Piezoelectric component

BACKGROUND OF THE INVENTION

The present invention relates generally to piezoelectric components and, more particularly, to a piezoelectric component comprising at least one piezoelectric resonator, such as an oscillator, a discriminator or a filter.

Fig. 13 is a perspective view of a conventional piezoelectric component 1. The piezoelectric component 1 includes a supporting substrate 2 on which pattern electrodes 3a and 3b are formed. A piezoelectric resonator 4 is supported on this supporting substrate 2. The piezoelectric resonator 4 includes, for example, a vibrating body 5 formed of a piezoelectric material and external electrodes 6a and 6b formed on two opposite surfaces of the vibrating body 5. When a signal is input between the external electrodes 6a and 6b, vibration in a longitudinal mode is excited in the vibrating body 5. A supporting member 7 is formed of, for example, an electroconductive material on the pattern electrode 3a. A central portion of the piezoelectric resonator 4 is supported by the supporting member 7. Simultaneously, the external electrode 6a of the piezoelectric resonator 4 and the pattern electrode 3a are electrically connected to each other by the supporting member 7. The other external electrode 6b of the piezoelectric resonator 4 is connected to the pattern electrode 3b by a lead wire 8.

In the thus-constructed piezoelectric component 1, to obtain desired electrical characteristics, the resonant frequency, the degree of polarization and the capacitance between the terminals of the piezoelectric resonator 4, etc., are designed according to the size design of the piezoelectric resonator 4 and working conditions.

However, the piezoelectric resonator has a specific mechanical quality factor Qm according to the piezoelectric material used. To set Qm to the desired value, therefore, it has been necessary to develop an optimal piezoelectric material for setting the desired value or a piezoelectric material capable of setting a value close to the desired value. A long period of time is usually taken to develop a piezoelectric material. If a piezoelectric material capable of setting a value not equal to but close to the desired value is used, it is necessary to tolerate a range not corresponding to the target characteristic.

US-A-5 406 682 discloses a method of compliantly mounting a piezoelectric device with a substrate. In this method, outer portions of a piezoelectric element are selectively metallized. Next, one layer of aluminum is selectively dispensed on the piezoelectric element. After this, an uncured conductive compliant material is placed and aligned on a substrate. Next, the piezoelectric element is placed and aligned on the conductive compliant material, such that upon curing the conductive compliant material forms a compliant mount connecting the outer metallized portions of the piezoelectric element with the substrate.

EP-A-0 626 212 discloses a holding structure for a piezoelectric element. In this publication reference is made to Japanese Utility Model laid open Publication 20422/1993 which proposes a holding structure of piezoelectric vibrator wherein protrusions made of electrically conductive rectangular rubber are formed around a node point of vibrations of a piezoelectric vibrator and the piezoelectric vibrator is held at the protrusions.

SUMMARY OF THE INVENTION

In view of the above-described problem, a main object of the present invention is to provide a piezoelectric component having a desired Qm by using a presently available piezoelectric material.

To achieve this object, according to the present invention, there is provided a piezoelectric component of above mentioned kind, which is characterized in that a rubber-like elastic material is provided as a filler between said piezoelectric resonator and a substracte supporting it.

In the above described piezoelectric component, the piezoelectric resonator may excite vibration in a longitudinal vibration mode.

In the above described piezoelectric component, the piezoelectric resonator may further comprise a supporting substrate for supporting the piezoelectric resonator, and the rubber-like elastic material is provided as a filler between said piezoelectric resonator and said supporting substrate.

In the above described piezoelectric component, the rubber-like elastic material may also be provided on a surface of said piezoelectric resonator opposite from another surface of said piezoelectric resonator facing said supporting substrate.

In the above described piezoelectric component, the rubber-like elastic material may comprise an electroconductive material.

In the above described piezoelectric component, the plurality of piezoelectric resonators may be connected on the supporting substrate in ladder shape for forming a ladder filter.

In the above described piezoelectric component, the piezoelectric resonator may be a piezoelectric resonator which comprises 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, whereby at least one pair of internal electrodes are disposed in the active section such that the internal electrodes are perpendicular to the longitudinal direction of the base member and are connected to the pair of external electrodes respectively, the active section is polarized in the longitudinal direction of the base member, and a longitudinal mode basic vibration is excited when an electric field is applied to the longitudinal direction of the base member via the internal electrodes.

In the above described piezoelectric resonator, an inactive section, in which vibration is not excited when the electric field is applied, may constitute the other part of the base member.

The vibration load on the piezoelectric resonator is increased by providing a rubber-like elastic material provided on the piezoelectric resonator, for example being between the piezoelectric resonator and the supporting substrate or on the surface of the piezoelectric resonator remote from the supporting substrate or both between the piezoelectric resonator and the supporting substrate and on the surface of the piezoelectric resonator remote from the supporting substrate. If an electroconductive material is used as the rubber-like elastic material provided between the piezoelectric resonator and the supporting substrate, the reliability of electrical connection between electrodes on the supporting substrate and the piezoelectric resonator can be improved.

In the ladder filter formed by connecting a plurality of the piezoelectric resonators in a ladder form, the vibration load on each piezoelectric resonator constituting the ladder filter can be adjusted by providing a rubber-like elastic material on the piezoelectric resonator.

According to the present invention, the mechanical quality factor Qm of the piezoelectric resonator can be changed substantially effectively by increasing the vibration load on the piezoelectric resonator by the rubber-like elastic material. Accordingly, a piezoelectric component having the desired Qm can be obtained by adjusting the amount of the rubber-like elastic material. The reliability of electrical connection between electrodes on the supporting substrate and the piezoelectric resonator can be improved by using an electroconductive material as the rubber-like elastic material between the piezoelectric resonator and the supporting substrate. Thus, improved characteristics of the piezoelectric component can be obtained. A ladder filter having a plurality of piezoelectric resonators and having improved characteristics can also be obtained by performing such Qm adjustment with respect to each piezoelectric resonator.

The above-described and other objects, features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments of the invention with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

<ul id="ul0001" list-style="none" compact="compact"><li>Fig. 1 is an exploded perspective view of a piezoelectric resonator which represents an embodiment of the present invention;</li><li>Fig. 2 is a perspective view of a piezoelectric resonator used in the piezoelectric component shown in Fig. 1;</li><li>Fig. 3 is a diagram showing the structure of the piezoelectric resonator shown in Fig. 2;</li><li>Fig. 4 is a plan view of a state in which insulating films are formed on a base member used in the piezoelectric resonator shown in Fig. 2;</li><li>Fig. 5 is a diagram of the structure using an electroconductive rubber-like elastic material between the piezoelectric resonator and the supporting substrate;</li><li>Fig. 6 is a plan view of an essential portion of a piezoelectric component using a plurality of piezoelectric resonators to form a ladder filter in accordance with the present invention;</li><li>Fig. 7 is an exploded perspective view of the essential portion of the ladder filter shown in Fig. 6;</li><li>Fig. 8 is an equivalent circuit diagram of the ladder filter shown in Fig. 6;</li><li>Fig. 9 is a graph showing the relationship between an attenuation characteristic and impedance characteristics of series and parallel resonators used in the ladder filter;</li><li>Fig. 10 is a graph showing characteristics of the ladder filter with respect to adjustment of Qm of both the series and parallel resonators;</li><li>Fig. 11 is a graph showing characteristics of the ladder filter with respect to adjustment of Qm of the series resonators;</li><li>Fig. 12 is a graph showing characteristics of the ladder filter with respect to adjustment of Qm of the parallel resonators; and</li><li>Fig. 13 is an exploded perspective view of a conventional piezoelectric component.</li></ul>

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fig. 1 shows a piezoelectric component 10 which represents an embodiment of the present invention. The piezoelectric component 10 includes a supporting substrate 12 formed of an insulating material such as alumina. Two recesses 14 are formed in each of two opposite side portions of the supporting substrate 12. Two pattern electrodes 16 and 18 are formed on one of two major surfaces of the supporting substrate 12. The pattern electrode 16 has a first portion formed between one opposite pair of the recesses 14 and a second portion formed into the shape of L such as to extend along one of the above-mentioned opposite sides of the supporting substrate 12 and then toward a central portion of the supporting substrate 12. The pattern electrode 18 has a first portion formed between the other opposite pair of the recesses 14 and a second portion formed into the shape of L such as to extend along the other of the opposite sides of the supporting substrate 12 and then toward the central portion of the supporting substrate 12. The ends of the two pattern electrodes 16 and 18 at the center of the supporting substrate 12 are formed so as to be opposed to and spaced apart from each other. The first portions of the pattern electrodes 16 and 18 are formed so as to extend to positions on the other major surface of the supporting substrate 12 via the recesses 14.

A piezoelectric resonator 20 is mounted on the ends of the pattern electrodes 16 and 18 at the center of the supporting substrate 12. The piezoelectric resonator 20 includes a base member 22 having, for example, the shape of a rectangular block, as shown in Fig. 2. The base member 22 is formed of, for example, a piezoelectric ceramic material. A plurality of internal electrodes 24 are formed in the base member 22, as shown in Fig. 3. Each internal electrode 24 is formed so that its major surfaces are perpendicular to the longitudinal direction of the base member 22. The base member 22 is polarized along its longitudinal direction so that a pair of its portions on the opposite sides of each internal electrode 24 are polarized in opposite directions, as indicated by the arrows in Fig. 3. However, the base member 22 is not polarized at the opposite ends in the longitudinal direction.

In one side face of the base member 22, a groove 25 is formed so as to extend in the longitudinal direction of the base member 22. The groove 25 is formed at the center in the widthwise direction of the base member 22 to bisect the side face of the base member 22. As shown in Fig. 4, on the side face divided by the groove 25, first insulating film 26 and second insulating film 28 are formed. Edges of the internal electrodes 24 exposed in one of the two portions of the side face of the base member 22 bisected by the groove 25 are alternately covered and left uncovered with the first insulating film 26. Edges of the internal electrodes 24 which are exposed in the other of the two portions of the side face of the base member 22 bisected by the groove 25 and which are opposite from those not covered with the first insulating film 26 are covered with the second insulating film 28.

Further, external electrodes 30 and 32 are formed on the portions of the base member 22 on which the first and second insulating films 26 and 28 are formed, i.e., on the opposite sides of the groove 25, so that the internal electrodes 24 not covered with the first insulating film 26 are connected to the external electrode 30 while the internal electrodes 24 not covered with the second insulating film 28 are connected to the external electrode 32. That <i>is,</i> in each adjacent pair of the internal electrodes 24, one is connected to the external electrode 30 or 32 while the other is connected to the external electrode 32 or 30.

In this piezoelectric resonator 20, the external electrodes 30 and 32 are used as input/output electrodes. When the piezoelectric resonator 20 is operated, an electric field is applied to the portion between each adjacent pair of the internal electrodes 24 defining a section of the base member 22 other than opposite end sections. The base member 22 is thereby made piezoelectrically active in this section. However, the base member 22 is not made piezoelectrically active in the opposite end sections because the base member 22 is not polarized in the opposite end sections and because no electric field is applied to the opposite end sections since no electrodes are formed on the opposite end surfaces of the base member 22. Thus, the central section of the base member 22 is formed as an active section 36 which is activated by an input signal while the opposite end sections of the base member 22 are formed as inactive sections 38 which are not activated by an input signal. Each inactive section 38 is defined as a section in which no driving force is generated by an input signal. Accordingly, an electric field may be applied to the portion between each adjacent pair of the internal electrodes in the inactive sections 38, if the portion between the internal electrodes is not polarized. Also, a structure for inhibiting application of an electric field to some of the polarized piezoelectric layers may be used. It is not always necessary to form such inactive sections 38; the entire base member 22 may be formed as an active section.

The thus-constructed piezoelectric resonator 20 is mounted on the pattern electrodes 16 and 18 on the supporting substrate 12. At this time, the piezoelectric resonator 20 are connected to the pattern electrodes 16 and 18 by two supporting members 40 formed of an electroconductive material. Portions of the external electrodes 30 and 32 of the piezoelectric resonator 20 defined at the center in the longitudinal direction are connected to the supporting members 40. Gaps between the piezoelectric resonator 20 and the supporting substrate 12 are filled with a rubber-like elastic material 42 such as silicone rubber or urethane. Further, rubber-like elastic material 44 is provided on the upper surface of the piezoelectric resonator 20. As these rubber-like elastic materials 42 and 44, a rubber-like elastic material having an insulating property, for example, is used.

A metallic cap 46 is placed on the supporting substrate 12. To prevent the metal cap 46 from being short-circuited to the pattern electrodes 16 and 18, an insulating resin is applied to the supporting substrate 12 and the pattern electrodes 16 and 18 in advance. By capping with the metallic cap 46, the manufacture of the piezoelectric component 10 is completed. In this piezoelectric component 10, the pattern electrodes 16 and 18 formed so as to extend to the reverse surface of the supporting substrate 12 via the side surfaces of the supporting substrate 12 are used as input/output terminals for connection to an external circuit.

When a signal is input to this piezoelectric component 10 through the pattern electrodes 16 and 18, a voltage is applied in opposite directions to the piezoelectric layers in the active section 36 polarized in opposite directions, so that the piezoelectric layers expand and contract in the same direction as a whole. Vibration is thereby excited in the longitudinal fundamental mode with a node corresponding to the center of the base member 22.

In this piezoelectric component 10, the direction of polarization of the active section 36, the direction of each electric field according to a signal and the direction of vibration of the active section 36 coincide with each other. That is, the piezoelectric resonator 20 is of the stiffened type. The stiffened piezoelectric resonator 20 has a larger electromechanical coupling coefficient than unstiffened piezoelectric resonators in which the direction of vibration does not coincide with the direction of polarization and the direction of an electric field. Therefore, the stiffened piezoelectric resonator 20 has a larger difference ΔF between the resonant frequency and the antiresonant frequency. This means that the piezoelectric resonator 20 obtains wide-frequency-band characteristics.

In the piezoelectric resonator 20, ΔF can be adjusted to a suitable value, for example, by changing the proportions of active section 36 and the inactive section 38 and/or by selecting the inactive section 38 formation position. The capacitance of the piezoelectric resonator 20 can be adjusted by changing the number of layers of the active section 36. Therefore, impedance matching between the piezoelectric component 10 and an external circuit can be achieved easily.

In any use of the piezoelectric component 10, e.g., use as a discriminator or an oscillator using one piezoelectric resonator 20 arranged as described above, a phase change at a frequency in the vicinity of the resonant frequency (Fr) and the antiresonant frequency (Fa) is utilized. However, if the mechanical quality factor Qm of the piezoelectric resonator 20 is unnecessarily large, a ripple due to unnecessary vibration occurs in the operating range to seriously affect characteristics the piezoelectric resonator 20. Therefore, suppression of Qm is important. Because Qm of the piezoelectric resonator 20 itself is determined by the piezoelectric material of the base member 22, it cannot be controlled by selecting the size of the element, the number of piezoelectric layers and so on.

In this piezoelectric component 10, therefore, Qm is controlled by the rubber-like elastic material 42 filling the gaps between the piezoelectric resonator 20 and the supporting substrate 12 and the rubber-like elastic material 44 applied on the upper surface of the piezoelectric resonator 20. That is, the vibration load on the piezoelectric resonator 20 is increased by these rubber-like elastic materials 42 and 44, thereby suppressing Qm of the piezoelectric resonator 20 substantially effectively. The desired value of Qm may be obtained by adjusting the amounts of the rubber-like elastic materials 42 and 44 while measuring characteristics of the piezoelectric component 10.

It is not always necessary to use both the rubber-like elastic materials 42 and 44; only one of them may be used. Use of only the rubber-like elastic material 42 filling the gaps between the piezoelectric resonator 20 and the supporting substrate 12 is more effective in suppressing Qm than use of only the rubber-like elastic material applied on the upper surface of the piezoelectric resonator 20. This is because the rubber-like elastic material 42 adheres both to the piezoelectric resonator 20 and to the supporting substrate 12 to effectively suppress vibration of the piezoelectric resonator 20. The rubber-like elastic material 42 reinforces the support for the piezoelectric resonator 20. However, adjustment of Qm by applying the rubber-like elastic material 44 on the upper surface of the piezoelectric resonator 20 is more easier to perform if Qm is adjusted while characteristics of the piezoelectric component 10 are being measured.

If the amounts of the rubber-like elastic materials 42 and 44 are increased, the vibration load on the piezoelectric resonator 20 becomes larger and the Qm suppression effect becomes higher. When the piezoelectric resonator 20 vibrates, it has a larger displacement at its opposite ends since it vibrates in the longitudinal mode with a node corresponding to its center. Accordingly, a portion of the rubber-like elastic material 42 or 44 closer to each of the opposite ends of the piezoelectric resonator 20 contributes more largely to suppression of Qm.

As the rubber-like elastic material 42 between the piezoelectric resonator 20 and the supporting substrate 12, an electroconductive rubber-like elastic material such as electroconductive silicone may be used. If such an electroconductive material is used as the rubber-like elastic material 42, the reliability of each of the electrical conduction between the external electrode 30 and the pattern electrode 16 and the electrical conduction between the external electrode 32 and the pattern electrode 18 can be improved. If such a material is used, an insulating rubber-like elastic material 50 is provided as a filler at the recess 25 such as to prevent electrical conduction between the two external electrodes 30 and 32, as shown in Fig. 5.

Fig. 6 is a plan view of an essential portion of a piezoelectric component 10 which represents another embodiment of the present invention, and which is arranged as a ladder filter by using a plurality of piezoelectric resonators. Fig. 7 is a perspective view of the essential portion of this piezoelectric component 10. In this piezoelectric component 10, four pattern electrodes 90, 92, 94, and 96 are formed on a supporting substrate 12. The pattern electrodes 90 to 96 have first to fifth lands arranged in a row in the direction from one end toward the other end of the supporting substrate 12 while being spaced apart from each other. The first land is formed as a portion of the pattern electrode 90, the second and fifth lands are formed as portions of the pattern electrode 92, the third land is formed as a portion of the pattern electrode 94, and the fourth land is formed as a portion of the pattern electrode 96.

External electrodes 30 and 32 of piezoelectric resonators 20a, 20b, 20c, and 20d are connected to these lands by supporting members 40. The piezoelectric resonators 20a, 20b, 20c, and 20d are mounted so as to form a ladder type circuit shown in Fig. 8. A metallic cap (not shown) is placed on the supporting substrate 12.

This piezoelectric component 10 is used as a ladder filter having a ladder type circuit such as shown in Fig. 8. To form such a ladder filter, two piezoelectric resonators 20a and 20c are used as series resonators while the other two piezoelectric resonators 20b and 20d are used as parallel resonators. Such a ladder filter is designed so that the parallel resonators 20b and 20d have much larger capacitances than the series resonators 20a and 20c. In a ladder filter, an attenuation characteristic is determined by an impedance characteristic of the series resonators 20a and 20c and an impedance characteristic of the parallel resonators 20b and 20d, as shown in Fig. 9.

In such a ladder filter, a group delay characteristic (group delay time (GDT)) can be controlled by controlling Qm of the piezoelectric resonators 20a to 20d through the rubber-like elastic members 42 and 44. Characteristics of a ladder filter formed in the above-described manner were examined. Fig. 10 shows the characteristics when Qm of each of the series and parallel piezoelectric resonators 20a to 20d was suppressed. Fig. 11 shows the characteristics when Qm of each of the series piezoelectric resonators 20a and 20c was suppressed. Fig. 12 shows the characteristics when Qm of each of the parallel piezoelectric resonators 20b and 20d was suppressed. In Figs. 10 to 12, the broken line indicates the group delay time before the application of the elastic materials while the solid line indicates the group delay time after the application of the elastic materials.

As can be understood from Figs. 10, 11, and 12, GDT deviations can be improved by suppressing Qm of the piezoelectric resonators 20a to 20d, and the effect of improving GDT deviations is particularly high if the factors Qm of all the piezoelectric resonators 20a to 20d are suppressed. A change in amplitude characteristic of each resonator due to a change in Qm will not be discussed since it is very small.

As described above, if rubber-like elastic materials 42 and 44 are used, the mechanical quality factor Qm can be controlled substantially effectively without changing the piezoelectric material. In the above described embodiments, a stiffened type laminated piezoelectric resonator is used as piezoelectric resonator 20 in the above-described piezoelectric components 10. However, an unstiffened piezoelectric resonator in which the direction of vibration does not coincide with the direction of polarization and the direction of an electric field may alternatively be used. Also in a case where such an unstiffened piezoelectric resonator capable of longitudinal vibration is used, Qm of the piezoelectric resonator can be controlled by the method of increasing the vibration load by using rubber-like elastic members 42 and 44.

Also in a piezoelectric component using a piezoelectric resonator having external electrodes formed on opposite surfaces of a base member and also using a lead wire as shown in Fig. 13, Qm of the piezoelectric resonator can be controlled by filling the gaps between the piezoelectric resonator and the supporting substrate with a rubber-like elastic material and/or by applying a rubber-like elastic material on the upper surface of the piezoelectric resonator.