Several designs that could produce significant wavelength selectivity in micromachined microbolometers are reviewed.
These frequency selective surfaces can be achieved using stacks of dielectric coated resistive sheets or by replacing the
normal uniform absorbing sheet used in IR microbolometers with true microbolometers (i.e., bolometers that are much
smaller than the wavelength) combined with an antenna. Here we discuss dielectric coated designs that can substantially
improve the wavelength selectivity of microbolometers.
The use of a patterned resistive sheet acting as an infrared frequency-selective absorber is discussed. These patterned resistive sheets are a modified form of classical Salisbury Screens that utilize a resistive absorber layer placed a quarter-wavelength in front of a mirror. In contrast with previously designed planar antenna-coupled microbolometers that consist of both resistive and highly conductive metal strips (acting as antennas), the absorption layer in these structures involves a single resistive layer with patterned holes.
Although the rapid development of 2-D focal plane arrays of thermal infrared (IR) detectors has led to remarkable progress in uncooled IR imaging technology, a major limitation of these sensors is the lack of true on-chip spectral discrimination. Multi-spectral detection capabilities enable rapid, efficient and multi-dimensional scene interpretation that is especially beneficial to advanced IR imaging systems for early threat warning and target recognition applications. We propose a novel design for a monolithic micromachined array of bolometric detectors capable of multi-spectral
imaging in the long-wave IR (7-14 μm) region. The central ingredient of this approach is to employ planar multi-mode antenna structures to efficiently couple incident electromagnetic radiation to a microbolometeric sensing element that is much smaller than the IR wavelength. The wavelength selectivity of such an antenna-coupled detector can be tuned by optimizing its multiple geometric parameters. We present a planar microbolometer design that can accomplish 3-color LWIR imaging with no moving parts analogous to solid-state color videography in the visible region. The proposed effort targets applications of uncooled color IR imaging where the benefits in space, power, weight and complexity will have a significant impact.
Multi-color narrow-band Salisbury Screen and Jaumann Absorbers using optimized thickness Si<sub>3</sub>N<sub>4</sub> layers are designed that produce wavelength selectivity in 7~14μm wavelength band. The Jaumann Absorbers can be used as a vertically stacked pixel structure to save space and enhance resolution compared to frequency selective Salisbury Screens pixels lying in a common plane.
Multimode microbolometers for wavelength-selective infrared detection have been designed using a Genetic Algorithm and an electromagnetic model of the planar antenna. Wavelength selectivity can be varied by changing the distance to a tuning mirror, or by changing only lithographically drawn parameters, with bandwidth narrower than Fabry-Perot microbolometers. The design of a three color system covering the 7-14 micron band is presented.