A thin-film surface acoustic wave resonator element structured to enable reduction of spurious transverse modes, and a related electronic apparatus

Abstract

The invention allows a structure for a thin-film surface acoustic wave (FSAW) resonator that enables reduction of spurious tranverse modes. More specifically, a disclosed layered FSAW res- onator element employing a piston topology based on a high-velocity split dummy fingers region may improve waveguiding while suppressing or at least reducing undesired transverse mode effects.

Description

[0001] A THIN-FILM SURFACE ACOUSTIC WAVE RESONATOR ELEMENT STRUCTURED TO ENABLE REDUCTION OF SPURIOUS TRANSVERSE MODES, AND A RELATED ELECTRONIC APPARATUS

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to the field of surface acoustic wave devices, and, more particularly, to a thin- film surface acoustic wave resonator structured to enable reduction of spurious tranverse modes, and a related electronic apparatus.

[0004] BACKGROUND

[0005] A SAW (surface acoustic wave) device is a device using propagation of elastic waves on a surface of a material or at an interface between several materials, and is commonly used in micro-mechanical resonators and filters. SAW devices use so-called interdigitated transducers (IDTs) to transform radio frequency (RF) signals into acoustic waves or acoustic waves into RF signals.

[0006] SAW devices are often used in RF filters. However, at least in some situations current SAW devices may suffer from spurious tranverse modes. For example, operation under 1 GHz may require SAW wavelengths of about 4 pm and longer which may in turn result in acoustically thin piezoelectric thin-film layers (e.g., of lithium tantalate (LT)) that may have a relative thickness of dfk < 0.15. The latter may result in significantly convex SAW dispersions within an interdigital transducer (IDT) arranged on the piezoelectric thin-film layer. Accordingly, the resulting slanted geometry may not effectively suppress transverse modes, while contributing to a significant waveguiding loss.

[0007] Accordingly, at least in some situations, there may be a need for solutions that allow reduction of spurious tranverse modes for SAW devices.

[0008] SUMMARY

[0009] 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 or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

[0010] It is an object of the invention to allow a thin-film surface acoustic wave resonator structured to enable reduction of spurious tranverse modes. The foregoing and other objects are achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures.

[0011] According to a first aspect, a thin-film surface acoustic wave, FSAW, resonator element is provided. The FSAW resonator element comprises a substrate layer. The FSAW resonator element further comprises an intermediate layer arranged on the substrate layer. The FSAW resonator element further comprises a piezoelectric thin-film layer arranged on the intermediate layer. The FSAW resonator element further comprises an interdigital transducer, IDT, arrangement arranged on the piezoelectric thin-film layer. The IDT arrangement comprises two conductive elements. The IDT arrangement further comprises an active area arranged between the two conductive elements. The IDT arrangement further comprises a plurality of interdigitating electrodes. Each interdigitating electrode extends from one of the conductive elements across the active area towards the other of the conductive elements. The IDT arrangement further comprises two sets of dummy fingers. Each dummy finger extends from one of the two conductive elements towards a respective tip of an opposing interdigitating electrode. Each dummy finger is separated by a gap from the respective tip of the opposing interdigitating electrode. The IDT arrangement further comprises two sets of electrode extensions. Each electrode extension extends from one of the two conductive elements towards a respective opposing interdigitating electrode. The dummy fingers and the electrode extensions alternate with each other in a lateral direction of the interdigitating electrodes and in a longitudinal direction of the interdigitating electrodes. At least one dummy finger of the two sets of dummy fingers or at least one electrode extension of the two sets of electrode extensions is at least partially split into two longitudinal sub-finger portions or sub-extension portions, respectively, along a longitudinal center axis of the respective dummy finger or electrode extension. The present disclosure allows a structure for a thin-film surface acoustic wave (FSAW) resonator that enables reduction of spurious tranverse modes.

[0012] In an implementation form of the first aspect, each dummy finger in a first group of the dummy fingers is completely split into the two longitudinal sub- finger portions, and / or each electrode extension in a first group of the electrode extensions is completely split into the two longitudinal sub-extension portions. This implementation form allows an effective SAW velocity increase in a region between the IDT and the conductive elements. This fast region promotes an effective waveguiding within the IDT.

[0013] In an implementation form of the first aspect, each dummy finger in a second group of the dummy fingers is partially split into the two longitudinal sub-finger portions, thereby each partially split dummy finger having a split dummy finger part and a non-split dummy finger part, and / or each electrode extension in a second group of the electrode extensions is partially split into the two longitudinal sub-electrode portions, thereby each partially split electrode extension having a split electrode extension part and a non-split electrode extension part. This implementation form allows for additional acoustic impedance matching between a fast region and the surrounding IDT and conductive elements.

[0014] In an implementation form of the first aspect, at least one non-split dummy finger part and / or at least one non-split electrode extension part extends from a respective one of the two conductive elements. This implementation form allows for additional acoustic impedance matching between the conductive elements and the fast region.

[0015] In an implementation form of the first aspect, at least one split dummy finger part and / or at least one split electrode extension part extends from a respective one of the two conductive elements. This implementation form allows for increasing the velocity of a region extending out from the conductive elements towards the IDT electrodes.

[0016] In an implementation form of the first aspect, at least one of one or more dummy fingers of the two sets of dummy fingers or one or more electrode extension of the two sets of electrode extensions has a length from one electrode pitch to six electrode pitches. This implementation form allows for sufficient acoustic wave decay in the fast region, such that acoustic wave propagation in the conductive elements is effectively suppressed.

[0017] In an implementation form of the first aspect, the IDT arrangement further comprises a piston mass loading structure in the vicinity of the tips of the interdigitating electrodes. This implementation form allows effectively matching the active area of the IDT arrangement with the faster region of the IDT arrangement.

[0018] In an implementation form of the first aspect, the piston mass loading structure comprises at least one of continuous mass loading dielectric layers on the tips of the interdigitating electrodes, mass loads on the tips of the interdigitating electrodes, or hammer heads on the tips of the interdigitating electrodes. This implementation form facilitates a desired velocity profile as well as a desired performance of the SAW resonator element.

[0019] In an implementation form of the first aspect, the continuous mass loading dielectric layers, mass loads, and hammer heads each have a length of no more than 2.5 * electrode pitch in the longitudinal direction of the interdigitating electrodes. This implementation form allows for more effective transverse mode suppression, avoiding additional responses coming from an acoustically different matching domain (e.g., mass loading layer, mass loads, and / or hammerheads).

[0020] In an implementation form of the first aspect, the mass loads each have a width of less than or equal to an electrode width. The hammer heads each have a width of less than or equal to 0.75 * electrode pitch. This implementation form allows an effective matching, with a minimum length of the structure.

[0021] In an implementation form of the first aspect, the piston mass loading structure comprises at least one of the continuous mass loading dielectric layers or the mass loads. At least one longitudinal sub- finger portion and / or sub-extension portion has a width dwl, such that mt * electrode pitch / 2 > dwl > mt * electrode pitch / 4, in which mt denotes a metallization ratio of each interdigitating electrode, and mt * electrode pitch represents a width of each interdigitating electrode. This implementation form allows for split electrode configurations suitable for fabrication via standard lithography techniques.

[0022] In an implementation form of the first aspect, the two longitudinal sub-finger portions of each at least partially split dummy finger and / or the two longitudinal sub-electrode portions of each at least partially split electrode extension have a distance gw between them, such that 2 * dwl + gw > mt * electrode pitch. This implementation form allows for split electrode configurations suitable for fabrication via standard lithography techniques.

[0023] In an implementation form of the first aspect, 0.85 > mt > 0.4. This implementation form allows designing SAW devices with a practical range of specific capacitances per IDT pair and per unit aperture.

[0024] In an implementation form of the first aspect, the piston mass loading structure comprises the hammer heads. At least one longitudinal sub-finger portion and / or sub-extension portion has a width dw2, such that rnh * electrode pitch / 2 > dw2 > mh * electrode pitch / 4, in which rnh denotes a metallization ratio of each hammer head, and rnh * electrode pitch represents a width of each hammer head. This implementation form allows for split electrode configurations suitable for fabrication via standard lithography techniques.

[0025] According to a second aspect, an electronic apparatus is provided. The electronic apparatus comprises the thin-film surface acoustic wave, FSAW, resonator element according to the first aspect. The present disclosure allows an electronic apparatus with a structure for a FSAW resonator that enables reduction of transverse modes.

[0026] In an implementation form of the second aspect, the FSAW resonator element is comprised in a radio frequency, RF, filter. This implementation form allows FSAW resonators for RF filter applications.

[0027] Many of the attendant features will be more readily appreciated as they become better understood by reference to the following detailed description considered in connection with the accompanying drawings.

[0028] DESCRIPTION OF THE DRAWINGS

[0029] In the following, example embodiments are described in more detail with reference to the attached figures and drawings, in which:

[0030] Fig. 1 is a diagram illustrating a thin-film surface acoustic wave resonator element according to an embodiment of the disclosure;

[0031] Fig. 2 is a block diagram illustrating an electronic apparatus according to an embodiment of the disclosure;

[0032] Fig. 3A is a diagram illustrating an interdigital transducer arrangement according to an embodiment of the disclosure;

[0033] Fig. 3B is a diagram illustrating an interdigital transducer arrangement according to another embodiment of the disclosure;

[0034] Fig. 3C is a diagram illustrating an interdigital transducer arrangement according to yet another embodiment of the disclosure;

[0035] Fig. 3D is a diagram illustrating an interdigital transducer arrangement according to yet another embodiment of the disclosure; and

[0036] Fig. 3E is a diagram illustrating an interdigital transducer arrangement according to yet another embodiment of the disclosure.

[0037] In the following, identical reference signs refer to identical or at least functionally equivalent features.

[0038] DETAILED DESCRIPTION

[0039] In the following description, reference is made to the accompanying drawings, which form part of the disclosure, and in which are shown, by way of illustration, specific aspects in which the invention may be placed. It is understood that other aspects may be utilized, and structural or logical changes may be made without departing from the scope of the invention. The following detailed description, therefore, is not to be taken in a limiting sense, as the scope of the invention is defined in the appended claims.

[0040] For instance, it is understood that a disclosure in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if a specific method step is described, a corresponding device may include a unit to perform the described method step, even if such unit is not explicitly described or illustrated in the figures. On the other hand, for example, if a specific apparatus is described based on functional units, a corresponding method may include a step performing the described functionality, even if such step is not explicitly described or illustrated in the figures. Further, it is understood that the features of the various example aspects described herein may be combined with each other, unless specifically noted otherwise.

[0041] The disclosure relates to, e.g., a sub 1GHz layered thin- film surface acoustic wave (FSAW) device employing a specific design of the electrode area between the electrical busbars and the active are of the IDT (interdigital transducer). At least some embodiments may aim at suppression or at least reduction of spurious transverse modes of such multi-layered SAW -based micro-mechanical resonators and filters.

[0042] Layered SAW resonators may suffer from transversal mode spurs. Usually, transversal modes in the IDT may be suppressed by decoupling them from an electric field of the IDT. For decoupling higher order transverse modes, a piston mode of operation may be needed. Ideally, a piston mode would be achieved naturally in the media with vertical inplane SAW dispersion. Alternatively, a piston mode may be achieved by matching the IDT to the outer media via various piston topologies in the vicinity of IDT ends.

[0043] A “piston region“ is a region with a SAW velocity smaller than SAW velocities in the surrounding media. It may also be desirable to have a convex shape of in-plane SAW dispersion in all matched regions in view of achieving simple piston mode topologies.

[0044] However, layered SAWs based on shear waves in piezoelectric lithium tantalate (LT) compounds (e.g., LiTaOa) may naturally exhibit a strong concave in-plane SAW dispersion which is often converted to convex under the IDT. Accordingly, a native piston mode may exist for a specific device topology at a specific frequency, which may limit its’ practical utility. On the other hand, achieving a piston mode through matching of different device regions may not be easy, since often a gap region may be too fast and with concave in-plane SAW dispersion, while an IDT region may be with convex in-plane SAW dispersion.

[0045] The term “piston mode” refers to a SAW mode that has uniform amplitude distribution along an IDT aperture, while decaying outside an active IDT area.

[0046] Accordingly, the piston mode may match to an external electric field pattern in an aperture direction. Owing to the mode-orthogonality in waveguides, the higher order waveguide modes (formed along IDT aperture) may be decoupled from the external electric field and thus may not appear as spurious content in an FSAW resonator admittance.

[0047] At least in some situations, achieving such a mode may require a matching region (piston region) that matches the active IDT region to the faster outer region.

[0048] A piston fundamental (0th) mode may be distinctive through its uniform amplitude distribution along the IDT aperture. Under such conditions, the higher order transverse waveguiding modes may not electrically couple to the IDT. Velocity in IDT region may be larger than velocity in a ‘piston load’ region but smaller than in an outer ‘gap’ region.

[0049] As will be discussed in more detail below, at least some of the disclosed embodiments may allow a structure for a thin-film surface acoustic wave (FSAW) resonator that enables reduction of spurious tranverse modes.

[0050] At least some of the disclosed embodiments may be used, e.g., for a low-band operation, i.e., for resonance frequencies < 1GHz.

[0051] At least some of the disclosed embodiments may allow a piston topology with an engineered SAW velocity profile along a device aperture.

[0052] At least some of the disclosed embodiments may allow using a split dummy fingers region instead of a gap region used in classical piston topologies. The velocity in the fast dummy finger region may be tuned by selecting a specific width (dwl or dw2) of and distance (gw) between corresponding split elements constituting the split dummy finger and split electrode extensions. The disclosed structure may fulfil the relation 2*dw+gw>mt*Pitch, where mt is the IDT mark to pitch ratio, while Pitch is the IDT period.

[0053] The split dummy finger topology may be designed with a threshold frequency for side radiation in the vicinity of the antiresonance of the IDT structure. At least some of the disclosed embodiments may allow a piston topology based on a high-velocity split dummy fingers region that may improve the waveguiding while suppressing or at least reducing undesired transverse mode effects. At least in some situations, this may result in an improved Q factor in the resonance - antiresonance frequency band of the resonator element.

[0054] The disclosed velocity increase in the dummy finger region may be implemented with, e.g., lithography and / or etching routines used in the IDT formation. I.e., additional processes may not be needed.

[0055] Next, example embodiments of thin-film SAW (FSAW) resonator element 100 are described based on Figs. 1 to 3E. Some of the features of the described devices are optional features which provide further advantages. Herein, an FSAW resonator element refers to a category of SAW resonator elements that use a thin- film material for a piezoelectric layer.

[0056] Fig. 1 is a diagram illustrating FSAW resonator element 100 according to an embodiment of the disclosure. Figs. 3A to 3E are diagrams illustrating various embodiments of an interdigital transducer (IDT) arrangement 140A-140E employed in FSAW resonator element 100 of Fig. 1.

[0057] FSAW resonator element 100 comprises a substrate layer (or base substrate) 110. Substrate layer 110 may comprise, e.g., silicon (Si).

[0058] FSAW resonator element 100 further comprises intermediate layer 120 arranged on substrate layer 110. Intermediate layer 120 may comprise, e.g., silicon dioxide (SiCh).

[0059] FSAW resonator element 100 further comprises piezoelectric thin-film layer 130 arranged on intermediate layer 120. For example, piezoelectric thin-film layer 130 may comprise a crystalline composition of a lithium tantalate (LT) compound (e.g., LiTaOa). At least in some embodiments, the crystalline composition may have a growth orientation of substantially 20°Y-X to 60°Y-X.

[0060] FSAW resonator element 100 further comprises interdigital transducer (IDT) arrangement 140, 140A, MOB, 140C, MOD, MOE arranged on piezoelectric thin-film layer 130.

[0061] IDT arrangement 140, 140A, MOB, 140C, MOD, MOE comprises two conductive elements (e.g., busbars) 141, 142. IDT arrangement 140, 140A, MOB, 140C, MOD, MOE further comprises active area 143 arranged between the two conductive elements 141, 142.

[0062] IDT arrangement 140, 140A, MOB, 140C, MOD, MOE further comprises a plurality of interdigitating electrodes 144, 145. Each interdigitating electrode extends from one ofthe conductive elements across active area 143 towards the other of the conductive elements. At least in some embodiments, longitudinal center axes of each two adjacent electrodes may have first distance 301 of an electrode pitch between them. For example, electrodes 144, 145 may comprise aluminum (Al), copper (Cu) or an AICu composite.

[0063] In other words, active area 143 forms a center area of IDT arrangement 140, 140A, MOB, 140C, MOD, MOE, and conductive elements 141, 142 form opposite edge areas of IDT arrangement 140, 140A, MOB, 140C, MOD, MOE.

[0064] By interdigitating it is meant that active area 143 comprises two sets of electrodes which are arranged such that the electrodes of the first set of electrodes alternate with the electrodes of the second set of electrodes, each first electrode being separated from a neighboring first electrode by a second electrode and, correspondingly, each second electrode being separated from a neighboring second electrode by a first electrode.

[0065] IDT arrangement 140, 140A, MOB, 140C, MOD, MOE further comprises two sets of dummy fingers 146, 147. Each dummy finger extends from one of the two conductive elements towards a respective tip of an opposing interdigitating electrode. Each dummy finger is separated by gap 150 from the respective tip ofthe opposing interdigitating electrode. At least in some embodiments, longitudinal center axes of each two adjacent dummy fingers may have second distance 302 of twice the electrode pitch between them.

[0066] IDT arrangement 140, 140A, MOB, 140C, MOD, MOE further comprises two sets of electrode extensions 148, 149. Each electrode extension extends from one of the two conductive elements towards a respective opposing interdigitating electrode. At least in some embodiments, longitudinal center axes of each two adjacent electrode extensions may have second distance 302 of twice the electrode pitch between them. At least in some embodiments, second distance 302 of twice the electrode pitch may be a characteristic of an IDT voltage periodicity due to the interdigitated topology. At least in some embodiments, the electrode pitch itself may vary along the structure in an IDT (i.e., the so-called chirping), but even then the two-pitch periodicity in the voltage would apply, since within a single pitch there is always an antiperiodicity.

[0067] The dummy fingers and the electrode extensions alternate with each other in a lateral direction of interdigitating electrodes 144, 145 and in a longitudinal direction of interdigitating electrodes 144, 145.

[0068] At least one dummy finger of the two sets of dummy fingers 146, 147 or at least one electrode extension of the two sets of electrode extensions 148, 149 is at least partially split into two longitudinal sub-finger portions 146A, 146B, 147A, 147B or sub-extension portions 148A, 148B, 149A, 149B, respectively, along a longitudinal center axis of the respective dummy finger or electrode extension.

[0069] At least in some embodiments, each dummy finger in a first group of the dummy fingers may be completely split into the two longitudinal sub- finger portions 146A, 146B, 147A, 147B, and / or each electrode extension in a first group of the electrode extensions may be completely split into the two longitudinal sub-extension portions 148A, 148B, 149A, 149B.

[0070] At least in some embodiments, each dummy finger in a second group of the dummy fingers may be partially split into the two longitudinal sub-finger portions 146A, 146B, 147A, 147B, thereby each partially split dummy finger having split dummy finger part 146_1, 147_1 and non-split dummy finger part 146_2, 147_2, and / or each electrode extension in a second group of the electrode extensions may be partially split into the two longitudinal sub-electrode portions 148A, 148B, 149A, 149B, thereby each partially split electrode extension having split electrode extension part 148_1 , 149_1 and non-split electrode extension part 148_2, 149_2.

[0071] At least in some embodiments, at least one non-split dummy finger part 146_2, 147_2 and / or at least one non-split electrode extension part 148_2, 149_2 may extend from a respective one of the two conductive elements (as shown in Fig. 3C).

[0072] At least in some embodiments, at least one split dummy finger part 146_1, 147_1 and / or at least one split electrode extension part 148_1 , 149_1 may extend from a respective one of the two conductive elements (as shown in Fig. 3D).

[0073] At least in some embodiments, a split dummy finger may interconnect an interdigitating electrode with one of the conductive elements.

[0074] At least in some embodiments, at least one of one or more dummy fingers of the two sets of dummy fingers 146, 147 or one or more electrode extension of the two sets of electrode extensions 148, 149 may have length 303 that is at least one electrode pitch and at most six electrode pitches.

[0075] At least in some embodiments, IDT arrangement 140A, 140B, 140C, 140D, 140E may further comprise piston mass loading structure 160 in the vicinity of the tips of interdigitating electrodes 144, 145. For example, piston mass loading structure 160 may comprise continuous mass loading dielectric layers on the tips of interdigitating electrodes 144, 145, mass loads 161 on the tips of interdigitating electrodes 144, 145, and / or hammer heads 162 on the tips of interdigitating electrodes 144, 145, and / or hammer heads 162 may be combined with mass loads and / or mass loading layers.

[0076] At least in some embodiments, the continuous mass loading dielectric layers, mass loads 161, and hammer heads 162 may each have length 304 of no more than 2.5 * electrode pitch in the longitudinal direction of interdigitating electrodes 144, 145.

[0077] At least in some embodiments, mass loads 161 may each have width 309 of less than or equal to electrode width 305. Hammer heads 162 may each have width 310 of less than or equal to 0.75 * electrode pitch.

[0078] At least in some embodiments, piston mass loading structure 160 may comprise the continuous mass loading dielectric layers and / or mass loads 161. At least one longitudinal sub-finger portion 146A, 146B, 147A, 147B and / or subextension portion 148A, 148B, 149A, 149B may have width dwl (an example of which is shown as 307 in Fig. 3A), such that mt * electrode pitch / 2 > dwl > mt * electrode pitch / 4, in which mt denotes a metallization ratio of each interdigitating electrode, and mt * electrode pitch represents width 305 of each interdigitating electrode. At least in some embodiments, the two longitudinal sub-finger portions 146A, 146B, 147A, 147B of each at least partially split dummy finger and / or the two longitudinal sub-electrode portions 148A, 148B, 149A, 149B of each at least partially split electrode extension may have distance gw (an example of which is shown as 308 in Fig. 3A) between them, such that 2 * dwl + gw > mt * electrode pitch. Fig. 3E shows an example in which 2 * dwl + gw > mt * electrode pitch, resulting in a partial overlap of dummy fingers 146, 147 and electrode extensions 148, 149 with respect to interdigitating electrodes 144, 145.

[0079] For example, 0.85 > mt > 0.4.

[0080] At least in some embodiments, nearest outer edges of a dummy finger and an electrode extension may have a distance W (an example of which is shown as 306 in Fig. 3A) between them, such that W = (1-mt) * electrode pitch. For example, at least some of the following may apply: W gw, dwl gw.

[0081] At least in some embodiments, piston mass loading structure 160 may comprise hammer heads 162. At least one longitudinal sub-finger portion 146A, 146B, 147A, 147B and / or sub-extension portion 148A, 148B, 149A, 149B may have width dw2 (an example of which is shown as 311 in Fig. 3B), such that mh * electrode pitch / 2 > dw2 > mh * electrode pitch / 4, in which mh denotes a metallization ratio of each hammer head, and mh * electrode pitch represents width 310 of each hammer head.

[0082] Fig. 2 is a block diagram illustrating electronic apparatus 200 according to an embodiment of the disclosure. Electronic apparatus 200 comprises FSAW resonator element 100 disclosed above. Electronic apparatus 200 may further comprise radio frequency (RF) filter 201. More specifically, at least in some embodiments, one or more of the FSAW resonator elements 100 may be comprised in RF filter 201.

[0083] Any range or device value given herein may be extended or altered without losing the effect sought. Also, any embodiment may be combined with another embodiment unless explicitly disallowed.

[0084] Although the subject matter has been described in language specific to structural features and / or acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as examples of implementing the claims and other equivalent features and acts are intended to be within the scope of the claims.

[0085] It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. It will further be understood that reference to 'an' item may refer to one or more of those items.

[0086] Aspects of any of the embodiments described above may be combined with aspects of any of the other embodiments described to form further embodiments without losing the effect sought.

[0087] The term 'comprising' is used herein to mean including the method, blocks or elements identified, but that such blocks or elements do not comprise an exclusive list and a method or apparatus may contain additional blocks or elements.

[0088] It will be understood that the above description is given by way of example only and that various modifications may be made by those skilled in the art. The above specification, examples and data provide a complete description of the structure and use of exemplary embodiments. Although various embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this specification.

Claims

CLAIMS1. A thin- film surface acoustic wave, FSAW, resonator element (100), comprising: a substrate layer (110); an intermediate layer (120) arranged on the substrate layer (110); a piezoelectric thin-film layer (130) arranged on the intermediate layer (120); and an interdigital transducer, IDT, arrangement (140, 140A, 140B, 140C, 140D, 140E) arranged on the piezoelectric thin-film layer (130), the IDT arrangement (140, 140A, 140B, 140C, 140D, 140E) comprising: two conductive elements (141, 142); an active area (143) arranged between the two conductive elements (141, 142); a plurality of interdigitating electrodes (144, 145) with each interdigitating electrode extending from one of the conductive elements across the active area (143) towards the other of the conductive elements; two sets of dummy fingers (146, 147) with each dummy finger extending from one of the two conductive elements towards a respective tip of an opposing interdigitating electrode and being separated by a gap (150) from the respective tip of the opposing interdigitating electrode; and two sets of electrode extensions (148, 149) with each electrode extension extending from one of the two conductive elements towards a respective opposing interdigitating electrode, the dummy fingers and the electrode extensions alternating with each other in a lateral direction of the interdigitating electrodes (144, 145) and in a longitudinal direction of the interdigitating electrodes (144, 145), wherein at least one dummy finger of the two sets of dummy fingers (146, 147) or at least one electrode extension of the two sets of electrode extensions (148, 149) is at least partially split into two longitudinal sub-finger portions (146A, 146B, 147A, 147B) or sub-extension portions (148A, 148B, 149A, 149B), respectively, along a longitudinal center axis of the respective dummy finger or electrode extension.

2. The FSAW resonator element (100) according to claim 1, wherein each dummy finger in a first group of the dummy fingers is completely split into the two longitudinal sub-finger portions (146A, 146B, 147A, 147B), and / or each electrode extension in a first group of the electrode extensions is completely split into the two longitudinal sub-extension portions (148A, 148B, 149A, 149B).

3. The FSAW resonator element (100) according to claim 1 or 2, wherein each dummy finger in a second group of the dummy fingers is partially split into the two longitudinal sub-finger portions (146A, 146B, 147A, 147B), thereby each partially split dummy finger having a split dummy finger part (146_1, 147_1) and a non- split dummy finger part (146_2, 147_2), and / or each electrode extension in a second group of the electrode extensions is partially split into the two longitudinal sub-electrode portions (148A, 148B, 149A, 149B), thereby each partially split electrode extension having a split electrode extension part ( 148_1 , 149_1) and a non-split electrode extension part (148_2, 149_2).

4. The FSAW resonator element (100) according to claim 3, wherein at least one non-split dummy finger part (146_2, 147_2) and / or at least one non-split electrode extension part (148_2, 149_2) extends from a respective one of the two conductive elements.

5. The FSAW resonator element (100) according to claim 3 or 4, wherein at least one split dummy finger part (146_1, 147_1) and / or at least one split electrode extension part (148_1, 149_1) extends from a respective one of the two conductive elements.

6. The FSAW resonator element (100) according to any of claims 1 to 5, wherein at least one of one or more dummy fingers of the two sets of dummy fingers (146, 147) or one or more electrode extension of the two sets of electrode extensions (148, 149) has a length (303) from one electrode pitch to six electrode pitches.

7. The FSAW resonator element (100) according to any of claims 1 to 6, wherein the IDT arrangement (140A, 140B, 140C, 140D, 140E) further comprises apistonmass loading structure (160) in the vicinity ofthe tips of the interdigitating electrodes (144, 145).

8. The FSAW resonator element (100) according to claim 7, wherein the piston mass loading structure (160) comprises at least one of continuous mass loading dielectric layers on the tips of the interdigitating electrodes (144, 145), mass loads (161) on the tips of the interdigitating electrodes (144, 145), or hammer heads (162) on the tips of the interdigitating electrodes (144, 145).

9. The FSAW resonator element (100) according to claim 8, wherein the continuous mass loading dielectric layers, mass loads (161), and hammer heads (162) each have a length (304) of no more than 2.5 * electrode pitch in the longitudinal direction of the interdigitating electrodes (144, 145).

10. The FSAW resonator element (100) according to claim 8 or 9, wherein the mass loads (161) each have a width (309) of less than or equal to an electrode width (305), and the hammerheads (162) each have a width (310) of less than or equal to 0.75 * electrode pitch.

11. The FSAW resonator element (100) according to any of claims 8 to 10, wherein the piston mass loading structure (160) comprises at least one of the continuous mass loading dielectric layers or the mass loads (161), and at least one longitudinal sub- finger portion (146A, 146B, 147A, 147B) and / or sub-extension portion (148A, 148B, 149A, 149B) has a width dwl (307), such that mt * electrode pitch / 2 > dwl > mt * electrode pitch / 4, in which mt denotes a metallization ratio of each interdigitating electrode, and mt * electrode pitch represents a width (305) of each interdigitating electrode.

12. The FSAW resonator element (100) according to claim 11, wherein the two longitudinal sub-finger portions (146A, 146B, 147A, 147B) of each at least partially split dummy finger and / or the two longitudinal sub-electrode portions (148A, 148B, 149A, 149B) of each at least partially split electrode extension have a distance gw (308) between them, such that 2 * dwl + gw > mt * electrode pitch.

13. The FSAW resonator element (100) according to claim 11 or 12, wherein 0.85 > mt > 0.4.

14. The FSAW resonator element (100) according to any of claims 8 to 10, wherein the piston mass loading structure (160) comprises the hammer heads (162), and at least one longitudinal sub- finger portion (146A, 146B, 147A, 147B) and / or sub-extension portion (148A, 148B, 149A, 149B) has a width dw2 (311), such that mh * electrode pitch / 2 > dw2 > mh * electrode pitch / 4, in which mh denotes a metallization ratio of each hammer head, and mh * electrode pitch represents a width (310) of each hammer head.

15. An electronic apparatus (200) comprising the thin- film surface acoustic wave, FSAW, resonator element (100) according to any one of the previous claims.

16. The electronic apparatus (200) according to claim 15, wherein the FSAW resonator element (100) is comprised in a radio frequency, RF, filter (201).

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