FR2991498A1 - METHOD AND SYSTEM FOR OBTAINING A SEMICONDUCTOR WAFER - Google Patents

Abstract

The invention relates to a method for obtaining a semiconductor wafer from a donor substrate made of a first semiconductor material and comprising in its thickness an embrittlement zone, characterized in that it comprises depositing (320) a second semiconductor material in the liquid state simultaneously over a plurality of zones (310) of a face (101) of the donor substrate; spreading the second semiconductor material in the liquid state to form a layer on said face (101) of the donor substrate; at least partially solidifying the layer of the second deposited semiconductor material to thicken the donor substrate; the separation of the donor substrate at the zone of weakening. The invention also relates to a method for producing a photovoltaic panel and to a system for implementing the method of obtaining the substrate.

Description

TECHNICAL FIELD OF THE INVENTION The present invention generally relates to the manufacture of substrates used by the semiconductor industry, microtechnologies and the photovoltaic industry. It more particularly describes a method for manufacturing substrates made of semiconductor material. The invention also relates to a system for manufacturing substrates made of semiconductor material. STATE OF THE ART In the field of manufacturing 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 under the trade name of "Smart CutTM" which allows to separate thin layers of a semiconductor material from a slice 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 helium, from the surface of a wafer 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, those commonly used in microelectronics, can develop acceleration energies up to 250 keV (kilo electron-volt) that can achieve maximum implantation depths of about 2 pm (pm or micrometer). micron = 10-6 meters).

Then subjected to a suitable heat treatment the slice 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 in the form of micro-cracks which finally induce, under the sole action of the heating or in combination with a mechanical stress, a rupture at the level of the implanted zone thus making it possible to detach a surface layer from the wafer, on a thickness corresponding to the implantation depth, the slice itself, 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 we observe the appearance of blisters or bubbles (exfoliation) of the material of the superficial layer that we try to to separate. The phenomenon of blistering is usually referred to as blistering in English. 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 stick a 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 be smooth without blistering over the entire surface of the wafer 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 field of obtaining substrates by embrittlement of an implanted area 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 layers in a thickness range compatible with the production of photovoltaic cells. This is achieved, however, 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 is closely dependent on that of the semiconductor material substrate and where the device produced must be at an extremely low cost to be competitive, the cost of the implementers at high energies 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 surface treatments after sawing which increases their cost. In addition, they do not make it easy to obtain thin plates, typically between the ten and hundred micrometers. To cope with the problem of excessive material consumption during sawing, other techniques provide for the production of plates of semiconductor material by epitaxial processes, for example in the liquid phase, or by chemical vapor deposition ( usually referred to as CVD, acronym for chemistry or vapor deposition) or molecular beam (usually referred to as MBE, acronym for molecular beam epitaxy). The high cost of the substrate or the slowness of these solutions makes them unsuitable for the industrial production of inexpensive slices.

There is therefore a need to obtain active layers of semiconductor material, in particular for photovoltaic applications, with a thickness preferably of between 10 and 70 microns, according to a low-cost method limiting at least some of the disadvantages that the solutions of the present invention exhibit. state of the art. The other objects, features, and advantages of the present invention will be 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 invention provides a method for obtaining a semiconductor material wafer from a donor substrate in a first semiconductor material and comprising in its thickness a zone of embrittlement, the method comprising: depositing a second semiconductor material in the liquid state simultaneously over a plurality of areas of a face of the donor substrate; spreading the second semiconductor material in the liquid state until a continuous deposited layer is formed on said face of the donor substrate; the at least partial solidification of the layer of the second deposited semiconductor material; the separation of the donor substrate at the zone of weakening. By providing a thin layer donor substrate recrystallization epitaxy method obtained by separating a donor substrate, the present invention provides freely adaptable thickness slices. In particular, slices of greater than 10 micrometers thickness can be obtained. Typically, the invention makes it easy to obtain self-supporting wafers, that is to say easily handled, whose thickness is between 10 and 70 microns, which is particularly advantageous for the production of photovoltaic cells.

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 an epitaxial growth of the given film from the donor substrate on one side of a donor substrate having a weakening zone, the invention allows the use of donor substrates whose weakening zone is located 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 a limited cost compared to known solutions. In addition, the invention makes it possible to obtain plate-shaped slices having large dimensions, typically from 20 cm 2 to 4 m 2, and preferably from 100 cm 2 to 1600 cm 2. Furthermore, and particularly advantageously, the invention provides for at least partial solidification of the deposited material in the liquid state before separation. The solidified layer thus acts as a stiffener on the surface of the donor substrate which prevents or at least reduces the risk of blistering of the latter. Separation means at least a microcracks propagation step in the implanted zone. Without this being limiting, this step may however go as far as detachment, i.e. a physical distance between the donor substrate and the slice obtained. Additional means, for example mechanical, can be implemented to achieve this detachment. The invention provides other 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 obtained, in particular by causing blistering on its surface. The invention, by providing localized inputs in various areas of small amounts of liquid material, and therefore small amounts of thermal energy per unit area, avoids excessive heating that can induce blistering of the donor substrate.

Advantageously, the method according to the invention may have at least any of the optional features and steps below. Deposition of the material in the liquid state is effected by pouring over a plurality of zones of the donor substrate. Depositing a second semiconductor material in the liquid state on a plurality of areas of the donor substrate comprises depositing a plurality of drops of the second semiconductor material on the donor substrate. The drops are deposited simultaneously on the donor substrate. Alternatively or cumulatively to the deposition of drops, the deposition of the second semiconductor material in the liquid state on a plurality of zones of the donor substrate comprises the deposition of a plurality of lines or streaks of the second semiconductor material on the donor substrate, which spread on the donor substrate to cover it.

Advantageously, the solidification is controlled so that the layer of the second semiconductor material forms a stiffener on said face of the donor substrate before said separation. More specifically, the quantity of the deposited semiconductor material, the temperature of the deposited semiconductor material, the temperature of the donor substrate, as well as the depth of the embrittlement zone are chosen so that the at least partial solidification of the semi-conducting material conductor occurs before sufficient heat to cause blistering to reach the weakening zone. Thus, the layer of deposited material acts as a stiffener, which prevents blistering on the surface and facilitates the separation at the weakening zone.

The solidification on the donor substrate is preferably effected by epitaxy, because of the heat transfer from the second semiconductor material in the liquid state to the donor substrate. Thus, the invention does not require any specific intervention to achieve the solidification.

Advantageously, the quantity of the second deposited semiconductor material, the temperature of the second deposited semiconductor material, the temperature of the donor substrate, as well as the depth of the embrittlement zone are chosen so that, following the deposition of the second semi material, conducting a liquid state simultaneously on a plurality of zones of the donor substrate, at least a portion of the heat supplied by the second semiconductor material in the liquid state propagates in the donor substrate to the zone of weakening , to participate in or to cause separation of the donor substrate at the zone of weakening. According to one embodiment, the separation of the donor substrate at the weakening zone is obtained solely by adding heat from the deposition of the second semiconductor material on said face of the donor substrate. Thus, the separation is obtained during the solidification of the second semiconductor material or at the end of the solidification and without additional intervention. Alternatively, the separation of the donor substrate at the weakening zone is obtained after the solidification and during an additional step comprising a heat treatment step and / or a mechanical step performed so as to apply a mechanical separation stress. The separation of the donor substrate results in the production of a self-supporting wafer of semiconductor material, the wafer obtained comprising a portion of the donor substrate and a layer formed by the deposited semiconductor material. Preferably, the process comprises, beforehand, a step of obtaining the semiconductor donor substrate comprising in its thickness an embrittlement zone, this obtaining step comprising the implantation of ionic species into the thickness of the donor substrate, using for example an implanter.

Preferably, the zone of weakness is formed within 5 micrometers of the face of the substrate on which the second semiconductor material is deposited in the liquid state and more preferably less than 3 micrometers. It is advantageously formed at about 2 micrometers from the surface, thanks to conventional implants. Preferably, the deposition is carried out so that the spreading of the second semiconductor material in the liquid state forms a layer of substantially constant thickness over the entire face of the donor substrate.

Thus, the pouring is performed so as to obtain a homogeneous distribution of the second semiconductor material on the donor substrate before solidification. Preferably, the layer covers the entire surface of the donor substrate. The pouring is performed so that the second semiconductor material poured over a plurality of areas of the donor substrate spills out to form a continuous surface of second semiconductor material. According to this option, all drops or streaks meet. Preferably, the pouring of the second semiconductor material over a plurality of areas of the donor substrate is carried out so that it spreads to cover the entire face of the donor substrate.

Advantageously, the layer formed by the second semiconductor material on the face of the donor substrate has a thickness of between 5 μm and 70 μm and preferably between 10 μm and 50 μm to obtain a manipulable wafer without melting the zone of embrittlement and preferably between 10 pm and 25 pm. This thickness is measured in a direction perpendicular to the face on which is deposited the layer of semiconductor material in the liquid state. Advantageously, the donor substrate has a crystalline structure at least at the level of said face on which the second semiconductor material is deposited and the solidification reproduces at least in certain zones the crystalline structure of the donor substrate.

Preferably, the donor substrate is monocrystalline or polycrystalline coarser, greater than 10 pm, typically of the order of a few tens of micrometers. More generally, the at least partial solidification of the semiconductor material deposited on the donor substrate reproduces the structure of said donor substrate. According to one embodiment, the first semiconductor material is silicon (Si), germanium (Ge) or silicon-germanium (Si-Ge) and the second semiconductor material is silicon (Si), germanium (Ge) or silicon-germanium (Si-Ge). More generally, the first semiconductor material and / or the second semiconductor material are taken from one of the type III-V composite semiconductor materials having a melting point of less than 2000 ° C.

Advantageously, the second semiconductor material in the liquid state is silicon having a temperature during deposition of between 1450 ° C. and 1700 ° C. and preferably between 1480 ° C. and 1650 ° C. and the donor substrate is made of silicon and has a temperature between 25 ° C and 450 ° C and more preferably between 150 ° C and 350 ° C during the deposition of the second semiconductor material in the liquid state. According to another aspect, the subject of the invention is a method for producing a photovoltaic cell comprising a method for obtaining a semiconductor wafer according to the invention. The present invention also relates to a method for obtaining a wafer made of a semiconductor material from a donor substrate comprising at least one face made of a first semiconductor material, characterized in that it comprises: depositing a second semiconductor material in the liquid state on a plurality of areas of said donor substrate face to form a layer on said donor substrate face; solidifying the second semiconductor material deposited on the donor substrate to thicken the donor substrate with a layer of deposited second semiconductor material. Optionally, the method also includes a subsequent step of detaching the donor substrate.

In a non-limiting embodiment, the semiconductor donor substrate comprises in its thickness an embrittlement zone containing implanted ionic species and the detachment of the donor substrate is carried out at the zone of weakening. Preferably, during deposition, the second semi-conductive material spreads on the face of the donor substrate to form said layer. The present invention also relates to a system for carrying out the method according to the invention comprising: a crucible; a support 15 disposed under the crucible and adapted to receive a substrate having a plate shape; the crucible having orifices for allowing the pouring on the substrate carried by the support of a liquid contained in the crucible, two adjacent orifices being separated from each other by a distance of between 5 and 50 mm and the minimum dimension an orifice being between 150 pm and 2 mm. Optionally, the system according to the invention may have at least any of the optional features below. Preferably, two adjacent orifices are separated from each other by a distance of between 20 and 40 mm and the minimum dimension of an orifice is between 180 μm and 500 μm. Advantageously, at least some of the orifices are holes of circular section, the minimum dimension being a diameter of the hole. The orifices may be arranged for example in line in the two directions of spreading or have a more compact arrangement (eg honeycomb). Alternatively or cumulatively, at least some of the orifices each form a line and the minimum dimension of these orifices is a width of the line. Advantageously, the system also comprises heating elements configured to heat the crucible and / or to heat the support.

Advantageously, the orifices are distributed over an area of between 20 cm 2 and 4 m 2, and more particularly between 100 cm 2 and 1600 cm 2. Such dimensions include square (or pseudo squares) wafers of -243 cm2 (156 * 156 mm), or circular wafers of -20 cm2 (50mm diameter plate) and -1963cm2 (500mm diameter plate), and more particularly between -79cm2 (plate of 100mm of diameter) and -707cm2 (plate of 300mm of diameter). Advantageously, the distance between the support and the orifices is between 5 centimeters and 1 meter and more preferably between 10 centimeters and 25 centimeters. Advantageously, the system comprises an enclosure configured to create a depressurization downstream of the orifices and / or means for creating an overpressure upstream of the orifices. Upstream and downstream are defined with respect to the path of the second semiconductor material in the liquid state.

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 illustrate the steps of an exemplary method according to the invention for obtaining thin self-supporting plates used in particular for the production of photovoltaic cells. FIGURES 2a and 2b illustrate a particular example of implementation of the method of the invention and an example of a system used for this implementation. 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 in an ingot. It is in the form of a plate having two parallel faces. 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 of the substrate through which the ions are implanted, this distance being measured in a direction perpendicular to said face. Under the action of a heating, possibly combined with a mechanical stress, will cause separation of the substrate at the weakened zone and the detachment of the wafer. Slices are not necessarily circular or square. The plate has, in view from above, that is to say in a vertical direction comprised in the plane of FIGS. 2a to 2e, any shape and most generally a form of disk, square, rectangle, hexagon or that of other polygons.

In the vocabulary commonly used in the microelectronics and nanotechnology industry and the photovoltaic industry, a slice, as described above, is also sometimes referred to as a plate or wafer, or even a wafer, a term borrowed from English with 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 detachment from a donor substrate are called slices.

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 those 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 non-perfectly crystalline material but having large crystallized grains is suitable for many applications. Typically, for an application to photovoltaic cells, the lateral 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. FIGS. 1a to 8b illustrate the steps of the method of the invention which consists in producing thin self-supporting slices, for example of silicon (Si), by a collective and coordinated deposition technique of drops of liquid silicon on a semiconductor donor substrate. , for example silicon, in which one has previously implanted from one of its surfaces, light ions, for example hydrogen and / or helium alone or in combination. Such an implantation technique used for the production of SOI substrates has been briefly described in the chapter on the state of the art. The main steps are as follows: FIG. 1a shows a substrate 100 of a semiconductor material in which light ions 210 are implanted from one of its faces 101 using an ion beam 200 generated by an implenter. Preferably, the implanter is a standard implanter used by the microelectronics industry. Implantation is carried out at a depth 111 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 of a few hundred keV for implementers who do not use high implantation energies. FIG. 1b shows the formation of drops 310 of liquid silicon from nozzles 300. The drops are formed at a Tdropplet temperature and are presented near the implanted face 101 of the donor substrate 100 which is maintained at a Tsubstrate temperature. less than Tdropp, and. FIG. 1c illustrates the drop 320 of the drops 310 of liquid silicon which are dropped or deposited on the face 101 of the substrate 100. FIG. 1d shows the spreading 330 of the deposited silicon drops. By spreading, the drops 310 meet and form a layer 340 on the surface 101. During this operation, the layer 340 solidifies due to the heat transfer of the drops to the substrate 100, the temperature of which is lower than that of the liquid material constituting the drops. FIG. 1e illustrates the separation 220 of the donor substrate 100, at the zone 210 weakened by the implantation of light ions, under the action of the heat provided by the drops. The implanted zone breaks, thereby detaching the layer 340 and the underlying layer 110 that has become integral to form a self-supporting wafer 360 obtained from the donor substrate 100. The substrate 100 can be reused immediately for the same operation. The underlying layer 110 can thus be described as a given film. The invention is not limited to the use of nozzles and sprays, nozzles, capillaries, or other orifices may be used. In general, the orifices make it possible to deposit semiconductor material in multiple zones on the surface of the implanted face 101 of the donor substrate 100. The spreading of these multiple deposits then forms a continuous layer 340 of semi-material -conductor deposited. In the main steps of the process described briefly above, it is necessary to make the following clarifications: The deposition of the drops 310 is advantageously collective to produce a continuous layer 340. The coordination of the deposit prevents any premature evolution of the implanted area in the vicinity of drops that would be deposited too early. Nonhomogeneous heat transfer laterally, that is to say in a direction parallel to the main face of the donor substrate on which the drops are deposited, would lead to a disparate modification of the embrittlement zone. The separation at the zone of weakening 210 would not be performed correctly. In addition, a non-homogeneous distribution of the drops would favor the blistering described in the chapter on the state of the art. The simultaneous deposition of all the drops prevents the appearance of these damaging phenomena. As the detachment of the drops 320 of the capillary nozzles 300 or of a pierced crucible is difficult to achieve by a valve-type control because of the dimensions involved, the volume of liquid is preferably checked, as represented in FIG. 1b. in the bath 350 which supplies the nozzles 310 directly. In practice, it is sufficient to control the height of liquid in this bath, the section being fixed by the dimensions of the deposition system. Preferably and optionally, the addition of a quantity of liquid from another source, for example a second bath (not shown in FIG. 1b), makes it possible to increase the weight on the drop at the end of the capillary nozzle to its detachment 320. In also optional implementations of the invention the detachment of the drops is obtained or is favored by applying an overpressure above the bath 350 and / or by placing the substrate 100 and the outlet nozzles in a confined space in depression. The development of the method of the invention, the steps of which have just been described briefly above, requires that from a physical point of view the concurrent kinetics of three phenomena be adjusted to obtain a section 360 comprising the deposited layer 340 and the given film 110. This resulting layer 360 has a thickness 111 and characteristics adapted to the application in question, for example to the production of wafers for the manufacture of photovoltaic cells. The physical phenomena in competition are: the spreading of the drop; solidification of the liquid silicon deposited and embrittlement of the implanted zone under the effect of heat propagated in the donor substrate. Concretely, it is necessary that at the end of the development of the process, one can be assured: - that the liquid will spread sufficiently before solidifying by resumption of epitaxy in liquid phase from the layer preferably monocrystalline donor substrate; the deposition of the liquid silicon does not cause the temperature of the implanted substrate to rise too much so that the separation in the implanted zone of the substrate is possible subsequently to the deposition; - A sufficient thickness of silicon is solidified to act as a stiffener so as to prevent the occurrence of blistering phenomenon described above. It is indeed necessary that the thickness of the layer provided is sufficiently large compared to the depth of the embrittlement zone so that this layer absorbs the mechanical stresses that may appear by heat input into the substrate so that these constraints do not occur. do not relax on the surface of the donor substrate to form blisters. The invention responds to these points by means of a method allowing the parallel deposition of a plurality of millimeter-sized drops on the substrate which has been previously weakened, thereby combining the advantages of "Smart Cut" technology and the epitaxy in the liquid phase. This is achieved by making a judicious choice of the key parameters of the process which must together achieve a compromise between the various mechanisms involved, those already mentioned above, that is to say: heat transfer between the hot parts and cold, kinetics of spreading and solidification of the drops, kinetics of embrittlement by cracking in order to achieve detachment of a thin plate after spreading and solidification of the deposited drops. The choice of parameters, some of which are interdependent, is made according to three main criteria which are explained below: - The first criterion concerns the size of each individual deposit on the surface of the donor substrate. The minimum size of each deposit, whether a drop as in the embodiment described with reference to the figures, or that it is for example trailing, is determined by a minimum dimension of orifice allowing the liquid semiconductor material to be deposited to escape. In the case of drops, the minimum dimension is the diameter of the hole, the nozzle or the capillary through which the drop 310 escapes to spread over the donor substrate 100. In what follows, and without this being limiting of the invention, reference will be made to a deposit of a drop 310 through a nozzle 300 as illustrated in the figures. At the moment of detachment, the relationship between the radius of the drop rg and the radius of the nozzle rc is given by Tate's law (neglecting the corrective factor F said of Harkins close to unity for capillaries or submillimeter nozzles): mg = 27Crc6 (a) or 6 represents the surface tension of the silicon, g the gravity and m the mass of the drop: m = p (4/3) 7c rg3 (b) p is the density of the silicon. This law specifies a balance between surface tension and the weight of the drop. It therefore comes: rg = (36 / 2pg) 1/3 rc1 / 3 By conventional machining methods, it is conceivable to make nozzles with a minimum radius of 100 μm. Thus, according to (c), for a nozzle of minimum diameter, a drop of 1.7 mm radius is obtained. - The second criterion concerns the distribution of the orifices (the nozzles in the illustrated example) as a function of the size of the drops and the thickness of the deposited layer. The choice of drop size parameters, nozzle arrangement and gap between the nozzles 300 should be made jointly from the desired wafer thickness which is typically between 5 μm and 50 μm for PV applications. Indeed, if we consider that the volume of a drop of radius rg should allow to create a layer of constant thickness eg on a surface of the square 2, where the square is the gap between adjacent nozzles, we can write: In the case where the nozzles are arranged to form a simple 'square lattice', the relation (1) makes it possible to determine the torque: size of the drop, gap between the nozzles. In an optimized configuration the nozzles are advantageously arranged to form a compact arrangement so that the spreading of the drops, in a cylindrical symmetry, rapidly covers the largest (1) possible surface. The coverage rate of the surface before contact between two drops may exceed 90% of the surface before filling the remaining interstices between the drops. In this optimized configuration, the nozzles form an array of equilateral triangles on the LneXa side and the relation between the drop radius rg and the thickness of the desired layer eg is then given by: 2 3 ~ rg = -g eg Lhexa In the In the case of this compact arrangement of the nozzles, the expression (2) allows the skilled person to adjust the above three parameters which are related. In the same way, for any other configuration of the nozzles these three parameters must be considered together, taking into account the equality of the volume of the drop and the fraction of layer that it must form. Thus, by adopting the compact arrangement with a gap of 3 cm Lhexa nozzles, the deposition of drops with a minimum radius of 1.7 mm provides a layer of 26.4 microns thick. - The third criterion concerns the melted thickness on the surface of the donor substrate and the size of the drops. Deposition by drop has the major advantage of providing a locally limited heat input. Indeed, a massive supply of molten material, and therefore of heat, could cause the fusion of the substrate beyond the implanted zone, making separation impossible at the level of the implanted zone. With the method of the invention a judicious choice of thermal and dimensional parameters allows to melt a substrate thickness lower than the implantation depth. Upon the impact of the drop on the surface of the donor substrate the excess of heat energy OQg of the drop relative to its melting temperature will be transmitted to the surface of the substrate in different forms that can be written: tQg-Qs + QTF + Qpertes (3) Qs is the heating energy of the substrate from its initial temperature (before deposition) TS up to the melting temperature of silicon TF, QTF is the heat required for the phase transformation (melting) of the melted thickness of the substrate e1. 4 (2) Qpertes represents the heat dissipation due to heat flows leaving the molten silicon volume (drop + thickness of molten substrate). The equilibrium described by equation (3) can be detailed as follows considering that the melted zone is at the melting temperature of silicon TF: 3rgJPLCP (Tg-TF) = pse1SCP (TF -Ts) + Ps Lfe1S + Qpertes (4) pi_ is the density of liquid silicon, pS the density of solid silicon, Cp the specific heat capacity of silicon (solid and liquid), Lf the heat of fusion of silicon, S the surface on which a drop spreads. Because of the observed rapidity of drop spreading, which is less than one hundredth of a second, and the melting of the superficial layer of the donor substrate, it is assumed as a first approximation that the thermal losses Qpexts which s' conduct either by conduction in the substrate or by radiation in the enclosure are weak. It should be noted, however, that these losses can nevertheless reduce the depth of molten substrate. By neglecting them, and considering that the drop spreads on the surface S to form a layer of thickness eg one can write: (eg S) pL Cp-1 "(Tg - TF) = Ps er S CP (TF - T5) + Ps Lf er S (5) Which makes it possible to obtain the ratio between the thickness of the layer deposited on the melted layer e, eg Ps Crs (TF -Ts) + Ps Lf ej PL CP (Tg -TF) (6) Depending on the thickness of the desired deposited layer, one skilled in the art knows how to deduce from the expression (6) the thickness of the theoretically melted substrate as a function of the thickness of the deposited drops. This value constitutes the upper limit of the thickness actually melted because of the thermal losses which have been neglected as seen above It should be remembered here that the melted thickness on the surface of the donor substrate must always be lower at the implantation depth, considering a drop at a temperature of 1500 ° C, which comes into contact with a silicon substrate at 300 ° C, the melted thickness will be 27.3 times lower than that deposited. Thus a deposited layer with a thickness of the order of 27 μm produces the melting of the surface of the donor substrate to a thickness of one micrometer. The parameters mentioned above are in this case: Lf = 1650 J / g; pL = 2.52 g / cm3; ps = 2.32 g / cm3; LC = 1.04 J / g.K-1; CPS = 0.9 J / g.K1; Tm = 1414 ° C.

The parameters involved in the implementation of the invention are preferably included in the range of values indicated below: the implanted species are light ions, in particular hydrogen at a dose of some 10 16 H + / cm 2 with or without helium, and with or without co-implantation of a species other than hydrogen or helium. The implantation energy is between 60 keV and 350 keV, and is preferably greater than 180 keV. For example, in the case of implantation of hydrogen ions alone, the implanted dose is between 2x1016 H + / cm2 and 1.2x1017 H + / cm2, preferably between 3x1016 H + / cm2 and 1.0x1017 H + / cm2 and better still between 4.5x1016 and 8x1016 H + / cm2. With energies of the order of 250keV, the maximum depth of implantation, corresponding to the maximum concentration of the implanted species, is thus of the order of 2.5 μm.s - the minimum temperature of the drops must be greater than at the melting temperature of the silicon to allow the drop to spread over the surface of the substrate before solidifying. The maximum temperature of the drop should not allow to melt a substrate thickness equal to or greater than the implantation depth as required by the third criterion discussed above. The temperature of the drops is therefore between 1450 ° C. and 1700 ° C., preferably between 1480 ° C. and 1650 ° C. - The maximum temperature of the substrate before contact with the drops should not allow the implanted area to develop significantly and especially should not allow the appearance of blisters. This temperature is controlled and between 25 ° C and 450 ° C, more preferably between 150 ° C and 350 ° C. This range of temperature higher than the ambient temperature has the advantage of facilitating the spreading of the drops. - The volume and the gap between the drops must allow sufficient spreading so that two neighboring drops on the substrate come into contact before solidification. The gap between the drops / nozzles is of the order of one centimeter, and can be advantageously determined by the second criterion described above. In the case of the use of nozzles, the inner diameter of the nozzles for depositing the drops is between 100 pm and 2 mm, preferably between 180 pm and 500 pm. As discussed above with the definition of the first criterion the size of the drops from such nozzles is determined by the law of Tate. It is typically in a range of 1.5 mm to 3 mm radius. As seen with the definition of the third criterion, the size of the drops must also be smaller than that producing the melting of a substrate thickness greater than the implantation depth. In addition, the deposition configuration must allow partial or complete solidification of the layer formed after spreading of the drops and before breaking of the implanted zone, that is to say that at least a thickness of 7 μm must be solidified before separation, preferably 10 μm, and even more preferably 20 μm must be solidified to fully play the role of stiffener.

In addition, for photovoltaic applications, the thickness of the slice obtained must be sufficient to act as a light absorber. That is, it should be between 5 pm and 70 pm, and preferably between 10 pm and 40 pm. - The method of the invention is also characterized in that the distance between the drop formation zone and the substrate is between 5 centimeters (cm) and 1 meter, preferably between 10 cm and 50 cm, more preferably between 10 cm and 25 cm. This distance makes it possible to avoid bursting of the drops in contact with the substrate by promoting their spreading, and to propose an ideally feasible configuration.

It should also be noted that the invention can be applied to other materials capable of forming a layer by epitaxy in the liquid phase. The silicon drops may be replaced by germanium to form a layer of this material. In this case, the temperature of the drops before deposition is modified to be between 939 ° C and 1600 ° C, preferably between 950 ° C and 1150 ° C. The ion implantation is also carried out at a dose of some 1016 H + / cm 2 in the case of hydrogen ions alone, for example a dose of 6 × 10 16 H + / cm 2 with an implantation energy of 180 keV.

The invention can also be implemented with a part of III-V composite semiconductor materials having a sufficiently low melting point, that is to say less than 2000 ° C, to be deposited at the same time. using nozzles. By way of example, mention may be made of indium arsenide (InAs), indium phosphide (InP), gallium phosphide (GaP), gallium antimonide (GaSb), gallium arsenide (GaAs). Figures 2a and 2b illustrate a particular example of implementation of the method of the invention and a system example used for this implementation.

FIG. 2a schematically shows the components of a standard implanter of the type used by the microelectronics industry which is intended to generate a stream of ions 200. The ions are extracted from a source 201 from a extractor 202 and separated according to their mass using an electromagnet system 203 to keep only those intended to be implanted. An acceleration and focusing of the selected ion beam is then performed 204. The beam is deflected 205 to scan the entire surface of the target which is constituted, in this particular example, a monocrystalline silicon substrate 100 having a diameter of 100 millimeters (mm) and a thickness of 525 pm. The substrate is implanted with a dose of 5 × 10 16 H + / cm 2 of ions having an energy of 210 keV. The native surface oxide layer was first removed by a hydrofluoric acid chemical treatment diluted to 50% concentration. The implanted substrate 100 is then placed in a system 500 for implementing the invention. This system 500 has a chamber 400 under secondary vacuum of the order of 5x10-5 millibar containing, on the one hand a support 410 adapted to receive the donor substrate 100. Preferably, the support 410 comprises a heating element arranged to control the temperature of the donor substrate 100 before deposition, typically around 250 ° C for a silicon substrate. The system 500 also comprises a drop deposit device 420. The latter consists of a vessel also designated crucible 430, preferably dense isotropic graphite coated with a thin layer of silicon carbide. The crucible 430 is pierced with a network of orifices 440 also designated openings, for example circular.

In this example, the network of orifices 440 is equipped with nozzles 300, with a diameter of 200 ± 10 μm. This network, which extends over a surface equivalent to that of the substrate 100, is arranged to form a compact drop formation having an inter-orifice distance of 30 mm. These orifices 5 act as spray nozzles. The underside of these orifices is advantageously coated with a layer of boron nitride to limit the wetting of the drop on the periphery of the orifice, and in fact the size of the drops. The silicon is melted at a temperature of 1500 ° C in a first crucible 450 feeding the crucible 430 pierced with a network of spray nozzles 10 placed at a height 460 15 cm from the substrate. The height of molten silicon 350 in the pierced crucible 440 is slowly increased by a supply 452 of molten silicon from the first crucible 450 to allow the detachment of a matrix of drops 310, each of which comes from a nozzle . In contact with the substrate 100 heated to 250 ° C., the silicon drops spread out until they come into contact and solidify, effectively forming a continuous layer (not shown). In such a configuration the drops have a diameter of the order of 1.7 mm which leads to a layer with a thickness of the order of 26 pm. The molten silicon thickness of the substrate before recrystallization does not exceed one micrometer. Thus, the implanted zone 210 is not damaged, and the heat diffusing drops towards the substrate weakens the implanted zone until it leads to the separation and detachment of the epitaxial layer on its seed formed by the given film 110. The method described above thus meets the objectives of the invention. It potentially has high productivity while being economical in the first case by taking each cycle only a thin surface layer of a donor substrate serving as seed during the solidification of the substrate to be produced. It allows the use of conventional implanters widely used by the microelectronics industry. The process is easily adaptable. For example, if the thermal contribution provided by the drops is insufficient to cause separation at the weakened zone of the donor substrate, additional heat treatment at a Trecuit temperature and / or mechanical biasing may be practiced to induce fracture and fracture. detachment of the slice.

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 the figures provides for the delivery of the semiconductor material in the form of drops, the invention extends to all forms of pouring of the semiconductor material. For example, and depending on the configuration of the system implementing the method according to the invention, lines may be poured onto the donor substrate in an alternative manner or combined with the pouring of drops, the objective being to obtain a homogeneous layer without temperature gradient at the surface of the donor substrate. From the foregoing, it is clear that the present invention proposes a particularly simple and inexpensive method for obtaining a slice of semiconductor material whose thickness can be freely chosen (typically between a few micrometers and the hundred micrometers) and of large lateral dimension (typically several hundred cm2). The invention is particularly advantageous for producing dedicated wafers for the production of photovoltaic cells, without being limited to this application.

Claims (21)

REVENDICATIONS1. A method for obtaining a wafer made of a semiconductor material from a donor substrate (100) of a first semiconductor material and comprising in its thickness an embrittlement zone (210), characterized in that it comprises depositing (320) a second semiconductor material in the liquid state simultaneously over a plurality of zones of a face (101) of the donor substrate (100); spreading the second semiconductor material in the liquid state until forming a layer (340) on said face (101) of the donor substrate (100); at least partial solidification of the layer (340) of the second deposited semiconductor material for thickening the donor substrate (100); the separation of the donor substrate (100) at the weakening zone (210). The method of claim 1, wherein depositing the second semiconductor material in the liquid state on a plurality of areas of the donor substrate (100) comprises depositing a plurality of drops (310) of the second semi material. -conductor on the donor substrate (100). A method according to any one of the preceding claims, wherein the solidification is controlled so that at least a portion of the layer (340) of the second semiconductor material forms a stiffener on said face (101) of the donor substrate ( 100) before said separation. 4. A method according to any one of the preceding claims, wherein the amount of the deposited second semiconductor material, the temperature of the second deposited semiconductor material, the temperature of the donor substrate (100), and the depth of the area. embrittlement (210) are chosen so that following deposition, heat supplied by the second semiconductor material in the liquid state propagates in the donor substrate (100) to the weakening zone (210), to participate in or to cause separation of the donor substrate (100) at the weakening zone (210). 5. Method according to any one of the preceding claims, wherein the separation of the donor substrate (100) at the weakening zone (210) is obtained solely by heat input from the deposition of the second semiconductor material on said face. (101) of the donor substrate (100). 6. Method according to any one of claims 1 to 4, wherein the separation of the donor substrate at the weakening zone is obtained after solidification and by an additional step comprising a heat treatment step and / or a mechanical step performed so as to apply a mechanical stress. 7. A method according to any one of the preceding claims, wherein the deposition is carried out so that the spreading (320) of the second semiconductor material in the liquid state forms a layer (340) of substantially constant thickness on all of said face (101) of the donor substrate (100). A method according to any one of the preceding claims, wherein the layer (340) formed by the second semiconductor material on the face (101) of the donor substrate (100) has a thickness of between 5 μm and 70 μm. and preferably between 10 pm and 50 pm. 9. Method according to any one of the preceding claims, wherein the donor substrate (100) has a crystalline structure at least at the level of said face (101) on which is deposited the second semiconductor material and in which the solidification reproduces at least in certain areas the crystalline structure of the donor substrate (100). 10. Method according to the preceding claim, wherein the donor substrate (100) is monocrystalline. 11. A method according to any one of the preceding claims, wherein the first semiconductor material is silicon (Si), germanium (Ge) or silicon-germanium (Si-Ge) and wherein the second semiconductor material is conductor is silicon (Si), germanium (Ge) or silicon-germanium (Si-Ge). The method according to any one of claims 1 to 10, wherein the first semiconductor material and / or the second semiconductor material are taken from one of the III-V composite semiconductor materials having a melting below 2000 ° C. 13. Process according to any one of the preceding claims, in which the second semiconductor material in the liquid state is silicon having a temperature during deposition of between 1450 ° C. and 1700 ° C. and preferably between 1480 ° C. and 1650 ° C. ° C and wherein the donor substrate (100) is silicon and has a temperature between 25 ° C and 450 ° C and more preferably between 150 ° C and 350 ° C during the deposition of the second semiconductor material in the liquid state . 14. A method of producing a photovoltaic cell comprising a method for obtaining a semiconductor wafer according to any one of the preceding claims. 15. System for carrying out the method according to any one of claims 1 to 13 comprising: - a crucible (350), - a support (410) disposed under the crucible (350) and adapted to receive a substrate (100). ) having a plate shape (100), - the crucible (350) having orifices (440) for allowing the substrate (100) carried by the support (400) to be poured on a liquid contained in the crucible (350) two adjacent ports (440) being separated from each other by a distance of between 5 and 50 mm and the minimum dimension of an orifice (440) being between 100 μm and 2 mm. 16. System according to the preceding claim, wherein two orifices (440) adjacent are separated from each other by a distance of between 20-28 and 40 mm and in which the minimum dimension of an orifice is between 180 pm and 500 pm. 17. System according to any one of the two preceding claims, wherein at least some orifices (440) are holes of circular section, the minimum dimension being a diameter of the hole. 18. System according to any one of the three preceding claims, wherein at least some of the orifices each form a line and the minimum dimension of these orifices is a width of the line. 19. System according to any one of the four preceding claims, also comprising heating elements configured to heat the crucible (350) and / or to heat the support (410). 20. System according to any one of the six preceding claims, wherein the distance between the support (410) and the orifices (440) is between 5 centimeters and 1 meter and more preferably between 10 centimeters and 25 centimeters. 21. System according to any one of the preceding claims, comprising an enclosure configured to create a depressurization downstream of the orifices and / or means for creating an overpressure upstream of the orifices.