Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1999-09-15
2002-09-03
Pyo, Kevin (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S207000, C250S338400, C073S774000
Reexamination Certificate
active
06444972
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to apparatus and methods for detecting electromagnetic radiation and relates more specifically to apparatus and methods for detecting electromagnetic radiation using electron photoemission in a micromechanical sensor.
BACKGROUND OF THE INVENTION
Photon detection and imaging has extensive medical, industrial, military, and commercial applications. The detection of infrared (IR) radiation, which is the second most intense radiation source in our environment, is very important for a variety of activities both commercial and military. Detectors that can sense electromagnetic radiation in the mid-infrared region (3 to 5 &mgr;m) and in the far-infrared region (8 to 14 &mgr;m) would allow the detection of unilluminated objects that are at room temperature.
Presently there are several families of available radiation detectors, including a number of various solid state radiation detectors, such as photon detectors and thermal detectors. Currently used photon detectors are based on the principle of converting photon energy to charge carriers (electron holes) that can be detected as current or voltage. Photon detectors can attain detectivities of approximately 10
13
cm Hz
1/2
/W. However, photon detectors which sense a current or voltage in an applied electric field suffer significant drawbacks in the form of “dark current” effects, i.e., the current that flows through a biased semiconductor when no photons are impinging upon it. Because dark current effects are temperature sensitive, the photodetector must be cooled in order to function accurately, or other elaborate measures must be taken to counteract dark current effects. However, this cryogenic operational requirement increases the complexity and cost of such devices.
Thermal detectors, including pyroelectric, thermoelectric, resistive microbolometers, and microcantilever thermal detectors, convert radiation into heat, which is subsequently sensed as changes in the detector temperature. Among the various electromagnetic radiation detectors, the photon detector class has fast response times and high detectivities, D*. Thermal detectors have a very broadband response, since they are based upon thermal conversion of the absorbed energy.
Thus there is a need for a method and apparatus for detecting electromagnetic radiation which does not suffer “dark current” effects and has fast response times and detectivity.
There is a further need for a method and apparatus for detecting electromagnetic radiation which does not require cooling to function accurately.
There is still another need for a method and apparatus for detecting electromagnetic radiation which is not complex or costly.
SUMMARY OF THE INVENTION
Stated generally, the present invention comprises a method and apparatus for detecting electromagnetic radiation which does not suffer “dark current” effects. The method and apparatus for detecting electromagnetic radiation does not require cooling to function accurately and is neither complex nor costly to manufacture. The device provides faster response times than existing thermal detectors. In addition, the device is at least one order of magnitude more sensitive than thermal detectors. The invention comprises a compact, light-weight, highly-sensitive micromechanical photon detector that is based on MEMS (micro-electro-mechanical systems) technology. It relies on the precise measurement of electronic stress due to internal photoemission in metal semiconductor micromechanical quantum detectors (MSMQD). When an MSMQD is exposed to photons with energies above the Schottky barrier, the excess charge carriers generated induce an electronic stress, which causes a silicon microcantilever to deflect. The extent of bending is directly proportional to the radiation intensity.
Stated somewhat more specifically, in a first respect the present invention relates to a micromechanical sensor which comprises a first layer of a semiconductor material having a second layer of metal or a second semiconductor material coated thereon. When exposed to electromagnetic radiation, the material of the second layer emits photoelectrons, which are absorbed by the semiconductor material of the first layer. The excess free charge carriers produce a local mechanical strain in the semiconductor material of the first layer, causing the micromechanical sensor to deflect. This deflection can be measured as a representation of the presence of electromagnetic radiation incident on the second layer.
Thus it is an object of the present invention to provide an improved electromagnetic sensor.
Another object of the present invention is to provide an electromagnetic sensor which does not suffer the effects of “dark current.”
It is another object of the present invention to provide an electromagnetic sensor which is less expensive to produce than existing sensors.
Still another object of the present invention is to provide an electromagnetic sensor which can operate at ambient temperatures and does not require expensive cooling equipment.
It is yet another object of the present invention to provide an electromagnetic sensor which provides faster response times and increased sensitivity relative to existing sensors.
Other objects, features, and advantages of the present invention will become apparent upon reading the following specification, when taken in conjunction with the drawings and the appended claims.
REFERENCES:
patent: 5923421 (1999-07-01), Rajic et al.
patent: 5977544 (1999-11-01), Datskos et al.
patent: 6118124 (2000-09-01), Thundat et al.
patent: WO97/26556 (1997-07-01), None
Datskos, et al., Photoinduced and thermal stress in silicon microcantilevers, Applied Physics Letters, US, American Institute of Physcias, New York, vol. 73, No. 16, pp. 2319-2321, Oct. 19, 1998.
Datskos, et al., Detection of infrared photons using the electronic stress in metal-semiconductor cantilever interfaces, Database Inspec Online!, The Institution of Electrical Engineers, Stevenage, GB, Database accession No. 6542761, Jun. 1, 1999.
P. G. Datskos, P. I. Oden, T. Thundat, E. A. Wachter, R. J. Warmack and S. R. Hunter,App. Phys. Letter,69, 2986, 1996.
Datskos Panagiotis G.
Datskou Irene C.
Egert Charles M.
Rajic Slobodan
Kilpatrick & Stockton LLP
Pyo Kevin
Roberts Larry A.
UT-Battelle LLC
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