TECHNICAL FIELD

The present disclosure relates to Bulk Acoustic Wave (BAW) devices. In particular, the disclosure is concerned with a BAW device having at least one BAW resonator and a frame formed by an electrically floating and isolated electrode. The frame may be arranged around the BAW resonator and/or may be arranged between two BAW resonators of the BAW device. The disclosure thereby aims at improving the spurious mode response of the at least one BAW resonator.

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 filter 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 typically the thickness-extensional (TE) mode. This mode bases on longitudinal waves that propagate in the thickness of the thin piezoelectric material. However, other classes of piezoelectric material can also be used (e.g. alternative piezoelectric crystal cuts), in which case the primary operating mode of the BAW resonator can also be the thickness-shear (TS) mode (or any alternative propagating mode in the piezoelectric medium). That is, the role of the TE and the TS may interchanged depending on the primary chosen operating mode of vibration of the piezoelectric material. The selection of the operating mode can be based on requirements such as high electro-mechanical coupling factor, low propagation loss, immunity to frequency drifts due to temperature changes or the like. In order to achieve a high quality electric signal, the mechanical resonance, which is generated within the BAW resonators, needs to be as efficient as possible with the least mechanical loss. Mechanical loss in a BAW resonator primarily arises from acoustic radiation of mechanical energy into the substrate (e.g., through the reflective element). Other routes of mechanical loss are radiation and scattering of energy at the resonator edges, particularly, at locations where no reflective element is present. One important metric for the mechanical loss is the equivalent mechanical quality factor (Q) of the resonator, designed as 2n times the stored energy divided by the energy dissipated per cycle.

Moreover, also lateral propagating modes can be generated at the different material interfaces, and can form additional standing waves inside the resonator region (or core region), i.e. the region of the piezoelectric material in the BAW resonator. This deteriorates the frequency characteristics of the BAW resonator. These lateral spurious modes are also sources of acoustic loss, as they can laterally escape out from the resonator region of the BAW resonator, and therefore limit the achievable Q of the BAW device. Furthermore, such spurious modes deteriorate the in-band characteristics of the BAW device and thus should be reduced.

Therefore, BAW resonators traditionally employ lateral boundary frame structures, in order to provide appropriate matching conditions to the external bulk substrate propagating media. These so-called “frames” are used to create a piston mode in the resonator region, and limit the coupling to lateral travelling spurious modes.

To date, spurious modes in BAW devices have been addressed by using the following frame designs:

1. Recess/ frame structures depending on the acoustic dispersion type of the resonator region.

However, this requires advanced lithography and processing to achieve very precise frame geometries.

2. Lateral frame regions with different electrical boundary conditions. For instance, a border ring composed of conductive and non-conductive portions.. However, improvements of such lateral frames used to suppress lateral spurious modes in BAW devices are required.

SUMMARY

In view of the above-mentioned, embodiments of the present invention aim to provide an improved frame design. An objective is, in particular, to provide a BAW device with at least one BAW resonator, and an improved frame for suppressing lateral spurious modes leaking from the BAW resonator (lateral leakage). The frame should be adapted to be arranged next to and/or around the BAW resonator and/or between two or more BAW resonators of the BAW device. The frame should efficiently limit the coupling to lateral travelling spurious modes which are building up in the BAW resonator. Another goal design of such a frame is that it can be easily formed in few processing steps and without any special processing required. Accordingly, an aim 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.

In particular, embodiments of the invention propose a frame, which is formed by an electrically floating and isolated electrode, optionally having an additional mass loading on top. The electrically floating electrode is provided on or beneath a piezoelectric material, which is also used for the resonator region of the at least one BAW resonator. The piezoelectric material is configured such that at least one of its acoustic propagating modes (modes of vibration) is piezoelectrically less coupled than the other modes, particularly, is un-coupled when applying an in-plane electric field.

A first aspect of this disclosure provides a BAW device, comprising: a piezoelectric layer having a first mode of vibration and a second mode of vibration, the piezoelectric layer being configured such that the second mode of vibration has a lower piezoelectric coupling coefficient than the first mode of vibration; a first electrode provided on the piezoelectric layer and a second electrode provided beneath the piezoelectric layer; wherein the first electrode and the second electrode sandwich the piezoelectric layer to form a BAW resonator, the BAW resonator comprising a resonator region, which is a region of the piezoelectric layer located between the first electrode and the second electrode; an electrically floating third electrode provided on or beneath the piezoelectric layer, the third electrode being electrically isolated from the first electrode and the second electrode; wherein the third electrode and the piezoelectric layer form a frame, the frame comprising a frame region, which is a region of the piezoelectric layer located on or beneath the third electrode.

The electrically floating boundary condition of the third electrode, and the electric isolation of the third electrode from the first and second electrode, may lead to the first mode of vibration having a different cut-off frequency in the frame region than in the resonator region. In other words, the first mode of vibration may experience a frequency shift going from the resonator region to the frame region. As a consequence, the second mode of vibration may be the only viable acoustic path in the frame region, whereas the BAW in the resonator region may be propagated in the first mode of vibration. Thus, the frame may suppress lateral leakage, i.e., it may efficiently limit the coupling of the BAW propagated in the resonator region to lateral spurious modes travelling in the frame region. Further, the frame can be easily formed in few processing steps and without any special processing required.

Notably, the second mode of vibration may be in the vicinity (i.e., in terms of frequency spacing) of the first mode of vibration.

In an implementation form of the first aspect, the cut-off frequency of the first mode of vibration is different in the resonator region than in the frame region.

That is, the first mode of vibration experiences a frequency shift between these two regions.

In an implementation form of the first aspect, the cut-off frequency of the first mode of vibration is lower in the resonator region than in the frame region.

In an implementation form of the first aspect, the BAW resonator is configured to propagate a BAW in the first mode of vibration in the resonator region.

In an implementation form of the first aspect, the piezoelectric coupling coefficient of the second mode of vibration is zero or is in a range of 0-0.05. That is, the second mode of vibration is not, or only very weakly, piezoelectrically coupled. Accordingly, the second mode of vibration is not influenced, at least not strongly, by electrical fields and different electrical boundary conditions.

In an implementation form of the first aspect, the third electrode is provided on the piezoelectric layer and the third electrode and the second electrode sandwich the piezoelectric layer to form the frame.

In an implementation form of the first aspect, the third electrode is provided on the piezoelectric layer; the BAW device further comprise a fourth electrode provided beneath the piezoelectric layer, the fourth electrode being electrically isolated from the first electrode and the second electrode; and the third electrode and the fourth electrode sandwich the piezoelectric layer to form the frame.

In an implementation form of the first aspect, an interleave region is arranged between the third electrode and the first electrode or the second electrode to electrically isolate the third electrode from the first electrode or the second electrode.

The interleave region may thus comprise an isolating material. For instance, the interleave region may be made of a dielectric material.

In an implementation form of the first aspect, the interleave region surrounds the first electrode in a top view of the BAW device.

Thus, the BAW resonator may be completely surrounded by the frame, which may suppress lateral leakage from the BAW resonator.

In an implementation form of the first aspect, the BAW device further comprises a mass loading layer provided on the third electrode.

The mass loading layer may further change the boundary conditions in the frame region, to the first order, modifying the cut-off frequencies of available modes in that region. The mass loading layer thus allows tuning the dispersion curve of the modes, particularly, of the first mode of vibration. In this way, the frame can be optimized. In an implementation form of the first aspect, the interleave region and the mass loading layer are made of the same material or are made of different materials.

For instance, both the interleave region and the mass loading layer may be formed by a single layer.

In an implementation form of the first aspect, the BAW device further comprises an interleave layer including the interleave region; wherein the interleave layer is arranged to form a top surface of the BAW device and/or embeds the first electrode and/or the third electrode.

The interleave layer may further form the mass loading layer, which is provided on the third electrode.

In an implementation form of the first aspect, the BAW device further comprises a passivation layer provided on the mass loading layer and/or on the interleave region.

The passivation layer may comprise a dielectric layer.

In an implementation form of the first aspect, a further interleave region is arranged between the fourth electrode and the second electrode to electrically isolate the fourth electrode from the second electrode.

The interleave region and the further interleave region may be made from the same material, or may be made from different materials.

In an implementation form of the first aspect, the third electrode is provided beneath the piezoelectric layer and the third electrode and the first electrode sandwich the piezoelectric layer to form the frame.

In an implementation form of the first aspect, the third electrode and the first electrode, or the third electrode and the second electrode, are formed by a common electrode layer that is separated into at least two electrically isolated parts. The common electrode layer simplifies the fabrication process of the BAW device, since fewer deposition steps are required, for instance.

In an implementation form of the first aspect, the interleave region arranged between the third electrode and the first or the second electrode is narrower than an acoustic quarter wavelength of the BAW in the first mode of vibration, which the BAW resonator is configured to propagate.

In an implementation form of the first aspect, the piezoelectric layer has a determined crystal cut orientation such that the second mode of vibration has the lower piezoelectric coupling coefficient than the first mode of vibration.

For instance, the piezoelectric layer may comprise or be Lithium-niobate oxide (LiNbCfi) with a Y + 0 cut orientation.

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

This reflective element suppresses vertical leakage, for instance, leakage to the substrate.

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 at the frequency of the BAW in the first mode of vibration, which the BAW resonator is configured to propagate.

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

In an implementation form of the first aspect, the frame is arranged next to or around the BAW resonator in a top view of the BAW device; and/or the frame is arranged between the BAW resonator and another BAW resonator in a top view of the BAW device.

Thus, two or more adjacent BAW resonators of the BAW device may be separated by a frame per BAW resonator pair, which reduces acoustic coupling between these BAW resonators. In an implementation form of the first aspect, the first mode of vibration is a fast-shear mode and the second mode of vibration is a slow-shear mode.

That is, both the first mode of vibration and the second mode of vibration may be TS mode.

A second aspect of this disclosure provides a method for fabricating BAW device, the method comprising: forming a piezoelectric layer having a first mode of vibration and a second mode of vibration , the piezoelectric layer being configured such that the second mode of vibration has a lower piezoelectric coupling coefficient than the first mode of vibration; forming a first electrode on the piezoelectric layer and a second electrode beneath the piezoelectric layer; wherein the first electrode and the second electrode sandwich the piezoelectric layer to form a BAW resonator, the BAW resonator comprising a resonator region, which is a region of the piezoelectric layer located between the first electrode and the second electrode; forming an electrically floating third electrode on or beneath the piezoelectric layer, the third electrode being electrically isolated from the first electrode and the second electrode; wherein the third electrode and the piezoelectric layer form a frame, the frame comprising a frame region, which is a region of the piezoelectric layer located on or beneath the third electrode.

In an implementation form of the second aspect, the cut-off frequency of the first mode of vibration is different in the resonator region than in the frame region.

In an implementation form of the second aspect, the cut-off frequency of the first mode of vibration is lower in the resonator region than in the frame region.

In an implementation form of the second aspect, the BAW resonator is configured to propagate a BAW in the first mode of vibration in the resonator region.

In an implementation form of the second aspect, the piezoelectric coupling coefficient of the second mode of vibration is zero or is in a range of 0-0.05.

In an implementation form of the second aspect, the third electrode is formed on the piezoelectric layer and the third electrode and the second electrode sandwich the piezoelectric layer to form the frame. In an implementation form of the second aspect, the third electrode is formed on the piezoelectric layer; the method further comprises forming a fourth electrode beneath the piezoelectric layer, the fourth electrode being electrically isolated from the first electrode and the second electrode; wherein the third electrode and the fourth electrode sandwich the piezoelectric layer to form the frame.

In an implementation form of the second aspect, an interleave region is arranged between the third electrode and the first electrode or the second electrode to electrically isolate the third electrode from the first electrode or the second electrode.

In an implementation form of the second aspect, the interleave region surrounds the first electrode in a top view of the BAW device.

In an implementation form of the second aspect, the method further comprises forming a mass loading layer on the third electrode.

In an implementation form of the second aspect, the interleave region and the mass loading layer are made of the same material or are made of different materials.

In an implementation form of the second aspect, the method further comprises forming an interleave layer including the interleave region; wherein the interleave layer is arranged to form a top surface of the BAW device and/or embeds the first electrode and/or the third electrode.

In an implementation form of the second aspect, the method further comprises forming a passivation layer on the mass loading layer and/or on the interleave region.

In an implementation form of the second aspect, a further interleave region is formed between the fourth electrode and the second electrode to electrically isolate the fourth electrode from the second electrode.

In an implementation form of the second aspect, the third electrode is formed beneath the piezoelectric layer and the third electrode and the first electrode sandwich the piezoelectric layer to form the frame. In an implementation form of the second aspect, the third electrode and the first electrode, or the third electrode and the second electrode, are formed by a common electrode layer that is separated into at least two electrically isolated parts.

In an implementation form of the second aspect, the interleave region arranged between the third electrode and the first or the second electrode is narrower than an acoustic quarter wavelength of the BAW in the first mode of vibration, which the BAW resonator is configured to propagate.

In an implementation form of the second aspect, the piezoelectric layer has a determined crystal cut orientation such that the second mode of vibration has the lower piezoelectric coupling coefficient than the first mode of vibration.

In an implementation form of the second aspect, the method further comprises forming an acoustic reflective element below the electrodes and the piezoelectric layer.

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 at the frequency of the BAW in the first mode of vibration, which the BAW resonator is configured to propagate.

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

In an implementation form of the second aspect, the frame is arranged next to or around the BAW resonator in a top view of the BAW device; and/or the frame is arranged between the BAW resonator and another BAW resonator in a top view of the BAW device.

In an implementation form of the second aspect, the first mode of vibration is a fast-shear mode and the second mode of vibration is a slow-shear mode.

The method of the second aspect and its implementation forms achieve the same advantages as described above for the BAW device of the first aspect and its respective implementation forms. BRIEF DESCRIPTION OF DRAWINGS

The above described aspects and implementation forms will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which

FIG. 1 shows a distribution of acoustic phase velocities for a LiNbO3 with a Y + 0 cut orientation.

FIG. 2 illustrates standard Euler angles (a, P, y) used for defining a crystal cut orientation.

FIG. 3a shows a dispersion curve characteristic of an exemplary piezoelectric stack.

FIG. 3b shows a dispersion curve characteristic of an exemplary piezoelectric stack.

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

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

FIG. 6 shows a dispersion curve characteristic of the exemplary BAW device shown in FIG. 5.

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

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

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

FIG. 10 shows an exemplary BAW device according to an embodiment of the invention. FIG. 11 shows an exemplary BAW device according to an embodiment of the invention.

FIG. 12 shows an exemplary BAW device according to an embodiment of the invention.

FIG. 13 shows an exemplary BAW device according to an embodiment of the invention.

FIG. 14 shows an exemplary BAW device according to an embodiment of the invention.

FIG. 15 shows a fabrication method for a BAW device according to an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the inventions, which all provide a BAW device including at least one BAW resonator and a frame, will be described in detail below and are based on the following considerations.

Since piezoelectricity involves the coupling of acoustic and electric tensors, electrical boundary conditions are equally important to mechanical boundary conditions in deciding the propagation nature of acoustic waves in a bulk material. In this disclosure, a frame including a frame region, which is electrically isolated from a resonator region of the BAW resonator, is proposed to provide the appropriate boundary conditions.

Moreover, it is possible to find piezoelectric materials, for instance, particularly crystal-cut orientations of certain piezoelectric materials, where one or more acoustic waves (i.e., modes of vibrations) are not piezoelectrically actuated, or significantly less piezoelectrically actuated that other acoustic waves. In particular, piezoelectric materials can be found, which allow forming a piezoelectric layer that has a first mode of vibration and a second mode of vibration, wherein the second mode of vibration has a lower piezoelectric coupling coefficient than the first mode of vibration. For example, as is shown in FIG. 1, this is the case for Y + 0 cut orientations LiNbCF. In particular, FIG. 1 shows, for the LiNbCh a Y + 0 cut orientation, a distribution of acoustic phase velocities. It can be seen from the right side of FIG. 1 that the slow shear-Y polarized waves (“shear Y-pol”) are un-coupled to external through-thickness electric fields, in contrast to fast shear-X polarized waves (shear X-pol”). That is, a slow-shear mode of vibration cannot be actuated for specific external electric-fields applied, but the fast-shear mode can. Accordingly, the above-mentioned second mode of vibration may the slow-shear mode and the above- mentioned first mode of vibration may a fast-shear mode.

FIG. 2 illustrates standard Euler angles (a, P, y) used for defining the crystal cut orientation. The cut orientation may be chosen such that the orthogonal basis vectors of the piezoelectric layer are aligned to the required crystal orientations, as defined by three successive rotations of the basis vectors. The thickness of the piezoelectric layer is then defined in the z ¹ '’ direction.

FIG. 3a shows an example of a piezoelectric stack, which includes a top electrode (TE) and a bottom electrode (BE) sandwiching a piezoelectric layer, thus forming a BAW resonator. Below the BAW resonator is provided a reflective element, which is based on a Bragg structure, and below the reflective element is provided a substrate. The thickness of the TE can have different values depending on the position in the stack. In one case, the thickness of the TE can be made smaller (i.e. “Recess” region) than that of the “Core” region. For a Type I resonator, the cutoff frequency of a first mode of vibration (TS-l)will be larger than that of the Core region. Furthermore, an additional mass-loading (ML) layer can be deposited on top of the TE to lower down the cut-off frequency of the first mode of vibration (TS-1 , e.g. the “Raised-frame” region). In order to suppress the lateral propagating modes, one can use a spurious suppression technique to produce a “Piston Mode” by using a well-defined lateral region as in the “Raised-Frame” region. At the cut-off frequency of the first mode of vibration (TS-1), positive lateral wavenumbers k are able to propagate in the Raised-Frame region, given the required condition for the Piston Mode to exist (still requires careful width optimization of the frame). On another hand, one can reduce the lateral leakage by implementing “Recess” regions surrounding the Core resonator region. In that case, the cut-off frequency of the first mode of vibration (TS-1 *) in the Recess will be higher than that of the first mode of vibration (TS-1) mode in the Core region. By carefully sizing this Recess thickness and width, significant enhancement in the Q factor at the anti-resonance frequency can be obtained. FIG. 3b shows a dispersion curve characteristic of the piezoelectric stack and at least some of its modes of vibration. The dispersion curve shows, in particular, curves for the first mode of vibration (TS-1 and TS-1*, respectively) of the piezoelectric layer, and a curve for a second mode of vibration of the piezoelectric layer (TS-2). For instance, the first mode of vibration (TS-1 and TS-1 *) may be a fast-shear mode and the second mode of vibration (TS-2) may be a slow- shear mode (thus both may be TS modes). The two curves for the first mode of vibration (TS-1 and TS-1*) are for a boundary condition where both electrodes are non-floating (TS-1) and for a boundary condition where at least one of the top electrode and the bottom electrode is floating (TS-1*). It can be seen that the cut-off frequency of the first mode of vibration is lower for the non-floating boundary condition (TS-1) than for the floating boundary condition (TS-1*). Since the second mode of vibration (TS-2) is, in this example, piezoelectrically uncoupled, it is not affected by the changing electrical boundary condition of the electrodes (i.e., it is the same for the floating and non-floating boundary condition).

FIG. 4 shows a BAW device 400 according to a basic embodiment of the invention. The BAW device 100 bases on the considerations made above. In particular, the BAW device 100 comprises a piezoelectric layer 401, which has a first mode of vibration and a second mode of vibration, wherein the piezoelectric layer 401 is configured such that the second mode of vibration has a lower piezoelectric coupling coefficient than the first mode of vibration. For instance, the piezoelectric layer 401 may have a determined crystal cut orientation, such that the second mode of vibration has the lower piezoelectric coupling coefficient than the first mode of vibration. As an example, the piezoelectric layer 401 may comprise or be made of LiNbCF with a Y + 0 cut orientation. The first mode of vibration may further be a fast-shear mode, and the second mode of vibration may be a slow-shear mode, thus both may be a TS mode.

The BAW device 400 further comprise a first electrode 402, which is provided on the piezoelectric layer 401, and a second electrode 403, which is provided beneath the piezoelectric layer 401. The first electrode 402 and the second electrode 403 sandwich the piezoelectric layer 401 to form a BAW resonator. The BAW resonator comprises a resonator region 405, which is a region of the piezoelectric layer 401 located between the first electrode 402 and the second electrode 403.

Further, the BAW device 400 comprises an electrically floating third electrode 404, which is provided on (as is exemplarily shown in FIG. 4) or beneath the piezoelectric layer 401. The third electrode 404 is electrically isolated from the first electrode 402 and the second electrode 403, respectively. The third electrode 404 and the piezoelectric layer 401 form a frame, wherein the frame comprises a frame region 406, which is a region of the piezoelectric layer 401 located on or beneath the third electrode 404. As it is shown, the frame region 506 of the piezoelectric layer may be arranged in the vicinity and next to the resonator region 405.

The floating boundary condition in the frame, i.e. experienced in the frame region 406, and the electrical isolation of the frame and the BAW resonator (by means of electrically isolating the respective electrodes 402, 403, 404) may result in the first mode of vibration having a different cut-off frequency in the frame region 406 than in the resonator region 405, in particular, in the first mode of vibration having a lower cut-off frequency in the resonator region 405 than in the frame region 406. That is, the first mode of vibration may be significantly “pushed away” in the dispersion curve characteristics in the frame region 406, so as to provide only the second mode of vibration as possible acoustic path for lateral leakage. Further, the BAW resonator may be configured to propagate a BAW in the first mode of vibration in the resonator region 405, for which there is accordingly no path in the frame region 406. Thus, the frame not only provides appropriate boundary conditions, but suppresses lateral leakage from the BAW resonator, i.e., the frame efficiently limits the coupling of the BAW in the resonator region 405 to lateral travelling spurious modes. Further, the frame also enables less electro-mechanical degradation, as there is no extra stray capacitance layer, which is being actively driven in order to create the frame.

FIG. 5 shows an example of a BAW device 400 according to an embodiment of the invention, which builds on the embodiment shown in FIG. 4. Same elements of the BAW devices 400 are labelled with the same reference signs, and are implemented likewise. Accordingly, also the BAW device 400 shown in FIG. 5 comprises the piezoelectric layer 401, the three electrodes 402, 403 and 404, the resonator region 405, and the frame region 406.

FIG. 5 shows that the BAW device 400 may further include an interleave region 503, which is arranged between the third electrode 404 and the first electrode 402 (since in this example the third electrode 404 is provided on the piezoelectric layer 401) to electrically isolate the third electrode 404 from the first electrode 402. The BAW device 400 may, in particular, comprise an interleave layer that includes the interleave region 503. The interleave layer may be arranged to form a top surface of the BAW device 400 and/or may cover (as shown) or even embed the third electrode 403. Accordingly, the interleave layer may not only form the interleave region 503, but may additionally function as a mass loading layer on the third electrode 404. That is, the interleave region 503 and this mass loading layer may be made of the same material, for instance, a dielectric material.

Notably, the interleave region 503, which is arranged between the third electrode 404 and the first electrode 402 (or between the third electrode 404 and the second electrode 403, if the third electrode 404 is provided beneath the piezoelectric layer 401) may beneficially be narrower than an acoustic quarter wavelength of the BAW in the first mode of vibration, which the BAW resonator of the BAW device 400 is configured to propagate. Further, the interleave region 503, which electrically isolates the frame from the BAW resonator by isolating the third electrode 404 from the first electrode 402 and the second electrode 403, does not necessarily have to be implemented as an acoustic frame element.

The BAW device 400 exemplarily shown in FIG. 5 further includes a substrate 501, and a reflective element, which is arranged between the substrate 501 and the resonator region 405 and frame region 406, respectively. In this example, the acoustic reflective element comprises a plurality of high and low acoustic impedance layers, which form a Bragg mirror structure 502 at the frequency of the BAW in the first mode of vibration.

An advantage of the device 400 shown in FIG. 5 is that the same deposition step can be used for forming both the first electrode 402 (for the BAW resonator) and the third electrode 404 (for the frame). For instance, the third electrode 404 and the first electrode 402 may be formed by a common electrode layer, which may then be separated into at least two electrically isolated parts (one part forming the third electrode 404, the other part forming the first electrode 402). A further advantage is that the interleave region 503 can be easily formed, and does not require very special processing or precision in terms of width and thickness. The interleave region 503 may be formed by depositing material between the separated parts of the common electrode layer. Preferably, the dimensions of the interleave region 503 are lower than the acoustic quarter wave-length of the BAW in the resonator region 405, and of laterally propagating acoustic modes.

FIG. 6 shows a dispersion curve characteristic of the devices 400 shown in FIG. 4 and FIG. 5, respectively. In particular, the modes of vibration in the frame region 405 are shown. For the device 400 of FIG. 4, the first mode of vibration (TS-1*) is higher in frequencies in the dispersion curve than the second mode of vibration (TS-2), due to the electrically floating third electrode 404 (similar as already shown in FIG. 2). In particular, the cut-off frequency (k=0) of the first mode of vibration is higher in the frame region 405 than the cut-off frequency of the second mode of vibration (TS-2). For the device 400 shown in FIG. 5, the additional mass loading provided on the third electrode 404 pushes the first mode of vibration even higher (i.e., TS-1**, similar as shown in FIG. 3). In particular, in both devices 400 of FIG. 4 and FIG. 5 the cut-off frequency of the first mode of vibration is lower in the resonator region 405 than it is in the frame region 406.

Another advantage of the devices 400 is that using the piezoelectrically weakly-coupled (or uncoupled) second mode of vibration enables using no (or at least much less) mass loading to get the appropriate frame boundary condition compared to, for instance, when using an isolated border ring in the case where such a mode is nonexistent. Indeed, since in this conventional case all the allowable lateral waves would have a dependence on the electrical boundary condition, much thicker mass loading layers would be required, in order to reach the boundary frame condition for optimal spurious mode reduction.

Notably, the width of the frame region 406 and the thickness of the mass loading layer on top of the third electrode 404 (here the mass loading layer is formed by the interleave layer, which also forms the interleave region 503) may be selected to tune the behavior of the frame, and to optimize the spurious mode response of the BAW device.

FIG. 7 shows another example of a BAW device 400 according to an embodiment of the invention, which builds on the embodiment shown in FIG. 5. Same elements of the BAW devices 400 are labelled with the same reference signs and are implemented likewise.

The BAW device 400 shown in FIG. 7 comprises a mass loading layer 701 provided on the third electrode 404. In this case, the mass loading layer 701 is not formed by the interleave layer. That is, the interleave region 503 and the mass loading layer 701 may be made of different materials in this BAW device 400. FIG. 8 shows another example of a BAW device 400 according to an embodiment of the invention, which builds on the embodiments shown in FIG. 5 and 7. Same elements of the BAW devices 400 are labelled with the same reference signs and are implemented likewise.

The BAW device 400 shown in FIG. 8 also comprises the mass loading layer 701 provided on the third electrode 404. Again, the interleave region 503 and the mass loading layer 701 may be made of different materials. The interleave region 503 is in this case formed by an interleave layer, wherein the interleave layer is arranged to form the top surface of the BAW device 400 and embeds the first electrode 402 (at least partly) and the third electrode 403 (entirely).

FIG. 9 shows another example of a BAW device 400 according to an embodiment of the invention, which builds on the embodiment shown in FIG. 5 and 7. Same elements of the BAW devices 400 are labelled with the same reference signs and are implemented likewise.

The BAW device 400 shown in FIG. 9 comprises again an interleave layer including the interleave region 503, wherein the interleave layer also covers the third electrode 403 to act a mass loading. However, the interleave layer does not form a top surface of the device 400. Instead, the device 400 further comprises also the mass loading layer 701, which is arranged on top of the interleave layer, and which embeds the first electrode 402 (at least partly), the third electrode 404 (entirely), and the interleave region 503. Further, the mass loading layer 701 forms the top surface of the device 400.

FIG. 10 shows another example of a BAW device 400 according to an embodiment of the invention, which builds on the embodiment shown in FIG. 4. Same elements of the BAW devices 400 are labelled with the same reference signs and are implemented likewise. Accordingly, also the BAW device 400 shown in FIG. 10 comprises the piezoelectric layer 401, the three electrodes 402, 403 and 404, the resonator region 405, and the frame region 406.

However, in this example, the third electrode 404 is arranged beneath the piezoelectric layer 401. In particular, the third electrode 404 and the first electrode 402 sandwich the piezoelectric layer 401 to form the frame. The third electrode 404 and the second electrode 403 may be formed by a common electrode layer that is separated into at least two electrically isolated parts (similar as described with respect to FIG. 5 for the third electrode 404 and the first electrode 402). The first electrode 402 may form the top surface of the BAW device 400. FIG. 11 shows another example of a BAW device 400 according to an embodiment of the invention, which builds on the embodiment shown in FIG. 4. Same elements of the BAW devices 400 are labelled with the same reference signs and are implemented likewise. Accordingly, also the BAW device 400 shown in FIG. 11 comprises the piezoelectric layer 401, the three electrodes 402, 403 and 404, the resonator region 405, and the frame region 406.

In this example, the third electrode 404 is again arranged on the piezoelectric layer 401. Further, the BAW device 400 comprises a fourth electrode 1101, which is provided beneath the piezoelectric layer 401. The fourth electrode 1101 is electrically isolated from the first electrode 402 and the second electrode 403. The third electrode 404 and the fourth electrode 1101 sandwich the piezoelectric layer 401 to form the frame. The BAW device 400 may also comprise a further interleave region 1102, which may be arranged between the fourth electrode 1101 and the second electrode 403 to electrically isolate the fourth electrode 1101 from the second electrode 403. The further interleave region 1102 and the interleave region 503 (here exemplarily formed by the interleave layer also formed on top of the third electrode 404) may be made of the same material, but their materials may also be different.

FIG. 12 shows another example of a BAW device 400 according to an embodiment of the invention, which builds on the embodiments shown in FIG. 4 and 5. Same elements of the BAW devices 400 are labelled with the same reference signs and are implemented likewise.

In this example, the BAW device 400 comprises a frame, and accordingly a frame region 406, arranged on either side of the BAW resonator and its resonator region 405. The BAW resonator comprises the first electrode 402 and second electrode 403 sandwiching the piezoelectric layer 401. The frame comprises a third electrode 404 provided on the piezoelectric layer 401 and sandwiching the piezoelectric layer 401 together with the second electrode 403. Notably, two separate (but possibly identical) frames may be arranged on either side of the BAW resonator in a top view of the BAW device 400 (i.e., the BAW device 400 may include two third electrodes 404 as derivable from the cross-sectional view shown in FIG. 12). However, one frame may also be arranged around the BAW resonator in a top view of the BAW device 400 (i.e., the BAW device 400 may include one third electrode 404 arranged around the first electrode 402 and frame region 503, as is also derivable from the cross-sectional view shown FIG. 12). Notably, the BAW device 400 may also comprise more than one BAW resonator, and one or more frames may be arranged next to each BAW resonator or between adjacent BAW resonators (in a top view of the BAW device 400).

FIG. 13 shows another example of a BAW device 400 according to an embodiment of the invention, which builds on the embodiment shown in FIG. 4, 5 and 12. Same elements of the BAW devices 400 are labelled with the same reference signs and are implemented likewise.

In contrast to the BAW device 400 shown in FIG. 12, in the BAW device 400 shown in FIG.

13 the reflective element is not a Bragg structure 502, but an acoustic cavity 1301.

FIG. 14 shows another example of a BAW device 400 according to an embodiment of the invention, which builds on the embodiment shown in FIG. 4, 5, 12 and 13. Same elements of the BAW devices 400 are labelled with the same reference signs and are implemented likewise.

In contrast to the BAW device 400 shown in FIG. 13, in the BAW device 400 shown in FIG.

14 the reflective element is a free-standing cavity 1301, and the substrate 501 has been fully etched away.

In all of the above-shown examples of the BAW device 400 according to various embodiment of the invention, the piezoelectric coupling coefficient of the second mode of vibration may be zero or may be in a range of 0-0.05. Further, a passivation layer may be provided on the mass loading layer 701, and/or on the interleave layer and region 503, respectively.

Further, in each BAW device 400, the substrate 501 may be made of one of silicon, glass, ceramic, and the like. In an example, the substrate 501 may include silicon, a silicon on insulator (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.

In each BAW device 400, the piezoelectric layer 401 may be a thin-film, for instance, may be made of one of lithium niobate, lithium tantalate, aluminum nitride, and the like. Thereby, the piezoelectric layer 401 may have a determined crystal cut orientation, such that the second mode of vibration has the lower piezoelectric coupling coefficient than the first mode of vibration (as described above). Further, in each BAW device 400, the electrodes 402, 403, 404, may respectively be made of a metal and/or metal alloy, such as copper, titanium, and the like, or may be made of a highly doped silicon layer. The electrodes 402, 403, 404 may be implemented by metallization layers, which may include, for example, copper, aluminum, tungsten, titanium, etc. Further, BEOL dielectric layers formed for contacting the electrodes 402, 403, 404, respectively, 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.

Further, the passivation layer and/or the interleave region 503 may comprise 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.

FIG. 15 shows a method 1500 for fabricating a BAW device, for instance, for fabricating the BAW device 400 according to the various embodiments of the invention shown in one of the previous figures. The method comprises a step 1501 of forming a piezoelectric layer 401, which has a first mode of vibration and a second mode of vibration, and which is configured such that the second mode of vibration has a lower piezoelectric coupling coefficient than the first mode of vibration. Further, the method 1500 comprises a step 1502 of forming a first electrode 402 on the piezoelectric layer 401 and forming a second electrode 403 beneath the piezoelectric layer 401. The first electrode 402 and the second electrode 403 may be formed so that they sandwich the piezoelectric layer 401 to form a BAW resonator, the BAW resonator comprising a resonator region 405, which is a region of the piezoelectric layer 401 located between the first electrode 402 and the second electrode 403. Further, the method 1500 comprises a step 1503 of forming an electrically floating third electrode 404 on the piezoelectric layer 401 or beneath the piezoelectric layer 401 , the third electrode 404 being electrically isolated from the first electrode 402 and the second electrode 403. The third electrode 404 and the piezoelectric layer 401 form a frame, the frame comprising a frame region 406, which is a region of the piezoelectric layer 401 located on or beneath the third electrode 404. 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 embodiments of the 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.