METHOD FOR PREPARING A THIN LAYER OF FERROELECTRIC MATERIAL

Abstract

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

Description

Title of the invention: METHOD FOR PREPARING A THIN LAYER OF FERROELECTRIC MATERIAL FIELD OF THE INVENTION

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

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

[0003] The present invention relates more particularly to the preparation of a ferroelectric thin film obtained by applying the Smart Cut™ technology, according to which a thin film is extracted from a bulk ferroelectric substrate by fracturing at a weak zone (or embrittlement plane) formed in the bulk substrate by implanting so-called "light" species, such as helium or hydrogen. A specific example of the implementation of this process can be found in document EP3646374B1.

[0004] According to this process, and after the layer sampling step, it is often necessary to apply treatments to improve its surface condition, its crystalline quality, or to modify its thickness. However, the applicant observed that these preparation steps, when applied to a ferroelectric thin film deposited on a silicon substrate, could lead to the formation of a plurality of ferroelectric domains within the thin film, thus giving it a multidomain character.

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

[0006] Document WO2020200986 reveals that the formation of ferroelectric domains in a surface portion of the layer is caused by the existence of hydrogen concentration gradients in the thin film during the application of a heat treatment. This hydrogen may correspond, in particular, to light species embedded in the bulk substrate to form the embrittlement zone from which the thin film can be extracted. The surface portion may have a thickness on the order of 150 nm to 200 nm or more. To permanently restore the single-domain character of the thin film, this document recommends thinning the ferroelectric thin film after the application of such a heat treatment.

[0007] This thinning can be achieved, in particular, by mechanochemical polishing of the layer, but removing a relatively large thickness of material by polishing tends to deteriorate the thickness uniformity of this layer. For example, removing a thickness on the order of 400 nm results in a layer whose thickness uniformity (i.e., the difference between the greatest and thinnest thicknesses when this thickness measurement is performed, for example, by reflectometry or ellipsometry, at multiple measurement points over the entire extent of the layer) is on the order of 100 nm. This variability in thickness is unacceptable, as it makes it impossible to collectively manufacture, from such a layer, devices having all the required characteristics.

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

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

[0010] This removal step also requires taking a layer of relatively significant thickness from the bulk ferroelectric material substrate, This necessitates the implantation of light species with significant energy. Beyond a certain threshold thickness, the energy required to define the extracted layer is not achievable with existing implantation equipment. Therefore, a prior art process for preparing a single-domain thin film in ferroelectric material presents limitations that should ideally be overcome. SUBJECT OF THE INVENTION

[0011] One object of the invention is to provide a method for preparing a thin film of ferroelectric material that addresses at least some of the aforementioned limitations. More particularly, one object of the invention is to provide a method for preparing a thin film of ferroelectric material that is simpler to implement than prior art methods. Another object of the invention is to provide a method for thinning a thin film obtained by detachment at a weakening plane of a donor substrate, in which the thickness of material removed to eliminate the multi-domain surface portion of the extracted layer is less than the thickness removed in prior art methods. BRIEF DESCRIPTION OF THE INVENTION

[0012] With a view to achieving one of these objectives, the object of the invention proposes a method for preparing a single-domain thin film of ferroelectric material, the method comprising: • a step of implanting so-called "light" species in a first face of a ferroelectric donor substrate to form a weakening plane and define a first layer between the weakening plane and the first face of the donor substrate; • an assembly step of the first face of the donor substrate to a support to form an intermediate assembly; • a fracture step of the intermediate assembly, including a first heat treatment, this step leading to the fracture of the donor substrate at the level of the weakening plane and to the formation of a free face of the first layer; • a first layer finishing sequence including an annealing step with a second heat treatment and, after the annealing step, a first layer thinning step to form the single-domain thin film.

[0013] According to the invention, the preparation process comprises, between the fracture step and the finishing sequence, the application of a free face treatment to produce a hydrogen concentration greater than 2.0 E21 at / cmA3 in a super- thickness strand of the first layer.

[0014] According to other advantageous and non-limiting features of the invention, taken alone or in any technically feasible combination: - the surface thickness, in which the hydrogen concentration is greater than 2.0 E21 at / cmA3, is greater than or equal to 100 nm; - the surface thickness is greater than or equal to 200 nm; - the free face treatment introduces a dose of hydrogen greater than or equal to 2.0 E16 at / cmA2 into the first layer; - the treatment of the free face includes immersing the first layer in a first solution having a temperature higher than the ambient temperature; - the first solution includes SCI and / or SC2; - the temperature of the first solution is above 50°C, preferably significantly higher than 65 °C; - the free surface treatment includes a sequence of cleaning steps using a second solution, the cleaning steps of the sequence being separated from each other by a waiting time greater than or equal to 24 hours; - the second solution includes deionized water, the cleaning steps simultaneously including brushing and dispensing deionized water onto the free side of the first layer; - the second solution is at room temperature. - the sequence of cleaning steps includes at least three cleaning steps, preferably at least five cleaning steps; - the treatment of the free face includes the deposition on the free face of a coating layer having a hydrogen concentration greater than 1.0 E20 at / cmA3; - the coating layer comprises silicon dioxide, silicon nitride or silicon oxynitride; - the cover layer has a thickness of 20nm or more; - the single-domain thin film is made of a piezoelectric material single crystal, such as lithium tantalate or lithium niobate; - the single-domain thin film is made of lithium niobate; - the assembly step includes the formation of an intercalated dielectric layer on the first face of the donor substrate and / or on the support. Brief description of the drawings

[0015] Other features and advantages of the invention will become apparent from the description detailed description of the invention which will follow with reference to the attached figures on which:

[0016] [Fig.1] The [Fig.1] represents a step in preparing a thin layer according to a first method of implementation;

[0017] [Fig.2] Fig.2 represents a step in the preparation of a thin film according to a second method of implementation;

[0018] [Fig.3] The [Fig.3] is a graph of an analysis of a sampled layer;

[0019] [Fig.4] Fig.4 represents a preparation method according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0020] The invention relates to a method for preparing a single-domain thin film 3 in a ferroelectric material transferred from a single-crystal donor substrate 1 onto a support substrate 7 by a transfer technique including the implantation of light species in a donor substrate 1. Several embodiments of this thin-film supply step exist.

[0021] According to a first embodiment, shown in Figures IA to 1F, the donor substrate 1 is composed of a solid, single-crystal, single-domain block of ferroelectric material, for example, LiTaO3, LiNbO3, LiA1O3, BaTiO3, PbZrTiO3, KNbO3, BaZrO3, CaTiO3, PbTiO3, or KTaO3. The donor substrate 1 may be in the form of a circular wafer of standardized dimensions, for example, 150 mm or 200 mm in diameter. However, the invention is in no way limited to these dimensions or this shape. The donor substrate may have been taken from an ingot of ferroelectric material, this taking having been carried out in such a way that the donor substrate 1 has a predetermined crystal orientation. The orientation is chosen according to the intended application. Thus, it is common to choose an orientation between 30° and 60°RY, or between 40° and 50°RY, in the case where one wishes to exploit the properties of a thin film in LiTaO3 to form a SAW filter.But the invention is by no means limited to a particular crystal orientation.

[0022] Regardless of the crystalline orientation of the donor substrate 1, the process includes introducing into the donor substrate 1 at least one so-called "light" species, in particular species selected from inert gases or hydrogen. This introduction may correspond to implantation, i.e., ion bombardment of a flat face 4 of the donor substrate 1 by light species such as hydrogen and / or helium ions.

[0023] In a manner known per se, and as shown in Figure IB, the implanted ions are intended to form a weakening plane 2 delimiting a first layer 3 of ferroelectric material to be transferred which is located on the side of the flat face 4 and another part 5 forming the rest of the substrate.

[0024] The nature, the dose of the implanted species and the implantation energy are chosen in depending on the thickness of the layer to be transferred and the physicochemical properties of the donor substrate. In the case of a donor substrate 1 in LiTaO3, it will thus be possible to choose to implant a dose of hydrogen between 1 E16 and 5 E17 at / cm2 with an energy between 30 and 300 keV to delimit a first layer 3 of the order of 200 to 2000 nm.

[0025] In a subsequent step, shown in Figure IC, the flat face 4 of the donor substrate 1 is assembled with a face 6 of a support substrate 7. The support substrate 7 may have the same dimensions and shape as the donor substrate 1. For reasons of availability and cost, the support substrate 7 is a silicon wafer, either monocrystalline or polycrystalline. However, more generally, the support substrate 7 may be made of any material, for example silicon, sapphire, or glass, and may have any shape.

[0026] In a particular embodiment, the support substrate comprises a base substrate 7b, for example of monocrystalline silicon, on which a charge-trapping layer 7a is disposed. The base substrate 7b may have a high resistivity, greater than 1000 ohms·cm, or a more conventional one, less than 1000 ohms·cm. The charge-trapping layer 7a, as is well known, may be formed of a layer of polycrystalline silicon and typically have a thickness between 500 nm and 10 microns.

[0027] Prior to the assembly step, it may be envisaged to prepare the faces of the substrates to be assembled by a step of cleaning, brushing, drying, polishing, or activation for example by plasma.

[0028] The assembly step may consist of bringing the donor substrate 1 into intimate contact with the support substrate 7 by molecular adhesion and / or electrostatic bonding. Optionally, to facilitate the assembly of the two substrates 1 and 7, particularly when assembled by direct bonding, at least one amorphous interlayer may be formed prior to assembly, either on the flat face 4 of the donor substrate 1, or on the flat face 6 of the support substrate 7 to be assembled, or on both. This interlayer is, for example, made of silicon dioxide, silicon nitride, or silicon oxynitride. It may have a thickness ranging from a few nanometers to a few microns.

[0029] An interlayer with a low hydrogen concentration or a barrier to hydrogen diffusion will be preferred in order to follow the teachings of document WO2020200986 and thus avoid the formation of a multi-domain zone at the interface between the first layer 3 and the amorphous interlayer, on the side of the second face of the first layer 3. The interlayer can be produced according to the various techniques known in the state of the art, such as heat treatments of oxidation or nitriding, chemical deposition (PECVD, LPCVD...), etc.

[0030] At the end of this assembly step, we have an assembly comprising the two associated substrates, the flat face 6 of the support substrate 7 adhering to the flat face 4 of the donor substrate 1.

[0031] The assembly is then treated to detach the first layer 3 of ferroelectric material from the donor substrate 1, for example by cleavage at the level of the embrittlement plane 2.

[0032] This detachment step can thus include the application to the assembly of a heat treatment in a temperature range of approximately 80°C to 300°C to allow the transfer of the first layer 3 onto the support substrate 7. As a replacement for or in addition to the heat treatment, this step can include the application of a sheet or a jet of gaseous or liquid fluid to the level of the embrittlement plane 2.

[0033] Following this detachment step, we obtain the structure 9 shown in Figure 1D. This structure 9 comprises the first layer 3 of single-crystal ferroelectric material comprising a first free face 8 and a second face 4 disposed on the support substrate 7.

[0034] Figures 2A to 2F represent a second embodiment leading to providing this same structure 9. This second embodiment is particularly suited to the realization of a heterogeneous structure 9, in which the first layer 3 has a coefficient of thermal expansion (in the principal plane defining this layer) very different from that of the support 7, for example having a difference of more than 10% (at room temperature).

[0035] This second embodiment differs mainly from the first embodiment by the nature of the donor substrate 1. For the sake of brevity, we therefore only present here the elements of this second embodiment which differ from the first, all the other characteristics of the first embodiment can therefore be foreseen.

[0036] With reference to Figure 2A, the donor substrate 1 is in this case composed of a thick layer of ferroelectric material la, possessing the same properties as those described for the bulk block of ferroelectric material in relation to the first embodiment, and a manipulator substrate 1b.

[0037] The manipulator substrate 1b is advantageously made of a material (or a plurality of materials) giving it a coefficient of thermal expansion close to that of the support substrate 7. By "close", we mean that the difference in coefficient of thermal expansion of the manipulator substrate 1b and that of the support is less, in absolute value, than the difference in thermal expansion of the solid block of the ferroelectric material and that of the support substrate 7.

[0038] Preferably, the manipulator substrate 1b and the support substrate have a identical coefficient of thermal expansion. When the donor substrate 1 and the support 7 are assembled, a unit is formed capable of withstanding heat treatment at a relatively high temperature. For ease of implementation, this can be achieved by choosing the manipulator substrate 1b to be made of the same material as the support substrate 7.

[0039] To form the donor substrate 1 of this embodiment, a solid block of ferroelectric material is first bonded to the manipulator substrate 1a, for example, using a molecular adhesion bonding technique as previously described or with the aid of an adhesive layer. Then, the ferroelectric material layer 1a is formed by thinning, for example, by grinding and / or mechanochemical polishing and / or etching. Before assembly, the formation of an adhesive layer (for example, by deposition of silicon oxide and / or silicon nitride, or an adhesive layer, for example, a polymer) may be planned on one or both of the contacting faces. The assembly may include the application of a low-temperature heat treatment (for example, between 50 and 300°C, typically 100°C) to sufficiently increase the bonding energy to allow the subsequent thinning step.

[0040] The manipulator substrate 1b is chosen to have a thickness substantially equivalent to that of the support substrate 7. The thinning step is carried out in such a way that the thick layer is sufficiently thin to reduce the stresses generated during the heat treatments applied later in the process. At the same time, this thickness is sufficient to allow the removal of the first layer 3, or a plurality of such layers. This thickness can, for example, be between 5 and 400 microns.

[0041] The following steps of the process in this second embodiment are equivalent to those of the steps described in the first embodiment. Lightweight materials are implanted within the thick layer 1a to generate a weakening plane 2 that marks the separation of the thin layer 3 from the remainder 5 of the donor substrate 1, as shown in Figure 2B. This step is followed by the step of assembling the donor substrate 1 onto the support substrate 7, as shown in Figure 2C. The first layer 3 is then detached from the remainder of the substrate 5 to obtain the structure 9 shown in Figure 2D.

[0042] This embodiment is advantageous in that the assembly formed by the donor substrate 1 and the support 7 can be exposed to a much higher temperature than that applied in the first embodiment, without risk of uncontrolled fracture of one of the substrates or delamination of the donor substrate 1 from the thin layer 3. The balanced structure, in terms of the coefficient of thermal expansion of this assembly, thus facilitates the detachment step of the first layer 3. by exposing the assembly to a relatively high temperature, for example between 100 and 500 °C.

[0043] Regardless of the embodiment chosen, and as stated in the introduction to this application, finishing steps for the first layer 3 are then necessary to improve its crystalline and surface quality and to provide a thin layer 3' whose thickness corresponds to, or approaches, a target thickness. These finishing steps, schematically represented in Figure 1E and Figure 2E, aim in particular to eliminate a rough, work-hardened surface layer resulting from the cleavage and detachment of the thin layer 3 from the rest of the donor substrate.

[0044] As described in document WO2020200986, a heat treatment step is first applied to the first sampled layer 3. This heat treatment helps to heal crystalline defects present in this layer 3, and may even reduce the roughness of its free face 8. Furthermore, it helps to consolidate its adhesion to the substrate 7. The heat treatment raises the structure to a temperature between 300°C and the Curie temperature of the ferroelectric material for a duration of between 30 minutes and 10 hours. This heat treatment is preferably carried out by exposing the free face of the first layer 3 to an oxidizing or neutral gaseous atmosphere, i.e., without covering this face of the thin film with a protective layer.

[0045] To avoid any ambiguity, it is emphasized that the finishing heat treatment step is quite distinct from the fracture heat treatment applied to the assembled structure. In particular, it is carried out in equipment different from that used to apply the fracture heat treatment.

[0046] A process according to the invention also includes, after the finishing heat treatment, a thinning step of the first thin film. This step aims in particular to remove the multi-domain surface portion of the first layer 3, this portion having been created during the preceding heat treatment step. It also aims to provide a single-domain thin film 3' whose thickness corresponds to a target thickness, as mentioned previously. This thinning can, generally speaking, correspond to polishing the first free face 8 of the thin film 3, for example by mechanical or mechanochemical thinning techniques. It can also be thinning carried out by ion etching, for example by reactive ion etching.

[0047] In all cases, this thinning leads to the elimination of at least the thickness of the multi-domain portion of the first layer 3, this thickness typically being on the order of at least 150 nm, which results in the drawbacks presented in the introduction to this application. Figures 1F and 2F represent the structure 9 obtained after these treatments, a single-domain thin layer 3' being arranged on support 7.

[0048] To characterize the thickness of the multi-domain surface portion, it is common practice to inspect a sample of this layer by transmission electron microscopy. This inspection technique makes it possible to visualize and distinguish, in the cross-section of the sample, the multi-domain surface portion and the underlying single-domain portion. However, the extent of this view remains limited to inspection fields of a few hundred microns on each side, which is not entirely representative of the quality of the first layer 3 over its entire extent. It is also a time-consuming and complex technique to implement.

[0049] To circumvent this analysis problem, the applicant has developed a faster characterization technique that allows for a more global evaluation of the thickness of the multi-domain portion.

[0050] This technique comprises a first step consisting of removing a predetermined thickness of the first layer 3 (for example, 125 nm) by chemical-mechanical polishing. Following this first step, the topography of the exposed face is measured: a high topography indicates the multi-domain quality of the material whose face is exposed, and conversely, a low topography indicates the single-domain quality of the material whose face is exposed. The topography measurement can be performed by atomic force measurement (AFM) on measurement fields of 5 micrometers by 5 micrometers. For the purposes of this description, a surface with a peak-to-valley roughness greater than or equal to 10 nm will be designated as "high topography," and conversely, a surface with a peak-to-valley roughness less than 10 nm will be designated as "low topography."

[0051] The rate of chemical or physicochemical etching of the thin film 3 occurring during the polishing step is made variable by the nature of the polarity of the removed piezoelectric material: the Z- side of this layer etches much faster than the Z+ side. Consequently, polishing a single-domain layer will result in a much lower surface topography than polishing a multi-domain layer.

[0052] Also, according to the proposed characterization technique, by evaluating the topography of the exposed face of the thin film after it has been thinned to a predetermined thickness, it is possible to determine very simply whether the multi-domain layer extended to a depth greater than the determined thickness (revealed by a significant topography) or whether it extended to a thickness less than the determined thickness (revealed by a shallow topography). This somewhat crude characterization of the multi-domain thickness nevertheless allows for an overall assessment of this thickness by repeating the topographic measurement at a plurality of locations. sampling the entire extent of this layer.

[0053] Using this global and rapid characterization technique, and by choosing a determined thickness of 125nm, the applicant was able to realize that the process for preparing a thin film 3 in a ferroelectric material which has just been presented (according to two different implementation methods) leads to the formation, after the heat treatment step of the transferred thin film 3 and before the thinning step, of a multi-domain surface layer of at least 125nm.

[0054] In the search for a solution aimed at reducing the thickness of this multi-domain surface portion, the applicant discovered quite surprisingly that the introduction of a sufficient dose of hydrogen into a surface thickness of the first layer 3, before the application of the heat treatment step of this layer 3, tended to reduce the thickness of the multi-domain surface portion which is revealed during this heat treatment.

[0055] This discovery resulted from experiments conducted in which, directly after the fracture step of a first lithium tantalate layer, an interlayer treatment was applied to the free face 8 of the first thin layer. Following this interlayer step, the heat treatment leading to the development of the multi-domain surface portion was applied, which was characterized using the technique described above, by reducing the thickness of the first layer by 125 nm through polishing and then measuring its topography.

[0056] These experiments included the following list of intercalated treatments of the first layer 3: • Cleaning 1: Cleaning of the free face 8 of the first thin layer by brushing and dispensing with deionized water at room temperature. Characterization of the layer after heat treatment revealed a thickness of the multi-domain surface portion greater than 125 nm. • Cleaning 2: Cleaning of the free face of the thin film 3 by successive immersion in baths of deionized water, SCI, and SC2, all at room temperature. Characterization of the layer after heat treatment revealed a thickness of the multi-domain surface portion greater than 125 nm. • Cleaning 3: cleaning of the free face by successive immersion in baths of deionized water, SCI, and SC2, the SCI solution being heated to 70°C this time. Characterization of the layer after heat treatment revealed a thickness of the multi-domain surface portion of less than 125nm.

[0057] To try to understand the reason for this phenomenon, the applicant carried out SIMS (secondary ion mass spectrometry) measurements of the first layer 3. To determine the hydrogen concentration profile according to the depth of the first layer, Figure 3 shows the hydrogen concentration profile of the first layer 3 directly after fracture (without cleaning application - NETO), after the application of cleaning 1 (NET1), and after the application of cleaning 3 (NET3). A superficial zone rich in hydrogen, approximately 200 nm thick, is observed. The application of cleaning 3 (which limits the thickness of the multi-domain superficial portion) produces a hydrogen concentration in the superficial zone of approximately 3.0 E21 at / cmA3, whereas it does not exceed 1.0 E21 at / cmA3 in the other cases.

[0058] By processing these hydrogen concentration data, it is noted that cleaning 1 introduces a hydrogen dose of 1.4 E16 at / cmA2 into the first layer 3 without, however, reducing the thickness of the superficial multi-domain portion below 125 nm. Cleaning 3, on the other hand, allows the introduction of a hydrogen dose of 4.3 E16 at / cmA2 and reduces the thickness of the multi-domain portion below 125 nm.

[0059] These preliminary results tending to link the thickness of the superficial multi-domain portion to the hydrogen concentration in the first layer 3 were confirmed by other experimental measurements.

[0060] According to one of them, the application of a treatment to the free face of the thin film 3 consisted of applying a sequence of 5 cleaning steps using a deionized water solution, the cleaning steps being separated from each other by a waiting time greater than or equal to 24 hours. The solution was at room temperature. This treatment resulted in a multi-domain thickness of less than 125 nm, starting from the 3rd cleaning in the sequence.

[0061] These cleaning steps have the effect of removing a thin surface layer of Li2CO3 from the thin layer 3 obtained directly after the fracture step. Its formation appears to be favored by the specific conditions under which the fracture step takes place. The presence of light species, hydrogen and / or helium, and the moderate temperature at which the fracture occurs seem to make the lithium in the thin layer 3 particularly mobile and the surface of this layer 3 particularly reactive. This surface layer of Li2CO3 is on the order of a nanometer, or even thicker. It is stable over time, that is, it does not change in consistency or thickness when the thin layer 3 is kept exposed to the atmosphere. This surface layer of Li2CO3 is, however, relatively fragile, and the applicant has observed that it can be removed by simple wet cleaning.The applicant also observed that thin layer 3, cleaned and devoid of its surface layer of Li2CO3, remained particularly reactive. By maintaining the free face of thin layer 3. When exposed to the atmosphere for an extended period, amorphous dendrites, rich in lithium and hydrogen (and other atmospheric species such as chlorine or fluorine), nucleate and regenerate. By carrying out successive cleaning steps of sufficient duration, a superficial layer of the thin layer 3 is depleted of its lithium (which migrates and accumulates on the surface) and, at the same time, hydrogen elements are introduced as replacements.When a sufficient dose of hydrogen was introduced into the thin film 3, the heat treatment applied to this layer led to the formation of a multi-domain thickness less than 125 nm, which confirms the hypothesis that an increase in the hydrogen concentration in a superficial thickness of the thin film 3 tends to reduce the thickness of the superficial multi-domain portion induced by the heat treatment of the finishing sequence of this layer.

[0062] According to a final experimental measurement, a 30 nm thick layer of hydrogen-rich silicon oxide was formed by PECVD deposition on the thin film 3. The finishing sequence was then annealed, resulting in the diffusion injection of a significant dose of hydrogen into the thin film 3. Characterization of this thin film 3, after removal of the silicon oxide layer, showed that the thickness of the multi-domain layer was less than 125 nm.

[0063] The applicant concluded from these experiments that a treatment of the free face of the first layer 3 aimed at forming a high hydrogen concentration in a superficial thickness of this layer, for example greater than 2.0 E21 at / cmA3 in a superficial thickness of lOOnm or 200 nm, made it possible to reduce the thickness of the superficial multi-domain portion significantly.

[0064] Advantageously, the treatment of the free face leads to the introduction of a dose of hydrogen greater than or equal to 2.0 E16 at / cmA2 in the first layer 3.

[0065] Under these conditions, the thickness of the superficial multi-domain portion revealed by the heat treatment of the finishing sequence is less than 125nm.

[0066] The invention therefore takes advantage of these results to propose a method for preparing a single-domain 3' thin film. This method is schematically represented in [Fig. 4]. It incorporates all the steps described in Figures IA to 1E and 2A to 2E. In particular, this method comprises: • a step of implanting so-called "light" species into a first face 4 of a ferroelectric donor substrate 1 to form a weakening plane 2 and define a first layer 3 between the weakening plane 2 and the first face 4 of the donor substrate 1; • an assembly step of the first face 4 of the donor substrate 1 to a support 7 to form an intermediate assembly; • a fracture stage of the intermediate assembly, comprising a first heat treatment, this step leading to the fracture of the donor substrate 1 at the level of the weakening plane 2 and to the formation of a free face 8 of the first layer 3; • a finishing sequence of the first layer 3 comprising an annealing step including a second heat treatment and, after the annealing step, a thinning step of the first layer 3 to form the single-domain thin layer 3'.

[0067] According to the invention, the preparation process comprises, between the fracture step and the finishing sequence, the application of a treatment to the free face 8 of the first layer to produce a hydrogen concentration greater than 2.0 E21 at / cmA3 in a superficial thickness of the first layer 3.

[0068] This surface thickness of the first layer, rich in hydrogen, can have a thickness of at least 100 nm from the free face 8 of this first layer. Advantageously, it has a thickness of at least 200 nm.

[0069] The hydrogen concentration can be obtained by choosing the free face treatment so that this treatment introduces a dose of hydrogen greater than or equal to 2.0 E16 at / cmA2 into the first layer 3.

[0070] According to a first approach, the treatment of the free face 8 comprises immersing the first layer 3 in a first solution having a temperature above ambient temperature, for example above 50°C, preferably above or equal to 65°C. This first solution may be or include SCI. The treatment of the first layer may, in particular, correspond to an RCA-type cleaning, which, as is well known, consists of a sequence formed by successive immersion of the structure comprising the first layer 3 in baths of deionized water, SCI, and SC2. When the treatment of the free face of the invention is implemented by such an RCA-type cleaning, the temperature of the SCI bath in which the structure is immersed is above ambient temperature.

[0071] According to another approach, the treatment of the free face 8 comprises a sequence of cleaning steps using a second solution, the cleaning steps being separated from each other by a waiting time greater than or equal to 24 hours. The cleaning steps may consist of brushing the free face 8 and, simultaneously, dispensing deionized water onto the free face 8 of the first layer 3, the deionized water then forming the second solution used during the cleaning steps. This second solution may be at room temperature or heated to a temperature above room temperature.

[0072] Advantageously, the sequence of cleaning steps includes at least three cleaning steps, preferably at least five cleaning steps.

[0073] According to yet another approach, the treatment of the free face 8 comprises depositing on the free face 8 a hydrogen-rich coating layer, for example, with a hydrogen concentration greater than 1.0 E20 at / cmA3. The coating layer may, in particular, comprise silicon dioxide, silicon nitride, or silicon oxynitride formed, for example, by a chemical vapor deposition technique at subatmospheric pressure. Regardless of its nature, the coating layer may have a thickness of 20 nm or more to contain a sufficient quantity of hydrogen.

[0074] By introducing hydrogen into the first layer 3 before applying the finishing sequence to this layer, it is possible to reduce the thickness of the multi-domain surface portion revealed by this sequence. In particular, it is possible to limit this thickness to less than 125 nm.

[0075] Of course the invention is not limited to the modes of implementation described and alternative embodiments can be made without departing from the scope of the invention as defined by the claims.

[0076] In particular, a treatment of the free face 8 can be applied, between the fracture stage and the finishing sequence, combining the three approaches shown in the illustration and according to any possible combination.

[0077] The treatment of the free face of the sampled layer 3 can also correspond to any other treatment of this face leading to the introduction of a sufficient dose of hydrogen into this layer.

Claims

Claims

1. Method for preparing a single-domain thin layer (3') of ferroelectric material, the method comprising: • a step of implanting so-called "light" species in a first face (4) of a ferroelectric donor substrate (1) to form a weakening plane (2) and define a first layer (3) between the weakening plane (2) and the first face (4) of the donor substrate (1); • a step of assembling the first face (4) of the donor substrate (1) to a support (7) to form an intermediate assembly; • a step of fracturing the intermediate assembly, comprising a first heat treatment, this step leading to the fracture of the donor substrate (1) at the weakening plane (2) and to the formation of a free face (8) of the first layer (3);• a finishing sequence of the first layer (3) comprising an annealing step including a second heat treatment and, after the annealing step, a step of thinning the first layer (3) to form the single-domain thin layer (3'); the preparation method being characterized in that it comprises, between the fracture step and the finishing sequence, the application of a treatment of the free face (8) to produce a hydrogen concentration greater than 2.0 E21 at / cmA3 in a surface thickness of the first layer (3).;

2. Preparation process according to the preceding claim wherein the surface thickness, in which the hydrogen concentration is greater than 2.0 E21 at / cmA3, is greater than or equal to 100 nm.

3. Preparation process according to the preceding claim in which the surface thickness is greater than or equal to 200 nm.

4. Preparation method according to one of the preceding claims in which the treatment of the free face (8) introduces a dose of hydrogen greater than or equal to 2.0 E16 at / cmA2 into the first layer (3).

5. Preparation process according to one of the preceding claims in wherein the treatment of the free face (8) comprises immersing the first layer (3) in a first solution having a temperature higher than ambient temperature.

6. Preparation process according to the preceding claim in which the first solution comprises SCI and / or SC2.

7. Preparation process according to one of the two preceding claims in which the temperature of the first solution is greater than 50°C, preferably greater than 65°C.

8. Preparation method according to one of the preceding claims in which the treatment of the free face (8) comprises a sequence of cleaning steps using a second solution, the cleaning steps of the sequence being separated from each other by a waiting time greater than or equal to 24 hours.

9. Preparation method according to the preceding claim in which the second solution comprises deionized water, the cleaning steps comprising simultaneously brushing and dispensing deionized water onto the free face (8) of the first layer (3).

10. Preparation process according to one of the two preceding claims in which the second solution is at room temperature.

11. Preparation method according to one of the three preceding claims in which the sequence of cleaning steps comprises at least three cleaning steps, preferably at least five cleaning steps.

12. Preparation method according to one of the preceding claims in which the treatment of the free face (8) comprises the deposition on the free face (8) of a covering layer having a hydrogen concentration greater than 1.0 E20 at / cmA3.

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

14. A preparation method according to the preceding claim wherein the covering layer has a thickness of 20nm or more.

15. Preparation method according to one of the preceding claims in which the single-domain thin layer (3) is made of a single-crystalline piezoelectric material, such as lithium tantalate or lithium niobate.

16. Preparation method according to the preceding claim in which the monodomain thin layer (3) is made of lithium niobate.

17. Preparation method according to one of the preceding claims in which the assembly step comprises the formation of an intercalary dielectric layer (3) on the first face of the donor substrate (1) and / or on the support (7).

Images