Patent Application
20250219611
The present invention relates to an acoustic multilayer film, a high frequency filter device, and a bulk acoustic wave filter device.
Use of high-frequency radio waves such as microwaves, millimeter waves, and terahertz waves enables high-speed broadband communications. According to the 5G mobile communication standard, a frequency band close to 6 GHZ, referred to as “sub-6”, and the 28 GHz-band are used, and future use of the 100 GHz-band is also studied. Thus, resonators and band-pass filters suited to high frequencies exceeding some gigahertz are demanded. For resonators of electronic devices such as smartphones and the like, and high-frequency filters of communication devices, BAW resonators utilizing Bulk Acoustic Waves (BAW) or BAW filters are used.
The BAW filter utilizes the piezoelectric effect of the piezoelectric layer interposed between the upper and lower electrodes to filter high frequencies. When the energy of resonance leaks to the substrate side during filtering, the wave reflected at the interface of the substrate adversely affects the resonance characteristics. In order to inhibit the leakage energy, an acoustic multilayer film in which a low acoustic impedance layer and a high acoustic impedance layer are alternately laminated is used as an acoustic mirror. In order to reduce the effect of the stress caused by the acoustic multilayer film, a method is known in which the acoustic multilayer film is divided in the in-plane direction by dicing and is separated into a plurality of regions (for example, see Patent Document 1). Further, a configuration is known in which an acoustic multilayer film is formed on one side of the substrate and a compressive stress film is provided on the opposite side of the substrate to offset the compressive stress generated by the acoustic multilayer (see, for example, Patent Document 2).
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2021-190794
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2008-22408
In the acoustic multilayer film, a metal thin film is used as a high acoustic impedance layer. In the 2 GHz band, the total thickness of the high acoustic impedance layer is 1 μm or more, and peeling from the substrate, cracking, and the like occur due to the surface roughness and the stress of the metal. In Patent Document 1, in order to relax such stress, the acoustic multilayer film is divided in the in-plane by dicing and is separated into a plurality of regions. There is concern that a residual diced film and re-adhesion of generated film fragments may cause electrode shorts and degradation of characteristics due to foreign matter when piezoelectric (or resonance) devices are formed.
In one aspect, an object of the present invention is to provide an acoustic multilayer film with reduced stress, and a high frequency filter device and a bulk acoustic wave filter device using such an acoustic multilayer film.
According to an aspect of an embodiment, an acoustic multilayer film includes:
In a preferred configuration example, the dividing layer inserted in the first layer has an acoustic impedance lower than the first specific acoustic impedance.
According to the embodiment, a stress-relaxed acoustic multilayer film, and a high frequency filter device using such a stress-relaxed acoustic multilayer film are provided.
According to an embodiment, in order to relax the stress generated in an acoustic multilayer film, a dividing layer is inserted into at least one of the high acoustic impedance layers forming the acoustic multilayer film such that the high acoustic impedance layer is divided in a laminated direction. The dividing layer is preferably amorphous. The dividing layer may be made of a metal oxide, a metal nitride, a metal oxynitride, or the like. From the viewpoint of stress relaxation and maintenance of crystallinity of the acoustic multilayer film, the number of layers of the dividing layer inserted into the high acoustic impedance layer is preferably two or more. By inserting the dividing layers into the high acoustic impedance layer, the effect of the crystalline state of the base is reset, and the stress is relaxed. Since this configuration does not require any fabrication steps, a damage-free, stress-relaxed acoustic multilayer film can be formed. Thus, it is possible to maintain good resonance characteristics of a device to which the acoustic multilayer film is applied.
The main role of the dividing layer 162 is to maintain the surface smoothness of sublayers 161 in a good condition and to improve the crystal orientation of an active element such as a resonator provided in an upper layer. Since the high acoustic impedance material is generally hard and has low shape followability, the high acoustic impedance material does not function as the dividing layer. Therefore, the specific acoustic impedance of the dividing layer 162 is desirably equal to or smaller than the specific acoustic impedance of the high acoustic impedance layer 16.
The sublayers 161 and the dividing layer 162 forming the high acoustic impedance layer 16 can be continuously formed by sputtering or the like. By inserting the dividing layer 162 into the high acoustic impedance layer 16, the crystalline states of the sublayers 161 acting as the base can be reset, and the good crystalline state can be maintained as the entirety of the laminated high acoustic impedance layer 16.
The sublayers 161 of the high acoustic impedance layer 16 are made of a material having a high bulk density or bulk modulus, such as tungsten (W), molybdenum (Mo), tantalum dioxide (Ta2O5), or zinc dioxide (ZnO). The high acoustic impedance layer 16 may have good thermal conductivity. The dividing layer 162 inserted into the high acoustic impedance layer 16 is made of a material having a lower density or a lower bulk modulus relative to that of the high acoustic impedance layer 16. From the viewpoint of resetting or improving the crystalline states of the sublayers 161, the dividing layer 162 is desirably an amorphous layer.
The dividing layer 162 may be made of amorphous SiO2, Al2O3, WO3, MoO3, Si, or the like.
The thickness of the dividing layer 162 is determined by the frequency of the acoustic wave to be reflected by the acoustic multilayer film 18, that is, the resonance frequency of the active element such as the piezoelectric element or the resonator provided on an upper part of the acoustic multilayer film 18. The “upper part” of the acoustic multilayer film 18 refers to a surface of the acoustic multilayer film 18 opposite the support substrate 11. When the wavelength of the acoustic wave 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 acoustic wave, and preferably 1/3000 or more and 1/66 or less. The basis of this range will be described in detail later with reference to
From the viewpoint of maintaining good crystallinity of the active element such as a resonator, the acoustic multilayer film 18 preferably has a surface roughness Ra of 3 nm or less. The surface roughness Ra is a deviation per unit area from the median of the surface roughness of the acoustic multilayer film 18. When the surface roughness Ra exceeds 3 nm, the piezoelectric layer provided on the acoustic multilayer film 18 tends to be less oriented. The disordered orientation of the piezoelectric layer generates an unnecessary vibration mode in the lateral direction, which causes noise. The basis of the surface roughness Ra will be described later with reference to
The low acoustic impedance layer 17 is made of a material having a bulk density or bulk modulus lower than the high acoustic impedance layer 16, such as SiO2, Al2O3, or alumina silicate glass. The low acoustic impedance layer 17 may be an amorphous layer or an oxide film, such as SiO2 or Al2O3 in which an amorphous phase is dominant.
The support substrate 11 is any substrate that can support the acoustic multilayer film 18. A substrate made of quartz, glass, or the like may be used, or a semiconductor substrate made of silicon (Si) or the like, or an inorganic dielectric substrate made of MgO, sapphire, or the like may be used. Alternatively, a plastic substrate may be used. In the case where a flexible plastic substrate is used as the support substrate 11, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), an acrylic resin, a cycloolefin polymer, polyimide (PI), thin film glass, or the like may be used.
The first electrode layer 12 and the second electrode layer 14 are made of conductive materials. For example, as the conductive materials, Mo, W, Pr, Au, Ru, Ir, Al, Cu, or the like may be used. As the material of the piezoelectric layer 13, wurtzite type crystal, perovskite type crystal, or the like can be used. These crystalline materials may be a main component, with a predetermined amount of impurity elements added as sub-components. As wurtzite-type piezoelectric materials, zinc oxide (ZnO), aluminum nitride (AlN), gallium nitride (GaN), and the like can be used.
When the resonance vibration is 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 wave (acoustic wave) propagates through the high acoustic impedance layer 16 is different from the speed at which the vibration wave propagates through the low acoustic impedance layer 17. By designing the film thickness so that the reflected wave is strengthened by interference at the interface of each layer forming the acoustic multilayer film 18, the vibration energy of resonance can be returned to the incident direction of the acoustic wave without being affected by the support substrate 11.
The resonance energy reflected by the acoustic multilayer film 18 and returned to the resonator 15 is confined between the first electrode layer 12 and the second electrode layer 14, which is extracted as an electric signal by the first electrode layer 12 and the second electrode layer 14. Since at least one of the high acoustic impedance layers 16 is divided into the plurality of sublayers 161 by the dividing layer 162, the surface smoothness of the high acoustic impedance layer 16 is improved and the crystal orientation of the piezoelectric layer 13 is improved; thus, the vibration mode in the film thickness direction is maintained and the high frequency filter device 10 with less noise is provided.
Assuming 2 GHz as the simulation condition, two pairs of a high acoustic impedance layer 16 of W having a thickness of 655 nm and a low acoustic impedance layer 17 of SiO2 having a thickness of 725 nm are laminated as the acoustic multilayer film 18 on the silica support substrate 11. Four layers of the dividing layer 162 are inserted into each of the high acoustic impedance layers 16, and the thickness of the dividing layer 162 is changed between 0 nm, 1 nm, 2 nm, 40 nm, 45 nm, and 55 nm. The dividing layer 162 having a thickness of “0 nm” indicates a configuration in which no dividing layer 162 is provided.
When the thickness of the dividing layer 162 is 1 nm and 2 nm, the same filter characteristics as those of the configuration in which no dividing layer 162 is provided are maintained, in conjunction with the surface smoothness of the upper sublayer 161 being improved. Thus, the crystal orientation of the piezoelectric layer 13 disposed over the upper sublayer 161 can be improved. When the thickness of the dividing layer 162 is 40 nm and 45 nm, the attenuation is smaller than 3 dB, and the filter characteristics are not affected substantially. When the thickness of the dividing layer 162 is 55 nm, the attenuation is 3 dB, which is an allowable limit.
The wavelength of the bulk acoustic wave propagating in a medium is defined by (propagation acoustic velocity V [m/s] in the medium)/(resonance frequency F [Hz]). In the case where W is used for the high acoustic impedance layer and SiO2 is used for the low acoustic impedance layer, the wavelength λ of the bulk acoustic wave propagating in the medium of W is approximately 2600 nm, and the wavelength of the bulk acoustic wave propagating in the medium of SiO2 is approximately 2979 nm at the resonance frequency of 2 GHz. When the wavelength λ of the acoustic wave propagating through the dividing layer 162 is defined by the propagation acoustic velocity in the medium of the dividing layer 162 and the resonant frequency, the preferable thickness of the dividing layer 162 is approximately 1/3000 or more and 1/50 or less of the wavelength λ, for example. The thickness range of the dividing layer 162 will be described in more detail with reference to
As samples, as in the simulation conditions of
The surface of the acoustic multilayer film 18 of each of the samples thus produced is observed in a tapping mode of an atomic force microscope (AFM), and the surface roughness Ra (arithmetic mean roughness) is measured. The measurement range is a region of 1.0 μm×1.0 μm. When the thickness of the dividing layer 162 is 1 nm and 2 nm in
In contrast, in the sample without the dividing layer 162 as illustrated in
The crystal orientation is indicated by the FWHM of a peak waveform obtained by measuring the surface of the piezoelectric layer 13 by the XRC method. Specifically, the piezoelectric layer 13 containing ZnO as a main component is formed on the acoustic multilayer film 18 via a metal electrode layer. The crystal orientation is represented by the value of FWHM of the peak waveform of a 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 crystal forming the piezoelectric material. Therefore, the FWHM of the peak waveform of the rocking curve obtained by the XRC method is an index of the c-axis orientation of the piezoelectric layer 13. The smaller the FWHM of the XRC peak waveform, the better the evaluation of the crystal orientation of the piezoelectric layer 13 in the c-axis direction.
The low acoustic impedance layer 17 is fixed to an amorphous SiO2 having a thickness 725 nm, and the specifications of the high acoustic impedance layer 16 are variously changed to produce a total of 10 samples, including samples of Examples 1 to 9 and Comparative Example 1. The low acoustic impedance layer 17 common to all the samples is formed by RF magnetron sputtering. After the samples are produced, the surface roughness Ra is measured by AFM. Each sample is connected to a network analyzer to measure the attenuation characteristics.
In Example 1, a silicon substrate was used as the substrate, the sublayers 161 of the high acoustic impedance layer 16 were made of W, and the dividing layer 162 were made of amorphous SiO2. The W layer was formed by RF magnetron sputtering as illustrated in
In Example 2, the conditions were the same as those of Example 1 except for the thickness of the dividing layer 162. That is, the sublayers 161 of the high acoustic impedance layer 16 were made of W, and a total thickness of the sublayers 161 was 650 nm. The dividing layers 162 were made of amorphous SiO2 having a thickness of 2 nm. The film forming conditions of the W layer and the SiO2 layers were the same as those in Example 1. Since the center frequency of the applied high frequency wave was 2 GHZ, the wavelength of the acoustic wave propagating through the W sublayers 161 was approximately 2600 nm, and the wavelength of the acoustic wave propagating through the SiO2 dividing layer 162 was approximately 2979 nm. The ratio of the thickness of the dividing layer 162 to the wavelength of the acoustic wave was 2/2979. The acoustic multilayer film 18 of the sample of Example 2 had a surface roughness Ra of 1.9 nm, and the attenuation of the filter was −1.0 dB. The surface roughness of the acoustic multilayer film 18 was small, and the attenuation of the filter was also small. The XRC FWHM of the piezoelectric layer 13 formed on the acoustic multilayer film 18 was 3.5°. Furthermore, the stress of the high acoustic impedance layer was relaxed by inserting the dividing layer 162, and the film peeling and cracking of the acoustic multilayer film could be inhibited, and therefore, the item of the film peeling/cracking in
In Example 3, the conditions were the same as those of the Example 1 and Example 2 except for the thickness of the dividing layer 162. That is, the sublayers 161 of the high acoustic impedance layer 16 were made of W and a total thickness of the sublayers 161 was 650 nm. The dividing layers 162 were made of amorphous SiO2 having a thickness of 45 nm. The film forming conditions of the W layer and the SiO2 layer were the same as those in Examples 1 and 2. Since the center frequency of the applied high frequency wave was 2 GHz, the wavelength of the acoustic wave propagating through the W sublayers 161 was approximately 2600 nm, and the wavelength of the acoustic wave propagating through the SiO2 dividing layer 162 was 2979 nm. The ratio of the thickness of the dividing layer 162 to the wavelength of the acoustic wave was 45/2979, that is, 5/331. The acoustic multilayer film 18 of the sample of Example 3 has a surface roughness Ra of 2.0 nm, and the attenuation of the filter was −1.8 dB. The surface roughness of the acoustic multilayer film 18 and the attenuation of the filter were sufficiently within allowable ranges. The XRC FWHM of the piezoelectric layer 13 formed on the acoustic multilayer film 18 was 3.7°, which was within the allowable range. Furthermore, the stress of the high acoustic impedance layer was relaxed by inserting the dividing layer 162, and the film peeling and cracking of the acoustic multilayer film could be inhibited, and therefore, the item of the film peeling/cracking in
In Example 4, the conditions were the same as those of Example 3 except for the material of the dividing layer 162. That is, the sublayers 161 of the high acoustic impedance layer 16 were made of W and a total thickness of the sublayers 161 was 650 nm. The dividing layer 162 was made of amorphous Al2O3 having a thickness of 45 nm. The Al2O3 layer was formed by RF magnetron sputtering with a O2 ratio of 30% as illustrated in
In Example 5, the conditions were the same as those of Example 3 and Example 4 except for the material of the dividing layer 162. That is, the sublayers 161 of the high acoustic impedance layer 16 were made of W and a total thickness of the sublayers 161 was 650 nm. The dividing layer 162 was made of amorphous WO3 having a thickness of 45 nm. The WO3 layers were formed by RF magnetron sputtering with a O2 ratio of 25%, as illustrated in
In Example 6, the conditions were the same as those of Example 3 except for the material of the sublayers 161 of the high acoustic impedance layer 16. That is, the sublayers 161 of the high acoustic impedance layer 16 were made of Mo, and the dividing layer 162 was made of amorphous SiO2 having a width of 45 nm. The Mo sublayers 161 were formed by DC magnetron sputtering as illustrated in
In Example 7, the frequency to be applied was set to 3 GHz. A silicon substrate was used as a substrate, the sublayers 161 of the high acoustic impedance layer 16 were made of W, and the dividing layer 162 was made of an amorphous SiO2 having a thickness 30 nm. The total thickness of the W sublayers 161 was 433 nm. In the case of 3 GHZ, the acoustic wave propagating through the W sublayers 161 had a wavelength of approximately 1733 nm, and the acoustic wave propagating through the amorphous SiO2 dividing layer 162 had a wavelength of 1986 nm. The ratio of the thickness of the dividing layer 162 to the wavelength of the acoustic wave was 30/1986, that is, 5/331. The surface roughness Ra of the acoustic multilayer film 18 of the sample of Example 7 was as small as 1.6 nm. The XRC FWHM of the piezoelectric layer 13 formed on the acoustic multilayer film 18 was as small as 3.1°, and the crystal orientation of the piezoelectric layer 13 was good. The attenuation of the filter was −2.0 dB, which was sufficiently within the allowable range. Furthermore, the stress of the high acoustic impedance layer was relaxed by inserting the dividing layer 162, and the film peeling and cracking of the acoustic multilayer film could be inhibited, and therefore, the item of the film peeling/cracking in
In Example 8, the frequency to be applied was set to 6 GHz. A silicon substrate was used as a substrate, the sublayers 161 of the high acoustic impedance layer 16 were made of W, and the dividing layer 162 was made of an amorphous SiO2 having a thickness 15 nm. The total thickness of the W sublayers 161 was 217 nm. In the case of 6 GHZ, the acoustic wave propagating through the W sublayers 161 had a wavelength of approximately 867 nm, and the acoustic wave propagating through the amorphous SiO2 dividing layer 162 had a wavelength of 993 nm. The ratio of the thickness of the dividing layer 162 to the wavelength of the acoustic wave was 15/993, that is, 5/331. The acoustic multilayer film 18 of the sample of Example 8 had a small roughness Ra of 1.2 nm. The XRC FWHM of the piezoelectric layer 13 formed on the acoustic multilayer film 18 was as small as 2.9°, and the crystal orientation of the piezoelectric layer 13 was good. The attenuation of the filter was −2.7 dB, which was within the allowable range. Furthermore, the stress of the high acoustic impedance layer was relaxed by inserting the dividing layer 162, and the film peeling and cracking of the acoustic multilayer film could be inhibited, and therefore, the item of the film peeling/cracking in
In Example 9, the conditions were the same as those of Example 1 except for the thickness of the dividing layer 162. That is, the sublayers 161 of the high acoustic impedance layer 16 were made of W and a total thickness of the sublayers 161 was 650 nm. The dividing layer 162 was made of amorphous SiO2 having a thickness of 55 nm. The thickness of the dividing layer 162 was the same condition as the characteristics of the simulation of
In Comparative Example 1, no dividing layer 162 was provided in the high acoustic impedance layer 16. Other conditions were the same as those in Example 1. The high acoustic impedance layer 16 was made of W, and the low acoustic impedance layer was made of SiO2. The center frequency of the applied high frequency wave was 2 GHZ, the thickness of the high acoustic impedance layers was set to be 650 nm. When no dividing layer 162 was provided, the attenuation of the filter was −1.0 dB, which is good, but the surface roughness of the acoustic multilayer film 18 was 3.6 nm. In this case, the stress generated in the acoustic multilayer film 18 increased, and the possibility of peeling or cracking was high. Further, the XRC FWHM of the piezoelectric layer 13 of the active element 15 was 6.1°, which exceeded the allowable range, and the crystal orientation was poor, so that good resonance characteristics were not expected. Further, since the film stress of the high acoustic impedance layer was very large, and the film peeling and cracking of the acoustic multilayer film was not inhibited, the item of film peeling/cracking in
Results of Examples 1 to 9 and Comparative Example 1 indicate that the surface roughness of the acoustic multilayer film 18 can be reduced and the surface smoothness can be maintained by inserting the dividing layer 162 having a thickness of 1/3000 or more and 1/55 or less, more preferably 1/3000 or more and 1/66 or less, of the wavelengths of the acoustic wave corresponding to the operating frequencies into at least one high acoustic impedance layers 16. As illustrated in
Although the present invention has been described based on the specific embodiments, the present invention is not limited to the above-described configuration examples. For example, the number of pairs of the high acoustic impedance layer 16 and the low acoustic impedance layer 17 that are repeatedly laminated is not limited to two, and a greater number of pairs of the high acoustic impedance layer 16 and the low acoustic impedance layer 17 may be laminated. In this case, the dividing layers 162 are inserted into one or more high acoustic impedance layers 16. The high acoustic impedance layers may be made of Zno, Ta2O5, Ru, and Ir, or a composite of these materials, in addition to W and Mo. In this case, the dividing layers are inserted into at least one high acoustic impedance layer. This can inhibit peeling and cracking due to stress, and can excellently maintain high frequency filter characteristics while efficiently reflecting resonance energy generated in an active element when connecting the active element such as a resonator and a piezoelectric element on an acoustic multilayer film. An active element 15 may be provided on a side of the acoustic multilayer film having the above-described configuration opposite the support substrate 11, and the active element 15 may include 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, thereby forming an SMR (Solid Mounted Resonator) type bulk acoustic wave filter device. In this case, the stress of the acoustic multilayer film is also relaxed, and the reliability of the device operation is improved.
The present application claims priority to Japanese Patent Application No. 2022-058814 filed on Mar. 31, 2022. The entire contents of Japanese Patent Application No. 2022-058814 are incorporated herein by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-058814 | Mar 2022 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2023/012784 | 3/29/2023 | WO |
