Elastic wave device

Abstract

Provided is an elastic wave device capable of more reliably suppressing diffusion of Cu constituting an IDT electrode into a piezoelectric substrate. The elastic wave device of the present invention includes: a piezoelectric substrate (2); and an IDT electrode (3) having a first layer (4) provided on the piezoelectric substrate (2) and a second layer (5) provided on the first layer (4) and having Cu as a main component, and having a plurality of electrode fingers. The first layer (4) includes a first main surface (4a) on the piezoelectric substrate (2) side and a second main surface (4b) in contact with the second layer (5). The second layer (5) includes a third main surface (5a) in contact with the first layer (4), a fourth main surface (5b) opposite the third main surface (5a), and a side surface (5c) connecting the third main surface (5a) and the fourth main surface (5b). The IDT electrode (3) further has a barrier layer (7) (outer layer) provided on the side surface (5c) of the second layer (5). A boundary (A) between the side surface (5c) of the second layer (5) and the barrier layer (7) is on the second main surface (4b) of the first layer (4), and the barrier layer (7) does not reach the piezoelectric substrate (2).

Description

TECHNICAL FIELD

[0001] The present application relates to an elastic wave device. BACKGROUND

[0002] Conventionally, elastic wave devices have been widely used for filters of portable telephones and the like. In Patent Literature 1 described below, an example of an elastic wave device is disclosed. In the elastic wave device, an IDT (Interdigital Transducer) electrode is provided on a piezoelectric substrate. The IDT electrode includes a Cu film. Further, a silicon oxide film is provided on the piezoelectric substrate so as to cover the IDT electrode. In order to prevent Cu in the Cu film from diffusing to the silicon oxide film, a protective film is provided on the piezoelectric substrate so as to cover the IDT electrode.

[0003] In the elastic wave device described in Patent Literature 2, the IDT electrode has a barrier layer provided on a piezoelectric substrate and a main electrode layer provided on the barrier layer. The barrier layer is provided in order to prevent metal of the main electrode layer from diffusing to the piezoelectric substrate.

[0004] PRIOR ART DOCUMENTS

[0005] PATENT LITERATURE

[0006] Patent Literature 1: Japanese Patent Application Publication No. 2019-068309

[0007] Patent Literature 2: Japanese Patent Application Publication No. 2017-157944 SUMMARY

[0008] PROBLEMS TO BE SOLVED BY THE INVENTION

[0009] If the protective film of Patent Literature 1 is provided on the piezoelectric substrate so as to cover the IDT electrode of Patent Literature 2, it is also considered that diffusion of metal constituting the IDT electrode to the silicon oxide film and the piezoelectric substrate can be suppressed. However, the present inventors have found that metal can diffuse to the piezoelectric substrate via a path of a boundary of the barrier layer and the protective film.

[0010] An object of the present application is to provide an elastic wave device capable of more reliably suppressing diffusion of Cu constituting an IDT electrode to a piezoelectric substrate.

[0011] MEANS FOR SOLVING THE PROBLEMS

[0012] The elastic wave device of the present invention includes: a piezoelectric substrate; and an IDT electrode having a first layer disposed on the piezoelectric substrate and a second layer disposed on the first layer and mainly composed of Cu, and having a plurality of electrode fingers. The first layer includes a first main surface located on the piezoelectric substrate side and a second main surface in contact with the second layer. The second layer includes a third main surface in contact with the first layer, a fourth main surface facing the third main surface, and a side surface connected to the third main surface and the fourth main surface. The IDT electrode also has an outer layer disposed on the side surface of the second layer. The boundary between the side surface of the second layer and the outer layer is located on the second main surface of the first layer, and the outer layer does not reach the piezoelectric substrate.

[0013] The effects of the invention

[0014] The elastic wave device according to the present invention can more reliably suppress the diffusion of Cu constituting the IDT electrode into the piezoelectric substrate. Attached Figure Description

[0015] Figure 1 This is a top view of the elastic wave device according to the first embodiment of the present invention.

[0016] Figure 2 It is along Figure 1 The cross-sectional view of the electrode finger of line II in the image.

[0017] Figure 3 This is a cross-sectional view of the electrode fingers in the first comparative example.

[0018] Figure 4 This is a cross-sectional view of the electrode fingers in the second comparative example.

[0019] Figure 5 This is a cross-sectional view of the electrode finger in a first variation of the first embodiment of the present invention.

[0020] Figure 6 This is a cross-sectional view of the electrode fingers in a second variation of the first embodiment of the present invention.

[0021] Figure 7 This is a cross-sectional view of the electrode fingers in the third variation of the first embodiment of the present invention.

[0022] Figure 8 This is a cross-sectional view of the electrode finger in the second embodiment of the present invention.

[0023] Figure 9 This is a cross-sectional view of the electrode fingers in a modified example of the second embodiment of the present invention.

[0024] Figure 10is a lateral sectional view of an electrode finger in the third embodiment of the present application.

[0025] Figure 11 is a lateral sectional view of an electrode finger in a modification of the third embodiment of the present application.

[0026] Figure 12 is a lateral sectional view of an electrode finger in the fourth embodiment of the present application.

[0027] Figure 13 is a front sectional view of an elastic wave device in the fifth embodiment of the present application.

[0028] Figure 14 is a front sectional view of an elastic wave device in the sixth embodiment of the present application.

[0029] Figure 15 is a front sectional view of an elastic wave device in the seventh embodiment of the present application. DETAILED DESCRIPTION

[0030] Hereinafter, specific embodiments of the present application will be described with reference to the drawings, thereby making the present application clear.

[0031] Note that each of the embodiments described in this specification is an exemplary embodiment, and it is foreseen that partial substitution or combination of structures can be made between different embodiments.

[0032] Figure 1 is a plan view of an elastic wave device in the first embodiment of the present application.

[0033] The elastic wave device 1 has a piezoelectric substrate 2. The piezoelectric substrate 2 of the present embodiment is a piezoelectric substrate including only a piezoelectric layer. The piezoelectric substrate 2 includes lithium niobate. In this specification, the piezoelectric substrate 2 including lithium niobate means also a case where the piezoelectric substrate 2 contains a trace amount of impurities. The same applies to other structures and materials of the elastic wave device of the present application. More specifically, the piezoelectric substrate 2 includes 128° rotated Y-cut X-propagation LiNbO3. However, the cut angle and the material of the piezoelectric substrate 2 are not limited to the above. As the material of the piezoelectric substrate 2, for example, lithium tantalate, zinc oxide, aluminum nitride, quartz, or PZT (lead zirconate titanate) or the like can be used. Note that the piezoelectric substrate 2 can also be a laminated substrate including a piezoelectric layer.

[0034] An IDT electrode 3 is provided on the piezoelectric substrate 2. The IDT electrode 3 has a first bus bar 18a, a second bus bar 18b, a plurality of first electrode fingers 19a, and a plurality of second electrode fingers 19b. The first bus bar 18a and the second bus bar 18b are opposed to each other. The plurality of first electrode fingers 19a are connected to the first bus bar 18a at one end thereof, respectively. The plurality of second electrode fingers 19b are connected to the second bus bar 18b at one end thereof, respectively. The plurality of first electrode fingers 19a and the plurality of second electrode fingers 19b are alternately inserted with each other. Note that a wavelength defined by an electrode finger pitch of the IDT electrode 3 is set to λ. The electrode finger pitch refers to a center-to-center distance between adjacent electrode fingers. In a case where the IDT electrode 3 has a portion in which the center-to-center distance is different, an average value of the center-to-center distance can be set to the electrode finger pitch.

[0035] An elastic wave is excited by applying an alternating voltage to the IDT electrode 3. A pair of reflectors 9A and 9B are provided on both sides of the IDT electrode 3 on the piezoelectric substrate 2 in the direction in which the elastic wave propagates. Thus, the elastic wave device 1 of the present embodiment is a surface acoustic wave resonator. However, the elastic wave device of the present application can also be an elastic boundary wave resonator. In addition, the elastic wave device of the present application is not limited to an elastic wave resonator, and can also be a filter device or a multiplexer having an elastic wave resonator.

[0036] Figure 2 is a cross-sectional view of the electrode fingers along the I-I line in Figure 1 .

[0037] The IDT electrode 3 has a first layer 4, a second layer 5, and a third layer 6. More specifically, the first layer 4 is provided on the piezoelectric substrate 2. The second layer 5 is provided on the first layer 4. The third layer 6 is provided on the second layer 5.

[0038] The first layer 4 has a first main surface 4a and a second main surface 4b. The first main surface 4a and the second main surface 4b are opposed to each other. The first main surface 4a is located on the piezoelectric substrate 2 side. The second main surface 4b is in contact with the second layer 5. In the present embodiment, the first layer 4 is a single-layered adhesion layer. The first layer 4 includes NiCr. However, the material of the first layer 4 is not limited to the above. Alternatively, the first layer 4 can also be a laminate.

[0039] The second layer 5 has a third main surface 5a and a fourth main surface 5b, and a side surface 5c. The third main surface 5a and the fourth main surface 5b are opposed to each other. The third main surface 5a is in contact with the first layer 4. The fourth main surface 5b is in contact with the third layer 6. The side surface 5c is connected to the third main surface 5a and the fourth main surface 5b. The second layer 5 includes Cu as a main component. In the present specification, the main component refers to a component having an occupancy rate of more than 50%. However, the second layer 5 preferably includes Cu.

[0040] The third layer 6 includes Ti. Note that the material of the third layer 6 is not limited to the above. Alternatively, the third layer 6 can be a laminate.

[0041] The IDT electrode 3 further has a barrier layer 7. The barrier layer 7 is an outer layer in the present application. The barrier layer 7 is provided to the side surface 5c of the second layer 5. More specifically, the barrier layer 7 covers the side surface 5c. In the present embodiment, the thickness of the portion of the barrier layer 7 provided to the side surface 5c of the second layer 5 is thicker than the thickness of the first layer 4. However, the thickness of the first layer 4 can be thicker than the thickness of the above-mentioned portion of the barrier layer 7. Alternatively, the thickness of the first layer 4 and the thickness of the above-mentioned portion of the barrier layer 7 can be the same. As shown in FIG. 1, the boundary A between the side surface 5c of the second layer 5 and the barrier layer 7 is located on the second main surface 4b of the first layer 4. More specifically, the boundary A is located inward from the end portion of the second main surface 4b. Figure 2

[0042] In the present embodiment, the barrier layer 7 includes an oxide of Mn. However, the barrier layer 7 can include an oxide of a metal. The barrier layer 7 preferably includes an oxide of a metal selected from the group consisting of Mn, Al, Mg, and Sn.

[0043] A protective film 8 is provided on the piezoelectric substrate 2 so as to cover the IDT electrode 3. The protective film 8 includes silicon oxide. More specifically, the protective film 8 includes SiO2. However, the protective film 8 is not limited to the above, and can include silicon oxynitride or the like, for example.

[0044] The present embodiment is characterized in that the boundary A between the side surface 5c of the second layer 5 and the barrier layer 7 is located on the second main surface 4b of the first layer 4, and the barrier layer 7 does not reach the piezoelectric substrate 2. Thus, diffusion of Cu constituting the IDT electrode 3 to the piezoelectric substrate 2 can be more reliably suppressed. Further, diffusion of the above-mentioned Cu to the protective film 8 can be effectively suppressed. The effects of the present embodiment will be described in more detail below.

[0045] An elastic wave filter device including the elastic wave device of the first embodiment was prepared. Further, an elastic wave filter device including the elastic wave device of the first comparative example, and an elastic wave filter device including the elastic wave device of the second comparative example were prepared. Note that a plurality of each of these elastic wave filter devices was prepared. As shown in FIG. 6, the elastic wave filter device of the first embodiment includes a plurality of the elastic wave devices of the first embodiment. Figure 3 As shown in FIG. 6, the elastic wave device of the first comparative example differs from the first embodiment in that the IDT electrode does not have a barrier layer. As shown in FIG. 6, the elastic wave device of the second comparative example differs from the first embodiment in that the barrier layer is provided to the side surface of the first layer. Figure 4 ​As shown, the second comparative example also differs from the first embodiment in that the IDT electrode does not have a barrier layer. In addition, in the second comparative example, the width of the first layer 104 is wider than the width of the second layer 5. This is the same as the structure described in Patent Literature 2 (Japanese Patent Application Publication No. 2017-157944). Here, the width of the electrode finger refers to the dimension of the electrode finger along the direction in which the elastic wave propagates.

[0046] The design parameters of the elastic wave device of the first embodiment are described below.

[0047] Material of piezoelectric substrate 2: 128° rotated Y-cut X-propagation LiNbO3

[0048] First layer 4: Material... NiCr, Thickness... 6 nm

[0049] Second layer 5: Material... Cu, Thickness... 300 nm

[0050] Third layer 6: Material... Ti, Thickness... 8 nm

[0051] Barrier layer 7: Material... oxide of Mn, Thickness... 15 nm, Position... formed only on the side surface 5c of the second layer 5.

[0052] Protective film 8: Material... SiO2, Thickness... 1110 nm

[0053] Wavelength λ in IDT electrode 3: 4 μm

[0054] The design parameters of the elastic wave devices of the first comparative example and the second comparative example are described below.

[0055] Material of piezoelectric substrate: 128° rotated Y-cut X-propagation LiNbO3

[0056] First layer: Material... NiCr, Thickness... 6 nm

[0057] Second layer: Material... Cu, Thickness... 300 nm

[0058] Third layer: Material... Ti, Thickness... 8 nm

[0059] Wavelength λ in IDT electrode: 4 μm

[0060] In the elastic wave filter device including the elastic wave device 1 of the first embodiment, the elastic wave device including the first comparative example, and the elastic wave device including the second comparative example, a high-temperature load test was performed. In this test, the temperature was set to 125°C, and a direct current voltage of 3 V was applied between the first bus bar and the second bus bar of the IDT electrode. In this state, the filter electrical characteristics were measured at a prescribed time interval.

[0061] As a result of performing the high-temperature load test, in the first comparative example, deterioration of the insertion loss was observed in all the samples at the time of measurement after the test time of 200 hours. In the second comparative example, deterioration of the insertion loss was observed in some of the samples at the time of measurement after the test time of 500 hours. Deterioration of the insulation resistance value was also confirmed in the samples of the second comparative example other than these. At the time of electrode pointing of the samples in which the deterioration of the insulation resistance value was observed, diffusion between Cu in the second layer 5 and LiNbO3 in the piezoelectric substrate 2 was confirmed. That is, according to the configuration of Patent Literature 2, although diffusion between Cu in the second layer 5 and LiNbO3 can be suppressed to some extent, it is not possible to obtain a level of strength that can satisfy market requirements.

[0062] On the other hand, in the elastic wave filter device including the elastic wave device 1 of the first embodiment, deterioration of the insertion loss and the insulation resistance value was not observed even at a test time of 1000 hours.

[0063] Thus, it was found that, by forming the barrier layer 7 on the side surface 5c of the second layer 5 and arranging the boundary A of the second layer 5 and the barrier layer 7 on the second main surface 4b of the first layer 4, it is possible to improve reliability. This is an effect that is produced by further cutting off the path of diffusion of Cu by the barrier layer 7 on the basis of the distance between the second layer 5 and the piezoelectric substrate 2 being separated.

[0064] In the elastic wave device 1, the first layer 4 functions as a close-contact layer. Here, the thickness of the first layer 4 is thin at 6 nm. Due to this, the insertion loss of the elastic wave filter device is ensured to be low. Further, the barrier layer 7 formed on the side surface 5c of the second layer 5 suppresses migration of Cu in the second layer 5. Thus, it is possible to improve the power resistance.

[0065] The thickness of the first layer 4 is desirably in a range of 2 nm or more and 10 nm or less. When the thickness of the first layer 4 is thinner than 2 nm, it can be difficult to suppress diffusion between Cu in the second layer 5 and the piezoelectric substrate 2, or deterioration of the power resistance can occur. On the other hand, in the case where the thickness of the first layer 4 is thicker than 10 nm, the insertion loss of the device can become large.

[0066] In the present embodiment, the first layer 4 is formed of NiCr. However, as the material of the first layer 4, for example, Ti, Ni, or Cr, or the like can also be used. The first layer 4 is desirably formed of an electric conductor including a metal. Due to this, it is difficult for the electromechanical coupling coefficient to decrease, and it is difficult for the characteristics of the device to deteriorate.

[0067] The thickness of the barrier layer 7 is desirably in a range of 10 nm or more and 20 nm or less. In a case where the thickness of the barrier layer 7 is thinner than 10 nm, the barrier property can be insufficient, and it can be difficult to suppress the diffusion of Cu. On the other hand, in a case where the thickness of the barrier layer 7 is thicker than 20 nm, the proportion of the barrier layer 7 with respect to the cross-sectional area of the electrode finger increases, and the proportion of the second layer 5 decreases, whereby the resistance of the electrode finger increases.

[0068] As described above, by making the thickness of the barrier layer 7 thicker than the thickness of the first layer 4, it is possible to realize a device that is excellent in reliability, has resistance to electric power, and has small insertion loss, by preventing diffusion.

[0069] As in the present embodiment, it is preferable that the third layer 6 be provided on the second layer 5. Thereby, it is possible to effectively suppress the diffusion of Cu in the second layer 5 to the protective film 8. Further, it is also possible to suppress the oxidation of Cu in the second layer 5.

[0070] Here, an example of a manufacturing method of the elastic wave device 1 in the first embodiment will be described. First, for example, by a lift-off method or the like, the electrode pattern for the first layer 4, the second layer 5, and the third layer 6 is formed on the piezoelectric substrate 2. The electrode pattern for the second layer 5 is formed of a metal obtained by adding 0.1 at% to 20 at% of Mn to Cu. Note that the electrode pattern for the first layer 4, the second layer 5, and the third layer 6 can also be patterned by etching each metal layer.

[0071] Next, heating treatment is performed for about 1 hour at 200°C to 400°C. At this time, the Mn in the electrode pattern combines with oxygen in the atmosphere to form an oxide film on the side surface of the electrode pattern. On the other hand, the side surface of the electrode pattern is covered with the first layer 4 and the third layer 6. Therefore, oxidation of Mn does not occur. Along with the oxidation of Mn, the Mn in the electrode pattern further moves to the side surface of the electrode pattern. Then, oxidation of Mn occurs. Thereby, the second layer 5 as a Cu layer is formed from the electrode pattern including a mixture of Cu and Mn. Further, the barrier layer 7 including an oxide of Mn is formed on the side surface 5c of the second layer 5. The barrier layer 7 is not directly formed in the electrode finger nonformation region between the electrode fingers on the piezoelectric substrate 2 and the like. Note that the reflector 9A and the reflector 9B are also formed simultaneously with the IDT electrode 3. After that, the protective film 8 is formed on the piezoelectric substrate 2 to cover the IDT electrode 3 by a sputtering method or the like.

[0072] Before the above-mentioned heat treatment, that is, in a state where a small amount of Mn is mixed in the Cu layer that is the electrode pattern, not only the resistivity of the Cu layer is high, but also the barrier property against diffusion of Cu is not present. By the heat treatment, Mn moves to the outside to form an oxide film, the barrier property is obtained, the Mn concentration of the Cu portion in the center portion decreases to approach pure Cu, and thus the resistivity decreases, and the effect of the present application is obtained.

[0073] The second layer 5 can also contain Mn in addition to Cu as the main component. The concentration of Mn in the second layer 5 is preferably 0.02 atomic% or less. In this case, the increase in the resistivity of pure Cu can be made small. However, as described above, the second layer 5 preferably contains Cu that does not contain impurities. In this case, the resistivity can be further reduced.

[0074] As the metal element that forms a stable oxide film exhibiting the same action as Mn, for example, Al, Mg, Sn, and the like are present. However, among the above-mentioned metal elements, Mn is the metal element that can minimize the resistance of the electrode finger when added to Cu in order to form the barrier layer 7. On the other hand, when Ag is added in addition to the above-mentioned metal, the withstand voltage of the IDT electrode 3 can be improved.

[0075] Note that, in a case where the protective film 8 is an oxide such as SiO2, the above-mentioned heat treatment can also be performed after the formation of the electrode pattern, instead of after the formation of the protective film 8. In this case, Mn combines with oxygen in the protective film 8, instead of oxygen in the atmosphere, to form an oxide film.

[0076] However, the protective film 8 can also not necessarily be formed. In a case where the protective film 8 is not formed, the above-mentioned heat treatment can also be performed after the formation of the electrode pattern. Figure 5 In the first modification of the first embodiment shown in FIG. 6, the protective film 8 is not formed on the piezoelectric substrate 2. In this case, as with the first embodiment, diffusion between Cu of the second layer 5 and the piezoelectric substrate 2 can be more reliably suppressed.

[0077] In the first embodiment, the boundary A of the second layer 5 and the barrier layer 7 is located at a position inside the end portion in the second main surface 4b of the first layer 4. However, the boundary A can be located on the second main surface 4b. In this case, as with the first embodiment, diffusion between Cu of the second layer 5 and the piezoelectric substrate 2 can be more reliably suppressed. Figure 6 In the second modification of the first embodiment shown in FIG. 7, the boundary A is located at the end portion of the second main surface 4b in the first layer 4A. In this case, the distance between the second layer 5 and the piezoelectric substrate 2 is further separated, and the path of diffusion of Cu is cut off by the barrier layer 7. Note that, since the barrier layer 7 does not reach the piezoelectric substrate 2, the path of Cu of the second layer 5 that reaches the piezoelectric substrate 2 through the barrier layer 7 is not present. Thus, as with the first embodiment, diffusion between Cu of the second layer 5 and the piezoelectric substrate 2 can be more reliably suppressed.

[0078] However, preferably, as in the first embodiment, the boundary A of the second layer 5 and the barrier layer 7 is located at a position inward from the end portion in the second main surface 4b of the first layer 4. Thereby, the path of diffusion of Cu between the second layer 5 and the piezoelectric substrate 2 can be further reliably cut off.

[0079] As described above, the first layer 4 can also be a laminate. In this case, the first layer 4 can include a plurality of layers. The first layer 4 can include a first metal layer 12 and a second metal layer 13. The first metal layer 12 is a tight contact layer including NiCr. The second metal layer 13 includes Ag. However, the number of layers of the first layer 4 and the material of each layer are not limited to the above. Figure 7

[0080] Figure 8 is a lateral cross-sectional view of an electrode finger in the second embodiment. Note that, in the present specification, the cross section of an electrode finger is a cross section in a direction orthogonal to the direction in which the electrode finger extends.

[0081] The present embodiment differs from the first embodiment in that the thickness of the first layer 24 is thicker than the thickness of the portion of the barrier layer 7 provided to the side surface 5c of the second layer 5. The elastic wave device of the present embodiment has the same structure as the elastic wave device 1 of the first embodiment except for the above aspect.

[0082] In the present embodiment as well as in the first embodiment, diffusion between Cu of the second layer 5 and the piezoelectric substrate 2 can be more reliably suppressed. Note that, hereinafter, the effects of the present embodiment will be described in more detail.

[0083] An elastic wave filter device including the elastic wave device of the second embodiment was prepared. Further, an elastic wave filter device including the elastic wave device of the third comparative example, and an elastic wave filter device including the elastic wave device of the fourth comparative example were prepared. Note that, a plurality of each of these elastic wave filter devices was prepared. The elastic wave device of the third comparative example differs from the second embodiment in that the IDT electrode does not have a barrier layer. The elastic wave device of the fourth comparative example also differs from the second embodiment in that the IDT electrode does not have a barrier layer. Further, in the fourth comparative example, the width of the first layer is wider than the width of the second layer, as in the second comparative example. Figure 4 As in the second comparative example, the width of the first layer is wider than the width of the second layer.

[0084] The design parameters of the elastic wave device of the second embodiment are as described below.

[0085] Material of the piezoelectric substrate 2: 128°-rotated Y-cut X-propagation LiNbO3

[0086] First layer 24: Material... NiCr, Thickness... 60 nm

[0087] ​Second layer 5: Material... Cu, Thickness... 300 nm

[0088] Third layer 6: Material... Ti, Thickness... 8 nm

[0089] Barrier layer 7: Material... Oxide of Mn, Thickness... 5 nm, Position... formed only on the side surface 5c of the second layer 5.

[0090] Protective film 8: Material... SiO2, Thickness... 1110 nm

[0091] Wavelength λ in the IDT electrode 23: 4 μm

[0092] Design parameters of the elastic wave device of the third and fourth comparative examples are described below.

[0093] Material of the piezoelectric substrate: 128° rotated Y-cut X-propagation LiNbO3

[0094] First layer: Material... NiCr, Thickness... 60 nm

[0095] Second layer: Material... Cu, Thickness... 300 nm

[0096] Third layer: Material... Ti, Thickness... 8 nm

[0097] Wavelength λ in the IDT electrode: 4 μm

[0098] In the elastic wave filter device including the elastic wave device of the second embodiment, the elastic wave filter device including the elastic wave device of the third comparative example, and the elastic wave filter device including the elastic wave device of the fourth comparative example, the same high-temperature load test as described above was performed. As a result of the high-temperature load test, in the third and fourth comparative examples, at the time of measurement after the test time of 500 hours, deterioration of the insertion loss was observed in a part of the samples, respectively. In the samples other than the samples of the third and fourth comparative examples in which the deterioration of the insertion loss was observed, deterioration of the insulation resistance value was also confirmed. At the time of observation of the electrode fingers of the samples in which the deterioration of the insulation resistance value was confirmed, diffusion between Cu in the second layer and LiNbO3 in the piezoelectric substrate was confirmed. From the result of the fourth comparative example, it was found that, by adopting the configuration of Patent Literature 2 and thickening the first layer, diffusion between Cu and LiNbO3 could be suppressed to some extent, but a level of performance capable of satisfying market requirements could not be obtained.

[0099] On the other hand, in the elastic wave filter device including the elastic wave device of the second embodiment, deterioration of the insertion loss and the insulation resistance value was not observed even at the test time of 1000 hours.

[0100] Therefore, it was confirmed that by increasing the distance between the second layer and the piezoelectric substrate by thickening the first layer, although some degree of effect related to improvement in reliability was obtained, the effect was still insufficient. In addition, it was found that, as in the second embodiment shown in Figure 8 by forming the barrier layer 7 on the side surface 5c of the second layer 5 and disposing the boundary A of the second layer 5 and the barrier layer 7 on the second main surface 4b of the first layer 24, it is possible to improve the reliability. This is an effect that occurs by further cutting off the diffusion path by the barrier layer 7 on the basis of the distance between the second layer 5 and the piezoelectric substrate 2 being further separated.

[0101] The thickness of the first layer 24 is preferably 20 nm or more. Thereby, it is possible to effectively suppress the diffusion of Cu in the second layer 5. On the other hand, in the thickness of the first layer 24, the upper limit is not particularly limited, but the thickness of the first layer 24 is preferably 30% or less of the wavelength λ. When the thickness of the IDT electrode 23 is too thick, it can be difficult to form the electrode pattern.

[0102] As described above, by making the thickness of the first layer 24 thicker than the thickness of the barrier layer 7, it is possible to effectively suppress the diffusion between the second layer 5 and the piezoelectric substrate 2, and therefore, it is possible to realize an elastic wave device that is excellent in reliability, has low resistance of the electrode finger, and is low in loss.

[0103] As described above, by making the thickness of the first layer 24 thicker than the thickness of the barrier layer 7, it is possible to effectively suppress the diffusion between the second layer 5 and the piezoelectric substrate 2, and therefore, it is possible to realize an elastic wave device that is excellent in reliability, has low resistance of the electrode finger, and is low in loss. Figure 7 As described above, by making the thickness of the first layer 24 thicker than the thickness of the barrier layer 7, it is possible to effectively suppress the diffusion between the second layer 5 and the piezoelectric substrate 2, and therefore, it is possible to realize an elastic wave device that is excellent in reliability, has low resistance of the electrode finger, and is low in loss. Figure 9 In the following embodiments from the third embodiment, the thickness of the first layer 4 can also be thicker than the portion of the barrier layer 37 provided to the side surface 5c of the second layer 5.

[0104]

[0105] is a lateral cross-sectional view of the electrode finger in the third embodiment. Figure 10 The first embodiment of the present embodiment differs from the first embodiment in that, in the IDT electrode 33, the barrier layer 37 is provided on both the side surface 5c of the second layer 5 and the fourth main surface 5b, and the third layer 6 is not provided. Other than the above, the elastic wave device of the present embodiment has the same structure as the elastic wave device 1 of the first embodiment.

[0106]

[0107] ​By covering the side surface 5c and the fourth main surface 5b of the second layer 5 with the barrier layer 37, diffusion of Cu of the second layer 5 to the protective film 8 can be suppressed. Also, as in the first embodiment, diffusion between Cu of the second layer 5 and the piezoelectric substrate 2 can be more reliably suppressed.

[0108] In forming the IDT electrode 33 of the present embodiment, for example, the third layer 6 is not provided, and heating treatment is performed as in the case of forming the IDT electrode 3 of the first embodiment. At this time, Mn in the electrode pattern for the second layer 5 binds with oxygen in the atmosphere to form an oxide film on the side surface and the upper surface of the electrode pattern. Note that the upper surface refers to the surface on the upper side in Figure 10 With the oxidation of Mn, Mn in the electrode pattern further moves to the side surface and the upper surface of the electrode pattern. Then, oxidation of Mn is performed. Thus, the second layer 5 as a Cu layer is formed from the electrode pattern including a mixture of Cu and Mn. Also, the barrier layer 37 including an oxide of Mn is formed on the side surface 5c and the fourth main surface 5b of the second layer 5.

[0109] As shown in Figure 10 , the barrier layer 37 has a connecting portion 37a. More specifically, the connecting portion 37a is a portion of the barrier layer 37 in which the portion provided on the side surface 5c of the second layer 5 and the portion provided on the fourth main surface 5b are connected. In the present embodiment, the shape of the connecting portion 37a in the cross section of the IDT electrode 33 is a shape in which a straight line and a straight line are connected. The connecting portion 37a is configured as a corner portion. Note that the connecting portion 37a is configured as a corner portion at both the boundary of the barrier layer 37 and the protective film 8 and the boundary of the barrier layer 37 and the second layer 5. However, the shape of the connecting portion 37a is not limited thereto. In a modification example of the third embodiment shown in Figure 11 , the connecting portion 37x of the barrier layer 37A has a curved surface shape. More specifically, the connecting portion 37x has a curved surface shape at both the boundary of the barrier layer 37A and the protective film 8 and the boundary of the barrier layer 37A and the second layer 5.

[0110] Figure 12 is a lateral cross-sectional view of an electrode finger in the fourth embodiment.

[0111] The present embodiment differs from the first embodiment in that a dielectric film 43 is provided between the piezoelectric substrate 2 and the IDT electrode 3. In aspects other than the above, the elastic wave device of the present embodiment has the same structure as the elastic wave device 1 of the first embodiment.

[0112] By providing the dielectric film 43, the electromechanical coupling coefficient can be adjusted to an appropriate value, and diffusion between the Cu of the second layer 5 and the piezoelectric substrate 2 can be effectively suppressed. The thickness of the dielectric film 43 is preferably 1% or less of the wavelength λ. Thus, the electromechanical coupling coefficient can be suppressed from becoming too small. Therefore, it is difficult for the insertion loss to become large.

[0113] Figure 13 is a front sectional view of an elastic wave device of a fifth embodiment.

[0114] The present embodiment differs from the first embodiment in that the piezoelectric substrate 52 is a laminated substrate of the support substrate 53 and the piezoelectric layer 54. The piezoelectric layer 54 is provided on the support substrate 53. The IDT electrode 3 is provided on the piezoelectric layer 54. The elastic wave device of the present embodiment has the same structure as the elastic wave device 1 of the first embodiment except for the above.

[0115] The piezoelectric layer 54 includes lithium niobate. However, the material of the piezoelectric layer 54 is not limited to the above, and for example, lithium tantalate, zinc oxide, aluminum nitride, quartz, PZT, or the like can also be used.

[0116] The support substrate 53 includes silicon. However, the material of the support substrate 53 is not limited to the above, and for example, a piezoelectric body of aluminum oxide, lithium tantalate, lithium niobate, quartz, or the like, various ceramics of alumina, sapphire, magnesium oxide, silicon nitride, aluminum nitride, silicon carbide, zirconia, cordierite, mullite, block talc, forsterite, or the like, a dielectric of diamond, glass, or the like, a semiconductor of gallium nitride, or the like, or a resin, or the like can also be used.

[0117] In the present embodiment, the IDT electrode 3 is also configured as in the first embodiment. Therefore, diffusion of Cu constituting the IDT electrode 3 to the piezoelectric substrate 52 and the protective film 8 can be more reliably suppressed.

[0118] The thickness of the piezoelectric layer 54 is preferably 1λ or less. In this case, the excitation efficiency of the elastic wave can be improved.

[0119] Figure 14 is a front sectional view of an elastic wave device of a sixth embodiment.

[0120] The present embodiment differs from the fifth embodiment in that the support substrate 63 has a hollow portion 63a and a support portion 63b. The elastic wave device of the sixth embodiment has the same structure as the elastic wave device of the fifth embodiment except for the above.

[0121] The hollow portion 63a of the support substrate 63 is surrounded by the support portion 63b and is open to the piezoelectric layer 54 side. The support substrate 63 supports the piezoelectric layer 54 at the support portion 63b. In this case, the excitation efficiency of the elastic wave can be effectively improved.

[0122] In the present embodiment, the IDT electrode 3 is configured similarly to the fifth embodiment. Thus, diffusion of Cu constituting the IDT electrode 3 to the piezoelectric substrate 62 and the protective film 8 can be more reliably suppressed.

[0123] Figure 15 is a front sectional view of an elastic wave device of a seventh embodiment.

[0124] The present embodiment differs from the first embodiment in that the elastic wave device 71 utilizes bulk waves of a thickness shear first mode. More specifically, in the present embodiment, in a case where the thickness of the piezoelectric layer 54 is set to d and the electrode finger pitch of the IDT electrode 3 is set to p, d / p is 0.5 or less. By adopting such a structure, bulk waves of the thickness shear first mode can be used as a main mode. Note that the elastic wave device 71 does not have a reflector. The elastic wave device 71 of the present embodiment has the same structure as the elastic wave device of the sixth embodiment except for the above.

[0125] In a case of utilizing bulk waves of a thickness shear first mode, even if the number of electrode fingers in the reflector is reduced, the propagation loss is less. Thus, the miniaturization of the elastic wave device 71 can be effectively promoted. Further, since the IDT electrode 3 is configured similarly to the sixth embodiment, diffusion of Cu constituting the IDT electrode 3 to the piezoelectric substrate 62 and the protective film 8 can be more reliably suppressed.

[0126] Explanation of Reference Numerals

[0127] 1... elastic wave device;

[0128] 2... piezoelectric substrate;

[0129] 3... IDT electrode;

[0130] 4, 4A, 4B... first layer;

[0131] 4a, 4b... first main surface, second main surface;

[0132] 5... second layer;

[0133] 5a, 5b... third main surface, fourth main surface;

[0134] 5c... side surface;

[0135] 6... third layer;

[0136] 7... barrier layer;

[0137] 8... protective film;

[0138] 9A, 9B... reflector;

[0139] 12, 13... first and second metal layers;

[0140] 18a, 18b... first and second bus bars;

[0141] 19a, 19b... first and second electrode fingers;

[0142] 23... IDT electrode;

[0143] 24, 24A... first layer;

[0144] 25 to 27... first to third metal layers;

[0145] 33... IDT electrode;

[0146] 37, 37A... barrier layer;

[0147] 37a, 37x... connecting portions;

[0148] 43... dielectric film;

[0149] 52... piezoelectric substrate;

[0150] 53... support substrate;

[0151] 54... piezoelectric layer;

[0152] 62... piezoelectric substrate;

[0153] 63... support substrate;

[0154] 63a... hollow portion;

[0155] 63b... support portion;

[0156] 71... elastic wave device;

[0157] 104... first layer;

[0158] A... boundary.

Claims

1. An elastic wave device, comprising: piezoelectric substrates; and An IDT electrode has a first layer disposed on the piezoelectric substrate and a second layer disposed on the first layer and mainly composed of Cu, and has multiple electrode fingers. The first layer includes a first main surface located on the piezoelectric substrate side and a second main surface in contact with the second layer. The second layer includes a third main surface that contacts the first layer, a fourth main surface that faces the third main surface, and a side surface that connects to the third main surface and the fourth main surface. The IDT electrode also has an outer layer disposed on the side of the second layer. The side surface of the second layer and the boundary of the outer layer are located on the second main surface of the first layer, and the outer layer does not reach the piezoelectric substrate. The outer layer comprises an oxide of a metal selected from the group consisting of Mn, Al, Mg and Sn.

2. The elastic wave device according to claim 1, wherein, The side surface of the second layer and the boundary of the outer layer are located inside the end of the second main surface of the first layer.

3. The elastic wave device according to claim 1 or 2, wherein, The elastic wave device also includes a protective film that is configured on the piezoelectric substrate to cover the IDT electrode.

4. The elastic wave device according to claim 1 or 2, wherein, The IDT electrode also has a third layer disposed on the fourth main surface of the second layer.

5. The elastic wave device according to claim 1 or 2, wherein, The outer layer is disposed on the side surface and the fourth main surface of the second layer.

6. The elastic wave device according to claim 1 or 2, wherein, The thickness of the portion of the outer layer disposed on the side of the second layer is greater than the thickness of the first layer.

7. The elastic wave device according to claim 1 or 2, wherein, The thickness of the first layer is greater than the thickness of the portion of the outer layer disposed on the side of the second layer.

8. The elastic wave device according to claim 1, wherein, The outer layer comprises an oxide of Mn.

9. The elastic wave device according to claim 1 or 2, wherein, The piezoelectric substrate has a support substrate and a piezoelectric layer disposed on the support substrate.

10. The elastic wave device according to claim 1 or 2, wherein, The elastic wave device further includes a dielectric film disposed between the piezoelectric substrate and the IDT electrode. When the wavelength defined by the electrode finger spacing of the IDT electrode is set as λ, the thickness of the dielectric film is less than 1% of the wavelength λ.

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