Electricity: electrical systems and devices – Electrostatic capacitors – Variable
Reexamination Certificate
1999-09-09
2001-04-10
Dinkins, Anthony (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Variable
C361S281000, C361S282000
Reexamination Certificate
active
06215644
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to tunable capacitors and associated fabrication methods and, more particularly, to high frequency tunable capacitors and associated fabrication methods.
BACKGROUND OF THE INVENTION
Microelectromechanical structures (MEMS) and other microengineered devices are presently being developed for a wide variety of applications in view of the size, cost and reliability advantages provided by these devices. For example, one advantageous MEMS device is a variable capacitor in which the interelectrode spacing between a pair of electrodes is controllably varied in order to selectively vary the capacitance between the electrodes. In this regard, conventional MEMS variable capacitors include a pair of electrodes, one of which is typically disposed upon and fixed to the substrate and the other of which is typically carried by a movable actuator or driver. In accordance with MEMS technology, the movable actuator is typically formed by micromachining the substrate such that very small and very precisely defined actuators can be constructed.
While a variable or tunable capacitor can be utilized for many applications, tunable filters frequently utilize variable capacitors in order to appropriately tune the filter to allow or reject signals having predetermined frequencies, while, correspondingly, allowing or rejecting signals having other frequencies. For tunable filters that are utilized for high frequency applications, such as applications involving radio frequency (RF) signals, the tunable filter preferably has low signal loss and a high Q, i.e., a high quality factor. Unfortunately, variable capacitors that include electrodes formed of conventional metals generally do not have a sufficiently high Q for high frequency applications. While electrodes formed of high temperature superconductor (HTS) materials would advantageously increase the Q of the resulting variable capacitor, the use of HTS materials is generally not compatible with the micromachining techniques, such as required to fabricate the actuator of a conventional MEMS variable capacitor. For example, the chemicals, i.e., the etchants, utilized during the micromachining of a substrate would likely damage the superconductor materials by altering their performance characteristics.
As such, MEMS variable capacitors that have improved performance characteristics are desired for many applications. For example, tunable filters having a higher Q so as to be suitable for filtering high frequency signals are desirable, but are currently unavailable.
SUMMARY OF THE INVENTION
A tunable capacitor is therefore provided that is micromachined so as to be precisely defined, extremely small and provide microelectromechanical actuation. In one embodiment the capacitor plates are formed of a high-temperature superconductor material. As such the tunable capacitor can be utilized for a wide variety of high performance applications having a high Q requirement. For example, a tunable filter using a tunable high Q capacitor and inductor can appropriately filter high frequency signals, such as radio frequency (rf) signals.
The tunable capacitor includes a first substrate having at least one first capacitor plate disposed thereon. The first capacitor plate may be formed of a high-temperature superconductor material. Additionally, the tunable capacitor includes a second substrate having a second capacitor plate disposed thereon. The second capacitor plate may be formed of a high-temperature superconductor material. The tunable capacitor also comprises a microelectromechanical (MEMS) actuator that is operably in contact with the second substrate so that when an electrostatic force is applied to the actuator it responds by displacing the second substrate, thereby varying the capacitance between the first capacitance plate and the second capacitance plate. Generally, the substrates may be comprised of a low signal loss material that is compatible with the high-temperature superconductor materials typically used to form the capacitor plates.
In one embodiment of the invention, the MEMS acuator comprises an electrostatic actuator that includes at least one first electrode formed on the surface of the first substrate and at least one cantilever structure that contacts the second substrate and provides for at least one second electrode. The electrodes can be fabricated from a variety of materials. For example, the first electrode may comprise a high-temperature superconductor material and the second electrode may comprise silicon or gold. The cantilever structure may be operably attached to the second substrate or alternatively, the cantilever structure may support, but be detached from, the second substrate. In the embodiment in which the cantilever structure is operably attached to the second substrate, spring-like structures may be patterned in the cantilever structure to facilitate elasticity in the cantilever structure and the second substrate. In the embodiment in which the cantilever structure is detached from the second substrate, spring-like structures may be attached to and connect the first and second substrates so as to facilitate elasticity in the second substrate.
In yet another embodiment of the invention the tunable capacitor includes a first substrate having at least one first capacitor plate disposed thereon. The first capacitor plate may be formed of a high-temperature superconductor material. Additionally, the tunable capacitor includes a second substrate having a second capacitor plate disposed thereon. The second capacitor plate may be formed of a high-temperature superconductor material. The tunable capacitor also comprises a MEMS actuator that is operably in contact with the second substrate so that when thermal actuation is applied the actuator responds by displacing the second substrate, thereby varying the capacitance between the first capacitor plate and the second capacitor plate. Generally, the substrates may be comprised of a low signal loss material that is compatible with the high-temperature superconductor materials typically used to form the capacitor plates.
In one embodiment of the invention, the MEMS acuator comprises a thermal bimorph actuator that includes at least two layers, the first layer disposed on the first substrate and the second layer disposed on the first layer with the second layer also being operably in contact with said second substrate. The layers can be fabricated from a variety of materials. For example, the first layer may comprise silicon and the second layer may comprise gold. Characteristically, the first and second layers will comprise materials that have different coefficients of thermal expansion so that actuation is effected upon changing the temperature of the thermal bimorph. The thermal bimorph structure may be operably attached to the second substrate or alternatively, the thermal bimorph may support, but be detached from, the second substrate. In the embodiment in which the thermal bimorph structure is operably attached to the second substrate, spring-like structures may be patterned in the thermal bimorph to facilitate elasticity in the bimorph structure and the second substrate. In the embodiment in which the thermal bimorph structure is detached from the second substrate, spring-like structures may be attached to and connect the first and second substrates so as to facilitate elasticity in the second substrate.
According to another embodiment the tunable capacitor may be comprised so that the MEMS acuator can serve as either or both an electrostatic actuator and/or a thermal bimorph actuator. In this embodiment the cantilever structure that is operably in contact with the second substrate comprises at least two layers in which the layers comprise materials having differing coefficients of thermal expansion so that the cantilever structure may serve as a thermal actuator. In addition, a first electrode is disposed on the first substrate and a second electrode is formed within a layer of the cantilever structure. This embodiment of t
Dinkins Anthony
JDS Uniphase Inc.
Myers Bigel & Sibley & Sajovec
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