Acoustic multilayer film, high frequency filter device, and bulk acoustic wave filter device

Abstract

Provided are a stress-relaxed acoustic multilayer film and a high-frequency filter device using the same. The acoustic multilayer film has two or more pairs of a first layer having a first specific acoustic impedance and a second layer having a lower specific acoustic impedance than the first specific acoustic impedance which are alternately stacked on a supporting substrate, and at least one of the first layers of the acoustic multilayer film is divided in the stacking direction by a dividing layer.

Description

Acoustic multilayer films, high frequency filter devices, and bulk acoustic wave filter devices

[0001] The present invention relates to an acoustic multilayer film, a high frequency filter device, and a bulk acoustic wave filter device.

[0002] High-speed, high-capacity communications are possible using high-frequency radio waves such as microwaves, millimeter waves, and terahertz waves. The 5G mobile communications standard uses a frequency band close to 6 GHz known as "sub-6" and the 28 GHz band, with the use of the 100 GHz band also being considered for future use. Therefore, resonators and bandpass filters suitable for high frequencies exceeding several GHz are required. Bulk Acoustic Wave (BAW) resonators or BAW filters utilizing BAW are used in resonators for electronic devices such as smartphones and high-frequency filters for communication devices.

[0003] BAW filters filter high frequencies by utilizing the piezoelectric effect of a piezoelectric layer sandwiched between an upper electrode and a lower electrode. If resonance energy leaks toward the substrate during filtering, waves reflected at the substrate interface adversely affect the resonance characteristics. To suppress this leakage energy, an acoustic multilayer film, consisting of alternating low acoustic impedance layers and high acoustic impedance layers, is used as an acoustic mirror. To reduce the effects of stress caused by the acoustic multilayer film, a method is known in which the acoustic multilayer film is divided into multiple regions in the in-plane direction by dicing (see, for example, Patent Document 1). Another known configuration involves forming an acoustic multilayer film on one side of a substrate and providing a compressive stress film on the opposite side of the substrate to offset the compressive stress generated in the acoustic multilayer film (see, for example, Patent Document 2).

[0004] JP 2021-190794 A JP 2008-22408 A

[0005] In acoustic multilayer films, metal thin films are used as high acoustic impedance layers. In the 2 GHz band, the total thickness of the high acoustic impedance layers is 1 μm or more, and peeling from the substrate, cracks, and other problems occur due to the surface roughness and stress of the metal. In Patent Document 1, the acoustic multilayer film is separated into multiple regions within the plane by dicing to relieve stress. When a piezoelectric (or resonant) element is formed, there is a concern that the remaining diced film and the resulting re-adhesion of film fragments may cause electrode shorts or degradation of characteristics due to foreign matter.

[0006] In one aspect, the present invention aims to provide a stress-relieved acoustic multilayer film, and a high-frequency filter device and a bulk acoustic wave filter device using the same.

[0007] In one embodiment, the acoustic multilayer film is formed by alternately stacking two or more pairs of first layers having a first specific acoustic impedance and second layers having a specific acoustic impedance lower than the first specific acoustic impedance on a support substrate, and at least one of the first layers of the acoustic multilayer film is divided in the stacking direction by a dividing layer.

[0008] In a preferred configuration example, the division layer included in the first layer has an acoustic impedance lower than the first specific acoustic impedance.

[0009] A stress-relieved acoustic multilayer film and a high-frequency filter device using the same are realized.

[0010] FIG. 2 is a schematic diagram of a laminate including an acoustic multilayer film of an embodiment. FIG. 3 is a schematic diagram of a high-frequency filter device using the acoustic multilayer film of FIG. 1. FIG. 4 is a diagram showing the relationship between the film thickness of a dividing layer and filter characteristics. FIG. 5 is a diagram showing the surface roughness of an acoustic multilayer film when there is no dividing layer. FIG. 6 is a diagram showing the surface roughness of an acoustic multilayer film when the dividing layer is 1 nm thick. FIG. 7 is a diagram showing the surface roughness of an acoustic multilayer film when the dividing layer is 2 nm thick. FIG. 8 is a diagram showing the surface roughness of an acoustic multilayer film when the dividing layer is 29 nm thick. FIG. 9 is a diagram showing the specifications and filter characteristics of acoustic multilayer films of examples and comparative examples. FIG. 10 is a diagram showing the film formation conditions for an acoustic multilayer film.

[0011] In an embodiment, in order to relieve stress generated in the acoustic multilayer, a dividing layer is inserted into at least one of the high acoustic impedance layers constituting the acoustic multilayer, dividing the high acoustic impedance layer in the stacking direction. The dividing layer is preferably amorphous. The dividing layer may be formed of a metal oxide, a metal nitride, a metal oxynitride, or the like. From the viewpoint of stress relief and maintaining the crystallinity of the acoustic multilayer, it is preferable that the number of dividing layers inserted into the high acoustic impedance layer be two or more. By inserting a dividing layer into the high acoustic impedance layer, the influence of the crystalline state of the base is reset and stress is relieved. This configuration does not require a processing step, so a damage-free acoustic multilayer with reduced stress can be produced. This allows the resonance characteristics of a device using the acoustic multilayer to be maintained at a good level.

[0012] FIG. 1 is a schematic diagram of a laminate 20 including an acoustic multilayer film 18 according to an embodiment. The laminate 20 includes a support substrate 11 and an acoustic multilayer film 18 disposed on the support substrate 11. The acoustic multilayer film 18 is formed by alternately stacking two or more pairs of first layers 16 having a predetermined specific acoustic impedance and second layers 17 having a lower specific acoustic impedance than the first layers. Because the first layers 16 have a higher specific acoustic impedance than the second layers 17, for convenience, the first layers are referred to as "high acoustic impedance layers 16" and the second layers are referred to as "low acoustic impedance layers 17." In this embodiment, a dividing layer 162 is inserted into at least one of the high acoustic impedance layers 16, dividing the high acoustic impedance layer 16 into multiple sublayers 161 in the stacking direction.

[0013] The main role of the dividing layer 162 is to maintain good surface smoothness of the sublayer 161 and to improve the crystal orientation of active elements such as resonators provided in the upper layer. High acoustic impedance materials are generally hard and have poor conformability, so they do not function as dividing layers. Therefore, it is desirable that the specific acoustic impedance of the dividing layer 162 be equal to or smaller than the specific acoustic impedance of the high acoustic impedance layer 16.

[0014] The sublayer 161 and the dividing layer 162 that constitute the high acoustic impedance layer 16 can be formed continuously by sputtering, etc. By inserting the dividing layer 162 into the high acoustic impedance layer 16, the crystalline state of the underlying sublayer 161 can be reset, and the laminated high acoustic impedance layer 16 as a whole can maintain a good crystalline state.

[0015] The sublayer 161 of the high acoustic impedance layer 16 is formed of a material with a high density or bulk modulus, such as tungsten (W), molybdenum (Mo), tantalum oxide (Ta2O5), or zinc oxide (ZnO). The high acoustic impedance layer 16 may have good thermal conductivity. The split layer 162 inserted into the high acoustic impedance layer 16 is formed of a material with a lower density or bulk modulus than the high acoustic impedance layer 16. From the viewpoint of resetting or improving the crystalline state of the sublayer 161, it is desirable that the split layer 162 be an amorphous layer. The split layer 162 may be formed of amorphous SiO ２ , Al ２ O ３ , W.O. ３ , MоO ３ , Si, etc. can be used.

[0016] The thickness of the dividing layer 162 is determined by the frequency of the elastic waves to be reflected by the acoustic multilayer film 18, i.e., the resonance frequency of the active elements such as piezoelectric elements and resonators provided on the upper part of the acoustic multilayer film 18. The "upper part" of the acoustic multilayer film 18 refers to the surface of the acoustic multilayer film 18 opposite to the support substrate 11. If the wavelength of the elastic waves propagating through the dividing layer 16 is λ, the thickness of the dividing layer 162 is 1 / 3000 or more and 1 / 55 or less of the wavelength λ of the elastic waves, preferably 1 / 3000 or more and 1 / 66 or less. The reason for this range will be described in detail later with reference to FIG. 5.

[0017] To maintain good crystallinity of active elements such as resonators, the surface roughness Ra of the acoustic multilayer 18 is preferably 3 nm or less. Here, the surface roughness Ra is the deviation per unit area from the median of the surface irregularities of the acoustic multilayer 18. If the surface roughness Ra exceeds 3 nm, the orientation of the piezoelectric layer provided on the acoustic multilayer 18 tends to decrease. Disturbance in the orientation of the piezoelectric layer generates unwanted lateral vibration modes, causing noise. The basis for this surface roughness Ra will be described later with reference to Figures 4A to 4D.

[0018] The low acoustic impedance layer 17 is made of a material having a lower density or bulk modulus than the high acoustic impedance layer 16, such as SiO ２ , Al ２ O ３ The low acoustic impedance layer 17 is made of an amorphous layer or a SiO 2 layer in which the amorphous phase is predominant. ２ , Al ２ O ３ The oxide film may be the same as above.

[0019] The support substrate 11 is any substrate capable of supporting the acoustic multilayer film 18. It may be a substrate such as quartz or glass, a semiconductor substrate such as silicon (Si), or an inorganic dielectric substrate such as MgO or sapphire. Alternatively, a plastic substrate may be used. When a flexible plastic substrate is used as the support substrate 11, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), acrylic resin, cycloolefin polymer, polyimide (PI), thin film glass, or the like may be used.

[0020] Fig. 2 is a schematic diagram of a high-frequency filter device 10 using the acoustic multilayer film 18 of Fig. 1. The high-frequency filter device 10 has a first electrode layer 12, a second electrode layer 14, and a piezoelectric layer 13 provided between the first electrode layer 12 and the second electrode layer 14 on the surface of the acoustic multilayer film 18 opposite the support substrate 11. The first electrode layer 12, the second electrode layer 14, and the piezoelectric layer 13 form a resonator 15, which is an active element.

[0021] The first electrode layer 12 and the second electrode layer 14 are formed of a conductive material. For example, Mo, W, Pr, Au, Ru, Ir, Al, Cu, etc. may be used as the conductive material. The piezoelectric layer 13 may be made of a wurtzite crystal, a perovskite crystal, etc. These crystal materials may be used as the main component, with a predetermined amount of impurity elements added as a secondary component. Examples of wurtzite piezoelectric materials include zinc oxide (ZnO), aluminum nitride (AlN), and gallium nitride (GaN).

[0022] When resonant vibrations are transmitted from the resonator 15 to the acoustic multilayer film 18, the vibration energy of the resonance is reflected by the acoustic multilayer film 18. The speed at which the vibration waves (elastic waves) propagate through the high acoustic impedance layer 16 is different from the speed at which they propagate through the low acoustic impedance layer 17. By designing the film thickness so that the reflected waves reinforce each other due to interference at the interfaces of the layers that make up the acoustic multilayer film 18, the vibration energy of the resonance can be returned in the direction of incidence of the elastic waves without being affected by the support substrate 11.

[0023] The resonance energy reflected by the acoustic multilayer film 18 and returned to the resonator 15 is trapped between the first electrode layer 12 and the second electrode layer 14 and is extracted as an electrical signal by the first electrode layer 12 and the second electrode layer 14. At least one of the high acoustic impedance layers 16 is divided into a plurality of sublayers 161 by the dividing layers 162, which improves the surface smoothness of the high acoustic impedance layer 16 and improves the crystal orientation of the piezoelectric layer 13, thereby maintaining the vibration mode in the film thickness direction and realizing a high frequency filter device 10 with less noise.

[0024] 3 shows the relationship between the film thickness of the split layer 162 and the filter characteristics. From the perspective of stress relaxation in the acoustic multilayer film 18, inserting a split layer 162 into the high acoustic impedance layer 16 may change the filter characteristics, so the thickness of the split layer 162 must be determined appropriately. It is desirable that the filter characteristics of the high frequency filter device 10 do not attenuate, but since commercially available high frequency filters generally specify a passband of approximately -3 dB or less due to the influence of wiring resistance, etc., up to around -3 dB is considered to be the acceptable range. The filter characteristics are estimated by simulation by changing the thickness of the split layer 162.

[0025] As a condition for the simulation, a frequency of 2 GHz was assumed, and a 655 nm thick W high acoustic impedance layer 16 and a 725 nm thick SiO ２ Two pairs of low acoustic impedance layers 17 are laminated. Four split layers 162 are inserted into each of the high acoustic impedance layers 16, and the thicknesses of the split layers 162 are varied as follows: 0 nm, 1 nm, 2 nm, 40 nm, 45 nm, and 55 nm. A split layer 162 with a thickness of "0 nm" is a configuration in which no split layer 162 is provided.

[0026] When the thickness of the split layer 162 is 1 nm or 2 nm, the same filter characteristics as in a configuration without the split layer 162 are maintained, and the surface smoothness of the upper sublayer 161 is improved, thereby improving the crystal orientation of the piezoelectric layer 13 provided thereon. When the thickness of the split layer 162 is 40 nm or 45 nm, the attenuation is less than 3 dB, and there is not much impact on the filter characteristics. When the thickness of the split layer 162 is 55 nm, the attenuation becomes 3 dB, which is the allowable limit.

[0027] The wavelength of a bulk acoustic wave propagating through a medium is defined as (sound propagation speed in the medium V [m / s]) / (resonance frequency F [Hz]). When W is used for the high acoustic impedance layer and SiO2 is used for the low acoustic impedance layer, at a resonant frequency of 2 GHz, the wavelength λ of the bulk acoustic wave propagating through the W medium is approximately 2600 nm, and the wavelength of the bulk acoustic wave propagating through the SiO2 medium is approximately 2979 nm. If the wavelength λ of the acoustic wave propagating through the split layer 162 is defined by the sound propagation speed in the split layer 162 medium and the resonant frequency, the thickness of the split layer 162 is preferably 1 / 3000 or more and 1 / 50 or less of the wavelength λ. The thickness range of the split layer 162 will be described in more detail with reference to Figures 4A to 4D based on an actually fabricated sample.

[0028] 4A to 4D show the relationship between the film thickness of the dividing layer 162 and the surface roughness of the acoustic multilayer 18. In FIGS. 4A, 4B, 4C, and 4, the thicknesses of the dividing layer 162 are 0 nm, 1 nm, 2 nm, and 29 nm, respectively. The surface condition of the acoustic multilayer 18 is shown for each thickness of the dividing layer 162. As described above, the dividing layer 162 is inserted to improve the surface smoothness of the upper sublayer 161. Therefore, the thickness range of the dividing layer 162 is considered from the viewpoint of surface smoothness.

[0029] As a sample, the same as the conditions of the simulation of FIG. 3, a W high acoustic impedance layer 16 having a thickness of 655 nm and a SiO ２ Two pairs of low acoustic impedance layers 17 are laminated to form an acoustic multilayer film 18. Four dividing layers 162 are inserted into each of the high acoustic impedance layers 16, and the thicknesses of the dividing layers 162 are varied to 0 nm, 1 nm, 2 nm, and 29 nm. The dividing layer 162 having a thickness of "0 nm" corresponds to the configuration in FIG. 4A where the dividing layer 162 is not provided.

[0030] The surface of the acoustic multilayer 18 of each sample was observed using an atomic force microscope (AFM) in tapping mode to measure the surface roughness Ra (arithmetic mean roughness). The measurement range was 1.0 μm × 1.0 μm. When the thickness of the dividing layer 162 was 1 nm and 2 nm in Figures 4B and 4C, the AFM images showed uniformity of the surface, and the surface roughness Ra of the acoustic multilayer 18 was small, at 2 nm or less. When the thickness of the dividing layer 162 was 29 nm in Figure 4D, the surface roughness Ra slightly exceeded 2 nm, but the surface remained smooth.

[0031] In contrast, in the sample without the dividing layer 162, as shown in FIG. 4A , surface roughness was observed in the AFM image, with a measured Ra of 3.58 nm. When the surface roughness of the acoustic multilayer 18 exceeds 3.0 nm, the orientation of the piezoelectric layer 13 tends to deteriorate. When the surface roughness of the acoustic multilayer 18 exceeds 3.5 nm, the full width at half maximum (FWHM) of ZnO (002) measured by X-ray rocking curve (XRD) increases by 3° or more. Disordered orientation of the piezoelectric layer 13 generates unwanted lateral vibration modes, causing noise. Therefore, it is desirable for the surface roughness of the acoustic multilayer 18 to be 3 nm or less. The relationship between the thickness of the dividing layer 162 and the surface roughness Ra of the acoustic multilayer 18 will be described in more detail with reference to FIG. 5 .

[0032] 5 shows the specifications and filter characteristics of the acoustic multilayer film 18 of the example and comparative example. The specifications of the acoustic multilayer film 18 include the center frequency of resonance (GHz), the wavelength (nm) of the elastic wave propagating in the sublayer 161 of the high acoustic impedance layer 16, the material of the sublayer 161, the total thickness of one high acoustic impedance layer 16, the thickness of the dividing layer 162, the wavelength (nm) of the elastic wave propagating in the dividing layer 162, the ratio of the thickness of the dividing layer to the wavelength of the elastic wave propagating in the dividing layer 162 (referred to as "wavelength ratio" in the figure for convenience), and the surface roughness Ra of the acoustic multilayer film 18. The filter characteristics are expressed in terms of attenuation (dB) and crystal orientation (°). The degree of film peeling or cracking is observed as a factor affecting the filter characteristics.

[0033] The crystal orientation is indicated by the FWHM of the peak waveform when the surface of the piezoelectric layer 13 is measured by the XRC method. Specifically, the piezoelectric layer 13, which is primarily composed of ZnO, is formed on the acoustic multilayer film 18 via a metal electrode layer. The crystal orientation is represented by the FWHM value of the peak waveform of the rocking curve obtained when the fluctuation of the plane orientation from the (002) plane of the ZnO crystal is measured by the XRC method. The ZnO contained in the piezoelectric layer 13 has a wurtzite crystal structure, and the FWHM value indicates the degree of orientation in the c-axis direction of the crystals constituting the piezoelectric material. Therefore, the FWHM of the peak waveform of the rocking curve obtained by the XRC method is an indicator of the c-axis orientation of the piezoelectric layer 13. The smaller the FWHM of the XRC peak waveform, the better the crystal orientation in the c-axis direction of the piezoelectric layer 13 is evaluated to be.

[0034] The low acoustic impedance layer 17 is made of amorphous SiO ２ The thickness of the high acoustic impedance layer 16 is fixed at 1000 nm, and the specifications of the high acoustic impedance layer 16 are variously changed to produce a total of 10 samples, including Examples 1 to 9 and Comparative Example 1. The low acoustic impedance layer 17, which is common to all samples, is formed by RF magnetron sputtering. After each sample is produced, the surface roughness Ra is measured using an AFM. Furthermore, each sample is connected to a network analyzer to measure the attenuation characteristics.

[0035] In Example 1, a silicon substrate was used as the substrate, the sublayer 161 of the high acoustic impedance layer 16 was made of W, and the dividing layer 162 was made of amorphous SiO ２ The W layer is formed by RF magnetron sputtering as shown in FIG. 6. Under these film forming conditions, a polycrystalline W layer is formed. The total film thickness of the sublayer 161 is 650 nm. The SiO ２ The layer was deposited by RF magnetron sputtering. ２ The ratio is 15%. Under these conditions, SiO ２ The thickness of the split layer 162 is 1 nm, and the total thickness of the high acoustic impedance layer 16 is 650 nm. When the center frequency of the applied high frequency is 2 GHz, the wavelength of the acoustic wave propagating through the W sublayer 161 is about 2600 nm, and the wavelength of the SiO ２The wavelength of the acoustic wave propagating through the split layer 162 is 2979 nm. The ratio of the thickness of the split layer 162 to the wavelength of the acoustic wave is 1 / 2979. The surface roughness Ra of the acoustic multilayer film 18 of the sample in Example 1 is 1.9 nm, and the filter attenuation is -1.0 dB. The surface roughness of the acoustic multilayer film 18 is small, and the filter attenuation is also low. The XRC FWHM of the piezoelectric layer 13 formed on the acoustic multilayer film 18 is 3.5°. It is estimated that even if the thickness of the split layer 162 is less than 1 nm, for example, a thickness of a few molecular layers, the surface roughness can be suppressed and high filter characteristics can be obtained. Furthermore, inserting the split layer 162 relieves stress in the high acoustic impedance layer 16, thereby suppressing peeling and cracking of the acoustic multilayer film 18. The peeling and cracking items in Figure 5 are represented by "O." The "O" symbol indicates that peeling and cracking are suppressed.

[0036] Example 2 Example 2 is set under the same conditions as Example 1 except for the thickness of the division layer 162. That is, the sublayer 161 of the high acoustic impedance layer 16 is formed of W with a total thickness of 650 nm, and the sublayer 161 is formed of amorphous SiO ２ The thickness of the split layer 162 is set to 2 nm. ２ The deposition conditions for the layer were the same as in Example 1. Since the center frequency of the applied high frequency was 2 GHz, the wavelength of the acoustic wave propagating through the W sublayer 161 was about 2600 nm, and the wavelength of the SiO ２ The wavelength of the elastic wave propagating through the split layer 162 is 2979 nm. The ratio of the thickness of the split layer 162 to the wavelength of the elastic wave is 2 / 2979. The surface roughness Ra of the acoustic multilayer film 18 of the sample of Example 2 is 1.9 nm, and the filter attenuation is -1.0 dB. The surface roughness of the acoustic multilayer film 18 is small, and the filter attenuation is also small. The XRC FWHM of the piezoelectric layer 13 formed on the acoustic multilayer film 18 is 3.5°. Furthermore, since the insertion of the split layer 162 relieves stress in the high acoustic impedance layer and can suppress peeling and cracking of the acoustic multilayer film, the peeling and cracking item in Figure 5 is evaluated as "Good".

[0037] Example 3 Example 3 is set under the same conditions as Examples 1 and 2, except for the thickness of the dividing layer 162. That is, the sublayer 161 of the high acoustic impedance layer 16 is formed of W with a total thickness of 650 nm, and the sublayer 161 is formed of amorphous SiO ２ The thickness of the split layer 162 is set to 45 nm. ２ The film formation conditions for the layers were the same as in Examples 1 and 2. Since the center frequency of the applied high frequency was 2 GHz, the wavelength of the acoustic wave propagating through the W sublayer 161 was about 2600 nm, and the wavelength of the SiO ２ The wavelength of the elastic wave propagating through the split layer 162 is 2979 nm. The ratio of the thickness of the split layer 162 to the wavelength of the elastic wave is 45 / 2979, or 5 / 331. The surface roughness Ra of the acoustic multilayer film 18 of the sample in Example 3 is 2.0 nm, and the attenuation of the filter is -1.8 dB. The surface roughness of the acoustic multilayer film 18 and the attenuation of the filter are well within the allowable range. The XRC FWHM of the piezoelectric layer 13 formed on the acoustic multilayer film 18 is 3.7°, which is within the allowable range. Furthermore, since the insertion of the split layer 162 alleviates the stress in the high acoustic impedance layer and prevents peeling and cracking of the acoustic multilayer film, the peeling and cracking item in Figure 5 is evaluated as "Good."

[0038] Example 4 In Example 4, the conditions are the same as those in Example 3, except for the material of the dividing layer 162. That is, the sublayer 161 of the high acoustic impedance layer 16 is formed of W with a total thickness of 650 nm, and the dividing layer 162 is formed of amorphous Al with a thickness of 45 nm. ２ O ３ It is formed by Al ２ O ３ The layer was deposited by RF magnetron sputtering, as shown in FIG. ２ The ratio is 30%. ２ O ３ The center frequency of the applied radio frequency is 2 GHz, the wavelength of the acoustic wave propagating through the W sublayer 161 is about 2600 nm, and the Al ２ O ３The wavelength of the elastic wave propagating through the split layer 162 in Example 4 is 2979 nm. The ratio of the thickness of the split layer 162 to the wavelength of the elastic wave is 45 / 2979, or 5 / 331. The surface roughness Ra of the acoustic multilayer film 18 of the sample in Example 4 is 2.0 nm, the same as in Example 3, but the filter attenuation is -1.0 dB, resulting in better filter characteristics than in Example 3. The XRC FWHM of the piezoelectric layer 13 formed on the acoustic multilayer film 18 is 3.7°, which is within the allowable range. Furthermore, since the insertion of the split layer 162 alleviates stress in the high acoustic impedance layer and suppresses peeling and cracking of the acoustic multilayer film, the peeling and cracking item in Figure 5 is evaluated as "Good."

[0039] Example 5 In Example 5, the conditions are the same as those in Examples 3 and 4, except for the material of the dividing layer 162. That is, the sublayer 161 of the high acoustic impedance layer 16 is formed of W with a total thickness of 650 nm, and the dividing layer 162 is formed of amorphous WO with a thickness of 45 nm. ３ Formed by WO ３ The layer was deposited by RF magnetron sputtering, as shown in FIG. ２ The ratio is 25%. ３ The center frequency of the applied radio frequency is 2 GHz, the wavelength of the acoustic wave propagating through the W sublayer 161 is about 2600 nm, and the Al ２ O ３ The wavelength of the elastic wave propagating through the split layer 162 is 2979 nm. The ratio of the thickness of the split layer 162 to the wavelength of the elastic wave is 45 / 2979, or 5 / 331. The surface roughness Ra of the acoustic multilayer film 18 of the sample in Example 5 is 2.0 nm, the same as in Examples 3 and 4. The filter attenuation is -1.1 dB, and filter characteristics similar to those in Example 4 are obtained. The XRC FWHM of the piezoelectric layer 13 formed on the acoustic multilayer film 18 is 3.7°, which is within the allowable range. Furthermore, since the insertion of the split layer 162 alleviates stress in the high acoustic impedance layer and prevents peeling and cracking of the acoustic multilayer film, the peeling and cracking item in Figure 5 is evaluated as "Good."

[0040] Example 6 In Example 6, the conditions are the same as those in Example 3, except for the material of the sublayer 161 of the high acoustic impedance layer 16. That is, the sublayer 161 of the high acoustic impedance layer 16 is formed of Mo, and the dividing layer 162 is formed of amorphous SiO 2 with a thickness of 45 nm. ２ The Mo layer of the sublayer 161 is formed by DC magnetron sputtering as shown in FIG. 6. Under these conditions, a polycrystalline Mo layer is formed. The total film thickness of the sublayer 161 is 781 nm. The center frequency of the applied high frequency is 2 GHz, and the wavelength of the elastic wave propagating through the Mo sublayer 161 is about 3125 nm. ２ The wavelength of the elastic wave propagating through the split layer 162 in this example is 2979 nm. The ratio of the thickness of the split layer 162 to the wavelength of the elastic wave is 45 / 2979, or 5 / 331. The surface roughness Ra of the acoustic multilayer film 18 of the sample in Example 6 is 2.1 nm, which is close to that of Examples 3 to 5. The attenuation of the filter is -1.7 dB, which is well within the acceptable range. The XRC FWHM of the piezoelectric layer 13 formed on the acoustic multilayer film 18 is 3.7°, which is also within the acceptable range. Furthermore, since the insertion of the split layer 162 alleviates stress in the high acoustic impedance layer and suppresses peeling and cracking of the acoustic multilayer film, the peeling and cracking item in Figure 5 is evaluated as "Good."

[0041] In Example 7, the applied frequency is 3 GHz. A silicon substrate is used as the substrate, the sublayer 161 of the high acoustic impedance layer 16 is made of W, and the dividing layer 162 is made of amorphous SiO 2 with a thickness of 30 nm. ２ The total thickness of the W sublayer 161 is 433 nm. At 3 GHz, the wavelength of the acoustic wave propagating through the W sublayer 161 is about 1733 nm, and the thickness of the amorphous SiO ２The wavelength of the elastic wave propagating through the split layer 162 is 1986 nm. The ratio of the thickness of the split layer 162 to the wavelength of the elastic wave is 30 / 1986, or 5 / 331. The surface roughness Ra of the acoustic multilayer film 18 of the sample in Example 7 is small at 1.6 nm. The XRC FWHM of the piezoelectric layer 13 formed on the acoustic multilayer film 18 is small at 3.1°, indicating good crystal orientation of the piezoelectric layer 13. The attenuation of the filter is -2.0 dB, which is well within the acceptable range. Furthermore, since the insertion of the split layer 162 alleviates stress in the high acoustic impedance layer and suppresses peeling and cracking of the acoustic multilayer film, the peeling and cracking item in Figure 5 is evaluated as "Good."

[0042] Example 8 In Example 8, the applied frequency is 6 GHz. A silicon substrate is used as the substrate, the sublayer 161 of the high acoustic impedance layer 16 is formed of W, and the dividing layer 162 is formed of amorphous SiO 2 with a thickness of 15 nm. ２ The total thickness of the W sublayer 161 is 217 nm. At 6 GHz, the wavelength of the acoustic wave propagating through the W sublayer 161 is about 867 nm, and the thickness of the amorphous SiO ２ The wavelength of the elastic wave propagating through the split layer 162 is 993 nm. The ratio of the thickness of the split layer 162 to the wavelength of the elastic wave is 15 / 993, or 5 / 331. The surface roughness Ra of the acoustic multilayer film 18 of the sample in Example 8 is small at 1.2 nm. The XRC FWHM of the piezoelectric layer 13 formed on the acoustic multilayer film 18 is small at 2.9°, indicating good crystal orientation of the piezoelectric layer 13. The attenuation of the filter is -2.7 dB, which is within the allowable range. Furthermore, since the insertion of the split layer 162 alleviates stress in the high acoustic impedance layer and suppresses peeling and cracking of the acoustic multilayer film, the peeling and cracking item in Figure 5 is evaluated as "Good."

[0043] Example 9 In Example 9, the conditions are the same as those in Example 1, except for the thickness of the dividing layer 162. That is, the sublayer 161 of the high acoustic impedance layer 16 is formed of W with a total thickness of 650 nm, and the sublayer 161 is formed of amorphous SiO ２The thickness of the split layer 162 is set to 55 nm. The thickness of this split layer 162 is the same as that of the characteristic f in the simulation of FIG. 3. The center frequency of the applied high frequency is 2 GHz, the same as in the simulation of FIG. 3, the wavelength of the acoustic wave propagating through the W sublayer 161 is about 2600 nm, and the thickness of the SiO ２ The wavelength of the elastic wave propagating through the split layer 162 is 2979 nm. The ratio of the thickness of the split layer 162 to the wavelength of the elastic wave is 55 / 2979, or 1 / 55. The surface roughness Ra of the sample of Example 9 is 2.0 nm, which is good, and the attenuation of the filter is -3.3 dB, which is within the allowable limit. The XRC FWHM of the piezoelectric layer 13 formed on the acoustic multilayer film 18 is 3.7°, which is close to the allowable limit. When the frequency is 2 GHz, SiO ２ It is estimated that increasing the thickness of the split layer beyond 55 nm would result in a deterioration in the filter characteristics. Furthermore, by inserting the split layer 162, the stress in the high acoustic impedance layer is alleviated, and peeling and cracking of the acoustic multilayer film can be suppressed. Therefore, the peeling and cracking item in Figure 5 is evaluated as "Good."

[0044] Comparative Example 1 In Comparative Example 1, the high acoustic impedance layer 16 does not have a split layer 162. All other conditions are the same as in Example 1. The high acoustic impedance layer 16 is formed of W, and the low acoustic impedance layer is formed of SiO2. The center frequency of the applied high frequency is 2 GHz, and the thickness of the high acoustic impedance layer is set to 650 nm. When the split layer 162 is not provided, the filter attenuation is good at -1.0 dB, but the surface roughness of the acoustic multilayer film 18 is 3.6 nm. In this case, stress generated in the acoustic multilayer film 18 is high, and peeling and cracking are likely to occur. Furthermore, the XRC FWHM of the piezoelectric layer 13 of the active element 15 is 6.1°, which exceeds the allowable range, resulting in poor crystal orientation and poor resonance characteristics. Furthermore, the film stress of the high acoustic impedance layer is very high, making it impossible to suppress peeling and cracking of the acoustic multilayer film. Therefore, the peeling and cracking items in Figure 5 are evaluated as "△" or "X." The mark "△" indicates that the prevention of film peeling and cracking is insufficient, and the mark "×" indicates that the prevention of film peeling and cracking is not sufficient.

[0045] Examples 1 to 9 and Comparative Example 1 demonstrate that inserting a dividing layer 162 having a thickness of 1 / 3000 to 1 / 55, and more preferably 1 / 3000 to 1 / 66, of the wavelength of the elastic wave corresponding to the operating frequency into at least one high acoustic impedance layer 16 reduces the surface roughness of the acoustic multilayer film 18 and maintains its surface smoothness. As shown in Figure 5, when the dividing layer 162 inserted into the high acoustic impedance layer 16 has a thickness of 1 nm to 55 nm, the thickness of the dividing layer 162 is 1 / 725 to 1 / 13 of the thickness of the low acoustic impedance layer 17 (a 725 nm thick SiO2 layer). This thickness range for the dividing layer 162 allows the filter characteristics to be maintained within an acceptable range.

[0046] Although the present invention has been described above based on specific embodiments, the present invention is not limited to the above-described configuration examples. For example, the number of pairs of repeatedly stacked high acoustic impedance layers 16 and low acoustic impedance layers 17 is not limited to two, and more pairs may be stacked. In this case, a dividing layer 162 is inserted into one or more high acoustic impedance layers 16. In addition to W and Mo, the high acoustic impedance layers may be formed of ZnO, Ta2O5, Ru, Ir, or composites thereof. In this case, a dividing layer is also inserted into at least one high acoustic impedance layer. When an active element such as a resonator or piezoelectric element is connected to the acoustic multilayer film, the resonant energy generated by the active element is efficiently reflected, while stress-induced peeling and cracking are suppressed, thereby maintaining excellent high-frequency filter characteristics. An SMR (Solid Mounted Resonator) type bulk acoustic wave filter device may be configured by providing an active element 15 on the side of the acoustic multilayer film configured above opposite the support substrate 11, and the active element 15 including a first electrode layer 12 provided on the acoustic multilayer film 18, a piezoelectric layer 13 provided on the first electrode layer 12, and a second electrode layer 14 provided on the piezoelectric layer. In this case as well, stress in the acoustic multilayer film is alleviated, improving the reliability of device operation.

[0047] This application claims priority from Patent Application No. 2022-058814 filed with the Japan Patent Office on March 31, 2022, the entire contents of which are incorporated by reference.

[0048] REFERENCE SIGNS LIST 10 High frequency filter device 11 Support substrate 12 First electrode layer 13 Piezoelectric layer 14 Second electrode layer 15 Resonator (active element) 16 High acoustic impedance layer (first layer) 17 Low acoustic impedance layer (second layer) 18, 28 Acoustic multilayer film 20 Laminate 161 Sublayer 162 Dividing layer

Claims

1. An acoustic multilayer film in which two or more pairs of first layers having a first specific acoustic impedance and second layers having a specific acoustic impedance lower than the first specific acoustic impedance are alternately stacked on a support substrate, 10. An acoustic multilayer film, wherein at least one of the first layers of the acoustic multilayer film is divided in the stacking direction by a dividing layer.

2. a dividing layer included in the first layer has an acoustic impedance lower than the first specific acoustic impedance; 2. The acoustic multilayer film according to claim 1.

3. Two or more of the division layers are inserted into one of the first layers.

3. The acoustic multilayer film according to claim 1.

4. the thickness of the dividing layer is 1 / 725 to 1 / 13 of the thickness of the second layer; 3. The acoustic multilayer film according to claim 1.

5. The film thickness of the division layer is 1 / 3000 or more and 1 / 55 or less of the wavelength of the resonance frequency.

3. The acoustic multilayer film according to claim 1.

6. The film thickness of the division layer is 1 / 3000 or more and 1 / 66 or less of the wavelength of the resonance frequency.

6. The acoustic multilayer film according to claim 5.

7. The surface smoothness of the acoustic multilayer film is 3 nm or less in terms of arithmetic mean roughness.

3. The acoustic multilayer film according to claim 1.

8. 3. The acoustic multilayer film according to claim 1, wherein the dividing layer is an amorphous layer.

9. 9. The acoustic multilayer film according to claim 8, wherein the dividing layer is made of a metal oxide or a metal nitride.

10. The dividing layer is made of SiO ２ , Al ２ O ３ , W.O. ３ , MоO ３ , SiN, or AlN; 10. The acoustic multilayer film according to claim 9.

11. The acoustic multilayer film according to claim 1 or 2; an active element provided on the opposite side of the acoustic multilayer film from the support substrate; the active element has a first electrode layer provided on the acoustic multilayer film, a piezoelectric layer provided on the first electrode layer, and a second electrode layer provided on the piezoelectric layer. High frequency filter device.

12. The acoustic multilayer film according to claim 1 or 2; an active element provided on the opposite side of the acoustic multilayer film from the support substrate; the active element has a first electrode layer provided on the acoustic multilayer film, a piezoelectric layer provided on the first electrode layer, and a second electrode layer provided on the piezoelectric layer. Bulk acoustic wave filter devices.