Abstract:
The invention concerns a method for making substrates, in particular for optics, electronics or optoelectronics. The method includes an operation which consists in implanting ( 100 ) atomic species beneath the surface of a material in the form of a cylindrical ingot ( 1 ), at a depth of implantation distributed about a certain value by bombardment of the atomic species on a zone of the ingot ( 1 ) cylindrical surface, and an operation which consists in removing ( 300 ), at a separation depth located proximate to the depth of implantation, the layer ( 2 ) of material located between the surface and the separation depth, to remove the layer ( 2 ) from the rest of the cylindrical ingot ( 1 ).

Description:
This is a nationalization of PCT/FR01/00976, filed Apr. 2, 2001 and published in French. 

   BACKGROUND ART 
   The invention relates to the fabrication of substrates, in particular for optics, electronics or opto-electronics. 
   Substrates for use in the aforementioned fields are generally obtained industrially by cutting up ingots. In the case of monocrystalline silicon, for example, the ingots are obtained from a bath of molten silicon by the Czochralski drawing method (referred to hereinafter as CZ drawing) or from a polycrystalline ingot by the zone fusion method (referred to hereinafter as FZ drawing). These growing methods produce cylindrical ingots which are then cut into slices perpendicular to the axis of the cylinder, generally using an internal cut circular saw. 
   However, the above methods do not produce substrates with satisfactory dimensions for some applications. This applies in particular in the field of fabrication of large substrates that can be used to make flat or other shape display screens or solar cells. 
   To obtain larger monocrystalline silicon substrates, the document FR 2 752 768 proposes cutting ingots parallel to their longitudinal axis. 
   BRIEF SUMMARY OF THE INVENTION 
   One object of the invention is to propose another way of fabricating substrates from ingots. 
   Another object of the invention is to obtain substrates of a monocrystalline material, such as monocrystalline silicon, for example, with lower production costs than with prior art methods. 
   That object is achieved by a method of fabricating substrates, in particular for optics, electronics or opto-electronics, the method being characterized in that it includes: 
   an operation of implanting atomic species under the surface of a material in the form of a cylindrical ingot at an implantation depth distributed around a particular value by bombarding said atomic species onto an area of the cylindrical surface of the ingot, and 
   a detachment operation at a detachment depth in the vicinity of the implantation depth of the layer of material between the surface and the detachment depth to detach that layer from the remainder of the ingot. 
   Thus in accordance with the invention a substrate is obtained by peeling off the surface layer of a cylindrical ingot parallel to the axis of the cylinder. 
   It must be understood that the term “substrate” is used throughout this text in the widest sense of the term, in other words to designate either an element of material able to serve as a support for another element or a thick or thin, rigid or flexible, etc. film or layer. 
   The terms “cylinder” and “cylindrical” must also be understood in their primary sense. In this sense, a cylinder is a solid body generated by a straight line that moves parallel to itself along the surface of a curve. In this text a cylinder can therefore have a round cross section or a polygonal cross section. 
   The method according to the invention advantageously enables continuous fabrication of substrates. 
   Because the method according to the invention can be implemented continuously, it increases the productivity of the fabrication of substrates and therefore reduces production costs. The method is particularly beneficial if it is required to fabricate monocrystalline silicon substrates at low cost, for example. 
   The method according to the invention has the following advantageous features, independently or in combination:
         the implantation of the atomic species is effected by continuously bombarding the cylindrical surface of the ingot with a localised beam that is swept in the longitudinal direction of the ingot while the ingot rotates about an axis parallel to the cylindrical surface;   the implantation of the atomic species is effected by continuously bombarding the cylindrical surface of the ingot with a beam of elongate cross section corresponding to a given area while the ingot rotates about an axis parallel to the cylindrical surface;   the implantation of the atomic species is effected by bombardment corresponding to a given area of successive zones adjacent the cylindrical surface of the ingot while the ingot rotates about an axis parallel to the cylindrical surface of the ingot between each bombardment and the next;   the implantation of the atomic species is effected by continuously bombarding the whole of the cylindrical surface of the ingot, for example by immersing the ingot in a plasma;   it further includes an operation of heating the cylindrical surface; the heating operation can be carried out before, during or after implantation; the heating operation can be carried out by the implantation itself; the heating operation reduces the necessary dose of atomic species implanted and/or encourages in situ healing of some implantation defects;   it includes an operation of transferring the layer of material between the cylindrical surface of the ingot and the detachment depth onto a support;   it includes an operation of pressing the layer of material between the cylindrical surface of the ingot and the detachment depth by means of a roller; this pressing operation causes a thermal shock if the roller is cooled or heats the ingot if it is heated and/or applies mechanical pressure and/or shear stresses to encourage and/or cause detachment from the ingot of the layer of material between the cylindrical surface of the ingot and the detachment depth;   the support is adhesive;   it includes an operation of covering the layer of material between the cylindrical surface of the ingot and the detachment depth with a liquid phase or gas phase deposit;   the material is silicon;   the atomic species comprise hydrogen ions;   the atomic species comprise doping ions such as phosphorus, arsenic or boron ions;   it includes operations of applying a layer to each face of the layer of material between the cylindrical surface of the ingot and the detachment depth by rolling those layers between rollers;   it includes an operation of transferring at least one layer comprising circuitry patterns onto the layer of material between the cylindrical surface of the ingot and the detachment depth.       

   For the fabrication of display screens and solar cells in particular, amorphous or polycrystalline silicon deposited on a glass substrate is often used, because monocrystalline silicon can be deposited on glass, on quartz, etc. at present only by layer transfer techniques using a monocrystalline silicon substrate to form the layer that is transferred onto the glass substrate. The diameter of monocrystalline silicon substrates is limited at present to 200 mm or even 300 mm. 
   Using the method according to the invention, it is possible to fabricate larger monocrystalline silicon substrates. Even if the monocrystalline silicon substrates obtained with some variants of the method according to the invention do not have a perfect crystalline orientation, they have better qualities than amorphous or polycrystalline silicon. Thus when they are used to make flat screens they offer an improvement in terms of integration density (number of pixels per unit surface area), screen refresh rate, etc. When they are used to make solar cells they increase the efficiency of photo-electric conversion. 
   Whether the substrates are small or large, the fact that the method of fabricating them can be implemented continuously reduces their cost. 
   Note that the initial diameter of the ingot is of little importance, but its length is more important. As a general rule, the greater the diameter of an ingot the shorter its length. Accordingly, depending on the intended applications, it is preferable to use an ingot of smaller diameter, because it is easier to obtain in a greater length. However, the layer made by the method according to the invention is obtained from the cylindrical surface of an ingot. In some embodiments of the method according to the invention, this layer can initially be given a curvature that can prove critical in some circumstances. For example, if the layer is to be stored on a cylinder with the curvature reversed, mechanical stresses can be generated that degrade the quality of the layer, or even cause it to break. As the diameter of the ingot increases, these curvature problems can become less severe. 
   It can also be beneficial to use large-diameter ingots if it is necessary to reduce the period in the resulting layer of variation of its crystalline orientation with the direction in which it is wound. 
   What is more, the curvature and crystallographic orientation problems referred to above are considerably reduced, or even non-existent, if the method according to the invention is used with ingots which have a square cross section, for example. In this case, the layer made by the method according to the invention is obtained from plane faces whose crystallographic orientation is clearly defined. 
   A 200 mm diameter ingot is typically 1.5 m long. As a general rule, before they are used in the method according to the invention, these ingots are cut into 40 to 50 cm lengths. 
   What is more, an ingot as drawn has an ill-defined exterior shape (undulating diameter, etc). During a preliminary step of the method according to the invention, the ingot is turned or machined to obtain an ingot in the form of a regular cylinder or having a polygonal cross section. The turning or machining operation is carried out before or after the cutting operations previously referred to. 
   With silicon ingots, a layer of the order of 10 μm thick is obtained by implanting hydrogen ions with energies of the order of 1 MeV. However, what is essential is to have a sufficiently rigid mass of material between the detachment depth and the surface of the ingot to avoid problems associated with the fragility and deformability of the layers. 
   The detachment depth is advantageously determined so that the continuous layers formed by the method according to the invention are self-supporting. 
   In an advantageous variant of the method according to the invention the layer is reinforced to prevent problems associated with its fragility or deformability by depositing a film before the detachment operation or even before the heating operation, if any, or even before the implantation operation. This variant is particularly advantageous when it is required to fabricate layers that are too thin to be self-supporting. In the case of silicon, for example, a 10 μm deposit of SiO 2  proves sufficient to reinforce the mechanical strength of the layer formed (a material other than SiO 2  can equally well be used). As a general rule, and therefore not only for applications of the method according to the invention to the fabrication of silicon layers, deposition methods of the epitaxial, atomisation, paint, spray, etc. type are also feasible. 
   The amounts of the atomic species implanted are advantageously of the order of 10 17  to 10 18 /cm 2 . With these amounts, and a depth of penetration of the order of one to several tens of microns, it is possible to separate the layer of material between the surface and the detachment depth from the remainder of the ingot with no additional heating operation and with the application of limited stresses or even no stresses. 
   As a general rule, if stresses are applied to the layer, they are advantageously mechanical stresses (shear, tension, compression, ultrasound, etc.), electrical stresses (electrostatic or electromagnetic field), thermal stresses (radiation, convection, conduction, etc.), etc. Applying stresses can also entail directing onto the layer/ingot detachment interface a jet of fluid (liquid or gas) that is either continuous or varies in time. Thermal stresses in particular can be derived from the application of an electromagnetic field, an electron beam, thermo-electric heating, a cryogenic fluid, a supercooled liquid, etc. 
   Another aspect of the invention provides a device for fabricating substrates, in particular for use in optics, electronics or opto-electronics, which device is characterized in that it includes: 
   means for implantation of atomic species under the surface of a material in the form of a cylindrical ingot at an implantation depth distributed around a particular value, 
   detaching means for detaching a layer of material at a detachment depth in the vicinity of the implantation depth, and 
   rotation means for rotating a cylindrical ingot of the material about an axis parallel to the cylindrical surface of the ingot. 
   The above device implements the method according to the invention as previously described. It advantageously includes means for holding the layer of material between the cylindrical surface of the ingot and the detachment depth to gather up said layer after it is detached from the ingot. The holding means advantageously include a plurality of reversible gripping means distributed over roller drive means. The principle of such gripping means is known in the art. Such gripping means employ pressure differences, electrostatic forces, etc., for example. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects, objects and advantages of the invention will become apparent on reading the following detailed description. The invention is explained with reference to the drawings, in which: 
       FIG. 1  is a diagrammatic perspective view of an ingot subjected to ion implantation and to the detachment of a layer of material by a first embodiment of the substrate fabrication method according to the invention; 
       FIG. 2  is a diagrammatic view of one example of a substrate fabrication device according to the present invention, in cross section relative to the axis of the cylinder of the ingot shown in  FIG. 1 ; 
       FIG. 3  is a diagrammatic view of the use of a second embodiment of the method according to the present invention, in cross section relative to the axis of the cylinder of an ingot like that shown in  FIGS. 1 and 2 ; 
       FIG. 4  is a diagrammatic view of the use of a third embodiment of the method according to the present invention, in cross section relative to the axis of the cylinder of an ingot like that shown in  FIGS. 1  to  3 ; 
       FIG. 5  is a diagrammatic view of the use of a variant of the embodiment of the method according to the present invention shown in  FIG. 4 , in cross section relative to the axis of the cylinder of an ingot such as that shown in  FIGS. 1  to  4 ; 
       FIG. 6  is a diagrammatic view of the use of another variant of the embodiment of the method according to the present invention shown in  FIG. 4 , in cross section relative to the axis of the cylinder of an ingot such as that shown in  FIGS. 1  to  5 ; 
       FIG. 7  is a diagrammatic view of the use of a fourth embodiment of the method according to the present invention, in cross section relative to the axis of an ingot like that shown in  FIGS. 1  to  6 ; 
       FIG. 8  is a diagrammatic view of the use of a fifth embodiment of the method according to the present invention, in cross section relative to the axis of an ingot like that shown in  FIGS. 1  to  7 ; 
       FIG. 9  is a diagrammatic view of the use of a variant of the fourth embodiment of the method according to the present invention, in cross section relative to the axis of an ingot like that shown in  FIGS. 1  to  7 ; 
       FIG. 10  is a diagrammatic view of the use of a sixth embodiment of the method according to the present invention, in cross section relative to the axis of an ingot like that shown in  FIGS. 1  to  9 ; 
       FIG. 11   a  is a diagrammatic view of a substrate obtained with a seventh embodiment of the method according to the invention, in section perpendicular to the surface subjected to bombardment by the method according to the invention, and  FIG. 11   b  is a diagram showing the concentration profile of the atomic species implanted as a function of the depth of implantation in the substrate shown in  FIG. 11   a;    
       FIG. 12   a  is a diagrammatic perspective view of three layers intended to be superposed in an eighth embodiment of the method according to the invention and  FIG. 12   b  shows the structure obtained after assembling the three layers shown in  FIG. 12   a;    
       FIG. 13  is a diagrammatic view of the use of a ninth embodiment of the method according to the present invention, in cross section relative to the axis of the cylinder of the ingot shown in  FIGS. 1  to  10 ; and 
       FIG. 14  is a diagrammatic view of a square cross section ingot used in a variant of the present invention, in cross section relative to the axis of the ingot. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The method according to the present invention is described hereinafter in the particular context of obtaining monocrystalline silicon substrates from an ingot obtained by CZ or FZ drawing. Silicon has been chosen because it is by far and away the most widely used material in the field of micro-electronics. However, the invention is not limited to this material. The invention applies generally to ingots of any monocrystalline, polycrystalline or amorphous materials, in particular semiconductors. 
     FIG. 1  shows an ingot  1  of monocrystalline silicon obtained by the CZ or FZ drawing process. It is approximately the shape of a circular cylinder with an axis X—X. The ingot  1  initially has a diameter of 200 mm and a length L=1.5 m and is usually cut into lengths. The drawing process is chosen to obtain an ingot  1  in which the faces perpendicular to the axis of the cylinder are oriented parallel to the &lt;1, 0, 0&gt; crystallographic plane. The &lt;0, 0, 1&gt; plane  3  and the &lt;0, 1, 0&gt; plane  5  are therefore parallel to the axis X—X of the ingot  1 . 
   Ten examples of implementation of the method according to the invention are described hereinafter. 
   EXAMPLE 1 
   In the first example, shown in  FIG. 1 , the method according to the invention includes an operation  100  of implanting atomic species and a detachment operation  300 . 
   In this example, the atomic species are H −  ions. They are implanted with a high energy. The beam made up of these ions is elongate in the longitudinal direction of the ingot. To obtain a 10 μm thick layer  2  of silicon the H −  ions are accelerated with an energy of 725 keV. The amount of H −  ions implanted is 1.21×10 17 /cm 2 . 
   The implantation operation  100  is carried out by sweeping a beam of accelerated atomic species over the surface of the ingot  1 , over the whole of its length, matching the rotation speed of the ingot  1  to the width of the beam and to the sweeping rate to obtain the appropriate dose. 
   The implantation depth Rp varies according to the crystallographic orientation of the implanted surface relative to the incident beam of atomic species. In applications in which variations in the thickness of the layer  2  are critical, modulation of the thickness of the layer  2  obtained is advantageously avoided by modulating the implantation energy as a function of the rotation angle. Note, however, that if an ingot  1  which has a square cross section is used, these problems of thickness variations can be reduced or even eliminated, because the implantation can be effected on faces whose crystallographic orientation is clearly defined. 
   The implantation operation  100  creates, within the volume of the ingot  1 , and at a depth close to the depth of penetration of the H −  ions, a fragile layer dividing the ingot  1  into a lower region constituting the mass of the ingot  1  and an upper region constituting a layer  2  of material destined to form the required substrate. 
   In the example described here the layer  2  is approximately 10 μm thick. That thickness is sufficient to avoid deformation of the layer (for example the formation of blisters) and the implantation conditions produce sufficient fragility at the detachment depth for the layer  2  to be detached from the ingot  1  with less force. 
   The separated layer  2  is advantageously held to enable it to be unwound. 
     FIG. 2  shows a device  50  in accordance with the invention for fabricating substrates which implements the method illustrated by FIG.  1 . It includes means  110  for implanting H −  ions, means  310  for holding the layer  2  when it has been separated from the ingot  1 , and rotation means  410 . 
   The implantation means  110  comprise an implanter which produces H −  ions accelerated to an energy of the order of 1 MeV. This type of implanter was initially developed by the Japan Atomic Energy Research Institute (JAERI). 
   The rotation means  410  cause the ingot  1  to rotate about the axis X—X. 
   The holding means  310  comprise a support  6 . The support  6  is advantageously an adhesive film. The layer  2  is brought directly into contact with the support  6 . The support  6  is pressed against the layer  2  by an applicator roller  315 . The applicator roller  320  is mounted on a shaft that is mobile so that it can track the movement of the surface of the ingot  1  as its diameter is decreased by the removal of material. Accordingly, when the ingot  1  begins to rotate about the axis X—X, before it has been implanted, no transfer of the layer  2  onto the support  6  is effected. Then, when the implantation operation  100  is started, and the implanted area comes into contact with the support  6 , the latter enables the layer  2  to be transferred onto the support  6 . After contact with the layer  2 , it separates the latter from the remainder of the ingot  1 . The transfer of the layer  2  onto the support  6  can then continue. 
   EXAMPLE 2 
   A second example of implementation of the method according to the invention, shown in  FIG. 3 , includes an implantation operation  100 , a heating operation  200  and a detachment operation  300 . 
   The species implanted are advantageously hydrogen ions. Hydrogen ions are implanted with an energy of the order of 700 keV and a dose of the order of 10 17 /cm 2 . 
   With the ingot  1  rotating continuously about its axis X—X in the device  50 , the area of the surface of the ingot  1  bombarded by the atomic species during the implantation operation  100  moves towards a heating area. 
   The heating operation  200  is carried out after the implantation operation  100  by heating means  210 . The heating operation  200  assists detaching the layer  2  between the surface and the detachment depth from the remainder of the ingot  1 . The heating operation  200  enables the doses of atomic species implanted to be reduced relative to the dose referred to in example 1. 
   The heating means  210  consist of a heating roller  215  in the form of a circular cylinder with its axis parallel to the rotation axis of the ingot  1 . The heating roller  215  is placed downstream of the implantation means  110  relative to the rotation direction of the ingot  1 . The heating roller  215  is in contact with the ingot  1 . The heating means  210  advantageously heat the surface of the ingot  1  locally to a temperature of around 500° C./600° C. The temperature is adjusted to suit the time of application of the heating means  210  and the implantation conditions, such as the implantation dose and energy. The dose and energy parameters also determine the temperature reached by the surface of the ingot  1  during the implantation operation  100 . This heating of the ingot  1  by implantation is taken into account in the thermal budget which determines the conditions of detachment of the layer  2  from the remainder of the ingot  1 . The time of application of the heating means  210  also depends on the application surface area, the rotation speed of the ingot  1 , etc. 
   The layer  2  is then transferred onto a support  6 , as described in example 1. 
   EXAMPLE 3 
   A third example of implementation of a device  50  for implementing a method according to the invention, shown in  FIG. 4 , includes an implantation operation  100  and a heating operation  200 , like those of example 2, plus a detachment operation  300  performed with the aid of holding means  310 . The holding means  310  can employ a pressure difference, an electrostatic force, a reversible adhesion force (by application of a low-tack adhesive), etc. 
   If the holding means  310  are of the suction type, they advantageously comprise a bar  315  with its length parallel to the axis X—X of the ingot  1 . The bar  315  is hollow. The pressure inside the bar is reduced to hold the layer  2  reversibly by suction. Detachment of the layer  2  from the remainder of the ingot  1  is encouraged by the heating operation  200 . 
   To begin peeling the layer  2  off the ingot  1 , the holding means  310  are applied to the first area of the ingot  1  that has been subjected to the implantation operation  100  and the heating operation  200 . Because of the suction in the bar  315 , mechanical stresses are applied to the layer  2 . Those mechanical stresses are reinforced by movement E of the bar  315  away from the ingot  1 . A separation front F is then obtained. 
   The holding means  310  are moved by the means shown in  FIGS. 5 and 6 , for example. 
   In the embodiment shown in  FIG. 5 , bars  315  are distributed over the periphery of a drive roller  316 . The principle of  FIG. 6  is the same as that of  FIG. 5 , but the bars  315  are distributed over a conveyor belt  317  moving in a straight line between two drive rollers  316 . 
   In the embodiments shown in  FIGS. 5 and 6  the pressure inside the bar  315  is reduced just before it comes into contact with the ingot  1 . At the moment of contact the bar  315  adheres to the surface of the ingot  1 . If the ingot has not been subjected to the implantation operation  100  and the heating operation  200 , in the area of contact between the ingot  1  and the bar  315  the latter move relative to each other and the contact is broken. On the other hand, if the ingot has been subjected to an implantation operation  100  in the area of contact between the ingot  1  and the bar  315 , the layer  2  is separated from the remainder of the ingot  1  and is held by the bar  315 . The vacuum in the bar  315  is broken when the area of the layer  2  held by the bar  315  reaches a take-up roller  8 . 
   This embodiment has the advantage over the previous embodiment that the layer  2  is subject to lower mechanical stresses because there is no reversing of the curvature of the layer  1  between the detachment operation  300  and the operation of storing it on the take-up roller  8 . holding means  310  and entrained by the support  6 . 
   In a variant of these embodiments of the method according to the invention, the layer  2  is transferred to a support (not shown in FIGS.  5  and  6 ). In this case, the layer  2  is moved away from the ingot  1  by the holding means  310 , as previously indicated, and then transferred to the support, to which it adheres. The layer  2  is then released by the holding means  310  and entrained by the support. 
   In this variant the layer  2  can be cut into sheets before or after it is transferred to the support  6 . 
   EXAMPLE 4 
   A fourth example of implementation of the method according to the invention includes an implantation operation  100 , an operation  400  of transfer onto a stiffener support  6 , and a heating operation  210  carried out before or during the transfer operation  400 . 
   This is shown in FIG.  7 . 
   In this example, the implantation operation  100  is carried out at an energy of 100 to 200 keV. This energy is insufficient to produce self-supporting layers  2 . The support  6  then acts as a stiffener. This prevents the layer  2  breaking and/or deforming (by forming blisters, for example). 
   The support  6  is advantageously an adhesive film. The adhesive film consists of a polymer resin, for example, or some other substance suited to this function, which becomes adhesive when it is heated or when it is irradiated with UV radiation. The adhesive film is stretched between two rollers  8 ,  10  between which the ingot  1  is pressed onto the support  6 . The axes of the rollers  8 ,  10  and the ingot  1  are parallel. The support  6  is initially wound onto a pay-out roller  10 . 
   The ingot  1  is rotated about its axis X—X by rotation means  410 . 
   In this example the heating means take the form of a roller  215 . The roller presses the support  6  and the layer  2  together and heats them at the same time. The support  6  serves as a stiffener which prevents deformation of the layer  2  (for example by blisters) that could otherwise occur during the heating operation that is virtually simultaneous with the coming into contact of the layer  2  and the support  6 . The heating operation  210  strengthens the adhesion between the support  6  and the layer  2  and contributes to making the ingot  1  fragile at the detachment depth. 
   After adhering to the support  6 , the layer  2  leaves the ingot  1  at the separation front F. Synchronizing the rotation of the ingot  1  with the movement of the support  6  propagates the separation of the layer  2  relative to the ingot  1  at the front F. At the separation front F, the material is sufficiently fragile for the mechanical stresses applied to the ingot  1  by the support  6  to complete detachment. 
   If the implantation and heating parameters are chosen accordingly, the detachment of the layer  2  from the ingot  1  has already occurred by the time of the heating operation  200  and the support  6  merely entrains the layer  2  away from the ingot  1 . 
   The combination of the layer  2 /support  6  is then wound onto a take-up roller  8  for storage. 
   As an alternative to the above, the support  6  can be preheated before it is brought into contact with the ingot  1  or the ingot  1  can be preheated before the support  6  is brought into contact with it. 
   In another variant of the embodiment of the method according to the invention described hereinabove a wedge, blade or some other type of mechanical contact or a jet of fluid, such as a gas, is used to initiate the separation of the layer  2  from the ingot  1 . 
   In a further variant of this embodiment, the support  6  takes the form of a plate  20  (see FIG.  9 ). The plate  20  is rigid. It is made of glass or quartz, for example. Thus an implantation operation  100  can be carried out after which the layer  2  is transferred to the plate  20 ; it can be heated to facilitate detachment and transfer of the layer  2  onto the plate  20  and adhesion of the layer to the plate. It is also possible to interrupt the implantation operation  100  to complete the transfer of a portion of the layer  2  already implanted onto the plate  20  and then start again with a new plate  20 . 
   EXAMPLE 5 
   A fifth example of implementation of the method according to the invention is derived from the fourth example, illustrated by FIG.  7 . The fifth example, illustrated by  FIG. 8 , includes an implantation operation  100 , a heating operation  200  and an operation  400  of transfer onto a stiffener support  6 , as in the fourth example, but further includes an operation that creates a thermal shock. After the implantation operation  100 , the ingot  1  is at a relatively high temperature. By pressing a cooling roller  216  against the ingot  1  in the areas that have been subjected to the implantation operation, a thermal shock is produced that facilitates separating the layer  2  and the ingot  1 . 
   EXAMPLE 6 
   A sixth example of the implementation of the method according to the invention includes an implantation operation  100 , a heating operation  200  and a detachment operation  300 , all of which are of the same kind as those of example 4. However, it further includes an operation of covering the layer  2  deposited on the support  6  with a covering material  12 . The covering material  12  is deposited in the form of a film or in the liquid or gas phase. This example is illustrated by  FIG. 10. A  system of three layers  6 ,  2  and  16  is then obtained. One example of use of an embodiment of this kind is described hereinafter in example 8. 
   EXAMPLE 7 
   A seventh example of implementation of the method according to the invention includes an implantation operation  100  which is advantageously effected simultaneously with H −  ions and phosphorus ions ( FIG. 11   a ). With the same acceleration energy, the H −  ions are implanted more deeply than the phosphorus ions, because they are not so heavy. The H −  ions therefore determine the depth at which detachment occurs. The phosphorus ions produce an n-doped doping layer  16 . The layer underlying the doping layer  16  forms a p-doped layer  17 .  FIG. 11   b  shows the profile of the concentration C of the atomic species H − , P 2 H 6  and PH 3  as a function of the depth of implantation in the layer  2  and the ingot  1  shown in  FIG. 11   a . Thus doping and implantation for the purpose of detachment can be carried out at the same time. 
   This example is advantageously completed by a heating operation  200 , a detachment operation  300  and a transfer operation  400  as described in example 4. 
   EXAMPLE 8 
   An eighth example of implementation of the method according to the invention is derived from the sixth and seventh examples described hereinabove. In the eighth example, shown in  FIGS. 12   a  and  12   b , the support  6  and the covering material  12  already incorporate patterns. 
     FIG. 12   a  shows a support  6 , a layer  2  and a covering material  12 . The support  6  includes interconnection patterns (not shown). The silicon layer  2  is obtained from a silicon ingot  1  by the method according to the invention. The layer  2  includes an n-doped layer  16  that is advantageously doped with phosphorus or arsenic and a p-doped layer  17 , as indicated hereinabove in example 7. The covering material  12  also includes interconnection patterns. The combination of the three layers  6 ,  2  and  12  is assembled by the method according to the present invention, for example the variant of the method illustrated by FIG.  6 . This produces a photovoltaic device like that shown in  FIG. 12   b , in which the face including the covering material  12  is exposed to the photons  18 . 
   The covering material  12  provides an antireflection coating. The surface of the layer  2  is rough because it has not been polished after the detachment operation  300  of the method according to the present invention. This enables light  18  to penetrate into the layer  2  with multiple reflections. 
   EXAMPLE 9 
   In a ninth example of implementation of the method according to the invention, shown in  FIGS. 13   a  and  13   b , the implantation operation  100  is carried out over the whole of the surface of the ingot  1 . To this end the ingot  1  is placed in a plasma implantation chamber in which the atomic species are accelerated to the required voltage ( FIG. 13   a ). 
   The ingot  1  is then optionally subjected to a heating operation  200 , depending on the conditions used in the preceding implantation operation  100 . 
   The ingot  1  is then withdrawn from the implantation chamber to peel off the layer  2 . The layer  2  is advantageously transferred onto a support  6 , as in any of the preceding examples ( FIG. 13   b ). 
   In a variant of this example the ingot  1  is subjected to the other operations leading to the formation of the layer  2  in the same chamber as the implantation operation  100 . 
   EXAMPLE 10 
   A tenth example of implementation of the method according to the invention by using a device  50 , shown in  FIG. 14 , includes an implantation operation  100 , a healing operation  200 , and a detachment operation  300 , conforming to those described in connection with example 2, but the ingot  1  has a square cross section relative to its longitudinal axis. The heating operation  200  and the detachment operation  300  are carried out simultaneously by means of a heating roller  215 . The heating roller  215  is mounted on a shaft that is mobile so that it can track the movement of the faces of the ingot  1  as it rotates and so that it can track the movement of the surface of the ingot  1  as its size is reduced by removing material. 
   Many variants of the method according to the invention can be obtained by combining the various embodiments described hereinabove. 
   In the embodiments described hereinabove, the implantation operation  100  is carried out by bombarding the surface of the ingot  1  either with a beam of atomic species or by immersion in a plasma. If a beam of atomic species is used, it can have a linear or rectangular shape or any other geometry. The ingot  1  can also be bombarded radially by more than one beam, simultaneously at several points on the surface, or even over the whole of its surface. 
   In the embodiments described hereinabove, a heating operation  200  can be carried out to encourage and/or cause detachment of the layer  2  from the ingot  1 . That operation can be complemented by the application of mechanical stresses to complete said detachment and separate the layer  2  from the remainder of the ingot  1 . However, the detachment of the layer  2  from the ingot  1  can be encouraged and/or caused entirely by the heating operation  200 . It can also be encouraged and/or caused entirely by mechanical stresses. 
   Similarly, in the embodiments described hereinabove, the atomic species implanted to create microcavities is hydrogen. Other atomic species can equally well be used. Examples are helium, boron, etc. Boron is advantageously used to dope the layer at the same time as encouraging or causing detachment. Boron can equally advantageously be used to reduce the doses of the atomic species implanted and/or the temperatures and/or the times of heating of the optional heating operation  200  (see U.S. Pat. No. 5,877,070, for example). 
   For some applications, and in particular if the surface of the ingot  1  exposed to implantation must be protected, a buffer layer can be deposited on the upstream side of the implantation  100 , relative to the direction of rotation of the ingot  1 . 
   Similarly, it can be beneficial to deposit a stiffener on the ingot  1 , even before the implantation operation  100 . 
   As a general rule, depending on the intended applications, it can be beneficial to deposit a support  6  (stiffener, reflecting layer, etc.) on one or both of its faces before or after the layer  2  is peeled off. 
   In a further variant of the method according to the invention, the layer  2  is transferred temporarily to a support  6  serving as a stiffener enabling a detachment operation  300  to be carried out, or even only a operation constituting a preliminary to the detachment operation, such as a heating operation  200 , preventing deformations such as those caused by the formation of blisters. The support  6  advantageously carries the layer  2  from the ingot  1  from which it has been obtained to storage means or carries the layer  2  before it is transferred to another support that confers the required mechanical strength on it. Thus one face of the layer  2  can adhere temporarily to the temporary support  6 , which possibly also serves as a stiffener, after which another support is caused to adhere to the other face, after which the temporary support  6  is finally removed. 
   A roller in contact with the ingot  1  downstream of the implantation means  110  can also serve as a temporary stiffener. This is advantageously combined with a heating operation  200 .