WO2024101338A1

WO2024101338A1 - Elastic wave resonator and communication device

Info

Publication number: WO2024101338A1
Application number: PCT/JP2023/039987
Authority: WO
Inventors: 惣一朗野添
Original assignee: 京セラ株式会社
Priority date: 2022-11-08
Filing date: 2023-11-07
Publication date: 2024-05-16

Abstract

In the present invention, an elastic wave resonator comprises a piezoelectric layer, an IDT electrode, and an acoustic reflection layer. The piezoelectric layer has piezoelectric properties, and has a first surface as well as a second surface that is on the opposite side from the first surface. The IDT electrode is positioned on the first surface, and has a plurality of electrode fingers. The acoustic reflection layer is positioned on the second surface, and includes a low-acoustic-impedance layer that has a lower acoustic impedance than the piezoelectric layer. The elastic wave resonator is such that 0.14≤(Ra1/T)×100, where Ra1 is the arithmetic mean roughness of at least a portion of the surfaces of the plurality of electrode fingers as seen in plan view is represented, and T is the thickness of the plurality of electrode fingers.

Description

Elastic wave resonator and communication device

The present invention relates to an elastic wave resonator, which is an electronic component that utilizes elastic waves, and a communication device that includes the elastic wave resonator.

Patent Document 1 below discloses an elastic wave device that includes a support substrate, an acoustic reflection layer formed on the support substrate, a piezoelectric layer formed on the acoustic reflection layer, and an IDT (interdigital transducer) electrode formed on the upper or lower surface of the piezoelectric layer. The acoustic reflection layer has a low acoustic impedance layer and a high acoustic impedance layer that has a higher acoustic impedance than the low acoustic impedance layer. With this configuration, plate waves propagating in the piezoelectric layer are reflected by the acoustic reflection layer, and the elastic wave energy is trapped in the piezoelectric layer, allowing the plate waves to propagate through the piezoelectric layer with high energy intensity.

Patent No. 5648695

An elastic wave resonator according to one embodiment of the present invention comprises a piezoelectric layer, an IDT electrode, and an acoustic reflection layer. The piezoelectric layer has piezoelectricity and has a first surface and a second surface opposite to the first surface. The IDT electrode is located on the first surface and has a plurality of electrode fingers. The acoustic reflection layer is located on the second surface side and includes a low acoustic impedance layer having a lower acoustic impedance than the piezoelectric layer. The arithmetic mean roughness of at least a portion of the surface of the plurality of electrode fingers in a planar view is Ra1, and the thickness of the plurality of electrode fingers is T, both of which have the same units. In this case, the elastic wave resonator satisfies the relationship 0.14≦(Ra1/T)×100.

Instead of or in addition to 0.14≦(Ra1/T)×100, Ra1 may be 0.20 nm or more.

A communication device according to one embodiment of the present invention has an antenna, an acoustic wave filter connected to the antenna, and an IC connected to the acoustic wave filter. The acoustic wave filter includes the acoustic wave resonator described above.

1 is a schematic cross-sectional view of an elastic wave resonator according to one embodiment of the present invention. 1 is a schematic plan view of an elastic wave resonator according to a preferred embodiment of the present invention; 1 is a schematic cross-sectional view of an elastic wave resonator according to one embodiment of the present invention. 4A, 4B, 4C, 4D, and 4E are diagrams showing simulation results of resonance characteristics of an elastic wave resonator according to one embodiment of the present invention and a comparative example. 13 is a diagram showing a change in average |S| when the arithmetic mean roughness of the surface of the electrode finger is changed. FIG. 13 is a diagram showing changes in the phase of spurious signals located on the higher frequency side than the main resonance when the arithmetic mean roughness of the electrode finger surfaces is changed relative to the thickness of the electrode fingers. 7A and 7B are schematic cross-sectional views of an elastic wave resonator according to another preferred embodiment of the present invention. FIG. 11 is a schematic cross-sectional view of an elastic wave resonator according to another embodiment of the present invention. FIG. 11 is a schematic cross-sectional view of an elastic wave resonator according to another embodiment of the present invention. FIG. 11 is a schematic cross-sectional view of an elastic wave resonator according to another embodiment of the present invention. 11A and 11B are schematic plan views of an elastic wave resonator according to another preferred embodiment of the present invention. 1 is a diagram illustrating a duplexer as an example of a use of an elastic wave resonator according to an embodiment of the present invention. 1 is a block diagram showing a configuration of a main part of a communication device as an example of a use of an elastic wave resonator according to an embodiment of the present invention.

Below, an elastic wave resonator and a communication device according to one embodiment of the present invention will be described with reference to the drawings. Note that the figures used in the following description are schematic diagrams, and the dimensional ratios and the like in the drawings do not necessarily match those of the actual elastic wave resonator and communication device.

For convenience, the drawings may be accompanied by an orthogonal coordinate system consisting of an X-axis, a Y-axis, and a Z-axis. In the elastic wave resonator 1 according to one embodiment of the present invention, either direction may be considered to be upward or downward. However, for convenience, the terms upper surface and lower surface may be used with the Z-axis direction being the up-down direction. Note that the X-axis is defined to be parallel to the arrangement direction of the electrode fingers 412 described below (which in the embodiment is also the propagation direction of the elastic wave used as the main resonance among the elastic waves propagating through the piezoelectric layer 2 described below), the Y-axis is defined to be parallel to the top surface of the piezoelectric layer 2 and perpendicular to the X-axis, and the Z-axis is defined to be perpendicular to the top surface of the piezoelectric layer 2.

Note that each embodiment described in this specification is merely illustrative, and different embodiments may be partially substituted with each other. Different embodiments may also be partially combined.

FIG. 1 is a schematic cross-sectional view of an elastic wave resonator 1 according to one embodiment of the present invention. As shown in FIG. 1, the elastic wave resonator 1 according to one embodiment of the present invention has a piezoelectric layer 2, an acoustic reflection layer 5, a support substrate 3, and an IDT electrode 4.

The piezoelectric layer 2 has an upper surface 2a and a lower surface 2b perpendicular to the Z axis, with the Z axis being the up-down direction. An acoustic reflection layer 5 and a support substrate 3, which will be described later, are located on the lower surface 2b side of the piezoelectric layer 2. An IDT electrode 4, which will be described later, is located on the upper surface 2a side of the piezoelectric layer 2.

Various materials having piezoelectricity can be used for the piezoelectric layer 2. Examples of materials having piezoelectricity include single crystals of lithium tantalate (LiTaO ₃ ; hereinafter referred to as LT) and single crystals of lithium niobate (LiNbO ₃ ; hereinafter referred to as LN). Specifically, in this embodiment, the piezoelectric layer 2 is made of single crystals of LT.

The piezoelectric layer 2 has piezoelectric properties, and when a high-frequency signal is applied to the IDT electrode 4 described later, an elastic wave propagating through the piezoelectric layer 2 is excited. The type of elastic wave to be used as the main resonance among the excited elastic waves may be set according to the desired frequency characteristics, etc. For example, the elastic wave used as the main resonance may be a plate wave or a bulk wave. Examples of plate waves include Lamb waves and SH waves. Examples of bulk waves include thickness shear waves. The elastic wave used as the main resonance may be a mixture of multiple types of elastic waves. Specifically, in the elastic wave resonator 1 of the embodiment, the elastic wave used as the main resonance includes a Lamb wave. In the description of the embodiment, for convenience, the description may be given on the assumption that the elastic wave is a plate wave (or even a Lamb wave) without any particular notice.

Just to be clear, the Lamb wave mainly has a component (P component) in the arrangement direction (X direction, propagation direction) of the electrode fingers 412 described later and/or a component (SV component) in the thickness direction of the piezoelectric layer 2. The A1 mode described later refers to a first-order asymmetric mode (A mode) (one node in the thickness direction). The SH wave mainly has a component (SH component) perpendicular to the arrangement direction (propagation direction) of the electrode fingers 412 and parallel to the surface of the piezoelectric layer 2. The thickness shear wave slides different parts (e.g., the upper and lower surfaces) in the thickness direction of the piezoelectric layer 2 in a direction along the upper and lower surfaces of the piezoelectric layer 2, and also propagates in the thickness direction of the piezoelectric layer 2. The order of the thickness shear wave is arbitrary, and is, for example, first-order (one node in the thickness direction).

The primary resonance refers to the resonance with the smallest minimum impedance value (or, from another point of view, the impedance at the resonant frequency) among multiple resonances that occur in the elastic wave resonator 1 and have different resonant frequencies. For convenience, the elastic wave (its main component) that causes the primary resonance or the frequency of the primary resonance may be abbreviated to primary resonance. For example, using a plate wave as the main component of the elastic wave that causes the primary resonance (for example, a component that accounts for 50% or more or 80% or more of the energy of the elastic wave at the resonant frequency) may be referred to as using a plate wave as the primary resonance.

The thickness of the piezoelectric layer 2 may be designed appropriately depending on the required frequency characteristics and the type of elastic wave used as the main resonance. Specifically, in this embodiment, the thickness of the piezoelectric layer 2 is 2λ or less, expressed using the wavelength λ described below. By setting the thickness of the piezoelectric layer 2 to 2λ or less, for example, Lamb waves can be effectively used as the main resonance. Furthermore, the thickness of the piezoelectric layer 2 may be 1λ or less or 0.5λ or less. The lower limit of the piezoelectric layer 2 is not particularly limited, but may be, for example, 0.01λ or 0.1λ.

In addition, in a plan view, the thickness of the piezoelectric layer 2 in the area overlapping with the IDT electrode 4 described later may be constant. With this configuration, for example, when Lamb waves are used as the main resonance, the loss of Lamb wave resonance is reduced, and an elastic wave resonator 1 having good frequency characteristics can be provided. Note that the thickness of the piezoelectric layer 2 being constant does not necessarily mean that it is strictly constant, but rather allows some variation within a range that does not significantly affect the characteristics of the elastic waves propagating through the piezoelectric layer 2. For example, when the thickness of the piezoelectric layer 2 is constant, the difference between the thinnest part and the thickest part may be smaller (or larger) than the arithmetic mean roughness Ra1 of the IDT electrode 4 described later.

The propagation mode of the elastic wave used in the elastic wave resonator 1 according to one embodiment of the present invention is not particularly limited and may be set according to the desired frequency characteristics. The Euler angles (φ, θ, ψ) of the piezoelectric single crystal used as the piezoelectric layer 2 may be appropriately designed according to the type and propagation mode of the elastic wave used as the main resonance. For example, if the piezoelectric layer 2 is LT, the A1 mode of the Lamb wave can be effectively used as the main resonance by setting the Euler angles (φ, θ, ψ) to (0°±10°, 0° to 55°, 0°±10°). If the piezoelectric layer 2 is LN, the A1 mode of the Lamb wave can be effectively used as the main resonance by setting the Euler angles (φ, θ, ψ) to (0°±10°, 0° to 55°, 0°±10°). If the piezoelectric layer 2 is Z-cut LN or LT, the thickness shear wave can be effectively used as the main resonance. Specifically, in this embodiment, the propagation mode of the Lamb wave used as the main resonance is the A1 mode, and the Euler angles (φ, θ, ψ) of LT are (0°, 24°, 0°).

The support substrate 3 is located on the underside of the piezoelectric layer 2 and the acoustic reflection layer 5 described below, and supports the piezoelectric layer 2 and the acoustic reflection layer 5. The thickness of the support substrate 3 is not particularly limited, and for example, the thickness of the support substrate 3 may be thicker than the thickness of the piezoelectric layer 2.

The material of the support substrate 3 is not particularly limited. For example, the material of the support substrate 3 may be a material having a smaller linear expansion coefficient than that of the piezoelectric layer 2. By using such a material for the support substrate 3, it is possible to reduce deformation of the piezoelectric layer 2 due to temperature changes and reduce changes in the resonance characteristics of the elastic wave resonator 1 due to temperature changes. Examples of materials for such a support substrate 3 include sapphire (Al ₂ O ₃ ), silicon carbide (SiC), and silicon (Si). In this embodiment, specifically, the support substrate 3 is Si.

The acoustic reflection layer 5 is located between the piezoelectric layer 2 and the support substrate 3. The acoustic reflection layer 5 has at least a low acoustic impedance layer 51. The acoustic impedance of the low acoustic impedance layer 51 is lower than the acoustic impedance of the piezoelectric layer 2. In this way, the low acoustic impedance layer 51 is located on the lower surface 2b side of the piezoelectric layer 2, so that the elastic waves propagating through the piezoelectric layer 2 are reflected by the low acoustic impedance layer 51 and are confined in the piezoelectric layer 2, thereby reducing leakage of the elastic waves from the lower surface 2b side. As such a low acoustic impedance layer 51, for example, silicon oxide (SiO ₂ ) can be exemplified. In this embodiment, specifically, the low acoustic impedance layer 51 is SiO ₂ .

The thickness of the acoustic reflection layer 5 is not particularly limited. For example, the acoustic reflection layer 5 may be thinner, the same thickness as, or thicker than the piezoelectric layer 2. Also, for example, the acoustic reflection layer 5 may be thick enough to prevent most (e.g., 95% or more or 100%) of the energy of the elastic wave used as the main resonance from leaking to the support substrate 3, or may be thick enough to allow a certain amount of the energy to leak to the support substrate 3. For example, the thickness of the acoustic reflection layer 5 may be 0.01λ or more and 4λ or less.

The IDT electrode 4 is located on the upper surface 2a of the piezoelectric layer 2. The IDT electrode 4 is made of a conductive material. The IDT electrode 4 may be made of various conductive materials such as aluminum (Al), copper (Cu), platinum (Pt), molybdenum (Mo), gold (Au), or alloys thereof. The IDT electrode 4 may also be made by laminating multiple layers of the various conductive materials described above. When the IDT electrode 4 is made by laminating multiple layers, a base layer may be interposed at the interface between the layers. In this embodiment, specifically, the IDT electrode 4 is made of Al.

FIG. 2 shows a schematic diagram of the shape of the IDT electrode 4 when the elastic wave resonator 1 according to this embodiment is viewed in a plan view from the Z-axis direction. As shown in FIG. 2, the IDT electrode 4 includes a pair of comb-shaped electrodes 41 (41a and 41b).

The comb-shaped electrode 41 includes, for example, a pair of bus bars 411 and a number of electrode fingers 412 extending from the bus bars 411. The electrode fingers 412 are arranged such that the electrode finger 412a connected to one bus bar 411a and the electrode finger 412b connected to the other bus bar 411b interdigitate with each other. The comb-shaped electrode 41 may also include a number of dummy electrode fingers 413 (413a and 413b). The dummy electrode finger 413 protrudes from one bus bar 411 between each of the electrode fingers 412 and faces the electrode finger 412 extending from the other bus bar 411.

The length of the electrode fingers 412 in the Y-axis direction may be set appropriately depending on the required electrical characteristics, etc. For example, the lengths of the electrode fingers 412 in the Y-axis direction are equal to each other. The IDT electrode 4 may be apodized, in which the length of the electrode fingers 412 in the Y-axis direction (or, from another perspective, the cross width) changes depending on the position in the X-axis direction.

The repetition interval of the electrode fingers 412 is defined as pitch P, and the width of the electrode fingers 412 is defined as width W. The pitch P and width W are designed as appropriate according to the desired frequency characteristics. In FIG. 2, the repetition interval of the electrode fingers 412 is constant, but is not limited to this example. For example, the repetition interval of the electrode fingers 412 may be designed to gradually increase, or may be designed to have multiple types of repetition intervals in stages. When there are multiple repetition intervals of the electrode fingers 412, the average value may be defined as pitch P, or the largest pitch may be defined as pitch P.

The specific values of these dimensions are not limited. For example, the pitch P may be 0.5 μm or more and 8.0 μm or less, or 0.5 μm or more and 2.0 μm or less. W/P may be 0.2 or more and 0.6 or less, or 0.3 or more and 0.6 or less. The thickness T described below may be 0.005λ or more and 0.3λ or less, 0.03λ or more and 0.3λ or less, or 0.05λ or more and 0.15λ or less. And/or the thickness T may be 50 nm or more and 600 nm or less, or 100 nm or more and 300 nm or less. Note that these ranges include the conditions of the simulation described below.

When a high-frequency signal is applied to the IDT electrode 4, an elastic wave having a wavelength roughly equivalent to the wavelength λ defined as twice the pitch P of the electrode fingers 412 is excited and propagates through the piezoelectric layer 2. The resonant frequency fr of the elastic wave resonator 1 is roughly equivalent to the frequency of the elastic wave used as the main resonance among the excited elastic waves. The anti-resonant frequency fa is determined by the resonant frequency fr and the capacitance ratio, and the capacitance ratio is determined mainly by the piezoelectric layer 2 and is adjusted by the number of electrode fingers 412, the crossing width, or the film thickness, etc.

In addition, in a case where the elastic wave used as the main resonance is a Lamb wave (more generally, an elastic wave propagating in the arrangement direction of the electrode fingers 412), the actual wavelength of the Lamb wave and the resonant frequency fr of the elastic wave resonator 1 are relatively highly dependent on the pitch P. For example, the actual wavelength of the Lamb wave is relatively close to λ defined as 2P as described above, and the resonant frequency fr is relatively close to the frequency of the Lamb wave whose actual wavelength is the above λ (= 2P).

On the other hand, in a configuration in which the elastic wave used as the primary resonance is a thickness shear wave, the actual wavelength of the thickness shear wave and the resonant frequency fr of the elastic wave resonator 1 are relatively less dependent on the pitch P and relatively more dependent on the thickness of the piezoelectric layer 2. However, unless otherwise specified and unless a contradiction arises, the explanation of dimensions using the wavelength λ (= 2P) (for example, the explanation of the thickness of the piezoelectric layer 2 described above) may be directly applied to the configuration in which the elastic wave is a thickness shear wave.

In the elastic wave resonator 1 of this embodiment, the thickness of the multiple electrode fingers 412 is defined as thickness T. The thickness of the electrode fingers 412 refers to the distance from the end of the electrode fingers 412 on the piezoelectric layer 2 side in the Z-axis direction to the surface 71 described below. The thickness of the electrode fingers 412 does not need to be strictly constant; for example, the thickness may be measured at several points near the center of the electrode fingers 412 in the X-axis direction, and the average of these values may be defined as thickness T.

The elastic wave resonator 1 in this embodiment may further include (or may not include) a pair of reflectors 42 on the upper surface 2a of the piezoelectric layer 2. The pair of reflectors 42 are located on both sides of the comb-shaped electrode 41 in the X-axis direction. The reflector 42 includes a pair of reflector bus bars 421 facing each other, and a plurality of strip electrodes 422 extending between the pair of reflector bus bars 421.

FIG. 3 is a schematic diagram showing a cross section of an elastic wave resonator 1 according to this embodiment. In this embodiment, if the arithmetic mean roughness of a surface 71 of the multiple electrode fingers 412 of the IDT electrode 4 is defined as Ra1, Ra1 has a value greater than 0 nm. In other words, the surface 71 of the electrode fingers 412 has a predetermined roughness. In this specification, the arithmetic mean roughness refers to the arithmetic mean roughness (Ra) defined in JIS (Japanese Industrial Standards) B 0601:2013. The surface 71 refers to the surface of the electrode fingers 412 located opposite the piezoelectric layer 2.

In the elastic wave resonator 1 of this embodiment, the low acoustic impedance layer 51 located on the lower surface 2b side of the piezoelectric layer 2 confines not only the elastic waves used as the main resonance but also unwanted elastic waves that cause spurious signals in the piezoelectric layer 2. Therefore, in the elastic wave resonator 1 of this embodiment, the surface 71 of the electrode fingers 412 has a predetermined roughness, so that, for example, unwanted waves trapped in the piezoelectric layer 2 propagate through the electrode fingers 412 and are scattered when reflected by the surface 71, thereby reducing spurious signals. Note that the effects of the elastic wave resonator 1 of this embodiment are not limited to those described above.

In addition, in the elastic wave resonator 1 according to this embodiment, the surface 71 of the electrode fingers 412 has a predetermined roughness, and for example, the weight of the electrode fingers varies depending on the part of the electrode fingers 412, which causes the frequency of the excited unwanted waves to shift, thereby reducing spurious emissions. While the resonance characteristics of Lamb waves depend on the thickness of the piezoelectric layer 2, they are less affected by the design of the electrode fingers 412. Therefore, for example, when the elastic wave resonator 1 according to this embodiment uses Lamb waves as the main resonance, spurious emissions can be reduced even more effectively. Note that the effects of the elastic wave resonator 1 according to this embodiment are not limited to those described above.

Figure 4A shows the simulated resonance characteristics at 4100 MHz to 5700 MHz of elastic wave resonators in Example 1, in which the Ra1 value is 0.8 nm, Example 2, in which the Ra1 value is 1.4 nm, and Example 3, in which the Ra1 value is 6.8 nm. Figure 4A also shows, as a comparative example, the simulated resonance characteristics of an elastic wave resonator in Comparative Example 1, in which the Ra1 value is 0 nm.

Furthermore, Figs. 4B to 4D show enlarged views of the resonance characteristics shown in Fig. 4A, particularly the resonance characteristics of the main resonance. Fig. 4B is a diagram comparing the resonance characteristics of the main resonance of Example 1 and Comparative Example 1. Fig. 4C is a diagram comparing the resonance characteristics of the main resonance of Example 2 and Comparative Example 1. Fig. 4D is a diagram comparing the resonance characteristics of the main resonance of Example 3 and Comparative Example 1.

As shown in Figures 4B to 4D, Examples 1 to 3, in which Ra1 is greater than 0 nm, are able to reduce spurious emissions within the band of the main resonance, compared to Comparative Example 1, in which Ra1 is 0 nm. Also, Examples 1 to 3 are able to reduce spurious emissions located at higher frequencies than the main resonance (its resonant frequency), compared to Comparative Example 1.

FIG. 4E shows the resonance characteristics of Example 2 and Comparative Example 1 using a Smith chart. In the Smith chart, the two intersections of the graph with the first center line extending horizontally of the circle indicate the resonance characteristics at the resonance frequency (fr) and the anti-resonance frequency (fa), respectively. In addition, in the Smith chart, the graph portion located above the first center line of the circle indicates the resonance characteristics within the band.

In the Smith chart, if the distance from the center of the circle to the graph is defined as |S|, then the better the resonance characteristics, the closer |S| is to the radius of the circle. In other words, the better the resonance characteristics, the closer the value of |S| is to 1. On the other hand, when spurious signals occur, |S| approaches 0. Therefore, in the Smith chart, the resonance characteristics can be evaluated by finding the average |S|, which is the average value of |S| within a band. For example, the closer the average |S| is to 1, the smaller the effect of spurious signals is, and the better the resonance characteristics are. In the elastic wave resonator 1 according to this embodiment, the resonance characteristics are evaluated by finding the average |S| in the range of (fr-40 MHz) to (fa+40 MHz).

The range of (fr-40MHz) to (fa+40MHz) is the range of fr to fa extended by approximately (fa-fr)/4 on both sides. If spurious emissions are reduced in this range, advantageous characteristics can be obtained in many devices. For example, in a ladder filter, the center frequency of the passband roughly coincides with fr or fa. Spurious emissions in the range of fr to fa are likely to affect the characteristics of the passband. Spurious emissions in a range adjacent to the range of fr to fa (here, the 40MHz range) are likely to affect the passband or a band adjacent to the passband.

In the elastic wave resonator 1 according to this embodiment, the arithmetic mean roughness Ra1 of the surface 71 of the electrode finger 412 has a value greater than 0 nm, but may be, for example, 0.20 nm or more. FIG. 5 is a diagram showing the change in average |S| when the thickness T of the electrode finger 412 is set to 140 nm and the arithmetic mean roughness Ra1 of the surface 71 of the electrode finger 412 is changed from 0.010 nm to 15 nm in the elastic wave resonator 1 according to this embodiment. The horizontal axis shows the value of Ra1 in logarithmic notation, and for example, the larger the value on the horizontal axis, the rougher the surface 71 of the electrode finger 412 becomes. The vertical axis shows the average |S| in the range of (fr-40 MHz) to (fa+40 MHz), and for example, the larger the value on the vertical axis, the better the resonance characteristics become.

As shown in FIG. 5, when the value of Ra1 is 0.20 nm or more, the average |S| value is large and the resonance characteristics are good. Therefore, in the elastic wave resonator 1 according to this embodiment, by setting Ra1 to 0.20 or more, it is possible to reduce spurious and obtain good frequency characteristics.

In the example of FIG. 5, the thickness T of the electrode finger 412 is 140 nm, so when the value of Ra1 (0.20 nm) is expressed as a percentage of the thickness T of the electrode finger 412, it is 0.14%. Therefore, the elastic wave resonator 1 according to this embodiment can reduce spurious and obtain good frequency characteristics when Ra1 and the thickness T satisfy the relationship shown in the following formula (1).

0.14≦(Ra1/T)×100 (1)
The lower limit of Ra1 may be even greater than the above. For example, instead of 0.20 nm (0.14%), 0.50 nm (0.36%) or 1.0 nm (0.71%) may be used. In the example of FIG. 5, the larger the lower limit is, the more spurious is reduced.

In addition, in the elastic wave resonator 1 according to this embodiment, Ra1 may be 12 nm or less. As shown in FIG. 5, when the value of Ra1 exceeds 12 nm, the average value of |S| decreases rapidly, and the resonance characteristics deteriorate. On the other hand, when the value of Ra1 is 12 nm or less, the average value of |S| remains high. Therefore, in the elastic wave resonator 1 according to this embodiment, by setting Ra1 to 12 nm or less, it is possible to reduce spurious and obtain good frequency characteristics.

In the example of FIG. 5, the thickness T of the electrode finger 412 is 140 nm, so when the value of Ra1 (12 nm) is expressed as a percentage of the thickness T of the electrode finger 412, it is 8.6%. Therefore, the elastic wave resonator 1 according to this embodiment can reduce spurious and obtain good frequency characteristics when Ra1 and the thickness T satisfy the relationship shown in the following formula (2).

(Ra1/T)×100≦8.6 (2)
In addition, in the elastic wave resonator 1 according to this embodiment, Ra1 may be 2.0 nm or less. Fig. 6 is a diagram showing the change in the phase of the spurious emission located on the higher frequency side than the main resonance (its resonant frequency; the same applies below) when the value of Ra1 is changed with respect to the thickness T. The horizontal axis shows the value of Ra1 in logarithmic notation. The vertical axis shows the phase of the spurious emission located on the higher frequency side than the main resonance, and the larger the value on the vertical axis, the greater the intensity of the spurious emission. In other words, the higher up the vertical axis in Fig. 6, the greater the intensity of the spurious emission, and the worse the frequency characteristics become.

As shown in FIG. 6, when the value of Ra1 is 0.20 nm or more, the phase of the spurious emission located on the higher frequency side than the main resonance becomes small. When the value of Ra1 exceeds 2.0 nm, the phase of the spurious emission located on the higher frequency side than the main resonance becomes large. Therefore, in the elastic wave resonator 1 according to this embodiment, by setting Ra1 to 2.0 nm or less, the phase of the spurious emission located on the higher frequency side can be made small, and even better frequency characteristics can be obtained.

6, the thickness T of the electrode fingers 412 is 140 nm, so that the value of Ra1 (2.0 nm) is 1.4% when expressed as a percentage of the thickness T of the electrode fingers 412. Therefore, in the elastic wave resonator 1 according to this embodiment, when Ra1 and the thickness T satisfy the relationship shown in the following formula (3), the phase of the spurious emission located on the high frequency side can be reduced, and even better frequency characteristics can be obtained.

(Ra1/T)×100≦1.4 (3)
In addition, in the elastic wave resonator 1 according to this embodiment, Ra1 may be equal to or less than 1.3 nm. When the value of Ra1 is equal to or less than 1.3 nm, the phase of the spurious component located on the higher frequency side than the main resonance can be further reduced, as shown in FIG. 6, thereby achieving better frequency characteristics.

In the example of FIG. 6, the thickness T of the electrode fingers 412 is 140 nm, so that the value of Ra1 (1.3 nm) is 0.93% when expressed as a percentage of the thickness T of the electrode fingers 412. Therefore, the elastic wave resonator 1 according to this embodiment can further reduce spurious and obtain better frequency characteristics when Ra1 and the thickness T satisfy the relationship shown in the following formula (4).

(Ra1/T)×100≦0.93 (4)
The multiple examples of the lower limit (0.20 nm (0.14%), 0.50 nm (0.36%), and 1.0 nm (0.71%)) and the multiple examples of the upper limit (12 nm (8.6%), 2.0 nm (1.4%), and 1.3 nm (0.93%)) may be combined in any combination (or may not be combined). That is, there are nine combinations of the three examples of the lower limit and the three examples of the upper limit, and Ra1 may be set so that any one of these combinations is satisfied. For example, 0.20 nm (0.14%) or more and 12 nm (8.6%) or less, 0.20 nm (0.14%) or more and 2.0 nm (1.4%) or less, or 1.0 nm (0.71%) or more and 1.3 nm (0.93%) or less may be satisfied.

As described above, the surface 71 of the electrode fingers 412 reduces spurious emissions by, for example, scattering unwanted waves or shifting the frequency of the unwanted waves due to its unevenness. As can be understood from this principle, there are no particular limitations on the material, pitch P, width W and thickness T of the electrode fingers 412, the material and thickness of the piezoelectric layer 2, and the material and thickness of the acoustic reflection layer 5 in obtaining the effect of reducing spurious emissions by setting Ra1.

However, for reference, specific values of the dimensions mentioned only within the scope of the simulation conditions related to Figures 4A to 6 are shown below. The pitch P is 1.20232 μm. The duty (W/P) is 0.4. The thickness of the piezoelectric layer 2 is 0.448 μm. The thickness of the acoustic reflection layer 5 is 0.448 μm.

In the elastic wave resonator 1 according to one embodiment of the present invention shown in FIG. 1, the piezoelectric layer 2 and the acoustic reflection layer 5 are in direct contact with each other, but this is not limiting. For example, in another embodiment of the present invention, the lower surface 2b of the piezoelectric layer 2 and the acoustic reflection layer 5 may be indirectly in contact with each other via an intermediate layer, an adhesive layer, or the like.

Examples of such intermediate layers include insulating materials such as silicon _nitride ( _Si3N4 ) and _aluminum oxide ( _Al2O3 ). By providing an insulating intermediate layer, it is possible to reduce the generation of unnecessary potential and unnecessary capacitance, thereby improving the electrical characteristics of the elastic wave resonator 1. Examples of adhesive layers include amorphous silicon. Both the adhesive layer and the intermediate layer may be present, or only one of them may be present.

In the elastic wave resonator 1 according to one embodiment of the present invention shown in FIG. 1, the acoustic reflection layer 5 has only one low acoustic impedance layer 51, but is not limited to this example. For example, as another embodiment of the present invention, as shown in FIG. 7A, the acoustic reflection layer 5 may be configured by alternately stacking a plurality of low acoustic impedance layers 51 and a plurality of high acoustic impedance layers 52 having a higher acoustic impedance than the low acoustic impedance layers 51.

By configuring the acoustic reflection layer 5 in this way, the elastic waves leaking from the lower surface 2b side of the piezoelectric layer 2 are reflected toward the piezoelectric layer 2 at the interface between the low acoustic impedance layer 51 and the high acoustic impedance layer 52, so that the leakage of the elastic waves can be more effectively reduced. Examples of such a high acoustic impedance layer 52 include hafnium oxide (HfO ₂ ), tantalum oxide (Ta ₂ O ₅ ), and zirconium oxide (ZrO ₂ ). The number of layers of the two types of layers is arbitrary, and may be, for example, 4 layers or more and 12 layers or less in total. The thickness of the high acoustic impedance layer 52 may be thinner, equal to, or thicker than the thickness of the low acoustic impedance layer 51. In any case, the above-mentioned explanation of the thickness of the low acoustic impedance layer 51 may be applied to the high acoustic impedance layer 52. Unlike the above explanation, three or more layers may be stacked, or the total number of layers of the two types of layers may be two or three.

In the elastic wave resonator 1 according to one embodiment of the present invention shown in FIG. 1, the low acoustic impedance layer 51 of the acoustic reflection layer 5 is a solid layer, but this is not limiting. For example, as another embodiment of the present invention, as shown in FIG. 7B, the low acoustic impedance layer 51 may be a gas present in a gap 8 provided in the support substrate 3. The gap 8 is on the piezoelectric layer 2 side of the support substrate 3 and is present at a position overlapping with the multiple electrode fingers 412 when viewed in a plan view. The gap 8 is covered by the piezoelectric layer 2 leaving an internal space, and a gas is present in the internal space. The gas may be normal air or an inert gas such as nitrogen or argon.

By configuring the low acoustic impedance layer 51 in this way, the gas present in the gap 8 acts as an acoustic reflection layer, effectively reducing the leakage of elastic waves from the lower surface 2b of the piezoelectric layer 2. The size and depth of the gap 8 may be set as appropriate.

In the elastic wave resonator 1 according to one embodiment of the present invention shown in FIG. 3, the thickness of the piezoelectric layer 2 is constant. However, the present invention is not limited to this example. The thickness of the piezoelectric layer 2 may vary. For example, as another embodiment of the present invention, as shown in FIG. 8, the thickness L1 of the first region 21 of the piezoelectric layer 2 may be greater than the thickness L2 of the second region 22 of the piezoelectric layer 2. The first region refers to the region of the piezoelectric layer 2 that overlaps with the electrode fingers 412 in a planar view. The second region refers to the region of the piezoelectric layer 2 that does not overlap with the electrode fingers 412 in a planar view. Note that the thickness of the piezoelectric layer 2 may be gradually changed at the boundary between the first region 21 and the second region 22.

In the elastic wave resonator 1 according to one embodiment of the present invention, the arithmetic mean roughness of the upper surface 2a of the piezoelectric layer 2 may be set as appropriate. For example, as another embodiment of the present invention, as shown in FIG. 9, the arithmetic mean roughness Ra2 of the upper surface 2a of the second region 22 of the piezoelectric layer 2 may be rougher than the arithmetic mean roughness Ra1 of the surface 71 of the electrode finger 412. With this configuration, spurious waves trapped in the piezoelectric layer 2 are scattered when reflected by the upper surface 2a of the second region 22, thereby reducing spurious. The specific value of Ra2 is not particularly limited, but may be, for example, 1 nm or more, 10 nm or more, 20 nm or more, or 50 nm or more, provided that (or without) Ra1<Ra2 is satisfied. Furthermore, when Ra1<Ra2 is satisfied, the difference between the two may be 1 nm or more, 10 nm or more, 20 nm or more, or 50 nm or more.

Note that, for example, when FIG. 8 and FIG. 9 are combined and Ra2 affects the comparison of L1 and L2, L1 and L2 may be determined, for example, based on the average line. The average line may be used as the reference not only for the upper surface 2a but also for the lower surface 2b. The difference between L1 and L2 is arbitrary, and may be, for example, larger than Ra2 (but does not have to be), and may be two or more times or five or more times Ra2. Also, for example, the difference between L1 and L2 may be 1 nm or more, 10 nm or more, 20 nm or more, or 50 nm or more, regardless of whether FIG. 8 and FIG. 9 are combined or not, and/or may be 0.5% or more, 1% or more, 2% or more, 5% or more, or 10% or more of the thickness of the piezoelectric layer 2.

In the elastic wave resonator 1 according to one embodiment of the present invention shown in FIG. 3, the side surface 72 of the electrode finger 412 is perpendicular to the upper surface 2a of the piezoelectric layer 2, but this is not limiting. For example, in another embodiment of the present invention, the side surface 72 of the electrode finger 412 may be inclined in the X-axis direction, as shown in FIG. 10. In this case, the cross section of the electrode finger 412 perpendicular to the Y-axis has a trapezoidal shape. The side surface 72 refers to the surface that connects the surface of the electrode finger 412 facing the piezoelectric layer 2 to the surface 71.

In addition, in FIG. 10, the roughness of the side surface 72 is omitted, but the arithmetic mean roughness Ra3 of the side surface 72 may be set appropriately. For example, the arithmetic mean roughness Ra3 of the side surface 72 may be the same as the arithmetic mean roughness Ra1 of the surface 71.

In the elastic wave resonator 1 according to one embodiment of the present invention, the surface 71 of all the electrode fingers 412 has a predetermined roughness. However, the present invention is not limited to this example. The surface 71 of only some of the electrode fingers 412 may have a predetermined roughness. For example, as another embodiment of the present invention, as shown in FIG. 11A, the surface 71 of only the electrode fingers 412 (hatched portion) located near both ends in the arrangement direction (X-axis direction) may have a predetermined roughness. In this case, the arithmetic mean roughness of the surface 71 of the electrode fingers 412 located near the center in the X-axis direction among the electrode fingers 412 is different from the arithmetic mean roughness of the surface 71 of the electrode fingers 412 located near both ends. With this configuration, it is possible to effectively reduce spurious while reducing deterioration of the main resonance characteristics.

In addition, in the elastic wave resonator 1 according to one embodiment of the present invention, an example has been shown in which the surface 71 of one electrode finger 412 has a predetermined roughness over the entirety, but this is not limiting. Only a portion of the surface 71 of the electrode finger 412 may have a predetermined roughness. For example, as another embodiment of the present invention, as shown in FIG. 11B, in an intersection region 9 where an electrode finger 412 intersects with an adjacent electrode finger 412, the surface 71 near both ends (hatched portions) in the extension direction (Y-axis direction) may have a predetermined roughness. In this case, in the intersection region 9, the arithmetic mean roughness of the surface 71 near the center in the Y-axis direction is different from the arithmetic mean roughness of the surface 71 near both ends. With this configuration, it is possible to effectively reduce spurious while reducing deterioration of the main resonance characteristics.

The example range of Ra1 according to the embodiment is derived by simulation. In other words, the example range of Ra1 described so far is not influenced by the measurement method of Ra1, and the example lower limit value and example upper limit value do not include any error (however, percentages are rounded off). When determining whether the Ra1 value of an actual elastic wave resonator 1 is within the range of Ra1 according to the embodiment, Ra1 may be measured with the required accuracy, taking into account the significant figures shown in the example lower limit value and example upper limit value.

For example, when determining whether Ra1 is 0.20 nm or more (valid to two decimal places), Ra1 may be measured using a measuring device with a resolution of 0.01 nm or less (e.g., an atomic force microscope). When it is clear that Ra1 is 0.20 nm or more, and when it is determined whether Ra1 is 2.0 nm or less or 1.2 nm or less (both valid to one decimal place), Ra1 may be measured using a measuring device with a resolution of 0.1 nm or less (e.g., a white light interferometer or a laser microscope). When it is clear that Ra1 is 0.20 nm or more, and when it is determined whether Ra1 is 12 nm or less (valid to one integer place), Ra1 may be measured using a measuring device with a resolution of 1 nm or less (e.g., a stylus measuring device). When determining whether Ra1/T x 100 is 0.14% or more, 8.6% or less, 1.4% or less, or 0.93% or less, Ra1 (and T) may be measured to two significant figures.

The relationship between accuracy and measurement principle varies depending on the measuring instrument (or manufacturer, from another perspective). As can be seen from the above examples of measuring instruments, the measuring instrument may be either a contact or non-contact type. The measuring instrument may conform to the JIS mentioned above and/or ISO (International Organization for Standardization) 25178 (it does not have to conform to these standards as long as accuracy is guaranteed). While Ra1 has been mentioned, the same applies to Ra2, Ra3, T, L1, L2, etc.

The method of forming the electrode fingers 412 having a certain degree of Ra1 is arbitrary. For example, the desired Ra1 may be obtained by roughening the surface of the electrode fingers 412 using an etching solution, an etching gas (plasma), a laser beam, or a blast. When an etching solution, an etching gas, or a laser beam is used, the Ra1 can be adjusted by the time the surface of the electrode fingers 412 is exposed to them. In addition, blasting, which controls the surface roughness with an accuracy of less than 1 nm, has been put to practical use. A resist mask may be formed in the area that is not desired to be roughened. The desired Ra1 may be obtained by smoothing the roughened surface or a surface that has been formed from the beginning to have a large Ra1. The smoothing may be performed, for example, by polishing. A technology for polishing the wafer so that the Ra is 0.2 nm or less has been put to practical use, and this may be applied. Although Ra1 has been described, the same may be applied to Ra2, Ra3, etc.

(Example of use of elastic wave resonator 1: duplexer)
12 is a circuit diagram showing a schematic configuration of a duplexer 101 as an example of the use of the elastic wave resonator 1. As can be seen from the reference numerals shown in the upper left corner of the figure, in this figure, the comb-shaped electrode 41 is shown typically in the shape of a two-pronged fork, and the reflector 42 is represented by a single line bent at both ends.

The splitter 101 has, for example, a transmit filter 105 that filters the transmit signal from the transmit terminal 103 and outputs it to the antenna terminal 102, and a receive filter 106 that filters the receive signal from the antenna terminal 102 and outputs it to the receive terminal 104.

For example, the elastic wave resonator 1 according to an embodiment of the present disclosure may be used in at least one of the transmit filter 105 and the receive filter 106. The elastic wave resonator 1 may also be used in both the transmit filter 105 and the receive filter 106.

Furthermore, the transmit filter 105 and the receive filter 106 are configured, for example, as ladder-type filters in which multiple resonators are connected in a ladder configuration. That is, the transmit filter 105 has multiple (or just one) resonators connected in series between the transmit terminal 103 and the antenna terminal 102, and multiple (or just one) resonators (parallel arms) that connect the series line (series arm) to a reference potential.

FIG. 12 is merely one example of the configuration of the splitter 101, and the splitter 101 is not limited to the configuration in FIG. 12. For example, the transmit filter 105 may be configured as a multimode filter. Also, in FIG. 12, both the transmit filter 105 and the receive filter 106 are elastic wave filters, but this configuration is not limiting. For example, either the transmit filter 105 or the receive filter 106 may be an elastic wave filter that uses an elastic wave resonator 1, and the other may be an LC filter that includes one or more inductors and one or more capacitors.

Note that although the above description has been given of a case in which splitter 101 is a duplexer equipped with transmit filter 105 and receive filter 106, splitter 101 is not limited to this configuration. For example, splitter 101 may be a diplexer or a multiplexer including three or more filters.

(Example of use of elastic wave resonator 1: communication device)
13 is a block diagram showing a main part of a communication device 111 as an example of a use of the acoustic wave resonator 1 (the duplexer 101). The communication device 111 includes the duplexer 101 and performs wireless communication using radio waves.

In the communication device 111, the transmission information signal TIS containing the information to be transmitted is modulated and frequency-raised (converted to a high-frequency signal of the carrier frequency) by the RF-IC (Radio Frequency Integrated Circuit) 113 to produce the transmission signal TS. Unnecessary components outside the transmission passband are removed from the transmission signal TS by the bandpass filter 115a, amplified by the amplifier 114a, and input to the splitter 101 (transmission terminal 103). The splitter 101 (transmission filter 105) then removes unnecessary components outside the transmission passband from the input transmission signal TS, and outputs the removed transmission signal TS from the antenna terminal 102 to the antenna 112. The antenna 112 converts the input electrical signal (transmission signal TS) into a wireless signal (radio wave) and transmits it.

In addition, in the communication device 111, a radio signal (radio waves) received by the antenna 112 is converted by the antenna 112 into an electrical signal (received signal RS) and input to the splitter 101 (antenna terminal 102). The splitter 101 (receiving filter 106) removes unnecessary components outside the receiving passband from the inputted received signal RS and outputs it from the receiving terminal 104 to the amplifier 114b. The outputted received signal RS is amplified by the amplifier 114b, and the unnecessary components outside the receiving passband are removed by the bandpass filter 115b. The received signal RS is then frequency-downshifted and demodulated by the RF-IC 113 to become the received information signal RIS.

The transmission information signal TIS and the reception information signal RIS may be low-frequency signals (baseband signals) containing appropriate information, such as analog audio signals or digitized audio signals. The passband of the wireless signal may be set appropriately, and in this embodiment, a relatively high-frequency passband (for example, 5 GHz or higher) is also possible. The modulation method may be phase modulation, amplitude modulation, frequency modulation, or a combination of two or more of these. Although the direct conversion method is exemplified in FIG. 13 as the circuit method, this is not limited to this example, and may be, for example, a double superheterodyne method. Also, FIG. 13 shows only the main parts in a schematic manner, and a low-pass filter or an isolator may be added at an appropriate position, and the position of the amplifier may be changed.

1: Acoustic wave resonator 2: Piezoelectric layer 2a: Upper surface 2b: Lower surface 21: First region 22: Second region 3: Support substrate 4: IDT electrode 41: Comb-shaped electrode 411: Bus bar 412: Electrode finger 42: Reflector 5: Acoustic reflection layer 51: Low acoustic impedance layer 52: High acoustic impedance layer 71: Surface 72: Side 8: Gap 9: Intersection region 101: Splitter 102: Antenna terminal 103: Transmitting terminal 104: Receiving terminal 111: Communication device 112: Antenna 113: RF-IC
114a, 114b:

Amplifiers

115a, 115b: Bandpass filters

Claims

a piezoelectric layer having piezoelectricity and having a first surface and a second surface opposite to the first surface;
an IDT electrode located on the first surface and having a plurality of electrode fingers;
an acoustic reflection layer located on the second surface side and including a low acoustic impedance layer having an acoustic impedance lower than that of the piezoelectric layer;
Equipped with
Assuming that the arithmetic mean roughness of at least a part of the surfaces of the plurality of electrode fingers in a plan view is Ra1 and the thickness of the plurality of electrode fingers is T, both of which are expressed in the same unit, then
The relationship of 0.14≦(Ra1/T)×100 is satisfied.
Elastic wave resonator.
a piezoelectric layer having piezoelectricity and having a first surface and a second surface opposite to the first surface;
an IDT electrode located on the first surface and having a plurality of electrode fingers;
an acoustic reflection layer located on the second surface side and including a low acoustic impedance layer having an acoustic impedance lower than that of the piezoelectric layer;
Equipped with
When the arithmetic mean roughness of at least a part of the surface of the plurality of electrode fingers in a plan view is Ra1, Ra1 is 0.20 nm or more.
Elastic wave resonator.
The acoustic reflection layer further includes a high acoustic impedance layer having an acoustic impedance higher than that of the low acoustic impedance layer,
The acoustic reflection layer is configured by alternately stacking a plurality of the low acoustic impedance layers and a plurality of the high acoustic impedance layers.
3. The elastic wave resonator according to claim 1 or 2.
The low acoustic impedance layer is a gas.
The elastic wave resonator according to any one of claims 1 to 3.
If the thickness of the plurality of electrode fingers is T and the units of Ra1 and T are the same, then
Further satisfying the relationship of (Ra1/T)×100≦8.6,
The elastic wave resonator according to any one of claims 1 to 4.
Further satisfying the relationship of (Ra1/T)×100≦1.4,
6. The elastic wave resonator according to claim 5.
Further satisfying the relationship of (Ra1/T)×100≦0.93,
The elastic wave resonator according to claim 6 .
The Ra1 is 12 nm or less.
The elastic wave resonator according to any one of claims 1 to 7.
The Ra1 is 2.0 nm or less.
The elastic wave resonator according to claim 8 .
The Ra1 is 1.3 nm or less.
The elastic wave resonator according to claim 9 .
The piezoelectric transducer is configured to utilize a Lamb wave excited by the IDT electrode and propagating through the piezoelectric layer as a primary resonance, the Lamb wave being in an A1 mode.
The elastic wave resonator according to any one of claims 1 to 10.
When twice the repeat pitch of the plurality of electrode fingers is defined as λ, the thickness of the piezoelectric layer is 2λ or less.
The elastic wave resonator according to any one of claims 1 to 11.
a first thickness of the piezoelectric layer at a portion overlapping with the plurality of electrode fingers in a plan view is greater than a second thickness of the piezoelectric layer at a portion not overlapping with the plurality of electrode fingers in a plan view;
The elastic wave resonator according to any one of claims 1 to 12.
When an arithmetic mean roughness of the first surface of the piezoelectric layer in a portion not overlapping with the plurality of electrode fingers in a plan view is Ra2, the Ra2 is larger than the Ra1.
The elastic wave resonator according to any one of claims 1 to 13.
The Ra1 is an arithmetic average roughness of the entire surface of the plurality of electrode fingers in a plan view.
The elastic wave resonator according to any one of claims 1 to 14.
The antenna,
an acoustic wave filter connected to the antenna;
an IC connected to the acoustic wave filter;
The acoustic wave filter includes an acoustic wave resonator according to any one of claims 1 to 15.
Communication device.