Method for preparing thin layers of ferroelectric material

Abstract

The invention relates to a method for producing a thin single domain layer (3 ') made of ferroelectric material, comprising between a step of splitting a donor substrate (1) at a weakening plane (2) to form a first layer (3) and a procedure for finishing the first layer (3), a treatment is applied to the free face (8) of the first layer (3) to produce a hydrogen concentration greater than 2.0 E21 at / cm3 in the surface thickness of the first layer (3).

Description

Technical Field

[0001] This invention relates to a method for preparing thin layers of ferroelectric materials. More specifically, this invention relates to a preparation method that preserves the single-domain properties of the ferroelectric material in the thin layer of the final product. This preparation method is applicable to fields such as microelectronics, micromechanics, and optoelectronics. Background Technology

[0002] As an introduction and explanation, ferroelectric materials are materials that are polarized in their natural state, and this polarization can be reversed by applying an external electric field. A ferroelectric domain refers to each continuous region in the material that is uniformly polarized (where all dipole moments are aligned parallel to each other in a given direction). Therefore, a ferroelectric material can be described as a "single domain" if it consists of a single region with uniform polarization, or as a "multi-domain" if it comprises multiple regions with potentially different polarities.

[0003] The present invention relates more specifically to applying Smart Cut TM The fabrication of thin ferroelectric layers using this technique involves removing a thin layer from a solid substrate of a ferroelectric material by fracturing it near a brittle region (or embrittled plane) formed by implanting a "light" substance such as helium or hydrogen into the solid substrate. Specific examples of implementing this method can be found in document EP 3646374 B1.

[0004] According to this method, after the layer removal step, it is usually necessary to apply a treatment to improve its surface finish, crystal quality, or change its thickness. However, the applicant observed that these fabrication steps, when applied to a thin ferroelectric layer attached to a silicon substrate, may result in the formation of multiple ferroelectric domains within the thin layer, thereby giving it multi-domain characteristics.

[0005] This characteristic makes the layer unsuitable for use because, for example, it can affect the performance capabilities of devices (such as surface acoustic wave (SAW) devices) that are to be formed on / in a thin layer.

[0006] Document WO 2020 / 200986 discloses the formation of ferroelectric domains in the surface portion of a thin layer due to a hydrogen concentration gradient within the layer during heat treatment. This hydrogen can specifically correspond to a light material implanted into a solid substrate to form an embrittlement region (from which the thin layer can be removed). The thickness of the surface portion can be approximately 150 nm to 200 nm or greater. To permanently restore the single-domain properties of the thin layer, this document suggests thinning the thin ferroelectric layer after applying this heat treatment.

[0007] This thinning can be achieved in particular by chemical mechanical polishing of the layer, but the removal of a relatively large thickness of material by polishing tends to reduce the thickness uniformity of the layer. By way of example, the removal of a thickness of about 400 nm results in a layer being formed having a thickness uniformity of about 100 nm (i.e. the difference between the maximum thickness and the minimum thickness when such thickness measurements are made at a plurality of measurement points over the entire extent of the layer by, for example, reflectometry or ellipsometry). This thickness variation is unacceptable because it makes it impossible to manufacture, from such a layer, devices having all the required properties collectively.

[0008] There are alternatives to the use of chemical mechanical polishing for thinning. In particular, it is possible to consider thinning the thin ferroelectric film by ion etching, for example by reactive ion etching (RIE). RIE is a dry etching which uses a plasma of chemically reactive ions to remove surface material from a wafer. The plasma is generated by electromagnetic fields at low pressure. The high-energy ions of the plasma attack the surface of the layer and react with it to pulverize it, thereby gradually thinning the layer. This method is described in particular in document FR 2111960.

[0009] However, whether it is carried out by chemical mechanical polishing or by ion etching, the removal of a relatively large thickness of material to remove the polydomain surface portion of the layer taken away is a drawback of the methods of the prior art, because this step tends to make the manufacturing method longer and more complex to implement.

[0010] This elimination step also requires the taking away of a layer having a relatively large thickness from the solid substrate of ferroelectric material, which requires the injection of a large amount of light material with energy. Beyond a certain threshold thickness, the energy required to define the layer taken away exceeds the capacity of existing implantation equipment. Thus, the methods of the prior art for preparing thin monodomain layers of ferroelectric material have limitations, overcoming these limitations is beneficial. SUMMARY

[0011] SUBJECT OF THE INVENTION

[0012] The object of the present invention is to propose a method for preparing a thin layer of ferroelectric material which at least partially solves some of the limitations described above. More particularly, the object of the present invention is to propose a method for preparing a thin layer of ferroelectric material which is easier to implement than the methods of the prior art. Another object of the present invention is to propose a method for thinning a thin layer obtained by separating a donor substrate at a plane of embrittlement, in which method the thickness of material removed to remove the polydomain surface portion of the layer taken away is lower than in the methods of the prior art.

[0013] BRIEF DESCRIPTION OF THE INVENTION

[0014] To achieve one of these aims, the subject of the present invention proposes a method for preparing a thin monodomain layer of ferroelectric material, said method comprising:

[0015] - a step of implanting a "light" substance in a first face of a ferroelectric donor substrate to form a plane of embrittlement and to define a first layer between the plane of embrittlement and the first face of the donor substrate;

[0016] - a step of assembling the first face of the donor substrate on a support to form an intermediate assembly;

[0017] - a step of breaking the intermediate assembly comprising a first heat treatment, which step causes the donor substrate to break at the plane of embrittlement and forms a free face of the first layer;

[0018] - a program for finishing the first layer, which comprises an annealing step and, after the annealing step, a step of thinning the first layer to form a thin monodomain layer, said annealing step comprising a second heat treatment.

[0019] According to the invention, the preparation method comprises, between the breaking step and the finishing program, a treatment applied to the free face to generate a hydrogen concentration greater than 2.0E21 at / cm^3 in the surface thickness of the first layer.

[0020] According to other advantageous and non-limiting features of the invention, the following features can be adopted, alone or according to any technically feasible combination:

[0021] - the surface thickness for which the hydrogen concentration is greater than 2.0E21 at / cm^3 is greater than or equal to 100 nm;

[0022] - the surface thickness is greater than or equal to 200 nm;

[0023] - the treatment of the free face introduces a hydrogen dose greater than or equal to 2.0E16 at / cm^2 into the first layer;

[0024] - the treatment of the free face comprises immersing the first layer in a first solution at a temperature higher than ambient temperature;

[0025] - the first solution comprises SC1 and / or SC2;

[0026] - the temperature of the first solution is greater than 50°C, preferably greater than 65°C;

[0027] - the treatment of the free face comprises a series of cleaning steps using a second solution, the series of cleaning steps being separated from each other by a waiting time of 24 hours or more;

[0028] - the second solution comprises deionized water, the cleaning steps simultaneously comprising brushing and dispensing deionized water onto the free face of the first layer;

[0029] - the second solution is at ambient temperature;

[0030] - the series of cleaning steps comprises at least three cleaning steps, preferably at least five cleaning steps;

[0031] - the treatment of the free surface comprises depositing on the free surface a capping layer having a hydrogen concentration greater than 1.0E20 at / cm^3;

[0032] - the capping layer comprises silicon dioxide, silicon nitride or silicon oxynitride;

[0033] - the capping layer has a thickness of 20 nm or more;

[0034] - the thin monodomain layer is composed of a single-crystal piezoelectric material such as lithium tantalate or lithium niobate;

[0035] - the thin monodomain layer is composed of lithium niobate;

[0036] - the assembly step comprises forming a dielectric intermediate layer on the first face of the donor substrate and / or on the support. BRIEF DESCRIPTION OF DRAWINGS

[0037] Other features and advantages of the application will become apparent in the course of the following detailed description, provided in reference to the attached drawings, which are made by way of non-limiting example and in which:

[0038] [ Figure 1 ]

[0039] Figure 1 shows the steps for preparing the thin layer according to the first embodiment;

[0040] [ Figure 2 ]

[0041] Figure 2 shows the steps for preparing the thin layer according to the second embodiment;

[0042] [ Figure 3 ]

[0043] Figure 3 is a graph of the analysis of the layer taken;

[0044] [ Figure 4 ]

[0045] Figure 4 shows the preparation method according to the application. DETAILED DESCRIPTION

[0046] The application relates to a method for preparing a thin monodomain layer 3 of ferroelectric material, which uses a transfer technique comprising implanting light species in a donor substrate 1 to transfer the thin monodomain layer 3 from the single-crystal donor substrate 1 to a support substrate 7. This step of providing the thin layer has several embodiments.

[0047] According to the diagram shown in Figure 1of 1A to Figure 1 of the first embodiment of 1F, the donor substrate 1 consists of a solid monocrystalline single domain block of a ferroelectric material, for example LiTaO3, LiNbO3, LiAlO3, BaTiO3, PbZrTiO3, KNbO3, BaZrO3, CaTiO3, PbTiO3 or KTaO3. The donor substrate 1 can present the form of a standard size circular wafer, for example with a diameter of 150 mm or 200 mm. However, the application is in no way limited to these sizes or this shape. The donor substrate can be extracted from a ingot of ferroelectric material in such a way as to form the donor substrate 1 with a predetermined crystal orientation. The orientation is chosen according to the intended application. Thus, when it is intended to take advantage of the properties of a thin layer of LiTaO3 to form a SAW filter, an orientation ranging between 30° and 60° RY or between 40° and 50° RY is generally chosen. However, the application is in no way limited to a particular crystal orientation.

[0048] Whatever the crystal orientation of the donor substrate 1, the method comprises introducing at least one "light" substance into the donor substrate 1, in particular a substance chosen from inert gases or hydrogen. This introduction can correspond to an implantation, i.e. an ion bombardment of the flat face 4 of the donor substrate 1 by light substances such as hydrogen and / or helium ions.

[0049] In a manner known per se, as illustrated in 1B of Figure 1 of the first embodiment of 1F, the donor substrate 1 consists of a solid monocrystalline single domain block of a ferroelectric material, for example LiTaO3, LiNbO3, LiAlO3, BaTiO3, PbZrTiO3, KNbO3, BaZrO3, CaTiO3, PbTiO3 or KTaO3. The donor substrate 1 can present the form of a standard size circular wafer, for example with a diameter of 150 mm or 200 mm. However, the application is in no way limited to these sizes or this shape. The donor substrate can be extracted from a ingot of ferroelectric material in such a way as to form the donor substrate 1 with a predetermined crystal orientation. The orientation is chosen according to the intended application. Thus, when it is intended to take advantage of the properties of a thin layer of LiTaO3 to form a SAW filter, an orientation ranging between 30° and 60° RY or between 40° and 50° RY is generally chosen. However, the application is in no way limited to a particular crystal orientation.

[0050] The type and dose of the implanted substance and the implantation energy are chosen according to the thickness of the layer to be transferred and the physicochemical properties of the donor substrate. In the case of a donor substrate 1 made of LiTaO3, it is thus possible to choose to implant a hydrogen dose ranging between 1 E 16 and 5 E 17 at / cm 2 between 30 and 300 keV, in order to delimit a first layer 3 of approximately 200 to 2000 nm.

[0051] In a subsequent step illustrated in 1C of Figure 1 of the first embodiment of 1F, the flat face 4 of the donor substrate 1 is assembled with the surface 6 of the support substrate 7. The support substrate 7 can present the same size and shape as the donor substrate 1. For reasons of availability and cost, the support substrate 7 is a monocrystalline or polycrystalline silicon wafer. However, more generally, the support substrate 7 can consist of any material such as silicon, sapphire or glass and can present any shape.

[0052] In a particular embodiment, the support substrate comprises a base substrate 7b made of, for example, monocrystalline silicon, on which a charge trapping layer 7a is arranged. The base substrate 7b can have a high resistivity, greater than 1000 ohms.cm or more generally less than 1000 ohms.cm. As is known per se, the charge trapping layer 7a can be formed of a polycrystalline silicon layer and can generally have a thickness ranging between 500 nanometers and 10 micrometers.

[0053] Prior to the assembly step, the surface of the substrates to be assembled can be prepared, for example, by a step of cleaning, brushing, drying, polishing or plasma activation.

[0054] The assembly step can involve bringing the donor substrate 1 into close contact with the support substrate 7 by molecular adhesion and / or electrostatic bonding. Optionally, 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 can be formed on the planar face 4 of the donor substrate 1 or on the planar face 6 of the support substrate 7 to be assembled or on both. This intermediate layer consists, for example, of silicon oxide, silicon nitride or silicon oxynitride. Its thickness can range between a few nanometers and a few micrometers.

[0055] An intermediate layer with a low concentration of hydrogen or forming a hydrogen diffusion barrier is preferred, to follow the teachings of document WO 2020 / 200986, so as to avoid the formation of a multidomain zone at the interface between the first layer 3 and the amorphous intermediate layer on one side of the second face of the first layer 3. The intermediate layer can be produced using various techniques known in the art, for example thermal oxidation or nitridation treatment, chemical deposition (PECVD, LPCVD, etc.), etc.

[0056] On completion of this assembly step, an assembly comprising the two bonded substrates is obtained, in which the planar face 6 of the support substrate 7 is adhered to the planar face 4 of the donor substrate 1.

[0057] The assembly is then subjected to a treatment to separate the first layer 3 of ferroelectric material from the donor substrate 1, for example by splitting at the fragilization plane 2.

[0058] This separation step can thus involve applying a heat treatment to the assembly in the temperature range of about 80°C to 300°C, to make the first layer 3 transferable to the support substrate 7. As an alternative or in addition to the heat treatment, this step can comprise applying a blade or a jet of gaseous or liquid fluid to the fragilization plane 2.

[0059] After this separation step, the structure 9 shown in Figure 1D is obtained. This structure 9 comprises a first layer 3 of monocrystalline ferroelectric material comprising a first free face 8 and a second face 4 arranged on a support substrate 7. Figure 1

[0060] ​Figure 2 of 2A to Figure 2 2F of the same structure 9. This second method is particularly suitable for producing a heterostructure 9 in which the thermal expansion coefficient (in the main plane defining the layer) of the first layer 3 is very different from the thermal expansion coefficient of the support 7, for example with a difference of more than 10% (at ambient temperature).

[0061] The main difference between this second embodiment and the first embodiment lies in the nature of the donor substrate 1. Thus, for the sake of brevity, only the components of this second embodiment that are different from the first embodiment are described herein, it being possible to provide all the other features of the first embodiment.

[0062] With reference to Figure 2 2A, in this case the donor substrate 1 is composed of a thick layer of ferroelectric material la having the same properties as those described for the ferroelectric material solid block involved in the first embodiment, and a handling substrate lb.

[0063] The handling substrate lb is advantageously composed of a material (or materials) that confers on it a thermal expansion coefficient close to that of the support substrate 7. The term "close" means that the difference between the thermal expansion coefficient of the handling substrate lb and that of the support is less than the difference between the thermal expansion of the ferroelectric material solid block and that of the support substrate 7, in absolute value.

[0064] Preferably, the handling substrate lb and the support substrate have the same thermal expansion coefficient. During assembly of the donor substrate 1 and the support 7, an assembly is formed that is able to withstand a thermal treatment at relatively high temperatures. To facilitate implementation, this can be obtained by choosing the handling substrate lb to be composed of the same material as the support substrate 7.

[0065] To form the donor substrate 1 in this embodiment, the ferroelectric material solid block is pre-assembled with the handling substrate la, for example by means of a molecular adhesion bonding technique as described above or using an adhesive layer. Next, the ferroelectric material layer la is formed by thinning (for example by grinding and / or chemical-mechanical polishing and / or etching). Before assembly, it is possible to consider forming an adhesive layer on one of the surfaces in contact and / or the other (for example by silicon oxide and / or silicon nitride deposition of an adhesive layer (for example a polymer)). The assembly can include the application of a low-temperature heat treatment (for example ranging between 50°C and 300°C, generally 100°C) that can sufficiently increase the bonding energy to make it possible to carry out the following thinning step.

[0066] The manipulating substrate 1b is selected to have a thickness substantially equal to that of the supporting substrate 7. The thinning step is performed such that the thickness of the thick layer 1a is low enough to reduce the intensity of the stresses generated during the heat treatment applied throughout the rest of the process. At the same time, this thickness is high enough to allow the removal of the first layer 3 or more such layers. For example, this thickness can be in the range of 5 to 400 micrometers.

[0067] The steps of this second embodiment are equivalent to those described in the first embodiment. A light material is injected into the thick layer 1a to create an embrittled plane 2 that defines the separation boundary between the thin layer 3 and the remainder 5 of the donor substrate 1, as shown below. Figure 2 As shown in 2B. Following this step is the assembly of the donor substrate 1 onto the support substrate 7, as follows... Figure 2 As shown in 2C. Next, the first layer 3 is separated from the remainder of the substrate 5 to obtain Figure 2 The 2D structure shown is 9.

[0068] The advantage of this embodiment is that the assembly formed by the donor substrate 1 and the support 7 can be exposed to temperatures much higher than those applied in the first embodiment without the risk of one of the substrates breaking uncontrollably or the donor substrate 1 delaminating from the thin layer 3. Therefore, the balanced structure facilitates the separation of the first layer 3 by exposing the assembly to relatively high temperatures (e.g., in the range of 100°C to 500°C) with respect to the coefficient of thermal expansion of such an assembly.

[0069] Regardless of the chosen implementation, as described in the introduction of this application, a subsequent finishing step is required for the first layer 3 to improve its crystal and surface quality and to provide a thin layer 3' with a thickness matching or close to the target thickness. Figure 1 1E and Figure 2 These finishing steps, schematically shown in 2E, are specifically designed to eliminate the hard, rough surface layer resulting from the splitting and separation of the thin layer 3 from the remainder of the donor substrate.

[0070] As disclosed in document WO 2020 / 200986, a heat treatment step is first applied to the first transfer layer 3. This heat treatment can correct any crystal defects present in the layer 3 and even reduce the roughness of its free surface 8. Furthermore, this heat treatment helps to strengthen its adhesion to the support 7. The heat treatment brings the structure to a temperature ranging from 300°C to the Curie temperature of ferroelectric materials, for a duration ranging from 30 minutes to 10 hours. This heat treatment is preferably performed by exposing the free surface of the first layer 3 to an oxidizing or neutral gas atmosphere (i.e., without a protective layer covering the surface of the thin layer).

[0071] For the avoidance of doubt, it should be noted that the finishing heat treatment step is very different from the breaking heat treatment applied to the assembled structure. It is in particular carried out in a different device from the one used to apply the breaking heat treatment.

[0072] The method according to the application also comprises, after the finishing heat treatment, a step of thinning the first thin layer. This step is in particular intended to eliminate the polydomain surface portion of the first layer 3, which is created in the previous heat treatment step. It is also intended to provide a thin monodomain layer 3’ whose thickness corresponds to the target thickness, as mentioned previously. This thinning can in particular correspond to a polishing of the first free face 8 of the thin layer 3 by, for example, mechanical or chemical-mechanical thinning techniques. It can also involve a thinning by ion etching, for example by reactive ion etching.

[0073] In all cases, this thinning gives rise to an elimination of the thickness of at least the polydomain portion of the first layer 3, which is generally of the order of at least 150 nm, which can lead to the drawbacks described in the introduction to the present application. Figure 1 of 1 F and Figure 2 of 2F shows the structure 9 obtained on completion of these treatments, in which a thin monodomain layer 3’ is arranged on the support 7.

[0074] In order to characterise the thickness of the polydomain surface portion, a sample of this layer is generally detected by transmission electron microscopy. This detection technique makes it possible to visualise and distinguish the polydomain surface portion and the underlying monodomain portion in a cross-section of the sample. However, the range of this view is still limited to a detection area of a few hundred microns, which does not fully represent the quality of the first layer 3 over its entire extent. It is also a technique which is long and complex to deploy.

[0075] In order to overcome this problem of analysis, the applicant has developed a faster characterisation technique which makes it possible to assess the thickness of the polydomain portion more globally.

[0076] This technique comprises a first step which involves taking a sample of the first layer 3 by chemical-mechanical polishing of a given thickness, for example 125 nm. After this first step, the topography of the surface thus revealed is measured: a high topography indicates that the material exposed by the surface is of polydomain nature, and conversely, a low topography indicates that the material revealed by the surface is of monodomain nature. The topography measurement can be carried out by atomic force measurement (AFM) over a measurement area of 5 microns by 5 microns. In the context of the present description, “high topography” is understood to mean a surface whose peak-to-valley roughness is 10 nm or more, and conversely, “low topography” is understood to mean a surface whose peak-to-valley roughness is less than 10 nm.

[0077] The speed of the chemical or physico-chemical etching of the thin layer 3 that occurs during the polishing step can vary as a function of the polar nature of the piezoelectric material removed: the etching of the Z- side of the layer is much faster than the Z+ side. Polishing of a mono-domain layer thus gives a much lower surface topography than polishing of a poly-domain layer.

[0078] Moreover, according to the proposed characterisation technique, by evaluating the topography of the surface of the thin layer exposed after thinning to a given thickness, it is very simple to determine whether the poly-domain layer extends to a depth greater than the given thickness (revealed by a high topography) or to a thickness less than the given thickness (revealed by a low topography). However, this slightly rough characterisation of the poly-domain thickness makes it possible to provide an overall evaluation of this thickness by repeating the topography measurement on a plurality of positions sampled over the entire extent of the layer.

[0079] Using this overall characterisation technique that can be implemented quickly, by choosing a specific thickness of 125 nm, the Applicant has realised that the method for preparing a thin layer 3 of ferroelectric material, as described above (according to two different embodiments), leads to the formation of a poly-domain surface layer of at least 125 nm after the step of heat treatment of the transferred thin layer 3 and before the thinning step.

[0080] In seeking a solution to reduce the thickness of this poly-domain surface portion, the Applicant has very surprisingly discovered that the introduction of a sufficient dose of hydrogen into the surface thickness of the first layer 3 before the step of heat treatment applied to this layer 3 tends to reduce the thickness of the poly-domain surface portion that appears during this heat treatment.

[0081] This discovery stems from experiments that were carried out involving the application of an inter-layer treatment to the free face 8 of the first thin layer immediately after the step of breaking the first layer of lithium tantalate. After this inter-layer treatment step, the heat treatment that leads to the formation of a poly-domain surface portion is applied, and the poly-domain surface portion is characterised using the technique described above by reducing the thickness of the first 125 nm layer by polishing and then measuring its topography.

[0082] These experiments include the following list of inter-layer treatments of the first layer 3:

[0083] • Cleaning 1 : the free face 8 of the first thin layer is cleaned by brushing and dispensing deionised water at ambient temperature. The characterisation of the layer after the heat treatment reveals a thickness of the poly-domain surface portion that exceeds 125 nm.

[0084] • Cleaning 2: the free face of the thin layer 3 is cleaned by successive immersion in baths of deionised water, an SC1 bath and an SC2 bath, all at ambient temperature. The characterisation of the layer after the heat treatment reveals a thickness of the poly-domain surface portion that exceeds 125 nm.

[0085] • Cleaning 3: cleaning the free face by successive immersion in a bath of deionized water, an SC1 bath and an SC2 bath, this time the SC1 solution being heated to 70°C. Characterization of the layer after the heat treatment revealed that the thickness of the polydomain surface portion was less than 125 nm.

[0086] To understand the reasons for this phenomenon, the Applicant carried out SIMS (Secondary Ion Mass Spectroscopy) measurements on the first layer 3 to determine the hydrogen concentration profile as a function of the depth of the first layer. Thus, Figure 3 The hydrogen concentration profile of the first layer 3 directly after the break (no application of a cleaning agent - NET0), the hydrogen concentration profile of the first layer 3 after application of cleaning agent 1 (NET1) and the hydrogen concentration profile of the first layer 3 after application of cleaning 3 (NET3) are shown. A hydrogen-rich surface region of about 200 nm in thickness can be seen. The application of cleaning 3, which limits the thickness of the polydomain surface portion, gives rise to a hydrogen concentration of about 3.0E21 at / cm^3 in the surface region, whereas in the other cases the concentration does not exceed 1.0E21 at / cm^3.

[0087] When these hydrogen concentration data are processed, it can be seen that cleaning 1 introduces a hydrogen dose of 1.4E16 at / cm^2 into the first layer 3, still without reducing the thickness of the surface polydomain portion to less than 125 nm. Cleaning 3, for its part, introduces a hydrogen dose of 4.3E16 at / cm^2 and reduces the thickness of the polydomain portion to less than 125 nm.

[0088] These preliminary results, which tend to link the thickness of the surface polydomain portion to the hydrogen concentration in the first layer 3, are confirmed by other experimental measurements.

[0089] According to one of these measurements, the treatment applied to the free face of the thin layer 3 involves the use of a deionized water solution to apply a series of 5 cleaning steps, the cleaning steps being separated from each other by a waiting time of 24 hours or more. The solution is at ambient temperature. From the third cleaning step in the series, this treatment makes the polydomain thickness less than 125 nm.

[0090] The role of these cleaning steps is to eliminate a thin surface layer of Li2CO3 on the thin layer 3 obtained directly after the breaking step. Its formation appears to be favored by the specific conditions under which the breaking step takes place. The presence of light species (i.e. hydrogen and / or helium) and the moderate temperature at which the breaking takes place appear to make the lithium in the thin layer 3 particularly mobile and the surface of this layer 3 particularly reactive. This Li2CO3 surface layer is of nanometric thickness or more. It is stable over time, i.e. its consistency or thickness does not change while the thin layer 3 remains exposed to the atmosphere. However, this Li2CO3 surface layer is relatively fragile and the Applicant has observed that it can be eliminated by simple wet cleaning. The Applicant has also observed that the cleaned thin layer 3, without the Li2CO3 surface thickness, remains particularly reactive. By leaving the free face of the thin layer 3 exposed to the atmosphere for a long time, amorphous dendrites rich in lithium and hydrogen (as well as other species present in the atmosphere, such as chlorine or fluorine) nucleate and form again. The successive cleaning steps, spaced apart from each other by sufficient time, tend to strip a certain surface thickness of lithium in the thin layer 3 (which migrates and accumulates on the surface), while tending to introduce the hydrogen element as a replacement. When a sufficient dose of hydrogen is introduced into the thin layer 3, the heat treatment applied to this layer makes the thickness of the surface domains formed less than 125 nm, which confirms the hypothesis that increasing the concentration of hydrogen in the surface thickness of the thin layer 3 tends to reduce the thickness of the surface domain portion resulting from the heat treatment of the finishing program on this layer.

[0091] As a final experimental measurement, a 30 nm thick hydrogen-rich silicon oxide was formed on the thin layer 3 by PECVD deposition. The finishing program was then annealed so as to inject a significant dose of hydrogen into the thin layer 3 by diffusion. The characterization of this thin layer 3 after removal of the silicon oxide layer revealed that the thickness of the domain layer was less than 125 nm.

[0092] The Applicant concludes from these experiments that the treatment of the free face of the first layer 3, which aims to form a high concentration of hydrogen in the surface thickness of this layer (for example greater than 2.0E21 at / cm^3 in a surface thickness of 100 nm or 200 nm), makes it possible to significantly reduce the thickness of the surface domain portion.

[0093] Advantageously, the treatment of the free face leads to the introduction of a dose of hydrogen greater than or equal to 2.0E16 at / cm^2 into the first layer 3.

[0094] Under these conditions, the thickness of the surface domain portion that appears as a result of the heat treatment of the finishing program is less than 125 nm.

[0095] The invention therefore benefits from these results and proposes a method for preparing a thin monodomain layer 3'. This method is illustrated schematically in Figure 4 . It uses the same steps as in Figure 1 1A toFigure 1 1E and Figure 2 2A to Figure 2 The description of 2E relates to all disclosed steps. Specifically, this method includes:

[0096] - The step of implanting a "light" material into the first surface 4 of the ferroelectric donor substrate 1 to form an embrittlement plane 2 and defining a first layer 3 between the embrittlement plane 2 and the first surface 4 of the donor substrate 1;

[0097] - The step of assembling the first surface 4 of the donor substrate 1 onto the support 7 to form an intermediate assembly;

[0098] - Including the step of a first heat treatment to break the intermediate component, which causes the donor substrate 1 to break at the embrittled plane 2 and form the free surface 8 of the first layer 3;

[0099] - A process for finishing the first layer 3, which includes an annealing step and a step of thinning the first layer 3 after the annealing step to form a thin single-domain layer 3', the annealing step including a second heat treatment.

[0100] According to the present invention, the preparation method includes treating the free surface 8 of the first layer between the fracture step and the finishing process to produce a hydrogen concentration greater than 2.0E21at / cm^3 in the surface thickness of the first layer 3.

[0101] The thickness of this hydrogen-rich surface of the first layer can be at least 100 nm from the free surface 8 of the first layer. Advantageously, it is at least 200 nm thick.

[0102] The hydrogen concentration can be obtained by selecting the treatment of the free surface such that the treatment introduces a hydrogen dose greater than or equal to 2.0E16 at / cm^2 into the first layer 3.

[0103] According to the first method, treating the free surface 8 involves immersing the first layer 3 in a first solution at a temperature higher than ambient temperature (e.g., above 50°C, preferably above or equal to 65°C). This first solution may be or may contain SC1. The treatment of the first layer may particularly correspond to RCA-type cleaning, which, as is known per se, involves a process of sequentially immersing a structure comprising the first layer 3 in a deionized water bath, an SC1 bath, and an SC2 bath. When treating the free surface of the invention by such RCA-type cleaning, the temperature of the SC1 bath into which the structure is immersed is higher than ambient temperature.

[0104] According to another method, the treatment of the free surface 8 comprises a series of cleaning steps using a second solution, these cleaning steps being separated from each other by a waiting time of 24 hours or more. The cleaning steps can correspond to brushing the free surface 8 and simultaneously dispensing deionized water onto the free surface 8 of the first layer 3, the deionized water then forming the second solution used in the cleaning steps. This second solution can be at ambient temperature or can reach a temperature higher than ambient temperature.

[0105] Advantageously, the series of cleaning steps comprises at least three cleaning steps, preferably at least five cleaning steps.

[0106] According to yet another method, the treatment of the free surface 8 comprises depositing a hydrogen-rich overlayer on the free surface 8, for example having a hydrogen concentration greater than 1.0E20 at / cm^3. The overlayer can in particular comprise silicon dioxide, silicon nitride or silicon oxynitride, formed for example by a chemical vapor deposition technique at sub-atmospheric pressure. Whatever the nature of the overlayer, it can be 20 nm or more thick so as to contain a sufficient amount of hydrogen.

[0107] By introducing hydrogen into the first layer 3 before applying the finishing procedure to this layer, it is possible to reduce the thickness of the polydomain surface portion resulting from this procedure. In particular, it is possible to limit this thickness to less than 125 nm.

[0108] Of course, the application is not limited to the described embodiments and alternative embodiments can be used without departing from the scope of the application as defined by the claims.

[0109] In particular, in combination with the three illustrated methods and according to any possible combination, it is possible to apply a treatment of the free surface 8 between the breaking step and the finishing procedure.

[0110] The treatment of the free surface of the layer 3 taken can also correspond to any other treatment of this surface that makes it possible to introduce a sufficient dose of hydrogen into this layer.

Claims

1. A method for preparing a thin single-domain layer (3') of a ferroelectric material, the method comprising: -The steps of implanting a "light" material into the first surface (4) of the ferroelectric donor substrate (1) to form an embrittlement plane (2) and defining a first layer (3) between the embrittlement plane (2) and the first surface (4) of the donor substrate (1); - The step of assembling the first surface (4) of the donor substrate (1) onto the support (7) to form an intermediate assembly; - Including the step of the first heat treatment to break the intermediate component, which causes the donor substrate (1) to break at the embrittled plane (2) and form the free surface (8) of the first layer (3); - A process for finishing the first layer (3) includes an annealing step and a step of thinning the first layer (3) after the annealing step to form a thin single-domain layer (3'), the annealing step including a second heat treatment; The preparation method is characterized in that the method includes applying a treatment to the free surface (8) between the fracture step and the finishing process to produce a hydrogen concentration greater than 2.0E21 at / cm^3 in the surface thickness of the first layer (3).

2. The preparation method according to the preceding claim, wherein, Surfaces with a hydrogen concentration greater than 2.0E21at / cm^3 and a thickness greater than or equal to 100nm.

3. The preparation method according to the preceding claim, wherein, The surface thickness is greater than or equal to 200 nm.

4. The preparation method according to any one of the preceding claims, wherein, The treatment of the free surface (8) introduces a hydrogen dose greater than or equal to 2.0E16 at / cm^2 into the first layer (3).

5. The preparation method according to any one of the preceding claims, wherein, The treatment of the free surface (8) involves immersing the first layer (3) in a first solution at a temperature higher than the ambient temperature.

6. The preparation method according to the preceding claim, wherein, The first solution contains SC1 and / or SC2.

7. The preparation method according to any one of the preceding claims, wherein, The temperature of the first solution is greater than 50°C, preferably greater than 65°C.

8. The preparation method according to any one of the preceding claims, wherein, The treatment of the free surface (8) includes a series of cleaning steps using a second solution, with each cleaning step spaced 24 hours or longer apart.

9. The preparation method according to the preceding claim, wherein, The second solution contains deionized water, and at least three cleaning steps include simultaneously brushing and distributing the deionized water onto the free surface (8) of the first layer (3).

10. The preparation method according to any one of the preceding claims, wherein, The second solution is at ambient temperature.

11. The preparation method according to any one of the three preceding claims, wherein, The series of cleaning steps includes at least five cleaning steps.

12. The preparation method according to any one of the preceding claims, wherein, The treatment of the free surface (8) includes depositing a capping layer with a hydrogen concentration greater than 1.0E20 at / cm^3 on the free surface (8).

13. The preparation method according to the preceding claim, wherein, The covering layer comprises silicon dioxide, silicon nitride, or silicon oxynitride.

14. The preparation method according to the preceding claim, wherein, The covering layer is 20 nm or thicker.

15. The preparation method according to any one of the preceding claims, wherein, The thin single-domain layer (3) is composed of a single-crystal piezoelectric material such as lithium tantalate or lithium niobate.

16. The preparation method according to the preceding claim, wherein, The thin single-domain layer (3) is composed of lithium niobate.

17. The preparation method according to any one of the preceding claims, wherein, The assembly steps include forming a dielectric interlayer (3) on the first side of the donor substrate (1) and / or the support (7).

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