JPH0590888A - Surface acoustic wave element - Google Patents

Abstract

(57) [Summary] [Object] To provide a surface acoustic wave device capable of reducing the propagation loss of a surface acoustic wave. [Structure] A substrate, a diamond thin film formed on the substrate, a piezoelectric thin film formed on the diamond thin film, and 1 for generating and extracting a surface acoustic wave of a specific wavelength.
And a pair of electrodes, and the diamond thin film is a diamond thin film in which (100) crystals are arranged.

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 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 utilizing surface waves propagating on the surface of an elastic body. Figure 5
Shows the general structure of a surface acoustic wave device.

Referring to FIG. 5, the surface acoustic wave element 24 is
It is configured by forming a pair of comb-shaped electrodes 22 and 23 on the piezoelectric body 21.

When an electric signal is applied to the comb-shaped electrode 22,
Strain occurs in the piezoelectric body 21, and this strain becomes a surface acoustic wave, propagates through the piezoelectric body 21, and is taken out as an electric signal by the other comb-shaped electrode 23. Thus, in the surface acoustic wave device,
The piezoelectric phenomenon of the piezoelectric body 1 is used to excite the surface wave.

As shown in FIG. 5, the frequency characteristic of this element is such that when the electrode period of the comb-shaped electrode is λ ₀ and the velocity of the surface acoustic wave is ν, the frequency f is defined by f ₀ = ν / λ _0. Bandpass characteristics centered on ₀ .

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 seed crystal such as LiNbO ₃ and LiTaO ₃ . However, 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, and is 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 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.

[0012]

However, in the conventional surface acoustic wave device using such a diamond thin film, the propagation loss of the surface acoustic wave is large, and it is not possible to make the surface acoustic wave device highly efficient. was there.

An object of the present invention is to provide a surface acoustic wave device which can reduce the propagation loss of surface acoustic waves and exhibits high efficiency.

[0014]

A surface acoustic wave device according to the present invention comprises a substrate, a diamond thin film formed on the substrate, and a piezoelectric thin film formed on the diamond thin film.
A diamond thin film is provided with a pair of electrodes for generating and extracting a surface acoustic wave having a specific wavelength.
It is characterized by being a diamond thin film in which crystals are arranged.

The substrate used in the present invention is not particularly limited as long as a polycrystalline diamond thin film can be formed thereon. For example, Si, Mo, W, Ga
As, LiNbO _{3 and the} like can be mentioned.

In the present invention, the diamond thin film is formed so as to have a (100) crystal arrangement. (10
0) As a method for arranging crystals, as will be described later in Examples, there is a method of arranging seed crystals having the same plane orientation on a substrate and growing a diamond thin film on the substrate. The diamond thin film growth method is, for example,
CVD method, microwave plasma CVD method, plasma CV
Conventionally known methods such as the D method, the PVD method, and the 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 layer used in the present invention
ZnO, AlN, Pb (Zr, Ti) O₃, (P
b, La) (Zr, Ti) O₃, LiTaO₃, LiN
bO ₃, SiO₂, Ta₂O_Five, Nb₂O_Five, BeO,
Li₂B_FourO₇, KNbO₃, ZnS, ZnSe and
It is possible to use one that has CdS as a main component.
It The piezoelectric layer is either a seed crystal or a polycrystal.
However, in order to use the device at higher frequencies, the
A seed 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.

The electrodes provided in the present invention are comb-shaped electrodes or interdigital transducers (ID
It is possible to use electrodes called T) electrodes. This electrode can be produced by etching, for example.

As the electrode material, an electrode having a low resistivity is preferable, and a metal that can be deposited at a low temperature such as Au, Ag, and Al, a refractory metal such as Ti, W, and Mo, and, for example, Al on Ti. It is possible to combine two or more kinds of metals to form Al and Ti are preferable from the viewpoint of easy production of the electrode. Further, W and Mo are preferable in terms of adhesion when the diamond thin film is used as a dielectric.

As a method of manufacturing the electrode, after forming the metal for the electrode, a resist is uniformly applied on the surface of the metal for the electrode, and a mask having a pattern of the electrode is placed on a transparent flat plate such as glass. There is a method of exposing using a mercury lamp or the like. It is also possible to directly form the electrodes with an electron beam.

The electrode etching method is, for example, Al.
In the case of a low melting point metal such as, wet etching with an alkaline solution such as sodium hydroxide solution or an acidic solution such as nitric acid is possible.

In the case of a refractory metal, etching can be performed using a mixed solution of hydrofluoric acid and nitric acid. Also BCl ₃
It is also possible to produce an electrode by a reactive ion etching method using such a gas.

The electrodes can also be formed using semiconductive diamond. Although diamond formed with high purity is insulating, impurities such as B, Al, P, and S are added by a method such as ion implantation, or lattice defects are introduced by using electron beam irradiation. A semiconductive diamond can be formed by performing hydrogenation treatment or the like.

[0030]

In the surface acoustic wave device according to the present invention, the diamond thin films are arranged in (100) crystals. Therefore, unlike the conventional diamond thin film, scattering of surface acoustic waves does not occur due to discontinuity at the grain boundary,
The propagation loss of surface acoustic waves can be reduced.

Therefore, the surface acoustic wave element according to the present invention can be a high efficiency surface acoustic wave element, and when used as a high frequency filter, it can be a high efficiency high frequency filter having excellent filter characteristics. ..

[0032]

EXAMPLE First, a seed crystal for forming a diamond thin film having a (100) crystal array on a silicon substrate is formed on the silicon substrate. FIG. 1 shows a silicon substrate having holes formed therein for selectively growing a diamond seed crystal. Referring to FIG. 1, a lattice-shaped resist 2 as shown in a plan view of FIG. 2 is formed on silicon substrate 1. In the presence of such a resist 2, the silicon substrate 1 is anisotropically etched to form seed crystal growth holes 3 of about 100 μm square having a (111) plane inclined surface 5 as shown in FIG. To form.

A commercially available diameter of 75-
A (111) face diamond seed crystal powder of 100 μm is placed and appropriately sieved to embed the seed crystal in the growth hole 3.

FIG. 3 shows a state in which a diamond seed crystal is embedded in the silicon substrate in this manner. Referring to FIG. 3, seed crystal 4 is embedded in each seed crystal growth hole 3.

Thus, a diamond thin film was grown on the silicon substrate 1 on which the seed crystals 4 were arranged at a predetermined interval, and a diamond thin film having a (100) crystal array was grown.

First, 20 H ₂ and CH ₄ were added to the reaction chamber.
The mixed gas mixed at a ratio of 0: 1 has a total flow rate of about 20 s.
It was introduced so as to be ccm. Next, the pressure in the reaction chamber was maintained at about 40 Torr, a microwave power of 400 W was discharged, and a plasma state was obtained, and a diamond thin film was grown to 25 μm on the above-mentioned silicon substrate on which seed crystals were arranged. The substrate temperature was about 850 ° C.

After growing a diamond thin film to a thickness of 100 μm, 500 liters of Al was vapor-deposited on the diamond thin film by a resistance heating method, and a comb-shaped electrode having an electrode width and an electrode interval of 2 μm was formed by photolithography. It was made. A wet etching method was used as a method for manufacturing the electrodes.

A ZnO thin film having a thickness of 0.9 μm was formed as a piezoelectric thin film on the diamond thin film on which the comb-shaped electrode was produced. The ZnO thin film was formed using a magnetron sputtering device.

For comparison, according to the conventional method, a randomly arranged polycrystalline diamond film was grown on a silicon substrate, and a surface acoustic wave device was produced in the same manner as in the above-mentioned embodiment.

When the propagation losses of the surface acoustic wave device of the above-mentioned embodiment and the comparative surface acoustic wave device are compared, the propagation loss of the surface acoustic wave device of the above-mentioned embodiment according to the present invention is found to be the comparative surface acoustic wave device. It was 1/4 of the propagation loss.

As is clear from the above, by arranging the diamond thin film in the (100) crystal according to the present invention, the propagation loss of the surface acoustic wave could be remarkably reduced.

FIG. 4 shows a silicon wafer, and the surface acoustic wave device according to the present invention divides the entire silicon wafer into a lattice shape as shown in FIG. By arranging the seed crystals 5 as described above, the diamond thin film can be grown only in this lattice-shaped region. By doing so, the diamond thin film is not formed in the lattice part corresponding to the boundary of the diamond thin film growth region 11, and after the surface acoustic wave device is produced on the silicon wafer, when separating each device, this boundary part Since the diamond thin film is not formed on, each element can be easily separated.

As described above, the method of growing the diamond thin film by using the substrate on which the seed crystals are arranged according to the method of the above embodiment can facilitate the manufacturing process.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a silicon substrate having a hole for selectively growing a diamond seed crystal.

FIG. 2 is a plan view of the silicon substrate shown in FIG.

FIG. 3 is a cross-sectional view showing a state where a diamond seed crystal is embedded in the silicon substrate of FIG.

FIG. 4 is a plan view showing a silicon wafer on which a diamond thin film is grown only in a region within a lattice.

FIG. 5 is a perspective view showing a general structure of a surface acoustic wave device.

[Explanation of symbols]

 1 Silicon Substrate 2 Resist 3 Seed Crystal Growth Hole 4 Seed Crystal 10 Silicon Wafer 11 Diamond Thin Film Growth Region

 ─────────────────────────────────────────────────── ─── 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 substrate, a diamond thin film formed on the substrate, a piezoelectric thin film formed on the diamond thin film, and a pair of electrodes for generating and extracting surface acoustic waves of a specific wavelength. A surface acoustic wave device comprising: a surface acoustic wave device, wherein the diamond thin film is a diamond thin film having a (100) crystal arrangement.