Elastic wave device

Abstract

The present invention relates to an elastic wave device comprising a piezoelectric material having first domains with a first polarization direction and second domains with a second polarization direction, the first direction being opposite to the second direction, wherein the first and second domains alternate periodically along a direction, called the periodic direction, perpendicular to the surface normal of the piezoelectric material, and a pair of interdigitated comb electrodes whose respective comb teeth extend essentially in the periodic direction, and to a method for manufacturing such an elastic wave device. Figure for the abstract: Fig. 1

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 variety 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 losses and small sizes 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, generally 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 (LiTaO3), Rayleigh waves have a phase speed of between 3,000 and 3,500 ms *, with a maximum coupling of the order of 2%. Rayleigh waves on lithium niobate (LiNbO3) have phase speeds of up to 3,900 ms 1 with a maximum coupling close to 5.6%. 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] Furthermore, by using the properties of Lline photolithography (365 nm), the standard industrial method for manufacturing SAW devices, operating frequencies are typically between approximately 1 GHz and 3 GHz. Operating devices at higher frequencies requires finer comb tooth dimensions, thus requiring 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.

[0006] The object of the invention is therefore to overcome these drawbacks by providing an elastic wave device, having a higher speed operating mode than Rayleigh or shear waves. This mode allows in particular operation of the elastic wave device according to the invention at frequencies su above 3 GHz with geometries achievable using I line photolithography.

[0007] To achieve the object, the invention provides an elastic 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 whose respective comb teeth extend essentially in the periodic direction or the comb teeth are deflected from the periodic direction by a maximum of ± 5°.

[0008] The consequence of the combination of the polarized domains with a pair of interdigitated comb electrodes is that the fundamental Rayleigh or shear modes are no longer excited but a mode appears at a frequency f higher than that deduced from the usual formula f=v / 2X with v the phase velocity of the mode and X the wavelength corresponding to twice the mechanical pitch of the pair of interdigitated electrodes.

[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 filters (LCRF), dual coupled mode filters (DMS) or coupled cavity resonance filters (SCAW) as well as sensors and delay lines.

[0010] Ferroelectricity is the property by which a material possesses an electric polarization in the spontaneous state. Ferroelectrics are a subclass of piezoelectric materials. The ferroelectric property can also be obtained artificially, for example by suitable growth conditions to create different polarization domains. In the following, we can therefore consider any piezoelectric material in which polarization domains can be produced as a ferroelectric material.

[0011] According to a variant, the comb teeth can be arranged periodically. According to a variant of the invention, the pitch of the electrodes pe - pe being the center-to-center distance between two neighboring electrodes, each belonging to a different comb, can be larger than the pitch of the domains pf - pf being the center-to-center distance between two domains of the same polarization. In particular pe is a multiple of pf, according to pe=nf*pf, nf being larger than 1, for example an integer larger than 1. By choosing a larger electrode pitch, it becomes possible to modify the mode at the desired frequency.

[0012] According to a variant of the invention, the electrodes are arranged directly on or at least partially, preferably entirely, buried in the ferroelectric material. By placing the electrodes directly on the ferroelectric material, the structure of the device becomes simple to produce. By burying the electrodes at least partially in the piezoelectric material, an increase in coupling is observed, which makes it possible to produce wider bands for the frequency filters.

[0013] According to a variant of the invention, the first direction and the second direction may be parallel to the surface normal of the piezoelectric material. Thus, fewer secondary modes are observed.

[0014] According to a variant of the invention, the width of the teeth of the interdigitated comb electrodes may be at least 280 nm, preferably at least 350 nm. Thus, the structure may be produced using I-line photolithography (365 nm) while taking advantage of the higher frequency mode.

[0015] 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 3 GHz, preferably 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 I-line lithography.

[0016] According to a variant of the invention, the metallization ratio a / pe of the pair of interdigitated comb electrodes may be between 0.3 and 0.75, preferably between 0.4 and 0.6, "a" corresponding to the width of the teeth of the electrodes and "pe" to the pitch of the electrodes.

[0017] 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 X / 2, and even more preferably less than X / 4, typically less than Ipm, above a base substrate, in particular a substrate of silicon, silicon oxide, silicon carbide (SiC), sapphire, silicon nitride (SiN), aluminum nitride (AIN), quartz, carbon, diamond, Yags, Yigs ("Ytrium iron gamet" in English terminology) or LiNbO3 under LiTaO3. It may be a bi-layer 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 losses.

[0018] According to a variant of the invention, a dielectric layer, in particular a layer of silicon oxide, SiN, Ta2O5, ZrO2, HfO2, SiON or a combination of these materials, may be arranged between the layer of piezoelectric material and the base substrate. The dielectric layer, preferably in particular with a thickness less than the wavelength and / or with 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 also increases the temperature stability of the device, which is the consequence of the opposite signs of the temperature coefficient of silicon and silicon oxide. 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.

[0019] According to a variant of the invention, a trapping layer, in particular a layer of polycrystalline silicon or polycrystalline AlN 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 0.3 μm and 2 μm. The presence of a trapping layer makes it possible to reduce leakage currents.

[0020] According to a variant of the invention, a metal layer may be arranged on the surface of the opposite piezoelectric material of the pair of interdigitated comb electrodes, in particular between the layer of piezoelectric material 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 applying an electric field.

[0021] When the metal layer is between the dielectric layer and the base substrate, the coupling of the new mode is higher, because the further the metal layer is distanced 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.

[0022] According to a variant of the invention, the piezoelectric material may be LiTaO3 or LiNbO3. It may also be KNbO3, PZT, PMnPt, PbTiO3 or AlScN. The modes observed for these materials have quality factors and electromechanical couplings allowing industrial use in filter or delay line type applications.

[0023] 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 acousto-electric device, in particular a filter, a sensor or a delay line, having operating frequencies of 3 GHz and above, in par particular of 6 GHz or more. Thus it becomes possible to provide filters, sensors or delay lines operating at higher frequencies than normally accessible using I-line lithography.

[0024] 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 a stronger electric field, in particular at least ten 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 on or at least partially, preferably entirely, buried in the piezoelectric material.

[0025] 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 AlCu, or AlSi, or AlTi 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.

[0026] According to a variant of the invention, the electric field can be applied in the form of parallel strips, in particular using electrolytic electrodes.

[0027] According to a variant of the invention, the field lines can be perpendicular to the surface of the piezoelectric material. Thus the polarization of the domains is also perpendicular to the surface, which makes it possible to reduce the presence of secondary modes.

[0028] According to a variant, 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. Thus, polarization can be obtained even in the absence of a counter electrode.

[0029] According to a variant, 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.

[0030] The invention will be better understood and other advantages will appear on reading the description which follows, given without limitation, and thanks to the attached figures among which:

[0031] [Fig.l] is a diagram schematically illustrating an elastic wave device according to a first embodiment of the invention.

[0032] [Fig.2] is a graph illustrating the harmonic conductance G and the susceptance harmonic B for an elastic wave device according to the first embodiment of the invention compared to an elastic wave device according to the state of the art.

[0033] [Fig.3] is a graph illustrating harmonic resistance and harmonic reactance for an elastic wave device according to the first embodiment of the invention compared to an elastic wave device according to the state of the art.

[0034] [Fig.4] is a graph illustrating the harmonic conductance G and the susceptance harmonic B for an elastic wave device according to the first embodiment of the invention.

[0035] [Fig.5] illustrates the mode observed for the elastic wave device according to the first embodiment of the invention.

[0036] [Fig.6] is a diagram schematically illustrating part of a wave device elastics according to a second embodiment of the invention.

[0037] [Fig.7] illustrates the mode observed for the elastic wave device according to the second embodiment of the invention.

[0038] [Fig.8] is a diagram illustrating the steps of a method of manufacturing a device elastic waves according to a third embodiment of the invention.

[0039] 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 practicing the present invention.

[0040] [Fig.l] illustrates an elastic wave device 1 according to a first embodiment of the invention.

[0041] 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 method. The composite substrate 11 is also called a piezoelectric-on-insulator substrate or POI substrate. In a layer transfer method, the dielectric layer 7 can act as a bonding layer between the base substrate 9 and the piezoelectric layer 5.

[0042] Ferroelectricity is the property by which a material possesses an electric polarization in the spontaneous state. Ferroelectrics are a subclass of piezoelectric materials. The ferroelectric property can also be obtained artificially, for example by suitable growth conditions to create different polarization domains. In the following, we can therefore consider any piezoelectric material in which polarization domains can be produced as a ferroelectric material.

[0043] According to the invention, the piezoelectric material of the layer 5 comprises first domains 3ai, with i ranging from 1 to j, of 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.

[0044] In this embodiment, the piezoelectric material of layer 3 may be monocrystalline lithium tantalate LiTaO3 or monocrystalline lithium niobate LiNbO3. Other piezoelectric materials may also be used. Layer 5 preferably has a thickness el less than the wavelength X, preferably less than X / 2, and even more preferably less than X / 4, typically less than 1 pm.

[0045] In the case of lithium tantalate (LiTaO3), the crystalline 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 (YXl) / 0 with the value of the angle 0 chosen between 30° and 110°. The crystalline orientation of the second direction 13b of the second domains 3bi of layer 3 is then (YX1) / 0 +180° according to the IEEE Std-176 version IRE 1949 standard.

[0046] In the case of lithium niobate (LiNbO3), the crystalline 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 (YXl) / 0 with the value of the angle 0 chosen in a range of ±15° around 128°. The crystalline orientation of the second direction 13b of the second domains 3bi of layer 3 is then (YXl) / 0+180° according to the IEEE Std-176 version IRE 1949 standard.

[0047] Depending on the orientation and nature of the crystals, shear waves or elliptical waves are excited. With the periodic polarization of the domains, two cases arise: in the first case, a combination of a shear wave with an elliptical wave is observed, in the second case, the result is the combination of an elliptical wave polarized in the direction of propagation and a wave elliptical polarized along the aperture.

[0048] 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, Ta2O5, ZrO2, HfO2, SiON or a combination of these materials.

[0049] The base substrate 9 is preferably a silicon substrate, for example silicon (100) or (110) or (111). The substrate may also be made of silicon oxide, silicon carbide (SiC), sapphire, silicon nitride (SiN), aluminum nitride (AIN), quartz, carbon, diamond, yttrium aluminum garnet (Yag), Yigs (“Ytrium ion gamet” in English) or LiNbO3 under LiTaO3.

[0050] 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 due to the fact that the temperature coefficients TCF of the shear modes in the constituent materials of the stack have opposite signs.

[0051] 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 17al, 17a2 and 17b 1, 17b2 extend essentially in the periodic direction d, i.e. perpendicular to the bands of the domains 3ai and 3bi. The number of teeth illustrated per comb electrode is here two, it being specified that each of the electrodes may have more than two teeth. The teeth 17al, 17a2 are connected to each other by a conductive bar 19a. The teeth 17b 1, 17b2 are connected to each other by a conductive bar 19b.

[0052] The teeth of the electrodes are arranged periodically with a mechanical pitch pe and therefore an electrical periodicity of 2pe. It nevertheless remains 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 pe is preferably larger than the pitch pf of the domains 3ai and 3bi. For example, pe is a multiple of pf, according to pe=nf*pf, nf being larger than 1, for example an integer, for example 2. Moreover, a reduction in the pitch of the domains pf leads to an increase in the speed, therefore in the frequency. A reduction in the pitch of the electrodes pe leads to a reduction in the wavelength, therefore an increase in the frequency.

[0053] 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 alloy of the copper aluminum type (AlCu) or an alloy mainly based on aluminum and another metal, titanium (Ti) for example. The pair of electrodes can in particular be made with AlCu, or AlSi, or AlTi 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 combinations Ti / Pt, Ti / Au, Ta / Pt, Cr / Au or with Zr. comb electrodes 15a and 15b preferably have thicknesses e3 of between 50 nm and 200 nm. The teeth 17al, 17a2, 17b 1 and 17b2 have a width a, perpendicular to the direction d and the normal n, 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 Lline photolithography.

[0054] The metallization ratio a / pe, a being the width of the teeth 17al, 17a2, 17b1 and 17b2 and pe their mechanical pitch, is preferably between 0.3 and 0.75, and in particular between 0.4 and 0.6. This ratio a / pe and / or the thickness e3 can be adjusted to regulate the electromechanical coupling and / or to reduce losses.

[0055] In this embodiment, the teeth all have the same shape; according to variants, the teeth can also be made with different lengths 1.

[0056] In the embodiment of [Fig.l], a power supply means 21 is connected to the interdigital comb electrodes 15a, 15b. Preferably, this power supply means 21 is configured to provide a radio frequency signal with a frequency of at least 3 GHz, preferably at least 6 GHz, to the interdigital comb electrodes 15a, 15b.

[0057] 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.

[0058] 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, AlN 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 ground, making it possible to reduce leakage currents. Such a trapping layer has a thickness preferably between 0.3 μm and 2 μm at the radio frequencies considered for telecoms, typically between 500 MHz and 6 GHz.

[0059] 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.

[0060] 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.l] the fundamental shear mode is no longer excited. Instead of this mode another mode appears at a frequency f higher than that deduced from the usual formula f=v / 2X with v the phase velocity of the fundamental shear mode for a homogeneous piezoelectric / ferroelectric layer and / . the wavelength corresponding to twice the mechanical pitch pe of the pair of interdigitated electrodes 15a, 15b.

[0061] The same phenomenon is observed for the Rayleigh-type mode and the super- positions mentioned above.

[0062] [Fig.2] illustrates the simulation results showing a comparison between a device without the presence of alternating domains and a device 1 as shown in [Fig. 1]. A composite substrate with a Si(100) base substrate, a 1 pm thick polycrystalline Si trapping layer, a 500 nm thick SiO2 dielectric layer and a 600 nm thick LiTaO3 layer was used for the comparison device. A composite substrate with a Si(100) base substrate, a 1 pm thick polycrystalline Si trapping layer, a SiO2 dielectric layer and a LiTaO3 layer of respective thicknesses of 500 nm and 600 nm with infinitely long and alternating domains with a pf pitch of 1 pm and with directions (YXI) / 42° and (YXI) / 222° was 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 an a / p ratio of 0.5 and a mechanical pitch pe of 2 pm.

[0063] [Fig. 2] illustrates the G harmonic conductance in Sm 1 on the left axis and the B harmonic susceptance in Sm 1 on the right axis as a function of the frequency in MHz. For the structure according to the invention with the periodically alternating polarization domains, the mode is observed at a frequency approximately three times higher, in particular around 3.850 MHz, see lines 101a and 101b in the G conductance and the B susceptance, compared to the mode at approximately 1.250 MHz for the structure without alternating domains, see lines 103a and 103b in the G conductance and the B susceptance. The mode of the structure according to the invention is characterized by a phase velocity of 12.5 km.s ', a coupling of 6.6% and a quality factor of more than 1.000.

[0064] The same phenomenon can be observed with LiNbO3 which for the same structural parameters shows a mode around 13 km.s', a coupling of around 13.3% and a quality factor also greater than 1,000.

[0065] [Fig.3] illustrates the results of the same simulation for LiTaO3 concerning the re harmonic resistance R in Qm' on the left axis and the harmonic reactance X in Qm 1 on the right axis as a function of frequency in MHz. For the structure according to the invention with the periodically alternating polarization domains, the mode is observed again at a frequency about three times higher, see lines 105a and 105b in the harmonic resistance in Q.m1 and the harmonic reactance compared to the mode for the structure without alternating domains, see lines 107a and 107b.

[0066] The same phenomenon can be observed with LiNbO3.

[0067] [Fig.4] is a graph illustrating the harmonic conductance G 109a and the harmonic susceptance B 109b as a function of frequency for the same elastic wave device 1 according to the first embodiment of the invention but over a wider frequency range. It is observed that there are no modes below the visible mode at around 3.850 MHz, showing the suppression of the existing standard modes in a device with the same properties but without the presence of alternating and periodic polarization domains. Beyond 3.850 MHz, some parasitic modes are observed, but sufficiently far from the 3.850 MHz mode to prevent their use, for example for the realization of a filtering function.

[0068] [Fig.5] illustrates the mode observed for the elastic wave device according to the first embodiment of the invention.

[0069] [Fig.5] illustrates the deformation obtained by the simulation for a cell of the device 1 with a part of a domain 3a 1 and a part of a domain 3b 1 which are crossed by a tooth 17al of the comb electrode 15a.

[0070] It is the domains 3al and 3bl as well as the electrode 15a which deform according to a shear mode with a phase speed three times greater than for the device without domains.

[0071] [Fig. 6] is a diagram schematically illustrating a part of an elastic wave device 41 according to a second embodiment of the invention. The elastic wave device 41 has the same structure and uses the same materials as the elastic wave device 1 of the first embodiment, except that the teeth 43 of the interdigital comb electrodes are at least partially, preferably entirely, buried in the layer 5 of piezoelectric material.

[0072] [Fig. 6] shows two bands of opposite polarization domains 3ax and 3bx crossed by a tooth 43 of one of the comb electrodes 15a, 15b (see [Fig. 1]). Here, the tooth 43 is entirely buried in the layer 5 and the thickness e3 of the tooth 43 is less than the thickness el of the layer 5. The surface 45 of the tooth 43 facing the outside of the device 41 is at the same level as the surface 47 of the layer 5. According to other variants of the second embodiment, the teeth are only partially buried. In this case, the surfaces 45 and 47 are at different levels.

[0073] The other teeth of the comb electrodes are buried in the same way. The connecting bars 19a and 19b (see [Fig.l]) may also be arranged in this way or arranged on the teeth to electrically connect the teeth of one electrode respectively.

[0074] The use of buried electrode teeth makes it possible to increase the electromechanical coupling.

[0075] According to variants of the first embodiment and the second embodiment of the invention, it is possible to envisage rotating the alignment of the electrodes by an angle a, with -5° < a < 5° around the normal n. Similarly, it is possible to have the polarization of the domains in a direction other than the direction normal to the surface of the layer 5. Even under these conditions with a misalignment between electrodes and periodic direction d or a misalignment between the polarization direction and the surface normal n, the higher frequency mode, the mode standard shear still remaining suppressed, but additional modes appear around the frequency of the main mode.

[0076] 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 filters (LCRF), dual coupled mode filters (DMS) or coupled cavity resonance filters (SCAW) as well as sensors and delay lines.

[0077] [Fig.7] illustrates the mode observed for the elastic wave device according to the second embodiment of the invention.

[0078] [Fig.8] illustrates the deformation obtained by the simulation for a cell of the device 41 with a part of a domain 3a 1 and a part of a domain 3b 1 which are crossed by a tooth 43 of the comb electrode.

[0079] [Fig. 8] is a diagram illustrating the steps of a method for manufacturing an elastic wave device according to a third embodiment of the invention, in particular an elastic wave device according to the first embodiment. The method begins with step 51 during which a piezoelectric material, in particular a ferroelectric material, is provided.

[0080] This step may consist of providing a solid substrate, such as a wafer made 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.

[0081] The piezoelectric material of layer 3 is monocrystalline lithium tantalate LiTaO3 or monocrystalline lithium niobate LiNbO3. Other piezoelectric materials may also be used.

[0082] In the case of lithium tantalate (LiTaO3), the crystalline 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 (YXl) / 0 with the value of the angle 0 chosen between 30° and 110°. The crystalline orientation of the second direction 13b of the second domains 3bi of layer 3 is then (YXl) / 0 +180° according to the IEEE Std-176 version IRE 1949 standard. It is also possible to have the domains in the orthogonal plane (YXlt) / 0 / 90° and (YXlt) / 0+180° / 90° relative to that described above while keeping the combs orthogonal.

[0083] In the case of lithium niobate (LiNbO3), the crystalline 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 (YXl) / 0 with the value of the angle 0 chosen in a range of ±15° around 128°. The crystalline orientation of the second direction 13b of the second domains 3bi of layer 3 is then (YXl) / 0+180° according to the IEEE Std-176 version IRE 1949 standard.

[0084] Depending on the orientation and nature of the crystals, shear waves or elliptical waves are excited. With the periodic polarization of the domains, two cases arise: in the first case, a combination of a shear wave with an elliptical wave is observed, in the second case, the wave results from the combination of an elliptical wave polarized in the direction of propagation and an elliptical wave polarized along the opening.

[0085] Such a composite substrate 111 can be obtained by a layer transfer process, such as a SmartCut process. During a SmartCut type process, ions are implanted in 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.

[0086] According to a variant, a trapping layer is produced 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.

[0087] Once the two substrates are attached, an input of mechanical and / or thermal energy takes the weakened zone to rupture. Thus a layer with a thickness of Ipm or less can be transferred onto the base substrate.

[0088] Then during step 53, an electric field is applied which is stronger than the coercive field of the piezoelectric material, in particular the ferroelectric material, to produce 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.mm 1 which represents the coercive field generally measured for lithium niobate and lithium tantalate.

[0089] Such a method 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, J. L. Decossas, J. C. Vareille, J. Hauden et A. Martinez : « Fer-roelectric domain inversion by électron beam on LiNbO3 and Ti : LiNbO3 » Journal of Applied Physics, 88:6665-6668, 2000, ou M. Yamada et K. Kishima : « Fabrication of periodically reversed domain structure for SHG in LiNbO3 by direct électron beam li-thography at room température », Electronics Letters, 27:828-829, 1991.

[0090] 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 during the next step. This layer can be made either on the dielectric layer 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 can be closer to 22 kV.mm1 and the temperature lower. 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.

[0091] Then during step 55, 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.

[0092] To produce the elastic wave device according to the second embodiment, a lithography and etching step is carried out to first create the trenches in the piezoelectric material, in particular the ferroelectric material, before these are filled with a metal, for example aluminum or molybdenum, or with a copper aluminum alloy or an alloy mainly based on aluminum and another metal, such as titanium for example.

Claims

Claims

1. Elastic wave device comprising a piezoelectric material, in particular a ferroelectric material (3), 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, in which the first and second domains (3a, 3bi) 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) of which the respective comb teeth (17al, 17a2 17bl, 17b2) extend essentially in the periodic direction (d) or the comb teeth are deflected from the periodic direction by a maximum of ± 5°.

2. An elastic wave device according to claim 1, wherein the comb teeth are arranged periodically.

3. An elastic wave device according to claim 1 or 2, wherein the mechanical pitch of the electrodes (pe) is larger than the pitch of the domains (pf).

4. Elastic wave device according to one of claims 1 to 3, wherein the interdigitated comb electrodes (15a, 15b) are arranged directly on or at least partially, preferably entirely, buried in the piezoelectric material (3).

5. Elastic wave device according to one of claims 1 to 4, wherein the first direction (13a) and the second direction (13b) are parallel to the surface normal (n) of the piezoelectric material (3).

6. Elastic wave device according to one of claims 1 to 5, wherein the width (a) of the teeth (17al, 17a2, 17b 1, 17b2) of the interdigitated comb electrodes (15a, 15b) is at least 280 nm, preferably at least 350 nm.

7. Elastic wave device according to one of claims 1 to 6, comprising a supply means (21) configured to supply a radio frequency signal of a frequency of at least 3 GHz, preferably at least 6 GHz, to the interdigitated comb electrodes (15a, 15b).

8. Elastic wave device according to one of claims 1 to 7, in in which the metallization ratio a / pe of the pair of interdigitated comb electrodes (15a, 15b) is between 0.3 and 0.75, preferably between 0.4 and 0.6, it being specified that “a” corresponds to the width of the teeth of the electrodes and “pe” to the pitch of the interdigitated comb electrodes (15a, 15b).

9. Elastic wave device according to one of claims 1 to 8, wherein the piezoelectric material (3) is arranged as a layer (5), in particular with a thickness (el) less than the wavelength, preferably less than X / 2, and even more preferably less than X / 4, above a base substrate (9), in particular a substrate of silicon, silicon oxide, silicon carbide (SiC), sapphire, silicon nitride (SiN), aluminum nitride (AIN), quartz, carbon, diamond, Yags, Yigs (Ytrium iron garnet) or LiNbO3 under LiTaO3.

10. An elastic wave device according to claim 9, wherein a dielectric layer (7), in particular a layer of silicon oxide, SiN, Ta2O5, ZrO2, HfO2, SiON 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.

11. Elastic wave device according to one of claims 9 or 10, wherein a layer, in particular a layer of polycrystalline silicon or polycrystalline AlN 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.

12. Elastic wave device according to one of claims 1 to 11, 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) or between the dielectric layer (3) and the base substrate (9).

13. An elastic wave device according to one of claims 1 to 12, the piezoelectric material (3) is at least one of LiTaO3, LiNbO3, KNbO3, PZT, PMnPt, PbTiO3 or AIScN.

14. Use of an elastic wave device according to claim 7, in an acousto-electric device, in particular a filter, a sensor or a delay line, having operating frequencies of 3 Ghz and above, especially 6 Ghz or above.

15. A method of manufacturing an elastic wave device according to one of claims 1 to 13 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 - producing the pair of interdigitated comb electrodes on or at least partially, preferably entirely, buried in the piezoelectric material.

16. A method of manufacturing an elastic wave device according to claim 15, wherein the electric field is applied in the form of parallel strips.

17. A method of manufacturing an elastic wave device according to claim 15 or 16, wherein the field lines are perpendicular to the surface of the piezoelectric material.

18. A method of manufacturing an elastic wave device according to at least one of claims 15 to 17, wherein the electric field is applied when the piezoelectric material has been heated to a temperature of at least 150°C, preferably to a temperature of at least 170°C.

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