Electro acoustic resonator and RF filter

The present invention refers to electro acoustic resonators that may be combined to establish RF filters that may be used in wireless communication devices. Electro acoustic resonators can be electrically combined, e.g. in a ladder-type like circuit topology or in a lattice-type like circuit topology, to establish RF filters such as bandpass filters or band rejection filters. Such filters can be used in wireless communication devices. The trend towards miniaturization demands for smaller spatial dimensions. The trend towards a higher number of wireless functions results in stricter specifications that have to be complied with. Thus, there is a general problem of providing resonators for filters with good electric and acoustic performance that comply with specifications.

Conventional electro acoustic resonators can comprise an acoustic track in which acoustic waves can propagate. An electrode structure is arranged on a piezoelectric material and converts - due to the piezoelectric effect - between electromagnetic RF signals and acoustic RF signals that propagate in the acoustic track. Typically, it is desired to have a single acoustic wave mode. However, in real transducers spurious modes can be excited that deteriorate the acoustic and electric performance of the resonator and, correspondingly, of the RF filter.

From US 2013/0051588 Ai electro acoustic transducers and corresponding resonators with reduced losses and with a reduced transversal emission of acoustic energy and improved performance and an improved suppression of transversal modes are known.

However, it was found that the technical measures disclosed therein may have reduced effects in a new type of electro acoustic resonator that use the piezoelectric material provided as a thin film. Thus, it is desired to have an improved electro acoustic resonator that provides RF filters with good electrical and acoustic performance and that is compatible with a thin film piezoelectric material.

Further, a corresponding transducer should have suppressed or eliminated spurious modes, reduced acoustic losses, and improved dielectric strength to prevent electrostatic discharge and improved power durability.

To that end, an electro acoustic resonator according to the independent claim is provided. Dependent claims provide preferred embodiments and preferred filters.

The electro acoustic resonator comprises an acoustic track with a first transversal side and a second transversal side. Further, the resonator has an electrode structure having a first electric connection, a second electric connection and a plurality of electrode fingers. Each of the plurality of electrode fingers is electrically connected to either the first or the second electric connection. Further, the resonator has a velocity

compensation structure. The velocity compensation structure is provided and adapted for establishing a homogenous transversal velocity profile.

Thus, the present electro acoustic resonator drastically deviates from the technical measures provided in US 2013/0051588 Ai which recommends a heterogeneous transversal velocity profile.

In the present resonator the acoustic track is the area of the piezoelectric material that is provided for the propagation of the acoustic waves. The direction of the propagation of the acoustic waves establishes the longitudinal direction of the acoustic track and of the resonator. The first transversal side and the second transversal side of the acoustic track flank the acoustic track and extend parallel to a central excitation area of the acoustic track, in the longitudinal direction. The first electric connection and the second electric connection can comprise bus bars that provide the electrode fingers with an

electromagnetic RF signal obtained from an external circuit environment. Bus bars can be arranged at the first transversal side and the second transversal side, respectively. The electrode fingers have an extension along the transversal direction. The transversal direction is essentially orthogonal to the longitudinal direction in the plane essentially defined by the surface of the piezoelectric material. The velocity compensation structure can comprise a material that is arranged at specific locations in the acoustic track such that the homogeneity of the transversal velocity profile is enhanced. The transversal velocity profile is defined as the velocity of the acoustic waves propagating in the acoustic track regarded along a path in the transversal direction from one to the respective other of the first transversal side and the second transversal side. Thus, the transversal velocity profile determines the wave velocity depending on the transversal position.

It is to be noted that“x” denotes a position along the longitudinal direction“y” denotes a position along the transversal direction and“z” denotes a position along the vertical direction orthogonal to the longitudinal direction and the transversal direction. Thus, the transversal velocity profile determines the relationship between the transversal position y and the velocity of the acoustic waves.

It was found that the propagation of acoustic waves is different in thin film based electro acoustic resonators compared to the propagation of waves in conventional electro acoustic resonators with an electrode structure arranged on a piezoelectric bulk material. The wave propagation determines acoustic losses that may be due to wave emission in a transversal direction or due to transversal modes. Unfortunately, measures for obtaining a piston mode as defined in the above-mentioned publication become less effective when a thin film piezoelectric material is employed.

The presented resonator bases on a counterintuitive two-step approach: the provided resonator is compatible with piezoelectric thin films and nevertheless can provide a reduction of acoustic losses. But to obtain a corresponding resonator a departure from the conventional means was found to be necessaiy. It is possible that the electro acoustic resonator comprises a first bus bar and a second bus bar. Further, the resonator can comprise a central excitation area between the first bus bar and the second bus bar. Additionally, the resonator can comprise a first gap region between the central excitation area and the first bus bar and a second gap region between the central excitation area and the second bus bar. The velocity compensation structure comprises dielectric material in the first and/or in the second gap region.

The gap region establishes an electrical gap between the two bus bars. The two bus bars - when the resonator is operating - are connected to opposite polarities. Without an electric gap between the bus bars the resonator would be short-circuited and without function. To obtain an electric isolation between the bus bars a corresponding gap between the conducting material of one electrode and the conducting material of the respective other electrode is provided. This can be obtained by maintaining a distance between finger ends of one electrode and conducting material of the respective other electrode. However, the situation in electro acoustic resonators is complex and each segment of conducting material of the electrode structure also has an impact on the acoustics of the resonator. A gap between conducting material of the two electrodes, thus, affects the propagation of the acoustic waves. Generally, a reduction of the mass loading of material arranged on the piezoelectric material, e.g. caused by a gap in the electrode structure, causes the acoustic velocity to be locally increased. A local increase of the acoustic velocity establishes a heterogeneity of the transversal velocity profile which follows the recommendation of US 2013/0051588 Ai. Further parameters that can alter the velocity are the stiffness values of a material. But in order to obtain the counterintuitive homogenous transversal velocity profile the velocity compensation structure can provide a local mass loading, specifically in the gaps, that provides a homogenous environment to the acoustic waves but that prevents a short-circuit of the opposite electrodes.

It is correspondingly possible that the compensation structure comprises a material and a location pattern such that a homogeneous acoustic impedance provided by the mass loading on the piezoelectric material is provided to the acoustic waves. Thus, the acoustic waves experience a homogenous acoustic environment between the bus bars.

It is possible that the material of the velocity compensation structure is provided as a plurality of dielectric patches arranged at the first and/or second gap region. Specifically, the patches can be located at the finger ends pointing towards the opposite electrode.

It is possible that the material of the velocity compensation structure is arranged - in a vertical position - below material of the electrode fingers, above material of the electrode fingers and/or next to material of the electrode fingers. When the material of the compensation structure is provided as patches arranged at the finger ends, then the material can be provided such that there is no overlap of material of the electrode fingers and of material of the compensation structure. However, it is possible that material of the electrode fingers or of the electrode structure overlaps material of the compensation structure or vice versa, e.g. while additional material of the compensation structure is arranged next to electrode fingers.

It is possible that material of the velocity compensation structure is provided as patches having an extension along the longitudinal direction that is smaller, equal to or larger than the width of the electrode fingers. The electrode fingers extend along the transversal direction. The width of the electrode fingers is defined as the thickness of the electrode fingers in the longitudinal direction. The patches can have a longitudinal extension that equals the width of the fingers or that deviates from the finger width.

When the acoustic impedance of the material of the compensation structure essentially equals the acoustic impedance of the material of the electrode fingers, then a longitudinal extension of the patches can be equal to the finger thickness. Especially, the height of the patches in the vertical direction can be equal to the height of the electrode fingers in the vertical direction.

If the material chosen for the compensation structure has a smaller (specific) acoustic impedance than the material of the electrode structure then the reduced impedance can be due to an increased mass loading and/or an increased volume. Thus an excess in height, width or length of the patches can be provided to obtain an overall homogenous acoustic impedance in the acoustic track.

However, different extensions in the longitudinal direction, in the transversal direction or in the vertical direction can be beneficial, if further effects on the acoustic wave propagation are wanted. For example, the extension in the longitudinal direction allows for adjusting the velocity for various resonators. For example series resonators and parallel resonators can be provided with different longitudinal extensions to

compensate for different pitches and/or metallization ratios. In this respect, a series resonator can be an electro acoustic resonator connected in series in a signal path of an RF filter. A parallel resonator establishes a resonator in a shunt path electrically connecting the signal path of the filter to a ground potential.

Although it is possible that the height of the material of the compensation structure is locally different, it is preferred that the material of the compensation structure is applied to the area of the transducer in a small number of production steps. Thus, it is preferred that the material of the compensation structure has a homogenous height over the entire chip.

A specific extension of the material of the compensation structure in the transversal direction allows for the compensation of edge angle effects at the finger ends which can occur in real transducers due to production-reduced limitations. Thus, fine-tuning of the continuous velocity profile is possible.

Further, the specific shape of the patches of the compensation structure may be modified for further optimization.

It is possible that the electro acoustic resonator comprises stub fingers. Material of the velocity compensation structure is arranged in a transversal direction between the stub fingers and electrode fingers of the respective opposite electrical connection.

Stub fingers (an alternative notation is dummy fingers) can be used to tune the wave propagation. In conventional resonators transversal gaps between structures of opposite electrodes are structured to be narrow. However, narrow gaps correspond to small distances between the electrodes.

The provision of the compensation structure allows increased distances between opposite electrodes such that the dielectric strength, e.g. against ESD pulses (ESD = electrostatic discharge) and the power durability are increased.

It is possible that the dielectric material used for the compensation structure has a smaller relative electric permittivity compared to the dielectric material in the vicinity of the compensation structure. Dielectric material in the vicinity of the compensation structure may be the piezoelectric material below the electrode structure or a temperature compensation layer or a trimming layer above or below the electrode structure. A reduced relative permittivity of the material of the compensation structure allows a reduction of the electric field in the transversal gaps such that the dielectric strength and the power durability is further enhanced. Additionally, a potentially deteriorating excitation or conversion of acoustic wave in the gap region may be reduced.

Thus, the provision of the compensation structure provides good compatibility with stub fingers.

It is possible that the electro acoustic resonator is selected from a SAW resonator (SAW = surface acoustic wave), a TC-SAW resonator (TC = temperature compensated), a GBAW resonator (GBAW = guided bulk acoustic wave) and a TF-SAW resonator (TF = thin film).

A TC-SAW resonator comprises a temperature compensation material above or below the electrode structure. The stiffness parameters of the material of the temperature compensation structure is chosen such that a temperature induced drift of characteristic frequencies of the resonator is reduced or eliminated. It is possible that a corresponding temperature compensation structure comprises an oxide such as a silicon oxide such as Si0 ₂ .

A GBAW resonator comprises a sagittal waveguiding structure arranged above and/or below the electrode structure such that the propagating waves are propagating at the interface between the piezoelectric material and a corresponding waveguiding layer.

A TF-SAW resonator utilizes a piezoelectric material provided as a thin film. The thin film may be provided utilizing thin film processing techniques like wafer bonding with subsequent mechanical polishing or smart cut and by conventional layer deposition techniques such as CVD (chemical vapor deposition), PVD (physical vapor deposition), sputtering, MBE (molecular beam epitaxy) and the like.

It is possible that the thin film piezoelectric material is arranged on a carrier substrate and functional layers, e.g. for temperature compensation, are added in the stack. It is possible that the electrode structure is selected from an unweighted transducer, an apodized transducer, a slanted transducer, a broken slanted transducer and a zigzag slanted transducer. In an unweighted transducer each pair of electrode fingers essentially contribute the same amount to the conversion between electromagnetic RF signals and acoustic RF signals. To that end, the overlap along the transversal direction of neighboring electrode fingers of opposite polarity can be equal along the longitudinal direction of the acoustic track.

In contrast, a weighted transducer provides different contributions to the overall excitation of acoustic waves for different pairs of neighboring electrode fingers of opposite polarity. To that end, the transversal overlap of the neighboring fingers can differ along the longitudinal direction. Such a weighted transducer can be an apodized transducer. An apodized transducer can be a sine weighted transducer or a cosine weighted transducer.

A slanted transducer has an angle between the propagation direction of the acoustic waves and the extension of the bus bars or the gaps such that each electrode finger is shifted in transversal direction compared to the adjacent electrode fingers. Typically, the electrode fingers are oriented orthogonal to the piezoelectric axis of the piezoelectric material. The electrode fingers are typically also orthogonal to the direction of the propagation of the acoustic waves of the wanted main acoustic mode. Thus, the extension of the bus bars and the gaps in a slanted transducer is not parallel to the direction of propagation of acoustic waves, i.e. to the longitudinal direction.

It was found that slanting or apodizing resonators can effectively reduce unwanted transversal modes even in a TF-SAW resonator.

Further, it was found that diffraction effects in the gap region of an apodized or slanted resonator have a more severe impact on the resonator’s performance than in unweighted resonators because the interaction between acoustic waves and the gap region is intensified in corresponding geometries. Thus, the counterintuitive approach of providing a homogenous transversal velocity profile that can correspond to a

homogeneous acoustic impedance even in the gap region minimizes unwanted acoustic effects in the gap region. Thus, improved electro acoustic resonators that are compatible with thin film piezoelectric materials can be obtained with the above-described measures.

A broken slanted transducer has segments along the acoustic track with different slanting angles. Thus, a broken slanted transducer has at least two segments. It is possible that in one segment the slanting angle is o°. Such a segment corresponds to a segment of a conventional, non-slanted resonator.

A zigzag slanted transducer comprises iteratively repeated slanted segments with slanting angles with alternating signs.

It is possible that the electro acoustic resonator is selected from a one-port resonator, a two-port resonator and a DMS resonator (DMS = dual mode SAW).

A one-port resonator has only one port to be connected to an external circuit environment. A two-port resonator has two ports to be connected to an external circuit environment. One of the two ports can be an input port for receiving electromagnetic RF signals. The respective other port can be an output port for providing

electromagnetic RF signals to an external circuit environment.

A DMS resonator can be established as a one-port resonator or as a two-port resonator. In a DMS resonator more than one acoustic main modes can propagate. A DMS resonator can comprises a first IDT (IDT = interdigital transducer) and a second IDT or more than two IDTs. The resonator can have a single transducer or a plurality of transducers. The one or more transducers of the resonator can be arranged between elements of an acoustic reflector, e.g. elements of Bragg reflectors.

One or more transducers can be weighted, apodized, slanted, broken slanted or zigzag slanted. However, it is also possible that several transducers are slanted such that a plurality of transducers in the acoustic track establish a broken slanted or zigzag slanted excitation structure.

The IDTs of resonators can be arranged between reflector structures of the resonator. lO

It is possible to use the described resonator in an RF filter.

Correspondingly, it is possible that an RF filter comprises an electro acoustic resonator as described above.

The RF filter can be a bandpass filter or a band rejection filter and can be used in a frontend circuit of a wireless communication device. It is possible that the RF filter has a ladder-type like filter topology or a lattice-type like filter topology.

In a ladder-type like filter topology one or more series resonators are electrically connected in series in a signal path between an input port and an output port. One or more parallel resonators can be arranged in one or more shunt paths electrically connecting the signal path to ground.

A lattice-type like filter topology can have an input port and an output port. The input port can comprise a first input connection and a second input connection. The output port can comprise a first output connection and a second output connection. A lattice- type like filter topology is obtained if one resonator electrically connects the first input connection to the second output connection. A signal crossing of signals propagating via a first resonator and a second resonator is obtained.

It is possible that such a filter is a filter of a multiplexer. The multiplexer can be a duplexer, a quadplexer or a multiplexer of a higher degree than two. One or many or all filters of the multiplexer can be provided as described above.

The above resonator reduces or smooths discontinuities in the transversal velocity profile via the compensation structure such that a resonator that is compatible with good electric and acoustic properties and thin film piezoelectric materials is obtained.

Central aspects of the provided resonator and details of preferred embodiments are shown and explained in the accompanying schematic figures.

Figure l shows a preferred acoustic impedance environment experienced by the acoustic waves; Figure 2 illustrates the corresponding electrode structure;

Figure 3 illustrates possible patch geometries in a top view;

Figure 4 illustrates corresponding cross-sections of the patches;

Figure 5 illustrates a slanted IDT structure of a resonator; Figures 6 and 7 show embodiments of broken slanted IDT structures;

Figures 8 and 9 illustrate zigzag slanted IDT structures;

Figure 10 illustrates the electrode structure of a cosine weighted transducer from a solely electric point of view;

Figure 11 shows a preferred acoustic impedance provided to the acoustic waves of the transducer shown in Figure 10; and

Figure 12 shows a possible application of the described resonators in a multiplexer.

Figure 1 shows a preferred acoustic impedance structure in a top view. The longitudinal direction extends along the x-direction. The transversal direction extends along the y- direction. In the upper part of Figure 1 the transversal acoustic velocity profile is shown. At the flanks of the acoustic track two bus bars BB are arranged that connect the electrode fingers EF to an external circuit environment. The electrode fingers EF do not short-circuit because Figure 1 illustrates the acoustic properties of the transducer. Electric properties can be obtained from Figure 2. The acoustic velocity is essentially homogenous and constant between the two bus bars BB and corresponds to the acoustic velocity denoted as v _tra ck. The bus bars have a higher mass loading.

Correspondingly, the acoustic velocity in the area of the bus bars BB is lower and denoted as v _met . Typically, the bus bars comprise metals and the metallization of the bus bars reduce the acoustic velocity locally. Figure l illustrates the desired structure when only the acoustics of the resonator are regarded. However, discontinuities in the electric structure are necessary to prevent a short-circuit and ensuring excitation of acoustic waves.

Correspondingly, Figure 2 shows a possible technical solution to provide an electrode structure without a short-circuit that can provide the acoustic structure shown in Figure l. Between the two bus bars electrode fingers EF and stub fingers SF can be arranged. The ends of the electrode fingers EF and the ends of the correspondingly opposing stub fingers SF are separated by a gap G. The gap G establishes an electric isolation of the two electrodes. In the area of the gap G material of the velocity compensation structure VCS is arranged, e.g. in the form of patches P. The patches P provide the homogeneity of the acoustic velocity between the two bus bars BB as shown in the top portion of Figure 2. Without the presence of the velocity compensation structure VCS the acoustic velocity in the gaps would be v _tg ap. By providing the compensation structure VCS the velocity in the gaps G can be reduced to the overall velocity in the central excitation area _rack . Thus, the acoustic velocity profile as shown in Figure l and as desired is obtained without short- circuiting the bus bars BB.

Figure 3 illustrates two possible arrangements of the patches P relative to the electrode finger EF and to the stub finger SF in a top view. In the left portion of Figure 3, where material of the electrode finger EF or of the stub finger SF is present, material of the patch P is arranged on the material of the corresponding finger. In an area where no finger material is present, the material of the patch P can be directly arranged on the piezoelectric material. Thus the patch P can overlap the electrode fingers EF and the stub fingers SF as shown in the left portion of Figure 3.

In contrast, the right portion of Figure 3 shows an arrangement where the material of the patch P is arranged below the material of the fingers. Thus, material of the patch P is always arranged directly on the piezoelectric material. Where material of the patch P is present the material of the fingers is arranged on the respective patch material.

The two arrangements shown in Figure 3 are illustrated in Figure 4 in a cross-section parallel to the yz-plane. The top left portion and the top right portion of Figure 4 correspond to the idealized arrangements indicated in Figure 3. The bottom left portion and the bottom right portion of Figure 4 illustrate a realistic distribution of the material with rounded edges as produced in reality. In the left portion of Figure 4 the material of the patch P is arranged between the fingers EF, SF where no material of the fingers is present. Where material of the fingers EF, SF is present the material of the patch P is arranged on the fingers.

In contrast, the right portion of Figure 4 illustrates the inverted arrangement where the material of the patch P is arranged on the piezoelectric material (not shown) while the material of the fingers EF, SF is arranged on the material of the patch P, respectively.

Figure 5 illustrates an important embodiment of the resonator where the transducer comprising the bus bars, the electrode fingers, the stub fingers and the patches is slanted. Thus, the extension of the bus bars BB and the gaps G deviates from an extension parallel to the longitudinal direction x while the electrode fingers essentially maintain their extension along the transversal direction y. The slanting angle a can be between 2 ⁰ and 20°, e.g. in the range of io°.

Slanting resonator is an effective means for reducing transversal modes even or especially when a thin film piezoelectric material is used. Together with the velocity structure a good performance of a TF-SAW resonator is obtained.

Figure 6 shows the footprint of a broken slanted transducer. The transducer comprises a first segment Si and a second segment S2. The first segment Si is slanted. The second segment S2 has a conventional orientation. In the slanted segment Si the bus bars are rotated compared to the bus bars of the conventionally arranged segment S2. The direction of the extension of the electrodes fingers is the same for all segments.

Figure 7 shows a possible layout of a broken slanted transducer with two slanted segments. A first segment Si is slanted. The second segment S2 is slanted, too. The rotation can be equal in absolute value and opposite in sign for both slanted segments Si, S2.

Figure 8 shows the layout of a zigzag slanted transducer with four segments. Segments Si and S3 have the same orientation. Segments S2 and S4 have the same orientation. Thus, the combination of segments of Si and S2 is repeated as segments S3 and S4. Further, Figure 8 shows a zigzag slanted resonator where each segment is slanted. In contrast, Figure 9 shows a zigzag slanted transducer where segments Si and S3 are slanted while segments S2 and S4 are conventionally oriented. Additionally, the overall structure is rotated in the xy plane.

Figure 10 shows a possible electrode layout for a cosine weighted transducer. The bus bars BB are connected to electrodes and the length of the local overlap region differs along the longitudinal direction such that a maximum value is obtained in the longitudinal center region of the transducer and a reduced overlap length is obtained in the rim regions of the transducer. Correspondingly, the doted curve COS follows the gaps and indicates the cosine weighting of the excitation.

It is to be noted that Figure 10 illustrates the configuration solely in terms of the electrical important structures without taking into account acoustically relevant parts like stub fingers. Thus, the transducer can also have stub fingers for the“upper” electrode.

In contrast, Figure 11 shows the acoustic configuration which is - due to stub fingers and/or the compensation structure - homogenous along the transversal direction between the bus bars BB. The cosine weighting is present for the electrical excitation only. The acoustic properties are homogenous - with respect to a l periodicity along the longitudinal direction.

Figure 12 illustrates possible applications of the above-described resonator. Figure 12 shows a duplexer as an embodiment of a multiplexer MUL to which the concept can be generalized. The duplexer comprises a transmission filter TXF and a first reception filter RXFi and a second reception filter RXF2. The transmission filter TXF, the first reception filter RXFi and the second reception filter RXF2 establish bandpass filters BPF. The transmission filter TXF and the first reception filter RXFi are realized in a ladder-type like filter topology with series resonators SR electrically connected in the signal path. Parallel resonators PR are electrically connected in shunt paths connecting the signal path to ground. The resonators in the ladder-type like circuit topologies can be one-port resonators or two-port resonators. The second reception filter RXF2 is realized as a DMS filter with an input port comprising three input connections and an output port comprising two output connections. Each input connection is connected to an input transducer. Each output connection is connected to an output transducer. Thus, five transducers are arranged - between reflectors - in the acoustic track of the DMS filter. At an antenna connection AC an antenna AN can be connected. The antenna connection AC is coupled to an output port of the transmission filter TXF and to an input port of the first reception filter RXFi. Between the AC and the input port of the first reception filter RXFi an impedance matching circuit IMC can be provided to match the antenna impedance and the output impedance of the transmission filter TXF, respectively, to the input impedance of the reception filter RXFi.

The resonator and the filter are not limited to the technical features described above or to the embodiments shown in the figures. Resonators can comprise further acoustically or electrically active elements such as reflector elements or impedance matching elements, e.g. for impedance conversion. Filters can comprise further resonators and acoustically active or inactive transducer structures and impedance elements.

List of Reference Signs

AC: antenna connection

AN: antenna

BB: bus bar

BPF: bandpass filter

COS: cosine weighting

DMS: DMS filter

EF: electrode finger

G: gap

IMC: impedance matching circuit

MUL: multiplexer, duplexer

P: patch

PR: parallel resonator

RXFl: first reception filter

RXF2: second reception filter

Si, S4: transducer segments

SF: stub finger

SR: series resonator

TXF: transmission filter

v: velocity

VCS: velocity compensation structure

Vmet· acoustic velocity in the bus bar

Vtgap· acoustic velocity in the gap

Vtrac _* velocity in the central excitation area x: longitudinal direction

y: transversal direction

z: vertical direction

a: slanting/shearing angle of bus bar