Thickness extensional vibration piezoelectric resonator and piezoelectric resonance device

A thickness-extensional piezoelectric resonator using a harmonic of a thickness-extensional vibration mode has a compact size, large electric capacity, is not affected by the stray capacitance of a circuit board, and has small variations in resonance characteristics. The thickness-extensional piezoelectric resonator includes a piezoelectric body, a first excitation electrode, a second excitation electrode, a terminal electrode and a connection electrode disposed on outer surfaces of the piezoelectric body, as well as an internal electrode inside of the piezoelectric body. Portions where the first excitation electrode, the second excitation electrode and the internal electrode overlap define a resonating portion. In the length direction of the piezoelectric body, a vibration-attenuating portion is defined at opposite sides of the resonating portion, whereas in the width direction, the vibration-attenuating portion is not provided at each side thereof. In addition, a relationship of Go/D.gtoreq.2.0 is satisfied, wherein Go represents a distance between the terminal electrode and the first excitation electrode, D is substantially equal to T/N, and wherein T is the thickness of the piezoelectric body and N is the order of harmonic vibrations.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a piezoelectric resonator and a
 piezoelectric resonance device for use as different kinds of resonators,
 oscillators, or similar apparatuses, and more particularly, to a thickness
 extensional piezoelectric resonator and a piezoelectric resonance device
 each operative to use higher harmonics of a thickness extensional
 vibration mode.
 2. Description of the Related Art
 Piezoelectric resonators are used in a variety of piezoelectric resonation
 devices such as piezoelectric oscillators, piezoelectric filters, and so
 forth.
 Japanese Unexamined Patent Publication No. 1-117409 discloses an
 energy-trap type piezoelectric resonator operative to utilize the second
 harmonic in a thickness extensional vibration mode. This piezoelectric
 resonator will be described with reference to FIGS. 22 and 23.
 The above-mentioned piezoelectric resonator is formed by laminating ceramic
 green sheets 61 and 62 made of a piezoelectric material, and firing them
 integrally, as shown in the exploded perspective view of FIG. 22. A
 circular excitation electrode 63 is provided on the ceramic green sheet 61
 in the center thereof. The excitation electrode 63 is led out to one of
 the side edges of the ceramic green sheet 61 through a lead electrode 64.
 Further, a circular excitation electrode 65 is provided in the center of
 the upper side of the ceramic green sheet 62. The excitation electrode 65
 is led out to one of the side edges of the ceramic green sheet 62 through
 a lead electrode 66. An excitation electrode 67 is provided on the
 underside of the ceramic green sheet 62, and is lead out to the side edge
 of the ceramic green sheet 62 through an lead electrode 68, as shown in
 the downward projection.
 The above-mentioned ceramic green sheets 61 and 62 are laminated, pressed
 in the thickness direction, and baked. The obtained sintered material is
 polarized whereby a piezoelectric resonator 70 as shown in FIG. 23 is
 produced.
 In the piezoelectric resonator 70, piezoelectric layers 71 and 72 are
 uniformly polarized in the thickness direction, namely, in the direction
 indicated by the arrows in FIG. 23.
 The piezoelectric resonator 70 can be resonated by connecting in common the
 excitation electrodes 63 and 67, and applying an AC voltage between the
 excitation electrodes 63, 67, and 65. In this case, the vibration energy
 is trapped in the area where the excitation electrodes 63, 65, and 67
 overlap, which defines a resonance portion A.
 The prior art piezoelectric resonator 70 which is operative to use higher
 harmonics in a thickness extensional vibration mode is provided as an
 energy-trapping piezoelectric resonator, as described above. Accordingly,
 it is necessary to provide a vibration-attenuating portion at the
 periphery of the resonance portion A for attenuation of the vibration.
 That is, it is necessary to provide the vibration attenuating portion
 having an area which is larger than that of the resonance portion A.
 Therefore, it becomes difficult to miniaturize the piezoelectric resonator
 70.
 Japanese Unexamined Patent Publication No. 2-235422 discloses an
 energy-trap type piezoelectric resonator containing a strip-type
 piezoelectric ceramic, which does not require a piezoelectric substrate
 portion having a large area located at the periphery of its resonance
 portion.
 In the energy-trap type piezoelectric resonator 80, an excitation electrode
 82a is provided on the upper side of an elongated piezoelectric substrate
 81, and an excitation electrode 82b on the underside thereof. Each of the
 excitation electrodes 82a and 82b is arranged to extend to a pair of the
 long sides of the piezoelectric substrate 81, that is, to extend over the
 entire width. Further, the back side of the excitation electrode 82a and
 the front side of the excitation electrode 82b are opposed to each other
 in the center in the longitudinal direction of the piezoelectric substrate
 81 whereby the resonance portion is defined by the overlapped portions.
 Further, these excitation electrodes 82a and 82b are extended to the ends
 81a and 81b in the longitudinal direction of the piezoelectric substrate
 81.
 In the piezoelectric resonator 80, unnecessary vibrations are generated
 during the excitation of the thickness extensional vibration mode.
 Japanese Unexamined Patent Publication No. 2-235422 describes that when a
 fundamental wave is utilized, the ratio of WIT (width/thickness) equal to
 about 5.33 at a resonance frequency of 16 MHz is preferred, and when the
 third harmonic is utilized, unnecessary spurious components between the
 resonance frequency and the anti-resonance frequency can be reduced by
 setting the ratio of W/T at about 2.87 when the resonance frequency is
 about 16 MHz. In other words, when higher harmonics in a thickness
 extensional vibration mode are practically utilized, various unnecessary
 spurious vibrations appear in addition to the spurious components between
 the resonance and anti-resonance frequencies. Accordingly, a problem
 occurs that effective resonance characteristics can not be obtained.
 Also referring to the piezoelectric resonator disclosed in Japanese
 Unexamined Patent Publication No. 2-235422, the electric capacity is
 relatively small so that the piezoelectric resonator is susceptible to a
 floating capacity generated from the circuit board other components.
 SUMMARY OF THE INVENTION
 To overcome the above described problems, preferred embodiments of the
 present invention provide a thickness extensional vibration piezoelectric
 resonator and a piezoelectric resonance device, each of which is operative
 to utilize higher harmonics in a thickness extensional vibration mode, is
 constructed to be miniaturized, has a large electric capacity, is not
 vulnerable to floating capacity generated from a circuit board,
 effectively suppresses the generation of undesired unnecessary spurious
 vibrations, and has excellent resonance characteristics.
 One preferred embodiment of the present invention preferably provides a
 thickness extensional vibration piezoelectric resonator, including a
 resonance portion, a vibration-attenuating portion disposed at both sides
 of the resonance portion and adapted to be vibrated in an N-order higher
 harmonic in a thickness extensional vibration mode. The piezoelectric
 resonator further includes a piezoelectric body, a first excitation
 electrode and a second excitation electrode respectively disposed on both
 sides of the piezoelectric body and opposed to each other with the
 piezoelectric body disposed therebetween, at least one layer of an
 internal electrode arranged inside of the piezoelectric body and at least
 partially opposed to the first and second excitation electrodes through
 the piezoelectric layers, first and second terminal electrodes disposed on
 the ends in a first direction of the of the piezoelectric body and
 electrically connected to the first and second excitation electrodes,
 respectively. The first direction being a direction passing through the
 vibration-attenuating portions located on both sides of the resonance
 portion, respectively. The first and second terminal electrodes being
 defined by the portion of the first and second excitation electrodes and
 the internal electrode overlapped in the thickness direction with the
 piezoelectric body disposed therebetween. A ratio Go/D being substantially
 equal to or higher than about 2.0, in which Go is each of the distances
 extending in the first direction between the first and second excitation
 electrodes and the terminal electrodes, and D is substantially equal to
 T/N, wherein T is the thickness of the piezoelectric body and N is the
 order of harmonics of thickness extensional vibration.
 According to the above described arrangement, the thickness extensional
 vibration piezoelectric resonator preferably includes a substantially
 rectangular plate-shaped piezoelectric body, first and second excitation
 electrodes disposed on both sides of the piezoelectric body, and at least
 one layer of an internal electrode arranged inside of the piezoelectric
 body and at least partially opposed to the first and second excitation
 electrodes. Accordingly, as compared with a prior art thickness
 extensional vibration piezoelectric resonator having no internal
 electrode, the electric capacity is greatly increased to such a degree as
 corresponds to the internal electrode, and thereby, hazardous influences
 caused by a floating capacity generated by the circuit board to which the
 thickness extensional vibration piezoelectric resonator is attached, is
 greatly reduced. That is, thickness extensional vibration piezoelectric
 resonators having high resonance characteristics can be provided. In
 addition, the vibration attenuating portions are preferably provided only
 on two opposite sides of the resonating portion. Therefore, the size of
 the thickness extensional vibration piezoelectric resonator can be
 reduced. Miniature thickness extensional vibration piezoelectric
 resonators can be thus provided. Moreover, the ratio Go/D is preferably
 about 2.0 or greater. Accordingly, unnecessary spurious vibrations between
 the resonance and anti-resonance frequencies are effectively suppressed.
 Another preferred embodiment of the present invention provides a thickness
 extensional vibration piezoelectric resonator, including a resonance
 portion, a vibration-attenuating portion disposed at opposite sides of the
 resonance portion and adapted to be vibrated in a higher order harmonic in
 a thickness extensional vibration mode, a piezoelectric body; a plurality
 of excitation electrodes disposed on the main surfaces of or inside of the
 piezoelectric body; the resonance portion being defined by the portion of
 the plural excitation electrodes overlapped through the piezoelectric
 layers in the thickness direction, the plural excitation electrodes
 containing at least one layer of an internal excitation electrode; first
 and second terminal electrodes disposed on the ends in the first direction
 of the piezoelectric body and electrically connected to the first and
 second excitation electrodes, the first direction being a direction
 passing through the vibration-attenuating portions located on both sides
 of the resonance portion, respectively, the first and second terminal
 electrodes constituting portions for external connection, and a ratio Gi/D
 is substantially equal to or greater than about 2.0, in which Gi is each
 of the distances extending in the first direction between the internal
 excitation electrode and the terminal electrodes connected to have the
 opposite potential for the internal excitation electrode, and D is
 substantially equal to T/N, wherein T is the thickness of the
 piezoelectric body and N is the order of harmonics of thickness
 extensional vibration.
 According to the above described arrangement, the thickness extensional
 vibration piezoelectric resonator preferably includes a substantially
 rectangular sheet-shaped piezoelectric body, at least one layer of the
 internal resonance electrode, and the plurality of excitation electrodes
 arranged to be overlapped through the piezoelectric body layers in the
 thickness direction. Accordingly, as compared with a prior art thickness
 extensional vibration piezoelectric resonator having no internal
 electrode, the electric capacity is greatly increased to such a degree
 that is similar to the internal electrode. As a result, hazardous
 influences caused by a floating capacity generated by the circuit board
 and other sources are minimized. Furthermore, the vibration attenuating
 portions are provided only on the two opposite sides of the resonating
 portion. Accordingly, the size of the thickness extensional vibration
 piezoelectric resonator is greatly reduced. At the same time, the ratio
 Gi/D is set at about 2.0 or greater. Accordingly, unnecessary spurious
 components are prevented from appearing between the resonance and
 anti-resonance frequencies.
 Another preferred embodiment of the present invention provides a
 piezoelectric resonance device including a substrate member, a casing
 member constituting a plurality of electrodes disposed on the substrate,
 and the above described thickness extensional vibration piezoelectric
 resonator bonded to the substrate so as to assure a space which allows for
 free vibration of the resonator and the casing member being fixed to the
 substrate so as to cover the thickness extensional vibration piezoelectric
 resonator bonded to the substrate.
 According to the above described arrangement, the piezoelectric resonance
 device defines an electronic device that can be surface-mounted onto a
 printed circuit substrate and other structures. Also, in the above
 described piezoelectric resonance device, the substrate may be a capacitor
 substrate containing a dielectric substrate and plural electrodes disposed
 on the dielectric substrate, and the thickness extensional vibration
 piezoelectric resonator may be electrically connected to a capacitor
 provided in the capacitor substrate.
 Other features and advantages of the present invention will become apparent
 from the following description of the invention which refers to the
 accompanying drawings.

DETAILED DESCRIPTIONS OF PREFERRED EMBODIMENTS
 A detailed explanation of the present invention will be provided below
 referring to some of the preferred embodiments of the present invention.
 FIGS. 1A and 1B are a perspective view and a longitudinal section of a
 thickness extensional piezoelectric resonator according to a first
 preferred embodiment of the present invention.
 The thickness extensional vibration piezoelectric resonator 1 preferably
 includes an elongated strip-shaped piezoelectric body 2. The piezoelectric
 body 2 is preferably made of a piezoelectric ceramic such as a lead
 titanate type ceramic and a lead titanate zirconate type ceramic, and is
 evenly polarized in the thickness direction.
 A first excitation electrode 3 is provided on the upper surface of the
 piezoelectric body 2, while a second excitation electrode 4 on the lower
 surface thereof. The excitation electrodes 3 and 4 are extended on the
 upper surface and the lower surface of the piezoelectric body 2 from one
 2a of the end surfaces of the piezoelectric body 2 toward the other end
 surface 2b thereof, respectively. The excitation electrodes 3 and 4 are
 preferably connected in common to a connecting electrode 5 provided on the
 end surface 2a of the piezoelectric body 2.
 An internal electrode 6 is provided inside of the piezoelectric body 2. The
 internal electrode 6 extends from the end surface 2b of the piezoelectric
 body 2, and is preferably electrically connected to a terminal electrode 7
 formed on the end surface 2b.
 During operation, the second harmonic in a thickness extensional vibration
 mode is intensely excited by application of an AC voltage between the
 first and second excitation electrodes 3 and 4 and the internal electrode
 6. Thus, the thickness extensional vibration piezoelectric resonator 1 can
 be operated to use the second harmonic.
 In this first preferred embodiment, the first and second excitation
 electrodes 3 and 4 and the internal electrode 6 are arranged so that they
 overlap through the piezoelectric layers in the center in the longitudinal
 direction of the piezoelectric body 2. Accordingly, the locations where
 the first and second excitation electrodes 3 and 4 and the internal
 electrode 6 overlap defines an energy-trapping resonance portion. The
 energy when the resonance portion is vibrated is attenuated in the
 piezoelectric body portions that are on the opposite sides of the
 resonance portion.
 The first and second excitation electrodes 3 and 4, and the internal
 electrode 6 are arranged to extend over the entire width of the
 piezoelectric body 2 only at the resonance portion, and do not need to be
 formed so as to have the same width in the dielectric body 2 outside of
 the resonance portion. For example, the excitation electrode 3 portion
 that is located on the end surface 2a side of the resonance portion may
 have a smaller width, since this excitation electrode 3 portion is used
 simply for electrical connection of the excitation electrode 3 to the
 connecting electrode 5.
 Also in this preferred embodiment, the vibration attenuating portions are
 provided on opposite sides of the resonance portion only in the
 longitudinal direction of the piezoelectric body 2. No vibration
 attenuating portion is provided in the width direction of the
 piezoelectric body 2. Accordingly, the size of the thickness extensional
 vibration piezoelectric resonator 1 is greatly reduced, and
 miniaturization of the piezoelectric resonator can be achieved.
 In addition, the thickness extensional vibration piezoelectric resonator of
 this preferred embodiment has the structure in which the first and second
 excitation electrodes 3 and 4, and the internal electrodes 6 are laminated
 through the piezoelectric body layers. Accordingly, the thickness
 extensional vibration piezoelectric resonator of this embodiment has a
 higher electric capacity as compared with a conventional thickness
 extensional vibration piezoelectric resonator 80 having no internal
 electrode, and is less vulnerable to floating capacity generated by a
 circuit board.
 The thickness extensional vibration piezoelectric resonator 1 is fixed
 preferably by bonding the terminal electrodes 5 and 7 to a case substrate
 or the like through a space for allowing free vibration of the resonance
 portion thereof. In some cases, it was observed that large spurious
 components in the frequency range between the resonance and anti-resonance
 frequencies were caused.
 In particular, as shown in the phase-frequency characteristics of FIG. 3, a
 large spurious component was observed in the frequency range between the
 resonance and anti-resonance frequencies, as indicated by the arrow B, in
 some cases. Further, the size of the thickness extensional vibration
 piezoelectric resonator was varied. There were some cases where the
 above-mentioned spurious component B was hardly generated. That is, as
 shown by the phase-frequency characteristics of FIG. 2, no spurious
 components were exhibited in the frequency range between the resonance and
 anti-resonance frequencies in some cases.
 Accordingly, the differences between the characteristics as shown in FIGS.
 2 and 3 were investigated from the various standpoints. As a result, it
 was discovered that when the distance Go extending in the first direction
 between the first and second excitation electrodes 3, 4 and the terminal
 electrode 7 connected to have the opposite potential for the excitation
 electrodes 3 and 4 is set to a desired value, spurious components can be
 effectively suppressed, as shown in FIG. 2. Based on this finding,
 preferred embodiments of the present invention has been developed.
 In particular, when the above-mentioned distance Go is excessively short,
 some other vibration occurs, due to the electrostrictive effect which is
 caused by the electric field between the excitation electrodes 3 and 4 and
 the terminal electrode 7. This causes a large spurious component to appear
 in the frequency range between the resonance and anti-resonance
 frequencies. Accordingly, various thickness extensional vibration
 piezoelectric resonators having different ratios Go/D were prepared.
 Investigation was conducted of the above-mentioned spurious component
 between the resonance and anti-resonance frequencies. FIG. 4 shows the
 results.
 In FIG. 4, the ratio Go/D is plotted as abscissa, and the phase value of
 the spurious component B appearing between the resonance and
 anti-resonance frequencies is plotted as the abscissa. The phase value is
 the minimum in the spurious component indicated by the arrow B in FIG. 3.
 The experimental piezoelectric body 2 has a size of about 2.0 mm by about
 0.4 mm by about 0.3 mm. That is, T equals 0.3 mm, and D equals 0.15 mm.
 Further, various thickness extensional vibration piezoelectric resonators
 were prepared at a constant value of D=0.15 mm and different Go values.
 As seen in FIG. 4, when the ratio Go/D is less than about 2.0, the minimum
 phase value of the spurious vibration B appearing between the resonance
 and anti-resonance frequencies is rapidly reduced. That is, the intensity
 of the spurious component B is steeply increased. On the other hand, it is
 shown that when the ratio Go/D is greater than about 2.0, the intensity of
 the spurious component appearing between the resonance and anti-resonance
 frequencies is not significantly increased. Accordingly, it is understood
 that the above-mentioned spurious component appearing between the
 resonance and anti-resonance frequencies can be suppressed by setting the
 ratio Go/D to about 2.0 or higher. The upper limit of the ratio Go/D is
 not restricted from the standpoints of the suppression of the spurious
 component B. However, for the purpose of the miniaturization of the
 thickness extensional vibration piezoelectric resonator 1, desirably, the
 ratio Go/D is set at about 20 or lower.
 The phase-frequency characteristics shown in FIG. 2 are at Go/D=2.0. The
 characteristics of FIG. 3 correspond to those at Go/D=1.5. That is, as
 seen in FIGS. 2 and 3, unnecessary spurious components appearing between
 the resonance and anti-resonance frequencies can be effectively suppressed
 by setting the ratio Go/D at about 2.0 or higher, which results in
 excellent resonance characteristics.
 Referring to the distance Go extending in the first direction between the
 first and second excitation electrodes 3, 4 and the terminal electrode 7
 connected so as to have the opposite potential for the excitation
 electrodes 3 and 4, the terminal electrode 7 is provided so as to extend
 onto the upper surface and lower surface of the piezoelectric body 2 in
 the first preferred embodiment, and accordingly, the distance Go is
 defined as a distance in the first direction between the tip of the
 terminal electrode 7 and the first or second excitation electrode 3 or 4.
 However, when a terminal electrode 7A is provided only on the end surface
 2b as in a modification example shown in FIGS. 5A and 5B, the distance Go
 is each of the distances in the first direction between the terminal
 electrode 7A and the excitation electrodes 3 and 4, in other words, each
 of the distances in the first direction between the end surface 2b and the
 excitation electrodes 3 and 4.
 In the thickness extensional vibration piezoelectric resonator 1 of the
 first preferred embodiment, the piezoelectric body 2 is polarized evenly
 in the thickness direction, and is a parallel-connection type
 piezoelectric resonator in which electric fields are applied in the
 opposite directions to the layers. According to another preferred
 embodiment, a series connection type piezoelectric resonator may be used
 that contains a plurality of piezoelectric layers polarized alternately
 oppositely in the thickness direction. FIG. 6 shows such a preferred
 embodiment of a series type thickness extensional vibration piezoelectric
 resonator.
 The thickness extensional vibration piezoelectric resonator 11 shown in
 FIG. 6 preferably includes an elongated, substantially rectangular
 plate-shaped strip type piezoelectric body 12. A first excitation
 electrode 13 is provided on the upper surface of the piezoelectric body
 12, while a second excitation electrode 14 on the lower surface thereof.
 The back side of the excitation electrode 13 and the front side of the
 excitation electrode 14 oppose each other with the piezoelectric body 12
 disposed therebetween. Further, the first and second excitation electrodes
 13 and 14 are opposed at central portions thereof in the longitudinal
 direction of the piezoelectric body 12. The resonator portion where the
 first and second excitation electrodes 13 and 14 are opposed to each other
 defines an energy-trapping type resonance portion.
 In this second preferred embodiment, the first and second excitation
 electrodes 13 and 14 stem from the end surfaces 12a and 12b of the
 piezoelectric body 12 to be connected to the terminal electrodes 15 and
 17, respectively. Further, the first and second excitation electrodes 13
 and 14 may be arranged so as to extend over the entire width of the
 piezoelectric body 12 besides the resonance portion.
 An internal electrode 16 is provided at a substantially central height in
 the piezoelectric body 12. The internal electrode 16 is arranged in order
 to polarize the piezoelectric body 12. In particular, a relatively high
 voltage is applied to the internal electrode 16, while a relatively low
 voltage applied to the excitation electrodes 13 and 14, so that the
 piezoelectric layers 12c and 12d are polarized oppositely in the thickness
 direction, as indicated by the arrows in FIG. 6.
 During operation, the second harmonic in a thickness extensional vibration
 mode can be excited by applying an AC voltage between the first and second
 excitation electrodes 13 and 14, without using the internal electrode 16.
 Also in the thickness extensional vibration piezoelectric resonator 11 of
 the second preferred embodiment, the vibration-attenuating portions are
 provided only on two opposite sides of the resonance portion in the
 longitudinal direction of the piezoelectric body 12. Thus, as in the
 thickness extensional vibration piezoelectric resonator 1 of the first
 preferred embodiment, a miniature thickness extensional vibration
 piezoelectric resonator is provided. Further, since the thickness
 extensional vibration piezoelectric resonator 11 contains the internal
 electrode 16, and the excitation electrodes 13 and 14 are arranged so as
 to extend to both of the side edges in the width direction of
 piezoelectric body 12, the electric capacity is greatly increased.
 Accordingly, the thickness extensional vibration piezoelectric resonator
 11 is much less vulnerable to influences from a floating capacity
 generated on a circuit substrate.
 In the thickness extensional vibration piezoelectric resonator 11 of the
 second preferred embodiment, the ratio Go/D is set at about 2.0 or higher,
 in which Go represents each of the distance between the first and second
 excitation electrodes 13 and 14, and the terminal electrodes 17 and 15
 connected to the opposite electrodes for the first and second excitation
 electrodes 13 and 14, respectively.
 In the first and second preferred embodiments, the thickness extensional
 vibration piezoelectric resonators 1 and 11 operate to use the second
 harmonic in a thickness extensional vibration mode are provided. However,
 the piezoelectric resonator according to preferred embodiments of the
 present invention may also be operative to use higher harmonics. FIGS. 7
 through 9 are cross-sections illustrating the piezoelectric resonators
 operative to utilize the higher harmonics other than the second harmonic,
 and are views corresponding to FIG. 1B illustrating the first preferred
 embodiment.
 FIG. 7 shows a parallel connection type thickness extensional vibration
 resonator 21 operative to utilize the third harmonic in a thickness
 extensional vibration mode. In particular, two plate internal electrodes
 22 and 23 are provided inside of the piezoelectric body 2. The
 piezoelectric resonator 21 operative to utilize the third harmonic in a
 thickness extensional vibration mode is provided by polarizing the
 piezoelectric body 2 evenly in the thickness direction as indicated by the
 arrow.
 A thickness extensional vibration piezoelectric resonator 24 shown in the
 cross-section of FIG. 8A is a parallel connection type piezoelectric
 resonator 24 operative to use the fourth harmonic in a thickness
 extensional vibration mode. In the thickness extensional vibration
 piezoelectric resonator 24, the piezoelectric body 2 is polarized evenly
 in the thickness direction. Three plate internal electrodes 25 through 27
 are provided at a substantially equal distances in the thickness direction
 inside of the piezoelectric body 2. An excitation electrode 3A is provided
 on the lower surface of the piezoelectric body 2, and connected to a
 terminal electrode 5. Accordingly, the fourth harmonic in a thickness
 extensional vibration mode can be effectively excited.
 FIG. 8B is a cross-section of a series connection type thickness
 extensional vibration resonator 28 operative to utilize the third harmonic
 in a thickness extensional vibration mode. In the thickness extensional
 vibration resonator 28, two plate internal electrodes 29 and 30 are
 provided in the piezoelectric body 12, and the inside of the piezoelectric
 body 12 is divided into three piezoelectric body layers 12e through 12g.
 Adjacent piezoelectric layers in the piezoelectric body 12 are polarized
 oppositely in the thickness direction by polarization treatment using
 these internal electrodes 29 and 30. Accordingly, the third harmonic in a
 thickness extensional vibration mode can be excited by applying an AC
 voltage to the first and second excitation electrodes 13 and 14.
 Similarly, FIG. 9 is a cross-section of a series connection type
 piezoelectric resonator 31 operative to utilize the fourth harmonic in a
 thickness extensional vibration mode. In this case, three plate internal
 electrodes 32 through 34 are provided in the piezoelectric body 12. The
 piezoelectric body 12 is polarized by use of these internal electrodes 32
 through 34, so that adjacent piezoelectric layers in the piezoelectric
 body 12 are polarized oppositely to each other in the thickness direction,
 as shown in FIG. 9.
 In the respective thickness extensional vibration piezoelectric resonators
 as shown in FIGS. 7 through 9, the vibration attenuating portions are
 provided only on two opposite sides of the resonance portion. Accordingly,
 a miniature thickness extensional vibration piezoelectric resonator is
 provided. Since the thickness extensional vibration piezoelectric
 resonators contains at least one internal electrode, the electric capacity
 is increased, and the resonator less vulnerable to influences from a
 floating capacity generated from the circuit board.
 Further, in the respective thickness extensional vibration piezoelectric
 resonators as shown in FIGS. 7 through 9, unnecessary spurious variations
 appearing between the resonance and anti-resonance frequencies are
 minimized and effectively suppressed if the above-described ratio Go/D is
 set at about 2.0 or higher.
 In the thickness extensional vibration piezoelectric resonator 35 shown in
 FIG. 10, piezoelectric layers 2A and 2B are laminated to the outsides of
 the first and second excitation electrodes 3 and 4, respectively. That is,
 the thickness extensional vibration piezoelectric resonator 35 is a
 modification example of the thickness extensional vibration piezoelectric
 resonator 1 as shown in FIGS. 1A and 1B. In this case, the first and
 second excitation electrodes 3 and 4 are provided in the form of an
 internal electrode, due to the formation of the piezoelectric layers 2A
 and 2B.
 Further, the terminal electrodes 5 and 7 are arranged on the end surfaces
 2a and 2b of the piezoelectric body 2 and further onto the upper surface
 and lower surface of the laminate formed by lamination of the
 piezoelectric body layers 2A and 2B to the piezoelectric body 2,
 respectively.
 In other respects, the thickness extensional vibration piezoelectric
 resonator 35 is configured in the same manner as the thickness extensional
 vibration piezoelectric resonator 1. It is noted that when the thicknesses
 of the piezoelectric layers 2A and 2B are excessively large, the vibration
 efficiency is reduced. On the other hand, when the thicknesses of the
 piezoelectric layers 2A and 2B are relatively small, the vibration
 efficiency is not significantly reduced.
 In addition, when the first, second excitation electrodes 3 and 4 are
 arranged as an internal electrode, the piezoelectric body 2, the
 piezoelectric layers 2A and 2B, the internal electrode 6, and the
 excitation electrodes 3 and 4 is provided by the well-known lamination
 ceramic integration firing technology. Accordingly, the excitation
 electrodes 3 and 4 can be formed by screen-printing conductive paste
 before firing, which enables the excitation electrodes 3 and 4 to be
 formed with very high precision, and the production process is greatly
 simplified.
 FIG. 11 is an exploded perspective view illustrating a chip type
 piezoelectric resonance device according to another preferred embodiment
 of the present invention. In the chip type piezoelectric resonance device
 of this preferred embodiment, the thickness extensional vibration
 piezoelectric resonator 1 as shown in FIG. 1 is bonded to a capacitor
 substrate 36 through conductive adhesives 37 and 38. The capacitor
 substrate 36 contains a rectangular dielectric substrate 36a made of a
 dielectric material such as dielectric ceramics or the like, and plural
 electrodes 36b through 36d provided on the outside of the dielectric
 substrate 36a. In the capacitor substrate 36, capacitors are arranged
 between the electrodes 36b and 36d, and the electrode 36c.
 Further, the thickness extensional vibration piezoelectric resonator 1 is
 bonded to the capacitor substrate 36 with extra space for allowing for
 free and unhindered vibration of the resonator. This is achieved by
 bonding the terminal electrodes 5 and 7 to the electrodes 36b and 36d with
 the conductive adhesives 37 and 38, respectively.
 As shown in FIG. 12, this piezoelectric resonance device is a circuit
 containing a resonator and two capacitors between the electrodes 36b, 36c,
 and 36d. Thus, a load-capacity containing type piezoelectric oscillator
 can be provided in the form of a single device.
 Finally, a cap 39 defining a casing member is preferably bonded to the
 capacitor substrate 36 with an insulating adhesive so as to cover the
 thickness extensional vibration piezoelectric resonator 1.
 In the piezoelectric resonance device according to preferred embodiments of
 the present invention, the substrate and casing member are not limited to
 the above-mentioned capacitor substrate 36 and the cap 39, respectively.
 That is, as the substrate, a container body having an opening on the upper
 side thereof may be prepared. In the container body, the thickness
 extensional vibration piezoelectric resonator 1 is placed, and then a lid
 to close the container body, as the casing member, is bonded to the
 substrate.
 FIGS. 13A and 13B are a perspective view and a longitudinal section
 illustrating a thickness extensional vibration piezoelectric resonator
 according to a fourth preferred embodiment of the present invention.
 The thickness extensional vibration piezoelectric resonator 1A of the
 fourth preferred embodiment is constituted in the same manner as the
 thickness extensional vibration piezoelectric resonator 1 of the first
 preferred embodiment except that an inward protuberance 5a, approximately
 positioned at the same height as the internal electrode 6, is electrically
 connected to the terminal electrode 5, and the ratio Gi/D, instead of the
 ratio Go/D, is set at about 2.0 or higher. The description of the other
 structure of the thickness extensional vibration piezoelectric resonator
 1A will be omitted since it is similar to that described with respect to
 the first preferred embodiment.
 As previously mentioned, the thickness extensional vibration piezoelectric
 resonator 1 of the first preferred embodiment, unnecessary spurious
 vibrations appearing between the resonance and anti-resonance frequencies
 are suppressed by setting the ratio Go/D at about 2.0 or higher in which
 Go represents each of the distances between the first and second
 excitation electrodes 3 and 4, and the terminal electrodes 7 and 5.
 Referring to the distance Gi between the internal electrode 6 defining an
 internal excitation electrode and the terminal electrode 5 connected to
 have the opposite potential for the internal electrode 6, the applicants
 of this invention have discovered that the above-described spurious
 vibrations are minimized suppressed by adjustment of the ratio Gi/D. In
 particular, thickness extensional vibration piezoelectric resonators were
 prepared with the ratio Gi/D being varied. Investigation of the resonance
 characteristics of the thickness extensional vibration piezoelectric
 resonators was conducted. FIGS. 14 through 16 show the results.
 In the experiment, the piezoelectric body 2 with a size of about 2.0 mm by
 about 0.4 mm by about 0.3 mm was used. The distance Gi was varied at D=0.3
 mm/2=0.15 mm. No significant spurious vibration C between the resonance
 and anti-resonance frequencies at Gi/D=0.2 were observed, as shown in FIG.
 14. On the other hand, as shown in FIG. 15, spurious vibrations between
 the resonance and anti-resonance frequencies, as indicated by the arrow C,
 appear at Gi/D=0.15. Further, it is observed that when the ratio Gi/D is
 less than about 2.0, the spurious vibration C appearing between the
 resonance and anti-resonance frequencies is increased, as seen in FIG. 16.
 In FIG. 16, the minimum phase value of the spurious component C is plotted
 as ordinate. A smaller phase value means a larger spurious component.
 Accordingly, in the thickness extensional vibration piezoelectric
 resonator 1A, spurious vibrations appearing between the resonance and
 anti-resonance frequencies are suppressed by selecting the distance Gi in
 the first direction between the internal electrode 6 and the tip of the
 inward protuberance 5a of the terminal electrode 5 so that the ratio Gi/D
 is at least 2.0.
 The upper limit of the ratio Gi/D is not particularly restrictive from the
 standpoint of the inhibition of the above-described spurious vibration.
 However, for the purpose of miniaturizing the piezoelectric resonator 1A,
 it is desirable that the ratio Gi/D is less than about 20.
 In the thickness extensional vibration piezoelectric resonator 1A, the
 inward protuberance 5a is provided on the terminal electrode 5.
 Accordingly, the distance Gi is the distance in the first direction
 between the internal electrode 6 and the tip of the inward protuberance
 5a. However, the inward protuberance may be omitted.
 In particular, when a thickness extensional vibration piezoelectric
 resonator 1B is constituted as shown in FIGS. 17A and 17B, the distance Gi
 is a distance between the internal electrode 6 and the terminal electrode
 5. Also in this case, spurious vibrations appearing between the resonance
 and anti-resonance frequencies can be effectively suppressed by setting
 the ratio Gi/D at about 2.0 or higher, as in the thickness extensional
 vibration piezoelectric resonator 1A.
 More preferably, the ratio Go/D is preferably set at about 2.0 or higher,
 so that spurious vibrations appearing between the resonance and
 anti-resonance frequencies can be suppressed more effectively.
 FIG. 18 is a perspective view of a thickness extensional vibration
 piezoelectric resonator according to a fifth preferred embodiment of the
 present invention.
 The thickness extensional vibration piezoelectric resonator 41 includes an
 elongated, substantially rectangular plate-shaped strip-type piezoelectric
 body 42. Internal excitation electrode 43 and 44 are arranged to overlap
 in the thickness direction in the piezoelectric body 42. Further, an
 internal electrode 45 used for polarization is provided between the
 internal excitation electrodes 43 and 44. The internal excitation
 electrode 43 is extended to one 42b of the end surfaces in the
 longitudinal direction of the piezoelectric body 42, while the internal
 excitation electrode 44 is extended to the other end surface 42a of the
 piezoelectric body 42. The portion where the internal excitation
 electrodes 43 and 44 are overlapped in the thickness direction through the
 piezoelectric body 42 constitutes a resonance portion. Thus, also in this
 preferred embodiment, the energy-trapping type piezoelectric resonator
 includes vibration attenuating portions arranged on two opposite sides of
 the resonance portion in the longitudinal direction.
 The thickness extensional vibration piezoelectric resonator 41 is a series
 connection type piezoelectric resonator as described above. The polarized
 piezoelectric body layer 43c and the piezoelectric body layer 43d provided
 on the upper side and the lower surface of the internal electrode 45 are
 polarized oppositely in the thickness direction.
 Further, terminal electrodes 15 and 17 are arranged to cover the end
 surfaces 42a and 42b, respectively. The terminal electrode 15 preferably
 has an inward protuberance 15a. The inward protuberance 15a is provided
 opposite to the tip of the internal excitation electrode 43 in the same
 plane, at the same height where the internal excitation electrode 43 is
 provided. Similarly, the terminal electrode 17 is preferably provided with
 an inward protuberance 17a. The inward protuberance 17a is provided to be
 opposed to the tip of the internal excitation electrode 44 in the same
 plane, at an interval relative to each other, and at the same height where
 the internal excitation electrode 44 is provided.
 The thickness extensional vibration piezoelectric resonator 41 of this
 preferred embodiment is a series connection type thickness extensional
 vibration piezoelectric resonator operative to utilize the second harmonic
 in a thickness extensional vibration mode. If the ratio Gi/D is set at
 about 2.0 or higher, spurious vibrations appearing between the resonance
 and anti-resonance frequencies are effectively suppressed. The distance Gi
 is defined as the distance between the internal excitation electrode 43
 and the inward protuberance 15a in the first direction, or as the distance
 between the internal excitation electrode 44 and the inward protuberance
 17a in the first direction.
 FIGS. 19 through 21 are longitudinal sections showing other modification
 examples of the thickness extensional vibration piezoelectric resonator of
 the present invention.
 A thickness extensional vibration piezoelectric resonator 51 as shown in
 FIG. 19 has a structure that the excitation electrodes 3 and 4 described
 in the thickness extensional vibration piezoelectric resonator 1 of FIG. 1
 are embedded in the piezoelectric body 52 to function as an internal
 excitation electrode, in which the inward protuberances 5a, and 7a and 7a
 are provided for the terminal electrodes 5 and 7, respectively.
 The internal electrode 6 need not be used for polarization, and is provided
 as an excitation electrode of FIG. 19. Also in this preferred embodiment,
 spurious vibrations can be effectively suppressed by setting the ratio
 Gi/D at about 2.0 or higher in which Gi represents each of the distances
 between the internal excitation electrodes 6, 53, and 54 and the terminal
 electrodes 5, 7a, and 7b, respectively.
 The thickness extensional vibration piezoelectric resonators of FIGS. 20
 and 21 correspond to the modification examples of the thickness
 extensional vibration piezoelectric resonators of FIGS. 7 and 8B,
 respectively. However, in each piezoelectric body, the first and second
 excitation electrodes are embedded as an internal excitation electrode. In
 addition, the internal excitation electrodes and the inward protuberances
 5a and 7a on the terminal electrodes 5 and 7 are connected to have the
 opposite potentials for the corresponding internal excitation electrodes.
 Also in the thickness extensional vibration piezoelectric resonators 58
 and 59 as shown in FIGS. 20 and 21, the above-described spurious
 vibrations can be effectively suppressed by setting the ratio Gi/D at
 about 2.0 or higher in which Gi represents each of the distances between
 the internal excitation electrodes and the terminal electrodes or the
 inward protuberances of the terminal electrodes connected to have the
 opposite potentials, correspondingly.
 With the thickness extensional vibration piezoelectric resonators of FIG.
 13, and FIGS. 17 through 21, the piezoelectric resonance device of FIG.
 11, for example, is provided by use of the substrate and a casing member,
 as in the thickness extensional vibration piezoelectric resonator of the
 first preferred embodiment.
 While the invention has been particularly shown and described with
 reference to preferred embodiments thereof, it will be understood by those
 skilled in the art that the forgoing and other changes in form and details
 may be made therein without departing from the spirit of the invention.