Upper electrode with step arrangement for bulk acoustic wave resonator - Patents.com

Abstract

To provide high quality factor and suppression of spurious modes, and also provide weakened BO modes that are shifted further away from a passband.SOLUTION: A bulk acoustic wave (BAW) resonator 38 includes a piezoelectric layer 18 on a lower electrode 22, an inner step 40 that forms an upper electrode 20 that includes a central region 36 that forms a first height H1 from the piezoelectric layer 18, and a boundary (BO) region 32 extending around the periphery of the central region 36, and in which the BO region 32 forms second height H2 from the piezoelectric layer 18, which is higher than the first height H1 of the central region 36, an outer step 42 that forms a third height H3 from the piezoelectric layer 18, which is higher than the first height H1 of the central region 36, an upper electrode 20, and a dielectric spacer layer 44 disposed between the outer step 42 and the piezoelectric layer 18.SELECTED DRAWING: Figure 8

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of Provisional Patent Application No. 62 / 792,113, filed January 14, 2019, the disclosure of which is incorporated herein by reference in its entirety.

[0002] The present disclosure relates to bulk acoustic wave (BAW) resonators, and more particularly to a top electrode with a step arrangement for a BAW resonator. [Background technology]

[0003] Acoustic resonators, especially bulk acoustic wave (BAW) resonators, are used in many high frequency communication applications. In particular, BAW resonators are often used in filter networks that operate at frequencies above 1.5 gigahertz (GHz), require a flat passband, have very steep filter skirts and squared shoulders at the upper and lower ends of the passband, and provide excellent rejection outside the passband. BAW-based filters also have relatively low insertion loss, tend to be small in size as the operating frequency increases, and are relatively stable over a wide temperature range. As such, BAW-based filters are the filters of choice for many third generation (3G) and fourth generation (4G) wireless devices, and are set to dominate filter applications in fifth generation (5G) wireless devices. Most of these wireless devices pose very stringent filtering requirements because they support cellular, wireless fidelity (Wi-Fi), Bluetooth, and / or near-field wireless communications on the same wireless device. As these demands continue to increase the complexity of wireless devices, there is a constant need to improve the performance and reduce the cost and size associated with BAW resonators and BAW-based filters. Summary of the Invention

[0004] The present disclosure relates to bulk acoustic wave (BAW) resonators, and in particular to a top electrode with a step arrangement for a BAW resonator. The BAW resonator disclosed herein includes a piezoelectric layer disposed between a top electrode and a bottom electrode. The top electrode is disclosed to include a boundary (BO) region with a dual step arrangement, with an inner step and an outer step formed to increase in height toward a peripheral edge of the top electrode. A dielectric spacer layer may be provided between the outer step and the piezoelectric layer. A passivation layer is disclosed extending on the top electrode either to the peripheral edge of the piezoelectric layer or from the peripheral edge of the piezoelectric layer. The piezoelectric layer may be disposed with a reduced thickness in the area not covered by the top electrode. A BAW resonator as disclosed herein is provided with a high quality factor and suppression of spurious modes, while also providing weakened BO modes that are shifted further away from the passband of such BAW resonators.

[0005] In one aspect, the BAW resonator includes a bottom electrode, a piezoelectric layer on the bottom electrode, and a top electrode on the piezoelectric layer, the top electrode forming a central region forming a first height from the piezoelectric layer, and a BO region extending around a periphery of the central region, the BO region including an inner step on the piezoelectric layer forming a second height from the piezoelectric layer that is higher than the first height in the central region, an outer step on the piezoelectric layer forming a third height from the piezoelectric layer that is higher than the first height in the central region, and a dielectric spacer layer disposed between the outer step and the piezoelectric layer. In certain embodiments, the third height is higher than the second height. In certain embodiments, the inner step and the outer step form a mass load portion of the top electrode. The dielectric spacer layer may include at least one of silicon dioxide, silicon nitride, or aluminum nitride. In certain embodiments, the bottom electrode is disposed to extend along a bottom of the piezoelectric layer a distance in the range of 200 nm to 1 μm beyond the lateral edges of the top electrode on top of the piezoelectric layer. The BAW resonator may further include a passivation layer on the top electrode. In certain embodiments, the passivation layer extends to a peripheral edge of the piezoelectric layer. In certain embodiments, the passivation layer extends to a peripheral edge of the outer step, the passivation layer being inserted from the peripheral edge of the piezoelectric layer. In certain embodiments, the top electrode forms an inner ring forming a fourth height from the piezoelectric layer, the fourth height being lower than the first height of the central region. In certain embodiments, the inner ring is spaced from the BO ring by a gap portion of the top electrode. In certain embodiments, the outer step includes a first outer step, and the BAW resonator further includes a second outer step, whereby the second outer step, the first outer step, and the inner step are arranged such that they form a step structure of a height that descends from the BO region toward the central region. The BAW resonator may include a rigidly mounted resonator (SMR) type BAW resonator, or a film bulk acoustic resonator (FBAR).

[0006] In another aspect, the BAW resonator includes a bottom electrode, a piezoelectric layer on the bottom electrode, and a top electrode on the piezoelectric layer, the top electrode forming a BO region extending around a central region and a periphery of the central region, the BO region including an inner step and an outer step, the inner step being disposed between the central region and the outer step, the outer region being defined by a peripheral boundary of the top electrode, and at least a portion of the piezoelectric layer in the outer region forming a reduced thickness compared to a portion of the piezoelectric layer aligned with the top electrode. In a particular embodiment, the central region forms a first height from the piezoelectric layer, the inner step forms a second height from the piezoelectric layer, and the outer step forms a third height from the piezoelectric layer, the second height and the third height being greater than the first height. The BAW resonator may further include a dielectric spacer layer disposed between the outer step and the piezoelectric layer. In a particular embodiment, the bottom electrode overlaps the portion of the piezoelectric layer forming the reduced thickness. The BAW resonator may further include a passivation layer on the top electrode. In certain embodiments, the passivation layer extends to a peripheral edge of the piezoelectric layer. In certain embodiments, the top electrode forms an inner ring that includes a mass-reduced region of the top electrode. In certain embodiments, the inner ring is spaced from the BO region by a gap portion of the top electrode. In certain embodiments, the inner ring is directly adjacent to the BO region of the top electrode. The BAW resonator may include an SMR-type BAW resonator or an FBAR.

[0007] In another embodiment, any of the above-mentioned embodiments and / or various separate embodiments and features as described herein may be combined for additional advantage. Any of the various features and elements as disclosed herein may be combined with one or more of the other disclosed features and elements, unless otherwise indicated herein.

[0008] Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in conjunction with the accompanying drawing figures. [Brief description of the drawings]

[0009] The accompanying drawings, which are incorporated in and form a part of this specification, illustrate several aspects of the present disclosure and, together with the description, serve to explain the principles of the disclosure.

[0010] [Figure 1] FIG. 1 illustrates a conventional bulk acoustic wave (BAW) resonator. [Diagram 2] FIG. 2 is a graphical depiction of the magnitude and phase of the electrical impedance as a function of frequency for a relatively ideal BAW resonator. [Figure 3A] FIG. 1 is a graphical representation of phase curves for various conventional BAW resonators. [Figure 3B] FIG. 1 is a graphical representation of phase curves for various conventional BAW resonators. [Figure 3C] FIG. 1 is a graphical representation of phase curves for various conventional BAW resonators. [Figure 4] FIG. 1 shows a conventional BAW resonator with a top electrode that includes a boundary (BO) ring. [Diagram 5] FIG. 13 is a graphical representation of the relationship between the anti-resonant frequency (Qp) and the quality factor at the relative strength of the formed BO modes versus the BO ring width. [Figure 6] FIG. 13 is a graphical representation of phase curves for a BAW resonator with and without a BO ring. [Figure 7] 5 is a cross-sectional view showing a BAW resonator in which a BO region is arranged in a similar configuration to the BAW resonator in FIG. 4. [Figure 8] 1A is a cross-sectional view of a BAW resonator in which the BO region forms a dual-step configuration around the periphery of the top electrode according to an embodiment disclosed herein. [Figure 9] 9 is a cross-sectional view showing a BAW resonator arranged similarly to the BAW resonator of FIG. 8, but having a larger area configuration of the central region of the top electrode according to embodiments disclosed herein. [Figure 10] 1A is a cross-sectional view of a portion of a BAW resonator including a passivation layer on a top electrode according to an embodiment disclosed herein. [Figure 11] FIG. 11 is a graphical illustration of simulation results showing the Qp response for various configurations of the BAW resonator as shown in FIG. [Figure 12] 1A is a cross-sectional view of a portion of a BAW resonator in which a passivation layer is inserted from a peripheral edge of a piezoelectric layer according to an embodiment disclosed herein. [Figure 13] FIG. 13 is a graphical illustration of simulation results showing the Qp response for various configurations of the BAW resonator as shown in FIG. [Figure 14] 1A is a cross-sectional view illustrating a portion of a BAW resonator in which a portion of the piezoelectric layer in an outer region is disposed at a reduced height according to an embodiment disclosed herein. [Figure 15] 14 is a graphical illustration of simulation results showing the Qp value response for the BAW resonator configuration of FIG. 13 compared to the BAW resonator of FIG. 8. [Figure 16A] FIG. 12 graphically illustrates a comparison of simulation results for the Qp value of various configurations of the BAW resonator of FIG. [Figure 16B] 16B is a graphical comparison of various performance characteristics of the same BAW resonator as in FIG. 16A. [Figure 16C] 16B is a graphical comparison of various performance characteristics of the same BAW resonator as in FIG. 16A. [Figure 16D] 16B is a graphical comparison of various performance characteristics of the same BAW resonator as in FIG. 16A. [Figure 17] 1 illustrates a cross-sectional view of a BAW resonator having a reduced-mass region disposed at an inner region of a top electrode according to an embodiment disclosed herein. [Figure 18] 1A is a cross-sectional view of a BAW resonator in which the top electrode includes multiple outer steps according to an embodiment disclosed herein. [Figure 19]1 illustrates a cross-sectional view of an exemplary strongly mounted resonator (SMR) type BAW resonator according to embodiments disclosed herein. [Figure 20] FIG. 1 illustrates a cross-sectional view of an exemplary film bulk acoustic resonator (FBAR) type BAW resonator according to embodiments disclosed herein. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The embodiments described below represent the necessary information to enable those skilled in the art to practice the embodiments and show the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawings, those skilled in the art will understand the concepts of the present disclosure and recognize applications of these concepts not specifically addressed herein. It is understood that these concepts and applications fall within the scope of the present disclosure and the appended claims.

[0012] It should be understood that, although terms such as first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are used only to distinguish one element from another. For example, a first element can be called a second element, and similarly, a second element can be called a first element, without departing from the scope of the present disclosure. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0013] It will be understood that when an element, such as a layer, region, or substrate, is referred to as being "on" or extending "onto" another element, the element can be directly on or extending directly onto the other element, or intervening elements may be present. In contrast, when an element is referred to as being "directly on" or extending "directly onto" another element, there are no intervening elements present. Similarly, it will be understood that when an element, such as a layer, region, or substrate, is referred to as being "over" or extending "over" another element, the element can be directly on or extending directly onto the other element, or intervening elements may be present. In contrast, when an element is referred to as being "directly over" or extending "directly over" another element, there are no intervening elements present. It should also be understood that when an element is referred to as being "connected" or "coupled" to another element, it may be directly connected or coupled to the other element, or there may be intervening elements present. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no intervening elements present.

[0014] Relative terms such as "lower" or "upper" or "upper" or "lower" or "horizontal" or "vertical" may be used herein to describe the relationship of one element, layer, or region to another element, layer, or region as shown in the figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the figures.

[0015] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the disclosure. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms unless the context clearly indicates otherwise. It is further understood that as used herein, the terms "comprises", "comprising", "includes" and / or "including" specify the presence of stated features, integers, steps, operations, elements and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof.

[0016] Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by those skilled in the art to which this disclosure belongs. It should be further understood that the terms used herein should be interpreted as having a meaning consistent with their meaning in the context of this specification and related art, and will not be interpreted in an idealized or overly formal sense unless expressly defined herein.

[0017] The present disclosure relates to bulk acoustic wave (BAW) resonators, and in particular to a top electrode with a step arrangement for a BAW resonator. The BAW resonator disclosed herein includes a piezoelectric layer disposed between a top electrode and a bottom electrode. The top electrode is disclosed to include a boundary (BO) region with a dual step arrangement, with an inner step and an outer step formed to increase in height toward a peripheral edge of the top electrode. A dielectric spacer layer may be provided between the outer step and the piezoelectric layer. A passivation layer is disclosed extending on the top electrode either to the peripheral edge of the piezoelectric layer or from the peripheral edge of the piezoelectric layer. The piezoelectric layer may be disposed with a reduced thickness in the area not covered by the top electrode. A BAW resonator as disclosed herein is provided with a high quality factor and suppression of spurious modes, while also providing weakened BO modes that are shifted further away from the passband of such BAW resonators.

[0018] Before delving into the details of these concepts, an overview of BAW resonators and filters utilizing BAW resonators will be provided. BAW resonators are used in many high frequency filter applications. An exemplary BAW resonator 10 is shown in FIG. 1. The BAW resonator 10 is a solidly mounted resonator (SMR) type BAW resonator 10 and generally includes a substrate 12, a reflector 14 mounted on the substrate 12, and a transducer 16 mounted on the reflector 14. The transducer 16 includes a piezoelectric layer 18 on the reflector 14 and sandwiched between a top electrode 20 and a bottom electrode 22. The top electrode 20 and the bottom electrode 22 may be formed of materials such as tungsten (W), molybdenum (Mo), platinum (Pt), and the piezoelectric layer 18 may be formed of aluminum nitride (AlN), zinc oxide (ZnO), or other suitable piezoelectric materials. Although shown in FIG. 1 as including a single layer, piezoelectric layer 18, top electrode 20, and / or bottom electrode 22 may include multiple layers of the same material, multiple layers where at least two layers are different materials, or multiple layers where each layer is a different material.

[0019] BAW resonator 10 is divided into an active region 24 and an outer region 26. Active region 24 generally corresponds to the section of BAW resonator 10 where top electrode 20 and bottom electrode 22 overlap, including layers below overlapping top electrode 20 and bottom electrode 22. Outer region 26 corresponds to the section of BAW resonator 10 that surrounds active region 24.

[0020] In the BAW resonator 10, application of an electrical signal across the top electrode 20 and bottom electrode 22 excites acoustic waves in the piezoelectric layer 18. These acoustic waves propagate primarily vertically. A primary goal in the design of a BAW resonator is to confine these vertically propagating acoustic waves within the transducer 16. Upwardly propagating acoustic waves are reflected back into the transducer 16 by the air-metal boundary on top of the top electrode 20. Downwardly propagating acoustic waves are reflected back into the transducer 16 by the reflector 14 or air cavity located directly below the film bulk acoustic resonator (FBAR) transducer 16.

[0021] The reflector 14 is typically formed by a stack of reflector layers (RL) 28A-28E (commonly referred to as reflector layers 28) that alternate in material composition to produce a significant reflection coefficient at the junction of adjacent reflector layers 28. Typically, the reflector layers 28 alternate between materials having high acoustic impedance and materials having low acoustic impedance, such as tungsten (W) and silicon dioxide (SiO2). Although only five reflector layers 28 are shown in FIG. 1, the number of reflector layers 28 and the structure of the reflector 14 will vary from design to design.

[0022] The magnitude (Z) and phase (φ) of the electrical impedance as a function of frequency (GHz) for a relatively ideal BAW resonator 10 are provided in FIG. 2. The magnitude (Z) of the electrical impedance is shown as a solid line, while the phase (φ) of the electrical impedance is shown as a dashed line. A unique feature of the BAW resonator 10 is that it has both a resonant frequency and an anti-resonant frequency. The resonant frequency is typically the series resonant frequency (f s) and the anti-resonance frequency is usually called the parallel resonance frequency (f p The series resonance frequency (f s ) occurs when the magnitude of the impedance or reactance of the BAW resonator 10 approaches zero. p ) occurs when the magnitude of the impedance or reactance of the BAW resonator 10 peaks at a very high level. In general, the series resonant frequency (f s ) is a function of the thickness or height of the piezoelectric layer 18 and the mass of the bottom and top electrodes 20, 22.

[0023] In terms of phase, the BAW resonator 10 has a series resonant frequency (f s ) and parallel resonance frequency (f p ) in the series resonant frequency (f s ) and the parallel resonant frequency (f p ), the BAW resonator 10 acts like a capacitance, providing a phase shift of −90°. s ) and exhibits very low, almost zero resistance at the parallel resonant frequency (f p ) The electrical properties of the BAW resonator 10 lend themselves to providing a very high quality factor (Q) inductance over a relatively short frequency range, which has proven to be very beneficial in high frequency filter networks, particularly those operating at frequencies above about 1.8 GHz.

[0024] Unfortunately, the phase (φ) curve in FIG. 2 represents an ideal phase curve. In practice, it is difficult to approach this ideal. A typical phase curve for the BAW resonator 10 of FIG. 1 is shown in FIG. 3A. Instead of being a smooth curve, the phase curve in FIG. 3A has a series resonant frequency (f s ) below the series resonant frequency (f s ) and parallel resonance frequency (f p ) and the parallel resonant frequency (f p) which are the result of spurious modes caused by spurious resonances occurring at corresponding frequencies. Although the majority of acoustic waves in the BAW resonator 10 propagate vertically, various boundary conditions on the transducer 16 result in the propagation of transverse (horizontal) acoustic waves, referred to as transverse standing waves. The presence of these transverse standing waves reduces the potential quality factor (Q) associated with the BAW resonator 10.

[0025] As shown in FIG. 4, a BO ring 30 is formed on or in the upper electrode 20 to suppress certain spurious modes. The spurious modes suppressed by the BO ring 30 are at the series resonance frequency (f s ) higher than the series resonant frequency (f s ) and parallel resonance frequency (f p ), and thus the suppression of spurious modes. Circle B is the parallel resonant frequency (f p ), and hence the suppression of spurious modes. In particular, the parallel resonant frequency (f p The spurious modes in the upper shoulder of the passband, just below the series resonant frequency (f s ) and parallel resonance frequency (f p ) and the parallel resonant frequency (f p ) as evidenced by a smooth or substantially ripple-free phase curve above

[0026] The BO ring 30 corresponds to a mass loading of a portion of the top electrode 20 that extends around the periphery of the active region 24. In this regard, the BO ring 30 with its mass loading forms a raised frame disposed around the periphery of the top electrode 20. The BO ring 30 may correspond to a thickened portion of the top electrode 20 or the application of an additional layer of a suitable material on the top electrode 20. The portion of the BAW resonator 10 that includes the BO ring 30 and is below it is referred to as the BO region 32. The BO region 32 thus corresponds to an outer peripheral portion of the active region 24 and is inside the active region 24.

[0027] The BO ring 30 has a series resonance frequency (f s ), but as shown in FIG. 3B, the BO ring 30 is effective in suppressing spurious modes above the series resonant frequency (f s ) has little or no effect on spurious modes below the series resonant frequency (f s ) to suppress spurious modes below

[0028] Apodization serves to avoid, or at least significantly reduce, any lateral symmetry in the BAW resonator 10, or at least its transducer 16. Lateral symmetry corresponds to the footprint of the transducer 16, and avoiding lateral symmetry corresponds to avoiding symmetry associated with the sides of the footprint. For example, one could choose a footprint that corresponds to a pentagon, instead of a square or rectangle. Avoiding symmetry helps to reduce the presence of lateral standing waves in the transducer 16. Circle C in FIG. 3C shows the series resonance frequency (f s ) is suppressed. Assuming that the BO ring 30 is not provided, in FIG. 3C, the apodization suppresses the spurious modes below the series resonant frequency (f sIt is easy to see that these spurious modes, which are above the apodization frequency, cannot be suppressed. Therefore, a typical BAW resonator 10 employs both apodization and a BO ring 30.

[0029] The thickness or height of the BO ring 30 may be measured perpendicular to or away from the piezoelectric layer 18, and the width of the BO ring 30 may be measured parallel to or laterally across the piezoelectric layer 18. The thickness and width of the BO ring 30 provide suppression of spurious modes and control the anti-resonant frequency (Q p ) can be simultaneously tuned to provide an improved quality factor at the BO ring 30. The additional mass associated with the BO ring 30 typically causes the BO region 32 to resonate at a slightly lower frequency than the rest of the active region 24. As a result, the presence of the BO ring reduces the series resonance frequency (f s ) can introduce unwanted modes or BO modes.

[0030] Figure 5 shows the anti-resonance frequency (Q p 5 is a graphical representation of the relationship between the BO ring width and the quality factor in terms of the relative intensity of the formed BO modes. In FIG. 5, the x-axis represents the BO ring width in micrometers (μm), while the linear y-axis represents the Q p 5, where the secondary y-axis represents the relative intensity or magnitude of the formed BO mode. The diagram in FIG. 5 plots data for three wafers, each containing BAW resonators with varying BO ring widths. As shown, Q p The values ​​generally increase with increasing BO ring width, and the highest Q p The values ​​correspond to a BO ring width of slightly more than 3 μm. As also shown, the intensity of the BO mode also generally increases with increasing BO ring width. For larger BO ring widths, the values ​​of the BO mode intensity remain high, but the Q p The value is decreasing. In this way, high Q pA BO ​​ring tuned to provide this can also introduce undesirable BO modes into the corresponding BAW device.

[0031] FIG. 6 is a graphical illustration of the phase curves of a BAW resonator with and without a BO ring. As shown, the spurious modes present within the passband of the BAW resonator without a BO ring are suppressed by adding a BO ring. However, the presence of the BO ring introduces undesirable BO modes below the passband. The BO modes may be introduced outside or even inside the passband of the resonator, limiting the design or use of the BAW resonator for wideband filtering applications. If the BO modes are within the passband, the insertion loss may be affected. The BO modes present outside the passband may be problematic in BAW multiplexing applications, where BAW filters of different frequency bands operate simultaneously. In such multiplexing applications, the BO modes of one BAW filter may fall within the passband of another BAW filter and introduce interference during multiplexing.

[0032] FIG. 7 is a cross-sectional view of a BAW resonator 34 in which the BO region 32 is disposed in a configuration similar to the BAW resonator 10 of FIG. 4. For illustrative purposes, FIG. 7 is a simplified diagram of the BAW resonator 34 and is not necessarily drawn to scale. However, it will be understood that the BAW resonator 34 may include many of the same components as the BAW resonator 10 of FIG. 4. As shown in FIG. 7, the BO region 32 is disposed around the periphery of the top electrode 20, with a central region 36 of the top electrode 20 defined laterally inward of the BO region 32. The BO ring 30 is formed on or within the top electrode 20 to suppress certain spurious modes, as described above with respect to FIG. 4. In FIG. 7, the active region 24 is shown as the section where the top electrode 20 and the bottom electrode 22 overlap on opposite sides of the piezoelectric layer 18. In this manner, the active region 24 generally corresponds to a portion of the piezoelectric layer 18 that is electrically driven to provide an electric field between the overlapping portions of the top electrode 20 and the bottom electrode 22. The outer region 26 generally corresponds to the portion of the piezoelectric layer 18 that is outside of the active region 24, and in this regard, the outer region 26 is generally not electrically driven. In the configuration of FIG. 7, the BO region 32 and corresponding BO ring 30 are disposed within the active region 24.

[0033] FIG. 8 is a cross-sectional view showing a BAW resonator 38 in which the BO region 32 forms a dual-step configuration around the periphery of the top electrode 20 according to an embodiment disclosed herein. For illustrative purposes, FIG. 8 is a simplified diagram of the BAW resonator 38 and is not necessarily drawn to scale. However, it is understood that the BAW resonator 38 may include many of the same components as the BAW resonator 10 of FIG. 4. The BO region 32 includes an inner step 40 and an outer step 42 that collectively form a stepped profile of raised height that decreases from the periphery of the BO region 32 toward the central region 36 of the top electrode 20. In a particular embodiment, the inner step 40 forms a first BO ring that is a first mass loading portion of the top electrode 20, and the outer step 42 forms a second BO ring that is a second mass loading portion of the top electrode 20. As shown in FIG. 8, the central region 36 forms a first height H1 of the top electrode 20 as measured from the piezoelectric layer 18, the inner step 40 forms a second height H2 as measured from the piezoelectric layer 18, and the outer step 42 forms a third height H3 as measured from the piezoelectric layer 18. In certain embodiments, the second height H2 and the third height H3 are both greater than the first height H1. In certain embodiments, the third height H3 is greater than the second height H2, which is greater than the first height H1, thereby forming a dual-step configuration of the BO region 32 that decreases in height toward the central region 36. The relative dimensions of the first height H1, the second height H2, and the third height H3, as well as the relative widths of the inner step 40 and the outer step 42, depend on the desired operating frequency for the BAW resonator 38. The BAW resonators disclosed herein may be suitable for operation in a variety of frequency ranges, including 1.5 GHz to 5.5 GHz. In certain embodiments, the second height H2 is greater than the first height H1 by a range of 40 nanometers (nm) to 80 nm. In certain embodiments, the third height H3 is greater than the first height H1 by a range of 50 nm to 200 nm. In this regard, the dual step configuration of the BO region 32 forms a dual frame structure disposed around the periphery of the top electrode 20.8, a BAW resonator as disclosed herein may have a configuration with additional steps, such as a three-step or four-step configuration, in which each additional step away from the central region 36 may be configured to increase in height.

[0034] By having a dual-step configuration in which the inner step 40 and the outer step 42 are disposed at different heights, the BO region 32 is configured to thereby provide two different acoustic impedances. This provides an acoustic impedance mismatch for Lamb waves propagating laterally within the BAW resonator 38, which would otherwise contribute to energy loss or energy leakage and a reduction in the Q factor of the BAW resonator 38. To achieve the desired large acoustic mismatch and corresponding large reflection coefficient, materials with large mass density are typically selected for the inner step 40 and the outer step 42. The height and width of each of the inner step 40 and the outer step 42 can be simultaneously adjusted to achieve high suppression of spurious modes and low anti-resonance frequency (Q p ) can be achieved. Thus, increasing the height of the inner step 40 and the outer step 42 can result in a decrease in the velocity of the laterally propagating Lamb waves and an increase in the average mass density. In this way, the dual-step configuration significantly reduces the lateral energy leakage as well as the mode conversion in the BO region 32, thereby providing a high Q factor for the BAW resonator 38. This allows the series resonant frequency (f s ) is provided, while weakening BO modes that are shifted further away from the passband of the BAW resonator 38 are also provided.

[0035] As further shown in FIG. 8, the BAW resonator 38 may include a spacer layer 44 in certain embodiments. The spacer layer 44 may be disposed between the outer step 42 and the piezoelectric layer 18. In this manner, the spacer layer 44 may be aligned with the outer step 42. In certain embodiments, the peripheral edge of the spacer layer 44 is aligned with the peripheral edge of the outer step 42. In certain embodiments, the spacer layer 44 may be formed of a material selected from the group consisting of silicon dioxide (SiO2), silicon nitride (SiN, Si x N y , Si3N4), aluminum nitride (AlN), or a combination thereof. Despite the presence of the spacer layer 44 between the outer step 42 and the piezoelectric layer 18, an electric field is still generated in the portion of the piezoelectric layer 18 aligned with the outer step 42 and the spacer layer 44. In certain embodiments, the dielectric nature of the spacer layer 44 serves to reduce some of the electric field in the piezoelectric layer 18 and reduces coupling in that region, thereby providing a reduction in the magnitude of the BO mode formed. The active region 24 of the BAW resonator 38 is formed where the top electrode 20 and the bottom electrode 22 overlap, and includes the outer step 42. In comparison to the BAW resonator 34 of FIG. 7, the active region 24 and the outer region 26 of the BAW resonator 38 may be configured with similar dimensions. In particular, the bottom electrode 22 may be disposed to extend laterally along the piezoelectric layer 18 such that the bottom electrode 22 extends beyond the peripheral boundary of the top electrode 20. As previously mentioned, the active region 24 is formed where the top electrode 20 and the bottom electrode 22 overlap. Thus, misalignment of the top electrode 20 and the bottom electrode 22 during fabrication can impair the performance of the BAW resonator 38. By disposing the bottom electrode 22 with a larger lateral dimension than the top electrode 20, the alignment tolerance of the placement of the top electrode 20 can be increased, thereby improving manufacturing tolerances. In certain embodiments, the bottom electrode 22 can extend along the bottom of the piezoelectric layer 18 beyond the lateral edges of the top electrode 20 on top of the piezoelectric layer 18 by a distance in the range of 200 nm to 1 μm.

[0036] FIG. 9 is a cross-sectional view of a BAW resonator 46 arranged similarly to the BAW resonator 38 of FIG. 8, but with a larger area configuration of the central region 36 of the top electrode 20 according to embodiments disclosed herein. For illustrative purposes, FIG. 9 is a simplified diagram of the BAW resonator 46 and is not necessarily drawn to scale. However, it is understood that the BAW resonator 46 may include many of the same components as the BAW resonator 10 of FIG. 4. In FIG. 9, the BO region 32 is configured with an inner step 40, an outer step 42, and a spacer layer 44 as described for FIG. 8. The top electrode 20 of the BAW resonator 46 is configured to extend a greater distance across the piezoelectric layer 18. Thus, the area of ​​the central region 36 bounded laterally by the BO region 32 is increased. Additionally, the area of ​​overlap between top electrode 20 and bottom electrode 22 is increased, thereby providing a larger active area 24' and smaller outer area 26' for BAW resonator 46 than BAW resonator 38 of Figure 8. For illustrative purposes, the relative sizes of active area 24 and outer area 26 of Figures 7 and 8 have been included in the diagram of Figure 9.

[0037] FIG. 10 is a cross-sectional view illustrating a portion of a BAW resonator 48 including a passivation layer 50 on the top electrode 20 according to an embodiment disclosed herein. For illustrative purposes, a close-up view of a portion of the BAW resonator 48 including the BO region 32 is also provided. As shown, the BAW resonator 48 includes a reflector 14 including a stack of reflector layers 28A-28E. In a particular embodiment, the reflector layers 28A, 28C, 28E include SiO2, while the reflector layers 28B, 28D include W. A thin layer of AlN may also be provided at one or more interfaces of the SiO2 / W. In some configurations, portions of the reflector layers 28A, 28C, 28E may be continuous beyond the peripheral edge of the bottom electrode 22. The passivation layer 50 is a silicon nitride (SiN, SiN) layer provided in a conformal arrangement over the top electrode 20 and the inner and outer steps 40, 42. x N y, Si3N4). In a particular embodiment, the passivation layer 50 extends to the peripheral edge of the piezoelectric layer 18 such that the passivation layer 50 is provided in a conformal arrangement on the top electrode 20 and on the portions of the piezoelectric layer 18 not covered by the top electrode 20 along the periphery of the BAW resonator 48. In this manner, the passivation layer 50 extends along the inner step 40, the outer step 42, and the sidewalls of the outer step 42. The top electrode 20 may include a single layer or multiple layers that collectively form the top electrode 20. In FIG. 10, the top electrode 20 includes multiple top electrode layers 20-1 to 20-3. In a particular embodiment, the top electrode layer 20-1 includes W, the top electrode layer 20-2 includes titanium tungsten alloy (TiW), and the top electrode layer 20-3 includes aluminum copper alloy (AlCu). As shown, the height or thickness of the top electrode layer 20-1 may be increased relative to the inner step 40 and the outer step 42.

[0038] Figure 11 shows the Q for various configurations of the BAW resonator shown in Figure 10. p11 is a graphical illustration of simulation results showing the response of values. Simulation data was collected for three different BAW resonator structures, including the BAW resonator 34 structure of FIG. 7, the BAW resonator 38 structure of FIG. 8, and the BAW resonator 46 structure of FIG. 9. Each of the structures was configured as a transmitter (TX) resonator operating in the Long Term Evolution (LTE) band 7, in an arrangement generally shown in FIG. 10. For the BAW resonators 38, 46 having a dual step configuration, for the simulations, the height or thickness of the top electrode layer 20-1 at the inner step 40 and the outer step 42 (e.g., above the spacer layer 44) was set to 80 nm, the height or thickness of the spacer layer 44 at the outer step 42 was set to 100 nm, and the width of the outer step 42 was set to 1.5 μm. The width of the inner step 40 was varied from less than 0.5 μm to 4 μm, as shown by the x-axis of FIG. 11. For comparison with the single step configuration of Figure 7, the BAW resonator 34 included the same dimensions as described for the simulations of the BAW resonators 38, 46, but the spacer layer 44 was omitted from the BAW resonator 34. In this regard, the inner step width plotted for the BAW resonator 34 in Figure 11 corresponds to a varied single raised step. As shown, the results for the BAW resonator 34 show the highest or optimal Q for an inner step width of about 3.5 μm. p value, and the second highest or second best Q for an inner step width of about 2.0 μm. p In particular, the dual-step configurations of the BAW resonators 38 and 46 have the highest or optimal Q values ​​for the narrower inner step width of about 1.0 μm, which is higher than either of the single-step configurations of the BAW resonator 34. p Furthermore, for an internal step width of about 1.0 μm, the Q values ​​corresponding to the leakage and scattering energies are pIn this regard, the dual-step configuration of the BAW resonators 38, 46 with the spacer layer (44 in Figs. 8 and 9) provides improved Q due to less lateral energy leakage into the outer region (26 in Figs. 8 and 9) and less mode conversion at the boundary with the BO region (32 in Figs. 8 and 9). p 12 is a cross-sectional view of a portion of a BAW resonator 52 in which a passivation layer 50 is inserted from a peripheral edge of the piezoelectric layer 18, according to an embodiment disclosed herein. The BAW resonator 52 includes a reflector 14 including a stack of reflector layers 28A-28E, a bottom electrode 22, a piezoelectric layer 18, a top electrode 20 with an inner step 40 and an outer step 42, a spacer layer 44, and a passivation layer 50, as described above with respect to FIG. 10. The top electrode 20 also includes top electrode layers 20-1-20-3. The passivation layer 50 is disposed in a conformal manner over the inner step 40 and the outer step 42 of the BO region 32. However, the passivation layer 50 does not extend to the peripheral edge of the piezoelectric layer 18, such that at least a portion of the piezoelectric layer 18 not covered by the top electrode 20 is devoid of the passivation layer 50. In this manner, the passivation layer 50 is inserted from the peripheral edge of the piezoelectric layer 18 in a cut-out arrangement on the piezoelectric layer 18. In certain embodiments, the passivation layer 50 may extend to the peripheral edge of the outer step 42 without extending beyond the outer step 42. In other embodiments, the passivation layer 50 may extend beyond the outer step 42 and over a portion of the piezoelectric layer 18 while still being inserted from the peripheral edge of the piezoelectric layer 18.

[0039] FIG. 13 shows the Q p13 is a graphical illustration of simulation results showing the response of values. Simulation data was collected for three different BAW resonator structures, including the structure of BAW resonator 34 of FIG. 7 with the arrangement of FIG. 10, the structure of BAW resonator 38 of FIG. 8 with the arrangement of FIG. 12, and the structure of BAW resonator 46 of FIG. 9 with the arrangement of FIG. 12. Each of the structures was configured as a TX resonator operating in LTE band 7, with an arrangement as generally shown in FIG. 12. For the dual-step configured BAW resonators 38, 46, for the simulations, the height or thickness of the top electrode layer 20-1 at the inner step 40 and the outer step 42 (e.g., above the spacer layer 44) was set to 80 nm, the height or thickness of the spacer layer 44 at the outer step 42 was set to 100 nm, and the width of the outer step 42 was set to 1.5 μm. The width of the inner step 40 was varied from less than 0.5 μm to 4 μm, as shown by the x-axis of FIG. 13. 7, the BAW resonator 34 included the same dimensions as described for the simulation of the BAW resonators 38, 46, but the spacer layer 44 was omitted from the BAW resonator 34. In this regard, the inner step width plotted for the BAW resonator 34 in FIG. 13 corresponds to a varied single raised step. By arranging the passivation layer 50 to be inserted from the peripheral edge of the piezoelectric layer 18, the dual step configuration of the BAW resonator 38 has its first optimum Q compared to FIG. p Moreover, the dual-step configuration of the BAW resonator 46 exhibits a higher overall Q value compared to FIG. p By demonstrating values, the strongest performance is maintained for an inner step width of about 1.0 μm. In this regard, BAW resonator 38 has an overall Q closer in magnitude to BAW resonator 46. p It demonstrates value.

[0040] FIG. 14 is a cross-sectional view of a portion of a BAW resonator 54 in which a portion 18' of the piezoelectric layer 18 in the outer region 26 is disposed at a reduced height, according to an embodiment disclosed herein. The BAW resonator 54 includes a reflector 14 including a stack of reflector layers 28A-28E, a bottom electrode 22, a piezoelectric layer 18, a top electrode 20 with an inner step 40 and an outer step 42, a spacer layer 44, and a passivation layer 50, as described above with respect to FIG. 10. The BAW resonator also includes a BO region 32 with an inner step 40, an outer step 42, and a spacer layer 44, as described above. As shown in FIG. 14, the piezoelectric layer 18 forms a reduced thickness portion 18' in or aligned with the outer region 26, as compared to a portion of the piezoelectric layer 18 aligned with the top electrode 20. In a particular embodiment, the thickness of the piezoelectric layer 18 is reduced in a range of 200 nm to 500 nm to form the reduced thickness portion 18'. In certain embodiments, the passivation layer 50 extends to the peripheral edge of the piezoelectric layer 18, which includes the reduced thickness portion 18'. By forming the reduced thickness portion 18' of the piezoelectric layer 18 in the outer region 26, lateral energy leakage into the outer region 26 and mode conversion in the BO region 32 may be reduced.

[0041] FIG. 15 shows the Q p FIG. 1 is a graphical representation of simulation results showing the response of the overall Q value of the structure configured as a TX resonator for operation in LTE band 7. p Values ​​were collected. With the exception of reduced thickness portion 18' of piezoelectric layer 18, the dimensions and arrangement for BAW resonator 54 were configured similarly to BAW resonator 38. For the purposes of the simulation, the thickness of piezoelectric layer 18 was reduced by 400 nm in reduced thickness portion 18' of BAW resonator 54. As shown, BAW resonator 54 exhibited a low overall Q, especially for an inner step width of about 1 μm. p This shows a significant improvement in values.

[0042] As disclosed herein, Q pImproved Q values ​​have been demonstrated in various dual-step configurations of BAW resonator structures. BAW resonator structures as disclosed herein may also demonstrate improved Q values ​​for all operating frequencies above the primary resonant frequency, thus improving Q p 16A shows the Q p FIG. 1 is a graphical comparison of simulation results for values ​​of 1 μm. As shown, an inner step width of 1 μm provides a high Q for the BAW resonators 38, 46. p values, and the inner (single) step width of 2 μm provides a high Q for the BAW resonator 34. p 16B-16D are a graphical comparison of various performance characteristics of the BAW resonator 34 with an internal step width of 2 μm and the BAW resonators 38, 46 with an internal step width of 1 μm of FIG. 16A. In FIG. 16B, the phase Z of the BAW resonators 34, 38, 46 is plotted against frequency in megahertz (MHz). Furthermore, there is a small difference in the effective electromechanical coupling coefficient (K2e in %) between the BAW resonators 34, 38, 46. In FIG. 16C, the y-axis is plotted as 1-(S11) 2 The BO mode has a series resonance frequency (f s ) are shown with various peaks below the series resonance frequency (f s In particular, the highest amplitude BO mode of the BAW resonators 38, 46 is shifted further away from the series resonance frequency (f s The series resonant frequency (f s The spurious modes seen just below the series resonant frequency (f) can be suppressed using apodization structures or by adding recessed or mass-reducing structures to the top electrode. FIG. 16D graphically compares the overall Q-factor for each of the BAW resonators 34, 38, 46. As shown, the Q-factor is approximately 100% for the BAW resonators 38, 46 compared to the series resonant frequency (f s ) and parallel resonance frequency (fp ) and Q p There has been a marked improvement in

[0043] FIG. 17 is a cross-sectional view showing a BAW resonator 56 in which a mass-reduced region is disposed in an inner region 58 of the top electrode 20 according to an embodiment disclosed herein. For illustrative purposes, FIG. 17 is a simplified diagram of the BAW resonator 56 and is not necessarily drawn to scale. However, it is understood that the BAW resonator 56 may include many of the same components as the BAW resonator 10 of FIG. 4. The BAW resonator includes the bottom electrode 22, the piezoelectric layer 18, and the top electrode 20, as previously described. In particular, the BO region 32 includes the inner step 40 or first BO ring, the outer step 42 or second BO ring, and the spacer layer 44. The top electrode 20 additionally includes an inner ring 60 formed in the inner region 58 that bounds the periphery of the central region 36. The inner ring 60 is a portion of the top electrode 20 that has reduced mass. In this regard, the inner ring 60 forms a fourth height H4 from the piezoelectric region 18, which is lower than the first height H1 of the central region 36 and lower than the third height H3 of the outer step 42 and the second height H2 of the inner step 40. In this manner, the outer step 42, the inner step 40, the gap 62, and the inner ring 60 may be arranged to form a descending step structure from the lateral edge of the top electrode 20 toward the central region 36. As shown in FIG. 17, the inner ring 60 may be formed by a channel embedded in the top electrode 20. Alternatively, the inner ring 60 may be formed simultaneously when the top electrode 20 is formed. In certain embodiments, the inner ring 60 (and the inner region 58) may be spaced from the BO region 32 by the gap 62. In certain embodiments, the presence of the gap 62 may enable the formation of a deeper inner ring 60, and thus better manufacturing control of its depth. In other embodiments, the gap 62 may be omitted, and thus the inner ring 60 is disposed directly adjacent to the inner step 40. By providing the inner ring 60 as a mass-reduced region, the series resonant frequency (f s) can be suppressed with significantly improved performance. In certain embodiments, apodization can be used in combination with the inner ring 60.

[0044] FIG. 18 is a cross-sectional view showing a BAW resonator 64 in which the top electrode 20 includes a plurality of outer steps 42-1, 42-2 according to an embodiment disclosed herein. For illustrative purposes, FIG. 18 is a simplified diagram of the BAW resonator 64 and is not necessarily drawn to scale. However, it is understood that the BAW resonator 64 may include many of the same components as the BAW resonator 10 of FIG. 4. The BAW resonator 64 includes a bottom electrode 22, a piezoelectric layer 18, and a top electrode 20, as described above. In a particular embodiment, the BO region 32 includes an inner step 40 and a plurality of outer steps 42-1, 42-2. Separate spacer layers 44-1, 44-2 (or spacer layer portions) are disposed between each of the outer steps 42-1, 42-2 and the piezoelectric layer 18 to provide a different height or thickness for each of the outer steps 42-1, 42-2 relative to the piezoelectric layer 18. In a particular embodiment, the spacer layers 44-1, 44-2 are continuous with each other. As shown, the second outer step 42-2 forms a height H5 from the piezoelectric layer 18 that is higher than each of the third height H3 of the first outer step 42-1, the second height H2 of the inner step 40, and the first height H1 of the central region 36. In this manner, the outer steps 42-1, 42-2, the inner step 40, and the central region 36 may be arranged to form a step structure of decreasing height from the lateral edges of the BO region 32 toward the central region 36.

[0045] The embodiments as described herein may be well suited for different arrangements of BAW resonators, including SMR and FBAR type BAW resonators. In particular, any of the various features and elements as disclosed above may provide improved performance for both SMR and FBAR type BAW structures. Figures 19 and 20 are provided to illustrate exemplary SMR and FBAR structures, respectively.

[0046] FIG. 19 is a cross-sectional view of an exemplary SMR-type BAW resonator 66 according to embodiments disclosed herein. The BAW resonator 66 includes a bottom electrode 22, a piezoelectric layer 18, and a top electrode 20, as previously described. As shown, the BO region 32 may include an inner step 40, an outer step 42, and a spacer layer 44, as previously described, with the BO region 32 laterally bounding the central region 36. Additionally, the BAW resonator 66 may include one or more reflectors 14 and a substrate 12, as described with respect to FIG. 4, to form an SMR structure. FIG. 20 is a cross-sectional view of an exemplary FBAR-type BAW resonator 68 according to embodiments disclosed herein. The BAW resonator 68 may include a bottom electrode 22, a piezoelectric layer 18, a top electrode 20, an inner step 40, an outer step 42, a spacer layer 44, and a central region 36, as previously described. Additionally, the BAW resonator 68 may include a support layer 70 on the substrate 12, with an air cavity 72 disposed therebetween.

[0047] Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present disclosure, and all such improvements and modifications are believed to be within the scope of the concepts disclosed herein and the claims that follow.

Claims

1. 1. A bulk acoustic wave (BAW) resonator comprising: A lower electrode; a piezoelectric layer on the lower electrode; a top electrode on the piezoelectric layer, the top electrode forming a central region forming a first height from the piezoelectric layer and a border (BO) region extending around a periphery of the central region, the BO region comprising: an inner step on the piezoelectric layer forming a second height from the piezoelectric layer that is greater than the first height in the central region; an outer step on the piezoelectric layer forming a third height from the piezoelectric layer that is greater than the first height of the central region; an upper electrode comprising: a dielectric spacer layer disposed between the outer step and the piezoelectric layer, the dielectric spacer layer contacting the piezoelectric layer below the outer step; a passivation layer on the top electrode, wherein a height profile including the top electrode and the passivation layer is defined across the central region and the BO region, the height profile decreasing from the outer step toward the inner step, and the height profile further decreasing from the inner step toward the central region; and Equipped with the top electrode forms an inner ring between the BO region and the central region and forming a fourth height from the piezoelectric layer, the fourth height being less than the first height in the central region, the height profile increasing from the inner ring towards the central region; The BAW resonator, wherein the inner ring is spaced from the BO region by a gap portion of the top electrode, the height profile decreasing from the gap portion towards the inner ring, and the gap portion is formed at the first height.

2. The BAW resonator of claim 1 , wherein the third height is greater than the second height.

3. The BAW resonator of claim 1 , wherein the inner step and the outer step form a mass loading portion of the top electrode.

4. The BAW resonator of claim 1 , wherein the dielectric spacer layer comprises at least one of silicon dioxide, silicon nitride, or aluminum nitride.

5. 2. The BAW resonator of claim 1, wherein the bottom electrode is disposed to extend along a bottom portion of the piezoelectric layer a distance in the range of 200 nm to 1 μm beyond lateral edges of the top electrode overlying the piezoelectric layer.

6. The BAW resonator of claim 1 , wherein the passivation layer extends to a peripheral edge of the piezoelectric layer.

7. The BAW resonator of claim 1 , wherein the passivation layer extends to a peripheral edge of the outer step, the passivation layer being inserted from a peripheral edge of the piezoelectric layer.

8. 2. The BAW resonator of claim 1, wherein the outer step comprises the first outer step, and the BAW resonator further comprises the second outer step, such that the second outer step, the first outer step, and the inner step are arranged to form a step structure of the height of the height profile that descends from the BO region toward the central region.

9. The BAW resonator of claim 1 , wherein the BAW resonator comprises a solidly mounted resonator (SMR) type BAW resonator.

10. The BAW resonator of claim 1 , wherein the BAW resonator comprises a film bulk acoustic resonator (FBAR).

11. 2. A BAW resonator according to claim 1, 1. A BAW resonator comprising: an outer region defined by a peripheral boundary of the top electrode; a portion of the piezoelectric layer within the outer region; and the portion of the piezoelectric layer in the outer region forming a reduced thickness compared to a portion of the piezoelectric layer aligned with the top electrode.

12. A bulk acoustic wave (BAW) resonator comprising: A lower electrode; a piezoelectric layer on the lower electrode; a top electrode on the piezoelectric layer, the top electrode forming a central region forming a first height from the piezoelectric layer and a border (BO) region extending around a periphery of the central region, the BO region comprising: an inner step on the piezoelectric layer, the inner step forming a second height from the piezoelectric layer that is greater than the first height in the central region; an outer step on the piezoelectric layer, the outer step forming a third height from the piezoelectric layer that is greater than the first height in the central region; an upper electrode comprising: a dielectric spacer layer disposed between the outer step and the piezoelectric layer, the dielectric spacer layer contacting the piezoelectric layer below the outer step; a passivation layer on the top electrode, wherein a height profile including the top electrode and the passivation layer is defined across the central region and the BO region, the height profile decreasing from the outer step toward the inner step, and the height profile further decreasing from the inner step toward the central region; and Equipped with The upper electrode is a gap portion on the piezoelectric layer disposed immediately adjacent to the inner step, the gap portion being between the BO region and the central region and formed at the first height; The BAW resonator further comprising an inner ring on the piezoelectric layer disposed directly adjacent to the gap portion, the inner ring being between the BO region and the central region and formed at a fourth height lower than the first height.

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