Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-12-21
2004-06-15
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S366000, C310S365000, C310S367000, C310S363000
Reexamination Certificate
active
06750593
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high frequency piezoelectric resonator and, more particularly, to a high frequency piezoelectric resonator that is adapted to suppress the occurrence of spurious.
2. Description of the Related Art
Following making higher the frequency, making higher the speed of data processing, and making larger the capacity, of communication apparatus, there has in recent years been a strong demand for making higher the frequency of a piezoelectric device that is used in each of these apparatus.
Development as to the increase in frequency of an AT cut crystal resonator has been being performed and, during this period of time, the frequency intended to be used therein has hitherto reached several hundreds of MHz. As well known, the waves in the vibration mode of the AT cut crystal resonator are in the thickness shear mode. Therefore, the frequency thereof is in inverse proportion to the thickness of the crystal plate. So, it is necessary to make thin the thickness of the crystal plate in order to make the frequency higher.
FIGS.
7
(
a
) and
7
(
b
) are views illustrating the construction of a conventional high frequency AT cut crystal resonator, FIG.
7
(
a
) being a plan view and FIG.
7
(
b
) being a sectional view. Part of a main surface of a crystal plate
21
is recessed using, for example, a photolithography technique and method of etching. The portion of the substrate
21
corresponding to this recess
22
is made to be an ultra-thin vibration portion. This portion of the plate
21
and a thick annular surrounding portion that retains the area around that vibration portion are formed integrally with each other. On one main surface (flat surface) of the crystal plate
21
there is disposed an electrode
23
. From the electrode
23
there is extended toward the edge of the plate
21
a lead electrode
25
. Further, on the other surface of the substrate
21
, i.e., on the surface having therein the recess
22
, there is formed an entire electrode
24
.
It is to be noted that the energy trapping of the AT cut high frequency crystal resonator having a structure illustrated in
FIG. 7
, as well known, depends only upon the mass loading of the electrode
23
and not upon the mass loading of the entire electrode
24
on the other surface.
FIG. 8
is a view illustrating a measured example of the frequency spectrum of the AT cut high frequency crystal resonator having the structure illustrated in FIG.
7
. The resonance frequency is set to be 156.6 MHz (the thickness of the crystal plate
21
is set to be approximately 10 &mgr;m); the dimension in the X-axial direction of the electrode
23
is set to be 0.55 mm; the dimension in the Z′-axial direction is set to be 0.435 mm; and as the electrode
23
there was adhered a thin film of gold having a thickness of 600 Å on a backing layer of nickel having a thickness of 50 Å. It is to be noted that the material of the film constituting the entire electrode
24
was also formed substantially in the same way.
As obvious from the frequency spectrum of
FIG. 8
, from the fundamental mode up to a large number of inharmonic modes fall within the range of the energy trapping mode. These modes, as seen, are sharply excited as spurious. Using this crystal resonator as one component of the oscillator, there is the possibility that the phenomenon of the frequency jump will occur.
As a method of analyzing a spurious mode in the thickness shear resonator such as that illustrated in
FIG. 8
, there is well known an energy trapping theory, which will briefly be explained below.
FIG. 9
is a sectional view illustrating the section of an ordinary AT cut crystal resonator. The diameter of a circular crystal plate
31
having a thickness of H is set to be
2
b
; the diameter of an electrode
32
that has been adhered thereto is set to be a; and the cutoff frequencies of the electrode
32
portion and the plate
31
portion are set to be f
1
and f
2
, respectively. Generally, the plate-back &Dgr;, the energy trapping coefficient &zgr; and the normalized frequency &psgr; are expressed as follows.
Δ
=
(
f
2
-
f
1
)
f
1
(
1
)
ζ
=
na
H
Δ
(
2
)
φ
=
(
f
-
f
1
)
(
f
2
-
f
1
)
(
3
)
In the equation No. 2, n represents the order of the overtone mode. When the mode is the fundamental mode, n=1.
FIG. 10
illustrates a frequency spectrum of the crystal resonator having the structure of
FIG. 9
, as determined by calculation, with the energy trapping coefficient &zgr; being plotted along the abscissa and the normalized frequency &psgr; being plotted along the ordinate. Generally, in order to design an resonator wherein spurious less occur, using the energy trapping coefficient &zgr;=0.707 that is immediately before the symmetrical 1st mode S
1
starts to be trapped is admitted as being preferable.
Here,
FIG. 10
illustrates the results that have been obtained by the calculation that has been performed on the premise that the substrate be an isotropic elastic body. However, if applying this method of determining the frequency spectrum to an anisotropic piezoelectric plate such as a crystal, it is well known that it is sufficient to multiply each length by an anisotropic constant to thereby correct this length. For instance, with respect to the thickness twist mode and the thickness shear mode of the crystal resonator, their respective optimum energy trapping coefficients &zgr; are said to be 2.4 and 2.8.
Determining the energy trapping coefficient &zgr; of the high frequency crystal resonator illustrated in
FIG. 8
according to the equation (2) as a trial, the &zgr;=5.7. It is seen that this value is the one that is much greater than the optimum value. Therefore, as stated above, a high order of vibration mode also becomes an energy-confining mode, whereby a large number of spurious are sharply excited.
On the other hand, in order to improve the spurious characteristic of the AT cut crystal resonator, a piezoelectric resonator that can somewhat arbitrarily control the frequency of the generated spurious regardless of the length, film thickness, etc. of the electrode has been proposed in Japanese Patent Application Laid-Open Nos. Hei-9-27729 and Hei-9-93076.
FIG.
11
(
a
) is a plan view illustrating a crystal resonator that is as proposed above and FIG.
11
(
b
) is a sectional view thereof. On both surfaces of the central parts of a crystal plate
41
there are disposed main electrodes
42
a
and
42
b
. Simultaneously, second electrodes
44
a
and
44
b
are disposed so as to surround the peripheral edges of those electrodes
42
a
and
42
b
and with a gap between the both. The respective cutoff frequencies of the main electrodes
42
a
,
42
b
, gap portion, and second electrodes
44
a
,
44
b
are set to be f
1
, f
2
, and f
3
, as illustrated in FIG.
11
(
b
). Here, the thickness of the electrode films are set so that the relationship among the cutoff frequencies of f
1
<f
3
<f
2
holds true among the cutoff frequencies.
Also, the second amount &Dgr;2 of decrease in frequency and the depth &ngr; of the gap (hereinafter referred to as “a groove depth”) are respectively defined as follows.
Δ
2
=
(
f
3
-
f
1
)
f
1
(
4
)
v
=
(
f
2
-
f
3
)
(
f
3
-
f
1
)
(
5
)
According to the above official gazettes, whatever value the mass loading (the film thickness) of the main electrode
42
a
,
42
b
is set to be at, only if appropriately setting the mass loading (the film thickness) of the second electrode
44
a
,
44
b
correspondingly, it becomes possible to decrease the amount &Dgr;2 of decrease in frequency, which participates in the energy trapping coefficient &zgr;, down to a desired value. The above official gazettes describe that therefore it becomes possible to easily control the energy trapping coefficient &zgr; and hence to manufacture a crystal resonator the spurious of that are less. Further, the above official gazettes describe that, by aptly setting the groove depth &ngr; and the ratios q/a
Dougherty Thomas M.
Gonzalez R. Julio
Koda & Androlia
Toyo Communication Equipment Co., Ltd.
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