Surface acoustic wave device

Abstract: Disclosed herein is a surface acoustic wave device which is sufficiently guaranteed in reliability and surface-mountable with no requirement for packaging. The surface acoustic wave device (20) has a pair of opposite interdigital electrodes (2a, 2b) and a piezoelectric member (4) being in close contact with the interdigital electrodes (2a, 2b) and having a portion located between the pair of interdigital electrodes (2a, 2b) for propagating surface acoustic waves. This device (20) is characterized in that an air bridge (13) is provided for covering the portion of the piezoelectric member (4) for propagating surface acoustic waves and the pair of interdigital electrodes (2a, 2b). On the air bridge (13), it is possible to provide an insulating film (14) which is not in contact with the piezoelectric member (4).

Description: BACKGROUND OF THE INVENTION Field of the Invention 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 30, a piezoelectric phenomenon which is caused by a piezoelectric member 31 is utilized for exciting surface waves. When an electric signal is applied to interdigital electrodes 32 which are provided on the piezoelectric member 31, the piezoelectric member 31 is distorted to generate surface acoustic waves, which in turn are propagated along the piezoelectric member 31 and fetched in other interdigital electrodes 33 as an electric signal. Assuming that λ 0 represents the electrode period in the interdigital electrodes 32 and 33 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 such as a portable telephone and a car telephone. In relation to the aforementioned surface acoustic wave device, a device employable in a higher frequency range of 1.5 to 3 GHz in response to frequency allocation 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. Japanese Patent Publication No. 54-38874 (1979) discloses a surface acoustic wave device which comprises a substrate, a piezoelectric thin film and an insulating layer, being made of a material (e.g., sapphire) having a larger surface acoustic wave propagation velocity than that in the piezoelectric thin film, provided between the substrate and the piezoelectric thin film. On the other hand, Japanese Patent Laying-Open No. 64-62911 (1989) in the name of the assignee 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 largest sound velocity among materials, and the same is physically and chemically stable. Each of the aforementioned surface acoustic wave devices has been generally stored in a package, with no passivation of its center portion which is defined by a pair of interdigital electrodes and a piezoelectric member. This is because the characteristics of the device, particularly the frequency characteristics, are inevitably changed if an insulating film for passivation is provided on the piezoelectric member for propagating surface acoustic waves. However, the device packaged with no passivation cannot be sufficiently guaranteed in reliability. When such a non-protected surface acoustic wave device is surface-mounted on an electronic device to be connected to a peripheral circuit such as an amplifying circuit, it is necessary to package the device to cause no damage on the piezoelectric member and the interdigital electrodes. In practice, however, it has been impossible to carry out such packaging, due to uncertain reliability of the surface acoustic wave device whose surface is not passivated. In "1988 PROCEEDINGS, ELECTRONIC COMPONENTS CONFERENCE, 9-11 May 1988; D.M. Kirk et al; SAW FILTERS USING ZNO THIN FILM"; pages 114-118, a SAW device having the features of the preamble of present claim 1 is disclosed. US-A-4 952 832 discloses another surface acoustic wave device having the feature of the preamble of present claim 1, but without an air bridge to cover the interdigital electrodes. SUMMARY OF THE INVENTION An object of the present invention is to solve the aforementioned problems and provide a surface acoustic wave device which is sufficiently guaranteed in reliability with no necessity for packaging in surface mounting. The object is achieved with a device as defined claim 1. Preferred embodiments are defined in the subclaims. According to the present invention, the piezoelectric member can be mainly composed of one or more compounds selected from a group of ZnO, AℓN, Pb(Zr,Ti)O 3 , (Fb,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 member can be made of either a single-crystalline or polycrystalline material, a polycrystalline material is preferably employed in order to use the device in a higher frequency range, due to small scattering of surface waves. As to the aforementioned materials, a piezoelectric member of ZnO, AℓO or Pb(Zr,Ti)O 3 can be formed by CVD or sputtering. In order to drive the device according to the present invention in a higher frequency range, a material, such as single-or poly-crystalline diamond, diamond-like carbon, boron nitride or sapphire, for example, having a higher acoustic wave propagation velocity than the piezoelectric member may be brought into close contact with the piezoelectric member. In order to improve reliability of the device according to the present invention, further, an insulating film may be provided on the air bridge. Such an insulating film may be prepared from an SiO 2 film, a PSG film, an Si 3 N 4 film or a synthetic resin film of polyimide or the like, for example. As to such films, an SiO 2 film and a PSG film can be grown by low-temperature CVD, for example, while an Si 3 N 4 film can be formed by plasma deposition, for example. In the surface acoustic wave device according to the present invention, the portion of the piezoelectric member for propagating surface acoustic waves and the pair of interdigital electrodes are covered with the air bridge. In other words, a center portion of the device, which is defined by the pair of interdigital electrodes and the piezoelectric member for electromechanical transduction, is protected by the air bridge. Further, no material being necessary for passivation comes into contact with the pair of interdigital electrodes and the portion located therebetween for propagating surface acoustic waves. Due to the structure of making passivation while leaving the center portion untouched, it is possible to passivate the device without changing its characteristics. In the device according to the present invention, further, the air bridge of a conductive material such as a metal may be so grounded that it is possible to suppress generation of direct waves between the pair of the interdigital electrodes. 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 the general structure of a surface acoustic wave device; Figs. 2A to 2J typically illustrate steps of manufacturing a surface acoustic wave device according to Example of the present invention; and Fig. 3 is a sectional view showing a surface acoustic wave device manufactured according to Example of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS A high frequency filter according to Example of the present invention was manufactured in practice. The manufacturing method is now described with reference to the drawings. Referring to the drawings, sectional views are shown on the left and plan views are shown on the right. Referring to Fig. 2A, a substrate 1 of single-crystalline diamond was first prepared. An Aℓ film was deposited on the substrate 1 by resistance heating in a thickness of 500 Å, and patterned by photolithography, to form a pair of interdigital electrodes 2a and 2b and a pair of leading electrodes 3a and 3b (Fig. 2B). Then, the substrate 1 was placed on a magnetron sputtering apparatus and regions other than those provided with the interdigital electrodes 2a and 2b were masked, so that a ZnO polycrystalline substance was sputtered by magnetron sputtering with a mixed gas of Ar:O 2 = 1:1 under conditions of a sputtering output of 100 W and a substrate temperature of 380°C, thereby depositing a ZnO piezoelectric layer 4 on the substrate 1 (Fig. 2C). Then the piezoelectric layer 4 was masked to deposit an insulating film 5 of SiO 2 by SiH 4 -N 2 O based ECR (electron cyclotron resonance)-CVD, to protect the leading electrodes 3a and 3b (Fig. 2D). Then, a resist film was deposited over the substrate 1, and a first resist pattern 6 having a ring-shaped groove 6a was formed around the piezoelectric layer 4 (Fig. 2E). Then, an Au layer 7 was formed by magnetron sputtering (Fig. 2F). Thereafter a portion of the Au layer 7 located on the piezoelectric layer 4 and the groove 6a was covered by photolithography, to form a second resist pattern 9 having a plurality of holes 8 on the groove 6a (Fig. 2G). Then, the Au layer 7 was etched by ion milling, to form an Au layer pattern 12 entirely covering a portion above the piezoelectric layer 4 and having a plurality of bridge pier portions 11 upwardly extending from the groove 6a (Fig. 2H). Thereafter the first and second resist patterns 6 and 9 were removed by an organic solvent, to obtain an air bridge 13 supported by the plurality of bridge pier portions 11 (Fig. 2I). Then, an insulating film 14 of SiO 2 was deposited by SiH 4 -N 2 O based plasma CVD, to passivate the substance. Thus obtained was a surface acoustic wave filter 20 comprising the diamond substrate 11 and the interdigital electrodes 2a and 2b as well as the piezoelectric layer 4 deposited thereon, which are covered with the insulating film 14 through the air bridge 13, as shown in Fig. 2J. Fig. 3 is an enlarged sectional view showing this filter 20. Since the insulating film 14 was deposited under decompression, the interior of the air bridge 13 is maintained in a state close to a vacuum. In such a surface acoustic wave filter 20, clearances between the air bridge pier portions 11 are filled up with the insulating film 14. However, the insulating film 14 thus partially penetrating into the interior of the air bridge 13 through the clearances between the bridge pier portions 11 will not reach the portion for propagating surface acoustic waves, since the same is sufficiently separated from the piezoelectric layer 4. Due to this structure, it is possible to obtain a surface acoustic wave filter being passivated with an insulating film which is not in contact with the portion for propagating surface acoustic waves, i.e., with no changes of characteristics of the device. When the insulating film is formed by a method having stronger directivity such as ECR-CVD, for example, it is possible to further reduce the amount of the insulating film penetrating through the air bridge pier portions. When the aforementioned surface acoustic wave filter was held under environment of 80°C in temperature and 85 % in humidity, no deterioration of characteristics was recognized. When a conventional surface acoustic wave filter obtained by depositing a ZnO piezoelectric layer in the aforementioned process was held under similar environment, on the other hand, its characteristics were deteriorated in one hour. Thus, it has been recognized that reliability of the inventive surface acoustic wave device can be maintained also under severe environment. Although the substrate was made of diamond in the aforementioned Example, a diamond layer may be formed on a substrate of a proper material such as Si, for example, so that the interdigital electrodes and the piezoelectric layer are deposited on the diamond layer. As hereinabove described, the inventive device is passivated with no change of its characteristics, whereby higher reliability is guaranteed upon packaging as compared with the prior art. Further, the inventive device is surface-mountable due to the passivation, to be applicable to a further miniaturized communication device.

Claims: A surface acoustic wave device comprising: a substrate (1); a pair of interdigital electrodes (2a, 2b) on said substrate; a piezoelectric layer (4) being in close contact with said interdigital electrodes and at least having a portion of propagating surface acoustic waves between said pair of interdigital electrodes characterized by an air bridge (13) which is made of conductive material, and at least covers said pair of interdigital electrodes and said portion of said piezoelectric layer for propagating said surface acoustic waves; for avoiding necessity for packaging in surface mounting. A surface acoustic wave device in accordance with claim 1, wherein said piezoelectric layer is essentially 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. A surface acoustic wave device in accordance with claim 1, wherein the substrate (1) or a layer of a material provided on the substrate has a higher acoustic wave propagation velocity than the piezoelectric layer and is brought into close contact with the piezoelectric layer. A surface acoustic wave device in accordance with claim 3, wherein said material is selected from a group of diamond, diamond like carbon, boron nitride and sapphire. A surface acoustic wave device in accordance with claim 1, wherein said air bridge is covered with an insulating film. A surface acoustic wave device in accordance with claim 5, wherein a space enclosed by said air bridge being covered with said insulating film and said piezoelectric layer being in close contact with said interdigital electrodes is substantially evacuated. A surface acoustic wave device in accordance with claim 1, wherein said air bridge is formed of a metal.

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, H03H3