Piezoelectric resonator and electronic component using the same

Abstract: A piezoelectric resonator (10) has a rectangular-parallelepiped-shaped base member (12). The base member (12) includes laminated 20 piezoelectric layers (12a). The piezoelectric layers(12a) are polarized in the longitudinal direction of the base member (12) such that the directions of polarization differ at adjacent piezoelectric layers (12a). Internal electrodes (14) are formed between the piezoelectric layers (12a). The internal electrodes (14) are formed in six stripes on main surfaces of the piezoelectric layers (12a). On opposing side faces of the base member (12), the internal electrodes (14) are alternately covered by insulating films (16,18). The insulating films (16,18) cover different internal electrodes (14). External electrodes (20,22) are formed on the opposing side faces of the base member (12) and are connected to internal electrodes (14).

Description: BACKGROUND OF THE INVENTION The present invention as it is defined in the appended claims relates to a piezoelectric resonator which uses the mechanical resonance of a piezoelectric member, and more particularly, to a piezoelectric resonator comprising a base member having a longitudinal direction, an active section composed of polarized piezoelectric member and constituting at least a part of the base member, and a pair of external electrodes provided with the active section. The present invention also relates to electronic components for use with the piezoelectric resonator, such as an oscillator, a discriminator, and a filter. Fig. 38 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 electric field is applied to the piezoelectric substrate 2 in the thickness direction and the piezoelectric substrate 2 vibrates in the longitudinal direction. In Fig. 39, 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 electric field is applied to the piezoelectric substrate 2 in the thickness direction and the piezoelectric substrate 2 vibrates in square-type vibration mode (in the plane direction). These piezoelectric resonators are of an unstiffened type, in which the vibration direction differs from the direction of polarization and the electric 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 electric 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-band width 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. 38 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 are considered, such as polishing, increasing mass, and changing the shape of the electrodes. These measures increase manufacturing cost. In addition, 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. Because a piezoelectric resonator has shape restriction described above, however, large attenuation cannot be obtained. The piezoelectric resonator shown in Fig. 39 uses square-type first-order resonance (in the plane direction). Due to its structure, large spurious resonances such as those in the thickness mode and in the triple-wave mode in the plane direction are also 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 band width of the filter cannot be increased. The inventors thought out a piezoelectric resonator having small spurious resonance and a large difference ΔF between the resonant frequency and the antiresonant frequency. In the piezoelectric resonator, a plurality of piezoelectric layers and a plurality of electrodes are alternately laminated to form a base member having a longitudinal direction, and the plurality of piezoelectric layers is polarized in the longitudinal direction of the base member. This laminated piezoelectric resonator is of a stiffened type, and has the piezoelectric layers in which the vibration direction, the direction of polarization, and the direction in which an electric 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 electric 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 a stiffened piezoelectric resonator. In a piezoelectric resonator having this lamination structure, each electrode is formed on the entire area of a surface perpendicular to the longitudinal direction of the base member in each piezoelectric layer. Therefore, it is necessary to change distances between electrodes and the number of electrodes to set ΔF to the desired value. Accordingly, it is the main object of the present invention to provide a piezoelectric resonator having a high degree of freedom in characteristics design and allowing the difference ΔF between the resonant frequency and the antiresonant frequency to be set to the desired value without changing distances between electrodes or the number of electrodes, and to provide an electronic component using the piezoelectric resonator. The foregoing object is achieved in one aspect of the present invention through the provision of a piezoelectric resonator of the above mentioned kind, which is characterized in that a plurality of internal electrodes connected to said external electrodes are disposed in/on said active section at intervals in the longitudinal direction of said base member, at least one of internal electrodes partially covers a cross section of said base member perpendicular to the longitudinal direction, and said base member excites a longitudinal basic vibration when an AC electric field in the longitudinal direction of said base member is applied to each of said active section via said plurality of internal electrodes. In the above described piezoelectric resonator, at least a part of said plurality of internal electrodes may partially cover a cross section of said base member perpendicular to the longitudinal direction in stripe shape. In the above described piezoelectric resonator, alternate layers of said plurality of internal electrodes may partially cover a cross section of said base member perpendicular to the longitudinal direction in stripe shape, and the other alternate layers of said plurality of internal electrodes may entirely cover a cross section of said base member perpendicular to the longitudinal direction. In the above described piezoelectric resonator, the piezoelectric resonator may further comprise a support member, and a mounting member disposed between said support member and a center section of said base member in the longitudinal direction. The forgoing object is achieved in another aspect of the present invention through the provision of an electric component for use with the above described piezoelectric resonator, which is characterized in that said support member is an insulating substrate on which a pattern electrode is provided, and said base member is mounted on said insulating substrate through said mounting member. The above described electric component may be a ladder filter in which a plurality of said base member is mounted on said insulating substrate through said mounting member and connected to each other in a ladder shape. 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 electric 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 electric 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 at least a part of a plurality of electrodes is partially formed on surfaces perpendicular to the longitudinal direction of the base member in the piezoelectric layers, the overlapping area of the opposing portions of adjacent electrodes as seen from the direction perpendicular to the electrodes can be changed and ΔF is thereby adjusted. This leads to a high degree of freedom in characteristics design. 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 (surface mount) electronic components. According to the present invention, the difference ΔF between the resonant frequency and the antiresonant frequency is large as compared with that of a conventional piezoelectric resonator, a wide-frequency-band piezoelectric resonator can be obtained. In addition, a piezoelectric resonator according to the present invention is unlikely to generate vibrations in modes other than the basic vibration mode and achieves superior characteristics. Furthermore, ΔF can be set to the desired value without changing distances between electrodes and the number of electrodes in a piezoelectric resonator according to the present invention. This leads to a high degree of freedom in characteristics design. Since a chip-type electronic component can be made using the piezoelectric resonator, it is easy to mount the component on a circuit board. The above-described object, other objects, other features, and other advantages of the present invention will be made clear in the following description noted by referring to the drawings. BRIEF DESCRIPTION OF THE DRAWINGS 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 is a plan showing electrodes used in the piezoelectric resonator shown in Fig. 1. Fig. 4 is a view showing a mother board and electrodes formed in stripes both used for manufacturing the piezoelectric resonator shown in Figs. 1 and 2. Fig. 5 is a view showing another mother board and another electrodes formed in stripes. Fig. 6 is a perspective view of an unstiffened piezoelectric resonator which vibrates in the longitudinal direction, shown for comparison. Fig. 7 is a perspective view of a stiffened piezoelectric resonator which vibrates in the longitudinal direction. Fig. 8 is a perspective view of an unstiffened piezoelectric resonator which vibrates in the plane direction (square-type vibration), shown for comparison. Fig. 9 is a view of another piezoelectric resonator according to the present invention. Fig. 10 is a plan showing electrodes used in the piezoelectric resonator shown in Fig. 9. Fig. 11 is a plan showing another electrode used in the piezoelectric resonator shown in Fig. 9. Fig. 12 is a view of still another piezoelectric resonator according to the present invention. Fig. 13 is a plan showing electrodes used in the piezoelectric resonator shown in Fig. 12. Fig. 14 is a plan showing other electrodes used in the piezoelectric resonator shown in Fig. 12. Fig. 15 is a plan showing still other electrodes used in the piezoelectric resonator shown in Fig. 12. Fig. 16 is a plan showing a further electrode used in the piezoelectric resonator shown in Fig. 12. Fig. 17 is a view indicating ideal driving force required for the base member during longitudinal basic vibration. Fig. 18 is a view indicating charges generated by the third-order spurious vibration in the base member of the piezoelectric resonator shown in Fig. 9. Fig. 19 is a view showing charges generated by the third-order spurious vibration in the base member of the piezoelectric resonator shown in Fig. 12. Fig. 20 is a view indicating charges generated by the fifth-order spurious vibration in the base member of the piezoelectric resonator shown in Fig. 9. Fig. 21 is a view showing charges generated by the fifth-order spurious vibration in the base member of the piezoelectric resonator shown in Fig. 12. Fig. 22 is a plan showing another electrode used in a piezoelectric resonator according to the present invention. Fig. 23 is a plan showing still another electrode used in a piezoelectric resonator according to the present invention. Fig. 24 is a view of an inactive section. Fig. 25 is a view of another inactive section. Fig. 26 is a view of still another inactive section. Fig. 27 is a perspective view of an electronic component using a piezoelectric resonator according to the present invention. Fig. 28 is a perspective view of an insulating substrate used in the electronic component shown in Fig. 27. Fig. 29 is an exploded perspective view of the electronic component shown in Fig. 27. Fig. 30 is a view indicating another structure for mounting the piezoelectric resonator to the insulating substrate. Fig. 31 is a side view showing the structure for mounting the piezoelectric resonator, shown in Fig. 30. Fig. 32 is a view indicating still another structure for mounting the piezoelectric resonator to the insulating substrate. Fig. 33 is a side view showing the structure for mounting the piezoelectric resonator, shown in Fig. 32. Fig. 34 is an exploded perspective view of a ladder filter using piezoelectric resonators according to the present invention. Fig. 35 is an exploded perspective view of an insulating substrate and the piezoelectric resonators used in the ladder filter shown in Fig. 34. Fig. 36 is an equivalent circuit diagram of the ladder filter shown in Figs. 34 and 35. Fig. 37 is an exploded perspective view of a two-terminal electronic component using a piezoelectric resonator according to the present invention. Fig. 38 is a perspective view of a conventional piezoelectric resonator. Fig. 39 is a perspective view of another conventional piezoelectric resonator. DESCRIPTION OF THE PREFERRED EMBODIMENTS 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. The base member 12 includes 20 laminated piezoelectric layers 12a made from, for example, piezoelectric ceramic. Each of these piezoelectric layers 12a has the same dimensions. The piezoelectric layers 12a are polarized such that adjacent layers have opposite directions of polarization as shown by arrows in Fig. 2. Electrodes 14 are formed between the 20 piezoelectric layers 12a in the base member 12. As shown in Fig. 3, the electrodes 14 are formed in six stripes on main surfaces perpendicular to the longitudinal direction of the base member 12 in the piezoelectric layers 12a. The electrodes 14 extend, for example, in straight belt shapes between opposing sides of the piezoelectric layers 12a in the direction in which external electrodes 20 and 22, which will be described later, oppose. These electrodes 14 are disposed perpendicularly to the longitudinal direction of the base member 12 at certain intervals in the longitudinal direction of the base member 12. The intervals between the electrodes 14 are equal to the dimensions of the corresponding piezoelectric layers 12a in the longitudinal direction of the base member 12. On the opposing side faces of the base member 12, a plurality of insulating films 16 and 18 is formed, respectively. On one side face of the base member 12, the insulating film 16 covers the exposed end section of every other electrode 14. On the other side face of the base member 12, the insulating film 18 covers the exposed end section of every other electrode 14 not covered by the insulating film 16 on the above-described side face. The two side faces of the base member 12 on which the insulating films 16 and 18 are formed serve as connection sections to the external electrodes, which will be described later. In these connection sections, namely, the side faces of the base member 12 on which the insulating films 16 and 18 are formed, external electrodes 20 and 22 are formed. The electrode 20 connects to electrodes 14 which are not covered by the insulating film 16, and the electrode 22 connects to electrodes 14 which are not covered by the insulating film 18. In other words, two adjacent electrodes 14 are connected to the electrodes 20 and 22, respectively. A method for manufacturing the piezoelectric resonator 10 will be described next. A plurality of mother boards 30 made from piezoelectric ceramic is prepared. On one main surface of each mother board 30, as shown in Fig. 4, electrodes 32 made from, for example, silver and palladium, are formed. Then, the plurality of mother boards 30 is laminated to complete a laminated member. Electrodes 32 are not formed on the mother board 30 disposed at the top. The laminated member is cut along a line with short dashes shown in Fig. 4. Then, the insulating films 16 and 18 and the external electrodes 20 and 22 are formed on it to complete the piezoelectric resonator 10. The mother boards 30 correspond to the piezoelectric layers 12a and the electrodes 32 correspond to the electrodes 14. It is preferred that the electrodes 32 be formed in stripes on a mother board 30 under the following conditions (a), (b), and (c) when the piezoelectric resonator 10 is manufactured as described above. (a) A plurality of electrodes 32 shall be formed in stripes on a mother board 30. (b) An electrode interval "d," which is the sum of the width of an electrode 32 and the width of a portion where no electrode is formed between adjacent electrodes 32 on a mother board, shall be constant and the width of each electrode 32 shall be also constant. (c) The width "c" of a portion where the laminated member is cut by a dicing machine to make a piezoelectric resonator shall be a multiple of the electrode interval "d." When a piezoelectric resonator is made with the foregoing conditions being satisfied, even if cut positions vary, the total area of the electrodes formed on a piezoelectric layer does not change and variations in characteristics are unlikely to occur. When the electrode interval "d" is set to 0.4 mm, the width "c" of a portion where the laminated member is cut by a dicing machine is set to 0.4 mm, and the width of an electrode is set to 0.2 mm, the foregoing conditions are satisfied and the total area of the electrodes formed on one piezoelectric layer is constant even if cut positions vary. Therefore, the characteristics of a piezoelectric resonator to be made do not vary either. In contrast, if the foregoing conditions are not satisfied, when the width "c" of a portion where the laminated member is cut is not a multiple of the electrode interval "d," for example, the total area of the electrodes formed on one piezoelectric layer differs depending on cut positions, and variations in characteristics may occur. The piezoelectric resonator 10 uses the external electrodes 20 and 22 as input and output electrodes. Piezoelectric layers 12a excluding the piezoelectric layers 12a disposed at both ends of the base member 12 are piezoelectrically active because an electric field is applied between adjacent layers of the electrodes 14 by applying a signal to the external electrodes 20 and 22. Since voltages are applied in opposite directions to the piezoelectric layers 12a polarized in opposite directions in the base member 12, the piezoelectric layers 12a expand and contract in the same direction as a whole. In other words, an AC electric field in the longitudinal direction of the base member 12 is applied to each piezoelectric layer 12a by use of the electrodes 14 connected to the external electrodes 20 and 22, and a driving force for expansion and contraction is generated in each piezoelectric layer. Therefore, the entire piezoelectric resonator 10 vibrates in the longitudinal direction in basic mode with the center of the base member 12 serving as a node. In the piezoelectric resonator 10, the polarization direction of the piezoelectric layer 12a, the applied electric field direction due to an input signal, and the direction of vibration in the piezoelectric layer 12a 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. 6, 7, and 8 were made. The piezoelectric resonator shown in Fig. 6 was made by forming electrodes on both surfaces in the thickness direction of a piezoelectric substrate measuring 4.0 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. 7 has the same dimensions as the piezoelectric resonator shown in Fig. 6. 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. 8 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. 6 and 8 were of unstiffened type and the piezoelectric resonator shown in Fig. 7 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. 6. Table 2 indicates the measured results of the piezoelectric resonator shown in Fig. 7. Table 3 indicates the measured results of the piezoelectric resonator shown in Fig. 8. Longitudinal basic vibration Longitudinal triple-wave vibration Width-mode vibration Resonant frequency (MHz) 0.460 1.32 1.95 Electromechanical coupling coefficient (%) 18.9 3.9 25.2 Longitudinal basic vibration Longitudinal triple-wave vibration Width-mode vibration Resonant frequency (MHz) 0.455 1.44 1.96 Electromechanical coupling coefficient (%) 42.9 12.2 4.0 Longitudinal basic vibration Square-type triple-wave vibration Thickness-mode vibration Resonant frequency (MHz) 0.458 1.25 5.65 Electromechanical 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 triple-wave 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 4.0%. In contrast, the electromagnetic coupling coefficient K during width-mode vibration is 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, at least a part of a plurality of electrodes 14 is partially formed on surfaces perpendicular to the longitudinal direction of the base member 12 in the piezoelectric layers 12a, the overlapping area of the opposing portions of adjacent electrodes as seen from the direction perpendicular to the electrodes can be changed and ΔF is thereby adjusted. This leads to a high degree of freedom in characteristics design. In the piezoelectric resonator 10, the capacitance of the resonator can be adjusted by changing the overlapping areas of the opposing portions of adjacent electrodes 14, the number of the piezoelectric layers 12a, or the electrodes 14, or the dimensions of the piezoelectric layers 12a in the longitudinal direction of the base member 12. In other words, the capacitance can be increased by increasing the overlapping areas of the opposing portions of adjacent electrodes 14, the number of the piezoelectric layers 12a, or the-electrodes 14, or by decreasing the dimensions of the piezoelectric layers 12a in the longitudinal direction of the base member 12. In contrast, the capacitance can be reduced by reducing the overlapping areas of the opposing portions of adjacent electrodes 14, the number of the piezoelectric layers 12a, or the electrodes 14, or by increasing the dimensions of the piezoelectric layers 12a in the longitudinal direction of the base member 12. 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 and used. Fig. 9 is a view showing another piezoelectric resonator according to the present invention. Unlike the piezoelectric resonator shown in Figs. 1 and 2, in the piezoelectric resonator shown in Fig. 9, each of alternate layers of electrodes 14 is formed on the entire area of a main surface perpendicular to the longitudinal direction of the base member 12 in the piezoelectric layers 12a. In other words, in the piezoelectric resonator 10 shown in Fig. 9 each of alternate layers of electrodes 14 including the second electrode layer from one end of the base member in the longitudinal direction is formed in six stripes on a main surface perpendicular to the longitudinal direction of the base member 12 in the piezoelectric layers 12a as shown in Fig. 10, and each of the other alternate layers of electrodes 14 including the first electrode layer from the end of the base member in the longitudinal direction is formed on the entire area of a main surface perpendicular to the longitudinal direction of the base member 12 in the piezoelectric layers 12a as shown in Fig. 11. The piezoelectric resonator shown in Fig. 9 achieves the same advantages as the piezoelectric resonator shown in Figs. 1 and 2. Since the piezoelectric resonator shown in Fig. 9 has a larger overlapping area of the opposing portions of each pair of adjacent electrode layers than the piezoelectric resonator shown in Figs. 1 and 2, the piezoelectric resonator shown in Fig. 9 has a larger ΔF and capacitance. Unlike the piezoelectric resonator shown in Figs. 1 and 2, since each of alternate layers of electrodes 14 is formed on the entire area of a main surface perpendicular to the longitudinal direction of the base member 12 in the piezoelectric layers 12a in the piezoelectric resonator shown in Fig. 9, when the piezoelectric layers 12a and stripe-shaped electrodes 14 are formed by cutting large piezoelectric layers of which electrodes have been formed in multiple stripes on main surfaces, even if cut positions shift, the overlapping area of the opposing portions of each pair of adjacent electrodes 14 changes little and the characteristics hardly vary. Fig. 12 is a view showing still another piezoelectric resonator according to the present invention. Unlike the piezoelectric resonator shown in Fig. 9, since the total area of electrodes 14 formed in stripes in one layer of electrodes is larger as the layer is disposed closer to the center of the base member 12 in the longitudinal direction in the piezoelectric resonator shown in Fig. 12, the overlapping area of the opposing portions of each pair of adjacent electrodes 14 becomes larger as the pair is disposed closer to the center of the base member 12 in the longitudinal direction. The other configurations are the same as in the piezoelectric resonator shown in Fig. 9. Only the difference therebetween will be described below. In the piezoelectric resonator 10 shown in Fig. 12, each of alternate layers of electrodes 14 disposed at the center of the base member 12 in the longitudinal direction is formed in six stripes on a main surface perpendicular to the longitudinal direction of the base member 12 in the piezoelectric layers 12a as shown in Fig. 13. Each of alternate layers of electrodes 14 disposed outside of the alternate layers described above is formed in four stripes on a main surface perpendicular to the longitudinal direction of the base member 12 in the piezoelectric layers 12a as shown in Fig. 14. Each of alternate layers of electrodes 14 disposed further outside of the alternate layers described above is formed in three stripes on a main surface perpendicular to the longitudinal direction of the base member 12 in the piezoelectric layers 12a as shown in Fig. 15. In this case, these electrodes 14 are formed, for example, in straight belt shapes between opposing sides of the piezoelectric layers 12a in the direction in which the external electrodes 20 and 22 oppose. These electrodes 14 formed in stripes have almost the same width. Each of the other layers of electrodes 14 is formed on the entire area of a main surface perpendicular to the longitudinal direction of the base member 12 in the piezoelectric layers 12a as shown in Fig. 16. Therefore, the overlapping area of the opposing portions of adjacent electrodes is larger as the electrodes are disposed closer to the center of the base member 12 in the longitudinal direction. The piezoelectric resonator shown in Fig. 12 achieves the same advantages as the piezoelectric resonator shown in Fig. 9. In longitudinal basic vibration, a stronger driving force is required at a portion located closer to the center of the base member in the longitudinal direction because of the large mass from this portion to an end of the base member in the longitudinal direction. Ideally, as shown in Fig. 17, a driving force different in strength corresponding to a cosine curve having the length of the base member as half the wavelength and a maximum amplitude at the center in the longitudinal direction of the base member is required. In contrast, in the piezoelectric resonator 10 shown in Fig. 12, since a plurality of electrodes 14 is formed such that the overlapping area of the opposing portions of adjacent electrodes 14 is larger as the electrodes are disposed closer to the center of the base member 12 in the longitudinal direction, a driving force different in strength, which is suited to longitudinal basic vibration, is obtained in the whole zone in the longitudinal direction of the base member 12. Therefore, the electromagnetic coupling coefficient and ΔF become larger. In the piezoelectric resonator shown in Fig. 12, since the capacitance between electrodes 14 becomes larger as the electrodes are disposed closer to the center of the base member in the longitudinal direction as compared with a piezoelectric resonator having a lamination structure in which the overlapping area of the opposing portions of adjacent electrodes is constant, such as the piezoelectric resonator shown in Fig. 9, charges generated in each piezoelectric layer 12a by odd-number-high-order-mode vibrations, such as the third-order and fifth-order vibrations, are canceled, and high-order-mode spurious vibrations are suppressed. The third-order-mode spurious vibration is considered, for example. As shown in Fig. 18, charges having a cyclic amplitude and the same maximum amplitude are generated from one end to the other end in the longitudinal direction of the base member in the piezoelectric resonator shown in Fig. 9 and the charges are not sufficiently canceled and thus, remain. In contrast, in the piezoelectric resonator shown in Fig. 12, charges are generated such that they have a larger amplitude as they are generated closer to the center of the base member in the longitudinal direction as shown in Fig. 19, and most of the charges are canceled. Next, the fifth-order-mode spurious vibration is examined. In the same way as in the third-order-mode spurious vibration, charges having a cyclic amplitude and the same maximum amplitude are generated as shown in Fig. 20 from one end to the other end in the longitudinal direction of the base member in the piezoelectric resonator shown in Fig. 9 and the charges are not sufficiently canceled and thus, remain. In contrast, in the piezoelectric resonator shown in Fig. 12, charges are generated such that they have a larger amplitude as they are generated closer to the center of the base member in the longitudinal direction as shown in Fig. 21, and most of the charges are canceled. In odd-number-order-mode spurious vibrations, such as the seventh-, ninth-, and eleventh-order vibrations, most charges are canceled as in the third- and fifth-order-mode spurious vibration. Therefore, the piezoelectric resonator shown in Fig. 12 suppresses high-order-mode spurious vibrations more than the piezoelectric resonator shown in Fig. 9. In the piezoelectric resonators described above, electrodes 14 are formed in multiple stripes on a main surface perpendicular to the longitudinal direction of the base member 12 in a piezoelectric layer 12a and electrodes 14 are formed on the entire area of a main surface perpendicular to the longitudinal direction of the base member 12 in a piezoelectric layer 12a. According to the present invention, electrodes 14 may be formed in one stripe on a main surface perpendicular to the longitudinal direction of the base member 12 in a piezoelectric layer 12a as shown in Fig. 22, or formed with holes 15 as shown in Fig. 23. Electrodes 14 may be formed in one or more stripes at an angle against the side face on which an external electrode is formed, on a main surface perpendicular to the longitudinal direction of the base member 12 in a piezoelectric layer 12a. In the above-described piezoelectric resonator 10, the base member 12 is formed such that the piezoelectric layers 12a disposed at both ends of the base member 12 in the longitudinal direction serve as piezoelectrically inactive sections and the other piezoelectric layers 12a serve as piezoelectrically active sections. Such inactive sections 24 may be formed such that an electric field is not applied by not forming electrodes 14 at the ends of the base member 12 as shown in Fig. 24. The ends of the base member 12 may be polarized or may not be polarized. As shown in Fig. 25, only the ends of the base member 12 may not be polarized. In this case, even if an electric field is applied between the electrodes 14, the portions not polarized are piezoelectrically inactive. The structure may be formed such that an electric field is not applied to the piezoelectric layers serving as the inactive sections 24 because the sections are insulated by the insulating films 16 and 18 even if the sections are polarized, as shown in Fig. 26. In other words, only when a piezoelectric layer is polarized and an electric field is applied, the layer becomes piezoelectrically active, otherwise it is inactive. In the configuration shown in Fig. 25, a capacitor is formed in an inactive section, and the capacitance of the piezoelectric resonator can be increased. A small electrode 26 may be formed on an end face of the base member 12 in the longitudinal direction as shown in Fig. 26 in order to adjust the frequency or to connect to an external circuit. A piezoelectric resonator according to the present invention may not necessarily include an inactive section. The base member may include active sections only. In each of the above-described piezoelectric resonators 10, one piezoelectric layer 12a is provided between two adjacent layers of electrodes 14. A plurality of piezoelectric layers may be provided therebetween. A dummy electrode which is not connected to an external electrode may be provided for the base member 12, Using such a piezoelectric resonator 10, electronic components such as oscillators and discriminators are produced. Fig. 27 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. 28. 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 ends 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. 29, 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 on the piezoelectric resonator 10 in advance. 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 complete the electronic component 60. 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 electronic component 60 uses the pattern electrodes 66 and 68, which are formed such that they are routed to the rear surface from ends of the insulating substrate 62, as input and output terminals for connecting to external circuits. Since 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 is sealed and protected by the metal cap 74, it can be used as a chip-type (surface mount) 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. 30 and 31. The piezoelectric resonator 10 may also be mounted on the insulating substrate 62 in a way shown in Figs. 32 and 33 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. A ladder filter can be made using a plurality of the piezoelectric resonators 10. As shown in Figs. 34 and 35, three pattern electrodes 76, 78, and 80 are formed on an insulating substrate 62 serving as a support member in an electronic component 60. Protrusions 82 and 86 serving as mounting members are formed with electrically conductive adhesive on both-end pattern electrodes 76 and 80. On the center pattern electrode 78, two protrusions 84 and 88 serving as mounting members are formed with electrically conductive adhesive. One external electrode 22 for each of piezoelectric resonators 10a, 10b, 10c, and 10d is mounted to each of the protrusions 82, 84, 86, and 88, respectively. The protrusions 82, 84, 86, and 88 may be formed on the piezoelectric resonators 10a, 10b, 10c, and 10d in advance. The other external electrodes 20 for piezoelectric resonators 10a, 10b, and 10c are connected to each other with electrically conductive wire 72. The other external electrode 20 of a piezoelectric resonator 10d is connected to the pattern electrode 80 with electrically conductive wire 72. A metal cap 74 is placed on the insulating substrate 62. The electronic component 60 is used as a ladder filter having a ladder-shaped circuit shown in Fig. 36. Two piezoelectric resonators 10a and 10c serve as series resonators and the other two piezoelectric resonator 10c 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, 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 the conventional piezoelectric resonator. A two-terminal electronic component 60 such as a ceramic oscillator and a ceramic discriminator can be produced with a piezoelectric resonator 10 as shown in Fig. 37. Two terminals 90 made from an electrically conductive material are prepared to produce such a two-terminal component 60. These terminals 90 are formed such that they extend from hoops 92. Practically, a plurality of terminals 90 are formed on each hoop 92 in line. A terminal 90 is provided with a fold section 94 at the intermediate portion and an H-shaped support member 96 at the end. The support member 96 is bent and is provided with a protruded mounting member 98 at the center. The two terminals 90 are disposed such that their mounting members 98 oppose each other. The piezoelectric resonator 10 is supported between the mounting members 98. The mounting members 98 abut against the external electrodes 20 and 22 at the center of the piezoelectric resonator in the longitudinal direction. Since the terminals 90 have fold sections 94, which serve as spring elements, the piezoelectric resonator 10 is spring-supported by the terminals 90. A case 100 having an opening at one end is placed on the piezoelectric resonator 10. The opening of the case 100 is closed with paper and then resin-sealed. The terminals 90 are cut from the hoops 92 to complete the electronic component 60. The electronic component 60 having a shape other than a chip-shape can thus be made.

Claims: A piezoelectric resonator (10), comprising a base member (12) having a longitudinal direction, an active section composed of polarized piezoelectric member and constituting at least a part of said base member (12), and a pair of external electrodes (20,22) provided with said active section, characterized in that a plurality of internal electrodes (14) connected to said external electrodes (20,22) are disposed in/on said active section at intervals in the longitudinal direction of said base member (12), at least one of internal electrodes (14) partially covers a cross section of said base member (12) perpendicular to the longitudinal direction, and said base member (12) excites a longitudinal basic vibration when an AC electric field in the longitudinal direction of said base member (12) is applied to said active section via said plurality of internal electrodes (14). A piezoelectric resonator (10) according to Claim 1, characterized in that at least a part of said plurality of internal electrodes (14) partially covers a cross section of said base member (12) perpendicular to the longitudinal direction in stripe shape. A piezoelectric resonator according to Claim 1, characterized in that alternate layers of said plurality of internal electrodes (14) partially covers a cross section of said base member (12) perpendicular to the longitudinal direction in stripe shape, and the other alternate layers of said plurality of internal electrodes (14) entirely covers a cross section of said base member (12) perpendicular to the longitudinal direction. A piezoelectric resonator (10) according to one of Claims 1 to 3, characterized in that the piezoelectric resonator (10) further comprises a support member (62), and a mounting member (70) disposed between said support member (62) and a center section of said base member (12) in the longitudinal direction. An electric component (60) for use with the piezoelectric resonator (10) according to Claim 4, characterized in that said support member (62) is an insulating substrate (62) on which a pattern electrode (66,68) is provided, and said base member (12) is mounted on said insulating substrate (62) through said mounting member (70). An electric component (60) according to Claim 5, characterized in that the electric component (60) is a ladder filter in which a plurality of said base member (10a, 10b, 10c, 10d) is mounted on said insulating substrate (62) through said mounting member (82,84,86,88) and connected to each other in a ladder shape.

Inventors: UNAMI TOSHIHIKO, UNAMI, TOSHIHIKO, Unami, Toshihiko c/o Murata Mfg Co. Ltd.
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1997
Main classification: H03H9
IPC codes: H03H9