the following description is given with regard to fig1 through 12 , which illustrate steps in a process for fabricating a semiconductor structure employing a handle substrate to support a carrier substrate . the expression “ semiconductor structure ” is understood to mean any structure that is used in the production of a semiconductor device . a semiconductor structure may comprise conductors , semiconductors and / or insulators . this may be a layer comprising or not comprising microcomponents or finished or partially finished microcomponents per se . the expression “ handle substrate ” is understood to mean a composite structure , the function of which is to act as a temporary mechanical support for a substrate or structure . the expression “ carrier substrate ” is understood to mean a substrate that will be joined , in particular , temporarily , to a handle substrate and that may be subjected to treatments . this may , for example , be a substrate comprising finished or partially finished microcomponents to be transferred to a host substrate . the expression “ host substrate ” is understood to mean a substrate intended to receive ( typically by transfer ) a substrate or a structure . the expression “ stop layer ” is understood to mean the first layer that is not removed during the recycling operation . in a process for fabricating a semiconductor structure , in a first step e 1 , a handle substrate 1 , 2 is provided comprising a seed substrate 1 and a sacrificial layer 2 covering the seed substrate 1 . the sacrificial layer 2 is weakened or has been weakened beforehand , so that , in a fabrication process , it is possible either to provide the handle substrate with the previously weakened sacrificial layer 2 or to weaken the sacrificial layer 2 in the fabrication process . the sacrificial layer 2 is typically weakened by introducing atomic species into the sacrificial layer 2 . the sacrificial layer 2 is preferably made of polysilicon , because detachment is particularly easy when this material is used . in this respect , the reader may refer to document c . h . yun , n . quitoriano , n . w . cheung : “ polycrystalline silicon layer transfer by ion - cut ,” applied physics letters , vol . 82 , no . 10 , march 2003 . furthermore , an intermediate layer 20 may be provided , placed between the seed substrate and the sacrificial layer 2 , this intermediate layer ensuring good adhesion of the sacrificial layer 2 to the seed substrate 1 . this intermediate layer 20 may serve both as a tie layer and as an etch - stop layer during optional recycling e 5 of the handle substrate , the recycling consisting in removing any residue of the sacrificial layer 2 present on the seed substrate 1 ( see below ). it will be noted that this layer is , in particular , necessary when the sacrificial layer 2 is made of the same material as the substrate 1 . the sacrificial layer 2 covers the seed substrate 1 . this sacrificial layer 2 may be covered with an additional layer 21 that makes it easier to subsequently join the handle substrate with the carrier substrate 3 . thus , this additional layer 21 may take the form of a superficial oxide bonding layer . whether this layer is present or not , it is important for the exposed surface of the handle substrate to be compatible with the subsequent assembly step e 2 . thus , if it is envisaged to temporarily join the handle substrate with the carrier substrate by molecular bonding , the rms surface roughness of the handle substrate must be about 10 ångströms or less . the introduction of atomic species has the objective of forming a weak zone 2 ′″ buried in the sacrificial layer 2 covering the seed substrate 1 . the atomic species introduced may be hydrogen or helium ions , inert - gas ions or even fluorine or boron ions , whether alone or in combination . hydrogen and helium are particularly advantageous because they are very commonly implanted . thus , the handle substrate is liable to separate in the weak zone 2 ′″ when it receives energy in this zone ( for example , when it is heated and / or a mechanical stress is applied ). the parameters of the atomic species introduction and , in particular , the dose of species introduced , may be adjusted so as to prevent the handle substrate from breaking or separating along the weak zone during assembly of the handle substrate with the carrier substrate 3 or during treatments carried out on the substrate 3 , in particular , if these treatments comprise a heat treatment step . this allows the handle substrate to be detached from the carrier substrate 3 during subsequent treatment steps , as will be described below . the depth to which the species are introduced into the handle substrate so as to produce the weak zone is mainly a function of the energy with which the species are introduced into the handle substrate . insofar as the species introduced are indeed essentially located in the sacrificial layer , the exact position of the weakened zone is not critical . by way of non - limiting example , the atomic species may be introduced into the sacrificial layer 2 to a depth of between 50 nm and several microns . the introduction of atomic species may consist in exposing an area of the handle substrate to atomic species implantation , at a dose of between 1 × 10 15 ions / cm 2 and 1 × 10 17 ions / cm 2 and at an energy of between 5 kev and 500 kev . alternatively , the introduction of atomic species may consist in diffusing atomic species into the handle substrate , i . e ., of bringing the surface of the handle substrate into contact with a chemical species that will penetrate into the handle substrate by chemical diffusion . this may be achieved using a plasma . this introduction may also be achieved during the formation of the sacrificial layer 2 , for example , by incorporating a large amount of hydrogen in the layer during its deposition . it will be noted that , in contrast to known implantation - based layer transfer techniques , it is not necessary , in the context of this invention , to precisely locate the implanted species so as to define a layer to be transferred . it is indeed sufficient to incorporate enough species in the layer to allow defects such as voids or platelets to form under the effect of a heat treatment , which defects will subsequently allow the seed substrate 1 to be detached . the h and / or he density in the sacrificial layer 2 lies between 1 × 10 16 at / cm 3 and 1 × 10 20 at / cm 3 . in the polysilicon sacrificial layer 2 , this density is about 1 × 10 18 at / cm 3 . furthermore , whatever method of introducing atomic species is used , the species introduction may be combined with a confinement in which : before the species introduction , a confinement layer is created in the handle substrate ; and after the species introduction , the handle substrate is exposed to a temperature of at least 100 ° c . with a view to promoting migration of the introduced species towards the confinement layer . as has already been mentioned , in a step e 2 , the handle substrate 1 , 2 is joined with a carrier substrate 3 . this assembly step e 2 allows the carrier substrate 3 to be provided with a mechanical support . advantageously , the material of the seed substrate 1 forming the handle substrate may be chosen so as to have a thermal expansion coefficient near the thermal expansion coefficient of the carrier substrate . preferably , | cte 1 − cte 2 |/ cte 1 & lt ; 50 %, where cte 1 is the thermal expansion coefficient of the seed substrate and cte 2 is the thermal expansion coefficient of the carrier substrate . again advantageously , the seed substrate 1 may be made of silicon or any other material that can be provided in the form of a substrate compatible with the treatments carried out on the carrier substrate 3 . thus , this seed substrate 1 must be able to withstand heat treatments at a few hundred degrees , for example , up to 500 ° c ., must be able to withstand mechanical stresses and must be chemically inert in order to withstand chemical - mechanical polishing ( cmp ) or grinding , and must be sufficiently flexible to be able to be strained during a molecular bonding step . in this respect , the seed substrate 1 will possibly be chosen from a single crystal ( silicon , sic , quartz , sapphire ) substrate , an amorphous or polycrystalline ( polysic , glass , glass - ceramic ) substrate , a ceramic ( aluminium or silicon nitride , mullite , alumina ) or a metal ( tungsten , molybdenum ). the assembly step e 2 may consist in bonding the handle substrate 1 , 2 to the carrier substrate 3 . it is the layer 2 ″ that then makes contact with the carrier substrate 3 . preferably , this is a molecular bonding operation , which , therefore , does not require an adhesive or any other form of adhesive layer , the limitations of which were mentioned in the introduction . once joined with the substrate 1 that supports it , the substrate 3 may undergo one or more treatments . for example , in the case where circuits are joined , the carrier substrate 3 is thinned from the back side and joined e 2 ′, for example , bonded , to a final host substrate 4 . in this case , the material of the seed substrate 1 may be chosen so as to have a thermal expansion coefficient near the thermal expansion coefficient of the final host substrate 4 . preferably | cte 1 − cte 3 |/ cte 1 & lt ; 50 %, where cte 1 is the thermal expansion coefficient of the seed substrate and cte 3 is the thermal expansion coefficient of the final host substrate . next , in a fourth step , the handle substrate is detached e 4 at the sacrificial layer 2 and , in particular , at the weak zone 2 ′″ in the case where the weakening was achieved by introducing atomic species . alternatively , or in a complementary way , before this detachment step has been carried out , a step may be implemented consisting in joining e 30 the carrier substrate 3 and the layer 2 ″ to a host substrate 4 . the detachment step e 3 especially consists in supplying energy by annealing at a temperature of at least 200 ° c . in addition to this heat treatment , a mechanical stress may be applied to the weakened zone in order to achieve this detachment . thus , in the case where the assembly step e 30 is carried out , the handle substrate allows the carrier substrate 3 to be placed on the host substrate 4 without damaging the carrier substrate 3 . furthermore , the remaining part of the handle substrate may easily be recycled by selectively etching any residue of the sacrificial layer 2 ′. for this purpose , a step e 5 of removing any residue of the sacrificial layer 2 from the seed substrate 1 is implemented . recycling of the remaining part of the carrier substrate does not reduce the thickness of the handle substrate since it is the residual layer 2 that is consumed . this makes it possible to reduce substrate consumption relative to known processes that involve transferring part of a silicon substrate before recycling it , using up part of the thickness of this substrate . a semiconductor structure is obtained using the process described above , the structure possibly consisting of the host substrate 4 , the carrier substrate 3 and possibly any residue of the layer 2 ′ originating from the sacrificial layer of the handle substrate . this residue will , for example , be removed by polishing or using a chemical treatment in the step e 5 of removing any residue of the sacrificial layer 2 present on the semiconductor structure . finally , the fabrication process may comprise a step of recycling the handle substrate , especially consisting in smoothing the free surface or removing the layer 2 ″. such smoothing or removal may be achieved using a grinding process , a wet - etching process or a chemical - mechanical polishing process . in the case where an intermediate layer 20 is placed between the seed substrate 1 and the sacrificial layer 2 , the intermediate layer 20 may be used as a stop layer . however , in the case where this intermediate layer is not present , the seed substrate 1 acts as a stop layer .