JP7493306B2 - Elastic Wave Device - Google Patents

Description

The present invention relates to an elastic wave device.

An acoustic wave device is disclosed in which a high acoustic velocity film, a low acoustic velocity film, a _LiTaO3 film, and an IDT electrode are laminated in this order on a support substrate (see Patent Document 1). The thickness of the _LiTaO3 film is disclosed to be 0.25λ, where λ is the wavelength determined by the period of the electrode fingers of the IDT electrode. With such an acoustic wave device, the acoustic wave can be confined in the _LiTaO3 film, so a high Q value can be obtained.

International Publication No. 2012/086639

In recent years, there has been a demand for a stable supply of elastic wave devices with high electrical characteristics to ensure that portable terminal devices used in mobile communications can stably achieve high communication quality.

The present invention was developed in consideration of these problems, and its purpose is to provide an elastic wave device that is highly productive and has excellent electrical characteristics.

The acoustic wave device of the present disclosure comprises an IDT electrode having a plurality of electrode fingers and exciting a surface acoustic wave, and a piezoelectric film made of an X-propagating rotated Y-cut lithium tantalate single crystal substrate, which is made of a piezoelectric crystal on whose upper surface the IDT electrode is located and has a thickness of more than 1.0λ and not more than 1.6λ, where λ is a wavelength defined as twice p, which is defined as the repetition interval of the plurality of electrode fingers. 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.

The above configuration makes it possible to provide an elastic wave device with high productivity and excellent electrical characteristics.

1 is a schematic cross-sectional view of an elastic wave device according to the present disclosure. 2 is a top view of the acoustic wave device of FIG. 1 . 1 is a diagram showing the relationship between the generation frequency of spurious bulk waves and the thickness of a piezoelectric film. 1 is a diagram showing the relationship between Δf and the cut angle of a piezoelectric film. 5A to 5C are graphs showing frequency characteristics of elastic wave devices according to a comparative example and an example. 5(a) and 5(b) are cross-sectional views showing modifications of the acoustic wave device shown in FIG. FIG. 13 is a diagram showing the condition under which the spurious intensity of mode 1-1 becomes 0.

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

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

The SAW device 1 comprises 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 layered in this order.

In this example, the support substrate 3 supports the piezoelectric film 7 laminated thereon, and is not particularly limited as long as it has a certain strength. For example, if it is made of a material with a smaller linear expansion coefficient than the piezoelectric film 7, it is possible to reduce the deformation of the piezoelectric film 7 due to temperature changes, thereby reducing changes in characteristics due to temperature changes. Furthermore, if a material is selected such that the transverse wave sound velocity of the elastic waves propagating in the support substrate 3 is faster than the transverse wave sound velocity of the elastic waves propagating in the piezoelectric film 7, the elastic waves can be confined in the piezoelectric film 7, and a SAW device 1 with excellent frequency characteristics can be provided.

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

The piezoelectric film 7 has a first surface 7a and a second surface 7b facing the first surface 7a. For convenience, the direction from the second surface 7b toward the first surface 7a is sometimes referred to as the upward direction. For this reason, the first surface 7a is sometimes referred to as the upper surface, and the second surface 7b is sometimes referred to as the lower surface.

The support substrate 3 is bonded to the second surface 7b of the piezoelectric film 7. The piezoelectric film 7 and the support substrate 3 may be bonded directly, or may be connected via an adhesive layer, intermediate layer, or the like. Examples of intermediate layers include insulating materials such as silicon oxide, aluminum oxide, silicon nitride, and aluminum nitride.

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

The thickness of the piezoelectric film 7 is greater than 1λ and less than or equal to 1.6λ, where λ is the wavelength of the elastic wave defined by the IDT electrode 9 described below. 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 made of a conductive material, and in this example, is made of an Al-Cu alloy in which Cu is added to Al. The IDT electrode 9 can also be made of various other conductive materials such as Al, Cu, Pt, Mo, Au, and alloys thereof. It may also be made by stacking multiple layers made of these conductive materials. Also, when it is made of a laminate of multiple layers, a base layer made of, for example, Ti may be interposed at the interface of the layers.

The film thickness of the IDT electrode 9, which will be described in detail later, is greater than 8% and less than 12% of the wavelength λ.

The shape of the IDT electrode 9 is shown in Fig. 2. Fig. 2 is a top view of the SAW device 1. As shown in Fig. 2, the IDT electrode 9 includes two bus bars 91 and a plurality of long electrode fingers 92 connected to either of the bus bars 91, which 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 arranged alternately. Also, a dummy electrode 93 is provided that faces the tip of the electrode finger 92 connected to one bus bar 91 and is connected to the other bus bar 91. In the figure, one of the configurations connected to one bus bar 91 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 is excited with the center-to-center spacing p of the electrode fingers 92 being half the wavelength. In other words, an elastic wave with a wavelength λ represented by 2p is excited.

Reflectors 11 are located on both sides of the arrangement direction of the electrode fingers 92 of the IDT electrode 9. As a result, the IDT electrode 9 and the reflectors 11 function as a one-port resonator. The SAW element 1 of the present disclosure only needs to include such IDT electrodes 9, and there are no particular limitations on the number, arrangement, etc. of the electrodes. For example, it is possible to configure a ladder-type filter or a vertically coupled filter that includes multiple such resonators.

According to the SAW device 1 of the present disclosure, in the above-mentioned configuration, by setting a specific relationship between the thickness of the piezoelectric film 7, the cut angle, and the film thickness of the IDT electrode 9, it is possible to achieve high productivity and excellent electrical characteristics. The mechanism behind this is described in detail below.

The elastic wave device described in Patent Document 1 uses an extremely thin piezoelectric film with a thickness of less than λ (0.25λ). This allows the elastic wave to be confined inside the piezoelectric film, and spurious responses due to bulk waves do not appear near the resonant frequency, achieving high electrical characteristics.

On the other hand, because the piezoelectric film is extremely thin, precise processing is required, which may reduce productivity. Furthermore, the thinner the piezoelectric film, the greater the fluctuation in frequency characteristics due to the variation in the thickness of the piezoelectric film, making it difficult to maintain stable characteristics. In addition, the elastic waves excited by the IDT electrode 9 are distributed on the support substrate 3 side as well, and may be affected by the support substrate itself and the interface between the piezoelectric film and the support substrate, resulting in increased loss and deterioration of the characteristics as a resonator.

In contrast, according to the SAW device 1 of the present disclosure, the thickness of the piezoelectric film 7 is set to less than 2λ, at which the effect of confining elastic waves can be expected, and to a value that exceeds the wavelength λ, thereby improving processability and reducing the sensitivity to characteristic variations due to thickness variations. In other words, it is possible to achieve both high electrical characteristics resulting from the effect of confining 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 due to bulk waves becomes large. Figure 3 shows the change in the frequency of the bulk wave spurious when the thickness of the piezoelectric film is changed. In Figure 3, the horizontal axis shows the thickness of the piezoelectric film normalized by λ, and the vertical axis shows the frequency. Figure 3 shows that various spurious with different modes and orders of bulk wave spurious are generated, and that each spurious shifts to the higher frequency side as the thickness of the piezoelectric film becomes thinner. Of these countless spurious, the bulk wave spurious that occurs at the lowest frequency is the first order of mode 1, which may be referred to as mode 1-1 below. The same rule is used for spurious of other modes and orders. 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, no bulk wave spurious occurs near the resonant frequency or anti-resonant frequency, but when the thickness exceeds 1λ, it can be seen that the bulk wave spurious of modes 1-1 and 2-1 may affect the resonance characteristics.

Therefore, according to the SAW device 1 of the present disclosure, the following is done to eliminate the influence of the spurious of mode 1-1 and mode 2-1. Note that if the thickness of the piezoelectric film 7 exceeds 1.6λ, spurious of mode 1-2 occurs. For this reason, the thickness of the piezoelectric film 7 is set to 1.6λ or less.

First, because the frequency at which spurious emissions occur is determined by the pitch p (or wavelength λ), by increasing the thickness of the IDT electrode 9, it is possible to shift the resonant frequency to a lower frequency while maintaining the frequency of the spurious emissions. This increases the spurious frequency of mode 2-1 relative to the resonant frequency (the relative frequency of mode 2-1), thereby reducing the influence of mode 2-1. In other words, it is possible to shift the spurious frequency of mode 2-1 away from the resonant frequency.

Next, we consider mode 1-1. The spurious intensity of mode 1, which is an SV wave, correlates with the bulk wave radiation from the IDT electrode 9. In other words, the spurious intensity can be changed by the cut angle of the LT crystals that make up the piezoelectric film 7, the thickness of the piezoelectric film 7, the thickness of the IDT electrode 9, etc. Here, by increasing the thickness of the IDT electrode 9, it is possible to move mode 2-1 away from the resonant frequency, but at the same time, it is estimated that the relative frequency of mode 1-1 also changes, increasing the spurious intensity. Therefore, for mode 1-1, the thickness and cut angle of the piezoelectric film 7 are adjusted so that bulk waves are not radiated (so that the spurious intensity is reduced).

Specifically, the thickness of the piezoelectric film 7 is adjusted to determine the resonant frequency and the frequency of mode 1-1, and a combination of the cut angle of the piezoelectric film 7 and the film thickness of the IDT electrode 9 is set so that the spurious intensity at that frequency is reduced.

As a result, by limiting the bulk wave spurious that significantly affects the frequency characteristics to mode 1-1 and reducing the strength of the bulk wave spurious of mode 1-1, it is possible to suppress the effects of the bulk wave spurious even when the thickness of the piezoelectric film 7 exceeds 1λ.

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 Figure 7 shows the results of simulating the combination of the thickness of the IDT electrode 9 and the cut angle of the LT crystal that results in the spurious intensity of mode 1-1 at this spurious generation frequency being 0. In the figure, "non-radiation" indicates that the spurious intensity is 0.

As is clear from Figure 7, in order to make the spurious intensity of mode 1-1 zero, if 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 must be. Also, if the thickness of the IDT electrode 9 is constant, the thicker the piezoelectric film 7, the smaller the cut angle of the LT crystal must be.

Figure 4 shows the relationship between the cut angle of the LT crystal and the difference between the resonant frequency and the anti-resonant frequency (hereinafter referred to as Δf) when the film thickness of the IDT electrode 9 is changed from 4% (λ ratio) to 12%. In Figure 4, the horizontal axis shows the cut angle of the LT crystal, and the vertical axis shows Δf.

As is clear from Figure 4, as the cut angle increases, Δf decreases. Furthermore, Δf is largest when the film thickness of the IDT electrode 9 is between 8% and 10%, and Δf decreases as the film thickness decreases or increases.

Since an LT crystal is used as the piezoelectric film 7, Δf may be ensured to be 75 MHz or more. In this case, the film thickness of the IDT electrode 9 may be 8% to 12%, and the cut angle of the LT crystal may be 46° or less. However, since the thickness of the IDT electrode 9 is made thick to adjust the relative frequency of mode 2-1, it may be 9% or more, more preferably 10% or more.

In addition, the thickness of the normal IDT electrode 9 is often set to about 8% in consideration of the efficiency of generating elastic waves. This is also evident from the fact that the electrical characteristics such as Δf are the best when the thickness of the IDT electrode 9 is set to 8% in FIG. 4.

Taking into account these limitations to maintain Δf, in order to bring the spurious intensity of mode 1-1 closer to 0, the thickness of the piezoelectric film 7 must be 1.2λ to 1.6λ, the cut angle must be 46° or less, and the thickness of the IDT electrode 9 must be 9% or more and 12% or less.

The cut angle Y of the LT crystal for making the spurious intensity of mode 1-1 shown in FIG. 7 zero is expressed by the following equation, where X is the thickness of the piezoelectric film 7 and Z is the film thickness of the IDT electrode 9:
Y = ^AX2 + BX + C
Where A = -4017.9Z ² + 1142.9Z - 45.893, B = 8625.0Z ² - 3065.0Z + 116.75, and 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 (i.e. ±1%), the spurious intensity of mode 1-1 can be brought close to 0.

As an example, the results of simulating the frequency characteristics of an actual resonator are shown in Figure 5. Figure 5(a) shows the resonator characteristics of a SAW device according to a comparative example, Figure 5(b) shows the resonator characteristics of a SAW device according to Example 1, and Figure 5(c) shows the resonator characteristics of a SAW device according to Example 2.

The cut angle and thickness of the piezoelectric film 7 and the thickness of the IDT electrode 9 in the comparative example, example 1, and example 2 are as follows, in that 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 indicates frequency, and the vertical axis indicates absolute impedance. As is clear from FIG. 5, the spurious intensity of mode 1-1 is reduced by approaching the resonant frequency, and the spurious intensity of mode 2-1 is shifted to the high frequency side, so that a region where no spurious exists can be secured over a wide frequency range even in the vicinity of the anti-resonant frequency and on the high frequency side. Note that in the resonator, spurious SH waves are generated at the high frequency end (band edge) of the reflection band of the IDT electrode (shown as "band edge" in FIG. 5). When the electrode thickness is thick, the frequency of this band edge shifts to the high frequency side relative to the resonant frequency. Therefore, the coupling between the SAW and the SH wave is weakened, and the spurious becomes smaller. In the SAW device of the present disclosure, the electrode thickness is set to be thicker than usual, which also has the effect of reducing the spurious at the band edge.

In the SAW device according to the first embodiment, the thickness of the IDT electrode 9 was fixed, the thickness of the piezoelectric film 7 was changed from 1.2λ to 1.6λ, and the cut angle was changed from 37° to 46°. As a result, it was confirmed that in each case, the influence of mode 1-1 was suppressed compared to the comparative example, and the spurious of mode 2-1 could be shifted to the higher frequency side.
It was confirmed that when the thickness of the piezoelectric film 7 was set to 1.6λ and the thickness of the IDT electrode 9 was set to 10%, the phase characteristic within the band could be stabilized.

Similarly, in the SAW device of Example 2, the thickness of the IDT electrode 9 was fixed, the thickness of the piezoelectric film 7 was changed to 1.5λ and 1.6λ, and the cut angle was changed from 44° to 46°. As a result, it was confirmed that in each case, the influence of mode 1-1 was suppressed compared to the comparative example, and the spurious of mode 2-1 could be shifted to the higher frequency side. In particular, it was confirmed that the phase characteristics within the band could be stabilized when the cut angle of the LT crystal was 44°, the thickness of the piezoelectric film 7 was 1.6λ, and the thickness of the IDT electrode 9 was 12%.

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

In this case, the insulating section 5 is made of an insulating material such as silicon oxide, silicon nitride, or aluminum oxide, and the crystallinity is not particularly limited. By providing the insulating section 5, it is possible to reduce the formation of unnecessary potential and unnecessary capacitance, thereby improving the electrical characteristics of the SAW device 1.

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

Also, as shown in FIG. 6(b), the support substrate 3 may not be present directly below the piezoelectric film 7. In FIG. 6(b), a recess is formed in the support substrate 3, and the piezoelectric film 7 is disposed so as to fill the recess.

Although not specifically shown, in FIG. 6(a), the insulating section 5 may be configured by stacking multiple different layers.

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

Claims (4)

an IDT electrode made of an Al-Cu alloy, having a plurality of electrode fingers, and exciting a surface acoustic wave;
a piezoelectric film made of an X-propagating rotated Y-cut lithium tantalate single crystal substrate, the piezoelectric film being made of a piezoelectric crystal on whose upper surface the IDT electrode is located, and having a thickness of more than 1.0λ and not more than 1.6λ, where λ is a wavelength defined as twice p, the wavelength being defined as the repetition interval of the plurality of electrode fingers;
The thickness of the IDT electrode is 9% or more and 12% or less,
The cut angle of the piezoelectric film is 46° or less;
An acoustic wave device that satisfies the following formula, where Y is the cut angle of the piezoelectric film, X is the thickness of the piezoelectric film, and Z is the thickness of the IDT electrode.
Y = AX2 ⁺ BX + C±1
A = -4017.9Z2 ⁺ 1142.9Z-45.893
B = 8625.0Z ² - 3065.0Z + 116.75
C = 5021.4Z2 ⁺ 503.57Z + 27.891 The acoustic wave device according to claim 1, wherein the thickness of the IDT electrode is 10% or more and 12% or less. 3. The elastic wave device according to claim 1 , further comprising a support substrate connected directly or indirectly to a lower surface side of the piezoelectric film, the support substrate having a shear wave velocity faster than the shear wave velocity propagating through the piezoelectric film. The acoustic wave device according to claim 3 , wherein the supporting substrate is a Si substrate.

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