Electro acoustic resonator with reduced spurious modes, RF filter and Multiplexer

The present invention refers to an electro acoustic resonator with reduced spurious modes. The resonator can be used in an RF filter and the filter can be used in a multiplexer.

Wireless communication devices need RF filters for separating wanted RF signals from unwanted RF signals. RF filters can comprise electro acoustic resonators employing an electrode structure being in contact with a piezoelectric material. The latter may be a bulk piezoelectric material or a layered material comprising at least one layer of a piezoelectric material. Due to the piezoelectric effect, the electrode structure can convert between acoustic and electric RF signals. With such resonators filter functionalities such as bandpass filters or band rejection filters can be obtained. The electrodes and/or the piezoelectric material may be covered with dielectric or other functional layers.

From WO 2011/088904 A1 electro acoustic resonators employing a piston mode are known.

Due to design restrictions, fabrication imperfections or competing performance targets it may be impossible to fully suppress all unwanted modes, e.g. transversal modes,

differently polarized surface modes or volume modes. A consequence of unwanted modes is a reduction of the filter performance. Specifically the insertion loss within a passband is increased and the out-of-band rejection is reduced. Additional unwanted effects are passband ripple, group delay ripple, a reduced power durability and a

compression degradation.

Thus, what is wanted is an electro acoustic resonator that allows RF filters where spurious modes are suppressed, where the insertion attenuation is improved, where smoother passband skirts are provided, where the group delay ripple is reduced and where the compression and the power durability are improved. Further, it is desired to have an alternative solution to spurious modes reduction and to have further degrees of freedom when designing transducer structures due to reduced design restrictions of known means for spurious modes reduction.

To that end, an electro acoustic resonator with reduced spurious modes according to the independent claim is

provided. Dependent claims provide preferred embodiments.

The electro acoustic resonator comprises a longitudinal direction and an interdigital transducer with two bus bars and interdigitated electrode fingers. Further, the electro acoustic resonator comprises a wave guide structure provided and configured to excite - together with the interdigital transducer - a piston mode in the resonator. The wave guide structure can be defined by a plurality of parameters. At least a first parameter pi of the wave guide structure varies along the longitudinal direction. The longitudinal direction x of the electro acoustic

resonator is essentially defined by the direction of

propagation of the acoustic waves of the acoustic main mode of the electro acoustic resonator. The interdigital

transducer comprises the two bus bars and a plurality of electrode fingers. Each of the electrode fingers is

electrically connected to one of the two bus bars. An overlap length of neighbored electrode fingers - between which an excitation center is arranged - essentially defines - in the transversal direction y - the aperture of the acoustic track of the resonator and the width of the active region of the electro acoustic resonator. Thus, the interdigital transducer comprises two comb-like structures that are interdigitated by means of their corresponding electrode fingers. The two bus bars essentially extend along the longitudinal direction x. The electrode fingers essentially extend along the

transversal direction y being mainly orthogonal to the two bus bars.

Thus, the direction of the bus bars and the direction of the electrode fingers define the xy plane that is the plane at which an acoustic main mode of the resonator can propagate in the longitudinal direction x.

A piston mode is an acoustic mode where the amplitude is nearly constant within an inner track and decays to zero at regions flanking the inner track.

The wave guide structure of the electro acoustic resonator can be described by means of defining its structural and physical properties or by explaining its impact on the wave propagation within the electro acoustic resonator. Physical structures and properties of wave propagation are related to one another. Thus, the first parameter pi of the wave guide structure can be a physical or a structural parameter or a parameter defining the wave propagation within the acoustic resonator .

The variation of the first parameter pi along the

longitudinal direction means that the value of the first parameter depends on the precise position along the

longitudinal direction x.

It is possible that the first parameter varies continuously along the longitudinal direction. However, it is also possible that the first parameter varies stepwise along the longitudinal direction.

The variation of the first parameter pi along the

longitudinal direction is a counterintuitive approach towards spurious modes reduction because the wave guide structure is provided to excite a piston mode in the resonator. For establishing a piston mode in the resonator, variations along the longitudinal direction are generally detrimental and if the variation along the longitudinal direction becomes too strong, then the piston mode may no longer be supported by the resonator.

However, it was found that the excitation strength and/or resonance frequency of unwanted spurious modes in a SAW resonator often depends on one or more parameters of the wave guide structure, e.g. the depth of the transversal trap, the position of the trap along the finger or the effective aperture. As an example, there may be spurious modes

localized in the trap region, whose resonance frequencies strongly depend on the width and depth of the trap region. Therefore, a variation of at least one of the parameters of the wave guide structure along the longitudinal direction of the resonator can avoid, or at least substantially reduce, unwanted resonant excitations of one or more spurious modes leading to an overall performance improvement of a

corresponding RF filter.

It is possible that the wave guide structure is provided and configured to establish a transversal velocity profile. The transversal velocity profile can have adjacent regions within the electro acoustic resonator' s area where the regions are selected from an inner track region, a trap region, a barrier region and a bus bar region. The acoustic velocity is different in adjacent regions.

The acoustic velocity is the velocity of the acoustic main mode of the electro acoustic resonator. An inner track region may be the area where electrode fingers that are electrically connected to opposite bus bars overlap one another such that excitation centers are arranged between the electrode fingers. A trap region can be defined as a region where the acoustic velocity is altered, e.g. increased or decreased, compared to the acoustic velocity of the inner track region or a barrier region. The barrier region can be defined as a region where the acoustic velocity is higher than the acoustic velocity in the trap region and/or in the inner track region. Also, it is possible that the barrier region can be defined as a region where the acoustic velocity is smaller than the acoustic velocity in the trap region and/or in the inner track region. Further, it is possible that the acoustic velocity of the barrier region is higher than the acoustic velocity of the inner track region. The bus bar region is defined by the position of the material of the bus bar.

It is possible that the inner track region is arranged between two trap regions. It is possible that a trap region is arranged between a barrier region and an inner track region. It is further possible that a barrier region is arranged between a bus bar region and a trap region.

Thus, the barrier of an electro acoustic resonator comprising an inner track region flanked by two trap regions which are flanked - at their outside interfaces - by two barrier regions which are flanked - at their outside interfaces - by two bus bar regions can be thought of as an acoustic track with two wave guide structures. One of the two wave guide structures is responsible for establishing a barrier region and a trap region at one side of the inner track region. The respective other wave guide structure is responsible for establishing the trap region and the barrier region on the respective other side of the inner track region.

It is possible that the wave guide structure locally raises or lowers the acoustic velocity.

Specifically, it is possible that the wave guide structure lowers the acoustic velocity in the trap region with respect to the barrier region and with respect to the inner track region. Further, the wave guide structure may locally raise the acoustic velocity of the barrier region with respect to the trap region.

A local increase or a local reduction of the acoustic velocity is a parameter that determines the effect of the transversal velocity profile. As already described, there are further types of parameters that determine the nature of the wave guide structure such as structural or physical

parameters related to the wave guide structure.

Correspondingly, it is possible that the wave guide structure locally adds or removes material from one or more layers of the resonator and thus locally modifies the mass loading of the resonator and/or locally lowers or increases a stiffness parameter of one or more layers of the resonator.

As described above, an interdigital transducer comprises an electrode structure. The electrode structure usually

comprises two electrodes. The material of the electrodes can be arranged on the surface of a piezoelectric material. The details of wave propagation at the surface of the acoustic material, e.g. of surface acoustic waves (SAW), depend on various parameters such as the local mass loading defined by the mass arranged at a specific position of the piezoelectric material. A further parameter is a stiffness parameter of the matter arranged on the piezoelectric material. The

propagation velocity usually increases with increasing stiffness parameters and decreases with an increasing mass loading .

Thus, the acoustic wave velocity can be increased by locally increasing the stiffness parameter, e.g. by adding a patch of matter with a higher stiffness parameter than the patch's environment. Another alternative is to locally replace a patch of matter by a material with higher stiffness. Correspondingly, the addition of a patch increases the mass loading and the provision of a recess within a layer above the piezoelectric material locally reduces the mass loading.

By providing corresponding layer thicknesses and well-chosen materials a transversal velocity profile of which at least one parameter varies along the longitudinal direction can be provided .

It is further possible that the first parameter pi is selected from the functional parameters: a width of an inner track region, a width of a trap region, a width of a barrier region;

a distance between a bus bar and an inner track region, a distance between a bus bar and a trap region, a distance between a bus bar and a barrier region;

a distance between two regions selected from: an inner track region, a trap region and a barrier region; and

the velocity of an inner track region, the velocity of a trap region and the velocity of a barrier region.

In a preferred version the electrode width and/or thickness in the trap region may be increased compared to the inner track region. The first parameter pi may be the transversal position of the trap region, the width and/or thickness of the electrodes in the trap region or the extent of the trap region .

The known relationship between mass loading, stiffness parameters and acoustic velocity easily provides the

correlation between such functional parameters and

structural/physical parameters. Thus, the provision of functional parameters and the provision of structural or physical parameters are equivalent.

Similarly, it is possible that the first parameter pi is selected from the structural parameters:

a stiffness of an inner track region, a stiffness of a trap region, a stiffness of a barrier region; and

a mass loading of an inner track region, a mass loading of a trap region, a mass loading of a barrier region.

It is possible that the first parameter pi has a variation within a specific range Drΐ . Thus, it is possible that the relative variation Drΐ/rq is ³ 0.01 and < 0.4, where pO is the average of the first parameter along the longitudinal direction .

The average can be the arithmetic average.

Thus, the relative variation of the first parameter pi is between 1% and 40% along the longitudinal direction.

The relative variation should be sufficiently large to effectively suppress the unwanted modes and sufficiently small to maintain the piston mode.

The number of varying parameters is not limited to one.

Correspondingly, it is possible that the wave guide structure has one or more additional parameters varying along the longitudinal direction.

For each of the one or more additional parameters, the above- stated properties are possible. Similarly, the number of wave guide structures is also not limited to one.

Correspondingly, it is possible that the electro acoustic resonator comprises one or more additional wave guide structures .

Also, for the one or more additional wave guide structures the above-stated properties are possible.

Specifically, it is possible that the resonator comprises two varying wave guide structures where each of the wave guide structures has at least one parameter varied along the longitudinal direction. It is preferred that both wave guide structures have the same number and the same types of varied parameters. The variation of the parameters of the two varying wave guide structures can be symmetric or anti symmetric with respect to a symmetry line extending along the longitudinal direction.

It is possible that the electro acoustic resonator is a one port resonator, a multi-port resonator or a DMS-resonator (DMS = dual mode SAW) .

A one port resonator comprises one input connection and one output connection. Specifically, it is possible that a one port resonator has not more than the two connections

establishing the one port of the resonator.

A multi-port resonator can comprise two or more ports. Specifically, a multi-port resonator can comprise a plurality of two or more transducers. At least one of the transducers has a wave guide structure as described above.

A DMS-resonator comprises two or more transducers that are acoustically coupled and arranged between acoustic reflectors such that a wide band bandpass filter can be obtained.

It is possible that the wave guide structure has a shape that is selected from a linear shape, a sinusoidal shape, a saw tooth shape, a triangular shape.

It is possible that an RF filter comprises one or more of the above-described electro acoustic resonators.

It is possible that the RF filter comprises a ladder-type like filter topology with one or more series resonators electrically connected in series in a signal path between an input port and an output port and one or more parallel resonators electrically connected in parallel paths

electrically connecting the signal path to a ground

potential .

However, it is also possible that the RF filter comprises a lattice-type like circuit topology where a circuit element, e.g. a resonator, electrically connects a first connection of a first port to a second connection of a second port.

Further, it is possible that a multiplexer comprises one or more of such RF filters.

Ihe multiplexer can be a duplexer, diplexer or a multiplexer of a higher order. A duplexer comprises a reception filter and a transmission filter. The reception filter is arranged between a common port and an output port and the transmission filter is arranged between an input port and the common port.

Central aspects of the electro acoustic resonator and details of preferred embodiments are shown in the schematic

accompanying figures.

In the figures:

Fig. 1 shows the general concept of the electro acoustic resonator with two varying wave guide structures;

Fig. 2 shows possible implementations of locally increasing and reducing the acoustic velocity;

Fig. 3 illustrates the relations between mass loading, velocity and piston mode;

Fig. 4 illustrates a specific example of a varying wave guide structure ;

Figs. 5 to 7 show additional specific examples of varying wave guide structures;

Fig. 8 shows the possibility of continuous wave guide structures ;

Fig. 9 shows the possibility of arranging more than two wave guide structures in the acoustic track;

Fig. 10 shows the possibility of phase shifted wave guide structures; Fig. 11 shows a duplexer topology; and

Fig. 12 shows a basic example of a DMS resonator.

Figure 1 shows an electro acoustic resonator EAR comprising an electrode structure on a piezoelectric material PM. The electrode structure comprises an interdigital transducer IDT comprising two bus bars and a plurality of electrode fingers EFI electrically connected to one of the two bus bars.

The interdigital transducer is arranged between a reflector R comprising a reflector grating, x denotes the longitudinal direction, i.e. the main propagation direction of the acoustic working mode of the electro acoustic resonator, y denotes the transversal direction along which the electrode fingers extend.

The electro acoustic resonator EAR comprises a first varying wave guide structure VWS1 and a second varying wave guide structure VWS2. The two wave guide structures essentially extend along the longitudinal direction x but have one parameter that changes along the longitudinal direction.

As shown in Figure 1, the parameter of each wave guide structure can be the distance between the corresponding wave guide structure and the bus bar. Thus, there are two

longitudinal positions where the distance at the specific position between the wave guide structure and the bus bar are different from one another.

It is possible that a wave guide structure comprises a continuous structure such as is shown in the first wave guide structure VWS1. However, it is also possible that the wave guide structure extends along a continuous path but the structure itself comprises a plurality of individual segments that have a certain distance between one another.

However, as long as the distance between segments of a wave guide structure is smaller than or approximately equal to the acoustic wavelength, the corresponding wave guide structure will be seen by the acoustic waves as a single entity. Thus, the terminology "one wave guide structure" although there is a plurality of segments, is justified.

Figure 2 illustrates a more detailed view of two neighbored electrode fingers, characteristic properties of parameters and their correspondence to the regions within the acoustic track. At an inner track region IT two electrode fingers that are electrically connected to opposite bus bars establish an excitation area. At locations where the finger width, i.e. the extension along the longitudinal direction x, is

increased, the local mass loading is increased and a trap region is obtained. In a barrier region the local mass loading is reduced because only one of the two electrode fingers contributes to the mass loading in the barrier region .

In the region of the bus bars the material of the bus bars is arranged .

Correspondingly, a sequence along the transversal direction y of: a bus bar region BB, a barrier region BA, a trap region TP, an inner track region IT, a trap region TP, a barrier region BA and a final bus bar region BB is obtained. The left barrier region BA and the left trap region TP can be associated with a first varying wave guide structure. The further trap region TP and the further barrier region BA on the right-hand side of Figure 2 can be associated with a second varying wave guide structure.

To complete the correlation between regions, mass loading and acoustic velocity, Figure 3 illustrates a cross-section through an electro acoustic resonator. Specifically, z denotes the vertical direction in which the piezoelectric material and the electrode system are stacked.

In the barrier regions BA the local mass loading is reduced because only one electrode finger contributes to the overall mass loading. In the trap regions the mass loading is increased due to an increased finger height along the vertical direction z. Within the inner track region IT the mass loading is essentially homogenous and between the mass loading of the trap region and the barrier region.

Correspondingly, the barrier region BA has the highest acoustic velocity v. The trap region has the lowest acoustic velocity and the inner trap region has an acoustic velocity between the velocity of the trap region and the barrier region. The transversal velocity profile, i.e. the

distribution of the acoustic velocity of the main mode along the longitudinal direction x along the transversal direction y establishes a wave guide such that a piston mode PMO

(dashed line) within the inner track region IT is established when the corresponding electro acoustic resonator is active.

Figure 4 shows a possible implementation of an electro acoustic resonator with a first varying wave guide structure VWS1 and a second varying wave guide structure VWS2. The wave guide structure comprises material patches PC arranged at distal ends of electrode fingers that extend towards the opposite electrode and patches at matched positions of the electrode fingers of the opposite electrode. The distance between the patches PC of the first varying wave guide structure VWS1 and a bus bar is varied along the longitudinal direction x. Specifically, the patches PC are arranged on a path PT defining the position of the wave guide structure.

The same holds true for the second varying wave guide structure VWS2. Thus, for both wave guide structures the varied parameter is the distance towards a bus bar which is equivalent to a position along the transversal direction y.

Thus, also the patches PC of the second wave guide structure follow a corresponding path. Further, the two paths have a wave-like shape corresponding to a sine function. Further, the two paths share the same wavelength of the sine function and have the same phase. Thus, patches arranged at the same electrode fingers but belonging to both wave guide structures essentially have the same distance with respect to one another .

Figure 5 illustrates a further possibility of establishing two wave guide structures within the acoustic track of an electro acoustic resonator to reduce spurious modes. The path PT of each wave guide structure essentially has a linear extension along the longitudinal direction. The varying parameter of the two wave guides may be regarded as the extension along the transversal direction y of the additional mass loading, i.e. the length of the patches along the transversal direction. Further, the distance between a corresponding patch PC of the wave guide structure and a bus bar is also varying along the longitudinal direction x.

Similar to the situation in Figure 4, the two wave guide structures have a wave-like function and share the same amplitude, wavelength and phase.

Figure 6 illustrates the possibility of increasing an extension of patches PC along the longitudinal direction x with varying position along the longitudinal direction x. The patches have a same extension along the transversal direction y but the mass loading is increased with an increasing value of the position x.

Figure 7 shows the possibility of arranging patches of the same dimension for the two wave guide structures. Further, the patches of each wave guide structure follow a sine function-like shape. Further, the wavelength of the sine shaped path of the two wave guide structures is equal.

However, the phase difference between the two paths is 180° such that distances between patches associated with a same electrode finger but distributed over the two wave guide structures vary along the longitudinal direction x.

Figure 8 illustrates in a more general fashion the

possibility of providing a plurality of two (or more, not shown) wave guide structures VWS that essentially extend in a parallel fashion along the longitudinal direction x.

In contrast thereto, non-parallel wave guide structures are shown in Figure 9. Further, Figure 9 shows the possibility of having three wave guide structures in the acoustic track extending along the longitudinal direction x.

Figure 10 shows the possibility of having two (or more, not shown) wavelengths of the path along which the two wave guide structures are arranged. A wavelength can be the same for both wave guide structures but a phase difference can exist between the two wave guide structures along the longitudinal direction x. Further, while the wave guide structures shown in Figures 8 and 9 comprise continuous material, the wave guide structures shown in Figure 10 comprise separated segments .

Electrode fingers electrically connected to opposite

electrodes must not be short-circuited. Thus, when patches or strips of material of the wave guide structures are in direct contact with an electrode finger and are electrically conductive, then the corresponding patches or strips must not be in direct contact with an adjacent electrode finger of the opposite electrode.

This can be obtained by using a dielectric material for the wave guide structures. Further it is possible to add an additional dielectric layer at least locally between the material of the conducting wave guide structure and the material of the conducting electrode finger.

Figure 11 illustrates the application of a corresponding resonator in a duplexer DU. The duplexer DU comprises a transmission filter TXF between an input port and a common port CP and a reception filter RXF between the common port CP and an output port. A further impedance-matching circuit IMC may be provided between the transmission filter TXF and the reception filter RXF to decouple the two filters in the respective transmission and reception frequency ranges. At the common port CP an antenna AN may be connected via which to-be-sent signals can be emitted and via which to-be- received signals can be received.

The filters shown in the duplexer of Figure 11 have a ladder- type like circuit topology with series resonators SR

electrically connected in series in a signal path and with parallel resonators PR electrically connected in shunt paths between the signal path and a ground potential.

Each of the resonators or one or a few of the resonators can be as described above.

Figure 12 shows details of a basic implementation of a DMS filter DMS where two transducers as described above are arranged between acoustic reflectors. DMS filters can provide a wide passband by acoustically coupling two or more

transducers to one another.

The electro acoustic resonator or corresponding filters or multiplexers are not limited to the technical details explained above or shown in the figures. Electro acoustic resonators can comprise further elements such as means for reducing a temperature-induced frequency drift or means for protecting the sensitive electrode structures from

detrimental external influences.

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List of Reference Signs

AN: antenna

BA: barrier region

BB: bus bar

CP: common port

DMS : dual mode SAW resonator

DU: duplexer

EAR: electro acoustic resonator

EFI : electrode finger

IDT : interdigital transducer

IMC : impedance-matching circuit

IT : inner track region

PC: patch

PM: piezoelectric material

PMO: piston mode

PR: parallel resonator

PT : path

R: acoustic reflector

RXF : reception filter

SR: series resonator

TP: trap region

TXF : transmission filter

v : acoustic velocity

VWS, VWS1, VWS2: varying wave guide structures x : longitudinal direction y: transversal direction z : vertical direction