Proceedings Article | 27 April 2020
KEYWORDS: Silicon photomultipliers, Atmospheric Cherenkov telescopes, Picosecond phenomena, Near infrared, Medical imaging, Biomedical optics, Medical imaging applications, Positron emission tomography, Crystals, Scintillators
Since 2005, Fondazione Bruno Kessler (FBK, Trento) has been improving and customizing its SiPM technologies for a wide variety of applications, ranging from medical imaging to big science experiments and industrial applications. Current-generation Near Ultra Violet, High Density (NUV-HD) SiPM technology is based on a p-on-n junction in which microcells are separated by deep trench isolation. The technology features peak photon-detection efficiency (PDE) higher than 60% at 410 nm, low primary (80 kcps/mm2) and correlated noise (less than 15% optical crosstalk probability at 50% PDE) and is very well suited for medical imaging applications, such as time-of-flight positron emission tomography (TOF-PET). Indeed, using an advanced high-frequency readout, a research group at CERN demonstrated state-of-the-art coincidence resolving time of 58 ps FWHM, employing 4x4 mm2 NUV-HD SiPMs coupled to small LSO:Ce:Ca crystals. With a similar, optimized setup, a single-photon time resolution (SPTR) of 90 ps FWHM was also demonstrated, which is even more important when faint Cherenkov emission is used to improve timing performance in relatively slow scintillators such as BGO.
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On the other hand, different applications require specific optimizations of SiPM parameters, especially in the field of big science experiments. To this end, several improvements of the NUV-HD technology are ongoing. Reduction of optical crosstalk probability by a factor of ~2.5 is achieved by introducing light absorbing/reflecting material inside trenches, providing CT probability of 5% at 50% PDE. This technology, called NUV-HD-LowCT, will be used for the CTA experiment (Cherenkov Telescope Array). Optimization of SiPM performance for operation at cryogenic temperatures, for the readout of very large areas, was carried out for experiments such as DarkSide-20k, in which approximately 15 m2 of SiPMs will be used for the readout of a dual-phase, liquid argon TPC operated at 87 K. Achievement of excellent performance at this temperature required three different changes to NUV-HD technology and fabrication process, the main challenges being the reduction of band to band tunneling and afterpulsing at low temperatures. The resulting NUV-HD-Cryo technology features an exceptionally low DCR of less than 10 mHz/mm2 and AP probability of less than 15% at 87 K. Using a single electronic channel for the readout of 24 cm2 photo-sensitive active area, composed of 24 SiPMs connected in a peculiar series-parallel configuration, it was possible to measure a few-photon spectrum with a remarkable SNR larger than 24 for the single photon peak at 77 K. Direct detection of vacuum ultra-violet (VUV) light, below 200 nm, is required by the nEXO experiment. In this context, FBK demonstrated the beyond state-of-the-art performance, with a PDE larger than 23% at 190 nm, measured at 170 K. Finally, recent interest in using SiPMs in harsh radiation environment, such as in space missions or in accelerator experiments, poses additional challenges in detector optimization and partially redefines the typical design trade-offs.
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At the lower-energy end of the sensitivity spectrum, we observe a growing interest from industry in using SiPMs for detection of near infrared light. Among different applications, the most important one is LIDAR, especially for advanced driver-assistance systems in the automotive field. NIR-sensitive SiPMs (NIR-HD) fabricated at FBK employ a thicker epitaxial layer to enhance detection efficiency at longer wavelengths, achieving high PDE of 18% at 850 nm and 12 % at 905 nm, without the use micro lenses. Ongoing research is aimed at understanding the ultimate limits to the design of small, fast cells with high NIR sensitivity, such as the effect of the virtual guard region on the dead area at the microcell borders. Experimental variations of the technology, based on this research, show improved detection efficiency by careful engineering of the SPAD structure.
