Coupled resonator bulk acoustic wave filter

Details Coupled resonator bulk acoustic wave filter Coupled resonator bulk acoustic wave filter

2001-11-16

2004-04-13

Summons, Barbara (Department: 2817)

Wave transmission lines and networks

Coupling networks

Electromechanical filter

C333S191000, C333S133000, C310S328000, C310S335000

Reexamination Certificate

active

06720844

ABSTRACT:

CROSS-REFERENCES TO RELATED APPLICATIONS
Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of a contract awarded by an agency of the U.S. Government.
REFERENCE TO A “MICROFICHE APPENDIX”
Not Applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention pertains to thin film acoustic devices. More particularly this invention pertains to thin film, bulk wave acoustic resonators for use as filters at microwave frequencies. A thin film, bulk wave acoustic resonator typically utilizes a thin layer of piezoelectric material that is sandwiched between two thin conducting layers of material to form a resonator. The conducting layers serve as electrodes and when an electrical voltage, at a microwave frequency, is applied between the two electrodes, the consequent electric field between the electrodes interacts with the piezoelectric material to generate acoustic waves within the piezoelectric material. In a bulk wave, acoustic resonator, acoustic waves propagate in the direction normal to the thin layers of material and the electrical impedance between the two electrodes exhibits a resonance when the acoustic thickness of the combination of the piezoelectric layer and of the two electrodes is approximately one-half of an acoustic wavelength or an odd multiple thereof. In some instances the acoustic waves are acoustic shear waves and in other instances the acoustic waves are acoustic longitudinal waves.
2. Description of the Prior Art
The fabrication of piezoelectric resonators for use at microwave frequencies is well known in the prior art. See, e.g., the descriptions of such devices in the specification of U.S. Pat. No. 5,894,647 for a “Method for Fabricating Piezoelectric Resonators and Product”, Lakin, and see the references to prior art cited therein. See also “Microwave Acoustic Resonators and Filters,” by Lakin, Kline and McCarron, IEEE Trans. on Microwave Theory and Techniques, Vol. 41, No. 12, December 1993, p. 2139; Guttwein, Ballato and Lubaszek, U.S. Pat. No. 3,694,677; and “Acoustic Bulk Wave Composite Resonators”, Applied Physics Letters 38(3) by Lakin and Wang, Feb. 1, 1981. Such resonators also may be fabricated on, and supported by a substrate by including a set of intervening layers of material having alternating high and low acoustic impedances, which layers have thicknesses of a quarter wavelength. The intervening layers act as an acoustic mirror that acoustically isolates the resonator from the underlying substrate. See, e.g., U.S. Pat. Nos. 3,414,832 and 5,373,268 and 5,821,833 and 6,291,931. For methods of analysis and further descriptions of reflectors and resonators see Lakin, “Solidly Mounted Resonators and Filters, 1995 IEEE Proc. Ultrasonics Symposium, pp. 905-908 and Lakin et al. “Development of Miniature Filters for Wireless Applications”, IEEE Trans. on microwave Theory and Techniques, Vol. 43, No. 12, December 1996, pp. 2933-2939.
As depicted in
FIG. 1
hereof, U.S. Pat. No. 5,821,833, for A Stacked Crystal Filter Device and Method of Making, Lakin, disclosed a bulk acoustic wave, stacked crystal filter
100
, a supporting substrate
112
and an acoustic reflector
113
located between stacked crystal filter
100
and the substrate. Acoustic reflector
113
was made of a sequence of layers
108
,
109
,
110
and
111
of material having alternately high and low acoustic impedance. The stacked crystal filter comprised two layers
102
and
106
of piezoelectric material separated by a conducting electrode layer
103
and bounded on the top and bottom by conducting electrode layers
104
and
107
. The top and middle electrodes provided a signal input port
101
and the middle and bottom electrodes provided a signal output port
121
with the middle electrode layer
103
being connected to signal ground
105
. The stacked crystal filter exhibited high transmission of signals from the input port to the output port for the signal frequency at which the combined thicknesses of the two piezoelectric layers and of the three electrode layers constituted approximately one-half an acoustic wavelength. The stacked crystal filter, by itself, also would have transmitted frequencies which were approximately an integral multiple of said frequency for which the combined thicknesses were approximately an integral multiple of one-half an acoustic wavelength.
The thickness of each layer of material in the reflector was one-quarter of an acoustic wavelength at the frequency at which the stacked acoustic resonator had a thickness of one acoustic wavelength and at this frequency the upper surface of the reflector exhibited a very low impedance that reflected substantially all of the acoustic wave from the resonator incident upon the reflector. As a consequence the reflector facilitated the transmission of signals by the stacked acoustic resonator at that frequency. However, the transmission by the filter of signals at higher frequencies for which the resonator thickness was two, three or more times a half acoustic wavelength, were inhibited because at these higher frequencies the layers of material underlying the stacked crystal filter did not operate as a reflector and did not isolate the acoustic vibrations of the stacked crystal filter from the underlying substrate.
A different example of prior art is depicted in FIG.
2
. In this example a pair of stacked crystal filters
200
and
201
were mounted side by side upon a reflector comprising layers
208
,
209
,
210
and
211
mounted on substrate
212
and connected electrically together in the manner depicted in
FIG. 2
to provide a filter in which the input port
205
to stacked filter
200
and the output port
221
from stacked filter
201
are both located on the upper surface of the device. Electrode
204
and the underlying portions of piezoelectric layers
202
and
206
and the underlying portions of electrodes
203
and
207
comprise the two resonators within stacked filter
200
. Electrode
224
and the underlying portions of piezoelectric layers
202
and
206
and the underlying portions of electrodes
203
and
207
comprise the two resonators within stacked filter
201
. As depicted in
FIG. 2
, electrodes
203
and
207
each constitute parts of both stacked crystal filters and provide direct electrical connections between the two stacked crystal filters. Because there are no intervening layers between the two resonators within each stacked crystal filter (other than the conducting electrode
203
, the degree of the coupling between the two resonators in each stack is fixed and may not be adjusted. As a consequence the range of filter characteristics that may be achieved by this configuration is limited.
U.S. Pat. No. 3,568,108 for a “Thin Film Piezoelectric Filter”, Poirier, disclosed the use of piezoelectric semiconductors for use in resonators. The patent places special importance upon the fact that the resonator in the patented device utilizes piezoelectric layers which are also semiconductors. The patent specification states that it is a characteristic of piezoelectric semiconductor materials that an acoustic wave propagating through the material generates a piezoelectric field which interacts and exchanges energy with mobile charge carriers driven through the medium by an external DC electric field and states that when a DC voltage is applied to the medium it creates a direct current [col. 1, ln. 48-57]. The sole independent claim of the patent recites resonators that include an epitaxial film having both piezoelectric and semiconductive properties [col. 4, ln. 25-26]. Accordingly, the '108 patent discloses resonators that utilize piezoelectric that are also semiconductors having semiconductive properties. The '108 patent does not disclose resonators that use piezoelectric materials that are insulators.
The '108 patent disclos

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