Patent Publication Number: US-6040651-A

Title: Surface acoustic wave device

Description:
This application is a continuation of U.S. patent application Ser. No. 08/948,060 filed Oct. 9, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a surface acoustic wave device having interdigital electrodes and, more particularly, to a surface acoustic wave device which is formed on a piezoelectric substrate and operates with a VHF (Very High Frequency) or an UHF (Ultra High Frequency). 
     In a conventional resonator type surface acoustic wave device, the transverse mode (fundamental mode), in which the energy is distributed in a direction perpendicular to the propagating direction of a surface acoustic wave, becomes the major propagating mode, while high-order modes sometimes considered as spurious. 
     In order to suppress such the spurious, as a conventional technique, Japanese Patent Laid-Open No. 62-219709 discloses a surface acoustic wave resonator in which, regarding the shape of the interdigital electrodes, the ratio of the overlap width of the electrode fingers to the aperture width of the electrode fingers is 65% to 75% and this overlap width is constant within the interdigital electrodes. 
     In this surface acoustic wave resonator, as shown in FIG. 8, the potential present in the +/- range of the high-order transverse mode is effectively canceled by setting a ratio W/W0 of an overlap width W to an aperture width W0 of interdigital electrodes 71 and 72 from 65% to 75%, thereby suppressing the higher-order transverse mode. The characteristic feature of this conventional technique resides in that the ratios of the overlap widths to the aperture widths of the interdigital electrodes which are present in the surface acoustic wave device are equal among all the interdigital electrodes. 
     In the conventional technique described above, however, when a plurality of high-order modes are present, the device characteristics are degraded by the spurious in the mode on an order different from the order of the mode which is to be suppressed. 
     The reason for this is as follows. As shown in FIG. 8, a surface acoustic wave device has one or more interdigital electrodes, and the ratio W/W0 of the overlap width of its interdigital electrodes is constant throughout the surface acoustic wave device. In a case wherein only one high-order transverse mode is present, this high-order transverse mode can be suppressed. 
     When a plurality of high-order transverse modes are present, as in a case wherein the aperture width W0 is increased or the thickness of the metal thin film that forms the interdigital electrodes is increased, all the high-order transverse modes cannot be suppressed simultaneously. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a surface acoustic wave device in which the high-order transverse mode is suppressed. 
     In order to achieve the above object, according to the present invention, there is provided a surface acoustic wave device comprising a piezoelectric substrate, an input first interdigital electrode formed on the piezoelectric substrate, and an output second interdigital electrode formed on the piezoelectric substrate to be adjacent to the first interdigital electrode, wherein a ratio of an overlap width to an aperture width of the first interdigital electrode and a ratio of an overlap width to an aperture width of the second interdigital electrode are set to different values, so that a transverse secondary mode is suppressed with one of the first and second interdigital electrodes while a transverse quaternary mode is suppressed with the other of the first and second interdigital electrodes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a surface acoustic wave filter according to the first embodiment of the present invention; 
     FIG. 2 is a view for explaining the transverse mode that occurs in the surface acoustic wave filter shown in FIG. 1; 
     FIG. 3 is a graph showing the theoretical calculation of the excitation efficiency of the high-order transverse mode according to the present invention; 
     FIG. 4 is a plan view of a surface acoustic wave filter according to the second embodiment of the present invention; 
     FIG. 5 is a graph showing the amplitude characteristics of the surface acoustic wave filter shown in FIG. 2; 
     FIG. 6 is a graph showing the amplitude characteristics of a surface acoustic wave filter that does not employ the present invention; 
     FIG. 7 is a plan view of a surface acoustic wave filter according to the third embodiment of the present invention; and 
     FIG. 8 is a plan view of a conventional surface acoustic wave filter. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described in detail with reference to the accompanying drawings. 
     FIG. 1 shows a resonator type surface acoustic wave filter according to the first embodiment of the present invention. Referring to FIG. 1, the surface acoustic wave filter has a piezoelectric substrate 11, an input interdigital electrode 12 and an output interdigital electrode 13 formed of thin metal films on the piezoelectric substrate 11 to be adjacent to each other, and two grating reflectors 14 and 15 formed on the two outer sides of the set of the interdigital electrodes 12 and 13 to trap the energy of the surface acoustic wave. 
     In this surface acoustic wave device, the ratios of an electrode finger overlap width Win and an electrode finger overlap width Wout to an aperture width W0 of the two interdigital electrodes 12 and 13, respectively, are set different from each other, so that a transverse secondary mode S2 is suppressed with one electrode while a transverse quaternary mode S4 is suppressed with the other electrode. 
     In this surface acoustic wave filter, in order to suppress the transverse secondary mode S2 and the transverse quaternary mode S4 with one and the other electrodes, respectively, the ratios W/W0 (Win/W0, Wout/W0) of the overlap widths Win and Wout to the aperture width W0 of the interdigital electrodes 12 and 13, respectively, are set to be different from each other. 
     The interdigital electrodes 12 and 13 are arranged such that the distal ends of the cross finger portions oppose each other. The aperture widths W0 of the interdigital electrodes 12 and 13 are equal to each other, and the overlap widths Win and Wout of the interdigital electrodes 12 and 13 are different from each other. 
     The characteristic feature of the present invention resides in that the ratios W/W0 of the overlap widths Win and Wout to the aperture width W0 of the interdigital electrodes 12 and 13 are set different between the input and output interdigital electrodes, and that the transverse secondary (S2) mode and the transverse quaternary (S4) mode are suppressed with one and the other electrodes, respectively. 
     The operation of the surface acoustic wave filter shown in FIG. 1 will be described in detail with reference to FIG. 2. 
     Referring to FIG. 2, the energy distribution of a transverse fundamental mode distribution 21 is entirely located in a positive region, and this mode is generally the major propagation mode. Each of a transverse secondary mode distribution 22 of the transverse secondary (S2) mode and a transverse quaternary mode distribution 23 of the transverse quaternary (S4) mode has both the positive and negative regions. If W/W0 is selected such that the areas of the + and - regions are substantially equal, excitation of each of the transverse secondary mode that is shown in distribution 22 and transverse quaternary mode that is shown in distribution 23 are suppressed simultaneously. 
     FIG. 2 shows a case wherein W/W0 is selected such that the transverse secondary mode distribution 22 and the transverse quaternary mode distribution 23 are suppressed with the input and output electrodes, respectively. 
     FIG. 3 shows the calculation result of the W/W0 dependency of the excitation efficiency [dB] of the each transverse mode. The calculation is performed based on an assumption that the substrate is an ST-cut quartz substrate, and the aperture width W0 is set to 40λ and a thickness H of the aluminum film that forms the interdigital electrode fingers is set to 0.98 μm. Note that λ is the wavelength of the SAW (Surface Acoustic Wave). In FIGS. 3, reference numerals 41, 42, and 43 denote the excitation efficiencies of the transverse fundamental (S0) mode, transverse secondary (S2) mode, and transverse quaternary (S4) mode, respectively. 
     As is apparent from FIG. 3, the values of W/W0, with which the excitation intensity becomes the smallest, differ among the three modes. In this calculation example, if W/W0 of the interdigital electrode 12 and that of the interdigital electrode 13 are set to about 0.71 and 0.86, respectively, the excitation efficiency of the transverse secondary (S2) mode and that of the transverse quaternary (S4) mode are suppressed by 50 dB or larger than that when compared to those in the transverse fundamental (S0) mode. 
     A surface acoustic wave filter according to the second embodiment of the present invention will be described with reference to FIGS. 4 to 6. 
     FIG. 4 shows a resonator type surface acoustic wave filter formed on an ST-cut quartz substrate, and FIGS. 5 and 6 show its amplitude characteristics. In FIG. 4, reference numerals 25 and 26 denote input and output interdigital electrodes, respectively. 
     The center frequency of this resonator type surface acoustic wave filter is about 90 MHz, and aluminum is used as the material to form the input and output interdigital electrodes 25 and 26 (thickness: 0.98 μm). FIG. 6 is a graph showing a case not using a surface acoustic wave filter according to the present invention. Electrode finger overlap widths Win and Wout, and an aperture width W are all set to 40λ. 
     FIG. 5 is a graph showing a case using a surface acoustic wave filter according to the present invention. An overlap width Win of the input interdigital electrode 25 is set to 28λ and an overlap width Wout of the output interdigital electrode 26 is set to 34λ, while the aperture width is kept at 40λ. In this case, Win/W0=0.7, and Wout/W0=0.86. In FIG. 6 that does not employ the present invention, the transverse secondary (S2) mode and transverse quaternary (S4) mode occur strongly on the high-frequency region of the fundamental mode. In contrast to this, in FIG. 5 that employs the present invention, the amplitude of each of the transverse secondary (S2) mode and transverse quaternary (S4) mode that are to be suppressed is decreased by 10 dB or larger than that. 
     In the surface acoustic wave filter employing the present invention, since the pair number of the input and output electrodes need not be equal, they can be selected to equalize the input and output impedances. 
     A surface acoustic wave filter according to the third embodiment of the present invention will be described with reference to FIG. 7. FIG. 7 shows a three-electrode type surface acoustic wave resonator filter. This filter has three interdigital electrodes 31, 32, and 33 in its structure. Grating reflectors 34 and 35 are arranged on the two sides of the set of the interdigital electrodes 31, 32, and 33 that are arranged adjacently. 
     Output-side electrode finger overlap widths W1 and W3 are set equal, while an input-side electrode finger overlap width W2 is set different from the electrode finger overlap width W1 or W3. In this structure as well, each of the input- and output-side electrodes can suppress one high-order transverse mode. 
     As has been described above, according to the present invention, the high-order transverse mode, i.e., the transverse secondary and quaternary mode, can be suppressed simultaneously by setting the ratios of the overlap widths to the aperture width of the input and output electrodes to be different from each other. 
     What poses a problem in the transverse mode of the surface acoustic wave filter is the high-order transverse mode with respect to the transverse fundamental mode, which is the major resonance mode. The distribution of the fundamental mode is entirely located in the positive region, whereas the distribution of the high-order transverse mode, which is the spurious, has both the positive and negative regions. When the overlap widths of the interdigital electrode fingers are appropriately selected such that the areas of the + and - regions are substantially equal to each other in the high-order mode distribution, the energy generated in the high-order mode is substantially canceled. 
     In the present invention, the ratios of the overlap widths w to the aperture width W0 of the interdigital electrodes are selected such that the transverse secondary mode is canceled with one electrode while the transverse quaternary mode is canceled with the other electrode. Therefore, two high-order spurious modes can be suppressed simultaneously.