1. Field of the Invention
The present invention relates to one-port surface acoustic wave resonators having reflectors disposed on both sides of an interdigital electrode transducer and relates to surface acoustic wave filters including the one-port surface acoustic wave resonators. More specifically, the present invention relates to one-port surface acoustic wave resonators and surface acoustic wave filters which include a rotated Y-cut LiTaO3 piezoelectric substrate.
2. Description of the Related Art
A variety of one-port surface acoustic wave resonators including a rotated Y-cut X-propagation LiTaO3 substrate have been proposed for use as bandpass filters for communication devices. A one-port surface acoustic wave resonator includes an interdigital electrode transducer and reflectors disposed on both sides in the surface acoustic wave propagation direction of the interdigital electrode transducer on a LiTaO3 substrate. A surface acoustic wave filter using the one-port surface acoustic wave resonator must have small fluctuations of frequency characteristics.
Japanese Unexamined Patent Application Publication No. 7-283682 (Patent Document 1) discloses that good characteristics of one-port surface acoustic wave resonators using the above-mentioned Y-cut X-propagation LiTaO3 substrate can be obtained by maintaining the ratio (h/λ) of an electrode film thickness (h) to a wavelength (λ) of the surface acoustic wave to the range of 0.06 to 0.10 and by maintaining the metallization ratio of the electrode to 0.6 or less.
Japanese Unexamined Patent Application Publication No. 9-93072 (Patent Document 2) discloses that a yield of ladder-type surface acoustic wave filters having a plurality of one-port surface acoustic wave resonators can be improved by maintaining the metallization ratio of the electrode at 0.6 or more, and preferably, in the range of 0.6 to 0.8.
The ladder-type surface acoustic wave filter having a plurality of one-port surface acoustic wave resonators is commonly used in duplexers as a low-frequency bandpass filter. The ladder-type surface acoustic wave filter of the low-frequency bandpass filter must have steep cut-off characteristics at the blocking band on the higher frequency side of the pass band. Therefore, in order to increase the cut-off steepness, the Q-factor of an antiresonance frequency must be improved in the one-port surface acoustic wave resonators defining a serial arm resonator of a ladder circuit.
Additionally, it is known that the one-port surface acoustic wave resonator is serially connected to a surface acoustic wave filter in order to sufficiently increase the attenuation level at a specific frequency outside of the pass band of the surface acoustic wave filter. Here, a trap is provided at the antiresonance frequency of the one-port surface acoustic wave resonator. With this structure, the Q-factor of the antiresonance frequency of the one-port surface acoustic wave resonator must also be improved.
U.S. Pat. No. 6,556,104 (Patent Document 3) discloses that the Q-factor of the antiresonance frequency can be improved by setting the cut angle of a rotated Y-cut X-propagation LiTaO3 substrate to 460 or more in the one-port surface acoustic wave resonator using the LiTaO3 substrate.
T. Matsuda, J. Tsutsumi, S. Inoue, Y. Iwamoto, Y. Satoh, “High-Frequency SAW Duplexer with Low-Loss and Steep Cut-Off Characteristics” IEEE International Ultrasonics Symposium, Oct. 8-11, 2002 (Non-Patent Document 1) discloses that the Q-factor of the antiresonance frequency is increased by controlling the metallization ratio to be less than 0.4 in the one-port surface acoustic wave resonator using a 36° to 42°-rotated Y-cut X-propagation LiTaO3 substrate.
In a one-port surface acoustic wave resonator using a rotated Y-cut X-propagation LiTaO3 substrate, the dependency of the acoustic velocity on the metallization ratio is the lowest at a metallization ratio of about 0.75. Specifically, when the metallization ratio is about 0.75, the frequency fluctuation caused by a fluctuation in the precision of electrode formation is the lowest. Therefore, as described in Patent Documents 1 and 2, it is recognized that a metallization ratio of 0.6 or more is preferable for decreasing the frequency fluctuation and for improving the yield.
According to Patent Document 3, in the one-port surface acoustic wave resonator using a rotated Y-cut X-propagation LiTaO3 substrate, the Q-factor of the antiresonance frequency can be improved by using a LiTaO3 substrate having a cut angle of 46° or more. However, the Q-factor of the antiresonance frequency sharply deteriorates as the metallization ratio of the electrodes increases, even when the one-port surface acoustic wave resonator including a 46° to 50°-rotated Y-cut LiTaO3 substrate is used.
According to Non-Patent Document 1, an improved Q-factor of the antiresonance frequency is achieved by decreasing the metallization ratio to be 0.4 or less.
Therefore, in the one-port surface acoustic wave resonator using a Y-cut X-propagation LiTaO3 substrate, the metallization ratio of the electrode must be increased to 0.5 or more in order to decrease the frequency fluctuation. On the other hand, a metallization ratio must be decreased to 0.4 or less in order to improve the Q-factor of the antiresonance frequency. Thus, it is very difficult to simultaneously improve the Q-factor of the antiresonance frequency and the frequency fluctuation.
To overcome the problems described above, preferred embodiments of the present invention provide a one-port surface acoustic wave resonator having a Y-cut X-propagation LiTaO3 substrate which simultaneously achieves both an improvement of the Q-factor of the antiresonance frequency and a decrease of the frequency fluctuation, and provide a surface acoustic wave filter including the one-port surface acoustic wave resonator.
The one-port surface acoustic wave resonator according to a preferred embodiment of the present invention includes a rotated Y-cut LiTaO3 substrate, an interdigital electrode transducer provided on the LiTaO3 substrate, and reflectors disposed at both sides of the interdigital electrode transducer in the surface acoustic wave propagation direction of the interdigital electrode transducer. When the electrode finger width of the interdigital electrode transducer is denoted by a and the gap between the electrode fingers is denoted by b, the metallization ratio, a/(a+b), is in the range of about 0.55 to about 0.85 and the interdigital electrode transducer is overlapping-length weighted.
According to another preferred embodiment of the present invention, the above-mentioned LiTaO3 substrate preferably has a cut angle of about 36° to about 60°. In the one-port surface acoustic wave resonator including a rotated Y-cut LiTaO3 substrate, an interdigital electrode transducer provided on the LiTaO3 substrate, and reflectors disposed at both sides of the interdigital electrode transducer in the surface acoustic wave propagation direction of the interdigital electrode transducer, the metallization ratio, a/(a+b), is in the range of about 0.45 to about 0.85 when the electrode finger width of the interdigital electrode transducer is denoted by a and the gap between the electrode fingers is denoted by b, the interdigital electrode transducer is weighted, and the cut angle of the LiTaO3 substrate is in the range of about 40° to about 60°. Additionally, in the one-port surface acoustic wave resonator according to a preferred embodiment of the present invention, the amount of the overlapping-length weighting is about 87.5% or less, preferably about 75% or less.
In the one-port surface acoustic wave resonator according to another preferred embodiment of the present invention, the film thickness of the interdigital electrode transducer is set such that the mass is equivalent to that of an aluminum electrode having a film thickness of about 8% to about 14% of the wavelength of the surface acoustic wave, preferably about 8.5% to about 11.5%, and more preferably about 9% to about 11%.
In the one-port surface acoustic wave resonator according to another preferred embodiment of the present invention, the film thickness of the interdigital electrode transducer is set such that the mass is equivalent to that of a copper electrode having a film thickness of about 2.4% to about 4.2% of the wavelength of the surface acoustic wave.
In the one-port surface acoustic wave resonator according to another preferred embodiment of the present invention, the film thickness of the interdigital electrode transducer is set such that the mass is equivalent to that of a gold electrode having a film thickness of about 1.1% to about 2.0% of the wavelength of the surface acoustic wave.
The surface acoustic wave filter according to another preferred embodiment of the present invention includes the one-port surface acoustic wave resonator according to preferred embodiments of the present invention. Examples of the surface acoustic wave filter include, but are not limited to, a ladder-type surface acoustic wave filter, a lattice-type surface acoustic wave filter, and a surface acoustic wave filter having a one-port surface acoustic wave resonator as a trap.
In the one-port surface acoustic wave resonator according to another preferred embodiment of the present invention, an interdigital electrode transducer and a pair of reflectors are disposed on a rotated Y-cut LiTaO3 substrate and the metallization ratio of the interdigital electrode transducer and the pair of reflectors is in the range of about 0.55 to about 0.85. Consequently, the frequency fluctuation is effectively decreased. Additionally, since the interdigital electrode transducer is overlapping-length weighted, not only is the frequency fluctuation decreased, but the Q-factor of the antiresonance frequency is also effectively increased.
Previously, in a one-port surface acoustic wave resonator, it has been very difficult to simultaneously achieve both a decrease in the frequency fluctuation and an improvement in the Q-factor of the antiresonance frequency. However, according to preferred embodiments of the present invention, the decrease in the frequency fluctuation and the improvement in the Q-factor of the antiresonance frequency are simultaneously achieved by maintaining the metallization ratio of the electrode in the above-mentioned particular range and overlapping-length weighting the interdigital electrode transducer.
Therefore, the cut-off steepness in the filter characteristics from the pass band to the blocking band is increased and the control of the trap using the one-port surface acoustic wave resonator is effectively improved in various surface acoustic wave filters which include the one-port surface acoustic wave resonator according to preferred embodiments of the present invention.
In particular, when the cut angle of the LiTaO3 substrate is in the range of about 36° to about 60°, the Q-factor of the antiresonance frequency is effectively improved. In the one-port surface acoustic wave resonator including a rotated Y-cut LiTaO3 substrate, an interdigital electrode transducer provided on the LiTaO3 substrate, and reflectors disposed at both sides of the interdigital electrode transducer in the surface acoustic wave propagation direction of the interdigital electrode transducer, the frequency fluctuation is effectively decreased by setting the metallization ratio, a/(a+b) to the range of about 0.45 to about 0.85 when the electrode finger width of the interdigital electrode transducer is denoted by a and a gap between the electrode fingers is denoted by b, weighting the interdigital electrode transducer, and also setting the cut angle of the LiTaO3 substrate to the range of about 40° to about 60°. Additionally, since the interdigital electrode transducer is overlapping-length weighted, not only is the frequency fluctuation decreased, but the Q-factor of the antiresonance frequency is also effectively increased.
The Q-factor of the antiresonance frequency is further effectively improved by setting the amount of the overlapping-length weight to about 87.5% or less, and more preferably to about 75% or less.
When the electrode film thickness is set such that the mass is equivalent to that of an aluminum electrode having a film thickness of about 8% to about 14% of the wavelength of the surface acoustic wave, the Q-factor of the antiresonance is further effectively improved.
Similarly, when the electrode film thickness is set such that the mass is equivalent to that of a copper electrode having a film thickness of about 2.4% to about 4.2% of the wavelength of the surface acoustic wave, the Q-factor of the antiresonance frequency is further effectively improved.
Similarly, when the electrode film thickness is set such that the mass is equivalent to that of a gold electrode having a film thickness of about 1.1% to about 2.0% of the wavelength of the surface acoustic wave, the Q-factor of the antiresonance frequency is further effectively improved.
The surface acoustic wave filter according to preferred embodiments of the present invention includes the one-port surface acoustic wave resonator according to preferred embodiments of the present invention. Therefore, the frequency fluctuation is decreased and the Q-factor of the antiresonance frequency of the one-port surface acoustic wave resonator is also improved. Consequently, cut-off steepness of the filter characteristics from the pass band to the blocking band of the surface acoustic wave filter is increased and the trap characteristics is effectively improved by using the one-port surface acoustic wave resonator as the trap.
These and other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.
Preferred embodiments of the present invention will be described with reference to the drawings.
An interdigital electrode transducer 3 is disposed on the LiTaO3 substrate and reflectors 4 and 5 are disposed on both sides of the interdigital electrode transducer 3 in the surface acoustic wave propagation direction of the interdigital electrode transducer 3. The interdigital electrode transducer 3 and the reflectors 4 and 5 are preferably formed by depositing a metal material, such as aluminum or an aluminum-based alloy, on the LiTaO3 substrate and then patterning the metal material. Metal materials other than aluminum and the aluminum-based alloy may also be used as the metal material.
The interdigital electrode transducer 3 includes a plurality of interdigitated electrode fingers 3a. Each of the reflectors 4 and 5 preferably includes a plurality of electrode fingers 4a and 5a, respectively.
In the one-port surface acoustic wave resonator 1 according to this preferred embodiment, the interdigital electrode transducer 3 and the reflectors 4 and 5 have a metallization ratio, a/(a+b), of about 0.55 to about 0.85, and the interdigital electrode transducer 3 is overlapping-length weighted as shown in the drawing. As shown in
In this preferred embodiment, as shown in
The amount of the overlapping-length weighting means the degree of the overlapping-length weighting. For example, when an envelope curve defined by joining the ends of the electrode fingers providing for the overlapping length is linear, as in the interdigital electrode transducer 3 shown in
In the interdigital electrode transducer 3, the envelope curve is a line connecting the ends of a plurality of the electrode fingers which are connected to each other at the same electric potential.
As described above, when the overlapping-length weighting is set such that the envelope curve is linear, the amount of the overlapping-length weighting is represented by (B/A)×100 (%). In preferred embodiments of the present invention, the overlapping-length weighting may be assigned such that the envelope curve has a shape other than a line, such as a sine curve. When the overlapping-length weighting is assigned such that the envelope curve has a shape other than a line, the dimensions of the area at which the overlapping-length weighting is assigned is determined on the basis of the dimensions of an area at which the overlapping-length weighting would be assigned if the envelope curve were linear. Specifically, when the dimensions of the area at which the weighting is assigned such that the envelope curve has a shape other than a line is Y, the dimensions of the area at which the weighting would be assigned if the envelope curve has a line is Y and (B/A)×100 (%) is Z; the amount of the overlapping-length weighting in the case that the envelope curve has the shape other than a line is assigned as Z.
In the one-port surface acoustic wave resonator 1, the interdigital electrode transducer 3 and the reflectors 4 and 5 are formed on the LiTaO3 substrate such that the metallization ratio is in the range of about 0.45 to about 0.85. Therefore, as will be apparent from the examples described below, the antiresonance frequency fluctuation caused by a fluctuation in the electrode precision is effectively decreased.
Additionally, the Q-factor of the antiresonance frequency is significantly improved because the interdigital electrode transducer 3 is overlapping-length weighted. This will be described with reference to the following concrete examples.
Rotated Y-cut X-propagation LiTaO3 substrates were prepared. A normal-type interdigital electrode transducer and a pair of reflectors were formed of aluminum on each of the LiTaO3 substrates at various metallization ratios. Then, resonance frequencies were determined.
One-port surface acoustic wave resonators having various metallization ratios were prepared by the same manner as described above. Resonance frequency fluctuation was determined when the size fluctuation in the width direction of the electrode fingers was about ±0.02 μm.
The frequency fluctuation on the vertical axis in
With reference to
A frequency fluctuation of about 4,000 ppm or less is preferable where a smaller frequency tolerance is required. With reference to
The inventors confirmed that the preferable range of the metallization ratio shown in
A one-port surface acoustic wave resonator included a normal-type interdigital electrode transducer and a pair of reflectors which were prepared as described in Example 1. In this example, the Y-cut X-propagation LiTaO3 substrate had a cut angle of about 46°, the wavelength was about 2 μm, the film thickness of the interdigital electrode transducer and the reflectors was about 10% of the wavelength, the number of electrode finger pairs of the interdigital electrode transducer was 125, the overlapping-length of the electrode fingers was about 32 μm, and the target resonance frequency was about 2 GHz. The metallization ratios of the one-port surface acoustic wave resonators were varied, and Q-factors of the antiresonance frequency were determined. The results are shown in
As shown by the solid line C in
Therefore, in view of the results of Example 1 and Example 2, it is observed that the frequency fluctuation exceeds about 7,000 ppm in the conventional one-port surface acoustic wave resonator, even if the metallization ratio is maintained to about 0.4 in order to obtain a good Q-factor of the antiresonance frequency. This frequency fluctuation is about 14 MHz if the resonance frequency is about 2 GHz. Therefore, this frequency fluctuation is a crucial defect in a device, such as a mobile phone, having a narrow frequency difference of about 20 MHz between a transmission band and a reception band. Additionally, it is also highly desirable in other applications to decrease this large frequency fluctuation.
However, as confirmed in Examples 1 and 2, it has been very difficult to simultaneously achieve both a decrease in the frequency fluctuation and a good Q-factor of the antiresonance frequency.
As described in the above-mentioned Patent Document 3, the Q-factor of the antiresonance frequency can be improved by maintaining the cut angle of the LiTaO3 substrate in the range of about 46° to about 54°. Two types of one-port surface acoustic wave resonators having a metallization ratio of about 0.4 and about 0.6 were prepared using Y-cut LiTaO3 substrates having various cut angles, as in Example 2.
As shown in
As shown in Example 3, the Q-factor of the antiresonance frequency cannot be improved because of the cut angle characteristics even if the metallization ratio is set to about 0.6 for improving the frequency fluctuation.
In the above-mentioned Non-Patent Document 1, the Q-factor of the antiresonance frequency is improved by decreasing the metallization ratio of electrodes. The cause of this improvement is thought to be due to the waveguiding effect. Specifically, the acoustic velocity at the surface acoustic wave propagation portion of the interdigital electrode transducer is sufficiently faster than that at a busbar when the metallization ratio is small. Consequently, the locked-in effect of the interdigital electrode transducer as the waveguide is improved. Thus, leakage of the surface acoustic wave from the busbar to the outside of the resonator is decreased so as to improve the Q-factor of the antiresonance frequency.
Therefore, the inventors believe that the Q-factor may be improved by decreasing the acoustic velocity at the busbar instead of by increasing the acoustic velocity in the interdigital electrode transducer. Therefore, an aluminum film having a thickness of about 1 μm was deposited on only the busbar of each of the surface acoustic wave resonators having electrodes of various metallization ratios as in Example 2.
From the results of Examples 3 and 4, the inventors do not believe that the main causes of the deterioration in the Q-factor of the antiresonance frequency when the metallization ratio is large are leakage components of the surface acoustic wave to the inside of the substrate and leakage components of the surface acoustic wave to the outside from the busbar. Furthermore, it was confirmed that the Q-factor of the antiresonance frequency was not improved by increasing the number of the reflector. Specifically, it is not thought that the cause is the leakage of the surface acoustic wave due to a shortage of reflectors.
The inventors have extensively studied and determined that the Q-factor of the antiresonance frequency can be improved by weighting, in particular, by overlapping-length weighting the interdigital electrode transducer 3.
In Example 5, one-port surface acoustic wave resonators were prepared in the same manner as in Example 2 except that the interdigital electrode transducers were overlapping-length weighted. In this case, various types of the one-port surface acoustic wave resonators were prepared by varying the above-mentioned amounts of overlapping-length weighting. The metallization ratio of the electrodes was about 0.6.
The Q-factors of the antiresonance frequency were determined by changing the metallization ratio of electrodes in the plurality of one-port surface acoustic wave resonators having overlapping-length weighting. The results are shown in the above-mentioned
As shown in
As shown in Example 5, even when the metallization ratio is large, such as in the range of about 0.45 to about 0.85, the Q-factor of the antiresonance frequency is effectively improved by overlapping-length weighting the interdigital electrode transducer. Next, influences of the electrode film thickness on this effect were investigated. In the one-port surface acoustic wave resonators using a 48°-rotated LiTaO3 substrate and including overlapping-length weighting of about 75%, improvement ratios (%) of the Q-factor of the antiresonance frequency were determined by varying the electrode film thickness. The metallization ratio was about 0.5.
As shown in
Therefore, in the present invention, the range of the electrode film thickness is preferably about 8% to about 14% of the wavelength, more preferably about 8.5% to about 11.5%, and most preferably about 9% to about 11%, when the electrode is made of aluminum.
Furthermore, when the electrode is made of a metal material other than aluminum, such as copper or gold, or when the electrode is formed by laminating a plurality of metal materials, similar results are obtained as long as the electrode has a thickness that is equivalent to the mass and the film thickness of the above-mentioned aluminum-electrode film thickness.
Specifically, an aluminum-electrode film thickness of about 8% to about 14% of the wavelength is equivalent to a copper-electrode film thickness of about 2.4% to about 4.2% of the wavelength and is equivalent to a gold-electrode film thickness of about 1.1% to about 2.0% of the wavelength. Similarly, an aluminum-electrode film thickness of about 8.5% to about 11.5% or about 9.0% to about 11.0% of the wavelength is equivalent to a copper-electrode film thickness of about 2.6% to about 3.5% or about 2.7% to about 3.3%, respectively, and is equivalent to a gold-electrode film thickness of about 1.2% to about 1.6% or about 1.3% to about 1.5%, respectively.
In Example 7, increase ratios of the Q-factor of the antiresonance frequency were determined by varying the cut angle of the LiTaO3 substrate. The interdigital electrode transducers were the same as those in Example 6, and the aluminum-electrode film thickness was about 10% of the wavelength of the surface acoustic wave.
As shown in
In the one-port surface acoustic wave resonator according to preferred embodiments of the present invention, the frequency fluctuation and the Q-factor of the antiresonance frequency are simultaneously improved by maintaining the metallization ratio in the range of about 0.45 to about 0.85, and preferably in the range of about 0.55 to about 0.85, and by using an interdigital electrode transducer with overlapping-length weighting. Therefore, the cut-off steepness of the filter characteristics is effectively improved with a providing a surface acoustic wave filter including the one-port surface acoustic wave resonator according to preferred embodiments of the present invention, and the attenuation level of the blocking band of a surface acoustic wave filter is effectively improved by using the one-port surface acoustic wave resonator as a trap. Examples of the one-port surface acoustic wave resonator according to preferred embodiments of the present invention and the surface acoustic wave filter using the one-port surface acoustic wave resonator include, but are not limited to, the surface acoustic wave filters shown in FIGS. 12 to 14.
The surface acoustic wave filter shown in
The surface acoustic wave filter shown in
While the surface acoustic wave device of the present invention has been described with respect to preferred embodiments thereof, it will be apparent to those skilled in the art that the disclosed invention may be modified in numerous ways and may assume many embodiments other than those specifically set out and described above. Accordingly, it is intended by the appended claims to cover all modifications of the invention which fall within the true spirit and scope of the present invention.
Number | Date | Country | Kind |
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2003-202837 | Jul 2003 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP04/08116 | 6/10/2004 | WO | 12/12/2005 |