Piezoelectric resonator and method of fabricating the same

A piezoelectric resonator which comprises a piezoelectric resonance element in which electrodes are formed on a pair of side surfaces, which are opposed to each other, of a piezoelectric substrate. The electrodes are opposed to each other while being separated by the piezoelectric substrate in the center of the piezoelectric substrate to form a vibrating portion of an energy-trapped type utilizing a thickness shear vibration mode in the center of the piezoelectric substrate. Spacers are respectively fixed to the side surfaces of the piezoelectric substrate with gaps being provided between the spacers and the above-mentioned vibrating portion, and a pair of cover sheets are respectively affixed to both major surfaces of a structure comprising the spacers and the piezoelectric resonance element so as to form gaps on top of and beneath the vibrating portion in the piezoelectric resonance element.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a piezoelectric resonator 
utilizing a thickness shear vibration mode and a method of fabricating the 
same, and more particularly, to a piezoelectric resonator having a 
structure in which the thickness of a product having the resonator is 
independent of the resonance frequency, and a method of fabricating the 
same. 
2. Description of the Prior Art 
Referring to FIGS. 11 and 12, one example of a conventional piezoelectric 
resonator will be described. A piezoelectric resonator 1 shown in FIG. 11 
is constructed using a piezoelectric resonance element 2 of an 
energy-trapped type utilizing a thickness shear vibration mode, spacers 3 
and 4, and cover plates or sheets 5 and 6, as shown in FIG. 12. 
The piezoelectric resonance element 2 has a structure in which electrodes 
2b and 2c are respectively formed on both major surfaces of a 
piezoelectric substrate 2a. The electrodes 2b and 2c are opposed to each 
other while being separated by the piezoelectric substrate 2a in the 
center of the piezoelectric substrate 2a, thereby to form a vibrating 
portion of an energy-trapped type utilizing a thickness shear vibrating 
mode in a region where the electrodes 2b and 2c are opposed to each other. 
The spacers 3 and 4 are respectively arranged in side parts of the 
piezoelectric resonance element 2. The spacers 3 and 4 are constituted by 
rectangular insulating plates and have cutout portions 3a and 4a formed by 
cutting away parts of side surfaces of the insulating plates, 
respectively. The cutout portions 3a and 4a are provided so as to form 
gaps for not preventing the vibration of the above described vibrating 
portion. 
In the fabrication of the piezoelectric resonator, the spacers 3 and 4 are 
first respectively fastened to both side surfaces of the piezoelectric 
resonance element 2 and then, the cover sheets 5 and 6 are respectively 
affixed to the piezoelectric resonance element 2 and the spacers 3 and 4 
so as to cover both major surfaces of the piezoelectric resonance element 
2 and the spacers 3 and 4. The cover sheets 5 and 6 are respectively 
constituted by rectangular insulating members having notches 5a and 5b and 
6a and 6b. The notches 5b and 6a are respectively provided so as to expose 
the electrodes 2b and 2c in the piezoelectric resonance element 2 to the 
exterior (see FIG. 11) to make it easy to make electrical connection to 
the exterior. 
Reference numerals 7 and 8 denote adhesive layers. As shown in FIG. 12, the 
adhesive layer 8 is provided in a peripheral region excluding a central 
region 6c on the upper surface of the cover sheet 6. This is for forming a 
gap so that the thickness of the adhesive layer 8 does not prevent the 
vibration of the vibrating portion in the piezoelectric resonance element 
2 after lamination. Similarly, the adhesive layer 7 is provided in a 
peripheral region excluding a central region on the lower surface of the 
cover sheet 5. 
In the piezoelectric resonator 1 shown in FIG. 11, the thickness t of the 
piezoelectric resonance element 2 utilizing a thickness shear vibration 
mode is determined by a desired resonance frequency. For example, when the 
resonance frequency is 3.58 MHz, the thickness t must be generally about 
0.35 mm. Consequently, when it is desired to limit the entire thickness T 
of the piezoelectric resonator 1 to be, for example, 0.5 mm or less, the 
total of the thickness of the adhesive layers 7 and 8 and the cover sheets 
5 and 6 must be 0.15 mm or less. As a result, the thicknesses of the cover 
sheets 5 and 6 must be substantially decreased, thereby to make it 
impossible to obtain a piezoelectric resonator having considerable 
mechanical strength. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a 
piezoelectric resonator which has considerable mechanical strength 
irrespective of a desired resonance frequency and thus, is suitable for 
being made thinner, and a method of fabricating the same. 
The present invention is directed to a piezoelectric resonator comprising a 
piezoelectric resonance element utilizing a thickness shear vibration mode 
which has a piezoelectric substrate and a pair of electrodes formed on a 
pair of side surfaces, which are opposed to each other, of the 
piezoelectric substrate and opposed to each other while being separated by 
the piezoelectric substrate in the center of the piezoelectric substrate 
to form a vibrating portion in a region where the above electrodes are 
opposed to each other, spacers respectively fastened to the above side 
surfaces of the piezoelectric substrate in the above piezoelectric 
resonance element with gaps being provided between the spacers and the 
above vibrating portion, and cover sheets respectively fastened to both 
major surfaces of the above piezoelectric resonance element and the above 
spacers so as to provide gaps for not preventing the vibration of the 
vibrating portion on and beneath the above vibrating portion. 
Furthermore, a method of fabricating a piezoelectric resonator according to 
the present invention comprises the steps of preparing a piezoelectric 
resonance element utilizing a thickness shear vibration mode which has a 
piezoelectric substrate and a pair of electrodes formed on a pair of side 
surfaces, which are opposed to each other, of the piezoelectric substrate 
and opposed to each other while being separated by the piezoelectric 
substrate in the center of the piezoelectric substrate to form a vibrating 
portion and a pair of spacers composed of plate-shaped members, 
respectively fixing the above spacers on side surfaces, on which the 
electrodes are formed, of said piezoelectric resonance element with gaps 
for not preventing the vibration of the vibrating portion being provided 
between the spacers and the vibrating portion, and respectively affixing a 
pair of cover sheets to both major surfaces of a structure comprising the 
above spacers and the above piezoelectric resonance element so as to leave 
gaps for not preventing the vibration of the vibrating portion in the 
above piezoelectric resonance element. 
In the present invention, electrodes are provided on a pair of side 
surfaces, which are opposed to each other, of a piezoelectric resonance 
element, a vibrating portion utilizing a thickness shear vibration mode is 
formed in the center of the pair of side surfaces, and spacers are 
fastened to the above side surfaces so as to have gaps between the spacers 
and the vibrating portion. In addition, cover sheets are affixed to both 
major surfaces of a structure comprising the piezoelectric resonance 
element and the spacers with gaps being provided on and beneath the 
vibrating portion in the piezoelectric resonance element. Consequently, 
the resonance frequency of the piezoelectric resonance element is 
determined by the distance between the electrodes, that is, the distance 
between the side surfaces and is independent of the thickness of the 
piezoelectric resonance element. Accordingly, the thickness of the 
piezoelectric resonance element can be arbitrarily set irrespective of the 
resonance frequency, thereby to make it possible to obtain piezoelectric 
resonators having various resonance frequencies without altering the 
thickness as well as to use as the cover sheets ones having considerable 
strength in terms of material and construction. Accordingly, even when the 
entire thickness is decreased, it is possible to construct a piezoelectric 
resonator having considerable mechanical strength. 
The foregoing and other objects, features, aspects and advantages of the 
present invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a perspective view showing a piezoelectric resonator according to 
a first embodiment of the present invention, and FIG. 2 is an exploded 
perspective view showing the above piezoelectric resonator. 
A piezoelectric resonator 11 shown in FIG. 1 has a structure in which 
spacers 13 and 14 are respectively fastened to side parts of a 
piezoelectric resonance element 12 having a long narrow rectangular plate 
shape, and cover sheets 15 and 16 are respectively affixed to both major 
surfaces of a structure comprising the piezoelectric resonance element 12 
and the spacers 13 and 14, as shown in FIG. 2. 
The piezoelectric resonance element 12 has a structure in which electrodes 
12b and 12c are formed on a pair of side surfaces, which are opposed to 
each other, of a piezoelectric substrate 12a having a long narrow 
rectangular plate shape, as shown in FIG. 3. The electrodes 12b and 12c 
are formed so as to be opposed to each other while being separated by the 
piezoelectric substrate 12a in a central region of the piezoelectric 
substrate 12a, thereby to form a vibrating portion of an energy-trapped 
type utilizing a thickness shear vibration mode in a region where the 
electrodes 12b and 12c are opposed to each other. 
In the present embodiment, terminal electrodes 12d and 12e electrically 
connected to the electrodes 12b and 12c are respectively formed on both 
end surfaces of the piezoelectric substrate 12a. The terminal electrodes 
12d and 12e are provided so as to make it easy to electrically connect the 
piezoelectric resonator 11 shown in FIG. 1 to the exterior. However, it is 
not always necessary to form the terminal electrodes 12d and 12e. 
The piezoelectric resonance element 12 shown in FIG. 3 resonates by 
applying an AC electric field to the electrodes 12b and 12c. With this 
configuration the piezoelectric substrate 12a expands and contracts, as 
represented by a broken line in FIG. 3. In addition, the resonance 
frequency of the piezoelectric resonance element 12 depends on not the 
wall thickness t of the piezoelectric substrate 12a but the width W 
thereof. Consequently, it is possible to obtain a piezoelectric resonance 
element 12 having a desired resonance frequency without altering the wall 
thickness t. 
Although in FIG. 3, the side surfaces, on which the electrodes 12b and 12c 
are formed, of the piezoelectric substrate 12a are in such a shape as to 
have a relatively smaller area than that of the upper and lower surfaces 
of the piezoelectric substrate 12a, the side surfaces, on which the 
electrodes 12b and 12c are formed, of the piezoelectric substrate 12a may 
be conversely in such a shape as to have a larger area than that of the 
upper and lower surfaces of the piezoelectric substrate 12a. That is, the 
thickness t may be larger than the width W, in FIG. 3. 
Turning to FIG. 2, the spacers 13 and 14, having a rectangular plate shape, 
and being made of an insulating material having the same wall thickness as 
that of the piezoelectric resonance element 12, are respectively fastened 
to the side parts of the piezoelectric resonance element 12 by adhesive 
layers 17 and 18. The adhesive layers 17 and 18 are shaped so as not to 
contact the above described vibrating portion in the piezoelectric 
resonance element 12. As a result, gaps 19a and 19b are respectively 
formed between the vibrating portion in the piezoelectric resonance 
element 12 and the spacers 13 and 14. The gaps 19a and 19b are provided so 
as not to prevent the vibration of the vibrating portion. 
The cover sheets 15 and 16 are respectively affixed to both major surfaces 
of a structure comprising the piezoelectric resonance element 12 and the 
spacers 13 and 14 fastened to the side parts thereof, as shown in FIG. 2. 
An adhesive layer 20 is provided on the upper surface of the cover sheet 
16 along the outer periphery of the cover sheet 16. Similarly, an adhesive 
layer is provided on the lower surface of the cover sheet 15. The adhesive 
layer 20 is provided on the upper surface of the cover sheet 16 along the 
outer periphery of the cover sheet 16 so as to form a gap for not 
preventing the vibration of the vibrating portion in the piezoelectric 
resonance element 12 beneath the piezoelectric resonance element 12 after 
affixing. Similarly, an adhesive layer 21 is provided on the lower surface 
of the cover sheet 15 only along the outer periphery of the cover sheet 
15, thereby to form a similar gap on the piezoelectric resonance element 
12. 
The cover sheets 15 and 16 are made of a suitable rigid material such as 
insulating ceramics. In the present embodiment, notches 15a and 15b and 
16a and 16b are respectively formed in both edges of the cover sheets 15 
and 16. Consequently, as seen in FIG. 1, both ends of the piezoelectric 
resonator 12 are exposed over a wide area by the notches 15a, 15b, 16a and 
16b. Accordingly, it is possible to electrically connect the terminal 
electrodes 12d and 12e (see FIG. 3) reliably to the exterior. 
The above described notches 15a to 16b are not indispensable constituent 
elements in the present invention. For example, a piezoelectric resonator 
in a shape obtained by cutting in the direction of thickness along 
portions represented by two-dot and dash lines A in FIG. 1 may be 
constructed. However, in the piezoelectric resonator obtained by cutting 
along the two-dot and dash lines A, it is preferably that the terminal 
electrodes 12d and 12e (see FIG. 3) are respectively provided to both end 
surfaces of the piezoelectric substrate 12a. In that case, the terminal 
electrodes 12d and 12e formed on the end surfaces of the piezoelectric 
resonance element 12 may be formed by sputtering along with the electrodes 
12b and 12c in the step of obtaining the piezoelectric resonance element 
12 shown in FIG. 3, or may be formed by sputtering or the like after a 
laminated body shown in FIG. 1 is constructed. In addition, the terminal 
electrodes 12d and 12e may be coated with solder by the dipping process or 
the like. 
Referring now to FIGS. 4 and 5, a method of fabricating the above described 
piezoelectric resonator 11 will be described. As shown in FIG. 4, a 
plurality of mother piezoelectric resonance element members 31 and a 
mother spacer plate 32 are first prepared. Each of the mother 
piezoelectric resonance element members 31 has a structure in which 
electrodes 33a and 33b are formed on both major surfaces of a 
piezoelectric substrate 31a subjected to polarization processing. The 
plurality of mother piezoelectric resonance element members 31 are 
disposed side by side as shown in FIG. 4 and each of them is prepared so 
as to construct the piezoelectric resonance element 12 shown in FIG. 3. 
More specifically, the mother piezoelectric resonance element member 31 
has such a shape that a plurality of piezoelectric resonance elements 12 
as shown in FIG. 3 are connected in the transverse direction with the 
side, on which one of the electrodes 12b and 12c is formed, of the 
piezoelectric resonance element 12 shown in FIG. 3 being pointed downward. 
That is, the mother piezoelectric resonance element member 31 is in such a 
shape that a lot of piezoelectric resonance elements 12 shown in FIG. 3 
are obtained by cutting in the direction at right angles to the 
longitudinal direction of the electrodes 33a and 33b. 
On the other hand, the mother spacer plate 32 is constituted by a 
rectangular insulating plate, and a plurality of adhesive layers 34a and 
34b are respectively provided in parallel to its upper and lower surfaces. 
The thickness of the mother spacer plate 32 is twice the width of the 
spacers 13 and 14 shown in FIG. 2. That is, the mother spacer plate 32 is 
so constructed that two spacers 13 and 14 are connected to each other on 
their side surfaces by cutting in the direction of thickness along one dot 
and dash lines C and one dot and dash lines D at right angles thereto. 
Although in the fabricating method according to the present embodiment, the 
plurality of mother piezoelectric resonance element plates 31 and mother 
spacer plates 32 as described above are alternately laminated, a mother 
spacer plate 35 in the uppermost part is so constructed as to have a 
thickness which is one-half that of the mother spacer plate 32. In 
addition, a plurality of adhesive layers 36a are printed in parallel with 
predetermined spacing only on the lower surface of the mother spacer plate 
35. 
Then, a laminated body obtained by laminating the plurality of mother 
piezoelectric resonance element members 31, the mother spacer plates 32 
and the mother spacer plate 35 is cut in the direction of thickness along 
one dot and dash lines D shown in FIG. 4, thereby to obtain a mother 
device substrate 37 shown in the center of FIG. 5. Mother cover sheets 38 
and 39 are respectively affixed to both major surfaces of the mother 
device substrate 37. Adhesive layers, which are not particularly shown, 
corresponding to the adhesive layers 20 and 21 shown in FIGS. 1 and 2 are 
provided on the surfaces, on the side of the device substrate 37, of the 
mother cover sheets 38 and 39 prior to affixing. In addition, a plurality 
of through holes 38a and 39a are respectively formed according to a 
predetermined pitch on the mother cover sheets 38 and 39. The through 
holes 38a and 39a are provided so as to form the notches 15a, 15b, 16a and 
16b shown in FIG. 2. 
Then, as shown below an arrow E in FIG. 5, a laminated body 40 obtained by 
laminating the mother cover sheets 38 and 39 on and beneath the mother 
device substrate 37 is cut in the direction of thickness along one dot and 
dash lines F and G, thereby to make it possible to efficiently massproduce 
the piezoelectric resonator 11 shown in FIG. 1. 
In the fabricating method described with reference to FIGS. 4 and 5, the 
laminated body 40 in which a plurality of piezoelectric resonators 11 are 
connected in the longitudinal and the transverse directions is finally 
obtained. Alternatively, a laminated body in which a plurality of 
piezoelectric resonators 11 are connected only in the longitudinal or the 
transverse direction may be obtained, to obtain a plurality of 
piezoelectric resonators 11 by cutting the laminated body only in one 
direction. For example, in FIG. 4, if mother piezoelectric resonance 
element members 31 and mother spacer plates 32 were prepared in which a 
plurality of piezoelectric resonance elements and a plurality of spacer 
plates were respectively connected only in the direction of the one dot 
and dash line C were prepared and laminated, and mother cover sheets 38 
and 39 in such a shape that a plurality of cover sheets are connected only 
in the same direction as the direction of the line C were used, it would 
be possible to finally obtain a laminated body in which a plurality of 
piezoelectric resonators were connected only in one direction. 
As described in the foregoing, in the piezoelectric resonator 11 shown in 
FIG. 1, the resonance frequency of the piezoelectric resonance element 12 
is determined by the distance between the electrodes 12b and 12c, that is, 
the width W of the piezoelectric resonance element 12. Consequently, it is 
possible to provide piezoelectric resonators 11 having various resonance 
frequencies with the wall thickness t of the piezoelectric resonance 
element 12 being constant. In addition, the thickness of the piezoelectric 
resonance element 12 is independent of the resonance frequency. 
Accordingly, a piezoelectric resonance element 12 having a higher 
resonance frequency can be constructed without decreasing the thickness t. 
Consequently, a piezoelectric resonator having considerable mechanical 
strength can be obtained irrespective of the resonance frequency. 
Experiments conducted by the inventors of the present application show that 
a piezoelectric resonator having substantially good properties can be 
obtained when the thickness t of the above described piezoelectric 
resonance element 12 is set to 0.15 mm and the width W thereof is set to 
0.35 mm so as to construct a piezoelectric resonator 11 having a resonance 
frequency of, for example, 3.58 MHz. Accordingly, the resonance frequency 
can be set irrespective of the thickness t of the piezoelectric resonance 
element 12, thereby to make it possible to select cover sheets 15 and 16 
having considerable strength in terms of material and construction. 
Consequently, it is found that further thinning can be promoted, as 
compared with the conventional piezoelectric resonator. 
The above described adhesive layers 17, 18, 20, 21, 34a, 34b and 36a can be 
formed by applying adhesives having fluidity before hardening, or by 
affixing a frame-shaped body composed of film-shaped adhesives containing 
a base material. 
In the piezoelectric resonator 11 shown in FIG. 1, the above described 
notches 15a to 16b are formed in the cover sheets 15 and 16. It was also 
pointed out that the piezoelectric resonator 11 may be cut in the 
direction of thickness along the two-dot and dash lines A shown in FIG. 1. 
However a pair of outer electrodes 41a and 41b may be preferably formed so 
as to cover both end surfaces of the piezoelectric resonator 11 in this 
case, as shown in FIG. 6. Similarly, a pair of outer electrodes may be 
formed on both end surfaces of the piezoelectric resonator 11 provided 
with the notches 15a to 16b shown in FIG. 1. 
FIG. 7 is an exploded perspective view showing a piezoelectric resonator 
according to a second embodiment of the present invention. The second 
embodiment is characterized in that gaps for not preventing the vibration 
of a piezoelectric resonance element 12 are formed not by adhesives but by 
the shapes of spacers and cover sheets. More specifically, as shown in 
FIG. 7, pairs of projections 53a and 53b and 54a and 54b are respectively 
formed on side surfaces, on the side of the piezoelectric resonance 
element 12, of the spacers 53 and 54, and the projections 53a to 54b are 
fastened to a pair of side surfaces, which are opposed to each other, of 
the piezoelectric resonance element 12. That is, gaps 19a and 19b 
dependent on the amounts of projection of the projections 53a to 54b are 
respectively formed between the projections 53a and 53b and 54a and 54b. 
Similarly, a cutout portion 56a is formed on the upper surface of a cover 
sheet 56, thereby to form a gap for not preventing the vibration of the 
piezoelectric resonance element 12. A similar cutout portion is also 
formed on the lower surface of the other cover sheet 55, thereby to form a 
gap for not preventing the vibration of the piezoelectric resonance 
element 12. The cover sheets 55 and 56 are fastened to the piezoelectric 
resonance element 12 and the spacers 53 and 54 in a region around the 
above cutout portion 56a. When the shapes for forming gaps are given to 
respective members such as the spacers 53 and 54 and the cover sheets 55 
and 56, the thickness of adhesive layers used for joining the members can 
be decreased. 
Meanwhile, the other structure of the piezoelectric resonator shown in FIG. 
7 is the same as that of the piezoelectric resonator shown in FIG. 1 and 
hence, the detailed description thereof is omitted by assigning the same 
reference numerals to the same portions to incorporate the description of 
the embodiment shown in FIG. 1. 
The piezoelectric resonator according to the second embodiment can be 
efficiently mass-produced by using mother members. For example, as shown 
in FIG. 8, mother spacer plates 65 and 66 are laminated on and beneath a 
mother piezoelectric resonance element member 31, and a laminated body 
obtained is cut along one dot and dash lines H shown in FIG. 8, thereby to 
make it possible to obtain a body in which a plurality of structures each 
comprising a piezoelectric resonance element 12d and spacers 53 and 54 
(see FIG. 7) fastened to both sides thereof are connected along the 
longitudinal direction of the piezoelectric resonance element 12. 
Accordingly, as shown in FIG. 9, mother cover sheets 67 and 68 in such a 
shape that a plurality of cover sheets 55 and 56 are connected in the 
direction of the length of the piezoelectric resonance element 12 are 
respectively affixed to both major surfaces of the body 69 obtained, and a 
laminated body obtained is cut for each length of each piezoelectric 
resonance element 12, thereby to make it possible to efficiently 
mass-produce a plurality of piezoelectric resonators according to the 
second embodiment. 
Although in the second embodiment, the spacers 53 and 54 provided with a 
plurality of projections 53a, 53b, 54a and 54b are used, a spacer 71 
constituted by a rectangular insulating plate and provided with a notch 
71a obtained by cutting away a central part of one long side of the 
insulating plate may be used, as shown in FIG. 10A. 
Additionally, as shown in FIG. 10B, the spacers 13 and 14 according to the 
first embodiment may be replaced with a spacer 72 provided with slits 72a 
and 72a. In this case, a side surface 72c, on the side of the slits 72a 
and 72a provided, of the spacer 72 is fastened to the piezoelectric 
resonance element 12. The spacer 72 is provided with the slits 72a and 
72a, thereby to make it possible to reliably and easily apply adhesives to 
side surface portions outside of the slits 72a and 72a. That is, adhesives 
can be prevented from being applied to a side surface portion sandwiched 
between the slits 72a and 72a and in contact with the vibrating portion in 
the piezoelectric resonance element 12. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.