Transducer structures for acoustic wave devices

Abstract

To provide a transducer structure for a novel surface elastic device.SOLUTION: The present invention relates to a transducer structure for a surface elastic device, comprises: a composite substrate including a piezoelectric layer; and a pair of interdigitated comb electrodes, comprising a plurality of electrode means with a pitch p satisfying a Bragg condition. In the transducer structure for the surface elastic device, The interdigitated comb electrodes are embedded in the piezoelectric layer, such that, in use, an excitation of a wave propagation mode occurs in a volume of each electrode means, and is a predominant propagating mode of the structure. The invention relates also to an elastic wave device comprising at least one transducer structure such as described above and to a method for manufacturing the transducer structure. The present invention also relates to use of a frequency of a bulk wave propagated in each electrode means of the transducer structure in the elastic wave device to generate contribution at a high frequency exceeding, In particular, 3GHz.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to acoustic wave devices, and more particularly to transducer structures for acoustic wave devices. [Background technology]

[0002] In recent years, surface acoustic wave (SAW) devices have been adopted in an increasing number of practical applications, such as filters, sensors, and delay lines. SAW filters are particularly interesting for mobile phone applications because they allow the creation of unprecedentedly compact, low-loss, high-order bandpass filters without the need for complex electrical circuits. SAW filters therefore offer significant advantages over other filter technologies in terms of performance and size.

[0003] A typical surface acoustic wave device contains one or more interdigitated transducers (IDTs) fabricated on a surface-propagating substrate, which converts acoustic waves to electrical signals and vice versa by utilizing the piezoelectric effect of the substrate. An IDT comprises opposing "electrode combs" of interdigitated metal fingers arranged on a piezoelectric substrate. Electrically exciting these fingers generates Rayleigh surface acoustic waves in the substrate. Other wave types, such as shear and longitudinally polarized waves, travel through the volume and are absorbed, necessitating the use of optimized metal grating thicknesses for filtering applications. Conversely, electrical signals can be induced across the fingers as surface acoustic waves propagate through the piezoelectric substrate material beneath the transducer.

[0004] SAW devices typically use wafers fabricated from monolithic crystals of quartz, LiNbO3, or LiTaO3 as the piezoelectric material. However, depending on the piezoelectric material used, the use of a piezoelectric substrate results in either high temperature sensitivity in the case of LiNbO3 or LiTaO3, or poor electromechanical coupling in the case of quartz.

[0005] Furthermore, acoustic wave velocities are generally limited by the material properties of single crystals, especially considering the phase velocity, which in most cases remains at 3000-4000 m / s. In fact, for quartz, Rayleigh surface acoustic waves are the most commonly used mode, and their phase velocity is 3000-3500 m / s. -1 Using shear waves, up to 5100 m.s -1 In quartz, the coupling is only 0.5%. In lithium tantalate, the Rayleigh wave has a phase velocity of 3000-3500 m.s. -1 The Rayleigh waves on lithium niobate exhibit phase velocities in the range of 3900 m.s. -1 and has a coupling coefficient of 5-6%, but 8% can be achieved by using a SiO2 passivation layer on top of the IDT.

[0006] Shear waves on LiTaO3 and LiNbO3, also called quasi-modes, exhibit radiation leakage, so-called leaky modes. In this case, the surface partially guides these waves. Therefore, the electrode grating plays a major role in capturing the energy near the surface. The phase velocity is 4000-4500 m.s for both materials. -1 The range is.

[0007] Finally, compressional modes can also be excited along certain crystal cuts on LiTaO3 and LiNbO3 substrates, but again the modes are inherently leaky and therefore require a specific electrode thickness for frequency to minimize leakage effects due to wave radiation into the bulk.

[0008] One approach to overcoming the leakage effect has been to use composite substrates, which comprise a piezoelectric layer formed on a base substrate. Composite substrates offer a wide range of materials for the base substrate, allowing the selection of base substrate materials with high acoustic wave propagation velocities, such as diamond, sapphire, silicon carbide, or silicon. As in optical systems, the use of such base substrates results in mode guidance.

[0009] The composite substrate has a strong electromechanical coupling, i.e., an electromechanical coefficient k greater than 1%. s 2 This can be combined with temperature stability, ie, a temperature coefficient of frequency (TCF) of less than 20 ppm / K, to improve the performance of SAW devices and provide design flexibility.

[0010] However, acoustic wave devices are limited to operating frequencies up to approximately 1–3 GHz. This is because, for a given phase velocity, the electrode pitch or mechanical period p of the interdigital transducer determines the wavelength λ of the acoustic wave, which is given by the equation p = λ / n (n ≥ 2, typically 2). Operation at frequencies above 2 GHz requires metal dimensions and thicknesses on the order of 100 nm or less, which poses structural stability problems. Therefore, in practice, it is difficult to further miniaturize interdigital transducers when higher operating frequencies are required. This is due, on the one hand, to the need to use higher-resolution lithography techniques compared to the I-line lithography currently used in the SAW industry, and, on the other hand, to electrical losses that arise in the structure.

[0011] Therefore, a significant technological effort is required to create SAW devices above 3 GHz. Summary of the Invention

[0012] It is therefore an object of the present invention to overcome the above-mentioned drawbacks by providing an interdigitated transducer structure for an acoustic wave device with improved parameters, and to provide an acoustic wave device that can function at frequencies above 3 GHz yet still be manufacturable using standard I-line lithography.

[0013] The object of the invention is achieved by a transducer structure for an acoustic wave device comprising a piezoelectric layer and a pair of interdigitated electrodes comprising a plurality of electrode means having a pitch p, the electrode means of the interdigitated electrodes being embedded in the piezoelectric layer and the acoustic impedance of the electrode means being less than the acoustic impedance of the piezoelectric layer. According to one variant, the interdigitated electrodes can be entirely embedded.

[0014] In the inventive device, because the electrode means are embedded in the piezoelectric layer and the acoustic impedance of the electrode means is smaller than the acoustic impedance of the piezoelectric layer, shear wave-like propagation modes (electrode modes) can be excited that are essentially confined to the volume of the electrode means. In practice, the boundary conditions are such that they allow the excitation of shear modes within the electrode means. Due to the difference in acoustic impedance, the reflectivity at the side edges of the electrode means is large enough to essentially confine the energy within the electrodes. Nevertheless, due to the grating configuration of the interdigitated electrodes, some vibrations are induced within the piezoelectric layer in the presence of alternating electrical polarities, which leads to coherence between the vibrations of the electrodes, which in turn leads to a resonance phenomenon with phase oscillations from one electrode to the next opposite adjacent electrode. The operating acoustic wavelength λ of the transducer is proportional to the resonant frequency f of the transducer structure. r and at this time f r = v / 2p = v / λ, where v is the acoustic wave propagation velocity in the acoustic wave propagation substrate. Therefore, for this given geometry, for example, 10,000 ms -1 Equivalent phase velocities much higher than those mentioned above, of the order of magnitude of 100 Hz, can be observed. Since the bulk acoustic waves within the electrode means involve resonances at higher frequencies compared to the waves guided in the piezoelectric layer of prior art arrangements such as those described above, this mode enables the transducer structure to function at frequencies above 3 GHz, higher than prior art transducer structures, without being limited by I-line lithography fabrication techniques.

[0015] In particular, the ratio of the acoustic impedances between the electrode means and the piezoelectric layer is preferably less than 0.5. Material combinations that yield an electromechanical coupling of more than 2%, preferably more than 3%, are advantageous for the establishment of modes within the electrodes and for coherent coupling between the electrodes.

[0016] According to one variant, the pitch p satisfies the Bragg condition given by p=λ / 2, where λ is the operating acoustic wavelength of the transducer. In this situation, the electrode modes are excited more efficiently.

[0017] According to a variant of the invention, the metallization ratio a / p of the electrode means, where "a" is the width and "p" is the pitch, may be comprised between 0.3 and 0.75, in particular between 0.4 and 0.65. Using an a / p ratio in this range is advantageous for the formation of bulk acoustic waves excited in the electrode means, reducing or suppressing the contribution of the acoustic surface modes of the piezoelectric layer.

[0018] According to a variant of the invention, the piezoelectric layer can be provided on a base substrate, the use of which is advantageous for confining the energy near the surface, and in particular within the electrode means.

[0019] According to one variant, the transducer structure may further comprise an attachment layer, in particular silicon dioxide (SiO2), between the piezoelectric layer and the base substrate. To optimize the transducer structure, various materials can be used for the piezoelectric layer and the underlying base substrate via the attachment layer. Silicon dioxide can be used to improve the temperature coefficient of frequency (TCF).

[0020] According to one variant, the transducer structure may further comprise a high velocity layer between the piezoelectric layer and the base substrate, the high velocity layer being made of a material that allows a higher shear wave phase velocity than the material and crystallographic orientation of the piezoelectric layer, which allows the fundamental shear mode to accelerate, but whose phase velocity may be greater than the slow shear bulk wave velocity of the base substrate, also called surface skimming bulk wave (SSBW), in which case the fundamental shear mode cannot be guided in the piezoelectric layer but will diffuse in the base substrate.

[0021] According to one variation, the high velocity layer can be disposed between the attachment layer and the base substrate. By disposing the attachment layer between the piezoelectric layer and the high velocity layer, acceleration properties can be exploited without having to change the attachment process of the piezoelectric layer to the underlying structure, resulting in a high quality piezoelectric layer independent of the material choice for the high velocity layer.

[0022] According to a variant, the transducer structure may further comprise a trap rich layer, in particular a polysilicon trap rich layer, between the piezoelectric layer and the base substrate. The trap rich layer makes it possible to suppress leakage currents.

[0023] According to one variation, the trap rich layer may be arranged between the high velocity layer and the base substrate, in such an order that the individual benefits of the various layers may be maintained in the overall structure.

[0024] According to one variant, the transducer structure may further comprise a cover layer over the embedded electrode means and the piezoelectric layer, which further improves the guidance of shear modes in the electrodes and further reduces the possibility of electromigration.

[0025] According to one variant, the cover layer may be made of a material and / or have a crystal orientation that allows a higher phase velocity of the shear waves than the material and / or crystal orientation of the piezoelectric layer. As a result, the material of the cover layer can be selected such that the velocity of the fundamental shear mode is accelerated compared to its velocity in the piezoelectric layer. This promotes diffusion into the volume of the base substrate, so that essentially only the electrode modes remain within the guided domain of the piezoelectric layer.

[0026] According to a variant of the invention, the transducer structure may further comprise a Bragg mirror below the piezoelectric layer and / or below the embedded electrode means, which reduces energy losses to the base substrate and can provide mechanical stability to the device structure.

[0027] According to a variant of the invention, the thickness of the embedded electrode means can be equal to or less than the thickness of the piezoelectric layer, which allows for the use of thicker electrodes compared to conventional transducer structures, thereby allowing the establishment of shear modes in the electrodes, increasing their stability and reducing resistive losses.

[0028] According to a variant of the invention, the thickness t of the electrode means e is λ>t e >0,1*λ. Within this thickness range, the electromechanical field near the surface can be concentrated within a region comparable in thickness to the electrode thickness in the case of single crystals, or the piezoelectric layer plus the electrode thickness in the case of composite wafers. This results in improved electromechanical coupling and spectral purity, allowing for a single, or at least limited, modal contribution to the device response.

[0029] According to a variant of the invention, the acoustic impedance of the base substrate of the composite substrate is of the order of magnitude of the acoustic impedance of the piezoelectric layer, in particular within plus or minus 25% of the acoustic impedance of the piezoelectric layer, more particularly within plus or minus 15% of the acoustic impedance of the piezoelectric layer. In particular, when the electrodes have the same thickness as the piezoelectric layer, the impedance matching makes it possible to confine the shear modes within the electrodes.

[0030] According to one variant, the embedded electrodes can be filled into grooves in the piezoelectric layer. The grooves can have a cross section with a pyramidal or trapezoidal, or a V- or U-shape, and / or the side walls and / or the bottom of the groove have a convex, concave or scalloped shape. In particular, pyramidal shapes or trapezoidal shapes with the shorter of their parallel sides at the surface of the transducer structure provide an improved quality factor compared to grooves with vertical walls.

[0031] According to a variant of the invention, a dielectric layer can be provided at the bottom of the groove. According to a variant of the invention, the side walls and bottom wall of the groove can be covered with a conductive material, and the remaining part of the groove can be charged with a dielectric material. According to a variant of the invention, the groove can extend into the piezoelectric layer, and the side walls of the groove can be covered with a conductive material, and the remaining part of the groove can be charged with a dielectric material. According to a variant of the invention, only the side walls facing the piezoelectric layer can be covered with a conductive material. According to a variant of the invention, the dielectric material can be a material that has a higher shear wave phase velocity than the conductive material. As already explained, this makes it possible to accelerate the phase velocity of the fundamental shear wave so that it can exceed the SSBW velocity, thereby enabling its diffusion within the base substrate.

[0032] According to a variant of the invention, the dielectric material may have a temperature coefficient frequency that is opposite in sign to that of the conductive material, thus allowing the device to be used over a wider temperature range.

[0033] According to a variant of the invention, the dielectric material of the cover layer and the dielectric material filled in the trenches can be the same, thus achieving both advantageous features in one process step.

[0034] According to one variant, the material of the electrode means can be made of a material lighter than manganese, in particular aluminum or an aluminum alloy containing Cu, Si or Ti. In particular, the combination of aluminum and lithium tantalate results in a coupling coefficient of more than 3%, with no fundamental shear mode present in the piezoelectric layer.

[0035] According to one variant, the piezoelectric layer can be lithium tantalate (LiTaO3) or lithium niobate (LiNbO3). With either material, composite substrates, in particular so-called piezo-on-insulator substrates (POI), can be obtained on an industrial scale.

[0036] According to one variant, the base substrate can be silica, silicon dioxide, glass quartz, fused silica, glass, LiTaO3, LiNbO3, or silicon, in particular Si(111). Using these substrates, the fundamental shear mode in the piezoelectric layer can be suppressed, while the shear mode within the electrode means remains. On Si(111), the SSBW velocity is particularly low compared to Si(100). At the same time, piezoelectric layers on glass or SiO2 substrates can be obtained on an industrial scale, for example, by using piezoelectric-on-insulator (POI) substrates.

[0037] According to one variation, the high-velocity layer is one of AlN, Al2O3, Si3N4, SiC, or carbon-based materials, particularly single-crystal diamond, amorphous carbide, and nanoparticle polycrystalline diamond. According to one variation, the cover layer can be one of AlN, Al2O3, Si3N4, SiC, or carbon-based materials, particularly single-crystal diamond, amorphous carbide, and nanoparticle polycrystalline diamond. According to one variation, the dielectric material can be carbon-based materials, particularly single-crystal diamond, amorphous carbide, nanoparticle polycrystalline diamond, AlN, or SiO2. These materials allow for a reduction in fundamental shear mode contribution. The use of SiO2 can improve TCF characteristics.

[0038] According to one variant, the pair of interdigitated comb electrodes comprises one or more regions in which two or more adjacent electrode means belong to the same comb electrode, the distance between them being the same as that of adjacent electrode means belonging to different comb electrodes, said distance being the edge-to-edge distance corresponding to the pitch p as defined above. According to one variant, two or more adjacent electrode means belonging to the same comb electrode have the same geometry as adjacent electrode means belonging to different comb electrodes. Linking adjacent electrode means to the same potential but still having the same mechanical periodicity removes the acoustic wave source from the system, resulting in a reduction in the electromechanical coupling. This can be used to adjust the electromechanical coupling and thus the transfer function of the filter during construction, by fine-tuning the width of the bandpass.

[0039] According to one variant, regions with two or more, in particular at least three, adjacent electrode means can belong to the interdigitated electrodes, which are not periodically distributed, in particular randomly distributed. They are characterized in particular in that the distances of adjacent regions relative to one another across the extent of the transducer structure are different. By reducing the symmetry of the system, spurious contributions of higher order periodicities can be reduced or even suppressed.

[0040] According to a variant, the regions with two or more adjacent electrode means belonging to the same interdigital electrode may have a different number of adjacent electrode means belonging to the same interdigital electrode. By connecting the regions with a different number of adjacent electrode means to the same potential, spurious contributions can be further reduced.

[0041] According to one variant, the electrode means of the transducer structure may have dimensions realizable by I-line lithography, in particular a width greater than 350 nm, thus making it possible to manufacture devices usable at frequencies above 3 GHz using cheaper lithographic means compared to lithographic tools using 248 nm or 193 nm or even shorter wavelengths.

[0042] The objectives of the present invention are also achieved by an acoustic wave device comprising at least one transducer structure, particularly an acoustic wave resonator and / or an acoustic wave filter and / or an acoustic wave sensor, as described above. This acoustic wave device, capable of operating at frequencies above 3 GHz, can be fabricated using I-line lithography techniques without the need for more advanced and therefore more expensive lithography tools. As a result, compared to Rayleigh surface waves using SAW devices, whose frequencies are limited to 2 GHz at most, the use of electrode shear wave modes allows for an extension of the frequency range without the need to change patterning techniques. The use of composite substrates allows for a reduced first-order temperature coefficient of frequency (TCF) of less than 20 ppm / K to be achieved, resulting in temperature-stable device performance. Using the present invention, acoustic wave bandpass filters with relative bandwidths greater than 5% or even 10% and up to 15% can be realized.

[0043] According to one variant, the elastic device may further comprise radio frequency (RF) supply means configured to drive the transducer structure with an RF signal above 3 GHz. In this way, I-line lithography can be used to realize devices that function above 3 GHz.

[0044] According to one variant, the acoustic wave device may comprise input and output transducer structures as already described.

[0045] The object of the present invention is also achieved by a method of using such a transducer structure, which includes the step of applying an alternating potential to two interdigitated electrodes to excite shear modes that occur primarily or exclusively in the electrode means compared to the piezoelectric layer and have a higher equivalent velocity than the fundamental shear wave mode of the piezoelectric layer. At least the vibration amplitude in the electrode means is greater than in the piezoelectric layer. The use of higher frequencies of the shear waves propagating in the electrode means of such a transducer structure, especially for ladder filters and / or impedance filters and / or coupled filters, leads to devices that function above 3 GHz, more particularly above 3.5 GHz. At the same time, I-line lithography can be used to form the device.

[0046] The object of the invention is also achieved by a method of using such a transducer structure, which comprises the step of applying an alternating potential to two interdigitated electrodes to excite in the electrode means a shear mode having a pair of neutral lines which exhibits a shear motion inside the electrodes and has an equivalent velocity no higher than that of the fundamental shear wave mode of the piezoelectric layer. The use of this mode makes it possible to exploit higher resonant frequencies so as to realize devices such as ladder filters and / or impedance filters and / or coupled filters operating above 3 GHz, in particular above 3.5 GHz, while using I-line lithography to form the device pattern.

[0047] The present invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which features of the invention are identified by reference characters. [Brief explanation of the drawings]

[0048] [Figure 1] 1A and 1B illustrate an interdigitated transducer structure on a composite substrate for an acoustic wave device according to a first embodiment of the present invention. [Figure 2]FIG. 2 shows a schematic representation of modes obtained using the transducer structure shown in FIG. 1; [Figure 3a] FIG. 10 shows the broadband harmonic admittance of a simulated excited mode according to a first example embodiment of the present invention. [Figure 3b] FIG. 10 is a mesh diagram showing the observed modes of vibration. [Figure 3c] FIG. 10 is a mesh diagram showing vibrations of modes with opposing phases. [Figure 3d] FIG. 3b is a close-up view of the broadband harmonic admittance of the simulated excited mode shown in FIG. 3a. [Figure 3e] FIG. 10 shows the conductance and resistance of the excitation mode. [Figure 3f] FIG. 10 is a diagram showing dispersion characteristics of an excitation mode. [Figure 3g] FIG. 10 is a diagram showing an internal modification of another example according to the first embodiment. [Figure 3h] FIG. 10 is a diagram showing an internal modification of another example according to the first embodiment. [Figure 4] 1A and 1B show three different electrode shapes in the piezoelectric layer, (a) according to the second embodiment, (b) according to the first embodiment, and (c) according to the third embodiment. [Figure 4d] FIG. 10 shows the harmonic susceptance corresponding to three different electrode geometries. [Figure 4e] FIG. 10 shows harmonic conductance corresponding to three different electrode geometries. [Figure 4f] 10A-10C show further variations in electrode shapes with concave sidewalls. [Figure 4g] 10A-10C show further variations in electrode shapes with convex sidewalls. [Figure 4h] 10A-10C show further variations in electrode shapes with scalloped sidewalls. [Figure 4i] 10A and 10B show further variations in electrode geometry with a dielectric layer in the groove. [Figure 4j] 10A and 10B show further variations in electrode geometry with a dielectric layer in the groove. [Figure 5] FIG. 10 illustrates the effect of electrode thickness on the occurrence of shear modes confined within the electrode. [Figure 6] FIG. 10 shows an interdigitated transducer structure for an acoustic wave device according to a fourth embodiment of the present invention. [Figure 7a] FIG. 10 illustrates an interdigitated transducer structure for an acoustic wave device according to a fifth embodiment of the present invention. [Figure 7b] FIG. 10 shows the broadband harmonic admittance of a simulated excited mode according to a fifth embodiment of the present invention. [Figure 7c] FIG. 10 shows the broadband harmonic admittance of a simulated excited mode according to a variant of the fifth embodiment of the present invention. [Figure 7d] FIG. 10 shows the broadband harmonic admittance of a simulated excited mode according to a variant of the fifth embodiment of the present invention. [Figure 8] FIG. 10 is a diagram showing a sixth embodiment of the converter structure according to any one of the first to fifth embodiments of the present invention. [Figure 9a] FIG. 10 is a diagram showing a seventh embodiment of the present invention, relating to a 5 GHz filter. [Figure 9b] FIG. 9b shows the transfer function of the filter according to FIG. 9a; [Figure 10a] FIG. 1 illustrates higher order modes according to the present invention. [Figure 10b] Figure 10 shows the harmonic conductance and resistance of this mode for LiTaO3 and SiO2 substrates. [Figure 10c] Figure 10 shows the harmonic conductance and resistance of this mode for Al2O3 and Si substrates. [Figure 11] FIG. 10 shows an eighth embodiment of the present invention having a suppressed acoustic wave source. [Figure 12a] 13 shows a variation of the ninth embodiment of the present invention. [Figure 12b] 13 shows another variation of the ninth embodiment of the present invention. [Figure 12c] 13 shows yet another variation of the ninth embodiment of the present invention. [Figure 13a] FIG. 12b shows part of the process for obtaining the electrode shape shown in FIG. 12a. [Figure 13b] FIG. 12b shows part of the process for obtaining the electrode shape shown in FIG. 12a. [Figure 13c] FIG. 12b shows part of the process for obtaining the electrode shape shown in FIG. 12a. [Figure 13d] FIG. 12b shows part of the process for obtaining the electrode shape shown in FIG. 12a. [Figure 14a] FIG. 13 shows the admittance and impedance of a transducer structure according to a ninth embodiment when using diamond-like carbon as the dielectric. [Figure 14b] FIG. 13 shows the admittance and impedance of a transducer structure according to a ninth embodiment when using diamond-like carbon as the dielectric. [Figure 14c] FIG. 13 is a mesh diagram showing vibration modes observed in a transducer structure according to a ninth embodiment. [Figure 15a] FIG. 13 shows the admittance and impedance of a transducer structure according to a ninth embodiment when using aluminum nitride as the dielectric. [Figure 15b] FIG. 13 shows the admittance and impedance of a transducer structure according to a ninth embodiment when using aluminum nitride as the dielectric. [Figure 16a] FIG. 10 shows the conductance G and resistance R of the transducer structure according to the ninth embodiment when using silicon dioxide as the dielectric. [Figure 16b] FIG. 1 is an enlarged view of the resonance. [Figure 16c] FIG. 1 is an enlarged view of an anti-resonance. [Figure 16d] FIG. 13 shows a fourth modified example of the converter according to the ninth embodiment. [Figure 17a]FIG. 23 is a diagram showing a modified example of the transducer structure according to the tenth embodiment. [Figure 17b] FIG. 23 is a diagram showing another modified example of the transducer structure according to the tenth embodiment. [Figure 18a] 1 is an image taken by an electron microscope showing an example of a transducer according to the present invention. [Figure 18b] 10 is another image taken by an electron microscope showing an example of a transducer according to the present invention. [Figure 18c] FIG. 18 shows the finite element mesh used to simulate the behavioral example shown in FIGS. 18a and 18b. [Figure 19a] FIG. 10 shows experimental measurements of susceptance and resistance for the above example. [Figure 19b] FIG. 18c shows susceptance and resistance results obtained by numerical simulation of a structure such as that shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0049] The invention will now be described in more detail using advantageous embodiments, by way of example, with reference to the drawings. It should be noted that the described embodiments are merely possible configurations according to the invention, and that the individual features as described above can be provided independently of one another or in combination to realize further embodiments according to the invention.

[0050] FIG. 1 shows an interdigitated transducer structure for an acoustic wave device according to a first embodiment of the present invention.

[0051] The transducer structure 100 comprises an acoustic wave propagating substrate 102. The acoustic wave propagating substrate may be a composite substrate 102 comprising a piezoelectric layer 104 formed on a base substrate 106. The composite substrate may be a so-called piezoelectric-on-insulator substrate or POI substrate. In other embodiments, the piezoelectric layer may be thick enough to form a bulk material.

[0052] The piezoelectric layer 104 described herein as an example may be lithium niobate (LiNbO3) or lithium tantalate (LiTaO3). The piezoelectric material layer 104 may be attached to the base substrate 106 by direct bonding, for example using the SmartCut® layer transfer technique. The thickness of the piezoelectric layer 104 formed on the base substrate 106 may be on the order of one wavelength λ or less, in particular about 2 μm or less, in particular less than 0.5λ, and even more in particular less than 0.4λ.

[0053] The piezoelectric layer described as an example in this specification may be lithium niobate (LiNbO3), in particular LiNbO3 with a crystal orientation defined in accordance with the standard IEEE 1949 Std-176 as (YXl) / θ, where 36°<θ<52° or 60°<θ<68° or 120°<θ<140°, and as (YXt) / Ψ, where 85°<Ψ<95°, and as (YXwlt) / ΦθΨ, where Φ=90°, -30°<θ<+45°, and 0°<Ψ<45°, or lithium tantalate (LiTaO3), in particular LiTaO3 with a crystal orientation defined in accordance with the standard IEEE 1949 Std-176 as (YXl) / θ, where 36°<θ<52°, and more particularly LiTaO3 with a crystal orientation defined in accordance with the standard IEEE 1949 Std-176 as (YXl) / θ, where 36°<θ<52°, It may be 42° Y-cut, X-propagating LiTaO3, defined as (YXl) / 42° cut according to Std-176.

[0054] As already mentioned, according to one variant, the thickness of the piezoelectric layer 104 can be greater than the wavelength λ, so that a piezoelectric layer thickness equivalent to that of bulk piezoelectric material can also be used.

[0055] The base substrate 106 used in the first embodiment of the present invention is a silica substrate, silica, quartz, fused silica, or glass. In this type of substrate, the slow shear bulk wave velocity (SSBW) is inferior to the SSBW of the fundamental elastic bulk shear mode in the piezoelectric layer, and therefore the bulk shear mode of the piezoelectric layer is suppressed by radiation and diffusion from the surface to the bulk.

[0056] Other substrates with high acoustic wave propagation velocities of 4500 m / s or greater, such as silicon, diamond, sapphire, silicon carbide, silicon nitride, or aluminum nitride, may also be used, in which case there may be a fundamental induced shear mode corresponding to the mode excited when electrodes are placed on the piezoelectric layer.

[0057] In this embodiment, the thickness of the base substrate 106 is greater than the thickness of the piezoelectric layer 104. A preferred situation corresponds to a base substrate thickness that is at least 10 times greater than the thickness of the piezoelectric layer 104, in particular 50 to 100 times greater.

[0058] Furthermore, the acoustic impedance of the base substrate 106 of the composite substrate 102 is of the order of magnitude of the acoustic impedance of the piezoelectric layer, in particular within a range of plus / minus 25%, more particularly within a range of plus / minus 15%. According to one variant, they are the same.

[0059] In a variant of the present invention, the base substrate 106 may further comprise a trap rich layer near the top layer of piezoelectric material. Such a trap rich layer may improve the insulating performance of the base substrate 106 and may be formed by at least one of polycrystalline, amorphous, or porous materials, such as polycrystalline silicon, amorphous silicon, or porous silicon. By the term "trap rich" is understood a layer that can absorb electric charges but does not form a conductive layer.

[0060] Composite substrates can be obtained by layer transfer methods, in which a piezoelectric layer is transferred to a base substrate. Methods such as bonding and thinning or layer transfer methods such as Smart-Cut®, which allow the transfer of subwavelength layers, can be used. Such composite substrates may also comprise additional layers or stacks of layers, such as bonding or attachment layers, in particular SiO2, or other functional layers, such as trap rich, Bragg mirror, low-speed / high-speed stacks, etc.

[0061] In one variant, the base substrate 106 may be a silicon-on-insulator (SOI) substrate obtained by molecular bonding using, for example, an intermediate SiO (bonding) layer and a means for transferring a silicon layer, such as the Smart-Cut® process mentioned above.

[0062] The transducer structure 100 further comprises a pair of opposing interdigitated comb-shaped electrodes 108 and 110, each of which has a plurality of interdigitated electrode means 112_i and 114_j (here 1<=1, j<=4) extending from their corresponding conductive portions 116 and 118. The comb-shaped electrodes 108 and 110, and in particular the electrode means 112_i, 114_j, are formed of any suitable conductive metal, for example pure aluminum or alloys such as Al doped with Cu, Si or Ti, as long as their acoustic impedance is lower than that of the piezoelectric layer 104. Generally, electrode materials lighter than manganese, and therefore lighter than manganese, including chromium, are suitable. According to this embodiment, the aspect ratio a / p of the electrode means 112_i, 114_j, where a is the width and p is the pitch, is comprised between 0.3 and 0.75, in particular between 0.4 and 0.65. The metallization or aspect ratio a / p of the electrode means 112_i, 114_j and the thickness t e is a parameter for controlling radiation losses and electromechanical coupling in the device.

[0063] An electrical load 120 is shown coupled across the interdigitated electrodes 108, 110. However, it will be understood that a power supply potential 120 may be coupled across the electrodes 108, 110 depending on whether the transducer 100 is utilized to excite acoustic waves in the substrate 102, or to convert received acoustic waves into electrical signals, or both.

[0064] The electrode means, e.g., 112_1 to 112_4 and 114_1 to 114_4, are interdigitated and connected to alternating potentials via their corresponding interdigitated electrodes 108 and 110. The alternating potentials can be +V and -V as shown, or mass and load / power potentials.

[0065] In this embodiment, the electrode means 112_i, 114_j all have the same length l, width a, and thickness t e According to a variant of the invention, the electrode means 112_i, 114_j may also have different lengths l, and / or widths a, and / or thicknesses t e It may also have

[0066] The electrode means 112_i and 114_j and their corresponding interdigital electrodes 108 and 110 are arranged in the same plane. According to a variant, the electrode means 112_i and 114_j are embedded in the piezoelectric layer 104, and the interdigital electrodes 108, 110 providing the electrical connection between the electrode means may be placed on top of the piezoelectric layer 104.

[0067] Furthermore, for the transducer structure 100, an electrode pitch p is used, defined as λ / 2, where λ is the operating wavelength of the acoustic wave, corresponding to the Bragg condition. The electrode pitch p corresponds to the distance between two adjacent electrode means from the opposing interdigital electrodes 108 and 110, e.g., between 112_3 and 114_3. In this case, the wavelength λ corresponds to the distance between two adjacent electrode means from the same interdigital electrode 108 or 110, e.g., between 112_3 and 112_4. In such a Bragg condition, the transducer operates at an operating frequency f r The transducer is said to operate in a tuned mode at frequency f, where all acoustic waves excited within the transducer structure are coherent and in phase. The electrode pitch p therefore determines the frequency of use of the transducer structure. r is fixed by the condition of phase matching, given by v / 2p, where v is the effective phase velocity of the acoustic wave propagating in the transducer structure 100 and p is the electrode pitch of the transducer structure 100.

[0068] The electrode means 112_i, 114_j are embedded in the piezoelectric layer 104 and have a thickness t e is preferably completely embedded in the piezoelectric layer 104 so that the thickness t of the piezoelectric layer 104 is equal to or less than the thickness t of the piezoelectric layer 104.

[0069] The thickness of the electrode means 112_i to 114_j is set to satisfy the following relationship with respect to the wavelength λ: <t e / λ<1.

[0070] Therefore, the electrode means 112_i to 114_j are thicker than the electrodes of prior art interdigitated transducer structures in which interdigitated electrodes are formed on a piezoelectric substrate. They therefore provide better stability and reduced electrical losses. Furthermore, power handling is improved because the buried electrodes limit the possibility of acousto-migration and electromigration due to power effects. The metal is placed in the grooves, which prevents direct metal contact due to surface diffusion and metal migration.

[0071] The conductive portions 116, 118 of the interdigital electrodes 108, 110 may be provided on the piezoelectric layer 104 and / or on the embedded electrode means 112_i, 114_j.

[0072] The charge distribution on the electrodes 108 and 110 of the transducer structure 100 excites acoustic waves in the electric field direction as indicated by arrow E in FIG. 1, which means perpendicular to the extension direction z of the electrode means 112_i, 114_j of the electrodes 108, 110.

[0073] In prior art SAW devices, various modes may exist, such as Rayleigh surface acoustic waves, Lamb waves, or shear waves. In contrast, the inventive transducer structure 100 enables new modes that result in shear-like modes that are primarily localized within the electrodes, hereafter referred to as electrode modes.

[0074] This is because the electrode means are embedded in the piezoelectric layer and the acoustic impedance of the electrode means is smaller than the acoustic impedance of the piezoelectric layer. In practice, the boundary conditions are such that they allow the excitation of shear modes within the electrode means. The maximum vibration occurs in the centre of the electrode and changes sign from one electrode means to the next adjacent electrode means.

[0075] Due to the difference in acoustic impedance, the reflectivity at the side edges of the electrode means is large enough to essentially confine the energy within the electrodes. Nevertheless, due to the grating configuration of the interdigitated electrodes, some vibrations are induced in the piezoelectric layer in the presence of alternating electrical polarities, which leads to coherence between the electrode vibrations, which leads to resonance phenomena, but still allows for guidance of modes.

[0076] The operating acoustic wavelength λ of the transducer is the resonant frequency f of the transducer structure. r and at this time f r = v / 2p = v / λ, where v is the acoustic wave propagation velocity in the acoustic wave propagation substrate.

[0077] This mode is shown diagrammatically in Figure 2, which shows a top view of the transducer structure 100a and two adjacent electrode means 112_3 and 114_3. Numerical simulations have been carried out with reference to S. Ballandras et al, Finite-element analysis of periodic piezoelectric transducers Journal of Applied Physics 93, 702 (2003); https: / / doi.org / 10.1063 / 1.1524711, to demonstrate the excitation of this mode, where the shear motion is localized within the electrode means. In fact, shear motions are also present within the piezoelectric layer 104, but with vibrations of a larger magnitude compared to the magnitude of the vibrations in the piezoelectric layer 104, occurring in alternating directions from one electrode to the other.

[0078] FIG. 3a shows the broadband harmonic admittance of a simulated excited mode according to a first example of the first embodiment of the present invention.

[0079] In this embodiment, the transducer structure comprises Al-Cu electrodes embedded in a LiTaO3 piezoelectric layer on a silica substrate. The wavelength λ of the transducer structure is equal to 2.8 μm, hence p=1.4 μm, and the aspect ratio a / p of the electrodes is equal to 0.43. In this embodiment, as shown in FIG. 1, the electrodes 112_i, 114_j are embedded in the piezoelectric layer 104 and represent a continuous material from a geometrical point of view, but the physical properties of the resulting layer are periodically distributed.

[0080] As can be seen from the results below, the unique feature of this mode is the equivalent phase velocity of approximately 9850 m.s -1 This is similar to a leaky SAW that propagates at a speed of typically about 3750 m.s. -1 is higher than the surface skimming bulk wave (SSBW) in silicon dioxide. In fact, considering a metallization ratio a / p of about 0.43 and a mechanical period of 2.8 μm for a structure with a resonant frequency of 3.45 GHz, it is about 9850 m.s -1 An equivalent velocity of 1000 Hz is achieved. The velocity of the mode is given by the product of the resonant frequency 3.45 GHz and the electrical period 2.8 μm. The velocity of the mode is calculated as the sum of the frequencies at the start and end of the stop band multiplied by the mechanical period. See further below.

[0081] Figure 3a shows simulation data obtained for a configuration in which the shear mode of the electrodes is excited, as previously shown in Figure 2. The graph represents the conductance G in S on the left Y-axis and the susceptance B in S on the right Y-axis as a function of frequency range in MHz on the X-axis, for both the G and B harmonics. As can be seen, a broadband harmonic admittance is observed, with a shear resonance at 3.45 GHz. The shear resonance at 3.45 GHz is the dominant excitation mode. Two additional, much smaller contributions are also observed at approximately 1.5 GHz and 7 GHz.

[0082] Figures 3b and 3c show the internal deformation of the transducer structure: the movement is concentrated only in the area of ​​the interdigitated electrodes 108, 110. Almost no vibration in the piezoelectric layer 104 was visible in the simulation.

[0083] In accordance with the present invention, the above parameters were such that shear bulk acoustic modes were excited primarily within the metal electrode means 112_i, 114_j, polarized in the y direction, and displaced in the z direction. The useful E-field extends along the x direction. In this case, the vibrations are primarily located within the electrodes, with deformation occurring near the fundamental shear bulk wave of the electrodes themselves. Shear motions are also observed within the piezoelectric layer 104, but the largest amplitude vibrations occur in alternating vibration directions in the electrodes 112_i, 114_j.

[0084] The shear displacement direction alternates from one electrode to another when the transducer 100 is excited by the +V / -V electric polarization structure. This electrode phase opposition increases charge accumulation at the electrode edges, resulting in enhanced excitation of bulk modes in the electrodes 112_i, 114_j. Because the piezoelectric layer 104 confines stress to the electrode edges and the transducer structure operates in the Bragg condition, vibration coherence occurs when the boundary conditions along the grating are satisfied. While shear motion is also present within the piezoelectric layer 104, the largest amplitude vibrations occur in the alternating vibration directions from one electrode to the other.

[0085] The dominant acoustic waves propagating within the transducer structure are in this case bulk shear waves that are essentially confined within the electrodes 112_i, 114_j. The resonant frequency f of the transducer structure 100 is r is f r = V / 2p = V / λ, where v is the acoustic wave propagation velocity in the acoustic wave propagation substrate and λ is the operating acoustic wavelength of the transducer.

[0086] Because the bulk acoustic waves in the electrode means have a higher frequency resonance than the fundamental shear mode guided waves in the piezoelectric layer, the transducer structure can function at higher frequencies, particularly above 3 GHz. Higher frequencies can therefore be utilized than in prior art devices, without the need to use more sophisticated lithography tools than those currently used in the SAW industry, particularly I-line lithography steppers, to move to smaller features.

[0087] A key aspect of the present invention is therefore that this type of structure allows the excitation of shear modes located primarily within the electrodes, similar to the shear bulk modes confined within the electrode means, as explained above. The electrode modes may qualify as spurious or leaky modes, i.e., confined with reduced but not completely suppressed leakage, resulting in tuning between the electrodes. This mode exhibits an equivalent velocity much greater than that achievable with prior art interdigitated transducer structures on composite substrates that utilize shear modes within the piezoelectric layer. Other modes are much less intense or suppressed, primarily due to the use of a silica substrate, which has a low SSBW velocity and therefore allows for the diffusion of standard shear modes.

[0088] This inventive configuration can be understood as a network or grating of individual resonators, where the resonators are coupled via a piezoelectric layer, thus preventing at least a large portion of diffusion into the substrate, even though the phase velocity is higher compared to the fundamental shear mode, making it comparable to a leaky mode.

[0089] FIG. 3d shows a zoomed-in view of the broadband harmonic admittance of the simulated excitation mode shown in FIG. 3a according to a first embodiment of the present invention.

[0090] The graph shown in Figure 3d represents conductance in S on the left Y-axis and susceptance in S on the right Y-axis for both the G and B harmonics, over the frequency range between 3400 and 3600 MHz on the X-axis. A resonance at approximately 3475 GHz and an anti-resonance at approximately 3525 GHz can be seen. The resonance and anti-resonance are well separated, resulting in a coupling coefficient of approximately 3%. The resonance occurs at the beginning of the stopband.

[0091] In this particular case, a reflection coefficient of about 9%, a quality factor Q of the resonance of about 500, and a quality factor Q of the anti-resonance of about 1000 are achieved.

[0092] The coupling coefficient can be improved by modifying the characteristics of the transducer structure, such as the aspect ratio a / p, the thickness of the electrodes, and the materials used. In particular, the aspect ratio a / p and thickness of the electrodes allow control of the speed, electromechanical coupling, quality factor or radiation loss, and reflection coefficient.

[0093] FIG. 3e shows the conductance and resistance of the excited mode for a variant of the first embodiment obtained with the same material choice but with an aspect ratio a / p of 0.57.

[0094] Furthermore, this configuration excites shear modes localized within the electrode. The graphs show conductance in S on the left Y-axis and susceptance in S on the right Y-axis for the frequency range between 3450 and 3750 MHz on the X-axis for both the G and R harmonics. A resonance at approximately 3550 GHz and an antiresonance at approximately 3700 GHz are obtained.

[0095] In this case, the increase in the aspect ratio a / p from 0.43 to 0.57 corresponds to a 10 km.s -1 This results in an increase in the equivalent phase velocity of the propagating mode to more than 10%, and an improvement in the coupling coefficient to more than 10%. However, the reflection coefficient is now less than 5%. At resonance the Q factor is still equal to 500, but at antiresonance the Q factor is now equal to 350.

[0096] FIG. 3f shows the dispersion characteristic of an excited mode as shown in FIG. 3a, according to a first embodiment of the present invention. In this case, the admittance is calculated for various normalized wavelengths around a value of 0.5, which corresponds to the edge of the first Brillouin zone, also known as the Bragg condition. For normalized wavelengths smaller than 0.5, the efficiency of excitation of the mode decreases, as can be seen on the curve and at lower frequencies. Taking this into account, by plotting the progression of the maximum admittance module versus frequency and normalized wavelength, a projected 2D representation is obtained, reminiscent of the dispersion curve of any wave propagating in a conventional periodic grating. As can be seen, resonance occurs at the beginning of the stopband.

[0097] The choice of substrate is important when modes other than the shear modes concentrated in the electrodes must be suppressed, or at least are simply weaker than the desired modes. As mentioned above, this condition is achieved when the SSBW velocity in the substrate is lower than the velocity of the fundamental shear mode in the piezoelectric layer. In this case, the fundamental shear mode will penetrate the substrate and its energy will be dissipated.

[0098] Furthermore, the acoustic impedance should be close to that of the piezoelectric layer to favor that mode.

[0099] 4500m.s -1 Other substrates with acoustic wave propagation velocities as high as or greater than 100 kV may still be used, such as silicon, diamond, sapphire, silicon carbide, or aluminum nitride, but in such cases bulk shear modes may exist in addition to the modes of interest localized within the electrodes.

[0100] This is also why the use of composite substrates is not essential: even bulk piezoelectric substrates of lithium tantalate or lithium niobate can be used with electrodes embedded in the surface area, and even in this case electrode modes can be observed.

[0101] Figures 3g and 3h show the internal deformation of another example of the first embodiment, where the shear motion is less concentrated in the area of ​​the interdigitated electrodes 108, 110, but is still the dominant and only one. Vibrations in the piezoelectric layer 104 were barely visible in the simulation.

[0102] In this example of embodiment, the transducer structure comprises an Al-Cu (2% Cu) electrode embedded in a LiTaO3(YX1) / 42° piezoelectric layer on a silica substrate. The wavelength λ of the transducer structure is equal to 2.8 μm, so p=1.4 μm, the aspect ratio a / p of the electrode is equal to 0.5, and the groove depth is t e / λ=20%. In this embodiment, the electrodes 112_i, 114_j are embedded in the piezoelectric layer 104, as in the first example shown in FIG.

[0103] Figure 3h is a diagram of the modes in the xz plane, showing that the electrode modes satisfy the tuning condition f = v / 2p described above, as indicated by reference numeral 130. Figure 3h shows the shear motion within the electrodes 108, 110, characterized by two neutral oscillation points 132 in each electrode 108, 110. Furthermore, the edges 134a, 134b, 136a, 136b of each electrode 108, 110 move in phase with each other. This indicates that within a single electrode 108 or 110, charges of the same sign exist at both interfaces.

[0104] Figures 4(a)-(c) show three variants of the implementation with three different electrode geometries considered for the excitation of the electrode-resonant modes. These figures show cuts in the xy plane of Figure 1 through one electrode means 112-i or 114-i. In all three figures, one mechanical period is represented. The embedded electrodes are filled into the grooves of the piezoelectric layer. For simulation purposes (results shown in Figures 4d and 4f), the grating is assumed to be infinitely long and excited by harmonic +V / -V excitation. The electrodes radiate energy into the bulk substrate from their bottom interface with the bulk substrate.

[0105] The Al electrodes 201a, 201b, 201c are each represented by horizontal crosshatching, the piezoelectric layer 203, here LiTaO3, is represented by diagonal crosshatching, and the bottom layer 205 is represented by vertical crosshatching, which can be either SiO2 or another interface material, or the same material as the piezoelectric layer 203.

[0106] 4(a) shows a second embodiment of the present invention. In this case, the grooves 203 in the piezoelectric layer have a pyramidal or trapezoidal cross section. The electrodes 201a embedded in the grooves therein have a pyramidal or trapezoidal type shape in their cross section according to the second embodiment of the present invention. In this embodiment, the shorter of the parallel sides of the trapezoid is aligned with the top surface of the piezoelectric layer 203.

[0107] The electrode 201b according to the first embodiment shown in FIG. 1 is of the vertical type shown in FIG. 4(b).

[0108] The electrode 201c according to the third embodiment of the invention is filled into a groove having a trapezoidal shape, but in contrast to the second embodiment, the short sides of the parallel sides are located inside the piezoelectric layer 203 rather than on the surface thereof, so that the electrode 201c is of the truncated V-shaped type.

[0109] In this case, the electrodes fill variously shaped grooves inside the piezoelectric layer 203, but behave like a bulk material depending on its thickness.

[0110] Figures 4d and 4e show the simulation results, providing a comparison of the electrode-resonant mode excitation efficiency for the three electrode geometries.

[0111] FIG. 4d shows the harmonic susceptance, and FIG. 4e shows the harmonic conductance.

[0112] The simulation clearly shows that the pyramidal shape is more interesting than the other two, but the coupling efficiency is relatively low.

[0113] Further design options are shown in Figures 4f-4h, which are based on the design shown in Figure 4b, but can be adapted to other embodiments.

[0114] FIG. 4f shows an electrode 201d inside a groove in the piezoelectric layer 203 with sidewalls 207a having a concave shape. FIG. 4g shows an electrode 201e inside a groove in the piezoelectric layer 203 with sidewalls 207b having a convex shape. FIG. 4h shows an electrode 201f inside a groove in the piezoelectric layer 203 with sidewalls 207c having a scalloped shape. Increasing the surface of the sidewalls allows more charges of the same sign to be present at the interface, thereby improving the charge distribution that contributes to the intrinsic characteristics of the mode and, therefore, the intensity of the mode excitation. It can also be noted that the Q factor of the mode, e.g., the capture efficiency, depends on the shape of the electrode. Therefore, to improve the operating conditions of the electrode mode, it is preferable to optimize both the active surface and the aspect ratio.

[0115] Additionally or alternatively, the bottom of the groove may also have a convex or concave or scalloped shape.

[0116] FIG. 4i shows a further embodiment based on the first embodiment shown in FIG. 4b, but which can be adapted to other embodiments and variations. A dielectric layer 211, e.g., SiN, is applied to the bottom 215 of the groove in the piezoelectric layer 203. A conductive layer 213 of the electrode is then applied to fill the groove. Due to the presence of the dielectric layer 211, no charge is present at the bottom of the electrode. Using a dielectric with a higher shear rate than the conductive material can accelerate the fundamental shear mode so that its phase velocity exceeds the SSBW velocity of the bottom layer, resulting in the fundamental shear mode being better absorbed by the bottom layer 205. This phenomenon will be described in more detail further below in connection with FIGS. 7 and 12.

[0117] 4j shows a further variation based on the previous variation, where conductive layer 217 is now bonded to bottom layer 205. Dielectric layer 211 again separates conductive layer 217 from piezoelectric layer 203, so that conductive layer 215 is bonded to the piezoelectric layer only via sidewalls 219 and 221.

[0118] When the conductive material of the electrode is not in contact with the upper and lower piezoelectric materials as shown in Figures 4i and 4j, the phase velocity, resonant Q factor, reflection coefficient, and coupling coefficient k 2 It has been found that parameters such as these can be optimized by comparing them with an equivalent structure lacking such features.

[0119] All the variants described above with respect to FIGS. 4 to 4h can be realized according to the variant shown in FIG. 4j, in which the electrodes are bonded to the bottom layer 205.

[0120] Figure 5 shows the effect of the thickness of electrode means 112_i and 114_j on the shear modes within the electrodes. Here, a harmonic analysis of an aluminum electrode embedded in a (YXl) / 42° lithium tantalate piezoelectric layer on a 325 nm thick silicon dioxide layer on Si(100) is shown for a 1 μm pitch, with a metal / piezoelectric a / p ratio set to 0.35. Figure 5 also shows the effect of the thickness of the electrode means 112_i and 114_j on the shear modes within the electrodes. Here, a harmonic analysis of an aluminum electrode embedded in a (YXl) / 42° lithium tantalate piezoelectric layer on a 325 nm thick silicon dioxide layer on Si(100) is shown for a 1 μm pitch, with a metal / piezoelectric a / p ratio set to 0.35. Figure 5 shows the effect of the thickness of the electrode means 112_i and 114_j on the shear modes within the electrodes. e / λ. t greater than 0.1 e It can be seen that resonance can be observed only starting from the / λ ratio.

[0121] Compared to the other examples shown above, the resonance occurs at a higher frequency of about 5.15 GHz, due to the smaller pitch.

[0122] FIG. 6 shows an interdigitated transducer structure for an acoustic wave device according to a fourth embodiment of the present invention.

[0123] The transducer structure 300 comprises a different acoustic wave propagating substrate 302 compared to the substrate 102 of the transducer structure 100 of the first embodiment, but this is the only difference with respect to the first embodiment. All other features remain the same and will therefore not be described again in detail, but reference is made to their above description.

[0124] The transducer structure 300 comprises a composite substrate 302 having a piezoelectric layer 104 formed on a base substrate 306, like the composite substrate 102, but also comprises an acoustic mirror 304, also known as a Bragg mirror, formed on the base substrate 306 and below the piezoelectric layer 104.

[0125] The Bragg mirror 304 comprises a plurality of stacked layers 306-309, where the layers with even reference numbers 306, 308 are of a first material and the layers with odd reference numbers 307, 309 are of a second material. The first and second materials have different acoustic impedances, and thus the Bragg mirror 304 comprises a stack of alternating high and low impedance layers.

[0126] The Bragg mirror 304 has a periodic repeat of alternating high impedance / low impedance layer pairs with a thickness of about a quarter wavelength to ensure reflection.

[0127] The first and second materials may be tungsten, molybdenum, LiTaO3, Al2O3, AlN, LiNbO3, Si3N4, and any combination of SiO2 and Si3N4 (also known as silicon oxynitride and SiO x N y where x and y control the amount of each element in the compound), and ZnO, aluminum, or SiO2.

[0128] In one variation, the first and second materials can be swapped so that the first material has a low impedance and the second material has a high impedance.

[0129] In this embodiment, Bragg mirror 304 is depicted as having four layers 306-309 forming a stack of alternating high and low impedance layers, however, in other variations, Bragg mirror 304 may have more or less than four layers forming a stack of alternating high and low impedance layers.

[0130] Increasing the number of pairs in the Bragg mirror 304 improves the reflectivity of the mirror, and increasing the impedance ratio between the materials in the Bragg pair improves both the reflectivity and the bandwidth. Commonly chosen stack materials are, for example, titanium dioxide and silica.

[0131] According to the present invention, the piezoelectric layer 104 and the Bragg mirror 304 are configured to reduce the contribution of additional modes present in the structure and promote unique modes within the transducer structure 100, thereby ensuring spectral purity and preventing spectral contamination of acoustic wave devices based on such transducer structure 100.

[0132] One approach is to optimize the thickness of the stack of the Bragg mirror 304 to promote a unique mode within the transducer structure and to achieve an efficient reflection coefficient for this mode. The Bragg mirror 304 then acoustically decouples the vibrations generated within the electrode means 112, 114 from the base substrate 106.

[0133] FIG. 7a shows an interdigitated transducer structure for an acoustic wave device according to a fifth embodiment of the present invention.

[0134] The only difference with respect to the first embodiment is that the transducer structure 400 comprises a cover layer 402 on top of the transducer structure 100 of the first embodiment, all other features remain the same and therefore will not be described in detail again, but reference is made to their above description.

[0135] The transducer structure 400 comprises a composite substrate 102 comprising a piezoelectric layer 104 formed on a base substrate 106 .

[0136] In this embodiment of the invention, layer 402 resides over the embedded electrodes 108, 110 and over the piezoelectric layer 104. Layer 402 may be a passivation layer or a dielectric substrate, including high speed, low loss materials such as silicon, sapphire Al2O3, garnet or yttrium based materials, aluminum nitride AlN, silicon carbide SiC, silicon nitride Si3N4, and the like.

[0137] According to a further variant, the layer 402 is a carbon-based layer, for example a monocrystalline diamond, an amorphous carbide layer, a nano-crystalline polycrystalline diamond (NCD) or a layer of a material with a compressional wave velocity of 15 km.s. -1 and shear wave speeds above 7km-s -1 The layer 402 may be any diamond-like carbon layer that can be pushed above the TCF value of the entire structure. In yet another embodiment, a layer of SiO may be used as layer 402. The SiO may act as a TCF corrector to improve the TCF value of the entire structure.

[0138] The cover layer 402 can also be made of glass, for example as a TCF corrector, and generally as a silicon-based substrate.

[0139] The use of high velocity, low loss materials can accelerate the phase velocity of the fundamental shear wave mode above or well above the SSBW velocity of the base substrate, resulting in the suppression of unwanted modes by diffusion into the base substrate 106.

[0140] In the embodiment shown in Figure 7a, the cover layer 402 is made of the same material as the base substrate 106 of the composite substrate 102. However, the cover layer 402 may be different from the base substrate of the composite substrate.

[0141] According to a further variant, the layer 402 may be present only on the electrode means 108 , 110 or only on the piezoelectric layer 104 .

[0142] 7b-7d show the broadband harmonic admittance of simulated excitation modes according to the fifth embodiment of the present invention for an acoustic wavelength of 2.8 μm, aluminum electrodes and an a / p ratio of 0.5 with (YXl / / 42°) lithium tantalate as the piezoelectric layer 104.

[0143] In Figure 7b, the cover layer 402 and base substrate 106 are silicon dioxide. The excited mode occurs at 3.5 GHz. In Figure 7c, the cover layer 402, base substrate 106, and piezoelectric layer 104 are (YXl / / 42°) lithium tantalate. The excited mode also occurs at approximately 3.5 GHz. An additional contribution is evident at approximately 6.5 GHz, which can be attributed to the third harmonic. However, this is essentially in the conductance, and there is no sign change in the corresponding susceptance, indicating low coupling.

[0144] In Figure 7d, the cover layer 402 and base substrate 106 are silicon substrates. The excited mode occurs at 3.5 GHz, but here the fundamental shear mode in the piezoelectric layer is also excited and can be seen at a lower frequency, i.e., around 1.8 GHz.

[0145] The present invention also relates to an acoustic wave device comprising two transducer structures, each of which is according to any one of the first to fifth embodiments of the present invention.

[0146] Alternatively, only one of the two transducer structures may be a surface acoustic device according to the present invention, the other according to the prior art.

[0147] The acoustic wave device may be an acoustic wave resonator, and / or an acoustic wave filter, and / or an acoustic wave sensor, and / or a high frequency source. The acoustic device may further comprise a radio frequency (RF) supply means configured to drive the transducer structure with an RF signal above 3 GHz.

[0148] FIG. 8 shows a sixth embodiment of the transducer structure.

[0149] The transducer structure 500 of FIG. 8 differs from that of FIG. 1 in that the interdigitated electrodes 512, 514 only partially fill the grooves 510 in the piezoelectric region 504.

[0150] Due to the manufacturing process, the thickness of the metal layer is not constant throughout the removed area 510. Due to surface energy characteristics, the thickness of the metal layer at the sidewalls 508 is greater than the thickness in the center.

[0151] The transducer structure 500 functions in the same manner as the transducer structures described above.

[0152] The use of bulk wave frequencies propagating within the electrode means of the transducer structures described above in acoustic wave devices, particularly for ladder filters and / or impedance filters and / or coupling filters, makes it possible to generate contributions at high frequencies, in particular above 3 GHz, more particularly above 3.5 GHz.

[0153] The use of such embedded electrode means for the transducer structure can improve the performance of acoustic wave devices and their range of applications compared to bulk piezoelectric substrates, without requiring changes to the manufacturing tools, i.e. I-line lithography.

[0154] 9a and 9b show, as a seventh embodiment, an example filter demonstrating an effective implementation of the present invention according to typical market requirements, e.g., 5G sub-6-GHz (C-band) filtering. This example is based on Al electrodes embedded in LiTaO3 on silica, with the following parameters: pitch p = 1 μm, which yields a wavelength λ of 2 μm, resulting in a resonance near 5 GHz as shown in FIG. 5, and electrode and piezoelectric layer thickness of 700 nm, hence t e The ratio of / λ is 0.35. According to the usual practice in SAW ladder filter design, the resonance of the series branch occurs at the anti-resonance of the parallel branch.

[0155] This embodiment of the present invention can be fabricated in a single batch using standard SAW fabrication techniques, for example using I-line lithography and single metal layer deposition. e Fine tuning of the resonant frequency using the / λ ratio can be achieved. To improve the performance, a passivation layer or a Bragg mirror may be used as shown in Figures 7a and 6.

[0156] The filter in this example is based on the basic transducer structure shown in Figure 1. Based on this approach, resonators are formed and combined in series and parallel to form cells that can be configured as cascades or ladder filter structures as known in the art.

[0157] In this example, two 35% gratings with material ratios a / p=0.6 and 0.65 were used. In both cases, the harmonic admittance and impedance were calculated and are shown in Figure 9a. The resonance of the a / p=0.65 grating is close to the antiresonance of the a / p=0.6 grating, which is a prerequisite for designing a ladder filter as described above.

[0158] By combining these responses, the transfer function of a 4π-cell filter as known in the art can be calculated, and the result is shown in Figure 9b. This transfer function demonstrates that when the electrodes are embedded in accordance with the present invention, an ultra-compact filter operating above 5 GHz with 700 nm thick electrodes allows for improved power handling. In this configuration, there is no physical migration. This inventive filter exhibits a coupling coefficient k of over 10% in LiTaO3. s 2 This can be improved by using LiNbO3, allowing for scaling between the two materials.

[0159] The proposed design exhibits a bandwidth of 300 MHz. By tuning the resonance-antiresonance conditions, improved matching can lead to higher bandwidths on the order of 400 MHz. As mentioned above, tuning can be achieved by adjusting the pitch and / or the ratio a / p.

[0160] Figure 10a shows that higher harmonic modes of the electrode shear mode can also be excited. Figure 10a actually shows the third-harmonic shear motion. Four neutral vibration points or lines are observed for this mode in each electrode. The electrode shear mode shown in Figure 10a was obtained for a transducer structure with 700 nm thick Al vertical electrodes, a metal ratio a / p of 0.5, a pitch of 1.4 μm, and a piezoelectric layer of LiTaO3(YX1) / 42°. In contrast to Figure 7b, here the third harmonic generates large vibrations at the edges of the electrodes, resulting in unique features of the mode that can characterize the coupling. Figure 10b compares LTO, Si, SiO2, and sapphire substrates, considering that the pyramidal electrode shape shown in Figure 4a provides a better Q than other electrode types. The other parameters are the same as in Figure 10a.

[0161] Figures 10b and 10c show the corresponding harmonic conductance G and susceptance B for various base substrates: LiTaO3, SiO2, Si, and sapphire. These modes are interesting because they exhibit high equivalent velocities due to their high resonant frequency of approximately 8.75 GHz. Excitation of the corresponding modes by use of the transducer structure of the present invention can be advantageously used to develop high-frequency sources. Figure 10c shows a sign change in the susceptance, suggesting that mode coupling is effective.

[0162] Figure 11 shows an eighth embodiment of the present invention. Figure 11 shows an interdigitated transducer structure 200 for a surface acoustic wave device according to a first embodiment of the present invention. The interdigitated transducer structure 200 comprises a pair of interdigitated comb-shaped electrodes 202 and 204, each comprising a plurality of electrode means 1206 and 208 embedded in a piezoelectric layer 212.

[0163] As in the first embodiment, the electrode means 206 and 208 have the shape of fingers 206, 208. In a variation of this embodiment, the electrode means may further comprise split fingers 206, 208, each consisting of two or more directly adjacent electrode fingers belonging to the same interdigital electrode.

[0164] The piezoelectric layer 212 is part of a composite substrate 210 that further comprises a base substrate 214. The piezoelectric layers are of the same material and have the same characteristics in terms of thickness as described in the other embodiments.

[0165] The thickness of the base substrate 214 can be greater than that of the piezoelectric layer 212 in order to accommodate its thermal expansion to the piezoelectric layer 212 and to reduce the sensitivity of the transducer to temperature changes. A preferred situation corresponds to a base substrate thickness that is at least 10 times greater than the thickness of the piezoelectric layer 212.

[0166] The base substrate 214 is of the same material as in the first embodiment.

[0167] The use of different materials for the base substrate 214 allows for greater design flexibility.

[0168] The pair of interdigitated interdigital electrodes 202 and 204 includes a plurality of electrode fingers 206 and 208. The electrode fingers, e.g., 206_1, 208_1 through 206_4, 208_4 and corresponding electrodes 208_5, 206_7 through 208_8, 206_10, are interdigitated and connected to an alternating potential via their interdigital electrodes 202 and 204 and embedded in the piezoelectric layer 212. The alternating potential may be +V and −V as shown, or in a variant, mass and load / power potentials. The electrode fingers are metallic and all have the same length l, width w, and thickness t. Also, here again, an electrode pitch p, defined as λ / 2, is used for the transducer structure 200. The number of electrode fingers is not fixed, and a device may include more or fewer than shown in FIG. 11 .

[0169] According to a variant of the invention, the electrode fingers 206, 208 also have different lengths l, widths w, and thicknesses t e It may also have

[0170] As in the first embodiment, the thickness t e is less than or equal to the thickness of the piezoelectric layer 212.

[0171] The eighth embodiment has the particularity in that the transducer structure 200 further comprises a region 218, also called the second region, in which two adjacent electrode fingers 208_4 and 208_5, also meaning directly adjacent electrode fingers, are connected to the same potential, here +V, without any electrode fingers 206 from the opposite interdigital electrode 202 between them. The two adjacent electrode fingers 208_4 and 208_5 are also connected to -V, or to the mass, or to the load / power supply potential V IN (not shown) In this context, a first region or regions are parts of the transducer structure in which directly adjacent electrode fingers belong to different interdigital electrodes.

[0172] In this variant, represented by two or more adjacent split fingers 206, 208 whose electrode means 206, 208 are at the same potential, two adjacent electrode means 206, 208 connected to the same potential may refer to all fingers of the split fingers 206 being connected to the same potential of the split fingers 208. However, it may also be the case that at least one electrode finger of the split fingers 206 is connected to the same potential of the split fingers 208.

[0173] 11, the region or second one 218 is actually placed in the center of the transducer structure 200, so that there are eight electrode fingers or four electrode finger pairs on each side, left and right, of the region 218. In a variation of this embodiment, the region 218 can be placed at a different location on the transducer structure, so that the electrode finger pairs are unevenly distributed on either side of the region 218. The region 218 can also be placed at either end of the transducer structure 200.

[0174] As already mentioned, the electrode fingers 206_1, 208_1 to 206_4, 208_4 and the corresponding electrode fingers 208_5, 206_5 to 208_8, 206_8 are interdigitated and have alternating potentials. Due to the presence of the region 218, it can indeed be seen that on the left side of the region 218, the interdigitated electrode fingers 206_1, 208_1 to 206_4, 208_4 are at alternating potentials -V / +V, respectively, while on the right side of the region 218, the interdigitated electrode fingers 208_5, 206_5 to 208_8, 206_8 are at alternating potentials +V / -V, respectively.

[0175] An electro-acoustic wave source is defined by a pair of adjacent electrode fingers connected at an alternating potential. For example, in FIG. 11 , adjacent interdigital electrode fingers 206_1 and 208_1 connected at an alternating potential −V / +V define an electro-acoustic wave source 220. Meanwhile, adjacent interdigital electrode fingers 208_1 and 206_2 connected at an alternating potential +V / −V define an electro-acoustic wave source 222. Thus, adjacent pairs of interdigital electrode fingers 206_2, 208_2 to 206_4, 208_4 also define an electro-acoustic wave source 220, and correspondingly, adjacent pairs of interdigital electrode fingers 208_2, 206_3 and 208_3, 206_4 also define an electro-acoustic wave source 220. In particular, in this case, on the left side of the region 218, there are four effective electro-acoustic wave sources 220 and three effective electro-acoustic wave sources 222 due to a total of eight interdigitated electrode fingers 206_1, 208_1 to 206_4, 208_4.

[0176] On the right side of region 218, a pair of adjacent interdigitated electrode fingers, for example, 208_5 and 206_5, connected at an alternating potential +V / −V also defines an electro-acoustic wave source 222, and a pair of adjacent interdigitated electrode fingers 206_5 and 208_6, which are at an alternating potential −V / +V, also defines an electro-acoustic wave source 220. On the right side of region 218, a total of eight interdigitated electrode fingers 208_5, 206_5 to 208_8, 206_8 result in four effective electro-acoustic wave sources 222 and three effective electro-acoustic wave sources 220. However, here, the electro-acoustic wave sources 220, 222 on the left side of region 218 are out of phase with the electro-acoustic wave sources 222, 220 on the right side of region 218, specifically by π.

[0177] However, since the electrode pitch p is defined as λ / 2, this means that the transducer structure 200 is operating in a tuned mode at the Bragg condition, so that the multiple electro-acoustic wave sources 220, 222 on the left side of the region 218 are all in phase and coherent with each other, while the multiple electro-acoustic wave sources 222, 220 on the right side of the region 218 are all in phase and coherent with each other.

[0178] In the region 218, there is no electro-acoustic wave source 220 or 222 between two adjacent electrode fingers 208_4 and 208_5, because they are both connected to the same potential.

[0179] According to a variant, the polarity of the potential may be switched between the first interdigital electrode 206 and the second interdigital electrode 208, or the mass of one interdigital electrode and the load / source potential V of the other interdigital electrode may be switched. IN may be connected to

[0180] Due to the presence of the second region 218 in the transducer structure 200, the electro-acoustic wave source on the left side of region 218 is in opposite phase to the electro-acoustic wave source on the right side of region 218, so that the phases of the electro-acoustic wave sources within the transducer are reversed by π. Thus, the energy radiated towards the transducer from either side of the two electrode fingers connected to the same interdigital electrode combines to create destructive interference between the electro-acoustic wave sources at the transducer, while the energy radiated towards the outside of the transducer is actually reflected by mirrors located on either side of the transducer structure within the SAW device.

[0181] Therefore, the amount of coherent, in-phase electro-acoustic wave sources present in the transducer structure 200 is reduced compared to a transducer structure of the same size in which all electrode fingers are at alternating potentials, as shown in Figure 1. As a result, the electromechanical coupling coefficient k s 2 is getting smaller.

[0182] In this case, in this particular embodiment, the right and left sides of region 218 in transducer structure 200 have exactly the same number of interdigitated electrode fingers 206 and 208, namely eight, since region 218 is centrally located in transducer structure 200, resulting in seven effective electro-acoustic wave sources. In this case, the electromechanical coupling coefficient k s 2is reduced by a factor of two. Again, the combination of energy radiated toward the transducer from each side of the two electrode fingers connected to the same interdigital electrode creates destructive interference between the electro-acoustic wave sources in the transducer structure, while the energy radiated toward the outside of the transducer is actually reflected by the mirror when it is emitted. This results in a factor of two reduction in the efficiency of the transducer.

[0183] Furthermore, the possibility of phase coherence of the modes reflected at the interface 216 of the composite substrate 212 is also modified compared to the situation in the prior art. In the case of a phase shift within the transducer structure, there is no possibility of detecting waves that do not meet the phase matching condition. Therefore, the detection of acoustic waves reflected at the interface 216 is reduced, which in turn leads to a reduction in parasitic resonances at undesired frequencies due to these reflections in the filtering operation of a SAW device based on the transducer structure 200.

[0184] Therefore, acoustic wave generation and / or detection in the transducer structure 200 of the present invention is controlled by the amount of in-phase electro-acoustic wave sources present within the transducer structure 200. Connecting two adjacent electrode fingers to the same potential results in a π phase change within the structure, which has a positive effect on the efficiency of the transducer structure by eliminating parasitic modes. While it is not necessary to change the dimensions of the transducer, such as the width or length of the electrode fingers or the inter-electrode distance, this would affect the manufacturing techniques for such structures and could significantly degrade the quality of resonance of resonators employing the above-described transducer structure.

[0185] According to a variant of the eighth embodiment, there may be more than one region 218 in the transducer structure, which results in an increase in the number of electro-elastic wave sources suppressed in the transducer structure, and thus the electromechanical coupling coefficient k s 2 can be made even smaller. This is an efficient way to control the filter bandwidth and gain more freedom to accommodate different filter bands.

[0186] According to another variant, more than just two, for example three or more, adjacent electrode means 208_4 and 208_5 may be linked to the same potential, so that further sources can be suppressed. Additionally or according to another variant, there may be more than one region with suppressed sources. In that case, it may be advantageous to distribute them randomly over the extent of the transducer structure. If there are more regions, the number of adjacent electrode means linked to the same potential will be different.

[0187] 12a-12c show three variants of the ninth embodiment of the present invention. Unlike the case described above, where the grooves in the piezoelectric layer are filled exclusively with a conductive material, in particular a metal such as Al or an Al alloy, as shown for example in Fig. 1 or Fig. 4a-c, the variants of the transducer structure of the ninth embodiment also include a dielectric material in the grooves of the piezoelectric layer. Besides this difference, the variants of the sixth embodiment have the same structural features and properties as the first embodiment and can be combined with any one or combination of the other embodiments 2-8.

[0188] 12a shows a partial cutaway view of a transducer structure 600 of a first variant of the ninth embodiment, in which two adjacent electrode means 612 and 614 each belong to a different interdigital electrode. The electrode means 612 and 614 are embedded in grooves 616 and 618 in a piezoelectric layer 604, which is provided on a base substrate 606 via an attachment layer 608. The side and bottom walls of the grooves 616 and 618 in this embodiment are covered with a conductive material 620 and 622, for example Al or an Al alloy as described above. The remaining parts of the grooves 616 and 618 are at least partly filled with a dielectric material 624 and 626, in particular diamond carbon.

[0189] Figure 12b shows a transducer structure 650 according to a variant of the ninth embodiment. Elements having the same reference numbers as those used in the first variant 600 will not be described again but will be referred to.

[0190] In this variation, grooves 652 and 654 extend entirely through the piezoelectric layer to mounting layer 608. Again, the sidewalls of grooves 652 and 654, as well as the bottoms of the grooves, which now contact mounting layer 608 of base layer 606, are covered with conductive materials 656 and 658. The remaining portions of grooves 652 and 654 are at least partially filled with dielectric materials 660 and 662. The same materials can be used as in the first variation.

[0191] The advantage of this embodiment is that it allows the use of materials with phase velocities greater than those of metallic materials, especially Al-based metals, resulting in higher frequencies being reached with this embodiment compared to embodiment 1.

[0192] 12c shows a third variant 690 of the transducer structure. The only difference from the second variant is that the dielectric materials 692 and 694 extend through the entire thickness of the grooves 696 and 698 to the mounting layer 608, so that only the sidewalls of the grooves 696 and 698 are covered by the conductive materials 656 and 658.

[0193] 13a to 13d show a method for obtaining a first variant of the ninth embodiment.

[0194] As shown in Figure 13a, grooves 616, 618 are etched into the piezoelectric layer 604. A metal deposition step is then performed, as shown in Figure 13b, to cover the piezoelectric layer 604 and the walls of the grooves 616, 618 with a metal layer 700. A dielectric layer 702 is subsequently deposited on the metal layer 700, so that the grooves 616 and 618 are at least partially filled with a dielectric material, as shown in Figure 13c. Finally, a polishing step, for example a CMP polishing step, is performed to obtain the transducer structure 600.

[0195] A second variation of the transducer structure 650 is obtained by configuring the etching step so that grooves extend through the piezoelectric layer 604 down to the base substrate 606 .

[0196] For the structure shown in Figure 12b, Figure 14a shows the numerical simulation results for the conductance G and susceptance B, and Figure 14b shows the numerical simulation results for the conductance G and susceptance B, as well as the resistance R and reactance X. In this simulation, diamond-carbon was used as the dielectric material and aluminum as the metal in the grooves. The piezoelectric layer 604 is LiTaO3(YX1) / 42°. The base substrate 606 is also lithium tantalate (same crystal cut) bonded to the piezoelectric layer 604 by an attachment layer 608 of silica SiO2.

[0197] This structure is also called a homotypic interface, meaning that the same material is used as the substrate and the piezoelectric layer is bonded using SiO2. SiO2 can be used as an etch stop layer to control the piezoelectric layer thickness and therefore the excitation layer thickness. SiO2 can further promote the reduction of TCF. The portion of the structure below the silica mounting layer is also called the emission domain, where unwanted modes diffuse, while the desired mode remains within the guiding domain above the mounting layer 608.

[0198] The mechanical period or electrode pitch p is 1.4 μm and the embedded electrode thickness is 500 nm with 100 nm thick metal layers 656, 658 and 400 nm thick AlN / carbon diamond as the dielectric fill material 660, 662. In this simulation, a piezoelectric / electrode aspect ratio close to a / p=0.5 was used. The observed modes correspond to leaky waves, but are 12 km.s -1 For a coupling coefficient of 2.2% with a phase velocity of r and anti-resonance Q a are equal to 400 and 670 respectively.

[0199] Figure 14c shows the electrode modes obtained by numerical simulation. Shear modes, such as vibrations, exist within the electrodes, i.e., the metal portion 656 and the dielectric portion 660, while the piezoelectric layer 604 exhibits less motion. Figure 14c shows the finite element mesh of the piezoelectric layer 604, the embedded electrode, and the mounting layer 608. The boundary conditions in this simulation considered wave behavior in the radiation domain below the mounting layer 608. Figure 15a shows the numerical simulation results for the harmonic conductance G and susceptance B, and Figure 15b shows the numerical simulation results for the harmonic conductance G and susceptance B, as well as the harmonic resistance R and reactance X, for a structure as shown in Figure 12b, with aluminum nitride AlN as the dielectrics 660 and 662. Apart from the dielectric material, all other structural parameters were the same. The phase velocity of AlN was reduced to 11.3 km s-1 compared to diamond-like carbon. This variant also exhibited a 4.4% coupling coefficient and a 0.2% quality factor Q. r =1850 and Q a =990 can be observed.

[0200] Figure 16a shows the numerical simulation results of the harmonic conductance G and resistance R, Figure 16b shows a close-up of the resonance, and Figure 16c shows a close-up of the anti-resonance. In this variant, in a structure as shown in Figure 12b, the dielectrics 660, 662 are silicon dioxide SiO2. Besides the dielectric material, all other structural parameters were the same as in the first and second variants of the ninth embodiment.

[0201] The use of SiO2 improves the TCF of the observed mode compared to transducer structures where the grooves are filled exclusively with metallic material. -1 This is due to the fact that the TCF coefficient of SiO2, which is equal to , is opposite to the TCF coefficient of metals.

[0202] In fact, in this case, when SiO2 is used as the dielectric 660, 662, the TCF value of the resonance is -11 ppm.K. -1 , and anti-resonance TCF value -14.7 ppm.K-1 The coupling coefficient is 6.7% and the resonant Q R The quality factor of is greater than 5000. Ar is smaller, about 650, but can be improved by optimizing the structural parameters of the design.

[0203] The number of observed TCFs can be further improved if an additional layer of SiO2 is provided on top of the transducer structure 650, as shown in Figure 16d. Figure 16d shows a transducer structure 670 according to a fourth variant of the ninth embodiment. The transducer structure 670 corresponds to the transducer structure 650 of Figure 12b, except for the presence of an additional layer of SiO2 672. The dielectric material may extend all the way down to the mounting layer 608, as shown in Figure 12c.

[0204] Another aspect of providing such a layer is to accelerate the phase velocity of the mode, as already described above in connection with FIG. 7, in order to accelerate the fundamental shear mode above the SSBW velocity of the substrate 606, if necessary to diffuse the observed fundamental mode.

[0205] Instead of using the same dielectric material to fill the trench and to provide the additional layer 672, two different materials can be used to further optimize the TCF and phase velocity.

[0206] Figures 17a and 17b show two variants of the tenth embodiment of the present invention. Situations may arise where electrode modes can be observed simultaneously with standard induced shear modes, for example the fundamental induced shear mode as shown in Figure 7d.

[0207] Such a situation can also arise when using a POI composite substrate with a LiTaO3 piezoelectric layer on SiO2. A typical example of such a POI substrate is a Si(100) base substrate with a 1 μm trap-rich poly-Si layer on top of a 500 nm thick SiO2 layer on top of a 600 nm thick LiTaO3 layer. This configuration produces a 3800-4200 m.s -1The persistence of the fundamental induced shear mode is observed at phase velocities between , which results in potential problems when addressing the high frequency band using electrode modes with frequencies approximately 2.5 times higher than the electrode modes.

[0208] As already mentioned, with reference to the description relating to Figures 3a-3f, the choice of base substrate 106, 205, 306, 406, 506, 606 plays an important role: in fact, if the SSBW velocity of the substrate is smaller than the SSBW velocity of the fundamental elastic bulk shear mode in the piezoelectric layer, the bulk shear mode will enter the base substrate and be dispersed there.

[0209] The contribution of the fundamental guided shear mode can be reduced by using a Si(111)-based substrate, but this involves a reduction in the SSBW velocity of 5650 m s , which is smaller than that on Si(100). -1 Typically 4700m.s -1 The Si orientation corresponding to (YXw) / 45° is particularly interesting for this purpose. However, in that case, although some higher-order modes are suppressed as already described, there is still the inherent signature of the fundamental shear-induced mode that can be observed.

[0210] To further reduce the remaining contribution of the fundamental guided shear mode, the transducer structure according to the tenth embodiment comprises at least one additional layer in the stack, selected to accelerate the fundamental guided shear wave and push its velocity well above the SSBW velocity of the substrate.

[0211] The transducer structure 700 as shown in Figure 17a comprises a LiTaO3 piezoelectric layer 704 with electrodes 712, 714, 716, here made of aluminum, embedded on a SiO2 layer 706. The structure further comprises a trap-rich poly-Si layer 708 on a Si(111) base substrate 710.

[0212] The transducer structure 700 further comprises an additional layer 718, also referred to as a high velocity low loss layer, sandwiched between the SiO2 layer 706 and the trap rich layer 708. The high velocity layer 718 is one of a layer of AlN, Al2O3, Si3N4, or SiC. All of these materials have a 10 km.s -1 Compressional bulk wave velocity values ​​exceeding 5 km.s -1 Alternatively, the high velocity layer 718 may be carbon-based, i.e., single crystal diamond, amorphous carbide layers, nano-particle polycrystalline diamond (NCD), and layers that exhibit shear bulk wave velocities exceeding 15 km.s. -1 and shear wave speeds above 7 km / s -1 The diamond-like carbon layer may be any layer that can be pushed up above the surface.

[0213] Figure 17b shows a second variant of the tenth embodiment. It is based on the first variant, but further includes a second additional layer 720, which is also a high-velocity, low-loss layer and can be of the same or a different material as the first additional layer 718. Both layers 718, 720 accelerate the velocity of the fundamental guided shear mode. Figure 18a shows an electron microscope image illustrating an example of a transducer according to the present invention. This photograph shows a side cutaway view of a transducer structure 800 corresponding to the structure of the first embodiment as shown in Figure 1. The transducer structure 800 has an aluminum electrode 802 embedded in a LiTaO3(YX1) / 42° bulk substrate 804. A layer 806 on the surface of the electrode 802 and the bulk substrate 804 was added after measuring the transducer properties and serves as a contrast enhancement layer for imaging purposes. The pitch was p = 3.4 μm, and the height of the electrode 802 was h = 510 nm. The aspect ratio a / p was 0.5. The structure had 50 pairs of electrode means.

[0214] Figure 18b is an enlarged view of the area highlighted by rectangle 808, thereby showing the shape of electrode 806. The groove in bulk substrate 804 has a trapezoidal shape, with the longer of its parallel sides aligned with surface 810 of bulk substrate 804. Sidewalls 812 and 814 are slightly concave, and bottom surface 814 is convex.

[0215] FIG. 18c shows a finite element mesh 820 that simulates the geometry of an example electrode 802 embedded in a piezoelectric substrate 804 used to simulate the behavior of the example shown in FIGS. 18a and 18b.

[0216] Figure 19a shows experimental measurements of the conductance and resistance of this example, and Figure 19b shows the conductance and resistance results obtained by numerical simulation of a structure such as that shown in Figure 18c.

[0217] The fabricated device shown in Figures 18a and 18b exhibited a 10950 m.s -1 Phase velocity of the mode, 1.85% coupling coefficient k s 2 , and the quality factor Q ar = 350. A mode was observed at a frequency of about 1.6 GHz, thus much higher than the fundamental shear mode at 580 MHz. The measured conductance and resistance are shown in Figure 19a.

[0218] Simulation results assuming an infinitely long transducer structure using the FEM mesh shown in Figure 18c yielded results in agreement with the experimental results for the electrode modes mentioned above. -1 speed, 0.62% coupling coefficient k s 2 , and a quality factor Q of the order of 100 ar Furthermore, this mode occurs at about 1.6 GHz and shows similar behavior in the conductance and resistance dependence.

[0219] Although several embodiments of the present invention have been described, it will be appreciated that various modifications and improvements can be made without departing from the scope of the following claims. [Explanation of symbols]

[0220] 104...piezoelectric layer, 108, 110...interdigital electrodes, 112_i, 114_j...electrode means, 100...transducer structure.

Claims

1. a piezoelectric layer (104); a pair of interdigitated electrodes (108, 110, 412, 414) comprising a plurality of electrode means (112_i, 114_j, 418, 420) having a pitch p; A transducer structure (100, 200, 300, 408, 410) for an elastic device, comprising: the electrode means of the interdigitated electrodes (108, 110, 412, 414) are embedded in the piezoelectric layer (104); the acoustic impedance of the electrode means is less than the acoustic impedance of the piezoelectric layer; The piezoelectric layer is disposed on a base substrate (106); the acoustic impedance of the base substrate (106) is within plus or minus 25% of the acoustic impedance of the piezoelectric layer (104); the embedded electrode means filling the grooves of the piezoelectric layer (104, 203); the grooves have a cross section with a pyramidal or trapezoidal shape (201a) or a V- or U-shape (201c), or at least one of the side walls and the bottom of the grooves has a convex (207b) or concave (207a) or scalloped (207c) shape, a transducer structure in which the grooves (696, 698) extend entirely through the piezoelectric layer (604), the sidewalls of the grooves (696, 698) are lined with a conductive material (656, 658), and the remainder of the grooves (696, 698) are filled with a dielectric material (692, 694).

2. 2. The transducer structure of claim 1, wherein the pitch p satisfies the Bragg condition given by p=λ / 2, where λ is the operating acoustic wavelength of the transducer structure.

3. 3. The transducer structure according to claim 1 or 2, wherein the electrode means (112_i, 114_j, 418, 420) have an aspect ratio a / p, where "a" is the width and "p" is the pitch, comprised between 0.3 and 0.75, in particular between 0.4 and 0.

65.

4. Between the piezoelectric layer (604, 704) and the base substrate (606, 710) is an attachment layer (608, 706), in particular silicon dioxide (SiO ２ 4. The transducer structure of claim 1, further comprising:

5. 5. The transducer structure of claim 1, further comprising a high velocity layer (718) between the piezoelectric layer (704) and the base substrate (710), the high velocity layer being made of a material that allows for a higher shear wave phase velocity than the material and crystal orientation of the piezoelectric layer (104).

6. 6. The transducer structure of claim 5 when dependent on claim 4, wherein the high velocity layer (718) is disposed between the mounting layer (706) and the base substrate (710).

7. The transducer structure according to any one of claims 1 to 6, further comprising a trap rich layer (708), in particular a polysilicon trap rich layer, between the piezoelectric layer (704) and the base substrate (710).

8. 8. The transducer structure of claim 7 when dependent on claim 6, wherein the trap rich layer (708) is disposed between the high velocity layer (718) and the base substrate (710).

9. The transducer structure of any one of claims 1 to 8, further comprising a cover layer (302) on top of said embedded electrode means (112_i, 114_j, 418, 420) and said piezoelectric layer (104).

10. 10. The transducer structure of claim 9, wherein the cover layer (302) is made of a material and / or has a crystal orientation that allows a higher shear wave phase velocity than the material and / or crystal orientation of the piezoelectric layer (104).

11. A transducer structure according to any one of the preceding claims, further comprising a Bragg mirror (204) beneath the piezoelectric layer (104) and / or the electrode means.

12. The transducer structure according to any one of the preceding claims, wherein the thickness of said embedded electrode means (112_i, 114_j, 418, 420) is equal to or less than the thickness of said piezoelectric layer (104).

13. The thickness t of the electrode means ｅ λ>t ｅ 13. The transducer structure of claim 12, wherein: >0,1*λ.

14. The transducer structure of any one of claims 1 to 13, wherein the acoustic impedance of the base substrate (106) is within plus or minus 15% of the acoustic impedance of the piezoelectric layer (104).

15. A converter structure as described in claim 1, wherein the dielectric material (692, 694) extends through the entire thickness of the grooves (696, 698), such that only the side walls are covered by the conductive material (656, 658).

16. 2. The transducer structure of claim 1, wherein the dielectric material is a material having a higher shear wave phase velocity than the conductive material.

17. 2. The transducer structure of claim 1, wherein the dielectric material has a temperature coefficient frequency that is opposite in sign to the temperature coefficient frequency of the conductive material.

18. 11. The transducer structure of claim 9 or 10, wherein the dielectric material of the cover layer (672) and the dielectric material (660, 662) filled in the grooves (652, 654) are the same.

19. A transducer structure according to any one of the preceding claims, wherein the electrode means are made of a material lighter than manganese, in particular aluminium or an aluminium alloy containing Cu, Si or Ti.

20. A transducer structure according to any preceding claim, wherein the piezoelectric layer is lithium tantalate or lithium niobate.

21. The base substrate (106) is made of silica, quartz, fused silica, or glass, or LiTaO ３ , or LiNbO ３ 21. The transducer structure according to any one of the preceding claims, which is one of the following: or silicon, in particular Si(111).

22. The high velocity layer (718) is made of AlN, Al ２ O ３ , Si ３ N ４ , SiC, or carbon-based, in particular one of monocrystalline diamond, amorphous carbide, nanoparticle polycrystalline diamond.

23. The cover layer (302, 672) is made of AlN, Al ２ O ３ , Si ３ N ４ , SiC, or carbon-based, in particular one of monocrystalline diamond, amorphous carbide, nanoparticle polycrystalline diamond.

24. 24. A transducer structure according to any one of claims 1 to 23, wherein the pair of interdigitated electrodes (202, 204) comprises one or more regions (218) in which two or more adjacent electrode means (206, 208) belong to the same interdigitated electrode (202, 204), the distances of said electrode means to each other being the same for said adjacent electrode means belonging to different interdigitated electrodes.

25. 25. A transducer structure according to claim 24, wherein the two or more adjacent electrode means belonging to the same interdigital electrode have the same geometric shape as the adjacent electrode means belonging to different interdigital electrodes.

26. 26. A transducer structure according to claim 24 or 25, having three or more regions (218) with two or more adjacent electrode means (206) belonging to the same interdigital electrode (202, 204), characterized in that the distances of adjacent regions to one another are different, in particular the adjacent regions are randomly distributed over the extent of the transducer structure.

27. 27. The transducer structure according to any one of claims 24 to 26, wherein the regions (218) having two or more adjacent electrode means (206, 208) belonging to the same interdigital electrode (202, 204) have different numbers of adjacent electrode means belonging to the same interdigital electrode.

28. Transducer structure according to any one of the preceding claims, wherein the electrode means have dimensions realizable by I-line lithography, in particular a width greater than 350 nm.

29. An acoustic wave device (400) comprising at least one transducer structure (100, 200, 300, 408, 410) according to any one of claims 1 to 28, wherein the acoustic wave device (400) is an acoustic wave resonator, and / or an acoustic wave filter, and / or an acoustic wave sensor, and / or a frequency source.

30. 30. The acoustic wave device of claim 29, further comprising a radio frequency (RF) supply configured to drive the transducer structure with an RF signal above 3 GHz.

31. 29. A method of using a transducer structure according to any one of claims 1 to 28, comprising the step of applying an alternating potential to said pair of interdigitated electrodes to excite shear modes having larger vibration amplitudes in said electrode means compared to said piezoelectric layer and having higher equivalent velocities than the fundamental shear wave mode of said piezoelectric layer.

32. 32. The method of claim 31 , wherein the shear modes occur primarily within the electrode means compared to the piezoelectric layer.

33. 29. A method of using a transducer structure according to any one of claims 1 to 28, comprising the step of applying an alternating potential to said pair of interdigitated electrodes to excite in said electrode means a shear mode having a pair of neutral lines which exhibits a shear motion within said electrode means and which has no higher equivalent velocity than the fundamental shear wave mode of said piezoelectric layer.

34. Method according to any one of claims 31 to 33, wherein the transducer structure is part of a filter, in particular a ladder filter and / or an impedance filter and / or a coupled filter, or a resonator, or a delay line, or a sensor.

35. 35. The method of claim 34, wherein the filter is used at frequencies greater than 3 GHz.

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