Method for manufacturing a ferroelectric layer transferred onto a substrate and for polarization with improved homogeneity

Abstract

A method for manufacturing a structure comprising a negatively polarized ferroelectric layer (P1), comprising the steps of: providing a ferroelectric material wafer (Ferrosub) having a polarization (-P1) oriented towards one face (Top); forming a weakening plane (Frgl) in the wafer; assembling the wafer to a support assembly (Sprt.Set); detaching a portion of the wafer so as to define a ferroelectric layer (Ferrolay) assembled to the support assembly (Sprt.Set) to obtain a structure (Struct), the ferroelectric layer having a negative polarization (P1); carrying out an additional full-field hydrogen implantation step, parameterized to correct or prevent the occurrence of polarization reversal in the volume of the ferroelectric layer and / or at its interface with the support assembly (Sprt.Ens); and applying at least one first heat treatment to the structure (Strct).Figure to be published with the abbreviation: Fig. 2.

Description

Title of the invention: Method for manufacturing a ferroelectric layer transferred onto a substrate and for improved homogeneity polarization TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to a method for manufacturing a ferroelectric layer transferred onto a substrate and polarized with improved homogeneity throughout its thickness. Such a structure can be used to form, for example, radio frequency (RF) components, in particular elastic wave components, especially surface acoustic wave (SAW) or body acoustic wave (BAW) 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 electroacoustic devices such as 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 polarization of a ferroelectric crystal with spontaneous polarization perpendicular to one of its faces, by application of an electric field according to 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 or its integration on a support.

[0007] In response to these shortcomings, international patent application WO 2020 / 200986 A1 and French patent applications FR2301220 and FR2301221 propose techniques for transferring ferroelectric layers onto a substrate. However, these techniques do not solve the problem entirely satisfactorily, in that, at the end of the manufacturing process, the transferred ferroelectric layers may exhibit uncontrolled polarization inhomogeneities within the layer thickness, particularly with regard to the transfer of negatively polarized ferroelectric layers onto the final substrate. Description of the invention

[0008] An object of the invention is to provide a method for manufacturing a ferroelectric layer transferred onto a substrate, the method correcting or preventing any inhomogeneities of polarization which could appear in their thickness, in particular for the situation where one seeks to obtain a ferroelectric layer of negative polarization, that is to say oriented towards the support of the ferroelectric layer.

[0009] With a view to the realization of these objects, an aspect of the invention is a method for manufacturing a structure comprising a negatively polarized ferroelectric layer, comprising the steps of: providing a ferroelectric material wafer having a first face and having a polarization oriented towards this face; forming a weakening plane in the ferroelectric material wafer by implanting hydrogen ions through the first face; assembling the ferroelectric material wafer comprising the weakening plane to a support assembly by bringing the first face into contact with a free surface of the support assembly;detach a portion of the ferroelectric material wafer to define a ferroelectric layer detached from the ferroelectric material wafer and assembled to the support assembly to obtain a structure formed from the ferroelectric layer and the support assembly, the ferroelectric layer having a negative polarization; proceed with an additional full-field hydrogen implantation step, parameterized to correct or prevent the occurrence of polarization reversal in the volume of the ferroelectric layer and / or at its interface with the support assembly; and apply at least one first treatment; thermal to the structure.

[0010] An advantage of the process according to the invention is its ability to produce a structure including a ferroelectric layer transferred onto a substrate and with a negative polarization, oriented towards the interface between the ferroelectric layer and its support, exhibiting improved homogeneity over its thickness compared to known manufacturing processes. Such a structure facilitates, for example, the integration of volume acoustic functions while maintaining a low cost and allowing a high level of performance.

[0011] According to additional, non-limiting features of the invention, considered individually or in any technically feasible combination:

[0012] - the additional hydrogen implantation step can be carried out after the less a first heat treatment, the process further comprising a step of applying a second heat treatment to the structure after the additional hydrogen implantation step;

[0013] - the process may further include a step of thinning the ferro- layer electrical, preferably between the application step of the first heat treatment and the application step of the second heat treatment;

[0014] - the additional hydrogen implantation step can be carried out before the minus an initial heat treatment;

[0015] - the process may further comprise a layer thinning step ferroelectric, preferably after the first heat treatment application step;

[0016] - the process may include a step of characterizing a structure of reference, an adjustment of parameters of the additional hydrogen implantation step can be defined in response to this characterization;

[0017] - the additional hydrogen ion implantation step may include a first corrective ion implantation in the ferroelectric layer, parameterized to generate a projected path of the implanted hydrogen ions such that a depth of its maximum reaches or exceeds a depth of interface between (i) a stable polarization volume of the ferroelectric layer and (ii) a reversed polarization volume due to the formation of the embrittlement plane or polarization that would reverse due to the formation of the embrittlement plane if a heat treatment were applied;

[0018] - the additional hydrogen ion implantation step may include a a second corrective ion implantation, parameterized to generate a projected path of the implanted hydrogen ions such that a depth of its maximum reaches or exceeds the depth of the interface between (i) the ferroelectric layer and (ii) the support assembly, preferably at a distance from this interface which is in less than the thickness of a polarization volume which is limited by this interface and which is multidomain or inverted with respect to a polarization of a stable polarization volume of the ferroelectric layer or which would invert or become multidomain if a heat treatment were applied;

[0019] - the additional full-field hydrogen implantation step can be parameterized in order to obtain a hydrogen concentration between 1019 and 1022 at / cm3 in a volume of the structure;

[0020] - the ferroelectric layer can extend in an extension plane and the polarization makes an angle with within an angular range from -20° to -160° with respect to the extension plane;

[0021] - the ferroelectric layer can be a lithium niobate or tantalate layer lithium. BRIEF DESCRIPTION OF THE FIGURES

[0022] 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:

[0023] [Fig.1] Fig.1 illustrates crystal sections of single crystals;

[0024] [Fig.2] [Fig.2] represents the fabrication of a structure comprising a layer ferroelectric material transferred to a support;

[0025] [Fig.3] The [Fig.3] illustrates the influence of the manufacture of the [Fig.2] on the polarization of the reported ferroelectric layer;

[0026] [Fig. 4] [Fig. 4] illustrates a polarization reversal mechanism in a layer of ferroelectric material;

[0027] [Fig.5] The [Fig.5] illustrates a method for correcting the polarization of a ferroelectric material by means of a first ion implantation;

[0028] [Fig.6] The [Fig.6] illustrates a method for correcting the polarization of a ferroelectric material by means of a second ion implantation;

[0029] [Fig. 7] Fig. 7 illustrates a structure comprising a ferroelectric layer polarization shift with improved homogeneity;

[0030] [Fig.8] Fig.8 illustrates a first manufacturing process according to the invention;

[0031] [Fig.9] Figure [Fig.9] illustrates a negative orientation polarization; and

[0032] [Fig. 10] The [Fig. 10] illustrates a second manufacturing process according to the invention. DETAILED DESCRIPTION OF THE INVENTION First embodiment of the invention

[0033] Depending on the piezoelectric device to be produced, it is necessary to choose an orientation for the polarization of the active layer of piezoelectric material, which consists of a layer of single-crystal ferroelectric material.

[0034] This ferroelectric material can be obtained by crystal growth according to the The so-called "Czochralski" method allows for the production of a solid single-crystal material in the form of an elongated cylinder, referred to as a "ball" or "ingot," by drawing a crystal seed of chosen orientation immersed in a molten material corresponding to the crystal to be formed, along a specific drawing direction. The single crystal grows along this drawing direction. Plates of the ferroelectric material are then obtained by cutting the ingot at a predetermined angle relative to its growth direction, and subsequently prepared and integrated as a ferroelectric layer in, for example, a piezoelectric device.

[0035] The polarization orientation of the layer depends on the crystal orientation of the cut wafer, which does not generally correspond to the growth direction of the single crystal. Of course, a person skilled in the art knows how to choose the most suitable seed crystal, close to the desired crystal orientation for ingot production, in order to reduce material loss during cutting and machining into circular wafers.

[0036] Figure 1 illustrates in (A) and (B) the orientations of LiTaO3 layers designated 42RY and -138RY, respectively. These designations are derived from the identification of the crystallographic orientation of these layers, corresponding to rotations of 42° and -138° of the y and z axes with respect to the counterclockwise direction around the x-axis, giving the x', y', and z' axes. The x, y, and z axes correspond to the proper crystallographic axes of the single crystal, and the y' axis is aligned with the direction normal to the cut and machined surface. More specifically, the x, y, and z axes correspond to the

[1120] ,

[0110] , and

[0001] axes, respectively.

[0037] After drawing along a chosen direction, it is known to those skilled in the art to achieve polarization by applying an electric field in the z direction when passing into temperature below the Curie temperature to determine the direction of polarization.

[0038] Conventionally, a so-called negative polarization is a polarization with an orientation directed towards the interior of the material or structure concerned, or in other words, the projection of the z-axis onto the y'-axis is negative.

[0039] Conventionally, for a ferroelectric layer fixed to a substrate, a layer whose polarization orientation is directed towards the free surface of the layer will be designated as a positive polarization layer.

[0040] Conversely, still for a ferroelectric layer fixed to a substrate, a layer whose polarization orientation is directed inwards, that is to say towards the interface between the layer and its support, will be designated as a negative polarization layer.

[0041] Figure (2) illustrates in (D) a situation in which the PI polarization of a Ferroiay ferroelectric layer is negative: it is oriented towards the interface between the The ferroelectric layer PI and its supporting structure, the latter here consisting of an intermediate layer Int on a support layer Sprt. We can also say that the component of the polarization along the vertical direction Vert, perpendicular to the ferroelectric layer and oriented from the support structure towards the ferroelectric layer, is negative. This component is obtained by projecting the polarization normally onto the Vert direction.

[0042] To obtain a piezoelectric device equipped, for example, with a negatively polarized ferroelectric layer, it is necessary to choose in advance the characteristics of the ferroelectric material wafer used to manufacture the device, and to take into account the manufacturing process and its potential impacts on the polarization of the wafer.

[0043] Figure 2 illustrates the general principle of a manufacturing process for the Struct structure shown in (D), which includes a step of transferring a ferroelectric layer Ferroiay onto a support assembly Sprt.Set formed by the intermediate layer Int on the support layer Sprt, the intermediate layer being interposed between the ferroelectric layer Ferroiay and the support layer. Notwithstanding, a layer of another material, in particular a silicon oxide layer, may also be found on the face of the ferroelectric layer Ferroiay before being arranged on the intermediate layer Int. The Struct structure is, for example, designed to be integrated into an electroacoustic device.

[0044] The characteristics of the support assembly can affect the acoustic properties of the Ferroiay layer, which is important in the case of a structure forming part of an acoustic device. The nature and thickness of these layers can therefore also be decisive in achieving the desired processing of an electrical signal, or at least influence this processing. In the example shown in [Fig. 2], the support assembly can include the Sprt support layer and the Int intermediate dielectric assembly layer, preferably comprising an oxide, in direct contact with the Ferroiay ferroelectric thin layer.

[0045] For reasons of availability and cost, the Sprt support layer can be made of silicon. It can be a support consisting of a solid base substrate of monocrystalline silicon, but the invention is not limited to this support, which can, more generally, be made of any material, for example silicon, even an electrically insulating material such as sapphire or glass. The Sprt support layer, when formed from a solid substrate, typically has a thickness of several hundred microns. This allows us to limit the density of electrical charges, holes or electrons, which are likely to move within the support layer and which could affect the proper functioning of an RF component formed on the basis of the Struct structure. The Sprt support layer can thus be consisting of a high-resistivity silicon substrate, i.e., one with a resistivity greater than 1000 ohm-centimeters, and more preferably greater than 3000 ohm-centimeters. To enhance the resistive nature of the Sprt support layer, a charge-trapping layer can be added to the ferroelectric layer, for example, one made of polycrystalline silicon. This charge-trapping layer can, of course, be formed using techniques other than polycrystalline silicon. This layer can also include carbon, or be composed of or comprise silicon carbide or a silicon-carbon alloy.Alternatively, the electrical traps can be created by bombarding a surface portion of the support layer with relatively heavy ions (e.g., argon) to create crystalline defects capable of trapping electrical charges. A charge-trapping layer made of a porous material can also be used, for example, by porosifying a surface portion of the support layer when it is made of silicon. However, the invention is not limited to a support layer with such characteristics.

[0046] By way of example, the intermediate layer Int can be made of silicon oxide, silicon nitride, or be formed from a stack of layers composed of these materials.

[0047] Alternatively, the intermediate layer can be an electrically conductive metallic layer interposed between the ferroelectric layer Ferroiay and the support layer Sprt. In this example, the metallic layer Int is in direct contact with both the support Sprt and the ferroelectric layer Ferroiay. The Int layer can then be used as a buried electrode to apply an electric field to the ferroelectric layer Ferroiay.

[0048] The presence of an intermediate Int layer is only an option. Not all applications require the presence of such a layer.

[0049] 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.

[0050] The Ferroiay ferroelectric layer can be made of a single-crystal ferroelectric material, such as lithium tantalate (LiTaO3) or lithium niobate (LiNbO3), or materials such as BaTiO3, PbZrTiO3, KNbO3, BaZrO3, 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 This does not preclude the use of different thicknesses, depending on the intended application. It is worth recalling 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 material's coercive field. As illustrated in this document, the ferroelectric layer preferably exhibits a single-domain polarization, meaning that all dipole moments are aligned parallel to each other along a given direction. Here, the given direction is inclined relative to the plane of the ferroelectric layer, that is, inclined relative to the free face of this layer.

[0051] With reference to Figures 2 to 9, the Struct structure can be produced by a manufacturing process 100 involving the transfer of a ferroelectric layer onto a support assembly, the process being summarized by [Fig. 8] and comprising: - a step S00 of selection and supply of a wafer of single-crystal ferroelectric material for its chemical composition and its crystalline orientation with respect to its extension plane, so that it may serve as a ferroelectric donor substrate Ferrosub, the chosen wafer has a single-domain positive polarization -PI, that is to say having a component oriented in the direction of its face designated by Top, that is to say that the wafer has a crystalline orientation between ORY and 180RY, as illustrated in (B) of [Fig.2]; - a step S10 of preparation of a support assembly Sprt.Ens, consisting here of a support layer Sprt provided with an intermediate layer Int on its surface as illustrated in (A) of [Fig.2]; - a step S20 of preparation of the ferroelectric donor substrate Ferrosub in order to form a weakening plane Frgl in view of the separation of a ferroelectric layer Ferroiay from the donor substrate Ferrosub in a later step, as illustrated in (B), the depth of the weakening plane Fgrl in the donor substrate Ferrosub defining the thickness of the ferroelectric layer Ferroky; - an S40 step of assembling the Sprt.Ens support assembly with the Ferrosub donor substrate, by bringing the free face Fr.Fac of the intermediate layer Int into contact with the Top face of the ferroelectric donor substrate Ferrosub in order to constitute an intermediate structure Structinter illustrated in (C), with the intermediate layer Int interposed between the Sprt support layer and the Top face of the donor of the ferroelectric substrate Ferrosub, the polarity orientation of the Ferrolay layer with respect to the support layer is reversed, changing from -PI to PI due to the inversion of the Ferrosub donor substrate for assembly; and - an S60 step of detachment of part of the Ferrosub donor substrate from the intermediate structure at the level of the embrittlement plane, leaving the ferroelectric layer Ferroiay fixed on the Sprt support and allowing to obtain the Struct structure illustrated in (D), the ferroelectric layer Ferroiay having as already mentioned a negative polarization with respect to the Vert normal of the free surface which served as the detachment interface.

[0052] The Ferrosub donor substrate illustrated in (B) is a substrate made of the ferroelectric material of the Ferroiay ferroelectric layer. Alternatively, it could comprise a surface layer of this material. Thus, the donor substrate could, for example, be formed from a solid substrate of lithium tantalate or lithium niobate, or from a composite substrate consisting of a primary substrate on which rests a layer (at least equal to that of the Ferroiay layer) of lithium tantalate or lithium niobate. The use of a composite substrate comprising a support substrate and a layer of ferroelectric material is necessary if the difference in the coefficient of thermal expansion of the ferroelectric material and the final substrate is too great to allow the application of the Smart Cut process. This approach is described in detail in documents WO2019002080 and WO2019186032, which are incorporated by reference.

[0053] The donor substrate therefore comprises at least one layer of ferroelectric material having a first positive polarization -PI single domain inclined with respect to the extension plane of the ferroelectric layer Ferroiay, which depends on the chosen crystal orientation of the wafer. This layer is intended to be attached to the intermediate layer Int before being separated from the donor substrate. Notwithstanding, a layer of another material, in particular a silicon oxide layer, may also be found on the face of the ferroelectric layer Ferroiay before being arranged on the intermediate layer Int.

[0054] Figure 2 illustrates a situation in which the ferroelectric layer originates from a 42RY type LiTaO3 section, with the orientation shown in (A) of Figure 1. The polarization of the layer is oriented upwards in (B) of Figure 2 as in (A) of Figure 1.

[0055] More generally, the process 100 is applied to a crystal cut within the range extending from 0RY to 180RY, preferably 20RY to 160RY, for the Ferrosub donor substrate. The polarization of the final structure is therefore negative, i.e. oriented from 0° to -180°, preferably within an angular range [Ang] from -20° to -160°, with respect to the plane of the ferroelectric layer Ferroiay, as illustrated by [Fig. 9].

[0056] The Ferroiay ferroelectric layer can be transferred from the Ferrosub ferroelectric donor substrate by implementing Smart Cut™ technology, in which case the The donor substrate must be prepared by introducing light species such as hydrogen and / or helium into it. This introduction can correspond to hydrogen implantation, i.e., hydrogen ion bombardment of the flat face Top of the donor substrate Ferrosub. As is known, and as illustrated in (B), the implanted hydrogen ions (H+) aim to form a weakening plane Frgl, delimiting the ferroelectric layer Ferroiay of the ferroelectric material to be transferred, located on the side of the Top face, from another part Ferrosep, forming the remainder of the substrate, which will be separated from the ferroelectric layer Ferroiay in a subsequent step.

[0057] 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.

[0058] In (C) of [Fig.2], the PI polarization is oriented towards the interface between the ferroelectric layer Ferroiay and the intermediate layer Int, so that the polarization of the ferroelectric layer is considered to be negative.

[0059] 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. Prior to assembly, the surfaces of the substrates to be assembled may be prepared by a cleaning, brushing, drying, polishing, or plasma activation step, or by applying an adhesive layer before or after cleaning.

[0060] The assembly may correspond to the intimate contact of the Ferrosub donor substrate with the Sprt support by molecular adhesion and / or electrostatic bonding.

[0061] 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.

[0062] 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.

[0063] In the present embodiment, the step of detaching a portion of the substrate The donor layer is produced using Smart Cut™ technology, whereby a layer intended to form the ferroelectric layer Ferroiay is delimited by the embrittlement plane Frgl defined by the implantation of hydrogen ions in the donor substrate, as illustrated in (B) of [Fig. 2]. After the assembly step, this layer is detached from the donor substrate by fracturing at the embrittlement plane Frgl and thus transferred to the Sprt support, as illustrated in (D) of [Fig. 2].

[0064] 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 portion of the ferroelectric layer Ferrolay from the substrate and thus complete its transfer to the entire support structure. 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.

[0065] Following step S60, which led to obtaining the structure illustrated in (D) of [Fig. 2] by separating the Ferroiay layer from the rest of the Ferrsub donor substrate, a stabilizing H.Treat-1 heat treatment is applied to the Struct structure at step S80. The stabilizing heat treatment heals crystalline defects present in the ferroelectric layer and helps to consolidate the bond between this Ferroiay ferroelectric layer and the intermediate Int layer. In the case of LiTaO3, this 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, up to 600°C) for a duration of between 5 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.

[0066] However, the inventors have found that the H.Treat-1 heat treatment, combined with the previous manufacturing steps and the Struct structure itself, has several effects locally modifying the polarization of the Ferroiay layer, as explained with the help of [Fig.3].

[0067] One of these effects is particularly troublesome, altering the polarization of the core of the ferroelectric layer, that is, a portion of the layer located at a distance from its surface or its interface with the support structure. Indeed, the presence of a hydrogen ion concentration gradient in a ferroelectric layer combined with heat treatment at a temperature of approximately 300° to 600°C, such as the H.Treat-1 treatment, causes a reversal of the polarization of the ferroelectric material in the volume where the gradient is sufficiently strong. It is interpreted that the gradient of implanted H+ ions causes the appearance of an electric field EH which, due to Thermal activation caused by a temperature increase can induce polarization reversal within a volume of the material. More specifically, during heat treatment at a sufficiently high temperature, the polarization of the ferroelectric material tends to align with the electrochemical field (EH), thus reversing the polarization when EH is oriented in the opposite direction to that of the material before heat treatment.

[0068] Figure 3 illustrates in four lines, from Line A to Line D, the impact of the heat treatment. Figure 1, part of step S80, illustrates the polarization within the volume of the Ferroiay layer, in relation to certain manufacturing steps. Each row corresponds to a manufacturing step and is divided into two columns. The first column, Col.1, illustrates the polarization of the Ferroiay layer in the absence of heat treatment and corresponds to the manufacturing steps of [Fig. 2], while the second column, Col.2, illustrates the effects of these different steps upon application of the heat treatment of step S80. Particular attention will be paid to the appearance and evolution of distinct polarization volumes within the Ferroiay layer.

[0069] Line Lin.A illustrates the ferroelectric donor substrate Ferrosub at step S00. Its polarization is indicated by -PI because it is opposite in direction to the PI polarization of the final Struct structure to be obtained, illustrated in (D) of [Fig. 2]. This polarization is homogeneous throughout the volume VB, which is defined at this point by the volume of the entire donor substrate Ferrosub. The two columns Col.1 and Col.2 show no difference: in the absence of hydrogen ions implanted with an inhomogeneous profile in the volume VB of Ferrosub, the heat treatment does not cause a polarization reversal.

[0070] Line Lin.B illustrates the state of the donor substrate Ferrosub after the formation of the embrittlement plane Frgl in step S20. Without the heat treatment (see Col.1), the polarization of the donor substrate is not modified. However, following the heat treatment, or if the heat treatment were applied immediately without proceeding to steps S40 and S60, the polarization would be reversed in a volume of material Vc derived from the material volume VB, in the vicinity of the embrittlement plane Frgl. The polarization of volume Vc then corresponds to that of a -138RY section of ferroelectric LiTaO3 material, as indicated by the number -138 in the figure. Outside volume Vc, the polarization remains unchanged.

[0071] This mechanism is illustrated by [Fig. 4], which shows the distribution of hydrogen ions implanted in the Ferroiay layer as a hydrogen concentration [H] according to the implantation depth Dpth from the surface of the Ferroiay layer (a representation also referred to as the "projected path" in the field of ion implantation). Such a distribution, which is highly inhomogeneous with respect to the depth of the Ferroiay layer, generates the electric field EH, which is capable of driving The polarization of the volume in question reverses when a sufficiently high-temperature heat treatment is applied. Indeed, under these conditions, the polarization tends to align with the electric field EH. Here, a portion of the volume VB, influenced by the hydrogen ion concentration gradient and the electric field it generates, has seen its polarization reversed, changing from PI to -PI in the volume Vc.

[0072] Line Lin.C illustrates the state of the donor substrate following its assembly with the support assembly in step S40. To achieve this, the donor substrate is inverted. By symmetry, this results in the situation that would occur if a -138RY section of ferroelectric LiTaO3 material had been used as the Ferrosub substrate without the inverting during assembly. In practice, the polarizations of the two volumes VB and Vc are reversed, as indicated by the polarization arrows PI and -PI and by the markings 42 and -138 corresponding to the 42RY and -138RY sections.

[0073] Line Lin.D illustrates, in addition to the presence of the two volumes VB and Vc already described, the presence of two other volumes VA and VD with reversed polarizations, resulting respectively from volumes VB and Vc following the heat treatment of step S80. These two volumes appear on both faces of the Ferrolay layer. Volume VD is defined by a portion of volume Vc as defined in line Lin.C, whose polarization is partially or totally reversed at the free surface of the Ferrolay layer. Volume VA is defined by a portion of volume VB as defined in line Lin.C, whose polarization is partially or totally reversed at the interface of the Ferrolay layer with the support assembly, and therefore with the Int layer of this example.These polarity reversals occur due to physicochemical surface and interface interactions related to the applied heat treatment temperature, and cause the transformation of surface portions of this ferroelectric layer. The layer transitions from a single-domain ferroelectric structure with one polarization to a ferroelectric structure with a second polarization opposite to the first, or to a multi-domain ferroelectric structure at its free surface and / or at its interface with the support structure. These surface portions are called multi-domain when they possess a ferroelectric structure exhibiting areas of different polarization orientations. These surface portions typically have thicknesses of approximately 150 nm or less and can extend throughout the entire extent of the ferroelectric layer.

[0074] We see that the manufacturing process of the Struct structure should preferably take into account the formation of volumes VA, Vc and VD from volume VB, in order to improve the homogeneity of the polarization of the ferroelectric layer Ferroiay, which is inhomogeneous in depth, and therefore leads to poorly controlled characteristics for the devices incorporating this layer. The portion of the VB volume not undergoing any polarization reversal is considered a stable polarization volume of the ferroelectric layer Ferroiay.

[0075] The VD volume can be simply eliminated by superficial polishing of the Ferroiay layer.

[0076] At a step S100 following step S80, during which the heat treatment is applied, 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 prepares the free face to have a low roughness, for example, less than 0.5 nm RMS 5x5 pm by atomic force measurement (AFM), and removes the volume VD of the Ferroiay ferroelectric layer. A removal of 50 to 300 nm is generally planned to achieve the target thickness of the Ferroiay ferroelectric layer, and in all cases, a thickness greater than that of the volume VD. This improves the homogeneity of the Ferroiay layer's polarization.

[0077] To improve the homogeneity of the Ferroiay layer's polarization in depth, it is proposed to perform, after step S100, a step S120 involving the implantation of corrective hydrogen ions (Corr.Imp) within the thickness of the Struct structure, followed by a second H.Treat-2 heat treatment during step S140, so as to correct the polarization of volumes VA and Vc. Step S120 may optionally include several ion implantation phases, but includes at least one. Of course, for other embodiments, and without limiting the invention, it is also possible to perform the Corr.Imp implantation of corrective hydrogen ions (S120) before the aforementioned thinning step S100.

[0078] Figure 5 illustrates a first corrective ion implantation leading to a projected path RP of the implanted hydrogen ions such that the depth D1(RP) of its maximum reaches or exceeds the depth of the interface between volumes VB and Vc and is located at or near this interface, either in volume Vc or in volume VB, preferably at a predetermined distance such that the hydrogen concentration in volume Vc exceeds a threshold of 10¹⁹ hydrogen atoms per cubic centimeter, for example, with a hydrogen concentration between 10¹⁹ and 10²² atoms / cm³ at a plane parallel to the free surface of the Ferroiay layer. This first corrective ion implantation will counteract the effect of the ion implantation that formed the embrittlement plane and will therefore correct the polarization of volume Vc by reversing its polarization again and reorienting it like that of volume VB.

[0079] Figure 6 illustrates a second corrective ion implantation leading to a projected path RP of the implanted hydrogen ions such that the depth D2(RP) of its maximum reaches or exceeds the depth of the interface between the VA volume and the Int layer, i.e., the interface between the Ferrolay ferroelectric layer and the Sprt.Ens support assembly, and is located at this interface or within the Int layer, preferably at a predetermined distance such that the hydrogen concentration in the VA volume exceeds a threshold of 10¹⁹ hydrogen atoms per cubic centimeter, for example, with a hydrogen concentration between 10¹⁹ and 10²² atoms / cm³ at a plane parallel to the free surface of the Ferrolay layer. This second corrective ion implantation, combined with the second H. heat treatment.Treat-2 will have a counteracting effect on the physicochemical effects occurring at the interface between VA and Int, and will therefore correct the polarization of the VA volume by reversing its polarization again and reorienting it like that of the VB volume.

[0080] The first and second implantations are carried out "full field," that is, over the entire surface of the Ferroiay layer, without a mask intended to create implantation patterns within a useful area of ​​the layer. A holding system can, however, be used to maintain the structure during operation and to mask certain parts of the Ferroiay layer, particularly the periphery; these masked parts are not considered to be part of the useful area of ​​the Struct structure. Implantation energies between 30 keV and 300 keV, and doses between 1 x 10¹⁵ and 9 x 10¹⁶ atoms / cm², can be considered.

[0081] Depending on the characteristics of the Ferroiay ferroelectric layer, and in particular its thickness, the second implantation may be sufficient to correct the polarization orientations of the two volumes VA and Vc. Indeed, the electric field generated by the hydrogen concentration gradient can be sufficiently extensive and intense to encompass the volume Vc within its zone of influence and affect the polarization during the H.Treat-2 heat treatment. It is therefore understandable that a relatively thin ferroelectric layer will more readily benefit from this advantage than a relatively thick ferroelectric layer.

[0082] The first correction implementation and the second correction implementation can be carried out successively during the correction step S120. Alternatively, the first or second implementation can be carried out alone.

[0083] An adjustment of the implementation parameters of the S120 correction step is preferably carried out according to the geometric characteristics (thickness, location) of the VA and Vc volumes of the Ferroiay layer. To this end, a first preparatory step Prep-SOO consists of manufacturing a reference Ref structure according to the same parameters as the Struct structure that one wishes to produce, therefore according to steps S00 to S100.

[0084] A second preparatory step, Prep-SlO, consists of performing a polarization analysis along a transverse plane of the Ferroiay layer of the reference sample, in order to obtain the Char characteristics of the polarization of the Ref sample. To do this, a beveled section of the Ferroiay layer is made, and this section is then analyzed by piezoelectric force microscopy (or PFM). Such an analysis makes it possible to determine the polarization orientation of the Ferroiay layer throughout its thickness, and thus to determine the absence or presence of reversed polarization volumes and, if so, to locate the volumes VA, VB, Vc, and VD, that is, the thicknesses of each of these volumes and therefore the depths at which the interfaces between VA and VB, VB and Vc, and Vc and VD are located, measured from the free surface of the Ferroiay layer.

[0085] Step S120 can be applied with implantation parameters adjusted in response to the analysis performed during the Prep-SlO step. Specifically, the implantation energy of the first ion implantation will be determined in such a way that the depth D1(RP) of the maximum of the projected path RP of the hydrogen ions reaches or is located near the depth of the interface between volumes VB and Vc or is located at the level of this interface or even in volume Vc or in volume VB, preferably at a predetermined distance already mentioned above to exceed the concentration threshold.The same principle applies to the depth D2(RP) of the second implantation, the implantation energy of which will be determined in such a way that the depth D2(RP) of the maximum of the projected path RP of the hydrogen ions reaches or is close to the depth of the interface between the volume VA and the Int layer, or is located at the level of this interface, in the volume VA or even in the Int layer.

[0086] The second H.Treat-2 heat treatment of step S140, applied to the Struct structure after the Corr.Imp correction implantation step S120, can be 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, up to 600°C) for a duration of between 5 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. H.Treat-2 leads to a reversal of the polarization of the VC and / or VA volumes and therefore to a homogenization of the polarization of the Ferrolay ferroelectric layer.Ideally, this layer is made, at the end of processing, of a single domain of negative polarization PI, oriented towards the interface between the Ferrolay layer and the support assembly, including here the intermediate layer Int and the support layer Sprt, as illustrated by [Fig.7]. Second embodiment of the invention

[0087] According to the first embodiment of the invention, a hydrogen implantation step and an associated heat treatment are applied to correct the polarization of the ferroelectric layer in response to unwanted polarization reversals caused by a first heat treatment. This is a polarization repair process.

[0088] The second embodiment consists, as an alternative to the first embodiment, in preventing polarization reversals which, although undesired, may be expected.

[0089] [Fig. 10] illustrates a process 200 for manufacturing the Struct structure illustrated by [Fig. 2]. Steps S00 to S60 are the same as those of process 100 of the first embodiment.

[0090] In the second embodiment, following the detachment of the Ferro^ layer from the Ferro^ donor substrate in step S60, an ion implantation step S270 is carried out. This step is similar to step S120 of the first embodiment, except that it must be taken into account that the Ferro^ layer has not yet been thinned or subjected to heat treatment. The principle remains the same, however, and can be applied after the preparation and characterization steps of a reference sample Ref'.

[0091] The reference sample Ref' is a sample obtained in a Prep-SOO step by means of steps S00 to S60, further subjected to a stabilizing heat treatment step such as that of step S80 of the first embodiment. This heat treatment reveals the effects on the polarization of the Ferro^ layer of the fabrication steps S00 to S60, as explained above, particularly in the comments with reference to [Fig. 3], concerning the inverted polarization volumes. The Prep-S1O analysis step provides the Char' characteristics of the volumes VA, VB, Vc, and VD, on the basis of which the implantation parameters of step S270 are chosen, as in the first embodiment.

[0092] Following step S270, a step S80 applying an H.Treat' heat treatment to the Struct structure is carried out during step S280. The H.Treat' treatment replaces the two H.Treat-1 and H.Treat-2 treatments of the first embodiment, with a stabilizing effect on the Struct structure similar to that of the H.Treat-1 treatment. Step S270 results in the formation of hydrogen ion concentration gradients that generate electric fields opposing polarization reversals within the ferroelectric layer. Thus, the VA and Vc volumes of polarization reversal of the VB volume in the first embodiment do not form. The S270 step helps to prevent polarization reversals that may occur in the volume of the ferroelectric layer, at the buried interface of the ferroelectric layer with the support assembly, or at the level of these two regions.

[0093] However, the H.Treat' heat treatment can cause the appearance of a multidomain volume on the surface of the Ferroiay layer, as previously explained. It is then necessary to perform a step S290 similar to step S100 of the first embodiment, which allows the removal of the surface multidomain layer and the Ferroiay layer to be brought to the desired thickness.

[0094] Process 200 has the advantage over process 100 of being simpler, with only one heat treatment step required instead of two.

[0095] 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.

[0096] 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. A method for manufacturing a structure comprising a negatively polarized ferroelectric layer (PI), comprising the steps of: - providing (S00) a ferroelectric material wafer (Ferrosub) having a first face (Top) and having a polarization (-P1) oriented towards this face; - forming (S20) a weakening plane (Frgl) in the ferroelectric material wafer (Ferrosub) by implanting hydrogen ions through the first face (Top); - assembling (S40) the ferroelectric material wafer (Ferro^) having the weakening plane (Frgl) to a support assembly (Sprt.Set) by contacting the first face with a free surface of the support assembly; - detach (S60) a part of the ferroelectric material wafer so as to define a ferroelectric layer (Ferro^) detached from the ferroelectric material wafer (Ferro^) and assembled to the support assembly (Sprt.Set) so as to obtain a structure (Struct) formed of the ferroelectric layer and the support assembly, the ferroelectric layer having a negative polarization (PI); - proceed (S 120; S270) to an additional full-field hydrogen implantation step, parameterized so as to correct or prevent the occurrence of polarization reversal in the volume of the ferroelectric layer and / or at its interface with the support assembly (Sprt.Ens); and - apply (S 80, S280) at least one first heat treatment (H.Treat-1; H. Treat') to the structure (Strct).

2. The method according to claim 1, wherein the step (S 120) of additional hydrogen implantation is carried out after at least one first heat treatment (S80, Htreat-1), the method further comprising a step (S 140) of applying a second heat treatment (H.Treat-2) to the structure after the step (S 120) of additional hydrogen implantation.

3. The process according to claim 2, further comprising a step (S100) of thinning (S.Pol) the ferroelectric layer, preferably between the step (S80) of applying the first heat treatment (H.Treat-1) and the step (S140) of applying the second heat treatment (H.Treat-2).

4. The process according to claim 1, wherein the additional hydrogen implantation step (S270) is carried out before at least one first heat treatment (S280, Htreat').

5. The process according to claim 4, further comprising a step (S290) of thinning (S.Pol') the ferroelectric layer, preferably after the step (S280) of applying the first heat treatment (H.Treat').

6. The method according to any one of the preceding claims 1 to 5, comprising a step (Prep-SlO) of characterizing a reference structure (Ref; Ref'), an adjustment of parameters of the additional hydrogen implantation step (S 120; S270) being defined in response to this characterization.

7. The method according to any one of the preceding claims 1 to 6, wherein the additional hydrogen ion implantation step (S 120; S270) comprises a first corrective ion implantation in the ferroelectric layer (Ferrolay), parameterized to generate a projected path of the implanted hydrogen ions such that a depth (D1(RP)) of its maximum reaches or exceeds an interface depth between (i) a volume (VB) of stable polarization of the ferroelectric layer (Ferrolay) and (ii) a volume (Vc) of reversed polarization due to the formation of the embrittlement plane or of polarization which would reverse due to the formation of the embrittlement plane if a heat treatment were applied.

8. The method according to any one of the preceding claims 1 to 7, wherein the additional hydrogen ion implantation step (S 120; S270) comprises a second corrective ion implantation, parameterized to generate a projected path of the implanted hydrogen ions such that a depth (D2(RP)) of its maximum reaches or exceeds the depth of the interface between (i) the ferroelectric layer (Ferro^) and (ii) the support assembly (Sprt.Ens), preferably at a distance from this interface that is less than the thickness of a polarization volume (VA) that is bounded by this interface and that is multidomain or inverted with respect to a stable polarization volume (VB) of the ferroelectric layer (Ferroiay) or that would invert or become multidomain if a heat treatment were applied.

9. A method according to any one of the preceding claims 1 to 8, wherein the full-field hydrogen implantation step adds- tionnelle is parameterized to obtain a hydrogen concentration between 1019 and 1022 at / cm3 in a volume (VA, VB, Vc) of the structure.

10. A method according to any one of the preceding claims 1 to 9, wherein the ferroelectric layer extends in an extension plane and the polarization (PI) makes an angle with included in an angular range ([Ang]) from -20° to -160° with respect to the extension plane.

11. A method according to any one of the preceding claims 1 to 10, wherein the ferroelectric layer (Ferroiay) is a layer of lithium niobate or lithium tantalate.

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