Elastic wave devices, filters, and multiplexers

Abstract

This invention provides an elastic wave device that can achieve both improved frequency temperature coefficient and suppression of variations in resonant frequency. [Solution] The elastic wave device 100 comprises a piezoelectric layer 14, a pair of comb-shaped electrodes 20 for exciting elastic waves in the piezoelectric layer 14, and an insulating film 12 provided in the region where the piezoelectric layer 14 and the pair of comb-shaped electrodes 20 overlap, which is an insulating film made of silicon oxide containing elements with greater oxide dissociation energy than silicon (for example, aluminum (Al)) and fluorine (F).

Description

[Technical Field]

[0001] The present invention relates to acoustic wave devices, filters, and multiplexers. [Background technology]

[0002] In high-frequency communication systems, such as mobile phones, high-frequency filters are used to remove unwanted signals outside the frequency band used for communication. Surface acoustic wave (SAW) resonators and bulk acoustic wave (BAW) resonators are used as high-frequency filters. In SAW resonators, it is known that a piezoelectric layer is bonded to a support substrate. It is known that a low-sonic-velocity film with a slower sound velocity than the piezoelectric layer is provided between the piezoelectric layer and the support substrate (for example, Patent Document 1). It is also known that a high-sonic-velocity film with a faster sound velocity than the piezoelectric layer is provided between the low-sonic-velocity film and the support substrate (for example, Patent Document 2). Furthermore, in acoustic wave devices, it is required to bring the temperature coefficient of frequency (TCF) close to zero in order to improve temperature characteristics. To improve TCF, it is known that a temperature compensation film having a TCE opposite to the temperature coefficient of elasticity (TCE) of the piezoelectric layer is used (for example, Patent Documents 3 and 4). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] U.S. Patent No. 10020796 [Patent Document 2] Japanese Patent Publication No. 2015-115870 [Patent Document 3] Japanese Patent Publication No. 2019-201345 [Patent Document 4] International Publication No. 2012 / 029354 [Overview of the project] [Problems that the invention aims to solve]

[0004] Fluorine-doped silicon oxide films are known to have a higher TCE (Temperature Ceiling Optimum) than undoped silicon oxide films and are therefore used as temperature compensation films to improve TCF (Temperature Control Factor). On the other hand, fluorine-doped silicon oxide films have a lower Young's modulus than undoped silicon oxide films. A decrease in Young's modulus leads to a decrease in the speed of sound, so fluorine-doped silicon oxide films have a slower speed of sound than undoped silicon oxide films. As a result, the difference in speed of sound between the piezoelectric layer and the fluorine-doped silicon oxide film is larger than that between undoped silicon oxide films. Consequently, when fluorine-doped silicon oxide films are used as temperature compensation films, variations in the resonance frequency due to variations in the thickness of the piezoelectric layer become larger.

[0005] This invention has been made in view of the above problems, and aims to achieve both improvement of the frequency temperature coefficient and suppression of variations in the resonant frequency. [Means for solving the problem]

[0006] The present invention is an elastic wave device comprising a piezoelectric layer, an electrode for exciting elastic waves in the piezoelectric layer, and a silicon oxide film containing an element whose oxide dissociation energy is greater than that of silicon and fluorine, provided in a region where the piezoelectric layer and the electrode overlap.

[0007] In the above configuration, the element with the large dissociation energy can be aluminum.

[0008] In the above configuration, the doping amount of the element with the large dissociation energy can be 5 atomic percent or more.

[0009] In the above configuration, the Young's modulus of the silicon oxide film is 60 GPa or higher, and the temperature coefficient of the elastic constant is 400 ppm / K or higher.

[0010] In the above configuration, the silicon oxide film may be configured to contain less than 8 atomic% of the aluminum.

[0011] In the above configuration, the silicon oxide film may be configured to contain 2 atomic% or less of the aluminum and 4 atomic% or less of the fluorine, or contain 4 atomic% or more and less than 8 atomic% of the aluminum and 7 atomic% or less of the fluorine.

[0012] In the above configuration, a value obtained by dividing the atomic% of aluminum by the atomic% of fluorine may be 1.6 or less.

[0013] The present invention is a filter including the acoustic wave device described above.

[0014] The present invention is a multiplexer including the filter described above.

Effects of the Invention

[0015] According to the present invention, both improvement of the frequency temperature coefficient and suppression of variation in resonance frequency can be achieved simultaneously.

Brief Description of the Drawings

[0016] [Figure 1] FIG. 1(a) and FIG. 1(b) are a plan view and a cross-sectional view, respectively, of the acoustic wave device according to Example 1. [Figure 2] FIG. 2(a) to FIG. 2(c) are cross-sectional views illustrating a method of manufacturing the acoustic wave device according to Example 1. [Figure 3] FIG. 3 is a cross-sectional view of a sample used in an experiment. [Figure 4] FIG. 4 is a diagram illustrating experimental results of Young's modulus and TCE of insulating films in samples A to G. [Figure 5] FIG. 5(a) and FIG. 5(b) are diagrams illustrating experimental results of density and TCE with respect to annealing temperature of the insulating film. [Figure 6]FIGS. 6(a) and 6(b) are diagrams showing experimental results of fluorine (F) concentration versus depth from the top surface of the insulating film. [Figure 7] FIG. 7 is a diagram showing an experimental result of absorption amount versus wave number of the insulating film. [Figure 8] FIG. 8 is a diagram showing simulation results of variation sensitivity Δfr / fr of resonant frequency for each model, and ΔTCF of each model when TCF of model A is used as a reference. [Figure 9] FIGS. 9(a) and 9(b) are diagrams showing simulation results of TCE and Young's modulus versus fluorine (F) concentration and aluminum (Al) concentration. [Figure 10] FIG. 10 is a cross-sectional view of an acoustic wave device according to a modified example of Embodiment 1. [Figure 11] FIG. 11(a) is a plan view of an acoustic wave device according to Embodiment 2, and FIG. 11(b) is a cross-sectional view taken along line A-A in FIG. 11(a). [Figure 12] FIGS. 12(a) and 12(b) are cross-sectional views of acoustic wave devices according to Modified Example 1 and Modified Example 2 of Embodiment 2, respectively. [Figure 13] FIGS. 13(a) and 13(b) are cross-sectional views of acoustic wave devices according to Modified Example 3 and Modified Example 4 of Embodiment 2, respectively. [Figure 14] FIG. 14(a) is a circuit diagram of a filter according to Embodiment 3, and FIG. 14(b) is a circuit diagram of a duplexer according to a modified example of Embodiment 3. DESCRIPTION OF EMBODIMENTS

[0017] Hereinafter, embodiments of the present invention will be described with reference to the drawings. EMBODIMENTS

[0018] Example 1 describes an example in which an elastic wave device has a surface acoustic wave resonator. Figures 1(a) and 1(b) are a plan view and a cross-sectional view of the elastic wave device 100 according to Example 1. The arrangement direction of the multiple electrode fingers 27 is the X direction, the longitudinal direction of the electrode fingers 27 is the Y direction, and the stacking direction of the support substrate 10 and piezoelectric layer 14 is the Z direction. The X and Y directions do not necessarily correspond to the X-axis and Y-axis directions of the crystal orientation of the piezoelectric layer 14. When the piezoelectric layer 14 is rotationally Y-cut X-propagation, the X direction is the X-axis direction of the crystal orientation.

[0019] As shown in Figures 1(a) and 1(b), a piezoelectric layer 14 is provided on a support substrate 10. An insulating film 11 is provided between the support substrate 10 and the piezoelectric layer 14. An insulating film 12 is provided between the insulating film 11 and the piezoelectric layer 14. An insulating film 13 is provided between the insulating film 12 and the piezoelectric layer 14. The thickness of the insulating film 11 is T1, the thickness of the insulating film 12 is T2, the thickness of the insulating film 13 is T3, and the thickness of the piezoelectric layer 14 is T4.

[0020] An elastic wave resonator 26 is provided on the piezoelectric layer 14. The elastic wave resonator 26 has an IDT (Interdigital Transducer) 22 and a reflector 24. The reflector 24 is provided on both sides of the IDT 22 in the X direction. The IDT 22 and the reflector 24 are formed by a metal film 16 on the piezoelectric layer 14.

[0021] The IDT22 comprises a pair of opposing comb-shaped electrodes 20. Each comb-shaped electrode 20 includes a plurality of electrode fingers 27 and a busbar 28 to which the plurality of electrode fingers 27 are connected. The region where the electrode fingers 27 of the pair of comb-shaped electrodes 20 intersect is the intersection region 25. The length of the intersection region 25 in the Y direction is the aperture length. In at least a portion of the intersection region 25, the electrode fingers 27 of the pair of comb-shaped electrodes 20 are alternately provided one at a time. The elastic wave that mainly excites the plurality of electrode fingers 27 in the intersection region 25 propagates mainly in the X direction. The pitch of the electrode fingers 27 of one of the pair of comb-shaped electrodes 20 is approximately equal to the wavelength λ of the elastic wave. The wavelength λ is approximately twice the average pitch D of the plurality of electrode fingers 27. The average pitch D of the electrode fingers 27 can be calculated by dividing the width of the IDT22 in the X direction by the number of electrode fingers 27. The reflector 24 reflects the elastic waves (surface acoustic waves) excited by the electrode fingers 27 of the IDT 22. As a result, the elastic waves are confined within the cross region 25 of the IDT 22.

[0022] The piezoelectric layer 14 is, for example, a single-crystal lithium tantalate (LiTaO3) layer or a single-crystal lithium niobate (LiNbO3) layer, and is, for example, a rotational Y-cut X-propagation lithium tantalate layer or a rotational Y-cut X-propagation lithium niobate layer.

[0023] The support substrate 10 is, for example, a sapphire substrate, alumina substrate, silicon substrate, spinel substrate, quartz substrate, silica substrate, or silicon carbide substrate. The sapphire substrate is a single-crystal Al2O3 substrate, the alumina substrate is a polycrystalline or amorphous Al2O3 substrate, and the silicon substrate is a single-crystal or polycrystalline Si substrate. The spinel substrate is a polycrystalline or amorphous MgAl2O4 substrate, the quartz substrate is a single-crystal SiO2 substrate, the silica substrate is a polycrystalline or amorphous SiO2 substrate, and the silicon carbide substrate is a polycrystalline or single-crystal SiC substrate. The coefficient of linear expansion in the X direction of the support substrate 10 is smaller than the coefficient of linear expansion in the X direction of the piezoelectric layer 14. This makes it possible to reduce the frequency-temperature dependence of the elastic wave resonator.

[0024] The speed of sound of bulk waves propagating through the insulating film 11 is faster than the speed of sound of bulk waves propagating through the insulating film 12 and the piezoelectric layer 14. This confines the energy of the main response elastic waves within the piezoelectric layer 14 and the insulating film 12. The speed of sound of bulk waves propagating through the insulating film 11 is preferably 1.1 times or more, and more preferably 1.2 times or more, than the speed of sound of bulk waves propagating through the insulating film 12. If the speed of sound of bulk waves in the insulating film 11 becomes too fast, the bulk waves are more likely to be reflected at the interface between the insulating film 11 and the insulating film 12. Therefore, the speed of sound of bulk waves propagating through the insulating film 11 is preferably 2.0 times or less, and more preferably 1.5 times or less, than the speed of sound of bulk waves propagating through the insulating film 12. The speed of sound of bulk waves propagating through the support substrate 10 is faster than the speed of sound of bulk waves propagating through the insulating film 11, for example, 1.1 times or more.

[0025] The insulating film 11 is, for example, polycrystalline or amorphous, and is an aluminum oxide film, silicon nitride film, aluminum nitride film, silicon carbide film, or polysilicon film. From the viewpoint of confining elastic waves within the insulating film 12 and piezoelectric layer 14, the thickness T1 of the insulating film 11 is preferably 0.3λ or more, and more preferably 1.0λ or more. From the viewpoint of improving properties, the thickness T1 is preferably 10.0λ or less. The sound velocity of the bulk wave in each film is the sound velocity of the transverse wave Vs, and if the shear modulus is G and the density is ρ, then Vs = (G / ρ) 1 / 2 The shear modulus G is given by G = E / 2(1+ν), where E is Young's modulus and ν is Poisson's ratio. Therefore, the speed of sound Vs is Vs = {E / 2ρ(1+ν)}. 1 / 2 This is expressed as follows: From this equation, we can see that as Young's modulus E decreases, the speed of sound Vs decreases. Note that Poisson's ratio ν is between 0.2 and 0.3, and is typically 0.25.

[0026] The insulating film 12 is a temperature-compensating film and has a temperature coefficient of elasticity (TCE) opposite in sign to that of the piezoelectric layer 14. For example, the TCE of the piezoelectric layer 14 is negative, and the TCE of the insulating film 12 is positive. In a plan view, the insulating film 12 overlaps with and is larger than the region where the pair of comb-shaped electrodes 20 are provided. The insulating film 12 is a silicon oxide (SiO2) film doped with fluorine (F) and an element whose oxide dissociation energy is greater than that of silicon (Si), and can be, for example, polycrystalline or amorphous. An example of an element whose oxide dissociation energy is greater than that of silicon is aluminum (Al). As a result, the TCE of the insulating film 12 can be made larger than that of an undoped silicon oxide film, thereby reducing the temperature coefficient of frequency (TCF) of the elastic wave resonator. In addition, the Young's modulus of the insulating film 12 can be made larger than that of a fluorine-doped silicon oxide film, thereby reducing the variation in resonance frequency caused by variations in the thickness of the piezoelectric layer 14. When the insulating film 12 is a silicon oxide film, the speed of sound of the bulk wave propagating through the insulating film 12 is slower than the speed of sound of the bulk wave propagating through the piezoelectric layer 14.

[0027] For the insulating film 12 to have a temperature compensation function, it is required that a certain amount of the energy of the main response elastic wave be present within the insulating film 12. The range in which the energy of the surface acoustic wave is concentrated depends on the type of surface acoustic wave, but is typically in the range of 2.0λ from the upper surface of the piezoelectric layer 14, and is particularly concentrated in the range of 1.0λ from the upper surface of the piezoelectric layer 14. Therefore, the distance (thickness T2 + T3 + T4) from the lower surface of the insulating film 12 to the upper surface of the piezoelectric layer 14 is preferably 2.0λ or less, and more preferably 1.0λ or less. In order for the energy of the surface acoustic wave to be present in the insulating film 12, the thickness T4 of the piezoelectric layer 14 is preferably 1.0λ or less, and more preferably 0.6λ or less. If the piezoelectric layer 14 becomes too thin, it becomes difficult to excite the elastic wave, so the thickness T4 is preferably 0.1λ or more, and more preferably 0.2λ or more.

[0028] The insulating film 13 is a bonding film that joins the insulating film 12 and the piezoelectric layer 14. The speed of sound of elastic waves propagating through the insulating film 13 is faster than the speed of sound of elastic waves propagating through the insulating film 12. The insulating film 13 is, for example, polycrystalline or amorphous, and is an aluminum oxide film, silicon film, aluminum nitride film, aluminum oxynitride film, diamond-like carbon film, silicon nitride film, or silicon carbide film. The thickness T3 of the insulating film 13 is preferably 20 nm or less, and more preferably 10 nm or less, from the viewpoint of not impairing the functions of the piezoelectric layer 14 and the insulating film 12. From the viewpoint of not impairing the function of the insulating film 13, the thickness T3 is preferably 1 nm or more, and more preferably 2 nm or more.

[0029] The metal film 16 is a film mainly composed of, for example, aluminum (Al), copper (Cu), or molybdenum (Mo). An adhesive film such as a titanium (Ti) film, a chromium (Cr) film, or a titanium nitride (TiN) film may be provided between the electrode finger 27 and the piezoelectric layer 14. The adhesive film is thinner than the electrode finger 27. Here, for a film to be mainly composed of a certain element, it is permissible for the film to contain intentional or unintentional impurities other than the main component. When a certain element is the main component in a film, the concentration of that element is, for example, 50 atomic percent or more, or for example, 80 atomic percent or more. In the case of a film with two main components, such as silicon oxide, the sum of the concentrations of silicon and oxygen is, for example, 50 atomic percent or more, or for example, 80 atomic percent or more, and the concentrations of silicon and oxygen are, for example, 10 atomic percent or more each.

[0030] [Manufacturing method] Figures 2(a) to 2(c) are cross-sectional views showing a method for manufacturing an elastic wave device 100 according to Example 1. As shown in Figure 2(a), an insulating film 11 and an insulating film 12 are formed on a support substrate 10 in this order. The insulating film 11 is formed by, for example, sputtering, CVD (Chemical Vapor Deposition), or vapor deposition. The insulating film 12 is, for example, a silicon oxide (SiO2) film doped with fluorine (F) and aluminum (Al), and is formed by, for example, a two-source vapor deposition method using SiO2 and AlF3 as vapor deposition sources, a vapor deposition method using a vapor deposition source of a mixture of SiO2 and AlF3, or a sputtering method using a target of SiO2 and AlF3. After forming the insulating film 12, the insulating film 12 may be annealed. If polishing is performed on the insulating film 12, the annealing may be performed before polishing or after polishing. The temperature for the annealing is, for example, 400°C to 800°C.

[0031] As shown in Figure 2(b), an insulating film 13 is formed on the insulating film 12. The insulating film 13 is deposited by, for example, sputtering, CVD, or vapor deposition. Next, a piezoelectric layer 14 is bonded to the insulating film 13 using, for example, a surface activation method, and then the piezoelectric layer 14 is polished to the desired thickness using, for example, CMP (Chemical Mechanical Polishing).

[0032] As shown in Figure 2(c), a metal film 16 is deposited on the piezoelectric layer 14, and then the metal film 16 is patterned into a desired shape. This forms the IDT 22 and reflector 24 on the piezoelectric layer 14. The metal film 16 is deposited using, for example, sputtering, CVD, or vapor deposition. The metal film 16 is patterned using, for example, photolithography and etching. This forms the acoustic wave device 100 according to Example 1.

[0033] [Experiment 1] Figure 3 is a cross-sectional view of the sample used in the experiment. As shown in Figure 3, the sample used in the experiment has an insulating film 72 provided on a support substrate 70. Multiple samples were prepared using different materials and deposition methods for the insulating film 72, and the Young's modulus and TCE of the insulating film 72 were measured for each sample. The Young's modulus was measured by nanoindentation. TCE was calculated using the formula TCE = ΔE / (E·ΔT), using the Young's modulus at 25°C measured by nanoindentation and the Young's modulus under applied temperature of 300°C. ΔE is the difference between the Young's moduli at 25°C and 300°C, ΔT is the temperature difference between 25°C and 300°C, and E is the Young's modulus at 25°C. Table 1 shows the prepared samples A to G. In all samples A to G, a silicon substrate was used for the support substrate 70, and the thickness of the insulating film 72 was 5000 nm. [Table 1]

[0034] As shown in Table 1, Sample A was an undoped SiO2 film deposited on insulating film 72 by CVD. Sample B was an undoped SiO2 film deposited on insulating film 72 by vapor deposition. Sample C was an undoped SiO2 film deposited on insulating film 72 by ion-assisted deposition (IAD). Sample D was a fluorine (F)-doped SiO2 film deposited on insulating film 72 by CVD. The amount of F doping was 5 atomic percent. Sample E was a fluorine (F) and aluminum (Al)-doped SiO2 film deposited on insulating film 72 by IAD. The amount of F and Al doping was both 5 atomic percent. Sample F was a fluorine (F) and aluminum (Al)-doped SiO2 film deposited on insulating film 72 by IAD, and after deposition, it underwent annealing at 500°C. The amount of F and Al doping was both 5 atomic percent. Sample G was a fluorine (F) and aluminum (Al) doped SiO2 film with insulating film 72 deposited by the IAD method, and was annealed at 700°C after deposition. The doping amounts of both F and Al were 5 atomic percent.

[0035] Figure 4 shows the experimental results of Young's modulus and TCE of the insulating film 72 for samples A to G. In Figure 4, the horizontal axis represents Young's modulus and the vertical axis represents TCE. As shown in Figure 4, compared to sample A, which used an undoped SiO2 film deposited by CVD, sample D, which used an F-doped SiO2 film deposited by CVD, showed a larger TCE but a smaller Young's modulus. By using an insulating film with a large TCE as a temperature compensation film, the TCF of the elastic wave resonator can be reduced. Therefore, by using an F-doped SiO2 film deposited by CVD as a temperature compensation film, the TCF of the elastic wave resonator can be reduced.

[0036] However, when an insulating film with a low Young's modulus is used as a temperature compensation film, variations in the resonance frequency occur due to manufacturing variations in the thickness of the piezoelectric layer. This is because: an insulating film with a low Young's modulus has the following characteristics: Vs = {E / 2ρ(1+ν)} 1 / 2 As is clear from the equation, the speed of sound of elastic waves decreases. For example, the speed of sound in an undoped silicon oxide film is about 3600 m / s, while the speed of sound in an F-doped silicon oxide film with a low Young's modulus is slower, at about 3100 m / s. Since the speed of sound in lithium tantalate and lithium niobate layers is about 4000 m / s, when an F-doped silicon oxide film with a low Young's modulus and slow speed of sound is used as a temperature compensation film, the difference in the speed of sound of elastic waves between the piezoelectric layer and the temperature compensation film becomes large. For the temperature compensation film to perform its temperature compensation function, the elastic waves of the main response must be within the temperature compensation film, and therefore the elastic waves are affected by the speed of sound of the temperature compensation film. If the difference in the speed of sound between the piezoelectric layer and the temperature compensation film is large, the variation in the speed of sound of elastic waves will increase when the thickness of the piezoelectric layer varies and the effect of the speed of sound on the elastic waves from the temperature compensation film changes. For this reason, when an insulating film with a low Young's modulus is used as a temperature compensation film, the resonance frequency will vary.

[0037] Samples E, F, and G, which used F and Al-doped SiO2 films as the insulating film 72, showed increased TCE and a smaller decrease in Young's modulus compared to sample D, which used an F-doped SiO2 film. In samples F and G, which underwent annealing treatment on the F and Al-doped SiO2 films, the TCE increased even further. Thus, F and Al-doped SiO2 films can achieve both increased TCE and suppression of the decrease in Young's modulus. Therefore, by using F and Al-doped SiO2 films as temperature compensation films, TCF can be improved and variations in resonance frequencies can be suppressed.

[0038] The reason why the decrease in Young's modulus was suppressed in F- and Al-doped SiO2 films compared to F-doped SiO2 films is not clear, but the following reasons are possible. It is known that Young's modulus E can be expressed by equation 1, where Vp is the ion packing efficiency and Di is the dissociation energy of the oxide (see, for example, "DIRECT CALCULATION OF YOUNG'S MODULUS OF GLASS", A. MAKISHIMA and JD MACKENZIE, Journal of Non-Crystalline Solids, 1973, Vol.12, pp35-45).

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[0039] [Experiment 2] As shown in FIG. 4, Samples F and G subjected to annealing treatment had a larger TCE than Sample E not subjected to annealing treatment. Therefore, an experiment was conducted on the change in TCE with respect to annealing temperature. In this experiment, samples having the structure shown in FIG. 3 were also used. In the experiment, after an insulating film 72 which is an F- and Al-doped SiO₂ film was formed on a support substrate 70, annealing was performed on the insulating film 72 at 200°C, 350°C, 500°C, and 700°C. The thickness of the insulating film 72 was 5000 nm, the F doping amount was 5 atomic% to 6 atomic%, and the Al doping amount was 6 atomic%. The density and TCE of the insulating film 72 were measured when no annealing treatment was performed and after the annealing treatment was performed. The density was measured by the X-ray reflectivity method. TCE was calculated by the aforementioned formula TCE=ΔE / (E·ΔT).

[0040] Figures 5(a) and 5(b) show experimental results of the density and TCE of the insulating film 72 as a function of annealing temperature. In Figures 5(a) and 5(b), the horizontal axis represents the annealing temperature. An annealing temperature of 0°C represents no annealing treatment. In Figure 5(a), the vertical axis represents the density of the insulating film 72. In Figure 5(b), the vertical axis represents the TCE of the insulating film 72. As shown in Figure 5(b), at annealing temperatures of 500°C and 700°C, the TCE of the insulating film 72 was greater compared to no annealing treatment (0°C). On the other hand, as shown in Figure 5(a), at annealing temperatures of 500°C and 700°C, the density of the insulating film 72 decreased compared to no annealing treatment (0°C).

[0041] The reason why the TCE increased after annealing is not clear, but the following reasons are possible. Partially differentiating equation 1 above with respect to temperature gives equation 2.

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[0042] The change in F concentration in an insulating film 72, which is a SiO2 film doped with F and Al, was evaluated before and after annealing at 700°C. The F concentration was measured using X-ray photoelectron spectroscopy (XPS).

[0043] Figures 6(a) and 6(b) show experimental results of fluorine (F) concentration as a function of depth from the top surface of the insulating film 72. Figure 6(a) shows the measurement results before annealing, and Figure 6(b) shows the measurement results after annealing at 700°C. In Figures 6(a) and 6(b), the horizontal axis represents the depth from the top surface of the insulating film 72, and the vertical axis represents the F concentration. As shown in Figures 6(a) and 6(b), annealing at 700°C significantly reduced the F concentration from the top surface of the insulating film 72 up to approximately 100 nm.

[0044] As shown in Figure 6(b), when a region with a reduced F concentration is formed by annealing, it is thought that the space available for thermal negative expansion of SiO2 increases. From this perspective, as shown in Figure 5(b), when annealing is performed at 500°C and 700°C, thermal negative expansion is more likely to occur, which is thought to increase the TCE. Note that, as shown in Figure 4, SiO2 is doped with F to increase the TCE. When F is doped during the SiO2 film deposition process, the member ring structure of SiO2 becomes larger, and as a result, the TCE increases. Even if F is removed from SiO2 after deposition, the effect on the member ring structure of SiO2 is small, so the change in TCE is suppressed. Therefore, even if the F concentration decreases due to annealing, the effect on TCE is small.

[0045] The stretching vibrations of the Si-O bonds in an insulating film 72, which is a SiO2 film doped with F and Al, were evaluated before and after annealing at 700°C. The stretching vibrations of the Si-O bonds were evaluated using Fourier transform infrared spectroscopy (FTIR).

[0046] Figure 7 shows the experimental results of the absorption amount of the insulating film 72 as a function of wavenumber. In Figure 7, the horizontal axis represents wavenumber, and the vertical axis represents the absorption amount at an arbitrary coordinate. 1100 cm -1 The peak in the vicinity is due to stretching vibrations of the Si-O bond. As shown in Figure 7, the peak wavenumber at which absorption is maximum before annealing is 1071 cm⁻¹. -1In contrast, after annealing at 700°C, the peak wavenumber was 1088 cm⁻¹. -1 The frequency shifted to the higher wavenumber side. From this, it can be said that annealing at 700°C increases the Si-O bond angle and enlarges the membered ring structure of SiO2. From this point of view, it can be considered that in the cases of annealing at 500°C and 700°C, thermal negative expansion is more likely to occur, and as shown in Figure 5(b), the TCE increased.

[0047] The results of Experiments 1 and 2 showed that silicon oxide films doped with fluorine and elements with higher oxide dissociation energies than silicon exhibited increased TCE and suppressed the decrease in Young's modulus. In particular, it was found that annealing silicon oxide films doped with fluorine and elements with high dissociation energies at temperatures above 400°C further increased TCE.

[0048] [Simulation 1] For multiple models of the structure shown in Figures 1(a) and 1(b), the TCF and the variation in resonant frequency with respect to the thickness of the piezoelectric layer 14 were evaluated using finite element method simulations. The simulations were performed for models A to E, which differed in the material of the insulating film 12 and whether or not annealing treatment was performed. The simulation conditions were as follows.

[0049] The common conditions for models A through E are as follows: Wavelength of elastic wave λ: 2.2 μm Support substrate 10: Sapphire substrate Insulating film 11: Aluminum oxide layer with a thickness T1 of 6000 nm Insulating film 13: None Piezoelectric layer 14: Lithium tantalate layer with a reference thickness T4 of 660 nm and a 42° rotation Y-cut X propagation pattern. Metal film 16: Aluminum film with a thickness of 170 nm

[0050] Table 2 shows the individual conditions for each of the models A through E. [Table 2] As shown in Table 2, Model A used an undoped SiO2 film for the insulating film 12. Model B used an F-doped SiO2 film for the insulating film 12. The amount of F doping was 5 atomic percent. Model C used an F and Al-doped SiO2 film for the insulating film 12. Model D used an F and Al-doped SiO2 film for the insulating film 12 and underwent annealing at 500°C. Model E used an F and Al-doped SiO2 film for the insulating film 12 and underwent annealing at 700°C. In Models C through E, the amount of F and Al doping was 5 atomic percent each. In addition, for the insulating film 12 of Models A through E, the thickness T2 was set to 440 nm, and the Young's modulus, TCE, and density were the values ​​shown in Table 2.

[0051] In the simulation, TCF was calculated using Equation 3. Specifically, the resonant frequencies fr(-35°C), fr(25°C), and fr(85°C) were determined for -35°C, 25°C, and 85°C respectively, and these values ​​were substituted into Equation 3 to calculate TCF.

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[0052] The variation in the resonant frequency with respect to the change in the thickness of the piezoelectric layer 14 was determined as the variation sensitivity Δfr / fr using Equation 4. Specifically, the resonant frequency fr (reference thickness) when the thickness of the piezoelectric layer 14 is the reference thickness, the resonant frequency fr (reference thickness × 0.95) when the thickness of the piezoelectric layer 14 is 0.95 times the reference thickness, and the resonant frequency fr (reference thickness × 1.05) when the thickness of the piezoelectric layer 14 is 1.05 times the reference thickness were determined, and the variation sensitivity Δfr / fr of the resonant frequency was calculated by substituting each value into Equation 4.

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[0053] Figure 8 shows the simulation results for the resonance frequency variation sensitivity Δfr / fr of each model and the ΔTCF of each model relative to the TCF of Model A. In Figure 8, the horizontal axis is the resonance frequency variation sensitivity Δfr / fr obtained by Equation 4, and the vertical axis is the ΔTCF of each model obtained by Equation 3, relative to the TCF of Model A (0). Model A is an undoped SiO2 film, and it is known that the TCF in this case is a negative value. Therefore, in Figure 8, when ΔTCF is large and positive, the TCF approaches 0 and improves.

[0054] As shown in Figure 8, models C, D, and E, which use F and Al-doped SiO2 films for the insulating film 12, showed a larger TCF compared to model A, which uses an undoped SiO2 film, and model B, which uses an F-doped SiO2 film. In addition, models C, D, and E showed a smaller resonance frequency variation sensitivity Δfr / fr compared to model B. Thus, models C, D, and E showed improved TCF and suppressed the variation in resonance frequency due to variations in the thickness of the piezoelectric layer 14 compared to models A and B.

[0055] From these simulation results, it was confirmed that using an F and Al-doped SiO2 film with high TCE and Young's modulus as the temperature compensation film for the insulating film 12 improves TCF and reduces the variation in resonance frequency.

[0056] [Simulation 2] For a model having the structure shown in Figure 3, the TCE and Young's modulus of the insulating film 72 were evaluated using finite element method simulations when the insulating film 72 was a fluorine (F) and aluminum (Al) doped SiO2 film with different amounts of F and Al doping. Figure 9(a) shows the simulation results of TCE for the concentrations of F and Al. Figure 9(b) shows the simulation results of Young's modulus for the concentrations of F and Al. As shown in Figures 9(a) and 9(b), TCE increased when the Al doping amount was 2 atomic percent or less and the F doping amount was 4 atomic percent or less, or when the Al doping amount was 4 atomic percent or more and less than 8 atomic percent and the F doping amount was 7 atomic percent or less. From the viewpoint of increasing TCE and suppressing the decrease in Young's modulus, the amount of Al doping may be 3 to 7 atomic percent and the amount of F doping may be 4 to 7.5 atomic percent, or the amount of Al doping may be 3.5 to 6.5 atomic percent and the amount of F doping may be 4.5 to 7 atomic percent, or the amount of Al doping may be 4 to 6 atomic percent and the amount of F doping may be 5 to 6.5 atomic percent, or the amount of Al doping may be 3 to 5 atomic percent and the amount of F doping may be 5 to 6.5 atomic percent.

[0057] [Differentiation] Figure 10 is a cross-sectional view of an elastic wave device 110 according to a modified example of Example 1. As shown in Figure 10, in the modified example of Example 1, the piezoelectric layer 14 is a thick piezoelectric substrate, and no support substrate is provided to support the piezoelectric layer 14. Therefore, no insulating film is provided below the piezoelectric layer 14. An insulating film 12a is provided on the piezoelectric layer 14, covering the elastic wave resonator 26. In a plan view, the insulating film 12a overlaps with and is larger than the region where the IDT 22 (a pair of comb-shaped electrodes 20) is provided. The insulating film 12a functions as a temperature compensation film and is an F and Al doped SiO2 film, the same as the insulating film 12 in Example 1. The other configurations are the same as in Example 1, so their description is omitted.

[0058] In Example 1 and its modified form, insulating films 12 and 12a are provided overlapping the region of the piezoelectric layer 14 where a pair of comb-shaped electrodes 20 for exciting elastic waves are provided. The insulating films 12 and 12a are silicon oxide (SiO2) films containing an element with a greater oxide dissociation energy than silicon (Si) (e.g., aluminum (Al)) and fluorine (F). An SiO2 film containing an element with a greater oxide dissociation energy than Si and F can increase the TCE and suppress the decrease in Young's modulus. Therefore, by providing insulating films 12 and 12a, which are SiO2 films containing an element with a greater oxide dissociation energy than Si and F, overlapping the pair of comb-shaped electrodes 20 and using them as a temperature compensation film, the TCF can be improved and the variation in resonance frequency with respect to the thickness variation of the piezoelectric layer 14 can be kept small.

[0059] Furthermore, in Example 1, aluminum is used as an element whose oxide dissociation energy is greater than that of Si. In this case, as shown in Figure 4, the TCE can be increased and the decrease in Young's modulus can be suppressed. Therefore, the TCF can be improved and the variation in resonance frequency with respect to the thickness variation of the piezoelectric layer 14 can be kept small. Since Figure 4 shows the experimental results when the amount of aluminum doping is 5 atomic%, it is preferable that the amount of aluminum doping is 5 atomic% or more in order to increase the TCE and suppress the decrease in Young's modulus. The amount of aluminum doping is more preferably 6 atomic% or more, and even more preferably 7 atomic% or more. Since it is possible that other adverse effects may occur if the amount of aluminum doping is too high, the amount of aluminum doping is preferably 15 atomic% or less, more preferably 12 atomic% or less, and even more preferably 10 atomic% or less.

[0060] Furthermore, the insulating films 12 and 12a, which are SiO2 films containing elements with greater oxide dissociation energy than Si and F, preferably have a Young's modulus of 60 GPa or more and a TCE of 300 ppm / K or more, as shown in Figure 4. This improves the TCF and reduces the variation in resonance frequency with respect to the thickness variation of the piezoelectric layer 14. From the viewpoint of improving TCF and reducing the variation in resonance frequency, it is preferable that the Young's modulus is 60 GPa or more and the TCE is 400 ppm / K or more. It is preferable that the Young's modulus is 60 GPa or more and the TCE is 500 ppm / K or more. It is preferable that the Young's modulus is 70 GPa or more and the TCE is 400 ppm / K or more.

[0061] In Example 1, as shown in Figures 1(a) and 1(b), the piezoelectric layer 14 is provided on the support substrate 10. The electrodes that excite the piezoelectric layer 14 with elastic waves are a pair of comb-shaped electrodes 20 provided on the piezoelectric layer 14. The insulating film 12 is located between the support substrate 10 and the piezoelectric layer 14 and overlaps the pair of comb-shaped electrodes 20. In this case, by using an SiO2 film containing an element with an oxide dissociation energy greater than that of Si and F as the insulating film 12, the TCF can be improved and the variation in resonance frequency with respect to the thickness variation of the piezoelectric layer 14 can be kept small.

[0062] When insulating films 12 and 12a contain Al as an element with a greater oxide dissociation energy than Si, it is preferable that the Al content be less than 8 atomic percent. This allows for a higher TCE, as shown in Figure 9(a). It is preferable that insulating films 12 and 12a contain 2 atomic percent or less of Al and 4 atomic percent or less of F, or 4 atomic percent or more and less than 8 atomic percent of Al and 7 atomic percent or less of F. This also allows for a higher TCE, as shown in Figure 9(a). Furthermore, as shown in Figures 9(a) and 9(b), from the viewpoint of increasing TCE and suppressing a decrease in Young's modulus, it is preferable that the value obtained by dividing the atomic percent of Al by the atomic percent of F (Al concentration / F concentration) is 1.6 or less. [Examples]

[0063] Example 2 describes an example in which the elastic wave device has a bulk wave resonator. Figure 11(a) is a plan view of the elastic wave device 200 according to Example 2, and Figure 11(b) is a cross-sectional view AA of Figure 11(a). Figure 11(a) mainly shows the support substrate 10, the lower electrode 32, and the upper electrode 36. The directions that are orthogonal to each other in the planar direction of the support substrate 10 are defined as the X direction and the Y direction, and the thickness direction of the support substrate 10 is defined as the Z direction.

[0064] As shown in Figures 11(a) and 11(b), an acoustic reflective film 40 is provided on the support substrate 10. A lower electrode 32 is provided on the acoustic reflective film 40. The lower electrode 32 includes a lower layer 32a and an upper layer 32b. The lower layer 32a is, for example, a chromium (Cr) film, and the upper layer 32b is, for example, a ruthenium (Ru) film. The thickness of the lower electrode 32 is, for example, 30 nm to 400 nm.

[0065] A piezoelectric layer 34 is provided on the acoustic reflective film 40 and the lower electrode 32. The piezoelectric layer 34 includes a lower layer 34a and an upper layer 34b. Both the lower layer 34a and the upper layer 34b of the piezoelectric layer 34 are aluminum nitride films mainly composed of aluminum nitride with the (002) direction as the principal axis. The thickness of the piezoelectric layer 34 is, for example, 100 nm to 1500 nm. The piezoelectric layer 34 is mainly composed of aluminum nitride and may contain other elements to improve resonance characteristics or piezoelectricity. Examples of additive elements include group 3 elements, group 2 elements, or combinations of group 12 and group 4 elements, or combinations of group 2 elements or group 12 and group 5 elements. Note that the piezoelectric layer 34 is not limited to an aluminum nitride film, but may also be a lithium tantalate layer or a lithium niobate layer, etc.

[0066] An insulating film 12b is provided between the lower layer 34a and the upper layer 34b of the piezoelectric layer 34. The insulating film 12b functions as a temperature compensation film and, like the insulating film 12 in Example 1, is an SiO2 film doped with F and an element whose oxide dissociation energy is greater than that of Si (for example, Al).

[0067] An upper electrode 36 is provided on the piezoelectric layer 34. The upper electrode 36 is provided on the piezoelectric layer 34 such that it has a region facing the lower electrode 32 across the piezoelectric layer 34. The region where the lower electrode 32 and the upper electrode 36 overlap in a plan view, with at least a part of the piezoelectric layer 34 in between, is the resonance region 50. The resonance region 50 has, for example, an elliptical shape in a plan view, and is the region where, for example, an elastic wave of the thickness longitudinal vibration mode resonates. The insulating film 12b overlaps with the resonance region 50 in a plan view and is larger than the resonance region 50. The upper electrode 36 includes a lower layer 36a and an upper layer 36b. The lower layer 36a is, for example, a Ru film, and the upper layer 36b is, for example, a Cr film. The thickness of the upper electrode 36 is, for example, 30 nm to 400 nm. Note that the resonance region 50 is not limited to an elliptical shape in a plan view, but may also be a quadrilateral or a polygon with pentagons or more.

[0068] An insulating film 38 is provided on the piezoelectric layer 34 and the upper electrode 36. The insulating film 38 covers at least a portion of the resonant region 50. The insulating film 38 is, for example, a silicon oxide film and may play a role in adjusting the frequency. The insulating film 38 may also function as a protective film. The wavelength λ of the elastic wave excited in the resonant region 50 is finely tuned by the insulating film 38, but is approximately twice the combined thickness of the piezoelectric layer 34 and the insulating film 12b.

[0069] The acoustic reflective film 40 is constructed by alternately stacking one or more low acoustic impedance films 42 and one or more high acoustic impedance films 44 with higher acoustic impedance than the low acoustic impedance films 42 in the Z direction. The low acoustic impedance films 42 are, for example, silicon oxide films. The high acoustic impedance films 44 are, for example, tungsten films. In a plan view, the acoustic reflective film 40 overlaps with the resonance region 50 and is larger than the resonance region 50. The thickness of the low acoustic impedance films 42 and the high acoustic impedance films 44 is, for example, about λ / 4 each. As a result, the acoustic reflective film 40 reflects elastic waves. The number of layers of low acoustic impedance films 42 and high acoustic impedance films 44 can be arbitrarily set.

[0070] [Differentiation] Figure 12(a) is a cross-sectional view of the elastic wave device 210 according to Modification 1 of Example 2. As shown in Figure 12(a), in Modification 1 of Example 2, the acoustic reflective film 40 is not provided on the support substrate 10. A recess is formed on the upper surface of the support substrate 10. This recess forms a gap 46 between the support substrate 10 and the lower electrode 32. The other configurations are the same as in Example 2, so their description is omitted. Note that the gap 46 is not limited to being formed by the recess on the upper surface of the support substrate 10, but may also be formed by a through hole penetrating the support substrate 10. An arch-shaped gap may also be formed on the flat upper surface of the support substrate 10 in cross-sectional view.

[0071] Figure 12(b) is a cross-sectional view of the elastic wave device 220 according to Modification 2 of Example 2. As shown in Figure 12(b), in Modification 2 of Example 2, two layers of insulating film 12b are provided, one in contact with the upper surface of the lower electrode 32 and the other in contact with the lower surface of the upper electrode 36. The other configurations are the same as in Example 2, so their description is omitted. Note that the insulating film 12b may be provided on only one of either the upper surface of the lower electrode 32 or the lower surface of the upper electrode 36.

[0072] Figure 13(a) is a cross-sectional view of the elastic wave device 230 according to Modification 3 of Example 2. As shown in Figure 13(a), in Modification 3 of Example 2, two layers of insulating film 12b are provided, one of which is provided between the lower layer 32a and the upper layer 32b of the lower electrode 32, and the other is provided between the lower layer 36a and the upper layer 36b of the upper electrode 36. The other configurations are the same as in Example 2, so their description is omitted. Note that the insulating film 12b may also be provided between the lower layer 32a and the upper layer 32b of the lower electrode 32, or between the lower layer 36a and the upper layer 36b of the upper electrode 36.

[0073] Figure 13(b) is a cross-sectional view of the elastic wave device 240 according to Modification 4 of Example 2. As shown in Figure 13(b), in Modification 4 of Example 2, an insulating film 12b is used for the low acoustic impedance film 42 of the acoustic reflective film 40. The other configurations are the same as in Example 2, so their description is omitted.

[0074] In Example 2 and its modified form, the piezoelectric layer 34 is provided on the support substrate 10. The insulating film 12b is provided overlapping the region of the piezoelectric layer 34 where the lower electrode 32 and upper electrode 36 for exciting elastic waves are provided. That is, the insulating film 12b is provided overlapping the resonance region 50 where the lower electrode 32 and upper electrode 36 face each other with the piezoelectric layer 34 in between. The insulating film 12b is a silicon oxide (SiO2) film containing an element whose oxide dissociation energy is greater than that of silicon (Si) (for example, aluminum (Al)) and fluorine (F). Thus, similar to Example 1 and its modified form, by providing the insulating film 12b, which is an SiO2 film containing an element whose oxide dissociation energy is greater than that of Si and F, overlapping the resonance region 50 in a plan view and using it as a temperature compensation film, the TCF can be improved and the variation in resonance frequency with respect to the thickness variation of the piezoelectric layer 34 can be kept small. [Examples]

[0075] Figure 14(a) is a circuit diagram of the filter 300 according to Embodiment 3. As shown in Figure 14(a), one or more series resonators S1 to S4 are connected in series between the input terminal Tin and the output terminal Tout. One or more parallel resonators P1 to P3 are connected in parallel between the input terminal Tin and the output terminal Tout. At least one of the series resonators S1 to S4 and the parallel resonators P1 to P3 can be an elastic wave device according to Embodiment 1 and its modified form, and Embodiment 2 and its modified form. The number of series resonators and parallel resonators can be set as appropriate. A ladder filter is shown as an example of the filter, but a multimode filter may also be used.

[0076] Figure 14(b) is a circuit diagram of a duplexer 310 according to a modified example of Embodiment 3. As shown in Figure 14(b), a transmit filter 60 is connected between the common terminal Ant and the transmit terminal Tx. A receive filter 62 is connected between the common terminal Ant and the receive terminal Rx. The transmit filter 60 allows the transmit band signal from the high-frequency signal input from the transmit terminal Tx to pass to the common terminal Ant as the transmit signal, and suppresses signals of other frequencies. The receive filter 62 allows the receive band signal from the high-frequency signal input from the common terminal Ant to pass to the receive terminal Rx as the receive signal, and suppresses signals of other frequencies. At least one of the transmit filter 60 and the receive filter 62 can be the filter of Embodiment 3. A duplexer is shown as an example of a multiplexer, but a triplexer or quadplexer may also be used.

[0077] Although embodiments of the present invention have been described in detail above, the present invention is not limited to these specific embodiments, and various modifications and changes are possible within the scope of the gist of the present invention as described in the claims. [Explanation of Symbols]

[0078] 10...Support substrate, 11...Insulating film, 12, 12a...Insulating film, 13...Insulating film, 14...Piezoelectric layer, 16...Metal film, 20...Comb-shaped electrode, 22...IDT, 24...Reflector, 25...Crossing region, 26...Elastic wave resonator, 27...Electrode fingers, 28...Busbar, 32...Lower electrode, 32a...Lower layer, 32b...Upper layer, 34...Piezoelectric layer, 34a...Lower layer, 34b...Upper layer, 36...Upper electrode, 36a...Lower Layer, 36b…Upper layer, 38…Insulating film, 40…Acoustic reflective film, 42…Low acoustic impedance film, 44…High acoustic impedance film, 46…Gap, 50…Resonance region, 60…Transmitting filter, 62…Receiver filter, 70…Support substrate, 72…Insulating film, 100, 110, 200, 210, 220, 230, 240…Elastic wave device, 300…Filter, 310…Duplexer

Claims

1. Piezoelectric layer and An electrode for exciting elastic waves is provided in the piezoelectric layer, An elastic wave device comprising a silicon oxide film containing an element whose oxide dissociation energy is greater than that of silicon and fluorine, provided in the region where the piezoelectric layer and the electrode overlap.

2. The elastic wave device according to claim 1, wherein the element with the large dissociation energy is aluminum.

3. The elastic wave device according to claim 2, wherein the doping amount of the element with high dissociation energy is 5 atomic percent or more.

4. The elastic wave device according to any one of claims 1 to 3, wherein the Young's modulus of the silicon oxide film is 60 GPa or more, and the temperature coefficient of the elastic constant is 400 ppm / K or more.

5. The elastic wave device according to claim 2, wherein the silicon oxide film contains less than 8 atomic percent of the aluminum.

6. The elastic wave device according to claim 2, wherein the silicon oxide film contains 2 atomic percent or less of aluminum and 4 atomic percent or less of fluorine, or contains 4 atomic percent or more but less than 8 atomic percent of aluminum and 7 atomic percent or less of fluorine.

7. The elastic wave device according to claim 5 or 6, wherein the value obtained by dividing the atomic percentage of aluminum by the atomic percentage of fluorine is 1.6 or less.

8. A filter comprising an elastic wave device according to any one of claims 1 to 3.

9. A multiplexer comprising the filter described in claim 8.

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