Surface acoustic wave device and method for producing the same

Abstract: A surface acoustic wave device having a diamond layer (2), a piezoelectric layer (3) and a comb-like electrode (4), in which the piezoelectric layer and the comb-like electrode are formed on a surface of the diamond layer, whose surface has been contacted to a substrate used in the formation of the diamond layer by a vapor phase growth method. The substrate is then removed chemically to produce a highly stable device in an economical way.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a highly stable surface acoustic wave device which operates in a high frequency range and is produced economically. Description of the Related Art Since a surface acoustic wave device utilizing a surface acoustic wave which propagates with energy concentrated on a surface of a solid material is a compact and stable device, it is practically used as an intermediate frequency filter in a TV set and the like. The surface acoustic wave device is usually excited by applying an alternating electric field on a piezoelectric material with comb-like electrodes. Such devices are, for example, disclosed in the US-patent No. 4 952 832 and the Japanese patent application No. 6 152 1158. As the piezoelectric material, bulk single crystals such as quartz crystal, LiNbO 3 , LiTaO 3 , etc. or a ZnO thin film which is vapor deposited on a substrate is used. In general, an operation frequency f of the surface acoustic wave device is determined by the equation: f = v/λ in which v is a propagation velocity of the surface acoustic wave and λ is a wavelength. The wavelength λ is determined from a period of the comb-like electrode as shown in Figs. 1 and 2. In Fig. 1, electrode tips each having a width d are integrally formed with a distance 3d. A pair of adjacent electrode tips are of the different electrodes and every other electrode tips are of the same electrodes. The comb-like electrode of Fig. 1 is one of the most commonly used electrodes. The wavelength with this type of electrodes is 4d. In the comb-like electrode of Fig. 2, two electrode tips each having a width d are repeatedly arranged with a distance 5d. The wavelength λ is 8d/3. With the comb-like electrode of Fig. 2, a three-times mode is strongly excited. The propagation velocity v depends on the piezoelectric material or the substrate material and also on the mode of surface acoustic wave. When the single crystal piezoelectric material made of LiNbO 3 is used, the propagation velocity v is from 3500 to 4000 m/sec., and when that made of LiTaO 3 is used, the propagation velocity v is from 3300 to 3400 m/sec. When the piezoelectric material comprising the ZnO thin film formed on the glass plate is used, the propagation velocity v is at most 3000 m/sec. To increase the operation frequency f, the propagation velocity v is increased and/or the wavelength λ is decreased. However, the propagation velocity v is limited by the characteristics of the material. The period size of the comb-like electrode has a lower limit due to limitation of fine processing technique. By the photolithography, the lower limit of the period size is 0.8 µm. With the electron beam exposure, the processing to submicron order is possible. However, the line width becomes smaller, an yield becomes worse. That is, because of the limitation of the processing technique, the wavelength λ cannot be reduced significantly. By the above reasons, the operation frequency of the practically used surface acoustic wave device is at most 900 MHz. By the way, as the frequency in the telecommunication such as satellite telecommunication or mobile telecommunication is increased, it is increasingly required to provide a surface acoustic wave device which can be used in a high frequency range (GHz band), and such surface acoustic wave device is being developed vigorously. In general, to apply the piezoelectric thin film grown on the substrate in the surface acoustic wave device, when the sound velocity through the substrate is larger than that through the piezoelectric material, plural surface acoustic waves having different propagation velocities (zeroth order mode, first order mode, second order mode and so on from the wave having the smaller propagation velocity) are generated. When the sound velocity through the substrate material is larger, the propagation velocity v becomes larger. Then, a prototype device comprising a substrate of sapphire through which the sound velocity is large (a velocity of transversal wave: 6000 m/sec., a velocity of longitudinal wave: 12,000 m/sec.) and a ZnO piezoelectric thin film formed thereon was produced (cf. Japanese Patent Kokai Publication No. 154088/1975). This prototype device achieved the propagation velocity of 5500 m/sec. Since the sound through diamond has the largest velocity (a velocity of transversal wave: 13,000 m/sec., a velocity of longitudinal wave: 16,000 m/sec.), a surface acoustic wave device comprising a diamond substrate will realize a propagation velocity of 10,000 m/sec. or larger. Since the sound velocity through a diamond-like carbon is substantially the same as that through the diamond, a device comprising the diamond-like carbon substrate will realize the same propagation velocity as in case of the device comprising the diamond substrate. Such device is described in Japanese Patent Kokai Publication Nos. 20714/1989 and 62911/1989). As an example of a laminated structure of a diamond layer, a piezoelectric layer and electrodes, a structure of Fig. 3 is contemplated, which may be prepared by growing a diamond film 2 on a substrate 1 such as a silicon plate by a vapor phase growth method, forming comb-like electrodes 4 and then forming a piezoelectric layer 3 such as a ZnO layer. Since the electrodes are formed at an interface between the diamond film through which the surface acoustic wave propagates at a high speed and the piezoelectric layer, the structure of Fig. 3 is advantageous because of low loss of signals. Usually, the diamond film grown by the vapor phase growth method has an irregularity of 10 % or more in relation to a thickness of the diamond film. To increase the propagation velocity of the surface acoustic wave through the diamond film to an inherent value for the diamond, a thickness of the diamond film should be the same as or larger than a wavelength of the surface acoustic wave. That is, the diamond film should have a thickness of 0.5 to 5 µm or larger for the surface acoustic wave device to be operated in a high frequency range of 1 GHz or higher. When the diamond film having such thickness is formed by the vapor phase growth method, the as-grown diamond film has an irregularity of 0.2 to 5 µm. Since the comb-like electrode to be formed on the diamond film consists of a precise pattern of a metal film having a thickness of 0.8 to 5.0 µm, a substrate on which the comb-like electrode is formed has preferably a surface roughness (R max ) of 0.1 µm or less in view of a production yield of the device. Therefore, the surface of the diamond film should be smoothened by, for example, abrasion before the formation of the comb-like electrode. Since the diamond is extremely hard and such surface smoothening by abrasion is time and cost-consuming, a surface acoustic wave device comprising the diamond film has not been produced at a low production cost. When a surface acoustic wave device has a structure of Fig. 4, since the piezoelectric layer 3 is formed on the diamond film 2, the layer 3 has substantially the same irregularity as the diamond film 2. SUMMARY OF THE PRESENT INVENTION One object of the present invention is to provide a surface acoustic wave device comprising a diamond layer having smaller irregularity on its surface on which a comb-like electrode or a piezoelectric layer is formed. Another object of the present invention is to provide a method for producing a surface acoustic wave device of the present invention. According to a first aspect of the present invention as defined in claim 1, there is provided a surface acoustic wave device comprising a diamond layer, a piezoelectric layer and a comb-like electrode, wherein said piezoelectric layer and said comb-like electrode are formed on a surface of said diamond layer, which surface has been contacted to a substrate used in the formation of said diamond layer by a vapor phase growth method and has a surface-roughness R max of 0.1 µm or less. According to a second aspect of the present invention as defined in claim 7, there is provided a method for producing a surface acoustic wave device comprising steps of: forming a diamond layer on a substrate by a vapor phase growth method, said diamond layer having a surface roughness R max of 0.1 µm or less, removing said substrate, and forming a piezoelectric layer and a comb-like electrode on a surface of said diamond layer which surface has been contacted to said substrate. According to a third aspect of the present invention, there is provided a method for producing a surface acoustic wave device comprising steps of: forming a diamond layer on a substrate by a vapor phase growth method, providing support means on a surface of said grown diamond layer, removing said substrate, and forming a piezoelectric layer and a comb-like electrode on a surface of said diamond layer which surface has been contacted to said substrate. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 and 2 are plane views of two examples of comb-like electrodes, Fig. 3 schematically shows a cross section of one example of a conventional surface acoustic wave device, Fig. 4 schematically shows a cross section of another example of a conventional surface acoustic wave device, Figs. 5, 6 and 7 schematically show cross sections of the surface acoustic wave devices according to the present invention. DETAILED DESCRIPTION OF THE DRAWINGS As a substrate on which the diamond layer is formed by the vapor phase growth method, a substrate having a surface roughness (R max ) of 0,1 µm or less is used. According to the present invention, a surface of the diamond layer which is present at an interface between the substrate and the diamond layer has substantially the same roughness as that of the substrate surface. Since it is easy to process a surface of the substrate to a surface roughness of 0.02 µm or less, the substrate to be used according to the present invention has a surface roughness of 0.02 µm preferably. Since the substrate should be removed after the formation of the diamond layer, a substrate material is preferably removed by, for example, dissolution with an acid, an alkali or other chemical materials. Preferred examples of the substrate material are semiconductive materials (e.g. Si, Ge, etc.) and metals (e.g. Mo, W, etc.). Also, oxides such as SiO 2 or oxide mixtures may be used. In some cases, the substrate is flawed with an abrasive grain such as diamond grain to control a crystal size of the grown diamond. In such cases, the surface roughness of the flawed substrate is 0.1 µm or less, and preferably 0.02 µm or less. As the vapor phase growth method of the diamond layer, a thermal CVD (chemical vapor deposition) method (cf. Japanese Patent Kokai Publication No. 91100/1983), a plasma CVD method (cf. Japanese Patent Kokai Publication Nos. 135117/1983 and 110494/1983), an ion beam method, a laser CVD method and a combustion flame method are known. Among them, the thermal CVD method and the plasma CVD method are preferred in view of uniform growth of the diamond layer. The diamond layer according to the present invention comprises carbon atoms mainly and contains at least partly crystalline diamond which is confirmed by an X-ray diffraction, an electron beam diffraction or a Raman spectroscopy. The diamond layer may contain other carbonaceous materials such as graphite, amorphous carbon or diamond-like carbon. As a content of diamond in the diamond layer increases, a propagation velocity of the surface acoustic wave increases. The diamond layer may contain, as an impurity, a minor amount of an element such as B, N, O, Al, Si, P, Ti, W, Ta, Fe, Ni and the like. When such impurity element or the above non-diamond carbonaceous material is contained, the diamond layer preferably has a resistivity of at least 10 6 Ω.cm. A thickness of the diamond layer is at least a half of a wavelength of the surface acoustic wave at a frequency of the surface acoustic wave device. In view of strength of the diamond layer, the thickness of the diamond layer is preferably at least 5 µm, more preferably from 10 to 100 µm. Since strength of the diamond layer having the thickness of 100 µm or less, in particular 10 µm or less may not be sufficient, its surface opposite to the surface on which the surface acoustic wave device is formed may be fixed to a support means by brazing or adhesion. In this case, the diamond layer is grown on the substrate of, for example, Mo, the support means is fixed to the grown surface of the diamond layer by brazing and the like, and the substrate is removed from the diamond layer. Examples of the piezoelectric material are ZnO, AlN, PZT, PLZT, SiO 2 , LiTaO 3 , LiNbO 3 and the like. In view of easiness of the formation of a c-axis orientated film, ZnO is preferred. A thickness of the piezoelectric layer is usually at least 0.1 µm, preferably 0.2 to 7 µm. The piezoelectric layer may be formed by any of conventional vapor phase synthesis methods such as sputtering, CVD, reactive vapor deposition and the like. By these methods, the piezoelectric layer which has large piezoelectricity and good orientation in the c-axis is grown. When a degree of the c-axis orientation is too small or a resistivity of the piezoelectric layer is too small, an electro-mechanical coupling factor K 2 is small. Preferably, a degree σ of the orientation which is calculated from an X-ray locking curve against the c-axis of ZnO is not larger than 3, and the resistivity is at least 10 8 Ω.cm. Three examples of the surface acoustic wave device according to the present invention are shown in Figs. 5, 6 and 7. In Fig. 5, the surface acoustic device comprises a diamond layer 2, a piezoelectric layer 3 and comb-like electrodes 4. In Fig. 6, the comb-like electrodes 4 are sandwiched between the diamond layer 2 and the piezoelectric layer 3. The embodiment of Fig. 7 has a support 6 which is fixed to the diamond layer 2 with a brazing layer 5. Example A surface acoustic wave filter having a structure of Fig. 6 and its filtering property was measured. The diamond layer 2 having a thickness of 40 µm was formed on a single crystal silicon substrate having a surface roughness (R max ) of 0.01 µm (a radius of 50 mm) by a thermal filament CVD method, which is disclosed, for example, in Japanese Patent Kokai Publication No. 9110/1983 under the following growing conditions: Raw material gases: CH 4 /H 2 = 1/100 Substrate temperature: 830°C Reaction pressure: 50 Torr. The silicon substrate was dissolved and removed with fluoronitric acid. The diamond layer had a resistivity of 10 10 Ω.m or larger. On a surface of the diamond layer which was contacted to to the silicon substrate, an aluminum film was vacuum deposited and then two pairs of comb-like electrodes were formed by photolithography. A line width of the electrode was 2 µm (λ = 8 µm) and the number of tip pairs was 25. On the surface of the diamond layer having the comb-like electrodes, a ZnO film was formed by an RF magnetron sputtering method under the following conditions: Substrate temperature: 400°C Sputtering gases: Ar/O 2 = 1/1 RF power: 250 W Pressure: 0.02 Torr. By controlling a sputtering time, the ZnO film having a thickness of 2.5 µm was formed. The ZnO film had the a value of 2.2, which means a good orientation in the c-axis, and a resistivity of 10 8 Ω.cm. The maximum resonance frequency of the produced surface acoustic wave filter was 1.1 GHz.

Claims: A surface acoustic wave device comprising a diamond layer, a piezoelectric layer and a comb-like electrode, wherein said piezoelectric layer and said comb-like electrode are formed on a surface of said diamond layer, which surface has been contacted to a substrate used in the formation of said diamond layer by a vapor phase growth method and has a surface roughness R max of 0.1 µm or less. The surface acoustic wave device according to claim 1, wherein said diamond layer has a thickness of at least 5 µm. The surface acoustic wave device according to claim 1, wherein said diamond layer has a resistivity of at least 10 6 Ω.cm. The surface acoustic wave device according to claim 1, wherein said piezoelectric layer is made of at least one material selected from ZnO, AlN, PZT, PLZT, SiO 2 , LiTaO 3 and LiNbO 3 . The surface acoustic wave device according to claim 1, wherein said piezoelectric layer has a resistivity of at least 10 4 Ω.cm. A surface acoustic wave device according to claim 1, wherein the surface roughness R max is 0.02 µm or less. A method for producing a surface acoustic wave device comprising steps of: forming a diamond layer on a substrate by a vapor phase growth method, said diamond layer having a surface roughness R max of 0.1 µm or less, removing said substrate, and forming a piezoelectric layer and a comb-like electrode on a surface of said diamond layer which surface has been contacted to said substrate. Method according to claim 7, which further comprises the step of providing support means on a surface of said grown diamond layer, before the step of removing the substrate.

Inventors: FUJIMORI NAOJI, HACHIGO AKIHIRO, IMAI TAKAHIRO, NAKAHATA HIDEAKI, FUJIMORI, NAOJI, HACHIGO, AKIHIRO, IMAI, TAKAHIRO, NAKAHATA, HIDEAKI, FUJIMORI, NAOJI, C/O ITAMI WORKS OF, HACHIGO, AKIHIRO, C/O ITAMI WORKS OF, IMAI, TAKAHIRO, C/O ITAMI WORKS OF, NAKAHATA, HIDEAKI, C/O ITAMI WORKS OF
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD
Application year: 1992
Main classification: H03H9
IPC codes: H03H9, H03H3