Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
2001-11-30
2004-12-07
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S339010
Reexamination Certificate
active
06828557
ABSTRACT:
FIELD
This disclosure pertains, inter alia, to radiation-detection devices, such as thermal infrared sensors and the like. The sensors can be of several types. One type encompasses so-called electrical-capacitance radiation detectors that convert incident radiation, such as infrared radiation, to a corresponding physical displacement of two opposing electrodes relative to each other and “read out” the displacement as a corresponding change in electrical capacitance. Another type encompasses so-called optical-readout radiation detectors that convert incident radiation, such as infrared radiation, to a corresponding physical displacement and “read out” the displacement as a corresponding change in readout light.
BACKGROUND
Radiation detectors are useful for a wide variety of contemporary applications in various fields of endeavor. Various types of conventional radiation detectors are sensitive to different wavelengths of incident radiation. For example, many conventional radiation detectors are sensitive to infrared (IR) radiation and are useful for surveillance, security, heat sensing, imaging, and other applications.
An exemplary IR-radiation detector is disclosed in U.S. Pat. No. 5,623,147 to Baert et al. This conventional detector is a so-called “electrical capacitance” type of detector. The detector comprises a substrate, a first bimetallic arm, and a second bimetallic arm. One end of each of the first and second bimetallic arms is mounted to the substrate. To the other end of the first bimetallic arm is affixed a first capacitor electrode. The second bimetallic arm is parallel to and structured similarly to the first bimetallic arm. To the other end of the second bimetallic arm is affixed a second capacitor electrode that faces the first electrode. A radiation-absorbing layer is thermally coupled to the first bimetallic arm but not to the second bimetallic arm. Each of the first and second bimetallic arms includes two layers made of respective materials having different coefficients of thermal expansion. Each layer extends in a respective plane that is parallel to the plane of the radiation-absorbing film. The first and second bimetallic arms (and respective electrodes) are separated from each other by an air gap extending in a “stacking direction” (i.e., a direction normal to the substrate and to the radiation-absorbing layer). Hence, when viewed from a direction normal to the plane of the radiation-absorbing layer (and thus normal to the electrodes of the first and second bimetallic arms), the first and second bimetallic arms overlap each other.
Further regarding the Baert et al. detector, whenever infrared radiation from an object is incident to the radiation-absorbing layer, the radiation is absorbed by the radiation-absorbing layer and converted to heat. The first bimetallic arm deforms (exhibits bending) in response to the heat. The second bimetallic arm exhibits substantially no deformation because essentially no heat absorbed by the radiation-absorbing layer is conducted or otherwise transmitted to the second bimetallic arm. Hence, as the first bimetallic arm deforms from absorption of heat, the gap between the first and second electrodes changes (typically becomes smaller). The magnitude of change of the gap is a function of the quantity of incident infrared radiation absorbed by the radiation-absorbing layer. As the gap changes, the electrical capacitance between the first and second electrodes correspondingly changes.
In the Baert et al. detector, since the second electrode is affixed to the second bimetallic arm (which is structured identically to the first bimetallic arm), whenever the first bimetallic arm deforms from a change in ambient temperature, the second bimetallic arm ideally deforms by the same amount. Thus, ideally, the relative positional relationship between the first and second electrodes is unchanged with a change in ambient temperature. Purportedly, with such a configuration, infrared radiation from an object is detected accurately without the electrical capacitance between the first and second electrodes changing or being affected adversely by changes in ambient temperature. Also, especially with control of the substrate temperature being unaffected by changes in ambient temperature, strict temperature control of the overall detector is unnecessary.
However, actual fabrication and operation experience with the Baert et al. detector reveals that the detector is not without significant problems. For example, in the Baert et al. detector, the first and second bimetallic arms are disposed so as to overlap each other when viewed from a normal direction, as noted above. As a result, during manufacture of the detector, one bimetallic arm must be produced displaced above the substrate and the other bimetallic arm must be produced displaced above the first bimetallic arm. Hence, the respective fabrication steps for forming the bimetallic arms must be performed separately, which substantially increases costs.
More importantly, because the first and second bimetallic arms are produced in separate fabrication steps, it is difficult to establish a desired “baseline gap” (existing gap whenever no infrared radiation is incident on the detector) between the first and second electrodes. It also is difficult to impossible to establish a consistent baseline gap from one pixel to another on the same radiation detector. The two film layers that comprise each bimetallic arm are formed extremely thin to decrease their thermal capacity and to increase responsiveness of the bimetallic arms. But, these conditions render the bimetallic arms susceptible to bending upward or downward relative to the substrate in response to residual internal stress in either film. Stress in a film can result from minute changes in film-forming conditions from one fabrication step to another that are extremely difficult to control to a sufficiently strict degree. Furthermore, because the bimetallic arms are produced in separate fabrication steps, the respective initial deformed conditions of the first and second bimetallic arms are different. Again, this difference makes it difficult to establish a desired positional relationship (e.g., gap) of the first and second electrodes. Hence, the desired sensitivity and dynamic range of infrared detection cannot be obtained on a sufficiently consistent basis to be practical.
Furthermore, the electrical capacitance between the first and second electrodes is inversely proportional to the distance between the first and second electrodes. Hence, the capacitance increases with decreasing gap. Also, the capacitance increases with changes in temperature caused by incident infrared radiation. Hence, the narrower the gap, the greater the sensitivity with which infrared radiation can be detected. But, if the electrodes were to touch each other, then any changes that may further increase the capacitance between the electrodes would not occur, thereby restricting the dynamic range.
It is preferable that the gap between the electrodes be as narrow as possible without allowing the electrodes to touch each other. But, due to the difficulty in setting the gap to a desired dimension, as discussed above, the gap usually is made larger than desired or it is accepted that the electrodes are vulnerable to touching each other, thereby decreasing detection sensitivity and undesirably limiting the dynamic range of the detector.
Another difficulty with manufacturing the first and second bimetallic arms in separate fabrication steps, as taught by Baert et al., is the difficulty in sufficiently suppressing changes in electrical capacitance between the electrodes due to changes in ambient temperature. Namely, in fabricating the first and second bimetallic arms by conventional methods, the characteristics of the films (e.g., film thickness) comprising the bimetallic arms cannot be maintained completely identical in both bimetallic arms. Since the characteristics of arm deformation caused by changes in temperature depend upon the film characteristics, the respective deformatio
Akagawa Keiichi
Ishizuya Tohru
Suzuki Junji
Gabor Otilia
Hannaher Constantine
Klarquist & Sparkman, LLP
Nikon Corporation
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