Infrared multispectral imaging with silicon-based multiband pass filter and infrared focal plane array

This paper reports the implementation of a silicon-based infrared (IR) multiband pass filter (BPF) on an uncooled microbolometer IR focal plane array (IR-FPA), both of which are fabricated by the standard CMOS process. IR spectroscopy has been widely used as a sample identification technique, exploiting the fact that each molecule structure has a unique spectral feature. Using IR-BPF and IR-FPA, a low-cost and compact IR-spectral imaging system is realized. The microbolometer IR-FPA exhibits broad spectral response sufficient to cover the IR-region from the mid-infrared (3 μm) to far-infrared (8 μm~), which is broader than the coverage of conventional non-silicon-based photodetectors such as mercury cadmium telluride. Single-band images of invisible gases such as ethanol vapor and CO₂ in breath are obtained with the IR-FPA. For multiband imaging, a guided-mode resonance IR filter is fabricated by patterning aluminum (Al) layer of 100 nm thickness on a silicon-on-insulator wafer. Measured peak transmittance wavelengths (λ_c) of square and hexagonal Al array are compared with results of rigorous coupled-wave analysis, as a function of the Al pattern period. The λ_c of 3.3, 3.9, and 4.4 μm are obtained at the pattern period of 1.8, 2.4, and 2.8 μm for the square array. In all cases, the λ_c slightly decreases for the hexagonal array. The full-width at half-maximum (FWHM) of each filter is approximately 200 nm for the λ_c of 4.4 μm, 400 nm for λ_c of 3.3 μm.