Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2001-11-02
2003-07-29
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C310S31300R, C310S31300R
Reexamination Certificate
active
06600391
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave apparatus for use as, for example, a resonator or band filter, and more particularly, the present invention relates to an end surface reflection type surface acoustic wave apparatus that utilizes reflection of a surface acoustic wave transmitted between two opposite end surfaces.
2. Description of the Related Art
Surface acoustic wave apparatuses have been widely used as resonators and resonating-type band filters. Now, a surface acoustic wave apparatus that uses a Rayleigh wave as a surface acoustic wave has a structure in which at least one interdigital transducer (hereinafter “IDT”) is provided on a piezoelectric substrate, and reflectors are disposed at both sides, in a surface acoustic wave propagation direction, of the region where the IDT is provided. That is, the reflectors are needed to reflect the surface acoustic wave between the reflectors and form a standing wave. As a result, the dimensions of the surface acoustic wave apparatus need to be large.
In contrast to this, there is a known end surface reflection type surface acoustic wave apparatus that uses a Shear Horizontal type (“SH-type”) surface acoustic wave. With an SH-type surface acoustic wave such as a BGS wave, etc., the reflectors can be omitted and the two opposite end surfaces of a piezoelectric substrate can be used as reflection end surfaces. Therefore the surface acoustic wave apparatus can be made compact.
Nevertheless, the acoustic velocity of an SH wave in a piezoelectric substrate is slow, so a surface acoustic wave apparatus that uses an SH wave has difficulty handling high frequencies. Also, the bandwidth of a surface acoustic wave filter depends on the electromechanical coupling coefficient k of the piezoelectric substrate, and utilizing an SH-type surface acoustic wave puts limits on the piezoelectric substrate material. Therefore it is not easy to obtain a desired bandwidth due to the restriction of the electromechanical coupling coefficient k.
BRIEF DESCRIPTION OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide an end surface reflection type surface acoustic wave apparatus that eliminates the defects of prior art, easily handles high frequencies, and easily provides a desired bandwidth.
Prior to the present invention, it was believed that one could not make an end surface reflection type surface acoustic wave apparatus that used a surface acoustic wave other than an SH wave. In contrast to this, the present inventors discovered that an end surface reflection type surface acoustic wave apparatus can be made by using a wave having a longitudinal wave or an Shear Vertical (“SV”) wave as the main component of the surface acoustic wave components and by reflecting such a wave between two opposite end surfaces of a surface acoustic wave substrate.
According to a first preferred embodiment of the present invention, an end surface reflection type surface acoustic wave apparatus includes a piezoelectric substrate having a pair of opposite main surfaces and a pair of opposite end surfaces that connect the pair of opposite main surfaces, and at least one interdigital transducer disposed on one main surface of the piezoelectric substrate, wherein a longitudinal wave surface acoustic wave is a main component of surface acoustic waves generated along the piezoelectric substrate.
A Rayleigh wave has two components including a longitudinal wave and an SV wave. These two components are always coupled and propagated. Therefore it was previously considered not possible to use a Rayleigh wave to make a surface acoustic wave apparatus that utilizes end surface reflection.
Nevertheless, with the present invention, a surface acoustic wave having a main component that is a longitudinal wave is reflected at the end surfaces of a substrate without causing mode conversion, as clearly shown in preferred embodiments described later, so it is possible to produce an end surface reflection type surface acoustic wave apparatus that utilizes a longitudinal wave. Moreover, the longitudinal wave has a faster acoustic velocity than an SH wave, so high frequencies can be handled easily. Also, since it is an end surface reflection type surface acoustic wave apparatus, reflectors are not necessary, so the apparatus can be very compact.
According to a preferred embodiment of the present invention, an SH wave component and an SV wave component, other than the longitudinal wave component, each occupy no more than approximately 20% when the energy strength of the entire excited surface acoustic wave is 100%. As a result, essentially only a surface acoustic wave having a longitudinal wave as the main component is reflected.
Also, it is preferred that the longitudinal wave component constitute approximately 70% or more when the energy strength of the entire excited surface acoustic wave is 100%. As a result, only a surface acoustic wave having a longitudinal wave as its main component is reflected, and characteristics deterioration caused by the occurrence of unnecessary modes is prevented. Even more preferred, when the displacements of the excited surface acoustic wave's longitudinal wave component, an SH component and an SV component are u
1
, u
2
, and u
3
respectively, is that a ratio between the substrate surface displacements u
1
and u
2
is about 0.015 or less when the substrate surface is electrically open, and/or about 0.1 or less when the substrate surface is electrically short-circuited, which achieves even better characteristics.
According to another preferred embodiment of the present invention, step differences are provided at intermediate height positions of the pair of opposite end surfaces, and reflection end surfaces are defined by end surface portions that are closer to the main surface side where the IDT is provided than the step differences. Also, the dimensions of the reflection end surfaces in the height direction are in the range of about 5&lgr; to about 20&lgr;, where &lgr; is the wavelength of the surface acoustic wave. By doing so, the surface acoustic wave having a longitudinal wave as a main component is efficiently reflected by the reflection end surfaces. Therefore, for example, grooves having a depth in the range of about 5&lgr; and about 20&lgr; are formed in the mother substrate and the substrate is divided at the centers of the grooves to efficiently manufacture a plurality of end surface reflection type surface acoustic wave apparatuses.
The piezoelectric substrate of preferred embodiments of the present invention is not especially limited as long as a surface acoustic wave having a longitudinal wave as its main component can be excited. Examples of this sort of piezoelectric substrate include an Li
2
B
4
O
7
substrate with Euler angles of approximately (0°, 40~60°, 90°) or approximately (90°, 90°, 20~50°), a crystal substrate with Euler angles of approximately (0°, 120~140°, 150°) or approximately (0°, 155.25°, 42°), an LiNbO
3
or LiTaO
3
substrate with Euler angles of approximately (90°, 90°, 10~90°), or other suitable substrates.
According to another preferred embodiment of the present invention, an end surface reflection type surface acoustic wave apparatus includes a piezoelectric substrate having a pair of opposite main surfaces and a pair of opposite end surfaces that connect the pair of opposite main surfaces, and at least one interdigital transducer disposed on one main surface of the piezoelectric substrate, wherein a shear vertical surface acoustic wave is a main component of surface acoustic waves generated along the piezoelectric substrate.
That is, the present inventors discovered that, among excited surface acoustic waves, a surface acoustic wave having an SV wave as its main component is reliably reflected at the opposing end surfaces of a substrate without causing mode conversion. Therefore, the present inventors discovered that it is possible to make an end surface reflection type surface acou
Hashimoto Ken-ya
Kadota Michio
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Summons Barbara
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