Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
2000-11-15
2003-01-14
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
Reexamination Certificate
active
06507021
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to infrared detectors and associated fabrication methods and, more particularly, to a reference bolometer and an associated fabrication method.
BACKGROUND OF THE INVENTION
Infrared detectors are used in a variety of applications to provide an electrical output which is a useful measure of the incident infrared radiation. For example, quantum detectors are one type of infrared detector that are often used for night vision purposes in a variety of military, industrial and commercial applications. Quantum detectors generally operate at cryogenic temperatures and therefore require a cryogenic cooling apparatus. As a result, quantum detectors that operate at cryogenic temperatures can have a relatively complex design and generally consume significant amounts of energy.
Another type of infrared detector is a thermal detector. Thermal detectors are typically uncooled and therefore generally operate at room temperature. One type of thermal detector that has been developed and is becoming increasingly popular is a microbolometer-based, uncooled focal plane array. A focal plane array generally includes a plurality of imaging pixels, each of which includes a bolometer disposed upon a common substrate. Each bolometer includes a transducer element that has an electrical resistance that varies as a result of temperature changes produced by the incident infrared radiation. By detecting changes in the electrical resistance, a measure of the incident infrared radiation can be obtained. Since the design of a bolometer-based uncooled focal plane array is generally less complex than cryogenically cooled quantum detectors and since these uncooled focal plane arrays generally require significantly less energy than cryogenically cooled quantum detectors, bolometer-based uncooled focal plane arrays are being increasingly utilized.
Each imaging pixel of a conventional uncooled focal plane array has a bolometer that includes an absorber element for absorbing infrared radiation and an associated transducer element having an electrical resistance that varies as its temperature correspondingly varies. Although the absorber and transducer elements can be separate layers of a multilayer structure, the absorber element and transducer element may sometimes be the same physical element. In operation, infrared radiation incident upon the absorber element will heat the absorber element. Since the absorber element and transducer element are in thermal contact, the heating of the absorber element will correspondingly heat the transducer element, thereby causing the electrical resistance of the transducer element to change in a predetermined manner. By measuring the change in electrical resistance of the transducer element, such as by passing a known current through the transducer element, a measure of the incident radiation can be obtained.
In order to provide thermal contact between the absorber and transducer elements while electrically insulating the transducer element from the absorber element, the bolometer also generally includes a thermally conductive, electrically insulating layer disposed between the absorber element and transducer element. In addition, the bolometer typically includes another insulating layer disposed on the surface of the bolometer facing the substrate which serves to structurally support the other layers and to protect the other layers during the fabrication process. See, for example, U.S. Pat. Nos. 5,286,976; 5,288,649 and 5,367,167 which describe the pixel structures of conventional bolometer-based focal plane arrays, the contents of each of which are incorporated herein by reference. However, the absorber and transducer elements can be spaced apart from one another as described in U.S. Pat. No. 6,307,194, the contents of which are also incorporated herein by reference. By spacing the absorber and transducer elements, these elements can be separately optimized even though the absorber and transducer elements remain in thermal contact.
In order to provide thermal contact between the absorber and transducer elements while electrically insulating the transducer element from the absorber element, the bolometer also generally includes a thermally conductive, electrically insulating layer disposed between the absorber element and transducer element. In addition, the bolometer typically includes another insulating layer disposed on the surface of the bolometer facing the substrate which serves to structurally support the other layers and to protect the other layers during the fabrication process. See, for example, U.S. Pat. Nos. 5,286,976; 5,288,649 and 5,367,167 which describe the pixel structures of conventional bolometer-based focal plane arrays, the contents of each of which are incorporated herein by reference. However, the absorber and transducer elements can be spaced apart from one another as described in U.S. patent application Ser. No. 09/326,937, the contents of which are also incorporated herein by reference. By spacing the absorber and transducer elements, these elements can be separately optimized even though the absorber and transducer elements remain in thermal contact.
In order to further improve the performance of conventional pixel structures, each bolometer can include a reflector disposed upon the surface of the substrate underlying the absorber and transducer elements. As such, infrared radiation that is incident upon the bolometer, but that passes through and is not absorbed by the absorber element, will be reflected by the reflector back towards the absorber element. At least a portion of the reflected radiation will therefore be absorbed by the absorber element during its second pass through the absorber element, thereby increasing the percentage of the incident radiation that is absorbed by the absorber element.
In operation, infrared radiation incident upon the imaging pixel will be absorbed by the absorber element of the bolometer and the heat generated by the absorbed radiation will be transferred to the transducer element. As the transducer element heats in response to the absorbed radiation, the electrical resistance of the transducer element will change in a predetermined manner. In order to monitor the change in resistance of the transducer element and, therefore, to indirectly measure the infrared radiation incident upon the bolometer of the imaging pixel, circuitry is generally formed upon the underlying substrate. The circuitry is generally electrically connected to the transducer element via a pair of conductive paths or traces defined by or upon the legs, pillars or the like that support the absorber and transducer elements above the surface of the substrate. By passing a known current through the transducer element, the change in electrical resistance of the transducer element can be measured and a corresponding measure of the incident infrared radiation can be determined.
In addition to the imaging pixels, a bolometer-based focal plane array also generally includes one or more reference pixels. As will be described, a reference pixel is responsive, not to incident radiation, but to changes in ambient temperature and other fluctuations in the operating characteristics of the focal plane array. Based upon the output of a reference pixel, the output of the imaging pixels can be interpreted to distinguish that portion of the output that is attributable to the incident radiation upon the imaging pixel from that portion of the output that is attributable to changes in ambient temperature and other operating conditions, thereby providing a more accurate measurement of the incident radiation.
A reference pixel typically has the same general construction as the imaging pixels described above. As shown in
FIG. 2
, a reference pixel
10
includes a bolometer formed upon the same substrate
12
as the imaging pixels. The bolometer of a reference pixel also includes a transducer element
14
and an absorber element
16
spaced from the substrate by two or more legs, pillars or the like
18
. The bolo
DRS Sensors & Targeting Systems, Inc.
Hannaher Constantine
Norris & McLaughlin & Marcus
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