Acoustic wave devices with embedded electrodes

Abstract

The acoustic wave device (400) includes a piezoelectric layer (206) and an interdigital electrode structure (412) over the piezoelectric layer (206). The interdigital electrode structure (412) includes a plurality of electrode fingers (410) that includes one or more first electrode fingers and one or more second electrode fingers extending in a width direction, the one or more first electrode fingers and the one or more second electrode fingers interleaved with one another along a length direction, the length direction being different from the width direction. At least one electrode finger of the plurality of electrode fingers comprises an electrode portion (410a) is at least partially embedded in the piezoelectric layer.

Description

Attorney Docket No. QID241599 / 62306.187WO01ACOUSTIC WAVE DEVICES WITH EMBEDDED ELECTRODESRELATED PATENT APPLICATIONS

[0001] The present application claims the benefit of U.S. Provisional Application No. 63 / 717,171, filed November 06, 2024, which is incorporated herein by reference in its entirety FIELD OF THE DISCLOSURE

[0002] The present disclosure relates to surface acoustic wave resonators, in particular, to an acoustic wave device with embedded electrodes.BACKGROUND

[0003] Acoustic wave devices are widely used in modern electronics. At a high level, an acoustic wave device often includes a piezoelectric material in contact with one or more electrodes. Piezoelectric materials acquire a charge when compressed, twisted, or distorted, and similarly compress, twist, or distort when a charge is applied to them. Accordingly, when an alternating electrical signal is applied to the one or more electrodes in contact with the piezoelectric material, a corresponding mechanical signal (i.e., an oscillation or vibration) is transduced therein. Based on the characteristics of the one or more electrodes on the piezoelectric material, the properties of the piezoelectric material, and other factors such as the shape of the acoustic wave device and other structures provided on the device, the mechanical signal transduced in the piezoelectric material exhibits a frequency dependence on the alternating electrical signal. Acoustic wave devices leverage this frequency dependence to provide certain functions.

[0004] As an example of acoustic wave devices, surface acoustic wave (SAW) resonators are increasingly used to form filters used in the transmission and reception of radio frequency (RF) signals for communication. Due to the stringent demands placed on filters for modem RF communications systems, acoustic wave devices for these applications are often desired to provide high quality factor, wide bandwidth (i.e., high electromechanical coupling coefficient), and are small in size.SUMMARY

[0005] Embodiments of the present disclosure provide an acoustic wave device. The acoustic wave device includes a piezoelectric layer and an interdigital electrode structure over the piezoelectric layer. The interdigital electrode structure includes a plurality of electrode fingers that includes one or more first electrode fingers and one or more second electrodeAttorney Docket No. QID241599 / 62306.187WO01 fingers extending in a width direction, the one or more first electrode fingers and the one or more second electrode fingers interleaved with one another along a length direction, the length direction being different from the width direction. At least one electrode finger of the plurality of electrode fingers comprises an electrode portion is at least partially embedded in the piezoelectric layer.

[0006] In some embodiments, the electrode portion comprises tungsten, platinum, iridium, ruthenium, molybdenum or a combination thereof.

[0007] In some embodiments, a thickness of the electrode portion is: equal to or greater than a quarter of a thickness of the piezoelectric layer; equal to or greater than half a thickness of the piezoelectric layer; or equal to or greater than half a width of the electrode finger.

[0008] In some embodiments, a duty factor of the plurality of electrode fingers is lower than about 40% or higher than about 60%.

[0009] In some embodiments, the at least one electrode finger of the plurality of electrode fingers further includes another electrode portion over the electrode portion.

[0010] In some embodiments, the other electrode portion includes aluminum, copper, or a combination thereof.

[0011] In some embodiments, the other electrode portion is at least partially above the piezoelectric layer.

[0012] In some embodiments, the other electrode portion is surrounded by the electrode portion the length direction and a vertical direction perpendicular to the width direction and the length direction.

[0013] In some embodiments, the interdigital electrode structure further includes an edge region, a first duty factor of the edge region being different from a second duty factor of a rest of the interdigital electrode structure.

[0014] In some embodiments, the electrode portion extends in an aperture of the interdigital electrode structure, and discontinues by a gap between the aperture and an opposing busbar.

[0015] In some embodiments, the electrode portion further extends to another gap between the aperture and a busbar from which the corresponding electrode finger extends from.Attorney Docket No. QID241599 / 62306.187WO01

[0016] In some embodiments, the electrode portion includes a first part in the aperture and a second part in the other gap, a thickness of the second part being less than that of the first part.

[0017] In some embodiments, the edge region is a slow region, and includes an added material that comprises at least one of a metal or a dielectric material.

[0018] In some embodiments, the piezoelectric layer comprises lithium tantalate with an orientation between about Y and about (Y+50°).

[0019] In some embodiments, the orientation is between about (Y+20°) and about (Y+40°).

[0020] In some embodiments, the orientation is between about (Y+25°) and about (Y+35°).

[0021] In some embodiments, the piezoelectric layer includes lithium niobate.

[0022] In some embodiments, the lithium niobate has an orientation between about (Y-10°) and about (Y+50°).

[0023] In some embodiments, the acoustic wave device further includes a cover layer in contact with the piezoelectric layer and the interdigital electrode structure, wherein the cover layer comprises a dielectric layer.

[0024] In some embodiments, a thickness of the cover layer is equal to or greater than twice a period of the interdigital electrode structure.

[0025] Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0026] The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description, serve to explain the principles of the disclosure.

[0027] FIG. 1 shows a top view of certain elements in an exemplary SAW structure, according to some embodiments of the present disclosure.

[0028] FIG. 2A shows a cross-sectional view of certain elements in an exemplary SAW structure, according to some embodiments of the present disclosure.

[0029] FIGS. 2B and 2C show simulation results of the SAW structure of FIG. 2A, according to some embodiments of the present disclosure.Attorney Docket No. QID241599 / 62306.187WO01

[0030] FIG. 3A shows a cross-sectional view of certain elements in another exemplary SAW structure, according to some embodiments of the present disclosure.

[0031] FIGS. 3B and 3C show simulation results of the SAW structure of FIG. 3A, according to some embodiments of the present disclosure.

[0032] FIG. 4A shows a cross-sectional view of certain elements in another exemplary SAW structure, according to some embodiments of the present disclosure.

[0033] FIGS. 4B-4D show simulation results of the SAW structure of FIG. 4A, according to some embodiments of the present disclosure.

[0034] FIGS. 5 A and 5B each illustrates an exemplary SAW structure in piston mode, according to some embodiments of the present disclosure.

[0035] FIG. 5C shows simulation results of the SAW structure of FIG. 5A, according to some embodiments of the present disclosure.

[0036] FIG. 6 shows a cross-sectional view of certain elements in another exemplary SAW structure, according to some embodiments of the present disclosure.

[0037] FIG. 7 shows a three-dimensional crystal orientation space used in some embodiments of the present disclosure.DETAILED DESCRIPTION

[0038] The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.

[0039] It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms may only be used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present disclosure. As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items.

[0040] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, theAttorney Docket No. QID241599 / 62306.187WO01 singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and / or “including” when used herein may specify the presence of stated features, integers, steps, operations, elements, and / or components, but may not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0041] Further, when a number or a range of numbers may be described with “about,” “approximate,” and the like, the term may be intended to encompass numbers that are within a reasonable range considering variations that inherently arise during manufacturing as understood by one of ordinary skill in the art. For example, the number or range of numbers may encompass a reasonable range including the number described, such as within + / - 10% of the number described, based on known manufacturing tolerances associated with manufacturing a feature having a characteristic associated with the number. For example, a material layer having a thickness of “about 5 nm” can encompass a dimension range from 4.25 nm to 5.75 nm where manufacturing tolerances associated with depositing the material layer are known to be + / — 15% by one of ordinary skill in the art. Still further, the present disclosure may repeat reference numerals and / or letters in the various examples. This repetition may be for the purpose of simplicity and clarity and may not in itself dictate a relationship between the various embodiments and / or configurations discussed.

[0042] Unless otherwise defined, all terms (including technical and scientific terms) used herein may have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. Additionally, like reference numerals may denote like features throughout specification and drawings.

[0043] In the present disclosure, a top surface may refer to a surface of a layer that is at least substantially parallel to and facing away from a substrate, and a bottom surface may refer to a surface of a layer that is at least substantially parallel to and facing the substrate. A side surface or a sidewall may refer to a surface that is at least substantially perpendicular to the substrate.

[0044] SAW resonators with layered substrates exhibit good quality factors and good temperature coefficient of frequency (TCF). Layered substrates often include a layer of lithium tantalate. The substrate is often silicon, but it can be other materials like for example, quartz,Attorney Docket No. QID241599 / 62306.187WO01 sapphire or silicon carbide. These devices may have good performances, but they are relatively large and have a coupling coefficient of about 10%. If the piezoelectric layer is lithium niobate, then the coupling coefficient is larger, but the device size can still be undesirably large. Lithium niobate also has a TCF which is often too high. It is possible to reduce the size by using IDTs with heavy electrodes. In this case, the size is reduced because the SAW velocity is reduced, but the coupling is not increased to a desired level, and it can be difficult to fabricate devices at higher frequencies. It can also be difficult to obtain good quality coefficients Q.

[0045] An existing SAW resonator often has electrodes deposited on the surface of a piezoelectric material to form an interdigitated transducer (IDT). When a voltage is applied between the busbars of the IDT, acoustic waves are generated and propagate at the surface of the substrate. The IDT is often disposed between two reflective gratings, creating a resonance. The equivalent circuit of a resonator can be a static capacitance in parallel with an inductorcapacitor (LC) series resonant circuit. The resonance frequency is defined as the frequency for which the admittance is maximum (e.g., the resonator is almost equivalent to a short circuit) and the antiresonance frequency is the frequency of maximum impedance (i.e., for which the resonator is close to an open circuit).

[0046] Resonators are often employed in ladder filters. The series and shunt resonators in a ladder filter have a different resonant frequency. Typically, the series resonators resonance frequency is close to the shunt resonators antiresonance frequency. At this frequency, the series resonators behave like short circuits while the shunt resonators behave like open circuits, meaning that the source and load impedance are directly connected, and the insertion loss is minimum. The transmission has zeros at the resonance frequency of the shunt resonators (below the passband) and at the antiresonance frequency of the series resonators (above the passband). The separation between the resonance frequency and the antiresonance frequency is characterized by the coupling factor. Large coupling factors correspond to large frequency separations and resonators with large coupling factors are often desired to design wide band filters. A large capacitance density can allow for smaller filter sizes.

[0047] Depending on the used acoustic mode, the SAW velocity may be larger than the velocity of some bulk acoustic mode in the substrate. This is often the case when a shear horizontal mode is used. For example, when the substrate is lithium tantalate, the shear vertical mode velocity is lower than the SAW velocity. This results in acoustic radiation in the bulk and ultimately in propagation losses and undesirable quality factors. It is known that it is possible to increase the quality factor of SAW resonators by using layered substrates. In this case, a piezoelectric layer is present on the surface of the substrate. One or more intermediateAttorney Docket No. QID241599 / 62306.187WO01 layers can be added between the substrate and the piezoelectric layer. If the bulk acoustic velocities in the substrate are larger than the surface wave velocity, then the acoustic mode is guided (mostly in the piezoelectric layer) and no bulk radiation loss is possible. In some embodiments, the piezoelectric layers include lithium tantalate and / or lithium niobate. In some embodiments, the substrates include silicon, silicon carbide, sapphire and / or quartz. Often, a silicon oxide layer is disposed between the substrate and the piezoelectric layer. The oxide layer results in a better TCF, a larger coupling factor, and a smaller capacitance.

[0048] Coupling factor can be increased by embedding electrodes in a piezoelectric substrate. For example, it is possible to obtain a coupling factor in the 40% range by using embedded copper electrodes in (Y+4°) lithium niobate, as described in by Kadota et al., (M. Kadota, T. Kimura, and Y. Ida, “Ultra wide band resonator composed of grooved Cu-electrode on LiNbOs and its application to tunable filter,” in 2009 IEEE International Ultrasonics Symposium, Sep. 2009, pp. 2668-2671). Kimura et al., (T. Kimura, M. Kadota, and Y. Ida, “High Q SAW resonator using upper-electrodes on grooved-electrodes in LiTaO ,” in Microwave Symposium Digest (MTT), 2010 IEEE MTT-S International, May 2010, pp. 1740- 1743) shows results obtained by using electrode partially embedded in (Y+42°) lithium tantalate. In this case, the embedded part of the electrode is copper and is 0.01 lambda / Z (i.e., the wavelength of the acoustic wave) deep while the upper part of the electrode is aluminum with a thickness of 0.08 lambda / Z. Electrode embedded resonators can be used to enhance the excitation of the 3rdor 5thharmonic, as shown by Clairet et al., (A. Clairet et al., “Electrode Confined Acoustic Wave (ECAW) devices for Ultra High Band applications,” in 2023 IEEE International Ultrasonics Symposium (1US), Montreal, QC, Canada: IEEE, Sep. 2023, pp. 1- 6) and Kadota et al., (M. Kadota, T. Kojima, and S. Tanaka, “2-8 GHz Range High Harmonic SAW Resonator with Grooved Electrodes in LiNbOi,” in 2021 IEEE International Ultrasonics Symposium (IUS), Sep. 2021, pp. 1-4). Both papers use aluminum electrodes and show embedded electrodes in a layered substrate.

[0049] Embodiments of the present disclosure provide SAW structures of desirably high coupling, and reduced sizes. The quality factors of the SAW structures are desirably high while using lithium tantalate layered (LRT type) substate. The SAW structures may include an interdigital electrode structure that has electrode fingers at least partially embedded in the piezoelectric layer of a SAW structure. In some embodiments, the electrode fingers are fully embedded in the piezoelectric layer. The electrode fingers may include a heavy metal, such as tungsten and / or copper, with improved electromechanical coupling. In some embodiments, the electrode fingers may also include a highly conductive metal such as aluminum to improveAttorney Docket No. QID241599 / 62306.187WO01 electrical conductivity. The embedded interdigital electrode structure may effectively reduce the size of the SAW structure. The use of a combination of a heavy metal and a highly conductive metal can achieve desirably high electromechanical coupling without sacrificing electrical conductivity.

[0050] In the present disclosure, a SAW structure (e.g., a SAW resonator) on a layered substrate with partially embedded electrodes, is provided. The duty factor of the SAW structure may be lower than 40% or higher than 60%. In some embodiments, the part / depth of the electrodes embedded in the layered substrate is more than a quarter of the thickness of the piezoelectric layer. In some embodiments, the part / depth of the electrodes embedded is more than a quarter of their width. In some embodiments, the part of the electrode embedded in the layered substrate is more than half of the piezoelectric layer thickness. In some embodiments, the electrodes are embedded more than half their width. In some embodiments, the electrode includes a heavy metal having one or more of tungsten, platinum, iridium, ruthenium, or alloys of these materials, is embedded in the piezoelectric layer. In some embodiments, a conductive metal is added on the electrodes, and the conductive metal includes one or more of aluminum, copper, or alloys of these metals. In some embodiments, the embedded electrodes include a heavy material and a conductive metal. In some embodiments, the surface of the SAW structure is flat, e.g., the top surface of the embedded electrodes being at least substantially coplanar with the top surface of the piezoelectric layer.

[0051] In the present disclosure, the piezoelectric layer may include lithium tantalate with an orientation between Y and (Y+50°). In some embodiments, the piezoelectric layer includes lithium tantalate with an orientation between (Y+20°) and (Y+40°). In some embodiments, the piezoelectric layer includes lithium tantalate with an orientation between (Y+25°) and (Y+35°). In the present disclosure, the piezoelectric layer may include lithium niobate. In some embodiments, the lithium niobate orientation is between (Y-10°) and (Y+50°) degrees.

[0052] In the present disclosure, the substrate may include silicon, silicon carbide, sapphire and / or quartz. A silicon oxide film may be disposed between the substrate and the piezoelectric layer. In some embodiments, at least one dielectric material is added on top of the piezoelectric layer. In some embodiments, the total thickness of the layers above the piezoelectric material is larger than two periods. In some embodiments, the piezoelectric layer thickness is smaller than the wavelength (twice the electrode period).

[0053] In some embodiments, the velocity is modified on the edges of the aperture of a SAW structure. For example, the velocity may be reduced in the edges. In some embodiments, the velocity is reduced in the edge by using a larger duty factor than in the active aperture. InAttorney Docket No. QID241599 / 62306.187WO01 some embodiments, the velocity is reduced by adding material in the edge. In some embodiments, the added material is metal and is discontinuous to avoid a short circuit. In some embodiments, the added material is dielectric. In some embodiments, the electrodes are not embedded in the gaps of a SAW structure, or are less embedded than those in the active aperture.

[0054] FIG. 1A illustrates an exemplary SAW structure 100 (e.g., a SAW resonator), according to embodiments of the present disclosure. SAW structure 100 may include a substrate structure 102, a pair of reflectors (106a, 106b) over substrate structure 102, and an interdigital electrode structure 104 disposed between reflectors 106. Substrate structure 102 may include a multi-layer structure, and may provide the base for forming interdigital electrode structure 104. As shown in FIG. 1 A, each reflector 106a / 106b may include a pair of reflector busbars each extending in the x-direction (e.g., the length direction) and a plurality of reflector bars extending between the pair of reflector busbars in the y-direction (e.g., the width direction).

[0055] Interdigital electrode structure 104 may be aligned with reflectors (106a, 106b) in the y-direction. Interdigital electrode structure 104 may include a first busbar 108A and a second busbar 108B each extending in the x-direction. Interdigital electrode structure 104 may include a plurality of first electrode fingers 110a extending from first busbar 108a in the y- direction, and a plurality of second electrode fingers 110b extending from second busbar 108b in the y-direction. An acoustic wave, having a wavelength of , may propagate in the first and second electrode fingers 110a and 110a along the x-direction. A first electrode finger 1 10a may overlap with at least one adjacent second electrode finger 110b (e.g., or both adjacent second electrode fingers 110b) in the y-direction, and a second electrode finger 110b may overlap with at least one adjacent first electrode finger 110a (e.g., or both adjacent first electrode fingers 110a) in the y-direction. In some embodiments, interdigital electrode structure 104 includes a heavy metal that includes one or more of tungsten, platinum, iridium, ruthenium, and / or alloys of one or more of these materials. A duty factor (DF) is defined as a ratio between a width (w) of an electrode finger HOa / l lOb and a pitch (L) between adjacent first / second electrode fingers. In some embodiments, large duty factors result in large capacitance densities while small duty factors result in very large coupling factors. DF may be greater than 0 and less than 1.

[0056] In some embodiments, the DF is less than 40%. In some embodiments, DF is greater than 60%. In some embodiments, the DF is less than 35%. In some embodiments, DF is greater than 65%. In some embodiments, lower DF (e.g., less than 40%) may improve theAttorney Docket No. QID241599 / 62306.187WO01 electromechanical coupling of interdigital electrode structure 104, while higher DF (e.g., higher than 60%) may effectively reduce the size of interdigital electrode structure 104. In some embodiments, the DF values are designed as a result of a reason shown in FIGS. 3B and 3C. For example, lower duty factors may result in larger coupling factors. For existing SAW devices, the larger couplings are obtained at about 50% duty factors. For example, for DF less than 40% and tungsten thickness larger than 0.15 pm, the coupling is higher than 16%. It becomes larger than 17% for tungsten of thickness greater than 0.25 pm. For duty factor lower than 35%, the coupling is larger than 17% for tungsten of thickness more than 0.2 pm.

[0057] Regarding larger duty factors, a larger capacitance density can be obtained than for a LRT while keeping a similar coupling coefficient. For example, for tungsten of about 0.25 pm and a duty factor larger than 60%, the capacitance density is larger than about 3.7 times the density without embedding the electrodes. For 50% duty factor, the capacitance density is 3.1 times the regular LRT density. The gain is about 10%. If the duty factor is 65%, the capacitance density may be about 4 times the regular LRT. The capacitance density may be about 30% more than for that of 50% duty factor. Also, the important thing is that the coupling factor may stay above 14% for duty factors higher than 60%, and stays above 13% even for duty factors below 70%. Existing LRT may have coupling factor is in a range of about 10% to about 11 % but large duty factors results in reduction of the coupling factor. This means that for filters which are not wide band, larger duty factors, which result in desirable size reduction, can be used. For LRT, large duty factors may also result in a size reduction but this is not possible for wide band filters.

[0058] The plurality of first electrode fingers 110a may extend to a first apodization edge, and the plurality of second electrode fingers 110b may extend to a second apodization edge. An aperture 112 may be the largest distance between the first apodization edge and the second apodization edge in the y-direction. In some embodiments, interdigital electrode structure 104 may include one or more of a heavy metal such as tungsten, platinum, iridium, ruthenium, and / or alloys of them.

[0059] Substate structure 102 may include a base substrate, an intermediate layer over the base substrate, and a piezoelectric layer over the intermediate layer. In some embodiments, the piezoelectric layer includes lithium tantalate (LiTaO) and / or lithium niobate (LiNbO). A thickness of the piezoelectric layer may be less than X, (or 2L). In some embodiments, the base substrate includes at least one of silicon, sapphire, quartz, silicon carbide, spinel, ceramics, and / or other insulation / semiconductor materials. In some embodiments, the intermediate layerAttorney Docket No. QID241599 / 62306.187WO01 includes an oxide material such as silicon oxide. In some embodiments, the intermediate layer is optional.

[0060] The piezoelectric layer may include lithium tantalate with an orientation between Y and (Y+50°). FIG. 7 shows the orientation of a structure in a three-dimensional (3D) space. For example, (i) shows a substrate 702 with Y normal, and (ii) shows substrate 702 with (Y+0) as the orientation. Specifically, substrate 702 is rotated by an angle 0 along the X axis. In some embodiments, the propagation of the acoustic wave is along the X axis. In some embodiments, the piezoelectric layer includes lithium tantalate with an orientation between about (Y+20°) and about (Y+40°). In some embodiments, the piezoelectric layer includes lithium tantalate with an orientation between about (Y+25°) and about (Y+35°). In some embodiments, the piezoelectric layer includes lithium niobate. In some embodiments, the lithium niobate has an orientation of between about (Y-10°) and about (Y+50°) degrees.

[0061] Interdigital electrode structure 104 may be at least partially embedded in substrate structure 102. In some embodiments, interdigital electrode structure 104 may be partially embedded in the piezoelectric layer such that at least a portion of interdigital electrode structure 104 is positioned below the top surface of the piezoelectric layer (or substrate structure 102) in the z-direction. In some embodiments, the thickness of the portion of interdigital electrode structure 104 embedded in the piezoelectric layer is equal to or greater than half the thickness of the piezoelectric layer. In some embodiments, the thickness of the portion of interdigital electrode structure 104 embedded in the piezoelectric layer is equal to or greater than half of width of an electrode finger (HOa / l lOb). In some embodiments, the portion of interdigital electrode structure 104 embedded in the piezoelectric layer includes a heavy metal such as tungsten, platinum, iridium, ruthenium, and / or alloys of them. The piezoelectric layer may have sufficient thickness to support the manufacturing and functioning of interdigital electrode structure 104. In various embodiments, the piezoelectric layer may be a piezoelectric wafer, or a piezoelectric membrane.

[0062] In the present disclosure, SAW structure 100 may have various structures, in which interdigital electrode structure 104 are at least partially embedded in the piezoelectric layer. FIGS. 2A, 3 A, and 4A each show an exemplary structure of SAW structure 100 along the AA’ direction.

[0063] FIG. 2A illustrates a cross-sectional view an exemplary SAW structure 200 in the AA’ direction, according to embodiments of the present disclosure. SAW structure 200 may be an example of SAW structure 100. SAW structure 200 may include a substrate structure 208, which includes a base substrate 202, an intermediate layer 204 on base substrate 202, andAttorney Docket No. QID241599 / 62306.187WO01 a piezoelectric layer 206 on intermediate layer 204. Piezoelectric layer 206 may include lithium tantalate (LiTaO) and / or lithium niobate (LiNbO). Base substrate 202 may include at least one of silicon, sapphire, quartz, silicon carbide, spinel, ceramics, and / or other insulation / semiconductor materials. Intermediate layer 204 may include an oxide material such as silicon oxide. SAW structure 200 may also include an interdigital electrode structure 212 that includes a heavy metal that includes one or more of tungsten, molybdenum, platinum, iridium, ruthenium, and / or alloys of one or more of these materials. In some embodiments, metals or alloys with a density larger than 10 kg / m3may be used.

[0064] Similar to interdigital electrode structure 104, interdigital electrode structure 212 may be at least partially embedded in piezoelectric layer 206. In some embodiments, interdigital electrode structure 212 is fully embedded in piezoelectric layer 206 such that a top surface of piezoelectric layer 206 is substantially coplanar with a top surface of interdigital electrode structure 212. It should be noted that, although in FIG. 2A and the following figures, the shape of the embedded part of the electrodes is shown as rectangular, the shape of the embedded part of the electrodes may have any suitable shapes, e.g., depending on the fabrication process. For example, the angle between a side surface and the bottom surface may be larger than, about, or less than 90 degrees. The shape of the embedded part of the electrodes may not change the working principle of the resonator. In this case, the duty factor is measured at the top surface of the piezoelectric film.

[0065] In various embodiments, one of the advantages of using embedded electrodes is the large increase of the capacitance density which allows to reduce the resonator size. It is advantageous to use embedded heavy metal. It is shown that the choice of the embedded electrode material is critical for the resonator design and that the duty factor impacts bo3th the coupling factor and the capacitance density.

[0066] As shown in FIG. 2 A, interdigital electrode structure 210 may include a plurality of electrode fingers 210 (e.g., similar to first / second electrode fingers 110a / l 10b). As described in FIG. 1, DF is defined as a ratio between a width (w) of an electrode finger 210 and a pitch / period (L) between adjacent electrode fingers 210. In some embodiments, the DF is less than 40%. In some embodiments, DF is greater than 60%. A thickness of the portion of electrode finger 210 embedded in piezoelectric layer 206 is represented by t, and a thickness of piezoelectric layer 206 is represented by to. In some embodiments, a ratio of t / tO is equal to or greater than 0.5. In some embodiments, a ratio of t / w is equal to or greater than 0.5. In some embodiments, to is about 0.5 pm. In some embodiments, base substrate 202 includesAttorney Docket No. QID241599 / 62306.187WO01 silicon. In some embodiments, intermediate layer 204 includes silicon oxide, and is about 0.5 pm thick in the z-direction. In some embodiments, L is about 1 |im and w is about is 0.5 |im. In some embodiments, the periods and layer thicknesses are homothetic to the previous thicknesses and periods. In other words, all the dimensions are multiplied by a factor “A” and the resonator is at a frequency divided by “A” and has similar electrical characteristics.

[0067] In some embodiments, better temperature coefficient frequencies (TCF)s can be obtained when using lithium tantalate as the piezoelectric material. One process to embed metal inside a material is the so-called damascene process. This process includes etching trenches in the substrate, depositing metal and removing the extra metal at the surface of the substrate using a CMP process.

[0068] FIG. 2B shows the simulated admittances and conductances using periodic finite element method (FEM) for embedded copper (Cu) electrodes (e.g., interdigital electrode structure 210) at depths (e.g., t) varying from 100 nm to 300 nm. The substrate may include a lithium tantalate layered (LRT) substrate. The piezoelectric layer may include lithium tantalate (LT) with a thickness of about 0.5 pm. The intermediate layer includes silicon oxide and has a thickness of about 0.5 m. The interdigital electrode structure may have a period L of about 1 pm and DF of about 50%. The arrows on the conductances show the lower edge of the stopband. For copper electrodes, the resonance is at the upper edge of the stopband, which is not desirable since this results in the antiresonance frequency being outside of the stopband. FIG. 2C shows the results for the same simulation but with electrodes (e.g., interdigital electrode structure 210) containing tungsten. In this case, the resonance is at the lower edge of the stopband, which is better.

[0069] Since the electrical conductivity of tungsten may not be desirably high, a second layer of a more conductive metal may be used. In some embodiments, a conductive layer on top of the embedded electrode may be desired to improve electrical conductivity, as shown in Kimura et al. This metal may be aluminum, copper or alloys of these metals. For processing reasons, a good option is to have some heavy metal embedded and the conductive material deposited on top in a second step. FIG. 3A shows a SAW structure 300 . The conductive metal width may be different from the heavy metal width. In some embodiments, both the heavy metal and the conductive metal are embedded.

[0070] FIG. 3A illustrates a cross-sectional view an exemplary SAW structure 300 in the AA’ direction, according to embodiments of the present disclosure. SAW structure 300 may be an example of SAW structure 100. Different from SAW structure 200, SAW structure 300 may include an interdigital electrode structure 312, which includes a plurality of electrodeAttorney Docket No. QID241599 / 62306.187WO01 fingers 310. Each electrode finger 310 may include a first electrode portion 310a and a second electrode portion 310b in contact with first electrode portion 310a. In some embodiments, first electrode portions 310 may be similar to electrode fingers 210. Second electrode portion 310b may include a conductive metal of desirably high conductivity. In some embodiments, second electrode portion 310 includes aluminum, copper, and / or alloy of aluminum and copper. Second electrode portion 310b may be in contact with the top surface of the corresponding first electrode portion 3I0a. In some embodiments, depending on the positions of first electrode portions 310a, second electrode portions 310b may be disposed fully above the top surface of piezoelectric layer 206, partially above the top surface of piezoelectric layer 206, or fully embedded in piezoelectric layer 206. In some embodiments, second electrode portions 310a are fully embedded in piezoelectric layer 206, and second electrode portions 310b are at least partially embedded in piezoelectric layer 206 such that at least part of second electrode portions 310b are below the top surface of piezoelectric layer 206 in the z-direction. In some embodiments, second electrode portions 310b are fully embedded in piezoelectric layer 206 such that the top surfaces of second electrode portions 310b are substantially coplanar with the top surface of piezoelectric layer 206. In some embodiments, the top surfaces of first electrode portions 310a are substantially coplanar with the top surface of piezoelectric layer 206, and second electrode portions 310b are fully above piezoelectric layer 206.

[0071] In some embodiments, piezoelectric layer 206 includes lithium tantalate. The orientation of lithium tantalate may be chosen accordingly for improved coupling factor while keeping the Rayleigh mode (below the resonance) coupling factor low. FIG. 3B shows the simulated coupling factor and the corresponding best orientation in a plot of embedded thickness t and duty factor. It is shown that the highest coupling factors are obtained for tantalate orientations between (Y+25°) and (Y+35°). FIG. 3C shows the increase of capacitance density when compared to non-embedded aluminum electrodes only. High capacitance densities may result in smaller SAW sizes. It can be shown that the best capacitance density can be obtained for duty factor larger than about 50% and preferably above about 60% or 65% while higher coupling factors can be obtained for duty factors lower than about 40% or 35%. Coupling factors greater than about 17% are obtained for duty factors lower than about 40% and tungsten depths larger than half the lithium tantalate thickness. The capacitance density may be greater than twice the one for aluminum only electrodes (nonembedded) and a 50% duty factor. For duty factors larger than about 60% and depth larger than half the lithium tantalate thickness, the capacitance density may be increased by a factorAttorney Docket No. QID241599 / 62306.187WO01 higher than 4 when compared to resonators with aluminum only electrodes and a 50% duty factor.

[0072] FIG. 4A illustrates a cross-sectional view an exemplary SAW structure 400 in the AA’ direction, according to embodiments of the present disclosure. SAW structure 400 may be an example of SAW structure 100. In some embodiments, an electrode finger includes a combination of a heavy metal and another metal of higher conductivity, with the heavy metal surrounding the other metal at the bottom and on the side. Different from SAW structures 200 and 300, SAW structure 400 may include an interdigital electrode structure 412, which includes a plurality of electrode fingers 410. Each electrode finger 410 may include a first electrode portion 410a and a second electrode portion 410b in contact with first electrode portion 410a. In some embodiments, the positions, dimensions, and materials of first electrode portions 410 may be similar to those of electrode fingers 210. Second electrode portion 410b may include a conductive metal of desirably high conductivity. In some embodiments, second electrode portion 410b includes aluminum, copper, and / or alloy of aluminum and copper.

[0073] Different from SAW structure 300, the top surfaces of first electrode portion 410a and the corresponding second electrode portion 410b may be substantially coplanar. As shown in FIG. 4A, first electrode portion 410a may partially surround the corresponding second electrode portion 410b such that first electrode portion may be in contact with the corresponding second electrode portion 410b in the x direction (at the bottom) an in the y- direction (on the side). In some embodiments, the top surfaces of first electrode portion 410a, the corresponding second electrode portion 410b, and piezoelectric layer 206 may be substantially coplanar.

[0074] FIGS. 4B-4C show periodic simulations results for various trench depths (t values). As an example, first electrode portion 410a includes tungsten, and second electrode portion 410b includes copper. A thickness of second electrode portion 410b varies from 100 nm to 200 nm in the z-direction, and the thickness of tungsten on the bottom is assumed to be substantially equal to the tungsten width on the side. The substrate may include a lithium tantalate layered (LRT) substrate. The piezoelectric layer may include lithium tantalate (LT) with a thickness of about 0.5 pm. The intermediate layer includes silicon oxide and has a thickness of about 0.5 pm. The interdigital electrode structure may have a period L of about 1 pm and DF of about 70%. The arrows on the conductances show the edge of the stop band which is not at the resonance frequency. It is shown that depending on the depth and the copper thickness, the resonance is on the lower edge of the stop band or on the upper edge of the stopAttorney Docket No. QID241599 / 62306.187WO01 band. The resonance is on the lower edge of the stop band when the tungsten thickness / width is large enough. In some embodiments, the duty factor is chosen to about 70% to be able to have sufficient copper and tungsten. The duty factor can be lower for lower frequencies. In some examples, the dimensions are proportional to the wavelength. This means that when the frequency is lowered all the dimensions are larger. In this case, there may be enough space for the heavy metal and the conductive metal even for lower duty factors.

[0075] One of the known issues for SAW or layered SAW resonators is the presence of several transverse modes. The SAW resonator can behave like a waveguide in the aperture direction and, depending on the aperture, several modes can be excited and propagate. Using SAW resonators with transverse modes to design filters can result in large pass-band ripples. To avoid this issue, “piston mode resonators” are often used. This technique may include adding a region with a different velocity at the edges of the resonator aperture. In some configurations in which the slowness curve is convex, the edges are designed to be slower than the center of the aperture. Also, a faster region, which is often the resonator gap, often exists outside of the active aperture. This fast region needs to be large enough to avoid transverse energy leakage. This technique can be used when electrodes (e.g., interdigital electrode structures) are embedded. The slow region can be formed by increasing the duty factor at the edges of the aperture or by adding some material on the electrode at the edge. The added material may be metallic or dielectric. If the material is metallic, it should not be continuous to avoid contact with each other to cause a short-circuit. If it is dielectric, the material can be continuous or not continuous.

[0076] FIGS. 5 A and 5B show SAW structures 500 and 501 of piston mode, according to embodiments of the present disclosure. SAW structures 500 and 501 may each include a pair of busbars 508a and 508b, similar to busbars 108a and 108b. SAW structures 500 and 501 may each include a plurality of first electrode fingers 510a and a plurality of second electrode fingers 510b altematingly disposed along the x-direction on substrate structure 208. The electrode fingers and the busbar on the other side may be separated by a gap 512a / 512b. An aperture 516 may be positioned between gaps 512a and 512b. As shown in FIGS. 5A and 5B, each electrode finger may include a region at the edge that has a different DF compared to the rest of the electrode fingers. For example, the rest of an electrode finger may have a width wl in the x-direction, and the region may have a width w2 in the direction. In some embodiments, w2 is different from wl. A slow region (514a and 514b) may be formed at the edges if w2 is greater than wl (e.g., w2 / L is greater than wl / L), and a fast region (514a and 514b) may beAttorney Docket No. QID241599 / 62306.187WO01 formed at the edge if w2 is less than wl (e.g., w2 / L is greater than wl / L). As an example, 514a and 514b may be slow regions, and w2 is greater than wl.

[0077] Each electrode finger 510a or 510b may have a double-layer structure, e.g., include a first electrode portion and a second electrode portion over the first electrode portion. For ease of illustration, FIG. 5 A shows a cross-sectional view of SAW structure 500 along the BB’ direction. The cross-section of a first electrode finger 510a is shown as an example. In some embodiments, as shown in FIG. 5A, first electrode finger 510a includes a first electrode portion 510a-l and a second electrode portion 510a-2 on first electrode portion 510a-l. First electrode portion 510a- 1 may be at least partially embedded in piezoelectric layer 206. Second electrode portion 510a-2 may be partially or fully above the top surface of piezoelectric layer 206. In some embodiments, first electrode portion 510a- 1 is fully embedded in piezoelectric layer 206, and the top surface of first electrode portion 510a-l is substantially coplanar with that of piezoelectric layer 206. In some embodiments, first electrode portion 510a-l extends in aperture 516 but discontinues in the y-direction by gaps 512a and 512b. In some embodiments, second electrode portion 510b may also include a first electrode portion 510b-l and a second electrode portion 510b-2 on first electrode portion 510b- 1. Similar to first electrode portion 510a-l , first electrode portion 510b- 1 may extend in aperture 516, and discontinues by gaps 512a and 512b.

[0078] FIG. 5B shows a cross-sectional view of SAW structure 500 along the CC’ direction. Different from SAW structure 500, electrode fingers of SAW structure 501 may extend in aperture 516 as well as in the gap 512a / 512b to the busbar that it extends from. In some embodiments, first electrode portion 510a-l extends in aperture 516 and gap 512a, and discontinues in the y-direction by busbar 508a. In some embodiments, second electrode portion 510b may also include a first electrode portion 510b- 1 and a second electrode portion 51 Ob-2 on first electrode portion 510b- 1. Similar to first electrode portion 510a-l, first electrode portion 510b- 1 may extend in aperture 516 and gap 512b, and discontinues by busbar 508b. In some embodiments, the part of first electrode portion 510a- 1 in the gap 512a / 512b is shallower (e.g., has a smaller thickness in the z-direction) than part of first electrode portion 510a-l in aperture 516. Also, other approaches know to suppress the transverse mode ripple may be used. For example, apodization techniques (shown on figurel) may be used and it may be combined with the presence of a slow region in the aperture. Also, as shown on figure 1, instead of separating the busbars and the active electrode, a dummy (not active) electrode region may be present between the gap and the busbar.Attorney Docket No. QID241599 / 62306.187WO01

[0079] In SAW structures 500 and 501, first electrode portions 510a-l and 510b-l may include a heavy metal such as tungsten, platinum, iridium, ruthenium, molybdenum or alloys of them. In some embodiments, second electrode portions 510a-2 and 51 Ob-2 may include a metal material of desirably high conductivity such as aluminum, copper, and / or alloys of aluminum and copper.

[0080] FIG. 5C shows a FEM simulation result for admittance as a function of frequency of SAW structure 500, according to some embodiments. DF is about 30% in the aperture (e.g., 516), and about 60% in the slow region (e.g., 514). In some embodiments, pitch L is about 1 pm (Z=2L), and slow region width may be about 0.4k (0.8 pm). In some embodiments, aperture 516 may be equal to about 16 . Intermediate layer (LRT) includes silicon oxide and has a thickness of about 0.5 pm. Piezoelectric layer 206 includes LT, has an orientation of about (Y+35°), and has a thickness of about 0.5 pm. The thickness of the embedded portion of first electrode portion (e.g., tungsten) 510a-l / 510b-l is about 0.4 pm deep. The thickness of second electrode portion 510a-2 / 510b-2 (e.g., aluminum) is about 0.2 pm. Coupling factor may be about 17.6%.

[0081] FIG. 6 illustrates a cross-sectional view an exemplary SAW structure 600 in the AA’ direction, according to embodiments of the present disclosure. SAW structure 600 may be an example of SAW structure 100. SAW structure 600 may include an interdigital electrode structure 612, which includes a plurality of electrode fingers 610. Interdigital electrode structure 612 may be at least partially embedded in piezoelectric layer 206. In some embodiments, interdigital electrode structure 612 may be fully embedded in piezoelectric layer 206 such that the top surface of interdigital electrode structure 612 is substantially coplanar with that of piezoelectric layer 206. In some embodiments, interdigital electrode structure 612 may be partially embedded in piezoelectric layer 206 (as shown in FIG. 6), such that part of an electrode finger 610 protrudes from piezoelectric layer 206. Different from SAW structures 200, 300, 400, 500, and 501, SAW structure 600 may include a cover layer 608 over and in contact with piezoelectric layer 206 and interdigital electrode structure 612. Cover layer 608 may provide passivation and / or for frequency trimming. For example, the thickness of cover layer 608 may be adjusted during fabrication to obtain the right frequency. The frequency of the resonator may be measured first, and cover layer 608 may be etched based on the measured frequency. In this case, the thickness of cover layer 608 may be relatively thin (thinner than the electrode typically) and may follow the shape of the interdigital electrode structure 612. In some embodiments, cover layer 608 are is sufficiently thick to prevent acoustic energy on top of SAW structure 600. In some embodiments, with the use of cover layer 608, interdigitalAttorney Docket No. QID241599 / 62306.187WO01 electrode structure 612 may not need to be packaged in a cavity. In some embodiments, cover layer 608 includes a suitable passivation material such as a dielectric layer, e.g., silicon oxide, silicon nitride, and / or silicon oxynitride. In some embodiments, cover layer 608 includes silicon oxide. In some embodiments, a thickness tl of cover layer 608 is equal to or greater than 2L. In some embodiments, the dimensions, and materials of interdigital electrode structure 612 may be similar to those of interdigital electrode structure 212.

[0082] In some embodiments, electrode fingers 610 may include heavy metal such as one or more of tungsten, platinum, iridium, ruthenium, and / or alloys of one or more of these materials. In some embodiments, although not shown, electrode fingers 610 may also include a conductive metal of desirably high conductivity, such as one or more of aluminum, copper, and / or alloy of aluminum and copper. For example, the metal of high conductivity may be over or partially surrounded by the heavy metal material, similar to those of SAW structures 300, 400, 500, and / or 501, respectively. Cover layer 608 may be over and in contact with electrode fingers 610. In various embodiments, cover layer 608 can be disposed over any of the interdigital electrode structures (e.g., 200, 300, 400, 500, and / or 501) of this present disclosure.

[0083] Even if it was not included specifically in the previous description, as it is well known, more than one transducer (e.g., SAW structure 600) may be inserted between the reflective gratings. In this case, some transducers are connected to the input and some to the output. This well known configuration, often called Coupled Resonator Filter (CRF) or Double Mode SAW (DMS) can be designed using the configurations described above.

[0084] Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.

Claims

Attorney Docket No. QID241599 / 62306.187WO01WHAT IS CLAIMED IS:

1. An acoustic wave device, comprising: a piezoelectric layer; and an interdigital electrode structure over the piezoelectric layer, the interdigital electrode structure comprising a plurality of electrode fingers that comprises one or more first electrode fingers and one or more second electrode fingers extending in a width direction, the one or more first electrode fingers and the one or more second electrode fingers interleaved with one another along a length direction, the length direction being different from the width direction, wherein at least one electrode finger of the plurality of electrode fingers comprises an electrode portion is at least partially embedded in the piezoelectric layer.

2. The acoustic wave device of claim 1 , wherein the electrode portion comprises tungsten, platinum, iridium, ruthenium, molybdenum or a combination thereof.

3. The acoustic wave device of claim 1 , wherein a thickness of the electrode portion is: equal to or greater than a quarter of a thickness of the piezoelectric layer; equal to or greater than half a thickness of the piezoelectric layer; or equal to or greater than half a width of the electrode finger.

4. The acoustic wave device of claim 1 , wherein a duty factor of the plurality of electrode fingers is: lower than about 40%; or higher than about 60%.

5. The acoustic wave device of claim 1, wherein the at least one electrode finger of the plurality of electrode fingers further comprises another electrode portion over the electrode portion.

6. The acoustic wave device of claim 5, wherein the other electrode portion comprises aluminum, copper, or a combination thereof.Attorney Docket No. QID241599 / 62306.187WO017. The acoustic wave device of claim 5, wherein the other electrode portion is at least partially above the piezoelectric layer.

8. The acoustic wave device of claim 5, wherein the other electrode portion is surrounded by the electrode portion the length direction and a vertical direction perpendicular to the width direction and the length direction.

9. The acoustic wave device of claim 1 , wherein the interdigital electrode structure further comprises an edge region, a first duty factor of the edge region being different from a second duty factor of a rest of the interdigital electrode structure.

10. The acoustic wave device of claim 9, wherein the electrode portion extends in an aperture of the interdigital electrode structure, and discontinues by a gap between the aperture and an opposing busbar.

11. The acoustic wave device of claim 10, wherein the electrode portion further extends to another gap between the aperture and a busbar from which the corresponding electrode finger extends from.

12. The acoustic wave device of claim 11, wherein the electrode portion comprises a first part in the aperture and a second part in the other gap, a thickness of the second part being less than that of the first part.

13. The acoustic wave device of claim 9, wherein the edge region is a slow region, and comprise an added material that comprises at least one of a metal or a dielectric material.

14. The acoustic wave device of claim 1, wherein the piezoelectric layer comprises lithium tantalate with an orientation between about Y and about (Y+50°).

15. The acoustic wave device of claim 14, wherein the orientation is between about (Y+20°) and about (Y+40°).

16. The acoustic wave device of claim 14, wherein the orientation is between about (Y+25°) and about (Y+35°).Attorney Docket No. QID241599 / 62306.187WO0117. The acoustic wave device of claim 1, wherein the piezoelectric layer comprises lithium niobate.

18. The acoustic wave device of claim 17, wherein the lithium niobate has an orientation between about (Y-lOo) and about (Y+50o).

19. The acoustic wave device of claim 1 , further comprising a cover layer in contact with the piezoelectric layer and the interdigital electrode structure, wherein the cover layer comprises a dielectric layer.

20. The acoustic wave device of claim 19, wherein a thickness of the cover layer is equal to or greater than twice a period of the interdigital electrode structure.

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