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

Abstract

A method of manufacturing a composite structure (EndStruct) comprising a negatively polarized ferroelectric layer (Ferrolay) (P-138) resting on a support assembly (Sprt.Set), comprising the steps of providing a starting composite structure (StartStruct) comprising a positively polarized ferroelectric layer (Ferrolay) (P42) resting on the support assembly (Sprt.Set); and 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 polarization with improved homogeneity TECHNICAL FIELD OF THE INVENTION

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

[0002] There are several types of applications that take advantage of or are influenced by the polarization properties of ferroelectric materials and the presence of polarization domains opposite each other. Examples include electroacoustic devices such as surface acoustic wave devices (SAW devices) or bulk acoustic wave devices (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 on 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 comprising electrodes only on the free face of the crystal, the control of a polarization which would be perpendicular to the substrate by means of an electric field, in the absence of a buried electrode, would require voltages that risk causing the breakdown of the silica layer.

[0004] Document EP 0 592 226 A1 describes an optical frequency conversion device obtained by the periodic juxtaposition of parallel bands of inverted polarization at 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 inversion of the polarization according to the bands is obtained by carrying out a proton exchange through a mask.

[0005] Document WO 2005 / 052682 A1 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, if they effectively make it possible to control the polarization domains of a ferroelectric layer on its surface, remain impractical and do not allow the control of the polarization in the thickness of this layer or its integration on a support.

[0007] In response to these shortcomings, the international patent application with publication number WO 2020 / 200986 A1 and the patent applications in France with filing numbers FR2301220 and FR2301221 propose techniques for transferring ferroelectric layers onto a substrate. However, these techniques do not solve the problem in a completely satisfactory manner, in that, at the end of manufacturing, the transferred ferroelectric layers may exhibit uncontrolled polarization inhomogeneities in the thickness of the layer, in particular with regard to the transfer of ferroelectric layers having a negative polarization onto the final support substrate. Disclosure 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 polarization inhomogeneities that could appear in their thickness, in particular for the situation where it is sought to obtain a ferroelectric layer of negative polarization, that is to say oriented towards the support of the ferroelectric layer.

[0009] In order to achieve these objects, one aspect of the invention is a method of 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 including a ferroelectric layer transferred onto a substrate and with a negative polarization, oriented towards the interface between the ferroelectric layer and its support, having 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 allowing a high level of performance.

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

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

[0013] - the heat treatment applied can be set to bring the ferro- electrical 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 pole positive orientation of the starting composite structure can make an angle within an angular range of 20° to 160° relative to this horizontal direction;

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

[0016] - the method may further comprise a step of polishing the ferro-layer electric;

[0017] - the method may further comprise obtaining the starting structure by means of of a method which may comprise the steps of providing the support assembly; providing a donor substrate of single-crystal ferroelectric material having a negative bias relative to a donor substrate face; forming a weakening plane in the donor substrate; joining the donor substrate to the support assembly by contacting the face with the support assembly; and detaching a portion of the donor substrate at the weakening plane so as to leave the ferroelectric layer attached to the support assembly and obtain the starting structure;

[0018] - the method may further comprise obtaining the starting structure by means of of a method comprising the steps of providing the support assembly; providing a donor substrate of single-crystal ferroelectric material having a positive polarization relative to a 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 contacting the face with the support assembly; detaching a portion of the donor substrate at the weakening plane so as to leave the ferroelectric layer attached 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 set 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 below 600°C. BRIEF DESCRIPTION OF THE FIGURES

[0020] Other characteristics and advantages of the invention will emerge from the detailed description of the invention which follows with reference to the appended figures in which:

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

[0022] [Fig.2] [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] [Fig.3] is a diagram of a method for correcting the polarization of a ferroelectric material by means of ion implantation, as illustrated by [Fig.2];

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

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

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

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

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

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

[0030] Depending on the piezoelectric device that one wishes to produce, it is appropriate to choose an orientation for the polarization of the active layer of piezoelectric material, which is made up of a layer of monocrystalline ferroelectric material.

[0031] This ferroelectric material can be obtained by crystal growth according to the so-called "Czochralski" method which makes it possible to obtain a solid monocrystalline material in the form of an elongated cylinder called a "ball" or "ingot" by pulling a crystal seed of chosen orientation immersed in a molten material corresponding to the crystal to be formed, according to a pulling direction. The monocrystal grows according to this pulling direction. Platelets of the ferroelectric material are then obtained by cutting the ingot at a determined angle relative to its growth direction, then prepared and integrated as a ferroelectric layer in, for example, a piezoelectric device.

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

[0033] Figure 1 illustrates in (A) and (B) the orientations of LiTaO3 layers designated by 42RY and -138RY, respectively. These names derive from the identification of the crystallographic orientation of these layers, corresponding to respective rotations of 42° and -138° of the y and z axes with respect to the trigonometric direction around the x axis to give the x', y' and z' axes, the x, y and z axes corresponding to the crystallographic axes proper to the single crystal and the y' axis being 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 pulling in a chosen direction, it is known to those skilled in the art to carry out polarization by applying an electric field in the z direction when the temperature falls below the Curie temperature to determine the direction of polarization.

[0035] Conventionally, a so-called negative polarization is a polarization of 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, i.e. towards the interface between the layer and its support, will be designated as a negative polarization layer.

[0038] [Fig.2] illustrates in (A) a situation in which a ferroelectric layer Ferroiay of LiTaO3 RY42 has a positive polarization P42: 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. We can also say that the component of the polarization in 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 on the Vert direction.

[0039] To obtain a piezoelectric device provided with a negative polarization ferroelectric layer, it is necessary to choose in advance the characteristics of the ferroelectric material plate used to manufacture the device, and, if necessary, to correct the orientation of the polarization.

[0040] [Fig.2] illustrates the general principle of a manufacturing process of the structure EndStruct illustrated in (B) and obtained by inversion of 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 influencing this processing. In the example shown in [Fig.2], the support assembly can comprise the support layer Sprt, and the intermediate dielectric assembly layer Int, preferably comprising an oxide directly in contact with the thin ferroelectric Ferroiay layer.

[0042] For reasons of availability and cost, the support layer Sprt may be made of silicon. It may be a support consisting of a solid base substrate made of monocrystalline silicon, but the invention is not limited to this support which may, more generally, be made of any material, for example silicon, even electrically insulating such as sapphire or glass. The support layer Sprt, when it is formed of a solid substrate, typically has a thickness of several hundred microns. The density of electrical charges, holes or electrons, which are likely to move in the support layer and which could affect the proper functioning of an RF component which would be formed on the basis of the structure Struct, may be limited.The Sprt support layer can thus be made up of a high-resistivity silicon substrate, i.e. having a resistivity greater than 1000 ohm-centimeters, and more preferably still greater than 3000 ohm-centimeters. To support the resistive nature of the Sprt support layer, it is possible to provide the Sprt support layer with a charge trapping layer on the side of the ferroelectric layer, for example formed of polycrystalline silicon. It is of course possible to provide this charge trapping layer by a technique other than that providing a layer formed of polycrystalline silicon. This layer can also comprise carbon or be made of or comprise silicon carbide or an alloy of silicon and carbon.Alternatively, for example, it may involve producing the electrical traps by ion bombardment of relatively heavy species (for example argon) in a surface part of the support layer in order to create crystalline defects capable of trapping electrical charges. It is also possible to provide a charge trapping layer formed from a porous material, for example by porosification of a surface part of the support layer Sprt when the latter is made of silicon. However, the invention is not limited to a support layer having 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 may be an electrically conductive metal layer interposed between the ferroelectric layer Ferroiay and the support layer Sprt. In this example, the metal layer Int is in direct contact with each of the support Sprt and the ferroelectric layer Ferroiay. The layer Int may in this case subsequently be used as a buried electrode intended to apply an electric field to the ferroelectric layer Ferroiay.

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

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

[0047] The ferroelectric layer Ferroiay may be made of a monocrystalline ferroelectric material, such as lithium tantalate LiTaO3 or lithium niobate LiNbO3, or materials such as BaTiO3, PbZrTiO3, KNbO3, BaZrO3, PbTiO 3 or KTaO3. These materials also have piezoelectric properties. Generally speaking, the ferroelectric layer may have a thickness of 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 exclude the use of different thicknesses, always depending on the intended application. It is recalled that a ferroelectric material is a material which has an electric polarization in its natural state, a polarization which can be reversed by the application of an external electric field greater than the coercive field of the material.As illustrated in this paper, the ferroelectric layer preferably has a single-domain polarization, i.e., all dipole moments are aligned parallel to each other along a given direction. Here, the given direction is inclined relative to the plane of the ferroelectric layer, i.e., inclined relative 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 inverting the polarization of the StartStruct structure illustrated in (A) of [Fig.2], according to a manufacturing method 100. The manufacturing method, summarized by the diagram of [Fig.3] comprises steps S110, S120, S130 and S140, carried out in this order.

[0049] Step SI 10 is a step of providing a starting structure StartStruct consisting of a support layer Sprt, an intermediate layer Int and a ferroelectric layer Ferroiay as defined above. The Ferroiay layer is in this example a layer of LiTaO3 RY42 having a positive polarization P42. More ge generally the layer could be made of another ferroelectric material having another polarization as long as this polarization is positive.

[0050] Step S120 is a step of implantation of hydrogen ions H+ in the structure StartStruct by the free face of the Ferroiay layer. The implantation energy is chosen so 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 close to the Int layer. This situation is illustrated schematically in (A') of [Fig.2] which represents the concentration [H] of hydrogen according to 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 depth D(RPMax) can be chosen to be about 200 nm to avoid implantation too deep in the oxide. In another embodiment, this distance can reach or exceed 500 nm, the majority of ions and damage are then located entirely in the Int layer, which can be advantageous. In yet another embodiment, the depth D(RPMax) can be located about 50 to 100 nm from the interface, but this time in the Ferrolay layer, which allows the interface and also the Int layer to be left intact and without implantation damage.

[0051] The distribution of hydrogen ions obtained, with a hydrogen concentration gradient, generates the appearance of an electric field EH directed towards the maximum of the hydrogen concentration. A sufficient gradient can be obtained for example by means of 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 step of heat treatment of the structure, this heat treatment being provided 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° or 550°, up to 600°) for a duration 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 a heat treatment at a temperature of the order of 300° to 600°C causes an inversion of the polarization of the ferroelectric material due to the thermal activation caused by the increase in temperature. More specifically, during the heat treatment, the polarization of the ferroelectric material tends to take the same orientation as the EH field, thus reversing the polarization when EH is oriented in a direction opposite to that of the material before application of the heat treatment. This is precisely the case here, with a positive polarization and an EH electric field directed from the free surface of the Ferroiay layer towards its interface with the Int layer (in fact, towards the plane of depth D(RPMax), as illustrated in (A')).

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

[0055] Step S140 is a step of polishing the free face of the ferroelectric Ferroiay layer, for example by mechanical, chemical-mechanical thinning and / or chemical etching techniques. This polishing makes it possible to prepare 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 Ferrolay layer is removed, which may have an inhomogeneous polarization and / or an oxide layer due to surface phenomena. For example, a removal of 50 nm of thickness may be provided. This improves the homogeneity of the Ferroiay layer and its polarization.

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

[0057] Obtaining the starting structure StartStruct

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

[0059] Alternative method 1

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

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

[0062] A step S220 of choosing and providing a wafer of monocrystalline ferroelectric material for its chemical composition and its crystalline orientation relative to its extension plane, so that it serves as a ferroelectric donor substrate Ferrosub. It is in this wafer that the Ferroiay layer will be formed. The chosen wafer has a monodomain negative polarization, that is to say having a component oriented opposite to the direction of its face designated by Top, that is to say that the wafer has a crystalline 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 of preparing the ferroelectric donor substrate Ferrosub in order to form therein 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] A step S250 of assembling the support assembly SprtSet 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 support layer Sprt and the ferroelectric donor substrate Ferrosub. the orientation of the ferroelectric donor substrate polarity relative to the support layer is reversed, going from P_[38 to P42 due to the turning over of the Ferrosub donor substrate for assembly.

[0065] A step S260 of detaching a portion of the Ferrosub donor substrate from the Structinter intermediate structure at the weakening plane, leaving the Ferroiay ferroelectric layer fixed on the Sprt support and making it possible to obtain the StartStruct structure as illustrated in (D). The Ferroiay ferroelectric layer has the positive polarization P42 relative to the Green normal of the free surface having served as a detachment interface.

[0066] A step 270 of applying a stabilizing heat treatment to the StartStruct structure. The stabilizing heat treatment makes it possible to cure crystalline defects present in the ferroelectric layer and contributes to consolidating the bonding between this ferroelectric layer Ferroiay and the intermediate layer Int. In the case of LiTaO3, this heat treatment is intended 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° or 550°, up to 600°) for a duration 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 stabilization heat treatment will not cause an inversion 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 embrittlement layer Frgl have a concentration gradient [H] of maximum value at the level of the plane of embrittlement and continuously reducing as one moves away 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 already both oriented towards the free surface of the Ferroiay layer, the polarization will remain unchanged. Thus, we can consider that the PreStabStruct structure that has not yet undergone the stabilization heat treatment already constitutes the starting structure StartStruct.

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

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

[0070] The Ferrosub donor substrate illustrated in (B) is a substrate made of the ferroelectric material of the Ferroiay ferroelectric layer. It could alternatively comprise a surface thickness of this material. Thus, the donor substrate may, for example, be formed of a solid substrate of lithium tantalate or lithium niobate, or of a composite substrate formed of a first substrate on which rests a thickness (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 proves 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 monodomain polarization P_n8 inclined relative to the extension plane of the ferroelectric layer Ferroiay which depends on the chosen crystalline orientation of the wafer. This layer is intended to be fixed 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 section of the -138RY type, with the orientation illustrated in (B) of [Fig.l].

[0073] More generally, the method 200 applies to a crystalline cut included in the range extending from 0RY to -180RY, preferably -20RY to -160RY, for the Ferrosub donor substrate. The polarization of the structure obtained StartStruct is therefore positive, i.e. oriented from 0° to 180°, preferably within an angular range [Ang] of 20° to 160°.

[0074] The ferroelectric layer Ferroiay can be transferred from the ferroelectric donor substrate Ferrosub by implementing the 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 a hydrogen implantation, i.e., an ion bombardment of hydrogen of the flat face lmp of the donor substrate Ferrosub. 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 of ferroelectric material to be transferred which is located on the side of the face lmp and another part Ferrosep forming the rest of the substrate and which will be separated from the ferroelectric layer Ferroiay at a later stage.

[0075] The nature, the dose of the implanted species and the implantation energy are chosen according to the thickness of the layer that one wishes to transfer and the physicochemical properties of the Ferrosub donor substrate. In the case of a LiTaO3 donor substrate, one can choose to implant a dose of hydrogen between 1016 and 5.1017 at / cm2 with an energy between 30 and 300 keV to delimit a Ferroiay ferroelectric layer of the order of 200 to 2000 nm thick.

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

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

[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 of the van der Waals or covalent type. The assembly of the two bodies is then obtained without the use of an adhesive.

[0080] The assembly may comprise the application of a low temperature heat treatment temperature (e.g. between 50 and 300°C, typically 100°C) to cure crystal defects present in the ferroelectric layer and to strengthen the bonding energy sufficiently to allow a possible subsequent thinning step.

[0081] In the present embodiment, the step of detaching a portion of the donor substrate is carried out by applying the Smart Cut™ technology, according to which a layer intended to form the ferroelectric layer Ferroiay is delimited by the embrittlement plane Frgl defined by 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 support Sprt, as illustrated in (D) of [Fig.2].

[0082] This detachment step may thus comprise the application to the intermediate structure Structinter of a heat treatment in a temperature range of the order of 80°C to 300°C to enable the detachment of the part of the donor substrate from the ferroelectric layer Ferrolay and thus complete the transfer of the latter onto the support assembly. As a replacement or in addition to the heat treatment, this step may comprise the application of a blade or a jet of gaseous or liquid fluid, or any other force of a mechanical nature at the level of the embrittlement plane Frgl.

[0083] Alternative method 2

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

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

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

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

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

[0089] However, as illustrated in (D') of [Fig.8], the concentration [He] of helium in the Ferroiay layer has a gradient with a maximum concentration at the free surface of the layer, as illustrated by [Fig.8], which represents the PreStabStruct structure obtained just after separation, the helium ions used to form the embrittlement layer Frgl have a concentration gradient [He] of maximum value at the embrittlement plane and continuously reducing when moving away 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 the polarization P_B8.

[0090] Thus, the application of a stabilization heat treatment in step S370, according to parameters that may be those described in relation to step S130, results in an inversion 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 inverting 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 method 2, the heat treatment step S370 following the separation step is necessary to obtain the StartStrct structure, unlike the heat treatment step S270 of alternative method 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 methods 1 and 2, the first using a donor substrate of negative polarity and a hydrogen implantation defining a weakening plane, and the second using a donor substrate of positive polarity and a helium implantation defining a weakening plane, make it possible to obtain the starting structure StartStruct of the method 100 for 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 crystalline cut layer, the method 100 applies more generally to crystalline cut layers included in the range extending from 0RY to 180RY, preferably 20RY to 160RY, for the ferroelectric layer Ferroiay of the starting structure StartStruct. In other words, considering that the ferroelectric layer extends in a horizontal direction Hnz perpendicular to the vertical direction Vert normal to the layer, the positive polarization P42 of the starting structure StartStruct makes an angle included in an angular range [Ang] going from 20° to 160° with respect to this horizontal direction Hnz, as illustrated by [Fig.9]. The polarization of the final structure EndStruct is negative, i.e. oriented from 0° to -180°, preferably within an angular range of -20° to -160°, relative to the plane of the ferroelectric layer Ferroiay. Of course, the methods 200 and 300 can be adapted so as to choose suitable crystal sections to obtain the structure StartStruct having a desired polarization of the ferroelectric layer Ferroiay.

[0094] In this document, the figures are not necessarily to scale. Some features and components may be shown exaggerated relative to other components or in a somewhat schematic form, and some 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 embodiments described and variant embodiments can be made without departing from the scope of the invention as defined by the claims.

Claims

Claims

1. Method (100) for manufacturing a composite structure (EndStruct) comprising a ferroelectric layer (Ferroiay) of negative polarization (P.i38) resting on a support assembly (Sprt.Set), comprising the steps of: - providing (SI 10) a starting composite structure (StartStruct) comprising a ferroelectric layer (Ferroiay) of positive polarization (P42) resting on the support assembly (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. The method (100) of claim 1, wherein the hydrogen ions are implanted such that the starting composite structure has a hydrogen atom concentration of between 1019 and 1022 at / cm3.

3. The method according to claim 1 or 2, wherein the applied heat treatment (S 130) is set 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, in which the ferroelectric layer extends in a horizontal direction (Hnz) and the positive polarization (P42) of the starting composite structure (StartStruct) makes an angle included in an angular range ([Ang]) from 20° to 160° relative to this horizontal direction (Hnz).

5. A method (100) according to any preceding claim, wherein the ferroelectric layer (Ferroiay) is a layer of lithium niobate or lithium tantalate.

6. A method (100) according to any preceding claim, further comprising a step (S 140) of polishing the ferroelectric layer (Ferroiay).

7. 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 (200) comprising the steps of: - providing (S 110) the support assembly (Sprt.Set); - providing (S220) a donor substrate (Ferrosub) of monocrystalline 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 - detaching (S260) a part of the donor substrate (Ferrosub) at the level of the weakening plane so as to leave the ferroelectric layer (Ferro lay) fixed on the support assembly (Sprt.Set) and obtain the starting structure (StartStruct).

8. 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 monocrystalline 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) a part of the donor substrate (Ferrosub) at the level of the weakening 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 method (100) according to claim 8, in which the heat treatment applied (S370) to the structure (PreStabStruct) is set 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 below 600°C.

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