Abstract:
A surface acoustic wave (SAW) device includes a piezoelectric substrate, a first and a second interdigital transducers (IDT) provided thereon. The second IDT has a side that is substantially aligned with a corresponding side of the first IDT, and another side of the second IDT is arranged so that the second IDT may adjust a leaked wave caused resulting from by a power-flow angle of the piezoelectric substrate.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention generally relates to a surface acoustic wave device that employs a piezoelectric material, and more particularly, to a surface acoustic wave device having multiple interdigital transducers (hereinafter simply referred to as IDT) on a piezoelectric material substrate (hereinafter simply referred to as piezoelectric substrate). 
   2. Description of the Related Art 
   In these years, the above-mentioned type of filter, which is composed of surface acoustic wave (hereinafter referred to as SAW) device having multiple IDTs on the piezoelectric substrate, has been employed for a bandpass filter in a television set having a frequency range of 30 MHz to 400 MHz and an RF filter in a mobile telephone having a frequency range of 800 MHz to several GHz. An IDT includes a pair of comb-like electrodes. Each comb-like electrode is composed of a bus bar and electrode fingers having first edges connected to the bus bar and second edges that are open. A pair of comb-like electrodes is arranged so that the electrode fingers of the comb-like electrodes are alternately crossed or interleaved at regular intervals. In other words, the interleaved electrode fingers are alternately connected to two bus bars. A SAW is generated by applying an alternating voltage across the pair of comb-like electrodes. The SAW has a frequency response by which a filter having a desired frequency characteristic is obtainable. 
     FIG. 1  shows a filter with the SAW. Japanese Patent Application Publication No. 10-41778 (hereinafter referred to as Document 1) discloses this type of filter. Referring to  FIG. 1 , there are arranged a first IDT  10 , a ground electrode  20 , and a second IDT  30  on a piezoelectric substrate  1 . The first IDT  10 , the ground electrode  20 , and the second IDT  30  are adjacently arranged in a direction of the SAW propagation. The ground electrode  20  is arranged between the first IDT  10  and the second IDT  30 , serving as a shield electrode. The first IDT  10  serves as an input electrode (or output electrode) and the second IDT  30  serves as an output electrode (or input electrode). The ground electrode  20  prevents electromagnetic coupling of the IDT  10  and the IDT  30 . Also, the ground electrode  20  is arranged on a tilt in order to prevent the SAW that travels from the IDT  10  (or the IDT  30 ) from being reflected by the ground electrode  20  and returning to the IDT  10  (or the IDT  30 ). 
   The IDT  10  includes a pair of comb-like electrode  10   a  and  10   b.  The comb-like electrode  10   a  includes a bus bar  12   a  and multiple electrode fingers  14   a.  The comb-like electrode  10   b  also includes a bus bar  12   b  and multiple electrode fingers  14   b.  The open edges of the electrode fingers  14   a  face those of the electrode fingers  14   b , which are referred to as crossing portions or overlapping parts. The crossing portions of the interleaved electrode fingers that face each other are involved in excitation of SAW. As shown in  FIG. 1 , an electrode finger pattern is weighted. The electrode finger pattern is defined as a pattern formed by the electrode fingers. The electrode finger pattern may be weighted by, for example, apodization. By this apodization, lengths of the electrode fingers in the overlapping parts (hereinafter referred to as aperture length) vary in the propagation direction. The aperture lengths are relatively small in the vicinity of both sides of the IDT  10 , which is defined as small overlapping parts. On the other hand, the aperture lengths are relatively large around the center of the IDT  10 . The aperture length is proportional to excitation intensity. Therefore, the strong SAWs are generated around the center of the IDT  10 , and weak SAWs are generated in the vicinity of both ends of the IDT  10 . The frequency characteristic may be altered by changing the weight by apodization. 
   The IDT  30  also includes a pair of comb-like electrodes. However, the IDT  30  is not weighted, which is different from the IDT  10 . In other words, the electrode fingers of the IDT  30  have an identical overlapping length. The above-mentioned IDT is defined as a normal IDT. 
   The bus bar  12   a  is connected to an electrode pad  15 , and the bus bar  12   b  is connected to an electrode pad  16 . The bus bars of the IDT  30  are respectively connected to electrode pads  17  and  18 . Thus, the filter with the above-mentioned configuration serves as a bandpass filter. 
   With the above-mentioned SAW device, it is necessary to consider a power-flow angle of the piezoelectric substrate  1 . The power-flow angle defines the propagation direction of the SAW. As shown in  FIG. 1 , the power-flow angle is created by an X-axis and the propagation direction of the SAW, where the X-axis is defined as the direction parallel to the central axes of the longer sides of the IDTs  10  and  30 , and a Y-axis is defined as the direction perpendicular to the X-axis. The power-flow angle is specific to the piezoelectric materials, and generally ranges from zero to a few degrees. For example, 112° LiTaO 3  has the power-flow angle of a few degrees.  FIG. 1  shows a case where the power-flow angle of the piezoelectric substrate  1  is not zero. The SAW travels from the IDT  10  at the power-flow angle. Therefore, the IDT  30  is unable to receive the entire SAW. The SAW that is not received by the IDT  30  is defined as leaked wave, which degrades the stopband characteristic. 
   The above-mentioned drawback has been well known, and some proposals have been made. International Publication Number WO 96/10293 (hereinafter referred to as Document 2), Japanese Patent Application Publication No. 10-209802 (hereinafter referred to as Document 3), and Japanese Patent Application Publication No. 11-205079 (hereinafter referred to as Document 4) have proposed that, in the case where the piezoelectric substrate having a non-zero power-flow angle, the electrodes are arranged so that the propagation direction of the SAW may be parallel to the power-flow angle. This is shown in  FIG. 2 . In addition, Japanese Patent Application Publication No. 53-114644 (hereinafter referred to as Document 5) has proposed that the aperture length of the output electrode is designed to be greater than that of the input electrode so that the leaked wave caused by the power-flow angle may be adjusted. This is shown in  FIG. 3 . Referring to  FIG. 3 , the IDT  30  extends from both sides of the IDT  10 , by a width A, in the direction perpendicular to the central axes. Similarly, the ground electrode  20  also extends from both sides of the IDT  10 , by the width A, in the direction perpendicular to the central axes. In Document 5, the aperture lengths of the output electrode in the Y direction are 1.05 to 1.50 times those of the input electrode. 
   However, with the above-mentioned techniques, it is to be noted that a larger piezoelectric substrate is required to arrange the IDT  10  on a tilt as disclosed in Documents 2, 3, and 4, or to arrange the IDT  30  and the ground electrode  20  having larger aperture lengths as disclosed in Document 5. The above-mentioned techniques cause a problem that the SAW device cannot be downsized. In particular, if the aperture lengths are made relatively large as disclosed in Document 5, the aperture lengths become larger than necessary, and the electrode finger resistance is increased. This may increase losses. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above circumstances and provides a small-sized and low-loss SAW device that is excellent in stopband characteristics. 
   According to an aspect of the present invention, there is provided a surface acoustic wave (SAW) device including a piezoelectric substrate, a first interdigital transducer (IDT) and a second IDT provided on the piezoelectric substrate. One side of the second IDT and its corresponding side of the first IDT are arranged in almost a line, and another side of the second IDT is arranged so that the second IDT may adjust a leaked wave caused resulting from by a power-flow angle of the piezoelectric substrate. The substrate can be downsized by arranging one side of the second IDT and its corresponding side of the first IDT. In addition, the second IDT has appropriate aperture lengths, which is low in losses. Excellent stopband characteristic is obtainable by arranging another side of the second IDT so that the second IDT may adjust a leaked wave caused resulting from by a power-flow angle of the piezoelectric substrate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will be described in detail with reference to the following drawings, wherein: 
       FIG. 1  is a plane view of a conventional SAW device; 
       FIG. 2  is a plane view of another conventional SAW device; 
       FIG. 3  is a plane view of yet another conventional SAW device; 
       FIG. 4  is a plane view of the SAW device in accordance with a first embodiment of the present invention; 
       FIG. 5  is a plane view of the SAW device in accordance with a second embodiment of the present invention; 
       FIG. 6  illustrates a simple electrode pattern; 
       FIG. 7  is a plane view of the SAW device in accordance with a third embodiment of the present invention; 
       FIG. 8  is a plane view of the SAW device in accordance with a fourth embodiment of the present invention; 
       FIG. 9  is a plane view of the SAW device in accordance with a fifth embodiment of the present invention; 
       FIG. 10  is a plane view of the SAW device in accordance with a sixth embodiment of the present invention; 
       FIG. 11  is a plane view of the SAW device in accordance with a seventh embodiment of the present invention; 
       FIG. 12  is a plane view of the SAW device in accordance with an eighth embodiment of the present invention; 
       FIG. 13  is a plane view of the SAW device in accordance with a ninth embodiment of the present invention; 
       FIG. 14  is a graph showing frequency characteristic of the conventional SAW device and the SAW device in accordance with the second embodiment of the present invention; and 
       FIG. 15  is a graph showing frequency characteristic of the SAW device in accordance with the second embodiment of the present invention and the SAW device in accordance with the eighth embodiment. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A description will now be given, with reference to the accompanying drawings, of embodiments of the present invention. 
   First Embodiment 
   A description will be given of a first embodiment of the present invention with reference to drawings.  FIG. 4  is a diagram showing a SAW device in accordance with the first embodiment of the present invention. The same components and configurations as those that have been described above have the same reference numerals. Referring to  FIG. 4 , the SAW device includes a piezoelectric substrate  1 A, an IDT  10  that is weighted by apodization, a ground electrode  20 , and an IDT  30 A. The IDT  30 A has a side (upper side) that is close to an imaginary straight line that extends from a corresponding side (upper side) of the IDT  10 . As shown in  FIG. 4 , the upper side of the IDT  10  is further in than that of the IDT  30 A and is further set back from the longitudinal edge of the substrate  1 A. The leaked wave at the power-flow angle does not travel to the upper side of the IDT  30 A, and it is unnecessary to extend the upper side of the IDT  30 A towards the longitudinal edge of the piezoelectric substrate  1 A. This arrangement of the IDT  30 A differs from the IDT  30  in  FIG. 3 . In contrast, the other side of the IDT  30 A is arranged so that the IDT  30 A may adjust the leaked wave caused by the power-flow angle. That is, the IDT  30 A extends in the direction perpendicular to the central axis of the SAW device and on the side that the leaked wave travels, namely, Y+ direction. The IDT  30 A thus arranged is capable of receiving the entire SAW that travels from the IDT  10  at the power-flow angle, and the excellent stopband characteristic is obtainable. In addition, the IDT  30 A in  FIG. 4  is smaller than the IDT  30  in  FIG. 3 . This makes it possible to make the piezoelectric substrate  1 A in FIG.  4  smaller than the piezoelectric substrate  1  in  FIG. 3 , and thereby to realize the downsized SAW device. 
   Second Embodiment 
     FIG. 5  is a diagram showing a SAW device in accordance with a second embodiment of the present invention. The IDT  30 A has a side (upper side) that is aligned with a corresponding side (upper side) of the IDT  10 . In other words, one side (upper side) of the IDT  30 A and its corresponding side of the IDT  10  are identically located on the Y-axis. The other side of the IDT  30 A extends beyond its corresponding side of the IDT  10 , by the width A 1 , in the Y+ direction. The width A 1  is provided for adjusting the leaked SAW. The IDT  30 A thus arranged is capable of receiving the entire SAW that travels at the power-flow angle from the IDT  10 , and the excellent stopband characteristic is obtainable. The IDT  30 A in  FIG. 5  is smaller than the IDT  30  in  FIG. 3 . This makes it possible to make the piezoelectric substrate  1 A in  FIG. 5  smaller than the piezoelectric substrate  1  in  FIG. 3 , and thereby to realize the downsized SAW device. 
   Here, for convenience of explanation, referring to  FIGS. 6A and 6B , the IDT  10  that is weighted by apodization is described.  FIG. 6A  is a pattern showing sizes of the aperture lengths and positions. The aperture lengths (lengths of the interleaved electrode fingers in the overlapping parts or the crossing portions) of this pattern are very small or zero at and around both ends. A tilt line is defined as a line connecting both ends of this pattern. Therefore, the tilt line represents an angle defined by apodization weighting. Referring to  FIG. 6B , generally, the weight by apodization is very small or zero at and around both ends of the IDT  10 . 
   Third Embodiment 
     FIG. 7  is a diagram showing a SAW device in accordance with a third embodiment of the present invention. An edge  44  is a part of an electrode finger of the IDT  30 A, and is arranged the farthest from the IDT  10 . The edge  44  is also arranged on an imaginary extended line of a tilt line  41  of the IDT  10 . The IDT  30 A thus arranged is capable of receiving the entire SAW that travels from the IDT  10  at the power-flow angle, and the excellent stopband characteristic is obtainable. The IDT  30 A in  FIG. 7  is smaller than the IDT  30  in  FIG. 3 . This makes it possible to make the piezoelectric substrate  1 A in  FIG. 7  smaller than the piezoelectric substrate  1  in  FIG. 3 , and thereby to realize the downsized SAW device. 
   Fourth Embodiment 
     FIG. 8  is a diagram showing a SAW device in accordance with a fourth embodiment of the present invention. An edge of an electrode finger of the IDT  30 A that is the farthest from the IDT  10  is arranged offset to the Y+ direction by the power-flow angle, from an electrode finger of the IDT  10  that is the closest to the IDT  30 A, that is, from a position  42  that has a small or zero weight by apodization. The IDT  30 A thus arranged has an extended portion  45  in the Y+ direction on the basis of the position  42 . The extended portion  45  is capable of receiving the leaked wave, and the excellent stopband characteristic is obtainable. The IDT  30 A in  FIG. 8  is smaller than the IDT  30  in  FIG. 3 . This makes it possible to make the piezoelectric substrate  1 A in  FIG. 8  smaller than the piezoelectric substrate  1  in  FIG. 3  and thereby to realize the downsized SAW device. As shown in  FIG. 8 , the SAW has a propagation direction parallel to a line  41 A that connects the position  42  and the edge of the electrode finger that is the farthest from the IDT  10 . 
   Fifth Embodiment 
     FIG. 9  is a diagram showing a SAW device in accordance with a fifth embodiment of the present invention. The edge  44  of an electrode finger of the IDT  30 A, which is the farthest from the IDT  10 , is arranged on an imaginary extended line of a straight line  48 . The edge  44  of the electrode finger is included in a very small or zero aperture length in the IDT  30 A. The straight line  48  passes through an edge  46  of an electrode finger that forms the largest aperture of the IDT  10  and is arranged in parallel with a propagation direction  47  at the power-flow angle. Thus, the IDT  30 A has an extended portion in the Y+ direction. The above-mentioned extended portion is capable of receiving the leaked wave, and the excellent stopband characteristic is obtainable. The IDT  30 A in  FIG. 9  is smaller than the IDT  30  in  FIG. 3 . This makes it possible to make the piezoelectric substrate  1 A in  FIG. 9  smaller than the piezoelectric substrate  1  in  FIG. 3  and thereby to realize the downsized SAW device. As shown in  FIG. 9 , the SAW has the propagation direction almost parallel to the line  48 . 
   Sixth Embodiment 
     FIG. 10  is a diagram showing a SAW device in accordance with a sixth embodiment of the present invention. The edge  44  of the electrode finger of the IDT  30 A, which is the farthest from the IDT  10 , is arranged on an imaginary extended line  51  of a straight line connecting an edge  49  and an edge  50 . The edge  49  forms the largest aperture of the IDT  10 . The edge  50  of the electrode finger is the closest to the IDT  30 A. As shown in  FIG. 10 , a tilt of the straight line  51  is bigger than the power-flow angle. The IDT  30 A has an extended portion in the Y+ direction, and is certainly capable of receiving the leaked wave, and the excellent stopband characteristic is obtainable. The IDT  30 A in  FIG. 10  is smaller than the IDT  30  in  FIG. 3 . This makes it possible to make the piezoelectric substrate  1 A in  FIG. 10  smaller than the piezoelectric substrate  1  in  FIG. 3  and thereby to realize the downsized SAW device. 
   Seventh Embodiment 
     FIG. 11  is a diagram showing a SAW device in accordance with a seventh embodiment of the present invention. A solid pattern  55  is added to the IDT  10  in  FIG. 5 . The solid pattern  55  is arranged to continue from one bus bar of the IDT  10 , which are the lower bus bar in  FIG. 11  and the bus bar  12   b  in  FIG. 1 . The width in the Y+ direction is equal to A 1  in  FIG. 5 . One side of the solid pattern  55  and its corresponding side of the IDT  30 A are arranged on a straight line. In other words, one side of the solid pattern  55  and its corresponding side of the IDT  30 A have identical positions on a Y-axis. The entire width of the IDT  10  including the solid pattern  55  in the Y-axis direction is almost equal to that of the IDT  30  in the Y-axis direction. 
   Eighth Embodiment 
     FIG. 12  is a diagram showing a SAW device in accordance with an eighth embodiment of the present invention. A dummy electrode  56  is arranged instead of the solid pattern  55  in  FIG. 11 . The adjacent electrode fingers are not overlapped in the dummy electrode, which does not excite the SAW. The same voltage is applied to pads  15  and  16 , and the ground potential is applied to the bus bar that is commonly connected from the IDT  10  and the dummy electrode  56  so that the dummy electrode  56  may cancel undesired waves generated on the IDT  10 . The undesired waves are particularly generated in small overlapping parts. The electrode finger pattern of the dummy electrode  56  is not limited to that shown in  FIG. 12 . Some electrode fingers of the dummy electrode  56  may form the small overlapping part. 
   Ninth Embodiment 
     FIG. 13  is a diagram showing a SAW device in accordance with a ninth embodiment of the present invention. Referring to  FIG. 13 , the solid pattern  55  in  FIG. 11  and the dummy electrode  56  in  FIG. 12  are both added. Dummy electrodes  57  and  58  are arranged in the propagation direction of one side of the IDT  10 . A solid pattern  59  is arranged around the center of the IDT  10 . Both sides of the propagation direction of the IDT  10  are small overlapping parts. Therefore, the dummy electrodes  57  and  58  are provided to cancel the undesired waves. 
   Referring back to  FIG. 5 , the patterns that can be formed in the lower part of the IDT  10  are not limited to the seventh through ninth embodiments of the present invention. Any other pattern may be formed. 
     FIG. 14  is a graph describing the frequency characteristic of the second embodiment of the present invention as shown in  FIG. 5  (solid line) and that of the conventional SAW device as shown in  FIG. 1  (thin line). The horizontal axis denotes frequency (MHz), and the vertical axis denotes attenuation (dB). As shown in  FIG. 14 , the stopband frequency characteristic has been improved in the second embodiment of the present invention. In other embodiments of the present invention, the same frequency characteristic is obtainable. 
     FIG. 15  is a graph describing the frequency characteristic of the eighth embodiment of the present invention as shown in  FIG. 12  (thin line) and that of the second embodiment of the present invention as shown in  FIG. 5  (solid line). The horizontal axis denotes frequency (MHz), and the vertical axis denotes attenuation (dB). The dummy electrode  56  in accordance with the eighth embodiment of the present invention is capable of canceling the undesired waves generated on the IDT  10 . Therefore, suppression in the stopband has been improved more than that of the second embodiment of the present invention. 
   The present invention is not limited to the above-mentioned first embodiment, and other embodiments, variations and modifications may be made without departing from the scope of the present invention. 
   The present invention is based on Japanese Patent Application No. 2003-388575 filed on Nov. 18, 2003, the entire disclosure of which is hereby incorporated by reference.