JP2000286668A - Piezoelectric resonator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a piezoelectric resonator which has higher mechanical strength than amorphous SiO2 and an extremely small temperature coefficient, shows a high Q value and can be easily produced by providing a vibrator having electrodes formed on both surfaces of a piezoelectric thin film on the surface opposite to a recess part of a substrate thin layer part. SOLUTION: A recess part 2 is formed on a substrate 1 to form a substrate thin layer part 3, and a vibrator 7 having a lower electrode 5 formed on the under surface of a piezoelectric thin film 4 and a pair of upper electrodes 6 formed on the top surface of the film 4 is provided on the surface opposite to a recess part 2 of the part 3. The substrate 1 consists of quartz, namely crystalline SiO2, and the part 2 is formed by etching the substrate 1 to provide a vibrating space A. By using quartz being crystalline SiO2 as the part 3 which supports the vibrator 7, satisfactory strength is attained for the part 3 without deteriorating the temperature characteristics of the resonance frequency and also a large Q value is obtained at high frequencies exceeding 1 GHz.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric resonator,
The present invention relates to a piezoelectric resonator utilizing resonance of thickness longitudinal vibration of a piezoelectric thin film.

[0002]

2. Description of the Related Art As the frequencies used for wireless communication and electric circuits have become higher, filters used for these electric signals have been developed to correspond to higher frequencies.

[0003] In particular, attention has recently been paid to an S-wave which uses the resonance of a surface acoustic wave, which is an acoustic wave transmitted on the surface of a solid.
This is a filter using an AW resonator. This filter uses the resonance of a high-frequency electric field applied between comb-shaped electrodes formed on a solid surface and surface acoustic waves, and
Filters having a resonance frequency up to about Hz have been manufactured.

However, in the SAW filter, the inter-electrode distance is inversely proportional to the resonance frequency. Therefore, in a frequency region exceeding 1 GHz, the inter-electrode distance is on the order of submicrons, and it is very difficult to manufacture electrodes. Was.

[0005] In the future, the frequency of electromagnetic waves used for wireless communication is expected to become higher and higher, and several GHs are already in use.
Since there is a movement to establish a standard of z or more, an inexpensive and high-performance filter corresponding to those frequencies is required.

In response to such demands, a resonator utilizing the resonance of a piezoelectric thin film has been proposed. This is a resonator using the fact that the piezoelectric thin film causes vibration in response to an input high-frequency electric signal, and the vibration causes resonance in the thickness direction of the piezoelectric thin film.

[0007] Since this resonator uses an elastic wave propagating in a solid instead of a surface acoustic wave, it is called a bulk acoustic wave resonator (hereinafter referred to as BAWR). Since it is possible to control the thickness of the piezoelectric thin film constituting the BAWR with an accuracy of submicron or less, it is expected that a resonator having a higher resonance frequency than the SAW filter can be manufactured. Development has been advanced.

As shown in FIG. 2, a conventional BAWR includes a base 11 and a support film 1 formed on the surface of the base 11.
3 and a buffer layer 15 formed on the support film 13
And a lower electrode 16 formed on the buffer layer 15
And a piezoelectric thin film 17 formed on the lower electrode 16;
It comprises a pair of upper electrodes 18 formed on the piezoelectric thin film 17 (see US Pat. No. 4,320,365). The support film 13 is formed on the upper surface of the base 11 so as to cover the vibration space A.

In a conventional BAWR, ZnO, AlN, CdS, etc. are used as a piezoelectric thin film material, Si is mainly used as a base material, Al and Au are used as electrode materials, and the piezoelectric thin film is supported. Amorphous SiO ₂ has been used as a support film.

For example, Japanese Patent Application Laid-Open No. 60-68710 discloses the use of ZnO, AlN, CdS as a piezoelectric thin film material, Si as a base material, Al and Au as electrode materials, and amorphous SiO ₂ as a support film material. I have.

[0011] Amorphous SiO ₂ is used as a support film. This is because amorphous SiO ₂ can be easily formed on a Si substrate and is described in the literature (Electronics Lett).
ersvol.17, No.14, pp.507-509 (1981)), amorphous SiO ₂ has a temperature coefficient with the opposite sign to the elastic temperature coefficient of the piezoelectric thin film, so that the resonance frequency of the resonator This can compensate for the change in.

[0012]

However, this B
Since the AWR obtains resonance by the propagation of vibration, not only the vibration characteristics of the piezoelectric thin film but also the vibration characteristics of the support film supporting the piezoelectric thin film greatly affect the characteristics of the resonator.

In order to operate at a frequency of 2 GHz, which is becoming mainstream at present, the thickness of the piezoelectric thin film regulated by the resonance frequency is about 1 μm, and the vibrating portion formed by the piezoelectric thin film and the supporting film has In order to generate a second-order standing wave that is most strongly excited and obtain a large electromechanical coupling coefficient, the amorphous SiO ₂ as the supporting film is also 1 μm in size.
Although a film thickness of about m is required, amorphous SiO ₂ has a low strength, so that it is difficult to stably form it at 1 μm or less, and it is difficult to form a resonator structure.

Further, in order to obtain strength as a support film, S
When a higher-order resonance is used as a composite resonator by increasing the film thickness of iO ₂ , for example, when a third-order resonance is used,
Although the thickness of O ₂ is about 2 μm, there is a problem that the Q value of the resonator is greatly reduced. This includes that the size of the ultrasonic absorption of amorphous SiO ₂ is fundamentally large, since the magnitude of the ultrasonic absorption increases in proportion to the square of the frequency, it size of SiO ₂ is at a high frequency such as 2GHz This is because the ultrasonic absorption power greatly reduces the Q value of resonance.

For example, when comparing with the attenuation constant α representing the magnitude of ultrasonic absorption, the attenuation constant α of fused silica is larger by two orders of magnitude than the attenuation constant α of quartz regardless of the frequency. Since the amorphous SiO ₂ film has a larger attenuation constant than that of fused quartz, a reduction in the Q value is essential.

Since the attenuation constant is proportional to the square of the frequency, it increases by two digits or more at 2 GHz as compared with the value at 100 MHz. As described above, an amorphous SiO ₂ film having a film thickness twice or more that of a piezoelectric thin film having a thickness of about 1 μm has a large absorption of ultrasonic waves at a high frequency such as 2 GHz, and the Q There is a problem that the value is greatly reduced.

Heretofore, there has been known a piezoelectric resonator in which a support film made of amorphous SiO ₂ is formed on a base made of Si, and the base is etched while leaving the support film thick enough to reinforce the support film. The vibrating part has a piezoelectric thin film,
Although the strength is improved by the support film and the Si film, the strength of the support film and the Si film is improved.
m, and the thickness of Si, which is a non-piezoelectric material, becomes large, so that the piezoelectric vibration is attenuated and the Q value of the resonator is reduced.

[0018]

Means for Solving the Problems As a result of intensive studies on the above-mentioned problems, the present inventors have found that a thin base layer on which a vibrating body is formed has a mechanical strength larger than that of amorphous SiO _{2 and} a temperature coefficient of resonance frequency. By using a crystal which is extremely small and has low ultrasonic absorption, it is easy to manufacture and has a high Q
The present inventors have found that a piezoelectric resonator exhibiting a value can be produced, and have reached the present invention.

Further, since the temperature coefficient of the resonance frequency can be reduced to zero and the electromechanical coupling coefficient is large as the piezoelectric thin film, the use of a Pb-containing perovskite oxide ferroelectric material capable of broadening the bandwidth is achieved. The present inventors have found that a piezoelectric resonator exhibiting a high Q value can be manufactured while keeping the temperature coefficient of the resonance frequency of the entire resonator small, and have reached the present invention.

That is, in the piezoelectric resonator of the present invention, a concave portion is provided in a substrate made of quartz, a thin substrate portion is formed on the bottom surface of the concave portion, and a piezoelectric thin film is formed on a surface of the thin substrate portion opposite to the concave portion. A vibrating body having electrodes formed on both surfaces of a body thin film is provided. Here, it is desirable that the piezoelectric thin film is a perovskite-type ferroelectric containing lead.

[0021]

[Action] In the piezoelectric resonator of the present invention, as the base thin-layer portion which supports a vibrating body consisting of a pair of electrodes and the piezoelectric thin film, by using a quartz crystal which is SiO ₂ crystalline, the temperature characteristics of the resonance frequency Without deterioration, sufficient strength can be realized as a thin base layer portion, and a large Q value can be realized at a high frequency exceeding 1 GHz as compared with an amorphous SiO ₂ film.

That is, since the base thin layer portion is made of crystalline quartz, which is crystalline SiO ₂ , the strength is high and a thin layer portion of about 2 μm can be formed. Also, the sound velocity is 2 to 3 of the piezoelectric thin film.
When used for high frequency of 2 GHz,
0.7 to 1 thickness of piezoelectric thin film defined from resonance frequency
The thickness can be 2 to 3 times the thickness of μm, and a secondary wave that is most strongly excited in the vibrating portion constituted by the base thin layer portion and the piezoelectric thin film can be generated. Further, quartz has a small attenuation of ultrasonic waves, so that the electromechanical coupling coefficient is hardly reduced, and a large Q value can be realized at a high frequency exceeding 1 GHz as compared with a conventional amorphous SiO ₂ film.

Conventionally, a support film is formed on a substrate,
Although the thin film piezoelectric resonator was formed by etching the base from the side where the support film was not formed, in the present invention, the base was formed of quartz, and the recess was formed by etching the base so as to leave the base thin layer portion. Is formed, the base and the support film (base thin layer portion) are integrated, and the manufacturing process is simplified.

Since it is difficult to form quartz on the surface of the substrate, the present invention employs a structure in which the substrate and the substrate thin layer are integrated. Thereby, the piezoelectric resonator can be easily manufactured.

In the piezoelectric resonator of the present invention, since the temperature characteristic of the resonance frequency of the thin-film base portion made of quartz is almost zero, the vibrating body is formed by using a lead-containing perovskite ferroelectric material as the piezoelectric thin film. The temperature change rate of the resonator can be made zero without compensation by the base thin layer portion that supports. It is known that a perovskite ferroelectric containing lead exhibits large piezoelectricity at a composition phase boundary called MPB. However, since it is a boundary between two different crystal forms at the same time, by controlling the composition, a negative effect is obtained. Can be shown. For this reason, not only the temperature coefficient of the ferroelectric film itself is reduced to zero, but also the temperature coefficient of the thin-film base layer made of quartz and the electrode, which has a slightly positive Can be set to zero.

Further, since the lead-containing perovskite ferroelectric has a large electromechanical coupling coefficient, ZnO, Al
A wider band can be achieved as compared with a conventional piezoelectric thin film such as N.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In a piezoelectric resonator according to the present invention, as shown in FIG. 1, a concave portion 2 is formed in a substrate 1 to form a substrate thin layer portion 3, and the concave portion 2 of the substrate thin layer portion 3 is formed. On the opposite surface, a lower electrode 5 is provided on the lower surface of the piezoelectric thin film 4 and a pair of upper electrodes 6 is provided on the upper surface.
Is provided.

The substrate 1 is made of crystalline quartz, which is crystalline SiO ₂ , and the concave portion 2 is formed by etching, so that the vibration space A is formed.

The piezoelectric thin film 4 includes ZnO, AlN, Cd
S, although PbTiO ₃ or the like is used, it is desirable that the main component PbTiO ₃ for reasons such as electromechanical coupling coefficient of thickness longitudinal vibration is large. For example, in the piezoelectric thin film 4,
A perovskite ferroelectric thin film containing Pb, Zr, and Ti is used. The piezoelectric thin film 4 containing PbTiO ₃ as a main component can exhibit large piezoelectricity by orienting the crystal axis in the c-axis direction at the time of film formation. When the piezoelectricity is weak, a DC voltage is applied. Alternatively, piezoelectricity may be imparted.

The electrodes 5 and 6 sandwiching the piezoelectric thin film 4 are made of a metal material having relatively low reactivity, such as Al, Pt, and Au, which are conventionally used. Piezoelectric thin film 4
Considering the reaction with Pt, Pt with low reactivity is desirable as the electrode material. Also, Al is desirable for the upper electrode 6 because the mass load on the vibrating body 7 is small and the resistivity is small.

In the piezoelectric resonator constructed as described above, a part of the single-crystal quartz base 1 is etched to form the base thin layer 3 having a thickness capable of supporting the vibrating body 7. Lower electrode 5, piezoelectric thin film 4, upper electrode 6 on the surface of thin layer portion 3.
Are sequentially formed to form the vibrating body 7.

The vibrating body may be one in which piezoelectric thin films and electrodes are alternately laminated.

[0033]

First, a quartz substrate is etched by a reactive ion etching method. C ₂ F ₆ (60%) and Ar (4
(0%), and a concave portion was formed in the substrate using a Si mask to form a substrate thin layer portion. The thickness of the base thin layer portion was 2.3 μm.

Next, a Pt film having a thickness of 100 nm and a thickness of 100 nm was formed on the surface of the base thin layer portion by magnetron sputtering at 500 ° C.
A film (lower electrode) was formed, and the composition was changed to Pb _1.06 Ba _0.09 {(Co _0.16 Yb _0.26 Nb) by the sol-gel method.
_0.58 ) _0.1615 Cr _0.0085 ｝ (Zr _0.41 Ti _0.59 ) _0.83 O
A PZT thin film of No. ₃ was formed.

The piezoelectric thin film is first coated with a solution having the above composition on a Pt film by spin coating, and then heat-treated at 380 ° C. for 60 seconds. 6 solution application and heat treatment
After repetition twice, it was baked at 700 ° C. for 15 minutes to crystallize. The process from solution application to crystallization was repeated once again to obtain a 0.72 μm thick ferroelectric thin film.

Next, an Al electrode (upper electrode) having a thickness of 50 nm was formed on the upper surface of the piezoelectric thin film at 200 ° C. by RF magnetron sputtering using a mask method.

Next, a polarization treatment was performed using a microprobe and a DC power supply. Since the resonator structure is equivalent to a structure in which two resonators are connected in series as shown in FIG. 1, it is necessary to apply an electric field that is at least twice the coercive electric field. A DC voltage of 30 V was applied at room temperature for 30 seconds to perform a polarization treatment to obtain a thin film piezoelectric resonator as shown in FIG.

The piezoelectric resonance characteristics were evaluated by impedance measurement. The anti-resonance Q value of the resonator was determined using resistance and reactance. The temperature coefficient of the resonance frequency is
The test was performed using an RF probe incorporating a hot chuck. The temperature coefficient is calculated by setting the resonance frequency at 25 ° C to fr ₂₅
When the resonance frequency of is set to fr ₈₅ , (fr ₈₅ −fr ₂₅ ) /
It was defined as fr ₂₅ · ΔT (ΔT = 85 ° C.-25 ° C.).

The resonator thus manufactured has a main resonance (2
Resonance frequency fr = 2.1 GHz, anti-resonance Q
= 620, temperature coefficient -2 ppm / ° C, and electromechanical coupling coefficient Kt = 0.25.

As a comparative example, an amorphous SiO ₂ film having a thickness of 2.5 μm was formed on a (100) plane Si substrate.
After the Si substrate was etched with KOH until the SiO ₂ film was exposed to form a concave portion, an electrode and a piezoelectric thin film were formed in the same manner as in the above embodiment to form a resonator.
Was 230.

[0041]

According to the piezoelectric resonator of the present invention, quartz is used as the base thin layer portion for supporting the vibrating body, because it has a high mechanical strength, a very small temperature coefficient of the resonance frequency, and a small ultrasonic attenuation. A piezoelectric resonator exhibiting a high Q value, which cannot be obtained with a conventional amorphous SiO ₂ support film, can be obtained. In addition, by using a lead-containing perovskite ferroelectric material having a large electromechanical coupling coefficient and a small temperature coefficient of the resonance frequency as the piezoelectric thin film, a piezoelectric resonance having a large frequency difference between the resonance frequency and the antiresonance frequency and a small temperature change rate is used. You can get a child.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a piezoelectric resonator of the present invention.

FIG. 2 is a cross-sectional view showing a conventional piezoelectric resonator.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Base 2 ... Depression 3 ... Base thin layer part 4 ... Piezoelectric thin film 5 ... Lower electrode 6 ... Upper electrode 7 ... Vibrating body

Claims (2)

[Claims] A concave portion is formed in a substrate made of quartz, a thin substrate portion is formed on the bottom surface of the concave portion, and electrodes are formed on both surfaces of the piezoelectric thin film on a surface of the thin substrate portion opposite to the concave portion. A piezoelectric resonator comprising a formed vibrator. 2. The piezoelectric resonator according to claim 1, wherein the piezoelectric thin film is a perovskite ferroelectric containing lead.