ACOUSTIC WAVE DEVICE

An acoustic wave device includes an IDT electrode on a piezoelectric layer. The IDT electrode includes first and second electrode fingers made of an alloy film including Al and at least one of Cu, Mg, Ag, or Nd, and an overlap region in which the first and second electrode fingers overlap when viewed in the direction of propagation of acoustic waves. The overlap region includes a central region and first and second edge regions outside the central region on opposite sides in the direction in which the first and second electrode fingers extend. In at least one of the first and second electrode fingers, a concentration of the at least one of Cu, Mg, Ag, or Nd in at least a portion of the first and second edge regions is higher than that in the central region.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to Japanese Patent Application No. 2020-006852 filed on Jan. 20, 2020 and is a Continuation Application of PCT Application No. PCT/JP2020/049270 filed on Dec. 29, 2020. The entire contents of each application are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an acoustic wave device including an IDT electrode made of an Al-based alloy film including at least one of Cu, Mg, Ag, or Nd.

2. Description of the Related Art

Japanese Unexamined Patent Application Publication No. 2019-102896 discloses an acoustic wave device whose IDT electrode is made of an Al-based alloy film. The alloy film is one made of an alloy of Al, Cu, and element(s) like Mg or Nd.

Such an Al-based alloy IDT electrode is disadvantageous in that its electric power handling capability is low when the concentrations of the ingredients of Cu and the element(s) such as Mg or Nd is low. Increasing the concentrations of Cu and the element(s) such as Mg or Nd helps enhance the electric power handling capability, but on the other hand, increases a loss of resistance at the electrode fingers of the IDT electrode, creating the disadvantage of increased loss in the acoustic wave device.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide acoustic wave devices each having an improved electric power handling capability and a limited increase in loss.

An acoustic wave device according to a preferred embodiment of the present invention includes a piezoelectric layer and an IDT electrode on the piezoelectric layer, the IDT electrode including multiple electrode fingers. The multiple electrode fingers are made of an alloy film including Al and at least one of Cu, Mg, Ag, or Nd. The multiple electrode fingers include interdigitated first and second electrode fingers, and an overlap region in which the first and second electrode fingers overlap when viewed in a direction of propagation of an acoustic wave. The overlap region includes a central region in a middle in a direction in which the electrode fingers extend, and first and second edge regions outside the central region on opposite sides in the direction in which the electrode fingers extend. In at least one of the multiple electrode fingers, a concentration of the at least one of Cu, Mg, Ag, or Nd in at least a portion of the first and second edge regions is higher than a concentration of the at least one of Cu, Mg, Ag, or Nd in the central region.

The acoustic wave devices according to preferred embodiments of the present invention each have an improved electric power handling capability and a limited increase in loss therein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following describes specific preferred embodiments of the present invention with reference to drawings to make the present invention clearly understood.

The preferred embodiments described herein are illustrative, and between different preferred embodiments, partial replacement and/or combination of elements, configurations, and arrangements are allowed.

FIG. 1Ais a front cross-sectional diagram for describing an acoustic wave device according to a preferred embodiment of the present invention, andFIG. 1Bis a plan view of its electrode structure.

The acoustic wave device1includes a piezoelectric substrate2. The piezoelectric substrate2includes a support substrate3, a layer of a high-acoustic-velocity material4on the support substrate3, a low-acoustic-velocity film5on the layer of a high-acoustic-velocity material4, and a piezoelectric layer6on the low-acoustic-velocity film5.

The piezoelectric layer6is made of, for example, a piezoelectric single crystal, such as lithium tantalate or lithium niobate. On the piezoelectric layer6are an IDT electrode8and reflectors9and10. The acoustic wave device1is, for example, a one-port acoustic wave resonator, including reflectors9and10on opposite sides of the IDT electrode8. In preferred embodiments of the present invention, however, the electrode structure of the acoustic wave device is not necessarily an acoustic wave resonator. The device may be, for example, an acoustic wave filter, which includes multiple IDT electrodes.

A dielectric film11covers the IDT electrode8and the reflectors9and10. In the present preferred embodiment, the dielectric film11is made of silicon oxide, for example. The dielectric film11is optional, but improves moisture resistance and protects the electrode structure. The frequency characteristics, furthermore, can be tuned by customizing the thickness of the dielectric film11. If the dielectric film11is made of, for example, silicon oxide or any similar material, it is also possible to adjust temperature characteristics therewith.

The IDT electrode8includes multiple first electrode fingers12and multiple second electrode fingers13. One end of the multiple first electrode fingers12is coupled to a first busbar14. One end of the multiple second electrode fingers13is coupled to a second busbar15. The multiple first electrode fingers12and the multiple second electrode fingers13are interdigitated.

The IDT electrode8propagates acoustic waves in the direction perpendicular or substantially perpendicular to that in which the first and second electrode fingers12and13extend. The region in which the first and second electrode fingers12and13overlap when viewed in this direction of propagation of acoustic waves is the overlap region K, and the length of this overlap region K in the direction in which the first and second electrode fingers12and13extend is the overlap width.

The overlap region K includes a central region C, which is in the middle in the direction in which the first and second electrode fingers12and13extend, and first and second edge regions E1and E2, which are outside the central region C on opposite sides in the direction in which the first and second electrode fingers12and13extend. Within the overlap region K, the first edge region E1is on the side on which the first electrode fingers12include their distal end. The second edge region E2is on the side on which the second electrode fingers13include their distal end. The width of the first and second edge regions E1and E2(length in the direction in which the first and second electrode fingers extend) is equal to or smaller than about2X, where X is the wavelength, determined by the finger pitch of the IDT electrode8.

In the first and second edge regions E1and E2, first and second mass-adding films16and17are respectively provided. In the present preferred embodiment, the first and second mass-adding films16and17are made of, for example, a dielectric material.

The first mass-adding film16, in the first edge region E1, extends from the end of the IDT electrode8closer to the reflector9to that closer to the reflector10for example. The second mass-adding film17extends, for example, from the end of the IDT electrode8closer to the reflector9to that closer to the reflector10. In other words, both of the first and second electrode fingers12and13include first and second mass-adding films16and17in the first and second edge regions E1and E2. It is, however, sufficient that at least one of the multiple electrode fingers includes a first or second mass-adding film16or17in the first or second edge region E1or E2.

The first and second mass-adding films16and17, furthermore, are provided the IDT electrode8and the piezoelectric layer6. That is, the device includes first and second electrode fingers12and13of the IDT electrode8such that they cover a portion of first and second mass-adding films16and17that have already been formed.

The first and second mass-adding films16and17in the present preferred embodiment are beneath the first and second electrode fingers12and13, but they may be above the first and second electrode fingers12and13. That is, the first and second mass-adding films16and17may be on top of the first and second electrode fingers12and13.

The first and second mass-adding films16and17are made of a dielectric material, for example, silicon oxide in the present preferred embodiment. Silicon oxide, however, is not the only dielectric material that can be used. Any suitable insulator can be used, such as, for example, niobium pentoxide, tungsten oxide, tantalum pentoxide, or hafnium oxide.

Alternatively, the first and second mass-adding films16and17may be made of an electrically conductive material, such as metal, for example. In that case, the first and second mass-adding films16and17are arranged so as not to extend to the regions between the first and second electrode fingers12and13. In other words, the first and second mass-adding films16and17are only positioned right above or right below the first and second electrode fingers12and13in the first and second edge regions E1and E2.

A feature of the acoustic wave device1is the material used for the IDT electrode8. Specifically, the first and second electrode fingers12and13of the IDT electrode8are made of an alloy film including Al and at least one of Cu, Mg, Ag, or Nd. In at least one of the multiple first and second electrode fingers12and13, furthermore, the concentration of the at least one of Cu, Mg, Ag, or Nd in at least a portion of the first and second edge regions E1and E2is higher than that in the central region C. This enables an increase in the electric power handling capability of the IDT electrode8with a limited increase in loss. Preferably, the central region C of the multiple first and second electrode fingers12and13is an epitaxial film. This helps further improve the electric power handling capability.

For the acoustic wave device1according to the present preferred embodiment, the first and second electrode fingers12and13include a higher concentration of Cu in their first and second edge regions E1and E2than in their central region C. In the first and second edge regions E1and E2, in which the device includes the first and second mass-adding films16and17, none or a small portion of the first and second electrode fingers12and is epitaxially grown. The high Cu concentration at non-epitaxially grown distal ends of the first and second electrode fingers12and13helps improve the electric power handling capability. The inventors of preferred embodiments of the present invention investigated cases of reduced electric power handling capability, and concluded that the cause is a relatively low concentration of Cu, for example, at the distal ends of the first and second electrode fingers12and13due to the absence of epitaxial growth there. As a solution to this, in preferred embodiments of the present invention, the concentration of Cu, for example, in the first and second edge regions E1and E2is higher than that in the central region C. This improves the electric power handling capability, and the associated increase in loss is limited due to a low concentration of Cu, for example, in the central region C.

The interfaces between the central region C and the first and second edge regions E1and E2are included in the first and second edge regions E1and E2. Preferred embodiments of the present invention are advantageous if the concentration of Cu, Mg, Ag, or Nd at the interfaces is higher than that in the central region C.

In preferred embodiments of the present invention, furthermore, the concentration of Cu, Mg, Ag, or Nd does not need to be higher in the entire edge regions than in the central region C. It only needs to be higher in at least a portion of the edge regions than in the central region C. Even this is sufficient for the electric power handling capability to be improved with a limited increase in loss in the acoustic wave device.

Preferably, in at least one of the multiple first and second electrode fingers12and13, the concentration of the ingredient of the at least one of Cu, Mg, Ag, or Nd in the central region C is about 10% by weight or less. This limits the increase in insertion loss more effectively. The following describes this with reference toFIG. 2.FIG. 2is a graphical representation of the relationship between the Cu content (% by weight) of an AlCu alloy in the central region C of the first and second electrode fingers12and13of the IDT electrode8and the specific resistance of the acoustic wave device.

The design parameters of the acoustic wave device1were as follows.

The support substrate3: Si

The layer of a high-acoustic-velocity material4: A SiN film, about 900 nm thick

The low-acoustic-velocity film5: A SiO2film, about 600 nm thick

The IDT electrode8and the reflectors9and10: The multilayer structure of a Ti film/an AlCu film/a Ti film. The thickness of the lower Ti film, about 4 nm; the thickness of the upper Ti film, about 12 nm. The thickness of the AlCu film, about 100 nm.

The first and second mass-adding films16and17: Dielectrics made of Ta2O5. Thickness about 30 nm.

The first and second electrode fingers12and13were observed by Electron BackScatter Diffraction Pattern (EBSP), finding that their central region C was an epitaxial film.

As for the first and second edge regions E1and E2, a portion of them was not an epitaxial film. At the distal ends of the first and second electrode fingers12and13, in particular, the electrode fingers were not epitaxially grown.

The composition of the first and second electrode fingers12and13in their central region C was primarily Al, with the percentage of Cu being about 1% by weight, about 7% by weight, about 8% by weight, about 10% by weight, about 12.5% by weight, about 15% by weight, about 17% by weight, about 20% by weight, about 26% by weight, or about 35% by weight assuming the amount of AlCu as a whole was 100% by weight.

An elemental analysis was done using energy-dispersive X-ray spectroscopy (EDX), revealing more Cu had precipitated in the first and second edge regions E1and E2than in the central region C. In other words, the concentration of Cu in the first and second edge regions E1and E2was higher than that in the central region C.

As clearly seen fromFIG. 2, the specific resistance increases, and the loss becomes greater accordingly, with increasing concentration of Cu in the central region C. Preferably, the Cu concentration is about 10% by weight or less, and more preferably about 5% by weight or less. This helps reduce the specific resistance significantly, thus limiting the increase in loss more effectively.

The alloy film of the first and second electrode fingers12and13in the above preferred embodiment is an Al-based one including Cu, but it may be an Al-based alloy film including at least one of Cu, Mg, Ag, or Nd.

In preferred embodiments of the present invention, therefore, the alloy film may have a composition that is primarily Al and includes Mg, may have a composition that is primarily Al and includes Ag, or may have a composition that is primarily Al and includes Nd. It may be an alloy film including two or more elements of Cu, Mg, Ag, and Nd. If the alloy film is primarily Al and includes two or more elements of Cu, Mg, Ag, and Nd, it is sufficient that the concentration of at least one of the two or more elements in the first and second edge regions E1and E2is higher than that in the central region C.

In addition, with the acoustic wave device1including the aforementioned first and second mass-adding films16and17, the acoustic velocity in the first and second edge regions E1and E2is lower than that in the central region C. Outside the first and second edge regions E1and E2on opposite sides in the direction in which the first and second electrode fingers12and13extend, there are first and second gap regions G1and G2. The acoustic velocity in the first and second gap regions G1and G2is higher than that in the first and second edge regions E1and E2. For the acoustic wave device1, therefore, ripples caused by the transverse mode are effectively reduced or prevented due to differences in acoustic velocity.

However, this lower acoustic velocity in the first and second edge regions E1and E2than in the central region C is not required.

The piezoelectric substrate2of the acoustic wave device1is a multilayer substrate including a support substrate3, a layer of a high-acoustic-velocity material4, a low-acoustic-velocity film5, and a piezoelectric layer6. The material for the support substrate3in the multilayer substrate does not need to Si, and may be a non-Si semiconductor or an insulator, such as alumina, for example. The layer of a high-acoustic-velocity material4is made of, for example, silicon nitride, but any suitable high-acoustic-velocity material can be used for the layer of a high-acoustic-velocity material4. The term high-acoustic-velocity material refers to a material through which bulk waves propagate faster than acoustic waves propagate through the piezoelectric layer6. A wide variety of high-acoustic-velocity materials can be used, such as, for example, aluminum oxide, silicon carbide, silicon nitride, silicon oxynitride, silicon, sapphire, lithium tantalate, lithium niobate, quartz, alumina, zirconia, cordierite, mullite, steatite, forsterite, magnesia, a DLC (diamond-like carbon) film, or diamond, a medium that is primarily any of these materials, or a medium that is primarily a mixture of these materials. The low-acoustic-velocity film5is made of a low-acoustic-velocity material. The term low-acoustic-velocity material refers to a material through which bulk waves propagate more slowly than through the piezoelectric layer6. A wide variety of low-acoustic-velocity materials can be used, such as, for example, silicon oxide, glass, silicon oxynitride, tantalum oxide, a compound derived by adding fluorine, carbon, boron, hydrogen, or silanol groups to silicon oxide, or a medium that is primarily any of these materials.

The piezoelectric layer6may be directly on the layer of a high-acoustic-velocity material.

Furthermore, the low-acoustic-velocity film5, which is a component of the piezoelectric substrate2, is optional. The support substrate3, on which the layer of a high-acoustic-velocity material4is disposed, may be a high-acoustic-velocity support substrate, made of a high-acoustic-velocity material. In that case, the layer of a high-acoustic-velocity material4is the high-acoustic-velocity support substrate.

The piezoelectric substrate, furthermore, does not need to be structured like the piezoelectric substrate2illustrated inFIGS. 1A and 1B. For example, it may have a structure in which a piezoelectric layer is on an alumina or similar support substrate, or the entire piezoelectric substrate2may be a piezoelectric single-crystal substrate or similar piezoelectric substrate.