Resonator device and method for mitigating spurious resonances

Abstract

The present disclosure relates to resonator devices associated with frequency filtering. An acoustic resonator device of this disclosure comprises a piezoelectric layer, and at least one pair of interdigital transducer electrodes formed on the piezoelectric layer, each of the interdigital transducer electrodes being connected to a respective busbar. In this regard, each of the interdigital transducer electrodes comprises at least one groove in a top surface of the interdigital transducer electrode, the top surface facing away from the piezoelectric layer.

Description

[0001]RESONATOR DEVICE AND METHOD FOR MITIGATING SPURIOUS RESONANCES TECHNICAL FIELD The present disclosure relates to resonator devices, for example, to transversely-excited film bulk acoustic resonators (XBAR) associated with frequency filtering. BACKGROUND Generally, radio frequency (RF) filters realized with XBAR resonators can be used as the key component in the frequency filtering at frequencies between 3 GHz and 7 GHz, which may fulfil the wide pass band requirements of novel 5G bands above 3.5 GHz. However, the XBAR resonators may create a large number of unwanted spurious resonances, which may destroy the XBAR resonator and filter performance. SUMMARY In view of the above, this disclosure aims to provide an improved acoustic resonator device and a method. An objective is to suppress the spurious resonances emerging in the resonator so that they do not disturb the filter performance at the pass band of the filter. These and other objectives are achieved by the embodiments of this disclosure as described in the independent claims. Advantageous implementations are further defined in the dependent claims. According to a first aspect of this disclosure, an acoustic resonator device is provided. The acoustic resonator device comprises a piezoelectric layer, and at least one pair of interdigital transducer electrodes formed on the piezoelectric layer, each of the interdigital transducer electrodes being connected to a respective busbar. In this regard, each of the interdigital transducer electrodes comprises at least one groove in a top surface of the interdigital transducer electrode, the top surface facing away from the piezoelectric layer. For example, the acoustic resonator device or the XBAR resonator may be realized from piezo on insulator (POI) wafers, where the top layer of the POI wafer may be the piezoelectric layer. The piezoelectric layer may comprise piezoelectric materials such as lithium niobate (LiNbO3), lithium tantalate (LiTaO3), scandium-doped aluminum nitride (ScAlN), aluminum nitride (AlN), zinc oxide (ZnO), zinc sulfide (ZnS), and so on. The XBAR resonator may be formed as a released structure with a cavity below the piezoelectric layer such that the active part of the structure can be released from the substrate. For instance, the interdigital transducer (IDT) electrodes may consist of two interlocking comb- shaped arrays of metallic electrodes to form a periodic structure deposited on the piezoelectric layer. Each of the two interlocking comb-shaped arrays of metallic electrodes may be connected to a respective busbar that may drive electrical signals, e.g., current, to the IDT electrodes. The released structure of the XBAR resonator having the IDT electrodes realized on the piezoelectric layer may allow different modes of Lamb waves to be propagated. Generally, the IDT electrodes of the XBAR resonator may be regarded as one of the main sources of the spurious resonances. Advantageously, the grooves on the top of the IDT electrodes may decrease the piezoelectric coupling coefficient of the spurious resonances emerging in the IDT electrodes, thereby mitigating the spurious resonances. In an implementation form of the first aspect, each of the interdigital transducer electrodes comprises a mesa structure comprising a first material formed on the piezoelectric layer, and at least one layer of a second material formed on the mesa structure. Furthermore, the at least one groove extends through the at least one layer of the second material. In this regard, the first material comprises a metal, and the second material comprises a metal and / or a passivation material. For example, the IDT electrodes may be processed on the piezoelectric layer such that the IDT electrodes may have slanted side walls, i.e., to realize the mesa structures. In this regard, the IDT electrodes, i.e., the mesa structures, may consist of only one material or may include several layers or regions of different materials. For instance, the mesa structure of the IDT electrodes may comprise a bottom electrode comprising a metal layer in contact with the piezoelectric layer, a metal layer on top of the bottom electrode with slanted side walls, and a top electrode comprising a metal layer on top of the metal layer with slanted side walls. The IDT electrodes, i.e., the mesa structure, may further comprise a passivation layer on top of the piezoelectric layer and the electrode layer stacks. The metal layers may comprise metals, such as aluminum (Al), gold (Au), copper (Cu), molybdenum (Mo), tungsten (W), chromium (Cr), and so on. The passivation layer may comprise passivation materials, such as scandium-doped aluminum nitride (ScAlN), silicon dioxide (SiO2), aluminum oxide (Al2O3), silicon nitride (Si3N4), and so on. In an implementation form of the first aspect, the at least one groove has a length along the interdigital transducer electrode, which corresponds to a length of the interdigital transducer electrode extending from the busbar. Alternatively, the at least one groove has a length along the interdigital transducer electrode, which is smaller than a length of the interdigital transducer electrode extending from the busbar. In an implementation form of the first aspect, the at least one groove has a length along the interdigital transducer electrode, which corresponds to a common length of the interdigital transducer electrodes of at least one pair of interdigital transducer electrodes between the respective busbars. Alternatively, the at least one groove has a length along the interdigital transducer electrode, which is smaller than a common length of the interdigital transducer electrodes of at least one pair of interdigital transducer electrodes between the respective busbars. In an implementation form of the first aspect, the at least one groove has a uniform width. In an implementation form of the first aspect, the at least one groove has a first section having a first uniform width, and a second section having a second uniform width, the first uniform width being smaller than the second uniform width. In an implementation form of the first aspect, the first section is formed at or around a mid- point of the length of the interdigital transducer electrode extending from the busbar. Alternatively, the second section is formed at or around a mid-point of the length of the interdigital transducer electrode extending from the busbar. In an implementation form of the first aspect, the at least one groove has a first section having a uniform width, and a second section having a non-uniform width. In an implementation form of the first aspect, the second section having the non-uniform width comprises one or more angled groove sections of the at least one groove. In an implementation form of the first aspect, the first section is formed at or around a mid- point of the length of the interdigital transducer electrode extending from the busbar. Alternatively, the second section is formed at or around a mid-point of the length of the interdigital transducer electrode extending from the busbar. In an implementation form of the first aspect, the second section has a length along the interdigital transducer electrode, which corresponds to the common length of the interdigital transducer electrodes of at least one pair of interdigital transducer electrodes between the respective busbars. Alternatively, the second section has a length along the interdigital transducer electrode, which is smaller than the common length of the interdigital transducer electrodes of at least one pair of interdigital transducer electrodes between the respective busbars. In an implementation form of the first aspect, each of the interdigital transducer electrodes comprises a number of N grooves in the top surface of the interdigital transducer electrode, where N is a positive integer. In this regard, each groove of the number of N grooves has a uniform width. Additionally or alternatively, the number of N grooves are parallel to each other. Advantageously, the dimensions and / or the number of grooves can be selected based on, for example, the width of the IDT electrodes and / or the order of the spurious resonances and / or the magnitude of the spurious resonances to be mitigated. According to a second aspect of this disclosure, a method is provided for forming an acoustic resonator device. The method comprises the steps of providing a piezoelectric layer, forming at least one pair of interdigital transducer electrodes on the piezoelectric layer such that each of the interdigital transducer electrodes is connected to a respective busbar, and forming at least one groove in a top surface of each of the interdigital transducer electrodes. In an implementation form of the second aspect, forming the at least one pair of interdigital transducer electrodes comprises the steps of forming a mesa structure comprising a first material on the piezoelectric layer, and forming at least one layer of a second material on the mesa structure. In an implementation form of the second aspect, forming the at least one groove comprises the step of etching the at least one layer of the second material from the top surface, to form the at least one groove through the at least one layer of the second material. For example, the grooves can be etched from the top of the IDT electrodes partially through the passivation layer, or completely through the passivation layer. In case the IDT electrodes may have multiple metal layers, the grooves can be etched from the top of the IDT electrodes through the passivation layer and the top contact of the mesa structure. Alternatively, forming the at least one groove comprises the step of etching the at least one layer of the second material and partially the mesa structure from the top surface, to form the at least one groove through the at least one layer of the second material and partially through the mesa structure. For example, the grooves can be etched from the top of the IDT electrodes through the passivation layer and partially through the mesa structure. In case the IDT electrodes may have multiple metal layers, the grooves can be etched from the top of the IDT electrodes through the passivation layer, the top contact, and partially through the metal layer with slanted side walls. Alternatively, forming the at least one groove comprises the step of etching the at least one layer of the second material, the mesa structure, and partially the piezoelectric layer from the top surface, to form the at least one groove through the at least one layer of the second material, the mesa structure, and partially through the piezoelectric layer. For example, the grooves can be etched from the top of the IDT electrodes through the passivation layer, the mesa structure, and partially through the piezoelectric layer. In case the IDT electrodes may have multiple metal layers, the grooves can be etched from the top of the IDT electrodes through the passivation layer, the top contact, the metal layer with slanted side walls, the bottom contact, and partially through the piezoelectric layer. It is to be noted that the method according to the second aspect corresponds to the acoustic resonator device according to the first aspect and its implementation forms. Further, the method of the third aspect achieves the same advantages and effects as the acoustic resonator device of the first aspect and its respective implementation forms. BRIEF DESCRIPTION OF THE DRAWINGS The above described aspects and implementation forms will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which: Fig.1A shows an exemplary diagram of a XBAR resonator according to this disclosure; Fig.1B shows a cross-sectional view of the XBAR resonator of Fig.1A; Fig.2A shows an IDT electrode pair according to this disclosure; Fig.2B shows a three-dimensional view of the IDT electrode pair of Fig.2A; Fig.3A shows an exemplary IDT electrode and its response in Smith chart; Fig.3B shows an IDT electrode according to this disclosure and its response in Smith chart; Fig. 3C shows a further IDT electrode according to this disclosure and its response in Smith chart; Fig.4A shows a first exemplary embodiment of the IDT electrode pair; Fig.4B shows a second exemplary embodiment of the IDT electrode pair; Fig.4C shows a third exemplary embodiment of the IDT electrode pair; Fig.4D shows a fourth exemplary embodiment of the IDT electrode pair; Fig.5A shows a fifth exemplary embodiment of the IDT electrode pair; Fig.5B shows a sixth exemplary embodiment of the IDT electrode pair; Fig.5C shows a seventh exemplary embodiment of the IDT electrode pair; Fig.6A shows an eighth exemplary embodiment of the IDT electrode pair; Fig.6B shows a ninth exemplary embodiment of the IDT electrode pair; Fig.6C shows a tenth exemplary embodiment of the IDT electrode pair; Fig.6D shows an eleventh exemplary embodiment of the IDT electrode pair; Fig.7A shows a twelfth exemplary embodiment of the IDT electrode pair; Fig.7B shows a thirteenth exemplary embodiment of the IDT electrode pair; Fig.7C shows a fourteenth exemplary embodiment of the IDT electrode pair; Fig.7D shows a fifteenth exemplary embodiment of the IDT electrode pair; Fig.8A shows a sixteenth exemplary embodiment of the IDT electrode pair; Fig.8B shows a seventeenth exemplary embodiment of the IDT electrode pair; Fig.8C shows an eighteenth exemplary embodiment of the IDT electrode pair; and Fig.9 shows an exemplary flow diagram of the method according to this disclosure. DETAILED DESCRIPTION OF EMBODIMENTS Reference will now be made in detail to exemplary embodiments of the present disclosure, which are illustrated in the accompanying drawings. However, the following embodiments of the present disclosure may be variously modified and the range of the disclosure is not limited by the following embodiments. Reference signs for similar entities in different embodiments are partially omitted. Fig. 1A and Fig. 1B show an exemplary diagram of a XBAR resonator 100 according to the first aspect of this disclosure. In particular, Fig.1A shows a top view of the XBAR resonator 100 and Fig.1B shows a cross-sectional view of the XBAR resonator 100. The XBAR resonator 100 may comprise a piezoelectric layer 101, and a plurality of IDT electrodes or fingers 102, 103 formed on the piezoelectric layer 101. The XBAR resonator 100 may further comprise busbars 104, 105 to drive electric current to the IDT electrodes 102, 103. For example, the IDT electrode 102 may be connected to the busbar 104 and the IDT electrode 103 may be connected to the busbar 105. The XBAR resonator 100 may be created from a POI wafer, where the top layer of the POI wafer may be the piezoelectric layer 101. The second layer 106 below the piezoelectric layer 101 may be a cavity. An insulating layer may be present below the piezoelectric layer, e.g., a layer of SiO2 or Si3N4. The third layer 107 below the second layer or cavity 106 may be a thick single silicon substrate. For instance, the piezoelectric layer 101 and the insulating layer may have a thickness below 2 micrometers, and the silicon substrate 107 may have a thickness of several hundreds of micrometers. Below the active area of the XBAR resonator 100, i.e., the area containing the IDT electrodes 102, 103, the insulating layer may be release etched or the backside with the insulating layer and the silicon substrate 107 may be fully etched off. In this example, the IDT electrodes 102, 103 are shown as two interlocking comb-shaped arrays of electrodes forming a periodic structure deposited on the piezoelectric layer 101. Each of the two interlocking comb-shaped arrays of electrodes may be respectively connected to the busbars 104, 105. For example, the IDT electrode array comprising the IDT electrode 102 may be connected to the busbar 104 and the IDT electrode array comprising the IDT electrode 103 may be connected to the busbar 105. Fig.2A and Fig.2B show the IDT electrode pair 102, 103 in detail. In particular, Fig.2A shows a two-dimensional view of the IDT electrode pair 102, 103 and Fig. 2B shows a three- dimensional view of the IDT electrode pair 102, 103. For example, the IDT electrodes 102, 103 may be formed on the piezoelectric layer 101 via deposition. In this regard, a respective mesa structure or a structure with slanted side walls 201 may be formed on the piezoelectric layer 101 in order to realize the IDT electrodes 102, 103. In this regard, the mesa structure 201 may be a single metal structure or may have several layers or regions of different metals. For instance, the mesa structure 201 may comprise a bottom electrode 203, e.g., a Cr layer, in contact with the piezoelectric layer 101, an Al layer on top of the bottom electrode with slanted side walls, and a top electrode 204, e.g., a Cr layer, on top of the Al layer. A passivation layer 205 may be added on top of the piezoelectric layer 101 and the mesa structure 201 of the IDT electrodes 102, 103. Furthermore, each of the IDT electrodes 102, 103 may comprise a groove 202 in a top surface of the mesa structure 201, i.e., the mesa surface facing away from the piezoelectric layer 101. In the case the mesa structure 201 of the IDT electrodes 102, 103 is a single metal structure, the grooves 202 may be formed or etched from the top surface partially or fully extending through the mesa structure 201 of the IDT electrodes 102, 103, and partially extending through the piezoelectric layer 101. In the case the mesa structure 201 of the IDT electrodes 102, 103 has several metal layers, the grooves 202 may be formed or etched from the top surface partially or fully extending through the passivation layer, partially or fully extending through the top electrode 204, partially or fully extending through the metal layer with slated side walls, partially or fully extending through the bottom electrode 203, and partially extending through the piezoelectric layer 101. In this example, the grooves 202 are formed on the top surface of the IDT electrodes 102, 103 such that the grooves extending through the passivation layer 205 on top of the mesa structure 201 and the top electrode 204. Fig.3A, Fig.3B, and Fig.3C show the effect of IDT electrodes on the resonator performance. In particular, Fig.3A shows an exemplary IDT electrode 300 and its S11 response in the Smith chart. For example, the IDT electrode 300 may be a multilayer electrode, i.e., comprising a bottom electrode connecting the piezoelectric layer 101, and a top electrode covered with a passivation layer. Wf may denote the width of the IDT electrode 300 at the top surface and We may denote the width of the top electrode. The spurious resonances below the resonance, and between the resonance and anti-resonance can be observed in the Smith chart. Fig.3B shows a first modification on the IDT electrode 102 according to this disclosure and its S11 response in the Smith chart. In this modification, a groove is formed on top of the IDT electrode 102 that may extend through the passivation layer and the top electrode. The width of the groove Wg may be defined as: ^^= 0.4 × ^^, or ^^= 0.3 × ^^As can be observed in the Smith chart, the S11 response improves compared to the S11 response shown in Fig.3A. Fig.3C shows a second modification on the IDT electrode 102 according to this disclosure and its S11 response in the Smith chart. In this modification, a groove is formed on top of the IDT electrode 102 that may extend through the passivation layer and the top electrode. The width of the groove Wg may be defined as: ^^= 0.6 × ^^, or ^^= 0.6 × ^^As can be observed in the Smith chart, the S11 response massively improves compared to the S11 response shown in Fig.3A. Therefore, in the spurious resonance suppression, the edges of the groove may create a mass load to the resonances emerging in the electrode region, by mitigating the emerging area of the resonance. In Fig. 4A, a first exemplary embodiment of the IDT electrode pair 1021, 1031 is illustrated. The IDT electrode 1021 may be connected to the busbar 104 and the IDT electrode 1031 may be connected to the busbar 105. Le may denote the length of the IDT electrodes 1021, 1031from its respective busbar 104, 105. A single respective groove 2021may be formed on top of each of the IDT electrodes 1021, 1031, which may extend through the entire length Le of the IDT electrodes 1021, 1031. The groove 2021may have a uniform width with respect to the width of the top surface of the IDT electrodes 1021, 1031 or to the width of the top electrode of the IDT electrodes 1021, 1031. In Fig.4B, a second exemplary embodiment of the IDT electrode pair 1022, 1032 is illustrated. The IDT electrode 1022 may be connected to the busbar 104 and the IDT electrode 1032 may be connected to the busbar 105. Lc may denote the common length of the IDT electrode pair 1022, 1032 on the piezoelectric layer. A single respective groove 2022 may be formed on top of each of the IDT electrodes 1022, 1032, which may extend through the entire common length Lc of the IDT electrode pair 1022, 1032. The groove 2022may have a uniform width with respect to the width of the top surface of the IDT electrodes 1022, 1032 or to the width of the top electrode of the IDT electrodes 1022, 1032. In Fig. 4C, a third exemplary embodiment of the IDT electrode pair 1023, 1033is illustrated. The IDT electrode 1023 may be connected to the busbar 104 and the IDT electrode 1033 may be connected to the busbar 105. A single respective groove 2023may be formed on top of each of the IDT electrodes 1023, 1033, where Lg may denote the length of the groove 2023.The groove length Lg may be smaller than Le, i.e., the length of the IDT electrodes 1023, 1033 form its respective busbar 104, 105. For example, the groove 2023 may start from the respective busbar 104, 105 and may extend through the length of the respective IDT electrodes 1023, 1033up to the length Lg. The groove 2023may have a uniform width with respect to the width of the top surface of the IDT electrodes 1023, 1033 or to the width of the top electrode of the IDT electrodes 1023, 1033. In Fig.4D, a fourth exemplary embodiment of the IDT electrode pair 1024, 1034 is illustrated. The IDT electrode 1024 may be connected to the busbar 104 and the IDT electrode 1034 may be connected to the busbar 105. A single respective groove 2024may be formed on top of each of the IDT electrodes 1024, 1034, where Lg may denote the length of the groove 2024. The groove length Lg may be smaller than Lc, i.e., the common length of the IDT electrode pair 1024, 1034on the piezoelectric layer. The groove 2024may have a uniform width with respect to the width of the top surface of the IDT electrodes 1024, 1034 or to the width of the top electrode of the IDT electrodes 1024, 1034. In Fig. 5A, a fifth exemplary embodiment of the IDT electrode pair 1025, 1035is illustrated. The IDT electrode 1025may be connected to the busbar 104 and the IDT electrode 1035may be connected to the busbar 105. A single respective groove 2025 may be formed on top of each of the IDT electrodes 1025, 1035. The groove 2025may have a first groove section 501 and a second groove section 502. The first groove section 501 may have a width smaller than a width of the second groove section 502. The first groove section 501 may be formed at or around the mid-point of Le, i.e., the length of the IDT electrodes 1025, 1035 form its respective busbar 104, 105. In other words, the groove 2025may be wider at the ends than at the center. The groove 2025 may extend through the entire length Le of the IDT electrodes 1025, 1035. Alternatively, the groove 2025may extend through the entire common length Lc of the IDT electrode pair 1025, 1035, as shown in Fig. 4B. Alternatively, the groove 2025 may start from the respective busbar 104, 105 and may extend through the length of the respective IDT electrodes 1025, 1035 up to the length Lg, as shown in Fig.4C. Alternatively, the groove 2025 may have a length smaller than the common length Lc of the IDT electrode pair 1025, 1035, as shown in Fig.4D. In Fig. 5B, a sixth exemplary embodiment of the IDT electrode pair 1026, 1036is illustrated. The IDT electrode 1026 may be connected to the busbar 104 and the IDT electrode 1036 may be connected to the busbar 105. A single respective groove 2026 may be formed on top of each of the IDT electrodes 1026, 1036. The groove 2026may have a first groove section 501 and a second groove section 502. The first groove section 501 may have a width smaller than a width of the second groove section 502. The second groove section 502 may be formed at or around the mid-point of Le, i.e., the length of the IDT electrodes 1026, 1036form its respective busbar 104, 105. In other words, the groove 2026may be narrower at the ends than at the center. The groove 2026 may extend through the entire length Le of the IDT electrodes 1026, 1036. Alternatively, the groove 2026may extend through the entire common length Lc of the IDT electrode pair 1026, 1036, as shown in Fig. 4B. Alternatively, the groove 2026 may start from the respective busbar 104, 105 and may extend through the length of the respective IDT electrodes 1026, 1036up to the length Lg, as shown in Fig.4C. Alternatively, the groove 2026may have a length smaller than the common length Lc of the IDT electrode pair 1026, 1036, as shown in Fig.4D. In Fig.5C, a seventh exemplary embodiment of the IDT electrode pair 1027, 1037is illustrated. The IDT electrode 1027may be connected to the busbar 104 and the IDT electrode 1037may be connected to the busbar 105. A single respective groove 2027 may be formed on top of each of the IDT electrodes 1027, 1037. The groove 2027may have a first groove section 501 and a second groove section 502. The first groove section 501 may have a width smaller than a width of the second groove section 502. The first groove section 501 may be formed from one end and the second groove section 502 may be formed from the other end of the IDT electrodes 1027, 1037such that the groove 2027may be wider at one end than the other end. For example, the first groove section 501 may extend from the respective busbar 104, 105 and the second groove section 502 may be formed at the end opposite to the busbar 104, 105, or vice versa. The groove 2027 may extend through the entire length Le of the IDT electrodes 1027, 1037. Alternatively, the groove 2027 may extend through the entire common length Lc of the IDT electrode pair 1027, 1037, as shown in Fig. 4B. Alternatively, the groove 2027 may start from the respective busbar 104, 105 and may extend through the length of the respective IDT electrodes 1027, 1037 up to the length Lg, as shown in Fig.4C. Alternatively, the groove 2027 may have a length smaller than the common length Lc of the IDT electrode pair 1027, 1037, as shown in Fig.4D. In Fig.6A, an eighth exemplary embodiment of the IDT electrode pair 1028, 1038 is illustrated. The IDT electrode 1028may be connected to the busbar 104 and the IDT electrode 1038may be connected to the busbar 105. A single respective groove 2028may be formed on top of each of the IDT electrodes 1028, 1038. The groove 2028 may have a first groove section 601 and a second groove section 602. The first groove section 601 may have a width smaller than a width of the second groove section 602. Moreover, the second groove section 602 may be an angled groove or an angled groove section, i.e., wider at one end than the other end. For instance, the wider end of the second groove section 602 may be formed at the end of the IDT electrodes 1028, 1038opposite to the respective busbars 104, 105. For example, the first groove section 601 may extend from the respective busbar 104, 105 and the second groove section 602 may be formed at the end opposite to the busbar 104, 105, or vice versa. The groove 2028 may extend through the entire length Le of the IDT electrodes 1028, 1038. Alternatively, the groove 2028 may extend through the entire common length Lc of the IDT electrode pair 1028, 1038, as shown in Fig. 4B. Alternatively, the groove 2028may start from the respective busbar 104, 105 and may extend through the length of the respective IDT electrodes 1028, 1038up to the length Lg, as shown in Fig.4C. Alternatively, the groove 2028may have a length smaller than the common length Lc of the IDT electrode pair 1028, 1038, as shown in Fig.4D. In Fig.6B, a ninth exemplary embodiment of the IDT electrode pair 1029, 1039 is illustrated. The IDT electrode 1029 may be connected to the busbar 104 and the IDT electrode 1039 may be connected to the busbar 105. A single respective groove 2029may be formed on top of each of the IDT electrodes 1029, 1039. The groove 2029 may have a first groove section 601 and a second groove section 602. The first groove section 601 may have a width smaller than a width of the second groove section 602. Moreover, the second groove section 602 may be an angled groove or an angled groove section, i.e., wider at one end than the other end. The first groove section 601 may be formed at or around the mid-point of Le, i.e., the length of the IDT electrodes 1029, 1039from its respective busbar 104, 105. In other words, the groove 2029 may be wider at the ends than at the center. For instance, the wider end of the second groove section 602 may be formed at the busbars 104, 105 as well as at the end of the IDT electrodes 1029, 1039opposite to the respective busbars 104, 105. The groove 2029 may extend through the entire length Le of the IDT electrodes 1029, 1039. Alternatively, the groove 2029may extend through the entire common length Lc of the IDT electrode pair 1029, 1039, as shown in Fig. 4B. Alternatively, the groove 2029may start from the respective busbar 104, 105 and may extend through the length of the respective IDT electrodes 1029, 1039up to the length Lg, as shown in Fig.4C. Alternatively, the groove 2029may have a length smaller than the common length Lc of the IDT electrode pair 1029, 1039, as shown in Fig.4D. In Fig.6C, a tenth exemplary embodiment of the IDT electrode pair 10210, 10310is illustrated. The IDT electrode 10210 may be connected to the busbar 104 and the IDT electrode 10310 may be connected to the busbar 105. A single respective groove 20210 may be formed on top of each of the IDT electrodes 10210, 10310. The groove 20210may have a first groove section 601 and a second groove section 602. The first groove section 601 may have a width smaller than a width of the second groove section 602. Moreover, the second groove section 602 may be an angled groove or an angled groove section, i.e., narrower at one end than the other end. The first groove section 601 may be formed at or around the mid-point of Le, i.e., the length of the IDT electrodes 10210, 10310 form its respective busbar 104, 105. In other words, the groove 20210may be wider at the ends than at the center. For instance, the narrower end of the second groove section 602 may be formed at the busbars 104, 105 as well as at the end of the IDT electrodes 10210, 10310 opposite to the respective busbars 104, 105. The groove 20210may extend through the entire length Le of the IDT electrodes 10210, 10310. Alternatively, the groove 20210 may extend through the entire common length Lc of the IDT electrode pair 10210, 10310, as shown in Fig.4B. Alternatively, the groove 20210 may start from the respective busbar 104, 105 and may extend through the length of the respective IDT electrodes 10210, 10310 up to the length Lg, as shown in Fig.4C. Alternatively, the groove 20210 may have a length smaller than the common length Lc of the IDT electrode pair 10210, 10310, as shown in Fig.4D. In Fig. 6D, an eleventh exemplary embodiment of the IDT electrode pair 10211, 10311 is illustrated. The IDT electrode 10211may be connected to the busbar 104 and the IDT electrode 10311may be connected to the busbar 105. A single respective groove 20211may be formed on top of each of the IDT electrodes 10211, 10311. The groove 20211 may have a first groove section 601 and a second groove section 602. The first groove section 601 may have a width smaller than a width of the second groove section 602. Moreover, the second groove section 602 may be an angled groove or an angled groove section, i.e., wider or narrower at one end than the other end. The first groove section 601 may be formed at or around the mid-point of Le, i.e., the length of the IDT electrodes 10211, 10311 from its respective busbar 104, 105. In other words, the groove 20211 may be wider at the ends than at the center. For instance, for the IDT electrode 10211, the narrower end of the second groove section 602 may be formed at the busbar 104 as well as at the end of the IDT electrode 10211 opposite to the busbar 104. Additionally, for the IDT electrode 10311, the wider end of the second groove section 602 may be formed at the busbar 105 as well as at the end of the IDT electrode 10311opposite to the busbar 105. The groove 20211 may extend through the entire length Le of the IDT electrodes 10211, 10311. Alternatively, the groove 20211may extend through the entire common length Lc of the IDT electrode pair 10211, 10311, as shown in Fig.4B. Alternatively, the groove 20211may start from the respective busbar 104, 105 and may extend through the length of the respective IDT electrodes 10211, 10311 up to the length Lg, as shown in Fig.4C. Alternatively, the groove 20211 may have a length smaller than the common length Lc of the IDT electrode pair 10211, 10311, as shown in Fig.4D. In Fig.7A, a twelfth exemplary embodiment of the IDT electrode pair 10212, 10312 is illustrated. The IDT electrode 10212may be connected to the busbar 104 and the IDT electrode 10312may be connected to the busbar 105. A single respective groove 20212 may be formed on top of each of the IDT electrodes 10212, 10312. The groove 20212 may comprise an angled groove 701 having wider ends than the center region. The narrower center region of the angled groove 701 may be formed at or around the mid-point of the length Le of the IDT electrodes 10212, 10312 or the mid-point of the common length Lc of the IDT electrode pair 10212, 10312. The angled groove 701 may cover the entire common length Lc of the IDT electrode pair 10212, 10312. In Fig. 7B, a thirteenth exemplary embodiment of the IDT electrode pair 10213, 10313 is illustrated. The IDT electrode 10213may be connected to the busbar 104 and the IDT electrode 10313may be connected to the busbar 105. A single respective groove 20213may be formed on top of each of the IDT electrodes 10213, 10313. The groove 20213 may comprise an angled groove 702 having narrower ends than the center region. The wider center region of the angled groove 702 may be formed at or around the mid-point of the length Le of the IDT electrodes 10213, 10313or the mid-point of the common length Lc of the IDT electrode pair 10213, 10313. The angled groove 702 may cover the entire common length Lc of the IDT electrode pair 10213, 10313. In Fig. 7C, a fourteenth exemplary embodiment of the IDT electrode pair 10214, 10314 is illustrated. The IDT electrode 10214 may be connected to the busbar 104 and the IDT electrode 10314may be connected to the busbar 105. A single respective groove 20214may be formed on top of each of the IDT electrodes 10214, 10314. The groove 20214 may comprise an angled groove 703 having wider ends than the center region. The narrower center region of the angled groove 703 may be formed at or around the mid-point of the length Le of the IDT electrodes 10214, 10314 or the mid-point of the common length Lc of the IDT electrode pair 10214, 10314. The angled groove 703 may have a length smaller than Lc, i.e., may partly cover the common length Lc of the IDT electrode pair 10214, 10314. In Fig. 7D, a fifteenth exemplary embodiment of the IDT electrode pair 10215, 10315is illustrated. The IDT electrode 10215 may be connected to the busbar 104 and the IDT electrode 10315may be connected to the busbar 105. A single respective groove 20215may be formed on top of each of the IDT electrodes 10215, 10315. The groove 20215may comprise an angled groove 704 having narrower ends than the center region. The wider center region of the angled groove 704 may be formed at or around the mid-point of the length Le of the IDT electrodes 10215, 10315or the mid-point of the common length Lc of the IDT electrode pair 10215, 10315. The angled groove 704 may have a length smaller than Lc, i.e., may partly cover the common length Lc of the IDT electrode pair 10215, 10315. In Fig. 8A, a sixteenth exemplary embodiment of the IDT electrode pair 10216, 10316 is illustrated. The IDT electrode 10216 may be connected to the busbar 104 and the IDT electrode 10316 may be connected to the busbar 105. A set of grooves 800 may be respectively formed on top of each of the IDT electrodes 10216, 10316. The set of grooves 800 may comprise two parallel grooves, each having a uniform width with respect to the width of the top surface of the IDT electrodes 10216, 10316or to the width of the top electrode of the IDT electrodes 10216, 10316. Furthermore, the set of grooves 800 may have equal or different uniform widths with respect to each other. The set of grooves 800 may extend through the entire length Le of the IDT electrodes 10216, 10316. Alternatively, the set of grooves 800 may extend through the entire common length Lc of the IDT electrode pair 10216, 10316, as shown in Fig. 4B. Alternatively, the set of grooves 800 may start from the respective busbar 104, 105 and may extend through the length of the respective IDT electrodes 10216, 10316up to the length Lg, as shown in Fig.4C. Alternatively, the set of grooves 800 may have a length smaller than the common length Lc of the IDT electrode pair 10216, 10316, as shown in Fig.4D. In Fig. 8B, a seventeenth exemplary embodiment of the IDT electrode pair 10217, 10317 is illustrated. The IDT electrode 10217 may be connected to the busbar 104 and the IDT electrode 10317may be connected to the busbar 105. A set of grooves 800 may be respectively formed on top of each of the IDT electrodes 10217, 10317. The set of grooves 800 may comprise three parallel grooves, each having a uniform width with respect to the width of the top surface of the IDT electrodes 10217, 10317or to the width of the top electrode of the IDT electrodes 10217, 10317. Furthermore, the set of grooves 800 may have equal or different uniform widths with respect to each other. The set of grooves 800 may extend through the entire length Le of the IDT electrodes 10217, 10317. Alternatively, the set of grooves 800 may extend through the entire common length Lc of the IDT electrode pair 10217, 10317, as shown in Fig. 4B. Alternatively, the set of grooves 800 may start from the respective busbar 104, 105 and may extend through the length of the respective IDT electrodes 10217, 10317up to the length Lg, as shown in Fig.4C. Alternatively, the set of grooves 800 may have a length smaller than the common length Lc of the IDT electrode pair 10217, 10317, as shown in Fig.4D. In Fig. 8C, an eighteenth exemplary embodiment of the IDT electrode pair 10218, 10318 is illustrated. The IDT electrode 10218 may be connected to the busbar 104 and the IDT electrode 10318 may be connected to the busbar 105. A set of grooves 800 may be respectively formed on top of each of the IDT electrodes 10218, 10318. The set of grooves 800 may comprise four parallel grooves, each having a uniform width with respect to the width of the top surface of the IDT electrodes 10218, 10318or to the width of the top electrode of the IDT electrodes 10218, 10318. Furthermore, the set of grooves 800 may have equal or different uniform widths with respect to each other. The set of grooves 800 may extend through the entire length Le of the IDT electrodes 10218, 10318. Alternatively, the set of grooves 800 may extend through the entire common length Lc of the IDT electrode pair 10218, 10318, as shown in Fig. 4B. Alternatively, the set of grooves 800 may start from the respective busbar 104, 105 and may extend through the length of the respective IDT electrodes 10218, 10318up to the length Lg, as shown in Fig.4C. Alternatively, the set of grooves 800 may have a length smaller than the common length Lc of the IDT electrode pair 10218, 10318, as shown in Fig.4D. It is to be noted that the set of grooves 800 formed on top of each of the IDT electrodes 10218, 10318 may comprise more than four grooves, where the number of grooves may be defined based on, e.g., the width of the IDT electrodes. In Fig.9, an exemplary embodiment of the method 900 according to the second aspect of this disclosure is illustrated. In a first step 901, a piezoelectric layer is provided. In a second step 902, at least one pair of interdigital transducer electrodes is formed on the piezoelectric layer such that each of the interdigital transducer electrodes is connected to a respective busbar. In a third step 903, at least one groove is formed in a top surface of each of the interdigital transducer electrodes. It should be understood that, several resonating modes emerging inside the XBAR resonator may create unwanted spurious resonance, where the IDT electrodes of the XBAR resonator may be regarded as one of the main sources for the spurious resonances. The embodiments of this disclosure facilitate the modification of the IDT electrode shapes, for example, by adding one or several grooves at the top of the IDT electrode structure, so that the unwanted spurious resonances can be partly or fully suppressed. It is important to note that, in the description as well as in the claims, the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. Moreover, the disclosure with regard to any of the aspects is also relevant with regard to the other aspects of the disclosure. Although the disclosure has been illustrated and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In addition, while a particular feature of this disclosure may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application.

Claims

CLAIMS 1. An acoustic resonator device (100) comprising: a piezoelectric layer (101), and at least one pair of interdigital transducer electrodes (102, 103) formed on the piezoelectric layer (101), each of the interdigital transducer electrodes (102, 103) being connected to a respective busbar (104, 105), wherein each of the interdigital transducer electrodes (102, 103) comprises at least one groove (202) in a top surface of the interdigital transducer electrode (102, 103), the top surface facing away from the piezoelectric layer (101).

2. The acoustic resonator device according to claim 1, wherein each of the interdigital transducer electrodes (102, 103) comprises: a mesa structure (201) comprising a first material formed on the piezoelectric layer (101), at least one layer (204, 205) of a second material formed on the mesa structure (201), wherein the at least one groove (202) extends through the at least one layer (204, 205) of the second material.

3. The acoustic resonator device according to claim 2, wherein: the first material comprises a metal, and the second material comprises a metal and / or a passivation material.

4. The acoustic resonator device according to any of claims 1 to 3, wherein: the at least one groove (202) has a length (Lg) along the interdigital transducer electrode (102, 103), which corresponds to a length (Le) of the interdigital transducer electrode (102, 103) extending from the busbar (104, 105), or the at least one groove (202) has a length (Lg) along the interdigital transducer electrode (102, 103), which is smaller than a length (Le) of the interdigital transducer electrode (102, 103) extending from the busbar (104, 105).

5. The acoustic resonator device according to any of claims 1 to 3, wherein: the at least one groove (202) has a length (Lg) along the interdigital transducer electrode (102, 103), which corresponds to a common length (Lc) of the interdigital transducer electrodes(102, 103) of at least one pair of interdigital transducer electrodes between the respective busbars (104, 105), or the at least one groove (202) has a length (Lg) along the interdigital transducer electrode (102, 103), which is smaller than a common length (Lc) of the interdigital transducer electrodes (102, 103) of at least one pair of interdigital transducer electrodes between the respective busbars (104, 105).

6. The acoustic resonator device according to any of claims 1 to 5, wherein the at least one groove (202) has a uniform width.

7. The acoustic resonator device according to any of claims 1 to 5, wherein the at least one groove (202) has a first section (501) having a first uniform width, and a second section (502) having a second uniform width, the first uniform width being smaller than the second uniform width.

8. The acoustic resonator device according to claim 7, wherein: the first section (501) is formed at or around a mid-point of the length (Le) of the interdigital transducer electrode (102, 103) extending from the busbar (104, 105), or the second section (502) is formed at or around a mid-point of the length (Le) of the interdigital transducer electrode (102, 103) extending from the busbar (104, 105).

9. The acoustic resonator device according to any of claims 1 to 5, wherein the at least one groove (202) has a first section (601) having a uniform width, and a second section (602) having a non-uniform width.

10. The acoustic resonator device according to claim 9, wherein the second section (602) having the non-uniform width comprises one or more angled groove sections (701, 702, 703, 704) of the at least one groove (202).

11. The acoustic resonator device according to claim 9 or 10, wherein: the first section (601) is formed at or around a mid-point of the length (Le) of the interdigital transducer electrode (102, 103) extending from the busbar (104, 105), or the second section (602) is formed at or around a mid-point of the length (Le) of the interdigital transducer electrode (102, 103) extending from the busbar (104, 105).

12. The acoustic resonator device according to any of claims 9 to 11, wherein: the second section (602) has a length along the interdigital transducer electrode (102, 103), which corresponds to the common length (Lc) of the interdigital transducer electrodes (102, 103) of at least one pair of interdigital transducer electrodes between the respective busbars (104, 105), or the second section (602) has a length along the interdigital transducer electrode (102, 103), which is smaller than the common length (Lc) of the interdigital transducer electrodes (102, 103) of at least one pair of interdigital transducer electrodes between the respective busbars (104, 105).

13. The acoustic resonator device according to any of claims 1 to 12, wherein: each of the interdigital transducer electrodes (102, 103) comprises a number of N grooves (800) in the top surface of the interdigital transducer electrode (102, 103), where N is a positive integer, and each groove of the number of N grooves (800) has a uniform width, and / or the number of N grooves (800) are parallel to each other.

14. A method (900) for forming an acoustic resonator device, comprising: providing (901) a piezoelectric layer, forming (902) at least one pair of interdigital transducer electrodes on the piezoelectric layer such that each of the interdigital transducer electrodes is connected to a respective busbar, and forming (903) at least one groove in a top surface of each of the interdigital transducer electrodes.

15. The method according to claim 14, wherein forming the at least one pair of interdigital transducer electrodes comprises: forming a mesa structure comprising a first material on the piezoelectric layer, and forming at least one layer of a second material on the mesa structure.

16. The method according to claim 14, wherein forming the at least one groove further comprises:etching the at least one layer of the second material from the top surface, to form the at least one groove through the at least one layer of the second material, or etching the at least one layer of the second material and partially the mesa structure from the top surface, to form the at least one groove through the at least one layer of the second material and partially through the mesa structure, or etching the at least one layer of the second material, the mesa structure, and partially the piezoelectric layer from the top surface, to form the at least one groove through the at least one layer of the second material, the mesa structure, and partially through the piezoelectric layer.

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