JPH0590874A - Surface acoustic wave element - Google Patents

Abstract

PURPOSE:To enable a surface acoustic wave element to reduce a conversion loss without thickening the interdigital electrode section by providing a diamond layer and a piezoelectric film in the element to form the interdigital electrodes using a super-conductor. CONSTITUTION:A substrate 1 is made from semiconductor materials such as Si, Mo, W GaAs, and LiNbO3 and inorganic material. A diamond layer 2 is a thin diamond film to be prepared on the substrate 1 made of Si or the like, and may be formed into a monocrystal or polycrystal. The thin diamond film is prepared on the substrate 1 by means of the CVD method, microwave plasma CVD method, or the like. Further, thin piezoelectric film 4 is made primarily of ZnO, AlN, Pb (Zr, Ti)O3, or the like by means of the CVD method. An electrode 3 is interdigital, which is wholly formed into super-conductor, followed by carrying out etching on the conductor for its patterning to prepare. Further, from the standpoint of the performance of an electrode the thickness of the superconductor 3 is made to fall in a range of 30Angstrom -300Angstrom . As a result, the resistance of the electrode can be reduced to zero, thereby being able to reduce the conversion loss.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave device that can be used for, for example, a high frequency filter, and more particularly to a surface acoustic wave device using diamond.

[0002]

2. Description of the Related Art A surface acoustic wave element is an electro-mechanical conversion element utilizing surface waves propagating on the surface of an elastic body. Figure 2
Shows the general structure of a surface acoustic wave device.

Referring to FIG. 2, the surface acoustic wave device 10 is
It is configured by forming a pair of comb-shaped electrodes 12 and 13 on the piezoelectric body 11.

When an electric signal is applied to the comb-shaped electrode 12,
Strain occurs in the piezoelectric body 11, and this strain becomes a surface acoustic wave, propagates through the piezoelectric body 11, and is taken out as an electric signal by the other comb-shaped electrode 13. Thus, in the surface acoustic wave device,
The piezoelectric phenomenon of the piezoelectric body is used to excite surface waves.

The frequency characteristic of this element is as shown in FIG.
And the electrode period in the comb electrode is λ₀, Surface acoustic wave
Let ν be the speed of₀= Ν / λ₀Frequency determined by
Number f ₀The band pass characteristic is centered around.

The surface acoustic wave device has a small number of parts, can be made compact, and can easily input and output signals on the propagation path of the surface wave. This element can be applied to filters, delay lines, oscillators, resonators, convolvers, correlators, and the like.

In particular, the surface acoustic wave filter has been put into practical use as an intermediate frequency filter for television from an early stage, and further V
It has been applied to TRs and filters for various communication devices.

This surface acoustic wave device has been manufactured by forming a comb-shaped electrode on a piezoelectric single crystal such as LiNbO ₃ and LiTaO ₃ . In recent years, a piezoelectric thin film such as ZnO formed on a substrate such as glass by a technique such as sputtering has been used. However, a piezoelectric thin film such as ZnO formed on glass is usually oriented polycrystalline and has a large loss due to scattering.
It was not suitable for use in the high frequency band above 100 MHz.

On the other hand, in the surface acoustic wave filter used in the field of mobile communication and the like, an element which can be used in a higher frequency band is desired. As described above, the electrode period λ
_{When 0} becomes smaller or the velocity ν of the surface wave becomes larger, the frequency characteristic of the element has a higher center frequency f ₀ .

Therefore, a surface acoustic wave device has been developed in which a piezoelectric film is laminated on a material that allows elastic waves to propagate faster, such as sapphire and diamond (see, for example, JP-A-54-38874 and JP-A-6
4-62911).

In particular, diamond has attracted attention as a substrate for forming a surface acoustic wave device because it has the highest sound velocity in diamond and is stable both thermally and chemically. The surface acoustic wave device using diamond is
From the viewpoint of productivity and cost, a method of forming a diamond thin film on a substrate and forming a piezoelectric thin film on the diamond thin film is mainly studied.

FIG. 3 is a sectional view showing a conventional surface acoustic wave device. Referring to FIG. 3, a diamond thin film 22 is formed on a substrate 21 made of, for example, silicon. On the diamond thin film 22, a metal comb-shaped electrode 23 patterned by etching a metal thin film is formed. A ZnO film 24 is formed as a piezoelectric layer on the diamond thin film 22 having the metal comb electrode 23 formed thereon.

[0013]

In such a conventional surface acoustic wave device, if the thickness of the comb-shaped electrode is increased in order to reduce the resistance in the portion of the comb-shaped electrode for the purpose of reducing the conversion loss. There is a problem that a large step is generated in the mold electrode portion, which causes reflection and scattering of the surface acoustic wave, resulting in a large propagation loss of the surface acoustic wave.

An object of the present invention is to solve the above-mentioned conventional problems and to increase the thickness of the comb-shaped electrode portion,
An object is to provide a surface acoustic wave device capable of reducing conversion loss.

[0015]

A surface acoustic wave element according to the present invention is a device for generating and extracting a diamond layer, a piezoelectric thin film formed on the diamond layer, and a surface acoustic wave of a specific wavelength. And a pair of electrodes, and at least one of the electrodes is formed of a superconductor.

In the present invention, the superconductor forming at least one of the electrodes is not particularly limited,
From the viewpoint of operating temperature, oxide-based superconducting materials exhibiting a high critical temperature are preferable. As the oxide-based superconducting material, all oxide-based superconducting materials including conventionally known superconducting materials such as lanthanum-based, yttrium-based, bismuth-based, and thallium-based materials can be used. Also,
Since diamond is used in the surface acoustic wave device of the present invention, a superconducting material such as C ₆₀ formed of the same carbon is also preferable. In addition to these superconducting materials, metallic superconducting materials and compound superconducting materials can be used.

In the present invention, the diamond layer may be a diamond thin film formed on a substrate such as silicon, or may be a single crystal diamond substrate.

The substrate on which the diamond thin film is formed is not particularly limited, but for example, Si, Mo, W,
Substrates of semiconductor and inorganic materials such as GaAs and LiNbO ₃ can be used.

In the case of a diamond thin film formed on a substrate, this diamond thin film may be a single crystal or a polycrystal. It may also be an amorphous diamond-like carbon film.

The method for forming the diamond thin film on the substrate is not particularly limited, but is, for example, CV.
D method, microwave plasma CVD method, plasma CVD
A method such as a method, a PVD method, and a hot filament method can be used.

As a method for decomposing and exciting the raw material gas to grow diamond by the vapor phase synthesis method, for example, 1) a method of activating the raw material gas by heating a thermoelectron emitting material to a temperature of 1500 K or higher, 2) There are a method of using discharge by a direct current, a high frequency or a microwave electric field, 3) a method of using ion bombardment, 4) a method of irradiating light such as a laser, and 5) a method of burning a source gas.

In the present invention, the raw material used is generally a carbon-containing compound. This carbon-containing compound is preferably used in combination with hydrogen gas. Further, if necessary, it may be used in combination with an oxygen-containing compound and / or an inert gas.

Examples of the carbon-containing compound include paraffinic hydrocarbons such as methane, ethane, propane and butane: olefinic hydrocarbons such as ethylene, propylene and butylene: acetylene hydrocarbons such as acetylene and allylene: diene such as butadiene Olefinic hydrocarbons: cycloaliphatic hydrocarbons such as cyclopropane, cyclobutane, cyclopentane, and cyclohexane: aromatic hydrocarbons such as cyclobutadiene, benzene, toluene, xylene, and naphthalene: ketones such as acetone, diethyl ketone, and benzophenone: Alcohols such as methanol and ethanol: amines such as trimethylamine and triethylamine: carbon dioxide gas, carbon monoxide and the like. These can be used individually by 1 type and can also use 2 or more types together. Alternatively, the carbon-containing compound may be a substance composed of only carbon atoms, such as graphite, coal or coke.

As the oxygen-containing compound, oxygen, water, carbon monoxide, carbon dioxide and hydrogen peroxide are preferable because they are easily available.

The inert gas is, for example, argon, helium, neon, krypton, xenon or radon.

As the piezoelectric thin film used in the present invention, Z
nO, A1N, Pb (Zr, Ti) O ₃ , (Pb, L
a) (Zr, Ti) O ₃ , LiTaO ₃ , LiNb
O ₃ , SiO ₂ , Ta ₂ O ₅ , Nb ₂ O ₅ , BeO, L
i ₂ B ₄ O ₇ , KNbO ₃ , ZnS, ZnSe and C
A material containing dS or the like as a main component can be used. .. The piezoelectric thin film may be either a single crystal or a polycrystal, but in order to use the device in a higher frequency range, a single crystal with less scattering of surface waves is more preferable.

ZnO, AlN and Pb (Zr, Ti)
The piezoelectric layer such as O ₃ can be formed by the CVD method.

In the present invention, the electrode formed of a superconductor is generally a comb-shaped electrode or an interdigital electrode.
There are electrodes called transducer (IDT) electrodes. This electrode can be manufactured, for example, by forming a thin film of a superconductor on the entire surface and then etching and patterning the thin film. Such etching can be performed by, for example, wet etching using phosphoric acid or the like, plasma etching using a halogen gas, reactive ion etching, or the like. Further, the superconductor thin film may be formed and patterned only on the portions where the electrodes are formed using a mask or the like. Furthermore, a method of forming a thin film of a superconductor on the whole surface and then patterning it by using electron beam irradiation so that only a predetermined electrode portion becomes a superconducting phase and the other portion becomes a non-superconducting phase may be adopted. Good.

The position of the electrode formed in the present invention is not particularly limited, and it may be formed between the diamond layer and the piezoelectric thin film, or may be formed on the piezoelectric thin film. Good. Further, it may be a surface acoustic wave device in which a short-circuit electrode is also provided.

The thickness of the superconductor electrode formed in the present invention is not particularly limited, but from the viewpoint of the performance as an electrode, it is preferably 30 Å or more. Also, in order to reduce the step caused by the electrode formation,
The thickness is preferably 300 Å or less.

[0031]

In the surface acoustic wave device according to the present invention, at least one of the electrodes is made of a superconductor. Therefore, by using the superconductor at a temperature not higher than the critical temperature, the resistance at the electrode can be made almost zero, and the conversion loss can be significantly reduced. Moreover, since the thickness of the electrode can be reduced, the propagation loss of the surface acoustic wave can be reduced.

[0032]

Example A diamond thin film was grown on a silicon substrate by the microwave CVD method. H as source gas
With ₂ and CH _4, were mixed to 0.5% of the total of CH _4, the mixed gas, the total flow rate of about 20scc
It is introduced so that it becomes m, and the pressure in the reaction chamber is about 4
Maintained at 0 Torr. With microwave power of 400W,
A diamond thin film was grown to a thickness of 25 μm in a plasma state by discharging. Substrate temperature is about 850 ° C
And

After growing the diamond thin film, YBa ₂ Cu ₃
A superconductor having a composition of O _7-x was grown by sputtering. When the diamond thin film grows to several tens of microns or more, the (110) orientation becomes predominant, so that the superconductor grown on this has the C axis orientation mainly. Therefore, the lattice mismatch rate is 1.9%.

A YBa ₂ Cu ₃ O _7-x superconductor was grown to a thickness of 10 unit cells, that is, about 120Å. The critical temperature of this thin film was 79K.

This superconducting thin film was patterned by photolithography and wet etching so as to form a comb-shaped electrode. A ZnO thin film as a piezoelectric thin film was formed thereon with a thickness of 1.9 μm. This ZnO thin film was formed using a magnetron sputtering device.

FIG. 1 is a sectional view showing the surface acoustic wave device according to the present invention manufactured as described above. Referring to FIG. 1, a diamond thin film 2 is formed on a substrate 1, and a superconductor comb-shaped electrode 3 is formed on the diamond thin film 2.
Are formed. A ZnO film is formed on the diamond thin film 2 on which the superconductor comb-shaped electrode 3 is formed.

The insertion loss at 77K of the surface acoustic wave device of the example according to the present invention obtained as described above is
It was 18 dB at 1.2 GHz.

For comparison, a surface acoustic wave device was produced in which the comb-shaped electrode was formed of aluminum metal which has been conventionally used instead of a superconductor. The aluminum comb electrode had a thickness of 700 Å, was grown by sputtering, and was patterned by reactive ion etching.
The comparative surface acoustic wave device thus obtained has a thickness of 77
The insertion loss was 22 dB at k and 1.2 GHz.

As is clear from the above, according to the present invention, the surface acoustic wave device in which the comb-shaped electrode is formed by the superconductor has lower insertion loss than the conventional surface acoustic wave device.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment according to the present invention.

FIG. 2 is a perspective view showing a general structure of a surface acoustic wave device.

FIG. 3 is a cross-sectional view showing a conventional surface acoustic wave device.

[Explanation of symbols]

 1 substrate 2 diamond thin film 3 superconductor comb-shaped electrode 4 ZnO film

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Naoji Fujimori 1-1-1 Kunyokita, Itami City, Hyogo Prefecture Sumitomo Electric Industries, Ltd. Itami Works

Claims (1)

[Claims] 1. A surface acoustic wave device comprising a diamond layer, a piezoelectric thin film formed on the diamond layer, and a pair of electrodes for generating and extracting a surface acoustic wave of a specific wavelength, wherein: A surface acoustic wave device, wherein at least one of the electrodes is formed of a superconductor.