SAW resonator with reduced spurious modes, electro acoustic filter and multiplexer

The present application refers to electro acoustic resonators that can be used in carrier aggregation applications and that have reduced spurious modes.

The ongoing trend towards a higher number of functionalities, larger data transmission rates and smaller spatial dimensions demands for improved devices for mobile communication and for improved components for such devices. The evolution of next generation mobile communication systems requires devices with outstanding performance.

In electro acoustic resonators an electrode structure in combination with a piezoelectric material convert - due to the piezoelectric effect - between electromagnetic and acoustic RF signals. However, in real devices unwanted, spurious modes maybe excited in addition to wanted acoustic modes. The unwanted, spurious acoustic modes deteriorate the corresponding filter’s performance, making it difficult or impossible for conventional resonators to comply with present or future specifications.

From US 9,190,981 B2 and from US 9,413,334 B2 layer constructions for electro acoustic resonators are known.

From US 2015/0102705 Ai electro acoustic resonators comprising dielectric functional layers are known. From DE 10 2017 111448 At the use of a silicon material as a carrier substrate is known.

However, conventional electro acoustic resonators, e.g. SAW resonators (SAW = surface acoustic wave) comprise additional functional layers, e.g. for temperature compensation, for passivation and the like that facilitate the creation of unwanted, spurious modes.

Thus, what is wanted is an electro acoustic resonator that enables RF filters, e.g. for mobile communication applications, having outstanding performance, providing a large bandwidth, having a low temperature coefficient of frequency (TCF), that are compatible with carrier aggregation applications, in which other performance parameters are not deteriorated, that comply with stringent specifications, that can be used with different frequency ranges and that have a reduced strength of unwanted, spurious modes such as higher order modes or bulk modes.

To that end, a SAW resonator according to independent claim l is provided. Dependent claims provide preferred embodiments.

The SAW resonator with reduced spurious modes can comprise a carrier substrate and an electrode structure above the carrier substrate. Further, the resonator can have a piezoelectric layer between the carrier substrate and the electrode structure.

It is possible that - during operation of the resonator - a spurious acoustic mode is separated from a wanted acoustic mode.

Further, it is possible that a wave mode separator is arranged between the carrier substrate and the electrode structure. Specifically, it is possible that the wave mode separator separates - during operation - the spurious acoustic mode from the wanted acoustic mode.

In SAW resonators a wanted acoustic mode is excited by the electrode structure utilizing the piezoelectric material. The electrode structure usually has interdigitated comb-like electrodes comprising two busbars and electrode fingers that are electrically connected to one of the two busbars. The wanted acoustic mode typically propagates in a direction perpendicular to the extension of the electrode fingers at the surface of the piezoelectric material.

Additional acoustic modes (spurious modes) can be excited as a result of, for example, linear effects of the piezoelectric material or by reflection effects within the

corresponding waveguiding structure for the wanted acoustic mode. Especially components of the resonators that maybe needed for complying with specific requirements, e.g. TCF layers and the like, can establish another source of excitation of unwanted modes. Thus, in conventional resonators spurious modes must be accepted as an unavoidable side effect.

The separation of a spurious mode from a wanted acoustic mode as described above removes the spurious mode at least partially such that detrimental effects are reduced and the performance of the resonator and the corresponding filter is improved.

Spurious modes can occur at frequency ranges that are sufficiently far away from the working frequencies of the resonator. However, when carrier aggregation systems are concerned then such spurious modes can disturb the operation of another frequency band. Thus, while such spurious modes may have been tolerated in systems without carrier aggregation, in new systems supporting carrier aggregation such modes cannot be accepted any longer and the above-described separation allows the present resonators to be used in carrier aggregation systems. It is possible that the resonator comprises a separation interface on or above the carrier substrate.

The separation interface can work as the wave mode separator for separating the spurious mode or several spurious modes from the wanted acoustic main mode.

The separation interface can be an interface between a material of the carrier substrate and a material of the electrode structure. Specifically, the separation interface can be located at the top side of the carrier substrate, at the bottom side of the piezoelectric layer or at an intermediate layer between the carrier substrate and the piezoelectric layer.

Specifically, the separation interface can be an interface between two different materials.

Further, it is possible that the two different materials between which the separation interface is located have different acoustic properties.

It is possible that the SAW resonator further comprises a first separation layer and/or a second separation layer.

The first separation layer can establish a wave mode separator or can contribute to the wave mode separation.

Also, the second separation layer can establish a wave mode separator or contribute to the wave mode separation. The provision of the wave mode separator has an influence on the propagation of acoustic modes at the surface of the SAW resonator and in the material below the surface of the SAW resonator.

The materials and the material system of the SAW resonator, specifically the provision of the wave mode separator (e.g. in the form of the separation interface, the first separation layer and/ or the second separation layer) can have a selective reflectivity for different wave modes. For wanted wave modes the reflectivity can be high such that wanted wave mode is kept at the surface of the SAW resonator. The reflectivity can be low for unwanted, spurious wave modes such that a separation takes place and unwanted, spurious wave modes are conducted away from the acoustic track of the resonator.

This can be obtained by choosing the materials of the system such that wanted and unwanted modes have different preferred directions of propagation.

The decoupling of wanted from unwanted modes keeps the wanted acoustic energy in the acoustic track while the energy of unwanted acoustic modes can dissipate in the bulk material.

Further, the materials and the material system and the layer arrangement can be chosen such that the excitation of wanted modes is enhanced while the excitation of unwanted modes is reduced.

The parameters of the materials of the corresponding layer construction of the resonator are chosen such that the above effects are obtained. For example by choosing the stiffness constants, the lattice constants and the lattice orientation of the layers, the above-described effects can be obtained. Correspondingly, it is possible that the first separation layer - when present - comprises or consists of a material selected from a dielectric material, a polycrystalline silicon, an amorphous material. Further, the first separation layer can have a thickness yi with 0.05 l < yi < l (while l is the acoustic wavelength of the wanted main mode).

The second separation layer - when present - can comprise or consist of a material selected from a dielectric material, a silicon oxide, a silicon dioxide, S1O2. The second separation layer can have a thickness y2 with 0.1 l < y2 < l.

The first separation layer - when present - can be arranged between the carrier substrate and the piezoelectric material.

The second separation layer - when present - can be arranged between the carrier substrate and the piezoelectric layer. The second separation layer can be arranged between the first separation layer and the piezoelectric layer. However, it is also possible that the second separation layer is arranged between the carrier substrate and the first separation layer.

It is possible that the carrier substrate comprises or consists of silicon, e.g.

monocrystalline silicon.

It is possible that the top surface of the carrier substrate is parallel to the [110] plane or parallel to the [111] plane. Further, it is possible that the carrier substrate has a crystal orientation with the Euler angles (-45°±io°; -54°±io°; 6o°±20°) or

(-45°±io°; -90°±io°; 30°±30°).

In this context the numbers 1, 1, o and the numbers 1, 1, 1 denote the Miller indices that define the orientation of planes of a crystal. By providing the Miller indices, the orientation of the plane, i.e. of the carrier substrate top surface, is unambiguously defined with respect to the crystallographic axes.

By providing the Euler angles, the orientation of the crystallographic axes relative to the top surface of the carrier substrate is also unambiguously clear. Additionally, the propagation direction of the acoustic wave mode relative to the crystallographic axes is also unambiguously defined by the Euler angles.

In this case, the Euler angles (l m, Q) are defined as follows: firstly, a set of axes x, y, z are taken as a basis, which are the crystallographic axes of the piezoelectric material.

The first angle, l’, specifies by what magnitude the x-axis and the y-axis are rotated about the z-axis, the x-axis being rotated in the direction of the y-axis. A new set of axes x', y', z' correspondingly arises, wherein z = z'.

In a further rotation, the z'-axis and y'-axis are rotated about the x'-axis by the angle m. In this case, the y'-axis is rotated in the direction of the z'-axis. A new set of axes x", y", z" correspondingly arises, wherein x' = x".

In a third rotation, the x"-axis and the y"-axis are rotated about the z"-axis by the angle Q. In this case, the x"-axis is rotated in the direction of the y"-axis. A third set of axes x'", y'", z'" thus arises, wherein z" = z'".

In this case, the x'"-axis and the y'"-axis are parallel to the surface of the substrate. The z'"-axis is the normal to the surface of the substrate. The x'"-axis specifies the propagation direction of the acoustic waves. The definition is in accordance with the International Standard IEC 62276, 2005-05, Annex At.

Euler angles with a reduced angle range (e.g. io° for all three angles, 5 ⁰ for all three angles, 2 ⁰ for all three angles) are also possible.

It is possible that the piezoelectric layer comprises or consists of a material selected from lithium tantalate (LiTa0 ₃ ) and lithium niobate (LiNb0 ₃ ). Further, the piezoelectric layer can have a thickness y3 with 0.2 l < y3 < l.

Further, the SAW resonator can be a resonator of an electro acoustic filter.

Further, the filter can be a filter of a multiplexer, e.g. for carrier aggregation (CA) applications.

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

In the figures:

Fig. 1 shows a cross-section through a possible layer construction;

Fig. 2 shows a layer construction comprising a first separation layer;

Fig. 3 shows a layer construction with a first and a second separation layer; Fig. 4 shows - in a top view - a basic layout of the electrode structure;

Fig. 5 illustrates possible circuit topologies of a duplexer;

Fig. 6 indicates the definition of the Euler angles;

Fig. 7 shows the real paths of admittance curves for different Euler angles of the carrier substrate (frequency-dependent);

Fig. 8 shows the corresponding magnitude values;

Fig. 9 shows the frequency-dependent real paths of admittance curves for different Miller indices; and

Fig. 10 shows the corresponding magnitude values.

Figure 1 illustrates a cross-section through a possible layer construction of the SAW resonator SAWR. The layer construction comprises a carrier substrate CS on which further layer elements are arranged. Especially the electrode structure ES is arranged above the carrier substrate CS. Between the carrier substrate CS and the electrode structure ES the piezoelectric layer PL comprising or consisting of a piezoelectric material is arranged. A wave mode separator WMS is arranged between the

piezoelectric layer PL and the carrier substrate CS. The wave mode separator WMS can be established as an interface I between the material of the carrier substrate CS and the material of the piezoelectric layer PL. The interface between the two materials can work as a wave mode separator particularly well when the material parameters of the carrier substrate CS and/or of the piezoelectric layer PL are correspondingly chosen. However, in a SAW resonator the crystallographic axes of the piezoelectric material are chosen such that an appropriate electro acoustic coupling coefficient is obtained. Thus, typically the piezoelectric axes is oriented parallel to the propagation direction of the acoustic main mode x. The layer construction has its layers arranged on one another in the y-direction. The electrode structure ES has electrode fingers of which the cross- section is shown in Figure l. The extension of the electrode fingers is orthogonal to the xy plane defining the cross-section shown in Figure 1.

Further, Figure 2 illustrates a possible layer construction where the wave mode separator WMS is realized by or augmented by a first separation layer SLi.

It is possible that an additional separation interface is present between the material of the carrier substrate CS and the material of the first separation layer SLi and/or between the material of the first separation layer SLi and the material of the piezoelectric layer PL.

Figure 3 indicates the possibility of arranging a second separation layer SL2 in the layer construction. The second separation layer SL2 can be arranged between the first separation layer SLi and the piezoelectric layer PL. However, it is also possible that the order of the first separation layer and the second separation layer is inverted.

Figure 4 illustrates a basic configuration of a SAW resonator in a top view. The surface of the SAW resonator is parallel to the xz plane. The direction of propagation of the acoustic waves is parallel to the x-direction. The electrode fingers EFI have their extension along the z-direction. The busbars BB have an extension along the longitudinal direction x. Electrode fingers EFI are electrically connected to one of two busbars and establish interdigitated structures IDS. The interdigitated structures IDS establish the electrode structure ES and are arranged between acoustic reflectors R to confine acoustic energy in the acoustic track. The electrode structure ES together with the reflectors R are arranged on the piezoelectric material PM.

Figure 5 illustrates a possible circuit topology of a duplexer as an example of a multiplexer. The duplexer comprises a transmission filter TXF and a reception filter RXF. Each of the two filters has electro acoustic resonators, e.g. SAW resonators. The resonators can be series resonators SR electrically connected in series in a signal path. Parallel resonators PR electrically connect the signal path to ground. A common port CP can be electrically connected to an antenna AN. An impedance matching circuit IMC can be provided between the transmission filter TXF and the reception RXF to match input and output impedances of the filters in accordance with the corresponding frequencies.

Figure 6 illustrates the definition of the Euler angles. The resulting axes x’”, y”’ and z’” correspond to the axes denoted by x, y, z in the figures above.

Figure 7 illustrates a comparison of the real paths of admittance curves of two resonators. Curve 1 corresponds to a resonator where the silicon substrate has Euler angles (o°, o°, o°). Curve 2 corresponds to a resonator where the carrier substrate has the Euler angles (o°, o°, 45 ⁰ ). It can clearly be seen that the orientation of the crystallographic axes of the carrier substrate substantially determines the performance of the resonator.

Correspondingly, Figure 8 shows the frequency-dependent magnitudes of the admittance curves of the resonators corresponding to Figure 7.

Figure 9 illustrates the frequency-dependent real path of the admittance curves for resonators where curve 1 corresponds to a resonator where the carrier substrate provides a top surface parallel to the plane defined by the Miller indices too. Curve 2 shows the real path of the admittance of a resonator where the carrier substrate provides a top surface parallel to the plane defined by the Miller indices 110.

Correspondingly, Figure 10 illustrates the frequency-dependent magnitude values of the admittance curves corresponding to Figure 9. The resonator is not limited to the details and configurations shown above. Additional elements such as TCF layers, passivation layers, waveguiding elements and similar elements can be present. Despite the possibility of the presence of a plurality of additional layers - that would lead to potential sources of unwanted spurious modes - spurious modes are reduced and the performance is improved.

List of Reference Signs

AN: antenna

BB: busbar

CP: common port

CS: carrier substrate

EFI: electrode finger

ES: electrode structure

I: interface

IDS: interdigitated structure

IMC: impedance matching circuit

PL: piezoelectric layer

PM: piezoelectric material

PR: parallel resonator

R: acoustic reflector

RXF: reception filter

SAWR: SAW resonator

SLi, SL2: first, second separation layer

SR: series resonator

TXF: transmission filter

WMS:wave mode separator