JP2020182137A - Acoustic wave device - Google Patents

Abstract

To provide an acoustic wave device with excellent electrical characteristics.SOLUTION: An acoustic wave device comprises: an IDT electrode that includes a plurality of electrode fingers and excites a surface acoustic wave; and a piezoelectric film that is composed of a piezoelectric crystal having the IDT electrode located on its top face, has a thickness of more than 1.0 λ and 1.6 λ or less with respect to a waveform λ that is defined by twice p that is defined by a repeating interval of the plurality of electrode fingers, and is formed of a rotated Y-cut X-propagating lithium tantalate single crystal substrate. The thickness of the IDT electrode is 9% or more and 12% or less, and the cut angle of the piezoelectric film is 46° or less.SELECTED DRAWING: Figure 2

Description

The present invention relates to an elastic wave device.

An elastic wave device in which a hypersonic film, a low sound velocity film, a LiTaO ₃ film, and an IDT electrode are laminated in this order on a support substrate is disclosed (see Patent Document 1). As the film thickness of the LiTaO ₃ film, 0.25λ or the like is disclosed when the wavelength determined by the period of the electrode finger of the IDT electrode is λ. According to such an elastic wave device, the elastic wave can be confined in the LiTaO ₃ film, so that a high Q value can be obtained.

International Publication No. 2012/0866639

In recent years, mobile terminal devices used for mobile communication are required to stably supply elastic wave devices having high electrical characteristics in order to stably realize high communication quality.

The present invention has been made in view of such a problem, and an object of the present invention is to provide an elastic wave device having high productivity and excellent electrical characteristics.

The surface acoustic wave device of the present disclosure comprises an IDT electrode having a plurality of electrode fingers and exciting an elastic surface wave, and a piezoelectric crystal in which the IDT electrode is located on an upper surface, and is defined by a repeating interval of the plurality of electrode fingers. A piezoelectric film made of an X-propagation-rotating Y-cut lithium tantalate single crystal substrate having a thickness of more than 1.0 λ and less than 1.6 λ with respect to a wavelength λ defined by twice p is provided. .. The thickness of the IDT electrode is 9% or more and 12% or less, and the cut angle of the piezoelectric film is 46 ° or less.

According to the above configuration, it is possible to provide an elastic wave device having high productivity and excellent electrical characteristics.

It is a schematic sectional view of the elastic wave apparatus which concerns on this disclosure. It is a top view of the elastic wave device of FIG. It is a diagram which shows the relationship between the generation frequency of bulk wave spurious and the thickness of a piezoelectric film. It is a diagram which shows the relationship between Δf and the cut angle of a piezoelectric film. 5 (a) to 5 (c) are diagrams showing the frequency characteristics of the elastic wave apparatus according to the comparative example and the embodiment. 5 (a) and 5 (b) are cross-sectional views showing modified examples of the elastic wave device according to FIG. 1, respectively. It is a figure which shows the condition which the spurious intensity of mode 1-1 becomes 0.

Hereinafter, the present invention will be clarified by explaining specific embodiments of the present disclosure with reference to the drawings.

FIG. 1 is a schematic cross-sectional view of an elastic wave device (hereinafter referred to as a SAW device) 1 according to the embodiment of the present disclosure.

The SAW device 1 includes a support substrate 3, a piezoelectric film 7, and an IDT electrode 9. The support substrate 3, the piezoelectric film 7, and the IDT electrode 9 are laminated in this order.

In this example, the support substrate 3 supports the piezoelectric film 7 laminated on the support substrate 3, and is not particularly limited as long as it has a certain strength. For example, when the material is made of a material having a coefficient of linear expansion smaller than that of the piezoelectric film 7, it is possible to reduce the characteristic change due to the temperature change by reducing the deformation of the piezoelectric film 7 due to the temperature change. Further, when the material is selected so that the transverse wave sound velocity of the elastic wave propagating in the support substrate 3 is faster than the transverse wave sound velocity of the elastic wave propagating in the piezoelectric film 7, the elastic wave is confined in the piezoelectric film 7. It is possible to provide the SAW device 1 having excellent frequency characteristics.

Examples of such a material include a sapphire substrate and a Si substrate. In this embodiment, a case where a Si substrate is used as the support substrate 3 will be described as an example.

The piezoelectric film 7 includes a first surface 7a and a second surface 7b facing the first surface 7a. For convenience, the direction from the second surface 7b to the first surface 7a may be referred to as upward. Therefore, the first surface 7a may be referred to as an upper surface, and the second surface 7b may be referred to as a lower surface.

The support substrate 3 is joined to the second surface 7b of the piezoelectric film 7. The piezoelectric film 7 and the support substrate 3 may be directly bonded to each other, or may be connected with an adhesive layer, an intermediate layer, or the like interposed between them. As the intermediate layer, an insulating material such as silicon oxide, aluminum oxide, silicon nitride, or aluminum nitride can be exemplified.

As the piezoelectric film 7, a single crystal substrate having piezoelectricity made of lithium tantalate (LiTaO ₃ ; hereinafter referred to as LT) crystal can be used. The cut angle of the LT crystal is 46 ° or less, which will be described in detail later.

The thickness of the piezoelectric film 7 is more than 1λ and 1.6λ or less with respect to the wavelength λ of the elastic wave defined by the IDT electrode 9 described later. The thickness of the piezoelectric film 7 will be described later.

The IDT electrode 9 is located on the first surface 7a of the piezoelectric film 7. The IDT electrode 9 is formed by using a conductive material, and in this example, it is formed of an Al—Cu alloy in which Cu is added to Al. For the IDT electrode 9, various conductive materials such as Al, Cu, Pt, Mo, Au and the like and alloys thereof can be adopted. Further, a plurality of layers made of these conductive materials may be laminated to form a structure. Also. When it is composed of a laminated body of a plurality of layers, a base layer made of, for example, Ti or the like may be interposed at the laminated interface.

The film thickness of the IDT electrode 9 will be described in detail later, but is set to a thickness of more than 8% and 12% or less with respect to the wavelength λ.

FIG. 2 shows the shape of the IDT electrode 9. FIG. 2 is a top view of the SAW device 1. As shown in FIG. 2, in the IDT electrode 9, a plurality of two bus bars 91 and a plurality of elongated electrode fingers 92 connected to any of the bus bars 91 are arranged in one direction. The electrode fingers 92 connected to one bus bar 91 and the electrode fingers 92 connected to the other bus bar 91 are alternately arranged. Further, a dummy electrode 93 facing the tip of the electrode finger 92 connected to one bus bar 91 and connected to the other bus bar 91 is provided. In the figure, one is shaded in order to distinguish between the configuration connected to one bus bar 91 and the configuration connected to the other bus bar 91.

When a high-frequency signal is applied to such an IDT electrode 9, a standing wave having a half-wavelength between the centers of the electrode fingers 92 is excited. In other words, an elastic wave of wavelength λ represented by 2p is excited.

Reflectors 11 are located on both sides of the IDT electrode 9 in the arrangement direction of the electrode fingers 92. As a result, the IDT electrode 9 and the reflector 11 function as a 1-port type resonator. The SAW element 1 of the present disclosure may include such an IDT electrode 9, and the number, arrangement, and the like thereof are not particularly limited. For example, a ladder type filter including a plurality of such resonators, a vertically coupled type filter, and the like can be configured.

According to the SAW apparatus 1 of the present disclosure, in the above configuration, the thickness of the piezoelectric film 7, the cut angle, and the film thickness of the IDT electrode 9 have a specific relationship, so that the productivity is high and the electrical characteristics are high. Will be excellent. The mechanics will be described in detail below.

The elastic wave device described in Patent Document 1 uses an extremely thin piezoelectric film having a thickness of less than λ (0.25λ). As a result, elastic waves can be confined inside the piezoelectric film, and spurious emissions due to bulk waves do not appear in the vicinity of the resonance frequency, so that high electrical characteristics can be realized.

On the other hand, since the thickness of the piezoelectric film is extremely thin, precise processing is required and there is a risk that productivity may decrease. Further, as the thickness of the piezoelectric film becomes thinner, the frequency characteristic fluctuation accompanying the thickness fluctuation of the piezoelectric film becomes large, and it is difficult to maintain stable characteristics. In addition, elastic waves excited by the IDT electrode 9 are also distributed on the support substrate 3 side, and are affected by the support substrate itself and the interface between the piezoelectric film and the support substrate, resulting in a large loss or as a resonator. There was a risk that the characteristics would deteriorate.

On the other hand, according to the SAW apparatus 1 of the present disclosure, the workability is improved by setting the thickness of the piezoelectric film 7 to less than 2λ, which can be expected to confine elastic waves, and to exceed the wavelength λ. At the same time, the sensitivity of characteristic fluctuations due to thickness fluctuations can be reduced. That is, it is possible to achieve both high electrical characteristics due to the confinement effect of elastic waves and high productivity by increasing the thickness of the piezoelectric film 7.

However, when the thickness of the piezoelectric film 7 exceeds 1λ, the influence of spurious caused by bulk waves becomes large. FIG. 3 shows how the frequency of the bulk wave spurious changes when the thickness of the piezoelectric film is changed. In FIG. 3, the horizontal axis represents the thickness of the piezoelectric film standardized by λ, and the vertical axis represents the frequency. From FIG. 3, it can be confirmed that various spurs having different bulk wave spurious modes and orders are generated, and each spurious shifts to the higher frequency side as the thickness of the piezoelectric film becomes thinner. The bulk wave spurious generated at the lowest frequency among these innumerable spurious is the primary of mode 1, and may be referred to as mode 1-1 below. The same rule is used for other modes and spurs of order. The bulk wave spurious located at the next lowest frequency after mode 1-1 is mode 2-1.

Here, when the thickness of the piezoelectric film is 1λ or less, bulk wave spurious is not generated near the resonance frequency and the antiresonance frequency, but when the thickness exceeds 1λ, the bulk wave spurious of modes 1-1, 2-1 is generated. It can be seen that may affect the resonance characteristics.

Therefore, according to the SAW device 1 of the present disclosure, in order to eliminate the influence of spurious between modes 1-1 and mode 2-1 as follows. When the thickness of the piezoelectric film 7 exceeds 1.6λ, spurious of mode 1-2 is generated. Therefore, the thickness of the piezoelectric film 7 is set to 1.6λ or less.

First, since the spurious generation frequency is determined by the pitch p (or wavelength λ), the resonance frequency can be shifted to the low frequency side while maintaining the spurious frequency by increasing the film thickness of the IDT electrode 9. .. As a result, the spurious frequency of mode 2-1 with respect to the resonance frequency (relative frequency of mode 2-1) is increased, and the influence of mode 2-1 is reduced. In other words, the spurious frequency of mode 2-1 can be shifted away from the resonance frequency.

Next, the mode 1-1 will be examined. The intensity of mode 1 spurious, which is an SV wave, correlates with the bulk wave radiation from the IDT electrode 9. That is, the strength of spurious can be changed by the cut angle of the LT crystal constituting the piezoelectric film 7, the thickness of the piezoelectric film 7, the thickness of the IDT electrode 9, and the like. Here, it was possible to separate the mode 2-1 from the resonance frequency by increasing the thickness of the IDT electrode 9, but it is estimated that the relative frequency of the mode 1-1 also changes at the same time and the spurious intensity increases. To. Therefore, for mode 1-1, the thickness and cut angle of the piezoelectric film 7 are adjusted so that the bulk wave is not radiated (so as to lower the spurious intensity).

Specifically, the thickness of the piezoelectric film 7 is adjusted to determine the resonance frequency and the frequency of mode 1-1, and the cut angle of the piezoelectric film 7 and the thickness of the IDT electrode 9 so that the spurious intensity at that frequency becomes small. Set the combination with.

From the above, the thickness of the piezoelectric film 7 exceeds 1λ by limiting the bulk wave spurious that greatly affects the frequency characteristics to mode 1-1 and reducing the intensity of the bulk wave spurious in that mode 1-1. Even in this case, the influence of bulk wave spurious can be suppressed.

Table 1 shows the spurious generation frequency of mode 1-1 when the thickness of the piezoelectric film 7 is 1λ to 1.6λ, and the thickness of the IDT electrode 9 at which the spurious intensity of mode 1-1 at this spurious generation frequency becomes 0. The result of simulating the combination with the cut angle of the LT crystal is shown in FIG. In the figure, "non-radiation" means that the spurious intensity becomes 0.

As is clear from FIG. 7, in order to make the spurious intensity of mode 1-1 0, when the thickness of the piezoelectric film 5 is constant, the larger the film thickness of the IDT electrode 9, the larger the cut angle of the LT crystal. There is a need to. Further, when the thickness of the IDT electrode 9 is constant, it is necessary to reduce the cut angle of the LT crystal as the thickness of the piezoelectric film 7 becomes thicker.

Here, FIG. 4 shows the difference between the cut angle of the LT crystal and the resonance frequency and the antiresonance frequency when the film thickness of the IDT electrode 9 is changed from 4% (λ ratio) to 12% (hereinafter, Δf). The relationship with) is shown. In FIG. 4, the horizontal axis represents the cut angle of the LT crystal, and the vertical axis represents Δf.

As is clear from FIG. 4, Δf decreases as the cut angle increases. Further, it can be seen that when the film thickness of the IDT electrode 9 is 8% to 10%, Δf becomes the largest, and when the film thickness becomes smaller or larger, Δf becomes smaller.

Since the LT crystal is used as the piezoelectric film 7, Δf may be secured at 75 MHz or more. In this case, the film thickness of the IDT electrode 9 may be 8% or more and 12%, and the cut angle of the LT crystal may be 46 ° or less. However, the thickness of the IDT electrode 9 may be 9% or more, more preferably 10% or more, because it is thickened for adjusting the relative frequency of the mode 2-1.

The thickness of the normal IDT electrode 9 is often set to about 8% in consideration of its elastic wave generation efficiency and the like. This is clear from the fact that in FIG. 4, when the thickness of the IDT electrode 9 is 8%, the electrical characteristics such as Δf are most excellent.

Taking into consideration such a limitation for maintaining Δf, in order to bring the spurious intensity of mode 1-1 close to 0, the thickness of the piezoelectric film 7 is 1.2λ to 1.6λ and the cut angle is 46 °. Hereinafter, it is necessary that the thickness of the IDT electrode 9 simultaneously satisfies 9% or more and 12% or less.

The cut angle Y of the LT crystal for setting the spurious intensity of mode 1-1 shown in FIG. 7 to 0 is as follows when the thickness of the piezoelectric film 7 is X and the thickness of the IDT electrode 9 is Z. It is represented by an expression.
Y = AX ² + BX + C
^{However, A = -4017.9Z 2 + 1142.9Z-} 45.893, B = 8625.0Z 2 -3065.0Z + 116.75, is a C = 5021.4Z 2 + 503.57Z + 27.891 .

From the above, by setting the cut angle to Y ± 1 ° and the film thickness of the IDT electrode 9 to Z ± 0.01 (that is, ± 1%), the spurious intensity of mode 1-1 can be brought close to 0.

Hereinafter, as an example, the result of simulating the frequency characteristics of an actual resonator is shown in FIG. FIG. 5A shows the resonator characteristics of the SAW apparatus according to the comparative example, FIG. 5B shows the resonator characteristics of the SAW apparatus according to the first embodiment, and FIG. 5C shows the resonator characteristics of the SAW apparatus according to the second embodiment. The resonator characteristics of the SAW device are shown.

The cut angle and thickness of the piezoelectric film 7 of Comparative Example, Example 1 and Example 2, and the thickness of the IDT electrode 9 are as follows in order.
Comparative Example 1: 42 °, 1.5λ, 8%
Example 1: 46 °, 1.2λ, 10%
Example 2: 46 °, 1.5λ, 12%
In FIG. 5, the horizontal axis represents the frequency and the vertical axis represents the absolute value of the impedance. As is clear from FIG. 5, the spurious of mode 1-1 reduces the spurious intensity by approaching the resonance frequency, and the spurious of mode 2-1 is shifted to the high frequency side to be near the anti-resonance frequency. It can be seen that a region in which spurious does not exist can be secured in a wide frequency range even on the high frequency side. In the resonator, SH wave spurious is generated at the high frequency side end (band edge) of the reflection band of the IDT electrode (described as "band edge" in FIG. 5). When the electrode thickness is thick, the frequency of this band edge shifts to the high frequency side with respect to the resonance frequency. Therefore, the coupling between the SAW and the SH wave becomes weak, and the spurious becomes small. In the SAW apparatus of the present disclosure, since the electrode thickness is set to be thicker than the general one, there is also an effect that the spurious of the band edge becomes small.

As a result of fixing the thickness of the IDT electrode 9, changing the thickness of the piezoelectric film 7 to 1.2λ to 1.6λ, and changing the cut angle from 37 ° to 46 ° in the SAW apparatus according to the first embodiment. In this case as well, it was confirmed that the influence of mode 1-1 can be suppressed and the spurious of mode 2-1 can be shifted to the high frequency side as compared with the comparative example. Above all, the cut angle of the LT crystal is 38 °,
It was confirmed that the phase characteristics in the band can be stabilized when the thickness of the piezoelectric film 7 is 1.6λ and the thickness of the IDT electrode 9 is 10%.

Similarly, in the SAW apparatus according to the second embodiment, the thickness of the IDT electrode 9 is fixed, the thickness of the piezoelectric film 7 is changed to 1.5λ and 1.6λ, and the cut angle is changed from 44 ° to 46 °. In each case, it was confirmed that the influence of mode 1-1 can be suppressed and the spurious of mode 2-1 can be shifted to the high frequency side as compared with the comparative example. Above all, it was confirmed that the phase characteristics in the band can be stabilized when the cut angle of the LT crystal is 44 °, the thickness of the piezoelectric film 7 is 1.6λ, and the thickness of the IDT electrode 9 is 12%.

(Other examples)
In the above example, the support substrate 3 and the piezoelectric film 7 are laminated, but the configuration is not limited to that. For example, as shown in FIG. 6A, an insulating portion 5 may be provided between the two.

In this case, the insulating portion 5 is made of a material having insulating properties such as silicon oxide, silicon nitrogen nitrogen, and aluminum oxide, and its crystallinity is not particularly limited. By providing the insulating portion 5, it is possible to reduce the formation of unnecessary potentials and unnecessary capacitances, so that the electrical characteristics of the SAW device 1 can be improved.

In particular, when a Si substrate, which is a semiconductor material, is used as the support substrate 3 as in this example, the influence of the support substrate 3 is reduced by providing an insulating portion 5 between the piezoelectric film 7 and the support substrate 3. Can be done. The thickness of the insulating portion 3 may be 0.01p or more and 2p or less in order to ensure the insulating property and to utilize the characteristics of the high sound velocity material of the supporting substrate 3. In particular, when the value is 0.1p to 0.4p, the influence of the conductivity of the support substrate 3 (Si in this case) can be avoided.

Further, as shown in FIG. 6B, the support substrate 3 may not exist directly under the piezoelectric film 7. In FIG. 6B, a recess is formed in the support substrate 3, and the piezoelectric film 7 is arranged so as to close the recess.

Although not particularly shown, in FIG. 6A, the insulating portion 5 may be configured by laminating a plurality of different layers.

1: SAW device 3: Support substrate 7: Piezoelectric film 9: IDT electrode

Claims (5)

An IDT electrode that has multiple electrode fingers and excites surface acoustic waves,
It consists of a piezoelectric crystal in which the IDT electrode is located on the upper surface, and exceeds 1.0λ and 1.6λ or less with respect to the wavelength λ defined by twice p defined by the repetition interval of the plurality of electrode fingers. A piezoelectric film made of a lithium tantalate single crystal substrate of X propagation rotation Y cut, which has a thickness of
The thickness of the IDT electrode is 9% or more and 12% or less.
An elastic wave device having a cut angle of 46 ° or less of the piezoelectric film. The elastic wave device according to claim 1, wherein the thickness of the IDT electrode is 10% or more and 12% or less. The elastic wave device according to claim 1 or 2, wherein when the cut angle of the piezoelectric film is Y, the thickness of the piezoelectric film is X, and the thickness of the IDT electrode is Z, the following equation is satisfied.
Y = AX ² + BX + C ± 1
A = -4017.9Z ² + 142.9Z-45.893
B ⁼ 8625.0Z 2 -3065.0Z + 116.75
C = 5021.4Z ² +503.57Z + 27.891 The one according to any one of claims 1 to 3, further comprising a support substrate which is directly or indirectly connected to the lower surface side of the piezoelectric film and whose transverse wave sound velocity is faster than the transverse wave sound velocity propagating in the piezoelectric film. Elastic wave device. The elastic wave device according to claim 4, wherein the support substrate is a Si substrate.

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