Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-11-10
2003-09-16
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S320000, C310S360000, C310S366000, C310S333000
Reexamination Certificate
active
06621194
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezoelectric element and a mobile communication device using the same.
2. Description of the Prior Art
The recent trends of speeding up information processing terminals and digitalization of communication require compact and high frequency vibrators and compact and wide band intermediate frequency filters. For these piezoelectric elements, energy trapping piezoelectric elements conventionally have been used, but the recent trends require more compact and wider band piezoelectric elements.
Crystal filters are used most widely as the energy trapping piezoelectric filters. However, it is difficult to form a wide band filter with quartz, because the material has a small electromechanical coupling factor.
For this reason, piezoelectric single crystals or piezoelectric ceramics that have a large electromechanical coupling factor are used for these applications. Among the piezoelectric single crystals, an X-cut LiTaO
3
single crystal, which has a high Q of vibration and excellent temperature characteristics, is used most commonly.
LiTaO
3
single crystal has large piezoelectricity and is suitable for a wide band filter, but undesired vibration tends to be excited so that large spurious (spurious response) is likely to occur. Therefore, thickness shear vibration having relatively small spurious is used as the vibration mode. The thickness shear vibration refers to vibration whose displacement direction is substantially identical to its propagation direction. Thickness twist vibration whose propagation direction is perpendicular to its displacement direction also is excited in the LiTaO
3
single crystals, which also causes spurious. Furthermore, reflections of these vibrations at the end faces of oscillating elements also cause spurious.
In order to reduce the influence of the reflections at the end faces of the thickness shear vibration and the thickness twist vibration for further compact piezoelectric vibrators, a method for forming a piezoelectric vibrator in a strip has been known.
When using a X plate of LiTaO
3
single crystal as a substrate of a strip vibrator, a substrate that is cut in a direction of −50°±2° from the Y-axis has the lowest resonance impedance, and therefore usually this direction is used as the displacement direction of vibration.
FIG. 8
is a plan view of an example of such a conventional LiTaO
3
piezoelectric vibrator.
Referring to
FIG. 8
, a conventional piezoelectric vibrator
1
includes a piezoelectric substrate
2
formed of LiTaO
3
and a pair of exciting electrodes
3
formed on one principal plane and another principal plane of the piezoelectric substrate
2
. In
FIG. 8
, an electrode for interconnection is not shown. As shown in
FIG. 8
, in the conventional LiTaO
3
resonator in general, the piezoelectric substrate is cut in such a manner that the displacement direction of the thickness shear vibration corresponds to the longitudinal direction, and the exciting electrodes
3
are formed along the entire width in the traverse direction of the piezoelectric substrate
2
.
In this vibrator, it is necessary to cut a substrate into a strip with a small width (about several times the thickness) in order to avoid undesired vibration in connection with the width. Therefore, when the direction in which the substrate is cut is significantly different from the direction of the cleavage of the crystal, the element is broken readily in the cleavage direction when cutting the substrate. Thus, miniaturization was difficult. Moreover, if chipping occurs in a cut plane, the chipping on the cut plane causes spurious. Thus, it was difficult to form a high precision vibrator or filter that operates at high frequencies.
As an attempt to solve this problem, a piezoelectric vibrator employing a piezoelectric substrate cut in the direction −57° from the Y-axis was proposed (see JP 6-303090A). The cleavage plane of the X plate of LiTaO
3
single crystal is in the direction −57° from the Y-axis, and therefore cutting along this cleavage plane can improve the processability.
However, there is still a problem in that the spurious caused by the shape of the element is likely to occur even if the piezoelectric substrate cut in the direction −57° from the Y-axis is used.
SUMMARY OF THE INVENTION
Therefore, with the foregoing in mind, it is an object of the present invention to provide a piezoelectric element that can suppress undesired spurious and allows operations at high frequencies, and to provide a mobile communication device using the same.
A piezoelectric element of the present invention includes a piezoelectric substrate, and a pair of electrodes formed on at least one principal plane selected from the group consisting of a first principal plane of the piezoelectric substrate and a second principal plane opposed to the first principal plane, wherein thickness shear vibration occurs, and the vibration direction of the thickness shear vibration is nonparallel to a side wall of the piezoelectric substrate. In the piezoelectric element of the present invention, undesired vibration such as thickness twist vibration can be suppressed, substantially without affecting the thickness shear vibration. Therefore, according to the above-described piezoelectric element, undesired spurious can be suppressed.
In the above-described piezoelectric element, it is preferable that the piezoelectric substrate has a shape of an elongated rectangular solid, and each of the pair of electrodes is formed along the entire width in the traverse direction on the at least one principal plane. This embodiment provides a strip-type piezoelectric element that allows miniaturization. In this specification, the traverse direction refers to a direction perpendicular to the longitudinal direction of the elongated rectangular solid-shaped substrate.
In the above-described piezoelectric element, it is preferable that the piezoelectric substrate is formed of LiTaO
3
single crystal, the pair of electrodes includes a first exciting electrode and a second exciting electrode, the first exciting electrode is formed on the first principal plane, the second exciting electrode is formed on the second principal plane, the side wall is a side wall in the longitudinal direction of the piezoelectric substrate, and the normal line of an edge of the first exciting electrode and the normal line of an edge of the second exciting electrode are parallel to each other and are nonparallel to the side wall in the longitudinal direction. This embodiment provides a piezoelectric vibrator that can be produced easily, has particularly small spurious, and allows operations at high frequencies. In this specification, the side wall in the longitudinal direction refers to a principal plane adjacent to the first principal plane in the traverse direction
In the above-described piezoelectric element, it is preferable that the first and second principal planes are X planes of the piezoelectric substrate, and the side wall in the longitudinal direction is parallel to a plane obtained by rotating the XY plane of the piezoelectric substrate for 56° to 58° clockwise about the X-axis. This embodiment provides a piezoelectric vibrator that can be produced particularly easily, has even smaller spurious, and allows operations at high frequencies. In this specification, when “clockwise rotation about the X-axis” is referred to, the substrate is viewed from the plus direction of the X-axis (see FIG.
1
A). The term “X plane of the substrate” as used in this specification includes substrates cut at planes that are inclined several degrees from the X plane of the substrate as equivalents.
In the above-described piezoelectric element, it is preferable that the direction of the normal line is a direction obtained by rotating the Y-axis of the piezoelectric substrate for 47° to 56° clockwise on the YZ plane. This embodiment provides a piezoelectric vibrator that has even smaller spurious.
In the above-described piezoelectric elemen
Moritoki Katsunori
Otsuchi Tetsuro
Sugimoto Masato
Gonzalez Julio C.
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P.C.
Ramirez Nestor
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