JP2019022093A - Elastic wave device - Google Patents

Abstract

To achieve high electric power proofness.SOLUTION: The elastic wave device includes: a piezoelectric substrate 10 and a plurality of electrode fingers 14 each having: a first metal film 12b which is formed on the piezoelectric substrate, has such an inclination surface as to have a larger width as it goes farther to the piezoelectric substrate and uses first metal as a main constituent; a second metal film 12e which is formed in contact with the first metal film to cover at least a part of the inclination surface of the first metal film and has a second metal film using second metal with a higher melting point and a higher resistivity than those of the first metal; and a third metal film 12b which is formed in contact with the second metal film to cover an inclination surface of the second metal film and uses the first metal as a main constituent.SELECTED DRAWING: Figure 13

Description

  The present invention relates to an acoustic wave device, for example, an acoustic wave device having electrode fingers.

  In a high-frequency communication system typified by a cellular phone, a high-frequency filter or the like is used to remove unnecessary signals other than the frequency band used for communication. An acoustic wave resonator such as a surface acoustic wave (SAW) resonator is used for a high-frequency filter or the like. In a surface acoustic wave resonator, an IDT (Interdigital Transducer) having a plurality of electrode fingers is provided on a piezoelectric substrate such as a lithium niobate substrate or a lithium tantalate substrate.

  It is known that Cu (copper) having high durability and voltage resistance and low resistance is used as an electrode finger material (for example, Patent Document 1). It is known to use a Ru (ruthenium) or RuO (ruthenium oxide) film so that the lithium tantalate substrate is not etched when a plurality of electrode fingers are formed on the lithium tantalate substrate using an etching method ( For example, Patent Document 2).

  It is known that an electrode finger is a laminated film in which Ti or Cr and Al or Cu are alternately laminated (for example, Patent Document 3). It is known that a laminated film in which Cu and Al are alternately laminated is used as an electrode finger (for example, Patent Document 4).

JP-A-9-199976 JP 2003-188673 A International Publication No. 2006/046545 Japanese Patent Laid-Open No. 7-122961

  When a high-power high-frequency signal is input to the IDT, surface acoustic waves are excited and the electrode fingers generate heat. Stress is applied to the electrode fingers due to thermal expansion. In addition, elastic stress accompanying deformation of the piezoelectric substrate is applied to the electrode fingers. As a result, stress migration occurs, and the power durability performance of the acoustic wave device deteriorates. In Patent Document 3, the power durability performance is improved by using a laminated film in which Ti or Cr and Al or Cu are alternately laminated on the electrode fingers. However, the improvement in power durability is not sufficient.

  This invention is made | formed in view of the said subject, and aims at improving power durability.

  The present invention includes a piezoelectric substrate, a first metal film having a sloped surface provided on the piezoelectric substrate and having a width that increases toward the piezoelectric substrate, the first metal as a main component, and the first metal. A second metal film mainly comprising a second metal having a melting point and a resistivity higher than that of the first metal, which is provided in contact with the first metal film so as to cover at least a part of the inclined surface of the film; A plurality of electrode fingers for exciting an elastic wave having a third metal film mainly composed of the first metal provided in contact with the second metal film so as to cover the inclined surface of the second metal film; An acoustic wave device comprising:

  The said structure WHEREIN: The said 2nd metal can be set as the structure which is a metal with melting | fusing point higher than Cr or Cr.

  The said structure WHEREIN: The said 1st metal can be set as the structure which is Cu.

  In the above configuration, the first metal may be Al.

  In the above configuration, the plurality of electrode fingers have a fourth metal film that is provided on the third metal film and has a third metal whose main component is a melting point and resistivity higher than those of the first metal. Can do.

  In the above-described configuration, the plurality of electrode fingers are provided with a fifth metal film that is provided between the piezoelectric substrate and the first metal film and has a fourth metal as a main component that has a melting point and a resistivity higher than those of the first metal. It can be set as the structure to comprise.

  In the above configuration, a dielectric film in contact with the side surfaces of the plurality of electrode fingers can be provided.

  In the above configuration, the piezoelectric substrate may be a lithium tantalate substrate or a lithium niobate substrate.

  The said structure WHEREIN: It can be set as the structure which comprises the filter containing these electrode fingers.

  In the above configuration, a multiplexer including the filter can be provided.

  According to the present invention, power durability can be improved.

FIG. 1A is a plan view of a surface acoustic wave resonator, and FIG. 1B is a cross-sectional view taken along line AA of FIG. FIG. 2 is a cross-sectional view of the vicinity of the electrode fingers in Comparative Examples 1 and 2. FIG. 3 is a diagram showing strain energy with respect to depth in Comparative Examples 1 and 2. FIG. 4A and FIG. 4B are schematic views of the interface between the electrode finger and the piezoelectric substrate in Comparative Example 1 and Comparative Example 2, respectively. FIG. 5 is a cross-sectional view of the vicinity of the electrode fingers in Comparative Examples 3 and 4. FIG. 6 is a diagram showing a power durability life in Comparative Examples 2 to 4. 7A and 7B are cross-sectional views of the vicinity of the electrode fingers in Samples A and B. FIG. FIG. 8A to FIG. 8D are cross-sectional views (part 1) illustrating the method for manufacturing the acoustic wave device according to the first embodiment. FIG. 9A to FIG. 9C are cross-sectional views (part 2) illustrating the method for manufacturing the acoustic wave device according to the first embodiment. FIG. 10 is a cross-sectional SEM image of the electrode finger in Sample B. FIG. 11 is a diagram illustrating a Q value with respect to frequency in Samples A and B and Comparative Example 2. FIG. 12 is a diagram illustrating the power durability with respect to input power in Samples A and B and Comparative Examples 2 and 3. 13A to 13C are cross-sectional views of the electrode fingers of sample B. FIG. FIG. 14A is a plan view of the filter according to the second embodiment, and FIG. 14B is a cross-sectional view taken along line AA of FIG. FIG. 15 is a circuit diagram of a duplexer according to the first modification of the second embodiment.

  FIG. 1A is a plan view of a surface acoustic wave resonator, and FIG. 1B is a cross-sectional view taken along line AA of FIG. As shown in FIGS. 1A and 1B, the surface acoustic wave resonator 24 includes an IDT 20 and a reflector 22. The IDT 20 and the reflector 22 are formed by the metal film 12 formed on the piezoelectric substrate 10. The IDT 20 includes a pair of opposing comb electrodes 18. The comb-shaped electrode 18 includes a plurality of electrode fingers 14 and a bus bar 16 to which the plurality of electrode fingers 14 are connected. The pair of comb-shaped electrodes 18 are provided to face each other so that the electrode fingers 14 are substantially staggered. A dielectric film 15 is provided on the piezoelectric substrate 10 so as to cover the electrode fingers 14.

  The elastic wave excited by the IDT 20 propagates mainly in the arrangement direction of the electrode fingers 14. The pitch of the electrode fingers 14 is approximately the wavelength λ of the elastic wave. The propagation direction of the elastic wave is the X direction, and the direction orthogonal to the propagation direction is the Y direction. The X direction and the Y direction do not necessarily correspond to the X axis direction and the Y axis direction of the crystal orientation of the piezoelectric substrate 10. The piezoelectric substrate 10 is a lithium tantalate substrate or a lithium niobate substrate. The dielectric film 15 is a temperature compensation film for suppressing the frequency temperature characteristics of the surface acoustic wave resonator, and is, for example, a silicon oxide film or a silicon oxide film to which an element such as fluorine is added.

  As the metal film 12, an Al film or a Cu film is used. For example, in a surface acoustic wave resonator in which the dielectric film 15 is not provided, an Al film is used as the metal film 12. In the surface acoustic wave resonator provided with the dielectric film 15, a Cu film is used as the metal film 12.

  Al has low acoustic impedance. For this reason, when the dielectric film 15 is provided, the difference in acoustic impedance between the dielectric film 15 and the electrode finger 14 is reduced. Thereby, the reflection coefficient of the elastic wave in the electrode finger 14 will become small. On the other hand, Cu has an acoustic impedance larger than that of Al. For this reason, the difference in acoustic impedance between the dielectric film 15 and the electrode finger 14 is increased, and the reflection coefficient of the elastic wave at the electrode finger is increased. Thereby, a broadband elastic wave device can be realized.

  Further, since Cu has a lower resistance than Al, the resistance of the electrode finger 14 can be reduced by using a Cu film. Furthermore, Cu has higher migration resistance than Al. Furthermore, since Cu has a higher density than Al, the electrode finger 14 can be made thinner and the elastic loss of the electrode finger 14 can be reduced.

  When a low-resistance Al film or Cu film is directly provided on the piezoelectric substrate 10, stress is applied to the Al film or Cu film and / or the Al film or Cu film generates heat during the operation of inputting a high-power high-frequency signal. For this reason, migration of Al or Cu occurs. Thereby, the power-proof performance of an elastic wave device falls. In order to improve the power durability, a metal film that is more heat resistant than Al or Cu is provided between the Al film or Cu film and the piezoelectric substrate 10. Thereby, the power-proof performance of an elastic wave device can be improved.

[Comparative Examples 1 and 2]
FIG. 2 is a cross-sectional view of the vicinity of the electrode fingers in Comparative Examples 1 and 2. As shown in FIG. 2, a metal film 12 is provided on the piezoelectric substrate 10. A dielectric film 15 is provided so as to cover the metal film 12. The metal film 12 includes stacked metal films 12a, 12b, and 12c. The metal film 12 a is provided on the piezoelectric substrate 10. The metal film 12b is provided on the metal film 12a. The metal film 12c is provided on the metal film 12b. The metal film 12a is a film for improving power durability. The metal film 12b is a Cu film. The metal film 12c is an etching stopper when an opening is provided in the dielectric film 15 by wet etching except for the electrode fingers 14. The upper surface of the dielectric film 15 is flat.

In Comparative Examples 1 and 2, a Ti (titanium) film and a Ru film were used as the film between the Cu film and the piezoelectric substrate 10, respectively. The distortion energy of the antiresonance frequency in the AA line of FIG. 2 was simulated for Comparative Examples 1 and 2. The simulation conditions are as follows.
Conditions common to Comparative Examples 1 and 2 Piezoelectric substrate 10: 127.86 ° rotated Y-cut X-propagating lithium niobate substrate Pitch of electrode fingers 14: 2.00 μm
Comparative Example 1
Metal film 12a: Ti film with a film thickness of 10 nm Metal film 12b: Cu film with a film thickness of 136 nm Metal film 12c: Cr film with a film thickness of 9 nm Dielectric film 15: Silicon oxide with a film thickness between electrode fingers 14 of 610 nm Membrane Comparative Example 2
Metal film 12a: Ru film with a thickness of 20 nm Metal film 12b: Cu film with a thickness of 125 nm Metal film 12c: Cr film with a thickness of 9 nm Dielectric film 15: Silicon oxide with a thickness of 593 nm between the electrode fingers 14 The Young's modulus in the bulk of the films Ru and Ti is about 420 GPa and 120 GPa, respectively. The Young's modulus of Ru is larger than Ti. In order to match the speed of sound in Comparative Examples 1 and 2, the Ru film of Comparative Example 2 was made thicker than the Ti film of Comparative Example 1.

FIG. 3 is a diagram showing strain energy with respect to depth in Comparative Examples 1 and 2. The unit of strain energy is J (joule). The depth of 0 μm indicates the interface between the piezoelectric substrate 10 and the metal film 12. A negative depth indicates the inside of the piezoelectric substrate 10, and a positive depth indicates the inside of the electrode finger 14. Ti, Cu, Cr (chromium), and SiO _{2 in} the upper part of the drawing indicate the Ti film, Cu film, Cr film, and dielectric film 15 in Comparative Example 1. Ru, Cu, Cr, and SiO _{2 at} the bottom of the figure indicate the Ru film, Cu film, Cr film, and dielectric film 15 in Comparative Example 2. LN represents a lithium niobate substrate.

  As shown in FIG. 3, in Comparative Example 2, the strain energy in the electrode finger 14 (particularly near the interface with the piezoelectric substrate 10) is smaller than in Comparative Example 1. This is presumably because Ru has a Young's modulus greater than that of Ti. When the strain energy is large, the energy of the elastic wave becomes the strain energy and the loss of the elastic wave becomes large. In Comparative Example 2, the strain energy in the electrode finger 14 is small, and the loss of elastic waves can be reduced. In order to reduce the strain energy in the electrode finger 14, the Young's modulus of the metal film 12a is preferably 300 GPa or more.

Next, surface acoustic wave resonators of Comparative Examples 1 and 2 were produced. The production conditions are as follows.
Number of pairs of electrode fingers 14: 100 Opening length: 20λ
Logarithm of reflector 22: 10 pairs Resonator: normal type Other conditions are the same as in the simulation, and the description thereof is omitted.

Q values were measured for Comparative Examples 1 and 2. The average Q value between the resonance frequency fr and the anti-resonance frequency fa is as follows.
Average Q value of Comparative Example 1: 1027
Average Q value of Comparative Example 2: 1271
As described above, in Comparative Example 2, the Q value can be improved by about 20% compared to Comparative Example 1. This is considered because the strain energy of Comparative Example 2 is smaller than that of Comparative Example 1 as in the simulation of FIG.

  The electrode finger 14 becomes a high temperature of several hundred degrees C. in the film forming process of the dielectric film 15 and the reflow process when mounted on the package. Further, the surface acoustic wave resonator is exposed to heat generation during operation. Due to such a high temperature, diffusion between the metal film 12a and the piezoelectric substrate 10 becomes a problem.

  FIG. 4A and FIG. 4B are schematic views of the interface between the electrode finger and the piezoelectric substrate in Comparative Example 1 and Comparative Example 2, respectively. As shown in FIG. 4A, in Comparative Example 1, a region 50 in which atoms of the piezoelectric substrate 10 are thermally diffused and a region 52 in which Ti atoms are thermally diffused are formed in the metal film 12a. The total thickness of the regions 50 and 52 is, for example, several nm. When the regions 50 and 52 are formed, the electrode fingers 14 are substantially embedded in the piezoelectric substrate 10. The regions 50 and 52 substantially reduce the insulation resistance on the surface of the piezoelectric substrate 10. As a result, when the acoustic wave device is operated, dielectric breakdown may occur between the electrode fingers 14 and cause a malfunction.

  As shown in FIG. 4B, in Comparative Example 2, a diffusion region having a thickness of 1 nm or more is hardly observed. The interface between the electrode finger 14 and the piezoelectric substrate 10 and the interface between the dielectric film 15 between the electrode finger 14 and the piezoelectric substrate 10 are substantially coplanar (for example, in the range of about 1 nm). Therefore, the malfunction by the insulation failure like the comparative example 1 is suppressed. The interface as shown in FIGS. 4A and 4B can be observed using a TEM (Transmission Electron Microscope).

  In order to investigate the influence of insulation failure, a high frequency signal having a frequency of 1780 MHz was applied to the surface acoustic wave resonators of Comparative Examples 1 and 2, and the instantaneous breakdown power that was instantaneously broken was measured. In Comparative Example 1, the instantaneous breakdown power was about 0.44 W, while in Comparative Example 2, it was about 1.10 W. In Comparative Example 2, the instantaneous breakdown power is improved about 2.5 times compared to Comparative Example 1. Thus, in Comparative Example 2, the power durability can be improved as compared with Comparative Example 1. One reason that the power durability performance can be improved in Comparative Example 2 is considered to be that there is almost no diffusion of Ru atoms into the piezoelectric substrate 10 as shown in FIG.

[Comparative Examples 3 and 4]
Thus, in Comparative Example 2, the power durability such as instantaneous breakdown power is improved as compared with Comparative Example 1. However, the power durability of Comparative Example 2 is not sufficient. In the following, Comparative Example 3 in which a metal film 12d is provided between the metal films 12b and 12c in order to improve the power durability over Comparative Example 2 will be described.

  FIG. 5 is a cross-sectional view of the vicinity of the electrode fingers in Comparative Examples 3 and 4. As shown in FIG. 5, a metal film 12d is provided between the metal films 12b and 12c. The metal film 12d is a film for improving power durability. Other configurations are the same as those of the comparative example 2, and the description is omitted.

Surface acoustic wave resonators of Comparative Examples 3 and 4 were produced. The production conditions are as follows.
Metal film 12a: Ru film with a film thickness of 10 nm Metal film 12b: Cu film with a film thickness of 125 nm Metal film 12d: Ru film with a film thickness of 10 nm (Comparative Example 3), Ti film (Comparative Example 4)
Metal film 12c: Cr film with a film thickness of 9 nm Dielectric film 15: Silicon oxide film with a film thickness of 593 nm between the electrode fingers 14 Other conditions are the same as in Comparative Example 2.

The Q values of the produced Comparative Examples 2 and 3 were measured. The average Q value between the resonance frequency fr and the anti-resonance frequency fa is as follows.
Average Q value of Comparative Example 2: 1271
Average Q value of Comparative Example 3: 1255
As described above, Comparative Example 3 has a Q value comparable to that of Comparative Example 2. This is presumably because Comparative Examples 2 and 3 have the same strain energy.

  Next, a high frequency signal having a frequency of 1780 MHz was applied to the surface acoustic wave resonator, and the breakdown time was defined as the power durability. The power (input power) of the applied high frequency signal is 29 dBm.

  FIG. 6 is a diagram showing a power durability life in Comparative Examples 2 to 4. As shown in FIG. 6, Comparative Example 3 in which the Ru metal film 12d is provided has a longer power durability than Comparative Example 2 in which the metal film 12d is not provided. However, Comparative Example 4 in which the metal film 12d is a Ti film has a shorter power durability than Comparative Example 2.

  As described above, the reason why the power durability is improved by providing the metal film 12d, which is a Ru film, on the metal film 12b is not clear, but may be considered as follows, for example.

The reason why the power durability is low is considered to be that, for example, Cu atoms in the metal film 12b move due to stress migration and the adjacent electrode fingers are short-circuited. Table 1 shows the melting point, lattice mismatch with Cu, standard electrode potential, and Young's modulus in Ti, Cr, Ru, Rh (rhodium), Re (rhenium), and Ir (iridium). The lattice mismatch is obtained by dividing the difference between the lattice constant of Cu and the lattice constant of the target metal by the lattice constant of Cu.

  As shown in Table 1, when compared with Ti, Cr and Ru, the melting point is the lowest for Ti, followed by Cr and Ru. This is the same as the order of Comparative Example 4, Comparative Example 2, and Comparative Example 3 in FIG. It is considered that the higher the melting point, the higher the bonding energy between the atoms, and the more stable the thermodynamics because the diffusion of atoms is suppressed. Thereby, the diffusion prevention property (barrier property) which suppresses the mutual diffusion between different elements is high. For this reason, it is considered that when a metal film 12d mainly composed of a refractory metal is provided on the metal film 12b, Cu migration is suppressed and power durability is improved. Thereby, it is considered that the power durability of Comparative Example 3 was improved as shown in FIG.

  Further, when the standard electrode potential showing an ionization tendency is high, impurities such as oxygen are hardly taken into the metal film 12d when the metal film 12d is formed. Thereby, it is hard to produce a defect in the metal film 12d. Furthermore, if the lattice mismatch is small, the adhesion at the interface between the metal films 12b and 12d is improved. Thereby, the diffusion of atoms at the interface between the metal films 12b and 12d is suppressed. As a result, Cu migration is suppressed.

  As shown in Table 1, the lattice mismatch is the largest for Ti and is in the order of Cr and Ru. The standard electrode potential is smallest for Ti and is in the order of Cr and Ru. Thereby, when the metal film 12d is a Ru film, it is considered that Cu migration is suppressed and power durability is improved.

  As shown in Table 1, there are Rh, Re, and Ir other than Ru as metals having a higher melting point than Cr, a small lattice mismatch, and a high standard electrode potential. These metals are considered to have a Young's modulus of 300 GPa or more, a low strain energy, and a high Q value.

  However, the power durability performance of Comparative Example 3 is not sufficient. Examples in which the power durability is further improved will be described below.

  Samples A and B were prepared as Example 1. 7A and 7B are cross-sectional views of the vicinity of the electrode fingers in Samples A and B. FIG. As shown in FIG. 7A, in Sample A, a single metal film 12e is inserted into the metal film 12b. In sample B, two layers of metal film 12e are inserted into metal film 12b. The metal film 12e is a film for improving power durability. Other configurations are the same as those of the comparative example 2, and the description is omitted.

The production conditions for Samples A and B are as follows.
Sample A
Metal film 12a: Ru film with a film thickness of 10 nm Metal film 12b: Cu film with a film thickness of 63 nm Metal film 12e: Ru film with a film thickness of 10 nm Metal film 12b: Cu film with a film thickness of 63 nm Metal film 12c: Film thickness 9 nm Cr film Dielectric film 15: Silicon oxide film having a film thickness of 593 nm between the electrode fingers 14 Other conditions are the same as in Comparative Example 2.

Sample B
Metal film 12a: Ru film with a film thickness of 10 nm Metal film 12b: Cu film with a film thickness of 42 nm Metal film 12e: Ru film with a film thickness of 5 nm Metal film 12b: Cu film with a film thickness of 42 nm Metal film 12e: Film thickness Ru film with a thickness of 5 nm Metal film 12b: Cu film with a thickness of 42 nm Metal film 12c: Cr film with a thickness of 9 nm Dielectric film 15: Silicon oxide film with a thickness of 593 nm between the electrode fingers 14 Other conditions are compared Same as Example 2.

[Production Method of Example 1]
FIG. 8A to FIG. 9C are cross-sectional views illustrating the method for manufacturing the acoustic wave device according to the first embodiment. As shown in FIG. 8A, a photoresist 40 is applied on the piezoelectric substrate 10. Then bake. The photoresist 40 is, for example, a positive type. As shown in FIG. 8B, the exposure light 43 is irradiated to the photoresist 40 through the photomask 42. The region 40a in the photoresist 40 is exposed. As shown in FIG. 8C, by developing the photoresist 40, the region 40a is removed and an opening 41 is formed. As shown in FIG. 8D, the metal film 12 is formed on the piezoelectric substrate 10 and the photoresist 40 in the opening 41 by using a vacuum deposition method.

  As shown in FIG. 9A, the metal film 12 on the photoresist 40 is lifted off by removing the photoresist 40. Thereby, the metal film 12 is formed on the piezoelectric substrate 10. As shown in FIG. 9B, a dielectric film 15 is formed on the piezoelectric substrate 10 so as to cover the metal film 12. The dielectric film 15 uses, for example, a CVD (Chemical Vapor Deposition) method or a sputtering method. As shown in FIG. 9C, the upper surface of the dielectric film 15 is flattened. For the planarization, for example, a CMP (Chemical Mechanical Polishing) method is used.

  FIG. 10 is a cross-sectional SEM (Scanning Electron Microscope) image of the electrode fingers in Sample B. As shown in FIG. 10, metal films 12a, 12b, 12e, 12b, 12e, 12b and 12c are laminated as electrode fingers 14 from the piezoelectric substrate 10 side. The electrode finger 14 is covered with a dielectric film 15.

  The side surface of the first metal film 12b closest to the piezoelectric substrate 10 is inclined. The first metal film 12e is provided so as to cover the side surface of the first metal film 12b. The second metal film 12b is provided so as to cover the first metal film 12e. The second metal film 12e is provided so as to cover the side surface of the second metal film 12b. The third metal film 12b is provided so as to cover the second metal film 12e.

  The angles of the side surfaces of the metal film 12a, the first to third metal films 12b, and the top surface of the piezoelectric substrate 10 are 13 °, 55 °, 65 °, and 70 °, respectively. As described above, the inner angle of the side surface of the metal film 12b increases as it goes to the upper layer. Thereby, the metal film 12e can cover the side surface of the lower metal film 12b.

  A structure as shown in FIG. 10 can be realized by appropriately selecting the deposition conditions of the metal film 12 in FIG. In principle, in the vacuum evaporation method, the flying direction of metal atoms is the direction of the evaporation source. Under such conditions, it is difficult for the metal film 12e to cover the side surface of the lower metal film 12b. For example, when the vapor deposition rate is increased, the degree of vacuum near the vapor deposition source in the chamber of the vacuum vapor deposition apparatus is deteriorated. As a result, the metal atoms are easily scattered, and the atoms fly from a plurality of directions as indicated by an arrow 44 in FIG. Therefore, the side surface of the metal film 12b becomes an inclined surface, and the metal film 12e easily covers the side surface of the metal film 12b.

The Q values of the produced samples A and B were measured. FIG. 11 is a diagram illustrating a Q value with respect to frequency in Samples A and B and Comparative Example 2. As shown in FIG. 11, samples A and B have a Q value comparable to that of Comparative Example 2. The average Q value between the resonance frequency fr and the anti-resonance frequency fa is as follows.
Average Q value of sample A: 1291
Average Q value of sample B: 1373
Average Q value of Comparative Example 2: 1282
As described above, samples A and B have a Q value comparable to that of Comparative Example 2. This is presumably because the samples A and B and Comparative Example 2 have the same strain energy.

  The power durability of samples A and B were measured. A high frequency signal having a frequency of 1780 MHz was applied to the surface acoustic wave resonator, and the time for destruction was defined as the power durability. The power durability against the power of the applied high frequency signal was measured. Note that the opening length of the surface acoustic wave resonator whose power durability is measured is 30λ.

  FIG. 12 is a diagram illustrating the power durability with respect to input power in Samples A and B and Comparative Examples 2 and 3. Dots are measurement points, and straight lines are approximate straight lines. As shown in FIG. 12, Comparative Example 3 has a longer power durability than Comparative Example 2. Sample A has a longer power durability life than Comparative Example 3. Sample B has a longer power durability than sample A.

  As described above, the reason why the power durability is improved by inserting the metal film 12e, which is a Ru film, into the metal film 12b is not clear, but may be considered as follows, for example.

  The metal film 12e has a higher melting point than the metal film 12b. For example, the melting point of Cu is 1085 ° C., and the melting point of Ru is 2334 ° C. as shown in Table 1. A metal having a high melting point has a high barrier property against mutual diffusion. As shown in FIG. 10, the metal film 12e covers the side and upper surfaces of the lower metal film 12b. Thereby, it is considered that the migration of atomic Cu in the metal film 12b is suppressed and the power durability performance is improved. When the number of layers of the metal film 12e is increased, the metal film 12b covered by the metal film 12e becomes thin. Therefore, it is considered that Cu migration can be further suppressed.

  The material of the metal film 12e is preferably a metal having a high standard electrode potential and / or a metal having a small lattice mismatch with the metal film 12b as shown in Table 1. Thereby, it is considered that Cu migration is suppressed. The material of the metal film 12e is preferably a metal having a large Young's modulus. For example, the Young's modulus is preferably 300 GPa or more. Thereby, it is considered that the strain energy of the electrode finger 14 is reduced and the Q value is increased.

  13A to 13C are cross-sectional views of the electrode fingers of sample B. FIG. As shown in FIG. 13A, the metal film 12e does not cover the side surface 54 of the lower metal film 12b. In such a structure, migration of atoms of the metal film 12b in the lateral direction (for example, in the dielectric film 15) cannot be suppressed. Therefore, it is considered that the improvement in power durability performance is small.

  As shown in FIG. 13B, the metal film 12e covers a partial region 56 on the side surface of the lower metal film 12b. Thereby, the horizontal migration of atoms in the metal film 12b can be suppressed. Therefore, the power durability performance is improved. As shown in FIG. 13C, the metal film 12e covers at least a part of the region 56 on the side surface of the lower metal film 12b. All regions 56 on the side surface of the lowermost metal film 12b are covered with the metal film 12e. Thereby, the horizontal migration of atoms in the metal film 12b can be further suppressed. Therefore, the power durability performance is further improved.

  The structure in which the metal film 12e covers at least a part of the side surface of the metal film 12b can be realized by appropriately selecting the vacuum deposition conditions. Further, this structure may be realized by using a sputtering method.

  The metal film 12a may be composed mainly of a metal having a melting point higher than that of the metal of the metal film 12b. As compared with Comparative Example 1 and Comparative Example 2, a Ru film is preferable to a Ti film. From the consideration of Table 1, the metal film 12a is mainly composed of Cr, or has a melting point higher than that of Cr, the standard electrode potential is higher than Cr, the lattice mismatch with the metal film 12b is smaller than Cr, and / or the Young's modulus. Is preferably composed mainly of a metal of 300 GPa or more.

  The metal film 12b is a layer through which current mainly flows, and preferably has a lower electrical resistivity than the metal films 12a, 12c, and 12e. For this reason, it is preferable that the metal film 12b has Cu or Al as a main component. The metal film 12b may contain an element that suppresses migration in addition to Al and Cu. When the metal film 12b contains Cu as a main component, the metal film 12b includes Ag, Al, Mg (magnesium), Mn (manganese), Sn (tin), Zr (zirconium), In (indium), Ta (tantalum). In addition, at least one of Ti and Mo may be contained. The thickness of the metal film 12b is preferably thicker than the thickness of each of the metal films 12a, 12c, and 12e, more preferably 2 times or more, and even more preferably 3 times or more.

  The metal film 12e may be composed mainly of a metal having a melting point higher than that of the metal of the metal film 12b. From the consideration of Table 1, the metal film 12e is mainly composed of Cr, or has a higher melting point than Cr, a standard electrode potential is higher than Cr, a lattice mismatch with the metal film 12b is smaller than Cr, and / or a Young's modulus. Is preferably composed mainly of a metal of 300 GPa or more. From the above viewpoint, the metal film 12e preferably contains at least one of Cr, Ru, Rh, Re, W, Mo, and Ir as a main component.

  The metal film 12c may be composed mainly of a metal having a melting point higher than that of the metal of the metal film 12b. From the consideration of Table 1, the metal film 12e is mainly composed of Cr, or has a higher melting point than Cr, a standard electrode potential is higher than Cr, a lattice mismatch with the metal film 12b is smaller than Cr, and / or a Young's modulus. Is preferably composed mainly of a metal of 300 GPa or more.

  Further, the metal film 12 c is an etching stopper when the dielectric film 15 is etched in a region other than the electrode fingers 14. From this point of view, the metal film 12c may be insoluble in an acidic etching solution such as buffered hydrofluoric acid. The metal film 12c may be, for example, a Ru film, Pt (platinum) film, Rh film, Au (gold) film, Ag (silver) film, and Mo (molybdenum) film. If the resistance value is low, a part of these films may be oxidized. The metal film 12c may not be provided.

  In order to improve the adhesion between the metal film 12 a and the piezoelectric substrate 10, an adhesion film such as a Cr film, a NiCr film, or a Ti film may be provided between the metal film 12 a and the piezoelectric substrate 10. In order to suppress diffusion as shown in FIG. 5A, the adhesion film is preferably thinner than the metal film 12a. The metal film 12a may not be provided.

  According to the first embodiment, the metal film 12 b (first metal film) has an inclined surface that is provided on the piezoelectric substrate 10 and has a width that increases toward the piezoelectric substrate 10. The metal film 12e (second metal film) is provided on and in contact with the metal film 12b so as to cover at least a part of the inclined surface of the metal film 12b. The metal film 12b (third metal film) is provided in contact with the metal film 12e so as to cover the inclined surface of the metal film 12e. The main component metal (second metal) of the metal film 12e has a higher melting point and resistivity than the main component metal (first metal) of the metal film 12b. Thereby, the power durability of an elastic wave device can be improved.

  The main component metal of the metal film 12e is Cr or a metal having a higher melting point than Cr. Thereby, the withstand power of an elastic wave device can be improved more. The main component metal of the metal film 12b is Cu or Al. Thereby, resistance of an electrode finger can be made low. It should be noted that “having a certain element as a main component” means that an element is included to such an extent that the effect of the first embodiment can be obtained. For example, an element is contained in 50 atomic% or more (or, for example, 90 atomic% or more).

  The electrode finger 14 has a metal film 12c (fourth metal film) provided on the uppermost metal film 12b. The main component metal (third metal) of the metal film 12c has a higher melting point and resistivity than the main component metal of the metal film 12b. Thereby, the withstand power of an elastic wave device can be improved more.

  The electrode finger 14 has a metal film 12a (fifth metal film) provided between the piezoelectric substrate 10 and the lowermost metal film 12b. The main component metal (fourth metal) of the metal film 12a has a higher melting point and resistivity than the main component metal of the metal film 12b. Thereby, the withstand power of an elastic wave device can be improved more.

  The dielectric film 15 is in contact with the side surfaces of the plurality of electrode fingers 14. Thus, when the dielectric film 15 is provided, the metal migration of the metal film 12b from the electrode finger 14 to the dielectric film 15 tends to occur. Therefore, the metal migration from the metal film 12b to the dielectric film 15 can be suppressed by providing the metal film 12e. The dielectric film 15 covers the plurality of electrode fingers 14 on the piezoelectric substrate 10 and is thicker than the plurality of electrode fingers 14. Thus, when the electrode finger 14 is embedded in the dielectric film 15, it is preferable that the metal film 12 b contains Cu as a main component in order to increase the acoustic impedance of the electrode finger 14. The dielectric film 15 contains at least one of silicon oxide and silicon nitride as a main component. Silicon oxide or silicon nitride may contain an element such as fluorine.

  When the piezoelectric substrate 10 is a lithium niobate substrate or a lithium tantalate substrate, the dielectric film 15 is a silicon oxide film or a silicon oxide film to which fluorine is added. Thereby, the frequency temperature coefficient of the acoustic wave device can be brought close to zero.

  In the rotated Y-cut X-propagating lithium niobate substrate, the Rayleigh wave electromechanical coupling coefficient is maximized when the rotated Y-cut angle is 127.86 °. When a simulation is performed using the Campbell & Jones method, the electromechanical coupling coefficient of the Rayleigh wave is larger than the electromechanical coupling coefficient of the leaky wave when the rotational Y cut angle is in the range of 120 ° to 140 °. Therefore, when the Rayleigh wave is the main mode and the leaky wave is an unnecessary wave, the rotation Y cut angle is preferably 120 ° or more and 140 ° or less. In consideration of manufacturing variations with respect to the rotational Y-cut angle of 127.86 ° at which the electromechanical coupling coefficient is maximized, the rotational Y-cut angle is more preferably from 126 ° to 130 °.

  Further, when the rotation Y cut angle is 0 °, the electromechanical coupling coefficient of the leaky wave is maximized. When the rotational Y cut angle is in the range of −10 ° to 10 °, the electromechanical coupling coefficient of the leaky wave is larger than the electromechanical coupling coefficient of the Rayleigh wave. Therefore, when the leaky wave is the main mode and the Rayleigh wave is the unnecessary wave, the rotation Y cut angle is preferably −10 ° or more and 10 ° or less. In consideration of manufacturing variations, the rotation Y cut angle is more preferably −3 ° or more and 3 ° or less.

  In a rotating Y-cut X-propagating lithium tantalate substrate, if the rotating Y-cut angle is smaller than 20 °, the electromechanical coupling coefficient becomes small. When the rotational Y cut angle is larger than 48 °, the temperature coefficient of frequency increases. Therefore, the rotation Y cut angle is preferably 20 ° or more and 48 ° or less.

  The piezoelectric substrate 10 is preferably a lithium niobate substrate, and the dielectric film 15 is preferably a silicon oxide film. Thereby, the distortion energy in the electrode finger 14 can be made small and a loss can be suppressed.

  Example 2 is an example of a filter having a surface acoustic wave resonator according to Example 1. FIG. FIG. 14A is a plan view of the filter according to the second embodiment, and FIG. 14B is a cross-sectional view taken along line AA of FIG. As shown in FIGS. 14A and 14B, the surface acoustic wave resonator 24, the wiring 26, and the pad 27 are provided on the piezoelectric substrate 10. A dielectric film 15 is provided so as to cover the surface acoustic wave resonator 24 and the wiring 26. Openings (not shown) are formed in the dielectric film 15 on the pads 27, and bumps 28 are provided in the openings. The surface acoustic wave resonator 24 includes an IDT 20 and a reflector 22.

  The plurality of surface acoustic wave resonators 24 include series resonators S1 to S3 and parallel resonators P1 and P2. The bump 28 includes terminals T1, T2, and Tg. The terminal T1 corresponds to an input terminal to which a high frequency signal is input. The terminal T2 corresponds to an output terminal that outputs a high-frequency signal. The terminal Tg corresponds to a ground terminal to which a ground potential is supplied. Series resonators S1 to S3 are connected in series between terminals T1 and T2, and parallel resonators P1 and P2 are connected in parallel.

  As in the second embodiment, the surface acoustic wave resonator according to the first embodiment is used as at least one resonator of the filter. Thereby, the characteristic of a filter can be improved. The number of series resonators and parallel resonators of the ladder filter can be arbitrarily set. The surface acoustic wave resonator according to the first embodiment may be used for a multimode filter.

  FIG. 15 is a circuit diagram of a duplexer according to the first modification of the second embodiment. As shown in FIG. 15, the transmission filter 60 is connected between the common terminal Ant and the transmission terminal Tx. A reception filter 62 is connected between the common terminal Ant and the reception terminal Rx. The transmission filter 60 passes signals in the transmission band among the high-frequency signals input from the transmission terminal Tx to the common terminal Ant and suppresses other signals. The reception filter 62 passes signals in the reception band among the high-frequency signals input to the common terminal Ant and suppresses other signals.

  The filter of the second embodiment can be used for at least one of the transmission filter 60 and the reception filter 62. Although an example of a duplexer has been described as a multiplexer, a triplexer or a quadplexer may be used.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to such specific embodiments, and various modifications and changes can be made within the scope of the gist of the present invention described in the claims. It can be changed.

DESCRIPTION OF SYMBOLS 10 Piezoelectric substrate 12, 12a-12e Metal film 14 Electrode finger 15 Dielectric film 20 IDT
22 reflector 24 surface acoustic wave resonator 60 transmission filter 62 reception filter

Claims (10)

A piezoelectric substrate;
At least one of a first metal film having a sloped surface provided on the piezoelectric substrate and having a width that increases toward the piezoelectric substrate, the first metal being a main component, and the sloped surface of the first metal film. A second metal film mainly comprising a second metal having a melting point and a resistivity higher than those of the first metal, and an inclined surface of the second metal film. A plurality of electrode fingers for exciting an elastic wave having a third metal film mainly composed of the first metal provided in contact with the second metal film so as to cover;
An elastic wave device comprising: 2. The acoustic wave device according to claim 1, wherein the second metal is Cr or a metal having a higher melting point than Cr.   The acoustic wave device according to claim 2, wherein the first metal is Cu.   The acoustic wave device according to claim 1, wherein the first metal is Al.   The plurality of electrode fingers include a fourth metal film that is provided on the third metal film and has a third metal as a main component, which has a melting point and a resistivity higher than those of the first metal. The acoustic wave device according to one item.   The plurality of electrode fingers include a fifth metal film that is provided between the piezoelectric substrate and the first metal film, and that includes a fourth metal having a melting point and a resistivity higher than those of the first metal as a main component. The elastic wave device according to any one of 1 to 5.   The acoustic wave device according to any one of claims 1 to 6, further comprising a dielectric film in contact with side surfaces of the plurality of electrode fingers.   The acoustic wave device according to any one of claims 1 to 7, wherein the piezoelectric substrate is a lithium tantalate substrate or a lithium niobate substrate.   The acoustic wave device according to any one of claims 1 to 8, further comprising a filter including the plurality of electrode fingers. The acoustic wave device according to claim 9, further comprising a multiplexer including the filter.

Images