BAW component and method for manufacturing a BAW component The present invention refers to BAW components (BAW = bulk acoustic wave), e.g. for RF filters, and to methods for manu ^¬ facturing BAW components.

BAW components usually comprise a layer stack with a piezo- electric material sandwiched between two electrodes. When an RF signal is applied to the electrodes, an acoustic resonance can be formed if means for confining acoustic energy in the stack are present. Mirror systems under the stack or a cavity under the stack are typical means for confining the acoustic energy. Utilizing acoustic resonances, band pass filters or band stop filters can be established.

Conventional BAW components comprise A1N (aluminum nitride) as a piezoelectric material. However, the band width of pass bands or stop bands of BAW components are limited. What is needed is a BAW component providing a larger band width.

Therefore, it is an object of the invention to provide a BAW component that allows a larger band width and to provide a method for manufacturing such a BAW component.

A BAW component comprises a bottom electrode, a top elec ^¬ trode, and a first piezoelectric layer between the bottom electrode and the top electrode. The first piezoelectric layer comprises a piezoelectric material having a higher pie ^¬ zoelectric coefficient c than A1N. The piezoelectric behavior of a BAW component or - more gen ^¬ erally - of a piezoelectric material is mainly determined by piezoelectric parameters. When an electric field is applied to a piezoelectric material, then a deformation of the mate- rial is obtained. Further, when a piezoelectric material is deformed, then an electric charge displacement takes place and an electric field is established. Piezoelectric parame ^¬ ters describe the relationships between deformation and elec ^¬ trical field or between strain and stress. Usually, the pie- zoelectric parameters are tensors as the directivity of force or of a force, a field or a deformation is relevant.

It is thus possible that the piezoelectric coefficient c is the coefficient d33 that describes the relationship between the deformation in a direction parallel to the piezoelectric axis of the piezoelectric material when the electrical field is parallel to the piezoelectric axis. In particular, the piezoelectric coefficient c can be a constant of proportion ^¬ ality when the material works in a linear regime.

It was found that a BAW component having a piezoelectric ma ^¬ terial with a higher piezoelectric coefficient c than AIN allows larger band widths. However, AIN provides good elastic components as AIN is hard enough to allow only relative small deformations. With only small deformations non-linear effects during the oscillation of the piezoelectric material can be neglected. Thus, when a material being different from AIN is utilized in a BAW component, e.g. to increase the band width, then tradeoffs with deteriorated elastic properties are a re- suit.

In one embodiment, the BAW component comprises Sc (Sc = scan ^¬ dium) doped AIN. Sc containing AIN has a higher piezoelectric coefficient and allows a higher piezoelectric coupling coef ^¬ ficient K ² . However, Sc doped A1N is softer than A1N and nonlinear effects due to a larger deformation can take place. Further, the insertion loss level of a pass band can be dete- riorated.

In one embodiment, the BAW component further comprises a sec ^¬ ond piezoelectric layer with a second piezoelectric material different from the piezoelectric material of the first piezo- electric layer. The second piezoelectric material is arranged between the bottom electrode and the first piezoelectric layer .

The second piezoelectric material can be chosen according to its piezoelectric properties or according to its elastic properties. Then, the piezoelectric material between the electrodes is a sandwich construction comprising different piezoelectric materials and an improved tradeoff between electric properties and elastic properties can be obtaind. However, manufacturing steps are more complex. In particular, the second piezoelectric material can comprise A1N having good elastic properties.

In one embodiment, the BAW component comprises a layer with a second piezoelectric material and the first piezoelectric layer is arranged on the second piezoelectric material. The mismatch of lattice parameters of the first piezoelectric layer' s material and the second piezoelectric layer' s mate ^¬ rial is less than 10%. The lattice mismatch can be in the range of 2 - 5%. When the first piezoelectric layer's

material is Sc doped A1N and the second piezoelectric layer' s material is A1N, then both materials have a similar lattice. Due to the different sizes of Sc atoms and of Al atoms, a lattice mismatch, however, is present. The doping level can be in the range of 1% to 25%, resulting in a lattice mismatch that is small enough to allow Sc doped A1N to be grown on the lower A1N layer with a good layer quality. In one example the doping level can be in the range of 5% to 7%.

In one embodiment, the second piezoelectric layer has a (002) texturation at the interface towards the first piezoelectric layer .

It is possible that the second piezoelectric material is a seed layer for the first piezoelectric material. It was found that Sc doped A1N grown on an A1N seed layer with a (002) texturation has a good crystalline quality and allows resona- tors with a high quality factor Q.

The (002) orientation can be easily obtained giving more de ^¬ grees of freedom in depositing processes. Sputtering can be utilized to deposit electrode or piezoelectric material lay- ers .

In one embodiment, the BAW component comprises a third piezo ^¬ electric layer. The third piezoelectric layer is arranged be ^¬ tween the first piezoelectric layer and the top electrode.

The third piezoelectric layer can have a piezoelectric mate ^¬ rial that is different from the piezoelectric material of the first piezoelectric layer and can be chosen according to elastic or piezoelectric properties. In particular in combi- nation with a second piezoelectric layer between the first piezoelectric layer and the bottom electrode, a threefold piezoelectric laminate between the electrodes can be obtained and fulfills modern requirements related to BAW components or RF filters.

Whether a further piezoelectric layer is below or above the first piezoelectric layer, the global piezoelectric coeffi ^¬ cient can be increased relative to the piezoelectric coeffi ^¬ cient of A1N. Utilizing the combination of A1N and the first material is a possibility to limit the reduction of the qual ^¬ ity factor of the respective resonator.

A method for manufacturing a BAW component comprises the steps :

- providing a bottom electrode (BE) ,

- depositing a first piezoelectric material having a higher piezoelectric coefficient c than A1N onto or above the bottom electrode (BE) ,

- structuring a top electrode (TE) onto or above the first piezoelectric material. In one embodiment, the method further comprises the step:

- depositing a second piezoelectric material onto or above the bottom electrode before depositing the first piezoelec ^¬ tric material.

In one embodiment of the method, the first piezoelectric ma ^¬ terial comprises Sc doped A1N. The first piezoelectric mate ^¬ rial can be deposited at a rate R with 5 μιη/η < R < 15 μιη/h. But higher deposition rates are also possible. The first piezoelectric material is deposited at a temperature T with 50 °C ≤ T ≤ 400 °C. Especially, it is possible to use a temperature between 150° C and 300° C. It was found that the Al/Sc nitride material system, especially an A1N - ScAIN - A1N laminate - provides good elastic and electric properties and can be deposited at a high rate and at a large

temperature interval which makes methods for manufacturing BAW components highly efficient.

Examples and working principles are shown in the schematic figures .

Short description of the figures

FIG. 1 shows a BAW component with a piezoelectric layer between two electrodes,

FIG. 2 shows the relationship between an electrical field an a mechanical distortion of a piezoelectric mate ^¬ rial, shows a BAW component comprising three

piezoelectric layers between two electrodes, shows a BAW component with a second piezoelectric layer between the first piezoelectric layer and a bottom electrode, shows a BAW component with a third piezoelectric layer between the first piezoelectric layer and top electrode,

FIG. 6 shows a BAW resonator with an acoustic mirror

Detailed description FIG. 1 shows a BAW component BAWC comprising a bottom elec ^¬ trode BE and a top electrode TE . Between the two electrodes, a first piezoelectric layer PLl is arranged. The first piezo ^¬ electric layer PLl comprises a piezoelectric material having a higher piezoelectric coefficient c than A1N.

FIG. 2 shows two versions of a piezoelectric material being arranged between two electrodes. An electric voltage is ap ^¬ plied to the electrodes resulting in an electric field caus- ing the piezoelectric material to expand in a vertical direc ^¬ tion which may be a direction parallel to the piezoelectric axis. In contrast, a reverse voltage, i.e. the same voltage with different sign, is applied to the piezoelectric material as shown in the piezoelectric material on the right-hand side. The electrical field causes the piezoelectric material to shrink in a vertical direction.

FIG. 3 shows a BAW component comprising a second piezoelec ^¬ tric layer PL2 between the first piezoelectric layer PLl and the bottom electrode. Further, a third piezoelectric layer

PL3 is arranged between the first piezoelectric layer PLl and the top electrode TE . The materials of the piezoelectric layer and the respective layer thicknesses can be chosen to provide an excellent BAW component as the materials and the thicknesses can be chosen to fulfill elastic and electric re ^¬ quirements .

FIG. 4 shows an embodiment of a BAW component where the third piezoelectric layer PL3 shown in FIG. 3 is omitted.

FIG. 5 shows the BAW component where the second piezoelectric layer PL2 of FIG. 3 is omitted. FIG. 6 shows a BAW component BAWC where an acoustic mirror AM is arranged between the layer stack with the electrodes and the piezoelectric material on one side and a carrier sub ^¬ strate CS on the other side. The acoustic mirror AM is a pos- sibility to confine acoustic energy in the stack so that a resonance can be established. The acoustic mirror AM can com ^¬ prise two or more layers with alternating acoustic impedance.

A BAW component or a method for manufacturing BAW components are not limited to the embodiments described in the specifi ^¬ cation or shown in the figures. Components comprising further elements such as layers or materials or methods comprising further deposition steps or structuring steps or combinations thereof are also comprised by the present invention.

List of reference signs

AM: acoustic mirror

BAWC: BAW component

BE : bottom electrode

CS : carrier substrate

PL1 : first piezoelectric layer

PL2 : second piezoelectric layer

PL3 : third piezoelectric layer

TE : top electrode