Substrate-mounted integrated structure, comprising a ferroelectric layer with selective reverse polarization in the thickness thereof, and method for manufacturing same

Abstract

Structure (Struct) comprising a ferroelectric layer (Ferro lay ) having a first polarization (P1) in a first volume (V1) and a second polarization (P2), opposite to the first polarization, in a second volume (V2) different from the first volume, the first polarization and the second polarization are oriented perpendicularly or obliquely to the ferroelectric layer; wherein a surface of the first volume (V1) forms a face (Sup) of the ferroelectric layer (Ferro lay) and the first volume (V1) is disposed between this face of the ferroelectric layer and the second volume (V2); and wherein the second volume (V2) has a hydrogen concentration, known as the polarity reversal concentration, greater than the first volume (V1), and the polarity reversal concentration is between 10 19 and 10 22 hydrogen atoms per cubic centimeter.

Description

INTEGRATED STRUCTURE TRANSFERRED TO A SUBSTRATE, COMPRISING A FERROELECTRIC LAYER WITH SELECTIVE POLARIZATION IN ITS THICKNESS, AND MANUFACTURING METHOD TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to an integrated structure comprising a ferroelectric layer transferred onto a support, and a method for manufacturing this structure. Such a structure can be used to form, for example, radiofrequency (RF) components, in particular bulk elastic wave components. TECHNOLOGICAL BACKGROUND

[0002] There are several types of applications that take advantage of or are influenced by the polarization properties of ferroelectric materials and the presence of polarization domains opposite each other. Examples include surface acoustic wave devices (SAW devices) or bulk acoustic wave devices (BAW devices). The existence of these applications has motivated the development of methods for controlling the polarization domains of ferroelectric layers.

[0003] The article “Seeing Is Believing – In-Depth Analysis by Co-Imaging of Periodically-Poled X-Cut Lithium Niobate Thin Films” by Sven Reitzig et al., published in Crystals 2021, 11, 288, describes a lithium niobate layer embedded on a silicon substrate via a silica layer, and the control of the polarization of this layer parallel to the plane in which it extends, by applying a voltage between electrodes arranged periodically on the free surface of the layer. It should be noted that, in such a structure with electrodes only on the free face of the crystal, controlling a polarization that would be perpendicular to the substrate by means of an electric field, in the absence of a buried electrode, would require voltages that risk causing the breakdown of the silica layer.

[0004] Document EP 0 592 226 A1 describes an optical frequency conversion device obtained by the periodic juxtaposition of parallel bands of inverted polarization at one face of a ferroelectric substrate having a spontaneous polarization perpendicular to the plane of extension of the substrate, i.e. perpendicular to this face. The inversion of the polarization according to the bands is obtained by carrying out a proton exchange through a mask.

[0005] Document WO 2005 / 052682 A1 describes the localized reversal of the polarization of a ferroelectric crystal with spontaneous polarization perpendicular to one of its faces, by applying an electric field along juxtaposed periodic bands, by means of gel electrodes arranged on this face and the opposite face of the crystal.

[0006] The structures and methods presented above, if they effectively allow the control of the polarization domains of a ferroelectric layer on its surface, remain impractical and do not allow the control of the polarization in the thickness of this layer and its integration on a support.

[0007] A first object of the invention is to provide a ferroelectric layer, possibly in an integrated form on a substrate, of polarization having polarization domains of opposite orientations and perpendicular or oblique to the plane of this layer, the domains being distributed in the thickness of the ferroelectric layer, the latter being suitable for use, for example, in applications based on bulk acoustic waves. A second object of the invention is a manufacturing method for obtaining the integrated ferroelectric layer mentioned above. A third object of the invention is a method for local and controlled inversion of the polarization of any ferroelectric element in its thickness.

[0008] In order to achieve these objectives, a first aspect of the invention is a structure comprising a ferroelectric layer having a first polarization in a first volume and a second polarization, opposite to the first polarization, in a second volume distinct from the first volume, in which the first polarization and the second polarization are oriented perpendicularly or obliquely to the ferroelectric layer, a surface of the first volume forms a face of the ferroelectric layer and the first volume is interposed between this face of the ferroelectric layer and the second volume, the second volume has a higher hydrogen concentration, called the polarity inversion concentration, than the first volume, and the polarity inversion concentration is between 10 19 and 10 22 hydrogen atoms per cubic centimeter.

[0009] An advantage of the structure according to the invention is the provision of an integrated structure, including a substrate on which is fixed a ferroelectric layer having a polarization perpendicular or oblique to the planes of extension of the layer and the substrate, this polarization being modulated in the thickness of the ferroelectric layer. Such a structure facilitates for example the integration of volume acoustic functions while maintaining a low cost.

[0010] According to additional non-limiting characteristics of the first aspect of the invention, considered individually or in any technically feasible combination: the structure may further comprise a support and a metal layer interposed between this support and the ferroelectric layer; the ferroelectric layer may be formed from a single crystal; the ferroelectric layer may comprise lithium niobate or lithium tantalate; and the support may comprise single-crystal silicon.

[0011] A second aspect of the invention relates to a method for manufacturing a structure whose polarization is locally controlled, comprising the steps of providing a ferroelectric element having a first polarization; enriching a given buried volume of the ferroelectric element with hydrogen ions by ion implantation of the hydrogen ions through a face of the ferroelectric element, a first volume of the ferroelectric element being interposed between the given buried volume and the face of the ferroelectric element; and applying an anneal to the ferroelectric element after introduction of the hydrogen ions, at a temperature between 500°C and 700°C, so as to switch the first polarization of the ferroelectric element in the given volume to a second polarization opposite in orientation to the first polarization, the first volume having, after the annealing, the first polarization.

[0012] The method according to the invention is advantageous in that it is simple, flexible, and easily integrated into a broader manufacturing process based on known and proven manufacturing techniques from the semiconductor industry. In addition, it allows the control of the polarization of a ferroelectric element in its thickness and is applicable to thin layers integrated on a substrate.

[0013] According to additional non-limiting characteristics of the second aspect of the invention, considered individually or in any technically feasible combination: the first polarization can be single-domain, perpendicular or oblique relative to a face of the ferroelectric element; during the selective enrichment of hydrogen ions, a hydrogen ion implantation dose can be adapted so as to obtain a hydrogen concentration of between 10 19 and 10 22hydrogen atoms per cubic centimeter;the hydrogen ion implantation may be carried out at an energy between 3 keV and 210 keV;the ferroelectric element may be a layer of lithium niobate or lithium tantalate;the method may further comprise assembling a ferroelectric substrate and a support, followed by separating or thinning the ferroelectric substrate so as to define the ferroelectric element; andthe support may be a monocrystalline silicon substrate, and a metal layer may be interposed between this monocrystalline silicon substrate and the ferroelectric substrate. BRIEF DESCRIPTION OF THE FIGURES

[0014] Other characteristics and advantages of the invention will emerge from the detailed description of the invention which follows with reference to the appended figures in which:

[0015] Larepresents a ferroelectric element with selectively modified polarization in its thickness and a corresponding manufacturing method according to the present description;

[0016] It represents variants of the ferroelectric element of the;

[0017] Illustrates a first method of integrating a ferroelectric layer on a support;

[0018] Illustrates a second method of integrating a ferroelectric layer on a support;

[0019] This is a schematic sectional view of a device according to the present description;

[0020] This is a first variation of the device of the ; and

[0021] This is a second variation of the device. DETAILED DESCRIPTION OF THE INVENTION

[0022] Depth-selective polarization inversion

[0023] Following a series of experiments, the applicant realized that it is possible to switch the polarization of polarization domains by hydrogen implantation followed by annealing. These experimental results are implemented in the manufacturing method described below, resulting in a ferroelectric element 10 having reversed polarization domains at depths chosen by the practitioner.

[0024] The ferroelectric element 10 may be a ferroelectric single crystal as such, this single crystal may be in the form of a ferroelectric single crystal layer, this ferroelectric crystal or this ferroelectric single crystal layer may be fixed on a support Sprt within a structure Struct, such as the ferroelectric layer Ferro lay illustrated in (D) of the.

[0025] Figures 1 and 2 illustrate a first embodiment, in which the ferroelectric element 10 is a ferroelectric single crystal as such, i.e. independent of any support.

[0026] Larepresente in (A) a section, along a plane ZX, of the ferroelectric element 10 having two opposite faces Sup and Inf perpendicular to a direction Z and having a first single-domain polarization P1 parallel to the direction Z and perpendicular to the upper faces Sup and lower faces Inf which are each parallel to the extension plane of the ferroelectric element 10, that is to say the crystalline plane in which its dimensions are the largest.

[0027] For explanatory purposes, this embodiment takes the example of a polarization perpendicular to the plane of extension of the ferroelectric element 10. However, the invention also applies to the situation where the polarization is not perpendicular to the plane of extension of the ferroelectric element 10 but oblique relative to it. The characteristic "perpendicular" means an inclination of 90° relative to the plane of extension of the ferroelectric element to within 10°, the characteristic "oblique" or "obliquely" means an inclination at least 5° away from the plane of extension of the ferroelectric element. A direction having an inclination in the range from 5° to 80°, or from 30° to 70°, or from 35° to 60° can thus be considered oblique.For example, a 50RY LiTaO3 layer and a 42RY LiTaO3 layer have polarizations inclined by 50° and 42° relative to the extension plane of the layer, respectively, thus oblique polarizations relative to the extension plane of the layer.

[0028] In a step S10, the ferroelectric element 10 is enriched with hydrogen to a depth chosen by selective ion implantation of hydrogen ion H + through the upper surface Sup, at an implantation energy between 3 keV and 210 keV. The depth of the implantation and its extent in the Z direction are controlled by the implantation energy and the dose of implanted hydrogen ions.

[0029] As illustrated by laen (B), this step S10 results in the formation of a volume V2 (indicated by the hatching in the figures) relatively rich in hydrogen in the ferroelectric element 10, directly under the upper surface Sup and on a volume V1 (white areas of the ferroelectric element 10) relatively poor in hydrogen, and of depth depending on the acceleration energy of the implantation step. The volume V2 is thus interposed between the non-implanted volume V1 and the upper surface Sup of the ferroelectric element 10. The implantation dose is adapted to the implantation energy so as to obtain a hydrogen density of between 10 19 and 10 22 atoms / cm 3 in volume V2. It is possible to carry out several successive implantations at different implantation energies and doses in order to better define volume V2 by homogenizing the distribution of the hydrogen implanted in this volume.

[0030] At this stage, the polarization of the entire ferroelectric element 10 remains unchanged in the implanted volumes, the implanted volume V2 and the regions of the non-implanted volume V1 still forming a single monodomain in which the polarization has only one orientation.

[0031] For example, a dose of implanted hydrogen ions of between 10 14 and 10 15 at / cm 2 accelerated by a voltage of 6 kV to create a polarization flip volume V2 extending over approximately 50 nm in the Z direction in thickness, volume V2 in which the hydrogen concentration is between 10 19 and 10 22 , preferably between 5.10 19 and 2.10 21 at / cm 3 In all cases, the volume V2, enriched in hydrogen, has a higher hydrogen concentration, called the polarity inversion concentration, than the first volume V1.

[0032] In this document, the YX and ZX planes are defined by the X, YZ axes of an orthogonal reference frame, the YX plane being defined by the Y and X axes of the reference frame, the ZX plane being defined by the Z and X axes of the reference frame. The Sup and Inf faces of the ferroelectric element 10 extend parallel to the YX plane, perpendicular to the Z axis which defines the direction of extension of the thickness of the ferroelectric element 10. Depth is understood to mean a distance along the Z axis considered from the upper surface Sup. In addition, a modulation of the polarization or of the hydrogen concentration of the ferroelectric element 10 according to its thickness is understood respectively as variations in hydrogen concentration or changes in the direction of the polarization in the volume of this element according to the Z direction.

[0033] In a step S20, the ferroelectric element 10 selectively enriched in hydrogen is subjected to annealing at a temperature between 500°C and 800°C, preferably between 500°C and 700°C, more preferably between 550°C and 600°C. This annealing step results in an inversion of the first polarization P1 of the ferroelectric element 10 only at the level of the volume V2 enriched in hydrogen to give a second polarization P2, of the same direction of alignment as the first polarization P1 but of the opposite direction thereto, as illustrated in (C0). Antiparallel polarizations are defined as polarizations aligned in parallel directions but in opposite directions. Conversely, parallel polarizations are defined as polarizations aligned in parallel directions but in the same direction.The definition of the volume V2 by hydrogen enrichment allows the selectivity of the polarization inversion, which propagates from the upper surface Sup to the entire volume V2 and thus passes from a parallel polarization between volumes V1 and V2 to an antiparallel polarization between volumes V1 and V2. Annealing is preferably carried out under an oxygen atmosphere to reduce the exodiffusion of oxygen from the ferroelectric layer, but can also be carried out, for example, under a nitrogen or air atmosphere, at atmospheric pressure for a duration of between 100 seconds and 10 hours.

[0034] The hydrogen concentration in volume V2 obtained following step S10 is necessary for the polarization inversion obtained in step S20, and can therefore be described as polarization inversion concentration.

[0035] Illustrated in (C1) and (C2) are variants for the volume V2, obtained by adjusting the number of implantations and their respective energies and doses. In (C1), the implantation energy was sufficient to enrich a part of the ferroelectric element 10 far from the surface Sup and reaching the lower surface Inf. The volume V2 therefore extends from the lower face Inf of the ferroelectric element, while leaving a non-implanted volume V1 on the side of the upper surface sup, so that a surface of the volume V1 forms a face (Sup) of the ferroelectric layer. In (C2), the enrichment in hydrogen at an adequate concentration has affected the entire volume of the ferroelectric element 10, the polarization inversion is effective over the entire thickness of the crystal so that the volume V1 disappears, entirely replaced by the volume V2. These variants can be obtained by using hydrogen ion implantation sequences of suitable parameters.

[0036] As already mentioned, this first embodiment is illustrative and is not limited to controlling the polarization of a simple ferroelectric crystal, but can also be applied to more complex structures such as a ferroelectric crystal attached to a support, as will become apparent in the remainder of this presentation. Integration on a support – Solution 1

[0037] The method of selective polarization inversion according to the depth in the ferroelectric layer described above is illustrated in Figures 1 and 2 in the situation where it is applied to a single ferroelectric crystal, used as the ferroelectric element 10. This method of selective inversion according to the depth can also be applied to an assembly formed by a ferroelectric crystal integrated on a substrate, this assembly then being able to constitute the ferroelectric element 10 illustrated by Figures 1 and 2.

[0038] Integration on a substrate can be carried out as explained below using, and with reference to, publication WO 2020 / 200986 A1 which describes in particular the transfer of a single-domain ferroelectric thin layer onto a substrate.

[0039] Larepresents a method of manufacturing an integrated Struct structure illustrated in (D), comprising a ferroelectric layer Ferro lay fixed on a Sprt support with an electrically conductive metal layer M1 interposed between these two elements. In this example, the metal layer M1 is in direct contact with each of the Sprt support and the ferroelectric layer Ferro lay . The M1 layer can subsequently be used as a buried electrode intended to apply an electric field to the ferroelectric layer Ferro layThe presence of a metallic M1 layer is only an option and applications other than those in this example do not require the presence of such a layer.

[0040] Conventionally, the Struct structure may be in the form of a circular plate whose diameter may be 100, 200, 300 or even 450 mm, but the invention is in no way limited to these dimensions or this shape.

[0041] The ferroelectric layer Ferro layis made of a monocrystalline ferroelectric material, such as lithium tantalate LiTaO3 or lithium niobate LiNbO3, or materials such as LiAlO3, BaTiO3, PbZrTiO3, KNbO3, BaZrO3, CaTiO3, PbTiO3 or KTaO3. These materials also have piezoelectric properties. Generally speaking, the ferroelectric layer may have a thickness of between 10 nanometers and 10 microns, depending on the intended application of the Struct structure and the expected performance of the components, but the invention does not exclude the use of different thicknesses, always depending on the intended application. It is recalled that a ferroelectric material is a material which has an electric polarization in its natural state, a polarization which can be reversed by the application of an external electric field greater than the coercive field of the material.As illustrated in this document, the ferroelectric layer preferably has a first single-domain P1 polarization, i.e. all the dipole moments are aligned parallel to each other along a given direction. Here, the given direction is perpendicular to the plane of the ferroelectric layer, i.e. perpendicular to the free face of this layer, or perpendicular to a surface plane of this layer.

[0042] For reasons of availability and cost, the Sprt support is preferably chosen, for its part, in silicon. It can be a support consisting of a solid base substrate in monocrystalline silicon, but the invention is not limited to this support which can, more generally, be made of any material, for example silicon, even electrically insulating such as sapphire or glass. The Sprt support, when it is formed of a solid substrate, typically has a thickness of several hundred microns. In monocrystalline silicon, the Sprt support is electrically conductive, but, preferably, has a high resistivity, greater than 1000 ohms. This limits the density of charges, holes or electrons, which are likely to move, which could affect the proper functioning of an RF component which would be formed on the basis of the Struct structure.But the invention is not limited to a support having such characteristics.

[0043] The metal layer may have a thickness of between a few nanometers and several microns, for example 100 nm or more. It may typically be formed of a metal such as chromium, nickel, aluminum, platinum, titanium, tungsten, gold or any combination of these elements with each other or with other metallic elements, in a single layer or in a combination of layers.

[0044] With reference to the, the structure Struct can be produced by a layer transfer manufacturing process comprising: the preparation of the support Sprt illustrated in (A) with here the formation of the metallic layer M1 on its surface; the possible preparation of a ferroelectric donor substrate Ferro sub illustrated in (B); the assembly of a first face of the Sprt support and a face of the ferroelectric donor substrate Ferro sub in order to constitute an intermediate structure Struct inter illustrated in (C) with the metal layer M1 interposed between the Sprt support and the donor face of the ferroelectric substrate Ferro sub ; and the detachment of a portion of the Ferro donor substrate sub of the intermediate structure to define the ferroelectric layer Ferro lay on the Sprt support and obtain the Struct structure illustrated in (D).

[0045] The Ferro donor substrate sub illustrated in (B) is a substrate made of the ferroelectric material of the ferroelectric layer Ferro lay , or comprising a surface thickness of this material. Thus, the donor substrate may, for example, be formed from a solid substrate of lithium tantalate or lithium niobate, or from a composite substrate formed from a first substrate on which rests a thickness (at least equal to that of the Ferro layer lay) of lithium tantalate or lithium niobate. The donor substrate preferably has a first monodomain polarization P1 perpendicular to the implantation face Imp through which hydrogen ions H + will be implanted into the donor substrate for a subsequent detachment step. This orientation is conventionally done by choosing the crystal growth mode and its cutting plane.

[0046] The ferroelectric layer Ferro lay can be transferred from the ferroelectric donor substrate Ferro sub by implementing Smart Cut technology TM , in which case the donor substrate must be prepared by introducing light species(s) such as hydrogen or helium into this donor substrate. This introduction may correspond to a hydrogen implantation, i.e., an ion bombardment of hydrogen of the flat face Imp of the Ferro donor substrate sub. In a manner known per se, and as illustrated in (B), the hydrogen ions H + implanted aim to form a weakening plane Frgl delimiting the ferroelectric layer Ferro lay of ferroelectric material to be transferred which is located on the Imp face side and another Ferro part sep forming the rest of the substrate and which will be separated from the ferroelectric layer Ferro lay at a later stage.

[0047] The nature, the dose of the implanted species and the implantation energy are chosen according to the thickness of the layer that one wishes to transfer and the physicochemical properties of the Ferro donor substrate. sub . In the case of a LiTaO3 donor substrate, we can choose to implant a dose of hydrogen between 10 16 and 5.10 17 at / cm² with an energy between 30 and 300 keV to delimit a ferroelectric layer Ferro layof the order of 200 to 2000 nm in thickness.

[0048] Following the preparations of the Sprt support and the Ferrosub donor substrate, including the formation of the M1 metal layer on the support or the donor substrate, these two elements are assembled by bringing them into contact, so that the M1 metal layer is interposed between the Sprt support and the Ferrosub ferroelectric layer. lay , the latter being located opposite the Sprt support, so as to obtain the intermediate structure Struct inter illustrated in (C) of the. The Sprt support substrate may have the same size and shape as the Ferro donor substrate sub , but the invention is not limited to such a configuration and different dimensions, shapes and configurations may be employed.

[0049] Prior to assembly, it may be considered to prepare the faces of the substrates to be assembled by a cleaning, brushing, drying, polishing step, or plasma activation.

[0050] The assembly may correspond to the intimate contact of the Ferro donor substrate sub with the Sprt support by molecular adhesion and / or electrostatic bonding, as mentioned for example in the French patent application published under No. 2,914,492.

[0051] As is well known, during a molecular adhesion process, the exposed surfaces of the Sprt support and the Ferro donor substrate sub , perfectly clean, flat and smooth, are brought into intimate contact to promote electrostatic bonding or the development of molecular bonds, for example van der Waals or covalent type. The assembly of the two bodies is then obtained without the use of an adhesive.

[0052] The assembly may include the application of a low temperature heat treatment (e.g., 50 to 300°C, typically 100°C) to cure crystalline defects present in the ferroelectric layer and to sufficiently enhance the bonding energy to allow a possible subsequent thinning step.

[0053] In the present embodiment, the step of detaching a portion of the donor substrate is performed by applying Smart Cut™ technology, according to which a layer intended to form the ferroelectric layer Ferro lay is delimited by the weakening plane Frgl. After the assembly step, this layer is detached from the donor substrate by fracture at the weakening plane Frgl and thus transferred to the Sprt support.

[0054] This detachment step can thus include the application to the intermediate structure Struct interof a heat treatment in a temperature range of the order of 80°C to 300° to allow the detachment of the part of the donor substrate from the ferroelectric layer Ferrol ay and the transfer of this onto the support substrate Sprt. As a replacement or in addition to the heat treatment, this step may include the application of a blade or a jet of gaseous or liquid fluid, or any other mechanical force at the level of the embrittlement plane Frgl.

[0055] As an alternative to implementing the Smart Cut process TM detailed above, the step of detaching a part of the donor substrate can be replaced by a step of mechano-chemical thinning of this donor ferroelectric substrate Ferro sub .

[0056] Whether the removal of part of the thickness of the donor substrate is achieved by thinning or by fracture, any type of finishing treatment can be applied to the structure Struct thus formed to shape the ferroelectric layer Ferro lay to specifications of thickness, thickness uniformity, roughness, crystal quality or any other type of specifications.

[0057] In particular, with a view to using a ferroelectric layer Ferro lay which would have a single-domain polarization, a special treatment can be applied to the ferroelectric layer Ferrolay transferred onto the substrate Subst, all with the aim of obtaining a single-domain polarization of the ferroelectric layer Ferro lay .

[0058] For example, we can apply to the ferroelectric layer Ferro lay a preparatory heat treatment followed by a thinning step.

[0059] The preparative heat treatment helps to cure crystal defects present in the ferroelectric layer. In addition, it can also help to consolidate the bonding between this ferroelectric layer Ferro layand the Sprt support. It also has the effect, if it has a sufficient temperature, of causing the diffusion of the hydrogen contained in the ferroelectric layer and the multidomain transformation of a surface portion of this ferroelectric layer. This surface portion may have a thickness of the order of 50 nm or less and develop over the entire extent of the ferroelectric layer. At the end of the preparation heat treatment, the ferroelectric layer has a relatively constant hydrogen concentration in its thickness. In the case of LiTaO3, this preparation heat treatment is intended to bring the ferroelectric layer to a temperature between 300°C and the Curie temperature of the ferroelectric material (and preferably greater than or equal to 450°C, 500° or 550° to promote the diffusion of hydrogen) for a period of between 30 minutes and 10 hours.This heat treatment is preferably carried out by exposing the free face of the dielectric layer to an oxidizing or neutral gas atmosphere, i.e. without covering this face of the thin layer with a protective layer which could prevent the exodiffusion of hydrogen.

[0060] Following the preparatory heat treatment, the ferroelectric layer is thinned. This thinning may correspond to the polishing of the free face of the ferroelectric layer Ferro lay , for example by mechanical, chemical-mechanical and / or chemical etching thinning techniques. It allows the free face to be prepared so that it has a low roughness, for example less than 0.5nm RMS 5x5 µm by atomic force measurement (AFM) and to remove the multi-domain surface portion of the ferroelectric layer Ferro lay. Typically, 50 to 300 nm of thickness removal is expected to achieve the target ferroelectric layer thickness. Ferro lay , and in all cases a thickness greater than that of the multi-domain surface portion. This creates a single-domain thin layer with the required qualities of surface state, crystalline quality and polarization.

[0061] Following the operations described above, a selective polarization inversion as illustrated by figures 1 and 2 applied to the structure Struct illustrated in (D) of the allows the structure Struct illustrated in (D) of the to be formed. Integration on a support – Solution 2

[0062] Solution 1 illustrates a process for assembling a Ferro donor substrate subpreferably having a first single-domain polarization P1 perpendicular to a planar face of the substrate. Subsequently, a selective polarization inversion can be applied to the ferroelectric layer Ferro lay of the Struct structure obtained to arrive at two vertical polarizations of opposite directions within this ferroelectric layer.

[0063] This section presents an alternative solution 2 to solution 1 in that instead of applying selective polarization reversal after assembly, selective polarization reversal is applied to a donor ferroelectric substrate before assembly, as illustrated by.

[0064] Thus, unless otherwise indicated, one can refer to solution 1 for all the steps of the assembly process illustrated by the, including the alternative by thinning the Ferro donor substrate subrather than by fracture of it. On the other hand, in this solution 2, the donor substrate Ferro sub has two vertical polarizations P1 and P2 of opposite directions. Such a substrate can for example be obtained from a single-domain polarization substrate by selective polarization inversion according to the depth as illustrated by figures 1 to 2 and the associated manufacturing process.

[0065] This solution makes it possible to obtain a ferroelectric layer Ferro lay whose surface polarization is controlled without having to use high-energy implantation

[0066] This solution also makes it possible to determine which of the volume V1 and the volume V2 will be on the surface of the ferroelectric layer Ferro layin the structure illustrated in (D) of the. Indeed, depending on the location in depth of the weakening plane Frgl, it could be found either (1) in the volume V1 as illustrated by the, or (2) in the volume V2 (situation not illustrated). In the first case, it is the volume V1, and therefore the polarization P1, which will be found on the free surface of the ferroelectric layer Ferro lay , the volume V2 being buried under the volume V1, as illustrated by the. In the second case, it is the volume V2, and therefore the polarization P2, which will be found on the free surface of the ferroelectric layer Ferro lay , volume V1 being buried under volume V2.

[0067] Alternatively, from the structure illustrated in (D) of the, a chemical-mechanical polishing step applied to the ferroelectric layer Ferro layallows it to be thinned, possibly to the point of exposing the volume V2, so that its surface constitutes the free face of the ferroelectric layer Ferro lay . Variants

[0068] The above examples illustrated by Figures 3 and 4 explicitly include only one metal layer M1, located between the Sprt support and the ferroelectric layer Ferro lay . Optionally, the structure may include an additional metal layer or optional dielectric layers at the interface between the Sprt support and the ferroelectric layer Ferro lay .

[0069] Illustrates a Struct structure comprising a second metal layer M2 located on the surface of the ferroelectric layer, the ferroelectric layer Ferro laybeing interposed between the two metal layers M1 and M2. This configuration can be used in the context of a bulk acoustic wave device, the metal layers M1 and M2 serving as electrodes for the application of an electric field to the ferroelectric layer Ferro lay .

[0070] Laillustrates a structure Struct further comprising a first dielectric layer such as an oxide layer Ox1 interposed between the substrate Sprt and the first metal layer M1. It may for example be the result of a manufacturing process of the structure Struct in which the metal layer M1 is formed on the dielectric layer Ferro lay , the dielectric layer Ox1 is formed on the support Sprt, then the ferroelectric layer Ferro layis assembled to the Sprt support by bringing the metal layer into intimate contact with the dielectric layer, according to a process similar to that explained in relation to the, with here an oxide-metal contact.

[0071] Laillustrates a Struct structure similar to that of the, further comprising a second dielectric layer such as an oxide layer Ox2 interposed between the first metal layer M1 and the first dielectric layer Ox1. It may for example be the result of a manufacturing process of the Struct structure in which the metal layer M1 and the second dielectric layer Ox2 are formed successively and in this order on the dielectric layer Ferro lay , the dielectric layer Ox1 is formed on the support Sprt, then the ferroelectric layer Ferro layis assembled to the Sprt support by bringing the two dielectric layers Ox1 and Ox2 into intimate contact, according to a process similar to that explained in relation to the, with here a dielectric-dielectric contact, and more specifically oxide-oxide in this example.

[0072] The two methods explained above with respect to Figures 6 and 7 are only two examples, and other methods are conceivable, leading to the same results or to other variants of the Struct structure. It is thus possible to form a Bragg mirror adjacent to the ferroelectric layer Ferrolay, preferably between this layer and the support Sprt, for example under the metal layer M1, by forming a stack of pairs of layers comprising a layer of SiO2 and a layer of HfSO2 or a layer of SiO2 and a layer of Mo or other structures and materials known for forming Bragg mirrors. The metal layer M2 may have the same composition as the layer M1 or may have a different composition. The oxide layers Ox1 and Ox2 may be, for example, layers of silicon oxide or silicon nitride oxide.

[0073] In addition, the Sprt support can be prepared by adding a dielectric layer that can be formed from a stack of dielectric layers of different types. The dielectric layer can be produced directly on the Sprt support using various techniques known in the state of the art, such as oxidation or nitriding heat treatments, chemical deposition using techniques called LPCVD (acronym for "Low Pressure Chemical Vapor Deposition" or chemical vapor deposition at subatmospheric pressure) or PECVD (acronym for "Plasma Enhanced Chemical Vapor Deposition" or plasma-assisted chemical vapor deposition) or PVD (Physical Vapor Deposition) or ALD (Atomic Layer Deposition).

[0074] The assembly of the support with the ferroelectric layer can cause an accumulation of hydrogen at the interface between them, constituting a hydrogen concentration gradient making possible the multidomain transformation of a portion of the ferroelectric layer near this interface, for example during the heat treatment of separation of the ferroelectric layer Ferro lay of Ferro ferroelectric substrate sub .

[0075] It can be ensured that the metallic layer and the optional dielectric layers adjacent to the ferroelectric layer Ferro lay have a lower hydrogen concentration than that present in the ferroelectric layer Ferro lay , so that excess hydrogen in the ferroelectric layer can be absorbed into these adjacent layers during diffusion caused by the heat treatment of Ferro layer separation sepThis prevents the accumulation of hydrogen at the assembly interface and avoids a multidomain transformation in the portion of the ferroelectric layer near this interface.

[0076] The hydrogen concentration of the metal layer and the optional dielectric layers can be reduced for example by means of an annealing step which aims to bring these adjacent layers to a temperature higher than that of the heat treatment for preparing the ferroelectric layer Ferro lay reported, which will be presented later in this description. This layer can thus be brought to an annealing temperature of 600°C, 700°C, or even 800°C or more. The average hydrogen concentration in the dielectric layer, after this exodiffusion step, can thus be lower 5.10 20 at / cm^3, or advantageously less than 10 18 at / cm 3 .

[0077] The examples illustrated in this description are limited to a continuous volume V2 forming a single layer of reversed polarization in the thickness of the ferroelectric layer, but it is possible to produce a pair of such volumes V2 extending respectively from two opposite surfaces of a piezoelectric layer, a volume V1 of non-reversed polarization and therefore opposite to that of the two volumes V2 being interposed between these two volumes V2. The two distinct volumes V2 can be formed by carrying out several successive implantations of hydrogen ions of different acceleration energies and therefore of different implantation depths, so as to define two volumes V2 each open on a surface of the ferroelectric layer and of thicknesses depending on the associated implantation energies. It is thus possible to obtain an alternation of opposite polarizations according to the direction of thickness of the ferroelectric layer.

[0078] Application examples

[0079] Each of the structures illustrated in Figures 5, 6 and 7 may constitute a bulk acoustic wave device or be integrated into such a device forming, for example, a BAW bandpass filter.

[0080] Numerical simulations have determined that such a device would exhibit reduced frequency sensitivity to thickness and improved power handling, with the energy density in the layer being distributed across the entire thickness of the ferroelectric layer.

[0081] The figures in this document are not necessarily to scale. Some features and components may be shown exaggerated in relation to other components or in a somewhat schematic form, and some details of conventional items may not be shown in the interest of clarity and conciseness.

[0082] Of course, the invention is not limited to the embodiments described and variant embodiments can be made without departing from the scope of the invention as defined by the claims.

Claims

Structure (Struct) comprising a ferroelectric layer (Ferro lay ) having a first polarization (P1) in a first volume (V1) and a second polarization (P2), opposite to the first polarization, in a second volume (V2) separate from the first volume,characterized in that the first polarization and the second polarization are oriented perpendicularly or obliquely to the ferroelectric layer, a surface of the first volume (V1) forms a face (Sup) of the ferroelectric layer (Ferro lay ) and the first volume (V1) is interposed between this face of the ferroelectric layer and the second volume (V2), in which the second volume (V2) has a hydrogen concentration, called polarity inversion concentration, higher than the first volume (V1), and in which the polarity inversion concentration is between 10 19 and 10 22 hydrogen atoms per cubic centimeter. Structure according to claim 1, further comprising a support (Sprt) and a metal layer (M1) interposed between this support and the ferroelectric layer. Structure according to any one of claims 1 to 2, in which the ferroelectric layer is formed from a single crystal. A structure according to any one of claims 1 to 3, wherein the ferroelectric layer comprises lithium niobate or lithium tantalate. A structure according to any one of claims 2, wherein the support comprises monocrystalline silicon. Method for manufacturing a ferroelectric element (10), comprising the steps of:- providing a ferroelectric element (10, Ferrolay) having a first polarization (P1);- enriching a given buried volume (V2) of the ferroelectric element with hydrogen ions (H+) by ion implantation of the hydrogen ions (H+) through a face (Sup) of the ferroelectric element, a first volume (V1) of the ferroelectric element (10) being interposed between the given buried volume (V2) and the face (Sup) of the ferroelectric element; and- applying an annealing to the ferroelectric element after introduction of the hydrogen ions, at a temperature between 500°C and 700°C, so as to switch the first polarization (P1) of the ferroelectric element in the given volume (V2) towards a second polarization (P2) opposite in orientation to the first polarization (P1), the first volume (V1) having, after the annealing, the first polarization (P1). Method according to claim 6, in which the first polarization (P1) is single-domain, perpendicular or oblique relative to a face (Sup) of the ferroelectric element. A method according to claim 6 or 7, wherein, during the selective enrichment of hydrogen ions, a hydrogen ion (H+) implantation dose is adapted so as to obtain a hydrogen concentration of between 10 19 and 10 22 hydrogen atoms per cubic centimeter in the given volume (V2). A method according to any one of claims 6 to 8, wherein the ion implantation of hydrogen ions is carried out at an energy between 3 keV and 210 keV. A method according to any one of claims 6 to 9, wherein the ferroelectric element (10, Ferro lay ) is a layer of lithium niobate or lithium tantalate. The method of any one of claims 6 to 10, further comprising assembling a ferroelectric (Ferro sub ) and a support (Sprt), followed by separation or thinning of the ferroelectric substrate so as to define the ferroelectric element (Ferro lay ). The method of claim 11, wherein the support (Sprt) is a monocrystalline silicon substrate, and a metal layer (M1) is interposed between this monocrystalline silicon substrate and the ferroelectric substrate (Ferro sub ).

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