Piezoelectric resonator, filter, and electronic communication device

A piezoelectric resonator includes a substrate, a vibration unit disposed on the substrate and having a structure in which at least one pair of an upper electrode and a lower electrode opposed to each other, the upper and lower electrodes sandwiching the upper and lower surfaces of an internal thin-film portion including at least one layer of a piezoelectric thin-film, and an external thin-film portion provided under the lower electrode and including at least one layer of a piezoelectric thin-film or a dielectric thin-film, the vibration unit being vibrated in an n-th harmonic (n is an integer of 2 or more), the upper electrode and the lower electrode being provided substantially in the positions of the loops of the n-th harmonic.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a piezoelectric resonator including a vibration unit having a multilayer structure including thin-films made of piezoelectric and dielectric materials, and more particularly, to a piezoelectric resonator which is used in a filter, an oscillator or other suitable apparatus and is operated so as to be thickness-longitudinally vibrated in a VHF band, a UHF band, and ultra-high frequency band of which the frequencies are higher than those of the these bands. Moreover, the present invention relates to a filter and an electronic communication device including the piezoelectric resonator.

2. Description of the Related Art

In some piezoelectric resonators using thickness-longitudinal vibration, a resonance response is achieved in an ultra-high frequency band by use of a structure in which a piezoelectric film with a very small film thickness is interposed between electrodes, utilizing an inversely proportional relationship between the resonance frequency and the thickness of the piezoelectric film.

Of the above-described piezoelectric resonators, some of the thickness-longitudinal vibration type are each provided with a substrate having a hole passing through the substrate from the front surface to the back surface, a diaphragm made of an SiO2thin film disposed on the substrate so as to cover the hole, and a vibration unit including a ZnO thin film interposed between a pair of opposed electrodes on the diaphragm.

Some of the above-described type piezoelectric resonators have a structure in which a piezoelectric film is sandwiched between a pair of upper and lower electrodes, and in particular, and have a structure in which the loops of a fundamental wave is positioned on the electrodes to eliminate the loss of resonance energy caused on the electrodes, so that the resonance characteristics are improved.

The loss of the resonance energy is reduced since the fundamental wave is used. However, in use of the fundamental wave, the resonance frequency temperature coefficient is considerably changed, due to the variation of the ratio of the thicknesses of the SiO2thin-film and the ZnO thin-film. Therefore, the resonance frequency tends to be significantly changed with the temperature. Thus, for the piezoelectric resonator, the stability of the resonance frequency to the change of temperature is low.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodiments of the present invention provide a piezoelectric resonator, a filter, and an electronic communication device, in which the loss of resonance energy on electrodes is greatly reduced, and the stability of resonance frequency to the change of temperature is significantly improved.

According to a preferred embodiment of the present invention, a piezoelectric resonator includes a substrate, a vibration unit disposed on the substrate and having a structure in which at least one pair of an upper electrode and a lower electrode are opposed to each other, the upper and lower electrodes sandwiching the upper and lower surfaces of an internal thin-film portion including at least one layer of a piezoelectric thin-film, and an external thin-film portion provided under the lower electrode and including at least one layer of a piezoelectric thin-film or a dielectric thin-film, the vibration unit being vibrated in an n-th harmonic (where n is an integer of 2 or higher), the upper electrode and the lower electrode being provided substantially in the positions of the loops of the n-th harmonic.

According to a preferred embodiment of the present invention, the upper electrode and the lower electrode are provided approximately in the positions of the loops. Thus, the resonance energy on the electrodes is greatly reduced. In addition, the vibration unit is vibrated in the vibration mode of an n-th harmonic. Thus, there is a film-thickness ratio range in which even if the film-thickness ratio of the thin-film portions is changed to be increased or decreased, the resonance frequency temperature coefficient is prevented from changing significantly. Therefore, the resonance frequency can be stabilized relative to the change of temperature by setting the film-thickness ratio to be in this range.

Preferably, the n-th harmonic is a second harmonic, and the film thickness ratio r=to/tiin which torepresents the thickness of the external thin-film, and tirepresents the thickness of the internal thin-film is set at a value at which the resonance frequency temperature coefficient of the entire piezoelectric resonator is nearly zero.

In this case, since the n-th harmonic is a second harmonic, the resonance frequency can be more effectively stabilized relative to the change of temperature.

More preferably, the thin-film portion that is at least one of the internal thin-film portion and the external thin-film portion has a combination in which the respective thin-films have different resonance frequency temperature coefficients.

The combination includes any one of the combination of the respective thin-films constituting the internal thin-film portion if the internal thin-film portion includes a plurality of the thin-films, the combination of the respective thin-films constituting the external thin-film portion if the external thin-film portion includes a plurality of the thin-films, and the combination of the respective thin-films constituting the internal thin-film portion and the external thin-film portion, respectively.

Thereby, the resonance frequency temperature coefficient of the piezoelectric resonator can be more effectively set at zero. Thus, the resonance frequency can be more stabilized relative to the change of temperature.

More preferably, the external thin-film portion includes at least one of a thin-film having a SiO2thin-film as a major component, a thin-film including an SiN thin-film as a major component, and a thin-film including an Al2O3thin-film as a major component.

More preferably, the internal thin-film portion includes a thin-film having ZnO as a major component, a thin-film including AlN as a major component, a thin-film including lead titanate zirconate (PZT) as a major component, a thin-film including lead titanate (PT) as a major component, and a thin-film including barium titanate (BT) as a major component.

More preferably, the substrate has a hole or a concavity, and the vibration unit is disposed above the hole or the concavity. Here, the hole indicates a space extending so as to pass through the substrate from the front surface to the back surface. The concavity indicates a space, which is a bottom depression formed in one surface of the substrate. The resonance characteristic is greatly improved since the vibration unit is disposed above the hole or the concavity.

Thus, the piezoelectric resonator of which the resonance frequency temperature coefficient is small with respect to variations of a film thickness, that is, the change of the resonance frequency, caused by the change of temperature, is greatly reduced, and the resonance response to the change of temperature is very stable, can be provided.

Japanese Unexamined Patent Application Publication No. 2001-203558 discloses that the resonance frequency temperature coefficient of the entire piezoelectric resonator is nearly zero by using a structure in which a piezoelectric film having a negative resonance frequency temperature coefficient and a piezoelectric film having a positive resonance frequency temperature coefficient are sandwiched between a pair of upper and lower electrodes, and thereby, the resonance response to the change of temperature is stabilized.

EP0963040A2 discloses that the resonance energy loss on the electrodes is eliminated by using the structure in which a piezoelectric film is sandwiched between a pair of the upper and lower electrodes and adopting a configuration in which the loops of a resonance wave are positioned on the electrodes, so that the resonance characteristic is greatly improved.

Japanese Examined Patent Application Publication No. 1-48694 discloses that between a pair of piezoelectric films provided between a pair of upper and lower electrodes, a thin-film having a resonance frequency temperature coefficient with a sign different from that of the piezoelectric films is laminated in the center of the piezoelectric films, so that the temperature characteristic of the resonance frequency is greatly improved.

However, in such conventional devices, second harmonics are not used, and the loops of a vibration wave are not positioned on the opposite electrodes, in contrast to the present invention. Thus, the effects of the present invention cannot be achieved.

The filter in accordance with another preferred embodiment of the present invention includes a plurality of the piezoelectric resonators according to other preferred embodiments of the present invention, the electrodes in the piezoelectric resonators being connected to the configuration of a filter circuit.

The filter in accordance with another preferred embodiment of the present invention includes a plurality of the piezoelectric resonators according to other preferred embodiments of the present invention, which are connected in a ladder arrangement.

The duplexer in accordance with yet another preferred embodiment of the present invention is configured using the filter in accordance with the above-described preferred embodiments of the present invention.

The electronic communication device in accordance with a further preferred embodiment of the present invention includes at least one piezoelectric resonator according to the above-described preferred embodiments of the present invention, the piezoelectric resonator being used in electronic communication operation.

Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The basic structure of a piezoelectric resonator of this preferred embodiment will be described with reference to FIG.1.

Reference numeral1designates the entire portion of a resonator. The piezoelectric resonator1is preferably a thickness-longitudinal vibration type resonator, and preferably includes a substrate2, a diaphragm3, and a vibration unit4.

The substrate2is preferably made of, e.g., Si (silicon) and has a hole5passing through the substrate2from the front surface to the back surface.

The diaphragm3functions as an external thin-film portion and is preferably made of a SiO2(silicon oxide) thin-film. The diaphragm3is arranged so as to cover the hole5.

The vibration unit4is provided on the diaphragm3, and includes a pair of upper and lower opposed electrodes which define an upper electrode4band a lower electrode4a, and an internal thin-film portion4chaving one or more layers. The internal thin-film portion4cincludes at least a piezoelectric film that is interposed between the upper electrode4band the lower electrode4a.

Both of the opposed electrodes4aand4bare preferably made of, e.g., Al (aluminum).

A production example of the piezoelectric resonator having the above-described structure will be briefly described. Both surfaces of the substrate2are heat-oxidized to form heat-oxidized SiO2thin-films. The heat-oxidized SiO2thin-film on the front surface of the substrate2constitutes the diaphragm3. The heat-oxidized SiO2thin-film on the back surface of the substrate2is patterned correspondingly to the hole5by photolithography. Thereby, the back surface of the substrate2is exposed. The exposed back surface of the substrate2is anisotropically etched using an alkali solution. This etching reaches the SiO2thin-film on the front surface of the substrate2, whereby a hole5is formed in the substrate2. Subsequently, the lower electrode4ais formed on the diaphragm3of the SiO2thin-film on the surface of the substrate2, preferably by lift-off vapor deposition. Thereafter, the internal thin-film portion4cincluding a ZnO thin-film is formed on the lower electrode4aand the diaphragm3by sputtering or another film-forming technique. Subsequently, the upper electrode4bis formed on the internal thin-film portion4cby lift-off vapor deposition.

Thus, the production of the piezoelectric resonator1is completed.

In this preferred embodiment, according to the above-described structure, the piezoelectric resonator1is vibrated in the second harmonic vibration mode of which the fundamental wave is illustrated by the broken line in FIG.1. Moreover, the upper electrode4band the lower electrode4aare disposed substantially in the positions of the loops of the second harmonic. The nodes of the second harmonic exist in the internal and external thin-film portions3and4c.

Thus, since the loops of the second harmonic are positioned in the upper electrode4band the lower electrode4a, the loss of resonance energy on the lower electrode4aand the upper electrode4bis greatly reduced, and thus, the resonance characteristic is significantly improved.

In addition, in this preferred embodiment, the diaphragm3and the respective thin-films of the vibration unit4are combined in such a manner that the resonance frequency temperature coefficients thereof have different signs. Moreover, the thickness ratio r=to/tiin which torepresents the thickness of the SiO2thin-film as the diaphragm3, and tirepresents the thickness of the ZnO thin-film of the vibration unit4is preferably set at a value at which the resonance frequency temperature coefficient of the entire piezoelectric resonator1becomes nearly zero.

The thickness ratio r will be further described with reference to FIG.2.

InFIG. 2, the film-thickness ratio r is plotted as abscissa, and the resonance frequency temperature coefficient TCF as the ordinate. Reference numerals (1) and (2) represent a fundamental wave and the second harmonic. The thicknesses of the respective thin-films are designed in such a manner that the vibration unit4of this preferred embodiment is excited in the thickness-longitudinal resonance mode of the twice harmonic. The film-thickness ratio r is preferably in the range of about 0.6 to about 1.3.

When the film-thickness ratio r is in this preferred range, the resonance frequency temperature coefficient TCF is about +10 ppm/° C. to about −10 ppm/° C. Thus, the resonance frequency temperature coefficient TCF can be set nearly at zero by setting the film-thickness ratio r to be in the above-described range. Thereby, the vibration frequency of the piezoelectric resonator1can be stabilized relative to the change of temperature.

As described above, the resonance frequency temperature coefficient can be set nearly at zero by adjustment of the film-thickness ratio r. This is due to the external thin-film portion3made of the SiO2thin-film having a positive resonance frequency temperature coefficient and the internal thin-film portion4cmade of the ZnO thin-film having a negative resonance frequency temperature coefficient. Referring toFIG. 2, when the film-thickness ratio toof the external thin-film portion3having a positive resonance frequency temperature coefficient is increased from a value of 1, relatively to the internal thin-film portion4chaving a negative resonance frequency temperature coefficient, so that the film-thickness ratio r becomes more than 1, the resonance frequency temperature coefficient TCF gradually approaches zero until the film-thickness ratio r becomes about 1.3. Moreover, when the film-thickness ratio toof the external thin-film portion3is further increased to be about 1.3 or more, the resonance frequency temperature coefficient TCF is further increased from zero. On the other hand, when the film-thickness toof the external thin-film portion3is decreased so that the film-thickness ratio r is reduced, the resonance frequency temperature coefficient TCF gradually approaches zero until the film-thickness ratio r is reduced to about 0.6. When the film-thickness toof the external thin-film portion3is further decreased to be about 0.6 or smaller, the resonance frequency temperature coefficient TCF is further increased from zero.

As described above, the resonance frequency of the piezoelectric resonator1can be stabilized relative to the change of temperature by adjustment of the resonance frequency temperature coefficient TCF to substantially zero which is carried out by setting of the film-thickness ratio r.

The present invention is not limited to the above-described preferred embodiments. Various applications and modifications cam be carried out without departing from the spirit and the scope of the present invention.

In any of the following preferred embodiments, it is premised that a second harmonic is used, and the loops of the second harmonic are positioned substantially on the upper electrode4band the lower electrode4a. Accordingly, the range shown inFIG. 2in which the change of the resonance frequency with temperature is small can be used by adjustment of the film-thickness ratio of the external thin-film portion3and the internal thin-film portion4c.

(1) The preferred embodiment shown inFIG. 1preferably uses a second harmonic. However, this is not restrictive. The vibration unit may be vibrated in the vibration mode of an N-th harmonic (n is an integer of 2 or greater), and the upper electrode4band the lower electrode4amay be provided in the positions of the loops of the n-th harmonic.

(2) As shown inFIG. 3, the external thin-film portion3may have a two layer structure which includes a heat oxidized SiO2film3aand an SiN (silicon nitride) film3b.

This external thin-film portion3includes two layers having different resonance frequency temperature coefficients, that is, the films3aand3b. Therefore, the resonance frequency temperature coefficient of the entire external thin-film portion3can be adjusted by appropriately changing the film-thickness ratio of the films3aand3b. By this adjustment, the temperature-dependent change-ratio of the resonance frequency temperature coefficient of the entire piezoelectric resonator1is significantly reduced. Thus, the stability to the change of temperature is greatly improved.

(3) As shown inFIG. 4, the external thin-film portion3may have two layer structure which includes a heat oxidized SiO2film3cand an SiO2film3dpreferably formed by sputtering.

According to this structure of the external thin-film portion3, the temperature characteristic of the resonance frequency of the internal thin-film portion4ccan be compensated by adjustment of the temperature characteristic of the resonance frequency of the entire portions of both of the films3cand3dconstituting the external thin-film portion3.

(4) As shown inFIG. 5, the internal thin-film portion4cmay have a two layer structure including an AlN (aluminum nitride) film4c1and an ZnO film4c2.

In this structure of the internal thin-film portion4c, the AlN film4c1has a positive resonance frequency temperature coefficient, while the ZnO film4c2has a negative resonance frequency temperature coefficient. Therefore, the internal thin-film portion4ccan be produced in such a manner that the resonance frequency temperature coefficient of the entire piezoelectric resonator1approaches zero by compensating for the resonance frequency temperature coefficient of the external thin-film portion3made of the heat oxidized SiO2utilizing the two layer structure including the AlN film4c1and the ZnO film4c2. As a result, the resonance frequency temperature coefficient of the piezoelectric resonator1is greatly reduced, so that the temperature characteristic becomes stable.

(5) As shown inFIG. 6, the external thin-film portion3may have the two layer structure including the heat oxidized SiO2film3cand the SiO2film3dpreferably formed by sputtering, and also, the internal thin-film portion4cmay have the two-layer structure including the AlN film4c1and the ZnO film4c2. Thereby, the same operation and effects as described above can be attained.

(6) As shown inFIG. 7, the external thin-film portion3may include an SiN film, and the internal thin-film portion4cmay include an AlN film. In this case, the same operation and effects as described in the Items (3) and (4) above can be attained at the same time.

(7) As shown inFIG. 8, the external thin-film portion3may have a two-layer structure including an AlN film3eand an Al2O3(aluminum oxide) film3f, and also, the internal thin-film portion4cmay have an AlN single layer structure. Also, in this case, the same operation and effects as described in Items (3) and (4) above can be attained at the same time.

(8) The internal thin-film portion4cof the vibration unit may include at least one of a thin-film containing PZT (lead titanate zirconate) as a major component, a thin-film including PT (lead titanate) as a major component, and a thin-film containing BT (barium titanate) as a major component, in addition to the thin-film containing ZnO as a major component and the thin-film containing AlN as a major component.

(9) The piezoelectric resonator1of this preferred embodiment may be incorporated as a filter element into a π type ladder filter shown inFIG. 9A, an L type ladder filter shown inFIG. 9B, a T type ladder filter shown inFIG. 9C, an L type ladder filter shown inFIG. 10A, and an L type ladder filter shown in FIG.10B. These filters have filter characteristics that are stable relative to the change of temperature. For the respective filters, a plurality of the above-described piezoelectric resonators1are provided on the substrate2. The piezoelectric resonators1on the substrate2can be connected via the respective electrodes thereof to provide a filter of which the operational characteristic is stabilized relative to the environmental temperature change.

(10) The piezoelectric resonator of this preferred embodiment may be mounted in a portable telephone, a radio-wave LAN, and other different kinds of communication devices. Thus, when the piezoelectric resonator is used in electronic communication operation of these electronic communication devices, the operational characteristic can be stabilized with respect to the environmental temperature-change.

(11) The piezoelectric resonator1of this preferred embodiment may be used as an element for a duplexer to be mounted onto a communication device. A duplexer31is provided with an antenna-terminal32, a reception side terminal33, and a transmission side terminal34as shown in FIG.11. The duplexer31is configured to include the piezoelectric resonator of preferred embodiments of the present invention or the filter described in Item9between the reception side terminal33, the transmission side terminal34and the antenna terminal32which permits a high frequency signal in a required frequency band to be transmitted.

(12) The structure shown inFIG. 12may be used as a modification of the piezoelectric resonator of other preferred embodiments of the present invention. In a piezoelectric resonator51shown inFIG. 12, a concavity53is formed on the upper surface of a silicon substrate52. A diaphragm54as the external thin-film potion, preferably including two layers, that is, a heat-oxidized SiO2film54aand an SiN film (silicon nitride)54b, is arranged so as to cover the upper surface of the silicon substrate52and the concavity53. A vibration unit55is arranged on the diaphragm54. The vibration unit55includes a pair of upper and lower opposed-electrodes, that is, an upper electrode58and a lower electrode56, and an internal thin-film portion57including at least one layer of a piezoelectric film sandwiched between the upper electrode58and the lower electrode56.