Abstract:
A semiconductor memory device has a SRAM memory cell comprising: a first inverter including a first nMOS transistor and a first pMOS transistor; a second inverter including a second nMOS transistor and a second pMOS transistor; a third nMOS transistor; and a fourth nMOS transistor, wherein a first diffusion region forming the first and third nMOS transistors and a second diffusion region forming the second and fourth nMOS transistors, respectively, are arranged in linear shapes without having any bent part, and driving capabilities of the first and second nMOS transistors are higher than those of the third and fourth nMOS transistors.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to memory cell layout of CMOS-type SRAM (Complementary Metal Oxide Semiconductor Static Random Access Memory) among semiconductor memory devices.  
           [0002]    The SRAM memory cell that comprises six transistors and is made by a typical semiconductor CMOS process is widely used for system LSIs and so on.  
           [0003]    The prior art layout pattern of the CMOS-type SRAM memory cell will be described below with reference to FIG. 8.  
           [0004]    The prior art SRAM memory cell comprises nMOS drive transistors TN 1  and TN 2 , nMOS access transistors TN 3  and TN 4 , pMOS load transistors TP 1  and TP 2 , polysilicon wires PL 1 , PL 2 , PL 3  and PL 4 , wiring layers AL 1  and AL 2 , and contacts CN 1 , CN 2 , CL 1  and CL 2 .  
           [0005]    The nMOS drive transistor TN 1  and the nMOS access transistor TN 3  are formed on an n-type diffusion region DN 1  and the nMOS drive transistor TN 2  and the nMOS access transistor TN 4  are formed on an n-type diffusion region DN 2 . The pMOS load transistor TP 1  is formed on a p-type diffusion region DP 1  and the pMOS load transistor TP 2  is formed on a p-type diffusion region DP 2 .  
           [0006]    Gates of the nMOS drive transistor TN 1  and the pMOS load transistor TP 1  are connected to each other with the polysilicon wire PL 1  and drains of them are connected to each other with the wiring layer AL 1  via contact, thereby forming a first inverter (CMOS structure). Gates of the nMOS drive transistor TN 2  and the pMOS load transistor TP 2  are connected to each other with the polysilicon wire PL 2  and drains of them are connected to each other with the wiring layer AL 2  via contact, thereby forming a second inverter (CMOS structure). The wiring layer AL 1  as an output node of the first inverter is connected to PL 2  as an input node of the second inverter, and the wiring layer AL 2  as an output node of the second inverter is connected to PL 1  as an input node of the first inverter. Thereby a latch circuit for holding data is formed.  
           [0007]    A drain of the nMOS access transistor TN 3  is connected to the wiring layer AL 1  as the output node of the first inverter and source thereof is connected to a bit line (not shown) extending longitudinally via the contact CN 1 . A drain of the nMOS access transistor TN 4  is connected to the wiring layer AL 2  as the output node of the second inverter and source thereof is connected to another bit line (not shown) extending longitudinally via the contact CN 2 . Gates of TN 3  and TN 4  are connected to a word line (not shown) extending transversally via the contacts CL 1  and CL 2 , respectively.  
           [0008]    With such memory cell layout, long lateral distance allows a wide interval between two bit lines so that coupling capacitance between bit lines, which may cause a problem in micro process, can be reduced. Therefore, such memory cell layout is advantageous to speeding-up.  
           [0009]    Next, relationship between capability ratio of drive transistors and access transistors and stability of data holding in the SRAM memory cell will be explained with reference to FIGS. 9, 10 and  11 .  
           [0010]    [0010]FIG. 9 shows a memory cell circuit diagram for evaluating stability of data holding. This circuit assumes the situation that the access transistors TN 3  and TN 4  turns on when the word line is in VDD level for a reading operation, and the bit line is raised to precharge level.  
           [0011]    [0011]FIG. 10 shows input/output characteristics of two inverter circuits (INV 1 , INV 2 ) in the latch circuit.  
           [0012]    Ain-Aout and Bin-Bout represent characteristics of INV 1  and INV 2 , respectively and it is plotted so as to be Ain=Bout and Bin=Aout. Cross points P 1  and P 2  in this drawing are stable points and each point corresponds to memory data  0  or  1 . In the plot, as area surrounded by two curved lines becomes larger, stability of data holding at P 1  and P 2  improves. Here, when driving capability of the access transistors TN 3  and TN 4  becomes greater than that of nMOS transistors (drive transistors) TN 1  and TN 2  in the inverter circuits, input/output characteristics of the inverter circuits change as shown in FIG. 11. The reason is that the access transistors transmit VDD level of the bit line to the latch nodes more easily, so that area surrounded by two curved lines becomes smaller. When noise voltage is applied into the memory cell having such characteristics, cross points are reduced to be only P 2 ′ and therefore the memory cell can hold only either data. That is, in the case where data other than P 2 ′ (i.e. P 1 ′) is held, the data is destroyed. Thus, maintaining a constant ratio of access transistors to drive transistors in driving capability is important for holding memory cell data stably. Generally, driving capability of access transistors is set to be 50 to 70% of that of drive transistors.  
           [0013]    In the prior art SRAM memory cell, channel width of drive transistors is set to be larger than that of access transistors, thereby generating a difference between them in driving capability.  
           [0014]    In the prior art SRAM memory cell in which channel width of the drive transistors is set to be larger than that of the access transistors, thereby generating a difference between them in driving capability, the diffusion regions necessarily include some bent parts and end parts. For example, in FIG. 8, the bent parts that produce round-offs as shown by dashed lines DL 3  and DL 4  are generated by difference between the nMOS drive transistors TN 1  and TN 2  and the corresponding nMOS access transistors TN 3  and TN 4  in channel width.  
           [0015]    With such layout, at the bent parts of the diffusion regions, finish pattern is rounded off as shown by dashed lines DL 1 , DL 2 , DL 3  and DL 4  in the figure. As a result, a problem arises that transistor width of the nMOS transistors TN 1 , TN 2 , TN 3  and TN 4  becomes larger than required. Moreover, at the end parts of the diffusion regions, finish pattern is retreated as shown by dashed-lines DL 5  and DL 6 . As a result, there arises a problem of reduction in overlap margin of the p-type diffusion region with respect to the contact as well as variation in channel width of the pMOS transistors TP 1  and TP 2 .  
           [0016]    Furthermore, system mounted on a semiconductor chip has increasingly become large scale. In connection with this, there is a tendency that the block of SRAM with a large scale in bit capacity is mounted on the chip. In order to meet these requests on the system side, it is desired to further reduce the size of SRAM memory cell. Although it is effective to use a MOS transistor with smaller channel width for the purpose of reducing cell size, such small-sized pattern is prone to undergo great variations in characteristics due to processing fluctuations. Therefore, reduction in cell size makes stable design by sufficient operational margin difficult. On the other hand, with recent micro process, it is more difficult to obtain desirable processed form and round-off or retreat of pattern tend to take place. Moreover, there often causes the phenomenon that even the same pattern form changes in finished form due to peripheral pattern form.  
           [0017]    To suppress such changes of processed form, it has already been implemented to correct mask pattern in consideration of bend up and bend down of layout pattern concerned and in consideration of the peripheral layout pattern in recent micro process. Such process, however, is sensitive to apparatus used in semiconductor diffusion process and processing conditions. Further, correction value must be modified each time processing conditions in diffusion process are changed, adding a burdensome operation.  
         SUMMARY OF THE INVENTION  
         [0018]    The present invention is made to solve the above-mentioned problems. With the memory cell layout of the semiconductor memory device according to the present invention, it is possible to lay out diffusion regions in linear shapes without any bent part by generating a difference between access transistors and drive transistors in driving capability without changing their channel width. As a result, processed form of diffusion areas of the SRAM memory cell is hard to change, thereby suppressing variations in characteristics of transistors so that transistors with narrow channel width can be used. An object of the present invention is to provide a highly integrated semiconductor memory device by use of such transistors with narrow channel width  
           [0019]    A semiconductor memory device according to one aspect of the present invention has a SRAM memory cell comprising: a first inverter including a first nMOS transistor and a first pMOS transistor; a second inverter including a second nMOS transistor and a second pMOS transistor; a third nMOS transistor; and a fourth nMOS transistor, wherein an input node of the first inverter is connected to an output node of the second inverter; and an input node of the second inverter is connected to an output node of the first inverter; either of drain and source of the third nMOS transistor is connected to the output node of the first inverter; the other of drain and source thereof is connected to a first bit line; and gate thereof is connected to a word line; either of drain and source of the fourth nMOS transistor is connected to the output node of the second inverter; the other of drain and source thereof is connected to a second bit line; and gate thereof is connected to the word line; a first diffusion region forming the first nMOS transistor and the third nMOS transistor and a second diffusion region forming the second nMOS transistor and the fourth nMOS transistor, respectively, are arranged in linear shapes without having any bent part, and current driving capability of the first and second nMOS transistors is higher than that of the third and fourth nMOS transistors.  
           [0020]    A semiconductor memory device according to another aspect of the present invention is an above-mentioned semiconductor memory device characterized in that gate length of the third and fourth nMOS transistors is longer than that of the first and second nMOS transistors.  
           [0021]    A semiconductor memory device according to another aspect of the present invention is an above-mentioned semiconductor memory device characterized in that threshold voltage characteristics of the third and fourth nMOS transistors are higher than those of the first and second nMOS transistors.  
           [0022]    A semiconductor memory device according to another aspect of the present invention is an above-mentioned semiconductor memory device characterized in that a gate oxide film of the third and fourth nMOS transistors are thicker than that of the first and second nMOS transistors.  
           [0023]    A semiconductor memory device according to another aspect of the present invention is an above-mentioned semiconductor memory device characterized in that driving voltage of the word line is lower than power supply voltage supplied to the first and second inverters.  
           [0024]    A semiconductor memory device according to another aspect of the present invention is an above-mentioned semiconductor memory device characterized in that a third diffusion region forming the first pMOS transistor and a fourth diffusion region forming the second pMOS transistor are arranged in linear shapes without having any bent part and located in parallel to the first and second diffusion regions.  
           [0025]    A semiconductor memory device according to another aspect of the present invention is an above-mentioned semiconductor memory device further comprising: a third pMOS transistor formed adjacent to drain of the first pMOS transistor on the third diffusion region; and a fourth pMOS transistor formed adjacent to drain of the second pMOS transistor on the fourth diffusion region, wherein gates of the first nMOS transistor, the first pMOS transistor and the fourth pMOS transistor are connected in succession via a first polysillicon wire; and gates of the second nMOS transistor, the second pMOS transistor and the third pMOS transistor are connected in succession via a second polysillicon wire; and absolute value of threshold voltage of the third and fourth pMOS transistors is higher than power supply voltage supplied to the first and second inverters.  
           [0026]    A semiconductor memory device according to another aspect of the present invention is an above-mentioned semiconductor memory device characterized in that n-type diffusion regions are provided on the opposite side of the diffusion region of the third pMOS transistor to the first pMOS transistor and on the opposite side of the diffusion region of the fourth pMOS transistor to the second pMOS transistor to fix potential of N-well region forming PMOS transistors.  
           [0027]    A semiconductor memory device according to another aspect of the present invention is an above-mentioned semiconductor memory device characterized in that, in a memory cell array in which the SRAM memory cells are arranged in a grid pattern, a substrate contact region for fixing well potential is provided at regular intervals in the extending direction of the first and second diffusion regions; a diffusion region forming the substrate contact region is arranged in a linear shape without any bent part on the extension of the first diffusion region and the second diffusion region of the memory cell; and a fifth nMOS transistor is provided at the boundary of the memory cell and the substrate contact region wherein gate potential thereof is fixed so as not to become ON state.  
           [0028]    A semiconductor memory device according to another aspect of the present invention is an above-mentioned semiconductor memory device characterized in that, in a memory cell array in which the SRAM memory cells are arranged in a grid pattern, a substrate contact region for fixing well potential is provided at regular intervals in the extending direction of the third and fourth diffusion regions;, a diffusion region forming the substrate contact region is arranged in a linear shape without any bent part on the extension of the third diffusion region and the fourth diffusion region of the memory cell; and a fifth pMOS transistor is provided at the boundary of the memory cell and the substrate contact region wherein gate potential thereof is fixed so as not to become ON state.  
           [0029]    A semiconductor memory device according to another aspect of the present invention is an above-mentioned semiconductor memory device characterized in that the first pMOS transistor is formed on the third diffusion region; the second pMOS transistor is formed on the fourth diffusion region; and the first, second, third and fourth diffusion regions are arranged at equally spaced intervals in the longitudinal direction of connect line of transistor gate.  
           [0030]    A semiconductor memory device according to another aspect of the present invention is an above-mentioned semiconductor memory device characterized in that the first pMOS transistor is formed on the third diffusion region; the second pMOS transistor is formed on the fourth diffusion region; and the first, second, third and fourth diffusion regions are arranged so as to have same width measured in the longitudinal direction of connect line of transistor gate.  
           [0031]    As a means of generating a difference in transistors&#39; capability without changing channel width, channel length of the access transistors is set to be longer than that of the drive transistors. This enables memory cell data to be held stably. In the prior art difference in capability has been generated from difference in channel width. Therefore, even if minimum width of diffusion region, which is possible in processing, can be applied to the access transistors, the drive transistors cannot be laid out at minimum width. According to the present invention, however, minimum width of diffusion region can also be applied to the drive transistors. In typical transistors, channel width is longer than channel length, and therefore reduction in channel width in the present invention probably can reduce cell size.  
           [0032]    However, in the prior art memory cell layout as shown in FIG. 8, because of its long sideways structure, longer channel length, which increases cell height, weakens the effect of reducing area by reduction in channel width of the drive transistor. Accordingly, in the present invention, as another means of generating a difference in transistors&#39; capability, threshold voltage of the access transistor is set to be greater than that of the drive transistor. Further, as other means, gate oxide film of the access transistor is set to be thicker than that of the drive transistor. By use of these means, the access transistor and the drive transistor have different capability while having the same channel length and width so that the effect of reducing area can be obtained. In the case that the above-mentioned means for changing characteristics of the access transistor are employed, it is necessary to add a supplementary process, suffering a slight increase in process costs. Therefore, as another means according to the present invention, power supply voltage supplied to a word line driving circuit for driving the gate of the access transistor is set to be lower than that supplied to a latch circuit in the memory cell. As a result, it is possible to generate a capability difference between the access transistor and the drive transistor having the same size and characteristics, thereby holding process costs down.  
           [0033]    As for the p-type diffusion region, removing end parts that exist in the prior art layout is employed. The p-type diffusion regions are arranged in linear shapes, and for the separation of pMOS load transistor devices between vertically adjoining cells, a pMOS separating transistor having an absolute value of threshold voltage greater than power supply voltage and being in OFF state at any time irrespective of gate potential is formed. This suppress changes of processed form in channel width of the load transistor due to retreat of pattern that occurs at the end part in the prior art, and also allows a sufficient overlap of the diffusion region with respect to the contact in the drain of load transistor. Looking from the aspect of the n-type diffusion region, the p-type diffusion region always lies next to the n-type diffusion region as its peripheral layout form. In the prior art layout, as the p-type diffusion region does not exist partly next to the n-type diffusion region, the processed form of the n-type diffusion region changes a part and the change must be corrected on the mask data. According to the present invention, the processed form of the n-type diffusion region can be improved without any correction.  
           [0034]    Furthermore, fixing potential of N well forming the pMOS transistors by applying n-type diffusion on the opposite side of a pMOS separating transistor to load transistor is employed. Therefore, vacant region in the memory cell can be used effectively as substrate contact region. The substrate contact region for fixing potential of P well may be separately formed outside of the memory cell. Nevertheless, as potential of N well can be definitely fixed, latch-up resistance is improved. Moreover, a potential-fixed region of P well that formed separately can be set at larger intervals, thereby enabling area of the memory cell array to be suppressed.  
           [0035]    Considering the layout of the whole memory cell array, it is necessary to provide the potential-fixed region of P well at regular intervals in the vertical direction of aligning memory cells. With the prior art layout, diffusion regions of memory cells above and under the potential-fixed region are necessarily separated. According to the present invention, by providing transistors whose gates are fixed in OFF state above and under the potential-fixed region both in n-type diffusion region and p-type diffusion region, the above-mentioned potential-fixed region of N well can be arranged in a linear shape in contiguous to the diffusion region of the memory cell. In this way, over the whole memory cell array, diffusion regions can be laid out in linear shapes without any bent part and partial separation, so that change in characteristics of transistors above and under the potential-fixed region can be suppressed.  
           [0036]    Furthermore, in the present invention, placing each diffusion region at the same interval is employed. As mentioned above, in recent micro process, finished processed form changes due to peripheral layout form. In the case of plural placement intervals, it becomes difficult to shape plural diffusion regions into desired interval and width. Maintaining a constant placement interval facilitates keeping forms of these diffusion regions constant. Similarly, placing each diffusion region at the same width is employed. It also facilitates keeping width and forms of these diffusion regions constant.  
           [0037]    The novel features of the invention are set forth with particularity in the appended claims. The invention as to both structure and content, and other objects and features thereof will best be understood from the detailed description when considered in connection with the accompanying drawings. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0038]    [0038]FIG. 1 is a diagram showing the layout of a CMOS-type SRAM memory cell of Embodiment 1.  
         [0039]    [0039]FIG. 2 is a diagram showing the layout of a CMOS-type SRAM memory cell of Embodiment 2.  
         [0040]    [0040]FIG. 3 is a circuit diagram of the CMOS-type SRAM memory cell of Embodiment 2.  
         [0041]    [0041]FIG. 4 is a circuit diagram of a CMOS-type SRAM memory cell and a word line driving buffer of Embodiment 3.  
         [0042]    [0042]FIG. 5 is a signal waveform diagram of the word line in the CMOS-type SRAM memory cell of Embodiment 3.  
         [0043]    [0043]FIG. 6 is a diagram showing the layout of a CMOS-type SRAM memory cell and a substrate contact cell of Embodiment 4.  
         [0044]    [0044]FIG. 7 is a diagram showing layout of a CMOS-type SRAM memory cell array of Embodiment 4.  
         [0045]    [0045]FIG. 8 is a diagram showing the layout of the prior art CMOS-type SRAM memory cell.  
         [0046]    [0046]FIG. 9 is a circuit diagram for estimating the stability of data holding of the memory cell.  
         [0047]    [0047]FIG. 10 is a diagram showing input/output characteristics of an inverter circuit in the memory cell.  
         [0048]    [0048]FIG. 11 is a diagram showing input/output characteristics of an inverter circuit in the memory cell in the case that capability of the access transistor is enhanced. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0049]    Embodiments 1 to 4 as preferred embodiments of a semiconductor memory device according to the present invention will be described below.  
         [0050]    &lt;&lt;Embodiment 1&gt;&gt; 
         [0051]    [0051]FIG. 1 shows the layout of a CMOS-type SRAM memory cell of Embodiment 1 according to the present invention.  
         [0052]    The CMOS-type SRAM memory cell of Embodiment 1 comprises nMOS drive transistors TN 1  and TN 2 , nMOS access transistors TN 3  and TN 4 , pMOS load transistors TP 1  and TP 2 , polysilicon wires PL 1 , PL 2 , PL 3  and PL 4 , wiring layers AL 1  and AL 2 , and contacts CN 1 , CN 2 , CL 1  and CL 2 .  
         [0053]    The nMOS drive transistor TN 1  and the nMOS access transistor TN 3  are formed on an n-type diffusion region DN 1  and the nMOS drive transistor TN 2  and the nMOS access transistor TN 4  are formed on an n-type diffusion region DN 2 . The pMOS load transistor TP 1  is formed on a p-type diffusion region DP 1  and the pMOS load transistor TP 2  is formed on a p-type diffusion region DP 2 .  
         [0054]    Gates of the nMOS drive transistor TN 1  and the pMOS load transistor TP 1  are connected to each other via the polysilicon wire PL 1  and drains of them are connected to each other with the wiring layer AL 1  via the contact, thereby forming a first inverter (CMOS structure). Gates if the nMOS drive transistor TN 2  and the pMOS load transistor TP 2  are connected to each other via the polysilicon wire PL 2  and drains of them are connected to each other with the wiring layer AL 2  via the contact, thereby forming a second inverter (CMOS structure). The wiring layer AL 1  as an output node of the first inverter is connected to PL 2  as an input node of the second inverter, and the wiring layer AL 2  as an output node of the second inverter is connected to PL 1  as an input node of the second inverter. This forms a latch circuit for holding data.  
         [0055]    The drain of the nMOS access transistor TN 3  is connected to the wiring layer AL 1  as the output node of the first inverter and the source thereof is connected to a bit line (not shown) extending longitudinally via the contact CN 1 . The drain of the nMOS access transistor TN 4  is connected to the wiring layer AL 2  as the output node of the second inverter and the source thereof is connected to another bit line (not shown) extending longitudinally via the contact CN 2 . The gates of TN 3  and TN 4  are connected to a word line (not shown) extending transversally. MN 1  and MN 2  are masks for determining n-type diffusion region.  
         [0056]    In this embodiment, the nMOS drive transistors TN 1  and TN 2  and the nMOS access transistors TN 3  and TN 4 , respectively, have a same channel width. On the contrary, the nMOS access transistors TN 3  and TN 4  are longer than the nMOS drive transistors TN 1  and TN 2  in channel length. Accordingly, the driving capability of the nMOS access transistors TN 3  and TN 4  becomes smaller than that of the nMOS drive transistors TN 1  and TN 2 , resulting in that memory data can be held stably. Owing to the uniform channel width, it is possible to lay out the n-type diffusion regions DN 1  and DN 2  in linear shapes without any bent part. As a result, no change of transistor width due to round-off takes place, thereby preventing variations in characteristics of these nMOS transistors.  
         [0057]    &lt;&lt;Embodiment 2&gt;&gt; 
         [0058]    [0058]FIG. 2 shows the layout of a CMOS-type SRAM memory cell of Embodiment 2 according to the present invention.  
         [0059]    In FIG. 2, elements that serve the similar function as in Embodiment 1 are assigned the same reference numerals and detailed explanations thereof are omitted.  
         [0060]    In this embodiment in contrast to Embodiment 1, all of the nMOS drive transistors TN 1  and TN 2  and the nMOS access transistors TN 3  and TN 4  have a same channel length. The gates of the nMOS access transistors TN 3  and TN 4  are covered with their respective masks HV 1  and HV 2  that determine an ion-injection region to increase the threshold voltage. The pMOS diffusion regions DP 1  and DP 2  are laid out in linear shapes so as to have no end part. The polysilicon wires PL 1  and PL 2  as input gates of a latch circuit cross the pMOS diffusion regions DP 2  and DP 1 , respectively, thereby forming pMOS transistors TP 4  and TP 3 . The gates of the pMOS transistors TP 3  and TP 4  are covered with their respective masks VV 1  and VV 2  that determine an ion-injection region so as to make an absolute value of threshold voltage greater than power supply voltage. Diffusion regions on the opposite side of TP 3  and TP 4  to TP 1  and TP 2 , respectively, are covered with their respective masks MN 3  and MN 4  that determine n-type diffusion region.  
         [0061]    With the memory cell layout of this embodiment, compared with the drive transistors TN 1  and TN 2 , the access transistors TN 3  and TN 4  have a larger threshold voltage while having the same channel width and length. Accordingly, the driving capability of the nMOS access transistors TN 3  and TN 4  become smaller than that of the nMOS drive transistors TN 1  and TN 2 , resulting in that memory data can be held stably. The memory cell of Embodiment 2 can be formed so as to be smaller in height than that of Embodiment 1.  
         [0062]    [0062]FIG. 3 shows a circuit diagram of the memory cell in this embodiment.  
         [0063]    The drain and gate of the pMOS transistor TP 3  are connected to the output node of the first inverter and the drain and gate of the pMOS transistor TP 4  are connected to the output node of the second inverter. Depending on data held in the memory cell, gate potential of TP 3  and TP 4  changes to GND level or power supply voltage (VDD) level. However, as TP 3  and TP 4  have a threshold voltage lower than −VDD, they are in OFF state all the time, and diffusion regions on both sides of each gate of TP 3  and TP 4  are electrically separated from each other. N-type diffusion is applied to one of the diffusion regions of TP 3  and TP 4  (i.e. diffusion region on the side which is not connected to the output node of the first inverter and the second inverter, respectively) so as to fix n-well potential via contacts CP 1  and CP 2 , respectively.  
         [0064]    In this embodiment, diffusion regions have neither bent part nor end part, and looking from the aspect of the n-type diffusion regions DN 1  and DN 2 , p-type diffusion regions DP 1  or DP 2  always lies next to the n-type diffusion regions DN 1  and DN 2 . For this reason, changes in processed form due to peripheral layout form occur in a similar manner at any place, facilitating finished form to be uniform. That is, it becomes easier to set processing conditions for keeping channel width of TN 1  and TN 3  as well as TN 2  and TN 4  same.  
         [0065]    &lt;&lt;Embodiment 3&gt;&gt; 
         [0066]    A CMOS-type SRAM memory of Embodiment 3 will be explained with reference to FIGS. 4 and 5. Layout of a memory cell of Embodiment 3 is similar to that of Embodiment 1 (FIG. 1). In the memory cell of Embodiment 1, channel length of the nMOS access transistors TN 3  and TN 4  are longer than that of nMOS drive transistors TN 1  and TN 2 . In the memory cell of Embodiment 3, channel length of the nMOS access transistors TN 3  and TN 4  are same as that of nMOS drive transistors TN 1  and TN 2 . Except for this point, both memory cells are identical with each other (refer to FIG. 1). Therefore, the memory cell of Embodiment 3 is shorter than that of Embodiment 1 in longitudinal direction.  
         [0067]    [0067]FIG. 4 shows a circuit of the CMOS-type SRAM memory cell and a word line driving buffer of Embodiment 3 of the present invention. In FIG. 4, elements that serve the similar function as in Embodiment 2 (FIG. 3) are assigned the same reference numerals and detailed explanations thereof are omitted.  
         [0068]    In FIG. 4, the drain of the nMOS access transistor TN 3  is connected to the output node of the first inverter and the source thereof is connected to a bit line extending longitudinally. The drain of the nMOS access transistor TN 4  is connected to the output node of the second inverter and the source thereof is connected to a bit line extending longitudinally. The gates of TN 3  and TN 4  are connected to a word line extending transversally.  
         [0069]    The gates of the nMOS access transistor TN 3  and TN 4  are activated by a word line driver  20 . Power supply potential VDDW of the word line driver  20  is set to be lower than power supply potential VDDM of the memory cell  10  (VDDW&lt;VDDM). When a Low level is input to an input node  21  of the word line driver  20 , the word line driver  20  outputs a High level with potential VDDW as shown in FIG. 5. At this time, the gates of the access transistors TN 3  and TN 4  are activated by VDDW. Meanwhile, the gate of either drive transistor TN 1  or TN 2  is activated by supply voltage VDDM of the memory cell.  
         [0070]    As the voltage VDDW applied to the gates of access transistors is set to be lower than the voltage VDDM supplied to the memory cell (VDDW&lt;VDDM), driving capability of the access transistors TN 3  and TN 4  is relatively smaller than that of the drive transistors TN 1  and TN 2 , thereby enabling memory data to be held stably.  
         [0071]    Also in the layout of memory cell of this embodiment as in Embodiment 2, the drive transistors TN 1  and TN 2  and the access transistors TN 3  and TN 4 , respectively, have a same size of channel width and channel length. This achieves such an effect as obtained in Embodiment 2. Furthermore, in the present embodiment, the drive transistors TN 1  and TN 2  and the access transistors TN 3  and TN 4 , respectively, have same threshold voltage characteristics.  
         [0072]    With the CMOS-type SRAM of this embodiment, driving capability of access transistors can be made lower than that of drive transistors without any additional process to access transistors TN 1  and TN 2  as shown in Embodiment 2, and therefore processed form of diffusion regions can be improved while suppressing process costs.  
         [0073]    &lt;&lt;Embodiment 4&gt;&gt; 
         [0074]    [0074]FIG. 6 shows layout of a CMOS-type SRAM memory cell and a substrate contact cell of Embodiment 4 according to the present invention.  
         [0075]    In FIG. 6 , numeral  11  designates the layout of memory cell and numeral  12  designates the layout of substrate contact cell. With the memory cell layout in FIG. 6, elements that serve the similar function as in Embodiment 2 (FIG. 2) are assigned the same reference numerals and detailed explanations thereof are omitted.  
         [0076]    Transistors with a higher threshold voltage characteristic are employed as the access transistors TN 3  and TN 4  in the memory cell of Embodiment 2, while transistors thicker than the drive transistors TN 1  and TN 2  in gate oxide film are employed as the access transistors TN 3  and TN 4  in the memory cell of this embodiment.  
         [0077]    HX 1  and HX 2  are masks for determining the region in which a thicker gate oxide film is formed. The access transistors TN 3  and TN 4  with a thicker gate oxide film have the same channel width and channel length as the drive transistors TN 1  and TN 2 , but their driving capability is suppressed. Therefore, the memory cell has an excellent data holding characteristic.  
         [0078]    Furthermore, in the memory cell of this embodiment, diffusion regions DN 1 , DP 1 , DP 2  and DN 2  are located at regular intervals and their width of the regions is same.  
         [0079]    Next, the layout of the adjoining substrate contact cell  12  will be explained below. Diffusion regions on the substrate contact cell are extensions of diffusion regions of the memory cell  11  and arranged in linear shapes in a same way.  
         [0080]    nMOS transistors TN 5  and TN 6 , and TN 7  and TN 8  are located on the n-type diffusion regions DN 1  and DN 2 , respectively, so as to electrically separating a transistor forming part of the memory cell from a substrate contact part. Similarly, pMOS transistors TP 5  and TP 6 , and TP 7  and TP 8  are located on the p-type diffusion regions DP 1  and DP 2 , respectively, so as to electrically separating a transistor forming part of the memory cell and a substrate contact part.  
         [0081]    Diffusion region between the nMOS transistors TN 5  and TN 6  as well as diffusion region between the nMOS transistors TN 7  and TN 8  are not covered with masks MN 1  and MN 2  for applying n-type diffusion and p-type diffusion is applied to these regions. Meanwhile, diffusion region between the pMOS transistors TP 5  and TP 6  as well as diffusion region between the pMOS transistors TP 7  and TP 8  are covered with a mask MN 5  for applying n-type diffusion so that n-type diffusion is applied to these regions. The gates of the nMOS transistors TN 5  and TN 6  are connected to each other. And, these gates as well as the p-type diffusion region between these transistors are connected to GND level. The same applies to the nMOS transistors TN 7  and TN 8 . Meanwhile, the gates of the pMOS transistors TP 5 , TP 6 , TP 7  and TP 8  are connected to each other. And these gates as well as the n-type diffusion regions between these transistors are connected to VDD level. Such structure makes it possible to fix the well potential while electrically separating diffusion regions in the memory cell in both N well and P well.  
         [0082]    [0082]FIG. 7 shows part of memory cell array in which memory cells as shown in FIG. 6 are arranged in a grid pattern. The substrate contact cell  12  is interposed at regular intervals between the memory cells aligning in a longitudinal direction. For convenience of explanation, only polysilicon wires PL 1 , PL 2 , PL 3  and PL 4  and diffusion regions DN 1 , DP 1 , DP 2  and DN 2  are shown in a simplified layout of the memory cell and substrate contact cell of FIG. 7.  
         [0083]    With the memory cell array of this embodiment, the diffusion regions of the substrate contact cell are arranged in linear shapes in contiguous to the memory cell array so that diffusion regions are placed with linear shapes over the whole memory array without breaking off. Further, each diffusion region is disposed at the uniform width and the uniform interval (S 1 =S 2 =S 3 =S 4 , W 1 =W 2 =W 3 =W 4 ).  
         [0084]    With the memory cell array of this embodiment, as peripheral layout form of diffusion regions is uniform at any part, changes of finished and processed form becomes uniform. As a result, changes in channel width of transistors can be suppressed, thereby minimizing variations in transistor&#39;s characteristics.  
         [0085]    As has been explained, with the semiconductor memory device of the present invention, it is possible to lay out the n-type diffusion regions in linear shapes without any bent part while ensuring data holding stability by generating a difference between the access transistors and the drive transistors in driving capability without making channel width of the access transistors and the drive transistors different. Accordingly, there causes no change in channel width of transistors due to round-off, and therefore variations in characteristics of the nMOS transistors can be suppressed. Moreover, as the drive transistors and access transistors can be constructed so as to have the same channel width, variations in characteristics are further suppressed. This allows for a narrower channel width, enabling reduction in cell area.  
         [0086]    Further, with the semiconductor memory device of the present invention, as the p-type diffusion regions are arranged in linear shapes without any end part, there causes no change in channel width of transistors due to round-off, thereby suppressing variations in characteristics of the pMOS transistors. Looking from the aspect of the n-type diffusion regions, since peripheral layout form of the n-type diffusion regions becomes uniform, changes of channel width in processed form of the access transistors and the drive transistors also become uniform, thereby suppressing variations in characteristics of the transistors. And, gates of two pMOS transistors for separating p-type diffusion regions are connected to gates of inverter circuits, and gate capacitance is added to data latch node, resulting in improvement of the stability of data holding against noise.  
         [0087]    Furthermore, in the semiconductor memory device according to the present invention, by applying n-type diffusion on the diffusion regions of pMOS separating transistors to pMOS load transistors, vacant region in the memory cell can be used effectively as substrate contact region.  
         [0088]    Furthermore, in the semiconductor memory device according to the present invention, by providing transistors whose gates are fixed in OFF state above and under the substrate contact region both in n-type diffusion region and p-type diffusion region,, the diffusion region of the substrate contact cell can be arranged in a linear shape in contiguous to the diffusion region of the memory cell. In this way, over the whole memory cell array, diffusion regions can be arranged in linear shapes without any bent part and partial separation, so that changes in characteristics of transistors above and under the substrate contact region can be suppressed.  
         [0089]    Furthermore, in the semiconductor memory device according to the present invention, by locating all diffusion regions at regular intervals, it is possible to keep changes in finished and processed form due to peripheral layout form at a constant value. Moreover, by locating all diffusion regions at a same width, it is possible to keep width and form of the diffusion regions constant.  
         [0090]    Clearly, numerous modifications and variations of the instant invention are possible in light of the above teachings. It is therefore understood that, within the scope and spirit of the claims made herein, the invention may be practiced otherwise than as specifically described herein, the invention may be modified in arrangement and detail without departing from such scope and spirit and, further, the utility described herein is by way of example.