Abstract:
The invention relates to a process for annealing a structure that includes at least one wafer, with the annealing process including conducting a first annealing of the structure in an oxidizing atmosphere while holding the structure in contact with a holder in a first position in order to oxidize at least portion of the exposed surface of the structure, shifting the structure on the holder into a second position in which non-oxidized regions of the structure are exposed, and conducting a second annealing of the structure in an oxidizing atmosphere while holding the structure in the second position. The process provides an oxide layer on the structure.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to the field of annealing semiconductor structures in oxidizing atmospheres used especially during the fabrication of multilayer semiconductor substrates (also called multilayer semiconductor wafers) obtained by transferring at least one layer onto a support substrate, such as Bonded Silicon-On-Insulator (BSOI) structures. 
       BACKGROUND OF THE INVENTION 
       [0002]    Fabricating substrates with oxide layers is well known in the art. An example of which is US Publication No. 2009/00170287 to Yuta Endo et al. (the &#39;787 application). The &#39;787 application relates to a process for fabricating an SOI substrate that uses a cleave technique to transfer a layer. Many processing steps (bonding, heat treatment, lift-off, etching, oxidation) are carried out between successive anneals so that, as a result, it is not the same structure that undergoes each anneal in the &#39;787 application. This document discloses localized oxidations at different moments in the fabrication process. 
         [0003]    Another example of a known process for fabricating a multilayer structure is also described with reference to  FIGS. 1A to 1F . 
         [0004]    As shown in  FIGS. 1A and 1B , a composite structure  100  is formed by assembling a first wafer  101  and a second wafer  102 , for example made of silicon. The first and second wafers,  101  and  102 , here have the same diameter. They could however have different diameters. 
         [0005]    An additional oxide layer (not shown) may be formed on one or both wafers,  101  and  102 , before placing them in contact, especially when fabricating an SOI (silicon-on-insulator) structure. 
         [0006]    The first wafer  101  has a chamfered edge, namely an edge comprising a top chamfer  104  and a bottom chamfer  105 . The role of these chamfers is to make handling the wafers easier and to prevent edge flaking that could occur if these edges were sharp, such flakes being a source of particulate contamination on the surfaces of the wafers. 
         [0007]    In the example described herein, the wafer assembly is produced using the direct bonding (molecular adhesion) technique, a process that is well known to those of ordinary skill in the art. 
         [0008]    Once the bonding is completed, the composite structure  100  undergoes a stabilizing anneal. The purpose this anneal is to strengthen the bond between the first wafer  101  and the second wafer  102 , and to provide these two wafers with a protective oxide cover layer. For this purpose, the stabilizing anneal is carried out in an oxidizing atmosphere so as to form an oxide layer  110  on the entire exposed surface of the composite structure  100  ( FIG. 1C ). The oxide layer  100  thus forms a protective layer that protects the composite structure from chemical etching, especially during subsequent processing operations. 
         [0009]    The structure  100  is then trimmed, which comprises essentially eliminating an annular portion of the layer  106  comprising the chamfer  105  ( FIGS. 1D and 1E ). Such trimming is necessary because the presence of the chamfer  105  prevents good contact from being achieved between the first and the second wafers at their periphery. There is therefore a peripheral region where the transferred layer is weakly bonded, or not at all bonded, to the second wafer. After transfer of the layer, this peripheral region of the transferred layer must be removed because it is liable to break in an uncontrolled way and to contaminate the structure with undesirable fragments or particles. 
         [0010]    Preferably, a trimming process called hybrid trimming is used. This process includes carrying out a first trimming by an entirely mechanical action or by mechanical machining ( FIG. 1D ), followed by a second, at least partially non-mechanical trimming for trimming the remaining thickness of the first wafer ( FIG. 1E ). This second trimming corresponds in general to chemical trimming that is selective with respect to the oxide layers formed on the wafer(s)  101  and/or  102 . The hybrid trimming especially prevents peel-off both at the bonding interface between the transferred layer and the second wafer and in the transferred layer itself. 
         [0011]    In the example described here, the chemical trimming comprises chemical etching or wet etching. The chemical etching solution is chosen depending on the material to be etched. In this example, for etching silicon, either of tetramethyl ammonium hydroxide (TMAH) or potassium hydroxide KOH solutions can be used, with both solutions being well known to those skilled in the art. The first wafer  101  may then be thinned so as to form a transferred layer  106  having a predetermined thickness e ( FIG. 1F ). This thickness e may for example be about 10 μm. 
         [0012]    Oxidation of the composite structure  100  ( FIG. 1C ) before the trimming operation prevents the second wafer  102  from being chemically etched by a TMAH solution during the chemical trimming. The oxide layer  110  also protects the surface of the first wafer  101  from the trimming chemical etch, before it is thinned. 
         [0013]    After chemical trimming, however, the appearance of defects has been observed at the edges of the composite structures. More specifically, the defects observed are nicks distributed over the edges (or edge face) of the second wafer of the composite structures. These nicks are undesirable for the fabricator since they may, for example, be the source of flakes coming from the second wafer, these flakes being liable to contaminate the exposed surface of the first wafer. More generally, these defects attest to a non-optimized fabrication process and as a result, the multilayer structures thus produced are less attractive. These defects may moreover cause various problems during supplementary technological steps on these defective composite structures, such as during the fabrication of microcomponents on the exposed surface of the first wafer. 
         [0014]    It is therefore necessary to produce composite structures without such defects even when the process for fabricating these structures comprises one or more steps implementing chemical etches. Such chemical etches may occur especially, but not exclusively, during a trimming or thinning operation on the first wafer. The present invention now provides such structures. 
       SUMMARY OF THE INVENTION 
       [0015]    The invention provides a process for annealing a structure comprising at least one wafer, with the annealing process comprising conducting a first annealing of the structure in an oxidizing atmosphere while holding the structure in contact with a holder in a first position in order to oxidize at least portion of the exposed surface of the structure, shifting the structure on the holder into a second position in which non-oxidized regions of the structure are exposed, and conducting a second annealing of the structure in an oxidizing atmosphere while holding the structure in the second position. 
         [0016]    The non-oxidized regions of the structure typically represent regions of contact between the structure and the holder in the first position. Thus, the structure is held by the holder in the second position by contact with oxidized regions of the structure in order to expose the non-oxidized regions to the oxidizing atmosphere. In this way, the first and second annealings of the structure in the oxidizing atmosphere provides an oxide layer on the structure. The process is advantageously conducted to form a uniform oxide layer on the entire surface of the substrate. 
         [0017]    The structure may be a heterostructure formed by bonding a first wafer to a second wafer. If so, after the second annealing, the structure may be subjected to a thinning step followed by chemical trimming step. 
         [0018]    Alternatively, the structure may be a first wafer such that the process further comprises forming a heterostructure after the second annealing by bonding the first wafer to a second wafer. In this embodiment, before bonding the wafers, at least one microcomponent can be formed in the second wafer. The heterostructure can then be subjected to an annealing step for stabilizing the bond between the wafers. After conducting the stabilizing annealing, the structure may be subjected to a thinning step followed by chemical trimming step. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The features and advantages of the present invention will become clear from the description given below with reference to the appended drawings that illustrate embodiments of the invention. In the figures: 
           [0020]      FIGS. 1A to 1F  are schematic views of a stabilizing annealing process known from the prior art; 
           [0021]      FIGS. 2A to 2C  illustrate schematically the mechanism forming the nicks on the edges of a heterostructure during its fabrication process; 
           [0022]      FIGS. 3A to 3D  show schematically a first embodiment of the invention for fabricating a heterostructure; 
           [0023]      FIG. 4  shows, in the form of a flow chart, the steps of the first process for fabricating a heterostructure illustrated in  FIGS. 3A to 3D ; 
           [0024]      FIGS. 5A to 5E  show schematically a second embodiment of the invention for fabricating a heterostructure; and 
           [0025]      FIG. 6  shows, in the form of a flow chart, the steps of the second process for fabricating a heterostructure illustrated in  FIGS. 5A to 5C . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    The annealing process of the invention is in particular applicable to multilayer structures or to a single wafer, the latter possibly being assembled with one or more other wafers. As indicated above, the expression “multilayer structure” is understood to mean a structure produced by transferring at least a first wafer onto a second wafer (or support substrate), such as, for example, BSOI structures. 
         [0027]    The process according to the invention allows an oxide layer to be grown on the region or regions of the structure that are in contact with the holder during the first annealing. In this way a uniform oxide layer is formed on the entire exposed surface of the substrate or substrates subjected to the annealing process according to the invention. 
         [0028]    This oxide layer protects the structure from subsequent chemical etches that may be carried out on the structure. 
         [0029]    For example, this oxide layer effectively protects the structure from chemical etches that may be implemented during a thinning or trimming operation on all or part of the structure. 
         [0030]    The oxide layer thus formed also protects the structure from chemical etches implemented during an operation of chemically treating all or part of the structure. 
         [0031]    Thus, the annealing process prevents the formation of nicks on the structure, especially on its edges, during a chemical etch. 
         [0032]    The holder may for example be a plurality of holding elements allowing the structure to be held in position. 
         [0033]    Moreover, the shifting may comprise rotating the structure through a predetermined angle relative to the holder so that all the regions of the structure in contact with the support during the first annealing are no longer in contact with the holder after the shifting. 
         [0034]    In this way it is possible to grow an oxide layer on all the regions of the structure that were in contact with the holder during the first annealing. This oxide layer is advantageous in that it protects all of the structure from the chemical etches that may be subsequently implemented. It is thus possible to prevent the formation of nicks on the surface of the structure when a chemical etch is implemented later on. 
         [0035]    The angle through which the structure is rotated during the shifting may for example be between 40° and 90°. 
         [0036]    The invention further comprises of bonding a first wafer to a second wafer to form the structure. 
         [0037]    The process for fabricating the structure in this embodiment forms a heterostructure, comprised of a first wafer and a second wafer, to be produced without nicks being formed on the second wafer during the chemical trimming. 
         [0038]    In particular, this process prevents the formation of nicks on the edges of the second wafer. 
         [0039]    The expression “chemical trimming” is understood here and after to mean a trimming operation which implements at least one chemical etch. 
         [0040]    The anneal carried out on the structure is preferably carried out in such a way that all the regions of the second wafer in contact with the holder during the first annealing are no longer in contact with the holder after the shifting. In this way, a protective oxide layer is formed over the entire exterior surface of the second wafer, this oxide layer protecting the second wafer from chemical etches that may be subsequently implemented. 
         [0041]    Moreover at least one of the following steps may be carried out after the structure has been annealed for providing uniformity of thickness and outline of the oxide layer: 
         [0042]    a thinning step, for thinning said first wafer; and 
         [0043]    a chemical trimming step, for trimming said first wafer. 
         [0044]    Another embodiment of the invention comprises bonding a first wafer to a second wafer to form the structure, wherein before the bonding, the second wafer is annealed according to the annealing process defined in one of the above-mentioned embodiments. 
         [0045]    This process for fabricating the structure allows an oxide layer to be formed on the surface of the second wafer; this layer effectively protecting the second wafer from chemical etches. 
         [0046]    This embodiment may further comprise, before the bonding of the wafers, forming at least one microcomponent in the first wafer. 
         [0047]    This process is advantageous because the first wafer does not undergo the first and second annealings. In this way, the components formed beforehand in the first wafer are not damaged during the first and second anneal ings. 
         [0048]    This embodiment may further comprise another annealing for stabilizing or strengthening the bonding interface between the wafers of the structure. 
         [0049]    This annealing for stabilizing or strengthening the bonding interface allows the bond between the first and second wafer to be strengthened. 
         [0050]    Moreover, at least one of the following steps may be carried out after this anneal for stabilizing or strengthening the bonding interface: 
         [0051]    a thinning step, for thinning said first wafer; and 
         [0052]    a chemical trimming step, for trimming said first wafer. 
         [0053]    The oxide layer formed on the second wafer thus serves as protection from chemical etches implemented especially during subsequent thinning and/or chemical trimming steps. 
         [0054]    The present invention is generally applicable to an annealing carried out in several steps on a structure comprising at least one wafer. 
         [0055]    The wafer or wafers making up the structure generally has/have a circular outline and may have various diameters, for example 100 mm, 200 mm and 300 mm. However, the wafers may also be of any shape, such as rectangular for example. 
         [0056]    When the structure comprises of multiple wafers, a composite structure, or heterostructure, is fabricated. Such is made by bonding a first wafer to a second wafer that acts as the support substrate for the first wafer. Thus the terms heterostructure, composite structure and structure are used interchangeably herein. 
         [0057]    As indicated above, the appearance of nicks on the edges of composite structures were noticed, when the structures have undergone a chemical etch, such as during a trimming or a thinning operation implementing a chemical phase, where the structure had previously undergone a standard stabilizing anneal in an oxidizing atmosphere. These nicks were especially observed on the edges of the second wafer of the composite structure. 
         [0058]    An in-depth study of these nicks allowed the mechanism forming these defects to be demonstrated and a process for preventing their formation to be developed. 
         [0059]    The mechanism that causes these defects will now be described with reference to  FIGS. 2A to 2C . 
         [0060]      FIG. 2A  shows an exemplary composite structure similar to that described in  FIG. 1B , namely a composite structure  200  formed by assembling a first wafer  201  and a second wafer  202 . 
         [0061]    Preferably at least one of the two wafers  201  and  202  has been oxidized before bonding. This oxidation makes it possible in particular for there to be an intermediate oxide layer between the two wafers once the bonding is completed. In the example described here, the first wafer  201  is oxidized before bonding so as to form an oxide layer (not shown in the figures) over the entire surface of the first wafer. 
         [0062]    In the example described here, the two wafers are made of silicon and the wafers are assembled using the technique of direct bonding, well known to those skilled in the art. Other bonding techniques can however be used, such as for example anodic, metallic or adhesive bonding. 
         [0063]    As a reminder, the principle of direct bonding is based on direct contact between two surfaces—that is to say with no specific material (adhesive, wax, braze, etc.) being used. To carry out such an operation, it is necessary for the bonding surfaces to be sufficiently smooth, particle and contamination-free, and for said surfaces to be placed so close that contact is initiated. Typically a distance of less than a few nanometers is required. In this case, the attractive forces between the two surfaces are strong enough that direct bonding occurs, i.e. bonding induced by the Van der Waals forces between the atoms or molecules of the two surfaces to be bonded. 
         [0064]    Moreover, the first wafer  201  has a chamfered edge, namely an edge comprising a top chamfer  204  and a bottom chamfer  205 . In  FIG. 2A  the wafers have rounded chamfers. However, the wafers may have various forms of chamfers or fillets, such as a bevel. 
         [0065]      FIG. 2B  shows schematically the composite structure  200  seen from the side of the first wafer  201 . The reference position “A” shown at the edge of the first wafer  201  corresponds for example to a flat or a notch. 
         [0066]    The composite structure  200  is placed on a holder  212  (sometimes called a boat) so as to continue with a standard anneal in an oxidizing atmosphere. The annealing here is a stabilizing anneal that has the objective of strengthening the bond between the first wafer  201  and the second wafer  202  and of forming a protective oxide layer. 
         [0067]    In this example, the holder  212  comprises four holding elements  212 A,  212 B,  212 C and  212 D configured to hold the composite structure  200  in position. The holding elements are in contact with the edges (or edge-face) of the composite structure  200  and more particularly in contact with the edges (or edge-face) of the second wafer  202 . The edge portions of the second wafer  202  in contact with the holding elements are referenced by the contact regions  213 A,  213 B,  213 C and  213 D, respectively (and denoted collectively as contact regions  213 ). 
         [0068]    The composite structure  200  and its holder  212  are placed in an oven or any other piece of equipment designed to carry out an annealing (not shown in the figures). A stabilizing anneal is carried out on the composite structure  200  at a temperature T 1  of between, for example, 900° C. and 1200° C., typically at 1100° C., for two hours. 
         [0069]    The anneal forms an oxide layer on the exposed surface of the first wafer  201  and of the second wafer  202 . 
         [0070]    Next, a hybrid trimming step is carried out on the first wafer  201  as described with reference to  FIGS. 1D and 1E . In the example described here, the hybrid trimming comprises a first partial, mechanical trimming using a grinding wheel or a blade followed by a selective chemical etching for etching the first wafer using a TMAH etching solution. 
         [0071]    This trimming operation allows the excess part at the edge of the first wafer  201  to be removed without damaging the second wafer  202 . It is possible to preserve the second wafer  202  because the oxide layer (not shown) intermediate between the two wafers acts as a stop layer for the chemical etching. The chemical etching is thus stopped at the periphery of the first wafer  201  on the oxide layer located at the bonding interface between the two wafers. 
         [0072]    Once the trimming is completed, the first wafer  201  may be thinned so as to form a transferred layer  206  having a predetermined thickness e, for example of about 10 μm. 
         [0073]    Thinning is carried out using a grinding wheel or any other tool capable of mechanically wearing away (or grinding) the material of the first wafer. 
         [0074]      FIG. 2C  shows schematically the structure  200  once the stabilizing anneal, the hybrid trimming and the thinning have been carried out. 
         [0075]    The applicant noticed the appearance of nicks  214 A,  214 B,  214 C and  214 D (collectively reference  214 ) on the edge of the second wafer  202  after the TMAH chemical etch. The distribution of these defects along the edge of the second wafer  202  corresponds to the positions of the abovementioned contact regions  213 . 
         [0076]    In fact, during the stabilizing anneal of the composite structure  200  an oxide layer (SiO 2  in the present example) forms on all the exposed surface of the composite structure  200 , except in the contact regions  213 . This is because, during the stabilizing anneal, the holding elements  212 A to  212 D locally mask the edge of the second wafer  202  so that the contact regions are not covered by an oxide layer. 
         [0077]    Added to this masking effect may be another effect, namely the possible deterioration of the oxide on the structure at the contact points (lift-off, scratching during removal or other movements of the wafer, etc.). The holding elements  212  may in fact locally damage the preliminary oxide layer formed on the first wafer  201  (or on the second wafer  202 ) before bonding and/or before the oxide layer that forms near the contact points  213  during the stabilizing anneal. 
         [0078]    As the contact regions  213  of the first wafer  201  have no protective oxide layer, these regions are directly subjected to the chemical etching action of the TMAH solution during the thinning and/or chemical trimming. The chemical etching of the contact regions  213  then generates the nicks  214 . The nicks  214 A,  214 B,  214 C and  214 D have thus been observed on the second wafer  202  in the positions of the contact regions  213 A,  213 B,  213 C and  213 D, respectively. The larger the regions not protected by the oxide are, the greater the etching effect of the TMAH solution. 
         [0079]    These surface defects generated on the edge face of the second wafer  202  are prejudicial for the reasons already mentioned above. 
         [0080]    For this purpose, the present invention proposes to carry out an anneal in several steps, shifting the structure on the support between two consecutive annealings so as to prevent the appearance of these nicks especially on the edge face (or edges) of the second wafer. 
         [0081]    One particular implementation of the annealing process and a first process for fabricating a heterostructure according to the invention will now be described with reference to  FIG. 3A to 3D  and  4 . 
         [0082]    As illustrated in  FIG. 3A , the first wafer  301 , having a top chamfer  304  and a bottom chamfer  305 , is firstly bonded to a second wafer  302  that also has chamfered edges, so as to form a composite structure  300  (step E 1 ). 
         [0083]    In the example described here, the first and second wafers  301  and  302  are made of silicon. The wafers may however be made of other materials. 
         [0084]    The first wafer  301  and/or the second wafer  302  may preferably be covered before bonding, with an oxide layer (not shown in the figures), in the context, for example, of the fabrication of BSOI multilayer structures. 
         [0085]    In the example described here, the wafers are assembled using direct bonding. However, as indicated above, other types of bonding may also be envisaged. 
         [0086]      FIG. 3B  shows schematically the composite structure  300  seen from one side of the first wafer  301 . The reference position “B” shown at the edge of the first wafer  301  corresponds for example to a flat or a notch. 
         [0087]    The composite structure  300  is placed on the holder  312 . In this first position, the four holding elements  312 A,  312 B,  312 C and  312 D of the holder  312  are thus in contact with the contact regions  313 A,  313 B,  313 C and  313 D, respectively (collectively denoted as contact regions  313 ), of the second wafer  302 . In this example, the contact regions  313  are distributed along the edge of the second wafer  302 . 
         [0088]    However, it is possible to envisage the contact regions being located for example on the backside of the second wafer  302 . 
         [0089]    The composite structure  300  and its holder  312  are then placed in an oven or any other piece of equipment (not shown in the figures) allowing a first annealing to be carried out on the composite structure  300  in an oxidizing (dry or wet) atmosphere (step E 2 ). 
         [0090]    In this example, the first annealing is carried out at a temperature T 2  of between 1000° C. and 1200° C. This first annealing serves as stabilizing anneal for the composite structure  300  since it allows the bond between the first wafer  301  and the second wafer  302  to be strengthened. 
         [0091]    Furthermore this first anneal allows an oxide layer  310  to grow on the exterior surface of the composite structure  300  and in particular on the exterior surface of the second wafer  302 . The expression “exterior surface of the second wafer” is here understood to mean the entire surface of the second wafer apart from the bonding interface. 
         [0092]    The purpose of this oxide layer is to protect the exterior surface of the second wafer  302  from subsequent chemical etches, especially during thinning and/or chemical trimming operations on the first wafer  301 . 
         [0093]    In the case of fabricating a BSOI multilayer structure, the oxide layer  310  thus formed may reach a thickness for example of about 1 μm. 
         [0094]    Once the first annealing E 2  is completed, the composite structure  300  is removed from the oven. An oxide layer  310  is then present over the entire exterior surface of the composite structure  300 , apart from the contact regions  314  distributed along the edge of the second wafer  302 . 
         [0095]    It should be noted that edge regions of the first wafer may also be in contact with the holder during the first annealing. In this case, these regions also have no protective oxide layer after the first annealing E 2 . 
         [0096]    As illustrated in  FIG. 3C , the composite structure  300  on the holder is then shifted by rotating it through a predetermined angle θ 1  with respect to the holder  312  (step E 3 ). This rotation may be carried out manually or by using a suitable positioning device (an aligner for example). The composite structure  300  is thus positioned on its holders  312  in a second position angularly offset by an angle θ 1  with respect to the first position ( FIG. 3B ). 
         [0097]    Once the rotation has been carried out, the contact regions  312  of the second wafer  302  are no longer in contact with the holding elements  312 . 
         [0098]    The angle of rotation θ 1  is therefore chosen depending on the geometry of the holder  312  used and, more particularly, depending on the placement of the holding elements  312 A to  312 D along the edge of the second wafer  302 . 
         [0099]    In one embodiment of the invention, the angle of rotation θ 1  is between 40° and 90°. 
         [0100]    In the example described here in  FIG. 3C , the composite structure  300  is turned through an angle of rotation θ 1  of about 40°. 
         [0101]    The composite structure  300  and its holder  312  are then put back in an oven (or any other suitable piece of equipment) so as to continue with a second annealing in an oxidizing (dry or wet) atmosphere (step E 4 ). 
         [0102]    This second annealing is carried out at a temperature T 3  of between 900° C. and 1200° C., for example. 
         [0103]    This second annealing E 4  allows an oxide layer to grow on the former (unoxidized) contact regions  314  of the second wafer  302 . Such an oxidation is possible because the holding elements of the holder  312  no longer protect the contact regions  314  from the oxidizing atmosphere. 
         [0104]    As necessary, this second annealing also allows the surface of the regions of the first wafer  301  that were in contact with the support  312  during the first annealing E 2  to be oxidized. 
         [0105]    It should be noted that it is not necessary to grow an oxide layer as thick as that grown during the first stabilizing annealing. The second annealing E 4  may be configured so as to form a layer a few hundreds of nanometers thick on the former contact regions  314 . 
         [0106]    In the context of a process for fabricating a BSOI multilayer structure for example, the second annealing E 4  may be carried out at a temperature T 3 =950° C. so as to form an oxide layer about 500 nm thick on the former contact regions  314 . 
         [0107]    After the second annealing E 4 , an oxide layer is then present on the entire exterior surface of the composite structure  300  and in particular on the entire exterior surface of the second wafer  302  (including the edge of the second wafer  302 ) 
         [0108]    In the example described here, the process then continues with the hybrid trimming (step E 5 ) and then with the thinning (step E 6 ) of the first wafer ( FIG. 3D ). 
         [0109]    The hybrid trimming operation (step E 5 ) comprises a mechanical first trimming followed by a chemical trimming, as explained with reference to  FIGS. 1D and 1E . 
         [0110]    More particularly, the mechanical trimming allows the top chamfered edge  304  to be removed from the first wafer  301 . Once the mechanical trimming is completed, the annular portion remaining on the periphery of the first wafer  201  is no longer protected by the oxide. It is then possible to continue with the chemical trimming of this remaining annular portion using a chemical etching solution, this step allowing the bottom chamfered edge  305  to be removed from the first wafer  301 . In the example described here, a TMAH solution is used to etch the silicon of the first wafer. Other chemical etching solutions may however be envisaged, these being chosen especially depending on the composition of the first wafer to trimmed. In certain cases, a KOH solution, well known to those skilled in the art, is used. 
         [0111]    The oxide layer  310  effectively protects the entire second wafer  302  from the chemical etch implemented during the chemical trimming and from chemical etches that may be implemented in other technological steps during the fabrication of the composite structure (especially during the fabrication of microcomponents on the exposed surface of the first thinned wafer). 
         [0112]    Once the trimming operation has been carried out, the first wafer  301  is thinned in an analogous way to the composite structure  200  (step E 6 ). Thus a transferred layer  306  having a predetermined thickness is formed on the second wafer  302 , this thickness possibly reaching about 10 μm for example ( FIG. 3D ). This thickness is measured between the top side and the bottom side of the first wafer, away from the chamfered edge. 
         [0113]    A second embodiment of the invention for fabricating a heterostructure will now be described with reference to  FIGS. 5A to 5E  and  6 . 
         [0114]      FIG. 5A  shows a first wafer  501  and a second wafer  502 . The first wafer  501  is different from the first wafer  301  described in  FIG. 3A  in that it further comprises microcomponents  503  on one of its sides. In addition, the first wafer  501  is preferably an SOI wafer comprising a buried insulating layer (not shown in the figures) corresponding, for example, to a buried layer of silicon oxide or silicon nitride or, alternatively, to a multilayer stack of these materials. 
         [0115]    The second wafer comprises a reference position “C” on the edge of one of its sides. 
         [0116]    The expression “microcomponents” is understood here to mean any elements made using materials different from that of the first wafer. These microcomponents correspond especially to elements forming all or part of an electronic component or a plurality of electronic microcomponents. They may for example be active or passive components, simple contacts or interconnects or even simple cavities. 
         [0117]    The first wafer may especially comprise microcomponents in the case of 3D-integration that requires the transfer of one or more layers of microcomponents onto a final support substrate, or even in the case of transferring circuits, as for example in the fabrication of backside-illuminated imagers. Of course, when such microcomponents are present they must be able to withstand the oxidation heat treatment according to the invention. This is the case for example for cavities. 
         [0118]    In this second embodiment, the annealing according to the invention is carried out only on the second wafer  502 . 
         [0119]    Steps E 11 , E 12  and E 13  differ therefore from steps E 2 , E 3  and E 4 , respectively, in that only the second wafer  502  undergoes a first annealing followed by a rotation and a second annealing. 
         [0120]    More precisely, the second wafer  502  is placed on a holder ( FIG. 5B ) and a first annealing E 11  in an oxidizing atmosphere is carried out on the second wafer  502 , in an analogous way to step E 2  described above. This first annealing is carried out at a temperature T 4 . 
         [0121]    As illustrated in  FIG. 5C , the second wafer  502  is then rotated through an angle θ 2  with respect to its holder (step E 12 ), in an analogous way to step E 3  described above. 
         [0122]    Once the rotation has been carried out, a second annealing E 13  is carried out on the second wafer  502  in an oxidizing atmosphere, in an analogous way to step E 4  described above. This second annealing is carried out at a temperature T 5 . 
         [0123]    After steps E 11  to E 13 , a protective oxide layer  510  is present over the entire surface of the second wafer  502 . This oxide layer covers in particular the regions of the second wafer  502  that were in contact with the holder during the first annealing E 11  ( FIG. 5D ). 
         [0124]    Next, the first wafer  501  is bonded to the second wafer  502  (step E 14 ), so that the microcomponents  503  are located at the bonding interface between the two wafers ( FIG. 5E ). The wafer  501  may have been provided with a smoothing layer (made of planarized SiO 2  for example) so as to supply a more suitable contact surface. 
         [0125]    In the example described here, the assembly is carried out using direct bonding. As explained above, other assembly techniques may however be envisaged. 
         [0126]    The second embodiment ensures the microcomponents  503  of the first wafer  501  do not undergo the first and second annealings of the invention. This is because the high temperatures that are involved during these annealings, they are liable to irreversibly damage the microcomponents  503 . 
         [0127]    In this second embodiment there is therefore no protective oxide layer on the exterior surface of the first wafer  501 . The first embodiment also differs from the second embodiment in that an oxide layer is present at the bonding interface between the first wafer  501  and the second wafer  502 . 
         [0128]    Once the bonding is completed, the composite structure  500  thus formed undergoes a consolidation annealing that has the objective of strengthening the bond between the first wafer  501  and the second wafer  502  (step E 15 ). This annealing is carried out at a temperature T 6  which is lower than the temperatures T 4  and T 5  during the first and second annealings E 11  and E 13 . The temperature T 6  is chosen in particular so as to not damage the microcomponents  503  of the first wafer  501 . It may be annealed between 400° C. and 500° C. for 30 minutes to 4 hours. 
         [0129]    In the example described here, thinning (step E 16 ) is then carried out comprising a first, mechanical (grinding) followed by a chemical treatment that is selective with respect to the buried insulating layer of the wafer  501 . An optional trimming (E 17 ) for trimming the first wafer  501  may then be carried out. 
         [0130]    The oxide layer  510  allows the entire surface of the second wafer  502  to be effectively protected from chemical etches especially implemented during thinning, and optionally trimming steps. 
         [0131]    Moreover, in certain cases it may be necessary to carry out more than one shifting (rotating) of the composite structure so as to allow all the regions of the second wafer and/or of the structure that were in contact with the holder during the first annealing to be oxidized. This may be the case, for example, when the contact regions represent a substantial portion of the edge of the second wafer and/or structure. 
         [0132]    According to another embodiment of the invention, a first annealing, conforming to step E 2  (or E 11 ), is carried out straight away. 
         [0133]    Next N rotations (N being an integer greater than or equal to 2), with each rotation are followed by an annealing analogous to the second annealing E 4  (or E 13 ) as described above. Each second annealing is configured to grow an oxide layer on at least some of the contact regions that are still unoxidized. 
         [0134]    It is also conceivable for the shifting of the invention to comprise shifting the structure relative to the holder in other ways, such as a translational movement for example. The latter can possibly be combined with a rotational movement.