Elastic wave device
In an elastic wave device that significantly reduces and prevents deterioration of a frequency characteristic without roughening an undersurface of a piezoelectric substrate, a structure is bonded to a surface of a piezoelectric substrate other than a main surface of the piezoelectric substrate on which IDTs are located. The structure is provided so that a path difference is defined between a first component and a second component of a bulk wave that is excited by the IDT and propagates in the piezoelectric substrate toward the bonding surface. The first component of the bulk wave is reflected from the bonding surface. The second component of the bulk wave enters the structure from the bonding surface, propagates in the structure, enters the piezoelectric substrate from the bonding surface, and propagates in the same direction as that of the first component reflected from the bonding surface in the piezoelectric substrate.
1. An elastic wave device comprising:
a piezoelectric substrate including a main surface on which an IDT is located and an elastic wave excited by the IDT propagates; and
a structure located on a bonding surface of the piezoelectric substrate other than the main surface; wherein
the structure is arranged such that a path difference is defined between a first component and a second component of a bulk wave that is excited by the IDT and propagates in the piezoelectric substrate toward the bonding surface, the first component being reflected from the bonding surface, the second component entering the structure from the bonding surface, propagating in the structure, and entering the piezoelectric substrate from the bonding surface, the first and second components propagating in the same direction in the piezoelectric substrate; and
the structure is bonded to a side surface of the piezoelectric substrate located between an outer periphery of the main surface and an outer periphery of the other main surface opposing the main surface.
2. The elastic wave device according to claim 1 , wherein the structure is bonded to another main surface of the piezoelectric substrate opposing the main surface.
3. The elastic wave device according to claim 1 , wherein d≠nλ is satisfied where an absolute value of the path difference between the first and second components of the bulk wave defined by the structure is d, a wavelength of the bulk wave is λ, and n is a positive integer.
4. The elastic wave device according to claim 1 , wherein (n+¼)λ<d<(n+¾) is satisfied where an absolute value of the path difference between the first and second components of the bulk wave defined by the structure is d, a wavelength of the bulk wave is λ, and n is an integer equal to or larger than zero.
5. The elastic wave device according to claim 1 , wherein d=(n+½)λ is satisfied where an absolute value of the path difference between the first and second components of the bulk wave defined by the structure is d, a wavelength of the bulk wave is λ, and n is a positive integer.
6. An elastic wave device comprising:
a piezoelectric substrate including a main surface on which an IDT is located and an elastic wave excited by the IDT propagates; and
a structure located on a bonding surface of the piezoelectric substrate other than the main surface; wherein
the structure is arranged such that a path difference is defined between a first component and a second component of a bulk wave that is excited by the IDT and propagates in the piezoelectric substrate toward the bonding surface, the first component being reflected from the bonding surface, the second component entering the structure from the bonding surface, propagating in the structure, and entering the piezoelectric substrate from the bonding surface, the first and second components propagating in the same direction in the piezoelectric substrate; and
an amplitude of the first component of the bulk wave obtained when the first component is reflected from the bonding surface and an amplitude of the second component of the bulk wave obtained when the second component enters the piezoelectric substrate from the bonding surface are the same or substantially the same.
7. The elastic wave device according to claim 1 , wherein the structure is a path difference defining film that is located on the bonding surface of the piezoelectric substrate and includes at least one layer.
8. The elastic wave device according to claim 7 , wherein the path difference defining film is an insulator.
9. The elastic wave device according to claim 7 , wherein the path difference defining film is made of a conductive material.
10. The elastic wave device according to claim 7 , wherein the path difference defining film and the piezoelectric substrate are made of acoustically different materials.
11. The elastic wave device according to claim 10 , wherein an acoustic impedance of the path difference defining film is different from an acoustic impedance of the piezoelectric substrate.
12. An elastic wave device comprising:
a piezoelectric substrate including a main surface on which an IDT is located and an elastic wave excited by the IDT propagates; and
a structure located on a bonding surface of the piezoelectric substrate other than the main surface; wherein
the structure is arranged such that a path difference is defined between a first component and a second component of a bulk wave that is excited by the IDT and propagates in the piezoelectric substrate toward the bonding surface, the first component being reflected from the bonding surface, the second component entering the structure from the bonding surface, propagating in the structure, and entering the piezoelectric substrate from the bonding surface, the first and second components propagating in the same direction in the piezoelectric substrate;
the structure is a path difference defining film that is located on the bonding surface of the piezoelectric substrate and includes at least one layer;
the path difference defining film and the piezoelectric substrate are made of acoustically different materials;
an acoustic impedance of the path difference defining film is different from an acoustic impedance of the piezoelectric substrate; and
Z 1 /Z 2 =⅓ is satisfied where Z 1 represents acoustic impedance of the piezoelectric substrate and Z 2 represents acoustic impedance of the path difference defining film.
13. An elastic wave device comprising:
a piezoelectric substrate including a main surface on which an IDT is located and an elastic wave excited by the IDT propagates; and
a structure located on a bonding surface of the piezoelectric substrate other than the main surface; wherein
the structure is arranged such that a path difference is defined between a first component and a second component of a bulk wave that is excited by the IDT and propagates in the piezoelectric substrate toward the bonding surface, the first component being reflected from the bonding surface, the second component entering the structure from the bonding surface, propagating in the structure, and entering the piezoelectric substrate from the bonding surface, the first and second components propagating in the same direction in the piezoelectric substrate;
the structure is a path difference defining film that is located on the bonding surface of the piezoelectric substrate and includes at least one layer;
the path difference defining film and the piezoelectric substrate are made of acoustically different materials;
an acoustic impedance of the path difference defining film is different from an acoustic impedance of the piezoelectric substrate; and
Z 1 /Z 2 =3 is satisfied where Z 1 represents acoustic impedance of the piezoelectric substrate and Z 2 represents acoustic impedance of the path difference defining film.
14. An elastic wave device comprising:
a piezoelectric substrate including a main surface on which an IDT is located and an elastic wave excited by the IDT propagates; and
a structure located on a bonding surface of the piezoelectric substrate other than the main surface; wherein
the structure is arranged such that a path difference is defined between a first component and a second component of a bulk wave that is excited by the IDT and propagates in the piezoelectric substrate toward the bonding surface, the first component being reflected from the bonding surface, the second component entering the structure from the bonding surface, propagating in the structure, and entering the piezoelectric substrate from the bonding surface, the first and second components propagating in the same direction in the piezoelectric substrate; and
the path difference defining film is provided only a portion of the other main surface of the piezoelectric substrate opposing the main surface or on only a portion of a side surface of the piezoelectric substrate that is present between an outer periphery of the main surface and an outer periphery of the other main surface opposing the main surface.
15. The elastic wave device according to claim 14 , wherein the path difference defining film is located at a position at which a bulk wave having an adverse effect on a device characteristic is reflected and in an area near the position on the other main surface and/or the side surface of the piezoelectric substrate.
16. The elastic wave device according to claim 15 , wherein, on the other main surface and/or the side surface of the piezoelectric substrate, an area in which the path difference defining film is provided and an area in which the path difference defining film is not provided are alternately arranged.
17. An elastic wave device comprising:
a piezoelectric substrate including a main surface on which an IDT is located and an elastic wave excited by the IDT propagates; and
a structure located on a bonding surface of the piezoelectric substrate other than the main surface; wherein
the structure is arranged such that a path difference is defined between a first component and a second component of a bulk wave that is excited by the IDT and propagates in the piezoelectric substrate toward the bonding surface, the first component being reflected from the bonding surface, the second component entering the structure from the bonding surface, propagating in the structure, and entering the piezoelectric substrate from the bonding surface, the first and second components propagating in the same direction in the piezoelectric substrate;
the structure is a path difference defining film that is located on the bonding surface of the piezoelectric substrate and includes at least one layer; and
the path difference defining film is made of one of a silicon oxide having an acoustic impedance of approximately 1×10 7 (Ns/m 3 ), an aluminum having an acoustic impedance of approximately 2×10 7 (Ns/m 3 ), a platinum having an acoustic impedance of approximately 7×10 7 (Ns/m 3 ), and a tungsten having an acoustic impedance of approximately 1×10 8 (Ns/m 3 ).
18. The elastic wave device according to claim 1 , wherein an undersurface of the piezoelectric substrate does not include any roughened area.
19. The elastic wave device according to claim 1 , wherein the elastic wave device is a surface acoustic wave device or a boundary acoustic wave device.