Abstract:
A method for making a thin-film element includes epitaxially growing a first crystalline layer on a second crystalline layer of a support where the second crystalline layer is a material different from the first crystalline layer, the first crystalline layer having a thickness less than a critical thickness. A dielectric layer is formed on a side of the first crystalline layer opposite to the support to form a donor structure. The donor structure is assembled with a receiver layer and the support is removed.

Description:
PRIORITY CLAIM 
     This application is a U.S. nationalization of PCT Application No. PCT/FR2006/002184, filed Sep. 25, 2006, and claims priority to French Patent Application No. 0509897, filed Sep. 28, 2005. 
     TECHNICAL FIELD 
     The invention concerns a method for making a thin-film element. 
     BACKGROUND 
     In such methods, a support is frequently used as a donor structure: a portion of the donor structure (generally a superficial layer) is used as an element of the target structure, for example by means of a layer transfer. 
     This solution is encountered in particular for making a germanium on insulator (GeOI) type structure, for example as described in the papers “Germanium-On-Insulator (GeOI) structure realized by the Smart Cut™ Technology”, by F. Letertre et al. in Mat. Res. Soc. Symp. Proc. Vol. 809, 2004 Materials Research Society, and “200 mm Germanium-On-Insulator (GeoI) Structures realized from epitaxial wafers using the Smart Cutt™ technology”, by C. Deguet et al., ECS 2005, Quebec, vol. 2005-05, page 78. 
     Those papers propose two alternative solutions for transferring the thin germanium layer that forms the superficial layer of the GeOI structure: the donor wafer used is either a bulk germanium wafer or a silicon wafer on which a germanium layer has been epitaxially deposited, at least a portion whereof will be transferred. 
     The solution using bulk germanium is advantageous from the electrical point of view in particular because of the very high crystalline quality of the transferred material (the proportion of dislocations in the bulk germanium is close to zero), but the manipulation of bulk germanium is relatively complex, in particular because of the high density of this material and its mechanical weakness. 
     This is why at present it is envisaged instead to use in practice as the donor wafer the germanium-based structure deposited epitaxially on silicon already mentioned. 
     However, using this solution as proposed in the papers already cited sometimes produces layers of germanium having a proportion of dislocations of the order of a few 10 6  cm −2 , essentially generated during the phase of epitaxial deposition of the germanium on the silicon because of the lattice parameter difference between the crystalline arrays of these two materials (Δ=4.2%). 
     Generally speaking, it can be beneficial to use as the donor wafer a structure consisting of a first material deposited epitaxially on a second material (for example to benefit from the electrical properties, or more generally the physical properties, of the one and the mechanical properties of the other). If these two materials have different lattice parameters, this type of solution leads, however, unless particular precautions are observed, to a relatively high proportion of dislocations in the layer of the first material, which can degrade the electrical and/or physical properties of that layer. 
     SUMMARY 
     In order to improve upon this state of affairs, and in particular to propose a method of making a thin-film element that can combine ease of manipulation of the donor wafer and a low proportion of dislocations in the layer donated thereby, the invention proposes a method for making a thin-film element characterized by the following steps: 
     epitaxial growth of a crystalline layer of a first material on a crystalline layer of a support formed in a second material different from the first material, said layer of the first material having a thickness such that its lattice parameter is determined by (and therefore generally corresponds to) that of the crystalline layer of the support; 
     formation of a dielectric layer on the side of said layer of the first material opposite the support to form a donor structure; 
     assembly of the donor structure with a receiver wafer; 
     elimination of the support. 
     Very few dislocations are produced in the layer of the first material by this method. The two materials can therefore be chosen independently for their respective properties, without this choice compromising the crystalline quality of the epitaxially deposited layer. 
     In one possible embodiment, there is effected, prior to the formation of the dielectric layer, a step of epitaxial growth of a layer of third material on said layer of the first material, the material and the thickness of the layer of third material being chosen so that the lattice parameter of the third material is determined by that of the crystalline layer of the support. 
     The layer of the first material is therefore encapsulated, and thus protected, between the layers of the second and third materials. Moreover, the layers are of good quality since the lattice parameter is uniform in this portion of the structure. 
     The third material is silicon, for example. 
     In this case the following steps can follow the step of elimination of the support: 
     local etching of the crystalline layer of the first material as far as the layer of the third material; 
     epitaxial growth of the third material on the regions of the layer of the third material exposed by the etching. 
     Regions of the first material and regions of the third material are therefore obtained on the surface, all of good crystalline quality. Each region can therefore be used with a specific aim according to its own properties. 
     The step of formation of the dielectric layer can comprise a step of at least partial oxidation of the epitaxially grown layer of the third material in order to obtain said dielectric layer. 
     This solution is particularly practical and generates an oxide layer of particularly good quality. It also underlines the benefit of the step of epitaxial growth of the layer of the third material referred to hereinabove. 
     Alternatively, the step of formation of the dielectric layer of the structure comprises a step of depositing a dielectric layer. 
     The dielectric layer is an oxide layer, for example. 
     In one embodiment, the support is a plate of the second material. 
     In another embodiment, the support is of the superficial layer on insulator type, the superficial layer whereof forms the crystalline layer of the second material. 
     The first material is germanium, for example. This particular case is beneficial because of the physical properties (in particular the electrical transport properties) of germanium and the problems of manipulating it. 
     The second material is bulk silicon, for example, which is relatively widely used and which can therefore be manipulated using standard means. 
     In one variant that can be envisaged, the second material is strained silicon or SiGe, which has in particular the effect of increasing the critical thickness of germanium. 
     In all cases, there may be envisaged a step of epitaxial growth of the first material on the face of the crystalline layer of the first material left free by eliminating the support. 
     Overall, the layer of first material finally obtained will be of better quality than a layer of first material of the same thickness deposited directly on the second material support. 
     Other features and advantages of the invention will become apparent on reading the following description with reference to the appended drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIGS. 1 to 7  represent the principal steps of a first example of a method for making a thin-film element according to the teachings of the invention; 
         FIGS. 8 and 9  represent two steps of a method of making a thin-film element according to a second embodiment of the invention; 
         FIGS. 10 and 11  represent two subsequent steps that can be applied to the structures obtained by the aforementioned methods. 
     
    
    
     The thicknesses of the layers represented in the Figures are purely illustrative and must not be understood as proportional to the thickness of the layers in reality. 
     DETAILED DESCRIPTION 
     A first embodiment of the invention is described next with reference to  FIGS. 1 to 7 . 
     This embodiment uses a plate  2  of crystalline silicon, as shown diagrammatically in  FIG. 1 . This is therefore perfectly relaxed silicon. Alternatively, it is possible to use a wafer having on the surface a silicon layer, for example an SOI (silicon on insulator) wafer. Such a structure comprises a bulk wafer (generally of silicon), a buried oxide (BOX) layer and a superficial silicon layer. Another variant can use a silicon wafer having a buried weak area obtained, for example, by implantation of gaseous species (for example hydrogen) favoring subsequent elimination of the wafer and its re-use. 
     A germanium layer  4  with a thickness less than the critical thickness is grown epitaxially on this silicon plate  2 . The critical thickness is the thickness up to which the crystalline quality of the epitaxially grown layer is guaranteed: above that thickness, defects appear in the epitaxially grown layer, especially dislocations. Below this critical thickness, the crystalline structure of the germanium layer  4  (and in particular the lattice parameter) is imposed by the superficial crystalline structure of the plate  2 : the lattice parameter of the germanium layer  4  is equal to that of the superficial crystalline structure of the plate  2  of silicon. The critical thickness depends of course on the wafer on which the epitaxial growth is effected. On the subject of the critical thickness, see the paper “Calculation of critical layer thickness considering thermal strain in Si 1-x Ge x /Si strained-layer hetero-structures”, by J. Huang et al. in J. Appl. Phys 83 (1), 1998 American Institute of Physics. 
     There is grown in the example described here, for a bulk silicon plate, a germanium layer  4  with a thickness less than 3 nm. This produces the stack represented in  FIG. 2 . 
     Because the thickness of the germanium layer  4  is less than the critical thickness and because its crystalline structure is imposed by that of the plate  2 , which is incidentally of good crystalline quality, the proportion of dislocations in the germanium layer is practically zero. 
     In the embodiment proposed here, there is then deposited (also epitaxially, for example) a very thin silicon layer  6 , typically with a thickness less than 1 nm, typically a few angstrom units. The structure represented in  FIG. 3  is then obtained. 
     Alternatively, it is possible to deposit epitaxially a slightly thicker silicon layer (typically between 1 nm and 20 nm thick, for example between 2 nm and 3 nm thick) and to oxidize all or part of that layer by chemical or physical surface treatment. A few Ångström units of silicon could remain after treatment, for example, advantageously 1 or 2 silicon monolayers. 
     Because of the crystalline quality of the silicon of the plate  2  and of the epitaxially deposited germanium, the silicon layer epitaxially deposited on the germanium will also have good crystalline quality and the interface between this silicon layer and the germanium layer will be of good quality, in particular from an electrical point of view. 
     Note that the lattice parameter of the germanium in the layer being determined by that of the silicon of the layer, it will also be that of the silicon layer produced and so dislocations will be avoided whatever its thickness. Note that, in the structure, the germanium layer  4  is encapsulated between the silicon plate  2  and the thin layer  6 , which provides good protection of the germanium layer in the subsequent steps of the method now being described. 
     After appropriate cleaning, a dielectric, for example Si 3 N 4 , a material of high permittivity (such as AlN or HfO 2 ), an oxide (such as Al 2 O 3  or SiO 2 ), is deposited on the thin layer  6 . In this example, the choice is made to deposit SiO 2 , for example by a PECVD (Plasma Enhanced Chemical Vapor Deposition) type process, here at a temperature of the order of 380° C., in order to obtain a thickness of 10 nm to 300 nm of oxide, densified or not. 
     Alternatively, it is possible to deposit this oxide layer directly on the germanium layer without making use of the epitaxially deposited silicon layer. In this case, the Ge/SiO 2  interface will be of lesser electrical quality, but may suffice for certain applications. 
     The oxide layer  8  deposited in this way is intended to form the buried oxide (BOX) layer of the GeOI structure finally obtained. 
     The stack obtained after deposition of the oxide layer  8 , represented diagrammatically in  FIG. 4 , is then transferred to a receiver wafer  10 , for example another silicon plate oxidized on the surface or not. Here this transfer is effected by molecular bonding of the oxide layer  8  to the receiver wafer  10 , after activation of the faces to be bonded (for example by chemical activation and/or mechanical and/or chemical polishing (CMP) and/or plasma). 
     The structure represented in  FIG. 5  is then obtained, which comprises the following stack: 
     the receiver wafer  10 ; 
     the oxide layer  8 ; 
     the thin silicon layer  6  (optional but advantageous); 
     the germanium layer  4  (the thickness of which is less than the critical thickness); 
     the silicon plate  2 . 
     There follows the elimination of the plate  2 , for example by mechanical action (lapping and polishing) and/or chemical etching of the silicon selectively relative to the germanium (for example using TMAH (tetramethylammonium hydroxide)). 
     The advantage of the above variant using an SOI wafer rather than the solid plate  2  of silicon as the starting wafer lies in easier elimination. With an SOI wafer, there are fewer precautions to observe: the silicon wafer is eliminated first by mechanical action to reach the buried oxide layer. This can then be removed, for example by selective chemical etching relative to the silicon. The thin silicon layer is then removed, for example by selective chemical etching relative to the germanium. 
     In the case of the variant proposed hereinabove and providing a buried weak zone, there follows fracture at the level of this buried weak zone, by thermal and/or mechanical treatment, for example. The residual layer of the silicon wafer that remains attached to the germanium layer after fracture can then be selectively eliminated, for example by chemical etching as in the previous case. 
     The required GeOI structure is then obtained, as represented in  FIG. 6 : 
     the wafer  10 , 
     the buried oxide layer  8 , 
     the thin silicon layer  6  (optional), 
     the germanium layer  4 , at this stage superficial. 
     Because of its thickness less than the critical thickness during epitaxial growth, the germanium layer  4  will include very few dislocations, as already mentioned. The electrical properties of this germanium layer  4  therefore conform to those expected from the use of germanium, with a quality equivalent to that which would have been obtained using a bulk germanium wafer. 
     The presence of a thin silicon layer  6 , typically less than 1 nanometer thick, between the germanium layer  4  and the buried oxide  8  does not call into question these good electrical properties (it even improves them), and provides a good quality interface with the germanium. 
     The GeOI structure can therefore be used as it stands in an electronic component. 
     It is equally possible, if it is required to use a GeOI structure with a thicker germanium layer, to proceed to further epitaxial growth of germanium on the germanium layer  4  of the above GeOI structure. To preserve the crystalline quality of the germanium layer, it will be necessary to ensure that the thickness of the epitaxially grown germanium remains less than the critical thickness for the new wafer now considered and formed of the Si/SiO 2 /Si (optional)/Ge (strained) stack. Moreover, if the constraints at the level of the crystalline quality of the germanium layer are relaxed, a greater thickness could be obtained epitaxially. 
     The proportion of dislocations obtained will in all cases be less than the proportion of dislocations that would have been obtained by growing the total thickness of germanium finally used directly on a bulk silicon plate  2 . 
     Given these hypotheses, there is obtained the structure represented in  FIG. 7 , which differs from that of  FIG. 6  by the greater thickness of the germanium layer  5 . 
     A second embodiment of the invention is described next with reference to  FIGS. 8 and 9 . 
     The second embodiment is close to the first embodiment that has just been described, and its description is essentially limited to the steps that distinguish it from the first embodiment. 
     This second embodiment also uses a silicon plate  12  (or an SOI wafer) on which is epitaxially grown a germanium layer  14  of thickness less than the critical thickness, as described with reference to  FIG. 2 . 
     In the second embodiment described here, there is epitaxially grown on the germanium layer  4  a silicon layer with a thickness of the order of a few nanometers, for example 5 nm (and generally speaking less than 10 nm). 
     There is obtained in this way a structure with a superficial silicon layer  17  having a relatively large thickness compared to that of the thin structure  6  of the first embodiment. 
     This is followed by the oxidation of all or part of the superficial silicon layer  17 , for example by plasma or chemical treatment, or by thermal oxidation (of course at temperatures where there is no risk of degrading the underlying germanium layer  14 ), in order to obtain at the surface a thermal oxide layer  18  that will form the buried oxide in the final GeOI structure, as represented in  FIG. 9 . 
     As can be seen in that FIG., it is proposed here to stop oxidation advantageously a few Ångström units before the interface between the germanium layer  14  and the silicon layer  17  in order to leave a thin silicon layer  16  between the germanium layer  14  and the oxide layer  18  for electrical reasons (the SiO 2 /Si/Ge interfaces being of better quality than the Ge/SiO 2  interface, as already indicated). 
     The structure obtained in this way and represented in  FIG. 9  is entirely comparable to that obtained and represented in  FIG. 4  using the first embodiment and could therefore be used in exactly the same way thereas (in accordance with the steps described hereinabove and represented in  FIGS. 5 and 6 , and where applicable  7 ) to form a GeOI structure. 
     The germanium layer  14  having a thickness less than the critical thickness, as in the first embodiment, the advantages obtained here are identical to those already described. 
     Furthermore, this solution produces a buried oxide (from the thermal oxide layer  18 ) of better quality than when this oxide is obtained by deposition (above first embodiment) and a better interface between that oxide and the thin silicon layer ( 16 ). It is nevertheless possible, if a thicker oxide layer is required, to deposit a complementary oxide layer on the thermal oxide layer. 
     From the structures obtained in  FIG. 6  or  7 , i.e. GeOI structures having under the thin germanium layer a very thin silicon layer (of good crystalline quality) it is possible to etch the germanium locally, as represented in  FIG. 10 , to expose locally the thin silicon layer  6 ,  16 , followed by local and selective epitaxial regrowth of silicon on the thin silicon layer  6 ,  16  to obtain a new silicon layer  7  in the previously etched region of the germanium layer  4 , as represented in  FIG. 11 . For example, after etching the germanium layer and before epitaxial regrowth of silicon, the germanium islets still present could be protected by a local protection layer leaving apertures in the silicon. 
     This layer could be selectively eliminated after the epitaxial deposition of silicon. 
     Thanks to the  FIG. 11  structure, it is possible to use each of the regions (residual portions of the layer  4  and layer  7  produced by the selective regrowth) and benefit from the properties of each of the two materials, for example to produce different components. 
     The invention is not limited to the embodiments that have just been described. 
     For example, although the embodiments described hereinabove propose depositing the germanium layer on relaxed silicon (bulk Si or SOI), it is equally possible to grow the germanium on strained silicon (obtained for example by epitaxial growth on SiGe or coming from a strained silicon on insulator (sSOI) type structure), which has the advantage of increasing the critical thickness of the germanium deposited by epitaxy (in particular, the possible thickness of germanium that can be deposited epitaxially without changing the lattice parameter of the crystalline structure that receives it, thereby minimizing the risk of dislocations). 
     The germanium can equally be grown on a wafer having at least at the surface a layer of Si x Ge 1-x , for example SiGeOI, with for example x=0.8, which yields 20% germanium. 
     This solution also increases the thickness of germanium that can be deposited without degrading crystalline quality. 
     For these last two examples (strained Si and SiGe), if a silicon layer is deposited epitaxially on the epitaxially deposited germanium layer, care must be taken that the thickness of this silicon layer (formed in this case in a material with a lattice parameter different from that of the superficial layer of the wafer) remains less than the critical thickness that can be deposited epitaxially, given the structure on which that epitaxial deposition is effected, in order to preserve the crystalline quality of this layer and the underlying germanium layer.