Abstract:
A semiconductor memory device has memory cells each of which has a MIS type of transistor capable of setting one of two kinds of threshold potentials, reference cells used for determining data stored in the memory cells, which have the same size, shape and electrical properties as those of the memory cells, word lines connected to gates of the memory cells, reference word lines connected to gates of the reference cells, source line contacts connected to sources of the memory cells and the reference cells, and bit line contacts connected to drains of the memory cells and the reference cells, arrangement order of the source line contact, the word line and bit line contact connected to each of the memory cells is equal to arrangement order of the source line contact, the reference word line and the bit line contact connected to the reference cell corresponding to the memory cell.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of priority under 35USC§119 to Japanese Patent Application No. 2004-314014, filed on Oct. 28, 2004, the entire contents of which are incorporated by reference herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a semiconductor memory device which determines logics of data stored in a memory cell by comparing a data potential of the memory cell with that of a reference cell. 
         [0004]    2. Related Art 
         [0005]    There is proposed a Floating Body Cell (FBC) which accumulates majority carriers in a floating body (hereinafter referred to as a body) of a transistor formed on Silicon On Insulator (SOI) and the like, as a memory cell alternative to a DRAM cell which stores data in a capacitor. 
         [0006]    In the FBC, at the time of writing “1”, the transistor performs a pentode operation to accumulate holes generated by impact ionization in a body, so that the potential of the body is raised to lower the threshold voltage of the transistor. At the time of writing “0”, the holes accumulated in the body are discharged by forward-biasing a PN diode between the body and the drain, so that the potential of the body is lowered to raise the threshold voltage of the transistor. 
         [0007]    In the FBC, a bit line contact and a source line contact are used in common between adjacent cells in order to reduce the cell area. More specifically, there are provided two types of FBCs in a memory cell array. In one type of these FBCs, source line contacts are arranged on the left side of word lines arranged in a row, and bit line contacts are arranged on right side of the word lines. In the other type of these FBCs, bit line contacts are arranged on the left side of word lines, and source line contacts are arranged on right side of the word lines. 
         [0008]    If such FBCs can be manufactured with the same accuracy as a design drawing, there is no problem, but in practice, alignment errors and shortening of a pattern occur in a lithography process, causing differences in gate-drain length and gate-source length between adjacent cells. The above described two kinds of FBCs in the memory cell array have different arrangement orders with respect to the source line contact, the word line and the bit line contact. Therefore, in these FBCs, a resistance of a diffusion layer between the source line contact and a channel and a resistance of a diffusion layer between the bit line contact and the channel are different from each other, and Vg-Id characteristics is also different from each other. 
         [0009]    Due to such positional deviation at the time of manufacture, FBCs having two different characteristics in the memory cell array are mixed up. 
         [0010]    In the conventional semiconductor memory in which a dummy cell is used to read data written in the body of FBC, the characteristics of the dummy cell and the FBC to be read are significantly different from each other due to the above described size difference. As a result, wrong data may be read (see Japanese Patent Laid-Open No. 2003-68877). 
         [0011]    Furthermore, in the conventional semiconductor memory in which a dummy cell is provided for each word line, the above described problem of alignment error at the time of manufacture can be eliminated, but a sense amplifier needs to be provided for each bit line, which increases the number of sense amplifiers, thereby making it difficult to have a practical use. 
       SUMMARY OF THE INVENTION 
       [0012]    According to one embodiment of a semiconductor memory device, comprising: 
         [0013]    memory cells each of which has a MIS type of transistor capable of setting one of two kinds of threshold potentials; 
         [0014]    reference cells used for determining data stored in the memory cells, which have the same size, shape and electrical properties as those of the memory cells; 
         [0015]    word lines connected to gates of the memory cells; 
         [0016]    reference word lines connected to gates of the reference cells; 
         [0017]    source line contacts connected to sources of the memory cells and the reference cells; and 
         [0018]    bit line contacts connected to drains of the memory cells and the reference cells, 
         [0019]    arrangement order of the source line contact, the word line and bit line contact connected to each of the memory cells is equal to arrangement order of the source line contact, the reference word line and the bit line contact connected to the reference cell corresponding to the memory cell. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a circuit diagram showing a schematic configuration of a semiconductor memory device according to a first embodiment of the present invention. 
           [0021]      FIG. 2  is a diagram showing one example of a layout of  FIG. 1 . 
           [0022]      FIG. 3  is a cross-sectional view showing a cross-sectional structure of an example of an FBC having a back gate. 
           [0023]      FIG. 4  is a diagram showing one example of a cross-sectional structure of an example of an FBC having a back gate. 
           [0024]      FIG. 5  is a layout diagram showing one example in which alignment error occurred. 
           [0025]      FIG. 6  is a circuit diagram showing a schematic configuration of a semiconductor memory device according to a second embodiment. 
           [0026]      FIG. 7  is a diagram showing one example of a layout of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    In the following, embodiments according to the present invention will be described with reference to the accompanying drawings. 
       First Embodiment 
       [0028]      FIG. 1  is a circuit diagram showing a schematic configuration of a semiconductor memory device according to a first embodiment of the present invention, and  FIG. 2  shows an example of a layout in  FIG. 1 . 
         [0029]    In the semiconductor memory device shown in  FIG. 1 , an OPEN-BL system is adopted, in which memory cell arrays  2  are arranged on both sides of sense amplifiers (S/A)  1 . The memory cell array  2  has a plurality of word lines WL 0 , WL 1 , WL 2 , WL 3 , a plurality of dummy word lines DWLLo, DWLLe, DWLRo, DWLRe, a plurality of bit lines BL 0  to BL 3  arranged in the direction which crosses to the word lines and the dummy word lines, and FBCs  3 ,  4  formed in the vicinity of intersections of the word lines and the bit lines. In the following, the FBC on the dummy word line is referred to as a dummy cell  3 , and the other cell for writing data is referred to as a memory cell  4 . The dummy cell  3  and the memory cell  4  are composed of MIS type of transistors, and have the same size, shape and electrical properties. 
         [0030]    In the example shown in  FIG. 1 , two word lines WL 0 , WL 1 , and two dummy word lines DWLLe, DWLLo are arranged on the left side of the sense amplifier  1 , and two word lines WL 2 , WL 3 , and two dummy word lines DWLRe, DWLRo are arranged on the right side of the sense amplifier  1 . 
         [0031]    The FBC  3 ,  4  have a cross-sectional structure, for example, as shown in  FIG. 3 . On a silicon substrate  11 , there is formed an insulating film  12 , such as a silicon oxide film, on the top surface of which n-diffusion layers  13 ,  14 , and a p-diffusion layer  15 , each separated from the silicon substrate  11 , are formed. The n-diffusion layers  13 ,  14  serve as a source region and a drain region respectively, and the p-diffusion layer  15  serves as a channel body. A gate electrode  17  is formed on the top surface of the p-diffusion layer  15  via a gate insulating film  16 . An SOI substrate is used as a material of the substrate in FBC  3 ,  4 . 
         [0032]      FIG. 4  is a figure showing an example of a cross-sectional structure of the FBC having a back gate. The FBC shown in  FIG. 4  is provided with a floating body  15  formed on the top surface of a back gate electrode  18  via an insulating film  19  such as SiO 2 . A gate electrode  17  is arranged via the gate insulating film  16  on the floating body  15 . 
         [0033]    The FBCs  3 ,  4  in  FIG. 1  may have the same structure as that of  FIG. 3 , or the same structure as that of  FIG. 4 . 
         [0034]    The FBCs  3 ,  4  shown in  FIG. 1  has bit line contacts  21  connected to the bit lines and gates  22  connected to the word lines, and source line contacts  23  connected to grounding conductors, as shown in  FIG. 2 . In one of the FBCs  3 ,  4  adjacent in the right and left directions in  FIG. 1 , the bit line contact  21 , the gate  22 , and the source line contact  23  are arranged in order. In the other of the FBCs, the source line contact  23 , gate  22 , and the bit line contact  21  are arranged in order. In this way, the FBCs  3 ,  4  adjacently arranged in the right and left directions are different from each other in arrangement order of the source, the gate and the drain. 
         [0035]      FIG. 2  shows an ideal layout arrangement of the FBCs  3 ,  4 . When alignment errors of a mask pattern and the like occur in a lithography process, a difference between a distance between the bit line contact  21  and the gate  22  and a distance between the gate  22  and the source line contact  23  occurs in the two adjacent FBCs, as shown in  FIG. 5 . 
         [0036]    Accordingly, in the present embodiment, the arrangement order of the bit line contact  21 , the gate  22 , and the source line contact  23  is the same in the dummy cell  3  and the memory cell  4 . Specifically, the memory cell  4  on the second word line WL 0  on the left side of the sense amplifier  1  shown in  FIG. 1 , is associated with the third dummy word line DWLRe on the right side. Similarly, the memory cell  4  on the first word line WL 1  on the left side of the sense amplifier  1  is associated with the fourth dummy word line DWLRo on the right side. 
         [0037]    Similarly, the memory cell  4  on the first word line WL 2  on the right side of the sense amplifier  1  is associated with the third dummy word line DWLLo on the left side. Similarly, the memory cell  4  on the second word line WL 3  on the right side of the sense amplifier  1  is associated with the fourth dummy word line DWLLe on the left side. 
         [0038]    In this way, the dummy cell  3  corresponding to the memory cell  4  connected to the word line at even number order crossing to one bit line of the bit line pair arranged on both sides of the sense amplifier  1  is connected to the dummy word line at odd number order crossing to the other bit line. The dummy cell corresponding to the memory cell  4  connected to the word line at odd number order crossing to one bit line is connected to the dummy word line at even number order crossing to the other bit line. 
         [0039]      FIG. 1  shows one example in which two word lines and two dummy word lines are arranged on each side of the sense amplifiers  1 . However, the number of the word lines and the number of the dummy word lines are not limited. It is desirable that the number of word lines is equal on each side of the sense amplifiers  1 , and the number of dummy word lines is equal on each side of the sense amplifiers  1 . The different memory cells  4  may share the same dummy cell  3 . 
         [0040]    As described above, in the first embodiment, when the memory cell  4  to be read out has the arrangement order of the bit line contact  21 , the gate  22  and the source line contact  23 , data is read out by using the dummy cell  3  which has the same arrangement order, i.e. the arrangement order of the bit line contact  21 , the gate  22  and the source line contact  23 . On the other hand, when the memory cell  4  to be read out has the arrangement order of the source line contact  23 , the gate  22  and the bit line contact  21 , data is read out by using the dummy cell  3  which has the same arrangement order, i.e. the arrangement order of the source line contact  23 , the gate  22  and the bit line contact  21 . 
         [0041]    By conforming the arrangement order, it is possible to conform electrical properties such as Vg-Id, in the memory cell  4  to be read out and the dummy cell  4 , thereby correctly reading out data of the memory cell  4 . 
       Second Embodiment 
       [0042]    A second embodiment is characterized in that a FOLDED-BL system is adopted. 
         [0043]      FIG. 6  is a circuit diagram showing a schematic configuration of a semiconductor memory device according to the second embodiment of the present invention, and  FIG. 7  shows an example of a layout of  FIG. 6 . 
         [0044]    The semiconductor memory device shown in  FIG. 6  has sense amplifiers  1  provided on both sides, a plurality of bit line pairs  31 ,  32  alternately arranged on the inside of the sense amplifiers, a plurality of word lines WL 0  to WL 3 , a plurality of dummy word lines DWLLo, DWLLe, DWLRo, DWLRe, which word lines are arranged in the direction which crosses to the bit line pairs, and FBCs  3 ,  4  formed in the vicinity of cross points of the bit lines and the word lines. The system in which bit line pairs are alternately arranged on the inside of the sense amplifiers  1  provided on both sides, as shown in  FIG. 6 , is referred to as the FOLDED-BL system. 
         [0045]    The FBCs  4  are memory cells to be read out, and the FBCs  3  are dummy cells used for determining data of the memory cells  4 . The dummy cell  3  and the corresponding memory cells  4  are connected to both of two bit lines composing of a bit line pair. The dummy cells  3  and the memory cells  4  have the same size, shape and electrical properties. 
         [0046]    The FBCs  3  and the dummy cells  4  connected to one bit line  31  of the bit line pair have the arrangement order of the bit line contact  21 , the word line (dummy word line) and the source line contact  23 , respectively. The FBCs  3  and the dummy cells  4  connected to the other bit line  31  of the bit line pair has the other arrangement order of the source line contact  23 , the word line (dummy word line) and the bit line contact  21 . 
         [0047]    In this way, one bit line  31  and the other bit line  32  of the bit line pair have different arrangement orders from each other. Therefore, it is impossible to conform the arrangement order in the bit lines  31 ,  23 . 
         [0048]    Because of this, in  FIG. 6 , the bit line pairs  31 ,  32  are crossed to each other in an intermediate point between the sense amplifiers  1  on both sides. The bit line pairs  31 ,  32  are connected to a wiring layer arranged on an upper layer or a lower layer via a contact so that both bit lines are not short-circuited. 
         [0049]    By crossing the bit lines  31 ,  32 , similarly to the first embodiment, it is possible to conform the arrangement order of the source line contact, the word line and the bit line contact in the dummy cells  3  and the memory cells to be read out. 
         [0050]    In  FIG. 6 , when the memory cell  4  on the word line WL 0  is read out, the dummy cell on the dummy word line DWLRe is used. When the memory cell  4  on the word line WL 1  is read out, the dummy cell  3  on the dummy word line DWLRo is used. The memory cells  4  connected to the word lines WL 0 , WL 1  and the dummy cells  3  connected to the dummy word lines DWLLo, DWLLe has the arrangement order of the bit line contact  21 , the word line (or reference word line) and the source line contact. 
         [0051]    When the memory cell  4  on the word line WL 2  is read out, the dummy cell  3  on the dummy word line DWLLe is used. When the memory cell  4  on the word line WL 3  is read out, the dummy cell  3  on the dummy word line DWLLo is used. The memory cells  4  connected to the word lines WL 2 , WL 3  and the dummy cells  3  connected to the dummy word lines DWLLo, DWLLe have the arrangement order of the source line contact  23 , the word line (or reference word line) and the bit line contact  21 . 
         [0052]    In this way, in terms of the bit line  31 , the memory cells  4  connected to the word line at odd number order on one side of the cross point are associated with the reference cells  3  connected to the reference word line at odd number order on the other side. The memory cells connected to the word line at even number order on one side of the cross point are associated with the reference cells  3  connected to the reference word line at even number order on the other side. 
         [0053]      FIG. 6  shows one example in which two word lines are arranged on each side of the cross point of the bit lines  31 ,  32 . The number of the word lines and the number of the dummy word lines are not limited. It is desirable to conform the number of the word lines and the number of the dummy word lines on both sides of the cross point. 
         [0054]    It is unnecessary to conform the number of the word lines and the number of the dummy word lines. The memory cells connected to the different word lines may share the same dummy cell. 
         [0055]    In this way, in a semiconductor memory device according to the second embodiment, the bit line pairs  31 ,  32  are crossed in the intermediate point between the sense amplifiers  1  on both sides, thereby conforming the arrangement order of the source line contact, the word line and the bit line contact in the dummy cells  3  and the memory cells  4 . Therefore, even if alignment errors occur in the manufacturing process, it is possible to correctly read out data of the memory cells  4  to be read out.