Monolithic Integration of Metalens in Silicon Photomultiplier for Improved Photodetection Efficiency

Abstract

Improving the SiPM performance, such as photon detection efficiency (PDE) and timing resolution, is key to high-accuracy measurements for these applications.A SiPM typically contains trench areas, which are located between the boundaries surrounding SPADs, to shield from optical crosstalk. However, trenches can be photo-insensitive areas. Thus, incident photons impinging the trench areas cause PDE degradation. To solve this problem, microlenses are commonly integrated with SPADs to guide the incident photons toward their photosensitive area. [10,11] The recent trend toward miniaturization of the SPAD has led to the development of a complex photosensitive area because electrode patterns and trench areas must be formed in a limited space. However, the fabrication of complex-shaped microlenses has been challenging. Furthermore, precise alignment between microlenses and small state-of-the-art SPADs has been technically difficult because microlenses are incompatible with the CMOS process.On the other hand, metalenses, [12][13][14][15][16][17][18] such as 2D sub-wavelength structures, are highly flexible in shape and size, and can be designed to focus incident photons on the complex and small photosensitive areas of SPADs. [19,20] In addition, metalenses can be easily integrated with such SPADs without alignment errors owing to their 2D ultrathin structure and suitability for CMOS processes. Based on these advantages, we introduce a monolithic metalens integrated with SPAD rather than using microlenses.Our previous study demonstrated that metalenses can improve the PDE and timing performance of the SiPM by focusing the incident photons toward the photosensitive area of the SPAD while avoiding illumination to the trench area. [21] However, as the metalens and SiPM are spatially separated, it is difficult to fix the relative position (e.g., tilt, distance, and alignment) between the metalens and the SiPM. Thus, a separate configuration is not practical. A metalens should be monolithically integrated with its corresponding SPAD to provide good alignment and stable performance enhancement while maintaining the miniaturization and flatness of the optical device, which benefits practical applications.A 300-µm-thick support glass or protection resin is typically mounted on a SiPM to protect its surface. [22] However, when metalenses are formed on the support glass, the propagation Silicon photomultipliers (SiPMs), consisting of 2D single-photon avalanche diode (SPAD) arrays, have been widely used in many applications. In a SiPM, trench structures surround every SPAD to minimize optical crosstalk; however, these structures are photo-insensitive areas. The previous study revealed that the use of metalenses for focusing incident photons on the photosensitive area of SPADs effectively improves the SiPM performance. However, ensuring good alignment between the metalens and SiPM has been difficult owing to its separated configuration. Herein the authors report the monolithic integration of a metalens with SPAD to provide stable pho...

Monolithic integration metalens in the entire matrix of the silicon photomultipliers for improving the performance of the silicon photomultipliers

“…1(b). HfO2 nanopillars whose period are 250 nm were fabricated as designed using electron beam and damascene lithography without collapsing [13]. Scanning electron microscopic image of the fabricated metalens.…”

“…Figure. To obtain the simulated results, angular spectrum method was conducted based on the phase distribution as shown in figure .3 (a) [13]. As a result, the calculated focusing efficiency was 92.4%.…”

“…However, the performance expected to be fluctuated due to misalignment caused by slight vibration because metalens and SPAD are spatially separated configuration. We thus monolithically integrated metalens with its corresponding SPAD to provide stable performance improvement while keeping the compact and flatness of the optical device [13]. In this study, we investigated the overall performance of the SiPM such as crosstalk and dark-current for further analysis.…”

Metalenses with Polarization‐Insensitive Adaptive Nano‐Antennas

“…While metalenses cannot completely replicate the properties of conventional objective lenses, they can reduce the cost, complexity, and volume of compact and integrated imaging systems. [ 4–18 ] Furthermore, metalenses have already shown equal or superior properties for some select applications, such as optical trapping [ 19–21 ] or narrow‐band wavelength confocal imaging. [ 22–25 ]…”

“…While metalenses cannot completely replicate the properties of conventional objective lenses, they can reduce the cost, complexity, and volume of compact and integrated imaging systems. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Furthermore, metalenses have already shown equal or superior properties for some select applications, such as optical trapping [19][20][21] or narrow-band wavelength confocal imaging. [22][23][24][25] The beauty of this simple design of placing meta-atoms into equally spaced units unfortunately reaches its limit for high numerical aperture (NA) operation, because the discretized phase profile is less and less able to replicate the continuous profile, so the deflection efficiency decreases for high deflection angles.…”