ +30 °C range. MORE DETAILS ON THE POSTER Dark count rate map obtained by biasing with an overvoltage of 20 % each SPAD array element and with an hold-off time of 6 µs. Only two pixels are out of the mean value of about 3000 cnts/s  At room temperature the array shows a dark quite uniform over all the array. Quantum Efficiency 600 nm Overvoltage 20% hold-off 6 µs Quantum Efficiency map of all the array elements at =600nm, obtained biasing each SPAD at 20% above breakdown and settling 6 µs as hold-off time. The mean value is about 57% and is evident the uniform sensitivity at 600 nm over all the array.

Taormina, 2005Scientific Detectors Workshop9 Thank you For your attention/patience