Process for preparing thin monodomain layers of lithium-containing ferroelectric materials

Abstract

The present invention relates to a process for preparing a thin monodomain layer (4) of a lithium-containing ferroelectric material, the process comprising providing a first layer (8) having a free surface (9). According to the invention, the preparation process comprises a surface treatment in which the free surface (9) of the first layer (8) is exposed to a treatment atmosphere containing at least 0.02% carbon dioxide to form a lithium-rich passivation layer, and a removal treatment in which the lithium-rich passivation layer is removed.

Description

[Technical Field]

[0001] The present invention relates to piezoelectric-on-insulator (POI) structures. Such structures are particularly applicable in the fields of microelectronics, microsystems, and photonics. Such structures can be used, in particular, to form radio-frequency (RF) components or to fabricate such components, in particular filters or resonators for acoustic waves, e.g., surface acoustic waves. [Background technology]

[0002] 1a and 1b, which show a conventional POI structure, the POI structure is typically formed of a thin piezoelectric layer 4 bonded to a first surface of a carrier 2. A dielectric interlayer 3 is disposed between and contacts the carrier 2 and the thin layer 4.

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

[0004] The carrier 2 is preferably made of silicon. This can be the case for a carrier consisting of a base substrate made of monocrystalline silicon, which may have a resistivity higher than 1000 Ω·cm. Alternatively, as shown in FIG. 1b, the carrier 2 can be formed by a base substrate 2a on which a layer 2b for trapping charges is arranged. In this alternative embodiment, a dielectric intermediate layer 3 is arranged in contact with the trapping layer 2b.

[0005] A more comprehensive description of the POI structure can be found in 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] For its part, WO 2020200986 A1 provides a process for producing such a POI substrate, which allows the monodomain properties of the thin layer to be maintained. This document provides for the transfer of a sampled layer from a donor substrate containing piezoelectric material to a carrier 2 by implanting light species according to the principles of Smart Cut® technology. Following this transfer, the sampled layer is subjected to a finishing sequence including a heat treatment followed by a polishing step, which results in the formation of a thin, single-crystalline, monodomain piezoelectric layer 4. It has been observed that during this sequence, the heat treatment results in the formation of multidomain surface segments on the sampled layer, which are then removed by a subsequent polishing operation, resulting in a thin layer 4 with the required monodomain properties.

[0007] However, under certain conditions, especially when light species are implanted at high doses and / or currents with the aim of increasing production rates, defects have been observed to appear in the thin layer 4. Referring to Figure 2, the first type of defect observed consists in the presence of pits or protrusions, which can sometimes take the form of craters, on the surface of the thin layer. These pits / protrusions D1 cause the thickness of the thin layer 4 to become non-uniform. These defects D1, referred to for simplicity as "pit defects" (an example of which is shown in the left inset of Figure 2), are circular or elliptical, have dimensions (diameter or major axis) on the order of 1 micron to 100 microns, and sometimes have a high aspect ratio. They typically have a depth or height relative to the exposed surface of the thin layer 4 that is comprised between 1 and 30 nanometers.

[0008] The second type of defect observed is the presence of "triangular defects" D2. These defects take the form of rods of ferroelectric domain inversions with a triangular cross section with sides between 0.1 microns and 10 microns long, as shown in the inset on the right side of Figure 2. The rods appear on the surface of the thin layer 4 and extend through the thickness of the thin layer 4, and in some cases, even through the thin layer 4. They are oriented in a direction antiparallel to the spontaneous polarization direction Ps of the thin piezoelectric layer 4. These triangular defects have a density of 10^3 / cm on the exposed surface of the thin layer 4. 2 It may have a higher density.

[0009] These two types of defects, pit defects and triangular defects, have a significant impact on the performance of devices, such as acoustic filters, formed on and in POI substrates.

[0010] Subject of the Invention An object of the present invention is to at least partially remedy this problem, or more precisely, to provide a piezoelectric-on-insulator structure having a thin piezoelectric layer that is free of pit and triangular defects, or at least has a lower density of these defects than thin piezoelectric layers obtained using prior art processes. Summary of the Invention

[0011] To this end, the present subject matter provides a process for preparing a thin monodomain layer of a lithium-containing ferroelectric material, the process comprising: - implanting light species into a first surface of the lithium-containing ferroelectric donor substrate to form an embrittlement plane and define a first layer between the embrittlement plane and the first surface of the lithium-containing dielectric donor substrate; - bonding a first side of a lithium-containing dielectric donor substrate to a carrier, preferably by means of a dielectric interlayer, to form an intermediate assembly; - dividing the intermediate assembly comprising a first heat treatment, which results in dividing the lithium-containing ferroelectric donor substrate along the embrittlement plane and forming a free surface of the first layer; - a finishing sequence applied to the first layer, the finishing sequence including an annealing step including a second heat treatment and, after the annealing step, thinning the first layer to form a thin monodomain layer; Includes:

[0012] According to the present invention, the preparation process comprises: - a surface treatment by exposing the free surface of the first layer to a treatment atmosphere containing at least 0.02% carbon dioxide to form a lithium-rich passivation layer; - a removal treatment to remove the lithium-rich passivation layer; Includes:

[0013] According to other advantageous and non-limiting features of the present invention, alone or in any technically feasible combination: the treatment atmosphere has a temperature comprised between 100°C and the Curie temperature of the lithium-containing ferroelectric material from which the first layer is made, the removal treatment is carried out by cleaning the free surface of the first layer using a wet treatment, - cleaning includes brushing the free surface of the first layer; - brushing is performed while dispensing deionized water onto the free surface of the first layer; - surface treatment and removal processes are carried out during intermediate steps between the dividing step and the finishing sequence, the surface treatment is carried out during the dividing step, and the exposure of the free surface of the first layer to the treatment atmosphere is carried out during or immediately after the first heat treatment, the treatment atmosphere comprises at least 0.02% carbon dioxide in a neutral gas such as argon or nitrogen, and the first heat treatment is carried out at a temperature comprised between 100°C and 700°C; - the surface treatment is carried out during the annealing step of the finishing sequence, and the exposure of the free surface of the first layer to the treatment atmosphere is carried out 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, and the second heat treatment is carried out at a temperature comprised between 300°C and the Curie temperature of the lithium-containing ferroelectric material from which the first layer is made; - the treatment atmosphere contains more than 0.05% carbon dioxide, - the carrier is formed from a conductive or semiconductive bulk substrate, the carrier includes a base substrate and a trapping layer, the trapping layer being disposed between the dielectric interlayer and the base substrate; the first layer and the thin monodomain layer are made of a single crystal piezoelectric material such as lithium tantalate or lithium niobate; The dielectric interlayer comprises at least one layer of silicon oxide, silicon oxynitride, or silicon nitride. [Brief explanation of the drawings]

[0014] Other features and advantages of the present invention will become apparent from the following detailed description of the invention which refers to the accompanying drawings.

[0015] [Figure 1a] 1 shows a prior art POI structure. [Figure 1b] 1 shows a prior art POI structure. [Figure 2] Defects present in the thin layers of the POI structure are shown. [Figure 3]The manufacturing process of the POI structure is shown. [Figure 4a] 1 shows the state during various steps of the preparation of the first layer of the POI structure. [Figure 4b] 1 shows the state during various steps of the preparation of the first layer of the POI structure. [Figure 4c] 1 shows the state during various steps of the preparation of the first layer of the POI structure. [Figure 4d] 1 shows the state during various steps of the preparation of the first layer of the POI structure. [Figure 5a] Three embodiments of the present invention are shown. [Figure 5b] Three embodiments of the present invention are shown. [Figure 5c] Three embodiments of the present invention are shown. DETAILED DESCRIPTION OF THE INVENTION

[0016] First, recall the process steps for manufacturing the POI structure 1 as presented in the introductory part of this patent application and as shown in Figures 1a and 1b.

[0017] Referring to FIG. 3, this process generally defines the transfer of a first ferroelectric layer 8 to a carrier 2. The first layer 8 is sampled from a monodomain ferroelectric donor substrate 5 using 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 contains lithium. It may be, for example, lithium tantalate or lithium niobate. Apart from its ferroelectric properties, the material of the donor substrate also has piezoelectric properties. Advantageously, the ferroelectric material of the substrate has a crystallographic orientation between 30° and 60° R, although other crystallographic orientations are also possible. The donor substrate 5 (as shown in FIG. 3) may correspond to a bulk substrate entirely composed of ferromagnetic and piezoelectric materials, or it may be a composite substrate formed from a bulk portion, e.g., silicon, on which a thick layer of ferroelectric and piezoelectric materials is deposited, from which the first layer 8 is sampled.

[0018] In certain embodiments, the carrier 2 consists of a conductive or semiconductive bulk substrate. In other embodiments, the carrier 2 comprises a base substrate 2a with a surface layer 2b for trapping charges. This trapping layer 2b is disposed on a first side of the carrier 2, the side intended to receive the thin layer 4. In these embodiments, a dielectric interlayer is in contact with the trapping layer 2b and the thin layer 4.

[0019] In the light species implantation-based transfer technique, referring to FIG. 3B, hydrogen and / or helium are implanted into the first surface 6 of the donor substrate 5 to form an embedded embrittlement plane 7 therein. Advantageously, to increase the manufacturing speed, the implantation dose is greater than 8E16 at / cm^2 and / or the implantation current is greater than 20 mA. However, the implantation current may also be more conventional, for example, between 10 mA and 15 mA. Thus, a first layer 8 is defined between the embrittlement plane 7 and the first surface 6 of the donor substrate 1. Next, as shown in FIG. 3C, during an assembly process, the first surface 6 of the donor substrate is assembled to the exposed surface 6' of the carrier 2 to form an intermediate assembly. A dielectric interlayer may be provided between the carrier 2 and the donor substrate 5, and this dielectric layer is formed on one and / or the other of the two substrates 2, 5. By way of example, the dielectric interlayer may include or consist of silicon oxide, silicon oxynitride, or silicon nitride.

[0020] During a subsequent splitting step, the donor substrate 5 is then split along the embrittlement plane 7, for example by applying a first moderate heat treatment of around 200°C in an atmosphere of neutral gas, and / or by applying a mechanical force. The first layer 8 is then peeled off from the donor substrate 5, exposing the free surface 9 of the first layer 8, while the other surface 4 of the first layer 8 is in direct contact with the carrier 2 or with a dielectric intermediate layer, if such a layer is present.

[0021] The residual segment 5' of the donor substrate 5 that remains after sampling the first layer 8 may be reconditioned to sample a new layer in a sampling cycle similar to that described above.

[0022] In general, it is necessary to prepare the first layer 8 transferred and bonded to the carrier 2 for forming the thin layer 4 of the POI structure 1 for finishing. This finishing can generally include a series of steps aimed at improving the crystalline quality of the first layer 8, adjusting its thickness to the desired thickness, and improving its surface condition (e.g. its roughness).

[0023] As mentioned in the introduction to this patent application, this finishing sequence includes an annealing step involving a second heat treatment of the free surface 9 of the first layer 8, followed by a step of thinning the first layer 8 to form a thin monodomain layer 4.

[0024] The second heat treatment of the free surface 9 of the first layer 8 may correspond to exposing this layer for a time comprised between 30 minutes and 10 hours in a neutral or oxygen-containing atmosphere raised to a temperature comprised between 300° C. and the Curie temperature of the ferroelectric material of which the first layer 8 is made, which is, for example, around 600° C. for lithium tantalate and around 1210° C. for lithium niobate.

[0025] The thinning step may be, for example, a chemical mechanical polishing step or an etching step.

[0026] At the end of these steps, the result is a substrate consisting of a thin layer 4 of lithium-containing ferroelectric material lying on and in contact with an intermediate layer 3, which itself is lying on and in contact with a carrier 2.

[0027] With a view to understanding why the thin layer 4 exhibits the defects described in the introduction, the Applicant has carried out a very thorough analysis of the first layer 8 obtained at the end of the process shown in FIG. 3, i.e. before this first layer 8 is completed to prepare the thin monodomain layer 4.

[0028] Thus, referring to FIG. 4a, Applicant observed the presence of a lithium-rich surface layer 11 on the first layer 8 obtained immediately after the splitting step. This surface layer 11 consists of Li2CO3. Its formation appears to be promoted by the specific conditions under which the splitting step is performed. The presence of light species (hydrogen and / or helium) and the moderate temperature at which the splitting occurs appear to make the lithium in 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 during splitting and the free surface 9 of the first layer 8 is exposed to the atmosphere of the first heat treatment or the ambient atmosphere, this surface reacts with carbon dioxide, hydrocarbons, and oxygen naturally present in these atmospheres to form a surface layer 11 of Li2CO3. This surface layer covering the first layer 8 is on the order of one nanometer. It remains stable over time, i.e., its composition or thickness does not change even if the first layer continues to be exposed to the atmosphere.

[0029] However, this surface thickness 11 is relatively fragile and applicant has observed that it can be removed by cleaning the first layer 8 using a wet process.

[0030] Applicant also observed that first layer 8, lacking lithium-rich surface thickness 11, remains particularly reactive. When the free surface 9 of this first layer 8 is exposed to the atmosphere for an extended period of time, amorphous dendrites 12 rich in lithium and hydrogen (as well as other elements present in the atmosphere, such as carbon, chlorine, or fluorine) nucleate and develop on the free surface 9 of first layer 8. This development is particularly pronounced at the end of a period potentially ranging from 50 to 75 hours in ambient atmosphere. As shown in Figure 4b, these dendrites 12 are distributed unevenly across the surface of first layer 8, and they densely accumulate in certain regions of the surface of layer 8, particularly around specific topological features of this surface, such as localized areas of roughness or topological features caused by the appearance of dislocations, while other regions are completely devoid of them.

[0031] Applicant applied the second heat treatment of the finishing sequence to the first layer 8 containing such high density regions of dendrites 12 and such other regions lacking these dendrites.

[0032] At the end of this heat treatment, the first layer 8 possessed a multi-domain surface layer 13 (FIG. 4c), as described in the references cited in the introduction to this patent application. The dendrites 12 had disappeared from the first layer 8, indeed having dissolved during the heat treatment. However, in areas of the first layer 8 where the dendrites 12 were initially densely packed, the multi-domain surface layer possessed an atypical morphology 14 that differed from the morphology of this multi-domain surface layer 13 in areas initially devoid of dendrites. This atypical morphology 14 was characterized by a smaller thickness of the multi-domain surface layer, as if the presence of dendrites 12 in the dense regions limited the effectiveness of the formation of this layer 13. Furthermore, Applicant has demonstrated that the thickness of the multi-domain surface layer is less than 10^4 / cm 2 At higher densities, the presence of triangular defects 15 was observed in the first layer.

[0033] The Applicant then applied a thinning step by chemical mechanical polishing to the first layer 8 obtained at the end of the heat treatment, thus obtaining the thin layer 4 (shown in FIG. 4d). It should be noted that after treating the first layer 8 by chemical mechanical polishing to remove the multi-domain surface layer 13, triangular defects 15 become visible on the surface of the thin layer 4. However, these defects 15 were present in the first layer 8 before the thinning step.

[0034] The resulting thin layer 4 also contained pit defects 16 in the areas of the atypical morphology 14 of the multi-domain surface layer 13 .

[0035] Based on these results and observations, the applicant has designed a process for preparing a thin monodomain layer 4 of lithium-containing ferroelectric material, which makes it possible to obtain a thin layer 4 that is free of or has few of the defects mentioned above. In particular, the applicant has the intuition that reducing or removing species that have become highly mobile at the surface of the first layer 8 by the splitting step, specifically lithium, can prevent the appearance of these defects in the thin layer 4.

[0036] To this end, the Applicant proposes to incorporate in the process for preparing the thin monodomain layer described above a surface treatment in which the free surface 9 of the first layer 8 is exposed to a treatment atmosphere consisting of carbon dioxide.

[0037] The purpose of this surface treatment is to incorporate the most mobile species, especially lithium, into a passivation layer at the surface of the first layer 8, making this surface highly reactive. This passivation layer (consisting of or containing Li2CO3) is therefore lithium-rich. It has a thickness at least equal to 2 nm and may be comprised between 5 nm and 10 nm. This thickness is much greater than the thickness (of the order of 1 nm) of the lithium-rich surface layer that naturally forms in conventional processes from traces of carbon oxides present in the atmosphere to which the first layer 8 may be exposed.

[0038] The preparation process according to the invention also includes a removal treatment to remove the passivation layer, for example by a cleaning step or during the step of thinning the first layer 8, depending on the embodiment selected.

[0039] The surface treatment can reduce or even eliminate the mobile lithium present at the surface of the first layer 8 by incorporating it into a passivation layer and by removing this passivation layer, thus making the surface less reactive and preventing or limiting the appearance of amorphous dendrites 12 on the first layer, which are believed to be the cause of the defects described in the introduction of this patent application.

[0040] Thus, by limiting the amount of mobile lithium superficially present in the first layer 8, this also limits the amount of lithium that can diffuse into the rest of the substrate, and in particular into the carrier 2, which may affect the electrical properties of this carrier 2, in particular its resistivity.

[0041] It should be noted that in order to limit the amount of lithium that reaches the carrier 2 by diffusion, it may be envisaged to provide a layer constituting a diffusion barrier in the substrate 1. This barrier may consist of a layer of silicon nitride incorporated in the dielectric interlayer 3, or of such a dielectric interlayer 3 enriched in silicon oxide with a certain proportion of nitrogen. A substrate that has undergone the surface treatment according to the invention does not require such a barrier layer, or if one is provided, the barrier effect of this layer need not be particularly high. For example, the proportion of nitrogen in the dielectric interlayer 3 can be reduced compared to conventional approaches without risking excessive contamination of the carrier 2 by lithium.

[0042] To obtain the above-described surface treatment benefits, the treatment atmosphere is selected to have an amount of carbon dioxide that exceeds the trace amounts present in the atmosphere to which the first layer 8 is exposed in conventional processes. Thus, the treatment atmosphere may contain at least 0.02% (by volume) carbon dioxide. This carbon dioxide may be incorporated into a neutral gas, such as argon or nitrogen, or an oxidizing gas, as explained below in the description of various embodiments. Advantageously, the treatment atmosphere contains more than 0.05% by volume carbon dioxide.

[0043] The surface treatment may be carried out by bringing the treatment atmosphere to a temperature that promotes the reaction that occurs on the free surface 9 of the first layer 8 and accelerates the formation of the passivation layer. This temperature may be comprised, for example, between 100° C. and the Curie temperature of the ferroelectric material from which the first layer 8 is made.

[0044] 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 is typically comprised between 10 and 90 minutes.

[0045] Generally, the duration of this exposure, the temperature to which the treatment atmosphere is brought, and the proportion of carbon dioxide present in this atmosphere are selected to form a passivation layer that is sufficiently thick and thus incorporates a significant proportion of the mobile lithium present in the surface thickness of the first layer 8. As mentioned above, the overall aim is to form a passivation layer having a thickness at least equal to 2 nm.

[0046] The formation of the passivation layer may be carried out in a chamber, for example a furnace chamber, in which the substrate including the first layer 8 is disposed. A treatment atmosphere is introduced into the chamber so as to expose the free surface 9 of the first layer 8 to this atmosphere.

[0047] The removal of the passivation layer, which typically consists of Li2CO3, may be carried out by simply cleaning the free surface 9 of the first layer 8, for example by using a wet process. This cleaning may include or consist of brushing the free surface 9 of the first layer 8 while spraying deionized water onto this free surface. Experiments carried out by the Applicant have demonstrated that this cleaning is able to completely remove the Li2CO3 passivation layer. Of course, other techniques for removing this layer may be provided, for example by etching or polishing.

[0048] In a first embodiment, shown diagrammatically in Fig. 5a, the surface treatment is carried out during an intermediate step between the dividing step and the finishing sequence. This intermediate step therefore comprises a surface treatment in which the free surface 9 of the first layer 8 is exposed to a processing atmosphere, as described above, followed by a treatment to remove the passivation layer, for example by simple cleaning. A finishing sequence (annealing followed by thinning of the first layer 8) may then be applied to the substrate thus treated.

[0049] Another embodiment proposes to integrate at least the surface treatment into an existing step of the process in order to avoid introducing an additional step as in the first embodiment.

[0050] Thus, in the second embodiment shown in Figure 5b, the surface treatment is carried out during the splitting step, and the exposure of the free surface 9 of the first layer 8 to the treatment atmosphere is then carried out during or immediately after the first heat treatment of this step.

[0051] 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 comprised between 100°C and 700°C, and the treatment atmosphere is therefore brought to this temperature.

[0052] In a first variant of this second embodiment, the splitting of the donor substrate is carried out during the first heat treatment itself, which is typically carried out in a splitting furnace. In this variant, a treatment atmosphere may be introduced into the splitting furnace throughout the first heat treatment or at the end of this first heat treatment. Once the splitting has taken place and the first layer 8 has been released from the donor substrate, the free surface 9 of this layer 8 is exposed to the treatment atmosphere, and a passivation layer is formed. In this variant, the surface treatment according to the invention is carried out "in situ" in the splitting furnace, in other words, during a single step of the process, without transferring the substrate from one apparatus to another.

[0053] In a second variant of this second embodiment, the splitting of the donor substrate 5 is carried out immediately after the first heat treatment, for example by applying a mechanical force to the intermediate assembly formed from the donor substrate 5 and the carrier 2. In this variant, the application of the mechanical force may not be carried out in a splitting furnace, but may be carried out in a chamber of a dedicated device filled with a processing atmosphere. Also in this variant, once the splitting has been carried out and the first layer 8 has been released from the donor substrate 5, the free surface 9 of this layer 8 is exposed to the processing atmosphere, where a passivation layer is formed.

[0054] Whether the preparation process is carried out according to the first or second variant, the passivation layer is removed in all cases, for example simply by washing as described above.

[0055] In a third embodiment shown in FIG. 5c, the surface treatment is performed during the finishing sequence, more specifically during the second heat treatment of the annealing step of this sequence. The free surface 9 of the first layer 8 is then exposed to the treatment atmosphere during or immediately after the second heat treatment included in this sequence by introducing the treatment atmosphere into the furnace implementing this second heat treatment. Thus, in this embodiment, the surface treatment is performed "in situ" in the annealing furnace, i.e., during a single step of the process, without transferring the substrate from one apparatus to another.

[0056] 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 comprised between 300°C and the Curie temperature of the ferroelectric material from which the first layer 8 is made. The treatment atmosphere is therefore brought to this temperature.

[0057] In this third embodiment, the removal of the passivation layer does not require the implementation of a dedicated step, e.g., a surface cleaning. This removal may be performed during the thinning step of the finishing sequence. However, it is possible to provide such a cleaning if it is deemed beneficial for reasons other than the removal of the passivation layer.

[0058] Of course, the invention is not limited to the described embodiments, and variations of the embodiments can be applied without departing from the scope of the invention as defined by the claims.

Claims

1. A process for preparing a thin monodomain layer (4) of lithium-containing ferroelectric material, comprising: - implanting light species into a first surface (6) of a lithium-containing ferroelectric donor substrate (5) to form an embrittlement plane (7) and to define a first layer (8) between said embrittlement plane (7) and said first surface (6) of said lithium-containing ferroelectric donor substrate (5); - bonding said first side (6) of said lithium-containing ferroelectric donor substrate (5) to a carrier (2), preferably by means of a dielectric interlayer (3), to form an intermediate assembly; - dividing the intermediate assembly, including a first heat treatment, which results in the division of the lithium-containing ferroelectric donor substrate (5) along the embrittlement plane (7) and the formation of a free surface (9) of the first layer (8); a finishing sequence applied to said first layer (8), comprising an annealing step including a second heat treatment and, after said annealing step, a step of thinning said first layer (8) to form said thin monodomain layer (4); Including, The preparation process comprises: a surface treatment of exposing said free surface (9) of said first layer (8) to a treatment atmosphere containing at least 0.02% carbon dioxide to form a lithium-rich passivation layer; a removal treatment to remove said lithium-rich passivation layer; A preparation process comprising:

2. the treatment atmosphere has a temperature comprised between 100°C and the Curie temperature of the lithium-containing ferroelectric material from which the first layer (8) is made, The preparation process according to claim 1.

3. the removal treatment is carried out by cleaning the free surface (9) of the first layer (8) using a wet treatment, The preparation process according to claim 1 or 2.

4. said cleaning step comprising brushing said free surface (9) of said first layer (8); The preparation process according to claim 3.

5. said brushing is carried out while squirting deionized water onto said free surface (9) of said first layer (8); The preparation process according to claim 4.

6. the surface treatment and the removal treatment are carried out during an intermediate step between the dividing step and the finishing sequence; The preparation process according to any one of claims 1 to 5.

7. the surface treatment is carried out during the dividing step, and the exposure of the free surface (9) of the first layer (8) to the treatment atmosphere is carried out during or immediately after the first heat treatment; The preparation process according to any one of claims 1 to 5.

8. the treatment atmosphere comprises at least 0.02% carbon dioxide in a neutral gas such as argon or nitrogen, and the first heat treatment is carried out at a temperature comprised between 100°C and 700°C; The preparation process according to claim 7.

9. the surface treatment is carried out during the annealing step of the finishing sequence, and the exposure of the free surface (9) of the first layer (8) to the treatment atmosphere is carried out during the second heat treatment. The preparation process according to any one of claims 1 to 5.

10. the treatment atmosphere comprises at least 0.02% carbon dioxide in oxygen or in a neutral gas such as argon or nitrogen, and the second heat treatment is carried out at a temperature comprised between 300°C and the Curie temperature of the lithium-containing ferroelectric material from which the first layer (8) is made; The preparation process according to claim 9.

11. The treatment atmosphere contains more than 0.05% carbon dioxide. The preparation process according to any one of claims 1 to 10.

12. The carrier (2) is formed from a conductive or semiconductive bulk substrate; The preparation process according to any one of claims 1 to 11.

13. The carrier (2) includes a base substrate (2a) and a trapping layer (2b), and the trapping layer (2b) is disposed between the dielectric intermediate layer (3) and the base substrate (2a). The preparation process according to any one of claims 1 to 12.

14. the first layer (8) and the thin monodomain layer (4) are made of a single crystal piezoelectric material such as lithium tantalate or lithium niobate; The preparation process according to any one of claims 1 to 13.

15. The dielectric intermediate layer (3) comprises at least one layer of silicon oxide, silicon oxynitride, or silicon nitride; The preparation process according to any one of claims 1 to 14.

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