JPH06252684A - Surface acoustic wave element - Google Patents

Abstract

PURPOSE:To obtain a surface acoustic wave element in which a stress at temperature rise, avoiding problems such as exfoliation and applying the element to applications for a GHz region. CONSTITUTION:The element has a 1st diamond thin film 2 formed on a substrate 1, a piezoelectric film 3 formed on the 1st diamond thin film 2, a couple of electrodes 4 whose pattern is formed interdigitally onto the piezoelectric film 3, and a 2nd diamond thin film 2 provided to cover the piezoelectric film 3 and the electrodes 4. The 2nd diamond thin film 2 is supported by the substrate 1 and a couple of the substrates 1 are fixed by a clamp 5 to press the 2nd diamond thin film 2 to the piezoelectric film 3 with the electrodes 4 provided thereto.

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 an extremely high frequency filter or the like.

[0002]

2. Description of the Related Art Diamond has advantages such as a large band gap and excellent chemical resistance and radiation resistance, and is expected to be applied to environment-resistant devices. In particular, attention has been paid to the fact that diamond has the highest sound velocity among various materials, and a surface acoustic wave device using a diamond film as a part of the substrate has been proposed (JP-A-2-299309, etc.). . It is expected that the use of a diamond thin film as a part of the substrate will increase the propagation velocity of the surface acoustic wave, and that it can be applied to the surface acoustic wave in the GHz region by using a general-purpose photolithography technique.

In a conventional surface acoustic wave device using diamond, a piezoelectric thin film is formed or adhered on a substrate on which a diamond thin film is formed, and the piezoelectric thin film or the interface between the diamond film and the piezoelectric thin film is formed. It has a structure in which a metal electrode is formed.

For example, as shown in FIG. 3, in a conventional surface acoustic wave device using diamond, a diamond thin film 12 is formed on a substrate 11, and this diamond thin film 12 is formed.
A piezoelectric body 13 made of a ZnO thin film and a pair of electrode pads 15 and 17 are formed on the top. Then, a pair of comb-shaped electrodes 14 and 16 are patterned on the piezoelectric body 13.

In the conventional surface acoustic wave device having such a structure, the comb-shaped electrode 14 is interposed via the electrode pad 15.
When a voltage is applied to the surface acoustic wave propagates in the direction of the arrow on the surface of the piezoelectric body 13, the surface acoustic wave is incident on the comb-shaped electrode 16 for reception and is detected as a voltage. The voltage of the comb electrode 16 is detected by connecting a measuring device to the electrode pad 17. As a result, surface acoustic waves are detected.

[0006]

However, since the conventional surface acoustic wave device using diamond has diamond formed on one side of the piezoelectric film or adhered thereto, when the device is heated, The stress at the interface between the piezoelectric body 13 and the diamond thin film 12 becomes large, and problems such as peeling easily occur. In addition, a research result has been reported that the bulk mode (a sound wave that propagates inside a solid) in a diamond film has a higher propagation speed than the Rayleigh wave (a sound wave that propagates along the surface of a solid, which is a type of surface sound wave). There is. For example, in diamond, the bulk mode longitudinal wave has a sound velocity of 18000 m / s, the transverse wave has a sound velocity of 12000 m / s, the surface acoustic wave has a sound velocity of 11000 m / s, and the bulk mode has a higher propagation velocity (K .Yamanouchi et.al., IEEE Ultrasonics Symp
osium Proceeding, p.351 (1989)). However, the surface acoustic wave device utilizing the bulk mode in the diamond film has not been developed by the conventional technique, and it cannot be said that the possibility of applying diamond to higher frequencies has been sufficiently investigated.

The present invention has been made in view of the above problems, and provides a surface acoustic wave device capable of relaxing stress at the time of temperature rise, avoiding problems such as peeling, and being applicable in the GHz range. The purpose is to provide.

[0008]

A first surface acoustic wave device according to the present invention comprises a first diamond thin film formed on a substrate, and a piezoelectric film formed on the first diamond thin film. It is characterized by having a pair of electrodes locally formed on the piezoelectric film, and at least a second diamond thin film provided on the piezoelectric film.

A second surface acoustic wave device according to the present invention comprises a first diamond thin film formed on a substrate, an electrode formed on the first diamond thin film, the first diamond thin film and the electrode. It is characterized in that it has a piezoelectric film formed thereon and a second diamond thin film provided on this piezoelectric film.

[0010]

In the present invention, heat is uniformly transferred from both sides of the piezoelectric film by covering the piezoelectric film with the diamond thin film or by pressing the diamond thin film onto the piezoelectric film to form a sandwich structure. To escape. Since diamond has the highest thermal conductivity among various substances, heat is rapidly conducted and stress at the time of temperature rise is relaxed. This makes it possible to form a surface acoustic wave device that does not cause problems such as peeling.

Further, depending on the thickness of the diamond film, bulk modes in the diamond film are coupled. That is, when the diamond film has only one surface as in the prior art, most of the excitation is surface waves. In the case of a diamond film having only one surface, if the film thickness is smaller than the film thickness of the piezoelectric body, the bulk mode of the substrate under the diamond film is coupled, and the advantage of using the diamond film is lost. However, as in the present invention, by sandwiching the piezoelectric film between the diamond films, the upper and lower diamond films are simultaneously excited, and not only the surface wave but also the bulk mode in the diamond film is coupled. It can be used as an elastic wave or an interfacial elastic wave.
In this case, the diamond film may be thicker than the piezoelectric film. Such an action occurs when the piezoelectric film is sandwiched between diamond films and cannot be obtained by the conventional structure. That is, this mode has not been conventionally used as a leak, but the present invention enables the use of this mode, and the application in the GHz region becomes easier.

Furthermore, the surface elastic element of the present invention can use a general-purpose photolithography technique even when it is used in the same frequency band as the conventional one, and the yield is improved. Center frequency f
Is the propagation velocity v and the wavelength λ (4 times the electrode interval (line width))
Is obtained as f = v / λ. Since conventional materials other than diamond have a slow propagation speed, it is necessary to reduce the electrode spacing in order to use them at high frequencies. However, it is difficult to process submicrons by photolithography using a general-purpose UV exposure device, and the yield of electrode formation of a size that can be used at high frequency is poor. On the other hand, when diamond is used, since the propagation speed is high, the electrode interval can be widened, the problem of fine processing is solved, and the yield is improved.

[0013]

Embodiments of the present invention will be specifically described below with reference to the accompanying drawings. As shown in FIG. 1, a first surface acoustic wave device according to an embodiment of the present invention includes a first diamond thin film 2 formed on a substrate 1 and a piezoelectric film formed on the first diamond thin film 2. The body film 3, a pair of electrodes 4 patterned in a comb shape (see FIG. 3) on the piezoelectric film 3, and the second diamond provided so as to cover the piezoelectric film 3 and the electrodes 4. And a thin film 2. This second
The diamond thin film 2 is also supported by the substrate 1, and the pair of substrates 1 are fixed by the clamp 5 to press the second diamond thin film 2 onto the piezoelectric film 3 provided with the electrode 4.

This surface acoustic wave device is manufactured by the following steps. First, an undoped diamond thin film 2 (first diamond thin film) is, for example, about 100 μm on the scratched Si substrate 1 by the microwave plasma CVD method.
The film is formed with a thickness of m. As the diamond synthesis gas, for example, hydrogen-diluted mixed gas of methane (0.5%) and oxygen (0.1%) is used, the gas pressure is 35 Torr, and the substrate temperature is 800 ° C. to form a film. After forming the diamond thin film 2, the surface of the diamond thin film 2 is polished and smoothed.

Next, Zn is deposited on the diamond thin film 2.
The piezoelectric film 3 is formed by forming an O thin film with a thickness of 0.8 μm. Then, after applying a photoresist on the piezoelectric film 3, the photoresist is exposed and developed with a predetermined electrode pattern by using a mask to form an electrode pattern on the photoresist. As a result, the region other than the portion where the metal electrode is formed is covered with the photoresist.

Then, Mo is sputtered on the entire surface.
After forming a thin film with a thickness of about 2000Å, the Mo thin film on the photoresist provided in the region other than the electrode portion is removed together with the photoresist by lift-off, thereby forming a regular comb-shaped electrode.

Then, a second diamond thin film 2 is formed on a new substrate 1 separately from the substrate on which the first diamond thin film 2 is formed and polished. The polished second diamond thin film produced in this manner is used as the piezoelectric film 3.
They are brought into close contact with each other and joined by pressing with the clamp 5.

In the surface acoustic wave device manufactured as described above, since the front and back surfaces of the piezoelectric film 3 are sandwiched by the diamond thin films 2 having extremely high thermal conductivity, the heat is dispersed rapidly and the temperature rises. The stress during warming is relieved. Actually, as a result of manufacturing the surface acoustic wave device under the above conditions,
A temperature rise test up to ° C was conducted, but no abnormality such as film peeling was observed.

Next, another embodiment of the present invention will be described with reference to FIG. An undoped diamond thin film 2 having a thickness of 100 μm is formed on the scratched Si substrate 1 by a microwave plasma CVD method under the same conditions as in the above-described embodiment by a selective growth technique. Next, the surface of the diamond thin film 2 is polished to make it smooth. A photoresist is applied on this diamond thin film, and a pattern is exposed and developed using a mask to form an electrode pattern.
Then, a Mo thin film having a thickness of about 2000 Å was formed by sputtering, and the Mo thin film on the photoresist was removed by lift-off to form a regular comb-shaped electrode.

Next, a ZnO thin film having a thickness of about 0.8 μm is selectively formed on the diamond growth region, and the separately prepared diamond thin film 2 after polishing is joined by applying pressure, and the clamp 5 is used. Fixed Then, each element was cut using a dicing saw. When the filter characteristics of the fabricated element were evaluated, the center frequency was 1.3 G
It showed a high value of Hz.

As the electrode material, M of the above embodiment is used.
In addition to o, W, conductive SiC, conductive Si (polycrystalline S
i), Ti, etc. can be used, and these laminated structures may be used as electrodes. Furthermore, as the piezoelectric film,
Inorganic compounds such as ALN, SiO ₂ , ZnS and SiC can also be used.

[0022]

According to the present invention, the heat generated in the piezoelectric film can be quickly dispersed through the pair of diamond films sandwiching the piezoelectric film, and the stress at the time of temperature rise is relaxed. be able to.

[Brief description of drawings]

FIG. 1 is a sectional view showing a surface acoustic wave device according to an embodiment of the present invention.

FIG. 2 is a sectional view showing a surface acoustic wave device according to another embodiment of the present invention.

FIG. 3 is a perspective view showing a conventional surface acoustic wave device.

[Explanation of symbols]

 1, 11; Substrate 2, 12; Diamond thin film 3, 13; Piezoelectric film 4, 14, 16; Electrode 5; Clamp

Claims (2)

[Claims] 1. A first diamond thin film formed on a substrate, a piezoelectric film formed on the first diamond thin film, and a pair of electrodes locally formed on the piezoelectric film. A surface acoustic wave element comprising at least a second diamond thin film provided on the piezoelectric film. 2. A first diamond thin film formed on a substrate, an electrode formed on the first diamond thin film, a piezoelectric film formed on the first diamond thin film and the electrode, and the piezoelectric film. A surface acoustic wave device having a second diamond thin film provided on a body film.