Micromachined tuned-band hot bolometer emitter

Details Micromachined tuned-band hot bolometer emitter Micromachined tuned-band hot bolometer emitter

2001-01-26

2004-06-29

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

Infrared responsive

Reexamination Certificate

active

06756594

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed to an micromachined infrared absorption emitter/sensor for detecting the presence of specific chemical and/or biological species.
BACKGROUND OF THE INVENTION
This invention relates in general to micromachined infrared emitter sensors or a “sensor-on-a-chip” and in particular to micromachined infrared emitter/bolometer sensors for detecting and discriminating the presence of specific biological, chemical, etc. substances comprising a heated bolometer integrated circuit element as a source of infrared emission, a filter for controlling the wavelength of emitted light and a detector of the absorption of the emitted light by a substance interacting with the emitted light.
There is a very serious demand and need for low-cost, mass market gas and chemical sensors, such as, for example, indoor natural gas, radon and carbon monoxide (CO) sensors. In the United States alone nearly 300 people die and thousands are injured from unintentional carbon monoxide poisoning every year. Such mass market sensors must be both hardy and sensitive. For example, CO concentrations of only 50 ppm can produce symptoms of carbon monoxide poisoning over a period of time while CO concentrations of 2000 to 2500 ppm will produce unconsciousness in about 30 minutes and higher CO concentrations can kill. As a comparison, typical gasoline-powered auto exhaust contains anywhere between 300 to 500 ppm concentrations of CO. The need for natural gas sensors, meanwhile, was highlighted most recently in the devastating fires that followed earthquakes in Northridge, Calif. and Kobe, Japan leading to a call for natural gas distribution systems to incorporate sensors in combination with automatic shut-off valves.
Currently the market for small, low-cost CO sensors is served by either catalytic or electrochemical sensors. Catalytic sensors use optical measurements to observe chemical, enzyme or bioengineered coatings that react, very specifically, to a substance of interest such as, for example, carbon monoxide. Despite the sensitivity and specificity of these detectors, inherent limitations reduce their utility in a mass market. For example, the catalytic element on these sensors requires periodic replacement, raising use cost and increasing the likelihood that the sensor will fail as a result of poor maintenance or high levels of contaminants.
Electrochemical sensors measure a change in output voltage of the sensing element due to interaction of the species of interest on the sensing element. While these electrochemical sensors are inexpensive and very sensitive, they are also historically subject to interference and false alarms due to chemical species other than that sought interacting with the sensing element. In addition, these sensors respond slowly and the response is not always reversible. Indeed, exposures to high concentrations of the species of interest can result in a permanent shift of the zero-point requiring a re-calibration of the unit. Furthermore, temperature and humidity changes frequently cause drift and false readings, and outgassing from the plastic and cardboard in which the detectors are packaged can also contaminate the sensors prior to actual sale to the consumer. Moreover, in many of these devices the detector element must be heated, and current consumer models require about 5 watts of continuous power. Although the cost of such usage per annum is low, these sensors' reliance on a steady source of power results in sensor failure if there is a power outage, when the sensor may be needed the most.
Despite these limitations, over 20 million American homes have installed CO monitors utilizing either a catalytic or electrochemical sensor. However, recently, a number of articles have appeared pointing out that a very high percentage of alarms triggered by available CO sensors are false alarms and that a very high percentage of sensors don't set off alarms when appropriate. See, e.g., “Home Alarms for Carbon Monoxide Recalled”,
Washington Post
, Mar. 19, 1999; “ULC Investigation Indicates Failures of Certain Lifesaver and Nighthawk CO Detectors”,
Canada Newswire, www.newswire.ca/releases/
Mar. 19, 1999/ c5815.html; “AmeriGas fined, must give free carbon monoxide detectors,”
Manchester
(N.H.)
Union Leader
, Apr. 9, 1999; “False Alarms”,
Forbes Magazine
, Jan. 13, 1997; “Carbon Monoxide Alarms Recalled”, USA Today, Mar. 19, 1999. The Gas Research Institute estimates that more than 80% of emergency calls triggered by CO sensors are false alarms and as many as 20% of the CO sensors sold in 1999 were recalled as defective.
One avenue of sensor development currently being investigated uses diode lasers for optical detection techniques. While this technique is again highly sensitive and less subject to contamination and false alarms than catalytic or electrochemical sensors, the units presently cost too much for home installation. In addition, because they depend on physical band-gaps, diode lasers can only be tuned with difficulty over a very narrow range. Moreover, there are no uncooled diode lasers and only low efficiency, low output (~5 &mgr;W), expensive (~$450) LEDs available at the wavelengths (~2-6 &mgr;m) for gas sensing.
Another detector technology currently under study utilizes infrared spectroscopy to detect species of interest. Many hazardous and pollutant gases (e.g., volatile organic compounds, carbon dioxide, nitrogen oxides, and sulfur dioxide) have unique infrared absorption signatures in the 2 to 12 &mgr;m region of the infrared. In general, infrared absorption is a function of the wavelength, gas concentration, temperature and pressure such that if the concentration of the species of interest is low enough to be considered dilute, then the absorption is directly proportional to the concentration. In addition, by observing a reference wavelength corrections can be easily made for contaminants, such as, for example, dust. While sensors designed to take advantage of the sensitivity and resolution of infrared spectroscopy are well-known in the art and are frequently used for industrial application, such as, for example, automotive exhaust, refrigerants and glucose monitors, the size and complexity of the infrared sensor unit has precluded their use in the mass-market. Conventional infrared gas and chemical sensors are expensive, high performance units consisting of a cabinet full of discrete components. For example, one type of conventional infrared sensor employs a multi-component design. In this design an infrared light source, usually a blackbody emitter, such as, for example, a Nernst glow bar or tungsten filament modulated by a mechanical chopper, serves as a source of infrared radiation. The radiation is directed through a sample compartment containing the sample gas or liquid to be measured or tested and then the radiation is directed to a separated monochromator and infrared detector and amplifier. The radiation is analyzed as intensity vs. wavelength, either by a spectrometer or by detectors with narrow-band interference filters. Much of the bulk and cost of these conventional infrared instruments is designed to maintain optical alignment in the face of varying ambient conditions and in spite of the expense and effort these instruments frequently require re-calibration and/or realignment.
Recently, photonic band gap structures, such as periodic dielectric arrays, have received much attention as optical and infrared filters with controllable narrow-band infrared absorbance. These photonic structures have been developed as transmission/reflection filters, low-loss light-bending waveguides, and for inhibiting spontaneous emission of light in semiconductors which could lead to zero-threshold diode lasers. In principle these photonic band gap structures operate as follows: electromagnetic waves with wavelength on the order of the period of the dielectric array propagate through this structure, the light interacts in a manner analogous to that for electrons in a periodic symmetric array of atoms. Thus the structure

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