Surface acoustic wave functional wave

Details Surface acoustic wave functional wave Surface acoustic wave functional wave

1999-12-02

2001-07-10

Budd, Mark O. (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

Reexamination Certificate

active

06259186

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave functional element, and moreover to a suitable thin film piezoelectric substrate thereto and a manufacturing method thereof. The surface acoustic wave functional element is a device utilizing a surface acoustic wave, and includes a filter, a resonator, a convolver or the like.
2. Description of Related Art
LiTaO
3
and LiNbO
3
have very superior properties in electromechanical coupling coefficient, electro-optical effect, non-linear optical effect or the like, and are practically used as a material for a surface acoustic wave (SAW) device. LiTaO
3
and LiNbO
3
have substantially equal material properties including crystal structure, lattice constant, thermal expansion coefficient or the like. A material represented by LiNb
x
Ta
1−x
O
3
(‘x’ is 0 or more and 1 or less) exhibits material properties similar to LiTaO
3
and LiNbO
3
.
An elastic wave is divided into a bulk wave and a surface acoustic wave. The bulk wave includes longitudinal and shear waves. The surface acoustic wave includes a Rayleigh wave, a Love wave, a Sezawa wave, a pseudo surface acoustic wave or the like. The pseudo surface acoustic wave has a propagation loss. The Rayleigh wave, Love wave, and Sezawa wave do not have such propagation loss. The Love wave is a wave mainly composed of a displacement component vertical to a propagation direction and parallel to a substrate surface. The Rayleigh wave and Sezawa wave are waves that have a less displacement of which the Love wave is mainly composed; and that are mainly composed of a displacement component in a propagation direction, or a depth direction to a substrate surface, respectively. Because the Love wave has shear-wave oriented properties as described above, it has a small propagation loss in a solution. Thus, an application to a surface acoustic wave sensor in the solution or the like is expected.
In creating a high-frequency wide-bandwidth SAW filter, it is preferable to employ a wave having a high surface acoustic wave velocity (V) and a large electromechanical coupling coefficient (K
2
) and being free of a propagation loss. In a commercially available single crystal material, V=4000 m/s and K
2
=5.5% are achieved with 128Y LiNbO
3
in a Rayleigh wave which is free of a propagation loss. For use of an SAW device such as SAW filter, assuming that a surface acoustic wave velocity is V and a wavelength of the surface acoustic wave is &lgr;, a use frequency is represented by f=V/&lgr;. Hence, in employing SAW filter in a high-frequency band, the low surface acoustic wave velocity results in reduced &lgr;. A pitch of a electrode of interdigital transducer is generally &lgr;/4 or less. When &lgr; is reduced, it becomes difficult to carry out an electrode fabrication process using lithography; and therefore, a material having higher V is desired. A technique for improving V includes a method for forming a thin film of LiNbO
3
, LiTaO
3
, ZnO or the like on a sapphire substrate having a high V and using it, which is practically used with a ZnO thin film. However, the ZnO thin film, has such is a disadvantage that K
2
is small, i.e., not greater than 5%. A material having high K
2
is required to form a filter with a wide bandwidth. In order to obtain a piezoelectric substrate material having high V together with large K
2
, there is a growing need for making a thin film of LiTaO
3
and LiNbO
3
having higher V and larger K
2
than ZnO, and a variety of studies have been made. The inventors have succeeded in making a thin film of piezoelectric (
012
) LiTaO
3
on a (
012
) sapphire substrate and (
001
) LiNbO
3
on a (
001
) sapphire substrate using a laser abrasion method, and has reported that its SAW velocity is significantly higher than that of a bulk material, and this material can be advantageously employed as a material for high frequency. However, the electromechanical coupling coefficients are obtained by theoretical calculation less than 5% for the LiTaO
3
film (Y. Shibata et al., Jpn. J. Appl. Phys., 34 (1995) 249-253.) and less than 6.9% for the LiNbO
3
film (Y. Shibata et al., J. Appl. Phys., 77 (1995) 1489-1503), so their use has been limited.
In a two-layer structure of a substrate and a piezoelectric film, it is known that when a velocity of the longitudinal wave in the substrate is greater than that in a bulk single crystal of a piezoelectric film material, the Sezawa wave or Love wave appears (Y. Shibata et al., Jpn. J. Appl. Phys., 34 (1995) 249-253, T. Mitsuyu et al., J. Appl. Phys., 51 (1980) 2464-2470, etc.). However, a surface acoustic wave functional element employing a Love wave or a Sezawa wave with a large electromechanical coupling coefficient and a high surface acoustic wave velocity is not practically used expect in a case that K
2
is about 4.3% or less at the Sezawa wave in the ZnO film.
Therefore, an object of the present invention is to achieve high V together with large K
2
for a wave free of a propagation loss with a LiNb
x
Ta
1−x
O
3
film. In particular, another object of the present invention is to generate a Love wave having a high surface acoustic wave velocity and a large electromechanical coupling coefficient.
SUMMARY OF THE INVENTION
To solve the aforementioned problems, as a result of through investigation by the inventors, the inventors found out that a propagation direction of a surface acoustic wave was controlled at a LiNb
x
Ta
1−x
O
3
(‘x’ denotes 0 or more and 1 or less) film on a (
012
) sapphire within a specific range, making it possible to employ a Love wave, and to provide superior properties including a high surface acoustic wave velocity and a large electromechanical coupling coefficient, and achieved the present invention.
1) In the first embodiment of the present invention, surface acoustic wave functional element comprises a (
012
) sapphire substrate and a LiNb
x
Ta
1−x
O
3
on said (
012
) sapphire substrate (‘x’ is 0 or more and 1 or less), wherein the LiNb
x
Ta
1−x
O
3
film is a (
012
) LiNb
x
Ta
1−x
O
3
(‘x’ is 0 or more and 1 or less), a crystal axis of said sapphire substrate and a crystal axis of said (
012
) LiNb
x
Ta
1−x
O
3
film are parallel to each other, a Love wave is propagated as a surface acoustic wave, and a propagation direction of said surface acoustic wave is within a range of ±15 degrees around an axis vertical to the C-axis projection line direction of the crystal axis of said sapphire substrate or said (
012
) LiNb
x
Ta
1−x
O
3
film.
2) In the surface acoustic wave functional element set forth in the above 1), h/&lgr; may be 0.05 or more and 0.7 or less, where film thickness of said LiNb
x
Ta
1−x
O
3
film is ‘h’, and a wavelength of a surface acoustic wave is &lgr;.
3) In the surface acoustic wave functional element set forth in the above 1) or 2), said (
012
) LiNb
x
Ta
1−x
O
3
film may be a (
012
) LiTaO
3
film.
4) In the surface acoustic wave functional element set forth in any of the above 1) to 3), said (
012
) LiNb
x
Ta
1−x
O
3
film may be a (
012
) LiNbO
3
film
5) In the second embodiment of the present invention, A surface acoustic wave functional element comprises a (
012
) sapphire substrate and a LiNb
x
Ta
1−x
O
3
film (‘x’ is 0 or more and 1 or less) on said (
012
) sapphire substrate, wherein said LiNb
x
Ta
1−x
O
3
film is (
100
) LiNb
x
Ta
1−x
O
3
(‘x’ is 0 or more and 1 or less); a C-axis projection line direction of a crystal axis of said sapphire substrate and a C-axis direction of a crystal axis of said (
100
) LiNb
x
Ta
1−x
O
3
film are parallel to each other; a surface acoustic wave propagation direction is within a range of ±20 degrees around an axis vertical to the C-axis projection line direction of the crystal axis of said sapphire substrate.
6) In the surface acoustic wave element set forth in the above 5), h/&lgr; may be 0.01 or more and 2 or less, where film thickness of said LiNb
x
Ta
1−x
O

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