JPH0590893A - Surface acoustic wave element - Google Patents

Abstract

PURPOSE:To reduce the propagation loss of surface acoustic wave by carrying out a predetermined treatment of the diamond layer to make the diamond layer electrically conductive, and by preparing tanden type electrodes. CONSTITUTION:When preparing a thin diamond film 2 on substrate 1 and the surface of this film is partly treated for hydrogeneration using hydrogen plasma, or for doping processing of H<+> by means of ion implantation method or the like, the relevant section of film 2 is made conductive, thereby preparing a diamond hydrogenated electrode 3 on the substrate 1. Further, when a thin ZnO film 4 of piezoelectric elements is laminated to prepare a surface acoustic wave element, the electrode 3 is not protruded from the thin film 2, the surface of the thin film 2 is flattened, the elastic wave reflection, scattering, or the like is reduced, and the propagation loss of the surface acoustic wave is reduced.

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 a diamond thin film.

[0002]

2. Description of the Related Art A surface acoustic wave element is an electro-mechanical conversion element having a surface wave propagating on the surface of an elastic body. Figure 2
1 shows a 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, distortion occurs in the piezoelectric body 11, and this distortion becomes a surface acoustic wave that propagates through the piezoelectric body 11 and the other comb-shaped electrode 13
It is taken out as an electric signal. As described above, in the surface acoustic wave element, the piezoelectric phenomenon of the piezoelectric body is used to excite the surface wave.

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 / λ₀Use defined by
Number f ₀The band pass characteristic is centered around.

The surface acoustic wave element 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.

Particularly, the surface acoustic wave filter has been put into practical use as an intermediate frequency filter for television from an early stage, and further, VT
It has been applied to filters for R and 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 , but in recent years, a piezoelectric thin film such as ZnO is formed on a substrate such as glass. What has been formed into a film by a technique such as sputtering has come to be used. However, a piezoelectric thin film such as ZnO formed on glass is
Usually, it is a polycrystalline material having an orientation, has a large loss due to scattering, and was not suitable for use in a high frequency band of 100 MHz or more.

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 acoustic waves to propagate faster, such as sapphire and diamond. Kaisho 64
-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 using a diamond thin film. Referring to FIG. 3, a diamond thin film 22 is formed on a substrate 21 such as silicon. A comb-shaped electrode 23 formed by patterning a metal by etching or the like is formed on the diamond 22. A ZnO film 24 is formed as a piezoelectric layer on the diamond thin film 22 on which the comb-shaped electrode 23 is formed.

[0013]

However, in the conventional surface acoustic wave device using such a diamond thin film, since the metal film is used as the comb-shaped electrode,
Since a step is formed on the diamond thin film, the propagating surface acoustic wave is reflected and scattered, which causes ripples in the filter characteristics and propagation loss.

An object of the present invention is to solve the problems of the conventional surface acoustic wave device, to reduce the propagation loss and ripple of the surface acoustic wave, and to show the surface acoustic wave exhibiting good characteristics with high efficiency. To provide a wave element.

[0015]

A surface acoustic wave element according to the present invention is a diamond layer, a piezoelectric thin film formed on the diamond layer, and a surface acoustic wave having a constant wavelength for generating and extracting the surface acoustic wave. A pair of electrodes is provided, and at least one of the electrodes is formed by subjecting a part of the diamond layer to a conductive treatment.

The diamond layer of the surface acoustic wave device of the present invention may be a diamond thin film formed on a substrate 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, a substrate made of a semiconductor material such as Si, Mo, W, GaAs, and LiNbO ₃ and an inorganic material can be used.

In the case of the diamond thin film thus formed, the 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 for example, a CVD method, a microwave plasma CVD method, a plasma CVD method, a PVD method, a hot filament method, or the like. Any method can be used.

As the method of 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 the 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 arylene: 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.

The piezoelectric thin film used in the present invention includes ZnO, AlN, Pb (Zr, Ti) O ₃ , and (P
b, La) (Zr, Ti) O ₃ , LiTaO ₃ , LiN
bO ₃ , SiO ₂ , Ta ₂ O ₅ , Nb ₂ O ₅ , BeO,
It is possible to use those containing Li ₂ B ₄ O ₇ , KNbO ₃ , ZnS, ZnSe, CdS and the like as main components. The piezoelectric thin film may be either a single crystal or a polycrystal, but a single crystal with less scattering of surface waves is more preferable because the element is used in a higher frequency range.

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

As an electrode for generating and extracting the surface acoustic wave, there is an electrode called a comb-shaped electrode or an interdigital transducer (IDT) electrode. In the present invention, the comb-shaped electrode is not formed of metal, but is formed by subjecting a part of the diamond layer to conductivity.

Such conductivity treatment can be performed by hydrogenating the surface of the diamond layer with hydrogen plasma or the like. The conductivity can also be obtained by doping H ⁺ on the surface of the diamond layer by an ion implantation method or the like. When the electrode is formed by hydrogenating with hydrogen activated by plasma, the diamond layer may be masked so as to have a comb-shaped electrode pattern and then hydrogenated. Also, H ⁺
Similarly, in the method of ion implantation, the patterning can be performed using a mask.

The mask is not particularly limited, but SiO ₂ or the like can be used, for example.

As a method of patterning the electrodes, conversely, the entire surface of the diamond layer is hydrogenated,
After that, the portion other than the comb-shaped electrode may be made highly resistive with oxygen or the like.

In the present invention, the electroconductivity treatment of the diamond layer is not limited to the above hydrogenation treatment, and other methods such as doping with impurities or ion implantation may be used. Any process may be used as long as it is a process.

[0031]

In the surface acoustic wave device according to the present invention, the electrodes are formed by subjecting a part of the diamond layer to electroconductivity. For this reason, since there is no step on the diamond layer due to the conventional electrode formation, reflection and scattering of surface acoustic waves can be reduced, and propagation loss can be significantly reduced. Therefore, it is possible to obtain a highly efficient surface acoustic wave device, and when used as a high frequency filter, a high frequency filter having excellent filter characteristics can be obtained.

[0032]

1 is a sectional view showing a surface acoustic wave device according to the present invention. Referring to FIG. 1, a diamond thin film 2 is formed on a silicon substrate 1. A SiO ₂ mask is formed on the diamond thin film 2 in a portion other than the electrode formation region. In this masked state, the diamond thin film 2 not masked by hydrogen plasma
Is hydrogenated to form a conductive portion in the pattern of a comb-shaped electrode, and the diamond hydrogenation electrode 3 is formed.
Next, SiO ₂ of the mask is removed by etching, and then a ZnO thin film as a piezoelectric thin film is formed on the diamond thin film 2 on which the hydrogenated diamond 3 is formed.

In this embodiment, the diamond thin film 2 has a thickness of 15 μm and the ZnO thin film has a thickness of 0.9 μm.
m. The conditions for hydrogenating the diamond hydrogenation electrode 3 with hydrogen plasma are as follows: pressure 20 Torr, microwave plasma power 300 W, H ₂ gas flow rate 200 s.
ccm was 3 minutes.

A surface acoustic wave device as shown in FIG. 1 obtained as described above and a conventional surface acoustic wave device having a comb-shaped electrode formed by etching a metal as shown in FIG. 3 were used as a filter. When comparing the characteristics,
The surface acoustic wave device according to the present invention shown in FIG. 1 has no ripple and the propagation loss is as small as half.

In the above embodiment, the diamond thin film formed on the substrate is partially electroconductively treated to form the electrode, but the present invention is not limited to such a diamond thin film, and the whole diamond layer is formed. May be a single crystal diamond substrate.

The conductivity treatment is not limited to the hydrogenation treatment of the above embodiment, but may be a conductivity treatment by doping with other boron or the like.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a surface acoustic wave device 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 diamond hydrogenation electrode 4 ZnO thin 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: At least one of the electrodes is formed by subjecting a part of the diamond layer to a conductive treatment, and a surface acoustic wave element.