Abstract:
A method of transferring a thin film onto a first support, includes supplying a structure comprising a film of which at least one part originates from a solid substrate of a first material and which is solidly connected to a second support having a thermal expansion coefficient that is different from that of the first material and close to that of the first support, forming an embrittled area inside the film that defines the thin film to be transferred, affixing the film that is solidly connected to the second support to the first support, and breaking the film at the embrittled area.

Description:
PRIORITY CLAIM 
       [0001]    This application is a U.S. nationalization of PCT Application No. PCT/FR2006/001945, filed Aug. 11, 2006, and claims priority to French Patent Application No. 0508555, filed Aug. 16, 2005. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention concerns a method of transferring a thin film, such as a layer with a thickness typically less than 1 μm, onto a support. 
       BACKGROUND 
       [0003]    In the context of the production of stacked structures that are formed, in particular thin layers carried by a support (for example a substrate), it has already been proposed to transfer a thin layer onto the substrate by means of a method that comprises the following main steps: 
         [0004]    the formation of a weakened zone at a given depth in a substrate consisting of the material that is to form the thin layer, for example by implantation of a gas at that depth; 
         [0005]    the bonding of the implanted substrate (referred to as the donor substrate) onto the support, for example by molecular bonding; and 
         [0006]    the separation of the donor substrate amputated from the thin layer (situated between the weakened zone and the initial surface of the donor substrate) and the support, which then carries the thin layer, by fracture (generally during a heat treatment step, usually between 200° C. and 600° C.) in the previously weakened zone. 
         [0007]    This kind of solution is described in French patent application No. FR 2 681 472, for example; it is used, for example, to deposit a thin layer of silicon onto a support consisting of a silicon substrate covered with a thin layer of insulative silicon oxide(SiO 2 ) in order to obtain an SOI (Silicon-On-Insulator) type structure. 
         [0008]    Although the method described briefly hereinabove can be applied as such in the situation that has just been referred to, certain problems can arise in the conventional application of this method in different contexts, for example if the donor substrate and the support have very different mechanical characteristics. 
         [0009]    This is the case in particular if it is required to replace the thin layer of silicon with a thin layer of germanium (Ge) that has certain advantageous electronic properties (such as the mobility of the electrical carriers, which improves the performance of circuits produced on germanium). 
         [0010]    The production of this kind of structure (referred to as GeOI, standing for Germanium-On-Insulator) by means of the method previously referred to is, for example, the subject of the paper “Germanium-On-Insulator (GeOI) Structure Realized by the Smart Cut™ Technology”, F. Letertre et al., in MRS proceedings, 809 B4.4 (2004). 
         [0011]    In this instance, the conventional application of the thin layer transfer method referred to hereinabove leads to bonding a silicon substrate onto a substrate implanted with germanium, with a view to their separation, in the zone weakened by implantation, by heat treatment. This solution is problematic, however, because of the large difference between the coefficients of thermal expansion of the two materials used (2.6×10 −6 /° C. for silicon and 5.8×10 −6 /° C. for germanium). The sudden releasing, at the moment of fracture, of the stresses stored in the structure can cause one or even both substrates to break. 
         [0012]    The paper referred to above also proposes carrying out implantation in a layer of germanium, the thickness of which can vary from one micron to a few microns and which is formed epitaxially on the surface of a standard silicon substrate (750 μm thick). The structure subjected to the separation heat treatment therefore behaves as a homostructure because of the small thickness of the germanium compared to the thickness of the two silicon substrates. 
         [0013]    This latter solution is nevertheless less advantageous from the electronic point of view because of the high number of dislocations and greater roughness in the epitaxial germanium. 
         [0014]    Another known solution for producing a structure including a layer of a first material on a substrate of a second material is, after assembly of a substrate of the first material with the substrate of the second material, to carry out chemical-mechanical thinning of the substrate in the first material. However, this technique cannot be used to obtain layers with a thickness of the order of one micron with a thickness of good homogeneity. Using this technique, the greater the thinning, the less homogeneous the thickness of the residual layer. 
       SUMMARY 
       [0015]    To solve these various problems, and to propose a solution that combines in particular simple implementation, high mechanical strength during the fracture heat treatment, and good electrical crystalline properties of the structure obtained, the invention provides a method for transferring a thin layer of a first material onto a first support formed of a second material, characterized by the following steps: 
         [0016]    providing a structure including a layer at least part of which comes from a bulk substrate of the first material and which is attached to a second support formed of a third material having a coefficient of thermal expansion different from that of the first material and close to that of the second material; 
         [0017]    forming in the layer a buried weakened zone at a given depth delimiting in the structure the thin layer to be transferred; 
         [0018]    bonding the layer attached to the second support to the first support; and 
         [0019]    fracturing the layer in the weakened zone, including at least one heat treatment step. 
         [0020]    In this kind of method, the second support provides good mechanical cooperation with the first support (similar temperature-related changes to the second and third materials), independently of the material of the thin layer to be transferred (first material). 
         [0021]    The thickness of the layer is preferably such that the mechanical behavior as a function of temperature of the structure obtained after bonding is imposed by the second support and the first support. The layer is then sufficiently thin not to be involved in the mechanical temperature behavior of the structure obtained after bonding. The fracture step therefore takes place under good conditions whatever the nature of this material, so that the material can be chosen freely, for example for its electrical properties. 
         [0022]    According to the invention, the materials and the thicknesses used, and in particular the thickness of the layer of the first material, are chosen so that the release of the stresses stored in the structure at the moment of fracture does not cause either of the structures obtained after fracture to break. 
         [0023]    The fracture step can also include a step of applying mechanical loads: mechanical forces (insertion of a blade, traction and/or bending and/or shear forces) and/or ultrasound or microwaves; the step of forming a weakened zone can be carried out by implantation of one or more gaseous species. 
         [0024]    The coefficient of thermal expansion of the first material differs from the coefficient of thermal expansion of each of the second and third materials by at least 10%, for example. 
         [0025]    The coefficient of thermal expansion of the second material can be chosen to differ by less than 10% from the coefficient of thermal expansion of the third material. The structure obtained after bonding can then be considered to constitute a homostructure. The second material is identical to the third material, for example. 
         [0026]    The thickness of the layer attached to the second support is less than 15% of the thickness of the second support, for example, which prevents any significant mechanical impact of this layer on the structure resulting from the bonding step, and in particular limits the elastic energy stored in this structure during heat treatment. This thickness must of course be chosen as a function of the difference between the coefficients of thermal expansion existing in the structure and the temperature that the structure must be able to withstand. The lower this temperature, the thicker the layer attached to the second support can be. Similarly, the smaller the coefficient of thermal expansion difference, the greater this thickness can be. 
         [0027]    The second material is silicon, for example. The first material can be germanium. 
         [0028]    The thickness of the layer of the first material (before fracture) is between 1 μm and 50 μm, for example. 
         [0029]    In an embodiment described hereinafter, the method can include a preliminary step of bonding a solid plate of the first material to the second support, for example at raised temperature (typically between 100° C. and 200° C.). In this case, the layer obtained from the plate and attached to the second support can be obtained by a step of thinning the plate of the first material, for example by chemical-mechanical thinning (which can be effected by a method known as grinding, followed by polishing). 
         [0030]    The method can also include a step of epitaxial deposition of the first material onto a portion of the layer (residual layer) remaining attached to the second support after fracture. The crystalline quality of the residual layer being good, that of the epitaxially deposited layer will also be good. 
         [0031]    Thus the epitaxially deposited layer can be used for further thin film transfer, for example by means of the following steps: 
         [0032]    forming a buried weakened zone in the epitaxial layer; 
         [0033]    bonding the epitaxial layer onto a third support; and 
         [0034]    fracturing the epitaxial layer in the weakened zone. 
         [0035]    In one possible implementation of the method, the layer attached to the second support is obtained entirely from the bulk substrate. This ensures that the whole of the layer is of very good crystalline quality. 
         [0036]    In another possible implementation, the layer attached to the second support includes an epitaxial layer of the first material. As already indicated, this enables continued transfer of thin layers based on a residual layer at the same time as retaining good crystalline quality by virtue of the portion obtained from the bulk substrate. 
         [0037]    In this case the layer attached to the second support can also include an epitaxial layer of a fourth material the thickness of which is such that its crystalline structure is imparted by the first material. This layer can then be used for other functions, without calling into question the crystalline quality of the layers of the first material. 
         [0038]    For example, the method can include a step of elimination of the epitaxial layer of the first material after fracture using the epitaxial layer of the fourth material as a stop layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0039]    Other features and advantages of the present invention will become apparent in the light of the following description, given with reference to the appended drawings, in which  FIGS. 1 to 6  illustrate various steps of one embodiment of a method in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    In these figures, the various layers are shown with diagrammatic thicknesses, not directly proportional to reality, in order to clarify their description. 
         [0041]    The various steps of one example of a method according to the invention are described next with reference to these figures. 
         [0042]    This example uses a plate  2  of bulk germanium (which therefore has good crystalline electrical properties), here with a typical diameter of 200 mm and a thickness of 750 μm, on which a surface layer  4  of silicon oxide (SiO 2 ) has been deposited, for example by PECVD (Plasma Enhanced Chemical Vapor Deposition) using SiH 4  chemistry at 380° C., as represented in  FIG. 1 . 
         [0043]    A germanium plate  2  with no surface layer, or with one or more surface layers of a different kind, could be used instead. 
         [0044]    The silicon oxide layer  4  can be prepared by densification (for example in nitrogen at 600° C. for one hour). The germanium plate  2 —silicon oxide layer  4  structure is then prepared for the bonding described hereinafter, for example by chemical cleaning and/or chemical-mechanical polishing for hydrophilic type bonding. 
         [0045]    This structure represented in  FIG. 1  is then bonded to a substrate  6  consisting of a material the coefficient of thermal expansion whereof is different from that of germanium, for example silicon (Si), which forms a support, on which may have been formed a layer, for example a layer  8  of silicon oxide (SiO 2 ) formed by thermal oxidation, to facilitate subsequent bonding (oxide/oxide bonding being a well proven technology). 
         [0046]    The assembly of the germanium plate  2  on the silicon substrate  6  (where applicable with interposed silicon oxide layers  4 ,  8 ) is represented during bonding in  FIG. 2 . 
         [0047]    This bonding can be consolidated in an oven, for example at 200° C. for two hours. 
         [0048]    This bonding can advantageously be carried out at raised temperature, for example between 100° C. and 200° C., which generates stresses in the structure that can compensate some of the stresses generated by subsequent heat treatments and in particular the fracture heat treatment, thereby reducing the risk of breakage. 
         [0049]    The germanium plate  2  is then thinned, preferably by the combination of grinding followed by chemical-mechanical polishing and where appropriate, chemical etching (polishing producing a good final roughness and chemical etching removing the defects created by grinding). Other thinning techniques could be used, provided that they guarantee the integrity of the structure (in particular provided that they do not necessitate too high a temperature increase of the structure). 
         [0050]    The final thickness of germanium must be such that the mechanical behavior of the germanium/silicon assembly is essentially dictated by the silicon  6 , so that the mechanical behavior of the assembly as a function of temperature is like that of a homostructure, to be more precise so that the elastic energy stored in the structure during subsequent heat treatments, and in particular during the fracture heat treatment (and even more particularly at the moment of fracture of the structure) does not lead to the structure breaking. A thickness of 1 μm to 50 μm, for example 20 μm, is typically chosen for this germanium layer. 
         [0051]    The structure represented in  FIG. 3  is then obtained, which thus essentially consists of a substrate  6  forming a support (here of silicon, with a typical thickness of 750 μm for a 200 mm diameter substrate), an intermediate layer  10  of silicon oxide (which corresponds to the assembly of the two silicon oxide layers  4 ,  8  referred to hereinabove), and a thinned germanium layer  3 , of reduced thickness, for example from a few micrometers to a few tens of micrometers thick, for example 20 μm thick. 
         [0052]    Because of the process used to produce it, the roughness and the crystalline structure of the thinned germanium layer  3  are close to those of a bulk germanium substrate and the crystalline and electrical properties of the thin layers formed as described hereinafter from the thinned layer  3  of germanium are therefore particularly good. 
         [0053]    As explained hereinafter, the reduced thickness of the thinned germanium layer  3  and its bonding to the silicon substrate  6  forming a support nevertheless produce a structure having a different mechanical behavior than a bulk germanium substrate, which will be advantageous when used in the separation step described hereinafter. 
         [0054]    What is more, because of the higher thermal conductivity of silicon than germanium, this (germanium/silicon) structure shows better evacuation of heat during subsequent technology steps than the solution using a germanium substrate. 
         [0055]    The structure represented in  FIG. 3  that has just been described therefore constitutes a particularly advantageous donor structure enabling the transfer of a thin layer, here of germanium, as described next. 
         [0056]    Before bonding to the support that is to receive the thin layer (essentially a silicon substrate in the example described here), the structure produced beforehand and represented in  FIG. 3  can be prepared by carrying out the following steps: 
         [0057]    depositing a layer of silicon dioxide (SiO 2 ), for example by PECVD, as before; 
         [0058]    optional densification of the silicon oxide layer in nitrogen at between 400° C. and 600° C. for one hour; and 
         [0059]    cleaning and/or chemical-mechanical polishing (to improve compatibility with hydrophilic bonding). 
         [0060]    Alternatively, it is of course possible not to deposit any oxide and to prepare the germanium surface directly for bonding it to the support that is to receive the thin layer. 
         [0061]    A weakened zone  14  is produced in the thinned germanium layer  3 , at a depth that corresponds to the thickness of the thin film to be transferred (generally of the order of a few hundred nanometers, for example between a few tens of nanometers and 1000 nm), for example by implantation of gaseous species, here hydrogen ions (H + ), with an energy between a few keV and 250 keV and at a dosage rate between 3.10 16  and 7.10 16 H + /cm 2 ; typically, with an implantation energy of 100 kev and a dosage rate of 5.10 16 H + /cm 2 , an implantation depth of approximately 700 nm is obtained. 
         [0062]    The implantation step is carried out after formation of the silicon oxide layer (SiO 2 ) layer and before cleaning the surface, for example. 
         [0063]    After this implantation step, and where applicable these preparation steps, the donor structure is therefore as shown in  FIG. 4 . 
         [0064]    That structure is then bonded (by the silicon oxide layer  12  deposited on the thinned germanium layer  3 , i.e. the surface that has been subjected to implantation), for example by hydrophilic bonding to the support onto which the thin layer is to be transferred, consisting here primarily of a silicon substrate  18  (generally of the order of 750 μm thick for a substrate of 200 mm thickness), covered by a silicon oxide (SiO 2 ) layer  16 . 
         [0065]    The assembly represented in  FIG. 5  is therefore obtained, that is to be subjected to a heat treatment step, generally between 200° C. and 500° C. (at 330° C. here, for example), in order to form a fracture in the weakened zone  14 . 
         [0066]    This separates the support formed by the silicon substrate  18  covered by the silicon oxide layer  16 , which henceforth carries a thin layer  22  of germanium (coming from the thinned layer  3 ), and the donor structure peeled off this transferred thin layer, as represented in  FIG. 6 . 
         [0067]    Because the thinned germanium layer  3  is thin compared to the silicon substrates  6 ,  18  (shown diagrammatically in the figures, in practice in a ratio of the order of at least 1 to 10) and the mechanical compatibility (here in terms of thermal expansion) of the two substrates  6 ,  18  (here made from the same material), the assembly referred to hereinabove (and represented in  FIG. 5 ) behaves essentially as a homostructure and therefore exhibits good mechanical behavior during the fracture heat treatment step, without serious risk of breakages. 
         [0068]    The release of the elastic energy stored in the structure at the moment of fracture is controlled and does not lead to breakage of the structures obtained after fracture. 
         [0069]    After the step of fracturing the weakened zone (and consequently separation of the  FIG. 5  assembly), the silicon substrate  18  covered with the silicon oxide layer  24  therefore carries a thin film  22  of germanium with good electrical properties because this thin film  22  is derived from the thinned germanium layer  3  the electrical properties whereof are close to those of the initial germanium plate as already mentioned. 
         [0070]    There is therefore obtained, where applicable after finishing treatments of the polishing and thermal annealing type, a plate of GeOI (i.e. of germanium on insulator) with electrical properties of the germanium layer that are particularly beneficial. 
         [0071]    The donor structure, consisting mainly of the silicon substrate  6  and the residual germanium layer  20  (thinned germanium layer  3  peeled from the thin layer  22 ), can then be recycled (for example by grinding and/or polishing techniques) in order to be used again as a donor structure for the transfer of a new thin layer of germanium, in this case obtained from the residual layer  20  (this is because, even when peeled from the thin layer  22 , the donor structure is essentially constituted as it was beforehand, and represented in  FIG. 3 ). 
         [0072]    According to one advantageous implementation possibility, the thinned layer  3  or the residual layer  20  of germanium of the donor structure can serve as a seed for the epitaxial growth of germanium on that structure. Because of the crystalline quality of the thinned (or residual) layer, the crystalline quality of this epitaxial layer will be close to that of a bulk germanium substrate. The thin layer transfer process can therefore be repeated using the epitaxial layer. 
         [0073]    Alternatively, there can be produced epitaxially (on the thinned layer  3  or the residual layer  20 ) successively and iteratively germanium (to a thickness of a few microns, for example 2 μm) and silicon (typically to a thickness of a few nanometers) to form an alternating stack of silicon and germanium. 
         [0074]    The epitaxial silicon layers are so thin that the germanium imposes its lattice parameter so that good crystalline quality is maintained in the epitaxial germanium layer. 
         [0075]    On the other hand, the combined thickness of the epitaxial layers and the initial germanium layer  3 ,  20  must remain sufficiently small for the mechanical behavior as a function of temperature of the structure obtained to be imposed by the silicon substrate. 
         [0076]    This variant can use the thin silicon layer as a stop layer during successive transfers. 
         [0077]    The following process can also be used: 
         [0078]    implantation is effected in the epitaxial layer of germanium situated on top of the stack (exterior layer) to define in that layer the thin film to be transferred; 
         [0079]    the thin film is transferred as indicated hereinabove; 
         [0080]    the rest of the exterior germanium layer is eliminated by selective etching (for example H 2 O 2  etching); 
         [0081]    the silicon stop layer is then eliminated by selective etching (for example using TMAH—tetramethylammonium hydroxide); and 
         [0082]    the process is repeated on the next germanium layer. 
         [0083]    This method avoids the use of polishing after fracture and therefore the non-homogeneous thickness that usually results. 
         [0084]    The examples that have just been described constitute only possible embodiments of the inventions which is not limited to them.