Method for preparing a single-domain thin layer made of ferroelectric material comprising lithium

Abstract

The invention relates to a method for preparing a single-domain thin film (4) of ferroelectric material comprising lithium, the method comprising the provision of a first layer (8) that has a free surface (9). According to the invention, the preparation method comprises a surface treatment that exposes the free face (9) of the first layer (8) to a treatment atmosphere comprising at least 0.02% carbon dioxide in order to form a lithium-rich passivation layer and a treatment for removing the lithium-rich passivation layer.

Description

Method for preparing a single-domain thin layer of ferroelectric material comprising lithium FIELD OF THE INVENTION

[0001] The invention relates to a piezoelectric-on-insulator (POI) structure. Such a structure finds application in particular in the fields of microelectronics, microsystems, and photonics. It can be used in particular to form radiofrequency (RF) components or to constitute such components, in particular filters or resonators based on elastic wave components, for example elastic surface waves. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] With reference to figures 1a and 1b which represent POI structures of the state of the art, a POI structure is typically formed of a piezoelectric thin layer 4 transferred onto a first face of a support 2. An interposed dielectric layer 3 is arranged between, and in contact with, the support 2 and the thin layer 4.

[0003] Thin layer 4 is made of a single-crystal piezoelectric material, such as lithium tantalate or lithium niobate. These materials also exhibit ferroelectric properties. It is recalled that a ferroelectric material is a material that has spontaneous electrical polarization in its natural state. Thin layer 4 of a POI structure must exhibit uniform polarization, i.e. all the dipole moments are aligned parallel to each other in a given direction.

[0004] The support 2 is preferably chosen, for its part, from silicon. It may be a support consisting of a base substrate made from monocrystalline silicon which may have a resistivity greater than 1000 Ohms.cm. Alternatively, and as shown in the, the support 2 may be formed from a base substrate 2a on which an electric charge trapping layer 2b is arranged. In this alternative, the intercalary dielectric layer 3 is arranged in contact with the trapping layer 2b.

[0005] A more complete presentation of a POI structure can be found in the document FR3068508 and in the publication by E. Butaud et al., "Smart Cut™ Piezo On Insulator (POI) substrates for high performance SAW components," 2020 IEEE International Ultrasonics Symposium (IUS), Las Vegas, NV, USA, 2020, pp. 1-4, doi: 10.1109 / IUS46767.2020.9251517.

[0006] Document WO2020200986A1 proposes a method for manufacturing such a POI substrate making it possible to preserve the single-domain character of the thin layer. This document provides for transferring onto the support 2 a layer taken from a donor substrate comprising a piezoelectric material, via a step of implanting light species in accordance with the principles of Smart Cut™ technology. Following this transfer, the taken layer is treated during a finishing sequence comprising a heat treatment followed by a polishing step, this finishing sequence leading to the formation of the piezoelectric, single-crystalline, single-domain thin layer 4.During this sequence, it was observed that the heat treatment led to the formation of a multi-domain surface portion on the sampled layer, this multi-domain surface portion then being eliminated by the following polishing treatment, which leads to providing the thin layer 4 having the required single-domain quality.

[0007] However, under certain conditions, particularly when the implantation of light species is carried out at a high dose and / or at a high current in order to increase production rates, the presence of defects in the thin layer 4 has been observed. A first type of defects observed, with reference to the, consists of the presence of depressions or protuberances, which can sometimes take the form of a volcano, on the surface of the thin layer, these depressions / protuberances D1 making the thickness of the thin layer 4 non-uniform. These D1 defects, designated by "depression type defect" for simplicity of expression, visible on the left insert of the, are of general circular or elliptical shapes whose dimension (the diameter or the major axis) is of the order of 1 micron to 100 microns, and which can sometimes have a high aspect ratio.They have a depth or elevation typically between 1 and 30 nanometers relative to the exposed surface of the thin layer 4.

[0008] A second type of observed defect consists of the presence of D2 "triangle defects". These defects appear as ferroelectric domain inversion bars with triangular sections of 0.1 microns to 10 microns on each side, as seen in the right-hand insert of the. The bars emerge at the surface of thin layer 4 and extend into the thickness of thin layer 4, in some cases even crossing it. They are oriented in a direction antiparallel to the spontaneous polarization direction Ps of piezoelectric thin layer 4. These triangle defects can have a density greater than 10^3 / cm² on the exposed surface of thin layer 4.

[0009] These two forms of defects, depression type and triangle defects, have a significant impact on the performance of devices, for example acoustic filters, formed on and in POI substrates. SUBJECT OF THE INVENTION

[0010] An aim of the invention is to remedy, at least in part, this problem. More specifically, an aim of the invention is to propose a structure of the piezoelectric on insulator type whose piezoelectric thin layer is free from depression-type defects or triangle defects or, at least, which has these defects in a lower density than a piezoelectric thin layer obtained by a method of the state of the art. BRIEF DESCRIPTION OF THE INVENTION

[0011] In order to achieve this aim, the subject of the invention proposes a method for preparing a single-domain thin layer of ferroelectric material comprising lithium, the method comprising:a step of implanting light species in a first face of a ferroelectric donor substrate comprising lithium to form a weakening plane and define a first layer between the weakening plane and the first face of the donor substrate;a step of assembling the first face of the donor substrate to the support, preferably by means of an intercalary dielectric layer to form an intermediate assembly;a step of fracturing the intermediate assembly, comprising a first heat treatment, this step leading to the fracture of the donor substrate at the weakening plane and to the formation of a free face of the first layer;a finishing sequence of the first layer comprising an annealing step comprising a second heat treatment and, after the annealing step, a step of thinning the first layer to form the single-domain thin layer.;

[0012] According to the invention, the preparation method comprises: a surface treatment exposing the free face (9) of the first layer (8) to a treatment atmosphere comprising at least 0.02% carbon dioxide to form a lithium-rich passivation layer; a treatment for removing the lithium-rich passivation layer.

[0013] According to other advantageous and non-limiting characteristics of the invention, taken alone or in any technically feasible combination: the treatment atmosphere has a temperature between 100°C and the Curie temperature of the ferroelectric material constituting the first layer; the removal treatment is carried out by wet cleaning of the free face of the first layer; the cleaning comprises brushing the free face of the first layer; the brushing is carried out while dispensing deionized water onto the free face of the first layer; the surface treatment and the removal treatment are carried out during an intermediate step arranged between the fracture step and the finishing sequence; the surface treatment is carried out during the fracture step, the exposure of the free face of the first layer to the treatment atmosphere being carried out during the first heat treatment or directly at the end thereof;the treatment atmosphere comprises at least 0.02% carbon dioxide in a neutral gas, such as argon or nitrogen, the first heat treatment being conducted at a temperature between 100°C and 700°C;the surface treatment is carried out during the annealing step of the finishing sequence, the exposure of the free face of the first layer to the treatment atmosphere being conducted during the second heat treatment;the treatment atmosphere comprises at least 0.02% carbon dioxide in oxygen or in a neutral gas, such as argon or nitrogen, the second heat treatment being conducted at a temperature between and 300°C and the Curie temperature of the ferroelectric material composing the first layer;the treatment atmosphere comprises more than 0.05% carbon dioxide;the support is formed of a solid electrically conductive or semiconductive substrate;the support comprises a base substrate and a trapping layer, the trapping layer being disposed between the interlayer dielectric layer and the base substrate;the first layer and the thin layer are made of a single-crystal piezoelectric material, such as lithium tantalate or lithium niobate;the interlayer dielectric layer comprises at least one layer of silicon oxide, silicon oxynitride or silicon nitride.; BRIEF DESCRIPTION OF THE FIGURES

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

[0015]

[0016] Figures 1a, 1b represent state-of-the-art POI structures;

[0017] It represents the defects present in a thin layer of a POI structure;

[0018] Represents the steps in manufacturing a POI structure;

[0019]

[0020]

[0021]

[0022] Figures 4a, 4b, 4c, 4d represent the state of the first layer of a POI structure during the different stages of its preparation;

[0023]

[0024]

[0025] Figures 5a, 5b, 5c represent three modes of implementation of the invention. DETAILED DESCRIPTION OF THE INVENTION

[0026] We first recall the steps of a method for manufacturing a POI 1 structure presented in the introductory part of this application and represented in figures 1a, 1b.

[0027] With reference to the, this method generally provides for transferring a first ferroelectric layer 8 onto a support 2, the first layer 8 being taken from a single-domain ferroelectric donor substrate 5 by a transfer technique based on the implantation of light species such as hydrogen and / or helium species. In the context of the present disclosure, the ferroelectric material of the donor substrate 5 comprises lithium. It may be, for example, lithium tantalate or lithium niobate. In addition to the ferroelectric properties, the material of the donor substrate also has piezoelectric properties. The ferroelectric material of the substrate advantageously has a crystalline direction of between 30° and 60°RY, without however excluding other crystalline directions.The donor substrate 5 may correspond to a solid substrate consisting entirely of the ferroelectric and piezoelectric material, as shown in the, or it may be a composite substrate formed of a solid part, for example made of silicon, on which rests a thick layer of ferroelectric and piezoelectric material from which the first layer 8 can be taken.

[0028] In certain embodiments, the support 2 consists of a solid conductive or semiconductive substrate. In other embodiments, the support 2 comprises a base substrate 2a provided with a superficial electric charge trapping layer 2b. This trapping layer 2b is arranged on the side of the first face of the support 2, which is intended to receive the thin layer 4. In these embodiments, an interposed dielectric layer is in contact with the trapping layer 2b and with the thin layer 4.

[0029] According to the transfer technique based on the implantation of light species, and with reference to Figure 3b, hydrogen and / or helium are implanted in a first face 6 of the donor substrate 5 to form a buried embrittlement plane 7 there. Advantageously, the implantation dose is greater than 8E16 at / cm^2 and / or the implantation current is greater than 20 mA, in order to increase the production rate. But the implantation current can also be more conventional, for example between 10 mA and 15 mA. In this way, the first layer 8 is defined between the embrittlement plane 7 and the first face 6 of the donor substrate 1. Then, as shown in Figure 3c, and during an assembly step, the first face 6 of the donor substrate is assembled to an exposed face 6' of the support 2 to form an intermediate assembly.An intermediate dielectric layer may be provided between the support 2 and the donor substrate 5, this dielectric layer having been formed on one and / or the other of the two substrates 2, 5. For example, the intermediate dielectric layer may comprise or be made of silicon oxide, silicon oxynitride or silicon nitride.

[0030] During a subsequent fracture step, the donor substrate 5 is then fractured at the weakening plane 7, for example using a first moderate heat treatment, of the order of 200°C in an atmosphere formed by a neutral gas, applied to the intermediate assembly and / or the application of a mechanical force. The first layer 8 of the donor substrate 5 is then released to expose a free face 9 of the first layer 8, the other face 4 of the first layer 8 being in direct contact with the support 2 or with the intercalary dielectric layer if such a layer is present.

[0031] A remaining portion 5' of the donor substrate 5, after the removal of the first layer 8, can be reconditioned in order to remove a new layer, in a removal cycle similar to that which has just been described.

[0032] It is generally necessary to provide for the finishing of the first layer 8 transferred and transferred onto the support 2, to form the thin layer 4 of the POI structure 1. This finishing may comprise a sequence of steps generally aimed at improving the crystalline quality of the first layer 8, adjusting its thickness to a desired thickness and improving its surface condition (for example its roughness).

[0033] As reported in the introduction to this application, this finishing sequence comprises an annealing step comprising a second heat treatment of the free face 9 of the first layer 8, followed by a step of thinning the first layer 8 to form the single-domain thin layer 4.

[0034] The second heat treatment of the free face 9 of the first layer 8 may correspond to exposing this layer to a neutral atmosphere or one comprising oxygen brought to a temperature between 300°C and the Curie temperature of the ferroelectric material making up the first layer 8, and for a duration between 30 minutes and 10 hours. This Curie temperature, for example, is of the order of 600°C for lithium tantalate, and of the order of 1210°C for lithium niobate.

[0035] The thinning step can, for example, be implemented by chemical-mechanical polishing or by etching.

[0036] At the end of these steps, we have a substrate composed of the thin layer 4 of ferroelectric material comprising lithium, on and in contact with the interlayer 3, itself on and in contact with the support 2.

[0037] In order to understand the origin of the defect in the thin layer 4 which was presented in the introduction, the applicant carried out a very careful analysis of the first layer 8 obtained at the end of the process presented on the, that is to say preceding the finishing of this first layer 8 in order to prepare the single-domain thin layer 4.

[0038] With reference to the, the applicant thus observed the presence of a surface thickness 11 rich in lithium on the first layer 8 obtained directly after the fracture step. This surface thickness 11 is made up of Li2CO3. Its formation seems to be favored by the particular conditions in which this fracture step takes place. The presence of light species, hydrogen and / or helium, and the moderate temperature at which the fracture occurs seem to make the lithium of the first layer 8 particularly mobile and the surface of this layer 8 particularly reactive.When the first layer 8 is released from the donor substrate at the time of its fracture, to expose the free face 9 of the first layer 8 to the atmosphere of the first heat treatment or in the ambient atmosphere, this surface reacts with the carbon dioxide, hydrocarbons and oxygen naturally present in these atmospheres, to form the surface thickness 11 of Li2CO3. This surface thickness, covering the first layer 8, is of the order of a nanometer. It is stable over time, that is to say that it does not change in consistency or thickness when the first layer is kept exposed to the atmosphere.

[0039] This surface thickness 11 is however relatively fragile, and the applicant has observed that it could be removed by wet cleaning of the first layer 8.

[0040] The applicant also observed that the first layer 8, devoid of its lithium-rich surface thickness 11, remained particularly reactive. By keeping the free face 9 of this first layer 8 exposed to the ambient atmosphere for an extended period of time, amorphous dendrites 12, rich in lithium and hydrogen (and other species present in the atmosphere such as carbon, chlorine or fluorine) nucleate and develop on the free surface 9 of the first layer 8. This development is particularly notable at the end of a period which can extend, at ambient temperature, between 50 hours and 75 hours.As illustrated in the, these dendrites 12 are distributed in a non-uniform manner on the surface of the first layer 8: they accumulate densely in certain areas on the surface of the layer 8, in particular at the level of certain topologies of this surface, such as local roughness or topologies caused by the emergence of dislocations, while other areas are entirely devoid of them.

[0041] The applicant applied the second heat treatment of the finishing sequence to the first layer 8 provided with these dendrite-dense areas 12 and these other areas devoid of these dendrites.

[0042] The first layer 8 presented (), at the end of this heat treatment, a multidomain surface layer 13, as documented in the reference cited in the introduction to this application. The dendrites 12 had disappeared from the first layer 8, certainly dissolved during the heat treatment. However, in the areas of the first layer 8 which were initially dense in dendrites 12, the multidomain surface layer presented an atypical morphology 14, and distinct from the morphology of this multidomain surface layer 13 in the areas initially devoid of dendrites. This atypical morphology 14 is characterized by a lesser multidomain surface layer thickness, as if the presence of dendrites 12 in the dense areas had limited the phenomenon of generation of this layer 13. Furthermore, the applicant observed the presence of triangle defects 15 in the first layer, in a density greater than 10^4 / cm².

[0043] The applicant then applied the thinning step, by chemical-mechanical polishing, to the first layer 8 obtained at the end of the heat treatment and thus provided the thin layer 4, shown in the. It will be noted that the triangle defects 15 are made visible on the surface of the thin layer 4, after the first layer 8 has been treated by chemical-mechanical polishing in order to eliminate the superficial multi-domain layer 13. These defects 15 were however present in the first layer 8 before the thinning step.

[0044] The thin layer 4 obtained also presented depression-type defects 16 at the level of the zones with atypical morphology 14 of the multi-domain surface layer 13.

[0045] From these results and observations, the applicant established a method for preparing a single-domain thin layer 4 made of ferroelectric material comprising lithium, making it possible to obtain a thin layer 4 which does not exhibit, or exhibits to a very limited extent, the defects noted above. The applicant in particular had the intuition that the reduction or elimination of the species made very mobile on the surface of the first layer 8 by the fracture step, and in particular lithium, could prevent the appearance of these defects in the thin layer 4.

[0046] To this end, the applicant proposes to introduce, into the process for preparing a single-domain thin layer which has just been recalled, a surface treatment exposing the free face 9 of the first layer 8 to a treatment atmosphere formed of carbon dioxide.

[0047] This surface treatment aims to incorporate into a passivation layer a maximum of mobile species on the surface of the first layer 8, and in particular lithium, which make this surface so reactive. This passivation layer (made up of or comprising Li2CO3) is therefore rich in lithium. It has a thickness at least equal to 2 nm and which can be between 5 nm and 10 nm. This thickness is much greater than that of the lithium-rich surface layer which forms natively from traces of carbon monoxide present in the atmospheres to which the first layer 8 can be exposed in the conventional process (of the order of 1 nm).

[0048] The preparation method according to the invention also comprises a treatment for removing the passivation layer, for example using a cleaning step or during the step of thinning the first layer 8, depending on the chosen embodiment.

[0049] The surface treatment therefore makes it possible, by incorporating the mobile lithium present superficially in the first layer 8 into a passivation layer, and by eliminating this passivation layer, to reduce or even eliminate this mobile lithium from the first layer. In this way, the surface is made less reactive and the appearance of amorphous dendrites 12 on the first layer, which appear to be the cause of the defects presented in the introduction to this application, is prevented or limited.

[0050] By thus limiting the quantity of mobile lithium present superficially in the first layer 8, the quantity of lithium capable of diffusing into the rest of the substrate, and in particular into the support 2, is also limited, which could affect the electrical characteristics of this support 2, in particular its resistivity.

[0051] It is noted that it is sometimes envisaged to place a diffusion barrier layer in the substrate 1, in order to limit the quantity of lithium reaching, by diffusion, the support 2. This barrier may consist of a layer of silicon nitride incorporated in the interlayer dielectric layer 3 or by enriching such an interlayer dielectric layer 3 with silicon oxide with a proportion of nitrogen. A substrate having received the surface treatment in accordance with the invention does not require such a barrier layer or, if it is nevertheless provided, it is not necessary for the barrier effect of this layer to be particularly high. For example, the proportion of nitrogen in the interlayer dielectric layer 3 can be reduced, in comparison with the conventional approach, without taking the risk of excessive contamination of the support 2 by lithium.

[0052] To obtain the benefits of the surface treatment that have just been presented, the treatment atmosphere is chosen to have an amount of carbon dioxide that exceeds the traces of carbon dioxide present in the atmospheres to which the first layer 8 is exposed in the conventional method. This treatment atmosphere can thus comprise at least 0.02% (by volume) of carbon dioxide. This carbon dioxide can be incorporated into a neutral gas, for example argon or nitrogen, or into an oxidizing gas, as will be specified later in the description of the different modes of implementation. Advantageously, the treatment atmosphere comprises more than 0.05%, by volume, of carbon dioxide.

[0053] The surface treatment can be carried out by bringing the treatment atmosphere to temperature, in order to promote the reaction occurring on the free surface 9 of the first layer 8, and accelerate the formation of the passivation layer. This temperature can be for example between 100°C and the Curie temperature of the ferroelectric material making up the first layer 8.

[0054] The duration of exposure of the free surface 9 of the first layer 8 to the treatment atmosphere is preferably chosen to be greater than 5 minutes and typically between 10 minutes and 90 minutes.

[0055] Generally speaking, the duration of this exposure, the temperature to which the treatment atmosphere is brought and the proportion of carbon dioxide present in this atmosphere will be chosen to form a sufficiently thick passivation layer and therefore incorporating a significant proportion of the mobile lithium present in a surface thickness of the first layer 8. As has been said, the aim is generally to form a passivation layer having a thickness at least equal to 2 nm.

[0056] The formation of the passivation layer can be carried out in a chamber, for example the chamber of an oven, in which the substrate comprising the first layer 8 will have been placed. The treatment atmosphere is introduced into the chamber in order to expose the free face 9 of the first layer 8 to this atmosphere.

[0057] The removal of the passivation layer typically formed of Li2CO3 can be implemented by simple cleaning, for example by wet cleaning, of the free face 9 of the first layer 8. This cleaning can comprise or consist of brushing the free face of the first layer 8 while dispensing deionized water onto this free face. The experiments carried out by the applicant have shown that this cleaning is entirely suitable for removing a passivation layer of Li2CO3. Other techniques can of course be provided for removing this layer, for example by etching or polishing.

[0058] In a first embodiment shown schematically in the, the surface treatment is carried out during an intermediate step arranged between the fracture step and the finishing sequence. This intermediate step therefore comprises, as has just been explained, the surface treatment exposing the free face 9 of the first layer 8 to the treatment atmosphere, then the treatment for removing the passivation layer, for example by simple cleaning. The finishing sequence (annealing followed by thinning of the first layer 8) can then be applied to the substrate thus treated.

[0059] Other implementation methods propose to integrate at least the surface treatment into the pre-existing steps of the process, in order to avoid introducing an additional step as in the first implementation method.

[0060] Thus, in a second embodiment shown in the, the surface treatment is carried out during the fracture step. The exposure of the free face 9 of the first layer 8 to the treatment atmosphere is then carried out during the first heat treatment of this step or directly at the end of it.

[0061] In this second embodiment, the treatment atmosphere comprises at least 0.02% carbon dioxide in a neutral gas, such as argon or nitrogen. The first heat treatment is carried out at a temperature between 100°C and 700°C, and the treatment atmosphere is therefore brought to this temperature.

[0062] In a first variant of this second embodiment, the fracture of the donor substrate occurs during the first heat treatment itself. This is typically carried out in a fracture furnace. In this variant, the treatment atmosphere can be introduced into the fracture furnace for the entire duration of the first heat treatment or at the end of this first heat treatment. When the fracture occurs and the first layer 8 is released from the donor substrate, the free surface 9 of this layer 8 is then exposed to the treatment atmosphere, and the passivation layer is formed. In this variant, the surface treatment in accordance with the invention is therefore carried out “in situ” in the fracture furnace, that is to say during a single step of the process and without moving the substrate from one piece of equipment to another.

[0063] In a second variant of this second embodiment, the fracture of the donor substrate 5 occurs directly following the first heat treatment, for example by applying mechanical force to the intermediate assembly formed by the donor substrate 5 and the support 2. In this variant, the application of the mechanical force, if it is not carried out in the fracture furnace, can be carried out in a chamber of dedicated equipment filled with the treatment atmosphere. In this variant also, when the fracture occurs and the first layer 8 is released from the donor substrate 5, the free surface 9 of this layer 8 is exposed to the treatment atmosphere, and the passivation layer is formed.

[0064] Whether the preparation process is carried out according to the first or second variant, in all cases the removal of the passivation layer is provided for, for example by means of simple cleaning as presented previously.

[0065] In a third embodiment shown in the, the surface treatment is carried out during the finishing sequence, and more precisely during the second heat treatment of the annealing step of this sequence. The exposure of the free face 9 of the first layer 8 to the treatment atmosphere is then carried out during the second heat treatment included in this sequence or directly at the end of it, by introducing the treatment atmosphere into the furnace implementing this second heat treatment. In this embodiment, the surface treatment is therefore carried out "in situ" in the annealing furnace, that is to say during a single step of the process and without moving the substrate from one piece of equipment to another.

[0066] In this third embodiment, the treatment atmosphere comprises at least 0.02% carbon dioxide in oxygen or in a neutral gas, such as argon or nitrogen. The second heat treatment is carried out at a temperature between 300°C and 300°C and the Curie temperature of the ferroelectric material making up the first layer 8. The treatment atmosphere is therefore brought to this temperature.

[0067] In this third embodiment, the removal of the passivation layer does not require a dedicated step, for example surface cleaning. This removal can take place during the thinning step of the finishing sequence. However, such cleaning can be provided if it appears useful for reasons other than the removal of the passivation layer.

[0068] Of course, the invention is not limited to the method of implementation described and variant embodiments can be made without departing from the scope of the invention as defined by the claims.

Claims

A method for preparing a single-domain thin layer (4) of ferroelectric material comprising lithium, the method comprising:a step of implanting light species in a first face (6) of a ferroelectric donor substrate (5) comprising lithium to form a weakening plane (7) and define a first layer (8) between the weakening plane (7) and the first face (6) of the donor substrate (5);a step of assembling the first face (6) of the donor substrate (5) to the support (2), preferably by means of an intercalary dielectric layer (3) to form an intermediate assembly;a step of fracturing the intermediate assembly, comprising a first heat treatment, this step leading to the fracture of the donor substrate (5) at the weakening plane (7) and to the formation of a free face (9) of the first layer (8);a finishing sequence of the first layer (8) comprising an annealing step comprising a second heat treatment and, after the annealing step, a step of thinning the first layer (8) to form the single-domain thin layer (4);the preparation method being characterized in that it comprises:a surface treatment exposing the free face (9) of the first layer (8) to a treatment atmosphere comprising at least 0.02% carbon dioxide to form a lithium-rich passivation layer;a treatment for removing the lithium-rich passivation layer.; Preparation method according to the preceding claim in which the treatment atmosphere has a temperature between 100°C and the Curie temperature of the ferroelectric material making up the first layer (8). Preparation method according to one of the preceding claims in which the removal treatment is carried out by cleaning the free face (9) of the first layer (8) by wet method. Preparation method according to the preceding claim in which the cleaning comprises brushing the free face (9) of the first layer (8). Preparation method according to the preceding claim in which the brushing is carried out while dispensing deionized water onto the free face (9) of the first layer (8). Preparation method according to one of the preceding claims in which the surface treatment and the removal treatment are carried out during an intermediate step arranged between the fracture step and the finishing sequence. Preparation method according to one of claims 1 to 5 in which the surface treatment is carried out during the fracture step, the exposure of the free face (9) of the first layer (8) to the treatment atmosphere being carried out during the first heat treatment or directly at the end thereof. Preparation process according to the preceding claim in which the treatment atmosphere comprises at least 0.02% carbon dioxide in a neutral gas, such as argon or nitrogen, the first heat treatment being carried out at a temperature between 100°C and 700°C. Preparation method according to one of claims 1 to 5 in which the surface treatment is carried out during the annealing step of the finishing sequence, the exposure of the free face (9) of the first layer (8) to the treatment atmosphere being carried out during the second heat treatment. Preparation method according to the preceding claim in which the treatment atmosphere comprises at least 0.02% carbon dioxide in oxygen or in a neutral gas, such as argon or nitrogen, the second heat treatment being carried out at a temperature between 300°C and 300°C and the Curie temperature of the ferroelectric material making up the first layer (8). Preparation process according to one of the preceding claims in which the treatment atmosphere comprises more than 0.05% carbon dioxide. Preparation method according to one of the preceding claims in which the support (2) is formed from a solid electrically conductive or semi-conductive substrate. Preparation method according to one of the preceding claims, in which the support (2) comprises a base substrate (2a) and a trapping layer (2b), the trapping layer (2b) being arranged between the intercalary dielectric layer (3) and the base substrate (2a). Preparation method according to one of the preceding claims in which the first layer (8) and the thin layer (4) are made of a monocrystalline piezoelectric material, such as lithium tantalate or lithium niobate. Preparation method according to one of the preceding claims in which the intercalary dielectric layer (3) comprises at least one layer of silicon oxide, silicon oxynitride or silicon nitride.

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