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

Abstract

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

Description

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

[0001] The invention relates to a method for manufacturing a ferroelectric layer transferred onto a substrate and polarization with improved homogeneity in its thickness. Such a structure can be used to form, for example, radiofrequency (RF) components, in particular elastic wave components, in particular surface acoustic wave (SAW) or volume acoustic wave (BAW) components. TECHNOLOGICAL BACKGROUND

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

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

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

[0005] Document WO 2005 / 052682 A1 describes the localized reversal of 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, if they effectively make it possible to control the polarization domains of a ferroelectric layer on its surface, remain impractical and do not allow the control of the polarization in the thickness of this layer or its integration on a support.

[0007] In response to these shortcomings, the international patent application with publication number WO 2020 / 200986 A1 and the patent applications in France with filing numbers FR2301220 and FR2301221 propose techniques for transferring ferroelectric layers onto a substrate. However, these techniques do not solve the problem in a completely satisfactory manner, in that, at the end of manufacturing, the transferred ferroelectric layers may exhibit uncontrolled polarization inhomogeneities in the thickness of the layer, in particular with regard to the transfer of ferroelectric layers having a negative polarization onto the final support substrate. Disclosure 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 polarization inhomogeneities that could appear in their thickness, in particular for the situation where it is sought to obtain a ferroelectric layer of negative polarization, that is to say oriented towards the support of the ferroelectric layer.

[0009] In order to achieve these objects, one aspect of the invention is a method of manufacturing a structure comprising a negatively polarized ferroelectric layer, comprising the steps of: providing a plate of ferroelectric material having a first face and having a polarization oriented towards this face; forming a weakening plane in the plate of ferroelectric material by implanting hydrogen ions through the first face; assembling the plate of ferroelectric material comprising the weakening plane to a support assembly by bringing the first face into contact with a free surface of the support assembly;detaching a portion of the ferroelectric material wafer so as to define a ferroelectric layer detached from the ferroelectric material wafer and assembled to the support assembly so as to obtain a structure formed from the ferroelectric layer and the support assembly, the ferroelectric layer having a negative polarization; carrying out an additional full-field hydrogen implantation step, configured so as to correct or prevent the occurrence of polarization inversion in the volume of the ferroelectric layer and / or at its interface with the support assembly; and applying at least a first treatment; thermal to the structure.

[0010] An advantage of the method 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, having improved homogeneity over its thickness compared to known manufacturing methods. 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 characteristics of the invention, considered individually or in any technically feasible combination:

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

[0013] - the method may further comprise a step of thinning the ferro-layer electrical, preferably between the step of applying the first heat treatment and the step of applying the second heat treatment;

[0014] - the additional hydrogen implantation step can be carried out before the at less a first heat treatment;

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

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

[0017] - the step of additional implantation of hydrogen ions may comprise a first correction ion implantation in the ferroelectric layer, parameterized so as 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 volume of stable polarization of the ferroelectric layer and (ii) a volume of reversed polarization due to the formation of the embrittlement plane or polarization which would reverse due to the formation of the embrittlement plane if a heat treatment were applied;

[0018] - the step of additional implantation of hydrogen ions may comprise a second corrective ion implantation, parameterized so as 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 to 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 so as to obtain a hydrogen concentration of 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 included in an angular range from -20° to -160° relative to the plane of extension;

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

[0022] Other characteristics and advantages of the invention will emerge from the detailed description of the invention which follows with reference to the appended figures in which:

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

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

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

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

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

[0028] [Fig.6] [Fig.6] illustrates a method of 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 reported from improved homogeneity polarization;

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

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

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

[0033] Depending on the piezoelectric device that one wishes to produce, it is appropriate to choose an orientation for the polarization of the active layer of piezoelectric material, which is made up of a layer of monocrystalline ferroelectric material.

[0034] This ferroelectric material can be obtained by crystal growth according to the method known as "Czochralski" which makes it possible to obtain a massive monocrystalline material in the form of an elongated cylinder called "ball" or "ingot" by pulling a crystal seed of chosen orientation immersed in a molten material corresponding to the crystal to be formed, in a pulling direction. The monocrystal grows in this pulling direction. Platelets of the ferroelectric material are then obtained by cutting the ingot at a determined angle relative to its growth direction, then prepared and integrated as a ferroelectric layer in, for example, a piezoelectric device.

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

[0036] Figure 1 illustrates in (A) and (B) the orientations of LiTaO3 layers designated by 42RY and -138RY, respectively. These names derive from the identification of the crystallographic orientation of these layers, corresponding to respective rotations of 42° and -138° of the y and z axes with respect to the trigonometric direction around the x axis to give the x', y' and z' axes, the x, y and z axes corresponding to the crystallographic axes proper to the single crystal and the y' axis being 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 pulling in a chosen direction, it is known to those skilled in the art to carry out polarization by applying an electric field in the z direction when the temperature falls below the Curie temperature to determine the direction of polarization.

[0038] Conventionally, a so-called negative polarization is a polarization of 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, i.e. 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 ferroelectric layer Ferroiay is negative: it is oriented towards the interface between the ferroelectric layer PI and the assembly supporting it, the latter here consisting of an intermediate layer Int on a support layer Sprt. We can also say that the component of the polarization in the vertical direction Vert perpendicular to the ferroelectric layer and oriented from the support assembly towards the ferroelectric layer, is negative. This component is obtained by a normal projection of the polarization on the Vert direction.

[0042] To obtain a piezoelectric device provided, for example, with a negative polarization 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] [Fig.2] illustrates the general principle of a method for manufacturing the structure Struct illustrated in (D), and which includes a step of transferring a ferroelectric layer Ferroiay onto a support assembly Sprt.Set formed of the intermediate layer Int on the support layer Sprt, the intermediate layer being interposed between the ferroelectric layer Ferroiay and the support layer. In a non-limiting manner, it is also possible to find a layer of another material, in particular a layer of silicon oxide, on the face of the ferroelectric layer Ferroiay before arranging it on the intermediate layer Int. The structure Struct 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 influencing this processing. In the example shown in [Fig.2], the support assembly can comprise the support layer Sprt, and the intermediate dielectric assembly layer Int, preferably comprising an oxide, directly in contact with the thin ferroelectric Ferroiay layer.

[0045] For reasons of availability and cost, the support layer Sprt may be made of silicon. It may be a support consisting of a solid base substrate made 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 electrically insulating such as sapphire or glass. The support layer Sprt, when it is formed of a solid substrate, typically has a thickness of several hundred microns. In this way, the density of electrical charges, holes or electrons, which are likely to move in the support layer and which could affect the proper functioning of an RF component which would be formed on the basis of the structure Struct, can be limited. The support layer Sprt may thus be consisting of a high resistivity silicon substrate, i.e. having a resistivity greater than 1000 ohm-centimeter, and more preferably still greater than 3000 ohm-centimeter. To support the resistive nature of the Sprt support layer, it may be provided to provide the Sprt support layer with a charge trapping layer on the side of the ferroelectric layer, for example formed of polycrystalline silicon. It may of course be provided to form this charge trapping layer by a technique other than that providing a layer formed of polycrystalline silicon. This layer may also comprise carbon or be made of or comprise silicon carbide or an alloy of silicon and carbon.Alternatively, it may involve producing the electrical traps by ion bombardment of relatively heavy species (for example argon) in a surface part of the support layer in order to create crystalline defects capable of trapping electrical charges. It is also possible to provide a charge trapping layer formed from a porous material, for example by porosification of a surface part of the support layer Sprt when the latter is made of silicon. However, the invention is not limited to a support layer having such characteristics.

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

[0047] Alternatively, the intermediate layer may be an electrically conductive metal layer interposed between the ferroelectric layer Ferroiay and the support layer Sprt. In this example, the metal layer Int is in direct contact with each of the support Sprt and the ferroelectric layer Ferroiay. The layer Int may in this case subsequently be used as a buried electrode intended to apply an electric field to the ferroelectric layer Ferroiay.

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

[0049] Conventionally, the structure Struct may be in the form of a circular plate whose diameter may be 100, 200, 300 or even 450 mm, but the invention is in no way limited to these dimensions or to this shape.

[0050] The ferroelectric layer Ferroiay may 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, PbTiO 3 or KTaO3. These materials also have piezoelectric properties. Generally, the ferroelectric layer may have a thickness of between 10 nanometers and 10 microns, depending on the intended application of the structure Struct and the expected performance of the components, but the invention does not exclude the use of different thicknesses, always depending on the intended application. It is recalled that a ferroelectric material is a material which has an electric polarization in its natural state, a polarization which can be reversed by the application of an external electric field greater than the coercive field of the material. As illustrated in this document, the ferroelectric layer preferably has a single-domain polarization, i.e. all the dipole moments are aligned parallel to each other in a given direction. Here, the given direction is inclined relative to the plane of the ferroelectric layer, i.e. inclined relative to the free face of this layer.

[0051] With reference to figures 2 to 9, the structure Struct can be produced by a manufacturing method 100 by transferring a ferroelectric layer onto a support assembly, the method being summarized by [Fig.8] and comprising: - a step S00 of choosing and providing a wafer of monocrystalline ferroelectric material for its chemical composition and its crystalline orientation relative to its extension plane, so that it serves as a ferroelectric donor substrate Ferrosub, the chosen wafer has a monodomain positive polarization -PI, i.e. having a component oriented in the direction of its face designated by Top, i.e. the wafer has a crystalline orientation between ORY and 180RY, as illustrated in (B) of [Fig.2]; - a step S10 of preparing 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 preparing the ferroelectric donor substrate Ferrosub in order to form therein a weakening plane Frgl with a view to separating a ferroelectric layer Ferroiay from the donor substrate Ferrosub at a subsequent 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; - a step S40 of assembling the support assembly Sprt.Ens 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 Ferrosub donor substrate in order to constitute an intermediate structure Structinter illustrated in (C), with the intermediate layer Int interposed between the support layer Sprt and the Top face of the donor of the ferroelectric Ferrosub substrate, the orientation of the polarity of the Ferrolay layer relative to the support layer is reversed, changing from -PI to PI due to the turning over of the Ferrosub donor substrate for the assembly; and - a step of S60 of detachment of a part of the Ferrosub donor substrate from the intermediate structure at the level of the weakening plane, leaving the Ferroiay ferroelectric layer fixed on the Sprt support and making it possible to obtain the Struct structure illustrated in (D), the Ferroiay ferroelectric layer having as already said a negative polarization with respect to the Green normal of the free surface having served as a detachment interface.

[0052] The Ferrosub donor substrate illustrated in (B) is a substrate made of the ferroelectric material of the Ferroiay ferroelectric layer. It could alternatively comprise a surface thickness of this material. Thus, the donor substrate may, for example, be formed of a solid substrate of lithium tantalate or lithium niobate, or of a composite substrate formed of a first substrate on which rests a thickness (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 proves 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 monodomain positive polarization -PI inclined relative to the extension plane of the ferroelectric layer Ferroiay which depends on the chosen crystalline orientation of the wafer. This layer is intended to be fixed to the intermediate layer Int before being separated from the donor substrate. In a non-limiting manner, it is also possible to find a layer of another material, in particular a layer of silicon oxide, on the face of the ferroelectric layer Ferroiay before arranging it on the intermediate layer Int.

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

[0055] More generally, the method 100 applies to a crystalline section included in 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, that is to say oriented from 0° to -180°, preferably included in an angular range [Ang] going from -20° to -160°, relative to the plane of the Ferroiay ferroelectric layer, as illustrated by [Fig.9].

[0056] 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(s) such as hydrogen and / or helium into this donor substrate. This introduction may correspond to a hydrogen implantation, i.e., an ion bombardment of hydrogen of the flat face Top of the donor substrate Ferrosub. 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 of ferroelectric material to be transferred which is located on the side of the face Top and another part Ferrosep forming the rest of the substrate and which will be separated from the ferroelectric layer Ferroiay at a later stage.

[0057] The nature, the dose of the implanted species and the implantation energy are chosen according to the thickness of the layer that one wishes to transfer and the physicochemical properties of the Ferrosub donor substrate. In the case of a LiTaO3 donor substrate, one can choose to implant a dose of hydrogen between 1016 and 5.1017 at / cm2 with an energy between 30 and 300 keV to delimit a Ferroiay ferroelectric layer of the order of 200 to 2000 nm thick.

[0058] In (C) of [Fig.2], the polarization PI 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 negative.

[0059] The Sprt support substrate may have the same size and shape as the Ferrosub donor substrate, but the invention is not limited to such a configuration and different sizes, shapes and configurations may be used. Prior to assembly, it may be envisaged to prepare the faces of the substrates to be assembled by a cleaning, brushing, drying, polishing, or plasma activation step, or before or after cleaning a deposit of a bonding layer.

[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 of the van der Waals or covalent type. The assembly of the two bodies is then obtained without the use of an adhesive.

[0062] The assembly may comprise the application of a low temperature heat treatment (for example between 50 and 300°C, typically 100°C) making it possible 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 donor is made by applying Smart Cut™ technology, according to which a layer intended to form the ferroelectric layer Ferroiay is delimited by the embrittlement plane Frgl defined by 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 fracture at the embrittlement plane Frgl and thus transferred to the support Sprt, as illustrated in (D) of [Fig.2].

[0064] This detachment step may thus comprise the application to the intermediate structure Structinter of a heat treatment in a temperature range of the order of 80°C to 300°C to enable the detachment of the part of the donor substrate from the ferroelectric layer Ferrolay and thus complete the transfer of the latter onto the support assembly. As a replacement or in addition to the heat treatment, this step may comprise the application of a blade or a jet of gaseous or liquid fluid, or any other force of a mechanical nature at the level of the embrittlement plane Frgl.

[0065] Following step S60 which led to obtaining the structure illustrated in (D) of [Fig.2] by separation of the Ferroiay layer from the rest of the Ferrsub donor substrate, a stabilization heat treatment H.Treat-1 is applied to the Struct structure, in a step S80. The stabilization heat treatment makes it possible to cure crystalline defects present in the ferroelectric layer and contributes to consolidating the bonding between this Ferroiay ferroelectric layer and the intermediate layer Int. In the case of LiTaO3, this heat treatment is intended to bring the ferroelectric layer to a temperature between 300°C and the Curie temperature of the ferroelectric material (and preferably greater than or equal to 450°C, 500° or 550°, up to 600°) for a duration 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 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, modifying the polarization of the core of the Ferroiay layer, that is to say a part of the layer located at a distance from its surface or its interface with the support assembly. Indeed, the presence of a hydrogen ion concentration gradient in a ferroelectric layer combined with a heat treatment at a temperature of the order of 300° to 600°C, such as the H.Treat-1 treatment, causes an inversion 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, thanks to Thermal activation caused by temperature increase can induce polarization reversal in a volume of the material. More specifically, during heat treatment at a sufficiently high temperature, the polarization of the ferroelectric material tends to take the same orientation as the EH field, thus reversing the polarization when EH is oriented in a direction opposite to that of the material before application of the heat treatment.

[0068] [Fig.3] illustrates in four lines Lin.A to Lin.D the impact of heat treatment H.Treat-1 of step S80 on the polarization in the volume of the Ferroiay layer, in connection with certain manufacturing steps. Each line corresponds to a manufacturing step and is divided into two columns. A first column Col.1 illustrates the polarization of the Ferroiay layer in the absence of the heat treatment and corresponds to the manufacturing steps of [Fig.2], while a second column Col.2 illustrates the effects caused by these different steps when applying the heat treatment of step S80. Particular attention will be paid to the appearance and evolution of volumes of distinct polarizations appearing in 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 time, by the volume of the entire Ferrosub donor substrate. The two columns Col.1 and Col.2 do not present any difference: in the absence of hydrogen ions implanted according to an inhomogeneous profile in the volume VB of Ferrosub, the heat treatment does not lead to a polarization reversal.

[0070] Line Lin.B illustrates the state of the Ferrosub donor substrate after the formation of the embrittlement plane Frgl in step S20. Without the heat treatment (see Col.l), the polarization of the donor substrate is not modified. On the other hand, 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 originating from the material volume VB, in the vicinity of the embrittlement plane Frgl. The polarization of the volume Vc then corresponds to that of a -138RY section of ferroelectric material LiTaO3, as indicated by the number -138 in the figure. Outside the 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 in the form of a hydrogen concentration [H] according to the implantation depth Dpth from the surface of the Ferroiay layer (representation also referred to as "projected path" in the field of ion implantation). Such a distribution, highly inhomogeneous according to the depth of the Ferroiay layer, generates the electric field EH, likely to cause the polarization inversion of the volume concerned when a heat treatment at a sufficiently high temperature is applied. Indeed, the polarization tends under these conditions to take the same orientation as the electric field EH. Here, a part of the volume VB, influenced by the concentration gradient of the hydrogen ions and the electric field that it generates, has seen its polarization reverse, passing from PI to -PI in the volume Vc.

[0072] Line Lin.C illustrates the state of the donor substrate following its assembly to the support assembly in step S40. To do this, the donor substrate is turned over. By symmetry, we find ourselves in the situation that we would have if we had used a -138RY cut of ferroelectric material LiTaO3 as the Ferrosub substrate without the turning over during assembly having been carried out. In practice, the polarizations of the two volumes VB and Vc are found to be reversed, as indicated by the polarization arrows PI and -PI and by the indications 42 and -138 corresponding to the 42RY and -138RY cuts.

[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 of reversed polarizations, originating respectively from the volume 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 part of the volume Vc as defined in line Lin.C, the polarization of which is partially or totally reversed at the free surface of the Ferrolay layer. Volume VA is defined by a part of the volume VB as defined in line Lin.C, the polarization of which 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 inversions occur due to surface and interface physicochemical interactions related to the applied heat treatment temperature, and cause the transformation of surface portions of this ferroelectric layer, which passes from a single-domain ferroelectric structure of a first polarization to a ferroelectric structure of a second polarization opposite to the first polarization or to a multi-domain ferroelectric structure at its free surface and / or at its interface with the support assembly. These surface portions are said to be multi-domain when they have a ferroelectric structure having zones of various polarization orientations. These surface portions typically have thicknesses of the order of 150 nm or less, and can develop over the entire extent of the ferroelectric layer.

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

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

[0076] In a step S100 following step S80 during which the heat treatment is applied, the ferroelectric layer is thinned. This thinning may correspond to the polishing of the free face of the Ferroiay ferroelectric layer, for example by mechanical, chemical-mechanical and / or chemical etching thinning techniques. It makes it possible to prepare the free face so that it has a low roughness, for example less than 0.5 nm RMS 5x5 pm by atomic force measurement (AFM) and to remove the volume VD of the Ferroiay ferroelectric layer. A removal of 50 to 300 nm of thickness is generally provided 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 polarization of the Ferroiay layer.

[0077] In order to improve the homogeneity of the polarization of the Ferroiay layer in depth, it is proposed to proceed, after step S100, to a step S120 of implantation Corr.Imp of correction hydrogen ions in the thickness of the structure Struct followed by a second heat treatment H.Treat-2 during a step S140, so as to correct the polarization of the volumes VA and Vc. Step S120 may optionally comprise several ion implantation phases, but comprises at least one. Of course, for other embodiments, and without this being limiting for the invention, it is also possible to proceed to step S120 of implantation Corr.Imp of correction hydrogen ions before said thinning step S100.

[0078] [Fig. 5] illustrates a first correction 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 this interface or close to the 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 correction ion implantation will have an effect counteracting that of the ion implantation having 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] [Fig.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 volumes VA and the layer Int, i.e. the interface between the ferroelectric layer Ferrolay and the support assembly Sprt.Ens, and is located at this interface or in the layer Int, preferably at a predetermined distance such that the hydrogen concentration in the volume VA 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 layer Ferroiay. This second correction ion implantation, combined with the second heat treatment H.Treat-2, will have an effect counteracting that of 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 implantation and the second implantation are carried out "full field", that is to say on the entire surface of the Ferroiay layer, without a mask intended to create implantation patterns in a useful area of ​​the layer. A holding system can however be used to hold the structure during operation and mask certain parts of the Ferroiay layer, in particular the periphery, these masked parts not being considered as part of the useful area of ​​the Struct structure. Implantation energies between 30 keV and 300 keV, and doses between 1.1015 and 9.1016 atoms / cm2 can be considered.

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

[0082] The first correction implantation and the second correction implantation may be performed successively during the correction step S120. Alternatively, the first implantation or the second implantation may be performed alone.

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

[0084] A second preparatory step Prep-S10 consists of carrying out an analysis of the polarization along a transverse plane of the Ferroiay layer of the reference sample, so as to obtain the Char characteristics of the polarization of the Ref sample. To do this, a bevel cut of the Ferroiay layer is carried out and then an analysis of this cut by piezoelectric force microscopy (or PFM in English terminology, for Piezoresponse Force Microscopy). Such an analysis makes it possible to determine the orientation of the polarization of the Ferroiay layer over its entire thickness, and therefore to determine the absence or presence of volumes of reversed polarizations and, if necessary, to locate the volumes VA, VB, Vc and VD, that is to say 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 may be applied with implantation parameters adjusted in response to the analysis performed during step Prep-S10. Specifically, the implantation energy of the first ion implantation will be determined such that the depth D1(RP) of the maximum of the projected path RP of the hydrogen ions reaches or is located close to the depth of the interface between volumes VB and Vc or is located at 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 for 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 located close to the depth of the interface between the volume VA and the layer Int, or is located at the level of this interface, in the volume VA or even in the layer Int.

[0086] The second heat treatment H.Treat-2 of step S140, applied to the structure Struct after the correction implantation step S120 Corr.Imp, may be provided to bring the ferroelectric layer to a temperature between 300°C and the Curie temperature of the ferroelectric material (and preferably greater than or equal to 450°C, 500° or 550°, up to 600°) for a duration 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 gas atmosphere. H.Treat-2 leads to an inversion of the polarization of the volumes VC and / or VA and therefore to a homogenization of the polarization of the ferroelectric layer Ferrolay.Ideally, this layer is made, at the end of treatment, of a single monodomain of negative polarization PI, oriented towards the interface between the Ferrolay layer and the support assembly, here comprising 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 intended to correct the polarization of the ferroelectric layer in response to unwanted polarization inversions caused by a first heat treatment. This is a polarization repair method.

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

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

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

[0091] The reference sample Ref' is a sample obtained in a Prep-S00 step by means of steps S00 to S60, further subjected to a step of applying a stabilizing heat treatment such as that of step S80 of the first embodiment. This heat treatment makes it possible to reveal the effects on the polarization of the Ferro^ layer of the manufacturing steps S00 to S60, as explained above, in particular in the comments with reference to [Fig. 3], concerning the inverted polarization volumes. The Prep-S10 analysis step makes it possible to obtain 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 of applying a heat treatment H.Treat' to the structure Struct is applied during a step S280. The treatment H.Treat' replaces the two treatments H.Treat-1 and H.Treat-2 of the first embodiment, with here a stabilizing effect of the structure Struct like the treatment H.Treat-1. Step S270 results in the formation of gradients of concentrations of hydrogen ions generating electric fields opposing the polarization inversions in the volume of the ferroelectric layer. Thus, the volumes VA and Vc of inversion of the polarization of the volume VB in the first embodiment are not formed. Step S270 prevents 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 both of these regions.

[0093] However, the heat treatment H.Treat' can cause the appearance of a multi-domain volume on the surface of the Ferroiay layer, as previously explained. It is then necessary to carry out a step S290 similar to step S100 of the first embodiment, which makes it possible to eliminate the superficial multi-domain layer and to bring the Ferroiay layer to the desired thickness.

[0094] The method 200 has the advantage over the method 100 of being simpler, only one heat treatment step being necessary instead of two.

[0095] In this document, the figures are not necessarily to scale. Some features and components may be shown exaggerated relative to other components or in a somewhat schematic form, and some 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 embodiments described and variant embodiments can be made without departing from the scope of the invention as defined by the claims.

Claims

Claims

1. Method for manufacturing a structure comprising a negative polarization ferroelectric layer (PI), comprising the steps of: - providing (S00) a plate of ferroelectric material (Ferrosub) having a first face (Top) and having a polarization (-P1) oriented towards this face; - forming (S20) a weakening plane (Frgl) in the plate of ferroelectric material (Ferrosub) by implanting hydrogen ions through the first face (Top); - assembling (S40) the plate (Ferro^) of ferroelectric material comprising the weakening plane (Frgl) to a support assembly (Sprt.Set) by bringing the first face into contact with a free surface of the support assembly; - detaching (S60) a portion 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 from the ferroelectric layer and the support assembly, the ferroelectric layer having a negative polarization (PI); - carrying out (S 120; S270) an additional full-field hydrogen implantation step, configured so as to correct or prevent the occurrence of polarization inversion in the volume of the ferroelectric layer and / or at its interface with the support assembly (Sprt.Ens); and - applying (S 80, S280) at least a 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 the 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 method 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 method according to claim 1, wherein the step (S270) of additional hydrogen implantation is carried out before the at least one first heat treatment (S280, Htreat').

5. The method 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-S10) of characterizing a reference structure (Ref; Ref'), an adjustment of parameters of the additional hydrogen implantation step (S120; S270) being defined in response to this characterization.

7. The method according to any one of the preceding claims 1 to 6, wherein the step of additionally implanting hydrogen ions (S 120; S270) comprises a first correction ion implantation in the ferroelectric layer (Ferrolay), parameterized so as 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 (Ferroiay) and (ii) a volume (Vc) of reversed polarization due to the formation of the embrittlement plane or 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 correction ion implantation, parameterized so as 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 to this interface which is less than the thickness of a polarization volume (VA) which is limited by this interface and which is multidomain or inverted with respect to a polarization of a stable polarization volume (VB) of the ferroelectric layer (Ferroiay) or which would invert or become multidomain if a heat treatment were applied.

9. A method according to any one of preceding claims 1 to 8, wherein the step of full-field implantation of addi- tional is set so as 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° relative to the extension plane.

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

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