Abstract:
A method for transferring a first substrate to a second substrate. First and second front faces of first and second substrates, respectively, are molecularly bonded to each other to provide a composite structure. The first front face has a first outline, the second front face has a second outline, and a peripheral side of the second substrate substantially borders the second front face and is oriented generally perpendicularly with respect thereto. The second outline has dimensions larger than the first outline, such that during bonding at least a portion of the first outline is disposed within the second outline for minimizing the size of a peripheral region about the first front face within an overlapping area at which the front faces overlap, in which peripheral region the bonding between the faces is weak or absent. A useful layer from a donor substrate, the useful layer comprising one of the first or second substrate adjacent the bonded face thereof.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. provisional application No. 60/473,137 filed May 27, 2003, the entire content of which is incorporated herein by reference thereto. 

   FIELD OF THE INVENTION 
   The present invention relates to a method of increasing the area of a useful layer of semiconductor material which is effectively transferred onto a support during the fabrication of substrates, in particular for use in electronics, optics, or optoelectronics. 
   BACKGROUND OF THE INVENTION 
   Currently, substrates fabricated using techniques combining bonding by molecular bonding (known as “wafer bonding”) and transferring a useful layer onto a support have a zone known as a “peripheral ring”. This peripheral ring is a zone located at the periphery of the support in which transfer of the useful layer has not occurred. The peripheral ring also includes a zone in which the useful layer has been transferred only partially or has disappeared during subsequent treatment due to its poor bonding to the support. 
   Accompanying  FIGS. 1 and 2  are respective cross-sectional and plan views of a substrate that is known to the skilled person under the acronym “SOI,” meaning “silicon on insulator”.  FIG. 1  shows a support  1  of silicon onto which a composite layer  2 , comprising a layer of silicon oxide  21  surmounted by a layer of silicon  22 , has been transferred by molecular bonding. 
   Peripheral ring  3  defines a substantially annular zone of support  1  onto which the composite layer  2  has either not been transferred or has been transferred poorly or incompletely to a substantial level during transfer of the layer. In the plan view of  FIG. 2 , it can be seen that this ring  3  is typically of varying width and/or shape, i.e. that the vertical side  200  of the composite layer  2  can be irregular and jagged Islands  2 ′ of composite layer may exist, which have been transferred onto the support  1 , but which are spaced apart from the remainder of the central portion of said composite layer  2 . It should be noted that  FIGS. 1 and 2  are diagrams that are not to scale both as regards the thickness of the different layers and of the support and as regards the width of the ring. 
   This ring phenomenon occurs with other substrates, for example those known under the acronym “SICOI” meaning “silicon carbide on insulator” or under the acronym “SOQ” meaning “silicon on quartz”. Other multi-layer substrates, such as those comprising gallium arsenide on silicon (AsGa/Si), also exhibit said ring. 
   Independently of the diameter of support  1 , which typically varies, for example, from 2 inches (50 millimeters (mm)) for silicon carbide to 12 inches (300 mm) for certain silicon substrates, the ring  3  generally varies from about 1 mm to about 4 mm in width, typically plus-or-minus 0.5 mm. Further, the width of the ring  3  can fluctuate, i.e., it can be smaller on one side of the substrate than on the other. 
   The appearance of the rings during layer transfer has a variety of origins and causes, as discussed below. Certain factors that can cause ring  3  to appear include chamfers on the substrates used, variations in bonding energy between layer  2  and the supports, and finally certain aggressive steps in substrate fabrication methods. 
   In order to explain how ring  3  typically appears, reference is made to the accompanying  FIG. 3  which is a diagrammatic cross-section of a portion of the sides of two substrates bonded together by molecular bonding. A source substrate  4  is preferably a layer from which the future useful layer is to be cut, and a support substrate  5  is selected to receive the useful layer. This figure illustrates the prior art. In the remainder of the description and drawings, the substrates are assumed to be circular in shape, as this is the shape used most frequently. Other shapes can alternatively be used. 
   The source substrate  4  has two opposite faces that are parallel in most desired applications. In  FIG. 3  reference numeral  400  designates what will be referred to herein as a front face of the opposite faces. The front face  400  is intended and prepared for bonding onto the support  5 . 
   The source substrate  4  has a side  41 , which can be a peripheral side. Further, the substrate  4  has preferably undergone treatment that forms a zone or region of weakness  42  that defines two portions: a rear portion  46  and a useful layer  43 . The useful layer  43  is intended for transfer to the support  5 . Throughout the remainder of the description, the expression “useful layer” refers to the transferred layer. The thickness of the useful layer typically depends on whether it is obtained, for example, by a method of implanting atomic species or by abrasive polishing and/or chemical etching as is described below. 
   In one embodiment, the substrates used both as the source substrate and as the support substrate are commercially available substrates satisfying standardized requirements (for example SEMI M1-0302 standards for a silicon substrate). Those standards are mainly concerned with ensuring that the substrates can be accepted by the equipment of as wide a range of users as possible. According to these standards, at the intersection between the side  41  and the front face  400 , the substrate  4  has an annular primary chamfer  44  or primary drop oriented at an angle α, which can be large and close to 45°, with the extension thereof, and more precisely with the central zone  40 , which is preferably flat to a high degree of precision, as will be explained below. The primary chamfer  44  extends over a width L in the radial direction parallel to the front face  400 . Width L generally varies from about 100 micrometers (μm) to about 500 μm, depending on the substrates used. The primary chamfer  44  is intended to limit the risk of mechanical breakage and notching of the source substrate  4 . 
   In a similar manner to that just described for the source substrate  4 , the support substrate  5  also has a front face  500 , a side  51  and a primary chamfer  54  according to typically present standards. When substrates  4  and  5  of the prior art are bonded to each other, bonding does not occur at chamfers  44  and  54  because of the magnitude of angle α. The width of the rings can thus be expected to generally correspond to the width L of the primary chamfers  44  and  54 . In practice, however, the width of  FIG. 3  is typically even wider. 
   It has been observed that the front face  400  of substrate  4  actually has two zones, namely a first flat zone  40  located substantially at the center of the substrate  4  and hereinafter also termed the “flat central zone,” and a second zone  45  surrounding the first zone  40 . 
   The second zone  45  is a secondary chamfer, which is generally annular, or a secondary drop, forming an angle β with the plane of the flat central zone  40 . The secondary chamfer of second zone  45  extends between the flat zone  40  and the primary chamfer  44 . 
   Throughout the remainder of the description and claims, the expression “flat” means a flatness that is suitable for bonding. The expression “central zone” designates a zone located substantially centrally or even at the center of the front face of the substrate and which can be located at various degrees of eccentrically, and most preferably, located slightly eccentrically on the front face  400 . 
   It should be noted that as  FIG. 3 , the following figures are only diagrammatic in nature and the magnitude of angle β has been considerably exaggerated in the figures for clarification purposes. More precisely, the secondary chamfer  45  a drop that is less sharp than the primary chamfer  44  and that appears during the various substrate shaping steps such as lapping, polishing, and chemical etching, which steps produce an etching and material-removal effect that is greater near the substrate side  41 . The secondary chamfer  45  is presently not subject to industry standards. Its width L′ taken in a radial direction varies from about 500 μm to 3000 μm on substrates that are commercially available on the market. 
   Further, the value of angle β also fluctuates, and the secondary chamfer  45  is not flat in cross-section as shown diagrammatically in  FIG. 3  but can be domed or irregular in places. 
   As a result, in practice, and in contrast to the diagrammatic representation of the figures, the side of the source substrate  4  is not formed by a plurality of beveled slopes, but instead by an overall convex shape, typically without edges between the secondary chamfer  45  and the primary chamfer  44  or between the primary chamfer and the side  41 . The purpose of the convex shape is to avoid any nicking of the substrate  4 . 
   In a manner similar to that just described for the source substrate  4 , the support substrate  5  has a flat central zone  50  and a substantially annular secondary chamfer  55 , but has similar irregularities to the secondary chamfer  45 . 
   Molecular bonding is a technique that does not tolerate substantial non-planar surfaces, the existence of secondary chamfers  45 ,  55  results in poor bonding and layer transfer in the zone of these surfaces, resulting in the appearance of a peripheral ring  3 . In addition, a second reason for the appearance of the ring  3  is that the bonding energy between two facing substrate faces fluctuates as a function of parameters such as roughness, flatness and the chemical nature of the surfaces in contact, the presence of particles and impurities, etc. These parameters can also vary in a less controlled manner at the sides of the substrates, thereby also contributing to the formation of the ring  3 . 
   Finally, another possible cause for formation of the ring  3  is the use of certain aggressive or vigorous steps during the substrate fabrication methods. 
   Methods of fabricating substrates known under the acronym BESOI (bond and etchback silicon on insulator) bond a source substrate onto a support substrate, at least one of the faces of the source substrate being coated with a layer of oxide. The exposed surface of the source substrate then undergoes an abrasive polishing and/or chemical attack etching treatment, followed by polishing until the source substrate becomes a useful layer. In this type of method involving chemical attack (with the risk of partial delamination of the bonding interface), oxidation affecting the lateral and frontal portions of the source substrate, and mechanical abrasive polishing forces, both tend to enlarge the ring  3 . 
   Similarly, in methods involving detachment of a layer by fracture along a zone of weakness, it has been observed that around the peripheral sides collective structure of the bonded substrates, detachment often tends to occur at the bonding interface instead of at the zone of weakness, resulting in the formation of an annular ring  3 , sometimes with a large surface area. 
   Referring again to  FIG. 3 , in the case in which the zone of weakness  42  is formed by implantation of atomic species, such as by hydrogen implantation, it has been observed that, during subsequent treatment to detach the useful layer  43  from the remainder of the source substrate  4 , expansion of hydrogen bubbles exerts a substantially perpendicular force on the surface of the secondary chamfer  45 . In the zone of the secondary chamber  45 , this force is often not compensated by sufficiently strong bonding since the secondary chamfer  55  of the support  5  is spaced from the secondary chamfer  45  of the useful layer  43  by an angle 2β, or the addition of the angles β if the angle of each opposing chamfer is different, and the bonding is thus ruptured. Thus, bubbles are formed at the surface of the secondary chamfer  45  instead of at the surface or edge of the layer  43  being transferred onto the support substrate  5 . In other words, bonding occurs but its quality is poor in this area. 
   A number of disadvantages are associated with the existence of said ring  3 . First, it is not possible to fabricate electronic components on the surface of this ring  3 . Unfortunately, from an economic point of view, each extra square millimeter of area can make it possible to fabricate a larger number of components per substrate. Furthermore, this ring is irregular as explained above, and its width can vary from one side to the other of the substrate, thus giving rise to problems of reproducibility in the various steps of an industrial process when such a substrate is used in a production facility. 
   The prior art includes methods of polishing the side of a substrate so as to make it possible to eliminate the ring  3 , see for example document U.S. Pat. No. 6,221,774. A method of mechanically polishing sides is also known as used by the supplier SEZ. This method is used on solid silicon substrates after deposition operations, which are known to run the risk of being associated with the effects of material being removed from the sides (known as “lift-off” or “peel-off”), i.e. leading to a high level of particulate contamination. Nevertheless, those methods tend to increase the size of the zone that has no transferred layer at the periphery, thereby restricting the useful area. In addition, finishing operations on the ring can lead to defects at the periphery of the transferred layer. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a method for transferring a first substrate or a portion thereof to a second substrate, which are preferably crystalline and of bulk material, to improve the bonding therebetween. In the preferred method, first and second front faces of first and second substrates, respectively, are bonded to each other to provide a composite structure. A peripheral side of the second substrate substantially borders the second front face and is oriented generally perpendicularly with respect thereto, preferably being perpendicular or quasi-perpendicular with respect to the second front face. Preferably, the second substrate is substantially free of a primary chamfer between the peripheral side and the second front face thereof, and more preferably is substantially free of any chamfer between the peripheral side and the second front face. The second front face has dimensions larger than the first front face, and the front faces are positioned during bonding so at least a portion of the outline of the first front face is disposed within the outline of the second front face. 
   The first front face is preferably completely disposed within the outline of the second front face during the bonding. This is done to minimize the size of a peripheral region about the first front face outline and within a region of overlay between the first and second front faces in which the bonding between the faces is weak or absent. Preferably the width of the peripheral ring at its maximum is less than about 0.5 mm, more preferably less than 0.2 mm. A useful layer is provided from a donor substrate, which comprises the first or second substrate adjacent the bonded face thereof. 
   Preferably, the useful layer is of semiconductor material. In a preferred embodiment, both the first and second substrates can be made of a semiconductor material, most preferably at least at the front faces. The preferred useful layer produced is useful for producing an electronic, optic, or optoelectronic component or substrate. 
   In the preferred method, the useful layer is separated from a donor portion of the donor substrate of the composite structure to provide a product wafer that includes the useful layer. The useful layer can be separated from the donor portion by splitting. The splitting, in turn, can be accomplished in several manners, including by forming a zone of weakness between the useful layer and the donor portion to facilitate and locate the splitting of the useful layer from the donor portion. The zone of weakness may be formed, for example, by implantation of atomic species or by providing a porous layer. The useful layer is preferably separated by applying electrical or mechanical stress to, supplying thermal energy to, or chemically etching the composite structure, or by combinations thereof. 
   The first substrate can comprises a first primary chamfer extending around the first front face and can have a primary chamfer outline that is at least partially disposed within the outline of the second front face during bonding. This primary chamfer outline is preferably disposed substantially entirely within the second front face during bonding. Additionally, the front faces are preferably flat, and at least one of the front faces can comprise an insulator. With round faces or cylindrical substrates, the second front face or second substrate preferably has a diameter 0.3 mm greater than the first front face or first substrate. 
   A preferred composite structure in accordance with the invention includes front faces of the first and second substrates that are molecularly bonded to each other. The dimensions of the second substrate outline are larger than the first substrate outline, and a peripheral side of the second substrate substantially bordering the second front face and being oriented generally perpendicularly with respect thereto. The front faces are molecularly bonded such that the outline of the first front face is disposed at least partially within the outline of the second front face. A peripheral ring extending around the first front face and facing the first substrate, in which bonding between the front faces is weak or absent, has a maximum width of less than about 0.5 mm. 

   
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     Other characteristics and advantages of the invention become apparent from the following description of several preferred implementations of the invention. This description is made with reference to the accompanying drawings, in which: 
       FIG. 1  is a diagrammatic view showing a vertical diametrical section through an SOI type substrate; 
       FIG. 2  is a diagrammatic plan view thereof; 
       FIG. 3  is a diagrammatic fragmentary, vertical section of a source substrate and a support substrate bonded together using a prior art technique; 
       FIG. 4  is a fragmentary diagrammatic view in vertical section of a source substrate and a support substrate in a first embodiment of the invention, with the substrates shown ready for bonding together; 
       FIGS. 5 and 6  are fragmentary diagrammatic views in vertical section showing a source substrate and a support substrate in the embodiment of  FIG. 4 , as well as in a second embodiment, with the substrates shown bonded together; and 
       FIG. 7  is a top view above of an angular portion of a support according to the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention reduces the area of the peripheral ring by increasing the quantity of material taken from the source substrate that is transferred onto a support during a layer transfer process. To this end, the invention provides a method of increasing the area of a useful layer of a material, in particular a semiconductor material, transferred onto a support substrate, during the fabrication of a composite substrate in particular for electronics, optics, or optoelectronics, the method comprising at least one step of molecular bonding a “front” face of a source substrate to a “front” face of a support substrate. A useful layer from the source substrate is transferred onto the support substrate with the useful layer including a portion of the source substrate extending to the front face. 
   In a preferred embodiment, before the bonding step, the front face of one of the support and source substrates, which is referred to herein as the “first” substrate, presents at least a flat central zone, the front face of the other substrate, referred to as the “second” substrate, has a flat zone bordered by an adjacent peripheral side face, substantially perpendicular or quasi-perpendicular thereto, preferably with substantially no, or without any, significant surface at another angle disposed between the flat zone and the peripheral side face. The outer outline of the flat zone has dimensions larger than the dimensions of the outer outline of the flat central zone of the first substrate. During bonding, the substrates are applied against each other preferably such that the outline of the flat central zone of the first substrate is inscribed within the outline of the flat zone of the second substrate. In this manner, a larger area of a larger area of the useful layer is transferred 
   According to other advantageous and non-limiting characteristics of the invention, taken singly or in combination:
         the first substrate can be surrounded by a primary chamfer. The dimensions of the outer outline of the flat zone of the second substrate are preferably greater than the dimensions of the inner outline of the primary chamfer of the first substrate, and during bonding, the substrates are applied one against the other such a manner that the inner outline of the primary chamfer of the first substrate is inscribed inside the outer outline of the flat zone of the second substrate;   the first substrate can be bonded to the second substrate such that its flat central zone is centered relative to the flat zone of said second substrate;   the second substrate can be the source substrate;   prior to the step of bonding the source substrate and the support substrate against each other, an embodiment of the method forming a zone of weakness inside said source substrate. The useful layer to be transferred in this embodiment extends between the zone of weakness and the front face of this substrate. After the bonding step, the method includes detaching the useful layer from the remainder of the source substrate along the zone of weakness;   the zone of weakness can be formed by implanting atomic species or by providing a porous layer in respective substrate;   the useful layer can be detached by at least one of the following techniques taken alone or in combination: applying stresses of mechanical or electrical origin; supplying thermal energy, and chemical etching; and   at least one of the faces of the front face of the source substrate and the front face of the support substrate maybe coated in a layer of an insulating material.       

   The invention improves the fabrication of a composite substrate. The method preferably includes at least one step of molecular bonding one of the faces of a source substrate to a facing face of a support substrate, and a step of transferring a useful layer from the source substrate onto the support substrate. A preferred composite substrate thus includes at least one useful layer transferred onto a support. 
   Referring to  FIG. 4 , in a first embodiment of the invention, a source substrate  6  (from which a useful layer  63  is to be taken subsequently) is ready for being molecularly bonded to a support substrate  7 . In this embodiment, the support substrate  7  can be the same as the support substrate  5  described above and shown in  FIG. 3 . It is therefore not described again in detail. The support substrate  7  presents a side  71 , a primary chamfer  74 , and a front face  700  for receiving the front face  600  of the source substrate  6 . This front face  700  covers both a flat central zone  70  and a secondary chamfer  75 . 
   As shown in  FIG. 4  and in the subsequent figures, the source and support substrates  6  and  7  are preferably substantially circular on cross-section. Similarly, their flat central zones are likewise substantially circular, and the primary and secondary chamfers (when they exist) are substantially annular. Nevertheless, the substrates  6  and  7  and/or their respective central zones can alternatively be of other shapes, such as oval, octagonal, rectangular, or of asymmetrical shapes within some embodiments. 
   As shown in  FIG. 7 , the secondary chamfer  75  is defined by an inner outline C 7  and by an outer outline C′ 7 . It should be observed that the inner outline C 7  of the secondary chamfer  75  is also the outer outline of the flat central zone  70  in this embodiment. Similarly, the outer outline C′ 7  of the secondary chamfer  75  the inner outline of the primary chamfer  74  in this embodiment. 
   In a first embodiment of the invention, shown in  FIGS. 4 and 5 , the front face  600  of the source substrate  6  is not machined prior to the bonding step. Thus, compared with conventional substrates, it has neither a primary chamfer nor a secondary chamfer. Consequently, the entire front face  600  is preferably a flat zone  67 , which is surrounded by a side face  66 . The outline referenced C″ 6  of said flat zone  67  constitutes the outer outline of the source substrate  6 , and the peripheral side face  66  is perpendicular or quasi-perpendicular to said flat zone  67  in the embodiment shown. The term “quasi-perpendicular” as used herein with reference to side face  66  means that by the side face  66  is substantially perpendicular to the zone  67 , even if certain subsequent polishing treatments might sometimes modify the angle to a small degree. 
   Referring to  FIG. 5 , in a first variant of the invention (shown in continuous lines), when the source substrate  6  is bonded to the support  7 , the side face  66  of the source substrate  6  is situated vertically in register with the side face  71  of the support  7 , and the two substrates  6  and  7  have substantially the same dimensions (or substantially the same diameter in the case that the substrates are circular). This variant is advantageous in that it enables traditional substrate-handling equipment to be used, since the diameter of the source substrate  6  as measured across its side face  66  is unchanged relative to the substrates presently in use in this type of method involving layer transfer and molecular bonding. 
   In a second variant (shown in chain-dotted lines), the side face  66  of the source substrate  6  is situated vertically facing or in register with the secondary chamfer  75  (between the outlines C 7  and C′ 7 ). In a third variant (shown in phantom lines), the diameter of the source substrate  6  is greater than the diameter of the support  7 , and the outline C″ 6  of the flat zone  67  then extends beyond the outside dimensions of the support  7 . 
   In the above-mentioned variants, care is preferably taken to ensure that the dimensions of the outline C″ 6  of the flat zone  67  of the source substrate  6  are greater than the dimensions of the outer outline C 7  of the flat central zone  70  of the support substrate  7 , and that during bonding, the source substrate  6  is applied against the support substrate  7  such that the outline C 7  is inscribed within the outline C″ 6 . Also, the dimensions of the outline C″ 6  of the flat central zone  67  of the source substrate  6  preferably are greater than the dimensions of the inner outline C′ 7  of the primary chamfer  74  of the support  7 . During bonding, the source substrate  6  is preferably applied against the support substrate  7  such that the inner outline C′ 7  is inscribed inside the outline C″ 6 . 
   In this manner, the secondary chamfer  75  of the support  7  is situated facing the flat zone  67  of the substrate  6  and forms a small angle β relative thereto. Bonding between these two faces is consequently improved, and a larger fraction of the useful layer  63  is transferred onto the chamfer  75  compared to prior art methods. Consequently, the area of the useful layer  63  that is actually transferred is increased, and conversely the area of the peripheral ring is decreased. 
   Given the usual fabrication tolerances on the substrates  6  and  7 , it is preferable in practice for the source substrate  6  to have a diameter D 61  that is at least 0.3 mm greater than the diameter D 71  of the support substrate  7  or than, more preferably at least 0.4 mm greater, and more preferably 0.5 mm greater. In one embodiment, the diameter D 61  is less than 2.5 mm greater than the diameter D 71 , more preferably less than 1.5 mm greater, and most preferably less than 1 mm greater. Furthermore, when commercially-available substrates  6  and  7  are used of the kind that are made available in the form of a range of substrates of discretely different increasing diameters, it is advantageous for the substrate  6  to be selected as having the diameter in the range that is the diameter immediately larger than that of the support substrate  7 . Standardized substrate diameters presently available include 2 inch, 3 inch, 4 inch, 5 inch, 6 inch, 8 inch, and 300 mm diameters. 
   Advantageously, the source substrate  6  is preferably bonded to the support substrate  7  so that the flat central zone  70  of the support  7  is centered relative to the central zone  67  of the source substrate  6 . Thus, the useful layer  63  as transferred is also centered or substantially centered relative to the support  7 . 
   In the embodiment of  FIG. 6 , the source substrate  6  has a flat central zone  60 , a primary chamfer  64 , and a secondary chamfer  65 , while the front face  700  of the support substrate  7  has a flat zone  77  substantially directly surrounded by a peripheral side face  76  extending perpendicularly or substantially perpendicularly. 
   Similarly, to the embodiment of  FIG. 5 , the dimensions of the outline C″ 7  of the flat zone  77  of the support  7  are greater than the dimensions of the outer outline C 6  of the flat central zone  60  of the source substrate  6 . During bonding, the source substrate  6  is preferably applied against the support substrate  7  such that outline C 6  is inscribed within the outline C″ 7 . Preferably, the dimensions of the outline C″ 7  are greater than the dimensions of the inner outline C′ 6  of the primary chamfer  64  of the source substrate  6 . During bonding the source substrate  6  is preferably applied against the support substrate  7  such that the inner outline C′ 6  is inscribed inside the outline C″ 7 . Additionally, the substrates  6  and  7  can be centered relative to each other. 
   Of the two implementations described above, the implementation of  FIG. 5 , in which the source substrate  6  presents an edge  66  perpendicular to the flat surface  67 , is generally preferred as it facilitates implementing the method on an industrial scale. Whichever implementation is selected for the substrates  6  and  7 , the useful layer  63  can be removed from the source substrate  6  in a variety of manners which are now described. 
   In a first variation, prior to the step of molecular bonding substrates  6  and  7  to each other, a zone of weakness  62  is formed within the source substrate  6  to define and delimit the useful layer  63  subsequently transferred to the support  7 . After bonding, the useful layer  63  is detached from the remainder of the source substrate  6  along this zone of weakness  62 . It should be observed that  FIGS. 4-6  are merely diagrammatic and, for clarification purposes, the useful layer  6  is shown therein as being much thicker than it is in reality. 
   Techniques for forming the zone of weakness  62  are known to the skilled person and are not all described in detail. Advantageously, this zone of weakness  62  can be formed by implanting atomic species from the front face  600 . The term “implanting atomic species” means any bombardment of atomic species, including molecular or ionic species, which can introduce the species into a material with a maximum concentration of said species located at a predetermined depth from the bombarded surface  600 . The molecular or ionic atomic species are introduced into the material with an energy that is also distributed about a maximum. 
   Atomic species can be implanted into the source substrate  6  using an ion beam implanter or a plasma immersion implanter, for example. Preferably, implantation is carried out by ionic bombardment. Preferably, the implanted ionic species is hydrogen. Other ionic species can advantageously be used alone or in combination with hydrogen, such as rare gases. 
   Implantation creates the zone of weakness  62  within the bulk of the source substrate  6  and at a mean ion penetration depth. The zone of weakness  62  is preferably substantially parallel to the plane of the front face  600 . The useful layer  63  extends from the front face  600  to this zone of weakness  62 . 
   A preferred method of transferring the useful layer  63  is known as SmartCut®. The zone of weakness  62  can alternatively be provided by a porous layer obtained, for example, using the method known as ELTRAN® of Canon, which is described in European patent EP-A-0 849 788. 
   After the step of molecular bonding of the faces  600  and  700 , the useful layer  63  is detached from the remainder of the source substrate  6 . Detachment of the useful layer  63  is preferably performed by at least one of the following techniques, alone or in combination: applying constraints of mechanical origin (inserting a blade or a jet of fluid under pressure into the zone of weakness  62 ) or of electrical origin, supplying heat energy, and chemical etching. These detachment techniques are known to the skilled person. This provides a composite substrate or product wafer having a useful layer  63  transferred onto a support  7 . 
   As mentioned above, the useful layer  63  is preferably detached horizontally, along the zone of weakness  62 . The thickness of the useful layer  63  detached is also in part determined by a “vertical self-limitation,” which is vertically in register with the zone where bonding to the support is of sufficient strength to transfer the useful layer  63 . 
   It is alternatively possible to produce the useful layer  63  by the technique known as “bond and etchback”, in which, after bonding the front face  600  of the source substrate  6  onto the front face  700  of the support substrate  7 , the rear face  601  of said source substrate  6  undergoes treatment by lapping and/or etching by chemical attack, typically followed by polishing, until only the thickness corresponding to said useful layer  63  remains on the support  7 . In the case of SOI (silicon on insulator) substrates, it is possible to obtain the useful layer  63  by the BESOI mentioned above. 
   Examples of materials to which said method can be applied follow. The support substrate  7  is preferably formed from a material that is optionally a semiconductor and that is selected, for example, from silicon, a transparent substrate (such as quartz or glass, for example), silicon carbide, gallium arsenide, indium phosphide, or germanium. 
   Preferably, the source substrate  6  is preferably formed from a semiconductor material selected, for example, from silicon, germanium, silicon carbide, silicon and germanium alloys or “compounds” (known as Si—Ge compounds), or alloys or compounds known as III/V compounds (i.e., compounds one element of which is from column IIIa of the periodic table and the other is from column Va, such as gallium nitride, gallium arsenide, or indium phosphide). 
   Finally, it should be noted that it is possible to cover the front face  700  of the support  7  with an insulating layer of the oxide type (for example SiO 2 ) or of the nitride type (for example Si 3 N 4 ). This insulating layer can then be interposed between the useful layer  63  and the support  7  after detaching said layer  63 . 
   It is possible to cover the front face  600  of the source substrate  6  with an insulating material of the type mentioned above; the transferred useful layer  63 , would then comprise two layers. It is even possible to deposit a plurality of layers onto the source substrate  6 , and the term “useful layer” could then designate a stack of layers. 
   The substrates are preferably of bulk material that is preferably commercially available, such as wafers sliced from ingots. An ingot is typically a mass of raw material, and the general form or shape of the ingot may vary. The ingot may be generally cylindrical in shape with two substantially conical ends, or it may be elongate, or tubular. The ingot may have a non-circular cross section, for example square, hexagonal, or octagonal, with or without two pointed ends, or it may be broadly spherical (known to the skilled person as a “boule”), or it may even be in the form of a cube. The substrates can be bulk material segments taken from the ingot. 
   When the ingot is elongate in shape, the thick segments can be cut substantially transversally or, in contrast, longitudinally. Typically, two pointed ends of the ingot are trimmed, and the remaining irregular lateral surface is ground and turned to obtain a cylinder of preferably circular cross section. Next, the ingot can be cut into slices using, for example, a circular saw or a wire saw. The rondelles or slices obtained then typically undergo finishing, which can include grinding to obtain a wafer with a uniform thickness, and then polishing at least one of the two opposite faces to obtain a flat surface. Each wafer can be immersed in a series of chemical baths to eliminate the dust and particles that may still subsist on the two faces and which could be a source of subsequent pollution. 
   While illustrative embodiments of the invention are disclosed herein, it will be appreciated that numerous modifications and other embodiments may be devised by those skilled in the art. Therefore, it will be understood that the appended claims are intended to cover all such modifications and embodiments that come within the spirit and scope of the present invention.