Piezoelectric resonator, manufacturing method thereof, and electronic component using the piezoelectric resonator

Abstract: A piezoelectric resonator has a base member formed by laminating piezoelectric layers and electrodes. The base member is polarized in different directions at both sides of an electrode. Electrodes are alternately covered by insulating film 16 and insulating film 18, respectively, at both sides of a groove formed on the base member. The insulating film 16 covers electrodes which are not covered by the insulating film 18, and vice versa. External electrodes 20 and 22 are formed at both sides of the groove on the base member and the electrodes are connected to them. A support member 24 made from an electrically conductive material is formed at the centers of the external electrodes in the longitudinal direction.

Description: BACKGROUND OF THE INVENTION The present invention, as it is defined in the appended claims relates to a piezoelectric resonator, manufacturing methods thereof, and electronic components using the piezoelectric resonators. More particularly, the piezoelectric resonator uses the mechanical resonance of a piezoelectric member and comprises a base member having a longitudinal direction, an active section composed of polarized piezoelectric member and constituting at least a part of said base member, and a pair of external electrodes provided with said active section. The electronic components using the piezoelectric resonator are, for example, an oscillator, a discriminator, and a filter. Fig. 28 is a perspective view of a conventional piezoelectric resonator. The document "Patent Abstract of Japan, vol. 009, no. 212 (E-339) 29 August 1985 discloses such 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. 29, 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 (in the plane direction). To produce an electronic component using the piezoelectric resonator 1, the piezoelectric resonator 1 is mounted on an insulating substrate 5 on which pattern electrodes 4 are formed, as shown in Fig. 30. The center of the piezoelectric resonator 1, which serves as a node, is supported by a support member 6 formed on a pattern electrode 4 so as not to interfere with the vibration of the piezoelectric resonator 1. The support member 6 is made from an electrically conductive material and electrically connects a pattern electrode 4 to one electrode 3 of the piezoelectric resonator 1. The other electrode 3 of the piezoelectric resonator 1 is connected to the other pattern electrode 4 with a lead 7. A metal cap 8 is placed on the insulating substrate 5. Since the support member 6 supports only the center of the piezoelectric resonator 1, which serves as a node, in the electronic component, the vibration of the piezoelectric substrate 2 is not hampered and the characteristics of the piezoelectric resonator 1 are prevented from deteriorating. 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-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. 28 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 electrode. 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. In the piezoelectric resonator shown in Fig. 29, large spurious resonances such as those in the thickness mode and in the triple-wave mode in the plane direction are 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. When an electronic component is produced using the piezoelectric resonator, it is necessary to use a lead to connect the piezoelectric resonator to a pattern electrode on the insulating substrate. This increases production cost. The support member is formed on the pattern electrode and the piezoelectric resonator is secured to the support member. Strict precision is required in placing the center of the piezoelectric resonator on the support member. If the support position shifts, the vibration of the piezoelectric resonator leaks and superior characteristics cannot be obtained. SUMMARY OF THE INVENTION Accordingly, it is the main object of the present invention to provide a piezoelectric resonator of the above mentioned kind which has a small spurious resonance, a large difference ΔF between the resonant frequency and the antiresonant frequency, adjustable capacitance and ΔF, and a large degree of freedom in characteristics design, and to provide a manufacturing method for the piezoelectric resonator. Another object of the present invention is to provide an electronic component using a piezoelectric resonator which is produced at low cost and which can suppress deterioration of characteristics. Still another object of the present invention is to provide a method for manufacturing such a piezoelectric resonator, which can easily mass-produce the piezoelectric resonators. The foregoing objects are 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 at least one pair of internal electrodes is disposed in said active section such that the internal electrodes are perpendicular to the longitudinal direction of said base member, ends of said internal electrodes are exposed at a side face of said base member, a first insulating film covers exposed portions of said internal electrodes at one end of said side face of said base member, a second insulating film covers exposed portions of said internal electrodes which are not covered by said first insulating film at the other end of said side face of said base member, said pair of external electrodes are respectively extended in the longitudinal direction at said one and the other ends of said side face of said base member and connected to said internal electrodes which are not covered by said first and second insulating films at each end of said side face of said base member, said active section is polarized in the longitudinal direction of said base member, and said base member excites a longitudinal mode basic vibration when an electric field is applied to the longitudinal direction of said base member via said internal electrodes. The other aspect of the present invention provides the above described piezoelectric resonator which further comprises a first insulating film which covers exposed portions of alternate said internal electrodes at one end of said side face of said base member, a second insulating film which covers exposed portions of alternate said internal electrodes which are not covered by said first insulating film at the other end of said side face of said base member. The piezoelectric resonator may have a groove on said side face of said base member between said two external electrodes. The piezoelectric resonator may have an electrically conductive support member provided on each external electrode at the center of said base member in the longitudinal direction. The piezoelectric resonator may have an electrically conductive support member provided on each external electrode at the center of said base member in the longitudinal direction. The foregoing objects are achieved in another aspect of the present invention through the provision of an electronic component using the above-described piezoelectric resonator, which is characterized in that an insulating substrate, and a pattern electrode provided on said insulating substrate and connected to said external electrodes of said piezoelectric resonator via said support member. The electronic component may serves as a ladder filter in which a plurality of said pattern electrodes are provided on said insulating substrate and connected to said external electrodes of a plurality of said piezoelectric resonators such that said piezoelectric resonators are connected each other in a ladder shape. The foregoing objects are achieved in still another aspect of the present invention through the provision of a manufacturing method of the piezoelectric resonator which comprises the steps of: 1)preparing a laminated member in which a plurality of piezoelectric layers and a plurality of internal electrodes are laminated; 2)forming an insulating film partially on a surface of said laminated member at which ends of said internal electrodes are exposed; 3)forming an external electrode on said surface of said laminated member; 4)cutting said laminated member perpendicular to said surface of said laminated member. In the manufacturing method of the piezoelectric resonator, the steps of forming a support member comprising an electrically conductive material on said external electrode may be included. In the above described manufacturing method of the piezoelectric resonator, said support member may be located at the center in the longitudinal direction of said laminated member. In the manufacturing method of the piezoelectric resonator, the steps of forming a groove on said surface of said laminated member on which said external electrode is formed, the direction of said groove being parallel to cutting direction of said laminated member may be included. In the manufacturing method of the piezoelectric resonator, said insulating film may be formed in a check design in which one set of alternate rows of said insulating film covers one set of alternate exposed portions of said internal electrodes and another set of alternate rows of said insulating film covers another set of alternate exposed portions of said internal electrodes. In the manufacturing method of the piezoelectric resonator, said laminated member may be prepared in such a way that said internal electrodes are alternately exposed at opposite sides of said piezoelectric layers, one pair of polarizing electrodes are formed on said opposite sides of said piezoelectric layers and electrically connected to every other said internal electrodes respectively, said piezoelectric layers are polarized by applying a DC voltage via said polarizing electrodes and said internal electrodes, and cut said piezoelectric member and said internal electrodes perpendicularly to a laminated direction thereof. The piezoelectric resonator according to the present invention is of a stiffened type, and has an active section 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 a stiffened piezoelectric resonator. Furthermore, the capacitance can be adjusted by changing the number of electrodes which are used for applying an electric field to the active section, the distances thereof, and the size thereof. The frequency difference ΔF and the resonant frequency can be adjusted by trimming a provided inactive section which does not vibrate unlike an active section or by adding mass to the section. When electronic components such as an oscillator, a discriminator, and a filter are made using the piezoelectric resonator, 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. Since the piezoelectric resonator has two external electrodes formed on one side face of the base member, it can be mounted to a pattern electrode by face bonding without the use of a lead. If a support member is formed at the center of the base member in advance, the node of the piezoelectric resonator can be positively supported when the piezoelectric resonator is mounted on a pattern electrode. The piezoelectric resonator according to the present invention can be mass-produced by forming insulating film, an external electrode, and a support member on a laminated member, forming a groove, and cutting the laminated member. According to the present invention, the frequency difference ΔF between the resonant frequency and the antiresonant frequency is large as compared with a conventional piezoelectric resonator, and thus a wide-frequency-band resonator is obtained. Since the frequency difference ΔF is adjusted by adjusting the inactive section, the frequency-band width of the piezoelectric resonator can be changed. In addition, vibrations in modes other than the basic-vibration mode are unlikely to occur in this piezoelectric resonator, and superior characteristics are achieved. Furthermore, since the capacitance of the piezoelectric resonator can be adjusted, it is easy to achieve impedance matching with an external circuit in a case when the piezoelectric resonator is mounted on a circuit board. An electronic component is produced with this piezoelectric resonator at low cost since the piezoelectric resonator can be connected to a pattern electrode on the insulating substrate without the use of a lead. An electronic component having superior characteristics can be obtained because the piezoelectric resonator is not prevented from vibrating, due to a support member formed on the piezoelectric resonator. When a ladder filter is made using a plurality of piezoelectric resonators, it can be manufactured at low cost while having superior characteristics. Since a chip-type electronic component can be made using the piezoelectric resonator, it is easy to mount the component on a circuit board. It is also easy to achieve impedance matching between such an electronic component and an external circuit by adjusting the capacitance of the piezoelectric resonator. In addition, in a ladder filter formed by connecting a plurality of piezoelectric resonators in series and in parallel alternately, attenuation in the filter can be adjusted by changing the ratio of the capacitance of the piezoelectric resonator connected in series to that of the piezoelectric resonator connected in parallel. According to the method for manufacturing a piezoelectric resonator specified by this invention, the piezoelectric resonator achieving the above-described advantages can be mass-produced. Therefore, such a piezoelectric resonator can be manufactured at low cost. 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 indicating a condition in which insulating film is formed on a base member used in the piezoelectric resonator shown in Fig. 1. Fig. 4 is a perspective view indicating how ceramic green sheets are laminated in order to produce the piezoelectric resonator shown in Fig. 1. Fig. 5 is a view showing a laminated base formed by the ceramic green sheets shown in Fig. 4. Fig. 6 is a view showing portions where the laminated base shown in Fig. 5 is cut. Fig. 7 is a view showing a laminated member made by cutting the laminated base shown in Fig. 6. Fig. 8 is a view showing a plan and side view of the laminated member shown in Fig. 7 to which insulating film is applied. Fig. 9 is a view showing a plan and side view of the laminated member shown in Fig. 8 to which an external electrode is applied. Fig. 10 is a view showing a plan and side view of the laminated member shown in Fig. 9 in which a support member is formed on the external electrode. Fig. 11 is a view showing a process for manufacturing a piezoelectric resonator, in which grooves are formed on the laminated member shown in Fig. 9 and the laminated member is cut. Fig. 12 is a perspective view of an unstiffened piezoelectric resonator which vibrates in the longitudinal direction, shown for comparison. Fig. 13 is a perspective view of a stiffened piezoelectric resonator which vibrates in the longitudinal direction. Fig. 14 is a perspective view of an unstiffened piezoelectric resonator which vibrates in the plane direction (square-type vibration), shown for comparison. Fig. 15 is a chart showing the relationship between the frequency and the impedance of a piezoelectric resonator according to the present invention. Fig. 16 is a chart showing the relationship between the frequency and the impedance of a conventional piezoelectric resonator. Fig. 17 is a view of a piezoelectric resonator in which the distribution of an active section and inactive sections is changed in a base member. Fig. 18 is a chart indicating the relationship between the distribution of an active section and capacitance, and ΔF/Fa. Fig. 19 is a chart showing the relationship between an active-section ratio and DF. Fig. 20 is a view showing a modified inactive section of a piezoelectric resonator. Fig. 21 is a view showing another modified inactive section of a piezoelectric resonator. Fig. 22 is a view showing still another modified inactive section of a piezoelectric resonator. Fig. 23 is an exploded perspective view of an electronic component using the above-described piezoelectric resonator. Fig. 24 is a side view showing how the piezoelectric resonator is mounted in the electronic component shown in Fig. 23. Fig. 25 is a plan of a main section of a ladder filter using the piezoelectric resonators according to the present invention. Fig. 26 is an exploded perspective view of the main section of the ladder filter shown in Fig. 25. Fig. 27 is an equivalent circuit diagram of the ladder filter shown in Fig. 25. Fig. 28 is a view of a conventional unstiffened piezoelectric resonator. Fig. 29 is a view of another conventional unstiffened piezoelectric resonator. Fig. 30 is an exploded perspective view of a structure in which a conventional unstiffened piezoelectric resonator which vibrates in the longitudinal mode is mounted. 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 includes a base member 12 having, for example, a cubic shape. The base member 12 is made from, for example, a piezoelectric, ceramic material. A plurality of internal electrodes 14 is formed in the base member 12 such that the surfaces of the internal electrodes 14 are perpendicular to the longitudinal direction of the base member 12. The base member 12 is polarized in the longitudinal direction such that the directions of polarization oppose each other at both sides of one internal electrode 14. On one side face of the base member 12, a groove 15 extends in the longitudinal direction of the base member 12. The groove 15 is formed at the center in the width direction of the base member 12 to divide the face into two sections. On the side face divided by the groove 15, first insulating film 16 and second insulating film 18 are formed as shown in Fig. 3. On one section made by dividing the side face of the base member 12 by the groove 15, the exposed portion of every other internal electrode 14 is covered by the first insulating film 16. On the other section, the exposed portion of every other internal electrode 14 which is not covered by the first insulating film 16 is covered by the second insulating film 18. Both ends of the base member 12 are not polarized. At portions where the first insulating film 16 and the second insulating film 18 are formed, namely, at both sides of the groove 15, external electrodes 20 and 22 are formed. Therefore, the external electrode 20 is connected to internal electrodes 14 which are not covered by the first insulating film 16, and the external electrode 22 is connected to internal electrodes 14 which are not covered by the second insulating film 18. In other words, adjacent internal electrodes 14 are connected to the external electrode 20 and the external electrode 22, respectively. At the centers of the external electrodes 20 and 22 in the longitudinal direction, support members 24 are formed, respectively, such that they are electrically conductive. The support members 24 may be made from an electrically conductive material. Electrical conductivity may be added to the support members 24 by forming electrode film on a surface of an insulating material constituting the support members. At both sides of the groove 15, the first insulating film 16 and the second insulating film 18 cover the internal electrodes 14, and the external electrodes 20 and 22 are formed on the first and second insulating film. The groove may not necessarily be formed. Electrodes may be formed, for example, such that, at both ends in the width direction on one side face of the base member 12, the internal electrodes 14 are alternately covered at their exposed portions by insulating film 16 and 18, and two rows of external electrodes 20 and 22 are formed thereon long in the longitudinal direction of the base member 12. In this case, although the base member 12 is not provided with a groove, adjacent internal electrodes 14 are connected to the external electrodes 20 and 22, respectively. The piezoelectric resonator 10 uses the external electrodes 20 and 22 as input and output electrodes. At portions except for both ends of the base member 12, since an electric field is applied to adjacent internal electrodes 14, the base member 12 is piezoelectrically active. At both ends of the base member 12, since the base member is not polarized and an electric field is not applied due to lack of electrodes formed at both ends, the base member is piezoelectrically inactive. Therefore, an active section 26 for input signals is formed at the center of the base member 12, and inactive sections 28 for input signals are formed at both ends of the base member 12. The inactive sections 28 do not generate driving force when receiving input signals. If an electric field is applied to internal electrodes, when the portion between the internal electrodes is not polarized, the portion is inactive. A structure may be used in which an electric field is not applied to polarized piezoelectric layers. To make the piezoelectric resonator 10, green sheets 30, made from piezoelectric ceramic, are first prepared as a piezoelectric layers, as shown in Fig. 4. On one surface of each green sheet 30, electrically conductive paste including, for example, silver, palladium, and an organic binder, is applied to form an electrically conductive paste layer 32 over almost the entire area of each green sheet 30 excluding an end portion. A plurality of green sheets 30 is laminated such that the end portions where the electrically conductive paste layers 32 are not formed on the green sheets are placed alternately in opposite directions. The laminated member with electrically conductive paste applied to opposite side faces is baked to form a laminated base 34 shown in Fig. 5. The laminated base 34 has a plurality of internal electrodes 36, which have been made by baking the electrically conductive layers 32. Polarizing electrodes 38 and 40 formed on opposite faces are connected to every other internal electrode 36, respectively, since the internal electrodes 36 are alternately exposed on opposite faces of the laminated base 34. When a DC voltage is applied to the polarizing electrodes 38 and 40, the laminated base 34 is polarized. Inside the laminated base 34, a high DC electric field is applied between adjacent internal electrodes 36 alternately in opposite directions. Therefore, the laminated base 34 is polarized in opposite directions at both sides of the internal electrodes 36 as shown by arrows in Fig. 5. The laminated base 34 is surface-ground to the desired thickness since the antiresonant frequency of the resonator is determined by the thickness of the laminated base 34. The laminated base 34 is cut by a dicing machine along dotted lines shown in Fig. 6 such that the cutting planes are perpendicular to the plurality of internal electrodes 36. Then, a laminated member 42 in which the ends of the internal electrodes 36 are exposed is obtained as shown in Fig. 7. Insulating film 44 is applied to one surface of the laminated member 42 in a check pattern as shown in Fig. 8. As compared with Fig. 7, Fig. 8 is a simplified figure and the number of laminated layers is reduced for simplicity. One row of the insulating film 44 covers alternate internal electrodes 36. The adjacent rows of the insulating film 44 cover the other alternate internal electrodes 36. An external electrode 48 is formed by sputtering on the surface where the insulating film 44 is formed on the laminated member 42 as shown in Fig. 9. A belt-shaped support member 50 made from an electrically conductive material is formed at the center of the laminated member 42 such that the support member is parallel to the internal electrodes 36 as shown in Fig. 10. Grooves 15 are formed between adjacent rows of insulating film 44 formed in a check fashion, such that the grooves are perpendicular to the surfaces of the internal electrodes 36 as shown in Fig. 11. The laminated member 42 is cut by the grooves 15 to form the piezoelectric resonator shown in Fig. 1. When a signal is applied to the external electrodes 20 and 22 in the piezoelectric resonator 10, since voltages are applied in opposite directions to the polarization of the piezoelectric layers in the active section 26, the piezoelectric layers expand and contract in the same direction as a whole. Therefore, the 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 active section 26, the applied electric field direction due to a signal, and the direction of vibration in the active section 26 are all the same. In other words, the piezoelectric resonator 10 is of stiffened type. The stiffened 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 piezoelectric resonator. This means that the piezoelectric resonator 10 obtains wide-frequency-band characteristics. To measure differences between stiffened and unstiffened piezoelectric resonators, piezoelectric resonators shown in Figs. 12, 13, and 14 were made. The piezoelectric resonator shown in Fig. 12 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. 13 had the same dimensions as the piezoelectric resonator shown in Fig. 12. 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. 14 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. 12 and 14 were of unstiffened type and the piezoelectric resonator shown in Fig. 13 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. 12. Table 2 indicates the measured results of the piezoelectric resonator shown in Fig. 13. Table 3 indicates the measured results of the piezoelectric resonator shown in Fig. 14. Longitudinal mode 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 mode 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 mode 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, the inactive section 28 is formed at both ends of the base member 12. The inactive section 28 is changed to adjust the resonant frequency and the difference ΔF between the resonant frequency and the antiresonant frequency. In other words, by grinding the end faces in the longitudinal direction of the base member 12 or by adding mass, the band width of the piezoelectric resonator 10 can be adjusted. In the piezoelectric resonator 10, the capacitance of the resonator can be adjusted by, for example, changing the number of layers in the active section 26. In the active section 26, piezoelectric layers and internal electrodes 14 are alternately stacked and electrically connected in parallel. When the number of layers is changed with the total thickness of the active section 26 remaining constant, the following relationship is satisfied since the thickness of one layer is inversely proportional to the number of layers. Capacitance of resonator is proportional to the ratio of (the number of layers in active section/thickness of a layer) which in turn is proportional to (the number of layers in active section) 2 . The capacitance of the resonator is proportional to the square of the number of layers in the active section 26. Therefore, the number of layers in the active section 26 is changed to adjust the capacitance of the piezoelectric resonator 10. This means that the piezoelectric resonator 10 has a large degree of freedom in 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. Electrically conductive paste including, for example, silver, palladium, and an organic binder, was applied to one surface of each green sheet 30 made from piezoelectric ceramic. A plurality of such green sheets were stacked alternately and baked integrally at 1200 °C to form a laminated base 34 measuring 20 mm by 30 mm by 3.9 mm. Polarizing electrodes 38 and 40 were formed by sputtering. A high DC electric field was applied between adjacent internal electrodes 36 to polarize the laminated base such that the directions of polarization in adjacent piezoelectric layers were alternately opposed. The thickness of the laminated base 34 was changed. The laminated base 34 was cut to form a laminated member 42 measuring 1.5 mm by 30 mm by 3.8 mm. Insulating film 44 was formed in a check pattern on the internal electrodes 36 which were exposed to one surface of the laminated member 42 and a silver electrode was formed on it by sputtering. A support member 50 was formed on the electrode and a groove 15 was also formed. The resultant block was cut by a dicing machine to obtain a piezoelectric resonator 10 measuring 1.5 mm by 1.5 mm by 3.8 mm. The piezoelectric resonator 10 had nineteen electrodes 14 in the base member 12, the electrodes 14 being disposed at an almost equal interval of 0.19 mm. First and second insulating films 16 and 18 were formed so that an electric field is not applied to three piezoelectric layers disposed at both ends of the base member 12. An active section 26 included 14 piezoelectric layers disposed at the center of the base member 12, and an inactive section 28 had three piezoelectric layers at both ends. The piezoelectric resonator 10 had a capacitance of 830 pF and the frequency characteristics shown in Fig. 15. For comparison, the frequency characteristics of a square-type vibration piezoelectric resonator is shown in Fig. 16. It is found from Figs. 15 and 16 that the piezoelectric resonator 10 according to the present invention has much less spurious vibration than the square-shaped piezoelectric resonator. Depending on the positions where active sections 26 and inactive sections 28 are formed, the frequency difference ΔF between the resonant frequency and the antiresonant frequency changes. Inactive sections 28 can be formed, for example as shown in Fig. 17, at both ends and the center of the base member 12. The finite element method was used to calculate changes in capacitance Cf and frequency difference ΔF in the piezoelectric resonator in a case in which the positions where active sections 26 were formed change, where "a" indicates the distance between the center and an end of the piezoelectric resonator 10, "b" indicates the distance between the center and the center of gravity of an active section 26, "c" indicates the length of the active sections 26, W indicates the width of the base member 12, and T indicates the thickness of the base member 12. Fig. 18 shows the relationship between b/a, and the ratio of ΔF to the antiresonant frequency Fa, ΔF/Fa, and the capacitance Cf with "a" being equal to 1.89 mm, W and T equal to 0.8 mm, "c" equal to 0.86 mm, and b/a changing. From Fig. 18, it is found that the capacitance Cf does not change irrespective of the positions where the active sections 26 were formed. In contrast, it was also found that ΔF decreases as the active sections 26 are disposed closer to both ends of the base member 12. The frequency difference ΔF can be changed in the piezoelectric resonator 10 by changing the ratio of the active sections 26 to the inactive sections 28. With a changing active-section ratio, which is a ratio of the length of the active section 26 to that of the base member 12 in the piezoelectric resonator 10 shown in Figs. 1 and 2, the resonant frequency Fr, the antiresonant frequency Fa, the frequency difference ΔF, and its rate of change were measured and are indicated in Table 4 and Fig. 19. Active-section length (mm) Active-section ratio (%) Fr (kHz) Fa (kHz) ΔF (kHz) ΔF change rate (%) 1.80 100.0 1047.4 1163.4 115.9 0.0 1.70 94.4 1042.4 1163.4 120.9 4.3 1.60 88.9 1038.6 1163.4 124.8 7.6 1.53 85.0 1036.6 1163.4 126.8 9.4 1.50 83.3 1035.9 1163.4 127.5 9.9 1.40 77.8 1034.5 1163.4 128.9 11.2 1.35 75.0 1034.3 1163.4 129.1 11.4 1.30 72.2 1034.3 1163.4 129.0 11.3 1.20 66.7 1035.5 1163.4 127.9 10.3 1.17 65.0 1036.1 1163.4 127.2 9.7 1.10 61.1 1038.1 1163.4 125.3 8.1 1.00 55.6 1042.0 1163.4 121.4 4.7 0.90 50.0 1047.4 1163.4 115.9 0.0 0.80 44.4 1054.3 1163.4 109.1 -5.9 0.70 38.9 1062.7 1163.4 100.6 -13.2 0.60 33.3 1072.7 1163.4 90.7 -21.8 0.50 27.8 1084.2 1163.4 79.1 -31.7 0.40 22.2 1097.3 1163.4 66.1 -43.0 0.30 16.7 1111.9 1163.4 51.5 -55.6 0.20 11.1 1127.9 1163.4 35.5 -69.4 0.10 5.6 1145.2 1163.4 18.2 -84.3 Fig. 19 shows the relationship between the active-section ratio and change in ΔF under the condition in which ΔF is set to 100% when the active-section ratio is 100%, namely when an inactive section does not exist. It is found from Fig. 19 that ΔF is large at an active-section ratio of 65% to 85% with the peak ΔF being obtained at an active-section ratio of 75%. The peak value is larger by about 10% than the ΔF obtained when the active-section ratio is 100%, in other words, when an inactive section does not exist. The same ΔF is obtained at active-section ratios of 50% and 100%. Therefore, to obtain a piezoelectric resonator having a large ΔF, it is necessary to set the active-section ratio to 50% or more. In the piezoelectric resonator 10, when 14 piezoelectric layers constituted the active section 24 among 20 layers, the capacitance was 830 pF. In contrast, when the active-section ratio was set to 100%, which means that only one piezoelectric layer was used, in other words, when electrodes were formed at both end faces of the base member 12, with the same material and the same dimensions, the capacitance was 3.0 pF. When all of the 20 piezoelectric layers constituted the active section 26, the capacitance was 1185.6 pF. By changing the number of piezoelectric layers in the active section 26 of the base member in the piezoelectric resonator 10, the capacitance can be changed within a range of about 400-times difference between the minimum and maximum. Therefore, by changing the lamination structure of the piezoelectric resonator 10, the capacitance can be selected from a wide range which provides a large degree of freedom in capacitance design. An inactive section 28 may be formed such that an electric field is not applied by not forming an electrode at an end of the base member 12 as shown in Fig. 20. The end of the base member 12 may be polarized or may not be polarized. As shown in Fig. 21, only the end of the base member 12 may not be polarized. In this case, even if an electric field is applied between the electrodes 14, a portion not polarized is piezoelectrically inactive. The structure may be formed such that the base member 12 is all polarized and an electric field is not applied to the internal electrodes 14 due to insulating films 16 and 18. 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 Figs. 21 and 22, capacitors are also formed in the inactive section, and the capacitance can be increased. A small electrode 52 may be formed on an end face of the base member 12 as shown in Fig. 22 in order to adjust the frequency or to connect to an external circuit. 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. 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. One pattern electrode 66 is formed between opposing indentations 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 in an L shape from a point near the other end toward the center of the insulating substrate 62. At the center of the insulating substrate 62, the two pattern electrodes 66 and 68 oppose with an interval. 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 ends of the pattern electrodes 66 and 68 disposed at the center of the insulating substrate 62, a support member 24 for the piezoelectric resonator 10 is connected by electrically conductive adhesive as shown in Fig. 24. The external electrodes 20 and 22 of the piezoelectric resonator 10 are thereby secured to the insulating substrate 62 and electrically connected to the pattern electrodes 66 and 68. 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 shortcircuited 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 support member 24 formed at the center in the longitudinal direction of the base member 12 supports the piezoelectric resonator 10 in this electronic component 60, the ends of the piezoelectric resonator 10 are disposed separately from the insulating substrate 62 and thus the vibration of the piezoelectric resonator 10 is not prevented. The center of the piezoelectric resonator 10, which serves as a node, is secured by the support member 24, and the external electrodes 20 and 22 are electrically connected to the pattern electrodes 66 and 68. Since the support member 24 is formed on the piezoelectric resonator 10 in advance, the node of the piezoelectric resonator 10 is accurately positioned. Therefore, as compared with a case in which the piezoelectric resonator is placed on protruded support members formed on the pattern electrodes 66 and 68, the node is more accurately supported. The vibration of the piezoelectric resonator 10 is prevented from leaking and superior characteristics are achieved. The electronic component 60 is manufactured at low cost because leads are not required to connect the external electrodes 20 and 22 of the piezoelectric resonator 10 to the pattern electrodes 66 and 68. 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. A ladder filter can be made using a plurality of the piezoelectric resonators 10. Fig. 25 is a plan of the main section of a ladder filter having a ladder-shaped circuit. Fig. 26 is an exploded perspective view of the main section. In the electronic component 60 shown in Figs. 25 and 26, four pattern electrodes 90, 92, 94, and 96 are formed on an insulating substrate 62. 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. Support members 24 formed at the external electrodes 20 and 22 of piezoelectric resonators 10a, 10b, 10c, and 10d are mounted on these lands to obtain a ladder-shaped circuit shown in Fig. 27. Then, a metal cap (not shown) 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. 27. Two piezoelectric resonators 10a and 10d serve as series resonators and the other two piezoelectric resonator 10b and 10c serve as parallel resonators. In such a ladder filter, the parallel piezoelectric resonators 10b and 10c are designed to have substantially larger capacitances than the series piezoelectric resonators 10a and 10d. 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. Since the present invention employs a stiffened piezoelectric resonator, the resonator has a larger ΔF and a wider frequency band than the conventional unstiffened piezoelectric resonator. In addition, the stiffened piezoelectric resonator has small spurious vibrations. Since the base member 12 has a laminated structure, the capacitance can be set to any desired value and it is easy to achieve impedance matching with an external circuit. Furthermore, by adjusting the sizes and positions of the active section and the inactive sections, ΔF can be changed. Since the electronic component 60 according to the present invention has a simple structure, it can be produced at a low cost while exhibiting the above-described features of the piezoelectric resonator 10. With the use of the method for manufacturing a piezoelectric resonator according to the present invention, the piezoelectric resonator 10 having the above-described characteristics is mass-produced. Therefore, the piezoelectric resonator 10 is offered at low cost. Since the piezoelectric resonator 10 according to the present invention includes more parameters which can be designed than the conventional piezoelectric resonator, various characteristics can be implemented.

Claims: Piezoelectric resonator (10), comprising a base member (12) having a longitudinal direction, an active section (26) 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 (26), characterized in that at least one pair of internal electrodes (14) is disposed in said active section (26) such that the internal electrodes (14) are perpendicular to the longitudinal direction of said base member (12), ends of said internal electrodes (14) are exposed at a side face of said base member (12), a first insulating film (16) covers exposed portions of said internal electrodes (14) at one end of said side face of said base member (12), a second insulating film (18) covers exposed portions of said internal electrodes (14) which are not covered by said first insulating film (16) at the other end of said side face of said base member (12), said pair of external electrodes (20,22) are respectively extended in the longitudinal direction at said one and the other ends of said side face of said base member (12) and connected to said internal electrodes (14) which are not covered by said first and second insulating films (16,18) at each end of said side face of said base member (12), said active section (26) is polarized in the longitudinal direction of said base member (12), and said base member (12) excites a longitudinal mode basic vibration when an electric field is applied to the longitudinal direction of said base member (12) via said internal electrodes (14). Piezoelectric resonator (10) according to Claim 1, characterized in that a first insulating film (16) which covers exposed portions of alternate said internal electrodes (14) at one end of said side face of said base member (12), a second insulating film (18) which covers exposed portions of alternate said internal electrodes (14) which are not covered by said first insulating film (16) at the other end of said side face of said base member (12). A piezoelectric resonator (10) according to Claim 1 or 2, characterized in that said piezoelectric resonator (10) has a groove (15) on said side face of said base member (12) between said two external electrodes (20,22). A piezoelectric resonator (10) according to one of Claims 1 to 3, characterized in that said piezoelectric resonator (10) has an electrically conductive support member (24) provided on each external electrode (20,22) at the center of said base member (12) in the longitudinal direction. A piezoelectric resonator according to one of Claims 1 to 4, characterized in that an inactive portion (28), in which vibration is not excited when said electric field is applied, constitutes the other part of said base member (12). An electronic component (60) for use with the piezoelectric resonator (10) according to one Claim 4 or 5, characterized in that an insulating substrate (62), and a pattern electrode (66,68) provided on said insulating substrate (62) and connected to said external electrodes (20,22) of said piezoelectric resonator (10) via said support member (24). An electronic component (60) according to Claim 6, characterized in that said electronic component (60) is a ladder filter in which a plurality of said pattern electrodes (90,92,94,96) are provided on said insulating substrate (62) and connected to said external electrodes (20,22) of a plurality of said piezoelectric resonators (10) such that said piezoelectric resonators (10) are connected each other in a ladder shape. A manufacturing method of the piezoelectric resonator (10) of one of Claims 1 to 5, comprising the steps of 1)preparing a laminated member (42) in which a plurality of piezoelectric layers and a plurality of internal electrodes (36) are laminated, 2)forming an insulating film (44) partially on a surface of said laminated member (42) at which ends of said internal electrodes (36) are exposed, 3)forming an external electrode (20,22) on said surface of said laminated member (42), 4)cutting said laminated member (42) perpendicular to said surface of said laminated member (42). A manufacturing method of the piezoelectric resonator (10) according to Claim 8, further comprising the steps of forming a support member (50) comprising an electrically conductive material on said external electrode (20,22). A manufacturing method of the piezoelectric resonator (10) according to Claim 9, characterized in that said support member (50) is located at the center in the longitudinal direction of said laminated member (42). A manufacturing method of the piezoelectric resonator (10) according to one of Claims 8 to 10, further comprising the steps of forming a groove (15) on said surface of said laminated member on which said external electrode (20,22) is formed, the direction of said groove (15) being parallel to cutting direction of said laminated member (42). A manufacturing method of the piezoelectric resonator (10) according to one of Claims 8 to 11, characterized in that said insulating film (44) is formed in a check design in which one set of alternate rows of said insulating film (44) covers one set of alternate exposed portions of said internal electrodes (36) and another set of alternate rows of said insulating film (44) covers another set of alternate exposed portions of said internal electrodes (36). A manufacturing method of the piezoelectric resonator (10) according to Claim 12, characterized in that said groove (15) is formed between adjacent rows of said insulating film (44) formed in the check design and perpendicular to said internal electrodes (36), and said laminated member (42) is cut on said rows of said insulating film (44) and parallel to said groove (15). A manufacturing method of the piezoelectric resonator (10) according to one of Claims 8 to 13, characterized in that said laminated member (42) is prepared in such a way that said internal electrodes (36) are alternately exposed at opposite sides of said piezoelectric layers, one pair of polarizing electrodes (38,40) are formed on said opposite sides of said piezoelectric layers and electrically connected to every other said internal electrodes (36) respectively, said piezoelectric layers are polarized by applying a DC voltage via said polarizing electrodes (38,40) and said internal electrodes (36), and cut said piezoelectric member and said internal electrodes (36) perpendicularly to a laminated direction thereof.

Inventors: BABA TOSHIYUKI, INOUE JIRO, KUSABIRAKI SHIGEMASA, TAKESHIMA TETSUO, UNAMI TOSHIHIKO, YOSHINO HIROHIDE, BABA, TOSHIYUKI, INOUE, JIRO, KUSABIRAKI, SHIGEMASA, TAKESHIMA, TETSUO, UNAMI, TOSHIHIKO, YOSHINO, HIROHIDE
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1997
Main classification: H03H9
IPC codes: H03H9, H01L41, H03H3