Abstract:
The invention relates to a method for fabricating a composite structure comprising a layer to be separated by irradiation, the method comprising the formation of a stack containing:
       a support substrate formed from a material that is at least partially transparent at a determined wavelength;   a layer to be separated; and   a separation layer interposed between the support substrate and the layer to be separated, the separation layer being adapted to be separated by exfoliation under the action of radiation having a wavelength corresponding to the determined wavelength.       
 
     Furthermore, the method comprises, during the step for forming the composite step, a treatment step modifying the optical properties in reflection at the interface between the support substrate and the separation layer or on the upper face of the support substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a national phase entry under 35 U.S.C. §371 of International Patent Application PCT/IB2012/001405, filed Jul. 18, 2012, designating the United States of America and published in English as International Patent Publication WO 2013/041926 A1 on Mar. 28, 2013, which claims the benefit under Article  8  of the Patent Cooperation Treaty and under 35 U.S.C. §119(e) to French Patent Application Serial No. 1158330, filed Sep. 20, 2011, the disclosure of which is hereby incorporated herein in its entirety by this reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention concerns the field of the fabrication of composite (or multilayer) semiconductor structures, and relates more particularly to methods of separation by exfoliation, making it possible to detach one or more layers of a composite structure, for example, in the scope of transferring layers from an initial support to a final support. 
       BACKGROUND 
       [0003]    In the field of the fabrication of composite structures, it is often useful to be able to assemble and/or separate films or layers, such as, for example, semiconducting or insulating layers. Such separations are, in particular, necessary in order to transfer a layer from an initial substrate to a final substrate. These transfers are carried out, for example, during the implementation of three-dimensional component technology, which involves the formation of electronic, photovoltaic and/or optoelectronic components on both faces (“front face” and “rear face”) of the same layer (3D integration). Layer transfers are also carried out in order to transfer circuits during the fabrication of rear-face illuminated imagers. The transfer of layers is also useful for changing the substrate on which one or more layers are formed, so that the new substrate meets requirements in terms particularly of cost, physical characteristics (cell size, thermal stability, . . . ), etc. 
         [0004]    A method of thin-film transfer is described, for example, in patent document EP 0 858 110. This method provides, in particular, the separation of a film with the aid of a technique of separation by exfoliation, this technique requiring, in particular, irradiation of a composite structure through a transparent substrate. 
         [0005]    An exemplary embodiment of a method for fabricating a composite structure (steps S 1  and S 2 ) and of a method of separation by exfoliation (steps S 3  and S 4 ) will now be described with reference to  FIG. 1 . 
         [0006]    First, a so-called separation layer  10  (or optical absorption layer) is assembled by bonding on one of the faces of a support substrate  5  (step S 1 ). As indicated in more detail below, the support substrate  5  is at least partially transparent at a predetermined wavelength. 
         [0007]    A layer  15  (also referred to as “layer to be separated”) is subsequently assembled by bonding on the face of the separation layer  10  on the opposite side from the one in contact with the support substrate  5 , so as to obtain a composite structure  25  (step S 2 ). 
         [0008]    It will be noted that the assembly of the layers  5 ,  10  and  15  during steps S 1  and S 2  may be carried out by means of any suitable assembly technique, such as, for example, a technique of molecular adhesion bonding or involving an intermediate bonding layer. 
         [0009]    Furthermore, the layers  10  and  15  are not necessarily assembled by bonding in order to form the composite structure  25 . As a variant, at least one of the layers  10  and  15  may be formed by means of a suitable deposition technique. The separation layer  10  may, for example, be formed by PECVD (“plasma-enhanced chemical vapor deposition”) or LPCVD (“low-pressure CVD”) deposition. 
         [0010]    Once the composite structure  25  has been formed, separation of the separation layer  10  may be carried out by exfoliation. This method makes it possible to detach the layer  15  from the support substrate  5 . 
         [0011]    To this end, the separation layer  10  is irradiated by means of electromagnetic radiation  20  through the support substrate  5  (step S 3 ). The radiation  20  is at a wavelength for which the support substrate  5  is at least partially transparent. Here, “partially transparent” is intended to mean a substrate whose transmittance at the wavelength in question is at least 10%, and preferably greater than or equal to 50%. As indicated below, the required level of transparency will vary according to the amount of energy of the electromagnetic beam  20 , which is received by the separation layer  10 . 
         [0012]    During this irradiation step S 3 , the separation layer  10  absorbs the incident light passing through the interface  8  between the support substrate  5  and the separation layer  10 . This irradiation leads to a reduction or elimination of the adhesion forces between the atoms or molecules in the material of the separation layer  10 . This is because, under the action of the radiation  20 , the material constituting the separation layer  10  is subjected to photochemical and/or thermal excitation, which leads to the breaking of a chain of atoms or molecules. These breaks thus cause separation of the separation layer  10  by exfoliation, either in the actual thickness of the layer  10  (so-called “internal” exfoliation) or at the interface  8  between the layer  10  and the support substrate  5  or at the interface  12  between the separation layer  10  and the layer  15  to be separated (“interfacial” exfoliation). This exfoliation phenomenon may also involve one or more gases released by the material of the separation layer  10  under the action of the radiation  20 . 
         [0013]    It should be noted that the separation induced by the radiation  20  does not necessarily lead to detachment or actual separation in the separation layer  10  (or at one of the interfaces  8  and  12 ), but may simply lead to weakening of the material of the separation layer  10 . In the latter case, the application of additional energy (for example, in the form of mechanical forces) is necessary in order to obtain the actual detachment between the support substrate  5  and the layer  15  (if such detachment is actually desired). 
         [0014]    Once the substrate  5  and the layer  15  have been fully separated (step S 4 ), the support substrate  5  may be recycled with a view to forming a new composite structure. 
         [0015]    Currently, the composite structures produced according to the layout of  FIG. 1  generally have one of the following compositions:
       GaN/Al 2 O 3 , which corresponds to a separation layer  10  consisting of GaN and a support substrate  5  consisting of sapphire;   Si 3 N 4 /Al 2 O 3 , which corresponds to a separation layer  10  consisting of Si 3 N 4  and a support substrate  5  consisting of sapphire.       
 
         [0018]    With regard to these compositions, the results in terms of quality of separation by exfoliation are, in general, satisfactory. When layers of GaN deposited on a sapphire substrate are separated, for example, the application of the radiation  20  (at a wavelength of typically between 190 nm and 250 nm) takes place under good conditions and the separation is obtained without any particular difficulty. 
         [0019]    The applicants have, however, observed that the results can be significantly degraded when this separation method is applied to other compositions of the composite structure  25 . For example, the separation by exfoliation is much more difficult for a composite structure  25  of the SiO 2 /Si type (i.e., silicon dioxide on silicon). The applicants have observed large variations in terms of quality of separation by exfoliation as a function of the batches studied and, in general, less uniform separations requiring more exposure to radiation. 
         [0020]    There is, therefore a current need for composite structures better suited to the method of separation by exfoliation, irrespective of the composition of the composite structures in question. There is, in particular, a need for composite structures of unconventional composition adapted to be separated by exfoliation under good conditions. 
       BRIEF SUMMARY 
       [0021]    To this end, the present invention relates to a method for fabricating a composite structure comprising a layer to be separated by irradiation, the method comprising the formation of a stack containing at least:
       a support substrate formed from a material that is at least partially transparent at a determined wavelength;   the layer to be separated; and   a separation layer interposed between the support substrate and the layer to be separated, the separation layer being adapted to be separated by exfoliation under the action of radiation having a wavelength corresponding to the determined wavelength;       
 
         [0025]    the fabrication method furthermore comprising, during the step for forming the composite stack, at least one treatment step modifying the optical properties in reflection at the interface between the support substrate and the separation layer or on the upper face of the support substrate on the opposite side from the face of the substrate in contact with the separation layer. 
         [0026]    It is then possible to apply a method of separation by exfoliation to the composite structure of the invention, in order to detach the layer to be separated from the support substrate. To this end, an electromagnetic wave is applied to the support substrate, which partially transmits it to the underlying separation layer. 
         [0027]    The treatment step modifying the optical properties in reflection advantageously makes it possible, during the method of separation by exfoliation, to limit or avoid reflection of the light beams at the interface or interfaces treated in this way. 
         [0028]    The composite structure of the invention thus makes it possible to reduce the variations in transmitted energy as a function of the thickness of the support substrate during the method of separation by exfoliation, and to maximize the amount of energy actually transmitted into the separation layer. The quality and reproducibility of the separation by exfoliation between the support substrate and the layer to be separated are thus greatly improved. The method makes it possible, in particular, to carry out separations by exfoliation on composite structures that have compositions different from those generally used and that are particularly sensitive to optical interference. 
         [0029]    According to a first embodiment of the invention, the treatment step comprises the formation of at least one antireflection layer at the interface between the support substrate and the separation layer and/or on the upper face of the support substrate. 
         [0030]    The effect of this antireflection layer is to combine two reflections in phase opposition so that they are destructive, which advantageously makes it possible to reduce or prevent any reflection of the light radiation transmitted by the support substrate during the method of separation by exfoliation. The transmission of the light energy through the support substrate to the separation layer is, therefore, maximized, which makes it possible to significantly improve the quality of the separation by exfoliation. 
         [0031]    The refractive index n 2  of the antireflection layer is preferably substantially equal to √{square root over (n 1 ×n 3 )}, where n 1  and n 3  are, respectively, the refractive index of the support substrate and of the separation layer. Even more preferably, n 2 =√{square root over (n 1 ×n 3 )}. 
         [0032]    Setting the optical index n 2  at a value close (for example, to within +10%, or even to within ±5%) or equal to √{square root over (n 1 ×n 3 )}, advantageously makes it possible to limit the undesirable reflections during the irradiation step of the method of separation by exfoliation to a minimum. 
         [0033]    Furthermore, the separation layer may comprise a heating sublayer and an exfoliation sublayer, the exfoliation sublayer being adapted to decompose under the action of heating induced by the heating sublayer. 
         [0034]    In a particular embodiment, the support substrate consists of silicon, the heating sublayer consists of silicon dioxide, the exfoliation sublayer consists of Si 3 N 4  and the antireflection layer consists of SiON, the refractive index of which is substantially equal to (or equal to) 2.61. 
         [0035]    According to a variant of the first embodiment, when an antireflection layer is formed at the interface between the support substrate and the separation layer, the composition of the antireflection layer varies gradually over its thickness so that its refractive index n 2  is equal to n 1  at its interface with the support substrate and is equal to n 3  at its interface with the separation layer. 
         [0036]    This variant advantageously makes it possible to avoid significant jumps in optical index at the interfaces involved in the composite structure. The reflections at the interface between the antireflection layer and the support substrate are thus minimized. 
         [0037]    According to this variant, the separation layer may comprise a heating sublayer consisting of silicon dioxide and an exfoliation sublayer consisting of Si 3 N 4 , the exfoliation sublayer being adapted to decompose under the action of heating induced by the heating sublayer, the support substrate consisting of silicon, and the composition of the antireflection layer may consist of SiO x , where x varies gradually between 0 and 2. 
         [0038]    As indicated above, an antireflection layer may be formed on the upper face of the support substrate. If this is the case, the composition of the antireflection layer may vary gradually over its thickness so that its refractive index n 2  is equal to n 1  at its interface with the support substrate and is equal to n 0  at its upper surface on the opposite side from the interface with the support substrate, n 0  being the refractive index of the medium in which the composite structure is placed during the irradiation. 
         [0039]    This gradual variation of the composition advantageously makes it possible to avoid significant jumps in optical index at the interface between the upper face of the support substrate and the surrounding medium in which the composite structure is placed. The reflections at the upper face of the support substrate will thus be minimized, which makes it possible to have maximum light energy transmitted to the separation layer during the separation method. 
         [0040]    Furthermore, when an antireflection layer is formed at the interface between the support substrate and the separation layer, the thickness (denoted e 1 ) of the antireflection layer is preferably substantially equal to (2M+1)·λ/4, M being an odd integer greater than or equal to 0 and λ being the determined wavelength. Even more preferably, e 1 =(2M+1)·λ/4. By adjusting the thickness of the antireflection layer in this way, it is possible to reduce its reflection to a minimum. 
         [0041]    According to a second embodiment of the invention, the treatment step comprises the formation of at least one rough layer at the interface between the support substrate and the separation layer and/or on the upper face of the support substrate, the rough layer having a mean roughness level greater than or equal to 50 Å rms over a measurement field equal to at least three times the dimension of the upper zone of the face to be irradiated with the radiation. 
         [0042]    The formation of such a rough layer advantageously makes it possible to modify the physical properties at the interface in question, the latter being converted from the specular state to the diffuse state. This makes the composite structure of the invention insensitive or not very sensitive to both constructive and destructive optical interference (see below). 
         [0043]    The formation of the rough layer may be carried out at least by mechanical grinding or chemical etching. 
         [0044]    In relation to this, the present invention also relates to a method for separating a layer from a composite structure of the invention as described above. The separation method comprises irradiation of the separation layer through the support substrate of the structure by means of incident light corresponding to the wavelength at which the support substrate is at least partially transparent, in order to induce weakening or separation by exfoliation of the separation layer. 
         [0045]    The invention also relates to a composite structure comprising a composite stack formed from at least:
       a support substrate formed from a material that is at least partially transparent at a determined wavelength;   a layer to be separated by irradiation; and   a separation layer interposed between the support substrate and the layer to be separated, the separation layer being adapted to be separated by exfoliation under the action of radiation having a wavelength corresponding to the determined wavelength,   the composite structure furthermore comprising at least one layer modifying the optical properties in reflection at the interface between the support substrate and the separation layer or on the upper face of the support substrate on the opposite side from the face of the substrate in contact with the separation layer.       
 
         [0050]    The embodiments and advantages explained above with reference to the fabrication method according to the invention apply similarly to the composite structure of the invention. 
         [0051]    According to a particular embodiment, each layer modifying the optical properties in reflection corresponds, respectively, to an antireflection layer and/or to a rough layer having a mean roughness level greater than or equal to 50 Å rms over a field of 5 μm×5 μm. 
         [0052]    Furthermore, the structure may comprise at least one layer modifying the optical properties in reflection corresponding to an antireflection layer, and at least one layer modifying the optical properties in reflection corresponding to a rough layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0053]    Other characteristics and advantages of the present invention will become apparent from the description given below with reference to the appended drawings, which illustrate an exemplary embodiment thereof, implying no limitation. In the figures: 
           [0054]      FIG. 1  schematically represents a known method (steps S 1  and S 2 ) for fabricating a composite structure, as well as a method (steps S 3  and S 4 ) of separation by exfoliation applied to the said structure; 
           [0055]      FIG. 2  schematically illustrates the mechanism of the creation of optical interference in the support substrate of a composite structure; 
           [0056]      FIGS. 3A and 3B  schematically represent a method (steps S 10 -S 14 ) for fabricating a composite structure according to a first embodiment of the invention, as well as a method (S 16 -S 18 ) of separation by exfoliation applied to the structure; 
           [0057]      FIGS. 4A and 4B  schematically represent a method (steps S 110 -S 114 ) for fabricating a composite structure according to a second embodiment of the invention, as well as a method (S 116 -S 118 ) of separation by exfoliation applied to the structure; and 
           [0058]      FIGS. 5A and 5B  schematically represent the irradiation of a composite structure according to two variants of the second embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0059]    The present invention relates to a composite structure that can be separated by exfoliation under good conditions, even for compositions that traditionally do not offer good results. 
         [0060]    The applicants have carried out a study that has made it possible to demonstrate the physical mechanism giving rise to the difficulties encountered for carrying out a method of separation by exfoliation. The study has, in particular, demonstrated the role of the optical interference occurring in the support substrate during the irradiation of the composite structure. 
         [0061]    This mechanism will now be described with reference to  FIG. 2 . This figure represents the composite structure  25  as described above with reference to  FIG. 1 . 
         [0062]      FIG. 2  represents, in particular, an incident light ray  22   a  reaching the exposed surface  5   a  of the support substrate  5  during the irradiation step S 3 . As for any light ray encountering a semi-absorbent medium, a part (not represented) of the light ray  22   a  is reflected at the upper surface  5   a  of the substrate  5  while a part  22   b  is transmitted into the support substrate  5 . During its passage through the thickness of the support substrate  5 , a part of the transmitted ray  22   b  is absorbed and the remaining part reaches the interface  8  between the support substrate  5  and the separation layer  10 . The interface  8  again functions as an optical dioptre so that the ray  22   b  is partially reflected (reflected ray  22   d ) and the remaining part  22   c  is transmitted into the separation layer  10 . The reflected ray  22   d  again encounters the upper surface  5   a  and leads to new processes of internal reflection within the support substrate  5 . 
         [0063]    However, the various light rays ( 22   b ,  22   d ,  22   e  . . . ) passing through the thickness of the support substrate  5  interact with one another in order to create, depending on their phase shift, waves with higher intensities (this is referred to as “constructive interference”) or waves with lower intensities (this is referred to as “destructive interference”). These interference phenomena give rise to the strong variations and significant reductions observed by the applicants in relation to the radiative energy transmitted by the support substrate to the separation layer. 
         [0064]    More particularly, the studies have shown that the level of the interference depends strongly on the jumps in the optical index (or refractive index) that are encountered by the radiation at the exposed surface  5   a  and at the interface  8 . Ideally, the optical index difference between the various materials involved should be minimized. With regard to the compositions of conventional composite structures (namely, GaN/Al 2 O 3  or Si 3 N 4 /Al 2 O 3 ), the situation is particularly favorable since the index jumps are at most 0.87 for radiation with a wavelength of between 150 nm and 300 nm (the optical indices of sapphire and Si 3 N 4  being 1.87 and 2.27, respectively). 
         [0065]    On the other hand, the situation is much less favorable for an SiO 2 /Si composition in which the optical indices of silicon dioxide and silicon are, respectively, 1.992 and /3.42 for radiation having a wavelength of more than 1.5 μm. If the exposed surface  5   a  is in contact with air (with an index equal to 1), then, for example, very large jumps in optical index are obtained (of the order of 2.4 or 1.4 between each of the layers) for radiation with a wavelength of 9.3 μm. These large jumps in optical index contribute to generating very strong variations in the light intensity transmitted to the separation layer  10  during step S 3 . 
         [0066]    Furthermore, the level of sensitivity to interference in relation to the thickness of the support substrate is extreme in the above case of a separation layer consisting of silicon dioxide and a support substrate consisting of silicon. Specifically, a maximum transmitted intensity is observed with a silicon thickness period of 1.35 μm. In other words, when considering a thickness L of the support substrate consisting of silicon for which a transmission maximum is reached, a transmission minimum will, in principle, be obtained for a thickness L+1.35/2 μm and a new transmission maximum will be reached for a thickness L+1.35 μm. This means that it would be necessary to control the thickness of the support substrate consisting of silicon with a level of /recision much better than 0.675 μm in order to avoid significant variations in transmission of the radiation into the separation layer. Such a level of control is not currently envisageable for support substrates consisting of silicon, the thickness of which generally varies with an amplitude of the order of 5 μm and 1.5 μm for a wafer diameter of 200 mm and 300 mm, respectively (Total Thickness Variation or “TTV”). 
         [0067]    The level of control over the thickness for substrates such as those consisting of silicon is, therefore, insufficient, which contributes to the problem of the variations in transmitted energy as explained above. 
         [0068]    The problems of transmission and optical interference have not hitherto been dealt with effectively, in particular, because they have a limited impact on composite structures of traditional composition. The Applicant has, therefore, developed a new composite structure making it possible to overcome the aforementioned drawbacks, and to do so irrespective of the composition of the structure in question. 
         [0069]    A composite structure according to a first embodiment of the invention will now be described with reference to  FIGS. 3A and 3B . These figures represent an example of a fabrication method (steps S 10  to S 14 ) making it possible to obtain this composite structure, as well as a method of separation by exfoliation (steps S 16  to S 18 ) applied to the structure. 
         [0070]    As represented in  FIG. 3A , an antireflection layer  106  of thickness e 1  is first formed by deposition on the surface  105   b  of the support substrate  105  (step S 10 ). The antireflection layer  106  may be produced by any suitable deposition technique (for example, PECVD or LPCVD deposition) or any other suitable layer formation technique. It is also conceivable to assemble the antireflection layer  106  by bonding on the surface  105   b  of the support substrate  105 . The purpose and the characteristics of this antireflection layer  106  will be explained in more detail below. 
         [0071]    In the example described here, the support substrate  105  consists of silicon. 
         [0072]    A separation layer  110  is subsequently assembled by bonding on the exposed face of the antireflection layer  106 , that is to say, the face on the opposite side from the one in contact with the support substrate  105  (step S 12 ). 
         [0073]    A layer  115  to be separated is also assembled by bonding on the exposed face of the separation layer  110 , so as to form the composite structure  125  (step S 14 ). 
         [0074]    The layers  110  and  115  are not necessarily assembled by bonding in order to form the composite structure  125 . As a variant, at least one of the layers  110  and  115  may be formed by means of a suitable deposition technique. The separation layer  110  may, for example, be formed by PECVD or LPCVD deposition. 
         [0075]    Once the structure  125  has been formed, the layer  115  may be detached from the support substrate  105  by separating the separation layer  110  by exfoliation (steps S 16  and S 18 ). It will be noted that the composite structure  125  may be subjected to complementary technology steps before the method of separation by exfoliation is carried out. One or more layers may, in particular, be formed or assembled on the exposed face of the layer  115  (for example, a final substrate), optionally after technology steps have been carried out on the rear face of the layer  115  (formation of components, etc.). 
         [0076]    In order to separate the layer  115  from the support substrate  105 , electromagnetic radiation  120  is first applied onto the separation layer  110  through the support substrate  105  (step S 16 ). This radiation, in the foam of a beam, has a wavelength  2  at which the support substrate  105  is at least partially transparent. At this wavelength, the substrate  105  has a transmittance of at least 10%, and preferably greater than or equal to 50%. It is, however, possible to compensate for a low transmittance by increasing the amount of energy of the beam applied during step S 16  (for example, by focusing the beam). 
         [0077]    In the example envisaged here, the separation layer  110  may comprise a first so-called heating sublayer (for example, consisting of silicon dioxide) and a second so-called exfoliation sublayer (for example, consisting of Si 3 N 4 ). The function of the heating sublayer is to induce heating under the effect of the irradiation during step S 16 . The function of the exfoliation sublayer is to lead to separation of the separation layer  110  by exfoliation under the effect of the heat energy transmitted from the heating sublayer (by thermal conduction). The exfoliation sublayer is adapted to decompose or weaken under the effect of the heating induced by the heating sublayer during the irradiation. 
         [0078]    In a variant, the separation layer  110  comprises at least one sublayer that simultaneously fulfills the function of heating and exfoliation. 
         [0079]    With regard to the antireflection layer  106 , it has the effect of combining two reflections in phase opposition so that they are destructive, which advantageously makes it possible to reduce or prevent any reflection of the light radiation transmitted by the support substrate  105 , while maximizing the transmission to the separation layer  110 . The function of the antireflection layer is, therefore, the opposite to that of a layer of the mirror type, which would maximize the reflection at the interface between the support substrate  105  and the separation layer  110 . In this regard, the layer  106  constitutes a layer modifying the optical properties in reflection at the interface between the support substrate  105  and the separation layer  110 . 
         [0080]    As indicated with reference to the composite structure  125 , the separation induced by the radiation  120  does not necessarily lead to detachment or actual separation in the separation layer  110 , but may simply result in weakening of the layer  110 , requiring the subsequent application of additional energy (for example, in the form of mechanical forces) in order to obtain actual detachment between the support substrate  105  and the layer  115 . 
         [0081]    Once the substrate  105  and the layer  115  have been fully separated (step S 18 ), the support substrate  105  may be recycled with a view to forming a new composite structure. This recycling may optionally involve removal of the antireflection layer  106 . 
         [0082]    In order to prevent the undesirable reflections as much as possible during the irradiation step S 16 , the antireflection layer  106  should have an optical index n 2  close, and preferably equal, to the square root of the product of the optical indices of the layers lying on either side. In other words, the index n 2  should satisfy the following condition (1): 
         [0000]        n 2=√{square root over ( n 1 ·n 3)}±10%
 
         [0083]    where n 1  and n 3 , respectively, correspond to the optical indices of the support substrate  105  and of the separation layer  110 . 
         [0084]    The index n 2  is preferably selected according to the following condition (2): 
         [0000]        n 2=√{square root over ( n 1· n 3)}
 
         [0085]    In the example envisaged here, it is assumed that n 1  and n 3  are 3.42 and 1.992, respectively. The optical index n 2  of the antireflection layer  106  is, therefore, of the order of 2.61, and preferably equal to this value. 
         [0086]    Furtheiinore, the reflectivity of the antireflection layer  106  depends on its thickness el. The ideal case in which its reflectivity is minimal is achieved for a layer thickness e 1 =(2M+1)λ/4, where M is an integer greater than or equal to 0. 
         [0087]    In the example envisaged here, the wavelength λ of the radiation  120  is 9.3 μm. In order for the reflectivity of the antireflection layer  106  to be minimal, it is expedient for its thickness to be about e 1 =4.6 μm (the case when n 1 &gt;n 3 ). 
         [0088]    Moreover, as indicated above, the level of the reflections depends strongly on the jumps in optical index between the materials involved (i.e., the differences between n 1 , n 2  and n 3 ). According to a variant of this first embodiment, the antireflection layer  106  is such that its optical index n 2  varies gradually over the thickness of the layer  106  so as to be equal to the value of n 1  at the interface with the support substrate  105  and equal to n 3  at the interface with the separation layer  110 . To this end, the antireflection layer  106  envisaged here is produced by depositing a layer of Si—O x  gradually richer and richer in silicon dioxide in order to reach the SiO 2  stoichiometry (x varying progressively between 0 and 2). 
         [0089]    This variant advantageously makes it possible to avoid significant jumps in optical index at the interfaces involved and, therefore, to minimize the reflections at the interface between the antireflection layer  106  and the support substrate  105 . 
         [0090]    In order to minimize the reflections while optimizing the transmission of the radiation to the separation layer  110 , it is possible to produce one or a plurality of nanostructured sublayers in the antireflection layer  106 , which are known by the term “moth-eye” and the principle of which is to contain patterns with dimensions very much less than the wavelength X of the radiation  120  used. These “moth-eye” layers have, in particular, a very low level of reflection for omnidirectional incident light. 
         [0091]    These layers of the moth-eye type, which are used particularly in the solar cells, are, for example, described in detail in the document “Characterization of antireflection moth-eye film on crystalline silicon photovoltaic module,” Noburu Yamada et al.,  Optics Express , Vol. 19, Issue S2, pp. A118-A125). Nanostructures of the moth-eye type may, for example, be produced in the form of thin films by plasma treatment or by a complex holographic optical method. 
         [0092]    For the different variants described above, it will be noted that it is possible to arrange the antireflection layer  106  on the exposed face  105   a  of the support substrate  105  instead of being positioned at the interface between the support substrate  105  and the separation layer  110 . This variant, however, is less preferred because it allows at least one internal reflection of the light ray in the support substrate  105 , namely, reflection of the ray reaching the interface between the support substrate  105  and the separation layer  110  for the first time. 
         [0093]    In order to be optimal, the latter variant should satisfy condition (1), and preferably condition (2), which were mentioned above in connection with the optical indices n 1 , n 2  and n 3 . In the example envisaged here, n 2  should, therefore, be equal to 2.61. Furthermore, the thickness (denoted e 1 ′) is preferably such that e 1 ′=(2P+1)λ/4, where P is an integer greater than or equal to 0. Thus, for a wavelength λ of 9.3 μm, the antireflection layer  106  arranged on the face  105   a  of the support substrate  105  is preferably an SiON alloy with an optical index of 2.61 and a thickness of 2.32 μm, 6.98 μm, etc. 
         [0094]    Alternatively, when an antireflection layer  106  is formed on the upper face  105   a  of the support substrate, the composition of the antireflection layer may vary gradually over its thickness so that its refractive index n 2  is equal to n 1  at its interface with the support substrate  105  and is equal to n 0  at its upper surface (i.e., its exposed surface) on the opposite side from the interface with the support substrate  105 , n 0  being the refractive index of the surrounding medium in which the composite structure is placed during the irradiation. 
         [0095]    This index gradient advantageously makes it possible to avoid significant jumps in optical index at the interfaces involved and, therefore, to minimize the reflections at the interface between the antireflection layer  106  and the support substrate  105 . 
         [0096]    A composite structure according to a second embodiment of the invention will now be described with reference to  FIGS. 4A ,  4 B,  5 A and  5 B. 
         [0097]    More particularly, these figures represent an example of a fabrication method (steps S 110  to S 114 ), making it possible to obtain a composite structure  225 , as well as a method of separation by exfoliation (steps S 116  to S 118 ) applied to the structure. 
         [0098]    As represented in  FIG. 4A , the support substrate  205  is first subjected to a roughening step S 110 , making it possible to increase the roughness of one of its surfaces (denoted  205   b ). The mean roughness reached on the surface  205   b  is preferably greater than or equal to 50 Å Luis (A for “angstrom” and rms for “root mean square”), and even more preferably at least equal to 100 Å rms over a measurement field (or surface) equal to at least three times the dimension of the zone irradiated (during the subsequent irradiation step S 116 ). Specifically, the level of roughness should be high over a field with, for example, the dimensions of a laser spot (i.e., 1 μm×1 μm, for example). This measurement field may, for example, be 5 μm×5 μm. 
         [0099]    This roughening is, for example, produced with the aid of a chemical method of the etching type (for example, TMAH etching when the support substrate  205  consists of silicon) or a method of the mechanical type, such as mechanical abrasion or grinding, sandblasting, polishing, etc. Alternatively, a combination of at least two of these methods may be used in order to achieve the desired level of roughness and surface condition. 
         [0100]    The separation layer  210  is then assembled by bonding on the rough face  205   b  of the support substrate  205  (step S 112 ). An interface  206  between the support substrate  205  and the separation layer  210  is thus obtained, which is characterized by an increased roughness at least equal to  50  A rms, or even 100 Å rms, over a field equal to at least three times the irradiated zone (for example, a field of 5 μm×5 μm). This high level of roughness extends over a layer (or zone)  207  of thickness e 2  lying at the interface  206  between the support substrate  205  and the layer  210  to be separated. This zone  207  corresponds to a layer consisting of the materials of the support substrate  205  and of the separation layer  210 , which lie on either side of the interface  206  over a thickness e 2 . This thickness e 2  may, for example, be more than  300  A and, for example, reach a value of 500 Å. 
         [0101]    In the example described here, the support substrate  205  consists of silicon. Furthermore, the separation layer  210  is identical to the separation layer  110 . It comprises, for example, a heating sublayer consisting of silicon dioxide and an exfoliation sublayer consisting of Si 3 N 4  (not represented in the figures). 
         [0102]    Once the step S 112  has been carried out, a layer  215  is assembled by bonding on the exposed face (i.e., the rear face on the opposite side from the face in contact with the support substrate  205 ) of the separation layer  210  (step S 114 ). 
         [0103]    It will be noted that the layers  210  and  215  are not necessarily assembled by bonding in order to form the composite structure  225 . As a variant, at least one of the layers  210  and  215  may be formed by means of a suitable deposition technique. The separation layer  210  may, for example, be formed by PECVD or LPCVD deposition. 
         [0104]    Once the structure  225  has been formed, it is possible to detach the layer  215  from the support substrate  205  (steps S 114  and S 116 ) by carrying out a method of separation by exfoliation identical to the one described above with reference to the composite structure  125 . 
         [0105]    The composite structure  225  may be subjected to complementary technology steps before the method of separation by exfoliation is carried out. One or more layers may, in particular, be formed or assembled on the exposed face of the layer  215  (for example, a final substrate), optionally after technology steps have been carried out on the rear face of the layer  215  (formation of components, etc.). 
         [0106]    In order to separate the layer  215  from the support substrate  205 , electromagnetic radiation  220  is first applied onto the separation layer  210  through the support substrate  205  (step S 116 ). This radiation has a wavelength λ at which the support substrate  205  is at least partially transparent. At this wavelength, the substrate  205  has a transmittance of at least 10%, and preferably greater than or equal to 50%. As indicated with reference to the substrate  105 , however, it is possible to compensate for a low transmittance of the support substrate  205  by increasing the energy of the beam applied during the irradiation step S 116 . 
         [0107]    The effect of the roughening previously carried out is to modify the physical properties at the interface  206 , the latter being converted from the specular state to the diffuse state. The rough layer  207  modifies, in particular, the optical properties in reflection at the interface  206  of the composite structure  225 . By virtue of this rough layer  207 , the composite structure  225  is insensitive, or not very sensitive, to both constructive and destructive optical interference. The layer  206 , therefore, constitutes a layer modifying the optical properties in reflection at the interface between the support substrate  205  and the separation layer  210 . 
         [0108]    It should be noted that, according to an alternative, it is possible to roughen the separation layer  210  before assembly with the support substrate  205 . The surface  205   b  of the support substrate and the surface of the separation layer  210 , which is intended to come into contact with the surface  205   b , may also be roughened. In all cases, the level of roughness obtained on the layer  207  is as described above. 
         [0109]    As represented in  FIG. 5A , when an incident light ray  222   a  reaches the exposed surface  205   a  of the composite structure  225  in step S 116 , a part  222   b  of the ray is transmitted in the support substrate  205  as far as the interface  206 . The effect of the interface  206  is to make the interface  206  diffuse, so that the ray  222   b  is scattered in different directions at the interface  206 . The scattered rays  222   c  are thus transmitted partially into the separation layer  210  along multiple directions, the remaining part being scattered into the support substrate  205 . This scattering makes it possible to avoid multiple reflections occurring in the support substrate  205  of the composite structure  225 , these reflections giving rise to undesirable optical interference. 
         [0110]    Although a part of the incident radiant energy is not transmitted to the separation layer  210 , this embodiment is nevertheless advantageous in that it makes it possible to suppress the sensitivity of the composite structure to variations in transmitted energy as a function of the thickness of the support substrate. 
         [0111]    As indicated with reference to the structures  25  and  125 , the separation induced by the radiation  220  does not necessarily give rise to detachment or actual separation in the separation layer  210 , but may simply give rise to weakening of the layer  210 , requiring the subsequent application of additional energy (for example, in the form of mechanical forces) in order to obtain actual detachment between the support substrate  205  and the layer  215 . 
         [0112]    Once the substrate  205  and the layer  215  have been fully separated (step S 118 ), the support substrate  205  may be recycled with a view to forming a new composite structure. This recycling may optionally involve removal of the rough layer  207  by polishing the surface  205   b  of the support substrate (or by any other suitable method). 
         [0113]    According to a variant, represented in  FIG. 5B , of this second embodiment, it is the exposed surface  205   a  of the support substrate  205  that is roughened before, during and/or after formation of the composite structure  225 . A layer with increased roughness is thus obtained (with the same value as that defined for the first variant) over a zone of thickness e 3  on the exposed surface  205   a . This zone constitutes a layer  227  modifying the optical properties in reflection of the composite structure. The thickness e 3  is of the same order of value as the thickness “e 2 ”. 
         [0114]    In this variant, the effect of the rough surface  205   a  (and more generally the rough layer  207 ) is to scatter each light ray  224   a  irradiated during step S 116  in multiple directions. Among the scattered rays  224   b , a ray  224   c  is transmitted to the interface  206 . A part  224   d  of this ray is then transmitted into the separation layer  210 . This variant, however, has a drawback in relation to that of  FIG. 5A , in that a reflection cannot be avoided at the interface  206  between the support substrate  205  and the separation layer  210 . The ray  224   e  reflected by the interface  206  is accordingly radiative energy not transmitted to the separation layer  210 . 
         [0115]    The ray  224   e  is subsequently scattered by the rough layer  227  on the surface  205   a , which makes it possible to avoid multiple reflections occurring in the support substrate  205 . This variant, therefore, also makes it possible to prevent the generation of detrimental optical interference in the thickness of the support substrate. 
         [0116]    The composite structure of the present invention, therefore, advantageously makes it possible to reduce the variations in energy transmitted during the irradiation step as a function of the thickness of the support substrate and to maximize the amount of energy actually transmitted into the separation layer. The quality and reproducibility of the separation by exfoliation between the support substrate and the layer to be separated are thereby thus greatly improved. The invention makes it possible, in particular, to carry out separations by exfoliation on composite structures that have compositions different from those generally used and that are particularly sensitive to optical interference. 
         [0117]    As other variants, it is conceivable to arrange an antireflection layer  106  on each of the faces of the support substrate of the composite structure, so as to modify the optical properties in reflection of the substrate on both of its faces (i.e., on its exposed surface and at the interface with the separation layer). 
         [0118]    It is also possible to roughen the two faces of the support substrate (in which case the support substrate comprises a rough layer  207  and a rough layer  227 ). This variant, however, leads to a more significant loss in terms of radiative energy transmission during the irradiation step. 
         [0119]    Alternatively, the support substrate may have an antireflection layer on one of its faces, the other surface being roughened as described above. 
         [0120]    It will furtheimore be noted that the support substrate and each of the layers forming the composite structure according to the invention may consist of a plurality of sublayers. In particular, as described above, the antireflection layer  106  may be formed by two or more sublayers. 
         [0121]    According to another variant of the invention, the composite structure may comprise both an antireflection layer  106  at the interface between the support substrate and the separation layer and a rough layer  227  on the upper (i.e., exposed) face of the support substrate, or conversely, a rough layer at the interface between the support substrate and the separation layer and an antireflection layer on the upper face of the support substrate.