JP7401999B2 - elastic wave element - Google Patents

Description

The present invention relates to an acoustic wave element.

Acoustic wave elements have conventionally been used as resonators and bandpass filters, and in recent years there has been a demand for them to be compatible with higher frequency bands. Under these circumstances, elastic wave devices using piezoelectric thin films have been proposed. For example, Patent Document 1 proposes an acoustic wave device that includes a support layer provided with a recess, a piezoelectric thin film disposed over the recess, and an IDT electrode formed on the piezoelectric thin film.

International Publication 2012/073871

In recent years, there has been an increasing demand for higher frequencies in communication equipment, and there is a need to provide acoustic wave elements that can achieve higher frequencies using other methods.

The acoustic wave device of the present disclosure includes an IDT electrode, a piezoelectric layer, and a substrate. The IDT electrode includes a plurality of electrode fingers that are repeatedly arranged at a repeating interval p and have a thickness of 0.08p or more and 0.12p or less. The IDT electrode is located on the top surface of the piezoelectric layer, the thickness is 0.5p or more and 0.65p or less, and the Euler angles (φ, θ, ψ) are (90°±2°, 90°±2° , 60° to 70°). The substrate includes a first surface directly or indirectly joined to the lower surface of the piezoelectric layer, and is made of silicon carbide and has a thickness of 1.6p or more.

An acoustic wave device according to another embodiment of the present disclosure includes an IDT electrode, a piezoelectric layer, and a substrate. The IDT electrode includes a plurality of electrode fingers arranged repeatedly at a repetition interval p. The piezoelectric layer has the above-mentioned IDT electrode located on the upper surface, and is made of lithium monoboxide having Euler angles (φ, θ, ψ) of (90°±2°, 90°±2°, 50° to 80°). Consists of crystals. The substrate includes a first surface directly or indirectly joined to the lower surface of the piezoelectric layer, and is made of silicon carbide and has a thickness of 1.6p or more. The relationship between the thickness of the piezoelectric layer and the thickness of the IDT electrode is within a region surrounded by a solid line in FIG.

An acoustic wave device according to another embodiment of the present disclosure includes an IDT electrode, a piezoelectric layer, and a substrate. The IDT electrode includes a plurality of electrode fingers arranged repeatedly at a repetition interval p. The piezoelectric layer has the above-mentioned IDT electrode located on the upper surface, and is made of lithium monoboxide having Euler angles (φ, θ, ψ) of (90°±2°, 90°±2°, 50° to 80°). Consists of crystals. The substrate includes a first surface directly or indirectly bonded to the lower surface of the piezoelectric layer, and is made of sapphire and has a thickness of 1.6p or more. The Euler angles (φ, θ, ψ) of the substrate are (90°±2°, 90°±2°, 130° to 160°). Further, the relationship between the thickness of the piezoelectric layer and the thickness of the IDT electrode is within a region surrounded by a solid line in FIG. 10 or 11.

According to the above configuration, it is possible to provide an elastic wave element that is compatible with higher frequencies.

FIG. 1 is a cross-sectional view of an acoustic wave element according to the present disclosure. FIG. 3 is a top view showing the structure of an IDT electrode. FIGS. 3(a) and 3(b) are diagrams showing the frequency characteristics of the acoustic wave element according to the present disclosure, respectively. FIG. 3 is a diagram showing the frequency characteristics of an acoustic wave element when changing the Euler angle of a piezoelectric layer. FIGS. 5(a) and 5(b) are diagrams showing frequency characteristics of elastic wave elements according to modified examples, respectively. FIGS. 6(a) and 6(b) are diagrams showing frequency characteristics of elastic wave elements according to modified examples, respectively. FIG. 2 is a diagram showing the relationship between the thickness of a piezoelectric layer of an acoustic wave element, the Euler angle of the piezoelectric layer, and the thickness of an IDT electrode. 2 is a cross-sectional view of a modification of the acoustic wave element shown in FIG. 1. FIG. FIGS. 9(a) and 9(b) are diagrams showing frequency characteristics of elastic wave elements according to modified examples, respectively. FIG. 3 is a diagram showing the relationship between the thickness of the piezoelectric layer of the acoustic wave element, the Euler angle of the substrate, and the thickness of the IDT electrode. FIG. 3 is a diagram showing the relationship between the thickness of the piezoelectric layer of the acoustic wave element, the Euler angle of the substrate, and the thickness of the IDT electrode. FIGS. 12(a) and 12(b) are diagrams showing frequency characteristics of elastic wave elements according to modified examples, respectively.

Hereinafter, an example of the acoustic wave device of the present disclosure will be described in detail using the drawings.

The acoustic wave device 1 (SAW device 1) of this embodiment includes a support substrate 10, a substrate 20, a piezoelectric layer 30, and an IDT electrode 4, as shown in FIG. The support substrate 10, the substrate 20, and the piezoelectric layer 30 are laminated in this order.

The support substrate 10 supports the substrate 20 and the piezoelectric layer 30 located thereon, and the material is not limited as long as it has the strength. For example, a ceramic substrate, an organic substrate, a dielectric substrate such as crystal or sapphire, a piezoelectric substrate, a semiconductor substrate, etc. may be used, and the substrate may be made of the same material as the piezoelectric layer 30 described later, or may be a multilayer substrate. In this example, a single crystal silicon substrate is used.

When a silicon substrate is used as the support substrate 10, the coefficient of thermal expansion is smaller than that of the material of the piezoelectric layer 30, which will be described later. Therefore, when a temperature change occurs, thermal stress is generated in the piezoelectric layer 30, and at this time, the temperature dependence of the elastic constant and the stress dependence cancel each other out, and as a result, temperature changes in the electrical characteristics of the SAW element 1 are suppressed (temperature special compensation).

Furthermore, by increasing the coefficient of linear expansion in the order of the support substrate 10, the substrate 20, and the piezoelectric layer 30, which will be described later, it is possible to suppress peeling of each layer and to effectively exhibit the temperature compensation effect of the support substrate 10. can.

The thickness of the support substrate 10 is not particularly limited, but may be, for example, about 100 μm to 250 μm. The thickness decreases in the order of the support substrate 10, the substrate 20, the piezoelectric layer 30, and the IDT electrode 4, which will be described later.

The substrate 20 includes a first surface 20A and a second surface 20B opposite to the first surface 20A. The second surface 20B is bonded to the upper surface of the support substrate 10, and the first surface 20A is bonded to the lower surface of the piezoelectric layer 30. In this example, the first surface 20A and the piezoelectric layer 30 and the second surface 20B and the support substrate 10 are both directly bonded, but this is not the case. In particular, when the substrate 20 has a thickness equal to or greater than the wavelength λ defined as twice the repetition interval Pt1 (hereinafter sometimes simply referred to as "pitch" or "p") of the electrode fingers 41 of the IDT electrode 4, which will be described later. The substrate 20 and the supporting substrate 10 may be bonded via a bonding layer or the like. The thickness of the bonding layer may be, for example, 0.2p or less. More preferably, the thickness may be 10 nm or less.

The substrate 20 is made of a material with a transverse sound velocity of 5800 m/s or more. Such materials include aluminum nitride (AlN), titanium nitride (TiN), silicon nitride (Si ₃ N ₄ ), sapphire, silicon carbide (SiC), alumina, boron nitride (BN), diamond, diamond-like carbon ( DLC), etc.

The thickness of the substrate 20 is, for example, 0.8λ (ie, 1.6p) or more. More preferably, it is 1λ or more. Although there is no particular upper limit to the thickness of the substrate 20, if the substrate 20 is formed by a thin film process or the like, it may be set to 10λ or less in consideration of film formability and the like.

A piezoelectric layer 30 is located on the first surface 20A of the substrate 20. The piezoelectric layer 30 is made of lithium niobate single crystal (LiNbO ₃ : hereinafter sometimes abbreviated as LN) with a thickness of less than 0.35λ. The Euler angles (φ, θ, ψ) are (90°±2°, 90°±2°, 50° to 80°).

The IDT electrode 4 is located on the top surface of the piezoelectric layer 30. The IDT electrode excites surface acoustic waves, and as shown in FIG. 2, constitutes a resonator consisting of, for example, a pair of comb-shaped electrodes 40A and 40B. The comb-shaped electrode includes a plurality of electrode fingers 41. The electrode fingers 41A connected to one potential and the electrode fingers 41B connected to the other potential are arranged so as to alternately cross each other, and the SAW propagates along the direction in which the electrode fingers 41 are arranged. . The interval between the centers of the widths of the electrode fingers 41A and 41B is defined as a pitch Pt1. Note that the width of the electrode finger 41 is assumed to be w1, and its thickness is assumed to be s. Reflector electrodes or the like may be provided on both sides of the IDT electrode 4 in the X direction.

Examples of materials constituting the IDT electrode 4 include Al--Cu alloy. The thickness is determined in consideration of the excitation efficiency of the SAW, the electromechanical coupling coefficient with the LN substrate, and the like.

A protective layer 6 is placed on the upper surface of the IDT electrode 4 to suppress its oxidation. Examples of the material for the protective layer 6 include inorganic insulating materials such as silicon oxide and silicon nitride.

FIG. 3 shows the results of simulating the resonance characteristics of the SAW element 1 having the above-described configuration. FIG. 3(a) shows impedance characteristics with respect to frequency, and FIG. 3(b) shows phase characteristics. In FIG. 3A, the horizontal axis represents frequency (unit: MHz), and the vertical axis represents impedance (unit: Ω). In FIG. 3(b), the horizontal axis represents frequency (unit: MHz), and the vertical axis represents impedance phase (unit: degree).

The basic configuration of simulation model 1 was as follows.
<Model 1>
Material of electrode finger 41: Al-Cu 1% addition alloy Thickness of electrode finger 41: 0.08p (0.04λ)
Pitch of electrode fingers 41: 1 μm (λ=2 μm)
Material of piezoelectric layer 30: LiNbO ₃
Euler angle of piezoelectric layer 30: (φ, θ, ψ) = (90°, 90°, 50°) (X-cut substrate/corresponds to 50°Y propagation)
Thickness of piezoelectric layer 30: 0.6p (0.3λ)
Material of substrate 20: SiC
Thickness of substrate 20: 2λ
Material of support substrate 10: Si
Thickness of support substrate 10: 250 μm
Note that this simulation is performed for an IDT with an infinite period, and the absolute value of impedance is converted when the intersection width x the number of IDTs is 2000λ.

The most common conventional SAW element uses a 128° rotated YX propagation LN substrate with a thickness of 1λ or more as the piezoelectric layer 30, with Euler angles of (0°, -48°, 0°). It is used in In this case, if the pitch of the electrode fingers 41 is 1 μm, the resonance frequency is about 2 GHz. On the other hand, according to the SAW element 1, by changing the conventional SAW element, the thickness of the piezoelectric layer 30, and the Euler angle, the pitch of the electrode fingers 41 is 1 μm in the model 1, as shown in FIG. It was confirmed that the resonant frequency could be set to 3.25 GHz.

In addition, in model 1, the value (sound velocity) obtained by multiplying the resonance frequency by λ is 6400 m/s. From this, it can be confirmed that the resonator functions using a faster mode of elastic waves than the mode of elastic waves used in conventional SAW elements.

Furthermore, from the waveform in Figure 3, Δf is wider than that of the conventional SAW element, and there is no spurious between the resonant frequency and the anti-resonant frequency, and the frequency characteristics are It was confirmed that an excellent SAW element 1 with low loss could be provided.

Model 1 was confirmed by applying all Euler angles over the entire angle range, and it was found that even if the Euler angles above were changed within ±2° for φ and ±2° for θ, compared to the conventional SAW element, It was confirmed that it is possible to provide a SAW device 1 with high frequency and low loss.

FIG. 4 shows changes in the impedance waveform of the resonator when the Euler angle ψ of the piezoelectric body is changed in Model 1. The horizontal axis is frequency (unit: MHz), and the vertical axis is impedance (unit: Ω).

As can be seen from FIG. 4, when ψ deviates from the above range, the frequency characteristics deteriorate. Specifically, as ψ becomes smaller, spurious waves located on the lower frequency side than the resonant frequency become larger, and when the angle is 40 degrees or less, the influence of the spurious waves becomes large. On the other hand, as ψ increases, the frequency difference (Δf) between the resonant frequency and the anti-resonant frequency decreases, and the spurious on the low frequency side also increases. Specifically, when ψ exceeds 90°, even though an LN substrate is used, the Δf becomes the same as when using a lithium tantalate substrate (approximately 0.4 times that when ψ = 50°). I end up. From the above, high frequency characteristics can be provided by setting ψ from the above-mentioned Euler angle to a range of 50° or more and 80° or less. More preferably, ψ may be set to 60° to 70°.

In addition, when ψ of the above-mentioned Euler angles is in the range of 50° to 80°, it is possible to provide a SAW element 1 with higher frequency and less loss than conventional SAW elements. There is an appropriate range depending on the thickness of the piezoelectric layer. This range will be discussed later.

Further, according to the SAW element 1, since the substrate 20 is located on the entire lower surface of the piezoelectric layer 30, it can be easily handled and has high reliability.

Note that by making the substrate 20 a material having a cutoff frequency higher than the resonance frequency band of the resonator realized by the SAW element 1, leakage to the substrate 20 side is suppressed, and the SAW element 1 with less loss is provided. can do. Specifically, the cutoff frequency of SiC used as the substrate 20 in this example is 3.9 GHz. Therefore, as shown in FIG. 3(b), a resonator with less loss can be achieved in the region below 3.9 GHz. Note that when the thickness of SiC becomes thinner, some of the acoustic waves reach the support substrate 10 and leak. Therefore, the thickness of the substrate 20 needs to be at least 0.8λ or more, preferably 1λ or more. Further, an adhesion layer or an adjustment layer for adjusting characteristics may be inserted between the substrate 20 and the piezoelectric layer 30.

Moreover, the Euler angles (φ, θ, ψ)=(90°, 90°, 50°) of the piezoelectric layer 30 of model 1 means that an X-cut LN−50°Y propagation substrate is used. Therefore, although vertical leakage can be suppressed by changing the material of the substrate 20, there is a risk that horizontal leakage will occur. For this reason, a dummy electrode may be provided in the IDT electrode 4, or the speed of sound may be made different in the area where the dummy electrode intersects with the electrode finger 41. Further, the IDT may be arranged tilted in the direction of power flow.

In this example, the substrate 20 has a diameter of 2λ, and the supporting substrate 10 made of Si is provided on the lower surface thereof. With this configuration, thermal stress due to Si, which has a small coefficient of thermal expansion, is applied to the piezoelectric body 30, so that the SAW element 1 can suppress changes in characteristics due to temperature changes.

<Correlation between impedance characteristics and Euler angle and thickness of piezoelectric layer 30>
The impedance characteristics of the SAW element 1 change depending on the Euler angle and thickness of the piezoelectric layer 30 and the thickness of the electrode 4. For example, for model 1, the Euler angle of the piezoelectric layer 30 is (90°, 9
0°, 50°) is fixed, and the impedance characteristics are shown in FIG. 5(a) when the thickness of the piezoelectric layer 30 is reduced to 0.4p. FIG. 5A is a diagram showing impedance characteristics with respect to frequency, where the vertical axis shows impedance (unit: Ω) and the horizontal axis shows frequency (unit: MHz). When the thickness of the piezoelectric layer 30 is 0.4p, the resonant frequency is higher than when it is 0.6p (FIG. 3). For this reason, if the thickness of the piezoelectric layer 30 is made thinner than this, it will approach the cutoff frequency of the substrate 20, and as a result, a large loss will occur in the frequency region near anti-resonance. Additionally, the spurious on the low frequency side is also large.

Furthermore, the impedance characteristics when the thickness of the piezoelectric layer 30 is 0.5p are shown in FIG. 5(b), the impedance characteristics when the thickness is 0.6p are shown in FIG. 6(a), and the impedance characteristics when the thickness is 0.8p. The characteristics are shown in FIG. 6(b). FIG. 5(b), FIG. 6(a), and FIG. 6(b) are drawings corresponding to FIG. 5(a).

As described above, by increasing the thickness of the piezoelectric layer 30, the resonant frequency becomes lower, and large spurious vibrations occur on the higher frequency side than the anti-resonance.

In this way, the thickness of the piezoelectric layer 30 has an appropriate range, and the range is related to the Euler angle of the piezoelectric layer 30. In the case of the Euler angle of the piezoelectric layer 30 of Model 1, by setting the thickness to less than 0.7p, resonance at high frequencies can be obtained without being affected by spurious components. In particular, when the thickness is set to 0.4p or more, deterioration of loss near the anti-resonance frequency can be suppressed. From the above, the thickness of the piezoelectric layer 30 may be greater than or equal to 0.4p and less than or equal to 0.7p, more preferably greater than or equal to 0.5p and less than or equal to 0.65p.

Figure 7 shows the simulation results of the range in which good impedance characteristics can be obtained when the Euler angle ψ of the piezoelectric layer 30, the thickness of the piezoelectric layer 30, and the thickness of the IDT electrode 4 are changed in Model 1. show. The horizontal axis is the thickness of the piezoelectric layer 30 (unit: x p), and the vertical axis is the electrode thickness (unit: x p). In addition, in the simulation, the thickness of the piezoelectric layer 30 was set to 0.3~
1.0p and an electrode thickness of 0.03 to 0.2p in a matrix format, and the points at which good impedance characteristics were obtained are plotted in FIG. The range surrounded by the solid line in FIG. 7 is the range in which good characteristics found from the above simulation results can be obtained.

The simulation was performed using the finite element method (FEM) with the duty of the electrode fingers set to 0.5 and the loss set to 0. In FEM, piezoelectric equations are solved using actual shapes and physical property values. That is, since there is no modeling, approximation, etc., it is possible to calculate results that are almost in line with actual measurements without software dependence. In addition, "good impedance characteristics" are those that satisfy conditions such as a desired resonance frequency, Δf of a certain width or more, and no spurious near the resonance frequency or anti-resonance frequency. Specifically, the following conditions are met. be.
・Δf≧120MHz
・Resonance frequency fr≧2700MHz
・Maximum spurious Δf≦18MHz
・Difference between fr and cutoff frequency ≧300MHz
Furthermore, although there is little dependence on software, for example, ANSYS Mechanical Ver. 19.0 may be used.

In addition, in FIG. 7, even if ψ was changed by ±2° from the specified value, there was no significant change in the range surrounded by the solid line.

As can be seen from FIG. 7, when ψ is 60° to 70°, the range in which good characteristics can be obtained is wide, but as ψ becomes smaller or larger, this range becomes narrower. When ψ is less than 40 degrees or more than 90 degrees, spurious appears in the impedance characteristics and Δf is small, making it impossible to obtain good characteristics.

From the above, good characteristics can be obtained when ψ=50° to 80°, the thickness of the piezoelectric layer 30 is 0.5p to 0.7p, and the thickness of the IDT electrode 4 is 0.07p to 0.15p. The range may be within the area indicated by the solid line in FIG. Furthermore, when the piezoelectric layer 30 has ψ=60° to 70°, its thickness is 0.5p to 0.65p, and the IDT electrode 4 has a thickness of 0.08p to 0.12p, the robustness to each parameter is improved. A resonator with excellent and stable characteristics can be obtained.

<Modification 1>
In the above example, the SAW element 1 having the support substrate 10 has been described, but since there is no upper limit to the thickness of the substrate 20 in terms of electrical characteristics, by making the substrate 20 thicker so as to have the function as the support substrate 10, The support substrate 10 may be omitted.

For example, as shown in FIG. 8, the substrate 20 may be a SiC substrate, a diamond-like carbon substrate, a diamond substrate, or the like having a thickness of about 50 μm to 250 μm. Further, an adhesion layer or an adjustment layer for adjusting characteristics may be inserted between the substrate 20 and the piezoelectric body 30.

Further, although the above example shows simulation results for the case where a non-oriented SiC substrate is used, it has been confirmed through simulation that similar results are obtained when oriented single-crystal SiC is used. Furthermore, materials having a sound velocity on the same level as SiC (such as BN in addition to the above-mentioned diamond-like carbon and diamond) can also obtain good characteristics within the same range as the results shown in FIG.

<Modification 2>
In the above example, the Euler angles of the piezoelectric layer 30 are (90°, 90°, 60°), but φ, θ, and ψ are each changed from 0° to 360° for every combination. S.A.
The frequency characteristics of the W element were simulated. As a result, as shown in FIG. 9, it was confirmed that a high resonant frequency could be obtained even for (30±2, 90±2, 120±10) without generating spurious in the vicinity of resonance and anti-resonance.

Note that with Euler angles other than the above, it was not possible to obtain resonance characteristics while suppressing spurious.

<Modification 3>
In the above example, the case where SiC was used as the substrate 20 was explained, but sapphire may also be used. When sapphire is used as the substrate 2, the speed of sound is not as high as that of SiC, so the antiresonance frequency and the cutoff frequency of sapphire itself become close, and large ripples appear on the slightly higher frequency side of antiresonance. On the other hand, since the sound speed of sapphire increases at a specific Euler angle, by adjusting the Euler angle it is possible to ensure a sufficient difference between the antiresonance and the cutoff frequency of sapphire.

Therefore, we simulated the frequency characteristics by varying the Euler angles of sapphire over all angles, and the results of finding the optimal Euler angles are shown in FIGS. 10 and 11. 10 and 11 correspond to FIG. 7 showing the results for the SiC substrate. FIG. 10 shows a case where the Euler angle of the piezoelectric layer 30 is (90, 90, ψ) and ψ=50°, and FIG. 11 shows a case where ψ=60°.

Also, for ψ=50° and 60°, the Euler angles of sapphire are (90, 90, Z
) shows the range of the thickness of the piezoelectric layer 30 and the thickness of the IDT electrode 4 in which good resonance characteristics without spurious waves can be obtained when Z=130° to 160°. Good resonance characteristics can be obtained within the range surrounded by the solid line in the figure.

Here, it has been confirmed that even if Z and ψ are varied by ±2 degrees, there is no large variation in the area surrounded by the solid line.

Note that good resonance characteristics were not obtained at Euler angles other than those shown in this figure (ψ is 40° or less and 70° or more, Z is 120° or less and 170° or more).

FIG. 12 shows an example of simulation results of resonance characteristics when a sapphire substrate is used as the substrate 20. The parameters are Z=150°, ψ=50°, the thickness of the piezoelectric layer 30 is 0.8p, and the thickness of the IDT electrode 4 is 0.09p.

As mentioned above, the interval between the anti-resonant frequency and the cut-off frequency is narrower than when SiC is used as the substrate 20, but there is no spurious between the resonant frequency and the anti-resonant frequency and on the lower frequency side, which is good. It can be seen that excellent resonance characteristics are obtained.

In addition, in the above example, both φ and θ of the Euler angles (φ, θ, ψ) of the substrate 20 were set to 90°±2°, but φ was set to 90°±0.5° and θ was set to 90°. It may be set to ±1°. In that case, spurious can be further reduced.

1: Acoustic wave element 20: Substrate 30: Piezoelectric layer 4: IDT electrode 41: Electrode finger

Claims (4)

an IDT electrode including a plurality of electrode fingers arranged repeatedly at a repetition interval p and having a thickness of 0.08p or more and 0.12p or less;
The IDT electrode is located on the top surface, the thickness is 0.5p or more and 0.65p or less, and the Euler angles (φ, θ, ψ) are (90° ± 2°, 90° ± 2°, 60° ~ a piezoelectric layer made of a lithium niobate single crystal whose angle is 70°);
An acoustic wave element comprising: a substrate made of silicon carbide and having a first surface bonded directly or indirectly to the lower surface of the piezoelectric layer and having a thickness of 1.6p or more. The substrate includes a second surface opposite to the first surface,
The acoustic wave device according to claim 1 , further comprising a support substrate bonded to the second surface and having a thickness exceeding 1λ. The acoustic wave device according to claim 2 , wherein the support substrate is made of silicon, and the thickness of the piezoelectric layer, the substrate, and the support substrate increase in this order. 4. The elastic wave element according to claim 1, wherein a value obtained by multiplying the resonance frequency of the elastic wave generated by the IDT electrode by 2p is 5800 m/s or more .

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