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., ) 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 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., to 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 are achieved for focal lengths in the substrate material as short as , representing a microlens f-number of 3.8 and providing a focal spot diameter of . 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.
Highlights
The use of complementary metal–oxide–semiconductor (CMOS) fabrication processes has enabled the manufacture and production of large-format, monolithically integrated single-photon imaging arrays
The vastly reduced coefficient of variation (CV) at all except the lowest illumination f-number ( f /2) in both instances demonstrates the high-quality, uniform performance achievable with diffractive microlenses fabricated in this manner
The largest concentration factor (CF) achieved with the MiSPIA detector array is 19.5, while the maximum CF achieved with the MF32 array is 33.8
Summary
The use of complementary metal–oxide–semiconductor (CMOS) fabrication processes has enabled the manufacture and production of large-format, monolithically integrated single-photon imaging arrays. Direct laser writing may be used [24], a method which enables a smooth progression in height across an etched structure by imposing a gradient to the laser power during exposure as opposed to the stepped structure produced by binary-mask-based photolithography This has the benefit of producing elements of higher diffraction efficiency in only a single stage of lithography, though it requires much more advanced and expensive technology to produce. Diffractive optical elements (DOEs) have a stronger wavelength dependence than their refractive counterparts [25]; the possibility to retain circular symmetry within a rectangular bound allows DOEs to achieve up to 100% coverage This larger achievable fill-factor provides a much greater benefit, especially in sparse photon applications, than would be possible using a reflow-type refractive component. The development of short-wave infrared SPADs, such as InGaAs/InP arrays [30,31] and Ge-on-Si detectors [32,33], justifies the further developments of stand-alone micro-optics, which can be integrated post fabrication
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