Method for preparing a thin layer of ferroelectric material

Abstract

The invention relates to a method for preparing a thin single-domain layer (3') made of ferroelectric material, the method comprising, between a step of splitting a donor substrate (1) at a weakened plane (2) in order to form a first layer (3) and a sequence for finishing the first layer (3), the application of a treatment to the free face (8) of the first layer (3) in order to produce a hydrogen concentration of greater than 2.0E21 at / cm^3 in a surface thickness of the first layer (3).

Description

PROCESS FOR PREPARING A THIN LAYER OF FERROELECTRIC MATERIAL FIELD OF THE INVENTION

[0001] The present invention relates to a method for preparing a thin layer of ferroelectric material. More particularly, it relates to a preparation method for preserving the single-domain character of the ferroelectric material in the thin layer of the final product. This preparation method is used, for example, in the fields of microelectronics, micromechanics, photonics, etc. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] As a preamble, we recall that a ferroelectric material is a material that has an electric polarization in its natural state, a polarization that can be reversed by the application of an external electric field. A ferroelectric domain is each region of a single piece of the material in which the polarization is uniform (all the dipole moments are aligned parallel to each other in a given direction). A ferroelectric material can therefore be characterized as "monodomain" in the case where this material consists of a single region in which the polarization is uniform or "multidomain" in the case where the ferroelectric material comprises a plurality of regions with polarities that may be different.

[0003] The present invention relates more particularly to the preparation of a ferroelectric thin layer obtained by the application of Smart Cut™ technology according to which a thin layer is removed from a solid substrate made of ferroelectric material by fracture at a fragile zone (or weakening plane) formed in the solid substrate by implantation of so-called "light" species, such as helium or hydrogen. A particular example of implementation of this method can be found in document EP3646374B1.

[0004] According to this method, and after the step of removing the layer, it is often necessary to apply treatments to it aimed at improving its surface condition, its crystalline quality or modifying its thickness. The applicant has however observed that these preparation steps, when applied to a thin ferroelectric layer transferred onto a silicon support, could lead to the formation of a plurality of ferroelectric domains within the thin layer, thus giving it a multidomain character.

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

[0006] Document WO2020200986 reveals that the formation of ferroelectric domains in a surface portion of the layer is caused by the existence of hydrogen concentration gradients in the thin layer during the application of a heat treatment. This hydrogen may in particular correspond to the light species implanted in the bulk substrate in order to form the embrittlement zone allowing the thin layer to be removed. The surface portion may have a thickness of the order of 150 nm to 200 nm or more. To permanently restore the single-domain character of the thin layer, this document recommends thinning the ferroelectric thin layer after the application of such a 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 deteriorate the thickness uniformity of this layer. For example, the removal of a thickness of the order of 400 nm results in the formation of a layer whose thickness uniformity (i.e. the difference between the greatest thickness and the thinnest thickness when this thickness measurement is carried out, for example by reflectometry or ellipsometry, at multiple measurement points over the entire extent of the layer) is of the order of 100 nm. This thickness variability is not acceptable, because it does not allow the collective manufacture, from such a layer, of devices having all the required characteristics.

[0008] Alternatives exist to thinning by chemical-mechanical polishing. One option is to thin the ferroelectric thin film by ion etching, for example by reactive ion etching (RIE). RIE is a type of dry etching that uses a plasma of chemically reactive ions to remove the surface material from a wafer. The plasma is generated under low pressure by an electromagnetic field. The high-energy ions in the plasma attack the surface of the film and react with it to pulverize it, thus gradually thinning it. Such an approach is described in the document filed under number FR2111960.

[0009] However, whether implemented by chemical-mechanical polishing or by ion etching, the removal of a relatively large thickness of material to remove the multi-domain surface portion of the sampled layer is a disadvantage of the prior art methods, because this step tends to make the manufacturing process longer and more complex to implement.

[0010] This elimination step also requires removing a layer of relatively large thickness from the bulk substrate of ferroelectric material, which requires implanting the light species with significant energy. Beyond a certain threshold thickness, the energy required to define the removed layer is not accessible to existing implantation equipment. A state-of-the-art method for preparing a thin single-domain layer of ferroelectric material therefore has limitations that it would be desirable to overcome. SUBJECT OF THE INVENTION

[0011] An aim of the invention is to propose a method for preparing a thin layer of ferroelectric material that addresses at least in part the aforementioned limitations. More particularly, an aim of the invention is to propose a method for preparing a thin layer of ferroelectric material that is simpler to implement than the methods of the prior art. Another aim of the invention is to propose a method for thinning a thin layer obtained by detachment at a weakening plane of a donor substrate, in which the thickness of materials removed to remove the multi-domain surface portion of the removed layer is smaller than the thickness removed in the methods of the prior art. BRIEF DESCRIPTION OF THE INVENTION

[0012] With a view to achieving one of these aims, the subject of the invention proposes a method for preparing a single-domain thin layer of ferroelectric material, the method comprising: a step of implanting 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; a step of assembling the first face of the donor substrate to a support to form an intermediate assembly; a step of fracturing the intermediate assembly, comprising a first heat treatment, this step leading to the fracture of the donor substrate at the weakening plane and to the formation of a free face of the first layer;a finishing sequence of the first layer comprising an annealing step comprising a second heat treatment and, after the annealing step, a step of thinning the first layer to form the single-domain thin layer.;

[0013] According to the invention, the preparation method comprises, between the fracture step and the finishing sequence, the application of a treatment of the free face to produce a hydrogen concentration greater than 2.0 E21 at / cm^3 in a surface thickness of the first layer.

[0014] According to other advantageous and non-limiting characteristics of the invention, taken alone or in any technically feasible combination: the surface thickness, in which the hydrogen concentration is greater than 2.0 E21 at / cm^3, is greater than or equal to 100 nm; the surface thickness is greater than or equal to 200 nm; the treatment of the free face introduces a dose of hydrogen greater than or equal to 2.0 E16 at / cm^2 in the first layer;the treatment of the free face comprises immersing the first layer in a first solution having a temperature higher than room temperature;the first solution comprises SC1 and / or SC2;the temperature of the first solution is higher than 50°C, preferably higher than 65°C;the treatment of the free face comprises a sequence of cleaning steps using a second solution, the cleaning steps of the sequence being separated from each other by a waiting time greater than or equal to 24 hours;the second solution comprises deionized water, the cleaning steps comprising simultaneously brushing and dispensing deionized water onto the free face of the first layer;the second solution is at room temperature.the sequence of cleaning steps comprises at least three cleaning steps, preferably at least five cleaning steps;the treatment of the free face comprises the deposition on the free face of a covering layer having a hydrogen concentration greater than 1.0 E20 at / cm^3;the covering layer comprises silicon dioxide, silicon nitride or silicon oxynitride;the covering layer has a thickness of 20nm or more;the single-domain thin layer is made of a single-crystalline piezoelectric material, such as lithium tantalate or lithium niobate;the single-domain thin layer is made of lithium niobate;the assembly step comprises the formation of an intercalary dielectric layer on the first face of the donor substrate and / or on the support.

[0015] Other characteristics and advantages of the invention will emerge from the detailed description of the invention which follows with reference to the appended figures in which:

[0016]

[0017] La represents a step of preparing a thin layer according to a first implementation method;

[0018]

[0019] La represents a step of preparation of a thin layer according to a second mode of implementation;

[0020]

[0021] This is a graph of an analysis of a sampled layer;

[0022]

[0023] La represents a preparation process in accordance with the invention. DETAILED DESCRIPTION OF THE INVENTION

[0024] The invention relates to a method for preparing a single-domain thin layer 3 made of a ferroelectric material transferred from a monocrystalline donor substrate 1 onto a support substrate 7 by a transfer technique including implantation of light species in a donor substrate 1. Several embodiments of this step of providing a thin layer exist.

[0025] According to a first embodiment, shown in Figures 1A to 1F, the donor substrate 1 is composed of a massive, monocrystalline, single-domain block of ferroelectric material, for example LiTaO3, LiNbO3, LiAlO3, BaTiO3, PbZrTiO3, KNbO3, BaZrO3, CaTiO3, PbTiO3 or KTaO3. The donor substrate 1 may take the form of a circular plate of standardized dimensions, for example 150 mm or 200 mm in diameter. However, the invention is in no way limited to these dimensions or this shape. The donor substrate may have been taken from an ingot of ferroelectric material, this taking having been carried out in such a way that the donor substrate 1 has a predetermined crystalline orientation. The orientation is chosen according to the intended application. Thus, it is usual to choose an orientation between 30° and 60°RY, or between 40° and 50°RY, in the case where one wishes to exploit the properties of a thin layer of LiTaO3 to form a SAW filter.But the invention is in no way limited to any particular crystal orientation.

[0026] Regardless of the crystalline orientation of the donor substrate 1, the method comprises the introduction into the donor substrate 1 of at least one so-called “light” species, in particular species chosen from inert gases or hydrogen. This introduction may correspond to an implantation, that is to say an ion bombardment of a flat face 4 of the donor substrate 1 by light species such as hydrogen and / or helium ions.

[0027] In a manner known per se, and as shown in Figure 1B, the implanted ions aim to 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.

[0028] The nature, the dose of the implanted species and the implantation energy are chosen according to the thickness of the layer that we wish to transfer and the physicochemical properties of the donor substrate. In the case of a LiTaO3 donor substrate 1, we can thus choose to implant a dose of hydrogen between 1 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.

[0029] In a next step, shown in Figure 1C, the flat face 4 of the donor substrate 1 is assembled with a face 6 of a support substrate 7. The support substrate 7 may have the same dimensions and the same shape as those of the donor substrate 1. For reasons of availability and cost, the support substrate 7 is a silicon wafer, monocrystalline or polycrystalline. But more generally, the support substrate 7 may be made of any material, for example silicon, sapphire or glass, and have any shape.

[0030] In a particular embodiment, the support substrate comprises a base substrate 7b, for example made of monocrystalline silicon, on which a charge trapping layer 7a is arranged. The base substrate 7b may have a high resistivity, greater than 1000 ohms.cm or more conventionally, less than 1000 ohms.cm. The charge trapping layer 7a, as is well known per se, may be formed from a polycrystalline silicon layer, and have a thickness typically between 500nm and 10 microns.

[0031] 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 step, or activation, for example by plasma.

[0032] The assembly step may correspond to 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, 7, in particular when they are assembled by direct bonding, at least one amorphous interlayer may be formed prior to the assembly, either on the flat face 4 of the donor substrate 1, or on the flat face 6 to be assembled of the support substrate 7, or on both. This interlayer is for example made of silicon oxide, silicon nitride, silicon oxynitride. It may have a thickness of between a few nanometers and a few microns.

[0033] An interlayer with a low hydrogen concentration or acting as a barrier to hydrogen diffusion will be preferred to follow the teachings of document WO2020200986 and thus avoid the formation of a multi-domain 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 in the state of the art, such as oxidation or nitriding heat treatments, chemical deposition (PECVD, LPCVD, etc.), etc.

[0034] 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.

[0035] 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 weakening plane 2.

[0036] This detachment step can thus comprise the application to the whole of a heat treatment in a temperature range of the order of 80°C to 300°C 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 can comprise the application of a blade or a jet of gaseous or liquid fluid at the level of the weakening plane 2.

[0037] Following this detachment step, the structure 9 shown in FIG. 1D is obtained. This structure 9 comprises the first layer 3 of monocrystalline ferroelectric material comprising a first free face 8 and a second face 4 arranged on the support substrate 7.

[0038] Figures 2A to 2F represent a second embodiment leading to providing this same structure 9. This second embodiment is particularly suitable for producing a heterogeneous structure 9, in which the first layer 3 has a coefficient of thermal expansion (in the main plane defining this layer) very different from that of the support 7, for example having a difference of more than 10% (at room temperature).

[0039] This second mode of implementation differs mainly from the first mode of implementation by the nature of the donor substrate 1. For the sake of brevity, we therefore only present here the elements of this second mode of implementation which differ from the first, all the other characteristics of the first embodiment can therefore be provided.

[0040] With reference to Figure 2A, the donor substrate 1 is in this case composed of a thick layer of ferroelectric material 1a, having the same properties as those described for the solid block of ferroelectric materials in relation to the first embodiment, and a manipulator substrate 1b.

[0041] 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 making up the support substrate 7. By “close”, we mean that the difference in the 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 ferroelectric material and that of the support substrate 7.

[0042] Preferably, the manipulator substrate 1b and the support substrate have an identical coefficient of thermal expansion. When assembling the donor substrate 1 and the support 7, an assembly is formed that is capable of withstanding heat treatment at a relatively high temperature. For reasons of simplicity of implementation, this can be achieved by choosing the manipulator substrate 1b to be made of the same material as that of the support substrate 7.

[0043] To form the donor substrate 1 of this embodiment, a solid block of ferroelectric material is first assembled with the manipulator substrate 1a, for example using a molecular adhesion bonding technique as described previously or using an adhesive layer. Then, the layer of ferroelectric material 1a is formed by thinning, for example by grinding and / or chemical-mechanical polishing and / or etching. Before assembly, provision may be made for the formation of an adhesion layer (for example by deposition of silicon oxide and / or silicon nitride, of an adhesive layer, for example a polymer) on one and / or the other of the faces brought into contact. The assembly may comprise the application of a low-temperature heat treatment (for example between 50 and 300°C, typically 100°C) making it possible to sufficiently strengthen the bonding energy to allow the following thinning step.

[0044] 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 in such a way that the thick layer 1a has a thickness that is sufficiently low so that the stresses generated during the heat treatments applied in the rest of the process are of reduced intensity. At the same time, this thickness is sufficiently large to be able to remove the first layer 3, or a plurality of such layers. This thickness may for example be between 5 and 400 microns.

[0045] The following steps of the method of this second embodiment are equivalent to those of the steps described in the first embodiment. Light species are implanted within the thick layer 1a to generate a weakening plane 2 which marks the separation of the thin layer 3 from the remainder 5 of the donor substrate 1, as shown in FIG. 2B. This step is followed by the step of assembling the donor substrate 1 on the support substrate 7, as shown in FIG. 2C. The first layer 3 is then detached from the remainder of the substrate 5 to obtain the structure 9 shown in FIG. 2D.

[0046] This embodiment is advantageous in that the assembly formed from the donor substrate 1 and the support 7 can be exposed to a temperature much higher than that applied in the context of the first embodiment, without risk of uncontrolled fracture of one of the substrates or delamination of the donor substrate 1 from the thin layer 3. The balanced structure, in terms of thermal expansion coefficient of this assembly, thus makes it possible to facilitate the step of detaching the first layer 3 by exposing the assembly to a relatively high temperature, for example between 100 and 500°C.

[0047] Whatever the chosen implementation method, and as specified in the introduction to this application, finishing steps of the first layer 3 are then necessary to improve its crystalline and surface quality, and to provide a thin layer 3' whose thickness corresponds to, or approaches, a target thickness. These finishing steps, shown schematically in Figure 1E and Figure 2E, aim in particular to eliminate a work-hardened and rough surface layer, resulting from the cleavage and detachment of the thin layer 3 from the rest of the donor substrate.

[0048] As set out in document WO2020200986, a heat treatment step is first applied to the first layer 3 taken. This heat treatment makes it possible to cure crystalline defects present in this layer 3, or even to reduce the roughness of its free face 8. In addition, it contributes to consolidating its adhesion to the support 7. The heat treatment brings the structure to a temperature between 300°C and the Curie temperature of the ferroelectric material for a period of between 30 minutes and 10 hours. This heat treatment is preferably carried out by exposing the free face of the first layer 3 to an oxidizing or neutral gaseous atmosphere, i.e. without covering this face of the thin layer with a protective layer.

[0049] To avoid any ambiguity, it is emphasized that the finishing heat treatment step is quite distinct from the fracture heat treatment applied to the assembled structure. In particular, it is carried out in equipment different from that used to apply the fracture heat treatment.

[0050] A method according to the invention also comprises, after the finishing heat treatment, a step of thinning the first thin layer. This step aims in particular to eliminate the multi-domain surface portion of the first layer 3, this portion having been created during the previous heat treatment step. It also aims to provide a single-domain thin layer 3' whose thickness corresponds to a target thickness, as mentioned previously. This thinning may, in general, correspond to the polishing of the first free face 8 of the thin layer 3, for example by mechanical, mechanochemical thinning techniques. It may also involve thinning carried out by ion etching, for example by reactive ion etching.

[0051] In all cases, this thinning leads to eliminating at least the thickness of the multi-domain portion of the first layer 3, this thickness typically being of the order of 150nm at least, which leads to the drawbacks presented in the introduction of this application. Figures 1F and 2F represent the structure 9 obtained at the end of these treatments, a thin single-domain layer 3' being arranged on the support 7.

[0052] To characterize the thickness of the multi-domain surface portion, it is usual to inspect a sample of this layer by transmission electron microscopy. This inspection technique makes it possible to visualize and distinguish, in the sample section, the multi-domain surface portion and the underlying single-domain portion. However, the extent of this view remains limited to inspection fields of a few hundred microns on each side, which is not entirely representative of the quality of the first layer 3 over its entire extent. It is also a long and complex technique to deploy.

[0053] To circumvent this analysis problem, the applicant has developed a faster characterization technique that allows for a more global evaluation of the thickness of the multi-domain portion.

[0054] This technique comprises a first step consisting of removing a determined thickness of the first layer 3 (for example 125 nm) by chemical-mechanical polishing. Following this first step, a measurement of the topography of the revealed face is carried out: a strong topography testifies to the multi-domain quality of the material whose face is exposed and, on the contrary, a weak topography testifies to the single-domain quality of the material whose face is revealed. The topography measurement can be carried out by atomic force measurement (AFM) on measurement fields of 5 micrometers by 5 micrometers. In the context of the present description, the term "strong topography" will be used to designate a surface having a roughness greater than or equal to 10 nm in peak-to-valley measurement and conversely, the term "weak topography" will be used to designate a surface having a roughness less than 10 nm in peak-to-valley measurement.

[0055] The speed of chemical or physicochemical etching of the thin layer 3 occurring during the polishing step is made variable by the nature of the polarity of the removed piezoelectric material: the Z- side of this layer is etched much faster than the Z+ side. Consequently, the polishing of a single-domain layer will have a much lower surface topography than the polishing of a multi-domain layer.

[0056] Also, according to the proposed characterization technique, by evaluating the topography of the exposed face of the thin layer after it has been thinned by a determined thickness, it is possible to determine very simply whether the multi-domain layer extended over a depth greater than the determined thickness (revealed by a significant topography) or whether it extended over a thickness less than the determined thickness (revealed by a weak topography). This somewhat crude characterization of the multi-domain thickness nevertheless allows for an overall assessment of this thickness by repeating the topography measurement at a plurality of locations sampling the entire extent of this layer.

[0057] Using this global and rapid characterization technique to implement, and by choosing a determined thickness of 125nm, the applicant was able to realize that the method for preparing a thin layer 3 in a ferroelectric material which has just been presented (according to two different implementation modes) leads to the formation, after the heat treatment step of the transferred thin layer 3 and before the thinning step, of a multi-domain surface layer of at least 125nm.

[0058] In the search for a solution aimed at reducing the thickness of this multi-domain surface portion, the applicant discovered, quite surprisingly, that the introduction of a sufficient dose of hydrogen into a surface thickness of the first layer 3, before the application of the heat treatment step of this layer 3, had a tendency to reduce the thickness of the multi-domain surface portion which is revealed during this heat treatment.

[0059] This discovery results from the experiments that were carried out and which consisted of applying, directly after the fracture step of a first layer of lithium tantalate, an intercalary treatment of the free face 8 of the first thin layer. After this intercalary step, the heat treatment leading to the development of the multi-domain surface portion was applied, which was characterized using the technique previously explained, by reducing the thickness of the first layer by 125nm by polishing and then measuring its topography.

[0060] These experiments included the following list of intercalary treatments of the first layer 3: Cleaning 1: cleaning the free face 8 of the first thin layer by brushing and dispensing deionized water at room temperature. Characterization of the layer after heat treatment revealed a thickness of the multi-domain surface portion greater than 125nm. Cleaning 2: cleaning the free face of the thin layer 3 by successive immersion in baths of deionized water, SC1, SC2, all at room temperature. Characterization of the layer after heat treatment revealed a thickness of the multi-domain surface portion greater than 125nm. Cleaning 3: cleaning the free face by successive immersion in baths of deionized water, SC1, SC2, the SC1 solution being brought this time to 70°C.Characterization of the layer after heat treatment revealed a thickness of the multi-domain surface portion less than 125nm.

[0061] In an attempt to understand the reason for this phenomenon, the applicant carried out SIMS (secondary ion mass spectrometry) measurements of the first layer 3 to determine the hydrogen concentration profile, depending on the depth of the first layer. The hydrogen concentration profile of the first layer 3 was thus shown directly after fracture (without cleaning application – NET0), after the application of cleaning 1 (NET1) and after the application of cleaning 3 (NET3). A hydrogen-rich surface zone with a thickness of around 200 nm is observed. The application of cleaning 3 (which leads to limiting the thickness of the multi-domain surface portion) produces a hydrogen concentration in the surface zone of around 3.0 E21 at / cm^3 whereas it does not exceed the concentration of 1.0 E21 at / cm^3 in the other cases.

[0062] By processing these hydrogen concentration data, we note that cleaning 1 introduces into the first layer 3 a hydrogen dose of 1.4 E16 at / cm^2 without however allowing the thickness of the superficial multi-domain portion to be reduced below 125 nm. Cleaning 3, for its part, allows the introduction of a hydrogen dose of 4.3 E16 at / cm^2 and the reduction of the thickness of the multi-domain portion below 125 nm.

[0063] These preliminary results tending to link the thickness of the superficial multi-domain portion to the hydrogen concentration in the first layer 3 were confirmed by other experimental measurements.

[0064] According to one of them, the application of a treatment of the free face of the thin layer 3 consisted of the application of a sequence of 5 cleaning steps using a deionized water solution, the cleaning steps being separated from each other by a waiting time greater than or equal to 24 hours. The solution was at room temperature. This treatment led to a multi-domain thickness less than 125nm, from the 3rd cleaning of the sequence.

[0065] These cleaning steps have the effect of removing a thin surface layer of Li2CO3 on the thin layer 3 obtained directly after the fracture step. Its formation appears to be favored by the particular conditions in which the fracture step is carried out. The presence of light species, hydrogen and / or helium, and the moderate temperature at which the fracture occurs appear to make the lithium of the thin layer 3 particularly mobile and the surface of this layer 3 particularly reactive. This surface thickness of Li2CO3 has a thickness of the order of a nanometer, or even more. It is stable over time, that is to say that it does not change in consistency or thickness when the thin layer 3 is kept exposed to the atmosphere. This surface thickness of Li2CO3 is, however, relatively fragile, and the applicant has observed that it could be removed by simple wet cleaning.The applicant also observed that the thin layer 3, cleaned and deprived of its surface thickness of Li2CO3, remained particularly reactive. By keeping the free face of the thin layer 3 exposed to the atmosphere for an extended period of time, amorphous dendrites, rich in lithium and hydrogen (and other species present in the atmosphere such as chlorine or fluorine) nucleate and develop again. By carrying out successive cleaning steps separated from each other by a sufficient duration, there is a tendency to deplete a surface thickness of the thin layer 3 of its lithium (which migrates and accumulates on the surface) and, at the same time, to introduce hydrogen elements as replacement.When a sufficient dose of hydrogen has been introduced into the thin layer 3, the heat treatment applied to this layer leads to the formation of a multi-domain thickness lower than 125 nm, which confirms the hypothesis that an increase in the hydrogen concentration in a superficial thickness of the thin layer 3 tends to reduce the thickness of the superficial multi-domain portion induced by the heat treatment of the finishing sequence of this layer.

[0066] According to a final experimental measurement, a 30nm thickness of hydrogen-rich silicon oxide was formed by PECVD deposition on thin layer 3. Then the finishing sequence was annealed, which led to the injection by diffusion of a significant dose of hydrogen into thin layer 3. The characterization of this thin layer 3, after removal of the silicon oxide layer, showed that the thickness of the multi-domain layer was less than 125nm.

[0067] The applicant concluded from these experiments that a treatment of the free face of the first layer 3 aimed at forming a high hydrogen concentration in a surface thickness of this layer, for example greater than 2.0 E21 at / cm^3 in a surface thickness of 100 nm or 200 nm, made it possible to reduce the thickness of the surface multi-domain portion significantly.

[0068] Advantageously, the treatment of the free face leads to the introduction of a dose of hydrogen greater than or equal to 2.0 E16 at / cm^2 into the first layer 3.

[0069] Under these conditions, the thickness of the superficial multi-domain portion revealed by the heat treatment of the finishing sequence is smaller than 125nm.

[0070] The invention therefore takes advantage of these results to propose a method for preparing a 3' mono-domain thin layer. This method is shown schematically in the. It includes all the steps set out in relation to the description of Figures 1A to 1E, 2A to 2E.In particular, this method comprises: a step of implanting 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; a step of assembling the first face 4 of the donor substrate 1 to a support 7 to form an intermediate assembly; a step of fracturing the intermediate assembly, comprising a first heat treatment, this step leading to the fracture of the donor substrate 1 at the weakening plane 2 and to the formation of a free face 8 of the first layer 3; a finishing sequence of the first layer 3 comprising an annealing step comprising a second heat treatment and, after the annealing step, a step of thinning the first layer 3 to form the single-domain thin layer 3'.

[0071] According to the invention, the preparation method comprises, between the fracture step and the finishing sequence, the application of a treatment of the free face 8 of the first layer to produce a hydrogen concentration greater than 2.0 E21 at / cm^3 in a surface thickness of the first layer 3.

[0072] This surface thickness of the first layer, rich in hydrogen, may have a thickness at least equal to 100nm from the free face 8 of this first layer. Advantageously, it has a thickness at least equal to 200nm.

[0073] The hydrogen concentration can be obtained by choosing the free face treatment so that this treatment introduces a hydrogen dose greater than or equal to 2.0 E16 at / cm^2 in the first layer 3.

[0074] According to a first approach, the treatment of the free face 8 comprises the immersion of the first layer 3 in a first solution having a temperature higher than ambient temperature, for example higher than 50°C, preferably higher than or equal to 65°C. This first solution may be or comprise SC1. The treatment of the first layer may in particular correspond to an RCA type cleaning, composed as is well known per se of a sequence formed by successive immersion of the structure comprising the first layer 3 in baths of deionized water, SC1, SC2. When the treatment of the free face of the invention is implemented by such an RCA type cleaning, the temperature of the SC1 bath in which the structure is immersed is higher than ambient.

[0075] According to another approach, the treatment of the free face 8 comprises a sequence of cleaning steps using a second solution, the cleaning steps being separated from each other by a waiting time greater than or equal to 24 hours. The cleaning steps may correspond to the brushing of the free face 8 and, simultaneously, to the dispensing of deionized water onto the free face 8 of the first layer 3, the deionized water then forming the second solution used during the cleaning steps. This second solution may be at room temperature or brought to a temperature above room temperature.

[0076] Advantageously, the sequence of cleaning steps comprises at least three cleaning steps, preferably at least five cleaning steps.

[0077] According to yet another approach, the treatment of the free face 8 comprises the deposition on the free face 8 of a hydrogen-rich covering layer, for example a hydrogen concentration greater than 1.0 E20 at / cm^3. The covering layer may in particular comprise silicon dioxide, silicon nitride or silicon oxynitride formed, for example, by a chemical vapor deposition technique at subatmospheric pressure. Whatever its nature, the covering layer may have a thickness of 20 nm or more to contain a sufficient quantity of hydrogen.

[0078] By introducing hydrogen into the first layer 3 before applying the finishing sequence to this layer, it is possible to reduce the thickness of the multi-domain surface portion revealed by this sequence. In particular, it is possible to limit this thickness to less than 125 nm.

[0079] Of course, the invention is not limited to the embodiments described and variant embodiments can be made without departing from the scope of the invention as defined by the claims.

[0080] In particular, a treatment of the free face 8 can be applied, between the fracture step and the finishing sequence, combining the three approaches taken in illustration and according to any possible combination.

[0081] The treatment of the free face of the sampled layer 3 can also correspond to any other treatment of this face leading to the introduction of a sufficient dose of hydrogen into this layer.

Claims

A method for preparing a single-domain thin layer (3') of ferroelectric material, the method comprising:a step of implanting 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);a step of assembling the first face (4) of the donor substrate (1) to a support (7) to form an intermediate assembly;a step of fracturing the intermediate assembly, comprising a first heat treatment, this step leading to the fracture of the donor substrate (1) at the weakening plane (2) and to the formation of a free face (8) of the first layer (3);a finishing sequence of the first layer (3) comprising an annealing step comprising a second heat treatment and, after the annealing step, a step of thinning the first layer (3) to form the single-domain thin layer (3');the preparation method being characterized in that it comprises, between the fracture step and the finishing sequence, the application of a treatment of the free face (8) to produce a hydrogen concentration greater than 2.0 E21 at / cm^3 in a surface thickness of the first layer (3).; Preparation process according to the preceding claim in which the surface thickness, in which the hydrogen concentration is greater than 2.0 E21 at / cm^3, is greater than or equal to 100 nm. Preparation process according to the preceding claim in which the surface thickness is greater than or equal to 200 nm. Preparation method according to one of the preceding claims in which the treatment of the free face (8) introduces a dose of hydrogen greater than or equal to 2.0 E16 at / cm^2 into the first layer (3). Preparation method according to one of the preceding claims in which the treatment of the free face (8) comprises immersing the first layer (3) in a first solution having a temperature higher than ambient temperature. Preparation process according to the preceding claim in which the first solution comprises SC1 and / or SC2. Preparation process according to one of the two preceding claims in which the temperature of the first solution is greater than 50°C, preferably greater than 65°C. Preparation method according to one of the preceding claims in which the treatment of the free face (8) comprises a sequence of cleaning steps using a second solution, the cleaning steps of the sequence being separated from each other by a waiting time greater than or equal to 24 hours. Preparation method according to the preceding claim in which the second solution comprises deionized water, and at least three cleaning steps comprising simultaneously brushing and dispensing deionized water on the free face (8) of the first layer (3). Preparation process according to one of the two preceding claims in which the second solution is at room temperature. Preparation method according to one of the three preceding claims in which the sequence of cleaning steps comprises at least five cleaning steps. Preparation method according to one of the preceding claims in which the treatment of the free face (8) comprises the deposition on the free face (8) of a covering layer having a hydrogen concentration greater than 1.0 E20 at / cm^3. Preparation process according to the preceding claim wherein the covering layer comprises silicon dioxide, silicon nitride or silicon oxynitride. Preparation method according to the preceding claim wherein the covering layer has a thickness of 20nm or more. Preparation method according to one of the preceding claims in which the single-domain thin layer (3) is made of a single-crystalline piezoelectric material, such as lithium tantalate or lithium niobate. Preparation method according to the preceding claim in which the monodomain thin layer (3) consists of lithium niobate. Preparation method according to one of the preceding claims in which the assembly step comprises the formation of an intercalary dielectric layer (3) on the first face of the donor substrate (1) and / or on the support (7).

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