JP2001102902A - Piezoelectric resonator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a piezoelectric resonator with a high Q value which is formed of a piezoelectric body with good electric insulation by employing as a base film a diamond thin film having superior sound characteristics and suitable for a thin film and forming a vibration body on the base film with good contactness. SOLUTION: The piezoelectric resonator is equipped with a base body 1 which has a vibration space, the base film 2 made of diamond formed on a top surface of the base body 1 so that the vibration space A is covered, a contact layer 3 which is formed in order thereupon, and the piezoelectric body 6 which is sandwiched between a pair of electrodes 5 and 7, and the contact layer 3 is made of an insulator.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a portable telephone and a wireless LAN.
It relates to a piezoelectric resonator used for AN and the like.
The present invention relates to a piezoelectric resonator utilizing resonance of a thickness longitudinal vibration of a piezoelectric body.

[0002]

2. Description of the Related Art In recent years, the frequency used for wireless communication and electric circuits has been increasing, and accordingly, filters used for these electric signals have been required to be compatible with high frequencies. Development is taking place.

Recently, a resonator called a bulk acoustic wave resonator (BAWR) has been developed. This is a resonator that uses a piezoelectric thin film to vibrate in response to an input high-frequency electric signal and causes the vibration to resonate in the thickness direction of the thin film. It is expected to be applied to resonators that have it.

[0004] As shown in FIG.
A support film 13 formed on the surface of the base 11; a metal layer 15 formed on the support film 13; a first electrode 16 formed on the metal layer 15; It comprises a piezoelectric body 17 formed thereon and two second electrodes 18 formed on the piezoelectric body 17 (for example, USP
4,320,365).

The support film 13 is formed on the upper surface of the base 11 so as to cover the space A, and the piezoelectric body 17 sandwiched between the electrodes 16 and 18 and the portion of the support film 13 in contact with the space A vibrate. Become. That is, since the metal layer 15, the first electrode 16, the piezoelectric body 17, and the second electrode 18 vibrate integrally, the supporting film 13 supporting these layers is required to have strength.

Therefore, considering the use in the high frequency range of GHz, a material having a thin supporting film 13 and a high sound speed is desired. A piezoelectric resonator using a supporting film made of diamond as a material having a high sound speed has been proposed. JP-A-10-209
No. 794.

[0007] In this publication, P is directly deposited on a diamond film.
When the t electrode is formed, the reactivity between diamond and Pt is low, so that the adhesion is low and the diamond is easily peeled. Therefore, a Ti thin film is formed as the metal layer 15 in order to improve the adhesion to diamond.

[0008]

However, Ti disclosed in Japanese Patent Application Laid-Open No. Hei 10-209794 is a metal and has conductivity, but the conductivity of Ti is lower than that of an electrode material made of Pt. Because of its high content, Ti substantially exists as a resistor, and the flowing current causes deterioration of characteristics. Therefore, there has been a problem that the Q value of the resonator decreases due to the resistance of Ti, and the characteristics of the piezoelectric resonator deteriorate.

Accordingly, it is an object of the present invention to provide a highly reliable piezoelectric resonator which can firmly attach a vibrator to a support film made of diamond, has a large Q value, and has a high Q value.

[0010]

According to the present invention, there is provided a piezoelectric resonator comprising:
A substrate having a vibration space, a support film made of diamond formed on the surface of the substrate so as to cover the vibration space, an adhesion layer formed on the support film, and a support film formed on the adhesion layer A piezoelectric resonator comprising a piezoelectric body sandwiched between a pair of electrodes, wherein the adhesion layer is made of an insulator. By adopting such a configuration, the insulator and the diamond form a reaction layer, and the adhesion strength between the support film and the adhesion layer is increased, and no current flows through the adhesion layer made of the insulator. It does not cause a decrease in the Q value, and high characteristics can be obtained as the piezoelectric resonator.

In particular, when the insulator is a metal oxide, the surface of the diamond reacts with oxygen when the metal oxide is formed on the diamond, and the surface roughness of the diamond surface increases. The adhesion strength of the metal oxide to the metal becomes high.

It is desirable that the metal oxide be a sintered thin film having an average particle size of 5 to 20 nm. Thereby, when forming the electrode thin film on the adhesion layer, the density of crystal nuclei is increased, and a dense and smooth electrode film is obtained. As a result, the crystallinity of the piezoelectric body is improved.

Further, it is desirable that the metal oxide is a perovskite-type composite oxide containing at least Pb and Ti as metal elements. The use of such metal oxide crystal particles as an adhesion layer has the function of enhancing the crystallinity of the electrode formed on the adhesion layer, and forms a piezoelectric film on such a dense and smooth electrode film. Accordingly, the insulating property of the piezoelectric body can be improved.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In a piezoelectric resonator according to the present invention, as shown in FIG. 1, a support film 2 is formed on a substrate 1, an adhesion layer 3 is formed on the upper surface of the support film 2, and an adhesion layer 3 is formed. A vibrating body 4 is formed on the upper surface of. The vibrating body 4 has electrodes 5, 7
And a piezoelectric body 6. This piezoelectric body 6 is polarized in the thickness direction. A vibration space A is provided on the side of the support film 2 opposite to the surface on which the vibration body 4 is formed.

The support film 2 is made of diamond. Since diamond has high hardness and can reduce residual stress, it is self-supporting even with a small film thickness. In addition, since the sound speed is high, deterioration of the resonator characteristics due to the support film such as a decrease in the resonance frequency, a decrease in the electromechanical coupling coefficient, and an increase in the loss can be reduced.

The thickness of the support film 2 is preferably 0.5 to 10 μm. This is because if it is larger than 10 μm, it takes a long time to form the film, and if it is smaller than 0.5 μm, there is a high probability that the support film 2 made of a free-standing diamond film is broken during the resonator manufacturing process. Because it becomes. The thickness of the support film 2 is 0.5 to 3 μm in terms of strength and production time.
m is desirable.

The adhesion layer 3 is an insulator, preferably an oxide. When an insulator is formed on diamond, carbides form at the interface between the diamond and the insulator,
Bonding between diamond and insulator at the atomic level can improve adhesion strength. Also, when oxide is used, part of the diamond surface is burned by oxygen and the surface shape becomes uneven, so adhesion to diamond The contact area of the layer is increased, the number of carriers for bonding at the atomic level is increased, and the adhesion is improved by the anchor effect.

As oxides, ZnO, SiO ₂ , Al
_{There are 2} O ₃ , TiO ₂ , MgO and PbTiO ₃ based ceramics and the like. A crystal film can be obtained at a relatively low temperature, a reaction occurs without excessive supply of oxygen to the diamond film, and the adhesion strength can be improved.

In particular, in PbTiO ₃ ceramics (Pb _1-x La _x ) TiO ₃ , PbTiO ₃
Forms a perovskite structure that is compatible with the crystal lattice of an electrode material such as Pt or Au at a low temperature such as 500 ° C.
In addition, since it is composed of fine particles, it is easy to form a smooth film surface, and since La is contained, the anisotropy of the crystal lattice is reduced. The performance is improved.

The thickness of the adhesion layer 3 is preferably 20 to 100 nm, more preferably 40 to 80 nm. This is the adhesion layer 3
However, since the piezoelectric structure does not cause piezoelectric vibration in the resonator structure,
This is because a thick film exceeding 100 nm suppresses vibration of the vibrating body and cannot be ignored as a cause of loss. Also, 20
This is because a film having a thickness of less than nm is difficult to form a continuous film, and the effect as the adhesion layer 3 is greatly reduced.

When the adhesion layer 3 is an oxide film, the oxide film is preferably made of a sintered thin film having an average particle size of 5 to 30 nm. That is, the surface of this oxide film is 5 to
Because of the very fine irregularities of 20 nm, the density of crystal nuclei generated in the electrode thin film is increased, and a dense and smooth electrode film is obtained. As a result, the crystallinity of the piezoelectric body is improved.

The piezoelectric body 6 is desirably formed of a ferroelectric. This is because the ferroelectric has a large electromechanical coupling coefficient and is useful for realizing a broadband filter. The thickness of the piezoelectric body 6 is desirably 2 μm or less. this is,
This is because, in a BAW resonator using longitudinal vibration in thickness, the resonance frequency, which is the operating frequency, is inversely proportional to the film thickness, and a film thickness of 2 μm or less is required for use at a frequency of 1 GHz or more.

The electrodes 5 and 7 are preferably made of Al, Pt or Au in order to improve the adhesion strength with the adhesion layer. Pt or Au is particularly preferable in terms of ease of forming the piezoelectric film.
The thickness of the electrodes 5 and 7 is preferably 20 to 200 nm. This is because, if it is larger than 200 nm, the vibration suppressing effect becomes large and the resonator characteristics are significantly deteriorated.
If the thickness is smaller than 20 nm, the continuity of the film becomes insufficient, the electrical resistivity increases, and the Q value of the resonator decreases. It is particularly preferable that the electrode has a thickness of 50 to 100 nm.

The adhesion layer 3 and the piezoelectric body 6 can be formed by a thin film preparation such as a sol-gel method or a sputtering method, or a lamination technique, and the electrode films 5 and 7 can be similarly formed by a sputtering method, a vacuum deposition method or the like. Further, the vibration space A for acoustically insulating the base 1 and the vibrating body 4 can be formed by a chemical etching method using KOH or the like, a reactive ion etching method, or the like.

As described above, in the piezoelectric resonator of the present invention,
A high Q value can be obtained by using an insulator as the adhesion layer, and the insulating property can be enhanced by using an oxide sintered body composed of fine particles.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, Si (10
0) A diamond thin film is formed on both sides of the substrate. The growth conditions were as follows: a mixed gas of CH ₄ , CO ₂ , and H ₂ was used under a reduced pressure, a microwave was input at 6 kW, and a film having a thickness of 1 μm was produced. 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, from the back side of the substrate, Si is etched using a 43 wt% KOH aqueous solution to form a via hole reaching the diamond thin film layer. The diamond thin film used here is characterized by being crystalline and having a small internal residual stress. Therefore, even with a film thickness of 1 μm, a free-standing film can be formed without self-destruction due to residual stress.

Next, an adhesion layer and a piezoelectric body were formed by the methods shown in Table 1. First, ZnO, SiO constituting the adhesion layer
_{For 2} , Al ₂ O ₃ , TiO ₂ and MgO, an RF magnetron sputtering method was used. A sintered body of each material was used as a target, and RF power of 300 W and Ar were used as sputtering gases at a substrate temperature of 500 ° C.

Thereafter, a Pt electrode was formed on the upper surface of the produced adhesion layer by using a magnetron sputtering method, and a piezoelectric material was formed on the Pt electrode film by a sol-gel method. This piezoelectric body
PbTiO ₃ , Pb _0.93 La _0.07 TiO ₃ , Pb _0.9 L
a _0.1 TiO ₃ or the like, which was produced using a sol-gel method. Finally, an Al electrode was formed on the piezoelectric body by a vacuum evaporation method. The film formation temperature was 300 ° C. for the Pt electrode layer and A
The temperature of the electrode layer is 200 ° C. The thickness of the adhesive layer is 5
The thickness was 0 nm, the Pt layer was 100 nm, and the Al layer was 80 nm.

The method for forming the piezoelectric body will be specifically described. First, a (Pb _1-x La _x ) TiO ₃ film was prepared as follows. A 1 M solution was prepared using lead acetate, Ti isopropoxide, and La isopropoxide as starting materials and 2-methoxyethanol as a solvent. After all the raw materials were mixed in a solvent at a predetermined molar ratio, reflux treatment was performed at 124 ° C. for 5 hours, and after cooling to room temperature, acetylacetone was added to prepare a coating solution. After applying and drying the sol solution on the lower electrode film, pyrolysis was performed at 280 ° C. for 1 minute to form a gel film, and the application and heat treatment were repeated 5 times. Thereafter, crystallization was performed at 520 ° C. for 5 minutes. The process from solution application / drying to crystallization is repeated again.
A crystal growth process for 0 minutes is performed, and a 700 nm thick (P
A piezoelectric body made of b _1-x La _x ) TiO ₃ film was produced.

The PZT film was produced as follows. A 1 M solution was prepared using lead acetate trihydrate, Zr isopropoxide, and Ti isopropoxide as starting materials, and 2-methoxyethanol as a solvent. Zr isopropoxide and Ti isopropoxide were mixed and dissolved in a solvent at a predetermined molar ratio, acetylacetone was added and mixed at room temperature, and then lead acetate trihydrate was mixed and dissolved to prepare a solution. The solution is applied and dried by spin coating,
A gel film was formed by pyrolysis at 0 ° C. The solution application / drying / pyrolysis was repeated seven times, and finally baked at 675 ° C. for 5 minutes. The obtained film thickness was 500 nm. The formation of the perovskite structure was confirmed by X-ray diffraction measurement.

In this manner, ten piezoelectric resonators were prototyped. Among them, defects due to peeling were examined.

Next, for comparison, a piezoelectric resonator using Ti as the adhesion layer was manufactured. The manufacturing method other than the Ti formation was the same as the above method. This Ti film was formed using a magnetron sputtering method. In addition, ten piezoelectric resonators were prototyped, and the number of peelings was examined.

The piezoelectric resonance characteristics, and RF network analyzer HP8719C (manufactured by Hewlett Packard) was carried out by measuring the S parameters S ₁₁ for resonator structure, using an RF wafer microprobe, measuring the frequency characteristics of S ₁₁ The resonance frequency was evaluated.

As the Q value, the Q value at the anti-resonance was measured. Before and after the anti-resonance frequency, the frequency was obtained from the frequency width at which the S ₁₁ value at the anti-resonance frequency changed by 3 dB.

In the dielectric breakdown test, a voltage of 20 V was applied to the first electrode and the second electrode, and then the capacitance was measured with an impedance analyzer. With this method, the one that could not be measured out of the thirty samples was regarded as having a dielectric breakdown, and the number thereof was examined. Table 1 shows the results.

[0037]

[Table 1]

Sample No. of the present invention The number of peelings of 1 to 18 is 2
The number Q of the quality factors of the piezoelectric resonator is 200 or more, the resonance frequency is 2.40 GHz or more, and the number of dielectric breakdowns is 1 or less.
There were three or less.

In particular, ZnO, SiO ₂ , Al ₂ O ₃ , M
Sample N of the present invention using gO and TiO ₂ for the adhesion layer
o. In 3 to 9, the Q value was 255 or more. Also, Pb
The sample No. of the present invention in which a TiO ₃ -based piezoelectric film or a PLT piezoelectric film was used for the adhesion layer. In Sample Nos. 10 to 18, the Q value was as high as 300 or more, and the particle size of Sample No. 5 of the present invention was 5 to 20 nm. 1
In 0 to 15, 17 and 18, the breakdown is 5 or less,
No. of the present invention having a particle size of 40 nm. At 16 the breakdown was 1
There were zero.

On the other hand, Sample 19 using Ti outside the scope of the present invention for the adhesion layer and Sample No. 19 having no adhesion layer. The number of peelings of 20 is 2 or more, and the Q value which is the quality factor of the resonator is 12
5, the resonance frequency is 2.53 and 2.60 GHz,
The number of dielectric breakdown was 12 or more.

[0041]

According to the piezoelectric resonator of the present invention, the adhesion between the support film and the adhesion layer and the adhesion between the adhesion layer and the electrode are improved by using diamond for the support film and forming an adhesion layer made of an insulator thereon. However, it is possible to provide a piezoelectric resonator having a high Q value by improving the electrical insulation of the piezoelectric body.

[Brief description of the drawings]

FIG. 1 is a sectional view of a basic structure of a piezoelectric resonator of the present invention.

FIG. 2 is a sectional view of a basic structure of a conventional piezoelectric resonator.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Base 2 ... Support film 3 ... Adhesion layer 4 ... Vibration body 5 ... 1st electrode 6 ... Piezoelectric body 7 ... 2nd electrode

Claims (4)

[Claims] A substrate having a vibration space, a support film made of diamond formed on the surface of the substrate so as to cover the vibration space, and an adhesion layer formed on the support film.
A vibrator formed on the contact layer and having a piezoelectric body sandwiched between a pair of electrodes, wherein the contact layer is an insulator. 2. The piezoelectric resonator according to claim 1, wherein the insulator comprises metal oxide crystal particles. 3. The piezoelectric resonator according to claim 2, wherein the metal oxide crystal particles have a size of 5 to 20 nm. 4. The piezoelectric resonator according to claim 2, wherein the metal oxide crystal particles are made of a perovskite-type composite oxide containing at least Pb and Ti as metal elements.