Surface acoustic wave device

Abstract: To obtain a surface acoustic wave device which can be produced with ease at a low cost and can be reduced in size without causing deterioration of characteristics, a surface acoustic wave device having input and output transducers (2, 3) arranged alternately on the surface of a substrate (1) for propagating a surface acoustic wave includes metal patterns (6) provided outside the transducers (2, 3) along the extension line of the propagation path of the surface acoustic wave and having at least one surface inclined at an angle Θ (5° < Θ < 85° or -85° < Θ < -5°) with respect to the propagation direction of the surface acoustic wave.

Description: The present invention relates to a surface acoustic wave device, and, more specifically, to a surface acoustic wave device having low loss and few ripples in in-band frequency characteristics. In the conventional method for making an excellent surface acoustic wave device having low loss and excellent out-band frequency characteristics, patterns made of a metal thin film are formed outside input and output transducers and grounded. Such a method is disclosed, for example, in CPM 81-20 of Vol.81 No.78 of Denshi Buhin and Zairyo (electronic parts and materials), a technical research report issued by the Institute of Electronics and Communication Engineers of Japan on July 20, 1981. The configuration of this type of surface acoustic wave device will be described as a prior art corresponding to the present invention with reference to FIG.3. FIG.3 is a schematic diagram of this prior art comprising three electrodes, which was drawn with reference to a photograph included in the above-described document. In FIG.3, the surface acoustic wave filter of this prior art is structured such that two input transducers 2 and one output transducer 3 are arranged alternately on a substrate 1 for propagating a surface acoustic wave along the propagating direction of the surface acoustic wave, the input transducers 2 are connected to an input terminal 4 while the output transducer 3 is connected to an output terminal 5, and metal patterns 6 which are grounded are arranged outside the input transducers 2. To describe the prior art briefly, the number of electrodes and the number of electrode fingers are reduced to explain the basic structure of the device and the role of the metal patterns 6 outside the transducers is the same as that of the prior art. FIG.5 is a schematic diagram of a surface acoustic wave device proposed, for example, in the Japanese Utility Model Application JP-2-28125. In the figure, reference numeral 1 represents a parallelogrammic substrate for propagating a surface acoustic wave. On the substrate, two input transducers 2 and one output transducer 3 are arranged alternately along the propagating direction of the surface acoustic wave and the input transducers 2 are connected to the input terminal 4 while the output transducer 3 is connected to the output terminal 5. A description is subsequently given of the operation of the above-described prior art. Electric power of an input signal supplied to the input terminal 4 is divided by two and supplied to the input transducers 2 which convert the electric power into a surface acoustic wave. The converted surface acoustic wave propagates toward both sides of the input transducers 2 as shown by the arrows of FIG.3 or FIG.5. The output transducer 3 receives the surface acoustic wave and converts it into an electric signal. The converted electric signal is then output from the output terminal 5 as an output electric signal. The surface acoustic wave propagating toward both end portions of the substrate 1 from the input transducers 2 is not received by the output transducer 3. Thus, half of the total electric power being directed toward both ends of the substrate 1 for propagating the surface acoustic wave is lost. Generally speaking, when the surface acoustic wave leaked toward both end portions of the substrate 1 propagates from the input transducers constructed by metal patterns toward both end portions of the substrate 1 where the metal patterns do not exist, reflection is caused by mismatching of acoustic impedance due to the existence and non-existence of the metal patterns, leading to such deterioration of characteristics as ripples in amplitude characteristics and phase characteristics. To overcome this problem, as shown in FIG.3, metal patterns 6 are formed along the extension line of the propagation path of the surface acoustic wave. Then, mismatching of acoustic impedance is minimal or rarely occurs, thus causing little reflection, whereby the effect of reflection at both end portions of the substrate 1 of the surface acoustic wave leaked toward both end portions of the substrate 1 can be minimized. Meanwhile, a surface acoustic wave device which makes use of irregular reflection at both end portions of the substrate 1 is shown in FIG.5. Electron beam lithography may be used for patterning of the surface acoustic wave devices. In the electron beam lithography, an electron beam is irradiated onto a photosensitive agent for exposure unlike photolithography in which a photomask pattern is transferred by light. This electron beam lithography is very effective especially for the formation of fine patterns because of the short wavelength of a radiation source. Electron beam exposure is applied in the production of a photomask used in the patterning of the above-described photolithography. The pattern configuration of a photomask is exactly the same as that of a surface acoustic wave device. However, since the prior art surface acoustic wave device is structured as described above, in the configuration of FIG.3, the surface acoustic wave leaked from end portions of the input transducers 2 is hardly reflected at the boundaries of the metal pattern 6 and the input transducer 2. However, when the surface acoustic wave propagates from an end portion of the metal pattern 6 toward an end portion of the substrate 1, it is reflected at an end portion of the metal pattern 6 due to mismatching of acoustic impedance caused by the existence and non-existence of the metal pattern. This is based on the fact that the effect of reflection of the surface acoustic wave propagating from the input transducers 2 at the boundaries of the metal pattern 6 and the input transducer 2 can be reduced by adjusting the space between the input transducer 2 and the metal pattern 6, whereas it is difficult to control the effect at the boundaries of the metal pattern 6 and an end portion of the substrate 1. A surface acoustic wave device having 13 input and output transducers arranged alternately on the surface of a 64° Y-rotation, X-propagation lithium niobate substrate like the above-described prior art was actually produced and it was found that the device was greatly affected by reflection, as shown in FIG.4. The expression 64° Y-rotation, X-propagation lithium niobate substrate, refers to a substrate over the surface of which a surface acoustic wave propagates and the plate normal of the surface of which is inclined at an angle of more than 59° and less than 69° from the Y axis of the crystalline axis toward the Z axis. The propagation direction of the surface acoustic wave forms an angle of more than -5° and less than +5° or more than 175° and less than 185° with respect to the X axis of the crystalline axis. This substrate has good performance in propagating a surface acoustic wave. T1 of FIG.4 represents a feedthrough signal (direct wave), t2 a main signal, t3 and t4 reflections from the metal pattern 6 of FIG.3, and t5 reflection from an end portion of the substrate 1. Frequency characteristics resulting from these signals are shown in FIG.6. Frequency characteristics obtained from the configuration of FIG.5 are shown in FIG.7. A number of ripples, although small, remain in both frequency characteristics. The configuration of the prior art device shown in FIG.5 in which the substrate 1 becomes bulky is not suitable to reduce the size of the device. An object of the present invention is to provide a surface acoustic wave device which has a simple structure that can be constructed by the same method as that used for input or output transducers, can prevent deterioration of frequency characteristics caused by reflections of a surface acoustic wave leaked from both ends of the transducers, and can be made small in size. According to the present invention, there is provided a surface acoustic wave device as defined in claim 1. In the surface acoustic wave device according to the present invention, since the surface acoustic wave leaked from the transducer propagates without being reflected at the boundaries of the transducer and the metal pattern and is refracted by an impedance gap, it deviates from the propagation direction of the surface acoustic wave in the transducer. As a result, most of the wave leaked from both ends of the transducer does not return to the inside of the transducer and accordingly, the effect of reflection from end portions of the substrate is small, thereby eliminating deterioration of frequency characteristics. The above and other objects and advantages of the invention will become more apparent from the following description when taken in conjunction with the accompanying drawings. FIG.1 is a block diagram of a surface acoustic wave device according to Embodiment 1 of the present invention; FIG.2 is a block diagram of another surface acoustic wave device according to Embodiment 1 of the present invention; FIG.3 is a block diagram of an exemplary surface acoustic wave device of the prior art; FIG.4 is a diagram of signal transmission characteristics of the prior art surface acoustic wave device of FIG.3; FIG.5 is a block diagram of another exemplary surface acoustic wave device of the prior art; FIG.6 is an enlarged view of a passband in the characteristic diagram of the prior art device of FIG.3; FIG.7 is an enlarged view of a passband in the characteristic diagram of the prior art device of FIG.5; FIG.8 is an enlarged view of a passband in the characteristic diagram of the Embodiment 1 of FIG.2(a); FIG.9 is a block diagram of a surface acoustic wave device according to Embodiment 2 of the present invention; FIG.10 is a block diagram of another surface acoustic wave device according to Embodiment 2 of the present invention; FIG.11 is a block diagram of another surface acoustic wave device according to Embodiment 2 of the present invention; FIG.12 is a block diagram of another surface acoustic wave device according to Embodiment 2 of the present invention; FIG.13 is a block diagram of another surface acoustic wave device according to Embodiment 2 of the present invention; FIG.14 is a block diagram of another surface acoustic wave device according to Embodiment 2 of the present invention; FIG.15 is a block diagram of another surface acoustic wave device according to Embodiment 2 of the present invention; FIG.16 is a block diagram of another surface acoustic wave device according to Embodiment 2 of the present invention; FIG.17 is a block diagram of another surface acoustic wave device according to Embodiment 1 of the present invention; and FIG.18 is a diagram of the configuration of another surface acoustic wave device according to Embodiment 1 of the present invention. Embodiment 1 Preferred embodiments will be described hereunder with reference to FIG.1 and FIG.2(a) and (b). A surface acoustic wave device according to the preferred embodiment is structured as shown in FIG.1 and FIG.2(a) and (b) in that two input transducers 2 and one output transducer 3 are arranged alternately along the propagation direction of a surface acoustic wave on a piezoelectric substrate 1, the input transducers 2 are connected to an input terminal 4 and the output transducer 3 to an output terminal 5, and metal patterns 6a and 6b made of a metal thin film are formed outside the input transducers 2. The metal thin film is formed by sputtering a metal such as Al, Au and Cu. As shown in FIG.2(a), the metal patterns 6a and 6b are spaced half a wavelength from the input transducers 2. FIG.2(b) is a schematic diagram of the phase of an incident wave and a reflected wave. In this embodiment, the metal patterns 6a and 6b adjacent to the input transducers are shaped like a trapezoid compared with the oblong metal patterns 6 of the prior art shown in FIG.3. In other words, the above-described metal patterns 6a and 6b are shaped like a trapezoid having a perpendicular side 6x that the surface acoustic wave propagating from the transducer reaches as a first side and an inclined side 6y that the surface acoustic wave reaches after it propagates through the location of the metal pattern, these sides being located at the boundary between the metal pattern and the substrate 1. Preferably, the angle  of the inclined side 6y is set at a range of more than 5° and less than 85°, or more than -85° and less than -5° with respect to the propagation direction of the surface acoustic wave. With the above-described structure, in the surface acoustic wave device of this embodiment shown in FIG.1 and FIG.2(a), the surface acoustic wave leaked from the input transducers 2 passes through the trapezoidal metal patterns 6a and 6b and is reflected and refracted by mismatching of impedance at end portions of the metal patterns 6a and 6b. Thereby, the surface acoustic wave which returns to the input transducers 2 is slight, and hence, the effect of reflection at end portions of the substrate is extremely small. In this instance, the sides x of the metal patterns 6a and 6b located outside the input transducers 2 may be inclined as shown in FIG. 17, or two sides y, y of the metal patterns 6a and 6b may be inclined as shown in FIG.18. In both cases, the same effect as described above can be obtained. In FIG.1, reflection which takes place at no-metal small spaces between the input transducers 2 and the metal patterns 6a and 6b is not taken into consideration. However, if it is taken into consideration, the spaces are set at half a wavelength (λo/2) as shown in FIG.2(a). Thereby, as shown in FIG.2(b), the wave reflected by the metal patterns 6a and 6b becomes different from the wave reflected by end portions of the input transducers 2 by a phase of π. Therefore, the waves are weakened by each other and the effect of reflections from end portions can be further reduced. With the structure shown in FIG.2(a), frequency characteristics having few ripples as shown in FIG.8 can be obtained. Embodiment 2 In the above-described embodiment, the metal patterns 6a and 6b located outside the transducers are trapezoidal. They may be pentagonal as shown in FIG.9, trapezoidal facing opposite directions as shown in FIG.10, or polygonal with a part of the metal patterns 6a and 6b uncovered with metal as shown in FIG.11. As shown in FIG.12, the metal patterns 6a and 6b may have a waved portion. The embodiment having these patterns produces the same effect as Embodiment 1. The above-described metal patterns 6a and 6b are provided between the input transducers 2 and end portions of the substrate 1 in Embodiment 1. Parts of the patterns may also surround the input transducers 2, the output transducer 3, the input terminal 4 and the output terminal 5 as shown in FIG.13. In this case, the same effect as in Embodiment 1 can be obtained. In Embodiment 1, the metal patterns 6a and 6b function as floating electrodes, but as shown in FIG.14, both may be grounded. Also, as shown in FIG.15, the transducers and the metal patterns 6a and 6b adjacent thereto may be combined together. In Embodiment 1, the metal patterns 6a and 6b are located outside the transducers, but, as shown in FIG.16, they may be located outside the grating reflectors 7. In this case, the same effect can be obtained. In each of the above embodiments, a surface acoustic wave device constructed by two input IDTs and one output IDT has been described, but each of the embodiments can be applied to a surface acoustic wave device which is constructed by a plurality of input IDTs and a plurality of output IDTs. In this case, the same effect can also be obtained. As described above since the metal patterns located outside the transducers are formed such that the surface acoustic wave is scattered, the surface acoustic wave device can be produced with ease at a low cost and does not deteriorate frequency characteristics even if it is made small in size.

Claims: A surface acoustic wave device comprising a substrate (1) for propagating a surface acoustic wave, input transducers (2) connected to an input terminal (4) for converting electrical signals to acoustic signals which are formed on the surface of said substrate (1), at least one output transducer (3) connected to an output terminal (5) for converting acoustic signals formed on the surface of said substrate (1) to electrical signals, said at least one output transducer (3) arranged alternately with said input transducers (2) in the propagation direction of the surface acoustic wave, and metal patterns (6) disposed on said substrate (1) outside of said input and output transducers (2, 3) along the extension line of the propagation direction of the surface acoustic wave, characterized in that each of said metal patterns (6) has a side (6y; x,y) inclined with respect to the propagation direction of the surface acoustic wave. The surface acoustic wave device according to Claim 1, wherein each of said metal patterns (6) has at least one side inclined at an angle  in the range of 50 <  < 85° or in the range - 85° <  < -5° with respect to the propagation direction of the surface acoustic wave. The surface acoustic wave device according to Claim 2, wherein each of said metal patterns (6) has a first side (6x) reached first by the surface acoustic wave propagating from said input transducers (2) and a second side (6y) reached by the surface acoustic wave after passing the area where said metal pattern (6) is located, both sides being disposed between the outermost input or output transducer (2, 3) and the edge portion of the substrate (1), said first side (6x) being perpendicular to the propagation direction of the surface acoustic wave and said second side being the inclined side (6y). The surface acoustic wave device according to Claim 3, wherein each of said metal patterns (6) is spaced from said transducer by (2n + 1) λ/2, where λ represents the wavelength of the surface acoustic wave, so that the phase of the surface acoustic wave reflected by the outermost end portion of said input or output transducer (2, 3) is different from the phase of the surface acoustic wave reflected by said first side (6x) of the metal pattern (6), the phase difference being (2n + 1) π where n is a natural number including zero. The surface acoustic wave device according to any one of Claims 1 to 4, further comprising grating reflectors (7) located outside said input and output transducers (2, 3) along the extension line of the propagation direction of the surface acoustic wave and inside said metal patterns (6). The surface acoustic wave device according to Claim 1 or 2, wherein each of said metal patterns (6) has a first side (x) reached first by the surface acoustic wave propagating from said input transducer (2) and a second side reached by the surface acoustic wave after passing the area where said metal pattern (6) is located, both sides being disposed between the outermost transducer (2, 3) and the edge portion of said substrate (1), said first side (x) being inclined with respect to said propagation direction of the surface acoustic wave. The surface acoustic wave device according to Claim 1 or 2, wherein each of said metal patterns (6) has a first side that the surface acoustic wave propagating from said transducer (2) reaches first and a second side that the surface acoustic wave reaches after passing through an area where said metal pattern (6) is located, both sides being disposed between the outermost input or output transducer (2, 3) and the edge portion of the substrate (1), said first and second sides (y, y) being inclined with respect to said propagation direction of the surface acoustic wave The surface acoustic wave device according to one of Claims 1 to 7, wherein said substrate for propagating the surface acoustic wave is made of lithium niobate.

Inventors: MORISHIMA KOUJI, SAKAI ATSUSHI, TERA YOSHIKO, MORISHIMA, KOUJI, SAKAI, ATSUSHI, TERA,YOSHIKO, Morishima, Kouji, c/o Mitsub. Denki KK, Tsushinki, Sakai, Atsushi, c/o Mitsubishi Denki KK, Tsushinki, Tera,Yoshiko, c/o Mitsubishi Denki KK, Tsushinki
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1993
Main classification: H03H9
IPC codes: H03H9