Thin-film piezoelectric resonators, filters and multiplexers

Abstract

To provide a piezoelectric thin film resonator capable of suppressing spurious, a manufacturing method thereof, a filter and a multiplexer.SOLUTION: A piezoelectric thin film resonator comprises: a substrate 10; a lower electrode 12 provided on the substrate 10; a piezoelectric layer 14 provided on the lower electrode; an upper electrode 16 provided on the piezoelectric layer while at least partially holding the piezoelectric layer between the upper electrode and the lower electrode; a plurality of low impedance layers 31a-31c provided between the substrate and the lower electrode; and a plurality of high impedance layers 32a and 32b having acoustic impedance higher than acoustic impedance of the plurality of low impedance layers and provided alternately with the plurality of low impedance layers. Provided is an acoustic reflection film in which roughness of a surface 34a on a lower electrode side in the first high impedance layer 32a closest to the lower electrode is smaller than roughness of a surface 34b on a first electrode side in at least one second high impedance layer 32b other than the first high impedance layer 32a.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a piezoelectric thin film resonator. 、 filter and Concerning multiplexers. [Background technology]

[0002] BAW (Bulk Acoustic Wave) resonators, such as SMR (Solid Mounted Resonators), are used as filters and duplexers for high-frequency circuits in wireless terminals such as mobile phones. BAW resonators are also known as film bulk acoustic resonators. Film bulk acoustic resonators have a structure in which a lower electrode and an upper electrode are provided with a piezoelectric layer sandwiched between them. The resonance region, where the lower electrode and the upper electrode face each other with at least a portion of the piezoelectric layer sandwiched between them, is a region in which acoustic waves resonate. It is known that in an acoustic reflection film in which a low acoustic impedance layer and a high acoustic impedance layer are stacked under the lower electrode as an acoustic reflection film that reflects acoustic waves, at least a portion of the surface is roughened (see, for example, Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2018 / 154950 [Patent Document 2] Japanese Patent Publication No. 2020-113954 Summary of the Invention [Problem to be solved by the invention]

[0004] In Patent Document 1, spurious emissions are suppressed by roughening the surface of the high-impedance layer facing the substrate. However, the suppression of spurious emissions is insufficient.

[0005] The present invention has been made in view of the above-mentioned problems, and has an object to suppress spurious signals. [Means for solving the problem]

[0006] The present invention provides a piezoelectric element comprising: a substrate; a first electrode provided on the substrate; a piezoelectric layer provided on the first electrode; a second electrode provided on the piezoelectric layer with at least a portion of the piezoelectric layer sandwiched between the first electrode and the second electrode; a plurality of low-impedance layers provided between the substrate and the first electrode; and a plurality of high-impedance layers having an acoustic impedance greater than that of the plurality of low-impedance layers and alternately provided with the plurality of low-impedance layers, wherein the roughness of a surface of a first high-impedance layer closest to the first electrode among the plurality of high-impedance layers on the first electrode side is smaller than the roughness of a surface of at least one second high-impedance layer other than the first high-impedance layer among the plurality of high-impedance layers on the first electrode side. The roughness of the surface of the second high-impedance layer on the substrate side is smaller than the roughness of the surface of the second high-impedance layer on the first electrode side. and an acoustic reflection film.

[0007] In the above configuration, the roughness of the surface of the first high-impedance layer facing the substrate may be smaller than the roughness of the surface of the second high-impedance layer facing the substrate.

[0008] In the above configuration, the second high-impedance layer may be configured to be second closest to the first electrode among the plurality of high-impedance layers.

[0009] In the above configuration, the roughness of the surface of the first high-impedance layer facing the substrate is greater than the roughness of the surface of the first high-impedance layer facing the first electrode, and the roughness of the surface of the second high-impedance layer facing the first electrode is greater than the roughness of the surface of the first high-impedance layer facing the substrate. stomach It can be configured as follows.

[0010] In the above configuration, the arithmetic mean roughness of the surface of the first high-impedance layer on the first electrode side may be less than 10 nm.

[0011] In the above configuration, the arithmetic mean roughness of the surface of the second high-impedance layer on the side of the first electrode may be 10 nm or more.

[0012] In the above configuration, the piezoelectric layer may be a rotated Y-cut lithium niobate substrate or an X-cut lithium tantalate substrate.

[0013] The present invention is a filter including the above-described thin film piezoelectric resonator.

[0014] The present invention is a multiplexer including the above filter. [Effects of the Invention]

[0016] According to the present invention, spurious signals can be suppressed. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a cross-sectional view of a piezoelectric thin film resonator according to a first embodiment. [Figure 2] FIG. 2(a) is an enlarged plan view of the vicinity of the resonance region of the piezoelectric thin film resonator according to the first embodiment, and FIG. 2(b) is a cross-sectional view taken along the line AA of FIG. 2(a). [Figure 3] FIG. 3 is a cross-sectional view of the resonator C in the simulation. [Figure 4] FIG. 4 is a diagram showing the absolute value |Y| of the admittance of the resonator C versus the frequency in the simulation. [Figure 5] 5(a) and 5(b) are cross-sectional views of the resonators A1 and B1, respectively, in the simulation. [Figure 6] 6(a) and 6(b) are diagrams showing the absolute value |Y| of the admittance versus frequency for the resonators A1 and B1, respectively, in the simulation. [Figure 7] 7(a) and 7(b) are cross-sectional views of the resonators A2 and B2, respectively, in the simulation. [Figure 8] 8(a) and 8(b) are diagrams showing the absolute value |Y| of the admittance versus frequency for the resonators A2 and B2, respectively, in the simulation. [Figure 9] 9(a) and 9(b) are cross-sectional views of resonators A3 and B3, respectively, in the simulation. [Figure 10] 10(a) and 10(b) are diagrams showing the absolute value |Y| of the admittance versus frequency for the resonators A3 and B3, respectively, in the simulation. [Figure 11] 11(a) and 11(b) are cross-sectional views of the resonators A4 and B4, respectively, in the simulation. [Figure 12] 12(a) and 12(b) are diagrams showing the absolute value |Y| of the admittance versus frequency for the resonators A4 and B4, respectively, in the simulation. [Figure 13] 13(a) and 13(b) are cross-sectional views of resonators A5 and B5, respectively, in the simulation. [Figure 14] 14(a) and 14(b) are diagrams showing the absolute value |Y| of admittance versus frequency for the resonators A5 and B5, respectively, in the simulation. [Figure 15] 15(a) to 15(d) are cross-sectional views illustrating a method for manufacturing the film bulk acoustic resonator according to the first embodiment. [Figure 16] 16(a) to 16(c) are cross-sectional views illustrating a method for manufacturing the film bulk acoustic resonator according to the first embodiment. [Figure 17] 17(a) to 17(c) are cross-sectional views illustrating a method for manufacturing the film bulk acoustic resonator according to the first embodiment. [Figure 18] 18(a) and 18(b) are cross-sectional views illustrating a method for manufacturing the film bulk acoustic resonator according to the first embodiment. [Figure 19] Fig. 19(a) is a circuit diagram of a filter according to Example 2. Fig. 19(b) is a circuit diagram of a duplexer according to Modification 1 of Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. [Example]

[0019] 1 is a cross-sectional view of a piezoelectric thin film resonator according to Example 1. The normal direction to the surface of the piezoelectric layer 14 as viewed in the thickness direction is defined as the Z direction, the leading direction of the upper electrode 16 in the planar direction of the piezoelectric layer 14 is defined as the +X direction, and the direction perpendicular to the X direction is defined as the Y direction. The piezoelectric thin film resonator on the left is, for example, a series resonator S of a ladder-type filter, and the piezoelectric thin film resonator on the right is a parallel resonator P.

[0020] As shown in FIG. 1, a lower electrode 12 (first electrode) is provided on a substrate 10. A piezoelectric layer 14 is provided on the lower electrode 12. An upper electrode 16 (second electrode) is provided on the piezoelectric layer 14. A resonance region 50 is defined by the region where the lower electrode 12 and the upper electrode 16 face each other, sandwiching at least a portion of the piezoelectric layer 14. In the parallel resonator P, additional films 20a and 20b are provided below the lower electrode 12 and on the upper electrode 16, respectively. By providing the additional films 20a and 20b, the resonance frequency of the parallel resonator P becomes lower than the resonance frequency of the series resonator S. It is sufficient that either the additional films 20a or 20b is provided.

[0021] An additional film 22 is provided on the upper electrode 16 in the peripheral region of the resonance region 50. The additional film 22 is provided to suppress spurious emissions. The additional film 22 need only be provided in at least a portion of the peripheral region of the resonance region 50, and does not necessarily have to be provided at all. When high-frequency power is applied between the lower electrode 12 and the upper electrode 16, an elastic wave is excited in the piezoelectric layer 14 in the resonance region 50. The wavelength of the elastic wave is approximately twice the total thickness of the lower electrode 12, the piezoelectric layer 14, and the upper electrode 16. The interface between the substrate 10 and the acoustic reflection film 30 is approximately flat, and the upper and lower surfaces of the piezoelectric layer 14 are approximately flat.

[0022] An acoustic reflection film 30 is provided in a resonance region 50 between the substrate 10 and the lower electrode 12. In the acoustic reflection film 30, a plurality of low-impedance layers 31 and a plurality of high-impedance layers 32 are alternately stacked. The acoustic impedance of the high-impedance layers 32 is higher than the acoustic impedance of the low-impedance layers 31. In a plan view, the acoustic reflection film 30 overlaps with and is large in the resonance region 50. The number of low-impedance layers 31 and the number of high-impedance layers 32 can be set appropriately. No high-impedance layers 32 are provided in regions other than the resonance region 50 and its vicinity, and only low-impedance layers 31 are provided.

[0023] A metal layer 24 is provided below the lower electrode 12 outside the resonance region 50. A metal layer 26 is provided which penetrates the piezoelectric layer 14 and contacts the metal layer 24. A metal layer 28 is provided which contacts the upper electrode 16 outside the resonance region 50. The metal layers 26 and 28 function as pads for electrically connecting the lower electrode 12 and the upper electrode 16 to the outside, respectively, and / or as wiring for electrically connecting the lower electrode 12 and the upper electrode 16 to other piezoelectric thin film resonators.

[0024] Fig. 2(a) is an enlarged plan view of the vicinity of the resonance region of the piezoelectric thin film resonator according to Example 1, and Fig. 2(b) is a cross-sectional view taken along line AA of Fig. 2(a). As shown in Fig. 2(a) and Fig. 2(b), the low-impedance layers are low-impedance layers 31a, 31b, and 31c from the side of the lower electrode 12, and the high-impedance layers are high-impedance layers 32a and 32b from the side of the lower electrode 12.

[0025] The surface of high-impedance layer 32a facing lower electrode 12 is surface 34a, and the surface of high-impedance layer 32a facing substrate 10 is surface 34b. The surface of high-impedance layer 32b facing lower electrode 12 is surface 34c, and the surface of high-impedance layer 32b facing substrate 10 is surface 34d. Surfaces 34a and 34b in resonance region 50 are flat surfaces, and surfaces 34c and 34d in resonance region 50 are rough surfaces.

[0026] The sum of the thickness of each of the low-impedance layers 31a-31c and the thickness of each of the high-impedance layers 32a and 32b is set to about 1 / 2 the wavelength of the elastic wave. For example, the thickness of each of the low-impedance layers 31a-31c is set to about 1 / 4 the wavelength of the elastic wave, and the thickness of each of the high-impedance layers 32a and 32b is set to about 1 / 4 the wavelength of the elastic wave. This allows the elastic wave excited in the piezoelectric layer 14 to be reflected by the acoustic reflecting film 30.

[0027] The substrate 10 may be, for example, a silicon substrate, sapphire substrate, alumina substrate, spinel substrate, quartz substrate, quartz substrate, glass substrate, ceramic substrate, or GaAs (gallium arsenide) substrate. The piezoelectric layer 14 may be, for example, a single-crystal lithium tantalate layer, a single-crystal lithium niobate layer, or a single-crystal quartz substrate. When the piezoelectric layer 14 is single-crystal lithium tantalate or single-crystal lithium niobate, thickness strain vibration elastic waves are excited in the piezoelectric layer 14. The piezoelectric layer 14 may also be, for example, a polycrystalline aluminum nitride layer, a zinc oxide layer, a lead zirconate titanate (PZT) layer, or a lead titanate (PbTiO) layer. In this case, thickness longitudinal vibration is excited in the piezoelectric layer 14.

[0028] The lower electrode 12 and the upper electrode 16 are, for example, aluminum (Al) films, and are single-layer films or laminated films of, for example, ruthenium (Ru), chromium (Cr), titanium (Ti), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), rhodium (Rh), iridium (Ir), etc. The additional film 22 may be a metal film exemplified for the lower electrode 12 and the upper electrode 16, or an insulating film such as a silicon oxide film, a silicon nitride film, an aluminum oxide film, a tantalum oxide film, or a niobium oxide film.

[0029] The low-impedance layers 31a to 31c are made of a material with low acoustic impedance, such as silicon oxide or silicon nitride. Materials with low acoustic impedance are mainly insulators. The high-impedance layers 32a and 32b are made of a material with high acoustic impedance, such as tungsten, tantalum, molybdenum, or ruthenium. Materials with high acoustic impedance have a high density, such as high-melting-point metals (for example, metals with a melting point higher than that of platinum). The metal layer 24 is a low-resistance layer, such as an aluminum layer, a gold layer, or a copper layer. The metal layers 26 and 28 are low-resistance layers, such as a gold layer, a copper layer, or an aluminum layer. The metal layers 24, 26, and 28 may include an adhesive film, such as a titanium film, a chromium film, or a nickel film, that contacts the lower electrode 12 or the upper electrode 16.

[0030] An example will be described in which an elastic wave excited in the piezoelectric layer 14 causes thickness-shear vibration. The piezoelectric layer 14 is a rotated Y-cut lithium niobate substrate. In this case, the normal direction (Z direction) of the top surface of the piezoelectric layer 14 is a direction within the YZ plane of the crystal orientation. This causes thickness-shear vibration in the planar direction of the piezoelectric layer 14. Furthermore, the X direction is the X-axis direction in the crystal orientation, and the Z direction is a direction rotated 105° from the Z-axis direction to the Y-axis direction around the X-axis direction within the YZ plane of the crystal orientation. This causes the direction of thickness-shear vibration to be the Y direction.

[0031] As another example, the piezoelectric layer 14 is an X-cut lithium tantalate substrate. In this case, the normal direction (Z direction) of the top surface of the piezoelectric layer 14 is the X-axis direction of the crystal orientation. As a result, thickness-shear vibration occurs in the planar direction of the piezoelectric layer 14. Furthermore, the Y direction is rotated 42° from the +Y axis direction of the crystal orientation to the -Z axis direction around the X axis. As a result, the direction of thickness-shear vibration becomes the Y direction.

[0032] For example, when the resonant frequency is set to 3.7 GHz, the piezoelectric layer 14 is a rotated Y-cut lithium niobate substrate with a thickness of 440 nm, and the bottom electrode 12 and the top electrode 16 are each aluminum layers with a thickness of 44 nm. The thicknesses of the bottom electrode 12 and the top electrode 16 are each 1% to 20% of the thickness of the piezoelectric layer 14. The width of the resonance region 50 in the X direction is, for example, 10 μm to 500 μm.

[0033] Although the planar shape of the resonance region 50 has been described as rectangular, the planar shape of the resonance region 50 may be a polygonal shape such as a pentagon, an elliptical shape, or the like.

[0034] [simulation] [Resonator C] The absolute value of the admittance of a piezoelectric thin film resonator was simulated using a two-dimensional finite element method. Figure 3 is a cross-sectional view of resonator C used in the simulation. As shown in Figure 3, in resonator C, low-impedance layers 31a to 31c and high-impedance layers 32a and 32b are alternately stacked on a substrate 10 to form an acoustic reflection film 30. A lower electrode 12 is provided on the acoustic reflection film 30, a piezoelectric layer 14 is provided on the lower electrode 12, and an upper electrode 16 is provided on the piezoelectric layer 14. Surfaces 34a to 34d of the high-impedance layers 32a and 32b are flat.

[0035] The simulation conditions for resonator C are as follows: Substrate 10: silicon substrate Low impedance layers 31a to 31c: tungsten layers with a thickness of 200 nm High-impedance layers 32a and 32b: silicon oxide layers with a thickness of 150 nm Bottom electrode 12: 44 nm thick aluminum layer Piezoelectric layer 14: Single crystal lithium niobate substrate with a thickness of 440 nm Top electrode 16: 44 nm thick aluminum layer Arithmetic mean roughness Ra of surfaces 34a to 34d: 0 nm The piezoelectric layer 14 had a crystal orientation that was rotated 105° from the +Y axis direction to the +Z axis direction around the X axis direction, with the X direction, Y direction, and Z direction being the +X axis direction, +Y axis direction, and +Z axis direction, respectively.

[0036] Figure 4 shows the absolute value of admittance |Y| versus frequency for resonator C in the simulation. As shown in Figure 4, ΔY is the difference between |Y| at the resonance frequency fr and |Y| at the anti-resonance frequency fa. ΔY corresponds to the magnitude of the main resonance response, and a larger value is better. For resonator C, ΔY is approximately 82 dB. The main resonance response is due to thickness-shear vibration. A spurious response Sp is observed between the resonance frequency fr and the anti-resonance frequency fa. There is a large difference between the maximum 61 and minimum 62 in the spurious response Sp. The spurious response Sp is thought to be caused mainly by elastic waves (transverse mode) propagating in the X direction.

[0037] [Resonator A1, B1] 5(a) and 5(b) are cross-sectional views of resonators A1 and B1, respectively, in the simulation. As shown in FIG. 5(a), in resonator A1, surfaces 34a and 34b of high-impedance layer 32a on the lower electrode 12 side are flat. Surfaces 34c and 34d of high-impedance layer 32b on the substrate 10 side are rough surfaces with irregularities. On surface 34c, convex portions 35c protrude toward the lower electrode 12 side in the resonance region 50. On surface 34d, concave portions 36d are recessed toward the lower electrode 12 side in the resonance region 50. Convex portions 35c and concave portions 36d are arranged to correspond to each other. The period of convex portions 35c is constant, and the period of concave portions 36d is also constant.

[0038] As shown in FIG. 5(b), in resonator B1, surfaces 34a and 34b of high-impedance layer 32a on the lower electrode 12 side are rough surfaces with projections and depressions. Surfaces 34c and 34d of high-impedance layer 32b on the substrate 10 side are flat. On surface 34a, a projection 35a protrudes toward the lower electrode 12 side. On surface 34b, a recess 36b is recessed toward the lower electrode 12 side. The projections 35a and the recesses 36b are provided to correspond to each other.

[0039] The simulation conditions for the resonators A1 and B1 are as follows. Resonator A1 Arithmetic mean roughness Ra of surface 34a: 0 nm Arithmetic mean roughness Ra of surface 34b: 0 nm Arithmetic mean roughness Ra of surface 34c: 47.7 nm Arithmetic mean roughness Ra of surface 34d: 47.7 nm Resonator B1 Arithmetic mean roughness Ra of surface 34a: 47.7 nm Arithmetic mean roughness Ra of surface 34b: 47.7 nm Arithmetic mean roughness Ra of surface 34c: 0 nm Arithmetic mean roughness Ra of surface 34d: 0 nm Other simulation conditions are the same as those for resonator C.

[0040] 6(a) and 6(b) are diagrams showing the absolute value |Y| of admittance versus frequency for resonators A1 and B1, respectively, in the simulation. As shown in FIG. 6(a), ΔY for resonator A1 is approximately 86 dB, which is slightly larger than that for resonator C. The difference between the maximum 61 and the minimum 62 in the spurious response Sp is smaller than that for resonator C in FIG. 4. As such, resonator A1 has a main response that is comparable to that of resonator C, but the spurious response Sp is smaller.

[0041] As shown in Figure 6(b), for resonator B1, ΔY is approximately 75 dB, which is smaller than that of resonator C. Also, the minimum at the anti-resonance frequency fa is less steep than that of resonator C. This indicates that the Q value near the anti-resonance frequency fa is lower. A lower Q value results in greater loss. The difference between the maximum 61 and the minimum 62 in the spurious response Sp is similar to that of resonator C in Figure 4, but a large response 64 is observed within the spurious response Sp. Furthermore, a large response 63 is observed at a frequency higher than the anti-resonance frequency fa.

[0042] [Resonator A2, B2] 7(a) and 7(b) are cross-sectional views of resonators A2 and B2, respectively, in the simulation. As shown in FIG. 7(a), in resonator A2, a recess 36c is recessed on the substrate 10 side on surface 34c. On surface 34d, a protrusion 35d protrudes on the substrate 10 side, and the recess 36c and the protrusion 35d are provided to correspond to each other. The other configurations are the same as those of resonator A1.

[0043] As shown in FIG. 7(b), in resonator B2, a recess 36a is recessed on the surface 34a toward the substrate 10. A protrusion 35b protrudes on the surface 34b toward the substrate 10. The recess 36a and the protrusion 35b are provided to correspond to each other. The other configurations are the same as those of resonator B1.

[0044] The simulation conditions for the resonators A2 and B2 are as follows. Resonator A2 Arithmetic mean roughness Ra of surface 34a: 0 nm Arithmetic mean roughness Ra of surface 34b: 0 nm Arithmetic mean roughness Ra of surface 34c: 47.7 nm Arithmetic mean roughness Ra of surface 34d: 47.7 nm Resonator B2 Arithmetic mean roughness Ra of surface 34a: 47.7 nm Arithmetic mean roughness Ra of surface 34b: 47.7 nm Arithmetic mean roughness Ra of surface 34c: 0 nm Arithmetic mean roughness Ra of surface 34d: 0 nm Other simulation conditions are the same as those for resonator C.

[0045] 8(a) and 8(b) are graphs showing the absolute value |Y| of admittance versus frequency for resonators A2 and B2, respectively, in the simulation. As shown in FIG. 8(a), ΔY for resonator A2 is approximately 82 dB, which is almost the same as that for resonator C. The spurious response Sp is similar to that of resonator A1 and smaller than that of resonator C.

[0046] As shown in Fig. 8(b), in resonator B2, ΔY is about 77 dB, which is smaller than that of resonator C, and the Q value is also smaller. Similar to resonator B1, responses 63 and 64 are observed.

[0047] [Resonators A3, B3] 9(a) and 9(b) are cross-sectional views of resonators A3 and B3, respectively, in the simulation. As shown in FIG. 9(a), in resonator A3, surfaces 34a to 34d are rough. On surfaces 34a and 34c, convex portions 35a and 35c protrude toward the lower electrode 12, and on surfaces 34b and 34d, concave portions 36b and 36d are recessed toward the lower electrode 12. Convex portions 35a, concave portions 36b, convex portions 35c, and concave portions 36d are provided so as to correspond to each other. The roughness increases from surface 34a to surface 34d. The rest of the configuration is the same as that of resonator A1.

[0048] 9(b), in the resonator B3, the unevenness decreases from the surface 34a to the surface 34d. The other configurations are the same as those of the resonator A3.

[0049] The simulation conditions for the resonators A3 and B3 are as follows: Resonator A3 Arithmetic mean roughness Ra of surface 34a: 6.4 nm Arithmetic mean roughness Ra of surface 34b: 19.1 nm Arithmetic mean roughness Ra of surface 34c: 31.8 nm Arithmetic mean roughness Ra of surface 34d: 44.6 nm Resonator B3 Arithmetic mean roughness Ra of surface 34a: 44.6 nm Arithmetic mean roughness Ra of surface 34b: 31.8 nm Arithmetic mean roughness Ra of surface 34c: 19.1 nm Arithmetic mean roughness Ra of surface 34d: 6.4 nm Other simulation conditions are the same as those for resonator C.

[0050] 10(a) and 10(b) are graphs showing the absolute value |Y| of admittance versus frequency for resonators A3 and B3, respectively, in the simulation. As shown in FIG. 10(a), ΔY for resonator A3 is approximately 82 dB, which is almost the same as that for resonator C. The difference between the maximum 61 and the minimum 62 in the spurious response Sp is smaller than that for resonator C, but slightly larger than that for resonators A1 and A2.

[0051] As shown in Figure 10(b), in resonator B3, ΔY is approximately 77 dB, which is smaller than that of resonator C. In addition, the minimum of the antiresonance frequency fa is less steep than that of resonator C, and the Q value is smaller. Similar to resonator B1, responses 63 and 64 are observed.

[0052] [Resonators A4, B4] 11(a) and 11(b) are cross-sectional views of resonators A4 and B4, respectively, in the simulation. As shown in FIG. 11(a), in resonator A4, recesses 36a and 36c on surfaces 34a and 34c are recessed toward the substrate 10, and protrusions 35b and 35d on surfaces 34b and 34d are protruding toward the substrate 10. Recesses 36a, protrusions 35b, recesses 36c, and protrusions 35d are provided in correspondence with each other. The unevenness increases from surface 34a to surface 34d. The rest of the configuration is the same as resonator A3.

[0053] 11(b), in the resonator B4, the unevenness decreases from the surface 34a to the surface 34d. The other configurations are the same as those of the resonator A4.

[0054] The simulation conditions for the resonators A4 and B4 are as follows. Resonator A4 Arithmetic mean roughness Ra of surface 34a: 6.4 nm Arithmetic mean roughness Ra of surface 34b: 19.1 nm Arithmetic mean roughness Ra of surface 34c: 31.8 nm Arithmetic mean roughness Ra of surface 34d: 44.6 nm Resonator B4 Arithmetic mean roughness Ra of surface 34a: 44.6 nm Arithmetic mean roughness Ra of surface 34b: 31.8 nm Arithmetic mean roughness Ra of surface 34c: 19.1 nm Arithmetic mean roughness Ra of surface 34d: 6.4 nm Other simulation conditions are the same as those for resonator C.

[0055] 12(a) and 12(b) are graphs showing the absolute value |Y| of admittance versus frequency for resonators A4 and B4, respectively, in the simulation. As shown in FIG. 12(a), ΔY for resonator A4 is approximately 82 dB, which is almost the same as that for resonator C. The difference between the maximum 61 and the minimum 62 in the spurious response Sp is smaller than that for resonator C, but slightly larger than that for resonators A1 and A2, and is similar to that for resonator A3.

[0056] As shown in FIG. 12(b), in resonator B4, ΔY is approximately 76 dB, which is smaller than that of resonator C. In addition, the minimum at the antiresonant frequency fa is not as steep as that of resonator C, and the Q value is smaller. As with resonators B1 to B3, response 63 is observed. Response 64 is not observed, but response 65 is observed near the antiresonant frequency fa.

[0057] As in resonators A1 to A4, when the roughness (arithmetic mean roughness Ra) of surfaces 34c and 34d of high-impedance layer 32b on the substrate 10 side is made larger than the roughness (arithmetic mean roughness) of surfaces 34a and 34b of high-impedance layer 32a on the lower electrode 12 side, the main response is approximately the same as that of resonator C, and the spurious response Sp can be suppressed. By roughening surfaces 34c and 34d as in resonators A1 and A2, the spurious response Sp can be further suppressed. This is thought to be because unwanted acoustic waves such as transverse modes are scattered by surfaces 34c and 34d.

[0058] On the other hand, when the roughness of surfaces 34a and 34b of high-impedance layer 32a on the lower electrode 12 side is made larger than the roughness of surfaces 34c and 34d of high-impedance layer 32b on the substrate 10 side, as in resonators B1 to B4, the main response is smaller than that of resonator C. This is thought to be because the acoustic waves in the main mode are affected by surfaces 34a and 34b.

[0059] [Resonators A5, B5] We investigated whether the suppression of spurious responses Sp in resonators A1 to A4 is due to the unevenness of surface 34c or 34d. Figures 13(a) and 13(b) are cross-sectional views of resonators A5 and B5, respectively, in the simulation. As shown in Figure 13(a), in resonator A5, surface 34d is a flat surface. Surface 34c is an uneven surface, and surface 34c has a recess 36c that is recessed toward the substrate 10 side, the same as resonator A2. The other configurations are the same as resonator A2.

[0060] 13(b), in resonator B5, surface 34c is a flat surface. Surface 34d is an uneven surface, and surface 34d is provided with a protrusion 35d that protrudes toward the substrate 10, the same as resonator A2. The other configurations are the same as those of resonator A2.

[0061] The simulation conditions for the resonators A5 and B5 are as follows. Resonator A5 Arithmetic mean roughness Ra of surface 34a: 0 nm Arithmetic mean roughness Ra of surface 34b: 0 nm Arithmetic mean roughness Ra of surface 34c: 47.7 nm Arithmetic mean roughness Ra of surface 34d: 0 nm Resonator B5 Arithmetic mean roughness Ra of surface 34a: 0 nm Arithmetic mean roughness Ra of surface 34b: 0 nm Arithmetic mean roughness Ra of surface 34c: 0 nm Arithmetic mean roughness Ra of surface 34d: 47.7 nm Other simulation conditions are the same as those for resonator C.

[0062] 14(a) and 14(b) are graphs showing the absolute value of admittance |Y| versus frequency for resonators A5 and B5, respectively, in the simulation. As shown in FIG. 14(a), for resonator A5, ΔY is approximately 85 dB, which is almost the same as for resonators A2 and C. The difference between the maximum 61 and the minimum 62 in the spurious response Sp is almost the same as for resonator A2.

[0063] 14(b), in the resonator B5, ΔY is about 85 dB, which is similar to that of the resonators A2 and C. The difference between the maximum 61 and the minimum 62 in the spurious response Sp is larger than that of the resonators A2 and A5 and similar to that of the resonator C.

[0064] Comparing resonators A5 and B5, it appears that the spurious response Sp is suppressed mainly by the unevenness of surface 34c on the lower electrode 12 side of high-impedance layer 32b on the substrate 10 side. This is because unwanted acoustic waves are mainly reflected from the surface where they enter from the low-impedance layer to the high-impedance layer, and are not reflected very much from the surface where they enter from the high-impedance layer to the low-impedance layer.

[0065] In the first embodiment, the low-impedance layers 31a to 31c are three layers, and the high-impedance layers 32a and 32b are two layers. However, the low-impedance layers and the high-impedance layers may each have multiple layers. The roughness of the surface 34a facing the lower electrode 12 of the first high-impedance layer 32a, which is closest to the lower electrode 12 among the multiple high-impedance layers, is smaller than the roughness of the surface 34c facing the lower electrode 12 of at least one second high-impedance layer 32b other than the first high-impedance layer 32a. This suppresses degradation of the main response and spurious responses. The roughness of the surfaces 34a to 34d is, for example, the arithmetic mean roughness Ra. The arithmetic mean roughness Ra of the surfaces 34a to 34d can be calculated from the unevenness of the surfaces 34a to 34d when a cross section of the acoustic wave device is observed with an electron microscope such as a scanning electron microscope (SEM). Other surface roughness measurements may also be used for the roughness of the surfaces 34a to 34d. The arithmetic mean roughness Ra of surface 34c is preferably at least twice, more preferably at least three times, and even more preferably at least five times the arithmetic mean roughness Ra of surface 34a. The arithmetic mean roughness Ra of surface 34c is, for example, 100 times or less the arithmetic mean roughness Ra of surface 34a.

[0066] The roughness of surface 34b of first high-impedance layer 32a facing the substrate 10 is smaller than the roughness of surface 34d of second high-impedance layer 32b facing the substrate 10. This suppresses degradation of the main response and spurious responses. When unevenness is formed on both surfaces 34c and 34d of high-impedance layer 32b, surface 34d does not need to be flattened. The arithmetic mean roughness Ra of surface 34d is preferably at least two times, more preferably at least three times, and even more preferably at least five times the arithmetic mean roughness Ra of surface 34b. The arithmetic mean roughness Ra of surface 34d is, for example, no more than 100 times the arithmetic mean roughness Ra of surface 34b.

[0067] When there are three or more high-impedance layers, the roughness of the surface of the second or subsequent high-impedance layer closest to the bottom electrode 12 facing the bottom electrode 12 should be greater than the roughness of the surface 34a of the first high-impedance layer 32a facing the bottom electrode 12. Providing an uneven surface closer to the bottom electrode 12 allows for more scattering of unwanted acoustic waves. From this perspective, it is preferable that the second high-impedance layer 32b be the second closest to the bottom electrode 12 among the multiple high-impedance layers.

[0068] The arithmetic mean roughness Ra of surface 34a of first high-impedance layer 32a facing lower electrode 12 is preferably less than 10 nm, and more preferably 5 nm or less, so that surface 34a is less likely to affect the main mode acoustic waves, thereby suppressing deterioration of the main response.

[0069] The arithmetic mean roughness of surface 34c of second high-impedance layer 32b facing lower electrode 12 is preferably 10 nm or more, more preferably 15 nm or more, and even more preferably 20 nm or more, so that unnecessary acoustic waves are scattered at surface 34c, thereby suppressing spurious responses.

[0070] A simulation was performed in which the main resonant response was due to thickness-shear vibration and the spurious responses were due to transverse modes. When the piezoelectric layer 14 was a rotated Y-cut lithium niobate substrate or an X-cut lithium tantalate substrate, thickness-shear vibration was the main resonant mode. Even when the main resonant response was due to thickness-extensional vibration, by making the roughness of surface 34a smaller than that of surface 34c, degradation of the main response and spurious responses can be suppressed.

[0071] [Manufacturing method of Example 1] Figures 15(a) to 18(b) are cross-sectional views showing a method for manufacturing the piezoelectric thin film resonator according to Example 1. In Figures 15(a) to 16(c), the Z direction is illustrated downward, which is upside down compared to Figure 1, and in Figures 17(a) to 18(b), the Z direction is illustrated upward, as in Figure 1.

[0072] As shown in FIG. 15(a), a piezoelectric substrate is prepared as the piezoelectric layer 14. The -Z side surface of the piezoelectric layer 14 is substantially flat. The arithmetic mean roughness Ra of the -Z side surface of the piezoelectric layer 14 is, for example, 1 nm or less. A lower electrode 12 is formed on the piezoelectric layer 14. A metal layer 24 is formed on the lower electrode 12. The lower electrode 12 and the metal layer 24 are formed using, for example, sputtering, vacuum deposition, or CVD (Chemical Vapor Deposition). The lower electrode 12 and the metal layer 24 are patterned into the desired shape using photolithography and etching.

[0073] As shown in FIG. 15(b), a low-impedance layer 31a is formed on the entire piezoelectric layer 14 so as to cover the lower electrode 12 and the metal layer 24. The low-impedance layer 31a is formed, for example, by sputtering, vacuum deposition, or CVD. When the low-impedance layer 31a is a silicon oxide film or silicon nitride, the low-impedance layer 31a is formed, for example, by CVD. When a silicon oxide layer with a thickness of 150 nm is formed using CVD, the arithmetic mean roughness Ra of the -Z side surface 34a of the low-impedance layer 31a is, for example, 4 nm.

[0074] As shown in FIG. 15(c), high-impedance layer 32a is formed on low-impedance layer 31a. For example, sputtering, vacuum deposition, or CVD is used to form high-impedance layer 32a. When high-impedance layer 32a is made of a high-melting-point metal such as tungsten, tantalum, molybdenum, or ruthenium, sputtering is used to form high-impedance layer 32a. Photolithography and etching are used to pattern high-impedance layer 32a into a desired shape. When a 200-nm-thick tungsten layer is formed using sputtering, the arithmetic mean roughness Ra of surface 34b on the −Z side of high-impedance layer 32a is, for example, 15 nm.

[0075] 15(d), low-impedance layer 31b is formed on low-impedance layer 31a and high-impedance layer 32a. The method for forming low-impedance layer 31b is the same as the method for forming low-impedance layer 31a. When a silicon oxide layer with a thickness of 150 nm is formed using a CVD method, the arithmetic mean roughness Ra of surface 34c on the -Z side of low-impedance layer 31b is, for example, 16 nm.

[0076] As shown in FIG. 16(a), high-impedance layer 32b is formed on low-impedance layer 31b. The method for forming high-impedance layer 32b is the same as the method for forming high-impedance layer 32a. High-impedance layer 32b is patterned into a desired shape. The patterning method for high-impedance layer 32b is the same as the patterning method for high-impedance layer 32a. When a tungsten layer with a thickness of 200 nm is formed using a sputtering method, the arithmetic mean roughness Ra of surface 34d on the -Z side of high-impedance layer 32b is, for example, 24 nm.

[0077] 16(b), low-impedance layer 31c is formed on low-impedance layer 31b and high-impedance layer 32b in the same manner as low-impedance layers 31a and 31b.

[0078] 16(c), the upper surface of the low-impedance layer 31c is planarized by, for example, chemical mechanical polishing (CMP), thereby forming an acoustic reflection film 30 in which the high-impedance layers 32a and 32b and the low-impedance layers 31a to 31c are alternately stacked.

[0079] 17(a), the substrate is turned upside down and the lower surface of low-impedance layer 31c is bonded to the upper surface of substrate 10. For bonding, a surface activation method, for example, is used. A bonding layer such as a silicon film or an aluminum oxide film may be provided between substrate 10 and low-impedance layer 31c.

[0080] As shown in Figure 17(b), the piezoelectric layer 14 is thinned. For example, grinding and / or CMP are used to thin the piezoelectric layer 14. For example, grinding is used to make the piezoelectric layer 14 approximately the desired thickness, and the upper surface is flattened using CMP. As a result, the upper surface of the piezoelectric layer 14 becomes approximately flat within the manufacturing tolerance.

[0081] As shown in Figure 17(c), an upper electrode 16 is formed on the piezoelectric layer 14. The upper electrode 16 is formed by, for example, sputtering, vacuum deposition, or CVD. The upper electrode 16 is patterned into a desired shape using photolithography and etching. This forms a resonance region 50.

[0082] As shown in FIG. 18(a), an additional film 22 is formed from the periphery of the resonance region 50 to the outside of the resonance region 50. The additional film 22 is formed by, for example, sputtering, vacuum deposition, or CVD. The additional film 22 is patterned into a desired shape by photolithography and etching. The additional film 22 has an opening 23 that exposes the top surface of the upper electrode 16. A through-hole 25 is formed outside the resonance region 50, penetrating the piezoelectric layer 14. The through-hole 25 penetrates the lower electrode 12 and reaches the metal layer 24. The through-hole 25 is formed by, for example, etching. The metal layer 24 functions as an etching stopper when the through-hole 25 is formed.

[0083] 18(b), a metal layer 26 electrically connected to the lower electrode 12 and the metal layer 24 is then formed in the through hole 25, and a metal layer 28 electrically connected to the upper electrode 16 is formed in the opening 23. In this way, the piezoelectric thin film resonator according to the first embodiment is manufactured.

[0084] When multiple layers are stacked on a flat surface, the roughness of the upper surface of each layer increases. This is especially true when layers are formed using sputtering. Furthermore, the roughness of the upper surface of each layer can be increased by adjusting the deposition conditions.

[0085] In the method for manufacturing the piezoelectric thin film resonator according to the first embodiment, as shown in Fig. 15(b), a first low-impedance layer 31a is formed on the surface of the piezoelectric layer 14 on which the lower electrode 12 is provided. As shown in Fig. 15(c), a first high-impedance layer 32a is formed on the first low-impedance layer 31a. The first high-impedance layer 32a has an acoustic impedance higher than that of the first low-impedance layer 31a and has a surface 34b opposite the lower electrode 12 that is rougher than the surface 34a on the lower electrode 12 side. As shown in Fig. 15(d), a second low-impedance layer 31b has an acoustic impedance lower than that of the first high-impedance layer 32a.

[0086] As shown in FIG. 16(a), a second high-impedance layer 32b is formed on the second low-impedance layer 31b. The second high-impedance layer 32b has an acoustic impedance higher than that of the second low-impedance layer 31b, a surface 34c facing the lower electrode 12 that is rougher than the surface 34b of the first high-impedance layer 32a opposite the lower electrode 12, and a surface 34d facing the lower electrode 12 that is rougher than the surface 34c facing the lower electrode 12. As shown in FIG. 16(b), a third low-impedance layer 31c is formed on the second high-impedance layer 32b. The third low-impedance layer 31c has an acoustic impedance lower than that of the second high-impedance layer 32b. As shown in FIG. 17(a), the low-impedance layer 31c is bonded to the substrate 10. As shown in FIG. 17(c), an upper electrode 16 (second electrode) is formed on the surface of the piezoelectric layer 14 opposite the surface on which the lower electrode 12 is provided. In this manner, an acoustic reflection film 30 is laminated on the piezoelectric layer 14, and then the acoustic reflection film 30 and the substrate 10 are bonded to each other. This allows the surface 34c to be rougher than the surface 34a without performing a step of roughening the surface 34a.

[0087] 15(a) to 18(b), the roughness of surface 34b of first high-impedance layer 32a facing substrate 10 is greater than the roughness of surface 34a of first high-impedance layer 32a facing bottom electrode 12. The roughness of surface 34c of second high-impedance layer 32b facing bottom electrode 12 is greater than the roughness of surface 34b of first high-impedance layer 32a facing substrate 10. The roughness of surface 34d of second high-impedance layer 32b facing substrate 10 is greater than the roughness of surface 34c of second high-impedance layer 32b facing bottom electrode 12. This makes it possible to suppress degradation of the main response and spurious responses.

[0088] The surfaces 34c and 34d may be roughened by etching, grinding, or blasting, and the surfaces 34a and 34b may be flattened by polishing such as CMP. [Example]

[0089] Example 2 is an example of a filter and a duplexer using the piezoelectric thin film resonator of Example 1. Fig. 19(a) is a circuit diagram of the filter according to Example 2. As shown in Fig. 19(a), one or more series resonators S1 to S4 are connected in series between the input terminal Tin and the output terminal Tout. One or more parallel resonators P1 to P4 are connected in parallel between the input terminal Tin and the output terminal Tout. The piezoelectric thin film resonator of Example 1 can be used for at least one of the one or more series resonators S1 to S4 and the one or more parallel resonators P1 to P4. The number of resonators in the ladder-type filter can be set as appropriate.

[0090] FIG. 19(b) is a circuit diagram of a duplexer according to a first modification of the second embodiment. As shown in FIG. 19(b), a transmit filter 40 is connected between a common terminal Ant and a transmit terminal Tx. A receive filter 42 is connected between the common terminal Ant and a receive terminal Rx. The transmit filter 40 passes signals in the transmit band, among signals input from the transmit terminal Tx, to the common terminal Ant as transmit signals, and suppresses signals of other frequencies. The receive filter 42 passes signals in the receive band, among signals input from the common terminal Ant, to the receive terminal Rx as receive signals, and suppresses signals of other frequencies. At least one of the transmit filter 40 and the receive filter 42 can be the filter of the second embodiment.

[0091] Although a duplexer has been described as an example of a multiplexer, a triplexer or a quadplexer may also be used.

[0092] 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 as defined in the claims. [Explanation of symbols]

[0093] 10 Substrate 12 Lower electrode 14 Piezoelectric layer 16 Upper electrode 24, 26, 28 metal layer 30 Acoustic reflective film 31, 31a to 31c Low impedance layer 32, 32a, 32b High impedance layers 34a~34c side 35a~35d Convex part 36a~36d Recess 40 Transmission Filter 42 Receive Filter 50 resonance area

Claims

1. A substrate; a first electrode provided on the substrate; a piezoelectric layer provided on the first electrode; a second electrode provided on the piezoelectric layer with at least a portion of the piezoelectric layer sandwiched between the second electrode and the first electrode; an acoustic reflection film comprising: a plurality of low-impedance layers provided between the substrate and the first electrode; and a plurality of high-impedance layers having an acoustic impedance greater than an acoustic impedance of the plurality of low-impedance layers and provided alternately with the plurality of low-impedance layers, wherein a roughness of a surface of a first high-impedance layer of the plurality of high-impedance layers closest to the first electrode on the first electrode side is smaller than a roughness of a surface of at least one second high-impedance layer other than the first high-impedance layer of the plurality of high-impedance layers on the first electrode side, and a roughness of a surface of the second high-impedance layer on the substrate side is smaller than a roughness of a surface of the second high-impedance layer on the first electrode side; A piezoelectric thin film resonator comprising:

2. 2. The piezoelectric thin film resonator according to claim 1, wherein the roughness of the surface of the first high-impedance layer facing the substrate is smaller than the roughness of the surface of the second high-impedance layer facing the substrate.

3. 3. The piezoelectric thin film resonator according to claim 1, wherein the second high-impedance layer is the second closest to the first electrode among the plurality of high-impedance layers.

4. 4. The piezoelectric thin film resonator according to claim 3, wherein the roughness of the surface of the first high-impedance layer facing the substrate is greater than the roughness of the surface of the first high-impedance layer facing the first electrode, and the roughness of the surface of the second high-impedance layer facing the first electrode is greater than the roughness of the surface of the first high-impedance layer facing the substrate.

5. 5. The piezoelectric thin film resonator according to claim 1, wherein the arithmetic mean roughness of the surface of the first high-impedance layer on the side of the first electrode is less than 10 nm.

6. 6. The piezoelectric thin film resonator according to claim 5, wherein the arithmetic mean roughness of the surface of the second high-impedance layer on the side of the first electrode is 10 nm or more.

7. 7. The piezoelectric thin film resonator according to claim 1, wherein the piezoelectric layer is a rotated Y-cut lithium niobate substrate or an X-cut lithium tantalate substrate.

8. A filter comprising the film bulk acoustic resonator according to claim 1 .

9. A multiplexer including the filter of claim 8.

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