Abstract
Snapshot multispectral image (MSI) sensors have been proposed as a key enabler for a plethora of multispectral imaging applications, from diagnostic medical imaging to remote sensing. With each application requiring a different set, and number, of spectral bands, the absence of a scalable, cost-effective manufacturing solution for custom multispectral filter arrays (MSFAs) has prevented widespread MSI adoption. Despite recent nanophotonic-based efforts, such as plasmonic or high-index metasurface arrays, large-area MSFA manufacturing still consists of many-layer dielectric (Fabry–Perot) stacks, requiring separate complex lithography steps for each spectral band and multiple material compositions for each. It is an expensive, cumbersome, and inflexible undertaking, but yields optimal optical performance. Here, we demonstrate a manufacturing process that enables cost-effective wafer-level fabrication of custom MSFAs in a single lithographic step, maintaining high efficiencies (∼75%) and narrow line widths (∼25 nm) across the visible to near-infrared. By merging grayscale (analog) lithography with metal–insulator–metal (MIM) Fabry–Perot cavities, whereby exposure dose controls cavity thickness, we demonstrate simplified fabrication of MSFAs up to N-wavelength bands. The concept is first proven using low-volume electron beam lithography, followed by the demonstration of large-volume UV mask-based photolithography with MSFAs produced at the wafer level. Our framework provides an attractive alternative to conventional MSFA manufacture and metasurface-based spectral filters by reducing both fabrication complexity and cost of these intricate optical devices, while increasing customizability.
Highlights
Complementary metal−oxide−semiconductor (CMOS) image sensors are low cost and compact, implemented in a plethora of applications from digital photography to medical imaging.[1−3] To resolve wavelength-specific information, spatially variant and spectrally distinct color filter arrays (CFAs) are deposited in mosaic-like patterns atop the image sensor with a pitch matched to the pixel size
A potential solution to this is to utilize analog lithographic techniques to control Fabry− Perot (FP) cavity height. Work suggests this is a promising approach, with reflective[35,36] and transmissive[37] MIM pixel arrays recently demonstrated. For practical realization these preliminary approaches are inadequate; they use nonscalable, direct-write electron beam lithography (EBL) over remarkably small lateral areas and provide relatively poor optical performance that falls short of state-of-the-art multispectral filter arrays (MSFAs)
We have demonstrated fabrication of highefficiency, narrowband, highly customizable MSFAs with full pixel coverage, using a simplified single lithographic processing step and demonstrated that it is scalable to wafer-level fabrication for practical application
Summary
Complementary metal−oxide−semiconductor (CMOS) image sensors are low cost and compact, implemented in a plethora of applications from digital photography to medical imaging.[1−3] To resolve wavelength-specific information, spatially variant and spectrally distinct color filter arrays (CFAs) are deposited in mosaic-like patterns atop the image sensor with a pitch matched to the pixel size. Letter spectral filtering have been proposed, including plasmonic arrays,[10,12−21] high-index dielectric metasurfaces,[22,23] diffractive elements,[24] and ultrathin dielectric coatings.[25] Such approaches overcome the need for multiple lithographic steps, but typically present additional problems: inherent polarization sensitivity due to 1D/2D grating unit cells; low transmission efficiencies, either through plasmonic losses or operating with linear polarization states; often infrared rather than visible spectral responses; broad full-width at halfmaximums (fwhm’s), yielding poor wavelength selectivity; many higher-order lattice-based resonances; and require expensive ultrahigh resolution (non-industry standard) lithographic patterning for commercialization. A potential solution to this is to utilize analog lithographic techniques to control FP cavity height Work suggests this is a promising approach, with reflective[35,36] and transmissive[37] MIM pixel arrays recently demonstrated. “subpixel” elements are utilized, with resultant lattice periods and effective fill factors.[35−37] As a result, their transmission efficiency is limited, additional diffractive orders are introduced along with polarization dependency (due to varying in-plane lattice constants), and inevitably ultrahighresolution lithographic patterning (e.g., EBL, deep UV, soft-Xray) is needed
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