Patent ID: 12261582

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be clarified below through the description of preferred embodiments of the present invention with reference to the accompanying drawings.

The preferred embodiments described in this specification are merely illustrative and the configurations to be described below may be partially replaced or combined between the different preferred embodiments.

FIG.1is a cross-sectional side view of an acoustic wave device according to a first preferred embodiment of the present invention (taken along line A-A inFIG.2B).FIG.2Ais a plan view of an IDT electrode in an acoustic wave device according to the first preferred embodiment of the present invention, andFIG.2Bis a schematic plan view of the acoustic wave device according to the first preferred embodiment of the present invention.

An acoustic wave device1includes a piezoelectric substrate2. The piezoelectric substrate2is made of, for example, a LiNbO3single crystal in this preferred embodiment, but may be made of another piezoelectric single crystal such as LiTaO3, for example.

An IDT electrode3is provided on the piezoelectric substrate2. A dielectric film4covers the IDT electrode3.

As illustrated inFIG.2B, a reflector5is provided on one side of the IDT electrode3in an acoustic wave propagation direction and a reflector6is provided on the other side of the IDT electrode3in the acoustic wave propagation direction. Accordingly, a one-port acoustic wave resonator is defined in the acoustic wave device1.

The details of the IDT electrode3are as illustrated inFIG.2A. The IDT electrode3includes a first busbar11and a second busbar12. The first busbar11and the second busbar12extend in an acoustic wave propagation direction. The first busbar11and the second busbar12are spaced apart from each other to face each other in a direction perpendicular or substantially perpendicular to the acoustic wave propagation direction.

First ends of a plurality of first electrode fingers13are connected to the first busbar11. First ends of a plurality of second electrode fingers14are connected to the second busbar12. The plurality of first electrode fingers13and the plurality of second electrode fingers14are interdigitated with each other. A region where the first electrode fingers13and the second electrode fingers14overlap in the acoustic wave propagation direction is an intersecting width region. The dimension of the intersecting width region along a direction in which the first electrode fingers13and the second electrode fingers14extend is an intersecting width.

The first electrode finger13includes a wide portion13bat the second end thereof. The second electrode finger14includes a wide portion14bat the second end thereof. The second electrode finger14includes a wide portion14cat a position overlapping the wide portion13bin the acoustic wave propagation direction. The first electrode finger13includes a wide portion13cat a position overlapping the wide portion14bin the acoustic wave propagation direction.

A region where the wide portion13cand the wide portion14bare alternately repeated along the acoustic wave propagation direction is a first edge region E1. A region where the wide portion13band the wide portion14care alternately repeated along the acoustic wave propagation direction is a second edge region E2. A region between the first edge region E1and the second edge region E2is a central region C. That is, the intersecting width region includes the central region C, the first edge region E1, and the second edge region E2. The first edge region E1and the second edge region E2are located on the outer side of the central region C in the direction in which the first electrode fingers13and the second electrode fingers14extend.

In the IDT electrode3, the first busbar11includes a plurality of openings11aalong the acoustic wave propagation direction. An inner busbar portion11bis provided between the region where the multiple openings11aare provided and the intersecting width region. An outer busbar portion11cis provided on the outer side of the openings11ain an intersecting width direction. The intersecting width direction is parallel or substantially parallel to the direction in which the first electrode fingers13and the second electrode fingers14extend. The outer busbar portion11cand the inner busbar portion11bare coupled by a coupling portion11d. The coupling portion11dis located between the adjacent openings11a. The coupling portion11dis located on an extension in the direction in which the first electrode fingers13and the second electrode fingers14extend.

The second busbar12also includes a plurality of openings12a, an inner busbar portion12b, an outer busbar portion12c, and a coupling portion12dlike the first busbar11.

A region where the openings11aare provided is a first high acoustic velocity region H1, and a region where the openings12aare provided is a second high acoustic velocity region H2.

The dimensions of the inner busbar portions11band12band the outer busbar portions11cand12calong the direction in which the first electrode fingers13and the second electrode fingers14extend are defined as widths. The width of the inner busbar portions11band12bis less than that of the outer busbar portions11cand12c.

An acoustic velocity in each region in the IDT electrode3is schematically illustrated on the right side ofFIG.2A. As represented by arrow V, an acoustic velocity increases toward the right side of the drawing. The relationship of V1>V2is satisfied where V1represents an acoustic velocity in the central region C and V2represents an acoustic velocity in the first edge region E1and the second edge region E2.

An acoustic velocity in gap regions G1and G2located on the outer side of the first edge region E1and the second edge region E2is represented by V3, an acoustic velocity in the inner busbar portions11band12bis represented by V4, an acoustic velocity in the first high acoustic velocity region H1in which the openings11aare provided and the second high acoustic velocity region H2in which the openings12aare provided is represented by V5, and an acoustic velocity in a region where the outer busbar portions11cand12care provided is represented by V6. The relationship of V2<V3<V5is satisfied. Accordingly, on the outer side of a low acoustic velocity region including the first edge region E1, the second edge region E2, the gap regions G1and G2, and the inner busbar portions11band12b, the first high acoustic velocity region H1and the second high acoustic velocity region H2are located where an acoustic velocity is V5. Accordingly, the formation of a piston mode can reduce or prevent the occurrence of a transverse-mode ripple.

For the reduction in the acoustic velocity V2, the wide portions13cand14bare provided in the first edge region E1and the wide portions13band14care provided in the second edge region E2. In addition, in the present preferred embodiment, the thickness of a dielectric film portion4bon the first edge region E1and the second edge region E2is greater than that of a dielectric film portion4aon the first high acoustic velocity region H1and the second high acoustic velocity region H2as illustrated inFIG.1. That is, the thickness of the dielectric film portion4bis greater than that of the dielectric film portion4a. Since the thickness of the dielectric film4on the first edge region E1and the second edge region E2is greater than that of the dielectric film4on the first high acoustic velocity region H1and the second high acoustic velocity region H2by ΔH, the acoustic velocity V5in the first high acoustic velocity region H1and the second high acoustic velocity region H2can be further increased as compared with the acoustic velocity V2in the first edge region E1and the second edge region E2. That is, the larger the difference ΔH in the film thickness of the dielectric film4illustrated inFIG.1, the larger the difference between the acoustic velocities V5and V2.

In the acoustic wave device1, an acoustic velocity difference can be obtained using the above difference ΔH in the film thickness of the dielectric film4. Accordingly, the difference between the acoustic velocities V2and V5can be increased without significantly increasing the width of the wide portions13b,13c,14b, and14c. Using the difference ΔH in the film thickness of the dielectric film4, the gap (dimension along the acoustic wave propagation direction) between the first electrode finger13and the second electrode finger14can be increased to some degree in the first edge region E1and the second edge region E2. The formation of a piston mode can therefore reduce or prevent the occurrence of a transverse-mode ripple while the occurrence of a surge breakdown is reduced or prevented.

FIG.3is an enlarged partially-broken cross-sectional view of the acoustic wave device1in the first high acoustic velocity region H1in the direction of arrow B inFIG.1. This drawing illustrates the cross section of the coupling portion11din the direction of the thickness.

In the first high acoustic velocity region H1, the coupling portions11dare disposed at regular intervals along the acoustic wave propagation direction. On the dielectric film4, protrusions4care provided above the respective coupling portions11dto cover the coupling portions11d. The height of the protrusion4cis located at a position higher than the upper surface of the dielectric film4between the protrusions4c. Since the protrusions4care provided at regular intervals along the acoustic wave propagation direction, the position of the upper end of the stop band of an acoustic wave resonator therefore moves to a higher-frequency side. This will be described below using resonance characteristics inFIG.4.

FIG.4is a diagram illustrating the impedance characteristics of an acoustic wave device defining and functioning as a resonator. The lower end of a stop band is located at a resonant frequency fr, and the upper end of the stop band is located at a position higher than an anti-resonant frequency fa. A ripple caused at the upper end of the stop band appears in resonance characteristics. A ripple represented by arrow SB inFIG.4is the ripple.

Since the protrusions4care provided at regular intervals in the present preferred embodiment, the ripple represented by the arrow SB is shifted from the anti-resonant frequency fa to a position at a distance from the anti-resonant frequency fa, that is, a higher-frequency side. Accordingly, the influence of a ripple caused at the upper end of a stop band upon resonance characteristics or the filter characteristics of an acoustic wave filter including an acoustic wave resonator can be reduced.

Referring toFIG.1, the dielectric film4includes the dielectric film portion4awhose upper surface is located at a relatively low height and the dielectric film portion4bwhose upper surface is located at a relatively high height. The dielectric film4on regions on the outer side of the first high acoustic velocity regions H1and H2is equal or substantially equal in film thickness to the dielectric film portion4a. However, the dielectric film4on the outer busbar portions11cand12cillustrated inFIG.2Adoes not necessarily have to be equal or substantially equal in film thickness to the dielectric film4on the first high acoustic velocity region H1and the second high acoustic velocity region H2. That is, the film thickness of the dielectric film4on the outer busbar portions11cand12cmay be greater than or less than that of the dielectric film portion4a.

The dielectric film4on the central region C does not necessarily have to be equal or substantially equal in thickness to the dielectric film portion4bof the dielectric film4on the first edge region E1and the second edge region E2. The thickness of the dielectric film4on the central region C may be greater than or less than that of the dielectric film portion4b. It is preferable that the thickness of the dielectric film4on the central region C is less than that of the dielectric film portion4bfor the acquisition of an acoustic velocity difference. However, when the upper surface of the dielectric film4on the central region C and the upper surface of the dielectric film4on the first edge region E1and the second edge region E2are flush with each other, that is, when a structure according to the present preferred embodiment is provided, the ease of manufacturing is achieved.

The dielectric film4on other regions, that is, the gap regions G1and G2where an acoustic velocity is V3and the region where the inner busbar portions11band12bare provided and an acoustic velocity is V4does not necessarily have to be equal or substantially equal in thickness to the dielectric film portion4bas in the present preferred embodiment and may be greater than or less than that of the dielectric film portion4b.

In the case of a structure according to the present preferred embodiment in which there are the two types of dielectric film portions4aand4b, the formation of the dielectric film4can be easily performed as described above.

FIG.5is an enlarged partially-broken plan view of an IDT electrode in an acoustic wave device according to a second preferred embodiment of the present invention. In an acoustic wave device according to the second preferred embodiment, an IDT electrode includes a first busbar21including no opening and a second busbar22including no opening. First ends of a plurality of first electrode fingers23are connected to the first busbar21. First ends of a plurality of second electrode fingers24are connected to the second busbar22. As in the first preferred embodiment, the first electrode finger23includes the central region C and wide portions23band23c. The second electrode finger24also includes the central region C and wide portions24band24c. The central region C, the first edge region E1, and the second edge region E2are therefore provided. In a gap region between the second ends of the first electrode fingers23and the second busbar22, the second high acoustic velocity region H2is provided. In a gap region between the second ends of the second electrode fingers24and the first busbar21, the first high acoustic velocity region H1is provided. The relationships of V11>V12and V12<V13are satisfied where V11represents an acoustic velocity in the central region C, V12represents an acoustic velocity in the first edge region E1and the second edge region E2, and V13represents an acoustic velocity in the first high acoustic velocity region H1and the second high acoustic velocity region H2.

Thus, the structure of an IDT electrode for the use of a piston mode is not limited to the structure according to the first preferred embodiment in which the openings11aand12aare provided. A structure of an IDT electrode according to the second preferred embodiment is the same or substantially the same as that of an IDT electrode according to the first preferred embodiment except for the above point. Accordingly, a dielectric film covers the IDT electrode. The thickness of the dielectric film on the first edge region E1and the second edge region E2is greater than that of the dielectric film on the first high acoustic velocity region H1and the second high acoustic velocity region H2. Accordingly, as in the first preferred embodiment, an acoustic wave device is provided in which a surge breakdown is less likely to occur.

Acoustic wave devices according to preferred embodiments of the present invention are widely applicable to, for example, various bandpass filters.

FIG.6is a circuit diagram of a ladder filter including an acoustic wave resonator that is an acoustic wave device according to a preferred embodiment of the present invention. A ladder filter31includes a plurality of series arm resonator S1to S4and a plurality of parallel arm resonators P1to P4. Each of the series arm resonator S1to S4and the parallel arm resonators P1to P4is an acoustic wave resonator. An acoustic wave device according to a preferred embodiment of the present invention can be used as at least one of the acoustic wave resonators, so that the occurrence of a transverse-mode ripple can be reduced or prevented and the occurrence of a surge breakdown can be effectively reduced or prevented.

In the acoustic wave device1according to the first preferred embodiment, the ripple at the upper end of a stop band can be shifted to a higher-frequency side. Accordingly, when the acoustic wave device is used as each of the parallel arm resonators P1to P4in the ladder filter, the influence on the passband can be further reduced.

FIG.7is a circuit diagram of a bandpass filter in which an acoustic wave device according to a preferred embodiment of the present invention is provided. An acoustic wave resonator33is connected in series with a longitudinally coupled resonator filter32. An acoustic wave resonator34is connected between a series arm and the ground. An acoustic wave device according to a preferred embodiment of the present invention can be used as each of the longitudinally coupled resonator filter32and the acoustic wave resonators33and34.

FIG.8is an elevational cross-sectional view of a piezoelectric substrate used in an acoustic wave device according to a modification of a preferred embodiment of the present invention. In an acoustic wave device according to a modification of a preferred embodiment of the present invention, a high-acoustic-velocity material layer43and a low-acoustic-velocity material layer44are laminated on a support substrate42. On the low-acoustic-velocity material layer44, a piezoelectric film45is laminated. A piezoelectric substrate46having such a laminated structure may be used.

The high-acoustic-velocity material layer43is made of a high acoustic velocity material through which a bulk wave propagates at an acoustic velocity higher than the acoustic velocity of a bulk wave that propagates through the piezoelectric film45. The low-acoustic-velocity material layer44is made of a low acoustic velocity material through which a bulk wave propagates at an acoustic velocity lower than the acoustic velocity of a bulk wave that propagates through the piezoelectric film45.

Examples of the above high acoustic velocity material include aluminum oxide, silicon carbide, silicon nitride, silicon oxynitride, silicon, sapphire, lithium tantalate, lithium niobate, quartz, alumina, zirconia, cordierite, mullite, steatite, forsterite, magnesia, a DLC (diamond-like carbon) film, diamond, media including the above materials as a principal component, and media including a mixture of the above materials as a principal component. Examples of the above low acoustic velocity material include silicon oxide, glass, silicon oxynitride, tantalum oxide, a compound obtained by adding fluorine, carbon, boron, hydrogen, or silanol group to silicon oxide, and media including the above materials as a principal component.

The support substrate42and the high-acoustic-velocity material layer43may be integrated to provide a substrate made of a high acoustic velocity material.

In the present invention, the piezoelectric substrate46having the above structure may be used. A structure may be used in which an acoustic reflection film including a high acoustic impedance layer and a low acoustic impedance layer is laminated between a piezoelectric film and a substrate, for example.

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.