A 512 × 512 SPAD Image Sensor With Integrated Gating for Widefield FLIM

Abstract

We report on SwissSPAD2, an image sensor with 512×512 photon-counting pixels, each comprising a single-photon avalanche diode (SPAD), a 1-bit memory, and a gating mechanism capable of turning the SPAD on and off, with a skew of 250ps and 344ps, respectively, for a minimum duration of 5.75ns. The sensor is designed to achieve a frame rate of up to 97,700 binary frames per second and sub-40ps gate shifts. By synchronizing it with a pulsed laser and using multiple successive overlapping gates, one can reconstruct a molecule’s fluorescent response with picosecond temporal resolution. Thanks to the sensor’s number of pixels (the largest to date) and the fully integrated gated operation, SwissSPAD2 enables widefield FLIM with an all-solid-state solution and at relatively high frame rates. This was demonstrated with preliminary results on organic dyes and semiconductor quantum dots using both decay fitting and phasor analysis. Furthermore, pixels with an exceptionally low dark count rate and high photon detection probability enable uniform and high quality imaging of biologically relevant fluorescent samples stained with multiple dyes. While future versions will feature the addition of microlenses and optimize firmware speed, our results open the way to low-cost alternatives to commercially available scientific time-resolved imagers.

Single-photon avalanche diode imagers in biophotonics: review and outlook

“…(f, g) A Convallaria FLIM measurement done with a linear 32×1 SPAD array [59]. Images reprinted from [25,[56][57][58][59].…”

“…Early characterization of SwissSPAD (without microlenses) for FLIM measurements was performed with reference data sets [69]; the sensor was found to be able to properly extract the lifetime of fluorophores in the nanosecond range. Ulku then designed SwissSPAD's successor, SwissSPAD2 [26,57], a 512×512 SPAD array -the largest time-resolved SPAD image sensor to date -with higher PDP and lower DCR, based on a similar architecture. A triple-color fluorescence intensity image is shown in Figure 3(c).…”

Quantum correlation measurement with single photon avalanche diode arrays

“…Recent progress in the design and manufacturing processes of CMOS based SPAD arrays led to sub-megapixel arrays [54], low DCR and improved photon detection efficiency (PDE) [24,57]. This has been achieved by a synergy between innovative SPAD designs, process improvements and 3D integrated circuit (IC) technology advancements.…”

High-throughput smFRET analysis of freely diffusing nucleic acid molecules and associated proteins

“…It is worth noting that alternative SPAD array designs using standard CMOS fabrication technology have also been developed during the past two decades [18]. While they afford larger scales (> 10 5 SPADs versus < 10 3 SPADs for the custom technology) ideal for wide-field imaging techniques, such as fluorescence lifetime imaging [19,20] or high-throughput fluorescence correlation spectroscopy [21], it is our experience [22] that CMOS SPAD arrays still have a lower photon detection efficiency (PDE) and generally higher dark count rates (DCR) than custom silicon SPAD detectors [14,23] making them poor detectors for freely-diffusing singlemolecule detection applications. Due to the fast pace of technological innovation in this field, this statement may become rapidly outdated.…”