Process for preparing thin films of ferroelectric materials

Abstract

The present invention relates to a method for preparing a thin film (3) of a ferroelectric material, the method comprising a step of providing a thin film (3), the thin film (3) exposing a first free surface (8). The method also comprises a step of thinning by ion etching and defined by etching parameters. According to the invention, the etching parameters are selected such that the free surface (8) of the thin film (3) has, at the end of the thinning step, a roughness not exceeding a threshold value.

Description

[Technical field]

[0001] FIELD OF THEINVENTION The present invention relates to a method for preparing thin films of ferroelectric material, more particularly to a preparation method which allows maintaining the monodomain nature of the ferroelectric material in the final thin film, for example in the fields of microelectronics, micromechanics, photonics, etc. [Background technology]

[0002] (Technical Background of the Invention) It should be noted in the preceding sentence that a ferroelectric material is a material that has an electric polarization in its natural state, which can be reversed by applying an external electric field. A "ferroelectric domain" refers to each region within a single piece of material in which the polarization is uniform (all dipole moments are aligned parallel to each other in a given direction). Thus, a ferroelectric material may be characterized as "monodomain" if the material is formed by a single region in which the polarization is uniform, or as "multidomain" if the ferroelectric material comprises multiple regions with polarities that may differ.

[0003] Various processes are known from the state of the art for forming thin films of ferroelectric material, which may be, for example, techniques using molecular beam epitaxy, plasma sputtering, laser pulse deposition, or even the application of the Smart Cut™ technology, in which a thin film is removed from a solid substrate of ferroelectric material by fracturing on a weakened zone (or embrittlement plane) formed in the solid substrate by implantation of light species.

[0004] The invention relates more particularly to the preparation of ferroelectric thin films obtained by application of such a method, specific exemplary embodiments of which can be found in document EP 3 646 374.

[0005] According to this method, after the film removal step, it is often necessary to apply a treatment to the film aimed at improving its surface state or its crystalline quality or modifying its thickness. However, the applicant has observed that these preparatory steps, when applied to a ferroelectric thin film transferred onto a silicon substrate, can lead to the formation of multiple ferroelectric domains in the thin film, thereby giving it a multi-domain nature.

[0006] Such characteristics make the films unsuitable for use in devices formed on / in the thin film, such as surface acoustic wave devices (SAW).

[0007] Document WO2020200986 discloses that the formation of ferroelectric domains in the surface portion of the film is caused by the presence of a hydrogen concentration gradient in the thin film during the application of a heat treatment. This hydrogen may correspond in particular to light species that are implanted in the solid substrate in order to form embrittled zones in the solid substrate and thereby make it possible to remove the thin film there. In order to permanently restore the monodomain nature of the thin film, this document recommends thinning the ferroelectric thin film after the application of such a heat treatment.

[0008] This thinning can be achieved, in particular, by chemical mechanical polishing of the film, but the relatively large removal of material by polishing tends to reduce the thickness uniformity of this film. As an example, removing a thickness of the order of 400 nm to achieve a target thickness of about 700 nm produces a film with a thickness uniformity of the order of 100 nm (i.e. the difference between the maximum and minimum thickness when this thickness measurement is performed at multiple measurement points over the entire range of the film, for example by reflectometry or ellipsometry). This thickness variation is unacceptable, as it does not make it possible to manufacture devices with all the required features altogether from such films.

[0009] There are alternative forms of thinning by chemical mechanical polishing. In particular, it is possible to envisage thinning thin ferroelectric films by ion etching, for example etching with reactive ions (i.e. RIE etching, "reactive ion etching" according to the expression usually adopted). RIE is a type of dry etching that uses a chemically reactive ion plasma to remove surface material from the wafer. The plasma is generated under low pressure by an electromagnetic field. High energy plasma ions attack the surface of the film and react with it, disrupting it and thereby gradually thinning it.

[0010] Thus, the document US Patent Application Publication No. 2018261756 proposes forming an amorphous piezoelectric film with a thickness of about 2 microns by CSD deposition of PZT. This film is then exposed to temperature to form polycrystalline. Then, a thickness of 50 nm is etched, for example by RIE etching, in order to reduce the roughness of the film and improve the dielectric constant of the film. Summary of the Invention

[0011] (Subject of the Invention) The present invention aims to determine the conditions for applying ion etching to the free surface of a thin film of a single crystal ferroelectric material in order to thin it. The object of the present invention is to propose a method for preparing a thin film of a ferroelectric material which at least partially addresses the aforementioned drawbacks. More specifically, the object of the present invention is to propose a method for ion etching of a thin film of a single crystal ferroelectric material obtained by detachment from a donor substrate in the embrittlement plane, which method maintains or restores the monodomain nature of the thin film. Another object of the present invention is to propose a method for thinning a thin film obtained by detachment in the embrittlement plane of a donor substrate, which thinning method makes it possible to form a thin film of a single crystal ferroelectric material which is monodomain and has a more uniform thickness than can be obtained by a thinning process implementing chemical mechanical polishing.

[0012] BRIEF DESCRIPTION OF THE DRAWINGS To achieve one of these objectives, the subject of the present invention is a method for preparing a thin film of a single-crystalline ferroelectric material, the method comprising the steps of: - providing a thin film, the thin film exposing a first free surface; - thinning the membrane by ion etching, the step being defined by etching parameters; Includes.

[0013] According to the invention, the providing step comprises assembling a donor substrate with a weakened plane and a support substrate, and detaching a thin film at the level of the weakened plane, the thin film having a second surface opposite to the free surface and positioned on the support substrate, and further, the etching parameters are selected such that the free surface of the thin film has a roughness at the end of the thinning step that does not exceed a threshold value.

[0014] According to other preferred and non-limiting features of the invention, the following are employed, either alone or in any technically feasible combination: The providing step includes forming an embrittlement plane by hydrogen ion implantation into the donor substrate. The donor substrate is a block of solid material. A donor substrate includes a thick layer of ferroelectric material disposed on a manipulator substrate. · The thinning step is followed by chemical-mechanical touch polishing of the free surface. The providing step includes a heat treatment step in which the free surface of the thin film is exposed to a particular gas atmosphere. The heat treatment is carried out at a temperature between 300° C. and the Curie temperature of the ferroelectric material constituting the thin film, for a period between 30 minutes and 10 hours. The heat treatment is carried out under an oxidizing or neutral gas atmosphere. The method includes, between the heat treatment step and the thinning step, a step of smoothing the free surface to reduce a surface roughness of the free surface to a value below a roughness threshold. The smoothing step involves chemical-mechanical touch polishing of the free surface. The roughness threshold is 3nm to 7nm.

[0015] According to another aspect, the invention relates to a substrate comprising a thin film of a single crystal and monodomain ferroelectric material disposed on a support, the thin film having a thickness of 700 nm or less and a thickness uniformity of 60 nm or less.

[0016] According to other preferred and non-limiting features of the invention, the following are employed, either alone or in any technically feasible combination:

[0017] The substrate comprises an amorphous intermediate layer positioned between the thin film and the support.

[0018] The amorphous intermediate layer is silicon oxide, silicon oxynitride, or silicon nitride.

[0019] The thin film has a free surface with a roughness of less than 0.5 nm.

[0020] · The thin film ferroelectric material is LiTaO3 or LiNbO3.

[0021] · A thin film ferroelectric material has a 42° RY crystallographic orientation.

[0022] The support has a charge trapping layer on the thin film side.

[0023] Other features and advantages of the present invention will become apparent from the following detailed description of the invention, which proceeds with reference to the accompanying drawings. [Brief description of the drawings]

[0024] [Figure 1] FIG. 1 shows a thin film providing step according to a first embodiment. [Diagram 2] FIG. 2 shows a thin film providing step according to a second embodiment. [Diagram 3] FIG. 3 illustrates a preparation method according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] (Detailed Description) To simplify the following description, the same reference numbers are used for identical elements or elements performing the same functions in different embodiments of the disclosed methods.

[0026] The figures are schematic, which are not drawn to scale for ease of reading, in particular the thicknesses of the layers are not drawn to scale relative to the lateral dimensions of these layers.

[0027] The expression "coefficient of thermal expansion" as used in the remainder of this specification with respect to a layer or a substrate refers to the coefficient of expansion in a direction defined in a major plane defining this layer or this substrate. If the material is anisotropic, the retained value of the modulus will be the value with the greatest amplitude. The values ​​of the modulus are measured at room temperature.

[0028] The present invention relates to a method for preparing a thin film 3 made of a ferroelectric material transferred from a monocrystalline donor substrate 1 to a support substrate 7 by a transfer technique involving implanting light species into the donor substrate 1. There are several embodiments of this providing step of the thin film.

[0029] According to a first embodiment shown in Fig. 1A to 1D, the donor substrate 1 is composed of a solid single crystal and monodomain block of ferroelectric material, for example LiTaO3, LiNbO3, LiAlO3, BaTiO3, PbZrTiO3, KNbO3, BaZrO3, CaTiO3, PbTiO3 or KTaO3. The donor substrate 1 may take the form of a circular wafer of standardized size, for example 150 mm or 200 mm diameter. However, the invention is in no way limited to these dimensions or to this shape. The donor substrate may have been removed from an ingot of ferroelectric material, this removal being carried out so that the donor substrate 1 has a predefined crystal orientation. The orientation is chosen according to the intended application. Thus, if it is intended to exploit the properties of thin films to form SAW filters, it is common practice to choose an orientation of 42° RY. However, the invention is in no way limited to a particular crystal orientation.

[0030] Regardless of the crystal orientation of the donor substrate 1, the method comprises the introduction of 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 by light species such as hydrogen and / or helium ions.

[0031] In a manner known per se, the purpose of the implanted ions is to form an embrittlement plane 2 which defines the boundary between a thin film 3 of ferroelectric material to be transferred, located on the side of plane 4, and another part 5 which forms the remainder of the substrate, as shown in FIG. 1B.

[0032] The nature of the implanted species, the dose and the implantation energy are selected depending on the thickness of the film intended to be transferred and on the physicochemical properties of the donor substrate. Thus, for a donor substrate made from LiTaO3, 1 is implanted with energies between 30 and 300 keV. E 16~5 E 17at / cm 2 By selecting to inject a hydrogen dose of about 200 to 2000 nm, it becomes possible to define the boundary of the thin film 3 at about 200 to 2000 nm.

[0033] In a next step, the flat surface 4 of the donor substrate 1 is assembled to the face 6 of the support substrate 7, as shown in FIG. 1C. The support substrate 7 may be the same size and shape as the donor substrate. For reasons of availability and cost, the support substrate 7 is a monocrystalline or polycrystalline silicon wafer. However, more generally, the support substrate 7 may be made of any material, for example silicon, sapphire, or glass, and may be of any shape.

[0034] In a particular embodiment, the support substrate comprises a base substrate 7b, for example of monocrystalline silicon, on which the charge trapping layer 7a is arranged. The base substrate 7b may have a high resistivity of more than 1000 ohm.cm or, more conventionally, less than 1000 ohm.cm. The charge trapping layer 7a may be formed, as known per se, from a layer of polycrystalline silicon and may have a thickness typically comprised between 500 nm and 10 microns.

[0035] Prior to the assembly step, it is possible to envisage preparing the surface of the substrate to be assembled by means of a cleaning, brushing, drying or polishing step, or by means of activation, for example by means of plasma.

[0036] The assembly step may correspond to positioning the donor substrate 1 in intimate contact with the support substrate 7 by molecular adhesion and / or electrostatic bonding. Optionally, in order to facilitate the assembly of the two substrates 1, 7, in particular when they are assembled by direct bonding, at least one amorphous intermediate layer may be formed before the assembly on the plane 4 of the donor substrate 1 or on the assembled plane 6 of the support substrate 7, or on both. This intermediate layer is for example made of silicon oxide, silicon nitride, silicon oxynitride. The intermediate layer may have a thickness comprised between a few nanometers and a few microns.

[0037] According to the teaching of document WO2020200986, it will be preferable to provide an intermediate layer which has a low hydrogen concentration or forms a barrier to hydrogen diffusion, thus avoiding the formation of a multi-domain zone at the interface between the thin film and the amorphous intermediate layer on the side of the second face of the thin film 3. The intermediate layer can be produced according to various techniques known in the state of the art, such as thermal oxidation or nitridation treatments, chemical deposition (PECVD, LPCVD...).

[0038] At the end of this mounting step, an assembly is obtained with two associated substrates, the planar surface 6 of the support substrate 7 bonded to the planar surface 4 of the donor substrate 1 .

[0039] The assembly is then processed to release the thin film 3 of ferroelectric material from the donor substrate 1, for example by cleaving at the embrittlement plane 2.

[0040] This release step may therefore involve applying a heat treatment to the assembly in a temperature range of around 80°C to 300°C to enable the thin film 3 to be transferred onto the support substrate 7. Alternatively or in addition to a heat treatment, this step may involve the application of a blade or jet of a gas or liquid fluid at the embrittlement plane 2.

[0041] Following this release step, the structure 9 shown in Figure 1D is obtained. The structure 9 comprises a thin film 3 of monocrystalline ferroelectric material with a first free surface 8 and a second surface 4 arranged on a supporting substrate 7.

[0042] 2A to 2D show a second embodiment, which is particularly suitable for producing heterogeneous structures 9, in which the thin film 3 has a thermal expansion coefficient very different from that of the support 7, for example having a difference of more than 10%. This second embodiment differs from the first embodiment mainly in the nature of the donor substrate 1. Therefore, for the sake of brevity, only the elements of this second embodiment which differ from the first embodiment are presented here, so that all other features of the first embodiment can be envisaged.

[0043] Referring to FIG. 2A, the donor substrate 1 in this case consists of a thick layer of ferroelectric material 1a having the same properties as described for the solid block of ferroelectric material in relation to the first embodiment, and a manipulator substrate 1b.

[0044] The manipulator substrate 1b is preferably made of a material (or materials) that provides the manipulator substrate 1b with a thermal expansion coefficient close to that of the support substrate 7. By "close" we mean that the difference between the thermal expansion coefficient of the manipulator substrate 1b and that of the support is smaller in absolute value than the difference between the thermal expansion of a solid block of ferroelectric material and that of the support substrate 7.

[0045] The manipulator substrate 1b and the support substrate preferably have the same thermal expansion coefficient. During assembly of the donor substrate 1 and the support 7, a structure is formed that is suitable for withstanding relatively high temperature heat treatments. For simplicity of implementation, this can be obtained by selecting the manipulator substrate 1b such that it is made of the same material as that of the support substrate 7.

[0046] To form the donor substrate 1 of this embodiment, a solid block of ferroelectric material is first assembled to the manipulator substrate 1a, for example with the aid of a bonding technique by molecular adhesion or an adhesive layer as described above. A film of ferroelectric material 1a is then formed, for example by thinning, for example by grinding and / or chemical-mechanical polishing and / or etching. Before assembly, provisions may be made to form an adhesive layer (for example a polymer, for example by deposition of silicon oxide and / or silicon nitride) on one and / or the other of the contacting surfaces. The assembly may include the application of a low-temperature heat treatment (for example 50-300 ° C, typically 100 ° C) that allows a sufficient strengthening of the bonding energy to allow the next step of thinning.

[0047] The manipulator substrate 1b is chosen to have a thickness substantially equivalent to that of the support substrate 7. The thinning step is carried out so that the thickness of the thick layer 1a is small enough so that the stresses that arise during the heat treatments applied in the remainder of the process are of lower intensity. At the same time, this thickness is large enough to be able to remove the thin film 3, or a number of such films. This thickness can be, for example, between 5 and 400 microns.

[0048] The following steps of the method of this second embodiment are equivalent to those described in the first embodiment: As shown in Fig. 2B, light species are implanted in the thick layer 1a to generate a plane of embrittlement 2, which indicates the separation of the thin film 3 from the remaining part 5 of the donor substrate 1. This step is followed by the assembly of the donor substrate 1 on a support substrate 7, as shown in Fig. 2C. The thin film 3 is then peeled off from the remaining part 5 of the substrate to obtain the structure 9 shown in Fig. 2D.

[0049] This embodiment is advantageous in that the assembly formed from the donor substrate 1 and the support 7 can be exposed to temperatures much higher than those applied in the first embodiment without any risk of uncontrolled fracture of one of the substrates or delamination of the donor substrate 1 from the thin film 3. The balanced structure of this assembly in terms of thermal expansion coefficients therefore makes it possible to facilitate the step of peeling off the thin film 3 by exposing the assembly to relatively high temperatures, for example 100°C to 500°C.

[0050] Regardless of the embodiment chosen, and as specified in the introduction of this application, a stage of finishing the thin film 3 is therefore necessary to improve the crystallinity and surface quality of the thin film 3 and to provide a thin film 3 whose thickness matches or approaches the target thickness. These finishing steps aim in particular to eliminate the work-hardened rough surface layer resulting from the cleavage and peeling of the thin film 3 from the rest of the donor substrate.

[0051] As explained in document WO 2020200986, a heat treatment step is first applied to the transferred thin film 3. This heat treatment makes it possible to repair crystalline defects present in the thin film 3 or even reduce the roughness of the free surface of the thin film. Furthermore, this heat treatment serves to strengthen the bond between the thin film 3 and the support 7. The heat treatment brings the structure to a temperature between 300 ° C and the Curie temperature of the ferroelectric material, for a period between 30 min and 10 h. This heat treatment is preferably carried out by exposing the free surface of the thin film 3 to an oxidizing or neutral gas atmosphere, i.e. without covering this surface of the thin film with a protective layer.

[0052] For the avoidance of doubt, it is emphasised that the finishing heat treatment step of the membrane 3 is different to the disruption heat treatment applied to the assembled structure.

[0053] The method according to the invention also comprises, after the final heat treatment, a step of thinning the thin film 3. This step aims in particular at eliminating any multi-domain surface layer that may have been created during the previous heat treatment step, and at providing a thin film 3 whose thickness corresponds to the target thickness, as mentioned above. This thinning may generally correspond to polishing the first free surface 8 of the thin film 3, for example by mechanical, chemical-mechanical thinning techniques. This thinning may result in a removal of a thickness between 100 nm and 1 micron, depending on the thickness of the transferred layer, the target thickness of the thin film 3, and the thickness of the multi-domain and / or work-hardened layers present on the surface from the final heat treatment. However, as presented in the introduction of this application, such a thinning approach by chemical-mechanical polishing tends to degrade the uniformity properties of the thin film 3, which is unacceptable. Thus, further experiments carried out by the applicant have shown that this uniformity decreases by about 25 nm for every 100 nm of removal.

[0054] In searching for an alternative solution to thinning by polishing, the applicant carried out the following experiments aimed at performing this step of thinning by ion etching.

[0055] The experimental sample consisted of a single crystal thin film of lithium tantalate transferred onto a silicon substrate (by the method just disclosed, including a finishing heat treatment), the thin film having not undergone a thinning step. The preparation method applied to this thin film therefore consisted only of a heat treatment step. The thin film therefore had a multi-domain surface thickness. The multi-domain surface thickness had a thickness of 680 nm and a roughness of the order of 10 nm RMS.

[0056] On this sample, a first RIE P1 type ion etching process and a second RIE P2 type ion etching process, each based on CHF3 reactive gas, were applied. As recalled in the introduction of the present application, the RIE etching process consists of exposing the surface to be treated (here the free surface 8 of the thin film 3) to a reactive ion plasma. This plasma results in surface etching of the material that comprises the free surface of the thin film.

[0057] The effect of ion etching, particularly RIE type ion etching, on the roughness of the film depends on the etching parameters, in particular the power of the radio frequency field formed to create the plasma of reactive ions, the flow rate of the reactive gases, the temperature of the thin film (which can be controlled by controlling the temperature of the support on which the substrate rests) and the pressure prevailing in the processing chamber of the RIE equipment.

[0058] The first RIE P1 process applied to the sample showed etching parameters that tended to increase the roughness of the thin film, in particular by choosing the flow rate of the reactive gas introduced into the chamber to be 65 sccm (from "standard cubic centimeters per minute"), i.e. 1.08 10^-6 m^3 / s, for standard conditions. This process was applied to the sample, reducing the thickness of the thin film by 200 nm, thus achieving a thickness of 480 nm.

[0059] The second RIE P2 process applied to the sample showed etching parameters that tended to reduce the roughness of the thin film, in particular by selecting the flow rate of the reactive gas introduced into the chamber at 20 sccm, i.e. 0.3 10^-6 m^3 / s under standard conditions. This process applied to the sample reduced the thickness of the thin film by 200 nm, thus resulting in a thickness of 480 nm.

[0060] The following table presents the results obtained at the end of the experiments carried out, the roughness being expressed as an RMS quantity measured in a field of 5x5 μm by atomic force measurements.

[0061] [Table 1]

[0062] It is noted that the application of the first process P1 (whose etching parameters tend to increase the roughness of the thin film) leads to obtaining a thinned film with high roughness. Moreover, contrary to what was expected, the monodomain nature of this thin film 3 is not restored.

[0063] Conversely, the application of a second process P2, whose etching parameters tend to reduce the roughness of the thin film, not only leads to obtaining a thinned film whose roughness is much less, but also makes it possible to restore the monodomain nature of the thin film 3.

[0064] It should be noted that in both cases the uniformity of the samples that were not subjected to thinning by polishing is of good quality.

[0065] According to a possible interpretation of these results, which in no way limits the performance of the preparation method of the invention, the ions of the plasma may interact with the piezoelectric material present within the roughness peaks in order to modify the polarization properties of said piezoelectric material when said roughness is excessive, resulting in the creation or maintenance of multi-domain thicknesses on the surface.

[0066] Therefore, to prevent this phenomenon, it appears necessary to select etching parameters that reduce the roughness of the thin film below the threshold at which this modification of the polarization properties is triggered.

[0067] The greater the initial roughness of the thin film, i.e., the roughness of the thin film immediately before the application of the thinning step, in particular if the initial roughness exceeds the roughness threshold, the more control must be exercised over the etching parameters during and at the end of this step in order to reduce and / or not exceed the roughness threshold. Conversely, the lower the initial roughness, in particular if the initial roughness is below the roughness threshold, the more freedom the etching parameters can be chosen, but without resulting in exceeding the roughness threshold at the end of the thinning step. In this way, the method can provide monodomain thin films of much better uniformity than that of films thinned by chemical mechanical polishing.

[0068] Depending on the nature of the ferroelectric material that constitutes the thin film, the roughness threshold may be between 3 nm and 7 nm, for example 5 nm or 6 nm.

[0069] The present invention therefore takes advantage of these results and proposes a method for preparing a thin film 3, as shown diagrammatically in FIG.

[0070] The preparation method therefore comprises a step of providing a monocrystalline thin film 3 as described above, for example according to any of the methods represented in figures 1 and 2, of which a first free face 8 is exposed. This providing step also comprises the application of a finishing heat treatment step aimed in particular at expelling hydrogen present in the thin film and at resulting in the formation of a multi-domain surface thickness (figure 3A). At the end of this step, the thin film has a determined roughness, typically of the order of 10 nm RMS, measured by atomic force measurement (AFM) over a field of 5x5 microns. The thin film has a relatively large thickness, of the order of 1000 nm or more.

[0071] After this providing step, the preparation method according to the invention includes a thinning step aimed at providing a selected layer, ie monocrystalline and monodomain, typically with a thickness of the order of 700 nm or less.

[0072] Very commonly, as shown in FIG. 3B, this thinning step is carried out by ion etching, i.e. by exposing the free surface 8 of the thin film to ions, preferably reactive ions. The reactive ions may in particular be ions of argon, ions of CHF3 or any other ions capable of chemically reacting with the material constituting the piezoelectric thin film. Depending on the thickness of the thin film 3 provided and the target thickness, the thinning step may result in eliminating thicknesses of the order of 50 nm to 900 nm, typically exceeding 100 nm, for example 100 nm to 400 nm. In all cases, the uniformity of the film is not much reduced than if the film was thinned by chemical mechanical polishing, this thickness uniformity may be less than 60 nm, or even less than 30 nm, or even reaching 20 nm or less, when measured by reflectometry or ellipsometry.

[0073] To achieve this, according to the invention, the etching parameters are selected such that the free surface 8 of the thin film 3 has a roughness at the end of the thinning step that does not exceed a threshold value. It is noted that, taking into account the relatively large thickness to be eliminated, it would have been more natural to select the etching parameters to favor the removal rate over the roughness.

[0074] As specified above, the "roughening" effect of the ion etching thinning step can be reduced by reducing at least one of the following etching parameters: the flow of reactive gas, the power of the radio field frequency formed to create the reactive ion plasma, the temperature of the thin film 3, and the pressure prevailing in the processing chamber of the etching equipment.

[0075] A person skilled in the art can very easily adjust these parameters during a series of routine experiments to determine, for a given material and initial surface state of the thin film 3, which combination of parameters results in etching while avoiding exceeding the roughness threshold.

[0076] If the providing step is not well mastered and this roughness varies from one application of the thin film to another, the preparation method may provide, between the providing step and the thinning step, a step of measuring the roughness of the thin film in order to adjust the etching parameters of the RIE etching process layer by layer to the measured roughness.

[0077] According to a particularly preferred embodiment, the preparation method of the invention comprises, between the finishing heat treatment step and the thinning step, a step of smoothing the free surface 8 to reduce the surface roughness of the free surface 8 to a relatively low initial value, for example less than 1 nm RMS.

[0078] For example, the smoothing step comprises a chemical-mechanical touch polishing of the free surface 8. This polishing aims to reduce the roughness of the thin film without significantly thinning this film. This very superficial smoothing step does not affect the uniformity of the film.

[0079] If the roughness of the free surface 8 of the thin film 3 is reduced, the etching parameters can be chosen more freely than in the absence of this smoothing step, by reducing the risk of exceeding the roughness threshold at the end of the thinning step and thus restoring the monodomain nature of the thin film.

[0080] Whether this preliminary smoothing step is implemented or not, the method for preparing a thin film may be followed by a chemical-mechanical touch polishing of the free surface after the thinning step, which makes it possible to prepare the free surface 8 to have a low roughness, for example less than 0.5 nm RMS, by atomic force measurement (AFM) of 5×5 μm (if this was not obtained at the end of the thinning step), without affecting the uniformity of this film.

[0081] Naturally, the invention is not limited to the described embodiments, but variations are possible without departing from the scope of the invention as defined by the claims.

Claims

1. A method for preparing a thin film (3) of a single-crystalline ferroelectric material, the method comprising the following steps: Providing the thin film (3), wherein the thin film (3) exposes a first free surface (8); Thinning the thin film by ion etching, and a step defined by etching parameters; comprising; The method includes assembling a donor substrate (1) having a brittle plane (2) and a support substrate (7) in the providing step, and peeling the thin film (3) at the height of the brittle plane (2), wherein the thin film (3) has a second surface positioned on the support substrate (7) opposite to the free surface (8), and the etching parameters are selected such that the free surface (8) of the thin film (3) has a roughness not exceeding a threshold value at the end of the thinning step. Method.

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

3. The method according to claim 1 or 2, wherein chemical mechanical touch polishing of the free surface (8) follows the thinning step.

4. The method according to claim 1 or 2, wherein the providing step includes a heat treatment step of exposing the free surface (8) of the thin film (3) to a specific gas atmosphere.

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

6. The method according to claim 4, wherein the heat treatment step is carried out in an oxidizing or neutral gas atmosphere.

7. The method according to claim 4, including a step of smoothing the free surface (8) between the heat treatment step and the thinning step to reduce the surface roughness of the free surface (8) to a value lower than the threshold value.

8. The method according to claim 7, wherein the smoothing step includes chemical mechanical touch polishing of the free surface (8).

9. The method according to claim 1 or 2, wherein the threshold value is 3 nm to 7 nm.

10. A substrate comprising a thin film (3) of a single-crystalline and monodomain ferroelectric material disposed on a support (7), wherein the thin film has a thickness of 700 nm or less and a thickness uniformity of 60 nm or less.

11. The substrate according to claim 10, comprising an amorphous intermediate layer positioned between the thin film (3) and the support (7).

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

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

14. The substrate according to any one of claims 10 to 12, wherein the ferroelectric material of the thin film (3) is LiTaO3 or LiNbO3.

15. The substrate according to any one of claims 10 to 12, wherein the support (7) comprises a charge trap layer on the side of the thin film (3).

Images