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 a piezoelectric-on-insulator (POI) structure. Such a structure is particularly applicable in the fields of microelectronics, microsystems and photonics. Such a structure may especially be used for forming radio-frequency (RF) components, or for fabricating such components, in particular filters or resonators for acoustic waves, for example surface acoustic waves. Background Art

[0002] Referring to FIGS. 1a and 1b showing 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 intermediate layer 3 is disposed between and in contact with the carrier 2 and the thin layer 4.

[0003] The thin layer 4 is made 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 spontaneous electric polarization in its natural state. The thin layer 4 of the POI structure needs to have uniform polarization, that is to say all the dipole moments must be aligned parallel to one another in a given direction.

[0004] The carrier 2 is preferably made of silicon. It may be a question of a carrier formed from a base substrate made of single-crystal silicon, which may have a resistivity higher than 1000 Ω·cm. Alternatively, as shown in FIG. 1b, the carrier 2 may be formed by a base substrate 2a provided with a layer 2b for trapping charges. In this alternative embodiment, the dielectric intermediate layer 3 is disposed in contact with the trapping layer 2b.

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

[0006] As part of this, International Publication No. 2020200986(A1) provides a process for manufacturing such POI substrates that enable the maintenance of monodomain properties in a thin layer. This document provides transferring a sampled layer from a donor substrate containing piezoelectric material to a carrier 2 by a step of injecting light species according to the principles of Smart Cut® technology. Following this transfer, the sampled layer is subjected to a finishing sequence including heat treatment and subsequent polishing steps, which result in the formation of a thin, single-crystal, monodomain piezoelectric layer 4. During this sequence, the heat treatment results in the formation of multidomain surface segments on the sampled layer, which are removed by a subsequent polishing operation, thereby obtaining a thin layer 4 with the desired monodomain quality.

[0007] However, under certain conditions, particularly when lighter species are injected at high doses and / or high 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 is the presence of pits or protrusions on the surface of the thin layer, which can sometimes take the form of craters, and these pits / protrusions D1 make the thickness of the thin layer 4 non-uniform. These defects D1 are referred to for brevity as “pit defects” (an example of which is shown in the inset on the left side of Figure 2), and are circular or elliptical, with dimensions (diameter or major axis) of about 1 to 100 microns, and sometimes have a high aspect ratio. They typically have a depth or height that is between 1 and 30 nanometers relative to the exposed surface of the thin layer 4.

[0008] The second type of defect observed is the presence of “triangular defects” D2. These defects take the form of ferroelectric domain inversion rods with a triangular cross-section having a side length of 0.1 to 10 microns, 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 to the thickness of the thin layer 4, and in some cases, penetrate 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 are present at a rate of 10^3 / cm² on the exposed surface of the thin layer 4. 2 It may have a higher density.

[0009] These two types of defects, namely pit defects and triangular defects, significantly affect the performance of devices formed on and within POI substrates, such as acoustic filters.

[0010] Subject of the invention One object of the present invention is to improve this problem at least partially. More precisely, one object of the present invention is to provide a piezoelectric on-insulator structure having a thin piezoelectric layer that is free of pit defects and triangular defects, or at least has a lower density of these defects than a thin piezoelectric layer obtained using a prior art process. [Overview of the project]

[0011] To achieve this objective, the subject of the present invention provides a process for preparing a thin monodomain layer made of a lithium-containing ferroelectric material, the process being - A step of injecting a light seed into the first surface of a lithium-containing ferroelectric donor substrate to form an embrittlement plane, and defining a first layer between the embrittlement plane and the first surface of the lithium-containing dielectric donor substrate, - Preferably, the process involves bonding the first surface of a lithium-containing dielectric donor substrate to a carrier using a dielectric intermediate layer to form an intermediate assembly. - A step of splitting an intermediate assembly, including a first heat treatment, the step of which results in the splitting of a lithium-containing ferroelectric donor substrate along a brittle plane and the formation of a free surface of the first layer. - A finishing sequence applied to the first layer, including an annealing step that includes a second heat treatment, and a step after the annealing step that thins the first layer to form a thin monodomain layer, Includes.

[0012] According to the present invention, the preparation process is - A surface treatment in which the free surface of the first layer is exposed to a treatment atmosphere containing at least 0.02% carbon dioxide to form a lithium-rich passivation layer, - Removal treatment to remove the lithium-rich passivation layer, Includes.

[0013] According to other advantageous and non-limiting features of the present invention, either alone or in any technically feasible combination, - The processing atmosphere has a temperature that falls between 100°C and the Curie temperature of the lithium-containing ferroelectric material on which the first layer is fabricated. - The removal process is carried out by cleaning the free surface of the first layer using a wet process. - Cleaning includes brushing the free surface of the first layer, - Brushing is performed while discharging deionized water onto the free surface of the first layer. - Surface treatment and removal processes are performed during intermediate processes that intervene between the splitting process and the finishing sequence. - Surface treatment is performed during the splitting process, and exposure of the free surface of the first layer to the treatment atmosphere is performed during or immediately after the first heat treatment. - The treatment atmosphere consists of a neutral gas such as argon or nitrogen containing at least 0.02% carbon dioxide, and the first heat treatment is performed at a temperature between 100°C and 700°C. - Surface treatment is performed during the annealing step of the finishing sequence, and exposure of the free surface of the first layer to the treatment atmosphere is performed during the second heat treatment. - The processing atmosphere is oxygen, or a neutral gas such as argon or nitrogen containing at least 0.02% carbon dioxide, and the second heat treatment is performed at a temperature between 300°C and the Curie temperature of the lithium-containing ferroelectric material on which the first layer is formed. - The processing atmosphere contains more than 0.05% carbon dioxide. - The carrier is formed from a conductive or semiconducting bulk substrate. - The carrier includes a base substrate and a trap layer, the trap 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 includes at least one layer of silicon oxide, silicon oxynitride, or silicon nitride. [Brief explanation of the drawing]

[0014] Other features and advantages of the present invention will become apparent from the following detailed description of the invention, which is given with reference to the accompanying drawings.

[0015] [Figure 1a] The POI structure of prior art is shown. [Figure 1b] The POI structure of prior art is shown. [Figure 2] This shows defects present in the thin layer of the POI structure. [Figure 3]Shows a manufacturing process of a POI structure. [Figure 4a] Shows states during various steps of preparing a first layer of a POI structure. [Figure 4b] Shows states during various steps of preparing a first layer of a POI structure. [Figure 4c] Shows states during various steps of preparing a first layer of a POI structure. [Figure 4d] Shows states during various steps of preparing a first layer of a POI structure. [Figure 5a] Shows three embodiments of the present invention. [Figure 5b] Shows three embodiments of the present invention. [Figure 5c] Shows three embodiments of the present invention.

Mode for Carrying Out the Invention

[0016] First, the steps of the process for manufacturing a POI structure 1 as presented in the introduction part of the present patent application and illustrated in FIGS. 1a and 1b are recalled.

[0017] Referring to FIG. 3, this process generally provides for transferring a first ferroelectric layer 8 onto a carrier 2, wherein the first layer 8 is sampled from a monodomain ferroelectric donor substrate 5 using a transfer technique based on 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, a matter of 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 crystal orientation comprised between 30° to 60° RY, but other crystal orientations are also possible. (As shown in FIG. 3,) the donor substrate 5 may correspond to a bulk substrate made entirely of ferromagnetic and piezoelectric material, or may be a composite substrate formed from a bulk portion, for example a bulk portion made of silicon, with a thick layer made of ferroelectric and piezoelectric material thereon, and the first layer 8 may be sampled from said thick layer.

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

[0019] In the transfer technique based on light seed injection, referring to Figure 3B, hydrogen and / or helium are injected into the first surface 6 of the donor substrate 5 to form an embedded embrittlement plane 7. Advantageously, to increase the manufacturing speed, the injection dose is greater than 8E16 at / cm^2 and / or the injection current is greater than 20 mA. However, the injection current may be more conventional, for example, between 10 mA and 15 mA. Thus, the first layer 8 is formed between the embrittlement plane 7 and the donor substrate. 5 It is defined between the first surface 6. Next, Figure 3 C As shown, during the 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 intermediate layer 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. As an example, the dielectric intermediate layer may contain or consist of silicon oxide, silicon oxynitride, or silicon nitride.

[0020] Next, during the subsequent splitting process, the donor substrate 5 is split along the brittle plane 7 by, for example, applying a moderate first heat treatment of about 200°C in an atmosphere consisting of a neutral gas to the intermediate assembly, and / or by applying mechanical force. Then, the first layer 8 is peeled off from the donor substrate 5 to expose the free surface 9 of the first layer 8, and the other surface 4 of the first layer 8 is in direct contact with the carrier 2, or, if such a layer exists, in direct contact with the dielectric intermediate layer.

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

[0022] Generally, in order to form a thin layer 4 of the POI structure 1, it is necessary to prepare the first layer 8, which has been transferred to and bonded to the carrier 2, for finishing. This finishing generally includes a series of steps aimed at improving the crystalline quality of the first layer 8, adjusting its thickness to a desired thickness, and improving its surface condition (e.g., its roughness).

[0023] As described in the introduction to this patent application, the finishing sequence includes an annealing step which includes a second heat treatment of the free surface 9 of the first layer 8, and a subsequent step which involves 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 to a neutral or oxygen-containing atmosphere for a period of 30 minutes to 10 hours, with the temperature raised to a temperature between 300°C and the Curie temperature of the ferroelectric material from which the first layer 8 is fabricated. This Curie temperature is, for example, around 600°C for lithium tantalate and around 1210°C for lithium niobate.

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

[0026] At the end of these processes, a substrate is obtained consisting of a thin layer 4 made of a lithium-containing ferroelectric material that is on and in contact with the intermediate layer 3, while the intermediate layer 3 itself is on and in contact with the carrier 2.

[0027] To understand why the thin layer 4 exhibits the defects described in the introduction, the applicant performed a very thorough analysis of the first layer 8 obtained at the end of the process shown in Figure 3, that is, before this first layer 8 was completed and the thin monodomain layer 4 was prepared.

[0028] Therefore, referring to Figure 4a, the applicant observed the presence of a lithium-rich surface thickness 11 on the first layer 8 obtained immediately after the splitting process. This surface layer thickness 11 consists of Li2CO3. Its formation appears to be facilitated by the specific conditions under which this splitting process is carried out. The presence of light species (hydrogen and / or helium) and a 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 its 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 naturally present carbon dioxide, hydrocarbons, and oxygen in these atmospheres to form the surface thickness 11 of Li2CO3. This surface thickness covering the first layer 8 is approximately 1 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 brittle, and the applicant observed that it can be removed by cleaning the first layer 8 using a wet treatment.

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

[0031] The applicant applied a second heat treatment of the finishing sequence to the first layer 8, which included such high-density regions of dendrites 12 and other regions lacking these dendrites.

[0032] The first layer 8 had a multi-domain surface layer 13 at the end of this heat treatment, as described in the reference cited in the introduction of this patent application (Figure 4c). The dendrites 12 had disappeared from the first layer 8 and had indeed dissolved during the heat treatment. However, in the regions of the first layer 8 where the dendrites 12 were initially densely concentrated, the multi-domain surface layer had an atypical form 14 that differed from the form of this multi-domain surface layer 13 in the regions where the dendrites were initially absent. This atypical form 14 is characterized by a multi-domain surface layer of smaller thickness, as if the presence of dendrites 12 in the denser regions limits the effect of the formation of this layer 13. Furthermore, the applicant states 10^4 / cm 2 At a higher density, the presence of triangular defects 15 was observed in the first layer.

[0033] Next, the applicant applied a thinning step by chemical mechanical polishing to the first layer 8 obtained at the end of the heat treatment, thereby obtaining a thin layer 4 (shown in Figure 4d). Note 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 region of the atypical morphology 14 of the multi-domain surface layer 13.

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

[0036] To this end, the applicant proposes incorporating a surface treatment in the process for preparing the thin monodomain layer described above, 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, particularly lithium, into the passivation layer on the surface of the first layer 8, thereby making this surface highly reactive. Therefore, this passivation layer (consisting of or containing Li2CO3) is lithium-rich. It has a thickness equal to at least 2 nm, and may be between 5 nm and 10 nm. This thickness is much greater than the thickness of the lithium-rich surface layer (around 1 nm) that naturally forms from trace amounts of carbon oxide present in the atmosphere to which the first layer 8 may be exposed in conventional processes.

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

[0039] By surface treatment, the mobile lithium present on the surface of the first layer 8 can be reduced or even removed from the first layer by incorporating it into a passivation layer and by removing this passivation layer. Thus, the surface becomes less reactive and prevents or limits the appearance of amorphous dendrites 12 on the first layer, which are thought to be the cause of the defects described in the introduction of this patent application.

[0040] Therefore, 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, particularly carrier 2, which may affect the electrical properties of this carrier 2, especially its resistivity.

[0041] To limit the amount of lithium that reaches carrier 2 through diffusion, the substrate 5 It should be noted that it may be conceivable to include a layer that constitutes a barrier to diffusion into the dielectric intermediate layer 3. This barrier may consist of a layer of silicon nitride incorporated into the dielectric intermediate layer 3, or of such a dielectric intermediate layer 3 concentrated in silicon oxide having a certain percentage of nitrogen. A substrate surface-treated according to the present invention may not require such a barrier layer, or if a barrier layer is provided, the barrier effect of this layer does not need to be particularly high. For example, the percentage of nitrogen in the dielectric intermediate layer 3 can be reduced compared to conventional approaches without risking excessive contamination of carrier 2 by lithium.

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

[0043] The surface treatment involves setting the treatment atmosphere to a certain temperature and free the first layer 8. surface This may be done by promoting the reaction occurring on layer 9 and accelerating the formation of the passivation layer. This temperature can be, for example, between 100°C and the Curie temperature of the ferroelectric material on which the first layer 8 is fabricated.

[0044] Freedom of the first layer (8) surface The duration of exposure to the treatment atmosphere in step 9 is preferably selected to be longer than 5 minutes, and is typically between 10 and 90 minutes.

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

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

[0047] Typically, the removal of the passivation layer, which consists of Li2CO3, may be carried out, for example, by simply cleaning the free surface 9 of the first layer 8 using a wet treatment. This cleaning may include, or consist of, brushing the free surface of the first layer 8 while discharging deionized water onto the free surface. Experiments conducted by the applicant have demonstrated that this cleaning can completely remove the Li2CO3 passivation layer. Of course, other techniques for removing this layer, such as etching or polishing, can be provided.

[0048] In the first embodiment schematically shown in Figure 5a, the surface treatment is performed during an intermediate step between the splitting step and the finishing sequence. Thus, this intermediate step includes, as described above, a surface treatment that exposes the free surface 9 of the first layer 8 to the treatment atmosphere, and then a treatment to remove the passivation layer, for example, by simply washing. The finishing sequence (annealing and the subsequent thinning of the first layer 8) may then be applied to the thus treated substrate.

[0049] Other embodiments propose integrating at least the surface treatment into an existing process to avoid introducing additional steps as in the first embodiment.

[0050] Therefore, in the second embodiment shown in Figure 5b, the surface treatment is performed during the splitting process. Subsequently, the exposure of the free surface 9 of the first layer 8 to the treatment atmosphere is performed during the first heat treatment of this process, or immediately after the first heat treatment.

[0051] In this second embodiment, the treatment atmosphere contains at least 0.02% carbon dioxide in a neutral gas such as argon or nitrogen. The first heat treatment is performed at a temperature between 100°C and 700°C, and therefore the treatment atmosphere is at this temperature.

[0052] In the first modification of this second embodiment, the splitting of the donor substrate is performed during the first heat treatment itself. This is typically done in a splitting furnace. In this modification, the treatment atmosphere may be introduced into the splitting furnace throughout the first heat treatment or at the end of the first heat treatment. Once the splitting is performed and the first layer 8 is released from the donor substrate, the freedom of this layer 8 surface 9 is exposed to the processing atmosphere, and a passivation layer is formed. In this modification, the surface treatment according to the present invention is performed "in-situ" within a divided furnace, in other words, during a single step of the process, without moving the substrate from one apparatus to another.

[0053] In a second modification of this second embodiment, the splitting of the donor substrate 5 is performed immediately after the first heat treatment, for example, by applying mechanical force to the intermediate assembly formed from the donor substrate 5 and the carrier 2. In this modification, if the application of mechanical force is not performed in the splitting furnace, it may be performed in a chamber of a dedicated apparatus filled with a processing atmosphere. In this modification as well, once the splitting is performed and the first layer 8 is released from the donor substrate 5, the freedom of this layer 8 surface 9 is exposed to the treatment atmosphere, and a passivation layer is formed.

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

[0055] In the third embodiment shown in Figure 5c, the surface treatment is performed during the finishing sequence, more specifically, during the second heat treatment of the annealing step in this sequence. Subsequently, exposure of the free surface 9 of the first layer 8 to the treatment atmosphere is performed during or immediately after the second heat treatment included in this sequence by introducing the treatment atmosphere into the furnace in which the second heat treatment is carried out. Thus, in this embodiment, the surface treatment is performed "in-situ" within the annealing furnace, in other words, during a single step of the process, without moving the substrate from one apparatus to another.

[0056] In this third embodiment, the treatment atmosphere contains at least 0.02% carbon dioxide in oxygen or a neutral gas such as argon or nitrogen. The second heat treatment is performed at a temperature between 300°C and the Curie temperature of the ferroelectric material on which the first layer 8 is fabricated. Therefore, the treatment atmosphere is set to this temperature.

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

[0058] Of course, the present invention is not limited to the embodiments described, and modifications 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) made of a lithium-containing ferroelectric material, - A step of injecting a light seed into the first surface (6) of the lithium-containing ferroelectric donor substrate (5) to form an embrittlement plane (7), and defining a first layer (8) between the embrittlement plane (7) and the first surface (6) of the lithium-containing ferroelectric donor substrate (5), - A step of dividing the intermediate assembly, including a first heat treatment, the step of which results in the division of the lithium-containing ferroelectric donor substrate (5) along the embrittlement plane (7) and the formation of a free plane (9) of the first layer (8), - A finishing sequence applied to the first layer (8), which includes an annealing step including a second heat treatment, and a step after the annealing step of thinning the first layer (8) to form the thin monodomain layer (4), Includes, The preparation process described above is - A surface treatment to form a lithium-rich passivation layer by exposing the free surface (9) of the first layer (8) to a treatment atmosphere containing at least 0.02% carbon dioxide, - A removal process for removing the lithium-rich passivation layer, A preparation process that includes this.

2. The processing atmosphere has a temperature that falls between 100°C and the Curie temperature of the lithium-containing ferroelectric material on which the first layer (8) is produced. The preparation process according to claim 1.

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

4. The cleaning includes brushing the free surface (9) of the first layer (8). The preparation process according to claim 3.

5. The brushing is performed while discharging deionized water onto the free surface (9) of the first layer (8). The preparation process according to claim 4.

6. The surface treatment and the removal treatment are performed during an intermediate process interposed between the division process and the finishing sequence. The preparation process according to any one of claims 1 to 5.

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

8. The treatment atmosphere contains at least 0.02% carbon dioxide in a neutral gas containing argon or nitrogen, and the first heat treatment is performed at a temperature between 100°C and 700°C. The preparation process according to claim 7.

9. The surface treatment is performed 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 performed during the second heat treatment. The preparation process according to any one of claims 1 to 5.

10. The treatment atmosphere is oxygen, or a neutral gas containing argon or nitrogen containing at least 0.02% carbon dioxide, and the second heat treatment is performed at a temperature between 300°C and the Curie temperature of the lithium-containing ferroelectric material on which the first layer (8) is produced. The preparation process according to claim 9.

11. The aforementioned treatment atmosphere contains more than 0.05% carbon dioxide. The preparation process according to any one of claims 1 to 5.

12. The carrier (2) is formed from a conductive or semiconducting bulk substrate. The preparation process according to any one of claims 1 to 5.

13. The process further includes the step of bonding the first surface (6) of the lithium-containing ferroelectric donor substrate (5) to the carrier (2) by a dielectric intermediate layer (3) to form an intermediate assembly. The preparation process according to any one of claims 1 to 5.

14. The carrier (2) includes a base substrate (2a) and a trap layer (2b), the trap layer (2b) being disposed between the dielectric intermediate layer (3) and the base substrate (2a). The preparation process according to claim 13.

15. The first layer (8) and the thin monodomain layer (4) are made of a single-crystal piezoelectric material containing lithium tantalate or lithium niobate. The preparation process according to any one of claims 1 to 5.

16. The dielectric intermediate layer (3) includes at least one layer of silicon oxide, silicon oxynitride, or silicon nitride. The preparation process according to claim 13.

Images