KR20250069872A - BAW resonator with no electromechanical coupling in the boundary region, flat and steep skirt, and square shoulders - Google Patents

Abstract

The present disclosure relates to a bulk acoustic wave (BAW) resonator including a lower electrode (56), an upper electrode (60), and a piezoelectric layer (58) sandwiched between the lower electrode and the upper electrode. Here, an active region of the BAW resonator is divided into a central region (82), a concave boundary (BO) region (92) laterally adjacent to and surrounding the central region, and a mass loading BO (94) region laterally adjacent to and surrounding the concave BO region. A height (H1) of the concave BO region is lower than a height (H3) of the central region and lower than a height (H2) of the mass loading region. A first portion (76) of the piezoelectric layer confined to the mass loading BO region has an electromechanical coupling tensor of zero, whereas a central portion (77) of the piezoelectric layer confined to the central region has an electromechanical coupling tensor that is not zero.

Description

BAW resonator with no electromechanical coupling in the boundary region, flat and steep skirt, and square shoulders

Related Applications

This application claims the benefit of U.S. Provisional Patent Application No. 63/407,459, filed September 16, 2022, the disclosure of which is incorporated herein by reference in its entirety.

Technical field

The present disclosure relates to a bulk acoustic wave (BAW) resonator having no coupling in the boundary (BO) region of the BAW resonator.

Acoustic resonators, particularly bulk acoustic wave (BAW) resonators, are used in many high-frequency communications applications. In particular, BAW resonators are often used in filter networks that operate at frequencies exceeding 1.5 gigahertz (GHz) and require a flat passband, have very 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, tend to decrease in size with increasing operating frequency, and are relatively stable over a wide temperature range. As such, BAW-based filters are suitable filters for many third-generation (3G) and fourth-generation (4G) wireless devices, and are expected to be used primarily in filter applications for fifth-generation (5G) wireless devices. Many of these wireless devices support cellular, Wi-Fi, Bluetooth, and/or short-range communications on the same wireless device, and therefore have very challenging filtering requirements. These demands are driving the continued increase in the complexity of wireless devices, while also driving a continued need to improve the performance of BAW resonators and BAW-based filters, and reduce their associated cost and size.

The present disclosure relates to a bulk acoustic wave (BAW) resonator having no coupling within the boundary (BO) region of the BAW resonator. The disclosed BAW resonator includes a lower electrode, an upper electrode, and a piezoelectric layer sandwiched between the lower electrode and the upper electrode. Here, the BAW resonator is divided into an active region and an outer region. The active region corresponds to a section of the BAW resonator where the upper electrode and the lower electrode overlap, while the outer 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-loaded BO region. The mass-loaded BO region is laterally adjacent to the outer region and around the periphery of the active region, the concave BO region is laterally adjacent to and surrounded by the mass-loaded BO region, and the central region is laterally adjacent to and 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-loaded BO region is greater than the height of the concave BO region. The first part of the piezoelectric layer, which is confined to the mass-loaded BO region, has an electromechanical coupling tensor of zero, whereas the central part of the piezoelectric layer, which is confined to the central region, has a non-zero electromechanical coupling tensor.

In one embodiment of the BAW resonator, the second part of the piezoelectric layer, which is confined to the concave BO region, has an electromechanical coupling tensor of zero. The third part of the piezoelectric layer, which is confined to the outer region, has an electromechanical coupling tensor of zero.

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.

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

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

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

In one embodiment of the BAW resonator, the second part of the piezoelectric layer, which is confined to the concave BO region, has a non-zero electromechanical coupling tensor. The third part of the piezoelectric layer, which is confined to the outer region, has a zero electromechanical coupling tensor. The BAW resonator is a type II resonator.

In one embodiment of the BAW resonator, the upper electrode includes a BO structure around a periphery of the upper electrode. In the present invention, the BO structure has a concave frame having a first height, a rising frame having a second height greater than the first height, and a transition section laterally between the concave frame and the rising frame. The height of the transition section varies from the first height of the concave frame to the second height of the rising frame to form a tapered wall. The rising frame is around the periphery of the upper electrode, such that an outer peripheral edge of the rising frame is a peripheral edge of the upper electrode. The concave frame is connected to the rising frame through the transition section and is surrounded by the rising frame. A central electrode portion of the upper electrode, which is confined to a 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 the section of the BAW resonator that includes the concave frame and is located above and below it, and the mass-loaded BO region corresponds to the section of the BAW resonator that includes the rising frame and the transition section and is located above and below it. The height variation between the central region, the concave BO region, and the mass-loaded BO region is formed due to the height variation of the upper electrode.

In one embodiment of the BAW resonator, the upper electrode comprises a first upper electrode layer formed directly on the piezoelectric layer, a second upper electrode layer formed on the first upper electrode layer, and a third upper electrode layer formed on the second upper electrode layer. The concave frame, the transition section, and the rising frame of the upper electrode are formed by a thickness variation of one or more of the first upper electrode layer, the second upper electrode layer, and the third upper electrode layer.

In one embodiment of the BAW resonator, the concave frame, transition section, and rising frame of the upper electrode are formed by variations in the thickness of the first upper electrode layer.

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

In one embodiment, the BAW resonator further comprises a passivation layer covering the upper electrode and a portion of the piezoelectric layer exposed through the upper electrode, wherein the passivation layer has a uniform thickness.

In one embodiment of the BAW resonator, the upper electrode comprises a BO structure around a periphery of the upper electrode. Here, the BO structure comprises a concave frame having a first height, an inner rising frame having a second height, a first transition section laterally between the concave frame and the inner rising frame, an outer rising frame having a third height, and a second transition section laterally between the inner rising frame and the outer rising frame. The second height is higher than the first height and lower than the third height. The height of the first transition section varies from the first height of the concave frame to the second height of the inner rising frame, while the height of the second transition section varies from the second height of the inner rising frame to the third height of the outer rising frame. The outer rising frame is around a periphery of the upper electrode, such that an outer periphery edge of the outer rising frame is a periphery edge of the upper electrode. The inner rising frame is connected to the outer rising frame via the second transition section and is surrounded by the outer rising frame. The concave frame is connected to the inner rising frame through the first transition section and is surrounded by the inner rising frame. The central electrode portion of the upper electrode, which is limited to the central region of the BAW resonator, has a fourth height higher than the first height of the concave frame. The concave BO region corresponds to a section of the BAW resonator that includes the concave frame, is positioned above it, and is positioned below it, and the mass load BO region corresponds to a section of the BAW resonator that includes a combination of the first transition section, the inner rising frame, the second transition section, and the outer rising frame, is positioned above it, and is positioned below it.

According to one embodiment, the BAW resonator further comprises a dielectric BO ring around the periphery of the active region. Here, the upper electrode is formed directly over the piezoelectric layer and comprises a plurality of upper electrode layers extending over the dielectric BO ring. The combination of the thickness variation of the upper electrode layers of the upper electrode and the thickness of the dielectric BO ring forms a height variation between the central region, the concave BO region, and the mass-loaded BO region.

In one embodiment, the BAW resonator further comprises a passivation layer covering the upper electrode and a portion of the piezoelectric layer exposed through the upper electrode, wherein the passivation layer is formed of silicon nitride (SiN), silicon dioxide (SiO ₂ ), or silicon oxynitride (SiON).

In one embodiment of the BAW resonator, the upper electrode comprises a BO structure having a rising frame and a transition section, and the passivation layer comprises a concave frame. The rising frame is around a periphery of the upper electrode, such that an outer periphery edge of the rising frame is a periphery edge of the upper electrode, and the transition section is laterally adjacent to and surrounded by the rising frame. The concave frame of the passivation layer is laterally adjacent to and limited by the transition section of the upper electrode. The thickness of the concave frame is thinner than the thickness of the passivation layer within the central region. The concave BO region corresponds to a section of the BAW resonator comprising the concave frame of the passivation layer and positioned beneath it, and the mass-loaded BO region corresponds to a section of the BAW resonator comprising the rising frame and the transition section and positioned above and below it.

In one embodiment of the BAW resonator, the upper electrode is composed of a plurality of upper electrode layers formed directly on the piezoelectric layer. The concave frame of the passivation layer and the BO structure of the upper electrode form a height variation between the central region, the concave BO region, and the mass-loaded BO region.

In one embodiment of the BAW resonator, the portion of the upper electrode defined within the central region and the concave BO region has a uniform thickness, and the height variation between the central region and the concave BO region is due to the thickness variation of the passivation layer. The height variation between the concave BO region and the mass-loaded BO region is due to the thickness variation of the upper electrode layer.

In other embodiments, any of the embodiments described above, either individually or together, and/or various separate embodiments and features as described herein may be combined for additional advantages. Any of the various features and elements disclosed herein may be combined with one or more other disclosed features and elements, unless indicated to the contrary herein.

Those skilled in the art will appreciate the scope of the present disclosure and realize further aspects thereof after reading the following detailed description of preferred embodiments in conjunction with the accompanying drawings.

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 present disclosure.
Figure 1 is a diagram illustrating a conventional bulk elastic wave (BAW) resonator.
Figure 2 is a diagram graphically depicting the magnitude and phase of the electrical impedance as a function of frequency for a relatively ideal BAW resonator.
Figures 3a to 3c are diagrams graphically illustrating phase curves for various conventional BAW resonators.
FIG. 4 is a diagram illustrating a conventional BAW resonator having an upper electrode including a boundary (BO) ring.
Figure 5 is a diagram showing the relationship of the BO ring width to the quality factor at the anti-resonant frequency (Q _p ) and the relative intensity of the formed BO mode.
Figure 6 is a diagram graphically illustrating phase curves for BAW resonators with and without BO rings.
FIG. 7 is a cross-sectional view illustrating an exemplary BAW resonator without coupling in the BO region according to an embodiment of the present disclosure.
Figure 8 is a cross-sectional view showing details of the upper electrode within the BAW resonator illustrated in Figure 7.
FIGS. 9 to 11 are cross-sectional views illustrating alternative BAW resonators without coupling in the BO region according to embodiments of the present disclosure.
Figures 12a and 12b show simulation results for various configurations of a type I BAW resonator.
Figures 13a and 13b show simulation results for various configurations of type II BAW resonators.
For clarity, it will be appreciated that Figures 1 through 13b may not be drawn to scale.

The implementations described below are intended to provide information necessary to enable those skilled in the art to perform the implementations and to illustrate the best mode of carrying out the implementations. 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 will recognize applications of these concepts that are not specifically mentioned herein. It is to be understood that these concepts and applications are within the scope of the present disclosure and the appended claims.

Although the terms first, second, etc. may be used herein to describe various elements, it will be understood that these elements are not limited by these terms. These terms are only used to distinguish one element from another. For example, a first element may be referred to as a second element, and similarly, a second element may be referred to as 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.

When an element, such as a layer, region, or substrate, is referred to as being "on" or "extending" "over" another element, it will be understood that it can be directly on or directly extending over the other element, or that intervening elements may also be present. In contrast, when an element is referred to as being "directly on" or "extending directly over" another element, no intervening elements are present. Likewise, when an element, such as a layer, region, or substrate, is referred to as being "on" or "extending over" another element, it will be understood that it can be directly on or directly extending over the other element, or that intervening elements may also be present. In contrast, when an element is referred to as being "directly on" or "extending directly over" another element, no intervening elements are present. Furthermore, when an element is referred to as being "connected" or "coupled" to another element, it will be understood that it can be directly connected or coupled to the other element, or that intervening elements may also be present. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, no intervening elements are present.

Relative terms such as "below" or "above" 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 depicted in the drawings. 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 drawings.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular forms "a," "an," and "a particular one" are intended to include the plural forms as well, unless the context clearly dictates otherwise. It will also be understood that the terms "comprises," "comprising," "comprises," and/or "comprising", when used herein, 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.

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

The embodiments are described herein with reference to schematic drawings of embodiments of the present disclosure. As such, actual dimensions of layers and elements may vary, and variations from the shapes of the drawings are expected, for example, as a result of manufacturing techniques and/or tolerances. For example, regions illustrated or described as square or rectangular may have rounded or curved features, and regions depicted as straight lines may have some irregularities. Accordingly, regions depicted in the drawings are schematic drawings, and their shapes are not intended to illustrate the precise shapes of regions of the elements, and are not intended to limit the scope of the present disclosure. In addition, the dimensions of structures or regions may be exaggerated relative to other structures or regions for illustrative purposes, and are thus provided to illustrate the general structure of the subject matter, and may or may not be drawn to scale. Common elements between the drawings may be indicated herein by common element numbers and may not be described again subsequently.

The present disclosure relates to bulk elastic wave (BAW) resonators, and more particularly to a BAW resonator having no coupling in the boundary (BO) region. The disclosed BAW resonator provides high quality factor, suppression of spurious modes, and suppression of BO modes.

Before going into the specifics of these concepts, an overview of BAW resonators and filters utilizing BAW resonators is provided. BAW resonators are used in many high frequency filter applications. An exemplary BAW resonator (10) is illustrated in FIG. 1. The BAW resonator (10) is a solid mounted resonator (SMR) type BAW resonator and typically 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) positioned on the reflector (14) and sandwiched between an upper electrode (20) and a lower electrode (22). The upper and lower electrodes (20, 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 material. Although depicted in FIG. 1 as comprising a single layer, the piezoelectric layer (18), the upper electrode (20), and/or the lower electrode (22) may comprise multiple layers of the same material, multiple layers where at least two layers are of different materials, or multiple layers where each layer is of a different material.

The BAW resonator (10) is divided into an active region (24) and an outer region (26). The active region (24) generally corresponds to the portion of the BAW resonator (10) where the upper and lower electrodes (20, 22) overlap, and also includes a layer beneath the overlapping upper and lower electrodes (20, 22). The outer region (26) corresponds to the portion of the BAW resonator (10) that surrounds the active region (24).

In the case of the BAW resonator (10), when an electrical signal is applied across the upper electrode (20) and the lower electrode (22), elastic waves are excited in the piezoelectric layer (18). These elastic waves mainly propagate vertically. A major goal in the design of the BAW resonator is to confine these vertically propagating elastic waves in the transducer (16). The upwardly traveling acoustic waves are reflected back into the transducer (16) by the air-metal boundary at the upper surface of the upper electrode (20). The downwardly traveling acoustic waves are reflected back into the transducer (16) by the reflector (14) or by the air cavity provided directly below the transducer (16) in a film bulk acoustic resonator (FBAR).

The reflector (14) is typically formed by a stack of reflective layers (RL) (28A through 28E) (generally referred to as reflective layers (28)) that alternate in material composition to produce a significant reflection coefficient at the junction of adjacent reflective layers (28). Typically, the reflective layers (28) alternate between materials having high and low acoustic impedance, such as tungsten (W) and silicon dioxide (SiO ₂ ). Although only five reflective layers (28) are illustrated in FIG. 1 , the number of reflective layers (28) and the structure of the reflector (14) may vary from design to design.

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 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 commonly referred to as the series resonant frequency (f _s ), and the anti-resonant frequency is commonly referred to as the parallel resonant frequency (f _p ). The series resonant frequency (f _s ) occurs when the magnitude of the impedance or reactance of the BAW resonator (10) approaches zero. The parallel resonant frequency (f _p ) occurs when the magnitude of the impedance or reactance of the BAW resonator (10) peaks at a relatively 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 lower and upper electrodes (22, 20).

For the phase (Φ), the BAW resonator (10) acts like an inductance providing a 90° phase shift between the series resonance frequency (f _s ) and the parallel resonance frequency (f _p ). In contrast, the BAW resonator (10) acts like a capacitance providing a -90° phase shift below the series resonance frequency (f _s ) and above the parallel resonance frequency (f _p ). The BAW resonator (10) provides a very low (almost zero) resistance at the series resonance frequency (f _s ) and a very high resistance at the parallel resonance frequency (f _p ). The electrical properties of the BAW resonator (10) are suitable for realizing a very high quality factor (Q) inductance over a relatively short frequency range, which has proven to be very advantageous for high-frequency filter networks, especially for operation at frequencies above about 1.8 GHz.

Unfortunately, the phase (Φ) curve of 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. The phase curve of Fig. 3a is not a smooth curve, but includes ripples below the series resonant frequency (f _s ), between the series resonant frequency (f _s ) and the parallel resonant frequency (f _p ), and above the parallel resonant frequency (f _p ). The ripples are a result of spurious modes, which are caused by spurious resonances occurring at the corresponding frequencies. Although most elastic waves in the BAW resonator (10) propagate vertically, various boundary conditions for the transducer (16) result in the propagation of lateral (horizontal) elastic 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).

As illustrated in FIG. 4, a BO ring (30) is formed on or inside the upper electrode (20) (not illustrated) to suppress a specific spurious mode. The spurious mode suppressed by the BO ring (30) is a mode higher than the series resonant frequency (f _s ), as highlighted by circles A and B in the phase curve of FIG. 3b. Circle A shows suppression of ripples and, as a result, suppression of spurious modes in the passband of the phase curve, which is located between the series resonant frequency (f _s ) and the parallel resonant frequency (f _p ). Circle B shows suppression of ripples and, as a result, suppression of spurious modes above the parallel resonant frequency (f _p ). In particular, spurious modes at the upper shoulder of the passband, just below the parallel resonant frequency (f _p ), and spurious modes above the passband are suppressed, as evidenced by flat or virtually ripple-free phase curves between the series resonant frequency (f _s ) and the parallel resonant frequency (f _p ) and above the parallel resonant frequency (f _p ).

The BO ring (30) corresponds to the mass loading of a portion of the upper electrode (20) extending around the periphery of the active region (24). In this regard, the BO ring (30) having the mass loading forms a rising frame arranged around the periphery of the upper electrode (20). The BO ring (30) may correspond to a thickened portion of the upper electrode (20) or may correspond to an additional layer of a suitable material applied over the upper electrode (20). The portion of the BAW resonator (10) including and positioned beneath the BO ring (30) is referred to as a BO region (32). Thus, the BO region (32) corresponds to an outer periphery portion of the active region (24) and is positioned 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). As illustrated in FIG. 3b, the BO ring (30) is effective in suppressing spurious modes above the series resonant frequency (f _s ), whereas the BO ring (30) has little or no effect on spurious modes below the series resonant frequency (f _s ). Spurious modes that fall below the series resonant frequency (f _s ) are suppressed using a technique often referred to as apodization.

Apodization acts to avoid, or at least significantly reduce, any lateral symmetry in the BAW resonator (10), or at least in the converter (16) thereof. Lateral symmetry corresponds to the footprint of the converter (16), and avoiding lateral symmetry corresponds to avoiding a symmetry associated with a side of the footprint. For example, a footprint corresponding to a pentagon may be selected instead of a square or a rectangle. Avoiding symmetry helps to reduce the presence of lateral standing waves in the converter (16). Fig. 3c illustrates the effect of apodization in suppressing spurious modes below the series resonant frequency (f _s ). Assuming that the BO ring (30) is not provided, it can be readily seen from Fig. 3c that the apodization does not suppress such spurious modes above the series resonant frequency (f _s ). As such, a typical BAW resonator (10) uses both apodization and BO rings (30).

The thickness or height of the BO ring (30) can be measured perpendicular to or away from the top surface of the piezoelectric layer (18), and the width of the BO ring (30) can be measured parallel to or transverse to the piezoelectric layer (18). Simultaneously adjusting the thickness and width of the BO ring (30) can suppress spurious modes and improve the quality factor at the anti-resonant frequency (Q _p ). 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 can cause undesirable modes, or BO modes, to occur at frequencies below the series resonant frequency (f _s ).

FIG. 5 is a diagram illustrating a graphical representation of the relationship between the BO ring width and the quality factor at the anti-resonant frequency (Q _p ) and the relative intensity of the formed BO mode. In FIG. 5 , the x-axis represents the BO ring width in microns (μm), while the primary y-axis represents Q _p and the secondary y-axis represents the relative intensity or magnitude of the formed BO mode. The diagram of FIG. 5 shows data for three wafers, each containing a BAW resonator with a different BO ring width. As shown, the Q _p values generally increase with increasing BO ring width, with the highest Q _p values corresponding to BO ring widths slightly exceeding 3 μm. Also as shown, the BO mode intensity also generally increases with increasing BO ring width. For even higher BO ring widths, the Q _p values decrease while the BO mode intensity values remain high. In this way, a BO ring tuned to provide high Q _p may also introduce undesirable BO modes in the corresponding BAW device.

Fig. 6 is a diagram illustrating phase curves for BAW resonators with and without a BO ring. As illustrated, spurious modes present in 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. The BO modes may be introduced outside or even inside the passband of the filter and may limit the design or use of the BAW resonator for wide bandwidth filtering applications. When the BO modes are within the passband, the insertion loss may be affected. The BO modes present outside the passband may be problematic for 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 may cause interference during multiplexing.

It is known that the main cause of the BO mode here is that the electromechanical coupling coefficient (k ² e≠0) of the piezoelectric layer within the BO region (e.g., the piezoelectric layer (18) within the BO region (32)) is not 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, when the polarization of the piezoelectric layer changes, the piezoelectric coefficient (d) will change accordingly, and consequently, the electromechanical coupling coefficient of the BAW resonator will also change. U.S. Provisional Patent Application No. 18/150,621 (U.S. Provisional Patent Application No. 6,332,4961) and U.S. Provisional Patent Application No. 18/150,627 (U.S. Provisional Patent Application No. 6,332,4968) describe methods to achieve zero/reduced electromechanical coupling coefficient (k ² e = 0 or k ² e → 0) in the BO region. However, zero/reduced electromechanical coupling coefficient in the BO region may increase lateral energy loss, resulting in a decrease in the Q _p value.

FIG. 7 is a cross-sectional view illustrating an exemplary BAW resonator (50) having no coupling in the BO region according to an embodiment of the present disclosure. For purposes of the present disclosure, the BAW resonator (50) is a BAW SMR and includes a reflector (54), a lower electrode structure (56) over the reflector (54), a piezoelectric layer (58) over the lower electrode structure (56), and an upper electrode (60) over the piezoelectric layer (58). In other applications, the BAW resonator (50) may be an FBAR in which the reflector (54) is omitted.

In detail, the reflector (54) includes a low acoustic impedance region (62), and a plurality of high acoustic impedance layers (64) are embedded within the low acoustic impedance region (62). In the present invention, 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 other applications, the number of acoustic impedance layers (64) embedded within the low acoustic impedance region (62) may be less or more. In the present invention, the lower high acoustic impedance layer (64-L) is located over a lower portion (62-B) of the low acoustic impedance region (62). The upper high acoustic impedance section (64-U) is vertically positioned above the lower high acoustic impedance section (64-L) and is separated from the lower high acoustic impedance section (64-L) by a middle portion (62-M) of the low acoustic impedance region (62). The upper portion (62-T) of the low acoustic impedance region (62) is positioned above the upper high acoustic impedance layer (64-U). The low acoustic impedance region (62) has lower acoustic impedance, lower density, and lower stiffness 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 lower 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 upper portion (62-T) of the low acoustic impedance region (62) is one RL of the reflector (54).

The lower electrode structure (56) is formed over the upper portion (62-T) of the low acoustic impedance region (62) and includes a lower electrode (66) and a planarizing oxide (68). The lower electrode (66) is vertically over the high acoustic impedance layer (64). The planarizing oxide (68) surrounds the lower electrode (66) and can electrically isolate the lower electrode (66) from an external lower electrode (not shown). The lower electrode (66) can include at least two lower electrode layers (70 and 72). The second lower electrode layer (72) is over the upper portion (62-T) of the low acoustic impedance region (62) and can be formed of aluminum copper (AlCu), while the first lower electrode layer (70) is over the second lower electrode layer (72) and can be formed of W, Mo, or Pt. The planarizing oxide (68) can be formed of SiO ₂ .

The piezoelectric layer (58) is formed on the lower electrode structure (56) and at least completely covers the lower electrode (66). The upper electrode (60) is formed on the piezoelectric layer (58) and is vertically positioned on the lower electrode (66). The upper electrode (60) includes a BO structure (76) positioned around the periphery of the upper electrode (60) and a central electrode portion (77) surrounded by the BO structure (76).

The BAW resonator (50) is divided into an active region (78) and an outer region (80). The active region (78) corresponds to the section of the BAW resonator (50) where the upper and lower electrodes (60, 66) overlap, and also includes layers above, middle, and below the overlapping upper and lower electrodes (60, 66). The outer region (80) corresponds to the 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 the section of the BAW resonator (50) that includes, is located above, and is located below the BO structure (76). The central region (82) is defined laterally within the BO region (84) and does not overlap the BO structure (76). The central electrode portion (77) of the upper electrode (60) is limited to the central region (82).

In particular, the lower electrode (66) may extend beyond the peripheral edge of the upper electrode (60). As described above, the active region (78) is formed where the upper and lower electrodes (60, 66) overlap. In this manner, misalignment of the upper electrode (60) and the lower electrode (66) during manufacturing may impair the performance of the BAW resonator (50). By arranging the lower electrode (66) having a larger lateral dimension than the upper electrode (60), the alignment tolerance for the placement of the upper electrode (60) may be increased, thereby improving the manufacturing tolerance.

In the present embodiment, the BO structure (76) includes a first concave frame (86) having a first height (H1) (measured from a top surface of the piezoelectric layer (58)), a rising frame (88) having a second height (H2) (measured from a top surface of the piezoelectric layer (58)), and a transition section (90) laterally between the first concave frame (86) and the rising frame (88). The rising frame (88) can extend around a periphery of the upper electrode (60), such that an outer periphery edge of the rising frame (88) can be a periphery edge of the upper electrode (60). The first concave frame (86) is connected to and confined to the rising frame (88) through the transition section (90). Additionally, the central electrode portion (77) defined in the central region (82) of the BAW resonator (50) has a third height (H3) higher than the first height (H1) of the first concave frame (86). Typically, the third height (H3) of the central electrode portion (77) is lower than the second height (H2) of the rising frame (88) (i.e., H2>H3>H1). In some applications, the third height (H3) of the central electrode portion (77) may be higher than the second height (H2) of the rising 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 rising frame (88) to form a tapered wall. The angle α formed between the tapered wall of the transition section (90) and the horizontal plane (e.g., parallel to the upper surface of the piezoelectric layer (58)) is 30 to 60 degrees. The height ratio between the first height (H1) of the first concave frame (86), the second height (H2) of the rising frame (88), and the third height (H3) of the central electrode portion (77) depends on the material used to form the first concave frame (86) and the rising frame (88).

Here, the BO region (84) includes a concave BO region (92) and a mass-loaded BO region (94), each of which has a ring shape. The concave BO region (92) includes the first concave frame (86), corresponds to a portion of the BAW resonator (50) positioned above and below it. Meanwhile, the mass-loaded BO region (94) corresponds to a section of the BAW resonator (50) including the rising frame (88) and the transition section (90), corresponds to a section of the BAW resonator (50) positioned above and below it. In this way, 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 concave BO region (92) is laterally adjacent to and surrounded by the mass-loaded BO region (94), and the central region (82) is laterally adjacent to and surrounded by the concave BO region (92).

FIG. 8 illustrates a detail of a BO structure (76) of the upper electrode (60). In this illustration, the upper electrode (60) is composed of a first upper electrode layer (96-1) formed directly on the piezoelectric layer (58), a second upper electrode layer (96-2) formed on the first upper electrode layer (96-1), and a third upper electrode layer (96-3) formed on the second upper electrode layer (96-2). The first upper electrode layer (96-1) may be formed of W, Mo, Pt, or other electrically conductive material having high acoustic impedance characteristics. The second electrode layer (96-2) may be formed of titanium tungsten (TiW) or titanium (Ti) and may function as a seed layer. The third upper electrode layer (96-3) may be formed of AlCu or other high electrically conductive material. The first concave frame (86) and the rising frame (88) are formed by a change in the thickness of one or more upper electrode layers (96).

For this urban purpose, the first concave frame (86) and the rising frame (88) are formed due to the change in the thickness of the first upper electrode layer (96-1). In the first concave frame (86), the first upper electrode layer (96-1) is reduced in thickness (compared to the thickness of the first upper electrode layer (96-1) in the central electrode portion (77), whereas the second and third upper electrode layers (96-2 and 96-3) are not changed in thickness (compared to the thickness of the second and third upper electrode layers (96-2 and 96-3) in the central electrode portion (77), respectively). In the rising frame (88), the first upper electrode layer (96-1) increases in thickness (compared to the thickness of the first upper electrode layer (96-1) within the first concave frame (86), whereas the second and third upper electrode layers (96-2 and 96-3) do not change in thickness (compared to the thicknesses of the second and third upper electrode layers (96-2 and 96-3) within the central electrode portion (77), respectively). In other applications, the first concave frame (86) and the rising frame (88) may be formed by a thickness change of the second upper electrode layer (96-2), a thickness change of the third upper electrode layer (96-3), or a thickness change of any combination of the first, second, and third upper electrode layers (96-1, 96-2, and 96-3) (not shown).

As a non-limiting example, the piezoelectric layer (58) can be formed of scandium aluminum nitride (Sc _x Al _1-x N), lead zirconate titanate (PZT), lead titanate (PTO), barium titanate (BTO), hafnium oxide (HfO ₂ ), or the like. In the present invention, a first portion of the piezoelectric layer (58) that is confined to the concave BO region (92) and aligned below the first concave frame (86) has an electromechanical coupling tensor of zero (e=0). A second portion of the piezoelectric layer (58) that is confined to the mass-loaded BO region (94) and aligned below the rising frame (88) and the transition section (90) has an electromechanical coupling tensor of zero (e=0). That is, the portion of the piezoelectric layer (58) that is confined to the BO region (84) and aligned below the BO structure (76) has an electromechanical coupling tensor of zero (e=0). Additionally, a third portion of the piezoelectric layer (58), which is confined to the outer region (80), has an electromechanical coupling tensor of zero (e=0). Additionally, a central portion of the piezoelectric layer (58), which is confined to the central region (82), aligned below the central electrode portion (77), and not covered by the BO structure (76), has a non-zero electromechanical coupling tensor (e≠0) and is configured to provide a main resonance of the BAW resonator (50). In some applications, the first, second, and third portions of the piezoelectric layer (58) may have a non-zero electromechanical coupling tensor and a very low electromechanical coupling tensor, wherein the very low electromechanical coupling tensor is at most 10% of the non-zero electromechanical coupling tensor in the central portion of the piezoelectric layer (58).

Additionally, the BAW resonator (50) may also include a passivation layer (100) to protect the exemplary BAW resonator (50) from the external environment. The passivation layer (100) covers the upper electrode (60) and a portion of the piezoelectric layer (58) exposed through the upper electrode (60). The passivation layer (100) may be formed of silicon nitride (SiN), SiO2, or silicon oxynitride (SiON).

In one embodiment, the passivation layer (100) can have a uniform thickness of 250 Å to 5000 Å and does not add any additional recessed portion or mass loading to the upper electrode (60). In another embodiment, the passivation layer (100) can also have a variation in thickness and can provide recessed portion and/or mass loading to the upper electrode (60). As illustrated in FIG. 9, the passivation layer (100) includes a second recessed frame (102), while the BO structure (76) of the upper electrode (60) does not include a first recessed frame (86) and only includes a transition section (90) and a rising frame (88). In this illustration, the mass loaded BO region (94) still includes the rising frame (88) and the transition section (90), and corresponds to a section of the BAW resonator (50) positioned above and below it. However, the concave BO region (92) adjacent to and confined within the mass loading BO region (94) corresponds to a section of the BAW resonator (50) that includes and is located beneath the second concave frame (102). That is, the second concave frame (102) of the passivation layer (100) is laterally adjacent to and confined to the transition section (90) of the upper electrode (60). The BO region (84) of the BAW50 still includes the concave BO region (92) and the mass loading BO region (94), while the central region (82) is still surrounded by the BO region (84).

In the present invention, a portion of the upper electrode (60) limited to the concave BO region (92) and a portion of the central electrode portion (77) limited to the central region (82) have the same third height (H3). Accordingly, the height of the transition section (90) varies from the third height (H3) of the rising frame (88) to the second height (H2). In the concave BO region (92), the passivation layer (100) (i.e., the second concave frame (102)) has a first thickness (T1), whereas in the central region (82), the passivation layer (100) has a second thickness (T2). The first thickness (T1) may be 70% to 90% of the second thickness (T2). Additionally, the passivation layer (100) in the mass load BO region (94) and the external region (80) may have a thickness equal to the first thickness (T1) or equal to the second thickness (T2).

A first portion of the piezoelectric layer (58) confined to the concave BO region (92) and aligned below the second concave frame (102) has an electromechanical coupling tensor of zero (e=0). A second portion of the piezoelectric layer (58) confined to the mass load BO region (94) and aligned below the BO structure (76) has an electromechanical coupling tensor of zero (e=0). Likewise, a portion of the piezoelectric layer (58) confined to the BO region (84) has an electromechanical coupling tensor of zero (e=0). A third portion of the piezoelectric layer (58) confined to the outer region (80) still has an electromechanical coupling tensor of zero (e=0). The central portion of the piezoelectric layer (58), which is confined to the central region (82) and not covered by the second concave frame (102) and the BO structure (76), is configured to have a non-zero electromechanical coupling tensor (e≠0) and to provide a main resonance of the BAW resonator (50). In some applications, the first, second and third portions of the piezoelectric layer (58) may have a non-zero electromechanical coupling tensor and a very low electromechanical coupling tensor, wherein the very low electromechanical coupling tensor is at most 10% of the non-zero electromechanical coupling tensor of the central portion of the piezoelectric layer (58).

It is noted that, regardless of whether the concave BO region (92) is determined by the second concave frame (102) of the passivation layer (100) (as illustrated in FIG. 9) or by the first concave frame (86) of the upper electrode (60) (as illustrated in FIGS. 7 to 8), the height of the concave BO region (92) is always lower than the height of the central region (82) and the height of the mass-loaded BO region (94) is always higher than the height of the concave BO region (92). Typically, the height of the mass-loaded BO region (94) is higher than the height of the central region (82) (as illustrated in FIGS. 7 to 9). In some applications, the height of the mass-loaded BO region (94) may be lower than the height of the central region (82).

In addition, the BAW resonator (50) including a zero/reduced portion of the coupling of the first/second concave frames (86/102), the rising frame (88), and the piezoelectric layer (58) can be a type I resonator or a type II resonator. Here, the frame combination of the first/second concave frames (86/102) and the rising frame (88) improves the quality factor of the BAW resonator (50) compared to that of a resonator having only the concave frames or the rising frames. Meanwhile, due to the zero/reduced electromechanical coupling tensor in the first and second portions of the piezoelectric layer (58) (limited to the BO region (84)), a negligible BO mode occurs in the BAW resonator (50). The zero/reduced electromechanical coupling tensor in the first and second portions of the piezoelectric layer (58) can also help to confine the energy inside the BAW resonator (50) and reduce lateral losses.

As illustrated in FIGS. 7-9, within the mass loading BO region (94), the BO structure (76) includes only one transition section (90) and one rising frame (88) forming a single-step configuration. In another application, within the mass loading BO region (94), the BO structure (76) may include two transition sections and two rising frames forming a double-step configuration, as illustrated in FIG. 10. Within the mass loading BO region (94), in addition to the rising frame (88) and the transition section (90), the BO structure (76) further includes a second rising frame (108) having a fourth height (H4) and a second transition section (110). Here, the rising frame (88) is an inner rising frame connected to the first concave frame (86) via the transition section (90). In another application, the first concave frame (86) of the upper electrode (60) may be replaced with a second concave frame (102) of the passivation layer (100) (not shown). The second rising frame (108) is an outer rising frame, which may extend around the periphery of the upper electrode (60) (e.g., an outer periphery edge of the second rising frame (108) may be a periphery edge of the upper electrode (60)) and is connected to the inner rising frame (88) via a second transition section (110). The height of the second transition section (110) varies from a second height (H2) of the inner rising frame (88) to a fourth height (H4) of the outer rising frame (108).

As described above, the inner rising frame (88) is formed by a thickness increment of one or more upper electrode layers (96) (e.g., a thickness increment of the first upper electrode layer (96-1) as illustrated in FIG. 8). In this example, the outer rising frame (108) is formed by a combination of a thickness increment of one or more upper electrode layers (96) and a dielectric BO ring (112) (thickness H4 to H2) around the outer periphery of the upper electrode (60). The dielectric BO ring (112) is positioned vertically between the upper electrode layer (96) and the piezoelectric layer (58). The dielectric BO ring (112) may be formed of SiO ₂ . Here, the portion of the piezoelectric layer (58) that is limited to the BO region (84) and aligned below the BO structure (76) (e.g., below the first concave frame (86), the transition section (90), the rising frame (88), the second transition section (110), and the second rising frame (108)) has an electromechanical coupling tensor of 0/extremely low (e=0 or e→0).

To achieve the desired high quality factor and effectively suppress the BO mode, the BAW resonator (50) (type I resonator or type II resonator) has zero/reduced coupling with both the concave frame and the rising frame in the first, second and third portions of the piezoelectric layer (58) (i.e., the portions of the piezoelectric layer (58) confined to the concave BO region (92), the mass loaded BO region (94) and the outer region (80), respectively (see FIGS. 7 to 10 ). For type I BAW resonators, if the BAW resonator does not include a concave frame but only a rising frame(s), and/or if the piezoelectric layer has only a small portion of zero/reduced electromechanical coupling tensor (e.g., zero/reduced coupling only in the second and third portions of the piezoelectric layer (58), confined to the mass loaded BO region (94) and the outer region (80) (not shown)), the quality factor of the BAW resonator will be significantly reduced due to the reduced lateral energy confinement (details are illustrated in FIGS. 12a-12b ). However, for type II BAW resonators, if the BAW resonator does not include a concave frame but only a rising frame(s), and/or if the piezoelectric layer has only a small portion where the electromechanical coupling tensor is zero/reduced (e.g., the coupling is zero/reduced only in the second and third portions of the piezoelectric layer (58), which are limited to the mass loaded BO region (94) and the outer region (80), as illustrated in FIG. 11 ), the quality factor of the BAW resonator can still achieve relatively high values (details are illustrated in FIGS. 13a to 13b ).

In FIG. 11, a first portion of the piezoelectric layer (58) defined in the concave BO region (92) and aligned below the first concave frame (86) (or aligned below the second concave frame (102) (not shown)), and a central portion of the piezoelectric layer (58) defined in the central region (82) have an electromechanical coupling tensor that is not zero (e≠0). A second portion of the piezoelectric layer (58) defined in the mass load BO region (94) and aligned below the rising frame (88) and the transition section (90), and a third portion of the piezoelectric layer (58) defined in the outer region (80) have an electromechanical coupling tensor that is zero/decreased (e=0 or e→0). In some applications, the third portion of the piezoelectric layer (58) defined in the outer region (80) may have an electromechanical coupling tensor that is not zero like the central portion of the piezoelectric layer (58) (not shown). Note that the value (zero or non-zero) of the electromechanical coupling tensor in the external region (80) has a negligible effect on the quality factor/BO mode suppression of the BAW resonator (50).

Figures 12a-12b illustrate simulation results for different configurations of a type I BAW resonator. Figure 12a shows quality factor simulation results for type I BAW resonators having three different configurations: 1) a first configuration of a type I BAW resonator is the same as that illustrated in Figure 4, 2) a second configuration of a type I BAW resonator is the same as that illustrated in Figure 9, and 3) a third configuration of a type I BAW resonator includes a mass-loaded BO region (e.g., mass-loaded BO region (94)) but does not include a concave BO region (e.g., concave BO region (92)), wherein the portions of the piezoelectric layer confined in the mass-loaded BO region and the outer region (e.g., mass-loaded BO region (94) and the outer region (80)) have zero coupling (not illustrated). Figure 12b illustrates 1-|S ₁₁ | for type I BAW resonators having the first and second configurations. Shows the response simulation results (1-|S ₁₁ | is equal to the power loss ratio in the resonator).

For type I resonators with either the first or second configuration, it is clearly shown that their quality factors can achieve relatively high values by choosing an optimized width of the mass-loaded BO region. In this example, the type I resonator with the first configuration can reach Qp of 2900 when the mass-loaded BO region has a width of 3.25 μm, whereas the type I resonator with the second configuration can reach Qp of 2500 when the mass-loaded BO region has a width of 3.6 μm. However, the type I resonator with the third configuration can reach Qp up to 1800, which is due to the reduction of the lateral energy limitation. On the other hand, the type I resonator with the first configuration has a very strong undesired BO mode, whereas the type I resonator with the second configuration has a good suppression of the BO mode. Therefore, the type I resonator with the second configuration (as shown in Fig. 9) has a relative quality factor with a well-suppressed BO mode.

Note that conventional type-I resonators do not typically require a concave frame since they do not have spurious modes below the series resonant frequency (fs). However, for type-I resonators, the proposed configuration (as illustrated in FIGS. 7 to 10 ) achieves excellent quality factor results by including a concave frame (e.g., a first concave frame (86) or a second concave frame (102)) and a piezoelectric layer portion with zero coupling below.

FIGS. 13a-13b illustrate simulation results for different configurations of a type-II BAW resonator. FIG. 13a shows quality factor simulation results for type-II BAW resonators having three different configurations: 1) a first configuration of the type-II BAW resonator is the same as that illustrated in FIG. 4, 2) a second configuration of the type-II BAW resonator is the same as that illustrated in FIG. 9, and 3) a third configuration of the type-II BAW resonator includes a mass-loaded BO region (e.g., the mass-loaded BO region (94)) but does not include a concave BO region (e.g., the concave BO region (92)), wherein the portions of the piezoelectric layer confined in the mass-loaded BO region and the outer region (e.g., the mass-loaded BO region (94) and the outer region (80)) have zero coupling (not illustrated). FIG. 13b shows 1-|S ₁₁ | response simulation results for type-II BAW resonators having the first, second and third configurations.

It is clearly shown that the type-II resonator with the second configuration has the maximum quality factor and the highest average quality factor over the width range of the mass-loaded BO region. In this figure, the type-II resonator with the second configuration can reach 3700 Qp when the mass-loaded BO region has a width of 4.3 μm, and can achieve an average Qp of 3450 over the width range of 0.2 μm to 5 μm. For the type-II resonator with the first or third configuration, its quality factor can achieve relatively high values by selecting an optimized width of the mass-loaded BO region. In this figure, the type-II resonator with the first configuration can reach 3000 Qp when the mass-loaded BO region has a width of 2.5 μm, whereas the type-II resonator with the third configuration can reach 3600 Qp when the mass-loaded BO region has a width of 2.7 μm. On the other hand, type II resonators with the first configuration have strong undesired BO modes, whereas type II resonators with the second or third configurations have well-suppressed BO modes (type II resonators with the second configuration have the least BO mode).

Note that to achieve similar 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). Thus, the second configuration (with zero electromechanical coupling tensor in the concave BO + mass loading BO + outer region) can have a smaller overall width where coupling is zero than the third configuration (with zero electromechanical coupling tensor in the mass loading BO+ outer region), which leads to less degradation in the overall resonator coupling (k ² e ). Thus, a type-II resonator with the second configuration (as illustrated in Fig. 9) can achieve the desired results in quality factor and BO mode suppression without significantly sacrificing the overall resonator coupling.

It is contemplated that any of the above-described aspects and/or various separate aspects and features as described herein may be combined for additional benefit. Any of the various embodiments disclosed herein may be combined with one or more other embodiments, unless otherwise indicated herein.

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 within the scope of the concepts disclosed herein and the claims that follow.

Claims (20)

As a bulk acoustic wave (BAW) resonator,
·Lower electrode;
·Upper electrode; and
· Including a piezoelectric layer sandwiched between the lower electrode and the upper electrode,
·The above piezoelectric layer is formed of a material having a box-shaped polarization-electric field (PE) curve;
·The BAW resonator is divided into an active region and an outer region, the active region corresponding to a section of the BAW resonator where the upper electrode and the lower electrode overlap, while the outer region corresponding to a section of the BAW resonator surrounding the active region;
·The active region is divided into a central region, a concave boundary (BO) region, and a mass loading BO region, wherein the mass loading BO region is laterally adjacent to the outer region and around the periphery of the active region, the concave BO region is laterally adjacent to and surrounded by the mass loading BO region, and the central region is laterally adjacent to and surrounded by the concave BO region, such that the concave BO region is in direct contact with the mass loading BO region and the central region;
·The height of the above concave BO region is lower than the height of the central region, and the height of the mass load BO region is higher than the height of the above concave BO region;
· A BAW resonator, wherein a first portion of the piezoelectric layer limited to the mass load BO region has an electromechanical coupling tensor of zero, while a central portion of the piezoelectric layer limited to the central region has an electromechanical coupling tensor that is not zero. A BAW resonator in the first aspect, wherein the second portion of the piezoelectric layer limited to the concave BO region has an electromechanical coupling tensor of 0. A BAW resonator in the second aspect, wherein the third portion of the piezoelectric layer limited to the external region has an electromechanical coupling tensor of 0. A BAW resonator in claim 1, wherein 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. A BAW resonator in the first aspect, wherein the height of the mass load BO region is higher than the height of the central region. A BAW resonator in the first aspect, wherein the height of the central region is higher than the height of the mass load BO region. In the third paragraph, the BAW resonator is a type I resonator or a type II resonator. In the first paragraph,
· The second part of the piezoelectric layer limited to the concave BO region has an electromechanical coupling tensor that is not 0;
·The above BAW resonator is a type II resonator. A BAW resonator in accordance with claim 8, wherein the third portion of the piezoelectric layer limited to the external region has an electromechanical coupling tensor of 0. In the first aspect, the upper electrode comprises a BO structure around the periphery of the upper electrode:
·The BO structure comprises a concave frame having a first height, a rising frame having a second height, and a transition section laterally between the concave frame and the rising frame, the second height being higher than the first height, and the height of the transition section changing from the first height of the concave frame to the second height of the rising frame to form a tapered wall;
·The rising frame is positioned around the periphery of the upper electrode, such that the outer peripheral edge of the rising frame is the peripheral edge of the upper electrode, and the concave frame is connected to the rising frame through the transition section and is surrounded by the rising frame;
· The central electrode portion of the upper electrode, which is limited to the central area of the BAW resonator, has a third height higher than the first height of the concave frame;
· A BAW resonator, wherein the concave BO region corresponds to a section of the BAW resonator including the concave frame, positioned above it, and positioned below it, and the mass loaded BO region corresponds to a section of the BAW resonator including the rising frame and the transition section, positioned above it, and positioned below it, and the height variation between the central region, the concave BO region, and the mass loaded BO region is formed due to the height variation of the upper electrode. In Article 10,
·The upper electrode is composed of a first upper electrode layer formed directly above the piezoelectric layer, a second upper electrode layer formed on the first upper electrode layer, and a third upper electrode layer formed on the second upper electrode layer;
· A BAW resonator, wherein the concave frame, transition section, and rising frame of the upper electrode are formed by a thickness variation of one or more of the first upper electrode layer, the second upper electrode layer, and the third upper electrode layer. A BAW resonator in accordance with claim 11, wherein the concave frame, transition section, and rising frame of the upper electrode are formed due to a change in the thickness of the first upper electrode layer. In Article 11,
·The above first upper 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);
· A BAW resonator, wherein the third upper electrode layer is formed of aluminum copper (AlCu). A BAW resonator in claim 11, further comprising a passivation layer covering the upper electrode and a portion of the piezoelectric layer exposed through the upper electrode, wherein the passivation layer has a uniform thickness. In the first aspect, the upper electrode comprises a BO structure around the periphery of the upper electrode:
·The BO structure includes a concave frame having a first height, an inner rising frame having a second height, a first transition section laterally between the concave frame and the inner rising frame, an outer rising frame having a third height, and a second transition section laterally between the inner rising frame and the outer rising frame, wherein the second height is higher than the first height and lower than the third height, and the height of the first transition section changes from the first height of the concave frame to the second height of the inner rising frame, while the height of the second transition section changes from the second height of the inner rising frame to the third height of the outer rising frame;
·The outer rising frame is located around the periphery of the upper electrode, and the outer peripheral edge of the outer rising frame is the peripheral edge of the upper electrode;
·The inner rising frame is connected to the outer rising frame through the second transition section and is surrounded by the outer rising frame;
·The above concave frame is connected to the inner rising frame through the first transition section and is surrounded by the inner rising frame;
· The central electrode portion of the upper electrode, which is limited to the central area of the BAW resonator, has a fourth height higher than the first height of the concave frame;
· A BAW resonator, wherein the concave BO region corresponds to a section of the BAW resonator that includes the concave frame, is positioned above it, and is positioned below it, and the mass load BO region corresponds to a section of the BAW resonator that includes a combination of the first transition section, the inner rising frame, the second transition section, and the outer rising frame, and is positioned above it and is positioned below it. In claim 15, further comprising a dielectric BO ring around the periphery of the active region:
·The upper electrode is formed directly above the piezoelectric layer and is composed of a plurality of upper electrode layers extending over the dielectric BO ring;
· A BAW resonator, wherein the height variation between the central region, the concave BO region, and the mass loading BO region is formed by a combination of the thickness variation of the upper electrode layer of the upper electrode and the thickness of the dielectric BO ring. A BAW resonator in accordance with claim 1, further comprising a passivation layer covering the upper electrode and a portion of the piezoelectric layer exposed through the upper electrode, wherein the passivation layer is formed of silicon nitride (SiN), silicon dioxide (SiO ₂ ), or silicon oxynitride (SiON). In Article 17,
·The upper electrode comprises a BO structure having a rising frame and a transition section, and the passivation layer comprises a concave frame;
·The rising frame is located around the periphery of the upper electrode, the outer periphery edge of the rising frame is the periphery edge of the upper electrode, and the transition section is laterally adjacent to the rising frame and surrounded thereby;
·The concave frame of the above passivation layer is laterally adjacent to and limited to the transition section of the upper electrode;
·The thickness of the above concave frame is thinner than the thickness of the passivation layer within the central region;
· A BAW resonator, wherein the concave BO region corresponds to a section of the BAW resonator that includes a concave frame of the passivation layer and is positioned below it, and the mass loaded BO region corresponds to a section of the BAW resonator that includes the rising frame and the transition section and is positioned above and below it. In Article 18,
·The upper electrode is composed of a plurality of upper electrode layers formed directly above the piezoelectric layer;
· A BAW resonator, wherein the height variation between the central region, the concave BO region, and the mass loading BO region is formed by the concave frame of the passivation layer and the BO structure of the upper electrode. In Article 19,
· The portion of the upper electrode defined within the central region and the concave BO region has a uniform thickness, and the change in height between the central region and the concave BO region is due to a change in the thickness of the passivation layer;
· A BAW resonator, wherein the height change between the concave BO region and the mass-loaded BO region is due to a change in the thickness of the upper electrode layer.