Surface acoustic wave device

Details Surface acoustic wave device Surface acoustic wave device

2002-08-05

2004-08-10

Tokar, Michael (Department: 2819)

Wave transmission lines and networks

Coupling networks

Electromechanical filter

C333S193000, C310S31300R

Reexamination Certificate

active

06774747

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a surface acoustic wave device which further reduces the fluctuation of frequency to temperature change by using an in-plane rotated ST cut quartz crystal plate around the Z′-axis (hereinafter “in-plane rotated ST cut quartz crystal plate”).
2. Description of Related Art
Related art surface acoustic wave devices exist in which IDT (Interdigital Transducer) electrodes are provided on the principal surface of a piezoelectric flat plate represented by a quartz crystal sheet, and multiple reflectors are provided at both ends of the IDT electrodes to oscillate a high frequency stably (hereinafter “SAW resonator”).
An ST cut SAW resonator can be provided in which an ST cut quartz crystal plate is used as a piezo-electric flat plate to reduce the fluctuation of frequency to temperature change, and the X-axis direction of the ST cut quartz crystal plate is taken as the propagation direction of the acoustic wave.
FIG. 6
is a schematic sectional view showing the structure of an ST cut SAW resonator. As shown in
FIG. 6
, in the ST cut SAW resonator
1
, an ST cut quartz crystal plate
2
is taken as the substrate, and IDT electrodes
3
are provided on the principal surface of the substrate. Comb teeth-like anodes
4
and cathodes
5
are alternately arranged in the IDT electrodes
3
, and a surface acoustic wave is excited due to the piezoelectric effect of the quartz crystal plate by adding a high-frequency electric field between the anodes
4
and cathodes
5
.
Multiple reflectors
6
are provided on both sides of the IDT electrodes
3
to reflect the surface acoustic wave, and the reflection of the surface acoustic wave emitted from the IDT electrodes
3
is performed by multiple short-circuit electrodes
7
formed in the reflectors
6
. The anodes
4
and cathodes
5
in the IDT electrodes
3
and the short-circuit electrodes
7
in the reflectors
6
are arrayed in the X-axis direction of the ST cut quartz crystal plate
2
, and the reflection of the surface acoustic wave in the short-circuit electrodes
7
is performed at positions of both edges of the electrodes.
In the ST cut SAW resonator
1
thus constructed, as shown in
FIG. 6
, the width and the pitch of the anodes
4
and cathodes
5
in the IDT electrodes
3
are defined as L
t
, P
t
, and the width and the pitch of the short-circuit electrodes
7
in the reflectors
6
are defined as L
r
, P
r
. The thickness of the anodes
4
and cathodes
5
is defined as H
t
, and the thickness of the short-circuit electrodes
7
is defined as H
r
.
FIG. 7
is a graph showing the reflection coefficient per short-circuit electrode of the ST cut SAW resonator. In the ST cut SAW resonator
1
, if the reflection coefficient of a surface acoustic wave can be increased, it is possible to reduce the number of reflectors
6
and to miniaturize the resonator itself.
FIG. 7
shows the value of L
t
/P
t
(hereinafter “L
r
/P
r
,” is referred to as “&eegr;”) on the horizontal axis, and shows the reflection coefficient per short-circuit electrode on the vertical axis, and shows how the reflection coefficient fluctuates with the value of the H
t
/2P
t
(≅H
r
/2P
r
).
When the reflection coefficient is considered, the H
t
/2P
t
and H
r
/2P
r
can be regarded as nearly the same value. 2P
t
and 2P
r
are nearly the same value, so 2P
t
and 2P
r
are defined as &lgr;. Therefore, in the present embodiment, H
t
/2P
t
and H
r
/2P
r
are not distinguished and are treated as the same value, i.e., H/&lgr;.
As shown in
FIG. 7
, for the ST cut SAW resonator
1
, the reflection coefficient also increases with an increase in the &eegr; value, and the higher the H/&lgr; value, the greater the reflection coefficient for the relationship of H/&lgr; and reflection coefficient, as disclosed in Japanese Laid-Open Patent Application H2-260908.
In the ST cut SAW resonator
1
, the thickness (H) of the anodes
4
, cathodes
5
and short-circuit electrodes
7
is commonly set up so that the H/&lgr; values become about 0.03 from a viewpoint of obtaining objective temperature characteristics. The &eegr; value is set to 0.5, so that a relation of P
t
=2L
t
is established.
In the SAW resonator, a quartz crystal plate cut from the in-plane rotated ST cut quartz crystal plate around the Z′-axis is sometimes used to further reduce the frequency fluctuation caused by temperature change. However, the relationship between the &eegr;, H/&lgr; value and the reflection coefficient have not yet been verified with the in-plane rotated ST cut quartz crystal plate around the Z′-axis.
SUMMARY OF THE INVENTION
The inventors studied the in-plane rotated ST cut quartz crystal plate around the Z′-axis, and discovered that it has characteristics quite different from a related art ST cut quartz crystal plate, and a regularity of the related art ST cut quartz crystal plate in which the reflection coefficient is increased by increasing the &eegr; and H/&lgr; values does not apply. Therefore, a problem arises that the reflection coefficient cannot be fully obtained, even if the regularity of the related art ST cut quartz crystal plate is applied to the in-plane rotated ST cut quartz crystal plate around the Z′-axis to increase the &eegr; and H/&lgr; values.
The present invention addresses the above problem, and provides a surface acoustic wave device which enables the reflection coefficient to be increased by grasping the characteristics of the in-plane rotated ST cut quartz crystal plate around the Z′-axis.
The present invention was discovered via various studies and the knowledge that the characteristics of the ST cut quartz crystal plate in a plane rotated around the Z′-axis differ from the characteristics of a related art ST cut quartz crystal plate.
Namely, the surface acoustic wave device relating to the present invention has one or more pairs of IDT electrodes to excite a Rayleigh wave arranged on a principal surface of an in-plane rotated ST cut quartz crystal plate and existing at a Euler angle (0, 113-135,±(40-49)), and the ratio L
t
/P
t
of width L
t
of the IDT electrodes to pitch P
t
of the IDT electrodes is less than 0.5. It is desirable that the L
t
/P
t
be 0.32±0.1, and it is further desirable that the thickness of the IDT electrodes be taken as H
t
and the H
t
/2P
t
be 0.06±0.01.
Another surface acoustic wave device relating to the present invention has one or more pairs of IDT electrodes to excite a Rayleigh wave, and one or more reflectors to trap the Rayleigh wave arranged on the principal surface of the in-plane rotated ST cut quartz crystal plate existing at a Euler angle (0, 113-135,±(40-49)), and either or both of the ratio L
t
/P
t
of width L
t
of the IDT electrodes to pitch P
t
in the IDT electrodes and the ratio L
r
/P
r
of width L
r
of the reflector to pitch P
r
in the reflectors are less than 0.5.
It is desirable that either or both of the L
t
/P
t
and the L
r
/P
r
be 0.32±0.1, and it is further desirable that the thickness of the IDT electrodes be taken as H
t
and the thickness of the reflectors be taken as H
r
, and either or both of H
t
/2P
t
and H
r
/2P
r
be 0.06±0.01 in another surface acoustic wave device relating to the present invention.
Thus, an acoustic wave device which has an in-plane rotated ST cut quartz crystal plate around the Z′-axis is different from a surface acoustic wave device to which a related art ST cut quartz crystal plate is applied. If the L
t
/P
t
value in the IDT electrodes is decreased, the value of reflection coefficient is enhanced.
More specifically, it is desirable that the L
t
/P
t
value be less than the L
t
/P
t
value (0.5) generally applied to a related art surface acoustic wave device to which a related art ST cut quartz crystal plate is applied. Thus, setting the L
t
/P
t
value to be less than 0.5, enables the value of reflection coefficient to be enhanced and miniaturization of the device itself to be achieved.
Setting t

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