Electricity: electrical systems and devices – Electrostatic capacitors – Variable
Reexamination Certificate
1999-12-15
2001-05-08
Dinkins, Anthony (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Variable
C361S277000, C361S298200
Reexamination Certificate
active
06229684
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to variable capacitors and associated fabrication methods and, more particularly, to microelectromechanical system (MEMS) variable capacitors having a high Q 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 on 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 capacitor can be utilized for many applications, tunable filters frequently utilize variable capacitors in order to appropriately tune the filter to pass signals having predetermined frequencies, while 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 a low 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 superconducting materials would advantageously increase the Q of the resulting variable capacitor, the use of superconducting 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 harm the acid and water sensitive superconducting materials. In addition, the elevated temperatures, in the range of 400° C. or greater, required for conventional micromachining will cause damage to the temperature-sensitive superconducting materials.
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 large in size, expensive to fabricate and have limited performance characteristics.
SUMMARY OF THE INVENTION
A variable capacitor is therefore provided that is micromachined so as to be precisely defined and extremely small, while also including electrodes formed of a low electrical resistance material. As such, the variable capacitor can be utilized for a wide variety of high performance applications including use as a tunable filter having a high Q. As such, the tunable filter can appropriately filter high frequency signals, such as radio frequency and microwave signals.
The variable capacitor includes a substrate and at least one substrate electrode formed of a low electrical resistance material, such as a high temperature superconducting (HTS) material or a thick metal layer, that is disposed upon the substrate. The variable capacitor also includes a bimorph member extending outwardly from the substrate and over the at least one substrate electrode. The bimorph member includes first and second layers formed of materials having different coefficients of thermal expansion. The first and second layers of the bimorph member define at least one bimorph electrode such that the establishment of a voltage differential between the substrate electrode and the bimorph electrode moves the bimorph member relative to the substrate electrode, thereby altering the interelectrode spacing. As such, the bimorph member serves as the actuator in order to controllably move the bimorph electrode relative to the fixed substrate electrode.
The variable capacitor can also include a substrate capacitor plate disposed upon the substrate. Preferably, the substrate capacitor plate is also formed of a low electrical resistance material, such as a HTS material or a thick metal layer. The bimorph member also preferably defines a bimorph capacitor plate that moves with the bimorph member in response to the voltage differential between the substrate and bimorph electrodes to thereby correspondingly alter the capacitance between the substrate and bimorph capacitor plates. By therefore selectively establishing a voltage differential between the substrate and bimorph electrodes, the bimorph member can be moved relative to the underlying substrate such that the spacing between the substrate and bimorph capacitor plates and the resulting capacitance established therebetween is selectively altered.
In one embodiment, the bimorph member defines the bimorph electrode and the bimorph capacitor plate as one continuous conductive layer of the bimorph member. As such, the one single conductive layer serves as both the bimorph electrode and the bimorph conductive plate. In another embodiment, the bimorph member defines the bimorph electrode and the bimorph capacitor plate to be discrete components. In particular, the bimorph member preferably defines the bimorph electrode to be in general alignment with the substrate electrode. In addition, the bimorph member of this embodiment preferably defines the bimorph capacitor plate to be spaced from the bimorph electrode and disposed in general alignment with the substrate capacitor plate.
The bimorph member can be fabricated from a variety of materials. For example, the first layer of the bimorph member can include a metal, such as gold, and the second layer of the bimorph member can include a metal, such as aluminum. Typically, the materials chosen for the layers of the bimorph member will have disparate coefficients of thermal expansion to facilitate proper thermal actuation of the bimorph member. Alternatively, the first layer of the bimorph member can include a dielectric material, such as silicon nitride, silicon oxide or a suitable polymer and the second layer of the bimorph member can include a metal, such as gold. The materials that form the first and second layers are preferably selected such that the bimorph member curls away from the substrate at a predetermined operating temperature in the absence of an applied voltage as a result of the different coefficients of thermal expansion of the materials that form the first and second layers of the bimorph member. As such, by appropriately applying voltages to the substrate and bimorph electrodes, the bimorph member can therefore be at least partially uncurled in order to control the spacing between the substrate capacitor plate and the bimorph capacitor plate.
A method of fabricating a variable capacitor is also provided according to another aspect of the present invention. In this regard, a low electrical resistance material, such as a HTS material, is initially deposited upon the substrate to define at least one substrate electrode and, more preferably, both the substrate electrode(s) and the substrate capacitor plate. Optionally, a dielectric layer may be disposed on the at least one substrate electrode and substrate capacitor plate to provide electrical insulation, as needed. Thereafter, a sacrificial layer, preferably formed of low temperature oxide, metal, or photoresist, is deposited over the substrate electrode, the substrate capacitor plate and the optional dielectric layer. A bimorph member that includes a bimorph electrode and, more preferably, both a bimorph electrode and a bimorph capacitor plate is then formed on at least a portion of the sacrificial layer and within a window through the sacrificial layer through which the underlying sub
Cowen Allen Bruce
Dhuler Vijayakumar Rudrappa
Hill Edward Arthur
Koester David Alan
Mahadevan Ramaswamy
Dinkins Anthony
JDS Uniphase Inc.
Myers Bigel & Sibley & Sajovec
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