JP3132515B2 - Method for manufacturing surface acoustic wave device - Google Patents

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 device is an electromechanical conversion device utilizing a surface wave propagating on the surface of an elastic body. FIG.
Shows a general structure of a surface acoustic wave device.

Referring to FIG. 5, a surface acoustic wave device 24 has
It is constituted by forming a pair of comb electrodes 22 and 23 on a piezoelectric body 21.

When an electric signal is applied to the comb electrode 22,
Distortion occurs in the piezoelectric body 21, and the distortion becomes a surface acoustic wave, propagates through the piezoelectric body 21, and is extracted 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 for exciting the surface wave.

As shown in FIG. 5, the frequency characteristics of this element are as follows: Assuming that the electrode period of the comb electrode is λ ₀ and the velocity of the surface acoustic wave is ν, the frequency f is determined by f ₀ = ν / λ _{0 The} bandpass characteristic is centered on ₀ .

The surface acoustic wave element has a small number of parts, can be miniaturized, and can easily enter and exit a signal on the propagation path of the surface acoustic 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 to practical use as an intermediate frequency filter of a television from an early stage.
It has been applied to filters for TRs and 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 thin film of a piezoelectric material 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 polycrystalline with orientation, 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 a 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
_The smaller the _{value of 0} or the higher the velocity ν of the surface wave, the more the frequency characteristic of the device has a higher center frequency f ₀ .

Therefore, a surface acoustic wave device in which a piezoelectric film is laminated on a material in which an elastic wave propagates faster, such as sapphire and diamond, has been developed (for example, Japanese Patent Laid-Open No. 54-38874 and JP 6
4-62911).

In particular, diamond has attracted attention as a substrate on which a surface acoustic wave device is formed, since it has the fastest sound speed in diamond and is thermally and chemically stable. A surface acoustic wave device using diamond is
From the viewpoints 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 has been mainly studied.

[0012]

However, the conventional surface acoustic wave device using such a diamond thin film has a problem that the propagation loss of the surface acoustic wave is large and it cannot be a highly efficient surface acoustic wave device. was there.

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

[0014]

According to the present invention, a substrate, a diamond thin film formed on the substrate, a piezoelectric thin film formed on the diamond thin film, and a surface acoustic wave having a specific wavelength are generated and generated. In a method of manufacturing a surface acoustic wave device including a pair of electrodes for taking out, a step of forming a lattice-like resist on a substrate, and forming a seed crystal growth hole by etching the resist-formed substrate. And a step of embedding a diamond seed crystal in the seed crystal growth hole and a step of forming a diamond thin film having a (100) crystal array on the seed crystal.

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 and LiNbO ₃ can be mentioned.

In the present invention, the diamond thin film is formed to have a (100) crystal arrangement. (10
0) As a method of arranging crystals, 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, as described later in Examples. For example, a method of growing a diamond thin film is as follows.
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 of growing diamond by a vapor phase synthesis method by decomposing and exciting a source gas, for example, 1) a method of activating a source gas by heating a thermoelectron emitting material to a temperature of 1500 K or more, 2). There are a method using discharge by direct current, high frequency or microwave electric field, 3) a method using ion bombardment, 4) a method of irradiating light such as a laser, and 5) a method of burning raw material gas.

In the present invention, a carbon-containing compound is generally used as a raw material. This carbon-containing compound is preferably used in combination with hydrogen gas. If necessary, they 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; acetylenic hydrocarbons such as acetylene and allylene; Olefinic hydrocarbons: alicyclic 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; and carbon dioxide and carbon monoxide. One of these can be used alone, or two or more can be used in combination. Alternatively, the carbon-containing compound may be a substance consisting of only carbon atoms, such as graphite, coal, coke, and the like.

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

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

The piezoelectric layer used in the present invention
, ZnO, AlN, Pb (Zr, Ti) O_Three, (P
b, La) (Zr, Ti) O_Three, LiTaO_Three, LiN
bO _Three, SiO_Two, Ta_TwoO_Five, Nb_TwoO_Five, BeO,
Li_TwoB_FourO₇, KNbO_Three, ZnS, ZnSe and
It is possible to use those containing CdS as the main component.
You. The piezoelectric layer is either a seed crystal or a polycrystal.
However, in order to use the device at higher frequencies,
A seed crystal with less surface wave scattering is more preferable.

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

The electrodes provided in the present invention may be comb electrodes or interdigital transducers (IDs).
T) An electrode called an electrode can be used. This electrode can be produced, for example, by etching.

The electrode material is preferably an electrode having a low resistivity, such as a metal which can be deposited at a low temperature such as Au, Ag and Al; a high melting point metal such as Ti, W and Mo; Can be used in combination of two or more metals. Al and Ti are preferable from the viewpoint of easy production of the electrode. Also, W and Mo are preferable from the viewpoint of adhesion when a diamond thin film is used as a dielectric.

As a method of manufacturing an electrode, after forming a metal for an electrode, a resist is uniformly applied on the surface of the metal for an electrode, and a mask having an electrode pattern formed on a transparent flat plate such as glass is placed. There is a method of exposing using a mercury lamp or the like. Further, the electrodes can be directly formed by an electron beam.

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

In the case of a high melting point metal, etching can be performed using a mixed solution of hydrofluoric acid and nitric acid. 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, By performing a hydrogenation treatment or the like, a semiconductive diamond can be formed.

[0030]

In the surface acoustic wave device according to the present invention, the diamond thin film has a (100) crystal arrangement. Therefore, unlike conventional diamond thin films, surface acoustic waves are not scattered due to discontinuities at grain boundaries,
The propagation loss of the surface acoustic wave can be reduced.

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

[0032]

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 in which holes for selectively growing a diamond seed crystal have been formed. Referring to FIG. 1, on a silicon substrate 1, a grid-like resist 2 as shown in a plan view in FIG. 2 is formed. In the presence of the resist 2, the silicon substrate 1 is subjected to anisotropic etching to form a seed crystal growth hole 3 having a (111) inclined surface 5 as shown in FIG. Is formed.

On the silicon substrate 1, a commercially available material having a diameter of 75 to
A seed crystal is buried in the growth hole 3 by placing a powder of (111) plane diamond seed crystal of 100 μm and sieving the powder appropriately.

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

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

First, 20 H ₂ and CH ₄ were introduced into the reaction chamber.
A mixed gas mixed at a ratio of 0: 1 is mixed at a total flow rate of about 20 seconds.
ccm was introduced. Next, the pressure in the reaction chamber was maintained at about 40 Torr, and a discharge was performed at a microwave power of 400 W to form a plasma state. A diamond thin film was grown to 25 μm on the silicon substrate on which seed crystals were arranged. The substrate temperature was about 850 ° C.

After a diamond thin film is grown to a thickness of 100 μm, Al is vapor-deposited on the diamond thin film at 500 ° by a resistance heating method, and a comb-shaped electrode having an electrode width and an electrode interval of 2 μm is formed by photolithography. Produced. As a method for manufacturing the electrode, a wet etching method was used.

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 formed. The ZnO thin film was formed using a magnetron sputtering device.

As a comparison, a polycrystalline diamond film randomly arranged on a silicon substrate was grown in accordance with a conventional method, and a surface acoustic wave device was manufactured in the same manner as in the above-described embodiment.

When the propagation loss of the surface acoustic wave device of the above embodiment was compared with that of the comparative surface acoustic wave device, the propagation loss of the surface acoustic wave device of the above embodiment according to the present invention was Was 1/4 of the propagation loss.

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

FIG. 4 shows a silicon wafer. In the surface acoustic wave device according to the present invention, for example, the entire silicon wafer as shown in FIG. By arranging the seed crystals 5 as described above, a diamond thin film can be grown only in this lattice-like region. By doing so, a diamond thin film is not formed on a lattice portion corresponding to the boundary of the diamond thin film growth region 11, and when a surface acoustic wave device is manufactured on a silicon wafer and then separated from each other, the boundary portion Since the diamond thin film is not formed on each element, each element can be easily separated.

As described above, in the method of growing a diamond thin film using a substrate on which seed crystals are arranged according to the method of the above embodiment, the manufacturing process can be further facilitated.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a silicon substrate in which holes for selectively growing a diamond seed crystal are formed.

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

FIG. 3 is a cross-sectional view showing a state in which 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]

 Reference Signs List 1 silicon substrate 2 resist 3 seed crystal growth hole 4 seed crystal 10 silicon wafer 11 diamond thin film growth area

Continuation of the front page (72) Inventor Naoji Fujimori 1-1-1, Kunyokita, Itami-shi, Hyogo Sumitomo Electric Industries, Ltd. Itami Works (56) References JP-A-54-101242 (JP, A) Electronic information Communication Research Institute Technical Report Vol. 88, No. 181 (US88-38), pp. 43-48 (58) Fields investigated (Int. Cl. ⁷ , DB name) H03H 9/25 H03H 3/08

Claims (1)

(57) [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 a surface acoustic wave having a specific wavelength. A method of manufacturing a surface acoustic wave device, comprising: forming a lattice-shaped resist on the substrate; etching the substrate on which the resist is formed to form seed crystal growth holes; A method of manufacturing a surface acoustic wave device, comprising: embedding a diamond seed crystal in a crystal growth hole; and forming a diamond thin film having a (100) crystal array on the seed crystal.