Infrared gas sensors that could be mass-produced at relatively low cost have been proposed for a variety of applications — for example, detecting carbon monoxide in air inside houses. A sensor of this type would include a housing into which ambient air could diffuse. The gas of interest would be detected via an infrared absorption measurement. The novel micromachined emitter/bolometer structure being developed is a heated strip of lithographically patterned, single-crystal silicon that will function as both an emitter of infrared radiation at a precise wavelength and as a bolometer. The emitted radiation would traverse a path within the sensor housing and would be reflected back along that path to the emitter/bolometer. In the presence of an infrared-absorbing gas, the amount of radiation returning to the emitter/ bolometer would decrease, causing the emitter/bolometer to come to thermal equilibrium at a lower temperature than it would in the absence of such a gas. The temperature-dependent electrical resistance of the emitter/bolometer would be measured and used to infer the concentration of the infrared-absorbing gas.
In addition to the emitter/bolometer, the sensor housing would contain a paraboloidal reflector, a lens, and a mirror that would direct the infrared radiation along the desired optical path beginning and ending at the emitter/bolometer (see figure). To maximize the sensitivity and signal-to-noise ratio of the sensor, the emitter/bolometer surface would be made (as described in more detail below) to emit and absorb maximally over a narrow band of wavelengths rather than over the whole black-body spectrum. Also to maximize sensitivity, the emitter/bolometer would be in the form of a suspended single-crystal silicon resistor with a high temperature coefficient of resistance.
The development of the proposed gas sensors and, in particular, the wavelength-selective emitter/bolometer components of the sensors, would exploit advances in (1) the understanding of the interactions between electromagnetic radiation and textured solid surfaces and (2) the wafer-scale fabrication of semiconductor devices. It has been observed in prior experiments that a surface textured microscopically by ion-beam etching exhibits increased optical absorption over a defined wavelength band, and, when heated, emits preferentially over the same wavelength band. In fabricating the proposed sensors, electron-beam lithography was used to texture the surfaces of the emitter/bolometers for a much more precise and narrow wavelength selectivity. The textures were designed according to the principles of photonic-band-gap (PBG) structures, which are characterized by spatially periodic variations of refractive index that give rise to narrow "forbidden" wavelength transmission bands. This effect was exploited to make two-dimensional PBG structures in high reflectivity (low emissivity) material that preferentially emit infrared light predominantly in the desired, narrow bands when heated.
An additional innovation was to take advantage of the exponential dependence of resistivity of single-crystal silicon on the temperature in the "intrinsic" conduction regime. This effect enhances the signal to- noise ratio of the bolometer considerably and enables the manufacture of a small, low-power, high-sensitivity gas sensor.
This Gas Sensor, shown here at approximately actual size, will contain a wavelength-selective infrared emitter/bolometer and associated optical components. Infrared absorption in the gas of interest would affect the temperature, and thus the electrical resistance, of the emitter/bolometer.
This work was done by Thomas George, Daniel Choi, and Eric Jones of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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Refer to NPO-20931, volume and number of this NASA Tech Briefs issue, and the page number.
This Brief includes a Technical Support Package (TSP).
Micromachined Emitter/Bolometer Structures for Infrared Gas Sensors
(reference NPO-20931) is currently available for download from the TSP library.
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