Vertical leaky surface acoustic wave device

Abstract

To improve the admittance ratio of LLSAW in LiNbO3. [Solution] The first piezoelectric layer 101 is Ca3Ta(Ga) in the direction of (0°, 90°, 90°) in Euler angle notation. 1-x Al x )3Si2O 14 It is composed of a single crystal (0≦x<1). The second piezoelectric layer 102 is composed of a LiNbO3 single crystal with an orientation of (0°,θ,90°) in Euler angle notation. θ is in the range of 30° to 95°. The crossed finger-shaped electrode 103 has a thickness of 0.07λ to 0.08λ.

Description

[Technical Field]

[0001] This invention relates to a vertical leaky surface acoustic wave device. [Background technology]

[0002] Surface Acoustic Wave (SAW) filters used in TV (Television), BS (Broadcasting Satellite) tuners, smartphones, and other devices have primarily utilized Rayleigh SAW and Leaky SAW (LSAW) filters.

[0003] A Rayleigh SAW consists of two components: a longitudinal wave (L) with displacement in the direction of propagation and a shear-vertical wave (SV) with displacement in the depth direction perpendicular to the longitudinal wave. Near the surface, it moves in an elliptical orbit that rotates backward with respect to the direction of propagation. As an example, Figure 12 shows the displacement distribution of each component at the resonant frequency of a 128° rotated Y-cut X-propagating LiNbO3 Rayleigh SAW.

[0004] The vibrational displacement is primarily composed of L and SV components, and these displacements are waves concentrated on the surface within one wavelength (λ: wavelength of Rayleigh SAW). Because the phase velocity of this SAW is slower than that of L, SV, or bulk waves of shear-horizontal (SH) waves with transverse displacement, it is a non-leakage mode, and theoretical propagation attenuation is zero. For this reason, Rayleigh SAWs have been used as SAW filters for low-frequency TV tuners.

[0005] Leaky SAW (LSAW) is a wave in which the wave energy is concentrated near the surface, but propagates while radiating SV waves into the interior of the crystal layer. As an example, Figure 13 shows the displacement distribution at the resonant frequency of a Rayleigh SAW of 42° rotated Y-cut X-propagating LiTaO3. Here, the grayscale representation of the displacement of each component shows the difference between the maximum and minimum displacements. The main component of the displacement is SH, and it can be seen that the L component and SH component are an order of magnitude smaller than the displacement of the SH component. Since the main component of the displacement in LSAW is SH, and its phase velocity is faster than that of SV bulk waves and Rayleigh-type SAWs, it is advantageous for high-frequency filtering.

[0006] Leaky SAW exhibits propagation attenuation due to bulk wave radiation, but crystal layer orientations have been found in materials such as 64° Y-cut X-propagating LiNbO3 and 36°~42° Y-cut X-propagating LiTaO3 where propagation attenuation is nearly zero. These crystal layer orientations yield relatively large electromechanical coupling coefficients and are widely used as components that form the relatively wide passband required for the transmitting and receiving sections of SAW filters and duplexers used in mobile communications such as smartphones at high frequencies. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] NF Naumenko, "Advanced Substrate Material for SAW Devices Combining LiNbO3 and Langasite", IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 67, no. 9, pp. 1909-1915, 2020. [Overview of the project] [Problems that the invention aims to solve]

[0008] On the one hand, a leakage mode having a high phase velocity close to the L wave is called a longitudinal leaky SAW (Longitudinal-type: LSAW, hereinafter LLSAW). As an example, Fig. 14 shows the displacement distribution at the resonance frequency of LLSAW in the (0°, θ, 90°) azimuth in the Euler angle representation of LiNbO3. In Fig. 14, the display (gray scale) of the displacement of each component shows the shade by the difference between the maximum and minimum displacements. In particular, the SH component has a displacement on the order of 10 -16 which is very small. The main component of the displacement is the L component, and it can be seen that it is a surface wave clearly different from the Rayleigh SAW and LSAW.

[0009] Although the main component of the displacement of LSAW is the L component, it is known that LSAW has a very large propagation attenuation because, in addition to the SV bulk wave, the SH bulk wave also propagates while radiating into the crystal layer (Non-Patent Document 1). Generally, since these two types of bulk waves do not radiate simultaneously, there is a drawback that there is no substrate azimuth at which the propagation attenuation of LLSAW becomes zero. However, since LLSAW is a wave faster than LSAW and is a propagation mode advantageous for high-frequencyization of filters, research on reducing the loss of LLSAW has been conducted.

[0010] In LiNbO3, it is known that there is LLSAW with a relatively large electromechanical coupling coefficient for X-cut 36°Y propagation. However, due to the large propagation loss, the admittance ratio (= 20log(Y r / Y a ) represented by the ratio of the admittance Y at resonance and the admittance Y at anti-resonance is small, and there has been a problem that it is not suitable for practical use of SAW filters. r / Y a )) is small, and there has been a problem that it is not suitable for practical use of SAW filters.

[0011] Here, regarding the admittance ratio, as shown in Fig. 15, taking the admittance Y r at resonance and the admittance Y a at anti-resonance, the admittance ratio = 20log(Y r / Y a) It is defined as follows. That is, it represents the difference in admittance between resonance and anti-resonance of the resonance characteristics in FIG. 15. The larger this difference is, the more it serves as an index for achieving both low insertion loss of the filter and large out-of-band attenuation.

[0012] The present invention has been made to solve the above problems, and aims to improve the admittance ratio of LLSAW in LiNbO3.

Means for Solving the Problems

[0013] The vertical leaky surface acoustic wave device according to the present invention is composed of a first piezoelectric layer made of a single crystal of Ca3Ta(Ga 1-x Al x )3Si2O 14 in the Euler angle representation with the orientation of (0°, 90°, 90°), a second piezoelectric layer made of a single crystal of LiNbO3 in the Euler angle representation with the orientation of (0°, θ, 90°) and joined to the first piezoelectric layer, and an interdigital electrode made of Al formed on the surface of the second piezoelectric layer for generating a vertical leaky surface acoustic wave with a wavelength λ on the surface of the second piezoelectric layer. Here, θ is in the range of 30° to 95°, and the interdigital electrode has a thickness of 0.07λ to 0.08λ.

[0014] In one configuration example of the above vertical leaky surface acoustic wave device, θ is in the range of 65° to 90°.

[0015] In one configuration example of the above vertical leaky surface acoustic wave device, the second piezoelectric layer has a thickness of 0.3λ to 0.8λ.

Effects of the Invention

[0016] As described above, according to the present invention, Ca3Ta(Ga 1-x Al x )3Si2O 14By bonding a second piezoelectric layer, composed of a LiNbO3 single crystal with an orientation of (0°,θ,90°) in Euler angle representation, to a first piezoelectric layer composed of a single crystal (0≦x<1), the admittance ratio of LLSAW in LiNbO3 can be improved. [Brief explanation of the drawing]

[0017] [Figure 1A] Figure 1A is a cross-sectional view showing a partial configuration of a vertical leaky surface acoustic wave device according to an embodiment of the present invention. [Figure 1B] Figure 1B is a plan view showing a partial configuration of a vertical leaky surface acoustic wave device according to an embodiment of the present invention. [Figure 2] Figure 2 is a characteristic diagram showing the θ dependence of the phase velocity of LLSAW in a LiNbO3 (hereinafter LN) crystal at the (0°, θ, 90°) orientation in Euler angle representation. [Figure 3] Figure 3 is a characteristic diagram showing the electromechanical coupling coefficient K2 eff of an LLSAW on a LN at the (0°, θ, 90°) orientation in Euler angle notation. [Figure 4] Figure 4 is a characteristic diagram showing the phase velocity change of LLSAW with respect to θ in a configuration in which LN is joined to a CTGS single crystal with the orientation (φ,θ,ψ) in Euler angle representation, with φ=0° and ψ=90°. [Figure 5] Figure 5 is a characteristic diagram showing the change in the electromechanical coupling coefficient of LLSAW with respect to θ in a configuration in which LN is bonded to CTGS single crystals with (φ,θ,ψ) orientation in Euler angle representation, with φ=0° and ψ=90°. [Figure 6] Figure 6 is a characteristic diagram showing the θ dependence of the electromechanical coupling coefficient k2 eff of the LLSAW on the upper surface of the second piezoelectric layer 102 in a structure in which the second piezoelectric layer 102 is bonded to the first piezoelectric layer 101, with the wavelength of the LLSAW being λ = 10 μm and the thickness of the cross-finger-shaped electrode 103 being hAl = 0.076 λ. [Figure 7] Figure 7 is a characteristic diagram showing the θ dependence of the admittance ratio of a vertical leaky surface acoustic wave device according to an embodiment. [Figure 8]Figure 8 is a distribution diagram showing the displacement distribution in a vertical leaky surface acoustic wave device according to an embodiment, where θ = 80°, the thickness of the second piezoelectric layer 102 is hLN = 0.57λ, and the thickness of the cross-finger-shaped electrode 103 is hAl = 0.076λ. [Figure 9] Figure 9 is a characteristic diagram showing the resonance characteristics of a resonator in a vertical leaky surface acoustic wave device according to an embodiment, where θ = 80°, the thickness of the second piezoelectric layer 102 is hLN = 0.57λ, and the thickness of the cross-finger-shaped electrode 103 is hAl = 0.076λ. [Figure 10] Figure 10 is a characteristic diagram showing the admittance ratio when the thickness hAl of the cross-finger-shaped electrode 103 is varied, with θ = 80°, in a vertical leaky surface acoustic wave device according to an embodiment. [Figure 11] Figure 11 is a characteristic diagram showing the resonance characteristics of a resonator in a vertical leaky surface acoustic wave device according to an embodiment, where θ = 80° and the thickness hAl of the cross-finger-shaped electrode 103 is varied. [Figure 12] Figure 12 is a distribution diagram showing the displacement distribution of each component at the resonant frequency of a Rayleigh SAW for 128° rotated Y-cut X-propagating LiNbO3. [Figure 13] Figure 13 is a distribution diagram showing the displacement distribution at the resonant frequency of a Rayleigh SAW of 42° rotated Y-cut X-propagating LiTaO3. [Figure 14] Figure 14 is a distribution diagram showing the displacement distribution at the resonant frequency of the LLSAW in the (0°, θ, 90°) orientation in Euler angle representation for LiNbO3. [Figure 15] Figure 15 is an explanatory diagram illustrating the admittance ratio. [Modes for carrying out the invention]

[0018] Hereinafter, a vertical leaky surface acoustic wave device according to an embodiment of the present invention will be described with reference to Figures 1A and 1B. This vertical leaky surface acoustic wave device comprises a first piezoelectric layer 101, a second piezoelectric layer 102 bonded to the first piezoelectric layer 101, and cross-finger-shaped electrodes 103 formed on the surface of the second piezoelectric layer 102.

[0019] The first piezoelectric layer 101 is Ca3Ta(Ga) in the direction of (0°, 90°, 90°) in Euler angle notation. 1-x Al x )3Si2O 14 It is composed of a single crystal (0≦x<1). The second piezoelectric layer 102 is composed of a LiNbO3 single crystal with an orientation of (0°,θ,90°) in Euler angle notation. θ is in the range of 30° to 95°. The crossed finger-shaped electrode 103 is composed of Al and generates a longitudinal leaky surface acoustic wave (LLSAW) of wavelength λ on the surface of the second piezoelectric layer 102. The crossed finger-shaped electrode 103 has a thickness of 0.07λ to 0.08λ.

[0020] Furthermore, θ can be in the range of 65° to 90°. Also, the thickness of the second piezoelectric layer 102 can be 0.3λ to 0.8λ.

[0021] Figure 2 shows the θ dependence of the phase velocity of LLSAW in a LiNbO3 (hereinafter LN) crystal at the (0°, θ, 90°) orientation in Euler angle notation. The wavelength λ of LSAW is λ = 10 μm, and the Al film used as the electrode (thickness h) is... Al The phase velocity when (=0.076λ) is present is shown by white circles, and the phase velocity on the LN surface without an Al film on the LN crystal is shown by black circles.

[0022] As shown in Figure 2, both the free surface and the Al film surface have phase velocities of 6000 m / s or more. The phase velocities of conventional Rayleigh SAWs and LSAWs are generally around 4000 m / s, although this depends on the crystal orientation. Therefore, it can be confirmed that LLSAWs have a phase velocity approximately 1.5 times faster than conventional SAWs. Furthermore, a higher phase velocity allows for a wider electrode spacing at the same frequency, which has the advantage of enabling higher frequency devices due to the limitations of manufacturing processes.

[0023] Next, the electromechanical coupling coefficient K of the LLSAW on the LN at the (0°, θ, 90°) direction in Euler angle representation. 2 eff This is shown in Figure 3. A large electromechanical coupling coefficient of 12% or more is obtained in the range of θ = 30° to 90°. The relative bandwidth shown in Figure 15 is proportional to this electromechanical coupling coefficient, so a larger electromechanical coupling coefficient is preferable. Currently, the electromechanical coupling coefficient of general SAWs in practical use is 5-7%, so it can be seen that LLSAW on LN has a large electromechanical coupling coefficient.

[0024] However, when the admittance of LN at the (0°, 80°, 90°) directions was determined, the resonant Q value was Q r =700, anti-resonance Q value is Q a The result was 400, and the admittance ratio was 70. According to this embodiment, it is possible to significantly improve the admittance.

[0025] Next, Ca3Ta(Ga 1-x Al x )3Si2O 14 This section describes the improvement of the admittance ratio by bonding a (0≦x<1) (hereinafter referred to as CTGS) single crystal with LN. A CTGS single crystal with the orientation (φ,θ,ψ) in Euler angle notation is set to φ=0° and ψ=90°, and LN is bonded to this CTGS single crystal to prevent coupling with transverse waves (SH component) parallel to the interface. Figure 4 shows the phase velocity change of LLSAW with respect to θ in this configuration.

[0026] Figure 4 shows the phase velocity changes of the free surface and the short-circuit (metal) surface, but they overlap and are indistinguishable. Both are around 7000 m / s at θ = 90°, indicating a high-speed LLSAW. This result is small for both the free surface and the short-circuit (metal) surface, meaning the electromechanical coupling coefficient is very small. Electromechanical coupling coefficient K in the above configuration, as determined from Figure 4. 2 This is shown in Figure 5. Compared to the electromechanical coupling coefficient of a single LN shown in Figure 3, it is four orders of magnitude smaller, making it an extremely small LLSAW.

[0027] From these results, we found that by setting the orientation of the CTGS joined to the LN to θ=90°, where the phase velocity is maximized, the SAW energy is concentrated within the LN in the configuration where the LN and CTGS are joined, and the admittance can be greatly improved.

[0028] Figure 6 shows the wavelength λ=10μm of the LLSAW and the thickness h of the crossed finger-shaped electrode 103. Al When k = 0.076λ, the electromechanical coupling coefficient of the LLSAW on the upper surface of the second piezoelectric layer 102 in a structure in which the second piezoelectric layer 102 is bonded to the first piezoelectric layer 101 is... 2 eff This is the θ-dependence. It can be seen that the electromechanical coupling coefficient of the LiNbO3 single crystal constituting the second piezoelectric layer 102 is larger than that of LN alone (see Figure 3) in the direction (0°,θ,90°) and θ = 30° to 70° in Euler angle representation. It can be seen that by using the first piezoelectric layer 101 made of CTGS as a support substrate, the electromechanical coupling coefficient becomes larger than that of LN alone.

[0029] Figure 7 shows the θ dependence of the admittance ratio of the vertical leaky surface acoustic wave device according to the embodiment. As can be seen from this figure, an admittance ratio of 70 or higher for the LN alone is achieved in the range θ = 65° to 90°. Thus, it can be seen that with the configuration of the embodiment, there exists a θ that exceeds the electromechanical coupling coefficient and admittance ratio of the LN alone.

[0030] In the vertical leaky surface acoustic wave device according to the embodiment, the electromechanical coupling coefficient is large and the admittance ratio is also large, with θ = 80°, and the thickness h of the second piezoelectric layer 102 is set. LN Let = 0.57λ, and the thickness of the crossed finger-shaped electrode 103 be h Al Figure 8 shows the displacement distribution when = 0.076λ. As can be seen from Figure 8, the vertical leaky surface acoustic wave device according to the embodiment is an LLSAW with a predominantly L component, and the surface waves targeted by the vertical leaky surface acoustic wave device according to the embodiment are different from Rayleigh SAW and LSAW waves.

[0031] Figure 9 shows the resonance characteristics of the resonator in the above configuration as solid lines. For comparison, the resonance characteristics of a single LN in the direction of (0°, 80°, 90°) in Euler angle notation are shown as dotted lines. While the resonance characteristics of a single LN are a resonance Q value of 700, an anti-resonance Q value of 380, and an admittance ratio of 70.1, according to the embodiment, the resonance Q value is 660, which is almost the same (the solid and dotted lines overlap), but the anti-resonance Q value becomes larger at 840, and the resonance response becomes larger (solid line), resulting in an admittance ratio of 75.7.

[0032] In the vertical leaky surface acoustic wave device according to the embodiment, the electromechanical coupling coefficient is large and the admittance ratio is also large, with θ = 80°, and the thickness h of the cross-finger-shaped electrode 103 is set. Al Figure 10 shows the admittance ratio when h is changed. Here, the thickness of the second piezoelectric layer 102 is h LN = 0.57λ was assumed to be constant. From this figure, the thickness of the crossed finger-shaped electrode 103 is h Al It is clear that an admittance ratio of 70 or higher can be achieved for a single LN in the range of 0.07λ to 0.08λ.

[0033] Similarly, in the vertical leaky surface acoustic wave device according to the embodiment, with θ = 80°, the thickness h of the second piezoelectric layer 102 is... LN Figure 11 shows the admittance ratio of the LLSAW on the upper surface of the second piezoelectric layer 102 when the value is changed. Here, the thickness of the crossed finger-shaped electrode 103 is h Al=0.076λ was assumed to be constant. As can be seen from this figure, the thickness of the second piezoelectric layer 102, h LN It can be seen that an admittance ratio of 70 or higher can be achieved for a single LN in the range of 0.3λ to 0.8λ.

[0034] As described above, according to the embodiment of the present invention, Ca3Ta(Ga) in the direction (0°,90°,90°) in Euler angle representation 1-x Al x )3Si2O 14 By bonding a second piezoelectric layer, composed of a LiNbO3 single crystal with an orientation of (0°,θ,90°) in Euler angle representation, to a first piezoelectric layer composed of a single crystal (0≦x<1), the admittance ratio of LLSAW in LiNbO3 can be improved. According to the embodiment of the present invention, it is possible to increase the admittance ratio, which can contribute to the commercialization and practical application of LLSAW devices.

[0035] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be implemented within the technical concept of the present invention by those with ordinary skill in the art. [Explanation of symbols]

[0036] 101...First piezoelectric layer, 102...Second piezoelectric layer, 103...Cross-finger shaped electrode.

Claims

1. Ca of the direction in Euler angle notation (0°, 90°, 90°) 3 Ta(Ga) 1-x Al x ) 3 Si 2 O 14 A first piezoelectric layer composed of a single crystal (0 ≤ x < 1), LiNbO in Euler angle notation (0°, θ, 90°) 3 A second piezoelectric layer, composed of a single crystal and bonded to the first piezoelectric layer, Cross-finger-shaped electrodes made of Al are formed on the surface of the second piezoelectric layer and generate a longitudinal leaky surface acoustic wave of wavelength λ on the surface of the second piezoelectric layer. Equipped with, θ is defined as being in the range of 30° to 95°. The aforementioned cross-finger-shaped electrodes are a vertical leaky surface acoustic wave device with a thickness of 0.07λ to 0.08λ.

2. In the vertical leaky surface acoustic wave device according to claim 1, A longitudinal leaky surface acoustic wave device in which θ is defined as being in the range of 65° to 90°.

3. In the vertical leaky surface acoustic wave device according to claim 1 or 2, The second piezoelectric layer is a vertical leaky surface acoustic wave device having a thickness of 0.3λ to 0.8λ.

Images