Piezoelectric resonator and electronic component using the same

Abstract: The invention provides a piezoelectric resonator having a small spurious resonance and a large difference ΔF between the resonant frequency and the antiresonant frequency, and which can be easily made compact and in which an external electrode is unlikely to break. A piezoelectric resonator 10 has a rectangular-parallelepiped base member 12. The base member 12 includes twelve laminated piezoelectric layers 14. The middle part of the piezoelectric layers 14 is polarized in opposing directions to each other. On main surfaces of the middle part of the piezoelectric layers 14, first and second inner electrodes 16 and 18 are formed such that they expose alternately at both side faces of the base member 12. On both side faces of the base member 12, first and second external electrodes 20 and 22 are formed so as to alternately connect to the first and second inner electrodes 16 and 18.

Description: The present invention relates to piezoelectric resonators and electronic components using the same, and more particularly, to a piezoelectric resonator which uses the mechanical resonance of a piezoelectric member, and electronic components using the piezoelectric resonator, such as an oscillator, a discriminator, and a filter. Fig. 37 of the drawings is a perspective view of a conventional piezoelectric resonator. A piezoelectric resonator 1 includes a piezoelectric substrate 2 having, for example, a rectangular plate shape viewed from the top. The piezoelectric substrate 2 is polarized in the thickness direction. On both surfaces of the piezoelectric substrate 2, electrodes 3 are formed. When a signal is input between the electrodes 3, an electrical field is applied to the piezoelectric substrate 2 in the thickness direction and the piezoelectric substrate 2 vibrates in the longitudinal direction. In Fig. 38, there is shown a piezoelectric resonator 1 in which electrodes 3 are formed on both surfaces of a piezoelectric substrate 2 having a square plate shape viewed from the top. The piezoelectric substrate 2 of the piezoelectric resonator 1 is polarized in the thickness direction. When a signal is input between the electrodes 3 in the piezoelectric resonator 1, an electrical field is applied to the piezoelectric substrate 2 in the thickness direction and the piezoelectric substrate 2 vibrates in square-type vibration mode. These piezoelectric resonators are of an unstiffened type, in which the vibration direction differs from the direction of polarization and the electrical field. The electromechanical coupling coefficient of such an unstiffened piezoelectric resonator is lower than that of a stiffened piezoelectric resonator, in which the vibration direction, the direction of polarization, and the direction in which an electrical field is applied are the same. An unstiffened piezoelectric resonator has a relatively small frequency difference ΔF between the resonant frequency and the antiresonant frequency. This leads to a drawback in which a frequency bandwidth in use is narrow when an unstiffened frequency resonator is used as an oscillator or a filter. Therefore, the degree of freedom in characteristics design is low in such a piezoelectric resonator and electronic components using the same. The piezoelectric resonator shown in Fig. 37 uses the first-order resonance in the longitudinal mode. It also generates due to its structure large spurious resonances in odd-number-order harmonic modes, such as the third-order and fifth-order modes, and in width mode. To suppress these spurious resonances, some measures applied to the piezoelectric resonator are considered, such as polishing, increasing mass, and changing the shape of the electrodes. These measures increase manufacturing cost. In addition, in the piezoelectric resonator shown in Fig. 37, since when viewed from the top the piezoelectric substrate has a rectangular plate shape, the substrate cannot be thinner due to restrictions in strength. Therefore, the distance between the electrodes cannot be reduced and a capacitance between terminals cannot be made large. This is extremely inconvenient for achieving impedance matching with an external circuit. To form a ladder filter by connecting a plurality of piezoelectric resonators in series and in parallel alternately, the capacitance ratio of the series resonator to the parallel resonator needs to be made large in order to increase attenuation in zones other than the pass band. Because a piezoelectric resonator has the shape restriction described above, however, a large attenuation may not be able to be obtained. The piezoelectric resonator shown in Fig. 38 uses first-order resonance in the square-type mode. Due to its structure, large spurious resonances such as those in the thickness mode and in the third-harmonic mode in the plane direction will be highly possibly generated. Since the piezoelectric resonator needs a large size as compared with a piezoelectric resonator using the longitudinal vibration in order to obtain the same resonant frequency, it is difficult to reduce the piezoelectric resonator in size. When a ladder filter is formed by a plurality of piezoelectric resonators, in order to increase the capacitance ratio between the series resonator and the parallel resonator, the resonators connected in series are made thick and electrodes are formed only on part of a piezoelectric substrate to make the capacitance small as well. In this case, since the electrodes are only partially made, the difference ΔF between the resonant frequency and the antiresonant frequency as well as the capacitance is reduced. The resonators connected in parallel are accordingly required to have small ΔF. As a result, the piezoelectricity of the piezoelectric substrate is not effectively used, and the transmission bandwidth of the filter cannot be increased. A piezoelectric resonator having a small spurious resonance and a large difference ΔF between the resonant frequency and the antiresonant frequency has been considered in Japanese patent application number 8-110475. Fig. 39 is a view of a piezoelectric resonator having such a lamination structure. In the piezoelectric resonator 4 shown in Fig. 39, a plurality of piezoelectric layers 6 and a plurality of electrodes 7 are alternately laminated to form a narrow base member 5, and the plurality of piezoelectric layers 6 are polarized in the longitudinal direction of the base member. This laminated piezoelectric resonator 4 is of a stiffened type, and has the piezoelectric layers 6 in which the vibration direction, the direction of polarization, and the direction in which an electrical field is applied are the same. Therefore, as compared with an unstiffened piezoelectric resonator, in which the vibration direction differs from the direction of polarization and electrical field, the stiffened piezoelectric resonator has a larger electromechanical coupling coefficient and a larger frequency difference ΔF between the resonant frequency and the antiresonant frequency. In addition, vibrations in modes such as the width and thickness modes, which are different from the basic vibration, are unlikely to occur in the piezoelectric resonator 4 with the lamination structure. In the piezoelectric resonator 4 having this lamination structure, the ends of the electrodes 7 are exposed at all side faces of the base member 5. Therefore, on one side face of the base member 5, the ends of alternate electrodes 7 are covered by insulating resin films 8a and an external electrode 9a is formed so as to be connected to the other alternate electrodes 7. On the other side face of the base member 5, which opposes the former side face, the ends of the second alternate electrodes 7 are covered by insulating resin films 8b and an external electrode 9b is formed so as to be connected to the first alternate electrodes 7, on which the insulating resin films 8a are formed. The capacitance C between the external electrodes 9a and 9b is expressed by C ∝ nS/T in the piezoelectric resonator 4 having this lamination structure, where S indicates the area of a cross section perpendicular to the longitudinal direction of the base member 5 or the area of the main surface of a piezoelectric layer 6, T indicates the thickness of a dielectric layer 6 or the distance between electrodes 7, and "n" indicates the number of layers between electrodes 7. Therefore, in the piezoelectric resonator 4 having this lamination structure, to obtain the same capacitance with the area S being made smaller to make the resonator compact, it is necessary to reduce T or increase "n." Since it is difficult to form those insulating resin films 8a and 8b precisely in position in a compact piezoelectric resonator having a distance of, for example, 100 µm or less between electrodes 7 by a method such as printing, however, it is difficult to make a piezoelectric resonator compact. Also in the piezoelectric resonator 4 having this lamination structure, since the external electrodes 9a and 9b formed on the insulating resin films 8a and 8b have a different thermal expansion coefficient from that of the insulating resin films 8a and 8b, the external electrodes may break on the insulation resin films 8a and 8b as shown in Fig. 39 due to thermal shock or thermal cycling in the subsequent process. EP 0 823 781 A2 which was published after the priority date of the present document and, therefore, constitutes a document in accordance with Art. 54 (3) EPC only, describes a piezoelectric resonator with the polarization direction in the longitudinal direction of the base member. The inner electrodes extend over the whole cross section of the base member perpendicular to the longitudinal direction. insulating films are provided in order to insulate every second inner electrode from the respective external electrode. EP 0 820 144 A2 which was published after:the priority date of the present document and, therefore, constitutes a document in accordance with Art. 54(3) EPC only, describes a piezoelectric resonator with a first group of inner electrodes extending over the whole cross section of the base member perpendicular to the longitudinal direction and a second group of inner electrodes extending from one surface of the base member carrying the first external electrode to the second surface of the base member carrying the second external electrode. Insulating films are provided in order to insulate every second inner electrode from the respective external electrode. EP 0 845 859 A2, EP 0 844 734 A2 and EP 0 818 881 A2, each of which was published after the priority date of the present document and, therefore, constitutes documents in accordance with Art. 54(3) EPC only; describes a piezoelectric resonator with a first group of inner electrodes being connected to the first external group and formed to not expose the portion where the second external electrode is formed on a surface of said base member and a second group of inner electrodes being connected to the second external electrode and formed to not expose to the portion where the first external electrode is formed an a surface of said base member. It is the main object of the present invention to provide a piezoelectric resonator having a small spurious resonance and a large difference ΔF between the resonant frequency and the anti resonant frequency which can be easily made compact and in which an external electrode is unlikely to break and to provide an electronic component using the piezoelectric resonator. This object is achieved with a piezoelectric resonator having the features of claim 1 and an electronic component having the features of one of the claims 5, 6, 11 or 12. Subclaims are directed to preferable embodiments. The piezoelectric resonator according to the present invention is of a stiffened type, and has piezoelectric layers in which the vibration direction, the direction of polarization, and the direction in which an electrical field is applied are the same. Therefore, as compared with an unstiffened piezoelectric resonator, in which the vibration direction differs from the direction of polarization and electrical field, the stiffened piezoelectric resonator has a larger electromechanical coupling coefficient and a larger frequency difference ΔF between the resonant frequency and the antiresonant frequency. In addition, vibrations in modes such as the width and thickness modes, which are different from longitudinal vibration, are unlikely to occur in the stiffened piezoelectric resonator. In a piezoelectric resonator according to the present invention, since the electrodes connected to the first electrode of the pair of external electrodes among the plurality of electrodes are formed so as not to expose to the portion where the second electrode of the pair of external electrodes is formed on a surface of the base member and the electrodes connected to the second electrode of the pair of external electrodes among the plurality of electrodes are formed so as not to expose to the portion where the first electrode of the pair of external electrodes is formed on a surface of the base member, it is unnecessary to form an insulating resin film for insulating the ends of the electrodes on the surfaces of the base member. Therefore, the distance between the electrodes can be made short and the resonator is easily made compact. In a piezoelectric resonator according to the present invention, since an insulating resin film is not formed between a first external electrode and the electrodes electrically connected to a second external electrode or between the second external electrode and the electrodes electrically connected to the first external electrode, the external electrodes are unlikely to break in thermal shock and thermal cycling. When electronic components such as an oscillator, a discriminator, and a filter are made using a piezoelectric resonator according to the present invention, the piezoelectric resonator is mounted on an insulating substrate on which pattern electrodes are formed and is covered by a cap to form chip-type electronic components. Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings in which Fig. 1 is a perspective view of a piezoelectric resonator according to the present invention, Fig. 2 is a view showing the structure of the piezoelectric resonator shown in Fig. 1, Fig. 3(a) and 3(c) are plans of inventive inner electrodes used in the piezoelectric resonator shown in Fig. 1 and Fig. 2, Fig. 3(C) is a view illustrating the center axis and the center point of the base member in the piezoelectric resonator shown in Fig. 1 and Fig. 2 and the center axis and the center line of opposing portions in electrodes, Fig. 4 is a perspective view of an unstiffened piezoelectric resonator which vibrates in the longitudinal direction, shown for comparison, Fig. 5 is a perspective view of a stiffened piezoelectric resonator which vibrates in the longitudinal direction, Fig. 6 is a perspective view of an unstiffened piezoelectric resonator which vibrates in the square-type vibration mode, shown for comparison, Fig. 7(a) and 7(b) are plans of electrodes used in a piezoelectric resonator shown for comparison Fig. 8(a) and 8(b) are plans of inventive modifications of the electrodes used in the piezoelectric resonator shown in Fig. 1 and Fig. 2, Fig. 9 is a view of another piezoelectric resonator, Fig. 10(a) and 10(b) are plans of electrodes shown for comparison and usable in the piezoelectric resonator shown in Fig. 9, Fig. 11 is a view of the main section of motherboards used for manufacturing the piezoelectric resonator shown in Fig. 9, Fig. 12 is a view of the main section of external electrodes used for manufacturing the piezoelectric resonator shown in Fig. 9, Fig. 13(a) and 13(b) are plans of inventive modifications of electrodes used in the piezoelectric resonator shown in Fig. 9, Fig. 13(c) is a view illustrating the center axis and the center point of the base member and the center axis and the center line of opposing portions of the first and second electrodes in the piezoelectric resonator using the first and second electrodes shown in Fig. 10, Fig. 13(d) and 13(e) are views of other inventive modifications of the first and second inner electrodes used in the piezoelectric resonator shown in Fig. 9, Fig. 14(a) and 14(b) are plans of modifications of electrodes shown for comparison and usable in the piezoelectric resonators shown in Fig. 1, Fig. 2, and Fig. 9, Fig. 15(a), 15(b), 15(c), 15(d), 15(e), 15(f), 15(g) and 15(h) are plans of other inventive modifications of electrodes used in the piezoelectric resonators shown in Fig. 1, Fig. 2, and Fig. 9, Fig. 16 is a view of still another piezoelectric resonator according to the present invention. Fig. 17(a) and 17(b) are views of electrodes used in the piezoelectric resonator shown in Fig. 16, Fig. 18(a) and 18(b) are views of other inventive modifications of the electrodes used in the piezoelectric resonator shown in Fig. 9, Fig. 19 is a view of yet another piezoelectric resonator according to the present invention, Fig. 20(a) and 20(b) are plans of electrodes used in the piezoelectric resonator shown in Fig. 19, Fig. 21(a) and 21 (b) are plans of modifications of the inner electrodes used in the piezoelectric resonator shown in Fig. 19, Fig. 22(a), 22(b), 22(c), 22(d), 22(e), 22(f), 22(g), 22(h), 22(i), 22(j), 22(k), 22(l), 22(m) and 22(n) are plans of other modifications of the electrodes used in the piezoelectric resonator shown in Fig. 19, Fig. 23 is a perspective view of an electronic component using a piezoelectric resonator according to the present invention, Fig. 24 is a perspective view of an insulating substrate used in the electronic component shown in Fig. 23, Fig. 25 is an exploded perspective view of the electronic component shown in Fig. 23, . Fig. 26 is a view indicating another structure for mounting the piezoelectric resonator to the insulating substrate, Fig. 27 is a side view showing the structure for mounting the piezoelectric resonator, shown in Fig. 26, Fig. 28 is a view indicating still another structure for mounting the piezoelectric resonator to the insulating substrate, Fig. 29 is a side view showing the structure for mounting the piezoelectric resonator, shown in Fig. 28, Fig. 30 is a view of another electronic component using a piezoerectric resonator according to the present invention, Fig. 31 is a side view showing the structure for mounting the piezoelectric resonator, shown in Fig. 30, Fig. 32 is a plan of the main portion of a ladder filter using piezoelectric resonators according to the present invention, Fig. 33 is an exploded perspective view of the main portion of the ladder filter shown in Fig. 32, Fig. 34 is an equivalent circuit diagram of the ladder filter shown in Figs. 32 and 33, Fig. 35 is a plan of the main portion of another ladder filter using piezoelectric resonators according to the present invention, Fig. 36 is an exploded perspective view of the main portion of the ladder filter shown in Fig. 35, Fig. 37 is a perspective view of a conventional piezoelectric resonator, Fig. 38 is a perspective view of another conventional piezoelectric resonator, Fig. 39 is a view of a piezoelectric resonator having lamination structure, which is a background of the present invention and Fig. 40 is a view of a state in which electrically conductive resin layers are formed on surfaces of the external electrodes of the piezoelectric resonator shown in Fig. 39. PREFERRED EMBODIMENTS OF THE PRESENT INVENTION Fig. 1 is a perspective view of a piezoelectric resonator according to an embodiment of the present invention. Fig. 2 shows the internal structure of the piezoelectric resonator. The piezoelectric resonator 10 shown in Figs. 1 and 2 includes a rectangular-parallelepiped-shaped base member 12 measuring, for example, 3.8 mm by 1 mm by 1 mm. The base member 12 includes twelve laminated piezoelectric layers 14 made from, for example, piezoelectric ceramic. These piezoelectric layers 14 are formed with the same dimensions. Eight intermediate piezoelectric layers 14 among these piezoelectric layers 14 are polarized in the longitudinal direction of the base member 12 such that adjacent piezoelectric layers 14 are polarized in opposing directions as shown by arrows in Fig. 2. In the eight intermediate polarized layers among the piezoelectric layers 14, first inner electrodes 16 and second inner electrodes 18 are alternately formed on main surfaces perpendicular to the longitudinal direction of the base member 12. Therefore, these inner electrodes 16 and 18 are perpendicular to the longitudinal direction of the base member 12 and are disposed in the longitudinal direction at certain intervals. The first inner electrodes 16 are formed on main surfaces of piezoelectric layers 14 except for a vertical strip section at one side as shown in Fig. 3(a). The second inner electrodes 18 are formed on main surfaces of piezoelectric layers 14 except for a vertical strip section at the other side as shown in Fig. 3(b). Therefore, the first inner electrodes 16 are formed so as to expose at three side faces of the base member 12, including a first side face, and so as not to expose at the face opposing the first side face. The second inner electrodes 18 are formed so as not to expose at the first side face and so as to expose at three side faces, including the face opposing the first side face. On the first side face and the other opposing side face of the base member 12, first and second external electrodes 20 and 22 are formed, respectively. Therefore, the external electrodes 20 connect to the electrodes 16, and the external electrode 22 connect to the electrodes 18. The piezoelectric resonator 10 uses the first and second external electrodes 20 and 22 as input and output electrodes. The eight intermediate layers among the piezoelectric layers 14, excluding four piezoelectric layers 14 at both ends, in the base member 12 are piezoelectrically active because an electric field is applied between adjacent inner electrodes 16 and 18 by applying a signal to the external electrodes 20 and 22. In this case, since voltages are applied in opposite directions to the piezoelectric layers 14 polarized in opposite directions in the base member 12, the piezoelectric layers 14 expand and contract in the same direction as a whole. In other words, with the use of the first and second inner electrodes 16 and 18 connected to the first and second external electrodes 20 and 22, an AC electric field is applied to each piezoelectric layer 14 in the longitudinal direction of the base member 12 to generate a driving force for expansion and contraction in each piezoelectric layer 14. Therefore, the basic vibration is excited in the entire piezoelectric resonator 10 in longitudinal vibration mode with the center of the base member 12 in the longitudinal direction serving as a node. In the piezoelectric resonator 10, the polarization direction of the piezoelectric layers 14, the applied electric field direction due to an input signal, and the direction of vibration in the piezoelectric layers 14 are all the same. In other words, the piezoelectric resonator 10 is of stiffened type. The piezoelectric resonator 10 has a larger electromagnetic coupling coefficient than an unstiffened piezoelectric resonator, in which the direction of vibration differs from the direction of polarization and electric field. Therefore, the piezoelectric resonator 10 has a larger frequency difference ΔF between the resonant frequency and the antiresonant frequency than the conventional unstiffened piezoelectric resonator. This means that the piezoelectric resonator 10 obtains wide-frequency-band characteristics as compared with the conventional unstiffened piezoelectric resonator. To measure differences between stiffened and unstiffened piezoelectric resonators, piezoelectric resonators shown in Figs. 4, 5, and 6 were made. The piezoelectric resonator shown in Fig. 4 was made by forming electrodes on both surfaces in the thickness direction of a piezoelectric substrate measuring 4.0 mm by 1.0 mm by 0.38 mm. This piezoelectric resonator was polarized in the thickness direction and vibrated in the longitudinal direction when a signal was applied to the electrodes. The piezoelectric resonator shown in Fig. 5 had the same dimensions as the piezoelectric resonator shown in Fig. 4. Electrodes were formed on both surfaces in the longitudinal direction of a piezoelectric substrate. The piezoelectric resonator was polarized in the longitudinal direction and vibrated in the longitudinal direction when a signal was applied to the electrodes. The piezoelectric resonator shown in Fig. 6 was made by forming electrodes on both surfaces in the thickness direction of a piezoelectric substrate measuring 4.7 mm by 4.7 mm by 0.38 mm. This piezoelectric resonator was polarized in the thickness direction and vibrated in the plane direction when a signal was applied to the electrodes. The piezoelectric resonators shown in Figs. 4 and 6 were of unstiffened type and the piezoelectric resonator shown in Fig. 5 was of stiffened type. The resonant frequency Fr and the electromechanical coupling coefficient K of each of these piezoelectric resonators were measured and the results are shown in Tables 1, 2, and 3. Table 1 indicates the measured results of the piezoelectric resonator shown in Fig. 4. Table 2 indicates the measured results of the piezoelectric resonator shown in Fig. 5. Table 3 indicates the measured results of the piezoelectric resonator shown in Fig. 6. Basic longitudinal vibration Longitudinal third-harmonic vibration Width-mode vibration Resonant frequency (MHz) 0.460 1.32 1.95 Electro-mechanical coupling coefficient(%) 18.9 3.9 25.2 Basic longitudinal vibration Longitudinal third-harmonic vibration Width-mode vibration Resonant frequency (MHz) 0.455 1.44 1.96 Electro-mechanical coupling coefficient(%) 42.9 12.2 4.0 Basic square-type vibration Square-type third-harmonic vibration Thickness-mode vibration Resonant frequency (MHz) 0.458 1.25 5.65 Electro-mechanical coupling coefficient(%) 35.0 11.5 23.3 It is understood from the measurement data that a stiffened piezoelectric resonator has a larger electromagnetic coupling coefficient K than an unstiffened piezoelectric resonator, and therefore has a larger frequency difference ΔF between the resonant frequency and the antiresonant frequency. The largest spurious vibration in a stiffened piezoelectric resonator is of longitudinal third-harmonic type and the electromagnetic coupling coefficient K is 12.2% during vibration. During width-mode vibration, which is different from basic vibration, the electromagnetic coupling coefficient K is as small as 4.0%. In contrast, the electromagnetic coupling coefficient K during width-mode vibration is as large as 25.2% in an unstiffened longitudinal-vibration piezoelectric resonator. In an unstiffened square-type vibration piezoelectric resonator, the electromagnetic coupling coefficient K is as large as 23.3% during thickness-mode vibration. Therefore, it is understood that a stiffened piezoelectric resonator has smaller spurious vibrations than an unstiffened piezoelectric resonator. In the piezoelectric resonator 10 shown in Fig. 1 and Fig. 2, as compared with the piezoelectric resonator 4 having the lamination structure shown in Fig. 39, since the first inner electrodes 16 connected to the first external electrode 20 are formed so as not to expose at the side face of the base member 12 where the second external electrode 22 is formed, and the second inner electrodes 18 connected to the second external electrode 22 are formed so as not to expose at the side face of the base member 12 where the first external electrode 20 is formed, it is unnecessary to form an insulating resin film for insulating the ends of the inner electrodes 16 and 18 on the side faces of the base member 12. Therefore, the distance between the inner electrodes 16 and 18 can be made short and the resonator is easily made compact. In the piezoelectric resonator 10, as compared with the piezoelectric resonator 4 having the lamination structure shown in Fig. 39, since an insulating resin film is not formed between the base member 12 and the external electrodes 20 and 22, the external electrodes 20 and 22 are unlikely to break in themal shock and thermal cycling. To make the external electrodes 9a and 9b formed on the insulating resin films 8a and 8b unlikely to break in the piezoelectric resonator 4 having the lamination structure shown in Fig. 39, it can be considered that electrically conductive resin layers 9c and 9d are formed on the surfaces of the external electrodes 9a and 9b as shown in Fig. 40. When the insulating resin films 8a and 8b and the electrically conductive resin layers 9c and 9d are formed on the side faces of the base member 5, however, a large load mass is formed on the side faces of the base member 5. It reduces the mechanical quality coefficient Qm and increases the voltage dependence of the resonant frequency. Conversely, in the piezoelectric resonator 10, since such a load mass is not formed on the side faces of the base member 12, the mechanical quality coefficient Qm does not deteriorate and the voltage dependence of the resonant frequency does not increase. Since the piezoelectric resonator 10 is formed such that the center axis L1 or the center point P1 of the shape formed by overlapping the electrodes 16 and 18 is identical with the center axis L2 or the center point P2 of a plane perpendicular to the longitudinal direction of the base member 12 as shown in Fig. 3(c), a driving force generated in the piezoelectric layers 14 is not misaligned with the center axis of the base member 12, and the base member 12 is hardly bent. Therefore, a spurious resonance due to bending in the base member 12 is unlikely to occur and unacceptable characteristics are also unlikely to be obtained. Since the inner electrodes 16 and 18 are partially formed on main surfaces of piezoelectric layers 14 in this piezoelectric resonator 10, ΔF can be adjusted by adjusting the opposing area of the inner electrodes 16 and 18. The degree of freedom in designing characteristics is large. In the piezoelectric resonator 10, the electric capacitance of the resonator can be adjusted by changing the opposing area of the inner electrodes 16 and 18, the number of the piezoelectric layers 14 and those of the inner electrodes 16 and 18, or the dimensions of the piezoelectric layers 14 in the longitudinal direction of the base member 12. In other words, the electric capacitance can be increased by increasing the opposing area of the inner electrodes 16 and 18, by increasing the number of the piezoelectric layers 14 or those of the inner electrodes 16 and 18, or by reducing the dimensions of the piezoelectric layers 14 in the longitudinal direction of the base member 12. In contrast, the electric capacitance can be reduced by reducing the opposing area of the inner electrodes 16 and 18, by reducing the number of the piezoelectric layers 14 or those of the inner electrodes 16 and 18, or by increasing the dimensions of the piezoelectric layers 14 in the longitudinal direction of the base member 12. Therefore, by changing the opposing area of the inner electrodes 16 and 18, the number of the piezoelectric layers 14 or those of the inner electrodes 16 and 18, or the dimensions of the piezoelectric layers 14 in the longitudinal direction of the base member 12, the electric capacitance is adjusted. This means that a high degree of freedom is given to capacitance design. Therefore, it is easy to achieve impedance matching with an external circuit when the piezoelectric resonator 10 is mounted on a circuit board. Even when the vertical dimension of the piezoelectric resonator 10 is reduced with the width of the portions where the inner electrodes 16 and 18 are not formed on main surfaces of the piezoelectric layers 14 being the same, in order to maintain insulation at the ends of the inner electrodes 16 and 18, the ratio of the opposing area of the inner electrodes 16 and 18 to the area of a main surface of the piezoelectric layers 14 does not change. Therefore, a low-profile piezoelectric resonator can be made without reducing the efficiency of the driving force generated in the piezoelectric layers 14. For comparison, the piezoelectric resonator 10 shown in Fig. 1 and Fig. 2 may be configured such that the inner electrodes 16 are formed on main surfaces of the piezoelectric layers 14 except for two side portions of the upper side and one vertical side adjacent thereto as shown in Fig. 7(a) and the inner electrodes 18 are formed on main surfaces of the piezoelectric layers 14 except for two side portions of the upper side and the other vertical side adjacentthereto as shown in Fig. 7(b). The inventive piezoelectric resonator 10 shown in Fig. 1 and Fig. 2 may be configured such that the electrodes 16 are formed on main surfaces of the piezoelectric layers 14 except for two adjacent side portions as shown in Fig. 8(a) and the electrodes 18 are formed on main surfaces of the piezoelectric layers 14 except for the other two adjacent side portions as shown in Fig. 8(b). Fig 9 is a view of another piezoelectric resonator according to the present invention. The piezoelectric resonator shown in Fig. 9 is different from the piezoelectric resonator shown in Fig. 1 and Fig. 2 in the inner electrodes 16 and 18 and the external electrodes 20 and 22. For comparison, in the piezoelectric resonator 10 shown in Fig. 9, the first inner electrodes 16 are formed on main surfaces of piezoelectric layers 14 except for the portion from the middle section at the upper side to one end as shown in Fig. 10(a) and the second inner electrodes 18 are formed on main surfaces of piezoelectric layers 14 except for the portion from the middle section at the upper side to the other end as shown in Fig. 10(b). In other words, the first inner electrodes 16 are formed so as not to expose from the middle to one end in the width direction on the upper side face of the base member 12 and so as to expose at the other end. The second inner electrodes 18 are formed so as to expose at one end in the width direction on the upper side face of the base member 12 and so as not to expose from the middle to the other end. First and second external electrodes 20 and 22 are formed in two lines in the longitudinal direction of the base member 12 on the upper side face of the base member 12 at one end and the other end viewed from the center in the width direction of the base member 12. In this case, the first external electrode 20 is connected to the first inner electrodes 16 and the second external electrode 22 is connected to the second inner electrodes 18. The piezoelectric resonator 10 shown in Fig. 9 has another advantage. As compared with the piezoelectric resonator shown in Fig. 1 and Fig. 2, even when the dimension of the piezoelectric resonator 10 is reduced in the width direction with the width of the portions where the first and second inner electrodes 16 and 18 are not formed on main surfaces of the piezoelectric layers 14 being the same, in order to maintain insulation at the ends of the first and second inner electrodes 16 and 18, since the ratio of the opposing area of the first and second inner electrodes 16 and 18 to the area of a main surface of the piezoelectric layers 14 does not change, the piezoelectric resonator can be reduced in the width direction without reducing the efficiency of the driving force generated in the piezoelectric layers 14. A manufacturing method for the piezoelectric resonator 10 shown in Fig. 9 will be described next. As shown in Fig. 11, twelve piezoelectric ceramic motherboards 15, which will serve as a plurality of piezoelectric layers 14, are prepared. On one main surface of each of five mother boards 15, each of mother electrodes 17, which will serve as a plurality of the first inner electrodes 16, is formed. On one main surface of each of another four mother boards 15, each of mother boards 19, which will serve as a plurality of second inner electrodes 18, is formed. No electrode is formed on the remaining three motherboards 15. These motherboards 15 are laminated to form a laminated member. The laminated member is cut along one-dot chain lines extending in the transverse direction in Fig. 11. On a cutting surface of the cut laminated member, as shown in Fig. 12, electrodes 21, which will serve as the external electrodes 20 and 22, are formed. Each motherboard 15 or each piezoelectric layer 14 is polarized by applying a high DC voltage to adjacent electrodes 21. This laminated member is cut along one-dot chain lines shown in Fig. 12 to form the piezoelectric resonators 10. Piezoelectric resonators other than that shown in Fig. 9 can be made in the same way. The piezoelectric resonator 10 shown in Fig. 9 may be configured such that the inner electrodes 16 are formed on main surfaces of piezoelectric layers 14 except for the portion from the middle section at the upper side to one end and the lower side as shown in Fig. 13(a), and the second inner electrodes 18 have the reflected image of the electrodes 16 and are formed on main surfaces of piezoelectric layers 14 except for the portion from the middle section at the upper side to the other end and the lower side as shown in Fig. 13(b). Since in the embodiment of Fig. 13(a) and Fig. 13(b), the piezoelectric resonator 10 having the first and second inner electrodes 16 and 18 is formed such that the center axis L1 or the center point P1 of the shape formed by overlapping the first and second electrodes 16 and 18 is identical with the center axis L2 or the center point P2 of a plane perpendicular to the longitudinal direction of the base member 12 as shown in Fig. 13(c), a driving force generated in the piezoelectric layers 14 is not misaligned with the center axis of the base member 12, and the base member 12 is hardly bent. Therefore, the resonator has another advantage. A spurious resonance due to bending in the base member 12 is unlikely to occur and unacceptable characteristics are also unlikely to be obtained. The piezoelectric resonator 10 shown in Fig. 1 may be configured such that the first inner electrodes 16 are not formed at three sides of piezoelectric layers 14, excluding the upper side, as shown in Fig. 13(d), and the second inner electrodes 18 have the reflected image of the first inner electrodes 16 and are not formed at three sides on main surfaces of piezoelectric layers 14, excluding the upper side where the external electrodes are formed, as shown in Fig. 13(e). When the first and second inner electrodes 16 and 18 are formed in this way, since the first and second inner electrodes 16 and 18 are not exposed on the surfaces of the piezoelectric resonator 10 at all, the resonator has another advantage. The moisture-proofness of the first and second inner electrodes 16 and 18 is increased and the insulation resistance between the first and second inner electrodes 16 and 18 is unlikely to decrease. For comparison, the piezoelectric resonator 10 shown in Fig. 1 may be configured such that the first inner electrodes 16 are formed on main surfaces of piezoelectric layers 14 except for the portion from the middle section at the upper side to one end and a vertical side portion adjacent thereto as shown in Fig. 14(a), and the second inner electrodes 18 have the reflected image of the first electrodes 16 and are formed on main surfaces of piezoelectric layers 14 except for the portion from the middle section at the upper side to the other end and the other vertical side portion adjacent thereto as shown in Fig. 14(b). The inventive piezoelectric resonator 10 shown in Fig. 1 may be configured such that the first inner electrodes 16 are formed on main surfaces of piezoelectric layers 14 from the middle section at the upper side to the middle section of a vertical side adjacent thereto as shown in Fig. 15(a), and the second inner electrodes 18 have the reflected image of the first inner electrodes 16 and are formed on main surfaces of piezoelectric layers 14 from the middle section at the upper side to the middle section of the other vertical side adjacent thereto as shown in Fig. 15(b). The piezoelectric resonator 10 shown in Fig. 1 may be formed such that the first inner electrodes 16 are formed on main surfaces of piezoelectric layers 14 from the center to the middle of each of three sides, the upper side, the lower side, and one vertical side, as shown in Fig. 15(c), and the second inner electrodes 18 have the reflected image of the first inner electrodes 16 and are formed on main surfaces of piezoelectric layers 14 from the center to the middle of each of three sides, the other vertical side, the upper side, and the lower side as shown in Fig. 15(d). The piezoelectric resonator 10 shown in Fig. 1 may be configured such that the first inner electrodes 16 are formed on main surfaces of piezoelectric layers 14 from the center to the middle of each of the upper and the lower sides without reaching the vertical portions at both sides of the piezoelectric layers 14 as shown in Fig. 15(e), and the second inner electrodes 18 have the reflected image of the first inner electrodes 16 and are formed on main surfaces of piezoelectric layers 14 from the center to the middle of each of the upper and lower sides without reaching the vertical portions at both sides of the piezoelectric layers 14 as shown in Fig. 15(f). The piezoelectric resonator 10 shown in Fig. 1 may be configured such that the first inner electrodes 16 are formed on main surfaces of piezoelectric layers 14 from the center to the vertical portions at both sides as shown in Fig. 15(g), and the second inner electrodes 18 have the reflected image of the first inner electrodes 16 and are formed on main surfaces of piezoelectric layers 14 from the center to the vertical portions at both sides as shown in Fig. 15(h). When the first and second inner electrodes 16 and 18 are formed as shown in Fig. 13(a),13(b) or Fig. 15(g),15(h), even if the dimension of the piezoelectric resonator is reduced in the width direction with the width of the portions where the first or second electrodes 16 or 18 are not formed on main surfaces of the piezoelectric layers 14 being the same, in order to maintain insulation at the ends of the first and second inner electrodes 16 and 18, since the ratio of the opposing area of the first and second inner electrodes 16 and 18 to the area of a main surface of the piezoelectric layers 14 does not change, the piezoelectric resonator can be reduced in the width direction without reducing the efficiency of the driving force generated in the piezoelectric layers 14. When the first and second inner electrodes 16 and 18 are formed as shown in Fig. 13(e), 13(f) and Fig. 15(a) - 15(h), since the piezoelectric resonator is formed such that the center axis or the center point of the shape formed by overlapping the first and second inner electrodes 16 and 18 is identical with the center axis or the center point of a plane perpendicular to the longitudinal direction of the base member 12, a driving force generated in the piezoelectric layers 14 is not misaligned with the center axis of the base member 12, and the base member 12 is hardly bent. Therefore, the resonator has another advantage. A spurious resonance due to bending in the base member 12 is unlikely to occur and unacceptable characteristics are also unlikely to be obtained. In these embodiments, the shapes of opposing portions of the first and second inner electrodes 16 and 18 are symmetric against a main surface of piezoelectric layers 14. In each piezoelectric resonator 10 described above, the middle portion in the longitudinal direction of the base member 12 is piezoelectrically active and vibrates. Both ends in the longitudinal direction of the base member 12 are formed of inactive sections, which are not piezoelectrically active. Only when the piezoelectric layers are polarized and an electric field is applied, the portion becomes piezoelectrically active and otherwise remains piezoelectrically inactive. If the inactive sections have such characteristics, it may have another structure. Fig 16 is a view of still another piezoelectric resonator according to the present invention. The piezoelectric resonator shown in Fig. 16 is different from the piezoelectric resonator shown in Fig. 1 and Fig. 2 in shape of the electrodes 16 and 18. In the piezoelectric resonator 10 shown in Fig. 16, the inner electrodes 16 are formed on main surfaces of piezoelectric layers 14 from the center to the middle of one vertical side as shown in Fig. 17(a), and the inner electrodes 18 have the reflected image of the inner electrodes 16 and are formed on main surfaces of piezoelectric layers 14 from the center to the middle of the other vertical side as shown in Fig. 17(b). Although not shown in Fig. 16, a first external electrode 20 is connected to the first inner electrodes 16 and a second external electrode 22 is connected to the second inner electrodes 18. In the piezoelectric resonator 10 shown in Fig. 16, since the inner electrodes 16 and 18 are not exposed at all at the surfaces of the resonator, the moisture-proofness of the inner electrodes 16 and 18 is increased and the insulation resistance between the inner electrodes 16 and 18 is unlikely to decrease. The piezoelectric resonator 10 shown in Fig. 9 may be configured such that the inner electrodes 16 are not formed at three sides of piezoelectric layers 14, excluding the upper side, as shown in Fig. 18(a), and the inner electrodes 18 have the reflected image of the inner electrodes 16 and are not formed at three sides on main surfaces of piezoelectric layers 14, excluding the upper side as shown in Fig. 18(b). Even when the inner electrodes 16 and 18 are formed in this way, the moisture-proofness of the inner electrodes 16 and 18 is increased and the insulation resistance between the inner electrodes 16 and 18 is unlikely to decrease in the same way as for the piezoelectric resonator 10 shown in Fig. 16. Fig 19 is a view of yet another piezoelectric resonator according to the present invention. The piezoelectric resonator shown in Fig. 19 is different from the piezoelectric resonators described above especially in the inner electrodes 16 and 18 and the external electrodes 20 and 22. In the piezoelectric resonator 10 shown in Fig. 19, first inner electrodes 16 are formed on main surfaces of piezoelectric layers 14 from the center to the middle of each of three sides, the upper side, the lower side, and one vertical side, as shown in Fig. 20(a), and second inner electrodes 18 have the reflected image of the first inner electrodes 16 and are formed on main surfaces of piezoelectric layers 14 from the center to the middle of each of three sides, the other vertical side, the upper side, and the lower side as shown in Fig. 20(b). First external electrodes 20 are formed on one side face, the upper side face, and the lower side face of the base member 12 so as to connect to the first inner electrodes 16. Second external electrodes 22 are formed on the other side face, the upper side face, and the lower side face of the base member 12 so as to connect to the second inner electrodes 18. In this case, the external electrodes 20 and 22 are formed in lines, respectively, on the upper side face and the lower side face of the base member 12. The piezoelectric resonator 10 shown in Fig. 19 has another advantage. Since the external electrodes 20 and 22 are formed on three different side faces of the base member 12, respectively, the resonator can be mounted, for example, to a circuit board with the current state, with a 90-degree-rotated state, and with the upside-down state. The piezoelectric resonator 10 shown in Fig. 19 may be configured such that the first inner electrodes 16 are not formed at vertical portions at either side of piezoelectric layers 14 as shown in Fig. 22(c), as compared with the electrode shape shown in Fig. 20(a), and the second inner electrodes 18 have the reflected image of the first inner electrodes 16 and are not formed at vertical portions at either side on main surfaces of piezoelectric layers 14 as shown in Fig. 22(d), as compared with the electrode shape shown in Fig. 20(b). The piezoelectric resonator 10 shown in Fig. 19 may be configured such that the first inner electrodes 16 are formed on main surfaces of piezoelectric layers 14 from the center to the vertical portions at both sides as shown in Fig. 22(e), and the second inner electrodes 18 have the reflected image of the first inner electrodes 16 and are formed on main surfaces of piezoelectric layers 14 from the center to the vertical portions at both sides as shown in Fig. 22(f). In this case, the piezoelectric resonator 10 shown in Fig. 19 has another advantage. Even when the shape of the resonator is compressed in the width direction, since the ratio of the opposing area of the inner electrodes 16 and 18 to the area of a main surface of the piezoelectric layers 14 hardly changes, the piezoelectric resonator can be reduced in the width direction without reducing the efficiency of the driving force generated in the piezoelectric layers 14. In the piezoelectric resonator 10 shown in Fig. 19, the inner electrodes 16 and 18 may have the shapes shown in Fig. 22(g) and Fig. 22(h), Fig. 22(i) and Fig. 22(j), Fig. 22(k) and Fig. 22(l), or Fig. 22(m) and Fig. 22(n). In the piezoelectric resonator 10 shown in Fig. 19, it is necessary to provide at least one first external electrode 20 connected only to the first inner electrodes 16 and at least one second external electrode 22 connected only to the second inner electrodes 18. In each piezoelectric resonator 10 described above, the middle portion in the longitudinal direction of the base member 12 is piezoelectrically active and vibrates. Both ends in the longitudinal direction of the base member 12 are formed of inactive sections, which are not piezoelectrically active. Only when the piezoelectric layers are polarized and an electric field is applied, the portion becomes piezoelectrically active and otherwise remains piezoelectrically inactive. If an inactive portion 24 has such characteristics, it may have another structure. Using such a piezoelectric resonator 10, electronic components such as oscillators and discriminators are produced. Fig. 23 is a perspective view of an electronic component 60. The electronic component 60 includes an insulating substrate 62 serving as a support member. At opposing end portions of the insulating substrate 62, two indentations 64 are formed, respectively. On one surface of the insulating substrate 62, two pattern electrodes 66 and 68 are formed as shown in Fig. 24. One pattern electrode 66 is formed between opposing indentations 64 and extends in an L-shaped manner from a point near one end toward the center of the insulating substrate 62. The other pattern electrode 68 is formed between opposing indentations 64 and extends straight from a point near the other end toward the center of the insulating substrate 62. The pattern electrodes 66 and 68 are formed such that they are routed in a roundabout fashion from the indentations 64 of the insulating substrate 62 to the opposite surface. At one end of the pattern electrode 66 disposed at the center of the insulating substrate 62, a protrusion 70 serving as a mounting member is formed with electrically conductive adhesive. As shown in Fig. 25, the above-described piezoelectric resonator 10 is mounted on the protrusion 70 such that the center of the base member 12 is disposed on the protrusion 70. An external electrode 22 of the piezoelectric resonator 10 is, for example, connected to the protrusion 70. The protrusion 70 may be formed in advance on the piezoelectric resonator 10. The other external electrode 20 is connected to a pattern electrode 68 with electrically conductive wire 72. The electrically conductive wire 72 is connected to the center of the external electrode 20 of the piezoelectric resonator 10. A metal cap 74 is placed on the insulating substrate 62. To prevent the metal cap 74 from being short-circuited to the pattern electrodes 66 and 68, insulating resin is applied to the insulating substrate 62 and the pattern electrodes 66 and 68 in advance. The metal cap 74 is put on to complete the electronic component 60. The electronic component 60 uses the pattern electrodes 66 and 68, which are formed such that they are routed to the rear surface from the indentations 64 of the insulating substrate 62, as input and output terminals for connecting to external circuits. Since the protrusion 70 is formed and the center of the piezoelectric resonator 10 is secured to the protrusion 70 in this electronic component 60, the ends of the piezoelectric resonator 10 are disposed separately from the insulating substrate 62 so vibration is not prevented. Excited longitudinal vibration is not weakened because the center of the piezoelectric resonator, which serves as a node, is secured to the protrusion 70 and is connected to the electrically conductive wire 72. The electronic component 60 is mounted on a circuit board together with IC chips and other components to form an oscillator or a discriminator. Since the electronic component 60 having this structure is sealed and protected by the metal cap 74, it can be used as a chip-type component, which can be mounted by reflow soldering. When the electronic component 60 is used in an oscillator, spurious vibrations are suppressed to a low level and unusual vibration caused by the spurious vibrations are prevented due to the features of the piezoelectric resonator 10 used in the electronic component 60. It is also easy to achieve impedance matching with an external circuit since the capacitance of the piezoelectric resonator 10 can be set to any desired value. Especially when the electronic component is used for an oscillator for voltage-controlled oscillation, a wide frequency range which cannot be obtained conventionally is acquired due to a large ΔF of the resonator. When the electronic component 60 is used for a discriminator, a wide peak-separation range is provided due to a large ΔF of the resonator. In addition, since the resonator provides a wide capacitance range, it is easy to achieve impedance matching with an external circuit. The piezoelectric resonator 10 may be mounted on the insulating substrate 62 so that two protrusions 70 made from an electrically conductive material such as electrically conductive adhesive are formed on both pattern electrodes 66 and 68, and the external electrodes 20 and 22 of the piezoelectric resonator 10 are connected to the two protrusions 70, as shown in Figs. 26 and 27. The piezoelectric resonator 10 may also be mounted on the insulating substrate 62 in a way shown in Figs. 28 and 29 in which two protrusions 70 made from an insulating material such as insulating adhesive are formed on the insulating substrate 62 and the external electrodes 20 and 22 are connected to the pattern electrodes 66 and 68 with electrically conductive wire 72. The protrusions 70 may be formed on the piezoelectric resonator 10 in advance. Fig. 30 is a view of another electronic component using a piezoelectric resonator according to the present invention. Fig. 31 is a side view showing how the piezoelectric resonator is mounted. The electronic component shown in Fig. 30 and Fig. 31 is different from that shown in Fig. 26 and Fig. 27 especially in that the piezoelectric resonator 10 in which the external electrodes 20 and 22 are formed on one side face of the base member 12, for example as shown in Fig. 9, is used. Fig. 32 is a plan of the main section of a ladder filter serving as an electronic component using a piezoelectric resonator according to the present invention. Fig. 33 is an exploded perspective view of the main section. In the electronic component 60 shown in Figs. 32 and 33, four pattern electrodes 90, 92, 94, and 96 are formed on an insulating substrate 62 serving as a support member. Five lands disposed in line at a certain interval are formed on the pattern electrodes 90, 92, 94, and 96. The first land, which is closest to an end of the insulating substrate 62, is formed on the pattern electrode 90, the second and fifth lands are formed on the pattern electrode 92, the third land is formed on the pattern electrode 94, and the fourth land is formed on the pattern electrode 96. Mounting members are formed using electrically conductive adhesive on the five lands: one protrusion 98 on the first land; two protrusions 100 and 102 on the second land; two protrusions 104 and 106 on the third land; two protrusions 108 and 110 on the fourth land, and one protrusion 112 on the fifth land. These protrusions 98, 100, 102, 104, 106, 108, 110, and 112 are formed in line at a certain interval. The external electrodes 20 and 22 of piezoelectric resonators 10a, 10b, 10c, and 10d are mounted on these protrusions 98, 100, 102, 104, 106, 108, 110, and 112. The piezoelectric resonator 10 in which the external electrodes 20 and 22 are formed on both side faces of the base member 12, for example as shown in Fig. 1 and Fig. 2, or Fig. 16, is used for these piezoelectric resonators 10a to 10d. The protrusions 98, 100, 102, 104, 106, 108, 110, and 112 may be formed in advance on the piezoelectric resonators 10a, 10b, 10c, and 10d. The protrusions 98, 100, 102, 104, 106, 108, 110, and 112 may be bonded to the lands of the pattem electrodes 90, 92, 94, and 96 and the external electrodes 20 and 22 of the piezoelectric resonators 10a to 10d with the same type of electrically conductive adhesive as that constituting the protrusions 98, 100, 102, 104, 106, 108, 110, and 112 or another type of electrically conductive adhesive. This also applies to electronic components other than that shown in Fig. 32 and Fig. 33. Then, a metal cap (not shown) is placed on the insulating substrate 62. The electronic component 60 shown in Fig. 32 and Fig. 33 is used as a ladder filter having a ladder-shaped circuit shown in Fig. 34. Two piezoelectric resonators 10a and 10c serve as series resonators and the other two piezoelectric resonator 10b and 10d serve as parallel resonators. In such a ladder filter, the parallel piezoelectric resonators 10b and 10d are designed to have substantially larger capacitances than the series piezoelectric resonators 10a and 10c. Attenuation in the ladder filter is determined by the capacitance ratio between the series resonators and the parallel resonators. In this electronic component 60 shown in Fig. 32 and Fig. 33, the capacitance can be adjusted by changing the number of laminated layers used in the piezoelectric resonators 10a to 10d. Therefore, a ladder filter having a larger attenuation with fewer resonators is implemented by changing the capacitances of the piezoelectric resonators, as compared with a case where the conventional unstiffened piezoelectric resonators are used. Since the piezoelectric resonators 10a to 10d have a larger ΔF than the conventional piezoelectric resonator, a wider transmission frequency band is implemented as compared with a case in which the conventional piezoelectric resonator is used. In the electronic component 60 shown in Figs. 32 and 33, since two electrodes of adjacent piezoelectric resonators are mounted on two protrusions formed on the same land, the two electrodes of adjacent piezoelectric resonators do not need to be insulated and thus adjacent resonators can be disposed closely, allowing the component to be made compact. Fig. 35 is a plan of the main section of another ladder filter serving as an electronic component using a piezoelectric resonator according to the present invention. Fig. 36 is an exploded perspective view of the main section. The electronic component shown in Figs. 35 and 36 differs from the electronic component shown in Fig. 32 and Fig. 33 especially in that the piezoelectric resonator 10 in which the external electrodes 20 and 22 are formed on one side face of the base member 12, for example as shown in Fig. 9 or Fig. 19, is used for piezoelectric resonators 10a, 10b, 10c, and 10d. The electronic component shown in Fig. 35 and Fig. 36 has the same advantage as the electronic component shown in Fig. 32 and Fig. 33. Each electronic component described above is formed in a chip shape. In the present invention, an electronic component may have a shape other than a chip shape. In each piezoelectric resonator 10 described above, the plurality of piezoelectric layers 14 are polarized in opposing directions alternately. The polarization directions of the plurality of piezoelectric layers 14 are not limited to this case. In each piezoelectric resonator 10 described above, the dimensions of the piezoelectric layers 14 in the longitudinal direction of the base member 12 or the distances between adjacent inner electrodes 16 and 18 are identical. The dimensions may not be identical and the distances may not be identical. In each piezoelectric resonator 10 described above, one piezoelectric layer 14 is formed between adjacent inner electrodes 16 and 18. A plurality of piezoelectric layers may be formed. In each piezoelectric resonator 10 described above, the electrodes 16 and 18 connected to the external electrodes 20 and 22 are alternately formed. The inner electrodes 16 and 18 may not be formed alternately. The invention has been particularly shown and described with reference to preferred embodiments thereof.

Claims: A piezoelectric resonator (10) comprising: a base member (12) having a longitudinal direction in which the base member has a larger extension than in the direction perpendicular to said longitudinal direction; a plurality of plane inner electrodes (16, 18) disposed perpendicularly to the longitudinal direction of said base member (12) at intervals in the longitudinal direction of said base member (12); a first and a second external electrode (20, 22) formed on said base member and connected to said plurality of inner electrodes, said base member (12) including a plurality of laminated piezoelectric layers (14); said plurality of piezoelectric layers (14) being polarized in the longitudinal direction of said base member (12); said plurality of inner electrodes being formed on surfaces of said plurality of piezoelectric layers (14) which are perpendicular to the longitudinal direction of said base member (12); a first group of said inner electrodes (16) being of identical shape and connected to the first external electrode (20) and formed to not expose to the portion where the second external electrode (22) is formed on a surface of said base member (12); and a second group of said inner electrodes (18) being of identical shape and connected to the second external electrode (22) and formed to not expose to the portion where the first external electrode (20) is formed on a surface of said base member (12), wherein the centre axis (L1) or the centre point (P1) of a shape formed by overlapping portion of projections of adjacent inner electrodes (14, 16) onto a plane perpendicular to the longitudinal direction of said base member (12) is identical with the centre axis (L2) or the centre point (P2) of said base member on a plane perpendicular to the longitudinal direction of said base member (12). The piezoelectric resonator according to claim 1, wherein said first and second external electrodes (20, 22) are provided on different side faces of said base member, respectively, said side faces extending along the longitudinal direction of said base member (12). The piezoelectric resonator according to claim 1, wherein said first and second external electrodes (20, 22) are provided on one side face of said base member, said side face extending along the longitudinal direction of said base member (12). The piezoelectric resonator according to one of claims 1 to 3, further comprising a support member (62), and a mounting member (70) disposed between said support member (62) and a centre portion of said base member (12) in its longitudinal direction. An electronic component (60) using the piezoelectric resonator of claim 4, said support member comprising an insulating substrate (62) with a patterned electrode (66, 68) thereon; said base member being mounted on said insulating substrate via said mounting member; and a cap (74) being disposed on said insulating substrate (62) so as to cover said base member. An electronic component using a plurality of piezoelectric resonators of claim 4, said support member comprising an insulating substrate with a patterned electrode (66, 68) thereon; a plurality of said base members being mounted on said insulating substrate (62) via said mounting member so as to constitute a ladder filter; and a cap (74) being disposed on said insulating substrate so as to cover said base member. A piezoelectric resonator according to claim 1, wherein: said piezoelectric resonator utilizes longitudinal vibration mode; and said first and second external electrodes (20, 22) are connected to the first and second groups of said inner electrodes (16, 18) respectively on one side face of said base member (12) extending along the longitudinal direction of said base member (12), said first and second external electrodes (20, 22) being formed near opposing ends of said side face, when viewed from the centre of said side face of said base member in the width direction. The piezoelectric resonator according to Claim 7, wherein a groove is formed in the longitudinal direction of said base member at the centre in the width direction on the side face of said base member on which said first and second external electrodes (20, 22) are formed, and said first and second external electrodes (20, 22) are formed on opposing sides of said groove on said side face of said base member. The piezoelectric resonator according to claim 7 or 8, wherein said first and second groups of said inner electrodes each comprise a plurality of said inner electrodes (16, 18). The piezoelectric resonator according to one of Claims 7 to 9, further comprising a mounting member disposed substantially at the centre of said base member when viewed in its longitudinal direction. An electronic component using the piezoelectric resonator of claim 10, said base member being mounted on a substrate with an electrode thereon, via said mounting member; and a cap being disposed on the substrate so as to cover said base member. An electronic component (60) using a plurality of piezoelectric resonators specified in Claim 10, a plurality of said base members being mounted on a substrate with an electrode thereon, via said mounting member so as to constitute a ladder filter; and a cap (74) being disposed on the substrate so as to cover said base member. The electronic component according to claim 12, wherein first, second, third and fourth base members (10a, 10b, 10c, 10d) are provided as said base members; first, second, third and fourth pattern electrodes (90, 92, 94, 96) are provided as said electrode on said substrate; said first base member (10a) is mounted on said first and second pattern electrodes (90, 92); said second base member (10b) is mounted on said second and third pattern electrodes (92, 94); said third base member (10c) is mounted on said second and fourth pattern electrodes (92, 96); and said fourth base member (10d) is mounted on said third and fourth pattern electrodes (94, 96). The electronic component according to one of claims 5, 6, 11, 12 and 13, wherein said mounting member is made of electrically conductive adhesive.

Inventors: BABA TOSHIYUKI, INOUE JIRO, KAWABATA SHOICHI, MIZUGUCHI TAKASHI, NISHIMURA TOSHIO, OGAWA MAMORU, UNAMI TOSHIHIKO, BABA, TOSHIYUKI, INOUE, JIRO, KAWABATA, SHOICHI, MIZUGUCHI, TAKASHI, NISHIMURA, TOSHIO, OGAWA, MAMORU, UNAMI, TOSHIHIKO
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1998
Main classification: H03H9
IPC codes: H03H9