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

Abstract

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 (λc) of square and hexagonal Al array are compared with results of rigorous coupled-wave analysis, as a function of the Al pattern period. The λc 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 λc slightly decreases for the hexagonal array. The full-width at halfmaximum (FWHM) of each filter is approximately 200 nm for the λc of 4.4 µm, 400 nm for λc of 3.3 µm.