Tech Briefs

These devices can be placed in handheld units, modified cell-phone cameras, rifle sights, and other devices for real-time chemical sensing and target recognition.

Uncooled infrared detectors have significant potential capabilities that have been little explored. Micro-machined uncooled detectors with tunable spectral characteristics across the long-wave infrared (LWIR, λ~8-12 μm) have been developed. In the middle wavelength infrared (MWIR) and LWIR regions, the fabrication of Fabry-Perot filters is more complex because the optical materials must be infrared-compatible and the layer thicknesses must be larger. It is very difficult to build filters for uncooled LWIR thermal detectors, which has limited previous researchers to demonstrations of discretely tunable 2- or 3-color thermal detectors rather than continuously tunable ones.

A conceptual diagram of a tunable absorption filter for LWIR Microbolometers. The primary actuation electrodes are on the support beams and substrate to delay snap-through and enhance tuning range.
The problem originates with the thermal isolation of the uncooled detector itself. Integrating a tunable Fabry-Perot cavity above a detector will create three air gaps, or two air gaps if one places the thermal detector layer inside the cavity. Multiple air gaps mean that there are multiple optical cavities, which may couple together to cause unwanted spectral artifacts and limited tuning. To negate these problems, a single air-gap coupled absorption filter was developed for thermal detectors using a micromachined weakly absorbing top mirror. Instead of filtering in transmission as a standard Fabry-Perot, these devices couple incident radiation into their own top plate, which serves as mirror, absorber, and thermistor all at once.

The devices can be continuously tuned, and a tuning range from 8.7 to 11.1 m with 0-42V of actuation voltage is demonstrated. Additionally, the devices can also be actuated to a broadband mode at 45V where the resonance width is increased to 2.83 μm. This mode is used to enhance its sensitivity in the presence of low-light signals with less spectral information. Experiments have shown that the devices have switching times of

about about 400-700 μsec, making them compatible with focal plane array frame rates.

The concepts apply to all thermal detectors, but in this work, they are demonstrated using a microbolometer. The IR absorbing material is deposited on the top of the upper plate of germanium (Ge). A thin layer of chromium (Cr) is chosen as the absorber because it is convenient for deposition and it has the most desirable optical constants of common metals. A key characteristic of Cr is that it produces a reasonably symmetric resonance with respect to wavelength. The bottom mirror is a modified quarter-wave distributed Bragg reflector (DBR), composed of Ge and zinc sulfide (ZnS) on top of an evaporated gold/chromium (Au/Cr) reflector with reflectivity centered around 10 μm.

The filter works in two modes. One is broadband IR reflection/absorption detection mode that is designed to maximize the thermal light absorbed. In this mode, a 45V actuation voltage pulls the top plate near the bottom mirror, creating a very small gap (<< λ/10). The top mirror itself does not touch the DBR mirror but instead the beam supports make contact so as not to thermally short the device. Although the contact area has not been measured in these devices yet, it is desirable to design the top movable structure such that the contact area is small enough to not affect the thermal performance.

The other mode is a reflection/absorption narrowband detection mode, achieved by using 0-42V actuation voltage to electronically control the air-gap over a distance of 4.3 to 6.4 μm. The position of the absorption resonance is continuously tuned, and a sharper resonance is obtained with the non-zero air gap. This mode can be used to recognize objects with subtle differences in emissivity spectrum, which are difficult to identify using standard bolometers.

This work was done by Joseph T. Talghader of the University of Minnesota-Minneapolis for the Army Research Office. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Physical Sciences category. ARL-0107

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Uncooled Tunable LWIR Microbolometers (reference ARL-0107) is currently available for download from the TSP library.

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