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.
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 xe2x80x9cMethod for Making Rolling Electrode for Electrostatic Devicexe2x80x9d, 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 Switchxe2x80x9d, 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 xe2x80x9cArc Resistant High Voltage Micromachined Electrostatic Switchxe2x80x9d, 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:
xcex94Pi=pixcex94T
where xcex94Pi is a change in polarization, pi is the pyroelectric coefficient and xcex94T 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 xe2x80x9chot spotsxe2x80x9d (i.e. an area of constant brightness) this area could be closed independent of the remaining detector/pixels to zero out the temperature spike. Localized detectivity could also be controlled by operating subsections of the FPA at a different chopping frequency. Lower chopping frequency could be used for areas of the image requiring higher sensitivity, and higher frequency could be used for faster image scan rates for less sensitive areas. Specific detectivity of IR detectors is known to be dependent on chopping frequency.
As such a need exists to develop an improved electromagnetic radiation chopping device, specifically an electrostatically activated MEMS device that will leverage the simplified MEMS fabrication techniques with the advantages of individual chopper/actuator design. Such a design will additionally provide signal noise reduction, sensitivity modulation, compensation for temperature fluctuations and temperature referencing capabilities.
The present invention provides for an improved electromagnetic radiation detector having an electrostatic chopping device and associated arrays incorporating a plurality of detectors and/or chopping devices. An electrostatically activated MEMS chopping device is provided that provides reliability, efficiency, noise reduction and temperature fluctuation compensation capabilities to the associated electromagnetic radiation detectors.
An electromagnetic radiation detector having an electrostatic MEMS chopping device according to the present invention comprises a detector material element, typically a pyroelectric or bolometer material element, and first and second electrodes in electrical contact with the detector material element and electrically isolated from one another. Additionally, the chopper device will incorporate a flexible film actuator overlying the detector material layer and moveable relative thereto. The flexible film actuator will typically include an electrode element and a biasing element such that the actuator remains in a fully curled, open state absent electrostatic voltage and moves to a fully uncurled, closed state upon the application of electrostatic voltage. Typically, the detector material element will be supported by a support surface, such as a microelectronic substrate or the like.
In one embodiment of the invention the flexible film actuator is fabricated directly on the detector or the pixels of a focal plane array having a fixed portion of the actuator attached to the underlying surface and a distal portion that is released from the underlying surface. The electrode element in the flexible film actuator and one of the electrodes associated with the detector material element interact to provide the electrostatic voltage necessary to move the flexible film actuator.
In an alternate embodiment of the invention a transparent substrate having a transparent electrode element are supported by a support structure, such as a microelectronic substrate and raised above the detector/pixel element. In this embodiment the flexible film actuator is supported by and attached to the transparent substrate. The transparent substrate and the transparent electrode are transparent to the electromagnetic radiation that is being transmitted to the detector, typically IR radiation. This embodiment of the invention reduces the effect of signal interference between the actuator drive voltage and the radiation detector output signal due to increased space between the electrodes in the flexible film actuator and the electrodes in the detector/pixel. Additionally, the transparent substrate and the resulting air gap between the transparent substrate and the detector/pixel provide for additional insulation.
In an alternate embodiment of the present invention, the electromagnetic radiation detector having an electrostatically driven MEMS chopper device is defined by a substrate, a thermal isolation bridge disposed on the substrate, a first electrode layer disposed on the thermal isolation bridge, a detector material layer overlying the thermal isolation bridge and in electrical contact with the first electrode layer and a second electrode layer overlying the thermal isolation bridge, electrically isolated from the first electrode layer and in electrical contact with the detector material layer. Additionally the detector device will comprise a flexible film actuator, serving as the chopper, overlying the detector material layer. The flexible film actuator having a third electrode layer and at least one biasing layer, the actuator having a fixed portion attached to the insulator layer and a distal portion movable with respect to the second electrode layer. The detector device may also include a radiation-absorbing layer and/or an insulator layer overlying the first and second electrode layers and the detector material layer.
In yet another embodiment of the invention the electromagnetic radiation detector having an electrostatically driven MEMS chopper device is defined by a substrate, a thermal isolation bridge disposed on the substrate, a first electrode layer disposed on the thermal isolation bridge, a detector material layer overlying the thermal isolation bridge and in electrical contact with the first electrode layer, and a second electrode layer overlying the thermal isolation bridge, electrically isolated from the first electrode layer and in electrical contact with the detector material layer. Additionally, the chopper device will include an radiation absorbing layer overlying the first and second electrode layers and the detector material layer, a support structure disposed on the substrate, a transparent substrate attached to the support structure, a transparent third electrode layer disposed on the transparent substrate; and a flexible film actuator overlying the transparent third electrode and the detector material layer. The flexible film actuator having a fourth electrode layer and at least one biasing layer, a fixed portion attached to the transparent substrate and a distal portion movable with respect to the third electrode layer.
Alternatively, the invention is embodied in an array of detectors having associated electrostatically activated chopper devices. The array includes a plurality of detector pixels, each detector pixel comprising a detector material layer and a first and second electrode in electrical contact with the detector material layer. In addition, the array includes a plurality of flexible film actuators, each actuator generally overlying the detector material layer of a corresponding detector pixel and moveable relative thereto to alternately allow passage of radiation to the detector material layer and block passage of radiation to the detector material.
A method for pixel modulation in an electromagnetic detector array is also provided for. The method comprises providing for a plurality of electrostatically activated flexible film actuators, each actuator corresponding to one or more detector pixels in an electromagnetic radiation array. Pulsing one or more flexible film actuators to block passage of electromagnetic radiation and to allow passage of electromagnetic radiation to the one or more detector pixels corresponding to the flexible film actuator. In turn, the pulsing of the actuators causes a temperature change in the one or more corresponding detector pixels. The change in temperature results in a change in an electrical property of the one or more detector pixels and, based on the property change, a read-out current is generated.