Patent Publication Number: US-2021189594-A1

Title: Inductively heatable susceptor and epitaxial deposition reactor

Description:
FIELD OF THE INVENTION 
     The present invention concerns a susceptor with heated stud and an epitaxial deposition reactor that comprises such a susceptor. 
     STATE OF THE ART 
     In the reaction chamber of an epitaxial deposition reactor the disc-shaped susceptor, which is used to horizontally support the substrates, can be heated by electromagnetic induction through a flat inductor arranged parallel to the susceptor in the vicinity of the lower horizontal wall of the reaction chamber; these are so-called epitaxial reactors with “induction heating”; in general, the susceptor is made to rotate about its axis of (substantial) symmetry during the epitaxial deposition processes through a drive shaft mechanically coupled with the susceptor. 
     The electromagnetic field generated by such a flat inductor is not uniform and, consequently, the heat generated inside the susceptor is not uniform. But the susceptor is made entirely of material that is a fair heat conductor, in general graphite, and therefore the temperature of the upper face of the susceptor (on which, in general, one or more substrates rest) is quite uniform. 
     However, the electromagnetic field generated by such a flat inductor is very weak at the axis of the inductor (which corresponds to the axis of the susceptor); as a result the temperature of the central zone of the upper face of the susceptor (on which, in general, one or more substrates rest) is a little lower than the rest of the upper face of the susceptor. 
     Such an effect is particularly troublesome in the case of susceptors adapted to support a single substrate. 
     The problem of the uniformity of the temperature of the susceptor is more felt when a higher quality is required of the layers of semi-conductive material deposited on the substrates. 
     Possible important and undesired effects of such lack of uniformity of temperature include: non-uniformity of the thickness of the layer deposited on the substrate, non-uniformity of the electrical properties of the layer deposited on the substrate, crystallographic defects in the layer deposited on the substrate, occurrence of stresses inside the substrate and/or the layer deposited on the substrate. 
     SUMMARY 
     The Applicant has thus set itself the task of improving the prior art and of solving the problems outlined above. 
     Such a task is substantially achieved thanks to the susceptor having the technical features set out in the attached claims that form an integral part of the present description. 
     The basic idea of the present invention is to provide a susceptor comprising a disc-shaped portion and a cylindrical or conical portion—the disc-shaped portion is used to (directly or indirectly) support substrates to be subjected to epitaxial deposition inside a reaction chamber of an epitaxial deposition reactor and the cylindrical or conical portion is used to contribute to the heating of the disc-shaped portion. 
     Thanks to this configuration of the susceptor, it is possible to heat the disc-shaped portion to a very uniform temperature; In fact, for example, the heating of the susceptor can be obtained through a first inductor adapted to directly heat (in particular by induction) the disc-shaped portion, in particular its outer annular zone, and a second inductor adapted to directly heat (in particular by induction) the cylindrical or conical portion and to indirectly heat (in particular by conduction) the disc-shaped portion, in particular its central zone. 
     An aspect of the present invention is also an epitaxial deposition reactor that comprises such a susceptor. 
    
    
     
       LIST OF FIGURES 
       The present invention will become clearer from the following detailed description to be considered together with the attached drawings in which: 
         FIG. 1  shows a schematic vertical section view of a susceptor according to the present invention inside a reaction chamber of an epitaxial reactor, 
         FIG. 2  shows an (ideal) temperature diagram relating to the susceptor of  FIG. 1  due to two distinct inductors, 
         FIG. 3  schematically shows the structure of a first embodiment of a susceptor according to the present invention, 
         FIG. 4  schematically shows the structure of a second embodiment of a susceptor according to the present invention, 
         FIG. 5  schematically shows the structure of a third embodiment of a susceptor according to the present invention, 
         FIG. 6  shows a (detailed) schematic vertical section view of a first implementation of the susceptor of  FIG. 1 , 
         FIG. 7  shows a (detailed) schematic vertical section view of a second implementation of the susceptor of  FIG. 1 , and 
         FIG. 8  shows (real) temperature diagrams relating to a susceptor very similar to that of  FIG. 1  in five different heating conditions. 
     
    
    
     As can be easily understood, there are various ways of practically carrying out the present invention that is defined in its main advantageous features in the attached claims. 
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic vertical section view of a susceptor according to the present invention in horizontal position inside a reaction chamber of an epitaxial reactor. 
     In this example, the susceptor comprises a disc-shaped portion  21  made entirely of graphite (possibly totally or partially coated with SiC and/or TaC) and a cylindrical portion  22  made entirely of graphite (possibly totally or partially coated with SiC or TaC), and is associated with a disc-shaped support  3  for substrates made entirely of graphite (possibly totally or partially coated with SiC and/or TaC)—such a support may be considered part of the susceptor. 
     The graphite that is used to make the susceptors and their components is isotropic and isostatic, often has a density of 1.5-2.5 g/cm 3 , an electrical resistivity of 5-15 microOhm*m (therefore it can be said to be a material with fair electrical resistivity), and heat conductivity of 50-200 W/m/K (therefore it can be said to be a material with fair heat conductivity). 
     In this example of  FIG. 1 , the reaction chamber is box-shaped and is made entirely from quartz (for example transparent quartz material possibly totally or partially coated with material that reflects infrared rays); in  FIG. 1 , it is possible to see partially an upper horizontal wall  11  of the chamber, partially a lower horizontal wall  12  of the chamber, entirely a vertical sleeve  13  of the chamber joined to the wall  12  at a hole thereof; in  FIG. 1 , it is also possible to partially see a horizontal “liner”  14  of the chamber that in part precedes and in part follows the portion  21  (this is a component that is used to guide the gaseous flow, of reaction gases and then of exhaust gases, in particular to confine it at its bottom). 
     The portion  21  and the support  3  are inside the chamber, in particular in its “reaction and deposition” zone, between the wall  11  and the wall  12 , but closer to the wall  12  so that the reaction gases flow between the upper surface of the support  3  and the lower surface of the wall  11 ; the “liner”  14  is horizontally aligned with the support  3  at its top and contributes to defining a flow channel of the reaction gases; the portion  22  is inside the sleeve  13 . 
     In this example, the axis of (substantial) symmetry of the portion  21  coincides with the axis of (substantial) symmetry of the portion  22 , with the axis of (substantial) symmetry of the support  3  and with a vertical axis Z of the reaction chamber, in particular the axis of (substantial) symmetry of the sleeve  13 . 
     In this example of  FIG. 1 , the portion  21  and the portion  22  are joined together directly. Alternatively, an intermediate portion (not shown in any of the figures) typically made of graphite can be interposed; this portion can, for example, be cylinder shaped, prism-shaped, frustum of cone shaped or frustum of pyramid shaped. In this way, heat can flow by conduction from the portion  22  to the portion  21 ; therefore, these portions must be made of material that is a heat conductor. 
     In this example of  FIG. 1 , the ratio between the average diameter of the portion  21  and the average thickness of the portion  21  is a number comprised between 10 and 100. 
     In this example of  FIG. 1 , the ratio between the average diameter of the portion  21  and the average diameter of the portion  22  is a number comprised between 5 and 20. 
     In this example of  FIG. 1 , the ratio between the height of the portion  22  and the thickness of the portion  21  is a number comprised between 2 and 6. 
     In this example of  FIG. 1 , graphite was chosen not only as heat conducting material, but also as electrically conductive material (in particular provided with good susceptance); in fact, in this example, both the portion  21  and the portion  22  are heated by electromagnetic induction. However, the portion  21  and/or the portion  22  could be heated in a different way, for example through irradiation by lamps or conduction by resistors (this is valid particularly for the portion  22 ), and therefore the property of the material of being electrically conductive (in particular provided with good susceptance) could not be necessary for one or both of these portions. 
     A reaction chamber like the one shown partially in  FIG. 1  is typically equipped with a cooling system, in particular, of the upper wall and of the lower wall; such cooling can be carried out through a flow of a fluid (for example liquid) for the lower wall and a flow of a fluid (for example gas and/or liquid) for the upper wall. 
     A reaction chamber like the one shown partially in  FIG. 1  is typically equipped or associated with reflection devices for reflecting, in whole or in part, in particular infrared radiation emitted by the susceptor when it is hot, in particular during the epitaxial deposition processes. 
     A reaction chamber like the one shown partially in  FIG. 1  can be equipped or associated with further temperature control devices (inside or outside the “reaction and deposition” zone). 
     An epitaxial reactor with a reaction chamber like the one shown partially in  FIG. 1  is typically equipped with a handling system of the substrates to be treated and of the treated substrates; the handling system can be adapted to directly handle substrates or supports for substrates, like for example the support  3  in  FIG. 1 . 
     With reference to the example of  FIG. 1 , ideally, the temperature T diagram of the upper face of the portion  21  as a function of the distance x from the axis Z is shown in  FIG. 2 ; in such a figure, R corresponds to the radius of the disc-shaped portion  21 . If the temperature of the upper face of the portion  21  is uniform (as shown in  FIG. 2 ), the temperature of the upper face of the support  3 , where the substrate(s) rest(s), is uniform. In any case, the mass of material of the support  3  (if present) can also contribute to making the temperature of the upper face of the support uniform. 
     It should be noted that, according to some embodiments, the preferred temperature T diagram of the upper face of the portion  21  as a function of the distance x from the axis Z could be, for example, slightly increasing or slightly decreasing or substantially constant. In  FIG. 2  there is a central zone CZ of the temperature diagram and a side zone LZ of the temperature diagram, on the right and on the left of the zone CZ; as will be understood hereinafter, the temperature of an outer annular zone of the upper face of the portion  21  (substantially aligned with the zone LZ) is more influenced by a first inductor  4 , in particular a flat inductor, and the temperature of a central zone of the upper face of the portion  21  (substantially aligned with the zone CZ) is more influenced by a second inductor  5 , in particular a cylindrical inductor (thanks also, but not only, to the cylindrical portion  22  of the susceptor that transmits heat by conduction to the disc-shaped portion  21 ). A typical embodiment of a flat inductor consists of an electrical conductor arranged in the form of a (flat) spiral; a typical embodiment of a cylindrical inductor consists of a plurality of circular turns wound on a cylinder. 
     In first approximation, when both the inductor  4  and the inductor  5  are active, the power density in the susceptor (first transmitted as electromagnetic field, then converted into induced electrical current in the susceptor, and finally transformed into heat by Joule) effect due to two inductors feeding electrical currents is added; of course, not all of the electrical power supplied to the inductors becomes heat in the susceptor. 
     The distribution of the temperature in the susceptor depends essentially on four elements, i.e. the disc-shaped portion of the susceptor ( 21  in  FIG. 1 ), the cylindrical portion of the susceptor ( 22  in  FIG. 2 ), the first inductor ( 4  in  FIG. 1 ) and the second inductor ( 5  in  FIG. 1 ), and on various parameters of these elements; such parameters are mainly geometric, electrical and chemical (i.e. the materials of the components). The temperature is influenced, for example, also by any heat emitted by irradiation and absorbed by irradiation by the susceptor through its surfaces. Moreover, when the susceptor is inside a reaction chamber of an epitaxial deposition reactor (see for example  FIG. 1 ), the temperature is influenced, for example, also by flows of gases in the reaction chamber and by flows of fluids (gaseous and/or liquid) outside of the reaction chamber. Finally, the temperature of the susceptor is influenced by the presence of a support with one or more substrates arranged on the susceptor. 
       FIG. 8  shows (real) temperature diagrams relating to a susceptor very similar to that of  FIG. 1  in five different heating conditions; precisely, it concerns the temperature in degrees Celsius of the upper face of the disc-shaped portion of the susceptor as a function of the distance from the central axis Z of symmetry of the susceptor. The diagrams of  FIG. 8  derive from a set of experiments carried out using a first disc-shaped inductor power-supplied by a first LF generator of alternating electric voltage/current at low frequency (for example a frequency comprised in the range 2-4 KHz) and a second cylindrical inductor power-supplied by a second HF generator of alternating electric voltage/current at high frequency (for example a frequency comprised in the range 20-40 KHz); the first generator was capable of supplying at its output a maximum power of 40-50 KW and the output power was able to be set; the second generator was capable of supplying at its output a maximum power of 10-20 KW and the output power was able to be set. The temperature was detected through a thermographic camera. 
     In the first experiment, the electrical power supplied to the flat inductor was about 55% of the maximum power of the first LF generator and the electrical power supplied to the cylindrical inductor was about 10% of the maximum power of the second HF generator. As can be seen in  FIG. 8A , the temperature diagram looks like an upturned Gaussian with a minimum at its centre of about 975° C. and a maximum at its ends of about 1025° C. 
     In the second experiment, the electrical power supplied to the flat inductor was about 65% of the maximum power of the first LF generator and the electrical power supplied to the cylindrical inductor was about 55% of the maximum power of the second HF generator. As can be seen in  FIG. 8B , the temperature diagram is like a portion of circumference with a maximum at its centre of about 1125° C. (slightly less) and a minimum at its ends of about 1100° C. (slightly more). 
     In the third experiment, the electrical power supplied to the flat inductor was about 50% of the maximum power of the first LF generator and the electrical power supplied to the cylindrical inductor was about 60% of the maximum power of the second HF generator. As can be seen in  FIG. 8C , the temperature diagram is like a Gaussian with a maximum at its centre of about 1030° C. and a minimum at its ends of about 990° C. 
     In the fourth experiment, the electrical power supplied to the flat inductor was about 45% of the maximum power of the first LF generator and the electrical power supplied to the cylindrical inductor was about 55% of the maximum power of the second HF generator. As can be seen in  FIG. 8D , the temperature diagram is like a triangle with a maximum at its centre of about 1050° C. and a minimum at its ends of about 1000° C. 
     In the fifth experiment, the electrical power supplied to the flat inductor was about 45% of the maximum power of the first LF generator and the electrical power supplied to the cylindrical inductor was about 50% of the maximum power of the second HF generator. As can be seen in  FIG. 8E , the temperature diagram is like a portion of straight and horizontal line with an average value of about 1020° C. 
     The use of two inductors to heat the portion  21  thus allows greater flexibility in the control of its temperature. 
     In  FIG. 1 , all of the inductors are arranged under the reaction chamber; according to alternative solutions, one or each of the inductors could be arranged above the reaction chamber. 
     Essentially, the susceptor according to the present invention comprises a first disc-shaped portion and a second cylindrical or conical portion; equivalently, the second portion could be prismatic or pyramid-shaped. The cylindrical or conical portion is used in particular to heat the central zone of the disc-shaped portion in particular by conduction; for example, the disc-shaped portion can be heated directly by induction through a first inductor (or for example resistors); for example the cylindrical or conical portion can be heated directly by induction through a second inductor (or for example resistors). 
     Of course, the (direct) heating by induction of a part (or many parts) requires that such a part (or many parts) be made of a material adapted for such a type of heating, i.e. equipped with good susceptance. 
       FIGS. 3, 4 and 5  schematically show three possible structures of the susceptor. 
     The disc-shaped portion of the susceptor is adapted to directly or indirectly support one or more substrates. 
     Typically and advantageously, the cylindrical or conical portion is coaxial to the disc-shaped portion; in particular, the axis of (substantial) symmetry of the cylindrical portion, the axis of (substantial) symmetry of the conical portion and the axis of (substantial) symmetry of the susceptor coincide. 
     Typically and advantageously, the cylindrical or conical portion is adjacent to the disc-shaped portion and in contact with it. 
     Typically and advantageously, the cylindrical or conical portion is located below the disc-shaped portion. 
     Typically and advantageously, the disc-shaped portion and the cylindrical or conical portion are made entirely of graphite (totally or partially coated with SiC and/or TaC). 
     In the susceptor  2  of  FIG. 3 , the disc-shaped portion  321  and the cylindrical or conical portion  322  are made in a single piece, i.e. the two portions are part of the same body. The portion  322  has a blind hole in which a drive shaft  6  screws that is used to make the susceptor  2  rotate; the shaft  6  can, for example, be made of steel (a material that is a good heat conductor and good electrical conductor) or of graphite (a material that is a fair heat conductor and a fair electrical conductor) or of ceramic (a material that is a poor heat conductor and a poor electrical conductor), taking due account of their mechanical properties; it should be noted that, if the shaft is made of graphite, this graphite could also be slightly different from the graphite of the susceptor, for example it could have lower heat conductivity. 
     In the susceptor  2  of  FIG. 4 , the disc-shaped portion  421  is made in a single first piece (for example made entirely of graphite totally or partially coated with SiC and/or TaC) and the cylindrical or conical portion  422  is made in a single second piece (for example made entirely of graphite totally or partially coated with SiC and/or TaC); the first piece and the second piece are fixed together. The portions  322  and  422  are substantially the same (axial) length. In such example, the attachment is carried out through a screw coupling; in particular, the portion  421  has a threaded through hole and the cylindrical or conical portion  422  is screwed in this hole. The portion  422  has a blind hole in which a drive shaft  6  is screwed that is used to make the susceptor  2  rotate; the shaft  6  can, for example, be made of steel (a material that is a good heat conductor and good electrical conductor) or of graphite (a material that is a fair heat conductor and a fair electrical conductor) or of ceramic (a material that is a poor heat conductor and a poor electrical conductor), taking due account of their mechanical properties; it should be noted that, if the shaft is made of graphite, this graphite could also be slightly different from the graphite of the susceptor, for example it could have lower heat conductivity. 
     In the susceptor  2  of  FIG. 5 , the disc-shaped portion  521  is substantially the same as the disc-shaped portion  321 , whereas the cylindrical or conical portion  522  differs from the cylindrical or conical portion  322  in that it is shorter (axially); the reduced length of the portion  522  facilitates the insertion of the susceptor in a closed reaction chamber, i.e. already assembled. In order to supply heat to the central zone of the disc-shaped portion  521 , it is useful to have a conical or cylindrical element that is long enough to be able to be coupled with a sufficiently long solenoid inductor; for this purpose, in addition to the cylindrical or conical portion  522  (for example made entirely of graphite totally or partially coated with SiC and/or TaC), a further cylindrical or conical portion  523  (for example made entirely of graphite totally or partially coated with SiC and/or TaC) has been provided in contact with it; the portions  522  and  523  are fixed together for example through a screw coupling. The portion  523  has a blind hole in which a drive shaft  6  is screwed that is used to make the susceptor  2  rotate; the shaft  6  can, for example, be made of steel (a material that is a good heat conductor and good electrical conductor) or of graphite (a material that is a fair heat conductor and a fair electrical conductor) or of ceramic (a material that is a poor heat conductor and a poor electrical conductor), taking due account of their mechanical properties; it should be noted that, if the shaft is made of graphite, this graphite could also be slightly different from the graphite of the susceptor, for example it could have lower heat conductivity. 
     The hole of the cylindrical or conical portions of  FIGS. 3, 4 and 5  is used to fix the drive shaft and to be able to transmit rotary movements to said disc-shaped portion through the cylindrical or conical portion. 
     However, a (blind or through) hole (for example a shaped hole) in the cylindrical or conical portion can also be used to influence the temperature profile of the upper face of the disc-shaped portion. 
     In the example of  FIG. 1 , there is a disc-shaped support  3  adapted to support one or more substrates and rested on the disc-shaped portion  21 ; in  FIG. 6 , such support, indicated with reference numeral  603  and rested on the disc-shaped portion  621 , is shown in greater detail according to an embodiment suitable for a single substrate  100  at a time; in  FIG. 7 , such support, indicated with reference numeral  703  and rested on the disc-shaped portion  721 , is shown in greater detail according to an embodiment suitable for a single substrate  100  at a time. 
     According to a variant of  FIG. 1  and  FIG. 6  and  FIG. 7 , the lower surface of the support  3  is perfectly flat, whereas the upper surface of the portion  21  is (slightly) concave centrally; this means that, when cold, although the support  3  is rested on the portion  21  the lower surface of the support  3  is not totally in contact with the upper surface of the portion  21 , and that, when hot (during the epitaxial deposition processes), the lower surface of the support  3  is more in contact with the upper surface of the portion  21  in particular perfectly in contact. 
     In  FIG. 6  and  FIG. 7 , the disc-shaped support  603 / 703  has a face that has a (thin) recess for a substrate  100  (alternatively a plurality of recesses for a corresponding plurality of substrates), substantially cylindrical in shape—the substrates can have so-called “flats”. 
     The recess can have a flat or (slightly) concave bottom. The recess typically has a solid bottom. It should be noted that a support that is the same as or similar to that of  FIG. 6  and  FIG. 7  can gradually deform, in particular become convex, process after process. 
     In  FIG. 6  and  FIG. 7 , the substrate  100  has a diameter of 300 mm. It should be noted that the present invention has been specifically devised for reactors adapted for treating a single substrate at a time of large diameter, for example 200 mm and above (up to 450 mm). 
     In  FIG. 7 , the disc-shaped support  703  has a lower edged shaped to allow handling of the disc-shaped support  703  by a tool; alternatively, the disc-shaped portion  721  can have an upper edge shaped to allow handling of the disc-shaped support  703  by a tool; alternatively again, both the lower edge of the support  703  and the upper edge of the portion  721  can be shaped. 
     According to the example of  FIG. 6 , the cylindrical or conical portion  622  of the susceptor  2  has a through hole, the disc-shaped portion  621  of the susceptor  2  has a seat, and there is a lifting device adapted for lifting substrates; the lifting device comprises a stem  682  and a plate  681  fixed together; the stem  682  is arranged in the through hole and adapted to slide along the through hole, and the plate  681  is arranged in the seat. 
     There is also a hollow drive shaft  606  fixed to the cylindrical or conical portion  622 . 
     For indicative purposes, the outer diameter of the portion  622  and/or of the shaft  606  is 40-60 mm. 
     For indicative purposes, the rotation speed of the shaft  606  (and therefore of the portions  621  and  622 ) is 5-50 RPM. 
     According to the example of  FIG. 7 , there is a drive shaft  706  fixed to the cylindrical or conical portion  722 . 
     For indicative purposes, the outer diameter of the portion  722  is 40-60 mm. 
     For indicative purposes, the rotation speed of the shaft  706  (and therefore of the portions  721  and  722 ) is 5-50 RPM. 
     Preferably, the shaft  706  is made entirely of graphite, in particular due to the heat resistivity and the ability to withstand high temperatures of this material. 
     The shaft  706  comprises an upper portion  706 A, a lower portion  706 D and a possible intermediate portion  706 C; the upper portion  706 A and the possible intermediate portion  706 C can be joined through a (smooth-) junction portion  706 B. It should be noted that the arrangement of the portion  722  and of the portion  706 A can be considered a single cylindrical element. In fact, in the example of  FIG. 7 , the inductor  5  can extend, for example, up to the lower end of the portion  706 A; in other words, the inductor  5  can extend up to the lower end of the cylindrical or conical portion ( 722  in  FIG. 7 ) or slightly beyond, for example beyond by 1-3 cm until it covers the portion  706 A of the shaft  706 . 
     The upper portion  706 A can have a cylindrical or conical protrusion ( 706 F in  FIG. 7 ) or recess (not foreseen in  FIG. 7 ) for the mechanical coupling with a corresponding cylindrical or conical recess or protrusion of the cylindrical or conical portion  722 . 
     According to the example of  FIG. 7 , the lower portion  706 D has radial protrusions  706 E for heat dissipation. 
       FIG. 1  shows some components that are not part of the reaction chamber in the strict sense, but that are typically part of an epitaxial reactor, in particular a first inductor  4  and a second inductor  5 . 
     The first inductor  4  is adapted to directly heat the disc-shaped portion  21 ; this is, in particular, “flat” (i.e. equating its turns to a curved line, this lies entirely in one plane). 
     The second inductor  5  is adapted to directly heat the cylindrical or conical portion  22  and to indirectly heat the disc-shaped portion  21 ; this is, in particular, “cylindrical” (i.e. equating the turns of each winding thereof to a curved line, this lies entirely in a cylindrical surface). The inductor  5  can comprise two (or more) coaxial windings  51  and  52  (in the same direction) of turns to create more electromagnetic field in less length. 
     According to variants of  FIG. 1 , the inductor  4  and/or the inductor  5  could be “shaped”; for example, the inductor  4  could not be perfectly flat and/or the inductor  5  could not be perfectly cylindrical. Moreover, the shape and/or the position of the inductor  4  and/or of the inductor  5  could be adjustable. 
     According to variants of  FIG. 1 , the inductor  4  and/or the inductor  5  could consist of a conductor having a section that varies as a function of the position. 
     The distribution of the electromagnetic field, and therefore the distribution of temperature, inside the disc-shaped portion of the susceptor, essentially depends on four elements, i.e. the disc-shaped portion ( 21  in  FIG. 1 ), the cylindrical portion ( 22  in  FIG. 2 ), the first inductor ( 4  in  FIG. 1 ) and the second inductor ( 5  in  FIG. 1 ), and on various parameters of these elements; such parameters are mainly geometric, electrical and chemical (i.e. the materials of the components). 
       FIG. 1  shows a separation element  7  adapted for reducing mutual induction between the first inductor  4  and the second inductor  5 ; this is arranged between the inductor  4  and the inductor  5 , in particular it surrounds the inductor  5  (as well as the sleeve  13  and the portion  22 ); materials that can be used for this purpose are ferrites and MDMs [Magneto Dielectric Materials]. 
     Typically and preferably, the inductors  4  and  5  are completely disconnected from one another, but they could also be connected electrically in series. Preferably, the first and the second inductor  4  and  5  are adapted to be power-supplied independently, in particular at different frequencies for example to limit mutual interactions; for example, the frequency of one could be 2-4 KHz and the frequency of the other 20-40 KHz; for example the ratio between the frequencies could be equal to a number comprised between 5 and 20. 
     The first inductor  4  can be adapted to be adjusted through modification of its position and/or through modification of the mutual position of its turns; the modification can be manual or motorised or automatic. 
     The second inductor  5  can be adapted to be adjusted through modification of its position and/or through modification of the mutual position of its turns; the modification can be manual or motorised or automatic.