Surface acoustic wave device, and surface acoustic wave resonator and radio frequency filter comprising the same

Abstract

A Surface Acoustic Wave (SAW) device is proposed, which is designed to provide efficient Rayleigh-type SAW (RSAW) suppression. The SAW device comprises an Interdigital Transducer (IDT) sandwiched between two (bottom and top) piezoelectric layers which are configured, in response to a Radio Frequency (RF) signal applied to the IDT, to excite RSAWs with opposite phases, resulting in the mutual cancellation of the RSAWs in the SAW device. In a preferred embodiment, the bottom piezoelectric layer is made of θ-rotated Y-cut 90ᵒ X-propagation LiNbO3, θ-rotated Y-cut X-propagation LiNbO3, θ- rotated Y-cut 90ᵒ X-propagation LiTaO3 or θ-rotated Y-cut X-propagation LiTaO3, and the top piezoelectric layer is made of c-axis oriented AlN. Alternatively, (θ+180ᵒ)-rotated Y-cut 90ᵒ X-propagation LiNbO3, (θ+180ᵒ)-rotated Y-cut X-propagation LiNbO3, (θ+180ᵒ)-rotated Y-cut 90ᵒ X-propagation LiTaO3 or (θ+180ᵒ)-rotated Y-cut X-propagation LiTaO3 may be used for the bottom piezoelectric layer, and reversed c-axis oriented AlN may be used for the top piezoelectric layer.

Description

[0001] SURFACE ACOUSTIC WAVE DEVICE, AND SURFACE ACOUSTIC WAVE RESONATOR AND RADIO FREQUENCY FILTER COMPRISING THE SAME

[0002] TECHNICAL FIELD

[0003] The present disclosure relates generally to the field of Surface Acoustic Wave (SAW) devices. In particular, the present disclosure relates to a multi-layered SAW device designed to provide Rayleigh-type SAW suppression, as well as to a SAW resonator and a Radio Frequency (RF) filter using one or more such multi-layered SAW devices.

[0004] BACKGROUND

[0005] RF filters using mechanical resonators, such as SAW resonators, are known in the prior art. The SAW resonators convert electrical signals to mechanical oscillations (or, in other words, SAWs), and vice versa (i.e., mechanical oscillations or SAWs to electrical signals). Besides certain SAW modes which are generated in such a SAW resonator and desired in terms of its applicability, there can be also spurious modes in a piezoelectric material included in the SAW resonator. These spurious modes are parasitic in the sense that they can adversely affect the operation of the RF filters comprising such SAW resonators. More specifically, high-frequency spurious modes of a given RF filter can interfere with RF filtering in a different (high-frequency) communication band. This makes it difficult to co-integrate different RF filters in a communication circuit (e.g., a multiplexer). Due to the undesired spurious modes, many of the existing SAW resonator-based RF filters and multiplexers are of limited use - they are applicable only when the spurious modes are not harmful for the communications circuit.

[0006] Rayleigh-type SAWs (RSAWs) are one type of suchspurious modes that can be harmful for RF filter applications. Forexample, the RSAWs can be observed in Shear SAW (SH-SAW)-based filters, in which Interdigital Transducers (IDTs) are formed on rotated Y-cut LiNbCh thin-film substrates with an X propagation direction perpendicular to the IDT fingers. The suppression of the RSAWs can be achieved only in specific cuts of LiNbCh (e.g., 41Y-X LiNbCh) or in specific topologies of 30Y-X LiNbCh thin-film substrate, which in turn limits the practical use of such LiNbO; -based SAW devices. Furthermore, in the absence of coupling to an SH-SAW mode, the RSAW must be suppressed to achieve predominantly capacitive performance of the SAW device, with a view to integrating it as a high-performance capacitor in a filter layout.

[0007] SUMMARY

[0008] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features of the present disclosure, nor is it intended to be used to limit the scope of the present disclosure.

[0009] It is an objective of the present disclosure to provide a SAW device design that enables efficient RSAW suppression.

[0010] The objective above is achieved by the features of the independent claims in the appended claims. Further embodiments and examples are apparent from the dependent claims, the detailed description, and the accompanying drawings.

[0011] According to a first aspect, a SAW device is provided. The SAW device comprises a first piezoelectric layer, an IDT comprising a set of interleaved fingers provided on the first piezoelectric layer, and a second piezoelectric layer covering the set of interleaved fingers of the IDT. The first piezoelectric layer is made of a first piezoelectric material configured, in response to a RF signal applied to the IDT, to excite a RSAW having a first phase. The second piezoelectric layer is made of a second piezoelectric material configured, in response to the RF signal applied to the IDT, to excite a RSAW having a second phase. The second phase is opposite to the first phase. By sandwiching the IDT between the two piezoelectric layers configured to excite RSAWs with opposite phases, it is possible to achieve the mutual cancellation of the RSAWs in the SAW device. The SAW device thus configured may be efficiently used in SAW resonators of RF filters. Furthermore, the SAW device according to the first aspect is not limited to a specific cut of a piezoelectric material; instead, the two piezoelectric layers may be made of any piezoelectric materials which provide RSAW excitation with opposite phases (or, in other words, any piezoelectric materials with opposite polarizations). Additionally, the fabrication process of the SAW device according to the first aspect does not require any complicated techniques, such as specific lithography and / or etching techniques, in order to create such a buried IDT.

[0012] In one exemplary embodiment of the first aspect, the first piezoelectric material comprises one of 0-rotated Y-cut 90° X- propagation LiNbCh. 0-rotated Y-cut X-propagation LiNbOs, 0-rotated Y-cut 90° X-propagation LiTaCh and 0-rotated Y-cut X-propagation LiTaCh. where 6 is a rotation angle. In this embodiment, the second piezoelectric material comprises one of AIN, Ali-xScxN, ZnO, PZT, LiNbCh. BaTiOs, and SrTiCh. These materials have piezoelectric properties suitable for the proper operation of the SAW device according to the first aspect.

[0013] In one exemplary embodiment of the first aspect, the first piezoelectric material comprises one of 0-rotated Y-cut 90° X- propagation LiNbCh. 0-rotated Y-cut X-propagation LiNbOs, 0-rotated Y-cut 90° X-propagation LiTaCh and 0-rotated Y-cut X-propagation LiTaCh. and the second piezoelectric material comprises c-axis oriented AIN. Alternatively, the first piezoelectric material comprises one of (0+18O°)-rotated Y-cut 90° X-propagation LiNbOs, (0+18O°)-rotated Y-cut X- propagation LiNbCh. (0+18O°)-rotated Y-cut 90° X-propagation LiTaCh and (0+18O°)-rotated Y-cut X-propagation LiTaCh. and the second piezoelectric material comprises reversed c-axis oriented AIN. Thus, any Y-rotated cut or its 180°-rotated counterpart may be used in accordance with the specific polarization of the second (top) piezoelectric layer which is usually grown by thin-film deposition techniques and is naturally poled in a certain (preferable) direction. All of this may make the SAW device according to the first aspect more flexible in fabrication.

[0014] In one exemplary embodiment of the first aspect, the second piezoelectric layer has a top surface having an arithmetic average roughness less than 100 nm. By providing such Ra of the top surface of the second (top) piezoelectric layer, it is possible to enhance the RSAW suppression and the distortion of high-frequency spurious modes beyond the RSAW frequency band.

[0015] Alternatively, in another exemplary embodiment of the first aspect, the second piezoelectric layer comprises a set of grooves having a pitch smaller than a pitch of the IDT. In this embodiment, each groove of the set of grooves has a depth less than or equal to h / 2, where h is a thickness of the second piezoelectric layer. The presence of such grooves may enhance the RSAW suppression and the distortion of high-frequency spurious modes beyond the RSAW frequency band.

[0016] In one exemplary embodiment of the first aspect, the SAW device further comprises an intermediate layer sandwiched between the first piezoelectric layer and the second piezoelectric layer. The intermediate layer is non-conductive, and the set of interleaved fingers of the IDT is included in the intermediate layer such that each finger of the set of interleaved fingers has a top surface being in contact with the second piezoelectric layer and a bottom surface being in contact with the first piezoelectric layer. Such an intermediate layer may facilitate smooth growth of the second (top) piezoelectric layer. Moreover, the interleaved fingers embedded in such an intermediate layer may reduce the reflectivity of the IDT.

[0017] In one exemplary embodiment of the first aspect, the intermediate layer is made of one of SiCh and Si3N4. These materials have dielectric properties suitable for the proper operation of the SAW device according to the first aspect. Moreover, SiCh has an opposite Temperature Coefficient of Elasticity (TCE) compared to LiTaCh and LiNbCh (which the two piezoelectric layers may be made of), thereby providing efficient temperature compensation of the SAW device according to the first aspect.

[0018] In one exemplary embodiment of the first aspect, the first piezoelectric layer has a thickness ranging from 200 nm to 10 pm, and the second piezoelectric layer has a thickness ranging from 200 nm to 3 pm. With such thicknesses of the piezoelectric layers, the SAW device according to the first aspect may be more compact in size, which allows it to be efficiently integrated (together with similar or other SAW devices) into a RF filter or multiplexer, for example.

[0019] In one exemplary embodiment of the first aspect, the SAW device further comprises a first underlying layer and a second underlying layer provided on the first underlying layer. The second underlying layer is non-conductive, and the first piezoelectric layer is provided on the second underlying layer. In this case, the two underlying layers may provide additional mechanical support to the first and second piezoelectric layers, and the first piezoelectric layer may be bonded to the second underlying layer using a wafer bonding process, or grown on the second underlying layer, or attached to the second underlying layer in some other manner. All of this may again provide flexibility in the fabrication process of the SAW device according to the first aspect. Furthermore, since the second underlying layer is non-conductive, this may also provide proper isolation of the IDT and the first and second piezoelectric layers.

[0020] In one exemplary embodiment of the first aspect, the first underlying layer is made of one of Si, YAG, SiC, sapphire and quartz, and the second underlying layer is made of one of SiCh and Si3N4. These materials may provide the above-mentioned mechanical support and isolation more efficiently.

[0021] According to a second aspect, a SAW resonator is provided. The SAW resonator comprises the SAW device according to the first aspect. The SAW resonator thus configured may be efficiently used for RF applications.

[0022] According to a third aspect, an RF filter is provided. The RF filter comprises at least one SAW resonator according to the second aspect. The RF filter thus configured may be co-integrated with one or more (similar or other) RF filters within a multiplexer in a single-die or multi-die fashion.

[0023] Other features and advantages of the present disclosure will be apparent upon reading the following detailed description and reviewing the accompanying drawings.

[0024] BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present disclosure is explained below with reference to the accompanying drawings in which:

[0026] FIG. 1 shows a schematic cross-sectional view of a Surface Acoustic Wave (SAW) device in accordance with a first exemplary embodiment;

[0027] FIG. 2 shows a schematic cross-sectional view of a SAW device in accordance with a second exemplary embodiment;

[0028] FIG. 3 shows simulated curves each illustrating the magnitudes of admittance and conductance versus a frequency for one of three different SAW devices, namely: the first SAW device comprising an Interdigital Transducer (IDT) on a regular Y-cut 90X LT substrate, the second SAW device is implemented as shown in FIG. 2 (i.e., with an IDT sandwiched between two piezoelectric layers with opposite polarizations), and the third SAW device is also implemented as shown in FIG. 2 but with the inverted polarization of one of the piezoelectric layers;

[0029] FIG. 4 shows a schematic cross-sectional view of a SAW device in accordance with a third exemplary embodiment;

[0030] FIG. 5 shows a simulated curve illustrating the magnitudes of admittance and conductance versus a frequency for the SAW device of FIG. 4;

[0031] FIG. 6 shows a schematic cross-sectional view of a SAW device in accordance with a fourth exemplary embodiment; and FIG. 7 shows a simulated curve illustrating the magnitudes of admittance and conductance versus a frequency for the SAW device of FIG. 6. DETAILED DESCRIPTION

[0032] Various embodiments of the present disclosure are further described in more detail with reference to the accompanying drawings. However, the present disclosure may be embodied in many other forms and should not be constmed as limited to any certain structure or function discussed in the following description. In contrast, these embodiments are provided to make the description of the present disclosure detailed and complete.

[0033] According to the detailed description, it will be apparent to the ones skilled in the art that the scope of the present disclosure encompasses any embodiment thereof, which is disclosed herein, irrespective of whether this embodiment is implemented independently or in concert with any other embodiment of the present disclosure. For example, the devices disclosed herein may be implemented in practice by using any numbers of the embodiments provided herein. Furthermore, it should be understood that any embodiment of the present disclosure may be implemented using one or more of the features presented in the appended claims.

[0034] The word “exemplary” is used herein in the meaning of “used as an illustration”. Unless otherwise stated, any embodiment described herein as “exemplary” should not be construed as preferable or having an advantage over other embodiments.

[0035] Any positioning terminology, such as “left”, “right”, “top”, “bottom”, “above” “below”, "upper", “lowed’, “horizontal”, “vertical”, etc., may be used herein for convenience to describe one element’s or feature's relationship to one or more other elements or features in accordance with the figures. It should be apparent that the positioning terminology is intended to encompass different orientations of the apparatus disclosed herein, in addition to the orientation(s) depicted in the figures. As an example, if one imaginatively rotates the apparatus in the figures 90 degrees clockwise, elements or features described as “left” and “right” relative to other elements or features would then be oriented, respectively, “above” and “below” the other elements or features. Therefore, the positioning terminology used herein should not be constmed as any limitation of the invention.

[0036] Although the numerative terminology, such as “first”, “second”, etc., may be used herein to describe various embodiments, elements or features, these embodiments, elements or features should not be limited by this numerative terminology. This numerative terminology is used herein only to distinguish one embodiment, element or feature from another embodiment, element or feature. Thus, a first piezoelectric layer discussed below could be called a second piezoelectric layer, and vice versa, without departing from the teachings of the present disclosure.

[0037] As used in the embodiments disclosed herein, a Surface Acoustic Wave (SAW) device may refer to a multi-layered structure that contains metal electrodes or fingers which are arranged in an interdigitated format in contact with one or more piezoelectric materials, thereby forming the so-called Interdigital Transducer (IDT). An input Radio Frequency (RF) signal applied to the IDT induces a deformation wave or SAW in the piezoelectric material(s). The IDT may also operate in an inverse manner, i.e., an input SAW applied to the IDT induces an output RF signal. In general, the IDT is well-known in the art, for which reason its detailed description is omitted herein. A SAW resonator is formed when the SAW device is, for example, combined with two or more reflectors such that the IDT is arranged between the reflectors. Such a SAW resonator may operate wirelessly on a received RF signal with no requirement for an additional power source.

[0038] The exemplary embodiments disclosed herein relate to a SAW device that is designed to provide efficient Rayleigh-type SAW (RS AW) suppression. The SAW device comprises an IDT sandwiched between two (bottom and top) piezoelectric layers which are configured, in response to an RF signal applied to the IDT, to excite RSAWs with opposite phases, resulting in the mutual cancellation of the RSAWs in the SAW device. For example, the piezoelectric layers may be made of any piezoelectric materials with opposite polarizations, or different cuts of the same piezoelectric material which lead to the excitation of opposite-phase RSAWs.

[0039] FIG. 1 shows a schematic cross-sectional view of a SAW device 100 in accordance with a first exemplary embodiment. The SAW device 100 comprises a first (bottom) piezoelectric layer 102 and a second (top) piezoelectric layer 104, with an IDT sandwiched therebetween. The first and second piezoelectric layers 102 and 104 are assumed to be made of piezoelectric materials which provide the excitation of RSAWs with opposite phases (which are schematically shown as solid and dashed arrows in FIG. 1). For example, the first piezoelectric layer 102 is made of 0-rotated Y-cut 90° X-propagation LiNbOs, 0-rotated Y-cut X-propagation LiNbOs, 0-rotated Y-cut 90° X-propagation LiTaOs, or 0-rotated Y-cut X-propagation LiTaOs, where 6 is a rotation angle relative to an y-axis. In this case, the second piezoelectric layer 104 may be made of AIN, Al i-xScxN, ZnO, PZT, LiNbO>. BaTiO>. and SrTiO;. In a preferred embodiment, the first piezoelectric layer 102 is made of 0-rotated Y-cut 90° X-propagation LiNbOs, 0-rotated Y-cut X-propagation LiNbOs, 0-rotated Y-cut 90° X-propagation LiTaOs or 0-rotated Y-cut X-propagation LiTaOs, and the second piezoelectric layer 104 is made of c-axis oriented AIN; alternatively, (0+18O°)-rotated Y-cut 90° X-propagation LiNbOs, (0+18O°)-rotated Y-cut X-propagation LiNbOs, (0+18O°)-rotated Y-cut 90° X-propagation LiTaO . or (0+18O°)-rotated Y-cut X-propagation LiTaOs may be used for the first piezoelectric layer 102, and reversed c-axis oriented AIN may be used for the second piezoelectric layer 104. The first piezoelectric layer 102 may have a thickness ranging from 200 nm to 10 pm, and the second piezoelectric layer 104 may have a thickness ranging from 200 nm to 3 pm. In the first exemplary embodiment, the first piezoelectric layer 102 may be considered as a substrate, since it is an underlying or support layer for the IDT and the second piezoelectric layer 104.

[0040] In one embodiment, the second piezoelectric layer 104 may have a rough top surface having an arithmetic average roughness (Ra) less than 100 nm. In another alternative embodiment, the second piezoelectric layer 104 may comprise a set of grooves having a pitch smaller than the pitch of the IDT, and each groove of the set of grooves may have a depth less than or equal to h / 2, where h is a thickness of the second piezoelectric layer 104. It should be noted that both Ra- and groove-related embodiments are optional and may be used to enhance the RSAW suppression and the distortion of high-frequency spurious modes beyond the RSAW frequency band.

[0041] The IDT used in the SAW device 100 comprises a set of interleaved electrodes or fingers 106 which is provided on the first piezoelectric layer 102 and covered by the second piezoelectric layer 104. Those skilled in the art would recognize that the set of interleaved fingers 106 comprises a first subset of fingers extending from one busbar and a second subset of fingers extending from another busbar. Both subsets of fingers are interleaved. The two busbars serve as two different-polarity terminals of the SAW device 100. During operation, an RF or microwave signal having a frequency comparable with the resonance frequency of the SAW device 100 is applied to the busbars. As a result of the piezoelectric effect, besides the spurious RSAWs, one or more desired SAW modes may be excited in the SAW device 100. For example, the desired SAW mode(s) may be mainly defined by the piezoelectric materials of the piezoelectric layers 102 and 104 (e.g., their cuts with certain Euler angles). The set of interleaved fingers 106 may be made of any suitable metal, such as Al, Cu, Au, etc., or any alloy of these and / or other metals. Each of the fingers 106 may also be configured as a multi-layered structure comprising different metal layers, such as Al / Cu / Ti, for example. For simplicity, only eight trapezoidal (which is mostly defined by the capabilities of the existing fabrication technologies, such as lithography) fingers 106 alternately connected to the separate busbars of the IDT are shown in FIG. 1. However, it is implied that the fingers 106 are periodically repeated on the surface of the first piezoelectric layer 102, so that there are hundreds of such fingers along an x-axis.

[0042] The piezoelectric layers 102, 104 and the fingers 106 included in the SAW device 100 may be provided by using any of the existing fabrication technologies, such as lithography (e.g., photolithography, electron lithography, Extreme ultraviolet (EUV) lithography, etc.), 3D-printing, atomic layer deposition (ALD), Chemical Vapor Deposition (CVD), Metal organic CVD (MOCVD), Ultra-High Vacuum CVD (UHV-CVD), Chemical Beam Epitaxy (CBE), laser ablation, sputtering, electrolytic deposition (e.g., electroplating), Molecular Beam Epitaxy (MBE), etc.

[0043] FIG. 2 shows a schematic cross-sectional view of a SAW device 200 in accordance with a second exemplary embodiment. Like the SAW device 100, the SAW device 200 comprises a first (bottom) piezoelectric layer 202, a second (top) piezoelectric layer 204, and an IDT comprising a set of interleaved fingers 206 and sandwiched between the piezoelectric layers 202 and 204. The piezoelectric layers 202 and 204 may be implemented in the same or similar manner as the piezoelectric layers 102 and 104, respectively. In other words, the piezoelectric layers 202 and 204 should be made of such piezoelectric materials which provide the excitation of opposite-phase RSAWs (see solid and dashed black arrows in FIG. 2). As for the fingers 206, they may be implemented in the same or similar manner as the fingers 106. At the same time, the SAW device 200 differs from the SAW device 100 in that the first piezoelectric layer 202 is not a substrate but a thin-film layer provided on two underlying layers 208 and 210. In other words, the SAW device 200 comprises a two-layered substrate that is composed of the underlying layers 208 and 210. Preferably, the first (bottom) underlying layer 208 is made of one of Si, YAG, SiC, sapphire and quartz, and the second (top) underlying layer is made of one of SiO2 and Si3N4.

[0044] FIG. 3 shows simulated curves each illustrating the magnitudes of admittance and conductance versus a frequency for one of three different SAW devices. More specifically, the solid curves correspond to the magnitudes of admittance and conductance which are obtained for the first SAW device comprising: an acoustically thick Si substrate having a thickness more than 200 pm, a 500 nm thick SiO2 layer provided on the Si substrate, a 600 nm thick 42Y -cut 90X LT layer provided on the SiO2 layer, and an IDT formed on the LT layer. The dotted curves correspond to the magnitudes of admittance and conductance which are obtained for the second SAW device implemented as the SAW device 200 (in particular, the second SAW device is implemented as the following multi-layered structure: the first underlying layer 208 has a thickness more than 200 jim and is made of Si, the second underlying layer 210 has a thickness of 500 nm and is made of SiO2, the first piezoelectric layer 202 has a thickness of 600 nm and is made of 42Y-cut 90X LT, the IDT has a pitch of 1 jim, each of the fingers 206 has a thickness of 170 nm and is made of Al, and the second piezoelectric layer 204 has a thickness of 575 nm and is made of c-axis oriented AIN). The dashed curves correspond to the magnitudes of admittance and conductance which are obtained for the third SAW device that is implemented as the second SAW device with but with the inverted polarization of the top piezoelectric layer 204 (i.e., reversed c-axis oriented AIN). As follows from FIG. 3, RSAW radiation is significantly suppressed by using the second SAW device (i.e., the SAW device 200 with the piezoelectric layers 202 and 204 which provide the excitation of oppositephase RSAWs) compared to the first SAW device. At the same time, the curves obtained for the third SAW device demonstrate the importance of the opposite polarizations of the piezoelectric layers 202 and 204 (in this case, the polarization of the second piezoelectric layer 204, i.e., the reversed c-axis oriented AIN, is improperly selected with respect to the polarization of the first piezoelectric layer 202, i.e., 42Y-cut 90X LT, thereby resulting in further enhancement in the RSAW response instead of its suppression).

[0045] FIG. 4 shows a schematic cross-sectional view of a SAW device 400 in accordance with a third exemplary embodiment. Like the SAW device 200, the SAW device 400 comprises a first (bottom) piezoelectric layer 402, a second (top) piezoelectric layer 404, and an IDT comprising a set of interleaved fingers 406 and sandwiched between the piezoelectric layers 402 and 404, as well as two underlying layers 408 and 410 which are used as a two-layered substrate for the piezoelectric layers 402 and 404. The piezoelectric layers 402 and 404 may be implemented in the same or similar manner as the piezoelectric layers 202 and 204, respectively. In other words, the piezoelectric layers 402 and 404 should be made of such piezoelectric materials which provide the excitation of opposite -phase RSAWs along the z-axis (see solid and dashed black arrows in FIG. 4). As for the fingers 406, they may be implemented in the same or similar manner as the fingers 206. The underlying layers 408 and 410 may be implemented in the same or similar manner as the underlying layers 208 and 210, respectively. At the same time, the SAW device 400 differs from the SAW device 200 in that there are periodic protrusions 412 on the top surface of the second piezoelectric layer 404, which have a shape like that of the fingers 406. Those skilled in the art would recognize that the protrusions 412 may be naturally formed during the process of depositing the second piezoelectric layer 404 on the IDT and the first piezoelectric layer 402.

[0046] FIG. 5 shows a simulated curve illustrating the magnitudes of admittance and conductance versus a frequency for the SAW device 400. For the simulation purposes, the SAW device 400 has been implemented as the following multi-layered structure: the first underlying layer 408 is a thick Si substrate like the one discussed above with reference to FIG. 3, the second underlying layer 410 has a thickness of 500 nm and is made of SiO2, the first piezoelectric layer 402 has a thickness of 500 nm and is made of 42Y-90X LT, the IDT has a pitch of 1 pm, each of the fingers 406 has a thickness of 170 nm and is made of Al, and the second piezoelectric layer 404 has a thickness of 675 nm and is made of c-axis oriented AIN. As can be seen from FIG. 5, RS AW radiation is again significantly suppressed by using the SAW device 400.

[0047] FIG. 6 shows a schematic cross-sectional view of a SAW device 600 in accordance with a fourth exemplary embodiment. Like the SAW device 200, the SAW device 600 comprises a first (bottom) piezoelectric layer 602, a second (top) piezoelectric layer 604, and an IDT comprising a set of interleaved fingers 606 and sandwiched between the piezoelectric layers 602 and 604, as well as two underlying layers 608 and 610 which are used as a two-layered substrate for the piezoelectric layers 602 and 604. The piezoelectric layers 602 and 604 may be implemented in the same or similar manner as the piezoelectric layers 202 and 204, respectively. In other words, the piezoelectric layers 602 and 604 should be made of such piezoelectric materials which provide the excitation of opposite -phase RSAWs along the z-axis (see solid and dashed black arrows in FIG. 6). As for the fingers 606, they may be implemented in the same or similar manner as the fingers 206. The underlying layers 608 and 610 may be implemented in the same or similar manner as the underlying layers 208 and 210, respectively. At the same time, the SAW device 600 differs from the SAW device 200 in that there is an intermediate non-conductive layer 612 between the piezoelectric layers 602 and 604. The IDT is embedded in the intermediate layer 612 such that each of the fingers 606 is in contact with each of the piezoelectric layers 602 and 604 (i.e., the thicknesses of the intermediate layer 612 and the fingers 606 are equal). The intermediate layer 612 may be made of any non-conductive material, such as any dielectric material like SiO2 and SiiNj. As can be seen, the presence of the intermediate layer 612 may allow one to avoid the formation of natural protrusions (like the protrusions 412) on the top surface of the second piezoelectric layer 604.

[0048] FIG. 7 shows a simulated curve illustrating the magnitudes of admittance and conductance versus a frequency for the SAW device 600. For the simulation purposes, the SAW device 600 has been implemented as the following multi-layered structure : the first underlying layer 608 is a thick Si substrate like the one discussed above with reference to FIG. 3, the second underlying layer 610 has a thickness of 500 nm and is made of SiO2, the first piezoelectric layer 602 has a thickness of 500 nm and is made of 42Y-90X LT, the IDT has a pitch of 1 pm, each of the fingers 606 has a thickness of 170 nm and is made of Al, and the second piezoelectric layer 604 has a thickness of 800 nm and is made of c-axis oriented AIN. As can be seen from FIG. 7, RSAW radiation is again significantly suppressed by using the SAW device 600.

[0049] It should be noted that there are two possible applications of the SAW devices 100, 200, 400 and 600, namely:

[0050] 1) If the RSAW is excited and the SH-SAW is not excited, the SAW device may be used as a capacitor with suppressed RASW radiation. In this case, reflectors are not needed as this is meant to be not an acoustic device in a given frequency range. Also, the IDT dimension is purely associated with the capacitance needed to be achieved. Such capacitances are used in a RF filter design in a manner complementary to the conventional resonators. The SAW designs disclosed herein enable integration of SH-SAW resonators (e.g., with the X-propagation) with capacitors (on the 90X direction) on the same 0Y-rotated LT piezoelectric layer.

[0051] 2) If the strongly coupled SH-SAW exists (e.g., along the X direction of 0Y-rotated LN) in the SAW device and the RSAW is a spurious mode, then a SAW resonator based on the SAW device is expected to have a cleaner spectrum and is formed as the conventional SAW resonator having a long IDT surrounded by (e.g., distributed) reflectors. The reflectors may be made as metal gratings. Such SAW resonators are used in ladder circuit filters as any other SAW resonators. Although the exemplary embodiments of the present disclosure are described herein, it should be noted that any various changes and modifications could be made in the embodiments of the present disclosure, without departing from the scope of legal protection which is defined by the appended claims. In the appended claims, the word “comprising” does not exclude other elements or operations, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS1. A Surface Acoustic Wave, SAW, device comprising: a first piezoelectric layer; an Interdigital Transducer, IDT, comprising a set of interleaved fingers provided on the first piezoelectric layer; and a second piezoelectric layer covering the set of interleaved fingers of the IDT; wherein the first piezoelectric layer is made of a first piezoelectric material configured, in response to a Radio Frequency, RF, signal applied to the IDT, to excite a Rayleigh-type SAW having a first phase; and wherein the second piezoelectric layer is made of a second piezoelectric material configured, in response to the RF signal applied to the IDT, to excite a Rayleigh-type SAW having a second phase, the second phase being opposite to the first phase.

2. The SAW device of claim 1, wherein the first piezoelectric material comprises one of 0-rotated Y-cut 90° X- propagation LiNbCh. 0-rotated Y-cut X-propagation LiNbOs, 0-rotated Y-cut 90° X-propagation LiTaCh and 0- rotated Y-cut X-propagation LiTaOs, where 6 is a rotation angle, and wherein the second piezoelectric material comprises one of AIN, Ali-xScxN, ZnO, PZT, LiNbCh. BaTiOs, and SrTiOs.

3. The SAW device of claim 2, wherein the first piezoelectric material comprises one of 0-rotated Y-cut 90° X-propagation LiNbCh. 0-rotated Y-cut X-propagation LiNbOs, 0-rotated Y-cut 90° X-propagation LiTaOs and 0-rotated Y-cut X-propagation LiTaCh. and the second piezoelectric material comprises c-axis oriented AIN; or the first piezoelectric material comprises one of (0+18O°)-rotated Y-cut 90° X-propagation LiNbCh. (0+180°)- rotated Y-cut X-propagation LiNbCh. (0+18O°)-rotated Y-cut 90° X-propagation LiTaC+and (0+18O°)-rotated Y-cut X-propagation LiTaOs, and the second piezoelectric material comprises reversed c-axis oriented AIN.

4. The SAW device of any one of claims 1 to 3, wherein the second piezoelectric layer has a top surface having an arithmetic average roughness less than 100 nm.

5. The SAW device of any one of claims 1 to 3, wherein the second piezoelectric layer comprises a set of grooves having a pitch smaller than a pitch of the IDT, and wherein each groove of the set of grooves has a depth less than or equal to h / 2, where h is a thickness of the second piezoelectric layer.

6. The SAW device of any one of claims 1 to 5, further comprising an intermediate layer sandwiched between the first piezoelectric layer and the second piezoelectric layer, the intermediate layer being non-conductive, and wherein the set of interleaved fingers of the IDT is included in the intermediate layer such that each finger of the set of interleaved fingers has a top surface being in contact with the second piezoelectric layer and a bottom surface being in contact with the first piezoelectric layer.

7. The SAW device of claim 6, wherein the intermediate layer is made of one of SiCh and Si3N4.

8. The SAW device of any one of claims 1 to 7, wherein the first piezoelectric layer has a thickness ranging from 200 nm to 10 jim, and the second piezoelectric layer has a thickness ranging from 200 nm to 3 jim.

9. The SAW device of any one of claims 1 to 8, further comprising a first underlying layer and a second underlying layer provided on the first underlying layer, the second underlying layer being non-conductive, and wherein the first piezoelectric layer is provided on the second underlying layer.

10. The SAW device of any one of claims 9, wherein the first underlying layer is made of one of Si, YAG, SiC, sapphire and quartz, and the second underlying layer is made of one of SiCh and Si3N4.

11. A Surface Acoustic Wave, SAW, resonator comprising the SAW device according to any one of claims 1 to 10.

12. A Radio Frequency, RF, filter comprising at least one SAW resonator according to claim 11.

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