Piezoelectric resonator

Abstract

Piezoelectric Resonator The invention relates to a piezoelectric resonator (10), particularly for a power converter, comprising: A piezoelectric layer (20), Two conducting electrodes (30, 40) extending on either side of the piezoelectric layer (20) such that the conducting layer (20) is sandwiched between the two conducting electrodes (30, 40), each conducting electrode (30, 40) being laterally delimited by an edge (35, 45) having a thickness different from that of the corresponding electrode (30, 40), the edges (35, 45) of the two electrodes (30, 40) being substantially symmetrical with respect to each other with respect to a median plane M extending through the midpoint of the piezoelectric layer (20). Figure for the abstract: Fig. 1

Description

Title of the invention: Piezoelectric resonator

[0001] The present invention relates to piezoelectric resonators, in particular for power converters, power converters comprising such resonators and a method of using such resonators in power converters. technical field

[0002] Power converters are now ubiquitous in our daily lives, powering all our electronic devices, whether portable or not. Essential for all our electronic devices, efforts are underway to make them smaller, less lossy, and more efficient.

[0003] One proposed solution is to increase their operating frequency in order to reduce the size of the transient energy storage components (i.e., the passive components). However, there are limitations to this frequency increase. These limitations are particularly noticeable with magnetic storage components, which have intrinsic iron losses and are subject to the skin effect, both of which worsen as the frequency increases. This hinders improvements in the efficiency of the power converters.

[0004] To address this problem, the use of piezoelectric resonators has been proposed. These resonators are used, in particular, between their resonant and antiresonant frequencies on a natural mode of the structure (for example, radial, longitudinal, thickness, shear, etc.). Between these two frequencies, they possess characteristics similar to those of the magnetic inductors that we seek to replace. The resonance modes exploited are mechanical in nature. Thus, the mechanical structure of the resonator must be taken into account, as it can cause wave leakage to the outside of the resonator (and therefore losses), but also contain unwanted resonance modes that can interfere with the system in its operating frequency band.

[0005] It is therefore crucial to find a resonator structure exhibiting a principal resonance mode with a high electromechanical coupling coefficient, in order to promote energy exchange between the resonator and the surrounding electrical circuit, and with a high quality factor and the minimum number of parasitic modes or the least coupled parasitic modes possible in the operating frequency band. Furthermore, the acoustic wave must remain confined within the resonator and propagate as little as possible to the outside.

[0006] Piezoelectric resonators are used in many fields, including for acoustic filters, particularly for radio frequency, the Body wave resonators, particularly those using materials like quartz as a time base (atomic clocks). The state of the art in the field of piezoelectric resonators has been largely influenced by previous work in these areas. Solutions have been proposed in the radio frequency domain. For example, those described in the work of J. Kaitila, "3C-1 Review of Wave Propagation in BAW Thin Film Devices - Progress and Prospects," 2007 IEEE Ultrasonics Symposium Proceedings, New York, NY, USA, 2007, pp. 120-129, doi: 10.1109 / ULTSYM.2007.43 which focus on the use of structures having a piezoelectric substrate covered on one side with an electrode of constant thickness extending over its entire length, and on the other side with an electrode of reduced extent covering only the active part of the resonator and having a thickened border (called frame or border-ring in English).Kaitila explains that the inertia provided by the electrode in the active zone lowers the resonant frequency of the resonator in this zone compared to the frequency of the same vibration mode outside the active zone, thus confining the vibration mode within the boundaries of the active zone. The thickened rim aims to facilitate the establishment of a resonant mode of homogeneous amplitude throughout the area inside this rim by easing the transition between the resonator's internal active zone and the external zone. Outside the resonator, the vibration is not completely zero, but drastically attenuated, limiting the propagation of waves outwards and therefore the associated energy loss, as well as the risks of wave reflection and the associated parasitic resonances.This border, present on only one side, also introduces mechanical constraints and manufacturing challenges, particularly due to differences in thermal expansion coefficients between the materials. Furthermore, the presence of thickened borders on only one side results in energy conversion to undesirable modes, thus generating the problem of additional parasitic modes, notably higher-order shear or longitudinal modes.

[0007] As an alternative to the thickened border, thinned borders are also described in the article by R. Thalhammer and JD Larson, "Finite element analysis of BAW devices: Principles and perspectives," in Proceedings of the 2015 IEEE International Ultrasonics Symposium. The analysis and issues are similar to those above in the case of the thickened border.

[0008] T. Wu, Hy Luo, Y.-w Feng, J.-f Bao and K.-y Hashimoto, “Ultra-wideband longitudinally coupled-resonator filters on lithium niobate using periodically slotted SiO 2 as an acoustic coupler”, Japanese Journal of Applied Physics vol. 63, 02SP87 (2024) also describes, as an alternative to the resonators described in the aforementioned Kaitila work, two resonator structures coupled between They are achieved through an intermediate structure and with separate electrical excitation for the two resonator structures. One of the two resonator structures is similar to the structure described by Kaitila in the aforementioned article, and the other has electrodes extending across the entire surface of the piezoelectric substrate on either side. The electrode opposite the other resonator structure has the same thickness as the edges of the other resonator structure. Such a resonator exhibits symmetrical thicknesses on both sides of the structure; this reduces the presence of additional parasitic modes due to the edges and avoids thick material deposition by distributing the thickness on both sides of the resonator. Nevertheless, the structure is complex due to the need for two resonator structures with separate electrical excitations.

[0009] These solutions were developed for the radio frequency domain, where the edge thickness issue remains minor due to the small dimensions of the resonators. The power converter domain requires piezoelectric layer thicknesses that are significantly greater than those of radio frequency filters, by a factor of 100 to 1000, related to the ratio of the operating frequencies of the components. Since the edge thickness is roughly proportional to the thickness of the piezoelectric layer, homothetic edge dimensioning is necessary, making the issue of thick material deposition more significant and problematic than in the radio frequency domain. It therefore appears that some of the issues detailed above are more or less important depending on the specific technical applications of the piezoelectric resonators.

[0010] Another solution proposed by E. Stolt et al., "A Spurious-Free Piezoelectric Resonator Based 3.2 kW DC-DC Converter for EV On-Board Chargers," seen in IEEE Transactions on Power Electronics, vol. 39, no. 2, pp. 2478-2488, Feb. 2024, doi: 10.1109 / TPEL.2023.3334211, involves the use of short-circuited rings around the resonators on one side. Although this method can reduce energy losses similarly to the thickened rim, it limits the voltage applicable to the resonator terminals to the breakdown voltage at the air gap(s) between the ring and the resonator electrode. This air gap must be small compared to the thickness for the ring to fully perform its function (here, 120 µm for a thickness of 500 µm). The electrical resistance is therefore even more limited. Furthermore, it is necessary to add an electrical connection between the upper and lower electrodes of the ring, which adds a manufacturing constraint.Furthermore, since the mechanical impedance difference between the resonator zone and the ring zone is small, the required ring width is quite large to achieve an effect similar to that of the previous frame by thickness. The ring therefore occupies a significant surface area compared to that of the [unclear]. resonator, thereby reducing the achievable power density and increasing the resonator's lateral footprint. Finally, the impedance mismatch of the piezoelectric structure between the resonator zone and the ring zone is imposed solely by the effect of the electrical short circuit; therefore, there are no degrees of freedom to optimize this impedance, particularly with regard to the surface area occupied or parasitic modes.

[0011] In summary, despite the progress made, current piezoelectric resonators still suffer from limitations in terms of performance, efficiency and compactness.

[0012] In particular, there is a need to further minimize the deformation energy transmitted outside the piezoelectric resonator and to promote energy transfer inside the resonator on a single resonance mode, while reducing parasitic modes and limiting the overall system size, which is important for the volume reduction of power converters in particular.

[0013] There is therefore a need for efficient, easy-to-manufacture and compact piezoelectric resonators, particularly in the case of use in power converters. Summary of the invention

[0014] The invention addresses this need, in particular by means of a piezoelectric resonator comprising: - A piezoelectric layer, - Two conductive electrodes extending on either side of the piezoelectric layer so that the piezoelectric layer is sandwiched between the two conductive electrodes, each conductive electrode being laterally delimited by a border having a different thickness than the corresponding electrode, the borders of the two electrodes being substantially symmetrical to each other with respect to a median plane extending through the middle of the thickness of the piezoelectric layer.

[0015] By "substantially symmetrical," we mean symmetrical with respect to each other, with tolerances, particularly manufacturing tolerances, in terms of dimensions, especially thickness and / or width, and / or alignment between them. Symmetry is typically within 10%, better to 5%, and even better to 2%. The thickness and width of the two edges can be identical to within 10%, better to 5%, and even better to 2%. The edges can be aligned with respect to each other symmetrically with respect to the median plane to within 10%, better to 5%, and even better to 2%. To preserve symmetry, the two edges are ideally made of identical materials.

[0016] By "laterally delimited," it is understood that the electrode is confined or limited at its edges by a border located nearby. This border is preferably in contact with, or overlaps, the edge of the electrode, thus forming a lateral boundary that determines the extent of the electrode's active surface. A proximity tolerance may be permitted, allowing the border to be located at a distance from the electrode edge while maintaining the integrity of the lateral delimitation according to the device specifications.

[0017] The absence of a gap between the electrode and the edge prevents an additional mechanical impedance break that would be associated with an intermediate gap. By thus limiting impedance breaks, the risks of energy transfer into undesired resonance modes are reduced.

[0018] The resonator thus comprises: - a working area defined by the overlap zone of the piezoelectric layer and the electrodes, - a boundary zone defined by the overlap zone of the piezoelectric layer and the edges, and having a thickness different from that of the resonator zone, and - an external zone defined outside of the other two zones.

[0019] The technical advantages of this configuration are numerous.

[0020] Such a resonator allows, in particular, the confinement of acoustic waves within the working area. The symmetry of the electrode edges allows the waves to be confined within the working area, thus reducing energy losses due to wave propagation outwards, particularly towards the external area. This improves the overall efficiency of the resonator.

[0021] By laterally delimiting the electrodes with substantially symmetrical edges, the invention minimizes the excitation of parasitic resonance modes. This makes it possible to obtain a purer signal and reduce unwanted interference, which is crucial for high-precision applications.

[0022] The sandwich configuration of the piezoelectric layer between two conducting electrodes, with substantially symmetrical edges, promotes a piston-like mode within the working area. This means that the deformation is substantially synchronous and of substantially homogeneous amplitude, which improves the electromechanical coupling and the efficiency of energy conversion.

[0023] Having edges on both sides makes manufacturing easier by reducing the required thickness difference compared to the case of an edge only on one electrode. This is because the thickness difference is then distributed on both sides, thus reducing the thickness of each edge.

[0024] In summary, the invention offers an innovative technical solution for improving the performance of piezoelectric resonators in terms of wave confinement, reduction of parasitic modes, electromechanical coupling, and ease of fabrication. These advantages make the invention particularly suitable for applications requiring compact and efficient devices, such as power converters and other advanced electronic systems.

[0025] Preferably, the piezoelectric resonator is for a power converter. Symmetry

[0026] Preferably, the external zone, in particular the piezoelectric layer, has a thickness less than that of the resonator zone and the boundary zone.

[0027] The electrodes can be substantially symmetrical with respect to each other with respect to the median plane extending through the middle of the thickness of the piezoelectric layer.

[0028] Preferably, the external zone and / or the working zone are substantially symmetrical with respect to the median plane extending through the middle of the thickness of the piezoelectric layer.

[0029] The resonator can be substantially symmetrical with respect to the median plane extending through the middle of the thickness of the piezoelectric layer. This symmetry helps, in particular, to avoid mechanical stresses and distortions at the interface, thereby improving the stability and performance of the resonator.

[0030] The resonator can also be substantially symmetrical with respect to a transverse plane extending perpendicularly to the median plane. This helps to reduce parasitic resonance modes that can interfere with the resonator's operation. Indeed, a structure symmetrical with respect to the transverse plane prevents the excitation of odd modes (n=1, 3, ...) due to the compensation of electrical charges over the entire surface of the electrodes. Symmetry with respect to the median plane reduces the excitation of even modes (n=2, 4, ...). Furthermore, the presence of the edges also reduces the excitation of these even modes. Secondly, the edges help to maintain a homogeneous deformation within the resonator, which promotes high electromechanical coupling and limits wave propagation outwards, thus reducing energy losses.These characteristics are particularly advantageous for the performance and efficiency of piezoelectric resonators in power conversion applications. Piezoelectric layer.

[0031] Preferably, the piezoelectric layer is single-layered. It can be made of a single material over its entire surface and be single-layered through its thickness. This allows in particular to avoid having interfaces within the piezoelectric layer that could generate energy losses.

[0032] The piezoelectric layer can be made of lithium niobate (LiNbO3), lithium tantalate (LiTaO3), potassium niobate (KNbO3), potassium sodium niobate (KNaNbO3), lead zirconate titanate (PbZrTiO3), quartz (SiO2), aluminum nitride (AIN), aluminum scandium nitride (AIScN), and zinc oxide (ZnO). In particular, in the case of power converters, the piezoelectric layer is made of lithium niobate (LiNbO3), lithium tantalate (LiTaO3), potassium niobate (KNbO3), potassium sodium niobate (KNaNbO3), lead zirconate titanate (PbZrTiO3), or quartz (SiO2).

[0033] The thickness of the piezoelectric layer can be constant to within 1%, better to within 0.1%, at least in the resonator area, better in all areas.

[0034] The piezoelectric layer may have a thickness greater than or equal to 10 pm, preferably greater than or equal to 25 pm, and even better greater than or equal to 50 pm. Such thicknesses are particularly advantageous for power converter applications. Electrodes

[0035] The electrodes can be of the same thickness to within 20%, better 10%, even better 5%.

[0036] The electrodes can each have a constant thickness to within 1%, or better, to within 0.1% over their entire length.

[0037] The electrodes may have a circular, oval, or polygonal outline, including a square. The invention is not limited to a particular shape of electrodes.

[0038] The thickness of the electrodes may be less than the thickness of the piezoelectric layer, in particular less than one third, or even better less than one fifth, of the thickness of the piezoelectric layer. Borders

[0039] Preferably, the edges are each in contact with the corresponding electrode over at least part of the electrode's periphery. They may be arranged laterally on the corresponding electrode at its edge or at a distance from the edge less than or equal to 5%, preferably 2% of the piezoelectric layer thickness, or they may border the corresponding electrode laterally around its entire periphery, being in lateral contact with it over at least part of its periphery. Each impedance break encloses a surface of a different size, and each size allows for the existence of additional resonance modes. Furthermore, each impedance break facilitates the transposition of one deformation mode onto another deformation mode, and thus a transfer

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[0048] energy transfer from one mode to another. Thus, the presence of a gap between the electrode and the edge increases the number of possible resonance modes and facilitates energy transfer from the desired main mode to other, undesired modes. Therefore, the absence of a gap between the electrode and the edge prevents an additional mechanical impedance break that would be associated with an intermediate gap. By limiting impedance breaks in this way, the risk of energy transfer to undesired resonance modes is reduced. The borders can delimit the electrodes over more than 80%, better more than 90%, even better more than 95%, of their periphery, in particular entirely. When using non-isotropic piezoelectric materials in the plane, such as lithium niobate (LiNbO3) or lithium tantalate (LiTaO3), it is advantageous to adapt the edge width according to the orientation of each elementary edge portion relative to the material's crystal orientation. Typically, the edge width can be non-constant and can depend at each point along the edge on the difference between the direction normal to that edge in the plane of the material and the material's crystal direction as defined in a single 3D coordinate system attached to the material plane. Preferably, the thickness of the borders is roughly constant. The width and thickness of the borders are preferably chosen so that approximately a quarter-wave propagates across the width of the borders at a predetermined operating frequency of the resonator. This promotes piston-like operation in the working region and allows for substantially synchronous deformation of substantially homogeneous amplitude within the working region. The borders can have a width of between 5 pm and 2000 pm, better between 50 pm and 1500 pm, better still between 50 pm and 300 pm in the particular case of a power converter. The borders in particular have a width less than or equal to four times the thickness of the piezoelectric layer, better less than or equal to three times the thickness of the piezoelectric layer, better still less than or equal to half the thickness of the piezoelectric layer. Advantageously, the border width can be defined to within 30%, better yet 15%, and even better still 5%, according to the following formula: kframet&^tkframeW'j — jkexf where W represents the width of the edges and kframe and kextremely the wavenumber of the edge region and the wavenumber of the outer region at a predetermined operating frequency of the resonator for predetermined thicknesses of the piezoelectric layer and edges. This characteristic allows Specifically, achieving optimal precision in the dimensioning of the resonator's edges is crucial, as it minimizes energy losses and unwanted resonance modes. By defining the edge widths with an accuracy of 30%, 15%, or even 5%, we ensure that the edges are sized to maximize the efficiency of electromechanical coupling and confine acoustic waves within the resonator. This also reduces the risk of deformation and distortion, thereby improving the resonator's stability and overall performance.

[0049] The predetermined working frequency of the resonator may be between the resonance frequency and the antiresonance frequency of the working area, in particular being substantially the geometric center between the resonance frequency and the weighted or unweighted antiresonance frequency.

[0050] The borders may be thicker than the electrodes. The cumulative thickness of the two borders may be greater than or equal to 1 pm, preferably greater than or equal to 10 pm.

[0051] Alternatively, the borders may be thinner than the electrodes. The cumulative sub-thickness of the two borders may be greater than or equal to 1 pm, preferably greater than or equal to 10 pm.

[0052] In one embodiment, the borders are made of an electrically insulating material, in particular silicon oxide or silicon nitride.

[0053] In the case where the edges are made of an electrically insulating material and are thicker than the electrodes, the thickness of the edges can be defined such that, at zero wavenumber, the resonant frequency of the edge zone is lower than the resonant frequency of the working zone, in particular by at least 3%, or preferably at least 6%, lower than the resonant frequency of the working zone. At the same time, in this case, the resonant frequency of the outer zone must be higher than the antiresonant frequency of the working zone, in particular by at least 3%, or preferably by at least 6%.

[0054] In the case where the borders are made of an electrically insulating material and where the borders are thinner than the electrodes, the thickness of the borders can be defined so that, at zero wavenumber, the resonance frequency of the border area is greater than the antiresonance frequency of the working area, in particular at least 3%, better at least 6%, greater than the antiresonance frequency of the working area.

[0055] In the case where the borders are made of an electrically insulating material, the thickness of the borders is preferably defined so that, at zero wavenumber, the resonance frequency of the border area is less than the resonance frequency of the external area.

[0056] In another embodiment, the edges may be made of an electrically conductive material; in particular, the material of the edges may be the same as the material of the corresponding electrode. They may be made of a metal, in particular copper, nickel, aluminum, molybdenum, tungsten, ruthenium, platinum, or gold. This makes it easier to manufacture and limits the presence of an interface that could generate energy loss, for example, resistive losses.

[0057] In the case where the edges are made of an electrically conductive material and where the edges are thicker than the electrodes, the thickness of the edges can be defined so that, at zero wavenumber, the resonance frequency of the edge area is lower than the resonance frequency of the resonator area, in particular at least 3%, better at least 6%, lower than the resonance frequency of the resonator area.

[0058] In the case where the borders are made of an electrically conductive material and where the borders are thinner than the electrodes, the thickness of the borders can be defined so that, at zero wavenumber, the antiresonance frequency of the border area is greater than the antiresonance frequency of the resonator area, in particular at least 3%, better at least 6%, greater than the antiresonance frequency of the resonator area.

[0059] In the case where the borders are made of an electrically conductive material, the thickness of the borders can be defined so that, at zero wavenumber, the antiresonance frequency of the border area is lower than the resonance frequency of the external area. External area

[0060] Preferably, the resonator is electrode-free outside the areas delimited by the borders on either side of the piezoelectric layer.

[0061] The thickness of the piezoelectric layer can be reduced in the external zone compared to its thickness in the boundary zone and in the resonator zone.

[0062] Preferably, the external zone is configured so that, at zero wavenumber, its resonance frequency is greater than the antiresonance frequency of the working zone, in particular at least 3%, better at least 6%, even better at least 18%, greater than the antiresonance frequency of the resonator zone. Electrical connection

[0063] Preferably, the resonator includes electrical contacts for connecting each electrode to an electrical circuit, in particular each contact with an electrode and / or an edge. The electrical contacts on either side of the piezoelectric layer are preferably not overlapping. This prevents vibration of the piezoelectric layer from occurring between these contacts in the area External. In the case of raised edges made of a conductive material, the electrical contacts can be integrated into the edges and have a thickness substantially equal to the raised edges. In the case of raised edges made of an insulating material, the electrical contacts can be integrated into the corresponding edge either by locally interrupting the edge, or by locally overlapping the edge by passing over or under it. Double border

[0064] The resonator may include additional borders on either side of the piezoelectric layer in the outer zone. The additional borders may be substantially symmetrical with respect to each other with respect to the median plane extending through the middle of the thickness of the piezoelectric layer. The additional borders may have a different width and / or thickness than the borders.

[0065] The invention also meets this need, at least in part, by means of a power converter comprising the piezoelectric resonator as described above. Method of use

[0066] The invention also meets the aforementioned need by a method of using the resonator as described above as a power converter.

[0067] The operating frequency range of the converter can be between the resonance frequency and the antiresonance frequency of the working area defined by the area of ​​overlap of the piezoelectric layer and the electrodes, the resonance frequency and the antiresonance frequency being defined at zero wavenumber. Brief description of the drawings

[0068] [Fig. 1] schematically represents in cross-section an example of a piezoelectric resonator,

[0069] [Fig.2] schematically represents in cross-section a variant of a piezoelectric resonator,

[0070] [Fig.3] schematically represents in cross-section a variant of a piezoelectric resonator,

[0071] [Fig.4] schematically represents in cross-section a variant of a piezoelectric resonator,

[0072] [Fig.5] schematically represents in cross-section a variant of a piezoelectric resonator,

[0073] [Fig.6] represents dispersion curves in the different zones of an example of a piezoelectric resonator.

[0074] [Fig.7] schematically represents in cross-section a variant of a piezoelectric resonator, and

[0075] [Fig-8] schematically represents, in top view, a variant of a piezoelectric resonator. Detailed description

[0076] Figure 1 illustrates a piezoelectric resonator 10 comprising a piezoelectric layer 20 sandwiched between two conductive electrodes 30 and 40. The two electrodes 30 and 40 are laterally delimited by borders, labeled 35 and 45 respectively, having different thicknesses than the electrodes 30 and 40. The borders 35 and 45 on either side of the piezoelectric layer 20 are substantially symmetrical with respect to a median plane M of the piezoelectric layer 20. They are shown here as perfectly symmetrical with respect to the median plane M, but there is obviously a tolerance in the symmetry which nevertheless allows for an interesting effect on the resonator 10. The borders can be identical to within 20%, better 10%, or even better 5%, and they can be positioned relative to each other to within 20%, better 10%, or even better, 5% to the nearest.

[0077] The zone T defined by the superposition of the piezoelectric layer 20 and the electrodes 30 and 40 (excluding edges) constitutes the working zone in which we wish to confine the waves and in which a synchronous deformation of homogeneous amplitude is sought. The zone(s) B defined by the superposition of the piezoelectric layer 20 and the edges 35 and 45 constitute the boundary zone(s) allowing a controlled transition of the deformation between the external zone and the working zone T in order to meet the requirements of the invention.

[0078] In the illustrated example, the piezoelectric layer 20 is a single layer made of a single material over its entire surface, which notably avoids internal interfaces that could generate energy losses. However, it could be otherwise. In particular, it could be multilayered, made of the same or different piezoelectric materials. In the case of a multilayer, it lacks electrodes in the multilayer structure, and the layers are preferably in contact with each other.

[0079] The piezoelectric layer 20 can be made of lithium niobate (LiNbO3), lithium tantalate (LiTaO3), potassium niobate (KNbO3), potassium sodium niobate (KNaNbO3), lead zirconate titanate (PbZrTiO3), quartz (SiO2), aluminum nitride (AIN), aluminum scandium nitride (AIScN), or zinc oxide (ZnO). Preferably, in the case of power converters, the piezoelectric layer 20 is made of lithium niobate (LiNbO3), lithium tantalate (LiTaO3), potassium niobate (KNbO3), potassium sodium niobate (KNaNbO3), lead zirconate titanate (PbZrTiO3), or quartz (SiO2). The thickness of the epiezo piezoelectric layer 20 is constant to within 1%, better to within 0.1%, at least in the working area T, preferably over its entire extent.

[0080] Each conductive electrode 30 or 40 can have a constant thickness eres to within 1%, or preferably 0.1%, over its entire length. In the illustrated example, the thickness eres is less than one-fifth of the ePiezo thickness of the piezoelectric layer 20. The electrodes 30 and 40 can have a circular, oval, or polygonal outline, including a square.

[0081] In the illustrated example, electrodes 30 and 40 are substantially identical and symmetrical with respect to the median plane M. There is, of course, a tolerance, as with the edges. The electrodes can be identical to within 20%, better 10%, even better 5%, and they can be positioned relative to each other to within 20%, better 10%, even better 5%.

[0082] The edges 35 and 45 have different eframe thicknesses from the eres thickness of the electrodes 30 and 40. This thickness of the edges eframe can be greater than that of the electrodes 30 and 40, as illustrated in figures 1 to 3, or less than that of the electrodes 30 or 40, as illustrated in figures 4 and 5.

[0083] The edges 35 and 45 are preferably each in contact with the corresponding electrode 30 or 40 over at least part of the periphery of the electrode 30 or 40. The edges 35 and 45 can delimit the electrodes 30 and 40 over more than 80%, better more than 90%, even better more than 95%, of their periphery, preferably entirely.

[0084] They can laterally border the corresponding electrode 30 or 40 on its entire periphery by being in lateral contact with the latter on at least part of its periphery, as illustrated in Figures 1 and 4. In an alternative illustrated in [Fig.2], they can advantageously be arranged laterally on the corresponding electrode at the edge of the latter or otherwise in the immediate vicinity of the edge, in particular at a distance from the edge of less than 5%, better 2% of the thickness of the piezoelectric.

[0085] The resonator 10 is preferably devoid of electrode 30 or 40 outside the areas delimited by the borders 35 and 45 on either side of the piezoelectric layer 20. The ePiezo thickness of the piezoelectric layer 20 can be reduced in the external area compared to its thickness in the border area B and in the working area T.

[0086] The electrodes 30 and 40 are preferably made of the same conductive material, in particular copper, nickel, aluminium, molybdenum, tungsten, ruthenium, platinum, gold.

[0087] Preferably, the electrodes and the edges are substantially symmetrical with respect to the median plane M and to a plane transverse to the median plane P. The whole of the

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[0095] a resonator can exhibit such symmetries with respect to the median plane M and to a transverse plane P. In the case of in-plane non-isotropic piezoelectric materials, such as lithium niobate (LiNbO3) or lithium tantalate (LiTaO3), it is advantageous to adapt the width W of the edges 35 and 45 according to the orientation of each elementary edge portion relative to the crystalline orientation of the material. Borders 35 and 45 are preferably of a thickness eframe that is substantially constant over the entire border. The edges 35 and 45 can be made of the same material as the corresponding electrode, as illustrated in Figures 3 and 5. This simplifies manufacturing and minimizes the presence of an interface that could generate energy loss. Alternatively, as illustrated in Figures 1, 2, and 4, they can be made of a different material. In this case, they can be either a conductive or an insulating material. The width W and thickness eframe of the borders 35 and 45 are preferably chosen so that a fraction of a wave corresponding approximately to a quarter wave propagates across the width of the borders at a predetermined operating frequency of the resonator fwork. This promotes piston-like mode in the working region T and allows for substantially synchronous deformation of substantially homogeneous amplitude in the working region T. The frame thickness of the edges 35 and 45 can be chosen to achieve a good compromise between reduced cost / manufacturing constraints, which are particularly important for thicker edges, and the fewest possible unwanted modes excited at the working frequency fwork. The frame thickness of the edges 35 and 45 can be substantially independent of the width L of the electrodes 30 and 40. Advantageously, the width W of borders 35 and 45 can be defined to within 30%, better yet 15%, and even better still 5%, according to the following formula: kframe^^J^frume^) — j^ext where kframe and kextremely are the wavenumber of the edge zone B and the wavenumber of the outer zone at a predetermined operating frequency of the resonator 10 for predetermined thicknesses of the piezoelectric layer epiezo and the edges e frame. The wavenumbers can be determined from predetermined dispersion curves, as illustrated in [Fig. 6], notably by periodic finite element simulation, particularly using Bloch-Floquet conditions imposing a phase relationship between the two edges of the mesh at the operating frequency for the different zones. These dispersion curves can be determined for different modes (here modes 1 and 2) for the working area (regular area), the The frame and the external area are considered. Determining the other modes also ensures that the transfer to other parasitic modes, particularly mode 2, remains negligible. This characteristic allows for an optimal approach to the resonator's frame dimensions, which in turn minimizes energy losses and parasitic resonance modes. By defining the frame widths with an accuracy of 30%, 15%, or even 5%, we ensure that the frames are sized to maximize the efficiency of electromechanical coupling and confine the acoustic waves within the resonator. This also reduces the risk of deformation and distortion, thus improving the resonator's stability and overall performance. The frame width can range from 5 µm to 2 mm.

[0096] Alternatively, for a predetermined border thickness eframe 35 and 45, the border width W as a function of the working frequency fwork can be determined from a pre-established curve of the width W determined according to the preceding equation as a function of the working frequency fwork. It is then possible to refer only to charts of the optimal width as a function of frequency for different border thicknesses. Thus, after determining the optimal thickness eframe, it is easy to determine the optimal width W at the working frequency fwork or the optimal width range within the working frequency range, in particular between the resonance frequency fres s and the antiresonance frequency fres p of the working zone T.

[0097] The working frequency of the resonator fwork can be between the resonance frequency fres s and the antiresonance frequency fres p of the working area, in particular being substantially the geometric center between the resonance frequency fresr and the antiresonance frequency fres p weighted or unweighted.

[0098] Preferably, the borders 35 and 45 have a width W less than or equal to four times the thickness of the piezoelectric layer epiezo, better less than or equal to three times the thickness of the piezoelectric layer epiezo, better still less than or equal to half the thickness of the piezoelectric layer epiezo.

[0099] In the case where the edges 35 and 45 are made of an electrically insulating material and are thicker than the electrodes 30 and 40, as illustrated in Figures 1 to 3, the thickness eframe of these edges is defined so that, at zero wavenumber, the resonance frequency fframe of the edge zone B is lower than the resonance frequency fext of the external zone. The thickness eframe of the edges can, moreover, be defined so that, at zero wavenumber, the resonance frequency fframe of the edge zone B is lower than the resonance frequency fres s of the working zone T, in particular at least 3%, preferably at least 6%, lower than the resonance frequency fres s of the working zone T. If they are thinner than the electrodes 30 and 40, as illustrated in Figures 4 and 5, the thickness of the borders eframe can be defined so that, at zero wavenumber, the resonance frequency of the border area fframe is greater than the antiresonance frequency of the working area fres p, in particular at least 3%, better at least 6%, greater than the antiresonance frequency of the working area fres p.

[0100] In the case where the borders 35 and 45 are made of an electrically conductive material and where the borders 35 and 45 are thicker than the electrodes 30 and 40, as illustrated in Figures 1 to 3, the thickness eframe of the borders is, preferably, defined so that, at zero wavenumber, the resonance frequency fframe_ s of the border area B is less than the resonance frequency of the resonator area fres s, in particular at least 3%, better at least 6%, less than the resonance frequency of the resonator area fres s. If they are thinner than electrodes 30 and 40, the thickness of the borders eframe can be defined so that, at zero wavenumber, the antiresonance frequency of the border area fframe p is greater than the antiresonance frequency of the working area fres p, in particular at least 3%, better at least 6%, greater than the antiresonance frequency of the working area fres p.

[0101] In both cases, preferably, the external zone is configured so that, at zero wavenumber, its resonant frequency fext is greater than the antiresonant frequency of the working zone fres p, in particular at least 3%, better at least 6%, even better at least 18%, greater than the antiresonant frequency of the working zone f xres_p*

[0102] The dimensions are also dependent on the working frequency fwork and therefore on the application. Typically, for a power converter, the piezoelectric layer 20 has a thickness ePiezo greater than or equal to 50 pm, the electrodes have a thickness eres greater than or equal to 1 pm, for example on the order of tens of micrometers, and the thickness and borders 35 and 45 can have an overthickness or underthickness eframe on the order of a few micrometers and a width W between 5 pm and 2000 pm, better between 50 pm and 1500 pm, better still between 50 pm and 300 pm.

[0103] The resonator 10 further comprises electrical contacts 60 and 70 for connecting each of the electrodes to an electrical circuit, in particular in contact with one of the electrodes 30 and 40 and / or an edge 35 and 45. The electrical contacts 60 and 70 on either side of the piezoelectric layer 20 are preferably not overlapping. This prevents vibration of the piezoelectric layer 20 from occurring between these contacts 60 and 70 in the outer zone. Such electrical contacts 60 and 70 are shown in [Fig. 8].

[0104] In the case of raised edges 35 and 45 made of a conductive material, the electrical contacts 60 and 70 can be integrated into the edges 35 and 45 and have a thickness substantially equal to the raised edges. In the case of raised edges made of an insulating material, the electrical contacts can be integrated into the corresponding edge either by locally interrupting the edge, or by locally overlapping the edge by passing over or under it.

[0105] The embodiment illustrated in [Fig. 7] differs from that of [Fig. 1] in that the resonator 10 may include additional borders 80 and 90 on either side of the piezoelectric layer 20 in the outer zone. The additional borders 80 and 90 may be substantially symmetrical with respect to each other with respect to the median plane M. The additional borders 80 and 90 may have the same width and thickness as the borders 35 and 45. Alternatively, they could have a different width and / or thickness than the borders 35 and 45. Example of sizing

[0106] In an embodiment of the invention illustrated in [Fig. 1] for a power converter, the piezoelectric layer is 500 µm thick and made of LNO (lithium niobate) with a Y36° crystal orientation relative to the electrodes. Electrodes 30 and 40 have a circular outline and are 15 µm thick. They are bordered by rims 35 and 45, 30 µm thick and 1.2 mm wide, joined to the electrodes. This resonator is used over an operating range around 6 MHz. The resonator is perfectly symmetrical with respect to a median plane of the piezoelectric layer and with respect to a median plane perpendicular to this median plane.

[0107] The invention is not limited to the embodiments just described. The features can be combined where technically feasible. Furthermore, as mentioned, there is a tolerance in the dimensions and / or positioning of the elements relative to each other that allows for interesting, if not equivalent, performance.

Claims

Demands

1. Piezoelectric resonator (10), in particular for a power converter, comprising: - A piezoelectric layer (20), - Two conducting electrodes (30, 40) extending on either side of the piezoelectric layer (20) so that the piezoelectric layer (20) is sandwiched between the two conducting electrodes (30, 40), each conducting electrode (30, 40) being laterally delimited by a border (35, 45) having a different thickness from the corresponding electrode (30, 40), the borders (35, 45) of the two electrodes (30, 40) being substantially symmetrical with respect to each other with respect to a median plane M extending in the middle of the piezoelectric thickness of the piezoelectric layer (20).

2. Resonator according to claim 1, comprising - a working zone T defined by the overlap zone of the piezoelectric layer (20) and the electrodes (30, 40), - a boundary zone B defined by the overlap zone of the piezoelectric layer (20) and the boundaries (35, 45) and having a thickness different from that of the working zone T, and - an external zone defined outside the other two zones B and T.

3. Resonator according to claim 2, wherein the external zone has an epiezo thickness less than that of the working zone T and the border zone B.

4. Resonator according to any one of the preceding claims, wherein the resonator is substantially symmetric with respect to the median plane M extending through the middle of the epiezo thickness of the piezoelectric layer (20).

5. Resonator according to any one of the preceding claims, wherein the epiezo thickness of the piezoelectric layer and the eres thickness of the electrodes is constant to within 1%, better to within 0.1%, at least in the resonator zone, better in all zones.

6. Resonator according to any one of the preceding claims, wherein the edges (35, 45) are each in contact with the corresponding electrode (30, 40) on at least part of the periphery of the electrode, in particular on their entire perimeter.

7. Resonator according to any one of the preceding claims, wherein the piezoelectric layer (20) has an epiezo thickness greater than or equal to 10 pm, better greater than or equal to 25 pm, even better greater than or equal to 50 pm and the borders (35, 45) have a width W less than or equal to four times the ePiezo thickness of the piezoelectric layer (20), better less than or equal to three times the epiezo thickness of the piezoelectric layer (20), better still less than or equal to half the epiezo thickness of the piezoelectric layer (20), in particular between 5 pm and 2000 pm, better between 50 pm and 1500 pm, better still between 50 pm and 300 pm.

8. Resonator according to any one of the preceding claims, wherein the width W and the thickness eframe of the borders (35, 45) are chosen such that a quarter wave propagates in the width W of the borders (35, 45) at a predetermined working frequency fwork of the resonator.

9. Resonator according to any one of the preceding claims, wherein the width W of the borders (35, 45) is defined to within 30%, better 15%, still better 5%, according to the following formula: ^frame^^J^frame^ ) — j^ext where W represents the width of the borders (35, 45) and k frame and k ext respectively the wavenumbers of the border area B and the external area respectively at a predetermined working frequency fwork of the resonator for predetermined thicknesses of the piezoelectric layer (20) and the borders (35, 45).

10. Resonator according to claim 8 or 9, wherein the predetermined working frequency fwork of the resonator is between the resonance frequency fres s and the antiresonance frequency fres p of the working area T, in particular being substantially the geometric center between the resonance frequency fres s and the antiresonance frequency fres_p weighted or unweighted.

11. Resonator according to any one of the preceding claims, wherein the edges (35, 45) are made of an electrically insulating material, the thickness eframe of the edges (35, 45) being defined in particular such that, at zero wavenumber, the frequency of resonance fframe of the border area is less than the resonance frequency fext of the outer area.

12. Resonator according to claim 11, wherein the borders (35, 45) are thicker than the electrodes (30, 40) and the thickness eframe of the borders (35, 45) is defined such that, at zero wavenumber, the resonance frequency fframe of the border area is less than the resonance frequency fres s of the working area, in particular at least 3%, preferably at least 6%, less than the resonance frequency fres s of the working area.

13. Resonator according to claim 11, wherein the borders (35, 45) are thinner than the electrodes (30, 40) and the thickness eframe of the borders (35, 45) is defined such that, at zero wavenumber, the resonance frequency fframe of the border area is greater than the antiresonance frequency fres p of the working area, in particular at least 3%, preferably at least 6%, greater than the antiresonance frequency fres p of the working area.

14. Resonator according to any one of claims 1 to 10, wherein the edges (35, 45) are made of an electrically conductive material, in particular the material of the edges (35, 45) may be the same as the material of the corresponding electrode, the thickness eframe of the edges (35, 45) being defined in particular so that, at zero wavenumber, the antiresonance frequency fframe s of the edge zone is less than the resonance frequency fext of the outer zone.

15. Resonator according to claim 14, wherein the borders (35, 45) are thicker than the electrodes and the thickness eframe of the borders (35, 45) is defined such that, at zero wavenumber, the resonance frequency fframe_s of the border area is less than the resonance frequency fres s of the resonator area, in particular at least 3%, preferably at least 6%, less than the resonance frequency fres s of the resonator area.

16. Resonator according to claim 14, wherein the borders (35, 45) are thinner than the electrodes and the thickness of the borders (35, 45) is defined such that, at zero wavenumber, the antiresonance frequency fframe_p of the border area is greater than the antiresonance frequency fres p of the resonator area, in particular at least 3%, preferably at least 6%, greater than the antiresonance frequency fres p of the resonator area.

17. Resonator according to any one of the preceding claims, comprising electrical contacts (60, 70) for connecting each of the electrodes (30, 40) to an electrical circuit, in particular each in contact with an electrode (30, 40) and / or an edge (35, 45), the electrical contacts on either side of the piezoelectric layer (20) not being superimposed on each other.

18. Resonator according to any one of the preceding claims, comprising additional borders (35, 45) on either side of the piezoelectric layer (20) in the outer zone substantially symmetrical to each other with respect to the median plane M extending in the middle of the thickness of the piezoelectric layer.

19. Power converter comprising a piezoelectric resonator (10) according to any one of the preceding claims.

20. Method of using the piezoelectric resonator according to any one of claims 1 to 18 in a power converter.

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