JPH1131921A - Voltage controlled oscillator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To vary the VHF through SHF bands in a wide range and also to secure the high stability for a voltage controlled oscillator by forming a lead zilconate titanate(PZT) thin film or a piezoelectric thin film of lead titanate(PT) on a monocrystal substrate and using a piezoelectric resonator obtained by forming a conductive film on the piezoelectric thin film. SOLUTION: A voltage controlled oscillator uses a 1-port or 2-port type surface acoustic wave resonator having a comb-line electrode consisting of a piezoelectric thin film formed on a monocrystal substrate and a conductive film formed on the piezoelectric thin film. The piezoelectric thin film consists of a PZT thin film or PT. For instance, a buffer layer(BST thin film) 52 of 0.2 μm thickness, a PZT thin film 53 of 0.8 μm thickness and Al electrodes 54 of 1500 Å thickness are formed on a sapphire(monocrystal) substrate 51 of 500 μm thickness to obtain a piezoelectric substrate 50. Then a gap W is set at 2 μm (1/2 surface acoustic wave length λ) between Al electrodes 54 (a repeller 54A, a comb-line electrode 54B).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to relates to a voltage controlled oscillator using the piezoelectric resonator, in particular, lead zirconate titanate as the piezoelectric thin film of the piezoelectric resonator (PZT) or lead titanate (PbTiO _3: PT ) Relates to a voltage controlled oscillator.

[0002]

2. Description of the Related Art Quartz resonators using piezoelectric substrates are made of quartz.
Vibrators, surface acoustic wave resonators, piezoelectric thin film resonators, etc.
However, when such a piezoelectric resonator is represented by an electric equivalent circuit,
Both can be represented as shown in FIG. Figure 1
And C₀Is the sum of the capacitance between the electrodes and the stray capacitance between the terminals
And the inductance L₁,capacitance
C ₁And the resistance R₁Series resonance circuit is the resonance phenomenon of the resonator
Is represented.

FIG. 2 is a basic configuration diagram showing a Colpitts-type voltage-controlled oscillator that is most frequently used in the prior art using the above-described resonator. In FIG. 2, 1 is a resonator, 3 is a circuit including an oscillation transistor Tr on the right side of a broken line,
_j is the capacitance of the varactor (variable capacitance diode) 2, C ₁₁
-C ₁₃ shows the respective capacitances, R ₁₁ to R ₁₄ each resistor. An equivalent circuit of this oscillator can be represented as shown in FIG. 3 when considered separately for the resonator 1 side and the other circuit 3. In such a Colpitts oscillator, the resonator 1 is used at a frequency between resonance and anti-resonance, and the dielectric reactance, that is, the inductance L
Working as In this case, the reactance on the circuit 3 side is capacitive and can be represented by a capacitance (load capacitance) C _L , and the oscillation frequency is approximately the series resonance frequency of the inductance L and the parameter C _j or the load capacitance C _L and the parameter C _j. Becomes

[0004] Therefore, by changing the reverse bias voltage (control voltage) V _r applied to Palatka 2 connected in series to the resonator 1, C _j is proportional to V _rX oscillation frequency f ₀
Can be varied according to the following equation (1).

[0005] Δf / f ₀ =-(1/2) · (1 / r) · (C ₀ · C _L / (C ₀ + C _L ) ² ) · (△ C / C _L ) 1) Here, r is the capacitance ratio C ₀ / C ₁ of the resonator, and ΔC is the amount of change in the capacitance of the parameter.

Here, in order to vary the oscillation frequency f ₀ over a wide frequency band, a resonator having a wide resonance-anti-resonance frequency, that is, a resonator having a small capacitance ratio r and a parameter having a large capacitance change are used. Must.

[0007] However, in the conventional oscillator, as shown in FIG. 3, the resonator and the load capacitance C _L of the capacitance C _j and circuits Palatka enters the series, the volume ratio of the apparent elevated resonator resonant frequency Therefore, as shown in FIG. 4, a circuit 4 in which the inductance L _s (extension L) is inserted in series with the resonator 1 as shown in FIG. 4 is used, the resonance frequency is reduced by the series capacitance, and the inductive region of the resonator is widened. I'm using

However, even in this case, since the amount of change in the capacitance of the varactor is finite and small, it is difficult to realize a variable varactor with a wide band.

Japanese Patent Application Laid-Open No. 61-135205 discloses that in order to solve the above problem, an inductive transistor circuit having a negative resistance is used as an oscillation circuit, thereby deteriorating the capacitance ratio of the resonator. However, with the recent digitization of mobile communications, further improvements in frequency broadband characteristics are required, and even voltage-controlled oscillators have a wider bandwidth and higher stability. At present, it is required to have a variable frequency.

By the way, a resonator having a small capacitance ratio r can be realized by using a piezoelectric material having a large electromechanical coupling coefficient k ² , as is clear from the relations shown in equations (2) and (3). Conceivable.

Capacity ratio r = C ₀ / C ₁ (2) Electromechanical coupling coefficient k ² = 1 / (1 + r) (3) Conventionally, as for the piezoelectric material of the resonator of the voltage controlled oscillator, Lithium tantalate (LiT) as a piezoelectric substrate
It has been proposed to use aO ₃ ) or lithium niobate (LiNbO ₃ ) (Japanese Patent Laid-Open Nos. 2-104120 and 2-128585).

[0012]

However, conventionally,
LiTaO ₃ or LiNbO ₃ proposed as a piezoelectric material of a resonator of a voltage controlled oscillator has a small electromechanical coupling coefficient k ² but good temperature characteristics, or has a large electromechanical coupling coefficient k ^2. as that temperature characteristics is poor, at least one of the electromechanical coupling coefficient k ² and the temperature characteristics are poor, not to both satisfy the both properties.

Therefore, a resonator having a small capacitance ratio r is realized by a piezoelectric material having a large electromechanical coupling coefficient k ² ,
It has been difficult to provide a wideband variable voltage controlled oscillator that satisfies the required characteristics.

The present invention solves the above-mentioned conventional problems, and has a larger electromechanical coupling coefficient k ² than the conventional piezoelectric material.
The object of the present invention is to provide a wide-band variable and highly stable voltage controlled oscillator that supports a VHF to SHF band by incorporating a resonator using a piezoelectric material having a large Q value and good temperature characteristics. I do.

[0015]

A voltage controlled oscillator according to the present invention comprises a single crystal substrate, a piezoelectric thin film formed on the single crystal substrate, and a conductive film formed on the piezoelectric thin film. In the voltage controlled oscillator using the provided piezoelectric resonator, the piezoelectric thin film is a lead zirconate titanate (PZT) thin film or a lead titanate (PT) thin film.

PZT and PT are the electromechanical coupling coefficients k ²
Is large, the Q value is large, and the temperature characteristics are good. Therefore, by using a piezoelectric resonator formed with a PZT thin film or a PT thin film, it is possible to cope with the VHF to SHF band,
It is possible to provide a wide-band variable and highly stable voltage controlled oscillator.

In the present invention, the following are examples of the piezoelectric resonator.

A surface acoustic wave resonator including a single crystal substrate, a piezoelectric thin film formed on the single crystal substrate, and a comb-shaped electrode formed of a conductive film formed on the piezoelectric thin film. A one-port surface acoustic wave resonator including a single crystal substrate, a piezoelectric thin film formed on the single crystal substrate, and a comb-shaped electrode formed of a conductive film formed on the piezoelectric thin film. A two-port surface acoustic wave resonator including a single crystal substrate, a piezoelectric thin film formed on the single crystal substrate, and a comb-shaped electrode formed of a conductive film formed on the piezoelectric thin film.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the voltage controlled oscillator according to the present invention will be described with reference to the drawings.

The voltage controlled oscillator according to the present invention comprises a single crystal substrate as a piezoelectric resonator and a PZ formed on the single crystal substrate.
Except for using a piezoelectric resonator including a piezoelectric thin film of a T thin film or a PT thin film and a conductive film formed on the piezoelectric thin film, a configuration similar to that of a conventional voltage controlled oscillator using a piezoelectric resonator is used. Therefore, a Colpitts type circuit can be used as the circuit, and a voltage controlled oscillator having a configuration as shown in FIG. 2 or FIG. 4 can be obtained.

As the single crystal substrate of the piezoelectric resonator according to the present invention, diamond, sapphire, MgO, SrTiO ₃
Etc. can be used.

PZT as Piezoelectric Thin Film on Single Crystal Substrate
Alternatively, the thickness of the PT thin film may be 10 μm if necessary.
The thin film formed as follows is formed by applying an electric field between the comb-shaped electrodes and performing a polarization process.

In order to form a piezoelectric thin film on such a single crystal substrate, it is preferable to employ a sol-gel method. For example, it can be formed by the following method.

That is, for forming the PZT thin film, a total concentration of a lead compound such as lead acetate, a zirconium compound such as zirconium butoxide and zirconium propoxide and a titanium compound such as titanium isopropoxide and titanium butoxide in a predetermined molar ratio is used. The composition for forming a PZT thin film dissolved in a solvent such as methoxyethanol or acetic ester is applied to the substrate so that the
The coating and drying are repeated at a temperature of 50 to 400 ° C. to obtain a predetermined film thickness. Finally at 500-800 ° C
Bake for minutes to 1 hour.

In forming the PT thin film, a total concentration of a lead compound such as lead acetate or the like and a titanium compound such as titanium isopropoxide or titanium butoxide is set to be about 10 to 20% by weight at a predetermined molar ratio. , A methoxyethanol, a composition for forming a PT thin film dissolved in a solvent such as acetic ester is applied to a substrate, dried at 150 to 400 ° C., and the application and drying are repeated to a predetermined thickness. Bake at 500-800 ° C for 1 minute to 1 hour.

In the present invention, PZ as a piezoelectric thin film is used.
T thin film or PT thin film has a thickness of 1
0 μm or less, preferably 0.1 μm or less.
The range is from 10 to 10 μm, more preferably from 0.2 to 3 μm. If the PZT thin film or the PT thin film is too thin, the piezoelectric effect cannot be obtained, and if it is too thick, good film quality cannot be obtained.

In the present invention, prior to the formation of the PZT or PT thin film, barium strontium titanate (BST) and strontium titanate (STO) are formed on a substrate.
Alternatively, a buffer layer of barium titanate (BTO) or the like may be formed. By forming such a buffer layer, an effect of preventing lead deficiency in the PZT thin film or the PT thin film by the effect of preventing lead diffusion can be achieved. This buffer layer can be formed by a sol-gel method, and its film thickness is usually 0.01 to 2 μm, especially 0.01 to 2 μm.
It is preferably from 0.2 to 0.2 μm.

As the conductive film on the piezoelectric thin film, Al,
A conductive metal film such as Cu, Ag, Au, and Pt is suitable. Such a conductive film can be formed by a sputtering method or the like, and its thickness is usually 1000 to 2000 μm.
m.

[0029]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The construction and effects of the voltage controlled oscillator of the present invention will be described more specifically with reference to embodiments.

Embodiment 1 A one-port type surface acoustic wave resonator is incorporated as the resonator 1 into the Colpitts circuit of FIG.
Example 1 was a voltage controlled oscillator in the 00 MHz band. FIG. 5A is a plan view showing the electrode configuration of the one-port type surface acoustic wave resonator used in the present embodiment, and FIG. 5B is an enlarged view of the same section. The piezoelectric substrate 50 of the one-port type surface acoustic wave resonator is a sapphire substrate (thickness: 500 μm).
0.2 μm thick buffer layer (BST thin film) 5 on 1
2. 0.8 μm thick PZT thin film 53 and 1500 thickness
ＡｌAl electrode (reflection electrode 54)
A, the gap W between the comb electrodes 54B) 54 is 2 μm (ｍ of the wavelength λ of the surface acoustic wave).

In this voltage controlled oscillator, the control voltage V
FIG. 8 shows a change in the oscillation frequency when _r is varied from 0 to 5V.

In this embodiment, the PZT thin film as the piezoelectric thin film has a total concentration of lead acetate, zirconium butoxide and titanium isopropoxide of 2 at a predetermined molar ratio.
Using a solution for forming a PZT thin film dissolved in 2-methoxyethanol so as to be 0% by weight, applying by spin coating, drying at 400 ° C., repeating this application and drying until a predetermined film thickness is reached. It was formed by firing at 650 ° C. for 60 minutes.

The conductive film was formed by a sputtering method.

[Embodiment 2] A one-port surface acoustic wave resonator is incorporated as the resonator 1 into the Colpitts circuit of FIG.
Example 2 was a voltage controlled oscillator in the 00 MHz band. FIG. 6A is a plan view showing an electrode configuration of the one-port type surface acoustic wave resonator used in this embodiment, and FIG. 6B is an enlarged view of the same section. The piezoelectric substrate 60 of the one-port type surface acoustic wave resonator includes a sapphire substrate (thickness: 500 μm) 6
0.2 μm thick buffer layer (BST thin film) 6 on 1
2. A PT thin film 63 having a thickness of 0.85 μm and a thickness of 1500
ＡｌAl electrode 64 (reflection electrode 64)
A, the gap W between the comb electrodes 64B) 64 is 2 μm (１ ／ of the wavelength λ of the surface acoustic wave).

In this voltage controlled oscillator, the control voltage V
FIG. 9 shows a change in the oscillation frequency when _r is varied from 0 to 5V.

In this embodiment, the PT thin film is a PT thin film forming solution in which lead acetate and titanium isopropoxide are dissolved in 2-methoxyethanol at a predetermined molar ratio to give a total concentration of 10% by weight. After coating by spin coating, the coating was dried at 400 ° C., and the coating and drying were repeated until a predetermined film thickness was reached, and finally, the coating was baked at 650 ° C. for 60 minutes to form a film.

The conductive film was formed by a sputtering method.

Embodiment 3 A two-port type surface acoustic wave resonator is incorporated as the resonator 1 in the Colpitts circuit of FIG.
Example 3 was a voltage controlled oscillator in the 00 MHz band. FIG. 7A is a plan view showing the electrode configuration of the two-port surface acoustic wave resonator used in the present embodiment, and FIG. 7B is an enlarged view of the same section. The piezoelectric substrate 70 of this two-port surface acoustic wave resonator is a single crystal diamond substrate (thickness 300).
0.2 μm thick buffer layer (BST)
A thin film 72, a PZT thin film 73 having a thickness of 0.8 μm, and an Al electrode 74 having a thickness of 1500 ° are formed. A gap W between the Al electrodes (reflection electrode 74 A, comb electrode 74 B) is 2 μm (one of the wavelength λ of the surface acoustic wave). / 2).

In this voltage controlled oscillator, the control voltage V
FIG. 10 shows a change in the oscillation frequency when _r is varied from 0 to 5V.

In this embodiment, the PZT thin film as the piezoelectric thin film is composed of lead acetate, zirconium butoxide, and titanium isopropoxide in a total concentration of 2 at a predetermined molar ratio.
Using a solution for forming a PZT thin film dissolved in 2-methoxyethanol so as to be 0% by weight, applying by spin coating, drying at 400 ° C., repeating this application and drying until a predetermined film thickness is reached. It was formed by firing at 650 ° C. for 60 minutes.

The conductive film was formed by a sputtering method.

In each of the embodiments, the piezoelectric resonator was incorporated in the Colpitts circuit shown in FIG. 2 and the characteristics were examined. For comparison, the voltage controlled oscillator described in Japanese Patent Application Laid-Open No. 61-135205 was used for comparison. FIG. 1 shows changes in the oscillation frequency measured in the same manner as in the following examples for (A) and the voltage-controlled oscillator (B) listed as a conventional example in the same publication.
1 is shown.

From the above results, it is clear that the voltage controlled oscillator of the present invention has a very wide band and high control sensitivity as compared with the conventional one using a piezoelectric substrate.

[0044]

According to the present invention, a lead zirconate titanate (PZT) or lead titanate (PT) thin film formed on a single crystal substrate and a conductive film formed on the piezoelectric thin film are formed. A voltage-controlled oscillator using a piezoelectric resonator uses lead zirconate titanate (PZT) or lead titanate (PT) having a large electromechanical coupling coefficient k and excellent Q characteristics, so that the effect is very wide band. Cover the frequency of
A voltage controlled oscillator having an excellent S / N ratio can be provided.

[Brief description of the drawings]

FIG. 1 is an electrical equivalent circuit diagram of a piezoelectric resonator.

FIG. 2 is a basic configuration diagram of a Colpitts type voltage controlled oscillator.

FIG. 3 is an equivalent circuit diagram of the voltage controlled oscillator of FIG.

FIG. 4 is a configuration diagram of a Colpitts-type voltage controlled oscillator that is made variable in a wide band.

FIG. 5A is a plan view showing an electrode configuration of the one-port type surface acoustic wave resonator used in Example 1, and FIG.
Is an enlarged view of the same section.

FIG. 6A is a plan view showing an electrode configuration of a one-port surface acoustic wave resonator used in Example 2, and FIG. 6B.
Is an enlarged view of the same section.

FIG. 7A is a plan view showing an electrode configuration of a two-port type surface acoustic wave resonator used in Example 3, and FIG.
Is an enlarged view of the same section.

FIG. 8 is a graph showing characteristics of the voltage controlled oscillator according to the first embodiment.

FIG. 9 is a graph illustrating characteristics of the voltage controlled oscillator according to the second embodiment.

FIG. 10 is a graph showing characteristics of the voltage controlled oscillator according to the third embodiment.

FIG. 11 is a graph showing characteristics of a voltage controlled oscillator according to a comparative example.

[Explanation of symbols]

 50, 60, 70 Piezoelectric substrate 51, 61, 71 Single crystal substrate 52, 62, 72 Buffer layer 53, 73 PZT thin film 54, 64, 74 Al electrode 63 PT thin film

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Kazuyasu Hikita 1-5-1, Otemachi, Chiyoda-ku, Tokyo Mitsubishi Materials Corporation

Claims (4)

[Claims] A voltage controlled oscillator using a piezoelectric resonator including a single crystal substrate, a piezoelectric thin film formed on the single crystal substrate, and a conductive film formed on the piezoelectric thin film, A voltage controlled oscillator, wherein the piezoelectric thin film is a lead zirconate titanate thin film or a lead titanate thin film. 2. The elastic body according to claim 1, wherein the piezoelectric resonator includes a single crystal substrate, a piezoelectric thin film formed on the single crystal substrate, and a comb-shaped electrode formed of a conductive film formed on the piezoelectric thin film. The voltage controlled oscillator according to claim 1, wherein the voltage controlled oscillator is a surface acoustic wave resonator. 3. The piezoelectric resonator according to claim 1, wherein the piezoelectric resonator includes a single crystal substrate, a piezoelectric thin film formed on the single crystal substrate, and a comb-shaped electrode made of a conductive film formed on the piezoelectric thin film. 2. The voltage controlled oscillator according to claim 1, wherein the voltage controlled oscillator is a port type surface acoustic wave resonator. 4. A piezoelectric resonator comprising: a single crystal substrate; a piezoelectric thin film formed on the single crystal substrate; and a comb-shaped electrode formed of a conductive film formed on the piezoelectric thin film. 2. The voltage controlled oscillator according to claim 1, wherein the voltage controlled oscillator is a port type surface acoustic wave resonator.