We review a wide range of absorbers based on patterned resistive sheets for use in mid-wave and long-wave infrared microbolometers. These structures range from wavelength selective dielectric coated Salisbury screens to patterned resistive sheets to stacked multi-spectral devices. For basic three color devices in the LWIR band we have designed and fabricated wavelength selective dielectric coated Salisbury screen (DSS) absorbers suitable for use in microbolometers. In order to produce wavelength selective narrowband absorption, the general design rules for DSS microbolometers show that the thickness of the air gap should be a half wavelength and the optical thickness of the dielectric support layer should be a quarter wavelength. This structure is also air gap tunable; i.e., by varying only air gap thickness, the center wavelength of the absorption curve is shifted. FTIR microscope measurements have been made on a number of the different devices demonstrating three color capability in the LWIR while maintain very high efficiency absorption. We have also shown that the use of a patterned resistive sheet consisting of a properly sized array of cross-shaped holes acts as a polarization independent frequency-selective absorber allowing a three-color system spanning the 7-14 micron band. For realistic metal layers the skin effect produces complex surface impedance that can be quite large in the LWIR band. We have shown that metal layers of thickness between one and three skin depths can act as the absorber layer, and have shown that thick metal layers can still produce excellent absorption in the LWIR. Holes in the dielectric support layer also reduce the thermal mass in the system without compromising spectral selectivity. Broadband designs using rectangular holes that produce substantially reduced thermal mass (over 50%) while maintaining efficient spectral absorption have also been found. Finally, we have considered multispectral stacked structures, including Jaumann absorbers and stacked dipole/slot patterned resistive sheets. These structures promise either two band (MWIR/LWIR) or two to three color LWIR in a multi-layer stacked pixel.
This paper describes the microfabrication process and characterization of wavelength selective germanium dielectric
supported microbolometers, which should be compatible with standard microbolometer fabrication processes. Here we
have demonstrated a micro fabricated robust germanium dielectric structure layer that replaces the usual silicon nitride
structural layer in microbolometers. The fabricated microbolometers consist of a chromium resistive sheet as an absorber
layer above an air-gap/germanium dielectric structure.
Past work has discussed infrared absorption using a patterned thin resistive sheet as the frequency-selective absorber for
use in wavelength-selective long wave infrared (LWIR) microbolometer focal planes arrays. These patterned resistive
sheets are essentially slot antennas formed in a lossy resistive ground plane layer placed a quarter-wavelength in front of
a mirror. Design studies have shown that for efficient IR absorption cross-shaped slots require a lossy sheet with the
optimized sheet resistance. For realistic metal layers, however, the skin effect produces a complex surface impedance
that can be quite large in the LWIR band. In this paper we consider metal layers of thickness between one and three skin
depths as the absorber layer instead of a thin resistive sheet layer, and show that the thick metal layers can still produce
excellent absorption in the LWIR.
The use of a cross-shaped patterned resistive sheet as an infrared-selective absorber, including the effects of a SiNx
mechanical support dielectric layer is discussed. These cross 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 comparison
with previously designed patterned resistive sheets that have only a single resistive layer with rectangular patterned
holes, here we consider a resistive absorber layer and a support dielectric layer with cross patterned holes through both
the resistive absorption layer and the support layer.
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.
Multi-color narrow-band Salisbury Screen and Jaumann Absorbers using optimized thickness Si3N4 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.