Method for transferring a thin film comprising a step of generating inclusions

Technical area

p0001The present invention relates to a method for transferring a thin film of solid material. This method enables in particular the transfer of a thin film of solid material on a support consisting of a solid material of the same nature or of different nature.

State of the prior art

p0002The document <patcit id="pcit0001" dnum="FR2681472A"><text>FR-A-2681472</text></patcit> (Corresponding to patent <patcit id="pcit0002" dnum="US5374564A"><text>US-A-5,374,564</text></patcit>) Discloses a method of manufacturing thin films of semiconductor material. This document discloses that the implantation of a rare gas or hydrogen in a substrate of semiconductor material is likely to cause the formation of a layer of microcavities or microbubbles (also denoted by the term "platelets" in English terminology -saxonne) at a depth of the average penetration depth (Rp) of the implanted ions. The concept of micro-cavities course includes microcracks. The thickness of the microcavity layer is determined by the implantation conditions. If the substrate is brought into intimate contact, via its implanted surface with a stiffener and a heat treatment is applied at a sufficient temperature, there occurs an interaction between the micro-cavities or micro-bubbles leading to a separation of the semiconductor substrate into two parts : a semiconductor thin film adhering to the stiffener on the one hand, the rest of the semiconductor substrate of the other. Separation occurs at the location where the micro-cavities or micro-bubbles are present. The heat treatment is such that the interaction between the microbubbles or microcavities created by implantation induces a separation between the thin film and the remainder of the substrate. So there is a transfer of a thin film from an initial substrate to a stiffener for supporting the thin film.

p0003This method can also be applied to the manufacture of a thin film of solid material other than semiconductor material (a conductive or dielectric material), lens or not.

p0004If the thin film defined in the substrate is sufficiently rigid by itself (due to its thickness or because of its mechanical properties) can be obtained after the transfer of annealing a self supporting film. This is taught document<patcit id="pcit0003" dnum="FR2738671A"><text>FR-A-2738671</text></patcit>.

p0005It was proposed by the document <patcit id="pcit0004" dnum="EP0767486A"><text>EP-A-0767486</text></patcit>, An improvement of the method disclosed in <patcit id="pcit0005" dnum="FR2681472A"><text>FR-A-2681472</text></patcit> cited above. According to the document<patcit id="pcit0006" dnum="EP0767486A"><text>EP-A-0767486</text></patcit> (See column 8), the method disclosed in document <patcit id="pcit0007" dnum="FR2681472A"><text>FR-A-2681472</text></patcit> has the following disadvantages. The choice of the film thickness to be transferred is of a low degree of freedom. The thickness of the film to be transferred (corresponding to Rp) and the separation conditions of the film from the initial substrate are linked. The flatness of the film surface obtained after the separation is not satisfactory and it is not possible to keep the homogeneity in thickness of a thin film during the transfer. Improvements proposed by the document<patcit id="pcit0008" dnum="EP0767486A"><text>EP-A-0767486</text></patcit> consists in carrying out the ion implantation depth Rp in a porous silicon layer formed on the surface of a silicon substrate. This ion implantation causes an increase of the porosity (pore density) insofar microcavities appear in the pore walls of the porous layer. This layer is then considered as a fine porous structure. Under certain conditions of implementation, the separation is caused in this thin porous layer, according to the mechanism described in the document<patcit id="pcit0009" dnum="FR2681472A"><text>FR-A-2-681 472</text></patcit>. There are therefore two effects zone by a zone of pores created by a porous silicon generation step, and by a cavity area between the pores generated in small perfect silicon regions as for the method according to document<patcit id="pcit0010" dnum="FR2681472A"><text>FR-A-2681472</text></patcit>. The proposed improvement is therefore to use a porous layer to obtain, after separation, a layer whose thickness homogeneity is well controlled.

p0006The method disclosed by the document <patcit id="pcit0011" dnum="EP0767486A"><text>EP-A-0767486</text></patcit> advocates the porous silicon formation (the porosity is a percentage of the order of several tens), which is equivalent to removing silicon or the material at the separation zone and resulting embrittlement of the material.

p0007A more significant improvement of the process revealed by the document <patcit id="pcit0012" dnum="FR2681472A"><text>FR-A-2681472</text></patcit> would be to reduce the thickness of the layer of microcavities obtained by ionic implantation. This is what provides the present invention.

Disclosure of Invention

p0008The improvement provided by the present invention is made possible by the creation of the initial substrate material an inclusion or a set of inclusions to confine the gaseous species implanted in the ion implantation step. Inclusion is a volume of material whose properties are different from those of the substrate material from which you want to transfer a thin film or thin films. The inclusions may be in the form of a layer extending substantially parallel to the surface through which the implantation is carried out. Forms that can take these volumes are different and their dimensions may vary from a few tenths of nanometers to several hundred micrometers.

p0009The role of the inclusions is being traps for the implanted gaseous species. The range of such traps depends upon the nature of the inclusions carried. There is then no material removal, as in the case of the method disclosed in document<patcit id="pcit0013" dnum="EP0767486A"><text>EP-A-0767486</text></patcit>.

p0010The method according to the present invention comprises a preliminary step of forming inclusions in the initial substrate material. A subsequent step consists in implanting gas species, rare gas or not, in this material. The presence of inclusions formed in the previous step leads to confinement of implanted gaseous species. The effectiveness of inclusions is related to their confinement to the gaseous species.

p0011The inclusions can be formed in the vicinity of a perfectly controllable depth. Their presence induces a containment of the implanted species in a disturbed layer thickness thinner than that which is obtained in the method of the prior art. This results in several advantages. The implanted gaseous species are preferentially trapped at and / or in the region influenced by the inclusions, said vicinity of these inclusions. This enables precise location of inducing fracture separation (transfer) at and / or in the vicinity of the inclusions. This results in a relatively low surface roughness at the level of the fracture. In addition, due to the containment of power, such a method allows the use of low doses implanted necessary to fracture. Finally, the confinement effect by the presence of inclusions reduces the thermal budget needed for the fracture to the extent where it promotes nucleation and growth of the cavities leading to fracture. The interest is evident in the case of transfer films in structures where a temperature rise limitation exists. Mention may be made, as an example, the heterogeneous bonding of materials having different coefficients of expansion of more than 10%.

p0012The invention therefore provides a method for transferring at least one thin film of solid material defined within an initial substrate, <b>characterized in that</b>It comprises the following steps:<ul><li>a step of forming a layer of inclusions in the initial substrate, to a depth corresponding to the thickness desired for the thin film, these inclusions being provided to constitute a confinement zone for the gaseous species which will then be implanted,</li><li>a subsequent step of implanting said gaseous species, so as to bring the gaseous species into the layer of inclusions, the dose of implanted gaseous species being sufficient to cause the formation of microcavities capable of constituting a fracture plane allowing the separation of the film thin the rest of the substrate.</li></ul>

p0013The step of implantation of gas species can be performed with an implantation energy of these gaseous species such that their mean depth of penetration into the substrate corresponds to the depth of the layer of inclusions. It can also be performed with an implantation energy of these gaseous species such that their mean depth of penetration into the substrate is adjacent the layer of inclusions, this implantation being associated with a thermal diffusion treatment to allow the migration of implanted species at the layer of inclusions.

p0014The implantation step can be made from one or more gaseous species implanted either simultaneously or successively.

p0015The initial substrate may be comprised of a solid part supporting a film structure (s) in which must be delimited said film of solid material. All or part of this structure may be obtained by epitaxy. This structure may be such that, after transfer of the thin film, the remainder of the substrate, carrier or not epitaxy, is reusable for another transfer thin film.

p0016The layer of inclusions may be formed by a film deposition technique. It can then consist of a column generation or a generation of grains.

p0017The inclusions may have a chemical affinity with said gas species.

p0018The inclusions may originate from a lattice mismatch of the material forming the layer of inclusions with the regions of the substrate which are adjacent to it. The lattice mismatch may be a gap of dimension lattice constants in crystal orientation differences in a plane parallel to the surface of the transferred structure, in difference in thermal expansion coefficient between one of the films and the starting material (and / or other films).

p0019The layer of inclusions can also be formed by a technique of etching a substrate layer.

p0020The layer of inclusions may be formed by implantation of elements in a substrate layer. These elements can be implanted at one or more times. The implantation of these elements may be assisted by a heat treatment adapted to increase trap efficiency, this heat treatment may be performed before, during and / or after implantation. This heat treatment is capable of modifying the morphology and / or composition of the inclusions, which promotes the further containment of the gaseous species. This heat treatment is performed at a temperature and for a time such that it does not achieve a fracture on the entire layer of inclusions.

p0021The layer of inclusions can also be obtained by thermal treatment of the films and / or by application of stresses to the (x) film (s) of a film structure (s).

p0022The layer of inclusions can also be obtained by a combination of the various techniques mentioned above.

p0023The introduction of the gaseous species can be achieved by implantation by bombardment of species chosen from neutral species and ions. It may also be produced by a method chosen from the plasma-assisted diffusion, thermal diffusion and the combined plasma-assisted diffusion with thermal diffusion and / or assisted by electrical polarization. Implantation can be performed in a normal manner relative to the implanted face of the substrate, or with a certain impact. It can be performed using different elements of rare gas or not.

p0024The method may comprise a heat treatment step capable of embrittling the substrate at the layer of inclusions to allow separation between the thin film and the remainder of the substrate. This heat treatment is conducted with a predetermined thermal budget and is based on various thermal budgets used during the process. In particular, this heat treatment takes into account the temperature rises or induced by heat treatments type out thermodynamic equilibrium such as may result from the step of forming the inclusions and / or gaseous species and the implantation step by heat treatment using a heating or cooling of the substrate, such as for example for the implementation, or a possible enhancement of binding forces in the case of bonding with a support. This heat treatment may be zero if the other steps of the process allow the said embrittlement. It can be achieved both with a positive temperature with a negative temperature. According to the invention, this weakening is such as to allow the separation of the thin film from the remainder of the substrate with or without the use of mechanical stresses. This heat treatment may be carried out by pulsed heating to obtain for example a rapid rise in temperature. This pulsed heating may be of RTA ( "Rapid Thermal Annealing") or RTP type ( "Rapid Thermal Process") for example.

p0025The method may further comprise an intimate contacting step of the thin film delimited in the substrate with a support to which the thin film will adhere after its separation from the rest of the substrate. The intimate contacting can be performed directly (by molecular adhesion, for example) or by means of an attached material. A heat treatment step can be implemented to enhance the adhesion between the thin film defined in the substrate and the insert holder.

p0026Mechanical stresses may be performed during and / or after and / or prior to heat treatment to help the separation between the thin film and the rest of the substrate.

p0027The method according to the invention advantageously applies to the transfer of a thin silicon film from an initial substrate. It can be applied also to the transfer of a thin film of III-V semiconductor material (e.g., GaAs) from an initial substrate. The thin film may be formed itself of a thin-film structure. It may have been at least partly treated before its transfer in order to constitute it, on all or part of the film to be transferred, an integrated circuit or to form it over all or part of the film to be transferred, an optoelectronic component.

Brief Description of Drawings

p0028The invention will be better understood from the following description given by way of non-limiting example, accompanied by appended drawings among which:<ul><li>the <figref idrefs="f0001">figure 1</figref> is a sectional view of a substrate formed from an initial support on which was grown by a sputtering technique, a film structure comprising a layer of inclusions due to columnar growth;</li><li>the <figref idrefs="f0001">2</figref> is a sectional view of a substrate formed from an initial support on which was grown by a sputtering technique, a film structure comprising a layer of inclusions due to grain growth;</li><li>the <figref idrefs="f0002">Figures 3 and 4</figref> are diagrams showing the evolution of the lattice parameter of a crystalline composition rate function of an implanted element in the composition;</li><li>the <figref idrefs="f0003">5</figref> is a sectional view of a substrate on which inclusions are generated by etching;</li><li>the <figref idrefs="f0004">6A to 6D</figref> are illustrative of the method according to the invention in the case where a thin film is transferred to a stiffener;</li><li>the <figref idrefs="f0003">7</figref> is a sectional view of a substrate for obtaining a different SOI structure of the method of the invention.</li></ul>

Detailed description of embodiments of the invention

p0029The substrate from which the thin film will be transferred can be a solid substrate (formed of a single material) or a composite substrate, that is to say formed by chemical natures of films and / or identical or different physical.

p0030The inclusions may be generated in the initial substrate in particular by:<ul><li>structural change in the original material (crystal structure, crystal orientation, locally amorphous areas, gaps ...)</li><li>a change of a physical (densification, inclusion of gas during the preparation, for example implantation of various ions, ion etching and / or selective chemical and / or electrochemical several layers ...)</li><li>a chemical change or chemical bonds (doping effect of composition variation effect, use of an interface of a previously bonded structure, nucleation and / or growth of precipitates ...),</li><li>more or less local deformation of the material (interface effects, heat treatment effect layers with different expansion coefficients, stress effect generated between layers, consecutive ...).</li></ul>

p0031A number of processing techniques or treatments film materials allows for inclusion in a relatively parallel area on the surface of the material.

p0032In terms of applications, the interest of such a process is to allow such a change of substrate for one or more stacked films, a structure partially or fully processed to achieve a microelectronic component, a sensor ... it will need such extremely important if the film or structure would be transferred to undergo thermal treatments that the final support could bear (too hot, too much thermal expansion difference ...).

p0033The various film deposition techniques used to make stacks of one or more films, in which one can easily vary the composition of the films, their stress state, structure, morphology. by depositing films it is meant to bring and / or develop. These various options allow the generation of inclusions in the starting material before the step of implantation of gaseous species. The interfaces (s) film (s) and (s) neighborhood (s) concerned shall be deemed thereafter as zone inclusions traps for gaseous species implanted during the second stage of the process.

p0034deposition techniques are many and chosen according to the type of materials to be developed. The materials may be amorphous materials, polycrystalline or monocrystalline. For some applications, deposits will be made of epitaxy (homogeneous or heterogeneous). Among the most commonly used deposition techniques include: deposits by ion sputtering, deposition by vapor phase reaction at high or low pressure, whether or not assisted by plasma, deposition by molecular beam deposition by epitaxy liquid phase deposition assisted by laser ablation.

p0035The ion spray technique growths columns, orientations and varying sizes. These sizes and orientations can be controlled according to the conditions of pressure, temperature and deposition of energy. During the columnar growth, some of the columns are stopped in their growth to other columns that widen. For example, in the embodiment of Co films (Zr, Nb), an argon pressure of about 30 mTorr during the deposition, promotes a columnar growth. This effect can be used to impose certain magnetic properties to the deposit by the plane of the initial support. Areas located at and / or adjacent the end of the columns arrested in their growth, are areas of inclusions.

p0036The <figref idrefs="f0001">figure 1</figref> illustrates a substrate thus obtained. It consists of an initial support 1, composite or not, on which is grown a thin film 2 by sputtering structure. Columnar growth was caused inside the structure 2 to form a layer of inclusions 3 which serve as traps zone for the gaseous species to be implanted. The location of the fracture surface in or around the trap area is a function of the efficiency of the traps created.

p0037This deposition technique also allows grain growth (monocrystalline, polycrystalline or amorphous aggregates) of medium size, very well controlled. For example, if Tm is the melting temperature of the material to be deposited, a deposition temperature T, such as T / Tm ratio is greater than 0.5, promotes growth of crystal grains. One can refer to in this article<nplcit id="ncit0001" npl-type="s"><text> AG and HJ DIRKS LEAMY in the journal Thin Solid Films, 47, 219, (1977</text></nplcit>). The joints between the grains are also inclusions zones for the process of the present invention.

p0038The <figref idrefs="f0001">2</figref> illustrates a substrate thus obtained. It consists of an initial support 5, composite or not, on which is grown a thin film 6 by sputtering structure. A granular growth was caused inside the structure 6 to form a layer of inclusions 7 which serve as traps zone for the gaseous species to be implanted. The location of the fracture surface at the area of ​​inclusions depends on the efficiency of the traps created.

p0039In general, the film deposition techniques can provide films with thicknesses can be perfectly controlled. It is then possible to produce structures of low thickness, consisting of single or multiple films. The film deposition (s) are made without crystalline relationship (with the original carrier and / or between films) or epitaxy (homogeneous or heterogeneous). In addition, the term deposits of movies, include deposition of multilayer films for buffering and / or adaptation effect (called "seed layer" and "buffer layer" in English) in order to achieve crystalline structures. Note that in the case of a homogeneous epitaxial growth of a film on a support of the same nature, the interface, if any, may be the location of inclusions. The gaseous species implanted subsequently will be located at and / or in the vicinity of this interface.

p0040These film structures (s) are all or part of areas of inclusions, since:<ul><li>the physical nature and / or chemical films (chemical interaction between the films, variation of crystal orientations in the case of multilayer structures, affinity for gaseous species to be implanted more ...),</li><li>stresses in these various films and interfaces generated (due to disagreement crystalline mesh of difference in thermal expansion coefficients, interface microroughness, inclusions of elements other than those of deposition material, inclusions heterogeneous phases ...).</li></ul>

p0041For example, it is possible to realize a multilayer structure wherein at least one crystal film is deposited, separated from the initial crystalline substrate by one or more films, said buffer layers and / or adaptation. The crystalline film has the same crystal orientations or not to those of the initial support. Role of buffer layers is to cause all or part of the surface of the wafer changes from the crystalline orientation, particularly in the plane relative to the initial support. In this case, generates a zone of constraints and / or dislocation allowing the adaptation of the crystalline mesh. This area is located in the vicinity of the films mentioned. This is so for depositing YBaCuO superconducting films by epitaxy on buffer layers of SrTiO₃ and / or CeO₂. These buffer layers are epitaxially on an R plane sapphire substrate (1<u>1</u>02). The mesh concordance imposes a 45 ° rotation of the crystalline type of axes <001> in the plan, along with a strong constraint in the vicinity of interfaces or in the volume of films mentioned. This 45 ° rotation can be eliminated in certain areas by the interposition in the same areas of a very thin film of MgO. One can refer in this regard to the article "<nplcit id="ncit0002" npl-type="s"><text>Bi-Epitaxial YBCO Grain Boundary Josephson Junctions on SrTiO3 and Sapphire Substrates "NICOLETTI S. et al., Published in the journal Physica C 269 (1996) 255-267</text></nplcit>.

p0042As another example related to the stresses due to disagreements crystal mesh include deposits by vapor deposition (CVD) of Si films_(1-x)Ge_x on silicon substrate. The constraint will be controlled according to the concentration x of germanium in the film composition. The<figref idrefs="f0002">3</figref> shows how changing the PR network parameter as a function of the concentration x of germanium in the composition. The slope of the line 10 is + 0.022 nm versus atomic percent Ge. One can also cite the effect of stresses-doping level of a silicon film (e.g., doping of boron at 10¹⁴ 10²⁰ atoms / cm³) Deposited on a silicon wafer lightly doped. The<figref idrefs="f0002">4</figref> shows how changing the PR network parameter as a function of the concentration x of atoms of boron in atomic percentage. The slope of the line is 11 nm -0.14. We can include here the notion of inclusions by chemical nature. And a Ti film deposited on a silicon substrate and then covered by an encapsulant film, keeps a high sensitivity to oxygen (effect "getter" in English) that can be included and can diffuse through the silicon during a subsequent thermal treatment. The induced effect is the generation of a stress area, said area of ​​inclusions.

p0043For the generation constraints during the deposition films include the use of deposition parameters such as deposition pressure, the deposition temperature, deposition power, deposition of atmospheric composition throughout the ratio of the partial pressures of carrier gases, inert gases and reactive gases. It is known that stress can result in the deposited films, a state of high compression or high tension following the pressure of film deposition. One can refer to in this article<nplcit id="ncit0003" npl-type="s"><text> MATERNE A. et al., Entitled "Changes in Stress and after-Coercivity Annealing of Amorphous Co (Zr, Bn) Thin Films Deposited by RF Sputtering" EMMA Conf., Salford, United Kingdom, 14-16 September 1987</text></nplcit>. Thus, in the case of sputter deposition of films of Co (Zr, Nb), a low pressure of the order of a few mTorr will cause a compression state of the film while a high pressure, of the order of a few tens of mTorr, will result in a state of tension of the same material. The cause of this development was attributed, according to chemical analysis, the density of argon and oxygen included in the film during deposition. The importance of stress is such that they can locally, in extreme cases, cause film adhesion defects.

p0044In the term of depositing films while heat treatment is included and / or physico-chemical, carried out before or after deposition, for inducing these effects in the deposited films.

p0045Inclusions can also be generated by etching. Etching, dry (ionic, reactive ion) and / or chemically "wet" (selective etching, anisotropic etching) and / or electrochemically allows the realization of selected sizes of cavities open on a very small surface. These cavities may or may not be fulfilled by following a capturing material for the gaseous species necessary for the transfer.

p0046To generate inclusions, multilayer structures of etching techniques may be used, more or less assisted by partial masking techniques on all or part of the surface of the wafer (conventional in microelectronics techniques). Thus, in a very thin surface film of silicon nitride, it is possible to etch a pattern of apertures very small (sub-micron). an exposure technique is used a resin film, positive or negative, through a mask. At select areas, the resin film may then be removed chemically by a developer suitable for the resin used. In these exposed areas, an etching technique can be used by accelerated ion beam, said ion etching to produce openings in the film of silicon nitride. This surface film being deposited on the surface of a silicon film, it is possible to attack the silicon plumb openings formed by etching to tetramethylammonium hydroxide. This chemical etch is very selective to the extent that the silicon etch rate is more than 100 times greater than that of nitride. Then it is possible to realize wider cavities that openings generated in the nitride film.

p0047The <figref idrefs="f0003">5</figref> shows such an embodiment. One recognizes a substrate 13 consisting of an initial support 14 covered with a silicon film 15. The film 15 is covered with a very thin film of silicon nitride 16 which are formed openings 17 of small dimensions. From the openings 17 were obtained cavities 18 in the silicon film 15. Depending on the size of the openings 17 made ​​in the silicon nitride film 16 and the thickness of this film 16, it is possible to deposit in the cavities 18 19 a material whose chemical nature is conducive to trapping (eg titanium for his trap effect, known as "getter" in English) of gaseous species (eg oxygen) implanted in step posterior implantation.

p0048Thereafter, one can possibly clog the openings made by depositing a layer. This deposit will not be necessary, such as in the case of transfer of a studded structure made of a crystalline silicon film. Likewise, under certain conditions, heat treatments under controlled atmosphere can facilitate, or even obtaining, closing such cavities. These etching zones will be considered, in the method according to the invention, such as inclusions, traps for the gas species implanted subsequently.

p0049Inclusions can also be generated by ion implantation.

p0050The implantation by bombardment of neutral species or ions of a material may cause inclusions rich layer at a specific depth of the implanted element. For the implanted species, we then take into account the electronic braking effects and nuclear by the target material. In the process according to the invention, the starting material is considered as the target material. The implantation process can be done in a number of locations. These implementations may optionally be assisted during or between each implantation by a heat treatment. We found the implanted species and associated defects in the vicinity of an average penetration depth Rp (named "projected range" in English). The inclusions generated appear as a small-scale disorder in the local order of the material. Morphology and size may be modified by a heat treatment and / or a single and / or multiple implementation of the same element or not.

p0051Examples include delivering semiconductor material on insulator (SOI) in the SIMOX process (separation by implantation of oxygen). The location at 120 keV oxygen is followed by a high-temperature heat treatment (eg, about 1300 ° C) to change the topology and morphology of inclusions generated. The low dose implantation (about 4.1017 W / cm²) Oxygen in a silicon wafer, allows an oxide layer to a typical depth of 250 nm, thin (typically 80 to 100 nm). This layer is defective: it is more or less continuously (presence of silicon named pipes "pipes" in English) and contains silicon islands (of several tens of nanometers as typical dimensions), depending on the implanted dose. One can refer to in this article<nplcit id="ncit0004" npl-type="s"><text> ASPAR B. et al., Entitled "Ultra Thin Buried Oxide Layers Formed by Low Dose SIMOX Processes", Proc. 6th International Conference on SOI Technology and Devices, Electroch. Soc., Vol. 94-11 (1994) 62</text></nplcit>. Similarly, the interfaces of the oxide layer with the top film are more or less rough as required to heat treatments. Typically, the interface roughness can be controlled within a range of a few tenths of nanometers to several nanometers as mentioned in the article entitled "<nplcit id="ncit0005" npl-type="s"><text>Characterization by Atomic Force Microscopy cf the SOI Topography Layer in Low-Dose SIMOX Materials "GUILHALMENC C. et al., Published in the journal Materials Science and Engineering B 46 (1997) 29-32</text></nplcit>. This implanted layer and its interfaces shall be regarded as an area of ​​inclusions, containment areas for the gaseous species implanted in the second step of the process according to the invention.

p0052Thermal treatment can also be used to generate inclusions in the initial material, carrier or in at least one of layers of the rope structure (s) to transfer.

p0053As an example may be mentioned, for silicon, heat treatment, called "high-low-high" in English, to precipitate at a certain depth, the oxygen present in the material. This depth is typically a few micrometers in the case of monocrystalline silicon obtained by the Czochralski pulling.

p0054For this, a temperature cycle will typically be made of a high temperature level, above 1000 ° C, followed by a low-temperature stage, below 900 ° C, again followed by a high-temperature stage, above 1000 ° C. An order of magnitude of the depth x can be estimated from the diffusion equation x H-alpha (Dt)^1/2 wherein D is the diffusion coefficient in the heat treatment temperature and t the time of diffusion at this temperature. This layer generated by thermal treatments is considered area of ​​inclusions.

p0055As another example, heat treatments are known to allow the adaptation of the level of stresses in films deposited by any one of the above methods. Thus, a thermal treatment above 500 ° C for a silicon oxide film deposited by CVD, can reduce the compressive stress to cancel or transform it into voltage. (Cf.<nplcit id="ncit0006" npl-type="s"><text>SLINTANI A. et al., J. Appi. Phys. 51 (8), p. 4197 (1980</text></nplcit>)]. Such behavior is attributed to the reactions of the oxide to the water vapor. It can be interpreted as a degassing effect or as a densification effect. Similarly, a large thermal expansion between a film and the initial support (or other films) can cause a large stress state and locally generate stresses inclusions favorable to trap gaseous species. Examples include the case of silicon film (100) produced on sapphire Plan A. The expansion coefficients are respectively in the range of 4.106 / 9.106 and K / K. Since the stress is very localized in the film thickness around the interface, this results in a local deformation of the material. Such a disturbed region is considered in the process of the invention as an area of ​​inclusions.

p0056Another way to induce a strain on a film plane structure (s) is deposited on the rear face of the initial support a forced film for a morphological deformation (convexity or concavity). The film structure is then distorted. Locally the stress area in the structure comprising the film or films to be transferred is in the method according to the invention an area of ​​inclusions for the gaseous species implanted subsequently.

p0057The method according to the invention comprises a second step after the generation of inclusions in the material concerned. This second stage consists of an implantation of gaseous species (atoms, ions) at a depth in the vicinity of the layer of inclusions generated in the previous step. These gaseous species are confined, thanks to the presence of inclusions. They participate in the nucleation and / or growth of microcavities, microbubbles (or "platelets") necessary to transfer fracture. This implantation may be carried out through the planar surface of the structure to be transferred by bombardment and / or plasma-assisted diffusion and / or by heat treatment and / or by electrical polarization.

p0058In the case of implantation by bombardment (neutral species and / or ions), these gas species are implanted to the mean penetration depth Rp. This depth is typical of the implantation energy of the element implanted into a target data. We will choose an implantation energy such as depth Rp corresponds to the level of inclusions or zone such that the depth is in the vicinity of the area of ​​inclusions, a diffusion heat treatment then being used to allow species migration located at the area of ​​inclusions. The gaseous species may be rare gas or not, as H, F, He. They may be implanted simultaneously or successively.

p0059The <figref idrefs="f0004">6A to 6D</figref> illustrate the process according to the invention in the case where the thin film is transferred onto a stiffener. The<figref idrefs="f0004">6A</figref> shows a substrate 20 (e.g. formed of a film structure (s) thin (s) of an initial support) having an area of ​​inclusions 21 formed by one of the methods described above. The area of ​​inclusions is situated at a distance from the surface 22 of the substrate corresponding to the thickness of the thin film to be transferred. The<figref idrefs="f0004">6B</figref> illustrates the ion implantation step. Gaseous species are implanted, for example by bombardment or by diffusion through the surface 22 of the substrate. The density of gaseous species as a function of the depth d is such that their mean depth of penetration Rp corresponds to the area of ​​inclusions 21 which becomes a trap area, dense gaseous species. The<figref idrefs="f0004">6C</figref> illustrates an adhering step of the surface 22 of substrate 20 to a stiffener 23 by providing an intermediate layer 24. Other technical adhesion between the surface 22 and the stiffener 23, without providing an intermediate layer, may also be used. The<figref idrefs="f0004">6D</figref> illustrates the subsequent separation step to an appropriate heat treatment depending on the thermal budget required as explained above. In this figure, the fracture separation pass into the trap area. The initial substrate is divided into a thin film member 25 to the stiffener 23 and a remaining portion 26. The trap area is shown here divided into two regions. However, depending on the case, it can remain in full or adhering to the thin film 25 or the remaining portion 26 of the substrate.

p0060In the case of implantation by gaseous diffusion, species can spread to a depth in the vicinity of the inclusions, adjusting time and diffusion temperature. conventional broadcasting laws in (Dt)^1/2 are applicable to adapt the diffusion depth. Thus, a heat treatment in an argon and hydrogen atmosphere, in the 9: 1 (called "forming gas" in English), allows the hydrogen diffusion in silicon, about 350 ° C.

p0061Whatever the implementation mode, the gaseous species to be implanted in an amount sufficient to participate in the nucleation and / or development of microcavities, microbubbles (or "platelets") from and in the vicinity of the inclusions described above. The implantation conditions (dose, energy, target temperature, time of implantation) depend in particular:<ul><li>the initial material (target)</li><li>the nature and location of inclusions, </li><li>the thermal budget provided by implantation,</li><li>the nature of the implanted gaseous,</li><li>the thermal budget provided subsequent to a possible collage,</li><li>the thermal budget (energy) supplied by the heat treatment, embrittlement,</li><li>any mechanical stress.</li></ul>The implanted doses are nevertheless lower than the maximum dose, dose defined by the occurrence of exfoliation in the material during the implantation of gaseous species. Efficacy is defined inclusions by their containment capacity of the gaseous species necessary for the transfer, considering the concentration of these species in the vicinity of the inclusions.

p0062In the case of ion implantation, this effect is illustrated by a decrease in the width of the implantation profile due to a higher concentration of the implanted species implantation around Rp. As an example, consider a structure composed of a film transfer Si02 0.4 .mu.m thick generated on a silicon substrate. A first ion implantation of hydrogen of 3.10¹⁶ H⁺/ cm², Energy of 100 keV for generating the inclusions, cause a hydrogen concentration in the average depth of 0.9 .mu.m. A heat treatment is performed, typically around 350 ° C for 2 hours, and is designed to change the morphology of inclusions (microcavities). It is found that the thickness of the layer containing the cavities is finer than if implantation was carried out with a higher dose as in the case of the method disclosed in document<patcit id="pcit0014" dnum="FR2681472A"><text>FR-A-2681472</text></patcit>. The area of ​​inclusions corresponds to this layer of microcavities during growth. A second implantation of 2.10¹⁶ H⁺/ cm² will be sufficient to allow a fracture in the vicinity of this area of ​​inclusions, during heat treatments of separation, for example at 500 ° C for 1 hour.

p0063It is therefore understood the advantage of a containment and a possible location of microcavities, microbubbles (or "platelets") on a very thin thickness due to the thickness of the zone of inclusions produced and / or structure films used. Similarly, the roughness of the fracture surface will also be reduced due to the containment of inclusions and therefore of the fracture zone.

p0064In general, it is possible to reduce the dose to implement, required for nucleation and / or development of microcavities and / or reduce the forces to exercise and / or reduce the energy budget of the heat treatment to induce the fracture.

p0065The transfer method to obtain a final film structure (s) on a support presupposes that the starting material is reported on a second support in a third step. The contact is either direct molecular adhesion (nominated for "wafer bonding" in English), or via an adhesion layer. It must allow the holder to have a final role of stiffener. In both cases of contacting, direct and indirect, a step of fixing by heat treatment at low temperature may be necessary. This treatment must be adapted not prevent the growth mechanisms of micro-cavities and fractures in the original material. It will be taken into account in the thermal budget required to induce fracture in a fourth process step. If the structure to be transferred is sufficiently rigid and / or thick and that this step is not necessary, will be obtained during the transfer, a so-called tractor.

p0066Thus, in the example of a structure coated with a Si02 film to be transferred on a silicon substrate, a temperature of the order of 200 ° C will be sufficient to reinforce the molecular adhesion. The bonding energy between the oxide film and the silicon substrate will be greater than 0.3 J / m².

p0067The fourth step of the structures of transfer process film (s) requires a heat treatment time and the temperature are defined, in particular, depending on the effectiveness of inclusions created, the dose of implanted gaseous species, thermal conditions of the implantation of gaseous species in the original material and the thermal conditions of membership to the final support plate. The heat treatment should be sufficient to cause a fracture in the original material. Thus causes a separation between a part of the unused starting material and the film structure (s) in contact with the final substrate. This separation takes place in the vicinity of the layer of trapped species. Under the conditions of the invention, the film structure (monolayer or multilayer) can be transferred with a thermal budget fracture reduced in comparison with the thermal budget needed in the process according to the prior art. To set the thermal budget of separation, to consider the efficiency of the generated inclusions and the overall thermal budget that is provided to the plates during the various stages of the process, namely during: the generation of inclusions, implantation gaseous species and the accession of the original material on the stiffening support.

p0068In addition, part of the energy necessary for transfer of the structures may be made by heat treatment and / or by using constraints, for example related to an effect of the final support stiffener, related to the application of stresses shear, bending, tensile, pressure, applied alone or in combination. The effect is the same as that described in document<patcit id="pcit0015" dnum="FR2748851A"><text>FR-A-2748851</text></patcit>. In this case, the minimum dose of gaseous species to be implanted, in the second process step is the one from which there is a creation and / or sufficient growth of micro-cavities to induce sufficient weakening of the wafer in parallel on the surface.

p0069The <figref idrefs="f0003">7</figref> illustrates a different application of the method according to the invention for obtaining an SOI structure. The initial substrate 30 is formed from a silicon wafer 31 on one face of which is deposited a film 32 of silicon about 50 nm thick, heavily doped (approximately 10¹⁹ atoms / cm3) with boron, developed by epitaxy. The film 32 is itself covered with a film 33 of silicon, approximately 350 nm thick, lightly doped (about 5.10¹⁵ atoms / cm3) with boron and also produced by epitaxy. The film 33 is finally covered with a film 34 of Si02, about 400 nm thick and having a free surface 35. The film highly doped silicon 32 will play the role of inclusions area.

p0070The substrate 30 is then subjected to the step of implantation of gaseous species through the surface 35. The implanted hydrogen in a dose 5.10¹⁶ atoms / cm³An energy of 80 keV and at room temperature.

p0071The surface 35 is then rendered adherent to a silicon wafer, by molecular adhesion reinforced by a heat treatment at 250 ° C for 30 minutes.

p0072The separation step in two parts of the initial substrate 30 includes a heat treatment whose effectiveness compared to the fracture is suitable for the thermal budget (time and temperature of the different heat inputs). This final heat treatment makes it possible to induce a fracture in the initial substrate, at and / or in the vicinity of the film 32. The final heat treatment may typically be from 2 hours to 250 ° C.

p0073One can thus obtain a structure formed of a lightly doped silicon film (the film 33 of the initial substrate) on a silicon oxide layer (film 34 of the original substrate), the latter being secured to a mass silicon. The film highly doped silicon 32 was used for containment of the fracture.

p0074The method of the invention is particularly interesting in the case of transfer structures in which one or more films should not be subjected to heat treatment at such a high temperature that involved in the process disclosed in the document <patcit id="pcit0016" dnum="FR2681472A"><text>FR-A-2681472</text></patcit>. It is also interesting to implement in the case where the structure to be transferred is made of materials having different thermal expansion coefficients.

p0075Finally, it is important to note the following advantage of the process according to the invention. The surface of the film structure (s) being transferred is disturbed area, obtained in the fracture. The thickness of the disturbed zone can be very reduced due to the use of a layer at and / or adjacent the inclusions for confining the dose of implanted gaseous species. Is thus obtained, a surface roughness of the transferred weak structure since they are directly related to the distribution of microbubbles or microcavities in the thickness of the material during transfer.