BAW resonator with improved crystalline quality, RF filter, multiplexer and method of manufacturing

The present invention refers to BAW resonators that provide an improved performance and to corresponding filters, multiplexers, and methods of manufacturing.

BAW resonators can be used in RF filters, e.g. for mobile communication devices. RF filters can be bandpass filters or band rejection filters and help separate wanted RF signals from unwanted RF signals. BAW resonators establish electro-acoustic resonators (BAW = bulk acoustic wave) and work with longitudinal wave modes propagating in a layer stack.

In a BAW resonator a piezoelectric material in a piezoelectric layer is sandwiched between a bottom electrode in a bottom electrode layer and a top electrode in a top electrode layer. The piezoelectric material, in combination with the electrodes, converts - due to the piezoelectric effect - between acoustic and electromagnetic RF signals. The performance of an electro-acoustic filter device depends on the performance of the piezoelectric resonators. The performance of a piezoelectric resonator depends on the quality factor which depends on the crystalline quality of the piezoelectric material and the quality of interfaces between different materials in the layer stack.

What is desired is an electro-acoustic resonator with improved performance.

To that end, the BAW resonator according to independent claim l is provided. The other claims provide preferred embodiments. The BAW resonator comprises a bottom electrode, a top electrode and a piezoelectric layer. The top electrode is arranged above the bottom electrode. The piezoelectric layer is arranged between the bottom electrode and the top electrode. Further, the BAW resonator comprises a first compensation layer between the bottom electrode and the top electrode.

Thus, a layer stack is provided where the piezoelectric material is sandwiched between the bottom electrode and the top electrode. A further compensation layer helps to improve the crystalline quality and is arranged below the piezoelectric material. In this context, the position (above, below) is relative to a conventional order of depositing material independent from an orientation in which the BAW resonator is used in a later application.

It was found that a mismatch between material parameters of the bottom electrode and of the piezoelectric material can cause a degradation of the resonator’s performance. Correspondingly, it is possible that the compensation layer is utilized for compensating the one or more different parameters with respect to one another.

However, it was also found that the physical interactions between different layers of a layer stack of a BAW resonator are complex. Electric parameters such as conductivity and dielectric constants and mechanical properties such as stiffness values, acoustic velocity and acoustic impedance must be considered. Thus, a BAW resonator stack is typically a complex arrangement where the stack’s parameters such as layer thickness and layer materials are adapted to one another such that a proper functioning of the resonator is obtained. Thus, adding a compensation layer to such a layer stack establishes a severe intervention and a new adaption of the stack’s parameters may be necessary.

Further, the electro-acoustic coupling coefficient K of the material of the compensation layer maybe substantially different from that of the piezoelectric material, jeopardizing the resonator’s performance. However, it was found that parameters for the layer stack’s materials and layer thicknesses can be found such that a resonator with overall improved performance is obtained.

It is possible that the first compensation layer compensates for a lattice mismatch between the bottom electrode and the piezoelectric layer.

During manufacturing the top surface of the bottom electrode layer is the place where - in conventional BAW resonator stacks - the material of the piezoelectric layer will be deposited. Typically, the piezoelectric material and the material of the bottom electrode layer have different lattice constants. A corresponding mismatch can cause a degradation of the crystalline quality of the piezoelectric material.

Thus, the compensation layer can be used to be arranged on the bottom electrode surface and provide an interface to the piezoelectric material where the piezoelectric material experiences a surface lattice constant that is closer or even equal to the piezoelectric material’s intrinsic lattice constant.

Thus, the material of the piezoelectric layer can be deposited such that a higher crystalline quality of the piezoelectric material is obtained, resulting in an overall performance gain of the BAW resonator.

It is possible that the BAW resonator further comprises a second compensation layer. The second compensation layer can be arranged between the piezoelectric layer and the top electrode. Further, the second compensation layer can compensate for a lattice mismatch between the material of the piezoelectric layer and the material of the top electrode for the same reasons as stated above. It is possible that the material of the bottom layer comprises or consists of tungsten (W), molybdenum (Mo), iridium (Ir), ruthenium (Ru) and/or gold (Au).

The material of the top electrode can comprise or consist of tungsten, molybdenum, iridium, ruthenium and/or gold.

The material of the piezoelectric layer can comprise or consist of aluminium, nitride or scandium doped aluminium nitride. Specifically, the main constituent of the piezoelectric layer can be Al - _x Sc _x N with 0.03 < x < 0.5. Thus, scandium atoms can replace one or sides of aluminium atoms in aluminium nitride.

The material of the first and/or second compensation layer can comprise or consist of indium (In), gallium (Ga) and/or nitrogen (N). Thus, the one or more compensation layers can comprise or consist of gallium-doped indium nitride or indium-doped gallium nitride.

Specifically, it is possible that the material of the piezoelectric layer comprises scandium-doped aluminium nitride with a doping level x being larger than equal to 0.07 and smaller than or equal to 0.41. A preferred doping range is: 0.25 < x < 0.30.

Further, the material of one of the compensation layers, e.g. the first compensation layer or the second compensation layer can comprise In - _x Ga _x N with o < x < 0.5 or 0.03 < x < 0.5 or with 0.07 and smaller than or equal to 0.41 or with 0.25 < x < 0.30 or with 0.2 < x < 0.8 or with 0.46 < x < 1.

In particular when tungsten and/or molybdenum are used for the material of the bottom electrode and/or for the material of the top electrode then a tailor-made indium/gallium nitride buffer layer provides an improved growth of a scandium-doped aluminium nitride as the material for the piezoelectric layer. Specifically, with varying the indium to gallium ratio of the compensation layer, the lattice constant of the compensation layer can be adjusted to the wanted lattice constant of the piezoelectric material. The material system of indium/gallium nitride provides the possibility of compensating the lattice mismatch between tungsten or molybdenum and scandium-doped aluminium nitride for a doping level between 7% (x = 0.07) and 41% (x = 0.41).

The provision of the compensation layer can significantly reduce pyramid defects and misaligned crystals of the piezoelectric material.

Specifically, the provision of the compensation layer provides a higher resonator performance compared to alternative provisions such as a plasma edge of the surface of the bottom electrode before the material of the piezoelectric layer is deposited.

Further, the provision of the compensation layer even allows an epitaxial growth of scandium-doped aluminium nitride as the material of the piezoelectric layer.

Correspondingly, it is possible that the BAW resonator has an epitaxially grown piezoelectric material between the electrodes.

It is possible that the first and/or second compensation layer comprises two or more sublayers.

The provision of a compensation layer comprising two or more sublayers has the advantage that the parameter compensation, e.g. the lattice mismatch compensation, can be performed in smaller steps such that the growth of the material of the compensation layer itself is improved. The material system of indium/gallium nitride provides the possibility of a variable lattice mismatch that depends on the indium/gallium ratio. Thus, it is possible that the compensation layer comprises a plurality of two or more sublayers wherein each sublayer - with respect to the material of the bottom electrode or to the material of a previous sublayer- the lattice mismatch is slightly changed such that large steps of lattice mismatches are prevented.

Further, it is possible that the compensation layer provides a continuous parameter compensation. Thus, it is possible that the corresponding parameter is varied continuously between the bottom side of the compensation layer to the top side of the compensation layer, e.g. by a continuous variation of the indium/gallium ratio when an indium/gallium nitride is used as the material of the compensation layer.

The above compensation layer is possible for different types of BAW resonators. Thus, the BAW resonator can be a FBAR-type resonator or a SMR-type resonator.

A FBAR-type resonator (FBAR = film bulk acoustic resonator) has the bottom electrode arranged above a cavity such that the layer stack is acoustically decoupled from its environment.

A SMR-type resonator (SMR = solidly mounted resonator) has the bottom electrode arranged on an acoustic mirror to confine acoustic energy to the resonator area. The acoustic mirror can comprise a plurality of layers with an alternating acoustic impedance such that a Bragg mirror-like structure is obtained to reflect acoustic waves back to the active area of the resonator where the conversion between electromagnetic and acoustic RF signals takes place.

Further, it is possible to utilize the BAW resonator in an electro-acoustic filter that comprises the BAW resonator and one or more other BAW resonators. Only one BAW resonator or several BAW resonators or all BAW resonators of the electro-acoustic filter can be of the improved resonator type as described above. However, electro-acoustic filters comprising BAW resonators as described above together with other resonators such as SAW resonators (SAW = surface acoustic wave) and the like is also possible.

The electro-acoustic filter can have a ladder-type like circuit topology or a lattice-type like circuit topology.

In a ladder-type like circuit topology series resonators are electrically in series in a signal path between a first port and a second port. Parallel shunt paths comprise further resonators that are coupled between the signal path and ground.

A lattice-type like circuit topology comprises a first port and a second port and a signal line crossing between the two ports.

Further, it is possible that a multiplexer, e.g. a duplexer, a triplexer, a quadplexer or a multiplexer of a higher order, comprises an electro-acoustic filter as described above.

A method of manufacturing a BAW resonator comprises the steps:

- providing a bottom electrode,

- providing a compensation layer on the bottom electrode, - providing a piezoelectric layer on the compensation layer,

- providing a top electrode on or above the piezoelectric layer.

Of course, the provision of a compensation layer comprising two or more sublayers is also possible. Further, it is possible to provide an additional compensation layer between the piezoelectric material and the top electrode. The additional compensation layer can also comprise two or more sublayers.

The compensation layer can have a thickness between to and too nm.

An acoustic mirror used for confining acoustic energy can comprise layers of high acoustic impedance such as tungsten or molybdenum and layers of low acoustic impedance such as a silicon oxide e.g. silicon dioxide. However, the use of molybdenum is possible but tungsten is preferred.

The acoustic mirror can be arranged on a carrier substrate, e.g. a silicon substrate.

The use of a compensation layer comprising a silicon oxide is possible. However, a compensation layer comprising indium/gallium nitride provides an improved quality factor compared to the use of silicon oxide for the compensation layer.

Different aspects of BAW resonator and details of preferred embodiments are shown in the accompanying schematic figures.

In the figures:

Fig. 1 shows a basic construction of a BAW resonator;

Fig. 2 shows a BAW resonator of the SMR-type;

Fig. 3 shows a BAW resonator of the FBAR-type; Fig. 4 illustrates a construction of the acoustic mirror;

Fig. 5 illustrates the use of a compensation layer comprising a plurality of sublayers;

Fig. 6 illustrates a stepwise parameter compensation;

Fig. 7 illustrates a continuous parameter compensation; and

Fig. 8 illustrates a possible use in a duplexer.

Figure l shows the schematics of the active area of a BAW resonator BAWR. The active region has a bottom electrode BE and a top electrode TE. Between the bottom electrode BE and the top electrode TE the piezoelectric material of the piezoelectric layer PL is arranged. For improving the performance the active area further has the compensation layer CL which can be a lattice-matching layer LML.

The compensation layer CL provides an improvement of the crystalline quality of the material of the piezoelectric layer PL.

Optionally, it is possible that between the piezoelectric layer PE and the top electrode TE a second compensation layer (not shown in Figure 1) is provided.

Figure 2 illustrates the arrangement of an SMR-type resonator. An SMR-type resonator has an acoustic mirror below the bottom electrode BE to confine acoustic energy to the active area of the resonator. The layer stack can be arranged on a carrier substrate CS. On the carrier substrate CS further layer stacks for further BAW resonators can be arranged. Further, on the carrier substrate CS signal lines for electrically connecting the resonator stacks with each other and with external circuit environments, e.g. via an input port, an output port or ground connections, can be arranged.

Figure 3 illustrates the working principle of an FBAR-type resonator. In a carrier substrate CS a cavity CAV is structured such that the cavity is arranged below the bottom electrode BE to confine acoustic energy to the active area.

Figure 4 illustrates the acoustic mirror AM having a plurality of layers of different acoustic impedance. First layers Li can have a high acoustic impedance. Second layers L2 can have a low acoustic impedance. A certain amount of acoustic energy is reflected at an interface between a material of a high acoustic impedance and of a low acoustic impedance. By arranging a plurality of such interfaces in the acoustic mirror AM a high overall degree of acoustic energy reflection is obtained.

In the case of an SMR-type resonator the acoustic mirror, at least the top levels of the acoustic mirror, are acoustically active and are part of the active area of the resonator.

Figure 5 illustrates the possibility of realizing the compensation layer CL with a plurality of sublayers SLi to SL6. The sublayers SLi to SL6 can be used to compensate for a mismatch, e.g. a lattice mismatch, in a more homogenous manner. Each of the sublayers SLi to SL6 can partially contribute to an overall compensation. Thus, a stepwise compensation can be obtained with a plurality of smaller steps compared to a single, large step.

The number of sublayers is not limited to 6. The number of sublayers can be 2, 3, 4, 5, 6, 7, 8, 9, 10 and higher.

The idea of a plurality of sublayers contributing in smaller steps can be extended to a substantially larger number of sublayers with a substantially smaller step size such that, practically, a homogenous transition from a first parameter between the bottom electrode and the compensation layer to a second parameter between the compensation layer and the piezoelectric layer is obtained. This can, for example, be obtained by gradually changing the indium to gallium ratio within the material of the compensation layer.

Correspondingly, Figure 6 illustrates a stepwise compensation from an initial lattice constant LCo at the top side of the bottom electrode to a final lattice constant LC3 at the bottom side of the piezoelectric material having three steps. The concept shown in Figure 6 would correspond to a compensation layer CL comprising three sublayers.

As an alternative, Figure 7 illustrates the situation where the number of sublayers is substantially increased while the step size is substantially decreased such that a homogenous transition between the initial lattice constant LCo and the final lattice constant LC3 is obtained. Of course, in reality the number of sublayers is limited to the number of lattice cells being arranged one above another in the compensation layer CL.

Figure 8 illustrates a possible application of a corresponding resonator in a duplexer DU. The duplexer has a transmission filter TXF and a reception filter RXF. The transmission filter TXF is arranged between an input port and a common port CP. The reception filter RXF is arranged between the common port CP and an output port. The transmission filter TXF and the reception filter RXF are established utilizing a ladder- type like circuit topology with a signal path with series resonators SR and with parallel paths including a parallel resonator PR. Further, at the common port CP an antenna AN can be connected. An impedance matching circuit IMC can be provided between the transmission filter TXF and the reception filter RXF to match the frequency-dependent impedances of the filter’s ports.

The BAW resonator is not limited to the embodiments and details shown in the figures and described above. Resonators can comprise further layers such as adhesion layers, passivation layers, trimming layers and structures supporting wanted acoustic modes. List of Reference Signs

AM: acoustic mirror

AN: antenna

BAWR: BAW resonator

BE: bottom electrode

CAV: cavity

CL: compensation layer

CP: common port

CS: carrier substrate

DU: duplexer

IMC: impedance matching circuit

Li, L2: layers of high, low acoustic impedance

LCo: initial lattice constant

LC3: final lattice constant

LML: lattice mismatch layer

PL: piezoelectric layer

PR: parallel resonator

RXF: reception filter

SLi,...; SL6: sublayer of the compensation layer

SR: series resonator

TΈ: top electrode

TXF: transmission filter