Elastic wave device

Abstract

The present invention relates to an elastic wave device, in particular a shear wave device, comprising a piezoelectric material (3), in particular a ferroelectric material with first domains (3a) having a first polarisation direction (13a) and second domains (3b) having a second polarisation direction (13b), the first direction (13a) being opposite to the second direction, wherein the first and second domains (3a, 3b) alternate periodically in a direction d, referred to as the periodic direction, which is perpendicular to the normal n to the surface of the piezoelectric material (3), and a pair of interdigitated comb electrodes (15a, 15b) above, in particular on, the piezoelectric material (3), the respective comb teeth (17a1 to 17a3 and 17b1 to 17b3) of which extend essentially perpendicular to the periodic direction d and to the normal n to the surface of the substrate.

Description

DESCRIPTION TITLE OF THE INVENTION: Elastic wave device

[0001] The invention relates to an elastic wave device and a method of manufacturing such an elastic wave device.

[0002] Surface acoustic wave (SAW) devices are used in a wide range of applications, such as filters, sensors, and delay lines. SAW radio frequency filters are used, for example, in mobile communication devices due to their simple structures with low insertion losses, typically less than 3 dB or even 2 dB in the filter bandwidth, and small sizes, less than a millimeter. 2 , at radio frequencies (100 MHz - 10 GHz).

[0003] In an elastic wave device, one or more interdigital comb transducers (IDTs) are formed on a single-crystal piezoelectric substrate. The direct piezoelectric effect converts an elastic wave, usually of the Rayleigh type, propagating on the surface of the piezoelectric substrate into electrical signals by electrically exciting the comb fingers. Conversely, an electrical signal can be induced through the comb fingers to create an elastic surface wave propagating in the piezoelectric substrate beneath the transducer.

[0004] The speed of elastic waves is generally limited by the properties of the piezoelectric material. In the case of lithium tantalate (LiTaCh), Rayleigh waves have a phase speed between 3,000 ms' 1 and 3,500 ms' 1, with a maximum coupling of the order of 2%. Rayleigh waves on lithium niobate (LiNbOa) exhibit phase velocities of up to 3,900 ms' 1 with a maximum coupling close to 5.6%. The other modes related to shear and compression waves are typically faster but are only partially guided on the surface and require complex guide structures.

[0005] Piezoelectric on insulator (POI) type wafers allow modes with higher phase speeds but which do not exceed 4,200 ms -1 on LiTaOa and 4,500 ms -1 on LiNbOa without radiation losses in the substrate.

[0006] The object of the invention is therefore to overcome these drawbacks by providing an elastic wave device, having a higher phase velocity operating mode than guided Rayleigh or shear waves.

[0007] To achieve the object, the invention provides an elastic wave device, in particular a shear wave device, comprising a piezoelectric material, in particular a ferroelectric material with first domains of a first polarization direction and second domains with a second polarization direction, the first direction being opposite to the second direction, in which the first and second domains are periodically alternated in a direction, called the periodic direction, perpendicular to the surface normal of the piezoelectric material, and a pair of interdigitated comb electrodes above, in particular on, the piezoelectric material whose respective comb teeth extend essentially perpendicular to the periodic direction and to the normal.

[0008] Due to the opposite polarization domains, modes appear with phase velocities greater than 4,200 ms' 1which propagate on the surface without radiation in the volume. This approach also makes it possible to create boundary conditions different from the operating modes without domains, giving rise to polarization modes more complex than the usual fundamental modes. In addition, when the implementation conditions allow it, the visualization of the alternating domains allows easy alignment of the combs with respect to the periodic ferroelectric structure. These modes are shear modes.

[0009] The elastic wave devices according to the invention can be used in a large number of applications, such as filters, in particular filters with impedance elements ("ladder" in English terminology) or acoustic coupling such as longitudinally coupled resonator filters (LCRF for "Longitudinally Coupled Resonator Filter" in English), double coupled modes (DMS for "Double-Mode-SAW" in English) or coupled cavity resonance filters (SCAW for "Surface Cavity Acoustic Wave" in English) as well as sensors and delay lines.

[0010] Ferroelectricity is the property that a material possesses spontaneous electrical polarization in its natural state. Ferroelectric materials are a subclass of piezoelectric materials. The ferroelectric property can also be achieved artificially, for example, by suitable growth conditions to create domains of different polarization. In the following, any piezoelectric material in which polarization domains can be produced can therefore be considered a ferroelectric material.

[0011] Alternatively, the comb teeth of the pair of interdigitated comb electrodes may be arranged periodically. The teeth preferably have a rectangular shape, i.e., a parallelepiped shape.

[0012] Alternatively, the comb teeth can be positioned over interfaces between the first domains and the second domains of opposite polarization directions. In such a configuration, acoustic modes are observed at frequencies twice as high compared to a configuration without domains but with the same electrodes and materials. These modes therefore have a phase velocity v e effective ff about twice as large, with Ven = Àxf = p e ixf of the mode, A being the coherence wavelength of the charges in the electrodes, p ei being the electrical period of the electrodes and f the frequency of the mode.

[0013] Alternatively, the comb teeth may be positioned symmetrically with respect to the interfaces between the first domains and the second domains. In this configuration, in which the central plane of the teeth coincides with the plane of the interfaces, contributions from parasitic modes may be reduced or even eliminated.

[0014] Alternatively, the directly adjacent comb teeth of the two interdigitated comb electrodes may be positioned above the interfaces between the first domains and the directly adjacent second domains. This provides a periodicity equal to the periodicity of the domains.

[0015] Operating devices at higher frequencies requires finer comb tooth dimensions, thus imposing the use of higher-resolution photolithography, for example using KrF or ArF excimer lasers, which is significantly more expensive. In addition, finer structures generate greater electrical, i.e. ohmic, losses.

[0016] Alternatively, the width of the teeth of the interdigitated comb electrodes may be between 25% and 75%, preferably between 40% and 60%, even more favorably 50%, of the width of the domains, and / or at least 280 nm, preferably at least 350 nm. Thus, it becomes possible to provide filters, sensors or delay lines operating at higher frequencies than normally accessible using l-line lithography.

[0017] Alternatively, each of the comb teeth may have a width corresponding to the width of a first domain and a second domain. Thus, particularly when the electrodes are arranged symmetrically with respect to the interfaces between the first domains and the second domains, the current flowing between the electrodes may be increased.

[0018] Alternatively, each of the comb teeth may be positioned entirely above only one of the first domains or second domains.

[0019] According to an alternative of this variant, the comb teeth directly adjacent to the electrodes can be positioned above domains with the same polarization direction, in particular domains with the same polarization direction only separated by a domain with the opposite polarization direction. In this configuration the second-order mode is suppressed and a third-order harmonic mode is favored over the fundamental shear mode with an effective phase velocity three times larger. It thus becomes possible to achieve phase velocities much higher than that of the fundamental mode; in particular, phase velocities larger than 10 km can be observed in simulations. -1 , which allows frequencies beyond the C band of the electromagnetic spectrum to be reached with l-line technology.

[0020] According to the further alternative of this variant, the comb teeth of one of the two comb electrodes may be positioned above the first domains and the comb teeth of the second comb electrode may be positioned above the second domains. In this embodiment, the second-order mode is favored over the fundamental mode and thus an increase in the effective phase velocity by a factor of two is observed.

[0021] Alternatively, the width of the comb teeth of the interdigitated comb electrodes may be the same as the width of the domains, and / or at least 280 nm, preferably at least 350 nm to be compatible with the limitations of l-line technology. Thus, it becomes possible to provide filters, sensors or delay lines operating at higher frequencies than normally accessible using l-line lithography. Furthermore, in the case of the teeth always positioned on domains of the same polarization, the electrodes can be used to create domains.

[0022] Alternatively, each of the comb teeth of one of the two electrodes may be positioned entirely over only one of the first and / or second domains while each of the comb teeth of the second electrode is positioned over interfaces between the first domains and the second domains. Multiple modes are observed with effective phase velocities greater than the phase velocity of the basic mode.

[0023] Alternatively, the device may be made such that there is at least one entire domain free of electrode fingers between two domains on which two teeth are positioned at least partially. By moving the electrodes apart, it becomes possible to vary the modes and frequencies and thus the effective phase velocities.

[0024] According to a variant of the invention, the piezoelectric material may be arranged in the form of a layer, in particular with a thickness less than the wavelength, preferably less than λ / 2, and even more preferably less than λ / 4, typically less than 1 μm, above a base substrate, in particular a substrate of silicon, amorphous silicon or polysilicon, silicon oxide, silicon carbide (SiC), sapphire, silicon nitride (SiN), aluminum nitride (AIN), quartz, carbon, diamond, Yags, Yigs (respectively "Ytrium aluminum garnet" and "Ytrium iron garnet" in English terminology), or LiNbOa and / or LiTaCh. It may be a two-layer structure or a stack with more than two layers. Thus it becomes possible to confine the energy in the piezoelectric thin layer, which makes it possible to reduce the overall losses of the mode.

[0025] According to a variant of the invention, a dielectric layer, in particular a layer of silicon oxide, SiN, TaaOs, ZrC>2, HfCh, SiON, polysilicon or a combination of these materials, can be arranged between the layer of piezoelectric material and the base substrate. The dielectric layer, preferably having a thickness less than the wavelength and / or a thickness between 0.2 pm and 2 pm, simplifies the production of the multilayer structure. Indeed, this layer can be used to bond the piezoelectric layer to the base substrate during a layer transfer process, for example by bonding, for example using molecular bonding or by adhesive bonding or by eutectic bonding.The process can be of the SmartCut™ type. The use of silicon oxide as a dielectric layer further allows the temperature stability of the device to be adjusted, which is a consequence of the opposite signs of the temperature coefficients of the elastic wave velocity of silicon and silicon oxide. A similar effect can be achieved with thallium pentoxide (Ta2Os). Thus, the thickness of the dielectric layer also depends on the thickness chosen for the piezoelectric layer and can preferably be chosen to reduce or even minimize the effect of temperature on the device.

[0026] According to a variant of the invention, a trapping layer, in particular a layer of polycrystalline silicon or polycrystalline AIN or SiOCH, may be arranged between the piezoelectric material and the base substrate or between the dielectric layer and the base substrate. This trapping layer preferably has a thickness of between 300 nm (i.e. 0.3 pm) and 2 pm. The presence of a trapping layer makes it possible to reduce leakage currents.

[0027] According to a variant of the invention, a metal layer may be arranged on the surface of the piezoelectric material opposite the pair of electrodes in interdigital combs, particularly between the piezoelectric material layer and the base substrate or between the dielectric layer and the base substrate. The presence of the metal layer simplifies the creation of the alternating domains by local application of an electric field opposite to the native polarization direction and exceeding the coercive field of the ferroelectric material.

[0028] When the metal layer is located between the dielectric layer and the base substrate, the coupling of the new mode is higher, because the further the metal layer is from the piezoelectric layer, the more the electric field lines in the piezoelectric layer are parallel to the surface. Preferably, the metal can be encapsulated to reduce or even avoid metal contamination, for example during the steps of a SmartCut™ process. It may also be advantageous to limit the location of the metal layer under the interpenetration zone of the electrodes of the interdigitated comb transducer in order to limit the parasitic elements linked to the presence of metal under the tracks and connection pads of the resulting device.

[0029] According to a variant of the invention, the piezoelectric material may be at least one of LiTaOs, LiNbOa, an ABO3 type perovskite, in particular KNbOaOr PbTiOa, PZT, PMnPt, or AIScN. The modes observed for these materials have quality factors and electromechanical couplings allowing industrial use in filter or delay line type applications.

[0030] According to a variant of the invention, the elastic wave device may comprise a first electrode which comprises first switches and a second electrode which comprises second switches, the first switches may be configured such that they can bring the first electrode into contact or not with teeth respectively and the second switches may be configured such that they can bring the second electrode into contact or not with teeth respectively, in particular to form the comb electrode pair. Thus, it becomes possible to adapt the excitation pattern of the teeth for example by switching from a pattern in which the teeth are connected alternately to the two electrodes to a pattern in which two directly adjacent teeth are connected to the first electrode and the next two directly adjacent teeth are connected to the second electrode.By changing the excitation scheme, the frequency of the modes can be moved and thus the same device can be adapted to different operating parameters.

[0031] Preferably, the switching of the teeth is applied so as not to short-circuit the first and second electrodes.

[0032] According to a variant of the invention, the device may comprise a power supply means configured to supply a radio frequency signal of a frequency of at least 2 GHz, in particular at least 3 GHz and even more in particular at least 6 GHz, to the interdigitated comb electrodes. Thus, it becomes possible to provide filters, sensors or delay lines operating at higher frequencies than normally accessible using l-line lithography.

[0033] According to a variant of the invention, the pair of interdigitated comb electrodes may be the only electrically conductive element in contact with the piezoelectric material. According to a variant of the invention, the elastic wave device may be devoid of any other electrode means apart from the pair of interdigitated comb electrodes, in particular devoid of any floating electrode on the opposite face of the piezoelectric material relative to the location of the pair of interdigitated comb electrodes. This prevents part of the energy introduced into the system from being used for the formation of modes other than the shear modes and the appearance of parasitic capacitances which may reduce the coupling of the mode of interest.

[0034] To achieve the object, the invention also proposes the use of an elastic wave device as described above, in particular with interdigitated comb electrode teeth having a width of at least 280 nm, preferably at least 350 nm, in an acoustoelectric device, in particular a filter, a sensor or a delay line, having operating frequencies of 2 GHz and above, in particular 3 GHz or above, and even more in particular 6 GHz or above. Thus it becomes possible to provide filters, sensors or delay lines operating at higher frequencies than normally accessible using l-line lithography.

[0035] To achieve the object, the invention also provides a method for manufacturing an elastic wave device as described above and comprising the following steps: providing a piezoelectric material, in particular a ferroelectric material, applying an electric field stronger, in particular at least 10 times stronger, than the coercive field of the piezoelectric material to produce the first and second domains alternating periodically and having opposite polarizations, and producing the pair of interdigitated comb electrodes above, in particular directly on, the piezoelectric material.

[0036] To achieve the object, the invention also provides a method of manufacturing an elastic wave device as described above and for which the directly neighboring comb teeth are positioned above domains with the same polarization direction and comprising the following steps: providing a material piezoelectric material, in particular a ferroelectric material, making the pair of interdigitated comb electrodes above, in particular directly on, the piezoelectric material and applying a stronger electric field, in particular at least ten times stronger, than the coercive field of the piezoelectric material using the pair of interdigitated comb electrodes to make the first and second domains alternating periodically and having opposite polarizations. Thus, the manufacturing process can be simplified.

[0037] According to a variant of the invention, the pair of electrodes can be made by depositing a metal layer, in particular aluminum (Al) or molybdenum (Mo) or gold (Au) or silver (Ag) or an alloy of the copper aluminum type (AlCu) or an alloy mainly based on aluminum and another metal, titanium (Ti) for example, followed by a step of forming the electrodes by lithography and etching. The pair of electrodes can in particular be made with AICu, or AISi, or AITi with Cu, Ti and / or Si dopings of between 0.5 and 5% and potentially with sub-layers of Ti, Ta, Mo, Pd, or Pt, or Ti / Pt, Ti / Au, Ta / Pt, Cr / Au combinations or with Zr.

[0038] According to a variant of the invention, the electric field can be applied in the form of parallel strips, in particular using electrolytic electrodes, preferably lithium chloride.

[0039] According to a variant of the invention, the electric field can be applied once the piezoelectric material has been heated to a temperature of at least 150°C, preferably to a temperature of at least 170°C, in order to lower the coercive field beyond which it is possible to flip the ferroelectric domains of the upper layer by applying an electric field oriented in the opposite direction of its native polarization. Thus, the polarization can be obtained even in the absence of a counter electrode.

[0040] According to a variant of the invention, the piezoelectric material is provided in the form of a layer on a base substrate. This type of composite substrate can be obtained by transferring a piezoelectric layer from a donor substrate onto the base substrate. This transfer can be done by a SmartCut™ type process with the creation of a weakening zone in the donor substrate, by implanting ions in the donor substrate. According to the invention, the formation of the domains can be carried out before or after the transfer of the piezoelectric layer.

[0041] The invention will be better understood and other advantages will appear on reading the description which follows, given without limitation, and thanks to the appended figures among which:

[0042] [Fig. 1] is a diagram schematically illustrating an elastic wave device according to a first embodiment of the invention.

[0043] [Fig. 2] is a diagram schematically illustrating a sectional view of a portion of the elastic wave device according to the first embodiment of the invention as well as a diagram of the shear mode.

[0044] [Fig. 3] is a graph illustrating the harmonic conductance G and harmonic admittance Y for a state-of-the-art elastic wave device.

[0045] [Fig. 4] is a graph illustrating the harmonic conductance G and harmonic admittance Y for an elastic wave device according to the first embodiment of the invention.

[0046] [Fig. 5] illustrates the Y harmonic admittance and B harmonic susceptance for the three pairs of angles 0 / 0+180°: 307210°, 427222° and 507230° of the piezoelectric material LiTaOs (YXI) / 0 and 0+180°.

[0047] [Fig. 6] is a diagram schematically illustrating a sectional view of a portion of the elastic wave device according to a variant of the first embodiment of the invention as well as a diagram of the two shear modes.

[0048] [Fig. 7] is a graph illustrating the harmonic conductance G and harmonic admittance Y for an elastic wave device according to the variant of the first embodiment of the invention.

[0049] [Fig. 8] is a diagram schematically illustrating a sectional view of a part of the elastic wave device according to a second variant of the first embodiment of the invention as well as a diagram of the two shear modes.

[0050] [Fig. 9] is a graph illustrating the harmonic conductance G and harmonic admittance Y for an elastic wave device according to the second variant of the first embodiment of the invention.

[0051] [Fig. 10] is a diagram schematically illustrating a part of an elastic wave device according to a second embodiment of the invention.

[0052] [Fig. 11] is a graph illustrating the harmonic conductance G and harmonic admittance Y for an elastic wave device according to the second embodiment of the invention.

[0053] [Fig. 12] is a diagram schematically illustrating a part of an elastic wave device according to a third embodiment of the invention.

[0054] [Fig. 13] is a graph illustrating the harmonic conductance G and harmonic admittance Y for an elastic wave device according to the third embodiment of the invention.

[0055] [Fig. 14] is a diagram schematically illustrating a part of an elastic wave device according to a fourth embodiment of the invention.

[0056] [Fig. 15] is a graph illustrating the harmonic conductance G and susceptance B for two embodiments of an elastic wave device according to the fourth embodiment of the invention.

[0057] [Fig. 16] is a diagram schematically illustrating an alternative interdigitated electrode pair for carrying out the fourth embodiment of the invention.

[0058] [Fig. 17] is a diagram schematically illustrating a second alternative interdigitated electrode pair for carrying out the fourth embodiment of the invention.

[0059] [Fig. 18] is a diagram illustrating the steps of a method of manufacturing an elastic wave device according to a fifth embodiment of the invention.

[0060] The invention will be described in more detail using advantageous embodiments, by way of example, and with reference to the drawings. The embodiments described are merely possible configurations so that individual features as described may be provided independently of one another or may be omitted when implementing the present invention.

[0061] [Fig. 1] illustrates an elastic wave device 1, in particular a shear wave device, according to a first embodiment of the invention.

[0062] The elastic wave device 1 according to the first embodiment comprises a piezoelectric material 3, in particular a ferroelectric material, arranged as a layer 5 above, in particular directly on a dielectric layer 7. The dielectric layer 7 is arranged above, in particular directly on a substrate 9. The layer 5, the dielectric layer 7 and the substrate 9 form a composite substrate 11, obtained for example by a SmartCut™ style layer transfer process. The composite substrate 11 is also referred to as a piezoelectric-on-insulator substrate or POI substrate. In a layer transfer process, the dielectric layer 7 may act as a bonding layer between the base substrate 9 and the piezoelectric layer 5.

[0063] Ferroelectricity is the property that a material possesses spontaneous electrical polarization in its natural state. Ferroelectric materials are a subclass of piezoelectric materials. The ferroelectric property can also be achieved artificially, for example, by suitable growth conditions to create domains of different polarization. In the following, any piezoelectric material in which polarization domains can be produced can therefore be considered a ferroelectric material.

[0064] According to the invention, the piezoelectric material of the layer 5 comprises first domains 3ai, with i ranging from 1 to j, with a first polarization direction 13a and second domains 3bi, with i ranging from 1 to j, with a second polarization direction 13b, the first direction 13a being opposite to the second direction 13b. In addition, the first and second domains 3ai and 3bi form bands of the same thickness which are periodically alternated with a pitch pf, in a direction d, called the periodic direction, perpendicular to the surface normal n of the piezoelectric material 3.

[0065] In this embodiment, the polarization axis Z of the domains forms a non-zero angle with the normal n. In addition, the polarization axis Z has at least one contribution in the periodic direction d. This contribution is opposite for the two types of domains. The polarization axis Z is preferably in the plane defined by the normal n and the direction d, so the angle (p) is equal to 0. Only single-rotation cuts defined around the crystal axis X are considered here without loss of generality. In particular, for these so-called single-rotation cuts, the angle (p) defined around the crystal axis Z is zero.

[0066] For such a configuration, shear waves can be induced in the piezoelectric layer, in particular for class 3m trigonal single crystal materials, such as single crystal lithium tantalate LiTaOs or lithium niobate LiN bOs, and perovskite materials of the ABO3 type. Due to their ferroelectric properties, they satisfy the conditions necessary for the excitation of the shear modes according to the invention. In these materials, for the implementation of the invention, particular attention will be paid in particular to the polarization only along the Z axis.

[0067] In this embodiment, the piezoelectric material of layer 3 may be monocrystalline lithium tantalate LiTaCh or monocrystalline lithium niobate LiNbCh. Other piezoelectric materials may also be used, as described above. Layer 5a preferably has a thickness e1 less than the wavelength λ, preferably less than λ / 2, and even more preferably less than λ / 4, typically less than 1 μm.

[0068] In the case of lithium tantalate (LiTaCh), the crystal orientation defined according to the IEEE Std-176 version IRE 1949 standard of the first direction 13a of the first domains 3ai of layer 3 is preferably (YXI) / 0 with the value of the angle 0 chosen between 30° and 110°. The crystal orientation of the second direction 13b of the second domains 3bi of layer 3 is then (YXI) / 0 +180° according to the IEEE Std-176 version IRE 1949 standard. In this reference frame, the normal n corresponds to the direction 90°- 0, and the direction d is orthogonal to n and forms an angle 0 with Z, and in a particular mode collinear with Z.

[0069] In the case of lithium niobate (LiNbCh), the crystal orientation defined according to IEEE Std-176 version IRE 1949 of the first direction 13a of the first domains 3ai of layer 3 is preferably (YXI) / 0 with the value of the angle 0 chosen in a range from -30° to +120° and preferably from 0 to 70°. The crystal orientation of the second direction 13b of the second domains 3bi of layer 3 is then (YXI) / 0+18O° according to IEEE Std-176 version IRE 1949.

[0070] For lithium tantalate and lithium niobate, the polarization direction is only along the Z axis. Under the influence of an electric field exceeding the coercive field, an atom in the crystal lattice moves from the barycenter in this direction to polarize the molecule.

[0071] The dielectric layer 7 is preferably a silicon oxide layer with a thickness e2 less than the wavelength and / or between 200 nm and 2 pm. The dielectric layer may also be made of SiN, Ta2Os, ZrC>2, HfC>2, SiON or a combination of these materials. It may also comprise, among other things, polysilicon.

[0072] The base substrate 9 is preferably a silicon substrate, for example silicon (100) or (110) or (111). The substrate may also be silicon oxide, silicon carbide (SiC), sapphire, silicon nitride (SiN), aluminum nitride (AIN), quartz, carbon, diamond, yttrium aluminum garnet (Yag), Yigs (“Ytrium ion garnet” in English), amorphous silicon or poly-silicon or LiNbOa and / or LiTaCh.

[0073] By using silicon and silicon oxide as the base substrate 9 and dielectric layer 7, the temperature stability of the device 1 can be improved. This is because the temperature coefficients of the velocity (TCV) of the shear modes in the materials constituting the stack have opposite signs and silicon whose thickness is greater than that of the dielectric and piezoelectric layers deposited on its surface, in particular greater than 50 times the thickness of the two layers, limits the thermal expansion effects of the piezoelectric layer. In particular, materials whose thermal expansion is less than 10 ppm.K' 1 , preferably 5 ppm.K -1 , are effective in limiting temperature effects on structure-guided modes.

[0074] A pair of interdigitated comb electrodes 15a, 15b is arranged above, in particular directly on, the layer 5 of ferroelectric material. The respective comb teeth 17a1 to 17a3 and 17b1 to 17b3 extend essentially perpendicular to the periodic direction d, i.e. parallel to the bands of the domains 3ai and 3bi. The number of teeth illustrated per comb electrode is here three, it being specified that each of the electrodes may have more than three teeth. teeth preferably have a rectangular shape (i.e. parallelepiped-shaped). The teeth 17a1 to 17a3 are connected to each other by a conductive bar 19a. The teeth 17b1 to 17b3 are connected to each other by a conductive bar 19b. The two bars 19a and 19b can be connected to the poles of a radiofrequency source for the excitation of the modes of interest.

[0075] The electrode teeth are arranged periodically with a mechanical pitch pe and therefore an electrical periodicity of 2p e . It is nevertheless possible to introduce one or more breaks in periodicities according to a variant, for example by omitting one or more teeth. The mechanical pitch of the electrodes p e is half the step pf of the 3ai and 3bi domains in this case.

[0076] The comb electrodes 15a and 15b are made of a metallic material, for example aluminum (Al) or molybdenum (Mo) or gold (Au) or silver (Ag) or an aluminum copper alloy (AlCu) or an alloy mainly based on aluminum and another metal, titanium (Ti) for example. The pair of electrodes 15a and 15b can in particular be made with AICu, or AISi, or AITi with Cu, Ti and / or Si dopings of between 0.5 and 5% and potentially with sub-layers of Ti, Ta, Mo, Pd, or Pt, or Ti / Pt, Ti / Au, Ta / Pt, Cr / Au combinations or with Zr. The comb electrodes 15a and 15b preferably have thicknesses e3 of between 50 nm and 200 nm. The teeth 17a1 to 17a3, 17b1 to 17b3 have a width a, in the direction d, which is preferably equal to or greater than 280 nm, preferably greater than 350 nm. Thus the comb electrodes 15a, 15b can be produced by l-line photolithography.

[0077] The a / p metallization ratio e , a being the width of teeth 17a1 to 17a3, 17b1 to 17b3 and p e their mechanical pitch, is preferably between 0.25 and 0.75, and in particular between 0.4 and 0.6. This ratio a / p e and / or the thickness e3 can be adjusted to adjust the electromechanical coupling and / or to reduce losses.

[0078] In this embodiment, the teeth all have the same shape. According to variants, the teeth can also be made with different lengths I.

[0079] Preferably there is no other electrode means than the interdigitated electrode pair. In particular there is no floating or planar electrode on the opposite side of the ferroelectric layer 5. This is to avoid the formation of elliptical polarization modes in the ferroelectric layer 5 which are detrimental to the spectral purity of the transducer.

[0080] In particular, any floating type electrode will create parasitic capacitances degrading the shear mode coupling and potentially degrading spectral purity.

[0081] In the embodiment of [Fig. 1], a feed means 21 is connected to the interdigital comb electrodes 15a, 15b. Preferably, this feed means 21 is configured to provide a radio frequency signal of a frequency of at least 2 GHz, preferably at least 6 GHz, to the interdigital comb electrodes 15a, 15b.

[0082] When the device 1 is configured to convert elastic waves into electrical signals, an electrical receiver is placed in place of the supply means 21.

[0083] According to variants of the first embodiment, the composite substrate 9 may comprise one or more other layers in its structure. A free charge trapping layer, for example polycrystalline silicon, AIN or polycrystalline SiOCH may be provided between the dielectric layer 7 and the base substrate 9 to be able to limit the mean free path of the free charges generated at the silicon / dielectric layer interface by the acoustoelectric field linked to the propagation of the wave, making it possible to reduce leakage currents. Such a trapping layer has a thickness preferably between 0.3 pm and 2 pm at the radio frequencies considered for telecoms, typically between 500 MHz and 6 GHz.

[0084] Instead of using a composite substrate 11, the invention can also be implemented with a solid substrate of ferroelectric material, therefore without the presence of a dielectric layer and without a base substrate.

[0085] [Fig. 2] is a diagram schematically illustrating a sectional view of a portion of the elastic wave device according to the first embodiment of the invention. [Fig. 2] shows two halves of the first domains 3a1 and 3a2 sandwiching a second domain 3b1 of opposite polarization. For each comb electrode 15a and 15b a tooth 17a1 and 17b1 is shown. The teeth 17a1 and 17b1 are arranged directly on the surface 31 of the layer 5 of piezoelectric material. [Fig. 2] also shows the dielectric layer 7 and the base substrate 9.

[0086] The teeth 17a1 and 17b1 are positioned on the interfaces 33a and 33b between the domains 3a1, 3b1 and 3a2 of opposite polarizations. The central planes 35a and 35b of each tooth 17a1 and 17b1 are preferably aligned with the interfaces 33a and 33b. Thus the teeth 17a1 and 17b1 are positioned symmetrically with respect to the interfaces 33a and 33b respectively in the periodic direction d.

[0087] Numerical simulations carried out according to the method described in S. Ballandras et al., “Finite element analysis of periodic piezoelectric transducers”, Journal of Applied Physics 93, 702 (2003), show that for a device 1 according to [Fig. 1] and [Fig. 2] the frequencies of the fundamental modes are twice as large compared to a device without opposite polarization domains which is due to a reduction of the wavelength by a factor of two. At the bottom of [Fig. 2] are illustrated the deformations of the mesh in a shear mode which has a wavelength of 2 pm therefore corresponding to the mechanical period p e teeth and not corresponding to the 2p electrical period e for a domain-free device. The mode exists only when the vibration of the electrodes is antisymmetric. The vibration of the electrode is antisymmetric and the two electrodes vibrate similarly given the equal and opposite boundary conditions to which they are subjected.

[0088] [Fig. 3] illustrates the simulation results for the device without alternating domains and [Fig. 4] a device 1 as shown in [Fig. 1] and [Fig. 2]. A composite substrate with a Si(100) base substrate, a 1 pm thick polycrystalline Si trapping layer, a 500 nm thick SiC>2 dielectric layer and a 600 nm thick LiTaCh layer were used for the comparison device. A composite substrate with a Si(100) base substrate, a 1 pm thick polycrystalline Si trapping layer, a SiC>2 dielectric layer and a LiTaCh layer with respective thicknesses of 500 nm and 600 nm with infinitely long and alternating domains with a pf periodicity of 4 pm and with directions (YXI) / 42° and (YXI) / 222° were used for the device according to the invention. In both cases, the electrodes are made of AlCu and have a thickness of 150 nm and a ratio a / p eof 0.5 and a mechanical step p e from 2 pm.

[0089] [Fig. 3] illustrates the modulus of the G harmonic conductance in dB on the left axis and the modulus of the Y harmonic admittance in dB on the right axis as a function of frequency in MHz. The Rayleigh mode 41 is observed around 700 MHz in the harmonic conductance only. The shear mode 43 is found around 1 GHz and the SSBW mode (acronym for the English name "surface skimming bulk wave") 45 around 1.15 GHz.

[0090] [Fig. 4] illustrates the modulus of the harmonic conductance G in dB on the left axis and the modulus of the harmonic admittance Y in dB on the right axis as a function of frequency in MHz for the device 1 according to the invention as illustrated in [Fig. 1] and [Fig. 2] with the alternating domains. Modes are observed at a frequency approximately twice as high, in particular the Rayleigh mode 51 at around 1400 MHz in the harmonic conductance only, the shear mode 53 at around 2 GHz and the SSBW mode 55 at around 2.3 GHz. The shear mode 53 observed at around 2 GHz of the structure according to the invention is characterized by a phase velocity effective 8 km. s -1, a coupling around 3.1% and a quality factor of more than 10,000 for resonance and more than 20,000 for anti-resonance. For orientations (YXI) / 30° and (YXI) / 210° the coupling rises to a value of 5%. Parasitic modes also appear which are due to the presence of the dielectric layer and the trapping layer.

[0091] The same phenomenon can be observed with LiNbCh which for the same structural parameters shows a mode with a coupling of about 10% and a quality factor also higher. By optimizing the thickness of the dielectric layer, here the silicon oxide layer, the temperature coefficient of frequency (TCF) defining the dependence of the frequency on the temperature Af / f=TCFx(T-To), with To the reference temperature generally taken at 25°C, can be minimized or even canceled. Indeed, for a thickness of the silicon oxide layer of about 1 pm, the CTF is minimum and close to zero. In addition, the resonance and anti-resonance CTF parameters have almost the same dependence on the temperature and a difference of only a few pprn.K' 1 between them.

[0092] [Fig. 5] illustrates the Y harmonic admittance on the left axis and the B harmonic susceptance on the right axis as a function of frequency in MHz for three pairs of angles 0 / 0+180° of the piezoelectric material LiTaOs (YXI) / 0 and 0+180°. Curves 57i and 59i correspond to the admittance in dB and the susceptance in S.rrr, respectively. 1 for 0=30° for the first domains and 0=210° for the second domains. Curves 572 and 592 correspond respectively to the admittance in dB and the susceptance in S.rrr 1 for 0=42° for the first domains and 0=222° for the second domains. Curves 57a and 59a correspond respectively to the admittance in dB and the susceptance in S.rrr 1for 0=50° for the first domains and 0=230° for the second domains. For all three orientations, the shear mode is observed around 2 GHz. We see that the coupling can be optimized depending on the orientation. The more the resonance and anti-resonance peaks are separated, the higher the coupling. This result also highlights that the resolution of the anti-resonance peaks varies with the orientation of the piezoelectric layer.

[0093] [Fig. 6] is a diagram schematically illustrating a sectional view of a portion of an elastic wave device 111 according to a variant of the first embodiment of the invention. [Fig. 6] shows the first domains 3a1 to 3a4, for domains 3a1 and 3a4 only half of the respective domain is shown and the second domains 3b1 to 3b3 of opposite polarization between domains 3a1 to 3a4. A tooth 113a1 and 113b1 is shown for each comb electrode 15a and 15b. [Fig. 6] also shows the dielectric layer 7 and the base substrate 9.

[0094] The teeth 113a1 and 113b1 are positioned centrally on the interfaces 115a and 115b between the oppositely polarized domains 3b1 and 3a2 and 3a3 and 3b3 respectively. Thus the teeth 113a1 and 113b1 are positioned symmetrically with respect to the interfaces 115a and 115b respectively in the periodic direction d.

[0095] The only difference between the variant of [Fig. 6] and the device 1 of [Fig. 2] is that the distance between the teeth 113a1 and 113b1 is greater than in the embodiment of [Fig. 1]. Indeed, the domain 3b2 is entirely devoid of electrode teeth. Thus the mechanical pitch p e corresponds to the width of three domains or 1.5 times the step pf of the periodicity of the domains instead of one time the domain width or 0.5 times the step pf for the variant of [Fig. 2],

[0096] The simulation as described above for the first embodiment was carried out for the present variant for a cell of the 111 device with a domain width of 1 pm and the orientations of LiTaCh (YXI) / 42° and (YXI) / 222° for the first and second domains and a base substrate in Si(111) (XZw / t) / - 457+35.3 / i with i the angular misalignment between the flats of the silicon and the LiTaOs chosen between 0 and 360°. It is noted that this choice is not restrictive and that the orientations of silicon (100), (111), (211) etc. can be used according to the invention. The choice of the angle i will follow the same rule as for the Si(111). The mechanical pitch p eis thus 3 pm. The width of the cell to make the simulations is 6 pm. Here the cell represents the unit of the structure which is repeated to infinity to carry out the simulation. Numerical simulations carried out highlight that two modes are observed for a 111 device which are illustrated at the bottom of [Fig. 6], One mode presents shear deformations with two shear wavelengths per electrical period 2p e and one mode exhibits deformations with three wavelengths per 2p electric period e .

[0097] [Fig. 7] illustrates the G harmonic conductance in dB on the left axis and the Y harmonic admittance in dB on the right axis as a function of frequency in MHz for the (YXI) / 42° and (YXI) / 222° orientations.

[0098] A mode appears around 1.4 GHz, visible in the admittance, see number 117, and the G harmonic conductance, see number 119. A second vibrational mode appears around 2.7 GHz in the Y harmonic admittance indicated by 121 and the G harmonic conductance indicated by 123. The equivalent phase velocities are 8.6 km.s -1 and 16 km. -1 respectively. The electromechanical couplings of these modes are 4.1 and 6.4% respectively. The quality coefficients of the resonance and anti-resonance of the first mode are clearly higher than 10,000 but remain lower than 500 for the second, where we see that the harmonic conductance peak G is notably flared. The first mode is directional, which results in a contribution at the input and output of the stop band.

[0099] [Fig. 8] is a diagram schematically illustrating a sectional view of a part of an elastic wave device 131 according to a second variant of the first embodiment of the invention. [Fig. 8] shows the first domains 3a1 to 3a4, for domains 3a1 and 3a4 only half of the respective domain is illustrated, and the second domains 3b1 to 3b3 of opposite polarization between domains 3a1 to 3a4. A tooth 133a1 and 133b1 is shown for each comb electrode 15a and 15b. [Fig. 8] also shows the dielectric layer 7 and the base substrate 9.

[0100] The teeth 133a1 and 133b1 are positioned centrally on the interfaces 135a and 135b between the opposite polarization domains 3b1 and 3a2 and 3a3 and 3b3 respectively. Thus the teeth 133a1 and 133b1 are positioned symmetrically with respect to the interfaces 135a and 135b respectively in the periodic direction d as for the first variant.

[0101] The only difference between the variant of [Fig. 8] and the variant of [Fig. 6] is that the teeth 133a1 and 133b1 are wider than in the device 111 of the first variant in the direction d. Indeed, each of the teeth 133a1 and 133b1 completely covers two areas, at least in the direction d.

[0102] [Fig. 9] illustrates the simulation results of the G harmonic conductance in dB on the left axis and the Y harmonic admittance in dB on the right axis as a function of frequency in MHz for the orientation (YXI) / 42° and (YXI) / 222°).

[0103] In particular, for the first mode 141 in Y harmonic admittance and 143 in G harmonic conductance around 1.35 GHz, we see that the widening of the teeth makes it possible to eliminate the directivity effect highlighted in the previous case of [Fig. 7] and allows us to understand that when the teeth do not completely cover the two adjacent domains, the boundary conditions at the entrance and exit of the stop band can actually be satisfied, which is not the case in the present case. The speed of the mode is naturally reduced by increasing the mass load to a value of approximately 8.2 km. s -1 but the electromechanical coupling increases and reaches 5%. The second mode 145 in admittance and 147 in conductance does not see its properties change significantly, its coupling and its quality coefficients are identical to the values ​​of the previous variant.

[0104] [Fig. 10] is a diagram showing a sectional view of a portion of an elastic wave device 61 according to a second embodiment of the invention. The elastic wave device 61 has the same structure and uses the same materials as the elastic wave device 1 of the first embodiment, except that the teeth 63a1 and 63b1 of the interdigital comb electrodes are entirely positioned above a domain and no longer straddling two domains as in the first embodiment. In this embodiment, each tooth 63a1 and 63b1 of the two comb electrodes 15a and 15b is positioned on the same type of domain, here the second domains 3b1 and 3b2. As illustrated in [Fig. 10], the two second domains 3b1 and 3b2 are each arranged between two first domains 3a1 and 3a2, and 3a2 and 3a3 respectively. For each comb electrode 15a and 15b a tooth 63a1 and 63b1 is shown.The teeth 63a1 and 63b1 have the same width a, preferably the same width as the domains 3b1 and 3b2. According to alternative embodiments, the width a of the teeth may be less than the width of the domains. In this case the teeth are preferably centered in the middle of a domain. [Fig. 10] also shows the dielectric layer 7 and the base substrate 9. The trapping layer is not illustrated as for the first embodiment of [Fig. 1] or [Fig. 2].

[0105] A vibration mode is illustrated at the bottom of [Fig. 10]. This vibration mode was obtained by the simulation as described above for the first mode, but for a cell of the device 61 with a domain width of 1 pm and a LiTaCh orientation (YXI) / 50° and (YXI) / 230°. The cell width is thus 4 pm. The observed results do not depend or only slightly on the crystal orientation of the base substrate 5.

[0106] [Fig. 11] illustrates the G harmonic conductance in dB on the left axis and the Y harmonic admittance in dB on the right axis as a function of frequency in MHz for the (YXI) / 50° and (YXI) / 230° orientations.

[0107] The number 71 in the harmonic admittance Y indicates the 1st order mode at 1 GHz with a wavelength λ equal to twice the mechanical step p e electrodes. The first-order mode is essentially visible only in the Y harmonic admittance. It is the fundamental shear mode propagating at a phase velocity of 4170 ms' 1 and having a coupling of 5%, without radiation losses in the substrate. The structure allows to excite the 3rd harmonic of the visible mode in the Y harmonic admittance indicated by 73 and the G harmonic conductance indicated by 75. This mode is found around 3 GHz. Excited beyond the guiding limit of the structure, therefore after the wave frequency SSBW, this contribution is affected by radiation losses and the resonance quality coefficient remains below 500 as a result. Nevertheless, its electromechanical coupling reaches 3.7%, which is significantly higher than in the general case of a harmonic contribution of order 3, where a reduction in coupling of an order of magnitude is generally observed compared to the fundamental mode. This phenomenon is due to the ferroelectric periodic structure which favors synchronism conditions different from those of a homogeneously polarized layer.

[0108] [Fig. 12] is a diagram schematically illustrating a sectional view of a portion of an elastic wave device 81 according to a third embodiment of the invention. The elastic wave device 81 has the same structure and uses the same materials as the elastic wave device 1 of the first embodiment, except that the teeth 83a1 and 83b1 of the interdigital comb electrodes are entirely positioned above a domain and no longer straddling two domains as in the first embodiment. In this embodiment, the teeth 83a1 and 83b1 of the two comb electrodes 15a and 15b respectively are positioned on domains of different polarizations. The tooth 83a1 of the first electrode 15a is positioned on the second domain 3b1 while the tooth 83b1 is positioned on the first domain 3a3. As illustrated in [Fig.12], each of the three illustrated second domains 3b1 to 3b3 is arranged between two first domains 3a1 to 3a3 respectively. One tooth 83a1 and 83b1 is shown for each comb electrode 15a and 15b. The teeth 83a1 and 83b1 have the same width a, preferably the same width as the domains 3a1 and 3b3. According to alternative embodiments, the width a of the teeth may be less than the width of the domains. In this case the teeth are preferably centered in the middle of a domain. [Fig. 12] also shows the dielectric layer 7 and the base substrate 9. The trapping layer is not shown as for the first embodiment of [Fig. 1] or [Fig. 2],.

[0109] A vibration mode is illustrated at the bottom of [Fig. 12]. This mode was obtained by the simulation as described above for the first embodiment, but for a cell of the device 81 with a domain width of 1 pm and a LiTaCh (YXI) / 42° and (YXI) / 222° orientation and a Si(111) base substrate. The mechanical step p e is thus 3 pm, the cell for the simulation has a width of 6 pm. The vibrations of the mode have a wavelength A equal to half a mechanical step p e electrodes.

[0110] [Fig. 13] illustrates the G harmonic conductance in dB on the left axis and the Y harmonic admittance in dB on the right axis as a function of frequency in MHz for the orientation (YXI) / 42° and (YXI) / 222°. Two vibration modes are visible.

[0111] A mode appears around 1.35 GHz, visible in the Y harmonic admittance, see number 91, and the G harmonic conductance, see number 93. A second vibrational mode appears around 2.7 GHz in the Y harmonic admittance indicated by 95 and the G harmonic conductance indicated by 97. Simulations show that for both observed modes, the motion of the electrodes is always in phase. The equivalent phase velocities are 8.24 and 15.73 km. s -1 respectively, associated with electromechanical couplings of 6.4% and 4.4%. The quality coefficients of the resonance and anti-resonance of the first mode are greater than 2,000 while those of the contribution near 2.7 GHz remain less than 100 due to guidance defect.

[0112] [Fig. 14] is a diagram schematically illustrating a sectional view of a portion of an elastic wave device 101 according to a fourth embodiment of the invention. The elastic wave device 101 has the same structure and uses the same materials as the elastic wave device 1 of the first embodiment, except that the tooth 103b1 of the comb electrode 15b is entirely positioned above the domain 3a2 while the tooth 103b2 of the comb electrode 15b is entirely positioned above the oppositely polarized domain 3b3. Between the two teeth 103b1 and 103b2 is the tooth 103a2 straddling the two domains 3b2 and 3a3 as in the first embodiment. The tooth 103a1 straddles the two domains 3a1 and 3b1 as in the first embodiment

[0113] As illustrated in [Fig. 14], the first domains 3a1 to 3a4 alternate with the second domains 3b1 to 3b3 respectively to create the cell. For each comb electrode 15a and 15b, two teeth 103a1, 103a2 and 103b1, 103b2 are shown. The teeth have the same width a, preferably the same width as the domains. According to alternative embodiments, the width a of the teeth may be less than the width of the domains. In this case the teeth are preferably centered with respect to the interfaces 105a and 105b or in the middle of a domain depending on the electrode. [Fig. 14] also shows the dielectric layer 7 and the base substrate 9. The trapping layer is not shown as for the first embodiment of [Fig. 1] or [Fig. 2],

[0114] [Fig. 15] is a graph illustrating the harmonic conductance G and harmonic admittance Y for two embodiments of an elastic wave device according to the fourth embodiment of the invention. The difference between the two The main advantage of these achievements is that the electrodes are subjected to different potential patterns. These patterns are shown above each graph.

[0115] The simulation was performed as described above for the first embodiment, but for a cell of the device 101 with a domain width of 1 pm and a LiTaCh (YXI) / 42° and (YXI) / 222° orientation and a Si(111) base substrate. The mechanical pitch p e is thus 1.5 pm, the cell for the simulation has a width of 6 pm. The observed results do not depend or depend only slightly on the crystalline orientation of the base substrate 5.

[0116] [Fig. 15] illustrates in a first graph the harmonic conductance G in dB on the left axis and the harmonic admittance Y in dB on the right axis as a function of the frequency in MHz for the orientation (YXI) / 42° and (YXI) / 222°. These results were obtained for an alternating +V / -V potential applied to the teeth 103a1, 103b1, 103a2 and 103b2 as illustrated above this first graph of [Fig. 15]. Several modes appear in the admittance and the conductance. In particular in the harmonic admittance Y with the number 105i towards 800 MHz, with the number 1052 towards 2 GHz and with the number 1053 towards 3.3 GHz.

[0117] [Fig. 15] illustrates in a second graph, positioned below the first graph, the G harmonic conductance in dB on the left axis and the Y harmonic admittance in dB on the right axis as a function of the frequency in MHz for the orientation (YXI) / 42° and (YXI) / 222°. These results were obtained for a +V potential applied to teeth 103a1 and 103a2' and a -V potential applied to teeth 103b1 and 103b2' as illustrated above this second graph of [Fig. 15]. Several modes appear in the admittance and conductance but at different frequencies compared to the previous realization. Particularly in the Y harmonic admittance with the number 107i towards 1.3 GHz, with the number 1072 towards 2.7 GHz and with the number 107a close to 4 GHz. Here it is in particular the 107a mode which is distinguished by a spectral purity superior to the 105a mode. The 107i mode, on the other hand, is doubled.

[0118] It may be advantageous to use this type of structure by favoring a particular contribution by coarse filtering or for applications of frequency sources which in practice only exploit a limited domain of the spectrum, which amounts to filtering the spectrum in one way or another by the electronic system exploiting the component. The advantage of the solution proposed here is once again to allow the exploitation of electrical responses notably higher in frequency than those conventionally achievable with non-polarized materials.

[0119] In this second embodiment, another type of interdigitated electrode pair is used, which is schematically illustrated in [Fig. 16], Here two teeth of the same potential are arranged directly adjacent followed by two teeth of the opposite potential which are also arranged directly adjacent. So 13a1' and 13a2', 13a3' and 13a4', 13a5' and 13a6' for electrode 15a' and 13bT and 13b2', 13b3' and 13b4', 13b5' and 13b6' for electrode 15b'.

[0120] Alternatively, a control circuit may be configured to change the arrangement of the electrodes from the configuration as shown in the top left to a configuration as shown in the top right, for example by using switches as shown in [Fig. 17],

[0121] [Fig. 17] illustrates an electrode 151 with switches 153a, 153b, 153c, 153d opposite a second electrode 155 with switches 157a, 157b, 157c and 157d. The switches 153a, 153b, 153c, 153d are configured such that they can contact or not contact the electrode 151 with teeth 159a, 159b, 159c and 159d respectively. Similarly, the switches 157a, 157b, 157c and 157d are configured such that they can contact or not contact the electrode 155 with teeth 159a, 159b, 159c and 159d respectively. Thus, potential application schemes such as those shown in the upper left and right of [Fig. 15] can be realized by a single device. The switching is preferably controlled such that a short circuit between electrodes is avoided. [Fig. 17] illustrates on the left the situation in which teeth 159a to 159d have a potential alternating from one tooth to the next. On the left of [Fig.17] two successive teeth therefore 159a and 159b and 159c and 159d have the same potential.

[0122] Electrodes 151 and 155 may also be used in the configurations of the other embodiments to allow the electrical excitation pattern to be changed.

[0123] [Fig. 18] is a diagram illustrating the steps of a method for manufacturing an elastic wave device according to one embodiment of the invention, in particular an elastic wave device according to the first embodiment. The method begins with step 201 during which a piezoelectric material, in particular a ferroelectric material, is provided.

[0124] This step may consist of providing a bulk substrate, such as a wafer of monocrystalline piezoelectric material or of providing a composite substrate 11 comprising a layer 5 of piezoelectric material 3, a dielectric layer 7, for example a layer of silicon oxide and a base substrate 9, for example made of silicon, for example Si (100), (110) or (111) or quartz. The orientations of the two crystals of the structure, visualized by the flats of the wafers used, may be misaligned or not.

[0125] The piezoelectric material of layer 3 is single-crystal lithium tantalate LiTaOs or single-crystal lithium niobate LiNbCh. Other piezoelectric materials may also be used, as defined above.

[0126] In the case of lithium tantalate (LiTaOs), the crystal orientation defined according to the IEEE Std-176 version IRE 1949 standard of the first direction 13a of the first domains 3ai of layer 3 is preferably (YXI) / 0 with the value of the angle 0 chosen between -30° and 120°, preferably between 0° and 70°. The crystal orientation of the second direction 13b of the second domains 3bi of layer 3 is then (YXI) / 0 +180° according to the IEEE Std-176 version IRE 1949 standard. It is also possible to have the domains in the orthogonal plane (YXIf) / 9 / 90° and (YXIt) / 0+ 180 90° relative to that described above by keeping the comb teeth parallel to the domains.

[0127] In the case of lithium niobate (LiNbCh), the crystal orientation defined according to IEEE Std-176 version IRE 1949 of the first direction 13a of the first domains 3ai of layer 3 is preferably (YXI) / 0 with the value of the angle 0 chosen in a range from -30° to +120° and preferably from 0 to 70°. The crystal orientation of the second direction 13b of the second domains 3bi of layer 3 is then (YXI) / 0+18O° according to IEEE Std-176 version IRE 1949.

[0128] Such a composite substrate 11 can be obtained by a layer transfer process, such as a SmartCut™ process. In a SmartCut™ type process, ions are implanted into a donor substrate, here a substrate of the piezoelectric material, to create a weakened zone inside the donor substrate. Then, the donor substrate is attached to a base substrate. Here the base substrate is a monocrystalline silicon substrate, for example Si(100) or Si(110) or Si(111) with its natural oxide layer or with a thermal silicon oxide or produced by chemical vapor deposition (“CVD”) or physical vapor deposition (“PVD”). This oxide layer facilitates the attachment of the two substrates, in particular by molecular bonding. The orientation in the plane of the silicon relative to the plane of the piezoelectric layer can be chosen to reduce, or even minimize, modes of order higher than the fundamental.

[0129] According to a variant, a trapping layer is made on the base substrate before the formation of the oxide. The trapping layer is typically polycrystalline silicon and generally any layer minimizing the mean free path of the charges generated at the interface between the silicon and the oxide layer by the passage of the acoustoelectric wave.

[0130] Once the two substrates are attached, an input of mechanical and / or thermal energy drives the weakened zone to failure. Thus, a layer with a thickness of 1 μm or less can be transferred onto the base substrate.

[0131] Then in step 203, an electric field is applied which is stronger than the coercive field of the piezoelectric material, in particular the ferroelectric material, to realize the first and second domains alternating periodically and having opposite polarizations. In this embodiment, the electric field is applied in the form of parallel strips, perpendicular to the surface of the piezoelectric material. The applied electric field must be stronger than 22 kV.mrrr 1 which represents the coercive field generally measured for congruent lithium niobate and lithium tantalate.

[0132] Such a process is for example described in Thorlabs, "Periodically Poled Lithium Niobate (PPLN) - Tutorial", page 686 to 687 available on www.thorlabs.com using structured electrodes which are then removed. Other alternatives use an electron beam, one can refer to the publication: C. Restoin, C. Darraud-Taupiac, JL Decossas, JC Vareille, J. Hauden and A. Martinez: "Ferroelectric domain inversion by electron beam on LiNbCh and Ti: LiNbOa" Journal of Applied Physics, 88:6665-6668, 2000, or M. Yamada and K. Kishima: "Fabrication of periodically reversed domain structure for SHG in LiNbOa by direct electron beam lithography at room temperature", Electronics Letters, 27:828-829, 1991.

[0133] To simplify the creation of the domains when applying the electric field, it may be advantageous to provide a metal layer on the surface of the piezoelectric material 3 opposite the surface on which the pair of interdigitated comb electrodes will be made in the next step. This layer can be made either on the dielectric layer 7 before attachment, or on the surface of the donor substrate on which the attachment will be made. In the presence of a metal layer, the applied electric field may be closer to 22 kV.mnr 1and the temperature close to ambient conditions (i.e. close to 25°C). In the absence of the metal layer, a stronger electric field must be applied, of the order of twice or more than the coercive field. Preferably, this step is carried out at a temperature of at least 150°C, or at least 170°C in the absence of the metal layer, which makes it possible to lower the field value to obtain the forced polarization effect.

[0134] Then, during step 205, the pair of interdigitated comb electrodes is produced on the piezoelectric material to obtain the elastic wave device according to the first embodiment using lithography, etching and deposition steps, known to those skilled in the art.

[0135] The method of step 205 can be adapted to the other embodiments described above by adapting the mask and the alignment of the mask during the lithography steps.

[0136] In the case of the third embodiment, steps 203 and 205 can be interchanged. In this case, teeth 83a1 and 83b1 can be used to form the domains by applying the same potential to both electrodes.

Claims

CLAIMS 1. Elastic wave device, in particular shear wave device, comprising a piezoelectric material (3), in particular a ferroelectric material with first domains (3a) of a first polarization direction (13a) and second domains (3b) with a second polarization direction (13b), the first direction (13a) being opposite to the second direction (13b), wherein the first and second domains (3a, 3b) are periodically alternated in a direction (d), called the periodic direction, perpendicular to the surface normal (n) of the piezoelectric material (3), and a pair of interdigitated comb electrodes (15a, 15b) above the, in particular on the, piezoelectric material whose respective comb teeth (17a1 to 17a3, 17b1 to 17b3) extend essentially perpendicular to the periodic direction (d) and to the normal (n).

2. An elastic wave device according to claim 1, wherein the comb teeth (17a1 to 17a3, 17b1 to 17b3) of the pair of interdigitated comb electrodes (15a, 15b) are arranged periodically.

3. An elastic wave device according to claim 1 or 2, wherein the comb teeth (17a1 to 17a3, 17b1 to 17b3) are positioned above interfaces between the first domains (3a) and the second domains (3b).

4. Elastic wave device according to claim 3, the comb teeth of which are positioned symmetrically with respect to the interfaces between the first domains (3a) and the second domains (3b).

5. Elastic wave device according to claim 3 or 4, wherein the directly adjacent comb teeth (17a1, 17b1) of the two interdigitated comb electrodes (15a, 15b) are positioned above the interfaces (33a, 33b) between the directly adjacent first domains (3a1, 3a2) and second domains (3b1).

6. Elastic wave device according to one of claims 1 to 5, wherein the width (a) of the teeth (17a1 to 17a3, 17b1 to 17b3) of the interdigitated comb electrodes (15a, 15b) is between 25% and 75%, preferably between 40% and 60%, even more preferably 50%, of the width of the domains (3a, 3b), and / or at least 280 nm, preferably at least 350 nm.

7. Elastic wave device according to one of claims 1 to 6, each of the comb teeth (133a1, 133b1) of which has a width corresponding to the width of a first domain and a second domain.

8. An elastic wave device according to claim 1 or 2, each of the comb teeth (63a1, 63b1, 83a1, 83b1) being entirely positioned above only one of the first domains (3a) or second domains (3b).

9. Elastic wave device according to claim 8, the directly adjacent comb teeth (63a1, 63b1) of the electrodes (15a, 15b) of which are positioned above domains with the same polarization direction (3b1, 3b2), in particular domains with the same polarization direction only separated by a domain (3a2) with the opposite polarization direction.

10. An elastic wave device according to claim 8, wherein the comb teeth (83b1) of one of the two comb electrodes are positioned above the first domains (3a3) and the comb teeth (83a1) of the second comb electrode are positioned above the second domains (3b1).

11. Elastic wave device according to one of claims 8 to 10, wherein the width (a) of the comb teeth (63a1, 63b1, 83a1, 83b1) of the interdigitated comb electrodes is the same as the width of the domains, and / or at least 280 nm, preferably at least 350 nm.

12. Elastic wave device according to claim 1 or 2, each of the comb teeth (103b1, 103b2) of one of the two electrodes being entirely positioned above only one of the first and / or second domains and each of the comb teeth (103a1, 103a2) of the second electrode being positioned above interfaces between the first domains (3a) and the second domains (3b).

13. Elastic wave device according to one of claims 1 to 12 for which there is at least one entire domain without electrode fingers between two domains on which two teeth are positioned at least partially.

14. Elastic wave device according to claim 1 to 13, wherein the piezoelectric material (3) is arranged as a layer (5), in particular with a thickness (e1) less than the wavelength, preferably less than λ / 2, and even more preferably less than λ / 4, above a base substrate (9), in particular a substrate of silicon, amorphous silicon or polysilicon, silicon oxide, silicon carbide (SiC), sapphire, silicon nitride (SiN), aluminum nitride (AIN), quartz, carbon, diamond, Yags (Ytrium aluminum garnet), Yigs (Ytrium iron garnet) or LiNbOa and / or LiTaCh.

15. Elastic wave device according to claim 14, wherein a dielectric layer (7), in particular a layer of silicon oxide, SiN, TaaOs, ZrC>2, HfCh, SiON, poly-silicon or a combination of these materials, is arranged between the layer (5) of piezoelectric material and the base substrate (9), the dielectric layer (7) preferably having a thickness less than the wavelength and / or between 200 nm and 2 pm.

16. Elastic wave device according to one of claims 14 or 15, wherein a trapping layer, in particular a layer of polycrystalline silicon or polycrystalline AIN or SiOCH, is arranged between the piezoelectric material (3) and the base substrate (9) or between the dielectric layer (7) and the base substrate (9), the trapping layer preferably having a thickness between 300 nm and 2 pm.

17. Elastic wave device according to one of claims 1 to 16, wherein a metal layer is arranged on the surface of the piezoelectric material (3) opposite the pair of interdigitated comb electrodes (15a, 15b), in particular between the layer (5) of piezoelectric material (3) and the base substrate (9).

18. Elastic wave device according to one of claims 1 to 17, the piezoelectric material (3) is at least one of LiTaOs, LiNbCh, ABO3 type perovskites, in particular KNbChou of PbTiCh, PZT, PMnPt, or AIScN.

19. An elastic wave device according to one of claims 1 to 18, wherein a first electrode (151) comprises first switches (153a, 153b, 153c, 153d) and a second electrode (155) comprises second switches (157a, 157b, 157c and 157d), the first switches (153a, 153b, 153c, 153d) are configured such that they can contact or not the first electrode (151) with teeth (159a, 159b, 159c and 159d) respectively and the second switches (153a, 153b, 153c, 153d) are configured such that they can contact or not the second electrode (151) with teeth (159a, 159b, 159c and 159d) respectively, in particular to form the comb electrode pair.

20. Elastic wave device according to one of claims 1 to 19, comprising a supply means (21) configured to supply a radio frequency signal of a frequency of at least 2 GHz to the interdigitated comb electrodes (15a, 15b).

21. Elastic wave device according to one of claims 1 to 20, wherein the pair of interdigitated comb electrodes is the only electrically conductive element in contact with the piezoelectric material.

22. Elastic wave device according to one of claims 1 to 21, devoid of any other electrode means apart from the pair of interdigitated comb electrodes, in particular devoid of any floating electrode.

23. Use of an elastic wave device according to claim 20, in an acousto-electric device, in particular a filter, a sensor or a delay line, having operating frequencies of 2 GHz and above.

24. Method of manufacturing an elastic wave device according to one of claims 1 to 22 and comprising the following steps: - providing a piezoelectric material, in particular a ferroelectric material, - applying a stronger electric field, in particular at least 10 times stronger, than the coercive field of the piezoelectric material to produce the first and second domains alternating periodically and having opposite polarizations, and - produce the pair of interdigitated comb electrodes above, in particular directly on, the piezoelectric material.

25. A method of manufacturing an elastic wave device according to claim 9 and comprising the following steps: - providing a piezoelectric material, in particular a ferroelectric material, - produce the pair of interdigitated comb electrodes above, in particular directly on, the piezoelectric material, - applying a stronger electric field, in particular at least ten times stronger, than the coercive field of the piezoelectric material by using the pair of interdigitated comb electrodes to realize the first and second domains alternating periodically and having opposite polarizations.

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