Acoustic wave sensor and method for manufacturing the same

Abstract

To provide an elastic wave sensor in which decrease in sensitivity is suppressed.SOLUTION: An elastic wave sensor 25 includes: a piezoelectric substrate 10; an elastic wave resonator 27 provided on the piezoelectric substrate and including a pair of interdigitated electrodes 16 each including a plurality of electrode fingers 14, in which at least a part of the electrode fingers of one interdigitated electrode in the arrangement direction of the electrode fingers and at least a part of the electrode fingers of the other interdigitated electrode are alternately arranged; and a sensitive film 24 provided on at least the piezoelectric substrate in a region between the electrode fingers in the region where the electrode fingers of one interdigitated electrode and the electrode fingers of the other interdigitated electrode are alternately arranged, the sensitive film 24 being thinner than the electrode fingers.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an acoustic wave sensor and a manufacturing method thereof, and more particularly, to an acoustic wave sensor having an acoustic wave resonator and a manufacturing method thereof. [Background technology]

[0002] There is known an environmental sensor that detects a change in the mass of a sensitive film to detect a change in the environment, such as the concentration of a specific atom or molecule in a gas or liquid, temperature, or humidity. There is also known an elastic wave sensor that detects a change in the mass of a sensitive film by detecting a change in frequency (e.g., Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2005-331326 A [Patent Document 2] JP 2006-313092 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, providing a sensitive film on the interdigital transducer lowers the resonance frequency, which in turn reduces the sensitivity of the acoustic wave sensor.

[0005] The present invention has been made in consideration of the above problems, and has an object to suppress a decrease in sensitivity. [Means for solving the problem]

[0006] The present invention is an elastic wave sensor comprising: a piezoelectric substrate; an elastic wave resonator having a pair of comb electrodes provided on the piezoelectric substrate, each of which has a plurality of electrode fingers, and in which at least a portion of the electrode fingers of one comb electrode and at least a portion of the electrode fingers of the other comb electrode are arranged alternately in the arrangement direction of the plurality of electrode fingers; and a sensitive membrane that is thinner than the plurality of electrode fingers and is provided at least on the piezoelectric substrate in a region between the plurality of electrode fingers in an arrangement region in which the electrode fingers of the one comb electrode and the electrode fingers of the other comb electrode are arranged alternately.

[0007] In the above configuration, a part of the sensitive film may be provided on the plurality of electrode fingers in the array region.

[0008] In the above configuration, the sensitive film provided on the piezoelectric substrate in the arrangement region and the sensitive film provided on the plurality of electrode fingers in the arrangement region may be formed not continuously.

[0009] In the above configuration, the sensitive film may be configured not to be provided on the plurality of electrode fingers in the array region.

[0010] The present invention is an elastic wave sensor comprising: a piezoelectric substrate; an elastic wave resonator having a pair of comb electrodes provided on the piezoelectric substrate, each of which has a plurality of electrode fingers, and in which at least a portion of the electrode fingers of one comb electrode and at least a portion of the electrode fingers of the other comb electrode are arranged alternately in the arrangement direction of the plurality of electrode fingers; and a sensitive membrane provided on the piezoelectric substrate in a region between the plurality of electrode fingers in an arrangement region in which the electrode fingers of the one comb electrode and the electrode fingers of the other comb electrode are arranged alternately, and which is not provided on the plurality of electrode fingers in the arrangement region.

[0011] In the above configuration, the sensitive film may be thicker than the plurality of electrode fingers.

[0012] In the above configuration, the resonant frequency of the acoustic wave sensor may be higher than the resonant frequency when the sensitive film is not provided.

[0013] The present invention is an elastic wave sensor comprising: a piezoelectric substrate; an elastic wave resonator having a pair of comb electrodes provided on the piezoelectric substrate, each of which has a plurality of electrode fingers, and in which at least a portion of the electrode fingers of one comb electrode and at least a portion of the electrode fingers of the other comb electrode are arranged alternately in the arrangement direction of the plurality of electrode fingers; and a sensitive membrane provided at least on the piezoelectric substrate in a region between the plurality of electrode fingers in an arrangement region in which the electrode fingers of the one comb electrode and the electrode fingers of the other comb electrode are arranged alternately, wherein the resonant frequency of the elastic wave resonator is higher than the resonant frequency of the elastic wave resonator when the sensitive membrane is not provided.

[0014] In the above configuration, the piezoelectric substrate may be a lithium tantalate substrate or a lithium niobate substrate, and the sensitive film may be made mainly of metal phthalocyanine.

[0015] The present invention is a method for manufacturing an elastic wave sensor, which includes preparing an elastic wave resonator having a pair of comb electrodes on a piezoelectric substrate, each of which has a plurality of electrode fingers, and in which at least a portion of the electrode fingers of one comb electrode and at least a portion of the electrode fingers of the other comb electrode are arranged alternately in the arrangement direction of the plurality of electrode fingers, and forming a sensitive film provided at least in a region between the plurality of electrode fingers on the piezoelectric substrate in an arrangement region in which the electrode fingers of one comb electrode and the electrode fingers of the other comb electrode are arranged alternately, so that the resonant frequency after forming the sensitive film is higher than the resonant frequency of the elastic wave resonator before forming the sensitive film. Effect of the Invention

[0016] An object of the present invention is to suppress a decrease in sensitivity. [Brief description of the drawings]

[0017] [Figure 1]FIG. 1(a) is a plan view showing an acoustic wave sensor in accordance with the first embodiment, and FIG. 1(b) is a cross-sectional view taken along line AA of FIG. 1(a). [Diagram 2] 2A to 2C are cross-sectional views illustrating a method for manufacturing the acoustic wave sensor in accordance with the first embodiment. [Diagram 3] 3(a) to 3(f) are cross-sectional views showing examples of the sensitive film in the first embodiment. [Figure 4] FIG. 4(a) is a plan view showing an acoustic wave sensor in accordance with a second embodiment, and FIG. 4(b) is a cross-sectional view taken along line AA of FIG. 4(a). [Diagram 5] 5(a) to 5(d) are cross-sectional views showing examples of the sensitive film in the second embodiment. [Figure 6] FIG. 6 is a cross-sectional view of an acoustic wave sensor in accordance with Comparative Example 1. As shown in FIG. [Figure 7] FIG. 7 is a graph showing admittance Y versus frequency of the acoustic wave sensor in Comparative Example 1. In FIG. [Figure 8] FIG. 8 is a graph illustrating admittance Y versus frequency of the acoustic wave sensor in accordance with the first embodiment. [Figure 9] FIG. 9 is a graph illustrating admittance Y versus frequency of the acoustic wave sensor in accordance with the first embodiment. [Figure 10] FIG. 10 is a graph illustrating admittance Y versus frequency of the acoustic wave sensor in accordance with the first embodiment. [Figure 11] FIG. 11 is a graph illustrating admittance Y versus frequency of the acoustic wave sensor in accordance with the second embodiment. [Figure 12] 12A to 12C are diagrams illustrating impedance Z versus frequency of the acoustic wave sensor in Example 1. FIG. [Figure 13] 13(a) and 13(b) are diagrams showing impedance |Z| versus frequency of acoustic wave sensors A and B in the experiment. [Figure 14] FIG. 14 is a cross-sectional view of an acoustic wave sensor in accordance with the third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, the embodiments will be described with reference to the drawings. EXAMPLES

[0019] Example 1 is an example in which an acoustic wave resonator is used as an acoustic wave sensor. Fig. 1(a) is a plan view showing the acoustic wave sensor, and Fig. 1(b) is a cross-sectional view taken along line AA of Fig. 1(a). In Fig. 1(a), a sensitive film 24 is indicated by a thick dashed line. The arrangement direction of the electrode fingers 14 is the X direction, the extension direction of the electrode fingers 14 is the Y direction, and the normal direction of the piezoelectric substrate 10 is the Z direction. Note that the X, Y, and Z directions do not necessarily coincide with the crystal orientation of the piezoelectric substrate 10.

[0020] As shown in FIG. 1(a) and FIG. 1(b), an acoustic wave sensor 25 includes an acoustic wave resonator 27 and a sensitive film 24. The acoustic wave resonator 27 includes an IDT 20 (interdigital transducer) and a reflector 22. The IDT 20 and the reflector 22 are provided on a piezoelectric substrate 10. The IDT 20 and the reflector 22 are formed of a metal film 12. The IDT 20 includes a pair of comb-shaped electrodes 16. Each of the pair of comb-shaped electrodes 16 includes a bus bar 15 to which a plurality of electrode fingers 14 and one end of the plurality of electrode fingers 14 extending in the Y direction are connected and which extends in the X direction. In the X direction, the electrode fingers 14 of one comb-shaped electrode 16 and the electrode fingers 14 of the other comb-shaped electrode 16 are at least partially staggered. A pad 17 is connected to the bus bar 15. When viewed from the X direction, the region where the electrode fingers 14 of one comb electrode 16 and the electrode fingers 14 of the other comb electrode 16 overlap is the intersection region 23. The intersection region 23 is an arrangement region where the electrode fingers 14 of one comb electrode 16 and the electrode fingers 14 of the other comb electrode 16 are arranged alternately. Reflectors 22 are formed on both sides of the IDT 20 in the X direction. The acoustic wave excited by the IDT 20 propagates mainly in the X direction, and the reflectors 22 reflect the acoustic wave. The pitch of one electrode finger 14 in a pair of comb electrodes 16 is λ. λ corresponds to the wavelength of the surface acoustic wave excited by the IDT 20. λ is twice the pitch D of the multiple electrode fingers 14. Note that λ may be other than twice the pitch D.

[0021] A sensitive film 24 is provided on the IDT 20 and on the piezoelectric substrate 10 between the electrode fingers 14. For example, when a substance such as a specific atom or molecule in a gas or liquid is adsorbed to the sensitive film 24, the mass of the sensitive film 24 increases. In this way, the mass of the sensitive film 24 changes due to a change in the environment, such as the concentration of a specific substance in the gas or liquid around the sensitive film 24. When the mass of the sensitive film 24 changes, the resonance frequency and anti-resonance frequency of the elastic wave resonator 27 change. By detecting the change in the resonance frequency or anti-resonance frequency of the elastic wave resonator 27, the change in the environment can be detected. Note that the resonance frequency or anti-resonance frequency changes when the temperature changes. In addition, when the humidity around the sensitive film 24 increases, moisture is adsorbed to the sensitive film 24 and the mass of the sensitive film 24 increases. This causes a change in the resonance frequency or anti-resonance frequency. These changes in the resonance frequency or anti-resonance frequency are noise.

[0022] The piezoelectric substrate 10 is, for example, a lithium tantalate substrate, a lithium niobate substrate, or a quartz substrate, for example, a single crystal rotated Y-cut X-propagation lithium tantalate substrate or a single crystal rotated Y-cut X-propagation lithium niobate substrate. The metal film 12 is, for example, an aluminum (Al) film, a copper (Cu) film, or a molybdenum (Mo) film. The metal film 12 may be a film whose main material is at least one metal selected from aluminum, copper, and molybdenum. The sensitive film 24 is, for example, made mainly of metal phthalocyanine.

[0023] 2(a) to 2(c) are cross-sectional views showing a method for manufacturing an acoustic wave sensor according to the first embodiment. As shown in FIG. 2(a), a piezoelectric substrate 10 is prepared. As shown in FIG. 2(b), a metal film 12 is formed on the piezoelectric substrate 10. The metal film 12 forms an acoustic wave resonator 27 including an IDT 20 and a reflector 22. The metal film 12 is formed by, for example, sputtering or vacuum deposition to cover one side of the piezoelectric substrate 10. A resist film is formed by photolithography and then patterned. The patterned resist film is used as a mask to etch the metal film 12, thereby patterning the metal film 12. As shown in FIG. 2(c), a sensitive film 24 is formed on the IDT 20 and on the piezoelectric substrate 10 between the electrode fingers 14. The sensitive film 24 is formed by, for example, vacuum deposition or sputtering. At this time, the sensitive film 24 is formed so that the resonant frequency and anti-resonant frequency of the elastic wave sensor 25 are higher than the resonant frequency and anti-resonant frequency of the elastic wave resonator 27 before the sensitive film 24 is formed (FIG. 2(b)). In this way, the elastic wave sensor 25 is manufactured.

[0024] 3(a) to 3(f) are cross-sectional views showing examples of the sensitive film in the first embodiment. Two electrode fingers 14 are shown in FIG. 3(a) to 3(f). As shown in FIG. 3(a), the sensitive film 24 is provided on the electrode fingers 14 and on the piezoelectric substrate 10 in a region 28 between the electrode fingers 14. There is a region in the upper part of the side surface of the electrode finger 14 that is not covered with the sensitive film 24. Therefore, the sensitive film 24 on the electrode finger 14 is separated from the sensitive film 24 in the region 28. The thickness of the electrode finger 14 is T1, the thickness of the sensitive film 24 in the region 28 is T2, and the thickness of the sensitive film 24 on the electrode finger 14 is T3. The thickness T2 is smaller than the thickness T1. The thicknesses T2 and T3 may be the same or different.

[0025] As shown in Fig. 3(b), electrode fingers 14 are provided on a piezoelectric substrate 10, and an insulating film 26 is provided to cover the upper and side surfaces of the electrode fingers 14 and the piezoelectric substrate 10 in regions 28 between the electrode fingers 14. The insulating film 26 is a protective film, and is, for example, an inorganic insulating film such as a silicon oxide film, a silicon nitride film, or an aluminum oxide film. The thickness of the insulating film 26 is smaller than the thickness T1 of the electrode fingers 14 and the thicknesses T2 and T3 of the sensitive film 24. The sensitive film 24 is provided on the insulating film 26. The other configuration is the same as in Fig. 3(a), and a description thereof will be omitted.

[0026] As shown in Fig. 3(c), the sensitive film 24 on the electrode fingers 14 and the sensitive film 24 in the region 28 are continuously formed by the sensitive film 24 provided on the side surface of the electrode fingers 14. The other configuration is the same as in Fig. 3(a), and the description will be omitted.

[0027] As shown in Fig. 3(d), an insulating film 26 is provided on the piezoelectric substrate 10 so as to cover the electrode fingers 14. The sensitive film 24 is provided on the insulating film 26. The other configuration is the same as in Fig. 3(c), and therefore a description thereof will be omitted.

[0028] As shown in Fig. 3(e), the thickness T2 of the sensitive film 24 in the region 28 is greater than the thickness T1 of the electrode fingers 14. The thickness T3 of the sensitive film 24 on the electrode fingers 14 is approximately T2-T1. This makes the upper surface of the sensitive film 24 approximately flat. The thickness T3 may be greater or less than T2-T1. The other configurations are the same as those in Fig. 3(a), and therefore a description thereof will be omitted.

[0029] As shown in Fig. 3(f), an insulating film 26 is provided on the piezoelectric substrate 10 so as to cover the electrode fingers 14. The sensitive film 24 is provided on the insulating film 26. The other configuration is the same as in Fig. 3(e), and therefore a description thereof will be omitted.

[0030] As shown in Figures 3(a) to 3(d), the thickness T2 of the sensitive film 24 in the region 28 may be smaller than the thickness T1 of the electrode finger 14, or as shown in Figures 3(e) and 3(f), the thickness T2 may be larger than the thickness T1. As shown in Figures 3(a) and 3(b), the sensitive film 24 in the region 28 and the sensitive film 24 on the electrode finger 14 may be separated, or as shown in Figures 3(c) and 3(d), the sensitive film 24 in the region 28 and the sensitive film 24 on the electrode finger 14 may be connected. EXAMPLES

[0031] Fig. 4(a) is a plan view showing an acoustic wave sensor in Example 2, and Fig. 4(b) is a cross-sectional view taken along line AA of Fig. 4(a). As shown in Fig. 4(a) and Fig. 4(b), sensitive film 24 is provided on piezoelectric substrate 10 in region 28 between electrode fingers 14 and on piezoelectric substrate 10 between electrode fingers 14 and bus bar 15, but not on electrode fingers 14. The other configurations are the same as those in Fig. 1(a) and Fig. 1(b) of Example 1, and therefore description thereof will be omitted.

[0032] Figures 5(a) to 5(d) are cross-sectional views showing examples of the sensitive film in Example 2. Two electrode fingers 14 are shown in Figures 5(a) to 5(d). As shown in Figure 5(a), the sensitive film 24 is provided on the piezoelectric substrate 10 in a region 28 between the electrode fingers 14, and is not provided on the electrode fingers 14. A thickness T2 of the sensitive film 24 is smaller than a thickness T1 of the electrode fingers 14.

[0033] As shown in Fig. 5(b), an insulating film 26 is provided on the piezoelectric substrate 10 so as to cover the electrode fingers 14. The sensitive film 24 is provided on the insulating film 26. The other configuration is the same as in Fig. 5(a), and therefore a description thereof will be omitted.

[0034] As shown in Fig. 5(c), the thickness T2 of the sensitive film 24 is greater than the thickness T1 of the electrode fingers 14. The other configurations are the same as those in Fig. 5(a), and therefore a description thereof will be omitted.

[0035] As shown in Fig. 5(d), an insulating film 26 is provided on the piezoelectric substrate 10 so as to cover the electrode fingers 14. The sensitive film 24 is provided on the insulating film 26. The other configuration is the same as in Fig. 5(c), and therefore a description thereof will be omitted.

[0036] [Comparative Example 1] Fig. 6 is a cross-sectional view of an acoustic wave sensor in accordance with Comparative Example 1. As shown in Fig. 6, in Comparative Example 1, sensitive film 24 is provided on electrode fingers 14, but not in region 28. Electrode fingers 14 and sensitive film 24 have thicknesses T1 and T3, respectively.

[0037] [simulation] For Example 1 and Comparative Example 1, the frequency characteristics of the surface acoustic wave resonator before and after forming the sensitive film, and the frequency characteristics of the acoustic wave sensor 25 after forming the sensitive film 24 were simulated. Piezoelectric substrate 10:42° rotated Y-cut X-propagation lithium tantalate substrate Electrode finger 14: Molybdenum film with T1 of 485 nm Sensitive film 24: Copper phthalocyanine (CuPc) Wavelength λ: 4.4μm The density ρ and Young's modulus E of copper phthalocyanine were measured to be 1.49 g / cm 3 and 55 GPa, and the Poisson's ratio σ was set to 0.35 according to the literature.

[0038] In the structure of Comparative Example 1 shown in FIG. 6, the thickness T3 of the sensitive film 24 was changed. FIG. 7 is a diagram showing the admittance Y versus frequency of the elastic wave sensor in Comparative Example 1. The resonant frequency fr0 before the sensitive film 24 was provided was 700 MHz (dashed line). The thickness T3 of the sensitive film 24 was 0.01 μm, 0.05 μm, 0.1 μm, 0.15 μm, 0.2 μm, and 0.485 μm. Both the resonant frequency fr and the anti-resonant frequency fa decreased as the thickness T3 increased.

[0039] In the structure of FIG. 3(a) of Example 1, the thicknesses T2 and T3 of the sensitive film 24 were changed to perform a simulation. FIGS. 8 to 10 are diagrams showing the admittance Y versus frequency of the elastic wave sensor in Example 1. The resonant frequency fr0 before the sensitive film 24 is provided is 700 MHz (dashed line). In FIG. 8, the thicknesses T2 and T3 of the sensitive film 24 are 0.01 μm, 0.05 μm, 0.1 μm, 0.15 μm, and 0.2 μm. T2=T3. When the sensitive film 24 with thicknesses T2 and T3 is provided, the resonant frequency is lower than the resonant frequency fr0 before the sensitive film 24 is provided. As the thicknesses T2 and T3 increase, the resonant frequency fr and the anti-resonant frequency fa increase. When the thicknesses T2 and T3 are between 0.15 μm and 0.2 μm, the resonant frequency fr is higher than the resonant frequency fr0 without the sensitive film 24. 9, the thicknesses T2 and T3 of the sensitive film 24 are set to 0.25 μm, 0.3 μm, 0.35 μm, and 0.4 μm. As the thicknesses T2 and T3 of the sensitive film 24 increase, the resonance frequency fr and the anti-resonance frequency fa increase.

[0040] In Figure 10, the thicknesses T2 and T3 of the sensitive film 24 are set to 0.181 μm, 0.182 μm, 0.183 μm, and 0.184 μm, and the vicinity of the resonant frequency is enlarged. When the thicknesses T2 and T3 of the sensitive film 24 are 0.183 μm, the resonant frequency fr is almost equal to the resonant frequency fr0 when the sensitive film 24 is not provided. When normalized with the wavelength λ of the surface acoustic wave of 4.4 μm, the resonant frequency fr is equal to or greater than fr0 when the normalized values ​​of the thicknesses T2 and T3 are 0.0416 or greater.

[0041] In the structure of FIG. 5(a) of Example 2, a simulation was performed by changing the thickness T2 of the sensitive film 24. FIG. 11 is a diagram showing the admittance Y versus frequency of the elastic wave sensor in Example 2. The resonant frequency fr0 before the sensitive film 24 is provided is 700 MHz (dashed line). In FIG. 11, the thickness T2 of the sensitive film 24 is 0.01 μm, 0.05 μm, 0.1 μm, 0.15 μm, 0.2 μm, and 0.485 μm. When the sensitive film 24 with the thickness T2 is provided, the resonant frequency is lower than the resonant frequency fr0 before the sensitive film 24 is provided. As the thickness T2 increases, the resonant frequency fr and the anti-resonant frequency fa increase. When the thickness T2 is between 0.05 μm and 0.1 μm, the resonant frequency fr is higher than the resonant frequency fr0 without the sensitive film 24. The thickness T2 of the sensitive film 24 at which the resonant frequency fr becomes higher than the resonant frequency fr0 when the sensitive film 24 is not provided is thinner than that of FIG. 10 of the first embodiment.

[0042] A simulation was performed for the structure in Fig. 3(a) to examine the reason for the change in resonant frequency due to the provision of the sensitive film 24. In the simulation, the density ρ, Young's modulus E, and Poisson's ratio σ of the sensitive film 24 were changed. The thicknesses T2 and T3 of the sensitive film 24 were 0.14 μm.

[0043] 12(a) to 12(c) are diagrams illustrating impedance Z versus frequency of the acoustic wave sensor in Example 1. As shown in FIG. 12(a), Young's modulus E and Poisson's ratio σ are fixed at 55 GPa and 0.35, respectively, and density ρ is set to 0.75 g / cm 3 , 1.49g / cm 3 and 3 g / cm 3 As the density ρ increases, the resonant frequency fr and the anti-resonant frequency fa decrease. As shown in FIG. 12(b), the density ρ and the Poisson's ratio σ are set to 1.49 g / cm 3 The density ρ and Young's modulus E were fixed at 1.49 g / cm3 and 0.35, respectively, and the Young's modulus E was set to 27.5 GPa, 55 GPa, and 110 GPa. As the Young's modulus E increases, the resonant frequency fr and the anti-resonant frequency fa increase. As shown in FIG. 12(c), the density ρ and Young's modulus E were set to 1.49 g / cm3 and 1.59 g / cm3, respectively. 3and 55 GPa, and the Poisson's ratio σ is set to 0.25, 0.35, and 0.49. Even if the Poisson's ratio is changed, the resonant frequency fr and the antiresonant frequency fa do not change much.

[0044] There are two factors that cause the resonance frequency fr and the anti-resonance frequency fa to change due to the provision of the sensitive film 24. The first is the effect of the added mass applied to the piezoelectric substrate 10 and the electrode fingers 14 by providing the sensitive film 24. As the added mass increases, the resonant frequency fr and the anti-resonant frequency fa decrease. The second is the effect of the sound speed of the surface acoustic wave propagating through the sensitive film 24. In the region 28, the surface acoustic wave propagates near the interface between the piezoelectric substrate 10 and the sensitive film 24. When the sound speed of the surface acoustic wave propagating through the sensitive film 24 increases, the speed of the surface acoustic wave propagating near the interface between the piezoelectric substrate 10 and the sensitive film 24 increases. This increases the resonance frequency fr and the anti-resonance frequency fa of the acoustic wave sensor 25. When the density ρ of the sensitive film 24 provided in the region 28 decreases, the load mass decreases, and the effect of decreasing the resonance frequency fr and the anti-resonance frequency fa decreases. On the other hand, when the density ρ of the sensitive film 24 decreases, it is considered that the sound speed of the surface acoustic wave propagating through the sensitive film 24 increases. Therefore, the resonance frequency fr and the anti-resonance frequency fa increase. It is believed that when the Young's modulus E of the sensitive film 24 provided in the region 28 becomes high, the sound velocity of the surface acoustic wave propagating through the sensitive film 24 becomes high, and the resonance frequency fr and the anti-resonance frequency fa become high.

[0045] In FIG. 12(a), when the density ρ of the sensitive film 24 increases, the mass loading increases and the sound velocity of the surface acoustic wave propagating through the sensitive film 24 decreases. Therefore, it is considered that the resonance frequency fr and the anti-resonance frequency fa decrease. In FIG. 12(b), when the Young's modulus E of the sensitive film 24 increases, the mass loading does not change and the sound velocity of the surface acoustic wave propagating through the sensitive film 24 increases. Therefore, it is considered that the resonance frequency fr and the anti-resonance frequency fa increase. In FIG. 12(c), since the Poisson's ratio σ of the sensitive film 24 does not have much effect on the mass loading and the sound velocity of the surface acoustic wave propagating through the sensitive film 24, it is considered that the resonance frequency fr and the anti-resonance frequency fa do not change much even if the Poisson's ratio σ changes.

[0046] Based on the above considerations, Comparative Example 1 and Examples 1 and 2 will be considered. From the results of Fig. 12(a) to Fig. 12(c), the following can be inferred. When the sensitive film 24 is not provided in the region 28 between the electrode fingers 14, as in Comparative Example 1, the change in the resonant frequency is only due to the effect of the addition of mass. For this reason, as shown in Fig. 7, when the thickness T3 of the sensitive film 24 increases, the resonant frequency fr and the antiresonant frequency fa decrease.

[0047] When the sensitive film 24 is provided in the region 28 as in the first and second embodiments, the change in the resonant frequency is influenced by the mass load and the sound velocity of the surface acoustic wave propagating through the sensitive film 24 .

[0048] First, when the thickness T2 of the sensitive film 24 is small, there is almost no effect on the sound velocity of the surface acoustic wave propagating through the sensitive film 24, but the effect of mass loading is large. Therefore, when a thin sensitive film 24 is provided, such as T2=0.01 μm in Figures 8 and 11, the resonance frequency fr and the anti-resonance frequency fa become lower than the resonance frequency and the anti-resonance frequency fa when the sensitive film 24 is not provided.

[0049] Next, when the thickness T2 of the sensitive film 24 is large, the effect of the sound speed of the surface acoustic wave propagating through the sensitive film 24 becomes greater than the effect of the mass load. Therefore, as shown in Figures 8 to 11, as the thickness T2 of the sensitive film 24 increases, the resonance frequency fr and the anti-resonance frequency fa become higher.

[0050] In comparison between Example 1 and Example 2, in Example 2, the sensitive film 24 is not provided on the electrode fingers 14. Therefore, the effect of the added mass is smaller in Example 2 than in Example 1. Therefore, in Example 1, the thickness T2 of the sensitive film 24 at which the resonant frequency fr becomes greater than fr0 is 0.183 μm as shown in FIG. 10. In contrast, in Example 2, the thickness T2 of the sensitive film 24 at which the resonant frequency fr becomes greater than fr0 is 0.05 μm to 0.1 μm as shown in FIG. 11. Thus, in Example 2, the thickness T2 at which the resonant frequency fr becomes greater than fr0 is smaller than that in Example 1.

[0051] [experiment] Elastic wave sensors A and B with different resonant frequencies were fabricated, and the frequency characteristics were measured before and after forming the sensitive film 24. A vector network analyzer was used to measure the frequency characteristics. The fabricated elastic wave sensor had the structure shown in FIG. 3(a) of Example 1, and the sensitive film 24 was copper phthalocyanine with thicknesses T2 and T3 of 0.14 μm. The wavelength λ of elastic wave sensor A was 1.81 μm, and the wavelength λ of elastic wave sensor B was 4.4 μm. The normalized thicknesses T2 / λ and T3 / λ were 0.077 and 0.032 for elastic wave sensors A and B, respectively.

[0052] 13(a) and 13(b) are diagrams showing impedance |Z| versus frequency for acoustic wave sensors A and B in the experiment. As shown in Fig. 13(a), in acoustic wave sensor A, the resonant frequency fr and the anti-resonant frequency fa after the sensitive film 24 is formed are higher than the resonant frequency fr and the anti-resonant frequency fa before the sensitive film 24 is formed. The peaks of the resonant frequency fr and the anti-resonant frequency fa are blunted, and the Q value is reduced.

[0053] 13(b), in the acoustic wave sensor B, the resonant frequency fr and the antiresonant frequency fa after the sensitive film 24 is formed are lower than the resonant frequency fr and the antiresonant frequency fa before the sensitive film 24 is formed. The peaks of the resonant frequency fr and the antiresonant frequency fa are somewhat dulled and the Q value is somewhat reduced, but not as much as in the acoustic wave sensor A.

[0054] After the sensitive film 24 is formed, the elastic wave sensor A has high fa and fr, while the elastic wave sensor B has low fa and fr. This is considered to be due to the difference between the normalized thicknesses T2 / λ and T3 / λ. In the simulation of FIG. 10, when the normalized thicknesses T2 and T3 are 0.0416λ, this is the boundary value at which fr and fa become low or high due to the formation of the sensitive film 24. This result does not contradict the results of the elastic wave sensors A and B. It is considered that the deterioration of the Q value increases as the normalized thicknesses T2 / λ and T3 / λ increase. The thickness T1 / λ of the general electrode finger 14 is 0.1 to 0.15. As shown in FIG. 13(a), the Q value is deteriorated when the thickness T2 / λ is 0.077. From the viewpoint of preventing the deterioration of the Q value, it is considered that the thickness T2 is preferably smaller than the thickness T1 of the electrode finger 14.

[0055] The simulation results and the experimental results in the above-mentioned first and second embodiments will be summarized.

[0056] In Comparative Example 1, since the sensitive film 24 is provided only on the electrode fingers 14, only the effect of mass load affects the change in the resonant frequency and the anti-resonant frequency. On the other hand, in Example 1, the sensitive film 24 is provided on the piezoelectric substrate 10 in the region 28 between the multiple electrode fingers 14. As a result, in addition to the effect of mass load, the effect of the sound speed of the surface acoustic wave propagating through the sensitive film 24 is added as a factor that affects the change in the resonant frequency and the anti-resonant frequency. Therefore, in Example 1, the resonant frequency fr and the anti-resonant frequency fa are higher than in Comparative Example 1, in which the sensitive film 24 is provided only on the electrode fingers 14. This makes it possible to suppress a decrease in the sensitivity of the acoustic wave sensor.

[0057] When the thickness T2 of the sensitive film 24 in the region 28 becomes large, the Q value decreases as shown in FIG. 13(a). Therefore, as shown in FIG. 3(a) to FIG. 3(d), FIG. 5(a) and FIG. 5(b), the thickness T2 is preferably smaller than the thickness T3 of the electrode finger 14. The thickness T2 is preferably 0.9 or less than T3, more preferably 0.8 or less, and more preferably 0.7 or less. On the other hand, the thicker the sensitive film 24 is, the greater the amount of a specific substance adsorbed, and the greater the increase in the mass of the sensitive film 24. Therefore, from this viewpoint, as shown in FIG. 3(e), FIG. 3(f), FIG. 5(c) and FIG. 5(d), the thickness T2 of the sensitive film 24 (height from the piezoelectric substrate 10) may be greater than the thickness T3 of the electrode finger 14.

[0058] 8 and 11, when the thickness T2 of the sensitive film 24 is small, the effect of increasing the speed of sound is reduced, the energy of the surface acoustic wave present in the sensitive film 24 is reduced, and the resonant frequency fr and the anti-resonant frequency fa are lowered. Therefore, when the average pitch of the electrode fingers 14 is D, the thickness T2 of the sensitive film 24 is preferably 0.02D (0.01λ) or more, more preferably 0.04D (0.02λ) or more, and even more preferably 0.08D (0.04λ) or more. The average pitch D of the electrode fingers 14 can be calculated by dividing the width of the IDT 20 in the X direction by the number of electrode fingers 14.

[0059] As shown in Figures 3(a) to 3(f) of the first embodiment, the sensitive film 24 may be provided on the electrode fingers 14 in the intersection region 23. If the sensitive film 24 is provided in the region 28 between the electrode fingers 14 and not on the electrode fingers 14, the manufacturing process becomes complicated. Therefore, in order to simplify the manufacturing process, it is preferable to provide the sensitive film 24 on the electrode fingers 14. If the thickness T3 of the sensitive film 24 on the electrode fingers 14 is large, the resonant frequency fr and the anti-resonant frequency fa become lower due to the effect of the added mass. Therefore, it is preferable that the thickness T3 is as small as possible.

[0060] 3(c) and 3(d), if the sensitive film 24 provided in the region 28 and the sensitive film 24 provided on the electrode fingers 14 are connected in the intersection region 23, the thickness of the sensitive film 24 may differ in the region 28, which may deteriorate the Q value. Therefore, as shown in Fig. 3(a) and 3(b), it is preferable that the sensitive film 24 provided on the piezoelectric substrate 10 in the region 28 and the sensitive film 24 provided on the electrode fingers 14 are not continuously formed in the intersection region 23. The fact that the sensitive film 24 is not continuously formed means that the sensitive film 24 provided on the piezoelectric substrate 10 in the region 28 and the sensitive film 24 provided on the electrode fingers 14 are separated from each other in the intersection region 23.

[0061] The sensitive film 24 provided on the electrode fingers 14 contributes only to adding mass. Therefore, it contributes only to lowering the resonance frequency fr and the anti-resonance frequency fa. From this viewpoint, it is preferable that the sensitive film 24 is not provided on the electrode fingers 14 in the intersection region 23.

[0062] Furthermore, the resonant frequency fr of the elastic wave sensor is higher than the resonant frequency fr when the sensitive film 24 is not provided. For example, the resonant frequency fr and the anti-resonant frequency fa of the elastic wave sensor 25 measured in FIG. 2(c) are higher than the resonant frequency fr0 and the anti-resonant frequency fa0 of the elastic wave resonator 27 measured in FIG. 2(b). This makes it possible to increase the resonant frequency fr and the anti-resonant frequency fa. This makes it possible to suppress a decrease in the sensitivity of the elastic wave sensor 25.

[0063] In order to increase the resonance frequency fr and the anti-resonance frequency fa, it is preferable that the sound velocity of the surface acoustic wave propagating through the sensitive film 24 is faster than the sound velocity of the surface acoustic wave propagating through the piezoelectric substrate 10. In lithium tantalate, the density ρ and Young's modulus E are 7.45 g / cm 3 and 230 GPa, and for lithium niobate, the density ρ and Young's modulus E are 4.65 g / cm 3 and 210 GPa. The density ρ and Young's modulus E of copper phthalocyanine are 1.49 g / cm, respectively. 3and 55 GPa. Therefore, when the piezoelectric substrate 10 is made of lithium tantalate or lithium niobate, by using copper phthalocyanine for the sensitive film 24, the sound velocity of the surface acoustic wave propagating through the sensitive film 24 can be made faster than the sound velocity of the surface acoustic wave propagating through the piezoelectric substrate 10. Therefore, the resonant frequency fr and the anti-resonant frequency fa can be made high.

[0064] In addition to copper phthalocyanine, copper phthalocyanine fluoride (CuPcF8, CuPcF 16 ), cobalt phthalocyanine (CoPc), manganese phthalocyanine (MnPc), iron phthalocyanine (FePc) or nickel phthalocyanine (NiPc) may be used as the main component. Copper phthalocyanine, copper phthalocyanine fluoride (CuPcF 16 ), cobalt phthalocyanine, manganese phthalocyanine and iron phthalocyanine have reported densities ρ of 1.641 g / cm, respectively. 3 , 2.054g / cm 3 , 1.625g / cm 3 , 1.651g / cm 3 and 1.649 g / cm 3 The densities ρ of copper phthalocyanine, iron phthalocyanine and nickel phthalocyanine measured by a dry densitometer were 1.62 g / cm3, respectively. 3 , 1.58g / cm 3 and 1.60 g / cm 3 The density ρ of copper phthalocyanine measured by X-ray reflectivity (XRR) is 1.49 g / cm 3 It is.

[0065] The density of these metal phthalocyanines is lower than that of the piezoelectric substrate 10. In addition, the Young's modulus of the metal phthalocyanines is high. Therefore, by using the metal phthalocyanines for the sensitive film 24, the sound velocity of the surface acoustic wave propagating through the sensitive film 24 can be increased, and the resonance frequency fr and the anti-resonance frequency fa can be increased. Thus, the metal phthalocyanines have a low density and a high Young's modulus. The metal of the metal phthalocyanines is a transition metal, such as copper, cobalt, manganese, iron, nickel, or titanium. The metal phthalocyanines may be halogenated phthalocyanines, such as fluorinated metal phthalocyanines, chlorinated metal phthalocyanines, or brominated metal phthalocyanines.

[0066] As described above, it is preferable that the piezoelectric substrate 10 is a lithium tantalate substrate or a lithium niobate substrate, and the sensitive film 24 is mainly made of metal phthalocyanine. This makes the sound velocity of the surface acoustic wave propagating through the sensitive film 24 faster than the sound velocity of the surface acoustic wave propagating through the piezoelectric substrate 10, and the resonance frequency fr and the anti-resonance frequency fa can be increased. Note that the sensitive film 24 being mainly made of metal phthalocyanine may contain impurities other than metal phthalocyanine, either intentionally or unintentionally. The proportion of metal phthalocyanine in the sensitive film 24 is, for example, 50% by weight or more or 80% by weight or more. EXAMPLES

[0067] FIG. 14 is a cross-sectional view of an acoustic wave sensor according to a third embodiment. As shown in FIG. 14, an insulating layer 11a is provided on a support substrate 11, and a piezoelectric substrate 10 is provided on the insulating layer 11a. The support substrate 11 is, for example, a sapphire substrate, an alumina substrate, a spinel substrate, a quartz substrate, or a silicon substrate. The insulating layer 11a is, for example, a single layer film such as a silicon oxide layer, a silicon nitride layer, an aluminum oxide layer, or an aluminum nitride layer, or a laminated film thereof. As an example, the insulating layer 11a is an aluminum oxide layer provided on the support substrate 11, and a silicon oxide layer provided on the aluminum oxide layer. The insulating layer 11a may not be provided, and the piezoelectric substrate 10 may be directly bonded onto the support substrate 11. The other configurations are the same as those of the first embodiment, and the description will be omitted. As in the third embodiment, the piezoelectric substrate 10 may be directly or indirectly bonded onto the support substrate 11 in the first and second embodiments.

[0068] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to such specific embodiments, and various modifications and variations are possible within the scope of the gist of the present invention described in the claims. [Explanation of symbols]

[0069] 10 Piezoelectric substrate 12 Metal Film 14 Electrode fingers 16 comb electrode 20 IDT 22 Reflector 24 Sensitive membrane 25 Elastic wave sensor 26 Insulating Film 27 Elastic Wave Resonator 28 areas

Claims

1. an elastic wave resonator including: a piezoelectric substrate; and a pair of comb electrodes provided on the piezoelectric substrate, each of the comb electrodes having a plurality of electrode fingers, the pair of comb electrodes being arranged such that at least a portion of the electrode fingers of one comb electrode and at least a portion of the electrode fingers of the other comb electrode are arranged in a staggered manner in a direction in which the plurality of electrode fingers are arranged; a sensitive film that is provided on the piezoelectric substrate at least in a region between the plurality of electrode fingers in an arrangement region in which the electrode fingers of the one comb electrode and the electrode fingers of the other comb electrode are arranged alternately, the sensitive film being thinner than the plurality of electrode fingers; An acoustic wave sensor comprising:

2. The acoustic wave sensor according to claim 1 , wherein a part of the sensitive film is provided on the plurality of electrode fingers in the arrangement region.

3. 3. The acoustic wave sensor according to claim 2, wherein the sensitive film provided on the piezoelectric substrate in the arrangement region and the sensitive film provided on the electrode fingers in the arrangement region are not formed continuously.

4. The acoustic wave sensor according to claim 1 , wherein the sensitive film is not provided on the plurality of electrode fingers in the arrangement region.

5. an elastic wave resonator including: a piezoelectric substrate; and a pair of comb electrodes provided on the piezoelectric substrate, each of the comb electrodes having a plurality of electrode fingers, the pair of comb electrodes being arranged such that at least a portion of the electrode fingers of one comb electrode and at least a portion of the electrode fingers of the other comb electrode are arranged in a staggered manner in a direction in which the plurality of electrode fingers are arranged; a sensitive film provided on the piezoelectric substrate in a region between the electrode fingers of the first comb electrode and the electrode fingers of the second comb electrode in an arrangement region in which the electrode fingers of the first comb electrode and the electrode fingers of the second comb electrode are arranged alternately, the sensitive film not being provided on the electrode fingers in the arrangement region; An acoustic wave sensor comprising:

6. The acoustic wave sensor according to claim 5 , wherein the sensitive film is thicker than the plurality of electrode fingers.

7. The acoustic wave sensor according to claim 1 , wherein a resonant frequency of the acoustic wave sensor is higher than a resonant frequency when the sensitive film is not provided.

8. An elastic wave resonator comprising: a support substrate, an insulating layer provided on the support substrate, a piezoelectric substrate provided on the insulating layer, and a pair of comb electrodes provided on the piezoelectric substrate, each of the comb electrodes having a plurality of electrode fingers, the pair of comb electrodes being arranged in a staggered manner in the arrangement direction of the plurality of electrode fingers, with at least a portion of the electrode fingers of one comb electrode and at least a portion of the electrode fingers of the other comb electrode being arranged in a staggered manner; a sensitive film that is provided on the piezoelectric substrate at least in a region between the plurality of electrode fingers in an arrangement region in which the electrode fingers of the one comb electrode and the electrode fingers of the other comb electrode are arranged alternately, the sensitive film being thinner than the plurality of electrode fingers; An acoustic wave sensor comprising:

9. the piezoelectric substrate is a lithium tantalate substrate or a lithium niobate substrate; The acoustic wave sensor according to claim 1 , wherein the sensitive film is made mainly of metal phthalocyanine.

10. an elastic wave resonator is provided on a piezoelectric substrate, the elastic wave resonator including a pair of comb electrodes, each of which has a plurality of electrode fingers, the pair of comb electrodes being arranged such that at least a portion of the electrode fingers of one comb electrode and at least a portion of the electrode fingers of the other comb electrode are staggered in an arrangement direction of the plurality of electrode fingers; A method for manufacturing an elastic wave sensor, comprising: in an arrangement region in which the electrode fingers of one comb electrode and the electrode fingers of the other comb electrode are arranged alternately, forming at least a sensitive film provided on the piezoelectric substrate in a region between the multiple electrode fingers so as to be thinner than the multiple electrode fingers.

11. An elastic wave sensor described in any one of claims 1 to 7, comprising an insulating film provided between the piezoelectric substrate and the sensitive film at least in the area between the plurality of electrode fingers and on the upper surfaces of the plurality of electrode fingers.

Images