Bulk acoustic wave resonator with a modified outside stack portion

Bulk Acoustic Wave (BAW) resonators that include a modified outside stack portion and methods for fabricating such BAW resonators are provided. One BAW resonator includes a reflector, a bottom electrode, a piezoelectric layer, and a top electrode. An active region is formed where the top electrode overlaps the bottom electrode and an outside region surrounds the active region. The piezoelectric layer includes a top surface adjacent to the top electrode and a bottom surface adjacent to the bottom electrode. The piezoelectric layer further includes an outside piezoelectric portion in the outside region with a bottom surface in the outside region that is an extension of the bottom surface of the piezoelectric layer, and the outside piezoelectric portion includes an angled sidewall that resides in the outside region and extends from the top surface of the piezoelectric layer to the bottom surface of the outside piezoelectric portion in the outside region.

FIELD OF THE DISCLOSURE

The present invention relates to Bulk Acoustic Wave (BAW) resonators.

BACKGROUND

Acoustic resonators, and particularly Bulk Acoustic Wave (BAW) resonators, are used in many high frequency communication applications. In particular, BAW resonators are often employed in filter networks that operate at frequencies above 1.5 GHz and require a flat passband, have exceptionally steep filter skirts and squared shoulders at the upper and lower ends of the passband, and provide excellent rejection outside of the passband. BAW-based filters also have relatively low insertion loss, tend to decrease in size as the frequency of operation increases, and are relatively stable over wide temperature ranges. As such, BAW-based filters are the filter of choice for many 3rd Generation (3G) and 4th Generation (4G) wireless devices, and are destined 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 as such, pose extremely challenging filtering demands. While these demands keep raising the complexity of the wireless devices, there is a constant need to improve the performance of BAW resonators and BAW-based filters as well as decrease the cost and size associated therewith.

SUMMARY

Bulk Acoustic Wave (BAW) resonators that include a modified outside stack portion and methods for fabricating such BAW resonators are provided. One BAW resonator comprises a reflector, a bottom electrode over the reflector, a piezoelectric layer over the bottom electrode, and a top electrode over the piezoelectric layer. An active region is formed where the top electrode overlaps the bottom electrode and an outside region surrounds the active region. The piezoelectric layer includes a top surface adjacent to the top electrode and a bottom surface adjacent to the bottom electrode. The piezoelectric layer further comprises an outside piezoelectric portion in the outside region with a bottom surface in the outside region that is an extension of the bottom surface of the piezoelectric layer, and the outside piezoelectric portion includes an angled sidewall that resides in the outside region and extends from the top surface of the piezoelectric layer to the bottom surface of the outside piezoelectric portion in the outside region.

In one embodiment, the bottom electrode comprises an outside bottom electrode (OBE) portion that extends into the outside region, the outside piezoelectric portion is formed on the OBE portion, and the OBE portion extends into the outside region past the outside piezoelectric portion. In various other embodiments, the BAW resonator further comprises an outside (OS) layer in the outside region that is laterally adjacent to the bottom electrode, and the outside piezoelectric portion resides on the OS layer in the outside region.

The OS layer, in one embodiment, extends into the outside region past the outside piezoelectric portion. In another embodiment, the OS layer and the outside piezoelectric portion extend the same lateral distance into the outside region. In other embodiments, the top layer of the reflector comprises a top reflector layer (RL) portion that resides in the outside region, the OS layer resides on the outside top RL portion, and the outside top RL portion, the OS layer, and the outside piezoelectric portion extend the same lateral distance into the outside region.

One method comprises providing a reflector, forming a bottom electrode over the reflector, forming a piezoelectric layer over the bottom electrode, and forming a top electrode over the piezoelectric layer. An active region is formed where the top electrode and the bottom electrode overlap and an outside region surrounds the active region. The piezoelectric layer comprises a top surface adjacent to the top electrode and a bottom surface adjacent to the bottom electrode, an outside piezoelectric portion of the piezoelectric layer with a bottom surface that is an extension of the bottom surface of the piezoelectric layer is provided in the outside region, and a portion of the outside piezoelectric portion is removed such that an angled sidewall that extends from the top surface of the piezoelectric layer to the bottom surface of the outside piezoelectric portion is provided in the outside region.

In one embodiment, an OBE portion that is an extension of the bottom electrode is provided in the outside region, the outside piezoelectric portion is provided on the OBE portion, and the portion of the outside piezoelectric portion is removed such that the OBE portion extends into the outside region past the outside piezoelectric portion. The method, in various other embodiments, comprises forming an OS layer that is laterally adjacent to the bottom electrode in the outside region.

In one embodiment, the portion of the outside piezoelectric portion is removed such that the OS layer extends into the outside region past the outside piezoelectric portion. In another embodiment, the portion of the outside piezoelectric portion is removed such that the OS layer extends the same lateral distance into the outside region as the outside piezoelectric portion. In yet another embodiment, a RL portion that resides in the outside region under the OS layer is provided and a portion of the outside top RL portion is removed, such that the outside top RL portion extends the same lateral distance into the outside region as the OS layer and the outside piezoelectric portion.

DETAILED DESCRIPTION

It should be understood that, although the terms “upper,” “lower,” “bottom,” “intermediate,” “middle,” “top,” and the like may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed an “upper” element and, similarly, a second element could be termed an “upper” element depending on the relative orientations of these elements, without departing from the scope of the present disclosure.

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 will be further understood that terms used herein should be interpreted as having meanings that are consistent with their meanings in the context of this Specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

Bulk Acoustic Wave (BAW) resonators that include a structure for confining lateral energy and methods for fabricating such BAW resonators are provided. One BAW resonator includes a reflector, a bottom electrode over a first portion of the reflector, a piezoelectric layer over the bottom electrode, and a top electrode over a first portion of the piezoelectric layer. An active region is formed where the top electrode overlaps the bottom electrode, an outside region surrounds the active region, and the piezoelectric layer includes a second portion with an angled sidewall in the outside region. The piezoelectric layer includes a top surface adjacent to the top electrode and a bottom surface adjacent to the bottom electrode. The bottom surface is wider than the top surface, the angled sidewall extends from the top surface to the bottom surface, and the angled sidewall forms an acute angle with respect to the bottom surface.

BAW resonators are used in many high-frequency filter applications. An exemplary BAW resonator10is illustrated inFIG. 1. The BAW resonator10is a solidly mounted resonator (SMR) type BAW resonator10and generally includes a substrate12, a reflector14mounted over the substrate12, and a transducer16mounted over the reflector14. The transducer16rests on the reflector14and includes a piezoelectric layer18, which is sandwiched between a top electrode20and a bottom electrode22. The top and bottom electrodes20and22may be formed of Tungsten (W), Molybdenum (Mo), Platinum (Pt), or like material, and the piezoelectric layer18may be formed of Aluminum Nitride (AlN), Zinc Oxide (ZnO) or other appropriate piezoelectric material. Although shown inFIG. 1as including a single layer, the piezoelectric layer18, the top electrode20, and/or the bottom electrode22may include 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.

The BAW resonator10is divided into an active region24and an outside region26. The active region24generally corresponds to the section of the BAW resonator10where the top and bottom electrodes20and22overlap and also includes the layers below the overlapping top and bottom electrodes20and22. The outside region26corresponds to the section of the BAW resonator10that surrounds the active region24.

For the BAW resonator10, applying electrical signals across the top electrode20and the bottom electrode22excites acoustic waves in the piezoelectric layer18. These acoustic waves primarily propagate vertically. A primary goal in BAW resonator design is to confine these vertically-propagating acoustic waves in the transducer16. Acoustic waves traveling upwardly are reflected back into the transducer16by the air-metal boundary at the top surface of the top electrode20. Acoustic waves traveling downwardly are reflected back into the transducer16by the reflector14, or by an air cavity, which is provided just below the transducer in a Film BAW Resonator (FBAR).

The reflector14is typically formed by a stack of reflector layers (RL)28A through28E, which alternate in material composition to produce a significant reflection coefficient at the junction of adjacent reflector layers28. Typically, the reflector layers28A through28E alternate between materials having high and low acoustic impedances, such as tungsten (W) and silicon dioxide (SiO2). While only five reflector layers28A through28E are illustrated inFIG. 1, the number of reflector layers28and the structure of the reflector14will vary from one design to another.

The magnitude (Z) and phase (ϕ)) of the electrical impedance as a function of the frequency for a relatively ideal BAW resonator10is provided inFIG. 2. The magnitude (Z) of the electrical impedance is illustrated by the solid line, while the phase (ϕ)) of the electrical impedance is illustrated by the dashed line. A unique feature of the BAW resonator10is that it has both a resonance frequency and an anti-resonance frequency. The resonance frequency is typically referred to as the series resonance frequency (fs), and the anti-resonance frequency is typically referred to as the parallel resonance frequency (fp). The series resonance frequency (fs) occurs when the magnitude of the impedance, or reactance, of the BAW resonator10approaches zero. The parallel resonance frequency (fp) occurs when the magnitude of the impedance, or reactance, of the BAW resonator10peaks at a significantly high level. In general, the series resonance frequency (fs) is a function of the thickness of the piezoelectric layer18and the mass of the bottom and top electrodes20and22.

For the phase, the BAW resonator10acts like an inductance that provides a 90° phase shift between the series resonance frequency (fs) and the parallel resonance frequency (fp). In contrast, the BAW resonator10acts like a capacitance that provides a −90° phase shift below the series resonance frequency (fs) and above the parallel resonance frequency (fp). The BAW resonator10presents a very low, near zero, resistance at the series resonance frequency (fs), and a very high resistance at the parallel resonance frequency (fp). The electrical nature of the BAW resonator10lends itself to the realization of a very high Q (quality factor) inductance over a relatively short range of frequencies, which has proven to be very beneficial in high frequency filter networks, especially those operating at frequencies around 1.8 GHz and above.

Unfortunately, the phase (ϕ) curve ofFIG. 2is representative of an ideal phase curve. In reality, approaching this ideal is challenging. A typical phase curve for the BAW resonator10ofFIG. 1is illustrated inFIG. 3A. Instead of being a smooth curve, the phase curve ofFIG. 3Aincludes ripple below the series resonance frequency (fs), between the series resonance frequency (fs) and the parallel resonance frequency (fp), and above the parallel resonance frequency (fp). The ripple is the result of spurious modes, which are caused by spurious resonances that occur in corresponding frequencies. While the vast majority of the acoustic waves in the BAW resonator10propagate vertically, various boundary conditions about the transducer16result 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 Q associated with the BAW resonator10.

As illustrated inFIG. 4, a border (BO) ring30is formed on or within the top electrode20to suppress certain of the spurious modes. The spurious modes that are suppressed by the BO ring30are those above the series resonance frequency (fs), as highlighted by circles A and B in the phase curve ofFIG. 3B. Circle A shows a suppression of the ripple, and thus the spurious mode, in the passband of the phase curve, which resides between the series resonance frequency (fs) and the parallel resonance frequency (fp). Circle B shows suppression of the ripple, and thus the spurious modes, above the parallel resonance frequency (fp). Notably, the spurious mode in the upper shoulder of the passband, which is just below the parallel resonance frequency fp, and the spurious modes above the passband are suppressed, as evidenced by the smooth or substantially ripple free phase curve between the series resonance frequency (fs) and the parallel resonance frequency (fp) and above the parallel resonance frequency (fp).

The BO ring30corresponds to a mass loading of the portion of the top electrode20that extends about the periphery of the active region24. The BO ring30may correspond to a thickened portion of the top electrode20or the application of additional layers of an appropriate material over the top electrode20. The portion of the BAW resonator10that includes and resides below the BO ring30is referred to as a BO region32. Accordingly, the BO region32corresponds to an outer, perimeter portion of the active region24and resides inside of the active region24.

While the BO ring30is effective at suppressing spurious modes above the series resonance frequency (fs), the BO ring30has little or no impact on those spurious modes below the series resonance frequency (fs), as shown inFIG. 3B. A technique referred to as apodization is often used to suppress the spurious modes that fall below the series resonance frequency (fs).

Apodization works to avoid, or at least significantly reduce, any lateral symmetry in the BAW resonator10, or at least in the transducer16thereof. The lateral symmetry corresponds to the footprint of the transducer16, and avoiding the lateral symmetry corresponds to avoiding symmetry associated with the sides of the footprint. For example, one may choose a footprint that corresponds to a pentagon instead of a square or rectangle. Avoiding symmetry helps reduce the presence of lateral standing waves in the transducer16. Circle C ofFIG. 3Cillustrates the effect of apodization in which the spurious modes below the series resonance frequency (fs) are suppressed. Assuming no BO ring30is provided, one can readily see inFIG. 3Cthat apodization fails to suppress those spurious modes above the series resonant frequency (fs). As such, the typical BAW resonator10employs both apodization and the BO ring30.

As further illustrated in the embodiment ofFIG. 4, the BAW resonator10comprises an outside stack portion34that resides in the outside region26. At least in the illustrated embodiment, the outside stack portion34comprises an outside RL portion28B′ that is an extension of the RL28B, an outside top RL portion28A′ that is an extension of the top RL28A, and an outside (OS) layer36. The OS layer36resides adjacent to and on the same lateral (horizontal) level as the bottom electrode22. Furthermore, the OS layer36is comprised of the same material as the top RL28A and the outside top RL portion28A′, and is formed over the outside top RL portion28A′. The outside stack portion34further comprises an outside piezoelectric portion18′ that is an extension of the piezoelectric layer18residing over the OS layer36.

A supplement to or alternative for apodization and the BO ring30is described below in which energy confinement in the active region24of the BAW resonator10can be improved by modifying the outside stack portion34in the outside region26. With reference toFIG. 5A, a modified outside stack portion34A in the outside region26is shown. The outside stack portion34A comprises a modified outside piezoelectric portion18″ that is essentially a triangular portion of the piezoelectric layer18that extends into the outside region26.

The outside piezoelectric portion18″ is a generalized right triangle and has three sides: a vertical side40, a horizontal side44, and an angled sidewall48. The triangular characterization of the outside piezoelectric portion18″ need not be perfectly triangular. The vertical side40is imaginary and resides on the boundary between the active region24and the outside region26, and extends vertically between a transition point38on the upper surface of the piezoelectric layer18and a vertex42on the bottom surface of the piezoelectric layer18. The horizontal side44extends laterally from the vertex42to an outside point46. The angled sidewall48extends between the transition point38to the outside point46.

The angled sidewall48forms an acute angle (β) with respect to the horizontal side44of the outside piezoelectric portion18″. Stated differently, the angled sidewall48forms an obtuse angle (α) with respect to a top surface50of the piezoelectric layer18.

In some embodiments, the acute angle (β) is in the range of about 45 degrees to about 80 degrees and, correspondingly, the obtuse angle (α) is in the range of about 100 degrees to about 135 degrees. In other embodiments, the acute angle (β) is in the range of about 60 degrees to about 70 degrees and, correspondingly, the obtuse angle (α) is in the range of about 110 degrees to about 120 degrees. In one embodiment, the acute angle (β) is 60 degrees and, correspondingly, the obtuse angle (α) is 120 degrees.

The outside piezoelectric portion18″ is formed over the OS layer36, which resides in the outside region26on the same lateral or horizontal level as the bottom electrode22. The OS layer36is formed on the outside top RL portion28A′, which is formed on the outside RL portion28B′.

With reference toFIG. 5B, a modified outside stack portion32B in the outside region26is shown. The modified stack portion32B comprises an outside bottom electrode (OBE) portion22′, which is an extension of the bottom electrode22that extends into the outside region26. The outside stack portion32B further comprises a modified outside piezoelectric portion18″ that is a portion of the piezoelectric layer18that extends into the outside region26similar to the modified outside piezoelectric portion18″ discussed above with reference toFIG. 5A.

The modified outside piezoelectric portion18″ comprises a transition point38, a horizontal side44, a vertex42, and an angled sidewall48similar to the modified outside stack portion34A discussed above with reference toFIG. 5A. However, the horizontal side44of the modified outside piezoelectric portion18″ in the modified outside stack portion32B resides on the OBE portion22′ and the OBE portion22′ extends laterally to at least the outside point46, and as illustrated inFIG. 5B, past the modified outside piezoelectric portion18″. In the embodiment ofFIG. 5B, the horizontal side44of the modified outside piezoelectric portion18″ is formed over the OBE portion22′, instead of being formed over the OS layer36as in the embodiment ofFIG. 5A.

The OS layer36in the modified outside stack portion34B resides in the outside region26. The OS layer36is on the same lateral or horizontal level as the bottom electrode22and the OBE portion22′. As illustrated inFIG. 5B, the modified outside piezoelectric portion18″ in the modified outside stack portion34B does not reside on the OS layer36.

Referring toFIG. 5C, a modified outside stack portion34C in the outside region26is shown. The modified outside stack portion34C comprises a modified outside piezoelectric portion18″ over an OS layer36similar to the modified outside piezoelectric portion18″ over the OS layer36in the modified outside stack portion34A discussed above with reference toFIG. 5A. In the embodiment illustrated inFIG. 5C, the modified outside piezoelectric portion18″ and the OS layer36extend about the same lateral distance into the outside region26. In other words, the modified outside piezoelectric portion18″ and the OS layer36both end at a vertical plane that coincides with the outside point46of the modified outside piezoelectric portion18″.

Referring toFIG. 5D, a modified outside stack portion34D in the outside region26is shown. The modified outside stack portion34D comprises a modified outside piezoelectric portion18″ over an OS layer36similar to the modified outside stack portion34C discussed above with reference toFIG. 5C. In the embodiment illustrated inFIG. 5D, the outside top RL portion28A′ extends the same lateral distance into the outside region26as the modified outside piezoelectric portion18″ and the OS layer36. In other words, the modified outside piezoelectric portion18″, the OS layer36, and the outside top RL portion28A′ each end at the vertical plane that coincides with the outside point46of the modified outside piezoelectric portion18″.

The modified outside stack portions34A through34D enable a BAW resonator, such as BAW resonator10, to operate more efficiently and effectively. Specifically, removing materials from an upper portion of an outside stack in the outside region26to form the modified outside stack portions34A through34D improves lateral energy confinement in the active region24, and thus provides a BAW resonator with a higher Q value.

FIGS. 6A-6Fare diagrams illustrating various embodiments of a method for fabricating a BAW resonator, such as BAW resonator10, with the modified outside stack portions34A-34D discussed with reference toFIGS. 5A-5D.

Referring toFIG. 6A, the method comprises depositing a reflector layer (RL)28B over another RL layer28C, which is not shown inFIG. 6A, but is shown inFIG. 4. A top RL28A is deposited over the RL28B. A bottom electrode22and an OS layer36are formed over different portions of the top RL28A and on the same lateral or horizontal level as one another. A piezoelectric layer18is deposited over the bottom electrode22and the OS layer36. A top electrode20is formed over a portion of the piezoelectric layer18.

The RL28B, the top RL28A, the bottom electrode22, the OS layer36, the piezoelectric layer18, and the top electrode20may be deposited using any deposition technique known in the art or developed in the future. Examples of deposition techniques include, but are not limited to, ion beam deposition (IBD), chemical vapor deposition (CVD), physical vapor deposition (PVD), molecular beam epitaxy (MBE), electrochemical deposition (ECD), and/or like deposition techniques.

As illustrated inFIG. 6B, a BO ring30is formed over a portion of the top electrode20to define a border between an active region24and an outside region26. The BO ring30may be deposited using any of the deposition techniques discussed above with reference toFIG. 6A.

The active region24comprises the BO ring30, the top electrode20, the piezoelectric layer18, the bottom electrode22, the top RL28A, and the RL28B. The outside region26comprises an outside piezoelectric portion18′, the OS layer36, an outside top RL portion28A′, and an outside RL portion28B′, which define an outside stack portion34.

With reference toFIG. 6C, a portion of the outside piezoelectric portion18′ is removed to create a modified outside piezoelectric portion18″, as discussed above with reference toFIGS. 5A-5D. That is,FIG. 6Cillustrates the formation of the modified outside stack portion34A as discussed above with reference toFIG. 5A.

The portion of the outside piezoelectric portion18′ may be removed to create the modified outside piezoelectric portion18″ using any removal or trimming technique known in the art or developed in the future. Examples of removal or trimming techniques include, but are not limited to, physical dry etching, wet etching, reactive ion etching and/or like trimming techniques.

Referring toFIG. 6D, when the bottom electrode22is deposited on the top RL28A, the bottom electrode22includes an OBE portion22′ that is an extension of the bottom electrode22that resides in the outside region26. The portion of the outside piezoelectric portion18′ is removed to create the modified outside piezoelectric portion18″. The portion of the outside piezoelectric portion18′ is removed such that the entirety of the modified outside piezoelectric portion18″ resides on the OBE portion22′. In other words, the OBE portion22′ extends into the outside region26past the modified outside piezoelectric portion18″. Specifically, the OBE portion22′ extends into the outside region26past a vertical plane that coincides with the outside point46of the modified outside piezoelectric portion18″.

The portion of the outside piezoelectric portion18′ and the portion of the OS layer36may be removed to create the modified outside stack portion34B using any removal or trimming technique discussed above with reference toFIG. 6C. Accordingly, the method illustrated inFIG. 6Dforms the modified outside stack portion34B discussed above with reference toFIG. 5B.

With reference toFIG. 6E, the portion of the outside piezoelectric portion18′ is removed to create the modified outside piezoelectric portion18″ similar to the embodiment discussed above with reference toFIG. 6D. A portion of the OS layer36is also removed such that the modified outside piezoelectric portion18″ and the OS layer36both extend into the outside region26the same amount or lateral distance. In other words, both the modified outside piezoelectric portion18″ and the OS layer36extend to or end at a vertical plane that coincides with the outside point46of the modified outside piezoelectric portion18″

The portion of the outside piezoelectric portion18′ and the portion of the OS layer36may be removed to create the modified outside stack portion34C using any removal or trimming technique discussed above with reference toFIG. 6C. Accordingly, the method illustrated inFIG. 6Eforms the modified outside stack portion34C discussed above with reference toFIG. 5C.

Referring toFIG. 6F, the portion of the outside piezoelectric portion18′ is removed to create the modified outside piezoelectric portion18″ and the portion of the OS layer36is removed similar to the embodiment discussed above with reference toFIG. 6E. A portion of the outside top RL portion28A′ is also removed such that the modified outside piezoelectric portion18″, the OS layer36, and the outside top RL portion28A′ extend into the outside region26the same amount or lateral distance. In other words, each of the modified outside piezoelectric portion18″, the OS layer36, and the outside top RL portion28A′ extend to or end at a vertical plane that coincides with the outside point46of the modified outside piezoelectric portion18″.

The portion of the outside piezoelectric portion18′, the portion of the OS layer36, and the portion of the outside top RL portion28A′ may be removed to create the modified outside stack portion34D using any removal or trimming technique discussed above with reference toFIG. 6C. Accordingly, the method illustrated inFIG. 6Fforms the modified outside stack portion34D discussed above with reference toFIG. 5D.

FIGS. 7A and 7Bare diagrams illustrating phase curves representing the various degrees to which spurious modes are suppressed by a BAW resonator, such as BAW resonator10, including the modified outside stack portions34A-34D discussed above with reference toFIGS. 5A-5D, respectively. In the examples shown inFIGS. 7A and 7B, the acute angle (β) formed by the slope of the sidewall48is 80°, 70°, 60°, and 50°. In addition,FIGS. 7A and 7Bshow various heights and widths for the BO ring30. In theory, an ideal phase curve that represents total energy confinement includes a smooth line with steep skirts and squared shoulders.

FIG. 7Aillustrates three combinations when the acute angle (β) is 80°, 70°, 60°, and 50° and the height of the BO ring30is 40 nm. Specifically, a first combination has a height of 40 nm and a width of 3.25 μm for the BO ring30, a second combination has a height of 40 nm and a width of 3.38 μm for the BO ring30, and a third combination has a height of 40 nm and a width of 3.50 μm for the BO ring30.

As illustrated inFIG. 7A, the phase curves closest to the ideal phase curve are produced by a BAW resonator that includes the modified outside stack portion34B. Specifically, the lines in these phase curves are the smoothest, the skirts the steepest, and the shoulders are the most square when the acute angle (β) is 50°, 60°, and 70°.

The smooth lines, steep skirts, and squared shoulders in the phase curve are indications that the BAW resonator including the outside stack portion34B is effectively suppressing spurious modes. Furthermore, fewer spurious modes are an indication that the BAW resonator including the modified outside stack portion34B is efficiently confining the signal energy to the active region24.

FIG. 7Billustrates three combinations when the acute angle (β) formed by the slope of the angled sidewall48is 80°, 70°, 60°, and 50° and the height of the BO ring30is 80 nm. Specifically, a first combination has a height of 80 nm and a width of 2.75 μm for the BO ring30, a second combination has a height of 80 nm and a width of 2.28 μm for the BO ring30, and a third combination has a height of 80 nm and a width of 3.0 μm for the BO ring30.

As illustrated inFIG. 7B, the phase curve closest to the ideal phase curve is produced by a BAW resonator including the modified outside stack portion34B. Specifically, the lines are the smoothest, the skirts the steepest, the shoulders are the most square when the acute angle (β) is 60°.

Again, the smooth lines, steep skirts, and square shoulders in the phase curve is an indication that the BAW resonator including the modified outside stack portion34B is effectively suppressing spurious modes. Furthermore, fewer spurious modes are an indication that the BAW resonator including the modified outside stack portion34B is efficiently confining the signal energy to the active region24.

FIG. 8is a diagram illustrating a comparison of the degree to which the spurious mode is suppressed by a BAW resonator including the modified outside stack portion34B and the conventional BAW resonator10. InFIG. 8, the acute angle (β) is 60°. Here, the BAW resonator including the modified outside stack portion34B has three width combinations, 3.25 μm, 3.38 μm, and 3.50 μm, for a 40 nm height of the BO ring30. The conventional BAW resonator10has three width combinations, 2.00 μm, 2.13 μm, and 2.25 μm, and a 60 nm height for the BO ring30.

As illustrated inFIG. 8, the phase curve for the BAW resonator including the modified outside stack portion34B includes significantly smoother lines, steeper skirts, and more square shoulders than the conventional BAW resonator10. The phase curve for the BAW resonator including the modified outside stack portion34B including smoother lines, steeper skirts, and more square shoulders than the conventional BAW resonator10is an indication that the structure of the modified outside stack portion34B enables a BAW resonator to better confine lateral energy to the active region24and thus, suppress spurious modes better than the conventional BAW resonator10.

Those skilled in the art will also 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.