Abstract:
An IC has cells placed in a cell row having a UTBOX-FDSOI pMOSFET including a ground beneath the pMOS, and an n-doped well beneath it and configured to apply a potential thereto, and a UTBOX-FDSOI nMOSFET including a ground beneath the nMOS, and a p-doped well beneath the ground and configured to apply a potential thereto, and cells, each including a UTBOX-FDSOI pMOSFET including a ground beneath the pMOS, and a p-doped well beneath the ground and configured to apply an electrical potential to the ground, and a UTBOX-FDSOI nMOSFET including a ground beneath the nMOS, and an n-doped well beneath the ground and configured to apply a potential thereto. The cells are placed so that pMOS&#39;s of standard cells belonging to a row align along it and a transition cell including a another well and contiguous with first row standard cells thus ensuring continuity with wells of those cells.

Description:
FIELD OF INVENTION 
     The present invention relates to semiconductor integrated circuits and, more specifically, integrated circuits manufactured on a SOI (Silicon on Insulator) substrate. 
     BACKGROUND 
     In SOI technology, a thin layer of silicon (typically featuring a thickness of a few nanometers) is separated from a semiconductor substrate by a relatively thick electrically insulating layer (typically featuring a thickness of a few tens of nanometers). 
     Integrated circuits in SOI technology offer a number of advantages compared to traditional “bulk” technology for CMOS (Complementary Metal Oxide Semiconductor) integrated circuits. SOI integrated circuits typically provide a lower power consumption for a same performance level. Such circuits also feature a reduced stray capacitance, allowing an increase of commutation speeds. Furthermore, the latch-up phenomena encountered in bulk technology can be mitigated. Such circuits are therefore particularly adapted to SoC (System on Chip) or MEMS (Micro electro-mechanical systems) applications. SOI circuits also are less sensitive to ionizing radiations, making them more reliable than bulk-technology circuits in applications where said radiations may induce operating problems, such as aerospace applications. SOI integrated circuits can include memory components such as SRAM (Static Random Access Memory), or logic gates. 
     Much research has been conducted on reducing the static power consumption of logic gates, while increasing their commutation speed. Some integrated circuits combine both logic gates with low power consumption, and logic gates with high commutation speed. In order to integrate two such logic gates on a same integrated circuit, it is known to lower the threshold voltage (typically noted V T  or V th ) of some transistors belonging to the high-speed logic gates, and to lower the threshold voltage of some other transistors of the low-consumption logic gates. In bulk technology, threshold voltage modulation is implemented by differentiating the doping level of the semiconductor canal of these transistors. However, FDSOI (Fully Depleted Silicon On Insulator) transistors have, by design, a depleted canal, featuring a low doping level (typically 10 15  cm −3 ). Due to this low doping level, it is not possible to modulate the threshold voltage of transistors with the method used in bulk technology. Some studies have proposed integrating different gate materials in otherwise identical transistors, in order to obtain differing threshold voltages. However, implementing this solution is technically challenging and economically prohibitive. 
     In order to obtain different threshold voltages for transistors in FDSOI technology, it is also known to include an electrically biased ground plane (also named back plane, or back gate), located between a thin electrically insulating oxide layer, and the silicon substrate. This technology is often known as UTBOX (for Ultra-Thin Buried OXide layer). By adjusting the doping levels of, and the electrical bias applied to these ground planes, it is therefore possible to define several ranges of threshold voltages for said transistors. For example, it is possible to define low-threshold voltage transistors (LVT), high-threshold voltage transistors (HVT) and medium or standard threshold voltage transistors (SVT). 
     Some publications have proposed modifying the structure of FDSOI integrated circuits. A practical problem, as with any such technological evolution, is that the software used to design said circuits may end up being incompatible with the modified circuits and may require substantive development. 
     As large-scale integrated circuits have become too complex to be designed by hand, circuit designers typically rely on computer-assisted design (CAD) software tools, also known as electronic design automation (EDA). For current technology nodes, numerous parameters must be taken into account in order to avoid a malfunction or a destruction of the circuits. 
     Many EDA tools use a functional specification as input. This functional specification describes the desired behaviour of the circuit, as well as non-functional design constraints (such as, for example, circuit surface, cost, and power consumption). ESD tools output a computer file describing a circuit at a physical level (usually in the GDSII file format or, more recently, the OASIS file format). This computer file defines layouts used to manufacture masks. Such masks are then used in semiconductor foundries, during photolithography steps of the integrated circuit fabrication process. 
     Standard EDA design flow typically comprises several steps. 
     First, starting with a user-specified functional specification of the circuit, the concept and the global architecture of the circuit is modelled at a high-level of abstraction. The performance of this modelled circuit is then validated. Typically, at this step, the circuit is modelled using a description language such as Verilog, VHDL, SPICE or other. 
     Then, during floorplanning step, the position of power connections and portions of the circuit is roughly mapped. 
     Then, a logic synthesis of the circuit is performed. The circuit is modelled at a register-transfer level (RTL). More specifically, the implementation of the circuit is modelled as a combination of several sequential elements as well as logic combinations between the respective inputs and outputs of the sequential elements and the primary inputs and outputs of the integrated circuit. This modelling provides a network, formed essentially of logic gates and hardware registers. This modelling is typically performed using a description language, such as Verilog or VHDL. For example, the RTL modelling is performed using elementary logic circuits (such as AND logic gates, OR logic gates, multiplexers . . . ) and sequential circuits (such as flip-flops . . . ) provided by a standard cell library. At this point of the process, the exact position of each element is not yet specified; the circuit is only represented as a list of elements required to implement the desired circuit functionality. 
     Then, a high-level synthesis (or algorithmic synthesis) of the circuit behaviour is performed, in order to simulate the time-dependent behaviour of the RTL model. 
     During a step of logic synthesis, or logic design, the circuit is implemented at a logic gate level, and described by a gate netlist. This gate netlist is generated from the RTL model and from a design library. Such design libraries usually include hundreds of logic circuit elements. Design libraries depend on the technology used for the fabrication process (such as the technology node, foundry-specific design rules . . . ). 
     The gate netlist outputted by the logic design is generally a computer file describing an instantiation of the logic gates of the circuit as well as their respective interconnections. This gate netlist may be described in a description language such as Verilog, VHDL or EDIF. 
     The logic design is followed by a step of placement and routing, or place-and-route. During this place-and-route step, the elements of the previously-defined gate netlist are automatically placed and connected, depending on the user specifications. 
     The logic design of UTBOX FDSOI circuits typically relies on commercially available EDA tools. As it is desirable to minimize the disruption of established EDA design flows and to avoid any extensive rewriting of existing EDA software, some steps of the design process may reuse elements initially defined for bulk technology. For example, the place-and-route step for UTBOX FDSOI circuits often reuses standard cell libraries containing bulk-technology elements. Additional automated transformations must then be performed, after said place-and-route step, in order to obtain a UTBOX FDSOI-compliant circuit layout. 
     However, said logic design may also be performed using a dedicated standard cell library containing UTBOX FDSOI-specific elements. 
     UTBOX FDSOI standard cells often include a nMOS transistor and a pMOS transistor, both formed in the thin silicon layer. This thin silicon layer lies onto the buried insulating oxide layer. The thickness of this oxide layer is typically smaller than 50 nanometers. A semiconductor ground plane, or back-gate, is established under each pMOS and nMOS, below the oxide layer. Each of these ground planes is electrically biased through a semiconductor well. The semiconductor well of each pMOS or nMOS transistor lies below the respective semiconductor ground plane belonging to said transistor, under a deep insulation trench. The threshold voltage of the transistors is adjusted by applying, among other parameters, an appropriate voltage on the respective semiconductor wells. In order to increase the possible combinations of threshold voltage ranges, the ground plane may be doped with either p-type or n-type impurities, for either the pMOS or the nMOS transistors. 
     In a first configuration, the ground planes of pMOS transistors are electrically biased through an n-doped well; the ground planes of nMOS transistors are electrically biased through a p-doped well. This configuration will henceforth be named regular. 
     Additionally, while the wells of pMOS transistors are usually biased at an electrical potential Vdd and the wells of nMOS transistors are usually biased at an electrical potential GND, said electrical potentials may be modulated in order to adjust the threshold voltages of said transistors. For example, a forward back biasing (FBB) scheme is commonly used. FBB includes applying a GND+ΔV electrical potential on the wells of nMOS transistors, and a Vdd−ΔV electrical potential on the wells of pMOS transistors. The value of ΔV is chosen smaller than Vdd/2, to avoid the formation of an undesirable forward bias between the n-doped and the p-doped wells of the respective pMOS and nMOS transistors. This forward bias would lead to a leakage current between said wells, which would have adverse consequences on the electrical properties of the circuit. 
     To remove this limitation on the value of ΔV, it is known to switch the doping type of the wells of pMOS and nMOS transistors. In that configuration, henceforth named flipped, pMOS transistors have a p-doped well, and nMOS transistors have a n-doped well. With this configuration, a different biasing scheme can be used. 
     In order to increase the performance of the circuit and get the benefits of both flipped and regular configurations, it has been proposed to co-integrate regular and flipped standard cells on a same circuit. This co-integration allows multiple ranges of threshold voltage for the transistors of said circuit, thus leading to a better flexibility of operation. 
     However, circuits comprising both cells of regular and flipped configurations may have electrical and design-related issues. The abrupt discontinuity between the respective n-doped and p-doped wells of two contiguous regular and flipped standard cells gives rise to so-called singularity points, which may not satisfy design rule checking steps of the design process and may cause mask design problems. This discontinuity may also prevent an adequate electrical biasing of the semiconductor wells. These design issues may affect the reliability of the circuit fabrication process. 
     SUMMARY 
     Therefore, there is a need for an integrated circuit of UTBOX FDSOI technology with multiple threshold voltage co-integration, featuring an increased level of performance and a more reliable design process. 
     It is therefore an object of the present invention to provide an integrated circuit, comprising a plurality of cells placed in a first row, said first row including
         a plurality of first cells, each including:
           a first pMOS field effect transistor of UTBOX FDSOI technology, including
               a first semiconductor ground plane, lying beneath the first pMOS transistor;   a first semiconductor well having a n-type doping, lying beneath the first semiconductor ground plane and able to apply an electrical potential to said first semiconductor ground plane;   
               a first nMOS field effect transistor of UTBOX FDSOI technology, including
               a second semiconductor ground plane, lying beneath the first nMOS transistor;   a second semiconductor well having a p-type doping, lying beneath the second semiconductor ground plane and able to apply an electrical potential to said second semiconductor ground plane;   
               
           a plurality of second cells, each including:
           a second pMOS field effect transistor of UTBOX FDSOI technology, including:
               a third semiconductor ground plane, lying beneath said second pMOS transistor;   a third semiconductor well having a p-type doping, lying beneath the third semiconductor ground plane and able to apply an electrical potential to said third semiconductor ground plane;   
               a second nMOS field effect transistor of UTBOX FDSOI technology, including:
               a fourth semiconductor ground plane, lying beneath the second nMOS transistor;   a fourth semiconductor well having a n-type doping, lying beneath the fourth semiconductor ground plane and able to apply an electrical potential to said fourth semiconductor ground plane;   
               
           said first and second cells being placed so that the pMOS transistors of said first and second standard cells belonging to said first row are aligned along said first row;   a first transition cell including a fifth semiconductor well,       

     in which said first transition cell is contiguous to a first and a second standard cells of said first row, so as to ensure electrical continuity with either one or the other of the n-doped first and fourth semiconductor wells or the p-doped second and third semiconductor wells of first and second standard cells. 
     In a first illustrative embodiment, the circuit contains a second and a third rows, both second and third rows being adjacent to the first row and comprising each a plurality of additional cells, said additional cells being placed adjacent to the contiguous first, second cells and first transition cell of the first row, said plurality of additional cells comprising either:
         first cells but no second cells, or   second cells but no first cells.       

     In another illustrative embodiment, said plurality of additional cells is devoid of first transition cells. 
     In another illustrative embodiment:
         the fifth semiconductor well has a p-type doping;   the first row includes a n-doped deep semiconductor well lying beneath the first to fifth semiconductor wells, said deep semiconductor well being able to ensure an electrical continuity between the second and third semiconductor wells.       

     In another illustrative embodiment:
         said circuit includes a p-doped substrate lying beneath the first to fourth semiconductor wells of the respective pMOS and nMOS of the first and second cells;   the fifth semiconductor well has an n-type doping.       

     In another illustrative embodiment:
         the circuit includes a p-doped substrate lying beneath the first to fourth semiconductor wells of the respective pMOS and nMOS of the first and second cells;   the fifth semiconductor well has a p-type doping;   the circuit includes a second transition cell comprising:
           a sixth semiconductor well, said sixth semiconductor well having a n-type doping;   an electrical contact for applying an electrical potential to the sixth semiconductor well;   
           the first and second transition cells of the first row are:
           each contiguous to a first and a second standard cells belonging to said first row, and   placed in an alternating pattern along said first row.   
               

     In another illustrative embodiment:
         the fifth semiconductor well has a p-type doping;   the first transition cell includes an electrical contact for applying an electrical potential to the fifth semiconductor well;   the circuit includes a second transition cell comprising a sixth semiconductor well having an n-type doping;   the first row includes a n-doped deep semiconductor well lying beneath the first to sixth semiconductor wells, said deep semiconductor well being able to ensure an electrical continuity between the second and third semiconductor wells;   the first and second transition cells of the first row are:
           each contiguous to a first and a second standard cells belonging to said first row, and   
               

     placed in an alternating pattern along said first row. 
     In yet another illustrative embodiment, is therefore an object of the present invention to provide a method for generating automatically an integrated circuit layout, comprising steps of:
         automatically placing a plurality of first and second standard cells in a first row of said circuit layout, each of the first standard cells including:
           a first pMOS field effect transistor of UTBOX FDSOI technology, including:   a first semiconductor ground plane, lying beneath the first pMOS transistor;   a first semiconductor well having a n-type doping, lying beneath the first semiconductor ground plane and able to apply an electrical potential to said first semiconductor ground plane;   a first nMOS field effect transistor of UTBOX FDSOI technology, including:   a second semiconductor ground plane, lying beneath the first nMOS transistor;   a second semiconductor well having a p-type doping, lying beneath the second semiconductor ground plane and able to apply an electrical potential to said second semiconductor ground plane;   
           and each of the second standard cells including:
           a second pMOS field effect transistor of UTBOX FDSOI technology, including:   a third semiconductor ground plane, lying beneath said second pMOS transistor;   a third semiconductor well having a p-type doping, lying beneath the third semiconductor ground plane and able to apply an electrical potential to said third semiconductor ground plane;   a second nMOS field effect transistor of UTBOX FDSOI technology, including:   a fourth semiconductor ground plane, lying beneath the second nMOS transistor;   a fourth semiconductor well having a n-type doping, lying beneath the fourth semiconductor ground plane and able to apply an electrical potential to said fourth semiconductor ground plane;   
               

     said first and second standard cells being automatically placed so that the pMOS transistors of said first and second standard cells belonging to said first row are aligned along said first row;
         generating a semiconductor mask layout for the fabrication of a UTBOX FDSOI integrated circuit including said first row;       

     wherein placing the first and second cells includes a step of inserting a first transition cell between first and second contiguous standard cells of the first row, so as to ensure electrical continuity with either one or the other of the n-doped first and fourth wells or the p-doped second and third wells of first and second standard cells belonging to said row, each first transition cell including a fifth semiconductor well. 
     In another illustrative embodiment, the method includes a step of placing a plurality of additional cells into a second and a third rows of the circuit, said second and third rows being adjacent to the first row, said additional cells being placed adjacent to the contiguous first, second cells and first transition cell of the first row, said plurality of additional cells comprising either:
         first cells but no second cells, or   second cells but no first cells.       

     In another illustrative embodiment, placing a plurality of second standard cells in the first row includes steps of:
         automatically placing first standard cells in said first row;   automatically switching the doping type of respective first and second ground planes of some of said first standard cells, in order to obtain second cells.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantage of the present invention will become apparent from the following description of several embodiments with reference to the accompanying drawings, in which: 
         FIG. 1  is a lateral cross-section view of a first standard cell of a UTBOX FDSOI technology integrated circuit; 
         FIG. 2  is a lateral cross-section view of a second standard cell of a UTBOX FDSOI technology integrated circuit; 
         FIG. 3  is a simplified view from above of a portion of an integrated circuit of UTBOX FDSOI technology according to a usually automatically rejected design, comprising the first and second standard cells of  FIGS. 1 and 2  placed into a first row; 
         FIG. 4  is a simplified top view of a portion of an integrated circuit of UTBOX FDSOI technology, comprising first transition cells and the first and second standard cells of  FIGS. 1 and 2  placed into a first row; 
         FIG. 5  is a simplified top view of another embodiment of the circuit of  FIG. 4 ; 
         FIG. 6  is a simplified top view of another embodiment of the circuit of  FIG. 4 ; 
         FIG. 7  is a simplified top view of another embodiment of the circuit of  FIG. 4 ; 
         FIG. 8  is a simplified top view of another embodiment of the circuit of  FIG. 4 ; 
         FIG. 9  is a flowchart detailing a method for generating an integrated circuit layout. 
         FIG. 10  illustrates a method for generating a layout of the circuit shown in  FIG. 5 ; 
         FIG. 11  illustrates a method for generating a layout of the circuit shown in  FIG. 6 ; and 
         FIG. 12  illustrates a method for generating a layout of the circuit shown in  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a first standard cell  2  typically used in SOI (Silicon On Insulator) CMOS integrated circuits. Cell  2  includes a pMOS  4  and a nMOS  6  field effect transistors of FDSOI (Fully Depleted Silicon On Insulator) technology. 
     Transistor  4  includes:
         an active silicon layer  8     an ultra-thin buried oxide (UTBOX) insulator layer  10 ,   a semiconductor ground plane  12 ,   a semiconductor well  14 ,   optionally a semiconductor substrate  16 .       

     The silicon layer  8  includes a source, a canal  18  and a drain. A gate oxide layer  20  covers canal  18 . Said layer  20  is covered by a gate stack comprising metal layers  22  and polysilicon layers  24 . Said stacks are laterally delimited by spacers  26 . Isolation trenches  28 ,  29  are placed on sides of the transistor  4 . The source and drain of layer  8  are doped with a p-type impurity. As known in FDSOI technology, canal  18  has a low doping level so as to be in a depleted state. For example, the doping concentration of canal  18  is lower than 10 16  cm −3 . 
     The oxide layer  10  lies below layer  8  and provides electrical insulation between the layer  8  and the substrate  16 . In the so-called UTBOX technology, the oxide layer  10  has a reduced thickness. For example, the thickness of the oxide layer  10  is comprised between 10 nm and 100 nm and, preferably, comprised between 10 nm and 50 nm. 
     The ground plane  12  (also named back plane, or back gate) lies beneath the oxide layer  10 , under the layer  8 . This ground plane  12  performs an electrical control of the threshold voltage of transistor  4 . 
     The well  14  lies beneath the ground plane  12 . This well  14  has an n-type doping. This well  14  is able to electrically bias the ground plane  12  when an electrical potential is applied to the well  14 . Here, a control circuit (not shown) is able to bias the well  14  at a first electrical potential. 
     The transistor  6  is identical to transistor  4 , except that:
         the layer  8  is replaced by an active silicon layer  9 , identical to the layer  8 , except that the source and drain of said layer  9  have a n-type doping.   the ground plane  12  is replaced by a semiconductor ground plane  30 , and   the well  14  is replaced by a semiconductor well  32 .       

     The ground plane  30  lies beneath the oxide layer  10 , under the transistor  6 . Ground plane  30  performs an electrical control of the threshold voltage of transistor  6 . 
     The well  32  lies beneath the ground plane  30 . This well  32  has a p-type doping. This well  32  is able to electrically bias the ground plane  30  when an electrical potential is applied to the well  32 . Here, another control circuit (not shown) is able to bias the well  32  at a second electrical potential. 
     The doping type of ground planes  12  and  30  can be chosen depending on the desired threshold voltage range of transistors  4  and  6 . In this example, transistors  4  and  6  are High Threshold Voltage (HVT) transistors, due to the combined biasing voltage. To this end, the ground planes  12  and  30  have, respectively, an n-type and a p-type doping. For example, said threshold voltage is at least equal to 500 mV and preferably comprised between 500 mV and 650 mV. 
     In this example, a Forward Back Biasing (FBB) scheme is used. The first electrical potential is chosen equal to Vdd−ΔV, where Vdd is a power supply voltage provided to the cell  2 . The second electrical potential is here chosen equal to GND+ΔV, where GND is a ground potential provided to the cell  2 . ΔV must be smaller than Vdd/2, to avoid the formation of an undesirable forward-biased diode between the n-doped well  14  and the p-doped well  32 . This forward bias would then lead to a leakage current between said wells, which would have adverse consequences on the electrical properties of the circuit. 
     In this example, the transistors  4  and  6  have a gate length smaller than 100 nm. Here, layers  8 ,  9  have a thickness at most equal to 50 nm and, preferably, at most equal to 40% of the gate length of, respectively, transistors  4  and  6 . 
     Here, the oxide layer  10  and the insulation trenches  28 ,  29  are made of silicon oxide (SiO 2 ). The ground planes  12 ,  30  and the wells  14 ,  32  have here doping levels comprised between 10 16  and 5*10 18  cm −3  and, preferentially, comprised between 5*10 16  and 5*10 17  cm −3 . The substrate  16  has a p-type doping with a doping level lower than 10 16  cm −3  and, preferentially, lower than 5*10 16  cm −3 . 
     The wells  14 ,  32  may extend to a depth of up to 800 nm or 700 nm below the oxide layer  10 . 
     Here, depths and thicknesses are measured along a vertical direction, perpendicular to the oxide layer  10 . 
       FIG. 2  shows a second cell  40  (also named flipped cell). This cell  40  includes a second pMOS  42  and a second nMOS  44  field effect transistors. 
     Transistor  42  is substantially identical to transistor  4 , except that:
         the ground plane  12  is replaced by a ground plane  46 , and   the well  14  is replaced by a semiconductor well  48  having a p-type doping.       

     Transistor  44  is substantially identical to transistor  6 , except that:
         the ground plane  30  is replaced by a ground plane  50 , and   the well  32  is replaced by a semiconductor well  52  having an n-type doping.       

     In this example, transistors  42  and  44  are High Threshold Voltage (HVT) transistors, due to the combined biasing voltage. For example, said threshold voltage at least equal to 500 mV. To this end, the ground planes  46  and  50  have, respectively, an n-type and a p-type doping. 
     By replacing wells  14  and  32  with wells  48  and  52  of opposite doping types, it is possible to use for the cell  40  a different biasing scheme. Here, the ground plane  46  is biased, through the well  48 , at the potential Vdd−ΔV; the ground plane  48  is biased, through the well  50 , at the potential GND+ΔV. Thus, a FBB scheme is applied on ground planes  46  and  48 . The value of ΔV may be increased up to Vdd in the flipped configuration, without the formation of an undesirable forward bias between wells  48  and  52 . 
     Co-integrating cells  2  and  40  into a single block of a same integrated circuit allows benefiting from the advantages of both respective biasing schemes. It can, however, lead to significant design problems. 
     An example of such design problems is illustrated on  FIG. 3 .  FIG. 3  shows a top view of a portion of an exemplar integrated circuit  53  that may be designed but would not be compliant with commonly used automatic design checking rules. This circuit  53  comprises a first row  54  including a plurality of standard cells. The first row  54  includes alternating cells  2  and  40 , co-integrated side by side into a single block. 
     To simplify the drawings, the doping type of the wells of cells  2 ,  40  is made visible and details of the transistors  4 ,  6 ,  42  and  44  are omitted, such as the insulation trenches  28  and  29 . For the same reasons, only three cells  2  and/or  40  are drawn, although row  54  can include more than three cells  2  and/or  40 . 
     Cells  2  and  40  are placed so that all pMOS transistors  4 ,  42  are aligned along a first direction parallel to row  54  and, all nMOS transistors  6 ,  44  are aligned along a second direction parallel to row  54 . This configuration allows the biasing of ground planes of cells  2 ,  40  by, respectively, power supply rails  56  and  58 . To this end, row  54  also includes a well tap cell  60 . This cell  60  includes a n-type  62  and a p-type  64  doped wells for applying an electrical bias to the p-doped and n-doped semiconductor wells of row  54 . 
     However, in this example, the arrangement of the cells  2  and  40  in row  54  leads to a discontinuity between the wells  14 ,  32 ,  48  and  52 , placed here in a checkerboard pattern. This disposition also gives rise to so-called singularity points  66 ,  67 , contiguous to the wells  14 ,  32 ,  48  and  52 . The singularity points  66 ,  67  may cause numerous design issues and are not compliant with usual design checking rules. Furthermore, due to discontinuities between the n-doped wells  14  and  52 , and the p-doped wells  32  and  48 , the wells  32  and  52  within row  54  cannot be biased by the cell  60 . 
       FIG. 4  illustrates an example of an integrated circuit in which such discontinuities and singular points may be avoided.  FIG. 4  shows a portion of an integrated circuit  70  including a row  72  containing a plurality of cells. On this figure, rails  56  and  58  are not drawn. 
     This row  72  is identical to row  54 , except that it also includes at least a first kind of transition cell  74 . Each cell  74  is placed contiguously to both a cell  2  and a cell  40 , at each interface between a cell  2  and a cell  40  belonging to row  72 . Each cell  74  includes a doped semiconductor well  75 . 
     By placing a cell  74  at each interface between cells  2  and  40 , the singularity points  66 ,  67  can be removed. The electrical continuities between p-doped wells and n-doped wells of the row  54  can also be maintained. In this example, the well  75  has an n-type doping. For example, the depth and the doping level of well  75  are essentially identical to the respective depth and doping level of wells  14  and/or  52 . Thus, the electrical continuity between the n-doped wells  14  and  52  of, respectively, cells  2  and  40  is maintained through said cells  74 . The electrical continuity between the p-doped wells  32  and  48  is maintained through the p-doped substrate  16 . Thus, the respective semiconductor wells of cells  2  and  40  of row  72  can be electrically biased by cell  60 . 
       FIG. 5  illustrates another embodiment of the circuit  70 .  FIG. 5  shows a portion of an integrated circuit  80 , including a row  82  containing a plurality of cells. This row  82  is substantially identical to row  72 , except that:
         cells  74  are replaced by a second kind of transition cell  84 ,   row  82  further includes a n-doped deep well  86 .       

     Cells  84  are identical to cells  74 , except that the well  75  is replaced by a well  85  having a p-type doping instead of an n-type doping, so as to ensure electrical continuity with contiguous p-doped wells  32  and  48 . 
     The deep well  86  lies beneath wells  14 ,  32 ,  48 ,  52  and  85 . 
     The electrical continuity between the p-doped wells  32  and  48  of, respectively, cells  2  and  40  is maintained through said cells  84 . The electrical continuity between the n-doped wells  14  and  32  is maintained through this n-doped well  86 . Thus, the respective semiconductor wells of cells  2  and  40  of row  82  can be electrically biased by cell  60 . 
       FIG. 6  illustrates another embodiment of the circuits  70  and  80 .  FIG. 6  shows a portion of an integrated circuit  90 , including a row  92  containing a plurality of cells. Row  92  is substantially identical to row  72 , except that cells  74  are replaced by first kind  94  and second kind  96  transition cells. For clarity, additional cells  2  and  40  have been drawn. Cells  94  and  96  are placed alternatively at interfaces between contiguous cells  2  and  40 . Thus, any cell  40 , or any plurality of contiguous cells  40 , is itself contiguous to one cell  94  and one cell  96 . 
     Cell  94  includes a semiconductor well  95  and an electrical contact  96  able to apply an electrical bias on well  95 . Cell  98  includes a semiconductor well  99  and an electrical contact  100  able to apply an electrical bias on well  99 . 
     Here, the wells  95  and  99  are identical, respectively, to wells  75  and  85 . 
     Thus, the semiconductor wells of cells  2  and  40  can be electrically biased by cell  60 . 
     Optionally, row  92  may include a n-doped deep semiconductor well  102  substantially identical to well  86 , said well  102  lying below wells  14 ,  32 ,  48 ,  52 ,  95  and  99 . 
     The electrical potentials applied to the electrical contacts of well tap cell  60  are applied to the contacts  96  and  100 . Thus, all the wells of row  92  can be electrically biased, regardless of the presence of the p-doped substrate  16  or the deep well  102 . 
     Integrated circuits may include a plurality of rows including cells  2  and  40 . In some specific cases, singularity points analogous to points  66 ,  67  may appear between adjacent rows, depending on the specific arrangement of said rows, as illustrated in  FIG. 7 . Such cells arrangements would not be compliant with commonly used design checking rules. For example, such arrangements would be rejected during CAD-to-mask steps of the design process.  FIG. 7  shows a portion of an integrated circuit  110 , comprising first  112 , second  114  and third  116  rows. Rows  114  and  116  are adjacent to row  112 . 
     In this example, rows  112 ,  114 ,  116  include a plurality of cells  2 ,  4 ,  60  and transition cells  118 . In each row  112 ,  114  and  116 , cells  2  and  40  are placed so that all pMOS transistors  4 ,  42  are aligned along a direction parallel to rows  112 ,  114  and  116 . The circuit  110  also includes an n-doped deep well  120 , substantially identical to well  102 . 
     Each cell  118  is placed contiguously to both cells  2  and  40 , at each interface between a cell  2  and a cell  40  that belongs to a same row  112 ,  114  or  116 . Cells  118  are here identical to cells  84  and have the same function as cells  84 . Cells  118  include here a p-doped well able to ensure an electrical continuity between the p-doped wells of cells  2  and  40  within each row  112 ,  114  and  116 . The electrical continuity between the n-doped wells  32  and  48  is maintained through the deep well  120 . 
     However, in some cases, singularity points  122  may appear between adjacent rows. In this example, singularity points  122  are present between cells  2 ,  40  and  118  of adjacent rows  112  and  114  and of adjacent rows  112  and  116 . 
       FIG. 8  illustrates an example of a circuit in which points  122  may be avoided.  FIG. 8  shows a portion of an integrated circuit  130  including a plurality of parallel rows  132 ,  134  and  136 , each comprising a plurality of cells. Rows  134  and  136  are adjacent to row  132 . 
     Row  132  is identical to row  112 . 
     Rows  134 ,  136  are, respectively, substantially identical to rows  114  and  116 , except that they do not include both cells  2  and  40 , at least in regions  138 ,  139 . These regions  138 ,  139  are adjacent to regions of the row  132  containing contiguous cells  2 ,  40  and  118 . For example, in each region  138 ,  139 , the rows  134  or  136  contain cells  2  but no cells  40 , or cells  40  but no cells  2 . 
     Additionally, rows  134 ,  136  may also be devoid of transition cells  118  in regions  138 ,  139 . 
     In this example, rows  134  and  136  comprise cells  2  but no cells  40 . 
     The occurrence of singularity points  120  between adjacent rows can thus be mitigated, by limiting the co-integration of cells  2  and  40  to only every other row. 
     A method for generating a layout of circuit  70  will henceforth be described, in reference to the flowchart of  FIG. 9  and to the circuit of  FIG. 4 . 
     During a step  150 , a plurality of standard cells  2 ,  40  and  60  are automatically placed in row  72 . Said cells  2 ,  40  are placed inside row  72  so that the pMOS transistors of said cells are aligned along said first row. Thus, all pMOS transistors  4 ,  42  are aligned along a first direction parallel to row  72  and, all nMOS transistors  6 ,  44  are aligned along a second direction parallel to row  72 . 
     For example, during a sub-step  152  of step  150 , some cells  2  are flipped, so as to obtain cells  40 . For example, said flipping may include automatically switching the doping type of the ground planes  12 ,  30 . 
     During a step  154 , transition cells  74  are automatically placed in row  72  at every interface between contiguous cells  2  and  40 , to separate said contiguous cells  2  and  40  from each other. 
     During a step  156 , a mask layout is automatically generated for the fabrication of circuit  70 . For example, this mask layout comprises a plurality of distinct photomasks. Each of said photomasks is able to be used during a specific step of a manufacturing process for the fabrication of circuit  70 . 
       FIG. 10  illustrates a method for generating a layout of circuit  80 . Said method includes steps  160 ,  162 ,  164  and  166  substantially identical, respectively, to steps  150 ,  152 ,  154  and  156  except that cells  2 ,  40  and  74  are replaced, respectively, by cells  3 ,  41  and  84 . 
       FIG. 11  illustrates a method for generating a layout of circuit  90 . Said method includes steps  170 ,  172  and  174 , substantially identical, respectively, to steps  160 ,  162  and  164  except that cells  74  are replaced, respectively, by cells  94  and  98 . Said cells  94  and  98  are placed in an alternating pattern. 
     For example, during step  170 , cells  94  are placed into row  92 . Then, during a step  171 , every other cell  94  is replaced by a cell  98 . 
     Optionally, during a step  176  of routing, electrical interconnections are placed in order to connect the electrical contacts  96  and  100  to electrical contacts of well tap cell  60 , so that all n-doped and p-doped wells of row  92  may be biased, respectively, at the first and second electrical potentials. 
     Then, during a step  178 , substantially identical to step  166 , a mask layout is automatically generated for the fabrication of circuit  90 . 
       FIG. 12  illustrates a method for generating a layout of circuit  130 . Said method includes steps  180 ,  182 ,  184 , substantially identical, respectively, to steps  160 ,  162 , and  164  except that cells  84  are replaced by cells  118 . 
     During a step  186 , a plurality of cells  2 ,  40 , are automatically placed into rows  134 ,  136  adjacent to row  132 . Especially, either cells  2  but no cell  40 , or cells  40  but no cell  2  are placed in regions  138  and  139  of, respectively, rows  134  and  136 . 
     Then, during a step  188 , substantially identical to step  166 , a mask layout is automatically generated for the fabrication of circuit  130 . 
     Other embodiments are possible. 
     The doping type of ground planes  12  and  30 ,  46  and  50  may be different than the one chosen, depending on the desired threshold voltage ranges for, respectively, transistors  4  and  6 ,  42  and  44 . 
     The biasing schemes (such as FBB) applied to a cell may be chosen independently of the threshold voltage range of the transistors of said cell. 
     The deep well  120  may be omitted from row  92 . 
     The pattern in which cells  2 ,  40  are placed may be different than the one used in the above examples. For example, the respective rows of circuits  53 ,  70 ,  80  and  90  are shown having cells  2  and  40  placed in a periodic pattern, but other patterns may be used instead, such as replacing one cell  40  by a plurality of contiguous cells  40 . In another example, the rows  134  or  136  may comprise cells  40  instead of cells  2 . Row  132  may also comprise a plurality of cells  2  and only one cell  40 . 
     The doping type of wells  95  and  99  may be switched. 
     The rows may include other types of standard cells not shown here but used in standard design libraries, such as diode protection cells. 
     Cells  118  of circuits  110  and  130  may be replaced by a cell having a p-doped semiconductor well, such as cell  74 . In that case, the deep wells  120  are omitted. Cells  118  may also be replaced by cells  94  and  98  placed in an alternating pattern.