Piezoelectric resonator and a filter

Details Piezoelectric resonator and a filter Piezoelectric resonator and a filter

2000-09-20

2002-12-10

Dougherty, Thomas M. (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

C310S366000

Reexamination Certificate

active

06492759

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezoelectric device. More particularly, the invention concerns a piezoelectric device that in case of a piezoelectric resonator suppresses the occurrence of spurious waves and in case of a two-pole monolithic filter makes the bandwidth larger and suppresses the spurious waves.
2. Description of the Related Art
Piezoelectric devices have been being used in many communication apparatus as electronic devices each of that enables obtaining excellent frequency/temperature characteristics over a wide range of frequency from several tens of kHz to several hundreds of MHz and is small in size and also is solid.
FIGS.
5
(
a
) and
5
(
b
) are a plan view and a sectional view taken along a line Q—Q, which illustrate the construction of an AT cut crystal resonator. Substantially at the centers of the both surfaces of an AT cut crystal substrate
31
, (hereinafter referred to as “a substrate”) there are disposed mutually opposing electrodes
32
a
and
32
b.
From these electrodes
32
a
and
32
b
there are extended toward the edges of the substrate
31
lead electrodes
33
a
and
33
b.
An AT cut crystal resonator element is thereby formed. This crystal resonator element is accommodated within a package (not illustrated), and the lead electrodes
33
a
and
33
b
are connected to the terminal electrodes of the package, respectively, using electrically conductive adhesive, etc. A crystal resonator is thereby formed.
Applying a high-frequency voltage across the lead electrodes
33
a
and
33
b
of the AT cut crystal resonator illustrated in FIGS.
5
(
a
) and
5
(
b
), two kinds of thickness vibrations are excited. One is a thickness twist mode of vibration that propagates in the Z′-axial direction and the other is a thickness shear mode of vibration that propagates in the X-axial direction. However, in general, these two kinds of modes of vibrations are called “the thickness shear mode” of vibration, generically.
While various methods of analyses have been used as those for analyzing the thickness shear mode of oscillation, it is well known that an energy trapping theory has been widely used on account of its brevity.
Assume that various parameters of the crystal resonator be set as illustrated in FIG.
5
(
c
). Namely, assume that H represents the thickness of the substrate; fs represents the cut-off frequency of the substrate; L represents the size of the electrode; and fe represents the cut-off frequency of the electrode part. Then,the resonance frequency fr of the crystal resonator is located between the cut-off frequencies fe and fs as illustrated in FIG.
5
(
d
). According to the energy trapping theory, the energy trapping coefficient P is defined as in the following equation.
P
=(&pgr;{square root over ( )}2) &mgr;
L/H
{square root over ( )}&Dgr;  (1)
Also, except for the constant (&pgr;{square root over ( )}2), the energy trapping coefficient P is sometimes defined as in the following equation.
P′=&mgr;L/H
{square root over ( )}&Dgr;  (2)
where &mgr; represents the constant that is primarily determined from the elastic constants of the substrate. Accordingly, the mass loading is defined as in the following equation.
&Dgr;=(
fs−fe
)
fs
  (3)
The energy trapping coefficient is an important parameter for determining up to which vibration mode should be set under the category of the “trapped mode”.
For example, the energy trapping coefficient P′ for which only a primary symmetric mode of the fundamental wave alone is set as the trapped mode is theoretically 2.17 and 2.75, respectively, for the thickness twist mode and for the thickness shear mode. However, actually, the energy trapping coefficient P′ is not as theoretically. Correcting each of these values experimentally so that the amount of energy confined may become the largest, it is well known that these values should be corrected, respectively, to values of 2.4 and 2.8.
FIGS.
6
(
a
) and
6
(
b
) are a plan view and a sectional view taken along a line Q—Q, which illustrate a two-pole monolithic filter (hereinafter referred to as “a two-pole monolithic filter”). On one surface of a substrate
41
there are disposed electrodes
42
and
43
closely to each other, and, an electrode
44
is disposed on the other surface thereof in such a way as to oppose the electrodes
42
and
43
. From the electrodes
42
,
43
, and
44
there are extended toward the edges of the substrate
41
lead electrodes
45
,
46
, and
47
, thereby constructing a two-pole monolithic filter.
Applying a high-frequency voltage to the lead electrodes
45
and
47
, as well known, a primary symmetrical mode of and a primary anti-symmetric mode are strongly excited in the electrodes
42
,
43
, and
44
. Utilizing these two modes of oscillation waves, a two-pole monolithic filter is constructed.
Assume that fs represents the cut-off frequency of the substrate
41
; and fe represents the cut-off frequency that prevails when having adhered the electrodes
42
,
43
, and
44
to the substrate
41
. Then, the frequencies f
1
and f
2
of the excited symmetrical primary mode and primary anti-symmetric mode of oscillation waves become spectrum as illustrated in FIG.
6
(
c
). Resultantly, the frequency bandwidth twice as large as that obtained as the difference between the frequencies f
1
and f
2
becomes a frequency bandwidth of the two-pole monolithic filter.
However, when attempting to design an oscillation device having a high frequency band of 200 MHz as the one for use in a crystal resonator or two-pole monolithic filter, even if using as the electrode materials an aluminum alloy that is light in mass, it is necessary to set the size of the electrodes to be very small in order to satisfy the above-described energy trapping coefficient. As a result, in case of a crystal resonator, there was the problem that the equivalent resistance was excessively high while, in case of a two-pole monolithic filter, there was the problem that the impedance was excessively high. Furthermore, at the time of the manufacture, because the electrode size is excessively small, there was also the problem that mask alignment was very difficult to make.
In order to solve these problems, an attempt has been made to use an entire-surface electrode as the electrode for use on one surface of a high-frequency crystal resonator or high-frequency two-polermonolithic filter. Through making this attempt, a device that has been arranged for mass loading not to contribute to the energy trapping has ever been put to practical use. However, when, for example, setting the electrode configuration on one side of a 200-MHz frequency-band two-pole monolithic filter of the fundamental-wave to be 0.15 mm×0.25 mm, the energy trapping coefficient becomes excessively large. As a result of this, the problem that an inharmonic mode of oscillation waves occurs still remains unsolved.
Also, FIGS.
7
(
a
) and
7
(
b
) are a plan view and a sectional view taken along a line Q—Q, both illustrating the construction of a monolithic crystal filter that is disclosed in Japanese Patent Application Laid-Open No. Hei10-32459. This publication describes a two-pole monolithic filter comprising a substrate
51
and electrodes
52
,
53
, and
54
. It describes that the entire remaining portion of the substrate
51
has disposed thereon electrodes for a suppression
55
b,
56
a,
and
56
b
in such a way as for these electrodes to be kept at a distance from those electrodes
52
,
53
, and
54
of the two-pole monolithic filter. And it describes thereby suppressing the occurrence of a higher harmonic mode of oscillation waves such as unnecessary flexible vibration, contour vibration, etc., whereby excellent pass-band characteristics have been obtained.
However, according to the width of the gap between the electrodes
52
,
53
, and
54
of the two-pole monolithic filter and the electrodes for a suppression
55
a,
55
b,
56
a,
and
56
b
dispose

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