Substrate-Independent Broad-Band Immersion Microlens Arrays with a High Coupling Efficiency for Infrared Focal Plane Arrays

Abstract

The sensitivity of infrared (IR) focal plane arrays (FPAs) is often limited by a low pixel fill factor. Solid immersion microlens arrays address this problem by focusing the light reaching each pixel into the most sensitive part of that pixel. This strategy is used in CMOS image sensors but has not been industrially adopted for IR FPAs due to significant difficulties in integration with compound semiconductors. Here, we present an all-in-one solution for producing solid immersion microlens arrays compatible with various IR FPAs regardless of their substrate material. Our strategy is to use refractive lenses made of SiO 2 and Si 3 N 4 with very broad-band and efficient focusing abilities. Notably, our strategy works across a broad range of wavelengths with little performance degradation, meaning it is scalable to various applications. We implemented our method in short-wavelength IR FPAs and demonstrated 7.4 times improvement in quantum efficiency. This is the first demonstration of an immersion microlens array in a non-silicon infrared FPA.

From Material to Cameras: Low‐Dimensional Photodetector Arrays on CMOS

“…For example, we developed a new method of fabricating an array of solid‐immersion micro‐lenses directly on our focal‐plane arrays to improve the fill factor of nano‐photodetector devices. [ 103 ] To provide an accurate evaluation of the effect of the immersion lens array on the response of the detectors, each lensed detector on this array is surrounded by four bare reference detectors in a checkered pattern. These bare detectors provide the means for accurate comparison of the enhancement of device response due to the microlenses (Figure 19c).…”

“…Recently, several groups have demonstrated new approaches in the integration of microlenses as an "immersion lens array" with such imaging sensors. [103,104] These methods are compatible with the realization of low-dimensional image sensors when the deposition of suitable dielectric layers on the low-dimensional material is possible. These approaches employ back-illuminated structures, where photons travel across a dielectric layer.…”

“…c) Checkered design of the micro lens array on a camera allows to quantify the 7.4 response improvement factor enabled by the micro lenses. Reproduced with permission [103]. Copyright 2022, American Chemical Society.…”

Toward sub-micron pixels for short-wave infrared imaging

“…Here, the operability is defined as the percentage of fully functional pixels with respect to all pixels in the FPA. While the fill factor issue can be addressed by using light concentrators as reported in several studies [15][16][17][18][19], the structural issue has not been addressed yet. Therefore, this study will focus mainly on the second challenge.…”