Method for fabricating thin layers of ferroelectric materials

Abstract

The present invention relates to a method for fabricating a thin single-domain layer (3') of a ferroelectric material, the method comprising the steps of forming a first layer (3) by fracturing a donor substrate (1) at a weakened surface (2) and applying a treatment to the free surface (8) of the first layer (3) to generate a hydrogen concentration greater than 2.0E21 at / cm^3 at the surface thickness of the first layer (3), between the first layer (3) and a finishing sequence for the first layer (3).

Description

【Technical Field】 【0001】 The present invention relates to a method for fabricating a thin layer of a ferroelectric material. More specifically, the present invention relates to a fabrication method that enables the single-domain property of the ferroelectric material to be maintained within the thin layer of the final product. This fabrication method is used in fields such as microelectronics, micromechanics, photonics, and the like. 【Background Art】 【0002】 As an introduction and reminder, a ferroelectric material is a material that has an electric polarization in its natural state, and this polarization can be reversed by applying an external electric field. A ferroelectric domain refers to each continuous region of the material where the polarization is uniform (all dipole moments are aligned parallel to each other in a given direction). Thus, a ferroelectric material can be characterized as "single-domain" if the material is composed of a single region where the polarization is uniform, or can be characterized as "multi-domain" if the ferroelectric material includes multiple regions with different polarities that may be present. 【0003】 More specifically, the present invention relates to the fabrication of thin ferroelectric layers obtained by applying Smart Cut (trademark) technology, according to which the thin layer is removed from a solid substrate of a ferroelectric material by fracturing it in the vicinity of a fragile zone (or a weakened plane) formed within this solid substrate by injecting a "light" elemental species such as helium or hydrogen. A specific example of the implementation of this method can be found in European Patent No. 3646374. 【0004】 According to this method, after the step of removing the layer, it is often necessary to apply a treatment to the layer to improve its surface finish, crystal quality, or thickness. However, the applicant has observed that when these fabrication steps are applied to a thin ferroelectric layer bonded to a silicon support, it can result in the formation of multiple ferroelectric domains within the thin layer, and therefore the layer has multi-domain properties. 【0005】 Such properties can affect the performance capabilities of devices that will be formed on or within a thin layer, such as surface acoustic wave (SAW) devices, making the use of the layer unsuitable. 【0006】 International Publication No. 2020 / 200986 discloses that the formation of ferroelectric domains in the surface portion of a layer is caused by the presence of a hydrogen concentration gradient within the thin layer when heat treatment is applied. This hydrogen can, in particular, correspond to light element species injected into the solid substrate to form a weakened zone within the substrate from which the thin layer can be removed. The thickness of the surface portion can be around 150 nm to 200 nm or more. To permanently restore the single-domain properties of the thin layer, this document recommends thinning the thin ferroelectric layer after applying such heat treatment. 【0007】 This thinning can be achieved, in particular, by chemical mechanical polishing of the layer, but removing a relatively large thickness of material by polishing tends to reduce the thickness uniformity of the layer. For example, removing a thickness of about 400 nm results in the formation of a layer with a thickness uniformity of about 100 nm (i.e., the difference between the maximum and minimum thickness when this thickness measurement is performed, for example, by reflectivity measurement or ellipsometry, at multiple measurement points over the entire range of the layer). This variation in thickness is unacceptable because it does not allow devices with all the required properties to be collectively manufactured from such layers. 【0008】 There are alternative forms of thinning using chemical mechanical polishing. In particular, thinning of thin ferroelectric films by ion etching can be attempted, for example, by reactive ion etching (RIE). RIE is a type of dry etching that uses a plasma of chemically reactive ions to remove surface material from a wafer. The plasma is generated under low pressure by an electromagnetic field. The high-energy ions of the plasma attack the surface of the layer, reacting with the surface of the layer and shattering it, thus gradually thinning the layer. Such a technique is described in particular in the literature filed under FR2111960. 【0009】 However, removing relatively large thicknesses of material, whether by chemical mechanical polishing or ion etching, and then removing the multi-domain surface portion of the removed layer is a drawback of conventional methods, as this step tends to make the implementation of the manufacturing method longer and more complex. 【0010】 This removal step also requires removing a relatively thick layer from a solid substrate of ferroelectric material, which necessitates the injection of light element species with a considerable amount of energy. Beyond a certain threshold thickness, the energy required to define the removed layer exceeds the range of existing injection equipment. Thus, conventional methods for fabricating thin single-domain layers of ferroelectric material have limitations that would be beneficial to overcome. 【0011】 Subject of the invention An object of the present invention is to propose a method for fabricating thin layers of ferroelectric material that at least partially addresses some of the limitations described above. More specifically, an object of the present invention is to propose a method for fabricating thin layers of ferroelectric material that is easier to implement than prior art methods. Another object of the present invention is to propose a method for thinning a thin layer obtained by exfoliating a donor substrate at a weakening surface, wherein the thickness of the material removed to remove the multi-domain surface portion of the removed layer is lower than the thickness removed by prior art methods. [Overview of the project] 【0012】 To achieve one of these objectives, the subject of this invention proposes a method for fabricating a thin single-domain layer of ferroelectric material, and this method is - The steps include: injecting "light" element species into the first surface of a ferroelectric donor substrate to form a weakened surface, and defining a first layer between the weakened surface and the first surface of the donor substrate; - The step of assembling the first surface of the donor substrate onto the support to form an intermediate assembly, - A step of crushing an intermediate assembly, including a first heat treatment, wherein this step results in the crushing of the donor substrate at the weakened surface and the formation of a free surface for the first layer. - A sequence for finishing the first layer, comprising an annealing step including a second heat treatment, and a step after the annealing step, thinning the first layer to form a thin single-domain layer. Includes. 【0013】 According to the present invention, the manufacturing method includes a step between the crushing step and the finishing sequence in which a treatment is applied to the free surface to generate a hydrogen concentration greater than 2.0E21 at / cm^3 at the surface thickness of the first layer. 【0014】 According to other preferred and non-limiting features of the present invention, viewed individually or in any technically feasible combination, - For surfaces with a hydrogen concentration exceeding 2.0E21 at / cm³, the surface thickness is 100 nm or more. - The surface thickness is 200 nm or more. - The treatment of the free surface involves introducing a hydrogen dose of 2.0E16 at / cm^2 or more into the first layer. -The treatment of the free surface includes immersing the first layer in a first solution having a temperature higher than the ambient temperature. -The first solution contains SC1 and / or SC2, - The temperature of the first solution exceeds 50°C, preferably exceeding 65°C. - The treatment of the free surface includes a sequence of cleaning steps using a second solution, and the cleaning steps of the sequence are separated from each other by a waiting time of 24 hours or more. - The second solution contains deionized water, and the cleaning step includes simultaneously brushing the free surface of the first layer and spraying deionized water onto the free surface of the first layer. - The second solution is at ambient temperature. - The sequence of cleaning steps includes at least three cleaning steps, preferably at least five cleaning steps. - The treatment of the free surface includes depositing a coating layer on the free surface at a hydrogen concentration exceeding 1.0E20 at / cm^3. - The coating layer includes silicon dioxide, silicon nitride or silicon oxynitride. - The thickness of the coating layer is 20 nm or more. - The thin single-domain layer is composed of a single crystal piezoelectric material such as lithium tantalate or lithium niobate. - The thin single-domain layer is composed of lithium niobate. - The assembly step includes forming a dielectric intermediate layer on the first surface of the donor substrate and / or on the support. 【0015】 Further features and advantages of the present invention will become apparent from the following detailed description of the present invention provided with reference to the accompanying drawings. 【Brief Description of the Drawings】 【0016】 [Figure 1] Shows the steps of fabricating a thin layer according to the first embodiment. [Figure 2] Shows the steps of fabricating a thin layer according to the second embodiment. [Figure 3] Is a graph of the analysis of the removed layer. [Figure 4] Shows the manufacturing method according to the present invention. 【Modes for Carrying Out the Invention】 【0017】 The present invention relates to a method for producing a thin single domain layer 3 of ferroelectric material transferred from a single crystal donor substrate 1 onto a support substrate 7 using a transfer technique that includes implanting a light element species into the donor substrate 1. There are several embodiments for this step of providing the thin layer. 【0018】 According to a first embodiment shown in FIGS. 1A - 1F, the donor substrate 1 is composed of a solid single crystal and single domain block of ferroelectric material, for example, LiTaO3, LiNbO3, LiAlO3, BaTiO3, PbZrTiO3, KNbO3, BaZrO3, CaTiO3, PbTiO3 or KTaO3. The donor substrate 1 can take the form of a standard - sized circular wafer having a diameter of, for example, 150 mm or 200 mm. However, the present invention is not at all limited to these sizes or this shape. The donor substrate may have been removed from an ingot of ferroelectric material so as to form a donor substrate 1 having a predetermined crystal orientation. The orientation is selected according to the intended application. Thus, when intending to use the properties of a thin layer of LiTaO3 to form a SAW filter, an orientation in the range of usually 30° to 60° RY, or in the range of 40° to 50° RY is selected. However, the present invention is not at all limited to a particular crystal orientation. 【0019】 Regardless of the crystal orientation of the donor substrate 1, the method includes introducing into the donor substrate 1 at least one "light" element species, particularly an element species selected from inert gases or hydrogen. This introduction can correspond to implantation, that is, ion bombardment of the flat surface 4 of the donor substrate 1 by light element species such as hydrogen and / or helium ions. 【0020】 In a method known per se, as shown in FIG. 1B, the implanted ions are intended to form a weakened surface 2 that defines a first layer 3 of the ferroelectric material to be transferred, located on one side of the flat surface 4, and another portion 5 that forms the remaining part of the substrate. 【0021】 The type and dose of the elemental species to be injected, as well as the injection energy, are selected according to the thickness of the layer to be transferred and the physicochemical properties of the donor substrate. In the case of donor substrate 1 made of LiTaO3, to define the first layer 3 of about 200-2,000 nm, an energy level in the range of 30 to 300 keV is used. E 16 to 5 E 17 at / cm 2 A range of hydrogen doses within this range may be selected for injection. 【0022】 In the subsequent step shown in Figure 1C, the flat surface 4 of the donor substrate 1 is assembled with the surface 6 of the support substrate 7. The support substrate 7 can be the same size and shape as the donor substrate 1. For reasons of availability and cost, the support substrate 7 is a single-crystal or polycrystalline silicon wafer. However, more generally, the support substrate 7 can be made of any material, such as silicon, sapphire, or glass, and can be any shape. 【0023】 In certain embodiments, the support substrate comprises a base substrate 7b made of, for example, single-crystal silicon, on which a charge trap layer 7a is disposed. The base substrate 7b can have a high resistivity of more than 1,000 ohms.cm, or more conventionally less than 1,000 ohms.cm. The charge trap layer 7a can be formed of a polycrystalline silicon layer, as is well known, and can typically have a thickness ranging from 500 nm to 10 microns. 【0024】 Prior to the assembly step, the surface of the substrate to be assembled may be prepared by steps such as cleaning, brushing, drying, polishing, or plasma activation. 【0025】 The assembly step may include bringing the donor substrate 1 into close contact with the support substrate 7 by molecular adhesion and / or electrostatic coupling. Optionally, to facilitate the assembly of the two substrates 1, 7, particularly when they are assembled by direct bonding, at least one amorphous intermediate layer may be formed on either the flat surface 4 of the donor substrate 1 or the flat surface 6 of the support substrate 7 to which it will be assembled, or both, before assembly. This intermediate layer may consist of, for example, silicon oxide, silicon nitride, or silicon oxynitride. Its thickness may range from a few nanometers to a few microns. 【0026】 In accordance with the teachings of International Publication No. 2020 / 200986, an intermediate layer having a low hydrogen concentration or forming a barrier to hydrogen diffusion is preferred, and therefore, the formation of a multi-domain zone at the interface between the first layer 3 and the amorphous intermediate layer on the second surface side of the first layer 3 is avoided. The intermediate layer can be produced using various known techniques of the prior art, such as thermal oxidation or nitriding, or chemical deposition (PECVD, LPCVD, etc.). 【0027】 Upon completion of this assembly step, an assembly comprising two related substrates is available, with the flat surface 6 of the support substrate 7 bonded to the flat surface 4 of the donor substrate 1. 【0028】 Next, the assembly is processed so that the first layer 3 of the ferroelectric material is peeled off from the donor substrate 1, for example, by cleavage at the weakened surface 2. 【0029】 Therefore, this peeling step may include applying heat treatment to the assembly in a temperature range of approximately 80°C to 300°C to enable the transfer of the first layer 3 to the support substrate 7. As an alternative to, or in addition to, heat treatment, this step may include applying a blade or a jet of gaseous or liquid fluid to the weakened surface 2. 【0030】 Following this peeling step, the structure 9 shown in Figure 1D is obtained. This structure 9 comprises a first layer 3 of a single-crystal ferroelectric material having a first free surface 8 and a second surface 4 disposed on the support substrate 7. 【0031】 Figures 2A to 2F illustrate a second embodiment that leads to the provision of this same structure 9. This second method is particularly suitable for generating a heterostructure 9 in which the first layer 3 has a coefficient of thermal expansion (in the main plane defining this layer) that is entirely different from that of the support 7, for example, by more than 10% (at ambient temperature). 【0032】 This second embodiment differs from the first embodiment primarily in terms of the properties of the donor substrate 1. Therefore, for brevity, this specification will describe only the elements of this second embodiment that differ from the first embodiment, and thus provide all the other features of the first embodiment. 【0033】 Referring to Figure 2A, in this case, the donor substrate 1 is composed of a thick layer 1a of a ferroelectric material having the same properties as those described for a solid block of ferroelectric material in relation to the first embodiment, and a handling substrate 1b. 【0034】 The handling substrate 1b is preferably composed of one (or more) materials that provide a coefficient of thermal expansion close to that of the support substrate 7. The term "close" means that the difference between the coefficient of thermal expansion of the handling substrate 1b and that of the support is, in absolute value, smaller than the difference between the thermal expansion of the solid block of ferroelectric material and that of the support substrate 7. 【0035】 It is preferable that the handling substrate 1b and the support substrate have the same coefficient of thermal expansion. During assembly of the donor substrate 1 and the support 7, an assembly capable of withstanding relatively high-temperature heat treatment is formed. To facilitate mounting, this can be achieved by selecting the handling substrate 1b so that it is made of the same material as the support substrate 7. 【0036】 To form the donor substrate 1 of this embodiment, a solid block of ferroelectric material is pre-assembled with a handling substrate 1a, for example, by molecular adhesive bonding techniques as described above, or by using an adhesive layer. Next, the layer 1a of ferroelectric material is formed by thinning, for example, by grinding and / or chemical mechanical polishing and / or etching. Prior to assembly, the formation of an adhesive layer (e.g., an adhesive layer, e.g., of a polymer, by silicon oxide and / or silicon nitride deposition) on one and / or the other of the contacting surfaces may be intended. Assembly may include applying low-temperature heat treatment (e.g., typically 100°C, in the range of 50 to 300°C) which allows the bonding energy to be sufficiently increased to enable the following thinning step. 【0037】 The handling substrate 1b is selected to have a thickness substantially equivalent to that of the support substrate 7. The thinning step is performed such that the thickness of the thick layer 1a is reduced to a sufficiently low level so that the stress generated during the heat treatment applied throughout the rest of the method is reduced to a lower strength. At the same time, this thickness is sufficiently high so that the first layer 3, or more such layers, can be removed from it. This thickness may be in the range of, for example, 5 to 400 microns. 【0038】 The following steps of the method in this second embodiment are equivalent to those described in the first embodiment. As shown in Figure 2B, light element species are injected into the thick layer 1a to create a weakening surface 2 that defines the boundary of separation of the thin layer 3 from the rest of the donor substrate 1 5. Following this step, as shown in Figure 2C, the donor substrate 1 is assembled onto the support substrate 7. Next, the first layer 3 is peeled off from the rest of the substrate 5 to obtain the structure 9 shown in Figure 2D. 【0039】 This embodiment is preferable in that the assembly formed from the donor substrate 1 and the support 7 can be exposed to temperatures much higher than those applied in the context of the first embodiment without jeopardizing uncontrolled fracture of one of the substrates or delamination of the donor substrate 1 from the thin layer 3. Thus, the balanced structure with respect to the coefficient of thermal expansion of this assembly facilitates the step of delaminating the first layer 3 by exposing the assembly to relatively high temperatures, for example, in the range of 100°C to 500°C. 【0040】 Regardless of the selected embodiment, as detailed in the introduction of this application, the finishing step of the first layer 3 is necessary later to improve its crystalline and surface quality and to provide a thin layer 3' having a thickness that matches or is close to the target thickness. These finishing steps, schematically shown in Figures 1E and 2E, are in particular intended to remove the hard, rough surface layer resulting from cleavage and delamination of the thin layer 3 from the rest of the donor substrate. 【0041】 As disclosed in International Publication No. 2020 / 200986, first, a heat treatment step is applied to the removed first layer 3. This heat treatment allows for the correction of crystalline defects present in this layer 3 and further reduces the roughness of its free surface 8. Furthermore, this helps to strengthen its adhesion to the support 7. The heat treatment raises the structure to a temperature ranging from 300°C to the Curie temperature of the ferroelectric material for a duration ranging from 30 minutes to 10 hours. This heat treatment is preferably carried out by exposing the free surface of the first layer 3 to an oxidizing or neutral gas atmosphere, i.e., without covering this surface of the thin layer 3 with a protective layer. 【0042】 To avoid misunderstanding, it should be noted that the finish heat treatment step is entirely different from the crushing heat treatment applied to the assembled structure. This is especially true because it is carried out using equipment different from that used for the crushing heat treatment. 【0043】 The method according to the present invention also includes a step of thinning the first thin layer after the finish heat treatment. This step is particularly aimed at removing the multi-domain surface portion of the first layer 3, which was created during the previous heat treatment step. This is also aimed at providing a thin single-domain layer 3' whose thickness corresponds to the target thickness, as described above. This thinning can generally correspond to polishing the first free surface 8 of the thin layer 3, for example, by mechanical or chemically mechanical thinning techniques. This can also include thinning carried out by ion etching, for example, by reactive ion etching. 【0044】 In all cases, this thinning results in the removal of thickness from at least the multi-domain portion of the first layer 3, which is typically at least 150 nm, resulting in the drawbacks described in the introduction to this application. Figures 1F and 2F show the structure 9 obtained upon completion of these processes, in which a thin single-domain layer 3' is placed on the support 7. 【0045】 To characterize the thickness of the multi-domain surface portion, samples of this layer are typically examined using a transmission electron microscope. This examination technique allows the multi-domain surface portion and the underlying single-domain portion to be visualized and distinguished in the cross-section of the sample. However, the field of view remains limited to an examination field of several hundred microns, which does not adequately represent the quality of the first layer 3 over its entire extent. This is also a long and complex technique to develop. 【0046】 To overcome this analytical problem, the applicant has developed a faster characterization technique that allows for a more comprehensive evaluation of the thickness of multi-domain portions. 【0047】 This technique includes a first step of removing a given thickness (e.g., 125 nm) of the first layer 3 by chemical mechanical polishing. Following this first step, the topography of the exposed surface is measured, where high topography indicates multi-domain quality of the material to which the surface is exposed, and conversely, low topography indicates single-domain quality of the material to which the surface is exposed. The topography measurement may be performed by atomic force measurement (AFM) in a measurement area of ​​5 micrometers × 5 micrometers. In the context of this description, "high topography" is understood to mean a surface with a peak-to-trough roughness of 10 nm or more, and conversely, "low topography" is understood to mean a surface with a peak-to-trough roughness of less than 10 nm. 【0048】 The rate of chemical or physicochemical etching of the thin layer 3 generated during the polishing step is variable depending on the polarity of the removed piezoelectric material, with the Z- side of this layer being etched much faster than the Z+ side. As a result, polishing a single-domain layer results in a much weaker surface topography than polishing a multi-domain layer. 【0049】 Furthermore, according to the proposed characterization technique, by evaluating the topography of the exposed surface of the thin layer after thinning by a given thickness, it is possible to very easily determine whether the multi-domain layer extends to a depth greater than a given thickness (revealed by high topography) or whether the multi-domain layer extends to a thickness less than a given thickness (revealed by low topography). Nevertheless, this somewhat rough characterization of the multi-domain thickness allows for a more comprehensive assessment of this thickness by repeating topographic measurements at multiple locations sampling the entire range of the layer. 【0050】 By using this rapidly implemented comprehensive characterization technique and by selecting a specific thickness of 125 nm, the applicant has recognized that the method for fabricating the ferroelectric material thin layer 3 described above (according to two different embodiments) results in the formation of a multi-domain surface layer that is at least 125 nm after the step of heat-treating the transferred thin layer 3 and before the thinning step. 【0051】 In exploring solutions to reduce the thickness of this multi-domain surface portion, the applicant surprisingly discovered that introducing a sufficient amount of hydrogen into the surface thickness of the first layer 3 before applying the heat treatment step to this layer 3 tended to reduce the thickness of the multi-domain surface portion revealed during this heat treatment. 【0052】 This discovery is the result of experiments conducted, which included applying an interlayer treatment to the free surface 8 of the first thin layer immediately after the step of fracturing the first layer of lithium tantalate. Following this interlayer treatment step, a heat treatment was applied, which resulted in the emergence of a multi-domain surface portion, characterized using the aforementioned technique by reducing the thickness of the first 125 nm layer by polishing and then measuring its topography. 【0053】 These experiments included the following list of interlayer processing steps for the first layer 3: • Cleaning 1: The free surface 8 of the first thin layer is cleaned by brushing and spraying with deionized water at ambient temperature. Characterization of the layer after heat treatment revealed a thickness of more than 125 nm for the multi-domain surface portion. • Cleaning 2: The free surface of the thin layer 3 is cleaned by continuously immersing it in deionized water, SC1, and SC2 tanks at ambient temperature. Characterization of the layer after heat treatment revealed a thickness of over 125 nm for the multi-domain surface portion. • Washing 3: The free surface was washed by continuous immersion in deionized water, SC1, and SC2 baths, with the SC1 solution being heated to 70°C this time. Characterization of the layer after heat treatment revealed a thickness of less than 125 nm for the multi-domain surface portion. 【0054】 To understand the reason for this phenomenon, the applicant performed SIMS (Secondary Ion Mass Spectrometry) measurements of the first layer 3 to determine the hydrogen concentration profile corresponding to the depth of the first layer 3. Thus, Figure 3 shows the hydrogen concentration profiles of the first layer 3 immediately after crushing (without washing -NET0), after applying washing 1 (NET1), and after applying washing 3 (NET3). A hydrogen-rich surface zone with a thickness of approximately 200 nm is observed. When washing 3 (which limits the thickness of the multi-domain surface portion) is applied, a hydrogen concentration of approximately 3.0E21 at / cm^3 is generated within the surface zone, but the hydrogen concentration does not exceed 1.0E21 at / cm^3 in other cases. 【0055】 When these hydrogen concentration data are processed, cleaning 1 is seen to introduce a hydrogen dose of 1.4E16 at / cm² into the first layer 3, without yet allowing the thickness of the surface multi-domain portion to be reduced to less than 125 nm. Cleaning 3 itself introduces a hydrogen dose of 4.3E16 at / cm², reducing the thickness of the multi-domain portion to less than 125 nm. 【0056】 These preliminary results, which tend to correlate the thickness of the surface multidomain portion with the hydrogen concentration in the first layer 3, were confirmed by other experimental measurements. 【0057】 According to one of these measurements, applying the treatment to the free surface of thin layer 3 involved applying a sequence of five washing steps using a deionized aqueous solution, with the washing steps separated from each other by a waiting time of more than 24 hours. The solution was at ambient temperature. This treatment resulted in a multi-domain thickness of less than 125 nm from the third washing step in the sequence. 【0058】 The effect of these washing steps is to remove a thin surface layer of Li2CO3 on the thin layer 3 obtained immediately after the crushing step. Its formation appears to be favored by the specific conditions under which the crushing step occurs. The presence of light element species, namely hydrogen and / or helium, as well as the moderate temperature at which the crushing occurs, appears to make the lithium in the thin layer 3 particularly mobile and the surface of this layer 3 particularly reactive. This surface thickness of Li2CO3 is about 1 nanometer or more. It is stable over time, i.e., its consistency or thickness does not change if the thin layer 3 is left exposed to the atmosphere. However, this surface layer of Li2CO3 is relatively fragile, and the applicant has observed that this surface layer can be removed by simple wet washing. The applicant has also observed that the washed thin layer 3, lacking the surface thickness of Li2CO3, remained particularly reactive. By leaving the free surface of thin layer 3 exposed to the atmosphere for an extended period, lithium and hydrogen (and other atmospheric elements such as chlorine or fluorine)-rich amorphous dendrites form nuclei and redevelop. Performing a series of washing steps separated from each other for a sufficient duration tends to strip away the surface thickness of that lithium thin layer 3 (which migrates and accumulates on the surface) and simultaneously introduce hydrogen as a substitution. When a sufficient dose of hydrogen is introduced into thin layer 3, the heat treatment applied to this layer results in the formation of multi-domain thicknesses of less than 125 nm, which supports the hypothesis that increasing the hydrogen concentration in the surface thickness of thin layer 3 tends to reduce the thickness of the surface multi-domain portion caused by the heat treatment in the finishing sequence of this layer. 【0059】 As a final experimental measure, a 30 nm thick hydrogen-rich silicon oxide layer was formed on thin layer 3 by PECVD deposition. Subsequently, a finishing sequence of annealing was performed, resulting in a significant amount of hydrogen being injected into thin layer 3 by diffusion. After removing the silicon oxide layer, characterization of thin layer 3 revealed that the thickness of the multi-domain layer was less than 125 nm. 【0060】 From these experiments, the applicant concluded that treating the free surface of the first layer 3, with the aim of forming a high hydrogen concentration of more than 2.0E21 at / cm^3 at a surface thickness of, for example, 100 nm or 200 nm, made it possible to significantly reduce the thickness of the surface multidomain portion. 【0061】 Preferably, a hydrogen dose of 2.0E16 at / cm² or more is introduced into the first layer 3 by treating the free surface. 【0062】 Under these conditions, the thickness of the surface multi-domain portion revealed by the heat treatment of the finishing sequence is less than 125 nm. 【0063】 Therefore, the present invention benefits from these results to propose a method for fabricating a thin single-domain layer 3'. This method is schematically shown in Figure 4. It uses all the steps disclosed in connection with the description of Figures 1A-1E and Figures 2A-2E. In particular, this method is - The steps include: injecting "light" element species into the first surface 4 of the ferroelectric donor substrate 1 to form a weakened surface 2, and defining a first layer 3 between the weakened surface 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, - A step of crushing an intermediate assembly, including a first heat treatment, wherein this step results in the crushing of the donor substrate 1 at the weakened surface 2 and the formation of a free surface 8 of the first layer 3. - A sequence for finishing the first layer 3, comprising an annealing step including a second heat treatment, and a step after the annealing step of thinning the first layer 3 to form a thin single-domain layer 3'. Includes. 【0064】 According to the present invention, the manufacturing method includes a step between the crushing step and the finishing sequence in which a treatment is applied to the free surface 8 of the first layer to generate a hydrogen concentration greater than 2.0E21 at / cm^3 at the surface thickness of the first layer 3. 【0065】 The hydrogen-rich surface thickness of the first layer may be at least 100 nm thick from the free surface 8 of the first layer. Preferably, the surface thickness is at least 200 nm thick. 【0066】 The hydrogen concentration can be obtained by selecting the treatment of the free surface such that a hydrogen dose of 2.0E16 at / cm² or more is introduced into the first layer 3. 【0067】 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 the ambient temperature, for example, higher than 50°C, preferably 65°C or higher. This first solution may or may contain SC1. The treatment of the first layer can correspond to an RCA-type wash, which in particular includes a sequence formed by successively immersing a structure containing the first layer 3 in tanks of deionized water, SC1, and SC2, as is well known in itself. When the treatment of the free surface of the present invention is carried out by such an RCA-type wash, the temperature of the SC1 tank into which the structure is immersed is higher than the ambient temperature. 【0068】 According to another method, the treatment of the free surface 8 includes a sequence of washing steps using a second solution, with these washing steps separated from each other by a waiting period of 24 hours or more. The washing steps can correspond to brushing the free surface 8 and simultaneously spraying deionized water onto the free surface 8 of the first layer 3, the deionized water then forming the second solution used during the washing steps. This second solution may be at ambient temperature or above ambient temperature. 【0069】 Preferably, the sequence of washing steps includes at least three washing steps, and more preferably at least five washing steps. 【0070】 Another method involves treating the free surface 8 by depositing a hydrogen-rich coating layer on the free surface 8 at a hydrogen concentration greater than, for example, 1.0E20 at / cm³. The coating layer may include silicon dioxide, silicon nitride, or silicon oxynitride, in particular, formed by, for example, chemical vapor deposition techniques at or below atmospheric pressure. Regardless of the properties of the coating layer, it may be 20 nm or thicker to contain a sufficient amount of hydrogen. 【0071】 By introducing hydrogen into the first layer 3 and then applying a 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. 【0072】 Naturally, the present invention is not limited to the embodiments described, and alternative embodiments may be used without departing from the scope of the invention as defined by the claims. 【0073】 In particular, the treatment of the free surface 8 can be applied between the crushing step and the finishing sequence by combining the three methods shown and according to any possible combination. 【0074】 The treatment of the free surface of the removed layer 3 can also be adapted to any other treatment of this surface that results in the introduction of a sufficient amount of hydrogen into this layer.

Claims

[Claim 1] A method for fabricating a thin single-domain layer (3') of a ferroelectric material, The steps include: injecting "light" element species into a first surface (4) of a ferroelectric donor substrate (1) to form a weakened surface (2), and defining a first layer (3) between the weakened surface (2) and the first surface (4) of the donor substrate (1); The steps include: forming an intermediate assembly by assembling the first surface (4) of the donor substrate (1) onto the support (7); A step of crushing the intermediate assembly, including a first heat treatment, wherein this step results in the crushing of the donor substrate (1) on the weakened surface (2) and the formation of a free surface (8) of the first layer (3), A sequence for finishing the first layer (3), comprising an annealing step including a second heat treatment, and a step after the annealing step of thinning the first layer (3) to form the thin single-domain layer (3'), A manufacturing method comprising the crushing step and the finishing sequence, wherein the manufacturing method includes a step of applying a treatment to the free surface (8) to generate a hydrogen concentration greater than 2.0E21 at / cm³ at the surface thickness of the first layer (3). [Claim 2] The manufacturing method according to claim 1, wherein the surface thickness of the hydrogen concentration is greater than 2.0E21 at / cm³ is 100 nm or more. [Claim 3] The manufacturing method according to claim 2, wherein the surface thickness is 200 nm or more. [Claim 4] The manufacturing method according to any one of claims 1 to 3, wherein the treatment of the free surface (8) introduces a hydrogen dose of 2.0E16 at / cm² or more into the first layer (3). [Claim 5] The method for producing according to any one of claims 1 to 4, wherein the treatment of the free surface (8) includes immersing the first layer (3) in a first solution having a temperature higher than the ambient temperature. [Claim 6] The method for producing according to claim 5, wherein the first solution comprises SC1 and / or SC2. [Claim 7] The method for producing according to claim 5 or 6, wherein the temperature of the first solution exceeds 50°C, preferably exceeding 65°C. [Claim 8] The manufacturing method according to any one of claims 1 to 7, wherein the treatment of the free surface (8) includes a sequence of washing steps using a second solution, and the washing steps in the sequence are separated from each other by a waiting time of 24 hours or more. [Claim 9] The method for producing according to claim 8, wherein the second solution contains deionized water, and at least three washing steps include simultaneously brushing the free surface (8) of the first layer (3) and spraying the deionized water onto the free surface (8) of the first layer (3). [Claim 10] The method for producing according to claim 8 or 9, wherein the second solution is at ambient temperature. [Claim 11] The manufacturing method according to any one of claims 8 to 10, wherein the sequence of the cleaning steps includes at least five cleaning steps. [Claim 12] The manufacturing method according to any one of claims 1 to 11, wherein the treatment of the free surface (8) includes depositing a coating layer on the free surface (8) at a hydrogen concentration of more than 1.0E20 at / cm³. [Claim 13] The method for producing according to claim 12, wherein the coating layer comprises silicon dioxide, silicon nitride, or silicon oxynitride. [Claim 14] The manufacturing method according to claim 13, wherein the coating layer has a thickness of 20 nm or more. [Claim 15] The manufacturing method according to any one of claims 1 to 14, wherein the thin single-domain layer (3) is composed of a single-crystal piezoelectric material such as lithium tantalate or lithium niobate. [Claim 16] The method for producing according to claim 15, wherein the thin single-domain layer (3) is composed of lithium niobate. [Claim 17] The manufacturing method according to any one of claims 1 to 16, wherein the assembly step includes a step of forming a dielectric intermediate layer (3) on the first surface of the donor substrate (1) and / or on the support (7).

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