JP2000278078A - Piezoelectric resonator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a piezoelectric resonator wherein a temperature change ratio is low and high resonance frequencies which cannot be obtained by any conventional supporting film constituted of SiO2 is obtained. SOLUTION: This piezoelectric resonator is provided with a substrate 1 having a vibrating space A, a supporting film 2 formed on the surface of the substrate 1 for covering the vibrating space A, and a vibrator 3 constituted by forming electrodes 5 and 6 on the both faces of a piezoelectric body thin film 4, and arranged at the position of the supporting film 2 faced to the vibrating space A. In this case, the supporting film 2 is formed as a laminated film constituted of an SiO2 film 8 and a diamond film 9, and the piezoelectric body film 4 is formed as a c-axially oriented ZnO2 film.

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 vibrating body having electrodes formed on both surfaces 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.

Further, in the above document, when ZnO is used as the piezoelectric thin film, when the thickness ratio of SiO _{2 to} ZnO is 0.5 for the fundamental wave, it is 0.25 for the secondary wave, and 0.7 for the secondary wave.
It has been reported that a zero temperature coefficient can be obtained at 5, 1.25 and a high electromechanical coupling coefficient kt can be obtained.

[0013]

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.

That is, in the conventional BAWR used at a frequency of 1 GHz or less, the thickness of ZnO is 2.5 μm or more. However, as reported in the above-mentioned document, SiO ₂ is used to realize a zero temperature coefficient. And amorphous SiO
The film thickness of ₂ is 0.25 to 1.25 times the film thickness of ZnO. However, if the film thickness of SiO ₂ is too small, the strength as a support film decreases, and the film cannot be used as a BAW resonator. The thickness of SiO ₂ used was as thick as about 2 to 3 μm.

Since SiO ₂ is amorphous, the attenuation of vibration, that is, insertion loss is large, and when the film thickness is increased to obtain a zero temperature coefficient, the path as an acoustic medium becomes large. There was a problem of becoming larger.

In order to operate at a frequency of 2 GHz, which is becoming mainstream at present, the thickness of ZnO is 1.3 μm.
m, but in order to compensate for the elastic temperature coefficient of ZnO and achieve a zero temperature coefficient, SiO ₂ must have a film thickness of 1 μm or less, and insertion loss is reduced, but it is amorphous. It was difficult to form SiO ₂ stably below 1 μm, and it was difficult to manufacture the resonator itself. On the other hand, when the thickness of ZnO is set to about 1.3 μm and the thickness of SiO ₂ is increased, S
There is a problem that the elastic property of iO ₂ becomes dominant, the loss becomes large, the temperature coefficient becomes large, and the electromechanical coupling coefficient becomes small due to the higher-order mode.

In order to solve the above problems, it is necessary to reduce the thickness of the amorphous SiO ₂ as much as possible to realize a zero temperature coefficient, increase the strength of the support film, and further reduce the temperature coefficient. Was.

[0018]

According to the present invention, there is provided a piezoelectric resonator comprising:
A substrate having a vibration space, a support film formed on the surface of the substrate and covering the vibration space, and a piezoelectric thin film formed on the support film so as to face the vibration space via the support film. And a vibrator having electrodes formed on both surfaces thereof, and the support film is a laminated film of a SiO ₂ film and a diamond film. Here, it is desirable that the diamond film faces the vibration space. Also,
It is desirable that the piezoelectric thin film is a ZnO film with c-axis orientation.

[0019]

In the piezoelectric resonator according to the present invention, the two-layer laminate of the SiO ₂ film and the diamond film is used as the supporting film. Therefore, the strength of the supporting film can be improved by the high-strength diamond film, Since the film has a very small temperature change rate of the resonance frequency, the temperature change rate of the resonance frequency as the support film is dominated by the SiO ₂ film, and the temperature change rate is almost equal to that of the SiO ₂ film, and a high-strength support film is obtained. be able to.

That is, for example, a piezoelectric thin film made of ZnO has a positive temperature coefficient, and amorphous SiO ₂ has a negative temperature coefficient. However, in the piezoelectric resonator of the present invention, the supporting film is made of a SiO ₂ film and a diamond. Since the diamond film itself has a large elastic constant and a very small elastic temperature change of 14 ppm / ° C., the temperature change rate of the resonance frequency of the diamond film is almost zero. The temperature coefficient is negatively influenced by the temperature coefficient of the SiO ₂ film, and becomes vibrated by the piezoelectric thin film made of ZnO having a positive temperature coefficient and the supporting film having a negative temperature coefficient (the supporting film and the vibrating body). ) Can bring the temperature coefficient of the resonance frequency close to zero.

Further, the conventional support film made of SiO ₂ has a limit of thinning in order to obtain sufficient mechanical strength. For example, a combination of a piezoelectric thin film made of ZnO and a support film made of SiO ₂ is required. A piezoelectric resonator having a resonance frequency exceeding 1.5 GHz could not be obtained,
_When a two-layer laminate of a two-layer film and a diamond film is used as a support film, the diamond film itself has high strength.
_{Even if the two} films are thinned, the structure as a support film can be maintained.

Further, since the diamond film has a sound speed approximately five times as high as the sound speed of the vibrating portion composed of the ZnO piezoelectric thin film and the SiO ₂ support film, the ZnO and S
When the total thickness of iO ₂ is about 1 μm, the diamond film may be about 5 μm. At this time, ZnO / SiO ₂ /
A tertiary standing wave is generated in the vibrating portion made of diamond, and the electromechanical coupling coefficient increases.

Further, in order to obtain a large electromechanical coupling coefficient, the thickness of the diamond film is small and ZnO / SiO
It is desirable that a secondary standing wave be generated in the vibrating portion constituted by ₂ . When the thickness of the diamond film is about 1 μm, the ZnO / Si
It is possible to generate a secondary wave that is most strongly excited in the vibrating section constituted by O ₂ . By using a diamond film, the critical thickness can be reduced while improving the mechanical strength of the support film. For this reason, a higher resonance frequency can be realized than in the case of the related art.

It is desirable that a diamond film faces the vibration space. This is because the diamond film has high strength and can sufficiently support the support film.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1, a piezoelectric resonator according to the present invention comprises a base 1 having a vibration space A, and a support disposed on the base 1 so as to cover the vibration space A. It comprises a membrane 2 and a vibrating body 3 arranged at the position of the supporting film 2 facing the vibration space A. The vibrating body 3 is composed of a lower electrode 5 on the lower surface of the piezoelectric thin film 4 and a pair of lower electrodes 5 on the upper surface. Upper electrode 6
Is formed.

The substrate 1 is made of, for example, silicon, and a vibration space A is formed by etching. The vibration space A of the base 1 refers to a space in which the vibration of the vibrating body 3 is not transmitted to the base 1, and a through hole is formed in the base 1 or a concave portion is formed in a portion of the base 1 where the support film is formed. It is formed by this.

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. In addition, c-axis oriented ZnO is desirable from the viewpoint that the temperature coefficient of the entire resonator approaches zero. Since a ZnO piezoelectric material develops piezoelectricity only in the c-axis direction, it needs to be a c-axis oriented film in a BAW resonator using thickness longitudinal vibration.

The piezoelectric thin film 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. It may be applied to impart piezoelectricity.

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 used more frequently than before. Piezoelectric thin film 4
Considering the reaction with Pt, Pt with low reactivity is desirable as the electrode material.

Further, in the piezoelectric resonator of the present invention, the support film 2 is constituted by a laminated film of the SiO ₂ film 8 and the diamond film 9. The thickness t ₁ of the SiO ₂ film 8 is equal to the thickness t _{2 of the} ZnO film.
It is desirable that the ratio (t ₁ / t ₂ ) of the thickness t ₁ of the SiO ₂ film to the ratio (t ₁ / t ₂ ) ≦ 0.25 ≦ (t ₁ / t ₂ ) ≦ 0.75. This is the case when (t ₁ / t ₂ ) <0.25 or 0.75 <(t
_{When 1} / t ₂ ) ≦ 1.25, ZnO / SiO ₂ has the same negative temperature coefficient as diamond and deteriorates temperature characteristics. When (t ₁ / t ₂ )> 1.25, SiO
_{Although 2} / ZnO shows a positive temperature coefficient, the thickness of the SiO ₂ film 8 is large, the absorption of ultrasonic waves is large, and the loss is large.

The SiO ₂ film of the present invention may be amorphous or crystalline, but amorphous is preferable because it is easier to manufacture. The diamond film 9 may be amorphous or crystalline, but is preferably crystalline in order to prevent a decrease in Q.

The thinner the film thickness t ₃ of the diamond film 9 is, the better it is.
The above is necessary. Since the RTH, ZnO piezoelectric thin film has a small relative dielectric constant, to match the impedance determined by the capacitance of the resonator to 50Ω, several hundreds of
A free-standing film having a size of μm □ is required, and a film thickness of 1 μm is required to stably produce a free-standing film of this size.

In the piezoelectric resonator of the present invention, since the two-layer laminate of the SiO ₂ film 8 and the diamond film 9 is used as the support film 2, the strength of the support film 2 can be improved by the high-strength diamond film 9. it is, because the diamond film 9 has an extremely small temperature coefficient of the resonant frequency, the temperature change rate is SiO ₂ film in the resonance frequency of the supporting film 2 8
And the temperature change rate is almost equal to that of the SiO ₂ film 8,
A high-strength support membrane can be obtained.

The piezoelectric thin film 4 made of ZnO has a positive temperature coefficient and amorphous SiO ₂ has a negative temperature coefficient. However, in the piezoelectric resonator of the present invention, the support film 2 is made of Si.
The diamond film 9 is a two-layer laminate of the O ₂ film 8 and the diamond film 9, and the temperature change rate of the resonance frequency of the diamond film 9 is almost zero, so that the temperature coefficient of the support film 2 is lower than that of the SiO ₂ film 8. Dominated and negative, Z with positive temperature coefficient
With the piezoelectric thin film 4 made of nO and the support film 2 having a negative temperature coefficient, the temperature coefficient of the resonance frequency of the vibrating part (the support film and the vibrator) can be made close to zero.

[0035]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, Si (10
0) A diamond thin film is formed on a substrate. The growth conditions are as follows, using a mixed gas of CH ₄ , CO ₂ , and H ₂ under reduced pressure.
A microwave having a thickness of 1 μm was produced by inputting microwaves at 6 KW.

The properties of natural diamond are reported to have a Young's modulus of 1.2 × 10 ¹² N / m ² , a density of 3.51 g / cm ³ , and a sound speed of 18,500 m / s. The characteristics of the prepared diamond thin film is that the density is 3.4 g / c.
m ³ , Young's modulus was 9.6 × 10 ¹¹ N / m ² , and sound velocity was 16800 m / s. Although this is slightly smaller than natural diamond, it is about three times as high as the sound velocity of SiO ₂ (fused quartz) of 5700 m / s, and is even higher than the amorphous SiO ₂ film. .

Next, Si is etched from the back side of the substrate to form a via hole reaching the diamond film.
The diamond film used here is characterized by being crystalline and having small internal residual stress. for that reason,
Even with a film thickness of 1 μm, a self-standing film can be formed without self-destruction due to residual stress.

An SiO ₂ film is formed on the diamond diaphragm thus manufactured by a thermal CVD method.
The thickness of the SiO ₂ film was 1.0 μm. Next, a Pt lower electrode layer, a ZnO piezoelectric thin film, and an Al upper electrode layer are sequentially stacked by magnetron sputtering. The growth temperature is 500 ° C. for the Pt electrode layer and 200 ° C. for both the piezoelectric thin film and the Al electrode layer.

The film thickness is 1 for both the lower electrode layer and the upper electrode layer.
00 nm, and the thickness of the piezoelectric thin film is 1.3 μm. Further, by controlling the thickness of these thin films, the magnitude of the resonance frequency can be controlled.

The evaluation was performed by impedance measurement in the resonator structure shown in FIG. 1.7 is measured by using an RF impedance analyzer and an RF wafer microprobe to measure the frequency characteristics of impedance.
A piezoelectric resonance (anti-resonance) was obtained at GHz.

A 1.5 μm SiO ₂ film was formed on a Si (100) substrate by thermal CVD, and the Si was etched from the back side of the substrate to form a via hole reaching the SiO ₂ film. A free-standing film could not be formed.

As described above, the thin-film piezoelectric resonator using the SiO ₂ / diamond support film of the present invention has a smaller thickness of the SiO ₂ film than the thin-film piezoelectric resonator using the SiO ₂ support film. Since the film can be formed, the thickness of the ZnO piezoelectric material that determines the resonance frequency can be reduced, and a resonator having a high resonance frequency can be formed.

[0043]

According to the piezoelectric resonator of the present invention, since the two-layer laminate of the SiO ₂ film and the diamond film is used as the support film, the strength of the support film can be improved by the high-strength diamond film. Since the temperature change rate of the resonance frequency of the diamond film is extremely small, the temperature change rate of the resonance frequency as the support film is dominated by the SiO ₂ film,
The temperature change rate is almost the same as that of the SiO ₂ film, and a high-strength support film can be obtained. Accordingly, for example, when a piezoelectric thin film made of ZnO is used, the vibrating portion (the supporting film and the vibration film) is formed by the piezoelectric thin film made of ZnO having a positive temperature coefficient and the support film having a negative temperature coefficient. The temperature coefficient of the resonance frequency of the body can be close to zero. Therefore, it is possible to obtain a piezoelectric resonator having a small temperature change rate and a high resonance frequency that cannot be obtained with a conventional support film made of SiO ₂ .

[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]

1 ... base 2 ... support film 3 ... vibrator 4 ... piezoelectric film 5 ... lower electrode 6 ... upper electrode 8 ... SiO ₂ film 9 ... diamond film A: Vibration space

Claims (3)

[Claims] 1. A base having a vibration space, a support film formed on the surface of the base and covering the vibration space, and formed on the support film so as to face the vibration space via the support film. And a vibrator having electrodes formed on both surfaces of a piezoelectric thin film, and the support film is a laminated film of a SiO ₂ film and a diamond film. 2. The piezoelectric resonator according to claim 1, wherein the diamond film faces the vibration space. 3. The piezoelectric resonator according to claim 1, wherein the piezoelectric thin film is a c-axis oriented ZnO film.