Piezoelectric material and piezoelectric device

The present invention refers to a piezoelectric material and to piezoelectric devices comprising the piezoelectric material.

Piezoelectric materials can be utilized to - due to their piezoelectric effect - convert mechanical energy to electrical energy and convert mechanical energy in response to applied electrical excitation. Piezoelectric materials can be used in a wide variety of devices. For example electro acoustic RF devices comprising electro acoustic resonators can have resonating structures whe33ere electrode structures and a piezoelectric material are combined. The performance of piezoelectric materials is determined by sets of elastic, dielectric, and piezoelectric parameters. Elastic parameters are, for example, Young’s modulus, C ₃₃ (a component of the material’s stiffness tensor), lattice density and the like. The electromechanical coupling coefficient, K , which is another important parameter determining the efficiency of acoustic wave excitation bridges the mechanical and electrical performance. Another parameter is the piezoelectric constant, e ₃₃ , a component of the material’s piezoelectric tensor. Others are the longitudinal stiffness, C33, and the dielectric permittivity, e ₃₃ .

For example, for electro acoustic applications it is preferred that the piezoelectric material have a high electromechanical coupling coefficient, K ² . A known piezoelectric material is the wurtzite-type AIN (aluminium nitride) that can be used in electro acoustic resonators, e.g. in BAW resonators (BAW = bulk acoustic wave). BAW resonators have a bottom electrode, a top electrode above the bottom electrode and the piezoelectric material sandwiched between the bottom electrode and the top electrode. It is preferred that the piezoelectric material of BAW resonators can be provided via a thin film deposition technique. A further known piezoelectric material is Sc doped AIN (scandium doped AIN). Sc doped AIN has the potential to provide a higher electromechanical coupling coefficient K ² , than pure aluminium nitride.

However, the desire for alternative materials suitable for use in piezoelectric devices exists. Further, it was found that Sc doping can deteriorate a corresponding device’s mechanical properties due to AlScN lattice softening, manifested in the reduction of the longitudinal stiffness coefficient, C ₃₃ , simultaneously, i.e., without having these performance parameters to have a trade-off effect.

What is wanted is a piezoelectric material that can be used in a wide variety of piezoelectric devices, that has improved piezoelectric properties, in particular an increased electromechanical coupling coefficient, K ² , and that has good mechanical properties, in particular a high longitudinal stiffness coefficient, C ₃₃ .

To that end, a piezoelectric material according to the independent claim and a piezoelectric device are provided. Dependent claims provide preferred embodiments.

In the following compositions for piezoelectric materials are provided. The tolerance level for the quantities of the atoms where the compositions can be regarded as equivalent can be ± 1 atomic % or ± 2 atomic %.

The piezoelectric material comprises as its main constituent (compound), structural formula, Al - _x [(Ca _a , Ru _b , Zi _Ci ,Z2 _c2 ) _y ] _x N, where a mole fraction is larger than or equal to 0.055 and smaller than or equal to 1.33. b mole fraction is larger than or equal to 0.055 and smaller than or equal to 1.33. ci is larger than or equal to o. C2 is larger than or equal to o. The sum c = ci + C2 is larger than or equal to o and smaller than or equal to 1.33. y is the reciprocal of the sum of a, b and c: y=i/ (a+b+c). x is larger than or equal to 0.03 and smaller than or equal to 0.75. Zi and Z2 are selected from B (boron) and Y (yttrium).

Thus, the piezoelectric material comprises Al (aluminium), Ca (calcium), Ru

(ruthenium) and N (nitrogen).

Further, the material can comprise Y (yttrium). Further, the material can comprise B (boron). Further, the material can comprise Y and B.

Ca, Ru - and if present Y and/or B - establish dopants that can replace Al. The value of y is chosen such that the dopants can be regarded as a group where each atom of the dopants group that can fractionally substitute for Al in the wurtzite lattice of Al - _x [(Ca _a , Ru _b , Zi _Ci ,Z2 _c2 ) _y ] _x N. Then, x denotes the total doping or replacement level of Al atoms

It was found that such a piezoelectric ma terial has good piezoelectric properties such as a good electromechanical coupling coefficient, K ² . Further, the piezoelectric material with the main constituent as described above also can have a higher stiffness, in particular a higher stiffness parameter C ₃₃ compared to pure AIN or Sc doped AIN, of a comparable

electromechanical coupling coefficients K ² .

Further, it was found that the piezoelectric material as described above allows piezoelectric resonators with an increased quality factor, Q, compared to electro acoustic resonators based on pure AIN or Sc doped AIN. Thus, corresponding RF filters or other piezoelectric components with an improved performance are possible. It is possible that the piezoelectric material has Al - _x [(Ca _a , Y _b , Z _c ) _y ] _x N as its main constituent(compound). Z is selected from B and Y.

Thus, the dopants consist of Ca, Ru and B or Ca, Ru and Y.

It is possible that the material has Al - _x [(Ca _a , Ru _b , Zi _cl , Z2 _c2 ) _y ] _x N as its main constituent (compound). Zi and Z2 are selected from B and Y or only B or only Y. 0.165 < a < 0.66, 0.165 < b < 0.66, o < ci, o < C2, o < c = ci+c2 < 0.66, y=i/(a+b+c) and 0.09 < x <

0.372.

It is possible that the doping level x is selected from 0.03, 0.06, 0.09, 0.12, 0.15, 0.18, 0.21, 0.24, 0.27, 0.30, 0.33, 0.36, 0.39, 0.42, 0.45, 0.48, 0.51, 0.54, 0.57, 0.60, 0.63, 0.66, 0.69, 0.72 and 0.75.

It is possible that the main constituent (compound) is Al _o.814 Ca _o.062 Ru _o.062 B _o.062 N. In this doping combination the dopants consist of Ca, Ru and B.

Also, it is possible that the main constituent (compound) is Al _o.814 Ca _o.062 Ru _o.062 B _o.124 N.

In this doping combination the dopants consist of Ca, Ru and B.

Also, it is possible that the main constituent (compound) is Al _o.876 Ca _o.062 Ru _o.062 B _o.124 N. In this doping combination the dopants consist of Ca, Ru and B.

Also, it is possible that the main constituent (compound) is Al _o.69 Ca _o.124 Ru _o.124 B _o.062 N. In this doping combination the dopants consist of Ca, Ru and B.

Also, it is possible that the main constituent (compound) is Al _0.814 Ca _0.062 Ru _0.062 Y _0.062 N. In this doping combination the dopants consist of Ca, Ru and Y. Also, it is possible that the main constituent (compound) is Al _o.870 Ca _o.062 Ru _o.062 N. In this doping combination the dopants consist of Ca and Ru. The dopants do not comprise Y or B.

It is possible that the at least 44.2 (atomic %), 53.5 (atomic %), 62.77(atomic %), 72.1 (atomic %), 8i.4(atomic %), 90.7(atomic %), 95.78 (atomic %), 98.25(atomic %) or 99(atomic %) are Al while the remaining balance is a combination of ternary or quaternary doped AIN piezoelectric material containing Ca (calcium), Ru (ruthenium), B (boron) andY (yttrium).

The above compositions of quaternary or quinary nitrides provide good electro mechanical properties and good mechanical properties which are shown in the following tables (Table 1 and Table 2). The Ab initio properties are derived from density functional perturbation theory calculations. Comparisons between calculations and experiments justify that the provided calculated values can be regarded close to the expected experimental values as sufficient number of quasi-random structures (SQS) and median statistical values had been obtained for each composition example.

Table 1

Table 2

Residual constituents can comprise other atoms that maybe unavoidable due to necessary manufacturing steps and the like.

It is possible that a piezoelectric device comprises a material as described above.

Such a device can be selected from

- an electro acoustic resonator, a SAW resonator, a SAW filter, a solidly mounted resonator (SMR-type resonator), a (SMR-)BAW filter, a guided BAW(GBAW) resonator, a GBAW filter, a film bulk acoustic wave (FBAR) resonator, a FBAR filter, or

- a resonator working with Lamb waves, acoustic plate waves (APW), Rayleigh SAW (R- SAW), Sezawa mode waves, shear-horizontal SAWs (SH-SAWs), Love mode waves, pseudo-surface acoustic waves (PSAW) or Leaky SAWs (LSAW) or

-an acoustic device, a multiplexer, a duplexer, a quadplexer, a hexaplexer based on any of the above types of resonators, a piezoelectric generator, a piezoelectric sensor, a mass sensor, a microfluidic sensor, a piezoelectric transducer, an energy harvester, an ultrasound devices, a transducer, atransmitter, a piezo (MEMS) microphone, a device that utilizes direct or reverse piezolectric effect in a thin film or bulk ceramic form.

A SAW filter is an RF filter that has at least one SAW resonator. A SAW resonator has a piezoelectric material and interdigitated electrode structures comprising electrode fingers arranged one next to another on the piezoelectric material. Each electrode finger is electrically connected to one of two bus bars. When an RF signal is applied to the bus bars then - due to the piezoelectric effect - the electrode structure converts between RF signals and acoustic waves. The wavelength of the acoustic wave is essentially determined by the distance between adjacent electrode fingers of the same polarity. A surface wave propagating at the surface of the piezoelectric material is established. The frequency depends on the wavelength and on the wave velocity. Utilizing such resonators, e.g. as series resonators and as parallel resonators in a ladder-type structure or as resonators in a lattice-type structure allows to create a bandpass filter or a band rejection filter, e.g. for wireless communication devices.

In a BAW resonator the piezoelectric material is sandwiched between a bottom electrode and a top electrode. While the acoustic waves in a SAW resonator propagate in a direction parallel to the surface of the piezoelectric material, in a BAW resonator the acoustic wave propagates in a vertical direction. To confine acoustic energy to the resonator structure the resonator structure must be acoustically decoupled from its environment. Correspondingly, it is possible that the BAW resonator is a SMR-type resonator (SMR = solidly mounted resonator) or a FBAR-type resonator (FBAR = film bulk acoustic wave resonator). In a SMR-type resonator the resonator structure is arranged on an acoustic mirror comprised of two or more layers of high and low acoustic impedance to act as an acoustic Bragg mirror to confine the acoustic energy. In a FBAR-type resonator the bottom electrode can be arranged above a cavity to acoustically isolate the resonator structure.

In a GBAW filter electrode structures are similar to that of a resonator in a SAW filter. However, the acoustic waves propagate in a longitudinal direction at an interface between the piezoelectric material and a cover layer such that a waveguiding structure is obtained.

Bandpass filters can be combined - possibly with additional impedance-matching circuits - to establish a multiplexer. For example in a duplexer a transmission filter and a reception filter are combined such that to-be-sent RF signals and to-be-received RF signals can share a common antenna port but propagate in separated signal paths, in a transmission signal path and in a reception signal path, respectively. Correspondingly, a multiplexer of a higher degree, e.g. a quadplexer comprises additional bandpass filters and additional signal paths. In an energy harvester the piezoelectric material can be utilized to convert mechanical energy into electric energy, e.g. to load a battery or a capacitor with mechanical energy obtained from the environment of the respective device.

Thus, an improved piezoelectric material that allows improved piezoelectric devices, especially resonator devices with an improved quality factor, is provided.

The piezoelectric devices are not limited to the devices stated above. Further devices are also possible.

In the figures:

Fig. 1 illustrates the arrangement of an interdigital structure of a SAW resonator;

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

Fig. 3 illustrates the combination of electro acoustic resonators to establish a duplexer;

Fig. 4 shows comparisons between calculated values of electromechanical coupling coefficient, K and measured values from actual SMR-BAW resonators with a piezoelectric layers made from different doping levels of Sc in AIN;

Fig. 5 shows comparison between extrapolated mechanical Quality Factor (Q _m ) for measured from SMR-BAW and obtained values from ab-initio calculations and it is showing how quickly Q _m is changing with different doping levels of Sc in AIN; Fig. 6 shows the dependence of C ₃₃ vs coupling coefficient K ² behaviour for an alternative to Ah- _x Sc _x N (o.o625£x£o.3i) material with a higher C ₃₃ for a range of coupling coefficients K ² ;

Fig. 7 shows the dependence of C ₃₃ vs coupling coefficient K ² behaviour for an alternative to Al - _x Sc _x N (o.o625<x£o.3i) material with a moderately higher C ₃₃ for a range of coupling coefficients K ² ;

Fig. 8 shows a C ₃₃ - K ² dependence;

Fig. 9 shows the dependence of C ₃₃ vs coupling coefficient K ² behaviour for an alternative to Ah- _x Sc _x N (o.o625<x£o.3i) material with a marginally equivalent C ₃₃ for a range of coupling coefficients K ² .

Figure 1 illustrates a basic arrangement of electrode structures on a piezoelectric material PM that can be provided as a single crystal piezoelectric substrate or by piezoelectric material provided as a thin layer. The electrode structure has an interdigitated structure, IDS, comprising electrode fingers, EFI, arranged one next to another. Each of the electrode fingers, EFI, is electrically connected to one of two bus bars. In the arrangement shown in Figure 3, the acoustic waves propagate at the surface of the piezoelectric material in a direction orthogonal to the electrode fingers.

Figure 2 illustrates the basic construction of a BAW resonator BAWR. The BAW resonator BAWR has the piezoelectric material, PM, sandwiched between a bottom electrode, BE, and a top electrode, TE. Figure 4 also illustrates an SMR-type resonator where the resonator structure comprising the two electrodes and the piezoelectric material is arranged on an acoustic mirror. The acoustic mirror has mirror layers, ML. Adjacent mirror layers, ML, have different acoustic impedance. At an interface between different mirror layers, ML, of different acoustic impedance, a part of the acoustic energy is reflected such that the combination of mirror layers, ML, establishes a Bragg mirror to confine the acoustic energy. Figure 3 illustrates the possibility of combining a transmission filter, TXF, and a reception filter, RXF, to establish a duplexer. The transmission filter, TXF, and the reception filter, RXF, comprise a signal path in which series resonators, SR, are electrically connected in series. Parallel resonators, PR, are electrically connected in shunt paths between the signal path and ground. An impedance matching circuit can be arranged between the transmission filter, TXF, and the reception filter, RXF, to provide matched frequency dependent impedances at the common port at which an antenna, AN, can be connected.

Figure 4 shows a comparison between measured and calculated data. Curve (1) shows the (experimentally) measured dependence of the coupling factor, K ² , on the Sc doping level of Sc doped AIN (Al - _x Sc _x N) for different doping levels, x. Curve (2) shows the results of calculations made in a simulation for determining a theoretical model of Sc doped AIN. It can be seen that the experiments essentially verifies the ab-initio calculated results.

Similarly, figure 5 shows a comparison between measured and calculated data. Curve (3) shows the measured dependence of the mechanical quality factors, Qm, derived from the impedance response of physical (experimentally fabricated) resonators on the Sc doping level of Sc doped AIN (Al - _x Sc _x N). Curve (4) shows the results of calculations made in the simulation. Again, the experimentally derived values essentially verify the calculated results.

Thus, the calculations on which the present compositions base are reliable.

Figure 6 shows a comparison between a plurality of parameters of Sc doped AIN and Al _o.814 Ca _o.062 Ru _o.062 B _o.062 N (corresponding to composition A) of the tables). The Sc doping level for the different Sc doped AIN composition essentially determines C ₃₃ as shown by curves (5). Curve (6) shows a polynomial interpolation of calculated data points indicating a C ₃₃ dependence on K for different quasi-random structures of Al _o.814 Ca _o.062 Ru _o.062 B _o.062 N. The different quasi-random structures of Al _o.814 Ca _o.062 Ru _o.062 B _o.062 N differ in an exact position of each dopant that substitutes the Al atoms. The calculations show that in a real composition a mixture of these quasi random structures is provided such that a K of approximately 0.18 and a C ₃₃ of 270.4 GPa are obtained. Thus, Al _o.814 Ca _o.062 Ru _o.062 B _o.062 N has a C ₃₃ that is approximately 65.3 GPa larger than that of Sc doped AIN baseline system with very similar K ² (close to 0.20), while the median value of Al _o.814 Ca _o.062 Ru _o.062 B _o.062 N has a C ₃₃ that is

approximately 42.3 GPa larger than that of Sc doped AIN baseline system with the very similar K ² (close to 0.15).

Figure 7 shows a comparison between a plurality of parameters of Sc doped AIN and Al _o.870 Ca _o.062 Ru _o.062 B _o.124 N (corresponding to composition B) of the tables). The Sc doping level for the different Sc doped AIN composition essentially determines C ₃₃ as shown by curves (7). Curve (8) shows a polynomial interpolation of calculated data points indicating a C ₃₃ dependence on K for different quasi-random structures of Al _o.876 Ca _o.062 Ru _o.062 B _o.124 N. The different quasi-random structures of

Al _o.876 Ca _o.062 Ru _o.062 B _o.124 N differ in an exact position of each dopant that substitutes the Al atoms. The calculations show that in a real composition a mixture of these quasi random structures is provided such that a K of approximately 0.139 and a C ₃₃ of 274.2 GPa are obtained. Thus, Al _o.876 Ca _o.062 Ru _o.062 B _o.124 N has a C ₃₃ that is approximately 14 GPa with very similar K (close to 0.13).

Figure 8 shows calculated parameters for doped AIN.

Figure 9 shows a comparison between a plurality of parameters of Sc doped AIN and Al _0.814 Ca _0.062 Ru _0.062 Y _0.062 N (corresponding to composition E) of the tables). The Sc doping level for the different Sc doped AIN composition essentially determines C ₃₃ as shown by curves (9). Curve (10) shows a polynomial interpolation of calculated data points indicating a C ₃₃ dependence on K for different quasi-random structures of Al _0.814 Ca _0.062 Ru _0.062 Y _0.062 N. The different quasi-random structures of

Al _o.876 Ca _o.062 Ru _o.062 B _o.124 N differ in an exact position of each dopant that substitutes the Al atoms. The calculations show that in a real composition a mixture of these quasi random structures is provided such that a K of approximately 0.179 and a C ₃₃ of 232 GPa are obtained. Thus, Al _o.870 Ca _o.062 Ru _o.062 B _o.124 N has a C ₃₃ that is approximately 18.8 GPa with very similar K ² (O.I8).

List of Reference Signs

AN: antenna

BAWR: BAW resonator

BE: bottom electrode

DU: duplexer

EFI: electrode finger

IDS: interdigitated electrode structure ML: acoustic mirror layer

PM: piezoelectric material

PR: parallel resonator

RXF: reception filter

SAWR: SAW resonator

SR: series resonator

TΈ: top electrode

TXF: transmission filter