A $256 \times 256$ SPAD Imager Architecture with Multi-Functional Pixel and High Temporal Aperture for Quantum Imaging Applications

In this paper we describe the architecture of a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{2 5 6} \times \mathbf{2 5 6}$</tex> Single Photon Avalanche Diode (SPAD) imager designed for quantum imaging applications in a 110 nm CMOS Sensor Imaging process. The chip features a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$256 \times 256$</tex> SPAD array with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$30 \mu \mathrm{m}$</tex> pitch and reconfigurable pixel multi-functionality i.e. photon counting with fast gating mode or continuous time stamping of time correlated events with an external trigger. The array is organized in macro-pixels composed of clusters of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2 \times 2$</tex> SPADs sharing the Time-to-Digital Converter (TDC), a logic arbiter circuit and other in-pixel circuitry reaching a remarkable <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\simeq 22 {\%}$</tex> Fill Factor despite the small pixel area. To optimize the usage of the available electronics, we designed an architecture which can share the in-pixel resources also among the neighboring macro-pixels laying on the same line. We split the chip in two independent halves with dedicated readout circuitry to improve the data throughput guaranteeing the possibility to sustain a maximum expected photon rate of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$100 \text{Mphotons} / \mathrm{s}$</tex> impinging on the focal plane array. To this end, a row and a column zero-suppression logic circuit have been envisaged to save time during the matrix readout phase increasing the chip frame rate.