Microlenses, and especially microlens arrays (MLA), are commonly used as stand-alone optical components, for beam homogenization and shaping. Or integrated as wafer-level optics (WLO), either on top of light sources for beam shaping, or on top of light or image sensors as light concentrators. Many techniques are available to originate the microlens shape: laser ablation, grayscale lithography, two photon absorption, etc. One common way is to pattern photoresist pillars by photolithography and to melt (reflow) them. We report new advances in thermal reflow mastering addressing its intrinsic limitations and expanding the design capabilities of reflow-based MLAs.
Microlenses, and especially microlens arrays (MLA), are commonly used as stand-alone optical components, for beam homogenization and shaping. Or integrated as wafer-level optics (WLO), either on top of light sources for beam shaping (e.g., on micro-LED or vertical-cavity surface-emitting laser – VCSEL), or on top of light or image sensors as light concentrators. In the latter case, each microlens of the MLA, also known in the photography domain as On-Chip Lens (OCL), redirects the light to the active volume of the pixel located underneath. This increases the external quantum efficiency (EQE) by increasing the pixel effective fill-factor, especially for front-illuminated image sensors and their limited fill-factors. We report various MLA optimizations and the concentration factors achieved when addressing challenges encountered with advanced photon detectors such as single-photon avalanche diodes (SPAD) or silicon photon multiplier (SiPM). For example: substrate size and type (wafer, bare or packaged die), optical transmission range from NUV to NIR, microlens geometrical parameter space (diameters from micrometers to millimeters) and stability to temperature, vibrations and irradiation (UV, gamma and proton).
Microlenses replicated on front-illuminated single-photon avalanche diodes (SPAD) or back-illuminated CMOS image sensor are found to be stable to temperature variations, exposure to humidity, mechanical shocks and vibrations, as well as irradiation by gamma rays (for space applications). They highly improve the effective fill-factor, on front-illuminated SPAD-based image sensors, and the parasitic light sensitivity on a back-illuminated CMOS image sensor. Their broad transmission spectrum from NUV to NIR, combined with the wide geometrical space available to fabricate microlenses on various active substrates (wafer or die down to 2×2 mm2), make them suitable to a wide range of quantum photonics applications.
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