Process for preparing a thin film of ferroelectric material

Abstract

This invention relates to a method for preparing a thin film (3) of a ferroelectric material, the method comprising the step of providing a thin film (3) that exposes a first free surface (8). The method further comprises an ion etching thinning step defined by etching parameters. According to the invention, the etching parameters are selected such that the roughness of the free surface (8) of the thin film (3) does not exceed a critical value at the end of the thinning step.

Description

Method for preparing ferroelectric material thin films This invention relates to a method for preparing ferroelectric material thin films. More specifically, this invention relates to a preparation method that can maintain the single-domain properties of ferroelectric materials in the final product thin film. This preparation method can be applied to fields such as microelectronics, micromechanics, and photonics. In the introduction, it was noted that ferroelectric materials are materials that possess electrical polarization in their natural state, which can be reversed by applying an external electric field. A "ferroelectric domain" refers to a region within a single material where the polarization is uniform (all dipole moments are aligned parallel to each other in a given direction). Therefore, when a ferroelectric material is formed from a single region with uniform polarization, it can be classified as "single-domain," while when a ferroelectric material comprises multiple regions with different polarities, it can be classified as "multi-domain." Various methods for forming thin films of ferroelectric materials are known in the prior art. For example, techniques such as molecular beam epitaxy, plasma sputtering, laser pulse deposition, or even the application of Smart Cut™ technology, which removes a thin film from a solid substrate by implanting light species to cause fracture in a fragile region (or embrittled surface) formed in the solid substrate of the ferroelectric material. More specifically, this invention relates to a method for preparing ferroelectric thin films by applying this method, and specific examples of such methods can be found in document EP3646374B1. According to this method, after the film removal step, it is usually necessary to apply a treatment to improve its surface condition or its crystal quality or modify its thickness. However, the applicant has observed that when these preparation steps are applied to ferroelectric thin films transferred onto silicon supports, it results in the formation of multiple ferroelectric domains within the film, thereby endowing it with multidomain properties. This characteristic makes the film unsuitable for use because it affects the performance of components to be formed on / in the film, such as surface acoustic wave (SAW) devices. Document WO2020200986 reveals that the formation of ferroelectric domains in the surface portion of the thin film is caused by a hydrogen concentration gradient within the film during heat treatment. This hydrogen specifically corresponds to a light species implanted in a solid substrate, which forms an embrittlement zone in the substrate, allowing for film removal at that location. To permanently restore the single-domain properties of the film, the document recommends thinning the ferroelectric film after this heat treatment. This thinning can be achieved through chemical mechanical polishing of the film, but removing a relatively large amount of material through polishing often disrupts the film's thickness uniformity. For example, removing approximately 400 nm to achieve a target thickness of approximately 700 nm results in a film with a thickness uniformity of approximately 100 nm (i.e., the difference between the maximum and minimum thickness when this thickness is measured at multiple points across the entire film, such as by reflection or ellipsometry). This thickness variation is unacceptable because it is impossible to fabricate a device with all the necessary characteristics from such a collective of films. Alternatives to thinning via chemical mechanical polishing exist. One particularly promising approach is thinning ferroelectric films through ion etching, such as reactive ion etching (RIE). RIE is a dry etching process that uses a chemically reactive ion plasma to remove surface material from the wafer. This plasma is generated by an electromagnetic field under low pressure. High-energy plasma ions erode and react with the film surface, breaking it down and thus gradually thinning the film. Therefore, document US2018261756 proposes to form an amorphous piezoelectric thin film by depositing PZT with a thickness of approximately 2 micrometers using CSD. This film is then exposed to a certain temperature to form a polycrystalline structure. A thickness of 50 nm is then etched, for example, through RIE etching, to reduce its roughness and increase its dielectric voltage. This invention aims to determine the conditions for thinning a single-crystal ferroelectric thin film by applying ion etching to its free surface. One object of this invention is to provide a method for preparing ferroelectric thin films that at least partially overcomes the aforementioned drawbacks. More specifically, one object of this invention is to provide a method for ion etching a single-crystal ferroelectric thin film obtained by separation from a brittle plane of a donor substrate, a method that preserves or restores the unidomain properties of the film. Another object of this invention is to provide a method for thinning a thin film obtained by separation from a brittle plane of a donor substrate, a thinning method that forms a single-crystal ferroelectric thin film that is unidomain and has a more uniform thickness than that obtained by chemical mechanical polishing. To achieve one of the above objectives, the present invention relates to a method for preparing single-crystal ferroelectric material thin films, the method comprising the steps of: providing a thin film exposing a first free surface; and thinning the thin film by ion etching defined by etching parameters. According to the present invention, the providing step includes bonding a donor substrate containing an embrittled plane to a support substrate, and separating the film at the embrittled plane, the film having a second surface relative to the free surface, and being disposed on the support substrate. Furthermore, etching parameters are selected such that the roughness of the free surface of the film does not exceed a critical value at the end of the thinning step. According to other advantageous and non-limiting features of the invention, individually or in any technically feasible combination: the providing step includes forming an embrittled plane by implanting hydrogen ions into a donor substrate; the donor substrate is a bulk solid material; the donor substrate includes a thick layer of ferroelectric material disposed on a manipulating substrate; the thinning step is followed by chemical mechanical contact polishing of the free surface; the providing step includes a heat treatment step of exposing the free surface of the film to a gaseous environment; the heat treatment is performed at a temperature between 300°C and the Curie temperature of the ferroelectric material constituting the film, for a time between 30 minutes and 10 hours; the heat treatment is performed in an oxidizing or neutral gaseous environment; the method includes a step of smoothing the free surface between the heat treatment step and the thinning step, reducing its surface roughness value to below the aforementioned roughness critical value; the smoothing step includes chemical mechanical contact polishing of the free surface; the roughness critical value is between 3 nm and 7 nm. According to another embodiment, the present invention relates to a substrate comprising a thin film of a single crystal and a single domain ferroelectric material disposed on a supporting substrate, the thin film having a thickness of less than or equal to 700 nm and a thickness uniformity of less than or equal to 60 nm. According to other advantageous and non-limiting features of the invention, individually or in any technically feasible combination: the substrate includes an amorphous intermediate layer disposed between the thin film and the supporting substrate; the amorphous intermediate layer is silicon oxide, silicon oxynitride, or silicon nitride; the thin film includes free surfaces with a roughness of less than 0.5 nm; the ferroelectric material of the thin film is LiTaO3 or LiNbO3; the ferroelectric material of the thin film has a 42°RY crystal orientation; the supporting substrate includes a charge trapping layer on the thin film side. To simplify the following description, in different embodiments of the disclosed method, the same element number is used for the same element or for elements that perform the same function. The diagrams are schematic and are not drawn to scale for ease of reading. In particular, the thickness of the layers is not drawn to scale relative to the lateral dimensions of these layers. In the remainder of this specification, the term "coefficient of thermal expansion" as used with respect to a layer or substrate refers to the coefficient of thermal expansion in a direction defined in the principal plane defining the layer or substrate. If the material is anisotropic, the retained value of the coefficient of thermal expansion is the value with the largest range. The value of the coefficient of thermal expansion is measured at room temperature. This invention relates to a method for preparing a thin film 3 of ferroelectric materials, wherein the thin film 3 is transferred from a single-crystal donor substrate 1 to a supporting substrate 7 by a transfer technique of implanting light species into a donor substrate 1. Several embodiments of the steps for providing this thin film are described below. According to a first embodiment, as shown in Figures 1A to 1D, the donor substrate 1 is composed of a solid, single-crystal, and single-domain bulk ferroelectric material, such as LiTaO3, LiNbO3, LiAlO3, BaTiO3, PbZrTiO3, KNbO3, BaZrO3, CaTiO3, PbTiO3, or KTaO3. The donor substrate 1 can be in the form of a standard-sized circular wafer, for example, with a diameter of 150 mm or 200 mm. However, the invention is not limited to these sizes or shapes. The donor substrate may have been cut from a ferroelectric material ingot, thereby giving the donor substrate 1 a predetermined crystallization orientation. This crystallization orientation is selected based on the intended application. Therefore, if the intention is to utilize the thin-film properties to form a SAW filter, a 42°RY orientation is typically selected. However, the invention is not limited to a specific crystallization orientation. Regardless of the crystal orientation of the donor substrate 1, the method includes introducing at least one light species into the donor substrate 1. This introduction may correspond to implantation, i.e., ion bombardment of the plane 4 of the donor substrate 1 with a light species such as hydrogen and / or helium ions. In a known manner, and as shown in Figure 1B, the purpose of implanting ions is to form a brittle plane 2, which defines the ferroelectric material film 3 to be transferred and is located on one side of the plane 4, while the other part forms the remaining part 5 of the substrate. The properties of the implanted species, dosage, and implantation energy were selected as a function of the thickness of the film to be transferred and the physicochemical properties of the donor substrate. In the case of LiTaO3 as the donor substrate 1, implantation energies between 30 keV and 300 keV can be selected. E 16 and 5 E A hydrogen dose between 17 at / cm² was used to define a thin film of approximately 200 nm to 2000 nm. In the next step, as shown in Figure 1C, plane 4 of the donor substrate 1 is joined to plane 6 of the support substrate 7. The support substrate 7 may have the same size and shape as the donor substrate. For usability and cost reasons, the support substrate 7 is a monocrystalline or polycrystalline silicon wafer. However, more generally, the support substrate 7 may be formed of any material such as silicon, sapphire, or glass, and may be of any shape. In a particular embodiment, the supporting substrate includes, for example, a substrate 7b of monocrystalline silicon, on which a charge-trapping layer 7a is disposed. The substrate 7b may have a high resistivity, greater than 1000 ohms·cm or more commonly less than 1000 ohms·cm. The charge-trapping layer 7a is known to be formed of a polycrystalline silicon layer and has a thickness typically between 500 nm and 10 micrometers. Prior to the bonding step, it is conceivable to use cleaning, brushing, drying or grinding steps, or to use activation (e.g., through plasma) to prepare the substrate surface to be bonded. The bonding step can correspond to setting the donor substrate 1 into close contact with the support substrate 7 through molecular adhesion and / or electrostatic bonding. If necessary, to facilitate the bonding of the donor substrate 1 and the support substrate 7, especially when bonding is performed by direct bonding, at least one amorphous intermediate layer can be formed prior to bonding on the plane 4 of the donor substrate 1, or on the plane 6 of the support substrate 7 to be bonded, or both. This intermediate layer is, for example, made of silicon oxide, silicon nitride, or silicon oxynitride. It can have a thickness between several nanometers and several micrometers. Following the instructions in document WO2020200986, the intermediate layer preferably has a low hydrogen concentration or forms a barrier layer for hydrogen diffusion to avoid the formation of multi-domain regions at the interface between the thin film and the amorphous intermediate layer (the second side of thin film 3). The intermediate layer can be fabricated using various techniques known in the art, such as thermal oxidation or nitriding, chemical deposition (PECVD, LPCVD, etc.). At the end of this joining step, a joint containing two related substrates is obtained, wherein the plane 6 of the supporting substrate 7 is adhered to the plane 4 of the donor substrate 1. The joint is then processed to separate the ferroelectric material film 3 from the donor substrate 1 (e.g., by splitting in the embrittled plane 2). For this purpose, the separation step may include applying a heat treatment to the joint in a temperature range of about 80°C to 300°C to allow the film 3 to transfer onto the support substrate 7. Instead of heat treatment or in addition to heat treatment, the step may include applying a blade or injecting a gaseous or liquid fluid at the embrittled plane 2. Following this separation step, structure 9 as shown in FIG1D is obtained. Structure 9 includes a thin film 3 of a single-crystal ferroelectric material, which includes a first free surface 8 and a second surface 4 disposed on a supporting substrate 7. Figures 2A to 2D illustrate a second embodiment, particularly suitable for fabricating heterogeneous structures 9, wherein the coefficient of thermal expansion of the thin film 3 differs significantly from that of the supporting substrate 7, for example, by more than 10%. The main difference between this second embodiment and the first embodiment lies in the properties of the donor substrate 1. For simplicity, only the elements that differ between the second and first embodiments are presented here; all other features of the first embodiment are conceivable. Referring to FIG2A, in this example, the donor substrate 1 is composed of a ferroelectric material thick layer 1a and a processed substrate 1b, which has the same properties as the solid bulk ferroelectric material described in the first embodiment. The substrate 1b is advantageously formed of a material (or a plurality of materials) whose coefficient of thermal expansion is close to that of the supporting substrate 7. "Close to" means that the absolute value of the difference between the coefficient of thermal expansion of the substrate 1b and the supporting substrate 7 is less than the difference between the coefficient of thermal expansion of the solid bulk ferroelectric material and the supporting substrate 7. Preferably, the treatment substrate 1b and the support substrate have the exact same coefficient of thermal expansion. During the joining process of the donor substrate 1 and the support substrate 7, a structure suitable for withstanding high-temperature heat treatment is formed. For ease of implementation, this structure can be obtained by selecting the treatment substrate 1b to be made of the same material as the support substrate 7. To form the donor substrate 1 of this embodiment, a solid bulk ferroelectric material is first bonded to the processed substrate 1b, for example, by bonding techniques via molecular adhesion as described above or by means of an adhesive layer. Then, the ferroelectric material film 1a is formed by thinning, such as grinding and / or chemical mechanical polishing and / or etching. Prior to bonding, an adhesive layer (e.g., deposition of silicon oxide and / or silicon nitride, or deposition of an adhesive layer, such as a polymer) may be formed on one of the surfaces to be contacted. Bonding may include applying a low-temperature heat treatment (e.g., between 50°C and 300°C, typically 100°C) to sufficiently strengthen the bond energy, thereby allowing for the next thinning step. The substrate 1b is selected to have a thickness substantially equal to that of the supporting substrate 7. A thinning step is performed so that the thickness of the thick layer 1a is small enough to minimize the stress generated during the heat treatment applied in the remainder of the process. Simultaneously, the thickness is large enough to allow the removal of film 3 or multiple such films. This thickness, for example, can be between 5 micrometers and 400 micrometers. The subsequent steps of this second embodiment are the same as those described in the first embodiment. A light species is implanted into the thick layer 1a to create a brittle plane 2, which marks the separation of the film 3 from the remaining portion 5 of the donor substrate 1, as shown in FIG2B. This step is followed by bonding the donor substrate 1 to the supporting substrate 7, as shown in FIG2C. The film 3 is then separated from the remaining portion 5 of the substrate to obtain the structure 9 shown in FIG2D. The advantage of this embodiment is that the joint formed by the donor substrate 1 and the support substrate 7 can be exposed to temperatures much higher than those applied in the first embodiment, without any risk of uncontrolled substrate breakage or delamination of the film 3 from the donor substrate 1. Regarding the coefficient of thermal expansion of the joint, the balanced structure allows for the promotion of the film 3 separation step by exposing the joint to relatively high temperatures, such as between 100°C and 500°C. Regardless of the chosen embodiment, and as described in the introduction, the finishing stage of film 3 is necessary to improve its crystallinity and surface quality, and to provide film 3 with a thickness that matches or approaches the target value. These finishing steps are designed to remove the work-hardened and rough surface layer resulting from the breakage and separation of film 3 from the remainder of the donor substrate. As described in document WO2020200986, a heat treatment step is first applied to the transferred film 3. This heat treatment can remove crystal defects present in the film 3, or even reduce the roughness of the free surface of the film. In addition, it helps to strengthen the bond between the film 3 and the supporting substrate 7. The heat treatment can bring the structure to a temperature between 300°C and the Curie temperature of ferroelectric materials, for a time between 30 minutes and 10 hours. This heat treatment is preferably carried out by exposing the free surface of the film 3 to an oxidizing or neutral gaseous environment, that is, without using a protective layer to cover the free surface of the film. To avoid any ambiguity, it is hereby emphasized that the finishing thermal treatment of film 3 is different from the fracture thermal treatment applied to the bonded structure. The method according to the invention also includes a step of thinning the film 3 after a finishing heat treatment. The specific purpose of this step is to remove the multi-domain surface layer that may have been generated in the previous heat treatment step. This step also aims to provide a film 3 with a thickness corresponding to the target thickness, as previously described. This thinning typically corresponds to polishing the first free surface 8 of the film 3 through techniques such as mechanical or chemical-mechanical thinning. This thinning may result in a thickness removal between 100 nm and 1 micrometer, depending on the thickness of the transfer layer, the target thickness of the film 3, and the thickness of the multi-domain and / or work-hardened surface layer present due to the finishing heat treatment. However, as stated in the introduction, this method of thinning through chemical-mechanical polishing results in a reduction in the uniformity of the film 3, which is unacceptable. Additional experiments conducted by the applicant also showed that, with a removal of 100 nm, the uniformity decreased by approximately 25 nm. In searching for an alternative to grinding and thinning, the applicant conducted the following experiments, which aimed to implement this thinning step through ion etching. The experimental sample consisted of a single-crystal lithium tantalate film transferred onto a silicon substrate (following the method described above and including finishing heat treatment); however, the film had not undergone any thinning stage. Therefore, the fabrication method applied to this film only included a heat treatment step. Consequently, the film exhibited a multi-domain surface thickness, with a thickness of 680 nm and a roughness of approximately 10 nm RMS. On this sample, a first RIE P1 type ion etching method and a second RIE P2 type ion etching method were applied, each based on CHF3 reactive gas. As mentioned above, the RIE etching method involves exposing the surface to be treated (here, the free surface 8 of film 3) to a reactive ion plasma. This plasma causes surface etching of the material constituting the free surface of the film. The effect of ion etching, especially RIE-type ion etching, on film roughness depends on the etching parameters, particularly the power of the radio frequency field formed to generate reactive ion plasma, the flow rate of the reactive gas, the temperature of the film (which can be controlled by controlling the temperature of the support on which the substrate is placed), and the main pressure in the processing chamber of the RIE equipment. The first RIE P1 method applied to the sample exhibited etching parameters that tended to increase film roughness, due to the selected reactive gas flow rate introduced into the chamber being 65 sccm ("standard cubic centimeters per minute"), which is 1.08 × 10⁻⁶ m³ / s under standard conditions. Applying this method to the sample reduced the film thickness by 200 nm, achieving a thickness of 480 nm. The second RIE P2 method applied to the sample exhibited etching parameters that tended to reduce film roughness, as it selected a reactive gas flow rate of 20 sccm introduced into the chamber, which is equivalent to 0.3 × 10⁻⁶ m³ / s under standard conditions. This method was applied to the sample to reduce the film thickness by 200 nm, achieving a thickness of 480 nm. The table below presents the results obtained at the end of the experiment. Roughness is expressed as RMS values ​​measured by atomic force measurement over a 5 × 5 µm scan range. It should be noted that applying the first method P1 (whose etching parameters tend to increase the roughness of the film) results in a film with high roughness. Furthermore, contrary to expectations, the single-domain properties of film 3 were not recovered. Conversely, applying the second method P2 (whose etching parameters tend to reduce the roughness of the film) not only yields a film with much lower roughness, but also makes it possible to restore the single-domain properties of film 3. It should be noted that both samples exhibited excellent thickness uniformity without undergoing grinding or thinning treatment. One possible explanation for these results (which does not limit the preparation method of the present invention in any way) is that when the roughness is too large, the plasma ions appear to interact with the piezoelectric material present at the roughness peaks, thereby altering its polarization characteristics and thus creating or maintaining multi-domain thickness on the surface. To prevent this phenomenon, it seems necessary to select etching parameters to reduce the film roughness below a critical value, as this critical value would trigger such a change in polarization characteristics. The greater the initial roughness of the film, i.e., the greater the film roughness before the thinning step, especially when it exceeds a roughness critical value, more etching parameters must be controlled to reduce the roughness and / or prevent it from exceeding the critical value during and at the end of the step. Conversely, the lower the initial roughness, especially when it is below the roughness critical value, the more freely the etching parameters can be selected without causing the roughness to exceed the critical value at the end of the thinning step. In this way, the method of the present invention can provide a single-domain film with uniformity far superior to that of films thinned by chemical mechanical polishing. Depending on the properties of the ferroelectric material constituting the thin film, the critical roughness value can be between 3 nm and 7 nm, for example, 5 nm or 6 nm. Therefore, the present invention utilizes these results to propose a method for preparing thin film 3, as shown in FIG3. The preparation method includes the step of providing a single-crystal thin film 3, which exposes the first free surface 8 of the film, as described above with reference to one of the methods shown in Figures 1 and 2. This providing step also includes applying a finishing heat treatment step, specifically designed to remove hydrogen present in the film and result in the formation of a multi-domain surface thickness (Figure 3A). At the end of this step, the film has a defined roughness, typically about 10 nm RMS, measured by atomic force measurement (AFM) within a 5 × 5 μm scanning range. The film has a relatively thick thickness of about 1000 nm or greater. Following the provision step, the preparation method according to the present invention includes a thinning step, which is intended to provide a layer of selected thickness, typically about 700 nm or less, i.e., single crystal and single domain. Generally, and as shown in Figure 3B, this thinning step is performed by ion etching, which exposes the free surface 8 of the film to ions, preferably reactive ions. In particular, these can be argon ions, CHF3 ions, or any other ions capable of chemically reacting with the materials constituting the piezoelectric film. Depending on the thickness of the film 3 provided and the target thickness, the thinning step can remove a thickness of approximately 50 nm to 900 nm, typically greater than 100 nm, for example, between 100 nm and 400 nm. In all cases, the degree of film uniformity degradation here is much smaller than that achieved by chemical mechanical polishing; this thickness uniformity, measured by reflectance or ellipsometry, can be less than 60 nm, or even less than 30 nm, or even 20 nm or less. To achieve this, and according to the present invention, the etching parameters are selected such that the roughness of the free surface 8 of the thin film 3 does not exceed a critical value at the end of the thinning step. It is worth noting that, in order to remove a relatively large thickness, it is more natural to select etching parameters that favor removal speed rather than roughness. As described above, the "roughening" effect of the ion etching thinning step can be mitigated by reducing at least one of the following etching parameters: the flow rate of the reactive gas, the power of the radio frequency field formed to generate the reactive ion plasma, the temperature of the thin film 3, and the pressure present in the processing chamber of the etching equipment. Those skilled in this art can easily adjust these parameters in a series of routine experiments to determine which combinations of parameters will result in etching while avoiding exceeding the roughness threshold, given the initial surface state of the material and film 3. If the steps are not fully understood and the roughness varies with the thin film, a step to measure the film roughness can be performed between the providing step and the thinning step to adjust the etching parameters of the RIE etching method layer by layer based on the measured roughness. According to a particularly advantageous embodiment, the preparation method of the present invention includes a step of smoothing the free surface 8 between the finishing heat treatment step and the thinning step, so as to reduce its surface roughness to a relatively low initial value, for example less than 1 nm RMS. For example, the smoothing step includes chemical mechanical contact polishing of the free surface 8. The purpose of this polishing is to reduce the roughness of the film without significantly thinning it. This surface smoothing step does not affect the uniformity of the film. Compared to the case without the aforementioned smoothing step, since the roughness of the free surface 8 of the thin film 3 has been reduced by decreasing the risk of exceeding the critical value at the end of the thinning step and thus restoring the single-domain properties of the thin film, we can select the etching parameters more freely. Regardless of whether the initial smoothing step is performed, the thin film preparation method of the present invention allows for chemical mechanical contact polishing of the free surface of the thin film after the thinning step. This polishing step enables the free surface 8 to ultimately have a low roughness (if not already obtained at the end of the thinning step), for example, less than 0.5 nm RMS 5 × 5 µm via atomic force measurement (AFM), without affecting the uniformity of the thin film. Of course, the present invention is not limited to the foregoing embodiments, and variations may be added without departing from the scope of the present invention as defined by the claims. 1: Donor substrate 1a: Thick layer 1b: Treated substrate 2: Embrittled plane 3: Thin film 4, 6: Plane 7: Supporting substrate 7a: Charge trapping layer 7b: Base substrate 8: First free surface 9: Structure Referring to the accompanying drawings, further features and advantages of the present invention will become apparent from the following detailed description of the invention, wherein: FIG1 illustrates the thin film providing steps according to the first embodiment; FIG2 illustrates the thin film providing steps according to the second embodiment; and FIG3 illustrates the preparation method according to the present invention. 9: Structure

Claims

1. A method for preparing a thin film (3) of a single-crystal ferroelectric material, the method comprising the steps of: providing the thin film (3) to expose a first free surface (8); thinning the thin film (3) by ion etching defined by etching parameters; characterized in that the providing step comprises joining a donor substrate (1) containing a brittle plane (2) to a support substrate (7) and separating the thin film (3) at the brittle plane (2), the thin film (3) having a second surface relative to the first free surface (8) and disposed on the support substrate (7), and characterized in that the etching parameters are selected such that the roughness of the first free surface (8) of the thinning step does not exceed a critical value.

2. The method of claim 1, wherein the providing step includes forming the brittle plane (2) by implanting hydrogen ions into the donor substrate (1).

3. The method of claim 1 or 2, wherein the thinning step is followed by chemical mechanical contact polishing of the first free surface (8).

4. The method of claim 1, wherein the providing step includes a heat treatment step of exposing the first free surface (8) of the film (3) to a gaseous environment.

5. The method of claim 4, wherein the heat treatment is performed at a temperature between 300°C and the Curie temperature of the ferroelectric material constituting the thin film (3), for a time between 30 minutes and 10 hours.

6. The method of claim 4 or 5, wherein the heat treatment step is carried out in an oxidizing or neutral gaseous environment.

7. The method of claim 4, which includes a step of smoothing the first free surface (8) between the heat treatment step and the thinning step, such that the surface roughness value is reduced to below the aforementioned roughness critical value.

8. The method of claim 7, wherein the smoothing step includes chemical mechanical contact grinding of the first free surface (8).

9. The method of claim 1, wherein the roughness threshold is between 3 nm and 7 nm.

10. A substrate comprising a thin film (3) of a single crystal and a single domain ferroelectric material disposed on a supporting substrate (7), characterized in that the thin film (3) has a thickness of less than or equal to 700 nm and a thickness uniformity of less than or equal to 60 nm.

11. The substrate of claim 10 includes an amorphous interlayer disposed between the thin film (3) and the supporting substrate (7).

12. The substrate of claim 11, wherein the amorphous intermediate layer is silicon oxide, silicon oxynitride, or silicon nitride.

13. The substrate of any one of claims 10 to 12, wherein the film (3) includes a free surface (8) with a roughness of less than 0.5 nm.

14. The substrate of claim 10, wherein the ferroelectric material of the thin film (3) is LiTaO3 or LiNbO3.

15. The substrate of claim 10, wherein the supporting substrate (7) includes a charge trapping layer on the side of the thin film (3).

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