Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
2001-04-13
2003-07-01
Porta, David (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S351000, C250S350000
Reexamination Certificate
active
06586738
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to microelectromechanical actuator structures, and more particularly to an electromagnetic radiation chopper device used in conjunction with an associated electromagnetic radiation detector.
BACKGROUND OF THE INVENTION
Advances in thin film technology have enabled the development of sophisticated integrated circuits. This advanced semiconductor technology has also been leveraged to create MEMS (Micro Electro Mechanical System) structures. MEMS structures are typically capable of motion or applying force. Many different varieties of MEMS devices have been created, including microsensors, microgears, micromotors, and other microengineered devices. MEMS devices are being developed for a wide variety of applications because they provide the advantages of low cost, high reliability and extremely small size.
Design freedom afforded to engineers of MEMS devices has led to the development of various techniques and structures for providing the force necessary to cause the desired motion within microstructures. For example, microcantilevers have been used to apply rotational mechanical force to rotate micromachined springs and gears. Electromagnetic fields have been used to drive micromotors. Piezoelectric forces have also been successfully been used to controllably move micromachined structures. Controlled thermal expansion of actuators or other MEMS components has been used to create forces for driving microdevices.
Various MEMS devices have been developed that implement electrostatic force to move structures. Traditional electrostatic devices were constructed from laminated films cut from plastic or mylar materials. A flexible electrode was attached to the film, and another electrode was affixed to a base structure. Electrically energizing the respective electrodes created an electrostatic force attracting the electrodes to each other or repelling them from each other. A representative example of these devices is found in U.S. Pat. No. 4,266,339, entitled “Method for Making Rolling Electrode for Electrostatic Device”, issued on May 12, 1981, in the name of inventor Kalt. These type of devices work well for typical motive applications, but these devices cannot be constructed in dimensions suitable for miniaturized integrated circuits, biomedical applications, or MEMS structures.
Micromachined MEMS devices have also utilized electrostatic forces to move microstructures. Some MEMS electrostatic devices use relatively rigid cantilever members, as found in U.S. Pat. No. 5,578,976, entitled Micro Electromechanical RF Switch”, issued on Nov. 26, 1996 in the name of inventor Yao. These types of cantilevered actuators fail to A disclose flexible electrostatic actuators with a radius of curvature oriented away from the substrate surface. Other MEMS devices disclose curved electrostatic actuators; however, some of these devices incorporate complex geometries using relatively difficult microfabrication techniques.
Recent developments have led to simplified MEMS devices that utilize electrostatic forces to move structures. These devices, which are based on flexible membranes that embody electrodes, provide for ease in fabrication and can be processed using conventional MEMS fabrication techniques. See for example, U.S. Pat. No. 6,057,520, entitled “Arc Resistant High Voltage Micromachined Electrostatic Switch”, issued on May 2, 2000, in the name of inventor Goodwin-Johansson. The Goodwin-Johansson '520 patent is herein incorporated by reference as if set forth fully herein. By modifying the biasing capabilities of the flexible film actuator disclosed in the Goodwin-Johansson '520 patent it is possible to fabricate actuators having a radius of curvature such that the actuator will fully curl prior to applying electrostatic voltage and fully uncurl upon the application of electrostatic voltage.
Current electromagnetic radiation imaging devices, typically infrared (IR) imaging devices, such as night vision devices, forward looking infrared devices (FLIRs) and the like, implement mechanical chopper wheels as the means by which radiation signals are pulsed for submission to the detectors/pixels. These chopping mechanisms are necessary for imaging device detectors to modulate or chop the incident electromagnetic radiation. The need for chopping of the signal is especially apparent in pyroelectric detectors since electrical charge is generated in the pyroelectric material by a change in temperature. The change in polarization of the pyroelectric material is defined in terms of the temperature change as:
&Dgr;
P
i
=p
i
&Dgr;T
where &Dgr;P
i
is a change in polarization, p
i
is the pyroelectric coefficient and &Dgr;T is the temperature change that the pyroelectric film detects corresponding to changes in the incident radiation.
Signal chopping is also beneficial for other electromagnetic radiation detector systems, preferably infrared detector systems, such as thermal bolometers that produce a change in resistance with temperature. The resistance change in a thermal bolometer is a direct current effect, versus the pyroelectric detector which is an alternating current effect, so a chopper device is not necessarily required for a bolometer detector. However, for systems needing high sensitivity, signal chopping is needed to periodically modulate the signal to prevent thermal drift and signal noise such that high sensitivities can be achieved.
The typical mechanical chopper wheel that is currently used in such imaging devices tend to be bulky in size (e.g., 1 to 4 inch diameter wheels made of patterned germanium or machined metal), consume significant electrical power and are typically constructed separate from the associated detectors and pixels. In addition, chopper wheels are potentially unreliable and inefficient in modulating the electromagnetic radiation signals. Additionally, since the chopping wheel will typically be responsible for chopping an entire focal plane array of detectors/pixels, if the chopping wheel fails, the entire FPA of detectors is rendered inoperable.
A need exists to develop a chopping device for electromagnetic radiation signal detection that is simple in design and fabrication, consumes less space and electrical power in the detector system, and is more reliable and efficient than current devices. By incorporating MEMS technology, and more specifically electrostatically activated flexible film actuators as chopping elements it is possible to design and fabricate a unitary structure that allows for further reduction in detector/pixel size as advances in the field of IR imaging devices occur. The electrostatic activation of such a device would provide significant size reduction and consume much less power compared with the typical chopping wheel and associated drive motor. Power consumed by the electrostatically activated MEMS chopper is about 2 mW at 100 Hz compared with a chopper wheel motor which consumes several Watts of power.
Additionally, such a device would provide for individual chopping elements (i.e., actuators) to be associated with an individual detector/pixel or, alternatively, a parsed portion of the overall FPA. This would allow the IR FPA to remain operational if only a single chopper element was to fail. In the same regard, it would be possible to close off individual detectors/pixels or small subsets of detectors/pixels could be closed while the remainder of detectors/pixels remain open. In this instance, the closed pixels could then be referenced as the background temperature to subtract out possible noise or temperature fluctuations occurring in the FPA. As such this would provide for a means of noise reduction and compensation for temperature fluctuations in the radiation detector. Current chopping wheel mechanisms are incapable of providing such noise reduction and/or temperature fluctuation compensation. In a similar fashion, if temperature spikes in the array result in “hot spots” (i.e. an area of constant brightness) this area could be closed independent of the remaining detector/pixel
Dausch David E.
McGuire Gary E.
Alston & Bird LLP
Berchem Troy A
MCNC
Porta David
LandOfFree
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