JP5882842B2 - Surface acoustic wave element and surface acoustic wave device - Google Patents

Description

The present invention relates to a surface acoustic wave element and a surface acoustic wave device using the same.

A surface acoustic wave element having a piezoelectric substrate and an IDT (InterDigital Transducer) electrode provided on a main surface of the piezoelectric substrate is known. The IDT electrode has a pair of comb electrodes. Each comb electrode includes, for example, a bus bar extending in the propagation direction of a surface acoustic wave (hereinafter may be abbreviated as “SAW”) and a plurality of electrode fingers extending from the bus bar in a direction perpendicular to the SAW propagation direction. The pair of comb electrodes are arranged so that the plurality of electrode fingers mesh with each other (cross each other).

In addition, in a SAW element using a quartz substrate, a LiNbO ₃ substrate, or the like as a piezoelectric substrate, spurious due to a high-order transverse mode may occur in its impedance characteristics.

  When a SAW device such as a SAW filter or a SAW duplexer is configured using a SAW element in which spurious due to a high-order transverse mode is not suppressed, the electrical characteristics of the SAW device deteriorate due to ripples generated in the passband.

  Therefore, as a configuration for suppressing high-order transverse mode spurious, a technique for applying cross width weighting to an IDT electrode has been conventionally known (see, for example, Patent Document 1).

JP 2007-60108 A

  However, with the conventional technique in which the IDT electrode is simply subjected to the cross width weighting, it is difficult to suppress the propagation loss while corresponding to the downsizing of the SAW element.

  An object of the present invention is to provide a surface acoustic wave element and a surface acoustic wave device that are compatible with miniaturization that can reduce SAW propagation loss.

A surface acoustic wave device according to an aspect of the present invention is a surface acoustic wave device including a piezoelectric substrate and an IDT electrode positioned on an upper surface of the piezoelectric substrate, wherein the IDT electrode is formed on the upper surface of the piezoelectric substrate. A first bus bar and a second bus bar positioned on the upper surface of the piezoelectric substrate so as to face each other in a direction orthogonal to the propagation direction of the surface acoustic wave propagating, and extends from the first bus bar toward the second bus bar. A plurality of first electrode fingers arranged in the propagation direction, and a plurality of first electrode fingers extending from the second bus bar toward the first bus bar and arranged in the propagation direction so as to alternate with the first electrode fingers. A tip of the second electrode fingers extending from the first bus bar toward the second bus bar and arranged in the propagation direction is opposed to the tips of the plurality of second electrode fingers via a first gap. Multiple A plurality of first dummy electrode fingers extending from the second bus bar toward the first bus bar and arranged in the propagation direction, the tips opposing the tips of the plurality of first electrode fingers via a second gap A second envelope electrode finger, the IDT electrode is subjected to cross width weighting, and a first envelope connecting tips of the plurality of first electrode fingers is provided for each first electrode finger. a zigzag bending the opposite side alternately, Ri zigzag der the second envelope connecting the tips of said plurality of second electrode fingers to bend to the opposite side alternately to one each of the second electrode finger, In addition, cross width weighting is performed so that the first envelope and the second envelope approach each other from the center to the end in the direction along the propagation direction of the surface acoustic wave .

  A surface acoustic wave device according to an aspect of the present invention includes the surface acoustic wave element and a circuit board on which the surface acoustic wave element is mounted.

  According to the above configuration, cross width weighting is applied to the IDT electrode, and the first envelope connecting the tips of the plurality of first electrode fingers is alternately bent to the opposite side for each first electrode finger. Since the second envelope connecting the tips of the plurality of second electrode fingers is zigzag alternately bent to the opposite side for each second electrode finger, the overall structure of the surface acoustic wave element is reduced. However, the propagation loss of the surface acoustic wave can be reduced.

It is a top view of the SAW element concerning the embodiment of the present invention. It is an enlarged view of the area | region II of FIG. It is sectional drawing in the III-III line of FIG. (A)-(e) is sectional drawing corresponding to FIG. 3 explaining the manufacturing method of a SAW element. It is sectional drawing which shows the example of the SAW apparatus to which the SAW element of FIG. 1 is applied. (A) is a top view of the SAW element of a comparative example, (b) is an enlarged view of the area | region VIb of (a). FIGS. 7A and 7B are diagrams for explaining a method for evaluating the propagation loss of the SAW element. It is a graph which shows the evaluation result of the propagation loss of an Example. It is a graph which shows the evaluation result of the spurious suppression of an Example. (A) is the top view which simplified the SAW element of the comparative example, (b) is the top view which simplified the SAW element of the Example.

  Hereinafter, SAW elements and SAW devices according to embodiments of the present invention will be described with reference to the drawings. Note that the drawings used in the following description are schematic, and the dimensional ratios in the drawings do not always match the actual ones.

(Configuration and manufacturing method of SAW element)
FIG. 1 is a plan view of a main part of a SAW element 1 according to an embodiment of the present invention. FIG. 2 is an enlarged view of region II in FIG. 3 is a cross-sectional view taken along line III-III in FIG.

  Note that the SAW element 1 may be either upward or downward, but in the following, the orthogonal coordinate system xyz is defined for convenience, and the upper side with the positive side in the z direction as the upper side, Terms such as bottom are used.

  The SAW element 1 includes a piezoelectric substrate 3, an IDT electrode 5 and a reflector 6 provided on the upper surface 3 a of the piezoelectric substrate 3, and a mass addition film 9 (FIG. 3) provided on the IDT electrode 5 and the reflector 6. The protective layer 11 (FIG. 3) covers the upper surface 3 a of the piezoelectric substrate 3 from above the mass addition film 9. In addition, the SAW element 1 has wirings, pads, and the like for inputting / outputting signals to / from the IDT electrode 5 (not shown).

The piezoelectric substrate 3 is composed of a single crystal substrate having piezoelectricity, such as a lithium tantalate (LiTaO ₃ ) single crystal or a lithium niobate (LiNbO ₃ ) single crystal.
More preferably, the piezoelectric substrate 3 is composed of a 128 ° ± 10 ° YX cut LiNbO ₃ substrate. The planar shape and various dimensions of the piezoelectric substrate 3 may be set as appropriate. As an example, the thickness (dimension in the z direction) of the piezoelectric substrate 3 is 0.2 to 0.5 mm.

  The IDT electrode 5 includes a first bus bar 21 extending in the SAW propagation direction (x direction) propagating through the piezoelectric substrate 3 and a second bus bar 22 extending in the propagation direction and facing the first bus bar 21. A plurality of first electrode fingers 7 extending from the first bus bar 21 toward the second bus bar 22, and a plurality of second electrode fingers 8 extending from the second bus bar 22 toward the first bus bar 21. Have. In the following, when referring to the structural members to which “first”, “second”, etc. are given together, “first”, “second” are omitted, and for example, “first bus bar” and “ The “second bus bar” may be simply referred to as “bus bar”.

  The first bus bar 21 and the second bus bar 22 extend in a substantially constant width and are formed in a long shape. The first bus bar 21 and the second bus bar 22 are arranged in parallel to the SAW propagation direction and face each other in a direction perpendicular to the SAW propagation direction.

  The first electrode fingers 7 and the second electrode fingers 8 are basically arranged alternately along the SAW propagation direction. That is, the first electrode finger 7 and the second electrode finger 8 are adjacent to each other.

  When the first bus bar 21 and the first electrode finger 7 are viewed integrally, a comb-like electrode is formed. Similarly, when the second bus bar 22 and the second electrode finger 8 are viewed integrally, the comb tooth is formed. Shaped electrodes. The IDT electrode 5 can also be regarded as being constituted by electrodes arranged so that these comb-like electrodes are engaged with each other.

  The plurality of first and second electrode fingers 7 and 8 are arranged at a substantially constant interval in the SAW propagation direction. This interval is defined, for example, by the center distance p (FIG. 3) between the first electrode finger 7 and the second electrode finger 8 adjacent thereto. The center-to-center distance p is provided so as to be equal to, for example, a half wavelength of the SAW wavelength λ at a frequency to be resonated. The wavelength λ (2p) is, for example, 1.5 to 6 μm. The width w1 (FIG. 3) of each electrode finger is appropriately set according to the electrical characteristics required for the SAW element 1, and is, for example, 0.4p to 0.6p with respect to the center distance p.

  The lengths of the first and second electrode fingers 7 and 8 change in the SAW propagation direction. Therefore, the length of the portion of the first electrode finger 7 that overlaps the adjacent second electrode finger 8 in the x direction (the length in the direction (y direction) orthogonal to the SAW propagation direction) varies in the SAW propagation direction. ing. Similarly, the length of the portion of the second electrode finger 8 that overlaps the adjacent first electrode finger 7 in the x direction changes in the SAW propagation direction. That is, the IDT electrode 5 is subjected to cross width weighting in which the cross width of the electrode fingers is changed along the SAW propagation direction. By applying such intersection width weighting, the occurrence of so-called high-order transverse mode spurious is suppressed.

  The IDT electrode 5 includes a plurality of first dummy electrode fingers 13 extending in the direction (y direction) perpendicular to the SAW propagation direction from the first bus bar 21 between the adjacent first electrode fingers 7, and a plurality of first dummy electrode fingers 13. A plurality of second dummy electrode fingers 14 extending in the y direction from the second bus bar 22 are provided between the two electrode fingers 8.

  The plurality of first and second dummy electrode fingers 13 and 14 are arranged at a substantially constant interval p together with the plurality of first and second electrode fingers 7 and 8.

The tip of the first dummy electrode finger 13 faces the tip of the second electrode finger 14 via the first gap G1. Similarly, the tip of the second dummy electrode finger 14 is opposed to the tip of the second electrode finger 8 via the second gap G2.

  The positions of the tips of the first and second dummy electrode fingers 13 and 14 correspond to the fact that the intersecting width of the first and second electrode fingers 7 and 8 changes in the SAW propagation direction. Is changing. Various dimensions of the first and second dummy electrode fingers 13 and 14 may be appropriately set. For example, the widths of the first and second dummy electrode fingers 13 and 14 are equal to the width w1 of the first and second electrode fingers 7 and 8, and the sizes of the first and second gaps G1 and G2 are Λ / 8 to λ / 2.

  In the SAW element 1, a contrivance is made in the method of weighting the cross width applied to the IDT electrode 5. Specifically, as shown by a broken line in FIG. 1, the first envelope 23 formed by connecting the tips of the adjacent first electrode fingers 7 in order with a straight line and the tips of the adjacent second electrode fingers 8 are connected to each other. Cross width weighting is performed so that the second envelope 34 formed by connecting in a straight line is zigzag-shaped. From another viewpoint, cross width weighting is performed so that the center position of the first gap G1 and the center position of the second gap are zigzag-shaped for each first electrode finger 7 and each second electrode finger 8. ing.

  The zigzag-shaped first envelope 23 shows the first electrode finger 7 in the center from the tip of the first electrode finger 7 at one end when the three first electrode fingers 7 arranged in succession are viewed. If the direction toward the tip of the first electrode finger 7 is to the right, the direction from the tip of the center first electrode finger 7 to the tip of the other first electrode finger 7 is to the left, in other words, the electrode finger 1 Each book is bent so that it bends to the opposite side alternately. The second envelope 24 is also bent in the same manner.

  Further, the IDT electrode 5 in the SAW element 1 has two third and fourth envelopes formed by connecting the tips of the plurality of first electrode fingers 7 every other one, as shown by a one-dot chain line in FIG. 25 and 26 are parallel to each other, and the intersection width is weighted so that the two fourth and fifth envelopes 27 and 28 formed by connecting every other tip of the second electrode finger 8 are parallel to each other. Is given.

  In the SAW element 1, cross width weighting is performed so that the first envelope 23 and the second envelope 24 approach each other from the center of the IDT electrode 5 in the direction (x direction) along the SAW propagation direction toward both ends. The figure formed by the first envelope 23 and the second envelope 24 is generally a rhombus. Therefore, in the SAW element 1, the intersection width at the center of the IDT electrode 5 is maximized, and the intersection width at the end of the IDT electrode 5 is minimized. The maximum intersection width at the center of the IDT electrode 5 is, for example, 20λ to 50λ, and the minimum intersection width at the end of the IDT electrode 5 is, for example, about 20% of the maximum value intersection width and is 4λ to 10λ.

  The lengths of the first and second dummy electrode fingers 13 and 14 are also different depending on the lengths of the first and second electrode fingers 7 and 8. By making the lengths of the first and second dummy electrode fingers 13 and 14 different depending on the lengths of the first and second electrode fingers 7 and 8, the SAW element 1 has a plurality of first and second dummy electrode fingers 13 and 14. The sizes of the two gaps G1 and G2 are constant.

  In the SAW element 1, as shown in FIG. 2, the first and second electrode fingers 7 are arranged so that the adjacent first gaps G1 and the adjacent second gaps G2 do not overlap in the SAW propagation direction (x direction). , 8, the lengths of the first and second dummy electrode fingers 13, 14 are changed.

  By applying cross width weighting as shown in FIGS. 1 and 2 to the IDT electrode 5, SAW propagation loss can be reduced without increasing the overall structure of the SAW element 1. This will be described with reference to the SAW element 500 of the comparative example of FIG.

  FIG. 6A is a plan view of a SAW element 500 of a comparative example, and FIG. 6B is an enlarged plan view of a region VIb in FIG. 6A. The SAW element 500 is obtained by applying cross width weighting to the IDT electrode 5 so that the first and second envelopes 23 and 24 are straight lines in the SAW element 1 according to the embodiment of the present invention described above. The configuration is the same as that of the SAW element 1 except for the width weighting configuration.

  In the SAW element 500, when the IDT electrode 5 is subjected to cross width weighting, as shown by a white arrow in FIG. 6B, a portion that overlaps adjacent gaps in the SAW propagation direction is likely to occur. When such gaps G overlap, the SAW propagation loss increases in this portion. This is thought to be because the periodic structure of the electrode fingers collapses at a portion where adjacent gaps G overlap, and a part of the SAW becomes a bulk wave and leaks into the piezoelectric substrate 3 at this portion.

  Therefore, as a method of reducing such an overlapping region of the gap G, an angle θ formed by a line connecting the tips of the first electrode fingers 7 (or second electrode fingers 8) adjacent to the propagation direction of the SAW (FIG. 6). It is conceivable to increase (refer to (b), hereinafter also referred to as apodization angle θ). However, if the apodization angle θ is increased without changing the capacitance of the IDT electrode, the width of the entire IDT electrode 5 in the y direction is increased, so that the IDT electrode 5 becomes larger and the SAW element 500 becomes larger. To do. That is, in the cross width weighting configuration like the SAW element 500, increasing the apodization angle θ in order to reduce the propagation loss leads to an increase in the size of the SAW element.

  On the other hand, in the SAW element 1 according to the embodiment of the present invention, since the IDT electrode 5 is subjected to cross width weighting so that the first and second envelopes 23 and 24 are zigzag, locally, While the apodization angle θ is increased to increase the displacement of the gap G, the cross width weighting can be performed at a gentle angle when viewed as a whole. With such a structure, an overlapping portion between adjacent gaps G can be reduced without increasing the IDT electrode 5, and thus the SAW element can be reduced in size while suppressing propagation loss.

  Further, as a method of reducing the overlapping region of the gap G, a method of reducing the gap G without changing the apodization angle θ can be considered. However, if the gap G is reduced, the electrode finger and the dummy electrode finger are brought closer, so that a short circuit is likely to occur between them. Therefore, the gap G cannot be made smaller than necessary, and when the SAW element 1 and the SAW element 500 having the same gap G are compared, the SAW element 1 is larger in size than the SAW element 500. While the IDT electrode 5 is small, the overlap between the adjacent gaps G can be eliminated.

  Here, as shown in FIG. 2, the size of the second gap G2 (or the first gap G1) is g, and the distance between the third envelope 25 and the fourth envelope 26 (or the fifth envelope 27 and the first envelope G1). When the distance from the six envelopes 28) is d, the IDT electrode 5 is weighted so as to satisfy at least d ≧ λtanθ. Assuming that the adjacent second gaps G2 overlap with each other, the overlapping of the adjacent second gaps G2 decreases as d increases from λtanθ, and the adjacent second gaps G are satisfied when d = (g + λtanθ) is satisfied. No overlap. Therefore, it is more preferable to weight the IDT electrode 5 so as to satisfy d ≧ (g + λtan θ).

The IDT electrode 5 is made of, for example, metal. The metal is made of, for example, Al or an alloy containing Al as a main component (Al alloy). The Al alloy is, for example, an Al—Cu alloy. The IDT electrode 5 may be composed of a plurality of metal layers. ID
Various dimensions of the T electrode 5 are appropriately set according to electrical characteristics required for the SAW element 1. As an example, the thickness e (FIG. 3) of the IDT electrode 5 is 100 to 300 nm.

  The IDT electrode 5 may be directly disposed on the upper surface 3a of the piezoelectric substrate 3, or may be disposed on the upper surface 3a of the piezoelectric substrate 3 via another member. Another member is, for example, Ti, Cr, or a laminate of these. Thus, when the IDT electrode 5 is disposed on the upper surface 3a of the piezoelectric substrate 3 via another member, the thickness of the other member has a thickness that does not substantially affect the electrical characteristics of the IDT electrode 5 (for example, Ti In this case, the thickness is set to 5% of the thickness of the IDT electrode 5).

  Although FIG. 1 shows an example in which one IDT electrode 5 is provided on the piezoelectric substrate 3, a ladder-type SAW filter in which a plurality of IDT electrodes 5 are connected by a system such as series connection or parallel connection is configured. Alternatively, a dual mode SAW resonator filter in which a plurality of IDT electrodes 5 are arranged along the X direction may be configured.

  When a voltage is applied to the piezoelectric substrate 3 by the IDT electrode 5, SAW propagating in the x direction along the upper surface 3a is induced near the upper surface 3a of the piezoelectric substrate 3. And the standing wave which makes the center distance p of the 1st electrode finger 7 and the 2nd electrode finger 8 a half wavelength is formed. The standing wave is converted into an electric signal having the same frequency as that of the standing wave, and is taken out by the first electrode finger 7 and the second electrode finger 8. In this way, the SAW element 1 functions as a resonator or a filter.

  The reflector 6 has a plurality of electrode fingers arranged at intervals substantially equal to the center distance p between the first electrode finger 7 and the second electrode finger 8 of the IDT electrode 5. For example, the reflector 6 is formed of the same material as the IDT electrode 5 and has a thickness equivalent to that of the IDT electrode 5.

  The protective layer 11 covering the IDT electrode 5 is provided, for example, over substantially the entire upper surface 3a of the piezoelectric substrate 3, covers the IDT electrode 5 and the reflector 6 provided with the mass addition film 9, and the upper surface 3a. The part exposed from the IDT electrode 5 and the reflector 6 is covered. The thickness T (FIG. 2) from the upper surface 3 a of the protective layer 11 is set to be larger than the thickness e of the IDT electrode 5 and the reflector 6. For example, the thickness T is 200 to 700 nm thicker than the thickness e by 100 nm or more. Further, from another viewpoint, the thickness T is, for example, 0.2λ to 0.5λ with respect to the wavelength λ of the SAW.

The protective layer 11 is made of an insulating material. Preferably, the protective layer 11 is made of a material such as SiO ₂ that increases the propagation speed of the surface acoustic wave when the temperature rises, thereby suppressing a change in electrical characteristics due to a change in temperature of the SAW element 1. be able to. Specifically, it is as follows.

When the temperature of the piezoelectric substrate 3 rises, the SAW propagation speed in the piezoelectric substrate 3 becomes slow, and the center-to-center distance p increases due to thermal expansion of the piezoelectric substrate 3. As a result, the resonance frequency is lowered and the desired characteristics may not be obtained. However, when the protective layer 11 is provided, the surface acoustic wave propagates not only in the piezoelectric substrate 3 but also in the protective layer 11. Since the protective layer 11 is made of a material (SiO ₂ ) that increases the propagation speed of the surface acoustic wave when the temperature rises, the temperature of the SAW propagating through the piezoelectric substrate 3 and the protective layer 11 rises as a whole. The change in speed due to is suppressed. The protective layer 11 also contributes to protecting the IDT electrode 5 from corrosion and the like.

It is desirable that the surface of the protective layer 11 has no large unevenness. Since the propagation speed of SAW propagating on the piezoelectric substrate 3 changes due to the influence of the irregularities on the surface of the protective layer 11, if there are large irregularities on the surface of the protective layer 11, the resonance frequency of each manufactured SAW element 1. A large variation will occur in this case. Therefore, if the surface of the protective layer 11 is made flat, the resonance frequency of each surface acoustic wave element is stabilized. Specifically, it is desirable that the flatness of the surface of the protective layer 11 is 1% or less of the wavelength of SAW propagating on the piezoelectric substrate 3.

  The mass addition film 9 is for improving the electrical characteristics of the IDT electrode 5 and the reflector 6. For example, the mass addition film 9 is provided over the entire upper surfaces of the IDT electrode 5 and the reflector 6. The mass addition film 9 has a rectangular cross-sectional shape orthogonal to the longitudinal direction (y direction) of the electrode fingers, for example. However, the cross-sectional shape of the mass addition film 9 may be a trapezoid or a dome shape. The thickness t (FIG. 3) of the mass addition film 9 may be appropriately set within a range in which the mass addition film 9 does not expose the protective layer 11. For example, the thickness of the mass addition film 9 is 0.01λ to 0.4λ with respect to the wavelength λ of the SAW.

  The material constituting the mass addition film 9 is a material having an acoustic impedance different from that of the material constituting the IDT electrode 5, the reflector 6, and the protective layer 11. The difference in acoustic impedance is preferably a certain amount or more, for example, 15 MRayl or more, more preferably 20 MRayl or more.

As such a material, for example, when the IDT electrode 5 is made of Al (acoustic impedance: 13.5 MRayl) and the protective layer 11 is made of SiO ₂ (12.2 MRayl), WC (102.5 MRayl) is used. TiN (56.0 MRayl), TaSiO ₂ (40.6 MRayl), Ta ₂ O ₅ (33.8 MRayl), and W ₅ Si ₂ (67.4 MRayl).

When the IDT electrode 5 is made of Al and the protective layer 11 is made of SiO ₂ , the acoustic impedance is close, so that the boundary between the electrode finger and the non-placed region of the electrode finger is acoustically ambiguous. Thus, the reflection coefficient at the boundary decreases. As a result, the reflected wave of SAW cannot be obtained sufficiently and desired characteristics may not be obtained. However, since the mass addition film 9 formed of a material having an acoustic impedance different from that of the material of the IDT electrode 5 and the protective layer 11 is provided on the upper surface of the IDT electrode 5, the boundary between the electrode finger and the electrode finger non-arrangement region is provided. The reflection coefficient at becomes high, and it becomes easy to obtain desired characteristics.

  The material of the mass addition film 9 preferably has a surface acoustic wave propagation speed slower than the materials of the IDT electrode 5, the reflector 6, and the protective layer 11. Due to the slow propagation speed, the vibration distribution tends to concentrate on the mass addition film 9, and the reflection coefficient at the boundary between the electrode finger and the non-arranged position of the electrode finger is effectively increased.

As such a material, for example, when the IDT electrode 5 is made of Al (propagation speed: 5020 m / s) and the protective layer 11 is made of SiO ₂ (5560 m / s), TaSiO ₂ (4438 m / s) is used. ), Ta ₂ O ₅ (4352 m / s), W ₅ Si ₂ (4465 m / s). As a material satisfying the condition that the propagation speed of the surface acoustic wave is slower than the material of the IDT electrode 5 and the protective layer 11, the acoustic impedance is lower than that of the material having a smaller acoustic impedance than the material of the IDT electrode 5 and the protective layer 11. Since many larger materials are known, the degree of freedom in material selection is high.

  FIG. 4A to FIG. 4E are cross-sectional views corresponding to FIG. 2 for each manufacturing process for explaining the outline of the method for manufacturing the SAW element 1. The manufacturing process proceeds in order from FIG. 4A to FIG. Note that the various layers change in shape and the like as the process progresses, but common symbols may be used before and after the change.

As shown in FIG. 4A, first, on the upper surface 3 a of the piezoelectric substrate 3, the conductive layer 15 that becomes the IDT electrode 5 and the reflector 6 and the additional layer 17 that becomes the mass addition film 9 are formed. Specifically, first, the conductive layer 15 is formed on the upper surface 3a by a thin film forming method such as a sputtering method, a vapor deposition method, or a CVD (Chemical Vapor Deposition) method. Next, the additional layer 17 is formed by the same thin film forming method.

  When the additional layer 17 is formed, a resist layer 19 is formed as a mask for etching the additional layer 17 and the conductive layer 15 as shown in FIG. Specifically, a negative or positive photosensitive resin thin film is formed by an appropriate thin film forming method, and a part of the thin film is removed at a non-arranged position such as the IDT electrode 5 and the reflector 6 by a photolithography method or the like. Is done.

  Next, as shown in FIG. 4C, the additional layer 17 and the conductive layer 15 are etched by an appropriate etching method such as RIE (Reactive Ion Etching). Thereby, the IDT electrode 5 and the reflector 6 provided with the mass addition film 9 are formed. Thereafter, as shown in FIG. 4D, the resist layer 19 is removed by using an appropriate chemical solution.

  Then, as shown in FIG. 4E, a thin film to be the protective layer 11 is formed by an appropriate thin film forming method such as a sputtering method or a CVD method. At this point, unevenness is formed on the surface of the thin film serving as the protective layer 11 due to the thickness of the IDT electrode 5 and the like. Then, if necessary, the surface is flattened by chemical mechanical polishing or the like, and a protective layer 11 is formed as shown in FIG. Note that a part of the protective layer 11 may be removed by a photolithography method or the like before or after the planarization in order to expose a pad 16 and the like described later.

(Configuration of SAW device)
FIG. 5 is a cross-sectional view showing an example of a SAW device 51 manufactured using the SAW element 1 described above.

  The SAW device 51 configures, for example, a filter or a duplexer. The SAW device 51 includes a SAW element 31, a circuit board 53 on which the SAW element 31 is mounted, and a sealing member 59 that covers the SAW element 31.

  The SAW element 31 is configured as, for example, a so-called wafer level package SAW element. The SAW element 31 includes the above-described SAW element 1, the cover 18 that covers the SAW element 1 side of the piezoelectric substrate 3, the terminal 20 that penetrates the cover 18, and the back surface layer that covers the opposite side of the piezoelectric substrate 3 from the SAW element 1. 12.

  The cover 18 is made of resin or the like, and a vibration space 18a for facilitating SAW propagation is formed above the IDT electrode 5 and the reflector 6 (on the positive side in the z direction). On the upper surface 3 a of the piezoelectric substrate 3, a wiring 10 connected to the IDT electrode 5 and a pad 16 connected to the wiring 10 are formed. The terminal 20 is formed on the pad 16 and is electrically connected to the IDT electrode 5. For example, although not particularly illustrated, the back surface portion 12 includes a back electrode for discharging charges charged on the surface of the piezoelectric substrate 3 due to a temperature change or the like, and a protective layer covering the back electrode.

  The circuit board 53 is constituted by, for example, a so-called rigid printed wiring board. A mounting pad 55 is formed on the mounting surface 53 a of the circuit board 53.

The SAW element 31 is arranged with the cover 18 side facing the mounting surface 53a. The terminals 20 and the mounting pads 55 are bonded by solder 57. Thereafter, the SAW element 31 is sealed with a sealing member 59. The sealing member 59 is made of resin, for example.

  According to the above embodiment, in the SAW element 1, the IDT electrode 5 is subjected to cross width weighting, and this cross width weight is applied to the first envelope 23 connecting the tips of the plurality of first electrode fingers 7. A zigzag shape that bends alternately to the opposite side for each electrode finger, and the second envelope 24 connecting the tips of the plurality of second electrode fingers 8 is bent to the opposite side alternately for each second electrode finger. Therefore, the SAW propagation loss can be reduced while reducing the overall structure of the SAW element 1. Further, by configuring the SAW device 51 using such a SAW element 1, a small SAW device 51 having excellent electrical characteristics can be obtained.

  The SAW element 1 shown in FIG. 1 was fabricated, and the electrical characteristics were examined to evaluate the SAW propagation loss and the spurious suppression effect. Further, the sizes of the resonators in the case where the characteristics are the same between the SAW element 1 shown in FIG. 1 and the SAW element 500 shown in FIG. 6 were compared. Table 1 shows the fabrication conditions of the SAW element 1 in the examples.

(SAW propagation loss evaluation method)
FIG. 7A and FIG. 7B are diagrams for explaining an evaluation method of SAW propagation loss.

FIG. 7A is a diagram illustrating impedance characteristics of the SAW element 1 as a resonator. In the figure, the horizontal axis indicates the frequency f (MHz), and the vertical axis indicates the absolute value | Z | (Ω) of the impedance and the phase θ (deg.) Of the impedance Z. The solid line Lz is the absolute value of the impedance | Z | shows the frequency changes, the solid line L _theta indicates the frequency change of the phase theta of the impedance.

In the graph of impedance characteristics, as indicated by a solid line Lz, a resonance point where the absolute value | Z | of the impedance becomes a minimum and an anti-resonance point where the absolute value | Z | of the impedance becomes a maximum appear. Further, between the resonance point and the antiresonance point, the impedance phase θ becomes the maximum phase θmax.

  FIG. 7B is a diagram showing the relationship between the maximum phase θmax and the SAW propagation loss LS. In the figure, the horizontal axis represents the propagation loss LS (dB / μm), and the vertical axis represents the maximum phase θmax.

  As shown in this figure, the smaller the loss of the resonator, the larger the maximum phase θmax. Therefore, the loss of the resonator can be evaluated by examining the maximum phase θmax. In the ideal state with no loss, the maximum phase θmax is 90 (deg.).

  As shown in FIG. 7A, the phase θ changes gently with respect to the change in the frequency f in the vicinity of the maximum phase θmax, while the absolute value | Z | In the vicinity, it changes rapidly with respect to the change of the frequency f. Therefore, the maximum phase θmax can be measured more stably than the absolute value | Z |, and the loss evaluation based on the maximum phase θmax is expected to have a smaller error than the loss evaluation based on the absolute value | Z |. Is done.

(SAW propagation loss evaluation results)
FIG. 8 is a graph showing the evaluation results of the propagation loss of the SAW element when the distance d between the two envelopes shown in FIG. 2 is changed. Note that the one-dot chain line in the graph is when there is no overlap in the propagation direction between adjacent gaps G, and the value of d at this time is 0.59 μm.

  As shown in the graph of FIG. 8, it has been confirmed that the SAW propagation loss can be improved as the distance d between the two parallel envelopes is increased.

(Evaluation result of spurious suppression effect)
FIG. 9 shows the evaluation result of the spurious suppression effect. The graph of FIG. 9 shows an enlarged view of the vicinity of the maximum phase θmax with respect to the phase θ of the impedance Z of the SAW element as a resonator. The evaluation is based on a SAW element having a normal type electrode structure in which cross width weighting is not applied (a crossing region of electrode fingers is rectangular), a SAW element of a comparative example having the electrode structure shown in FIG. 6, and an electrode structure shown in FIG. This was done by comparing three types of SAW elements of the example. In FIG. 9, the thin line indicates the result for the normal type SAW element, the broken line indicates the result for the comparative example SAW element, and the two-dot chain line indicates the result for the example SAW element.

  First, paying attention to the impedance characteristics of a normal type SAW element that is not subjected to cross width weighting, it can be seen that high-order mode spurious is generated. Next, when attention is paid to the SAW elements of the comparative example and the example, it can be seen that any spurious generated in the normal type SAW element is suppressed. This is an effect due to the intersection width weighting.

  Further, when comparing the SAW element of the comparative example and the SAW element of the example, the maximum phase θmax of the SAW element of the example is larger than that of the SAW element of the comparative example. That is, it can be seen that the SAW element of the example has improved propagation loss compared to the SAW element of the comparative example.

  From these results, it was confirmed that the SAW element of the example can improve the propagation loss while maintaining the spurious suppression effect.

(Evaluation result of SAW element miniaturization)
The size of the SAW element having the electrode structure shown in FIG. 1 and the comparative example having the electrode structure shown in FIG. 6 were compared in terms of electrical characteristics. Specifically, the area (occupied area) required to form the SAW element was compared when the capacitance formed on the IDT electrode was made equal between the SAW element of the example and the SAW element of the comparative example. .

  A comparison result of the occupied area between the SAW element of the comparative example and the SAW element of the example will be described with reference to FIG. 10A is a simplified plan view of the SAW element of the comparative example of FIG. 6, and FIG. 10B is a simplified plan view of the SAW element of the embodiment of FIG. In the same figure, the area surrounded by the dotted line is an area necessary for forming the SAW element, and when the cross width weighting is performed so that the crossing area of the electrode fingers is approximately a rhombus as in the comparative example and the example. Is a rectangular region having the long side of the IDT electrode in the propagation direction of the surface acoustic wave and the short side of the maximum crossing width of the electrode fingers.

  In FIG. 10, the filled area is the intersection area of the electrode fingers, and the capacity of the IDT electrode is basically determined by the area (effective area) of this area. Since the main characteristics of the SAW element are determined by this capacity, the comparison of the sizes of the SAW elements of the comparative example and the example is performed under the condition that the capacity is equal, that is, the effective area is equal. It was. Further, in both the SAW element of the comparative example and the SAW element of the example, the adjacent first gaps G1 are shifted from each other so that there is no overlap when viewed in the propagation direction of the surface acoustic wave. The second gaps G2 are also shifted from each other so that there is no overlap when viewed in the propagation direction of the surface acoustic wave.

  As a result of comparing the occupied area of the SAW element of the comparative example and the SAW element of the example under such conditions, the occupied area of the SAW element of the example was 0.81 times that of the SAW element of the comparative example. Met. From this result, it was confirmed that when the electrical characteristics were the same, the occupied area could be reduced by about 20% according to the SAW element of the example.

  From another point of view, when the occupied areas are made equal, it can be said that the effective area of the SAW element of the embodiment can be made larger than that of the SAW element of the comparative example. When the occupied area is the same, the larger effective area is expected to improve the power durability. Therefore, if the same size, the SAW element of the example is considered to improve the power durability.

  The present invention is not limited to the above embodiment, and may be implemented in various aspects.

  As described above, the intersection width weighting method is one in which the outer periphery of the entire intersection area is generally a rhombus, the rhombus side part is a cosine curve, or a plurality of rhombuses are connected. Various forms are possible.

As the piezoelectric substrate 3, in addition to the above-described 128 ° ± 10 ° YX cut LiNbO ₃ substrate, for example, 38.7 ° ± YX cut LiTaO ₃ can be used. The material of the IDT electrode 5 is not limited to Al and an alloy containing Al as a main component. For example, Cu, Ag, Au, Pt, W, Ta, Mo, Ni, Co, Cr, Fe, Mn, Zn, Ti It may be. The material of the protective layer is not limited to SiO _2, and may be silicon oxide other than SiO ₂ , for example.

1 ... SAW element (surface acoustic wave element)
DESCRIPTION OF SYMBOLS 3 ... Piezoelectric substrate 5 ... IDT electrode 6 ... Reflector 7 ... 1st electrode finger 8 ... 2nd electrode finger 9 ... Mass addition film | membrane 13 ... 1st dummy electrode finger 14 ... 2nd dummy electrode finger 21 ... 1st bus bar 22 ... 2nd bus bar 23 ... 1st envelope 24 ... 2nd envelope

Claims (3)

A surface acoustic wave device comprising a piezoelectric substrate and an IDT electrode located on the upper surface of the piezoelectric substrate,
The IDT electrode is
A first bus bar and a second bus bar located on the upper surface of the piezoelectric substrate so as to face each other in a direction orthogonal to the propagation direction of the surface acoustic wave propagating on the upper surface of the piezoelectric substrate;
A plurality of first electrode fingers extending from the first bus bar toward the second bus bar and arranged in the propagation direction;
A plurality of second electrode fingers extending from the second bus bar toward the first bus bar and arranged in the propagation direction so as to alternate with the first electrode fingers;
A plurality of first dummy electrodes extending from the first bus bar toward the second bus bar and arranged in the propagation direction, the tips of which are opposed to the tips of the plurality of second electrode fingers via a first gap. Fingers,
A plurality of second dummy electrode fingers extending from the second bus bar toward the first bus bar and arranged in the propagation direction, the tips facing the tips of the plurality of first electrode fingers via a second gap; Have
The IDT electrode is cross-weighted, and has a zigzag shape in which a first envelope connecting tips of the plurality of first electrode fingers is alternately bent to the opposite side for each first electrode finger. , wherein the plurality of zigzag der the second envelope connecting the tip of the second electrode finger is bent toward the opposite side alternately every one second electrode fingers is, and,
2. The elasticity according to claim 1, wherein cross width weighting is performed such that the first envelope and the second envelope approach each other from the center to the end in the direction along the propagation direction of the surface acoustic wave. Surface wave device . The adjacent first gaps are in positions that do not overlap with each other in the propagation direction of the surface acoustic wave,
2. The surface acoustic wave device according to claim 1, wherein the adjacent second gaps are positioned so as not to overlap each other in the propagation direction of the surface acoustic wave. A surface acoustic wave device according to claim 1 or 2 ,
A surface acoustic wave device comprising: a circuit board on which the surface acoustic wave element is mounted.

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