DIFFERENTIAL ACCOUSTIC WAVE SENSORS

Abstract

An acoustic wave sensor device, comprising an interdigitated transducer; a first reflection structure arranged on one side of the interdigitated transducer, and a second reflection structure arranged on another side of the interdigitated transducer; a first resonance cavity comprising a first upper surface and formed between the interdigitated transducer and the first reflection structure; a second resonance cavity comprising a second upper surface and formed between the interdigitated transducer and the second reflection structure; and wherein the second upper surface comprises a physical and/or chemical modification as compared to the first upper surface.

Claims

1. Acoustic wave sensor device, comprising: an interdigitated transducer; a first reflection structure arranged on one side of the interdigitated transducer; a second reflection structure arranged on another side of the interdigitated transducer; a first resonance cavity comprising a first upper surface and formed between the interdigitated transducer and the first reflection structure; a second resonance cavity comprising a second upper surface and formed between the interdigitated transducer and the second reflection structure; and wherein the second upper surface comprises a physical and/or chemical modification as compared to the first upper surface.

2. The acoustic wave sensor device of claim 1, wherein the physical and/or chemical modification comprises a metallization layer formed on the second upper surface.

3. The acoustic wave sensor device of claim 2, wherein the metallization layer comprises or consists of at least one of AlCu and Ti.

4. The acoustic wave sensor device of claim 1, wherein the physical and/or chemical modification comprises a passivation layer formed on the second upper surface, wherein the passivation layer comprises or consists of at least one of Si.sub.3N.sub.4, Al.sub.2O.sub.3, AlN, Ta.sub.2O.sub.5 and SiO.sub.2.

5. The acoustic wave sensor device of claim 1, wherein the physical and/or chemical modification comprises a recess of the second upper surface.

6. The acoustic wave sensor device of claim 2, wherein the first upper surface is modified by formation of a passivation layer on the first upper surface and/or formation of a recess of the first upper surface.

7. The acoustic wave sensor device of claim 4, wherein the first upper surface is modified by formation of a metallization layer on the first upper surface and/or formation of a recess of the first upper surface.

8. The acoustic wave sensor device of claim 5, wherein the first upper surface is modified by formation of a metallization layer or passivation layer on the first upper surface.

9. The acoustic wave sensor device of claim 1, wherein the first reflection structure comprises or consists of a Bragg mirror and the second reflection structure comprises or consists of another Bragg mirror.

10. The acoustic wave sensor device of claim 1, wherein the first reflection structure comprises or consists of an edge reflection structure.

11. The acoustic wave sensor device of claim 1, wherein the first resonance cavity comprises first resonance sub-cavities separated from each other by first reflection sub-structures of the first reflection structure, and the second resonance cavity comprises second resonance sub-cavities separated from each other by second reflection sub-structures of the second reflection structure.

12. The acoustic wave sensor device of claim 1, further comprising: a bulk substrate; a dielectric layer formed on the bulk substrate; and a piezoelectric layer; and wherein the interdigitated transducer and the first and second reflection structures are formed on the piezoelectric layer and the first and second resonance cavities comprise the piezoelectric layer.

13. The acoustic wave sensor device of claim 1, wherein extension lengths of the first resonance cavity and the second resonance cavity differ from each other.

14. The acoustic wave sensor device of claim 1, wherein the interdigitated transducer is split into two parts and further comprising an additional reflection structure positioned between the two parts.

15. The acoustic wave sensor device of claim 1, wherein the acoustic wave sensor device is a passive surface acoustic wave sensor device and/or an ambient parameter is one of a temperature, chemical species, strain, pressure or torque of a rotating axis.

16. The acoustic wave sensor device of claim 1, wherein the acoustic wave sensor device is a passive surface acoustic wave sensor device configured to sense an ambient parameter selected from among the group consisting of a temperature, a chemical species, a strain, a pressure, or a torque.

17. The acoustic wave sensor device of claim 12, wherein the bulk substrate comprises a silicon bulk substrate.

18. The acoustic wave sensor device of claim 12, wherein the dielectric layer comprises an SiO.sub.2 layer.

19. The acoustic wave sensor device of claim 12, wherein the piezoelectric layer comprises a LiNbO.sub.3 or LiTaO.sub.3 layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Additional features and advantages of the present disclosure will be described with reference to the drawings. In the description, reference is made to the accompanying figures that are meant to illustrate preferred embodiments of the disclosure. It is understood that such embodiments do not represent the full scope of the invention.

[0036] FIG. 1 represents an example of a surface acoustic wave sensor according to the art.

[0037] FIG. 2 represents a principle sketch illustrating an essential feature in form of a modified resonance cavity of an embodiment of an acoustic wave sensor device according to the present disclosure.

[0038] FIG. 3 represents an illustration of the effect of the formation of a metallization layer on one of two resonance cavities of an acoustic wave sensor device according to an embodiment.

[0039] FIG. 4 represents an illustration of differences in the temperature coefficients of velocity obtained by metallization of one of two resonance cavities of an acoustic wave sensor device according to an embodiment.

[0040] FIG. 5 represents an illustration of differences in the temperature coefficients of velocity obtained by either metallization or recessing a piezoelectric layer of one of two resonance cavities of an acoustic wave sensor device according to an embodiment.

[0041] FIG. 6 represents illustrations of embodiments of an inventive acoustic wave sensor device.

[0042] FIG. 7 represents illustrations of further embodiments of an inventive acoustic wave sensor device.

[0043] FIG. 8 represents an illustration of an embodiment of an inventive acoustic wave sensor device comprising side reflection structures.

[0044] FIG. 9 represents an illustration of an embodiment of an inventive acoustic wave sensor device comprising cascaded resonance cavities.

[0045] FIG. 10 represents an illustration of a cascaded resonance cavity configuration of an acoustic wave sensor device wherein the transducer does not operate at the Bragg condition according to an embodiment of the present invention.

[0046] FIG. 11 represents an illustration of another cascaded resonance cavity configuration of an acoustic wave sensor device wherein the transducer does not operate at the Bragg condition.

DETAILED DESCRIPTION

[0047] The present disclosure provides acoustic wave sensors, in particular, passive SAW sensors, that are characterized by a high signal-to-noise ratio, sensitivity and reliability. With respect to temperature measurements, for example, the obtainable resonance frequency sensitivity allows for a measurement sensitivity of more than 1 ppm per Kelvin. The acoustic wave sensors can be interrogated by any interrogators that are configured to determine a response spectrum from an interrogated acoustic wave sensor. The interrogated acoustic wave sensor can, for example, be a resonator device, for example, a differential SAW sensor. It goes without saying that the disclosure can be implemented in any devices employing acoustic wave sensors or dielectric resonators, RLC circuits, etc.

[0048] The interrogation device interrogating one of the inventive acoustic wave sensor devices may comprise a transmission antenna for transmitting an interrogation radiofrequency signal to the sensor device and a reception antenna for receiving a response radiofrequency signal from the sensor device. The interrogation radiofrequency signal transmitted by the transmission antenna may be generated by a signal generator that may comprise a radiofrequency synthesizer or oscillator as well as optionally some signal shaping module providing a suitable frequency transposition and/or amplification of the signal to be transmitted by the transmission antenna. The interrogation radiofrequency signal generated by the signal generator may be a pulsed or bursty signal with a frequency selected according to the resonance frequency of the acoustic wave sensor device.

[0049] Furthermore, the interrogation device may comprise a processing means connected to the reception antenna. The processing means may comprise filtering and/or amplification means and be configured for analyzing the response radiofrequency signal received by the reception antenna. For example, the sensor device operates at a resonance frequency of 434 MHz or 866 MHz or 915 MHz or 2.45 GHz (the said ISM bands).

[0050] The interrogation device may transmit a long radiofrequency pulse and after the transmission has been stopped, the resonance cavities of the sensor device discharge at their resonant eigenfrequencies with time constants τ equal to Q.sub.f/πF wherein F is the central frequency and Q.sub.f is the quality factor of the resonance, Q.sub.f corresponding to the ratio between the resonance central frequency and the width at half maximum of the band pass used in the interrogation process. Spectral analysis performed by the processing means of the interrogation device allows for calculating the resonator frequency(ies) and, thereby, the sensing of an ambient parameter. The received response radiofrequency signal may be mixed by the processing means with interrogation radiofrequency signal according to the so-called I-Q protocol as known in the art to extract the real and imaginary parts (in-phase components I=Y cos φ and quadrature components Q=Y sin φ with the signal amplitude Y and the phase φ) from which the modulus and phase can then be derived.

[0051] FIG. 2 illustrates an exemplary embodiment of an inventive surface acoustic wave (SAW) sensor device 10. The SAW sensor device 10 shown in FIG. 2 comprises an interdigitated (a comb) transducer T connected to an antenna (not shown in FIG. 2) for receiving an electromagnetic wave E.sub.1 and converting the electromagnetic wave E.sub.1 into a surface acoustic wave S.sub.1. The comb transducer T comprises interdigitated electrodes as described with reference to FIG. 1, for example. Two SAW resonance cavities R.sub.1 and R.sub.2 with extensions (gaps) g.sub.1 and g.sub.2, respectively, are provided between the comb transducer T and Bragg mirrors M.sub.1 and M.sub.2, respectively. It is noted that here and in the following a sensor device comprising one transducer and two resonance cavities is described but more than one transducer and more than two resonance cavities may also be provided.

[0052] The sensor device 10 may comprise a SAW-based sensor built on single-crystal piezoelectric materials like quartz (SiO.sub.2), lithium niobate (LiNbO.sub.3), lithium tantalate (LiTaO.sub.3), langasite (LGS) or poly-crystal piezoelectric materials like aluminum nitride (AlN) or zinc oxide (ZnO), in particular, deposited on silicon, or even on a Piezo-On-Insulator (POI) composite material comprising a layer of piezoelectric material, in particular, a single-crystal material, such as, for example, lithium tantalate or lithium niobate, bonded to a support substrate such as, for instance, silicon, if necessary by means of a bonding layer, such as, for instance, a silicon oxide layer. The transducer T converts an interrogation radiofrequency signal E.sub.1 received by the antenna into a surface acoustic wave S.sub.1 that is reflected back by the Bragg mirrors M.sub.1 and M.sub.2 of the resonance cavities R.sub.1 and R.sub.2 and converted back into a radiofrequency signal that in course is transmitted by the antenna (or another antenna) as a response radiofrequency signal. The surface acoustic wave sensor 10 (as well as the devices described below with reference to other one of the figures) may operate at Bragg conditions with wavelengths of the excited surface acoustic waves of some multiples of the pitches of the comb electrodes of the comb transducer T. When operation is performed at Bragg conditions the comb transducer T itself substantially functions as a mirror (with a reflectivity of at least 80%) thereby definitely separating the resonance cavities R.sub.1 and R.sub.2 from each other.

[0053] According to the disclosure, the provided resonance cavities R.sub.1 and R.sub.2 differ from each other in that one of them (R.sub.2 in the example shown in FIG. 2) exhibits some physical and/or chemical modification ρ as compared to the other one. By the physical and/or chemical modification ρ different propagation conditions and, thus, different resonance characteristics, are provided in the different resonance cavities R.sub.1 and R.sub.2. Multiple resonances according to multiple resonance cavities R.sub.1 and R.sub.2 and, thus, differential effects when comparing resonances characterizing each of the multiple resonance cavities R.sub.1 and R.sub.2 thereby arise.

[0054] There is a variety of means for providing the physical and/or chemical modification ρ in order to achieve propagating wave modes that exhibit differential parametric sensitivities. These means, for example, include realization of the physical and/or chemical modification ρ by the formation of a metallization layer and/or passivation layer.

[0055] An effect of the provision of the physical and/or chemical modification ρ of one of the resonance cavities R.sub.1 and R.sub.2 is illustrated in FIG. 3. The physical and/or chemical modification ρ is realized by forming a metallization layer of 100 nm thickness on the region of the resonance cavity R.sub.2; no metallization layer is formed on the resonance cavity R.sub.1. In some detail, an AlCu metallization layer is formed on the region of the resonance cavity R.sub.2, AlCu is also used for the formation of the comb transducer T and electrodes of the Bragg mirrors M.sub.1 and M.sub.2, and it is used an LiTaO.sub.3 (XY)/42° layer of thickness of 600 nm on an SiO.sub.2 layer of thickness of 500 nm on (100) Si, a comb transducer with 125 finger pairs, mirrors with 60 electrodes and an acoustic aperture of 70 μm, which translates to 42 wavelengths λ of the excited surface acoustic waves. The obtained reflection coefficient S.sub.11 is illustrated on the left-hand side of FIG. 3 and the obtained impedance is illustrated on the right-hand side of FIG. 3. Varying the thickness of the metallization layer or the length of one of the cavities allows for clearly identifying which modes belong to the metallized resonance cavity R.sub.2 and modes belong to the non-metallized one R.sub.1.

[0056] In particular, FIG. 3 shows what happens when, for instance, the R.sub.2 cavity length is varied from 100 to 80 μm. One can see that only peaks related to resonances established in the R.sub.2 cavity are affected by this change. Actually, when both cavities exhibit the same length, only the solid line signature is predicted as resonances of cavity R.sub.1 and R.sub.2 are superimposed and thus contribute to the reflection coefficient S.sub.11 or the impedance Z.sub.11 response of the resonator at identical frequencies (for instance, the most marked at 2.35, 2.37 and 2.39 GHz). Changing the length of one cavity translates to locally modifying the wave propagation properties and thus actually simulates the sensor operation principle. It turns out that when the cavity length is reduced, the above mentioned mode resonances split in two contributions (the dashed line) with preservation of the initial ones corresponding to R.sub.1 cavity resonances and occurrence of a second set of resonances related to cavity R.sub.2. The quasi-independence of the two resonance sets demonstrates the actual operation of the disclosure.

[0057] The associated temperature coefficients of velocity of the metallized and the free surfaces of the resonance cavities differ from each other as it is illustrated in FIG. 4. For an operation frequency of about 2.45 GHz a detectable difference of about 10 ppm per Kelvin can be obtained (see left-hand side of FIG. 4). The dependency of the difference of the temperature coefficients of velocity on the thickness of the metallization layer is illustrated on the right-hand side of FIG. 4.

[0058] When the same material is used for the metallization and the formation of the comb transducer T and electrodes of the Bragg mirrors M.sub.1 and M.sub.2, all of these elements can be deposited in the same deposition process. In other embodiments, a different material is used for the metallization. In other embodiments, one metallization layer or passivation layer of one material is formed on the first resonance cavity R.sub.1 and another metallization layer or passivation layer of another material is formed on the second resonance cavity R.sub.2. For example, a positive-temperature shifting material, for example, SiO.sub.2 or Ta.sub.2O.sub.5, is formed on one of the resonance cavities and a negative-temperature shifting material, for example, Si.sub.3N.sub.4 or AlN, is formed on the other one of the resonance cavities.

[0059] According to an alternative embodiment, realization of the physical and/or chemical modification ρ can be achieved by recessing a piezoelectric layer of one of the two resonance cavities. FIG. 5 comparatively illustrates the effects of a passivation of one of the resonance cavities by an SiO.sub.2 overlay up to a thickness of 250 nm (left-hand side) and recessing the piezoelectric layer of one of the resonance cavities from 600 nm down to 350 nm (right-hand side), respectively, on the temperature coefficient of velocity. Again, it is used (or started from) an LiTaO.sub.3 (XY)/42° layer of thickness of 600 nm on an SiO.sub.2 layer of thickness of 500 nm on (100) Si, a comb transducer with 125 finger pairs, mirrors with 60 electrodes and an acoustic aperture of 70 μm. In the case of passivation by an SiO.sub.2 layer a detectable difference of the temperature coefficients of velocity of about 10 ppm per Kelvin can be obtained, whereas in the case of recessing the piezoelectric layer a detectable difference of the temperature coefficients of velocity of about 14 ppm per Kelvin can be obtained, both for an operation frequency of about 2450 MHz.

[0060] Furthermore, according to different embodiments, different physical and/or chemical modifications ρ can be performed for the first resonance cavity R.sub.1 and the second resonance cavity R.sub.2. Additionally, the extension lengths of the resonance cavities R.sub.1 and R.sub.2 may differ from each other in order to more clearly separate the spectral responses of the resonances on each side of the comb transducer from each other. FIG. 6 illustrates a variety of exemplary embodiments in cross-sectional views. In all of the embodiments shown in FIG. 6 a (100) silicon bulk substrate is provided for the sensor device and a SiO.sub.2 layer is formed on the (100) silicon bulk substrate and an LiTaO.sub.3 layer is formed on the SiO.sub.2 layer, for example. It goes without saying that other suitable materials may be used.

[0061] In the upper row of FIG. 6, an embodiment is illustrated in that the left-hand side resonance cavity is metallized whereas the right-hand side resonance cavity is not metallized but has a larger extension than the left-hand side one. In the second row from the top, an embodiment is illustrated in that the left-hand side resonance cavity is passivated whereas the right-hand side resonance cavity is not passivated but has a larger extension than the left-hand side one. Passivation may be realized by forming a passivation layer made of or comprising Si.sub.3N.sub.4, Al.sub.2O.sub.3 or AlN. In the third row from the top, an embodiment is illustrated in that the left-hand side resonance cavity is recessed (etched) whereas the right-hand side resonance cavity is not recessed or otherwise treated but has a larger extension than the left-hand side one. In the bottom row, an embodiment is illustrated in that the left-hand side resonance cavity is passivated whereas the right-hand side resonance cavity is not passivated but has a larger extension than the left-hand side one. Different from the embodiment shown in the second row from top of FIG. 6, a passivation layer is not only formed on the left-hand side resonance cavity but also on the comb transducer and the Bragg mirrors. For example, the passivation layer may be initially formed on the entire structure and, then, removed from the region of the right-hand side resonance cavity. In all embodiments shown in FIG. 6, the left-hand side and right-hand side resonance cavities exhibit different sensitivities to measurands due to the provided different resonance characteristics and, thus, allow for differential measurements.

[0062] According to other embodiments, material layers can be formed on both resonance cavities. Moreover, material layers formed on one or more of the resonance cavities may have inhomogeneous thicknesses along the direction of propagation of the acoustic waves. Further, multi-layers may be formed on one or more of the resonance cavities. In this context, it should be noted that, in general, provision of a material layer on a resonance cavity may result in a reduction of the phase velocity of acoustic waves due to mass loading effects, particularly, if layers of a material of a high atomic number, as Pt, Au or W, are used. This effect can be compensated by adding a layer exhibiting a relatively high acoustic velocity, for example, AlN, Si.sub.3N.sub.4, Al.sub.2O.sub.3.

[0063] FIG. 7 illustrates an embodiment wherein a left-hand side resonance cavity is metallized and a right-hand side resonance cavity is passivated (upper row of FIG. 7) and an embodiment wherein a left-hand side resonance cavity is covered by a multi-layer, for example, a metal layer formed on a dielectric layer, and a right-hand side resonance cavity is recessed (lower row of FIG. 7).

[0064] It should also be noted that the passivation, the recessed structure or the metallization or any combination of such surface modifications may comprise or consist of some grating, in particular, periodic gratings operating out of the Bragg condition of the wave emitted by the IDT (i.e., non-reflective gratings). Moreover, the surface modification may be partially applied to the cavity concerned.

[0065] In all of the above-described embodiments, Bragg mirrors are provided in order to form the resonance cavities. However, according to alternative embodiments, the Bragg mirrors may be replaced by side/edge reflection structures for pure shear modes guidance. Thereby, very compact configurations can be achieved in that the Bragg reflection is replaced by a flat surface reflection without any energy loss or mode conversion. Configurations with side/edge reflection structures for pure shear modes guidance are particularly useful for sensing ambient parameters in liquids. Shear waves are very suitable for in-liquid probing. Particularly, highly coupled modes (>5%) together with high-k materials (with a dielectric constant k larger than 30, for example) are attractive for in-liquid applications.

[0066] An exemplary embodiment making use of side reflection structures rather than Bragg mirrors is show in FIG. 8. An IDT is formed on a stack comprising an SiO.sub.2 layer that is formed on an (100) silicon bulk substrate and an LiTaO.sub.3 layer that is formed on the SiO.sub.2 layer. Resonance cavities of different extensions are provided left and right of the IDT. The extensions may be multiple integers of the wavelength λ of the excited acoustic waves. The left-hand side resonance cavity is metallized and the right-hand side resonance cavity is passivated. The sensor device comprises side/edge reflection structures, i.e., recesses with depth t at the edges extending into the Si bulk substrate, for generating resonances. The depth of the recesses t (or thickness of the edges) has to be larger than one wavelength λ of the excited acoustic waves, particularly, larger than 5λ.

[0067] In all of the above-described embodiments comprising Bragg mirrors, simple resonance cavities are employed. However, all of these embodiments may employ cascaded resonance cavities comprising multiple mirror electrode structures. An exemplary embodiment comprising cascaded resonance cavities is shown in FIG. 9. In this embodiment, three-mirror electrode structures separated by gaps g.sub.1 and g.sub.2 resulting in resonance sub-cavities are provided on each side of the IDT. Different widths of the cavities g.sub.1 and g.sub.2 may result in a limitation of the number of 50Ω matched resonances to only two, which is different to more than two resonances that arise in the previously described embodiments. The distance between the two resonances as well as the coupling coefficient of the resonances can be controlled by the number of the mirror electrode structures and resonance sub-cavities.

[0068] In the case of using cascaded resonator cavities, it is possible to use a transducer, which does not operate at the Bragg condition. For instance, the transducer may exhibit three or four fingers per wavelength or even 5 fingers per two wavelength and in general all suitable structures allowing to excite waves at a given synchronism without wave reflection on the IDT electrodes.

[0069] Two examples for possible configurations in this respect are shown in FIGS. 10 and 11. According to one configuration (see FIG. 10) smaller mirrors are arranged close to the transducer and larger mirrors separated by distances g.sub.1 and g.sub.2, respectively, from the smaller mirrors are additionally provided to guarantee resonances in the resonance cavities. FIG. 11 illustrates a configuration in that on the-left-hand side no Bragg condition is fulfilled for the operation of the transducer. One or more of the resonance cavities may be physically and/or chemically modified as it is described above. It is worth noting that in the examples shown in FIGS. 10 and 11, some supplementary resonances are established in the IDT region itself, i.e., the IDT operates as a supplementary cavity, which could potentially be used to complete the measurements.

[0070] According to further embodiments, the acoustic wave sensor can be used for chemical detection. In this case, on one of the resonance cavities a material layer is formed that exhibits a particular chemical activity with respect to some chemical species that is to be detected whereas the other one of the resonance cavities maintains free of such a material layer or is covered by a material layer without that particular chemical activity. With the exception of sensitivity to the chemical species both resonance cavities should exhibit the same sensitivity to ambient parameters as, for example, the temperature or water loading properties. The chemical activity may result in some specific deposition of material of the chemical species and, thus, mass or elastic loading effects that in course change the resonance frequency of the resonance cavity. An example is hydrogen absorption by a layer made of or comprising palladium.

[0071] Furthermore, it is noted that it is also envisaged herein an operation situation in that the reflection coefficient of the transducer T is not strong enough to allow for a clear enough separation between the resonances of the cavities. In that case, (for example, in all of the above-described embodiments) the IDT may be split into two parts with an additional reflector provided in the middle of two parts in order to improve the cavity resonance separation. This will be particularly useful for Rayleigh or more generally elliptically polarized waves on quartz, langasite, lithium tantalate single-crystal substrates and composite substrates including GaN, AlN and ZnO layers because the corresponding modes are generally exhibiting a coupling factor smaller than 1% and a reflection coefficient on a unique electrode of less than 5%, typically less than 3% and even less or equal 1%. In some extend, the reflection coefficient is related to the coupling factor as it is generally composed of a mechanical part (elastic and mass loading effect) and an electric part (electrical load effect).

[0072] For POI configurations using LiTaO.sub.3 layers with an electromechanical coupling in excess of 5%, it can be demonstrated by computation that the reflection coefficient is larger than 5% and may reach 10% or even more (15% measured, >20% computed considering metal of high atomic number). For quartz, it can be shown that adding a central mirror inside the transducer allows for obtaining the separation of the two cavity modes. This consideration is particularly true for AlCu based electrodes. The use of electrode with high atomic number like, for instance, molybdenum or gold or platinum or tungsten may allow for large reflection coefficients (particularly for single crystals, not for AlN or GaN based layered substrates). It is also of interest in such a configuration to use single phase unidirectional transducers (SPUDT) in order to promote the emission of the waves in a given direction, again, in order to facilitate the separation between the resonances of the cavities.

[0073] For example, for Rayleigh waves on lithium niobate, particularly, on the LiNbO.sub.3 (YX1)/128° cut the coupling factor is larger than 5% but the electrical and mechanical parts of the reflection coefficient are exhibiting an opposite sign in short-circuit mode (the mode used for exciting and detecting the elastic waves), thus yielding moderate reflection coefficient for usual metallization (relative metallization h/λ between 1 and 5%), particularly, for devices operating close to 2.45 GHz. Furthermore, using such a combination of material and mode yields relatively short transducers meeting the 50Ω condition.

[0074] For instance, a transducer with 15 electrode pairs does exhibit the said condition at its synchronism frequency. With a limited reflection coefficient per electrode (i.e., <5% or even 3% or even less), such a length may not be sufficient to provide a global reflection coefficient larger than 50% and preferentially larger than 80%. As a consequence, one might split here also the transducer in two parts and insert a reflecting grating in between the said two parts in order to increase the cavity separation. In particular, it is of interest to use a mirror grating operating at the open circuit mode, meaning that each electrode is electrically isolated, thus meeting the condition of global neutral charge. According to this condition, the sign of the mechanical and electrical parts of the reflection coefficient is similar and the resulting reflection effect is twice or three-times larger than for the short-circuit condition. As in the case of other single crystals, the use of SPUDT effects can be useful to improve the device operation. The SPUDT effect will be obtained by introducing a gap between the transducer and the open circuit mirror grating reflector. These considerations are particularly true for AlCu electrodes.

[0075] All previously discussed embodiments are not intended as limitations but serve as examples illustrating features and advantages of the disclosure. It is to be understood that some or all of the above described features can also be combined in different ways.