One of the main issues of Single Photon Avalanche Diode arrays is optical crosstalk. Since its intensity increases with reducing the distance between devices, this phenomenon limits the density of integration within arrays. In the past optical crosstalk was ascribed essentially to the light propagating from one detector to another through direct optical paths. Accordingly, reflecting trenches between devices were proposed to prevent it, but they proved to be not completely effective. In this paper we will present experimental evidence that a significant contribution to optical crosstalk comes from light reflected internally off the bottom of the chip, thus being impossible to eliminate it completely by means of trenches. We will also propose an optical model to predict the dependence of crosstalk on the distance between devices.

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One of the main drawbacks of Single Photon Avalanche Diode arrays is the optical crosstalk between adjacent detectors. This phenomenon represents a fundamental limit to the density of arrays, since the crosstalk increases with reducing the distance between adjacent devices. In the past, crosstalk was mainly ascribed to the light propagating from one detector to another through a direct optical path. Accordingly, deep trenches coated with metal were introduced as optical isolation barriers between pixels. This solution, however, was unable to completely prevent the crosstalk. In this paper we present experimental evidence that a significant contribution to crosstalk comes from photons reflected internally at the bottom of the chip. These photons can bypass trenches making them ineffective. We also propose an optical model suitable to predict the dependence of crosstalk on the position within the array

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