BAW RESONATOR WITH REDUCED LOSSES, RF FILTER, MULTIPLEXER AND METHOD OF MANUFACTURING A BAW RESONATOR

The present invention refers to a BAW resonator with reduced losses and to a corresponding RF filter, multiplexer and a method of manufacturing.

RF filters can be used in wireless communication devices to separate wanted RF signals from unwanted RF signals. RF filters are parts of multiplexers of a frontend circuit, e.g. of a mobile communication device. Parameters such as insertion loss, out-of-band rejection, roll-off, isolation, linearity, durability, etc., determine the quality of an RF filter. The filter insertion loss parameter is directly impacted by the losses of the individual components cascaded to construct the filter. Such components in an RF filter are resonators, with some being detuned with respect to others to form an overall band-pass or band-stop filter function after being cascaded. Therefor resonator quality factor is a typical key performance indicator for filter performance.

Resonators used in RF filters can be electro-acoustic employing acoustic waves and comprising a piezoelectric material. Due to the piezoelectric effect a transducer in a resonator converts between electromagnetic and acoustic RF signals. To that end a BAW resonator (BAW = bulk acoustic wave) typically comprises a piezoelectric material arranged between a bottom electrode and top electrode that are in direct mechanical contact with the piezoelectric. The overall conductive losses of the resonator are determined by the top and bottom electrode material conductivity, and thickness. One cannot simplify increase the electrode thickness and expect the acoustic performance to remain the same. In practice, there is a trade-off - one can improve effective electrode conductivity at the expense of acoustic quality factor, electromechanically coupling coefficient (related to pole/zero distance and achievable filter bandwidth), and spurious mode content/level. As filter passband frequency increase this tradeoff becomes more and more of an issue as acoustically the electrode thickness should scale down to have similar acoustic properties as a lower-frequency device, while to have similar effective conductivity as a lower-frequency device the electrode film thickens should be roughly the same.

A method to decouple this conductivity / acoustic performance trade-off is provided that permits simultaneous improvement in acoustic performance and effective electrical conductivity of resonator and overall filter performance. A BAW resonator and a method of manufacturing a BAW resonator according to the independent claims are provided. Dependent claims provide preferred embodiments.

The BAW resonator with reduced losses comprises a piezoelectric material. The piezoelectric material is arranged in a piezoelectric layer. Further, the resonator comprises a bottom electrode arranged and structured in a bottom electrode layer.

Further, the BAW resonator comprises a top electrode structured and arranged in a top electrode layer. The piezoelectric layer is arranged on or above the bottom electrode layer. The top electrode layer is arranged on or above the piezoelectric layer. A first gap is arranged between the piezoelectric material and a first electrode. The first electrode is selected from the bottom electrode and the top electrode.

Thus, a BAW resonator is provided that has a stacked layer structure that is different from the sandwich structure of conventional BAW resonators where the piezoelectric material is sandwiched between the bottom electrode and the top electrode. Specifically the first gap that is arranged between the piezoelectric material and the first electrode (which can be the bottom electrode or the top electrode) allows a significant reduction of losses as conductivity can be improved by increased thickness without impacting the acoustic loading the high quality (low acoustic loss) piezoelectric material.

A reduction of losses can be obtained because ohmic losses and acoustic losses can be reduced. The first gap can reduce acoustic losses because the confinement of acoustic energy to the resonator area comprising the piezoelectric material is improved because no acoustic energy can drain via the gap.

Further, the RF signal can be coupled via the first electrode and via the gap to the piezoelectric material via a wide area such that currents, current eddies and ohmic losses are reduced.

Further, the acoustic decoupling of the piezoelectric material from the first electrode reduces the number of possible spurious modes. Thus, the number of acoustic channels draining acoustic energy from the resonator is further reduced.

Thus, the provided BAW resonator has significantly reduced losses and therefore a higher quality factor. Such a BAW resonator allows RF filters with reduced losses and corresponding RF filter components such as multiplexers with reduced losses and with improved electrical properties.

The reduced insertion loss of the corresponding RF filter, e.g. a bandpass filter, corresponds to an improved signal-to-noise ratio and needs less signal amplification resulting in an overall longer battery life of the corresponding wireless communication device due to the simultaneous improvement of acoustic and electric properties of the resonator.

The bottom electrode and/or the top electrode can comprise or consist of conventional electrode materials for BAW resonators, e.g. gold, molybdenum, silver, tungsten, copper, aluminum or alloys thereof.

The piezoelectric material in the piezoelectric layer can comprise conventional piezoelectric materials used for electro-acoustically active devices of electro-acoustic filter components. Thus, the piezoelectric material can comprise or consists of lithium tantalate, lithium niobate, aluminium nitride, scandium-doped aluminium nitride, quartz or combinations thereof.

The first gap is a vertical gap between a material below and a material above the gap. Thus, the gap has an extension in horizontal (x, y) directions and being essentially orthogonal to the vertical (z) direction. The gap has a thickness in the vertical direction that can be 10 nm or larger and 100 nm and smaller.

The bottom electrode and/or the top electrode can have extensions in horizontal directions that are of the order of 100 pm. The gap, thus, also has such horizontal extensions. Thus, the gap has a quite large aspect ratio of approximately 100000 x 10 to 100000 xioo.

To obtain such a gap in such a way that the acoustics of the piezoelectric material are undisturbed by the first electrode, special conditions during manufacturing must be complied with. Thus, warping or other disturbances reducing the vertical thickness of the gap locally to zero must be avoided.

The steps during manufacturing of the corresponding layer stack must be controlled with a high precision.

Then, a stress-free and optionally charge-free interface on one or both sides of the piezoelectric material is possible that leads to the above-mentioned advantages of the provided BAW resonator.

It is not only possible to provide one gap between the piezoelectric material and the first electrode. It is also possible to provide a corresponding second gap on the respective other side of the piezoelectric material between the piezoelectric material and the corresponding other electrodes selected from the bottom electrode and the top electrode such that the piezoelectric material is isolated from both electrodes via a corresponding gap. The piezoelectric material is separated from the first electrode via the first gap and from the second electrode via the second gap.

Correspondingly, all technical features described with respect to the first gap can also be applied - mutatis mutandis - to the second gap if present.

Specifically, with respect to the arrangement of the piezoelectric material and the gaps, the structures in the vicinity of the piezoelectric material can have - at least with respect to the layer construction - a mirror symmetry with the extension of the piezoelectric material in the horizontal directions establishing a mirror plane.

It is possible that the gap (e.g. the first gap and/or the second gap) mechanically and/or electrically isolates the piezoelectric material from the first (and/or second) electrode.

The mechanical isolation improves the acoustics of the transducer because energy loss channels are removed and the possibilities for the excitation of spurious modes - due to the simplified geometry and symmetry the transducer structure - is reduced.

The electrical isolation reduces the possibilities of ohmic losses as their thickness can be increased without affecting the acoustics. The RF signal can be coupled capacitively from the corresponding electrode via the gap to the piezoelectric material. Charge transfer from the piezoelectric material to the corresponding first (and/ or second) electrode or vice versa is prevented such that the overall ohmic losses of the BAW resonator are reduced.

Correspondingly, it is possible that the first gap is provided for and suited to capacitively couple the first electrode to the piezoelectric material. Thus, also the first electrode and the piezoelectric material are provided for and suited to capacitively couple the transducer structure comprising the piezoelectric material to a signal path that is electrically connected to the BAW resonator. It is possible that the first electrode (and/or the second electrode), the first gap (and/or the second gap, if present) and the corresponding interface between the gap and the piezoelectric material establish a series capacitance element.

The provision of the series capacitance element establishes an additional degree of freedom of adjusting the electro-acoustic coupling corresponding to the coupling coefficient K of the BAW resonator.

Correspondingly, when two gaps are present then the transducer structure comprising the piezoelectric material is electrically connected in series between the two series capacitance elements established via the two gaps.

It is possible that the BAW resonator comprises a first (and/or second) coupling layer between the piezoelectric material and the first (and/ or second) gap.

Specihcally, the first (and/or second) coupling layer can be arranged at the top or bottom surface of the piezoelectric material.

The first and/or second coupling layer can comprise a conductive or dielectric material. The corresponding first and/or second coupling layer improves the capacitive coupling of the RF signal to and from the transducer comprising the piezoelectric material.

The capacitive coupling can essentially be a quasi-electrostatic coupling.

Further, the material of the coupling layer can be used during manufacturing to act as an etch stop layer, .e.g. when the gap is provided utilizing a sacrificial material that is removed during manufacturing. As indicated above, it is possible that the BAW resonator comprises a second gap in addition to the first gap. The second gap is arranged between the piezoelectric material and a second electrode. The second electrode is selected from the bottom electrode and the top electrode and the second electrode is the respective other electrode of the bottom electrode and the top electrode compared to the first electrode. Thus, when the first electrode is the bottom electrode then the second electrode is the top electrode. When the first electrode is the top electrode then the second electrode is the bottom electrode.

It is possible that the BAW resonator comprises a second coupling layer between the piezoelectric material and the second gap.

It is possible that the BAW resonator comprises a horizontal acoustic reflection structure arranged at the top side and/or at the bottom side of the piezoelectric material.

The horizontal acoustic reflection structure reflects acoustic energy that would otherwise drain from the transducer area along a lateral, i.e. horizontal, direction. The horizontal acoustic reflection structure is different from and can be provided in addition to a vertical Bragg mirror structure that prevents the drain of acoustic energy in a vertical direction as known from SMR-type BAW resonators (SMR = solidly mounted resonator).

It is possible that the horizontal acoustic reflection structure comprises a vertical interface between areas of different acoustic impedances.

Acoustic energy is reflected at an interface between materials of different acoustic impedance. The acoustic impedance of a material depends on the stiffness parameters of the material and on the density of the material. When several such interfaces are iteratively arranged in a horizontal direction, specifically when the horizontal distance is an integer multiple of l/2 (l being the acoustic wavelength), then a Bragg mirror structure reflecting a large fraction of the acoustic energy can be obtained.

Such a horizontal acoustic reflection structure can have structures at the top side of the piezoelectric material and structures at the bottom side of the piezoelectric material.

It is possible that the horizontal acoustic reflection structure comprises one or more recesses in the piezoelectric material. Additionally or as an alternative it is possible that the horizontal acoustic reflection structure comprises one or more patches of an additional material arranged on a top or at a bottom side of the piezoelectric material.

In addition to the horizontal acoustic reflection structure the BAW resonator can comprise a Bragg reflector arranged below or above the piezoelectric material. The Bragg reflector can be acoustically coupled to the piezoelectric material.

Then, the BAW resonator is a resonator of the SMR-type (SMR = solidly mounted resonator).

The Bragg reflector can comprise two or more iteratively stacked layers of materials of different acoustic impedances.

Conventional SMR-type BAW resonator mirror material such as tungsten for a high acoustic impedance and silicon oxide for a low acoustic impedance are possible.

Further, the BAW resonator can also be a resonator of the TF-BAR type. A TF-BAR (thin-film-bulk acoustic resonator) has a cavity arranged above or below the piezoelectric material to confine acoustic energy to the transducer area. The BAW resonator as described above has at least one gap between an electrode and the piezoelectric material that establishes an acoustic isolation and that confines acoustic energy by preventing the drain of acoustic energy. When the BAW resonator has a second gap on the respective other side then a good isolation is already contained.

However, if the BAW resonator has only one gap as described above, then the respective other side of the piezoelectric material can be acoustically isolated from its environment utilizing the Bragg mirror of an SMR-type resonator or utilizing a cavity of a TF-BAR.

It is further possible that the BAW resonator comprises a first and/or a second additional layer. The first and/or second additional layer is selected from a carrier material layer and a cap material layer. The carrier material layer can be arranged below the piezoelectric material. The cap material layer can be arranged above the piezoelectric material. The first additional layer or the second additional layer or the first and the second additional layers can contain a through connection.

Then, the carrier material layer can establish a layer of the carrier material on which the stacked layer construction of the BAW resonator is arranged. The carrier material establishes a physical carrier and provides protection of the sensitive electro-acoustic transducer at the bottom side of the BAW resonator.

The cap material layer establishes a cap that protects the sensitive transducer from detrimental influences at the top side of the BAW resonator. Specifically, the cap material layer can establish a cap covering a cavity in which the piezoelectric material and the first and/or gap are arranged such that the transducer structure comprising the piezoelectric material is hermetically sealed from its environment. io

Spacer elements can be used to mechanically separate the carrier material layer and/or the cap material layer from the piezoelectric material.

The through connection through the carrier material layer and/or through the cap material layer allows an electrical contact of the corresponding bottom or top electrode to electrically couple the transducer structure to a signal path of the corresponding RF filter. The cross-section of the through connection can be of the order of the bottom or the top electrode such that ohmic losses - due to the large cross-section of the conductor - are reduced. Correspondingly, the through connection utilizes a material with a high conductivity such as silver, gold, aluminium, copper or similar metals or alloys.

Correspondingly, it is possible that an RF filter comprises a BAW resonator as described above.

The RF filter can have a ladder-type circuit topology or a lattice-type circuit topology.

In a ladder-type like circuit topology series resonators are electrically connected in series between a first port and a second port. Parallel resonators can be arranged in one or more shunt paths electrically connecting the signal path to ground.

In a lattice-type circuit topology, electro-acoustic resonators are electrically connected between a first port comprising two connections and a second port comprising two connections. At least one circuit element is connected between a first connection of the first port and first connection of the second port and one circuit element is electrically connected between one connection of the first port and the corresponding other connection of the second port such that a cross connection between connections of the two ports is obtained. Of course, it is possible that two or more or all of the resonators of the RF filter have technical features described above.

Further, it is possible that a multiplexer comprises an RF filter as described above.

The multiplexer can be a duplexer, a triplexer, a quadplexer or a multiplexer of a higher order. Specifically, the multiplexer can be provided for working in a carrier aggregation operation mode.

A method of manufacturing a BAW resonator comprises the step of arranging a first gap between a piezoelectric material and a first electrode. The piezoelectric material can be the piezoelectric material as described above and the first electrode can be one of the first electrodes as described above.

The provision of the piezoelectric material can comprise steps of thin-film layer deposition techniques, e.g. sputtering. The provision of the piezoelectric material using thin-film layer deposition techniques is specifically possible when aluminium nitride or scandium-doped aluminium nitride is used as the piezoelectric material.

When the piezoelectric material is lithium niobate or lithium tantalate then techniques such as Smart Cut™ or similar techniques are possible. The carrier material layer can be provided via a carrier wafer and the cap material layer can be provided via a cap wafer. Wafer bonding methods for connecting the cap wafer and/or the carrier wafer to one another or to the piezoelectric material, e.g. via spacer elements, are possible.

The coupling layers can be used to control the lateral wave dispersion properties of the piezoelectric material. In particular, by placing an electrically shorting film atop the piezoelectric the lateral mode cut off frequency will reduce and can reduce spurious mode content around the device resonance frequency - especially for type II dispersion where modes display regions of negative group velocity (typically just below the cut-off frequency).

The coupling layers can comprise or consist of aluminium, molybdenum or tungsten or other low acoustic loss and electrically conductive materials. The thickness in a vertical direction of the coupling layers can be 20 nm or larger and 500 nm or smaller.

The piezoelectric material can consist of a single crystal material or comprise a plurality of grains and domains. However, it is preferred that the piezoelectric material has essentially a piezoelectric axis oriented in parallel to the vertical direction.

Central aspects and working principles of the BAW resonator and details of preferred embodiments are shown by the accompanying schematic figures.

In the figures:

Figures 1 to 3 show construction possibilities of basic resonator structures;

Fig. 4 shows a resonator with two gaps and with a carrier and a cap;

Fig. 5 shows the use of a cavity below the piezoelectric material;

Fig. 6 shows the use of a Bragg mirror below the piezoelectric material;

Fig. 7 shows the use of a conducting Bragg mirror below the piezoelectric material; Fig. 8 shows possible structures of horizontal acoustic reflection structures;

Fig. 9 shows additional or alternative possible details of horizontal acoustic reflection structures.

Figures 10 and 11 show possible details of horizontal acoustic reflection structures; and

Fig. 12 illustrates a possible circuit topology of a duplexer.

Figure 1 illustrates a possibility of providing a BAW resonator BAWR having a first gap

Gi between the piezoelectric material PM and the first electrode ELi. Specifically, the BAW resonator with reduced losses has a piezoelectric material PM in a piezoelectric layer PL. Further, the resonator has a top electrode TE in a top electrode layer TEL. A bottom electrode BE is arranged and structured in a bottom electrode layer BEL.

The piezoelectric material PM is arranged between the bottom electrode BE and the top electrode TE. The first gap Gi is arranged between the piezoelectric material PM and the top electrode TE as the first electrode ELi. Thus, a capacitive coupling of an RF signal to the piezoelectric material is provided from the top side of the piezoelectric material PM. The provision of the first gap Gi between the first electrode ELi and the piezoelectric material permits reduction of ohmic losses (by thickening electrodes) without disturbing the acoustics performance.

Figure 2 illustrates the possibility of arranging the first gap Gi below the piezoelectric material. Thus, the first electrode ELi and the piezoelectric material PM between which the first gap is arranged is the bottom electrode BE.

Figure 3 illustrates the possibility of providing a gap above and a gap below the piezoelectric material PM. The top electrode TE establishes the first electrode ELi. The bottom electrode BE establishes the second electrode EL2. The first gap Gi is arranged between the piezoelectric material PM and the top electrode TE. The second gap Gi is arranged between the piezoelectric material and the bottom electrode BE.

In Figure l the first gap in combination with the top electrode and the piezoelectric material PM establishes a capacitive element that is electrically connected in series between the top side of the BAW resonator BAWR and the piezoelectric material.

In Figure 2 the first gap Gi in combination with the first electrode ELi and the piezoelectric material PM establishes a capacitance element arranged at the bottom side of the resonator BAWR.

In Figure 3 the first gap in combination with the top electrode and the piezoelectric material establishes a first series capacitance element. The second gap G2 establishes - combination with the bottom electrode and the piezoelectric material a second capacitance element such that the electro-acoustic transducer comprising the piezoelectric material PM is electrically connected in series between the two capacitance elements.

The coupling of the RF signal from an external circuit environment, e.g. of an RF filter, to the piezoelectric material PM is via the first gap Gi and/or via the first and the second gap Gi, G2.

Figure 4 illustrates the use of coupling layers CLi, CL2 to allow tuning of the lateral mode dispersion properties of the active-area as well as resonator frequency detuning. Specifically, a first coupling layer CLi is arranged on the top side of the piezoelectric material. The second coupling layer CL2 is arranged at the bottom side of the piezoelectric material. The first coupling layer CLi augments the coupling via the first gap Gi. The second coupling layer CL2 augments the coupling via the second gap G2. Further, the BAW resonator BAWR schematically shown in Figure 4 illustrates the use of spacer elements SP surrounding the electro-acoustically active transducer region and maintaining the necessary gaps Gi, G2 between the piezoelectric material and the environment to confine the acoustic energy to the piezoelectric material and the coupling layer CLi, CL2. The resonator BAWR has a cap material layer CPML and a carrier material layer CRML. The cap material CPML establishes a cap and the carrier material layer CRML establishes a carrier substrate such that the transducer structure can be hermetically sealed in a closed volume such that the sensitive transducer structure is protected against detrimental influences of the environment of the BAW resonator BAWR.

Electrical connections are provided utilizing through connections TC through the cap material layer CPML and through the carrier material layer CRML. Via the through connections TC the transducer comprising the piezoelectric material PM is electrically coupled to signal lines SL electrically connecting the BAW resonator BAWR to an external circuit environment, e.g. to other circuit elements or resonators of an RF filter.

Figure 5 illustrates the use of a cavity CAV arranged below the piezoelectric material PM. Specifically, the cavity CAV is arranged below the bottom electrode BE that can be directly mechanically connected to the piezoelectric material PM. The cavity CAV provides an improved decoupling of the piezoelectric material PM from its

environment such that acoustic energy is confined to the transducer area to further reduce losses. Thus, Figure 5 shows a BAW resonator of the TF-BAR type.

In contrast, Figure 6 illustrates a BAW resonator of the SMR-type having a Bragg mirror BM arranged below the piezoelectric material PM to confine acoustic energy to the resonator structure. The piezoelectric material at its bottom side can be directly connected to the bottom electrode BE. Then, non-conducting materials can be used in the Bragg mirror BM. The Bragg mirror BM comprises iteratively arranged layers of different acoustic impedance to maintain a high degree of energy reflection back to the piezoelectric material.

In contrast, Figure 7 shows the use of a Bragg mirror BM that establishes a part of the bottom electrode BE. This is possible if each of the layers of the Bragg mirror BM comprises a conductive material.

Figure 8 shows - in a cross-sectional view corresponding to Figures 1 to 11 - structures of a horizontal acoustic reflection structure HARS. The horizontal acoustic reflection structure comprises structures at the top side of the piezoelectric material. However, the structures can be mirrored to the bottom side of the piezoelectric material PM, too. The structures HARS comprise vertical interfaces between areas of different acoustic impedances. Specifically, the embodiments shown in Figure 8 base on two recesses. A first recess Ri is structured in the piezoelectric material. An additional second recess R2 of a smaller lateral dimension is additionally structured in the top side of the piezoelectric material such that the vertical interfaces between air or vacuum and the piezoelectric material at the lateral positions LP2 and LP3 are obtained. The lateral position LPi is defined by the lateral end of the electrode and/or of the coupling layer CLi. The second lateral position LP2 is defined by the vertical section corresponding to the second recess. The third lateral position LP3 in the cross-section shown in Figure 8 is defined by the vertical interface corresponding to the perimeter of the first recess Ri. The fourth lateral position LP4 is defined by the lateral end of the piezoelectric material PM. It can be preferred that the distance between the second lateral position LP2 and the third lateral position LP3 and/ or the distance between the third lateral position LP3 and the fourth lateral position LP4 is an integer multiple of l/4 wherein l is the corresponding acoustic wavelength such that a Bragg mirror-like structure reflecting acoustic energy back to the active transducer structure is obtained. Thus, the horizontal acoustic reflection structure HRAS establishes a horizontal acoustic mirror HAM.

Further, the depth of the first and the second recess Ri, R2 are provided by the height differences of the first, second and third vertical position. Specifically, the depth of the first recess Ri is defined by the height difference of the third vertical position VP3 and the second vertical position VP2. The depth of the second recess R2 is defined by the height difference between the first vertical position VPi and the second vertical position VP2.

Figure 9 illustrates a possible alternative to the structures shown in Figure 8 which base on recesses. In contrast to the embodiment shown in Figure 8, the structures HARS of Figure 9 base on the provision of additional material in the corresponding sections between the second, third and fourth lateral positions LP2, LP3, LP4.

Specifically, the material arranged in the layer of the first coupling layer CLi can have a different acoustic impedance compared to that of the first coupling layer CLi between the second and the third lateral position. An additional step in acoustic impedance can be arranged at the third lateral position, i.e. between the material between the second and the third lateral position and the material arranged between the third and the fourth lateral position LP3, LP4 such that a horizontal acoustic mirror reflecting acoustic energy back in a horizontal direction to the transducer structure is obtained.

Figure 10 illustrates the possibility of providing a fifth lateral position LP5 at which the thickness of the material between the third and the fourth lateral position LP3, LP4 is changing with respect to the lateral direction.

Figure 11 shows the possibility of adding a sixth lateral position LP6 at which a thickness of the material in the section between the second lateral position LP2 and the third lateral position LP3 provides a step for improving the reflection of acoustic energy back to the active transducer structure.

Figure 12 shows a possible implementation of the BAW resonator as a series resonator SR and/or as a parallel resonator PR of a multiplexer MUL as indicated by the duplexer shown in Figure 12. The duplexer comprises a transmission filter TXF arranged between an input port and a common port CP and a reception filter RXF arranged between the common port CP and an output port. The transmission filter TXF and the reception filter RXF have a ladder-type like circuit topology with series resonators SR electrically connected in series in the signal path between the ports. Parallel resonators PR are arranged in parallel paths electrically connecting the signal path to ground. Between the common port CP and the reception filter RXF an impedance matching circuit IMC is provided to match the frequency-dependent impedances of the ports of the transmission filter TXF and of the reception filter RXF. An antenna AN can be electrically connected to the common port CP.

The BAW resonator, the RF filter, the multiplexer and the method of manufacturing a BAW resonator are not limited to the technical details described above or shown in the figures. The resonator can comprise further acoustic and/or electric means for optimization.

List of Reference Signs

AN: antenna

BAWR: BAW resonator

BE: bottom electrode

BEL: bottom electrode layer

BM: Bragg mirror

CAV: cavity

CP: common port

CPML: cap material layer

CRML: carrier material layer

ELl, EL2: first, second electrode

Gi, G2: first, second gap

HAM: horizontal acoustic mirror

HARS: horizontal acoustic reflection structure

IMC: impedance matching circuit

LPi, LP2, LP3, first, second, ..., sixth lateral

LP4, LP5, LP6: position

MUL: multiplexer

PL: piezoelectric layer

PM: piezoelectric material

PR: parallel resonator

Ri, R2: first, second recess

RXF: reception filter SL: signal line

SP: spacer element

SR: series resonator

TC: through-connection

TΈ: top electrode

TEL: top electrode layer

TXF: transmission filter

VPi, VP2, VP3: first, second, third vertical position