Elastic wave devices, filters, and multiplexers

Abstract

The present invention provides elastic wave devices, filters, and multiplexers capable of suppressing degradation of power handling capacity. [Solution] The elastic wave device 100 comprises a piezoelectric layer 15, a pair of comb-shaped electrodes 22 including electrode fingers 23 having a first layer 31 which is a titanium nitride layer provided on the piezoelectric layer 15, a second layer 32 which is a tungsten layer provided on the first layer 31 and is thicker than the first layer 31, and a third layer 33 which is an aluminum layer or an aluminum alloy layer provided on the second layer 32. The filter comprises the elastic wave device. The multiplexer comprises the filter.

Description

Technical Field

[0001] The present invention relates to an elastic wave device, a filter, and a multiplexer.

Background Art

[0002] In a high-frequency communication system typified by a mobile phone, a high-frequency filter is used to remove unnecessary signals outside the frequency band used for communication. For example, a surface acoustic wave (SAW) resonator is used in the high-frequency filter. In the surface acoustic wave resonator, an interdigital transducer (IDT) having a plurality of electrode fingers is provided on a piezoelectric layer such as a lithium tantalate layer or a lithium niobate layer. It is known to use an aluminum layer or an aluminum alloy layer as the electrode fingers (for example, Patent Documents 1-6). A configuration in which the aluminum alloy layer is divided into two and an adhesion layer such as a titanium layer or a tungsten layer is provided therebetween is known (for example, Patent Document 4). A configuration in which a titanium alloy layer is provided between the piezoelectric layer and the aluminum alloy layer is also known (for example, Patent Document 5). A configuration in which a tungsten layer is provided between the piezoelectric layer and the aluminum layer or the aluminum alloy layer is also known (for example, Patent Document 6).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

[0004] To miniaturize elastic wave devices, it is conceivable to use high-density metallic materials for the electrode fingers. For example, a laminated film consisting of a high-density tungsten layer and a low-resistance aluminum layer or aluminum alloy layer could be used for the electrode fingers. However, in this case, the power handling capacity may deteriorate.

[0005] This invention has been made in view of the above problems, and aims to suppress the deterioration of power withstand capability. [Means for solving the problem]

[0006] The present invention is an elastic wave device comprising a pair of comb-shaped electrodes including electrode fingers, each having a piezoelectric layer, a first layer provided on the piezoelectric layer which is a titanium nitride layer, a second layer provided on the first layer which is a tungsten layer and is thicker than the first layer, and a third layer provided on the second layer which is an aluminum layer or an aluminum alloy layer.

[0007] In the above configuration, the first layer can be a titanium nitride layer having a (111) orientation.

[0008] In the above configuration, the thickness of the second layer can be 5% or more and 60% or less of the thickness of the electrode finger.

[0009] In the above configuration, the thickness of the first layer can be 5 nm or more.

[0010] In the above configuration, the piezoelectric layer can be a lithium tantalate layer or a lithium niobate layer.

[0011] The present invention is a filter that includes the elastic wave device described above.

[0012] The present invention is a multiplexer including the filter described above.

Effect of the Invention

[0013] According to the present invention, deterioration of the power resistance can be suppressed.

Brief Description of the Drawings

[0014] [Figure 1] Fig. 1(a) is a plan view of the surface acoustic wave device according to Example 1, and Fig. 1(b) is a cross-sectional view of the electrode fingers in Example 1. [Figure 2] Fig. 2(a) is a cross-sectional view of Samples A and B used in Experiment 1, and Figs. 2(b) and 2(c) are schematic views showing the results of pole measurement of the Al(111) orientation of the aluminum copper alloy layer in Samples A and B. [Figure 3] Fig. 3 is a diagram showing the results of the power resistance test of the surface acoustic wave resonator fabricated using Samples A and B. [Figure 4] Figs. 4(a) to 4(d) are cross-sectional views of Samples C to F used in Experiment 3. [Figure 5] Figs. 5(a) to 5(d) are schematic views showing the results of pole measurement of the Al(111) orientation of the aluminum copper alloy layer in Samples C to F. [Figure 6] Figs. 6(a) to 6(e) are cross-sectional views of the surface acoustic wave devices according to Modification Examples 1 to 5 of Example 1. [Figure 7] Figs. 7(a) and 7(b) are cross-sectional views of the surface acoustic wave devices according to Modification Examples 6 and 7 of Example 1. [Figure 8] Fig. 8(a) is a circuit diagram of the filter according to Example 2, and Fig. 8(b) is a circuit diagram of the diplexer according to the modification example of Example 2.

Mode for Carrying Out the Invention

[0015] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

Examples

[0016] FIG. 1(a) is a plan view of the elastic wave device 100 according to Example 1, and FIG. 1(b) is a cross-sectional view of the electrode finger 23 in Example 1. The arrangement direction of the electrode fingers 23 is the X direction, the extending direction of the electrode fingers 23 is the Y direction, and the thickness direction of the piezoelectric layer 15 is the Z direction. The X direction, Y direction, and Z direction do not necessarily correspond to the X-axis direction and Y-axis direction of the crystal orientation of the piezoelectric layer 15. When the piezoelectric layer 15 is a piezoelectric layer of rotation Y-cut X-propagation, the X direction is the X-axis direction of the crystal orientation.

[0017] As shown in FIG. 1(a), an elastic wave resonator 20 is provided on the piezoelectric layer 15. In Example 1, the piezoelectric layer 15 is a piezoelectric substrate. The piezoelectric layer 15 is a single crystal lithium tantalate layer or a single crystal lithium niobate layer, and may be, for example, a rotation Y-cut X-propagation lithium tantalate layer or a rotation Y-cut X-propagation lithium niobate layer, and may be, for example, a 30° - 50° rotation Y-cut X-propagation lithium tantalate layer. The elastic wave resonator 20 has an IDT 21 and reflectors 25. The reflectors 25 are provided on both sides of the IDT 21 in the X direction. The IDT 21 includes a pair of comb-shaped electrodes 22 facing each other. The comb-shaped electrode 22 includes a plurality of electrode fingers 23 and a bus bar 24 to which the plurality of electrode fingers 23 are connected. The region where the electrode fingers 23 of the pair of comb-shaped electrodes 22 intersect is the intersection region 26. The length in the Y direction of the intersection region 26 is the aperture length.

[0018] In the pair of comb-shaped electrodes 22, the electrode fingers 23 are alternately provided one by one in at least a part of the intersection region 26. The elastic wave mainly excited by the plurality of electrode fingers 23 in the intersection region 26 mainly propagates in the X direction. The pitch of the electrode fingers 23 of one of the pair of comb-shaped electrodes 22 is approximately equal to the wavelength λ of the elastic wave. Approximately twice the average pitch D of the plurality of electrode fingers 23 is the wavelength λ. The average pitch D can be calculated by dividing the width of the IDT 21 in the X direction by the number of electrode fingers 23. The reflector 25 reflects the elastic wave (elastic surface wave) excited by the electrode fingers 23 of the IDT 21. Thereby, the elastic wave is confined within the intersection region 26 of the IDT 21.

[0019] As shown in Figure 1(b), the IDT 21, such as the electrode fingers 23, and the reflector 25 are formed by a conductive film 30 provided on the piezoelectric layer 15. The conductive film 30 comprises a first layer 31 provided on the piezoelectric layer 15, a second layer 32 provided in contact with the upper surface of the first layer 31, and a third layer 33 provided in contact with the upper surface of the second layer 32. The thicknesses of the first layer 31, the second layer 32, and the third layer 33 are T1, T2, and T3, respectively. The thickness of the conductive film 30 is T4. T4 = T1 + T2 + T3.

[0020] The first layer 31 is a conductive titanium nitride (TiN) layer, which is, for example, polycrystalline. The first layer 31 may contain impurities other than titanium (Ti) and nitrogen (N), either intentionally or unintentionally. The combined titanium and nitrogen content in the first layer 31 is, for example, 80 atomic% or more and 90 atomic% or more. The first layer 31 is, for example, a titanium nitride layer having a crystalline state dominated by (111) orientation. It is known that by depositing a titanium nitride layer on a lithium tantalate layer or a lithium niobate layer under appropriate deposition conditions, the titanium nitride layer can have a crystalline state dominated by (111) orientation. The orientation can be obtained, for example, by measuring by X-ray diffraction or by measuring with an electron microscope. The thickness T1 of the first layer 31 is, for example, 5 nm to 10 nm.

[0021] The second layer 32 is a tungsten (W) layer, and is, for example, polycrystalline. The second layer 32 may contain impurities other than tungsten (W), either intentionally or unintentionally. The tungsten content in the second layer 32 is, for example, 80 atomic percent or more, and 90 atomic percent or more. The thickness T2 of the second layer 32 is greater than the thickness T1 of the first layer 31. The thickness T2 of the second layer 32 varies depending on the operating band in which the elastic wave device 100 is used, but for example, it is 150 nm to 350 nm when used in the low band (less than 1 GHz), 100 nm to 200 nm when used in the middle band (1 GHz to 7 GHz), and 80 nm to 120 nm when used in the high band (greater than 7 GHz). For example, the thickness T2 of the second layer 32 is 5% to 60% of the thickness T4 of the electrode finger 23.

[0022] The third layer 33 is an aluminum (Al) layer or an aluminum (Al) alloy layer, and is, for example, polycrystalline. The third layer 33 has a crystalline state in which Al(111) orientation is dominant. If the third layer 33 is an aluminum alloy layer, the third layer 33 contains at least one element other than aluminum (Al) from among copper (Cu), magnesium (Mg), scandium (Sc), zirconium (Zr), titanium (Ti), neodymium (Nd), and silicon (Si). If the third layer 33 is an aluminum layer, the third layer 33 may contain impurities other than aluminum, either intentionally or unintentionally. If the third layer 33 is an aluminum alloy layer, the third layer 33 may contain impurities other than aluminum and the metallic elements constituting the aluminum alloy, either intentionally or unintentionally. The aluminum content in the third layer 33 is, for example, 80 atomic% or more, and 90 atomic% or more. The thickness T3 of the third layer 33 is greater than the thickness T1 of the first layer 31. The thickness T3 of the third layer 33 varies depending on the operating band in which the elastic wave device 100 is used. For example, it is 150 nm to 350 nm when used in the low band (less than 1 GHz), 100 nm to 200 nm when used in the middle band (1 GHz to 7 GHz), and 80 nm to 120 nm when used in the high band (greater than 7 GHz). For example, the thickness T3 of the third layer 33 is 38% to 93% of the thickness T4 of the electrode finger 23.

[0023] [Manufacturing method] The elastic wave device 100 according to Example 1 is manufactured by the following method. First, the surface of the piezoelectric layer 15 is cleaned with an organic solvent. Then, a first layer 31, which is a titanium nitride layer, is deposited on the piezoelectric layer 15 using a sputtering method. For example, a Ti target, argon (Ar) gas, and nitrogen (N2) gas are used, and the film is deposited under the conditions of power: 2.0 kW, gas pressure: 0.2 Pa, and gas flow rate ratio Ar:N2=3:5. By cleaning the surface of the piezoelectric layer 15 with an organic solvent before deposition and then depositing the film under the above deposition conditions, a titanium nitride layer having a crystalline state dominated by (111) orientation is formed.

[0024] Next, a tungsten layer, the second layer 32, is deposited on the first layer 31 using a sputtering method under general deposition conditions. For example, a W target and Ar gas are used, and the film is deposited under the conditions of power: 1.0 kW and gas pressure: 0.2 Pa.

[0025] Next, a third layer 33, which is an aluminum layer or an aluminum alloy layer, is deposited on the second layer 32 using a sputtering method under general deposition conditions. For example, if the third layer 33 is an aluminum layer, an Al target and Ar gas are used, and the film is deposited under the conditions of power: 2.0 kW and gas pressure: 0.1 Pa. If the third layer 33 is an aluminum alloy layer, an Al alloy target with other elements added to aluminum and Ar gas are used, and the film is deposited under the conditions of power: 2.0 kW and gas pressure: 0.1 Pa.

[0026] Next, the first layer 31, the second layer 32, and the third layer 33 are shaped into the desired form using photolithography and etching to form the IDT 21 and the reflector 25. Thus, the elastic wave device 100 according to Example 1 is formed.

[0027] [Experiment 1] Figure 2(a) is a cross-sectional view of Sample A and Sample B used in Experiment 1. As shown in Figure 2(a), both Sample A and Sample B have a structure in which a titanium nitride layer 40 and an aluminum-copper alloy layer 42, in which 0.43 atomic percent copper is added to aluminum, are stacked in that order on a piezoelectric layer 15, which is a single-crystal lithium tantalate layer. The thickness of the titanium nitride layer 40 is 40 nm, and the thickness of the aluminum-copper alloy layer 42 is 100 nm.

[0028] In Sample A, the piezoelectric layer 15 was subjected to reverse sputtering (etch-back) treatment before the titanium nitride layer 40 was deposited, and then the titanium nitride layer 40 was deposited under the above-described deposition conditions. The surface of the piezoelectric layer 15 becomes uneven due to reverse sputtering. When the titanium nitride layer 40 is deposited on such a surface of the piezoelectric layer 15 under the above-described deposition conditions, the titanium nitride layer 40 does not become (111) oriented but becomes randomly oriented. In Sample B, the surface of the piezoelectric layer 15 was cleaned with an organic solvent before the titanium nitride layer 40 was deposited, and then the titanium nitride layer 40 was deposited under the above-described deposition conditions. As a result, the titanium nitride layer 40 has a crystalline state in which (111) orientation is dominant. In both Sample A and B, the aluminum copper alloy layer 42 was deposited under the above-described deposition conditions.

[0029] Pole measurements were performed on the Al(111) orientation of the aluminum-copper alloy layer 42 in samples A and B. Figures 2(b) and 2(c) are schematic diagrams showing the results of the pole measurements on the Al(111) orientation of the aluminum-copper alloy layer 42 in samples A and B. As shown in Figure 2(b), in sample A, the change in diffraction intensity with respect to orientation was small, confirming that the aluminum-copper alloy layer 42 does not have Al(111) orientation and is randomly oriented. This is thought to be because the titanium nitride layer 40 is randomly oriented, and the aluminum-copper alloy layer 42 formed on the titanium nitride layer 40 also became randomly oriented following the titanium nitride layer 40.

[0030] As shown in Figure 2(c), in sample B, diffraction intensity was observed only in a specific direction, confirming that the aluminum-copper alloy layer 42 has an Al(111) orientation. This is thought to be because the interatomic distance of the (111) plane of titanium nitride (0.2999 nm) and the interatomic distance of the (111) plane of aluminum (0.2863 nm) are close, and by forming the aluminum-copper alloy layer 42 on the titanium nitride layer 40 which has an (111) orientation, the aluminum-copper alloy layer 42 came to have an Al(111) orientation.

[0031] [Experiment 2] Elastic wave resonators were fabricated using samples A and B, and power withstand tests were conducted. A power withstand test is a test in which a constant power is continuously applied to the sample and the time until the sample fails is measured; it is also called a life test. The fabrication conditions for the elastic wave resonators are as follows. Piezoelectric layer 15:42° rotation Y-cut X propagation lithium tantalate layer Titanium nitride layer 40:60nm Aluminum-copper alloy layer 42:90nm Wavelength of elastic waves λ: 1.5 μm

[0032] Figure 3 shows the results of power withstand tests for elastic wave resonators fabricated using Sample A and Sample B. In Figure 3, the horizontal axis represents the input power applied to the elastic wave resonator, and the vertical axis represents the time until the elastic wave resonator fails (lifetime). The experimental results are shown as dots, along with an approximate straight line. As shown in Figure 3, the elastic wave resonator using Sample B had a longer time until failure compared to the elastic wave resonator using Sample A. Observation of the surfaces of the elastic wave resonators using Sample A and Sample B after the power withstand tests revealed that the elastic wave resonator using Sample B showed suppressed electromigration compared to the elastic wave resonator using Sample A. Therefore, it is considered that the elastic wave resonator using Sample B had a longer lifespan because electromigration was suppressed. From this, it can be seen that by using an aluminum-copper alloy layer 42 having Al(111) orientation, electromigration can be suppressed and power withstand capability can be improved.

[0033] [Experiment 3] Figures 4(a) to 4(d) are cross-sectional views of samples C to F used in Experiment 3. As shown in Figure 4(a), sample C has a structure in which a titanium nitride layer 40, a tungsten layer 41, and an aluminum-copper alloy layer 42 are stacked in that order on a piezoelectric layer 15, which is a single-crystal lithium tantalate layer. The titanium nitride layer 40 was deposited under the above-described deposition conditions after cleaning the surface of the piezoelectric layer 15 with an organic solvent. Therefore, the titanium nitride layer 40 has a crystalline state in which (111) orientation is dominant. The tungsten layer 41 and the aluminum-copper alloy layer 42 were deposited under the above-described deposition conditions. The thickness of the titanium nitride layer 40 is 10 nm, the thickness of the tungsten layer 41 is 200 nm, and the thickness of the aluminum-copper alloy layer 42 is 200 nm.

[0034] As shown in Figure 4(b), sample D has a structure in which a tungsten layer 41 and an aluminum-copper alloy layer 42 are stacked in that order on a piezoelectric layer 15 which is a single-crystal lithium tantalate layer. The tungsten layer 41 and the aluminum-copper alloy layer 42 were deposited on the surface of the piezoelectric layer 15 after cleaning with an organic solvent, under the deposition conditions described above. The thickness of the tungsten layer 41 is 200 nm, and the thickness of the aluminum-copper alloy layer 42 is also 200 nm.

[0035] As shown in Figure 4(c), sample E has a structure in which a tungsten layer 41, a titanium nitride layer 40, and an aluminum-copper alloy layer 42 are stacked in that order on a piezoelectric layer 15 which is a single-crystal lithium tantalate layer. The tungsten layer 41, titanium nitride layer 40, and aluminum-copper alloy layer 42 were deposited on the surface of the piezoelectric layer 15 after cleaning with an organic solvent, under the deposition conditions described above. The thickness of the tungsten layer 41 is 200 nm, the thickness of the titanium nitride layer 40 is 10 nm, and the thickness of the aluminum-copper alloy layer 42 is 200 nm.

[0036] As shown in Figure 4(d), sample F has a structure in which a titanium layer 43, a tungsten layer 41, and an aluminum-copper alloy layer 42 are stacked in that order on a piezoelectric layer 15, which is a single-crystal lithium tantalate layer. The titanium layer 43 was deposited after cleaning the surface of the piezoelectric layer 15 with an organic solvent, using a Ti target and Ar gas under the conditions of power: 2.0 kW and gas pressure: 0.1 Pa. The titanium layer 43 has a crystalline state in which (002) orientation is dominant. The tungsten layer 41 and the aluminum-copper alloy layer 42 were deposited under the above-described deposition conditions. The thickness of the titanium layer 43 is 10 nm, the thickness of the tungsten layer 41 is 200 nm, and the thickness of the aluminum-copper alloy layer 42 is 200 nm.

[0037] The copper content of the aluminum-copper alloy layer 42 in samples C to F is the same as in Example 1.

[0038] Pole measurements were performed on the Al(111) orientation of the aluminum-copper alloy layer 42 in samples C through F. Figures 5(a) through 5(d) are schematic diagrams showing the results of the pole measurements on the Al(111) orientation of the aluminum-copper alloy layer 42 in samples C through F.

[0039] As shown in Figure 5(a), in sample C, diffraction intensity was observed only in a specific direction, confirming that the aluminum-copper alloy layer 42 has Al(111) orientation. On the other hand, as shown in Figures 5(b) and 5(c), in samples D and E, the change in diffraction intensity with respect to orientation was small, confirming that the aluminum-copper alloy layer 42 does not have Al(111) orientation and is randomly oriented. As shown in Figure 5(d), in sample F, the diffraction intensity has a ring-shaped distribution, confirming that the aluminum-copper alloy layer 42 has weak (insufficient) Al(111) orientation.

[0040] The reason why the aluminum-copper alloy layer 42 in sample D did not have Al(111) orientation is not clear, but the following are possible explanations. When a tungsten layer 41 is formed on the piezoelectric layer 15, which is a lithium tantalate layer, the tungsten layer 41 is likely to have a random orientation. This is likely due to the large difference between the lattice constants of lithium tantalate (a=5.154 Å, c=13.783 Å) and tungsten (3.165 Å), and / or the poor compatibility between the crystal structure of lithium tantalate (trigonal corundum structure) and the crystal structure of tungsten (body-centered cubic structure). Since the aluminum-copper alloy layer 42 was formed on the randomly oriented tungsten layer 41, it is thought that the aluminum-copper alloy layer 42 also followed the tungsten layer 41 and became randomly oriented.

[0041] Similarly, in sample E, the titanium nitride layer 40 and the aluminum-copper alloy layer 42 were formed on a randomly oriented tungsten layer 41, so it is considered that the titanium nitride layer 40 and the aluminum-copper alloy layer 42 also became randomly oriented following the tungsten layer 41.

[0042] In sample C, the aluminum-copper alloy layer 42 had Al(111) orientation, whereas in sample F, the Al(111) orientation of the aluminum-copper alloy layer 42 was insufficient. The reason for this is not clear, but the following is one possible explanation. In sample C, the tungsten layer 41 is formed on the titanium nitride layer 40, whereas in sample F, the tungsten layer 41 is formed on the titanium layer 43. Titanium nitride has a face-centered cubic crystal structure, titanium has a hexagonal close-packed crystal structure, and tungsten has a body-centered cubic crystal structure. The crystal structure of titanium nitride (face-centered cubic) and the crystal structure of tungsten (body-centered cubic) are compatible, but the crystal structure of titanium (hexagonal close-packed) and the crystal structure of tungsten (body-centered cubic) are not compatible. Therefore, in sample F, the orientation of the tungsten layer 41 was poor, and as a result, the aluminum-copper alloy layer 42 had insufficient Al(111) orientation.

[0043] [Differentiation] Figure 6(a) is a cross-sectional view of the elastic wave device 110 according to Modification 1 of Example 1. As shown in Figure 6(a), in Modification 1 of Example 1, the piezoelectric layer 15 is provided on the substrate 10. The other configurations are the same as in Example 1, so their description is omitted.

[0044] Figure 6(b) is a cross-sectional view of the elastic wave device 120 according to Modification 2 of Example 1. As shown in Figure 6(b), in Modification 2 of Example 1, the piezoelectric layer 15 is provided on the substrate 10. An insulating layer 12 is provided between the substrate 10 and the piezoelectric layer 15. An insulating layer 13 is provided between the insulating layer 12 and the piezoelectric layer 15. The interface between the substrate 10 and the insulating layer 12 is roughened. The other configurations are the same as in Example 1, so their description is omitted.

[0045] Figure 6(c) is a cross-sectional view of the elastic wave device 130 according to Modification 3 of Example 1. As shown in Figure 6(c), in Modification 3 of Example 1, the piezoelectric layer 15 is provided on the substrate 10. An insulating layer 11 is provided between the substrate 10 and the piezoelectric layer 15. An insulating layer 12 is provided between the insulating layer 11 and the piezoelectric layer 15. An insulating layer 13 is provided between the insulating layer 12 and the piezoelectric layer 15. The other configurations are the same as in Example 1, so their description is omitted.

[0046] Figure 6(d) is a cross-sectional view of the elastic wave device 140 according to Modification 4 of Example 1. In Modification 3 of Example 1, the interface between the substrate 10 and the insulating layer 11 was mirror-finished, as shown in Figure 6(c), but in Modification 4 of Example 1, the interface between the substrate 10 and the insulating layer 11 is roughened, as shown in Figure 6(d). The other configurations are the same as in Modification 3 of Example 1, so their explanation is omitted. The arithmetic mean roughness Ra of the roughened surface is, for example, greater than 10 nm and 100 nm or less, while the arithmetic mean roughness Ra of the mirror-finished surface is, for example, 10 nm or less and about 1 nm.

[0047] Figure 6(e) is a cross-sectional view of the elastic wave device 150 according to Modification 5 of Example 1. In Modifications 3 and 4 of Example 1, the interface between the insulating layer 11 and the insulating layer 12 was mirror-finished, as shown in Figures 6(c) and 6(d), but in Modification 5 of Example 1, the interface between the insulating layer 11 and the insulating layer 12 is roughened, as shown in Figure 6(e). The other configurations are the same as in Modification 4 of Example 1, so their description is omitted.

[0048] In Modifications 1 to 5 of Example 1, the substrate 10 is, for example, a sapphire substrate, an alumina substrate, a silicon substrate, a spinel substrate, a quartz substrate, a silica substrate, or a silicon carbide substrate. The speed of sound of the bulk wave propagating through the substrate 10 may be faster or slower than the speed of sound of the bulk wave propagating from the piezoelectric layer 15 and the insulating layer 11 to the insulating layer 13.

[0049] In Modifications 3 to 5 of Example 1, the speed of sound of the bulk wave propagating through the insulating layer 11 is faster than the speed of sound of the bulk wave propagating through the insulating layer 12 and the piezoelectric layer 15. As a result, the energy of the main response elastic wave is confined within the piezoelectric layer 15 and the insulating layer 12. The insulating layer 11 is, for example, polycrystalline or amorphous, and is an aluminum oxide layer, a silicon nitride layer, an aluminum nitride layer, a silicon layer, or a silicon carbide layer.

[0050] In Modifications 2 to 5 of Example 1, the insulating layer 12 is a temperature compensation layer and has a temperature coefficient of elasticity with the opposite sign to the sign of the temperature coefficient of elasticity of the piezoelectric layer 15. For example, the temperature coefficient of elasticity of the piezoelectric layer 15 is negative, and the temperature coefficient of elasticity of the insulating layer 12 is positive. The insulating layer 12 is an insulating layer mainly composed of silicon oxide, for example, a silicon oxide layer with no additives or containing additive elements such as fluorine, and for example, polycrystalline or amorphous. This makes it possible to reduce the frequency temperature coefficient of the elastic wave resonator. When the insulating layer 12 is a silicon oxide layer, the speed of sound of bulk waves propagating through the insulating layer 12 is slower than the speed of sound of bulk waves propagating through the piezoelectric layer 15.

[0051] The insulating layer 13 is a bonding layer that joins the insulating layer 12 and the piezoelectric layer 15. When the insulating layer 12 is a silicon oxide layer, it is difficult to directly bond the piezoelectric layer 15 and the insulating layer 12 using a surface activation method. In such cases, an insulating layer made of a different material from the insulating layer 12 is provided as the insulating layer 13. The insulating layer 13 is, for example, polycrystalline or amorphous, and is an aluminum oxide layer, a silicon nitride layer, an aluminum nitride layer, a silicon layer, or a silicon carbide layer.

[0052] Figure 7(a) is a cross-sectional view of the elastic wave device 160 according to Modification 6 of Example 1. As shown in Figure 7(a), in Modification 6 of Example 1, a protective film 16 is provided on the piezoelectric layer 15 so as to cover the electrode fingers 23. The thickness of the protective film 16 is less than the thickness of the electrode fingers 23. The other configurations are the same as in Example 1, so their description is omitted.

[0053] Figure 7(b) is a cross-sectional view of the elastic wave device 170 according to Modification 7 of Example 1. As shown in Figure 7(b), in Modification 7 of Example 1, a protective film 16 is provided on the piezoelectric layer 15 so as to cover the electrode fingers 23. The thickness of the protective film 16 is greater than the thickness of the electrode fingers 23, and the upper surface of the protective film 16 is flattened. The other configurations are the same as in Example 1, so their description is omitted.

[0054] In modified examples 6 and 7 of Example 1, the protective film 16 is an inorganic insulating film such as a silicon oxide film or a silicon nitride film.

[0055] According to Example 1 and its modified form, the electrode finger 23 has a first layer 31 provided on the piezoelectric layer 15, a second layer 32 provided on the first layer 31, and a third layer 33 provided on the second layer 32. The first layer 31 is a titanium nitride layer. The second layer 32 is a tungsten layer and is thicker than the first layer 31. The third layer 33 is an aluminum layer or an aluminum alloy layer. In this way, by providing a second layer 32 which is a tungsten layer thicker than the first layer 31 which is a titanium nitride layer, the elastic wave device can be miniaturized. By providing a second layer 32 which is a tungsten layer on the first layer 31 which is a titanium nitride layer, and a third layer 33 which is an aluminum layer or an aluminum alloy layer on the second layer 32, the third layer 33 comes to have an Al(111) oriented crystalline state, as shown in Figures 4(a) and 5(a). For example, if the second layer 32, which is a tungsten layer, is provided on the piezoelectric layer 15 without the first layer 31, which is a titanium nitride layer, then, as shown in Figures 4(b) and 5(b), the third layer 33 will have a random orientation without Al(111) orientation. When the third layer 33 has a random orientation, the power withstand capability deteriorates, as shown in Figure 3. However, by having the third layer 33 have Al(111) orientation, even when the second layer 32, which is a tungsten layer, is provided to miniaturize the elastic wave device, the deterioration of power withstand capability can be suppressed, as shown in Figure 3.

[0056] Furthermore, in Example 1 and its modified form, the first layer 31 is a titanium nitride layer having (111) orientation. In this case, as shown in Figures 4(a) and 5(a), the third layer 33 is more likely to adopt an Al(111) oriented crystalline state. Therefore, the deterioration of power resistance can be suppressed.

[0057] Furthermore, in Example 1 and its modified form, the thickness T2 of the second layer 32, which is a tungsten layer, is 5% to 60% of the thickness T4 of the electrode finger 23. By setting the thickness T2 of the second layer 32 to this size, the elastic wave device can be miniaturized, and the thinning of the thickness T3 of the third layer 33 is suppressed, thereby suppressing an increase in the electrical resistance of the electrode finger 23. Even when the thickness T2 of the second layer 32 is such, the third layer 33 has an Al(111) oriented crystalline state. From the viewpoint of miniaturizing the elastic wave device, the thickness T2 of the second layer 32 is preferably 10% or more, more preferably 15% or more, and even more preferably 20% or more, of the thickness T4 of the electrode finger 23. From the viewpoint of suppressing an increase in the electrical resistance of the electrode finger 23, the thickness T2 of the second layer 32 is preferably 55% or less, more preferably 53% or less, and even more preferably 50% or less, of the thickness T4 of the electrode finger 23.

[0058] Furthermore, in Example 1 and its modified form, the thickness T1 of the first layer 31 is 5 nm or more. When the thickness T1 of the first layer 31 is 5 nm or more, the third layer 33 is more likely to take on an Al(111) oriented crystalline state. From the viewpoint of making the third layer 33 an Al(111) oriented crystalline state, the thickness T1 of the first layer 31 is preferably 6 nm or more, more preferably 8 nm or more, and even more preferably 10 nm or more. As the thickness T1 of the first layer 31 increases, the thickness T2 of the second layer 32 and the thickness T3 of the third layer 33 become relatively smaller. Therefore, from the viewpoint of miniaturizing the elastic wave device and reducing the electrical resistance of the electrode fingers 23, the thickness T1 of the first layer 31 is preferably 20 nm or less, more preferably 15 nm or less, and even more preferably 10 nm or less.

[0059] Furthermore, in Example 1 and its modified form, the piezoelectric layer 15 is a lithium tantalate layer or a lithium niobate layer. In the above experiment, when the piezoelectric layer 15 is a lithium tantalate layer, providing a titanium nitride layer 40 between the piezoelectric layer 15 and the tungsten layer 41 resulted in the aluminum-copper alloy layer 42 on the tungsten layer 41 having an Al(111) orientation. Lithium niobate has the same crystal structure as lithium tantalate (trigonal corundum structure), and its lattice constant is also close to that of lithium tantalate (lithium tantalate: a=5.154 Å, c=13.783 Å; lithium niobate: a=5.148 Å, c=13.863 Å). Therefore, even when a lithium niobate layer is used for the piezoelectric layer 15, the third layer 33, which is an aluminum layer or aluminum alloy layer, will have an Al(111) oriented crystal state, similar to the case where a lithium tantalate layer is used.

[0060] In Example 1 and its modified examples, the second layer 32 is shown as a tungsten layer, but it may also be a molybdenum (Mo) layer, a ruthenium (Ru) layer, a rhodium (Rh) layer, a platinum (Pt) layer, or a silver (Ag) layer. [Examples]

[0061] Figure 8(a) is a circuit diagram of the filter 200 according to Embodiment 2. As shown in Figure 8(a), one or more series resonators S1 to S4 are connected in series between the input terminal Tin and the output terminal Tout. One or more parallel resonators P1 to P3 are connected in parallel between the input terminal Tin and the output terminal Tout. At least one of the series resonators S1 to S4 and the parallel resonators P1 to P3 can be an elastic wave device according to Embodiment 1 and its modified form. The number of series resonators and parallel resonators can be set as appropriate. A ladder filter is shown as an example of the filter, but a multimode filter may also be used.

[0062] Figure 8(b) is a circuit diagram of a duplexer 210 according to a modified example of Embodiment 2. As shown in Figure 8(b), a transmit filter 50 is connected between the common terminal Ant and the transmit terminal Tx. A receive filter 52 is connected between the common terminal Ant and the receive terminal Rx. The transmit filter 50 allows the transmit band signal from the high-frequency signal input from the transmit terminal Tx to pass to the common terminal Ant as the transmit signal, and suppresses signals of other frequencies. The receive filter 52 allows the receive band signal from the high-frequency signal input from the common terminal Ant to pass to the receive terminal Rx as the receive signal, and suppresses signals of other frequencies. At least one of the transmit filter 50 and the receive filter 52 can be the filter of Embodiment 2. A duplexer is shown as an example of a multiplexer, but a triplexer or quadplexer may also be used.

[0063] Although embodiments of the present invention have been described in detail above, the present invention is not limited to these specific embodiments, and various modifications and changes are possible within the scope of the gist of the present invention as described in the claims. [Explanation of Symbols]

[0064] 10...Substrate, 11...Insulating layer, 12...Insulating layer, 13...Insulating layer, 15...Piezoelectric layer, 16...Protective film, 20...Elastic wave resonator, 21...IDT, 22...Comb-type electrode, 23...Electrode fingers, 24...Busbar, 25...Reflector, 26...Crossing region, 30...Conductive film, 31...First layer, 32...Second layer, 33...Third layer, 40...Titanium nitride layer, 41...Tungsten layer, 42...Aluminum copper alloy layer, 43...Titanium layer, 50...Transmitting filter, 52...Receiving filter, 100, 110, 120, 130, 140, 150, 160, 170...Elastic wave device, 200...Filter, 210...Duplexer

Claims

1. Piezoelectric layer and An elastic wave device comprising a pair of comb-shaped electrodes including electrode fingers, each having a first layer which is a titanium nitride layer provided on the piezoelectric layer, a second layer which is a tungsten layer provided on the first layer and is thicker than the first layer, and a third layer which is an aluminum layer or an aluminum alloy layer provided on the second layer.

2. The elastic wave device according to claim 1, wherein the first layer is a titanium nitride layer having a (111) orientation.

3. The elastic wave device according to claim 1 or 2, wherein the thickness of the second layer is 5% or more and 60% or less of the thickness of the electrode finger.

4. The elastic wave device according to claim 1 or 2, wherein the thickness of the first layer is 5 nm or more.

5. The elastic wave device according to claim 1 or 2, wherein the piezoelectric layer is a lithium tantalate layer or a lithium niobate layer.

6. A filter comprising the elastic wave device according to claim 1 or 2.

7. A multiplexer comprising the filter described in claim 6.

Images