Abstract:
Provided is a semiconductor memory having a memory cell structure capable of reducing soft error without complicating a circuit configuration. Specifically, an inverter (I 1 ) consists of a NMOS transistor (N 1 ) and a PMOS transistor (P 1 ), and an inverter (I 2 ) consists of a NMOS transistor (N 2 ) and a PMOS transistor (P 2 ). The inverters (I 1,  I 2 ) are subjected to cross section. The NMOS transistor (N 1 ) is formed within a P well region (PW 0 ), and the NMOS transistor (N 2 ) is formed within a P well region (PW 1 ). The P well regions (PW 0,  PW 1 ) are oppositely disposed with an N well region (NW) interposed therebetween.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a semiconductor memory and, in particular, to a memory cell structure having improvements in resistance to soft error of a MOS static RAM.  
           [0003]    2. Description of the Background Art  
           [0004]    As the miniaturization of memory cells proceeds, the following soft error problem becomes noticeable. Specifically, the data stored in a storage node is inverted due to electrons generated from alpha rays released from a package and neutron beams from outer space. Particularly, as power supply voltage is lowered, malfunction becomes more significant. Attempts to reduce soft error are being pursued.  
           [0005]    [0005]FIG. 37 is a circuit diagram illustrating a structure equivalent to a SRAM memory cell disclosed in, for example, Japanese Patent No. 2589949. As shown in FIG. 37, a memory cell  100  is made up of PMOS transistors PT 1  and PT 2 , and NMOS transistors NT 5  to NT 8 , NT 11 , NT 12 , NT 21  and NT 22 .  
           [0006]    The sources of the PMOS transistors PT 1  and PT 2  are both connected to a power supply voltage V cc . The drain of the PMOS transistor PT 1  is connected through a node  101  to the gate of the PMOS transistor PT 2  and to the gates of the NMOS transistors NT 21  and NT 22 . The drain of the PMOS transistor PT 2  is connected through a node  111  to the gate of the PMOS transistor PT 1  and to the gates of the NMOS transistors NT 11  and NT 12 .  
           [0007]    The sources of the NMOS transistors NT 11  and NT 12  are both grounded. The drain of the NMOS transistor NT 11  is connected through the node  101  to the drain of the PMOS transistor PT 1 . The drain of the NMOS transistor NT 12  is connected through the nodes  101  and  102  to the drain of the PMOS transistor PT 1 .  
           [0008]    The sources of the NMOS transistors NT 21  and NT 22  are both grounded. The drain of the NMOS transistor NT 21  is connected through the node  111  to the drain of the PMOS transistor PT 2 . The drain of the NMOS transistor NT 22  is connected through the nodes  111  and  112  to the drain of the PMOS transistor PT 2 .  
           [0009]    The NMOS transistor NT 5  is interposed between a bit line BL 50  and the node  101 , and its gate is connected to a word line WL 50 . The NMOS transistor NT 6  is interposed between a bit line BL 60  and the node  101 , and its gate is connected to a word line WL 60 . The NMOS transistor NT 7  is interposed between a bit line BL 51  and the node  111 , and its gate is connected to the word line WL 50 . The NMOS transistor NT 8  is interposed between a bit line BL 61  and the node  111 , and its gate is connected to the word line WL 60 .  
           [0010]    In such a configuration, the word line WL 50  or WL 60  is brought into the active state and the NMOS transistors NT 5  and NT 6 , or the NMOS transistors NT 6  and NT 8  are brought into the on state, thereby to provide access to the nodes  101  and  111 , each being a storage node. This enables to obtain the data from the paired bit lines BL 50  and BL 51  or the paired bit lines BL 60  and BL 61 .  
           [0011]    In this configuration, a NMOS driver transistor that is usually made up of a single NMOS transistor is divided into two NMOS transistors (which is divided into the NMOS transistors NT 11  and NT 12 , and NT 21  and NT 22 ).  
           [0012]    In order to divide the storage node serving as the drain of the PMOS transistor PT 1  (PT 2 ) into the node  101  ( 111 ) and the node  102  ( 112 ), the NMOS transistor NT 11  (NT 21 ) and the NMOS transistor NT 12  (NT 22 ) are oppositely disposed so as to interpose therebetween an N well region where the PMOS transistor PT 1  is to be formed.  
           [0013]    Therefore, the N well region prevents that a depletion region on the opposite side of the N well region is adversely affected by electrons or holes generated from energy particles colliding with one side of the N well region. This enables to lower incidence of soft error.  
           [0014]    However, even with the foregoing SRAM memory cell, a reduction in soft error is insufficient. Further, there is the problem that the circuit configuration is complicated by using two driver transistors, although it can be generally configured by using one.  
         SUMMARY OF THE INVENTION  
         [0015]    According to a first aspect of the invention, a semiconductor memory having a memory cell containing first and second inverters subjected to cross connection, a first conductivity type being defined by first kind, and a second conductivity type being defined by second kind, is characterized in that: the first inverter consists of a first field effect transistor of the first kind and a first field effect transistor of the second kind; that the second inverter consists of a second field effect transistor of the first kind and a second field effect transistor of the second kind; and that the first and second field effect transistors of the first kind are disposed in separate first and second well regions of the second kind, respectively.  
           [0016]    According to a second aspect of the invention, the semiconductor memory of the first aspect is characterized in that: an output part of the first inverter includes a connecting part between one electrode of the first field effect transistor of the first kind and one electrode of the first field effect transistor of the second kind, an input part thereof includes a connecting part between a control electrode of the first field effect transistor of the first kind and a control electrode of the first field effect transistor of the second kind; an output part of the second inverter includes a connecting part between one electrode of the second field effect transistor of the first kind and one electrode of the second field effect transistor of the second kind, and an input part thereof includes a connecting part between a control electrode of the second field effect transistor of the first kind and a control electrode of the second field effect transistor of the second kind; that the memory cell further includes: (i) a third field effect transistor of the first kind, one electrode of which is connected to a first storage terminal electrically connected to the output part of the first inverter and the input part of the second inverter, and the other electrode of which is connected to a first bit line, and a control electrode of which is connected to a word line; and (ii) a fourth field effect transistor of the first kind, one electrode of which is connected to a second storage terminal electrically connected to the output part of the second inverter and the input part of the first inverter, and the other electrode of which is connected to a second bit line, and a control electrode of which is connected to a word line; and that the third and fourth field effect transistors of the first kind are disposed in second and first well regions of the second kind, respectively.  
           [0017]    According to a third aspect of the invention, the semiconductor memory of the second aspect is characterized in that the respective one electrodes in the first to fourth field effect transistors of the first kind are disposed separately.  
           [0018]    According to a fourth aspect of the invention, the semiconductor memory of the second aspect is characterized in that: the first and third field effect transistors of the first kind and the first field effect transistor of the second kind are arranged in an approximately straight line along the direction of formation of the word line; and that the second and fourth field effect transistors of the first kind and the second field effect transistor of the second kind are arranged in an approximately straight line along the direction of formation of the word line.  
           [0019]    According to a fifth aspect of the invention, the semiconductor memory of the first aspect is characterized in that the first and second field effect transistors of the first kind are arranged so as to be point symmetry with respect to the central point of the memory cell.  
           [0020]    According to a sixth aspect of the invention, the semiconductor memory of the second aspect is characterized in that the third and fourth field effect transistors of the first kind are arranged so as to be point symmetry with respect to the central point of the memory cell.  
           [0021]    According to a seventh aspect of the invention, the semiconductor memory of the second aspect is characterized in that the width of the control electrode of the first and second field effect transistors of the first kind is set so as to be larger than the width of the control electrode of the third and fourth field effect transistors of the first kind.  
           [0022]    According to an eighth aspect of the invention, the semiconductor memory of one of the foregoing aspects is characterized in that the memory cell further includes (i) a first resistance component interposed between the input part of the first inverter and the second storage terminal, and (ii) a second resistance component interposed between the input part of the second inverter and the first storage terminal.  
           [0023]    According to a ninth aspect of the invention, the semiconductor memory of the eighth aspect is characterized in that the first and second resistance components include a high resistance metal wiring formed from a metal material having a higher resistivity than CoSi.  
           [0024]    According to a tenth aspect of the invention, the semiconductor memory of the eighth aspect is characterized in that the first and second resistance components include a high resistance polysilicon wiring formed from polysilicon having a higher resistivity than CoSi.  
           [0025]    According to an eleventh aspect of the invention, the semiconductor memory of the second aspect is characterized in that the control electrodes of the third and fourth field effect transistors of the first kind and the word line are formed by using a single polysilicon.  
           [0026]    According to a twelfth aspect of the invention, the semiconductor memory of the second aspect is characterized in that: the word line includes separate first and second word lines; that the control electrode of the third field effect transistor of the first kind is connected to the first word line; and that the control electrode of the fourth field effect transistor of the first kind is connected to the second word line.  
           [0027]    According to a thirteenth aspect of the invention, the semiconductor memory of the twelfth aspect is characterized in that: the first bit line includes first and second partial bit lines forming a pair of bit lines; that the second bit line includes third and fourth partial bit lines forming a pair of bit lines; that the third field effect transistor of the first kind includes fifth and sixth field effect transistors of the first kind, the fifth field effect transistor of the first kind being interposed between the first partial bit line and the second storage terminal, the sixth field effect transistor of the first kind being interposed between the second partial bit line and the first storage terminal; and that the fourth field effect transistor of the first kind includes seventh and eighth field effect transistors of the first kind, the seventh field effect transistor of the first kind being interposed between the third partial bit line and the first storage terminal, the eighth field effect transistor of the first kind being interposed between the fourth partial bit line and the second storage terminal.  
           [0028]    According to a fourteenth aspect of the invention, the semiconductor memory of the thirteenth aspect is characterized in that the width of the control electrode of the first and second field effect transistors of the first kind is set so as to be larger than the width of the control electrode of the fifth to eighth field effect transistors of the first kind.  
           [0029]    According to a fifteenth aspect of the invention, the semiconductor memory of the second, twelfth or thirteenth aspect is characterized in that a region for forming the control electrode of the first and second field effect transistors of the first kind is disposed so as to form a portion of the second and first storage terminals, respectively.  
           [0030]    According to a sixteenth aspect of the invention, the semiconductor memory of one of the foregoing aspects is characterized in that: the first and second field effect transistors of the second kind are disposed in a well region of the first kind; and that the well region of the first kind is disposed between the first and second well regions of the second kind.  
           [0031]    In the semiconductor memory of the first aspect, the first and second field effect transistors of the first kind are disposed in the separate first and second well region of the second kind, respectively. Therefore, if carriers generated from alpha rays, etc. are collected into one or the other electrode region of one of the first and second field effect transistor of the first kind, such carriers are cancelled by being released from one or the other electrode region of the other of the first and second field effect transistor of the first kind on which no influence of the carriers is exerted. This enables to increase resistance to soft error.  
           [0032]    In addition, since the first and second inverters each consists of a combination of a field effect transistor of the first kind and that of the second kind, a complementary type can be realized by at least sufficient circuit configuration.  
           [0033]    In the semiconductor memory of the second aspect, the third and fourth field effect transistors of the first kind are disposed in the second and first well regions of the second kind, respectively. Thereby, the memory cell selecting operation by means of the word line, and the write/read operation to the memory cell via the first and second bit lines, are executable while improving resistance to soft error.  
           [0034]    In the semiconductor memory of the third aspect, resistance to soft error can be increased by separately forming one electrode to be connected to the first or second storage terminal in the first to fourth field effect transistors of the first kind.  
           [0035]    In the semiconductor memory of the fourth aspect, the degree of integration can be increased by virtue of the layout of the first to fourth field effect transistors of the first kind and the first and second field effect transistors of the second kind.  
           [0036]    In the semiconductor memory of the fifth aspect, by disposing the first and second MOS transistors so as to be point symmetry with respect to the central portion of the memory cell, arrangement between adjacent memory cells can be facilitated to increase the degree of integration.  
           [0037]    In the semiconductor memory of the sixth aspect, by disposing the third and fourth MOS transistors so as to be point symmetry with respect to the central portion of the memory cell, arrangement between adjacent memory cells can be facilitated to increase the degree of integration.  
           [0038]    In the semiconductor memory of the seventh aspect, the stability of the memory cell can be increased by setting such that the control electrode width of the first and second field effect transistors of the first kind is larger than that of the third and fourth field effect transistors of the first kind.  
           [0039]    In the semiconductor memory of the eighth aspect, by signal propagation delay due to the first and second resistance components, the response characteristic for inverting the data held in the first and second storage terminals of the memory cell can be elongated, thereby soft error is hard to occur.  
           [0040]    The semiconductor memory of the ninth aspect realizes the first and second resistance components by using the high resistance polysilicon wiring.  
           [0041]    The semiconductor memory of the tenth aspect realizes the first and second resistance components by using the high resistance polysilicon wiring.  
           [0042]    In the semiconductor memory of the eleventh aspect, by using a single polysilicon common to the control electrodes and word lines of the third and fourth MOS transistors, the number of layers to be formed can be reduced, thereby allowing for a reduction in the cost of the semiconductor memory.  
           [0043]    In the semiconductor memory of the twelfth aspect, by the presence of two memory cell selecting means composed of the first and second word lines, the memory cell can be used for FIFO memory.  
           [0044]    The semiconductor memory of the thirteenth aspect realizes a two-port memory cell composed of the first to fourth partial bit lines and the first and second word lines.  
           [0045]    In the semiconductor memory of the fourteenth aspect, the stability of the memory cell can be increased by setting such that the control electrode width of the first and second field effect transistors of the first kind is larger than that of the fifth to eighth field effect transistors of the first kind.  
           [0046]    In the semiconductor memory of the fifteenth aspect, with the arrangement such that the region for forming the control electrode of the first and second field effect transistors of the first kind forms a portion of the second and first storage terminals, respectively, the region for forming memory cell can be narrowed to increase the degree of integration.  
           [0047]    In the semiconductor memory of the sixteenth aspect, by the well region of the first kind disposed between the first and second well regions of the second kind, it is avoided that carriers generated in the first or second well region of the second kind exert influence on the other well region of the second kind.  
           [0048]    It is an object of the present invention to overcome the foregoing problem by providing a semiconductor memory having a memory cell structure capable of reducing soft error without complicating a circuit configuration.  
           [0049]    These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0050]    [0050]FIG. 1 is an explanatory diagram viewed from above the layout configuration in all layers of a SRAM memory cell according to a first preferred embodiment of the invention;  
         [0051]    [0051]FIG. 2 is an explanatory diagram viewed from above mainly the layout configuration beneath a first aluminum wiring layer in FIG. 1;  
         [0052]    [0052]FIG. 3 is an explanatory diagram viewed from above mainly the layout configuration over a second aluminum wiring layer in FIG. 1;  
         [0053]    [0053]FIG. 4 is a circuit diagram illustrating an equivalent circuit of the memory cell in the first preferred embodiment;  
         [0054]    [0054]FIG. 5 is an explanatory diagram viewed from above the layout configuration in all layers of a SRAM memory cell according to a second preferred embodiment of the invention;  
         [0055]    [0055]FIG. 6 is an explanatory diagram viewed from above mainly the layout configuration beneath a first aluminum wiring layer in FIG. 5;  
         [0056]    [0056]FIG. 7 is an explanatory diagram viewed from above the layout configuration beneath the first aluminum wiring layer between adjacent memory cells;  
         [0057]    [0057]FIG. 8 is an explanatory diagram viewed from above the layout configuration in all layers of a SRAM memory cell according to a third preferred embodiment of the invention;  
         [0058]    [0058]FIG. 9 is an explanatory diagram viewed from above mainly the layout configuration beneath a first aluminum wiring layer in FIG. 8;  
         [0059]    [0059]FIG. 10 is a circuit diagram illustrating an equivalent circuit of the memory cell in the third preferred embodiment;  
         [0060]    [0060]FIG. 11 is an explanatory diagram viewed from above the layout configuration in all layers of a SRAM memory cell according to a fourth preferred embodiment of the invention;  
         [0061]    [0061]FIG. 12 is an explanatory diagram viewed from above mainly the layout configuration beneath a first aluminum wiring layer in FIG. 11;  
         [0062]    [0062]FIG. 13 is an explanatory diagram viewed from above the layout configuration in all layers of a SRAM memory cell according to a fifth preferred embodiment of the invention;  
         [0063]    [0063]FIG. 14 is an explanatory diagram viewed from above mainly the layout configuration over a second aluminum wiring layer in FIG. 13;  
         [0064]    [0064]FIG. 15 is a circuit diagram illustrating an equivalent circuit of the memory cell in the fifth preferred embodiment;  
         [0065]    [0065]FIG. 16 is an explanatory diagram viewed from above the layout configuration in all layers of a SRAM memory cell according to a sixth preferred embodiment of the invention;  
         [0066]    [0066]FIG. 17 is an explanatory diagram viewed from above mainly the layout configuration beneath a first aluminum wiring layer in FIG. 16;  
         [0067]    [0067]FIG. 18 is an explanatory diagram viewed from above mainly the layout configuration above a second aluminum wiring layer in FIG. 16;  
         [0068]    [0068]FIG. 19 is an explanatory diagram viewed from above the layout configuration in all layers of a SRAM memory cell according to a seventh preferred embodiment of the invention;  
         [0069]    [0069]FIG. 20 is an explanatory diagram viewed from above mainly the layout configuration beneath a first aluminum wiring layer in FIG. 19;  
         [0070]    [0070]FIG. 21 is an explanatory diagram viewed from above mainly the layout configuration over a second aluminum wiring layer in FIG. 19;  
         [0071]    [0071]FIG. 22 is an explanatory diagram viewed from above the layout configuration in all layers of a SRAM memory cell according to an eighth preferred embodiment of the invention;  
         [0072]    [0072]FIG. 23 is an explanatory diagram viewed from above mainly the layout configuration beneath a first aluminum wiring layer in FIG. 22;  
         [0073]    [0073]FIG. 24 is an explanatory diagram viewed from above mainly the layout configuration over a second aluminum wiring layer in FIG. 22;  
         [0074]    [0074]FIG. 25 is a circuit diagram illustrating an equivalent circuit of the memory cell in the eighth preferred embodiment;  
         [0075]    [0075]FIG. 26 is an explanatory diagram viewed from above the layout configuration in all layers of a SRAM memory cell according to a ninth preferred embodiment of the invention;  
         [0076]    [0076]FIG. 27 is an explanatory diagram viewed from above mainly the layout configuration beneath a first aluminum wiring layer in FIG. 26;  
         [0077]    [0077]FIG. 28 is an explanatory diagram viewed from above mainly the layout configuration over a second aluminum wiring layer in FIG. 26;  
         [0078]    [0078]FIG. 29 is an explanatory diagram viewed from above the layout configuration in all layers of a SRAM memory cell according to a tenth preferred embodiment of the invention;  
         [0079]    [0079]FIG. 30 is an explanatory diagram viewed from above mainly the layout configuration beneath a first aluminum wiring layer in FIG. 29;  
         [0080]    [0080]FIG. 31 is an explanatory diagram viewed from above mainly the layout configuration over a second aluminum wiring layer in FIG. 29;  
         [0081]    [0081]FIG. 32 is an explanatory diagram viewed from above the layout configuration in all layers of a SRAM memory cell according to an eleventh preferred embodiment of the invention;  
         [0082]    [0082]FIG. 33 is an explanatory diagram viewed from above mainly the layout configuration beneath a first aluminum wiring layer in FIG. 32;  
         [0083]    [0083]FIG. 34 is an explanatory diagram viewed from above mainly the layout configuration over a second aluminum wiring layer in FIG. 32;  
         [0084]    [0084]FIG. 35 is an explanatory diagram viewed from above the layout configuration in all layers of a SRAM memory cell according to a twelfth preferred embodiment of the invention;  
         [0085]    [0085]FIG. 36 is an explanatory diagram viewed from above mainly the layout configuration over a second aluminum wiring layer in FIG. 35; and  
         [0086]    [0086]FIG. 37 is a circuit diagram illustrating a conventional SRAM memory cell. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0087]    First Preferred Embodiment  
         [0088]    FIGS.  1  to  4  are diagrams illustrating a memory cell structure of a SRAM according to a first preferred embodiment of the invention. FIG. 1 is an explanatory diagram viewed from above the layout configuration in all layers. FIG. 2 is an explanatory diagram viewed from above mainly the layout configuration beneath a first aluminum wiring layer in FIG. 1. FIG. 3 is an explanatory diagram viewed from above mainly the layout configuration over a second aluminum wiring layer in FIG. 1. Some reference numerals used in FIG. 2 or  3  are omitted in FIG. 1.  
         [0089]    [0089]FIG. 4 is a circuit diagram illustrating an equivalent circuit of the SRAM memory cell of the layout configuration shown in FIGS.  1  to  3 . As seen from FIG. 4, the SRAM memory cell of the first preferred embodiment is made up of NMOS transistors N 1  to N 4  and PMOS transistors P 1  and P 2 .  
         [0090]    The PMOS transistors P 1  and P 2 , each being a driver transistor, are disposed within an N well region NW. The NMOS transistor N 1  that is a driver transistor and the NMOS transistor N 4  that is an access transistor are disposed within a P well region PW 0 . The NMOS transistor N 2  that is a driver transistor and the NMOS transistor N 3  that is an access transistor are disposed within a P well region PW 1 . The P well regions PW 0  and PW 1  are oppositely disposed with the N well region NW interposed therebetween.  
         [0091]    A first CMOS inverter I 1  is made up of the NMOS transistor N 1  and PMOS transistor P 1 . That is, the gates of the PMOS transistor P 1  and NMOS transistor N 1  are both connected to a storage terminal Nb, and their drains are both connected to a storage terminal Na. The source of the PMOS transistor P 1  is connected to a power supply voltage V dd , and the source of the NMOS transistor N 1  is grounded.  
         [0092]    A second CMOS inverter I 2  is made up of the NMOS transistor N 2  and PMOS transistor P 2 . That is, the gates of the PMOS transistor P 2  and NMOS transistor N 2  are both connected to the storage terminal Na, and their drains are both connected to the storage terminal Nb. The source of the PMOS transistor P 2  is connected to the power supply voltage V dd , and the source of the NMOS transistor N 2  is grounded.  
         [0093]    Thus, an output part of the inverter I 1  and an input part of the inverter I 2  are electrically connected to the storage terminal Na, and an input part of the inverter I 1  and an output part of the inverter I 2  are electrically connected to the storage terminal Nb, so that the CMOS inverters I 1  and I 2  are subjected to cross section. Thereby, the storage terminals Na and Nb can store information of mutually inverted logical level.  
         [0094]    The NMOS transistor N 3  is interposed between a bit line BLA and the storage terminal Na, and its gate is connected to a word line WL. The NMOS transistor N 4  is interposed between a bit line BLB and the storage terminal Nb, and its gate is connected to the word line WL.  
         [0095]    In such a configuration, the word line WL is brought into the active state and the NMOS transistors NT 3  and NT 4  are brought into the on state, thereby to provide access (i.e., read or write) to the storage terminals Na and Nb. This enables to obtain the data from the bit line BLA or BLB.  
         [0096]    Referring to FIGS.  1  to  3 , description will proceed to the memory cell structure of the first preferred embodiment.  
         [0097]    In the N well region NW, the PMOS transistor P 1  is made up of P +  diffusion regions FL 110 , FL 111 , and a polysilicon wiring PL 1 , and the PMOS transistor P 2  is made up of P +  diffusion regions FL 120 , FL 121 , and a polysilicon wiring PL 2 .  
         [0098]    In the P well region PW 0 , the NMOS transistor N 1  is made up of N +  diffusion regions FL 210 , FL 211 , and the polysilicon wiring PL 1 , and the NMOS transistor N 4  is made up of N +  diffusion regions FL 240 , FL 241 , and a polysilicon wiring PL 4 . The polysilicon wiring PL 1  extends from the N well region NW to the P well region PW 0 , so as to be used as a gate common to the NMOS transistor N 1  and PMOS transistor P 1 .  
         [0099]    In the P well region PW 1 , the NMOS transistor N 2  is made up of N +  diffusion regions FL 220 , FL 221 , and the polysilicon wiring PL 2 , and the NMOS transistor N 3  is made up of N +  diffusion regions FL 230 , FL 231 , and a polysilicon wiring PL 3 . The polysilicon wiring PL 2  extends from the N well region NW to the P well region PW 1 , so as to be used as a gate common to the NMOS transistor N 2  and PMOS transistor P 2 .  
         [0100]    The foregoing diffusion regions FL 110 , FL 111 , FL 120 , FL 121 , FL 210 , FL 211 , FL 220 , FL 221 , FL 230 , FL 231 , FL 240  and FL 241  are obtainable by implanting and diffusing impurity.  
         [0101]    A ground wiring LG 1  (first layer aluminum wiring) over the diffusion region FL 210  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 210 . An aluminum wiring AL 11 , which is a first layer aluminum wiring extending over the diffusion region FL 211 , FL 111  and FL 231 , is electrically connected through a diffusion contact hole  1 C to the diffusion regions FL 211 , FL 111  and FL 231 , respectively. The aluminum wiring AL 11  is also disposed over part of the polysilicon wiring PL 2 , and is electrically connected through a gate contact hole GC to the polysilicon wiring PL 2 . The aluminum wiring AL 11  can be electrically connected with low impedance, and it corresponds to the storage terminal Na.  
         [0102]    The diffusion contact hole  1 C means a contact hole between a diffusion region and a first layer (aluminum) wiring. The gate contact hole GC means a contact hole between a polysilicon wiring and a first layer wiring.  
         [0103]    The polysilicon wiring PL 4  is electrically connected through a gate contact hole GC to the word line WL 1  (first layer aluminum wiring). A bit line BLB 1  (first layer aluminum wiring) over the diffusion region FL 241  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 241 .  
         [0104]    An aluminum wiring AL 12 , which is a first layer aluminum wiring extending over the diffusion regions FL 240 , FL 120  and FL 220 , is electrically connected through a diffusion contact hole  1 C to the diffusion regions FL 240 , FL 120  and FL 220 , respectively. The aluminum wiring AL 12  is also disposed over part of the polysilicon wiring PL 1 , and is electrically connected through a gate contact hole GC to the polysilicon wiring PL 1 . The aluminum wiring AL 12  can be electrically connected with low impedance, and it corresponds to the storage terminal Nb.  
         [0105]    A power supply wiring LV 1  (first layer aluminum wiring) over the diffusion region FL 110  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 110 . The power supply wiring LV 1  over the diffusion region FL 121  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 121 .  
         [0106]    A bit line BLA 1  (first layer aluminum wiring) over the diffusion region FL 230  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 230 . A word line WL 1  over the polysilicon wiring PL 3  is electrically connected through a gate contact hole GC to the polyslicon wiring PL 3 . A ground wiring LG 1  over the diffusion region FL 221  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 221 .  
         [0107]    A ground wiring LG 1  is electrically connected through a via hole  1 T to a ground wiring LG 2  (second layer aluminum wiring ( 2 AL)), and the ground wiring LG 2  is electrically connected through a via hole  2 T to a ground wiring LG 3  (third layer aluminum wiring ( 3 AL)).  
         [0108]    A word line WL 1  is electrically connected through a via hole  1 T to a word line WL 2  (second layer aluminum wiring), and the word line WL 2  is electrically connected through a via hole  2 T to a word line WL 3  (third layer aluminum wiring). The word line WL shown in FIG. 4 is made up of these word lines WL 1  to WL 3 .  
         [0109]    The via hole  1 T means a via hole to make connection between a first layer wiring and a second layer (aluminum) wiring. The via hole  2 T means a via hole to make connection between a second layer wiring and a third layer (aluminum) wiring.  
         [0110]    The word line WL 3  and ground wiring LG 3  are disposed parallel with each other, across the P well regions PW 0 , PW 1 , and the N well region NW. Two ground wirings LG 3  are disposed with the word line WL 3  interposed therebetween.  
         [0111]    A bit line BLA 2  (second layer aluminum wiring) is electrically connected through a via hole  1 T to a bit line BLA 1  (not shown in FIG. 3). A bit line BLB 2  (second layer aluminum wiring) is electrically connected through a via hole  1 T to a bit line BLB 1  (not shown in FIG. 3). A power supply wiring LV 2  (second layer aluminum wiring) is electrically connected through a via hole  1 T to a power supply wiring LV 1  (not shown in FIG. 3). The bit lines BLA and BLB shown in FIG. 4 are made up of the bit lines BLA 1  and BLA 2 , and the bit lines BLB 1  and BLB 2 , respectively.  
         [0112]    The bit lines BLA 2 , BLB 2  and the power supply wiring LV 2  are disposed over the P well regions PW 1 , PW 0  and the N well region NW, respectively, so as to be parallel to each other in the longitudinal direction viewing the drawing.  
         [0113]    Thus, in the memory cell structure of the SRAM of the first preferred embodiment, with the N well region NW interposed between the P well regions PW 0  and PW 1 , the NMOS transistors N 1  and N 4  are disposed in the P well region PW 0 , and the NMOS transistors N 2  and N 3  are disposed in the P well region PW 1 . Thereby, the N +  diffusion region FL 211  and the N +  diffusion region FL 220  that are electrically connected to the storage terminals Na and Nb, respectively, can be separately formed in the different P well regions PW 0  and PW 1 .  
         [0114]    As a result, there are the following effects. Firstly, when electrons generated from alpha rays and neutron beams are collected into the N +  diffusion region formed in one of the P well regions PW 0  and PW 1 , such electrons are released from the N +  diffusion region formed in the other P well region where the influence of the generated electrons can be avoided by the presence of the N well region NW. This cancels out the occurrence of electrons acting to invert the hold data of the storage terminals Na and Nb, and thus the inversion of data is hard to occur. That is, there is the effect of improving resistance to soft error (This is hereinafter referred to as the first effect.).  
         [0115]    Secondly, since the P well regions PW 0  and PW 1  are separately formed in a direction vertical to the direction of formation of the bit lines BLA and BLB, the formation of the two P well regions PW 0  and PW 1  exerts no influence on the wiring length of the bit lines BLA and BLB. Hence, there is no possibility that the formation of the P well regions PW 0  and PW 1  increases the wiring length of the bit lines, thus maintaining a good access time (This is hereinafter referred to as the second effect.).  
         [0116]    Thirdly, since the NMOS transistors N 1  and N 2 , and the NMOS transistors N 3  and N 4 , are respectively arranged so as to be point symmetry with respect to the central part of the memory cell (the central part of the N well region NW), the degree of integration can be increased when a plurality of the memory cells of the first preferred embodiment are disposed adjacent each other (This is hereinafter referred to as the third effect.).  
         [0117]    Fourthly, the formation of the polysilicon wirings PL 1  to PL 4  in the same direction (the lateral direction viewing the drawing) facilitates the control of the gate dimension. Further, because the polysilicon wirings PL 1  and PL 3  (NMOS transistors N 1 , N 3 , and PMOS transistor P 1 ), and the polysilicon wirings PL 2  and PL 4  (NMOS transistors N 2 , N 4 , and PMOS transistor P 2 ) are respectively arranged in a straight line, no waste region is present and a reduction in the circuit area increases the degree of integration (This is hereinafter referred to as the fourth effect.).  
         [0118]    Fifthly, by separately forming a region serving as a drain (i.e., a region electrically connected to the storage terminal Na or Nb) in the NMOS transistors N 1  to N 4 , resistance to soft error can be maintained at a high level (a fifth effect).  
         [0119]    Sixthly, with the arrangement that each of inverters I 1  and I 2  of a CMOS structure is made up of a combination of a NMOS transistor and a PMOS transistor, the memory cell can be realized by at least sufficient circuit configuration as a CMOS structure (a sixth effect).  
         [0120]    Second Preferred Embodiment  
         [0121]    [0121]FIGS. 5 and 6 are diagrams illustrating a memory cell structure of a SRAM according to a second preferred embodiment of the invention. FIG. 5 is an explanatory diagram viewed from above the layout configuration in all layers. FIG. 6 is an explanatory diagram viewed from above mainly the layout configuration beneath a first aluminum wiring layer in FIG. 5. An explanatory diagram viewed from above the layout configuration over a second aluminum wiring layer in FIG. 5 is similar to that of FIG. 3 in the first preferred embodiment. A circuit diagram illustrating an equivalent circuit of the second preferred embodiment is similar to that of FIG. 4. Some reference numerals used in FIG. 6 or  3  are omitted in FIG. 5.  
         [0122]    As seen from these figures, over a rectangular N +  diffusion region for a NMOS transistor N 1 , a polysilicon wiring PL 1  is formed by bending it from the central part of the N +  diffusion region, so that a relatively wide diffusion region FL 212  and a relatively narrow diffusion region FL 213  are formed on the outside and inside of the polysilicon wiring PL 1 , respectively. The NMOS transistor N 1  is made up of the diffusion regions FL 212 , FL 213 , and the polysilicon wiring PL 1 .  
         [0123]    Likewise, over a rectangular N +  diffusion region for a NMOS transistor N 2 , a polysilicon wiring PL 2  is formed by bending it from the central part of the N +  diffusion region, so that a relatively wide diffusion region FL 223  and a relatively narrow diffusion region FL 222  are formed on the outside and inside of the polysilicon wiring PL 2 , respectively. The NMOS transistor N 2  is made up of the diffusion regions FL 222 , FL 223 , and the polysilicon wiring PL 2 .  
         [0124]    A ground wiring LG 1  over the diffusion region FL 212  is electrically connected through two diffusion contact holes  1 C to the diffusion region FL 212 . An aluminum wiring AL 11  over the diffusion region FL 213  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 213 .  
         [0125]    Likewise, a ground wiring LG 1  over the diffusion region FL 223  is electrically connected through two diffusion contact holes  1 C to the diffusion region FL 223 . An aluminum wiring AL 12  over the diffusion region FL 222  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 222 . Otherwise, the layout configuration is similar to that of the first preferred embodiment, and the description thereof is thus omitted.  
         [0126]    The second preferred embodiment having the foregoing layout configuration produces the following effects in addition to the first, second, fifth and sixth effects of the first preferred embodiment.  
         [0127]    It is able to increase the gate width (channel width) of the NMOS transistors N 1  and N 2  that are driver transistors. As a result, the operation speed can be increased by quickly removing the carriers of the bit lines BLA and BLB.  
         [0128]    Additionally, the ratio of a gate width W to the NMOS transistors N 3  and N 4 , which are the respective access transistors of the NMOS transistors N 1  and N 2  that are driver transistors, can be increased to improve the stability of the memory cell.  
         [0129]    [0129]FIG. 7 is an explanatory diagram viewed from above the layout configuration between adjacent cells. Like FIG. 6, FIG. 7 illustrates mainly the layout configuration beneath a first aluminum wiring layer in FIG. 5.  
         [0130]    In FIG. 7, there are shown an N well region NW and a P well region PW 0  of a memory cell MC 1 , and an N well region NW and a P well region PW 0  of a memory cell MC 2 .  
         [0131]    The NMOS transistors N 1  and N 2  are respectively arranged so as to be point symmetry with respect to the central part of the memory cell (the central part of the N well region NW). This corresponds to the third effect of the first preferred embodiment. Referring to FIG. 7, between the adjacent memory cells MC 1  and MC 2 , the NMOS transistors N 1  (N 2 ), each being a driver transistor, can be disposed adjacent each other in line symmetric relation, thereby to increase the gate width W of the NMOS transistors N 1  and N 2 , while increasing the degree of integration by having the diffusion region FL 212 , word line WL 1 , ground wiring LG 1 , diffusion contact hole  1 C and gate contact hole GC share at least their respective portions.  
         [0132]    Thus, there is little or no increase of area due to the bending of the polysilicon wirings PL 1  and PL 2  that become the gates of the NMOS transistors N 1  and N 2 , respectively. It is therefore able to obtain a high-density memory cell structure similar to that of the first preferred embodiment.  
         [0133]    In addition, the degree of integration can be increased by disposing the NMOS transistors N 1 , N 3  and PMOS transistor P 1 ; and the NMOS transistors N 2 , N 4  and PMOS transistor P 2 , in an approximately straight line, respectively. This corresponds to the fourth effect of the first preferred embodiment.  
         [0134]    Third Preferred Embodiment  
         [0135]    FIGS.  8  to  10  are diagrams illustrating a memory cell structure of a SRAM according to a third preferred embodiment of the invention. FIG. 8 is an explanatory diagram viewed from above the layout configuration in all layers. FIG. 9 is an explanatory diagram viewed from above mainly the layout configuration beneath a first aluminum wiring layer in FIG. 8. An explanatory diagram viewed from above the layout configuration over a second aluminum wiring layer in FIG. 8 is similar to that of FIG. 3 in the first preferred embodiment. Some reference numerals used in FIG. 9 or  3  are omitted in FIG. 8.  
         [0136]    [0136]FIG. 10 is a circuit diagram illustrating an equivalent circuit of the SRAM memory cell having the layout configuration shown in FIGS. 8, 9 and  3 . Referring to FIG. 10, a resistance R 1  is interposed between a storage terminal Nb and the gate of a NMOS transistor N 1  and a PMOS transistor P 1 . A resistance R 2  is interposed between a storage terminal Na and the gate of a NMOS transistor N 2  and a PMOS transistor P 2 . Otherwise, the configuration is similar to that of the first preferred embodiment described with respect to FIG. 4, and the description thereof is thus omitted.  
         [0137]    Referring to FIGS. 8, 9 and  3 , description will proceed to the memory cell structure of the third preferred embodiment.  
         [0138]    As shown in these figures, a polysilicon wiring PL 13  (corresponding to the polysilicon wiring PL 1  of the first preferred embodiment), which becomes the gate of the NMOS transistor N 1  and PMOS transistor P 1 , is electrically connected to a high resistance metal wiring M 00  that becomes the resistance R 1 . The high resistance metal wiring M 00  is electrically connected through a via hole  0 T to an aluminum wiring AL 12  that is the storage terminal Nb. The via hole  0 T means a via hole to make connection between the high resistance metal wiring M 00  formed in the same layer as the polysilicon wiring, and a first layer wiring.  
         [0139]    Likewise, a polysilicon wiring PL 14  (corresponding to the polysilicon wiring PL 2  of the first preferred embodiment), which becomes the gate of the NMOS transistor N 2  and PMOS transistor P 2 , is electrically connected to a high resistance metal wiring M 01  that becomes the resistance R 2 . The high resistance metal wiring M 01  is electrically connected through a via hole  0 T to the aluminum wiring AL 11  that is the storage terminal Na.  
         [0140]    Examples of material of the high resistance metal wirings M 00  and M 01  are tungsten, etc., having a higher resistivity than CoSi (cobalt silicon). Otherwise, the configuration is similar to that of the first preferred embodiment described with respect to FIGS.  1  to  3 , and the description thereof is thus omitted.  
         [0141]    The third preferred embodiment having the foregoing memory cell structure produces the following effect in addition to the first to sixth effects of the first preferred embodiment.  
         [0142]    In the memory cell of the third preferred embodiment, the response characteristic for inverting the data held in the cell is elongated due to signal delay propagating the resistances R 1  and R 2 . As a result, even if the potential of one of the storage terminals Na and Nb is inverted by electrons generated from alpha rays and neutron beams, it returns to the initial hold state before the data of the other storage terminal is inverted, thereby soft error becomes much rare.  
         [0143]    Fourth Preferred Embodiment  
         [0144]    [0144]FIGS. 11 and 12 are diagrams illustrating a memory cell structure of a SRAM according to a fourth preferred embodiment of the invention. FIG. 11 is an explanatory diagram viewed from above the layout configuration in all layers. FIG. 12 is an explanatory diagram viewed from above mainly the layout configuration beneath a first aluminum wiring layer in FIG. 11. An explanatory diagram viewed from above the layout configuration over a second aluminum wiring layer in FIG. 11 is similar to that of FIG. 3 in the first preferred embodiment. Some reference numerals used in FIG. 12 or  3  are omitted in FIG. 11. An equivalent circuit of the SRAM memory cell having the layout configuration of the fourth preferred embodiment is similar to that of the third preferred embodiment described with respect to FIG. 10.  
         [0145]    Referring to FIGS. 11, 12 and  3 , description will proceed to the memory cell structure of the fourth preferred embodiment.  
         [0146]    Of polysilicon wirings PL 13  and PL 17  (corresponding to the polysilicon wiring PL 1  of the first preferred embodiment), which become the gate of a NMOS transistor N 1  and a PMOS transistor P 1 , the polysilicon wiring PL 17  that becomes a resistance R 1  is formed from a material having a higher resistance than the polysilicon wiring PL 13 . For instance, when the polysilicon wiring PL 13  is formed from CoSi, the polysilicon wiring PL 17  is formed from a material having a higher resistivity than CoSi.  
         [0147]    The polysilicon wiring PL 17  is electrically connected through a gate contact hole GC to an aluminum wiring AL 12  that is a storage terminal Nb.  
         [0148]    Likewise, of polysilicon wirings PL 14  and PL 18  (corresponding to the polysilicon wiring PL 2  of the first preferred embodiment), which become the gate of a NMOS transistor N 2  and a PMOS transistor P 2 , the polysilicon wiring PL 18  that becomes a resistance R 2  is formed from a material having a higher resistance than the polysilicon wiring PL 14 . The polysilicon wiring PL 18  is electrically connected through a gate contact hole GC to an aluminum wiring AL 11  that is a storage terminal Na. Otherwise, the configuration is similar to that of the first preferred embodiment described with respect to FIGS.  1  to  3 , and the description thereof is thus omitted.  
         [0149]    The fourth preferred embodiment having the foregoing memory cell structure produces the following effect in addition to the first to sixth effects of the first preferred embodiment.  
         [0150]    In the memory cell of the fourth preferred embodiment, the response characteristic for inverting the data held in the cell is elongated due to signal delay propagating the resistances R 1  and R 2 . As a result, even if the potential of one of the storage terminals Na and Nb is inverted by electrons generated from alpha rays and neutron beams, it returns to the initial hold state before the data of the other storage terminal is inverted, thereby soft error becomes much rare.  
         [0151]    Fifth Preferred Embodiment  
         [0152]    FIGS.  13  to  15  are diagrams illustrating a memory cell structure of a SRAM according to a fifth preferred embodiment of the invention. FIG. 13 is an explanatory diagram viewed from above the layout configuration in all layers. FIG. 14 is an explanatory diagram viewed from above mainly the layout configuration over a second aluminum wiring layer in FIG. 13. An explanatory diagram viewed from above the layout configuration beneath a first aluminum wiring layer in FIG. 13 is similar to that of FIG. 2 in the first preferred embodiment, except that the word line WL 2  is divided into word lines WLA 2  and WLB 2 . Some reference numerals used in FIG. 14 or  2  are omitted in FIG. 13.  
         [0153]    [0153]FIG. 15 is a circuit diagram illustrating an equivalent circuit of the SRAM memory cell having the layout configuration shown in FIGS. 13, 14 and  2 . Referring to FIG. 15, a word line WLA is connected to the gate of a NMOS transistor N 3 , and a word line WLB that is independent of the word line WLA is connected to the gate of a NMOS transistor N 4 . Otherwise, the configuration is similar to that of the first preferred embodiment described with respect to FIG. 4, and the description thereof is thus omitted.  
         [0154]    Referring to FIGS. 13, 14 and  2 , description will proceed to the memory cell structure of the fifth preferred embodiment.  
         [0155]    A polysilicon wiring PL 3  is electrically connected through a gate contact hole GC to a word line WLA 1  (first layer aluminum wiring). The word line WLA 1  is electrically connected through a via hole  1 T to a word line WLA 2  (second layer aluminum wiring). The word line WLA 2  is electrically connected through a via hole  2 T to a word line WLA 3  (third layer aluminum wiring). The word line WLA of FIG. 15 is made up of these word lines WLA 1  to WLA 3 .  
         [0156]    Likewise, a polysilicon wiring PL 4  is electrically connected through a gate contact hole GC to a word line WLB 1  (first layer aluminum wiring). The word line WLB 1  is electrically connected through a via hole  1 T to a word line WLB 2  (second layer aluminum wiring). The word line WLB 2  is electrically connected through a via hole  2 T to a word line WLB 3  (third layer aluminum wiring). The word line WLB of FIG. 15 is made up of these word lines WLB 1  to WLB 3 .  
         [0157]    The word line WLA 3 , WLB 3  and a ground wiring LG 3  are disposed parallel with each other, across P well regions PW 0 , PW 1  and an N well region NW. Two ground wirings LG 3  are disposed with the word lines WLA 3  and WLB 3  interposed therebetween. Otherwise, the layout configuration is similar to that of the first preferred embodiment, and the description thereof is thus omitted.  
         [0158]    The fifth preferred embodiment having the foregoing memory cell structure produces the following effect in addition to the first to sixth effects of the first preferred embodiment.  
         [0159]    As shown in the equivalent circuit of FIG. 15, the word line connected to the gate of the NMOS transistors N 3  and N 4  that are access transistors can be divided into the word lines WLA and WLB. This enables to realize a memory cell structure usable in a FIFO memory.  
         [0160]    Sixth Preferred Embodiment  
         [0161]    FIGS.  16  to  18  are diagrams illustrating a memory cell structure of a SRAM according to a sixth preferred embodiment of the invention. FIG. 16 is an explanatory diagram viewed from above the layout configuration in all layers. FIG. 17 is an explanatory diagram viewed from above mainly the layout configuration beneath a first aluminum wiring layer in FIG. 16. FIG. 18 is an explanatory diagram viewed from above the layout configuration over a second aluminum wiring layer in FIG. 16. Some reference numerals used in FIG. 17 or  18  are omitted in FIG. 16. An equivalent circuit of the SRAM memory cell having the layout configuration of the sixth preferred embodiment is similar to that of FIG. 15 described in the fifth preferred embodiment.  
         [0162]    Referring to FIGS.  16  to  18 , description will proceed to the memory cell structure of the sixth preferred embodiment.  
         [0163]    In an N +  diffusion region for NMOS transistors N 3  and N 4 , the direction of formation of a source/drain region is located at an angle of 90° to the direction of formation of a source/drain region of NMOS transistors N 1 , N 2  and PMOS transistors P 1 , P 2 . That is, diffusion regions FL 242  and FL 243  for the NMOS transistor N 3 , and diffusion regions FL 232  and FL 233  for the NMOS transistor N 4 , are disposed in the lateral direction viewing the drawing.  
         [0164]    A bit line BLB 1  over the diffusion region FL 243  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 243 . A bit line BLB 2  (second layer aluminum wiring) is electrically connected through a via hole  1 T to the bit line BLB 1  (not shown in FIG. 18).  
         [0165]    Likewise, a bit line BLA 1  over the diffusion region FL 232  constituting the NMOS transistor N 3  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 232 . A bit line BLA 2  (second layer aluminum wiring) is electrically connected through a via hole  1 T to the bit line BLA 1  (not shown in FIG. 18).  
         [0166]    The bit lines BLA 2  and BLB 2  are disposed parallel with each other, across P well regions PW 0 , PW 1  and an N well region NW.  
         [0167]    A ground wiring LG 1  is electrically connected through a diffusion contact hole  1 C to diffusion regions FL 210  and FL 221 . A ground wiring LG 2  is electrically connected through a via hole  1 T to the ground wiring LG 1  (not shown in FIG. 18). A ground wiring LG 3  is electrically connected through a via hole  2 T to the ground wiring LG 2 .  
         [0168]    A power supply wiring LV 1  is electrically connected through a diffusion contact hole  1 C to diffusion regions FL 110  and FL 121 . A power supply wiring LV 2  is electrically connected through a via hole  1 T to the power supply wiring LV 1  (not shown in FIG. 18). A power supply wiring LV 3  is electrically connected through a via hole  2 T to the power supply wiring LV 2 .  
         [0169]    A word line WLA 1  is electrically connected through a gate contact hole GC to a polysilicon wiring PL 23 . A word line WLA 2  is electrically connected through a via hole  1 T to the word line WLA 1  (not shown in FIG. 18). A word line WLA 3  (third layer aluminum wiring) is electrically connected through a via hole  2 T to the word line WLA 2 .  
         [0170]    Likewise, a word line WLB 1  is electrically connected through a gate contact hole GC to a polysilicon wiring PL 24 . A word line WLB 2  is electrically connected through a via hole  1 T to the word line WLB 1  (not shown in FIG. 18). A word line WLB 3  (third layer aluminum wiring) is electrically connected through a via hole  2 T to the word line WLB 2 .  
         [0171]    The (first) ground wiring LG 3 , word line WLB 3 , power supply wiring LV 3 , word line WLA 3  and (second) ground wiring LG 3  are disposed parallel with each other in the longitudinal direction viewing the drawing. The (first) ground wiring LG 3  and word line WLB 3  are disposed over the P well region PW 0 . The power supply wiring LV 3  is disposed over the N well region NW. The word line WLA 3  and (second) ground wiring LG 3  are disposed over the P well region PW 1 .  
         [0172]    The sixth preferred embodiment having the foregoing memory cell structure produces the effect equivalent to that inherent in the fifth preferred embodiment, in addition to the first to third, fifth and sixth effects of the first preferred embodiment.  
         [0173]    Seventh Preferred Embodiment  
         [0174]    FIGS.  19  to  21  are diagrams illustrating a memory cell structure of a SRAM according to a seventh preferred embodiment of the invention. FIG. 19 is an explanatory diagram viewed from above the layout configuration in all layers. FIG. 20 is an explanatory diagram viewed from above mainly the layout configuration beneath a first aluminum wiring layer in FIG. 19. FIG. 21 is an explanatory diagram viewed from above the layout configuration over a second aluminum wiring layer in FIG. 19. Some reference numerals used in FIG. 20 or  21  are omitted in FIG. 19. An equivalent circuit of the SRAM memory cell having the layout configuration of the seventh preferred embodiment is similar to that of FIG. 4 in the first preferred embodiment.  
         [0175]    Referring to FIGS.  19  to  21 , description will proceed to the memory cell structure of the seventh preferred embodiment.  
         [0176]    A common polysilicon wiring PL 5  of NMOS transistors N 3  and N 4  extends over a P well region PW 0 , N well region NW and P well region PW 1 . The common polysilicon wiring PL 5  is used as the word line WL of FIG. 4.  
         [0177]    Otherwise, the configuration is similar to that of the second preferred embodiment described with respect to FIGS. 5, 6 and  3 , except for the pattern shape of polysilicon wirings PL 1  and PL 2 , the position of a gate contact hole GC between a polysilicon wiring PL 1  and an aluminum wiring AL 12 , and the position of a gate contact hole GC between a polysilicon wiring PL 2  and an aluminum wiring AL 11 .  
         [0178]    The seventh preferred embodiment having the foregoing memory cell structure produces the same effects as the second preferred embodiment. In addition, since the word line WL does not require any of via holes  1 T,  2 T and word lines WL 2 , WL 3 , the number of necessary layers is reduced to lower the cost.  
         [0179]    Eighth Preferred Embodiment  
         [0180]    FIGS.  22  to  25  are diagrams illustrating a memory cell structure of a SRAM according to an eighth preferred embodiment of the invention. FIG. 22 is an explanatory diagram viewed from above the layout configuration in all layers. FIG. 23 is an explanatory diagram viewed from above mainly the layout configuration beneath a first aluminum wiring layer in FIG. 22. FIG. 24 is an explanatory diagram viewed from above the layout configuration over a second aluminum wiring layer in FIG. 22. Some reference numerals used in FIG. 23 or  24  are omitted in FIG. 22.  
         [0181]    [0181]FIG. 25 is a circuit diagram illustrating an equivalent circuit of the SRAM memory cell having the layout configuration shown in FIGS.  22  to  24 . Referring to FIG. 25, the SRAM memory cell of the eighth preferred embodiment is made up of NMOS transistors N 1 , N 2 , N 5  to N 8 , and PMOS transistors P 1  and P 2 .  
         [0182]    The NMOS transistor N 5  is interposed between a bit line BLA and a storage terminal Nb. The NMOS transistor N 6  is interposed between a bit line BLA and a storage terminal Na. The gates of the NMOS transistors N 5  and N 6  are both connected to a word line WLA.  
         [0183]    The NMOS transistor N 7  is interposed between a bit line BLB and a storage terminal Na. The NMOS transistor N 8  is interposed between a bit line BLB and a storage terminal Nb. The gates of the NMOS transistors N 7  and N 8  are both connected to a word line WLB.  
         [0184]    The PMOS transistors P 1  and P 2  that are driver transistors are disposed within an N well region NW. The NMOS transistor N 1  that is a driver transistor and the NMOS transistors N 7  and N 8  that are access transistors are disposed within a P well region PW 0 . The NMOS transistor N 2  that is a driver transistor and the NMOS transistors N 5  and N 6  that are access transistors are disposed within a P well region PW 0 . The P well regions PW 0  and PW 1  are oppositely disposed with the N well region NW interposed therebetween. Otherwise, the configuration is similar to that of the equivalent circuit of FIG. 15 described in the fifth preferred embodiment.  
         [0185]    Referring to FIGS.  22  to  24 , description will proceed to the memory cell structure of the eighth preferred embodiment.  
         [0186]    In the N well region NW, the PMOS transistor P 1  is made up of P +  diffusion regions FL 110 , FL 111  and a polysilicon wiring PL 17 , and the PMOS transistor P 2  is made up of P +  diffusion regions FL 120 , FL 121  and a polysilicon wiring PL 18 .  
         [0187]    In the P well region PW 0 , the NMOS transistor N 1  is made up of N +  diffusion regions FL 212 , FL 213  and the polysilicon wiring PL 17 . The NMOS transistor N 7  is made up of N +  diffusion regions FL 244 , FL 245  and a polysilicon wiring PL 20 . The NMOS transistor N 8  is made up of N +  diffusion regions FL 246 , FL 247  and the polysilicon wiring PL 20 . The polysilicon wiring PL 17  extends from the N well region NW to the P well region PW 0 , so as to be used as a gate common to the NMOS transistor N 1  and PMOS transistor P 1 . The polysilicon wiring PL 20  is common to the NMOS transistors N 7  and N 8 .  
         [0188]    In the P well region PW 1 , the NMOS transistor N 2  is made up of N +  diffusion regions FL 222 , FL 223  and the polysilicon wiring PL 18 . The NMOS transistor N 5  is made up of N +  diffusion regions FL 234 , FL 235  and a polysilicon wiring PL 19 . The NMOS transistor N 3  is made up of N +  diffusion regions FL 236 , FL 237  and a polysilicon wiring PL 19 . The polysilicon wiring PL 18  extends from the N well region NW to the P well region PW 1 , so as to be used as a gate common to the NMOS transistor N 2  and PMOS transistor P 2 . The polysilicon wiring PL 18  is common to the NMOS transistors N 5  and N 6 . The foregoing diffusion regions are obtainable by implanting and diffusing impurity.  
         [0189]    A ground wiring LG 1  over the diffusion region FL 212  is electrically connected through a diffusion contact hole  11 C to the diffusion region FL 212 . A bit line BLB 1  over the diffusion region FL 245  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 245 . A bit line {overscore (BLB 1 )} over the diffusion region FL 247  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 247 .  
         [0190]    An aluminum wiring AL 15 , which is a first layer aluminum wiring extending over the diffusion regions FL 244 , FL 213 , FL 111  and FL 237 , is electrically connected through a diffusion contact hole  1 C to the diffusion regions FL 244 , FL 213 , FL 111  and FL 237 , respectively. The aluminum wiring AL 15  is also disposed over part of the polysilicon wiring PL 18 , and is electrically connected through a gate contact hole GC to the polysilicon wiring PL 18 . The aluminum wiring AL 15  can be electrically connected with low impedance, and it corresponds to the storage terminal Na.  
         [0191]    A polysilicon wiring PL 20  is electrically connected through a gate contact hole GC to a word line WLB 1 .  
         [0192]    A power supply wiring LV 1  over the diffusion region FL 110  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 110 . A power supply wiring LV 1  over the diffusion region FL 121  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 121 .  
         [0193]    The ground wiring LG 1  is electrically connected through two diffusion contact holes  1 C to the diffusion region FL 223 . A bit line BLA 1  over the diffusion region FL 234  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 234 . A bit line {overscore (BLA 1 )} over the diffusion region FL 236  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 236 .  
         [0194]    An aluminum wiring AL 16 , which is a first layer aluminum wiring extending over the diffusion regions FL 235 , FL 222 , FL 120  and FL 246 , is electrically connected through a diffusion contact hole  1 C to the diffusion regions FL 235 , FL 222 , FL 120  and FL 246 , respectively. The aluminum wiring AL 16  is also disposed over part of the polysilicon wiring PL 17 , and is electrically connected through a gate contact hole GC to the polysilicon wiring PL 17 . The aluminum wiring AL 16  can be electrically connected with low impedance, and it corresponds to the storage terminal Nb.  
         [0195]    A word line WLA 1  over a polysilicon wiring PL 19  is electrically connected through a gate contact hole GC to the polysilicon wiring PL 19 .  
         [0196]    A ground wiring LG 1  is electrically connected through a via hole  1 T to a ground wiring LG 2 , and the ground wiring LG 2  is electrically connected through a via hole  2 T to a ground wiring LG 3 .  
         [0197]    The word line WLA 1  is electrically connected through a via hole  1 T to a word line WLA 2 , and the word line WLA 2  is electrically connected through a via hole  2 T to a word line WLA 3 . The word line WLA of FIG. 25 is made up of these word lines WLA 1  to WLA 3 .  
         [0198]    Likewise, a word line WLB 1  is electrically connected through a via hole  1 T to a word line WLB 2 , and the word line WLB 2  is electrically connected through a via hole  2 T to a word line WLB 3 . The word line WLB of FIG. 25 is made up of these word lines WLB 1  to WLB 3 .  
         [0199]    The word lines WLA 3 , WLB 3 , and ground wiring LG 3  are disposed parallel with each other, across the P well regions PW 0 , PW 1 , and the N well region NW. Two ground wirings LG 3  are disposed with the word lines WLA 3  and WLB 3  interposed therebetween.  
         [0200]    A bit line BLA 2  is electrically connected through a via hole  1 T to a bit line BLA 1 , and a bit line BLB 2  is electrically connected through a via hole  1 T to a bit line BLB 1 .  
         [0201]    Likewise, a bit line {overscore (BLA 2 )} is electrically connected through a via hole  1 T to a bit line {overscore (BLA 1 )}, and a bit line {overscore (BLB 2 )} is electrically connected through a via hole  1 T to a bit line {overscore (BLB 1 )}.  
         [0202]    A power supply wiring LV 2  is electrically connected through a via hole  1 T to a power supply wiring LV 1 . The bit lines BLA, {overscore (BLA)}, BLB and {overscore (BLB)}, are made up of the bit lines BLA 1  and BLA 2 ; {overscore (BLA 1 )} and {overscore (BLA 2 )}; BLB 1  and BLB 2 ; and {overscore (BLB 1 )} and {overscore (BLB 2 )}, respectively.  
         [0203]    The paired bit lines BLA 2  and {overscore (BLA 2 )}, paired bit lines BLB 2  and {overscore (BLB 2 )}, and the power supply wiring LV 2  are disposed over the P well regions PW 1 , PW 0  and N well region NW, respectively, so that these are parallel with each other in the longitudinal direction viewing the drawing.  
         [0204]    Thus, in the memory cell structure of the SRAM of the eighth preferred embodiment, with the N well region NW interposed between the P well regions PW 0  and PW 1 , the NMOS transistors N 1 , N 7  and N 8  are disposed in the P well region PW 0 , and the NMOS transistors N 2 , N 5  and N 6  are disposed in the P well region PW 1 . Thereby, the N +  diffusion region FL 213  and the N +  diffusion region FL 222  that are electrically connected to the storage terminals Na and Nb, respectively, can be separately formed in the different P well regions PW 0  and PW 1 .  
         [0205]    As a result, it is able to increase resistance to soft error, which is the first effect of the first preferred embodiment.  
         [0206]    Since the P well regions PW 0  and PW 1  are separately formed in a direction vertical to the direction of formation of the paired bit lines BLA and {overscore (BLA)}, and the paired bit lines BLB and {overscore (BLB)}, the formation of the two P well regions PW 0  and PW 1  exerts no influence on the wiring length of the paired bit lines BLA and {overscore (BLA)}, and the paired bit lines BLB and {overscore (BLB)}. Hence, there is no possibility that the formation of the P well regions PW 0  and PW 1  increases the wiring length of the bit lines, thus maintaining a good access time. This corresponds to the second effect of the first preferred embodiment.  
         [0207]    Since the NMOS transistors N 1  and N 2 , the NMOS transistors N 5  and N 7 , and the NMOS transistors N 6  and N 8 , are respectively arranged so as to be point symmetry with respect to the central part of the memory cell (the central part of the N well region NW), the degree of integration can be increased when a plurality of the memory cells of the eighth preferred embodiment are disposed adjacent each other. This corresponds to the third effect of the first preferred embodiment.  
         [0208]    The formation of the polysilicon wirings PL 17  to PL 20  in the same direction (the lateral direction viewing the drawing) facilitates the control of the gate dimension. Further, since the polysilicon wirings PL 17  and PL 19 , and the polysilicon wirings PL 18  and PL 20 , are respectively arranged in a straight line, no waste region is present and a reduction in the circuit area increases the degree of integration. This corresponds to the fourth effect of the first preferred embodiment.  
         [0209]    By separately forming a region serving as a drain in the NMOS transistors N 1 , N 2  and N 5  to N 8 , resistance to soft error can be maintained at a high level. This corresponds to the fifth effect of the first preferred embodiment.  
         [0210]    With the arrangement such that each of inverters I 1  and I 2  of a CMOS structure is made up of a combination of a NMOS transistor and a PMOS transistor, the memory cell can be realized by at least sufficient circuit configuration as a CMOS structure. This corresponds to the sixth effect of the first preferred embodiment.  
         [0211]    In addition, the memory cell of the eighth preferred embodiment realizes a two-port memory cell which employs two word lines WLA and WLB, and two pairs of bit lines (the paired bit lines BLA and {overscore (BLA)}, and paired bit lines BLB and {overscore (BLB)}), as shown in FIG. 25.  
         [0212]    Ninth Preferred Embodiment  
         [0213]    FIGS.  26  to  28  are diagrams illustrating a memory cell structure of a SRAM according to a ninth preferred embodiment of the invention. FIG. 26 is an explanatory diagram viewed from above the layout configuration in all layers. FIG. 27 is an explanatory diagram viewed from above mainly the layout configuration beneath a first aluminum wiring layer in FIG. 26. FIG. 28 is an explanatory diagram viewed from above the layout configuration over a second aluminum wiring layer in FIG. 26. Some reference numerals used in FIG. 27 or  28  are omitted in FIG. 26.  
         [0214]    An equivalent circuit of the SRAM memory cell having the layout configuration of the ninth preferred embodiment is similar to that of FIG. 25 in the eighth preferred embodiment.  
         [0215]    Referring to FIGS.  26  to  28 , the memory cell structure of the ninth preferred embodiment will be described particularly with regard to the different points from the eighth preferred embodiment.  
         [0216]    In a P well region PW 0 , a NMOS transistor N 1  is made up of N +  diffusion regions FL 214 , FL 215  and a polysilicon wiring PL 31 . Here, a considerably large gate width than that of other NMOS transistors N 5  to N 8  can be set by forming the polysilicon wiring PL 31  by bending it 90° two times over the N +  diffusion regions (FL 214 , FL 215 ) for the NMOS transistor N 1 .  
         [0217]    The NMOS transistor N 7  is made up of N +  diffusion regions FL 270 , FL 271  and a polysilicon wiring PL 37 . The NMOS transistor N 8  is made up of N +  diffusion regions FL 280 , FL 281  and a polysilicon wiring PL 38 .  
         [0218]    The polysilicon wiring PL 31  extends from an N well region NW to the P well region PW 0 , so as to be used as a gate common to the NMOS transistor N 1  and a PMOS transistor P 1 .  
         [0219]    In a P well region PW 1 , the NMOS transistor N 2  is made up of N +  diffusion regions FL 224 , FL 225  and a polysilicon wiring PL 32 . Here, a considerably large gate width than that of the other NMOS transistors N 5  to N 8  can be set by forming the polysilicon wiring PL 32  by bending it 90° two times over the N +  diffusion regions (FL 224 , FL 225 ) for the NMOS transistor N 2 .  
         [0220]    The NMOS transistor N 5  is made up of N +  diffusion regions FL 250 , FL 251  and a polysilicon wiring PL 35 . The NMOS transistor N 6  is made up of N +  diffusion regions FL 260 , FL 261  and a polysilicon wiring PL 36 .  
         [0221]    The polysilicon wiring PL 32  extends from the N well region NW to the P well region PW 0 , so as to be used as a gate common to the NMOS transistor N 2  and a PMOS transistor P 2 . The foregoing diffusion regions are obtainable by implanting and diffusing impurity.  
         [0222]    Each of two ground wirings LG 1  over the diffusion region FL 214  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 214 . A bit line BLB 1  over the diffusion region FL 271  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 271 . A bit line {overscore (BLB 1 )} over the diffusion region FL 280  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 280 .  
         [0223]    An aluminum wiring AL 17 , which is a first layer aluminum wiring extending over the diffusion regions FL 281 , FL 215 , FL 111 , and FL 251 , is electrically connected through a diffusion contact hole  1 C to the diffusion regions FL 281 , FL 215 , FL 111  and FL 251 , respectively. The aluminum wiring AL 17  is also disposed over part of the polysilicon wiring PL 32 , and is electrically connected through a gate contact hole GC to the polysilicon wiring PL 32 . The aluminum wiring AL 17  can be electrically connected with low impedance, and it corresponds to a storage terminal Na.  
         [0224]    The polysilicon wirings PL 37  and PL 38  are both electrically connected through a gate contact hole GC to a word line WLB 1 .  
         [0225]    A power supply wiring LV 1  over the diffusion region FL 110  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 110 . A power supply wiring LV 1  over the diffusion region FL 121  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 121 .  
         [0226]    Each of two ground wirings LG 1  over the diffusion region FL 224  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 224 . A bit line BLA 1  over the diffusion region FL 250  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 250 . A bit line {overscore (BLA 1 )} over the diffusion region FL 261  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 261 .  
         [0227]    An aluminum wiring AL 18 , which is a first layer aluminum wiring extending over the diffusion regions FL 260 , FL 224 , FL 120  and FL 270 , is electrically connected through a diffusion contact hole  1 C to the diffusion regions FL 260 , FL 224 , FL 120  and FL 270 , respectively. The aluminum wiring AL 18  is also disposed over part of the polysilicon wiring PL 31 , and is electrically connected through a gate contact hole GC to the polysilicon wiring PL 31 . The aluminum wiring AL 18  can be electrically connected with low impedance, and it corresponds to a storage terminal Nb.  
         [0228]    A word line WLA 1  over the polysilicon wirings PL 35  and PL 36  is electrically connected through a gate contact hole GC to the polysilicon wirings PL 35  and PL 36 , respectively.  
         [0229]    A ground wiring LG 1  is electrically connected through a via hole  1 T to a ground wiring LG 2 , and the ground wiring LG 2  is electrically connected through a via hole  2 T to a ground wiring LG 3 .  
         [0230]    A word line WLA 1  is electrically connected through a via hole  1 T to a word line WLA 2 , and the word line WLA 2  is electrically connected through a via hole  2 T to a word line WLA 3 . Likewise, a word line WLB 1  is electrically connected through a via hole  1 T to a word line WLB 2 , and the word line WLB 2  is electrically connected through a via hole  2 T to a word line WLB 3 .  
         [0231]    A bit line BLA 2  is electrically connected through a via hole  1 T to a bit line BLA 1 . A bit line BLB 2  is electrically connected through a via hole  1 T to a bit line BLB 1 .  
         [0232]    Likewise, a bit line {overscore (BLA 2 )} is electrically connected through a via hole  1 T to a bit line {overscore (BLA 1 )}, and a bit line {overscore (BLB 2 )} is electrically connected through a via hole  1 T to a bit line {overscore (BLB 1 )}. A power supply wiring LV 2  is electrically connected through a via hole  1 T to a power supply wiring LV 1 .  
         [0233]    Thus, in the memory cell structure of the SRAM of the ninth preferred embodiment, with the N well region NW interposed between the P well regions PW 0  and PW 1 , the NMOS transistors N 1 , N 7  and N 8  are disposed in the P well region PW 0 , and the NMOS transistors N 2 , N 5  and N 6  are disposed in the P well region PW 1 . This enables to increase resistance to soft error, which is the first effect of the first preferred embodiment.  
         [0234]    By separately forming the P well regions PW 0  and PW 1  in a direction vertical to the direction of formation of the paired bit lines BLA and {overscore (BLA)} and the paired bit lines BLB and {overscore (BLB)}, it is able to maintain a good access time, which is the second effect of the first preferred embodiment.  
         [0235]    Further, in the ninth preferred embodiment, as in the eighth preferred embodiment, the NMOS transistors N 1  and N 2 , the NMOS transistors N 5  and N 7 , and the NMOS transistors N 6  and N 8 , are respectively arranged so as to be point symmetry with respect to the central part of the memory cell. It is therefore able to increase the degree of integration when a plurality of the memory cells of the ninth preferred embodiment are disposed adjacent each other. This corresponds to the third effect of the first preferred embodiment.  
         [0236]    Furthermore, by separately forming a region serving as a drain in the NMOS transistors N 1 , N 2  and N 5  to N 8 , resistance to soft error can be maintained at a high level. This corresponds to the fifth effect of the first preferred embodiment.  
         [0237]    By arranging such that each of inverters I 1  and I 2  of a CMOS structure is made up of a combination of a NMOS transistor and a PMOS transistor, the memory cell can be realized by at least sufficient circuit configuration as a CMOS structure. This corresponds to the sixth effect of the first preferred embodiment.  
         [0238]    Like the eighth preferred embodiment, the memory cell of the ninth preferred embodiment can be used as a two-port memory cell.  
         [0239]    Additionally, as in the second preferred embodiment, it is able to increase the operation speed and the stability of the memory cell by increasing the gate width (channel width) of the NMOS transistors N 1  and N 2 , each being a driver transistor.  
         [0240]    Tenth Preferred Embodiment  
         [0241]    FIGS.  29  to  31  are diagrams illustrating a memory cell structure of a SRAM according to a tenth preferred embodiment of the invention. FIG. 29 is an explanatory diagram viewed from above the layout configuration in all layers. FIG. 30 is an explanatory diagram viewed from above mainly the layout configuration beneath a first aluminum wiring layer in FIG. 29. FIG. 31 is an explanatory diagram viewed from above the layout configuration over a second aluminum wiring layer in FIG. 29. Some reference numerals used in FIG. 30 or  31  are omitted in FIG. 29.  
         [0242]    An equivalent circuit of the SRAM memory cell having the layout configuration of the tenth preferred embodiment is similar to that of FIG. 25 in the eighth preferred embodiment.  
         [0243]    Referring to FIGS.  29  to  31 , description will proceed to the memory cell structure of the tenth preferred embodiment.  
         [0244]    In an N well region NW, a PMOS transistor P 1  is made up of P +  diffusion regions FL 110 , FL 111  and a polysilicon wiring PL 41 , and a PMOS transistor P 2  is made up of P +  diffusion regions FL 120 , FL 121  and a polysilicon wiring PL 42 .  
         [0245]    In a P well region PW 0 , a NMOS transistor N 1  is made up of N +  diffusion regions FL 210 , FL 211  and the polysilicon wiring PL 41 , a NMOS transistor N 7  is made up of N +  diffusion regions FL 270 , FL 271  and a polysilicon wiring PL 47 , and a NMOS transistor N 8  is made up of N +  diffusion regions FL 280 , FL 281  and the polysilicon wiring PL 47 . The polysilicon wiring PL 41  extends from the N well region NW to the P well region PW 0 , so as to be used as a gate common to the NMOS transistor N 1  and PMOS transistor P 1 . The polysilicon wiring PL 47  is common to the NMOS transistors N 7  and N 8 .  
         [0246]    In a P well region PW 1 , a NMOS transistor N 2  is made up of N +  diffusion regions FL 220 , FL 221  and the polysilicon wiring PL 42 , a NMOS transistor N 5  is made up of N +  diffusion regions FL 250 , FL 251  and a polysilicon wiring PL 45 , and a NMOS transistor N 6  is made up of N +  diffusion regions FL 260 , FL 261  and the polysilicon wiring PL 45 . The polysilicon wiring PL 42  extends from the N well region NW to the P well region PW 1 , so as to be used as a gate common to the NMOS transistor N 2  and PMOS transistor P 2 . The polysilicon wiring PL 42  is common to the NMOS transistors N 5  and N 6 . The foregoing diffusion regions are obtainable by implanting and diffusing impurity.  
         [0247]    A ground wiring LG 1  over the diffusion region FL 210  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 210 . A bit line BLB 1  over the diffusion region FL 271  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 271 . A bit line {overscore (BLB 1 )} over the diffusion region FL 281  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 281 .  
         [0248]    An aluminum wiring AL 17 , which is a first layer aluminum wiring extending over the diffusion region FL 270  (FL 211 ) and the diffusion region FL 111 , is electrically connected through a diffusion contact hole  1 C to the diffusion regions FL 270  (FL 211 ) and FL 111 , respectively.  
         [0249]    The aluminum wiring AL 17  is also electrically connected to the polysilicon wiring PL 42 . The polysilicon wiring PL 42  is electrically connected through a shared contact SC to the diffusion regions FL 111  and FL 261 , respectively. As used herein, the term “shared contact” means a common contact electrically connecting a diffusion region and polysilicon.  
         [0250]    The aluminum wiring AL 17  can be electrically connected with low impedance. The aluminum wiring AL 17 , two shared contacts SC and polysilicon wiring PL 42  correspond to a storage terminal Na.  
         [0251]    The polysilicon wiring PL 47  is electrically connected through a gate contact hole GC to a word line WLB 1 .  
         [0252]    A power supply wiring LV 1  over the diffusion region FL 110  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 110 . A power supply wiring LV 1  over the diffusion region FL 121  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 121 .  
         [0253]    A ground wiring LG 1  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 221 . A bit line BLA 1  over the diffusion region FL 250  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 250 . A bit line {overscore (BLA 1 )} over the diffusion region FL 260  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 260 .  
         [0254]    An aluminum wiring AL 18 , which is a first layer aluminum wiring extending over the diffusion region FL 251  (FL 220 ) and the diffusion region FL 120 , is electrically connected through a diffusion contact hole  1 C to the diffusion regions FL 251  (FL 220 ).  
         [0255]    The aluminum wiring AL 18  is also electrically connected to the polysilicon wiring PL 41 . The polysilicon wiring PL 41  is electrically connected through a shared contact SC to the diffusion regions FL 120  and FL 280 , respectively.  
         [0256]    The aluminum wiring AL 18  can be electrically connected with low impedance. The aluminum wiring AL 18 , two shared contacts SC and polysilicon wiring PL 41  correspond to a storage terminal Nb.  
         [0257]    A word line WLA 1  over the polysilicon wiring PL 45  is electrically connected through a gate contact hole GC to the polysilicon wiring PL 45 .  
         [0258]    A word line WLA 1  is electrically connected through a via hole  1 T to a word line WLA 2 , and the word line WLA 2  is electrically connected through a via hole  2 T to a word line WLA 3 . Likewise, a word line WLB 1  is electrically connected through a via hole  1 T to a word line WLB 2 , and the word line WLB 2  is electrically connected through a via hole  2 T to a word line WLB 3 .  
         [0259]    The word lines WLA 3  and WLB 3  are disposed parallel with each other, across the P well regions PW 0 , PW 1 , and the N well region NW.  
         [0260]    A bit line BLA 2  is electrically connected through a via hole  1 T to a bit line BLA 1 . A bit line BLB 2  is electrically connected through a via hole  1 T to a bit line BLB 1 .  
         [0261]    Likewise, a bit line {overscore (BLA 2 )} is electrically connected through a via hole  1 T to a bit line {overscore (BLA 1 )}. A bit line {overscore (BLB 2 )} is electrically connected through a via hole  1 T to a bit line {overscore (BLB 1 )}.  
         [0262]    A power supply wiring LV 2  is electrically connected through a via hole  1 T to a power supply wiring LV 1 . A ground wiring LG 1  is electrically connected through a via hole  1 T to a ground wiring LG 2 .  
         [0263]    The paired bit lines BLA 2  and {overscore (BLA 2 )}, paired bit lines BLB 2  and {overscore (BLB 2 )}, ground wiring LG 2  and power supply wiring LV 2  are disposed parallel with each other in the longitudinal direction viewing the drawing.  
         [0264]    The paired bit lines BLA 2  and {overscore (BLA 2 )} and the ground wiring LG 2  are disposed over the P well region PW 1 . The paired bit lines BLB 2  and {overscore (BLB 2 )} and the ground wiring LG 2  are disposed over the P well region PW 0 . The power supply wiring LV 2  is disposed over the N well region NW.  
         [0265]    Thus, in the memory cell structure of the SRAM of the tenth preferred embodiment, with the N well region NW interposed between the P well regions PW 0  and PW 1 , the NMOS transistors N 1 , N 7  and N 8  are disposed in the P well region PW 0 , and the NMOS transistors N 2 , N 5  and N 6  are disposed in the P well region PW 1 . This enables to increase resistance to soft error, which is the first effect of the first preferred embodiment.  
         [0266]    By separately forming the P well regions PW 0  and PW 1  in a direction vertical to the direction of formation of the paired bit lines BLA and {overscore (BLA)} and the paired bit lines BLB and {overscore (BLB)}, it is able to maintain a good access time, which is the second effect of the first preferred embodiment.  
         [0267]    Further, in the tenth preferred embodiment as in the eighth preferred embodiment, the NMOS transistors N 1  and N 2 , the NMOS transistors N 5  and N 7 , and the NMOS transistors N 6  and N 8 , are respectively arranged so as to be point symmetry with respect to the central part of the memory cell. It is therefore able to increase the degree of integration when a plurality of the memory cells of the tenth preferred embodiment are disposed adjacent each other. This corresponds to the third effect of the first preferred embodiment.  
         [0268]    The memory cell of the tenth preferred embodiment realizes a two-port memory cell, as in the eighth preferred embodiment.  
         [0269]    The formation of the polysilicon wirings PL 41 , PL 42 , PL 47  and PL 48  in approximately the same direction (the lateral direction viewing the drawing) facilitates the control of the gate dimension. Further, because the polysilicon wirings PL 41  and PL 45 , and the polysilicon wirings PL 42  and PL 47 , are respectively disposed in a straight line, no waste region is present and a reduction in the circuit area increases the degree of integration. This corresponds to the fourth effect of the first preferred embodiment.  
         [0270]    By arranging such that each of inverters I 1  and I 2  of a CMOS structure is made up of a combination of a NMOS transistor and a PMOS transistor, the memory cell can be realized by at least sufficient circuit configuration as a CMOS structure. This corresponds to the sixth effect of the first preferred embodiment.  
         [0271]    In addition, with the arrangement that the storage terminal Na is made up of the aluminum wiring AL 17 , shared contacts SC and polysilicon wiring PL 42 , and the storage terminal Nb is made up of the aluminum wiring AL 18 , shared contacts SC and polysilicon wiring PL 41 , it is able to increase the degree of integration by the amount that the well forming width in the longitudinal direction viewing the drawing can be formed by a two-transistor pitch.  
         [0272]    Eleventh Preferred Embodiment  
         [0273]    FIGS.  32  to  34  are diagrams illustrating a memory cell structure of a SRAM according to an eleventh preferred embodiment of the invention. FIG. 32 is an explanatory diagram viewed from above the layout configuration in all layers. FIG. 33 is an explanatory diagram viewed from above mainly the layout configuration beneath a first aluminum wiring layer in FIG. 32. FIG. 34 is an explanatory diagram viewed from above the layout configuration over a second aluminum wiring layer in FIG. 32. Some reference numerals used in FIG. 33 or  34  are omitted in FIG. 32.  
         [0274]    An equivalent circuit of the SRAM memory cell having the layout configuration of the eleventh preferred embodiment is similar to that of FIG. 4 in the first preferred embodiment.  
         [0275]    Referring to FIGS.  32  to  34 , description will proceed to the memory cell structure of the eleventh preferred embodiment.  
         [0276]    In an N well region NW, a PMOS transistor P 1  is made up of P +  diffusion regions FL 110 , FL 111  and a polysilicon wiring PL 51 , and a PMOS transistor P 2  is made up of P +  diffusion regions FL 120 , FL 121  and a polysilicon wiring PL 52 .  
         [0277]    In a P well region PW 0 , a NMOS transistor N 1  is made up of N +  diffusion regions FL 210  (FL 210 A, FL 210 B), FL 211  and the polysilicon wiring PL 51 , and a NMOS transistor N 4  is made up of N +  diffusion regions FL 240 , FL 241  and a polysilicon wiring PL 54 . The polysilicon wiring PL 51  extends from the N well region NW to the P well region PW 0 , so as to be used as a gate common to the NMOS transistor N 1  and PMOS transistor P 1 .  
         [0278]    In a P well region PW 1 , a NMOS transistor N 2  is made up of N +  diffusion regions FL 220  (FL 220 A, FL 220 B), FL 221  and the polysilicon wiring PL 52 , and a NMOS transistor N 3  is made up of N +  diffusion regions FL 230 , FL 231  and a polysilicon wiring PL 53 . The polysilicon wiring PL 52  extends from the N well region NW to the P well region PW 1 , so as to be used as a gate common to the NMOS transistor N 2  and PMOS transistor P 2 . The foregoing diffusion regions are obtainable by implanting and diffusing impurity.  
         [0279]    A ground wiring LG 1  over the diffusion region FL 210 A and FL 210 B is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 210 A and FL 210 B, respectively. A bit line BLB 1  over the diffusion region FL 241  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 241 .  
         [0280]    An aluminum wiring AL 17 , which is a first layer aluminum wiring extending over the diffusion region FL 211  and the diffusion region FL 111 , is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 211 .  
         [0281]    The aluminum wiring AL 17  is also electrically connected to the polysilicon wiring PL 52 . The polysilicon wiring PL 52  is electrically connected through a shared contact SC to the diffusion regions FL 111  and FL 231 , respectively.  
         [0282]    The aluminum wiring AL 17  can be electrically connected with low impedance. The aluminum wiring AL 17 , two shared contacts SC and polysilicon wiring PL 52  correspond to a storage terminal Na.  
         [0283]    The polysilicon wiring PL 54  is electrically connected through a gate contact hole GC to a word line WL 1 .  
         [0284]    A power supply wiring LV 1  over the diffusion region FL 110  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 110 . A power supply wiring LV 1  over the diffusion region FL 121  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 121 .  
         [0285]    A ground wiring LG 1  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 221 . A bit line BLA 1  over the diffusion region FL 230  is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 230 .  
         [0286]    An aluminum wiring AL 18 , which is a first layer aluminum wiring extending over the diffusion region FL 220  and the diffusion region FL 120 , is electrically connected through a diffusion contact hole  1 C to the diffusion region FL 220 .  
         [0287]    The aluminum wiring AL 18  is also electrically connected to the polysilicon wiring PL 51 . The polysilicon wiring PL 51  is electrically connected through a shared contact SC to the diffusion regions FL 120  and FL 240 , respectively.  
         [0288]    The aluminum wiring AL 18  can be electrically connected with low impedance. The aluminum wiring AL 18 , two shared contacts SC and polysilicon wiring PL 51  correspond to a storage terminal Nb.  
         [0289]    A word line WL 1  over the polysilicon wiring PL 53  is electrically connected through a gate contact hole GC to the polysilicon wiring PL 53 .  
         [0290]    A word line WL 1  is electrically connected through a via hole  1 T to a word line WL 2 , and the word line WL 2  is electrically connected through a via hole  2 T to a word line WL 3 . The word line WL 3  is disposed across the P well regions PW 0 , PW 1  and the N well region NW.  
         [0291]    A bit line BLA 2  is electrically connected through a via hole  1 T to a bit line BLA 1 . A bit line BLB 2  is electrically connected through a via hole  1 T to a bit line BLB 1 .  
         [0292]    A power supply wiring LV 2  is electrically connected through a via hole  1 T to a power supply wiring LV 1 . A ground wiring LG 1  is electrically connected through a via hole  1 T to a ground wiring LG 2 .  
         [0293]    The bit lines BLA 2 , BLB 2 , ground wiring LG 2  and power supply wiring LV 2  are disposed parallel with each other in the longitudinal direction viewing the drawing.  
         [0294]    The bit lines BLA 2  and ground wiring LG 2  are disposed over the P well region PW 1 . The bit lines BLB 2  and ground wiring LG 2  are disposed over the P well region PW 0 . The power supply wiring LV 2  is disposed over the N well region NW.  
         [0295]    Thus, in the memory cell structure of the SRAM of the eleventh preferred embodiment, with the N well region NW interposed between the P well regions PW 0  and PW 1 , the NMOS transistors N 1  and N 4  are disposed in the P well region PW 0 , and the NMOS transistors N 2  and N 3  are disposed in the P well region PW 1 . This enables to increase resistance to soft error, which is the first effect of the first preferred embodiment.  
         [0296]    By separately forming the P well regions PW 0  and PW 1  in a direction vertical to the direction of formation of the bit lines BLA and BLB, it is able to maintain a good access time, which is the second effect of the first preferred embodiment.  
         [0297]    Further, in the eleventh preferred embodiment, as in the first preferred embodiment, the NMOS transistors N 1  and N 2 , and the NMOS transistors N 3  and N 4 , are respectively arranged so as to be point symmetry with respect to the central part of the memory cell. It is therefore able to increase the degree of integration when a plurality of the memory cells of the eleventh preferred embodiment are disposed adjacent each other. This corresponds to the third effect of the first preferred embodiment.  
         [0298]    The formation of the polysilicon wirings PL 51  to PL 54  in approximately the same direction (the lateral direction viewing the drawing) facilitates the control of the gate dimension. Further, because the polysilicon wirings PL 51  and PL 53 , and the polysilicon wirings PL 52  and PL 54 , are respectively disposed in a straight line, no waste region is present and a reduction in the circuit area increases the degree of integration. This corresponds to the fourth effect of the first preferred embodiment.  
         [0299]    By separately forming a region serving as a drain in the NMOS transistors N 1  to N 4 , resistance to soft error can be maintained at a high level. This corresponds to the fifth effect of the first preferred embodiment.  
         [0300]    With the arrangement such that each of inverters I 1  and I 2  of a CMOS structure is made up of a combination of a NMOS transistor and a PMOS transistor, the memory cell can be realized by at least sufficient circuit configuration as a CMOS structure. This corresponds to the sixth effect of the first preferred embodiment.  
         [0301]    In addition, with the arrangement that the storage terminal Na is made up of the aluminum wiring AL 17 , shared contacts SC and polysilicon wiring PL 52 , and the storage terminal Nb is made up of the aluminum wiring AL 18 , shared contacts SC and polysilicon wiring PL 51 , it is able to increase the degree of integration by the amount that the well forming width in the longitudinal direction viewing the drawing can be formed by a two-transistor pitch.  
         [0302]    Twelfth Preferred Embodiment  
         [0303]    [0303]FIGS. 35 and 36 are diagrams illustrating a memory cell structure of a SRAM according to a twelfth preferred embodiment of the invention. FIG. 35 is an explanatory diagram viewed from above the layout configuration in all layers. FIG. 36 is an explanatory diagram viewed from above mainly the layout configuration over a second aluminum wiring layer in FIG. 35. An explanatory diagram viewed from above the layout configuration beneath a first aluminum wiring layer in FIG. 35 is similar to that of FIG. 33 described in the eleventh preferred embodiment, except that the word line WL 2  is divided into word lines WLA 2  and WLB 2 . Some reference numerals used in FIG. 36 or  33  are omitted in FIG. 35. An equivalent circuit of the SRAM memory cell having the layout configuration of the twelfth preferred embodiment is similar to that of FIG. 15 in the fifth preferred embodiment.  
         [0304]    Referring to FIGS. 35, 36 and  33 , description will proceed to the memory cell structure of the twelfth preferred embodiment.  
         [0305]    A polysilicon wiring PL 53  is electrically connected through a gate contact hole GC to a word line WLA 1  (corresponding to the word line WL 1  at the right end in FIG. 33). The word line WLA 1  is electrically connected through a via hole  1 T to the word line WLA 2 . The word line WLA 2  is electrically connected through a via hole  2 T to a word line WLA 3 . The word line WLA of FIG. 15 is made up of these word lines WLA 1  to WLA 3 .  
         [0306]    Likewise, a polysilicon wiring PL 54  is electrically connected through a gate contact hole GC to a word line WLB 1  (corresponding to the word line WL 1  at the left end in FIG. 33). The word line WLB 1  is electrically connected through a via hole  1 T to a word line WLB 2 . The word line WLB 2  is electrically connected through a via hole  2 T to a word line WLB 3 . The word line WLB of FIG. 15 is made up of these word lines WLB 1  to WLB 3 .  
         [0307]    The word lines WLA 3  and WLB 3  are disposed parallel with each other, across P well regions PW 0 , PW 1  and an N well region NW. Otherwise, the layout configuration is similar to that of the eleventh preferred embodiment, and the description thereof is thus omitted.  
         [0308]    The twelfth preferred embodiment having the foregoing memory cell structure produces the effects of the eleventh preferred embodiment, and also realizes a memory cell structure usable in FIFO memory, as in the fifth preferred embodiment.  
         [0309]    Other Embodiments  
         [0310]    If every conductivity type is reversed in the foregoing first to twelfth preferred embodiments, the same effects are obtainable. Further, these embodiments are applicable with the same effects to field effect transistors such as MIS transistors, without limiting to MOS transistors.  
         [0311]    While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention.