WO2024117061A1

WO2024117061A1 - Acoustic wave device and communication device

Info

Publication number: WO2024117061A1
Application number: PCT/JP2023/042298
Authority: WO
Inventors: 俊哉木村; 優平井; 達也堂本
Original assignee: 京セラ株式会社
Priority date: 2022-11-28
Filing date: 2023-11-27
Publication date: 2024-06-06

Abstract

This acoustic wave device comprises a piezoelectric substrate and an IDT electrode disposed on the piezoelectric substrate, the IDT electrode having a first layer including a conductive material and a second layer including a CuAl alloy and disposed between the first layer and the piezoelectric substrate, wherein, when the atomic ratio of Cu : Al in the second layer is defined as 1 : x, the average value of x is greater than 2.1 at least on a first surface side, which is on the first layer side of the second layer.

Description

Acoustic wave devices and communication devices

This disclosure relates to an elastic wave device and a communication device.

In recent years, acoustic wave devices have been used in communication devices such as mobile terminals as duplexers that filter signals transmitted and received from antennas. An acoustic wave device is composed of a piezoelectric substrate and an IDT (Interdigital Transducer) electrode formed on the piezoelectric substrate. An acoustic wave device utilizes the property that allows conversion between electrical signals and surface acoustic waves in the relationship between the IDT electrode and the piezoelectric substrate.

In such a duplexer, it is required to improve the power durability. In order to improve the power durability of the duplexer, it is effective to improve the power durability of the acoustic wave device constituting the duplexer. For example, Patent Document 1 discloses an acoustic wave device in which an electrode is formed on a piezoelectric substrate, the electrode having a first layer containing Al and Cu, and a second layer having Al crystals and at least a part of _CuAl2 crystal grains.

International Publication No. 2021/85465 Brochure

An elastic wave device according to one aspect of the present disclosure includes a piezoelectric substrate and an IDT electrode located on the piezoelectric substrate, the IDT electrode having a first layer including a conductive material and a second layer including a CuAl alloy located between the first layer and the piezoelectric substrate, the second layer having a first surface located on the first layer side, and when the atomic ratio of Cu:Al in the second layer is defined as 1:x, the average value of x is greater than 2.1 at least on the first surface side of the second layer.

1 is a plan view illustrating a configuration of an elastic wave device according to an embodiment of the present invention. 2 is an enlarged cross-sectional view of a main portion of the elastic wave device of FIG. 1 taken along line II-II. 3A to 3C are cross-sectional views each showing a cross section of an IDT electrode in a lamination direction during a manufacturing process of the acoustic wave device of FIG. 1 . 2A to 2C are plan views each showing a schematic state of an IDT electrode before and after heating in the acoustic wave device of FIG. 1 . 10 is a graph showing the correlation between the minimum insertion loss and the number of heating cycles for a Tx filter and an Rx filter formed using the acoustic wave device of FIG. 1 . 1 is a block diagram showing a main part of a communication device according to an embodiment; 11A to 11C are diagrams illustrating results of a power resistance test for an elastic wave device according to an example or a comparative example. 13 is a diagram illustrating the relationship between the area ratio of spots and the Al ratio in a second layer in an elastic wave device according to an example or a comparative example. FIG. 1 is a cross-sectional view illustrating a cross section of an elastic wave device according to an embodiment of the present invention taken in a lamination direction using a TEM. 10A to 10C are diagrams showing the results of EDX analysis at the measurement points shown in FIG. 9 of an elastic wave device according to an example. 1 is a cross-sectional view illustrating a cross section of an elastic wave device according to an embodiment of the present invention taken in a lamination direction using a TEM. 12A and 12B are diagrams showing the results of EDX analysis at measurement points P3 and P4 shown in FIG. 11 of an elastic wave device according to an example. 12A and 12B are diagrams showing the results of EDX analysis at measurement points P5 and P6 shown in FIG. 11 of an elastic wave device according to an example.

An elastic wave device according to one embodiment of the present disclosure will be described with reference to the drawings. The figures used in the following description are schematic, and the dimensional ratios and the like in the drawings do not necessarily correspond to the actual ones. In addition, in this specification, "A to B" indicates greater than or equal to A and less than or equal to B.

In an elastic wave device, either direction may be considered to be up or down. In the following description, for convenience, a Cartesian coordinate system xyz is defined, with the x direction being the left-right direction, the y direction being the front-back direction, and the z direction being the up-down direction. In the elastic wave device 1 shown in FIG. 1, the front side as viewed from the page is considered to be up, the left side is considered to be leftward, and the bottom side is considered to be forward, but this is not limited to the above.

<Configuration of the Acoustic Wave Device>
Fig. 1 is a plan view showing a configuration of an acoustic wave (SAW: Surface Acoustic Wave) device 1 according to an embodiment of the present disclosure. Fig. 2 is an enlarged cross-sectional view of a main portion taken along line II-II in Fig. 1. As shown in Fig. 1, the acoustic wave device 1 has a piezoelectric substrate 2 and an IDT (Interdigital Transducer) electrode 3. The acoustic wave device 1 may also have a reflector 4, a protective layer 5, and a resin layer 6.

The piezoelectric substrate 2 is composed of a single crystal substrate having piezoelectric properties. The single crystal substrate having piezoelectric properties may be, for example, a substrate made of lithium niobate crystal or lithium tantalate crystal. The planar shape and various dimensions of the piezoelectric substrate 2 may be set appropriately. For example, the length in the z direction indicating the thickness of the piezoelectric substrate 2 may be 0.2 mm or more and 0.5 mm or less. The piezoelectric substrate 2 may have an upper surface 2a, which is the upper surface.

The IDT electrode 3 is located on the piezoelectric substrate 2. The top surface 2a of the piezoelectric substrate 2 may be the surface on which the IDT electrode 3 is located. The IDT electrode 3 may have two comb-tooth electrodes 30. The comb-tooth electrode 30 may have two bus bars 31 facing each other and a number of electrode fingers 32 extending from one bus bar 31 toward the opposing bus bar 31. The pair of comb-tooth electrodes 30 are positioned so that the electrode fingers 32 connected to one bus bar 31 and the electrode fingers 32 connected to the opposing bus bar 31 interdigitate with each other in the propagation direction of the elastic wave, in other words, so that the electrode fingers 32 cross each other. The comb-tooth electrode 30 may also have dummy electrode fingers 33 facing each of the electrode fingers 32.

The busbar 31 may have a long, linear shape. The electrode fingers 32 may have a long, linear shape in a direction intersecting the busbar 31, and may be aligned at approximately regular intervals in the direction of propagation of the elastic wave.

The multiple electrode fingers 32 may be positioned side by side so that the pitch Pt, which is the distance between the centers of adjacent electrode fingers 32 in the left-right direction, is roughly uniform. In other words, the pitch Pt indicates the distance from the center of one electrode finger 32 to the center of the electrode finger 32 adjacent to that electrode finger 32 in the propagation direction of the elastic wave. The pitch Pt may be, for example, a length equivalent to half the wavelength λ of the elastic wave at the frequency at which resonance is desired. The wavelength λ may be, for example, 1.5 to 6 μm. Since the IDT electrode 3 is positioned side by side so that the pitch Pt between most of the electrode fingers 32 is uniform, the multiple electrode fingers 32 are arranged at a constant period, and elastic waves can be generated efficiently.

By positioning the electrode fingers 32 in this manner, the IDT electrode 3 generates an elastic wave that propagates in a direction perpendicular to the multiple electrode fingers 32. Therefore, taking into consideration the crystal orientation of the piezoelectric substrate 2, the two bus bars 31 may be positioned to face each other in a direction intersecting the direction in which the elastic wave is desired to propagate. The multiple electrode fingers 32 may be positioned to extend in a direction perpendicular to the direction in which the elastic wave is desired to propagate. The propagation direction of the elastic wave is determined by the orientation of the multiple electrode fingers 32, but in this embodiment, for convenience, the orientation of the multiple electrode fingers 32 may be described based on the propagation direction of the elastic wave.

The number of electrode fingers 32 on each side may be 50 to 350. The lengths of the electrode fingers 32 in the extension direction may be set to be, for example, approximately the same. The interdigitation width, which is the interdigitation length between opposing electrode fingers 32, may be, for example, 0 to 300 μm.

When a voltage is applied, the IDT electrode 3 excites an elastic wave that propagates in the x-direction near the top surface 2a of the piezoelectric substrate 2. The excited elastic wave is reflected at the boundary between the elongated region between adjacent electrode fingers 32, which is an area where no electrode fingers 32 are arranged. A standing wave is then formed with the pitch Pt of the electrode fingers 32 being half the wavelength. The standing wave is converted into an electrical signal of the same frequency as the standing wave and is extracted by the electrode fingers 32. In this way, the elastic wave device 1 may function as a one-port resonator.

The reflectors 4 may be positioned so as to sandwich the IDT electrode 3 in the propagation direction of the elastic wave. The reflectors 4 may be formed in a roughly slit shape.

As shown in FIG. 2, the protective layer 5 may be located on the piezoelectric substrate 2 so as to cover the IDT electrode 3 and the reflector 4 from above. Specifically, the protective layer 5 may cover the surfaces of the IDT electrode 3 and the reflector 4, and may also cover the exposed portion of the upper surface 2a of the piezoelectric substrate 2 other than the IDT electrode 3 and the reflector 4. The thickness of the protective layer 5 may be, for example, 1 nm or more, and may be 20% or less of the thickness of the IDT electrode 3.

Protective layer 5 may be made of an insulating material and may be a layer that contributes to protection against corrosion, etc. Protective layer 5 may be a layer containing a material such as _SiO2 , which increases the propagation speed of elastic waves as the temperature increases. In this case, changes in electrical characteristics of elastic wave device 1 due to changes in temperature can be kept small. Protective layer 5 may also contain a material such as SiNx to improve the moisture resistance of elastic wave device 1.

The resin layer 6 is located on the piezoelectric substrate 2 and may be a layer that covers at least a portion of the surface of the piezoelectric substrate 2. Specifically, the resin layer 6 may cover the surfaces of the IDT electrode 3 and the reflector 4, or may cover the exposed portion of the upper surface 2a of the piezoelectric substrate 2 other than the IDT electrode 3 and the reflector 4. From the viewpoint of the efficiency of generating an elastic wave by the IDT electrode 3, the resin layer 6 may cover only the exposed portion of the upper surface 2a of the piezoelectric substrate 2.

The resin layer 6 may be a layer containing a thermosetting resin. The thermosetting resin that is the material of the resin layer 6 may be a thermosetting resin that hardens at 300°C or higher. An example of such a thermosetting resin is polyimide. The thermosetting resin that is the material of the resin layer 6 may be acrylic. The resin layer 6 may contain at least one of polyimide and acrylic. The presence of the resin layer 6 allows the wiring to cross over in a multi-level manner.

The elastic wave device 1 having such a resin layer 6 is subjected to a heat treatment in the manufacturing process to harden the resin layer 6. Such a heat treatment may cause the solid solution state of Cu contained in the IDT electrode 3 to change, and may also affect the power resistance of the IDT electrode 3. The elastic wave device 1 according to this embodiment exhibits stable power resistance even when subjected to a heat treatment of, for example, 300°C or higher to harden the resin layer 6. The specifics are described below.

<Configuration of IDT electrode 3>
(overview)
Conventional acoustic wave devices generally have an IDT electrode made of Al, taking into consideration the excitation efficiency of acoustic waves, radiation loss, electrical resistance, etc. However, in recent years, the power of high-frequency signals input to acoustic wave devices has been increasing, and so there is a demand for electrodes with better power durability than electrodes made of Al.

However, simply replacing the electrode material with one with high strength such as Mo would not have been sufficient to replace the Al electrodes, as there is a trade-off between the excitation efficiency of the elastic waves, radiation loss, and electrical resistance.

Therefore, in the elastic wave device 1 according to this embodiment, Cu was selected as a material that could improve power resistance and potentially exhibit characteristics equivalent to those of current Al electrodes. In addition, a configuration was found in which the IDT electrode 3 containing Al and Cu can stably maintain its power resistance even when subjected to heat treatment at, for example, 300°C or higher.

As shown in FIG. 2, the IDT electrode 3 has a first layer 34 and a second layer 35. FIG. 2 illustrates an example of the layered structure of the electrode fingers 32, but the busbar 31, electrode fingers 32, and dummy electrode fingers 33 of the IDT electrode 3 may each have such a layered structure.

(First layer)
The first layer 34 is a layer containing a conductive material. The first layer 34 may contain Al as a conductive material. When the first layer 34 contains a component other than Al, the first layer 34 may contain the component other than Al such that the content of the component other than Al is 10% or less in terms of the atomic ratio to the atomic number of Al. Al is a material having high electrical conductivity. When the first layer 34 contains such an amount of Al, the IDT electrode 3 can exhibit good electrical characteristics.

Examples of components other than Al contained in the first layer 34 include Cu and Mg. If the first layer 34 contains such components other than Al, it is possible to reduce the diffusion of components such as Cu from the second layer 34 due to heating of the IDT electrode 3 during the manufacturing process, etc.

(Second layer)
The second layer 35 is located between the first layer 34 and the piezoelectric substrate 2. The second layer 35 contains a CuAl alloy. The CuAl alloy may be, for example, _CuAl2 . The acoustic wave device 1 has good power durability because it includes the second layer 35 containing a CuAl alloy having high strength. The second layer 35 may also contain impurities to the extent that they segregate to grain boundaries or are solid-soluble in _CuAl2 crystals. The concentration of such impurities in the second layer 35 may be less than 5% or less than 2% in terms of atomic ratio.

The second layer 35 has a first surface 35a located on the first layer side. The first surface 35a may be a surface that contacts the intermediate layer 36. Also, if the IDT electrode 3 does not have an intermediate layer 36, the first surface 35a may be a surface that contacts the first layer 34.

In order to ensure stable and good power durability for the IDT electrode 3, it was discovered that, when the atomic ratio of Cu:Al in the second layer 35 is defined as 1:x, it is effective to adjust the range of x, which led to the present disclosure.

Here, the elastic wave device 1 can be manufactured by a conventional method. FIG. 3 is a schematic diagram showing a cross section of the IDT electrode 3 during a part of the manufacturing process of the elastic wave device 1, with the left diagram showing the state during film formation and the right diagram showing the state after annealing of the second layer 35. As shown in FIG. 3, the second layer 35 may be a layer made of a CuAl alloy by laminating a layer made of Cu and a layer made of Al, and performing an annealing treatment, for example, by heating at 260° C. for 70 minutes. The x may be adjusted, for example, by changing the thicknesses of the layer made of Cu and the layer made of Al during film formation. The second layer 35 may also be formed by directly depositing a film of the CuAl alloy.

If the atomic ratio of Cu:Al in the second layer 35 is defined as 1:x, then by depositing the second layer 35 in a range where x is greater than 2.1, it is possible to form a second layer 35 having an atomic ratio of Cu:Al that satisfies the various numerical ranges shown below.

In addition, in the manufacturing process of the elastic wave device 1, appropriate heat treatments are also performed after the formation of the second layer 35, such as forming the protective layer 5, thermally curing the resin layer 6, and solder reflow. In these heat treatments, heating at 300°C or higher may be performed.

Such heating may cause some of the Cu contained in the second layer 35 to diffuse into the first layer 34. Figure 4 is a plan view that shows a schematic state of the IDT electrode 3 after it has been heated at 300°C for 70 minutes after annealing. As shown in Figure 4, the diffusion of Cu from the second layer 35 to the first layer 34 may cause spots to appear on the IDT electrode 3. At the spots, precipitation of Cu is observed in the first layer 34.

If Cu diffuses into the first layer 34, the filter loss characteristics of the IDT electrode 3 will deteriorate. Figure 5 shows a graph examining the correlation between the minimum insertion loss and the number of heating cycles for the Tx filter and Rx filter composed of the IDT electrode 3. Heating was performed using a hot plate at 330°C for 60 minutes. As shown in Figure 5, it was shown that the filter loss characteristics of the IDT electrode 3 deteriorate more as the number of heating cycles increases.

This deterioration in the filter loss characteristics of the IDT electrode 3 is believed to be due in part to an increase in the electrical resistance of the first layer 34 caused by the diffusion of Cu into the first layer 34. Such diffusion of Cu also leads to a reduction in the power resistance of the IDT electrode 3. The deterioration in the filter loss characteristics is also believed to be a factor in the reduction in power resistance. In this disclosure, it has been discovered that the diffusion of Cu into the first layer 34 can be reduced by adjusting the range of x in the second layer 35.

In the IDT electrode 3, the second layer 35 has an average value of x greater than 2.1, at least on the first surface 35a side. If the second layer 35 has such a configuration, it is possible to reduce the diffusion of Cu from the second layer 35 to the first layer 34 due to heating. This is thought to be because the portion of the second layer 35 where the average value of x is greater than 2.1 has a relatively low Cu concentration, making it difficult for Cu to diffuse, and therefore serves the function of reducing the diffusion of Cu to the first layer 34.

The "first surface 35a side" of the second layer 35 may be a portion including the first surface 35a and closer to the first surface 35 than the piezoelectric substrate 2. The value of x on the first surface 35a side of the second layer 35 may be, for example, a value obtained by measuring the first surface 35a from above the second layer 35. When observing a cross section of the IDT electrode 3 in the lamination direction in measuring the second layer 35, the value obtained by measuring the first surface 35a or its vicinity in the cross section may be the value of x on the first surface 35a side of the second layer 35. The vicinity of the first surface 35a of the second layer 35 may be, for example, a portion 10 nm from the first surface 35a in the lamination direction of the IDT electrode 3, but is not limited to this. Such a vicinity of the first surface 35a of the second layer 35 is an example of a portion on the first surface 35a side of the second layer 35.

The average value of x may be the average value of measurements taken at two or more randomly selected locations in the measurement target portion of the second layer 35. Two or more measurement locations are sufficient, but the average value of measurements taken at more measurement locations may be evaluated as a more reliable value. This is not limited to the average value of x on the first surface 35a side of the second layer 35, but can also be applied to the average value of x in the entire second layer 35, etc.

At least on the first surface 35a side of the second layer 35, the average value of x may be 2.7 or more, or may be 8 or more. Also, at least on the first surface 35a side of the second layer 35, the average value of x may be 20 or less, or may be 15 or less.

The second layer 35 may have x greater than 2.1 in 90% or more of the first surface 35a side. Also, the second layer 35 may have x less than 20, or less than 15, in 90% or more of the first surface 35a side. Also, the second layer 35 may have x greater than 2.1 over the entire first surface 35a side. Also, the second layer 35 may have x less than 20, or less than 15, over the entire first surface 35a side.

The second layer 35 may have an average value of x greater than 2.1. This indicates that it is sufficient that the average value of x in the entire second layer 35 is greater than 2.1. Furthermore, the second layer 35 may have an average value of x greater than or equal to 2.7. Furthermore, the second layer 35 may have an average value of x less than or equal to 5.6, less than or equal to 4.7, or less than or equal to 4.0.

The second layer 35 may have x greater than 2.1 in 90% or more by volume. The second layer 35 may have x less than 5.6 in 90% or more by volume. The second layer 35 may have x greater than 2.1 throughout. The second layer 35 may have x less than 5.6 throughout.

If the value of x in the second layer 35 is within this range, the inclusion of Cu maintains the power resistance of the second layer 35, while effectively reducing the diffusion of Cu from the second layer 35 to the first layer 34 due to heating. This results in an IDT electrode 3 that is stably equipped with excellent power resistance and reduces the generation of loss due to electrical resistance.

By using such an IDT electrode 3, it is possible to realize an acoustic wave device 1 that is highly efficient and has a long life. This effect contributes to achieving, for example, Goal 7.3 "Improve energy efficiency" and Goal 12.5 "Reduce waste generation" of the Sustainable Development Goals (SDGs) advocated by the United Nations.

As shown in FIG. 2 and FIG. 8 described later, the second layer 35 may include a third layer 35b located on the first layer 34 side. The third layer 35b may be located on the side of the second layer 35 closer to the first layer 34 than the piezoelectric substrate 2. The third layer 35b may include a portion of the first surface 35a. The third layer 35b is an example of a portion of the second layer 35 on the first surface 35a side.

The second layer 35 may also include a fifth layer 35d located on the piezoelectric substrate 2 side. The fifth layer 35d may be located on the side of the second layer 35 closer to the piezoelectric substrate 2 than the first layer 34. In this case, the portion of the second layer 35 between the third layer 35b and the fifth layer 35d can be defined as the fourth layer 35c. The second layer 35 may have a layered structure in which, from the first layer 34 side, the third layer 35b, the fourth layer 35c, and the fifth layer 35d are located in this order.

The third layer 35b may have a smaller atomic ratio of Cu to Al than the first portion 35e of the second layer 35 located between the center of the second layer 35 and the third layer 35b. The first portion 35e may be at least a part of the fourth layer 35c, and may be a portion of the second layer 35 closer to the first layer 34 than the center of the second layer 35.

The third layer 35b and the fifth layer 35d may be portions of the second layer 35 that have a relatively low Cu concentration. In other words, the third layer 35b and the fifth layer 35d may be layers that have a lower Cu concentration and a higher Al concentration than the fourth layer 35c. If the second layer 35 includes such a third layer 35b, the diffusion of Cu from the second layer 35 to the first layer 34 can be effectively reduced.

The third layer 35b may have an average value of x of 8 or more. The third layer 35b may have an average value of x of 20 or less, or may have an average value of 15 or less. If the third layer 35b has such a configuration, the diffusion of Cu from the second layer 35 to the first layer 34 can be more effectively reduced.

The crystallinity of the second layer 35 is not particularly limited. The second layer 35 may be amorphous or polycrystalline. This may be applied to any of the third layer 35b, fourth layer 35c, and fifth layer 35d contained in the second layer 35.

The third layer 35b, the fourth layer 35c, and the fifth layer 35d can be formed, for example, by stacking layers made of Al and layers made of Cu and performing an annealing treatment so that the average value of x in the second layer 35 is greater than 2.1 when the second layer 35 is formed. The third layer 35b and the fifth layer 35d may be formed by oxidizing aluminum in the surface layer of the second layer 35 to become an aluminum oxide layer such as _Al2O3 _, which remains in the surface layer of the second layer 35 when Cu and Al are alloyed by annealing.

The thickness of the second layer 35 is not particularly limited. For example, the thickness T2 of the second layer 35 may be 30% or less of the total thickness T1+T2 of the thickness T1 of the first layer 34 and the thickness T2 of the second layer 35. In other words, the thickness T2 of the second layer 35 may be a thickness that satisfies the relationship "T2≦0.3(T1+T2)". Furthermore, the thickness T2 of the second layer 35 may be 5% or more of the total thickness T1+T2.

In the IDT electrode 3, as the proportion of the second layer 35 increases, the power resistance increases. However, as the proportion of the second layer 35 increases, the electrical resistance of the electrode increases, and electrical loss is likely to increase. If the thickness T2 of the second layer 35 is within the above-mentioned range, an IDT electrode 3 with excellent power resistance and reduced loss due to electrical resistance can be obtained.

The second layer 35 may be located closer to the piezoelectric substrate 2 than the first layer 34. If the second layer 35 is located on the piezoelectric substrate 2 side, the second layer 35, which has higher strength, can be provided on the side closer to the piezoelectric substrate 2, which has stronger vibrations. This makes it possible to provide an elastic wave device 1 with excellent power durability. In addition, the center of gravity of the IDT electrode 3 can be moved downward, making it possible to reduce the electromechanical coupling coefficient and thus the propagation loss. The first layer 34 may be located closer to the piezoelectric substrate 2 than the second layer 35.

(Middle class)
The IDT electrode 3 may have an intermediate layer 36. The intermediate layer 36 is a layer located between the first layer 34 and the second layer 35. The intermediate layer 36 may be a layer containing a material that has low reactivity with Cu and Al, is stronger and more chemically stable than Al, and has electrical conductivity. For example, the intermediate layer 36 may be a layer containing one selected from the group consisting of Ti, Cr, Mo, a NiCr alloy, and an AlTi alloy as a main component. The intermediate layer 36 may be a layer containing any one of these components, or may be a layer containing two or more of them.

The intermediate layer 36 may be a layer having a thickness of 5% or less than the thickness of the IDT electrode 3. The intermediate layer 36 may also be a layer having a thickness of 20 to 100 Å, or may be a layer having a thickness of 60 Å. If the intermediate layer 36 has such a thickness, it can exhibit a function of reducing the diffusion of components such as Cu between adjacent layers. Furthermore, an intermediate layer 36 of such a thickness is unlikely to adversely affect the excitation of acoustic waves and the electrical characteristics as an electrode. By having such an intermediate layer 36, the IDT electrode 3 can reduce the diffusion of Cu and the like between the first layer 34 and the second layer 35, ensuring stable characteristics such as power resistance.

The IDT electrode 3 may have a plurality of first layers 34 and second layers 35. In this case, the first layers 34 and the second layers 35 may be alternately stacked, with the intermediate layer 36 positioned entirely between them, or at least partially between them.

(Base layer)
The IDT electrode 3 may have an underlayer 37. The underlayer 37 is a layer located between the second layer 35 and the piezoelectric substrate 2. The underlayer 37 may be a layer made of a material different from that of the piezoelectric substrate 2. The underlayer 37 may be a layer made of, for example, Ti, Cr, or an alloy thereof. The thickness of the underlayer 37 may be such that it does not affect the electrical characteristics of the IDT electrode 3, and may be, for example, 5% or less of the thickness of the IDT electrode 3.

The area of the underlayer 37 in contact with the piezoelectric substrate 2 may be larger than the area of the underlayer in contact with the electrode fingers 32 of the IDT electrode 3. In this case, the underlayer 37 can also increase the power resistance of the IDT electrode 3.

In this way, the IDT electrode 3 may be located in contact with the upper surface 2a of the piezoelectric substrate 2, or may be located on the upper surface 2a of the piezoelectric substrate 2 via the base layer 37.

(Another embodiment)
2, the cross-sectional shape of the first layer 34 and the second layer 35 in the stacking direction may be rectangular, but is not limited to this and may be, for example, trapezoidal. The second layer 35 may have a thin portion at a position away from the outer edge in the width direction of the electrode finger 32. With this configuration, it is possible to provide an acoustic wave device 1 that can reduce loss due to electrical resistance while improving the power durability of the IDT electrode 3.

In the example shown in FIG. 2, a sufficiently thick piezoelectric substrate 2 is used, but a so-called laminated substrate may be used in which a thin piezoelectric substrate 2 is used and a support substrate is bonded to the underside of the thin piezoelectric substrate 2. In this case, the thickness of the piezoelectric substrate 2 may be, for example, about 0.5 μm to 20 μm, and the support substrate may be thick enough to support the piezoelectric substrate 2. In particular, when a sapphire substrate or a Si substrate is used as the support substrate, deformation of the piezoelectric substrate 2 due to temperature changes can be reduced, and an elastic wave device 1 with excellent temperature characteristics can be provided. The thickness of the piezoelectric substrate 2 may be 0.2 λ or more and 10 λ or less in wavelength ratio. A bonding layer or the like may be located between the piezoelectric substrate 2 and the support substrate.

In addition, in the above example, the IDT electrode 3 has been described as having a uniform layer structure and thickness, but this is not limited to the above. For example, the bus bar 31 may be thicker than the electrode fingers 32, or may have a layer structure different from that of the electrode fingers 32.

<Communication Device>
6 is a block diagram showing a main part of a communication device 101 according to an embodiment of the present disclosure. The communication device 101 includes an elastic wave device 1. The communication device 101 may be a device that performs wireless communication using radio waves. A branching filter 7 included in the communication device 101 may have a function of branching a signal of a transmission frequency and a signal of a reception frequency in the communication device 101. The branching filter 7 includes an elastic wave device 1, and the elastic wave device 1 may be used to branch the signal by the branching filter 7. It is sufficient that the communication device 101 includes the elastic wave device 1 according to an embodiment of the present disclosure, and other configurations may be general configurations.

An example of a communication device 101 is described below. In communication device 101, a transmission information signal TIS containing information to be transmitted is modulated and frequency-raised by RF-IC 103, converted into a high-frequency signal of a carrier frequency, and becomes a transmission signal TS. Unwanted components outside the transmission passband are removed from the transmission signal TS by bandpass filter 105, amplified by amplifier 107, and input to splitter 7. Splitter 7 uses elastic wave device 1 to remove unwanted components outside the transmission passband from the input transmission signal TS, and outputs the signal to antenna 109. Antenna 109 converts the input electrical signal (transmission signal TS) into a radio signal and transmits it.

In communication device 101, a radio signal received by antenna 109 is converted by antenna 109 into a received signal RS, which is an electrical signal, and input to splitter 7. Splitter 7 uses elastic wave device 1 to remove unnecessary components outside the receiving passband from the received signal RS, and outputs the signal to amplifier 111. The output received signal RS is amplified by amplifier 111, and unnecessary components outside the receiving passband are removed by bandpass filter 113. The received signal RS is then frequency-downshifted and demodulated by RF-IC 103 to produce received information signal RIS.

〔summary〕
An elastic wave device according to a first aspect of the present disclosure comprises a piezoelectric substrate and an IDT electrode located on the piezoelectric substrate, the IDT electrode having a first layer including a conductive material and a second layer located between the first layer and the piezoelectric substrate and including a CuAl alloy, the second layer having a first surface located on the first layer side, and where an atomic ratio of Cu:Al in the second layer is defined as 1:x, the average value of x is greater than 2.1 at least on the first surface side of the second layer.

The elastic wave device according to aspect 2 of the present disclosure may be the same as in aspect 1, in which x is greater than 2.1 in 90% or more of the second layer on the first surface side.

The elastic wave device according to aspect 3 of the present disclosure is the elastic wave device according to claim 1, in which in

aspect

1 or 2, the average value of x in the second layer is greater than 2.1.

In the elastic wave device according to aspect 4 of the present disclosure, in any one of aspects 1 to 3, x may be greater than 2.1 in at least 90 volume % of the second layer.

The elastic wave device according to aspect 5 of the present disclosure may be such that in any one of aspects 1 to 4, the average value of x in the second layer is 5.6 or less.

The elastic wave device according to aspect 6 of the present disclosure may be any one of aspects 1 to 5, in which x is 5.6 or less in 90 volume % or more of the second layer.

The elastic wave device according to aspect 7 of the present disclosure may be such that in any one of aspects 1 to 6, the average value of x in the second layer is 4.0 or less.

The elastic wave device according to aspect 8 of the present disclosure may be such that in any one of aspects 1 to 6, the average value of x in the second layer is 2.7 or more and 4.7 or less.

The elastic wave device according to aspect 9 of the present disclosure is any one of aspects 1 to 8, in which the second layer includes a third layer located on the first layer side, and the third layer may have a smaller atomic ratio of Cu to Al than a first portion of the second layer that is located between the center of the second layer and the third layer.

The elastic wave device according to aspect 10 of the present disclosure may be the same as aspect 9, in which the average value of x in the third layer is 8 or greater.

In an elastic wave device according to aspect 11 of the present disclosure, in any one of aspects 1 to 10, the IDT electrode may further include an intermediate layer located between the first layer and the second layer.

The elastic wave device according to aspect 12 of the present disclosure is the same as in aspect 11, except that the intermediate layer may be mainly composed of one selected from the group consisting of Ti, Cr, Mo, NiCr alloy, and AlTi alloy, and may have a thickness of 20 to 100 Å.

The elastic wave device according to aspect 13 of the present disclosure is any one of aspects 1 to 12, in which the first layer contains Al and the content of components other than Al is 10% or less in atomic ratio.

The elastic wave device according to aspect 14 of the present disclosure may be any of aspects 1 to 13, further comprising a resin layer located on the piezoelectric substrate, the resin layer including a thermosetting resin.

The elastic wave device according to aspect 15 of the present disclosure may be the same as in aspect 14, in which the resin layer contains a thermosetting resin that hardens at 300°C or higher.

The elastic wave device according to aspect 16 of the present disclosure may be the same as that according to aspect 14, except that the resin layer contains at least one of polyimide and acrylic.

The elastic wave device according to aspect 17 of the present disclosure may be any one of aspects 1 to 16, in which the thickness of the second layer is 30% or less of the total thickness of the first layer and the second layer.

The communication device according to aspect 18 of the present disclosure includes an elastic wave device according to any one of aspects 1 to 17.

One embodiment of this disclosure is described below.

<Relationship between Cu:Al atomic ratio and power durability>
Acoustic wave devices 1 were manufactured by varying the Cu:Al atomic ratio in the second layer, and their power durability was evaluated. Each acoustic wave device 1 is designated as sample S1 to S6. Each sample has a layered structure in which resin layer 6, protective layer 5, first layer 34, intermediate layer 36, second layer 35, underlayer 37, and piezoelectric substrate 2 are located in this order in the portion of electrode fingers 32 of IDT electrode 3.

The first layer 34 was a layer mainly composed of Al and containing 1% (atomic ratio) Cu. The second layer 35 was a layer made of a CuAl alloy. The intermediate layer 36 and the underlayer 37 were layers made of Ti. The total thickness of each sample was 1500 Å, the thickness of the first layer 34 was 1104 Å, the thickness of the second layer 35 was 276 Å, and the thickness of the intermediate layer 36 and the underlayer 37 was 60 Å. The second layer 35 was formed by changing the thicknesses of the Al layer and the Cu layer during film formation, and heating and annealing at 260°C for 70 minutes to form a layer made of a CuAl alloy. Table 1 below shows the atomic ratio of Cu:Al in the second layer 35 and the thickness (Å) of each layer during film formation. The average electrode pitch of each sample was 1.13 μm.

Each of the obtained samples was further heated on a hot plate at 330°C for 60 minutes. After heating, each sample was evaluated for the occurrence of spots.

The evaluation of spots was carried out by calculating the area ratio of spots that occurred on the busbar 31 in a plan view. The range of each spot was determined by area calculation processing using the CAD software "AutoCAD (registered trademark)". The ratio of the total area of all spots to the entire area of the busbar 31 was calculated as the spot area ratio.

A power resistance test was also conducted on each sample. The power resistance test was conducted at an environmental temperature of 50°C, with a reference power of 31 dBm for a CW signal, and an accelerated test with input power was performed to obtain an estimate of the relationship between the input power (dBm) and lifespan (h) for each sample. The "power resistance strength" of each sample was defined as the magnitude of the input power corresponding to a lifespan of 5000 hours, and used as an evaluation index for power resistance.

Figure 7 shows the results of the power resistance test for each sample. Figure 8 is a graph showing the relationship between the area ratio of spots and the Al ratio in the second layer for each sample. As shown in Figures 7 and 8, the area ratio of spots in the busbar 31 decreased as the value of x increased. Regarding the power resistance of each sample, the power resistance strength increased rapidly as the number of spots decreased until the value of x reached approximately 4. This is a novel finding presented in this disclosure that spots caused by Cu diffused into the first layer 34 significantly affect the power resistance of the IDT electrode 3.

On the other hand, since the decrease in the area ratio of spots becomes gradual when the value of x is around 4, it was shown that even if the ratio of Al is increased so that the value of x is greater than 4, the effect of reducing the occurrence of spots is small. In addition, it was found that when the value of x is increased beyond 4, the power resistance strength decreases with the increase in the value of x. This is thought to indicate that the effect of increasing power resistance by adding Cu to the second layer containing Al decreases with a decrease in the Cu ratio.

<Cu distribution in the second layer>
The Cu distribution in the second layer 35 of the IDT electrode 3 according to an embodiment of the present disclosure was evaluated. Fig. 9 is a schematic diagram of a cross section in the lamination direction of sample S5 shown in Table 1, observed by a transmission electron microscope (TEM). Fig. 10 is a diagram of the results of energy dispersive X-ray fluorescence analysis (EDX analysis) at measurement points P1 and P2 shown in Fig. 9. Fig. 9 and Fig. 11 described later show a schematic representation of the Cu concentration distribution in the second layer 35, and the hatching of the cross section of each layer is omitted.

As shown in FIG. 9, the second layer 35 contained the third layer 35b, the fourth layer 35c, and the fifth layer 35d. The fourth layer 35c also showed variations in shade, suggesting that the distribution of Cu and Al was not uniform. Therefore, the ratio of elements at each measurement point was measured by EDX analysis for the dark area P1 and the light area P2 in the fourth layer 35c in the TEM image. The results of the EDX analysis are shown in FIG. 10 and Table 2 below.

Table 2 below shows the measured atomic ratio values (%) of each element, as well as the Cu:Al atomic ratio at each measurement point calculated based on the measured values. For reference, Table 2 below also shows the results of EDX analysis of sample S2, which was manufactured so that x was 2. Sample S2 had almost no shading in the second layer 35, so EDX analysis was performed on only one point.

As shown in Table 2, in sample S2 manufactured so that x was 2, the atomic ratio of the entire second layer 35 was almost the same as the atomic ratio at the measurement point. On the other hand, as shown in FIG. 10 and Table 2, in sample S4 manufactured so that x was 3.6, the atomic ratio of Cu:Al in the fourth layer 35c was different from the atomic ratio of Cu:Al in the entire second layer 35. Specifically, the atomic ratio of Al tended to be smaller in the fourth layer 35c compared to the entire second layer 35. In regard to this, in sample S4, the third layer 35b and the fifth layer 35d were formed, and it was considered possible that Al was segregated in these layers.

Therefore, for sample S3 manufactured so that x was 2.7, the element ratios in each of the third layer 35b, the fourth layer 35c, and the fifth layer 35d were measured by EDX analysis. Figure 11 shows a schematic view of a cross section in the stacking direction of sample S3 shown in Table 1, observed by TEM. Figure 12 shows the results of EDX analysis at measurement points P3 and P4 shown in Figure 11. Figure 13 shows the results of EDX analysis at measurement points P5 and P6 shown in Figure 11. For each measurement point, in the TEM image, the light-colored area in the fourth layer 35c is measurement point P3, and the dark-colored area is measurement point P4. Measurement point P5 was selected from the third layer 35b, and measurement point P6 was selected from the fifth layer 35d.

The measurement results of the EDX analysis are shown in Figures 11 to 13 and Table 3 below.

As shown in Figures 11 to 13 and Table 3, in sample S3, the value of x in the third layer 35b and the fifth layer 35d was significantly larger than the value of x in the fourth layer 35c. In other words, it was shown that when the second layer 35 is formed so that the atomic ratio of Al in the entire second layer 35 is large, Al segregates in the third layer 35b and the fourth layer 35d. A large atomic ratio of Al may mean, for example, that the value of x is greater than 2.1.

[Additional Notes]
The invention according to the present disclosure has been described above based on the drawings and examples. However, the invention according to the present disclosure is not limited to the above-mentioned embodiments. In other words, the invention according to the present disclosure can be modified in various ways within the scope of the present disclosure, and the embodiments obtained by appropriately combining the technical means disclosed in the different embodiments are also included in the technical scope of the invention according to the present disclosure. In other words, it should be noted that a person skilled in the art can easily make various modifications or corrections based on the present disclosure. It should also be noted that these modifications or corrections are included in the scope of the present disclosure.

REFERENCE SIGNS LIST 1 Acoustic wave device 2 Piezoelectric substrate 3 IDT electrode 34 First layer 35 Second layer 35b Third layer 36 Intermediate layer 6 Resin layer 101 Communication device

Claims

A piezoelectric substrate;
an IDT electrode located on the piezoelectric substrate,
The IDT electrode is
a first layer comprising a conductive material;
a second layer located between the first layer and the piezoelectric substrate, the second layer including a CuAl alloy;
the second layer has a first surface located on the first layer side,
An elastic wave device, wherein, when an atomic ratio of Cu:Al in the second layer is defined as 1:x, an average value of x is greater than 2.1 at least on the first surface side of the second layer.
The elastic wave device of claim 1, wherein x is greater than 2.1 in 90% or more of the second layer on the first surface side.
The elastic wave device according to claim 1 or 2, wherein the average value of x in the second layer is greater than 2.1.
The elastic wave device according to any one of claims 1 to 3, wherein x is greater than 2.1 in at least 90% by volume of the second layer.
The elastic wave device according to any one of claims 1 to 4, wherein the average value of x in the second layer is 5.6 or less.
The elastic wave device according to any one of claims 1 to 5, wherein x is 5.6 or less in 90% or more by volume of the second layer.
The elastic wave device according to any one of claims 1 to 6, wherein the average value of x in the second layer is 4.0 or less.
The elastic wave device according to any one of claims 1 to 6, wherein the average value of x in the second layer is 2.7 or more and 4.7 or less.
the second layer includes a third layer located on the first layer side,
9. The elastic wave device according to claim 1, wherein the third layer has a smaller atomic ratio of Cu to Al than a first portion of the second layer that is located between a center of the second layer and the third layer.
The elastic wave device according to claim 9, wherein the average value of x in the third layer is 8 or more.
The acoustic wave device according to any one of claims 1 to 10, wherein the IDT electrode further includes an intermediate layer located between the first layer and the second layer.
The elastic wave device according to claim 11, wherein the intermediate layer is mainly composed of one selected from the group consisting of Ti, Cr, Mo, NiCr alloy, and AlTi alloy, and has a thickness of 20 to 100 Å.
The elastic wave device according to any one of claims 1 to 12, wherein the first layer contains Al and the content of components other than Al is 10% or less in terms of atomic ratio.
Further comprising a resin layer located on the piezoelectric substrate,
The acoustic wave device according to claim 1 , wherein the resin layer includes a thermosetting resin.
The elastic wave device according to claim 14, wherein the resin layer contains a thermosetting resin that hardens at 300°C or higher.
The elastic wave device according to claim 14, wherein the resin layer includes at least one of polyimide and acrylic.
The elastic wave device according to any one of claims 1 to 16, wherein the thickness of the second layer is 30% or less of the total thickness of the first layer and the second layer.
A communication device comprising the elastic wave device according to any one of claims 1 to 17.