Abstract:
An electronic subassembly and associated method for the production of an electronic subassembly include a semiconductor layer bearing at least a first transistor having an adjustable threshold voltage is joined to an insulator layer and a in which a first trapping zone is formed at a predetermined first depth. The first trapping zone extends at least beneath a channel of the first transistor and includes traps of greater density than the density of traps outside the first trapping zone, in such a way that the semiconductor layer and the first trapping zone are capacitively coupled. The useful information from the first transistor includes the charge transport within this transistor. A second trapping zone can be formed that extends at least beneath a channel of a second transistor that is formed by a second implantation with an energy and/or a dose and/or atoms that differ from those used to form the first trapping zone.

Description:
PRIORITY CLAIM 
       [0001]    This application is a nationalization under 35 U.S.C. 371 of PCT Application No. PCT/FR2009/050212, filed Feb. 11, 2009, which claims priority to French Patent Application No. 0851073, filed Feb. 19, 2008, and incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention concerns an electronic subsystem and a method for producing it. It applies in particular to adjusting the threshold voltage of an MOS (metal-oxide semiconductor) transistor by means of a buried trapping layer. 
       BACKGROUND 
       [0003]    The threshold voltage is a key parameter of an MOS transistor. In an electronic circuit, transistors with different threshold voltages are generally required because transistors do not all have the same functions and must offer different performance. For example, some transistors in a circuit have a high threshold voltage and thus a low “blocked” state current (low-consumption transistors) whereas others have a low threshold voltage and thus a high “passing” state current (high-performance transistors). Different families of transistors are therefore used in circuit design. These transistors are produced on bulk silicon (Si), silicon on insulator (SOI) or silicon on nothing (SON), as described in the paper “Silicon-on-Nothing MOSFETs: Performance, Short-Channel Effects, and Backgate Coupling” by Jérémy Pretet et al, IEEE Transactions on Electron Devices, Vol. 51, No. 2, February 2004. 
         [0004]    The threshold voltage generally depending on the doping of the channel of the transistor, differently implanted transistor channels are generally used. Another technique, known as the fully depleted (FD) technique, is used to produce transistors on undoped films. This technique may be used on SOI, as described in the paper “Threshold Voltage of Thin-Film Silicon-on-Insulator (SOI) MOSFETs” by Kyung-Kyu LIM et al., IEEE Transactions on Electron Devices, Vol. ED-30, No. 10, October 1983. This technique is referred to as the FDSOI (fully-depleted SOD or SON technique. It provides better electrostatic control, reduced variability and increased mobility. In this case, the threshold voltage is essentially dependent on the output work of the gate of the transistor (and little dependent on the doping). 
         [0005]    Accordingly, to obtain different threshold voltages in this architecture, one solution is to co-integrate gates with different output works. This approach has numerous drawbacks. Adjusting the output work of the gates and the co-integration remains a real technological and physical challenge. Adjusting the threshold voltages in FD devices generally also remains a major challenge blocking their further development. 
         [0006]    An MOS transistor with adjustable threshold voltage is known from the document European Patent Publication No. EP 0 409 697. In that document, a floating gate transistor has a threshold voltage that is adjusted continuously by trapping. Compared to the usual floating gate structure, the tunnel oxide in the above document is thinned above the drain to facilitate programming. However, in this case, the gate stack must at one and the same time be compatible with memory applications and logic applications, which is difficult to achieve. In logic applications, there is a benefit in increasing the gate/channel coupling (by reducing the thickness of the gate oxide), whereas in memory applications, a minimum tunnel oxide size must be maintained to guarantee good retention. The oxide is therefore necessarily thick, which is not the optimum for logic transistors. 
         [0007]    There is also known the document by H. Silva and S. Tiwari “A Nanoscale Memory and Transistor Using Backside Trapping”, IEEE Trans. on Nanotechnology Vol. 3 No. 12 June 2004. In that document, the threshold voltage of the transistor is modulated by trapping in the buried insulation and by capacitive coupling. The threshold voltage is always modulated under the same (binary) conditions. The curves in the above document are typical of the memory application, in terms of endurance and retention time. 
         [0008]    The corresponding U.S. Pat. No. 7,057,234 concerns the structure: silicon nanocrystals are referred to as possible trapping sites. The above document indicates the use of this structure either for transistors in logic mode or for memory points, which modes are distinguished by different bias sets. The above document proposes to use the trapping mechanism for memory points only. 
         [0009]    Thus the above document does not relate in any way to producing transistors with an adjustable threshold voltage, and notably transistors in which the threshold voltage is intentionally adjusted as a function of the logic or analog target application of the transistor (low threshold voltage transistor, standard threshold voltage transistor or high threshold voltage transistor). 
         [0010]    Thus there is at present no technological solution that makes it possible to produce FD transistors that are optimized, notably from an electrostatic point of view (and more particularly for short channels) and simultaneously have a threshold voltage that is adjustable as a function of the intended logic or analog application of the transistor. 
       SUMMARY 
       [0011]    The present invention aims to remedy these drawbacks. 
         [0012]    To this end, a first aspect of the present invention provides an electronic subsystem, characterized in that it includes:
       a semiconductor layer carrying at least one first transistor with an adjustable threshold voltage, and   an insulation layer including, at a predetermined first depth, a first trapping area extending at least under the channel of said first transistor and including traps with a density greater than the density of traps outside said first trapping area, the semiconductor layer and the first trapping area being capacitively coupled and the useful information of said first transistor being the transport of charges in that transistor.       
 
         [0015]    Because of the coupling of the traps with the gate, the present invention provides for a standard transistor application in which it is the transport of charges (in particular, the current) that constitutes the information, the voltage being one parameter among others of the current/voltage characteristic of the device, on the same terms as the mobility of the charge carriers, for example. Likewise, in this application, the state of charge of the structure is not the principal information. Thus in the “transistor” target application of the present invention, the information varies according to the function of the transistor. In analog or radio-frequency (RF) applications, the whole of a portion of the current/voltage characteristic of the transistor is used and constitutes the information. Elementary devices are not necessarily in matrix form but divided into blocks providing a particular function. 
         [0016]    Of the advantages of the present invention, the following may also be cited:
       the threshold voltage may be adjusted by bias of the area of the substrate under the transistor concerned (this is known as “Fowler Nordheim” writing),   the threshold voltage may be adjusted transistor by transistor by combining a substrate bias and a drain bias (this is known as hot carrier writing). In this case, the substrate terminal may be shared/pooled for a plurality of transistors without this assigning them a particular function relative to the value of the threshold voltage; this adjustment necessitates at least one terminal for controlling the voltage Vds at the terminals of the transistor for which the threshold voltage is to be adjusted or indirect control of the voltage Vds of the transistor via accessible terminals of the circuit in which the transistor is incorporated,   the threshold voltage may be adjusted by writing more or fewer buried traps quasi-continuously whereas, if using a plurality of gate output works or a plurality of channel dopings, only a discrete number of threshold voltage states is allowed, and the threshold voltage of a transistor may be reconfigured by bias (of the drain and substrate) after fabrication of the transistors. In the prior art, transistor threshold voltages are generally fixed (by the channel doping and the output work of the gate) during fabrication of the transistors, are not reconfigurable afterwards and may evolve in an uncontrolled manner, notably as a function of temperature or irradiation.       
 
         [0020]    The last two advantages cited above make it possible to envisage the following applications:
       dynamic allocation of consumption/speed parameters of blocks of circuitry,   compensation of threshold voltage variability caused by technological fluctuations,   compensation of threshold voltage drift caused by a change of external temperature (and control of consumption),   compensation of threshold voltage drift caused by use in an irradiated environment (and increase in system service life),   reconfigurable circuits, and   production of MOS transistors with a variable threshold voltage.       
 
         [0027]    Note that the useful information of each transistor is preferably the transport of charges within the transistor and not the state of charge of the transistor, as in the prior art. Thus the threshold voltage is adapted to optimize the operation of the transistor as a function of the logic or analog application of the transistor. 
         [0028]    According to particular features, the first trapping area extends only under the channel of said first transistor. 
         [0029]    According to particular features, the electronic subsystem of the present invention, as succinctly defined hereinabove, further includes a conductive area placed, relative to the insulation layer, on the other side of the semiconductor layer carrying at least one first transistor. 
         [0030]    This conductive area makes it possible to control the threshold voltage of each transistor it faces, this threshold voltage being modulated by the presence of the traps in an intermediate position between the transistor and the conductive area. 
         [0031]    According to particular features, the conductive area is an electrically conductive substrate. 
         [0032]    According to particular features, said conductive area is an area of a substrate under the insulation layer and including dopants, for example implanted dopants. 
         [0033]    According to particular features, the electronic subsystem of the present invention, as succinctly defined hereinabove, includes at least one second trapping area. 
         [0034]    According to particular features, the second trapping area is at a second depth different from the first depth. 
         [0035]    According to particular features, the second trapping area has a trap density distribution different from that of the first trapping area. 
         [0036]    According to particular features, said second trapping area is at least partly under a channel of a second transistor. 
         [0037]    According to particular features, said traps consist of nanocrystals. 
         [0038]    According to particular features, said nanocrystals include atoms of Si, Ge, metal or semiconductor/metal alloy in an insulation layer. 
         [0039]    According to particular features, said traps are formed by implanting atoms of N or F and heat treatment. 
         [0040]    According to particular features, the insulation layer is formed of a stack of two oxide layers, a continuous trapping layer being formed at the interface of the two oxide layers. 
         [0041]    According to particular features, the insulation layer is formed of a stack comprising an oxide layer, a continuous nitride or polysilicon trapping layer and an oxide layer. 
         [0042]    In this case, the upper oxide layer is used to inject charges by the tunnel effect into the trapping layer, the trapping layer is used to store charges in traps and the lower oxide layer is used to provide coupling between the trapping layer and a substrate or the conductive layer. 
         [0043]    A second aspect of the present invention provides a method of producing an electronic subsystem, characterized in that it includes:
       a step of assembling a semiconductor layer carrying at least one first transistor with an adjustable threshold voltage and an insulation layer, and   a step of forming in the insulation layer, at a predetermined first depth, a first trapping area extending at least under a channel of said first transistor and including traps with a density greater than the density of traps outside said first trapping area so that the semiconductor layer and the first trapping area are capacitively coupled, the useful information of said first transistor being the transport of charges in that transistor.       
 
         [0046]    According to particular features, during the step of forming the first trapping area, a first implantation of atoms of Ge, Si, metal or semiconductor/metal alloy in the insulation layer is effected. 
         [0047]    According to particular features, during the step of forming the first trapping area, annealing to form nanocrystals is effected. 
         [0048]    According to particular features, the method of the present invention, as succinctly defined hereinabove, includes a step of forming a second trapping area extending at least under a channel of a second transistor by a second implantation with an energy and/or a dose and/or atoms different from those used in the first implantation. 
         [0049]    According to particular features, during the step of forming the first trapping area, masking to position the first trapping area is effected. 
         [0050]    According to particular features, during the step of forming the first trapping area, direct gate alignment through the gate stack is effected, the transistor gate being used as a mask to align traps with the channel of the transistor. 
         [0051]    According to particular features, during the step of forming the first trapping area, a gate last alignment of the transistors is effected by emptying a gate last cavity of at least one transistor, effecting implantations in the cavities and then filling each cavity with a new gate stack. 
         [0052]    According to particular features, the method of the present invention, as succinctly defined hereinabove, includes a step of FN writing a transistor of the electronic subsystem. 
         [0053]    According to particular features, the method of the present invention, as succinctly defined hereinabove, includes a step of hot carrier writing a transistor of the electronic subsystem. 
         [0054]    Note that it is beneficial to use a buried oxide (or a buried oxide stack) that is thin or has a high dielectric constant to reduce the bias of the substrate to be applied to charge the traps of the oxide or oxide stack. Stacks allowing deletion/writing at around 10 volts have been produced by the inventors for buried stacks of 5 nm tunnel oxide/70 nm nitride/70 nm oxide. 
         [0055]    According to particular features, the method of the present invention, as succinctly defined hereinabove, includes a step of bonding stacks of layers to associate the semiconductor layer, the insulation layer and a conductive layer. 
         [0056]    The particular advantages, objects and features of this method being similar to those of the electronic subsystem of the present invention, as succinctly defined hereinabove, they are not repeated here. 
     
    
     
       BRIEF DESCRIPTION OF DRAWING 
         [0057]    Other advantages, objects and features of the present invention emerge from the following description given by way of nonlimiting explanation with reference to the appended drawings, in which: 
           [0058]      FIGS. 1 to 5  represent different steps in the production of the electronic subsystem of a first particular embodiment of the present invention, 
           [0059]      FIG. 6  represents diagrammatically an alternative step in the production of the electronic subsystem shown in  FIG. 5 , 
           [0060]      FIGS. 7 to 9  represent diagrammatically steps of the production of the electronic subsystem of a second particular embodiment of the present invention, 
           [0061]      FIGS. 10 to 12  represent diagrammatically steps of the production of the electronic subsystem of a third particular embodiment of the present invention, 
           [0062]      FIGS. 13 to 15  represent diagrammatically steps of the production of the electronic subsystem of a fourth particular embodiment of the present invention, 
           [0063]      FIGS. 16 and 17  represent diagrammatically steps of the production of the electronic subsystem of a fifth particular embodiment of the present invention, and 
           [0064]      FIG. 18  represents as seen from above a configuration allowing two modes of writing an electronic subsystem of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0065]    As seen in  FIG. 1 , to fabricate the electronic subsystem of the first embodiment of the present invention, the starting point is an assembly of layers including:
       a substrate  101 ,   a buried insulation layer  102 , referred to as a “trapping layer”, capable of trapping charges through the effect of an electric field, and   an upper semiconductor  103 , etched or not.       
 
         [0069]    The trapping layer  102  and the upper semiconductor layer  103  are capacitively coupled to enable production in the semiconductor layer of an FD (Fully Depleted) transistor. 
         [0070]    Then, as shown in  FIG. 2 , a continuous or non-continuous conductive layer  104  is produced under the buried insulation  102 , for example by implanting dopant in the substrate  101  using masks. It is seen that this conductive layer  104  can be formed in an analogous manner to a box terminal or substrate terminal in the case of PDSOI (partially depleted SOI) transistors. This conductive layer  104  may be omitted if the substrate  101  is itself sufficiently conductive to provide that function, as explained hereinafter. 
         [0071]    Then, as shown in  FIG. 3 , the active area of the upper semiconductor  103  is etched. 
         [0072]    As shown in  FIG. 4 , MOS transistors  110  are then produced on the surface of the semiconductor layer  103  in a manner that is known in itself. During the process of fabricating the MOS transistors, a step of etching the buried insulation  102  using masks is carried out. This step is advantageously carried out before any silicidation of the source/drain and the gate of the transistors. 
         [0073]    Finally, as shown in  FIG. 5 , an insulator  112  is deposited on the transistors  110 , after which contacts (not shown) are formed on the gate and on the active areas, including at least one contact  111  on the conductive area  104 . 
         [0074]    The step shown in  FIG. 2  that relates to the production of a conductive layer  104  may, in the case of a continuous conductive layer  114 , be replaced by a step of bonding two stacks as seen in  FIG. 6 . The first stack includes the substrate  101  associated with the conductive layer  114 , for example formed of metal or a doped semiconductor. The second stack includes at least the insulation or trapping layer  102  and the semiconductor layer  103 . Alternatively, the bonding relates to, on the one hand, the substrate  101  and, on the other hand, the semiconductor layer  103 , the trapping insulation layer  102  and the conductive layer  114 . Alternatively, if the trapping insulation layer  102  is itself formed by a stack of a plurality of layers  121 ,  122  and  123  (see  FIGS. 16 and 17 ), for example as in the case of a nitride or polysilicon trapping layer disposed between two oxide layers, the bonding may be effected between two of those layers. In all cases, it is also possible to provide between the substrate  101  and the conductive layer  114  an insulation, for example SiO 2 , layer, with bonding taking place at the level of this layer or not. 
         [0075]    Alternatively, the insulation layer is formed of a stack of two oxide layers, a continuous trapping layer being formed at the interface of the two oxide layers. Accordingly, to form the first trapping area, it is possible to molecularly bond two insulation layers, the trapping area appearing at the bonding interface. 
         [0076]    Whether the step of producing a conductive layer is effected as shown in  FIG. 2  or as shown in  FIG. 6 , the trapping insulation layer  102  may be a stack of a type known in the memory application art. For example, as shown in  FIGS. 16 and 17 , the trapping insulation layer  102  may be a stack comprising an oxide layer  121 , a trapping layer  122  and an oxide layer  123 . In this case, the upper oxide layer  121  is called the tunnel oxide layer. The upper oxide layer  121  is used to inject charges into the trapping layer  122  by the tunnel effect. The thickness of the tunnel oxide layer  121  is of the order of a few nanometers, typically 10 nm. The tunnel oxide layer  121  is typically formed in SiO 2  and/or in a high-K material such as HfO 2  or Al 2 O 3 . Using a high-K material makes it possible to reduce the thickness of the tunnel oxide layer  121  and the programming voltages without excessively penalizing retention. 
         [0077]    The trapping layer  122  stores charges in traps. Its thickness is of the order of a few nanometers to a few tens of nanometers. It may be in polysilicon, nitride, Al 2 O 3 , sub-stoichiometric SiO 2  with nanocrystals of Si, Ge, metal or semiconductor/metal alloy, produced by deposition and annealing or by implantation as disclosed in the document US Publication No. 2007 0105310. The last three options, using nitride, Al 2 O 3  or SiO 2  with localized trapping, make it possible to reduce the thickness of the tunnel oxide layer  121  relative to polysilicon. 
         [0078]    Alternatively, forming the trapping sites includes implanting atoms of N (nitrogen) or F (fluorine), which do not create nanocrystals but also make it possible to create or heal traps. 
         [0079]    Accordingly, discrete trapping sites are generally formed, for example after implantation of atoms of N or F and heat treatment and/or forming nanocrystals as explained above. 
         [0080]    The oxide layer  123  is referred to as the “control oxide” or “interpoly” layer. The oxide layer  123  provides coupling between the trapping layer and the control “gate” that consists of the substrate  101  or the conductive layer  104  when present. Note that it is possible for there to be no conductive layer  104  if the substrate  101  is sufficiently conductive. In this case, the transistors cannot be controlled individually, unless control via the drain (known as “hot carrier” control) is used. The control oxide layer  123  may consist of SiO 2 , Si 3 N 4  or Al 2 O 3 , the latter material being better in terms of the equivalent oxide thickness (EOT) vs. leakage trade-off, or an SiO 2 /Al 2 O 3  or high-K/SiO 2  stack, which has the advantage of a better EOT vs. leakage trade-off. 
         [0081]    The insulation layer  102  may be continuous or non-continuous. It may be partly discontinuous if it consists of a stack, as in the fifth embodiment described with reference to  FIGS. 16 and 17 . 
         [0082]    To produce the electronic subsystem of the second particular embodiment of the present invention shown in  FIG. 7 , the starting point is an SOI or SON structure including:
       the substrate  101 ,   a buried insulation layer  105 , preferably in SiO 2 ,   the upper semiconductor  103 , etched or not.       
 
         [0086]    Then, as shown in  FIG. 8 , atoms of Si, Ge, metal or a metal/semiconductor alloy are implanted in the insulation layer  105  followed by annealing to form nanocrystals (also known as “traps” or “trap centers”)  220 . In this case the insulation layer  105  has the same function as the layers  121  and  123  described above and the trapping centers  220  assume the role of the layer  122 . Thus the layers  121  and  123  may be defined by the regions without implantation of the insulation layer  105  and the layer  122  may be defined by the region with implanted trapping centers  220 . It is seen that the implantation depth is controlled by the implantation energy. The trapping center density is controlled by the implantation dose. The distribution of the trapping centers is produced by masking. 
         [0087]    As shown in  FIG. 9 , MOS transistors  110  are then produced on the surface of the semiconductor layer  103 , in a manner known in itself, an insulator  112  is deposited on the transistors  110 , and contacts are formed on the gate and/or the active areas (not shown), a contact  111  being produced on the conductive area  104 . 
         [0088]    The second embodiment is advantageous in that:
       it necessitates no substrate other than a standard SOI or SON substrate,   it makes it possible to create a plurality of trapping regions in the insulation layer  105 , with positions (at a depth controlled by the implantation energy) or trap densities (controlled by the implanted dose) that are different and by effecting successively or simultaneously a plurality of implantations with different depths and/or doses and/or elements. This in particular makes it possible to modify the possible threshold voltages for the same writing condition. It is thus possible to define more or less sensitive writing regions. For example, regions may be provided in which the threshold voltage can vary very greatly (thanks to a high density of traps  220  or to traps  220  closer to the surface) and others where the modulation of the threshold voltage is weaker for given bias conditions (lower density of traps  220 ′ or traps  220 ′ implanted more deeply), and   it makes possible a local approach: masking may be used to localize the trapping areas  220  in the insulation layer  105 . In particular, the trapping areas  220  may be placed under the transistors  110  for which the threshold voltage must be changed. To be more precise, it is possible to localize this implantation under the channel only (and not also under the source and drain), which makes it possible to reduce trap discharge currents and stray capacitances.       
 
         [0092]    In the third embodiment, which is described with reference to  FIGS. 10 to 12  and constitutes a variant of the second embodiment, implantation is effected through the gate stack (this is known as “direct gate implantation”). As shown in  FIG. 11 , the trapping centers  220  are implanted using the gate of the transistor  110  as a mask to obtain self-alignment of the trapping centers  220  on the channel of the transistor  110 . Implantation is effected throughout the structure. Given the additional thickness of the stack at the gate, implantation at the gate is less deep than in the rest of the structure. The species implanted outside the channel are therefore implanted deeper. As shown in  FIGS. 11 and 12 , they may even be returned to the conductive area  104 .  FIG. 12  shows the final structure. 
         [0093]    In the fourth embodiment, which is described with reference to  FIGS. 13 to 15  and also constitutes a variant of the second embodiment, gate last integration of the transistors  110  is effected and implantation is effected in the cavities or through the gate in a direct gate (gate first) integration. 
         [0094]      FIG. 13  shows again the structure described with reference to  FIGS. 4 and 5 . As shown in  FIG. 14 , for some transistors (here the transistors  132  and  133  on the right-hand side of  FIG. 14 ) the gate last cavity  151  is then emptied in the future gate areas, using a mask. The mask  141  makes it possible to avoid this operation at the level of the transistor  131 , for example. It also masks subsequent implantations. It is advantageously aligned to the gates of the transistors  151  under the channel of which the implanted atoms are to be positioned. 
         [0095]    Implantation follows, to form nanocrystals  220 . The structure being finer at the openings  151 , implantation in this area is deeper than in the rest of the structure. As shown for the transistor  132 , it is advantageously possible to provide for implantation through a mask  142  to limit implantation solely to the level of the channel of the transistor  132 . 
         [0096]    As shown in  FIG. 15 , the cavities are afterwards filled with a stack of gates based on polysilicon, for example. The contact terminals are then produced to form the final structure represented in  FIG. 15  (not all the contacts are shown, in particular the source, drain, gate contacts). 
         [0097]    The third and fourth embodiments are advantageous through not creating stray currents or capacitances. It is principally the region under the channel that is charged, because of the shielding by the source/drain, and that controls the threshold voltage, for the same reason. The trap regions placed under the sources and the drains may induce stray capacitance between the source/drain and the trapping areas and stray currents that may lead to discharging of the traps, in particular for a continuous polysilicon layer and, to a lesser degree, a discontinuous trapping area. 
         [0098]    The third and fourth embodiments are also advantageous as they make it possible, as in the second embodiment, to adjust different transistors (or groups of transistors) with the same write or delete bias by locally implanting with different species and/or doses and/or energies, depending on the transistors (or groups of transistors), making it possible to obtain traps that are more or less deep and traps that are more or less dense, depending on the transistors (or groups of transistors). 
         [0099]    In the electronic subsystem of the fifth particular embodiment of the present invention described with reference to  FIGS. 16 and 17 , a stack of layers  121 ,  122 ,  123  as described above is used, notably if the layer  122  is a continuous polysilicon or nitride trapping layer. In this embodiment, it is preferable to etch at least the layers  121  and  122  and, where applicable, the layer  123 , in the insulation areas of the transistors  110 , when forming this insulation, to define localized layers  122  under only some transistors  110 . Accordingly, on writing, only some of the transistors  110  may have a controlled threshold voltage, thanks to this insulation. 
         [0100]    As shown in  FIG. 16 , starting from the structure shown in  FIG. 2 , with a stack of layers  121 ,  122  and  123 , to form the insulation layer  102 , the layer  122  is etched to insulate it laterally, which is especially advantageous in the case of a continuous trapping layer. 
         [0101]    At the end of each way of producing an electronic subsystem of the present invention, the final steps correspond to the production of the transistors and the contact terminals as explained with reference to the first embodiment. 
         [0102]      FIG. 17  thus represents the final structure for the fifth particular embodiment. 
         [0103]    There follows a description of the general principle of modifying the transistor threshold voltage. 
         [0104]    For an SOI or SON FD transistor, the trapping of charges fixed by the buried insulation induces offsetting of the threshold voltage of the transistor by capacitive coupling. Applying certain biases then enables writing by injecting charges into the insulation layer  102  or  105 , to be more precise into the layer  122 , where necessary. 
         [0105]    Once this writing is done, the charges remain in this layer for a time depending on the retention time of the structure (more than 10 years in the case of stacks used in non-volatile memories). No bias is necessary to maintain the charges. Writing biases may be applied once and for all. It is possible, as an alternative, to delete at any time (which entails reduction or even disappearance of the stored charges) or if necessary to rewrite at any time (to modify the quantity of charges or to refresh the memory). 
         [0106]    As shown from above in  FIG. 18 , on the upper surface of the electronic subsystem are located a substrate terminal  301 , sets  302  of transistors or logic blocks under the influence of the substrate terminal  301  and sets  303  of transistors or logic blocks that are not under the influence of the substrate terminal  301 . This configuration enables an FN writing mode in which all or part of the circuit is controlled by bias of the buried electrode (or by direct bias of the substrate if there is no buried electrode and if the substrate is sufficiently conductive). Thus the Vbs of the transistor is controlled, in particular, the potential difference between the substrate or bulk and the source of the transistor. 
         [0107]    In a “hot carrier” second writing mode, only the transistors selected by a high Vds, in particular, a high potential difference between the drain and the source, and a high potential difference between the substrate and the source of the transistor have their threshold voltage adjusted. This writing mode enables transistor by transistor control requiring fewer substrate terminals. If the conductivity of the conductive area is perfect, only one substrate terminal is necessary on the electronic subsystem, or possibly no terminals are necessary if the substrate is sufficiently conductive, the bias being effected by the substrate itself. 
         [0108]    Among the advantages of the present invention there may be cited:
       the threshold voltage may be adjusted by bias of the area of the substrate under the transistor concerned (this is known as Fowler Nordheim writing),   the threshold voltage may be adjusted transistor by transistor by combining a substrate bias and a drain bias (this is known as “hot carrier writing”). In this case, the substrate terminal may be shared/pooled for a plurality of transistors without this assigning them a certain function in relation to the value of the threshold voltage. This adjustment necessitates at least one terminal for controlling the voltage Vds at the terminals of the transistor the threshold voltage of which is to be adjusted or indirect control of the voltage Vds of the transistor via accessible terminals of the circuit in which the transistor is included,   the threshold voltage may be adjusted by writing more or fewer buried traps, in quasi-continuous manner, whereas only a discrete number of threshold voltage states is permitted if using a plurality of gate output works or a plurality of channel dopings, and   the threshold voltage of a transistor may be reconfigured by (drain and substrate) bias after fabrication of the transistors. In the prior art, the threshold voltages of the transistors are generally fixed (by the channel doping and the gate output work) during fabrication of the transistors, are not reconfigurable afterwards and may evolve in an uncontrolled manner, notably as a function of temperature or irradiation.       
 
         [0113]    The last two advantages cited above make it possible to envisage the following applications:
       dynamic allocation of consumption/speed of blocks of circuitry parameters,   compensation of variability of the threshold voltage because of technology fluctuations,   compensation of the drift of the threshold voltage caused by a change of external temperature (and control of consumption),   compensation of the drift of the threshold voltage caused by use in an irradiated medium (and increased system service life),   reconfigurable circuits and   production of MOS transistors with variable threshold voltage.       
 
         [0120]    It is seen that the useful information of each transistor consists of the transport of charges within the transistor and not the state of charge of the transistor, as in the prior art. Thus the threshold voltage is adapted to optimize the operation of the transistor as a function of the logic or analog application of the transistor. 
         [0121]    Because of the coupling of the traps with the gate, the present invention enables a standard transistor application in which it is the transport of charges (in particular, the current) that constitutes the information, the voltage being one parameter among others of the current/voltage characteristic of the device, on the same terms as the mobility of the charge carriers, for example. Similarly, in this application, the state of charge of the structure is not the principal information. Thus in the “transistor” application targeted by the present invention, the information varies according to the function of the transistor. In an analog or radio-frequency (RF) application, the whole of a portion of the current/voltage characteristic of the transistor is used and constitutes the information. 
         [0122]    It is seen that it is beneficial to use a buried oxide (or buried oxide stack) that is thin or that has a high dielectric constant to reduce the substrate bias to be applied to charge the traps of this oxide or oxide stack. The inventors have produced stacks enabling deletion/writing at around 10 volts.