JP3723697B2 - Piezoelectric resonator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a piezoelectric resonator used in a mobile phone, a wireless LAN, and the like, and more particularly, to a piezoelectric resonator using resonance of a thickness longitudinal vibration of a piezoelectric body.
[0002]
[Prior art]
In recent years, with an increase in frequency used for wireless communication and electric circuits, filters used for these electric signals have been developed corresponding to high frequencies. In particular, in wireless communication, microwaves near 2 GHz are becoming mainstream, and there is already a movement to formulate standards of several GHz or more, so an inexpensive and high-performance filter corresponding to those frequencies is required. .
[0003]
Recently, a filter using a surface acoustic wave resonator (SAWR) that uses resonance of a surface acoustic wave, which is an acoustic wave transmitted on the surface of a solid, has attracted attention. This filter uses resonance of high frequency electric field and surface acoustic wave applied between comb-shaped electrodes formed on a solid surface, has high frequency selectivity, and is widely used as an excellent bandpass filter. .
[0004]
However, since the distance between the comb electrodes is inversely proportional to the resonance frequency of the SAW filter, the distance between the comb electrodes is 1 μm or less in a frequency region exceeding 1 GHz, and it is very difficult to fabricate the electrodes.
[0005]
Thus, in recent years, a resonator using a thickness longitudinal vibration mode of a thin film exhibiting piezoelectricity has been proposed in order to cope with a high frequency of several GHz or more.
[0006]
This is a resonator that uses the piezoelectric thin film to cause longitudinal vibration in the thickness of the input high-frequency electric signal, and the vibration causes resonance in the thickness direction of the thin film. BAWR). The BAWR is expected to be capable of producing a resonator having a higher resonance frequency than the SAWR, and has been developed.
[0007]
As shown in FIG. 10, the conventional BAWR includes a base 1, a support film 2 formed on the surface of the base 1, a buffer layer 3 formed on the support film 2, and the buffer layer 3. A first electrode 4 formed on the first electrode 4, a piezoelectric thin film 5 formed on the first electrode 4, and two second electrodes 6 formed on the piezoelectric thin film 5 (USP 4,320). , 365). The support film 2 is formed of the support film 2 on the surface of the base 1 so as to cover the space A formed in the base 1, thereby oscillating the first electrode 4, the piezoelectric thin film 5, and the second electrode 6. The portion of the support film 2 in contact with the substrate also vibrates.
[0008]
Japanese Laid-Open Patent Publication No. 9-55998 discloses a piezoelectric resonator in which a plurality of piezoelectric bodies are arranged at a constant interval between a pair of electrodes to form a plurality of vibrating portions. In this piezoelectric resonator, a gas or a liquid is filled between the vibrating part, and this structure increases the mechanical coupling constant, and the realizable band is doubled compared to the conventional case. .
[0009]
[Problems to be solved by the invention]
However, in the piezoelectric resonator disclosed in US Pat. No. 4,320,365, since a piezoelectric body having a small relative dielectric constant such as ZnO or AlN is used, the electromechanical coupling coefficient of these piezoelectric materials is small. It has been difficult to realize a broadband GHz band filter. In addition, since ZnO and AlN have a small relative dielectric constant, a large electrode area with one side exceeding 100 μm is required to match the impedance required for the microwave circuit.
[0010]
Thus, when a piezoelectric body having a small relative dielectric constant such as ZnO or AlN is used, there is a problem that unnecessary vibration is likely to occur because the electrode size is as large as 100 times or more than the film thickness. This is because, when the electrode area is sufficiently larger than the film thickness, there is an in-plane propagation mode, which is one of the energy confinement modes, and it is reflected at the boundary with the non-electrode part and slightly more than the thickness longitudinal vibration. This is because standing waves having different frequencies are generated and unnecessary vibrations are generated.
[0011]
Further, in the piezoelectric resonator disclosed in Japanese Patent Application Laid-Open No. 9-55998, an attempt is made to improve the mechanical coupling coefficient of the resonator by dividing the vibration portion. However, since the periphery of each vibration portion is air or liquid. Of the atoms constituting the free end and constituting the piezoelectric body between the electrodes, atoms in contact with air or liquid are easy to move, and there is a problem that an unnecessary vibration mode occurs.
[0012]
The object of the present invention is to be able to configure a high frequency filter in the GHz band, enable effective energy confinement in the thickness longitudinal vibration mode, suppress unnecessary vibrations other than main vibrations, and prevent spurious generation, An object of the present invention is to provide a piezoelectric resonator having good impedance characteristics.
[0013]
[Means for Solving the Problems]
The piezoelectric resonator of the present invention is in contact with a base having a vibration space, a support film formed on the surface of the base so as to cover the vibration space, and a surface of the support film opposite to the vibration space. A piezoelectric resonator comprising a vibrating body sandwiched between a first electrode and a second electrode, wherein the first electrode and the second electrode are each composed of a plurality of elongated electrodes arranged substantially in parallel. The elongated electrode of the first electrode and the elongated electrode of the second electrode intersect with each other when viewed from the substrate side, and are formed so as to be electrically connected. Is 3 to 10 μm .
[0014]
In addition, the substrate, the vibration reflection layer formed on the surface of the substrate and formed by stacking a plurality of insulators, and the vibration formed on the surface of the vibration reflection layer and sandwiching the piezoelectric body between the first electrode and the second electrode The first electrode and the second electrode are formed by electrically connecting a plurality of elongated electrodes arranged substantially in parallel to each other, and the first electrode The long electrode of the electrode and the long electrode of the second electrode intersect with each other when viewed from the substrate side.
[0015]
In particular, the piezoelectric body is preferably made of a ferroelectric material, and the width of the long electrode is preferably 10 μm or less.
[0016]
[Action]
In the piezoelectric resonator of the present invention, the first electrode and the second electrode are each composed of a plurality of elongated electrodes arranged substantially in parallel, and the elongated electrodes of the first electrode and the second electrode. Are crossed when viewed from the substrate side, the overlapping portion of the first electrode and the second electrode becomes a vibrating region (vibrating portion), and the electrode portion of the overlapping region is an electrode (vibrating portion) for one vibrating portion. Electrode).
[0017]
Each vibration part defined by the overlapping region of the first electrode and the second electrode is composed of the vibration electrode of the first electrode, the piezoelectric body, and the vibration electrode of the second electrode, and the electrode size is about the same as the film thickness. Since the size can be increased, the in-plane propagation mode is eliminated, and the generation of the reflected standing wave at the boundary with the non-electrode portion can be prevented and the generation of unnecessary vibration can be suppressed. This is because the surrounding of the piezoelectric body of each vibration part is fixed by the piezoelectric body between the vibration parts, and thus unnecessary vibration modes that occur at the time of the free end as in the conventional case can be suppressed. is there.
[0018]
In general, when the vibrating body becomes small, the impedance increases, and the impedance of one vibrating body cannot be matched with the transmission impedance of the microwave circuit. However, since each vibration part in the present invention is synchronized in vibration, impedance matching with the circuit to be used can be achieved by controlling the number of vibration parts. In other words, since impedance matching is caused by the total area of the electrodes, the necessary impedance can be obtained by forming the number of vibrating electrodes corresponding to the required area.
[0019]
In addition, since the piezoelectric body forming the vibration part is mainly composed of a ferroelectric substance having a large relative dielectric constant, the capacitance of the vibration part can be realized in a small area compared to a non-ferroelectric substance.
[0020]
In particular, by setting the width of the long electrode to 3 to 10 μm , each vibration part can be made sufficiently small to be 10 μm or less. Therefore, a small-sized GHz band resonator suitable as a broadband filter that exhibits resonance characteristics without unnecessary vibration can be obtained. Can be provided.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
The piezoelectric resonator of the present invention is a thin film piezoelectric resonator in which a support film, a piezoelectric thin film, and an electrode thin film are formed on a base. As shown in FIGS. 1 and 2, a base 11 having a vibration space A, A support film 12 formed on the surface of the substrate 11, a first electrode 13 formed on the support film 12, a piezoelectric body 14 formed on the first electrode 13, and on the piezoelectric body 14 The second electrode 15 is formed.
[0022]
The first electrode 13 has five elongated electrodes 16a arranged substantially in parallel at equal intervals, and these are electrically connected by a base electrode 17a to form a comb shape. The second electrode 15 is also configured by electrically connecting five elongated electrodes 16b similar to the first electrode 13 by a base electrode 17b.
[0023]
In the piezoelectric resonator of the present invention, as is apparent from FIG. 2, the long electrode 16a of the first electrode 13 and the long electrode 16b of the second electrode 15 intersect each other when viewed from the base 11. A portion where these long electrodes 16 a and 16 b overlap is a vibrating portion 18. Since the vibrating portion 18 is limited to a portion where the long electrodes 16a and 16b intersect and overlap each other, the type of standing wave is limited, and unnecessary vibration is less likely to occur.
[0024]
The substrate 11 is an inorganic material such as Si, and is not particularly limited as long as it has a flat surface on which a ferroelectric thin film can be formed. The vibration space A of the base 11 is a space for not transmitting the vibration of the piezoelectric body 14 excited by the voltage applied to the first electrode 13 and the second electrode 15 to the base 11, and has a through hole in the base 11. It is formed by forming or forming a concave recess 19 in a portion of the base 11 where the support film 12 is formed as shown in FIG.
[0025]
The support film 12 is formed by a method such as a sputtering method or a CVD method, and a film having a high sound speed and a small residual stress is desired. Although the thickness depends on the sound speed of the material, a thickness of 2 μm or less is desirable in order to use low-order piezoelectric resonance.
[0026]
The piezoelectric body 14 can be formed by a sol-gel method, a method in which a ceramic slurry is applied and heated and fired, or a gas phase synthesis method typified by a sputtering method. The film thickness of the piezoelectric body 14 is desirably 2 μm or less. This is because the distance between the first electrode 13 and the second electrode 15 must be 2 μm or less in order to obtain a resonance frequency of 1 GHz or more by thickness longitudinal vibration.
[0027]
The first electrode 13 and the second electrode 15 are produced by a vapor phase synthesis method such as a CVD method, a sputtering method, a vacuum deposition method, or an ion plating method, and then the formed electrodes are elongated by a reactive ion etching method or the like. It is obtained by etching into a shape. The thickness of the electrode is desirably 0.05 to 0.2 μm, and particularly preferably 0.05 to 0.1 μm.
[0028]
The width of the long electrodes 16a and 16b is preferably 10 μm or less. In particular, 7 μm or less is desirable. This is because unnecessary vibration tends to occur when the thickness exceeds 10 μm. Even if the width of the long electrode 16a of the first electrode 13 is different from the width of the long electrode 16b of the second electrode 15, there is no problem as long as the width of the electrode is 10 μm or less.
[0029]
The substrate 11 is an inorganic substance such as silicon or alumina, and the material is not particularly limited as long as it has a flat surface on which a piezoelectric body can be formed.
[0030]
The support film _{12, SiO 2, Al 2 O} 3, ZrO 2, _{diamond, BN, B 4 C, SiC} , etc. Si ₃ N ₄ is used. Among these, a high-strength film having a high sound speed and a small residual strain is desirable. Among them, diamond is particularly desirable because it has high strength, high sound velocity, and a small rate of temperature change of the elastic constant.
[0031]
Further, when a resonator is formed using a non-ferroelectric piezoelectric material such as ZnO or AlN, the area becomes large and a small resonator cannot be realized. Therefore, the piezoelectric material 14 of the present invention has a relative dielectric constant. A ferroelectric having a large thickness is particularly used.
[0032]
In particular, a ferroelectric thin film having a perovskite type structure such as Pb (Zr _1-X Ti _X ) O ₃ has a large spontaneous polarization and can provide a large piezoelectricity, and is not a ferroelectric substance such as ZnO or AlN. Since the relative dielectric constant is about 100 times larger than that of the piezoelectric body and the electrode area, which is one of the factors determining the impedance, can be reduced to about 1/100, the size of the resonator can be reduced.
[0033]
Examples of the ferroelectric having a perovskite structure include Pb (Zr _1-X Ti _x ) O ₃ , PbTiO ₃ , and BaTiO ₃ . Further, a ferroelectric thin film other than the perovskite structure, for example, Sr ₂ KNb ₅ O ₁₅ having a tungsten bronze structure can be used.
[0034]
Among the ferroelectric materials, those having Pb (Zr _1-X Ti _X ) O ₃ as a basic composition are particularly preferable. This is because Pb (Zr _1-X Ti _X ) O ₃ having a basic composition exhibits large piezoelectricity and relative dielectric constant even when it is a thin film. Ferroelectric materials such as BaTiO ₃ and Sr ₂ KNb ₅ O ₁₅ have a large size effect that the ferroelectricity is reduced when the crystal grain size is reduced, and it is difficult to obtain large piezoelectricity due to the ferroelectricity when a thin film is used. It is.
[0035]
For the first electrode 13 and the second electrode 15 that sandwich the piezoelectric body 14, a metal material such as Al and Au that has been used more often than before is used. Al is desirable because the mass load on the piezoelectric body 14 is small and it is difficult to suppress piezoelectric vibration, and the electrical resistivity is small and the decrease in the Q value of the resonator is small.
[0036]
In FIGS. 1 to 3, it goes without saying that electrodes for extraction are formed on the base electrodes 17a and 17b of the first electrode 13 and the second electrode 15 in order to actually use them as resonators.
[0037]
In the piezoelectric resonator configured as described above, a ferroelectric is used for the piezoelectric body 14, and the electrode area can be reduced and the number of electrodes can be arbitrarily determined. The impedance can also be controlled.
[0038]
Further, since each vibration electrode is small and the periphery of the piezoelectric body 14a of each vibration part is fixed by the piezoelectric body 14b between the vibration parts, unnecessary vibration is improved, and the energy of thickness longitudinal vibration is reduced to a pair of vibration electrodes. It can be sufficiently confined between them and spurious in the vicinity of the resonance frequency and anti-resonance frequency used can be suppressed.
[0039]
FIG. 4 shows another example of the piezoelectric resonator of the present invention. This piezoelectric resonator is composed of a base 11 and a laminate of a plurality of insulators (32a, 32b) formed on the surface of the base 11. The vibration reflection layer 32, the first electrode 13 formed on the vibration reflection layer 32, the piezoelectric body 14 formed on the first electrode 13, and the second electrode formed on the piezoelectric body 14 15.
[0040]
The insulators 32 a and 32 b are two kinds of materials having different acoustic impedances, and the thickness of each layer may be about half of the thickness of the piezoelectric body 14. This is because a half-wave standing wave exists in the thickness direction of the piezoelectric body 14, and reflection is performed by laminating an insulator with a thickness corresponding to a quarter wavelength of the standing wave. It is because it can do. When such a vibration reflection layer 32 is used, a resonator can be formed on the substrate without forming a vibration space, the strength of the resonator can be increased, the manufacturing process can be simplified, and the manufacturing cost can be reduced. Can be reduced.
[0041]
In FIG. 1, the vibration portion 18 and the base body 11 are formed with a vibration space A so that vibration is not transmitted. In FIG. 4, insulators 32 a and 32 b having different acoustic impedances are stacked. The vibration reflection layer 32 is formed, the vibration is reflected by the vibration reflection layer 32, and the vibration from the vibration part does not reach the base. The structure of FIG. 4 does not have a hole processing of the substrate, which is a wet process, compared to FIG. 1, simplifies the manufacturing process, and eliminates the support film that vibrates with the vibration part. The vibration frequency can be increased, making it easier to cope with higher frequencies.
[0042]
Furthermore, as shown in FIG. 5, the piezoelectric body 14 may be a two-layered laminate of piezoelectric bodies 14 c and 14 d made of a ferroelectric material whose temperature coefficient of sound velocity has an opposite sign. Piezoelectric bodies 14c and 14d made of a ferroelectric material whose temperature coefficient of sound velocity has an opposite sign can be realized by using, for example, Pb (Zr _1−X Ti _X ) O ₃ having a composition near the composition phase boundary. By laminating the piezoelectric bodies 14c and 14d made of a ferroelectric material whose temperature coefficient has the opposite sign, it is possible to realize a resonator in which the temperature coefficient of the resonance frequency is close to zero.
[0043]
Further, as shown in FIG. 6, the piezoelectric body 14 may be a two-layered laminate of piezoelectric bodies 14e and 14f made of ferroelectric materials whose polarization directions are opposite to each other. Such a piezoelectric resonator can be realized by applying an electric field larger than the coercive electric field of the piezoelectric bodies 14 e and 14 f made of a ferroelectric material between the intermediate electrode 20 and the first electrode 13 and the second electrode 15. A high-frequency electric field is applied between the first electrode 13 and the second electrode 15, but since a laminate of two layers of piezoelectric bodies 14 e and 14 f having opposite polarization directions is used, the first electrode 13 and the second electrode 15 A standing wave of one wavelength is generated during this period, and high frequency can be promoted.
[0044]
【Example】
Example 1
First, a silicon nitride film was formed on the back surface of the Si substrate by a CVD method. Next, a diamond film was formed on the surface of the Si substrate by a CVD method. The silicon nitride film was patterned for via hole formation by reactive ion etching, and then the Si substrate was anisotropically etched using a KOH solution to form a through hole (vibration space).
[0045]
Next, a first electrode made of Au having a thickness of 100 nm was formed on the diamond film at 500 ° C. by using a magnetron sputtering method.
[0046]
A long electrode and a base electrode were formed using a photolithographic method and processed into a comb shape. Next, as described below, three types of resonators having a long electrode width different from 3 μm, 7 μm, and 10 μm were manufactured.
[0047]
First, five long electrodes having a width of 3 μm and a length of 70 μm are arranged at intervals of 5 μm and connected by a base electrode, and then three long electrodes having a width of 7 μm and a length of 80 μm are connected to a 10 μm Disposed at intervals and connected by a base electrode, and finally, two long electrodes having a width of 10 μm and a length of 100 μm were arranged at an interval of 10 μm and connected by a base electrode, respectively.
[0048]
A PbZr _0.53 Ti _0.47 O ₃ thin film having a thickness of 1 μm was formed thereon by a sol-gel method. This piezoelectric body was produced as follows. First, a 1M concentration coating solution is formed and dried on the first electrode of the Si substrate by spin coating, heat treatment (420 ° C., 1 minute) and film formation are repeated 10 times, followed by crystallization at 650 ° C. for 30 minutes. Processing was performed to obtain a ferroelectric thin film having a thickness of 1 μm. Next, unnecessary portions of the ferroelectric film were removed by photolithography and etching to expose the base electrode.
[0049]
Finally, a second electrode was produced. A Pt electrode film having a thickness of 100 nm was formed at 500 ° C. by sputtering. It processed into the comb shape using the lift-off method. The shape of the elongated electrodes is the same as that of the first electrode, but the number of elongated electrodes was as follows. The number of elongated electrodes having widths of 3, 7, and 10 μm was set to 5, 2, and 2, respectively. The piezoelectric resonator shown in FIGS. 1 and 2 was produced by forming each long electrode so that the direction of the long electrode was orthogonal to the long electrode of the first electrode.
[0050]
Next, using an RF impedance analyzer (HP4291A) and an RF probe, a 20V DC voltage was applied between the electrodes for 10 seconds to polarize the piezoelectric body.
[0051]
Next, when the piezoelectric resonance characteristics were evaluated by impedance measurement, similar impedance characteristics were shown. FIG. 7 shows a case where the long electrode width is 7 μm as a representative. From this figure, a resonance peak at which the impedance is minimized and an anti-resonance peak at which the impedance is maximized were observed at 1.4 GHz to 1.5 GHz.
[0052]
In FIG. 7, no peaks due to other spurious vibrations were observed, and only single mode vibrations were obtained. The same applies to the peaks of the long electrode width of 3, 10 μm.
[0053]
Example 2
First, five layers (about 0.5 μm) of tungsten film having high acoustic impedance and silicon oxide film having low acoustic impedance are alternately formed on the surface of the Si substrate by sputtering, and a vibration reflecting layer (¼ wavelength reflector) is formed. ) Was formed. Next, in the same manner as in Example 1, the first electrode, the piezoelectric body, and the second electrode were formed on the surface of the vibration reflecting layer, and the piezoelectric resonator shown in FIG. 4 was manufactured. The first electrode and the second electrode had a long electrode with a width of 7 μm and a length of 70 μm.
[0054]
Similarly to Example 1, a 20 V DC voltage was applied between the electrodes for 10 seconds to polarize the piezoelectric body, and the piezoelectric resonance characteristics were evaluated by impedance measurement.
[0055]
FIG. 8 shows the impedance characteristics. In 1.4 GHz to 1.5 GHz, a resonance peak at which the impedance is minimized and an anti-resonance peak at which the impedance is maximized were observed. No peaks due to other spurious vibrations were observed, and only single mode vibrations were obtained.
[0056]
Comparative Example The resonator manufactured in Example 1 has the same manufacturing method and evaluation method, but the upper electrode and the lower electrode are not comb-shaped, and the first and second squares are usually 15 μm square. A piezoelectric resonator having two electrodes was produced.
[0057]
FIG. 9 shows the impedance characteristics. A peak due to spurious vibration was observed between the resonance and anti-resonance peaks.
[0058]
【The invention's effect】
In the piezoelectric resonator of the present invention, the first and second electrodes are configured by electrically connecting a plurality of long electrodes arranged in parallel with each other, and the first electrode and the second electrode are connected to each other. Since the long electrode of the electrode is crossed when viewed from the substrate side, it vibrates only at the overlapping part of the electrode, and the thickness of the piezoelectric body and the electrode size can be made comparable. Generation of vibration can be suppressed, and a small resonator without unnecessary vibration can be realized.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a piezoelectric resonator according to the present invention.
2 is a plan view of FIG. 1. FIG.
FIG. 3 is a cross-sectional view showing another example of the piezoelectric resonator of the present invention.
FIG. 4 is a cross-sectional view showing a piezoelectric resonator of the present invention having a vibration reflecting layer.
FIG. 5 is a cross-sectional view of a piezoelectric resonator according to the present invention having two layers of ferroelectric materials whose signs of temperature coefficient of sound velocity are opposite to each other.
FIG. 6 is a cross-sectional view of a piezoelectric resonator of the present invention having two layers of ferroelectric materials whose polarization directions are opposite to each other.
7 is a diagram showing impedance characteristics of a piezoelectric resonator having a long electrode width of 7 μm in Example 1. FIG.
8 is a diagram showing impedance characteristics of the piezoelectric resonator of Example 2. FIG.
FIG. 9 is a diagram illustrating impedance characteristics of a piezoelectric resonator according to a comparative example. .
FIG. 10 is a cross-sectional view of the structure of a conventional piezoelectric resonator.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 ... Base | substrate 12 ... Support film 13 ... 1st electrode 14 ... Piezoelectric body 15 ... 2nd electrode 16a, 16b ... Elongated electrode 18 ... Vibrating part 32a, 32b ... Insulator 32 ... Vibration reflection layer A ... Vibration space

Claims (3)

A base having a vibration space; a support film formed on the surface of the base so as to cover the vibration space; and a surface of the support film opposite to the vibration space; A piezoelectric resonator including a vibrating body sandwiched between two electrodes, wherein the first electrode and the second electrode are formed by electrically connecting a plurality of long electrodes arranged in parallel with each other. In addition, the elongated electrode of the first electrode and the elongated electrode of the second electrode intersect with each other when viewed from the substrate side, and the width of the elongated electrode is 3 to 10 μm. A piezoelectric resonator characterized by the above. A substrate, a vibration reflecting layer formed on the surface of the substrate and formed by laminating a plurality of insulators, and a vibrating member formed on the surface of the vibration reflecting layer and sandwiching the piezoelectric body between the first electrode and the second electrode, The first electrode and the second electrode are each formed by electrically connecting a plurality of elongated electrodes arranged substantially in parallel, and the first electrode and the second electrode The long electrode and the long electrode of the second electrode intersect with each other when viewed from the substrate side, and the width of the long electrode is 3 to 10 μm . 3. The piezoelectric resonator according to claim 1, wherein the piezoelectric body is made of a ferroelectric material.

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