There is a growing interest for high-sensitivity parallel fluorescence detection in life science, especially for techniques that require short acquisition times, such as parallel FCS (Fluorescence Correlation Spectroscopy) and FLIM (Fluorescence Lifetime IMaging). All these techniques require an array of high-sensitive photodetectors and would greatly benefit from miniaturization of the array itself.

In order to fulfill this request, we designed a compact bi-dimensional array of SPAD detectors, sporting single-photon-level sensitivity. The detectors are arranged in a 6x8 matrix with a 240-µm pitch. Each element has a large active area with diameter of 50 µm, and is connected to an independent integrated AQC (Active Quenching Circuit) for proper operation.

The detectors are biased at 5 V above the breakdown voltage, and are moderately cooled at 15 °C. In this condition, we obtained remarkable performances in terms of photon detection efficiency, dark counting rate, afterpulsing probability and optical crosstalk probability.

In particular, we achieved a peak detection efficiency of 48 % at 530 nm, and values above 30 % for all the visible spectrum; all the while the PDE remains above a few percent even in the near infrared up to 950 nm.

With regards to the dark counting rate, when the array is cooled at -15 °C the DCR is within 60 cps for about 40 % of the individual pixels, and remains below 6000 cps for all of them. Afterpulsing is negligible if the hold-off time is set to 250 ns, and a total aferpulsing probability of 0.5 % has been measured when reducing the hold-off time to 50 ns.

Thanks to the particular technological solutions implemented, optical crosstalk is kept below 0.2 % even for adjacent pixels.

In order to reduce the electrical crosstalk among pixels, the matrix has been partitioned into four interleaved sectors, each one composed of 12 SPAD detectors. The four sectors are cyclically activated, and the elements are arranged in such a way that the distance between two active detectors is 480 µm in both horizontal and vertical directions.

To take advantage of the characteristics of this SPAD array, we also developed a versatile and compact photon counting module that can be interfaced to a PC through an USB link. The modules measures only 20 x 8 x 4 cm, and all its working parameters can be set directly from the PC.

Publications

• Rech, I., Marangoni, S., Gulinatti, A., Ghioni, M., Cova, S.:
  "Portable genotyping system: four-colour microchip electrophoresis."
  Sensors and Actuators B: Chemical, B143(2), 583–589.
  2010
• Marangoni, S., Rech, I., Ghioni, M., Maccagnani, P., Chiari, M., Cretich, M., et al.:
  "A 6 × 8 photon-counting array detector system for fast and sensitive analysis of protein microarrays."
  Sensors and Actuators B: Chemical, 149, 420–426.
  2010
• Rech, I., Marangoni, S., Gulinatti, A., Ghioni, M., Cova, S.:
  "Toward single-molecule detection with very compact dna sequencer based on single-photon avalanche diode array."
  Paper presented at SPIE Photonics West 2008 - Single Molecule Spectroscopy and Imaging, San Jose, California, USA.
  2008, January 19—24
• Restelli, A., Rech, I., Maccagnani, P., Ghioni, M., Cova, S.:
  "Monolithic silicon matrix detector with 50μm photon counting pixels."
  Journal of Modern Optics, 54(2), 213–223.
  2007

Contacts

Massimo Ghioni - ghioni@elet.polimi.it
Ivan Rech - rech@elet.polimi.it
Angelo Gulinatti - gulinatt@elet.polimi.it