Surface acoustic wave element

Abstract: A surface acoustic wave element according to the present invention has a structure including a hard layer containing a composition component essentially consisting of at least one of diamond and a diamond-like carbon film [2], a piezoelectric layer formed on the hard layer [4], a silicon dioxide (SiO₂) [5] layer formed on the piezoelectric layer, and electrodes combined with the piezoelectric layer to perform electro-mechanical conversion. The surface acoustic wave element having the above structure according to the present invention has a larger electro-mechanical coupling coefficient and a higher surface acoustic wave propagation velocity than those of a conventional surface acoustic wave element having no silicon dioxide layer, thereby obtaining a surface acoustic wave element operated in a high-frequency range. In particular, the electro-mechanical coupling element is increased. The silicon dioxide (SiO₂) layer is an electric insulator and rarely reacts with moisture or an acid. The silicon dioxide layer can protect the piezoelectric layer and the electrodes from an external environment, thereby providing a surface acoustic wave element having good high-frequency characteristics and a high resistance to environment.

Description: The present invention relates to a surface acoustic wave device which is operated in a high-frequency range including several hundreds of MHz to GHz and has a high resistance to environment and, more particularly, to the structure of the surface acoustic wave element. A surface acoustic wave element is an electro-mechanical conversion element utilizing a surface wave propagating on the surface of an elastic body and has a basic structure shown in Fig. 14. In a surface acoustic wave element 40 in Fig. 14, the piezoelectric phenomenon of a piezoelectric body 44 is used in exciting a surface acoustic wave. When an electrical signal is applied to one comb-like electrode 43 formed on the piezoelectric body 44, the piezoelectric body 44 is stressed, and this stress becomes a surface acoustic wave. The surface acoustic wave propagates on the piezoelectric body 44 and is extracted as an electrical signal at the other comb-like electrode 43'. The frequency characteristics of the surface acoustic wave element are given as band passing characteristics having, as a center frequency, a frequency f 0 proportional to a ratio V/λ 0 of a propagation speed V of the surface acoustic wave to a pitch period λ 0 of the segments of each comb-like electrode. A surface acoustic wave element requires a smaller number of parts and can be made compact. In addition, signals can be easily input and output on a surface acoustic wave propagation path. This element can be used in a variety of applications such as a filter, a delay line, an oscillator, a resonator, a convolver, or a correlator. In particular, a surface acoustic wave filter has been used as an IF television filter. Such surface acoustic wave filters have also been applied in VTRs and various communication equipment filters as of mobile and portable telephones. A typical conventional surface acoustic filter is an element having a structure in which comb-like electrodes are formed on a crystalline piezoelectric material such as LiNbO 3 or LiTaO 3 . Another surface acoustic wave element having a piezoelectric thin film made of ZnO or the like sputtered on a substrate as of glass is used. It is, however, difficult to manufacture an element operated in a high-frequency (Ghz band) range using the conventional element structure described above. An element obtained by forming comb-like electrodes on a monocrystalline piezoelectric body cannot have a high center frequency of 1 Ghz or more because it has a low surface acoustic wave propagation speed V. As indicated by the above equation, to cause a surface acoustic wave element to have band passing characteristics having a higher center frequency, the pitch period λ 0 must be shortened, or the surface acoustic wave propagation speed V must be increased. A decrease in electrode pitch period λ 0 of the comb-like electrode to increase the center frequency is limited by micropatterning techniques such as photolithography. For this reason, various techniques for increasing the propagation speed V of the surface acoustic wave have been examined, and techniques disclosed in Japanese Patent Laid-Open Nos. 54-38874, 64-62911, 1-103310, and 3-198412 are known. A surface acoustic wave element used as a filter in the fields of mobile communications is required to be used at a higher frequency of 1.5 to 3 Ghz in accordance with frequency assignment. To realize such a filter, an element structure capable of highly efficiently performing electro-mechanical conversion and having a higher surface acoustic wave propagation speed V is required. A surface acoustic wave element as a high-frequency filter in particularly a portable telephone or the like has been considered. As is known well, an element incorporated in this telehone is adversely affected by humidity and impurities because the portable telephone is used in various environments. Therefore, the surface acoustic wave element used in the portable telephone must always maintain stable characteristics in various application environments. EP-A-0,303,495 discloses a surface acoustic wave device comprising a hard layer essentially consisting of diamond and/or diamond-like carbon; a piezoelectric layer formed directly on said hard layer; a silicon dioxide layer formed directly on said piezoelectric layer; and an electrode group. However, this device does not have stable characteristics in different environments. The present invention has been made in consideration of these conventional technical problems, and has as its object to provide a surface acoustic wave element which can be used in a high-frequency range and can maintain stable performance (high resistance to environment) to an external environment. SUMMARY OF THE INVENTION In order to achieve the above object of the present invention, a surface acoustic wave element according to the present invention has a structure comprising the features of the independent claims 1 or 5. According to the invention, a hard layer contains a composition component essentially consisting of at least one of diamond and a diamond-like carbon film, a piezoelectric layer is formed on the hard layer, a silicon dioxide (SiO 2 ) layer is formed on the piezoelectric layer, and electrodes are combined with the piezoelectric layer to perform electro-mechanical conversion. As will be described with reference to a preferred embodiment of the present invention, in a surface acoustic wave element having a piezoelectric layer and comb-like electrodes sandwiched between the layer essentially consisting of at least one of diamond and the diamond-like carbon film and the silicon dioxide (SiO 2 ) layer, when the thicknesses of the piezoelectric and silicon dioxide (SiO 2 ) layers are appropriately set, an electro-mechanical coupling coefficient and a surface acoustic wave propagation speed, and particularly, the electro-mechanical coupling coefficient can be increased as compared with a case in which the silicon dioxide (SiO 2 ) layer is not formed. That is, according to the present invention, there is provided a surface acoustic wave element which can perform electro-mechanical conversion with a higher efficiency than that of a conventional surface acoustic wave element having no silicon dioxide (SiO 2 ) layer and has a higher center frequency. In the surface acoustic wave element, the silicon dioxide (SiO 2 ) layer is formed on the piezoelectric layer and the comb-like electrodes. Silicon dioxide is an electric insulator and rarely reacts with moisture and an acid. For this reason, the silicon dioxide (SiO 2 ) layer serves as a passivation layer for protecting the main part of the element which has the piezoelectric layer and the comb-like electrodes from the external environment. The influences of humidity and impurities from the external environment can be minimized. The sign of the change coefficient of the surface acoustic wave propagation speed as a function of the temperature of a material constituting the silicon dioxide (SiO 2 ) layer is opposite to that of the change coefficient of the surface acoustic wave propagation speed as a function of the temperatures of a material constituting the hard layer containing a composition component essentially consisting of at least one of diamond and the diamond-like carbon layer and a material constituting the piezoelectric layer. When the temperature is increased, the surface acoustic wave propagation speed in the silicon dioxide (SiO 2 ) layer is increased, while the surface acoustic wave propagation speed in the hard layer containing the composition component essentially consisting of at least one of diamond and the diamond-like carbon film and the piezoelectric layer tends to be decreased. In the surface acoustic wave element according to the present invention, when the silicon dioxide (SiO 2 ) layer consisting of the material having the change coefficient with an opposite sign is stacked on the hard layer containing the composition component essentially consisting of at least one of diamond and the diamond-like carbon film and the piezoelectric layer, a canceling effect is obtained. Therefore, characteristic variations of the element against changes in temperatures, and particularly, variations in surface acoustic wave propagation speed can be minimized. According to the present invention, there is provided a surface acoustic wave element which can be used to have a high electro-mechanical conversion efficiency in a higher-frequency range and has stable characteristics. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention. Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art form this detailed description. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view showing the structure of a surface acoustic wave element according to the first embodiment of the present invention; Fig. 2 is a sectional view showing the structure of a conventional surface acoustic wave element manufactured as a comparative example; Fig. 3 is a sectional view showing the structure of a surface acoustic wave element according to the second embodiment of the present invention; Fig. 4 is a sectional view showing the structure of a surface acoustic wave element according to the third embodiment of the present invention; Fig. 5 is a sectional view showing the structure of a surface acoustic wave element according to the fourth embodiment of the present invention; Fig. 6 is a sectional view showing the structure of a surface acoustic wave element according to the fifth embodiment of the present invention; Fig. 7 is a graph showing a relationship between the thickness of an SiO 2 film and an electro-mechanical coupling coefficient in the surface acoustic wave element of the second embodiment; Fig. 8 is a graph showing a relationship between the thickness of an SiO 2 film and an electro-mechanical coupling coefficient in the surface acoustic wave element of the third embodiment; Fig. 9 is a graph showing a relationship between the thickness of an SiO 2 film and an electro-mechanical coupling coefficient in the surface acoustic wave element of the fourth embodiment; Fig. 10 is a graph showing a relationship between the thickness of an SiO 2 film and an electro-mechanical coupling coefficient in the surface acoustic wave element of the fifth embodiment; Fig. 11 is a graph showing a relationship between the thicknesses of the SiO 2 films and the surface acoustic wave propagation velocity in the surface acoustic wave elements of the second, third, fourth, and fifth embodiments; Figs. 12 and 13 are plan views showing the shapes of comb-like electrodes according to the embodiments; and Fig. 14 is a perspective view showing the basic structure of a surface acoustic wave element; Fig. 15 is a graph showing a relationship between the thicknesses of the SiO 2 and ZnO films and the electro-mechanical coefficient in the surface acoustic wave elements of the sixth embodiment; Fig. 16 is a graph showing a relationship between the thicknesses of the SiO 2 and ZnO films and the electro-mechanical coefficient in the surface acoustic wave elements of the seventh embodiment; Fig. 17 is a graph showing a relationship between the thicknesses of the SiO 2 films and the electro-mechanical coefficient in the surface acoustic wave elements of the eighth embodiment; Fig. 18 is a graph showing a relationship between the thicknesses of the SiO 2 and ZnO films and the mechanical coefficient in the surface acoustic wave elements of the ninth embodiment; Fig. 19 a sectional view of the surface acoustic wave element according to the tenth embodiment; Fig. 20 is a graph showing a relationship between the thicknesses of the SiO 2 and ZnO films and the electro-mechanical coefficient in the surface acoustic wave elements of the tenth embodiments; Fig. 21 is a graph showing a relationship between the thicknesses of the SiO 2 and ZnO films and the surface wave propagation velocity in the surface acoustic wave elements of the sixth to tenth embodiments ; and Fig. 22 is a graph showing a relationship between the thicknesses of the SiO 2 films and the temperature coefficient in the surface acoustic wave elements of the sixth to tenth embodiments. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior to a description of preferred embodiments, the structural feature, operation, and effect of a surface acoustic wave element according to the present invention will be described below. A surface acoustic wave element according to the present invention has a structure comprising a hard layer essentially consisting of diamond, a piezoelectric layer formed on the hard layer, a silicon dioxide layer formed on the piezoelectric layer, and an electrode group, sandwiched between the hard layer and the piezoelectric layer or between the piezoelectric layer and the silicon dioxide layer, for performing electro-mechanical conversion. As described above, the piezoelectric layer is sandwiched between the hard layer containing a composition component essentially consisting of diamond or a diamond-like carbon film and the silicon dioxide (SiO 2 ) layer. The main part of the element which comprises the piezoelectric layer and the electrodes can be protected, and a high electro-mechanical coupling level can be obtained. According to the present invention, the silicon dioxide (SiO 2 ) layer can be grown by, e.g, a sputtering method, an ion plating method, a CVD method, an electron beam deposition method, or a low-temperature CVD method. Silicon dioxide (SiO 2 ) may be of crystalline or amorphous. According to the present invention, the diamond-like carbon film consists of diamond-like carbon called i-carbon. The diamond-like carbon film is found in the process of studies on epitaxial synthesis of diamond. The physical properties of the i-carbon have been examined and clarified by a lot of researchers. Although no theory which defines the diamond-line carbon film as one material is found, this film is apparently different from diamond, graphite, or amorphous carbon and has the following properties. (1) The diamond-like carbon is a composition consisting of carbon and hydrogen and has a smaller number of hydrogen atoms than that of carbon atoms. (2) The crystalline state of the diamond-like carbon is amorphous. (3) The diamond-like carbon has a larger hardness value than that of at least a general metal. (4) The diamond-like carbon is an electric insulator. (5) The diamond-like carbon transmits light therethrough. Diamond has a hardness value of 10,000 (Hv), while the diamond-like carbon film has a hardness value of 1,000 to 5,000 (Hv). The diamond-like carbon film is manufactured by an epitaxial process such as plasma CVD, ion beam deposition, or sputtering as in diamond synthesis. According to the present invention, the layer containing at least diamond or a diamond-like carbon film may contain a small amount of impurity. A hard layer consisting of diamond is preferably used in favor of a higher surface acoustic wave propagation velocity and a higher electro-mechanical conversion efficiency. That is, the hard layer must contain a composition component essentially consisting of diamond. On the other hand, the diamond-like carbon film is epitaxially synthesized even at room temperature, while diamond requires a synthesis temperature of 500°C or more. For this reason, the diamond-like carbon film makes it possible to widen the selection of a substrate on which the diamond-like carbon film is to be formed. A diamond-like carbon film having a large area can be easily formed and has a smoother surface than that of diamond. According to the present invention, the hard layer containing the composition component essentially consisting of at least one of diamond and the diamond-like carbon film may consist of natural monocrystalline diamond or monocrystalline diamond synthesized at a very high pressure. Alternatively, the hard layer may be a diamond thin film formed on a substrate or a diamond-like carbon film. The thickness of the hard layer essentially consisting of at least one of diamond and the diamond-like carbon film is 2πHd/λ > 3 and, more preferably 2πHd/λ > 4, as will be described in the following embodiments. The substrate on which the diamond thin film is to be formed is not limited to a specific one, but may consist of a semiconductor material (e.g., Si, Mo, W, GaAs, or LiNbO 3 ) or an inorganic material. The substrate on which the diamond-like carbon film makes it possible to select a substrate in a wider range than that for diamond. Such a substrate includes a substrate consisting of an organic compound such as a synthetic resin. The diamond thin film formed on the substrate may be monocrystalline or polycrystalline diamond. The diamond thin film formed by epitaxial synthesis generally has a polycrystalline structure. A method of forming a diamond thin film on a substrate can be exemplified by a CVD method (e.g., a thermal CVD method, a plasma CVD method, a microwave CVD method, a photo-CVD method, or a laser CVD method), sputtering, and ion beam deposition. The diamond-like carbon film may be formed on a substrate using the same method as in formation of the diamond thin film. A method of decomposing and exciting a source gas to grow a diamond thin film in accordance with an epitaxial synthesis method is as follows: (1) a method of heating a thermoelectron radiant material to a temperature of 1,500 K or more to activate a source gas; (2) a method using discharge generated by a DC, RF, or microwave electric field; (3) a method using ion bombardment; (4) a method of irradiating light such as a laser beam; or (5) a method of causing combustion of a source gas. A source material used to epitaxially synthesize a diamond thin film and a diamond-like carbon film is generally a carbon-containing compound and preferably used in combination with hydrogen gas. An oxygen-containing compound and/or an inert gas may be added to the source gas, as needed. Examples of the carbon-containing compound are a paraffin-based hydrocarbon (e.g., methane, ethane, propane, or butane), an olefin-based hydrocarbon (e.g., ethylene, propylene, or butylene), an acetylene-based hydrocarbon (e.g., acetylene or allylene), a diolefin-based hydrocarbon (e.g., butadiene), an alicyclic hydrocarbon (e.g., cyclopropane, cyclobutane, cyclopentane, or cyclohexane), an aromatic hydrocarbon (e.g., cyclobutadiene, benzene, toluene, or cyclohexane), ketones (e.g., acetone, diethyl ketone, and benzophenone), alcohols (e.g., methanol and ethanol), amines (trimethylamine and triethylamine), carbon dioxide gas, and carbon monoxide. These materials can be used singly or in a combination of at least two materials. Alternatively, the carbon-containing compound may be a material consisting of only carbon atoms, such as graphite, coal, or coke. As the oxygen-containing compound added to the source gas, oxygen, water, carbon monoxide, carbon dioxide, or hydrogen peroxide is preferable because it is easily accessible. Examples of the inert gas which can be added to the source gas are argon, helium, neon, krypton, xenon, and radon. The hard layer containing a composition component essentially consisting of at least one of diamond and the diamond-like carbon film may be constituted by a substrate essentially consisting of monocrystalline diamond. The monocrystalline diamond substrate is preferably used to increase the surface acoustic wave propagation velocity and the electro-mechanical coupling coefficient. For example, such a substrate may consist of natural diamond or synthetic diamond synthesized at a very high pressure. According to the present invention, the piezoelectric layer essentially consists of a piezoelectric material such as ZnO, AlN, Pb(Zr,Ti)O 3 , (Pb,La)(Zr,Ti)O 3 , LiTaO 3 , LiNbO 3 , SiO 2 , Ta 2 O 5 , Nb 2 O 5 , BeO, Li 2 B 4 O 7 , KNbO 3 , ZnS, ZnSe, or CdS. The piezoelectric layer may consist of a monocrystalline or polycrystalline piezoelectric material. To use an element in a high-frequency range, the piezoelectric layer preferably consists of a monocrystalline material in which a surface acoustic wave is scattered less than that of a polycrystalline material. A piezoelectric layer essentially consists of ZnO, AlN, or Pb(Zr,Ti)O 3 may be formed by CVD or sputtering. According to the present invention, the electrode which performs the electro-mechanical conversion, i.e., conversion between an electrical signal and a surface acoustic wave, is typically a comb-like electrode or an interdigital transducer (IDT) electrode. The comb-like electrode can be manufactured to have an electrode pitch period of about 1.2 µm using the state-of-the-art photolithographic technique. A material for forming the electrode is preferably a metal having a low resistivity and is exemplified as a metal (e.g., Au, Ag, or Al) which can be deposited at a low temperature, and a refractory metal (e.g., Ti, W, or Mo). Al and Ti are preferably used to facilitate formation of electrodes. W and Mo are preferably used to improve adhesion properties with diamond. The comb-like electrode may be made of a single metal material or a combination of two or more metal materials such that Al is stacked on Ti. The process for forming a comb-like electrode will be described below. A metal layer selectively consisting of the above materials is formed at a predetermined location. A resist film is formed on the metal layer, a mask having a comb-like electrode pattern formed on a transparent flat plate as of glass is placed above the resist film, and the resist film is exposed with a mercury lamp. The exposed resist film is developed to obtain a resist pattern. The resist pattern may be formed using a method directly sensitizing a resist film with an electron beam in place of the above-mentioned method. After the resist pattern is formed, the metal layer is patterned into a predetermined shape by etching. To etch a layer consisting of a low-melting metal such as Al, an alkaline solution such as a sodium hydroxide solution or an acidic solution such as a nitric acid solution is used. On the other hand, to etch a refractory metal, a solution mixture of hydrofluoric acid and nitric acid is used. Reactive ion etching using a gas such as BCl 3 may be used to etch the metal layer. According to the present invention, the electrode may also be formed of electrically conductive diamond. The conductive diamond may be formed by a method of epitaxially growing diamond while doping an impurity such as P, Al, P, or S, a method of doping such an impurity to insulating diamond by ion implantation, a method of radiating insulating diamond with an electron beam to introduce lattice defects, or a method of hydrogenating insulating diamond. According to the present invention, the comb-like electrodes can be formed between the piezoelectric layer and the layer essentially consisting of at least one of diamond and the diamond-like carbon film or between the piezoelectric layer and the silicon dioxide (SiO 2 ) layer. In addition, the comb-like electrodes may be formed together with short-circuiting electrodes. When the comb-like electrodes are formed between the piezoelectric layer and the layer essentially consisting of at least one of diamond and the diamond-like carbon film, the short-circuiting electrodes may be formed between the piezoelectric layer and the silicon dioxide (SiO 2 ) layer and/or on the silicon dioxide (SiO 2 ) layer. When the comb-like electrodes are formed between the piezoelectric layer and the silicon dioxide (SiO 2 ) layer, the short-circuiting electrodes can be formed between the piezoelectric layer and the layer essentially consisting of at least one of diamond and the diamond-like carbon film and/or on the silicon dioxide (SiO 2 ) layer. The comb-like electrodes have a shape shown in Fig. 12 or 13. In a surface acoustic wave element in which a piezoelectric layer is formed on a substrate, when the sound velocity of a substrate material is higher than that of a piezoelectric material, a plurality of surface acoustic waves having different propagation velocity are excited. In this case, the excitation modes are defined as a 0th-order mode, a first-order mode, a second-order mode, a third-order mode,... in a propagation velocity ascending order. A surface acoustic wave element according to the present invention can be used as, for example, an element using the first-order mode. According to the present invention, there can be provided a surface acoustic wave element in which the propagation velocity in the first-order mode is about 8,000 m/s or more and the electro-mechanical coupling coefficient is 3% or more. In this case, the electro-mechanical coupling coefficient is an index representing conversion efficiency in conversion of an electrical energy into a mechanical energy. As a conventional surface acoustic wave element similar to that of the present invention, a surface acoustic wave element having a structure in which a silicon dioxide (SiO 2 ) layer is deposited on an LiTaO 3 layer is found to have an electro-mechanical coupling coefficient as large as 5.2% when the thickness of the SiO 2 layer is 2πHs/λ = 1.3(Jpn. J. Appl. Phys. Suppl. No. 29-1, pp. 157 - 158, 1990, SATOH K., FUJIWARA Y., and HASHIMOTO K.) The surface acoustic wave element according to the present invention is characterized in that the surface acoustic wave propagation velocity can be twice that of the above-mentioned surface acoustic wave element and that the surface acoustic wave element of the present invention can maintain a large electro-mechanical coupling coefficient. According to the present invention, to realize such a surface acoustic wave element, the effective ranges of the thicknesses of the layer essentially consisting of at least one of diamond and the diamond-like carbon film, the piezoelectric layer, and the silicon dioxide (SiO 2 ) layer are clarified. As will be described in the following embodiments, in a surface acoustic wave element having a piezoelectric layer and comb-like electrodes sandwiched between a layer essentially consisting of at least one of diamond and a diamond-like carbon film and a silicon dioxide (SiO 2 ) layer, when the thicknesses of the piezoelectric and silicon dioxide (SiO 2 ) layers are appropriately set, an electro-mechanical coupling coefficient and a surface acoustic wave propagation velocity, and particularly, the electro-mechanical coupling coefficient can be increased according to the finding of the present inventors as compared with a case in which the silicon dioxide (SiO 2 ) layer is not formed. That is, according to the present invention, there is provided a surface acoustic wave element which can perform electro-mechanical conversion with a higher efficiency than that of a conventional surface acoustic wave element having no silicon dioxide (SiO 2 ) layer and which has a higher center frequency. In the surface acoustic wave element, the silicon dioxide (SiO 2 ) layer is formed on the piezoelectric layer and the comb-like electrodes. Silicon dioxide is an electric insulator and rarely reacts with moisture and an acid. For this reason, the silicon dioxide (SiO 2 ) layer serves as a passivation layer for protecting the main part of the element which has the piezoelectric layer and the comb-like electrodes from the external environment. The influences of humidity and impurities from the external environment can be minimized. The sign of the change coefficient of the surface acoustic wave propagation velocity as a function of the temperature of a material constituting the silicon dioxide (SiO 2 ) layer is opposite to that of the change coefficient of the surface acoustic wave propagation velocity as a function of the temperatures of a material constituting the hard layer essentially consisting of at least one of diamond and the diamond-like carbon layer and a material constituting the piezoelectric layer. When the temperature is increased, the surface acoustic wave propagation velocity in the silicon dioxide (SiO 2 ) layer is increased, while the surface acoustic wave propagation velocity in the hard layer essentially consisting of at least one of diamond and the diamond-like carbon film and the piezoelectric layer tends to be decreased. In the surface acoustic wave element according to the present invention, when the silicon dioxide (SiO 2 ) layer consisting of the material having the change coefficient with an opposite sign is stacked on the hard layer essentially consisting of at least one of diamond and the diamond-like carbon film and the piezoelectric layer, a canceling effect is obtained. Therefore, characteristic variations of the element against changes in temperatures, and particularly, variations in surface acoustic wave propagation velocity can be minimized. According to the present invention, there is provided a surface acoustic wave element which can be used to have a high electro-mechanical conversion efficiency in a higher-frequency range and has stable characteristics. EMBODIMENT 1 An Si substrate having dimensions of 10 x 10 x 1 mm was prepared and placed in a plasma CVD apparatus. The reaction chamber of the apparatus was evacuated, and a gas mixture of H 2 : CH 4 = 200 : 1 was supplied thereto. Plasma CVD was performed at a vacuum chamber pressure of about 40 Torr, a substrate temperature of 850°C, and a microwave power of 400 W to grow a 25-µm thick diamond thin film on the Si substrate. The diamond thin film was left in the outer atmosphere at 450°C for 10 minutes to increase the resistance of the diamond thin film. After the surface of the diamond thin film was polished, a 500-Å thick Al layer was deposited on the polished surface by a resistance heating method. Using a photolithographic method, the Al layer was etched to form comb-like electrodes each having line and space of 2 µm. The shape of the comb-like electrodes was identical to that in Fig. 12. Magnetron sputtering for sputtering a polycrystalline ZnO body using a gas mixture of Ar : O 2 = 1 : 1 at a sputtering output of 150 W and a substrate temperature of 380°C was performed to deposit a 0.93-µm thick ZnO film. An amorphous SiO 2 film was formed using an SiO 2 target and a gas mixture of Ar : O 2 = 1 : 1 at a substrate temperature of 150°C, an RF power of 200 W, and a pressure of 0.01 Torr in accordance with a sputtering method. Fig. 1 is a sectional view showing a surface acoustic wave element based on EMBODIMENT 1. As shown in Fig. 1, a diamond thin film 2 is formed on an Si substrate 1. Comb-like electrodes 3 are formed on the diamond thin film 2. A ZnO thin film 4 is formed on the comb-like electrodes 3. An SiO 2 film 5 is formed on the ZnO thin film 4. To compare the above surface acoustic wave element with a comparative example, the comparative example as a surface acoustic wave element having no SiO 2 film, as shown in Fig. 2, was prepared. As shown in Fig. 2, a diamond thin film 12 is formed on an Si substrate 11. Comb-like electrodes 13 are formed on the diamond thin film 12, and an ZnO thin film 14 is formed on the comb-like electrodes 13. The surface acoustic wave element as the comparative example was manufactured following the same procedures as in the surface acoustic wave element based on EMBODIMENT 1, except that the SiO 2 film is not formed. A 5% diluted hydrochloric acid solution was dripped on the surfaces of the surface acoustic wave elements of EMBODIMENT 1 and the comparative example, and filter characteristics were evaluated. As a result, no change was observed in the filter characteristics of the surface acoustic wave element 1 of EMBODIMENT 1 shown in Fig. 1. However, the ZnO thin film 14 was etched in the surface acoustic wave element as the comparative example shown in Fig. 2, and the filter characteristics were lost. In this manner, it is found that the SiO 2 film 5 formed on the ZnO thin film 4 protects the surface acoustic wave element from the external environment. EMBODIMENT 2 An element having a structure shown in Fig. 3 was manufactured. As shown in Fig. 3, a diamond thin film 22 is formed on an Si substrate 21. Short-circuiting electrodes 26a are formed on the diamond thin film 22, and a ZnO thin film 24 is formed on the short-circuiting electrodes 26a. Comb-like electrodes 23 are formed on the ZnO thin film 24, and an SiO 2 film is formed on the comb-like electrodes 23. In the manufacture of the element having the structure shown in Fig. 3, the diamond thin film 22 was formed by a plasma CVD method under the same conditions as the synthesis conditions of EMBODIMENT 1. The thickness of the diamond thin film was about 25 µm. The ZnO thin film 24 and the comb-like electrodes 23 were formed under the same conditions as in EMBODIMENT 1. On the other hand, the short-circuiting electrodes 26a were formed by a photolithographic method after a 500-Å thick Al layer was deposited on the polished surface of the diamond thin film 22 by the resistance heating method. In the element having the structure shown in Fig. 3, the short-circuiting electrodes 26a were formed in a region where the comb-like electrodes 23 were to be formed, so as to improve the excitation efficiency of the element. Various SiO 2 films 25 were formed by the low-temperature CVD method under the same conditions as the synthesis conditions of EMBODIMENT 1 so as to examine element characteristics. The structure of the surface acoustic wave element shown in Fig. 3 is defined as an A structure. Changes in characteristics upon changes in thicknesses of the SiO 2 film 25 are shown later. EMBODIMENT 3 An element having a structure shown in Fig. 4 was manufactured. As shown in Fig. 4, a diamond thin film 22 is formed on an Si substrate 21, and comb-like electrodes 23 are formed on the diamond thin film 22. A ZnO thin film 24 is formed to cover the diamond thin film 22 and the comb-like electrodes 23. Short-circuiting electrodes 26a are formed on the ZnO thin film 24, and an SiO 2 film 25 is formed to cover the short-circuiting electrodes 26a. The element having the structure shown in Fig. 4 was formed under the same conditions as in EMBODIMENTs 1 and 2. The structure of the surface acoustic wave element shown in Fig. 4 is defined as a B structure. Changes in characteristics upon changes in thicknesses of the SiO 2 film 25 are shown later. EMBODIMENT 4 An element having a structure shown in Fig. 5 was manufactured. As shown in Fig. 5, a diamond thin film 22 is formed on an Si substrate 21. Short-circuiting electrodes 26a are formed on the diamond thin film 22, and a ZnO thin film 24 is formed on the short-circuiting electrodes 26a. Comb-like electrodes 23 are formed on the ZnO thin film 24, and an SiO 2 film 25 is formed on the comb-like electrodes 23. Short-circuiting electrodes 26b are further formed on the SiO 2 film so as to cover a region in which the comb-like electrodes 23 are formed. The element having the structure shown in Fig. 5 was formed under the same conditions as in EMBODIMENTs 1 to 3. The structure of the surface acoustic wave element shown in Fig. 5 is defined as a C structure. Changes in characteristics upon changes in thicknesses of the SiO 2 film 25 are shown later. EMBODIMENT 5 An element having a structure shown in Fig. 6 was manufactured. As shown in Fig. 6, a diamond thin film 22 is formed on an Si substrate 21. Comb-like electrodes 23 are formed on the diamond thin film 22, and a ZnO thin film 24 are formed on the comb-like electrodes 23. Short-circuiting electrodes 26a are formed on the ZnO thin film 24 so as to cover a region in which comb-like electrodes 23 are formed. An SiO 2 film 25 is formed on the short-circuiting electrodes 26a. Short-circuiting electrodes 26b are formed on the SiO 2 film 25. The element having the structure shown in Fig. 6 was formed under the same conditions as in EMBODIMENTs 1 to 4. The structure of the surface acoustic wave element shown in Fig. 6 is defined as a D structure. Changes in characteristics upon changes in thicknesses of the SiO 2 film 25 are shown later. In the surface acoustic wave elements having the A to D structures, the ZnO thin films 24 were defined to have equal thicknesses, and the thicknesses of the SiO 2 films 25 formed as the uppermost layers were changed to measure electro-mechanical coupling coefficients K 2 . The changes in electro-mechanical coupling coefficients were plotted as a graph. The following parameters were defined so as to represent the relationship between the thicknesses of the SiO 2 films and the electro-mechanical coupling coefficients K 2 . The wavelength of the surface acoustic wave is defined as λ. To set the thickness of the ZnO thin film as a zero-dimensional parameter, the thickness is expressed as 2πHz/λ obtained by dividing a film thickness Hz by the wavelength λ and multiplying the quotient with 2π. Similarly, to set the thickness of the SiO 2 film as a zero-dimensional parameter, the thickness is expressed as 2πHs/λ obtained by dividing a film thickness Hs by the wavelength λ and multiplying the quotient with 2π. 2πHz/λ and 2πHs/λ are used herein because ratios of the film thicknesses Hz and Hs to the wavelength apparently have larger influences on the surface acoustic wave propagation velocity V and the electro-mechanical coupling coefficient K 2 than the absolute values of the film thicknesses Hz and Hs. Fig. 7 is a graph showing the relationship between the 2πHs/λ and the electro-mechanical coupling coefficient K 2 in the element having the A structure for 2πHz/λ = 0.7. As shown in Fig. 7, in the element having the A structure, if 2πHz/λ = 0.7 and 2πHs/λ = 0.8, then the electro-mechanical coupling coefficient K 2 in the first-order mode can be increased up to about 4%. Judging from the result of Fig. 7, it is found that the electro-mechanical coupling coefficient K 2 can be increased by forming the SiO 2 film on the ZnO thin film 24. In the relationship between the electro-mechanical coupling coefficient K 2 and 2πHs/λ, if 2πHz/λ falls within the range of 0.65 to 0.75, the same tendency as described above can be confirmed. Fig. 8 is a graph showing the relationship between the 2πHs/λ and the electro-mechanical coupling coefficient K 2 in the element having the B structure for 2πHz/λ = 0.7. As shown in Fig. 8, in the element having the B structure, if 2πHz/λ = 0.7 and 2πHs/λ = 0.8, then the electro-mechanical coupling coefficient K 2 in the first-order mode can reach about 6%. In this manner, the electro-mechanical coupling coefficient K 2 can be increased by 2% as compared with the structure in which any SiO film is not formed (2πHs/λ = 0). In the relationship between the electro-mechanical coupling coefficient K 2 and 2πHs/λ, if 2πHz/λ falls within the range of 0.65 to 0.75, the same tendency as described above can be confirmed. Fig. 9 is a graph showing the relationship between the 2πHs/λ and the electro-mechanical coupling coefficient K 2 in the element having the C structure for 2πHz/λ = 0.7. As shown in Fig. 9, in the element having the C structure, if 2πHz/λ= 0.7 and 2πHs/λ = 0.8, then the electro-mechanical coupling coefficient K 2 in the first-order mode can reach about 3%. In this manner, the electro-mechanical coupling coefficient K 2 can be increased by 3% as compared with the structure in which any SiO 2 film is not formed (2πHs/λ = 0). In the relationship between the electro-mechanical coupling coefficient K 2 and 2πHs/λ, if 2πHz/λ falls within the range of 0.65 to 0.75, the same tendency as described above can be confirmed. Fig. 10 is a graph showing the relationship between the 2πHs/λ and the electro-mechanical coupling coefficient K 2 in the element having the D structure for 2πHz/λ = 0.7. As shown in Fig. 10, in the element having the D structure, if 2πHz/λ = 0.7 and 2πHs/λ = 0.8, then the electro-mechanical coupling coefficient K 2 in the first-order mode can reach about 6%. In this manner, the electro-mechanical coupling coefficient K 2 can be increased by 2% as compared with the structure in which any SiO 2 film is not formed (2πHs/λ = 0). In the relationship between the electro-mechanical coupling coefficient K 2 and 2πHs/λ, if 2πHz/λ falls within the range of 0.65 to 0.75, the same tendency as described above can be confirmed. In each of the elements having the A to D structures described above, the thickness of the diamond thin film 22 formed on the Si substrate was about 25 µm. When a thickness Hd of the diamond thin film 22 is increased, the surface acoustic wave propagation velocity V is increased. However, if 2πHd/λ ≥ 4.0, the surface acoustic wave propagation velocity V is not much increased. Therefore, the diamond thin film 22 sufficiently has the thickness Hd satisfying 2πHd/λ = 4.0. The relationships between the surface acoustic wave propagation velocity and 2πHs/λ in the elements having the A to D structures are shown in Fig. 11. As shown in Fig. 11, it is apparent that high propagation velocity are obtained in both the first- and second-order modes. In the first-order mode, when the thickness of the SiO 2 film 25 is increased, the propagation velocity is slightly decreased. A sufficiently high propagation velocity of about 8,000 m/s is found to be obtained at 2πHs/λ = 0.5. As shown in Figs. 7 to 10, in all the elements having the A, B, C, and D structures, when 2πHs/λ is increased, the electro-mechanical coupling coefficient in the 0th-order mode is found to be suppressed to be small. Therefore, in these elements, oscillation of unnecessary waves can be suppressed, and good spurious characteristics can be obtained. In each of the surface acoustic wave elements shown in EMBODIMENTs 1 to 4, when the thickness of the SiO 2 film 25 has a 2πHs/λ value falling within the range of 0.2 to 0.8, the element can be applied as an element having a first-order mode oscillation structure. EMBODIMENT 6 An surface acoustic wave element having the same structure as that of the elements shown in Fig. 1 and different from the element of Fig. 1 in only the thickness of the ZnO thin film and the SiO 2 thin film was manufactured. In the manufactured element, a structure of a Comb-like electrode is the same type as that of the element of Fig. 1, and both of the width of the electrodes and the distance between the electrodes are 2 µm. Further, 2πHd/λ in a Diamond layer is set to be 4 , and the element was formed under the same conditions as in EMBODIMENTs 1 to 5. Fig. 15 shows an estimation result of the characteristics in the manufactured element. Fig. 15 is a graph showing the relationship between 2πHz/λ and 2πHs/λ and electro-mechanical coupling coefficient K 2 . In Fig. 15, a vertical line and horizontal line respectively show 2πHz/λ and 2πHs/λ and numeral in the graph represents the value of K 2 . As shown in Fig. 15, in the element having this structure, the value of electro-mechanical coupling coefficient K 2 increases with the increase in the thickness (2πHz/λ) of the ZnO thin film in spite of the thickness (2πHs/λ) of the SiO 2 thin film. Especially when 2πHs/λ ≦ 0.8 and 2πHz/λ ≧ 1.4, the surface acoustic wave element having K 2 = 3.5 % can be obtained. In consideration of the propagation velocity of such a surface acoustic wave element and the stability of the element against the temperature, a surface acoustic wave element having the ZnO thin film and the SiO 2 thin film at the thickness corresponding to an area surrounded by 4 circles in Fig. 15 is preferable in practical use. The surface acoustic wave element has the propagation velocity of about 8000-10000 m/s, and a temperature coefficient o of 0±10 ppm/°C. Thus, in the surface acoustic wave element having the SiO 2 according to this EMBODIMENT, the thicknesses of the ZnO thin film or the SiO 2 thin film can be selected to realize good characteristics of the element in actual use. EMBODIMENT 7 An surface acoustic wave element having the same structure as that of the elements shown in Fig. 3 and different from the element of Fig. 3 in only the thickness of the ZnO thin film and the SiO 2 thin film was manufactured. In the manufactured element, a structure of a Comb-like electrode is the same type as that of the element of Fig. 13, and both of the width of the electrodes and the distance between the electrodes are 2 µm. Further, 2πHd/λ in a Diamond layer is set to be 4 , and the element was formed under the same conditions as in EMBODIMENTs 1 to 5. Fig. 16 shows an estimation result of the characteristics in the manufactured element. Fig. 16 is a graph showing the relationship between 2πHz/λ and 2πHs/λ and electro-mechanical coupling coefficient K 2 . In Fig. 16, a vertical line and horizontal line respectively show 2πHz/λ and 2πHs/λ and numeral in the graph represents the value of K 2 . As shown in Fig. 16, in the element having this structure, the value of electro-mechanical coupling coefficients K 2 increases with the increase in the thickness (2πHz/λ) of the ZnO 2 thin film in spite of the thickness (2πHs/λ) of the SiO 2 thin film. Especially when 2πHs/λ falls within the range of 0.7 to 0.8 and 2πHz/λ falls within the range of 0.7 to 0.8, the surface acoustic wave element having K 2 < 4 % can be obtained. In consideration of the propagation velocity of such a surface acoustic wave element and the stability in the element to the temperature, a surface acoustic wave element having the ZnO thin film and the SiO 2 thin film at the thickness corresponding to an area surrounded by 4 circles in Fig. 16 is preferable in practical use. The surface acoustic wave element has the propagation velocity of about 8000-10000 m/s, and a temperature coefficient σ of 0±10 ppm/°C. Thus, in the surface acoustic wave element having the SiO 2 according to the EMBODIMENT 7, the thicknesses of the ZnO thin film or the SiO 2 thin film can be selected to realize good characteristics of the element in actual use. EMBODIMENT 8 An surface acoustic wave element having the same structure as that of the elements shown in Fig. 4 and different from the element of Fig. 4 in only the thickness of the ZnO thin film and the SiO 2 thin film was manufactured. In the manufactured element, a structure of a Comb-like electrode is the same type as that of the element of Fig. 13, and both of the width of the electrodes and the distance between the electrodes are 2 micro-meters. Further, 2πHd/λ in a Diamond layer is set to be 4 , and the element was formed under the same conditions as in EMBODIMENTs 1 to 5. Fig. 17 shows an estimation result of the characteristics in the manufactured element. Fig. 17 is a graph showing the relationship between 2πHz/λ and 2πHs/λ and electro-mechanical coupling coefficient K 2 . In Fig. 16, a vertical line and horizontal line respectively show 2πHz/λ and 2πHs/λ and numeral in the graph represents the value of K 2 . As shown in Fig. 17, in the element having this structure, the value of electro-mechanical coupling coefficients K 2 increases with the increase in the thickness (2πHz/λ) of the ZnO 2 thin film in spite of the thickness (2πHs/λ) of the SiO 2 thin film. Especially when 2πHs/λ ≧ 0.4 and 2πHz/λ ≧ 0.65, the value of K 2 in the surface acoustic wave element can reach at least about 5 % and when 2πHs/λ ≧ 1.2, the value of K 2 in the surface acoustic wave element can reach about value not less than 6 %. In consideration of the propagation velocity of such a surface acoustic wave element and the stability in the element to the temperature, a surface acoustic wave element having the ZnO thin film and the SiO 2 thin film at the thickness corresponding to an area surrounded by 4 circles in Fig. 16 is preferable in practical use. The surface acoustic wave element has the propagation velocity of about 8000-10000 m/s, and a temperature coefficient σ of 0±10 ppm/°C. Thus, in the surface acoustic wave element having the SiO 2 according to the EMBODIMENT 8, the thicknesses of the ZnO thin film or the SiO 2 thin film can be selected to realize good characteristics of the element in actual use. EMBODIMENT 9 An surface acoustic wave element having the same structure as that of the elements shown in Fig. 5 and different from the element of Fig. 4 in only the thickness of the ZnO thin film and the SiO 2 thin film was manufactured. In the manufactured element, a structure of a Comb-like electrode is the same type as that of the element of Fig. 13, and both of the width of the electrodes and the distance between the electrodes are 2 µm. Further, 2πHd/λ in a Diamond layer is set to be 4 , and the element was formed under the same conditions as in EMBODIMENTs 1 to 5. Fig. 18 shows an estimation result of the characteristics in the manufactured element. Fig. 18 is a graph showing the relationship between 2πHz/λ and 2πHs/λ and electro-mechanical coupling coefficients K 2 . In Fig. 16, a vertical line and horizontal line respectively show 2πHz/λ and 2πHs/λ and numeral in the graph represents the value of K 2 . As shown in Fig. 18, in the element having this structure, the value of electro-mechanical coupling coefficient K 2 increases with the increase in the thickness (2πHs/λ) of the SiO 2 thin film in spite of the thickness (2πHz/λ) of the ZnO thin film. Especially when 2πHs/λ ≧ 0.4 and 2πHz/λ ≧ 0.65, the value of K 2 in the surface acoustic wave element can reach at least about 5 % and when 2πHs/λ ≧ 1.2, the value of K 2 in the surface acoustic wave element can reach a value not less than 6 %. In consideration of the propagation velocity of such a surface acoustic wave element and the stability of the element against the temperature, a surface acoustic wave element having the ZnO thin film and the SiO 2 thin film at the thickness corresponding to an area surrounded by 4 circles in Fig. 18 is preferable in practical use. The surface acoustic wave element has the propagation velocity of about 8000-10000 m/s, and a temperature coefficient a of 0±10 ppm/°C. Thus, in the surface acoustic wave element having the SiO 2 according to the EMBODIMENT 9, the thicknesses of the ZnO thin film or the SiO 2 thin film can be selected to realize good characteristics of the element in actual use EMBODIMENT 10 An element having a structure shown in Fig. 19 was manufactured. As shown in Fig. 19, a diamond thin film 22 is formed on an Si substrate 21. Comb-like electrodes 23 are formed on the diamond thin film 22, and a ZnO thin film 24 are formed on the comb-like electrodes 23. Short-circuiting electrodes 26a are formed on the ZnO thin film 24 so as to cover a region in which comb-like electrodes 23 are formed. A structure of the Comb-like electrode 23 is as the same type as that shown in Fig. 13, and both of the width of the electrode and the distance d between the electrodes is 2 µm, and 2πHd/λ of the diamond layer is 4. An element having a structure shown in Fig. 19 was manufactured under the same condition of the EMBODIMENTs 1-9. Fig. 20 shows an estimation result of the characteristics in the manufactured element. Fig. 20 is a graph showing the relationship between 2πHz/λ and 2πHs/λ and electro-mechanical coupling coefficients K 2 . In Fig. 20, a vertical line and horizontal line respectively show 2πHz/λ and 2πHs/λ and numeral represents in the graph shows K 2 . As shown in Fig. 20, in the element having this structure, the value of electro-mechanical coupling coefficients K 2 increases with the increase in the thickness (2πHs/λ) of the SiO 2 thin film in spite of the thickness (2πHz/λ) of ZnO thin film. In consideration of the stability of a propagation velocity and the temperature in such a surface acoustic wave element to transmission, a surface acoustic wave element having ZnO thin film and SiO 2 thin film at the thickness corresponding to an area surrounded by 4 circle marks in Fig. 20 is preferable in practical use. The surface acoustic wave element has a propagation velocity of about 8000-10000 m/s, and a temperature coefficient o of 0±10 ppm/C. Thus, in a surface acoustic wave element having a SiO 2 according to the EMBODIMENT 10, the thicknesses of ZnO thin film or SiO 2 thin film can be selected to realize good characteristics of the element in actual. Thus, since in the surface acoustic wave element according to the EMBODIMENTs 6-10, since a SiO 2 thin film is applied in 0.3≦2πHz/λ ≦ 1.0, the element has a propagation velocity not less than 7000 m/s, high electric-mechanical coupling coefficient K 2 and high stability in high temperature. Besides, in the surface acoustic wave element according to EMBODIMENTS 6-10, the thickness of a diamond thin film provided on a Si substrate was about 25 µm. The propagation velocity v increases with the increase in the thickness Hd of the diamond thin film 22, but when 2πHd/λ ≧ 4.0, the propagation velocity of the surface acoustic wave does not increase so much. Accordingly, the diamond thin film sufficiently has the thickness satisfying 2πHd/λ <4.0. As described in the above, in the surface acoustic wave element of the EMBODIMENTs 6-10, the relationship between 2πHz/λ and 2πHs/λ and electro-mechanical coupling coefficients K 2 is shown in Fig. 21. In Fig. 21, a vertical line and horizontal line respectively show 2πHz/λ and 2πHs/λ and numeral in the graph represents the value of v. As shown in Fig. 21, the propagation velocity v decreases with the increase in 2πHz/λ and 2πHs/λ, but even if 2πHs/λ = 0.8, when 2πHz/λ ≦ 0.4, the propagation velocity not less than 8000 m/s can be obtained in the element. Further, in the surface acoustic wave element according to the above EMBODIMENTs 6-10, the relationship between 2πHz/λ and 2πHs/λ and temperature coefficient σ (ppm/°C) in a first mode is investigated. The investigated result is shown in Fig. 22. As shown in Fig. 22, in spite of the value of 2πHz/λ, the temperature coefficient σ increases with the increase in 2πHs/λ, and when 2πHs/λ falls within the range of 0.5 to 0.8, it is possible to realize σ = 0, by setting 2πHz/λ to be a proper value not larger than 0.6. From the above result, in consideration of the electro-mechanical coupling coefficient K 2 , the propagation velocity v and the temperature coefficient σ, it is understood that the effective range of 2πHz/λ and 2πHs/λ in actual use are respectively 0.4 to 0.8 and 0.2 and 0.6. In the above surface acoustic wave element according to EMBODIMENTS 6-10, by setting 2πHs/λ of the thickness of SiO 2 layer 25 to fall within the range of 0.2 to 0.8, an element having the first mode oscillation structure can be realized. In each of these embodiments described above, a surface acoustic wave element can be used as a high-frequency filter used in a communication equipment such as a mobile or portable telephone. In each of these embodiments described above, the standard comb-like electrodes shown in Fig. 12 are used, but comb-like elements having another shape shown in Fig. 13 or any other shape can be used to obtain the same effect as described above. From the invention thus described, it will be obvious that the invention may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims: A surface acoustic wave device comprising: a hard layer (2, 22) essentially consisting of diamond and/or diamond-like carbon; a piezoelectric layer (4, 24) formed directly on said hard layer (2, 22); a silicon dioxide layer (5, 25) formed directly on said piezoelectric layer; and an electrode group (3, 23); characterized in that said electrode group (3, 23) is sandwiched between said hard layer (2, 22) and said piezoelectric layer (4, 24) or between said piezoelectric layer (4, 24) and said silicon dioxide layer (5, 25), for performing electro-mechanical conversion; said piezoelectric layer (4, 24) essentially consists of ZnO, and said hard layer (2, 22) has a thickness Hd such that 3<2πHd/λ, where λ denotes a wavelength of said acoustic wave device in the first-order mode. The surface acoustic wave device of claim 1, wherein said piezoelectric layer (4, 24) has a thickness Hz such that 0.3≤2πHz/λ≤1.0, and said silicon dioxide layer (5, 25) has a thickness Hs such that 0.4≤2πHs/λ≤0.8, where λ denotes a wavelength of said acoustic wave device in the first-order mode. The surface acoustic wave device of any one of the preceding claims, wherein said piezoelectric layer (4, 24) has a thickness Hz such that 0,3≤2πHz/λ≤0.8, where λ denotes a wavelength of said acoustic wave device in the first-order mode. The surface acoustic wave device of any one of the preceding claims, wherein said silicon dioxide layer (5, 25) has a thickness Hs such that 0.2≤2πHs/λ≤0.6, where denotes a wavelength of said acoustic wave device in the first-order mode. The surface acoustic wave device of claim 1, wherein said piezoelectric layer (4, 24) has a thickness Hz, and said silicon dioxide layer (5, 25) has a thickness Hs, such that the ratios 2πHz/λ and 2πHs/λ, are in a range wherein a propagation velocity v of said acoustic wave device is between about 8,000 m/s and about 10,000 m/s and a temperature coefficient σ of said acoustic wave device is about 0 ± 10 ppm/°C.

Inventors: HACHIGO AKIHIRO, HIGAKI KENJIRO, NAKAHATA HIDEAKI, SIKATA SHINICHI, HACHIGO, AKIHIRO, HIGAKI, KENJIRO, NAKAHATA, HIDEAKI, SIKATA, SHINICHI, HACHIGO, AKIHIRO, C/O ITAMI WORKS OF SUMITOMO, HIGAKI, KENJIRO, C/O ITAMI WORKS OF SUMITOMO, NAKAHATA, HIDEAKI, C/O ITAMI WORKS OF SUMITOMO, Sikata, Shinichi, c/o Itami Works of Sumitomo
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1993
Main classification: H03H9
IPC codes: H03H9