Abstract:
A memory cell array includes a plurality of memory cells arranged at intersections of bitline pairs and word lines. Each memory cell includes a first transistor having one main electrode connected to a first bit line, a second transistor having one main electrode connected to a second bit line, a first node electrode for data-storage connected to the other main electrode of the first transistor, a second node electrode for data-storage connected to the other main electrode of the second transistor, and a shield electrode formed surrounding the first and second node electrodes. The first and second transistors have respective gates both connected to an identical word line, and the first and second bit lines are connected to an identical sense amp. The first and second node electrodes, the first and second bit lines, the word line and the shield electrode are isolated from each other using insulating films.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on and claims the benefit of priority from prior Japanese Patent Application No. 2006-348870, filed on Dec. 26, 2006, the entire contents of which are incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a semiconductor memory device, and more particularly to a DRAM (dynamic random access memory). 
       BACKGROUND OF THE INVENTION 
       [0003]    There are various kinds of commercially-available semiconductor memory devices. Memories with relatively larger capacity and lower power consumption include an SRAM (static random access memory). The SRAM has problems, however, because variations in threshold voltage result in unstable operation, and leakage current flowing in turned-off transistors increases power consumption. 
         [0004]    On the other hand, semiconductor memory devices suitable for high integration include a DRAM. A method of forming a DRAM with multi-layered wiring for much higher integration is disclosed in Japanese Patent Laying-open No. 2002-133892, which is incorporated herein by reference. The DRAM thus formed usually requires formation of capacitors. Since the capacitors needs processes different from a CMOS processes. Accordingly, it is difficult to manufacture the DRAM through CMOS process only. 
         [0005]    Therefore, it is advantageous in a semiconductor memory device that a DRAM can be formed through a general CMOS process, and to provide a DRAM-combined semiconductor memory device manufacturable at a lower production cost. 
       SUMMARY OF THE INVENTION 
       [0006]    In an aspect the present invention provides a semiconductor memory device, comprising: a memory cell array including a plurality of memory cells arranged at intersections of bit line pairs and word lines, wherein each of the memory cells includes a first transistor having one main electrode connected to a first bit line, a second transistor having one main electrode connected to a second bit line, a first node electrode for data-storage connected to the other main electrode of the first transistor, a second node electrode for data-storage connected to the other main electrode of the second transistor, and a shield electrode formed surrounding the first and second node electrodes, wherein the first transistor and the second transistor have respective gates both connected to an identical word line, wherein the first bit line and the second bit line are connected to an identical sense amp, wherein the first node electrode, the second node electrode, the first bit line, the second bit line, the word line and the shield electrode are isolated from each other using insulating films. 
         [0007]    In another aspect the present invention provides a semiconductor memory device, comprising: a memory cell array including a plurality of memory cells arranged at intersections of bit line pairs and word lines, wherein each of the memory cells includes a transistor having one main electrode connected to one bit line of the bit line pair, a node electrode for data-storage connected to the other main electrode of the transistor, and a shield electrode formed surrounding the node electrode, wherein the transistor has a gate connected to the word line, wherein the bit line pair is connected to an identical sense amp, wherein the bit line pair, the word line, the node electrode and the shield electrode are isolated from each other using insulating films. 
         [0008]    In another aspect the present invention provides a semiconductor memory device, comprising: a memory cell array including a plurality of memory cells arranged at intersections of bit line pairs and word lines, the memory cell including a first electrode layer containing a first transistor having one main electrode connected to a first bit line, a second transistor having one main electrode connected to a second bit line, a first node electrode connected to the other main electrode of the first transistor, and a second node electrode connected to the other main electrode of the second transistor, a second electrode layer formed on the first electrode layer and containing a third and a fourth node electrode connected via respective interlayer contact electrodes to the first and second node electrodes, and a power supply electrode surrounding the third and fourth node electrodes, a third electrode layer formed on the second electrode layer and containing a fifth and a sixth node electrode connected via respective interlayer contact electrodes to the third and fourth node electrodes, and an electrode surrounding the fifth and sixth node electrodes at least in part and serving as a word line, wherein the first transistor and the second transistor have respective gates both connected to an identical word line, wherein the first bit line and the second bit line are connected to an identical sense amp, wherein said first node electrode, said second node electrode, said third node electrode, said fourth node electrode, said fifth node electrode, said sixth node electrode, said bit lines, said power supply electrode, and said electrode serving as said word line are isolated from each other using insulating films. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a diagram showing a circuit of a semiconductor memory device in accordance with one embodiment. 
           [0010]      FIG. 2  is a cross-sectional view of a memory cell in the semiconductor memory device in accordance with one embodiment taken in a direction vertical to a substrate. 
           [0011]      FIG. 3  is a cross-sectional view of the memory cell in the semiconductor memory device in accordance with one embodiment taken in a direction vertical to the substrate. 
           [0012]      FIG. 4  is a plan view of a semiconductor substrate for the memory cell in the semiconductor memory device in accordance with one embodiment. 
           [0013]      FIG. 5  is a cross-sectional view of the memory cell in the semiconductor memory device in accordance with one embodiment taken in a direction parallel with  FIG. 4 . 
           [0014]      FIG. 6  is a cross-sectional view of the memory cell in the semiconductor memory device in accordance with one embodiment taken in a direction parallel with  FIG. 4 . 
           [0015]      FIG. 7  is a cross-sectional view of the memory cell in the semiconductor memory device in accordance with one embodiment taken in a direction parallel with  FIG. 4 . 
           [0016]      FIG. 8  is a cross-sectional view of the memory cell in the semiconductor memory device in accordance with one embodiment taken in a direction parallel with  FIG. 4 . 
           [0017]      FIG. 9  is a cross-sectional view of the memory cell in the semiconductor memory device in accordance with one embodiment taken in a direction parallel with  FIG. 4 . 
           [0018]      FIG. 10  is a surface arrangement diagram of a memory cell array in the semiconductor memory device in accordance with one embodiment. 
           [0019]      FIG. 11  is a cross-sectional view of a memory cell in a semiconductor memory device in accordance with another embodiment taken in a direction vertical to a substrate. 
           [0020]      FIG. 12  is a cross-sectional view of the memory cell in the semiconductor memory device in accordance with another embodiment taken in a direction vertical to the substrate. 
           [0021]      FIG. 13  is a surface diagram of the memory cell in the semiconductor memory device in accordance with another embodiment. 
           [0022]      FIG. 14  is a cross-sectional view of the memory cell in the semiconductor memory device in accordance with another embodiment taken in a direction parallel with  FIG. 13 . 
           [0023]      FIG. 15  is a cross-sectional view of the memory cell in the semiconductor memory device in accordance with another embodiment taken in a direction parallel with  FIG. 13 . 
           [0024]      FIG. 16  is a cross-sectional view of the memory cell in the semiconductor memory device in accordance with another embodiment taken in a direction parallel with  FIG. 13 . 
           [0025]      FIG. 17  is a cross-sectional view of the memory cell in the semiconductor memory device in accordance with another embodiment taken in a direction parallel with  FIG. 13 . 
           [0026]      FIG. 18  is a cross-sectional view of the memory cell in the semiconductor memory device in accordance with another embodiment taken in a direction parallel with  FIG. 13 . 
           [0027]      FIG. 19  is a surface arrangement diagram of a memory cell array in the semiconductor memory device in accordance with another embodiment. 
           [0028]      FIG. 20  is a diagram showing a circuit of a semiconductor memory device in accordance with yet another embodiment. 
           [0029]      FIG. 21  is a cross-sectional view of a memory cell in a semiconductor memory device in accordance with yet another embodiment taken in a direction vertical to a substrate. 
           [0030]      FIG. 22  is a cross-sectional view of the memory cell in the semiconductor memory device in accordance with yet another embodiment taken in a direction vertical to the substrate. 
           [0031]      FIG. 23  is a surface diagram of a semiconductor substrate for the memory cell in the semiconductor memory device in accordance with yet another embodiment. 
           [0032]      FIG. 24  is a cross-sectional view ( 1 ) of the memory cell in the semiconductor memory device in accordance with yet another embodiment taken in a direction parallel with  FIG. 23 . 
           [0033]      FIG. 25  is a cross-sectional view of the memory cell in the semiconductor memory device in accordance with yet another embodiment taken in a direction parallel with  FIG. 23 . 
           [0034]      FIG. 26  is a cross-sectional view of the memory cell in the semiconductor memory device in accordance with yet another embodiment taken in a direction parallel with  FIG. 23 . 
           [0035]      FIG. 27  is a cross-sectional view of the memory cell in the semiconductor memory device in accordance with yet another embodiment taken in a direction parallel with  FIG. 23 . 
           [0036]      FIG. 28  is a cross-sectional view of the memory cell in the semiconductor memory device in accordance with yet another embodiment taken in a direction parallel with  FIG. 23 . 
           [0037]      FIG. 29  is a surface arrangement diagram of a memory cell array in the semiconductor memory device in accordance with yet another embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0038]    One embodiment of the present invention will now be described below. 
         [0039]      FIG. 1  shows a circuit diagram of memory cells in the present embodiment. A memory cell array in the present embodiment includes two types of complementary bit lines. The number of the bit lines is (N+1), respectively. Specifically, it includes bit lines BLtk, BLck (k=0-N). The number of word lines is (M+1). Specifically, it includes word lines WLj (j=0-M). The memory cells  11  in the present embodiment are formed in regions at intersections of the complementary bit lines BLtk, BLck (k=0-N) and the word lines WLj (j=0-M). For example, a memory cell  11  is formed in a region at an intersection of complementary bit lines BLt 0 , BLc 0  and a word line WL 0  as shown in  FIG. 1 . 
         [0040]    A memory cell  11  includes two N-type MOS transistors T 1 , T 2  and three capacitors C 1 , C 2 , C 3 . The N-type MOS transistor T 1  has a source connected to the bit line BLt 0 , and the N-type MOS transistor T 2  has a source connected to the bit line BLc 0 . The N-type MOS transistor T 1  and the N-type MOS transistor T 2  have respective gates, which are both connected to the word line WL 0 . 
         [0041]    The N-type MOS transistor T 1  and the N-type MOS transistor T 2  have respective drains, between which both electrodes of the capacitor C 1  are connected. In addition, the drain of the N-type MOS transistor T 1  is connected to the capacitor C 2 . And the drain of the N-type MOS transistor T 2  is connected to the capacitor C 3 . Thus, the connection region between the drain of the N-type MOS transistor T 1  and the capacitor C 2  forms a data-storage node SNt. The connection region between the drain of the N-type MOS transistor T 2  and the capacitor C 3  forms a data-storage node SNc. The complementary bit lines BLt 0 , BLc 0  are connected to a sense amp (SA)  12 , which can read out stored information. 
         [0042]      FIGS. 2-9  show a specific structure of the memory cell  11  for one bit shown in  FIG. 1 .  FIGS. 2 and 3  are cross-sectional views taken in a direction vertical to a semiconductor substrate  21 .  FIG. 2  is a cross-sectional view taken vertical to the semiconductor substrate  21  along the line  3 A- 3 B.  FIG. 3  is also a cross-sectional view taken along the line  3 A- 3 B but has an angle of 90 degrees from the cross-sectional view of  FIG. 2 .  FIGS. 4-9  are cross-sectional views taken in a direction parallel with the semiconductor substrate  21 . Namely, they are cross-sectional views vertical to the sections of  FIGS. 2 and 3 .  FIG. 4  is a cross-sectional view taken along the line  4 A- 4 B in  FIG. 2  and the line  4 C- 4 D in  FIG. 3 .  FIG. 5  is a cross-sectional view taken along the line  5 A- 5 B in  FIG. 2  and the line  5 C- 5 D in  FIG. 3 .  FIG. 6  is a cross-sectional view taken along the line  6 A- 6 B in  FIG. 2  and the line  6 C- 6 D in  FIG. 3 .  FIG. 7  is a cross-sectional view taken along the line  7 A- 7 B in  FIG. 2  and the line  7 C- 7 D in  FIG. 3 .  FIG. 8  is a cross-sectional view taken along the line  8 A- 8 B in  FIG. 2  and the line  9 C- 9 D in  FIG. 3 . 
         [0043]    The present embodiment is directed to a semiconductor memory device having a multi-layered structure, which includes interlayer insulators  101  formed on the surface of the semiconductor substrate  21 , and wiring patterns serving as electrodes formed between the interlayer insulators  101  three-dimensionally. This structure is described on the basis of  FIGS. 2 and 3 , layer by layer to be formed, based on  FIGS. 4-9 . A region surrounded by a dashed-chain line in the figures shows a memory cell region for one bit. 
         [0044]    In the present embodiment, active regions  24  and  25  are formed in the semiconductor substrate  21  as shown in  FIG. 4 . The active region  24  is used to form a source region, a drain region and a channel region (not shown) of the transistor T 1 . The active region  25  is used to form a source region, a drain region and a channel region (not shown) of the transistor T 2 . Electrodes are formed on these regions. 
         [0045]    The semiconductor substrate  91  includes a device isolation layer (STI)  22  formed therein. The isolation layer  22  contributes to isolation between plural memory cells  11  and also contributes to isolation between the transistors T 1  and T 2  in one memory cell  11 . Further, the word line WL 0  is formed over the semiconductor substrate  21  and the isolation layer (STI)  22  formed in the surface thereof. Specifically, the word line WL 0  is formed on the channel region of the transistor T 1  in the active region  24  and on the channel region of the transistor T 2  in the active region  25 , with a gate insulator, not shown, interposed therebetween. The word line WL 0  serves as gate electrodes of the transistors T 1  and T 2 . 
         [0046]    An interlayer insulator  101  is formed on the semiconductor substrate  21 , and an interlayer contact electrode  26  is formed through the interlayer insulator  101  down to the active region  24  used to form the transistor T 1 . The bit line BLt and the source region of the transistor T 1  formed in the active region  24  are connected to each other via the interlayer contact electrode  26 . The data-storage node SNt and the transistor T 1  are connected to each other via an interlayer contact electrode  27 . Similarly, an interlayer contact electrode  28  is formed through the interlayer insulator  101  down to the active region  25  used to form the transistor T 2 . The bit line BLc and the transistor T 2  are connected to each other via the interlayer contact electrode  28 . The data-storage node SNc and the transistor T 2  are connected to each other via an interlayer contact electrode  29 . 
         [0047]      FIG. 5  is a cross-sectional view taken along the line  5 A- 5 B in  FIG. 2  and the line  5 C- 5 D in  FIG. 3 . In the section of  FIG. 5 , an electrode BLtM 1  serving as the bit line BLt, an electrode BLcM 1  serving as the bit line BLc, an electrode SNtM 1  serving as the data-storage node SNt, and an electrode SNcM 1  serving as the data-storage node SNc are formed. 
         [0048]    As described above, the electrode BLtM 1  serving as the bit line BLt is connected to the transistor T 1  via the interlayer contact electrode  26 , and the electrode SNtM 1  serving as the data-storage node SNt is connected to the transistor T 1  via the interlayer contact electrode  27 . 
         [0049]    In addition, the electrode BLcM 1  serving as the bit line BLc is connected to the transistor T 3  via the interlayer contact electrode  28 , and the electrode SNcM 1  serving as the data-storage node SNc is connected to the transistor T 2  via the interlayer contact electrode  29 . 
         [0050]    In the section of  FIG. 5 , the electrode SNtM 1  serving as the data-storage node SNt, the electrode SNcM 1  serving as the data-storage node SNc, and the interlayer insulator  101  sandwiched between these two electrodes SNtM 1  and SNcM 1  form the capacitor C 1 . 
         [0051]    In addition, the region sandwiched between the electrode SNtM 1  serving as the data-storage node SNt and the electrode BLtM 1  serving as the bit line BLt, and the interlayer insulator  101  sandwiched between the electrode SNtM 1  serving as the data-storage node SNt and the electrode BLcM 1  serving as the bit line BLc form the capacitor C 2 . 
         [0052]    Further, the region sandwiched between the electrode SNcM 1  serving as the data-storage node SNc and the electrode BLtM 1  serving as the bit line BLt, and the interlayer insulator  101  sandwiched between the electrode SNcM 1  serving as the data-storage node SNc and the electrode BLcM 1  serving as the bit line BLc form the capacitor C 3 . An interlayer insulator  101  is formed over these electrodes BLtM 1 , BLcM 1 , SNtM 1 , SNcM 1 , and then electrode patterns in the next layer are formed thereon. 
         [0053]      FIG. 6  is a cross-sectional view taken along the line  6 A- 6 B in  FIG. 2  and the line  6 C- 6 D in  FIG. 3 . In the section of  FIG. 6 , an electrode VM 2  for supplying power, an electrode SNtM 2  serving as the data-storage node SNt, and an electrode SNcM 2  serving as the data-storage node SNc are formed. The electrode SNtM 2  serving as the data-storage node SNt is connected to the electrode SNtM 1  via an interlayer contact electrode  30 , and the electrode SNcM 2  serving as the data-storage node SNc is connected to the electrode SNcM 1  via an interlayer contact electrode  31 . The electrode VM 2  is formed in a closed-loop surrounding the two electrodes SNtM 2  and SNcM 2 . 
         [0054]    In the section of  FIG. 6 , the interlayer insulator  101  sandwiched between the electrode SNtM 2  serving as the data-storage node SNt and the electrode SNcM 2  serving as the data-storage node SNc forms the capacitor C 1 . In addition, the interlayer insulator  101  sandwiched between the electrode SNtM 2  serving as the data-storage node SNt and the electrode VM 2  for supplying power forms the capacitor C 2 . Further, the interlayer insulator  101  sandwiched between the electrode SNcM 2  serving as the data-storage node SNc and the electrode VM 2  for supplying power forms the capacitor C 3 . An interlayer insulator  101  is formed over these electrodes SNtM 2 , SNcM 2 , VM 2 , and then electrode patterns in the next layer ( FIG. 7 ) are formed. 
         [0055]      FIG. 7  is across-sectional view taken along the line  7 A- 7 B in  FIG. 2  and the line  7 C- 7 D in  FIG. 3 . In this section, an electrode WL 0 M 3  serving as the word line WL 0 , an electrode WL 1 M 3  serving as an adjacent word line WL 1 , an electrode SNtM 3  serving as the data-storage node SNt, and an electrode SNcM 3  serving as the data-storage node SNc are formed. The electrode SNtM 3  serving as the data-storage node SNt is connected to the electrode SNtM 2  via an interlayer contact electrode  32 . The electrode SNcM 3  serving as the data-storage node SNc is connected to the electrode SNtM 2  via an interlayer contact electrode  33 . The electrode WL 0 M 3  serving as the word line WL 0  and the electrode WL 1 M 3  serving as an adjacent word line WL 1  are formed surrounding the two nodes SNtM 3 , SNcM 3 . 
         [0056]    In the section of  FIG. 7 , the interlayer insulator  101  sandwiched between the electrode SNtM 3  serving as the data-storage node SNt and the electrode SNcM 3  serving as the data-storage node SNc forms the capacitor C 1 . In addition, the region sandwiched between the electrode SNtM 3  serving as the data-storage node SNt and the electrode WL 0 M 3  serving as the word line WL 0 , and the interlayer insulator  101  sandwiched between the electrode SNtM 3  serving as the data-storage node SNt and the electrode WL 1 M 3  serving as the word line WL 1  form the capacitor C 2 . Further, the region sandwiched between the electrode SNcM 3  serving as the data-storage node SNc and the electrode WL 0 M 3  serving as the word line WL 0 , and the interlayer insulator  101  sandwiched between the electrode SNcM 3  serving as the data-storage node SNc and the electrode WL 1 M 3  serving as the word line WL 1  form the capacitor C 3 . An interlayer insulator  101  is formed over these electrodes SNtM 3 , SNcM 3 , WL 0 M 3 , WL 1 M 3 , and then electrode patterns in the next layer ( FIG. 8 ) are formed. 
         [0057]      FIG. 8  is a cross-sectional view taken along the line  8 A- 8 B in  FIG. 2  and the line  8 C- 8 D in  FIG. 3 . In the section of  FIG. 8 , an electrode VM 4  for supplying power, an electrode SNtM 4  serving as the data-storage node SNt, and an electrode SNcM 4  serving as the data-storage node SNc are formed. The electrode SNtM 4  serving as the data-storage node SNt is connected to the electrode SNtM 3  via an interlayer contact electrode  34 , and the electrode SNcM 4  serving as the data-storage node SNc is connected to the electrode SNcM 3  via an interlayer contact electrode  35 . Although the present embodiment forms two interlayer contact electrodes, that is, the interlayer contact electrodes  34  and  35 , only one may be sufficient. By forming plural interlayer contact electrodes  34  and  35 , capacitive components formed between the interlayer contact electrodes  34  and  35  serves to increase the capacities of the capacitors C 1 , C 2  and C 3 . In this way, it is possible to additionally increase the capacities of the capacitors C 1 , C 2  and C 3 . The electrode VM 4  is formed in a closed-loop surrounding the two electrodes SNtM 4  and SNcM 4 . 
         [0058]    In the section of  FIG. 8 , the interlayer insulator  101  sandwiched between the electrode SNtM 4  serving as the data-storage node SNt and the electrode SNcM 4  serving as the data-storage node SNc forms the capacitor C 1 . 
         [0059]    In addition, the interlayer insulator  101  sandwiched between the electrode SNtM 4  serving as the data-storage node SNt and the electrode VM 4  for supplying power forms the capacitor C 2 . Further, the interlayer insulator  101  sandwiched between the electrode SNcM 4  serving as the data-storage node SNc and the electrode VM 4  for supplying power forms the capacitor C 3 . An interlayer insulator  101  is formed over these electrodes, and then electrode patterns in the next layer ( FIG. 9 ) are formed. 
         [0060]      FIG. 9  is a cross-sectional view taken along the line  9 A- 9 B in  FIG. 2  and the line  9 C- 9 D in  FIG. 3 . In this section, an electrode VM 5  for supplying power, an electrode SNtM 5  serving as the data-storage node SNt, and an electrode SNcM 5  serving as the data-storage node SNc are formed. The electrode SNtM 5  serving as the data-storage node SNt is connected to the electrode SNtM 4  via an interlayer contact electrode  36 , and the electrode SNcM 5  serving as the data-storage node SNc is connected to the electrode SNcM 4  via an interlayer contact electrode  37 . The electrode VM 5  is connected to the electrode VM 4  via an interlayer contact electrode  38 . The electrode VM 5  is formed in a closed-loop surrounding the two electrodes SNtM 5  and SNcM 5 . 
         [0061]    Although the present embodiment forms a plurality of the interlayer contact electrodes  36 ,  37  and  38 , only one may be sufficient. If a plurality of the interlayer contact electrodes  36 ,  37  and  38  are formed, capacitive components formed between the interlayer contact electrodes contribute for increasing capacities of the capacitors C 1 , C 2  and C 3  to additionally increase the capacities of the capacitors C 1 , C 2  and C 3 . 
         [0062]    In the section of  FIG. 9 , the interlayer insulator  101  sandwiched between the electrode SNtM 5  serving as the data-storage node SNt and the electrode SNcM 5  serving as the data-storage node SNc forms the capacitor C 1 . In addition, the interlayer insulator  101  sandwiched between the electrode SNtM 5  serving as the data-storage node SNt and the electrode VM 5  for supplying power forms the capacitor C 2 . Further, the interlayer insulator  101  sandwiched between the electrode SNcM 5  serving as the data-storage node SNc and the electrode VM 5  for supplying power forms the capacitor C 3 . 
         [0063]    Although not shown, through formation of an interlayer insulator on the top surface of the three-dimensionally structured memory cell thus formed, and then a metal film thereon, an additional increase in capacity can be achieved. For the bit lines BLtk (k=0-N) and BLck (k=0-N), formation of paired bit lines as intersecting makes it possible to suppress noises between the bit lines. It is also possible to prevent a phenomenon in which bit lines have uneven capacities due to alignment and so forth during exposure on production. 
         [0064]    In the present embodiment, the memory cell  11  is surrounded by the electrodes VM 2 , VM 4 , and VM 5  for forming power supplies. The electrodes are not limited to such the electrodes but rather may include those other than for power supplies so long as they can increase the capacities of the capacitors C 1 , C 2 , and C 3 . 
         [0065]      FIG. 10  shows an arrangement of a memory cell array including the memory cells  11  formed therein in the present embodiment superimposing  FIG. 4  on  FIG. 5 . The memory cells  11  are formed two-dimensionally. In each of the memory cells  11 , the transistors T 1 , T 2  are formed in the active regions  24  and  25 . 
         [0066]    The interlayer insulators  101  sandwiched between the electrodes BLtM 1 , BLcM 1 , SNtM 1 , and SNcM 1  form the capacitors C 1 , C 2 , and C 3 . Three-dimensional formation of the two-dimensionally formed memory cells  11  allows the interlayer insulators  101  sandwiched between the electrodes in the upper and lower layers to form the capacitors C 1 , C 2 , and C 3 . As a result, the total capacity of the whole memory cells becomes sufficient for DRAM operation. In a usual CMOS process, a silicon oxide (such as SiO 2 ) having a relative permittivity of 5 or below is used as the interlayer insulator for a reduction in parasitic capacity. Even in such the case, the memory cells three-dimensionally structured in accordance with the present embodiment make it possible to ensure necessary and sufficient capacities for DRAM operation. 
         [0067]    The present embodiment allows the interlayer insulators to be used as capacitors in a DRAM. As a result, the usual CMOS process can be used to obtain a DRAM having a cell area equal to 60% or below of that of the SRAM. 
         [0068]    A second embodiment is directed to the memory cell  11  having the circuitry shown in  FIG. 1  and relates to an additionally space-reduced, three-dimensionally structured memory cell. 
         [0069]      FIGS. 11-18  show a specific structure of the memory cell  11  for one bit shown in  FIG. 1 .  FIGS. 11 and 12  are cross-sectional views taken in a direction vertical to a semiconductor substrate  121 .  FIG. 11  is a cross-sectional view taken vertical to the semiconductor substrate  121  along the line  12 A- 12 B.  FIG. 12  is also a cross-sectional view taken along the line  12 A- 12 B but has an angle of 90 degrees from the cross-sectional view of  FIG. 11 .  FIGS. 13-18  are cross-sectional views taken in a direction parallel with the semiconductor substrate  121 . Namely, they are cross-sectional views vertical to the sections of  FIGS. 11 and 12 .  FIG. 13  is a cross-sectional view taken along the line  13 A- 13 B in  FIG. 11  and the line  13 C- 13 D in  FIG. 12 .  FIG. 14  is a cross-sectional view taken along the line  14 A- 14 B in  FIG. 11  and the line  14 C- 14 D in  FIG. 12 .  FIG. 15  is a cross-sectional view taken along the line  15 A- 15 B in  FIG. 11  and the line  15 C- 15 D in  FIG. 12 .  FIG. 16  is a cross-sectional view taken along the line  16 A- 16 B in  FIG. 11  and the line  16 C- 16 D in  FIG. 12 .  FIG. 17  is a cross-sectional view taken along the line  17 A- 17 B in  FIG. 11  and the line  17 C- 17 D in  FIG. 12 .  FIG. 18  is a cross-sectional view taken along the line  18 A- 18 B in  FIG. 11  and the line  18 C- 18 D in  FIG. 12 . 
         [0070]    The present embodiment is directed to a semiconductor memory device having a multi-layered structure, which includes interlayer insulators  101  formed on the surface of the semiconductor substrate  121 , and wiring patterns serving as electrodes formed between the interlayer insulators  101  three-dimensionally. This structure is described on the basis of  FIGS. 11 and 12 , layer by layer to be formed, based on  FIGS. 13-18 . A region surrounded by a dashed-chain line in the figures shows a memory cell region for one bit. 
         [0071]    In the present embodiment, active regions  124  and  125  are formed in the semiconductor substrate  121  as shown in  FIG. 13 . The active region  124  is used to form a source region, a drain region and a channel region (not shown) of the transistor T 1  therein. The active region  125  is used to form a source region, a drain region and a channel region (not shown) of the transistor T 2  therein. 
         [0072]    The semiconductor substrate  121  includes a device isolation layer (STI)  122  formed therein. The isolation layer  122  contributes to isolation between plural memory cells  11  and also contributes to isolation between the transistors T 1  and T 2  in one memory cell  11 . Further, the word line WL 0  is formed over the semiconductor substrate  121  and the isolation layer (STI)  122  formed in the surface thereof. Specifically, the word line WL 0  is formed on the channel region of the transistor T 1  in the active region  124  and on the channel region of the transistor T 2  in the active region  125 , with a gate insulator, not shown, interposed therebetween. The word line WL 0  serves as gate electrodes of the transistors T 1 , T 2 . 
         [0073]    An interlayer insulator  101  is formed on the semiconductor substrate  121 , and an interlayer contact electrode  126  is formed through the interlayer insulator  101  down to the active region  124  used to form the transistor T 1 . The bit line BLt and the source region of the transistor T 1  formed in the active region  124  are connected to each other via the interlayer contact electrode  126 . The data-storage node SNt and the transistor T 1  are connected to each other via an interlayer contact electrode  127 . Similarly, an interlayer contact electrode  128  is formed through the interlayer insulator  101  down to the active region  125  used to form the transistor T 2 . The bit line BLc and the transistor T 2  are connected to each other via the interlayer contact electrode  128 . The data-storage node SNt and the transistor T 2  are connected to each other via an interlayer contact electrode  129 . 
         [0074]      FIG. 14  is a cross-sectional view taken along the line  14 A- 14 B in  FIG. 11  and the line  14 C- 14 D in  FIG. 12 . In the section of  FIG. 14 , an electrode BLtM 1  serving as the bit line BLt, an electrode BLcM 1  serving as the bit line BLc, an electrode SNtM 1  serving as the data-storage node SNt, and an electrode SNcM 1  serving as the data-storage node SNc are formed. 
         [0075]    As described above, the electrode BLtM 1  serving as the bit line BLt is connected to the transistor T 1  via the interlayer contact electrode  126 , and the electrode SNtM 1  serving as the data-storage node SNt is connected to the transistor T 1  via the interlayer contact electrode  127 . 
         [0076]    In addition, the electrode BLcM 1  serving as the bit line BLc is connected to the transistor T 1  via the interlayer contact electrode  128 , and the electrode SNcM 1  serving as the data-storage node SNc is connected to the transistor T 1  via the interlayer contact electrode  129 . The electrode BLtM 1  serving as the bit line BLt is connected to the transistor T 2  via the interlayer contact electrode  126 , and the electrode SNtM 1  serving as the data-storage node SNt is connected to the transistor T 2  via the interlayer contact electrode  127 . 
         [0077]      FIG. 15  is a cross-sectional view taken along the line  15 A- 15 B in  FIG. 11  and the line  15 C- 15 D in  FIG. 12 . In the section of  FIG. 15 , an electrode SNtM 2  serving as the data-storage node SNt, and an electrode SNcM 2  serving as the data-storage node SNc are formed. 
         [0078]    As described above, the electrode SNcM 2  serving as the data-storage node SNc is connected to the electrode SNcM 1  via an interlayer contact electrode  131 , and the electrode SNtM 2  serving as the data-storage node SNt is connected to the electrode SNtM 1  via an interlayer contact electrode  130 . 
         [0079]    In the section of  FIG. 15 , the electrode SNtM 2  serving as the data-storage node SNt, the electrode SNcM 2  serving as the data-storage node SNc, and the interlayer insulator  101  sandwiched between these two electrodes SNtM 2  and SNcM 2  form the capacitor C 1 . 
         [0080]      FIG. 16  is a cross-sectional view taken along the line  16 A- 16 B in  FIG. 11  and the line  16 C- 16 D in  FIG. 12 . In this section of  FIG. 16 , an electrode WL 0 M 3  serving as the word line WL 0 , an electrode WL 1 M 3  serving as the word line WL 1 , an electrode SNtM 3  serving as the data-storage node SNt, and an electrode SNcM 3  serving as the data-storage node SNc are formed. 
         [0081]    The electrode SNtM 3  serving as the data-storage node SNt is connected to the electrode SNtM 2  via an interlayer contact electrode  132 . The electrode SNcM 3  serving as the data-storage node SNc is connected to the electrode SNcM 2  via an interlayer contact electrode  133 . The electrode WL 0 M 3  serving as the word line WL 0  is formed surrounding the two electrodes SNtM 3 , SNcM 3  at least in part and, opposite to the electrode WL 0 M 3 , the electrode WL 1 M 3  serving as the word line WL 1  is formed surrounding the two electrodes SNtM 3 , SNcM 3  at least in part. 
         [0082]    In the section of  FIG. 16 , the interlayer insulator  101  sandwiched between the electrode SNtM 3  serving as the data-storage node SNt and the electrode SNcM 3  serving as the data-storage node SNc forms the capacitor C 1 . In addition, the interlayer insulator  101  sandwiched between the electrode SNtM 3  serving as the data-storage node SNt and the electrode WL 0 M 3  serving as the word line WL 0  forms the capacitor C 2 , and the interlayer insulator  101  sandwiched between the electrode SNtM 3  serving as the data-storage node SNt and the electrode WL 1 M 3  serving as the word line WL 1  forms the capacitor C 2 . Further, the interlayer insulator  101  sandwiched between the electrode SNcM 3  serving as the data-storage node SNc and the electrode WL 0 M 3  serving as the word line WL 0  forms the capacitor C 3 , and the interlayer insulator  101  sandwiched between the electrode SNcM 3  serving as the data-storage node SNc and the electrode WL 1 M 3  serving as the word line WL 1  forms the capacitor C 3 . An interlayer insulator  101  is formed over these electrodes SNcM 3 , SNtM 3 , WL 0 M 3 , and WL 1 M 3 , and then electrode patterns in the next layer ( FIG. 17 ) are formed. 
         [0083]      FIG. 17  is a cross-sectional view taken along the line  17 A- 17 B in  FIG. 11  and the line  17 C- 17 D in  FIG. 12 . In the section of  FIG. 17 , an electrode VM 4  for supplying power, an electrode SNtM 4  serving as the data-storage node SNt, and an electrode SNcM 4  serving as the data-storage node SNc are formed. The electrode SNtM 4  serving as the data-storage node SNt is connected to the electrode SNtM 3  via an interlayer contact electrode  134 , and the electrode SNcM 4  serving as the data-storage node SNc is connected to the electrode SNcM 3  via an interlayer contact electrode  135 . The electrode VM 4  is formed in a closed-loop surrounding the two electrodes SNtM 4  and SNcM 4 . 
         [0084]    In the section of  FIG. 17 , the interlayer insulator  101  sandwiched between the electrode SNtM 4  serving as the data-storage node SNt and the electrode SNcM 4  serving as the data-storage node SNc forms the capacitor C 1 . In addition, the interlayer insulator  101  sandwiched between the electrode SNtM 4  serving as the data-storage node SNt and the electrode VM 4  for supplying power forms the capacitor C 2 . Further, the interlayer insulator  101  sandwiched between the electrode SNcM 4  serving as the data-storage node SNc and the electrode VM 4  for supplying power forms the capacitor C 3 . An interlayer insulator  101  is formed over these electrodes SNcM 4 , SNtM 4 , VM 4 , and then electrode patterns in the next layer ( FIG. 18 ) are formed. 
         [0085]      FIG. 18  is a cross-sectional view taken along the line  18 A- 18 B in  FIG. 11  and the line  18 C- 18 D in  FIG. 12 . In this section of  FIG. 18 , an electrode VM 5  for supplying power, an electrode SNtM 5  serving as the data-storage node SNt, and an electrode SNcM 5  serving as the data-storage node SNc are formed. The electrode SNtM 5  serving as the data-storage node SNt is connected to the electrode SNtM 4  via an interlayer contact electrode  136 , and the electrode SNcM 5  serving as the data-storage node SNc is connected to the electrode SNcM 4  via an interlayer contact electrode  137 . The electrode VM 5  for supplying power is connected to the electrode VM 4  via an interlayer contact electrode  138 . The electrode VM 5  is formed in a closed-loop surrounding the two electrodes SNtM 5  and SNcM 5 . 
         [0086]    Although the present embodiment forms a plurality of the interlayer contact electrodes  138 , only one may be sufficient. If the interlayer contact electrodes  138  are formed plural, capacitive components formed between the interlayer contact electrodes contribute to increases in capacity of the capacitors C 2 , C 3  to additionally increase the capacities of the capacitors C 2 , C 3 . 
         [0087]    In the section of  FIG. 18 , the interlayer insulator  101  sandwiched between the electrode SNtM 5  serving as the data-storage node SNt and the electrode SNcM 5  serving as the data-storage node SNc forms the capacitor C 1 . In addition, the interlayer insulator  101  sandwiched between the electrode SNtM 5  serving as the data-storage node SNt and the electrode VM 5  for supplying power forms the capacitor C 2 . Further, the interlayer insulator  101  sandwiched between the electrode SNcM 5  serving as the data-storage node SNc and the electrode VM 5  for supplying power forms the capacitor C 3 . 
         [0088]      FIG. 19  shows an arrangement of a memory cell array including the memory cells  11  formed therein in the present embodiment superimposing  FIG. 13  on  FIG. 14 . The memory cells  11  are formed two-dimensionally. In each of the memory cells  11 , the transistors T 1 , T 2  are formed in the active regions  124  and  125  and the interlayer insulators  101  sandwiched between the electrodes BLtM 1 , BLcM 1 , SNtM 1 , and SNcM 1  form the capacitors C 1 , C 2 , and C 3 . Three-dimensional formation of the two-dimensionally formed memory cells  11  allows the interlayer insulators  101  sandwiched between the electrodes in the upper and lower layers to form the capacitors C 1 , C 2 , and C 3 . As a result, the total capacity of the whole memory cells becomes sufficient for DRAM operation. In a usual CMOS process, for a reduction in parasitic capacity, a silicon oxide (such as SiO 2 ) having a relative permittivity of 5 or below is used as the interlayer insulator. Even in such the case, the memory cells three-dimensionally structured in accordance with the present embodiment make it possible to ensure necessary and sufficient capacities for DRAM operation. 
         [0089]    The present embodiment allows the interlayer insulators to be used as capacitors in a DRAM. As a result, the usual CMOS process can be used to obtain a DRAM having a cell area equal to 40-60% of that in the SRAM. 
         [0090]    A third embodiment of the present invention is described next. The present embodiment relates to a DRAM that uses one transistor and one capacitor to configure a memory cell for one bit.  FIG. 20  shows a circuit diagram of memory cells in the present embodiment. A memory cell array in the present embodiment includes two types of complementary bit lines. The number of the bit lines is (N+1), respectively. Specifically, it includes bit lines BLtk, BLck (k=0-N). 
         [0091]    Word lines are provided. The number of word lines is M+1. Specifically, it includes word lines WLj (j=0-M). Further, dummy word lines DWL 0 , DWL 1  are provided. 
         [0092]    The memory cells  211  in the present embodiment are formed in regions at intersections of the complementary bit lines BLtk, BLck (k=0-N) and the word lines WLj (j=0-M). Such the arrangement of the memory cells is called the folded bit-line arrangement. For example, a memory cell  211  is formed in a region at an intersection of complementary bit lines BLt 0 , BLc 0  and a word line WL 1  as shown in  FIG. 20 . In addition, a dummy cell  213  is formed in a region at an intersection of the complementary bit lines BLt 0 , BLc 0  and the dummy word line DWL 1 . 
         [0093]    A memory cell  211  includes an N-type MOS transistor T and a capacitor C. The N-type MOS transistor T has a source connected to the bit line BLt 0 . The N-type MOS transistor T has a gate connected to the word line WL 1 . The N-type MOS transistor T has a drain connected to the capacitor C. Thus, a data-storage node SNs is formed in the connection region between the drain of the N-type MOS transistor T and the capacitor C. The complementary bit lines BLt 0  and BLc 0  are connected to a sense amp (SA)  212 , which can read out stored information. Dummy cells  213  are formed in the regions at intersections of the complementary bit line BLtk or BLck (k=0-N), the dummy word line DWL 0  or DWL 1 , and lines EQL, VBL for supplying voltages required for driving the dummy cells. 
         [0094]      FIGS. 21-28  show a specific structure of the memory cell  211  for one bit shown in  FIG. 20 .  FIGS. 21 and 22  are cross-sectional views taken in a direction vertical to a semiconductor substrate  221 .  FIG. 21  is a cross-sectional view taken vertical to the semiconductor substrate  221  along the line  22 A- 22 B.  FIG. 22  is also a cross-sectional view taken along the line  22 A- 22 B but has an angle of 90 degrees from the cross-sectional view of  FIG. 21 .  FIGS. 23-28  are cross-sectional views taken in a direction parallel with the semiconductor substrate  221 . Namely, they are cross-sectional views vertical to the sections of  FIGS. 21 and 22 .  FIG. 23  is a cross-sectional view taken along the line  23 A- 23 B in  FIG. 21  and the line  23 C- 23 D in  FIG. 22 .  FIG. 24  is a cross-sectional view taken along the line  24 A- 24 B in  FIG. 21  and the line  24 C- 24 D in  FIG. 22 .  FIG. 25  is a cross-sectional view taken along the line  25 A- 25 B in  FIG. 21  and the line  25 C- 25 D in  FIG. 22 .  FIG. 26  is a cross-sectional view taken along the line  26 A- 26 B in  FIG. 21  and the line  26 C- 26 D in  FIG. 22 .  FIG. 27  is a cross-sectional view taken along the line  27 A- 27 B in  FIG. 21  and the line  27 C- 27 D in  FIG. 22 .  FIG. 28  is a cross-sectional view taken along the line  28 A- 28 B in  FIG. 21  and the line  28 C- 28 D in  FIG. 22 . 
         [0095]    The present embodiment is directed to a semiconductor memory device having a multi-layered structure, which includes interlayer insulators  101  formed on the surface of the semiconductor substrate  221 , and wiring patterns serving as electrodes formed between the interlayer insulators  101  three-dimensionally. This structure is described on the basis of  FIGS. 21 and 22 , layer by layer to be formed, based on  FIGS. 23-28 . A region surrounded by a dashed-chain line in the figures shows a memory cell region for one bit. 
         [0096]    In the present embodiment, an active region  224  is formed in the semiconductor substrate  221  as shown in  FIG. 23 . The active region  224  is used to form a source region, a drain region and a channel region (not shown) of the transistor T therein. 
         [0097]    The semiconductor substrate  221  includes a device isolation layer (STI)  222  formed therein. The isolation layer  222  contributes to isolation between plural memory cells  211  and also contributes to isolation between the memory cell  211  and the dummy cell  213 . Further, the word lines WL 0 , WL 1  are formed over the semiconductor substrate  221  and the isolation layer (STI)  222  formed in the surface thereof. Specifically, the word line WL 1  is formed on the channel region of the transistor T in the active region  224  with a gate insulator, not shown, interposed therebetween. 
         [0098]    An interlayer insulator  101  is formed on the semiconductor substrate  221 , and an interlayer contact electrode  226  is formed through the interlayer insulator  101  down to the active region  224  used to form the transistor T therein. The bit line BLt and the transistor T are connected to each other via the interlayer contact electrode  226 . The data-storage node SNs and the transistor T are connected to each other via an interlayer contact electrode  227 . 
         [0099]      FIG. 24  is a cross-sectional view taken along the line  24 A- 24 B in  FIG. 21  and the line  24 C- 24 D in  FIG. 22 . In the section of  FIG. 24 , an electrode BLtM 1  serving as the bit line BLt, an electrode BLcM 1  serving as the bit line BLc, and an electrode SNsM 1  serving as the data-storage node SNs are formed. The electrode BLtM 1  serving as the bit line BLt is connected to the transistor T via the interlayer contact electrode  226 , and the electrode SNsM 1  serving as the data-storage node SNs is connected to the transistor T via the interlayer contact electrode  227 . 
         [0100]    In the section of  FIG. 24 , the interlayer insulator  101  sandwiched between the electrode SNsM 1  serving as the data-storage node SNs and the electrode BLtM 1  serving as the bit line BLt forms the capacitor C. In addition, the interlayer insulator  101  sandwiched between the electrode SNsM 1  serving as the data-storage node SNs and the electrode BLcM 1  serving as the bit line BLc forms the capacitor C. An interlayer insulator  101  is formed over these electrodes SNsM 1 , BLtM 1 , BLcM 1 , and then electrode patterns in the next layer ( FIG. 25 ) are formed. 
         [0101]      FIG. 25  is a cross-sectional view taken along the line  25 A- 25 B in  FIG. 21  and the line  25 C- 25 D in  FIG. 22 . In this section of  FIG. 25 , an electrode WL 0 M 2  serving as the word line WL 0 , an electrode WL 2 M 2  serving as a word line WL 2 , and an electrode SNsM 2  serving as the data-storage node SNs are formed. The electrode SNsM 2  serving as the data-storage node SNs is connected to the electrode SNsM 1  via an interlayer contact electrode  228 . 
         [0102]    The electrode WL 0 M 2  serving as the word line WL 0  is formed surrounding the node SNsM 2  at least in part. In addition, opposite to the electrode WL 0 M 2 , the electrode WL 2 M 2  serving as the word line WL 2  is formed surrounding the node SNsM 2  at least in part. 
         [0103]    In the section of  FIG. 25 , the interlayer insulator  101  sandwiched between the electrode SNsM 2  serving as the data-storage node SNs and the electrode WL 0 M 2  serving as the word line WL 0  forms the capacitor C. In addition, the interlayer insulator  101  sandwiched between the electrode SNsM 2  serving as the data-storage node SNs and the electrode WL 2 M 2  serving as the word line WL 2  forms the capacitor C. An interlayer insulator  101  is formed over these electrodes SNsM 2 , WL 0 M 2 , WL 2 M 2 , and then electrode patterns in the next layer ( FIG. 26 ) are formed. 
         [0104]      FIG. 26  is a cross-sectional view taken along the line  26 A- 26 B in  FIG. 21  and the line  26 C- 26 D in  FIG. 22 . In the section of  FIG. 26 , an electrode WL 1 M 3  serving as the word line WL 1 , an electrode WL 3 M 3  serving as a word line WL 3 , an electrode SNsM 3  serving as the data-storage node SNs are formed. The electrode SNsM 3  serving as the data-storage node SNs is connected to the electrode SNsM 2  via an interlayer contact electrode  229 . The electrode WL 1 M 3  serving as the word line WL 1  is formed surrounding the electrode SNsM 3  in part. In addition, opposite to the electrode WL 1 M 3 , the electrode WL 3 M 3  serving as the word line WL 3  is formed surrounding the node SNsM 3  at least in part. 
         [0105]    In the section of  FIG. 26 , the interlayer insulator  101  sandwiched between the electrode SNsM 3  serving as the data-storage node SNs and the electrode WL 1 M 3  serving as the word line WL 1  forms the capacitor C. In addition, the interlayer insulator  101  sandwiched between the electrode SNsM 3  serving as the data-storage node SNs and the electrode WL 3 M 3  serving as the word line WL 3  forms the capacitor C. An interlayer insulator  101  is formed over these electrodes SNsM 3 , WL 1 M 3 , WL 3 M 3 , and then electrode patterns in the next layer ( FIG. 27 ) are formed. 
         [0106]      FIG. 27  is a cross-sectional view taken along the line  27 A- 27 B in  FIG. 21  and the line  27 C- 27 D in  FIG. 22 . In the section of  FIG. 27 , an electrode VM 4  for supplying power, and an electrode SNsM 4  serving as the data-storage node SNs are formed. The electrode SNsM 4  serving as the data-storage node SNs is connected to the electrode SNsM 3  via an interlayer contact electrode  230 . The electrode VM 4  is formed in a closed-loop surrounding the electrode SNsM 4 . 
         [0107]    In the section of  FIG. 27 , the interlayer insulator  101  sandwiched between the electrode SNsM 4  serving as the data-storage node SNs and the electrode VM 4  for supplying power forms the capacitor C. An interlayer insulator  101  is formed over these electrodes SNsM 4 , VM 4 , and then electrode patterns in the next layer ( FIG. 28 ) are formed. 
         [0108]      FIG. 28  is a cross-sectional view taken along the line  28 A- 28 B in  FIG. 21  and the line  28 C- 28 D in  FIG. 22 . In this section of  FIG. 28 , an electrode VM 5  for supplying power, and an electrode SNsM 5  serving as the data-storage node SNs are formed. The electrode SNsM 5  serving as the data-storage node SNs is connected to the electrode SNsM 4  via an interlayer contact electrode  231 . The electrode VM 5  for supplying power is connected to the electrode VM 4  via an interlayer contact electrode  232 . The electrode VM 5  is formed in a closed-loop surrounding the electrode SNsM 5 . 
         [0109]    In the present embodiment the interlayer contact electrodes  232  are formed plural though only one may be sufficient. If the interlayer contact electrodes  232  are formed plural, capacitive components formed between the interlayer contact electrodes contribute to increases in capacity of the capacitor C to additionally increase the capacity. 
         [0110]    In the section of  FIG. 28 , the interlayer insulator  101  sandwiched between the electrode SNsM 5  serving as the data-storage node SNs and the electrode VM 5  for supplying power forms the capacitor C. 
         [0111]      FIG. 29  shows an arrangement of a memory cell array including the memory cells  211  formed therein in the present embodiment superimposing  FIG. 23  on  FIG. 24 . The memory cell array is formed such that the memory cells  211  are alternately rotated 180 degrees in the direction vertical to the page and arranged laterally and longitudinally. The memory cells  211  are formed two-dimensionally. Each of the memory cells  211  includes the transistor T and the capacitor C formed in a respective active region  224 . 
         [0112]    Three-dimensional formation of the two-dimensionally formed memory cell array allows the interlayer insulators  101  sandwiched between the electrodes in the upper and lower layers to form the capacitor C. As a result, the total capacity of the whole memory cells becomes sufficient for DRAM operation. In a usual CMOS process, for a reduction in parasitic capacity, a silicon oxide (such as SiO 2 ) having a relative permittivity of 5 or below is used as the interlayer insulator. Even in such the case, the memory cells three-dimensionally structured in accordance with the present embodiment make it possible to ensure necessary and sufficient capacities for DRAM operation. 
         [0113]    The present embodiment allows the interlayer insulators to be used as the capacitor in a DRAM. As a result, the usual CMOS process can be used to obtain a DRAM having a cell area equal to 30-50% of that in the SRAM. 
         [0114]    Several embodiments of the semiconductor memory device in accordance with the present invention have been described in detail above by way of example only. The present invention is not limited to the above embodiments but rather can be variously modified and varied without departing from the scope and spirit of the invention as recited in the appended claims as has been known by the skilled person in the art.