Abstract:
A method for transferring a thin layer from an initial substrate includes forming an assembly of the initial substrate with one face of a silicone type polymer layer, this face having been treated under an ultraviolet radiation, and processing the initial substrate to form the thin layer on the silicone type polymer layer.

Description:
RELATED APPLICATIONS 
     The present patent document claims the benefit of priority to French Patent Application No. 07 59893, filed Dec. 17, 2007, which is incorporated herein by reference. 
     TECHNICAL DOMAIN AND PRIOR ART 
     The invention relates to the domain of thin layers, and particularly the domain of semiconducting materials. 
     In particular it relates to a technique for transferring a thin layer onto an easily removable host substrate, but with sufficient bonding energy between the thin layer and the substrate. 
     A fracture method, known under the name of Smart Cut™ is described in the article by A. J. Auberton-Hervé et al. “Why can Smart-Cut change the future of microelectronics?” that was published in International Journal of High Speed Electronics and Systems, Vol. 10, No. 1 (2000), p. 131-146. 
     This method is used in particular for making SOI (Silicon On Insulator) components or elements. 
     An SOI structure ( FIG. 1C ) comprises a stack composed of a support substrate  7  (for example made of silicon), a buried dielectric layer  8 , for example silicon oxide or a nitride such as Si3N4 and a silicon layer  5 , in which components may be located. 
     The buried layer  8  forms an insulation from parasite currents and charges originating from ionised particles. It thus enables good insulation of adjacent components made in the same silicon layer and a particularly significant reduction in parasite capacitances between such adjacent components, and insulation of the thin layer  5  made of semiconducting material from the subjacent support  7 . 
       FIGS. 1A to 1C  show an example implementation of a layer transfer method using the Smart Cut™ method in order to make an SOI structure. These figures are sectional views. 
       FIG. 1A  shows a silicon substrate  1  provided with a surface oxide layer  8 , during an ionic or atomic implantation step  3  of one or several gaseous species through the face  2  of this substrate. A buried layer  4  forming a weakened zone is then formed separating the substrate  1  into two parts: a thin layer  5 , located between the implanted face  2  and the weakened zone  4  and the remaining part  6  of the substrate located under the weakened zone  4 . 
       FIG. 1B  shows a step in fixing the layer  5  of the substrate  1 , through its face  2 , onto a support substrate  7  or a stiffener. This fixing may be obtained by direct bonding (also called molecular bonding). 
       FIG. 1C  shows the result of a step induced by a thermal and/or mechanical effect, to separate the thin layer  5  and the remaining part  6  (not shown) of the substrate  1  along the weakened zone  4 . The result that can be obtained is a silicon on insulator (SOI) type of structure. As a variant, this structure could be obtained by molecular bonding of a silicon substrate (possibly oxidised on the surface) and a host substrate (also possibly covered with an oxide layer on the surface) and mechanical-chemical thinning of the silicon substrate. 
     These techniques are not limited to the production of a silicon on insulator substrate, but may be more generally applied to production of a “semiconductor on insulator” structure. In particular, apart from silicon, this concerns germanium, gallium nitride, silicon carbide, gallium arsenide and indium phosphate. 
     Substrate  7  keeps the previous two thin layers  5 ,  8  stacked together to create the final structure. It must be sufficiently rigid to facilitate fracture in the case of the Smart Cut process in the implantation zone  4  of the substrate  1 , rather than the occurrence of blisters on the surface of the structure. Similarly, bonding between the two substrates ( 1 , 7 ) must be sufficiently strong to prevent any separation at this bonding interface, particularly during application of the fracture heat treatment in the case of the Smart Cut process and to enable mechanical-chemical thinning. 
     In most cases, the bond of the detached film  5  on its new support  7  is permanent. This is the case particularly for SOI made by previously described techniques. 
     But in some cases, a double transfer of the layer  5  is required. For example, it might be desirable for the free surface  5 ′ of the thin layer  5  to form a buried surface after the double transfer. In other words, the transfer described above with reference to  FIGS. 1A-1C  is then temporary, and an additional transfer step is made, the face  5 ′ of the layer  5  being fixed to another substrate and the substrate  7  being eliminated. 
     This is the case particularly for polar materials for example such as SiC or GaN. When a thin film  5  of this material is to be transferred to a second support, there is a need once again to separate this thin film from the first transfer substrate  7  (or temporary support), after having bonded it onto a new substrate through its free face  5 ′, or before bonding it onto this new substrate. 
     In order to easily separate the substrate  7 , it would be useful to have a low bonding energy between this substrate and the thin layer  5 . But as described previously, such energy is not always compatible with the thinning method used to obtain the thin film. Furthermore, a double transfer cannot be made using flexible supports because:
         they are too soft and they deform during the thinning method,   or they are thermodeformable or thermodegradable and do not resist temperatures imposed by the fracture method without softening or degrading, and therefore they no longer retain the rigidity necessary for the method.       

     Furthermore, it is impossible to increase the rigidity of such flexible materials using an oxide deposited on it, because the differences in the coefficients of expansion between this material and the deposited oxide are such that the deposited oxide is stressed and ripples appear on the surface of said material, which increases the roughness and prevents bonding. 
     Therefore, the problem that arises is to find a host substrate that is sufficiently rigid to obtain a thin layer by thinning (mechanical-chemical or Smart Cut) of an initial substrate that can easily be removed. 
     SUMMARY 
     The invention relates to a method for bonding a substrate to be thinned onto a temporary easily removable support, but that is capable of producing the thinning method in question. 
     A transfer onto a host or temporary substrate takes place by bonding an initial substrate onto the host substrate and then thinning the initial substrate to obtain the required thin layer. Thinning may be mechanical-chemical thinning, or it may be the result of a fracture method like that known under the name Smart Cut™. 
     The invention relates to a method for transferring a thin film from an initial substrate, for example made of semiconducting material, comprising the following steps:
         a) assembly by direct bonding of the initial substrate with a face of a silicone type polymer layer, this face having been treated under ultraviolet radiation,   b) thinning of the initial substrate to form the thin layer.       

     The invention can be used to create a thick oxide, for example between 10 μm and 20 μm thick, on the surface of the polymer, after UV treatment of the polymer. After direct bonding, this oxide creates a strong and rigid assembly between the polymer and the initial substrate compatible with the thinning step of this substrate, either using a substrate fracture technique of the Smart Cut™ type (without applying temperatures beyond the temperature at which the polymer is stable) or by mechanical and/or chemical thinning. Simultaneously, this oxide makes the polymer sufficiently rigid so that this thinning can be done. 
     Thinning may be done by mechanical and/or chemical polishing. 
     As a variant, it can be done by creating a buried fragile zone in the initial substrate before the assembly step a) by the implantation of ionic and/or atomic species delimiting the thin layer to be transferred in this initial substrate, and application of a heat and/or mechanical fracture treatment of the initial substrate along the buried fragile zone after the assembly step a). 
     Advantageously, the thin layer is transferred onto another substrate called the transfer substrate. 
     Therefore, the invention relates particularly to a method for transferring a thin layer from an initial substrate, for example made of a semiconducting material, onto a transfer substrate for example a final substrate, this method comprising the following steps:
         a) creation of a buried fragile zone in the initial substrate by the implantation of ionic and/or atomic species, delimiting the thin layer to be transferred in this substrate,   b) assembly of this initial substrate with a face of a silicone type polymer layer, this face having been subjected to a treatment under ultraviolet radiation,   c) fracture of the initial substrate along the buried fragile zone, to leave the layer to be transferred on the silicone type polymer layer,   d) transfer of the layer onto the transfer substrate.       

     The invention also relates to a method for making a transfer of a thin layer of a first substrate to a second substrate, also called the transfer substrate, comprising:
         a first transfer of said layer onto a temporary substrate made of silicone material treated on the surface by a UV radiation,   a second transfer of said layer, from the temporary substrate made of a silicone material onto the second substrate,   a temporary substrate peeling step.       

     The first transfer may take place by assembly by direct bonding of the first substrate with the face of the temporary substrate treated under UV. A thinning can then take place by mechanical and/or chemical polishing. As a variant, it may be done by creating a buried fragile zone in the first substrate by the implantation of ionic and/or atomic species, delimiting the thin layer to be transferred in this substrate, and the application of a heat and/or mechanical fracture treatment of the first substrate along the buried fragile zone. 
     According to the invention, regardless of the embodiment, the polymer used as a temporary or support substrate has been treated under ultraviolet radiation, which has the advantages already described above (particularly the formation of a thick oxide, which then results in a strong rigid assembly). 
     A temporary bonding is then made on this support or temporary substrate that can subsequently be easily separated. 
     Regardless of the planned embodiment, the transfer substrate or the second substrate may be made from a rigid material or from an elastic material, for example a silicone material not treated under UV. 
     After transferring onto the transfer substrate, or onto the second substrate, a step to peel all or some of the silicone type polymer may be done. 
     The peeling step of the polymer layer can induce a separation at the interface between the silicone and the silicone transformed into oxide, or in the silicone layer. In the latter case, a step could be included to chemically eliminate the residual polymer. 
     An additional step can be made to eliminate the part of the polymer transformed into oxide. 
     Advantageously, the treatment of the polymer layer or the silicone layer under ultraviolet radiation is done under an ozone atmosphere. 
     For example, the silicone may be polymerised PDMS. Heat treatments may be used, particularly at low temperature or at temperatures below the silicone decomposition temperature. In particular, a fracture step may be applied by a thermal effect, along a plane of weakness or in a weakened zone, at a temperature lower than the silicone decomposition temperature. 
     A mechanical treatment can also be used, for example by insertion of a blade or by bending or imposing a curvature for stressing, to participate firstly in the separation of the thin film bonded to the silicone, and secondly the remaining initial substrate. 
     Before the assembly step, a preliminary weakening annealing can be performed to limit the thermal and/or mechanical budget necessary to obtain the fracture after the assembly. 
     Prior to the assembly step, and possibly before the implantation step, a step may be included to deposit an oxide layer or a nitride layer on the initial substrate. 
     The material from which the layer to be transferred is made may be semiconducting, for example Si or Germanium or SiC or GaN. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1C  show a layer transfer method, 
         FIGS. 2A to 2F  show an example of a method according to the invention, 
         FIG. 3  shows the chemical structure of the PDMS, a silicone type polymer that can be used in the framework of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     We will now describe the invention in the case of a substrate or layer made of polydimethylsiloxane (PDMS) . But it can be done with any silicone type polymer, these polymers presenting a chain based on Si—O—Si—O that can be transformed into oxide under the effect of UV radiation. 
       FIG. 3  shows the chemical formula of PDMS. PDMS polymerises by polyaddition, in other words by a chain reaction with combination of monomer, the reaction taking place by means of a cross-linking agent. The reaction between the polymer and the cross-linking agent occurs under the action of a catalyst (contained in the cross-linking agent) and heat or the drying time, in accordance with the following scheme: 
     
       
                 
         
             
             
         
      
     
     This polymerisation method does not generate any derivatives (water, releases of gases, etc.), which makes the polymer compatible with the microelectronics. The catalyst is disinhibited under the action of heat. Thus, the PDMS can be polymerised at different temperatures from ambient temperature up to 150° C. The polymerisation time is longer when the temperature is lower. For example, the polymerisation time at 150° C. is 15 minutes, whereas at ambient temperature it is 7 days. PDMS can be deposited on the surface of the material in a layer several millimeters thick and is therefore easily manipulated. 
     The presence of covalent bonds between silicon and oxygen results in silicones having a higher decomposition temperature than other polymers. Thus, PDMS remains stable at between −50° C. and 250° C. 
     PDMS is naturally hydrophobic after polymerisation (a contact angle θ greater than 90° can be measured) due to its CH 3  terminations 
     In order to make hydrophilic bonding, which is required in the case of the required transfer layer, an attempt will be made to bond the two surfaces with (OH) terminations. 
     PDMS has the special feature that it becomes hydrophilic on the surface under the influence of the different treatments, such as a plasma treatment or a UV/Ozone treatment. 
       FIG. 2A  shows a substrate  20  made from this PDMS polymer material. A UV treatment  22  is applied to this substrate (for example under an ozone atmosphere), which in particular changes the hydrophobic properties of PDMS into hydrophilic properties. Silicone reacts to form free radicals under the effect of UV radiation and form SiO x  bonds as illustrated below: 
     
       
                 
         
             
             
         
      
     
     Therefore, an oxide film  25  is formed on the surface of the substrate  20  ( FIG. 2B ) . The thickness of this SiOX film  25  can reach 10 μm or even 20 μm depending on the exposure time. This layer or this film  25  is rigid, while the material subjacent to this layer keeps the flexible properties of the initial polymer. 
     It will be noted that this layer or this film  25  is the result of a transformation of the polymer, and not the production of a deposit on the polymer. This thus avoids all problems related to the difference between coefficients of thermal expansion of the polymer and an oxide layer, when the oxide layer is simply deposited on the polymer. 
     As indicated above, other treatments of the polymer can be applied to obtain a hydrophilic layer  25 . This is the case of a treatment by an oxygen plasma. Such a plasma treatment can cause the appearance of Si—OH groups that are substituted for Si—CH3 groups on the surface of the PDMS. This modification of the PDMS structure takes place over a thickness that can be as much as 130 to 160 nm and which makes it particularly hydrophilic (angle of contact less than 3°). Furthermore, for a treatment applied with oxygen plasma, the reaction is faster than under UV (20 s to reach an angle of 3° under a plasma compared with 60 min in the case of a treatment under UV) . But this plasma treatment is not permanent and the hydrophilic nature of the surface changes after 45 min. Therefore, treatment under UV has the advantage that it makes the surface hydrophilic more permanently than treatment under oxygen plasma. Finally, an AFM analysis of the polymer surface after plasma treatment shows the presence of ripples, such that this surface is rough and prevents hydrophilic bonding. 
     Unlike plasma treatment, exposure to UV enables a more durable transformation than a plasma treatment, does not create these ripples on the surface, and therefore does not create any roughness. 
     A measurement made on the surface of a PDMS substrate  25  shows that its roughness is:
         1) 17.8 nm after plasma treatment,   2) while it is 0.57 nm after treatment under UV, therefore equivalent to the roughness after polymerisation.       

     Bonding can then be done on the polymer substrate  25  to which the UV treatment mentioned above had been applied. 
     As mentioned above, the superficial portion  25  of the substrate  20  is rigid. Therefore, a stiffener (the oxide layer) is obtained on the surface of the polymer after the treatment has been applied, which is favourable for a subsequent thinning method, for example to produce a Smart Cut™ type method. 
     Nevertheless, it is important to emphasize that the PDMS substrate  20  maintains some elasticity due to its non-oxidized part. 
     Furthermore, a substrate  1  can be implanted or have been implanted as explained above with reference to  FIG. 1A . For example, it may be a semiconducting substrate of Silicon or Germanium or GaN, or SiC or even LTO (LiTaO3) . Such a substrate may for example be implanted with a hydrogen beam at doses of between 5×10 16  at/cm 3  and a few 10 17  at/cm 3  for example 5×10 17  at/cm 3  or 10 18  at/cm 3 , and with an energy of the order of 50 keV to 200 keV, for example between 70 keV to 180 keV. 
     Advantageously, a thin layer  8  of oxide (or nitride, for example SiON), that is a few nm thick for example 5 nm to about 1 μm, will have been deposited before implantation. This oxide will make polishing possible if the roughness of the semiconducting surface  1  is high, or prepare the surface for bonding with preparations such as the plasma treatment or mechanical-chemical polishing or a wet treatment, these treatments having the purpose of leaving the surface hydrophilic. 
     A weakening heat treatment is possible at this stage, the thermal budget of this step not introducing any blister on the surface of the substrate, which would prevent any subsequent bonding. 
     The surface of the PDMS substrate  20 , treated as explained above, and the surface  2  of an implanted semiconducting substrate  1  of the type shown in  FIG. 1A  (possibly provided with a layer  8  as explained above) can then be brought into contact ( FIG. 2C ). Bonding is of the direct bonding or molecular bonding type, a bonding technique that is described particularly by Q. Y. Tong in “Silicon Wafer Bonding Technology for VLSI and MEMS applications”, edited by S. S. Iyer and A. J. Auberton—Hervé, 2002, INSPEC, London, Chapter 1, pages 1-20. 
     A solidarisation annealing of the bonding can then be applied, for example at between 100° C. and 200° C. The duration of this annealing is between a few minutes and a few hours. This annealing is done with a thermal budget, and therefore at a temperature and for a duration such that it does not cause any blistering or cleavage conditions in the semiconductor  1 . 
     The next step can be annealing of the fracture, for example between 200° C. and 250° C., assisted or not assisted by mechanical treatment (insertion of the blade, and/or bending or imposing a curvature of semiconductor  1  for stressing, etc.). The PDMS remains stable within this temperature range. The result is then a thin semiconducting layer  5  on the surface of the PDMS substrate  20  ( FIG. 2D ). 
     As a variant, the thin layer  5  may be obtained by a mechanical-chemical thinning step of the non-implanted substrate  1  after assembly with the surface hardened polymer layer, instead of by implantation/fracture as described above. 
     Regardless of the method used, the layer  5  can then be transferred and bonded onto a support  30  ( FIG. 2E ), by a hydrophilic or hydrophobic type of molecular bonding. This support  30  may be a final support. 
     The next step can then be peeling of the polymer substrate  20  ( FIG. 2F ). The high elasticity of the PDMS substrate  20  (that it retained in its part not transformed into oxide) will enable this retraction operation by peeling. This peeling is easier if the substrate  20  is thick. For example, a thickness of a few mm, for example between 1 mm and 3mm or 3 mm, enables manual retraction. 
     During this retraction by peeling operation, the rupture can take place at the interface between the non-transformed polymer and the polymer transformed into oxide, in which case it is an adhesive rupture. 
     Or the rupture may take place within the polymer  20 , in its non-transformed part, and it is then a cohesive rupture. In this case, the remainder of the layer  20  of non-transformed polymer can then be removed chemically, for example by means of a Tetra-n-butylammonium fluoride solution, 1M solution in Tetrahydrofuran (TBAF). This solution dissolves the PDMS. The etching rate of this solution is estimated at 3 μm/min. 
     The polymer layer  25  transformed into an oxide can also be eliminated in turn, for example by an HF treatment or selective etching. 
     The substrate  30  may itself have some stiffness. In this case, this substrate may be a final substrate. 
     As a variant, this substrate  30  can also have some flexibility, for example to enable stress relaxation in the transferred layer or film  5 ; in this case, it is not a final substrate. In particular, it may be a silicone substrate, for example also made of PDMS but not treated under UV. 
     In one example embodiment, the PDMS used is Sylgard 184 made by Dow Corning. It is composed of a monomer and a thermal primer. 
     The primer is mixed in the proportion 1:10. The mix generates air bubbles that are removed by placing the mix under a vacuum (10 −1  bars) . The polymer can then be conditioned for 2 h before the beginning of polymerisation. The mix is then poured into moulds and polymerised at ambient temperature to prevent stresses that could develop during cooling after polymerisation at high temperature, due to its high coefficient of thermal expansion. 
     The result obtained is then a PDMS support  20  with a thickness between a few μm and a few mm, for example between firstly 1 μm or 5 μm or 10 μm, and secondly 1 mm or 5 mm or 10 mm. This support can be left bonded on the plate that was used as a mould or it can be separated from this plate. 
     The PDMS support is then treated under UV (advantageously under an ambient atmosphere, the UV radiation then transforming oxygen in the air into ozone: this treatment is referred to as UV/Ozone). The next step is to transform the surface of the PDMS substrate  20  into an oxide over a thickness of between 1 μm and 15 μm, for a treatment duration of between 20 or 30 minutes and 120 minutes. The result is then a PDMS support with a surface stiffened by a transformation, which results in a high oxide thickness. 
     The semiconducting substrate  1  for which a thin layer  5  is to be transferred ( FIG. 1A ) is implanted with gaseous species  3  such as hydrogen and/or helium that enable production of the “Smart Cut™” method. 
     Example implantations, energies and doses have already been given above. 
     Advantageously, implantation conditions are chosen that enable cleavage at a temperature below 250° C., the PDMS decomposition temperature, for example as can be done by co-implantations of hydrogen and helium ions at the doses mentioned above. 
     Preferably, a fairly deep implantation will be made in the material to add the stiffness of the layer to the stiffness of the oxide created to facilitate the cleavage. 
     Advantageously, the material  1  can be implanted as described above and pre-weakened by applying preliminary annealing to it, to obtain cleavage at a temperature of below 250° C. This annealing leads to the creation of a fragile zone, but cleavage is not initiated. 
     Bonding can then be done between the PDMS treated surface  20  and the semiconducting surface  1  or the layer  8  deposited on this surface, and then the substrate  1  can then be thinned, in this case by fracture by means of a heat treatment (that may or may not be assisted by a mechanical treatment). The thin layer obtained can then be transferred onto a final substrate, the PDMS layer being eliminated by peeling.