Method for preparing a thin layer of ferroelectric material

Abstract

The invention relates to a method for preparing a single-domain thin layer (3') of monocrystalline ferroelectric material. The method comprises implanting "light" species in a first face (4) of a ferroelectric donor substrate (1), joining the first face (4) of the donor substrate (1) to a support substrate (7), and fracturing the donor substrate (1) to transfer the first layer (3) onto the support substrate and expose a free face (8) of the first layer (3). The method also comprises heat-treating the free face (8) of the first layer (3) and thinning the first layer (3) to provide the single-domain thin layer (3'). The method does not involve any treatment that exposes the first layer (3) to an aqueous solution between the step of fracturing the donor substrate (1) and heat-treating the first layer (3).

Description

METHOD FOR PREPARING A THIN LAYER OF FERROELECTRIC MATERIAL FIELD OF INVENTION

[0001] The present invention relates to a method for preparing a thin film of single-crystal ferroelectric material. More particularly, it relates to a preparation method that preserves the single-domain character of the ferroelectric material in the final thin film. This method can notably be used to form a piezoelectric-on-insulator (POI) structure, comprising the thin film transferred to a substrate via an interlayer. Such a structure finds application in the fields of microelectronics, microsystems, and photonics. It can be used to form radio frequency (RF) components, in particular filters or resonators based on elastic wave components, for example, surface elastic wave components.The electro-optical properties of the ferroelectric material can be exploited in particular to form optical modulators, integrated photonic components and / or components of quantum technology. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] As a preliminary point, it is important to remember that a ferroelectric material is a material that possesses an electrical polarization in its natural state, a polarization that can be reversed by the application of an external electric field. A ferroelectric domain is defined as each contiguous region of the material in which the polarization is uniform (all dipole moments are aligned parallel to each other along a given direction, called the ferroelectric axis). A ferroelectric material can therefore be described as "single-domain" if it consists of a single region with uniform polarization, or as "multi-domain" if it comprises multiple regions with potentially different polarities.

[0003] A process exists for preparing a ferroelectric thin film using Smart Cut™ technology. In this process, a thin film is extracted from a bulk substrate of single-crystal ferroelectric material (referred to as the "donor substrate" in this field) by fracturing it at a weak zone (or embrittlement plane) formed by the implantation of so-called "light" species, such as helium and / or hydrogen. A specific example of the implementation of this process can be found in document EP3646374B1.

[0004] According to this process, and after the layer sampling step, it is often necessary to apply treatments to improve its surface condition, crystalline quality, or modify its thickness. However, the applicant observed that these preparation steps, when applied to a ferroelectric thin film transferred using Smart Cut technology, could lead to the formation of multiple ferroelectric domains within the thin film, thus giving it a multi-domain character.

[0005] Such a characteristic renders the layer unsuitable for its intended use, as it affects the performance of devices intended to be formed on / in the thin layer, such as surface acoustic wave (SAW) devices.

[0006] Document FR2914492 describes a process for manufacturing a thin film of ferroelectric material using Smart Cut technology. This document applies an electric field to the thin film to improve or restore its ferroelectric properties. However, for easy application, this process requires electrodes on each face of the thin film, which is not always the case.

[0007] Document WO2024022723 reveals that a layer extracted using Smart Cut technology is particularly reactive. The presence of light species, hydrogen and / or helium, and the moderate temperature at which the fracture step occurs appear to make the lithium in this layer highly mobile. When a free face of this extracted layer is exposed to the atmosphere, the lithium near this face tends to migrate and react with hydrocarbons and oxygen naturally present in the atmosphere, forming a surface layer of Li2CO3 and / or amorphous dendrites, rich in lithium, hydrogen, and other atmospheric species such as chlorine or fluorine. To address these phenomena and prevent the formation of triangular or depression-type defects, the document recommends, in particular, cleaning the free face of the extracted layer by brushing and using deionized water.

[0008] Document WO2020200986 states that the formation of ferroelectric domains in a surface portion of the film is caused by the presence of hydrogen concentration gradients within the thin film during heat treatment. This hydrogen may correspond to light species embedded in the bulk substrate, forming the embrittlement zone that allows the thin film to be extracted. The surface portion can be on the order of 150 nm to 200 nm or more in thickness. To permanently restore the single-domain character of the thin film, this document recommends thinning the ferroelectric film after applying such heat treatment.

[0009] Document WO2024223276 observes that saturating a surface layer of the sampled thin film with hydrogen before the heat treatment step reduces the thickness of the multidomain surface portion. This document specifically describes a series of cleaning steps using an aqueous solution to deplete a surface layer of lithium and replace it with hydrogen. According to this document, this method can reduce the thickness of the multidomain portion to less than 125 nm.

[0010] However, regardless of the technique used to limit the thickness of the multidomain surface portion, its removal by thinning represents a limitation of prior art processes, as this step tends to make the manufacturing process longer and more complex. The thinning step, when it aims to remove a relatively large thickness, can also lead to deterioration of the uniformity of the extracted thin layer, particularly when this step is performed by polishing.

[0011] A process for preparing a single-domain thin film in ferroelectric material from the prior art therefore presents limitations that it would be desirable to overcome. SUBJECT OF THE INVENTION

[0012] One object of the invention is to provide a method for preparing a thin film of ferroelectric material that addresses at least some of the aforementioned limitations. More specifically, one object of the invention is to provide a method for preparing a thin film of ferroelectric material, particularly a lithium-based ferroelectric film, that is faster to implement than prior art methods. Another object of the invention is to provide a method for preparing a ferroelectric thin film obtained by detachment at a weakening plane of a donor substrate, in which the thickness of material removed to eliminate the multidomain surface portion of the extracted layer is less than the thickness removed in prior art methods. BRIEF DESCRIPTION OF THE INVENTION

[0013] With a view to achieving one of these goals, the object of the invention proposes a method for preparing a single-domain thin film in single-crystal ferroelectric material, comprising the following steps: implantation of so-called "light" species in a first face of a ferroelectric donor substrate to form a weakening plane and define a first layer between the weakening plane and the first face of the donor substrate; assembly of the first face of the donor substrate to a support substrate; fracturing of the donor substrate at the level of the weakening plane to transfer the first layer onto the support substrate and expose a free face of the first layer; finishing of the first layer, this finishing comprising a heat treatment of the free face of the first layer and a thinning of the first layer to provide the single-domain thin film.

[0014] According to the invention, the process is free of any treatment exposing the first layer to an aqueous solution between the step of fracturing the donor substrate and the heat treatment of the first layer.

[0015] According to other advantageous and non-limiting features of the invention, taken alone or in any technically feasible combination: the heat treatment is applied directly, without any other intermediate treatment step, after the fracture step; the process includes, before or during the heat treatment, a step of introducing lithium into a superficial thickness of the first layer; the introduction step is carried out before the heat treatment; the introduction step includes immersing the first layer in a non-aqueous solution comprising lithium; the solution comprises LiCl, LiNO3 or LiBF4 in an organic solvent, such as acetonitrile (CH3CN), dimethyl sulfoxide (DMSO), ethanol, isopropanol; the solution contains Li2CO3 or LiOH suspended in an organic solvent, such as ethanol, toluene or acetonitrile;The introduction step includes immersing the first layer in a molten lithium salt, such as LiNO3-KNO3 or LiCl-KCl; the introduction step is carried out during heat treatment; the preparation process includes deposition of a lithium-containing reservoir layer on the first layer prior to heat treatment; the lithium introduction step includes annealing the first layer in a lithium-containing atmosphere; the lithium-containing atmosphere is obtained by sublimation from a solid source; the solid source comprises powdered LiTaO3, Li2O, Li2CO3, or LiCl; the lithium-containing atmosphere is obtained by a gaseous stream containing lithium; the gaseous stream comprises lithium in organometallic form, such as butyllithium (BuLi) or phenyllithium (PhLi); the thinning of the first layer follows the heat treatment of its free face.the supporting substrate includes a layer for trapping electrical charges.

[0016] According to another aspect and in an alternative, the finishing step of the first layer includes, before or during the heat treatment, a step of introducing lithium into a superficial thickness of the first layer.

[0017] According to other advantageous and non-limiting features of this alternative, taken alone or in any technically feasible combination: the introduction step is carried out before the heat treatment; the introduction step includes the immersion of the first layer in a non-aqueous solution comprising lithium; the solution comprises LiCl, LiNO3 or LiBF4 in an organic solvent, such as acetonitrile (CH3CN), dimethyl sulfoxide (DMSO), ethanol, isopropanol; the solution contains Li2CO3 or LiOH suspended in an organic solvent, such as ethanol, toluene or acetonitrile.The introduction step includes immersing the first layer in a molten lithium salt such as LiNO3-KNO3, LiCl-KCl; the introduction step is carried out during the heat treatment; the preparation process includes deposition of a reservoir layer comprising lithium on the first layer before heat treatment; the lithium introduction step includes annealing the first layer in an atmosphere comprising lithium; the atmosphere comprising lithium is obtained by sublimation from a solid source; the solid source comprises powder of LiTaO3, Li2O, Li2CO3 or LiCl; the atmosphere comprising lithium is obtained by a gas stream containing lithium; the gas stream comprises lithium in organometallic form such as butyllithium (BuLi) or phenyllithium (PhLi).the process is free of any treatment exposing the first layer to an aqueous solution between the donor substrate fracture step and the heat treatment of the first layer; the heat treatment is applied directly, without any other intermediate treatment step, after the fracture step; the support substrate includes an electrical charge trapping layer.

[0018]

[0019] Figures 1 and 2 represent two methods for preparing a single-domain thin film;

[0020]

[0021] Lare represents a substrate comprising a first layer prepared for its characterization;

[0022]

[0023] Larepresents PFM images of first layers that have or have not undergone cleaning treatment;

[0024]

[0025] Larepresents a measurement by secondary ion mass spectrometry (SIMS) of the concentration of certain elements in the depth of a first layer that has undergone cleaning;

[0026]

[0027] Figures 6, 7a, 7b represent secondary ion mass spectrometry (SIMS) measurements and PFM phase images of a first layer that has undergone acid cleaning and a first layer that has undergone acid cleaning compensated by cleaning in a solution including lithium. DETAILED DESCRIPTION OF THE INVENTION

[0028] The invention relates to a method for preparing a single-domain thin film 3' of single-crystal ferroelectric material transferred from a donor substrate 1 onto a support substrate 7 by a transfer technique including the implantation of light species in the donor substrate 1. Several embodiments of this thin-film supply step are possible.

[0029] According to a first embodiment, shown in steps 1A to 1F, the donor substrate 1 is composed of a solid, single-crystal, single-domain block of ferroelectric material, for example, LiTaO3, LiNbO3, LiAlO3, BaTiO3, PbZrTiO3, KNbO3, BaZrO3, CaTiO3, PbTiO3, or KTaO3. Preferably, this material is lithium-based, for example, lithium tantalate or lithium niobate. The donor substrate 1 may be in the form of a circular wafer of standardized dimensions, for example, 150 mm or 200 mm in diameter. However, the invention is not limited to these dimensions or this shape. The donor substrate may have been extracted from an ingot of ferroelectric material, this extraction having been carried out in such a way that the donor substrate 1 has a predetermined crystal orientation. The orientation is chosen according to the intended application.Thus, it is common to choose an orientation between 30° and 60°RY, or between 40° and 50°RY, when one wishes to exploit the properties of a LiTaO3 thin film to form a SAW filter. However, the invention is by no means limited to a particular crystal orientation or application.

[0030] Regardless of the crystalline orientation of the donor substrate 1, the process includes the introduction into the donor substrate 1 of at least one so-called "light" species, in particular species chosen from among inert gases or hydrogen. This introduction may correspond to implantation, that is to say, ion bombardment of a planar face 4 of the donor substrate 1 by light species, such as hydrogen and / or helium ions.

[0031] As is known in itself, and as shown in Figure 1B, the implanted ions form a weakening plane 2 delimiting a first layer 3 of ferroelectric material to be transferred which is located on the side of the flat face 4 and another part 5 forming the rest of the substrate.

[0032] The nature and dose of the implanted species, as well as the implantation energy, are chosen according to the thickness of the layer to be transferred and the physicochemical properties of the donor substrate. For example, in the case of a LiTaO3 donor substrate, a hydrogen dose between 1 and 100% can be implanted. E 16 and 5 E 17 at / cm² with an energy between 30 and 300keV to delimit a first layer 3 of the order of 200 to 2000 nm.

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

[0034] In one particular embodiment, the support substrate 7 comprises a base substrate 7b, for example made of monocrystalline silicon, on which is disposed an electrical charge trapping layer 7a (hereafter referred to simply as the "trapping layer"). The base substrate 7b may have a high resistivity, greater than 1000 ohms·cm, or a more conventional one, less than 1000 ohms·cm.

[0035] The trapping layer 7a can be of highly varied nature. Generally, it is a non-single-crystal layer exhibiting structural defects, such as dislocations, grain boundaries, amorphous zones, interstices, inclusions, pores, etc. These structural defects trap charges that may circulate within the material, for example, at incomplete or dangling chemical bonds. This prevents conduction in this layer 7a, which consequently exhibits high resistivity.

[0036] In general, the trapping layer 7a can have a thickness between 10 nm and 30 micrometers, preferably between 50 nm and 5 micrometers.

[0037] According to one possible approach, and for reasons of simplicity of implementation, the trapping layer 7a is formed from a layer of polycrystalline silicon. This layer can be formed by deposition on a first face of the base substrate 7b. In this case, and in order to preserve the polycrystalline quality of this layer during the thermal treatments that the support substrate 7 may undergo, an amorphous layer, of silicon dioxide for example, can advantageously be placed on the base substrate 7b before the deposition of the trapping layer 7a.

[0038] Alternatively, the trapping layer 7a can be formed by implanting a relatively heavy material, such as argon, into a surface layer of the base substrate 7b, thereby creating the structural defects that constitute the electrical traps. This trapping layer 7a can also be formed by porosifying a surface layer of the base substrate 7b on its first face, or by any other method capable of creating structural defects in a surface layer of the substrate, these structural defects being capable of trapping electrical charges.

[0039] According to a particularly advantageous approach, the trapping layer 7a consists of a silicon-rich oxide deposited on the first face of the base substrate 7b. A "silicon-rich oxide" is defined as an oxide with an atomic concentration of silicon between 50% and 99.9%. Oxygen and, optionally, nitrogen are present in this layer at substoichiometric atomic concentrations. Preferably, this oxide has an atomic concentration of silicon between 70% and 90% and an atomic concentration of oxygen between 10% and 30%. It may contain nitrogen at an atomic concentration between 8% and 45%.

[0040] A silicon-rich oxide layer is usually formed from an amorphous matrix. This amorphous matrix may include crystalline silicon inclusions or crystalline silicon grains, the density and size of these inclusions and / or grains within the amorphous matrix depending on the relative proportions of oxygen, nitrogen, and silicon in the layer and the heat treatments it has undergone. In extreme cases, a silicon-rich oxide layer may be entirely amorphous or polycrystalline, the amorphous matrix in the latter case being reduced to amorphous inclusions present between the polycrystalline grains. Such a layer has the advantage of exhibiting a particularly high defect density, forming traps for electrical charges. It is therefore likely to exhibit very high resistivity, up to approximately 10^12 ohm·cm.Further details on the formation of the trapping layer 7a in the form of a silicon-rich oxide layer can be found in publications WO2024115410, WO2024115411 and WO2024115414.

[0041] Returning to the general description of the supply process shown on the diagram, and prior to the assembly stage, it may be considered to prepare the faces of the substrates to be assembled by a cleaning, brushing, drying, polishing, or activation step, for example by plasma.

[0042] The assembly step may involve bringing the donor substrate 1 into intimate contact with the support substrate 7 by molecular adhesion and / or electrostatic bonding. Optionally, to facilitate the assembly of the two substrates 1 and 7, particularly when assembled by direct bonding, at least one amorphous interlayer may be formed prior to assembly, either on the flat face 4 of the donor substrate 1, or on the flat face 6 of the support substrate 7 to be joined, or on both. This interlayer is, for example, composed of silicon dioxide, silicon nitride, or silicon oxynitride. It may have a thickness ranging from a few nanometers to a few microns.

[0043] An interlayer with a low hydrogen concentration or a barrier to hydrogen diffusion will be preferred to follow the teachings of document WO2020200986 and thus avoid the formation of a multidomain zone at the interface between the first layer 3 and the amorphous interlayer, on the side of the second face of the first layer 3. The interlayer can be produced using the various techniques known from the state of the art, such as heat treatments of oxidation or nitriding, chemical deposition (PECVD, LPCVD…), etc.

[0044] At the end of this assembly step, we have an assembly comprising the two associated substrates, the flat face 6 of the support substrate 7 adhering to the flat face 4 of the donor substrate 1.

[0045] The assembly is then treated to detach the first layer 3 of ferroelectric material from the donor substrate 1, for example by cleavage at the level of the embrittlement plane 2.

[0046] This fracture step may therefore include the application to the whole of a heat treatment in a temperature range of the order of 80°C to 300° to allow the transfer of the first layer 3 onto the support substrate 7. As a replacement or in addition to the heat treatment, this step may include the application of a blade or a jet of gaseous or liquid fluid at the level of the embrittlement plane 2.

[0047] Following this fracture step, we obtain the structure 9 represented in figure 1D. This structure 9 comprises the first layer 3 of single-crystal ferroelectric material comprising a first free face 8 and a second face 4 arranged on the support substrate 7.

[0048] Lare represents a second mode of implementation leading to providing this same structure 9. This second mode of embodiment is particularly suited to the realization of a heterogeneous structure 9, in which the first layer 3 has a coefficient of thermal expansion (in the principal plane defining this layer) very different from that of the support substrate 7, for example showing a difference of more than 10% (at room temperature).

[0049] This second implementation differs primarily from the first implementation in the nature of the donor substrate 1. For the sake of brevity, only the elements of this second implementation that differ from the first are presented here; all other characteristics of the first implementation can therefore be assumed. Further details on this second implementation can be found in document EP3646374.

[0050] With reference to step 2A, the donor substrate 1 in this case comprises a layer of single-crystal ferroelectric material 1a arranged on a manipulator substrate. The ferroelectric material layer has the same properties as those described for the bulk block of ferroelectric material in relation to the first embodiment.

[0051] The manipulator substrate 1b is advantageously made of a material (or a plurality of materials) giving it a coefficient of thermal expansion close to that of the support substrate 7. By "close", we mean that the difference in coefficient of thermal expansion of the manipulator substrate 1b and that of the support is less, in absolute value, than the difference in thermal expansion of the solid block of the ferroelectric material and that of the support substrate 7.

[0052] Preferably, the manipulator substrate 1b and the support substrate 7 have the same coefficient of thermal expansion. When the donor substrate 1 and the support substrate 7 are joined, a unit is formed that can withstand heat treatment at a relatively high temperature. For ease of implementation, this can be achieved by choosing the manipulator substrate 1b to be made of the same material as the support substrate 7.

[0053] To form the donor substrate 1 of this embodiment, a solid block of ferroelectric material is first bonded to the manipulator substrate 1b, for example, using a molecular adhesion technique as previously described or with an adhesive layer, such as a polymer adhesive. Prior to bonding, the formation of an adhesive layer (for example, by deposition of silicon oxide and / or silicon nitride, or an adhesive layer, such as a polymer) on one or both of the contacting faces may be planned. Then, the ferroelectric material layer 1a is formed by thinning, for example, by grinding and / or chemical polishing and / or etching. The bonding process may include the application of a low-temperature heat treatment (for example, between 50 and 300°C, typically 100°C) to sufficiently increase the bonding energy to allow the subsequent thinning step.

[0054] 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 such that the ferroelectric material layer 1a is sufficiently thin to reduce the stresses generated during the subsequent heat treatments. At the same time, this thickness is sufficient to allow the removal of the first layer 3, or a plurality of such layers. This thickness can, for example, be between 5 and 400 microns.

[0055] The subsequent steps of the process in this second embodiment are equivalent to those described in the first embodiment. Lightweight species are implanted within the ferroelectric material layer 1a to generate a weakening plane 2, which marks the separation of the thin layer 3 from the remaining portion 5 of the donor substrate 1, as shown in Figure 2B. This step is followed by the assembly of the donor substrate 1 onto the support substrate 7, as shown in Figure 2C. The first layer 3 is then detached from the remaining portion 5 of the donor substrate 1 to obtain the structure 9 shown in Figure 2D.

[0056] This method of implementation is advantageous in that the assembly formed by the donor substrate 1 and the support substrate 7 can be exposed to a much higher temperature than that applied in the first method of implementation, without risk of uncontrolled fracture of one of the substrates or delamination of the donor substrate 1 of the first layer 3. The balanced structure, in terms of the coefficient of thermal expansion of this assembly, thus makes it easier to detach the first layer 3 by exposing the assembly to a relatively high temperature, for example between 100 and 500 °C.

[0057] Regardless of the implementation method chosen, and as stated in the introduction to this application, finishing steps of the first layer 3 are then necessary to improve its crystalline and surface quality, and provide a thin layer 3' whose thickness corresponds to, or approaches, a target thickness.

[0058] For the sake of accuracy, in this application the term "first layer" refers to the layer taken from the donor substrate 1 and transferred to the support substrate 7 after the fracturing step, and the term "thin layer" refers to the single-domain layer obtained at the end of the finishing steps.

[0059] These finishing steps 1E,2E represented schematically on laet la, aim in particular to eliminate a hardened and rough surface layer, resulting from the cleavage and detachment of the thin layer 3 from the rest of the donor substrate 1.

[0060] Within the scope of this description, the finishing steps include a heat treatment of the first sampled layer 3. This heat treatment helps to cure crystalline defects present in this layer 3, and may even reduce the roughness of its free face 8. Furthermore, it helps to consolidate its adhesion to the supporting substrate 7. The heat treatment raises the structure to a temperature between 300°C and the Curie temperature of the ferroelectric material for a duration of between 30 minutes and 10 hours. It can be carried out in a neutral atmosphere or one containing oxygen.

[0061] To avoid any ambiguity, it is emphasized that the finishing heat treatment stage is distinct from the fracture heat treatment applied to the assembled structure. In particular, it is carried out using different equipment than that used to apply the fracture heat treatment.

[0062] After the finishing heat treatment, a thinning step of the first layer 3 is also planned. This step aims to eliminate a multi-domain surface portion of the first layer 3, this portion may appear during the previous heat treatment step.

[0063] Thinning also allows for the production of a single-domain thin film 3 whose thickness corresponds to a target thickness, as previously mentioned. This thinning can generally involve polishing the first free face 8 of the thin film 3, for example, using mechanical or mechanochemical thinning techniques. It can also involve thinning achieved through ion etching, for example, by reactive ion etching or any other technique.

[0064] Steps 1F and 2F in Figures 1 and 2 represent the structure 9 obtained at the end of these treatments, with a single-domain thin layer 3' being placed on the support substrate 7.

[0065] To characterize the thickness of any potential multidomain surface portion of the first layer 3 after heat treatment, the Applicant has developed an original characterization technique. This technique comprises a first step consisting of removing a bevel from the structure 9 (and more specifically from the first layer 3 of this structure 9) by chemical-mechanical polishing, as illustrated in Figure 1. The bevel exposes this first layer 3 in its entire thickness down to the underlying substrate 7. Following this first step, piezoelectric force microscopy (PFM) imaging is performed on an inspection zone ZI encompassing the entire exposed surface of the bevel.The images produced by this technique therefore incorporate a portion of the free surface of the first layer 3, the entire exposed surface of the bevel in the thickness of the layer, and a portion of the support 7. The exposed surface of the bevel in the thickness of the layer has been delimited by the boundary lines I1, I2.

[0066] In piezoelectric force microscopy (PFM), a conductive AFM tip is brought into contact with the surface of a ferroelectric or piezoelectric material. An alternating voltage is then applied between the tip and the sample, creating an electric field within the material. Under the influence of this field, the sample undergoes local expansion or contraction, depending on the orientation of the ferroelectric polarization of the domain beneath the tip. This movement causes deformation of the AFM tip, which is then detected to produce topography and phase images by scanning the sample with this tip. In PFM imaging, the phase difference between the applied voltage and the piezoelectric response is directly related to the orientation of the ferroelectric domains. The amplitude of this response, in turn, allows the position of the domain walls to be identified.

[0067] In the phase image, the multidomain nature of the sample leads to a mottled, "leopard-skin" pattern, revealing the presence of multiple domains. Conversely, a single-domain sample appears as a relatively uniform phase image.

[0068] By measuring the extent of the spotted portion on the PFM phase image of the bevel, we can determine the thickness of the multidomain surface portion.

[0069] We have thus represented on the diagram such analyses carried out on a first layer 3 after its transfer onto a support 7 and after applying a heat treatment at 500°C in a neutral atmosphere. In the experiments carried out, the first layer 3 was made of lithium tantalate with a 42RY crystal orientation (left part of the diagram) or a 50RY crystal orientation (right part of the diagram).

[0070] For each prepared first layer, the PFM characterization technique described above was applied. In some preparations, the process lacked any treatment step for the first layer 3 between the fracture step and the heat treatment (column A). In other preparations, the process included a cleaning step between the fracture step and the heat treatment (column B). This cleaning step involved brushing the free face 8 of the first layer 3 and, simultaneously, dispensing deionized water onto this free face 8.

[0071] In each column A and B, we have represented the intensity image PFM (top row I) and the phase image PFM (bottom row P) of the wedge previously formed in the structures 9 under test.

[0072] It is quite visible on the PFM images of the, that the first 3 layers having undergone cleaning (columns B) show a mottled phase image to the left of the I1 boundary line (SA zone on the), which testifies to the multidomain character of the surface of the first 3 layer. We see that this mottled image area extends to the right of the I1 boundary line, which indicates that the multidomain character extends in depth into the first layer (SB zone on the).

[0073] In comparison, the PFM images corresponding to the first layers that have not undergone cleaning (columns A) appear relatively smooth, without a mottled appearance, which testifies to the entirely single-domain nature of this layer.

[0074] It therefore appears that a lack of cleaning of the thin layer between the fracture stage and the heat treatment tends to prevent the appearance of a multi-domain surface thickness in the first layer, or at least reduce its extension in depth.

[0075] The Applicant knew, prior to these results, that the Li+ ions in the first layer 3, due to its prior treatment (implantation, fracture annealing), were particularly mobile and likely to move within the layer, for example, to agglomerate on the surface. Further experiments, such as those presented in the previous section, led the Applicant to believe that the exchanges between H+ ions from the environment (e.g., cleaning water) and the Li+ ions available in the first layer 3 were particularly pronounced in a first layer that had been implanted and annealed during the fracture step. These exchanges tend to deplete the first layer 3 of lithium at its surface and, at the same time, enrich it with hydrogen. "At its surface" refers to a thickness of the first layer that can extend under the free face 8 of this first layer 3 to a few tens, or even a few hundred, nanometers.

[0076] A secondary ion mass spectrometry (SIMS) measurement of the concentration C of certain elements in depth D of the first layer 3 (depth 0 corresponding to the free face of the first layer 3) was thus represented on the graph. This measurement was performed on a first layer that, between the fracture and heat treatment steps, underwent brushing of the free face 8 of the first layer 3 and, simultaneously, the application of deionized water to this free face 8 (thus corresponding to a sample identical to that of column B on the graph). The graph shows that the treated layer appears depleted in lithium at its free face, with the concentration of LiO- and Li- decreasing towards this face. At the same time, the graph shows that the first layer is enriched in hydrogen towards this free face.

[0077] The Applicant had the intuition that this phenomenon, which can be likened to a proton exchange and its result of lithium depletion at the level of the free face 8 of the first layer 3, was the driving force behind the formation of the polarity reversals caused by the heat treatment, these reversals being at the origin of the multidomain surface portion of this first layer.

[0078] It is worth recalling that the proton exchange phenomenon involves the partial or complete substitution of lithium ions (L-) by protons (H+), for example, protons supplied by an acid bath. However, it was not expected that such a phenomenon would occur with such intensity when the material was exposed to deionized water, as demonstrated in the image. It is generally recognized that such exchange is highly dependent on the strength of the acid in the bath (as measured by its acidity constant, pKa). It therefore appears that the first layer (3), due to the steps taken to extract it, exhibits an extraordinary capacity to substitute the lithium it contains with hydrogen from its environment.

[0079] After the heat treatment of the finishing step, the first layer 3 no longer exhibits this property. Subsequent steps following this heat treatment, particularly cleaning steps, no longer lead to domain nucleation, even after the application of another heat treatment.

[0080] The present invention takes advantage of the results, observations and these intuitions to propose a method for preparing the first layer 3 aimed at limiting the thickness of the multidomain surface portion, or even making it disappear.

[0081] According to the invention, the process of preparing the ferroelectric thin film is carried out in a controlled manner, this control aiming to temper this phenomenon of lithium depletion / hydrogen enrichment, that is to say to prevent occurrence or compensate for effects, between the fracture stage and the heat treatment, as well as during the heat treatment itself.

[0082] Thus, according to a first approach which constitutes an aspect of the present invention, the thin-film preparation process 3' is conducted, between the fracture step and the heat treatment, as well as during the heat treatment, in such a way as to prevent the lithium depletion / hydrogen enrichment phenomenon. To this end, the first layer 3 is not exposed to an aqueous solution or a humid environment, as the water present in this solution or environment promotes superficial lithium depletion in the first layer 3, and therefore ultimately leads to the creation of a multidomain surface portion within the first layer 3.

[0083] In this first approach, the preparation process is devoid, between the fracture of the donor substrate 1 and the heat treatment of the first layer 3, of any treatment exposing the first layer to an aqueous solution, in particular of aqueous treatment for cleaning the free face.

[0084] In particular, we do not perform so-called "standard" cleaning or RCA cleaning ("RCA clean" in industry terminology, an acronym for Radio Corporation of America). Such cleaning, very common in the microelectronics field, involves the successive immersion of the substrate in a wet chemistry solution consisting of SC1 ("Standard Clean 1") containing ammonium hydroxide (NH4OH), hydrogen peroxide (H2O2), and ultrapure water, and then in a wet chemistry solution consisting of SC2 ("Standard Clean 2") containing hydrochloric acid (HCl), hydrogen peroxide (H2O2), and ultrapure water. These immersions are preceded and / or followed by rinsing the substrate by immersion in a rinsing solution composed of ultrapure water or based on ultrapure water.

[0085] In a specific implementation of this initial approach aimed at preventing lithium depletion / hydrogen enrichment, the heat treatment is applied directly, without any intermediate treatment steps, after the fracturing stage. This avoids any risk of superficial lithium depletion in the first layer. This direct sequence can, for example, be performed "in situ" using the same equipment for both the fracturing and the heat treatment of the finishing stage.

[0086] In this implementation method, the heat treatment of the finishing step is therefore applied to the first layer 3 while it is covered with the surface thickness of Li2CO3 which develops there as soon as it is released from the donor substrate, after the fracture step.

[0087] By taking such precautions between the fracture stage and the end of the heat treatment, it is possible to obtain a first layer with a multidomain surface portion with a thickness of less than nm, or even devoid of any multidomain surface portion.

[0088] Following the heat treatment, and as presented in the previous section, the first layer 3 can be thinned to eliminate, if necessary, this multi-domain surface portion and provide a single-domain thin layer 3' with a desired thickness.

[0089] It should be noted that the absence of any aqueous treatment between the fracture step and the heat treatment deviates from the practices implemented in semiconductor processes. Indeed, water-based cleaning steps are usually performed between successive technological steps. Reference can be made to the abundant literature on the subject, such as the document by Binoy Bera, "Silicon Wafer Cleaning: A Fundamental and Critical Step in Semiconductor Fabrication Process," International Journal of Applied Nanotechnology, 2019; 5 (1): 8–13p. This is particularly relevant after a fracture step in a process conforming to Smart Cut™ technology, as this step is likely to generate a large number of particles on the substrate surface, which should be removed.In the case of lithium-based layer transfer, the prior art mentioned in the introduction to this application recommends the removal of surface species such as Li₂CO₃ before any heat treatment in order to limit the appearance of triangle or depression type defects.

[0090] The Applicant, however, highlighted that such exposure promotes proton-type exchanges in the first fractured layer. It contributes to the formation of a multidomain surface portion during heat treatment. The elimination of any aqueous step between the fracture and heat treatment stages, quite unexpectedly given the prior art and the general knowledge of a person skilled in the art, nevertheless contributes to obtaining a layer with an improved single-domain character.

[0091] According to a second approach, which in itself forms a second aspect of the invention (but which can be combined with the first approach), the process of preparing the single-domain thin film is conducted, between the fracture step and the heat treatment as well as during the heat treatment itself, in such a way as to compensate for the effects of lithium depletion of the first layer at the level of its free face.

[0092] Lithium depletion of the first layer may have occurred during steps preceding the heat treatment, for example, during a cleaning step using an aqueous solution of the first layer 3 after the fracture step. This may be a standard cleaning step, the purpose of which is, in particular, to remove particles that may have been deposited on the first layer 3 during the fracture. Alternatively, this cleaning step may conform to that described in document WO2024223276, using deionized water. It has been observed that such a step tends to deplete the first layer of lithium on its free face.

[0093] By "compensating for the effects" we mean that a step is planned, before or during the heat treatment, to introduce lithium into a surface thickness of the first layer 3. This introduction step aims to increase the lithium concentration in this surface thickness, in order to at least partially compensate for any depletion that may have occurred. The surface thickness into which this lithium is introduced can extend to a depth of several hundred nanometers, for example, more than 200 nm or more than 300 nm, from the free face of the first layer. The lithium introduction step can be carried out in accordance with the teachings of document US11398595.

[0094] This introductory step can be carried out to target a specific lithium concentration within the surface layer. Alternatively, the aim can be to reduce the lithium concentration gradient within this surface layer to a target gradient.

[0095] The introduction step can be carried out before heat treatment, for example by immersing the first layer 3 in a lithium-containing solution. This solution can be an aqueous or non-aqueous solution of LiNO3. It can be a solution of LiCl, LiNO3, or LiBF4 in an organic solvent such as acetonitrile, dimethyl sulfoxide, ethanol, or isopropanol. Alternatively, it can be Li2CO3 or LiOH suspended in an organic solvent such as ethanol, toluene, or acetonitrile.

[0096] The introduction step may also include immersing the first layer 3 in a molten lithium salt, such as LiNO3-KNO3 or LiCl-KCl. This may involve immersing the structure containing the first layer 3 in a bath at a temperature that ensures the lithium salt remains liquid (350°C to 450°C for LiCl-KCl or 130°C to 300°C for LiNO3-KNO3) for a period typically between 10 minutes and 2 hours.

[0097] Rather than immersing the layer in a lithium-containing fluid, the introduction step can be more simply performed by depositing a lithium reservoir layer onto and in contact with the first layer 3. This reservoir layer can be composed of or contain metallic lithium or a lithium precursor. Examples include Li₂CO₃, Li₃N, Li₂O, and / or LiNO₃. The lithium contained in this reservoir layer can migrate to the first layer 3 by diffusion. This diffusion can be promoted by moderately raising the temperature of the reservoir-first layer stack.

[0098] According to a very advantageous implementation method, the deposition can be carried out before the heat treatment of the finishing stage, this heat treatment being carried out with the reservoir layer present on the first layer 3. The step of introducing lithium into the first layer 3 is then carried out during the heat treatment itself.

[0099] There are other ways to put the introduction step during the heat treatment itself, for example by operating the heat treatment annealing the first layer 3 in an atmosphere including lithium.

[0100] This atmosphere can be produced by a gas stream containing a lithium-based organometallic compound such as butyllithium (BuLi) or phenyllithium (PhLi). The gas stream can be introduced into a chamber containing the structure comprising the first layer 3, in order to expose the free face 8 of this layer.

[0101] This lithium-containing atmosphere can be obtained by sublimation from a solid lithium source. In this approach, the solid source can be arranged as a bed. A carrier gas flows over the bed containing the solid source, which sublimates to incorporate lithium-based species into the gas stream. The bed can be located upstream of the chamber or within the chamber containing the solid source bed. The bed can be composed of a powder, for example, a powder containing LiTaO3, Li2O, Li2CO3, and / or LiCl.

[0102] The temperature and duration of the heat treatment can be chosen to incorporate a defined quantity of lithium into the first layer. This can involve, as already stated, achieving a predefined lithium concentration, or a lithium concentration gradient, within a specific surface thickness of this first layer.

[0103] We can naturally combine the two approaches described, that is to say both prevent the lithium depletion of the first layer 3 and provide a step of introducing lithium into this first layer 3.

[0104] In such a case, the first layer 3 is not exposed to an aqueous solution during the lithium introduction step. Therefore, if the first layer 3 is immersed in a lithium-containing solution, this solution must be non-aqueous. It could be a solution of a lithium-containing material in an organic solvent, as previously suggested.

[0105] To illustrate a benefit of the invention, an experiment was conducted to prepare a first lithium tantalate layer 3 according to the process described at the beginning of this description. Between the fracture step and the heat treatment, the first lithium tantalate layer was immersed in an acidic solution (based on HCl) at a relatively high temperature (80°C) to promote proton exchange. A SIMS measurement performed after this immersion step, and shown in Figure 6a, demonstrates that this exchange led to the formation of a surface layer with a high hydrogen concentration (curve labeled H) and a high lithium depletion (curve labeled L).

[0106] Figure 1 represents a PFM phase image of the first layer 3 (produced using the same bevel technique described previously) that underwent immersion in an acidic medium and was then annealed during the heat treatment of the finishing step. The free surface 8 of the layer and the bevel surface within the thickness of the first layer 3 are delimited on this image by the boundary line I1. A multidomain portion is present on the free face and below the free face of this first layer.

[0107] In another experiment, the immersion step of a first lithium tantalate layer in an acidic solution at temperature was also replicated. Then, before proceeding with the heat treatment, the lithium depletion was compensated for by immersing the first layer in a saturated aqueous LiNO3 solution at 80°C. The SIMS measurements performed after this second immersion are presented in Figure 3. These measurements show that, following this treatment in a saturated LiNO3 solution, the surface thickness exhibits a reduced hydrogen concentration (curve labeled H') and a less pronounced lithium depletion (curve labeled L').

[0108] Figure 1 represents a PFM phase image of this first layer 3 after this layer has been annealed during the heat treatment. The free surface 8 of the layer and the surface of the bevel in the thickness of the first layer 3 are also delimited on this image by the boundary line I1. Note that the multidomain portion extends under the free face of the first layer 7, in a very limited way, and much less deeply than in the case of the first layer 3.

[0109] It is therefore clear that by controlling the preparation process of the monomania thin film, both between the fracturing step and the heat treatment, as well as during the heat treatment itself, in order to prevent or compensate for the lithium depletion / hydrogen enrichment effects that can occur on the free face of the first layer 3, the presence of a multidomain surface portion is significantly reduced (or even its formation is avoided) after the heat treatment of the finishing step. The first layer thinning step and the preparation process as a whole can be shortened and simplified.

[0110] In some cases, the first layer 3 does not develop a multi-domain surface portion during the finishing heat treatment. In this case, it is not essential to follow this heat treatment with a thinning step. If such thinning is necessary, for example, to remove a rough, work-hardened surface layer resulting from cleavage and fracture of the thin layer 3, this thinning can be carried out prior to the heat treatment.

[0111] Of course the invention is not limited to the implementation methods described and alternative embodiments can be made without departing from the scope of the invention as defined by the claims.

Claims

A process for preparing a single-domain thin film (3') of single-crystal ferroelectric material, comprising the following steps: implantation of so-called "light" species in a first face (4) of a ferroelectric donor substrate (1) to form a weakening plane (2) and define a first layer (3) between the weakening plane (2) and the first face (4) of the donor substrate (1); assembly of the first face (4) of the donor substrate (1) to a support substrate (7); fracturing the donor substrate (1) at the level of the weakening plane (2) to transfer the first layer (3) onto the support substrate and expose a free face (8) of the first layer (3); finishing the first layer (3), this finishing comprising a heat treatment of the free face (8) of the first layer (3) and a thinning of the first layer (3) to provide the single-domain thin film (3');the process being characterized in that it is devoid of any treatment exposing the first layer (3) to an aqueous solution between the step of fracturing the donor substrate (1) and the heat treatment of the first layer (3).; Preparation method according to the preceding claim, wherein the heat treatment is applied directly, without any further intermediate treatment step, after the fracture step. Preparation process according to claim 1, comprising, before or during heat treatment, a step of introducing lithium into a superficial thickness of the first layer (3). Preparation process according to the preceding claim, wherein the introduction step is carried out before the heat treatment. Preparation method according to the preceding claim, wherein the introduction step includes immersing the first layer (3) in a non-aqueous solution comprising lithium. A preparation method according to the preceding claim, wherein the solution comprises LiCl, LiNO3 or LiBF4 in an organic solvent, such as acetonitrile (CH3CN), dimethyl sulfoxide (DMSO), ethanol, isopropanol. A preparation method according to claim 5, wherein the solution contains Li2CO3 or LiOH suspended in an organic solvent, such as ethanol, toluene, or acetonitrile. Preparation method according to claim 5, wherein the introduction step comprises immersing the first layer (3) in a molten lithium salt such as LiNO3-KNO3, LiCl-KCl. Preparation method according to claim 3, wherein the introduction step is carried out during the heat treatment. Preparation method according to the preceding claim, comprising the deposition of a reservoir layer comprising lithium on the first layer (3) prior to heat treatment. Preparation process according to claim 9, wherein the lithium introduction step includes annealing the first layer (3) in an atmosphere including lithium. A preparation method according to the preceding claim, wherein the atmosphere comprising lithium is obtained by sublimation from a solid source. A preparation method according to claim 11, wherein the atmosphere comprising lithium is obtained by a gaseous stream containing lithium. Preparation method according to any one of the preceding claims, wherein the thinning of the first layer (3) follows the heat treatment of its free face (8). A preparation method according to any one of the preceding claims, wherein the support substrate (7) comprises an electrical charge trapping layer (7a).

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