Bulk acoustic wave resonator with recessed structure in piezoelectric layer, filter, and electronic device

Abstract

The present application relates to a bulk acoustic wave resonator, comprising: a substrate; an acoustic mirror; a bottom electrode disposed above the substrate; a top electrode; and a piezoelectric layer disposed above the bottom electrode and between the bottom electrode and the top electrode, wherein: an overlapping area of the acoustic mirror, the bottom electrode, the piezoelectric layer and the top electrode in a resonator thickness direction constitutes an effective area of the resonator; the piezoelectric layer is provided with a recess structure, the recess structure has an inner edge and an outer edge, and in a vertical projection, the inner edge of the recess structure coincides with or is outside the edge of the top electrode. The present application also relates to a filter having the above-mentioned resonator, and an electronic device having the above-mentioned resonator or filter.

Description

Technical Field

[0001] Embodiments of the present invention relate to the semiconductor field, and more particularly to a bulk acoustic resonator with a piezoelectric layer recessed structure, a filter having the resonator, and an electronic device having the resonator or the filter. Background Technology

[0002] In recent years, silicon-based semiconductor devices, especially integrated circuit chips, have developed rapidly and have firmly occupied the mainstream position in the industry. Thin-film bulk wave resonators, which utilize the longitudinal resonance of piezoelectric thin films in the thickness direction, have become a feasible alternative to surface acoustic wave devices and quartz crystal resonators in wireless communication systems.

[0003] like Figure 1 As shown, a film bulk acoustic resonator (FBAR) includes: a substrate P00, an acoustic reflection structure P10 (which can be a cavity, a Bragg reflector layer, or other equivalent structures) located on or embedded in the substrate, a bottom electrode P20 located above the acoustic reflection structure P10 and the substrate P00, a piezoelectric layer film P30 covering the bottom electrode P20 and the upper surface of the substrate P00, and a top electrode P40 located above the piezoelectric layer, etc., wherein the overlapping area of ​​the acoustic reflection structure P10, the bottom electrode P20, the piezoelectric layer P30, and the top electrode P40 in the thickness direction constitutes the effective acoustic region AR of the resonator, and the top electrode, the piezoelectric layer, and the bottom electrode constitute a sandwich structure.

[0004] When the bulk acoustic resonator is in ideal operating condition, only piston-mode acoustic waves propagate within the sandwich structure, and the energy of this vibration mode is confined within the effective acoustic region AR. However, in reality, the sandwich structure of the resonator contains not only piston-mode vibrations but also laterally propagating vibration modes. The energy of the latter dissipates laterally from the piezoelectric layer within the sandwich structure to the piezoelectric layer and other structures outside the sandwich structure (indicated by arrow PE), resulting in a decrease in the resonator's quality factor (Q value) and thus degrading the resonator's performance. Summary of the Invention

[0005] To alleviate or solve the above-mentioned problems in the prior art, the present invention is proposed.

[0006] According to one aspect of an embodiment of the present invention, a bulk acoustic resonator is provided, comprising:

[0007] Base;

[0008] Acoustic mirror;

[0009] The bottom electrode is positioned above the substrate;

[0010] Top electrode, having an electrode connection portion; and

[0011] A piezoelectric layer is disposed above the bottom electrode and between the bottom electrode and the top electrode.

[0012] in:

[0013] The overlapping area of ​​the acoustic mirror, bottom electrode, piezoelectric layer and top electrode in the thickness direction of the resonator constitutes the effective area of ​​the resonator.

[0014] The piezoelectric layer is provided with a recessed structure, the recessed structure having an inner edge and an outer edge, and in vertical projection, the inner edge of the recessed structure coincides with the edge of the top electrode or is outside the edge of the top electrode.

[0015] Optionally, in the vertical projection, the inner edge of the recessed structure coincides with the edge of the acoustic mirror; or, in the vertical projection, the edge of the acoustic mirror is located within the recessed structure, or the outer edge of the recessed structure coincides with the edge of the acoustic mirror; or, in the vertical projection, the recessed structure is located between the edge of the acoustic mirror and the edge of the top electrode, or the inner edge of the recessed structure coincides with the edge of the top electrode. Further optionally, in the vertical projection, the radial distance X between the inner edge of the recessed structure and the edge of the top electrode is not greater than 10 μm. Even further, in the vertical projection, the radial distance X between the inner edge of the recessed structure and the edge of the top electrode is: 0 μm ≤ X ≤ 1 μm, or 3 μm ≤ X ≤ 4 μm, or 6 μm ≤ X ≤ 8 μm.

[0016] Optionally, the recessed structure is disposed on the upper side, the lower side, or between the upper and lower sides of the piezoelectric layer, or penetrates the piezoelectric layer in the thickness direction of the resonator.

[0017] Optionally, the recessed structure includes a single recess. The recess may be a stepped recess.

[0018] Optionally, the recessed structure has at least two recesses, and further, the at least two recesses are spaced apart from each other in the radial direction.

[0019] Optionally, in a vertical projection, the outer edge of the recessed structure is located inside the edge of the bottom electrode.

[0020] Optionally, in vertical projection, the outer edge of the recessed structure is located inside the edge of the acoustic mirror.

[0021] Optionally, the electrode connection portion is formed with a bridge portion; and the recessed structure is an annular recessed structure.

[0022] Optionally, the recessed structure is filled with a filler material. The filler material may be selected from the following materials: monocrystalline silicon, polycrystalline silicon, silicon dioxide, silicon nitride, silicon carbide, doped aluminum nitride, and metal oxides.

[0023] Embodiments of the present invention also relate to a filter, including the aforementioned bulk acoustic resonator.

[0024] Embodiments of the present invention also relate to an electronic device, including the filter or the resonator described above. Attached Figure Description

[0025] The following description and accompanying drawings will better aid in understanding these and other features and advantages of the various embodiments disclosed herein, wherein the same reference numerals in the drawings always denote the same parts, wherein:

[0026] Figure 1 This is a schematic cross-sectional view of a bulk acoustic resonator in the prior art.

[0027] Figure 2 This is a top view schematic diagram of a bulk acoustic resonator according to an exemplary embodiment of the present invention;

[0028] Figure 2A A schematic diagram illustrating the sound wave reflection effect of the recessed structure;

[0029] Figures 3A to 3L respectively along Figure 2 A partial cross-sectional view of the left side of boundary S1, obtained by cutting along A1-A2, according to an exemplary embodiment of the present invention;

[0030] Figures 4A to 4H respectively along Figure 2 A partial cross-sectional view of the portion to the right of boundary S2 obtained by cutting along A1-A2 according to an exemplary embodiment of the present invention;

[0031] Figure 5 This is a schematic diagram of a bulk acoustic resonator according to an exemplary embodiment of the present invention, wherein the width of the recessed structure is D1, the depth is H1, and the distance between the inner edge of the recessed structure and the edge of the top electrode is X1.

[0032] Figure 6 To illustrate the relationship between the parallel resonant impedance (Rp) and the radial distance X1 between the recessed structure and the edge of the top electrode;

[0033] Figure 7 To illustrate the relationship between the parallel resonant impedance (Rp) and the radial distance X1 between the recessed structure and the edge of the top electrode;

[0034] Figure 8 The dispersion curve of the S1 mode at the parallel resonant frequency of the bulk acoustic resonator. Detailed Implementation

[0035] The technical solution of the present invention will be further described in detail below through embodiments and in conjunction with the accompanying drawings. In this specification, the same or similar reference numerals indicate the same or similar components. The following description of the embodiments of the present invention with reference to the accompanying drawings is intended to explain the overall inventive concept of the present invention and should not be construed as a limitation thereof.

[0036] The following description, with reference to the accompanying drawings, exemplarily describes a bulk acoustic resonator with a piezoelectric layer and a recessed structure according to an embodiment of the present invention.

[0037] Figure 2 A top view schematic diagram of a bulk acoustic resonator of an exemplary embodiment of the present invention is provided, as follows: Figure 2 As shown, the resonator includes a substrate 00, a bottom electrode 20 on the substrate, a piezoelectric layer 30 on the bottom electrode and the substrate, a recessed structure 31 (the channel portion shown in shaded) on the upper surface of the piezoelectric layer, a top electrode 40 on the piezoelectric layer, and a pin (i.e., an electrode connection portion) 43 of the top electrode.

[0038] Figure 2 The acoustic reflection structure (acoustic mirror) and the pins of the bottom electrode located on the upper surface of the substrate are not shown.

[0039] The following reference Figure 2A Exemplary illustration of the function of the recessed structure. For example... Figure 2A As shown, the upper surface of the piezoelectric layer 30 has a recessed structure 31, which forms two acoustic impedance mismatch boundaries, B1 and B2, within the piezoelectric layer. When a sound wave propagates laterally from the effective acoustic region (not shown in the figure) located to the right of B1 to the B1 or B2 region, it will be reflected multiple times back to the effective region of the resonator at the mismatch interface. Under the superposition of sound wave interference, the leakage of sound wave energy is reduced, thereby improving the Q value of the resonator.

[0040] The embodiments of the present invention correspondingly propose the following technical solutions, such as... Figure 2 , Figures 3A to 3L as well as Figures 4A to 4H As shown:

[0041] A bulk acoustic resonator, comprising:

[0042] Base 00;

[0043] Acoustic mirror 10;

[0044] Bottom electrode 20 is disposed above substrate 00;

[0045] Top electrode 40, having electrode connection portion 43; and

[0046] The piezoelectric layer 30 is disposed above the bottom electrode and between the bottom electrode and the top electrode.

[0047] in:

[0048] The overlapping region of the acoustic mirror, bottom electrode, piezoelectric layer, and top electrode in the thickness direction of the resonator constitutes the effective region AR of the resonator (see [link]). Figure 1 );

[0049] The piezoelectric layer is provided with a recessed structure 31, which has an inner edge (the side of the recessed structure close to the effective area) and an outer edge (the side of the recessed structure away from the effective area). In vertical projection, the inner edge of the recessed structure coincides with the edge of the top electrode or is outside the edge of the top electrode.

[0050] In this invention, when the recessed structure is located outside or on the outer side of the top electrode, the processing technology is simpler compared to structures where the recess overlaps with the top electrode. (When the recessed structure is located on the outer side of the top electrode, only photolithography and etching steps are required, eliminating processes such as sacrificial layer filling, grinding, and sacrificial layer release.) This reduces the adverse effects of complex processes on the resonator structure and performance.

[0051] In this invention, the material of the substrate 00 can be, but is not limited to, single-crystal silicon, gallium arsenide, quartz, sapphire, silicon carbide, etc.

[0052] In this invention, the materials of electrodes 20 and 40 may be, but are not limited to, molybdenum, ruthenium, gold, aluminum, magnesium, tungsten, copper, titanium, iridium, osmium, chromium, or a composite or alloy of the above metals.

[0053] In this invention, the material of the piezoelectric layer 30 may be selected from, but is not limited to, aluminum nitride, zinc oxide, lead zirconate titanate (PZT), lithium niobate, etc. Optionally, a certain proportion of rare earth element impurities may be added to the material.

[0054] In this invention, the piezoelectric layer is a thin film with a thickness of less than 10 micrometers, has a single crystal or polycrystalline microstructure, and can be fabricated by sputtering or deposition processes.

[0055] In this invention, the acoustic mirror 10 is not limited to the acoustic mirror structure shown in the example.

[0056] Figure 3A For along Figure 2 A partial cross-sectional view of the left side of boundary S1 obtained by cutting A1-A2 according to an exemplary embodiment of the present invention.

[0057] Figure 3AIn the structure, the acoustic mirror (or acoustic reflection structure) 10 is located on the upper surface of the substrate 00 and has a left boundary C1. The top electrode 40 has a left boundary T1. A recessed structure 31 is embedded on the upper surface of the piezoelectric layer 30. The recessed structure is rectangular ABCD. It should be noted that the shape of the recessed structure 31 is not limited to this. Depending on the actual application or manufacturing process, it can be, for example, […]. Figure 3F The inverted trapezoidal cross section is shown.

[0058] The recessed structure 31 has a width W30 and a depth H30. Furthermore, in Figure 3A In the middle, the right side CD (inner edge) of the concave structure 31 coincides with the boundary C1.

[0059] The width of the recessed structure is W30 (see Figure 3A The value of ) ranges from 0.5 micrometers to 4 micrometers, and further from 1 to 3 micrometers. In addition to the above endpoint values, it can also be 2 micrometers; or it can be one-quarter of the wavelength of the Lamb wave of the S1 mode at the parallel resonant frequency or an odd multiple thereof.

[0060] The depth of the recessed structure is H30 (see Figure 3A The range of the piezoelectric layer thickness is 0.02 micrometers to 0.5 micrometers, further 0.1 micrometers to 0.3 micrometers, and in addition to the above endpoint values, it can also be 0.2 micrometers, or 5% to 100% of the piezoelectric layer thickness, further 10% to 40%, and in addition to the above endpoint values, it can also be 20%.

[0061] In this invention, the depth of the recessed structure is the maximum depth of the recessed structure; and the width of the recessed structure is the width of the top opening of the recessed structure.

[0062] The following is a brief explanation of the Lamb wave wavelength λ of the S1 mode at the parallel resonant frequency of the resonator. When the bulk acoustic resonator is operating, a large number of vibrations are generated in the sandwich structure. If these vibrations are plotted as dispersion curves according to the relationship between their frequency (f) and wavenumber (k), curves for multiple modes can be obtained. The curve for one of these modes is called the S1 mode (the curves for the other modes are not shown in the diagram). Figure 8 (as shown in the image), it has Figure 8 The figure shows the dispersion curve, where the horizontal axis represents the wave number and the vertical axis represents the vibration frequency. The vibration frequency is the parallel resonant frequency f. p When the wave number is k, p The wavelength λ of the S1 mode is defined by the following equation:

[0063] exist Figure 3A In the vertical projection, the inner edge of the recessed structure coincides with the edge of the acoustic mirror; however, the recessed structure can also be located in other positions.

[0064] like Figure 3BAs shown, in the vertical projection, the edge of the acoustic mirror is located within the recessed structure.

[0065] like Figure 3C As shown, in the vertical projection, the outer edge of the recessed structure coincides with the edge of the acoustic mirror.

[0066] like Figure 3D As shown, in vertical projection, the recessed structure is located between the edge of the acoustic mirror and the edge of the top electrode.

[0067] like Figure 3E As shown, in the vertical projection, the inner edge of the recessed structure coincides with the edge of the acoustic mirror.

[0068] In addition, such as Figure 3G As shown, the recessed structure can also be filled with other materials, such as non-metals like silicon dioxide, silicon carbide, silicon nitride, etc., or metals like titanium, molybdenum, magnesium, aluminum, etc.

[0069] Figure 5 This is a schematic diagram of a bulk acoustic resonator according to an exemplary embodiment of the present invention, wherein the width of the recessed structure is D1, the depth is H1, and the radial distance between the inner edge of the recessed structure and the edge of the top electrode is X1. Figure 6 and Figure 7 The graphs show the relationship between the parallel resonant impedance (Rp) and the radial distance X1 between the edge of the recessed structure and the top electrode.

[0070] exist Figure 6 In this model, X1 varies from 0 to 7 micrometers, with each variation increment being 0.5 micrometers. The other two parameters, D1 and H1, are fixed at three sets. Each time X1 changes, D1 and H1 remain constant. Specifically... Figure 6 The following three sets of change data are shown:

[0071] (1) D1 = 1.5um, H1 = 1000A, data on the variation of parallel resonant impedance Rp1 with X1.

[0072] (2) D1 = 2.5um, H1 = 1000A, data on the variation of parallel resonant impedance Rp2 with X1.

[0073] (3) D1=3um, H1=1000A, data on the variation of parallel resonant impedance Rp3 with X1.

[0074] By comparing the above data with the known parallel resonant impedance Rp0 of a resonator without a recessed structure and plotting the result, we can obtain... Figure 6 The curve shown (a higher Rp value indicates a higher Q value for the resonator, and better performance) is derived from... Figure 6The results show that the resonator with the recessed structure outperforms the conventional resonator without the recessed structure in terms of Q value, over most of the X1 range. Furthermore, the recessed structure significantly improves the Q value of the resonator in certain X1 ranges, such as at X1 = 0 micrometers and near X1 = 3 micrometers.

[0075] based on Figure 6 In the embodiments of the present invention, the value of X1 is in the range of 0-10 micrometers, and more specifically, X1 is 0-0.5 micrometers or 3-3.5 micrometers.

[0076] exist Figure 7 In this model, X1 varies from 0 to 5.5 micrometers, with each variation increment being 0.5 micrometers. The other two parameters, D1 and H1, are fixed at three sets. Each time X1 changes, D1 and H1 remain constant. Specifically... Figure 7 The following three sets of change data are shown:

[0077] (1) D1=1um, H1=1000A, data on the variation of parallel resonant impedance Rp4 with X1.

[0078] (2) D1 = 1um, H1 = 2000A, data on the variation of parallel resonant impedance Rp5 with X1.

[0079] (3) D1=1um, H1=3000A, data on the variation of parallel resonant impedance Rp6 with X1.

[0080] By comparing the above data with the known parallel resonant impedance Rp0 of a resonator without a recessed structure and plotting the result, we can obtain... Figure 7 The curve shown (a higher Rp value indicates a higher Q value for the resonator and better performance).

[0081] Depend on Figure 7 The results show that the resonator with the recessed structure outperforms the conventional resonator without the recessed structure in terms of Q value, over most of the X1 range. Furthermore, the recessed structure significantly improves the Q value of the resonator in certain X1 ranges, such as at X1 = 0 micrometers and near X1 = 3.5 micrometers.

[0082] based on Figure 7 In the embodiments of the present invention, the value of X1 is in the range of 0-5.5 micrometers, further X1 is 0-0.5 micrometers, or X1 is 3-4 micrometers.

[0083] from Figure 6 and Figure 7 It can be seen that the value of X1 has a significant impact on the Q value of the resonator.

[0084] Based on the above, the value range of X1 can be 0μm≤X1≤0.5μm, or 3μm≤X1≤4μm.

[0085] Based on the above, in the vertical projection, the radial distance X between the inner edge of the recessed structure and the edge of the top electrode is: 0μm≤X≤1μm, or 3μm≤X≤4μm, or 6μm≤X≤8μm.

[0086] It should be noted that the recessed structure is not limited to being disposed on the upper side of the piezoelectric layer (e.g., Figure 3B As shown), it can also be placed on the underside of the piezoelectric layer (e.g. Figure 3I As shown), or between the upper and lower sides (as shown). Figure 3H As shown), or penetrating the piezoelectric layer in the thickness direction of the resonator (as shown). Figure 3J (As shown).

[0087] In addition, see Figure 3K The recessed structure can also be a stepped recess. Specifically, the recessed structure 31 has components with different depths. The stepped recess not only increases the number of acoustic impedance mismatch boundaries but also enriches the reflected wavelengths.

[0088] exist Figures 3A to 3K In the example, the recessed structure is a single recessed structure, but the invention is not limited thereto. See also Figure 3L The recessed structure may also include at least two recesses. Figure 3L In the example, the two recesses 31 and 32 are spaced apart from each other by a distance W33 in the radial direction. It should be noted that the widths W31 and W32 of the recesses 31 and 32 can be the same or different; in addition, the depths H31 and H32 of the two recesses can also be different from each other.

[0089] Figure 4A For along Figure 2 A partial cross-sectional view of the right side of boundary S2, obtained by sectioning A1-A2 according to an exemplary embodiment of the present invention. As shown, the electrode connection portion 43 is formed with a bridge portion (i.e., the arched portion in the figure); and the recessed structure 31 is an annular recessed structure (see Figure 1). Figure 2 (The ring shape in the middle).

[0090] like Figure 4A As shown, the acoustic mirror 10 has a right boundary C2, the top electrode 40 has a right boundary T2, the top electrode has an electrode connection structure (i.e., a pin) 43, the electrode connection structure 43 has an arched bridge structure, and the upper surface of the piezoelectric layer 30 is provided with a recessed structure 31. The left edge of the recessed structure 31 (the inner edge of the recessed structure) coincides with the boundary C2.

[0091] exist Figure 4AIn this case, the inner edge of the recessed structure coincides with the edge of the acoustic mirror; however, the recessed structure can also be located in other positions.

[0092] like Figure 4B As shown, the vertical projection of the edge of the acoustic mirror 10 is located within the recessed structure 31.

[0093] like Figure 4C As shown, the vertical projection of the outer edge of the recessed structure coincides with the edge of the acoustic mirror.

[0094] like Figure 4D As shown, the recessed structure is located between the edge of the acoustic mirror and the edge of the top electrode.

[0095] like Figure 4E As shown, the vertical projection of the inner edge of the recessed structure coincides with the edge of the acoustic mirror.

[0096] Furthermore, although not shown, the inner edge of the recessed structure may be located outside the edge of the acoustic mirror.

[0097] See Figures 3A-3L In an optional embodiment, in vertical projection, the outer edge of the recessed structure is located inside the edge of the bottom electrode; or the outer edge of the recessed structure is located inside the edge of the acoustic mirror.

[0098] In this invention, the term "vertical projection" is used, as shown in the appendix. Figure 3A As shown, this should be understood as a projection along the thickness direction of the resonator, for example, in Figure 3A In this invention, the dashed lines or boundaries C1 and T1 can also be considered as vertical projection lines. The term "coincidence" in this invention refers to points on the same vertical projection line, or points substantially on the same vertical projection line. The term "edge" in this invention refers to the outermost or innermost edge of the corresponding component.

[0099] Although not shown, embodiments of the present invention also relate to a filter including the aforementioned bulk acoustic resonator.

[0100] Embodiments of the present invention also relate to an electronic device, including the resonator or the filter described above.

[0101] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that variations may be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A bulk acoustic resonator, comprising: Base; Acoustic mirror; The bottom electrode is positioned above the substrate; The top electrode has an electrode connection portion; and A piezoelectric layer is disposed above the bottom electrode and between the bottom electrode and the top electrode. in: The overlapping area of ​​the acoustic mirror, bottom electrode, piezoelectric layer and top electrode in the thickness direction of the resonator constitutes the effective area of ​​the resonator. The piezoelectric layer is provided with a recessed structure, the recessed structure having an inner edge and an outer edge, and in vertical projection, the inner edge of the recessed structure coincides with the edge of the top electrode or is outside the edge of the top electrode. The recessed structure includes at least two recesses, at least some of which are connected or spaced apart in the radial direction, and the at least two recesses are located on the same side of the top electrode to increase the number of acoustic impedance mismatch boundaries on the same side of the top electrode.

2. The resonator according to claim 1, wherein: In vertical projection, the inner edge of the recessed structure coincides with the edge of the acoustic mirror; or In vertical projection, the edge of the acoustic mirror is located within the recessed structure, or the outer edge of the recessed structure coincides with the edge of the acoustic mirror; or In vertical projection, the recessed structure is located between the edge of the acoustic mirror and the edge of the top electrode, or the inner edge of the recessed structure coincides with the edge of the top electrode.

3. The resonator according to claim 2, wherein: In vertical projection, the radial distance X between the inner edge of the recessed structure and the edge of the top electrode is no greater than 10 μm.

4. The resonator according to claim 3, wherein: In the vertical projection, the radial distance X between the inner edge of the recessed structure and the edge of the top electrode is: 0μm≤X≤1μm, or 3μm≤X≤4μm, or 6μm≤X≤8μm.

5. The resonator according to claim 1, wherein: The recessed structure is filled with a filling material.

6. The resonator according to claim 5, wherein: The filling material is selected from the following materials: monocrystalline silicon, polycrystalline silicon, silicon dioxide, silicon nitride, silicon carbide, doped aluminum nitride, and metal oxides.

7. The resonator according to claim 1, wherein: The recessed structure is disposed on the upper side, lower side, or between the upper and lower sides of the piezoelectric layer, or penetrates the piezoelectric layer in the thickness direction of the resonator.

8. The resonator according to claim 1, wherein: The depression is a stepped depression.

9. The resonator according to claim 1, wherein: In vertical projection, the outer edge of the recessed structure is located inside the edge of the bottom electrode.

10. The resonator according to claim 1, wherein: In vertical projection, the outer edge of the recessed structure is located inside the edge of the acoustic mirror.

11. The resonator according to claim 1, wherein: The electrode connection portion has a bridge portion; and The recessed structure is a ring-shaped recessed structure.

12. The resonator according to claim 1, wherein: The width of the recessed structure ranges from 0.5 μm to 4 μm, or is one-quarter of the wavelength of the Lamb wave in the S1 mode at the parallel resonant frequency, or an odd multiple thereof; and The depth of the recessed structure ranges from 0.02 μm to 0.5 μm, or is 5% to 100% of the thickness of the piezoelectric layer.

13. The resonator according to claim 12, wherein: The depth of the recessed structure ranges from 10% to 40% of the thickness of the piezoelectric layer.

14. A filter comprising a bulk acoustic resonator according to any one of claims 1-13.

15. An electronic device comprising the filter according to claim 14 or the resonator according to any one of claims 1-13.

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