Piezoelectric thin film resonator, filter, and multiplexer

Abstract

To provide a piezoelectric thin film resonator capable of suppressing spurious.SOLUTION: A piezoelectric thin film resonator 100 comprises a substrate 10, a lower electrode 12 provided on the substrate 10, a piezoelectric film 14 provided on the lower electrode 12, an upper electrode 16, and an insertion film 28. The upper electrode 16 forms a resonance region 50 opposed to the lower electrode 12 while interposing at least a part of the piezoelectric film 14 therebetween. The insertion film 28 is inserted to the resonance region 50, is not provided in a central region 54 of the resonance region 50 and is provided along an outer periphery of the resonance region 50 so as to enclose the central region 54 and has an inner periphery 29 in the resonance region 50. In the inner periphery 29, in a planar view, a plurality of recesses 58 and a plurality of projections 60 are alternately provided where the depth in a plane direction is larger than H / 2 and smaller than 3H / 2 when a total thickness of the lower electrode 12, the piezoelectric film 14 and the upper electrode 16 in the resonance region 50 is defined as H.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a thin film piezoelectric resonator, a filter, and a multiplexer. [Background technology]

[0002] Bulk acoustic wave resonators are used as filters and multiplexers for high-frequency circuits in wireless terminals such as mobile phones. A bulk acoustic wave resonator has a structure in which a lower electrode and an upper electrode are provided with a piezoelectric film sandwiched therebetween, and a resonance region in which the lower electrode and the upper electrode face each other with at least a part of the piezoelectric film sandwiched therebetween is a region in which elastic waves resonate. In order to improve the Q value and suppress spurious, it is known to insert an insertion film into the piezoelectric film in the outer peripheral region of the resonance region and to provide a notch on the inner circumference of the insertion film (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2015-95728 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the structure in which the notch is provided only in the vicinity of the minor axis of the elliptical resonance region as described in Patent Document 1, there is still room for improvement in terms of suppressing spurious emissions.

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

[0006] The present invention is a piezoelectric thin-film resonator comprising: a substrate; a lower electrode provided on the substrate; a piezoelectric film provided on the lower electrode; an upper electrode provided on the piezoelectric film and forming a resonance region facing the lower electrode across at least a portion of the piezoelectric film; and an insertion film inserted into the resonance region, not provided in a central region of the resonance region, but provided along the outer periphery of the resonance region so as to surround the central region, having an inner periphery within the resonance region, and in which, when viewed in a plane, a plurality of concave portions and a plurality of convex portions are alternately provided on the inner periphery, the depth in the planar direction being greater than H / 2 and smaller than 3H / 2, where H is the total thickness of the lower electrode, the piezoelectric film, and the upper electrode in the resonance region.

[0007] In the above configuration, a depth of the plurality of recesses may be set to be not less than 3H / 4 and not more than 5H / 4.

[0008] In the above configuration, the width of the plurality of recesses and the width of the plurality of protrusions may be set to be equal to or greater than H / 2 and equal to or less than 4H.

[0009] In the above configuration, the insertion film may be configured such that two or more recesses constituting the plurality of recesses and two or more protrusions constituting the plurality of protrusions are alternately provided at each portion of the inner circumference facing the center of the resonance region.

[0010] In the above configuration, the insertion membrane may be configured such that the plurality of recesses and the plurality of protrusions are alternately provided around substantially the entire inner periphery.

[0011] In the above configuration, the resonance region may be configured to have a circular, elliptical, or oval shape in a plan view.

[0012] In the above configuration, a bottom surface of a recess among the plurality of recesses that is located at a position corresponding to a curved portion of the outer periphery of the resonance region may be curved along the outer periphery of the resonance region.

[0013] In the above configuration, bottom surfaces of the plurality of recesses may be substantially flat.

[0014] In the above configuration, a gap may be formed between the substrate and the lower electrode in the resonance region, or an acoustic reflection film may be provided on the opposite side of the lower electrode from the piezoelectric film to reflect elastic waves propagating through the piezoelectric film.

[0015] The present invention is a filter including the above-described thin film piezoelectric resonator.

[0016] The present invention is a multiplexer comprising the filter described above. Effect of the Invention

[0017] According to the present invention, spurious signals can be suppressed. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1(a) is a plan view of a piezoelectric thin film resonator according to a first embodiment, FIG. 1(b) is a plan view of the vicinity of a resonance region of an insertion film in the first embodiment, and FIG. 1(c) is a cross-sectional view taken along line AA of FIG. 1(a). [Diagram 2] FIG. 2 is an enlarged plan view of the vicinity of the inner periphery of the insertion membrane in the first embodiment. [Diagram 3] 3(a) to 3(d) are cross-sectional views illustrating a method for manufacturing the piezoelectric thin film resonator according to the first embodiment. [Figure 4] 4(a) is a plan view of a piezoelectric thin film resonator according to a comparative example, FIG. 4(b) is a plan view of the vicinity of the resonance region of an insertion film in the comparative example, and FIG. 4(c) is a cross-sectional view taken along line AA of FIG. 4(a). [Diagram 5] FIG. 5 shows experimental results illustrating the amount of attenuation versus frequency for the piezoelectric thin film resonators according to Example 1 and the comparative example. [Figure 6] FIG. 6 is a schematic diagram of an IFFT (Inverse Fast Fourier Transform) image obtained by optical observation measurement of an elliptical resonance region. [Figure 7] 7(a) to 7(c) are plan views showing other examples of the insertion membrane. [Figure 8] 8(a) and 8(b) are plan views showing other examples of the resonance region. [Figure 9] 9(a) and 9(b) are plan views showing examples of recesses provided on the inner periphery of the insertion membrane. [Figure 10] FIG. 10(a) is a cross-sectional view of a piezoelectric thin film resonator according to a first modified example of the first embodiment, and FIG. 10(b) is a cross-sectional view of a piezoelectric thin film resonator according to a second modified example of the first embodiment. [Figure 11] FIG. 11 is a circuit diagram of a filter in accordance with the second embodiment. [Figure 12] FIG. 12 is a block diagram of a duplexer in accordance with the third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Hereinafter, embodiments of the present invention will be described with reference to the drawings. EXAMPLES

[0020] FIG. 1(a) is a plan view of a piezoelectric thin film resonator 100 according to a first embodiment, FIG. 1(b) is a plan view of the vicinity of a resonance region 50 of an insertion film 28 in the first embodiment, and FIG. 1(c) is a cross-sectional view taken along line AA of FIG. 1(a). FIG. 1(a) mainly illustrates a substrate 10, a lower electrode 12, and an upper electrode 16. As shown in FIGS. 1(a) to 1(c), the piezoelectric thin film resonator 100 according to the first embodiment includes a substrate 10, a lower electrode 12, a piezoelectric film 14, an upper electrode 16, an additional film 24, and an insertion film 28. The substrate 10 has a recess formed on its upper surface. The lower electrode 12 is formed flat on the substrate 10. The recess formed on the upper surface of the substrate 10 forms a gap 30 between the substrate 10 and the lower electrode 12. The gap 30 may be formed so as to penetrate the substrate 10. The substrate 10 is, for example, a silicon substrate, and has a thickness of 100 μm to 1000 μm. The lower electrode 12 includes a lower layer 12a and an upper layer 12b, and has a thickness of 30 nm to 400 nm. The lower layer 12a is, for example, a chromium (Cr) film, and the upper layer 12b is, for example, a ruthenium (Ru) film.

[0021] A piezoelectric film 14 is provided on the lower electrode 12. The piezoelectric film 14 is, for example, an aluminum nitride film mainly composed of aluminum nitride with a main axis in the (002) direction, and has a thickness of 800 nm to 1500 nm. The piezoelectric film 14 includes a lower piezoelectric film 14a provided on the lower electrode 12, and an upper piezoelectric film 14b provided on the lower piezoelectric film 14a. "Mainly composed" may mean that the total of aluminum atoms and nitrogen atoms is 50 atomic % or more, 80 atomic % or more, or 90 atomic % or more.

[0022] An upper electrode 16 is provided on the piezoelectric film 14. The upper electrode 16 is provided on the piezoelectric film 14 so as to have an area facing the lower electrode 12 with the piezoelectric film 14 in between. The area where the lower electrode 12 and the upper electrode 16 face each other with the piezoelectric film 14 in between is a resonance area 50. The resonance area 50 has, for example, an elliptical shape in a plan view, and is an area where an elastic wave in a thickness longitudinal vibration mode resonates. In a plan view, the size of the gap 30 is the same as or larger than the resonance area 50. In a plan view, the resonance area 50 may have a polygonal shape such as a rectangle or a pentagon. The upper electrode 16 includes a lower layer 16a and an upper layer 16b, and has a thickness of 30 nm to 400 nm. The lower layer 16a is, for example, a Ru film, and the upper layer 16b is, for example, a Cr film.

[0023] An additional film 24 is provided on the upper electrode 16 to cover at least a portion of the resonance region 50. The additional film 24 is, for example, a silicon oxide film, and may play a role in adjusting the frequency. The additional film 24 may also function as a protective film. The total thickness of the lower electrode 12, the piezoelectric film 14, and the upper electrode 16 in the resonance region 50 is H. The wavelength λ of the elastic wave in the thickness longitudinal vibration mode resonated in the resonance region 50 is finely adjusted by the additional film 24, but is generally twice (2H) the total thickness H of the lower electrode 12, the piezoelectric film 14, and the upper electrode 16.

[0024] A hole 35 is provided in the lower electrode 12. The hole 35 communicates with the void 30 via an introduction path 33 below the lower electrode 12. The hole 35 and the introduction path 33 are for introducing an etching liquid into the sacrificial layer used to form the void 30 when etching the sacrificial layer.

[0025] The insertion film 28 is provided between the lower piezoelectric film 14a and the upper piezoelectric film 14b. The insertion film 28 is preferably provided at approximately the center in the thickness direction of the piezoelectric film 14. The insertion film 28 is provided in the outer peripheral region 52 in the resonance region 50, but not in the central region 54. The outer peripheral region 52 is a region in the resonance region 50 that includes the outer periphery of the resonance region 50 and is along the outer periphery. The outer peripheral region 52 is, for example, strip-shaped or ring-shaped. The central region 54 is a region in the resonance region 50 that includes the center of the resonance region 50. The central region 54 does not have to be the geometric center. Therefore, the insertion film 28 is not provided in the central region 54, but is provided along the outer periphery of the resonance region 50 so as to surround the central region 54. The insertion film 28 has an inner periphery 29 in the resonance region 50. The inner periphery 29 of the insertion film 28 is approximately along the outer periphery of the resonance region 50, and is, for example, approximately elliptical in plan view. The insertion film 28 is provided, for example, in the outer peripheral region 52 and continuously up to the region 56 surrounding the resonance region 50, but may be provided only within the resonance region 50. The insertion film 28 is, for example, a silicon oxide film. The insertion film 28 is provided to suppress the energy of the elastic wave from leaking out of the resonance region 50.

[0026] The insertion film 28 is not limited to being inserted in the piezoelectric film 14, but may be inserted in the resonance region 50. For example, the insertion film 28 may be inserted between the lower electrode 12 and the upper electrode 16, such as between the lower electrode 12 and the piezoelectric film 14 or between the piezoelectric film 14 and the upper electrode 16, or may be provided below the lower electrode 12 or above the upper electrode 16.

[0027] In the region where the lower electrode 12 is drawn out from the resonance region 50, the outer periphery of the upper piezoelectric film 14b is located outside the outer periphery of the resonance region 50, and the outer periphery of the lower piezoelectric film 14a is located outside the outer periphery of the upper piezoelectric film 14b. The outer periphery of the insertion film 28 approximately coincides with the outer periphery of the lower piezoelectric film 14a. This forms a step in the piezoelectric film 14.

[0028] The insertion film 28 has a plurality of recesses 58 and a plurality of protrusions 60 arranged alternately and repeatedly on the inner circumference 29. The depth and width of the recesses 58 are, for example, substantially constant, and the height and width of the protrusions 60 are, for example, substantially constant. The recesses 58 of substantially constant width and the protrusions 60 of substantially constant width are arranged alternately and repeatedly over substantially the entire circumference of the inner circumference 29 of the insertion film 28. The bottom surface of the recesses 58 may be located in the outer periphery region 52 within the resonance region 50, may be substantially coincident with the outer periphery of the resonance region 50, or may be located in the region 56 surrounding the resonance region 50.

[0029] FIG. 2 is an enlarged plan view of the vicinity of the inner circumference 29 of the insertion membrane 28 in the first embodiment. As shown in FIG. 2, the recess 58 and the protrusion 60 provided on the inner circumference 29 of the insertion membrane 28 are, for example, substantially rectangular in plan view. The depth L of the recess 58 is greater than H / 2 and smaller than 3H / 2. In other words, the depth L of the recess 58 is greater than λ / 4 and smaller than 3λ / 4. The width W1 of the recess 58 and the width W2 of the protrusion 60 are, for example, H / 2 or more and 4H or less, in other words, λ / 4 or more and 2λ or less. For example, the width W1 of the recess 58 and the width W2 of the protrusion 60 are substantially the same size. "Substantially the same" means that the difference between the width W1 and the width W2 is 5% or less. When the bottom surface of the recess 58 is curved, the depth L of the deepest part of the recess 58 is the depth L. The width W1 of the recess 58 is defined as the width closest to the center of the resonance region 50 when the two side surfaces of the recess 58 are inclined toward each other. Similarly, the width W2 of the protrusion 60 is defined as the width closest to the center of the resonance region 50 when the two side surfaces of the protrusion 60 are inclined toward each other. The center of the resonance region 50 is, for example, the center of gravity of the resonance region 50.

[0030] As the substrate 10, in addition to a silicon substrate, for example, an insulating substrate or a semiconductor substrate such as a sapphire substrate, a spinel substrate, an alumina substrate, a quartz substrate, a glass substrate, a ceramic substrate, or a gallium arsenide substrate can be used. As the lower electrode 12 and the upper electrode 16, in addition to Ru and Cr, for example, a single layer film of aluminum (Al), titanium (Ti), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), rhodium (Rh), iridium (Ir), or the like, or a laminated film thereof can be used.

[0031] The piezoelectric film 14 may be made of, for example, zinc oxide (ZnO), gallium nitride (GaN), lead zirconate titanate (PZT), or lead titanate (PbTiO3) in addition to aluminum nitride (AlN). The piezoelectric film 14 may contain other elements, mainly made of aluminum nitride, to improve resonance characteristics or piezoelectricity. The piezoelectricity of the piezoelectric film 14 is improved by using, for example, scandium (Sc), two elements of group 2 and group 4, or two elements of group 2 and group 5 as an additive element. This improves the effective electromechanical coupling coefficient of the piezoelectric thin film resonator. The group 2 element is, for example, calcium (Ca), magnesium (Mg), strontium (Sr), or zinc (Zn). The group 4 element is, for example, titanium (Ti), zirconium (Zr), or hafnium (Hf). The group 5 element is, for example, tantalum (Ta), niobium (Nb), or vanadium (V). Furthermore, the piezoelectric film 14 may be mainly composed of aluminum nitride and may contain fluorine (F) or boron (B).

[0032] The insertion film 28 is preferably formed of a material having a smaller Young's modulus and / or acoustic impedance than the piezoelectric film 14. This can improve the Q value. The insertion film 28 can be a single layer film of aluminum (Al), gold (Au), copper (Cu), titanium (Ti), platinum (Pt), tantalum (Ta), chromium (Cr), or the like, or a laminated film of these, in addition to silicon oxide. By using a metal film as the insertion film 28, the effective electromechanical coupling coefficient can be improved. In addition to a silicon oxide film, the additional film 24 can be, for example, a silicon nitride film or an aluminum nitride film.

[0033] [Manufacturing method] 3(a) to 3(d) are cross-sectional views showing a method for manufacturing the piezoelectric thin film resonator 100 according to the first embodiment. As shown in FIG. 3(a), a recess is formed on the upper surface of the substrate 10 by, for example, etching, and then a sacrificial layer 38 is embedded in the recess. The depth of the recess is, for example, 10 nm to 100 nm. The sacrificial layer 38 is selected from materials that can be easily dissolved in an etching solution or etching gas, such as MgO, ZnO, Ge, or SiO2. Next, a lower layer 12a and an upper layer 12b are formed as the lower electrode 12 on the sacrificial layer 38 and the substrate 10. The lower electrode 12 is formed by, for example, sputtering, vacuum deposition, or CVD (Chemical Vapor Deposition). Then, the lower electrode 12 is patterned into a desired shape by photolithography and etching. The lower electrode 12 may be formed by a lift-off method.

[0034] As shown in FIG. 3(b), the lower piezoelectric film 14a and the insertion film 28 are formed on the lower electrode 12 and the substrate 10 by, for example, sputtering, vacuum deposition, or CVD. The insertion film 28 is patterned into a desired shape by photolithography and etching. The insertion film 28 may be formed by a lift-off method. A recess 58 and a protrusion 60 (not shown in FIG. 3(b)) are formed on the inner periphery 29 of the insertion film 28.

[0035] As shown in FIG. 3(c), the upper piezoelectric film 14b and the lower layer 16a and upper layer 16b of the upper electrode 16 are formed on the lower piezoelectric film 14a by, for example, sputtering, vacuum deposition, or CVD. The piezoelectric film 14 is formed from the lower piezoelectric film 14a and the upper piezoelectric film 14b. The upper electrode 16 is patterned into a desired shape by photolithography and etching. The upper electrode 16 may be formed by a lift-off method. The additional film 24 is formed on the piezoelectric film 14 by, for example, sputtering, vacuum deposition, or CVD. The additional film 24 is patterned into a desired shape by photolithography and etching. Then, the piezoelectric film 14 is patterned into a desired shape by photolithography and etching.

[0036] As shown in FIG. 3(d), an etching solution for the sacrificial layer 38 is introduced into the sacrificial layer 38 below the lower electrode 12 through the hole 35 and the introduction path 33 (see FIG. 1(a)). This removes the sacrificial layer 38. The medium for etching the sacrificial layer 38 is preferably a medium that barely etches the materials constituting the resonator other than the sacrificial layer 38. In particular, the etching medium is preferably a medium that barely etches the lower electrode 12 with which it comes into contact. By removing the sacrificial layer 38, a gap 30 is formed between the lower electrode 12 and the substrate 10. In this manner, the piezoelectric thin film resonator 100 according to the first embodiment is formed.

[0037] [Comparative Example] Fig. 4(a) is a plan view of a piezoelectric thin film resonator 500 according to a comparative example, Fig. 4(b) is a plan view of the vicinity of the resonance region 50 of the insertion film 28 in the comparative example, and Fig. 4(c) is a cross-sectional view taken along line AA of Fig. 4(a). As shown in Figs. 4(a) to 4(c), in the piezoelectric thin film resonator 500 according to the comparative example, one recess 58 is provided on the inner circumference 29 of the insertion film 28, located in the vicinity of the minor axis of the resonance region 50. The other configurations are the same as those in the first embodiment, and therefore will not be described.

[0038] [experiment] An experiment was carried out to evaluate the spurious response of the piezoelectric thin film resonators of Example 1 and Comparative Example under the following conditions. Substrate 10: Silicon substrate Lower piezoelectric film 14a, upper piezoelectric film 14b: Aluminum nitride (AlN) film with a thickness of 1152 nm The lower layer 12a of the lower electrode 12: a chromium (Cr) film having a thickness of 55 nm Upper layer 12b of bottom electrode 12: 205 nm thick ruthenium (Ru) film Lower layer 16a of upper electrode 16: Ruthenium (Ru) film with a thickness of 205 nm Upper layer 16b of upper electrode 16: 55 nm thick chromium (Cr) film Additional film 24: 70 nm thick silicon oxide (SiO2) film Insertion film 28: 122 nm thick silicon oxide (SiO2) film Elastic wave wavelength λ: 2400nm Depth L of the recess 58: λ / 2 Width W1 of the recess 58 and width W2 of the protrusion 60: λ / 2 Resonance region 50: Elliptical shape with minor axis length of 122 μm and major axis length of 146 μm

[0039] 5 shows experimental results showing the attenuation versus frequency of the piezoelectric thin film resonators according to Example 1 and the comparative example. The resonant frequency of the piezoelectric thin film resonator 100 according to Example 1 is 2021 MHz, and the resonant frequency of the piezoelectric thin film resonator 500 according to the comparative example is 2018 MHz. As shown in FIG. 5, the piezoelectric thin film resonator 100 according to Example 1 has more suppressed spurious and improved attenuation than the piezoelectric thin film resonator 500 according to the comparative example.

[0040] The mechanism by which spurious is suppressed in Example 1 will be considered. FIG. 6 is a schematic diagram of an IFFT (Inverse Fast Fourier Transform) image of an elliptical resonance region 50. In the piezoelectric thin film resonator in FIG. 6, the side surface of the inner circumference 29 of the insertion film 28 is flat, and no recess is formed. In FIG. 6, standing waves caused by transverse mode elastic waves propagating in the resonance region 50 are mainly shown, and elastic waves in the thickness-extensional vibration mode, which is the main mode, are hardly shown. In FIG. 6, the region where the standing waves are generated is illustrated by hatching, and the large amplitude region 27 where the standing waves reinforce each other and the amplitude is large is illustrated by fine hatching. Note that it is believed that a plurality of large amplitude regions 27 exist in the resonance region 50, but only a part of the large amplitude region 27 is illustrated due to the measurement accuracy of the optical observation measurement.

[0041] 6, the outer circumferential region within the resonance region 50, which is the region near the inner circumference 29 of the insertion film 28, is the large amplitude region 27. Furthermore, four regions located near the major axis and the minor axis in the central region of the elliptical resonance region 50 are also large amplitude regions 27. Since the large amplitude region 27 exists over the entire inner circumference 29 of the insertion film 28, it is considered that standing waves exist in various directions within the resonance region 50.

[0042] Here, the inventors assumed that the wavelength of the elastic wave in the thickness-extensional vibration mode, which is the main mode resonating in the resonance region 50, is approximately the same as the wavelength of the elastic wave in the transverse mode propagating through the resonance region 50, and that the wavelength of the standing wave caused by the elastic wave in the transverse mode is also approximately the same as the wavelength of the elastic wave in the thickness-extensional vibration mode. Therefore, in the above experiment, a recess 58 with a depth L of λ / 2 was provided on the inner circumference 29 of the insertion film 28 so that the standing waves reflected on the inner circumference 29 of the insertion film 28 cancel each other out.

[0043] In the comparative example, there was no significant difference in spurious responses compared to a piezoelectric thin film resonator in which no recess was provided on the inner circumference 29 of the insertion film 28. This is believed to be because providing only one recess 58 with a depth of λ / 2 on the inner circumference 29 of the insertion film 28 did not cause much cancellation of standing waves reflected on the inner circumference of the insertion film 28. Therefore, in Example 1, recesses 58 with a depth of λ / 2 were provided around the entire circumference of the inner circumference 29 of the insertion film 28. This caused standing waves in various directions to cancel each other out, and as a result, it is believed that in Example 1, spurious responses were suppressed and attenuation was improved compared to the comparative example.

[0044] In the above experiment, the depth L of the recess 58 was set to λ / 2. In order to effectively cancel and weaken the standing waves reflected at the inner circumference 29 of the insertion film 28, the depth L of the recess 58 is preferably set to λ / 2. However, even if the depth L of the recess 58 is set to be greater than λ / 4 and smaller than 3λ / 4, the standing waves reflected at the inner circumference 29 of the insertion film 28 can be canceled. As described above, the wavelength λ of the elastic wave in the thickness longitudinal vibration mode resonating in the resonance region 50 is approximately twice (2H) the total thickness H of the lower electrode 12, the piezoelectric film 14, and the upper electrode 16. Therefore, when the depth L of the recess 58 is set to be greater than H / 2 and smaller than 3H / 2, the standing waves reflected at the inner circumference 29 of the insertion film 28 can be canceled.

[0045] As described above, according to the first embodiment, the insertion film 28 has a plurality of recesses 58 and a plurality of protrusions 60, each having a depth L in the planar direction greater than H / 2 and smaller than 3H / 2, alternately provided on the inner circumference 29 in a plan view. This allows standing waves reflected on the inner circumference 29 of the insertion film 28 to be cancelled out and weakened, thereby suppressing spurious signals. From the viewpoint of weakening standing waves reflected on the inner circumference 29 of the insertion film 28, the depth L of the recesses 58 is preferably 3H / 4 or more and 5H / 4 or less, more preferably 4H / 5 or more and 6H / 5 or less, and even more preferably H.

[0046] In addition, in Example 1, the width W1 of the recess 58 and the width W2 of the protrusion 60 are H / 2 or more and 4H or less. When the width W1 of the recess 58 is narrow and the width W2 of the protrusion 60 is wide, or when the width W1 of the recess 58 is wide and the width W2 of the protrusion 60 is narrow, the ratio of the standing waves reflected by the recess 58 and the standing waves reflected by the protrusion 60 in the inner circumference 29 of the insertion film 28 is greatly different. Therefore, the effect of weakening the standing waves is reduced. By setting the width W1 of the recess 58 and the width W2 of the protrusion 60 to H / 2 or more and 4H or less, it is possible to suppress a large difference in the ratio of the standing waves reflected by the recess 58 and the standing waves reflected by the protrusion 60, and it is possible to effectively weaken the standing waves. From the viewpoint of weakening the standing waves, the width W1 of the recess 58 and the width W2 of the protrusion 60 are preferably H or more and 3H or less, and more preferably 3H / 2 or more and 5H / 2 or less.

[0047] In the first embodiment, the insertion film 28 has a plurality of recesses 58 and a plurality of protrusions 60 alternately provided over substantially the entire circumference of the inner circumference 29. This effectively weakens standing waves reflected at the inner circumference 29 of the insertion film 28. "Substantially the entire circumference of the inner circumference 29" refers to 90% or more of the inner circumference 29, but may also refer to 95% or more. It is preferable that the recesses 58 and the protrusions 60 are alternately provided over the entire inner circumference 29.

[0048] 7(a) to 7(c) are plan views showing other examples of the insertion film 28. As shown in FIG. 7(a), the insertion film 28 may have two or more recesses 58 and two or more protrusions 60 alternately provided in the vicinity of the major axis of the inner circumference 29 facing the center of the elliptical resonance region 50, and no recesses 58 may be provided in the vicinity of the minor axis. As shown in FIG. 7(b), the insertion film 28 may have two or more recesses 58 and two or more protrusions 60 alternately provided in the vicinity of the minor axis of the inner circumference 29 facing the center of the resonance region 50, and no recesses 58 may be provided in the vicinity of the major axis. As shown in FIG. 7(c), the insertion film 28 may have two or more recesses 58 and two or more protrusions 60 alternately provided in the vicinity of the major axis and the minor axis of the inner circumference 29 facing the center of the resonance region 50, and no recesses 58 may be provided between them. As such, the insertion membrane 28 may have a flat surface on the inner circumference 29 that is wider than the protrusions 60 between closely adjacent recesses 58. The number of recesses 58 near each of the opposing major axes and / or near each of the opposing minor axes is preferably 5 or more, and more preferably 10 or more.

[0049] As shown in Fig. 7(a) to Fig. 7(c), the insertion film 28 is preferably provided with two or more recesses 58 and two or more protrusions 60 alternately at each of the locations of the inner circumference 29 facing the center of the resonance region 50. This effectively weakens the standing waves reflected at the inner circumference 29 of the insertion film 28. Since transverse mode elastic waves propagating in the direction of the minor axis of the resonance region 50 are difficult to attenuate, the insertion film 28 is preferably provided with two or more recesses 58 and two or more protrusions 60 alternately at each of the locations of the inner circumference 29 near the minor axis of the resonance region 50. The recesses 58 and the protrusions 60 are preferably formed on 50% or more of the inner circumference 29 of the insertion film 28, more preferably on 70% or more, and even more preferably on 80% or more.

[0050] In the first embodiment, the recess 58 is substantially rectangular in plan view. This effectively weakens the standing waves reflected at the inner circumference 29 of the insertion film 28. The substantially rectangular shape allows for curved sides and bottom surfaces and rounded corners. The recess 58 may also be substantially triangular in plan view.

[0051] In the first embodiment, the resonance region 50 has an elliptical shape in plan view, but the shape is not limited to this. FIGS. 8(a) and 8(b) are plan views showing other examples of the resonance region 50. As shown in FIG. 8(a), the resonance region 50 may have a circular shape in plan view. In this case, the inner circumference 29 of the insertion film 28 is approximately circular along the outer circumference of the resonance region 50. As shown in FIG. 8(b), the resonance region 50 may have an oval shape in plan view. In this case, the inner circumference 29 of the insertion film 28 is approximately oval along the outer circumference of the resonance region 50. Even in this case, as in the case of FIG. 6, the large amplitude region 27 exists over the entire inner circumference 29 of the insertion film 28, and standing waves exist in various directions in the resonance region 50, so it is preferable to alternately provide a plurality of recesses 58 and a plurality of protrusions 60 on the inner circumference 29 of the insertion film 28.

[0052] 9(a) and 9(b) are plan views showing examples of recesses 58 provided on the inner circumference 29 of the insertion film 28. As shown in FIG. 9(a), the bottom surface of the recess 58 located at a position corresponding to the curved portion of the outer circumference of the resonance region 50 among the multiple recesses 58 may be a curved surface along the outer circumference of the resonance region 50. In other words, the distance between the bottom surface of the recess 58 and the outer circumference of the resonance region 50 may be approximately constant. The term "approximately constant" means that a difference of the order of manufacturing error is allowed. As shown in FIG. 9(b), the bottom surface of the multiple recesses 58 including the recess 58 corresponding to the curved portion of the outer circumference of the resonance region 50 may be an approximately flat surface. The term "approximately flat surface" means that a curved surface of the recess 58 is allowed to be curved to the order of manufacturing error. In either case of FIG. 9(a) or FIG. 9(b), the standing wave reflected at the inner circumference 29 of the insertion film 28 can be effectively weakened.

[0053] The depth L of the multiple recesses 58 may be approximately the same, or may be different from one another among at least two or more recesses 58. Even when the depth L of at least two or more recesses 58 is different, standing waves reflected by the inner circumference 29 of the insertion film 28 can be canceled out.

[0054] [Variations] Fig. 10(a) is a cross-sectional view of a piezoelectric thin film resonator 110 according to a first modified example of the first embodiment. As shown in Fig. 10(a), in the piezoelectric thin film resonator 110, a gap 30 having a dome-shaped bulge is formed between the flat main surface of the substrate 10 and the lower electrode 12. The dome-shaped bulge is, for example, a bulge in a shape such that the height of the gap 30 is small around the periphery of the gap 30 and the height of the gap 30 increases toward the inside of the gap 30. The other configurations are the same as those of the first embodiment, and therefore description thereof will be omitted.

[0055] FIG. 10(b) is a cross-sectional view of a piezoelectric thin film resonator 120 according to a second modification of the first embodiment. As shown in FIG. 10(b), in the piezoelectric thin film resonator 120, an acoustic reflection film 31 is formed under the lower electrode 12 in the resonance region 50. The acoustic reflection film 31 is formed by alternately providing a film 31a having a low acoustic impedance and a film 31b having a high acoustic impedance. The thicknesses of the films 31a and 31b are, for example, λ / 4. The number of layers of the films 31a and 31b can be set arbitrarily. The acoustic reflection film 31 may be formed by laminating at least two types of layers having different acoustic characteristics with a gap therebetween. The substrate 10 may be one of the at least two types of layers having different acoustic characteristics of the acoustic reflection film 31. For example, the acoustic reflection film 31 may be configured such that one layer of a film having a different acoustic impedance is provided in the substrate 10. The other configurations are the same as those in the first embodiment, and therefore will not be described.

[0056] As in the first embodiment and its first modification, the piezoelectric thin film resonator may be an FBAR (Film Bulk Acoustic Resonator) in which a gap 30 is formed between the substrate 10 and the lower electrode 12 in the resonance region 50. As in the second modification of the first embodiment, the piezoelectric thin film resonator may be an SMR (Solidly Mounted Resonator) that includes an acoustic reflection film 31 below the lower electrode 12 in the resonance region 50, which reflects an elastic wave propagating through the piezoelectric film 14. EXAMPLES

[0057] FIG. 11 is a circuit diagram of a filter 200 according to a second embodiment. As shown in FIG. 11, in the filter 200, one or more series resonators S1 to S4 are connected in series between an input terminal Tin and an output terminal Tout. One or more parallel resonators P1 to P3 are connected in parallel between the input terminal Tin and the output terminal Tout. The parallel resonators P1 to P3 are connected between a path between the input terminal Tin and the output terminal Tout and a ground terminal. At least one of the one or more series resonators S1 to S4 and the one or more parallel resonators P1 to P3 can be a piezoelectric thin film resonator according to the first embodiment and its first and second modifications. The number of resonators in the ladder filter can be set appropriately. EXAMPLES

[0058] FIG. 12 is a block diagram of a duplexer 300 according to a third embodiment. As shown in FIG. 12, in the duplexer 300, a transmission filter 40 is connected between a common terminal Ant and a transmission terminal Tx. A reception filter 42 is connected between the common terminal Ant and a reception terminal Rx. The transmission filter 40 passes a signal in a transmission band among signals input from the transmission terminal Tx to the common terminal Ant as a transmission signal, and suppresses signals of other frequencies. The reception filter 42 passes a signal in a reception band among signals input from the common terminal Ant to the reception terminal Rx as a reception signal, and suppresses signals of other frequencies. At least one of the transmission filter 40 and the reception filter 42 can be the filter of the second embodiment. Although a duplexer has been described as an example of a multiplexer, a triplexer or a quadplexer may be used.

[0059] The acoustic wave device is not limited to the above, but may be a MEMS or a sensor.

[0060] Although the embodiment of the present invention has been described in detail above, the present invention is not limited to such specific embodiment, and various modifications and variations are possible within the scope of the gist of the present invention described in the claims. [Explanation of symbols]

[0061] 10 Substrate 12 Lower electrode 14 Piezoelectric film 16 Upper electrode 24 Additional membrane 27 Large amplitude region 28 Insertion Membrane 29 Inner Circumference 30 void 31 Acoustic reflective film 33 Introductory path 35 Hole 38 Sacrificial Layer 40 Transmission Filter 42 Receiving Filter 50 resonance area 52 Outer area 54 Central area 56 areas 58 Recess 60 Convex 100, 110, 120, 500 Piezoelectric thin film resonators 200 Filters 300 Duplexer

Claims

1. A substrate; a lower electrode provided on the substrate; a piezoelectric film provided on the lower electrode; an upper electrode provided on the piezoelectric film and forming a resonance region facing the lower electrode with at least a portion of the piezoelectric film interposed therebetween; a piezoelectric thin-film resonator comprising: an insertion film that is inserted into the resonance region, is not provided in a central region of the resonance region, but is provided along the outer periphery of the resonance region so as to surround the central region, has an inner periphery within the resonance region, and has a plurality of concave portions and a plurality of convex portions alternately provided on the inner periphery in a planar view, the depth of which is greater than H / 2 and smaller than 3H / 2, where H is a total thickness of the lower electrode, the piezoelectric film, and the upper electrode in the resonance region.

2. 2. The piezoelectric thin film resonator according to claim 1, wherein the depth of the plurality of recesses is not less than 3H / 4 and not more than 5H / 4.

3. 3. The piezoelectric thin film resonator according to claim 1, wherein the width of each of the recesses and the width of each of the protrusions are equal to or larger than H / 2 and equal to or smaller than 4H.

4. 4. The piezoelectric thin film resonator according to claim 3, wherein the insertion film has two or more concave portions constituting the plurality of concave portions and two or more convex portions constituting the plurality of convex portions alternately provided at each of the locations of the inner circumference facing the center of the resonance region.

5. 4. The piezoelectric thin film resonator according to claim 3, wherein the insertion film has the plurality of recesses and the plurality of protrusions alternately provided around substantially the entire inner periphery.

6. The piezoelectric thin film resonator according to claim 1 , wherein the resonance region has a circular, elliptical or oval shape in a plan view.

7. 7. The piezoelectric thin film resonator according to claim 6, wherein a bottom surface of the recess located at a position corresponding to a curved portion of the periphery of the resonance region is a curved surface that follows the periphery of the resonance region.

8. The piezoelectric thin film resonator according to claim 6 , wherein the bottom surfaces of the plurality of recesses are substantially flat.

9. 9. The piezoelectric thin film resonator according to claim 1, wherein in the resonance region, a gap is formed between the substrate and the lower electrode, or an acoustic reflection film is provided on the opposite side of the lower electrode from the piezoelectric film, the acoustic reflection film reflecting elastic waves propagating through the piezoelectric film.

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

11. A multiplexer comprising the filter of claim 10.

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