Elastic wave device

Abstract

Provided is an elastic wave device in which variation in the size of SH wave dispersion is unlikely to occur even when the film thickness of a frequency adjustment film has changed. An elastic wave device (1) is provided with an IDT electrode (3), a dielectric film (6), and a frequency adjustment film (7) on a LiNbO3 substrate (2). The Euler angle of the LiNbO3 substrate (2) is within a range of 0°±5°, within a range of θ±1.5°, or within a range of 0°±10°. The IDT electrode (3) has a main electrode that is Cu or an alloy in which Cu is the main component. When the film thickness of the main electrode, normalized by a wavelength λ determined by the electrode finger pitch of the IDT electrode (3), is T, and the density ratio of the material of the main electrode to Pt is r, the film thickness T of the main electrode and the Euler angle θ satisfy formula (1). Formula (1): θ = -0.05° / (T / r-0.04) + 3135°.

Description

[0001] This application is a divisional application of the invention patent application filed on August 8, 2016, with application number 201680059542.3 and invention title "Elastic Wave Device". Technical Field

[0002] This invention relates to an elastic wave device having an IDT electrode, a dielectric film, and a frequency adjustment film stacked on a LiNbO3 substrate. Background Technology

[0003] Patent Document 1 discloses an elastic wave device utilizing Rayleigh waves. In this elastic wave device, a SiO2 film is stacked on a LiNbO3 substrate, covering an IDT electrode. Furthermore, a SiN film for frequency adjustment is provided on the SiO2 film. By adjusting the thickness of the SiN film, the frequency of the elastic wave device can be adjusted.

[0004] Prior art literature

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2012-186808 Summary of the Invention

[0007] The problem that the invention aims to solve

[0008] In the elastic wave device described in Patent Document 1, if the thickness of the SiN film changes, the response of the stray SH wave will change. Therefore, when the thickness of the SiN film changes in order to adjust the frequency, the suppressed SH wave stray wave may sometimes be generated at a greater rate.

[0009] The purpose of this invention is to provide an elastic wave device that does not easily generate variations in the magnitude of SH wave strays even when the thickness of the frequency-adjusting membrane changes.

[0010] Technical solutions for solving the problem

[0011] The elastic wave device of the present invention comprises: a LiNbO3 substrate; an IDT electrode disposed on the LiNbO3 substrate; a dielectric film disposed on the LiNbO3 substrate such that it covers the IDT electrode; and a frequency adjustment film disposed on the dielectric film. The Euler angle of the LiNbO3 substrate is (within the range of 0°±5°, θ±1.5°, and 0°±10°). The IDT electrode has a main electrode. When the thickness of the main electrode, which is obtained by normalizing the wavelength λ determined by the distance between the electrode fingers of the IDT electrode, is set to T, and the density ratio of the material of the main electrode to Pt is set to r, the thickness T of the main electrode and the Euler angle θ satisfy the following equation (1).

[0012] Equation (1): θ = -0.05° / (T / r - 0.04) + 31.35°

[0013] In a particular aspect of the elastic wave device of the present invention, θ is in the range of 25° or more and 31° or less. In this case, even with variations in the thickness of the frequency-adjusting membrane, variations in SH wave strays can be suppressed more effectively.

[0014] In another specific aspect of the elastic wave device of the present invention, the main electrode is a metal selected from the group consisting of Pt, Au, W, Ta, Mo and Cu, or an alloy thereof as the main body.

[0015] In another specific aspect of the elastic wave device according to the present invention, the thickness of the frequency adjustment film is greater than 0 and less than 0.025λ. In this case, a region with high frequency adjustment sensitivity can be used. Therefore, the cost of the frequency adjustment process can be reduced.

[0016] In another specific aspect of the elastic wave device of the present invention, the thickness of the frequency adjustment diaphragm is 0.005λ or less.

[0017] In another specific aspect of the elastic wave device involved in this invention, the dielectric film comprises silicon oxide such as SiO2. In this case, the absolute value of the frequency temperature coefficient (TCF) can be reduced.

[0018] In another specific aspect of the elastic wave device of the present invention, the frequency adjustment film comprises silicon nitride such as SiN.

[0019] In another specific aspect of the elastic wave device of the present invention, the IDT electrode has the main electrode and other electrode layers comprising metals other than the main electrode.

[0020] In another specific aspect of the elastic wave device involved in this invention, there is a bandpass filter having the IDT electrodes.

[0021] In another specific aspect of the elastic wave device of the present invention, the total film thickness of the IDT electrodes is 0.25λ or less.

[0022] In another specific aspect of the elastic wave device involved in this invention, the silicon oxide is thicker than the IDT electrode.

[0023] Invention Effects

[0024] In the elastic wave device of the present invention, even if the thickness of the frequency adjustment membrane changes, it is not easy to generate changes in the magnitude of stray waves caused by SH waves. Attached Figure Description

[0025] Figures 1(a) and 1(b) are a top view of an elastic wave device according to an embodiment of the present invention and a partially enlarged front cross-sectional view showing the main parts.

[0026] Figure 2 This is an enlarged cross-sectional view showing the electrode fingers of the IDT electrode in an elastic wave device according to a modified embodiment of the present invention.

[0027] Figure 3 This is a graph showing the frequency characteristics of the S-parameters in an elastic wave resonator with an Euler angle θ of 25° on a LiNbO3 substrate and a Pt film thickness of 0.0475λ in the IDT electrode.

[0028] Figure 4 This is a graph showing the impedance characteristics of an elastic wave resonator with an Euler angle θ of 25° on a LiNbO3 substrate and a Pt film thickness of 0.0475λ in the IDT electrode.

[0029] Figure 5 This is a graph showing the relationship between the thickness of the SiN film, which serves as a frequency adjustment film, and the relative frequency band of the SH wave in an elastic wave device with an Euler angle θ of 25° on a LiNbO3 substrate and a Pt film thickness of 0.0475λ in the IDT electrode.

[0030] Figure 6 This is a graph showing the relationship between the thickness of the SiN film, which serves as a frequency adjustment film, and the relative frequency band of the SH wave in an elastic wave device with an Euler angle θ of 28° on a LiNbO3 substrate and a Pt film thickness of 0.055λ in the IDT electrode.

[0031] Figure 7 It is Figure 6 The graph shows a magnified view of the relative frequency bands along the vertical axis.

[0032] Figure 8 This is a graph showing the relationship between the thickness of the SiN film, which serves as a frequency adjustment film, and the relative frequency band of the SH wave in an elastic wave device with an Euler angle θ of 30° on a LiNbO3 substrate and a Pt film thickness of 0.0775λ in the IDT electrode.

[0033] Figure 9 It is Figure 8 The diagram is shown with the vertical axis scale enlarged.

[0034] Figure 10This is a graph showing the relationship between the thickness of the SiN film, which serves as a frequency adjustment film, and the relative frequency band of the SH wave in an elastic wave device with an Euler angle θ of 38° on a LiNbO3 substrate and a Pt film thickness of 0.02λ in the IDT electrode.

[0035] Figure 11 This is a graph showing the relationship between the thickness of the SiN film and the speed of Rayleigh waves.

[0036] Figure 12 This is a graph showing the relationship between θ, where the relative frequency band of the SH wave becomes minimal, and the thickness of the Pt film.

[0037] Explanation of reference numerals in the attached figures

[0038] 1: Elastic wave device;

[0039] 2: LiNbO3 substrate;

[0040] 3: IDT electrode;

[0041] 3a: Electrode finger;

[0042] 4, 5: Reflectors;

[0043] 6: Dielectric film;

[0044] 7: Frequency adjustment membrane;

[0045] 11: NiCr film;

[0046] 12: Pt membrane;

[0047] 13: Ti film;

[0048] 14: AlCu alloy film. Detailed Implementation

[0049] Hereinafter, specific embodiments of the present invention will be described with reference to the accompanying drawings, thereby making the present invention clear.

[0050] In addition, it should be noted in advance that the embodiments described in this specification are illustrative and that partial substitutions or combinations of structures can be made between different embodiments.

[0051] Figures 1(a) and 1(b) are a top view of an elastic wave device according to an embodiment of the present invention and a partially enlarged front cross-sectional view showing the main parts.

[0052] The elastic wave device 1 has a LiNbO3 substrate 2. An IDT electrode 3 is disposed on the LiNbO3 substrate 2. The IDT electrode 3 has multiple electrode fingers 3a. Reflectors 4 and 5 are disposed on both sides of the elastic wave propagation direction of the IDT electrode 3. Thus, a single-port elastic wave resonator is formed. The elastic wave device 1 utilizes Rayleigh waves.

[0053] In the elastic wave device 1, a dielectric film 6 is disposed on a LiNbO3 substrate 2, covering the IDT electrode 3. In this embodiment, the dielectric film 6 comprises SiO2.

[0054] A SiN film is disposed on the dielectric film 6 as a frequency adjustment film 7.

[0055] Because the dielectric film 6 contains SiO2, the absolute value of the temperature coefficient of frequency (TCF) is set small in the elastic wave device 1. However, the dielectric film 6 can also be formed from other dielectric materials such as SiON.

[0056] By adjusting the thickness of the frequency adjustment film 7, the frequency of the elastic wave device 1 can be adjusted. That is, by reducing the thickness of the frequency adjustment film 7, the frequency can be adjusted in the direction of lowering the resonant frequency and the anti-resonant frequency.

[0057] The frequency tuning film 7 is not limited to SiN, and may also include other materials such as SiON. Because the film has appropriate sound velocity and is easy to process, SiN film is preferred.

[0058] The elastic wave device 1 is characterized in that the IDT electrode 3 has a main electrode, and when the film thickness of the main electrode is set to T and the density ratio of the main electrode material to Pt is set to r, the Euler angle of the LiNbO3 substrate 2 is within the range of 0°±5°, θ±1.5°, and 0°±10°, and the thickness T of the main electrode and the Euler angle θ satisfy the following equation (1). Therefore, even if the thickness of the frequency adjustment film 7 changes, the relative frequency band of the stray SH wave is set to be small.

[0059] Equation (1): θ = -0.05° / (T / r - 0.04) + 31.35°

[0060] In the elastic wave device 1, if the thickness of the SiN film changes, the relative frequency band of the stray SH waves will change. The relative frequency band of the SH waves is expressed as the ratio of the frequency that produces the response caused by the SH waves to the difference between the resonant frequency and the anti-resonant frequency of the SH waves, corresponding to the electromechanical coupling coefficient. If the relative frequency band increases, the response caused by the SH waves will be larger. (Refer to...) Figures 3 to 10 Explain it.

[0061] The frequency characteristics of the S-parameters of the elastic wave resonator fabricated by setting θ to 25° in the Euler angle (0°, θ, 0°) of the LiNbO3 substrate, setting the wavelength λ determined by the electrode finger spacing of the IDT electrode 3 to 4.0 μm, and setting the thickness of the Pt film constituting the IDT electrode to 190 nm (0.0475λ) are shown below. Figure 3 The impedance characteristics are shown in Figure 4 In the various figures, the solid lines represent the case where the SiN film thickness as a frequency tuning film is 40 nm (0.01λ), and the dashed lines represent the case where it is 10 nm (0.0025λ). In the solid lines, only the Rayleigh wave response is observed, but in the dashed lines, spurious signals caused by SH waves are generated near 810 MHz. This is because changing the SiN film thickness increases the relative bandwidth of the SH waves.

[0062] Next, various elastic wave devices were fabricated by varying the Euler angle (0°, θ, 0°) of the LiNbO3 substrate and the thickness of the Pt film constituting the IDT electrode 3. In these various elastic wave devices, the thickness of the SiN film, which serves as a frequency adjustment film, was varied, and the changes in the relative frequency band of the SH wave were determined. Figure 5 The results are shown when the Euler angle θ is 25° and the Pt film thickness is 0.0475λ. Furthermore, λ is the wavelength determined by the electrode finger spacing of the IDT electrode 3.

[0063] according to Figure 5 It is clearly known that if the SiN film thickness exceeds 0.005λ, the relative bandwidth of the SH wave becomes approximately zero. On the other hand, when the SiN film thickness is below 0.005λ, the relative bandwidth increases as the SiN film thickness decreases, which can sometimes lead to spurious signals caused by the SH wave in the device characteristics. Therefore, when θ = 25° and the Pt film thickness is 0.0475λ, it is preferable to adjust the frequency by varying the SiN film thickness within a range of 0.005λ or greater.

[0064] Figure 6 The results are shown when the Euler angle θ is 28° and the Pt film thickness is 0.055λ. Figure 7 It is Figure 6 The diagram is shown with the vertical axis scale enlarged. According to... Figure 6 as well as Figure 7 It is clear that when θ is 28° and the Pt film thickness is 0.055λ, the relative bandwidth of the SH wave becomes 0 when the SiN film thickness is 0.0075λ. Furthermore, according to... Figure 7 It is clear that within the range of SiN film thickness above and below 0.0025λ, the relative bandwidth of SH waves is approximately below 0.002%, which is very small.

[0065] Figure 8 This is a graph showing the relationship between the SiN film thickness and the relative frequency band of the SH wave when the Euler angle θ is 30° and the Pt film thickness is 0.0775λ. Figure 9 It is Figure 8 The diagram is shown with the vertical axis scale enlarged.

[0066] according to Figure 8 as well as Figure 9 It is clear that even under these conditions, within the range of SiN film thickness above and below 0.0025λ, the relative frequency band of the SH wave becomes approximately 0.

[0067] Figure 10 The part shown, excluding the relationship between θ and the electrode film thickness, is the relationship between the SiN film thickness and the relative frequency band of the SH wave under the same conditions disclosed in Patent Document 1, with the Euler angle θ being 38° and the Pt film thickness being 0.02λ. According to... Figure 10 It is clearly known that when the SiN film thickness is below 0.01λ, the relative bandwidth of the SH wave increases as the SiN film thickness decreases. It is also known that if the frequency is adjusted to reduce the SiN film thickness to below 0.01λ, the relative bandwidth of the SH wave changes drastically. Therefore, the structure in Patent Document 1 cannot reliably provide an elastic wave device with minimal characteristic deviation.

[0068] In contrast, Figures 3-9 In the example shown, it can be seen that even with frequency adjustment to thin the SiN film, down to below 0.025λ, and especially below 0.005λ, the change in the relative bandwidth of the SH wave is very small. Furthermore, when the SiN film thickness is 0λ, the frequency adjustment function is lost; therefore, the SiN film thickness must be at least greater than 0.

[0069] on the other hand, Figure 11 This graph illustrates the relationship between the SiN film thickness and the Rayleigh wave velocity. It clearly shows that the relationship between the SiN film thickness and the Rayleigh wave velocity is a convex upward curve; the thinner the SiN film, the greater the frequency variation relative to the SiN film thickness, meaning the higher the frequency adjustment sensitivity. In recent years, there has been a demand for further cost reduction in elastic wave devices. Therefore, the frequency adjustment process requires high sensitivity, meaning frequency adjustment must be performed within the thinnest possible range of the SiN film thickness.

[0070] As mentioned earlier, by selecting the Euler angle θ of the LiNbO3 substrate and the thickness of the Pt film, even if the SiN film thickness is reduced to below 0.025λ, especially below 0.005λ, and frequency adjustment is performed, the variation in the magnitude of spurious emissions caused by SH waves can be reduced.

[0071] The inventors of this application have considered the foregoing aspects, not only those mentioned above. Figures 3-9 Furthermore, the film thickness of the Pt film and the Euler angle θ of the LiNbO3 substrate 2 were varied in various ways, and the relative frequency band of the SH wave under these conditions was determined. Table 1 below shows the combination of Euler angle θ and Pt film thickness that minimizes the relative frequency band of the SH wave.

[0072] [Table 1]

[0073]

[0074] If we depict the results shown in Table 1, then as Figure 12 As shown.

[0075] In addition, Figure 12 In the above, point A1 corresponds to the case where the Euler angle θ = 25° and the thickness of the Pt film is 0.0475λ, that is, it corresponds to... Figure 5 The results shown indicate that point A2 corresponds to the case where the Euler angle θ = 28° and the Pt film thickness is 0.055λ, i.e., equivalent to... Figure 6 The results shown are as follows. Point A3 corresponds to the case where the Euler angle θ = 30° and the Pt film thickness is 0.0775λ, that is, it corresponds to... Figure 8 The situation is shown.

[0076] Therefore, if in Figure 12 On the solid line A shown, the relative frequency band of the SH wave can be minimized. If we express this solid line A as a formula, it becomes formula (1A).

[0077] Equation (1A): θ=-0.05° / (T) Pt -0.04)+31.35°

[0078] That is, when the thickness of the Pt film is set to T... Pt At that time, at film thickness T Pt When the Euler angle θ satisfies the above equation (1A), the relative bandwidth of the SH wave can be minimized. The inventors of this application have also conducted various studies on main electrode materials other than Pt. That is, similar studies were conducted on Au, W, Ta, Mo, and Cu. As a result, it was confirmed that when using these metals, if the film thickness, which is normalized to the wavelength determined by the distance between the electrode fingers of the IDT electrode 3, is set to T, then simply replacing T in the above equation (1A) will achieve the desired result. PtT / r can be used. Furthermore, r = the ratio of the density of the main electrode material to the density of Pt. Therefore, when the main electrode contains Pt, r = 1; when it contains a metal with a higher density than Pt, r > 1. Thus, if the Euler angle θ and the film thickness T of the main electrode are chosen to satisfy the aforementioned equation (1), the relative frequency band of the SH wave can be minimized. Therefore, the influence of the SH wave response caused by variations in the SiN film thickness can be effectively suppressed. Therefore, frequency adjustment can be performed with high precision, and deviations in the characteristics of the elastic wave device are less likely to occur.

[0079] Preferably, the Euler angle θ is set to a range of 25° or higher and 31° or lower. According to... Figures 5-9 It is known that the smaller the Euler angle θ, the greater the variation in the relative frequency band of SH waves when the SiN film thickness decreases. However, if θ is greater than 25°, the relative frequency band of SH waves near a SiN film thickness of 0.005λ becomes sufficiently small. Therefore, spurious emissions caused by SH waves can be suppressed more effectively. On the other hand, according to... Figure 12 It is known that if θ increases, the film thickness of the main electrode of the IDT increases. If the film thickness of the main electrode becomes too thick, the cost may increase, or the characteristic deviation may increase if there is a deviation in the width of the electrode fingers. Therefore, based on the above reasons, it is more preferable to have T... Pt Let it be set to 0.1λ or less. Therefore, more preferably, θ is set to a range of 30.5° or less.

[0080] On the other hand, such as Figure 11 As shown, the thinner the SiN film, the greater the change in sound velocity caused by variations in the SiN film thickness. That is, when the SiN film is thin, the sensitivity of frequency adjustment is high. Therefore, as in the above embodiment, by performing frequency adjustment within a thin range where the SiN film thickness is 0.01λ or less, a large frequency adjustment range can be obtained.

[0081] Furthermore, frequency adjustment can be achieved by etching the SiN film or by adjusting the thickness of the SiN film during deposition. As described above, by using a thinner region of the SiN film, the etching and deposition times can be shortened. Therefore, manufacturing costs can also be reduced.

[0082] As described above, in the above embodiment, frequency adjustment can be performed using a region of SiN film thickness with high frequency adjustment sensitivity. Regarding the SiN film thickness, it is preferable to have a thickness greater than 0 and less than 0.025λ, but more preferably less than 0.01λ, and even more preferably less than 0.005λ. This allows for more effective improvement in frequency adjustment sensitivity.

[0083] Furthermore, the thickness of the SiO2 film used as the dielectric film is not particularly limited, but it only needs to be thicker than the IDT electrode and less than 0.6λ. If the thickness of the SiO2 film is within this range, good frequency-temperature characteristics can be obtained. In addition, it is less likely to cause a drop in the resonant frequency.

[0084] Furthermore, the IDT electrode 3 can be a single-layer electrode containing a main electrode material such as Pt, or it can be formed from a stacked metal film. For example, like in... Figure 2 As shown in the modified example with the portion of electrode 3a enlarged, it can also have a structure in which a NiCr film 11, a Pt film 12 as the main electrode, a Ti film 13 as a diffusion prevention film, and an AlCu alloy film 14 for improving conductivity are stacked.

[0085] The aforementioned Pt film 12 is the main electrode. The main electrode refers to the electrode layer that constitutes the largest mass in the IDT electrode, and is an electrode that, when disposed on the LiNbO3 substrate 2, can produce a sufficiently large Rayleigh wave response and reflection. Preferably, the material constituting such a main electrode is a metal selected from the group consisting of Pt, Au, W, Ta, Mo, and Cu, or an alloy containing that metal as the main component. An alloy containing a metal as the main component refers to an alloy containing that metal in a proportion exceeding 50% by weight.

[0086] The NiCr film 11 described above is provided to firmly bond the Pt film 12, which serves as the main electrode, to the LiNbO3 substrate 2. That is, the NiCr film 11 is provided as a close-fitting layer. As a close-fitting layer, it is not limited to a NiCr film; Ti films, Ni films, Cr films, etc., can also be used.

[0087] The Ti film 13 is provided to suppress interdiffusion between the Pt film 12, which serves as the main electrode, and the AlCu alloy film 14. It is not limited to Ti films; Ni films, Cr films, NiCr films, etc., can also be used.

[0088] The AlCu alloy film 14 is provided to reduce the resistance of the IDT electrode 3. A suitable metal with higher conductivity than the main electrode can be used instead of the AlCu alloy film 14.

[0089] The total thickness of the IDT electrode, including the main electrode, the bonding layer, the diffusion prevention film, and the AlCu film, is not particularly limited. However, if the electrode becomes thicker, the aspect ratio of the electrode becomes larger, making it difficult to form. Therefore, it is desirable to set it to be below 0.25λ.

[0090] Furthermore, even when using the stacked metal film as described above, as long as the thickness T of the main electrode and the Euler angle θ satisfy the above equation (1), the variation in response caused by the SH wave due to the variation in the thickness of the frequency adjustment film can be effectively suppressed, just like in the above embodiment.

[0091] Furthermore, although a single-port elastic wave resonator was described in the above embodiments, the elastic wave device of the present invention is not limited to a single-port elastic wave resonator. For example, it can also be a longitudinally coupled resonator type elastic wave filter with multiple IDT electrodes, a trapezoidal filter with multiple elastic wave resonators, or a bandpass filter. In addition, the elastic wave device of the present invention can also be applied to band-stop filters and notch filters.

Claims

1. An elastic wave device, comprising: LiNbO3 substrate; An IDT electrode is disposed on the LiNbO3 substrate; A dielectric film is disposed on the LiNbO3 substrate, thereby covering the IDT electrode; and A frequency adjustment film is disposed on the dielectric film. The Euler angles of the LiNbO3 substrate are (within the range of 0°±5°, θ±1.5°, and 0°±10°). The IDT electrode has a main electrode. When the film thickness of the main electrode, which is normalized by the wavelength λ determined by the electrode finger spacing of the IDT electrode, is set as T, and the density ratio of the main electrode material to Pt is set as r, The main electrode is Cu or an alloy with Cu as the main component. The film thickness T of the main electrode and the Euler angle θ satisfy the following equation (1). Equation (1): θ = -0.05° / (T / r-0.04)+31.35°.

2. The elastic wave device according to claim 1, wherein, The value of θ is in the range of 25° or higher and 31° or lower.

3. The elastic wave device according to claim 1 or 2, wherein, The thickness of the frequency adjustment film is greater than 0 and less than 0.025λ.

4. The elastic wave device according to claim 3, wherein, The thickness of the frequency adjustment film is less than 0.005λ.

5. The elastic wave device according to claim 1 or 2, wherein, The dielectric film contains silicon oxide.

6. The elastic wave device according to claim 1 or 2, wherein, The frequency adjustment film contains silicon nitride.

7. The elastic wave device according to claim 1 or 2, wherein, The IDT electrode has the main electrode and other electrode layers containing metals other than the main electrode.

8. The elastic wave device according to claim 1 or 2, wherein, The elastic wave device is a bandpass filter having the IDT electrodes.

9. The elastic wave device according to claim 1 or 2, wherein, The total film thickness of the IDT electrodes is less than 0.25λ.

10. The elastic wave device according to claim 5, wherein, The silicon oxide is thicker than the IDT electrode.

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