WO2020200478A1

WO2020200478A1 - Saw device and method for its fabrication

Info

Publication number: WO2020200478A1
Application number: PCT/EP2019/058674
Authority: WO
Inventors: Andreja ERBES; Peng Liu
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2019-04-05
Filing date: 2019-04-05
Publication date: 2020-10-08
Also published as: CN113615082A

Abstract

The present invention relates to the field of Surface Acoustic Wave (SAW) devices. In particular, a SAW device of the invention has suppressed spurious modes, in particular transversal modes. The SAW device of the invention comprises a piezoelectric layer (101) configured to propagate a SAW, and at least one electrode (102) provided on the piezoelectric layer and configured to couple an electrical signal to a SAW propagating in the piezoelectric layer (101). The SAW device further includes at least one bus bar (103) configured to route the electrical signal to the electrode (102). The SAW device further includes a decoupling layer (104) separating the piezoelectric layer (101) and the electrode (102) from the bus bar (103). The decoupling layer (104) has an acoustic impedance different than the electrode (102) and the bus bar (103), respectively.

Description

SAW DEVICE AND METHOD FOR ITS FABRICATION

TECHNICAL FIELD

The present invention relates to the technical field of Surface Acoustic Wave (SAW) devices. The invention presents in particular a SAW device with a new design that suppresses spurious transverse modes. Further, the invention relates to a method for fabricating the SAW device.

BACKGROUND

Acoustic wave devices are key components used in modem electronic circuits. Their desired high frequency selectivity, while maintaining a low electronic insertion loss, requires high quality factor mechanical resonators coupled in a filter topology.

SAW devices couple an electrical, time varying signal, to a mechanical wave that travels on the surface of a piezoelectric material (layer). Conventional SAW resonators are produced on a piezoelectric layer, with alternating metal interdigital transducer (IDT) electrodes on the surface of the piezoelectric layer.

While the excited waves trigger the required modes of vibration, SAW resonators suffer from the generation of in-band spurious vibration modes. These are caused by the arrangement of the IDT electrodes over the piezoelectric layer, the geometry of metallic bus bars routing the electrical signal and confining the resonators, as well as by bilateral acoustic confinement with Bragg mirrors. All these causes enhance the growth of spurious modes within the frequency band of interest. The finite dimensions in the transverse direction of wave propagation also contribute to the development of spurious electro-acoustic modes within the resonator.

Having such spurious modes close to the desired operating frequency can impact the achievable mechanical quality factor Q, due to modal coupling. When the SAW resonators are arranged in a filter configuration (e.g. ladder structures, lattice, dual-mode resonators, etc.), the existence of in-band spurious modes also affect the in-band ripple metric of these electrical filter circuits. To date, the unwanted spurious modes have been addressed in the following ways: • Apodization of IDT electrodes: This may result in a reduction of the excitation efficiency of higher modes. Further, lower mode shapes may be matched by the excitation. However, the loss is significantly increased by scattering at discontinuities.

• Dummy electrode weighting: Using dummy electrodes, spurious modes penetrate and leak in bus bar regions. However, reflection at gaps creates a strong wave confinement in the gap region, which guides the vibrations and again allows for spurious modes to appear.

• Piston mode design: For instance, devices on quartz (or a high coupling substrate), combined with apodization of dummy electrodes, in order to maintain a mode shape that is flat. The shape of the excitation stays constant along the aperture, and the mode shape may be matched to it. The electro-mechanical coupling to these spurious modes is then greatly reduced. However, this approach leads to more complex resonator designs.

SUMMARY

In view of the above-mentioned disadvantages, embodiments of the present invention aim to improve conventional SAW devices and the approaches addressing spurious modes.

An objective is to provide a SAW device, in which spurious transverse modes are suppressed. Thus, a goal of the invention is to reduce passband ripples, and to increase the achievable off hand rejection. The spurious modes should be suppressed without requiring a more complex SAW resonator/device design. Furthermore, the SAW device should have low loss.

A first aspect of the invention provides a SAW device, comprising: a piezoelectric layer configured to propagate a SAW, at least one electrode provided on the piezoelectric layer and configured to couple an electrical signal to a SAW propagating in the piezoelectric layer, at least one bus bar configured to route the electrical signal to the electrode, and a decoupling layer separating the piezoelectric layer and the electrode from the bus bar, wherein the decoupling layer has an acoustic impedance different than the electrode and the bus bar, respectively.

In particular, the decoupling layer can comprise piezoelectric material, dielectric material, or any semiconductor material. The decoupling layer decouples the bus bar and the SAW resonator (piezoelectric layer and electrode(s)). Thus, spurious modes can be suppressed in the SAW device of the first aspect, and an improved, low-loss SAW device is provided.

In an implementation form of the first aspect, the at least one bus bar is arranged at least partly above the piezoelectric layer and electrode, and the decoupling layer vertically separates the piezoelectric layer and electrode from the bus bar.

The vertical decoupling suppresses the spurious transverse modes effectively.

In an implementation form of the first aspect, the SAW device further comprises: a vertical interconnect access, VIA, formed through the decoupling layer for electrically connecting the bus bar to the electrode.

The VIA this allows controlling the SAW resonator(s). Multiple VIAs may be provided. Multiple VIAs may also be used to form a periodic arrangement in the decoupling layer, e.g. to form a phononic crystal.

In an implementation form of the first aspect, the decoupling layer is a patterned layer and/or a passivation layer.

In particular, if the SAW resonator comprises the optional passivation layer (e.g. over the IDT arrangement), it is not used for spurious mode reduction in this case.

In an implementation form of the first aspect, one or more material elements are embedded in the decoupling layer, the embedded elements having an acoustic impedance and/or having different electrical and/or optical properties than the surrounding material of the decoupling layer.

In an implementation form of the first aspect, a plurality of the material elements is embedded in a periodic arrangement in the decoupling layer. In an implementation form of the first aspect, a spatial periodicity of the periodic arrangement of the embedded elements is in the order of a wavelength of the SAW propagating in the decoupling layer at the working frequency of the SAW device.

The embedded elements could be the same, or may include, the above-described VIA(s) contacting the bus bar to the electrode(s).

In an implementation form of the first aspect, a plurality of embedded elements forms a phononic crystal in the decoupling layer.

In particular, the dimension(s) of the members of this plurality of embedded elements is chosen base on Bloch’s Theorem, the operating frequency of the resonator and operating bandwidth of the filter.

Thus, an acoustic bandgap may be formed, supporting the decoupling and the suppression of the spurious modes. Moreover, the phononic crystal can be sized in such a way to allow for partial penetration of acoustic energy within the frame structure to produce a“piston- like” mode of vibration in the core resonator. This greatly reduces the triggering of spurious modes.

In an implementation form of the first aspect, the phononic crystal in the decoupling layer has a bandgap centered around an operating frequency of the SAW device.

Thus, spurious modes are particularly suppressed close to the desired operating frequency, and therefore have no impact on the achievable mechanical quality factor Q, due to modal coupling.

In an implementation form of the first aspect, the phononic crystal (which may also be referred to as a phononic crystal frame structure) has a length in the range of a quarter- wavelength at the operating frequency of acoustic waves it is able to support.

In an implementation form of the first aspect, the decoupling layer has a temperature coefficient having an opposite sign than the temperature coefficient of the piezoelectric layer. The precise geometry and implementation of the phononic crystal (as e.g. described below with reference to the figures) is merely illustrative and non-symmetric periodic arrangements can be implemented in different sections of the SAW device.

Ideally the temperature coefficients of the decoupling layer and the piezoelectric layer have the same absolute value and opposite sign. Thus, a temperature compensation is achieved.

In an implementation form of the first aspect, the decoupling layer includes a dielectric material.

In an implementation form of the first aspect, the SAW device further comprises: a plurality of SAW resonators, each SAW resonator being formed by at least one IDT electrode and at least part of a piezoelectric layer.

In an implementation form of the first aspect, the SAW device further comprises: one or more Bragg reflectors arranged adjacent to a SAW resonator, wherein a plurality of embedded elements in the decoupling layer is configured to guide acoustic energy from the SAW resonator to the one or more Bragg reflectors.

This implementation form prevents diffractive scattering along the transverse direction.

In an implementation form of the first aspect, the decoupling layer is configured to acoustically couple at least two SAW resonators, wherein the coupling direction is perpendicular to the SAW propagating direction.

The coupling strength between SAW resonators may be tuned by varying geometry and structure of the decoupling layer. SAW propagating direction (in the implementation a longitudinal direction) means the main direction of propagation.

In an implementation form of the first aspect, the distance between adjacent SAW resonators is chosen to be between a quarter- wavelength and half- wavelength of the induced acoustic waves at the operating frequency of the resonators. In an implementation form of the first aspect, the piezoelectric layer (which may also be called substrate) comprises at least one of the piezoelectric materials lithium-Tantalate oxide (LiTa03) or lithium-niobate (LiNb03).

In an implementation form of the first aspect, the at least one IDT electrode comprises metal material (e.g. at least one of copper (Cu), tungsten (W), argentum (Ag), platinum (Pt), titanium (Ti) or aluminium (Al)).

In an implementation form of the first aspect, the decoupling layer can comprise dielectric material (e.g. at least one of Si02, SiCOH).

In an implementation form of the first aspect, the at least one electrode is an interdigital transducer (IDT) electrode.

A second aspect of the invention provides a method for fabricating a SAW device, the method comprising: providing a piezoelectric layer configured to propagate a SAW, forming at least one electrode on the piezoelectric layer, wherein the electrode is configured to couple an electrical signal to a SAW propagating in the piezoelectric layer, forming a decoupling layer, and forming a bus bar configured to route the electrical signal to the electrode, wherein the decoupling layer separates the piezoelectric layer and the electrode from the bus bar, and wherein the decoupling layer has an acoustic impedance different than the electrode and the bus bar, respectively.

The method of the second aspect can be implemented further according to the implementation forms of the device of the first aspect. Accordingly, the method of the second aspect and its implementation forms achieve all effects and advantages of the first aspect and its respective implementation forms.

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

BRIEF DESCRIPTION OF DRAWINGS

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

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

FIG. 2 shows a SAW device according to an embodiment of the invention in (a) a top view, and (b) a side view.

FIG. 3 shows steps of fabricating a SAW device according to an embodiment of the invention.

FIG. 4 shows steps of fabricating a SAW device according to an embodiment of the invention.

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

FIG. 6 shows parts of a SAW device according to embodiments of the invention.

FIG. 7 shows a SAW device according to embodiments of the invention.

FIG. 8 shows a method for fabricating a SAW device according to embodiments of the invention.

DETAIFED DESCRIPTION OF EMBODIMENTS

In particular, embodiments of the invention propose the creation of a decoupling layer (also called“resonator frame”) around the core electrodes, through the use of a certain deposited material. This frame decouples the SAW resonator electrodes from the metal routing and bus bar of the SAW device. The frame may be patterned (e.g. apodized geometries may be created). Further, the frame may be embedded with material regions to form an acoustic band-gap near the operating frequency of the SAW device. The frame temperature coefficient of frequency (TCF) may be chosen to reduce the overall TCF of the SAW device. Further, two distinct SAW resonators may be mechanically coupled through a frame in the transverse direction, wherein an acoustic coupling strength may optimized by modifying the geometry of the frame. The following aspects and implementation forms describe embodiments of the invention.

FIG. 1 shows a section of a SAW device 100 according to an embodiment of the invention. The SAW device 100 includes a piezoelectric layer 101, which is configured to propagate a SAW, and includes at least one electrode 102 provided on the piezoelectric layer 101, wherein the electrode 102 is configured to couple an electrical signal to a SAW propagating in the piezoelectric layer 101. The electrode 102 and the piezoelectric layer 101, or at least a part of the piezoelectric layer 101, may form a SAW resonator of the SAW device 100. Multiple electrodes 102 and the piezoelectric layer 101 may form multiple SAW resonators.

Further, the SAW device 100 of FIG. 1 includes at least one bus bar 103, which is configured to route the electrical signal to the electrode 102. Further, the SAW device 100 includes a decoupling layer 104, which separates the piezoelectric layer 101 and the electrode 102 from the bus bar 103. The decoupling layer 104 has an acoustic impedance, which is different than that of the electrode 102 and the bus bar 103, respectively.

In the SAW device 100, the actuating electrode 102, which may be an IDT electrode, and the routing bus bar 103 are decoupled by the decoupling layer 104. The decoupling layer 104 may particularly be a passivation layer, and may be referred to as a“resonator frame”. The decoupling layer 104 may be further engineered/pattemed, for instance, to modify the acoustic surroundings of the electrode(s) 102 (IDT structures). Accordingly, additional“optimization” is possible for the SAW device 100, particularly when compared to traditional ID metal design.

The decoupling layer 104 (which has preferably an intrinsic surface-wave reflectivity which is different than the metal electrode 102) can be patterned, for example, to obtain a specific band- gap shape/acoustic mirror, e.g. except at the central resonance frequency. The decoupling layer 104 can also be designed to specifically leak out energy of transverse modes. In terms of process complexity, the good quality of the electrode metallization is kept. Patterning the decoupling layer 104 is less dependent on process variations (i.e. it does not need as precise of lithography as required by conventional approaches). The decoupling layer 104 has also the advantage of modifying the temperature characteristics of the SAW device 100, for example, in the case of a TC-SAW design.

FIG. 2 shows a SAW device 100 according to an embodiment of the invention, which builds on the SAW device 100 shown in FIG. 1. Same elements are labelled with the same reference signs and function likewise. The SAW device 100 of FIG. 2 is particularly shown in (a) in a top view, and in (b) in a side view of a longitudinal section.

The SAW device 100 of FIG. 2 further includes one or more VIAs 200, which are formed through the decoupling layer 104, e.g. by metal. The VIA(s) are configured to electrically connect the bus bar 103 to the electrode(s) 102, so that electrical signals can be routed from the bus bar 103 to the electrode(s) 102. FIG. 2 also shows that the SAW device 100 may be provided on a bulk wafer or substrate 201.

As further exemplarily shown in FIG. 2, the one or more electrodes 102 may be IDT electrodes, which are formed by/in a first metal layer Ml . The decoupling layer 104 may be a passivation layer. The bus bar 103 may be formed by/in a second metal layer M2. The passivation layer 104 is used to reduce the creation of the spurious modes. This is achieved by the decoupling of the electrode(s) 102/SAW resonator(s) from the boundaries of the SAW device 100 (usually where the bus bar(s) 103 is located). The passivation layer 104, with the one or more VIAs 200, may be formed by depositing a specific, for instance dielectric, material.

FIG. 3 and FIG. 4 show how the SAW device 100 of FIG. 2 may be fabricated. FIG. 3 and FIG. 4 thereby show the SAW device 100 in (a) a side view, (b) in a front view (at the boundary where the bus bar 103 is), and (c) in a back view (where the SAW resonators are). A general fabrication method 800 of a SAW device 100 is additionally shown in FIG. 8. Steps I/IV to IV/IV will be described below.

The general fabrication method 800 shown in FIG. 8 includes: a step 801 of providing a piezoelectric layer configured to propagate a SAW; a step 802 of forming at least one electrode 102 on the piezoelectric layer 101, wherein the electrode 102 is configured to couple an electrical signal to a SAW propagating in the piezoelectric layer 101; a step 803 of forming a decoupling layer 104, and a step 804 of forming a bus bar 103 configured to route the electrical signal to the electrode 102, wherein the decoupling layer 104 separates the piezoelectric layer 101 and the electrode 102 from the bus bar 103, and wherein the decoupling layer 104 has an acoustic impedance different than the electrode 102 and the bus bar 103, respectively.

In FIG. 3, in step I/IV, the piezoelectric layer 101 is provided on a wafer 201, and a plurality of electrodes 102 are formed on the piezoelectric layer. This step I/IV thus relates to steps 801 and 802 of FIG. 8 Then, in step II/IV, the decoupling layer 104 is deposited on the piezoelectric layer 101 and electrodes 102, respectively, and VIAs 200 are formed. This step II/IV thus relates to step 803 of FIG. 8.

In FIG. 4, in step III/IV, the decoupling layer 104 is removed over parts of the electrodes (where the core SAW resonators are formed). In step IV/IV, the bus bar 103 is formed on the remaining decoupling layer 104. These steps thus relate to step 804 of FIG. 8.

FIG. 5 shows a SAW device 100 according to an embodiment of the invention, which builds on the SAW device 100 shown in FIG. 1 and FIG. 2. Same elements are labelled with the same reference signs and function likewise. The SAW device 100 of FIG. 5 is shown in a side view.

The SAW device 100 of FIG. 5 shows that the decoupling layer 104 (“resonator frame”) may be patterned by embedding one or more material elements 500 inside the decoupling layer 104. The embedded elements 500 may have an acoustic impedance and/or may have different electrical and/or optical properties, than the surrounding material of the decoupling layer 104. A plurality of the material elements 500 may be embedded in a periodic arrangement in the decoupling layer 104. In particular, the decoupling layer 104 may be provided with periodic replications of metal structures, e.g. of the VIAs 200, (or any other material deposition within the decoupling layer 104 that has an acoustic impedance lower or greater than the decoupling layer 104). Material islands embedded in the decoupling layer 104 may have unit lengths, e.g. in the order/fraction of the acoustic wavelength at the frequency of the SAW device 100. Thus, a plurality of the embedded elements /islands 500 may form a phononic crystal in the decoupling layer 104. FIG. 6 shows a part of a SAW device 100 according to an embodiment of the invention, which builds on the SAW device 100 shown in FIG. 1 and FIG. 5. Same elements are labelled with the same reference signs and function likewise. The SAW device 100 of FIG. 5 is shown in a top view.

In the SAW device 100 of FIG. 6, outer Bragg reflectors 600 are used to confine the acoustic energy within the core SAW resonator(s). In this case the metal bus bar(s) 103 may be removed, and the decoupling layer 104 may be patterned with two-dimensional periodic replicated structures 500 (e.g. using the metal VIAs 200 or another material), to effectively guide the acoustic vibration in the longitudinal direction, and to prevent diffractive scattering along the transverse direction.

FIG. 7 shows a SAW device 100 according to an embodiment of the invention, which builds on the SAW device 100 shown in FIG. 1 and FIG. 6. Same elements are labelled with the same reference signs and function likewise. The SAW device 100 of FIG. 7 is shown in a top view.

The SAW device 100 of FIG. 7 particularly has a hierarchical design of mechanically coupled SAW resonators (i.e. electrode 102 and part of piezoelectric layer 101 forming a SAW resonator), located in the vicinity of each other, e.g. for filter applications. The decoupling layer 104 (which can be patterned and optimized as described above) may be used in this case as a transverse acoustic coupler between adjacent SAW resonators. Varying the geometry and structure of the decoupling layer 104 may tune the acoustic coupling strength between adjacent SAW resonators.

In the above embodiments of the SAW device 100, a substrate (bulk wafer 201) may be one including silicon, glass, ceramic, and the like. The piezoelectric layer 101 may be a thin film piezoelectric layer including one of: lithium niobate, lithium tantalate, and the like. The electrode(s) 102 may be a low resistivity layer including a metal and/or metal alloy layer such as copper, titanium, and the like, or may be a highly doped silicon layer.

In the above embodiments of the SAW device 100, the decoupling layer 104 may include, or may be made of, a dielectric material. For example, the dielectric material of the decoupling layer 104 may 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.

In the above embodiments of the SAW device 100, the metallization layers Ml and M2 (for bus bar(s) 103 and/or electrode(s) 102, respectively) may include: copper, aluminum, tungsten, titanium, etc. BEOL dielectric layers may include: 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 may include: copper metallization, tungsten, low-k dielectrics, silicon dioxides, copper capping layers, etch stop layers, anti-reflecting coatings, etc.

In the above embodiments of the SAW device 100, a substrate may include silicon, a SOI technology substrate, gallium arsenide, gallium phosphide, gallium nitride, and/or indium phosphide or other example substrate, an alloy semiconductor including GaAsP, AlInAs, GalnAs, GalnP, or GalnAsP or combinations thereof.

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.

Claims

1. Surface acoustic wave, SAW, device (100), comprising:

a piezoelectric layer (101) configured to propagate a SAW,

at least one electrode (102) provided on the piezoelectric layer (101) and configured to couple an electrical signal to a SAW propagating in the piezoelectric layer (101),

at least one bus bar (103) configured to route the electrical signal to the electrode (102), and

a decoupling layer (104) separating the piezoelectric layer (101) and the electrode (102) from the bus bar (103),

wherein the decoupling layer (104) has an acoustic impedance different than the electrode (102) and the bus bar (103), respectively.

2. SAW device (100) according to claim 1, wherein:

the at least one bus bar (103) is arranged at least partly above the piezoelectric layer (101) and electrode (102), and

the decoupling layer (104) vertically separates the piezoelectric layer (101) and electrode (102) from the bus bar (103).

3. SAW device (100) according to claim 1 or 2, comprising:

a vertical interconnect access (200), VIA, formed through the decoupling layer (104) for electrically connecting the bus bar (103) to the electrode (102).

4. SAW device (100) according to one of the claims 1 to 3, wherein:

the decoupling layer (104) is a patterned layer and/or a passivation layer.

5. SAW device (100) according to one of the claims 1 to 4, wherein:

one or more material elements (500) are embedded in the decoupling layer (104), the embedded elements (500) having an acoustic impedance and/or having different electrical and/or optical properties than the surrounding material of the decoupling layer (104).

6. SAW device (100) according to claim 5, wherein:

a plurality of the material elements (500) is embedded in a periodic arrangement in the decoupling layer (104).

7. SAW device (100) according to claim 6, wherein:

a spatial periodicity of the periodic arrangement of the embedded elements (500) is in the order of a wavelength of the SAW propagating in the decoupling layer (104) at the working frequency of the SAW device (100).

8. SAW device (100) according to one of the claims 5 to 7, wherein:

a plurality of embedded elements (500) forms a phononic crystal in the decoupling layer

(104).

9. SAW device (100) according to claim 8, wherein:

the phononic crystal in the decoupling layer has a bandgap centered around an operating frequency of the SAW device (100).

10. SAW device (100) according to one of the claims 1 to 9, wherein:

the decoupling layer (104) has a temperature coefficient having an opposite sign than the temperature coefficient of the piezoelectric layer (101).

11. SAW device (100) according to one of the claims 1 to 10, wherein:

the decoupling layer (104) includes a dielectric material.

12. SAW device (100) according to one of the claims 1 to 11, comprising:

a plurality of SAW resonators, each SAW resonator being formed by at least one electrode (102) and at least part of a piezoelectric layer (101).

13. SAW device (100) according to claim 12, comprising:

one or more Bragg reflectors (600) arranged adjacent to a SAW resonator,

wherein a plurality of embedded elements (500) in the decoupling layer (104) is configured to guide acoustic energy from the SAW resonator to the one or more Bragg reflectors (600).

14. SAW device (100) according to claim 12 or 13, wherein:

the decoupling layer (104) is configured to acoustically couple at least two SAW resonators, wherein the coupling direction is perpendicular to the SAW propagating direction.

15. SAW device (100) according to any one of the preceding claims, wherein the at least one electrode (102) is an interdigital transducer, IDT, electrode. 16. Method (800) for fabricating a surface acoustic wave, SAW, device (100), the method

(800) comprising:

providing (801)a piezoelectric layer (101) configured to propagate a SAW,

forming (802) at least one electrode (102) on the piezoelectric layer (101), wherein the electrode (102) is configured to couple an electrical signal to a SAW propagating in the piezoelectric layer (101),

forming (803) a decoupling layer (104), and

forming (804) a bus bar (103) configured to route the electrical signal to the electrode

(102),

wherein the decoupling layer (104) separates the piezoelectric layer (101) and the electrode ( 102) from the bus bar (103), and