Below 100 ps CTR using FastIC+, an ASIC including on-chip digitization for ToF-PET and beyond

This work presents the 8-channel FastIC+, a low-power consumption and highly configurable multi-channel front-end ASIC with internal digitization, for the readout of photo-sensors with picosecond time resolution and intrinsic gain. This ASIC, manufactured in 65 nm CMOS technology, can readout positive or negative polarity sensors and provides a digitized measurement of the arrival time and energy of the detected events with a power consumption of 12.5 mW per channel. On-chip digitization is executed by a Time-to-Digital Converter (TDC) based on a Phase-Locked Loop (PLL) generating 16 phases at 1.28 GHz. The internal TDC introduces a jitter contribution of 31.3 ps FWHM, with minimal impact on timing measurements. When evaluating FastIC+ to readout 3$\times$3 mm$^2$ silicon photomultipliers (SiPMs) with a pulsed laser, we achieved a single-photon time resolution (SPTR) of (98 $\pm$ 1) ps FWHM. We also performed time-of-flight positron emission tomography (ToF-PET) experiments using scintillator crystals of different sizes and materials. With LYSO:Ce,Ca crystals of 2.8$\times$2.8$\times$20 mm$^3$ we obtained a coincidence time resolution (CTR) of (130 $\pm$ 1) ps FWHM. With LGSO crystals of 2$\times$2$\times$3 mm$^3$, a CTR of (85 $\pm$ 1) ps FWHM. To the best of our knowledge, this is the first time that a CTR below 100 ps using on-chip digitization is reported.