TECHNICAL FIELD

The present disclosure relates generally to the field of Surface Acoustic Wave (SAW) devices. In particular, the present disclosure relates to an Yttrium Aluminum Garnet (YAG)-based multilayered SAW device, as well as to a Radio Frequency (RF) filter and a multiplexer using one or more such YAG-based multi-layered SAW devices.

BACKGROUND

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). The SAW resonators typically use <100> or <111> Si substrates. 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 between the opposing faces of 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.

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 high-frequency spurious mode suppression in SAW devices, thereby allowing the SAW devices to be efficiently used in SAW resonators of RF filters and multiplexers. 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 SAW device is provided. The SAW device comprises an YAG substrate having a top surface and a back surface, a dielectric layer provided on the top surface of the YAG substrate, a piezoelectric layer provided on the dielectric layer, and an Interdigital Transducer (IDT) provided on the piezoelectric layer. The IDT is configured to convert an input RF signal to a SAW propagating in the piezoelectric layer. The back surface of the YAG substrate has an arithmetic mean roughness Ra more than 50 nanometers (nm). By using this YAG substrate (i.e., with such Ra) in the SAW device, it is possible to provide efficient spurious mode suppression within a frequency range from 0.5 GHz to 2.7 GHz. It should be also noted that, compared to the existing SAW devices (e.g., SAW resonators) based on <100> Si substrates, the SAW device based on the YAG substrate may be especially beneficial (in terms of the spurious mode suppression) in a frequency sub-range from 0.5 GHz to 1.8 GHz.

In one exemplary embodiment of the first aspect, the top surface of the YAG substrate has an arithmetic mean roughness Ra less than 50 nm. By providing such Ra of the top surface of the YAG substrate, the spurious mode suppression may be performed more efficiently.

In one exemplary embodiment of the first aspect, the YAG substrate has a thickness ranging from 200 micrometers (pm) to 600 pm. Preferably, the thickness of the YAG substrate is 350 pm ± 100 nm. With such a thickness of the YAG substrate, the SAW device 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.

In one exemplary embodiment of the first aspect, a ratio of a thickness of the piezoelectric layer to a thickness of the dielectric layer ranges from 0.6 to 1 .4. Preferably, this ratio ranges from 0.7 to 0.85. Such thicknesses of the piezoelectric and dielectric layers additionally make the SAW device more compact in size, which makes it suitable for use in different RF filters and/or multiplexers.

In one exemplary embodiment of the first aspect, the piezoelectric layer is made of LiTaOs or LiNbOa. These materials have piezoelectric properties suitable for the proper operation of the SAW device. In one exemplary embodiment of the first aspect, the dielectric layer is made of SiC>2. This material has dielectric properties suitable for the proper operation of the SAW device. Moreover, SiC>2 has an opposite Temperature Coefficient of Elasticity (TCE) compared to LiTaO3 and LiNbO3, which provides efficient temperature compensation of the SAW device.

According to a second aspect, a RF filter is provided, which comprises at least one SAW resonator. Each of the at least SAW resonator comprises at least one SAW device according to the first aspect, as well as two SAW reflectors arranged such that the IDT of each of the at least one SAW device is between the two SAW reflectors. The RF filter thus configured may be co-integrated with one or more (similar or other) RF filters within a multiplexer in a singledie or multi-die fashion.

In one exemplary embodiment of the second aspect, the at least one SAW resonator comprises a plurality of SAW resonators each comprising the single SAW device according to the first aspect. In this embodiment, the plurality of SAW resonators is arranged in a ladder scheme. By using the ladder scheme, it is possible to electrically connect the (one-port) SAW resonators both in series and in shunt within the RF filter. The ladder scheme also provides a close-packed arrangement of the SAW resonators.

In one exemplary embodiment of the second aspect, the RF filter comprises, in addition to the plurality of SAW resonators, an additional SAW resonator which comprises at least two SAW devices according to the first aspect and two reflectors arranged such that the IDT of each of the at least two SAW devices is between the two reflectors of the additional SAW resonator. In this embodiment, the RF filter is composed of two filtering sections: one with the (one-port) SAW resonators arranged in the accordance with the ladder scheme and another with the (multi-port) SAW resonator comprising the acoustically coupled IDTs of the two or more SAW devices according to the first aspect. In other words, the former corresponds a ladder SAW filter structure, and the latter corresponds to a Coupled Resonator Filter (CRF) structure. The combination of these filter structures within one RF filter may be beneficial in some RF applications.

In one exemplary embodiment of the second aspect, the at least one SAW resonator comprises, as an alternative to the plurality of SAW resonators, a single SAW resonator comprising at least two SAW devices according to the first aspect. In this embodiment, the RF filter is configured only as a CRF. According to a third aspect, a multiplexer is provided. The multiplexer comprises a circuit card, and at least two RF filters according to the second aspect. Each of the at least two RF filters is mounted on the circuit card in an inverted manner such that the IDT of each of the at least one SAW device in each of the at least one SAW resonator faces the circuit card. The multiplexer further comprises at least one 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. By using a combination of two or more (similar or different) RF filters, the multiplexer may provide efficient RF filtering in different high-frequency bands.

In one exemplary embodiment of the third aspect, the circuit card comprises a plurality of metal interlayers with mutual connections. The mutual connections may ensure proper signal routing between different parts of the circuit card. Thus, they may insure proper (low-parasitic) connections between the RF filters themselves, as well as between the impedance-matching component(s) and the RF filters.

In one exemplary embodiment of the third aspect, the at least one impedance-matching component comprises at least one of an inductor and a capacitor. These types of the impedance-matching components may provide proper impedance-matching between the antenna and the RF filters.

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 Si-based SAW device in accordance with the prior art;

FIG. 2 shows a schematic cross-sectional view of an YAG-based SAW device in accordance with one exemplary embodiment;

FIG. 3 shows a schematic top view of a SAW resonator in accordance with one exemplary embodiment;

FIG. 4 shows a schematic block diagram of an RF filter in accordance with one exemplary embodiment;

FIG. 5 shows one possible implementation of the RF filter of FIG. 4 on a single die in accordance with one exemplary embodiment; FIG. 6 shows a flowchart of a method for fabricating the RF filter of FIG. 4 on the single die in accordance with one exemplary embodiment;

FIGs. 7A and 7B show different schematic views of a multiplexer in accordance with one exemplary embodiment, namely: FIG. 7A shows an isometric view of the multiplexer, while FIG. 7B shows a cross-sectional view of the multiplexer, as taken along line A-A in FIG. 7A;

FIG. 8 shows a flowchart of a method for fabricating the multiplexer of FIG. 7 in accordance with one exemplary embodiment;

FIG. 9 shows a schematic block diagram of a diplexer in accordance with one exemplary embodiment; and

FIG. 10 shows dependences of a transmission coefficient S21 on frequency for two diplexers, namely: one with YAG-based RF filters and another with Si-based RF filters.

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.

As used in the embodiments disclosed herein, a Surface Acoustic Wave (SAW) device may refer to a multi-layered structure that contains metal electrodes which are arranged in an interdigitated format on the surface of a piezoelectric layer or substrate, 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 layer or substrate. 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 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.

FIG. 1 shows a schematic cross-sectional view of a Si-based SAW device 100 in accordance with the prior art. The Si-based SAW device 100 is a multi-layered structure that comprises a <100> Si substrate 102, a SiC>2 layer 104 provided on the substrate 102, a LiTaCh layer 106 provided on the SiC>2 layer 104, and an IDT formed on the surface of the LiTaCh layer 106. For simplicity, only two metal (e.g., Al, Au, or Cu) trapezoidal electrodes (or, in other words, fingers) 108 and 110 connected to separate electrical busbars of the IDT are shown in FIG. 1. It is implied that the electrodes 108 and 110 are periodically repeated on the surface of the LiTaOs layer 106, so that there are hundreds of such electrodes along an X direction. LiTaOs is known to have excellent piezoelectric properties, so that an input RF signal applied to the IDT will induce a SAW propagating in the LiTaOs layer 106. Preferably, LiTaOs has the following properties: GY-X LiTaOs, which means a ©-rotated (relative to a crystallographic X axis) Y-cut, where 0 = 20° - 65°, and the SAW propagation direction aligns with the crystallographic X axis. Thicknesses t of the SiO2 layer 104 and the LiTaOs layer 106 may be selected based on an inter-electrode spacing (the so-called “pitch”) 112 or a SAW wavelength A to be obtained. For example, the following relationships may be used: A = 2 x pitch, tsio2 = 0.1 A -0.6 A, and tuTaO3 = 0.1 A -O.6 A. The Si-based SAW device 100 may be used in RF filers and multiplexers; however, it does not provide efficient suppression of spurious modes that may adversely affect the operation of the RF filters and multiplexers. 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 SAW device with high- frequency spurious mode suppression, which may be efficiently used (either alone or together with similar or other SAW devices) in RF filters and multiplexers. Unlike the existing SAW devices (like the SAW device 100), the SAW device uses an Yttrium Aluminum Garnet (YAG) substrate rather than a Si substrate. Dielectric and piezoelectric layers are successively provided on the top surface of the YAG substrate, and an IDT is provided on the piezoelectric layer. The YAG substrate is characterized in that its back surface has an arithmetic mean roughness Ra more than 50 nm. By using this YAG substrate (i.e. , with such Ra) in the SAW device, it is possible to provide efficient spurious mode suppression (at least) within a frequency range from 0.5 GHz to 2.7 GHz.

FIG. 2 shows a schematic cross-sectional view of an YAG-based SAW device 200 in accordance with one exemplary embodiment. The YAG-based SAW device 200 is a multilayered structure that, unlike the Si-based SAW device 100, has a substrate 202 made of YAG rather than Si. The YAG substrate 202 may have a thickness ranging from 200 pm to 600 pm (preferably its thickness is 350 pm ± 100 nm). The YAG substrate 202 has a top surface 204 and a back surface 206. The back surface 206 is assumed to have Ra more than 50 nm, while the top surface 204 may optionally have Ra less than 50 nm. The YAG-based SAW device 200 further comprises a dielectric layer 208 provided on the top surface 204 of the YAG substrate 202, and a piezoelectric layer 210 provided on the dielectric layer 208. The dielectric and piezoelectric layers 208 and 210 may be implemented similar to the dielectric and piezoelectric layers 104 and 106, respectively. That is, the dielectric layer 208 may be made of SiO2 and have a thickness of 0.1 A -0.6 A, and the piezoelectric layer 210 may be made of LiTaOs and have a thickness of 0.1 A -O.6 A. At the same time, those skilled in the art would recognize that the present disclosure is not limited to these materials and thicknesses, and any other materials and thickness of the layers constituting the SAW device 200 may be used depending on particular applications. For example, the piezoelectric layer 210 may be also made of LiNbOa, and the thicknesses of the piezoelectric layer 210 and the dielectric layer 208 may be selected such that their ratio, i.e., tuTaos/tsice, ranges from 0.6 to 1.4, preferably from 0.7 to 0.85.

As also follows from FIG. 2, the YAG-based SAW device 200 further comprises an IDT formed on the piezoelectric layer 210; for simplicity, only two trapezoidal electrodes (or IDT fingers) 212 and 214 connected to separate electrical busbars of the IDT are shown in FIG. 2. Each electrode of the IDT may be made of a thin layer of electrode material (e.g., Al or Cu) which is deposited on the piezoelectric layer 210 via a thin adhesion Ti or Cr layer. The electrodes 212 and 214 are spaced from each other by a pitch 216 which may be defined similar to the pitch 112. The layers and electrodes included in the YAG-based SAW device 200 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.

FIG. 3 shows a schematic top view of a SAW resonator 300 in accordance with one exemplary embodiment. The SAW resonator 300 comprises an YAG-based SAW device 302 and two (e.g., distributed) reflectors 304 and 306. The YAG-based SAW device 302 may be implemented similar to the YAG-based SAW device 200. The reflectors 304 and 306 may be made as metal gratings. The reflectors 302 and 304 are formed such that an IDT 308 of the YAG-based SAW device 302 is placed therebetween. More specifically, the reflectors 304 and 306 are arranged on the piezoelectric layer of the YAG-based SAW device 302 along the propagation direction of a SAW excited by the IDT 308 (i.e., on the left and right sides of the IDT 308 in FIG. 3). In this configuration, the SAW excited by the IDT 308 is reflected by each of the reflectors 304 and 306 towards the IDT 308, thereby resulting in energy storage in the vicinity of the acoustic resonance. The SAW resonator 300 is also referred to as a one-port SAW resonator in the art.

It should be noted that the present disclosure is not limited to the single-IDT design of the SAW resonator 300, as shown in FIG. 3. In some other embodiments, the SAW resonator 300 may comprise two or more YAG-based SAW devices (each like the YAG-based SAW device 200) which are arranged such that the IDT of each of the YAG-based SAW devices is between the reflectors 304 and 306. In this case, there is an array of two or more acoustically coupled IDTs placed between the reflectors 304 and 306, which may be used to form the so-called Coupled Resonator Filter (CRF) structure.

FIG. 4 shows a schematic block diagram of an RF filter 400 in accordance with one exemplary embodiment. The RF filter 400 comprises a plurality of SAW resonators, each of which is assumed to be implemented as the (one-port) SAW resonator 300. The plurality of SAW resonators may be conveniently divided into two sub-pluralities: one with series-connected SAW resonators S1-S5, and another with shunt-connected SAW resonators P1-P6. In the configuration shown in FIG. 4, the plurality of SAW resonators is arranged in accordance with a ladder scheme, for which reason the RF filter 400 may be also referred to as a ladder filter. It should be apparent to those skilled in the art that the number, arrangement and configuration of the SAW resonators S1-S5, P1-P6 are illustrative only and should not be construed as any limitation of the present disclosure. In some other embodiments, the RF filter 400 may comprise SAW resonators of different types; for example, the RF filter 400 may comprise one or more SAW resonators each implemented as the SAW resonator 300 and one or more other SAW resonators each implemented as the above-discussed CRF structure (i.e., with two or more IDTS between the reflectors 304 and 306).

FIG. 5 shows one possible implementation of the RF filter 400 on a single die 500 in accordance with one exemplary embodiment. As shown in FIG. 5, the SAW resonators S1-S5, P1-P6 may be closely packed on the surface of the die 500 together with contact pads 502 and interconnecting pads 504. The contact pads 502 are electrical lines between the SAW resonators towards the input and output of the RF filter 400. An extra metal layer (up to 2 pm of Al or Au) is usually deposited over the contact pads 502 and the interconnecting pads 504 to reduce parasitic resistance.

FIG. 6 shows a flowchart of a method 600 for fabricating the RF filter 400 on the single die 500 in accordance with one exemplary embodiment. The method 600 starts with a step S602, in which a YAG-based multi-layered substrate is fabricated. Referring to FIG. 2, the step S602 consists in depositing the dielectric layer 208 first and then the piezoelectric layer 210 on the YAG substrate 202. Then, the method 600 goes on to a step S604, in which the SAW resonators S1-S5, P1-P6 are formed on the YAG-based multi-layered substrate. In other words, the step 604 involves forming the IDTs and the reflectors on the surface of the YAG- based multi-layered substrate. The step 604 also involves forming the contact pads 502 and the interconnecting pads 504 between the SAW resonators S1-S5, P1-P6. The thickness of the fingers of the IDTs and the reflectors may be less than 500 nm. The method 600 ends up with a step S606, in which a relatively thick metal layer is deposited over the contact pads 502 and the interconnecting pads 504 to minimize parasitic resistance. The thickness of the contact pads 502 and the interconnecting pads 504 may range from 500 nm to 5 pm.

FIGs. 7A and 7B show different schematic views of a multiplexer 700 in accordance with one exemplary embodiment. More specifically, FIG. 7A shows an isometric view of the multiplexer 700, while FIG. 7B shows a cross-sectional view of the multiplexer 700, as taken along line A- A in FIG. 7A. The multiplexer 700 comprises a circuit card 702, and an array 704 of five RF filters RF F1 - RF F5. In one embodiment, each of the RF filters RF F1 - RF F5 may be implemented as the RF filter 400. In other embodiments, some of the RF filters RF F1 - RF F5 may be implemented as the RF filter 400, while others may be implemented as RF filters based on the existing SAW devices (like the SAW device 100). Irrespective of their implementation, the RF filters RF F1 - RF F5 are mounted on the circuit card 702 in an inverted manner such that the IDT of the SAW device(s) included in each RF filter faces the circuit card 702 (see FIG. 7B). This may be done by using the standard flip-chip technology. The multiplexer 700 further comprises two impedance-matching components 706 (e.g., implemented as inductors and/or capacitors), which are arranged on the circuit card 702 and configured to provide impedance-matching between the RF filters RF F1 - RF F5 and an antenna to which each of the RF filters RF F1 - RF F5 is to be coupled. As shown in FIG. 7B, the circuit card 702 may optionally comprise a plurality of metal interlayers 708 with mutual connections, as well as contact pads 710 on its back surface. The mutual connections may ensure proper signal routing between different parts of the circuit card 702.

FIG. 8 shows a flowchart of a method 800 for fabricating the multiplexer 700 in accordance with one exemplary embodiment. The method 800 starts with steps S802-S806, which may be performed in parallel or in sequence. The step S802 consists in fabricating the RF filters RF F1 - RF F5 on the YAG-based substrate. The step S804 is optional and may be performed if there should be at least one RF filter based on the existing Si-based SAW device (like the SAW device 100). The step S806 consists in fabricating the circuit card 702 (optionally with the metal interlayers 708). Then, the method 800 proceeds to a step S808, in which the RF filters RF F1 - RF F5 and the impedance-matching components 706 are mounted on the circuit card 702, thereby forming the multiplexer 700. The method 800 ends up with a step S810, in which the resulting multiplexer 700 is packaged.

FIG. 9 shows a schematic block diagram of a diplexer 900 in accordance with one exemplary embodiment. The diplexer 900 comprises two ladder RF filters 902 and 904, each of which is assumed to be implemented as the above-described YAG-based RF filter (i.e., the RF filter 400 in which each SAW resonator uses the YAG-based SAW device(s) 200 only). The RF filter 902 is configured to operate at lower frequencies (e.g., band B75), while the RF filter 904 is configured to operate at higher frequencies (e.g., band B1). An input RF signal is fed to port 1 , filtered by the RF filters 902 and 904, and outputted via port 2 and port 3, respectively.

To demonstrate the advantage of the YAG-based RF filters over the conventional Si-based RF filters (i.e., the RF filters in which each SAW resonator uses the Si-based SAW device(s) 100 only), two diplexers have been fabricated, with one using the Si-based RF filters and another using the YAG-based RF filters. Both diplexers have a design like the one shown in FIG. 9. FIG. 10 shows dependences of a transmission coefficient S21 on frequency for each of the above-mentioned two diplexers. More specifically, in FIG. 10, the dotted curve relates to the diplexer with the <100> Si-based RF filters, and the solid curve relates to the diplexer with the YAG-based RF filters. As can be seen, the use of the YAG-based RF filters at the lower frequencies (i.e. , in band B75) may allow one to mitigate the influence of the spurious mode effects which in a better way as compared to the Si-based RF filters.

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.