FR2991499A1 - METHOD AND SYSTEM FOR OBTAINING A SEMICONDUCTOR WAFER - Google Patents

Abstract

The subject of the present invention is in particular a method for obtaining a self-supporting semiconductor wafer (500) from a donor substrate (100) made of a first semiconductor material and comprising in its thickness a zone of embrittlement (210), the method being characterized in that it comprises: depositing an intermediate layer (300) of a second semiconductor material on a face (101) of the donor substrate (100); supplying a third semiconductor material (400) in fusion on the intermediate layer (300) to form an additional layer; separating (510) the donor substrate (100) at the weakening zone (210) to form a self-supporting wafer (500) comprising a portion (110) of the donor substrate (100), the intermediate layer (300) and the additional layer.

Description

TECHNICAL FIELD OF THE INVENTION The present invention generally relates to the manufacture of substrates used by the semiconductor industry, the microtechnology industry and the photovoltaic industry and more particularly discloses a method of making slices of semi material. -driver. STATE OF THE ART In the field of the manufacture of substrates used by the microelectronics industry, a very important technological progress has been made with the introduction in the 90's of the technology known as commercial "Smart CutTM". which makes it possible to separate thin layers of a semiconductor material from an initial plate, usually called a donor substrate, cut in an ingot made of this type of material and in particular from monocrystalline silicon ingots. This technology is for example described in the European patent application published under the number EP0533551. This technology is based on the implantation of light ions, in particular hydrogen and / or helium, from the surface of the donor substrate of the semiconductor material. These ions are inserted into the crystalline structure of the material at a depth determined by the implantation energy. Conventional implanters, such as those commonly used in microelectronics, can develop acceleration energies up to 250 keV (kilo electron volts) which make it possible to reach implantation depths, for example of hydrogen in silicon, maximum of about 2 pm (pm or micrometer = 10-6 meter). Then subject to an appropriate heat treatment the donor substrate sees, at the level of the implanted zone, the development of cavities filled with the implanted gas (s). Pressurized by the heat treatment, under optimal conditions of implementation of the process, the cavities develop essentially laterally. They create an embrittlement zone 30 in the form of microcracks which finally induce, under the sole action of heating or in combination with a mechanical stress, detachment (rupture) at the implanted zone thus making it possible to detach a superficial wafer from the donor substrate , to a thickness corresponding to the implantation depth of the donor substrate, which is immediately reusable. The technique briefly described above is used in particular to produce so-called SOI substrates, the acronym for "silicon on insulator", that is to say "silicon on insulator". As mentioned above, the development of the weakened zone must essentially be lateral. However, if the pressurized cavities are close to a free surface, there is a pressure threshold beyond which there is bubbling or blistering of the material of the superficial edge that we are trying to separate. The phenomenon of blistering is also commonly referred to as "blistering" which has the same meaning. Only if the cavities are sufficiently far from a free surface can they actually extend laterally in the form of cracks that will allow separation at the level of the implanted area. The Smart CutTM process avoids this pitfall by providing, for example, to glue a second rigid substrate on the side of the implanted surface that will serve, for the SOI structure being created, final mechanical support and handling substrate. Before separation, the bonded rigid substrate also advantageously acts as a stiffener and will allow the separation to proceed smoothly without blistering over the entire surface of the donor substrate, leaving in place on the SOI substrate a uniform layer, generally of monocrystalline silicon, in and from which we will be able to realize the active components.

The light ion implantation separation method, however, makes it possible to obtain, as previously indicated, with conventional implanters used by the microelectronics industry, only depths of up to about 2 μm. While these thicknesses are well suited to the production of SOI substrates for the manufacture of integrated circuits, where the transistors are produced in increasingly thin functional layers, this is not the case for other applications. For example, for the production of photovoltaic cells, in particular based on silicon, it is necessary to have, for the functional layer, thicknesses of semiconductor material typically ranging from 10 to 100 μm in order to obtain a sufficient efficiency of converting the received light energy into produced electrical energy. In the area of obtaining slices by embrittlement of a zone implanted in light ions the North American company "Silicon Genesis" located in the state of California, has developed in recent years a process similar to the "Smart CutTM" process and whose trade name is "PolyMaxTM" which makes it possible to obtain functional slices in a thickness range compatible with the production of photovoltaic cells.

However, this is achieved by employing high-energy implants of the order of a few million electron volts, that is to say, to develop implantation energies an order of magnitude higher than those of conventional implants. used by the microelectronics industry. The cost of these implants is extremely high. In a field of application such as photovoltaic cells where the price of the final device depends closely on that of the wafer made of semiconductor material and where the device produced must be at an extremely low cost to be competitive, the cost of High energy implants is clearly problematic. Moreover, the conventional techniques for obtaining plates of semiconductor materials by sawing an ingot obtained from a liquid bath have several disadvantages. In particular, these techniques have the disadvantage of consuming a significant portion of the ingot mass during sawing. In addition, they require post-sawing surface treatments which increases their cost and do not allow to easily obtain thin plates, typically between the ten and hundred micrometers. There is therefore a need to obtain active layers (layers) of semiconductor material, in particular for photovoltaic activation, with a thickness of greater than 5 micrometers and preferably of between 10 and 100 micrometers, according to a low cost method limiting at least some disadvantages presented by the solutions of the state of the art.

Other objects, features and advantages of the present invention will become apparent from the following description and accompanying drawings. It is understood that other benefits may be incorporated. SUMMARY OF THE INVENTION According to one embodiment, the subject of the present invention is a process for obtaining a self-supporting semiconductor wafer from a donor substrate made of a first semiconductor material and comprising in its thickness an embrittlement zone, the method being characterized in that it comprises: depositing an intermediate layer of a second semiconductor material on one side of the donor substrate; - The provision of a third molten semiconductor material on the intermediate layer to form an additional layer. Preferably, the method also comprises a step of at least partial solidification of the additional layer to produce an active layer.

Preferably, it also comprises separating the donor substrate at the weakening zone to form a self-supporting wafer comprising a portion of the donor substrate, the intermediate layer and the additional layer.

Thus, the self-supporting wafer obtained is a multilayer structure formed of layers of semiconductor materials, and whose thickness can be easily adapted, in particular as a function of the amount of semiconductor material molten on the intermediate layer. In particular, slices of thickness greater than 5 microns can be obtained.

Typically, the invention makes it possible to easily obtain slices whose thickness is between 10 and 1000 micrometers, which is particularly advantageous for the production of photovoltaic cells or for the manufacture of microstructures. In addition, the process according to the invention makes it possible to reuse the donor substrate to form several slices, thereby avoiding excessive consumption of material, as is the case in solutions requiring sawing for each slice. By providing a donor substrate recrystallization epitaxy having an embrittlement zone, the invention allows the use of donor substrates having a weakened zone of weakness at a shallow depth. Thus, in the preferred case where the zone of weakness is obtained by implantation of ionic species in the thickness of the donor substrate, the invention allows a shallow implantation which avoids having to resort to high energy implants. The invention thus has the advantage of making wafers at a limited cost compared to known solutions. In addition, the invention makes it possible to obtain slabs in the form of slabs having large dimensions, typically from 20 cm 2 to 4 m 2, and preferably from 100 cm 2 to 1600 cm 2. The invention provides further advantages in terms of controlling the heat provided by the semiconductor material deposited on the donor substrate. It is recalled that an excessive supply of heat to an embrittlement zone can activate phenomena liable to deteriorate the substrate, in particular by causing bubbling or blistering on its surface. The invention, by providing an intermediate layer, makes it possible to control the propagation of heat transferred to the donor substrate from the molten semiconductor material, thus avoiding too much and / or rapidly heating the donor substrate at the level of the weakened area and thus reducing the risk of causing blistering. In addition, the presence of the intermediate layer makes it possible to better control the total thickness of solid materials, located between the zone of weakness and the free surface after deposition of the third liquid material, and which can then serve as a stiffener thus facilitating the rupture / separation in the zone of weakening.

Separation means at least a microcracks propagation step in the implanted zone. This step may however go as far as detachment, i.e. a physical distance between the donor and the slice. Additional means, for example mechanical, can be implemented to achieve this detachment. The detachment of the donor substrate thus generates a self-supporting wafer comprising a portion of the donor substrate, the intermediate layer and the additional layer formed by the third semiconductor material brought in the liquid and solidified state.

Optionally, the method according to the invention may have at least any of the features and optional steps below. Advantageously, the intermediate layer acts as a stiffener during the separation of the donor substrate. Advantageously, the intermediate layer remains solid at least partially after the step of supplying the third molten semiconductor material or the intermediate layer solidifies at least partially after the step of supplying the third semiconductor material in fusion and before the appearance of blisters it avoids, and before the separation step, thus acting as a stiffener during the development of microcracks and during separation. Thus, the intermediate layer is solid at least over part of its thickness and acts as a stiffener preventing blistering phenomena or exfoliation (bursting bubbles).

Advantageously, the intermediate layer liquefies at least partially under the effect of the heat provided by the deposition of the third molten semiconductor material. Thus, the intermediate layer absorbs at least partly the heat energy provided by the third molten semiconductor material.

Advantageously, the zone of weakening delimits in the donor substrate a surface layer extending from the zone of weakness to the face of the donor substrate on which the intermediate layer is deposited. The at least partial liquefaction of the intermediate layer and the resulting solidification preserves a solid layer serving as a stiffener at the time of separation of the donor substrate to release the self-supporting wafer. One of the advantages of the invention is to control the progress of interface / solid liquid. It is necessary to have a solid film sufficiently thick to have the stiffening effect, at the time of the separation. The stiffener, i.e., the solid layer is preferably thicker than the surface layer to have the stiffening effect.

According to one embodiment, the solid layer serving as a stiffener comprises at least the entire surface layer. According to one embodiment, the solid layer serving as stiffener comprises the entire surface layer and at least a portion of the intermediate layer. Said portion of the intermediate layer is a portion remained solid after the supply of the third molten semiconductor material. According to one embodiment, this portion of the intermediate layer remaining solid is completed by a portion of the liquefied intermediate layer and then solidified after the introduction of the third semiconductor material and before the appearance of blisters, thus allowing the separation. According to one embodiment, the solid layer serving as a stiffener comprises the entire surface layer, the intermediate layer and at least a portion of the additional layer having partly at least solidified before the appearance of blisters and before the separation of the substrate donor, thus enabling the donor substrate to be separated at the weakening zone. Advantageously, the solid layer serving as a stiffener has a thickness greater than or equal to 3 micrometers, and preferably greater than or equal to 7 micrometers and preferably greater than or equal to 10 micrometers and preferably greater than or equal to 20 micrometers. Advantageously, the supply of the third molten semiconductor material causes the complete melting of the intermediate layer, then the intermediate layer re-solidifies at least partially before the appearance of blisters and separation. Thus, the complete melting of the intermediate layer makes it possible to obtain crystallization of the intermediate layer and then of the additional layer by epitaxy from the crystalline seed formed by the implanted substrate. Advantageously, the method comprises a prior step of obtaining the weakening zone in the thickness of the donor substrate, this obtaining step may comprise the implantation of species into the thickness of the donor substrate. The separation of the donor substrate at the zone of weakening is obtained by adjusting: the temperature of the third molten semiconductor material during the supply step, the preheating temperature of the donor substrate, the type, the dose and the energy of implanted species during implantation. Advantageously, the homogeneous separation in the donor substrate at the level of the embrittlement zone is obtained by further choosing the mode of deposition of the third molten semiconductor material. Advantageously, the semiconductor material and the thickness of the intermediate layer are chosen to obtain a total separation in the zone of weakening induced automatically under the sole effect of the heat energy provided by the deposition of the third molten semiconductor material. . Alternatively, the separation of the donor substrate at the zone of weakening occurs automatically before the third molten semiconductor material comes into contact with the intermediate layer or is completely brought on the intermediate layer. Thus, according to a first embodiment, the separation of the donor substrate at the zone of weakening is induced solely by the supply of heat from the supply of the third molten semiconductor material on said face of the donor substrate and by the heat propagation to the zone of weakness. According to an alternative embodiment, the separation of the donor substrate at the zone of weakening is obtained before the contact of the third molten semiconductor material, solely by convection or heat radiation from the third molten semiconductor material and heat propagation to the zone of weakness.

According to an alternative embodiment, the separation of the donor substrate at the zone of weakening is obtained after the solidification of the additional layer and by a subsequent step comprising a heat treatment step and / or a mechanical step performed so as to apply mechanical stress. Preferably, the embrittlement zone is formed within 5 micrometers of the face of the substrate on which the intermediate layer is deposited and more preferably less than 3 microns. It is advantageously formed at about 2 micrometers from the surface, thanks to conventional implants. The supply of the third molten semiconductor material to the intermediate layer is performed by liquid phase deposition. According to one embodiment, the supply of the third molten semiconductor material is carried out by a single casting on the intermediate layer. According to another embodiment, the supply of the third molten semiconductor material on the intermediate layer is carried out by depositing the third molten semiconductor material on a plurality of zones of the intermediate layer, the zones being distinct from each other. others. According to another embodiment, the deposition of the third molten semiconductor material comprises the deposition of a plurality of drops and optionally the sprinkling of a free face of the intermediate layer. The drops can be deposited using nozzles, capillaries, orifices, injectors or sprays. According to another embodiment, the supply of the third molten semiconductor material to the intermediate layer is carried out by contacting a face of the intermediate layer remaining free with a melt of the third semiconductor material. . According to another embodiment, the supply of the third molten semiconductor material on the intermediate layer is carried out by placing the donor substrate covered with the intermediate layer in the right of a crucible containing the third molten semiconductor material. and having a retractable bottom, for example sliding, and removing the bottom of the crucible so as to release the third molten semiconductor material on the intermediate layer.

Advantageously, these embodiments make it possible to induce a contact between the third semiconductor material and the intermediate layer only at the level of the free face of the latter. In particular, the vertical walls, that is to say the edges perpendicular to the remaining free face are not in contact with the third semiconductor material. The propagation of heat is better controlled. In addition, this technique allows a particularly fast deposit. Advantageously, the intermediate layer is deposited on the donor substrate at a temperature below 500 ° C, preferably below 400 ° C and more preferably below 300 ° C. Thus, the risk of separation or bubbling or exfoliation during the deposition step of the intermediate layer is reduced. Advantageously, the intermediate layer is deposited directly on the face of the donor substrate.

Advantageously, the third semiconductor material is deposited directly on the intermediate layer. Advantageously, after separation, the self-supporting wafer is subjected to a heat treatment, typically an annealing, of crystallization. This annealing makes it possible to improve the crystalline quality of the structure of the self-supporting wafer formed of the stack of layers. Advantageously, the first, second and third semiconductor materials are chosen from: silicon (Si), germanium (Ge) and silicon-germanium (Si-Ge). In one embodiment, the first, second and third semiconductor materials are made of the same material. In an alternative embodiment, at least two of the first, second and third semiconductor materials are formed of different materials. Advantageously, the self-supporting wafer has a thickness of between 5 and 1000 micrometers and preferably 10 and 100 microns and even more preferably between 20 and 100 microns.

Advantageously, the thickness eL of the third semiconductor material provided is determined, so that a thickness eF of the intermediate layer and possibly of a layer of the donor substrate are melted under the effect of the heat provided by the third semi material. -conducteur is less than the distance between the weakening zone and the face of the intermediate layer on which the third semiconductor material is brought. This distance being the depth of the embrittlement zone from the face of the intermediate layer on which the third semiconductor material is brought. This prevents the weakening zone from melting, which would prevent any possibility of separation and detachment. Preferably, the first, the second and the third semiconductor material are in the same semiconductor material. Advantageously, the thickness eF is determined by applying the following equation: PL CP (TL-TF) in which eF represents the maximum thickness of material which melts under the effect of heat propagated from the third semiconductor material brought in the liquid state, this material being taken in the intermediate layer and optionally in the portion of the donor substrate; eL represents the thickness of the additional layer formed by the third semiconductor material brought to the liquid state once distributed on the surface; pi_ is the density of the liquid semiconductor material; 25 μs is the density of the solid semiconductor material; Cps represents the specific heat capacity of the semiconductor solid phase material; Cp 'represents the specific heat capacity of the semiconductor material in the liquid phase; Lf is the heat of fusion of the semiconductor material; eF-eL PsCP (TF -Ts) + PsL TL the deposition temperature of the semiconductor material brought to the liquid state; TF the melting temperature of the semiconductor material; TS the temperature, before supply of the third semiconductor material, of the donor substrate covered with the intermediate layer The subject of the invention is also a method for producing a photovoltaic cell comprising a method for obtaining a self-supporting semiconductor wafer according to the invention.

The subject of the invention is also a process for obtaining a self-supporting semiconductor wafer from a donor substrate made of a first semiconductor material, characterized in that it comprises: - the deposition of an intermediate layer of a second semiconductor material on one side of the donor substrate; providing a third molten semiconductor material on the intermediate layer to form an additional layer; solidification of the third semiconductor material. Optionally and preferably, the method also comprises a step of detaching the donor substrate to form a self-supporting wafer comprising a portion of the donor substrate, the intermediate layer and the additional layer. Advantageously and optionally, the donor substrate comprises in its thickness an embrittlement zone. The detachment step consists of breaking the embrittlement zone. BRIEF DESCRIPTION OF THE FIGURES The objects, objects, as well as the features and advantages of the invention will become more apparent from the detailed description of an embodiment thereof which is illustrated by the following accompanying drawings in which: FIGURES la illustrate the main steps of the method according to the invention for obtaining self-supporting slices used in particular for the production of photovoltaic cells. FIGURE 2 illustrates a method of depositing a liquid phase semiconductor material on the surface of an intermediate layer-covered donor substrate for adjusting the process of separating and detaching self-supporting wafers from a donor substrate . FIG. 3 illustrates a mode of depositing a semiconductor material in the liquid phase in the form of drops.

FIGURE 4 illustrates a method of depositing a semiconductor material in the liquid phase by contact with a molten silicon bath. FIGURE 5 illustrates a method of depositing a semiconductor material in the liquid phase from a crucible with a removable bottom. The drawings are given by way of examples and are not limiting of the invention. They constitute schematic representations of principle intended to facilitate the understanding of the invention and are not necessarily at the scale of practical applications. In particular, the relative thicknesses of the different layers and films are not representative of reality.

DETAILED DESCRIPTION OF THE INVENTION In the description which follows of the invention, as in the introductory chapter on the state of the art, the term "zone of weakness" is understood to mean a zone which extends under a main face of a donor substrate. The donor substrate is, most generally, a slice cut into an ingot or a brick. It is in the form of a flat plate having two parallel faces. The flat plates or slices are not necessarily circular or square, they can have any shape in their plane and most generally a form of disk, square, rectangle, hexagon or that of other polygons. The plate is shown in Figures la to the in a section perpendicular to its plane. The weakened zone is developed under one of the faces of the plate, parallel to this face, at a substantially constant depth in the thickness of the donor substrate. The depth of the embrittlement zone is the distance between this zone and the face 101 of the donor substrate 100 through which the ions are implanted, this distance being measured in a direction perpendicular to said face 101. Under the action of a heating , possibly combined with a mechanical stress, it will cause the separation of the substrate 100 at the weakened zone. In the vocabulary commonly used in the semiconductor, microtechnology and photovoltaic industries, a slice, as described above, is also sometimes referred to as a plate or wafer, or even a "wafer", a term borrowed from English having the same meaning. The term substrate is a generic term that is applicable to any support or multilayer structure from which a device can be developed and which, by implication, has the mechanical qualities necessary to be manipulated in a production line. In the following description of the invention, the thickened surface layers obtained by separation and detachment of a donor substrate are referred to as slices (sometimes also referred to as substrate, a term which we will avoid using in the present description to avoid any confusion with the donor substrate). These slices are "self-supporting" in the sense that they can be handled as such in a production line.

It is specified that in the context of the present invention, the term "over", "overcomes" or "underlying" or their equivalent do not necessarily mean "in contact with". For example, the deposition of a first layer on a second layer does not necessarily mean that the two layers are in direct contact with one another, but that means that the first layer at least partially covers the second layer. being either directly in contact with it or separated from it by another layer or another element. As we have seen, to increase the conversion efficiency of the light energy received by a photovoltaic cell, it is advantageously made from a monocrystalline semiconductor and more particularly from silicon (Si) monocrystalline. Other semiconductor materials are also likely to be used in a monocrystalline form such as germanium (Ge) or alloys of these two materials (Si-Ge). It will be noted here immediately that the solidification in a substantially crystalline form, from a liquid phase, between the preceding semiconductor materials can be done if the material in liquid form is of the same nature as the material of the donor substrate, or if the it exists in a crystalline form adapted to the crystal lattice of the donor substrate, that is to say that it has, for example, mesh parameters which differ by less than 1% and preferably by less than 0.5% of that of the material of the donor substrate in the plane parallel to the face of the donor substrate (epitaxial plane). Under these conditions the solidification will be particularly orderly and will produce a crystalline material of very good quality. Obtaining a layer of a material not perfectly crystalline but having large grains crystallized in an amorphous matrix is suitable for many applications. Typically, for an application to photovoltaic cells, the size of the grains must be greater than or equal to three times the thickness of the electrically active wafer, preferably five times greater, which can easily be obtained by implementing the present invention. In general, the invention describes a method for producing self-supporting wafers, also referred to as semiconductor multilayer structures, obtained from a liquid semiconductor material which is solidified on a substrate comprising an intermediate layer capable of promoting solidification operation avoiding the occurrence of bubbling phenomena (or blistering). The invention thus makes it possible to produce self-supporting slices of desired thickness, thin or thick. The method comprises the following main steps: FIGS. 1a-1c illustrate the steps of the method of the invention which consists in producing self-supporting semiconductor wafers, for example silicon (Si), from liquid silicon on the one hand and on the other hand, a crystalline donor substrate. FIG. 1a illustrates the first step and shows a donor substrate 100 of a semiconductor material in which light ions are implanted from one of its faces 101, typically with the aid of an ion beam 200 generated by an implanter, to create an embrittlement zone 210. The substrate is for example weakened by implantation of light gas ions such as hydrogen (H) or helium (He) used alone or in combination, or in co-implantation with other species such as boron (B). Preferably, the implanter is a standard implanter used by the microelectronics industry. Implantation is then performed at a depth which does not typically exceed 2 μm because, as discussed in the chapter on the state of the art, implantation energies used which are in a range of values from a few tens to a few hundred keV so as not to have to implement very expensive high energy implants.

Optionally, the implantation of the weakened zone can be done by bombarding ions from a plasma formed in a confined space. Thus, the donor substrate has an embrittlement zone 210 in its thickness, which zone defines a surface layer 110 between the embrittlement zone 210 and the face 101 of the donor substrate 100 closest to the embrittlement zone 210. Typically, this face 101 is the face through which species have been implanted. The thickness of the surface layer 110 is thus defined by the depth of the embrittlement zone 210. This thickness is measured perpendicularly to the face 101. FIG. 1b illustrates the second step and shows the production on the surface 101 of the substrate donor 100, at least one intermediate layer 300, also called intermediate film, which is advantageously but not limited to amorphous silicon or polycrystalline silicon or nanocrystallized silicon. - Figure 1c illustrates the next step where it comes to bring, on a face 301 remained free of the intermediate layer 300, an appropriate amount of semiconductor material 400 liquid, typically liquid silicon. This material in the liquid state will also contribute to forming the self-supporting slice in progress. The invention proposes several embodiments to bring the liquid semiconductor material 400 onto the intermediate layer 300. These embodiments are described in detail in the following description and some of them are illustrated in FIGS. and FIG. 1d illustrates an optional melting step 410 of all or part of the intermediate layer 300 because of the heat provided by the liquid silicon 400 and a step of (re) solidification of the latter. It may be noted that in the case where the melting temperature of the third semiconductor material 400 is lower than that of the intermediate layer 300, the intermediate layer may not melt at all. Such would be for example the case of germanium deposited at 1100 ° C on silicon. FIG. 1c shows the detachment step 510 of the slice of the self-supporting slice 500 consecutive to the separation (fracture) occurring at the weakened zone 210 by implantation of the donor substrate 100. Preferably, this fracture is activated by the propagation of the heat provided by the liquid silicon 400 to the embrittlement zone 210. The implementation of the invention is based on the use of the intermediate layer 300, potentially fusible at the temperature of the liquid deposited, in such a way that the various adjustment parameters of the process contribute to the automatic detachment of a thin self-supporting wafer 500 constituted on the one hand by silicon brought in liquid and then solidified form 400, and on the other hand by the intermediate layer 300 and the surface layer 110 which has been separated from the donor substrate 100. In the case of application of the method for the production of wafers for photovoltaic cells their thickness must be sufficient to play the role of light absorber. It is typically in this case in a range from 10 pm to 1000 pm and preferably in a thickness range between 10 pm and 100 pm and even more preferably between 20 pm and 100 pm.

The intermediate layer 300 is intended to play several roles. In particular: - A first role is to serve at least partially, if not totally, stiffener to promote fracture in the zone of weakness 210 avoiding blistering or exfoliation. A second role is to prevent the temperature of the donor substrate 100 from rising too high or too fast at the weakening zone 210, in particular by preventing a fusion from occurring prematurely at this zone when contacting the intermediate layer 300 with the semiconductor material 400 supplied in the liquid state. It will be noted here the necessary presence of a stiffener in the form of a solidified layer, typically silicon, above the embrittlement zone of a sufficiently large thickness to serve as mechanical support and allow the separation and detachment of the Donor substrate 100 can be done in optimal conditions, that is to say in particular without blistering. To fully play its role of stiffener it is advantageous that the solid layer has a minimum thickness of 3 pm before fracturing (separation). It is preferably of a minimum thickness of 7 μm, and even more preferably of 10 μm. The solid layer acting as a stiffener consists of: the surface layer 110, typically made of monocrystalline or polycrystalline silicon with large grains (greater than 10 μm, typically of the order of a few tens of micrometers), which is not melted (or which has partially melted during the deposition of the third liquid material and resolidified before separation) located between the embrittlement zone 210 and the initial surface 101 of the donor substrate 100, that is to say the free surface of the substrate before deposition of the layer intermediate 300; and - a solid part of the intermediate layer 300 which is either unmelted or resolidified if partially melted during the deposition of the third liquid material; and optionally - a solidified part of the silicon film 400 deposited in the liquid phase. Preferably, the solidified layer comprises at least the surface layer 110 and at least part of the intermediate layer 300. In an advantageous embodiment, the solidified layer comprises the entire surface layer 110. In an advantageous embodiment, the layer solidified part also comprises a part of the additional layer formed by the silicon 400 supplied in the liquid state and which solidifies before rupture of the embrittlement zone 210.

The method of depositing the molten silicon 400 on the intermediate layer 300, examples of which are described in the following figures, may allow partial or total solidification before rupture (separation, fracture) of the weakening zone.

The fracturing of the embrittled zone 210 occurs: either before the deposition of the liquid silicon 400 on the intermediate layer 300, for example in a temperature rise / thermalization phase of the donor substrate 100 due to the thermal radiation during the approach furnace for depositing the silicon in a liquid form; during or after the deposition of the liquid silicon, for example because of the heat flux supplied by the liquid silicon itself, or for example by conjugation of the heat flux brought by the liquid silicon and heat coming from a external source. The detached self-supporting wafer may consist, for example, of a stack of layers comprising: a crystalline layer formed by a portion of the surface layer 110 of the donor substrate 100 and which, for example, has not melted, or which has partially melted during the deposition of the third liquid material and resolidified before separation) - a layer in a more or less crystalline state formed by a portion of the intermediate layer 300 which has been wholly or partially melted and resolidified, - an additional layer more or less crystalline formed by the third semiconductor material 400 provided in the liquid state and solidified. To improve the crystalline quality of the detached wafer, at least one additional heat treatment may optionally be used. For example, a heat treatment or annealing in the solid phase is used. Typically, this is carried out at temperatures in the range of 400 ° C to 1300 ° C for silicon. It is adapted to obtain layers of better crystalline quality possibly allowing the production of monocrystalline or coarse-grain polycrystalline silicon. In the specific case where the molten layer during the deposition of the liquid silicon has time to solidify before the fracture (separation) is induced, it is advantageous to melt the intermediate layer 300 at least up to the initial face 101 of the donor substrate 100 made of crystalline silicon so as to promote epitaxy. An additional heat treatment as described above can make it possible to improve the crystalline structure obtained. Here it should be noted that it must be avoided in all cases that the embrittlement zone is melted during the deposition of the liquid semiconductor material before the fracturing could have occurred. As already mentioned above, one of the roles of the intermediate layer 300 is to protect the weakening zone 210 against a melting. The intermediate layer 300 must therefore be sufficiently thick. It can be melted partially or totally under the effect of the heat of the molten semiconductor material. The maximum thickness that can be melted during the deposition of a liquid layer 400 of semiconductor material can be calculated as indicated below. In the following expressions, eF represents the maximum thickness of film which melts during the deposition of the third semiconductor material which forms a liquid layer of thickness eL. The layer 400 of liquid semiconductor material of thickness eL is deposited on the intermediate layer 300 at a temperature TL. With respect to its melting temperature, TF, it has an excess of heat energy AQg. This excess energy is transmitted to the surface 301 of the intermediate layer 300 and induces several physical mechanisms such as: AQg -Qs + QTF + Q losses (1) expression in which: - Qs represents the heating energy of the substrate of its initial temperature (before deposition) TS up to the melting temperature of the semiconductor material 25 TF; QTF is the latent heat necessary for the phase transformation (melting) of the thickness eF which melts when the film of liquid semiconductor material of thickness eL is brought into contact; Qpertes represents the heat dissipations of heat flows leaving the volume of liquid and molten semiconductor material (eL + eF).

The equilibrium described by equation (1) can be rewritten per unit area, considering that the melted zone is at the melting temperature of the semiconductor material TF: eL PL cf- '(TL-TF) - PS eF CP ( TF-Ts) + Ps L f eF S + -pertes (2) where: pL is the density of the liquid semiconductor material; pS is the density of the solid semiconductor material; - Cps represents the specific heat capacity of the solid semiconductor material (S) - CP 'represents the specific heat capacity of the liquid semiconductor material (L); Lf is the heat of fusion of the semiconductor material. The heat dissipations, Qpertes, have the effect of reducing the thickness of molten semiconductor material during the deposition of the liquid semiconductor material. These losses are carried out either by conduction in the substrate or by radiation in the enclosure. If we consider that they are weak in the time scale of the induced fusion, we can then neglect this term. In this case, it is possible to calculate a maximum thickness value of semiconductor material which will be melted during the deposition of the liquid semiconductor material.

Equation 2 then becomes: eL PL CP (TL-TF) - Ps eF CPS (TF -Ts) + Ps Lf eF (3) From which the expression of the maximum thickness value of semiconductor material can be deduced which will be melted: e _e PL CP (TL -TF) (4) FL PSCP (TF - TS j + Ps L This value constitutes the upper limit of the thickness actually melted because of the thermal losses mentioned above and which n have not been taken into account in the calculation, it should be noted here that this melted thickness may consist only of a part of the intermediate layer 300, or its entire thickness, and may even include part of the surface layer 110 of the substrate donor 100 to the extent that the fusion does not reach the zone of weakening 210 as already noted above.

This result can also be expressed by the ratio of the melted and deposited liquid thicknesses: eF / eL. For example, considering the following values for silicon: Lf = 1650 J / g, pL = 2.52 g / cm3, ps = 2.32 g / cm3, CpL = 1.04 J / gK- 1, CpS = 0.9 J / g.K1, Tf = 1414 ° C (the melting temperature of the silicon) and considering a liquid film deposited at 1600 ° C which comes into contact with a silicon substrate at 300 ° C the melted thickness will be at most about 12 times lower than that deposited. Thus a deposited liquid layer, of a thickness of the order of 35 μm, can produce the melting to a maximum thickness of about 3 μm.

It will be noted here that the crystalline or amorphous structure of the intermediate layer 300 as deposited does not matter. Preferably, the silicon deposition techniques known to those skilled in the art for their high growth rate, for example by spray, by plasma torch, by PVD (acronym for "physical vapor deposition"), are used. that is, "physical vapor deposition") or CVD, the acronym for "chemical vapor deposition", that is, "chemical vapor deposition". Under optimal conditions, these deposition techniques make it possible to treat the implanted plates without causing damage such as bubbling and exfoliation. In particular, low deposition temperatures can reduce the risk of fracture and other damage that may otherwise result from this operation. The deposition of the intermediate layer 300 is carried out, for example, below 500 ° C., preferably below 400 ° C. and even more preferably below 300 ° C. The implementation of the process includes other preferential but non-limiting conditions among which: the implanted species are, as already mentioned, preferably light gas ions, in particular hydrogen with or without helium, at a dose of a few 1016 atoms / cm2, alone or in combination and with or without co-implantation of species other than hydrogen or helium. - Implantation is done with conventional ion implanters used by the microelectronics industry in a range of energy from 20 keV to 300 keV. The embrittlement zone is preferably implanted deep enough in order to be able to transfer a maximum thickness of monocrystalline or coarse-grain polycrystalline silicon during fracture separation and / or if it is desired for the thickness of the superficial layer to participate in the formation; stiffener.

To obtain this result energies higher than 100 keV are applied. They are preferably greater than 150 keV and more preferably greater than 180 keV. In the case of implantation of hydrogen ions (H +) alone, the implanted dose will preferably be between 1x1016 H + / cm 2 and 1.2 × 10 17 H + / cm 2, and more preferably between 3 × 10 16 and 8 × 10 16 H + / cm 2 and better still between 4.5x1016 and 8x1016 H + / cm2. The following figures very briefly describe different liquid phase silicon deposition methods known to those skilled in the art and which may be used for the implementation of the invention. As shown in FIG. 2, the liquid silicon 400 is deposited on the surface of the intermediate layer 300 integral with the implanted substrate 100. As discussed above, the liquefied portion 410 by the liquid silicon deposit 400 extends over a greater or lesser thickness. large of the intermediate layer 300, preserving a solid portion 416. The liquefied portion possibly extends 414 into the surface layer of the donor substrate 100 without however reaching the weakened zone 210 by implantation. At the time of fracturing (separation) of the weakened zone of the donor substrate 100, the unmelted solid portion of the superficial layer and of the intermediate layer 412 optionally completed with the re-solidified part of the superficial layer and the intermediate layer serve as stiffener. For the deposition of liquid silicon, use is made, for example, of single silicon casting systems of the type used by the RGS technology, the acronym for "ribbon growth on substrate", that is to say "Ribbon growth on substrate". Alternatively, it is possible to use point distribution systems where the liquid semiconductor material is deposited on a plurality of zones of the face 301 of the intermediate layer 300. The liquid semiconductor material spreads out to form an additional layer which thickens the multilayer structure comprising the donor substrate 100 and the intermediate layer 300. Typically, the semiconductor material 400 is spread on the intermediate layer 300 in the form of drops 421 by means of injectors, capillaries, orifices, sprays or of nozzles 420 as illustrated in FIG. 3. In this case, after formation of the weakened zone 210, the drops 421 are formed from nozzles 420 and dropped 430 on the face 301 of the intermediate layer 300, where they spread 440 uniformly and solidify before detachment of a new slice 500. Alternatively, the semiconductor material 400 is spread on the intermediate layer 30 0 in linear form or any other pattern providing localized deposits in multiple areas. Whatever the method of deposition of the liquid silicon, the distribution of the liquid silicon on the intermediate layer 300 must be carried out as uniformly as possible over its entire surface. This makes it possible to avoid the establishment of strong lateral thermal gradients on the surface of the implanted substrate and in the weakened plane by implantation. A strong thermal gradient in the fracturing zone (separation) 210 could cause damage in the transferred film in the form of cracks, partial bubbling or localized exfoliations and tearing of the intercavities. Preferably, the additional layer formed by the contribution of the third semiconductor material covers the entire face 310 of the intermediate layer 300. The pouring is carried out so that the third semiconductor material 400 is poured over a plurality of zones of the intermediate layer 300 spills out to form a continuous surface. Preferably, the pouring of the third semiconductor material 400 on a plurality of zones of the intermediate layer 300 is carried out so that it spreads over to cover the whole of one face of the intermediate layer 300. third semiconductor material 400 on the donor substrate preferably takes place naturally. Thus, the invention does not require any specific intervention to carry out the spreading.

In another mode of deposition of the liquid silicon illustrated in FIG. 4, the free surface of the intermediate layer 300, ie the face 301, is brought into contact with a bath of molten semi-conducting material 600 of small volume. Note that in this case, it is the volume of the bath of semiconductor material, typically liquid silicon, that must be retained to calculate the heat input and the maximum thickness that can be melted and deduce the amount of thickness of the intermediate layer 300 to be deposited. Preferably any technique is used in which, at any time, only a portion of the implanted plate is brought into contact with the liquid bath. Advantageously, the technique used must allow contact to be established only between the free face 301 of the intermediate layer 300 and the bath of molten semiconductor material 400. For example, a technique allowing the rapid sliding of the implanted substrate covered with intermediate layer 300 in a crucible 700 with a removable base 701, a crucible containing liquid semiconductor material, is illustrated in FIG. 5. The effacement of the base 701 must be fast enough for the third semiconductor material 400 to reach the layer intermediate 300 substantially at the same time. As already noted, to achieve the fracturing (separation) of the substrate at the zone of weakening 210 and the detachment of a continuous wafer, without producing bubbling or blistering or even exfoliations on the surface of the implanted substrate, it is necessary that during the operation a sufficient thickness of stiffener film is formed above the embrittlement zone 210. Under these conditions, after fracturing (separation) of the embrittlement zone 210 by implantation, we obtain a slice self-supporting semiconductor material 500 corresponding to the desired thickness which takes into account the implantation depth 110 of the donor substrate, the thickness of the intermediate layer 300 and that of the film corresponding to the deposited liquid semiconductor material 400 In an alternative embodiment of the invention at least part of the liquid semiconductor material 400 may be deposited after the detachment is produced. Specific examples of realization of self-supporting slices are described below: In a first example, the implantation conditions are chosen so that at 415 ° C. no bubbling occurs in the donor substrate 100 during the waiting period. liquid silicon deposition. For example, we implant a hydrogen dose of 5x1016 atom / cm3 at an energy higher than 180 keV. In this example, an intermediate layer 300 of 12 μm of amorphous silicon is deposited on the implanted donor substrate 100. Then a 20 μm layer of liquid silicon 400 heated at a temperature of 1615 ° C. is deposited on the silicon substrate covered by the intermediate layer, which is preheated to a temperature of 415 ° C. The structure is that of FIG. 2. The intermediate layer 300 is partially melted on a maximum thickness 410 of 1.8 μm. The stiffener, which consists of unmelted thicknesses 414, is greater than 10 μm. Fracturing 510 is induced by the heat propagated in the weakened zone 210 by implantation. After fracturing, the detachment of a silicon film forming a self-supporting wafer 500 is obtained, the thickness of which, as desired in this example, is greater than 33 μm, which is very significantly greater than the thickness obtained with a method of Implantation using conventional implants and postponement of the surface layer.

The table below summarizes the example above and three other examples of implementations of the method of the invention making it possible to obtain different thicknesses of self-supporting slices. Preheating temperature of the donor substrate and intermediate layer in ° C: 415 415 400 400 Hydrogen implantation dose in 5x10 16 4 5x10 16 5x10 16 5 5x10 16 atom / cm3: implantation energy in keV: 180 120 180 180 Thickness of the intermediate layer of 12 11 10 10 Si amorphous in pm: Thickness of the liquid Si layer 20 31 10 5 enpm: Temperature of the liquid Si in ° C: 1615 1515 1515 1515 Maximum thickness of the partially melted intermediate layer in pm: 1.8 1.3 0.45 0.25 Thickness of the stiffener in μm:> 10> 10> 10> 11 Thickness of the self-supporting wafer 33 43 21 16.5 obtained in pm: The process described above thus meets the objectives of the invention. It potentially has a high productivity while being economical in raw material by taking each cycle only a thin surface layer of a donor substrate advantageously serving seed during the solidification of the multilayer structure to produce. It allows the use of conventional implanters widely used by the microelectronics industry. The process is easily adaptable in particular because of the presence of the intermediate layer which makes it possible to regulate the thermal contribution provided by the deposited liquid semiconductor material in order to cause the separation at the weakened zone of the donor substrate without any blistering. no other damage is involved. Subsequent heat treatments, which may for example be carried out up to temperatures of 1300 ° C., make it possible to adapt the crystalline quality of these slices thus produced. The invention is not limited to the previously described embodiments and extends to variant embodiments.

In particular, although the embodiment which is the subject of the detailed description and of the figures already provides for several methods of depositing the semiconductor material, the invention extends to all forms of supply of the liquid semiconductor material allowing to obtain a homogeneous layer. With the above, it is clear that the present invention provides an inexpensive method for obtaining a slice of semiconductor material whose thickness can be freely selected (typically, between a few micrometers and several hundred micrometers) and large. lateral dimensions (typically having a lateral dimension of several tens of centimeters making it possible to obtain 20 cm 2 and 4 m 2 surfaces, and more particularly between 100 cm 2 and 1600 cm 2). The invention is particularly advantageous for the realization of dedicated wafers for the realization of photovoltaic cells, without being limited to this application.

Claims (24)

REVENDICATIONS1. A method for obtaining a self-supporting semiconductor wafer (500) from a donor substrate (100) made of a first semiconductor material and comprising in its thickness an embrittlement zone (210), the method being characterized in that it comprises: depositing an intermediate layer (300) of a second semiconductor material on one side (101) of the donor substrate (100); supplying a third semiconductor material (400) in fusion on the intermediate layer (300) to form an additional layer; separating (510) the donor substrate (100) at the weakening zone (210) to form a self-supporting wafer (500) comprising a portion (110) of the donor substrate (100), the intermediate layer (300) and the additional layer. 2. Method according to the preceding claim wherein the intermediate layer (300) acts as a stiffener during the separation of the donor substrate (100) at the weakening zone (210). 3. Method according to the preceding claim wherein the intermediate layer (300) remains solid at least partially after the step of supplying the third semiconductor material (400) melt or wherein the intermediate layer (300) solidifies at less partially after the step of supplying the third molten semiconductor material (400) and before the separation step forming a self-supporting wafer (500), thus acting as a stiffener of the separation. 4. Method according to any one of the preceding claims, wherein the intermediate layer (300) is liquefied (410) at least partially under the effect of the heat provided by the deposition of the third semiconductor material (400) melt. 5. Method according to the preceding claim, wherein the weakening zone (210) delimits in the donor substrate (100) a superficial layer (110) extending from the weakening zone (210) to the face (101) of the donor substrate (100) on which is deposited the intermediate layer (300) and wherein the at least partial liquefaction of the intermediate layer (300) preserves, at the time of separation of the donor substrate (100), a solid layer (412) as a stiffener, the solid layer (412) comprising at least a portion of the surface layer (110). 6. Method according to the preceding claim, wherein the solid layer (412) serving as a stiffener comprises the entire surface layer (110). 7. Method according to the preceding claim, wherein the solid layer serving as a stiffener (412) comprises the entire surface layer (110) and at least a portion (416) of the intermediate layer (300). 8. Method according to the preceding claim, wherein the solid layer serving as a stiffener (412) comprises the entire surface layer (110), the intermediate layer (300) and at least a portion of the additional layer (400) being less solidified before the appearance of blisters and before the separation of the donor substrate (100). 9. A method according to any one of the preceding claims wherein the solid layer (412) serving as a stiffener has a thickness greater than or equal to 3 micrometers and preferably greater than or equal to 10 micrometers. A method according to any one of the preceding claims, wherein the supply of the third semiconductor material (400) in melt causes complete melting of the intermediate layer (300), then the intermediate layer (300) recovers solidifies at least partially before separation. 25 The method according to any one of the preceding claims wherein the semiconductor material and the thickness of the intermediate layer (300) are selected to automatically separate the embrittlement zone (210) under the sole effect of the heat energy provided by the deposition of the third molten semiconductor material (400). 12. Method according to any one of the preceding claims, comprising a preliminary step of obtaining the weakening zone (210) in the thickness of the donor substrate (100), this obtaining step comprising the implantation (200) of species in the thickness of the donor substrate (100), and wherein the separation of the donor substrate (100) at the embrittlement zone (210) is obtained by adjusting: the temperature of the third semiconductor material (400) melt during the supply step, a preheating temperature of the donor substrate (100), the type, the dose and the energy of the implanted species during implantation (200). A method according to any one of the preceding claims wherein the separation of the donor substrate (100) at the zone of weakness (210) occurs automatically after the third molten semiconductor material (400) has been completely brought . The method of any one of claims 1 to 12 wherein the separation of the donor substrate (100) at the zone of weakness (210) occurs automatically before the third molten semiconductor material (400) is entered. in contact with the intermediate layer (300) or has been completely provided on the intermediate layer (300). 15. Method according to the preceding claim wherein the supply of the third semiconductor material (400) melt on the intermediate layer (300) is carried out by a single casting on the intermediate layer (300). The method according to any one of claims 1 to 14, wherein the supply of the third semiconductor material (400) in fusion on the intermediate layer (300) is carried out by point deposits of the third semiconductor material ( 400) in a plurality of zones of the intermediate layer (300). 17. Method according to the preceding claim wherein the deposition of the third molten semiconductor material (400) comprises depositing a plurality of drops on the intermediate layer (300). 18. A method according to any one of claims 1 to 14, wherein the supply of the third semiconductor material (400) melt on the intermediate layer (300) is carried out by contacting the intermediate layer (300). ) with a melt bath of the third semiconductor material (400). The process according to any of claims 1 to 14, wherein the supply of the third semiconductor material (400) in fusion on the intermediate layer (300) is carried out by placing the donor substrate (100) covered with the intermediate layer (300) in line with a crucible (700) containing the third molten semiconductor material (400) and having a retractable bottom (701), and removing the bottom (701) of the crucible so as to release the third semiconductor material (400) melt on the intermediate layer (300). 20. A method according to any one of the preceding claims wherein after separation of the donor substrate 100 is practiced on the self-supporting wafer (500) a crystallization annealing. 21. Process according to any one of the preceding claims, in which the first, second and third semiconductor materials are chosen from: silicon (Si), germanium (Ge) and silicon-germanium (Si-Ge ). 22. Method according to any one of the preceding claims, wherein the self-supporting wafer (500) has a thickness of between 5 and 1000 micrometers and preferably 10 and 100 micrometers. 23. Method according to any one of the preceding claims wherein the thickness eL of the third semiconductor material (400) provided is determined, so that a thickness eF of the intermediate layer (300) and possibly a portion of the donor substrate (100) melted under the effect of the heat provided by the third semiconductor material (400) is less than the distance between the weakening zone (210) and the face (301) of the intermediate layer (300) ) on which is provided the third semiconductor material (400), wherein the first, second and third semiconductor materials are of the same semiconductor material and wherein the thickness eF is determined by applying the equation following: PL CP (TL-TF) in which eF represents the maximum thickness of material which melts under the effect of heat propagated from the third semiconductor material (400) brought to the liquid state, this the material being taken in the intermediate layer (300) and optionally in the portion of the donor substrate (100); eL represents the thickness of the additional layer formed by the third semiconductor material brought into the liquid state; pL is the density of the liquid semiconductor material; pS is the density of the solid semiconductor material; Cps represents the specific heat capacity of the semiconductor material in the liquid phase; Cp 'represents the specific heat capacity of the solid phase semiconductor material; Lf is the heat of fusion of the semiconductor material; TL the deposition temperature of the semiconductor material brought to the liquid state; TF the melting temperature of the semiconductor material; Ts the temperature of the donor substrate (100) covered with the intermediate layer (300) before supplying the third semiconductor material (400). 24. A method of producing a photovoltaic cell comprising a method for obtaining a self-supporting semiconductor wafer (500) according to any one of the preceding claims. eF = eLPSCP (TF -TS) + PSL