High concentration factor diffractive microlenses integrated with CMOS single-photon avalanche diode detector arrays for fill-factor improvement

Abstract

Large-format single-photon avalanche diode (SPAD) arrays often suffer from low fill-factors—the ratio of the active area to the overall pixel area. The detection efficiency of these detector arrays can be vastly increased with the integration of microlens arrays designed to concentrate incident light onto the active areas and may be refractive or diffractive in nature. The ability of diffractive optical elements (DOEs) to efficiently cover a square or rectangular pixel, combined with their capability of working as fast lenses (i.e., 3 ) makes them versatile and practical lens designs for use in sparse photon applications using microscale, large-format detector arrays. Binary-mask-based photolithography was employed to fabricate fast diffractive microlenses for two designs of 32 32 SPAD detector arrays, each design having a different pixel pitch and fill-factor. A spectral characterization of the lenses is performed, as well as analysis of performance under different illumination conditions from wide- to narrow-angle illumination (i.e., 2 to 22 optics). The performance of the microlenses presented exceeds previous designs in terms of both concentration factor (i.e., increase in light collection capability) and lens speed. Concentration factors greater than 33 are achieved for focal lengths in the substrate material as short as 190 , representing a microlens f-number of 3.8 and providing a focal spot diameter of 4 . These results were achieved while retaining an extremely high degree of performance uniformity across the 1024 devices in each case, which demonstrates the significant benefits to be gained by the implementation of DOEs as part of an integrated detector system using SPAD arrays with very small active areas.

Single Photon Avalanche Diode Arrays for Quantum Imaging and Microscopy

“…A good solution to mitigate the problem, while maintaining the same pixel pitch, is employing microlens arrays mounted on top of the detector chip. [ 83 ]…”

“…A good solution to mitigate the problem, while maintaining the same pixel pitch, is employing microlens arrays mounted on top of the detector chip. [83] Microlenses are miniaturized lenses placed over each pixel to collect photons from the entire pixel area and convey them to the SPAD active area. The sensitivity enhancement due to microlenses is measured by means of a parameter known as concentration factor (CF), which is defined as the ratio of light intensity detected with and without microlenses.…”

Simultaneous multi‐spectral, single‐photon fluorescence imaging using a plasmonic colour filter array

“…The relatively small active area of this device presents an obvious disadvantage in terms of overall detection efficiency; however, there are several approaches to increase low fill-factors in SPAD arrays. High concentration-factor microlens arrays are a simple and effective strategy [39,40], while advances in 3D stacking the in-pixel electronics [41,42] is a highly effective long-term solution.…”

High performance InGaAs/InP avalanche photodiode integrated with metal-insulator-metal microcavity

“…In this case, optical lenses are used to compensate for the loss of optical response (Wen et al 2016;Ou et al 2018;Sun et al 2012;Ni et al 2013). However, in order to obtain a smaller optical spot, the traditional optical lens usually requires a larger diameter and a smaller focal length (Piotrowski et al 2003;Li et al 2013;Giuseppe et al 2015;Petter et al 2020). At the same time, due to the separation from the APD, it is a huge challenge to achieve a high coupling e ciency (Giuseppe et al 2015;Petter et al 2020).…”

“…However, in order to obtain a smaller optical spot, the traditional optical lens usually requires a larger diameter and a smaller focal length (Piotrowski et al 2003;Li et al 2013;Giuseppe et al 2015;Petter et al 2020). At the same time, due to the separation from the APD, it is a huge challenge to achieve a high coupling e ciency (Giuseppe et al 2015;Petter et al 2020). In this paper, a metal insulator metal (MIM) optical microcavity is directly integrated on the APD, which is reported in detail in (Wen et al 2017).…”