Method for manufacturing a ferroelectric layer transferred onto a substrate and for polarization with improved homogeneity

Abstract

A method for manufacturing a composite structure (EndStruct) comprising a negatively polarized ferroelectric layer (Ferrolay) (P-138) resting on a support set (Sprt.Set), comprising the steps of providing a starting composite structure (StartStruct) comprising a positively polarized ferroelectric layer (Ferrolay) (P42) resting on the support set (Sprt.Set); and of implanting hydrogen ions (H+) into the starting composite structure (StartStruct); and then applying a heat treatment to the starting composite structure (StartStruct) so as to reverse the polarization (P42) of the ferroelectric layer (Ferrolay) of the starting composite structure (StartStruct). Figure to be published with the abstract: Fig. 2

Description

Title of the invention: Method for manufacturing a ferroelectric layer transferred onto a substrate and for improved homogeneity polarization TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to a method for manufacturing a ferroelectric layer transferred onto a substrate and polarized with improved homogeneity throughout its thickness. Such a structure can be used to form, for example, radio frequency (RF) components, in particular elastic wave components, especially surface acoustic wave (SAW) or body acoustic wave (BAW) components. TECHNOLOGICAL BACKGROUND

[0002] Several types of applications utilize or are influenced by the polarization properties of ferroelectric materials and the presence of opposing polarization domains. Examples include electroacoustic devices such as surface acoustic wave (SAW) devices and bulk acoustic wave (BAW) devices. The existence of these applications has motivated the development of methods for controlling the polarization domains of ferroelectric layers.

[0003] The article ““Seeing Is Believing”—In-Depth Analysis by Co-Imaging of Periodically-Poled X-Cut Lithium Niobate Thin Films”, by Sven Reitzig et al., published in Crystals 2021, 11, 288, describes a lithium niobate layer integrated onto a silicon substrate via a silica layer, and the control of the polarization of this layer parallel to the plane in which it extends, by applying a voltage between electrodes arranged periodically on the free surface of the layer. It should be noted that, in such a structure with electrodes only on the free face of the crystal, controlling a polarization that would be perpendicular to the substrate by means of an electric field, in the absence of a buried electrode, would require voltages that could cause the silica layer to break down.

[0004] Document EP 0 592 226 A1 describes an optical frequency conversion device obtained by the periodic juxtaposition of parallel bands of inverted polarization on one face of a ferroelectric substrate having a spontaneous polarization perpendicular to the plane of extension of the substrate, i.e., perpendicular to this face. The polarization inversion along the bands is achieved by performing a proton exchange through a mask.

[0005] Document WO 2005 / 052682 Al describes the localized reversal of polarization of a ferroelectric crystal with spontaneous polarization perpendicular to one of its faces, by application of an electric field according to juxtaposed periodic bands, by means of gel electrodes arranged on this face and the opposite face of the crystal.

[0006] The structures and methods presented above, while they do allow control of the polarization domains of a ferroelectric layer at its surface, remain impractical and do not allow control of the polarization in the thickness of this layer or its integration on a support.

[0007] In response to these shortcomings, international patent application WO 2020 / 200986 A1 and French patent applications FR2301220 and FR2301221 propose techniques for transferring ferroelectric layers onto a substrate. However, these techniques do not solve the problem entirely satisfactorily, in that, at the end of the manufacturing process, the transferred ferroelectric layers may exhibit uncontrolled polarization inhomogeneities within the layer thickness, particularly with regard to the transfer of negatively polarized ferroelectric layers onto the final substrate. Description of the invention

[0008] An object of the invention is to provide a method for manufacturing a ferroelectric layer transferred onto a substrate, the method correcting or preventing any inhomogeneities of polarization which could appear in their thickness, in particular for the situation where one seeks to obtain a ferroelectric layer of negative polarization, that is to say oriented towards the support of the ferroelectric layer.

[0009] For the purpose of producing these objects, one aspect of the invention is a method for manufacturing a composite structure comprising a negatively polarized ferroelectric layer resting on a support assembly, comprising the steps of providing a starting composite structure comprising a positively polarized ferroelectric layer resting on the support assembly; and implanting hydrogen ions into the starting composite structure and then applying a heat treatment to the starting composite structure so as to reverse the polarization of the ferroelectric layer of the starting composite structure.

[0010] An advantage of the method according to the invention is its ability to produce a structure comprising a ferroelectric layer transferred onto a substrate and with a negative polarization, oriented towards the interface between the ferroelectric layer and its support, exhibiting improved homogeneity over its thickness compared to known manufacturing methods. Such a structure facilitates, for example, the integration of volume acoustic functions while maintaining a low cost and enabling a high level of performance.

[0011] According to additional non-limiting features of the invention, considered individually or in any technically feasible combination:

[0012] - hydrogen ions can be implanted in such a way that the structure the starting composite has a concentration of hydrogen atoms between 10 19 and 1022 at / cm3;

[0013] - the applied heat treatment can be parameterized to bring the ferro- layer electric at a temperature between 300°C and the Curie temperature of this ferroelectric layer, and preferably greater than or equal to 450°C, 500° or 550° and less than 600°C;

[0014] - the ferroelectric layer can extend in a horizontal direction and the polarization positive orientation of the initial composite structure can make an angle within an angular range from 20° to 160° with respect to this horizontal direction;

[0015] - the ferroelectric layer can be a lithium niobate or tantalate layer lithium;

[0016] - the process may further include a step of polishing the ferro- layer electric;

[0017] - the process may further include obtaining the starting structure by means of of a process which may include the steps of providing the support assembly; of providing a donor substrate of single-crystal ferroelectric material having a negative polarization with respect to a face of the donor substrate; of forming a weakening plane in the donor substrate; of assembling the donor substrate to the support assembly by bringing the face into contact with the support assembly; and of detaching a portion of the donor substrate at the level of the weakening plane so as to leave the ferroelectric layer fixed to the support assembly and obtain the starting structure;

[0018] - the process may further include obtaining the starting structure by means of of a process comprising the steps of providing the support assembly; providing a donor substrate of single-crystal ferroelectric material having a positive polarization with respect to one face of this wafer; forming a weakening plane in the donor substrate by implanting helium through the face; assembling the donor substrate to the support assembly by bringing the face into contact with the support assembly; detaching a portion of the donor substrate at the weakening plane so as to leave the ferroelectric layer fixed to the support assembly to obtain an intermediate structure; and applying a heat treatment to the intermediate structure so as to reverse the polarization of the ferroelectric layer and obtain the starting structure; and

[0019] - the heat treatment applied to the structure can be parameterized to bring the ferroelectric layer at a temperature between 300°C and the Curie temperature of this ferroelectric layer, and preferably greater than or equal to 450°C, 500° or 550° and less than 600°C. BRIEF DESCRIPTION OF THE FIGURES

[0020] Other features and advantages of the invention will become apparent from the detailed description of the invention which follows with reference to the accompanying figures in which:

[0021] [Fig.1] Fig.1 illustrates crystal sections of single crystals;

[0022] [Fig.2] Figure [Fig.2] represents the reversal of the polarization of a structure of starting point comprising a ferroelectric layer transferred onto a support and of positive polarization;

[0023] [Fig.3] The [Fig.3] is a diagram of a method for correcting the polarization of a ferroelectric material by means of an ion implantation, as illustrated by the [Fig.2];

[0024] [Fig.4] The [Fig.4] is a diagram of a first manufacturing process of the starting structure of the [Fig.2];

[0025] [Fig.5] Fig.5 illustrates the first manufacturing process;

[0026] [Fig.6] Figure 6 illustrates the absence of influence of hydrogen concentration implanted on the polarization of a ferroelectric layer;

[0027] [Fig.7] The [Fig.7] is a diagram of a second manufacturing process of the starting structure of the [Fig.2];

[0028] [Fig.8] Fig.8 illustrates the second manufacturing process;

[0029] [Fig.9] The [Fig.9] illustrates an angular range of polarizations. DETAILED DESCRIPTION OF THE INVENTION First embodiment of the invention

[0030] Depending on the piezoelectric device to be produced, it is necessary to choose an orientation for the polarization of the active layer of piezoelectric material, which consists of a layer of single-crystal ferroelectric material.

[0031] This ferroelectric material can be obtained by crystal growth using the so-called "Czochralski" method, which produces a solid single-crystal material in the form of an elongated cylinder, referred to as a "ball" or "ingot," by drawing a crystal seed of chosen orientation immersed in a molten material corresponding to the crystal to be formed, along a specific drawing direction. The single crystal grows along this drawing direction. Plates of the ferroelectric material are then obtained by cutting the ingot at a predetermined angle relative to its growth direction, and subsequently prepared and integrated as a ferroelectric layer in, for example, a piezoelectric device.

[0032] The polarization orientation of the layer depends on the crystalline orientation of the cut wafer, which does not generally correspond to the growth direction of the single crystal. Of course, a person skilled in the art knows how to choose the seed crystal. more suitable and closer to the desired crystallographic orientation for ingot casting in order to reduce material losses during cutting and machining into circular wafers.

[0033] Figure 1 illustrates in (A) and (B) the orientations of LiTaO3 layers designated 42RY and -138RY, respectively. These designations are derived from the identification of the crystallographic orientation of these layers, corresponding to rotations of 42° and -138° of the y and z axes with respect to the counterclockwise direction around the x-axis, giving the x', y', and z' axes. The x, y, and z axes correspond to the proper crystallographic axes of the single crystal, and the y' axis is aligned with the direction normal to the cut and machined surface. More specifically, the x, y, and z axes correspond to the

[1120] ,

[0110] , and

[0001] axes, respectively.

[0034] After drawing along a chosen direction, it is known to those skilled in the art to achieve polarization by applying an electric field in the z direction when passing into temperature below the Curie temperature to determine the direction of polarization.

[0035] Conventionally, a so-called negative polarization is a polarization with an orientation directed towards the interior of the material or structure concerned, or in other words, the projection of the z-axis onto the y'-axis is negative.

[0036] Conventionally, for a ferroelectric layer fixed to a substrate, a layer whose polarization orientation is directed towards the free surface of the layer will be designated as a positive polarization layer.

[0037] Conversely, still for a ferroelectric layer fixed to a substrate, a layer whose polarization orientation is directed inwards, that is to say towards the interface between the layer and its support, will be designated as a negative polarization layer.

[0038] Figure 2 illustrates in (A) a situation in which a ferroelectric layer Ferroiay of LiTaO3 RY42 exhibits a positive P42 polarization: it is oriented towards the free surface, opposite its substrate, or support assembly SprtSet, the latter here consisting of an intermediate layer Int on a support layer Sprt. It can also be said that the component of the polarization along the vertical direction Vert, perpendicular to the ferroelectric layer and oriented from the support assembly towards the ferroelectric layer, is positive. This component is obtained by a normal projection of the polarization onto the Vert direction.

[0039] To obtain a piezoelectric device with a negatively polarized ferroelectric layer, the characteristics of the ferroelectric material wafer used to manufacture the device must be chosen in advance, and, if necessary, the polarization orientation must be corrected.

[0040] Figure 2 illustrates the general principle of a manufacturing process for the structure EndStruct illustrated in (B) and obtained by reversing the polarization of the StartStruct structure illustrated in (A).

[0041] The characteristics of the support assembly can affect the acoustic properties of the Ferroiay layer, which is important in the case of a structure forming part of an acoustic device. The nature and thickness of these layers can therefore also be decisive in achieving the desired processing of an electrical signal, or at least influence this processing. In the example shown in [Fig. 2], the support assembly can include the Sprt support layer and the Int intermediate dielectric assembly layer, preferably comprising an oxide in direct contact with the Ferroiay ferroelectric thin layer.

[0042] For reasons of availability and cost, the Sprt support layer can be made of silicon. It may be a support consisting of a solid base substrate of monocrystalline silicon, but the invention is not limited to this support, which can, more generally, be made of any material, for example silicon, even an electrically insulating material such as sapphire or glass. The Sprt support layer, when formed from a solid substrate, typically has a thickness of several hundred microns. The density of electrical charges, holes or electrons, which are likely to move within the support layer and which could affect the proper functioning of an RF component formed on the basis of the Struct structure, can be limited.The Sprt support layer can thus be made of a high-resistivity silicon substrate, i.e., one with a resistivity greater than 1000 ohm-centimeters, and even more preferably greater than 3000 ohm-centimeters. To enhance the resistive nature of the Sprt support layer, a charge-trapping layer can be added to the ferroelectric layer, for example, one made of polycrystalline silicon. This charge-trapping layer can, of course, be formed using techniques other than polycrystalline silicon. This layer can also include carbon, or be made of or comprise silicon carbide or a silicon-carbon alloy.Alternatively, for example, electrical traps could be created by bombarding a surface area of ​​the support layer with relatively heavy ions (e.g., argon) to create crystalline defects capable of trapping electrical charges. A charge-trapping layer made of a porous material could also be used, for example, by porosifying a surface area of ​​the support layer when it is made of silicon. However, the invention is not limited to a support layer with such characteristics.

[0043] By way of example, the intermediate layer Int may be made of silicon oxide, silicon nitride, or be formed from a stack of layers composed of these materials.

[0044] Alternatively, the intermediate layer can be an electrically conductive metallic layer interposed between the ferroelectric layer Ferroiay and the support layer Sprt. In this example, the metallic layer Int is in direct contact with both the support Sprt and the ferroelectric layer Ferroiay. The Int layer can then be used as a buried electrode to apply an electric field to the ferroelectric layer Ferroiay.

[0045] The presence of an intermediate Int layer is only an option. Not all applications require the presence of such a layer.

[0046] Conventionally, the StartStruct structure and the EndStruct structure can be in the form of a circular plate whose diameter can be 100, 200, 300 or even 450 mm, but the invention is in no way limited to these dimensions or this shape.

[0047] The Ferroiay ferroelectric layer can be made of a single-crystal ferroelectric material, such as lithium tantalate (LiTaO3) or lithium niobate (LiNbO3), or materials such as BaTiO3, PbZrTiO3, KNbO3, BaZrO3, PbTiO3, or KTaO3. These materials also exhibit piezoelectric properties. Generally, the ferroelectric layer can have a thickness between 10 nanometers and 10 microns, depending on the intended application of the EndStruct structure and the expected performance of the components, but the invention does not preclude the use of different thicknesses, again depending on the intended application. It should be noted 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 greater than the coercive field of the material.As illustrated in this document, the ferroelectric layer preferably exhibits a single-domain polarization, meaning that all dipole moments are aligned parallel to each other along a given direction. Here, the given direction is inclined with respect to the plane of the ferroelectric layer, that is, inclined with respect to the free face of this layer.

[0048] With reference to Figures 2 and 3, the EndStruct structure illustrated in (B) can be obtained by reversing the polarization of the StartStruct structure illustrated in (A) of [Fig.2], according to a manufacturing process 100. The manufacturing process, summarized by the diagram in [Fig.3] comprises the steps S10, S120, S130 and S140, carried out in that order.

[0049] Step SI 10 is a step for providing a starting structure StartStruct consisting of a support layer Sprt, an intermediate layer Int, and a ferroelectric layer Ferroiay as defined above. In this example, the Ferroiay layer is a LiTaO3 RY42 layer exhibiting a positive P42 polarization. [Further details omitted] Generally the layer could be made of another ferroelectric material having a different polarization as long as that polarization is positive.

[0050] The S120 step is a hydrogen ion implantation step (H+) into the StartStruct structure via the free face of the Ferroiay layer. The implantation energy is chosen such that the projected path RP of the implanted hydrogen ions is such that the depth D(RPMax) of its maximum RPMax reaches or exceeds the depth of the interface between the Ferrolay layer and the Int layer, and is preferably located in the Int layer, or in the Ferroiay layer, or even near the Int layer. This situation is schematically illustrated in (A') of [Fig. 2], which represents the hydrogen concentration [H] as a function of the depth Dpth in the structure considered from the free surface of the Ferroiay layer, in particular for a configuration for which the depth D(RPMax) of the maximum RPMax exceeds the depth of the interface between the Ferrolay layer and the Int layer.The distance between the interface depth and the maximum ion depth (D(RPMax)) can be chosen to be approximately 200 nm to avoid excessively deep implantation in the oxide. In another embodiment, this distance can reach or exceed 500 nm; the majority of the ions and damage are then located entirely within the Int layer, which can be advantageous. In yet another embodiment, the maximum ion depth (D(RPMax)) can be located approximately 50 to 100 nm from the interface, but this time within the Ferrolay layer, thus leaving both the interface and the Int layer intact and free from implantation damage.

[0051] The resulting hydrogen ion distribution, with a hydrogen concentration gradient, generates an electric field EH directed towards the maximum hydrogen concentration. A sufficient gradient can be obtained, for example, by hydrogen doping resulting in a hydrogen concentration exceeding a threshold of 10¹⁹ hydrogen atoms per cubic centimeter, for example with a hydrogen concentration between 10¹⁹ and 10²² atoms / cm³ at a plane parallel to the free surface of the Ferroiay layer.

[0052] Step S130 is a heat treatment step for the structure. This heat treatment is intended to bring the ferroelectric layer to a temperature between 300°C and the Curie temperature of the ferroelectric material of the Ferroiay layer (and preferably greater than or equal to 450°C, 500°C, or 550°C, up to 600°C) for a duration of between 30 minutes and 10 hours. This heat treatment is preferably carried out by exposing the free face of the dielectric layer to an oxidizing or neutral gaseous atmosphere.

[0053] The presence of the electric field EH combined with heat treatment at a temperature of approximately 300° to 600°C causes a reversal of the polarization of the ferroelectric material due to thermal activation caused by the temperature increase. More specifically, during heat treatment, the polarization of the Ferroelectric material tends to align with the electric field (EH), thus reversing the polarization when EH is oriented in the opposite direction to that of the material before heat treatment. This is precisely the case here, with a positive polarization and an electric field (EH) directed from the free surface of the Ferroiay layer towards its interface with the Int layer (in fact, towards the depth plane D(RPMax), as illustrated in (A')).

[0054] In the case of a P42 polarization of the ferroelectric layer, the reversal of the polarization will lead to the appearance of a P_B8 polarization, opposite to the initial P42 polarization.

[0055] Step S140 is a polishing step of the free face of the Ferroiay ferroelectric layer, for example by mechanical, chemical-mechanical, and / or chemical etching techniques. This polishing prepares the free face so that (i) it has a low roughness, for example less than 0.5 nm RMS 5x5 pm by atomic force measurement (AFM), and (ii) a surface layer of the Ferroiay layer is removed, which may exhibit inhomogeneous polarization and / or an oxide layer due to surface phenomena. For example, a removal thickness of 50 nm can be planned. This improves the homogeneity of the Ferroiay layer and its polarization.

[0056] Following step S140, the desired EndStruct structure was obtained, which is ready to be integrated into the manufacturing process of a complex Dev component such as a surface acoustic wave (SAW) or volume acoustic wave (BAW) component.

[0057] Obtaining the starting structure StartStruct

[0058] Figures 4 to 8 illustrate two alternative processes 1 and 2 of obtaining the starting structure StartStrcut of process 100.

[0059] Alternative method 1

[0060] Figures 4 to 6 illustrate the alternative process 1 for obtaining the StartStruct structure, process 200 summarized by the diagram in [Fig.4] and comprising the following steps.

[0061] A step S210 of preparing a SprtSet support assembly, here consisting of the Sprt support layer provided with the Int intermediate layer on its surface as illustrated in (A) of [Fig.5].

[0062] A step S220 of selecting and supplying a wafer of single-crystal ferroelectric material for its chemical composition and its crystal orientation relative to its extension plane, so that it may serve as a ferroelectric donor substrate Ferrosub. It is in this wafer that the Ferroiay layer will be formed. The selected wafer has a single-domain negative polarization, that is to say, having a component oriented opposite to the direction of its face designated by Top, i.e., the wafer has a crystal orientation between 0RY and - 180RY, as illustrated by the P_n8 polarization in (B) of [Fig.6]. This is a polarization opposite in direction to that of the polarization of the desired structure EndStruct.

[0063] A step S240 prepares the ferroelectric donor substrate Ferrosub to form a weakening plane Frgl for the purpose of separating the ferroelectric layer Ferroiay from the donor substrate Ferrosub in a subsequent step, as illustrated in (B). The depth of the weakening plane Fgrl in the donor substrate Ferrosub defines the thickness of the ferroelectric layer Ferroiay. Obtaining this plane will be detailed below.

[0064] An S250 step of assembling the SprtSet support assembly with the Ferrosub donor substrate, by bringing the free face Fr.Fac of the intermediate layer Int into contact with the Top face of the ferroelectric donor substrate Ferrosub. The intermediate structure Structinter thus formed is illustrated in (C), with the intermediate layer Int interposed between the Sprt support layer and the ferroelectric donor substrate Ferrosub. The orientation of the polarity of the ferroelectric donor substrate with respect to the support layer is reversed, changing from P_[38 to P42 due to the inversion of the Ferrosub donor substrate for assembly.

[0065] A step S260 involves detaching a portion of the donor substrate Ferrosub from the intermediate structure Structinter at the embrittlement plane, leaving the ferroelectric layer Ferroiay fixed to the support Sprt and yielding the StartStruct structure as illustrated in (D). The ferroelectric layer Ferroiay exhibits the positive polarization P42 with respect to the Vert normal of the free surface that served as the detachment interface.

[0066] A step 270 of applying a stabilizing heat treatment to the StartStruct structure. The stabilizing heat treatment heals crystalline defects present in the ferroelectric layer and helps to consolidate the bond between this ferroelectric layer (Ferroiay) and the intermediate layer (Int). In the case of LiTaO3, this heat treatment is designed to bring the ferroelectric layer to a temperature between 300°C and the Curie temperature of the ferroelectric material (and preferably greater than or equal to 450°C, 500°C, or 550°C, up to 600°C) for a duration of between 30 minutes and 10 hours. This heat treatment is preferably carried out by exposing the free face of the dielectric layer to an oxidizing or neutral gaseous atmosphere.

[0067] It should be noted that in the present case, the stabilizing heat treatment will not cause a reversal of the polarization of the Ferroiay layer. Indeed, as illustrated by [Fig. 6], which represents the StartStruct structure after detachment, the hydrogen ions used to form the embrittling layer Frgl exhibit a concentration gradient [H] with a maximum value at the plane of The concentration gradient weakens and decreases continuously with increasing distance from this plane, i.e., from the free face of the Ferroiay layer. The electric field generated by this concentration gradient is therefore directed towards the free surface of the Ferroiay layer. Thus, even if a heat treatment is applied that could cause a polarization reversal, since the existing polarization and the electric field are both already oriented towards the free surface of the Ferroiay layer, the polarization will remain unchanged. Therefore, the PreStabStruct structure, which has not yet undergone the stabilizing heat treatment, can be considered the initial StartStruct structure.

[0068] At a polishing step S280, a mechano-chemical polishing step, similar to step S140 of process 100, is preferably applied to the free surface of the Ferroiay layer, which has been rough since the detachment step S260. However, considering that the StartStruct structure is in any case intended to be subjected to polishing at step S140 of process 100, this step S270 remains optional.

[0069] Following steps S210 to S270 and, where applicable, step S280, the starting structure StartStruct of process 100 is obtained.

[0070] The Ferrosub donor substrate illustrated in (B) is a substrate made of the ferroelectric material of the Ferroiay ferroelectric layer. Alternatively, it could comprise a surface layer of this material. Thus, the donor substrate could, for example, be formed from a solid substrate of lithium tantalate or lithium niobate, or from a composite substrate consisting of a primary substrate on which rests a layer (at least equal to that of the Ferroiay layer) of lithium tantalate or lithium niobate. The use of a composite substrate comprising a support substrate and a layer of ferroelectric material is necessary if the difference in the coefficient of thermal expansion of the ferroelectric material and the final substrate is too great to allow the application of the Smart Cut process. This approach is described in detail in documents WO2019002080 and WO2019186032, which are incorporated by reference.

[0071] The donor substrate therefore comprises at least one layer of ferroelectric material having a first positive single-domain polarization P_n8 inclined with respect to the plane of extension of the ferroelectric layer Ferroiay, which depends on the chosen crystal orientation of the wafer. This layer is intended to be attached to the intermediate layer Int before being separated from the donor substrate.

[0072] Fig. 5 illustrates a situation in which the ferroelectric layer comes from a LiTaO3 cut of the -138RY type, with the orientation illustrated in (B) of Fig. 1.

[0073] More generally, process 200 is applicable to a crystal cut within the range extending from 0RY to -180RY, preferably -20RY to -160RY, for the Ferrosub donor substrate. The polarization of the resulting StartStruct structure is therefore positive, that is to say oriented from 0° to 180°, preferably within an angular range [Ang] from 20° to 160.

[0074] The ferroelectric layer Ferroiay can be transferred from the ferroelectric donor substrate Ferrosub by implementing Smart Cut™ technology, in which case the donor substrate must be prepared by introducing light species such as hydrogen or helium into this donor substrate. This introduction can correspond to hydrogen implantation, i.e., hydrogen ion bombardment of the lmp face of the Ferrosub donor substrate. In a manner known per se, and as illustrated in (B), the implanted hydrogen ions H+ are intended to form a weakening plane Frgl delimiting the ferroelectric layer Ferroiay from the ferroelectric material to be transferred, which is located on the lmp face side, and another part Ferrosep forming the remainder of the substrate, which will be separated from the ferroelectric layer Ferroiay in a subsequent step.

[0075] The nature and dose of the implanted species and the implantation energy are chosen according to the thickness of the layer to be transferred and the physicochemical properties of the Ferrosub donor substrate. In the case of a LiTaO3 donor substrate, a hydrogen dose of between 10¹⁶ and 5 x 10¹⁷ at / cm² can be implanted with an energy between 30 and 300 keV to delimit a Ferroiay ferroelectric layer on the order of 200 to 2000 nm thick.

[0076] In (C) of [Fig.5], the P42 polarization is oriented opposite to the interface between the ferroelectric layer Ferroiay and the intermediate layer Int, so that the polarization of the ferroelectric layer is considered to be positive.

[0077] The Sprt support substrate may have the same dimensions and shape as the Ferrosub donor substrate, but the invention is not limited to such a configuration, and different dimensions, shapes, and configurations may be used. Prior to assembly, the surfaces of the substrates to be assembled may be prepared by a cleaning, brushing, drying, polishing, or plasma activation step.

[0078] The assembly may correspond to the intimate contact of the Ferrosub donor substrate with the Sprt support by molecular adhesion and / or electrostatic bonding, as mentioned for example in the French patent application published under No. 2 914 492.

[0079] As is well known, during a molecular adhesion process, the exposed surfaces of the Sprt support and the Ferrosub donor substrate, which are perfectly clean, flat, and smooth, are brought into intimate contact to promote electrostatic bonding or the development of molecular bonds, for example, van der Waals or covalent bonds. The two bodies are then joined without the use of an adhesive.

[0080] The assembly may include the application of a low-temperature heat treatment temperature (for example between 50 and 300°C, typically 100°C) allowing to heal crystalline defects present in the ferroelectric layer and to sufficiently strengthen the bonding energy to allow a possible subsequent thinning step.

[0081] In this embodiment, the step of detaching a portion of the donor substrate is performed by applying Smart Cut™ technology, according to which a layer intended to form the ferroelectric layer Ferroiay is delimited by the embrittlement plane Frgl defined by the implantation of hydrogen ions in the donor substrate, as illustrated in (B) of [Fig. 2]. After the assembly step, this layer is detached from the donor substrate by fracture at the embrittlement plane Frgl and thus transferred to the Sprt support, as illustrated in (D) of [Fig. 2].

[0082] This detachment step may thus include applying heat treatment to the intermediate structure Structinter in a temperature range of approximately 80°C to 300°C to allow the detachment of the donor portion of the ferroelectric layer Ferrolay from the substrate and thus complete its transfer to the entire support structure. As an alternative to or in addition to heat treatment, this step may include the application of a blade or jet of gaseous or liquid fluid, or any other mechanical force, to the embrittlement plane Frgl.

[0083] Alternative method 2

[0084] Figures 7 and 8 illustrate the alternative process 2 for obtaining the StartStruct structure, process 300 summarized by the diagram in [Fig.7].

[0085] Alternative process 2 is very close to alternative process 1, with steps S310 to S380 substantially identical to steps S210 to S280, respectively, apart from the points detailed below.

[0086] At step S320, instead of choosing a negative polarization donor substrate as in step S220, a positive polarization donor substrate, P42 on [Fig.8], is chosen.

[0087] At step S340 of formation of the embrittlement plane Frgl, instead of implanting hydrogen ions as in step S240, helium ions He+ will be implanted this time to form this plane.

[0088] The Ferroiay layer of the intermediate Structinter structure obtained in step S350 exhibits a negative polarization P_B8, opposite to the positive polarization P42 due to the inversion of the donor substrate for its fixation onto the Sprt.Set support assembly. Thus, step S360, which separates a portion of the donor substrate Ferrosep to leave the Ferroiay layer on the support assembly, results in a PreStabStruct structure. This PreStabStruct structure is distinct from the desired StartStruct structure in that the polarization of the Ferroiay layer is oriented opposite to the one desired, as illustrated in (D) of [Fig.8].

[0089] However, as illustrated in (D') of [Fig. 8], the helium concentration [He] in the Ferroiay layer exhibits a gradient with a maximum concentration at the free surface of the layer, as illustrated by [Fig. 8], which represents the PreStabStruct structure obtained immediately after separation. The helium ions used to form the embrittling layer Frgl exhibit a concentration gradient [He] with a maximum value at the embrittling plane and decreasing continuously with distance from this plane, i.e., from the free face of the Ferroiay layer. The electric field generated by this concentration gradient is therefore directed towards the free surface of the Ferroiay layer, opposite to the P_B8 polarization.

[0090] Thus, applying a stabilizing heat treatment at step S370, according to parameters that may be those described in relation to step S130, results in a reversal of the polarization P_n8 to give the polarization P42, for the reasons explained above in relation to step S130. Indeed, the concentration of helium implanted to form the embrittlement plane is sufficient to generate an electric field EHe that reverses the polarization during the application of the heat treatment. In this way, the PreStabStruct structure is modified to give the desired StartStruct structure. In other words, in alternative process 2, the heat treatment step S370 following the separation step is necessary to obtain the StartStruct structure, unlike the heat treatment step S270 of alternative process 1, where it is only optional and does not influence the polarization of the Ferroiay layer.

[0091] Of course, a polishing step S380 similar to step S280 can be applied to smooth the free surface of the Ferroiay layer.

[0092] As explained above, each of the two alternative processes 1 and 2, the first employing a negative polarity donor substrate and a hydrogen implantation defining a weakening plane, and the second employing a positive polarity donor substrate and a helium implantation defining a weakening plane, makes it possible to obtain the starting structure StartStruct of the process 100 of obtaining a composite structure comprising a ferroelectric layer of negative polarity and homogeneous in the thickness of the layer.

[0093] Although illustrated here for a 42RY crystal cut layer, the process 100 is more generally applicable to crystal cut layers in the range from 0RY to 180RY, preferably 20RY to 160RY, for the ferroelectric layer Ferroiay of the StartStruct. In other words, considering that the ferroelectric layer extends along a horizontal direction Hnz perpendicular to the vertical direction Vert normal to the layer, the positive polarization P42 of the StartStruct makes an angle within an angular range [Ang] from 20° to 160° with respect to this horizontal direction Hnz, as illustrated by [Fig. 9]. The polarization of the final EndStruct structure is negative, i.e., oriented from 0° to -180°, preferably within an angular range of -20° to -160°, relative to the plane of the Ferroiay ferroelectric layer. Of course, processes 200 and 300 can be adapted to select suitable crystal sections to obtain the StartStruct structure exhibiting the desired polarization of the Ferroiay ferroelectric layer.

[0094] In this document, the figures are not necessarily to scale. Certain features and components may be shown exaggerated in relation to other components or in a somewhat schematic form, and certain details of conventional elements may not be shown in the interest of clarity and conciseness.

[0095] 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.

Claims

Demands

1. A method (100) for manufacturing a composite structure (EndStruct) comprising a negatively polarized ferroelectric layer (Ferroiay) (P.i38) resting on a support set (Sprt.Set), comprising the steps of: - providing (SI 10) a starting composite structure (StartStruct) comprising a positively polarized ferroelectric layer (Ferroiay) (P42) resting on the support set (Sprt.Set); and - implanting (S 120) hydrogen ions (H+) into the starting composite structure (StartStruct) and then applying (S 130) a heat treatment to the starting composite structure (StartStruct) so as to reverse the polarization (P42) of the ferroelectric layer (Ferroiay) of the starting composite structure (StartStruct).

2. A method (100) according to claim 1, wherein the hydrogen ions are implanted so that the starting composite structure has a hydrogen atom concentration between 1019 and 1022 at / cm3.

3. The process according to claim 1 or 2, wherein the applied heat treatment (S 130) is parameterized to bring the ferroelectric layer (Ferroiay) to a temperature between 300°C and the Curie temperature of this ferroelectric layer (Ferroiay), and preferably greater than or equal to 450°C, 500° or 550° and less than 600°C.

4. Method (100) according to any one of the preceding claims, wherein the ferroelectric layer extends along a horizontal direction (Hnz) and the positive polarization (P42) of the starting composite structure (StartStruct) makes an angle within an angular range ([Ang]) from 20° to 160° with respect to this horizontal direction (Hnz).

5. Method (100) according to any one of the preceding claims, wherein the ferroelectric layer (Ferroiay) is a lithium niobate or lithium tantalate layer.

6. Method (100) according to any one of the preceding claims, further comprising a step (S 140) of polishing the ferroelectric layer (Ferroiay).

7. A method (100) according to any one of the preceding claims 1 to 6, further comprising obtaining the starting structure (StartStruct) by means of a process (200) comprising the steps of: - providing (S 110) the support assembly (Sprt.Set); - providing (S220) a donor substrate (Ferrosub) of single-crystal ferroelectric material having a negative polarization (P_i38) with respect to a face (Top) of the donor substrate (Ferrosub); - forming (S240) a weakening plane in the donor substrate (Ferro sub); - assembling (S250) the donor substrate (Ferrosub) to the support assembly (Sprt.Set) by bringing the face (Top) into contact with the support assembly (Sprt.Set); and - detach (S260) part of the donor substrate (Ferrosub) at the level of the embrittlement plane so as to leave the ferroelectric layer (Ferro lay) fixed on the support assembly (Sprt.Set) and obtain the starting structure (StartStruct).

8. A method (100) according to any one of the preceding claims 1 to 6, further comprising obtaining the starting structure (StartStruct) by means of a method (300) comprising the steps of: - providing (3110) the support assembly (Sprt.Set); - providing (S320) a donor substrate (Ferrosub) of single-crystal ferroelectric material having a positive polarization (P42) with respect to a face (Top) of this wafer; - forming (S340) a weakening plane in the donor substrate (Ferro sub) by implanting helium (He+) through the face (Top); - assembling (S350) the donor substrate (Ferrosub) to the support assembly (Sprt.Set) by bringing the face (Top) into contact with the support assembly (Sprt.Set); - detach (S360) part of the donor substrate (Ferrosub) at the level of the embrittlement plane so as to leave the ferroelectric layer (Ferro lay) fixed on the support assembly (Sprt.Set) to obtain an intermediate structure (PreStabStruct); and - apply (S370) a heat treatment to the intermediate structure (PreStabStruct) so as to reverse the polarization (P.i38) of the ferroelectric layer (Ferrolay) and obtain the starting structure (StartStruct).

9. The process (100) according to claim 8, wherein the heat treatment applied (S370) to the structure (PreStabStruct) is parameterized to bring the ferroelectric layer (Ferroiay) to a temperature between 300°C and the Curie temperature of this ferroelectric layer (Ferroiay), and preferably greater than or equal to 450°C, 500° or 550° and less than 600°C.

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