Thin film preparation process

Abstract

The present invention relates to a method for preparing a thin film (1) comprising a weakening step (S2) for forming a weakened zone (3) in a central portion (2c) of a donor substrate (2), the weakened zone (3) not extending into a peripheral portion (2p) of the donor substrate (2); an assembly step (S3) of the main face (2a) of the donor substrate (2) with a receiving substrate (5) to form a unit to be fractured; and a separation step (S4) of the unit to be fractured, the separation step comprising a heat treatment leading to the release of the thin film (1) from the donor substrate (2) only at its central portion (2c). The method also comprises, after the separation step, a detachment step (S6) comprising treating the unit to be fractured to detach the peripheral portion of the donor substrate (2) from the receiving substrate (5). (Figure 1)

Description

Title of the invention: Method for preparing a thin layer FIELD OF THE INVENTION

[0001] The present invention relates to a method for preparing a thin layer transferred onto a support substrate by applying Smart Cut™ technology. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] In certain embodiments of the Smart Cut™ technology, a thin layer is prepared by introducing light species through a main face of a donor substrate to form a buried weakened zone. The thin layer is thus defined, in this donor substrate, between the buried weakened zone and the main face of the substrate. Then, the donor substrate is assembled on a second substrate, called the “receiver”, and a separation heat treatment is applied to this assembly, possibly assisted by mechanical stresses, in order to cause the initiation and propagation of a fracture wave at the buried weakened zone to release the thin layer which is thus transferred to the recipient substrate.

[0003] The heat separation treatment has the effect of promoting the growth and pressurization of microcavities whose development is linked to the presence of light species in the weakened zone. This effect is at the origin of the initiation and propagation of the fracture wave leading to the release of the thin layer.

[0004] The state of the exposed surface of the thin layer transferred to the receiving substrate is irregular, which is generally not entirely satisfactory. A method implementing Smart Cut technology therefore generally provides for an additional finishing step aimed at reducing these surface irregularities. This may thus involve treating the exposed surface of the thin layer, during this finishing step, by chemical-mechanical polishing or by high-temperature annealing, typically of the order of 1100°C, by exposing it to a reducing or neutral atmosphere.

[0005] Document FR3061988 proposes a method for finishing a thin layer prepared using Smart Cut technology, as just presented. In this document, it is proposed to apply a heat treatment of moderate amplitude to the thin layer, below 950 degrees and in a reducing or neutral atmosphere, directly after it has been released from the donor substrate. Since the free surface of the thin layer has not been exposed to the ambient atmosphere, it has not oxidized and the atoms that constitute it are very mobile to reorganize and smooth the surface. This heat treatment is therefore particularly effective, even at relatively low temperatures, for smoothing the surface released from the thin layer. It should be noted that in the method presented in FR3061988, the fracture and smoothing must be carried out without re-contact with the ambient atmosphere between the two stages, i.e. in the same equipment. This imposes drastic constraints in terms of thermal capacities and control of the purity of the equipment atmosphere

[0006] Document EP2933828 indicates that the surface irregularities originate from several phenomena of different natures which combine. These irregularities may correspond to the imprints of the microcavities at the origin of the fracture of the donor substrate. They contribute to forming a surface roughness of the thin layer whose amplitude and wavelength are of the order of one nm or tens of nm.

[0007] Irregularities can also originate from the interaction between the propagation of the fracture wave and acoustic vibrations encountered in the assembly to be fractured at the time of fracture. These vibrations are caused by the sudden release of energy during the fracture of the thin layer. The fracture wave is then likely to be deflected vertically from its plane of progression depending on the instantaneous stress state of the material it passes through. Reference may be made to the article “Crack Front Interaction with Self-Emitted Acoustic Waves”, Physical Review Letters, American Physical Society, 2018, 121. (19), pp.195501, for a detailed analysis of this mechanism. The irregularities caused by this phenomenon are relatively large in size, and constitute a non-uniformity in the thickness of the thin layer, the amplitude of which can be of the order of a nanometer and the wavelength of a millimeter or even a centimeter.

[0008] Furthermore, and as documented in the publication by Massy et al. "Fracture dynamics in implanted Silicon", Applied Physic Letters 107.9 (2015), the sudden release of energy, oscillations and beating of the two substrates generated during the fracture can also lead to damage to the separated surfaces. This damage can in particular be caused by impacts between these two surfaces or their relative displacements.

[0009] More generally, handling the support substrate carrying the thin layer and the residue of the donor substrate, after the embrittlement step and after these two parts have been separated from each other, can also cause damage, for example scratches or particles, to the surface released from the thin layer.

[0010] It is difficult to eliminate surface irregularities of the thin layer, other than those appearing in the form of roughness, using a simple heat treatment. The dimensions of these irregularities can in fact exceed the average mobility distance, under the effect of the heat treatment, of the atoms forming this surface. This is particularly the case in the smoothing process proposed by the aforementioned document FR3061988.

[0011] Consequently, a thin layer prepared by a prior art method may have an imperfect surface condition, in which all irregularities could not be reduced, or may be damaged.

[0012] The present invention seeks to remedy at least in part the limitations which have just been mentioned of this state of the art. BRIEF DESCRIPTION OF THE INVENTION

[0013] In order to achieve this aim, the subject of the invention proposes a method for preparing a thin layer comprising the following steps: - a weakening step comprising the introduction of light species through a main face of a donor substrate, the step aiming to form a weakened zone in a central portion of the donor substrate to define the thin layer with the main face of the donor substrate, the weakened zone not extending into a peripheral portion of the donor substrate; - a step of assembling the main face of the donor substrate with a recipient substrate to form an assembly to be fractured; - a step of separating the assembly to be fractured, the separation step comprising a heat treatment leading to the release of the thin layer of the donor substrate at its central zone only, the donor substrate and the receiving substrate remaining integral with each other at the peripheral portion of the donor substrate; - a detachment step, applied after the separation step, the detachment step comprising the treatment of the assembly to be fractured to detach the peripheral portion of the donor substrate from the recipient substrate and thus transfer the thin layer to the recipient substrate.

[0014] By locating the weakened zone in the central portion of the donor substrate only, the thin layer can be released without causing complete detachment of the donor substrate and the recipient substrate, which makes it possible to improve the surface condition of this thin layer.

[0015] According to other advantageous and non-limiting characteristics of the invention, taken alone or in any technically feasible combination: - The method comprises a finishing step, this step comprising a heat treatment aimed at bringing the assembly to be fractured to a temperature higher than the heat treatment temperature of the separation step in order to smooth the surface released from the thin layer; - the finishing step is applied between the separation step (S4) and the detachment step; - the finishing step involves a heat treatment exposing the donor substrate to a temperature below 1000°C, 950°C, 900°C, or 600°C; - the weakening stage and the finishing stage are carried out in the same equipment; - the weakening stage and the finishing stage are carried out in separate equipment; - the finishing step is applied after the detachment step; - the donor substrate is a circular wafer and the peripheral portion is an annular portion, the width of which taken from the edge of the substrate is between 1mm and 5cm; - the donor substrate comprises a peripheral chamfer on the side of its main face, and the peripheral portion extends from the chamfer over a width of between 1 mm and 5 cm; - light species are chosen from the list consisting of hydrogen and helium; - the donor substrate is made of monocrystalline silicon; - the separation step implements a heat treatment exposing the donor substrate at a temperature between 250°C and 500°C; - the detachment step is carried out by chemical etching or by applying a disassembly force to the support substrate and the donor substrate. Brief description of the drawings

[0016] 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:

[0017] [Fig-1] [Fig.l] represents the steps of a method according to the invention;

[0018] [Fig.2] [Fig.2] represents the central portion and the peripheral portion of a donor substrate. DETAILED DESCRIPTION OF THE INVENTION

[0019] With reference to [Fig.l], a method for preparing a thin layer 1 in accordance with the present description is now presented.

[0020] This method comprises providing, during a preliminary step SI, a donor substrate 2, this substrate having a main face 2a. The donor substrate 2 may consist of a monocrystalline silicon wafer, in particular in the case where the method for preparing the thin layer aims to manufacture a silicon-on-insulator substrate. This wafer may have undergone prior treatments, such as than oxidation, the formation of a surface layer by epitaxy or deposition or any other surface preparation step.

[0021] The invention is however in no way limited to a donor substrate 2 consisting of a silicon wafer, and this may be freely chosen in its nature and in its shape according to the intended application. It may thus be a substrate comprising any single-crystal semiconductor material, for example silicon carbide or germanium. It may also be an insulating material such as a piezoelectric material, such as lithium tantalate or lithium niobate. The donor substrate may thus take the form of a circular wafer, the diameter of which may be of any size, from a few tens of millimeters to 300 mm or 450 mm or more. It may take another shape, square or rectangular. As already specified, the donor substrate 2 may have undergone prior steps of polishing, deposition, oxidation, and therefore be formed from a stack of heterogeneous layers.

[0022] The donor substrate 2 may have a chamfered contour 2b as is common in the field of semiconductor material wafers in order to avoid chipping it during handling. The chamfer 2b may extend over several millimeters at the periphery of the substrate.

[0023] As shown in [Fig. 2], in a donor substrate 2 according to the present description, two portions are distinguished: a central portion 2c and a peripheral portion 2p. These portions 2c, 2p extend from one face of the substrate to the other and project onto the main face 2a of the substrate in any shape, insofar as the peripheral portion 2p completely surrounds the central portion 2c. The central portion 2c can thus be projected, as shown in [Fig.2], onto the main face 2a of a donor substrate 2 (here in the form of a circular plate) to form a disc, centered on this face, and the peripheral portion 2p, to form an annular surface, concentric with the disc defining the central portion 2c, and extending to the edge of the substrate 2. The central portion 2c and the peripheral portion 2p are intended to be brought into contact with a receiving substrate, so as to assemble the two substrates to each other.When the donor substrate 2 has a chamfer 2b, the peripheral portion 2p therefore extends radially from the edge of the substrate towards its center, beyond this chamfer, in order to allow this contact.

[0024] By way of example, the peripheral portion 2p of the donor substrate 2 may have a width of between 1 mm and several centimeters, for example 5 cm, taken from the edge of the chamfer when this chamfer exists, or from the edge of the substrate in the opposite case. Thus, in the case of the example shown in [Fig. 2], in which the donor substrate 2 is a circular wafer having a chamfer of approximately 1 mm width on its extreme edge, the peripheral portion 2p is defined as an annular portion, the width of which, taken from the inner edge of the chamfer, is between 1 mm and 5 cm, so that this peripheral zone can come into contact with a receiving substrate having at least the same dimension. The portion arranged inside the annular portion forms the central portion 2c of the donor substrate 2.

[0025] Continuing the description of [Fig. 1], a method in accordance with the present description comprises a weakening step S2 comprising the introduction of light species through a main face 2a of a donor substrate 2. This step aims to form a weakened zone 3 buried in the donor substrate 2. The weakened zone 3 extends in a plane of the donor substrate 2 essentially parallel to the main face 2a of this substrate 2. The weakened zone 3 defines, with the main face 2a of the donor substrate 2, at least a portion of the thin layer 1 which is intended to be prepared. As is well known per se, the light species are typically chosen from the list formed by hydrogen and helium, without this being limiting. These light species are usually introduced into the donor substrate 2 by ion implantation, although other methods of introduction are also possible (by plasma, by diffusion).In the case of implantation, the implantation energy defines the depth of penetration of the ions into the donor substrate material 2, and therefore the depth of the buried plane in which the weakened zone 3 is located.

[0026] When the introduction is carried out by ion implantation, it can be provided that this introduction is carried out in a single step, for example to implant a single dose of hydrogen or a single dose of helium, or in a plurality of successive steps, for example to implant a dose of hydrogen then another dose of helium. Whatever the nature and the manner in which the species are introduced into the donor substrate, the term "dose" will be used to designate the total quantity of species introduced into a unit area of ​​the donor substrate, for example the number of light species implanted per cmA2.

[0027] In the context of the present description, the embrittlement step S2 is carried out so that, at the end of this step, the embrittled zone 3 is located in the central portion 2c of the donor substrate 2 and so that it does not extend into the peripheral portion 2p. As will be made apparent in the remainder of this description, by locating the embrittled zone in the central portion 2p only, it will be possible to release the thin layer, without causing the complete detachment of the donor substrate and the receiving substrate with which the donor substrate will be assembled, which makes it possible to improve the surface condition of this thin layer.

[0028] Several approaches are possible to locate the weakened zone 3 in the central portion 2c only. Thus, and according to a first approach, the weakening step S2 is carried out by differentiating the dose introduced into the central portion 2c from that introduced in the peripheral portion 2p. In this regard, reference may be made to application FR3063176.

[0029] The embrittlement step S2 can thus comprise the introduction of a first dose of light species into the central portion 2c of the donor substrate 2 and the introduction of a second dose of light species, lower than the first dose, into the peripheral portion 2p of this substrate 2.

[0030] The first dose of light species is chosen so that it is sufficient, during a subsequent separation step S4, to release the thin layer 1 from the donor substrate 1 at the weakened zone arranged in the central portion 2c. This dose therefore leads to the formation of the weakened zone 3 in the central zone 2c. The second dose of light species introduced into the peripheral portion 2p is chosen so that it does not allow this detachment in the peripheral portion 2p. This dose therefore does not lead to the formation of a weakened zone within the meaning of the present description.To be completely clear, the peripheral portion 2p of the donor substrate 2 may comprise light species, implanted or introduced in any other way, but these species are not present in a sufficient concentration in this portion 2p to form a zone of weakening within the meaning of the present application, and in particular it cannot cause the separation of the donor substrate 2 at the level of this portion 2p.

[0031] By way of example, and according to a preferred embodiment which is that shown in [Fig.l], the second dose of light species can be zero, and in this case the light species are introduced only into the central portion 2c of the donor substrate 2 and not into the peripheral portion 2p.

[0032] To achieve this, the peripheral portion 2p can be masked using a screen material that acts as a barrier to the penetration of light species, when the introduction of these species is carried out by ion implantation. This mask can be made of Teflon, aluminum or carbon. It can also be formed from a sacrificial mask 4 made of resin, hard oxide or nitride placed at the peripheral portion 2p of the main face 2a of the donor substrate 2 at least, it being understood that this mask will be removed at the end of this embrittlement step S2.

[0033] When the embrittlement step S2 provides for several successive phases of ion implantations, as is the case with co-implantation of hydrogen and helium, the mask may be present during only some of the implantation phases.

[0034] According to a variant of this first approach, the introduction of the light species into the donor substrate 2 can be carried out by scanning a beam comprising such light species on the main face 2a of the donor substrate 2. In such an embodiment of the embrittlement step S2, the movement of the beam can be controlled in such a way that the peripheral portion 2p of the donor substrate 2 is excluded from any implantation or that a lower dose of species is introduced into this portion 2p than species introduced into the central portion 2c. In this variant, it is therefore possible to do without a mask, which is advantageous.

[0035] The nature of the species and the exact doses of these species introduced into the central portion 2c and into the peripheral portion 2p are chosen according to the nature of the donor substrate and according to the characteristics of the subsequent separation step S4.

[0036] By way of example, when the donor substrate 2 is made of monocrystalline silicon, and the separation step implements a heat treatment exposing the donor substrate 2 to a temperature between 250°C and 500°C, the following conditions are possible: Hydrogen alone

[0037] Central portion: between 4E16 and 1E17 at / cm2

[0038] Peripheral portion: between 0 and 3E16 at / cm2 Hydrogen and Helium

[0039] Central portion: H: between 0.5E16 and 2E16 at / cm2; He between 0.5E16 and 2E16 at / cm2.

[0040] Peripheral portion: H: between 0 and 1E16 at / cm2; He between 0.5E16 and 2E16 at / cm2.

[0041] According to an alternative approach, the embrittlement step S2 can comprise the introduction, during a first sub-step, of the same dose of light species into the central portion 2c of the donor substrate 2 and into the peripheral portion 2p of this substrate 2. This conventional dose, for example of the order of 5E16 at / cmA2 of hydrogen when the donor substrate 2 is made of silicon, is sufficient to form a weakened zone 3 extending into the central zone 2c and into the peripheral zone 2p. In a second sub-step, the peripheral portion 2p of the donor substrate 2 is treated to render the light species introduced into the peripheral portion 2p inoperative. This may involve treating this peripheral portion, for example using a laser, to diffuse the light species previously introduced into this zone, and effectively eliminate the weakened zone 3 from this peripheral portion 2p.Alternatively, the peripheral portion can be treated to damage the weakened zone located in this portion, for example by implanting relatively heavy species, such as silicon species. At the end of the treatment carried out during this second sub-step, the weakened zone 3 is well located only in the central zone 2c of the donor substrate 2, and it no longer extends to the peripheral zone 2p.

[0042] Regardless of the manner in which this embrittlement step S2 was implemented, at the end of it there is a donor substrate 2 comprising a buried embrittled zone 3 located in a central portion 2c of the donor substrate 2, the weakened zone 3 defining with the main face 2a of the donor substrate 2 the thin layer 1, and the weakened zone 3 not extending into the peripheral portion 2p.

[0043] In a following assembly step S3 of a method according to the present description, the donor substrate thus prepared is assembled to a receiver substrate 5 by bringing the main face 2a of the donor substrate 2 into contact with a face of the receiver substrate 5. The receiver substrate 5 can be of any nature and of any suitable shape insofar as it has a face of sufficient size to be brought into contact with the central portion 2c and the peripheral portion 2p of the donor substrate 2.

[0044] This assembly can be carried out by any method, for example by molecular adhesion, by application on at least one of the faces to be assembled of an adhesive material, by eutectic bonding when the faces to be assembled have been previously prepared to present a metallic surface. The assembly step S3 can also provide for conditioning the faces of the two substrates, by cleaning, activation or any other preparation step, to facilitate this assembly and to secure the two substrates to each other with sufficient adhesion energy.

[0045] Alternatively, the assembly may consist of the progressive formation of the receiving substrate 5 by deposition, at moderate temperature, of the material constituting said substrate 5 on the main face 2a of the donor substrate 2.

[0046] In the remainder of this description, the assembly formed from the donor substrate 2 and the receiver substrate 5 at the end of this step S3 will be referred to as “assembly to be fractured”.

[0047] In a following step of a method according to the present description, the assembly to be fractured is heat-treated during a separation step S4. The heat treatment causes the development of microcavities, the coalescence of these microcavities when they are sufficiently numerous and their pressurization by the available light species, as is conventional in the implementation of Smart Cut technology. As mentioned previously, the buried weakened zone 3 does not, however, extend over the entire extent of the donor substrate 2. The heat treatment therefore leads to the release of the thin layer 1 from the donor substrate 2 at its central zone 2c only, but not at the peripheral zone 2p.

[0048] The heat treatment of the separation step S4, typically between 200°C and 500°C when the donor substrate 2 is made of silicon, can be applied by placing the assembly to be fractured in a conventional furnace for a period which can be between several minutes and several hours. In practice, and for the sake of productivity, a large number of assemblies to be fractured are generally placed in a nacelle, and this nacelle is placed in a furnace or in an oven of large capacity to collectively apply heat treatment to the assemblies to be fractured, at the temperature and for the duration chosen.

[0049] The fracture wave that is initiated during this separation step S4 does not propagate from one end of the donor substrate 2 to the other, and the residue of the donor substrate (i.e. the donor substrate from which the thin layer has been removed) is not suddenly released from the receiving substrate, as is the case in the separation steps of the prior art. In the context of the method described in [Fig. 1], the receiving substrate 5 and the donor substrate 2 remain assembled to each other at the end of the separation step S4, although the thin layer 1 has actually been released. In the method of the present description, the generation of acoustic waves, the oscillations and the beating of these substrates are therefore limited.The harmful interactions between the fracture wave and the acoustic waves are reduced or eliminated, and shocks or sliding of the substrates are also avoided, which, in the processes of the state of the art, lead to the creation of irregularities or damage to the surface of the thin layer which is difficult to reduce by the application of a simple heat treatment.

[0050] Furthermore, the space 6 between the released surfaces of the thin layer 1 and the donor substrate 2 is filled with the light species at the origin of the release of the thin layer 1. These species are confined in this closed and sealed space 6, because the fracture has not propagated in the peripheral portion 2p of the donor substrate 2, which therefore remains firmly assembled to the receiving substrate 5. This closed and sealed space 6 is therefore also isolated from the surrounding atmosphere by the peripheral portion 2p of the donor substrate 2. This prevents external contaminants, for example oxygen, from penetrating into this space 6 and passivating the released surface of the thin layer 1, which would make it more difficult to smooth it.It is noted in this regard that the light species introduced during the embrittlement step S2 which allow the fracture of the donor substrate 2 (typically hydrogen or helium) are extremely pure, they do not interact with the atoms forming the surface of the thin layer released to passivate it and to limit the surface mobility of the atoms which makes it an atmosphere particularly suitable for thermal smoothing.

[0051] To take advantage of this very particular state of the assembly to be fractured following the separation step S4, a preferred embodiment of a method in accordance with the present description provides for applying, during a finishing step S5, a heat treatment aimed at bringing the assembly to be fractured to a temperature higher than the temperature of the heat treatment of the separation step S4, in order to smooth the surface released from the thin layer 1.

[0052] In this preferred embodiment, the heat treatment exposes the assembly to be fractured to a plateau temperature higher than the temperature, between 200°C and 500°C when the donor substrate is made of silicon, of the separation step S4. Advantageously, this heat treatment remains moderate, below 1000°C, or 950°C, or even below 900°C or 600°C. It can extend over a period of time between a few seconds at the chosen plateau temperature, and several hours at this plateau temperature.

[0053] This heat treatment of the finishing step S5 leads to bringing the assembly to be fractured to the treatment temperature, including the light species confined in the closed space 6 delimited by the released surfaces of the thin layer 1 and the donor substrate 2. These surfaces, not contaminated by reactive species coming from the external atmosphere for example, are composed of atoms that the heating makes very mobile, which makes it possible to smooth said surfaces, in particular the free surface of the thin layer 1, very effectively, even at a low temperature, less than or equal to 1000°C or 950°C. It is noted that this treatment of the assembly to be fractured can be implemented in equipment whose atmosphere can be any, since it is not this atmosphere which contributes to the smoothing of the surface of the thin layer 10. This equipment, for example an oven, only aims to bring the assembly to temperature.

[0054] Experiments carried out by the applicant have thus shown that it was possible to obtain, after a heat treatment of the finishing step S5 consisting of exposing the assembly to be fractured for 2 minutes at 950°C, a thin layer 1 of silicon having a surface whose roughness was of the order of 1.6 nm RMS. In the absence of any finishing step, and therefore of any heat treatment higher in temperature than the temperature reached during the separation step S4, the roughness of the thin layer was measured at 5.5 nm RMS. The effectiveness of the finishing step which has just been described is clearly seen.

[0055] The finishing step S5 can be carried out in the same equipment as that used for the separation step S4, for example the same furnace or the same oven. But unlike the solution of the prior art set out in the introduction to this application, it is also possible to carry out the finishing step S5 in another piece of equipment, separate from the equipment used to carry out the separation step S4. The two substrates forming the assembly to be fractured are in fact still integral with each other, and the risk of damaging the thin layer 1 during the handling of this assembly to store it or move it to another piece of equipment is reduced. Furthermore, since the free surface of the thin layer 1 is entirely confined, it cannot be exposed to the ambient atmosphere during this change of equipment.It therefore does not oxidize or passivate, and its full smoothing potential is retained, even at low temperatures, by carrying out the finishing step in an environment other than that used during the separation step. And as already mentioned, . the equipment in which the finishing step S5 is carried out in this implementation mode can be chosen very freely.

[0056] In a following step, a detachment step S6 is applied to the assembly to be fractured, aiming to detach the donor substrate 2 (or more precisely the residue of this donor substrate 2, after the thin layer 1 has been removed therefrom) from the receiving substrate 5 and thus finalize the transfer of the thin layer 1 onto the receiving substrate 5. The treatment implemented to obtain this detachment can be varied: it can involve chemically etching, for example by wet method, the contour of the assembly to be fractured, to disassemble the two substrates at their assembly interface. For this, the assembly to be fractured can be positioned vertically to soak a portion of its contour in a KOH or TMAH solution (when the donor substrate is made of silicon) or in a solution comprising HF when a silicon oxide layer is arranged at the interface between the donor substrate 2 and the receiving substrate 5.By rotating the assembly, this entire contour is successively treated to disassemble the two substrates from each other. Alternatively, and as shown diagrammatically in [Fig.l], this treatment may include the application of a blade at the assembly interface, and its introduction between the two substrates to cause the assembly to disassemble. More generally, it may involve applying by any suitable means a force to disassemble the support substrate 5 and the donor substrate 2.

[0057] In all cases, this detachment step S6 can be perfectly controlled, unlike the fracture step of a method of the state of the art, so that the risks of damage to the thin layer 1 can be minimized.

[0058] According to another embodiment, the finishing step is not carried out directly after the separation step S4, between this step S4 and the detachment step S6, but after the detachment step S6.

[0059] The finishing step S5 of this embodiment may be a conventional finishing step, for example exposing the released surface of the thin layer 1, when the latter is made of silicon, to a reducing or neutral atmosphere at a temperature between 950 or 1000°C and 1100°C. More generally, the exposed surface of the thin layer will be brought to a neutral or reducing atmosphere brought to a temperature higher than the temperature of the heat treatment of the separation step.

[0060] In this embodiment, advantage is taken of the absence or low presence of large irregularities or damage to the surface of the thin layer 1. The roughness that this thin layer 1 presents can be smoothed effectively by means of the heat treatment which has just been described.

[0061] At the end of these treatments, whatever the chosen implementation mode, and as represented by the final step S7 of [Fig.l], the thin layer 1 is provided, transferred onto the support substrate 5. The thin layer 1 has an improved surface condition compared to a thin layer 1 obtained directly after fracture by a method of the state of the art. In particular, it has a relatively low roughness, thanks to the smoothing effect of the finishing step S5. It also has little or no non-uniformity linked to the interactions of the fracture wave with acoustic waves propagating in the receiving substrate 5 and in the donor substrate 2 when they are entirely detached from each other by the propagation of the fracture wave. Damage to the thin layer 1, the origin of which is the impacts between the two substrates or their relative sliding on each other, is also limited.

[0062] 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.

[0063] In particular, other steps may be provided in addition to those described. In particular, this may involve applying additional finishing treatments to the thin layer, for example a sacrificial oxidation step, polishing to perfect the qualities of this layer or adjust its thickness.

[0064] It is also possible to combine the two implementation modes presented, and to carry out the finishing step S5 during a first phase, between the separation step S4 and the detachment step S6 by means of a first heat treatment, and to complete this treatment with a second phase, for example a second heat treatment, applied to the thin layer 1 after the detachment step S6.

Claims

Claims

1. A method of preparing a thin layer (1) comprising the following steps: - a weakening step (S2) comprising the introduction of light species through a main face (2a) of a donor substrate (2), the step aiming at forming a weakened zone (3) in a central portion (2c) of the donor substrate (2) to define the thin layer (1) with the main face (2a) of the donor substrate (2), the weakened zone (3) not extending into a peripheral portion (2p) of the donor substrate (2); - a step of assembling (S3) the main face (2a) of the donor substrate (2) with a receiving substrate (5) to form an assembly to be fractured;- a separation step (S4) of the assembly to be fractured, the separation step comprising a heat treatment leading to the release of the thin layer (1) from the donor substrate (2) at its central portion (2c) only, the donor substrate (2) and the receiving substrate (5) remaining integral with each other at the peripheral portion (2p) of the donor substrate (2); - a detachment step (S6), applied after the separation step, the detachment step comprising the treatment of the assembly to be fractured to detach the peripheral portion of the donor substrate (2) from the receiving substrate (5) and thus transfer the thin layer (1) onto the receiving substrate (5).;

2. Method for preparing a thin layer (1) according to the preceding claim comprising a finishing step (S5), this step comprising a heat treatment aimed at bringing the assembly to be fractured to a temperature higher than the temperature of the heat treatment of the separation step (S4) in order to smooth the surface released from the thin layer (1).

3. A method of preparing a thin layer (1) according to the preceding claim wherein the finishing step (S5) is applied between the separation step (S4) and the detachment step (S6).

4. A method of preparing a thin film (1) according to claim 3 wherein the finishing step (S5) involves a heat treatment exposing the donor substrate (2) to a temperature below 1000°C, 950°C, 900°C, or 600°C.

5. A method of preparing a thin layer (1) according to one of claims 2 to 4, wherein the separation step (S4) and the finishing step (S5) are carried out in the same equipment.

6. Method for preparing a thin layer (1) according to one of claims 2 to 4 in which the separation step (S4) and the finishing step (S5) are carried out in separate equipment.

7. A method of preparing a thin film (1) according to claim 2, wherein the finishing step (S5) is applied after the detaching step (S6).

8. Method for preparing a thin layer (1) according to one of the preceding claims in which the donor substrate (2) is a circular-shaped wafer and the peripheral portion (2p) is an annular portion, the width of which taken from the edge of the substrate is between 1mm and 5cm.

9. Method for preparing a thin layer (1) according to one of claims 1 to 7 in which the donor substrate (2) comprises a peripheral chamfer (2b) on the side of its main face (2a), and the peripheral portion (2p) extends from the chamfer over a width of between 1mm and 5cm.

10. Method for preparing a thin layer (1) according to one of the preceding claims in which the light species are chosen from the list formed by hydrogen and helium.

11. Method for preparing a thin layer (1) according to one of the preceding claims in which the donor substrate (2) is made of monocrystalline silicon.

12. Method for preparing a thin layer (1) according to one of the preceding claims in which the separation step (S4) implements a heat treatment exposing the donor substrate (2) to a temperature between 250°C and 500°C.

13. Method for preparing a thin layer (1) according to one of the preceding claims in which the detachment step is carried out by chemical etching or by applying a force for disassembling the support substrate (5) and the donor substrate (2).

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