JPH10276059A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the broad band surface acoustic wave device having provision for a high frequency that incorporates an IC with a desired resonance frequency without the need for very fine processing of interdigital electrodes through the use of an inexpensive single crystal Si substrate. SOLUTION: The device is formed with a piezoelectric thin film via a buffer layer on a single crystal Si substrate, interdigital electrodes (1a, 1b, 2a, 2b) made of a conductive film are formed on the piezoelectric thin film. The piezoelectric thin film is made of a lead titanate thin film that is manufactured by the sol gel method and whose thickness is 0.03-5 μm, the buffer layer is a thin film made of strontium barium titanate, strontium titanate or barium titanate and whose thickness is 0.01-0.2 μm manufactured by the sol gel method as required, and the piezoelectric thin film subject to polarization processing by applying an electric field among the interdigital electrodes 1a, 1b, 2a, 2b is used.

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, and more particularly, to a high-frequency surface acoustic wave device formed by depositing lead titanate (PT) as a piezoelectric material on a single-crystal Si substrate.

[0002]

2. Description of the Related Art Oxide-based piezoelectric materials are used for applications such as ultrasonic vibrators and mechanical filter elements, but when applied to surface acoustic wave elements, they are compared with single-crystal piezoelectric materials. As a result, the temperature coefficient of the surface wave velocity and the propagation loss of the surface wave are large, and application to a high frequency range is difficult.

Conventionally, generally used UHF, VH
In the F-band surface acoustic wave device, in order to obtain a desired resonance frequency from the relationship between the sound velocity of the surface acoustic wave and H / λ,
It is necessary to finely process the electrode interval of the comb-shaped electrode for determining the wavelength λ to a submicron unit, and an extremely advanced process is required for forming the electrode of the fine pattern. Japanese Patent Application Laid-Open No. 5-283970 describes a patterning process for microfabrication of a comb-shaped electrode, which makes it possible to form a comb-shaped electrode in sub-micron units. This is disadvantageous in terms of manufacturing efficiency, manufacturing cost, and the like.

[0004] In recent mobile communications, the use of higher frequencies and the spread of digital communications are remarkable. In order to increase the frequency and digitization of such mobile communication, the surface acoustic wave device also requires high frequency and wide band characteristics.

In the conventional surface acoustic wave device, since a single crystal substrate is used, it has to be externally provided.

[0006]

As described above, in a conventional general UHF and VHF band surface acoustic wave device,
In order to obtain a desired resonance frequency, it is necessary to process the inter-electrode interval of the comb-shaped electrode to a submicron unit, but an extremely sophisticated process is required for forming an electrode of a fine pattern. Therefore, the realization of a desired resonance frequency has a technical limit depending on the improvement of the fine processing technology of the comb-shaped electrode. In the microwave band, it is difficult to provide a surface acoustic wave device having a wide frequency characteristic.

The present invention solves the above-mentioned conventional problems and uses an inexpensive single-crystal Si (silicon) substrate and does not require fine processing of a comb-shaped electrode. It is an object of the present invention to provide a possible and inexpensive high-frequency surface acoustic wave device.

[0008]

The surface acoustic wave device according to the present invention comprises: (1) a single crystal substrate, a piezoelectric thin film formed on the single crystal substrate, and a conductive thin film formed on the piezoelectric thin film. A surface acoustic wave device comprising a comb-shaped electrode made of a conductive film, wherein the single crystal substrate is a single crystal Si substrate, and the piezoelectric thin film is formed on a buffer layer of a dielectric thin film formed on the substrate. A piezoelectric thin film made of a lead titanate (PT) thin film having a thickness of 0.03 to 5 μm formed by a sol-gel method, and applying an electric field between the comb-shaped electrodes to form the piezoelectric thin film. A surface acoustic wave device in which a thin film is subjected to polarization treatment to form a piezoelectric thin film, (2) the buffer layer
0.01-0.2 μm thickness formed by sol-gel method
Barium strontium titanate (BST), strontium titanate (STO) or barium titanate (B
(1) The surface acoustic wave device according to (1), which is made of a dielectric thin film such as TO), and (3) the single crystal Si substrate has an oxide film in which an oxide film is formed on at least the surface on which the piezoelectric thin film is formed. The surface acoustic wave device according to any one of (1) and (2), which is a single crystal Si substrate, is characterized.

In the surface acoustic wave device of the present invention, since the piezoelectric thin film is made of a PT thin film, it is possible to realize a high-frequency wide-band surface acoustic wave device having a desired resonance frequency without requiring fine processing of a comb-shaped electrode.

In the present invention, since the piezoelectric thin film made of the PT thin film is formed on the substrate via the buffer layer, an inexpensive single crystal Si substrate can be used as the substrate. That is, if it is an Si substrate with an oxide film,
The surface is smooth and the diffusion of elements during heat treatment can be prevented.
In addition, the buffer layer and the PT thin film having good film quality can be formed by the sol-gel method due to the sufficient mechanical strength. According to the present invention, the PT thin film is formed on the single-crystal Si substrate via the buffer layer of the dielectric thin film.
The effect of preventing lead deficiency in the T film is exhibited, and a good PT thin film can be formed. If this buffer layer is not formed, PT lead diffuses into Si, so that a PT thin film cannot be formed on a single crystal Si substrate.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a surface acoustic wave device according to the present invention will be described below. In the present invention, a single crystal Si substrate is usually used as the single crystal substrate, but a single crystal Si substrate having an oxide film formed on the surface is used as necessary. If the thickness of the oxide film of the oxide film single crystal Si substrate is too small, the diffusion preventing effect is not sufficient. If the thickness is too large, it causes cracks and warpage of the substrate. It is preferable that the thickness of the oxide film is about 0.3 to 2.0 μm for a single crystal Si substrate having a thickness of about 0.3 to 2.0 μm.

On the other hand, a film containing Al, Pt, Au or the like as a main component can be used as the conductive film constituting the comb-shaped electrode.

The surface acoustic wave device of the present invention comprises a buffer substrate, such as BST,
After forming a dielectric thin film such as STO or BTO, a PT thin film is formed, an upper electrode layer of a comb electrode is formed on the PT thin film, and a polarization process is performed by applying an electric field between the comb electrodes. Is done.

The BST, STO or BTO thin film as the buffer layer and the PT thin film of the piezoelectric thin film are preferably formed as follows.

First, barium compounds such as barium carboxylate such as barium 2-ethylhexanoate; strontium compounds such as strontium carboxylate such as strontium 2-ethylhexanoate; tetraethoxytitanium, tetraisopropoxytitanium, tetrabutoxytitanium; Titanium compounds such as titanium alkoxides such as dimethoxydiisopropoxytitanium; and 2-ethylhexanoic acid;
BST obtained by dissolving a carboxylic acid such as ethyl butyric acid in a solvent such as isoamyl acetate at a predetermined molar ratio and in a total concentration of about 0.5 to 8% by weight in terms of metal oxide. , An STO or BTO thin film forming composition is applied on a single crystal Si substrate provided with an oxide film by a spin coater or the like and dried at 400 to 600 ° C. This coating and drying are repeated until a thin film having a desired thickness is obtained.
BST, STO or BT fired at 00 ° C for 1 minute to 1 hour
Obtain an O thin film.

The BST and STO thus formed
Or, if the thickness of the BTO thin film is too large, it affects the piezoelectric properties, and if the thickness is too thin, the effect of preventing lead diffusion is lost, so that the thickness is usually 0.01 to 0.2 μm, especially 0.01 to 0.2 μm.
It is preferably 15 μm.

Next, on the buffer layer thus formed, a lead compound such as lead carboxylate such as lead acetate, lead alkoxide such as diisopropoxy lead and the like, and tetraethoxytitanium, tetraisopropoxytitanium, tetrabutoxytitanium A titanium compound such as a titanium alkoxide such as dimethoxydiisopropoxytitanium is mixed with a solvent such as 2-methoxyethanol at a predetermined molar ratio and a total concentration of metal oxides is about 10 to 20% by weight. The composition for forming a PT thin film obtained by dissolving the above is coated on a single crystal substrate by a spin coater or the like, and dried at 400 to 600 ° C. This coating and drying are repeated until a PT thin film having a desired film thickness is obtained, and finally, it is baked at 600 to 700 ° C. for 1 minute to 1 hour to obtain a PT thin film.

The thickness of the PT thin film thus formed is appropriately set according to a desired resonance frequency, an electromechanical coupling coefficient, and the like, but is usually 0.03 to 5 μm, and is preferably. Is 0.03 to 3 μm. If the film thickness is less than 0.03 μm, it is difficult to excite the surface acoustic wave due to the effect of the substrate, and if it exceeds 5 μm, there is a problem that a defect occurs in the film quality and the loss of the surface acoustic wave increases.

The polarization treatment of the PT thin film can be performed by applying a DC voltage of 2 to 50 V to the comb electrode formed on the PT thin film for 1 to 60 minutes. Here, by performing sufficient polarization processing, the film functions as a piezoelectric thin film. However, if the film quality of the PT thin film is insufficient, the electric field of the polarization processing cannot be sufficiently applied, and the film does not function as a piezoelectric thin film. become.

In the present invention, the comb-shaped electrode formed on such a PT thin film is made of a conductive material such as Al according to a conventional method in accordance with a desired line width and spacing (Line).
& Space, hereinafter abbreviated as "LS". ).

The surface acoustic wave device of the present invention has a desired resonance frequency by forming a PT thin film as a piezoelectric thin film on a Si substrate without performing fine processing of a comb-shaped electrode as described above. This realizes a surface acoustic wave device in a several GHz band (UHF band). However, the comb-shaped electrode may be finely processed to a submicron unit.
Further, it is possible to realize a surface acoustic wave device capable of coping with an electromagnetic wave region in a high frequency SHF band.

[0022]

The present invention will be described more specifically with reference to the following examples. [Examples 1 and 2] Single-crystal Si having an oxide film formed on the surface
Substrate (4 inch diameter x 250 µm thickness; oxide film thickness 1 µm
m) was used as a single crystal substrate, and a BST thin film having a thickness of 0.15 μm and a PT thin film having a thickness of 0.8 μm were sequentially formed on the surface of the single crystal substrate by a sol-gel method.

For forming a BST thin film, a BST having the following composition is used.
Using the composition for forming a thin film, the composition was applied by a spin coater and dried at 400 ° C. After repeating this application and drying, the composition was finally baked at 650 ° C. for 1 hour.

Composition for forming BST thin film (total concentration in terms of metal oxide: 7% by weight) Barium 2-ethylhexanoate: 9.514% by weight Strontium 2-ethylhexanoate: 3.598% by weight Tetraisopropoxytitanium : 9.205 wt% 2-ethylhexanoic acid: 18.50 wt% isoamyl acetate: balance To form a PT thin film, a composition for forming a PT thin film having the following composition was used. After drying and repeating this coating and drying, finally 6
It was baked at 50 ° C. for 1 hour.

Composition for forming PT thin film (total concentration in terms of metal oxide: 20% by weight) Lead acetate: 23.985% by weight Tetraisopropoxytitanium: 7.842% by weight 2-methoxyethanol: balance After that, FIG. After forming a comb-shaped electrode (LS values are shown in Table 1 respectively) made of an Al film having a pattern shown in FIG.
(Hereinafter, referred to as devices 1 and 2 of the present invention).

This polarization treatment is performed by the comb-shaped electrodes 1a and 1b.
, And a DC voltage (30 V) was applied for 10 minutes between 2a and 2b.

In the surface acoustic wave device, when an electric signal is applied to the electrodes 1a and 1b, the surface is distorted by a piezoelectric phenomenon, and the surface acoustic wave is excited. When the period of the electrodes 2a and 2b coincides with half of the wavelength of the surface acoustic wave, the surface acoustic wave is strongly excited and operates as a surface acoustic wave device.

When an electric signal was applied to the electrodes 1a and 1b of the surface acoustic wave device and the signals output from the electrodes 2a and 2b were measured, the resonance frequencies in the basic mode were output to the values shown in Table 1, respectively. Was done.

[Examples 3, 4, and 5] In the formation of the buffer layer, an STO thin film having a thickness of 0.15 μm was formed as a buffer layer using a composition for forming an STO thin film having the following composition. In the same manner as in Examples 1 and 2, the surface acoustic wave devices 3, 4, and 5 of the present invention comprising comb electrodes having the L · S values shown in Table 1 (hereinafter, devices 3, 4, and 5 of the present invention)
Was made).

Composition for forming STO thin film (total concentration in terms of metal oxide: 7% by weight) Strontium 2-ethylhexanoate: 14.269% by weight Tetraisopropoxytitanium: 10.950% by weight 2-ethylhexanoic acid: 22.015% by weight isoamyl acetate: balance In this surface acoustic wave device, when an electric signal is applied to the electrodes 1a and 1b, the surface is distorted due to the piezoelectric phenomenon, and the surface acoustic wave is excited. When the period of the electrodes 2a and 2b coincides with half of the wavelength of the surface acoustic wave, the surface acoustic wave is strongly excited and operates as a surface acoustic wave device.

When an electric signal was applied to the electrodes 1a and 1b of the surface acoustic wave device and the signals output from the electrodes 2a and 2b were measured, the resonance frequencies, which are the basic modes of the values shown in Table 1, were output. Was done.

[Examples 6 and 7] In Example 1, except that a BTO thin film having a thickness of 0.15 μm was formed as a buffer layer using a composition for forming a BTO thin film having the following composition in forming a buffer layer. 2 and L ·
The surface acoustic wave devices 6 and 7 of the present invention (hereinafter, referred to as the present devices 6 and 7) each including a comb-shaped electrode having an S value were manufactured.

Composition for forming BTO thin film (total concentration in terms of metal oxide: 7% by weight) Barium 2-ethylhexanoate: 12.722% by weight Titanium isopropoxytitanium: 8.616% by weight 2-ethylhexanoic acid: 17.31% by weight isoamyl acetate: balance In this surface acoustic wave device, when an electric signal is applied to the electrodes 1a and 1b, the surface is distorted due to a piezoelectric phenomenon, and the surface acoustic wave is excited. When the period of the electrodes 2a and 2b coincides with half of the wavelength of the surface acoustic wave, the surface acoustic wave is strongly excited and operates as a surface acoustic wave device.

When an electric signal was applied to the electrodes 1a and 1b of the surface acoustic wave device and the signals output from the electrodes 2a and 2b were measured, the resonance frequencies, which are the basic modes of the values shown in Table 1, were output. Was done.

[0035]

[Table 1]

In the above embodiment, since the polarization directions are alternated between the opposing comb-shaped electrodes formed on the piezoelectric thin film, the polarization direction is constant in the direction perpendicular to the comb-shaped electrodes. In contrast, the frequency is twice as high.

[0037]

As described in detail above, according to the surface acoustic wave device of the present invention, the inter-electrode spacing of the comb-shaped electrodes is made larger than before so that processing to the submicron unit is not required, and therefore, Without employing advanced processes, U
An HF surface acoustic wave device can be provided.

In addition, BST, ST
By forming a dielectric thin film of O, BTO, or the like, an inexpensive single-crystal Si substrate can be used, which is advantageous not only in terms of cost but also incorporation of IC.

[Brief description of the drawings]

FIG. 1 is a plan view showing a comb electrode pattern of a surface acoustic wave device according to the present invention.

[Explanation of symbols]

 1a, 1b, 2a, 2b: comb-shaped electrode

Claims (3)

[Claims] 1. A surface acoustic wave device 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. Wherein the single crystal substrate is a single crystal Si substrate, and the piezoelectric thin film is a surface acoustic wave device formed on a buffer layer of a dielectric thin film formed on the substrate, wherein the piezoelectric thin film is Thickness formed by sol-gel method 0.0
A surface acoustic wave device comprising a lead titanate thin film having a thickness of 3 to 5 [mu] m, wherein an electric field is applied between the comb-shaped electrodes, and the lead titanate thin film is polarized to form a piezoelectric thin film. 2. The method according to claim 1, wherein the buffer layer is a thin film of barium strontium titanate, strontium titanate or barium titanate having a thickness of 0.01 to 0.2 μm formed by a sol-gel method. 2. The surface acoustic wave device according to claim 1. 3. The single-crystal Si substrate according to claim 1, wherein said single-crystal Si substrate is an oxide-film-attached single-crystal Si substrate having an oxide film formed on at least a surface on which a piezoelectric thin film is formed. The surface acoustic wave device according to any one of the above.