JP2005223409A - Surface acoustic wave device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface acoustic wave device for reducing spurious emission of longitudinal modes generated in out-band, and reducing impedance. <P>SOLUTION: The surface acoustic wave device is one, in which interdigital transducers 20, each having a plurality of electrode fingers 22 and one end of each of the short-circuited electrode fingers 22 are provided on a piezoelectric substrate 12. Each of the electrode fingers 22 is provided with an obliquely-bent connection 24, the electrode fingers 22 are divided into a plurality of straight portions 26, and the divided straight portions 26 each have different lengths. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a surface acoustic wave device, and more particularly to a surface acoustic wave device that obtains desired filter characteristics.

  Examples of devices that use surface acoustic waves that propagate along the surface or interface of a piezoelectric substrate include surface acoustic wave (hereinafter referred to as SAW) filters and SAW resonators. FIG. 6 shows a schematic plan view of the SAW chip used in these devices. FIG. 6A shows a SAW chip used for a SAW filter, and FIG. 6B shows a SAW chip used for a SAW resonator. The SAW chip 1 (SAW element piece) is provided with interdigital electrodes (hereinafter referred to as IDT) on the surface of a piezoelectric substrate 2 made of a material having a piezoelectric effect, and reflectors 4 on both sides of the IDT 3. Is the main structure. The IDT 3 has a plurality of electrode fingers 5 and is formed by short-circuiting one end of the electrode fingers 5. Then, the two IDTs 3 engage each other with the electrode fingers 5 to form a pair of IDTs 3. When forming a SAW filter, an input electrode 6 made of IDT 3 and an output electrode 7 are provided on the piezoelectric substrate 2.

By the way, the SAW chip 1 uses a piezoelectric substrate having a small electromechanical coupling coefficient (for example, a quartz substrate) to increase the impedance. However, the required characteristic requires a low impedance value, so that the width of the electrode fingers 5 meshing with each other ( Although it is necessary to increase the crossing width a, it is known that when the crossing width a is increased, a resonance in a transverse mode perpendicular to the propagation direction of the surface acoustic wave is generated, and spurious is generated by the resonance in the transverse mode. ing. Patent Document 1 discloses a SAW filter that does not generate spurious. In the present invention, a plurality of IDTs are arranged in parallel between two reflectors and symmetrical with respect to the center line of the reflector width. By adopting this configuration, even if the crossing width of the IDT is increased to just before the secondary transverse mode appears, spurious due to the primary transverse mode does not appear, and considering the fact that the logarithm of the IDT can be increased, impedance design The degree of freedom can be increased.
Japanese Patent Laid-Open No. 3-19816

  However, the configuration of the SAW chip according to the prior art cannot suppress the occurrence of spurious in the transverse mode and cannot reduce the impedance.

  Further, in the configuration of the invention disclosed in Patent Document 1, when three IDTs are connected in parallel, the IDT located outside the center line is electrically connected. When four IDTs are connected in parallel, the inner two IDTs and the outer two IDTs at the target position with respect to the center line are electrically connected to each other. At this time, if the connecting portion between the IDTs is thin, the wiring resistance loss becomes large. Therefore, the connecting portion must be thickened. Therefore, there is a problem that the area of the SAW chip becomes large.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a surface acoustic wave device that reduces spurious in a transverse mode that occurs outside a passband and reduces impedance.

  In order to achieve the above object, a surface acoustic wave device according to the present invention includes a plurality of electrode fingers, and an interdigital electrode formed by short-circuiting one ends of the plurality of electrode fingers on a piezoelectric substrate. The wave device is characterized in that the electrode finger is provided with an obliquely bent connection portion, the electrode finger is divided into a plurality of linear portions, and the lengths of the divided linear portions are different. When an electric signal is applied to the interdigital electrode, a periodic mechanical strain is generated in the linear portion, and a surface acoustic wave is excited. This surface acoustic wave propagates along the direction perpendicular to the straight line portion (the direction in which one end of the electrode finger is short-circuited). For this reason, each linear part becomes a track which propagates a surface acoustic wave, and the crossing width of each track is different. By changing the track crossing width, the position of spurious generated in each track can be changed. Therefore, spurious can be reduced in the characteristics of the surface acoustic wave chip that combines the characteristics of each track. Although the track crossing width of each track is narrow, since there are a plurality of tracks, the track crossing width of the entire surface acoustic wave chip can be widened. Therefore, the impedance can be reduced.

  A plurality of the interdigital electrodes are provided in series along the propagation direction of the surface acoustic wave, and one interdigital electrode is used as an input electrode, and the other interdigital electrode is used as an output electrode. Since the surface acoustic wave filter has an input electrode and an output electrode on a piezoelectric substrate, spurious can be generated outside the passband in the filter characteristics of each track, and the spurious generation position can be changed in each track. Can do. Therefore, the spurious outside the pass band can be reduced with the filter characteristics of the SAW chip that combines the filter characteristics of each track.

  In addition, a reflector is provided on both sides of the interdigital electrode along the propagation direction of the surface acoustic wave, and a plurality of connecting portions bent obliquely are provided on the electrode fingers of the reflector, so that the electrode fingers have a plurality of different lengths. The straight line portion has the same length as the straight line portion of the adjacent interdigital electrode. As a result, both ends of each track can be sandwiched by the reflectors in accordance with the shape of the interdigital electrode having the connection portion, so that a standing wave can be generated in each track. Therefore, a low-loss surface acoustic wave chip can be obtained.

  Hereinafter, preferred embodiments of the surface acoustic wave device according to the present invention will be described. FIG. 1 shows a plan view of a surface acoustic wave chip according to the present embodiment. FIG. 1 shows only main components of the surface acoustic wave chip (surface acoustic wave element piece). The main structure of the surface acoustic wave (SAW) chip 10 is such that an input electrode 14, an output electrode 16 and a reflector 18 are provided on a piezoelectric substrate 12 made of a material that generates a piezoelectric effect.

  The input electrode 14 and the output electrode 16 are composed of interdigital electrodes 20 (IDT 20). The IDT 20 includes a plurality of electrode fingers 22 and is formed by short-circuiting one end of the electrode fingers 22. The two IDTs 20 alternately engage the electrode fingers 22 to form a pair of IDTs 20. The electrode finger 22 has a connection portion 24 that is bent obliquely with respect to the meshing direction of the electrode finger 22, and a linear portion 26 that is divided into a plurality of portions by the connection portion 24 is formed. In addition, the connection part 24 provided in each electrode finger 22 is provided so that the electrode which short-circuits the electrode finger 22 may be provided, and the one where the length of the connection part 24 is short is preferable. Moreover, when providing the some connection part 24 in the one electrode finger 22, what is necessary is just to provide the connection part 24 by alternating the direction to be bent. That is, in FIG. 1, the connection part 24a on the upper side of the drawing is bent to the right side, and the connection part 24b on the lower side is bent to the left side to be bent alternately on the left and right. If the bending directions are alternated, the outer shape of the SAW chip 10 does not increase. Note that the connecting portion 24 may be bent only in one direction, but in this case, the outer shape of the SAW chip is larger than when the connecting portions 24 are alternately bent.

  The straight portions 26a, 26b, and 26c have different lengths, and the order in which the straight portions 26a, 26b, and 26c are provided is arbitrary. That is, in FIG. 1, the upper straight line portion is the longest, the middle step is the middle length, and the lower step is the shortest. However, the present invention is not limited to this. The length may be the shortest at the middle and the longest at the bottom. The distance L1 between the straight portions 26 divided by the connecting portion 24, that is, the straight portion 26a and the straight portion 26b and between the straight portion 26b and the straight portion 26c (in the meshing direction of the electrode fingers 22) is 3λ / 8 ≦ L1 ≦. They are separated by a distance that satisfies the condition of 5λ / 8. Here, λ is the wavelength of the surface acoustic wave excited by the piezoelectric substrate 12 when an electric signal is applied to the input electrode 14. In FIG. 1, it is described as L1 = λ / 2. The length of the straight portion 26 is set so as to obtain a desired filter characteristic.

  A pair of IDTs 20 having the same configuration as the IDT 20 described above is provided on the side of the pair of IDTs 20, that is, in one of the meshing directions of the electrode fingers 22. It has become. Such a SAW chip 10 is mounted in a package to form a SAW device. The SAW device on which this SAW chip 10 is mounted becomes a SAW filter.

  Reflectors 18 are provided at positions on both sides sandwiching the input electrode 14 and the output electrode 16. The reflector 18 has a plurality of electrode fingers 30 and both ends of the electrode fingers 30 are short-circuited. The electrode fingers 30 constituting the reflector 18 are formed along the electrode fingers 22 provided on the IDT 20. And the electrode finger 30 which comprises the reflector 18 is the structure which has the connection part 32 bent diagonally with respect to the meshing direction of the electrode finger 30, and formed the linear part 34 divided | segmented into plurality by the connection part 32. . The connecting portion 32 is on an extension line of the connecting portion 24 provided in the IDT 20, and the lengths of the linear portions 34 constituting the reflector 18 are different. And it is preferable that the length of the connection part 32 is as short as the connection part 24 provided in IDT20. The connecting portion 32 may be bent in the same direction as the connecting portion 24 provided in the IDT 20.

  The distance L2 between the straight portions 34 divided by the connecting portion 32, that is, the straight portions 34a and 34b, and the straight portions 34b and 34c in the lateral direction (the meshing direction of the electrode fingers 30) is 3λ / 8 ≦ L2 ≦. They are separated by a distance that satisfies the condition of 5λ / 8. In FIG. 1, it is described as L2 = λ / 2. Moreover, the length of the linear part 34 of the reflector 18 adjacent to the linear part 26 of IDT20 is the same, respectively. Therefore, the lengths of the straight portion 26a of the IDT 20 and the straight portion 34a of the reflector 18, the straight portion 26b of the IDT 20 and the straight portion 34b of the reflector 18, the straight portion 26c of the IDT 20 and the straight portion 34c of the reflector 18 are the same. Become.

  When an electric signal is applied to the input electrode 14 of the SAW chip 10 described above, a periodic mechanical strain is generated in each linear portion 26 due to the piezoelectric effect of the piezoelectric substrate 12, and a surface acoustic wave is excited. The surface acoustic wave propagates along the surface of the piezoelectric substrate 12 along the meshing direction of the electrode fingers 22 and is converted into an electric signal when reaching the output electrode 16. Here, a region where the surface acoustic wave is propagated is defined as a track, and this track is formed in each of the straight portions 26 and 34. In each track, the width at which the straight portions 26 face each other is defined as a track crossing width W. In FIG. 1, the surface acoustic wave propagation region corresponding to the straight portion 26a is defined as track 1, and the cross width of track 1 is defined as W1. Similarly, a region corresponding to the straight line portion 26b is defined as a track 2, and an intersection width is defined as W2. Further, an area corresponding to the straight line portion 26c is a track 3, and an intersection width is W3.

  FIG. 2 shows a plan view of a SAW filter having the same track crossing width W. When all the track crossing widths W are the same, the track crossing width W can reduce the track crossing width in one track as compared with the SAW chip according to the prior art in which no connection portion is provided. It has been confirmed that it moves away from the passband of. However, even if the filter characteristic can be moved away from the pass band, spurious may occur outside. Since the filter characteristics are required to have a pass band with a small attenuation and an attenuation region with a large attenuation, it is not preferable that spurious is generated in the attenuation region. This spurious is generated largely in the attenuation region by combining the spurious generated in track 1, track 2 and track 3, respectively. Therefore, if the position of the spurious generated in each of the tracks 1 to 3 is changed, spurious will not be generated greatly in the attenuation region. In order to change the spurious position, the track crossing widths W1, W2, and W3 may be changed.

  Next, a method for setting the intersection widths W1 to W3 in each track will be described. By combining the filter characteristics of the tracks 1 to 3, the filter characteristics of the SAW chip 10 can be obtained. Therefore, it is only necessary to use filter characteristics that have the same passband in each of the tracks 1 to 3 and have different spurious positions in the attenuation region. This is because the relationship between the shift amount of the center frequency of the passband when the track crossing width W is changed and the position (frequency) where spurious is generated when the track crossing width W is changed is experimentally examined in advance. What is necessary is just to set based on a relationship. FIG. 3 is an explanatory diagram of the relationship between the track intersection width W, the shift amount of the center frequency, and the position where spurious occurs. Note that the horizontal axis of FIG. 3 indicates the track crossing width normalized by λ, and the vertical axis indicates the center frequency of the passband and the position where spurious is generated in frequency. The graph shown on the upper side of FIG. 3 represents the position where spurious is generated, and the graph shown on the lower side represents the shift amount of the center frequency. In FIG. 3, the center frequency when the track crossing width W is 6.7λ is used as a reference, and the position where the spurious is generated is deviated from the center frequency. It can be seen from FIG. 3 that even if the track crossing width W changes, the shift amount of the center frequency does not change much. On the other hand, it can be seen that the position where the spurious is generated varies greatly compared to the shift amount of the center frequency.

  Then, the filter characteristics of the tracks 1 to 3 are predicted from the relationship between the track intersection width W and the spurious position in FIG. 3, and the track intersection width W is set in each track so that the spurious positions do not overlap. Good. Further, if the track crossing width W is changed in each track, the center frequency of the pass band is changed. For this reason, the center frequency of the pass band may be adjusted by changing the wavelength of the surface acoustic wave propagated in each track.

  For example, when the center frequency of the passband is 400 MHz and the SAW chip 10 is composed of three tracks, the track crossing widths are set to 6.7λ, 5.4λ, and 4.9λ from FIG. 3 so that the spurious generated in each track does not overlap. To do. When the track crossing width W is set to 6.7λ, the propagation speed of the surface acoustic wave is 3130 m / s, and in order to set the center frequency of the pass band to 400 MHz, the wavelength λ1 of the surface acoustic wave is 7.825 μm. Similarly to the above, when the track crossing width W is set to 5.4λ, the propagation speed of the surface acoustic wave is 3132.74 m / s, and the wavelength λ2 of the surface acoustic wave is 7.832 μm. Further, when the track crossing width W is set to 4.9λ, the propagation speed of the surface acoustic wave is 3134.40 m / s, and the wavelength λ3 of the surface acoustic wave is 7.836 μm. In each track, the SAW chip 10 may be formed so as to satisfy the track crossing width W and the surface acoustic wave wavelength. In addition, what is necessary is just to change the space | interval of an electrode finger, for example in order to change the wavelength of a surface acoustic wave.

  FIG. 4 shows the filter characteristics of the SAW chip 10 formed and set as described above. The horizontal axis in FIG. 4 indicates the amount of deviation from the center frequency of the passband, and the vertical axis indicates the attenuation. 4A shows the filter characteristics when the track crossing width W is 6.7λ and 5.4λ, and FIG. 4B shows the filter characteristics when the track crossing width W is 5.4λ and 4.9λ. FIG. 4C shows the filter characteristics of the SAW chip 10 formed with track crossing widths W of 6.7λ, 5.4λ, and 4.9λ. 4A, when the track crossing width W is 6.7λ, spurious is generated around ΔF = 7.5 MHz, and when the track crossing width W is 5.4λ, spurious is generated around ΔF = 11 MHz. ing. Further, as shown in FIG. 4B, spurious is generated in the vicinity of ΔF = 12 MHz when the track crossing width W is 4.9λ. Accordingly, it can be seen that when the track crossing width W is shortened, the position where the spurious is generated moves away from the pass band, and the position of the spurious generated in each track does not overlap. From FIG. 4C, it can be seen that the spurious positions generated in the tracks do not overlap, so that the filter characteristic of the SAW chip 10 is an attenuation region with a large attenuation amount. It can be seen that no large spurious is generated.

  As described above, since the connection portions 24 and 32 are provided on the electrode fingers 22 and 30 and divided into a plurality of tracks, the track crossing width W in each track becomes narrow, and spurious due to the transverse mode can be kept away from the passband. By changing the track crossing width W of each track, it is possible to change the appearance position of spurious generated outside the passband in each track. Therefore, with the filter characteristics of the SAW chip 10 that combines the filter characteristics of each track, it is possible to reduce spurious generated outside the passband, that is, in the attenuation band.

Although the track crossing width W in each track is narrow, since there are a plurality of tracks, the track crossing width of the entire SAW chip 10 can be widened. Therefore, the impedance can be reduced.
Further, when adjusting the frequency of the pass band in each track, for example, the interval between the electrode fingers 22 is adjusted. Since the adjustment amount is very small as described above, the frequency of the pass band is easily adjusted. be able to.

In the above-described embodiment, the number of tracks is three. However, the number of tracks is not limited to this, and the number of tracks may be two or four or more.
Further, the SAW chip 10 of the above-described embodiment is a resonator filter that is a longitudinally coupled double mode filter, but is not limited to this, and the electrode finger has a connection portion and a plurality of linear portions having different lengths. And a resonator filter or a transversal filter having a configuration in which are provided.

In the above-described embodiment, the SAW filter is described. However, a SAW resonator having a connection portion and a plurality of linear portions having different lengths may be provided on the electrode finger. FIG. 5 shows a plan view of the SAW chip constituting the SAW resonator. The SAW chip 40 has a pair of IDTs 42 and reflectors 44 are provided on both sides sandwiching the IDTs 42. The electrode fingers 46 and 48 of the IDT 42 and the reflector 44 are provided with connection portions 50 and 52 and linear portions 54 and 56 similar to those of the above-described embodiment. With such a configuration, the spurious can be moved away from the vicinity of the resonance frequency and is not affected by the spurious. Depending on the configuration of the SAW resonator, the SAW chip 40 may be configured without the reflector 44. Further, an oscillation circuit may be mounted on the above-described SAW resonator, and the SAW chip and the oscillation circuit may be electrically connected to form a SAW oscillator.
In addition, the above-described SAW chip can be used in a SAW device such as a mass sensor that detects the mass of a substance from a change in resonance frequency, or an acceleration sensor that detects the magnitude of acceleration or impact applied from the outside.

It is a top view of the surface acoustic wave chip concerning this embodiment. It is a top view of the surface acoustic wave filter which comprised track crossing width W all the same. It is explanatory drawing of the relationship between the track | truck crossing width, the deviation | shift amount of a center frequency, and the position where a spurious occurs. It is a filter characteristic of a surface acoustic wave chip. 1 is a plan view of a surface acoustic wave chip constituting a surface acoustic wave resonator. It is the schematic top view of the surface acoustic wave chip concerning a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ......... Surface acoustic wave (SAW) chip | tip, 12 ......... Piezoelectric substrate, 18 ..... Reflector, 20 ..... Interdigital electrode (IDT), 22, 30 ...... Electrode finger, 24, 32 ... ...... Connection part, 26, 34 ......... Linear part, 40 ......... SAW chip.

Claims (4)

In a surface acoustic wave device having a plurality of electrode fingers, and interdigital electrodes formed by short-circuiting one end of the plurality of electrode fingers on a piezoelectric substrate,
Providing a connection portion bent obliquely to the electrode finger, the electrode finger is divided into a plurality of linear portions,
Each of the divided straight portions has a different length.
A surface acoustic wave device characterized by that.   2. The interdigital transducers are provided in series along the propagation direction of surface acoustic waves, and one interdigital electrode is used as an input electrode, and the other interdigital electrode is used as an output electrode. The surface acoustic wave device as described. Along with the propagation direction of the surface acoustic wave, a reflector is provided on both sides sandwiching the interdigital electrode,
Providing a connecting portion bent obliquely on the electrode finger of the reflector, the electrode finger is divided into a plurality of linear portions having different lengths,
The straight portion has the same length as the straight portion of the adjacent interdigital electrode.
The surface acoustic wave device according to claim 1 or 2. 4. The surface acoustic wave device according to claim 1, wherein the piezoelectric substrate is mounted on a package.

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