WO2004038915A1

WO2004038915A1 - Tunable acoustic wave device

Info

Publication number: WO2004038915A1
Application number: PCT/SE2003/001648
Authority: WO
Inventors: Sture Petersson; Alexander M. Grishin
Original assignee: Sture Petersson; Grishin Alexander M
Priority date: 2002-10-24
Filing date: 2003-10-23
Publication date: 2004-05-06
Also published as: SE524009C2; SE0203155D0; AU2003274861A1; SE0203155L

Abstract

The invention discloses a tunable acoustic wave device (100; 200) comprising a piezoelectric material (120; 220) with a tunable dielectric permittivity. The dielectric permittivity of the material is tuned by applying a tuning electric field (190), preferably a DC-bias field, low frequency AC field, pulsed electric field or AC electric field superimposed onto an electric field pulse, thereto. By tuning the dielectric permittivity, the operation characteristics of the device (100; 200), including the acoustic wave velocity in the material (120; 220) and the resonance frequency and bandwidth of the device (100; 200), may be tuned. The tuning electric field (190) may applied by superimposing it onto the input high frequency electric field signal of the device (100; 200) or by applying it over at least a portion of the piezoelectric material (120; 220). A piezoelectric material (120; 220) with a tunable dielectric permittivity can be found in ceramic crystalline superparaelectric materials, e.g. perovskite niobates-tantalates.

Description

TUNABLE ACOUSTIC WAVE DEVICE

TECHNICAL FIELD

The present invention relates to tunable acoustic wave devices and to a method of tuning such devices. In particular, the invention relates to acoustic wave devices comprising a piezoelectric material with an electrically tunable dielectric permittivity.

BACKGROUND ART The explosive growth of wireless broadband communications has increased the demand for miniaturized acoustic wave devices operating in the microwave region. Traditionally, such acoustic wave devices are divided into surface acoustic wave (SAW) devices and bulk acoustic wave (BAW) devices.

A typical SAW device consists of a pattern of electrodes and possibly also grooves fabricated by photolithography on the top of a piezoelectric substrate. The generation of SAW is based on the piezoelectric effect, i.e. coupling of mechanical and electric fields in certain crystals with no center of symmetry. In such SAW devices, surface waves propagate as a mixture of longitudinal and shear waves along a surface, whereby the wave energy extending only a few wavelengths into the bulk of the piezoelectric substrate.

The present BAW and SAW devices feature very small size and good functional characteristics. In RF circuitry applications, they compete with ceramic microwave devices, e.g. filters, which have both lower price and provide excellent characteristics, but tend to have relatively large overall size. Both monolithic and multilayered BAW devices are actively explored, but at present the BAW technology is not competitive against the SAW and ceramic technologies. Although the recent advances in SAW technology primarily have taken place and been driven by the vast expansion of the cellular phone markets, SAW technology has also been used in high-performance delay lines and radar systems. Emerging new applications of SAW can be found in SAW sensors and SAW tags.

For many high frequency applications, quartz has long been the first choice, due to its negligible temperature coefficient of frequency, relatively high velocities of bulk and surface acoustic waves and low propagation losses. However, quartz has a substantial disadvantage due to its small, as in the most common materials, electromechanical coupling coefficient, K² ~ 0.1% - 0.17%. This significantly limits the bandwidth that can be achieved in acoustic wave filters and the frequency shifts in oscillator circuits. In addition, low K² materials are not suitable for some applications, including mobile systems, because of high power consumption in these devices due to their low electrical/ mechanical energy conversion efficiencies.

To achieve appropriate K² and simultaneously utilize the advantages of quartz, thin films of high K² materials are deposited on the quartz substrates. The use of piezoelectric micrometer thick films on nonpiezoelectric substrates enables the overall K² to increase up to several percent. However, since a SAW penetrates to a depth of around two wavelengths beneath the surface of piezoelectric film and thus propagates primarily in the substrate, the propagation loss and high acoustic quality are to a large extent still dictated by substrate material.

Compared to the SAW technology, BAW devices presently have larger design size yet pioneering the field of high frequency applications. BAW devices, operating in the range between 500 MHz and 10 GHz, require thin piezoelectric layers from a few μm down to 100 nm, which host an acoustic wave (thickness extensional mode) excited by AC electric field. In order to reduce energy loss, an acoustic reflector may be provided underneath the resonator. Such a reflector could include free micromachined membranes, air gaps and acoustic interference filters. Use of piezoelectric thin films is known both for SAW and BAW devices. When being prepared at highly c-axis-oriented state, they provide high electromechanical coupling coefficient as well as low inserted acoustic losses.

Most of the prior art SAW and BAW devices suffer from a significant drawback of not being frequency tunable. This is due to that the operational frequency is defined by device geometry and is therefore fixed. For BAW devices the operational frequency depends on the thickness of the piezoelectric film, whereas for SAW devices the frequency is determined by the period of the lithographically patterned interdigital transducers.

Recently, some solutions have been proposed for obtaining tunable surface acoustic wave devices. US Patent 4,078,186 discloses a surface acoustic wave device having a thin magnetostrictive film deposited on the surface between its input and output transducers. A variable magnetic field is applied to the film for varying its characteristics and thereby correspondingly for varying the delay or phase shift of the surface acoustic wave.

In US Patent 4,342,971, Clyde et al describes a surface acoustic wave device secured in a housing. An electrostrictive element is also secured within the housing between an end of the device and an interior wall of the housing. A control signal is applied to electrodes connected to the element for varying the size thereof. The element applies a mechanical force to the surface acoustic wave device to alter the distance between its input and output transducers and consequently a control of the delay time between an applied input signal and an output signal is provided.

US Patent 5,343, 175 discloses a surface acoustic wave device having a piezoelectric substrate and a mechanism mechanically coupled to the substrate for mechanically deforming the substrate in order to modify the output of the device. A SAW device being magnetically tuned is disclosed in US Patent 5,959,388. In the device, a piezoelectric layer is mechanically coupled to a magnetostrictive material. Strains magnetically induced in the magnetostrictive substrate are coupled to the piezoelectric layer, altering the velocity of acoustic waves propagating in the layer.

All the SAW devices discussed above are mechanically tuned by deforming the piezoelectric layer through which surface acoustic waves propagate. However, such mechanically variable devices are very slow to operate and quite difficult to fabricate. The tunability of acoustic filters may instead be obtained by introducing voltage dependent capacitors in the acoustic filters, as is illustrated by the two following patents.

US Patent 5,291,159 describes an acoustic resonator filter having a shunt coupling consisting of an inductor and voltage variable capacitor matrix. A pair of acoustic resonators each having a parallel inductor is connected to each other and the shunt coupling network. One of a pair of voltage variable capacitor matrices each having a center frequency command input voltage is connected to each acoustic resonator and an associated inductor.

In US Patent 6,018,281 a surface acoustic wave filter connected in a ladder type is disclosed. The SAW filter has an element that changes in capacity by applying a voltage. This element is connected in series with a resonator in turn being connected in parallel to the signal line.

The two above-identified patents try to achieve tuning of acoustic resonators or SAW filters by introducing voltage variable capacitor matrices or elements. Thus, extra elements and units have to be arranged in the filters, increasing the design complexity and cost.

In US Patent 5,438,554 Yoshida et al disclose a ultrasonic probe for non- invasively viewing of the interior of an object, including the human body. The probe comprises an ultrasonic transducer that in turn includes a body of a first piezoelectric material acoustically coupled to a body of a second acoustic material. At operation temperature, an oscillating voltage is applied to the first piezoelectric material having a fixed polarization. The oscillating frequency generates acoustic signals in the first material through the electromechanical properties of the material. However, the second piezoelectric material has a Curie temperature of about 25 °C and therefore is randomly poled and electromechanically inert. By applying a DC bias voltage to the body of this second material, the material becomes polarized and electromechanically active.

SUMMARY OF THE INVENTION The present invention overcomes these and other drawbacks of the prior art arrangements.

It is a general object of the invention to provide a tunable acoustic wave device.

It is another object of the invention to provide an acoustic wave device that can be electrically tuned.

It is a further object of the invention to provide a method of electrically tuning operation characteristics of an acoustic wave device.

The above objects are achieved by devices and methods according to the enclosed claims. In general words, an acoustic wave device comprises a piezoelectric material with a tunable dielectric permittivity. The dielectric permittivity of the piezoelectric material is tunable by applying an electric field to the material. By being able to tune and vary the dielectric permittivity of the material, the operation characteristics of the acoustic wave device may be tuned. These operation characteristics comprises, among others, the velocity of the acoustic wave propagating through the piezoelectric material, the resonance frequency and the bandwidth of the device and the direction of the acoustic wave power flux propagation in the material, sometimes referred to as beam steering.

The electric field is preferably a DC, low frequency AC (with a frequency of up to 100kHz -1 MHz) or pulsed electric field. In addition, a low frequency AC field superimposed onto a pulsed electric field may used for tuning the acoustic wave device according to the invention. The electric field may be applied to the piezoelectric material by being superimposed onto the input high frequency electric field signal, which is to be converted to an acoustic wave signal by the acoustic device. Alternatively, or as a combination, the electric field may be applied all over or partly over the portion of the piezoelectric material where the acoustic wave signal propagates.

The piezoelectric material with an electrically tunable dielectric permittivity is preferably a ceramic crystalline material for enhancing the piezoelectric effect and obtaining a strong dependence of the dielectric permittivity of the applied tuning electric field.

Typical materials that fulfill the requirements of the invention are superparaelectric materials. Regarding the electrical properties, such materials are composed of clusters of electrical dipoles. The dipoles within each cluster are polarized in one direction. However, the directions of the polarization of different clusters are statistically random. Therefore, at zero electric field the dipole moments cancel each other resulting in zero net polarization. By applying an electric field, the dipole moments of the clusters rotate in the direction of electric field. At the same time the total polarization increases and becomes saturated at strong applied electric fields. By providing such superparaelectric material with large cluster sizes and avoiding polarization of all clusters in one direction, through preventing interaction between the clusters, high dielectric permittivity (susceptibility) can be achieved at moderately low electric fields. In addition, the risk for hysteresis and high losses in the material can be reduced. Superparaelectric materials with both piezoelectric effect and variable dielectric permittivity can be produced by tailoring Curie temperature and optimizing the (nano) crystalline structure of piezoelectric ceramics. Various perovskite niobates-tantalates, including potassium niobate (KNbO₃) could be the candidates for such materials.

The piezoelectric material according to the present invention is preferably provided as a film in the acoustic wave device. For surface acoustic wave devices, this means that the piezoelectric and electrically tunable film is mounted on a substrate of for example quartz. Any relevant transducers (e.g. input and output transducer) may be fabricated using photolithography atop the film. In a bulk acoustic wave device the film is arranged between the top and bottom (ground) electrodes. By employing the recent method of physical vapor deposition, arbitrary alloying of various compatible perovskite compositions according to the invention can be provided as epitaxial films for use in acoustic wave devices.

The invention offers the following advantages:

Tuning the operation characteristics of an acoustic wave device over wide ranges;

No extra elements or units connected to the device are required, thereby allowing a small overall device size;

Allowing fast tuning of the acoustic wave device;

Allowing dynamic tuning of the acoustic wave device; - Reducing hysteresis and power losses;

Providing extended power-handling capabilities.

Other advantages offered by the present invention will be appreciated upon reading the following description of the embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

The invention together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings in which:

Fig. 1 illustrates the electric field dependence of the polarization in a superparaelectric material;

Fig. 2 illustrates the electric field dependence of the polarizability in a superparaelectric material; Fig. 3 is a schematic perspective view of a surface acoustic wave device provided with a piezoelectric material with an electrically tunable dielectric permittivity according to the invention;

Fig. 4 is a cross sectional view of the surface acoustic wave device of Fig. 3 provided with control equipment for dynamically tuning the operation of the wave device;

Fig. 5 is a cross sectional view of a bulk acoustic wave device provided with a piezoelectric material with an electrically tunable dielectric permittivity according to the invention;

Fig. 6 is a flow diagram of a method of tuning an acoustic wave device according to the invention; and

Fig. 7 is a flow diagram of the electric field applying step of Fig. 6.

DESCRIPTION OF THE INVENTION

Throughout the drawings, the same reference characters will be used for corresponding or similar elements.

The present invention refers to tunable acoustic wave devices. Such devices are employed for converting, in an input transducer or resonator, an input electric field signal to an acoustic wave signal, including SAW and BAW signals. Subsequently, the acoustic wave signal may analogously be converted, in an output transducer or resonator, to an output electric field signal. Such devices may be employed for a wide range of applications including filters, resonators, oscillators, delay lines and high frequency generators.

The tunability of an acoustic wave device according to the invention is accomplished by arranging, in the wave device, a mechanical and electrical field coupling material that has both piezoelectric effect and a tunable dielectric permittivity.

The piezoelectric effect provides an effective coupling between electric and mechanical fields, i.e. converting electric field signals to acoustic wave signals and vice versa. A tunable dielectric permittivity allows the acoustic wave device to be tuned by applying a modulating electric field to the piezoelectric material. Such an electric field is preferably a DC, low frequency AC, pulsed electric field, or a low frequency AC field superimposed onto a pulsed electric field or electric field pulse with an arbitrary waveform applied to the wave device. This applied modulating electric field allows control of the functional or operational characteristics of the acoustic wave device, which is discussed on more detail below. The DC-bias field can be used to keep a constant value of the device characteristics, whereas the low frequency AC field can be used to modulate device characteristics at frequencies up to 100 kHz - 1 MHz. In addition, a pulsed electric field can be used to switch between two or several different values of operation characteristics of the device. Combination of a low frequency AC field with pulsed electric field can be used to achieve both switching between different channels and modulating the acoustic wave device within a certain channel. The pulsed electric field may be a pulse of arbitrary waveform to obtain the desired switching of values of operation characteristics.

The modulating electric field may be superimposed onto the high frequency electric field signal that is to be converted to an acoustic signal in the acoustic wave device. High frequency electric field has in most acoustic wave devices of today an operational frequency of up to 1-2 GHz, but it may also be up to several GHz, e.g. 10 GHz. Alternatively, or as a combination, the electric field may be applied all over or at least partly over the portion of the piezoelectric material where the acoustic wave signal propagates. In such a case, the wave signal may be modulated along its propagation between an input transducer or resonator and output transducer or resonator, resulting in tuning of the operation characteristics of the device.

The dielectric permittivity of the piezoelectric material effects several of the operation or acoustic characteristics of the device, including the acoustic wave velocity in the material, the resonator frequency (operation frequency) and bandwidth of the device, and the direction of the acoustic wave power flux propagation ("beam steering"), which will be discussed in more detail below. Thus, by tuning the dielectric permittivity it is possible to tune and modulate the above-identified acoustic characteristics of the device and consequently the operation of the device.

"Superparaelectric" materials meet the material requirements according to the invention, i.e. piezoelectric effect and tunable dielectric permittivity. As a person skilled in the art knows, a paraelectric material is an ensemble of noninteracting small electric dipoles, e.g. molecules. Such materials therefore have a nonlinear dielectric permittivity. Compared to paraelectric materials, in superparaelectric materials the small molecular dipoles are joined together in bigger ensembles or clusters. In each such cluster the dipoles spontaneously are polarized in one and the same direction. Although dipoles are polarized up to saturation in one direction within a cluster, the different clusters have statistically random orientation of polarization until they interact with each other. Furthermore, the clusters are statistically distributed regarding their size and position. The characteristics of such cluster distributions can be tailored by technological parameters in the process of the fabrication of the material for acoustic wave devices. In particular, in order to manufacture superior superparaelectric material the dipole moments in the material should be gathered into clusters with as large cluster size as possible. In addition, any interaction between the clusters should be minimized as far as possible to avoid spontaneous polarization of all clusters in one direction. Thus, the superparaelectric materials can be produced by optimizing the (nano) structure of the materials and by tailoring the Curie temperature.

Consequently, superparaelectric materials are composed of big clusters weakly interacting with other through long-range Coulomb dipole-dipole and/ or electro strictive interaction, resulting in a strongly nonlinear dielectric permittivity.

The dielectric permittivity ε in superparaelectric materials follows equation (1):

ε = l + χ (1)

where χ is the dielectric susceptibility of the material. The susceptibility can be expressed through the permittivity in vacuum ε₀ and the differential dP polarizability of the material a = — according to equation (2): dE

l_ dP_

X = (2) ε Ό_n dE

The polarizability may in turn be obtained from the polarization P of superparaelectric materials, which follows the Langevin law:

where n [πr³] is the concentration of the electric dipoles, p_Q [Cm] is an electric dipole moment, k is the Boltzmann constant, T is the temperature and E is an electric field. At zero applied electric field there is no net polarization, i.e. all the clusters of molecular dipoles are randomly orientated and cancel each other. An applied electric field orients them in one direction by rotation until full saturation.

Fig. 1 illustrates polarization of a superparaelectric material with the corresponding polarizability illustrated in Fig. 2. The dependence of the polarizability on the electric filed follows equation (4):

From equation (4) it follows that the highest obtainable polarizability at low

electric fields a increases as a power of two of the dipole

moment of the cluster. The corresponding dependence of the dielectric permittivity for the superparaelectric material is:

Therefore, by applying an electric field E the dielectric permittivity ε of the material may be tuned.

Although the material of the piezoelectric material of the invention is preferably a superparaelectric material, it is not limited thereto. Also ferroelectric materials can have a piezoelectric effect and an electrically tunable dielectric permittivity. In particular, ferroelectric materials with strongly correlated dipole moments can be produced with special crystalline structures in order to achieve superparaelectric properties. Thus, also such ferroelectric materials may be used in an acoustic wave device according to the invention. Typical ferroelectric materials have a permanent polarization on account of cooperative shift of some of its atoms or molecules in a given direction. Thus, this results in a net (remnant) polarization even at zero applied field. Furthermore, this permanent polarization could give rise to hysteresis and high losses in the material. This drawback of ferroelectric material is lessened for materials with low coercive fields and slimming P-E hysteresis loops. Actually, an optimal superparaelectric material would be a strong ferroelectric material with zero coercive field and where the ascending and descending P-E loop branches collapse into one smooth curve, as is illustrated in Fig. 1.

Therefore, by employing (superparaelectric) materials with large enough dipole clusters separated by distances larger than what is required to obtain a ferroelectrically ordered state two goals can be achieved simultaneously.

Firstly, the nonlinear dielectric permittivity (susceptibility) is enhanced since the clusters have a large dipole moment. Secondly, no hysteresis arises in the material. In addition, according to equation (3) the electric field needed to kT saturate the superparaelectric material E_sat and thus to achieve

P maximum tunability decreases with the size (dipole moment) of the clusters. As a consequence, superparaelectric materials with comparatively large cluster sizes (dipole moments) can be tuned with low electric fields.

Also, in order to achieve higher dielectric permittivity (susceptibility) for paraelectric material, the paraelectric material should be sintered resulting in bigger dipole clusters.

As was discussed above, by electrically tuning the dielectric permittivity of the piezoelectric material in an acoustic wave device, the operation and acoustic characteristics of the device and consequently the device itself may be tuned.

Starting with the velocity S of acoustic waves, which in highly polarizable crystals depends on the dielectric permittivity as: S oc (6)

^β)

Since the dielectric permittivity ε(E) is electrically tunable, also the acoustic wave velocity is electrically tunable.

In a SAW device, the resonance frequency / is a function of the wave velocity and the period L of the interdigital planar transducer structure according to:

where n is an integer, n = 0, 1, 2, ... As the wave velocity is tunable according to equation (6), the resonance frequency, including the center frequency f₀ at n = 0 , is electrically tunable.

Similarly, the bandwidth of a SAW device is determined both by the finite number of the electrodes in the transducer, which is a fixed number for a chosen transducer geometry, and by the acoustic wave velocity. Therefore, also a tunability of this quantity is obtainable.

Finally, electrically beam steering is obtainable according to the invention. The effect of beam steering is caused by the anisotropy of the crystals used for SAW and BAW devices. The effect arises due to that the direction of the acoustic power flow, in general, does not coincide with the direction of the phase velocity of the acoustic waves. Instead, the power flux streams along the outward normal of the "slowness curve" (1/ s) in the propagation direction. Since an electric field is a vector, it affects only certain components of the polarizability tensor. Thus, by applying an electric field only some components of the acoustic wave velocity will be affected, which results in a change of the shape of the slowness curve during field application. As a result, the electric field can tune the direction of the acoustic wave power flux propagation and beam steering is obtained.

The piezoelectric material of the invention may be a polycrystalline or amorphous material, preferably a polycrystalline or amorphous ceramic material. Such materials include nonmetallic materials with covalent bonds. However, crystalline ceramic materials have turned out to be particular advantageous according to the invention. Such, crystalline materials enhance the effects of the invention, i.e. provide a high piezoelectricity and strong electrically tunability of dielectric permittivity. These effects are smaller in polycrystalline and amorphous material.

Perovskite niobates-tantalates are promising candidates for tunable acoustic devices. For example, potassium niobate (KNbOe) single crystals have attracted substantial attention due to their ultrahigh electromechanical coupling coefficient K² ~ 53% [1]. The modern methods of physical vapor deposition (PVD) enable arbitrary alloying of various compatible perovskite compositions. As a result, continuous series of solid solutions in the form of epitaxial films can be obtained [2]. Therefore, epitaxial films can be tailored from the ferroelectric to superparaelectric state to engineer the required piezoelectric and tunable acoustic properties. Thus, an acoustic wave device of the invention may be include such a perovskite niobate-tantalate film with high piezoelectric effect and electrically tunable dielectric permittivity. Further examples of niobates-tantalates that can be used according to the teaching of the present invention are found in reference [3-5].

Fig. 3 illustrates a surface acoustic device 100 provided with a layer 120 of piezoelectric material with a tunable dielectric permittivity according to the present invention. The device 100 comprises a body 110, onto which the layer 120 is deposited. The body 100 could be a quartz crystal, with the layer 120 as a thin film, e.g. a ceramic superparaelectric pervoskite niobate- tantalate film. On the film 120 an input 130 and output 140 electrode or interdigital transducer are provided for example by photolithography. An input electric field signal applied to the input transducer 130 generates a surface acoustic wave in the film 120. The wave 120 propagates towards the output transducer 140, where an output electric field signal is generated in the transducer 140 based on the surface acoustic wave. According to the invention, in order to tune the operation characteristics of the surface acoustic device 100, e.g. the resonance frequency and/or bandwidth of the device 100, a modulating electric field, e.g. low frequency AC electric field, is applied to the piezoelectric film 120. As was discussed above, the electric field may be applied to the input transducer 130 and/or applied over at least a portion of the film 120 between the transducers 130 and 140. This electric field will tune the dielectric permittivity of the piezoelectric film 120 and consequently change the acoustic characteristics, including the velocity of the surface acoustic wave, of the film 120, which in turn will tune the resonance frequency and/ or the bandwidth of the device 100.

The surface acoustic wave device of Fig. 3 should be viewed as a schematic overview of a SAW device to which the teaching of the present invention can be applied. Thus, the SAW device may be equipped with other elements and units to enhance its operation, e.g. reflector electrodes positioned at respective side of the transducers in the film. Likewise, the actual design of the transducers may differ from what is illustrated in the figure.

Fig. 4 is a cross sectional view of the surface acoustic wave device 100 in Fig. 3 provided with control equipment for dynamically tuning operation of the acoustic wave device 100. An output detector 160 is arranged for measuring the output electric field signal from the output transducer 140 of the device 100. Based on the measured signal, the output detector 160 generates a control signal 180 that is transmitted to a tuning electric field source 170. The control signal 180 causes the source 170 to change the magnitude and/ or frequency of the tuning electric field 190. The electric field is then applied to the film 120 e.g. through the input transducer 130. A bulk acoustic wave device 200 is schematically illustrated in a cross sectional view in Fig. 5. The BAW device 200 generally includes one or more acoustically mismatched layers 250 mounted on a body 210 and acting as a reflector for acoustic waves. Similar reflection effect is obtained by exchanging the layers 250 with a layer of a dielectric substance, micromachined membranes, air gaps or acoustic interference filters. On the reflecting layer(s) 250, a ground electrode or conductor 240 is provided. Upper electrodes 230, 235 are separated from the ground electrode 240 by a thin ceramic, preferably superparaelectric, piezoelectric layer 220 with an electrically tunable dielectric permittivity. In the BAW device 200 of Fig. 5 the conversion of electric field signals to acoustic wave signals is achieved by the piezoelectric layer 200 between the electrodes 230, 235 and 240, respectively. Each upper electrode 230, 235 defines an individual resonator with the underlying piezoelectric layer 220 and the ground electrode 240. These two resonators are effectively electrically connected in series, with the common electrode 240 at the junction between them.

A flow diagram of the method of tuning the operation characteristics of an acoustic wave device according to the invention is illustrated in Fig. 6. The method starts in step SI. In step S2 a modulating or tuning electric field is applied to the piezoelectric material according to the invention. This electric field will affect the dielectric permittivity of the material and thereby change or tune the acoustic characteristics of the device, including the resonance frequency and bandwidth of the device, through a change in the velocity of the acoustic wave in the piezoelectric material. The method is ended in step S7.

Fig. 7 is a flow diagram illustrating the field applying step S2 of Fig. 6 in more detail. Starting with the optional step S3, the output electric field signal from the acoustic wave device is measured. Based on this measured output signal, a control signal is provided in the optional step S4. In step S5 the tuning electric field is generated based on the obtained control signal. Thereafter in step S6, the tuning electric field is provided to the piezoelectric material, allowing tuning of the dielectric permittivity of the piezoelectric material in the device. The method then continues to step S7.

The embodiments described above are merely given as examples, and it should be understood that the present invention is not limited thereto. Further modifications, changes and improvements which retain the basic underlying principles disclosed and claimed herein are within the scope of the invention.

REFERENCES

[1] K. Yamanouchi, H. Odagawa, T. Kojima and T. Matsumura, "Theoretical and experimental study of super-high electromagnetic coupling surface acoustic wave propagation in KNbO₃ single crystal", Electronics Letters, Vol. 33, No. 3, pp. 193-194 (1997).

[2] M.A. Grishin, A.M. Grishin, S.I. Khartsev, and U.O. Karlsson, "High performance films of binary system SrTiO3-PbZro.52Tio.4sO3 on sapphire", Integrated Ferroelectrics 39 (1-4), pp. 1301-1308 (2001).

[3] US Patent 3,437,597

[4] US Patent 6,083,415

[5] US Patent 6,093,339

Claims

1. A tunable acoustic wave device (100; 200) comprising a piezoelectric substrate (120; 220) adapted for coupling of mechanical and electrical fields, characterized in that said piezoelectric substrate (120; 220) presents a tunable dielectric permittivity for enabling tuning of operation characteristics of said device.

2. The device according to claim 1, characterized in that said dielectric permittivity is electrically tuned by applying an electric field (190) to said material (120; 220).

3. The device according to claim 2, characterized in that said electric field (190) is selected from at least one of the items in a list of: - a low frequency AC field with a frequency up to 1 MHz; a DC bias electric field; a pulsed electric field; or a low frequency AC field with a frequency up to 1 MHz superimposed onto a pulsed electric field.

4. The device according to any of the preceding claims, characterized in that said tunable dielectric permittivity enables tuning of at least one of said operation characteristic in a list of: acoustic wave velocity of said acoustic wave (100; 200); - resonance frequency of said acoustic wave device (100; 200); bandwidth of said acoustic wave device (100; 200); or direction of acoustic wave power flux propagation.

5. The device according to any of the preceding claims, characterized in that said piezoelectric substrate (120; 220) is a crystalline ceramic material.

6. The device according to any of the preceding claims, characterized in that said piezoelectric substrate (120; 220) is a superparaelectric material.

7. The device according to claim 6, characterized in that said piezoelectric substrate (120; 220) is made of a superparaelectric material forming clusters of dipoles, where said dipoles of a cluster being polarized in one direction; the dipole moment of each cluster being statically random oriented regarding the orientation of polarization in noninteracting clusters.

8. The device according to any of the preceding claims, characterized in that said piezoelectric substrate (120; 220) is provided as a film.

9. The device according to any of the preceding claims, characterized in that said piezoelectric substrate (120; 220) is a perovskite niobate-tantalate.

10. The device according to claim 2, characterized in that said device (100; 200) further comprising: an output detector (160) adapted for measuring an output electric field signal from said acoustic wave device (100; 200) and generating a control signal (180) based on said measured output signal; and an electric field source (170) connected to said detector (160) and arranged for applying said electric field (190) to said piezoelectric substrate (120; 220) based on said control signal (180).

11. A method of tuning operation characteristics of an acoustic wave device (100; 200) comprising a piezoelectric substrate (120; 220) adapted for coupling of mechanical and electrical fields, characterized by the step of: applying an electric field (190) to said piezoelectric substrate (120; 220) for tuning the dielectric permittivity of said piezoelectric substrate (120; 220), thereby allowing tuning of said operation characteristics.

12. The method according to claim 11, characterized in that said electric field applying step in turn comprises the step of: superimposing said electric field (190) to an input electric field signal, which is to be converted by said piezoelectric substrate (120; 220) to an acoustic wave signal.

13. The method according to claim 11, characterized in that said electric field applying step in turn comprises the step of: applying said electric field (190) over at least a portion of said piezoelectric substrate (120; 220).

14. The method according to any of the claims 11 or 13, characterized by the further steps of: measuring an output electric field signal from said acoustic wave device (100; 200); generating said electric field (190) based on said measured output signal.

15. The method according to any of the claims 11 to 14, characterized in that said electric field (190) is selected from at least one of the items in a list of: - a low frequency AC field with a frequency up to 1 MHz; a DC bias electric field; a pulsed electric field; or a low frequency AC field with a frequency up to 1 MHz superimposed onto a pulsed electric field.

16. The method according to any of the claims 11 to 15, characterized in that said tunable dielectric permittivity enables tuning of at least one of said operation characteristic in a list of: acoustic wave velocity of said acoustic wave (100; 200); - resonance frequency of said acoustic wave device (100; 200); bandwidth of said acoustic wave device (100; 200); or direction of acoustic wave power flux propagation.

17. The method according to any of the claims 11 to 16, characterized in that said piezoelectric substrate (120; 220) is a crystalline ceramic material.

18. The method according to any of the claims 11 to 17, characterized in that said piezoelectric substrate (120; 220) is a superparaelectric material.

19. The method according to any of the claims 11 to 18, characterized in that said piezoelectric substrate (120; 220) is a perovskite niobate-tantalate.