Abstract:
The invention relates to an integrated circuit comprising a semi-conducting substrate and first and second cells. Each cell comprises first and second transistors of nMOS and pMOS type including first and second gate stacks including a gate metal. There are first and second ground planes under the first and second transistors and an oxide layer extending between the transistors and the ground planes. The gate metals of the nMOS and of a pMOS exhibit a first work function and the gate metal of the other pMOS exhibiting a second work function greater than the first work function. The difference between the work functions is between 55 and 85 meV and the first work function Wf1 satisfies the relation Wfmg−0.04−0.005*Xge&lt;Wf1&lt;Wfmg−0.03−0.005*Xge.

Description:
RELATED APPLICATIONS 
       [0001]    Under 35 USC 119, this application claims the benefit of the Jun. 19, 2013 priority date of French application FR 1355796, the contents of which are herein incorporated by reference. 
       FIELD OF INVENTION 
       [0002]    The invention relates to an integrated circuit, and in particular an integrated circuit produced on a substrate of Silicon-On-Insulator (SOI) type and comprising rows of nMOS transistors and of pMOS transistors. 
       Background 
       [0003]    SOI technology consists of separating a fine silicon layer (a few nanometres) on a silicon substrate by a relatively thick insulator layer (a few tens of nanometres as a general rule). 
         [0004]    Integrated circuits produced in SOI technology exhibit a certain number of advantages. Such circuits generally exhibit lower electrical consumption for equivalent performance. Such circuits also induce lower parasitic capacitances, which make it possible to improve switching speed. Moreover, the phenomenon of parasitic triggering (known as “latchup”) encountered by metal-oxide-semiconductor (MOS) transistors in “bulk” technology can be avoided. Such circuits therefore turn out to be particularly suitable for applications of SoC (“Systems on Chip”) or MEMS (“Micro Electro-Mechanical Systems”) type. It is also noted that SOI integrated circuits are less sensitive to the effects of ionizing radiations and thus turn out to be more reliable in applications where such radiations may induce operating problems, in particular in space applications. SOI integrated circuits may in particular comprise random-access memories of SRAM (“Static Random Access Memory”) type or logic gates. 
         [0005]    The reduction in the static consumption of logic gates while increasing their tripping speed forms the subject of much research. Certain integrated circuits in the course of development integrate logic gates with low consumption and also logic gates with high tripping speed. To generate these two types of logic gates on one and the same integrated circuit, the threshold voltage (abbreviated V T ) of certain transistors is lowered to form logic gates with high tripping speed, and the threshold voltage of other transistors is increased to form logic gates with low consumption. In “bulk” technology, the modulation of the threshold voltage level of transistors of the same type is performed by differentiating their channel doping level. However, in fully depleted channel semi-conductor on insulator technology, better known by the acronym FDSOI (for “Fully Depleted Silicon On Insulator”), the doping of the channel is almost zero (10 15  cm −3 ). Thus, the doping level of the channel of the transistors cannot therefore exhibit any significant variations, thereby preventing differentiation of the threshold voltages in this fashion. 
         [0006]    In order to have distinct threshold voltages for various transistors in FDSOI technology, it is known to use an electrically biased ground plane (or “back-plane”) disposed between a thin isolating oxide layer and the silicon substrate. By altering the doping of the ground planes and their electrical bias, it is possible to improve the electrostatic control of these transistors, thereby making it possible to define various ranges of threshold voltages for these transistors. It is thus possible to have low-threshold-voltage transistors termed LVT (for “Low VT”, typically 400 mV), high-threshold-voltage transistors termed HVT (for “High VT”, typically 550 mV) and intermediate-threshold-voltage transistors termed RVT (for “Regular VT”, typically 450 mV). 
         [0007]    In a known manner, such transistors exhibiting different threshold voltages can be integrated within one and the same integrated circuit. Such co-integration makes it possible in particular to benefit from several threshold voltage spans, together with better operating flexibility of the circuit. 
         [0008]      FIGS. 1 and 2  illustrate an example of integrated circuit of a known type in 28 nm technology.  FIGS. 1 and 2  are schematic sectional views of an integrated circuit at the level of nMos/pMos pairs respectively of RVT type and of LVT type. The integrated circuit comprises a bias circuit, for applying electrical biases Vdd and Gnd. 
         [0009]      FIG. 1  represents a first cell of RVT type of the integrated circuit. The first cell comprises a semi-conducting substrate  44 . This substrate  44  here exhibits a doping of type p, with a concentration of dopants of less than or equal to 10 16  cm −3 . 
         [0010]    The first cell comprises: 
         [0000]    a buried layer  43  of an electrically insulating material;
 
field-effect MOS transistors, namely an nMOS transistor ns and a pMOS transistor ps. The source, the drain and the channel of the transistor ns are formed in a semiconducting layer  11   s  disposed on the buried layer  43 . The source, the drain and the channel of the transistor ps are formed in a semi-conducting layer  21   s  disposed on the buried layer  43 ;
 
semi-conducting ground planes  41   s  and  42   s  are situated under the layer  43  and placed plumb, respectively, with the transistors ns and ps. The ground planes  41   s  and  42   s  respectively exhibit dopings of type p and n;
 
semi-conducting wells extend depth-wise in continuity respectively with the ground planes  41   s  and  42   s.  The wells form a separation between the ground planes  41   s  and  42   s  and the substrate  44 .
 
         [0011]    The ground plane  41   s  and the well disposed in continuity therewith are biased to Gnd by way of a P+ implanted zone  32   s.  The ground plane  42   s  and the well disposed in continuity therewith are biased to Vdd by way of an N+ implanted zone  33   s.  One and the same bias is applied to the gates of the transistors ns and ps. 
         [0012]    The first cell is isolated at its periphery by isolation trenches  61  and  66 . The implanted zones  31   s  and  32   s  are separated by an isolation trench  62 . The implanted zone  32   s  is separated from the transistor ns by way of an isolation trench  63 . The transistor ns is separated from the transistor ps by an isolation trench  64 . The transistor ps is separated from the implanted zone  33   s  by way of an isolation trench  65 . 
         [0013]      FIG. 2  represents a second cell of LVT type of the integrated circuit. The second cell comprises: 
         [0000]    a buried layer  43  of an electrically insulating material;
 
field-effect MOS transistors, namely an nMOS transistor nl and a pMOS transistor pl. The source, the drain and the channel of the transistor nl are formed in a semi-conducting layer  11   l  disposed on the buried layer  43 . The source, the drain and the channel of the transistor pl are formed in a semi-conducting layer  21   l  disposed on the buried layer  43 ;
 
semi-conducting ground planes  41   l  and  42   l  are situated under the layer  43  and placed plumb, respectively, with the transistors nl and pl. The ground planes  41   l  and  42   l  respectively exhibit dopings of type n and p;
 
semi-conducting wells extend depth-wise in continuity respectively with the ground planes  41   l  and  42   l.  The wells form a separation between the ground planes  41   l  and  42   l  and the substrate  44 .
 
         [0014]    The ground plane  41   l  and the well disposed in continuity therewith are biased to Gnd by way of an N+ implanted zone  32   l.  The ground plane  42   l  and the well disposed in continuity therewith are biased to Gnd by way of a P+ implanted zone  33   l.  One and the same bias is applied to the gates of the transistors nl and pl. 
         [0015]    The second cell is isolated at its periphery by isolation trenches  61  and  66 . The implanted zones  31   l  and  32   l  are separated by an isolation trench  62 . The implanted zone  32   l  is separated from the transistor nl by way of an isolation trench  63 . The transistor nl is separated from the transistor pl by an isolation trench  64 . The transistor pl is separated from the implanted zone  33   l  by way of an isolation trench  65 . 
         [0016]    The transistors ns, ps, nl and pl are produced according to FDSOI technology. The transistor ns comprises a semi-conducting layer  11   s,  surmounted by a gate stack  12   s.  This layer  11   s  forms a channel  13   s  between a source and a drain of the transistor ns. The transistor nl comprises a semi-conducting layer  11   l,  surmounted by a gate stack  12   l.  This layer  11   l  forms a channel  13   l  between a source and a drain of the transistor nl. The transistor ps comprises a semi-conducting layer  21   s,  surmounted by a gate stack  22   s.  This layer  21   s  forms a channel  23   s  between a source and a drain of the transistor ps. The transistor pl comprises a semi-conducting layer  21   l,  surmounted by a gate stack  22   l.  This layer  21   l  forms a channel  23   l  between a source and a drain of the transistor pl. In a manner known in FDSOI technology, the channels  13   s,    13   l,    23   s  and  23   l  are in a depleted state and exhibit a very low doping level, of less than or equal to 10 15  cm −3 . The layer  43  exhibits a thickness of 25 nm, the layers  11   s,    11   l,    21   s  and  2   l  exhibit a thickness of 7 nm and the electrical thickness of the gate oxide in the gate stacks  12   s,    12   l,    22   s  and  22   l  is 1.5 nm. The gate stacks  12   s  and  12   l  comprise one and the same gate metal whose work function is 4.565 eV (middle of the forbidden band of the silicon, termed the midgap, of 4.6 eV−35 meV). 
         [0017]    The gate stacks  22   s  and  22   l  comprise one and the same gate metal whose work function is 4.635 eV (midgap of 4.6 eV+35 meV). 
         [0018]    The diagram of  FIG. 3  illustrates the leakage currents and the conduction currents for the transistors ns, ps, nl and pl, when the first cell is optimized so that the transistors ns and ps exhibit one and the same threshold voltage level. It is then noted that, for the same doping levels of the wells and ground planes of the first and second cells, the discrepancy in threshold voltage between the transistors ns and nl is 70 mV whereas the discrepancy in threshold voltage between the transistors ps and pl is 140 mV. The optimization of the leakage current is therefore not the same for the transistors nl and pl and the threshold voltage levels for these transistors are not identical. 
         [0019]    The diagram of  FIG. 4  illustrates the leakage currents and the conduction currents for the transistors ns, ps, nl and pl, when the second cell is optimized so that the transistors nl and pl exhibit one and the same threshold voltage level. It is then noted that, for the same doping levels of the wells and ground planes of the first and second cells, the discrepancy in threshold voltage between the transistors ns and nl is 70 mV whereas the discrepancy in threshold voltage between the transistors ps and pl is 140 mV. The optimization of the leakage current is therefore not the same for the transistors ns and ps and the threshold voltage levels for these transistors are not identical. 
       SUMMARY 
       [0020]    A need therefore exists for an integrated circuit comprising MOS transistors of FDSOI technology exhibiting different threshold voltage levels, in which the threshold voltage distortions of an nMOS and of a pMOS of one and the same level are removed or substantially reduced. 
         [0021]    The invention thus pertains to an integrated circuit such as defined in the appended claims. 
         [0022]    Other characteristics and advantages of the invention will emerge clearly from the description given thereof hereinafter, by way of wholly nonlimiting indication, with reference to the appended drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a schematic transverse sectional view of a cell with standard threshold voltage according to an example of the prior art; 
           [0024]      FIG. 2  is a schematic transverse sectional view of a cell with low threshold voltage according to an example of the prior art; 
           [0025]      FIGS. 3 and 4  are diagrams illustrating the conduction and leakage currents in two typical cases for the cells of the prior art; 
           [0026]      FIG. 5  is a schematic transverse sectional view of a cell with standard threshold voltage according to an embodiment of the invention; 
           [0027]      FIG. 6  is a schematic transverse sectional view of a cell with low threshold voltage according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]      FIG. 5  represents a transverse sectional view of a first cell of RVT type of an integrated circuit according to an embodiment of the invention. The first cell comprises a semi-conducting substrate  440 . This substrate  440  here exhibits a doping of type p, with a concentration of dopants of less than or equal to 10 16  cm −3 , and, preferably, less than 5*10 15 cm −3 . 
         [0029]    The first cell comprises: 
         [0000]    a buried layer  430  of an electrically insulating material. By electrically insulating material what is meant is a material whose electrical resistivity, measured at a temperature of 20° C., is greater than or equal to 10 5  Ω·m or 10 6  Ω·m. For example, this layer  430  is made of silicon oxide (SiO 2 );
 
field-effect MOS transistors, namely an nMOS transistor ns and a pMOS transistor ps. The source, the drain and the channel of the transistor ns are formed in a semi-conducting layer  110   s  disposed on the buried layer  430 . The source, the drain and the channel of the transistor ps are formed in a semi-conducting layer  210   s  disposed on the buried layer  430 ;
 
semi-conducting ground planes  410   s  and  420   s  are situated under the layer  430  and placed plumb, respectively, with the transistors ns and ps. The ground planes  410   s  and  420   s  respectively exhibit dopings of type p and n;
 
semi-conducting wells extend depth-wise in continuity respectively with the ground planes  410   s  and  420   s.  In this description, the thickness and the depth are defined with respect to a direction perpendicular to a plane in which the substrate  440  extends. The wells form a separation between the ground planes  410   s  and  420   s  and the substrate  440 .
 
         [0030]    The first cell will typically be disposed in a row of cells, in which the nMOSs of various cells are mutually aligned and in which the pMOSs of various cells are mutually aligned. 
         [0031]    The ground plane  410   s  and the well disposed in continuity therewith are biased to Gnd (with Gnd=0V) by way of a P+ implanted zone  320   s.  The ground plane  420   s  and the well disposed in continuity therewith are biased to Vdd (with Vdd=1V) by way of an N+ implanted zone  330   s.  One and the same bias can be applied to the gates of the transistors ns and ps. 
         [0032]    The first cell is isolated at its periphery by isolation trenches  610  and  660 . The implanted zones  310   s  and  320   s  are separated by an isolation trench  620 . The implanted zone  320   s  is separated from the transistor ns by way of an isolation trench  630 . The transistor ns is separated from the transistor ps by an isolation trench  640 . The transistor ps is separated from the implanted zone  330   s  by way of an isolation trench  650 . The wells extend more deeply than the isolation trenches,  630  and  650 , in particular so as to allow a biasing of the ground planes by a lateral contact. 
         [0033]    In a manner known per se, the combination: 
         [0000]    of a fine buried insulator layer  430 ;
 
of a given bias on the ground planes  410   s  and  420   s,  
 
of a given type of doping of these ground planes makes it possible to influence the threshold voltage of the transistors ns and ps. The ground planes actually make it possible to control the electrostatic properties of the channel of the transistors.
 
         [0034]      FIG. 6  represents a transverse sectional view of a second cell of LVT type of the integrated circuit. The second cell comprises: 
         [0000]    a buried layer  430  of an electrically insulating material;
 
field-effect MOS transistors, namely an nMOS transistor nl and a pMOS transistor pl. The source, the drain and the channel of the transistor nl are formed in a semi-conducting layer  110   l  disposed on the buried layer  430 . The source, the drain and the channel of the transistor pl are formed in a semi-conducting layer  210   l  disposed on the buried layer  430 ;
 
semi-conducting ground planes  410   l  and  420   l  are situated under the layer  430  and placed plumb, respectively, with the transistors nl and pl. The ground planes  410   l  and  420   l  respectively exhibit dopings of type n and p. The dopings of the ground planes  410   l  and  420   l  are therefore opposite to the dopings of the ground planes  410   s  and  420   s  respectively;
 
semi-conducting wells extend depth-wise in continuity respectively with the ground planes  410   l  and  420   l.  The wells form a separation between the ground planes  410   l  and  420   l  and the substrate  440 .
 
         [0035]    The second cell will typically be disposed in a row of cells, in which the nMOSs of various cells are mutually aligned and in which the pMOSs of various cells are mutually aligned. 
         [0036]    The ground plane  410   l  and the well disposed in continuity therewith are biased to Gnd by way of an N+ implanted zone  320   l.  The ground plane  420   l  and the well disposed in continuity therewith are biased to Gnd by way of a P+ implanted zone  330   l.  One and the same bias can be applied to the gates of the transistors nl and pl. 
         [0037]    The second cell is isolated at its periphery by isolation trenches  610  and  660 . The implanted zones  310   l  and  320   l  are separated by an isolation trench  620 . The implanted zone  320   l  is separated from the transistor nl by way of an isolation trench  630 . The transistor nl is separated from the transistor pl by an isolation trench  640 . The transistor pl is separated from the implanted zone  330   l  by way of an isolation trench  650 . The wells extend more deeply than the isolation trenches,  630  and  650 , in particular so as to allow a biasing of the ground planes by a lateral contact. 
         [0038]    In a manner known per se, the combination: 
         [0000]    of a fine buried insulator layer  430 ;
 
of a given bias on the ground planes  410   l  and  420   l,  
 
of a given type of doping of these ground planes makes it possible to influence the threshold voltage of the transistors nl and pl. The ground planes actually make it possible to control the electrostatic properties of the channel of the transistors.
 
         [0039]    First and second cells can be co-integrated in the same rows or in distinct rows sharing one and the same buried insulator layer. The transistors ns, ps, nl and pl are produced according to FDSOI technology. The transistor ns comprises a semi-conducting layer  110   s,  surmounted by a gate stack  120   s.  This layer  110   s  forms a channel  130   s  between a source and a drain of the transistor ns. The transistor nl comprises a semi-conducting layer  110   l,  surmounted by a gate stack  120   l.  This layer  110   l  forms a channel  130   l  between a source and a drain of the transistor nl. The transistor ps comprises a semi-conducting layer  210   s,  surmounted by a gate stack  220   s.  This layer  210   s  forms a channel  230   s  between a source and a drain of the transistor ps. The transistor pl comprises a semi-conducting layer  210   l,  surmounted by a gate stack  220   l.  This layer  210   l  forms a channel  230   l  between a source and a drain of the transistor pl. In a manner known in FDSOI technology, the channels  130   s,    130   l,    230   s  and  230   l  are in a depleted state and exhibit a very low doping level, typically less than or equal to 10 15  cm −3 . 
         [0040]    The doping level of the ground planes  410   l,    420   l,    410   s  and  420   s  is advantageously at least equal to 10 18  cm  −3 , for example between 10 18  cm −3  and 10 20  cm −3 . This doping level is for example formed on a ground plane thickness of at least 10 nm under the buried insulator layer  430 . The doping level of the wells is for example at least equal to 10 16 , and for example between 10 16  and 10 18  cm −3 . 
         [0041]    The difference between the work functions of the gate metals of the gate stacks  220   s  and  220   l  is between 55 and 85 meV, preferably between 60 and 80 meV, advantageously between 65 and 75 meV and preferably equal to 70 meV. The channels  130   s,    130   l,    230   s  and  230   l  contain a negligible quantity of germanium. The gate stacks  120   s,    120   l  and  220   l  comprise one and the same gate metal whose work function is between 4.555 and 4.575 eV, and preferably equal to 4.565 eV. The gate stack  220   s  comprises a gate metal whose work function is between 4.625 and 4.645 eV, and preferably equal to 4.635 eV. The following materials can for example be used for the gate metals of the transistors: titanium nitride (TiN), tantalum nitride (TaN) with or without the presence of aluminium in the composition (TiAlN or TaAlN), or else a layer of aluminium interposed between two layers of TiN: TiN/Al/TiN. 
         [0042]    In combination with other parameters detailed subsequently, the discrepancy between the threshold voltages ns and nl is substantially 70 mV. Likewise, the discrepancy between the threshold voltages ps and pl is substantially 70 mV. The threshold voltages are therefore balanced for the nMOS and pMOS transistors of one and the same threshold voltage level (LVT or RVT) of the integrated circuit. Such a discrepancy of the order of 70 mV between the transistors of LVT type and the RVT transistors turns out to be desirable. Likewise, for one and the same threshold voltage level, the nMOS and pMOS transistors exhibit homogeneous leakage currents and conduction currents. These results are obtained without making it necessary to bias the ground planes with different voltage levels from the levels of the supply voltages of the transistors ns, ps, nl and pl. Such a result is furthermore obtained by using a gate material for the transistor pl already used for the transistors ns and nl, not therefore inducing any further complication of the process for fabricating the integrated circuit. Moreover, the difference between the work functions of the transistors pl and ps is relatively reduced, thereby rendering the method of fabrication easier to implement. 
         [0043]    For a 28 nm technological node: 
         [0000]    the layer  430  is of UTBOX type and exhibits a thickness of between 20 and 30 nm, and advantageously of 25 nm;
 
the layers  110   s,    110   l,    210   s  and  20   l  exhibit a thickness of between 6 and 8 nm, and advantageously of 7 nm;
 
the gate oxide in the gate stacks  120   s,    120   l,    220   s  and  220   l  exhibits an electrical thickness of between 1.3 and 1.7 nm, and advantageously of 1.5 nm.
 
         [0044]    Simulations for this technological node with the parameters mentioned above have made it possible to determine threshold voltages of a level of 0.48V for the transistors ns and ps, and threshold voltages of a level of 0.41V for the transistors nl and pl. 
         [0045]    For a 14 nm technological node: 
         [0000]    the layer  430  is of UTBOX type and exhibits a thickness of between 15 and 25 nm, and advantageously of 20 nm;
 
the layers  110   s,    110   l,    210   s  and  20   l  exhibit a thickness of between 5 and 7 nm, and advantageously of 6 nm;
 
the gate oxide in the gate stacks  120   s,    120   l,    220   s  and  220   l  exhibits an electrical thickness of between 1.1 and 1.5 nm, and advantageously of 1.3 nm.
 
         [0046]    For a 10 nm technological node: 
         [0000]    the layer  430  is of UTBOX type and exhibits a thickness of between 10 and 20 nm, and advantageously of 15 nm;
 
the layers  110   s,    110   l,    210   s  and  20   l  exhibit a thickness of between 4 and 6 nm, and advantageously of 5 nm;
 
the gate oxide in the gate stacks  120   s,    120   l,    220   s  and  220   l  exhibits an electrical thickness of between 1 and 1.2 nm, and advantageously of 1.1 nm.
 
         [0047]    In the example detailed above, the channel of the transistors comprises a negligible quantity of germanium and the middle of the forbidden band of the silicon (termed the midgap value) corresponds to the mean value between the work function value of the gate metal of the gate stack  220   s  and the work function value of the gate metal of the gate stack  220   l.    
         [0048]    To have greater flexibility in the value of the respective work functions of the gate metals of the stacks  220   s  and  220   l,  it is possible to include germanium in the channel of the pMOS transistors ps and pl. 
         [0049]    The concentration of germanium atoms in the channels of the transistors ps and pl is for example between 0 and 40%. Designating by Xge the percentage of germanium in these channels, simulations have made it possible to deduce parameters for the work function of the metals of the gate stacks such as to preserve equilibrium of the threshold voltages for the nMOS and pMOS transistors of one and the same threshold voltage level (LVT or RVT) of the integrated circuit. Preserving a difference between the work functions of the gate metals of the gate stacks  220   s  and  220   l  of between 60 and 80 meV (or of between 65 and 75 meV), the work function Wf1 of the gate metal of the gate stacks  120   s,    120   l  and  220   l  must comply with the following condition: 
         [0000]      Wfmg−0.04−0.005*Xge&lt;Wf1&lt;Wfmg−0.03−0.005*Xge
 
         [0050]    Wfmg being the work function corresponding to the middle of the forbidden band of the silicon (about 4.6 eV). 
         [0051]    For example, with Xge=12, a work function Wfmg of 4.505 eV will for example be taken. 
         [0052]    For a 28 nm technological node, preserving the parameters mentioned above (thickness of the layer  430 , thickness of the layers  110   s,    110   l,    210   s  and  20   l,  thickness of the gate oxide in the gate stacks  120   s,    120   l,    220   s  and  220   l ), simulations have made it possible to determine threshold voltages of a level of 0.42V for the transistors ns and ps, and threshold voltages of a level of 0.35V for the transistors nl and pl (discrepancy between the threshold voltages maintained at 70 mV).