Electrical generator or motor structure – Non-dynamoelectric – Charge accumulating
Reexamination Certificate
2000-01-13
2001-06-19
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Charge accumulating
Reexamination Certificate
active
06249073
ABSTRACT:
1. Technical Field
This invention relates to devices including micromechanical resonators and, in particular, to devices including micromechanical resonators having operating frequencies and methods of extending the operating frequencies.
2. Background Art
Vibrating mechanical tank components, such as crystal and SAW resonators, are widely used in current wireless communication sub-systems for frequency selection and reference due to their high quality factor, Q, and extraordinary temperature stability. However, these devices are bulky, normally require precise machining, and, therefore, are expensive. Most importantly, being off-chip components, these mechanical devices must interface with integrated electronics at the board level, and this constitutes an important bottleneck to miniaturization and performance of heterodyning transceivers.
In recent years, micromechanical resonators (abbreviated “&mgr;resonators”) with similar performance to their macro-counterparts have been demonstrated using IC-compatible, polysilicon surface-micromachining technology. With Q's of over 80,000 under vacuum and center frequency temperature coefficients in the range of −10 ppm/° C. (several times less with nulling techniques), polycrystalline silicon &mgr;resonators can serve well as miniaturized substitutes for crystals in a variety of high-Q oscillator and filtering applications. To date, high-Q, folded-beam &mgr;resonators in the frequency range from several to hundreds of kilohertz have been demonstrated. However, for communication applications, higher frequency resonators are required, such as IF filters in the VHF range.
Vibrating beam micromechanical (or “&mgr;mechanical”) resonators constructed in a variety of materials, from polycrystalline silicon to plated-nickel, have recently emerged as potential candidates for use in a variety of frequency-selective communications applications. In particular, provided the needed VHF and UHF frequencies can be attained, both low loss IF and RF filters and high-Q oscillators stand to benefit from the tiny size, virtually zero DC power consumption, and integrability of such devices.
To date, due to the relative ease with which they attain both small mass and high stiffness, clamped-clamped beam &mgr;mechanical resonators have been intensively investigated for VHF range applications. The ability to simultaneously achieve high-Q and high stiffness is paramount for communications-grade resonators, since stiffness directly influences the dynamic range of circuits comprised of such resonators. However, for the case of clamped-clamped beam designs, larger stiffness often comes at the cost of increased anchor dissipation, and thus, lower resonator Q.
U.S. Pat. No. 5,640,133 to MacDonald et al. discloses a capacitance-based tunable micromechanical resonator. The resonator includes a movable beam which holds a plurality of electrodes. The resonator also includes a plurality of stationary electrodes. In operation, an adjustable bias voltage, applied to the beam electrodes and the stationary electrodes, is used to adjust the resonant frequency of the resonator.
U.S. Pat. No. 5,550,516 to Burns et al. discloses an integrated resonant microbeam sensor and transistor oscillator. The sensor and oscillator, capable of providing high-Q values, utilizes various circuitry, electrode placement, and various configurations of microbeam geometry to vary the operating resonant frequency.
U.S. Pat. No. 5,399,232 to Albrecht et al. discloses a microfabricated cantilever stylus with an integrated pyramidal tip. The pyramidal tip, integrally formed on the cantilever arm, limits the movement of the arm in the direction of the tip.
U.S. Pat. No. 4,262,269 to Griffin et al. discloses a Q-enhanced resonator which utilizes resonator positioning to provide a desired performance. Resonators are separated by one-quarter-wavelength distances to obtain desired loss characteristics.
U.S. Pat. No. 4,721,925 to Farace et al. discloses a micromechanical electronic oscillator etched from a silicon wafer. The patent discusses the configuration and the circuitry which enables the oscillator to perform according to desired characteristics.
The following U.S. patents are generally related to this invention: U.S. Pat. Nos. 4,081,769; 4,596,969; 4,660,004; 4,862,122; 5,065,119; 5,191,304; 5,446,729; 5,428,325; 5,025,346; 5,090,254; 5,455,547; 5,491,604; 5,537,083; and 5,589,082.
DISCLOSURE OF INVENTION
An object of the present invention is to provide a device including a micromechanical resonator having a high-Q factor formed on a substrate wherein the device has a frequency range of current commercial transceivers and to further provide a method for extending the operating frequency of the resonator.
Another object of the present invention is to provide a device including a micromechanical resonator having a high-Q factor formed on a substrate wherein the device retains the basic flexural-mode beam design of previous resonators, but strategically altering their supports so that anchors and their associated losses are virtually eliminated from the design; using this approach, free-free beam &mgr;mechanical resonators have been demonstrated with center frequencies from 30 MHz to 90 MHz and above, high stiffness, and Q's as high as 8,400.
Yet another object of the present invention is to provide device including a micromechanical resonator having a high-Q factor formed on a substrate wherein the device: a) is compatible with silicon-based planar IC technology; b) is reduced by orders of magnitude in size compared to SAW and quartz resonators; c) achieves high-Q in wide VHF range; d) has a yield-enhancing design; and (d) has good temperature stability. This deice has potential applications in wireless transceivers (e.g., cellular phones, cordless phones, GPS, etc.) and resonator-based sensors systems.
In carrying out the above objects and other objects of the present invention, a device including a micromechanical resonator having an operating frequency and a resonator beam formed on a substrate is provided. The device includes a non-intrusive support structure anchored to the substrate to support the resonator beam above the substrate. The support structure includes at least one torsional beam dimensioned to correspond to an effective quarter-wavelength of the operating frequency of the resonator. The at least one torsional beam is attached at at least one flexural nodal point of the resonator beam so that the resonator beam sees substantially no resistance to transverse motion. The resonator is a high-Q resonator.
In an embodiment of the present invention, at least one drive electrode is formed on the substrate to allow electrostatic excitation of the resonator beam. The resonator beam and the at least one drive electrode define a capacitive transducer gap therebetween.
At least one spacer having a height extends between the resonator beam and the substrate at the at least one flexural nodal point. The size of the gap is based on the height of the at least one spacer during pull down of the resonator beam.
Preferably, the resonator is a silicon-based resonator, but the resonator may also be nickel or diamond-based.
Further in carrying out the above objects and other objects of the present invention, a high-Q flexural mode micromechanical resonator device is provided. The device includes a substrate and a resonator beam having at least one flexural nodal point. The device also includes at least one torsional beam for supporting the resonator beam at the at least one flexural nodal point and at least one rigid anchor for anchoring the at least one torsional beam to the substrate. The device further includes at least one drive electrode to cause the resonator beam to oscillate upon application of an electrical signal to the at least one drive electrode. The at least one torsional beam is dimensioned so as to effect an impedance transformation that substantially isolates the oscillating resonator beam from the at least one rigid anchor.
Still further in carrying out the above objects
McCorquodale Michael
Nguyen Clark T.-C.
Wang Kun
Brooks & Kushman P.C.
Dougherty Thomas M.
The Regents of the University of Michigan
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