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.
We present fabrication and characterization of wavelength selective germanium dielectric supported microbolometers
using a self-alignment technique to help insure a flat microbolometer membrane. The fabricated microbolometer consists
of a resistive absorber sheet on a quarter wavelength germanium layer above a half wavelength air gap, producing
dielectric interference [1]. We use a self-aligned process without a polyimide patterning process that helps eliminate
deformation and stress in the structure membrane. We demonstrated that the fabricated wavelength selective
microbolometers have flat, robust membranes and produce excellent tunable narrowband absorption in MWIR/LWIR
band with efficient multi-color IR spectral response using wavelength selective pixels.
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.
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.
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.
This paper describes the fabrication and characteristics of an ionic polymer-metal composite (IPMC) membrane-shaped micro-actuator and its application to the fabrication of a micro-pump. After fabricating two 8mm×8mm IPMC membrane-shaped actuators using a Nafion film, their displacements were measured. The fabricated IPMC membrane-shaped micro-actuators showed displacement of 14~27μ at the applied voltage ranging from 4VP-P to 10VP-P at 0.5Hz. Displacement of the IPMC actuator fabricated with a commercially available Nafion is large enough to make the IPMC actuator a membrane-shaped micro-actuator for fabricating an IPMC micro-pump. IPMC micro-pump was fabricated by assembling IPMC membrane-shaped micro-actuator and PDMS(polydimethylsiloxane) micro-channel together. PDMS micro-channel was designed to have nozzle/diffuser structures which make the fluids flow from inlet to outlet when the IPMC membrane-shaped micro-actuator is deflected up and down by the applied voltages. The measured flow rate of the fabricated IPMC micro-pump was about 9.9μℓ/min at 0.5Hz when the input voltage and duty ratio were 8V P-P and 50%, respectively. The test results illustrate that the fabricated IPMC micro-pump is suitable for pumping fluid through micro-channel on a PDMS substrate. Mechanical performances of beam-shaped and bridge-shaped conductive polymer actuator in aqueous solution and in solid electrolyte have been measured and analyzed. The optimum thickness of polypyrrole for the best bending performance is about 17-19 μm which has been polymerized at the current density of 5.4 μA/mm2 for 120 minutes. For the application of conductive polymer actuator to a micropump, silicon bulk micromachining process has been combined.
A 3D Feed horn shape MEMS antenna has some attractive features for array application, which can be used to improve microbolometer performance. Since MEMS technology have been faced many difficulties to fabrication of 3D feed horn shape MEMS antenna array itself. The purpose of this paper is to propose a new fabrication method to realize a 3D feed horn shape MEMS antenna array using a MRPBI(Mirror Reflected Parallel Beam Illuminator) system with an ultra-slow-rotated and inclined x-y-z stage. A high-aspect-ratio 300 micrometers sidewalls had been fabricated using SU-8 negative photo resist. It can be demonstrated to feasibility of realize 3D feed horn shape MEMS antenna array fabrication. In order to study the effect of this novel technique, the 3D feed horn shape MEMS antenna array had been simulated with HFSS(High Frequency Structure Simulator) tools and then compared with traditional 3D theoretical antenna models. As a result, it seems possible to use a 3D feed horn shape MEMS antenna at the tera hertz band to improve microbolometer performance and optical MEMS device fabrication.
In the paper, we improved the performance of the microbolometer using coupled feed horn antenna. The response time of the device was improved by reducing thermal time constant as the area of the absorption layer was reduced. We designed the shape of an absorption layer as circular structure in order to reduce the coupling loss between the antenna and the bolometer. A supporting leg for thermal isolation also has circular structure and its length increased up to 82micrometers , it reduced the thermal conductance to 4.65x 10-8[W/K]. The directivity of the designed antenna has 20.8dB. So the detectivity of the bolometer was improved to 2.37x 10-9 [cm ROOT(Hz)/W] as the noise characteristics of the bolometer was enhanced by coupling feed horn antenna. The fabrication of the bolometer are carried out by a surface micromachining method that uses a polyimide as a sacrificial layer. The absorption layer material of the bolometer is VOx and its TCR value has above 2%/K. The 3-D feed horn antenna structure can be constructed by using a PMER negative photoresist. The antenna and the bolometer can be bonded by Au-Au flip chip bonding method.
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