Abstract:
The invention relates to a process for fabricating a semiconductor that comprises providing a handle substrate comprising a seed substrate and a weakened sacrificial layer covering the seed substrate; joining the handle substrate with a carrier substrate; optionally treating the carrier substrate; detaching the handle substrate at the sacrificial layer to form the semiconductor structure; and removing any residue of the sacrificial layer present on the seed substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of right of priority under 35 U.S.C. §119(a) based on French Patent Application No. FR 1155548, filed Jun. 23, 2011 and entitled “Process for Fabricating a Semiconductor Structure Employing A Temporary Bond,” the disclosure of which is hereby incorporated herein by this reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to the production of semiconductor structures for electronic, optical or microelectronic applications. 
         [0003]    More precisely, the invention relates to a process for fabricating a semiconductor structure by temporarily bonding one substrate to another. 
         [0004]    The invention also relates to a semiconductor assembly employed in such a process. 
       BACKGROUND 
       [0005]    In a process for fabricating a semiconductor structure, layers, for example, comprising integrated circuits may be transferred. Such transfers especially allow circuits to be attached to substrates other than those used to produce them, or else allow circuits to be stacked so as to form “3D” components. 
         [0006]    If the thin layer to be transferred is of a small thickness (i.e., below 200 μm), it may, during transfer, be liable to crack or split or, more generally, it may be damaged. 
         [0007]    A solution disclosed in EP 0,786,801 for reinforcing the layer to be transferred or the substrate to be treated comprises temporarily bonding a handle substrate to the substrate comprising the layer to be transferred. The layer to be transferred or the substrate to be treated may thus be freely handled and undergo all the fabrication steps necessary for its transfer or treatment. 
         [0008]    In EP 0,786,801, the handle substrate comprises a cleavage zone that allows, at the end of the process, the handle substrate to be removed along this cleavage zone. 
         [0009]    One problem is that such a handle substrate consumes material. Furthermore, it is not easy to recycle the remaining part in order to reuse it. This is because it is necessary to implement a polishing operation, thereby increasing the duration and cost of the process. 
         [0010]    Another known solution, which does not consume material, consists in temporarily bonding, by means of an adhesive, the handle substrate to the substrate comprising the layer to be transferred. 
         [0011]    In this case, during the transfer or treatment, the force associated with the attachment of the layer to be transferred and the handle substrate is withstood by the adhesive. 
         [0012]    Once the transfer or treatment has been carried out, the handle substrate may be removed. 
         [0013]    A problem arises from the use of an adhesive. 
         [0014]    This is because adhesives can become unstable if exposed to the high temperatures employed in the treatments carried out on the substrate or during fabrication of semiconductor structures using a transfer. 
         [0015]    Moreover, an adhesive layer does not allow sufficiently stable attachment of the substrate for certain treatments to be carried out thereon. This is the case when the substrate, for example, is thinned by grinding to below a thickness threshold, for example, of 200, 50 or 40 microns. The mechanical stress exerted in this step leads to strain in the layer resting on the insufficiently rigid adhesive layer, thereby, in turn, leading to non-uniform thinning of the substrate. 
         [0016]    Furthermore, once the treatment has been carried out, the adhesive is completely removed by means of a chemical removal technique (dissolving in a solvent, for example). Such removal increases the duration of the fabrication process and risks damaging the semiconductor structure obtained. 
       BRIEF SUMMARY 
       [0017]    The invention allows the drawbacks mentioned above to be alleviated. 
         [0018]    Thus, according to a first aspect, the invention relates to a process for fabricating a semiconductor structure, characterized in that it comprises the following steps:
       providing a handle substrate comprising a seed substrate and a weakened sacrificial layer covering the seed substrate;   joining the handle substrate with a carrier substrate;   optionally treating the carrier substrate;   detaching the handle substrate at the sacrificial layer so as to form the semiconductor structure; and   removing any residue of the sacrificial layer present on the seed substrate.       
 
         [0024]    By virtue of the process of the invention, on the one hand, the material of the seed substrate of the handle substrate may be chosen from a wide range of materials and, on the other hand, the remaining part of this substrate may be recycled in a particularly easy way in order to be reused in the same way. 
         [0025]    Specifically, recycling of the remaining part of the handle substrate after the detachment step can be easily carried out: a simple selective etch of the sacrificial layer is enough to allow recycling of the handle substrate. 
         [0026]    This type of recycling process is much less expensive than a polishing-based recycling technique, such as is necessarily the result of the process described in EP 0,786,801, for example. 
         [0027]    Another advantage of the invention is that the recycling does not reduce the thickness of the handle substrate since the handle substrate is covered with a sacrificial layer, it being this layer alone that is consumed in the recycling. The handle substrate is, therefore, theoretically infinitely reusable. It is thus possible to make cost savings relative to known processes that involve transfer of part of a silicon handle substrate and its recycling, part of the thickness of this substrate being consumed in these processes. 
         [0028]    The following are other aspects of the process according to the first aspect of the invention:
       the sacrificial layer is weakened by introducing atomic species into the sacrificial layer of the handle substrate;   the seed substrate has a thermal expansion coefficient CTE 1  near the thermal expansion coefficient CTE 2  of the carrier substrate such that |CTE 1 −CTE 2 |/CTE 1 &lt;50%;   the handle substrate comprises an intermediate layer placed between the seed substrate and the sacrificial layer in order to promote adhesion of the material forming the sacrificial layer to the seed substrate;   the sacrificial layer has a weak zone and defines a layer located between the surface of the handle substrate and the weak zone;   the process comprises, before the detachment step, a step consisting in joining the carrier substrate to a host substrate;   the seed substrate is chosen so as to have a thermal expansion coefficient CTE 1  near the thermal expansion coefficient CTE 2  of the host substrate;   the carrier substrate comprises an integrated-circuit part;   the detachment step consists in supplying energy by annealing at a temperature of at least 200° C.;   the introduction of atomic species consists 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;   the introduction of atomic species consists in diffusing atomic species into the handle substrate by bringing the surface of the handle substrate into contact with a chemical species that will penetrate into the handle wafer by chemical diffusion;   the introduction of atomic species involves:
           before the species introduction, creating a confinement layer in the handle substrate; and   after the species introduction, exposing the handle substrate to a temperature of at least 200° C. with a view to promoting migration of the introduced species towards the confinement layer;   
           the joining consists in bonding the handle substrate to the carrier substrate;   the joining is achieved by molecular adhesion;   the sacrificial layer is a polysilicon layer; and   the seed substrate is: a single crystal substrate, or an amorphous or polycrystalline substrate, or a ceramic, or a metal.       
 
         [0046]    According to a second aspect, the invention relates to a handle substrate comprising a seed substrate and a weakened sacrificial layer. 
         [0047]    The following are other aspects of the handle substrate according to the second aspect of the invention:
       the sacrificial layer contains a density of H and/or of He lying between 1×10 16  at/cm 3  and 1×10 20  at/cm 3 ;   the sacrificial layer is made of polysilicon;   the seed substrate is a single crystal substrate, an amorphous or polycrystalline substrate, a ceramic or a metal;   it has an RMS surface roughness of 10 Ångströms or less;   it has an additional surface layer that simplifies subsequent joining of the handle substrate with the carrier substrate; and   the additional layer is made of silicon oxide.       
 
         [0054]    In addition, according to a third aspect, the invention relates to a process for fabricating a handle substrate comprising the following steps: forming a sacrificial layer on a seed substrate and introducing an atomic species into the sacrificial layer. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0055]    Other features and advantages of the invention will become more clear from the following description, which is purely illustrative and non-limiting and which must be read with regard to the appended drawings in which: 
           [0056]      FIG. 1  illustrates the steps of a process according to one embodiment of the invention; and 
           [0057]      FIGS. 2 through 12  illustrate configurations found in a process according to one embodiment of the invention. 
       
    
    
       [0058]    In all the figures, similar elements have been given identical reference numbers. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0059]    The following description is given with regard to  FIGS. 1 through 12 , which illustrate steps in a process for fabricating a semiconductor structure employing a handle substrate to support a carrier substrate. 
         [0060]    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. 
         [0061]    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. 
         [0062]    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. 
         [0063]    The expression “host substrate” is understood to mean a substrate intended to receive (typically by transfer) a substrate or a structure. 
         [0064]    The expression “stop layer” is understood to mean the first layer that is not removed during the recycling operation. 
         [0065]    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 . 
         [0066]    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. 
         [0067]    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. 
         [0068]    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 . 
         [0069]    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. 
         [0070]    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. 
         [0071]    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). 
         [0072]    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. 
         [0073]    This allows the handle substrate to be detached from the carrier substrate  3  during subsequent treatment steps, as will be described below. 
         [0074]    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. 
         [0075]    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. 
         [0076]    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. 
         [0077]    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. 
         [0078]    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 . 
         [0079]    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.       
 
         [0082]    As has already been mentioned, in a step E 2 , the handle substrate  1 ,  2  is joined with a carrier substrate  3 . 
         [0083]    This assembly step E 2  allows the carrier substrate  3  to be provided with a mechanical support. 
         [0084]    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. 
         [0085]    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 . 
         [0086]    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). 
         [0087]    The assembly step E 2  may consist in bonding the handle substrate  1 ,  2  to the carrier substrate  3 . 
         [0088]    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. 
         [0089]    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 . 
         [0090]    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. 
         [0091]    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. 
         [0092]    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 . 
         [0093]    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. 
         [0094]    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 . 
         [0095]    Furthermore, the remaining part of the handle substrate may easily be recycled by selectively etching any residue of the sacrificial layer  2 ′. 
         [0096]    For this purpose, a step E 5  of removing any residue of the sacrificial layer  2  from the seed substrate  1  is implemented. 
         [0097]    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. 
         [0098]    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. 
         [0099]    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. 
         [0100]    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 ″. 
         [0101]    Such smoothing or removal may be achieved using a grinding process, a wet-etching process or a chemical-mechanical polishing process. 
         [0102]    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.