Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
1999-11-01
2001-10-23
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S191000, C333S201000, C029S025350, C310S312000, C310S364000
Reexamination Certificate
active
06307447
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electrical filters employing a mechanical transducer and more particularly to a method for fine tuning such filters following batch fabrication of the filters.
2. Description of Related Art
The need to reduce the cost and size of electronic equipment has led to a continuing need for ever smaller filter elements. Consumer electronics such as cellular telephones and miniature radios place severe limitations on both the size and cost of the components contained therein. Many such devices utilize filters that must be tuned to precise frequencies. Hence, there has been a continuing effort to provide inexpensive, compact filter units.
One class of filter element that meets these needs is constructed from mechanical resonators such as acoustic resonators. These devices use acoustic waves, bulk longitudinal waves for example, in thin film material, typically but not exclusively piezoelectric (PZ) material. In one simple configuration, a layer of PZ material is sandwiched between two metal electrodes. The resonator may be suspended in air, supported along its rim, or may be placed on an acoustic mirror comprised of a plurality of alternating layers of high and low acoustic impedance (the product of speed and density), usually silicon dioxide and aluminum nitride. When an electric field is applied between the two electrodes via an impressed voltage, the PZ material converts some of the electrical energy into mechanical energy in the form of sound waves. For certain crystal orientations, such as having the c axis parallel to the thickness of an Aluminum Nitride film, the sound waves propagate in the same direction as the electric field and reflect off of the electrode/air or electrode/mirror interface.
At a certain frequency which is a function of the resonator thickness the forward and returning waves add constructively to produce a mechanical resonance and because of the coupling between mechanical strain and charge produced at the surface of a piezoelectric material, the device behaves as an electronic resonator; hence, such devices combined in known architectures can act as a filter. The fundamental mechanical resonant frequency is that for which the half wavelength of the sound waves propagating in the device is equal to the total thickness of the piezoelectric plus electrode layers. Since the velocity of sound is many orders of magnitude smaller than the velocity of light, the resulting resonator can be more compact than dielectric cavity resonators. Resonators for 50 Ohm matched applications in the GHz range may be constructed with physical dimensions approximately 100 micrometers in diameter and few micrometers in thickness.
Combinations of such resonators may be used to produce complex filters for band pass applications as disclosed inter alia in U.S. Pat. No. 5,910,756 issued to Ella. This patent describes the use of multiple acoustic resonators in constructing ladder and T type band pass filters.
The resonant frequency of the resonator is a function of the acoustic path of the resonator. The acoustic path is determined by the distances between the outer surfaces of the electrodes. When batch producing resonators on a substrate, the thickness of the transducing material and the electrodes is fixed at fabrication; hence, the resultant resonance frequency is also fixed. Since there are variations in thickness from device to device resulting from manufacturing tolerances, some method for fine tuning the resonance frequency of each device is needed.
To compensate for this inability to reliably and inexpensively mass produce resonators and therefore filters with the proper resonance characteristics, it is known to intentionally produce resonators having a lesser thickness than the thickness indicated to achieve a desirable resonant frequency, and then deposit excess material on at least one of the electrodes to change the overall thickness of the device and thereby fine tune the device. As this deposition of material may be done while the device is subjected to an input signal and simultaneously tested for resonance this method has produced acceptable results.
This method is not, however without problems as the presence of a mask needed to control the deposition over the desired electrodes creates problems of its own. If the mask, for instance is in contact with the electrode, the mask mass is added to the device mass and alters the resonance characteristics of the device. On the other hand if the mask is not in contact with the device the control of the deposition area suffers. Such masking techniques have been successful with quartz type resonators that are much larger, but have not been as successful with resonators of the order of less than one millimeter.
It has also been proposed to remove material from the device in order to adjust its resonant frequency.
Whether deposition or removal of electrode material is used in fine tuning a resonator, in producing a filter that uses more than one resonator as is the typical case, more than one resonator frequency must be adjusted and that involves a multiplicity of steps wherein each resonator is masked and fine tuned in separate process steps.
There is thus still a need for a process to fine tune more than one resonator to different desired frequencies without need to move and re-mask the resonators.
SUMMARY OF THE INVENTION
The above object is obtained in accordance with this invention by a method for adjusting different resonant frequencies of a plurality of mechanical resonators formed on a common substrate, wherein the resonant frequencies of said resonators are a function of each resonator thickness, the method comprising forming said resonators with an etchable layer comprising a material having different etching properties for each of said resonators having different resonant frequencies and selectively etching said etchable layers to adjust the resonant frequencies of said resonators.
The terms different etching properties and selective etching as used herein mean that the materials used may be etched using an etching process for one that does not effect the other, or that effects the other at a different rate so that one material can be etched for the purpose of this invention while both are exposed to the same etching process without effecting the other, or effecting the other to a degree that does not interfere with the purpose of this invention. Thus selective etching is the process of subjecting two or more materials to an etching process that effects only one of the materials, or that effects one of the materials differently, i.e. at a different rate, than the others.
In somewhat more detail, the method comprises adjusting the resonant frequencies of at least two mechanical resonators comprising a filter, to a first and a second desired frequency. Each of the resonators comprises a mechanical energy transducer between a top and a bottom electrode, and its resonant frequency is a function of the overall resonator thickness. The frequency adjustment comprises the following process:
Forming the top electrode over the transducer material for each of the resonators, having a thickness selected such that the combined bottom electrode thickness, transducer thickness and top electrode thickness is in excess of the required total thickness for each resonator to resonate at a desired frequency.
The top electrode of the first resonator comprises a first material and the top electrode for the second resonator comprises a second material. These materials are selected to have different etching properties, that is each is etched by a different etchant that does not effect the other.
The process next comprises selectively etching the top electrodes of the first and second resonators while applying a signal and monitoring each of the resonator frequencies, and stopping the etching process for each of resonators when the monitored resonant frequency is the desired resonant frequency for each resonator. Thus there is no need to either move or mask the resonato
Barber Bradley Paul
Gammel Peter L.
Wong Yiu-Huen
Agere Systems Guardian Corp.
Pascal Robert
Summons Barbara
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