TECHNICAL FIELD The present invention relates to a Bulk Acoustic Wave (BAW) device. In particular, the invention is concerned with suppressing spurious modes in a BAW device. To this end, the invention proposes a BAW device that comprises a piezoelectric layer including embedded material elements, and proposes a method for fabricating such a BAW device. BACKGROUND

Acoustic wave devices are key components for modem electronic circuits. In particular, high frequency selectivity, while maintaining low electronic insertion loss, requires high quality factor mechanical resonators coupled in a fdter topology.

BAW resonators are configured to couple an electrical, time varying signal, to a mechanical wave traveling in the bulk of a piezoelectric material, i.e. they are configured to couple an electrical signal to a BAW. Conventional BAW resonators are produced using a thin piezoelectric layer, which is disposed over a reflective element. In the case of thin-film bulk acoustic resonators (FBAR), the reflective element is a cavity, while in the case of solidly mounted acoustic resonators (SMR), the reflective element is an acoustic mirror or Bragg reflector comprising alternating layers of high and low acoustic impedance materials. The fundamental mode of vibration, which a BAW resonator can electromechanically couple to, is the thickness-extensional mode (TE). This mode bases on longitudinal waves that propagate in the thickness of the thin piezoelectric material. However, in addition to the desired TE mode, also lateral plate modes (Rayleigh-Lamb waves) may be triggered, which are generated in the same structure. These spurious Rayleigh-lamb modes have wave-vectors k, which are perpendicular to the longitudinal TE modes, and are highly sensitive to boundary condition(s) in the (x, y) plane of the BAW resonator stack. In addition to a degradation of the pass-band ripple, when such BAW resonators are arranged in a fdter topology (e.g. ladder, lattice combination), these spurious modes also couple to the fundamental TE modes, and thereby reduce the quality factor Q of the BAW resonator. The energy of the Rayleigh-Lamb modes are not confined to the piezoelectric layer, and usually escape the resonant cavity, leading to a path of energy reduction and ultimately, to a degradation in Q.

Spurious modes physically arise from a mechanical mismatch at the boundary of the BAW resonator on the one hand, and anchor points supporting the structure to the surrounding bulk material/substrate on the other hand. When operating in the TE mode, the anchor region needs to satisfy the displacement continuity, which in turn generates additional bulk and surface waves. These waves can be polarized longitudinally or in the shear direction. These waves, when confined within a finite structure, are further responsible for the generation of additional spurious modes.

Since the BAW resonator, in the SMR configuration, is composed of a finite number of material layers with different acoustic properties, the combination of reflection /transmission of longitudinal/share waves at each of the material interfaces in the thickness direction, interact and give rise to lateral propagating plate modes of vibration, as described earlier (Rayleigh-Lamb).

So far the above-described problems have been addressed in the following conventional solutions:

1. Apodization of top electrodes of a BAW resonator: a. Spurious modes are formed by standing waves in the lateral direction. b. The BAW resonator is therefore shaped in an irregular way to imperfect the reflection coefficients and prevent the spurious modes.

2. Piston mode design with resonator frame/recess structure: a. The surrounding area of the BAW resonator top electrode is patterned, or additional material is deposited, in order to modify the acoustic property of the anchor region. By choosing appropriate thicknesses/widths of this resonator frame, a flatter mode-shape can be generated in the core of the resonator. Since the resonator is piezoelectrically driven, having a flatter mode shape enhances the electro-mechanical coupling of the fundamental TE mode, compared to the unwanted, higher harmonic spurs. b. This results in an overall lower coupling of spurious modes to the applied electric field. c. These mentioned techniques require careful and complex fabrication process to produce the correct material deposition / material etch. SUMMARY

In view of the above-described problems, embodiments of the invention aim to improve the conventional solutions. An objective is to provide a BAW device, in which spurious modes are effectively suppressed. The BAW device may include one or more BAW resonators. Another goal is to provide a low-complex fabrication method for such a BAW device.

The objective is achieved by the embodiments of the invention as described in the enclosed independent claims. Advantageous implementations of the embodiments of the invention are further defined in the dependent claims.

A first aspect of the invention provides a BAW device comprising: a piezoelectric layer configured to propagate a BAW, a top electrode and a bottom electrode sandwiching the piezoelectric layer and configured to couple an electrical signal to a BAW propagating in the piezoelectric layer, wherein the piezoelectric layer includes a core region located between the top electrode and the bottom electrode and a frame region located below and/or at a side edge of the top electrode, wherein the frame region comprises one or more material elements embedded in the piezoelectric layer and wherein the one or more embedded material elements have acoustic and/or material properties different from those of the surrounding material of the piezoelectric layer.

In the BAW device of the first aspect, the top electrode and/or the bottom electrode is not necessarily continuous, but may comprise several pieces or patches, which do not necessarily contact each other directly. In this way, one or more BAW resonators may be formed in the BAW device. The BAW device may accordingly comprise one or more BAW resonators, wherein each BAW resonator is formed by a piece/patch of the top electrode, a piece/patch of the bottom electrode, and a part of the piezoelectric layer. Each BAW resonator may also be formed by a different top and/or bottom electrode, i.e. the BAW device may include more than one top electrode and/or more than one bottom electrode.

Top“ and „bottom“, as used herein, are defined with respect to a „body-fixed“ reference frame/coordinate system, i.e. a reference frame that is fixed to the BAW device and rotates with the latter in case the latter is rotated.

Acoustic waves travel in the piezoelectric layer, rather than just in the core region. In fact the inclusion of the frame region enables a much more uniform vibration distribution within the whole structure (i.e. the vibration is fully spread-out in the core region of each BAW resonator and the frame region).

In the BAW device of the first aspect, spurious modes are effectively suppressed, at least as good as in BAW devices according to the conventional solutions, and at the same time the BAW device can be manufactured with lower complexity.

In particular, embedded into the piezoelectric layer is material, which has different acoustic/material properties than the core piezoelectric layer. The wave-dispersion characteristics of this frame region can, for instance, be tuned to have an equivalent acoustic quarter-wavelength at the operating frequency of the resonator. The mode-shape can thus be optimized to be much more uniform within the resonator region. This reduces the electro-mechanical coupling to higher harmonics and the spurious modes.

In an implementation form of the first aspect, the one or more embedded material elements comprise two or more embedded material elements placed one after the other in a direction away from the core region.

The direction away from the core region may be orthogonal to a boundary or transition zone between the core region and the frame region. In an implementation form of the first aspect, the spacing between any neighboring material elements among the two or more embedded material elements increases or decreases monotonically in said direction away from the core region.

In an implementation form of the first aspect, the two or more embedded material elements form a phononic crystal in the piezoelectric layer.

The phononic crystal may be used to effectively suppress the spurious modes in the BAW device.

In an implementation form of the first aspect, the two or more embedded material elements include a first periodic arrangement of identical material elements and a second periodic arrangement of identical material elements, wherein the second arrangement is placed after the first arrangement in said direction away from the core region, and wherein the first periodic arrangement and the second periodic arrangement differ from each other in or more of the following characteristics: shape of the material elements, size of the material elements, material composition of the material elements, and spacing between neighboring material elements.

In an implementation form of the first aspect, the two or more embedded material elements include an intermediary arrangement of material elements located between the first periodic arrangement and the second periodic arrangement, wherein the intermediary arrangement provides a gradual or stepwise transition in one or more or all of said characteristics of the first and the second periodic arrangement.

In an implementation form of the first aspect, the frame region is arranged adjacent to the top electrode and/or surrounds the top electrode, in a top view of the BAW device.

In an implementation form of the first aspect, the frame region is arranged inside and/or outside the area of the piezoelectric layer covered by the top electrode, in a top view of the BAW device. In an implementation form of the first aspect, a width of the frame region is approximately an integer multiple of an equivalent acoustic quarter wavelength of the BAW at an operating frequency of the BAW device.

“Approximately” can mean „within a certain range comprising the nominal value". That range may be +- 10 %, +-5% or +-2 % of the nominal value.

In an implementation form of the first aspect, the embedded material elements have a different acoustic impedance and/or have different electrical properties than the surrounding material of the piezoelectric layer.

In an implementation form of the first aspect, the embedded material elements have a temperature coefficient of frequency opposite to that of the surrounding material of the piezoelectric layer.

In other words, the signs of the two temperature coefficients may be opposite (one positive, one negative). This reduces the impact of thermal drift on the operating frequency of the resonator. In a filter configuration, this may translate to less margin required on the filter pass-band for a given spec as there is less spread in frequency due to temperature variations.

In an implementation form of the first aspect, the one or more embedded material elements are identical in shape and in their material composition.

In an implementation form of the first aspect, the one or more embedded material elements provide an acoustic waveguide.

In an implementation form of the first aspect, the BAW device further an acoustic reflective element located below the top electrode, the piezoelectric layer and the bottom electrode.

In an implementation form of the first aspect, the acoustic reflective element comprises a plurality of high and low acoustic impedance layers, which form a Bragg mirror structure. The acoustic reflective element is arranged below the piezoelectric layer and the bottom electrode, respectively.

In an implementation form of the first aspect, the acoustic reflective element comprises a cavity.

A second aspect of the invention provides a method for fabricating a BAW device, the method comprising: providing a piezoelectric layer configured to propagate a BAW, forming a top electrode and a bottom electrode sandwiching the piezoelectric layer and configured to couple an electrical signal to a BAW propagating in the piezoelectric layer, forming a core region located between the top electrode and the bottom electrode, forming a frame region located below and/or at a side edge of the top electrode, by embedding one or more material elements into the piezoelectric layer, wherein the one or more embedded material elements have acoustic and/or material properties different from those of the surrounding material of the piezoelectric layer.

In an implementation form of the second aspect, the one or more embedded material elements comprise two or more embedded material elements placed one after the other in a direction away from the core region.

In an implementation form of the second aspect, the spacing between any neighboring material elements among the two or more embedded material elements increases or decreases monotonically in said direction away from the core region.

In an implementation form of the second aspect, the two or more embedded material elements form a phononic crystal in the piezoelectric layer.

In an implementation form of the second aspect, the two or more embedded material elements include a first periodic arrangement of identical material elements and a second periodic arrangement of identical material elements, wherein the second arrangement is placed after the first arrangement in said direction away from the core region, and wherein the first periodic arrangement and the second periodic arrangement differ from each other in or more of the following characteristics: shape of the material elements, size of the material elements, material composition of the material elements, and spacing between neighboring material elements.

In an implementation form of the second aspect, the two or more embedded material elements include an intermediary arrangement of material elements located between the first periodic arrangement and the second periodic arrangement, wherein the intermediary arrangement provides a gradual or stepwise transition in one or more or all of said characteristics of the first and the second periodic arrangement.

In an implementation form of the second aspect, the frame region is arranged adjacent to the top electrode and/or surrounds the top electrode, in a top view of the BAW device.

In an implementation form of the second aspect, the frame region is arranged inside and/or outside the area of the piezoelectric layer covered by the top electrode, in a top view of the BAW device.

In an implementation form of the second aspect, a width of the frame region is approximately an integer multiple of an equivalent acoustic quarter wavelength of the BAW at an operating frequency of the BAW device.

In an implementation form of the second aspect, the embedded material elements have a different acoustic impedance and/or have different electrical properties than the surrounding material of the piezoelectric layer.

In an implementation form of the second aspect, the embedded material elements have a temperature coefficient of frequency opposite to that of the surrounding material of the piezoelectric layer.

In an implementation form of the second aspect, the one or more embedded material elements are identical in shape and in their material composition.

In an implementation form of the second aspect, the one or more embedded material elements provide an acoustic waveguide. In an implementation form of the second aspect, the method further comprises forming an acoustic reflective element below the top electrode, the piezoelectric layer and the bottom electrode.

In an implementation form of the second aspect, the acoustic reflective element comprises a plurality of high and low acoustic impedance layers, which form a Bragg mirror structure.

In an implementation form of the second aspect, the acoustic reflective element comprises a cavity.

In summary, embodiments of the invention base on engineering a BAW resonator frame region adjacent to and/or around a BAW resonator core region, wherein in the frame region embedded material units are embedded within the piezoelectric layer. This resonator frame region modifies the boundary condition of the corresponding BAW resonator (i.e. the boundary condition to the outside substrate), and thus greatly reduces the triggering of spurious modes. The material can be embedded in a replicated manner, in order to form one or more acoustic phononic crystals for an efficient suppression of the spurious modes.

It has to be noted that all devices, elements, units and means described in the present application could be implemented in the software or hardware elements or any kind of combination thereof. All steps which are performed by the various entities described in the present application as well as the functionalities described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if, in the following description of specific embodiments, a specific functionality or step to be performed by external entities is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented in respective software or hardware elements, or any kind of combination thereof.

BRIEF DESCRIPTION OF DRAWINGS The above described aspects and implementation forms of the present invention will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which:

FIG. 1 shows a BAW device according to an embodiment of the invention.

FIG. 2 shows a BAW device according to an embodiment of the invention, in a top view.

FIG. 3 shows a BAW device according to an embodiment of the invention.

FIG. 4 shows a BAW device according to an embodiment of the invention.

FIG. 5 shows a BAW device according to an embodiment of the invention.

FIG. 6 shows a BAW device according to an embodiment of the invention.

FIG. 7 shows a BAW device according to an embodiment of the invention.

FIG. 8 shows a BAW device according to an embodiment of the invention.

FIG. 9 shows a BAW device according to an embodiment of the invention.

FIG. 10 shows a BAW device according to an embodiment of the invention, in a top view.

FIG. 11 shows a method for fabricating a BAW device according to an embodiment of the invention.

FIG. 12 shows shows a computed wave-dispersion plot of an AIN based SMR-BAW resonator. FIG. 13 shows a simulated plot of a one-port resonator admittance, with different spurious modes.

FIG. 14 shows different dispersion curve characteristics of a resonator stack and that of a unit-cell replicated structure of a BAW device according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows a BAW device 100 according to an embodiment of the invention, in a cross- sectional view. In particular, the different layers of the BAW device arranged on top of each other in the z-direction, and their extension into the perpendicular x-direction are shown. The y-direction is into the x-z-plane.

The BAW comprises a piezoelectric layer 101, a top electrode 102, and a bottom electrode 103, wherein the two electrodes 102 and 103 sandwich the piezoelectric layer 101, i.e. the piezoelectric layer 101 is arranged between the top electrode 102 and the bottom electrode 103. The piezoelectric layer 101 is configured to propagate a BAW. In particular, the BAW can be excited in the piezoelectric layer 101 by means of the two electrodes 102 and 103, e.g. by applying an electrical signal (e.g. alternating voltage) to these electrodes 102, 103, and the excited BAW can travel in and along the piezoelectric layer 101. That means, the top electrode 102 and bottom electrode 103 are configured to couple an electrical signal to a BAW propagating in the piezoelectric layer 101.

The piezoelectric layer 101 includes a core region 104, which is located between the top electrode 102 and the bottom electrode 103, and further includes a frame region 105, which is located below and/or at a side edge of the top electrode 102. That means, the frame region 105 can be arranged below the top electrode 102, at a side edge of the top electrode 102, or (both) below and at a side edge of the top electrode 102.

The frame region 105 comprises one or more material elements 106, which are embedded in the piezoelectric layer 101. That means, the frame region 105 may be defined by the part of the piezoelectric layer 101 that includes the embedded material elements 106. Multiple material elements 106 may thereby be embedded, periodically or in one or more periodic arrangements, in the frame region 105. The core region 104 may be designed such that it does not include any (of the) embedded material elements.

The one or more embedded material elements 106 have acoustic and/or material properties, which are different from those of the surrounding material of the piezoelectric layer 101, i.e. they are different than the acoustic and/or material properties of the surrounding material of the piezoelectric layer 101.

Notably, the BAW device 100 may comprise one or more BAW resonators, wherein each BAW resonator is formed by a top electrode 102 (or by a piece/patch of a common top electrode 102), a part of the piezoelectric layer 101, and a bottom electrode 103 (or by a piece/patch of a common bottom electrode 103).

FIG. 2 shows a BAW device 100 according to an embodiment of the invention that builds on the embodiment shown in FIG. 1. That means, FIG. 2 shows more, optional, details of the BAW device 100 shown in FIG. 1. FIG. 2 is in particular a top view of the BAW device 100, i.e. shows the device along the x-direction and y-direction. The z-direction is into the x-y plane.

FIG. 2 shows the different areas of interest are included in the design of the BAW device 100. The region of the top electrode 102 is the region, where a conductive material, e.g. a metal, is deposited on the top surface of the piezoelectric layer 101. The frame region 105 may be any region englobing the top electrode 102 region. For instance, the frame region 105 may be arranged adjacent to the top electrode 102 and/or may surround the top electrode 102, in the top view of the BAW device 100. The frame region 105 is where the embedded material elements 106 are included in the piezoelectric layer 101, in order to prevent the creation of spurious modes. The frame region 105 plus the top electrode 102 region (i.e. plus the core region when considering the piezoelectric layer 101) may be considered as an active region of the BAW device 100, which active region may correspond to one BAW resonator of the BAW device 100. The frame region 105 does not need to be uniformly arranged around the top electrode 102 region (hence the dimensions a, b, c, d given in FIG. 1 can be selected flexibly). In other words, the frame region 105 can also be non-uniformly arranged around the top electrode region 102. For instance, a lateral dimension a of the frame region 105 on one side of the top electrode 102 region may be different from a lateral dimension c of the frame region 105 on another side of the top electrode 102 region (e.g. in x-direction). Likewise, a transversal dimension b of the frame region 105 on one side of the top electrode 102 region may be different from a transversal dimension d on another side of the top electrode 102 region (e.g. in y-direction, perpendicular to the x-direction).

FIG. 3 shows a BAW device 100 according to an embodiment of the invention that builds on the embodiment shown in FIG. 1. That means, FIG. 3 shows more, optional, details of the BAW device 100 shown in FIG. 1. FIG. 3 is a cross-sectional view of the BAW device 100, i.e. showing the x-direction and z-direction of the BAW device 100 (as in FIG. 1).

In particular, FIG. 3 shows a unit cell of the frame region 105, as it may be formed in the BAW device 100. Multiple such unit cells (or a repetition of the unit cell) may together form the frame region 105 with an arrangement of multiple material elements 106, with a pitch as indicated in FIG. 3. The x-direction replication pitch dimension should be carefully chosen, in order to provide a lateral acoustic waveguide to the BAW traveling in the piezoelectric layer 101. The material element(s) 106 within the piezoelectric layer 101 may be of different acoustic impedance than the surrounding piezoelectric layer 101.

An acoustic reflective element may be located below the top electrode 102, the piezoelectric layer 101, and the bottom electrode 103, respectively. The acoustic reflective element may comprise a plurality of high acoustic and low acoustic impedance layers (here layers 300 and 301), which form a Bragg mirror structure, e.g. on a substrate 302. In FIG. 3, the lower acoustic impedance material 300 and the higher acoustic impedance material 301, respectively, are used to form the reflective element of the Bragg mirror.

FIG. 4 shows a BAW device 100 according to an embodiment of the invention that builds on the embodiment shown in FIG. 1 and FIG. 3. That means, FIG. 4 shows more, optional, details of the BAW device 100 shown in FIG. 1 and FIG. 3. FIG. 4 is a cross-sectional view of the BAW device 100 (x-direction and z-direction are shown).

In particular, FIG. 4 shows one half of a BAW resonator of the BAW device 100 used with a Bragg mirror (e.g. SMR-BAW configuration) as in FIG. 3.

The location of the frame region 105 is in this embodiment at a side edge of the top electrode 102, below the top electrode 102. The material elements 106 embedded in the piezoelectric layer 101 are in a periodic arrangement. In particular, the piezoelectric layer 101 comprises two or more embedded material elements 106 (three are exemplarily shown), which are placed one after the other in a direction away from the core region 104. A spacing between neighboring material elements 106 among the two or more embedded material elements 106 may be constant, or it may increases or decreases monotonically in said direction away from the core region 104.

The top electrode 102 can have additionally deposited layers over it (e.g. other conductive or metal layers like a Ti layer, or a dielectric layer, e.g. S13N4 for passivation and/or mass load). These are not shown in the figure.

FIG. 5 shows a BAW device 100 according to an embodiment of the invention that builds on the embodiment shown in FIG. 1 and FIG. 4. That means, FIG. 5 shows more, optional, details of the BAW device 100 shown in FIG. 1 and FIG. 4. FIG. 5 is a cross-sectional view of the BAW device 100 (x-direction and z-direction are shown).

In particular, FIG. 5 shows that the BAW device 100 may comprise the two or more embedded material elements 106, which may include a first periodic arrangement 500 of identical material elements 106, and a second periodic arrangement 501 of identical material elements 106 (the material elements 106 in the second periodic arrangement being different from the material elements 106 in the first periodic arrangement). The second periodic arrangement 501 may be placed after the first arrangement 500 in the direction away from the core region 104. Further, the first periodic arrangement 500 and the second periodic arrangement 501 may differ from each other, e.g. in or more of the following characteristics: shape of the material elements 106, size of the material elements 106, material composition of the material elements 106, and spacing between neighboring material elements 106.

FIG. 6 shows a BAW device 100 according to an embodiment of the invention that builds on the embodiment shown in FIG. 1 and FIG. 5. That means, FIG. 6 shows more, optional, details of the BAW device 100 shown in FIG. 1 and FIG. 5. FIG. 6 is a cross-sectional view of the BAW device 100 (x-direction and z-direction are shown).

In particular, the BAW device 100 may comprise the two or more embedded material elements 106, which may include an intermediary arrangement 600 of material elements 106, which may be located between the first periodic arrangement 500 and the second periodic arrangement 501 of material elements 106 (as previously shown in FIG. 5). The intermediary arrangement 600, as shown in FIG. 6, may provide a gradual or stepwise transition in one or more or all of said characteristics of the first and the second periodic arrangement 500, 501, respectively, i.e. in one or more of: shape of the material elements 106, size of the material elements 106, material composition of the material elements 106, and spacing between neighboring material elements 106.

Any periodic arrangement 500, 501, and/or 600 of the embedded material elements 106 can be composed of one or more distinct phononic crystal regions within the frame region 105.

FIG. 7 shows a BAW device 100 according to an embodiment of the invention that builds on the embodiment shown in FIG. 1 and FIG. 3. That means, FIG. 7shows more, optional, details of the BAW device 100 shown in FIG. 1 and FIG. 3. FIG. 7 is a cross-sectional view of the BAW device 100 (x-direction and z-direction are shown).

In particular, in FIG. 7 the frame region 105 of the BAW device 100 is located outside of the top electrode 102 region (in a top view of the BAW device 100), i.e. is not directly below the top electrode 102 (in the top view). That is, the frame region 105 is arranged outside of the area of the piezoelectric layer 101, which is covered by the top electrode 102, (in the top view of the BAW device 100).

FIG. 8 shows a BAW device 100 according to an embodiment of the invention that builds on the embodiment shown in FIG. 1, FIG. 3 and FIG. 4. That means, FIG. 8 shows more, optional, details of the BAW device 100 shown in FIG. 1, FIG. 3 and FIG. 4. FIG. 8 is a cross-sectional view of the BAW device 100 (x-direction and z-direction are shown).

In particular, in FIG. 8, the BAW device 100 comprises an acoustic reflective element located below the top electrode 102, the piezoelectric layer 101, and the bottom electrode 103, respectively. As shown in FIG. 8, the acoustic reflective element of the BAW device 100 may comprise a cavity, specifically a buried cavity 802. The buried cavity 802 may be formed in, or by, a substrate 803 arranged below the bottom electrode 103 of the BAW device 100.

Furthermore, as shown in FIG. 8, each side of the BAW device 100 can include one or more frame regions 105, wherein each frame region 105 may be configured as described above (e.g. each frame region 105 may have a distinct distribution of the material elements 106 embedded into the piezoelectric layer 101, pitch between the material elements 106, material choice of the material elements 106, etc.).

FIG. 9 shows a BAW device 100 according to an embodiment of the invention that builds on the embodiment shown in FIG. 1, FIG. 3 and FIG. 4. That means, FIG. 9 shows more, optional, details of the BAW device 100 shown in FIG. 1, FIG. 3 and FIG. 4. FIG. 9 is a cross-sectional view of the BAW device 100 (x-direction and z-direction are shown).

In particular, compared to FIG. 8, the acoustic reflective element of the BAW device 100 shown in FIG. 9 comprises a cavity 900, which is not disposed over a handle substrate 803, but rather when the back-side of the substrate 803 is etched away. That is, the cavity 900 is not a buried cavity 802 as in FIG. 8, but can be referred to as a back-side cavity 900.

FIG. 10 shows a BAW device 100 according to an embodiment of the invention that builds on the embodiment shown in FIG. 2. That means, FIG. 10 shows more, optional, details of the BAW device 100 shown in FIG. 2. FIG. 10 is a top view of the BAW device 100 (x- direction and y-direction are shown).

In particular, FIG. 10 shows that the BAW device 100 can include more than one BAW resonator. Here two BAW resonators are shown, and the BAW resonators are arranged adjacent to another. One BAW resonator is associated with top electrode 102a and frame region 105a, while the other BAW resonator is associated with top electrode 102b and frame region 105b. The top electrodes 102a and 102b may be different pieces/patches of the same top electrode 102.

The adjacent BAW resonators can be acoustically coupled via the adjacent frame regions 105a and 105b. The strength of mechanical coupling can be tuned via the design of a coupling region 1000, where the frame regions 105 a and 105b overlap, e.g. in the x- direction, i.e. by the width of that coupling region 1000. That means, the adjacent BAW resonators can be either moved closer together or further apart, which influences the modes of vibration of the overall mechanically coupled structure.

In the above-described embodiments, the substrate 302 or 803 may be one of silicon, glass, ceramic, and the like. The (e.g. thin fdm) piezoelectric layer 101 may be one of lithium niobate, lithium tantalate, and the like. The (low resistivity) top electrode 102 and/or bottom electrode 103 may be a metal and/or metal alloy layer, such as copper, titanium, and the like, or may be a highly doped silicon layer.

In an example, the acoustic resonator frame material, i.e. the embedded material elements 106, may be a dielectric material. For example, the dielectric material can include SiCOH, a phosphosilicate glass, an oxide or a nitride of aluminum, silicon, germanium, gallium, indium, tin, antimony, tellurium, bismuth, titanium, vanadium, chromium, manganese, cobalt, nickel, copper, zinc, zirconium, niobium, molybdenum, palladium, cadmium, hafnium, tantalum, or tungsten, or any combination thereof.

The metallization layers, i.e. the top electrode 102 and/or bottom electrode 103 may include, for example, copper, aluminum, tungsten, titanium, etc. The BEOL dielectric layers may include, for example, copper capping layers (CCL), etch stop layers (ESL), diffusion barriers (DB), antireflection coating (ARC) and low-k dielectrics such as, for example, SiCOH, SiOCN, SiCN, SiOC, SiN.

The materials available in the CMOS BEOL layers include, for example, copper metallization, tungsten, low-k dielectrics, silicon dioxides, copper capping layers, etch stop layers, anti-reflecting coatings, etc.

In an example, the substrate 302 or 803 can include silicon, a SOI technology substrate, gallium arsenide, gallium phosphide, gallium nitride, and/or indium phosphide or other example substrate, an alloy semiconductor including GaAsP, AlInAs, GalnAs, GalnP, or GalnAsP or combinations thereof.

FIG. 11 shows schematically a method 1100 according to an embodiment of the invention. The method 1100 is in particular for fabricating a BAW device 100, particularly the BAW device 100 of the embodiments described above.

The method 1100 include: providing 1101 a piezoelectric layer 101, which is configured to propagate a BAW; forming 1102 a top electrode 102 and a bottom electrode 103 sandwiching the piezoelectric layer 101 and configured to couple an electrical signal to a BAW propagating in the piezoelectric layer 101; forming a core region 104 located between the top electrode 102 and the bottom electrode 103; and forming a frame region 105 located below and/or at a side edge of the top electrode 102, by embedding one or more material elements 106 into the piezoelectric layer 101. The one or more embedded material elements 106 have acoustic and/or material properties different from those of the surrounding material of the piezoelectric layer 101.

FIG. 12 shows a computed wave-dispersion plot of an AIN based SMR-BAW resonator. These are extracted from numerically computed top surface amplitude vibration distribution. The main TE1 vibration branch is clearly visible, with in this particular case a type II behavior. From the diagram, spurious modes below the series frequency are expected to significantly affect the admittance measurements of the resonators.

FIG. 13 shows a simulated plot of a one-port resonator admittance, with different spurious modes clearly visible on the admittance data. The inset figure shows the vibration amplitude distribution of the resonator top surface (x-axis being a horizontal spatial cut of the resonator, y-axis is the frequency). At the series frequency, the top surface of the resonator has a maximum displacement, and this vibration extends in the whole lateral dimension of the resonator. Below the series frequency, numerous spurious modes where lateral standing waves are triggered, which correspond to the spurious modes measured in the admittance 1 -port data. FIG. 14 shows different dispersion curve characteristics of a resonator stack (square data- points) and that of a unit-cell replicated structure (e.g. FIG. 3) of a BAW device 100 according to an embodiment of the invention (Phononic Crystal (PnC) W triangular data points). At the series frequency (horizontal line around 2620MHz), the PnC region supports lateral waves, of which the quarter-wavelength is around 15 pm. This plot can be used to size the frame region 105 appropriately, in order to mitigate the effect of the spurious modes.

The present invention has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed invention, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.