Grayscale-to-color: Single-step fabrication of bespoke multispectral filter arrays

TL;DR

This paper introduces a novel single-step lithographic process for fabricating customizable multispectral filter arrays using MIM cavities, enabling compact, efficient, and low-cost multispectral imaging.

Contribution

It presents the first single-step fabrication method for bespoke multispectral filter arrays based on MIM cavities, simplifying production and enhancing customization.

Findings

Achieved high transmission efficiency (~75%)

Demonstrated narrow linewidths (~50 nm)

Enabled multispectral imaging with customized filters

Abstract

Conventional cameras, such as in smartphones, capture wideband red, green and blue (RGB) spectral components, replicating human vision. Multispectral imaging (MSI) captures spatial and spectral information beyond our vision but typically requires bulky optical components and is expensive. Snapshot multispectral image sensors have been proposed as a key enabler for a plethora of MSI applications, from diagnostic medical imaging to remote sensing. To achieve low-cost and compact designs, spatially variant multispectral filter arrays (MSFAs) based on thin-film optical components are deposited atop image sensors. Conventional MSFAs achieve spectral filtering through either multi-layer stacks or pigment, requiring: complex mixtures of materials; additional lithographic steps for each additional wavelength; and large thicknesses to achieve high transmission efficiency. By contrast, we show here for the first time a single-step grayscale lithographic process that enables fabrication of bespoke MSFAs based on the Fabry-Perot resonances of spatially variant metal-insulator-metal (MIM) cavities, where the exposure dose controls insulator (cavity) thickness. We demonstrate customizable MSFAs scalable up to N-wavelength bands spanning the visible and near-infrared with high transmission efficiency (~75%) and narrow linewidths (~50 nm). Using this technique, we achieve multispectral imaging of several spectrally distinct target using our bespoke MIM-MSFAs fitted to a monochrome CMOS image sensor. Our unique framework provides an attractive alternative to conventional MSFA manufacture, by reducing both fabrication complexity and cost of these intricate optical devices, while increasing customizability.