This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2016-123972, filed on Jun. 22, 2016, the entire contents of which are incorporated herein by reference.
A certain aspect of the present invention relates to an acoustic wave resonator, a filter, and a multiplexer.
In high-frequency communication systems typified by mobile phones, high-frequency filters have been used to remove unnecessary signals other than signals in the frequency band used for communications. Acoustic wave resonators such as surface acoustic wave (SAW) resonators have been used for the high-frequency filters. In the SAW resonator, a metal grating electrode is formed on a piezoelectric substrate such as a lithium tantalate (LiTaO3) substrate or a lithium niobate (LiNbO3) substrate.
The grating electrode excites a Shear Horizontal (SH) wave, a Rayleigh wave, or a boundary acoustic wave, which is a type of surface acoustic wave. Reflectors are located at both sides in the propagation direction of the acoustic wave excited by the grating electrode to confine the acoustic wave in the vicinity of the grating electrode. Ladder-type filters and multimode filters can be made with acoustic wave resonators. There have been known acoustic wave resonators in which the width in the direction perpendicular to the propagation direction of the acoustic wave is weighted in the grating electrode as disclosed in, for example, Japanese Patent Application Publication Nos. 9-270667 and 2008-78883 (hereinafter, referred to as Patent Documents 1 and 2, respectively).
In acoustic wave resonators having a grating electrode, lateral-mode spurious, which is unnecessary response, is generated. The lateral-mode spurious is generated when the acoustic waves each having a component in the direction perpendicular to the propagation direction of the acoustic wave amplify each other at a certain wavelength. In Patent Documents 1 and 2, since the overlap width changes in the propagation direction of the acoustic wave, the frequency at which the lateral-mode spurious is generated changes in the propagation direction. Accordingly, the frequencies at which the acoustic waves in the lateral mode amplify each other are averaged, and thereby, the lateral-mode spurious is reduced. However, this does not mean that the generation of the acoustic wave in the lateral mode is inhibited. Therefore, the acoustic wave in the lateral mode leaks to the outside of the grating electrode, causing loss.
According to a first aspect of the present invention, there is provided an acoustic wave resonator including: a piezoelectric substrate; and a pair of grating electrodes that is formed on the piezoelectric substrate, one of the pair of grating electrodes including a plurality of first electrode fingers having electric potentials equal to each other, another of the pair of grating electrodes including a plurality of second electrode fingers having electric potentials that differ from the electric potentials of the plurality of first electrode fingers and are equal to each other, two second electrode fingers of the plurality of second electrode fingers being located between at least a pair of adjacent first electrode fingers of the plurality of first electrode fingers, Pg differing from λ/4 where λ represents a wavelength of an acoustic wave excited by the plurality of first electrode fingers and the plurality of second electrode fingers and Pg represents a distance between centers of the two second electrode fingers.
According to a second aspect of the present invention, there is provided an acoustic wave resonator including: a piezoelectric substrate; a pair of grating electrodes that is formed on the piezoelectric substrate and includes a plurality of electrode fingers arranged in an arrangement direction, the plurality of electrode fingers exciting an acoustic wave; and a metal additional film that is formed on the piezoelectric substrate between the plurality of electrode fingers in an overlap region where the plurality of electrode fingers overlap in the arrangement direction, the metal additional film being not electrically coupled to the plurality of electrode fingers.
According to a third aspect of the present invention, there is provided a filter including the above acoustic wave resonator.
According to a fourth aspect of the present invention, there is provided a multiplexer including the above filter.
A description will be given of a structure of an acoustic wave resonator in accordance with comparative examples and embodiments of the present invention.
The region where the electrode fingers 14 of a pair of the comb-shaped electrodes 20 overlap in the arrangement direction is an overlap region 15. The acoustic wave excited by the grating electrode 16 in the overlap region 15 mainly propagates in the arrangement direction of the electrode fingers 14. The pitch of the electrode fingers 14 approximately corresponds to the wavelength λ of the acoustic wave. The region between the edges of the electrode fingers 14 of one of the comb-shaped electrodes 20 and the bus bar 18 of the other of the comb-shaped electrodes 20 is a gap region 17. When dummy electrode fingers are provided, the gap region is a region between the edges of the electrode fingers of one of the comb-shaped electrodes 20 and the edges of the dummy electrode fingers of the other of the comb-shaped electrodes 20. The propagation direction of the acoustic wave is defined as an X direction, and the direction perpendicular to the propagation direction is defined as a Y direction. The arrangement direction corresponds to the X direction. The X direction and the Y direction do not necessarily correspond to the X-axis direction and the Y-axis direction of the crystal orientation of the piezoelectric substrate 10, respectively. The piezoelectric substrate 10 is, for example, a lithium tantalate substrate or a lithium niobate substrate. The metal film 12 is, for example, an aluminum film or a copper film.
Next, a description will be given of an anisotropy coefficient.
In
The anisotropy coefficient γ is determined by the material of the piezoelectric substrate 10, and the material, the film thickness, and the pitch of the grating electrode 16. For example, when the piezoelectric substrate 10 is a rotated Y-cut X-propagation lithium niobate substrate, the anisotropy coefficient γ is positive. When the piezoelectric substrate 10 is a rotated Y-cut X-propagation lithium tantalate substrate, the anisotropy coefficient γ is negative. When a rotated Y-cut X-propagation lithium tantalate substrate is used, and the grating electrode 16 is made of a heavy material and has a large film thickness, the anisotropy coefficient γ may be positive.
The case where the anisotropy coefficient γ is positive will be considered. In this case, when the anisotropy coefficient γ in the overlap region 15 decreases and becomes close to zero, the Y-direction component of the acoustic wave is hardly generated. When γ becomes zero, the Y-direction component of the acoustic wave becomes zero. Accordingly, the lateral-mode spurious disappears. As described above, as γ is reduced, the lateral-mode spurious is reduced. The inventors focused on the anisotropy coefficient γ and the reflectance in the grating electrode of the acoustic wave propagating in the X direction to reduce the lateral-mode spurious.
The second comparative example reduces the reflection of the acoustic wave by the grating electrode. As illustrated in
One-port resonators in accordance with the first and second comparative examples were fabricated to measure the reflection characteristic S11. The fabricated one-port resonators have the following structure.
Piezoelectric substrate 10: Rotated 42° Y-cut X-propagation lithium tantalate substrate
Metal film 12: Molybdenum (Mo) film with a film thickness Tg of 0.11λ
Pitch P=λ: 4.4 μm
Overlap width: 20λ
Number of pairs: 100 pairs
Number of reflector electrode fingers: 10
The pitch P represents a pitch of the electrode fingers of the same comb-shaped electrode. The overlap width represents a width of the overlap region 15 in the Y direction (a width along which the electrode fingers 14 overlap). The number of pairs is the number of pairs of the electrode fingers 14 in the grating electrode 16. The number of reflector electrode fingers represents the number of electrode fingers of the reflector 22.
As illustrated in
As illustrated in
The reason why the loss increases in the second comparative example is considered because the energy of the acoustic wave cannot be confined in the IDT 21 since the acoustic wave are hardly reflected by the grating electrode 16.
In the first embodiment, the pitch Pg is configured to differ from λ/4. In this case, the reflection of the acoustic wave does not meet Bragg's condition. The acoustic waves 50c and 50d generated when the acoustic waves 50 propagating in the X direction are reflected by the adjacent electrode fingers 14b have different phases that are not opposite or identical. Thus, the reflectance of the acoustic wave by the grating electrode 16 in the first embodiment is less than that in the first comparative example. Therefore, the anisotropy coefficient γ is less than that of the first comparative example, and the lateral-mode spurious is therefore reduced. Additionally, the reflectance of the acoustic wave of the first embodiment is greater than that of the second comparative example. Therefore, the energy of the acoustic wave is confined in the IDT 21, and the increase in loss is reduced.
An acoustic wave resonator having the structure of
As illustrated in
In the first embodiment and the variations thereof, the grating electrode 16 includes a plurality of electrode fingers 14a (first electrode fingers) having electric potentials equal to each other and a plurality of electrode fingers 14b (second electrode fingers) having electric potentials equal to each other. The electrode fingers 14a and 14b are respectively included in different comb-shaped electrodes 20a and 20b. Therefore, the electrode fingers 14a have electric potentials different from the electric potentials of the electrode fingers 14b. Two electrode fingers 14b are located between at least a pair of the adjacent electrode fingers 14a. When the distance between the centers of the two electrode fingers 14b is represented by Pg, Pg differs from λ/4. By using the double electrode, the reflectance of the acoustic wave by the grating electrode 16 is reduced compared to the first comparative example. Accordingly, the anisotropy coefficient γ becomes smaller, and the lateral-mode spurious is therefore reduced. Furthermore, by configuring the pitch Pg to be different from λ/4, the no-reflection state described in the second comparative example disappears. Therefore, the loss is reduced and the Q-value is improved.
In
In
Both the electrode fingers 14a and 14b are preferably the double electrodes, but one of them may be the single electrode. All the electrode fingers 14a (or 14b) are preferably the double electrodes, but it is sufficient if at least one electrode finger is the double electrode.
A second embodiment provides a metal additional film between the electrode fingers 14a and 14b in the overlap region 15 to reduce the reflectance of the acoustic wave by the grating electrode 16.
The acoustic wave resonator of the second embodiment was fabricated to measure the reflection characteristic S11.
As illustrated in
The acoustic impedance Z [kg/(m2·s)] of each material is expressed by the following equation.
Z=V×ρ=√{square root over (ρ×E)}
Here, E [Pa] represents Young's modulus, ρ [kg/m3] represents density, and V [kg/(m2 s)] represents acoustic velocity. As described above, the acoustic impedance is the square root of the product of Young's modulus E and density ρ.
Young's modulus, the density, and the acoustic impedance of each of Mo, the air, and Au are listed in Table 1.
The acoustic impedance Z1 and Z2 in
In the second embodiment and the variations thereof, the metal additional film 30 is formed between a plurality of the electrode fingers 14a and 14b in the overlap region 15, and is not electrically coupled to the plurality of the electrode fingers 14a and 14b. This structure reduces the reflectance of the acoustic wave by the grating electrode 16. Thus, the anisotropy coefficient γ decreases, and the lateral-mode spurious is thereby reduced. The metal additional film 30 has a greater density than the insulating film. Thus, the acoustic impedance is greater. Thus, even though the film thickness is not increased, the acoustic impedance Z2 can be increased. In addition, the reflectance of the acoustic wave becomes greater than zero by making the acoustic impedance Z2 and Z1 different from each other, and therefore, the loss is reduced.
The preferable range of Z2 is Zf<Z2<Z1 or Z1<Z2<Z1+(Z1−Zf). To avoid the increase in the film thickness of the metal additional film 30, Zf<Z2<Z1 is preferable. It is sufficient if the metal additional film 30 is located in at least a part of the overlap region 15. To elicit the effect more, the metal additional film 30 is preferably located across the entire of the overlap region 15.
When the metal additional film 30 comes in contact with the electrode fingers 14a and 14b, they are electrically connected. On the other hand, when the distance between the metal additional film 30 and the electrode finger 14a or 14b is large, the effect produced by the provision of the metal additional film 30 becomes small. Thus, the distance Ws between the metal additional film 30 and the electrode finger 14a (or 14b) adjacent to the metal additional film 30 is preferably less than λ/16. The distance Ws is more preferably less than λ/32.
A third embodiment is an exemplary filter and an exemplary duplexer using the acoustic wave resonator in accordance with any of the first and second embodiments and the variations thereof.
A duplexer has been described as an example of the multiplexer, but the multiplexer may be a triplexer or a quadplexer.
Although the embodiments of the present invention have been described in detail, it is to be understood that the various change, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.
Number | Date | Country | Kind |
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2016-123972 | Jun 2016 | JP | national |
Number | Name | Date | Kind |
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9654084 | You | May 2017 | B2 |
20110063047 | Okuda | Mar 2011 | A1 |
20130234805 | Takahashi | Sep 2013 | A1 |
Number | Date | Country |
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S61-6917 | Jan 1996 | JP |
9-270667 | Oct 1997 | JP |
2000-124763 | Apr 2000 | JP |
2003-298383 | Oct 2003 | JP |
2008-78883 | Apr 2008 | JP |
Entry |
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Trzcinski, “Surface Acoustic Wave (SAW) filter technology”, Torino, 2008. |
Tyagi et al., “Saw and Interdigital Transducers”, a research paper published by International Journal of Scientific & Engineering Research, IJSER 2012, pp. 1-5. |
Nakamura et al., “A review of SiO2 thin film technology for temperature compensated SAW devices”, Sixth International Symposium on Acoustic Wave Devices for future Mobile Communication Systems, Nov. 2015, Chiba, Japan, pp. 67-71. |
Japanese Office Action dated Dec. 4, 2018, in a counterpart Japanese patent application No. 2016-123972. (A machine translation (not reviewed for accuracy) attached.). |
Number | Date | Country | |
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20170373669 A1 | Dec 2017 | US |