Abstract:
A method for preparing a thin layer of GaN from a starting substrate in which at least one thick surface area extending along a free face of the starting substrate includes GaN, where the method includes bombarding the free face of the substrate with helium and hydrogen atoms, the helium being implanted first into the thickness of the thick surface area and the hydrogen being implanted thereafter, and where the helium and hydrogen doses each vary between 1.1017 atoms/cm2 and 4.1017 atoms/cm2. The starting substrate is subjected to a rupture process in order to induce the separation, relative to a residue of the starting substrate, of the entire portion of the thick area located between the free face and the helium and hydrogen implantation depth. The helium is advantageously implanted in a dose at least equal to that of hydrogen, and can also be implanted alone.

Description:
PRIORITY CLAIM 
       [0001]    This application is a nationalization under 35 U.S.C. 371 of PCT Application No. PCT/FR2007/002100 filed Dec. 18, 2007, which claims priority to French Patent Application No. 0655664, filed Dec. 19, 2006, and incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention concerns a method of fabrication of thin layers (also referred to as thin films) of GaN for applications of microtechnology. 
       BACKGROUND 
       [0003]    The method known by the name “Smart Cut®” is used to detach a thin film and to transfer it onto a support, sometimes called a stiffener, by performing the following steps: 
         [0004]    1. bombardment of one face of an initial substrate with gaseous species or ions (H or rare gases), in order to implant those ions (or atoms) in a concentration sufficient to create a layer of microcavities, 
         [0005]    2. bringing of this face of the substrate into intimate contact (typically by molecular bonding) with a second substrate called the support or stiffener, 
         [0006]    3. fracturing of the layer of microcavities by the application of a heat treatment and/or a detachment stress (for example, the insertion of a blade between the two substrates and/or application of traction and/or bending and/or shear forces and/or application of ultrasound or of microwaves of judiciously chosen power and frequency), and 
         [0007]    4. recycling of the substrate. 
         [0008]    In the case of the production of heterostructures, for example in the case of the transfer of a thin film of a material A onto a substrate of material B, if the step 2 of bringing the two substrates into intimate contact is followed by a heat treatment (consolidation of the intimate contact (bonding) or thermal fracture), unbonding or rupture of the two bonded substrates can be observed. Because of their intrinsic properties, the different materials A and B generally have different coefficients of thermal expansion (CTE). The more different the CTE, the less readily can the integrity of the bonded structure be maintained at high temperatures. Accordingly, in the case of a self-supporting GaN substrate of approximately 325 microns thickness (denoted ˜325 μm) bonded to a sapphire substrate of thickness ˜330 μm, the two bonded substrates must not in practice be heated to a temperature beyond approximately 230° C. (i.e. not beyond ˜230° C. using the above notation): beyond 230° C., unbonding of the two substrates is observed, i.e. their detachment from each other at the bonding interface. This low temperature resistance is particularly problematic for the fracture step 3, since this step generally consists in whole or in part of a heat treatment, and heat treatments are in practice conducted at much higher temperatures. The bonding temperature resistance therefore limits the fracture heat treatment. 
         [0009]    It is known from U.S. Pat. No. 5,877,070 (primarily concerning silicon, silicon carbide, germanium or diamond) that a sensitization step by heat treatment (at high temperature) of the implanted plate before the bonding step reduces the subsequent fracture heat treatment. The problem is that this sensitization step is of limited effect: it must not induce deformation of the surface in the form of blisters or even exfoliated areas. This in practice implies that the sensitization cannot represent more than about 10% of the fracture thermal budget; as a result of this the fracture treatment proper, after bonding to the stiffener, must therefore rely on approximately 90% of the fracture thermal budget. This limited sensitization therefore does not enable a significant reduction in the fracture treatment as such, with the result that the limitation imposed by the bonding temperature resistance remains even after such sensitization treatment. It is appropriate to mention here that the fracture thermal budget corresponds to the annealing time necessary to produce the fracture for a given annealing temperature (it is clear that the fracture time depends on the annealing temperature); the fracture thermal budget depends on the implantation conditions, notably on the nature of the ions (or atoms) implanted, their dose, their energy, the substrate implanted, etc. 
         [0010]    The step 2 of bringing the implanted substrate into intimate contact with the stiffener must in principle be effected with plane and perfectly clean surfaces. The problem is that, when it takes place, this bringing into intimate contact cannot be produced effectively over all of the surface of the substrates: 
         [0011]    firstly, the edges of the plates forming the substrates are generally chamfered and therefore cannot be brought into contact; this problem of non-bonding at the edge of plates is encountered for all materials (Si, Ge, GaAs, GaN, sapphire, SiGe, LiTaO 3 , LiNbO 3 , SiC, InP, etc.) and for all plate diameters between 5 cm and 30 cm (in practice between 2 inches and 12 inches); 
         [0012]    in the case of substrates or layers structured intentionally (for example by patterns produced photolithographically) or unintentionally (for example by growth defects in the case of epitaxial layers or by defects linked to the deposition of a layer on the initial substrate), patterns or defects that are recessed in the surface give rise to non-bonded areas (NBA); 
         [0013]    finally, in the case of insufficiently effective cleaning, the presence of particles (“dust”) at the bonding interface also gives rise to NBA. 
         [0014]    In the fracture step, if the dimension of the NBA is large relative to the thickness of the film to be transferred (for example with a ratio (NBA lateral dimension)/(film thickness) of approximately 10), the thin film remains locally fastened to the initially implanted substrate. These areas are called non-transferred areas (NTA). 
         [0015]    These bonding defects can even lead to highly localized (over dimensions of a few square microns) lifting or even detachment of the thin film, in the form of blisters or exfoliated areas, which has to be avoided completely. 
         [0016]    By way of example,  FIG. 1  represents a substrate  1 , here of sapphire, onto which a layer  2 , here of GaN, has been deposited and has then undergone implantation that has resulted in the formation of an implanted area  2 A. On this layer  2  is represented an optional bonding layer  3 . This layer  2  is in intimate contact at an interface  9  with another substrate  4 , here also of sapphire, and also provided with an optional bonding layer  5 , for example similar to the layer  3 . 
         [0017]    It is seen that, the substrates being chamfered, the peripheral areas P are not bonded. Moreover, because of a defect that has occurred when depositing the GaN layer, there is a recess in the bonding layer  3  and therefore a non-bonded area C. Finally, the reference I represents dust trapped between the bonding surfaces (remaining after ineffective cleaning) and locally reducing or even eliminating the mechanical strength of the bonding interface  9 . 
         [0018]    It is seen in  FIG. 2  that, at the time of fracture in the implanted area, there remain non-transferred areas in vertical alignment with the peripheral areas, the growth defect and the dust. 
         [0019]    According to a notable advantage of the “Smart Cut®” technology, the initially implanted substrate can, after peeling a thin film during the fracture step, be recycled for other, analogous transfer cycles. However, it has just been seen that the thin film remains locally attached to the substrate in the non-transferred areas, therefore forming steps with a thickness typically between 10 and 1000 nm (corresponding to the thickness of the thin film). Moreover, implantation followed by fracture generally cause roughness to appear at the bared surface of the substrate. It follows from this that recycling the substrate from which a thin film has just been detached generally necessitates particular leveling steps, notably by mechanical polishing and/or chemical attack (this is explained in particular in the documents EP-A-1 427 002 and EP-A-1 427 001). 
       SUMMARY 
       [0020]    The invention overcomes the drawbacks cited above in the case of gallium nitride, and in particular, to enable the detachment by fracture of a thin layer of GaN, or its transfer onto a host substrate, in times that are realistic on an industrial scale, at the same time as forming, at the time of such fracture, free surfaces involving no significant leveling treatment (because of not having the steps described above), so that the remainder of the starting substrate can then be reused (recycled), if required, substantially as it stands, for a new cycle of formation of a thin film. Another object of the invention is to enable transfer to a host substrate at temperatures sufficiently low for there to be no risk of unbonding between the starting substrate and the host substrate because of too great a difference between the coefficients of thermal expansion of those substrates. 
         [0021]    To this end the invention proposes a method of fabrication of a thin layer of GaN from a starting substrate of which at least a thick surface area along a starting substrate free face is in GaN, comprising the following steps: 
         [0022]    bombardment of said free face of the starting substrate with helium and hydrogen ions, the helium being implanted first in the thickness of said thick area and the hydrogen being implanted second, the doses of helium and of hydrogen each being between 10 17  atoms/cm 2  and 4.10 17  atoms/cm 2 , 
         [0023]    application to the starting substrate of a fracture treatment adapted to cause the detachment from a remainder of the starting substrate of all the portion of the thick area of GaN situated between the free face and the implantation depth of the helium and of the hydrogen. 
         [0024]    It has become apparent that the conditions of the invention, in relation to the detachment phenomenon, enable surfaces of low roughness (without significant steps) to be obtained at temperatures sufficiently low for there to be no risk of unbending in the case of transfer onto a host substrate. 
         [0025]    It is to be noted that the implantation conditions determine the implantation depths. 
         [0026]    According to advantageous features of the invention, where appropriate combined: 
         [0027]    the implantation dose of the helium is at least equal to the implantation dose of the hydrogen, 
         [0028]    the remainder of the starting substrate after fracture (or detachment) is recycled; in this case, the method advantageously further includes a treatment of preparation of the remainder of the starting substrate, before its recycling, involving at most one polishing of at most 1 micron thickness (typically of the order of 0.2 micron), or even no treatment at all, 
         [0029]    the starting substrate is, before the detachment, brought into intimate contact with a host substrate via said free face; this bringing into intimate contact is advantageously a molecular bonding; at least one layer is preferably deposited on the free surface of the starting substrate before bringing it into intimate contact with the host substrate, in particular to protect one or the other of these surfaces or to facilitate bonding, 
         [0030]    the fracture treatment includes the application to the starting substrate, before this bringing into intimate contact, of an embrittlement treatment including a heat treatment and corresponding to at least  85 % of the thermo-mechanical budget necessary for obtaining the fracture of this starting substrate at the implantation depth of the hydrogen, 
         [0031]    this embrittlement treatment advantageously includes a step of depositing a layer in vapor form, 
         [0032]    this embrittlement treatment advantageously includes the application of mechanical stresses (for example application of light tension, because of the contact with a layer having different thermal properties but a small thickness (typically less than the order of one micron)), 
         [0033]    this embrittlement heat treatment is preferably effected at a temperature of at most 400° C., preferably at most equal to 300° C., 
         [0034]    this embrittlement treatment consists for example mainly of an embrittlement heat treatment carried out for at least 85% of the time necessary to obtain thermal fracture, 
         [0035]    this embrittlement treatment advantageously corresponds to a thermo-mechanical budget of at least 95% of the budget necessary to obtain the detachment of the thin layer; it can theoretically be chosen greater than 99%, or even 99.99%, but to guarantee that the starting substrate retains sufficient mechanical strength for bringing it into intimate contact with a host substrate, where applicable, this budget is preferably at most equal to 99% of the budget necessary for fracture, 
         [0036]    the residual fracture treatment is advantageously effected at a temperature at most equal to 250° C., for example at room temperature, 
         [0037]    this treatment advantageously includes the application of mechanical stresses, preferably at room temperature, 
         [0038]    the residual fracture treatment includes the application of heat treatment and/or a detachment stress, for example by the application of ultrasound and/or microwaves (of judiciously chosen power and frequency), 
         [0039]    the implantation dose of helium is preferably between 1.10 17  and 2.10 17  He + /cm 2  (preferably of the order of 2.10 17  He + /cm 2 ), and the hydrogen implantation dose is advantageously in the same range (between 1.10 17  and 2.10 17  H + /cm 2 ), preferably also of the order of 2.10 17  H + /cm 2 ; thus the implantation doses of hydrogen and helium are advantageously each substantially equal to 2.10 17  atoms/cm 2 , 
         [0040]    the cumulative implantation dose is between 3.10 17  atoms/cm 2  and 4.5 10 17  atoms/cm 2 , preferably between  3 . 5   10   17  atoms/cm 2  and 4 10 17  atoms/cm 2 , approximately, 
         [0041]    the implantation energy for the helium implantation step is greater than the implantation energy for the hydrogen implantation step, 
         [0042]    the helium implantation energy is between 90 keV and 210 keV and the hydrogen implantation energy is between 60 keV and 120 keV, 
         [0043]    the method advantageously further includes a finishing heat treatment for perfecting the detachment of all the thin layer, including in any non-bonded areas. 
         [0044]    It is to be noted that the principle of co-implantation of hydrogen and helium has already been proposed, notably with a view to obtaining for the thin film a free surface of good roughness. There may be cited in this regard PCT Patent Application Publication Nos. WO-A-2004/044976, WO-A-2004/042779, WO-A-2005/013318 and WO-A-2005/043615. However, these documents in practice propose to profit from the presence of the two implantation elements to reduce as much as possible the implantation dose of each of the elements below 5.10 16  at/cm 2 , bonding of the first substrate to a second substrate before any significant heat treatment, and usually heat treatments at high temperatures. 
         [0045]    Moreover, U.S. Patent Publication Nos. 2004/0262686 and 2005/0269671 have already proposed steps of transfer from gallium nitride using co-implantation of hydrogen and helium in the gallium nitride, but with doses less than those proposed by the invention and without specifying the order of implantation (the second document cited above merely mentions the possibility of co-implantation without specifying how to perform this). These documents are not concerned with the quality of the surface resulting from the detachment of the thin film and can therefore in no way have identified how the order and doses of implantation can impact on the quality of this surface quality. 
         [0046]    It must be noted in this regard that, given that implantation leads to a disturbance of the crystal lattice over a certain thickness, the requirement to have for the thin film a rear surface that is very clean and plane had the a priori consequence that the surface of the starting substrate bared by the detachment of the thin layer was significantly degraded, especially if the doses were high. However, the invention is founded on the observation that appropriate choices of implantation conditions enable the advantages mentioned above to be obtained. 
         [0047]    Without the physical mechanisms having been elucidated, it seems that the helium atoms diffuse toward the defects created by the hydrogen to tension them; given the high implantation dose of the hydrogen, comparable to that routinely used to cause fracture by itself, the additional implantation of helium confers on the defects generated by the implantation of hydrogen an optimum efficacy, with the result that their coalescence is produced very neatly, whence the formation of a very clean separation interface. 
         [0048]    It has even become apparent that, as soon as the cumulative implantation dose was high, that is to say at least equal to 3.10 17  atoms/cm 2 , the presence of hydrogen was not necessary. 
         [0049]    Thus, another aspect of the invention proposes a method of fabrication of a thin layer of GaN from a starting substrate of which at least a thick surface area along a free face of the starting substrate is in GaN, comprising the following steps: 
         [0050]    bombardment of said free face of the starting substrate with helium ions and then, optionally, hydrogen ions, the helium being implanted first in the thickness of said thick area, the cumulative implantation dose being between 3.10 17  atoms/cm 2  and 4.5 10 17  atoms/cm 2  inclusive, consisting at least in the most part of helium, 
         [0051]    application to the starting substrate of a fracture treatment adapted to cause the detachment from a remainder of the starting substrate of all the portion of the thick area of GaN situated between the free face and the implantation depth of the helium. 
         [0052]    The implantation dose advantageously consists entirely of helium, of the order of approximately 4.10 17  atoms/cm 2  (that is to say between 3.5 10 17  atoms/cm 2  and 4.5 10 17  atoms/cm 2 , even between 3.75 10 17  atoms/cm 2  and 4.25 10 17  atoms/cm 2 ). 
         [0053]    Because there has been co-implantation of helium and hydrogen or implantation of only helium, the invention leads to delamination that is easily extended to the whole of the section of the starting substrate (even if it means lengthening the fracture treatment), and is therefore independent of the existence of localized bonding defects or the absence of bonding at the periphery; this contributes to guaranteeing that the bared surface of the remainder of the starting substrate is neat and clean. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0054]    Objects, features and advantages of the invention emerge from the description that follows, given with reference to the appended drawings, in which: 
           [0055]      FIG. 1  is a theoretical diagram representing a combination of a GaN substrate deposited on a sapphire substrate and that has been implanted, bonded to another sapphire substrate but featuring non-bonded areas, 
           [0056]      FIG. 2  is a theoretical diagram representing the combination from  FIG. 1 , after separation in the implanted area but featuring non-transferred areas, 
           [0057]      FIG. 3  is a diagram of a first implantation step of a method of the invention, 
           [0058]      FIG. 4  is a diagram of a second implantation step of that method, 
           [0059]      FIG. 5  is a diagram of a heat processing step of that method, 
           [0060]      FIG. 6  is a diagram showing bringing into intimate contact with a second substrate, and 
           [0061]      FIG. 7  is a diagram showing the separation of the starting substrate into a thin layer and a substrate remainder ready to be recycled. 
       
    
    
     DETAILED DESCRIPTION 
       [0062]      FIGS. 3 to 7  represent the main steps of one example of a method of the invention for fabrication of a thin layer of GaN: 
         [0063]    1) first implantation of a GaN starting substrate  11 , by means of bombardment with helium, symbolized by arrows  12 , to form an implanted layer  13 , this implantation being carried out at a dose between of the order of 10 17 /cm 2  and of the order of 4.10 17 /cm 2 , 
         [0064]    2) second implantation of this starting substrate  11 , by means of bombardment with hydrogen, symbolized by the arrows  12 ′, this implantation being carried out at a dose between of the order of 10 17 /cm 2  and 4.10 17 /cm 2 , 
         [0065]    3) embrittlement treatment step, in practice including a heat treatment, preferably at low temperature, for a given time, this treatment corresponding to the application of a thermal (or thermo-mechanical if mechanical stresses are applied) budget advantageously representing at least 85% of the thermal (or thermo-mechanical) budget that would be necessary to obtain separation or fracture by this heat treatment alone, 
         [0066]    4) bringing this implanted substrate into intimate contact with a second substrate  17 , also called the host substrate, 
         [0067]    5) application of a residual fracture treatment to separate the starting structure into a thin layer  15  (possibly including remainder fragments  15 A) and a substrate remainder  16  ready to be recycled and to undergo the aforementioned implantation steps again. 
         [0068]    The steps 3 to 5 conjointly constitute a fracture treatment adapted to cause detachment from the remainder of the starting substrate of all the portion of the GaN area situated between the free face through which the implantation was effected and the implantation depth of the hydrogen. A particularly simple version of the method of the invention is limited, for example, to conducting the step 3 until detachment of the required thin layer. 
         [0069]    The starting substrate  11  here consists entirely of GaN. Alternatively, as in the case of  FIG. 1 , it can be a substrate of which only a thick surface area, along the free face of the substrate, is of GaN, this area being a layer carried by a support that can consist of sapphire. This area is sufficiently thick for the implantations to be effected within the thickness of this area. 
         [0070]    The thermal (or thermo-mechanical) budget concept corresponds to the energy input in thermal or thermo-mechanical form leading to fracture, and the percentage indicated is in practice given with reference to the time that would be necessary to attain that energy at constant temperature and (where applicable) stress. 
         [0071]    The heat treatment of step  3 , combined or not with the application of stresses, causes the defects introduced by the implantations to evolve so as to form an embrittled layer  13 ″ substantially at the level of the layer  13  separating the future thin layer  15  and the future remainder  16 . The surface  14  can be covered with a protective layer  18 , for example of oxide, before or after the implantation steps, which layer  18  can be removed or not before the bringing into contact with the second substrate (step 4). 
         [0072]    The invention resides particularly in the particular choice of the conditions of implantation in the GaN, the effect of which is that the steps 1 and 2 lead to the formation of defects that in one particular embodiment evolve with temperature. Thus the person skilled in the art knows that implantation of gaseous ions (H, He, etc.) in a substrate (Si, Ge, GaN, etc.) leads to the formation of defects and microcavities at a depth that depends primarily on the ions implanted, the substrate and the implantation energy. If a heat treatment is applied to the implanted substrate (without bonding beforehand to a stiffener), the defects normally evolve in size and in density until they form, substantially at the implantation depth, microcracks containing a gaseous phase of the implanted ion. Under the effect of the pressure of the gas, these microcracks then lead to local deformation of the free surface of the substrate in the form of blisters, or even local detachment in the form of exfoliation. 
         [0073]    On the other hand, the implantation conditions specified for the invention lead to the formation in the GaN of defects that evolve differently with temperature. These original conditions, following appropriate heat treatment, and even without the application of a stiffener substrate, lead to total detachment of the implanted surface film  15 , without localized exfoliation or formation of blisters. This delamination has appeared to be linked to the particular nature of the implantation defects generated by the conditions of the invention, and not to a limited implantation energy (see PCT Patent Application Publication No. WO-A-2003/063213 which teaches a relationship between dose and implantation energy to obtain substantially complete exfoliation during a fracture treatment executed entirely after bonding to a stiffener). 
         [0074]    It follows from this that the conditions of implantation and heat treatment of the invention are capable of provoking for GaN complete fracture of the starting substrate in the region of the hydrogen implantation area without the presence of an effective stiffener being necessary (as the person skilled in the art has previously thought). 
         [0075]    Because the presence of a stiffener to prevent local exfoliation or to maintain the flatness of the delaminated layer is not necessary, the invention teaches that it is advantageous to carry out the heat treatment (known as “superembrittlement”) with no stiffener substrate, in particular, with no massive stiffener (one with a thickness greater than several microns), and therefore without limits associated with any difference in coefficient of thermal expansion between the two substrates, and this treatment can be carried out “virtually to the end”, in such a manner as to enable, if required, and just before separation, fixing to any substrate to facilitate manipulation of the thin layer after separation (remember that, in practice, the separation of a thin layer from the remainder of the starting substrate, after the starting substrate has been fixed to a second substrate, is defined as a transfer step because the thin layer, initially forming part of the starting substrate, is in the end attached to the second substrate). Thanks to the invention, even the areas not bonded to the second substrate (in particular the peripheral areas, the areas of defects in the substrate, and the areas of lack of bonding corresponding to areas P, C, and I from  FIG. 1 ) are detached thanks to an appropriate fracture treatment. 
         [0076]    According to one aspect of the invention, this heat treatment can be accompanied by mechanical stresses. 
         [0077]    If a transfer is to be effected, this “superembrittlement” step must not be conducted until it ends in fracture in the microcavities layer, but can advantageously represent at least 85%, even 85% or at least 99% (for example 99.99%) of the fracture thermal (and/or thermo-mechanical) budget, even though, to avoid untimely fracture, it may appear prudent not to exceed 99%, for example. 
         [0078]    The step of bringing into intimate contact with the second substrate  7  forming a host is advantageously carried out by bonding (preferably molecular bonding), and can involve the use of a bonding layer deposited on the surface  14  of the GaN substrate as well as or instead of the protection layer  18 . 
         [0079]    Heat treatment to strengthen this bonding can be provided, and either participate or not in the evolution of the defects in the layer  13 ″ (thus the thermal budget includes two steps). 
         [0080]    The final fracture step consists in heat and/or mechanical treatment. The thermo-mechanical budget to be applied is inversely proportional to the super-embrittlement treatment. The step of transferring the thin film onto the second substrate can be carried out at room temperature, and thus without heat treatment, simply by application of mechanical stresses, for example by insertion of a blade. 
         [0081]    However, as it may prove difficult to apply mechanical treatment to some non-bonded areas, a heat treatment is advantageously chosen for the detachment of the areas in vertical alignment with those non-bonded areas. 
         [0082]    As a consequence of the fracture step, the thin film  15  is detached entirely from the substrate initially implanted, including in locally non-bonded areas such as those labeled C or I in  FIG. 1  and including those at the periphery (reference P in  FIG. 1 ). The detachment of these areas P (or even these areas C or I) can be simultaneous with detachment of the usable part (in practice the central part of the film) transferred to the second substrate (for example after appropriate heat treatment) or necessitate a specific fracture treatment before or after detachment of this usable portion. Recycling of the initial substrate  1  after complete fracture therefore necessitates no costly step of mechanical and/or chemical leveling to remove non-transferred areas (as is the case at present in a standard process). 
         [0083]    According to the applications, the recycling can be prepared through a simple step of light mechanical-chemical polishing that reduces surface roughness after fracture with minimum removal of material (at most one micron, typically of the order of 0.01 nm or of the order of 0.2 microns). Depending on requirements, the substrates can be recycled directly after the fracture step, with no particular surface treatment, for example to carry out further GaN transfers. 
         [0084]    Particular embodiments of the method of the invention are described hereinafter. 
       Example 1  
       [0085]    A GaN ( 70 Ga  14 N) crystalline substrate is implanted with He ions under the following conditions: energy=90 keV, dose=2.10 17  cm 2 , then with H ions under the following conditions: energy=60 keV, dose=2.10 17  cm 2 . A protection layer of SiO 2  of thickness that can be chosen between 500 nm and 1 μm is then deposited by the PECVD (Plasma Enhanced Chemical Vapor Deposition) process onto the implanted substrate. 280° C.-1 h superembrittlement annealing is then applied (i.e. for 1 hour at 280° C. with no additional mechanical stress). 
         [0086]    Note that 280° C.-1 h annealing represents ˜85% of the fracture thermal budget at 280° C., since under these conditions fracture is obtained after 280° C.-1 h10. 
         [0087]    It has been noted that the implantation depths obtained in this way were 390 nm for He and 450 nm for hydrogen, i.e. were substantially equal (to within 60 nm). 
         [0088]    Moreover, the very operation of PECVD deposition of the protection layer can induce heating and thus contribute to superembrittlement of the substrate. 
         [0089]    The GaN substrate is then bonded by molecular adhesion to a bulk sapphire stiffener, by the following steps: 
         [0090]    chemical cleaning of the GaN and sapphire substrates, 
         [0091]    mechanical-chemical polishing, 
         [0092]    bringing of the substrates into contact. 
         [0093]    Fracture is then caused in the implanted layer by a heat treatment. GaN and sapphire having different coefficients of thermal expansion (GaN: ˜5.8.10 −6 /K, sapphire: ˜8.10 −6 /K), the temperature of the fracture heat treatment must be sufficiently low to prevent unbonding of the structure. It has been considered here that the fracture heat treatment must be carried out at T°&lt;230° C. The superembrittlement then enables fracture to be obtained at 230° C. in ˜13 h. Without the superembrittlement step, the fracture time (i.e. the fracture thermal budget) would have been 88 h at 230° C. 
         [0094]    This 230° C.-13 h fracture step leads to the detachment of a film (or thin layer) of GaN ˜480 nm thick, in principle over the whole of the surface of the implanted GaN substrate. Nevertheless, it can happen that a central portion of the GaN film is first transferred to the sapphire substrate (thus a GaN/SiO 2 /sapphire structure is obtained that can be used to produce light-emitting diodes (LED), for example). Another portion localized to the ring of the GaN substrate is detached from the substrate in the form of an annular self-supporting membrane (symbolized by the ring  15 A in  FIG. 7 ). The remainder  16  of the initially implanted GaN substrate  11  can then be recycled directly after fracture to effect a number of successive transfers, with no intermediate step of specific surface preparation between each transfer cycle, the aforementioned steps being nevertheless applied to this remainder  6 , and then to the remainder of that remainder after a new transfer, and so on. 
         [0095]    It has been verified that photos representing the remainder of a standard transfer (after hydrogen implantation) show very clearly a change of appearance in the peripheral portion, as well as at localized places (within the central area), while photos taken under the same conditions have revealed an entirely homogeneous appearance after fracture by the steps described above. 
       Example 2  
       [0096]    A GaN ( 70 Ga  14 N) crystalline substrate is covered with a bonding layer of SiO 2  of 500 nm thickness, deposited by the LPCVD (Low Pressure Chemical Vapor Deposition) process at 900° C. This substrate is implanted with He ions under the following conditions: energy=210 keV, dose=2.10 17  cm −1 , then with H ions under the following conditions: energy=120 keV, dose=2.10 17  cm −2 . Superembrittlement annealing of 300° C.-44 min is then applied. 
         [0097]    Note that a 300° C.-44 min annealing represents ˜97.7% of the fracture thermal budget at 300° C. since, under these conditions, fracture is obtained after 300° C.-45 min. 
         [0098]    It has been found that the depths of implantation obtained in this way were 750 nm for He and 700 nm for hydrogen, i.e. that they were substantially equal (to within 50 nm). 
         [0099]    The GaN substrate with the bonding layer is then bonded by molecular adhesion to a sapphire substrate. Fracture is then caused in the implanted layer by a heat treatment at T°&lt;230° C. The superembrittlement then enables fracture to be obtained at 220° C. in ˜16 h. Without the superembrittlement step, the fracture time (i.e. the thermal budget of this final fracture step) would have been 30 days at 220° C. 
         [0100]    The 220° C.-16 h fracture step leads to the detachment of a film of GaN of ˜480 nm thickness and of the 500 nm SiO 2  bonding layer over the whole of the area of the implanted GaN substrate. As indicated above with reference to example 1, a portion of the GaN film is transferred onto the sapphire substrate; a GaN/SiO 2 /sapphire structure is thus obtained which can be used for example to produce light-emitting diodes (LED). Another portion, localized to the ring of the GaN substrate, is detached from the substrate in the form of a self-supporting membrane. The initially implanted GaN substrate is then recycled for further transfers, after a step of simple resurfacing by CMP (for example a few seconds of mechanical-chemical polishing) to eliminate the roughness associated with the fracture. 
       Example 3  
       [0101]    A GaN ( 70 Ga  14 N) crystalline substrate is covered with an SiO 2  bonding layer of 250 nm thickness, deposited by the PECVD (Plasma Enhanced Chemical Vapor Deposition) process at 300° C. This substrate is implanted with He ions under the following conditions: energy=90 keV, dose=3.10 17  cm −2 , and then with H ions under the following conditions: energy=60 keV, dose=10 17  cm −2 . Superembrittlement annealing of 495° C.-4 h10 min is then applied. Note that a 495° C.-4 h10 min annealing represents ˜98% of the fracture thermal budget at 495° C. since under these conditions fracture is obtained after 495° C.-4 h15. 
         [0102]    It was found that the implantation depths obtained in this way were 390 nm for He and 450 nm for hydrogen, i.e. that they were substantially equal (to within 60 nm). 
         [0103]    The GaN substrate with the bonding layer is then bonded by molecular adhesion to a sapphire substrate. The bonding is then consolidated by annealing at low temperature, for example 150° C.-2 h. This type of annealing does not contribute significantly to superembrittlement of the implanted GaN substrate, this annealing temperature being too low; it therefore makes no contribution to the energy budget already expended on the starting support. 
         [0104]    Fracture is then caused in the implanted layer by application of mechanical stresses. For this purpose a blade is inserted between the two bonded substrates, for example (see the arrow L in  FIG. 7 ). A portion of the GaN film is transferred onto the sapphire substrate, and there is obtained in this way a GaN/SiO 2 /sapphire structure that can be used to produce light-emitting diodes (LED), for example. Because the residual fracture treatment is essentially mechanical, portions insufficiently bonded (exclusion areas, NBA, etc.) are not detached from the GaN substrate, but annealing at 400° C.-1 min eliminates these residual areas on the GaN substrate in the form of a self-supporting membrane. The whole of the GaN film is therefore detached, and there is obtained on the remainder of the starting substrate a plane surface with no reliefs. This remainder of the initially implanted GaN substrate can then be recycled for further transfers. 
         [0105]    Quality delamination in a GaN substrate has in fact been observed, under the aforementioned implantation conditions, for very varied pairs of helium and hydrogen doses, respectively, between 10 17  atoms/cm 2  and 4.10 17  atoms/cm 2 , notably the following pairs, with regard to which note that they correspond to total doses between 3 and 4, preferably between 3.5 and 4 (expressed in units corresponding to 10 17  atoms/cm 2 ): 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 He = 2 
                 H = 1 
                 i.e. a total of 3, 
               
               
                   
                 He = 2 
                 H = 1.5 
                 i.e. a total of 3.5, 
               
               
                   
                 He = 1.75 
                 H = 2 
                 i.e. a total of 3.75, 
               
               
                   
                 He = 2 
                 H = 2 
                 i.e. a total of 4, and 
               
               
                   
                 He = 3 
                 H = 1 
                 i.e. a total of 4. 
               
               
                   
                   
               
             
          
         
       
     
         [0106]    Quality delamination, under the same implantation conditions as before, has even been found with implantation of only helium, i.e. with He=4 and H=0. 
         [0107]    This makes it possible to conclude that, in this range of implantation doses, if the helium implantation dose is sufficiently high (at least equal to approximately 3.10 17  atoms/cm 2 , typically between 3.5 10 17  atoms/cm 2  and 4.5 17  atoms/cm 2 , even between 3.75 10 17  atoms/cm 2  and 4.25 10 17  atoms/cm 2 ), very good delamination is also obtained, with a good surface quality on the faces obtained by the fracture, in the absence of complementary hydrogen implantation.