FR3158015A1 - Elastic wave device - Google Patents

Abstract

The present invention relates to an elastic 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, in which the first and second domains 3a, 3b are periodically alternated in a direction d, called the periodic direction, perpendicular to the normal n of the surface of the piezoelectric material 3, and a pair of interdigitated comb electrodes 15a, 15b buried in the piezoelectric material 3 whose respective comb teeth 17a1 to 17a3 and 17b1 to 17b3 extend essentially perpendicular to the periodic direction d and to the normal n. Figure for abstract: Fig. 1

Description

Elastic wave device

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

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 losses and small sizes at radio frequencies (100 MHz - 10 GHz).

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.

The speed of elastic waves is generally limited by the properties of the piezoelectric material. In the case of lithium tantalate (LiTaO ₃ ), Rayleigh waves exhibit a phase velocity between 3,000 ms ^-1 and 3,500 ms ^-1 , with a maximum coupling of the order of 2%. Rayleigh waves on lithium niobate (LiNbO ₃ ) exhibit phase velocities 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 surface guided and require complex guiding structures.

Piezoelectric on insulator (POI) wafers allow modes with higher phase velocities but not exceeding 4,200 ms ^-1 on LiTaO ₃ and 4,500 ms ^-1 on LiNbO ₃ without radiation losses in the substrate.

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.

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 d, called periodic direction, perpendicular to the surface normal n of the piezoelectric material and a pair of interdigitated comb electrodes whose respective comb teeth extend essentially perpendicular to the periodic direction d and to the normal n and whose comb teeth are arranged at least partially, preferably entirely, buried in the piezoelectric material.

Due to the opposite polarization domains, modes appear with phase velocities greater than 4,200 ms ^-1 which propagate mainly on the surface. The radiation effects in the volume associated with the fact that the wave propagates beyond the SSBW speed (acronym for "surface skimming bulk wave"), speed beyond which the surface of the substrate no longer naturally guides the modes, are less than 10 ^-3 dB/λ, thus conditions allowing to neglect said effects.

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 for “Longitudinally Coupled Resonator Filter” in English), with double coupled modes (DMS for “Double-Mode-SAW” in English) or with coupled cavity resonances (SCAW for “Surface Cavity Acoustic Wave” in English) as well as sensors and delay lines.

Ferroelectricity is the property that a material possesses an electric polarization in its spontaneous 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.

According to a variant of the invention, the comb teeth can be arranged periodically.

According to a variant of the invention, the piezoelectric material (3) can be arranged in the form of a layer, 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, in particular a substrate of silicon, silicon oxide, silicon carbide (SiC), sapphire, silicon nitride (SiN), aluminum nitride (AlN), quartz, carbon, diamond, Yags ("Ytrium aluminum garnet" in English), Yigs ("Ytrium iron garnet" in English), amorphous silicon or poly-silicon or LiNbO ₃ and/or LiTaO ₃ . It can be a bi-layer or a stack with more than two layers. This makes it possible to confine the energy in the thin piezoelectric layer, which reduces losses.

According to a variant of the invention, a dielectric layer, in particular a layer of silicon oxide, SiN, Ta ₂ O ₅ , ZrO ₂ , HfO ₂ , SiON, polysilicon or a combination of these materials, may 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 between 200 nm and 2 µm. Indeed, this layer may 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 may be of the SmartCut ^TM type. The use of silicon oxide as a dielectric layer further increases the temperature stability of the device, which is a consequence of the opposite signs of the temperature coefficients of the elastic wave velocity 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.

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. The trapping layer preferably has a thickness between 300 nm and 2 µm. The presence of a trapping layer makes it possible to reduce leakage currents.

According to one embodiment of the invention, the comb teeth extend between regions of the piezoelectric material layer from the interface with the base substrate or the dielectric layer. According to yet another embodiment, the thickness of the comb teeth may be at least equal to or less than the thickness of the piezoelectric material layer. By optimizing the thickness of the comb teeth, the spectral purity of the observed mode(s) can be improved. This makes it possible to produce electrodes with a significant thickness, which allows the use of an incoming power of approximately 26dB to 33dB, or even more. This type of device is thus suitable for use as filters in transmission mode (TX), requiring higher powers than in reception mode. For thinner electrode teeth, the purity of the observed modes can be optimized.

Alternatively, the comb teeth are arranged in recesses in the piezoelectric layer and therefore do not extend to the interface with the base substrate or the dielectric layer. Preferably, the thickness of the teeth is at least more than half the thickness of the piezoelectric layer. Thus, it becomes possible to modulate the coupling of the mode used as a function of the thickness of the teeth.

According to a variant of the invention, the comb teeth can be positioned between domains of different polarization direction. In this case, the electrodes can be used to create the domains before using the electrodes in filters, sensors or delay lines. In addition, larger effective phase velocities are observed than for standard POIs.

According to a variant of the invention, the comb teeth can be positioned such that the polarization direction on both sides of each tooth is the same, in particular such that the polarization direction is different for different electrode teeth. For this embodiment only one mode is excited which makes its use for filters, sensors or delay lines etc. particularly suitable.

According to a variant of the invention, the comb teeth of one of the two electrodes can be positioned such that the polarization direction of both sides of each tooth is the same, and the comb teeth of the second electrode can be positioned between domains of different polarization direction.

Alternatively, the width a of the teeth of the interdigitated comb electrodes is between 40% and 60%, preferably 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 I-line lithography.

According to a variant of the invention, the piezoelectric material may be LiTaO ₃ or LiNbO ₃ . It may also be KNbO ₃ , PZT, PMnPt, PbTiO ₃ or AlScN. The modes observed for these materials have quality factors and electromechanical couplings allowing industrial use in filter or delay line type applications.

According to a variant, the elastic wave device may comprise a first electrode which comprises first switches and a second electrode which comprises second switches, the first switches are configured such that they can bring the first electrode into contact or not with teeth respectively and the second switches are 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 pattern, the frequency of the modes can be moved and thus the same device can be adapted to different operating parameters.

Preferably, the switching of the teeth is applied so as not to short-circuit the first electrode and the second electrode.

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, 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.

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, sensor or delay line, having operating frequencies of 2 GHz and above, 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 I-line lithography.

To achieve the object, the invention also provides a method for manufacturing an elastic wave device as described above and comprising the steps: providing a piezoelectric material, in particular a ferroelectric material, making the pair of interdigitated comb electrodes at least partially, preferably entirely, buried in the piezoelectric material and then applying a stronger electric field, in particular at least ten times stronger, than the coercive field of the piezoelectric material to make the first and second domains alternating periodically and having opposite polarizations. Advantageously, the electrodes can be used to create the domains, which simplifies the manufacturing process.

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. Thus, polarization can be obtained even in the absence of a counter electrode with a lower applied field than at room temperature.

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:

There FIG. 1 is a diagram schematically illustrating an elastic wave device according to a first embodiment of the invention.

There FIG. 2 is a diagram schematically illustrating a sectional view of a part of the elastic wave device according to the first embodiment of the invention as well as a diagram of the shear mode.

There FIG. 3 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.

There FIG. 4 is a graph illustrating the harmonic conductance G and the harmonic admittance Y for two different thicknesses of the dielectric layer of an elastic wave device according to the first embodiment of the invention.

There FIG. 5 is a diagram schematically illustrating a sectional view of a part of the elastic wave device according to a variant of the first embodiment of the invention.

There FIG. 6 is a graph illustrating the harmonic conductance G and the harmonic admittance Y for three different thicknesses of the teeth of an elastic wave device according to the variant of the first embodiment of the invention.

There FIG. 7 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.

There FIG. 8 is a graph illustrating the harmonic conductance G and the harmonic admittance Y for three different thicknesses of the teeth of an elastic wave device according to the second variant of the first embodiment of the invention.

There FIG. 9 is a diagram schematically illustrating a part of an elastic wave device according to a second embodiment of the invention.

There FIG. 10 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.

There FIG. 11 is a diagram schematically illustrating a part of an elastic wave device according to a third embodiment of the invention.

There FIG. 12 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 for two electrical excitation schemes.

There FIG. 13 is a diagram schematically illustrating an alternative pair of interdigitated electrodes for carrying out the fourth embodiment of the invention.

There FIG. 14 is a diagram schematically illustrating a second alternative pair of interdigitated electrodes for carrying out the fourth embodiment of the invention.

There FIG. 15 is a diagram illustrating the steps of a method of manufacturing an elastic wave device according to a fifth embodiment of the invention.

The invention will be described in more detail using advantageous embodiments, by way of example, and with reference to the drawings. The described embodiments 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.

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 ^TM 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.

Ferroelectricity is the property that a material possesses an electric polarization in its spontaneous 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.

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 p _f , in a direction d, called the periodic direction, perpendicular to the surface normal n of the piezoelectric material 3.

In this embodiment, the piezoelectric material of layer 3 may be monocrystalline lithium tantalate LiTaO ₃ or monocrystalline lithium niobate LiNbO _3. Other piezoelectric materials may also be used. Layer 5 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.

In the case of lithium tantalate (LiTaO ₃ ), 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 (YX/)/θ with the value of the angle θ chosen between -30° and 110°. The crystalline orientation of the second direction 13b of the second domains 3bi of layer 3 is then (YX/)/θ +180° according to the IEEE Std-176 version IRE 1949 standard. The polarization axis being the Z axis, starting from a plate oriented (YX), this axis is collinear with the width of the plate. After rotation of θ which gives the orientation (YX/)/θ, this axis forms an angle of θ-90° with the normal to the plate and of θ with the axis collinear with the surface of the plate. In this frame, the normal n therefore corresponds to the direction 90°- θ, and the direction d is orthogonal to n and collinear to Z.

In the case of lithium niobate (LiNbO ₃ ), the crystal orientation defined according to the IEEE Std-176 version IRE 1949 standard of the first sense13a of the first domains 3ai of layer 3 is preferably (YX/)/θ with the value of the angle θ chosen in a range from -40 to +110° and preferably from -10 to +50°. The crystal orientation of the second sense 13b of the second domains 3bi of layer 3 is then (YX/)/θ+180° according to the IEEE Std-176 version IRE 1949 standard.

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 µm. The dielectric layer may also be made of SiN, Ta ₂ O ₅ , ZrO ₂ , HfO ₂ , SiON or a combination of these materials. It may also comprise, among other things, polysilicon.

The base substrate 9 is preferably a silicon substrate, for example (100) or (110) or (111) silicon. The orientation deviation between the flats of the two crystals of silicon and lithium tantalate of the structure can vary from 0 to 180° without loss of generality on the fundamental properties of the transducer. Essentially the same results are also observed for the three orientations of the silicon. The substrate can also be made of silicon oxide, silicon carbide (SiC), sapphire, silicon nitride (SiN), aluminum nitride (AlN), quartz, carbon, diamond, yttrium aluminum garnet (Yag), Yigs ("Ytrium ion garnet" in English), amorphous silicon or poly-silicon or LiNbO ₃ or LiTaO ₃ .

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 velocity (TCV) of the shear modes in the constituent materials of the stack have opposite signs.

A pair of interdigitated comb electrodes 15a, 15b may be arranged such that they are at least partially, preferably entirely, buried in the piezoelectric material 3. 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. The 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 electrode teeth are arranged periodically with a mechanical pitch p _e 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 pitch p _f of the domains 3ai and 3bi. This data can be varied to increase, or even maximize, for example the coupling or adjust the speed of the mode to reach a specified frequency value.

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 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. 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 I-line photolithography.

The metallization ratio a/p _e , a being the width of the 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 regulate the electromechanical coupling and/or to reduce losses.

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

In the embodiment of the FIG. 1 , 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 2 GHz, preferably at least 6 GHz, to the interdigital comb electrodes 15a, 15b.

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.

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 wave, 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.

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.

There 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 representing a periodic mesh. The 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 buried in the layer 5 of piezoelectric material. FIG. 2 also represents the dielectric layer 7 and the base substrate 9.

The tooth 17a1 is positioned between two opposite polarization domains, namely 3a1 and 3b1. The tooth 17b1 is also positioned between two opposite polarization domains, namely 3b1 and 3a2. In this embodiment, the teeth 17a1 and 17b1 extend from the surface/interface 21 of the dielectric layer 7 through the entire thickness e1 of the piezoelectric layer 5. The thickness e3 of the teeth 17a1 and 17b1 is therefore the same as the thickness e1 of the piezoelectric layer 5. The width a of the teeth 17a1 and 17b1 is 50% of the width a2 of the domains. In variants, the width may have a value between 40% and 60% of a2.

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 the 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 in wavelength by a factor of two. At the bottom of the FIG. 2 are illustrated the deformations of the mesh in a shear mode which has a wavelength of 2 µm therefore corresponding to the mechanical period p _e of the teeth and not to the electrical period 2p _e for a device without domains. The mode exists only when the vibration of the electrodes is antisymmetric.

There FIG. 3 illustrates the simulation results for a device 1 as shown in FIG. 1 and to the FIG. 2 . A composite substrate with a Si(100) base substrate with the flat oriented at 45° to that of the LiTaO ₃ layer, a 1 µm thick polycrystalline Si trapping layer, a 500 nm thick SiO ₂ dielectric layer and a 600 nm thick LiTaO ₃ layer was used for the simulation. The domains are considered infinitely long and alternating with a periodicity p _f of 4 µm coinciding with the electrical period of the lattice and with directions (YX/)/42°/30° and (YX/)/222°/210°. In both cases, the electrodes are made of AlCu and have a thickness of 400 nm and a ratio a/p _e of 0.5 and a mechanical pitch p _e of 2 µm.

On the FIG. 3 , which shows the simulation results with the Y harmonic admittance modulus in dB on the left axis and the G harmonic conductance modulus in dB on the right axis as a function of frequency in MHz. We observe the fundamental shear mode 31 around 2 GHz which dominates and three other contributions at higher frequencies. A 33 mode around 3.3 GHz, a 35 mode around 5.8 GHz and a 37 mode around 6.9 GHz.

Mode 33 can be reduced by reducing the thickness of the piezoelectric layer. Modes 35 and 37 are highly dependent on the thickness of the dielectric layer 7. The FIG. 4 shows this effect by comparing the simulation results for two dielectric layer thicknesses with the Y harmonic admittance modulus in dB on the left axis and the G harmonic conductance modulus in dB on the right axis as a function of frequency in MHz. For a dielectric layer 7 thickness of 300 nm instead of 500 nm, the modes are less pronounced. Arrow 41 illustrates the drop in the G harmonic conductance of the mode around 6.8 GHz and reference number 43 illustrates the drop in the Y harmonic admittance when going from 500 nm to 300 nm thickness.

Mode 31 therefore has an apparent effective phase velocity v _eff of about 8 km ^-1 and therefore about twice as large compared to a piezoelectric layer without domains with alternating polarizations, with v _eff = λ*f =p _el *f of the mode, λ being the coherence wavelength of the charges in the electrodes, p _el being the electrical period of the electrodes and f the frequency of the mode. Using a pair of interdigitated comb electrodes deposited on the surface of the piezoelectric layer with alternating domains therefore makes it possible to increase the phase velocity while preserving the advantages of an electromechanical coupling greater than 7% or even 10%, quality coefficients of several thousand beyond 1 GHz and controlled temperature effects.

Moreover, for a configuration with electrodes having the same thickness as the lithium tantalate piezoelectric layer, in this case between 300 nm and 1 µm, a crystalline orientation of the single rotation layer (YX/)/θ with θ between +10 and +60° and a thickness of the SiO ₂ dielectric layer between 300 nm and 2 µm, it is possible to minimize the temperature coefficient of the frequency (CTF) of the resonance or anti-resonance to a value lower than 20 ppm.K ^-1 in absolute value and ideally lower than 10 ppm.K ^-1 .

There FIG. 5 is a diagram schematically illustrating a sectional view of a portion of an elastic wave device 51 according to a variant of the first embodiment of the invention. The only difference from the embodiment illustrated in the FIG. 1 and the FIG. 2 is that the teeth 17a1 and 17b1 do not extend from the interface 21 between the dielectric layer 7 and the piezoelectric layer 5 but the teeth 17a1 and 17b1 are embedded in recesses 53a and 53b of the piezoelectric layer 5. The surfaces 55a and 55b of the teeth 17a1 and 17b1 are aligned with the surface 57 of the piezoelectric layer 5. According to other variants, the teeth 17a1 and 17b1 may protrude from the surface 57 or be set back from the surface 57. FIG. 5 also represents the dielectric layer 7 and the base substrate 9. The trapping layer is not illustrated as for the first embodiment of the FIG. 1 or of the FIG. 2 .

There FIG. 6 is a graph illustrating 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 of an elastic wave device according to the variant of the first embodiment of the invention. A mode 59 appears below 2 GHz.

For the simulation, the electrode thickness was 500 nm for a piezoelectric layer thickness of 600 nm. The LiTaO ₃ was of orientation (YX l )/42°-222°, the dielectric layer was SiO ₂ with a thickness of 500 nm, the trapping layer was 1 µm thick, and the base substrate was Si(111) with a flat with a misalignment of 45° relative to the LiTaO ₃ . The electrical period was 4 µm which gives rise to a mode with a speed greater than 7200 ms ^-1 for an electromechanical coupling of more than 10%. Essentially the same results are observed for the other silicon orientations.

The contribution at 3.5 GHz is related to the layers under the tantalate. It can be reduced by varying the thickness ratios of the different layers.

Simulations show that if the teeth are completely buried in the layer and do not reach the upper surface of the piezoelectric layer, the spectral purity and the electromechanical coupling of the mode can be further improved. The resonance quality is also improved compared to the previous case.

According to a variant, the elastic wave device 51, the part of the piezoelectric layer 5 underlying the teeth 17a1, 17b1 remains in its initial polarization state, with or without domain. The simulations show that the vibration mode remains essentially the same as illustrated in the FIG. 6 .

There FIG. 7 is a diagram schematically illustrating a sectional view of a portion of an elastic wave device 61 according to a second variant of the first embodiment of the invention. The only difference from the embodiment illustrated in the FIG. 1 and the FIG. 2 and that the surfaces 63a and 63b of the teeth 17a1 and 17b1 are no longer aligned with the surface 65 of the piezoelectric layer 5 but set back. Thus, e3 < e1. According to another variant, the teeth 17a1 and 17b1 can protrude from the surface 65. Thus e3 > e1. FIG. 7 also represents the dielectric layer 7 and the base substrate 9. The trapping layer is not illustrated as for the first embodiment of the FIG. 1 or of the FIG. 2 .

There FIG. 8 is a graph illustrating 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 of the elastic wave device 61 according to the second variant of the first embodiment of the invention.

As in the variant of the FIG. 6 , the simulation shows that the spectral purity and resonance quality improve compared to the mode shown in FIG. 3 if the teeth do not reach the upper surface of the piezoelectric layer. The mode is also better coupled than when the electrode completely fills the cavity. [Fig. 7] also illustrates that for reasons of boundary conditions, the teeth 17a1 and 17b1 of the electrodes vibrate synchronously for this mode 67, as for the modes 31 and 59 illustrated in the FIG. 3 and the FIG. 6 .

There FIG. 9 is a diagram schematically illustrating a sectional view of a portion of an elastic wave device 71 according to a second embodiment of the invention. The elastic wave device 71 has the same structure and uses the same materials as the elastic wave device 1 of the first embodiment, except that the teeth 73a1 and 73b1 of the interdigital comb electrodes are positioned such that the polarization direction of both sides of each tooth is the same, but the polarization direction of neighboring domains is different for the teeth 73a1 and 73b1 which are of different electrodes. Thus, the tooth 73a1 of the first electrode 15a is in the middle of a first domain 75a1, while the tooth 73b1 of the second electrode 15b is in the middle of a second domain 75b1 of opposite polarization. In this embodiment the teeth are positioned centrally in a domain, in a variant the teeth can also be positioned outside the center of a domain. FIG. 9 also represents the dielectric layer 7 and the base substrate 9. The trapping layer is not illustrated as for the first embodiment of the FIG. 1 or of the FIG. 2 .

In this embodiment the thickness e3 of the teeth 73a1 and 73b1 is greater than the thickness e1 of the domains 75a1 and 75b1. According to variants e1 can also be equal to or smaller than e3. In addition, as for the first variant of the first embodiment illustrated in the FIG. 5 the electrodes 73a1 and 73b1 may also be positioned in recesses in the areas 75a1 and 75b1 and not extend to the interface 77 between the piezoelectric layer 5 and the dielectric layer 7.

The width a of teeth 73a1 and 73b1 is about half of the widths a2_1 and a2_2 of both parts of domains 75a1 and 75b1 together. In other variants, the width a can have a value between 40% and 60% of the sum of the widths a2_1 and a2_2.

A vibration mode is illustrated at the bottom of the FIG. 9 . This mode was obtained by the simulation as described above for the first embodiment, but for a cell of the device 71 with a domain width of 1 µm and a LiTaO ₃ orientation (YX/)/42° and (YX/)/222° and a base substrate in Si(111). Essentially the same results are observed for the other silicon orientations. The mechanical pitch p _e is thus 3 µm, the cell for the simulation therefore has a width of 6 µm. The vibrations of the mode have a wavelength λ equal to a mechanical pitch p _e of the electrodes.

There FIG. 10 illustrates the G harmonic conductance in dB on the left axis and the Y harmonic admittance on the right axis as a function of frequency in MHz for the orientation (YX/)/42° and (YX/)/222°). Only one very pronounced mode of vibration compared to the others is visible.

The mode appears around 1.3 GHz, visible in the Y harmonic admittance, see issue 81, and the G harmonic conductance, see issue 83. Simulations show that for the observed mode, the electrode motion is always in phase. The equivalent phase velocity is 7.8 km.s ^-1 , thus close to twice the velocity of the shear mode in a comparable POI substrate but without alternating opposite polarization domains.

The simulations also show that for thicknesses e3 > e1 the spectral purity of the mode improves compared to smaller thicknesses.

There FIG. 11 is a diagram schematically illustrating a sectional view of a portion of an elastic wave device 91 according to a third embodiment of the invention. The elastic wave device 91 has the same structure and uses the same materials as the elastic wave device 1 of the first embodiment, except that the teeth 93a1, 93a2 of the first interdigital comb electrode are positioned such that the polarization direction of the domains 3a1 and 3b1 respectively 3b1 and 3a2 on both sides of each tooth 93a1, 93a2 is opposite while the teeth 93b1, 93b2 of the second electrode are positioned such that the polarization direction of both sides of the teeth is the same. In fact, the tooth 93b1 is arranged in the middle of the domain 3b1 and the tooth 93b2 is arranged in the middle of the domain 3a2. Only a part of the first domain 3a1 is illustrated as for domain 3a2 to find the same boundary conditions at the beginning and at the end of the cell. FIG. 11 also represents the dielectric layer 7 and the base substrate 9. The trapping layer is not illustrated as for the first embodiment of the FIG. 1 or of the FIG. 2 .

There FIG. 12 is a graph illustrating at the top and bottom the harmonic conductance G and the harmonic admittance Y for two embodiments of an elastic wave device according to the third embodiment of the invention. The difference between the two embodiments lies in the fact that the electrodes are subjected to different potential patterns. These patterns are shown above each graph of the FIG. 12 with the “+” and “-” signs in the electrode teeth.

The simulation was performed as described above for the first embodiment for a cell of the device 91 with a cell width of 6 µm with an electrode width of 750 nm and equal domain width. A structure repeat period of 6 µm is found. The orientation of LiTaO ₃ is (YX/) /42° and (YX l )/222° and the base substrate 9 is Si(111). Essentially the same results are observed for the other silicon orientations.

There FIG. 12 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 (YX/)/42° and (YX/)/222°. These results were obtained for an alternating +V/-V potential applied to teeth 93a1, 93b1, 93a2 and 93b2 as illustrated above the first graph of the FIG. 12 .

Several modes appear in the admittance and conductance. In particular, in the Y harmonic admittance with number 95 ₁ towards 600 MHz and in the G harmonic conductance with number 95 ₂ towards 2 GHz, with number 95 ₃ towards 3 GHz and with 95 ₄ towards 4 GHz.

The simulation shows that mode 95 ₁ is a Rayleigh mode with its phase velocity of about ~3600 ms ^-1 . The other contributions around 2 and 3 GHz exhibit more radiation losses but have larger phase velocities.

The second mode 95 ₂ with radiation losses, has a quality factor of Q< 100 and corresponds to the combination of a shear wave and a vibration mode of the electrode teeth.

The third contribution 95 ₃ towards 3 GHz has an effective phase velocity of about ~18 km.s ^-1 and shows less loss than the previous mode 95 ₂ is also a combination of a shear wave and a vibrational mode of the electrode teeth as shown in the FIG. 11 .

The fourth contribution 95 ₄ towards 4 GHz has an effective phase velocity of about 24 km.s ^-1 and shows in the simulation a full wavelength in the electrode. This is a second-order electrode vibrational mode.

There FIG. 12 illustrates in a second graph, below the 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 (YX/)/42° and (YX/)/222°. These results were obtained for a +V potential applied to teeth 93a1 and 93a2' and a –V potential applied to teeth 93b1 and 93b2' as illustrated above the second graph of the FIG. 12 . Three modes appear in the admittance and conductance at different frequencies compared to the previous realization. In particular in the Y harmonic admittance with number 97 ₁ towards 1.5 GHz and with number 97 ₂ towards 2.5 GHz and in the G harmonic conductance with number 97 ₃ close to 3.15 GHz.

Mode 97 ₁ at 1.5 GHz with an effective phase velocity of about ~9 km.s ^-1 is almost on the threshold of crossing the SSBW frequency. It can be further improved with respect to losses by increasing the mass. The simulation shows that it is a combination of an electrode tooth vibration mode and the classical shear wave. According to the simulation results, it is likely that only two of the four electrodes contribute significantly to the resonance.

The second mode 97 ₂ towards 2.5 GHz with an effective velocity close to 15 km.s ^-1 is an anti-symmetric shear mode at the electrodes which all vibrate in phase. In fact, as before, only two electrodes actually contribute to the electrical response. According to the simulation results, it is likely that only the electrodes located between two domains of opposite polarization can actually contribute to the resonance, the other two must have a charge balance that balances.

Mode 97 ₃ at 3.15 GHz with an effective speed close to 20 km.s ^-1 is a vibrational mode of the electrode teeth, as illustrated at the bottom of the FIG. 11 .

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.

In this second embodiment, another type of interdigitated electrode pair is used, which is schematically illustrated in the FIG. 13 . 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 13b1' and 13b2', 13b3' and 13b4', 13b5' and 13b6' for electrode 15b'.

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 the FIG. 14 .

There FIG. 14 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 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 the electrode 155 with teeth 159a, 159b, 159c and 159d respectively. Thus, one can make diagrams of application of potentials such as shown at the top left and right of the FIG. 12 by a single device. The switching is preferably applied such that a short circuit between electrodes is avoided. The FIG. 14 illustrates on the left the situation in which teeth 159a to 159d have a potential alternating from one tooth to the next. To the left of the FIG. 14 two successive teeth therefore 159a and 159b and 159c and 159d have the same potential.

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

There FIG. 15 is a diagram illustrating the steps of a method of 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 101 during which a piezoelectric material, in particular a ferroelectric material, is provided.

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), Si(110) or Si(111) or quartz.

The piezoelectric material of layer 3 is single-crystal lithium tantalate LiTaO ₃ or single-crystal lithium niobate LiNbO _3. Other piezoelectric materials can also be used.

In the case of lithium tantalate (LiTaO ₃ ), 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 (YX/)/θ with the value of the angle θ chosen between 30° and 110°. The crystal orientation of the second direction 13b of the second domains 3bi of layer 3 is then (YX/)/θ +180° according to the IEEE Std-176 version IRE 1949 standard. It is also possible to have the domains in the orthogonal plane (YX/ t )/θ/90° and (YX/ t )/θ+180°/90° relative to that described above by keeping the comb teeth parallel to the domains.

In the case of lithium niobate (LiNbO ₃ ), 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 (YX/)/θ with the value of the angle θ chosen in a range -40 to +110° and preferably from -10 to +50°. The crystal orientation of the second direction 13b of the second domains 3bi of layer 3 is then (YX/)/θ+180° according to the IEEE Std-176 version IRE 1949 standard.

Such a composite substrate 11 can be obtained by a layer transfer process, such as a SmartCut ^TM process. In a SmartCut ^TM 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.

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.

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.

Then, during step 103, the pair of interdigitated comb electrodes is produced in the piezoelectric material to obtain the elastic wave device according to the first embodiment using lithography, etching, deposition and possibly polishing steps, known to those skilled in the art.

The method of step 103 can be adapted to the other embodiments described above by adapting the mask and the alignment of the mask during the lithography steps.

Then in step 105, 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. 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.

Thus, the teeth of the interdigital comb electrode pair can also be used to create the structure of the opposite polarization domains.

To carry out the second and third embodiments, it is sufficient to carry out step 105 before step 103 using electrodes to create an electric field in the form of parallel bands, preferably perpendicular to the surface of the piezoelectric material.

Such a process is for example described in Thorlabs, “Periodically Poled Lithium Niobate (PPLN) – Tutorial”, pages 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 and A. Martinez: “Ferroelectric domain inversion by electron beam on LiNbO3 and Ti: LiNbO3” Journal of Applied Physics, 88:6665–6668, 2000, or M. Yamada and K. Kishima: “Fabrication of periodically reversed domain structure for SHG in LiNbO3 by direct electron beam lithography at room temperature”, Electronics Letters, 27:828–829, 1991.

Claims (13)

Elastic 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),
in which 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)
whose respective comb teeth (17a1 to 17a3, 17b1 to 17b3) extend essentially perpendicular to the periodic direction (d) and to the normal (n) and
whose comb teeth (17a1 to 17a3, 17b1 to 17b3) are arranged at least partially, preferably entirely, buried in the piezoelectric material (3) and in which the comb teeth are positioned such that the polarization direction on both sides of each tooth is the same, in particular such that the polarization direction is different for teeth of different electrodes. An elastic wave device according to claim 1, wherein the comb teeth (17a1 to 17a3, 17b1 to 17b3) are arranged periodically. Elastic wave device according to claim 1 or 2, 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 (AlN), quartz, carbon, diamond, Yags (Ytrium aluminum garnet), Yigs (Ytrium iron garnet) or LiNbO ₃ and/or LiTaO ₃ . Elastic wave device according to claim 3, wherein a dielectric layer (7), in particular a layer of silicon oxide, SiN, Ta ₂ O ₅ , ZrO ₂ , HfO ₂ , SiON, polysilicon 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 µm. Elastic wave device according to one of claims 3 or 4, wherein a trapping 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 µm. Elastic wave device according to one of claims 3 to 5, wherein the comb teeth extend between regions of the layer (5) of piezoelectric material from the interface (21) with the base substrate (9) or the dielectric layer (7). Elastic wave device according to one of claims 3 to 6, wherein the thickness of the comb teeth is at least equal to or less than the thickness of the layer (5) of the piezoelectric material (3). Acoustic wave device according to one of claims 1 to 7, wherein the width (a) of the teeth of the interdigitated comb electrodes (15a, 15b) is between 40% and 60%, preferably 50%, of the width of the domains, and/or at least 280 nm, preferably at least 350 nm. An acoustic wave device according to one of claims 1 to 8, the piezoelectric material (3) is at least one of LiTaO ₃ , LiNbO ₃ , KNbO ₃ , PZT, PMnPt, PbTiO ₃ or AlScN. An elastic wave device according to one of claims 1 to 9, 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. Acoustic wave device according to one of claims 1 to 10, comprising a supply means (21) configured to supply a radio frequency signal of a frequency of at least 2 GHz, preferably at least 6 GHz, to the interdigitated comb electrodes (15a, 15b). Use of an elastic wave device according to claim 11 in an acoustoelectric device, in particular a filter, a sensor or a delay line, having operating frequencies of 2 GHz and above, in particular 6 GHz or above. Method of manufacturing an elastic wave device according to one of claims 1 to 11 and comprising the following steps:
- providing a piezoelectric material, in particular a ferroelectric material,
- making the pair of interdigitated comb electrodes at least partially, preferably entirely, buried in the piezoelectric material (3).and then
- 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.