Our team are exploring the potential for using advanced 3D stacking technologies to design ultra-miniature SPAD arrays for time-resolved endoscopy applications.
Common challenges faced by SPAD image sensor designers are trade-offs between pixel size (scalability), sensitivity (light sensitive area in-pixel) and integrated functionality. SPAD pixels are relatively large compared with conventional pixels because, in addition to counting photons, processing capability is needed to extract temporal information. Miniaturising the pixels can involve an undesirable compromise between these different modes of operation.
In order to miniaturise SPAD pixels without compromising their capabilities, advanced integrated circuit fabrication technologies, such as 3D-stacking, can be used to better optimise the pixel design. As shown in the figure below, 3D-stacking allows the detector to be decoupled from its electronic circuitry and to be integrated on top, rather than side by side. This way, not only can the pixel size decrease thus increasing the density of pixels, but both the detector and the electronics can be optimised separately during fabrication.