Method for preparing thin layer of ferroelectric material

Abstract

To provide a method for preparing a monodomain thin layer of ferroelectric material.SOLUTION: The method for preparing a monodomain thin layer of ferroelectric material comprises: implanting light species in a first face of a ferroelectric donor substrate to form an embrittlement plane and define a first layer between the embrittlement plane and the first face of the substrate; assembling the first face of the donor substrate and a support substrate using a dielectric assembly layer (7b), and fracturing the donor substrate at the embrittlement plane, where the dielectric assembly layer comprises an oxide either having a hydrogen concentration lower than that of the first layer (3) or preventing the diffusion of hydrogen to the first layer, or the dielectric assembly layer (7b) comprises a barrier preventing the diffusion of hydrogen; and performing a heat treatment of a free face of the first layer to diffuse the hydrogen contained in the first layer, cause the multidomain transformation of a surface portion of the first layer, and subsequently thin the first layer to remove at least the surface portion.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for preparing thin layers of ferroelectric material, more particularly to a preparation method that allows maintaining the monodomain nature of the ferroelectric material in the thin layer of the final product, which is used, for example, in the fields of microelectronics, micromechanics, photonics, etc. [Background technology]

[0002] It is noted in the preceding sentence that a ferroelectric material is a material that has an electric polarization in its natural state, which polarization can be reversed by applying an external electric field. A "ferroelectric domain" refers to each continuous region of a material in which 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 "monodomain" if the material is formed by a single region in which the polarization is uniform, or as "multidomain" if the ferroelectric material contains multiple regions with different polarizations.

[0003] Various methods are known from the state of the art for forming thin layers of ferroelectric material, for example using molecular beam epitaxy, plasma sputtering, laser pulse deposition or application of the SmartCut™ technology, in which a thin layer is removed from a solid substrate of ferroelectric material by fracturing at weak zones (or embrittled surfaces) formed in the solid substrate by injection of optical species.

[0004] The invention more specifically relates to the preparation of thin ferroelectric layers obtained by applying such a method. US Pat. No. 5,399,433 notes that ferroelectric layers of lithium tantalate transferred by a method implementing hydrogen implantation lead to the formation of multi-domain layers. Such features affect the performance of devices formed on / in the thin layer, for example surface acoustic wave devices (SAW), making the layer unsuitable for use.

[0005] US Pat. No. 5,399,633 discloses the finishing of a thin ferroelectric layer transferred onto a support by an intermediate layer and by application of the Smart Cut method. In one example, this finishing involves a heat treatment of the thin layer followed by polishing. According to this document, this sequence allows the initial monodomain properties of the thin ferroelectric layer to be restored. However, additional analyses carried out by the applicant showed that this finishing sequence alone does not guarantee the monodomain nature of the layer throughout its thickness, i.e. from the interface with the intermediate layer to its free surface. More specifically, these analyses using PFM ("Piezo Response Force Microscopy") measurements were generally carried out over a depth of about 50 nm from the measurement surface, confirming that the surface portion of the thin layer is indeed monodomain, while the buried portion juxtaposed with the intermediate layer is multidomain. To perform this "buried" PFM measurement, the layer was gradually thinned by chemical mechanical polishing in order to approach the measurement surface of this intermediate layer.

[0006] US Pat. No. 5,399,433 also discloses a method for producing thin layers of ferroelectric material using the Smart Cut technique. In this document, as in the aforementioned US Pat. No. 5,399,433, the layer to be removed and transferred onto a support substrate is prepared by exposing it to an electric field in order to improve or restore its monodomain ferroelectric properties. Such a treatment requires, for example, the presence of a pair of electrodes arranged on either side of the ferroelectric layer to be treated in electrical contact with this layer. It is not always possible to have such a pair of electrodes, especially when the ferroelectric layer is transferred onto an electrically insulating substrate or onto an electrically insulating layer. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] US20100088868 [Patent Document 1] FR3068508 [Patent Document 3] FR2914492 [Patent Document 4] FR2860341 [Patent Document 5] FR2933233 [Patent Document 6] FR2953640 [Patent Document 7] US2015115480 Summary of the Invention

[0008] The object of the present invention is to propose a method for preparing a thin layer of ferroelectric material that at least partially addresses the aforementioned drawbacks. It more particularly relates to a method for transferring a layer of ferroelectric material carrying out a step of implanting optical species, a method that allows the monodomain quality of the transferred layer to be preserved or restored throughout its thickness, thus allowing the layer formation method to transfer it onto an electrically insulating substrate or to provide a dielectric layer, without the need to expose the latter to an electric field to restore its monodomain quality.

[0009] To this end, the subject of the present invention is a method for preparing a thin monodomain layer of a ferroelectric material comprising the steps of: implanting optical species into a first surface of a ferroelectric donor substrate to form an embrittled surface and defining a first layer between the embrittled surface and the first surface of the donor substrate; assembling the first surface of the donor substrate to a support substrate using a dielectric assembly layer; fracturing the donor substrate at the embrittled surface to transfer the first layer onto the support substrate and expose a free surface of the first layer; finishing the first layer, the finishing comprising heat treating the free surface of the first layer followed by thinning the first layer to form the thin monodomain layer; Includes.

[0010] According to the invention, the dielectric assembly layer comprising an oxide has a lower hydrogen concentration than the first layer, or prevents diffusion of hydrogen into the first layer, or the dielectric assembly layer comprises a barrier that prevents diffusion of the hydrogen into the first layer.

[0011] Surprisingly, this precise sequence of preparatory steps, combined with the specific properties of the assembly layers, leads to the formation of a final thin layer with monodomain polarization throughout its thickness and with satisfactory crystallographic and surface qualities.

[0012] According to other advantageous and non-limiting characteristics of the present invention, taken alone or in any technically feasible combination, the following are provided: The thinning includes a chemical mechanical polish applied to the free surface of the first layer.

[0013] The heat treatment is carried out at a temperature between 300° C. and the Curie temperature of the ferroelectric material constituting the first layer, for a period between 30 minutes and 10 hours.

[0014] The heat treatment is carried out under an oxidizing or neutral gaseous atmosphere.

[0015] The donor substrate comprises a ferroelectric material selected from LiTaO3 or LiNbO3.

[0016] The ferroelectric material has a crystal orientation between 30° RY and 60° RY.

[0017] The material of the supporting substrate is silicon.

[0018] The donor substrate is a block of solid material.

[0019] The donor substrate includes a thick layer of ferroelectric material disposed on a manipulator substrate.

[0020] Here, the manipulator substrate has a thermal expansion coefficient that is the same as or close to that of the support.

[0021] The material of the first layer and the material of the supporting substrate have different coefficients of thermal expansion.

[0022] The support substrate includes a charge trapping layer.

[0023] The dielectric assembly layer comprises an oxide having a nitrogen / oxygen ratio of 0.01 or 0.05 or greater.

[0024] The dielectric assembly layer comprises silicon oxide with a nitrogen / oxygen ratio between 0.01 and 0.25, or between 0.05 and 0.01.

[0025] The dielectric assembly layer is formed by a stack including a first oxide layer disposed on a side of the support substrate and a layer of silicon nitride disposed on a side of the first layer.

[0026] According to another aspect, the present invention provides a method for producing a method for manufacturing a pharmaceutical composition comprising: a thin, completely monodomain layer of ferroelectric material; a dielectric assembly layer in contact with said thin layer and comprising an oxide that prevents diffusion of hydrogen into said thin layer or a barrier that prevents diffusion of hydrogen into said first layer; a support in contact with the dielectric layer; The present invention proposes an apparatus including:

[0027] According to other advantageous, non-limiting features of this aspect of the invention, taken alone or in any technically feasible combination, the support substrate comprises a charge trapping layer disposed between the solid portion of the support and the dielectric assembly layer; The dielectric assembly layer comprises an oxide having a nitrogen / oxygen ratio of 0.01 or 0.05 or greater.

[0028] The dielectric assembly layer comprises silicon oxide with a nitrogen / oxygen ratio between 0.01 and 0.25, or between 0.05 and 0.01.

[0029] The dielectric assembly layer is formed by a stack including a first oxide layer disposed on a side of the support substrate and a layer of silicon nitride disposed on a side of the first layer. [Brief description of the drawings]

[0030] Other features and advantages of the present invention will become apparent from the following detailed description of the invention, which description is given with reference to the accompanying drawings. [Figure 1] 1A is an explanatory diagram showing a first embodiment of the method according to the present invention; 1B is an explanatory diagram showing a first embodiment of the method according to the present invention; 1C is an explanatory diagram showing a first embodiment of the method according to the present invention; and 1D is an explanatory diagram showing a first embodiment of the method according to the present invention. [Diagram 2] 2A is a diagram showing a second embodiment of the method according to the present invention; 2B is a diagram showing a second embodiment of the method according to the present invention; 2C is a diagram showing a second embodiment of the method according to the present invention; and 2D is a diagram showing a second embodiment of the method according to the present invention. [Diagram 3] 3A and 3B are schematic diagrams illustrating a method for preparing a layer according to the present invention; [Figure 4] FIG. 1 is an illustration of a surface acoustic device that can be manufactured using the method according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] To simplify the following description, the same reference symbols are used for identical elements or elements that perform the same function in different embodiments of the methods described.

[0032] The figures are schematic and, for ease of reading, are not necessarily drawn to scale, in particular the thicknesses of the layers are not drawn to scale with respect to the lateral dimensions of these layers.

[0033] The expression "coefficient of thermal expansion" used in the remainder of this description with respect to a layer or substrate refers to the coefficient of expansion in a direction defined by the main plane defining this layer or substrate. If the material is anisotropic, the retained value of the modulus is the value of maximum amplitude. The values ​​of the modulus are measured at room temperature.

[0034] FIG. 4 shows a surface acoustic device that can be manufactured using the method according to the invention.

[0035] The thin ferroelectric layer 10 is arranged in direct contact with the support 7 via the dielectric layer 7b. Metal electrodes 11a, 11b are arranged on the thin ferroelectric layer 10. In a manner well known per se, a high-frequency electric signal applied to one of the electrodes propagates in the thin ferroelectric layer 10 in the form of a surface wave, where it is processed (e.g. filtered) and recovered on the other electrode. The characteristics of the thin ferroelectric layer 10, and more generally of all the layers forming the stack in which the thin ferroelectric layer 10 is placed, are decisive for carrying out the desired processing of the electric signal, in particular the thickness and the monodomain crystal quality of the thin ferroelectric layer 10.

[0036] The characteristics of the support 7 can affect the propagation of waves in the thin layer 10 by acoustic coupling. The nature and thickness of these layers are therefore also decisive to perform the desired processing of the electrical signal or at least to influence this processing. In the example shown in FIG. 4, the support 7 comprises a solid silicon part 7a and a dielectric assembly layer 7b, preferably comprising an oxide, which is in direct contact with the thin ferroelectric layer 10. By way of example, the assembly layer 7b is made of silicon oxide or silicon nitride or is formed by a stack of layers composed of these materials. Preferably, the support 7 is electrically resistive to avoid electrical coupling with the high-frequency signals applied to the electrodes 11a, 11b, which could affect the propagation of the electrodes 11a, 11b. The solid part 7a can therefore be formed by a silicon substrate of high resistivity, i.e. having a resistivity of more than 1000 ohm-cm, more preferably more than 3000 ohm-cm.

[0037] To support the resistivity of the support 7, in the example of FIG. 4, provision is made for inserting a charge trapping layer 7c, for example made of polycrystalline silicon, between the solid part 7a and the assembly layer 7b. The formation of such a layer and its role in contributing to the resistivity of the support substrate 7 is well known per se and a detailed description thereof can be found in US Pat. No. 5,399,633, US Pat. No. 5,499,663, US Pat. No. 5,499,673 and US Pat. No. 5,523,366. Of course, it is possible for this charge trapping layer 7c to be formed using techniques other than those providing a layer made of polycrystalline silicon. This layer may also comprise carbon or consist of or contain silicon carbide or an alloy of silicon and carbon. Alternatively, it is a matter of generating electric traps in the layer 7c by ion bombardment of the surface portion of the support with a relatively heavy species (for example argon) in order to generate therein crystalline defects capable of trapping charges. It is also possible to provide a charge trapping layer 7c made of a porous material, for example by porosifying the surface portion of the substrate 7, if the latter is made of silicon.

[0038] It should be noted that the device shown in Figure 4 does not provide an electrode underneath the thin ferroelectric layer 10. The presence of the dielectric assembly layer 7b does not allow contact with the back surface of the thin ferroelectric layer 10. It is therefore not possible to pole the thin ferroelectric layer 10 using the presence of such an electrode or the formation of an electrical contact, as described in the state of the art cited in the introduction to this application.

[0039] It is therefore important to have a method for preparing a thin ferroelectric layer 10 that can preserve or restore the monodomain character of this layer 10 without the need for the application of a repolarization voltage.

[0040] With reference to Figures 1 and 2 and to be able to manufacture the device shown in Figure 4, the invention generally provides for the transfer of a first ferroelectric layer 3 onto a support substrate 7, the first layer 3 being taken from a monodomain ferroelectric donor substrate 1 by a transfer technique based on the implantation of light species such as hydrogen species.

[0041] According to this technique, hydrogen is implanted into the first side 4 of the donor substrate 1 in order to form an embrittled surface 2 embedded therein. A first layer 3 is thus defined between the embrittled surface 2 and the first side 4 of the donor substrate 1. The first side 4 of the donor substrate 1 is then assembled with a support 7, in this case by means of a dielectric assembly layer 7b. The donor substrate 1 is then fractured at the embrittled surface 2, for example using a moderate heat treatment and / or the application of mechanical forces. The first layer 3 is then released from the donor substrate 1, exposing the free side 8 of the first layer, while the other side 4 of the thin layer is in direct contact with the assembly layer 7b of the support substrate 7.

[0042] In order to form a "useful" thin layer 10, it is generally necessary to provide steps for preparing the first transferred layer 3 thus transferred onto the support 7. These steps are generally aimed at improving the crystalline quality of the thin layer 10 and its surface condition (for example its roughness). These preparation steps may include a step of thinning the first layer 3 (for example by chemical mechanical polishing, indicated in the table below with "CMP") and / or a step of heat treatment (in the case of a ferroelectric material, it can be a heat treatment of the order of 500 ° C in a neutral atmosphere or a heat treatment with oxygen, indicated in the table below with "TTH"). It should be noted that this step of heat treating the free surface 8 of the layer 3 is separate from the step that caused the donor substrate to fracture. In fact, this heat treatment cannot treat its free surface, since the layer 3 has not yet been completely freed during the fracture annealing.

[0043] However, the applicant has observed that the method of preparing the first layer 3 transferred onto the supporting substrate 7 for the purpose of forming the thin layer 10 may result in the creation of multiple ferroelectric domains within the thin layer 10, giving it a multi-domain nature. As already mentioned, such a feature affects the performance of devices formed on / in the transferred thin layer, for example a surface acoustic device as shown in FIG. 4, making the layer unsuitable for that use.

[0044] The present inventors have observed that the nature and sequence of the steps of preparing the first layer 3 can significantly affect the ferroelectric properties of the thin layer 10. From these initial observations, the present inventors have carried out analyses of various sequences of preparation steps using piezoelectric force microscopy (PFM) techniques to characterize the monodomain or multidomain nature of the surface of the thin layer 10. These experiments are summarized in the table below.

[0045] [Table 1]

[0046] It can be seen that the first layer 3 obtained at the end of the exfoliation step and before the preparation exhibits a conformable ferroelectricity, i.e. monodomain (first row of the table). However, the surface state and the crystalline quality of this layer require the application of a preparation step.

[0047] The fourth row of the table corresponds to the application of a method linking thinning and thermal treatment (CMP+TTH), and at the end of this treatment it can be seen that the thin layer has a multi-domain ferroelectric property.

[0048] Rows 2 and 3 represent the application of a single step of thinning CMP or thermal treatment TTH, respectively. Both of these steps lead to non-compliant properties of thin layer 3.

[0049] Applicant has realised that by sequentially applying a thermal treatment step TTH followed by a thinning step CMP, the resulting thin layer 10 has a monodomain distribution of its ferroelectric polarization and acceptable crystal and surface quality, as observed on the surface of this layer by PFM.

[0050] Additional analyses carried out by the Applicant have made it possible to identify that the formation of multiple ferroelectric domains in the thin layer 10 can be linked to the presence of a strong hydrogen concentration gradient in the first layer 3 prior to its preparation. Indeed, it has been observed that the application of a heat treatment to a layer with such a gradient leads to the formation of multiple ferroelectric domains. The hydrogen implanted in the donor substrate 1 during the step of defining the first layer 3 is distributed in this substrate according to a profile with a concentration peak at the embrittled surface 2. Thus, after fracturing, the first ferroelectric layer 3 transferred onto the support substrate 7 has, at its free surface 8, a concentration gradient of 10 21 at(atomic) / cm 3 This concentration decreases with the thickness of the thin layer in the direction of the support 7, reaching a value of 10 19 at / cm 3 Reach a level of the order of.

[0051] If the preparation step consists of a first step of thinning the first layer 3 to the target thickness of the thin layer 10 followed by a heat treatment, the latter being applied while the thin layer still has a strong hydrogen gradient, especially in the surface part of the thin layer, this results in the transformation of the thickness or surface part of the thin layer 10 where the hydrogen gradient is substantial into a plurality of ferroelectric domains.

[0052] If these two steps are reversed, the surface portion of the first layer 3 with the strong hydrogen concentration gradient is transformed to have multiple ferroelectric domains. However, this multi-domain surface portion of the first layer 3 is removed by the subsequent thinning step. This makes it possible to provide a thin layer 10 with the required monodomain quality.

[0053] It should be noted that the heat treatment, whether performed before or after the thinning step, diffuses the hydrogen with which the layer is saturated and reduces the thickness gradient of this layer, and therefore heat treatments that may be applied later in the preparation of thin layer 10 are unlikely to affect the monodomain nature of this layer.

[0054] A very detailed analysis using piezoelectric force microscopy of thin layer 10 also revealed that after thin layer 10 is thinned, it may have embedded multi-domain portions in contact with the assembly layers that are not easily visible using simple PFM surface observations, where measurements are limited to a depth of about 50 nm.

[0055] From PFM observations of the thin layers of several samples, applicants have determined that the properties of the dielectric assembly layer 7b play an important role with regard to the redistribution of hydrogen in the ferroelectric layer during the heat treatment of the method for preparing the thin layer 10.

[0056] If the assembly layer 7b is rich in hydrogen, as is the case for certain deposited oxides or dielectrics, it forms a hydrogen source that the thermal treatment can cause migration into the ferroelectric layer with which it is in contact. This hydrogen can accumulate in the ferroelectric layer, especially at the interface between this layer and the assembly layer 7b. This migration and this accumulation can constitute a hydrogen concentration gradient that allows a multi-domain transformation of the part of the ferroelectric layer in contact with the assembly layer during the thermal treatment.

[0057] Conversely, if the assembly layer 7b is relatively hydrogen depleted, i.e. has a lower hydrogen concentration than that present in the transferred ferroelectric layer, the excess hydrogen in the ferroelectric layer will be absorbed into the assembly layer 7b during diffusion by thermal treatment, thus preventing hydrogen accumulation at the assembly interface and avoiding multi-domain transformation in the portion of the ferroelectric layer near this interface.

[0058] The present invention takes advantage of these results and observations to propose a method for preparing a thin layer 10. More specifically, the invention relates to a method for preparing a thin layer 10 of ferroelectric material which is transferred from a donor substrate 1 to a support substrate 7 by a transfer technique involving the implantation of optical species such as hydrogen species, as described very generally above. There are several embodiments for carrying out this transfer.

[0059] According to a first embodiment shown in Fig. 1A to 1D, the donor substrate 1 consists of a solid monodomain 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 circular wafer of standardized size, for example 150 mm or 200 mm diameter. However, the invention is in no way limited to these dimensions or to this shape. The donor substrate 1 may have been removed from an ingot of ferroelectric material, the removal being carried out so that the donor substrate 1 has a predefined crystal orientation. The orientation is selected depending on the intended application. Thus, if the aim is to exploit the properties of thin layers to form SAW filters, it is common practice to select an orientation between 30°RY and 60°RY or between 40°RY and 50°RY. However, the invention is in no way limited to a particular crystal orientation.

[0060] Irrespective of the crystal orientation of the donor substrate 1, the method comprises the introduction of hydrogen into this donor substrate 1. This introduction can correspond to an implantation of hydrogen, i.e. an ion bombardment of the plane 4 of the donor substrate 1 with hydrogen. Naturally, this introduction of hydrogen, for example by implantation, can be expected to be complemented by the introduction of other optical species, such as helium.

[0061] In a manner known per se, as shown in FIG. 1B, the purpose of the implanted ions is to form an embrittlement plane 2 which defines the boundary between a first layer 3 of the ferroelectric material to be transferred flanking face 4 and another part 5 which forms the remainder of the substrate.

[0062] The nature, the dose of the implanted species and the implantation energy are chosen as a function of the thickness of the layer intended to be transferred and of the physicochemical properties of the donor substrate 1. Thus, for a donor substrate 1 made from LiTaO3, an energy of between 30 keV and 300 keV and a dose of 100 keV are chosen in order to define a first layer 3 on the order of 200 nm to 2000 nm. E 16 and 5E 17at / cm 2 It is then possible to choose to implant a dose of hydrogen between 0.1 and 0.5.

[0063] In the next step shown in FIG. 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 may be of the same size and shape as that of the donor substrate 1. For reasons of availability and cost, the support substrate 7 is a silicon, monocrystalline or polycrystalline wafer. More generally, however, the support substrate 7 may be made of any material, for example silicon, or even an electrically insulating material such as sapphire or glass, and may be of any shape. To form the device shown in FIG. 4, a support substrate is chosen that is formed by a solid portion 7a made of highly resistive silicon with a charge trapping layer 7c made of polycrystalline silicon, for example.

[0064] Prior to the assembly process, it is possible to consider preparing the surface of the substrate to be assembled using cleaning, brushing, drying, polishing processes or using, for example, plasma activation based on oxygen or nitrogen.

[0065] The assembly process can correspond to intimate contact of the donor substrate 1 and the support substrate 7 by molecular adhesion and / or electrostatic bonding. To facilitate the assembly of the two substrates 1, 7, provision is made for forming at least one dielectric assembly layer 7b on the face 6 of the support substrate 7 to be assembled. It is also or alternatively possible to form an assembly layer on the face 4 of the donor substrate 1 to be assembled, preferably before the formation of the embrittled face 2. In other words, the dielectric assembly layer 7b can be formed, at least partially, on one and / or the other of the two substrates 1, 7.

[0066] The dielectric assembly layer 7b is formed, for example, by silicon oxide or, more generally, by an oxide such as Ta2O5, ZrO2 or HfO (these oxides, like SiO2, can be formed by deposition) or silicon nitride, and has a thickness between a few nanometers and a few microns. The assembly layer 7b can consist of a stack of different types of dielectric layers. The dielectric assembly layer 7b can be produced by various well-known techniques, for example, thermal oxidation or nitridation processes, chemical deposition (PECVD, LPCVD, etc.).

[0067] Generally, techniques for depositing or forming the assembly layer are preferred, leading to the incorporation of small amounts of hydrogen therein. It should be noted that the choice of the material forming this assembly layer, its thickness and its forming technique are not completely free. This choice is determined in particular by the electrical insulation, acoustic wave propagation and / or adhesion properties of this layer in the final structure and according to the intended application. Finally, it should be noted that forming the assembly layer by deposition, in particular oxide deposition, is generally easier and more universal, since this forming method can be carried out independently of the nature of the support and at relatively moderate temperatures, lower than 1000° C. or even lower than 950° C. or even lower than 900° C., which may be imposed by the nature of the solid part of the support 7a or by the presence of the trapping layer 7c.

[0068] According to an important feature of one embodiment, care is taken to ensure that the dielectric assembly layer 7b has a lower average hydrogen concentration than the first layer 3. By assuming that the majority of the hydrogen in the first layer 3 comes from the hydrogen implanted to form the embrittled surface 2, the average concentration of hydrogen is determined to be a function of the implanted dose (at / cm 2 ) by the thickness (cm) of the first layer 3. 20 at / cm 3 and 10 22 at / cm 3By way of example, if the assembly layer 7b is formed by a silicon oxide deposited on a support substrate, this oxide is annealed before the assembly step so as to exodiffuse most of the hydrogen it contains.

[0069] In general, the step of annealing the assembly layer 7b can be provided with the aim of subjecting this dielectric layer to a higher temperature than the heat treatment for preparing the first transfer layer described later in this description. This layer can therefore be at temperatures of 600° C., 700° C. or even 800° C. or higher. The average hydrogen concentration in the assembly layer 7b after this exodiffusion step is therefore 5.10 20 at / cm 3 or preferably less than 10 18 at / cm 3 or more preferably less than 10 18 at / cm 3 It should be noted that annealing can modify features of the assembly layer other than the hydrogen concentration. In particular, it can reduce the diffusivity of hydrogen, i.e., the ability of this species to diffuse through the materials forming the assembly layer, so that hydrogen can be present in relatively high concentrations (10 20 at / cm 3 Even if the amount of the impurities is small (eg, about 100 μm), the impurities are less likely to diffuse toward the first layer 3.

[0070] Alternatively, or in addition to this embodiment, provisions can be made to provide the dielectric assembly layer 7b with a barrier layer preventing the diffusion of hydrogen towards the first layer 3. By way of example, the assembly layer 7b can be formed by a stack formed by a first layer of oxide, for example silicon oxide, arranged on the side of the support substrate 7a (or, in the latter case, the trapping layer 7c) and a layer of silicon nitride arranged on the side of the first ferroelectric layer 3. This nitride layer prevents hydrogen that may be present in the silicon oxide layer from diffusing towards the first ferroelectric layer 3.

[0071] According to yet another approach, the dielectric assembly layer can be provided with any concentration of hydrogen, which has a very low diffusivity and therefore remains sufficiently confined not to diffuse significantly in the direction of the thin layer 3.

[0072] Such a layer can be formed by an oxide, for example silicon oxide SiON, with a nitrogen / oxygen ratio of 0.01, 0.05 or 0.1 or more. If the assembly dielectric layer 7b is based on silicon oxide SiON, which is very common since this material has well-known characteristics of formation, adhesion, electrical insulation and propagation of acoustic waves, it can be determined not to exceed a nitrogen / oxygen ratio of more than 0.1 or 0.25 in order not to excessively modify these functions and to maintain a behavior equivalent or close to that of simple silicon oxide SiO2. It is noted that such nitrogen-rich oxide layers can be easily formed by deposition techniques, for example PECVD, and at least one of the carrier gases can be selected to be nitrogen, which can be incorporated in the oxide layer in a controlled way. The nitrogen / oxide ratio can be established from measurements of nitrogen and oxygen measured by a technique called EDX ("energy dispersive X-ray spectroscopy") or measured in a SIMS (secondary ion mass spectrometry) measurement of the oxide layer 4.

[0073] The dielectric assembly layer 7b may be formed over its entire thickness by a material having low diffusivity, for example a nitrogen-containing oxide as described above.

[0074] Alternatively, as explained in the previous alternative, only a barrier layer can be provided that prevents the diffusion of hydrogen into the first layer 3 made of this material with low diffusivity. For example, a layer of silicon oxide with a high concentration of hydrogen can be formed by deposition on a support substrate, this layer being prepared using a nitrogen-based plasma in order to superficially incorporate this nitrogen. A barrier layer of nitrogen-rich SiO is thus formed in this superficial thickness, which is able to prevent the diffusion of hydrogen contained in the remaining oxide.

[0075] If the solid part of the support 7a is provided on its surface with a charge trapping layer 7c comprising silicon, it is conceivable to form the dielectric assembly layer 7b by thermal oxidation of the surface part of this trapping layer by treatment in an oxidizing atmosphere at a moderate temperature below 950° C., preferably between 800° C. and 900° C. This therefore avoids damage, for example by recrystallization, caused by exposing the charge trapping layer 7c to high temperatures above 950° C. or 900° C. Optionally, a step of polishing the surface of the trapping layer 7c thus oxidized can be introduced to make it compatible with subsequent assembly steps.

[0076] Regardless of the nature of the dielectric assembly layer 7b and the treatment it has undergone, at the end of this assembly step, an assembly is provided as shown in FIG. 1C, comprising two associated substrates, the flat surface 6 of a support substrate 7 adhered to the flat surface 4 of a donor substrate 1 by the dielectric assembly layer 7b.

[0077] The assembly is then treated to release the first layer of ferroelectric material 3 from the donor substrate 1 by cleaving at the embrittlement plane 2 .

[0078] This detachment step may therefore comprise applying a heat treatment to the assembly, within a temperature range of the order of 80° C. to 300° C., in order to enable the first layer 3 to be transferred onto the support substrate 7. Instead of or in addition to a heat treatment, this step may comprise the application of a blade, or a jet of a gas or liquid fluid, or any other force of a mechanical nature, at the embrittled surface 2.

[0079] Following this separation step, the structure 9 shown in Figure 1D is obtained, which comprises a first layer 3 of ferroelectric material comprising a first free face 8 and a second face 4 (corresponding to the first face of the donor substrate) arranged on a support substrate 7, with a dielectric assembly layer 7b being arranged between the first layer 3 and the remainder of the support 7.

[0080] Figures 2A to 2D show a second embodiment, which is particularly suitable for producing a heterogeneous structure 9, in which the first layer 3 has a thermal expansion coefficient that is very different from that of the support, for example having a difference of more than 10%.

[0081] Referring to FIG. 2A, the donor substrate 1 in this case consists of a thick layer of ferroelectric material 1a and a manipulator substrate 1b, with the same properties as described in relation to the first embodiment for a solid block of ferroelectric material.

[0082] The manipulator substrate 1b is advantageously formed from a material (or materials) that provides a coefficient of thermal expansion close to that constituting the support substrate 7. By "close" we mean that the difference in the coefficient of thermal expansion between the manipulator substrate 1b and the support is smaller in absolute value than the difference in thermal expansion between a solid block of ferroelectric material and the support substrate 7.

[0083] Preferably, the manipulator substrate 1b and the support substrate 7 have the same thermal expansion coefficient. During assembly of the donor substrate 1 and the support 7, a structure is formed that can withstand heat treatment at relatively high temperatures. A simple implementation can be obtained by selecting the manipulator substrate 1b to be made by the same material as that of the support substrate 7.

[0084] To form the donor substrate 1 of this embodiment, a solid block of ferroelectric material is first assembled with the manipulator substrate 1b, for example according to a bonding technique by molecular adhesion as described above. A layer 1a of ferroelectric material is then formed by thinning, for example by grinding and / or chemical-mechanical polishing and / or etching. Before assembly, one and / or the other of the contacted surfaces can be prepared to form an adhesion layer (for example by depositing silicon oxide and / or silicon nitride). The assembly includes the application of a low-temperature heat treatment (for example 50° C. to 300° C., typically 100° C.), allowing sufficient strengthening of the bonding energy to allow the following thinning step.

[0085] The manipulator substrate 1b is selected to have a thickness substantially equivalent to that of the support substrate 7. The thinning step is carried out so that the thickness of the thick layer 1a is thin enough that the stresses generated during the heat treatments applied in the remainder of the method have a lower intensity. At the same time, this thickness is sufficient to be able to remove the first layer 3, or several such layers. This thickness may be, for example, between 5 and 400 microns.

[0086] The following steps of the process of this second implementation are comparable to those described in the first embodiment. As shown in FIG. 2B, hydrogen is implanted in the thick layer 1a to generate an embrittlement surface 2, which indicates the separation of the first layer 3 from the remaining part 5 of the donor substrate 1. This step is followed by a step of assembling the donor substrate 1 on a support substrate 7, as shown in FIG. 2C. A dielectric assembly layer 7b is provided between the donor substrate 1 and the support substrate 7, this assembly layer having the same characteristics as those provided within the context of the first embodiment. If it is decided to form a layer of silicon oxide (or a dielectric of other nature that requires exodiffusion annealing of hydrogen) on the face of the donor substrate 1 of this embodiment to be assembled, the thickness is limited to a few nanometers, for example less than 10 nm or 50 nm. This is because it may be difficult to apply exodiffusion annealing to this layer without risking damage to the donor substrate 1, which is composed of an assembly of materials with very different thermal expansion coefficients.

[0087] The first layer 3 is then separated from the remainder of the substrate 5 to obtain the structure 9 shown in FIG. 2D.

[0088] This embodiment is advantageous in that the assembly formed from the donor substrate 1 and the support 7 can be subjected to much higher temperatures than those applied in the first embodiment, without the risk of uncontrolled destruction of one of the substrates or delamination of the donor substrate 1. That is to say, the balanced structure of this assembly in terms of the thermal expansion coefficient makes it possible to facilitate the step of separating the thin layer 3 by subjecting the assembly to relatively high temperatures, for example between 100° C. and 600° C.

[0089] Regardless of the embodiment selected, and as specified above, a step of preparing the first layer 3 is then required to form a thin layer 10 having satisfactory crystallographic and surface quality.

[0090] As shown in Fig. 3A, the method first involves a heat treatment of the free surface 8 of the transferred first layer 3. This heat treatment can correct the crystal defects present in the first layer 3. Furthermore, it helps to strengthen the bond between this layer 3 and the support 7. The above studies also show that, if this heat treatment has a sufficient temperature, it has the effect of causing the diffusion of hydrogen contained in the first layer 3, especially in its surface portion, and causing multi-domains in this surface portion.

[0091] The surface portion is about 50 nm thick or less and can be established throughout the entire extent of layer 3. At the end of the heat treatment, ferroelectric layer 10 has a relatively constant concentration of hydrogen throughout its thickness, 10 18 at / cm 3 and 10 19 at / cm 3 It is noted that due to the above-mentioned properties of the assembly layer 7b, the hydrogen contained in this assembly layer is less likely to diffuse towards the thin layer 3 during annealing and / or to be trapped at the interface present between the assembly layer 7b and this layer 3. This therefore avoids the formation of a hydrogen concentration gradient in this buried zone near this interface, thus preventing this zone from being transformed to have multi-domain properties.

[0092] The heat treatment for preparing the compliant thin layer 10 brings the structure to a temperature between 300° C. and the Curie temperature of the ferroelectric material (preferably 450° C., 500° or 550° or higher to promote hydrogen diffusion) for between 30 minutes and 10 hours. The heat treatment can provide a gradual gradient. This heat treatment is preferably carried out by exposing the free face 8 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 capable of preventing exodiffusion of hydrogen.

[0093] With reference to FIG. 3B, the preparation method also includes a thinning of the thin layer 3 after the heat treatment. This thinning can correspond to a polishing of the free surface 8 of the thin layer 3, for example by mechanical, chemical, mechanical thinning techniques and / or chemical etching. It allows the free surface 8 to be prepared to exhibit a low roughness, for example by atomic force measurement (AFM), less than 0.5 nm RMS 5×5 μm, and the multi-domain surface portion of the thin layer 10 to be removed. To reach the target thickness of the thin layer 10, and in all cases a thickness thicker than the thickness of the multi-domain surface portion, a removal of a thickness between 100 nm and 300 nm is generally provided.

[0094] It therefore constitutes a thin monodomain layer throughout its thickness, which, for crystalline quality, has the necessary qualities for surface state quality. The dielectric assembly layer 7b, which has a lower hydrogen concentration than that of the first layer 3 or which contains a barrier to the diffusion of hydrogen into the first layer 3, avoids the accumulation of excess hydrogen in the buried zone of the thin layer 10 located near the assembly interface. This prevents a multidomain transformation of this zone, which is impossible to eliminate.

[0095] Of course, the invention is not limited to the embodiments and examples described, but modified embodiments are possible without departing from the scope of the invention as defined by the claims.

Claims

1. A method for preparing a thin monodomain layer (10) of a ferroelectric material, comprising the steps of: Injecting optical species into a first surface (4) of a ferroelectric donor substrate (1) to form an embrittled surface (2) and defining a first layer (3) between said embrittled surface (2) and said first surface (4); assembling said first side (4) of said donor substrate (1) to a support substrate (7) using a dielectric assembly layer (7b); - fracturing the donor substrate (1) at the embrittled surface (2) in order to transfer the first layer (3) to the support substrate (7) and to expose a free surface (8) of the first layer (3); finishing said first layer (3), said finishing comprising a heat treatment of said free surface (8) of said first layer (3), followed by thinning said first layer (3) to form a thin monodomain layer (10); Including, The method according to claim 1, wherein the dielectric assembly layer (7b) comprises an oxide that prevents diffusion of hydrogen into the first layer, or the dielectric assembly layer (7b) comprises a barrier that prevents diffusion of the hydrogen towards the first layer (3).

2. A method according to claim 1, characterized in that said thinning comprises a chemical-mechanical polishing applied to said free surface (8) of said first layer (3).

3. 3. Method according to claim 1 or 2, characterized in that the heat treatment is carried out at a temperature between 300° C. and the Curie temperature of the ferroelectric material constituting the first layer (3), for a period between 30 minutes and 10 hours.

4. 4. The method according to claim 1, wherein the heat treatment is carried out under an oxidizing or neutral gaseous atmosphere.

5. The donor substrate (1) is LiTaO 3 Or LiNbO 3 5. The method of claim 1, further comprising the step of:

6. 6. The method of claim 1, wherein the ferroelectric material has a crystal orientation between 30° and 60° RY.

7. 7. The method according to claim 1, wherein the material of the support substrate (7) is silicon.

8. 8. The method according to any one of the preceding claims, characterized in that the donor substrate (1) is a block of solid material.

9. 9. The method according to any one of the preceding claims, characterized in that the donor substrate (1) comprises a thick layer (1a) of ferroelectric material arranged on a manipulator substrate (1b).

10. 10. The method according to claim 9, characterized in that the manipulator substrate (1b) has a coefficient of thermal expansion that is the same as or close to the coefficient of thermal expansion of the support (7).

11. 11. A method according to any one of the preceding claims, characterized in that the material of the first layer (3) and the material of the support substrate (7) have different thermal expansion coefficients.

12. 12. A method according to any one of the preceding claims, characterized in that the dielectric assembly layer (7b) comprises an oxide having a nitrogen-containing nitrogen / oxygen ratio of 0.01 or 0.05 or more.

13. 13. The method according to any one of claims 1 to 12, characterized in that the dielectric assembly layer (7b) comprises silicon oxide with a nitrogen / oxygen ratio between 0.01 and 0.25, or between 0.05 and 0.

01.

14. 14. The method according to claim 1, wherein the dielectric assembly layer (7b) is formed by a stack including a first oxide layer arranged on a side of the support substrate (7a) and a layer of silicon nitride arranged on a side of the first layer (3).

15. a thin full monodomain layer of ferroelectric material (10); a dielectric assembly layer (7b) in contact with said thin layer (10) and comprising an oxide preventing the diffusion of hydrogen into said thin layer (10) or comprising a barrier preventing the diffusion of hydrogen into said first layer (3); a support (7) in contact with said dielectric layer; An apparatus comprising:

16. 16. Apparatus according to claim 15, characterized in that the carrier substrate (7) comprises a charge trapping layer (7c) arranged between the solid part (7a) of the carrier and the dielectric assembly layer (7b).

17. 17. A device according to claim 15 or 16, characterized in that the dielectric assembly layer (7b) comprises an oxide having a nitrogen / oxygen ratio of 0.01 or 0.05 or more.

18. 18. A device according to any one of claims 15 to 17, characterized in that the dielectric assembly layer (7b) comprises silicon oxide with a nitrogen / oxygen ratio between 0.01 and 0.25, or between 0.05 and 0.

01.

19. 19. The device according to claim 15, wherein the dielectric assembly layer (7b) is formed by a stack including a first oxide layer arranged on a side of the support substrate (7a) and a layer of silicon nitride arranged on a side of the first layer (3).

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