JPH08316782A - Surface acoustic wave element - Google Patents

Abstract

PURPOSE: To improve the performance by forming a protection film made of a material more stable chemically than a high sound velocity thin film between the high sound velocity thin film and a piezoelectric thin film so as to prevent deterioration in the high sound velocity thin film. CONSTITUTION: The surface acoustic wave resonator is formed by sequentially forming a high sound velocity thin film 2 made of a polycrystal diamond, a protection film 5 made of platinum, a piezoelectric thin film 3 made of an aluminum nitride, and an electrode 4 made of aluminum onto a silicon substrate 1. Through the constitution above, since the thin film 2 is covered by the chemically stable film 5, when the thin film 3 is formed by using various thin film forming technologies, even when a substrate temperature at film forming is increased or the film is exposed to plasma, oxidation of the thin film 2 or deterioration in the film is not caused. Thus, the substantial performance of the film 2 is obtained and the element with an expected characteristic is acquired.

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 usable in a high frequency band and a method for manufacturing the same.

[0002]

2. Description of the Related Art In communication equipment such as portable telephones,
Surface acoustic wave devices are used as circuit devices such as resonator filters and signal processing delay lines. The surface acoustic wave element is one in which, for example, a comb-shaped electrode or a reflector is formed on the surface of a substrate having piezoelectricity, and an electric signal and a surface acoustic wave are mutually converted. Generally, the piezoelectric substrate of the surface acoustic wave device is required to have a large electromechanical coupling coefficient and a small propagation loss.

By the way, with the recent increase in the frequency of communication equipment, there is an increasing need for a surface acoustic wave element that can be used in the gigahertz band. Center frequency f _{0 of the} surface acoustic wave element
Is expressed by the following equation in the relationship between the propagation velocity V of the surface acoustic wave and the wavelength λ.

F ₀ = V / λ where the wavelength λ is determined by the electrode pitch d (λ
= 4d). Therefore, in order to cope with the higher frequency of the surface acoustic wave element, a method of using a material having a high sound velocity (surface acoustic wave propagation velocity) or a method of narrowing the electrode pitch can be considered.

Conventionally, lithium tantalate (LiTaO ₃ ) or lithium niobate (LiNbO ₃ ) has been used as the substrate material of the surface acoustic wave element, but a sufficient sound velocity has not yet been obtained.

Therefore, in a structure in which a piezoelectric thin film for exciting surface acoustic waves is formed on the substrate surface using a material that does not have piezoelectricity but can obtain a higher sound velocity than lithium tantalate or lithium niobate. Surface acoustic wave devices have been studied. In addition, as materials for such a high sonic substrate, silicon, sapphire, magnesium oxide (MgO) are used.
Are being considered.

Further, as shown in FIG. 7, a substrate such as silicon is used.
On the surface of (1), a high acoustic velocity thin film (2) composed of a diamond or a diamond-like (hereinafter collectively referred to as diamond) carbon layer is formed, and a piezoelectric thin film (3) is formed on the surface of the high acoustic velocity thin film (2). A surface acoustic wave element having electrodes and electrodes (4) has been studied. According to the surface acoustic wave element, it is possible to configure a communication device that can be used in a high frequency band exceeding 1 GHz.

[0007]

By the way, in the manufacturing process of the surface acoustic wave device having the structure shown in FIG. 7, the piezoelectric thin film (3) is used.
A thin film forming technique such as sputtering, CVD, vacuum deposition, or ion plating is used to form the film. Here, in order to obtain a high-quality piezoelectric thin film, it is desirable that the substrate temperature during film formation is high.

However, if the substrate temperature is raised, the temperature of the carbon layer of diamond, which becomes the high acoustic velocity thin film (2), is also raised. The carbon layer of diamond easily oxidizes at high temperatures, and as a result, the quality of the film deteriorates.

Further, when the piezoelectric thin film is formed by the RF sputtering method or the like, there is a problem that the carbon layer of diamond is exposed to plasma and the film quality is similarly deteriorated.

Further, as the diamond carbon layer, polycrystalline diamond is used because it is difficult to produce a large-diameter single crystal thin film, but a high-quality piezoelectric thin film is formed on the polycrystalline diamond. It is extremely difficult to do.

Therefore, in the structure shown in FIG.
It is difficult to bring out the original performance of (2),
A surface acoustic wave device having desired characteristics cannot be obtained. An object of the present invention is to provide a surface acoustic wave element and a method for manufacturing the same, which can bring out the original performance of the high acoustic velocity thin film.

[0012]

In a surface acoustic wave device according to the present invention, a high acoustic velocity thin film (2) made of a hard material and a thin film (3) made of a piezoelectric material are formed on a surface of a substrate (1). Then, an electrode for exciting a surface acoustic wave on the surface of the piezoelectric thin film (3).
(4) is formed, and a protective film (5) made of a material more chemically stable than the high sonic thin film (2) is provided between the high sonic thin film (2) and the piezoelectric thin film (3). It is characterized by being formed.

Further, the method of manufacturing a surface acoustic wave device according to the present invention comprises a high acoustic velocity thin film made of a hard material on the surface of the substrate (1).
A first film forming step of forming (2), a second film forming step of forming a thin film (3) made of a piezoelectric material on the high acoustic velocity thin film (2), and a surface of the piezoelectric thin film (3). And an electrode forming step of forming an electrode (4) for exciting the surface acoustic wave. In the second film forming step, the piezoelectric thin film (3) is formed on the surface of the high acoustic velocity thin film (2).
The method is characterized in that the piezoelectric thin film (3) is formed after the protective film (5) for protecting the high acoustic velocity thin film (2) is formed in the film forming process (1).

[0014]

In the above-described surface acoustic wave device and the method for manufacturing the same, a high quality piezoelectric material is used when the piezoelectric thin film (3) is formed by using a thin film forming technique such as sputtering, CVD, vacuum deposition or ion plating. Even if the substrate temperature during film formation is raised in order to obtain a thin film, the high-sonic velocity thin film (2) is covered with a chemically stable protective film (5) such as platinum, so contact with oxygen is prevented. And its oxidation is prevented.

Even when the piezoelectric thin film is formed by the RF sputtering method or the like, the high acoustic velocity thin film (2) is a protective film.
Since it is covered by (5), it is not exposed to plasma, and therefore the film quality does not deteriorate.

Further, even when the high acoustic velocity thin film (2) is a polycrystalline diamond thin film, if the protective film (5) is formed from a material having good crystallinity such as platinum, the protective film (5) will be formed. By forming the piezoelectric thin film (3) on the surface, high quality film quality can be obtained.

It should be noted that the protective film (5) does not impede the sonic performance that the high sonic velocity thin film (2) should exhibit, and is of a thickness sufficient to exert a protective function for the high sonic velocity thin film (2), for example, platinum. Protective film
In the case of (5), it is formed to a thickness of several to several hundred Å.

[0018]

According to the surface acoustic wave device and the method for manufacturing the same of the present invention, the high acoustic velocity thin film is not deteriorated, so that it is possible to bring out its original performance. A surface acoustic wave device having characteristics can be obtained.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An example in which the present invention is applied to a surface acoustic wave resonator will be described in detail below with reference to the drawings. As shown in Figure 1,
The surface acoustic wave resonator of the present invention comprises a substrate (1) made of (110) silicon, a high acoustic velocity thin film (2) made of polycrystalline diamond and having a thickness of 7500Å, and a thickness of 100 made of platinum.
Å protective film (5), thickness 1.2 made of aluminum nitride
Electrodes made of piezoelectric thin film (3) of 3 μm and aluminum
(4) is formed.

In the process of manufacturing the surface acoustic wave resonator, first, a polycrystalline diamond thin film having a thickness of 7500Å is formed on the surface of the substrate (1) by the ECR plasma CVD method to obtain a high acoustic velocity thin film ( 2). At this time, as the film forming conditions, the source gas CH ₄ + H ₂ was used, and the reaction pressure was 30
Torr and bias voltage were set to 100V.

Next, a thin film of platinum (Pt) having a thickness of 100 Å was formed on the surface of the high acoustic velocity thin film (2) by using an ion beam sputtering method to form a protective film (5). At this time, the deposition rate was set to 20 Å / min at room temperature. X-ray diffraction analysis of the protective film (5) revealed that the platinum (111)
It was found that the planes were oriented.

Then, a thin film of aluminum nitride (AlN) having a thickness of 1.2 μm was formed on the surface of the protective film (5) by the ECR dual ion beam sputtering method. At this time, as film forming conditions, the substrate temperature is 300 ° C., the energy of the nitride ion beam is 100 eV, and the film forming rate is 3
It was set to 6Å / min.

After that, an electrode (4) having a pattern shown in FIG. 2 was formed on the surface of the protective film (5). At this time, first, an aluminum film is formed to 2000 using an ion beam sputtering method.
After forming the aluminum film to a thickness of Å, a photolithography method is used to show the aluminum film.
(6) (7) and the comb-shaped reflectors (8) and (9) on both sides thereof were molded. In addition, the comb transducers (6) and (7) and the comb reflector
(8) The line width and the space between lines (9) are set to 1.0 μm, the number of electrode pairs of the comb-shaped transducers (6) and (7) is 40, and the number of comb-shaped reflectors (8) and (9) is 200, respectively. did.

FIG. 3 shows the result of X-ray diffraction analysis and the rocking curve of the aluminum nitride piezoelectric thin film (3) formed on the surface of the platinum protective film (5) as described above. On the other hand, FIG. 4 shows the result of the X-ray diffraction analysis and the rocking curve in the case where the piezoelectric thin film (3) of aluminum nitride was directly formed on the surface of the high acoustic velocity thin film (2) of diamond as in the conventional case. Is shown.

In each case, with respect to aluminum nitride, only the (0002) diffraction line and the (0004) diffraction line were observed, indicating that an aluminum nitride thin film oriented in the C axis was obtained. However, comparing Figure 3 and Figure 4,
In the invention of FIG. 3, the diffraction intensity of the (0002) diffraction line and the (0004) diffraction line of aluminum nitride is higher.
Further, when the half-widths of the rocking curves of the two are compared, they are 2.4 degrees and 8.6 degrees, respectively, which is smaller in the present invention of FIG. This shows that the crystallinity of the piezoelectric thin film (3) was improved by forming the piezoelectric thin film (3) on the surface of the protective film (5).

FIG. 5 shows the pass characteristic of the surface acoustic wave resonator of the above embodiment, and FIG. 6 shows the pass characteristic of the conventional surface acoustic wave resonator having no protective film. However, the conventional resonator was manufactured by the same method as that of the resonator of this example except for the formation of the protective film.

As shown in FIG. 5, the center frequency of the surface acoustic wave resonator of the present invention is 1.72 GHz, and the speed of sound calculated from this is about 8600 m / s. In this case, when the electrode pitch is 0.6 μm, the center frequency is 2.87 GH
It is possible to raise to z.

On the other hand, in the conventional surface acoustic wave resonator having no protective film, the center frequency is 1.2 as shown in FIG.
The frequency is 7 GHz, which is about 5090 m / s when converted to the speed of sound. The reason for the decrease in the sound velocity is that the diamond thin film was exposed to plasma and deteriorated during the formation of the aluminum nitride thin film, and the characteristics relating to the sound velocity of diamond were impaired.

Further, comparing the losses of both, the surface acoustic wave resonator of the present invention has a value of 7.2 dB, while the conventional surface acoustic wave resonator has a large value of 22.5 dB. This is because in the conventional surface acoustic wave resonator, the surface acoustic wave is greatly attenuated due to the deterioration of the diamond thin film layer and the deterioration of the crystallinity of the aluminum nitride thin film layer. In the surface acoustic wave resonator of the present invention, the diamond thin film is prevented from being deteriorated by the formation of the protective film, and the aluminum nitride thin film having good crystallinity can be obtained. Therefore, good characteristics can be realized.

The above description of the embodiments is for explaining the present invention, and should not be construed as limiting the invention described in the claims or reducing the scope. Further, the configuration of each part of the present invention is not limited to the above embodiment, but various modifications can be made within the technical scope described in the claims.

For example, in the above embodiment, diamond is used as the material for the high-sonic velocity thin film (2), but boron nitride is used.
Other hard materials such as (BN) can be used.
Although aluminum nitride is used as the material of the piezoelectric thin film (3), ZnO, LiTaO ₃ , LiNbO ₃ , Li ₂ B ₄ O are used.
Other piezoelectric materials such as ₇ can be used. Furthermore, it goes without saying that the present invention can be implemented not only in the above-mentioned resonator but also in any type of surface acoustic wave device such as a filter, a delay line, a convolver and the like.

[Brief description of drawings]

FIG. 1 is a sectional view showing an example in which the present invention is applied to a surface acoustic wave resonator.

FIG. 2 is a plan view showing an electrode pattern in the surface acoustic wave resonator.

FIG. 3 is a graph showing a result of an X-ray diffraction analysis and a rocking curve of aluminum nitride forming the surface acoustic wave resonator of the present invention.

FIG. 4 is a graph of the same in the conventional surface acoustic wave resonator.

FIG. 5 is a graph showing pass characteristics of the surface acoustic wave resonator of the present invention.

FIG. 6 is a graph showing pass characteristics of a conventional surface acoustic wave resonator.

FIG. 7 is a sectional view of a conventional surface acoustic wave resonator.

[Explanation of symbols]

 (1) Substrate (2) High-sonic velocity thin film (3) Piezoelectric thin film (4) Electrode (5) Protective film (6) Comb transducer (7) Comb transducer (8) Comb reflector (9) Comb reflector

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kenichi Shibata 2-5-5 Keihan Hondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd.

Claims (3)

[Claims] 1. A high acoustic velocity thin film (2) made of a hard material and a thin film (3) made of a piezoelectric material are formed on the surface of a substrate (1),
In a surface acoustic wave device in which an electrode (4) for exciting a surface acoustic wave is formed on the surface of the piezoelectric thin film (3), between the high acoustic velocity thin film (2) and the piezoelectric thin film (3). A surface acoustic wave device characterized in that a protective film (5) made of a material that is chemically more stable than the high acoustic velocity thin film (2) is formed. 2. The elastic surface according to claim 1, wherein the high-sonic velocity thin film (2) is composed of a diamond or diamond-like carbon layer, and the piezoelectric thin film (3) is composed of a platinum layer having a thickness of several hundreds of liters. Wave element. 3. A first film forming step of forming a high acoustic velocity thin film (2) made of a hard material on a surface of a substrate (1), and the high acoustic velocity thin film.
(2) A second film forming step of forming a thin film (3) made of a piezoelectric substance on the electrode, and an electrode for forming an electrode (4) for exciting a surface acoustic wave on the surface of the piezoelectric thin film (3) In the method of manufacturing a surface acoustic wave device, which comprises a forming step, in the second film forming step, the high acoustic velocity thin film (2) is formed on the surface of the high acoustic velocity thin film (2) during the film forming process of the piezoelectric thin film (3). 1. A method of manufacturing a surface acoustic wave device, comprising forming a protective film (5) to protect the piezoelectric thin film (3).