In-depth analysis of optical crosstalk in single-photon avalanche diode arrays

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.