JP5053616B2 - Surface acoustic wave filter - Google Patents

Description

  The present invention relates to a surface acoustic wave filter using a natural single-phase unidirectional transducer that generates a surface acoustic wave in one direction combined with anisotropy of a piezoelectric substrate.

  As the performance of communication devices increases, the surface acoustic wave filter is required to have low loss and high frequency. In order to meet this requirement, conventionally, a unidirectional converter that achieves a reduction in insertion loss of a surface acoustic wave filter and good phase and frequency characteristics has been used.

  There are several types of unidirectional transducers: asymmetry of interdigital electrode structure, internal reflection due to mass load effect, reflection bank, reflection using floating electrode, piezoelectric Those utilizing the anisotropy of the substrate are known. In the surface acoustic wave filter using these unidirectional transducers, the phase difference between the excitation wave and the reflected wave is the same in the forward direction and is opposite in the opposite direction. I try to have it.

  In particular, Patent Document 1 discloses a natural single-phase unidirectional converter that has a unidirectional property using a normal electrode having an electrode pitch and an electrode width of λ / 4 due to anisotropy of a langasite single crystal substrate. NSPUDT (Natural Single Phase Unidirectional Transducer: hereinafter referred to as a unidirectional transducer) is disclosed.

In general, depending on which angle the substrate surface is set with respect to the single crystal of the piezoelectric substrate and in which direction the surface acoustic wave is propagated, for example, the surface acoustic wave velocity (Vs), the electromechanical coupling coefficient (k ² ) The characteristics of the piezoelectric substrate such as the delay time temperature coefficient (TCD), the anisotropy constant (γ), and the power flow angle (PFA) change.

  The power flow angle is an angle representing the difference between the direction of the phase velocity and the group velocity that propagates when a surface acoustic wave is excited by the interdigital transducer. Since the propagation velocity (sound speed) of propagation of surface acoustic waves is non-uniform in an anisotropic substrate, for example, surface acoustic waves propagating in the direction of high sound velocity are dragged by surface acoustic waves propagating in the direction of low sound velocity. Therefore, a phenomenon occurs in which the phase velocity propagation direction of surface acoustic waves differs from the group velocity propagation direction, which is the energy propagation direction of surface acoustic waves.

  Here, the Langasite single crystal is the same trigonal system as quartz, has an electromechanical coupling coefficient larger than that of quartz (smaller than that of lithium tantalate and lithium niobate), is excellent in frequency temperature stability, and has an interdigital electrode. Surface processing suitable for forming is easy. For example, a Langasite single crystal substrate cut out at a predetermined cut angle has a property (anisotropy constant γ = −1) that a wave does not spread when a surface acoustic wave propagates. Since the flow angle is small and the phase velocity propagation direction and the group velocity propagation direction are easily aligned in the same direction, it has been attracting attention as a substrate for a surface acoustic wave filter.

  Patent Document 2 shows that, for example, a Langasite single crystal substrate cut out by Euler angle display (0 °, 135 °, 28 °) under a condition close to the middle of a specified range can provide favorable results. It is also shown that γ = −1 and the power flow angle is approximately 0 °.

JP 2000-101383 A JP 2000-503189 A FIG. Mitobe et al., “Theoretical Calculation of Equivalent Circuit Constant of Metal Grating SAW Reflector”, IEICE Transactions 86/7, VolJ69-C, No7, pp884-891.

  In addition, with the miniaturization of portable communication devices, it is desired to reduce the loss of surface acoustic wave filters that contribute to circuit miniaturization. As a method for realizing a reduction in insertion loss, the present inventor uses a fan-shaped interdigital electrode finger (tilted interdigital electrode finger) in which the pitch of the electrode finger changes in a direction orthogonal to the propagation direction to implement a low-loss filter. Realized. In addition, the slanted interdigital electrode finger (Slanted Finger IDT: SFIT) is hereinafter referred to as an oblique electrode finger.

  In a surface acoustic wave filter having oblique electrode fingers, a unidirectional electrode is sometimes used as one of techniques for reducing loss.

  The langasite single crystal substrate has natural unidirectionality, and by forming a single electrode finger of 1 / 4λ pitch on the substrate, it is possible to easily transmit (input) a unidirectional surface acoustic wave. . However, since a surface acoustic wave travels only in the same direction with a single electrode finger, the surface acoustic wave cannot be received (output) using the single electrode finger. Therefore, the electrode finger on the receiving side needs to be a double electrode finger or an inverted one-way electrode finger, and surface acoustic wave filters having different electrode structure fingers on the input side and the output side are required.

  Generally, when the electrode finger structure is different, the propagation speed (average sound speed) of the surface acoustic wave changes. The cause of this may be, for example, the energy storage effect shown in Non-Patent Document 1. Due to this energy storage effect, non-radiating bulk waves from the end portions of the electrodes formed on the substrate surface reduce the average velocity of the surface acoustic waves. However, although the change in the average sound speed does not change in the characteristics when the number of electrode finger pairs is about several tens, there is a problem that the characteristics deteriorate when the number of electrode finger pairs is increased to several hundreds.

  Accordingly, an object of the present invention is to provide a surface acoustic wave filter that can increase the number of electrode finger pairs to about several hundred pairs.

This onset Ming retrieves the interdigital input electrode fingers becomes natural single-phase type unidirectional transducer for generating combined surface acoustic wave in one direction and the piezoelectric substrate anisotropically, the surface acoustic wave generated as a signal In a surface acoustic wave filter having interdigital output electrode fingers serving as directional reversal converters, the difference in average sound velocity and average wavelength generated for surface acoustic waves of the same frequency due to differences in input and output electrode finger structures And the interdigital input electrode finger before correction is an oblique electrode finger in which positive electrode fingers and negative electrode fingers are alternately arranged at a pitch of λ / 2. a single electrode fingers forming the a single electrode fingers distance between adjacent electrode fingers is changed to the direction intersecting the direction of propagation of surface acoustic waves, to the pre-correction It is shaped output electrode fingers, a double electrode fingers that form the oblique electrode fingers disposed in interdigital positive electrode fingers and negative electrode fingers alternately at a pitch of each two increments lambda / 2, adjacent electrode fingers Double electrode finger der interval between changes with respect to the direction intersecting the propagation direction of a surface acoustic wave is, the pitch of the positive electrode finger and negative electrode finger of the interdigital input electrode fingers after correction, blind after correction The pitch of each of the positive electrode fingers and the negative electrode fingers of the output electrode fingers is different .

In addition, the present invention provides interdigital input electrode fingers that are natural single-phase unidirectional transducers that generate surface acoustic waves in one direction combined with anisotropy of the piezoelectric substrate, and the generated surface acoustic waves as signals. In a surface acoustic wave filter having a comb-like output electrode finger serving as a directional reversal converter to be extracted, the difference in average sound speed and average wavelength generated for surface acoustic waves of the same frequency due to the difference in input and output electrode finger structure, It is corrected by changing the pitch of at least one of the input and output electrode fingers, and the interdigital input electrode fingers before correction are diagonal electrodes in which positive electrode fingers and negative electrode fingers are alternately arranged at a pitch of λ / 2 a single electrode fingers forming a finger, a single electrode fingers distance between adjacent electrode fingers is changed to the direction intersecting the direction of propagation of surface acoustic waves, bEFORE correction Rusudare shaped output electrode fingers, positive electrode fingers and negative electrode fingers, across the floating electrode extending align each electrode finger and the direction, lambda / 2 in the structure of the oblique electrode fingers arranged alternately with a pitch there are, have a structure in which the interval between adjacent electrode fingers is changed to the direction intersecting the direction of propagation of surface acoustic waves, the pitch of the positive electrode finger and negative electrode finger of the interdigital input electrode fingers after correction The pitch of the positive electrode finger and the negative electrode finger of the interdigital output electrode finger after the correction is different .

Also, the surface acoustic wave filter of the present invention desirably, and metallization ratio of interdigital input electrode fingers before correction, are identical and metallization ratio interdigital output electrode fingers before correction, O The difference between the average sound speed and the average wavelength caused by the surface acoustic wave of the same frequency due to the difference in the electrode finger structure is corrected by changing the metallization ratio of at least one of the input and output electrode fingers . The metallization ratio of the input electrode finger is different from the metallization ratio of the interdigital output electrode finger after correction .

Also, the surface acoustic wave filter of the present invention desirably, the average speed of sound and the difference in mean wavelength resulting surface acoustic wave of the same frequency by the difference of the electrode fingers structure of input and output, at least one electrode of the input and output The film thickness of the interdigital input electrode finger after the correction is made different from the film thickness of the interdigital output electrode finger after the correction .

  By using the present invention, it is possible to solve the problem of the conventional surface acoustic wave filter that the characteristics are deteriorated when the number of electrode finger pairs is increased to several hundred pairs. Even when the number of electrode finger pairs is increased, the characteristics are deteriorated. There is an effect that can be prevented.

  Hereinafter, embodiments of the present invention (hereinafter referred to as embodiments) will be described with reference to the drawings.

  FIG. 1 shows a schematic configuration of a surface acoustic wave filter 10 according to the first embodiment. The surface acoustic wave filter 10 has input side oblique electrode fingers 11 and 12 and output side oblique electrode fingers 13 and 14 on a langasite single crystal substrate 18.

  The input-side oblique electrode fingers 11 and 12 are single electrode fingers in which positive electrode fingers and negative electrode fingers each having a width of λ / 4 are alternately arranged with an edge interval of λ / 4. Further, the output-side oblique electrode fingers 13 and 14 are alternately arranged in an interdigital manner so that the edge interval of each of the positive electrode fingers and the negative electrode fingers each having a width of λ / 8 is λ / 8. It is a double electrode finger which forms the formed oblique electrode finger. The oblique electrode fingers 11 and 13 are input / output positive electrode fingers, and the oblique electrode fingers 12 and 14 are grounded negative electrode fingers.

  Further, as shown in FIG. 1, the input side oblique electrode fingers 11 and 12 and the output side oblique electrode fingers 13 and 14 are such that the center distance between adjacent electrode fingers (11 and 12, 13 and 14) is a surface acoustic wave. The surface acoustic wave excited by the input signal varies in accordance with the frequency of the input signal, and is changed in a direction (Y-axis direction in the figure) orthogonal to the propagation direction (X-axis direction in the figure). (ΔY1, ΔY2,... ΔYS) are propagated.

  FIG. 2 is a filter characteristic diagram showing a horizontal axis frequency and a vertical axis amplitude composed of a plurality of channels parallel to the propagation direction of the surface acoustic wave. In the vertical direction of the figure, filter characteristic diagrams of 1-ch, i-ch,..., S-ch are shown, and in the horizontal direction, the input-side oblique electrode fingers 11 and 12 and the output-side oblique electrodes are shown. The sub-filter obtained by the fingers 13 and 14 and the oblique electrode fingers and the sum of the sub-filters are shown in the lower right SFIT. The characteristics of the surface acoustic wave filter (SFIT) in the present embodiment are the characteristics indicated by the sum of the sub-filters, and the frequencies f1 to fS indicate the passbands of the filter characteristics.

  Next, changes in filter characteristics due to the difference in the number of diagonal electrode finger pairs will be described with reference to FIGS. In the figure, the upper graph shows a large insertion loss scale to show all the characteristics of the filter, and the lower graph enlarges the insertion loss scale. The solid line in the figure shows the case where the average sound velocity of the surface acoustic wave passing through the input side oblique electrode finger is equal to the average sound velocity of the surface acoustic wave passing through the output side oblique electrode finger, and the broken line shows the input side oblique electrode finger. The filter characteristics when the average sound speed of the surface acoustic wave passing through and the average sound speed of the surface acoustic wave passing through the output-side oblique electrode finger are shifted by 0.5% are shown.

  FIG. 3 shows the “small number of electrode finger pairs” filter characteristic in which the number of pairs of input side oblique electrode fingers is 200 and the number of pairs of output side oblique electrode fingers is 180. FIG. 4 shows the “multi-electrode finger pair” filter characteristics when the number of electrode finger pairs is increased, the number of input side oblique electrode fingers is 900, and the number of output side oblique electrode fingers is 700. 3 and 4, the center frequency is 380 MHz, and the characteristics of each filter are shown in Table 1. The relationship between the average sound speed and the center frequency can be obtained by the average wavelength λ = average sound speed (v) / center frequency (f).

  When the dynamic range is 90 dB, the pass frequency bandwidth is ± 8 MHz with respect to the center frequency in the case of the few electrode finger pairs in FIG. 3, but the insertion loss is slightly inferior in the number of multi-electrode finger pairs in FIG. The frequency bandwidth is as narrow as ± 4 MHz, and the shoulder characteristics of the filter are improved.

  Next, considering the influence of the deviation of the average sound speed, the number of small electrode finger pairs in FIG. 3 is in a state where there is almost no difference between the solid line and the broken line in insertion loss (about 0.5 dB), but the number of multi-electrode finger pairs in FIG. Shows a value where the insertion loss is 3.5 dB lower than the solid line due to the deviation of the average sound velocity. Since this deviation occurs even when the power flow angle is corrected, it is considered that this deviation is different from the conventional characteristic deterioration factor.

  Therefore, the present inventor makes the pitch between the electrode fingers constituting the average wavelength of the surface acoustic wave different between the input side oblique electrode finger and the output side oblique electrode finger, and the propagation speed of the surface acoustic wave is 0.25. By adopting a structure that corrects the deviation of 0.75% to 0.75%, the reduction in insertion loss shown in FIG. 4 is prevented. This is because a subtle difference in sound speed of the average wavelength affects the characteristics of the surface acoustic wave filter.

  FIG. 5 shows a schematic configuration of the surface acoustic wave filter 10 according to the second embodiment. The surface acoustic wave filter 10 has input side oblique electrode fingers 11, 12, output side oblique electrode fingers 15, 16, and floating electrodes 17 formed of direction-inverted electrodes on a langasite single crystal substrate 18. ing. Here, both the oblique electrode fingers 12 and 16 are grounded. The relationship between the electrode width and the interval of the direction-reversal electrodes is λ / 10, λ / 10, λ / 5, λ from the left as shown in the output side oblique electrode fingers (15, 16, 17) in FIG. / 10, λ / 10, λ / 10, λ / 5, and λ / 10, and the pitch between the electrode fingers constituting the average wavelength of the surface acoustic wave is changed between the input-side oblique electrode fingers and the output-side oblique electrode fingers. The structure is such that the propagation speed of surface acoustic waves is deviated from 0.25% to 0.75%. In general, the propagation speed of the surface acoustic wave is shifted by about 0.5%.

  As described above, by using this embodiment as described above, it is possible to solve the problem of the conventional surface acoustic wave filter that the characteristics deteriorate when the number of electrode finger pairs is increased to several hundred pairs, and the number of electrode pairs is increased. Even in the case, the deterioration of the characteristics can be prevented. In addition, by changing the metallization ratio or the film thickness (height) of the oblique electrode fingers by etching or laser processing, it is also possible to correct by changing the average sound velocity of the surface acoustic wave between the input side and the output side. . In the case of using laser processing, the material used for the electrode fingers is preferably a material (Cr, Ni, Ta, Fr, Cu, etc.) that has a good absorption rate of laser light and a large heat conduction by the laser light. An ArF maxima laser, a YAG laser, or the like that can obtain a large power in a short time is suitable.

  In the present embodiment, the pitch between the electrode fingers constituting the average wavelength of the surface acoustic wave is changed so that the average sound speed is the same between the input side oblique electrode finger and the output side oblique electrode finger. However, the present invention is not limited to this, and the pitch of the input interdigital transducer or the output interdigital transducer may be fixed and the other pitch may be increased or decreased to correct the average sound velocity, or the thickness of the electrode may be changed. You may respond with. Also, the amount of change is not limited to this value, and an appropriate value may be set for each surface acoustic wave filter.

  Furthermore, in the present embodiment, description has been made using a single electrode finger, a double electrode finger, a direction-inverted electrode finger, etc. using a langasite substrate, but the present invention is not limited to this, and other substrates and apodized electrode fingers are used. Etc. can also be suitably implemented.

1 is a plan view showing a schematic configuration of a surface acoustic wave filter according to a first embodiment of the present invention. In the surface acoustic wave filter which concerns on embodiment of this invention, it is explanatory drawing explaining the filter characteristic comprised by the some channel parallel to the propagation direction of a surface acoustic wave. FIG. 10 is a characteristic diagram showing a change in insertion loss characteristic due to a change in average sound velocity when the number of diagonal electrode finger pairs on the input side is 200 and the number of diagonal electrode finger pairs on the output side is 180. FIG. 10 is a characteristic diagram showing a change in insertion loss characteristic due to a change in average sound velocity when the number of diagonal electrode finger pairs on the input side is 900 and the number of diagonal electrode finger pairs on the output side is 700; It is a top view which shows schematic structure of the surface acoustic wave filter which concerns on 2nd embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 Surface acoustic wave filter, 11, 12 Input side diagonal electrode finger, 13, 14, 15, 16 Output side diagonal electrode finger, 17 Floating electrode, 18 Langasite single crystal substrate.

Claims (4)

Interdigital input electrode fingers that form a natural single-phase unidirectional transducer that generates a surface acoustic wave in one direction combined with the anisotropy of the piezoelectric substrate, and a directional inversion transducer that extracts the generated surface acoustic wave as a signal In a surface acoustic wave filter having interdigital output electrode fingers,
The difference of the average sound velocity and the mean wavelength occurs for a surface acoustic wave of the same frequency by the difference of the electrode fingers structure input and output, changing at least one of the pitch of the electrode fingers of the input and output are corrected by Rukoto,
The interdigital input electrode finger before correction is a single electrode finger forming an oblique electrode finger in which positive electrode fingers and negative electrode fingers are alternately arranged at a pitch of λ / 2, and the interval between adjacent electrode fingers is It is a single electrode finger that changes with respect to the direction intersecting the propagation direction of the surface acoustic wave ,
Interdigital output electrode fingers before correction is a double electrode fingers that form the oblique electrode fingers disposed in interdigital positive electrode fingers and negative electrode fingers alternately at a pitch of each two increments lambda / 2, next Double electrode finger der the distance between the electrode fingers fit changes with respect to the direction intersecting the propagation direction of a surface acoustic wave is,
The pitch of the positive and negative electrode fingers of the interdigital input electrode fingers after correction is different from the pitch of each of the positive and negative electrode fingers of the interdigital output electrode fingers after correction. A characteristic surface acoustic wave filter. Interdigital input electrode fingers that form a natural single-phase unidirectional transducer that generates a surface acoustic wave in one direction combined with the anisotropy of the piezoelectric substrate, and a directional inversion transducer that extracts the generated surface acoustic wave as a signal In a surface acoustic wave filter having interdigital output electrode fingers,
Differences in average sound speed and average wavelength that occur for surface acoustic waves of the same frequency due to differences in input and output electrode finger structures are corrected by changing the pitch of at least one of the input and output electrode fingers,
The interdigital input electrode finger before correction is a single electrode finger forming an oblique electrode finger in which positive electrode fingers and negative electrode fingers are alternately arranged at a pitch of λ / 2, and the interval between adjacent electrode fingers is It is a single electrode finger that changes with respect to the direction intersecting the propagation direction of the surface acoustic wave ,
The interdigital output electrode fingers before correction are diagonal electrode fingers alternately arranged at a pitch of λ / 2, with the positive electrode fingers and the negative electrode fingers sandwiching floating electrodes extending in the same direction as each electrode finger. a structure, the interval between adjacent electrode fingers have a structure that changes the direction intersecting the direction of propagation of surface acoustic waves,
Elastic surface characterized in that the pitch of the positive and negative electrode fingers of the interdigital input electrode finger after correction is different from the pitch of the positive and negative electrode fingers of the interdigital output electrode finger after correction Wave filter. The surface acoustic wave filter according to claim 1 ,
The metallization ratio of the interdigital input electrode finger before correction and the metallization ratio of the interdigital output electrode finger before correction are the same,
The difference between the average sound velocity and the average wavelength that occurs for surface acoustic waves of the same frequency due to the difference in the input and output electrode finger structures is corrected by changing the metallization ratio of at least one of the input and output electrode fingers, and the corrected sandals The surface acoustic wave filter is characterized in that the metallization ratio of the interdigital input electrode finger is different from the metallization ratio of the interdigital output electrode finger after correction . The surface acoustic wave filter according to any one of claims 1 to 3 ,
The difference between the average sound velocity and the average wavelength that occurs for surface acoustic waves of the same frequency due to the difference in the input and output electrode finger structures is corrected by changing the film thickness of at least one of the input and output electrode fingers, and the corrected sandals A surface acoustic wave filter characterized in that the film thickness of the input electrode finger having a shape different from the film thickness of the interdigital output electrode finger after correction is made different .

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