Elastic wave device

Abstract

The present invention relates to an elastic wave element, particularly a shear wave element, comprising a piezoelectric material (3), particularly a ferroelectric material, having a plurality of first polarization domains (3a) in a first polarization direction (13a) and a plurality of second polarization domains (3b) in a second polarization direction (13b), the first polarization direction (13a) being opposite to the second polarization direction, wherein the first and second polarization domains (3a, 3b) alternate periodically along a periodic direction (d) perpendicular to the surface normal (n) of the piezoelectric material (3), and a pair of forked comb-shaped electrodes (15a, 15b) located above, particularly on, the piezoelectric material (3), wherein the comb teeth (17a1 to 17a3 and 17b1 to 17b3) extend primarily perpendicular to the periodic direction (d) and the surface normal (n) of the substrate.

Description

[Technical Field]

[0001] This invention relates to an elastic wave element and a method for manufacturing such an elastic wave element. [Previous Technology]

[0002] Surface acoustic wave (SAW) devices are widely used, such as filters, sensors, and delay lines. For example, SAW RF filters can be used in mobile communication devices because of their simple structure, low insertion loss, filter bandwidth (typically less than 3 dB, or even 2 dB), and small size (less than 1 mm²) in the RF range (100 MHz – 10 GHz).

[0003] In an elastic wave element, one or more interdigitated comb transducers (IDTs) are formed on a single-crystal piezoelectric substrate. The direct piezoelectric effect can convert elastic waves (typically Rayleigh waves) propagating along the surface of the piezoelectric substrate into electrical signals by electrically exciting the comb-like fingers. Conversely, an electrical signal can be applied through the finger structure of the comb electrodes to induce surface elastic waves in the piezoelectric substrate under the transducer.

[0004] The velocity of elastic waves is generally limited by the properties of the piezoelectric material. For lithium tantalate (LiTaO3), the Rayleigh wave phase velocity is between 3000 ms⁻¹ and 3500 ms⁻¹, with a maximum coupling degree of approximately 2%. On lithium niobate (LiNbO3), the Rayleigh wave phase velocity reaches as high as 3900 ms⁻¹, with a maximum coupling degree approaching 5.6%. Other modes associated with shear and compression waves are generally faster, but are only partially guided by the surface and require complex guiding structures.

[0005] Piezoelectric-on-insulator (POI) wafers can achieve modes with larger phase velocities, but without radiation loss in the substrate, their velocities do not exceed 4200 ms-1 on LiTaO3 and 4500 ms-1 on LiNbO3. [Summary of the Invention]

[0006] Therefore, the object of the present invention is to overcome these disadvantages by providing an elastic wave element whose operating mode has a phase velocity higher than that of a guided Rayleigh wave or shear wave.

[0007] To achieve this objective, the present invention proposes an elastic wave element, particularly a shear wave element, comprising a piezoelectric material, particularly a ferroelectric material, having a plurality of first polarization domains in a first polarization direction and a plurality of second polarization domains in a second polarization direction, the first polarization direction being opposite to the second polarization direction, wherein the first and second polarization domains are periodically alternating along a periodic direction perpendicular to the surface normal of the piezoelectric material and located above, particularly above, the piezoelectric material, a pair of forked comb electrodes, each of whose comb teeth extends substantially perpendicular to the periodic direction and the surface normal, and each comb tooth of one of the forked comb electrodes may be completely located above a single first and / or second polarization domain, while each comb tooth of the second forked comb electrode is located above the interface between the first and second polarization domains.

[0008] Because the polarization directions of the polarization domains are opposite, modes with phase velocities greater than 4200 ms⁻¹ emerge, propagating on the surface without volumetric radiation effects. This method also makes it possible to create polarization-free conditions under different operating mode limits, resulting in more complex polarization modes than the usual basic modes. Furthermore, when implementation conditions permit, alternating polarization domains can easily align the comb teeth with the periodic ferroelectric structure. These modes are shear modes.

[0009] The elastic wave element according to the present invention can be used in a wide range of applications, such as filters, especially ladder filters, or filters with acoustic coupling, such as longitudinally coupled resonator filters (LCRF), dual-mode SAW (DMS) filters or surface cavity acoustic (SCAW) filters, as well as sensors and delay lines.

[0010] Ferroelectricity is the property of a material to spontaneously polarize. Ferroelectric materials are a subclass of piezoelectric materials. Ferroelectric properties can also be obtained artificially, for example, by creating polarization domains with different polarization directions through suitable growth conditions. Therefore, the following discussion may consider any material capable of generating polarization domains as a piezoelectric material.

[0011] According to a variation, the teeth of the pair of finger-shaped comb electrodes can be arranged periodically. These teeth are preferably rectangular in shape, i.e., parallelepiped in shape.

[0012] According to a variation, the comb teeth can be disposed above the interface between the first polarization domains and the second polarization domains having opposite polarization directions. In this configuration, compared with a configuration without polarization domains but having the same electrodes and materials, the frequency of the acoustic mode is observed to be twice as high. Therefore, the effective phase velocity veff of these modes is approximately twice that of the mode, veff = λ × f = pel × f, where λ is the coherence wavelength of the charge in the electrode, pel is the electrical period of the electrode, and f is the frequency of the mode.

[0013] According to a variation, the comb teeth can be symmetrically arranged with respect to the interface between the first polarization domains and the second polarization domains. In this configuration, the central plane of the comb teeth coincides with the plane of the interface, thereby reducing or even eliminating the parasitic pattern distribution.

[0014] According to a variation, the directly adjacent comb teeth of the two finger-shaped comb electrodes can be positioned above the interface directly adjacent to the first polarization domain and the second polarization domain. This yields a period equal to the period of the polarization domains.

[0015] The operation of higher frequency components requires thinner comb teeth, thus necessitating the use of higher resolution photolithography, such as KrF or ArF excimer lasers, which are obviously more expensive. In addition, thinner structures result in greater electrical losses, i.e., ohmic losses.

[0016] According to a variation, the width of the comb teeth of the pair of finger-shaped electrodes can be between 25% and 75% of the width of the polarization domains, more specifically between 40% and 60% of the width of the polarization domains, and more specifically 50% of the width of the polarization domains, and / or at least 280 nm, preferably at least 350 nm. Therefore, filters, sensors, or delay lines operating at frequencies higher than normally available frequencies can be provided by using I-line lithography.

[0017] According to a variation, the width of each comb tooth may correspond to the width of a first polarization domain and a second polarization domain. Therefore, in detail, when the electrodes are symmetrically arranged with respect to the interface between the first polarization domain and the second polarization domain, the current passing through the electrodes can be increased.

[0018] According to a variation, each comb tooth may be located entirely above a single first polarization domain or a single second polarization domain.

[0019] According to an alternative variation, the directly adjacent comb teeth of the pair of finger-shaped comb electrodes can be located above polarization domains with the same polarization direction, especially polarization domains with the same polarization direction separated only by a polarization domain with opposite polarization directions. In this configuration, the second-order mode is removed, and the third-order harmonic mode is preferred compared to the basic shear mode, with an effective phase velocity three times that of the basic shear mode. Therefore, a phase velocity much greater than that of the basic mode can be achieved, especially a phase velocity greater than 10 km·s⁻¹ observed in simulations, which allows frequencies outside the C-band of the electromagnetic spectrum to be reached using I-line lithography.

[0020] According to other alternative variations, the teeth of one of the pair of finger-shaped comb electrodes may be located above the first polarization domains, while the teeth of the second finger-shaped comb electrode may be located above the second polarization domains. In this embodiment, the second-order mode is preferred compared to the basic mode, resulting in a twofold increase in the effective phase velocity.

[0021] According to a variation, the width of the comb teeth of the pair of forked comb electrodes may be the same as the width of the polarization domains, and / or at least 280 nm, preferably at least 350 nm, to be compatible with the limitations of I-line lithography. Therefore, filters, sensors, or delay lines operating at frequencies higher than normally available frequencies can be provided by using I-line lithography. Furthermore, these electrodes can be used to generate polarization domains when the comb teeth are always located on polarization domains with the same polarization direction.

[0022] According to a variation, each tooth of one of the pair of finger-shaped comb electrodes may be located entirely above a single first and / or second polarization domain, while each tooth of the second finger-shaped comb electrode is located above the interface between the first and second polarization domains. It was observed that the effective phase velocity of several modes was greater than the phase velocity of the fundamental mode.

[0023] According to a variation, the element can be manufactured such that between the two polarization domains where the two comb teeth are at least partially located, there is at least one complete polarization domain without any electrode fingers. By moving these electrodes apart from each other, the mode and frequency can be changed, thereby changing the effective phase velocity.

[0024] According to one variation of the present invention, the piezoelectric material can be disposed as a single layer on a substrate. The thickness of the layer is less than the wavelength, preferably less than λ / 2, more preferably less than λ / 4, and typically less than 1 µm. The substrate is silicon, amorphous silicon or polycrystalline silicon, silicon oxide, silicon carbide (SiC), sapphire, silicon nitride (SiN), aluminum nitride (AlN), quartz, carbon, diamond, yttrium aluminum garnet (Yags), yttrium iron garnet (Yigs), or LiNbO3 and / or LiTaO3. This can be a two-layer structure or a stack of two or more layers. Therefore, energy can be confined within the piezoelectric thin layer, thereby reducing overall energy loss.

[0025] According to one variation of the invention, a dielectric layer, particularly a layer of silicon oxide, silicon nitride, Ta₂O₅, ZrO₂, HfO₂, SiON, polycrystalline silicon, or a combination of these materials, may be disposed between the piezoelectric material layer and the substrate. The thickness of the dielectric layer is preferably less than the wavelength and / or between 0.2 µm (i.e., 200 nm) and 2 µm. In practice, the dielectric layer can be used during a layer transfer method to bond the piezoelectric layer to the substrate, for example, through bonding, such as by using molecular bonding, by adhesive bonding, or by eutectic bonding. The layer transfer method may be the SmartCut™ method. Using silicon oxide as the dielectric layer can further improve the temperature stability of the device because the temperature coefficients of the elastic wave velocities of silicon and silicon oxide have opposite signs. Thallium pentoxide (Ta₂O₅) can also achieve similar effects. Therefore, the thickness of the dielectric layer also depends on the thickness of the piezoelectric layer selected, and it can preferably be selected to reduce or even minimize the effect of temperature on the device.

[0026] According to one variation of the present invention, a trapping layer, particularly a layer of polycrystalline silicon, polycrystalline aluminum nitride, or SiOCH, may be disposed between the piezoelectric material and the substrate, or between the dielectric layer and the substrate. The thickness of this trapping layer is preferably between 300 nm (i.e., 0.3 µm) and 2 µm. The presence of the trapping layer reduces leakage current.

[0027] According to one variation of the present invention, a metal layer may be disposed on the surface of the piezoelectric material relative to the interdigitated comb electrodes, specifically between the piezoelectric material layer and the substrate, or between the dielectric layer and the substrate. The presence of the metal layer simplifies the fabrication of the alternating polarization domain by locally applying an electric field that is opposite in direction to the original polarization direction and exceeds the coercive field of the ferroelectric material.

[0028] When the metal layer is located between the dielectric layer and the substrate, the coupling rate of the new mode is higher because the farther the metal layer is from the piezoelectric layer, the more parallel the electric field lines in the piezoelectric layer are to the surface. Preferably, the metal can be encapsulated to reduce or even avoid metal contamination, for example, during steps of the SmartCut™ method. It may also be advantageous to limit the location of the metal layer below the interpenetration zone of the finger comb converter electrodes in order to limit parasitic elements associated with the metal beneath the wires and connection pads of the resulting device.

[0029] According to one variation of the present invention, the piezoelectric material may be at least one of LiTaO3, LiNbO3, ABO3-type perovskite (especially KnbO3 or PbTiO3), PZT, PMnPt, or AlScN. The modes observed in these materials (especially shear modes) have a quality factor and electromechanical coupling, enabling them to be used industrially in filter or delay line type applications.

[0030] According to one variation of the present invention, the elastic wave element may include a first electrode comprising a plurality of first switches, and a second electrode comprising a plurality of second switches. The first switches may be configured to individually contact or not contact the comb teeth, and the second switches may be configured to individually contact or not contact the comb teeth, thereby forming the pair of finger-like comb electrodes. Therefore, the excitation configuration of the comb teeth can be adjusted, for example, from a configuration where the comb teeth are alternately connected to two electrodes to a configuration where two directly adjacent comb teeth are connected to the first electrode and the next two directly adjacent comb teeth are connected to the second electrode. By changing this excitation scheme, the frequency of the mode can be changed, thereby allowing the same element to adapt to different operating parameters.

[0031] Preferably, the applied comb switching should avoid short-circuiting the first electrode and the second electrode.

[0032] According to one variation of the present invention, the element may include a supply device configured to provide the pair of interdigitated comb electrodes with a radio frequency of at least 2 GHz, preferably at least 3 GHz, and more preferably at least 6 GHz. Therefore, filters, sensors, or delay lines operating at frequencies higher than normally available frequencies can be provided using I-line lithography.

[0033] According to one variation of the present invention, the interdigitated comb electrode may be the only conductive element in contact with the piezoelectric material. According to another variation of the present invention, the elastic wave element may have no other electrode elements besides the interdigitated comb electrode; specifically, no floating electrode is located on the opposite surface of the piezoelectric material relative to the position of the interdigitated comb electrode. Therefore, it is possible to avoid the use of some of the energy introduced into the system to form modes other than shear modes, and to avoid the occurrence of parasitic capacitance that reduces the coupling of the mode of interest.

[0034] To achieve this objective, the present invention also proposes an acousto-electric device using the aforementioned elastic wave element (especially the comb-like electrode with a tooth width of at least 280 nm, preferably at least 350 nm), particularly a filter, sensor, or delay line, with an operating frequency of 2 GHz or higher, particularly 3 GHz or higher, and even particularly 6 GHz or higher. Therefore, filters, sensors, or delay lines operating at frequencies higher than normally available can be provided by using I-line lithography.

[0035] In order to achieve this objective, the present invention also proposes a method for manufacturing an elastic wave element as described above, comprising the following steps: providing a piezoelectric material, especially a ferroelectric material, applying an electric field that is stronger than the coercive field of the piezoelectric material, especially at least ten times stronger, to generate a first and a second polarization domain that are periodically alternating and have opposite polarization directions, and manufacturing the pair of finger-shaped comb electrodes above the piezoelectric material, specifically directly on the piezoelectric material.

[0036] The present invention also proposes a method for manufacturing an elastic wave element as described above, wherein directly adjacent comb teeth are located above polarization domains having the same polarization direction. The method includes the following steps: providing a piezoelectric material, particularly a ferroelectric material, and, specifically, directly on the piezoelectric material, manufacturing the pair of finger-shaped comb electrodes; and applying an electric field stronger than, particularly at least ten times stronger than, the coercive field of the piezoelectric material using the pair of finger-shaped comb electrodes to generate periodically alternating first and second polarization domains having opposite polarization directions. Thus, the manufacturing method can be simplified.

[0037] According to one variation of the present invention, the comb-shaped electrodes can be fabricated by depositing a metal layer, specifically, made of aluminum (Al), molybdenum (Mo), gold (Au), silver (Ag), or a copper-aluminum alloy (AlCu), or an alloy primarily based on aluminum and other metals (e.g., titanium (Ti)), followed by the steps of forming these electrodes by photolithography and etching. Specifically, the comb-shaped electrodes can be made of AlCu, AlSi, or AlTi, doped with 0.5% to 5% copper, titanium, and / or silicon, and may have sublayers of titanium, tantalum, molybdenum, palladium, or platinum, or combinations of titanium / platinum, titanium / gold, tantalum / platinum, chromium / gold, or zirconium.

[0038] According to one variation of the present invention, the electric field can be applied in the form of parallel bands, especially by using an electrolytic electrode, preferably lithium chloride.

[0039] According to one variation of the present invention, once the temperature of the piezoelectric material is heated to at least 150°C, preferably to at least 170°C, the electric field can be applied to reduce the coercive field, thereby enabling the upper ferroelectric polarization domain to be reversed by applying an electric field with a direction opposite to its original polarization direction. Therefore, polarization can be obtained even in the absence of a counter electrode.

[0040] According to one variation of the present invention, the piezoelectric material can be disposed as a single layer on a substrate. Such a composite substrate can be obtained by transferring a piezoelectric layer from a donor substrate to the substrate. This transfer can be accomplished using a SmartCut™ type method, which creates a weakened region in the donor substrate by implanting ions into the donor substrate. According to the present invention, the formation of these polarization domains can be performed before or after the transfer of the piezoelectric layer.

Implementation Method

[0060] The invention is described in more detail below by way of advantageous embodiments and with reference to drawings. These embodiments are merely possible configurations, and therefore the various features described may be provided independently or omitted when implementing the invention.

[0061] FIG1 illustrates an elastic wave element 1 according to a first embodiment of the present invention, particularly a shear wave element.

[0062] According to the first embodiment, the elastic wave element 1 includes a piezoelectric material 3, particularly a ferroelectric material, disposed on top in the form of a layer 5, particularly directly on a dielectric layer 7. The dielectric layer 7 is disposed on top, particularly directly on a substrate 9. The layer 5, dielectric layer 7, and substrate 9, obtained through a layer transfer method such as SmartCut™, can form a composite substrate 11. This composite substrate 11 is also referred to as a piezoelectric substrate on insulator or a POI substrate. In the layer transfer method, the dielectric layer 7 can serve as a bonding layer between the substrate 9 and the piezoelectric layer 5.

[0063] Ferroelectricity is the property of a material to be electrically polarized in a spontaneous state. Ferroelectric materials are a subclass of piezoelectric materials. Ferroelectric properties can also be obtained artificially, for example, by creating polarization domains with different polarization directions through suitable growth conditions. Therefore, any material capable of generating polarization domains can be considered as a piezoelectric material in the following text.

[0064] According to the present invention, the piezoelectric material of layer 5 includes a first polarization domain 3ai, wherein i ranges from 1 to j and has a first polarization direction 13a, and a second polarization domain 3bi, wherein i ranges from 1 to j and has a second polarization direction 13b, wherein the first polarization direction 13a and the second polarization direction 13b are opposite. Furthermore, the first and second polarization domains 3ai and 3bi form bands of the same thickness, which periodically alternate along a periodic direction d and a spacing pf, the periodic direction being perpendicular to the surface normal n of the piezoelectric material 3.

[0065] In this embodiment, the polarization axis Z of the polarization domain forms a non-zero angle with the normal n. Furthermore, the polarization axis Z has at least one component in the periodic direction d. This component is opposite for both types of polarization domains. The polarization axis Z preferably lies on the plane defined by the normal n and the periodic direction d, thus the angle φ is equal to 0. In this case, only sections undergoing a single defined rotation about the crystal axis X are considered without loss of generality. Specifically, for these so-called single-rotation sections, the angle φ defined about the crystal axis Z is 0.

[0066] This configuration can induce shear waves in the piezoelectric layer, especially for trigonal monocrystalline materials of the 3m class, such as single-crystal lithium tantalate (LiTaO3) or lithium niobate (LiNbO3), and ABO3-type perovskite materials. These materials satisfy the conditions required for exciting shear modes according to the present invention due to their ferroelectric properties. In these materials, particular attention will be paid to single polarization along the Z-axis for carrying out the present invention.

[0067] In this embodiment, the piezoelectric material of layer 3 can be single-crystal lithium tantalate LiTaO3 or single-crystal lithium niobate LiNbO3. Other piezoelectric materials as described above can also be used. The thickness e1 of the piezoelectric layer 5 is preferably less than the wavelength λ, preferably less than λ / 2, even more preferably less than λ / 4, and typically less than 1 μm.

[0068] For lithium tantalate (LiTaO3), according to the standard IEEE Std-176 version IRE 1949, the crystal orientation of the first polarization direction 13a of the first polarization domain 3ai of layer 3 is preferably (YXl) / θ, where the selected angle θ is between -30° and 110°. Therefore, according to the standard IEEE Std-176 version IRE 1949, the crystal orientation of the second polarization direction 13b of the second polarization domain 3bi of layer 3 is (YXl) / θ+180°. Therefore, in this system, the normal n corresponds to the direction 90°-θ, and the periodic direction d is orthogonal to the normal n and forms an angle θ with the Z-axis, and in a specific mode, it is collinear with the Z-axis.

[0069] For lithium niobate (LiNbO3), according to the standard IEEE Std-176 version IRE 1949, the crystal orientation of the first polarization direction 13a of the first polarization domain 3ai of layer 3 is preferably (YXl) / θ, wherein the selected angle θ is between -30° and +120°, and preferably between 0° and 70°. Therefore, according to the standard IEEE Std-176 version IRE 1949, the crystal orientation of the second polarization direction 13b of the second polarization domain 3bi of layer 3 is (YXl) / θ+180°.

[0070] For lithium tantalate and lithium niobate, the polarization direction is only along the Z-axis. Under the action of an electric field exceeding the coercive field, atoms in the crystal lattice will move from the barycentre along this direction, causing the molecules to become polarized.

[0071] The dielectric layer 7 is preferably a silicon oxide layer, the thickness e2 of which is less than the wavelength and / or between 200 nm and 2 μm. The dielectric layer may also be made of silicon nitride, Ta2O5, ZrO2, HfO2, SiON or a combination of these materials. The dielectric layer may also include other components such as polycrystalline silicon.

[0072] The substrate 9 is preferably a silicon substrate, such as silicon (100) or (110) or (111). The substrate may also be made of silicon oxide, silicon carbide (SiC), sapphire, silicon nitride (SiN), aluminum nitride (AlN), quartz, carbon, diamond, yttrium aluminum garnet (Yags), yttrium iron garnet (Yigs), amorphous silicon or polycrystalline silicon, or LiNbO3 and / or LiTaO3.

[0073] By using silicon as the substrate 9 and silicon oxide as the dielectric layer 7, the temperature stability of the elastic wave element 1 can be improved. This is because the velocity temperature coefficient (VTC) of the shear modes has opposite signs among the stacked constituent materials, and the thickness of silicon is greater than the total thickness of the dielectric and piezoelectric layers deposited on its surface. In particular, when its thickness is 50 times the thickness of the two layers, the thermal expansion effect of the piezoelectric layer can be suppressed. Specifically, materials with a coefficient of thermal expansion of less than 10 ppm·K⁻¹ (preferably 5 ppm·K⁻¹) have proven to effectively reduce the influence of temperature on structure-guided modes.

[0074] A pair of forked comb-shaped electrodes 15a and 15b are directly disposed on, in particular, the ferroelectric material layer 5. The comb teeth (17a1~17a3 and 17b1~17b3) extend primarily perpendicular to the periodic direction d (i.e., parallel to the frequency bands of polarization domains 3ai and 3bi). The illustrated comb-shaped electrodes have three comb teeth; in this case, each electrode may have more than three comb teeth. The comb teeth are preferably rectangular (i.e., parallelepiped) in shape. The comb teeth 17a1~17a3 are connected to each other via conductive strip 19a. The comb teeth 17b1~17b3 are connected to each other via conductive strip 19b. The two conductive strips 19a and 19b can be connected to the poles of the radio frequency source to excite modes of interest.

[0075] The electrode comb teeth are periodically spaced with a mechanical pitch pe, resulting in an electrical cycle of 2pe. However, according to a variation, one or more periodic interruptions can still be introduced, for example, by omitting one or more comb teeth. In this case, the mechanical pitch pe of the electrodes is half the pitch pf of the polarization domains 3ai and 3bi.

[0076] The comb electrodes 15a and 15b are made of metallic materials, such as aluminum (Al), molybdenum (Mo), gold (Au), silver (Ag), or copper-aluminum alloys (AlCu), or alloys primarily based on aluminum and other metals (e.g., titanium (Ti)). Specifically, the comb electrodes 15a and 15b may be made of AlCu, AlSi, or AlTi, doped with 0.5% to 5% copper, titanium, and / or silicon, and may have sublayers of titanium, tantalum, molybdenum, palladium, or platinum, or combinations of titanium / platinum, titanium / gold, tantalum / platinum, chromium / gold, or zirconium. The thickness e3 of the comb electrodes 15a and 15b is preferably between 50 nm and 200 nm. The comb teeth 17a1~17a3, 17b1~17b3 have a width a in the periodic direction d, which is preferably equal to or greater than 280 nm, and more preferably greater than 350 nm. Therefore, comb-shaped electrodes 15a and 15b can be manufactured using I-line lithography.

[0077] In the metallization ratio a / pe, a is the width of the comb teeth 17a1~17a3, 17b1~17b3, and pe is the mechanical pitch of the comb teeth, preferably between 0.25 and 0.75, especially between 0.4 and 0.6. This ratio a / pe and / or the thickness e3 can be adjusted to set the electromechanical coupling degree and / or reduce losses.

[0078] In this embodiment, the comb teeth are all the same shape. According to variations, the comb teeth can also be made to different lengths l.

[0079] Preferably, there are no other electrode elements besides the pair of finger electrodes. Specifically, there are no floating or planar electrodes opposite to the ferroelectric layer 5. This is to avoid the formation of elliptical polarization modes in the ferroelectric layer 5, which would adversely affect the spectral purity of the converter.

[0080] In detail, any floating electrode will generate parasitic capacitance, which will degrade the coupling of shear modes and may degrade spectral purity.

[0081] In the embodiment of FIG1, the supply device 21 is connected to the finger comb electrodes 15a, 15b. This supply device 21 is preferably configured to provide radio frequency signals to the finger comb electrodes 15a, 15b at a frequency of at least 2 GHz, and preferably at least 6 GHz.

[0082] When the elastic wave element 1 is configured to convert elastic waves into electrical signals, the supply device 21 will be replaced by an electrical receiver.

[0083] According to a variation of the first embodiment, the composite substrate 9 may include one or more other layers in its structure. A layer for capturing free charges (e.g., a layer of polycrystalline silicon, AlN, or polycrystalline SiOCH) may be disposed between the dielectric layer 7 and the substrate 9 to limit the average free path of free charges generated at the silicon / dielectric layer interface through an acousto-electric field associated with wave propagation, thereby reducing leakage current. Considering radio frequency applications for telecommunications (typically between 500 MHz and 6 GHz), the thickness of such capturing layer is preferably between 0.3 μm and 2 μm.

[0084] In addition to using composite substrate 11, the present invention can also be realized using block substrate of ferroelectric material, so that there is no dielectric layer and no substrate.

[0085] Figure 2 schematically illustrates a cross-sectional view of a portion of the elastic wave element according to the first embodiment of the present invention. Figure 2 shows two halves 3a1 and 3a2 of the first polarization domain, in which a second polarization domain 3b1 with opposite polarization directions is sandwiched. Each comb-shaped electrode 15a and 15b is represented by comb teeth 17a1 and 17b1, respectively. Comb teeth 17a1 and 17b1 are directly disposed on the surface 31 of the piezoelectric material layer 5. Figure 2 also shows the dielectric layer 7 and the substrate 9.

[0086] The comb teeth 17a1 and 17b1 are located on the interfaces 33a and 33b between polarization domains 3a1, 3b1 and 3a2 with opposite polarization directions. The center planes 35a and 35b of each comb tooth 17a1 and 17b1 are preferably aligned with the interfaces 33a and 33b. Therefore, the comb teeth 17a1 and 17b1 are symmetrically positioned relative to the interfaces 33a and 33b in the periodic direction d, respectively.

[0087] Numerical simulations performed according to the method described by S. Ballandras et al. in "Finite element analysis of periodic piezoelectric transducers", Journal of Applied Physics 93, 702 (2003) highlight that, for the elastic wave element 1 in Figures 1 and 2, the frequencies of its fundamental modes are twice as high as those of the element without opposite polarization domains, due to the wavelength being halved. The lower half of Figure 2 illustrates the deformation of the lattice in the shear mode, with a wavelength of 2 µm, corresponding to the mechanical period pe of the comb teeth, not the electrical period 2pe of the element without polarization domains. This mode exists only when the electrode vibrations are antisymmetric. When the electrode vibrations are antisymmetric, and because they are subject to equal and opposite boundary conditions, the two electrodes vibrate in a similar manner.

[0088] Figure 3 shows the simulation results of the device without alternating polarization domains, and Figure 4 shows the simulation results of the elastic wave device 1 shown in Figures 1 and 2. The devices were compared using a composite substrate with a silicon (100) substrate, a polycrystalline silicon trapping layer with a thickness of 1 µm, a silicon dioxide dielectric layer with a thickness of 500 nm, and a LiTaO3 layer with a thickness of 600 nm. The device of the present invention uses a composite substrate with a silicon (100) substrate, a polycrystalline silicon trapping layer with a thickness of 1 µm, a silicon dioxide dielectric layer with thicknesses of 500 nm and 600 nm, and a LiTaO3 layer, which have infinitely long and alternating polarization domains, wherein the period pf is 4 µm and the orientation is (YXl) / 42° and (YXl) / 222°. In both cases, the electrodes were made of AlCu with a thickness of 150 nm, a metallization ratio a / pe of 0.5, and a mechanical spacing pe of 2 µm.

[0089] Figure 3 shows the harmonic conductance modulus G (in dB) on the left axis and the harmonic admittance modulus Y (in dB) on the right axis, both as a function of frequency (in MHz). A Rayleigh mode 41 at approximately 700 MHz is observed only in the harmonic conductance. A shear mode 43 is observed at approximately 1 GHz, and a surface skimming bulk wave (SSBW) mode 45 is observed at approximately 1.15 GHz.

[0090] Figure 4 illustrates the harmonic conductance modulus G (in dB) on the left axis and the harmonic admittance modulus Y (in dB) on the right axis, both as a function of frequency (in MHz). This is for the elastic wave element 1 of the present invention with alternating polarization domains shown in Figures 1 and 2. These modes are observed at approximately twice the frequency, including Rayleigh mode 51 at approximately 1400 MHz only in harmonic conductance, shear mode 53 at approximately 2 GHz, and SSBW mode 55 at approximately 2.3 GHz. The shear mode 53 observed at approximately 2 GHz in the structure of the present invention is characterized by an effective phase velocity of 8 km·s⁻¹, a coupling ratio of approximately 3.1%, a resonant quality factor greater than 10000, and an anti-resonant quality factor greater than 20000. For orientations (YXl) / 30° and (YXl) / 210°, the coupling value increases to 5%. Parasitic modes also appear due to the presence of the dielectric layer and the trapping layer.

[0091] The same phenomenon can be observed using LiNbO3, which exhibits a mode with a coupling rate of approximately 10% and a larger quality factor for the same structural parameters. By optimizing the thickness of the dielectric layer (in this case, a silicon oxide layer), the frequency temperature coefficient (FTC) can be minimized or even eliminated. This coefficient defines the relationship between frequency and temperature as Δf / f = FTC × (T - T0), where the reference temperature T0 is typically taken as 25°C. In fact, for a silicon oxide layer with a thickness of approximately 1 µm, the FTC is minimal and close to zero. Furthermore, the resonant and anti-resonant parameters of the FTC have almost the same dependence on temperature, with a difference of only a few ppm·K⁻¹.

[0092] Figure 5 illustrates three pairs of angles θ / θ+180° for LiTaO3(YXl) / θ and θ+180° piezoelectric materials. The left axis represents the harmonic admittance Y and the right axis represents the harmonic susceptance B, both as functions of frequency (in MHz). Curves 571 and 591 correspond to admittance (in dB) and susceptance (in Sm⁻¹), respectively, where θ = 30° in the first polarization domain and θ = 210° in the second polarization domain. Curves 572 and 592 correspond to admittance (in dB) and susceptance (in Sm⁻¹), respectively, where θ = 42° in the first polarization domain and θ = 222° in the second polarization domain. Curves 573 and 593 correspond to admittance (in dB) and susceptance (in Sm⁻¹), respectively, where θ = 50° in the first polarization domain and θ = 230° in the second polarization domain. For these three orientations, a shearing mode was observed to occur at approximately 2 GHz. It can be seen that the coupling degree can be optimized according to the orientation. The greater the separation between the resonance peak and the anti-resonance peak, the greater the coupling degree. This result also highlights that the resolution of the anti-resonance peak varies with the orientation of the piezoelectric layer.

[0093] FIG6 schematically illustrates a cross-sectional view of a portion of an elastic wave element 111 according to a variation of the first embodiment of the present invention. FIG6 illustrates first polarization regions 3a1-3a4, where only half of each polarization region is shown, and second polarization regions 3b1-3b3 have opposite polarization directions between polarization regions 3a1-3a4. Comb electrodes 15a and 15b are represented by comb teeth 113a1 and 113b1, respectively. FIG6 also illustrates the dielectric layer 7 and the substrate 9.

[0094] The comb teeth 113a1 and 113b1 are located on the interfaces 115a and 115b between polarization domains 3b1 and 3a2 and 3a3 and 3b3 with opposite polarization directions, respectively. Therefore, the comb teeth 113a1 and 113b1 are symmetrically positioned relative to the interfaces 115a and 115b in the periodic direction d, respectively.

[0095] The only difference between the variation in Figure 6 and the elastic wave element 1 in Figure 2 is that the distance between the comb teeth 113a1 and 113b1 is greater than that in the embodiment of Figure 1. In fact, the polarization domain 3b2 has no electrode comb teeth at all. Therefore, the mechanical spacing pe is equivalent to the width of three polarization domains or 1.5 times the polarization domain spacing pf, rather than only 0.5 times the width of one polarization domain or the polarization domain spacing pf as in the variation in Figure 2.

[0096] The cell of the elastic wave element 111 in this variation is simulated as described in the first embodiment above, wherein the width of the polarization domain is 1 μm, and the orientations of the first and second polarization domains are LiTaO3(YXl) / 42° and (YXl) / 222°, and the substrate is made of silicon (111)(XZwlt) / -45° / +35.3 / ψ, wherein the misalignment angle ψ of the selected land region between silicon and LiTaO3 is between 0 and 360°. It is worth noting that this selection is not limiting, and the orientations of silicon (100), (111), (211), etc., can be operated according to the present invention. The selection of the angle ψ will follow the same rules as silicon (111). Therefore, the mechanical spacing pe is 3 µm. The cell width for simulation is 6 µm. In this case, the cell represents a structural unit for infinitely repeated simulation. Multiple simulations revealed two modes observed in the elastic wave element 111, as shown in the lower half of Figure 6. One mode exhibits shear deformation of two shear wavelengths 2pe per electrical cycle, while the other mode exhibits deformation of three shear wavelengths 2pe per electrical cycle.

[0097] Figure 7 illustrates the harmonic conductance G (in dB) on the left axis and the harmonic admittance Y (in dB) on the right axis for orientations (YXl) / 42° and (YXl) / 222°, both as functions of frequency (in MHz).

[0098] A mode 117 can be observed at approximately 1.4 GHz in the admittance, and mode 119 can be observed in the harmonic conductance G. A second vibrational mode appears at approximately 2.7 GHz, with its harmonic admittance Y designated as 121 and harmonic conductance G designated as 123. The equivalent phase velocities are 8.6 km·s⁻¹ and 16 km·s⁻¹, respectively. The electromechanical coupling of these modes is 4.1% and 6.4%, respectively. The quality factors of the resonance and anti-resonance of the first mode are significantly greater than 10,000, but the quality factor of the second mode remains less than 500, indicating a significant broadening of the peak value of the harmonic conductance G. The first mode is directional, indicating that it contributes to the stopband at both the input and output.

[0099] FIG8 schematically illustrates a cross-sectional view of a portion of the elastic wave element 131 according to a second variation of the first embodiment of the present invention. FIG8 illustrates the first polarization regions 3a1 to 3a4, where only half of each polarization region is shown. The second polarization regions 3b1 to 3b3 have opposite polarization directions between polarization regions 3a1 to 3a4. The comb-shaped electrodes 15a and 15b are represented by comb teeth 133a1 and 133b1, respectively. FIG8 also illustrates the dielectric layer 7 and the substrate 9.

[0100] The comb teeth 133a1 and 133b1 are located on the interfaces 135a and 135b between polarization domains 3b1 and 3a2 and 3a3 and 3b3, respectively, which have opposite polarization directions. Therefore, the comb teeth 133a1 and 133b1 are symmetrically positioned relative to the interfaces 135a and 135b in the periodic direction d, as shown in the first variation example.

[0101] The only difference between the variation in Figure 8 and the variation in Figure 6 is that the width of the comb teeth 133a1 and 133b1 in the periodic direction d is greater than that of the elastic wave element 111 in the first variation. In fact, each comb tooth 133a1 and 133b1 completely covers at least two polarization domains in the periodic direction d.

[0102] Figure 9 shows the simulation results for orientations (YXl) / 42° and (YXl) / 222°, with the left axis being harmonic conductance G (in dB) and the right axis being harmonic admittance Y (in dB), both as functions of frequency (in MHz).

[0103] It can be seen that, for the harmonic admittance Y of the first mode 141 and the harmonic conductance G of the second mode 143 at approximately 1.35 GHz, increasing the number of comb teeth can eliminate the directivity effect highlighted in Figure 7 above. It can also be understood that when the comb teeth do not completely cover the two adjacent polarization domains, the boundary conditions for stopband input and output can actually be satisfied, but this condition cannot be achieved in the current case. By increasing the mass load to approximately 8.2 km·s⁻¹, the speed of this mode will naturally decrease, but the electromechanical coupling will increase to 5%. The characteristics of the second admittance and conductance modes 145 and 147 do not change significantly, and their coupling and quality factor are the same as those in the previous variation example.

[0104] FIG10 shows a cross-sectional view of a portion of an elastic wave element 61 according to a second embodiment of the present invention. The elastic wave element 61 has the same structure and uses the same material as the elastic wave element 1 of the first embodiment, except that the comb teeth 63a1 and 63b1 of the fork-shaped electrodes are completely positioned above the polarization domains and are no longer installed across the two polarization domains as described in the first embodiment. In this embodiment, each comb tooth 63a1 and 63b1 of the two comb electrodes 15a and 15b is located on the same type of polarization domain, which is the second polarization domain 3b1 and 3b2. As shown in FIG10, the two second polarization domains 3b1 and 3b2 are respectively disposed between the two first polarization domains 3a1 and 3a2 and 3a2 and 3a3. The comb electrodes 15a and 15b are respectively represented by comb teeth 63a1 and 63b1. The comb teeth 63a1 and 63b1 have the same width a, preferably the same width as the polarization domains 3b1 and 3b2. According to an alternative embodiment, the comb tooth width 'a' may be smaller than the polarization domain width. In this case, the comb teeth are preferably located in the middle of the polarization domain. Figure 10 also illustrates the dielectric layer 7 and the substrate 9. As with the first embodiment of Figure 1 or Figure 2, Figure 10 does not illustrate the trapping layer.

[0105] The lower half of Figure 10 illustrates a vibrational mode. This vibrational mode was obtained through simulation as described in the first mode above, but for the unit cell of element 61, its polarization domain width is 1 μm, and the LiTaO3 orientation is (YXl) / 50° and (YXl) / 230°. Therefore, the width of this unit cell is 4 µm. The observed results do not depend on (or depend only slightly on) the crystal orientation of the substrate 5.

[0106] Figure 11 illustrates the harmonic conductance G (in dB) on the left axis and the harmonic admittance Y (in dB) on the right axis for orientations (YXl) / 50° and (YXl) / 230°, both as functions of frequency (in MHz).

[0107] Harmonic admittance Y, designated 71, represents a first-order mode at 1 GHz with a wavelength λ equal to twice the mechanical spacing pe between the electrodes. This first-order mode is primarily observed only in harmonic admittance Y. This is a fundamental shear mode propagating at a phase velocity of 4170 ms⁻¹ with a coupling coefficient of 5%, and without radiation loss in the substrate. This structure can excite a visible mode of harmonic 3 in harmonic admittance Y, designated 73, and harmonic conductance G, designated 75. This mode is approximately 3 GHz. Because the excitation exceeds the conduction limit of this structure, i.e., beyond the frequency of the SSBW wave, the distribution is affected by radiation loss, and the quality factor of the resonance remains less than 500. However, its electromechanical coupling reaches 3.7%, significantly higher than the typical third-order harmonic contribution, where the coupling decreases by an order of magnitude relative to the fundamental mode. This phenomenon is caused by the ferroelectric periodic structure, which promotes synchronization conditions different from those of the homogeneous polarization layer.

[0108] FIG12 schematically illustrates a cross-sectional view of a portion of an elastic wave element 81 according to a third embodiment of the present invention. The elastic wave element 81 has the same structure and uses the same materials as the elastic wave element 1 of the first embodiment, except that the comb teeth 83a1 and 83b1 of the fork-shaped electrodes are completely positioned above the polarization domains and are no longer mounted across the two polarization domains as described in the first embodiment. In this embodiment, each comb tooth 83a1 and 83b1 of the two comb electrodes 15a and 15b is located on a polarization domain with different polarization directions. The comb tooth 83a1 of the first electrode 15a is located on the second polarization domain 3b1, and the comb tooth 83b1 is located on the first polarization domain 3a3. As shown in FIG12, each of the three second polarization domains 3b1 to 3b3 is respectively disposed between the two first polarization domains 3a1 to 3a3. The comb electrodes 15a and 15b are respectively represented by comb teeth 83a1 and 83b1. The comb teeth 83a1 and 83b1 have the same width 'a', preferably the same width as the polarization regions 3a1 and 3b3. According to an alternative embodiment, the comb tooth width 'a' may be smaller than the polarization region width. In this case, the comb teeth are preferably located in the middle of the polarization region. Figure 12 also illustrates the dielectric layer 7 and the substrate 9. As with the first embodiment in Figure 1 or Figure 2, Figure 12 does not show the trapping layer.

[0109] The lower half of Figure 12 illustrates a vibration mode. This vibration mode was obtained through simulation as described in the first mode above, but for the unit cell of element 81, the polarization domain width is 1 μm, and the LiTaO3 orientation is (YXl) / 42° and (YXl) / 222°, and the substrate is silicon (111). Therefore, the mechanical spacing pe is 3 µm, and the width of the unit cell used for simulation is 6 µm. The resonant wavelength λ of these modes is equal to the mechanical half-spacing pe of these electrodes.

[0110] Figure 13 illustrates the harmonic conductance G (in dB) on the left axis and the harmonic admittance Y (in dB) on the right axis for orientations (YXl) / 42° and (YXl) / 222°, both as functions of frequency (in MHz). Two vibration modes can be observed.

[0111] A mode appears at approximately 1.35 GHz and can be observed in harmonic admittance Y (number 91) and harmonic conductance G (number 93). A second vibration mode appears at approximately 2.7 GHz and can be observed in harmonic admittance Y (number 95) and harmonic conductance G (number 97). These simulations show that the electrode motion is always in phase for both observed modes. The equivalent phase velocities are 8.24 and 15.73 km·s⁻¹, respectively, with associated electromechanical coupling of 6.4% and 4.4%. The resonant and anti-resonant quality factors of the first mode are greater than 2000, while the quality factor distributed near 2.7 GHz remains less than 100 according to the guidance preset value.

[0112] FIG14 schematically illustrates a cross-sectional view of a portion of the elastic wave element 101 according to a fourth embodiment of the present invention. The elastic wave element 101 has the same structure and uses the same material as the elastic wave element 1 of the first embodiment, except that the comb teeth 103b1 of the comb electrode 15b are completely positioned above the polarization domain 3a2, and the comb teeth 103b2 of the comb electrode 15b are completely positioned above the polarization domain 3a3 having opposite polarization directions. The comb teeth 103a2 are located between the two comb teeth 103b1 and 103b2, and are mounted to span across the two polarization domains 3b2 and 3a3, as described in the first embodiment. The comb teeth 103a1 are mounted to span across the two polarization domains 3a1 and 3b1, as described in the first embodiment.

[0113] As shown in FIG14, the first polarization domains 3a1-3a4 and the second polarization domains 3b1-3b3 are alternately formed as units. The comb-shaped electrodes 15a and 15b are represented by two sets of comb teeth 103a1, 103a2 and 103b1, 103b2, respectively. These comb teeth have the same width a, preferably the same width as the polarization domains. According to an alternative embodiment, the comb tooth width a may be smaller than the polarization domain width. In this case, these comb teeth are preferably centered or located in the middle of the polarization domains of the electrodes relative to the interfaces 105a and 105b. FIG14 also shows the dielectric layer 7 and the substrate 9. As with the first embodiment of FIG1 or FIG2, FIG14 does not show the trapping layer.

[0114] Figure 15 illustrates the harmonic conductance G and harmonic admittance Y of the elastic wave element according to the fourth embodiment of the present invention for two embodiments. The difference between the two embodiments lies in the different potential configurations of the electrodes. These configurations are shown above each graph.

[0115] Simulations were performed using the first embodiment described above, but for the unit cell of the elastic wave element 101, the polarization domain width was 1 μm, the LiTaO3 orientation was (YX1) / 42° and (YX1) / 222°, and the substrate was silicon (111). Therefore, the mechanical spacing pe was 1.5 µm, and the unit cell width used for simulation was 6 µm. The observed results did not depend on (or depended only slightly on) the crystal orientation of the substrate 5.

[0116] The first graph in Figure 15 shows the harmonic conductance G (in dB) on the left axis and the harmonic admittance Y (in dB) on the right axis for orientations (YXl) / 42° and (YXl) / 222°, both as functions of frequency (in MHz). These results were obtained by applying alternating +V / -V potentials to comb teeth 103a1, 103b1, 103a2, and 103b2, as shown at the top of the first graph in Figure 15. Several modes appear in the admittance and conductance. Specifically, the harmonic admittance Y of mode 1051 is at approximately 800 MHz, the harmonic conductance G of mode 1052 is at approximately 2 GHz, and mode 1053 is at approximately 3.3 GHz.

[0117] The second graph in Figure 15 (located below the first graph) illustrates the harmonic conductance G (in dB) on the left axis and the harmonic admittance Y (in dB) on the right axis for orientations (YXl) / 42° and (YXl) / 222°, both as functions of frequency (in MHz). These results were obtained by applying a +V potential to comb teeth 103a1 and 103a2' and a -V potential to comb teeth 103b1 and 103b2', as shown above the second graph in Figure 15. Several modes appear in the admittance and conductance, but at different frequencies compared to the aforementioned embodiment. Specifically, the harmonic admittance Y of mode 1071 is at approximately 1.3 GHz, mode 1072 is at approximately 2.7 GHz, and mode 1073 is close to 4 GHz. In this case, mode 1073 is characterized by a higher spectral purity than mode 1053. However, mode 1071 exhibits a double spectrum.

[0118] Such a structure may have advantages, particularly in cases where the contribution of a specific mode is enhanced through coarse filtering, or in frequency source applications where only a limited segment of the spectrum needs to be used. In other words, the electronic system filters the spectrum in some way when using this component. The advantage of the solution proposed here again lies in its ability to achieve an operating frequency significantly higher than that achievable with conventional nonpolar materials.

[0119] In this second embodiment, another type of finger-fork electrode pair is used, as shown in FIG16. In this case, two comb teeth with the same potential are arranged directly adjacent to each other, and then two comb teeth with opposite potentials are also arranged directly adjacent to each other. Therefore, electrode 15a' has comb teeth 13a1' and 13a2', 13a3' and 13a4', 13a5' and 13a6', and electrode 15b' has comb teeth 13b1' and 13b2', 13b3' and 13b4', 13b5' and 13b6'.

[0120] According to a variation, a control circuit can be configured to change the electrode arrangement from the configuration shown in the upper left corner to the configuration shown in the upper right corner, for example, by using a switch as shown in FIG17.

[0121] Figure 17 illustrates a first electrode 151 having switches 153a, 153b, 153c, and 153d, located before a second electrode 155 having switches 157a, 157b, 157c, and 157d. Switches 153a, 153b, 153c, and 153d are configured to be in contact with or not in contact with electrode 151 via comb teeth 159a, 159b, 159c, and 159d, respectively. Similarly, switches 157a, 157b, 157c, and 157d are configured to be in contact with or not in contact with electrode 155 via comb teeth 159a, 159b, 159c, and 159d, respectively. Therefore, the potential application configuration shown in the upper left and upper right corners of Figure 15 can be generated by a single element. The switching is preferably controlled to avoid short circuits between these electrodes. Figure 17 on the left shows comb teeth 159a to 159d, where the potential alternates between the comb teeth. On the right side of Figure 17, two adjacent comb teeth, namely 159a and 159b, and 159c and 159d, have the same potential.

[0122] Electrodes 151 and 155 can also be used in the configuration of other embodiments, so that the electroexcitation configuration can be changed.

[0123] Figure 18 illustrates the steps of a method for manufacturing an elastic wave element (especially an elastic wave element of the first embodiment) according to an embodiment of the present invention. The method begins at step 201, which involves providing a piezoelectric material, especially a ferroelectric material.

[0124] This step may include providing a bulk substrate, such as a wafer made of a single-crystal piezoelectric material, or providing a composite substrate 11, which includes a piezoelectric layer 5 of the piezoelectric material 3, a dielectric layer 7 (e.g., a silicon oxide layer), and a substrate 9 (e.g., a silicon substrate, such as silicon (100), silicon (110), or silicon (111), or quartz). The orientation of the two crystals in this structure can be seen from the connection region of the wafer used; these orientations may be aligned or misaligned.

[0125] The piezoelectric material of layer 3 is a single-crystal lithium tantalate LiTaO3 or a single-crystal lithium niobate LiNbO3. Other piezoelectric materials as defined above may also be used.

[0126] For lithium tantalate (LiTaO3), according to the standard IEEE Std-176 version IRE 1949, the crystal orientation of the first polarization direction 13a of the first polarization domain 3ai of layer 3 is preferably (YXl) / θ, wherein the selected angle θ is between -30° and 120°, and preferably between 0° and 70°. Therefore, according to the standard IEEE Std-176 version IRE 1949, the crystal orientation of the second polarization direction 13b of the second polarization domain 3bi of layer 3 is (YXl) / θ+180°. By keeping the comb teeth parallel to the polarization domain, it is also possible to set the polarization domain on an orthogonal plane, that is, relative to the situation described above, to adopt the configuration of (YXlt) / θ / 90° and (YXlt) / θ+180° / 90°.

[0127] For lithium niobate (LiNbO3), according to the standard IEEE Std-176 version IRE 1949, the crystal orientation of the first polarization direction 13a of the first polarization domain 3ai of layer 3 is preferably (YXl) / θ, wherein the selected angle θ is between -30° and +120°, and preferably between 0° and 70°. Therefore, according to the standard IEEE Std-176 version IRE 1949, the crystal orientation of the second polarization direction 13b of the second polarization domain 3bi of layer 3 is (YXl) / θ+180°.

[0128] This composite substrate 11 can be obtained by a layer transfer method (e.g., the SmartCut™ method). In the SmartCut™ process, ions are implanted into a donor substrate (in this case, a piezoelectric substrate) to create a weakened region within the donor substrate. The donor substrate is then bonded to a base substrate. In this case, the base substrate is a monocrystalline silicon substrate, such as silicon (100), silicon (110), or silicon (111), and has a native oxide layer or silicon oxide with thermal or chemical vapor deposition ("CVD") or physical vapor deposition ("PVD"). The oxide layer facilitates the bonding of the two substrates, especially bonding through molecular bonding. The orientation of the silicon plane relative to the piezoelectric layer plane can be selected to reduce or even minimize higher-than-fundamental order modes.

[0129] According to a variation, a trapping layer is first formed on the substrate before the oxide is formed. This trapping layer is usually a polycrystalline silicon layer, and its main function is to minimize the mean free path of the generated charge when an acoustic-electric wave passes through the interface between the silicon and the oxide layer.

[0130] Once the two substrates are bonded together, the supply of mechanical and / or thermal energy can cause the weakened area to fracture. Therefore, a layer with a thickness of 1 μm or less can be transferred to the substrate.

[0131] Then, during step 203, an electric field stronger than the coercive field of the piezoelectric material (especially the ferroelectric material) is applied to generate periodically alternating first and second polarization domains with opposite polarization directions. In this embodiment, the electric field is applied in the form of a parallel frequency band perpendicular to the surface of the piezoelectric material. The applied electric field must be higher than 22 kV·mm⁻¹, which represents the coercive field typically measurable in congruent lithium niobate and lithium tantalate.

[0132] This method is described, for example, in Thorlabs' "Periodically Poled Lithium Niobate (PPLN) – Tutorial", pages 686-687 (available at www.thorlabs.com), which uses patterned electrodes and then removes them. Other alternatives use electron beams, as seen in: C. Restoin, C. Darraud-Taupiac, JL Decossas, JC 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.

[0133] To simplify the generation of polarization domains during the application of an electric field, it may be advantageous to provide a metal layer on the surface of the piezoelectric material 3, which is opposite to the surface of the pair of forked comb electrodes to be generated during the following steps. This layer may be generated on the dielectric layer 7 before bonding, or on the surface of the donor substrate to be bonded. With the presence of the metal layer, the applied electric field can be close to 22 kV·mm⁻¹, and the temperature can be close to ambient conditions (i.e., close to 25°C). Without the metal layer, a stronger electric field must be applied, approximately twice or more than the coercive field. This step is preferably performed at a temperature of at least 150°C (or even at least 170°C) without the presence of the metal layer, thus reducing the value of the electric field to obtain a forced polarization effect.

[0134] Then, during step 205, the pair of finger comb electrodes can be fabricated on the piezoelectric material by using lithography, etching, deposition and polishing steps known to those skilled in the art to obtain an elastic wave element as in the first embodiment.

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

[0136] In the third embodiment, steps 203 and 205 can be interchanged. In this case, the polarization domains can be formed by applying the same potential to the two electrodes using the comb teeth 83a1 and 83b1. [Simplified Explanation of the Diagram]

[0041] The invention and other advantages will be better understood by reading the following non-limiting description in conjunction with the accompanying drawings, wherein:

[0042] Figure 1 illustrates the elastic wave element according to the first embodiment of the present invention.

[0043] Figure 2 schematically illustrates a cross-sectional view and shearing mode of a portion of an elastic wave element according to the first embodiment of the present invention.

[0044] Figure 3 illustrates the harmonic conductance G and harmonic admittance Y of an elastic wave element according to the prior art.

[0045] Figure 4 illustrates the harmonic conductance G and harmonic admittance Y of the elastic wave element according to the first embodiment of the present invention.

[0046] Figure 5 illustrates the harmonic admittance Y and harmonic susceptance B for three pairs of angles θ / θ+180° for the piezoelectric material LiTaO3(YXl) / θ and θ+180°: 30° / 210°, 42° / 222° and 50° / 230°.

[0047] FIG6 schematically illustrates a cross-sectional view of a portion of an elastic wave element according to a variation of the first embodiment of the present invention and two shear modes.

[0048] Figure 7 illustrates the harmonic conductance G and harmonic admittance Y of an elastic wave element according to a variation of the first embodiment of the present invention.

[0049] FIG8 schematically illustrates a cross-sectional view of a portion of an elastic wave element according to a second variation of the first embodiment of the present invention and two shear modes.

[0050] Figure 9 illustrates the harmonic conductance G and harmonic admittance Y of the elastic wave element according to a second variation of the first embodiment of the present invention.

[0051] FIG10 schematically illustrates a portion of an elastic wave element according to a second embodiment of the present invention.

[0052] Figure 11 illustrates the harmonic conductance G and harmonic admittance Y of the elastic wave element according to the second embodiment of the present invention.

[0053] Figure 12 schematically illustrates a portion of an elastic wave element according to a third embodiment of the present invention.

[0054] Figure 13 illustrates the harmonic conductance G and harmonic admittance Y of the elastic wave element according to the third embodiment of the present invention.

[0055] Figure 14 schematically illustrates a portion of an elastic wave element according to a fourth embodiment of the present invention.

[0056] Figure 15 illustrates the harmonic conductance G and harmonic susceptance B of the elastic wave element according to the fourth embodiment of the present invention.

[0057] Figure 16 schematically illustrates the alternative finger fork electrode used to implement one of the fourth embodiments of the present invention.

[0058] Figure 17 schematically illustrates the second pair of alternative finger fork electrodes used to implement the fourth embodiment of the present invention.

[0059] Figure 18 illustrates the steps of a method for manufacturing an elastic wave element according to the fifth embodiment of the present invention.

Claims

1. An elastic wave element, particularly a shear wave element, comprising a piezoelectric material (3), particularly a ferroelectric material, having a plurality of first polarization domains (3a) in a first polarization direction (13a) and a plurality of second polarization domains (3b) in a second polarization direction (13b), the first polarization direction (13a) being opposite to the second polarization direction (13b), wherein the first and second polarization domains (3a, 3b) alternate periodically along a periodic direction (d) perpendicular to the surface normal (n) of the piezoelectric material (3), and a pair of finger-shaped comb electrodes (15a, 15b) located above, particularly on, the piezoelectric material (3). Each of its comb teeth (103b1~103b2, 103a1~103a2) extends primarily perpendicular to the periodic direction (d) and the surface normal (n), wherein each comb tooth (103b1, 103b2) of one of the pair of finger-shaped comb electrodes is completely located above a single first and / or second polarization domain, while each comb tooth (103a1, 103a2) of the second finger-shaped comb electrode is located above the interface between the first polarization domains (3a) and the second polarization domains (3b).

2. The elastic wave element of claim 1, wherein the teeth of the pair of forked comb electrodes (15a, 15b) are arranged periodically.

3. The elastic wave element of claim 1 or 2, wherein the width (a) of the comb teeth (17a1~17a3, 17b1~17b3) of the pair of finger-shaped comb electrodes (15a, 15b) is between 25% and 75% of the width of the polarization domains (3a, 3b), more specifically between 40% and 60% of the width of the polarization domains, more specifically 50% of the width of the polarization domains, or at least 280 nm, preferably at least 350 nm.

4. The elastic wave element of claim 1, wherein between two polarization regions where the two comb teeth are at least partially positioned, there is at least one polarization region without any electrode fingers.

5. The elastic wave element of claim 1, wherein the piezoelectric material (3) is disposed in the form of a layer (5) on a substrate (9), the thickness (e1) of the piezoelectric layer (5) is less than the wavelength (λ), preferably less than λ / 2, more preferably less than λ / 4, and the substrate (9) is silicon, amorphous or polycrystalline silicon, silicon oxide, silicon carbide (SiC), sapphire, silicon nitride (SiN), aluminum nitride (AlN), quartz, carbon, diamond, yttrium aluminum garnet (Yags), yttrium iron garnet (Yigs), or LiNbO3 and / or LiTaO3 substrate.

6. The elastic wave element of claim 5, wherein a dielectric layer (7), particularly a layer of silicon oxide, silicon nitride, Ta2O5, ZrO2, HfO2, SiON, polycrystalline silicon or a combination thereof, is disposed between the piezoelectric layer (5) and the substrate (9), wherein the thickness of the dielectric layer (7) is preferably less than the wavelength (λ) or between 200 nm and 2 µm.

7. The elastic wave element of claim 6, wherein a trapping layer, in particular a layer of polycrystalline silicon or polycrystalline aluminum nitride or SiOCH, is disposed between the piezoelectric material (3) and the substrate (9), or between the dielectric layer (7) and the substrate (9), and the thickness of the trapping layer is preferably between 300 nm and 2 µm.

8. The elastic wave element of claim 1, wherein a metal layer is disposed on the surface of the piezoelectric material (3) relative to the pair of interdigitated comb electrodes (15a, 15b), specifically between the piezoelectric layer (5) and the substrate (9).

9. The elastic wave element of claim 1, wherein the piezoelectric material (3) is selected from at least one of LiTaO3, LiNbO3, ABO3 type perovskite (especially KnbO3 and PbTiO3), PZT, PMnPt, or AlScN.

10. The elastic wave element of claim 1, wherein a first electrode (151) includes a plurality of first switches (153a, 153b, 153c, 153d) and a second electrode (155) includes a plurality of second switches (157a, 157b, 157c, 157d), the first switches (153a, 153b, 153c, 153d) being configured to contact or not contact comb teeth (159a, 159b, 159c, 159d) respectively, and the second switches (157a, 157b, 157c, 157d) being configured to contact or not contact comb teeth (159a, 159b, 159c, 159d) respectively, to form the pair of finger-shaped comb electrodes.

11. The elastic wave element of claim 1, comprising a supply device (21) configured to provide a radio frequency signal of at least 2 GHz to the pair of finger comb electrodes (15a, 15b).

12. The elastic wave element of claim 1, wherein the pair of finger-shaped comb electrodes is the only conductive element in contact with the piezoelectric material.

13. The elastic wave element of claim 1 has no other electrode elements besides the pair of forked comb electrodes, specifically, no floating electrodes.

14. An acoustic-electric device using an elastic wave element as claimed in claim 11, particularly a filter, sensor, or delay line, having an operating frequency of 2 GHz or higher.

15. A method for manufacturing an elastic wave element as claimed in any one of claims 1 to 13, comprising the steps of: providing a piezoelectric material, particularly a ferroelectric material; applying an electric field stronger than, particularly at least ten times stronger than, the coercive field of the piezoelectric material to generate periodically alternating first and second polarization domains having opposite polarization directions; and fabricating the pair of finger-shaped comb electrodes above, specifically directly on, the piezoelectric material.

Images