Grayscale-to-Color: Scalable Fabrication of Custom Multispectral Filter Arrays

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.

Scalable and High-Throughput Top-Down Manufacturing of Optical Metasurfaces

“…Mask-based photolithography can selectively control the size of each array using a spatially variant photomask. Spatially different 3D metal-insulator-metal cavity arrays could be realized by employing two grayscale lithography techniques ( Figure 2 a,b) [ 69 ]. Transmissive multispectral filter arrays (MSFAs) with low volumes generated through grayscale electron beam lithography (EBL) have been realized as proof-of-concept, and finally grayscale UV mask-based photolithography can be used to fabricate large-volume MSFAs on a 3 inch wafer scale in a single step.…”

“…(a) Grayscale photolithography process of a multispectral filter array (MSFA)[69]. (i) Calculated profile of grayscale exposure dose, (ii) profile of a resist after exposure, (iii) the spatially variant three-dimensional (3D) resist profile after development and metal deposition, (iv) calculated transmittance spectra, and (v) spatially variant grayscale exposure dose matrix.…”

“…(g) (i) Photograph of a centimeter-scale perfect absorber fabricated by photolithography, (ii) simulated and experimental spectra of the perfect absorber[74]. Reproduced with permission from (a,b)[69]; ACS Publications, 2019, (c-e)[70]; Copyright 2019 American Chemical Society, (f)[71]; De Gruyter, 2012, and (g)[74]; Royal Society of Chemistry, 2015.…”

Engineering Microneedles for Therapy and Diagnosis: A Survey

“…The photomask is made up of flat transparent substrate and a patterned absorber layer. During the UV exposure period, the grayscale distribution of the absorber layer determines the illumination dose of different areas, subsequently resulting in 3D microstructure [78]. Except for application of the photomask, other alternative strategies, including switchable, movable laser source [77], and variable UV intensity [79], are also proposed to achieve 3D structure.…”

Multilevel nanoimprint lithography with a binary mould for plasmonic colour printing

“…In previous studies, a grayscale technique involving electron beam lithography (EBL), [13][14][15] focused ion beams (FIB) milling 16 and photolithography (PL) 17 have been utilised to produce nanostructures with varying vertical dimension (i.e. grayscale) for structural colouration and 3-dimensional plasmonic devices.…”