Multi-spectral uncooled infrared sensors fabricated using encapsulated metal-insulator-metal plasmonic metamaterial absorbers

Multispectral uncooled infrared (IR) sensors are attracting interest for use in applications such as gas analysis, hazardous material recognition, and biological analysis. However, conventional sensors require additional filters or a resonant structure in the vertical direction. To address this challenge, we investigated plasmonic metamaterial absorbers (PMAs) such as plasmonic crystals (PCs), metal–insulator–metal (MIM), and mushroom-type PMAs. Such PMAs exhibit wavelength-selective absorption by surface plasmon resonance (SPR, and the absorption wavelength can be controlled by modifying the features of the surface pattern (e.g., the periodicity of dimples and the micropatch size). Previously, we fabricated thermopiles with PC-type PMAs and used them for wavelength-selective detection. Although PC-type PMAs have advantages such as easy fabrication, robustness against structural fluctuations, and a wide operating wavelength range, they suffer from an incidence-angle dependence and require a large absorber volume. Here, we fabricated thermopiles with encapsulated MIM (EMIM)-type PMAs using a complementary metal oxide semiconductor process. The devices consist of a top encapsulated layer, periodic top micropatches, a flat insulator layer, and a bottom reflector. The encapsulated layer functions as a protective layer. Like MIM-type PMAs, the EMIM-type PMAs exhibit incidence-angle independence and a thin absorber volume, where the top micropatch induces localized SPR with the bottom reflector, and its size mainly determines the absorption wavelength. Spectral measurements showed that wavelength-selective IR detection can be performed in the mid-IR wavelength range without any additional filters. We expect the obtained results to contribute to the realization of advanced functional uncooled IR sensors and expand their range of applications.