JP2021027401A - Acoustic wave device, filter, and multiplexer - Google Patents

Abstract

To suppress deterioration of an electrode finger.SOLUTION: An acoustic wave device comprises: a piezoelectric substrate 10; and a plurality of electrode fingers 14 comprising a first metal layer 12a having a first metal that is provided onto the piezoelectric substrate 10 and includes a fusion point larger than that of a platinum as a main component and a second metal layer 12b that includes a second metal having an extension intensity higher than that of the first metal as a main component, and is contacted to a whole surface of an upper surface of the first metal layer 12a.SELECTED DRAWING: Figure 3

Description

The present invention relates to elastic wave devices, filters and multiplexers, for example elastic wave devices, filters and multiplexers having multiple electrode fingers.

In a high frequency communication system typified by a mobile phone, a high frequency filter or the like is used to remove unnecessary signals other than the frequency band used for communication. A surface acoustic wave device having a surface acoustic wave (SAW) element or the like is used as a high frequency filter or the like. The SAW element is an element in which an IDT (Interdigital Transducer) having a pair of comb-shaped electrodes is formed on a piezoelectric substrate (for example, Patent Documents 1 to 4). It is known that the speed of sound of surface acoustic waves excited by IDT is made slower than the speed of sound of bulk waves propagating in a piezoelectric substrate to reduce loss (for example, Patent Document 3). It is known that a molybdenum layer and an aluminum layer are laminated as electrode fingers, and a tungsten layer is provided on the aluminum layer (for example, Patent Document 4).

Japanese Unexamined Patent Publication No. 2008-244523 Japanese Unexamined Patent Publication No. 2008-28980 Japanese Unexamined Patent Publication No. 2016-136712 JP-A-2018-23110

When a refractory metal is used for the comb-shaped electrode in order to slow down the sound velocity of the surface acoustic wave as in Patent Document 3, the electrode finger vibrates when a large amount of electric power is applied. Therefore, deterioration such as peeling or deformation of the electrode finger may occur.

The present invention has been made in view of the above problems, and an object of the present invention is to suppress deterioration of electrode fingers.

The present invention provides a piezoelectric substrate, a first metal layer provided on the piezoelectric substrate and containing a first metal having a melting point equal to or higher than the melting point of platinum as a main component, and a tensile strength higher than the tensile strength of the first metal. It is an elastic wave device including a plurality of electrode fingers having a second metal as a main component and each having a second metal layer in contact with the entire upper surface of the first metal layer.

In the above configuration, the specific resistance of the first metal layer can be lower than the specific resistance of the second metal layer.

In the above configuration, the second metal can have a Young's modulus higher than the Young's modulus of the first metal.

In the above configuration, the density of the second metal layer can be higher than the density of the first metal layer.

In the above configuration, the first metal layer can be thicker than the second metal layer.

In the above configuration, the second metal layer can be configured to be provided on the first metal layer.

In the above configuration, the second metal layer may be provided on the side surface of the first metal layer.

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

In the above configuration, the second metal may be tungsten.

The present invention comprises a piezoelectric substrate, a first metal layer provided on the piezoelectric substrate and containing any one of molybdenum, ruthenium, rhodium and iridium as a main component, and a first metal layer containing tungsten as a main component. An elastic wave device comprising a plurality of electrode fingers, each comprising a second metal layer in contact with the entire upper surface of the surface.

The present invention is a filter including the elastic wave device.

The present invention is a multiplexer including the above filter.

According to the present invention, deterioration of the electrode finger can be suppressed.

1 (a) is a plan view showing an elastic wave resonator in the first embodiment, and FIG. 1 (b) is a sectional view taken along the line AA of FIG. 1 (a). 2 (a) and 2 (b) are enlarged cross-sectional views of the elastic wave resonator according to the first embodiment and the first comparative example, respectively. 3 (a) to 3 (c) are enlarged cross-sectional views of the elastic wave devices according to the first to third modifications of the first embodiment, respectively. FIG. 4A is a circuit diagram of the filter according to the second embodiment, and FIG. 4B is a circuit diagram of the duplexer according to the first modification of the second embodiment.

Hereinafter, examples of the present invention will be described with reference to the drawings.

An elastic wave resonator will be described as an example of an elastic wave device. 1 (a) is a plan view showing an elastic wave resonator in the first embodiment, and FIG. 1 (b) is a sectional view taken along the line AA of FIG. 1 (a). The arrangement direction of the electrode fingers 14 is the X direction, the stretching direction of the electrode fingers 14 is the Y direction, and the normal direction of the piezoelectric substrate 10 is the Z direction. The X, Y and Z directions do not always match the crystal orientation of the piezoelectric substrate 10.

As shown in FIGS. 1 (a) and 1 (b), the elastic wave resonator 24 has an IDT 20 and a reflector 22. The IDT 20 and the reflector 22 are provided on the piezoelectric substrate 10. The piezoelectric substrate 10 is, for example, a lithium tantalate substrate, a lithium niobate substrate, or a quartz substrate. The IDT 20 and the reflector 22 are formed of a metal layer 12. The IDT 20 has a pair of comb-shaped electrodes 18. Each of the pair of comb-shaped electrodes 18 has a plurality of electrode fingers 14 and a bus bar 16 to which the plurality of electrode fingers 14 are connected. The electrode fingers 14 of one comb-shaped electrode 18 and the electrode fingers 14 of the other comb-shaped electrode 18 are provided alternately at least in part. Reflectors 22 are formed on both sides of the IDT 20 in the X direction. The elastic wave excited by the IDT 20 propagates mainly in the X direction, and the reflector 22 reflects the elastic wave. Let λ be the pitch of the electrode fingers 14 in the same comb-shaped electrode 18. λ corresponds to the wavelength of the surface acoustic wave excited by the IDT 20. An insulating film such as a silicon oxide film or a silicon nitride film may be provided so as to cover the metal layer 12. The film thickness of the insulating film may be thicker or thinner than the film thickness of the metal layer 12.

FIG. 2A is an enlarged cross-sectional view of the elastic wave resonator according to the first embodiment. As shown in FIG. 2A, the metal layer 12 includes a metal layer 12a provided on the piezoelectric substrate 10 and a metal layer 12b provided on the metal layer 12a. No other layer (metal layer and insulating layer) is provided between the metal layers 12a and 12b, and the metal layers 12a and 12b are in contact with each other. The main component of the metal layer 12a is metal M1 (for example, molybdenum), and the main component of the metal layer 12b is metal M2 (for example, tungsten). The melting point of metal M1 is higher than that of platinum. The tensile strength of the metal M2 is higher than the tensile strength of the metal M1. The thickness of the metal layer 12 is, for example, 0.05λ to 0.15λ. The thickness T1 of the metal layer 12a is, for example, 300 nm, and the thickness T2 of the metal layer 12b is, for example, 100 nm. The metal layer 12a may be thicker or thinner than the metal layer 12b.

[Comparative Example 1]
FIG. 2B is an enlarged cross-sectional view of the elastic wave resonator according to Comparative Example 1. As shown in FIG. 2B, in Comparative Example 1, the electrode finger 14 is a single-layer metal layer 12. When the sound velocity of the surface acoustic wave excited by the IDT 20 is faster than the speed of sound of the bulk wave (for example, the slowest transverse wave bulk wave) propagating in the piezoelectric substrate 10, the surface acoustic wave radiates the bulk wave and radiates the surface of the piezoelectric substrate 10. Propagate. Therefore, a loss occurs. In particular, the speed of sound of SH (Shear Horizontal) waves, which are a type of surface acoustic waves, is faster than the speed of sound of bulk waves. Therefore, the loss becomes large in the elastic wave resonator whose main mode is the SH wave. For example, in a Y-cut X-propagated lithium tantalate substrate having a cut angle of 20 ° or more and 48 ° or less, the SH wave is the main mode.

In order to slow down the speed of sound of surface acoustic waves, a metal having a large acoustic impedance is used for the metal layer 12. The acoustic impedance Z is expressed by the following equation, where ρ is the density, E is the Young's modulus, and Pr is the Poisson's ratio.

Since the Poisson's ratio does not differ greatly between metal materials, a metal with a large acoustic impedance is a metal with a large density x Young's modulus. The density is large for metals with a large atomic number, and the Young's modulus is large for hard metals. Such a metal is a high melting point metal having a high melting point. As described above, when the refractory metal is used for the metal layer 12, the sound velocity of the surface acoustic wave becomes slow and the loss becomes small.

Further, the refractory metal has a large number of electrons and a small atomic radius, so that the metal bond becomes strong. Electromigration and stress migration are phenomena in which metal atoms move due to electric fields and stress, respectively. Therefore, these migrations are unlikely to occur in refractory metals with strong metal bonds. Therefore, when the refractory metal is used for the metal layer 12, the migration becomes small.

For example, aluminum, which is generally used as the metal layer 12, has a melting point of 660 ° C., and has a density, Young's modulus, Poisson's ratio, and acoustic impedance of 2.7 g / cm ³ , 68 GPa, 0.34, and 8.3 GPa · s, respectively. / M. Molybdenum, a refractory metal, has a melting point of 2622 ° C. and a density, Young's modulus, Poisson's ratio and acoustic impedance of 10.2 g / cm ³ , 329 GPa, 0.31 and 35.9 GPa · s / m, respectively. As described above, molybdenum has a melting point 2000 ° C. higher than that of aluminum, a density of about 4 times, a Young's modulus of about 5 times, and an acoustic impedance of about 4 times.

Table 1 is a table showing the melting point, density, resistivity, Young's modulus, and tensile fracture strength of metals. As for the specific resistance, the bulk specific resistance described in the literature and the like and the specific resistance measured by forming a thin film are described.

As shown in Table 1, the densities of molybdenum (Mo), tungsten (W), ruthenium (Ru), rhodium (Rh), iridium (Ir) and platinum (Pt) having a melting point equal to or higher than the melting point of platinum (Pt) × Young's modulus is greater than the density x Young's modulus of gold (Au) and silver (Ag) having a melting point lower than that of platinum.

As described above, a refractory metal having a melting point of platinum or higher has a high density and a high acoustic impedance. Therefore, by forming these metals into the metal layer 12, the speed of sound of surface acoustic waves becomes slower, and loss can be suppressed. Moreover, since the melting point is high, migration is less likely to occur.

As the metal layer 12 used for the electrode finger 14, a metal having a low resistivity, low cost, and easy processing is preferable. Tungsten has a lower bulk resistivity than molybdenum, but when formed by a sputtering method or the like, the resistivity is higher than that of molybdenum. Further, tungsten is more difficult to etch when forming the electrode finger 14 than molybdenum. Ruthenium and platinum have high resistivity. Rhodium, iridium and platinum are expensive. The metal of the metal layer 12 is determined in consideration of these. For example, the main component of the metal layer 12 is molybdenum.

When a high-power high-frequency signal (for example, 45 MHz to 2 GHz) is applied to the IDT 20, the electrode finger 14 vibrates and stress is applied to the electrode finger 14. For example, when a molybdenum layer having a thickness of 485 nm is used as the metal layer 12 and a high frequency power of 1 W is applied to the IDT 20, a stress of 1500 MPa is applied to the electrode finger 14. As a result, the electrode finger 14 is peeled off from the piezoelectric substrate 10 and / or the electrode finger is deformed such as refraction. Therefore, the withstand power performance is lowered.

According to the first embodiment, the metal layer 12a (first metal layer) provided on the piezoelectric substrate 10 contains a metal M1 (first metal) having a melting point equal to or higher than the melting point of platinum as a main component. As a result, loss can be suppressed and migration can be suppressed. However, the electrode finger 14 is peeled off and / or deformed, and the power resistance is lowered. Therefore, no other layer (for example, a metal layer or an insulating layer) is provided between the metal layer 12a, and the metal layer 12b is provided so as to be in contact with the entire upper surface of the metal layer 12a. The metal layer 12b (second metal layer) contains a metal M2 (second metal) having a tensile strength higher than the tensile strength of the metal M1 as a main component.

As a result, the metal layer 12b reinforces the metal layer 12a, so that peeling and / or deformation of the metal layer 12a can be suppressed. The tensile strength of the metal M2 is preferably 1.5 times or more, more preferably 2 times or more the tensile strength of the metal M1. Further, when the tensile strength of the metal M1 is smaller than 1500 MPa, the tensile strength of the metal M2 is preferably 1500 MPa or more in order to make it larger than the stress applied to the electrode finger 14.

The specific resistance of the metal layer 12a is lower than the specific resistance of the metal layer 12b. If a metal layer having a small specific resistance is selected as the metal layer 12a, the tensile strength may be low. In such a case, it is preferable that the tensile strength of the metal M2, which is the main component of the metal layer 12b, is higher than the tensile strength of the metal M1. The specific resistance of the metal M1 is preferably 0.9 times or less, more preferably 0.8 times or less the specific resistance of the metal M2.

From the viewpoint of reinforcing the metal layer 12a with the metal layer 12b, the Young's modulus of the metal M2 is preferably higher than the Young's modulus of the metal M1. The Young's modulus of the metal M2 is preferably 1.1 times or more, more preferably 1.2 times or more, the Young's modulus of the metal M1.

The density of the metal layer 12b is higher than the density of the metal layer 12a. As a result, the acoustic impedance of the metal layer 12b can be made larger than the acoustic impedance of the metal layer 12a. Therefore, the electrode finger 14 can be made thin. The density of the metal layer 12b is preferably 1.1 times or more, more preferably 1.2 times or more the density of the metal layer 12a.

The metal layer 12a is thicker than the metal layer 12b. As a result, the characteristics (for example, resistance) required for the electrode finger 14 are almost determined by the metal layer 12a, and the metal layer 12b can be used for reinforcement. The thickness of the metal layer 12a is preferably 1.5 times or more, more preferably 2 times or more the thickness of the metal layer 12b. Since the metal layer 12b functions as a reinforcement, the thickness of the metal layer 12a is preferably 10 times or less, more preferably 5 times or less the thickness of the metal layer 12b.

As shown in Table 1, molybdenum is preferable as the metal M1 in consideration of acoustic impedance, actual resistivity and price. At this time, tungsten is preferable as the metal M2 in consideration of tensile strength. If a molybdenum layer that is easy to etch is used for the metal layer 12a and a tungsten layer that is difficult to etch is used for the metal layer 12b, the tungsten layer can be made thin. Therefore, the etching of the metal layer 12 becomes easier as compared with the case where the entire metal layer 12 is made into a tungsten layer.

The metal layer 12b is provided on the metal layer 12a. At this time, the metal layer 12b reinforces the metal layer 12a when the electrode finger 14 vibrates in either the Z direction or the X direction. When the vibration direction of the electrode finger 14 is the Z direction, the metal layer 12b reinforces the metal layer 12a, and the deterioration of the electrode finger 14 can be further suppressed.

When the metal layer 12a contains any one of molybdenum, ruthenium, rhodium and iridium as a main component in consideration of characteristics and the like, the metal layer 12b preferably contains tungsten as a main component. As a result, the metal layer 12b reinforces the metal layer 12a, and deterioration of the electrode finger 14 can be further suppressed.

The fact that the metal layers 12a and 12b contain the metals M1 and M2 as main components, respectively, means that the metals M1 and M2 are contained to the extent that the effects of the present application are exhibited, respectively, and impurities contained intentionally or unintentionally are contained. The atomic concentrations of the metals M1 and M2 in the metal layers 12a and 12b are, for example, 50% or more, for example 80% or more, and for example 90% or more.

[Modification 1 of Example 1]
FIG. 3A is an enlarged cross-sectional view of the elastic wave device according to the first modification of the first embodiment. As shown in FIG. 3A, in addition to the upper surface of the metal layer 12a, a metal layer 12b is provided between the metal layer 12a and the piezoelectric substrate 10. In this way, a plurality of metal layers 12b may be provided. The metals M2, which are the main components of the plurality of metal layers 12b, may be the same as or different from each other. By sandwiching the metal layer 12a between the metal layers 12b, the metal layer 12b reinforces the metal layer 12a, and deterioration of the electrode fingers 14 can be further suppressed. By sandwiching the metal layer 12a from the Z direction by the metal layer 12b, deterioration of the electrode finger 14 due to vibration of the electrode finger 14 in the Z direction can be particularly suppressed. Other configurations are the same as those in the first embodiment, and the description thereof will be omitted.

A plurality of metal layers 12b may be provided in the Z direction as in the first modification of the first embodiment. Further, a plurality of metal layers 12a may be provided in the Z direction. Each of the metal layers 12b is preferably thinner than the metal layer 12a, and the total thickness of the metal layers 12b is preferably smaller than the total thickness of the metal layers 12a.

[Modification 2 of Example 1]
FIG. 3B is an enlarged cross-sectional view of the elastic wave device according to the second modification of the first embodiment. As shown in FIG. 3B, in addition to the upper surface of the metal layer 12a, the metal layer 12b is provided on the side surface of the metal layer 12a. In this way, the metal layer 12b may be provided on the side surface of the metal layer 12a. The metal M2, which is the main component of the metal layer 12b provided on the upper surface and the side surface of the metal layer 12a, may be the same as or different from each other. The metal layer 12b may be provided on at least one of the two side surfaces. By providing the metal layer 12b on the side surface of the metal layer 12a, when the vibration direction of the electrode finger 14 is in the X direction, the metal layer 12b reinforces the metal layer 12a, and deterioration of the electrode finger 14 can be further suppressed. By sandwiching the metal layer 12a from the X direction, the metal layer 12b reinforces the metal layer 12a, and deterioration of the electrode finger 14 can be further suppressed. In particular, deterioration of the electrode finger 14 can be suppressed when the electrode finger 14 vibrates in the X direction. Other configurations are the same as those in the first embodiment, and the description thereof will be omitted.

As in the first and second modifications of the first embodiment, the metal layer 12b may be provided on any surface of the metal layer 12a.

"Modification 3 of Example 1]
FIG. 3C is an enlarged cross-sectional view of the elastic wave device according to the third modification of the first embodiment. As shown in FIG. 3C, the piezoelectric substrate 10 is joined on the support substrate 11. The support substrate 11 is, for example, a sapphire substrate, an alumina substrate, a spinel substrate, a quartz substrate, a crystal substrate, or a silicon substrate. The coefficient of linear expansion of the support substrate 11 is smaller than the coefficient of linear expansion of the piezoelectric substrate 10. This makes it possible to reduce the temperature coefficient of frequency of the elastic wave device. An intermediate layer such as a silicon oxide layer may be provided between the piezoelectric substrate 10 and the support substrate 11. Other configurations are the same as those in the first embodiment, and the description thereof will be omitted.

The piezoelectric substrate 10 may be provided directly or indirectly on the support substrate 11 in the first embodiment and the first and second modifications thereof, as in the third modification of the first embodiment.

A metal film thinner than the electrode finger 14, for example, 1/5 times or less the thickness of the electrode finger 14, is provided on at least one of the electrode finger 14 and between the electrode finger 14 and the piezoelectric substrate 10. You may be. This metal film is, for example, a chromium (Cr) film, a Ni (nickel) film, and a Ti (titanium) film, and functions as an adhesion layer, an etching stop layer in a manufacturing process, and / or a layer that suppresses migration.

Example 2 is an example of a filter and a duplexer using an elastic wave resonator of Example 1 and its modifications. FIG. 4A is a circuit diagram of the filter according to the second embodiment. As shown in FIG. 4A, 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 P4 are connected in parallel between the input terminal Tin and the output terminal Tout. The elastic wave resonators of Example 1 and its modifications can be used for at least one resonator of one or more series resonators S1 to S4 and one or more parallel resonators P1 to P4. Although the ladder type filter has been described as an example of the filter, the filter may be a multiple mode type filter.

FIG. 4B is a circuit diagram of the duplexer according to the first modification of the second embodiment. As shown in FIG. 4B, a transmission filter 40 is connected between the common terminal Ant and the transmission terminal Tx. A reception filter 42 is connected between the common terminal Ant and the reception terminal Rx. The transmission filter 40 passes a signal in the transmission band among the high-frequency signals input from the transmission terminal Tx to the common terminal Ant as a transmission signal, and suppresses signals of other frequencies. The reception filter 42 passes a signal in the reception band among the high frequency signals input from the common terminal Ant to the reception terminal Rx as a reception signal, and suppresses signals of other frequencies. At least one of the transmission filter 40 and the reception filter 42 can be the filter of the second embodiment.

Although the duplexer has been described as an example as the multiplexer, a triplexer or a quadplexer may be used.

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

10 Piezoelectric board 11 Support board 12, 12a, 12b Metal layer 14 Electrode finger 18 Comb type electrode 20 IDT
22 Reflector 24 Elastic wave resonator 40 Transmit filter 42 Receive filter

Claims (12)

Piezoelectric board and
A first metal layer provided on the piezoelectric substrate and containing a first metal having a melting point equal to or higher than the melting point of platinum as a main component and a second metal having a tensile strength higher than the tensile strength of the first metal as main components. A plurality of electrode fingers each comprising a second metal layer in contact with the entire upper surface of the first metal layer, and
An elastic wave device equipped with. The elastic wave device according to claim 1, wherein the specific resistance of the first metal layer is lower than the specific resistance of the second metal layer. The elastic wave device according to claim 1 or 2, wherein the second metal has a Young's modulus higher than the Young's modulus of the first metal. The elastic wave device according to any one of claims 1 to 3, wherein the density of the second metal layer is higher than the density of the first metal layer. The elastic wave device according to any one of claims 1 to 4, wherein the first metal layer is thicker than the second metal layer. The elastic wave device according to any one of claims 1 to 5, wherein the second metal layer is provided on the first metal layer. The elastic wave device according to any one of claims 1 to 6, wherein the second metal layer is provided on a side surface of the first metal layer. The elastic wave device according to any one of claims 1 to 7, wherein the first metal is molybdenum. The elastic wave device according to claim 8, wherein the second metal is tungsten. Piezoelectric board and
A first metal layer provided on the piezoelectric substrate and containing any one of molybdenum, ruthenium, rhodium and iridium as a main component, and a second metal layer containing tungsten as a main component and in contact with the entire upper surface of the first metal layer. A plurality of electrode fingers, each comprising a metal layer,
An elastic wave device equipped with. A filter comprising the elastic wave device according to any one of claims 1 to 10. A multiplexer containing the filter according to claim 11.

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