INTEGRATED STRUCTURE ON A SUBSTRATE, COMPRISING A FERROELECTRIC LAYER WITH SELECTIVE POLARIZATION THROUGH ITS THICKNESS, AND MANUFACTURING METHOD

Abstract

Structure comprising a ferroelectric layer (Ferrolay) 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) distinct from the first volume. The first and second polarizations are oriented perpendicularly or obliquely to the ferroelectric layer. A surface of the first volume (V1) forms a face (Sup) of the ferroelectric layer (Ferrolay), and the first volume (V1) is interposed between this face of the ferroelectric layer and the second volume (V2). The structure further comprises a support (Sprt) and a metallic layer (M1) interposed between this support and the ferroelectric layer. Figure to be published with the abbreviation: Fig. 5

Description

Title of the invention: Integrated structure mounted on a substrate, comprising a ferroelectric layer with selective polarization throughout 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, radio frequency (RF) components, in particular bulk elastic wave components. TECHNOLOGICAL BACKGROUND

[0002] Several types of applications utilize or are influenced by the polarization properties of ferroelectric materials and the presence of opposing polarization domains. Examples include surface acoustic wave (SAW) devices and bulk acoustic wave (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 integrated onto 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 could cause the silica layer to break down.

[0004] Document EP 0 592 226 A1 describes an optical frequency conversion device obtained by the periodic juxtaposition of parallel bands of inverted polarization on 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 polarization inversion along the bands is achieved by performing a proton exchange through a mask.

[0005] Document WO 2005 / 052682 Al 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, while they do allow control of the polarization domains of a ferroelectric layer at its surface, remain impractical and do not allow control of the polarization in the thickness of this layer and its integration on a support. Description of the invention

[0007] A first object of the invention is to provide a ferroelectric layer, optionally in an integrated form on a substrate, with polarization having polarization domains of opposite orientations and perpendicular or oblique to the plane of this layer, the domains being distributed throughout the thickness of the ferroelectric layer, the latter being suitable for use, for example, in applications based on body wave acoustics. 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 locally and controlled polarization inversion of any ferroelectric element throughout its thickness.

[0008] In view of achieving 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.

[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 extension planes 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 features of the first aspect of the invention, considered individually or in any technically feasible combination: - the structure may further include a support and a metallic layer interposed between this support and the ferroelectric layer; - the second volume may exhibit a concentration of hydrogen, known as polarity reversal concentration, higher than the first volume; - the first volume may have a higher concentration of hydrogen, known as the polarity reversal concentration, than the second volume; - the polarity reversal concentration can be between 1019 and 1022 hydrogen atoms per cubic centimeter; - the ferroelectric layer can be formed from a single crystal; - the ferroelectric layer may include lithium niobate or lithium tantalate; and - the support may include monocrystalline 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 through a face of the ferroelectric element; and applying an anneal to the ferroelectric element after the introduction of the hydrogen ions, at a temperature between 500°C and 700°C, so as to switch the first polarization of the ferroelectric layer in the given volume to a second polarization opposite in orientation to 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 in the semiconductor industry. Furthermore, it allows for the control of the polarization of a ferroelectric element throughout its thickness and is applicable to thin films integrated onto a substrate.

[0013] According to additional, non-limiting features 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 with respect to a face of the ferroelectric element; - During the selective enrichment of hydrogen ions, a hydrogen concentration of between 10¹⁹ and 10²² hydrogen atoms per cubic centimeter can be used - Hydrogen ion implantation can be implemented at an energy between 3 keV and 210 keV; - the ferroelectric element can be a layer of lithium niobate or lithium tantalate; - the process may further comprise the assembly of a ferroelectric substrate and a support, followed by separation or thinning of the ferroelectric substrate so as to define the ferroelectric element; and - the support can be a monocrystalline silicon substrate, and a metallic layer can be interposed between this monocrystalline silicon substrate and the ferroelectric substrate. BRIEF DESCRIPTION OF THE FIGURES

[0014] Other features and advantages of the invention will become apparent from the detailed description of the invention which follows with reference to the accompanying figures in which:

[0015] [Fig.1] Fig.1 represents a ferroelectric element of selectively modified polarization in its thickness and a corresponding manufacturing process according to the present description;

[0016] [Fig.2] The [Fig.2] represents variants of the ferroelectric element of the [Fig.1];

[0017] [Fig.3] Fig.3 illustrates a first method of integrating a ferro- layer electric on a support;

[0018] [Fig.4] Fig.4 illustrates a second method of integrating a ferroelectric layer onto a support;

[0019] [Fig.5] The [Fig.5] is a schematic cross-sectional view of a device according to the present description;

[0020] [Fig.6] [Fig.6] is a first variation of the device of [Fig.5]; and

[0021] [Fig.7] The [Fig.7] is a second variation of the device of the [Fig.5]. DETAILED DESCRIPTION OF THE INVENTION

[0022] Depth-dependent selective polarization inversion

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

[0024] The ferroelectric element 10 can be a ferroelectric single crystal as such, this single crystal can be in the form of a ferroelectric single crystal layer, this ferroelectric crystal or this ferroelectric single crystal layer can be fixed on a Sprt support within a Struct structure, such as the ferroelectric layer Ferroiay illustrated in (D) of [Fig.3].

[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] Fig. 1 represents in (A) a cross-section, along a plane ZX, of the ferroelectric element 10 having two opposite faces Sup and Inf perpendicular to a direction Z and exhibiting a first single-domain PI polarization parallel to the direction Z and perpendicular to the upper face Sup and lower face 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 the sake of clarity, this embodiment uses the example of a polarization perpendicular to the plane of extension of the ferroelectric element 10. However, the invention also applies to situations where the polarization is not perpendicular to the plane of extension of the ferroelectric element 10 but oblique to it. The characteristic "perpendicular" means an inclination of 90° to the plane of extension of the ferroelectric element, to within 10°, while the characteristic "oblique" or "obliquely" means an inclination at least 5° away from the plane of extension of the ferroelectric element. Thus, an oblique direction can be considered to have an inclination within the range of 5° to 80°, or 30° to 70°, or 35° to 60°.For example, a 50RY LiTaO3 layer and a 42RY LiTaO3 layer exhibit inclined polarizations of 50° and 42° with respect to the layer's extension plane, respectively, therefore oblique polarizations with respect to the layer's extension plane.

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

[0029] As illustrated in [Fig. 1] (B), this step S10 results in the formation of a volume V2 (indicated by hatching in the figures) relatively rich in hydrogen within the ferroelectric element 10, directly beneath the upper surface Sup and over a volume VI (white areas of the ferroelectric element 10) relatively poor in hydrogen, and whose depth depends on the acceleration energy of the implantation step. Volume V2 is thus interposed between the unimplanted volume VI and the upper surface Sup of the ferroelectric element 10. The implantation dose is adjusted to the implantation energy to obtain a hydrogen density of between 10¹⁹ and 10²² atoms / cm³ in volume V2. It is possible to perform several successive implantations at different implantation energies and doses in order to better define volume V2 by homogenizing the distribution of the implanted hydrogen within 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 unimplanted volume V1 still forming a single monodomain in which the polarization has only one orientation.

[0031] By way of example, a dose of implanted hydrogen ions between 10¹⁴ and 10¹⁵ at / cm² accelerated by a voltage of 6 kV can be used to create a volume V² of polarization reversal extending over approximately 50 nm in the Z direction of thickness, volume V2 in which the hydrogen concentration is between 1019 and 1022, preferably between 5.1019 and 2.1021 at / cm3. In all cases, the hydrogen-enriched volume V2 has a higher hydrogen concentration, called the polarity reversal concentration, than the first volume VI.

[0032] In this document, the YX and ZX planes are defined by the X and YZ axes of an orthogonal coordinate system, with the YX plane defined by the Y and X axes of the system, and the ZX plane defined by the Z and X axes of the system. 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 the thickness of the ferroelectric element 10. The term "depth" refers to a distance along the Z axis from the upper surface Sup. Furthermore, a modulation of the polarization or hydrogen concentration of the ferroelectric element 10 with respect to its thickness is understood, respectively, as variations in hydrogen concentration or changes in the direction of polarization within the volume of this element along the Z direction.

[0033] In a step S20, the ferroelectric element 10, selectively enriched with hydrogen, is subjected to annealing at a temperature between 500°C and 800°C, preferably between 500°C and 700°C, and more preferably between 550°C and 600°C. This annealing step results in a reversal of the first polarization PI of the ferroelectric element 10 only in the hydrogen-enriched volume V2, giving a second polarization P2, with the same alignment direction as the first polarization PI but in the opposite direction, as illustrated in (Co). Antiparallel polarizations are defined as polarizations aligned along parallel directions but in opposite directions. Conversely, parallel polarizations are defined as polarizations aligned along parallel directions but in the same direction.Defining volume V2 by hydrogen enrichment allows for the selectivity of the polarization reversal, which propagates from the upper surface Sup throughout the entire volume V2, thus changing from a parallel polarization between volumes VI and V2 to an antiparallel polarization between volumes VI and V2. Annealing is preferably carried out under an oxygen atmosphere to reduce oxygen exodiffusion of the ferroelectric layer, but can also be done, 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 as a result of step S10 is necessary for the polarization reversal obtained in step S20, and can therefore be described as the polarization reversal concentration.

[0035] Figure 2 illustrates in (Cl) and (C2) variants for the volume V2, obtained by adjusting the number of implantations and their respective energies and doses. In (Cl), the implantation energy was sufficient to enrich a portion of the ferro- element The electric ion 10 extends away from the surface (Sup) and reaches the lower surface (Inf). The volume V2 thus extends from the lower face (Inf) of the ferroelectric element, while leaving an unimplanted volume VI on the side of the upper surface (Sup), so that a surface of volume VI forms a face (Sup) of the ferroelectric layer. In (C2), hydrogen enrichment at a suitable concentration has affected the entire volume of the ferroelectric element 10; the polarization reversal is effective over the entire thickness of the crystal, so that volume VI disappears, entirely replaced by volume V2. These variants can be obtained by employing hydrogen ion implantation sequences with 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 mounted on a support, as will become apparent later in this discussion. Integration on a support - Solution 1

[0037] The depth-selective polarization inversion process 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 depth-selective inversion process can also be applied to an assembly consisting of a ferroelectric crystal integrated on a substrate, this assembly then constituting the ferroelectric element 10 illustrated by Figures 1 and 2.

[0038] Integration on a substrate can be carried out as explained below with reference to [Fig.3], and with reference to publication WO 2020 / 200986 Al which describes in particular the transfer of a single-domain ferroelectric thin film onto a substrate.

[0039] Figure 3 represents a method for manufacturing an integrated Struct structure illustrated in (D), comprising a ferroelectric layer Ferroiay fixed to a support Sprt with an electrically conductive metallic layer Ml interposed between these two elements. In this example, the metallic layer Ml is in direct contact with both the support Sprt and the ferroelectric layer Ferroiay. The layer Ml can subsequently be used as a buried electrode to apply an electric field to the ferroelectric layer Ferroiay. The presence of a metallic layer Ml is only an option, and applications other than those in this example do not require such a layer.

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

[0041] The Ferroiay ferroelectric layer is made of a single-crystal ferroelectric material, such as lithium tantalate LiTaO3 or lithium niobate LiNbO3, or materials such as LiA1O3, BaTiO3, PbZrTiO3, KNbO3, BaZrO3, CaTiO3, PbTiO3 or KTaO3. These materials also exhibit piezoelectric properties. Generally, the ferroelectric layer can have a thickness 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 preclude the use of different thicknesses, again depending on the intended application. It should be noted that a ferroelectric material is a material that possesses an electrical polarization in its natural state, a polarization that 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 exhibits a first single-domain PI polarization, meaning that all the dipole moments are aligned parallel to each other in a given direction.Here, the given direction is perpendicular to the plane of the ferroelectric layer, that is, 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 made of silicon. It may be a support consisting of a solid base substrate of monocrystalline silicon, but the invention is not limited to this support, which may more generally be made of any material, for example silicon, even an electrically insulating material such as sapphire or glass. The Sprt support, when formed from 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, that are likely to move, which could affect the proper functioning of an RF component based on the Struct structure.But the invention is not limited to a support exhibiting such characteristics.

[0043] The metallic layer can have a thickness ranging from a few nanometers to several microns, for example 100 nm or more. It can 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 [Fig. 3], the Struct structure can be produced by a layer transfer manufacturing process comprising: - the preparation of the Sprt support illustrated in (A) with here the formation of the metallic layer Ml on its surface; - the possible preparation of a ferroelectric donor substrate Ferrosub illustrated in (B); - the assembly of a first face of the Sprt support and a face of the Ferrosub ferroelectric donor substrate to constitute an intermediate Structinter structure illustrated in (C) with the metallic layer Ml interposed between the Sprt support and the donor face of the Ferrosub ferroelectric substrate; and - the detachment of part of the Ferrosub donor substrate from the intermediate structure to define the ferroelectric layer Ferroiay on the Sprt support and obtain the Struct structure illustrated in (D).

[0045] The Ferrosub donor substrate illustrated in (B) is a substrate made of the ferroelectric material of the Ferroiay ferroelectric layer, or comprising a surface layer of this material. Thus, the donor substrate may, for example, be formed from a bulk substrate of lithium tantalate or lithium niobate, or from a composite substrate consisting of a first substrate on which rests a layer (at least equal to that of the Ferroiay layer) of lithium tantalate or lithium niobate. The donor substrate preferably has a first single-domain PI polarization perpendicular to the implantation face lmp, through which hydrogen ions H+ will be implanted in the donor substrate for a subsequent detachment step. This orientation is conventionally achieved by choosing the crystal growth mode and its cutting plane.

[0046] The ferroelectric layer Ferroiay can be transferred from the ferroelectric donor substrate Ferrosub by implementing Smart Cut™ technology, in which case the donor substrate must be prepared by introducing light species such as hydrogen or helium into this donor substrate. This introduction can correspond to hydrogen implantation, i.e., hydrogen ion bombardment of the lmp face of the Ferrosub donor substrate. In a manner known per se, and as illustrated in (B), the implanted hydrogen ions H+ are intended to form a weakening plane Frgl delimiting the ferroelectric layer Ferroiay from the ferroelectric material to be transferred, which is located on the lmp face side, and another part Ferrosep forming the remainder of the substrate, which will be separated from the ferroelectric layer Ferroiay in a subsequent step.

[0047] The nature and dose of the implanted species and the implantation energy are chosen according to the thickness of the layer to be transferred and the physicochemical properties of the Ferrosub donor substrate. In the case of a LiTaO3 donor substrate, a hydrogen dose of between 10¹⁶ and 5 x 10¹⁷ at / cm² can be implanted with an energy between 30 and 300 keV to delimit a Ferroiay ferroelectric layer of the order of 200 to 2000 nm in thickness.

[0048] Following the preparation of the Sprt support and the Ferrosub donor substrate, including the formation of the metallic layer Ml on the support or the donor substrate, these two elements are assembled by bringing them into contact, so that the metallic layer Ml The metallized Ml is interposed between the Sprt support and the ferroelectric Ferroiay layer, the latter being located opposite the Sprt support, so as to obtain the intermediate Structinter structure illustrated in (C) of [Fig. 4]. The Sprt support substrate may have the same dimensions and shape as the Ferrosub donor substrate, but the invention is not limited to such a configuration and different dimensions, shapes and configurations may be used.

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

[0050] The assembly may correspond to the intimate contact of the Ferrosub donor substrate 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 Ferrosub donor substrate, which are 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 bonds. The two bodies are then joined without the use of an adhesive.

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

[0053] In this 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 Ferroiay is delimited by the embrittlement plane Frgl. After the assembly step, this layer is detached from the donor substrate by fracture at the embrittlement plane Frgl and thus transferred to the Sprt support.

[0054] This detachment step may thus include applying heat treatment to the intermediate structure Structinter in a temperature range of approximately 80°C to 300°C to allow the detachment of the donor substrate portion of the ferroelectric layer Ferrolay and its transfer to the support substrate Sprt. As an alternative to or in addition to heat treatment, this step may include the application of a blade or jet of gaseous or liquid fluid, or any other mechanical force, to the embrittlement plane Frgl.

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

[0056] Whether the removal of part of the thickness of the donor substrate is carried out by thinning or by fracturing, any type of finishing treatment can be applied to the Struct structure thus formed, enabling the ferroelectric layer Ferro lay to conform to specifications of thickness, uniformity of thickness, roughness, crystalline quality or any other type of specifications.

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

[0058] For example, a heat treatment for preparation followed by a thinning step can be applied to the Ferroiay ferroelectric layer.

[0059] The preparatory heat treatment helps to heal crystalline defects present in the ferroelectric layer. Furthermore, it can also contribute to strengthening the bond between this ferroelectric layer (Ferroiay) and the substrate (Sprt). If the temperature is sufficient, it also induces the diffusion of hydrogen contained in the ferroelectric layer and the multidomain transformation of a surface portion of this layer. This surface portion can be approximately 50 nm thick or less and extend over the entire thickness of the ferroelectric layer. Following the preparatory heat treatment, the ferroelectric layer exhibits a relatively constant hydrogen concentration throughout its thickness.In the case of LiTaO3, this preparatory heat treatment is designed 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°C, or 550°C to promote hydrogen diffusion) for a duration 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 gaseous atmosphere, that is, without covering this face of the thin film with a protective layer that could prevent hydrogen exodiffusion.

[0060] Following the heat treatment preparation, the ferroelectric layer is thinned. This thinning can correspond to polishing the free face of the Ferroiay ferroelectric layer, for example by mechanical, chemical-mechanical, and / or chemical etching techniques. It allows the free face to be prepared to have a low roughness, for example less than 0.5 nm RMS 5x5 pm by atomic force measurement (AFM), and to remove the multidomain surface portion of the Ferroiay ferroelectric layer. It is anticipated that... Generally, a removal of 50 to 300 nm of thickness is required to achieve the target thickness of the Ferroiay ferroelectric layer, and in all cases a thickness greater than that of the multidomain surface portion. This creates a single-domain thin film exhibiting the required surface condition, crystalline quality, and polarization.

[0061] Following the operations described above, a selective polarization inversion as illustrated by Figures 1 and 2 applied to the Struct structure illustrated in (D) of [Fig.3] allows the Struct structure illustrated in (D) of [Fig.4] to be formed. Integration onto a support - Solution 2

[0062] Solution 1 illustrates a method for assembling a Ferrosub donor substrate preferably having a first single-domain PI polarization perpendicular to a planar face of the substrate. Subsequently, a selective polarization inversion can be applied to the ferroelectric layer Ferroiay of the resulting Struct structure to obtain two vertical polarizations of opposite directions within this ferroelectric layer.

[0063] The present section presents an alternative solution 2 to solution 1 in that instead of applying a selective polarization reversal after assembly, a selective polarization reversal is applied to a donor ferroelectric substrate before assembly, as illustrated by [Fig.4].

[0064] Thus, unless otherwise indicated, reference can be made to solution 1 for all the steps of the assembly process illustrated in [Fig. 4], including the alternative of thinning the Ferrosub donor substrate rather than fracturing it. However, in this solution 2, the Ferrosub donor substrate exhibits 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 along the depth, as illustrated in Figures 1 to 2, and the associated manufacturing process.

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

[0066] This solution also makes it possible to determine which of volume VI and volume V2 will be located on the surface of the Ferroiay ferroelectric layer in the structure illustrated in (D) of [Fig. 4]. Indeed, depending on the depth location of the Frgl weakening plane, it can be located either (1) in volume V1 as illustrated by [Fig. 4], or (2) in volume V2 (situation not illustrated). In the first case, it is volume VI, and therefore polarization PI, that will be located on the free surface of the Ferroiay ferroelectric layer, volume V2 being buried under volume VI, as illustrated by [Fig. 4]. In the second case, it is volume V2, and therefore polarization P2, that will be located on the free surface of the Ferroiay ferroelectric layer, volume V1 being buried under volume V2.

[0067] Alternatively, starting from the structure illustrated in (D) of [Fig.4], a mechano-chemical polishing step applied to the Ferroiay ferroelectric layer makes it possible to thin it, possibly to expose the volume V2, so that its surface constitutes the free face of the Ferroiay ferroelectric layer. Variants

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

[0069] Figure 5 illustrates a Struct structure comprising a second metallic layer M2 located on the surface of the ferroelectric layer, the ferroelectric layer Ferroiay being interposed between the two metallic layers M1 and M2. This configuration can be used in a body-wave acoustic device, the metallic layers M1 and M2 serving as electrodes for applying an electric field to the ferroelectric layer Ferroiay.

[0070] Fig. 6 illustrates a Struct structure further comprising a first dielectric layer such as an Oxl oxide layer interposed between the Sprt substrate and the first metallic layer ML. II may, for example, be the result of a manufacturing process of the Struct structure in which the metallic layer Ml is formed on the Ferroiay dielectric layer, the Oxl dielectric layer is formed on the Sprt support, and then the Ferroiay ferroelectric layer is assembled to the Sprt support by bringing the metallic layer into intimate contact with the dielectric layer, according to a process analogous to that explained in relation to Fig. 3, with here an oxide-metal contact.

[0071] Figure 7 illustrates a Struct structure similar to that of Figure 6, further comprising a second dielectric layer such as an oxide layer Ox2 interposed between the first metallic layer Ml and the first dielectric layer Oxl. This may, for example, be the result of a manufacturing process for the Struct structure in which the metallic layer Ml and the second dielectric layer Ox2 are formed successively and in that order on the dielectric layer Ferroiay, the dielectric layer Oxl is formed on the support Sprt, and then the ferroelectric layer Ferroiay is assembled to the support Sprt by bringing the two dielectric layers Oxl and Ox2 into intimate contact, according to a process analogous to that explained in relation to Figure 3, with here a dielectric-dielectric contact, and more specifically an oxide-oxide contact in this example.

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

[0073] Furthermore, the Sprt support can be prepared by adding a dielectric layer which can be formed from a stack of dielectric layers of different natures. The dielectric layer can be produced directly on the Sprt support using various techniques known in the state of the art, such as heat treatments of oxidation or nitriding, chemical deposition using techniques known as LPCVD (acronym for the Anglo-Saxon expression "Low Pressure Chemical Vapor Deposition" or chemical vapor deposition at subatmospheric pressure) or PECVD (acronym for the Anglo-Saxon expression "Plasma Enhanced Chemical Vapor Deposition" or plasma-enhanced chemical vapor deposition) or even 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 Ferroiay from the ferroelectric substrate Ferrosub.

[0075] It can be ensured that the metallic layer and the optional dielectric layers adjacent to the Ferroiay ferroelectric layer have a lower hydrogen concentration than that present in the Ferroiay ferroelectric layer, so that the excess hydrogen in the ferroelectric layer can be absorbed into these adjacent layers during diffusion induced by the heat treatment separating the Ferrosep layer. This 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 metallic layer and the optional dielectric layers can be reduced, for example, by means of an annealing step which aims to raise these adjacent layers to a temperature higher than that of the heat treatment for preparing the Ferroiay ferroelectric layer, 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 less than 5 x 10²⁰ at / cm³, or advantageously less than 10¹⁸ at / cm³.

[0077] The examples illustrated in this description are limited to a continuous volume V2 forming a single layer of reversed polarization within 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, with a volume VI of non-reversed polarization, and therefore polarization opposite to that of the two volumes V2, interposed between these two volumes V2. The two distinct volumes V2 can be formed by carrying out several successive implantations of hydrogen ions with different acceleration energies and therefore different implantation depths, so as to define two volumes V2, each open onto a surface of the ferroelectric layer and with thicknesses that are functions of the associated implantation energies. Alternating polarizations of opposite polarizations can thus be obtained along the thickness direction of the ferroelectric layer.

[0078] Examples of applications

[0079] Each of the structures illustrated in Figures 5, 6 and 7 can constitute a volume 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 over the entire thickness of the ferroelectric layer.

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

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

Claims

Demands

1. Structure comprising a ferroelectric layer (Ferroiay) having a first polarization (PI) in a first volume (VI) and a second polarization (P2), opposite to the first polarization, in a second volume (V2) distinct 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 (VI) forms a face (Sup) of the ferroelectric layer (Ferroiay) and the first volume (VI) is interposed between this face of the ferroelectric layer and the second volume (V2), in which the second volume (V2) has a higher concentration of hydrogen, called the polarity reversal concentration, than the first volume (VI), and in which the polarity reversal concentration is between 1019 and 1022 hydrogen atoms per cubic centimeter.

2. Structure according to claim 1, further comprising a support (Sprt) and a metallic layer (Ml) interposed between this support and the ferroelectric layer.

3. Structure according to any one of claims 1 to 2, wherein the ferroelectric layer is formed of a single crystal.

4. Structure according to any one of claims 1 to 3, wherein the ferroelectric layer comprises lithium niobate or lithium tantalate.

5. Structure according to any one of claims 2, wherein the support comprises monocrystalline silicon.

6. A method for manufacturing a ferroelectric element (10), comprising the steps of: - provide a ferroelectric element (10, Ferroiay) exhibiting a first polarization (PI); - to enrich a given buried volume (V2) of the ferroelectric element with hydrogen ions (H+) by ionic implantation of hydrogen ions (H+) through a face (Sup) of the ferroelectric element, a first volume (VI) of the ferroelectric element (10) being interposed between the given buried volume (V2) and the face (Sup) of the ferroelectric element; and - to apply annealing to the ferroelectric element after the introduction of hydrogen ions, at a temperature between 500°C and 700°C, so as to switch the first polarization (PI) of the ferroelectric element in the given volume (V2) towards a second polarization (P2) opposite in orientation to the first polarization (PI), the first volume (VI) having, after annealing, the first polarization (PI).

7. Method according to claim 6, wherein the first polarization (PI) is single-domain, perpendicular or oblique with respect to a face (Sup) of the ferroelectric element.

8. A method according to claim 6 or 7, wherein, during the selective enrichment of hydrogen ions, a hydrogen concentration of between 1019 and 1022 hydrogen atoms per cubic centimeter is used.

9. 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.

10. A method according to any one of claims 6 to 9, wherein the ferroelectric element (10, Ferroiay) is a layer of lithium niobate or lithium tantalate.

11. A method according to any one of claims 6 to 10, further comprising the assembly of a ferroelectric substrate (Ferrosub) and a support (Sprt), followed by a separation or thinning of the ferroelectric substrate so as to define the ferroelectric element (Ferroiay).

12. Method according to claim 11, wherein the support (Sprt) is a monocrystalline silicon substrate, and a metallic layer (Ml) is interposed between this monocrystalline silicon substrate and the ferroelectric substrate (Ferrosub).

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