EP4508748A1

EP4508748A1 - Device for producing an acoustic wave, and radio frequency filter and multiplexer comprising the same

Info

Publication number: EP4508748A1
Application number: EP23705525.6A
Authority: EP
Inventors: Ventsislav Yantchev; Kiryl KUSTANOVICH
Original assignee: Huawei Technologies Co Ltd
Current assignee: Huawei Technologies Co Ltd
Priority date: 2023-02-15
Filing date: 2023-02-15
Publication date: 2025-02-19
Also published as: CN119586003A; WO2024170071A1

Abstract

The present disclosure relates to a cubic Boron Arsenide, c-BAs, based multi-layered device configured to produce an acoustic wave. The device comprises a c-BAs-based substrate (102), a piezoelectric layer (104) provided on the c-BAs-based substrate, and an Interdigital Transduce, IDT (108, 110), provided on the piezoelectric layer. The IDT is configured to excite the acoustic wave in response to an applied RF signal. The device further comprises a temperature compensation, TC, layer (106) either provided between the c-BAs-based substrate and the piezoelectric layer or covering the piezoelectric layer and the IDT. By using the c-BAs-based substrate, it is possible to minimize the number of propagating spurious acoustic waves at different material interfaces in the device. Thus, the suppression of the spurious acoustic wave modes is provided without having to use any external components (e.g., combinations of capacitors, inductors, transformers, etc.) or additional resonators in a RF circuit in which the device is to be used.

Description

DEVICE FOR PRODUCING AN ACOUSTIC WAVE, AND RADIO FREQUENCY FIETER AND MUETIPEEXER COMPRISING THE SAME

TECHNICAL FIELD

The present disclosure relates generally to the field of acoustic devices. In particular, the present disclosure relates to a cubic Boron Arsenide (c-BAs)-based multi-layered device configured to produce an acoustic wave, as well as to a Radio Frequency (RF) filter and multiplexer each using one or more such c-BAs-based multi-layered devices.

BACKGROUND

Wireless communication devices (e.g., mobile phones) heavily rely on high-performance RF filters which are used to reject any undesired incoming RF signals and keep only a desired transmitted signal. In some wireless communication applications, the RF filters must have exceptionally good selectivity of the incoming RF signals, passing only a very narrow band of an incoming frequency spectrum. More specifically, the key technological requirements of the RF filters can include, but are not limited to, a high frequency, high selectivity for the suppression of spurious signals, a large bandwidth for high data rates, a low insertion loss factor for low power consumption, and a form-factor suitable for the RF filters to be used in small hand-held devices.

The above-mentioned requirements have caused one to use acoustic devices, such as Surface Acoustic Wave (SAW) devices, in the design of the RF filters. Additionally, the acoustic devices are currently key components not only for signal filtering in different wireless communication systems (e.g., 2G/3G/4G/LTE/5G, Bluetooth, etc.), but also for other signal processing, frequency generation and sensing applications. To reduce the inherently high insertion loss of the SAW devices, a variety of special low-loss techniques have been developed, each optimized for a specific application.

The latest generation of the SAW devices is referred to as Layered SAW (LSAW) or Film SAW (FSAW) devices. An FSAW device has a multi-layered design in which a piezoelectric layer is provided on a stack of layers each made of a different material. The combination of these different materials in the stack, as well as the appropriate choice of their thicknesses and material properties can significantly improve acoustic energy confinement in the piezoelectric layer. This effect lowers acoustic losses/radiation into a supporting substrate of the FSAW device and improves the overall response of a resulting RF filter.

However, the presence of multiple “buried” layers in the stack of the FSAW device also increases the number of allowable propagating acoustic waves and mechanical modes. These waves can also include those produced at frequencies located away from the main operating frequency band of the RF filter. According to the current radio communication standards, an out-of-band response needs to be tightly controlled as front-end circuits rely on stringent out- of-band channel attenuation levels to prevent any interaction between different communication channels and /or standards. As a result, it is important to be able to minimize or even eliminate the out-of-band spurious mode content of the RF filters comprising such SAW devices.

SUMMARY

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.

It is an objective of the present disclosure to provide a technical solution that allows desired acoustic wave modes to be effectively guided for a wide frequency range, while minimizing the generation of spurious acoustic wave modes.

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.

According to a first aspect, a device for producing an acoustic wave is provided. The device comprises a cubic Boron Arsenide (c-BAs)-based substrate, a piezoelectric layer provided on the c-BAs-based substrate, and an Interdigital Transducer (IDT) provided on the piezoelectric layer. The IDT is configured to excite the acoustic wave in response to an applied RF signal. The device further comprises a temperature compensation (TC) layer provided between the c- BAs-based substrate and the piezoelectric layer or covering the piezoelectric layer and the IDT. By using the c-BAs-based substrate, it is possible to minimize the number of propagating spurious (i.e., out-of-band) acoustic waves at different material interfaces in the device, i.e., to break the boundary condition required for spurious acoustic wave modes to exist and propagate in the device. Thus, the suppression or attenuation of the spurious acoustic wave modes is provided without having to use any external components (e.g., combinations of capacitors, inductors, transformers, etc.) or additional resonators in a RF circuit in which the device is to be used. The other benefits provided by the device thus configured are as follows:

- a high mechanical quality factor (Q) is achieved due to the guidance of desired acoustic wave modes only;

- a high electro-mechanical coupling coefficient (k_t ²) is achieved due to fast longitudinal/shear bulk acoustic waves in the device;

- the mismatch in the coefficient of thermal-expansion (CTE) of c-BAs relative to a piezoelectric material may improve the passive temperature compensation (or, in other words, reduce the frequency drift) of the device due to temperature variations;

- the presence of the TC layer may provide additional temperature compensation, which may make the device applicable in a wide temperature range;

- the device thus configured is less prone to packaging/filter architecture issues and may be implemented in a variety of filter products and bands; and

- additionally, the device thus configured may be used in different micro-electro-mechanicalsystem (MEMS)-based phase-locked loop (PLL) timing circuits (oscillators), allowing one to reduce the number of available spurious acoustic wave modes on which an oscillator may lock- in.

In one exemplary embodiment of the first aspect, the c-BAs-based substrate has a crystallographic orientation <111>, <110>, or <100>. By using c-BAs with such crystallographic orientations, the core region (i.e., the one with the piezoelectric layer and the IDT) of the device may be isolated electrically and acoustically from the substrate more efficiently. This, in turn, may contribute to better suppression of the spurious acoustic wave modes.

In one exemplary embodiment of the first aspect, the c-BAs-based substrate comprises an underlaying supporting layer and a c-BAs layer provided on the underlaying supporting layer. This embodiment may be beneficial when it is difficult to fabricate a thick c-BAs substrate.

In one exemplary embodiment of the first aspect, the underlaying supporting layer in the c- BAs-based layered substrate is made of quartz, diamond, fused silica, sapphire, yttrium aluminum garnet (YAG), silicon (Si), gallium nitride (GaN), gallium arsenide (GaAs), or silicon carbide (SiC). Any of these materials are well-known in the art, for which reason techniques for fabricating thick substrates based thereon are well-established.

In one exemplary embodiment of the first aspect, the c-BAs layer is a polycrystalline layer or has a crystallographic orientation <111>, <110>, or <100>. By such c-BAs layer, the core region (i.e., the one with the piezoelectric layer and the IDT) of the device may be isolated electrically and acoustically from the substrate more efficiently. This, in turn, may contribute to better suppression of the spurious acoustic wave modes.

In one exemplary embodiment of the first aspect, the c-BAs layer has a thickness defined based on a wavelength of a primary mode of the acoustic wave in the piezoelectric layer. The c-BAs layer with such thickness values may provide desired electro-acoustic characteristics of the device (e.g., acoustic energy confinement or waveguiding).

In one exemplary embodiment of the first aspect, the thickness of the c-BAs layer falls within a range of 0.25k to 2k, where k is the wavelength of the primary mode of the acoustic wave in the piezoelectric layer. By using this range of thickness values, it is possible to achieve the desired electro-acoustic characteristics of the device.

In one exemplary embodiment of the first aspect, the TC layer has a shear acoustic impedance lower than a shear acoustic impedance of the piezoelectric layer or the c-B As-based substrate. By using this relationship between the shear acoustic impedances of the TC layer, the piezoelectric layer and the c-BAs-based substrate, it is possible to confine the acoustic wave in the TC and piezoelectric layers.

In one exemplary embodiment of the first aspect, the TC layer is provided between the c-BAs- based substrate and the piezoelectric layer. In this embodiment, the device further comprises a passivation layer covering the piezoelectric layer and the IDT. The TC layer thus arranged may provide, in addition to TC, better adhesion or bonding between the piezoelectric layer and the c-BAs-based substrate. As for the passivation layer, it protects the core region of the device from the surrounding environment, thereby extending the service life of the device.

In one exemplary embodiment of the first aspect, the passivation layer is made of a dielectric material comprising at least one of silicon oxide (SiO ) and silicon nitride (SiaN- , or both. These materials are less readily affected or corroded by the surrounding environment. In one exemplary embodiment of the first aspect, the acoustic wave (e.g., its primary mode) is one of a Surface Acoustic Wave (SAW), a Lamb wave and a shear-horizontal plate wave. This means that the device may be configured to excite different types of acoustic waves (depending on the configuration parameters of the IDT, the thickness and crystallographic orientation of the piezoelectric layer, etc.), thereby making the device more flexible in use.

In one exemplary embodiment of the first aspect, the piezoelectric layer is made of lithium tantalate (LiTaOs), lithium niobate (LiNbOs), aluminum nitride (AIN), scandium (Sc) or yttrium (Y) doped AIN (AlScxN, A1N:Y), lithium iodate (LilOs), zinc oxide (ZnO), lead zirconate titanate (PZT), potassium niobate (KNbO₃), sodium niobate (NaNbO₃), gadolinium doped cerium oxide (GdiCeCL), or quartz. These materials have piezoelectric properties suitable for the proper operation of the device.

In one exemplary embodiment of the first aspect, the piezoelectric layer is made of: 0Y-X LiTaO₃, where 0 = (20°-65°) U (115°-135°); or 0Y-X LiNbO₃, where 0 = (0°-90°) U (115°- 135°). These cuts of LiTaO₃ or LiNbO₃ may provide different electromechanical coupling coefficients (K²) and wave polarizations, thereby making the device more flexible in use (e.g., if the device is intended to be used in a narrow bandwidth filter, large K² may be problematic, and vice versa, for which reason the angles 0 for 0Y-X LiTaO₃ or 0Y-X LiNbO₃ should be properly selected from the above-indicated angular ranges).

In one exemplary embodiment of the first aspect, each of the piezoelectric layer and the TC layer has a thickness defined based on a wavelength of a primary mode of the acoustic wave in the piezoelectric layer. The TC layer with such thickness values may provide proper TC.

In one exemplary embodiment, the TC layer is provided between the c-BAs-based substrate and the piezoelectric layer. In this embodiment, the thickness of each of the piezoelectric layer and the TC layer falls within a range of 0. Ik to 0.5k, where k is the wavelength of the primary mode of the acoustic wave in the piezoelectric layer. The combination of such thicknesses of the TC and piezoelectric layers may allow one to achieve the lowest temperature coefficient of frequency (TCF) (ideally equal to 0 ppm/K).

In one exemplary embodiment of the first aspect, the TC layer covers the piezoelectric layer and the IDT. In this embodiment, the thickness of the piezoelectric layer falls within a range of 0.15k to k, and the thickness of the TC layer falls within a range of 0.15k to 0.5k, where k is the wavelength of the primary mode of the acoustic wave in the piezoelectric layer. The combination of such thicknesses of the TC and piezoelectric layers may allow one to achieve the lowest temperature coefficient of frequency (TCF) (ideally equal to 0 ppm/K).

In one exemplary embodiment of the first aspect, the TC layer is provided between the c-BAs- based substrate and the piezoelectric layer. In this embodiment, the device further comprises a layer of trap-rich material provided between the c-BAs-based substrate and the TC layer. The layer of trap-rich material may be used to prevent charge accumulation at the material interfaces, which may adversely affect the operation of the device.

In one exemplary embodiment of the first aspect, the TC layer covers the piezoelectric layer and the IDT. In this embodiment, the device further comprises a layer of trap-rich material provided between the c-BAs-based substrate and the piezoelectric layer. The layer of trap-rich material may be used to prevent charge accumulation at the material interfaces, which may adversely affect the operation of the device.

In one exemplary embodiment of the first aspect, the layer of trap-rich material has a thickness falling within a range of 0. Ik to X, where X is the wavelength of a primary mode of the acoustic wave in the piezoelectric layer. This range of thickness values for the layer of trap-rich material may provide, together with the other layers of the device, the proper electro-acoustic characteristics of the device.

According to a second aspect, an RF filter is provided. The RF filter comprises at least one acoustic resonator each comprising at least one device according to the first aspect and two reflectors arranged such that the IDT of each of the at least one device is between the two reflectors. The RF filter thus configured may provide wide-band spurious-free responses. Furthermore, 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.

According to a third aspect, a multiplexer is provided. The multiplexer comprises a circuit card, at least two RF filters according to the second aspect, at least one first impedance-matching component, and at least one second impedance-matching component. Each of the at least two RF filters is mounted on the circuit card. Each of the at least one first impedance-matching component is arranged on the circuit card and configured to provide impedance-matching between the at least two RF filters and an antenna to which each of the at least two RF filters is to be coupled. Each of the at least one second impedance-matching component is arranged on the circuit card and configured to provide the impedance-matching between the at least two RF filters and a Radio Frequency Integrated Circuit (RFIC) to which each of the at least two RF filters is to be coupled. By using a combination of two or more RF filters, the multiplexer may provide efficient RF filtering in different high-frequency bands.

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

BRIEF DESCRIPTION OF THE DRAWINGS

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

FIG. 1 shows a schematic cross-sectional view of a device for producing an acoustic wave in accordance with a first exemplary embodiment;

FIG. 2 shows a schematic cross-sectional view of a device for producing an acoustic wave in accordance with a second exemplary embodiment;

FIG. 3 shows a schematic top view of a device for producing an acoustic wave in accordance with a third exemplary embodiment;

FIGs. 4A and 4B show comparison results for the simulated admittance of a FSAW device based on a <100> Si substrate (FIG. 4A) and the device of FIG. 1 (FIG. 4B);

FIG. 5 shows a schematic block diagram of an acoustic resonator in accordance with one exemplary embodiment; and

FIG. 6 shows a schematic block diagram of a multiplexer in accordance with one exemplary embodiment.

DETAILED DESCRIPTION

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 construed 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. 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 apparatuses 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.

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.

Any positioning terminology, such as “left”, “right”, “top”, “bottom”, “above” “below”, “upper”, “lower”, “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 construed as any limitation of the invention.

Furthermore, although the numerative terminology, such as “first”, “second”, etc., may be used herein to describe various embodiments, elements or features, it should be understood that 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. For example, a first exemplary embodiment discussed below could be called a second exemplary embodiment, and vice versa, without departing from the teachings of the present disclosure.

Currently, Film Surface Acoustic Wave (FSAW) devices are widely used in different wireless communication applications (e.g., in RF filters, multiplexers, etc.). As a rule, an FSAW device has a multi-layered structure. The properties of the FSAW device depend on a piezoelectric material, a supporting substrate and other layers which are used in the multi-layered structure of the FSAW device. Preferably, piezoelectric materials with high K² are used, such as different crystal cuts of lithium tantalate ( LiTaOs), lithium niobate ( LiNbOs) or aluminum nitride (AIN). The supporting substrate is usually made of a high acoustic velocity supporting material, such as Si. However, the main disadvantage of using only such substrates in the multi-layered structure of the FSAW device is the natural build-up of higher order/out-of-band spurious modes. Furthermore, an additional layer (e.g., Si, silicon oxide (SiCF), etc.) is included in the multi-layered structure of the FSAW device for temperature compensation (TC) purposes. However, the Si (supporting substrate)/SiO2 (TC layer) interface is known to generate additional interface charges which can significantly degrade the RF performance of the FSAW devices. Optionally, a rough or poly-Si thin layer (e.g., lum thick) may be added to the multilayered structure of the FSAW device and used as the so-called “trap-rich” layer to prevent unnecessary RF coupling through the substrate which may lower the performance of the RF filter based on the FSAW device. However, the addition of the trap-rich layer significantly increases the whole manufacturing complexity and, therefore, limits the choice of alternative deposition technique s/optimization of the other layers composing the multi-layered stack.

The exemplary embodiments disclosed herein provide a technical solution that allows mitigating or even eliminating the above- sounded drawback peculiar to the prior art. In particular, the exemplary embodiments disclosed herein relate to a cubic Boron Arsenide (c- BAs)-based multi-layered device configured to produce an acoustic wave. More specifically, the device comprises a c-BAs-based substrate, a piezoelectric layer provided on the c-BAs- based substrate, and an Interdigital Transducer (IDT) provided on the piezoelectric layer. The IDT is configured to excite the acoustic wave in response to an applied RF signal. The device further comprises a temperature compensation (TC) layer either provided between the c-B As- based substrate and the piezoelectric layer or covering the piezoelectric layer and the IDT. By using the c-BAs-based substrate, it is possible to minimize the number of propagating spurious acoustic waves at different material interfaces in the device. Thus, the suppression or attenuation of the spurious acoustic wave modes is provided without having to use any external components (e.g., combinations of capacitors, inductors, transformers, etc.) or additional resonators in a RF circuit (e.g., RF filter or multiplexer) in which the device is to be used.

FIG. 1 shows a schematic cross-sectional view of a device 100 for producing an acoustic wave in accordance with a first exemplary embodiment. The device 100 is intended to be used in RF filers and multiplexers. As can be seen, the device 100 has a multi-layered structure that comprises a c-BAs-based substrate 102, a piezoelectric layer 104 provided on the c-BAs-based substrate 102, a TC layer 106 provided between the piezoelectric layer 104 and the c-BAs- based substrate 102, and an IDT formed on the surface of the piezoelectric layer 104.

For simplicity, only two metal trapezoidal electrodes (or, in other words, fingers) 108, 110 connected to separate electrical busbars of the IDT are shown in FIG. 1. Those skilled in the art would recognize that the IDT may comprise multiple (e.g., hundreds of) electrodes arranged in an interdigitated format on the surface of the piezoelectric layer 104 along an X direction. The electrodes of the IDT may be made of aluminum, an aluminum-copper-alloy, copper, a copper- aluminum-alloy, chromium, titanium, tungsten, gold, palladium, molybdenum, or any combination thereof. A pitch 112 between the electrodes 108 and 110 is selected to provide a desired wavelength of the acoustic wave excited by the IDT in response to an applied RF signal. The IDT is well-known in the art, for which reason the details of its operation are omitted herein. For example, the relationship between the wavelength and the pitch 112 may be expressed as follows: A = 2 X pitch, where A is the wavelength of a primary mode of the acoustic wave in the piezoelectric layer 104. In general, the shape of the electrodes and their pitch in the IDT is determined by the capabilities of the existing fabrication technologies, such as lithography (e.g., photolithography, electron-beam lithography), 3D-printing, Physical Vapor Deposition (PVD), Chemical Vapor Deposition (CVD), wet and dry etching, laser ablation, electrolytic deposition (e.g., electroplating), etc. The IDT may be configured to excite the acoustic wave in the form of a Surface Acoustic Wave (SAW), a Lamb wave or a shearhorizontal plate wave.

As for the c-BAs-based substrate 102, it may be relatively thick (e.g., its thickness may be defined based on the compactness, strength and heat dissipation requirements applied for the device 100 and/or the capabilities of the existing fabrication technologies). Preferably, the c- BAs-based substrate 102 has a crystallographic orientation <111>, <110>, or <100>. If the crystallographic orientation <111> is used, Euler angles are preferably defined as (45°, 54.74°, 0°) with some range around the angles (e.g., +/- 15 deg). c-BAs is a material which is now of particular interest in the semiconductor industry. It has a high ambipolar carrier mobility and simultaneously an ultrahigh thermal conductivity of 1300 W/(m K). The main advantage of c- BAs for the FSAW device applications is that it has a shear wave phase velocity higher than that of a typical piezoelectric material (LiNbOs and LiTaOs) that is arranged above it in the multi-layered structure of the device 100. This allows one to confine the acoustic wave in the above layer (waveguiding) of the device 100, minimize bulk radiation losses and, therefore, improve the performance of the device. Some parameters of c-BAs relevant to acoustic device applications are presented below in Table 1 in comparison with Si and Yttrium Aluminum Garnet (YAG) (in Table 1, V_Sh is the phase velocity, p is the material density, s_r is the relative permittivity, and CTE stands for the coefficient of thermal expansion). Furthermore, the c-B As- based substrate 102 may be made in a practical and economic way, since B, As are abundant minerals and the crystal fabrication technologies are inexpensive. In some embodiments, the c- BAs-based substrate 102 may be made n- and p-type doped.

Table 1: Comparison of parameters for Si, BAs, and YAG.

The piezoelectric layer 104 may be made of any one of the following piezoelectric materials: lithium tantalate (LiTaOs), lithium niobate (LiNbOs), aluminum nitride (AIN), scandium (Sc) or yttrium (Y) doped AIN (AlScxN, A1N:Y), lithium iodate (LilOs), zinc oxide (ZnO), lead zirconate titanate (PZT), potassium niobate (KNbO₃), sodium niobate (NaNbO₃), gadolinium doped cerium oxide (GdiCeOi), and quartz. Preferably, the piezoelectric layer 104 may be made of 0Y-X EiTaO₃, where 0 = (20°-65°) U (115°-135°), or 0Y-X LiNbO₃, where 0 = (0°-90°) U (115°— 135°). Those skilled in the art would recognize that the notation “0Y-X LiTaO₃” or “0Y-X LiNbO₃” means a 0-rotated (relative to a crystallographic X axis) Y-cut of LiTaO₃ or LiNbO₃, and the propagation direction of the excited acoustic wave aligns with the crystallographic X axis. The piezoelectric layer 104 may have a thickness defined based on the wavelength A of the primary mode of the acoustic wave.

The TC layer 106 may be made of a material having a shear acoustic impedance lower than that of the material of the piezoelectric layer 104 or the c-BAs-based substrate 102. For example, the TC layer 106 may be made of silicon oxide (S iO 2). An acoustic impedance may be defined as the opposition that a system presents to an acoustic flow resulting from an acoustic pressure applied to the system and may be calculated by multiplying a material density by a wave phase velocity of interest. The main function of the TC layer 106 is to provide an effective temperature compensation (TC) of the device 100. Changes in the ambient temperature, as well as the operation of the device 100 at high input power values may affect the temperature of the device 100 itself. A frequency stability as a function of temperature is described in terms of a temperature coefficient of frequency (TCF). The temperature drift of the device 100 is related to changes in the stiffness coefficients of a piezoelectric material as a function of temperature, but also to the thermal expansion that affects both the dimensions of the device 100 and the density of the piezoelectric material. Most materials become “softer” upon exposure to elevated temperatures, thereby causing a decrease in acoustic wave velocities (which corresponds to the negative first-order TCF), while other become “stiffer” and the acoustic wave velocities increase as the temperature increases (which corresponds to the positive first-order TCF). For example, LiNbOs, LiTaOs, and AIN have negative TCF values, while silicon oxide has a positive TCF value. By combining these materials together and carefully choosing their dimensions, the device 100 with a zero or near-zero TCF value may be designed. In addition to its main TC function, the TC layer 106 also serves to provide boding or adhesion between the piezoelectric layer 104 and the c-BAs-based substrate 102. Similarly, the TC layer 106 may have a thickness defined based on the wavelength 2. of the primary mode of the acoustic wave. In a preferred embodiment, the thickness of each of the piezoelectric layer 104 and the TC layer 106 falls within a range of 0.12. to 0.52..

Optionally, the device 100 may comprise an additional layer 114 of trap-rich material provided between the TC layer 106 and the c-BAs-based substrate 102. As used in the exemplary embodiments disclosed herein, a trap-rich layer may refer to a layer having a high density of electrically active carrier traps (e.g., the density of such traps may be selected depending on the properties of the other layers constituting the multi-layered structure of the device 100). The trap-rich material may be represented by any poly crystalline material, such as poly-Si and its doped versions. Again, the layer 114 of trap-rich material may have a thickness defined based on the wavelength 2. of the primary mode of the acoustic wave. In a preferred embodiment, the layer 114 of trap-rich material has a thickness falling within a range of 0. Ik to X.

Although not shown in FIG. 1, the device 100 may comprise an optional passivation layer covering the piezoelectric layer 104 and the IDT (i.e., the electrodes of the IDT and the interelectrode spacing). The passivation layer may be made of SiCh, silicon nitride (SiaN-i), or their combination. FIG. 2 shows a schematic cross-sectional view of a device 200 for producing an acoustic wave in accordance with a second exemplary embodiment. The device 200 is intended to be used in RF filers and multiplexers. Similar to the device 100, the device 200 has a multi-layered structure that comprises a c-BAs-based substrate 202, a piezoelectric layer 204 provided on the c-BAs-based substrate 202, a TC layer 206 provided between the piezoelectric layer 204 and the c-BAs-based substrate 202, and an IDT formed on the surface of the piezoelectric layer 104. For simplicity, FIG. 2 again shows only two metal trapezoidal electrodes or fingers 208, 110 of the IDT which are connected to separate electrical busbars of the IDT. In general, the piezoelectric layer 204, the TC layer 206 and the IDT of the device 200 may be configured in the same or similar manner as the piezoelectric layer 104, the TC layer 106 and the IDT of the device 100, respectively. Therefore, a pitch 212 between the electrodes 208, 210 may be defined in the same or similar manner as the pitch 112.

At the same time, the c-BAs-based substrate 202 differs from the c-BAs-based substrate 102 in that it is also a layered structure comprising an underlaying supporting layer 214 and a c-BAs layer 216. The underlaying supporting layer 214 may be made of quartz, diamond, fused silica, sapphire, yttrium aluminum garnet (YAG), silicon (Si), gallium nitride (GaN), gallium arsenide (GaAs), or silicon carbide (SiC). The c-BAs layer 216 may be a polycrystalline layer or may have a crystallographic orientation < 111 >, <110>, or <100>. The c-BAs layer 216 may have a thickness that is again defined based on the wavelength of a primary mode of the acoustic wave in the piezoelectric layer 204. In a preferred embodiment, the thickness of the c-BAs layer 216 falls within a range of 0.252 to 22, where 2 is the wavelength of the primary mode of the acoustic wave in the piezoelectric layer 204.

Optionally, the device 200 may comprise an additional layer 218 of trap-rich material provided between the TC layer 206 and the c-BAs-based substrate 202 (i.e., between the TC layer 206 and the c-BAs layer 216). The layer 218 of trap-rich material may have the same thickness as the layer 114 of trap-rich material.

Also, an optional passivation layer covering the piezoelectric layer 204 and the IDT (i.e., the electrodes of the IDT and the inter-electrode spacing) may be added to the multi-layered structure of the device 200. The optional passivation layer of the device 200 may be made in the same or similar manner as the optional passivation layer of the device 100.

FIG. 3 shows a schematic cross-sectional view of a device 300 for producing an acoustic wave in accordance with a third exemplary embodiment. The device 300 is intended to be used in RF filers and multiplexers. Similar to the device 100, the device 300 has a multi-layered structure that comprises a c-B As-based substrate 302, a piezoelectric layer 304 provided on the c-BAs- based substrate 302, an IDT formed on the surface of the piezoelectric layer 304, and a TC layer 306 covering the piezoelectric layer 304 and the IDT.

For simplicity, FIG. 3 again shows only two metal trapezoidal electrodes or fingers 308, 310 of the IDT which are connected to separate electrical busbars of the IDT. In general, the c-BAs- based substrate 302 and the IDT of the device 300 may be configured in the same or similar manner as the c-BAs-based substrate 102 and the IDT of the device 100, respectively. Therefore, a pitch 312 between the electrodes 308, 310 may be defined in the same or similar manner as the pitch 112. In another embodiment, the c-BAs-based substrate 302 may be implemented in the same or similar manner as the c-BAs-based substrate 202.

As for the piezoelectric layer 304 and the TC layer 306, they may be made of the same materials as the piezoelectric layer 104 and the TC layer 106, respectively, but their thicknesses preferably fall within the following different ranges: 0. 152 to 2 for the piezoelectric layer 304, and 0.152 to 0.52 for the TC layer 306. Furthermore, since the TC layer 306 covers the piezoelectric layer 304 and the TC layer 306, the TC layer 306 also serves as a passivation layer in the multi-layered structure of the device 300.

Optionally, the device 300 may comprise an additional layer 314 of trap-rich material provided between the piezoelectric layer 304 and the c-BAs-based substrate 302. The layer 314 of traprich material may be made in the same or similar manner as the layer 114 of trap-rich material.

It should be noted that some other embodiments are possible, in which any other substrate having an acoustic velocity from 4000 m/s to 5500 m/s and/or a shear acoustic impedance from 10 MRayl to 40 MRayl may be used instead of the c-BAs-based substrates 102, 202, or 302. In other words, it is possible to replace c-BAs with any material having a shear acoustic velocity and/or shear acoustic impedance falling within the above-indicated ranges.

FIGs. 4A and 4B show comparison results for the simulated admittance of a FSAW device based on a < 100> Si substrate (FIG. 4A) and the device 100 (FIG. 4B). In particular, the < 111 > c-BAs-based substrate 102 was used in the device 100. The simulated admittance is represented by two curves: ‘G’ relating to a conductance (i.e., the real part of the admittance) and ‘Y’ relating to an absolute admittance. The effect of choosing the suitable crystal orientation can be observed: spurious modes exist in the range between 2500 - 3000 MHz for the FSAW device based on the <100> Si substrate, while in the case of the device 100 with the <111> c-B As- based substrate they are effectively suppressed.

FIG. 5 shows a schematic block diagram of an acoustic resonator 500 in accordance with one exemplary embodiment. The acoustic resonator 500 comprises a device 502 for producing an acoustic wave and two (e.g., distributed) reflectors 504 and 506. The device 502 may be implemented as one of the devices 100, 200, 300. The reflectors 504 and 506 may be made as metal gratings. The reflectors 502 and 504 are formed such that the IDT of the device 502 is placed therebetween. More specifically, the reflectors 502 and 504 are arranged on the piezoelectric layer of the device 502 (i.e., on the piezoelectric layer 104, 204, or 304) along the propagation direction of the acoustic wave excited by the IDT (i.e., on the left and right sides of the IDT in FIG. 5). In this configuration, the acoustic wave excited by the IDT is repeatedly reflected by each of the reflectors 502 and 504 towards the IDT, thereby resulting in resonance generation. It should be noted that an RF filter may comprise one or more acoustic resonators 500 which may be series-connected and/or shunt-connected.

FIG. 6 shows a schematic block diagram of a multiplexer 600 in accordance with one exemplary embodiment. The multiplexer 600 comprises a circuit card 602, and an array 604 of five RF filters RF Fl - RF F5 each having one or more acoustic resonators 500. In one embodiment, each of the RF filters RF Fl - RF F5 may be the same. In other embodiments the RF filters RF Fl - RF F5 may be implemented differently (e.g., with a different number of acoustic resonators). The multiplexer 600 further comprises two impedance-matching components 606 (e.g., implemented as inductors and/or capacitors) which are arranged on the same surface of the circuit card 602 as the RF filters RF Fl - RF F5. One of the components 606 may be configured to provide impedance-matching between the RF filters RF Fl - RF F5 and an antenna to which each of the RF filters RF Fl - RF F5 is to be coupled, while another of the components 606 may be configured to provide the impedance-matching between the RF filters RF Fl - RF F5 and a Radio Frequency Integrated Circuit (RFIC) to which each of the RF filters RF Fl - RF F5 is to be coupled. It should be noted that the number of the RF filters and the impedance-matching components are shown in FIG 6 for illustrative purposes only and should not be construed as any limitation of the present disclosure.

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

1. A device for producing an acoustic wave, comprising: a cubic Boron Arsenide (c-BAs)-based substrate; a piezoelectric layer provided on the c-BAs-based substrate; an Interdigital Transducer (IDT) provided on the piezoelectric layer, the IDT being configured to excite the acoustic wave in response to an applied Radio Frequency (RF) signal; and a temperature compensation (TC) layer provided between the c-BAs-based substrate and the piezoelectric layer or covering the piezoelectric layer and the IDT.

2. The device of claim 1, wherein the c-BAs-based substrate has a crystallographic orientation < 111 >, <110>, or <100>.

3. The device of claim 1, wherein the c-BAs-based substrate comprises an underlaying supporting layer and a c-BAs layer provided on the underlaying supporting layer.

4. The device of claim 3, wherein the underlaying supporting layer is made of quartz, diamond, fused silica, sapphire, yttrium aluminum garnet (YAG), silicon (Si), gallium nitride (GaN), gallium arsenide (GaAs), or silicon carbide (SiC).

5. The device of claim 3 or 4, wherein the c-BAs layer is a polycrystalline layer or has a crystallographic orientation < 111 >, <110>, or <100>.

6. The device of any one of claims 3 to 5, wherein the c-BAs layer has a thickness defined based on a wavelength of a primary mode of the acoustic wave in the piezoelectric layer.

7. The device of claim 6, wherein the thickness of the c-BAs layer falls within a range of 0.252 to 22, where 2 is the wavelength of the primary mode of the acoustic wave in the piezoelectric layer.

8. The device of any one of claims 1 to 7, wherein the TC layer has a shear acoustic impedance lower than a shear acoustic impedance of the piezoelectric layer or the c- BAs-based substrate.

9. The device of any one of claims 1 to 8, wherein the TC layer is provided between the c- BAs-based substrate and the piezoelectric layer, and wherein the device further comprises a passivation layer covering the piezoelectric layer and the IDT.

10. The device of claim 9, wherein the passivation layer is made of a dielectric material comprising at least one of silicon oxide (SiC ) and silicon nitride (Sia i).

11. The device of any one of claims 1 to 10, wherein the acoustic wave is one of a Surface Acoustic Wave (SAW), a Lamb wave and a shear-horizontal plate wave.

12. The device of any one of claims 1 to 11, wherein the piezoelectric layer is made of lithium tantalate (LiTaOs), lithium niobate (LiNbOs), aluminum nitride (AIN), scandium (Sc) or yttrium (Y) doped AIN (AlScxN, A1N:Y), lithium iodate (LilCL), zinc oxide (ZnO), lead zirconate titanate (PZT), potassium niobate (KNbCL), sodium niobate (NaNbC ), gadolinium doped cerium oxide (GdiCeCL), or quartz.

13. The device of claim 12, wherein the piezoelectric layer is made of:

0Y-X LiTaO₃, where 0 = (20°-65°) U (115°-135°); or

0Y-X LiNbO₃, where 6 = (0°-90°) U (115°- 135°).

14. The device of any one of claims 1 to 13, wherein each of the piezoelectric layer and the TC layer has a thickness defined based on a wavelength of a primary mode of the acoustic wave in the piezoelectric layer.

15. The device of claim 14, wherein the TC layer is provided between the c-BAs-based substrate and the piezoelectric layer, and wherein the thickness of each of the piezoelectric layer and the TC layer falls within a range of 0.12 to 0.52, where 2 is the wavelength of the primary mode of the acoustic wave in the piezoelectric layer.

16. The device of claim 14, wherein the TC layer covers the piezoelectric layer and the IDT, and wherein the thickness of the piezoelectric layer falls within a range of 0.152 to 2, and the thickness of the TC layer falls within a range of 0.152 to 0.52, where 2 is the wavelength of the primary mode of the acoustic wave in the piezoelectric layer.

17. The device of any one of claims 1 to 16, wherein the TC layer is provided between the c-BAs-based substrate and the piezoelectric layer, and wherein the device further comprises a layer of trap-rich material provided between the c-BAs-based substrate and the TC layer.

18. The device of any one of claims 1 to 16, wherein the TC layer covers the piezoelectric layer and the IDT, and wherein the device further comprises a layer of trap-rich material provided between the c-BAs-based substrate and the piezoelectric layer.

19. The device of claim 17 or 18, wherein the layer of trap -rich material has a thickness falling within a range of 0.1 to , where is the wavelength of a primary mode of the acoustic wave in the piezoelectric layer.

20. A Radio Frequency (RF) filter comprising: at least one acoustic resonator each comprising: at least one device according to any one of claims 1 to 19; and two reflectors arranged such that the IDT of each of the at least one device is between the two reflectors.

21. A multiplexer comprising: a circuit card; at least two RF filters according to claim 20, each of the at least two RF filters being mounted on the circuit card; at least one first impedance- matching component arranged on the circuit card and configured to provide impedance- matching between the at least two RF filters and an antenna to which each of the at least two RF filters is to be coupled; and at least one second impedance-matching component arranged on the circuit card and configured to provide the impedance-matching between the at least two RF filters and a Radio Frequency Integrated Circuit (RFIC) to which each of the at least two RF filters is to be coupled.