Surface acoustic wave device

Abstract: A layer of boron nitride being relatively easily workable is brought into close contact with a piezoelectric member, to provide a surface acoustic wave device which can be driven in a higher frequency range. A surface acoustic device (10) comprises a substrate (1), a boron nitride film (2) which is formed on the substrate (1), and a pair of interdigital electrodes (3a, 3b) which are formed on the boron nitride film (2). The interdigital electrodes (3a, 3b) are covered with a piezoelectric film (4), which is in close contact with the boron nitride film (2).

Description: The present invention relates to a surface acoustic wave device, which is applied to a high frequency filter or the like. Description of the Background Art A surface acoustic wave device, which is an electromechanical transducer device utilizing surface waves propagated on the surface of an elastic body, has a general structure shown in Fig. 1, for example. In such a surface acoustic wave device 60, a piezoelectric phenomenon which is caused by a piezoelectric member 61 is utilized for exciting surface waves. When an electric signal is applied to a interdigital electrodes 62 which are provided on the piezoelectric member 61, the piezoelectric member 61 is distorted to generate surface acoustic waves, which in turn are propagated along the piezoelectric member 61 and fetched in other interdigital electrodes 63 as an electric signal. Assuming that λ 0 represents the electrode period in the interdigital electrodes 62 and 63 and v represents the velocity of the surface acoustic waves in relation to frequency characteristics, this device has a bandpass characteristic with a center frequency f 0 , which is expressed as f 0 = v/λ 0 . The surface acoustic wave device can be miniaturized with a small number of components, while it is possible to input/output signals on a surface wave propagation path. This device can be applied to a filter, a delay line, an oscillator, a resonator, a convolver, a correlator etc. In particular, a surface acoustic wave filter has been practically applied to an intermediate frequency filter for a television over a long time, while the same is now being applied to filters for VTR and various communication devices. In relation to the aforementioned surface acoustic wave device, a device employable in a higher frequency range is awaited for a surface acoustic wave filter which is used in the field of mobile communication, for example. As understood from the above expression, the device has a higher center frequency f 0 as to the frequency characteristics when the electrode period λ 0 is reduced or the velocity v of the surface waves is increased. In relation to this, Japanese Patent Laying-Open No. 64-62911 (1989) in the name of the assignee, for example, discloses a surface acoustic wave device comprising a piezoelectric member which is stacked on a diamond layer for increasing the velocity v of surface acoustic waves. Diamond has the highest sound velocity among currently known substances, and is physically and chemically stable. Thus, diamond is suitably employed as a material for increasing the center frequency f 0 of the device. On the other hand, it is possible to improve the device in reliability by increasing the velocity v in the aforementioned manner while relatively increasing the electrode spacing λ 0 . A diamond layer can be formed by polishing single-crystalline diamond which is prepared by extra-high pressure synthesis, or the surface of a diamond film which is grown by CVD, for example. In such a polishing step being necessary for forming the diamond layer, however, a considerably long time is required for working, since it is difficult to work diamond. US-A-4 952 832 discloses a surface accoustic wave device with the features of the preamble of claim 1. SUMMARY OF THE INVENTION An object of the present invention is to provide a surface acoustic wave device which can be driven in a high frequency range, by employing an easily workable material. The inventors have made deep study as to an easily workable material having a sound velocity which is close to that of diamond, to complete the present invention with discovery of boron nitride. According to the present invention, provided is a surface acoustic wave device which is formed by stacking a boron nitride layer, a piezoelectric layer and an electrode layer. The boron nitride layer can serve as a substrate to be provided with the surface acoustic wave device, while the same may alternatively be formed on a substrate which is made of another appropriate material. Further, while either single-crystalline or polycrystalline boron nitride is employable, single-crystalline boron nitride is more preferably employed for a device which is used in a high frequency range, due to small scattering of surface waves. The boron nitride layer can be formed on a substrate by vapor deposition. Methods of such vapor deposition include a method of decomposing/exciting a raw material gas with a laser beam (refer to Japanese Patent Laying-Open No. 63-134662 (1988), for example), a method of exciting a raw material gas with plasma (refer to Japanese Patent Laying-Open Nos. 63-199871 (1988) and 63-128179 (1988), for example), a method of passing a raw material gas through nonpolar-discharged microwaves for depositing boron nitride on a heated substrate (refer to Japanese Patent Laying-Open No. 63-134661 (1988), for example), and a method of heating a substrate with a thermoionic emission material and applying high frequency plasma to the substrate for exciting/activating a raw material gas (refer to Japanese Patent Laying-Open No. 1-119673 (1989), for example). While the boron nitride layer can be made of either hexagonal boron nitride or cubic boron nitride, cubic boron nitride is more preferable, in consideration of hardness. A harder boron nitride layer brings a higher center frequency f 0 . Further, it is possible to mix various elements into the boron nitride layer, to change its electric characteristics. The piezoelectric layer can be mainly composed of one or more compounds selected from a group of 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 and CdS. While the piezoelectric layer can be made of either a single-crystalline or polycrystalline material, a single-crystalline material is more preferably employed in order to use the device in a higher frequency range, due to small scattering of surface waves. A piezoelectric layer of ZnO, AℓN or Pb(Zr,Ti)O 3 can be formed by CVD or sputtering. According to the present invention, the aforementioned interdigital electrodes is employed in general. These electrodes can be generally formed by depositing a metal through photolithography. Alternatively, the electrode may be formed on the boron nitride layer by introducing an impurity into the surface of the boron nitride layer having a high dielectric constant, or introducing a lattice defect by ion implantation or application of an electron beam. Further, a boron nitride layer having a high dielectric constant may be deposited on a substrate by vapor deposition, to thereafter deposit another boron nitride layer, being mixed with an impuritiy, having a lower dielectric constant as the electrode. The boron nitride layer, the piezoelectric layer and the electrodes can be stacked in order of the boron nitride layer, the electrodes and the piezoelectric layer, the boron nitride layer, the piezoelectric layer and the electrodes, or the piezoelectric layer, the electrodes and the boron nitride layer, as the lower layer, the intermediate layer and the upper layer respectively. Among materials capable of increasing the propagation velocity of surface acoustic waves, boron nitride is the hardest material next to diamond. Thus, the surface acoustic waves are propagated in boron nitride at a velocity close to that in diamond. The present invention implements a device which can be used in a higher frequency range by stacking a boron nitride layer and a piezoelectric member for increasing the velocity of the surface acoustic waves. Further, boron nitride can be further easily worked by polishing etc. as compared with diamond. Thus, the inventive device can be easily worked by polishing, in particular, as compared with the conventional device employing diamond, whereby it is possible to reduce the time required for working. According to the present invention, therefore, it is possible to provide a miniature mass-produceable surface acoustic wave device, which can be used in an extra-high frequency range. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view for illustrating a general structure of a surface acoustic wave device; Fig. 2 is a sectional view showing an embodiment of the inventive surface acoustic wave device; Fig. 3 is a sectional view showing another embodiment of the inventive surface acoustic wave device; Fig. 4 is a sectional view showing still another embodiment of the inventive surface acoustic wave device; Fig. 5 is a sectional view showing a further embodiment of the inventive surface acoustic wave device; Fig. 6 is a sectional view showing a further embodiment of the inventive surface acoustic wave device; and Fig. 7 is a plan view showing an embodiment of electrodes according to the present invention. Fig. 8 is a typical diagram showing a plasma CVD apparatus for forming a boron nitride film according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figs. 2 to 6 illustrate preferred embodiments of a surface acoustic wave device according to the present invention. Referring to Fig. 2, a surface acoustic wave device 10 comprises a substrate 1 for film formation, a boron nitride layer 2 which is formed on the substrate 1, and interdigital electrodes 3a and 3b which are formed on the boron nitride layer 2. The boron nitride layer 2 and the interdigital electrodes 3a and 3b are covered with a piezoelectric thin film 4. Referring to Fig. 3, a surface acoustic wave device 20 comprises a substrate 1 for film formation, a boron nitride layer 2 and a piezoelectric thin film 4 which are successively deposited on the substrate 1, and interdigital electrodes 3a and 3b which are formed on the piezoelectric thin film 4. Referring to Fig. 4, a surface acoustic wave device 30 being similar to that shown in Fig. 2 further comprises short-circuit electrodes 5a and 5b which are formed on a piezoelectric thin film 4. Referring to Fig. 5, a surface acoustic wave device 40 being similar to that shown in Fig. 3 further comprises short-circuit electrodes 5a and 5b which are formed between a boron nitride layer 2 and a piezoelectric thin film 4. Referring to Fig. 6, a surface acoustic wave device 50 comprises a boron nitride substrate 6, a piezoelectric thin film 4 which is deposited on the substrate 6, and interdigital electrodes 3a and 3b which are held between the substrate 6 and the film 4. The interdigital electrodes have shapes shown in Fig. 7, for example. Referring to Fig. 7, a pair of electrodes 21 and 22 are formed by alternately arranged reed-screen type electrodes, while the electrodes 21 are adapted to receive electric signals through terminals 21a and 21b and covert the same to surface acoustic waves. The other electrodes are adapted to output electric signals from the surface acoustic waves through terminals 22a and 22b. A surface acoustic wave device having the same structure as that shown in Fig. 2 was prepared in practice in the following manner, for example: First, an apparatus 70 shown in Fig. 8 was employed to form a boron nitride thin film on a molybdenum substrate as follows: A molybdenum substrate 75 was set on a substrate support 76 of the apparatus 70, and thereafter a reactor 74 was evacuated to supply boron fluoride, which was contained in hydrogen by 1 percent, and ammonia from a boron material supply system 71 and a nitrogen material supply system 72 at 10 cc/min. and 30 cc/min. respectively. At this time, the pressure in the reactor 74 was adjusted to 0.3 Torr by an exhauster 81. A thermoionic emission material 73 formed by a tungsten filament was set at a temperature of 2200°C, to raise up the temperature of the molybdenum substrate 75 to 950°C. DC discharge was caused by a DC power source 77 across the thermoionic emission material 73, which was heated by a power source 79, and the substrate support 76, while high frequency plasma of 13.56 MHz was applied to the substrate 75 from a high frequency power source 78 of 400 W through an RF coil 80, to form a boron nitride film. This film was evaluated by X-ray analysis, whereby wide peaks were detected in the vicinity of 2 = 26.7° and 43.2° respectively. This result has proved that the as-formed film was composed of a mixture of cubic boron nitride and hexagonal boron nitride. Then, the surface of the boron nitride film was polished with a diamond electro-deposited wheel. It was easy to polish the surface. Then, an Aℓ film was deposited on the boron nitride film by resistance heating in a thickness of 500 Å, and thereafter interdigital electrodes were formed by photolithography. Then, a ZnO polycrystalline substance was sputtered by magnetron sputtering with a mixed gas of Ar:O 2 = 1:1 under conditions of sputtering power of 100 W and a substrate temperature of 380°C, to deposit a ZnO piezoelectric film. It was possible to drive the as-formed surface acoustic wave device at a high frequency of about 1 GHz with an electrode width of 2 µm. On the other hand, a single crystal of cubic boron nitride, which was grown for about 40 hours under a high pressure of about 50 kb and a high temperature of about 1600 to 1700°C by a temperature difference method, was polished. Then, interdigital electrodes were formed on the polished boron nitride single crystal similarly to the above, and a ZnO polycrystalline substance was sputtered by magnetron sputtering with a mixed gas of Ar:O 2 = 1:1 under conditions of sputtering power of 100 W and a substrate temperature of 380°C, to deposit a ZnO piezoelectric film. It was possible to drive the as-formed surface acoustic wave device at a high frequency of about 1 GHz with an electrode width of 2 µm. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being limited only by the terms of the appended claims.

Claims: A surface acoustic wave device being formed by stacking a first layer (2), a piezoelectric layer (4) and an electrode layer (3a,3b) characterized in that the first layer is a boron nitride layer. A surface acoustic wave device in accordance with claim 1, wherein said boron nitride layer (2) is essentially made of either a single-crystalline or polycrystalline material. A surface acoustic wave device in accordance with claim 1, wherein said boron nitride layer (2) is a thin film formed by vapor deposition. A surface acoustic wave device in accordance with claim 1, wherein said piezoelectric layer (4) is essentially made of either a single-crystalline or polycrystalline material. A surface acoustic wave device in accordance with claim 1, wherein said boron nitride layer (2) is essentially made of cubic boron nitride. A surface acoustic wave device in accordance with claim 1, wherein said piezoelectric layer (4) is essentially made of one or more compounds selected from a group of ZnO, AlN, Pb(zr,T)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 and CdS. A surface acoustic wave device in accordance with claim 1, wherein said electrode layer (3a,3b) is formed by a portion of said boron nitride layer (2) containing an impurity being added thereto. A surface acoustic wave device in accordance with claim 1, wherein said electrode layer (3a,3b) is essentially made of boron nitride containing an impurity being added thereto. A surface acoustic wave device in accordance with claim 1, comprising a substrate (1); said first layer (2) formed on said substrate (1), said layer being essentially made of boron nitride; said piezoelectric layer (4) being in close contact with said layer (2), and a pair of interdigital electrode (3a,3b) layers being in close contact with said piezoelectric layer (4). A surface acoustic wave device in accordance with claim 1, comprising: a substrate (1) being essentially made of boron nitride; said piezoelectrid layer (4) being formed on said substrate (1); and a pair of interdigital electrode (3a,3b) layers being in close contact with said piezoelectric layer (4).

Inventors: FUJIMORI NAOJI, HACHIGO AKIHIRO, NAKAHATA HIDEAKI, SHIKATA SHINICHI, FUJIMORI, NAOJI, HACHIGO, AKIHIRO, NAKAHATA, HIDEAKI, SHIKATA, SHINICHI, FUJIMORI, NAOJI, C/O ITAMI WORKS OF SUMITOMO, HACHIGO, AKIHIRO, C/O ITAMI WORKS OF SUMITOMO, NAKAHATA, HIDEAKI, C/O ITAMI WORKS OF SUMITOMO, Shikata, Shinichi, c/o Itami Works of Sumitomo
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1992
Main classification: H03H9
IPC codes: H03H9