JPH04341005A - Surface acoustic wave element using diamond film - Google Patents

Abstract

PURPOSE:To improve the orientation and smoothness of a piezoelectric film and to realize the surface acoustic wave element compatible with a high frequency with excellent mechanical strength by selecting a specific Vickers hardness for the diamond film when a diamond layer, a piezoelectric layer and an electrode layer form the surface acoustic wave element. CONSTITUTION:A diamond layer 2 including carbon as a major component and including hydrogen as a trace impurity by the ion vapor-deposition method and a piezoelectric layer 3 made of ZnO, AlN, LiNbO3 or the like and an electrode 4 made of Au or the like are laminated on a substrate 1 made of glass and alumina. Thus, the film strength is sufficiently high, the close adhesion to the substrate 1 is excellent, and the smoothness is high, then the layer 2 with an excellent C axis orientation and high refractive index is obtained and the element with high performance at a high sound speed is obtained. The Vickers hardness of the diamond layer is specified to 6500-9000kb/mm<2> while varying the current.

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 element used in high frequency filters and the like, and more particularly to a surface acoustic wave element using a diamond-like layer, which is advantageous for forming a piezoelectric layer, and a method for manufacturing the same.

0002

[Prior Art and Problems to be Solved by the Invention] A surface acoustic wave device is a device that applies elastic waves propagating on a solid surface, and conventionally, a surface acoustic wave device uses a crystal substrate such as LiNbO3 that has high piezoelectricity and low sound wave absorption loss. It was manufactured by forming a blind electrode on top. Since the frequency propagated in such a surface acoustic wave element depends on the wavelength and sound speed of the surface wave, it is conceivable to improve these to correspond to a high frequency of GHz. The wavelength of the surface wave is determined by the pitch of the arrangement of the electrodes of the blind electrode, and the finer the pitch, the higher the frequency of the surface wave can be achieved. However, due to limitations in microfabrication techniques such as etching techniques, it is currently difficult to increase the frequency by making the pitch of the electrode arrangement finer. On the other hand, since the speed of sound is determined by the substrate material, technology has been developed in recent years to increase the frequency by using a sapphire or diamond-like layer with a sound speed of over 10,000 m/s in combination with a piezoelectric film.

[0003] JP-A-64-20714, filed on July 16, 1988, discloses a surface acoustic wave device equipped with a combination of a diamond thin film, an interdigital transducer, and a piezoelectric film on a substrate. There is. This application describes that the diamond thin film is formed by methods such as sputtering, vacuum evaporation, ion plating, and CVD.

[0004] Japanese Unexamined Patent Publication No. 1983, filed on September 3, 1986
No. 2-220937 discloses a surface acoustic wave device having a single-crystal or polycrystalline diamond layer, a piezoelectric layer, and an electrode layer. No. 1-103310 discloses a surface acoustic wave device having a diamond-like carbon film layer, a piezoelectric layer and an electrode layer. However, the physical properties and production conditions of such a diamond-like carbon film layer have not been specified at all.

[0005] Japanese Patent Application Laid-Open No. 1-233819 filed on March 15, 1988 and Japanese Patent Application Laid-open No. 2-20910 filed on September 1, 1988 disclose that on a single crystal semiconductor substrate, A surface acoustic wave device with a three-layer composite structure in which a diamond crystal thin film and an aluminum nitride thin film are successively formed is disclosed in JP-A-2-137413 filed by the same applicant on November 18, 1988. A method for manufacturing a surface acoustic wave device is described in which a diamond single crystal on which a dielectric thin film and a piezoelectric thin film are formed is embedded in a substrate. These applications all target crystalline diamonds such as single crystals.

[0006] Furthermore, Japanese Patent Application Laid-open No. 2-239715, filed on March 14, 1999, discloses a surface acoustic wave substrate in which a piezoelectric thin film is formed on a diamond substrate, and artificial diamond is used as a surface acoustic wave substrate. and natural diamonds, but there are no details of manufacturing methods or physical properties.

[0007] In the above-mentioned prior art, surface acoustic wave devices using single-crystal or polycrystalline diamond do not have easy crystal growth of the piezoelectric layer due to the low smoothness of the diamond film. There were also problems with the surface smoothness and bond strength of the piezoelectric layer.

On the other hand, the diamond-like layer, which is amorphous diamond, varies in refractive index, hardness, and
A variety of surface acoustic wave devices with different physical properties such as density can be obtained, and the frequency characteristics and ease of manufacture of surface acoustic wave devices are significantly dependent on the physical properties of the diamond-like layer. For example, Japanese Patent Application No. 2-83221 describes that diamond-like layers can be mainly classified into four types.

However, among the above-mentioned conventional techniques, JP-A-1-103310, which uses a diamond-like layer, does not describe its physical properties or manufacturing conditions, and the diamond-like layer cannot be specified. According to the applicant's investigation and research, it is not possible to obtain a surface acoustic wave device with good characteristics simply by using a diamond-like layer. In particular, crystal growth and post-processing of the piezoelectric film are difficult unless the layer has a diamond-like layer having a certain physical property value such as Vickers hardness.
It was found that a good element could not be obtained.

Therefore, an object of the present invention is to form a piezoelectric film,
It is an object of the present invention to provide a surface acoustic wave device using a diamond-like layer and a method for manufacturing the same, which is easy to post-process and has high smoothness of the device, thereby providing good surface acoustic wave device functionality.

[0011]

[Means for Solving the Problems] The present inventor fabricated surface acoustic wave devices using various diamond-like layers having different physical property values, and after intensively studying their characteristics and ease of manufacture, the inventors found that The present inventors have discovered that a surface acoustic wave device suitable for manufacturing a surface acoustic wave device and having good characteristics can be obtained only when a diamond-like layer having Vickers hardness, density, etc. is used, and the present invention has been completed. reached.

That is, the present invention provides a diamond-like layer;
A surface acoustic wave element including a piezoelectric layer and an electrode layer,
The Vickers hardness of the diamond-like layer is 6500-900
The present invention provides a surface acoustic wave element characterized in that the surface acoustic wave element is 0 kg/mm2.

[0013] The diamond-like layer used in the present invention is a thin film with an amorphous structure mainly composed of carbon and containing a trace amount of impurity such as hydrogen. Such a diamond-like layer has sufficiently high film strength and is easy to handle, and has sufficient adhesion to the substrate, as compared to single-crystal and other-crystalline diamond crystal bodies. Furthermore, since the film has high smoothness, the C-axis orientation of the piezoelectric film is improved. Further, since the diamond-like layer has a high refractive index, the film is dense, which is also advantageous for the C-axis orientation of the piezoelectric material.

According to the present invention, the Vickers hardness of the diamond-like layer must be in the range of 6,500 to 9,000 kg/mm 2 . The reason why the Vickers hardness is limited here is that the hardness strongly depends on the heat resistance of the diamond-like layer, and a diamond-like layer within this range has sufficient heat resistance. As a result, the diamond-like layer can sufficiently withstand the substrate temperature required when depositing the piezoelectric film in vacuum, and also enables post-processing of the deposited piezoelectric film to improve the crystallinity of the piezoelectric film. can do. In particular, this post-treatment requires a high temperature of about 500°C. If the Vickers hardness is not within the above range, the C-axis orientation of the piezoelectric layer, such as a ZnO2 layer, formed on the diamond-like layer will be low.

[0015] Furthermore, the density of the diamond-like layer is 2.3
It is preferable that the refractive index is in the range of 5 to 2.41, and the refractive index is 3.
．． It is preferably between 45 and 3.50. If the density and refractive index are within the above range, the film will be smoother and the density will be higher.Higher refractive index means that the film is denser, and the smoothness of the piezoelectric film formed on it will be higher. The bondability with the diamond-like layer also improves, and the C-axis orientation of the piezoelectric film becomes easier. Furthermore, it is very important that having physical property values in this range makes the diamond-like layer dense and facilitates crystal growth of the piezoelectric film.

[0016] The diamond-like layer used in the present invention can be formed by various methods, but the ionization vapor deposition method is particularly preferred because it yields a diamond-like layer with high smoothness. Although the film thickness is not particularly limited, it is preferably about 1 to 20 μm. Furthermore, Nakayama et al. have found that there is a correlation between the Vickers hardness, density, and refractive index (Japanese Patent Application No. 144480/1999), and by using this relationship, the physical property values can be easily determined. Diamond-like layers within the range of .

A diamond-like layer as described above is generally formed on a substrate. The substrate material is not particularly limited, and glass, alumina, etc. can be used.

The piezoelectric film used in the present invention is made of ZnO, Al
N, ZnS, LiNbO3, Pb(Zr,Ti)O3
, LiTaO3, SiO2, Ta2O5, etc., but are not particularly limited thereto. Al
N is advantageous because it can improve the sound velocity more than ZnO. These piezoelectric materials can be produced by sputtering, vacuum deposition, CVD
It can be formed by various methods such as. These piezoelectric films are generally formed on a diamond-like layer, but it is preferable to raise the substrate temperature during film formation and to perform additional heat treatment after film formation to improve the orientation of the piezoelectric material. . In particular, the diamond-like layer of the present invention having a predetermined Vickers hardness has good heat resistance and is therefore suitable for such post-treatment.

The electrode layer used in the present invention is made of aluminum,
Metals such as gold are used. Photolithography is suitable for forming the electrode layer. The position where the electrode layer is formed is not particularly limited, and may be either on the piezoelectric layer or the diamond-like layer. Furthermore, the position of the counter electrode is not particularly limited as described above.

Further, the present invention provides a method for manufacturing a surface acoustic wave device including a diamond-like layer, a piezoelectric layer, and an electrode layer, the surface acoustic wave device having a Vickers hardness of 6500 to 9000 kg.
The present invention also provides a method for manufacturing a surface acoustic wave device, characterized in that a diamond film of /mm2 is formed by an ionized vapor deposition method. The piezoelectric layer and the electrode layer can be manufactured by the methods described above, and the order in which the layers are formed is not particularly limited.

Examples of the present invention are shown below, but the present invention is not limited thereto.

[0022]

【Example】

Formation of a diamond-like layer by ionization vapor deposition method
A diamond-like layer was formed on a silicon wafer substrate using the ionization vapor deposition apparatus described in Japanese Patent No. 1-234397 and Japanese Patent No. 2-196095. The formation conditions are as follows.

[0023] Methane gas is introduced and the gas pressure is set to 0.1 Torr.
A discharge was caused in the hot cathode filament as r. The magnetic flux density was 400 Gauss, the substrate voltage Va=-300V, and the substrate temperature was 200°C. Also, the filament has a current If=2
5A was flowed. The filament is coiled and its width is 3m.
m, and the gap between it and the surrounding electrode 36 was 8 mm. The grid was vibrated at a speed of 5 mm/min. Note that Vickers hardness can be adjusted by changing the current value.

[0024] Furthermore, RF plasma CVD was performed on the same substrate.
A diamond-like layer was formed on the same substrate by a method and a microwave plasma CVD method. The results are shown in Table 1.

[0025]

[Table 1] From the same table, 6500 to 9000 kg/mm2 of the present invention
A diamond film with a Vickers hardness of 500℃
It can be seen that it has the above-mentioned allowable temperature limit, and that it is possible to raise the temperature of the substrate on which a film is formed, and that the heat treatment of the formed piezoelectric layer can be stopped. Further, the smoothness of the formed diamond-like layer is shown in the same table. The smoothness was measured using DEKTAK IIA (ULVAC). It can be seen that the diamond-like layer formed by the ionized vapor deposition method has good smoothness.

Measurement of refractive index and density The refractive index was measured using an ellipsometer. Density is Mg
The film formed on O was peeled off with hydrochloric acid and measured.

An AlN film was formed on the above diamond-like layer by a piezoelectric layer forming sputtering method. The film forming temperatures are shown in Table 1. The film forming conditions were gas pressure 2.2 x 10-3 Torr and sputtering RF power supply 2.
00W, Ar/N2 gas ratio 1/1, film thickness 2.
It was set to 4 μm. (1) Measurement of adhesion force, etc. Measured using a SEBASTIAN device. (2) Heat treatment of AlN Place in a heat treatment furnace for heat treatment. (3) Measure the X-ray diffraction of oriented AlN in the AlN layer before and after heat treatment. The angle of the (002) plane is fixed and its rocking curve is measured. The C-axis orientation can be evaluated from the standard deviation σ.

Formation of electrodes Electrodes are formed by vapor depositing Al. A cross-sectional view of the surface acoustic wave device of the present invention thus obtained is shown in FIG. In the figure, 1
2 represents a substrate, 2 represents a diamond-like layer, 3 represents a piezoelectric layer, and 4 represents an electrode. The structure of the surface acoustic wave device of the present invention is not limited to the structure shown in the figure, and within the scope of the present invention, those skilled in the art can arbitrarily select the positions of the electrodes, the presence or absence of counter electrodes, etc. Included within the scope of.

[0029]

[Effects of the Invention] According to the method of the present invention, the crystal orientation of the piezoelectric layer can be sufficiently improved, the bond between the diamond layer and the piezoelectric layer is also good, and a smooth piezoelectric layer can be obtained. can be formed. The surface acoustic wave device of the present invention obtained by this method is a surface acoustic wave device with high performance and high quality in terms of mechanical strength, sound velocity, etc., and is therefore suitable for applications such as high frequency filters.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between Vickers hardness and allowable temperature limit of a diamond-like film formed on a substrate.

FIG. 2 is a diagram showing a surface acoustic wave device structure that is a specific example of the present invention.

[Explanation of symbols]

1　Substrate, 2 Diamond-like layer, 3 piezoelectric layer, 4 electrode,

Claims (4)

[Claims] 1. A surface acoustic wave device comprising a diamond-like layer, a piezoelectric layer, and an electrode layer, wherein the diamond-like layer has a Vickers hardness of 6500 to 9000 kg/mm2.
A surface acoustic wave device characterized by: 2. A surface acoustic wave device comprising a diamond-like layer, a piezoelectric layer, and an electrode layer, wherein the diamond-like layer has a refractive index of 2.35 to 2.41 and a density of 3.
45 to 3.50. 3. A method for manufacturing a surface acoustic wave device comprising a diamond-like layer, a piezoelectric layer, and an electrode layer, the method comprising:
Vickers hardness of diamond film is 6500-9000
A method for manufacturing a surface acoustic wave element, characterized in that the surface acoustic wave element is formed so as to have a weight of 1 kg/mm2. 4. The surface acoustic wave device according to claim 1, wherein the diamond-like layer is formed by an ionized vapor deposition method.