SURFACE ELASTIC WAVE DEVICE

Abstract

The invention relates to a surface elastic wave device (1) which comprises: - a composite substrate (2) which comprises a layer of piezoelectric material (5), a layer of dielectric material (7), a trapping layer (8) and a support substrate (S); - at least one electroacoustic transducer formed on the layer of piezoelectric material (5); the surface elastic wave device (1) being characterized in that the composite substrate comprises an additional layer (6) interposed between the layer of piezoelectric material (5) and the layer of dielectric material (7), the additional layer (6) being chosen so that the phase velocity of an elastic wave is greater than the phase velocities of the same elastic wave in either of the layer of dielectric material and the layer of piezoelectric material. Figure 1

Description

Description Title of the invention: ELASTIC WAVE DEVICE SURFACE FIELD OF INVENTION

[0001] — The present invention relates to the field of elastic wave devices surface. More specifically, the present invention relates to a wave device surface elastics formed on a composite substrate that includes a layer of piezoelectric material. More particularly, the present invention relates to a elastic surface wave device and for which parasitic modes are attenuated. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Initially manufactured on solid substrates of mono- piezoelectric material crystalline (hereinafter "bulk substrate"), surface elastic wave filters (SAW filter or Surface Acoustic Wave filter, according to Anglo- Saxone) now implement piezoelectric substrates on insulation (hereinafter designated "POI" for Piezoelectric-On-Insulator). These include in particular, from a front face to a rear face, a layer of piezo- material electrical, a layer of dielectric material, a trapping layer and a substrate support. The trapping layer, usually made of polycrystalline silicon, allows for to limit radio frequency losses in the substrate. The trapping layer advantageously includes a semiconductor material that exhibits resistivity electrical resistance greater than 1kΩ2.cm, advantageously between 2kΩ.cm and 5kΩ2.cm.

[0003] — These POI substrates, unlike solid substrates, make it possible to meet the requirements of certain requirements in terms of temperature sensitivity of guided modes in the layer of piezoelectric material.

[0004] — In particular, the supporting substrate acts as a stiffener and helps to limit the di- thermal latation and to influence the thermoelastic behavior of the layer of piezoelectric material.

[0005] — Thus, in general, a surface elastic mode propagating on a POI substrate exhibits a temperature coefficient of frequency ("TCF" or "Temperature coefficient of Frequency" (according to Anglo-Saxon terminology) rela- very low, and in particular less than 10 ppm.K!, in a range of frequencies which extends from 1.4 GHz to 2.4 GHz.

[0006] — The manufacture of POI substrates can implement a process called “SmartCutTM” well known in the field of microelectronics, particularly for the manufacture of silicon-on-insulator substrates.

[0007] — In this regard, document [1] cited at the end of the description proposes to transfer a layer of piezoelectric material, and more particularly of LiNbO; (hereinafter “LNO”), on a silicon substrate according to the SmartCutTM process, Document [2], cited at the end of the description, also discloses the implementation of a POI substrate for the fabrication of a surface elastic wave filter. The surface elastic wave filter described in this document comprises a mirror layer formed by alternating layers with low and high acoustic impedances. However, the implementation of a trapping layer has consequences for the spectral behavior of a surface elastic wave filter formed on such a substrate. Indeed, parasitic modes are likely to be observed during the operation of the filter in question. One aim of the present invention is therefore to propose a surface elastic wave filter formed from a POI substrate which makes it possible to limit the appearance of parasitic modes, BRIEF DESCRIPTION OF THE INVENTION The present invention relates to a surface elastic wave device comprising: - a composite substrate which comprises, from a front face to a rear face, a layer of piezoelectric material, a layer of dielectric material, a trapping layer and a support substrate, the trapping layer comprising a defect density greater than a predetermined defect density, the predetermined defect density being a defect density for which the resistivity of the trapping layer (8) is greater than or equal to 10 kΩ for temperatures between -20°C and 120°C; - at least one electroacoustic transducer formed on and / or in the layer of piezoelectric material; the surface elastic wave device being remarkable in that the composite substrate further comprises an additional layer intercalated between the layer of piezoelectric material and the layer of dielectric material, the additional layer being chosen such that the phase velocity of an elastic wave in said additional layer is greater than the phase velocities of the same elastic wave in both the layer of dielectric material and the layer of piezoelectric material. As an example, the predetermined defect density may be between 10'° and 10°! cm-3 (for example, in the case of a polycrystalline silicon trapping layer). Advantageously, the trapping layer comprises a porous or polycrystalline material. According to one implementation method, the additional layer is associated with a co- efficient temperature of the frequency of opposite sign to that associated with the dielectric material layer. Advantageously, the additional layer is associated with a negative frequency temperature coefficient. According to one implementation method, the additional layer has, for frequencies below 1 GHz, an acoustic impedance greater than or equal to that of the dielectric material layer. Depending on the implementation method, the additional layer has a thickness between 20 nm and 300 nm. According to one implementation method, the additional layer of dielectric material comprises at least one of the materials chosen from: SiN, Al,O;, AIN, Carbon diamond, GaN, LiNbO,, SiC. The invention also relates to another surface elastic wave device which comprises: - a composite substrate which comprises, from a front face to a rear face, a layer of piezoelectric material, a layer of dielectric material, a trapping layer and a support substrate, the trapping layer comprising a defect density greater than a predetermined defect density, the predetermined defect density being a defect density for which the resistivity of the trapping layer (8) is greater than or equal to 10 kΩ for temperatures between -20°C and 120°C; - at least one electroacoustic transducer formed on and / or in the layer of piezoelectric material; the surface elastic wave device is remarkable in that the support substrate comprises a silicon substrate with a 110 type orientation. As an example, the predetermined defect density may be between 10'° and 10°! cm-3 (for example, in the case of a polycrystalline silicon trapping layer). Advantageously, the trapping layer comprises a porous or polycrystalline material. According to one implementation method, the trapping layer comprises polycrystalline silicon and has a thickness between 400 nm and 2000 nm, advantageously greater than 1000 nm. According to one embodiment, the dielectric material layer has a thickness greater than 300 nm, advantageously a thickness between 300 nm and 600 nm, even more advantageously a thickness between 300 nm and 400 nm. According to one embodiment, the dielectric material layer comprises silicon dioxide. According to one implementation method, the layer of piezoelectric material has a thickness between 300 nm and 1000 nm, advantageously greater than 700 nm, even more advantageously greater than 800 nm. According to one implementation method, the piezoelectric material layer comprises either LITaOQ, and LINbO. According to one implementation method, the material forming the piezoelectric material layer has a cutting angle (YX) / 0 with 6 between 10° and 60°, advantageously between 15° and 50°. Brief description of the drawings Other features and advantages of the invention will become apparent from the detailed description that follows, with reference to the accompanying figures in which: [Fig.1] The [Fig.1] is a schematic representation of a surface elastic wave device along a cutting plane perpendicular to a main face of the substrate supporting the POI substrate according to the principles of the present invention; [Fig.2] [Fig.2] represents the theoretical conductance “G th” and experimental conductance “G exp” (left vertical axis in S), as well as the theoretical resistance “R th” and experimental resistance “R exp” (right vertical axis in Ohm) of a first resonator whose resonance frequency is equal to 750 MHz, as a function of the frequency (horizontal axis in MHz); [Fig.3] The [Fig.3] represents the theoretical susceptances “X th” and experimental “X exp” (left vertical axis in S), as well as the theoretical “B th” and experimental “B exp” (right vertical axis in Ohm) reactances of the first resonator as a function of frequency (horizontal axis in MHz); [Fig.4] Fig.4 represents the theoretical conductance "G th" and experimental conductance "G exp" (left vertical axis in S), as well as the theoretical resistance "R th" and experimental resistance "R exp" (right vertical axis in Ohm) of a second resonator whose resonance frequency is equal to 940 MHz, as a function of frequency (horizontal axis in MHz); [Fig.5] The [Fig.5] represents the theoretical susceptances “X th” and experimental “X exp” (left vertical axis in S), as well as the theoretical “B th” and experimental “B exp” (right vertical axis in Ohm) reactances of the second resonator as a function of frequency (horizontal axis in MHz); [Fig.6] Fig.6 graphically represents the conductance G (left vertical axis, in S / m) and the susceptance B (right vertical axis, in S / m) of a surface elastic wave resonator formed from the standard POI substrate and a POI substrate according to the present invention, as a function of frequency (horizontal axis, in MHz), focused on the parasitic mode from which we wish to free ourselves according to the invention. DETAILED DESCRIPTION OF THE INVENTION For the sake of simplicity in the following description, the same references are used for identical elements or elements performing the same function in the different modes of implementation described of the invention and in the prior art. The present invention relates to a surface elastic wave device formed from a POI substrate. According to the present invention, the surface elastic wave device can be, at will: a filter, a resonator, or a sensor. It is understood, however, that the present invention is not limited to these aspects, and those skilled in the art may adapt its principles to any type of surface elastic wave device formed from a POI substrate. In particular, the present invention relates to a surface elastic wave device made on a POI substrate. In particular, the POI substrate comprises, from a front face to a rear face, a layer of piezoelectric material, a layer of dielectric material, a trapping layer and a support substrate. In the implementation of a surface skimming bulk wave (SSBW) device known from the prior art, a bulk elastic wave propagates within the piezoelectric material layer if its phase velocity is lower than the velocity the same wave would have in the substrate. Consequently, there is a velocity, called the threshold velocity (SSBW or Surface Skimming Bulk Wave), beyond which the wave is likely to radiate towards the substrate. This radiation occurs at a specific angle, called the radiation angle, which depends on its phase velocity. Depending on the value of the radiation angle and the nature of the layers underlying the piezoelectric layer, the radiated energy excites a propagation mode that is consistent with the charge distribution under the electrodes of the piezoelectric transducer.A spurious signal is then likely to be detected, particularly when the phase velocity in the trapping layer differs from that in the support substrate, the difference being in absolute value strictly greater than 0% and less than or equal to 10% relative to the phase velocity in the support substrate. Such a case arises when the POI substrate includes a trapping layer made of polycrystalline or amorphous silicon interposed between the dielectric material layer and a silicon support substrate. Conversely, the dielectric material layer, when made of SiO₂, is not likely to generate such a spurious signal in the presence of a silicon support substrate because the phase velocity of the elastic bulk wave in this layer is much lower than that observed in a silicon support substrate. The layers above the dielectric material layer have no impact on the generation of parasitic modes following this mechanism. However, the nature of the substrate can have an impact. In particular, the amplitude of the parasitic mode is greater when the threshold velocity is high. This problem is present for frequencies f below 1 GHz but can also occur beyond this limit. To address this problem, a thinner trapping layer could be considered. Such a solution would, without further consideration, change the equivalent phase velocity of the parasitic mode, thereby invalidating the coherence condition mentioned above. However, using a thinner trapping layer would also increase radiofrequency losses in the supporting substrate. According to a first aspect of the present invention, the POI substrate may comprise an additional layer interposed between the piezoelectric material layer and the dielectric material layer. The additional layer is chosen such that the phase velocity of an elastic wave in said additional layer is greater than the phase velocities of the same elastic wave in both the dielectric material layer and the piezoelectric material layer. According to another aspect, complementary or alternative, the support substrate includes a silicon substrate with a 110 type orientation. Furthermore, the surface elastic wave device comprises at least one electroacoustic transducer formed on and / or within the piezoelectric material layer. For example, and without limiting the scope of the present invention, the surface elastic wave device comprises at least one input electroacoustic transducer and at least one output electroacoustic transducer formed on and / or within the piezoelectric material layer. The at least one electroacoustic transducer according to the present invention may be equipped with electrode Bragg mirrors. These electrodes may be arranged periodically, and in particular may have a repetition period close to that of electrodes of the at least one transducer located on either side of the transducer. The structure may include at least one free surface interval within it.The resonator thus formed is used as an impedance element of a filter combining one or more identical cells incorporating at least one resonator as described above. It is understood that the orientation of a substrate or layer must be associated with its crystal orientation, as defined by IEEE 1949 Std-176. It is further understood that an angular deviation from a crystal orientation may be permitted. In particular, whatever the crystal orientation considered, it will generally be accepted that the latter is specified to within 10°, preferably to within 5°, around said orientation. Fig. 1 is a schematic illustration of a surface elastic wave device according to a first aspect of the present invention (in particular, in this Fig. 1, the device is a resonator). In particular, the surface elastic wave device 1] includes a POI substrate 2. The POI 2 substrate comprises, in this respect, from a front face 3 to a rear face 4, a layer of piezoelectric material 5, a layer of dielectric material 7, a trapping layer 8 and a support substrate S. The trapping layer is adapted to limit the mean free path of electric carriers at the interface formed between the dielectric material layer 7 and said trapping layer 8 with regard to the mean free path of electric carriers at an interface formed between the dielectric material layer 7 and the support substrate S. In particular, the trapping layer 8 comprises a defect density greater than a predetermined defect density, the predetermined defect density being a defect density for which the resistivity of the trapping layer (8) is greater than or equal to 10 k® for temperatures between -20°C and 120°C. As an example, the predetermined defect density may be between 10⁻¹² and 10⁻¹³ cm⁻³ (for example, in the case of a polycrystalline silicon trapping layer). Advantageously, the trapping layer comprises a porous or polycrystalline material. The trapping layer may include polycrystalline silicon or amorphous silicon, and have a thickness between 400 nm and 2000 nm, advantageously greater than 1000 nm. The dielectric material layer 7 has, for example, a thickness greater than 300 nm, advantageously a thickness between 300 nm and 600 nm, even more advantageously a thickness between 300 nm and 400 nm. As another example, the dielectric material layer 7 comprises silicon dioxide. The piezoelectric material layer 5 can have a thickness between 300 nm and 1000 nm, advantageously greater than 700 nm, even more advantageously greater than 800 nm. In particular, the piezoelectric material layer 5 comprises either LiTaO or LINbO-. The material forming the piezoelectric material layer has a cutting angle (YX) / ® with © between 10° and 60°, advantageously between 15° and 50°. As an example, the elastic wave device with surface area 1 can be a bandpass filter that has a center frequency fo and a bandwidth Af which can be expressed as a fraction of this center frequency, typically between 0.1% and 12% with a POI substrate equipped with a piezoelectric layer 5 made of lithium tantalate (LiTaO). Considering a piezoelectric layer 5 based on LiNbO allows the upper limit of this fraction to be pushed back to 15% and more by choosing the crystalline orientation of said layer so as to maximize the electromechanical coupling of the mode thus exploited. The surface elastic wave device 1 comprises at least one electroacoustic transducer. For example, and as illustrated in [Fig. 1], said device may comprise at least one input electroacoustic transducer 9 and at least one output electroacoustic transducer 10. In another embodiment, the surface elastic wave device consists of at least one cell equipped with at least one surface elastic wave resonator forming a quadrupole with an input port and an output port. Several cells are generally arranged in cascade to form a filter designated as an impedance element filter. In operation, the electromagnetic signal to be filtered is applied to an input port of the filter, and the filtered electromagnetic signal is taken from the filter's output port. It should be noted, however, that the designations "input" and "output" are entirely arbitrary, and the surface elastic wave device, and in particular the filter, can be operated by applying / taking an electromagnetic signal from either of the two ports. In a very general way, the input electroacoustic transducer 9 and the output electroacoustic transducer 10 conform to those known from the prior art, some characteristics of which are recalled below. Each transducer 9 and 10 comprises two interdigitated comb electrodes. Such transducers 9 and 10 are each formed of a network of metallic fingers that are alternately connected to two buses between which an electrical potential difference is applied / taken. With this device, a surface elastic wave can thus be directly generated / detected in the piezoelectric material layer 5. The input electroacoustic transducer 9 and the output electroacoustic transducer 10 are generally configured to be identical. However, the invention is not limited to this aspect; those skilled in the art may consider two different transducers. The filters thus formed are generally referred to as transverse filters. Indeed, their transfer function can be advantageously modulated by the interpenetration of comb-shaped electrodes along the direction of wave propagation. For this type of component, the transducers are capable of operating outside the Bragg condition. These filters are considered to have a finite impulse response. Those skilled in the art can also implement... Transducers operating under Bragg conditions are advantageously fitted with mirrors at each end of the filter to reflect the in-phase energy back into the structure, thus resulting in filters with infinite impulse response. The impedance element filters mentioned earlier also correspond to filters with infinite impulse response insofar as they are composed of resonant elements. The electrode metal is typically aluminum-based, for example, pure aluminum or an aluminum alloy such as aluminum doped with Cu, Si, or Ti. However, it is possible to use other materials, for example, to achieve a higher reflection coefficient with a smaller electrode thickness than would be required with aluminum to achieve similar characteristics. In this regard, preferred electrode materials include copper (Cu), molybdenum (Mo), nickel (Ni), platinum (Pt), or gold (Au) with an adhesive layer such as titanium (Ti), tantalum (Ta), chromium (Cr), zirconium (Zr), palladium (Pd), iridium (Ir), tungsten (W), etc. The period p of the electrodes is generally chosen such that p = A / 2, where A is the wavelength of the elastic wave propagating in the piezoelectric material layer 5 at the synchronous frequency f of the transducer, given by Σ = V / Ve, with Ve representing the phase velocity of the operating mode. The transducer then operates under the Bragg condition mentioned above. Other configurations are possible, and more generally the period p can be written as p = M(nb_elec), where nb_elec is the number of fingers (electrodes) per wavelength. These parameters are related by the equation Σ = V / f, in which V represents the phase velocity of the elastic wave under the transducer.The speed of the elastic wave on a free surface or under an electroacoustic transducer depends in particular on the nature of the material in which the wave propagates, and it is generally known or accessible to a person skilled in the art by specifically using the elastic, piezoelectric and dielectric constants as well as the density tabulated in the literature in suitable models. In the case of a bandpass filter, particularly one with a given frequency and bandwidth, it may be advisable to use transducers whose synchronous frequencies are chosen to meet the aforementioned requirements. The methods for implementing this choice(s) are known to those skilled in the art and are therefore not described here. The electrode metallization width, denoted a, is generally chosen so that the a / p ratio is around 0.5 for technological control reasons, but other ratios are possible to optimize the characteristics of the guided mode. The number of finger pairs on each transducer is usually around 20 or 100, but this value must be adapted to the conditions imposed by the system. host in particular the electrical circuit integrating the filter) of the filter. In general, increasing the number of finger pairs increases the rejection of frequencies out of the bandwidth and reduces the spectral range of the transducer's admittance or impedance response around its synchronous frequency. The elastic wave resonator with surface 1 may also include two external mirrors 11 and 12 arranged on either side of the pair of electroacoustic input 9 and output 10 transducers. As is well known, these mirrors 11 and 12 allow for maximum confinement of the elastic energy between the two electroacoustic input 9 and output 10 transducers. Accordingly, they are designed to exhibit a very high reflection coefficient, as close as possible to 1 (total reflection of the incident energy), by selecting the metallic thickness of the mirror fingers and the number of these fingers, typically a few dozen per mirror in the case of a POI substrate for a shear wave. It should be noted, however, that these two external mirrors are by no means essential to the invention, and that a filter according to the invention and fully functional may do without them. The implementation of the trapping layer, as indicated in the section "technological background of the invention", is not without consequence on the functioning of the surface elastic wave filter considered. In particular, trapping layer 8 contributes to the emergence of parasitic modes. To illustrate this effect, [Fig.2], [Fig.3], [Fig.4] and [Fig.5] illustrate, theoretically and experimentally, the frequency behavior of surface elastic wave resonators formed on a known POI substrate from the state of the art and implementing a trapping layer made of polycrystalline silicon. Specifically, the known POI substrate from the prior art implemented in these illustrations includes: - a layer of LiTaO, (YX1) / 50° of 620 nm; - a layer of SiO, 500 nm thick; - a 1 µm layer of polycrystalline silicon; - a silicon substrate. In particular, [Fig.2] represents the theoretical conductance “G th” and experimental conductance “G exp” (left vertical axis in S), as well as the theoretical resistance “R th” and experimental resistance “R exp” (right vertical axis in Ohm) of a first resonator whose resonance frequency is equal to 750 MHz, as a function of the frequency (horizontal axis in MHz). Fig. 3 represents the theoretical susceptances "Xth" and experimental susceptances "Xexp" (left vertical axis in S), as well as the theoretical reactances "Bth" and experimental reactances "Bexp" (right vertical axis in Ohm) of the first resonator as a function of frequency (horizontal axis in MHz). Figure 4 represents the theoretical conductance "Gth" and experimental conductance "Gexp" (left vertical axis in S), as well as the theoretical resistance "Rth" and experimental resistance "Rexp" (right vertical axis in Ohm) of a second resonator whose resonance frequency is equal to 940 MHz, as a function of frequency (horizontal axis in MHz). Fig. 5 represents the theoretical susceptances "Xth" and experimental susceptances "Xexp" (left vertical axis in S), as well as the theoretical reactances "Bth" and experimental reactances "Bexp" (right vertical axis in Ohm) of the second resonator as a function of frequency (horizontal axis in MHz). Each of these figures shows a resonance peak "A" as well as a peak "B" at a frequency equal to 1.5 times the resonance frequency of the peak considered. To overcome this effect, according to a first aspect of the present invention, it is proposed to interpose an additional layer 6 between the piezoelectric material layer 5 and the dielectric material layer 7. The additional layer 6 is in particular chosen so that the phase velocity of an elastic wave in said additional layer is greater than the phase velocities of the same elastic wave in both the dielectric material layer and the piezoelectric material layer. The additional layer 6 can have a thickness between 20 nm and 300 nm. As an example, the additional layer 6 includes at least one of the materials chosen from: SiN, Al.O, AIN, Diamond Carbon, GaN, LiNbO4, SiC. It is understood without needing to specify that the silicon nitride (SIN) considered in the present invention is not necessarily stoichiometric. Figure 6 illustrates the effect of implementing an additional layer 6 made of silicon nitride (SIN). Specifically, in this illustration, the surface elastic wave resonator is formed on a POI substrate which comprises: - a layer of LiTaO, (YX1) / 50° of 620 nm; - a 100 nm SiN layer; - a layer of SiO, 500 nm thick; - a 1 µm layer of polycrystalline silicon; - a silicon substrate. Figure 6 graphically represents the conductance G (left vertical axis, in S / m) and susceptance B (right vertical axis, in S / m) of a surface elastic wave filter formed from the POI substrate described above. More specifically, the susceptance and conductance of this filter are denoted, respectively, BSiN+TR | µm and GSiN+TR | µm. For comparison, this graph also illustrates the conductance GTR | µm and susceptance BTR | µm of a resonator with elastic surface waves formed from a known POI substrate from the prior art and comprising a 1 µm polycrystalline silicon layer. In both cases considered, a parasitic mode (called a first-order mode) can be observed (area "B" in [Fig. 6]). However, the amplitude of this first-order parasitic mode is attenuated when a SiN layer is considered. A second-order parasitic mode can also be observed in both cases (area "C" in [Fig. 6]). The inventors were able to establish that the effect of alumina (AlO3), diamond carbon, silicon carbide (SiC) is similar to that observed with the implementation of silicon nitride. The present invention also covers a second alternative or complementary aspect to the first aspect described above. The second aspect essentially replicates all the characteristics of the first aspect. However, the second aspect does not necessarily require consideration of the additional layer 6. According to the second aspect, the support substrate S comprises at least one of the elements chosen from: a silicon substrate having either of the orientations 110 (CYXt) / 45°) and 111, a sapphire substrate having an orientation XXX. The inventors have demonstrated that the implementation of a silicon substrate having either of the orientations 110 ((YXt) / 45°) and 111, or of a sapphire substrate having an orientation XXX makes it possible to reduce the amplitude of parasitic modes. The implementation of the present invention according to either of the first and second aspects allows the amplitude of the parasitic modes likely to be observed when a trapping layer is implemented. Furthermore, these two aspects also allow the frequency behavior in temperature of surface elastic wave filters formed on POI substrates. Of course, the invention is not limited to the embodiments described and alternative embodiments can be made without departing from the scope of the invention as defined by the claims. references [1] T. Pastureaud, et al, “High-Frequency Surface Acoustic Waves Excited on Thin-Oriented LiNbO; Single-Crystal Layers Transferred Onto Silicon”, IEEE Trans. on UFFC, vol. 54, no. 4, pp.870-876, 2007; [2] US 2014 / 0152146 A1; [3] S. Ballandras, et al, “Simulations of surface acoustic wave devices built on stratified media using a mixed finite element / boundary integral formulation”, Journal of Applied Physics, n 12, pp. 7731-7741, 2004.

Claims

Claims

1. A surface elastic wave device (1) which comprises: - a composite substrate (2) which comprises, from a front face (3) to a rear face (4), a layer of piezoelectric material (5), a layer of dielectric material (7), a trapping layer (8) and a support substrate (S), the trapping layer (8) comprising a defect density greater than a predetermined defect density, the predetermined defect density being a defect density for which the resistivity of the trapping layer (8) is greater than or equal to 10 kΩ for temperatures between -20°C and 120°C; - at least one electroacoustic transducer (9, 10) formed on and / or in the layer of piezoelectric material (5);the surface elastic wave device (1) being characterized in that the composite substrate further comprises an additional layer (6) interposed between the layer of piezoelectric material (5) and the layer of dielectric material (7), the additional layer (6) being chosen so that the phase speed of an elastic wave in said additional layer is greater than the phase speeds of the same elastic wave in both the layer of dielectric material and the layer of piezoelectric material, and the additional layer (6) comprises at least one of the materials chosen from: Carbon diamond, LiNbO3.;

2. Surface elastic wave device (1) according to claim 1, wherein the additional layer (6) is associated with a temperature coefficient of the frequency of opposite sign to that associated with the layer of dielectric material (7).

3. Surface elastic wave device (1) according to claim 1 or 2, in which the additional layer (6) has, for frequencies below 1 GHz, an acoustic impedance greater than or equal to that of the layer of dielectric material (7).

4. Surface elastic wave device (1) according to one of claims 1 to 3, in which the additional layer (6) has a thickness of between 20 nm and 300 nm.

5. Surface elastic wave device (1) according to one of claims 1 to 4, wherein the trapping layer (8) comprises polycrystalline silicon and has a thickness of between 400 nm and 2000 nm, advantageously greater than 1000 nm.

6. Surface elastic wave device (1) according to one of claims 1 to 5, in which the layer of dielectric material (7) has a thickness greater than 300 nm, advantageously a thickness of between 300 nm and 600 nm, even more advantageously a thickness of between 300 nm and 400 nm.

7. Surface elastic wave device (1) according to one of claims 1 to 6, wherein the layer of dielectric material (7) comprises silicon dioxide.

8. Surface elastic wave device (1) according to one of claims 1 to 7, in which the layer of piezoelectric material (5) has a thickness of between 300 nm and 1000 nm, advantageously greater than 700 nm, even more advantageously greater than 800 nm.

9. The surface elastic wave device (1) according to one of claims 1 to 8, wherein the piezoelectric material layer (5) comprises either LiTaO3 or LiNbO3.

10. Surface elastic wave device (1) according to one of claims 1 to 9, in which the material forming the layer of piezoelectric material (5) has a cutting angle (YX) / 0 with 0 between 10° and 60°, advantageously between 15° and 50°.

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