BAW RESONATOR WITH ZERO-ELECTROMECHANICAL COUPLING IN BOUNDARY REGIONS AND FLAT, STEep SHAPES AND Square

Abstract

The present disclosure relates to a bulk acoustic wave (BAW) resonator including a bottom electrode (56), a top electrode (60), and a piezoelectric layer (58) sandwiched between the bottom electrode and the top electrode. Herein, an active region of the BAW resonator is divided into a central region (82), a recessed boundary BO region (92) laterally adjoining and surrounding the central region, and a mass-loaded BO region (94) laterally adjoining and surrounding the recessed BO region. A height (H1) of the recessed BO region is less than a height (H3) of the central region and less than a height (H2) of the mass loading region. A first portion (76) of the piezoelectric layer confined in the mass load BO region has a zero electromechanical coupling tensor, while a central portion (77) of the piezoelectric layer confined in the central region has a non-zero electromechanical coupling tensor.

Description

[0001] Related Applications

[0002] This application claims the benefit of provisional patent application serial number 63 / 407,459 filed on September 16, 2022, the disclosure of which is hereby incorporated by reference in its entirety. Technical Field

[0003] The present disclosure relates to a bulk acoustic wave (BAW) resonator having zero coupling in a boundary (BO) region of the BAW resonator. Background Art

[0004] Acoustic resonators, especially bulk acoustic wave (BAW) resonators, are used in many high-frequency communication applications. Specifically, BAW resonators are often used in filter networks that operate at frequencies above 1.5 gigahertz (GHz) and require a flat passband, have unusually steep filter skirts and square 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, which tends to decrease in size as the operating frequency increases, and are relatively stable over a wide temperature range. Therefore, BAW-based filters are the preferred filters for many 3rd generation (3G) and 4th generation (4G) wireless devices, and are intended to dominate filter applications for 5th generation (5G) wireless devices. Most of these wireless devices support cellular, wireless fidelity (Wi-Fi), Bluetooth, and / or near-field communications on the same wireless device, and therefore present extremely challenging filtering requirements. Although these requirements continue to increase the complexity of wireless devices, there is always a need to improve the performance of BAW resonators and BAW-based filters, as well as reduce the cost and size associated with them. Summary of the invention

[0005] The present disclosure relates to a bulk acoustic wave (BAW) resonator having zero coupling within a boundary (BO) region of the BAW resonator. The disclosed BAW resonator includes a bottom electrode, a top electrode, and a piezoelectric layer, wherein the piezoelectric layer is sandwiched between the bottom electrode and the top electrode. In this article, the BAW resonator is divided into an active region and an external region. The active region corresponds to a section of the BAW resonator in which the top electrode overlaps the bottom electrode, and the external region corresponds to a section of the BAW resonator that surrounds the active region. The active region is divided into a central region, a concave boundary (BO) region, and a mass load BO region. The mass load BO region is laterally adjacent to the external region and surrounds the periphery of the active region, the concave BO region is laterally adjacent to the mass load BO region and is surrounded by the mass load BO region, and the central region is laterally adjacent to the concave BO region and is surrounded by the concave BO region. The height of the concave BO region is less than the height of the central region, and the height of the mass load BO region is greater than the height of the concave BO region. A first portion of the piezoelectric layer confined in the mass loaded BO region has a zero electromechanical coupling tensor, while a central portion of the piezoelectric layer confined in the central region has a non-zero electromechanical coupling tensor.

[0006] In one embodiment of the BAW resonator, the second portion of the piezoelectric layer confined in the recessed BO region has a zero electromechanical coupling tensor. The third portion of the piezoelectric layer confined in the outer region has a zero electromechanical coupling tensor.

[0007] In one embodiment of the BAW resonator, the piezoelectric layer is formed from one of the group consisting of scandium aluminum nitride, lead zirconate titanate, lead titanate, barium titanate, and hafnium oxide.

[0008] In one embodiment of the BAW resonator, the height of the mass loaded BO region is greater than the height of the central region.

[0009] In one embodiment of the BAW resonator, the height of the central region is greater than the height of the mass loaded BO region.

[0010] In one embodiment of the BAW resonator, the BAW resonator is a type I resonator or a type II resonator.

[0011] In one embodiment of the BAW resonator, the second portion of the piezoelectric layer confined in the recessed BO region has a non-zero electromechanical coupling tensor. The third portion of the piezoelectric layer confined in the outer region has a zero electromechanical coupling tensor. The BAW resonator is a Type II resonator.

[0012] In one embodiment of the BAW resonator, the top electrode includes a BO structure around the periphery of the top electrode. In this article, the BO structure has a concave frame, a convex frame and a transition section, the concave frame has a first height, the convex frame has a second height greater than the first height, and the transition section is laterally located between the concave frame and the convex frame. The height of the transition section changes from the first height of the concave frame to the second height of the convex frame to form a tapered wall. The convex frame surrounds the periphery of the top electrode so that the outer peripheral edge of the convex frame is the peripheral edge of the top electrode. The concave frame is connected to the convex frame via the transition section and is surrounded by the convex frame. The central electrode portion of the top electrode confined in the central region of the BAW resonator has a third height greater than the first height of the concave frame. The concave BO region corresponds to a section of the BAW resonator that includes a concave frame, resides above the concave frame, and resides below the concave frame, and the mass load BO region corresponds to a section of the BAW resonator that includes a convex frame and a transition section, resides above the convex frame and the transition section, and resides below the convex frame and the transition section. The height variation among the central region, the concave BO region and the mass-loaded BO region is due to the height variation of the top electrode.

[0013] In one embodiment of the BAW resonator, the top electrode is composed of a first top electrode layer, a second top electrode layer, and a third top electrode layer, wherein the first top electrode layer is formed directly above the piezoelectric layer, the second top electrode layer is formed above the first top electrode layer, and the third top electrode layer is formed above the second top electrode layer. The concave frame, transition section, and convex frame of the top electrode are formed due to the thickness variation of one or more of the first top electrode layer, the second top electrode layer, and the third top electrode layer.

[0014] In one embodiment of the BAW resonator, the concave frame, the transition section and the convex frame of the top electrode are formed due to thickness variations of the first top electrode layer.

[0015] In one embodiment of the BAW resonator, the first top electrode layer is formed of one of tungsten (W), molybdenum (Mo), platinum (Pt), the second electrode layer is formed of titanium tungsten (TiW) or titanium (Ti), and the third top electrode layer is formed of aluminum copper (AlCu).

[0016] According to one embodiment, the BAW resonator further comprises a passivation layer covering the top electrode and the portion of the piezoelectric layer exposed by the top electrode. Herein, the passivation layer has a uniform thickness.

[0017] In one embodiment of the BAW resonator, the top electrode includes a BO structure around the periphery of the top electrode. In this article, the BO structure includes a concave frame, an inner convex frame, a first transition section, an outer convex frame, and a second transition section, wherein the concave frame has a first height, the inner convex frame has a second height, the first transition section is laterally located between the concave frame and the inner convex frame, the outer convex frame has a third height, and the second transition section is laterally located between the inner convex frame and the outer convex frame. The second height is greater than the first height and less than the third height. The height of the first transition section changes from the first height of the concave frame to the second height of the inner convex frame, and the height of the second transition section changes from the second height of the inner convex frame to the third height of the outer convex frame. The outer convex frame surrounds the periphery of the top electrode so that the outer peripheral edge of the outer convex frame is the peripheral edge of the top electrode. The inner convex frame is connected to the outer convex frame via the second transition section and is surrounded by the outer convex frame. The concave frame is connected to the inner convex frame via the first transition section and is surrounded by the inner convex frame. The central electrode portion of the top electrode confined in the central region of the BAW resonator has a fourth height greater than the first height of the concave frame. The concave BO region corresponds to a section of the BAW resonator including the concave frame, residing above the concave frame, and residing below the concave frame, and the mass-loaded BO region corresponds to a section of the BAW resonator including a combination of the first transition section, the inner convex frame, the second transition section, and the outer convex frame, residing above the combination, and residing below the combination.

[0018] According to one embodiment, the BAW resonator further comprises a dielectric BO ring around the periphery of the active region. In this context, the top electrode is composed of a plurality of top electrode layers formed directly above the piezoelectric layer and extending above the dielectric BO ring. The height variation between the center region, the recessed BO region, and the mass-loaded BO region is due to a combination of the thickness variation of the top electrode layer of the top electrode and the thickness of the dielectric BO ring.

[0019] According to one embodiment, the BAW resonator further includes a passivation layer covering the top electrode and a portion of the piezoelectric layer exposed by the top electrode, wherein the passivation layer is formed of silicon nitride (SiN), silicon dioxide (SiO 2 ) or silicon oxynitride (SiON).

[0020] In one embodiment of the BAW resonator, the top electrode includes a BO structure having a raised frame and a transition section, and the passivation layer includes a recessed frame. The raised frame surrounds the periphery of the top electrode so that the outer peripheral edge of the raised frame is the peripheral edge of the top electrode, and the transition section is laterally adjacent to the raised frame and surrounded by the raised frame. The recessed frame of the passivation layer is laterally adjacent to the transition section of the top electrode and is confined in the transition section of the top electrode. The thickness of the recessed frame is less than the thickness of the passivation layer in the central region. The recessed BO region corresponds to the recessed frame of the BAW resonator including the passivation layer and the section residing below the recessed frame of the passivation layer, and the mass-loaded BO region corresponds to the section of the BAW resonator including the raised frame and the transition section, residing above the raised frame and the transition section, and residing below the raised frame and the transition section.

[0021] In one embodiment of the BAW resonator, the top electrode is composed of multiple top electrode layers formed directly above the piezoelectric layer. The height variation between the central region, the recessed BO region and the mass-loaded BO region is due to the recessed frame of the passivation layer and the BO structure of the top electrode.

[0022] In one embodiment of the BAW resonator, the portion of the top electrode confined within the central region and the recessed BO region has a uniform thickness, and the height variation between the central region and the recessed BO region is due to the thickness variation of the passivation layer. The height variation between the recessed BO region and the mass-loaded BO region is due to the thickness variation of the top electrode layer.

[0023] On the other hand, any of the aforementioned aspects, and / or various individual aspects and features as described herein, may be combined individually or together to obtain additional advantages. Unless otherwise indicated herein, any of the various features and elements disclosed herein may be combined with one or more other disclosed features and elements.

[0024] Those skilled in the art will appreciate the scope of the present disclosure and become aware of additional aspects thereof after reading the following detailed description of the preferred embodiments and the associated drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The accompanying drawings incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure.

[0026] FIG. 1 is a diagram showing a conventional bulk acoustic wave (BAW) resonator.

[0027] Figure 2 FIG. 1 is a graph showing graphically the magnitude and phase of electrical impedance as a function of frequency relative to an ideal BAW resonator.

[0028] Figures 3A-3C is a diagram graphically illustrating phase curves of various conventional BAW resonators.

[0029] FIG. 4 is a diagram showing a conventional BAW resonator having a top electrode including a boundary (BO) ring.

[0030] Figure 5 To graphically illustrate the relationship between BO loop width and anti-resonance frequency (Q p ) and the relative intensity of the formed BO mode.

[0031] Figure 6 is a diagram graphically illustrating the phase curve of a BAW resonator with and without a BO ring.

[0032] Figure 7 is a cross-sectional view showing an exemplary BAW resonator with zero coupling in the BO region according to an embodiment of the present disclosure.

[0033] Figure 8 It is shown Figure 7 Cross-sectional view of a detail of the top electrode within the BAW resonator shown in FIG.

[0034] Figure 9-11 is a cross-sectional diagram illustrating an alternative BAW resonator with zero coupling in the BO region according to an embodiment of the present disclosure.

[0035] Figures 12A-12B Simulation results for various configurations of type I BAW resonators are shown.

[0036] Figures 13A-13B Simulation results for various configurations of Type II BAW resonators are shown.

[0037] It should be understood that for purposes of clarity of illustration, FIGS. 1-13B may not be drawn to scale. DETAILED DESCRIPTION

[0038] The embodiments set forth below represent information necessary to enable those skilled in the art to practice the embodiments and to illustrate the best mode for practicing the embodiments. After reading the following description with reference to the accompanying drawings, those skilled in the art should understand the concepts of the present disclosure and will appreciate the applications of these concepts not specifically set forth herein. It should be understood that these concepts and applications are within the scope of the present disclosure and the appended claims.

[0039] It should be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish different elements. For example, without departing from the scope of the present disclosure, the first element may be referred to as the second element, and similarly, the second element may be referred to as the first element. As used herein, the term "and / or" includes any and all combinations of one or more items in the associated listed items.

[0040] It should be understood that when an element such as a layer, a region or a substrate is referred to as being "on another element" or "extending onto another element", it may be directly on another element or directly extending onto another element, or there may also be an intermediate element. In contrast, when an element is referred to as being "directly on another element" or "extending directly onto another element", there are no intermediate elements. Similarly, it should be understood that when an element such as a layer, a region or a substrate is referred to as being "above another element" or "extending above another element", it may be directly above another element or directly extending above another element, or there may also be an intermediate element. In contrast, when an element is referred to as being "directly above another element" or "extending directly above another element", there are no intermediate elements. 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 another element, or there may be an intermediate element. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no intermediate elements.

[0041] Relative terms such as "below" or "above" or "upper" or "lower" or "horizontal" or "vertical" may be used herein to describe one element, layer or region's relationship to another element, layer or region as illustrated in the figures. It should 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.

[0042] The terms used herein are only used for the purpose of describing specific embodiments and are not intended to limit the present disclosure. As used herein, unless the context clearly indicates otherwise, the singular forms "a", "an", and "the" are also intended to include the plural forms. It should also be understood that when 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 exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0043] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present disclosure belongs. It should be further understood that, unless explicitly defined herein, the terms used herein should be interpreted as having a meaning consistent with their meaning in the context of this specification and the relevant technology, and will not be interpreted in an idealized or overly formal sense.

[0044] Embodiments are described herein with reference to schematic illustrations of embodiments of the present disclosure. In this way, the actual sizes of layers and elements may be different, and it is expected that they will be different from the illustrated shapes due to, for example, manufacturing techniques and / or tolerances. For example, a region shown or described as a square or rectangle may have a circular or curved feature, and a region shown as a straight line may have some irregularities. Therefore, the region shown in the figure is schematic, and its shape is not intended to illustrate the precise shape of the region of the device, and is not intended to limit the scope of the present disclosure. In addition, for illustrative purposes, the size of a structure or region may be enlarged relative to other structures or regions, and therefore a structure or region is provided to illustrate the general structure of this theme, and may be drawn to scale or may not be drawn to scale. Common elements between figures may be displayed with common element numbers in this article, and may not be repeatedly described subsequently.

[0045] The present disclosure relates to a bulk acoustic wave (BAW) resonator, and in particular to a BAW resonator with zero coupling in a boundary (BO) region. The disclosed BAW resonator has a high quality factor, suppression of parasitic modes, and suppression of BO modes.

[0046] Before delving into the details of these concepts, an overview of BAW resonators and filters employing BAW resonators is provided. BAW resonators are used in many high frequency filter applications. An exemplary BAW resonator 10 is shown in FIG. The BAW resonator 10 is a solidly mounted resonator (SMR) type BAW resonator and generally includes a substrate 12, a reflector 14 mounted above the substrate 12, and a transducer 16 mounted above the reflector 14. The transducer 16 rests on the reflector 14 and includes a piezoelectric layer 18 sandwiched between a top electrode 20 and a bottom electrode 22. The top electrode 20 and the bottom electrode 22 may be formed of tungsten (W), molybdenum (Mo), platinum (Pt), or similar materials, 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 comprising a single layer, the piezoelectric layer 18, the top electrode 20, and / or the bottom electrode 22 may comprise multiple layers of the same material, multiple layers in which at least two layers are different materials, or multiple layers in which each layer is a different material.

[0047] The BAW resonator 10 is divided into an active region 24 and an outer region 26. The active region 24 generally corresponds to a section of the BAW resonator 10 where the top electrode 20 overlaps the bottom electrode 22, and also includes layers located below the overlapping top and bottom electrodes 20, 22. The outer region 26 corresponds to a section of the BAW resonator 10 surrounding the active region 24.

[0048] For the BAW resonator 10, applying an electrical signal across the top electrode 20 and the bottom electrode 22 excites acoustic waves in the piezoelectric layer 18. These acoustic waves propagate primarily vertically. A primary goal in BAW resonator design is to confine these vertically propagating acoustic waves to the transducer 16. The upwardly traveling acoustic waves are reflected back into the transducer 16 by the air-metal boundary at the top surface of the top electrode 20. The downwardly traveling acoustic waves are reflected back into the transducer 16 by the reflector 14 or by an air cavity disposed just below the transducer 16 in a film bulk acoustic resonator (FBAR).

[0049] The reflector 14 is typically formed of a stack of reflector layers (RL) 28A to 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 with high and low acoustic impedances, 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.

[0050] Figure 2 1 and 1 , which provide a plot of the magnitude (Z) and phase (φ) of the electrical impedance as a function of frequency (GHz) relative to an ideal BAW resonator 10. The magnitude (Z) of the electrical impedance is shown by the solid line, while the phase (φ) of the electrical impedance is shown by the 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 often referred to as the series resonant frequency (f s ), and the anti-resonance frequency is often referred to as the parallel resonant frequency (f p When the magnitude of the impedance or reactance of the BAW resonator 10 approaches zero, a series resonant frequency (f s The parallel resonant frequency (f ) occurs when the magnitude of the impedance or reactance of the BAW resonator 10 peaks at a significantly higher level. p ). Usually, the series resonant frequency (f s ) varies with the thickness or height of the piezoelectric layer 18 and the mass of the bottom electrode 22 and the top electrode 20.

[0051] With respect to the phase (φ), the BAW resonator 10 behaves like a series resonant frequency (f s ) and the parallel resonant frequency (f p). In contrast, the BAW resonator 10 acts like an inductor at the series resonant frequency (f s ) and below the parallel resonant frequency (f p ) provides a -90° phase shift. The BAW resonator 10 is at the series resonant frequency (f s ) exhibits extremely low, near-zero resistance and at the parallel resonant frequency (f p ) presents a very high resistance at low frequencies. The electrical properties of the BAW resonator 10 are suitable for achieving 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 of about 1.8 GHz and above.

[0052] Unfortunately, Figure 2 The phase (φ) curve of represents the ideal phase curve. In practice, it is challenging to approach this ideal. Figure 3A A typical phase curve of the BAW resonator 10 of FIG. 1 is shown in FIG. Instead of being a smooth curve, Figure 3A The phase curve contains the series resonant frequency (f s ) or less, at the series resonant frequency (f s ) and the parallel resonant frequency (f p ) and at the parallel resonant frequency (f p ) above. The ripple is the result of spurious modes, which are caused by parasitic resonances occurring at corresponding frequencies. Although the vast majority of acoustic waves in the BAW resonator 10 propagate vertically, various boundary conditions around the transducer 16 result in the propagation of lateral (horizontal) acoustic waves, which are referred to as lateral standing waves. The presence of these lateral standing waves reduces the potential quality factor (Q) associated with the BAW resonator 10.

[0053] As shown in FIG4, a BO ring 30 is formed on or within the top electrode 20 (not shown) to suppress certain parasitic modes. The parasitic modes suppressed by the BO ring 30 are at the series resonant frequency (f s ) above, such as Figure 3B This is highlighted by circles A and B in the phase curve of FIG. Circle A shows the phase curve in the passband that resides at the series resonant frequency (f s ) and the parallel resonant frequency (f p ) and hence the suppression of parasitic modes. Circle B shows the p ) and hence the suppression of parasitic modes. It is worth noting that the parasitic modes in the upper shoulder of the passband (just at the parallel resonant frequency (f p) and above the passband are suppressed, such as at the series resonant frequency (f s ) and the parallel resonant frequency (f p ) and at the parallel resonant frequency (f p ) as evidenced by the smooth or essentially ripple-free phase curve above.

[0054] The BO ring 30 corresponds to mass loading of a portion of the top electrode 20 extending around the periphery of the active region 24. In this regard, the BO ring 30 with mass loading forms a raised frame arranged 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 above the top electrode 20. The portion of the BAW resonator 10 that includes the BO ring 30 and resides below the BO ring is referred to as a BO region 32. Thus, the BO region 32 corresponds to an outer peripheral portion of the active region 24 and resides inside the active region 24. In addition, a central region 36 of the BAW resonator 10 is laterally defined inside the BO region 32 and is not covered by the BO ring 30. Although the BO ring 30 effectively suppresses the series resonance frequency (f s ) above the parasitic mode, but the BO ring 30 is at the series resonant frequency (f s )The following parasitic modes have little or no effect, such as Figure 3B A technique called apodization is often used to suppress the drop to the series resonant frequency (f s ) below the parasitic mode.

[0055] Apodization is used to avoid or at least significantly reduce any lateral symmetry in the BAW resonator 10 or at least in its transducer 16. The 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, a footprint corresponding to a pentagon instead of a square or rectangle may be chosen. Avoiding symmetry helps reduce the presence of lateral standing waves in the transducer 16. Figure 3C The circle C shows the effect of the apodization method, where at the series resonant frequency (f s ) below the parasitic mode is suppressed. Assuming that the BO ring 30 is not provided, then Figure 3C It can be easily seen that the apodization method cannot suppress the series resonant frequency (f s Therefore, the typical BAW resonator 10 employs both the apodization method and the BO ring 30.

[0056] The thickness or height of the BO ring 30 may be measured in a direction perpendicular to or away from the top surface of the piezoelectric layer 18, and the width of the BO ring 30 may be measured in a direction parallel to or transversely across the piezoelectric layer 18. The thickness and width of the BO ring 30 may be tuned simultaneously to suppress parasitic modes and improve the quality factor (Q) at the anti-resonance frequency. p The additional mass associated with the BO ring 30 generally causes the BO region 32 to resonate at a slightly lower frequency than the rest of the active region 24. Thus, the presence of the BO ring can provide a resonance frequency below the series resonant frequency (f s ) introduces undesirable modes or BO modes at frequencies below .

[0057] Figure 5 To graphically illustrate the relationship between BO loop width and anti-resonance frequency (Q p ) and the relative intensity of the resulting BO mode. Figure 5 In the figure, the x-axis represents the BO ring width in micrometers (μm), and the primary y-axis represents Q p , and the secondary y-axis represents the relative intensity or magnitude of the BO pattern formed. Figure 5 The graph of Figure 1 plots data from three wafers, each containing BAW resonators with different BO ring widths. As shown, Q p The value generally increases with the BO ring width, and the highest Q p The values ​​correspond to BO ring widths just over 3 μm. As also shown, the BO mode intensity generally increases with increasing BO ring width. For higher BO ring widths, Q p The value of the BO mode intensity is reduced while the value of the BO mode intensity remains high. In this way, the high Q p The BO ring may also introduce undesirable BO modes into the corresponding BAW device.

[0058] Figure 6 Graphs that graphically illustrate phase curves of BAW resonators with and without BO rings. As shown, parasitic modes present within the passband of a 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. BO modes can be introduced outside the band or even inside the filter and can limit the design or use of BAW resonators for wide bandwidth filtering applications. If the BO mode is within the passband, the insertion loss can be affected. BO modes present outside the passband can be problematic for BAW multiplexing applications, where BAW filters of different frequency bands operate simultaneously. In such multiplexing applications, the BO mode of one BAW filter can fall into the passband of other BAW filters and introduce interference during multiplexing.

[0059] As is well known, the main reason for exciting the BO mode is the non-zero electromechanical coupling coefficient (k 2 e≠0). The electromechanical coupling coefficient of the piezoelectric layer is a function of the piezoelectric coefficient d of the piezoelectric layer, and the piezoelectric coefficient d is proportional to the polarization of the piezoelectric layer. Therefore, once the polarization of the piezoelectric layer changes, the piezoelectric coefficient d will change accordingly, and therefore, the electromechanical coupling coefficient of the BAW resonator will also change. U.S. Patent Application No. 18 / 150,621 (U.S. Patent Provisional Application No. 63324961) and U.S. Patent Application No. 18 / 150,627 (U.S. Patent Provisional Application No. 63324968) describe methods for achieving a zero / reduced electromechanical coupling coefficient (k) in the BO region. 2 e=0 or k 2 However, the zero / reduced electromechanical coupling coefficient in the BO region may lead to increased lateral energy losses, resulting in Q p The value decreases.

[0060] Figure 7 A cross-sectional view of an exemplary BAW resonator 50 with zero coupling in the BO region according to an embodiment of the present disclosure is shown. For the purposes of this illustration, the BAW resonator 50 is a BAW SMR and includes a reflector 54, a bottom electrode structure 56 above the reflector 54, a piezoelectric layer 58 above the bottom electrode structure 56, and a top electrode 60 above the piezoelectric layer 58. In a different application, the BAW resonator 50 may be a FBAR in which the reflector 54 is omitted.

[0061] In detail, the reflector 54 includes a low acoustic impedance region 62, and a plurality of high acoustic impedance layers 64 are embedded in the low acoustic impedance region 62. Herein, there are two high acoustic impedance layers 64: an upper high acoustic impedance layer 64-U, and a lower high acoustic impedance section 64-L. In different applications, there may be fewer or more high acoustic impedance layers 64 embedded in the low acoustic impedance region 62. Herein, the lower high acoustic impedance layer 64-L resides above the bottom portion 62-B of the low acoustic impedance region 62. The upper high acoustic impedance layer 64-U is vertically located above the lower high acoustic impedance layer 64-L and is separated from the lower high acoustic impedance layer 64-L by the middle portion 62-M of the low acoustic impedance region 62. The top portion 62-T of the low acoustic impedance region 62 is located above the upper high acoustic impedance layer 64-U. The low acoustic impedance region 62 has a lower acoustic impedance, a lower density, and a lower hardness than the high acoustic impedance layer 64, and may be formed of SiO2. Each high acoustic impedance layer 64 is formed of a high acoustic impedance material such as W, Mo or Pt. Each of the bottom portion 62-B of the low acoustic impedance region 62, the lower high acoustic impedance layer 64-L, the middle portion 62-M of the low acoustic impedance region 62, the upper high acoustic impedance layer 64-U, and the top portion 62-T of the low acoustic impedance region 62 is an RL of the reflector 54.

[0062] The bottom electrode structure 56 is formed over the top portion 62-T of the low acoustic impedance region 62 and includes a bottom electrode 66 and a planarization oxide 68. The bottom electrode 66 is vertically located over the high acoustic impedance layer 64. The planarization oxide 68 surrounds the bottom electrode 66 and is capable of electrically separating the bottom electrode 66 from an external bottom electrode (not shown). The bottom electrode 66 may include at least two bottom electrode layers 70 and 72. The second bottom electrode layer 72 is located over the top portion 62-T of the low acoustic impedance region 62 and may be formed of aluminum copper (AlCu), while the first bottom electrode layer 70 is located over the second bottom electrode layer 72 and may be formed of W, Mo, or Pt. The planarization oxide 68 may be formed of SiO2.

[0063] The piezoelectric layer 58 is formed over the bottom electrode structure 56 and at least completely covers the bottom electrode 66. The top electrode 60 is formed over the piezoelectric layer 58 and is vertically located over the bottom electrode 66. The top electrode 60 includes a BO structure 76 positioned around the periphery of the top electrode 60, and a central electrode portion 77 surrounded by the BO structure 76.

[0064] The BAW resonator 50 is divided into an active region 78 and an outer region 80. The active region 78 corresponds to a section of the BAW resonator 50 where the top electrode 60 overlaps the bottom electrode 66, and also includes layers located above, between, and below the overlapping top and bottom electrodes 60, 66. The outer region 80 corresponds to a section of the BAW resonator 50 that surrounds the active region 78. The active region 78 is further divided into a central region 82 and a BO region 84. The BO region 84 corresponds to a section of the BAW resonator 50 that includes the BO structure 76, resides above the BO structure, and resides below the BO structure. The central region 82 is laterally confined within the BO region 84 and does not overlap the BO structure 76. The central electrode portion 77 of the top electrode 60 is confined in the central region 82.

[0065] Notably, the bottom electrode 66 may extend beyond the peripheral edge of the top electrode 60. As previously described, an active region 78 is formed in which the top electrode 60 overlaps the bottom electrode 66. In this manner, misalignment of the top electrode 60 with the bottom electrode 66 during fabrication may impair the performance of the BAW resonator 50. By arranging the bottom electrode 66 with a larger lateral dimension than the top electrode 60, the alignment tolerance for placing the top electrode 60 may be increased, thereby improving fabrication tolerances.

[0066] In this illustration, the BO structure 76 includes a first concave frame 86 having a first height H1 (as measured from the top surface of the piezoelectric layer 58), a convex frame 88 having a second height H2 (as measured from the top surface of the piezoelectric layer 58), and a transition section 90 laterally disposed between the first concave frame 86 and the convex frame 88. The convex frame 88 may extend around the periphery of the top electrode 60 such that the outer peripheral edge of the convex frame 88 may be the peripheral edge of the top electrode 60. The first concave frame 86 is connected to the convex frame 88 via the transition section 90 and is confined in the convex frame 88. In addition, the central electrode portion 77 confined in the central region 82 of the BAW resonator 50 has a third height H3 that is greater than the first height H1 of the first concave frame 86. Typically, the third height H3 of the central electrode portion 77 is less than the second height H2 of the convex frame 88 (i.e., H2>H3>H1). In some applications, the third height H3 of the center electrode portion 77 may be greater than the second height H2 of the convex frame 88 (i.e., H3>H2>H1). The height of the transition section 90 changes from the first height H1 of the first concave frame 86 to the second height H2 of the convex frame 88 to form a tapered wall. The angle α formed between the tapered wall of the transition section 90 and a horizontal plane (e.g., parallel to the top surface of the piezoelectric layer 58) is between 30 degrees and 60 degrees. The height ratio between the first height H1 of the first concave frame 86, the second height H2 of the convex frame 88, and the third height H3 of the center electrode portion 77 depends on the materials used to form the first concave frame 86 and the convex frame 88.

[0067] Herein, the BO region 84 includes a recessed BO region 92 and a mass-loaded BO region 94, each of which has an annular shape. The recessed BO region 92 corresponds to a section of the BAW resonator 50 that includes a first recessed frame 86, resides above the first recessed frame, and resides below the first recessed frame. On the other hand, the mass-loaded BO region 94 corresponds to a section of the BAW resonator 50 that includes a raised frame 88 and a transition section 90, resides above the raised frame and the transition section, and resides below the raised frame and the transition section. Therefore, the mass-loaded BO region 94 is laterally adjacent to the outer region 80 and extends around the periphery of the active region 78, the recessed BO region 92 is laterally adjacent to the mass-loaded BO region 94 and is surrounded by the mass-loaded BO region, and the central region 82 is laterally adjacent to the recessed BO region 92 and is surrounded by the recessed BO region.

[0068] Figure 8Detail of the BO structure 76 of the top electrode 60 is shown. In this illustration, the top electrode 60 is composed of a first top electrode layer 96-1, a second top electrode layer 96-2, and a third top electrode layer 96-3, wherein the first top electrode layer is formed directly above the piezoelectric layer 58, the second top electrode layer is formed above the first top electrode layer 96-1, and the third top electrode layer is formed above the second top electrode layer 96-2. The first top electrode layer 96-1 can be formed of W, Mo, Pt or other conductive materials with high acoustic impedance properties. The second electrode layer 96-2 can be formed of titanium tungsten (TiW) or titanium (Ti) and acts as a seed layer. The third top electrode layer 96-3 can be formed of AlCu or other highly conductive materials. The first concave frame 86 and the convex frame 88 are formed due to the thickness variation of one or more top electrode layers 96.

[0069] For purposes of the present illustration, the first recessed frame 86 and the raised frame 88 are formed due to a thickness variation of the first top electrode layer 96-1. In the first recessed frame 86, the first top electrode layer 96-1 has a thickness reduction (compared to the thickness of the first top electrode layer 96-1 within the central electrode portion 77), while the second top electrode layer 96-2 and the third top electrode layer 96-3 do not have a thickness variation (compared to the thickness of the second top electrode layer 96-2 and the third top electrode layer 96-3 within the central electrode portion 77, respectively). In the raised frame 88, the first top electrode layer 96-1 has a thickness increase (compared to the thickness of the first top electrode layer 96-1 within the first recessed frame 86), while the second top electrode layer 96-2 and the third top electrode layer 96-3 do not have a thickness variation (compared to the thickness of the second top electrode layer 96-2 and the third top electrode layer 96-3 within the central electrode portion 77, respectively). In different applications, the first recessed frame 86 and the raised frame 88 may be formed by thickness variations of the second top electrode layer 96-2, thickness variations of the third top electrode layer 96-3, or thickness variations of any combination of the first top electrode layer 96-1, the second top electrode layer 96-2, and the third top electrode layer 96-3 (not shown).

[0070] For non-limiting example, the piezoelectric layer 58 may be made of scandium aluminum nitride (Sc x Al 1-xN), lead zirconate titanate (PZT), lead titanate (PTO), barium titanate (BTO), hafnium oxide (HfO2), etc. In this article, the first portion of the piezoelectric layer 58 confined in the recessed BO area 92 and aligned under the first recessed frame 86 has a zero electromechanical coupling tensor (e=0). The second portion of the piezoelectric layer 58 confined in the mass-loaded BO area 94 and aligned under the raised frame 88 and the transition section 90 has a zero electromechanical coupling tensor (e=0). In other words, the portion of the piezoelectric layer 58 confined in the BO area 84 and aligned under the BO structure 76 has a zero electromechanical coupling tensor (e=0). In addition, the third portion of the piezoelectric layer 58 confined in the external area 80 has a zero electromechanical coupling tensor (e=0). In addition, the central portion of the piezoelectric layer 58 has a non-zero electromechanical coupling tensor (e≠0) and is configured to provide a primary resonance of the BAW resonator 50, wherein the central portion is confined in the central region 82, aligned under the central electrode portion 77, and not covered by the BO structure 76. In some applications, the first portion, the second portion, and the third portion of the piezoelectric layer 58 may have an extremely low electromechanical coupling tensor instead of a zero electromechanical coupling tensor, wherein the extremely low electromechanical coupling tensor is at most 10% of the non-zero electromechanical coupling tensor in the central portion of the piezoelectric layer 58.

[0071] In addition, the BAW resonator 50 may further include a passivation layer 100 to protect the exemplary BAW resonator 50 from the external environment. The passivation layer 100 covers the top electrode 60 and the portion of the piezoelectric layer 58 exposed by the top electrode 60. The passivation layer 100 may be formed of silicon nitride (SiN), SiO2, or silicon oxynitride (SiON).

[0072] In one embodiment, the passivation layer 100 may have a thickness between and and does not add further recess or mass loading to the top electrode 60. In another embodiment, the passivation layer 100 may also have a thickness variation and provide recess and / or mass loading to the top electrode 60. Fig. 9As shown in , the passivation layer 100 includes a second recessed frame 102, while the BO structure 76 of the top electrode 60 does not include the first recessed frame 86, but only includes a transition section 90 and a raised frame 88. In this illustration, the mass-loaded BO region 94 still corresponds to the section of the BAW resonator 50 that includes the raised frame 88 and the transition section 90, resides above the raised frame and the transition section, and resides below the raised frame and the transition section. However, the recessed BO region 92 adjacent to the mass-loaded BO region 94 and confined in the mass-loaded BO region corresponds to the section of the BAW resonator 50 that includes the second recessed frame 102 and resides below the second recessed frame. In other words, the second recessed frame 102 of the passivation layer 100 is laterally adjacent to the transition section 90 of the top electrode 60 and is confined in the transition section. The BO region 84 of the BAW 50 still includes both the recessed BO region 92 and the mass-loaded BO region 94, and the central region 82 is still surrounded by the BO region 84.

[0073] Herein, a portion of the top electrode 60 confined in the concave BO region 92 and the central electrode portion 77 confined in the central region 82 have the same third height H3. Therefore, the height of the transition section 90 changes from the third height H3 to the second height H2 of the raised frame 88. In the concave BO region 92, the passivation layer 100 (i.e., the second concave frame 102) has a first thickness T1, while in the central region 82, the passivation layer 100 has a second thickness T2. The first thickness T1 may be 70%-90% of the second thickness T2. In addition, the passivation layer 100 in the mass-loaded BO region 94 and in the outer region 80 may have a thickness that is the same as the first thickness T1 or the same as the second thickness T2.

[0074] The first portion of the piezoelectric layer 58 that is confined in the recessed BO region 92 and aligned below the second recessed frame 102 has a zero electromechanical coupling tensor (e=0). The second portion of the piezoelectric layer 58 that is confined in the mass-loaded BO region 94 and aligned below the BO structure 76 has a zero electromechanical coupling tensor (e=0). Therefore, the portion of the piezoelectric layer 58 that is confined in the BO region 84 has a zero electromechanical coupling tensor (e=0). The third portion of the piezoelectric layer 58 that is confined in the outer region 80 still has a zero electromechanical coupling tensor (e=0). The central portion of the piezoelectric layer 58 that is confined in the central region 82 and not covered by the second recessed frame 102 and the BO structure 76 has a non-zero electromechanical coupling tensor (e≠0) and is configured to provide the main resonance of the BAW resonator 50. In some applications, the first, second, and third portions of the piezoelectric layer 58 may have extremely low electromechanical coupling tensors rather than zero electromechanical coupling tensors, where the extremely low electromechanical coupling tensors are at most 10% of the non-zero electromechanical coupling tensor of the central portion of the piezoelectric layer 58 .

[0075] It should be noted that no matter whether the recessed BO region 92 is defined by the second recessed frame 102 of the passivation layer 100 (eg Fig. 9 ) or is determined by the first concave frame 86 of the top electrode 60 (as shown in Figure 7-8 ), the height of the concave BO region 92 is always less than the height of the central region 82, and the height of the mass-loaded BO region 94 is always greater than the height of the concave BO region 92. Typically, the height of the mass-loaded BO region 94 is greater than the height of the central region 82 (as shown in FIG. Figure 7-9 In some applications, the height of the mass loading BO region 94 may be less than the height of the central region 82.

[0076] In addition, the BAW resonator 50 may be a Type I resonator or a Type II resonator, the BAW resonator comprising a first / second concave frame 86 / 102, a convex frame 88, and a zero / reduced coupling portion of the piezoelectric layer 58. In this context, the frame combination of the first / second concave frame 86 / 102 and the convex frame 88 facilitates an improvement in the quality factor of the BAW resonator 50 compared to a resonator having only a concave frame or a convex frame. On the other hand, due to the zero / reduced electromechanical coupling tensor in the first and second portions of the piezoelectric layer 58 (confined in the BO region 84), there will be negligible BO modes in the BAW resonator 50. The zero / reduced electromechanical coupling tensor in the first and second portions of the piezoelectric layer 58 may also help confine the energy inside the BAW resonator 50 and reduce lateral losses.

[0077] like Figure 7-9 As shown in FIG. 1 , within the mass-loaded BO region 94, the BO structure 76 includes only one transition section 90 and one raised frame 88 forming a single-step configuration. Fig.10 As shown in , within the mass loading BO region 94, the BO structure 76 may include two transition sections and two raised frames forming a double step configuration. Within the mass loading BO region 94, in addition to the raised frame 88 and the transition section 90, the BO structure 76 further includes a second raised frame 108 having a fourth height H4 and a second transition section 110. Herein, the raised frame 88 is an inner raised frame connected to the first recessed frame 86 via the transition section 90. In different applications, the first recessed frame 86 of the top electrode 60 may be replaced by the second recessed frame 102 (not shown) of the passivation layer 100. The second raised frame 108 is an outer raised frame that may extend around the periphery of the top electrode 60 (e.g., the outer peripheral edge of the second raised frame 108 may be the peripheral edge of the top electrode 60) and is connected to the inner raised frame 88 via the second transition section 110. The height of the second transition section 110 changes from the second height H2 of the inner raised frame 88 to the fourth height H4 of the outer raised frame 108.

[0078] As described above, the inner raised frame 88 is due to the thickness increment of one or more top electrode layers 96 (e.g., the thickness increment of the first top electrode layer 96-1, such as Figure 8 ). In this illustration, the outer raised frame 108 is formed due to a combination of a thickness increment of one or more top electrode layers 96 and a dielectric BO ring 112 (having a thickness H4-H2) surrounding the outer periphery of the top electrode 60. The dielectric BO ring 112 is vertically located between the top electrode layer 96 and the piezoelectric layer 58. The dielectric BO ring 112 can be formed of SiO2. In this article, the portion of the piezoelectric layer 58 that is confined in the BO region 84 and aligned below the BO structure 76 (for example, below the first recessed frame 86, the transition section 90, the raised frame 88, the second transition section 110, and the second raised frame 108) has a zero / very low electromechanical coupling tensor (e=0 or e→0).

[0079] In order to achieve the desired high quality factor and effectively suppress the BO mode, the BAW resonator 50 (type I resonator or type II resonator) may have concave and convex frames and zero / reduced coupling in the first, second, and third portions of the piezoelectric layer 58 (i.e., the portions of the piezoelectric layer 58 confined in the concave BO region 92, the mass-loaded BO region 94, and the outer region 80, respectively) (see Figure 7-10 ). For a type I BAW resonator, when the BAW resonator does not include a concave frame but only a convex frame, and / or the piezoelectric layer has fewer portions with zero / reduced electromechanical coupling tensors (e.g., only in the second and third portions of the piezoelectric layer 58 confined in the mass-loaded BO region 94 and the external region 80 with zero / reduced coupling, not shown), the quality factor of the BAW resonator will be significantly reduced (details such as Figures 12A-12B ), which is due to reduced lateral energy confinement. However, for a type II BAW resonator, when the BAW resonator does not include a concave frame but only a convex frame, and / or the piezoelectric layer has fewer portions with zero / reduced electromechanical coupling tensors (e.g., only in the second and third portions of the piezoelectric layer 58 confined in the mass-loaded BO region 94 and the external region 80 with zero / reduced coupling, as shown in Fig.11 ), the quality factor of the BAW resonator can still achieve relatively high values ​​(details as shown in Figures 13A-13B ).

[0080] exist Fig.11, a first portion of the piezoelectric layer 58 confined in the recessed BO region 92 and aligned under the first recessed frame 86 (or aligned under the second recessed frame 102, not shown), and a central portion of the piezoelectric layer 58 confined in the central region 82 have a non-zero electromechanical coupling tensor (e≠0). A second portion of the piezoelectric layer 58 confined in the mass-loaded BO region 94 and aligned under the raised frame 88 and the transition section 90, and a third portion of the piezoelectric layer 58 confined in the outer region 80 have a zero / reduced electromechanical coupling tensor (e=0 or e→0). In some applications, the third portion of the piezoelectric layer 58 confined in the outer region 80 may have a non-zero electromechanical coupling tensor, as does the central portion of the piezoelectric layer 58 (not shown). It should be noted that the value of the electromechanical coupling tensor (zero or non-zero) in the outer region 80 has a negligible effect on the quality factor / BO mode suppression of the BAW resonator 50.

[0081] Figures 12A-12B Simulation results for various configurations of type I BAW resonators are shown. Fig. 12A The quality factor simulation results of a type I BAW resonator with three different configurations are shown: 1) the first configuration of the type I BAW resonator is the same as shown in FIG4 , 2) the second configuration of the type I BAW resonator is the same as shown in FIG4 , Fig. 9 and 3) a third configuration of the type I BAW resonator includes a mass-loaded BO region (e.g., mass-loaded BO region 94) but does not include a recessed BO region (e.g., recessed BO region 92), wherein portions of the piezoelectric layer confined in the mass-loaded BO region and the external region (e.g., mass-loaded BO region 94 and external region 80) are zero-coupled (not shown). Fig. 12B Demonstrating 1-|S of a Type I BAW resonator with a first configuration and a second configuration 11 |Response simulation results (1-|S 11 | is equal to the power ratio lost in the resonator).

[0082] It is clearly shown that for the I-type resonator with the first configuration or the second configuration, its quality factor can achieve relatively high values ​​by selecting the optimized width of the mass-loaded BO region. In this illustration, the I-type resonator with the first configuration can reach 2900Qp when the mass-loaded BO region has a width of 3.25μm, while the I-type resonator with the second configuration can reach 2500Qp when the mass-loaded BO region has a width of 3.6μm. However, the I-type resonator with the third configuration can only reach a maximum of 1800Qp, which is caused by the reduced lateral energy confinement. On the other hand, the I-type resonator with the first configuration has a very strong undesirable BO mode, while the I-type resonator with the second configuration has a superiorly suppressed BO mode. Therefore, the I-type resonator with the second configuration (such as Fig. 9) has a relatively high quality factor and excellent suppression of the BO mode.

[0083] It should be noted that a conventional I-type resonator usually does not require a recessed frame because it does not have parasitic modes below the series resonant frequency (fs). Figure 7-10 ) includes a concave frame (eg, first concave frame 86 or second concave frame 102) and a zero-coupling piezoelectric layer portion below to achieve superior quality factor results.

[0084] Figures 13A-13B Simulation results for various configurations of Type II BAW resonators are shown. Fig.13A The quality factor simulation results of a type II BAW resonator with three different configurations are shown: 1) the first configuration of the type II BAW resonator is the same as shown in FIG4, 2) the second configuration of the type II BAW resonator is the same as shown in FIG4, Fig. 9 and 3) a third configuration of the Type II BAW resonator includes a mass-loaded BO region (e.g., mass-loaded BO region 94) but does not include a recessed BO region (e.g., recessed BO region 92), wherein portions of the piezoelectric layer confined in the mass-loaded BO region and the external region (e.g., mass-loaded BO region 94 and external region 80) are zero-coupled (not shown). Fig. 13B 1-|S of a type II BAW resonator with a first configuration, a second configuration, and a third configuration are shown. 11 |Response simulation results.

[0085] It is clearly shown that the Type II resonator with the second configuration has the maximum quality factor and the highest average quality factor within the width range of the mass-loaded BO region. In this illustration, the Type II resonator with the second configuration can reach 3700Qp when the mass-loaded BO region has a width of 4.3μm, and can achieve an average of 3450Qp within the width range of 0.2μm-5μm. For the Type II resonator with the first configuration or the third configuration, its quality factor can achieve a relatively high value by selecting the optimized width of the mass-loaded BO region. In this illustration, the Type II resonator with the first configuration can reach 3000Qp when the mass-loaded BO region has a width of 2.5μm, and the Type II resonator with the third configuration can reach 3600Qp when the mass-loaded BO region has a width of 2.7μm. On the other hand, the Type II resonator with the first configuration has a strong undesirable BO mode, while the Type II resonator with the second configuration or the third configuration has a superiorly suppressed BO mode (the Type II resonator with the second configuration has the smallest BO mode).

[0086] Note that in order to achieve comparable desired Qp values ​​and BO suppression results, the third configuration requires a much larger width (e.g., 2.7 μm for 3600 Qp) than the second configuration (e.g., 0.5 μm for 3500 Qp). Therefore, the second configuration (with zero electromechanical coupling tensor in the recess BO + mass loading BO + external region) can have a smaller total zero coupling width than the third configuration (with zero electromechanical coupling tensor in the mass loading BO + external region), which will cause the coupling k of the entire resonator to be smaller. 2 Therefore, the type II resonator with the second configuration (such as Fig. 9 ) can achieve the desired results of quality factor and BO mode suppression without significantly sacrificing coupling across the resonator.

[0087] It is contemplated that any of the aforementioned aspects, and / or the various individual aspects and features described herein, may be combined to obtain additional advantages. Unless otherwise indicated herein, any of the various embodiments disclosed herein may be combined with one or more other disclosed embodiments.

[0088] Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present disclosure. All such improvements and modifications are considered to be within the scope of the concepts disclosed herein and the following claims.

Claims

1. A bulk acoustic wave (BAW) resonator, comprising: Bottom electrode; Top electrode; as well as A piezoelectric layer sandwiched between the bottom electrode and the top electrode, wherein: The BAW resonator is divided into an active region and an external region, wherein the active region corresponds to a section of the BAW resonator in which the top electrode overlaps the bottom electrode, and the external region corresponds to a section of the BAW resonator surrounding the active region; the active region is divided into a central region, a recessed boundary (BO) region, and a mass-loaded BO region, wherein the mass-loaded BO region laterally adjoins the outer region and surrounds the periphery of the active region, the recessed BO region laterally adjoins and is surrounded by the mass-loaded BO region, and the central region laterally adjoins and is surrounded by the recessed BO region; The height of the concave BO region is less than the height of the central region, and the height of the mass-loaded BO region is greater than the height of the concave BO region; and • A first portion of the piezoelectric layer confined in the mass loaded BO region has a zero electromechanical coupling tensor, whereas a central portion of the piezoelectric layer confined in the central region has a non-zero electromechanical coupling tensor. 2 . The BAW resonator of claim 1 , wherein the second portion of the piezoelectric layer confined in the recessed BO region has a zero electromechanical coupling tensor. 3 . The BAW resonator of claim 2 , wherein a third portion of the piezoelectric layer confined in the outer region has a zero electromechanical coupling tensor.

4. The BAW resonator of claim 1, wherein the piezoelectric layer is formed of one of the group consisting of: scandium aluminum nitride, lead zirconate titanate, lead titanate, barium titanate, and hafnium oxide. 5 . The BAW resonator of claim 1 , wherein the height of the mass-loaded BO region is greater than the height of the central region. The BAW resonator of claim 1 , wherein the height of the central region is greater than the height of the mass-loaded BO region. 7 . The BAW resonator of claim 3 , wherein the BAW resonator is a type I resonator or a type II resonator.

8. The BAW resonator of claim 1, wherein: a second portion of the piezoelectric layer confined in the recessed BO region has a non-zero electromechanical coupling tensor; and • The BAW resonator is a Type II resonator.

9. The BAW resonator of claim 8, wherein the third portion of the piezoelectric layer confined in the outer region has a zero electromechanical coupling tensor.

10. The BAW resonator of claim 1 , wherein the top electrode comprises a BO structure surrounding a periphery of the top electrode, wherein: The BO structure comprises a concave frame, a convex frame and a transition section, wherein the concave frame has a first height, the convex frame has a second height, the transition section is laterally located between the concave frame and the convex frame, wherein the second height is greater than the first height, and the height of the transition section changes from the first height of the concave frame to the second height of the convex frame to form a tapered wall; The raised frame surrounds the perimeter of the top electrode such that an outer peripheral edge of the raised frame is a peripheral edge of the top electrode, and the recessed frame is connected to and surrounded by the raised frame via the transition section; a central electrode portion of the top electrode confined in the central region of the BAW resonator has a third height greater than the first height of the recessed frame; and ●The recessed BO region corresponds to a segment of the BAW resonator including the recessed frame, residing above the recessed frame, and residing below the recessed frame, and the mass-loaded BO region corresponds to a segment of the BAW resonator including the raised frame and the transition segment, residing above the raised frame and the transition segment, and residing below the raised frame and the transition segment, wherein the height variation among the central region, the recessed BO region, and the mass-loaded BO region is formed due to the height variation of the top electrode.

11. The BAW resonator according to claim 10, wherein: The top electrode is composed of a first top electrode layer, a second top electrode layer, and a third top electrode layer, wherein the first top electrode layer is formed directly above the piezoelectric layer, the second top electrode layer is formed above the first top electrode layer, and the third top electrode layer is formed above the second top electrode layer; and The concave frame, the transition section and the convex frame of the top electrode are formed due to thickness variations of one or more of the first top electrode layer, the second top electrode layer and the third top electrode layer. 12 . The BAW resonator of claim 11 , wherein the concave frame, the transition section, and the convex frame of the top electrode are formed due to a thickness variation of the first top electrode layer.

13. The BAW resonator of claim 11, wherein: ● the first top electrode layer is formed of one of the group consisting of tungsten (W), molybdenum (Mo), and platinum (Pt); The second electrode layer is formed of titanium tungsten (TiW) or titanium (Ti); and • The third top electrode layer is formed of aluminum copper (AlCu). 14 . The BAW resonator of claim 11 , further comprising a passivation layer covering the top electrode and a portion of the piezoelectric layer exposed by the top electrode, wherein the passivation layer has a uniform thickness.

15. The BAW resonator of claim 1 , wherein the top electrode comprises a BO structure surrounding a perimeter of the top electrode, wherein: ●The BO structure comprises a concave frame, an inner convex frame, a first transition section, an outer convex frame and a second transition section, wherein the concave frame has a first height, the inner convex frame has a second height, the first transition section is laterally located between the concave frame and the inner convex frame, the outer convex frame has a third height, the second transition section is laterally located between the inner convex frame and the outer convex frame, wherein the second height is greater than the first height and less than the third height, the height of the first transition section changes from the first height of the concave frame to the second height of the inner convex frame, and the height of the second transition section changes from the second height of the inner convex frame to the third height of the outer convex frame; The outer raised frame surrounds the perimeter of the top electrode such that an outer peripheral edge of the outer raised frame is a peripheral edge of the top electrode; The inner lug frame is connected to the outer lug frame via the second transition section and is surrounded by the outer lug frame; The female frame is connected to and surrounded by the inner male frame via the first transition section; a central electrode portion of the top electrode confined in the central region of the BAW resonator has a fourth height greater than the first height of the recessed frame; and ●The recessed BO region corresponds to a segment of the BAW resonator including the recessed frame, residing above the recessed frame, and residing below the recessed frame, and the mass-loaded BO region corresponds to a segment of the BAW resonator including a combination of the first transition segment, the inner raised frame, the second transition segment, and the outer raised frame, residing above the combination, and residing below the combination.

16. The BAW resonator of claim 15, further comprising a dielectric BO ring around the perimeter of the active region, wherein: The top electrode is composed of a plurality of top electrode layers formed directly over the piezoelectric layer and extending over the dielectric BO ring; and • The height variation between the central region, the recessed BO region and the mass loaded BO region is due to a combination of the thickness variation of the top electrode layer of the top electrode and the thickness of the dielectric BO ring.

17. The BAW resonator according to claim 1, further comprising a passivation layer covering the top electrode and a portion of the piezoelectric layer exposed by the top electrode, wherein the passivation layer is formed of silicon nitride (SiN), silicon dioxide (SiO2) or silicon oxynitride (SiON).

18. The BAW resonator of claim 17, wherein: The top electrode comprises a BO structure having a convex frame and a transition section, and the passivation layer comprises a concave frame; The raised frame surrounds the periphery of the top electrode such that an outer peripheral edge of the raised frame is a peripheral edge of the top electrode and the transition section laterally abuts and is surrounded by the raised frame; The recessed frame of the passivation layer is laterally adjacent to and confined in the transition section of the top electrode; ●The thickness of the recessed frame is less than the thickness of the passivation layer in the central area; and ●The recessed BO area corresponds to the recessed frame of the BAW resonator including the passivation layer and the segment residing below the recessed frame of the passivation layer, and the mass load BO area corresponds to the segment of the BAW resonator including the raised frame and the transition segment, residing above the raised frame and the transition segment, and residing below the raised frame and the transition segment.

19. The BAW resonator of claim 18, wherein: The top electrode is composed of a plurality of top electrode layers formed directly above the piezoelectric layer; and • The height variation between the central region, the recessed BO region and the mass loaded BO region is due to the recessed frame of the passivation layer and the BO structure of the top electrode.

20. The BAW resonator of claim 19, wherein: The portion of the top electrode confined within the central region and the recessed BO region has a uniform thickness, and the height variation between the central region and the recessed BO region is due to the thickness variation of the passivation layer; and • The height variation between the recessed BO region and the mass loading BO region is due to the thickness variation of the top electrode layer.

Images