Process for preparing thin layers

Abstract

The present invention relates to a process for preparing a thin layer (1), comprising a weakening step (S2) aimed at forming a weakened zone (3) in a central portion (2c) of a donor substrate (2), the weakened zone (3) not extending to the peripheral portion (2p) of the donor substrate (2), a step (S3) of bonding a main surface (2a) of the donor substrate (2) to a receiver substrate (5) to form an assembly to be divided, and a separating step (S4) of the assembly to be divided, comprising a heat treatment resulting in cleavage of the thin layer (1) from the donor substrate (2) only in its central portion (2c). After the separating step, the process also comprises a detaching step (S6) comprising treating the assembly to be divided in order to remove the peripheral portion of the donor substrate (2) from the receiver substrate (5).

Description

[Technical field]

[0001] The present invention relates to a process for preparing a thin layer to be transferred to a carrier substrate through the application of Smart Cut™ technology. [Background technology]

[0002] In some implementations of the Smart Cut™ technology, a thin layer is cut through a major surface of the donor substrate to form a buried weakened zone. Light seeds The donor substrate is prepared by introducing (especes legeres) into the substrate. In this way, a thin layer is defined in this donor substrate between the buried zone of weakness and the main surface of the substrate. The donor substrate is then bonded to a second substrate, called the "receiver" substrate, and a separation heat treatment is applied to this assembly, potentially assisted by mechanical stress, in order to cause the initiation and propagation of a splitting wave in the buried zone of weakness in order to cleave the thin layer, which is then transferred to the receiver substrate.

[0003] The effect of the separation heat treatment is to promote the growth and compression of microcavities, and the development of microcavities leads to the formation of a weak zone. Light seeds This effect is the source of the initiation and propagation of the splitting waves that lead to the cleavage of the thin layer.

[0004] The condition of the exposed surface of the thin layer transferred to the receiver substrate is uneven, which is generally not entirely satisfactory. The process implementing the Smart Cut technology therefore generally envisages an additional finishing step aimed at reducing these surface irregularities. This may mean treating the exposed surface of the thin layer by chemical mechanical polishing or by annealing at high temperatures, typically of the order of 1100° C., during this finishing phase, while exposing it to a reducing or neutral atmosphere.

[0005] As mentioned above, a process has been proposed for finishing the thin layer prepared using the Smart Cut technology (see, for example, US Pat. No. 5,399,636). It has been proposed to subject the thin layer immediately after it has been cleaved from the donor substrate to a heat treatment of moderate amplitude, lower than 950° C., in a reducing or neutral atmosphere (see, for example, US Pat. No. 5,399,636). As the cleaved surface of the thin layer is not exposed to the ambient atmosphere, it is not oxidized, and the atoms forming it are highly mobile and rearrange themselves to smooth the surface. This heat treatment is therefore particularly effective for smoothing the cleaved surface of the thin layer, even at relatively low temperatures. It should be noted that in the process (presented, for example, in US Pat. No. 5,399,636), the splitting and smoothing treatments must be performed without contact with the ambient atmosphere between the two steps, i.e. in the same apparatus. This imposes extreme constraints on the heat capacity in the apparatus and on the control of the purity of the atmosphere.

[0006] A similar heat treatment is provided immediately after the thin layer is cleaved from the donor substrate (see, for example, US Pat. No. 5,399,313).

[0007] It has been shown that the surface irregularities are the result of several phenomena of different nature combined (see, for example, US Pat. No. 5,399,633). These irregularities may correspond to the imprint of microcavities that led to the splitting of the donor substrate. They contribute to the formation of undulations on the surface of the thin layer, the amplitudes and wavelengths of which are of the order of nm or tens of nm.

[0008] Roughness can also arise in the interaction between the propagation of splitting waves and acoustic vibrations encountered in the assembly to be split at the moment of splitting. These vibrations are caused by the sudden cleavage of energy at the splitting of the thin layer. The splitting wave is then prone to be bent perpendicularly from its plane of progression depending on the instantaneous state of stress of the material in which it is moving. For a detailed analysis of this mechanism, reference may be made to "Analyzing the Behavior of Acoustic Waves in Thin Films," IEEE Transactions on Materials Science, vol. 1, No. 1, pp. 1111-1115, 2003. The roughness caused by this phenomenon is relatively large in size, constituting an inhomogeneity in the thickness of the thin layer, the amplitude of which may be of the order of nanometers, and the wavelength of which may be of the order of millimeters, or even centimeters.

[0009] In addition, as documented (see, for example, Non-Patent Document 2), the sudden cleavage, oscillation and impact of the two substrates caused during splitting can also result in damage to the separate surfaces, which can be caused in particular by the impact between these two surfaces or their relative motion.

[0010] More generally, handling of the carrier substrate carrying the thin layer and the remainder of the donor substrate after the weakening step and after these two parts have been separated from each other can also cause damage, e.g. scratches or particles, on the cleaved surface of the thin layer.

[0011] It is difficult to remove surface irregularities, except in the form of relief, from thin layers using heat treatment alone. In particular, the dimensions of these irregularities, under the effect of heat treatment, can exceed the average distance of mobility of the atoms forming this surface. This is especially the case for the smoothing process proposed by the above-cited US Pat. No. 5,399,363.

[0012] Thus, thin layers prepared using prior art processes may have a damaged or imperfect surface condition from which it was not possible to remove all of the irregularities.

[0013] The present invention seeks to at least partially overcome the limitations of the prior art mentioned above. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] French Patent Application Publication No. 3061988 [Patent Document 2] French Patent Application Publication No. 2876307 [Patent Document 3] French Patent Application Publication No. 2933828 [Patent Document 4] French Patent Application Publication No. 3063176 [Non-patent literature]

[0015] [Non-Patent Document 1] “Crack Front Interaction with Self-Emitted Acoustic Waves”, Physical Review Letters,” American Physical Society, 2018, 121.(19), pp.195501 [Non-Patent Document 2] Massy, ​​et al., “Fracture dynamics in implanted silicon,” Applied Physics Letters 107.9(2015) Summary of the Invention

[0016] Aiming to achieve this object, the subject of the present invention is a process for preparing a thin layer, comprising: - Through the major surface of the donor substrate Light seeds a weakening step including the introduction of a thin layer of the first metal oxide, the first metal oxide being formed on the first surface of the donor substrate, the first metal oxide being formed on the second surface of the donor substrate, the first metal oxide being formed on the second surface of the donor substrate, the second metal oxide being formed on the first ... - bonding a major surface of a donor substrate to a receiver substrate to form an assembly to be divided, the central portion of the donor substrate and a peripheral portion of the major surface being in contact with a surface of the receiver substrate; The present invention relates to a process comprising:

[0017] According to the invention, the process comprises: - separating the assembly to be separated, comprising a heat treatment resulting in the initiation and propagation of a splitting wave in a weakened zone in order to cleave the thin layer from the donor substrate only in its central part, the splitting wave not propagating completely through the peripheral part so that the donor substrate and the receiver substrate remain attached to each other at the peripheral part of the donor substrate; a detaching step, applied after this step and distinct from the separating step, which involves the processing of the assembly to be divided in order to detach a peripheral portion of the donor substrate (2) from the receiver substrate (5) and thus to transfer the thin layer (1) to the receiver substrate (5); Also includes.

[0018] By localizing the weakened zone only in the central portion of the donor substrate, it is possible to cleave the thin layer without causing complete detachment of the donor and receiver substrates, which makes it possible to improve the surface condition of this thin layer.

[0019] According to other advantageous, non-limiting features of the present invention, taken alone or in any technically feasible combination, the process includes a finishing step, which involves a heat treatment aimed at bringing the assembly to be divided to a temperature higher than that of the heat treatment of the separating step, in order to smooth the cleaved surfaces of the thin layers; - a finishing step is applied between the separating step (S4) and the removing step, - the finishing step comprises performing a heat treatment exposing the donor substrate to a temperature less than 1000°C, less than 950°C, less than 900°C, or less than 600°C; - the separating and finishing steps are carried out in the same apparatus; - the separating and finishing steps are carried out in different apparatus; - The finishing step is applied after the removing step, - the donor substrate is a circular wafer, the peripheral portion being an annular portion, the width of which, taken from the edge of the substrate, is between 1 mm and 5 cm; - the donor substrate comprises a peripheral chamfer on a major surface thereof, the peripheral portion extending from the chamfer over a width between 1 mm and 5 cm; - Light seeds is selected from the list consisting of hydrogen and helium, - The donor substrate is made of single crystal silicon, - the step of separating comprises performing a thermal treatment by exposing the donor substrate to a temperature between 250°C and 500°C; The detaching step is carried out by chemical etching or by applying a mechanical force to separate the carrier substrate and the donor substrate. [Brief description of the drawings]

[0020] Further features and advantages of the present invention will become apparent from the following detailed description of the invention, which proceeds with reference to the accompanying drawings. [Figure 1] FIG. 2 illustrates steps of a process according to the present invention. [Diagram 2] FIG. 2 shows a central portion and a peripheral portion of a donor substrate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] With reference to FIG. 1, a process for preparing a thin layer 1 according to the present invention is now presented.

[0022] The process comprises, in a preliminary step S1, the preparation of a donor substrate 2, which has a main surface 2a. The donor substrate 2 may consist of a wafer of monocrystalline silicon, in particular if the process for preparing a thin layer is aimed at producing a silicon-on-insulator substrate. This wafer may have undergone a pretreatment, such as oxidation, formation of a surface layer by epitaxy, or deposition or any other surface preparation step.

[0023] However, the invention is in no way limited to a donor substrate 2 consisting of a silicon wafer, and the nature and shape of this substrate may be freely chosen depending on the target application. It may therefore be a substrate comprising any monocrystalline 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 therefore take the shape of a circular wafer, of any diameter from a few tens of millimeters to 300 mm or 450 mm or more. It may also take another shape, such as square or rectangular. As specified above, the donor substrate 2 may have undergone previous steps such as polishing, deposition, oxidation, etc., and may therefore be formed from a stack of heterogeneous layers.

[0024] The donor substrate 2 may have a chamfered edge 2b, as is customary in the field of semiconductor wafers, to prevent chipping thereof during handling. The chamfer 2b may extend for several millimeters around the periphery of the substrate.

[0025] As shown in FIG. 2, in the donor substrate 2 according to the present invention, a distinction is made between two parts: a central part 2c and a peripheral part 2p. These parts 2c, 2p extend from one side of the substrate to the other and are projected onto the main surface 2a of the substrate, which may have any shape, as long as the peripheral part 2p completely surrounds the central part 2c. The central part 2c is therefore projected onto the main surface 2a of the donor substrate 2 (here in the shape of a circular wafer) to form a disk centered on this surface, and onto the peripheral part 2p to form an annular surface concentric with the disk that defines the central part 2c, which may extend to the edge of the substrate 2, as shown in FIG. 2. The central part 2c and the peripheral part 2p are intended to be brought into contact with a receiver substrate, so as to join the two substrates together. When the donor substrate 2 has a chamfer 2b, the peripheral part 2p therefore extends radially from the edge of the substrate towards its center, beyond this chamfer, to allow this contact.

[0026] By way of example, the peripheral portion 2p of the donor substrate 2 may have a width between 1 mm and several centimetres, for example 5 cm, taken from the edge of the chamfer, if this chamfer is present, or from the edge of the substrate if this is not the case. Thus, in the case of the example shown in Fig. 2, where the donor substrate 2 is a circular wafer with a chamfer with a width of about 1 mm at its outer 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, such that this peripheral zone may come into contact with a receiver substrate having at least the same size. The portion aligned inside the annular portion forms the central portion 2c of the donor substrate 2.

[0027] According to the illustration of FIG. 1, the process according to the present invention is carried out through a major surface 2a of a donor substrate 2. Light seedsThe method comprises a weakening step S2, which involves the introduction of a weakening agent, the weakening step having the purpose of forming a weakened zone 3 embedded in the donor substrate 2, which extends in a plane of the donor substrate 2 that is essentially parallel to a main surface 2a of this substrate 2. The weakened zone 3, together with the main surface 2a of the donor substrate 2, defines at least a part of the thin layer 1 that is sought to be prepared. As is known per se, Light seeds is typically selected from the list consisting of, but not limited to, hydrogen and helium. Light seeds are generally introduced into the donor substrate 2 by ion implantation, although other methods of introduction are possible (by plasma, by diffusion). In the case of implantation, the implantation energy defines the depth to which the ions penetrate into the material of the donor substrate 2 and thus the depth of the buried plane in which the weakened zone 3 is located.

[0028] When the introduction is carried out by ion implantation, it is possible to envisage 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 several successive steps, for example to implant a dose of hydrogen followed by another dose of helium. Whatever the nature and the means by which the species is introduced into the donor substrate, the term "dose" refers to the total amount of species introduced into a unit area of ​​the donor substrate, for example the amount of species implanted per cm^2. Light seeds Refers to the number of.

[0029] In the present context, the weakening step S2 is performed in such a way that, at the completion of this step, the weakened zone 3 is localized in the central portion 2c of the donor substrate 2 and does not extend towards the peripheral portion 2p. As will become clear in the remainder of this specification, by localizing the weakened zone only in the central portion 2p, it is possible to cleave the thin layer without causing a complete detachment of the donor substrate and of the receiver substrate to which it is bonded, thereby making it possible to improve the surface condition of this thin layer.

[0030] Several approaches are possible for localizing the weakened zone 3 only to the central portion 2c. Thus, according to a first approach, the weakening step S2 is performed by differentiating the dose introduced in the central portion 2c from the dose introduced in the peripheral portion 2p. In this respect, reference may be made to US Pat. No. 5,399,633.

[0031] The weakening step S2 therefore involves the weakening of the central portion 2c of the donor substrate 2. Light seeds and introducing a first amount of the compound into the peripheral portion 2p of the substrate 2, the compound being lower than the first amount. Light seeds and introducing a second amount of

[0032] Light seeds The first quantity of is chosen such that it is sufficient to cleave the thin layer 1 from the donor substrate 1 at the weakened zones aligned in the central portion 2c in the subsequent separation step S4. This quantity therefore results in the formation of the weakened zones 3 in the central zone 2c. Light seeds The second quantity of is chosen such that it does not allow this detachment in the peripheral portion 2p. This quantity therefore does not result in the formation of a weakened zone within the meaning of the present specification. For complete clarity, the peripheral portion 2p of the donor substrate 2 is not implanted or introduced by any other means. Light seeds however, these species are not present in this portion 2p in a sufficient concentration to form a weakened zone in the sense of the present application, which may in particular not cause separation of the donor substrate 2 in this portion 2p.

[0033] As an example, according to one preferred implementation shown in FIG. Light seeds may be zero, in which case Light seeds is introduced only into the central portion 2c of the donor substrate 2, and is not introduced into the peripheral portion 2p.

[0034] To achieve this, when the introduction of these species is carried out by ion implantation, Light seedsIt is possible to mask the peripheral portion 2p with a shielding material that forms a barrier against the penetration of the ions. This mask may be made of Teflon, of aluminum or of carbon. It may also be formed of a sacrificial mask 4 made of a resin, of a hard oxide or of a nitride, aligned at least to the peripheral portion 2p of the main face 2a of the donor substrate 2, it being understood that this mask is removed upon completion of this weakening step S2.

[0035] When the weakening step S2 envisages multiple successive phases of ion implantation, such as simultaneous implantation of hydrogen and helium, the mask may be present only during some of the phases of implantation.

[0036] According to a variant of this first approach, the donor substrate 2 Light seeds The introduction of such a Light seeds In such an implementation of the weakening step S2, the movement of the beam may be controlled such that the peripheral portion 2p of the donor substrate 2 is excluded from the implantation or the amount of species introduced in this portion 2p is less than the amount of species introduced in the central portion 2c. In this variant, the mask may therefore be omitted, which is advantageous.

[0037] The nature of the seeds and the exact quantities of these seeds 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.

[0038] By way of example, when the donor substrate 2 is made of monocrystalline silicon and the separation step involves a heat treatment exposing the donor substrate 2 to a temperature between 250° C. and 500° C., the following conditions can be envisaged:

[0039] Hydrogen only Center part: 4E16~1E17at / cm 2 Surrounding area: 0~3E16at / cm 2 Hydrogen and Helium Center part: H:0.5E16~2E16at / cm 2 , He 0.5E16~2E16at / cm 2 Surrounding area: H:0~1E16at / cm 2 , He 0.5E16~2E16at / cm 2 According to an alternative approach, the weakening step S2 consists of applying equal amounts of ions to the central portion 2c of the donor substrate 2 and to the peripheral portion 2p of this substrate 2 during a first sub-step. Light seeds This conventional amount, for example of the order of 5E16 per cm^2 of hydrogen, is sufficient to form a weakened zone 3 in the central zone 2c extending to the peripheral zone 2p when the donor substrate 2 is made of silicon. In a second sub-step, the peripheral portion 2p of the donor substrate 2 is doped with hydrogen. Light seeds This is done by using a laser to nullify any previously introduced Light seeds This may involve treating this peripheral portion 2p in order to diffuse the weak zone 3 therein and effectively remove the weak zone 3 from this peripheral portion 2p. Alternatively, the peripheral portion may be treated in order to damage the weak zone present in this portion, for example by implanting relatively heavy species such as silicon species. Upon completion of the treatment carried out in this second sub-step, the weak zone 3 is localized only in the central zone 2c of the donor substrate 2 and no longer extends into the peripheral zone 2p.

[0040] Whatever the means by which this weakening step S2 is carried out, what results therefrom is a donor substrate 2 with a buried weakened zone 3 localized in a central portion 2c of the donor substrate 2, which, together with the main surface 2a of the donor substrate 2, defines a thin layer 1, and which weakened zone 3 does not extend towards the peripheral portions 2p.

[0041] In a subsequent bonding step S3 of the process according to the present invention, the donor substrate thus prepared is bonded to a receiver substrate 5 by contacting a major surface 2a of the donor substrate 2 with a surface of the receiver substrate 5. The receiver substrate 5 may be of any nature and of any shape required, so long as it has a surface of sufficient size to be contacted with at least a part of the central portion 2c and the peripheral portion 2p of the donor substrate 2.

[0042] This bonding operation may be carried out by any method, for example by molecular adhesion, by applying an adhesive material to at least one of the surfaces to be bonded, or by eutectic bonding, when the surfaces to be bonded have been previously prepared to have a metallic surface. The bonding step S3 may also envisage preparing the surfaces of the two substrates by cleaning, activating or any other preparation step in order to facilitate this bonding operation and to fix the two substrates relative to each other with sufficient adhesive energy.

[0043] Alternatively, the bonding operation is present in the gradual formation of the receiver substrate 5 by deposition of the constituent material of the substrate 5 on the main surface 2a of the donor substrate 2 at moderate temperatures.

[0044] The term "assembly to be split" will be used in the remainder of this specification to refer to the assembly formed from the donor substrate 2 and the receiver substrate 5 upon completion of this step S3.

[0045] In the next step of the process according to the present invention, the assembly to be divided is heat treated during the separating step S4. The heat treatment, as is conventional in the implementation of the Smart Cut technology, is carried out to prevent the development of microcavities and to prevent these microcavities from being broken down into the available Light seedsWhen the number of weak zones 3 and their compression via the heat treatment is sufficient, it causes their coalescence. As mentioned above, the buried weak zone 3 does not extend over the entire extent of the donor substrate 2. Therefore, the heat treatment leads to the cleavage of the thin layer 1 from the donor substrate 2 only in its central zone 2c and not in the peripheral zones 2p.

[0046] When the donor substrate 2 is made of silicon, the heat treatment of the cleaving step S4, typically between 200° C. and 500° C., may be applied by placing the to-be-cleaved assembly in a conventional oven for a duration that may be between a few minutes and a few hours. In practice, for productivity reasons, a large number of to-be-cleaved assemblies are typically arranged in a cassette, which is placed in a high-volume oven or stove to collectively apply a heat treatment to the to-be-cleaved assemblies at a chosen temperature and for a chosen duration.

[0047] As in the prior art detachment steps, the splitting wave initiated in this detachment step S4 does not propagate from one end of the donor substrate 2 to the other, and the remainder of the donor substrate (i.e. the donor substrate from which the thin layer was taken) is not suddenly cleaved from the receiver substrate. In particular, the splitting wave does not propagate completely to the peripheral portion 2p of the donor substrate 2, which therefore remains firmly attached to the receiver substrate 5 at this portion 2p. In the context of the process described by FIG. 1, the receiver substrate 5 and the donor substrate 2 therefore remain attached to each other at the completion of the detachment step S4, even if the thin layer 1 has been effectively cleaved. In the process herein, the generation of acoustic waves, the oscillation and the beating of these substrates are therefore limited. The adverse interaction of the splitting wave with the acoustic wave is reduced or eliminated, and also the impact or sliding of the substrates is prevented, which in the prior art processes lead to the creation or damage of irregularities in the surface of the thin layer, which are difficult to reduce only by applying a heat treatment. It should be noted that in this case, the feature of the process in which the splitting wave does not propagate completely to the peripheral portion 2p of the donor substrate 2 (the carrier substrate and the receiver substrate therefore remain joined to each other) is entirely verifiable, since it is not possible to remove the remainder of the donor substrate from the receiver substrate, i.e. to separate them from each other. This entirely verifiable character of this feature will enable the skilled person to easily, using some simple experiments, introduce into the parameters of the steps of the process, in particular the extent of the peripheral zone, the central zone and the peripheral zone respectively, together with the examples provided herein. Light seeds This makes it possible to determine the amount of, and the strength of the heat treatment in the separation step S4.

[0048] The space 6 between the cleaved surface of the thin layer 1 and the cleaved surface of the donor substrate 2 is Light seeds, which leads to the cleavage of the thin layer 1. These species are confined to this enclosed and sealed space 6, since the splitting does not propagate to the peripheral portion 2p of the donor substrate 2, which therefore remains firmly bonded to the receiver substrate 5. This enclosed and sealed space 6 is therefore also isolated from the surrounding atmosphere by the peripheral portion 2p of the donor substrate 2. External contaminants, e.g. oxygen, which would make it more difficult to smooth, are therefore prevented from penetrating this space 6 and passivating the cleaved surface of the thin layer 1. In this regard, the weakening step S2 introduced during the weakening step S2 that allows the splitting of the donor substrate 2 is Light seeds It should be noted that the gases (typically hydrogen or helium) are very pure, they do not interact with the atoms forming the cleaved surface of the thin layer so as to inactivate it and limit the surface mobility of the atoms, making this an atmosphere particularly suitable for thermal smoothing.

[0049] In order to take advantage of this very particular state of the assembly to be divided following the separating step S4, one preferred implementation of the process according to the present specification envisages applying a heat treatment during the finishing step S5 aimed at bringing the assembly to be divided to a temperature higher than that of the heat treatment of the separating step S4, in order to smooth the cleaved surface of the thin layer 1.

[0050] In this preferred implementation, the heat treatment exposes the assembly to be split to a plateau temperature higher than the temperature of the splitting step S4, between 200° C. and 500° C. when the donor substrate is made of silicon. Advantageously, this heat treatment remains moderate, below 1000° C., or below 950° C., or even below 900° C. or 600° C. It may last for a duration between a few seconds at the chosen plateau temperature and a few hours at this plateau temperature.

[0051] This heat treatment of the finishing step S5 divides the assembly to be divided into two parts, one part being a cleaved surface of the thin layer 1 and the other part being a cleaved surface of the donor substrate 2, and the other part being a cleaved surface of the thin layer 1. Light seeds These surfaces, which are not contaminated with reactive species from the external atmosphere, are composed of atoms that are made highly mobile by the increase in temperature, for example, which makes it possible to smooth the surfaces, in particular the cleavage surfaces of the thin layer 1, in a very effective manner, even at low temperatures lower than or equal to 1000° C. or 950° C. It should be noted that this treatment of the assembly to be split may be carried out in an apparatus with any atmosphere, since it is not this atmosphere that contributes to the smoothing of the surface of the thin layer 10. This apparatus, for example a stove, is only intended to increase the temperature of the assembly to be split.

[0052] Experiments carried out by the Applicant have shown that after a heat treatment of the finishing step S5, which consists in exposing the assembly to be divided to 950° C. for 2 minutes, it is possible to obtain a thin layer of silicon 1 having a surface whose relief is of the order of 1.6 nm RMS. In the absence of a finishing step, and therefore of a heat treatment at a higher temperature than that reached during the separating step S4, the relief of the thin layer was measured at 5.5 nm RMS. The effectiveness of the finishing step just described is clearly evident.

[0053] The finishing step S5 may be carried out in the same equipment as that used for the separating step S4, for example in the same oven or the same stove. However, in contrast to the prior art solutions summarized in the introduction of this application, it is also possible to carry out the finishing step S5 in another equipment different from the equipment used to carry out the separating step S4. In particular, the two substrates forming the assembly to be divided are still attached to each other, reducing the risk of damaging the thin layer 1 during handling of this assembly to store it or to move it to another equipment. In addition, since the cleavage surface of the thin layer 1 is completely confined, it cannot be exposed to the ambient atmosphere during this change of equipment. It is therefore not oxidized or passivated and all of its smoothing possibilities, even at low temperatures, are preserved when carrying out the finishing step in an environment other than that used in the separating step. Moreover, as mentioned above, the equipment in which the finishing step S5 is carried out in this implementation may be chosen very freely.

[0054] In a subsequent step, the assembly to be split undergoes a detaching step S6 aimed at detaching the donor substrate 2 (or, more precisely, the remainder of this donor substrate 2 after it has been removed) from the receiver substrate 5, thus completing the transfer of the lamina 1 to the receiver substrate 5. It should be noted that this detaching step S6 is clearly different from the separating step S4. In fact, one very advantageous feature of the process herein is that of clearly distinguishing the propagation of the splitting wave, which is caused in the separating step S4 and leads to the cleavage of the lamina 1, from this detaching step S5, which leads to the donor substrate being completely detached from the receiver substrate. In the conventional splitting steps of the prior art, the propagation of the splitting wave propagates completely through the plane of the donor substrate, detaching the donor substrate from the receiver substrate in this single step.

[0055] The process carried out to obtain the detachment during step S6 may vary. It may involve chemically etching the edges of the assembly to be split, for example wet etching, in order to detach the two substrates at their bonding interface. For this, the assembly to be split may be placed vertically to immerse a part of its edge in a solution of KOH or TMAH (when the donor substrate is made of silicon), or in a solution containing HF when a layer of silicon oxide is present at the interface between the donor substrate 2 and the receiver substrate 5. By rotating the assembly, it is processed continuously throughout in order to detach the two substrates from each other. Alternatively, as shown diagrammatically in FIG. 1, this process may involve applying a blade at the bonding interface and inserting it between the two substrates to cause separation of the assembly. More generally, it may involve any means or applying any force that results in the detachment of the carrier substrate 5 and the donor substrate 2 at their contact interface.

[0056] In any case, this detaching step S6, unlike the parting steps of prior art processes, may be perfectly controlled so that the risk of damaging the lamina 1 may be minimized.

[0057] According to another implementation, the finishing step is not performed immediately after the separating step S4, between this step S4 and the detaching step S6, but after the detaching step S6.

[0058] The finishing step S5 of this implementation may be a conventional finishing step, for example a step of exposing the cleaved surface of the thin layer 1, when it is made of silicon, to a reducing or neutral atmosphere at a temperature between 950 or 1000° C. and 1100° C. More typically, the exposed surface of the thin layer is exposed to a reducing or neutral atmosphere brought to a temperature higher than the temperature of the heat treatment of the detaching step.

[0059] This implementation takes advantage of the absence or minor presence of large sized irregularities or damage on the surface of thin layer 1. The undulations exhibited by this thin layer 1 may be effectively smoothed out via heat treatment, as explained above.

[0060] At the completion of these treatments, whatever the chosen implementation, the result is a thin layer 1 transferred to a carrier substrate 5, as illustrated by the final step S7 of FIG. 1. The thin layer 1 exhibits an improved surface condition with respect to the thin layer 1 obtained immediately after splitting using the prior art process. In particular, it exhibits a relatively low undulation due to the smoothing effect of the finishing step S5. It also exhibits little or no inhomogeneity due to the interaction of the splitting wave with the acoustic wave propagating through the receiver substrate 5 and the donor substrate 2 when they are completely detached from each other by the propagation of the splitting wave. Damage to the thin layer 1 caused by impacts between the two substrates or their relative sliding over each other is also limited.

[0061] Of course, the invention is not limited to the described implementations, variations of which may fall within the scope of the invention as defined by the claims.

[0062] In particular, other steps may be envisaged in addition to those described, in particular these may involve applying additional finishing treatments to the thin layer, such as a sacrificial oxidation step or polishing, in order to improve the quality of this layer or to adjust its thickness.

[0063] It is also possible to combine the two presented implementation forms and carry out in a first phase a finishing step S5 via a first heat treatment between the separating step S4 and the detaching step S6, and to supplement this treatment with a second heat treatment applied to the thin layer 1 in the second phase, for example after the detaching step S6.

Claims

1. A process for preparing a thin layer (1), comprising the steps of: a weakening step (S2) comprising the introduction of light species through a main surface (2a) of a donor substrate (2) with the aim of forming a weakened zone (3) in a central portion (2c) of said donor substrate (2) in order to define said thin layer (1) together with said main surface (2a) of said donor substrate (2), said weakened zone (3) not extending towards a peripheral portion (2p) of said donor substrate (2); - bonding (S3) of said main surface (2a) of said donor substrate (2) to a receiver substrate (5) to form an assembly to be divided, said central portion (2c) of said donor substrate (2) and said peripheral portion (2p) of said main surface (2a) being in contact with a surface of said receiver substrate (5); In a process including a step (S4) of separating the assembly to be separated, comprising a heat treatment resulting in the initiation and propagation of a splitting wave in the weakened zone in order to cleave the thin layer (1) from the donor substrate (2) only in its central portion (2c), said splitting wave not propagating completely through said peripheral portion (2p), said donor substrate (2) and said receiver substrate (5) remaining attached to each other in said peripheral portion (2p) of the donor substrate (2); a detaching step (S6), applied after this step and different from said separating step (S4), which comprises a processing of the assembly to be divided in order to detach said peripheral portion of the donor substrate (2) from the receiver substrate (5) and thus to transfer said thin layer (1) to said receiver substrate (5); 1. A process for preparing a thin layer (1), comprising:

2. 2. A process for preparing a thin layer (1) according to claim 1, comprising a finishing step (S5) including a heat treatment aimed at smoothing the cleaved surface of the thin layer (1) at a temperature higher than the temperature of the heat treatment of the separating step (S4).

3. The process for preparing a thin layer (1) according to claim 2, wherein said finishing step (S5) is applied between said separating step (S4) and said removing step (S6).

4. 4. The process for preparing a thin layer (1) according to claim 3, wherein the finishing step (S5) comprises carrying out a heat treatment exposing the donor substrate (2) to a temperature lower than 1000°C, lower than 950°C, lower than 900°C or lower than 600°C.

5. A process for preparing a thin layer (1) according to any one of claims 2 to 4, wherein said separating step (S4) and said finishing step (S5) are carried out in the same apparatus.

6. A process for preparing a thin layer (1) according to any one of claims 2 to 4, wherein said separating step (S4) and said finishing step (S5) are carried out in different apparatus.

7. The process for preparing a thin layer (1) according to claim 2, wherein said finishing step (S5) is applied after said removing step (S6).

8. 8. The process for preparing a thin layer (1) according to any one of claims 1 to 7, wherein the donor substrate (2) is a circular wafer and the peripheral portion (2p) is an annular portion, the width of which, taken from the edge of the donor substrate, is between 1 mm and 5 cm.

9. 8. The process for preparing a thin layer (1) according to any one of claims 1 to 7, wherein the donor substrate (2) is provided with a peripheral chamfer (2b) on its main surface (2a) side, the peripheral portion (2p) extending from the chamfer over a width between 1 mm and 5 cm.

10. Process for preparing a thin layer (1) according to any one of claims 1 to 9, wherein said light species is selected from the list formed by hydrogen and helium.

11. Process for preparing a thin layer (1) according to any of the preceding claims, wherein the donor substrate (2) is made of monocrystalline silicon.

12. 12. A process for preparing a thin layer (1) according to any one of claims 1 to 11, wherein the separating step (S4) comprises carrying out a heat treatment exposing the donor substrate (2) to a temperature between 250°C and 500°C.

13. 13. A process for preparing a thin layer (1) according to any one of claims 1 to 12, wherein the detaching step is performed by chemical etching or by applying a mechanical force to separate the carrier substrate (5) and the donor substrate (2).

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