POLARIZATION UNIFORMITY

TECHNICAL FIELD

The present disclosure relates to Bulk Acoustic Wave (BAW) devices, in particular to the fabrication of BAW devices. The disclosure provides accordingly a BAW device and a method for fabricating the BAW device, wherein the BAW device has an improved piezoelectric polarization uniformity. The improved piezoelectric polarization uniformity is due to a piezoelectric layer of the BAW device, which has an enhanced crystallographic quality, for example, shows low c-axis disorientation, in particular, in locations between different BAW resonators of the BAW device.

BACKGROUND

Acoustic wave devices are key components for modern electronic circuits. In particular, high- frequency selectivity, while maintaining low electronic insertion loss, requires high quality factor mechanical resonators that are 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 the electrical signal to a BAW. Conventional BAW resonators are produced using a thin piezoelectric layer, which is typically 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 (TE) mode. This mode bases on longitudinal waves that propagate in the thickness of the thin piezoelectric material. In some cases, other classes of piezoelectric material can be used, in which case the fundamental mode of vibration can also be the thickness-shear (TS) mode. That is, the role of the TE and TS may be interchanged depending on the core mode of vibration of the chosen piezoelectric material. In order to achieve a high-quality electric signal, the mechanical resonance generated within a BAW resonator should be as efficient as possible, and should generate the least amount of mechanical loss. Mechanical loss in a BAW resonator arises primarily from acoustic radiation of mechanical energy into the substrate, e.g., through a reflective element like a Bragg reflector. Other routes of mechanical loss are radiation and scattering of the energy at the BAW resonator edges, or at other locations where the Bragg reflectors or similar reflective elements are not present.

Furthermore, poor growth of the piezoelectric material can also enhance the acoustic loss, due to wave scattering caused by multiple domains or grain boundaries in the piezoelectric material itself. Thermal-elastic damping can also be a significant source of mechanical loss in a BAW resonator, which is again promoted by bad piezoelectric crystal growth.

Thus, high-quality piezoelectric growth is generally desired, in order to improve the mechanical quality factor of a BAW resonator, as well as the piezoelectric electro-mechanical coupling strength of two or more BAW resonators (ultimately dictating the achievable filter bandwidth).

As the number of filters and devices increases in front-end modules, more stringent requirements on the overall implementation size of filters are required. Therefore, also a reduced floor-plan area of BAW resonators is required, and a decrease of the inter-resonator distance. However, as the separation between different BAW resonators of a BAW device is reduced, any small variation in the crystallographic and c-axis orientation of the piezoelectric material can become detrimental for the overall BAW device performance. Indeed, the c-axis of the piezoelectric layer may need a finite horizontal distance, in order to re-align itself to the correct vertical axis. This dead zone, in which the c-axis is not orientated correctly, may ultimately degrade the performance of the BAW device.

To date, some of these issues have been addressed in the following approaches:

• In one approach, the bottom electrode of an exemplary BAW resonator, and its edge shape, is chosen to be as smooth as possible, i.e., avoiding sharp edges and creating a slanted profile. However, the disadvantage is that a highly-optimized etching formula for the bottom electrode is required, which increases the overall device cost. Further, there is always a risk of creating cracks and strong grain boundaries at these edges, even with large slanted angles of the electrodes.

• In another approach, a separation between exemplary BAW resonators is kept large enough to avoid any impact of the misaligned piezoelectric on the edges. However, a disadvantage is that this approach is unable to target ultra-compact filter topologies, in order to avoid close proximity effects of adjacent resonators. A further disadvantage is that there is no clear methodology to simulate the impact of various polarization directions in the piezoelectric at the device edge, e.g., the impact on the electrostatics, mechanics, etc.

SUMMARY

In view of the above-mentioned problems and disadvantages, embodiments of the present invention aim to improve conventional BAW devices, and in particular the above-described approaches and exemplary BAW resonators. An objective is to improve the crystallographic quality of the piezoelectric material in a BAW device, in order to improve the polarization quality, i.e., to enhance the piezoelectric polarization uniformity. Specifically, the quality of the piezoelectric material located between different BAW resonators should be improved. To this end, higher quality growth of the piezoelectric material is desired. Thereby, large piezoelectric electro-mechanical coupling figures are targeted.

A particular issue taken into account by the inventors for embodiments of the invention, is that deposition of a piezoelectric layer, and the resulting quality of the piezoelectric material at the edges of a BAW resonator (area), are highly dependent on the edge termination seed layer, or the substrate, on which the piezoelectric material is deposited, i.e., on the type of material beneath the piezoelectric layer in the 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 base on the above-mentioned particular issue of the seed layer beneath the piezoelectric material. A first aspect of the present disclosure provides a BAW device comprising: a substrate; an interleave layer provided on the substrate; a piezoelectric layer provided on the interleave layer and configured to propagate a BAW; a first top electrode and a first bottom electrode sandwiching at least a part of the piezoelectric layer to form a first BAW resonator, wherein the first BAW resonator comprises a first resonator core region, which is a region of the piezoelectric layer located between the first top electrode and the first bottom electrode, and wherein the first bottom electrode is arranged in the interleave layer or in the piezoelectric layer in contact with the interleave layer; and one or more seed elements arranged in the interleave layer and in contact with the piezoelectric layer, wherein the one or more seed element are arranged below an intermediate region, which is a region of the piezoelectric layer located next to the first resonator core region.

The one or more seed elements, which are in contact with the piezoelectric layer (i.e., during the fabrication of the BAW device, the piezoelectric layer may be deposited on the one or more seed elements), help to promote an improved quality of the piezoelectric layer (in particular, they may enhance single crystal growth), and thus help to improve the polarization uniformity of the piezoelectric material, i.e., of the piezoelectric layer. Thereby, an improved BAW device is provided, as the above-described problems are mitigated.

The choice of an appropriate seed layer beneath the piezoelectric layer (in particular, the choice of the one or more seed elements) may depend on the crystallographic structure of the piezoelectric material chosen as the core material of the BAW resonator. There is preferably an appropriate matching of the atom, ions and molecules arrangement between the seed layer and the piezoelectric layer to achieve the best possible crystallographic growth, during the thin-film deposition. In particular, the choice of the seed layer may allow promoting a single-crystal, near perfect c-axis growth of the thin-film piezoelectric layer.

In an implementation form of the first aspect, the BAW device further comprises: a second top electrode and a second bottom electrode sandwiching at least a part of the piezoelectric layer to form a second BAW resonator, wherein the second BAW resonator comprises a second resonator core region, which is a region of the piezoelectric layer located between the second top electrode and the second bottom electrode, wherein the second bottom electrode is arranged in the interleave layer or in the piezoelectric layer in contact with the interleave layer; and wherein at least one of the one or more seed elements is arranged below a part of the intermediate region located between the first resonator core region and the second resonator core region.

Thus, in particular, the quality of the piezoelectric material in between the first and second BAW resonators is improved by using the one or more seed elements. Consequently, large piezoelectric electro-mechanical coupling figures are achievable.

In an implementation form of the first aspect, the first and/or second bottom electrode are arranged in the interleave layer and at the same level as the one or more seed elements.

The first and/or second bottom electrode, and the one or more seed elements, may be formed in the same layer of the BAW device, and/or may be formed in the same process during the fabrication of the BAW device. The bottom electrodes and the seed elements may, respectively, seed/promote the growth of the piezoelectric layer during the fabrication of the BAW device. This leads to an improved piezoelectric layer quality and, in particular, to an improved piezoelectric polarization uniformity.

In an implementation form of the first aspect, the first and/or second bottom electrode and the one or more seed elements have the same layer structure.

This leads to an improved piezoelectric layer quality and, in particular, to an improved piezoelectric polarization uniformity, due to the identical, or at least the very similar, seeding of the piezoelectric layer by the bottom electrodes and seed elements, respectively.

In an implementation form of the first aspect, the first and/or second bottom electrode and the one or more seed elements each comprise one or more metal layers, and a seed layer provided on the one or more metal layers and in contact with the piezoelectric layer.

In an implementation form of the first aspect, the one or more seed elements comprise two or more seed elements placed one after the other in a direction away from the first bottom electrode and/or the second bottom electrode.

Thus, a larger area of the piezoelectric layer can be qualitatively improved, leading to a home homogeneous piezoelectric polarization uniformity across the BAW device. In an implementation form of the first aspect, the one or more seed elements comprise two or more seed elements placed one after the other along one or more side edges of the first BAW resonator and/or the second BAW resonator.

Thus, the piezoelectric layer along and/or around one or more or each of BAW resonator of the BAW device can be improved.

In an implementation form of the first aspect, the one or more seed elements comprise a first periodic arrangement of two or more seed elements.

In an implementation form of the first aspect, the one or more seed elements comprise a second periodic arrangement of two or more seed elements, and the first periodic arrangement and the second periodic arrangement differ from each other in or more of the following characteristics: shape of the seed elements, size of the seed elements, and spacing between neighboring seed elements, layer stack of the seed elements, material properties of the seed elements.

In an implementation form of the first aspect, the one or more seed elements comprise at least two seed elements that are identical in their shape and/or size; and/or the one or more seed elements comprise at least two seed elements that differ from each other in their shape and/or size.

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

In an implementation form of the first aspect, each of the one or more seed elements is distanced laterally from the first bottom electrode and/or from the second bottom electrode by at least a minimum distance.

Thus, the one or more seed elements do not disturb the bottom electrode, and thus the performance of the corresponding BAW resonator(s), while the one or more seed elements promote the piezoelectric layer quality next to the BAW resonator(s) and in between different BAW resonators. In an implementation form of the first aspect, the piezoelectric layer has a free upper surface in the intermediate region.

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

This minimizes acoustic loss, particularly, to the substrate.

In an implementation form of the first aspect, the acoustic reflective element comprises a plurality of higher and lower acoustic impedance layers, which form a Bragg mirror structure; and/or the acoustic reflective element comprises a plurality higher and lower dielectric constant layers, which form a Bragg mirror structure.

For instance, in the case of the BAW device being a SMR device (i.e., including one or more SMRs), the interleave layer may be a region, in which the lower and higher index materials are deposited, e.g., to form a reflective element beneath the BAW resonator(s). In that case, the interleave layer may form the lower index material.

In an implementation form of the first aspect, the one or more seed elements are provided on the Bragg mirror structure.

In an implementation form of the first aspect, the Bragg mirror structure is formed by the interleave layer and a plurality of Bragg layers, wherein the Bragg layers are arranged in the interleave layer one above the other.

In an implementation form of the first aspect, the interleave layer comprises a silicon oxide (SiC ) and the Bragg layers comprise a material having a higher acoustic impedance than the silicon oxide (SiC ).

In an implementation form of the first aspect, the piezoelectric layer comprises AIN or comprises AIN with additional doping of rare earth elements, in particular scandium (Sc). In an implementation form of the first aspect, wherein at least one of the one or more seed elements comprises platinum (Pt).

In an implementation form of the first aspect, the first and/or second bottom electrode comprises at least one of tungsten (W), molybdenum (Mo), and aluminum (Al).

A second aspect of the present disclosure provides a method for fabricating a BAW device, the method comprising: forming a substrate; forming an interleave layer on the substrate; forming one or more seed elements arranged in the interleave layer; forming a piezoelectric layer on the interleave layer and on the one or more seed elements, respectively; forming a first bottom electrode in the interleave layer or in the piezoelectric layer in contact with the interleave layer, forming a first top electrode on the piezoelectric layer, wherein the first top electrode and the first bottom electrode sandwich at least a part of the piezoelectric layer to form a first BAW resonator, the first BAW resonator comprising a first resonator core region, which is a region of the piezoelectric layer located between the first top electrode and the first bottom electrode; wherein the one or more seed elements are in contact with the piezoelectric layer, and wherein the one or more seed elements are arranged below an intermediate region, which is a region of the piezoelectric layer located next to the first resonator core region.

In an implementation form of the second aspect, the piezoelectric layer is grown simultaneously on the interleave layer and on the one or more seed elements, for example, by sputtering, epitaxy or vapor deposition.

In an implementation form of the second aspect, the first bottom electrode and the one or more seed elements are formed in the same one or more process steps of the method.

In an implementation form of the second aspect, the first bottom electrode and the one or more seed elements comprise the same seed layer that seeds the growth of the piezoelectric layer.

The method of the second aspect may further have other implementation forms, which produce the respective implementation forms of the BAW device of the first aspect. The method of the second aspect accordingly provides at least the same advantages as the BAW device of the first aspect. The above-described aspects and implementation forms (embodiments of the invention) allow the creation of a smooth(er) piezoelectric c-axis growth, in particular at the BAW resonator edge(s), through the use of the one or more seed elements, which may be embedded at the same layer as the bottom electrode(s) of one or more BAW resonators of the BAW device. Thus, the same high-quality seed layer (provided by the one or more seed elements) may be used next to and/or between BAW resonator(s), as for the piezoelectric material that is deposited onto the bottom electrode(s) of these BAW resonator(s) of the BAW device. Some beneficial features may include:

• The one of more seed elements may be deposited at a given distance from the BAW resonator edge(s).

• The same material(s) may be used for the one or more seed elements seeding the piezoelectric layer in the intermediate region, as is used for the bottom electrode(s) seeding the piezoelectric layer in the resonator core region.

• The distance(s) between the one or more seed elements and the bottom electrode(s) may be optimized, to promote the best quality piezoelectric layer polarization (uniformity).

• The same (patterned) one or more seed elements may be used between two or more adjacent BAW resonators in a filter configuration.

• Periodically replicated structures of multiple seed elements may be used, for instance, with different dimensions and replications pitches, to promote further the polarization uniformity in the piezoelectric layer.

• The effect of band-gap creation from the replicated seed elements may be used, in order to further enhance the mechanics at the BAW resonator edge(s).

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 will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which

FIG. 1 shows a piezoelectric c-axis of an exemplary BAW resonator, and shows possible polarization disorientation in the exemplary BAW resonator.

FIG. 2 shows an exemplary BAW device.

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

FIG. 4 shows an example of a seed element for a BAW device according to an embodiment of the invention.

FIG. 5 shows an example of a seed element for a BAW device according to an embodiment of the invention.

FIG. 6 shows an example of a seed element for 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.

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

FIG. 12 shows a BAW device according to an embodiment of the invention. FIG. 13 shows a comparison of a shows a BAW device according to an embodiment of the invention with an exemplary BAW device.

FIG. 14 shows BAW devices according to embodiments of the invention.

FIG. 15 shows BAW devices according to embodiments of the invention.

FIG. 16 shows a BAW device according to an embodiment of the invention.

FIG. 17 shows a method according to an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a piezoelectric c-axis of an exemplary BAW resonator, and shows polarization disorientation in the example of the BAW resonator. In particular, FIG. 1(a) shows how the polarization vector direction (indicated by the arrows) would ideally align with an external electric field, e.g., as established between the top electrode and the bottom electrode of the exemplary BAW resonator. FIG. 1(b) shows, however, c-axis disorientation that often occurs in the exemplary BAW resonator, i.e., here the polarization vector direction does not align well with the external electric field. This typically occurs next to a BAW resonator or between two BAW resonators of a conventional BAW device, in particular when there are no bottom electrodes. This is due to the fact that the bottom electrode typically include a seed layer, which promotes a high-quality piezoelectric layer deposited onto it.

The electro-mechanical efficiency of any BAW device is linked to the co-linearity of the piezo polarization with the electric field. Variations in the polarization vector direction (i.e., the polarization disorientation) may be induced by multi-domain or grain boundaries, and reduces the achievable electro-mechanical coupling (kt2).

FIG. 2 shows a cross sectional diagram of an example of a BAW device including a BAW resonator, which is formed by a top electrode and a bottom electrode sandwiching at least a part of the piezoelectric layer. The BAW resonator comprises a resonator core region, which is a region of the piezoelectric layer located between the top electrode and the bottom electrode of said BAW resonator. FIG. 2 shows further that the BAW device includes a Bragg-reflector, which is formed by Bragg layers 1 and 2 embedded an interleave layer, as a reflective element arranged between the resonator core region and the substrate (bulk).

FGI. 2 shows also that the bottom electrode can comprise a seed layer, on which the piezoelectric layer is formed. In this example, the bottom electrode comprises two metal layers 1 and 2, and the seed layer provided on the metal layers. The seed layer typically leads to deposition of the piezoelectric layer with an acceptable or good quality, and thus an acceptable or good alignment of the polarization vector direction to the external electric field, as established between the top and bottom electrode.

However, FIG. 2 also shows a polarization disorientation region (dashed box), wherein the piezoelectric layer is deposited directly on the interleave layer, and wherein the piezoelectric layer has a free upper surface. In this disorientation region, the deposition of the piezoelectric layer typically results in a poorer piezoelectric material quality, resulting in the disorientation of the polarization vector direction. In particular, the piezoelectric layer, which may be grown over the interleave layer, may have a non-homogenous polarization vector distribution.

FIG. 3 shows a BAW device 100 according to an embodiment of the invention, which addresses the drawback described with respect to the example BAW device of FIG. 2.

The BAW device 100 comprises a bulk or substrate 101 (e.g., comprising silicon or silicon based), and an interleave layer 102 (e.g., comprising a silicon oxide layer) provided on the substrate 101. Further, a piezoelectric layer 103 is provided on the interleave layer 102. The piezoelectric layer 103 is configured to propagate a BAW. A first top electrode 106 and a first bottom electrode 104 are provided, and sandwich at least a part of the piezoelectric layer 103, i.e., they sandwich the piezoelectric layer 103 in a certain area of the BAW device 100, to form a first BAW resonator. The first BAW resonator comprises a first resonator core region, which is a region of the piezoelectric layer 103, which is located between the first top electrode 106 and the first bottom electrode 104.

In the BAW device 100 the first bottom electrode 104 may be arranged in the interleave layer 102 (as shown in FIG. 1), or may be arranged in the piezoelectric layer 103 in contact with the interleave layer 102 (alternative, not shown in FIG. 1). Further, the BAW device 200 comprises one or more seed elements 105 (one is shown in FIG. 1), which is/are arranged in the interleave layer 102 and are in contact with the piezoelectric layer 103. This means, the piezoelectric layer 103 is provided on the one or more seed elements 105. For instance, during fabrication of the BAW device 100, the piezoelectric layer 102 may be deposited onto/over the one or more seed elements 105. Notably, if the first bottom electrode 104 is in the interleave layer 102 (as shown in FIG. 1), the piezoelectric layer 103 may also be deposited - for instance, at the same time - on or over the first bottom electrode 104. The one or more seed elements 105 are arranged below an intermediate region, which is a region of the piezoelectric layer 103 located next to the first resonator core region. The one or more seed elements 105 result in a better quality of the piezoelectric layer 103 in the intermediate region, and thus lead to an improved polarization uniformity. That is, the piezoelectric layer 103 deposited (e.g., grown) over the one or more seed elements 105 may have more homogenous polarization vector distribution than the piezoelectric layer grown over the interleave layer of the exemplary BAW device shown in FIG. 2.

The BAW device 100 may further comprise an acoustic reflective element located below the first top electrode 106, the piezoelectric layer 103, and the first bottom electrode 104.

FIG. 4 shows a cross-sectional view of an example seed element 105, which may be used for the BAW device 100 according to an embodiment of the invention. This example shows a simple seed element 105, which may be formed by a material element (e.g., comprising a single material), wherein the seed element 105 is embedded into the interleave layer 102. The material of the seed element 105 may be specifically selected so as to promote single crystal growth of the piezoelectric material deposited over it, during the fabrication of the BAW device 100. The part of the BAW device 100 shown in FIG. 4 may be regarded as a “unit cell”, which may be repeated one or more times and along one or more directions of the BAW device 100, to form, for instance, one or more arrangements, e.g., periodic arrangements, of multiple seed elements 105. Thereby, each seed element 105 can be the same, but the seed elements 105 may also differ from another in one or more of: a size, a shape, a material composition, a number of material layers, a width, a thickness (i.e., a height), or generally by one or more structural parameters.

FIG. 5 shows a cross-sectional view of another example seed element 105, in a similar manner than shown in FIG. 4, which may be used for the BAW device 100 according to an embodiment of the invention. Again, the seed element 105 may be considered a “unit cell”, and may be replicated as described above one or more times, to form an arrangement or structure comprising multiple seed elements 105. In FIG. 5, the seed element 105 comprises two metal layers 105b and 105c, and comprises a seed layer 105a as the top layer, i.e., the seed layer 105a is the layer, on which the piezoelectric layer 103 is provided. Generally, the seed elements 105 may comprise one or more metal layers 105b, 105c, and the seed layer 105a may be provided on the one or more metal layers 105b, 105c, wherein the seed layer 105 is in contact with the piezoelectric layer 103.

FIG. 6 shows a cross-sectional view of another example seed element 105 forming a “unit cell”, which example builds on the example shown in FIG. 5, and may be used for the BAW device 100 according to an embodiment of the invention. In this example, the “unit cell” also includes Bragg layers 600, which are embedded in the interleave layer 102. Whether the “unit cell” for the BAW device 100 is as shown in FIG. 5 or as shown in FIG. 6 may depend on which position of the BAW device 100 the one or more seed elements 105 are arranged at. Generally, a Bragg mirror structure (as an acoustic reflective element) may be formed by the interleave layer 102 and a plurality of Bragg layers 600 (e.g., two Bragg layers 6000), wherein the Bragg layers 600 may be arranged in the interleave layer 102, one above the other.

FIG. 7 shows a cross-sectional diagram of a BAW device 100 according to an embodiment of the invention, which builds on the embodiment shown in FIG. 3. In particular, the BAW device 100 shown in FIG. 7 includes one or more seed elements 105 according to the “unit cell” shown in FIG. 5. One of the one or more seed elements 105 is illustrated next to the first bottom electrode 104, i.e., below the intermediated region of the piezoelectric layer 103 (which is next to the resonator core region of the illustrated BAW resonator).

In the BAW device 100 shown in FIG. 7, the first bottom electrode 104 and at least one of the one or more seed elements 105 each comprise one or more metal layers 104b, 104c, 105b, 105c, and each comprise a seed layer 104a, 105a provided on the one or more metal layers 104b, 104c, 105b, 105c, respectively, and in contact with the piezoelectric layer 103. The first bottom electrode 104 and the at least one seed element 105 have the same layer structure 104a, 104b, 104c, 105a, 105b, 105c. A same layer structure is likewise possible with other structures, e.g., both the at least one seed element 105 and the bottom electrode 104 may comprise the same single material, as e.g. shown in FIG. 3 or 4. The bottom electrode 104 and the one or more seed elements 105 may be formed in the same one or more process steps during the fabrication of the BAW device 100. FIG. 7 shows further that the BAW device 100 may comprise a Bragg mirror structure formed by the interleave layer 102 and a plurality of Bragg layers 600, wherein the Bragg layers 600 are arranged in the interleave layer 102, one above the other. Further, FIG. 7 shows that the BAW device 100 may include a via 601 extending from the first bottom electrode 104 to the top surface of the BAW device 100, in particular to the top surface next to the first top electrode 106.

FIG. 8 shows a BAW device 100 according to an embodiment of the invention, which builds on the embodiment shown in FIG. 3, and is similar to the embodiment shown in FIG. 7. Compared to the embodiment of FIG. 7, the one or more seed elements 105 comprise at least one seed element 105 (as illustrated), which has a different structure than the bottom electrode 104. Here, as an example, the one or more seed elements 105 are designed as shown in FIG. 4, and the bottom electrode 104 is designed as shown in FIG. 7.

FIG. 9 shows a BAW device 100 according to an embodiment of the invention, which builds on the embodiment shown in FIG. 3. In particular, the BAW device 100 of FIG. 8 comprises two adjacent BAW resonators. The first BAW resonator (BAW 1) is as described with respect to FIG. 3, in particular, like in FIG. 7. Further, the BAW device 100 comprises a second top electrode 906 and a second bottom electrode 904 sandwiching at least a part of the piezoelectric layer 103 to form a second BAW resonator (BAW 2). The second BAW resonator comprises a second resonator core region, which is a region of the piezoelectric layer 103 located between the second top electrode 906 and the second bottom electrode 904. The second bottom electrode 904 is arranged in the interleave layer 102 (as illustrated), or in the piezoelectric layer 103 in contact with the interleave layer 102 (alternative, not illustrated).

In the case of two adjacent BAW resonators, at least one of the one or more seed elements 105 (here, as an example, multiple seed elements 105) are arranged below a part of the intermediate region located between the first resonator core region and the second resonator core region. Thus, the piezoelectric layer 103 between the two BAW resonators has an improved quality, with a more homogeneous polarization vector distribution than achievable without the one or more seed elements 105. In particular, a homogenous polarization is expected in this area, compared to the case without seed elements 105, which leads to an improved coupling figure between the BAW resonators. Notably, in the example of FIG. 9, the first and the second BAW resonator both have multiple disconnected Bragg layers 600.

Fig. 10 show a cross-sectional diagram of a BAW device 100 according to an embodiment of the invention, which builds on the embodiment of FIG. 3, and is similar to the embodiment shown in FIG. 9. Two adjacent BAW resonators (BAW 1 and BAW 2) are formed and one or more seed elements 105 are included in an intermediated region between the resonator core regions of these adjacent BAW resonators. The one or more seed elements 105 may comprise two or more seed elements 105 placed one after the other in a direction away from the first bottom electrode 104 and the second bottom electrode 904, respectively. The two or more seed elements 105 may thereby be arranged regularly, in particular with a constant pitch 1000. The pitch 1000 may, however, also may be non-constant.

Compared to the embodiment of FIG. 9, the BAW device 100 shown in FIG. 10 comprises Bragg layers 600, which are shared by both adjacent BAW resonators. Thus, they may be referred to as Bragg lines.

Furthermore, the BAW device 100 may comprise one or more top materials provided on the first top electrode 106 and/or on the second top electrode 906, e.g., including a passivation layer and/or a protective layer and/or a loading layer. In FIG. 10, a first top material 1002 is provided on both the first and the second top electrode 106, 906, and a second top material 1001 is provided on the first top material 1002.

FIG. 11 show a cross-sectional diagram of a BAW device 100 according to an embodiment of the invention, which builds on the embodiment of FIG. 3, and is similar to the embodiment shown in FIG. 10. The BAW device 100 of FIG. 11 comprises one or more cavities 1100 (two cavities 1100 are shown as example), as one or more acoustic reflective elements, and provided instead of one or more Bragg structures formed by the Bragg layers 600. In particular, a cavity 1100 may be provided below the resonator core region of each BAW resonator (BAW 1 and BAW 2), i.e. below the respective top electrodes 106, 906, the piezoelectric layer 103, and the respective bottom electrodes 104, 904. The BAW resonators of the BAW device 100 are now FBARs. That is, the BAW device 100 is a FBAR device. The one or more seed elements 105 are now included into the central support region 1101 arranged between the adjacent BAW resonators, in particular arranged between the two cavities 1100

FIG. 12 show a cross-sectional diagram of a BAW device 100 according to an embodiment of the invention, which builds on the embodiment of FIG. 3, and is similar to the embodiment shown in FIG. 11. Compared to the embodiment of FIG. 11, the BAW device 100 comprises only one cavity 1100, which are applied for both the adjacent BAW resonators. The BAW device 100 of FIG. 12 may comprise fully suspended membrane resonators. Accordingly, there is no space for the central support region 1101 shown in FIG. 11. Accordingly, the one or more seed elements 105 may be embedded into the bottom metal layer, which forms the first bottom electrode 104 and the second bottom electrode 904, respectively.

FIG. 13 shows a comparison of a BAW device 100 according to an embodiment of the invention in FIG. 13(b) and FGI. 13(d), and an exemplary BAW device in FIG. 13(a) and FIG. 13(c). FIG. 13 shows in particular a top view (FIG. 13(a) and FIG. 13(b)) of these BAW devices, and a corresponding cross-sectional view (FIG. 13(c) and FIG. 13(d)) of these BAW devices 100. The exemplary BAW device does not comprise any seed elements 105 (see FIG. 13(a)), and has thus c-axis polarity disorientation (see FIG. 13(c)). The BAW device 100 comprises the one or more seed elements 105 (see FIG. 13(b)), and shows thus an improved piezoelectric polarization uniformity outside the resonator core region (see FIG. 13(d)).

In particular, the BAW device 100 comprises two or more seed elements 105 placed one after the other along one or more side edges of the BAW resonator, in particular along each side edge of the BAW resonator (i.e., in the top view, the seed elements 105 are placed adjacent to the first top electrode 106 and/or the first bottom electrode 104). That is, the intermediate region may be arranged adj acent to the first top electrode 106 and/or the first bottom top electrode 104; or the intermediate region may surround the first top electrode 106 and/or the first bottom electrode 104.

The one or more seed elements 105 may thereby comprise one or more periodic arrangements of two or more seed elements 105. For example, one periodic arrangement may be placed along each of the edges of the BAW resonator. In case of more than one periodic arrangement (as shown), a first periodic arrangement and a second periodic arrangement may differ from each other in or more of the following characteristics: a shape of the seed elements 105, a size of the seed elements 105, a spacing between neighboring seed elements 105, a layer stack of the seed elements 105, material properties of the seed elements 105.

FIG. 14 shows BAW devices 100 according to embodiments of the invention, in a top view and a cross-sectional view, respectively, which build on the embodiment shown in FIG. 3.

FIG. 14(a) and FIG. 14(c) show a BAW device 100 according to an embodiment, where different seed elements 105 are provided. In particular, an elongated seed element 105 is placed along one edge of a BAW resonator of the BAW device 100. Further, a periodic arrangement of differently-shaped seed elements 105 (particularly, not as elongated) is placed along another edge of the BAW resonator of the BAW device 100.

FIG. 14(b) and FIG. 14(d) show a BAW device 100 according to an embodiment, with two BAW resonators, which share a bottom electrode layer forming the first bottom electrode and second bottom electrode. Different seed elements 105 are arranged along the edges of the two BAW resonators, in particular along a top edge and a bottom edge of the common bottom electrode layer, as shown in the top view (FIG. 14(b)). The multiple seed elements 105 include an elongated seed element 105 (in Region C), a first periodic arrangement of identical seed elements 105 (in Region A), a second periodic arrangement of identical seed elements 105 (in Region B), and a third periodic arrangement of non-identical seed elements 105 (in Region D).

FIG. 15 shows BAW devices 100 according to embodiments of the invention FIG. 15(a) and FIG. 15(c) show a BAW device 100, which is similar to the BAW device 100 shown in FIG. 14(b) and FIG. 14(d). However, the BAW device 100 comprises another periodic arrangement of seed elements 105, which is located between the two adjacent BAW resonators, which in this case do not have a common bottom electrode layer, but which comprise, respectively, a first bottom electrode and a second bottom electrode. The another periodic arrangement comprises seed elements 105 arranged one after the other in a direction perpendicular to the direction from the first to the second BAW resonator.

FIG. 15(b) and FIG. 15(d) show a BAW device 100 with a more complex shape of a common top electrode 106 of two BAW resonators. Seed elements 105 of different shapes and/or sizes are, in the top view, arranged along the edges of the more complex shape of the top electrode 106.

FIG. 16 shows a BAW device 100 according to embodiments of the invention. In particular, FIG. 16(b), FIG. 16(c) and FIG. 16(d) show BAW resonators with different top electrode shapes, and different arrangements of seed elements 105 along the edges of the BAW resonators. FIG. 16 demonstrates, in particular, that the present disclosure and the embodiments of the invention should not be limited to any specific type, shape, size or arrangement of the one or more seed elements 105, and also not to any type or shape of BAW resonator(s).

In the above-described embodiments of the invention, i.e. for the BAW device 100 described with respect to the various figures, the following selection of materials is possible.

The substrate 101 may comprise silicon, glass, ceramic, and the like. The (e.g., thin film) piezoelectric layer 103 may comprise lithium niobate, lithium tantalate, aluminum nitride, and the like. The top electrode(s) 106, 906 and/or the bottom electrode(s) 104, 904 may comprise 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, an acoustic resonator frame provided around one or more BAW resonators (passivation layer) 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.

Metallization layers, e.g. forming the bottom electrode(s) 104, 904 may include, for example, copper, aluminum, tungsten, titanium, etc. Back end of line (BEOL) dielectric layers, e.g. forming the interleave layer 102, 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. Materials available in the CMOS BEOL layers, e.g., forming the top electrode(s) 106, 906, 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 101 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, Alin As, GalnAs, GalnP, or GalnAsP or combinations thereof.

In a particular embodiment, the BAW device 100 may comprise an interleave layer 102 comprising silicon dioxide, and may comprise Bragg layers 600 comprising a material having a higher acoustic impedance than the silicon dioxide. In another particular embodiment the BAW device 100 may comprise a piezoelectric layer 103 comprising AIN or comprises AIN with additional doping of rare earth elements, in particular Sc. In another particular embodiment the BAW device 100 may comprise at least one of the one or more seed elements 105 comprising Pt. In another particular embodiment the BAW device 100 may comprise a first and/or second bottom electrode 104, 904 comprising at least one of W, Mo, and Al. Two or more or all of these particular embodiments can be combined.

FIG. 17 shows a method 1700 according to an embodiment of the invention. The method 1700 is for fabricating a BAW device 100, according to the above-described embodiments. The method 1700 may comprise steps of: forming 1701 a substrate 101; forming 1702 an interleave layer 102 on the substrate 101; forming 1703 one or more seed elements 105 arranged in the interleave layer 102; forming 1705) a piezoelectric layer 103 on the interleave layer 102 and on the one or more seed elements 105, respectively; forming a first bottom electrode 104 in the interleave layer 102 or in the piezoelectric layer 103 in contact with the interleave layer 102; and forming 1706 a first top electrode 106 on the piezoelectric layer 103. The first top electrode 106 and the first bottom electrode 104 sandwich at least a part of the piezoelectric layer 103 to form a first BAW resonator, the first BAW resonator comprising a first resonator core region, which is a region of the piezoelectric layer 103 located between the first top electrode 106 and the first bottom electrode 104. Further, the one or more seed elements 105 are in contact with the piezoelectric layer 103. Further, the one or more seed elements 105 are arranged below an intermediate region, which is a region of the piezoelectric layer 103 located next to the first resonator core region. The embodiments of the invention use the one or more seed elements 105, in order to maintain the correct piezoelectric polarization growth of the piezoelectric layer 103 at BAW resonator edges. Furthermore, to aid on the processing uniformity, these seed elements 105 may use the same fabrication deposition mask as the bottom electrodes 104, 904 of the BAW resonator(s).

Some of the benefits of the various embodiments of the invention are:

• C-axis disorientation can be avoided, even when the piezoelectric layer 103 is grown away from the bottom electrode(s) 104, 904.

• There is no need to pattern large bottom electrodes 104, 904, due to the seed elements 105 provided.

• Parasitic capacitances can be avoided.

• The same mask as for the bottom electrode(s) 104, 904 patterning step can be used for forming the one or more seed elements 105.

• Stress-distribution at the BAW resonator edges can be avoided, by minimizing the creation of grain boundaries in the crystal of the piezoelectric layer 103.

• Better polarization control of, e.g., a doped AIN piezoelectric layer 103 is enabled, which is highly dependent on seed layer optimization.

• A closer interaction between adjacent BAW resonators is possible, due to a better control of the piezoelectric effects in close proximity regions.

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.