Abstract:
The invention relates to a layout structure of a semiconductor device and more particularly to a layout structure of column pass transistors in a semiconductor memory device where the area occupied with the transistors is reduced to the minimum allowable. Thus, in spite of high integration of the semiconductor memory device and miniaturization of memory cells, the column path transistors can be kept in an efficient arrangement. In the aforementioned layout structure, the active regions of the column path transistors are longitudinally in perpendicular to the bit line pairs, thereby making it possible to reduce the area occupied in terms of the total number of memory cells.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a layout structure of a semiconductor memory device and more particularly to a layout structure and method of a column path of a semiconductor memory device for accomplishing high integration on a minimized layout area of column pass transistors connected to memory cells, thereby effectively constructing a layout structure of the column pass transistors.  
           [0003]    2. Description of the Prior Art  
           [0004]    Since the metal oxide semiconductor (MOS) transistor was invented to replace a bipolar transistor, semiconductor memory device technology has made amazing progress all over the world very recently.  
           [0005]    Such noteworthy progress in the semiconductor memory device has also triggered another advance in the high integration technology by which a great number of elements are integrated on a single wafer. Such a technological improvement in a highly integrated device has been attributed to a revolutionary technology called “very large scale integration (VLSI).” The VLSI revolutionary technology takes a lead in the field of micro-electronics represented by ultra-fine process techniques, sub-micron element techniques, circuit designing techniques in dynamic random access memory (hereinafter referred to as DRAM) and static random access memory (hereinafter referred to as SRAM).  
           [0006]    Among these advances, the technical progress in the ultra-fine process techniques and the sub-micron element techniques leads to high integration and large capacity of a semiconductor memory device characterized by memory cells of a smaller size.  
           [0007]    However, the area occupied with the interface or peripheral circuits of the memory cells becomes relatively large in contrast to miniaturization of the memory cells, so that this interface or peripheral circuit area becomes one of the important factors in determining the size of a chip. This remains as a problem to all semiconductor manufacturers that develop a miniature chip under high density. In other words, development of a miniaturized chip layout structure becomes an important factor in miniaturizing various electronic products including the semiconductor memory device, thereby to improve the competitiveness of those products.  
           [0008]    Among them, the area of the column path to provide a read/write data path of a memory cell is an important factor in determining the size of a chip, so that it is necessary to make a layout structure wherein the area of the column path is a minimum allowable in accordance with the design rule of the chip.  
           [0009]    In general, the layout structure of the cell interface has been determined according to the shape of the memory cell, which heretofore as been made with a long Y-axis. Due to limitations of the design rule, linear column pass transistors, e.g. NMOS or PMOS transistors, have been arranged in parallel to the bit line pairs BL/BLB. However, if the X-axis of a memory cell gets too short, there may be a reduction in the area of the column pass transistors to be arranged in the bit line pairs.  
           [0010]    Moreover, there may be a limitation in the conventional layout structure due to the reduction to be made in the tiny area for column pass transistors. In other words, scaling down of each memory cell can lead to a reduction in the area to be occupied by all memory cells, so that it becomes impractical to use the conventional layout structure of the column pass transistors. Thus, in order to accommodate smaller memory cells, the layout structure of a cell interface, for instance, the column pass transistors, should be improved as the scaling down of the chip continues.  
           [0011]    Therefore, the conventional layout structure of the column pass transistors may no longer be properly applied to the miniature memory cells in the near future. Thus, it is required to develop a layout structure of the column pass transistors that is different from the conventional one since it is easily predicted that memory cells will be smaller and smaller.  
           [0012]    However, if a proper layout structure cannot be made for the column pass transistors, all the efforts focused on miniaturization of a memory cell will be in vain in spite of a success in making a smaller memory cell. In other words, if a more efficient layout structure of column pass transistors is developed along with a progress in miniaturization of a memory cell, it will make a contribution to miniaturization of a chip. If a newly developed layout structure of the column pass transistors can further reduce the area for the column pass transistors, it will be advantageous in miniaturization of a semiconductor chip and enhancement of all manufacturers&#39; efforts to reduce the size of a chip.  
           [0013]    Besides, there has been another problem in the conventional column path layout structure in that the bit lines and the section data lines commonly combining inputs and outputs are made of different materials. For example, the bit lines typically have been made of a first metal layer and the section data lines have been made of a different metal, e.g., tungsten. Thus, undesirable loading of the inputs and outputs increases.  
         SUMMARY OF THE INVENTION  
         [0014]    The present invention to solves the aforementioned problems and it is an object of the present invention to provide a layout structure of column pass transistors which can be efficiently arranged in the area of memory cells which may be manufactured much smaller than the conventional layout structure.  
           [0015]    It is another object of the present invention to provide a column path layout structure of a semiconductor memory device not only to reduce the size of memory cells, but also effectively and efficiently to make a contribution to miniaturization of chips for a semiconductor memory device.  
           [0016]    It is still another object of the present invention to provide a column path layout structure of a semiconductor memory device and a method related thereto that can solve the aforementioned problem of increased loads on bit lines and section data lines made of different materials.  
           [0017]    In order to accomplish the aforementioned objects of the present invention, there is provided a column path layout structure of a semiconductor memory device wherein the longitudinal direction of active regions of the same conductivity type of first and second transistors respectively connected to bit lines further connected with a plurality of memory cells is approximately perpendicular to that of the bit line pairs. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    For a more complete understanding of the nature and objects of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings in which:  
         [0019]    [0019]FIG. 1 is an equivalent circuit diagram for illustrating conventional column pass transistors of a semiconductor memory device;  
         [0020]    [0020]FIG. 2 is an equivalent circuit diagram for illustrating a write path of a conventional semiconductor memory device for a single bit line;  
         [0021]    [0021]FIG. 3 includes diagrams explaining certain layout structures of column pass transistors shown in FIGS. 4 through 7;  
         [0022]    [0022]FIG. 4 is a top plan view for illustrating a partial layout structure of column pass transistors of a semiconductor memory device in accordance with a first embodiment of the present invention;  
         [0023]    [0023]FIG. 5 is a top plan view for illustrating an arrangement of a single transistor in the layout structure of column pass transistors in accordance with the first embodiment of the present invention;  
         [0024]    [0024]FIG. 6 is a top plan view illustrating a non-shared, active region of transistors in accordance with an embodiment of the present invention;  
         [0025]    [0025]FIG. 7 is a top plan view for illustrating a layout structure in accordance with an embodiment of the present invention;  
         [0026]    [0026]FIG. 8 is a top plan view illustrating the layout structure of column pass transistors applied to sixteen column memory cells of a semiconductor memory device in accordance with a second embodiment of the present invention; and  
         [0027]    [0027]FIG. 9 (FIG. 9 a - 9   b ) are top plan views illustrating a layout structure of column pass transistors applied to thirty-two column memory cells of an SRAM in accordance with a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]    Objects and aspects of the invention will become apparent from the following detailed description of preferred embodiments with reference to the accompanying drawings. Like reference numerals and symbols are used for designation of like or equivalent parts or portions for simplicity of illustration and explanation, repetitive detailed descriptions of which will be omitted. In addition, a number of particularly detailed descriptions of specific layout processes are provided for a further understanding of the present invention. It should also be noted that detailed descriptions about widely recognized techniques or structures unnecessary to clarify the key points of the present invention will be omitted.  
         [0029]    A conventional circuit arrangement of column pass transistors on a given column path of a semiconductor memory device shown in FIGS. 1 and 2.  
         [0030]    [0030]FIG. 1 is an equivalent circuit for illustrating a given column path of a semiconductor memory device, specifically an SRAM. FIG. 2 is an equivalent circuit for illustrating a write path of a semiconductor memory device relating to a single bit line. Bit line pairs BL, BLB are connected to provide write and read data paths onto a column path in a memory cell. Each of the bit lines includes NMOS transistors used as column pass transistors on the write column path of a section data line and PMOS transistors used as column pass transistors on the read column path of a section data line.  
         [0031]    As shown in FIG. 1, the equivalent circuit of the given column pass transistors comprises: bit line pairs BL 1 /BLB 1 , BLn/BLBn respectively connected to a plurality of memory cells; identical conductivity type column pass transistors (NMOS transistors) [ 100 - n - a/b ] connected between bit line pairs and write section data lines SDL_W/SDLB_W; and an identical conductivity type of column pass transistors (PMOS transistors) [ 300 - n - a/b ] connected between bit line pairs and read section data lines SDL_R/SDLB_R. The section data line SDL_R of the read path commonly belong to the column pass transistors (PMOS transistors) [ 300 - i - a ] connected to the bit lines BL of all memory cells, and the section data line bars SDLB_R commonly belong to the column pass transistors (PMOS transistors) [ 300 - i - b ] connected to the bit line bars BLB of all memory cells. For a predetermined memory cell number, a first transistor for bit lines BL and a second transistor for bit line bars BLB are respectively designated by ‘a’ and ‘b’ in the drawings, ‘i’ being the generic designator (index). In addition, the column pass transistors of the write path and those of the read path are respectively designated by reference numerals  100 ,  300 . Furthermore, the column path of the bit lines will be understood generally to be referred to as the “Y-Path.” 
         [0032]    On the other hand, the column pass transistors applied to a column path layout structure of the present invention can be applied to all the column pass transistors constructed on the read and write paths. It should be noted that detailed description of the present invention will be limited to the column pass transistors (NMOS transistors) constructed on the write path. For other cases, Table 1 will be referenced for further descriptions.  
         [0033]    [0033]FIG. 2 illustrates the layout structure of a memory cell interface, as including column pass transistors on the write path. In other words, FIG. 2 is a circuit diagram for illustrating the layout structure of a NMOS transistor (designated by [ 100 - i - a/b ] in FIG. 1) constructed on the write path of each bit line BL or BLB. The bit line BL/BLB is connected to an active region of NMOS transistor (drain)  20  through a first contact  40 - 1 , and the other active region of the NMOS transistor (source)  30  is connected with a section data line SDL_W/SDLB_W through second contact  40 - 2 . Furthermore, a gate  10  of the NMOS transistor is connected to a gate signal Yi, where ‘i’ will be understood to refer to any given instant of such a gate signal.  
         [0034]    In practice, there are specific layers and operational sequences for arranging a bit line, an active region, a gate and a section data line described above. Since the specific layers and operational sequences are identical to those in the prior art, there is no need to describe them in detail. Only a brief description will be made about the layout structure of those layers applied to the present invention with reference to FIG. 7.  
         [0035]    As shown in FIG. 7, the layout structure includes a substrate-active region-gate polysilicon-tungsten layer-a first metal layer vertically stacked from the bottom to the top. An oxide layer and an insulating layer required in the process of manufacturing layers of a semiconductor memory device are omitted in FIG. 7. In addition, the first and second contacts show the contacts between the first metal layer and a metal conductivity type layer, e.g. a tungsten layer. The third contact shows the contact between a metal conductivity type layer, e.g. a tungsten layer and an active region. On the other hand, it should be noted that FIG. 7 is mainly presented to show vertical positions of layers to help understand the layout structure of layers, not necessarily to indicate horizontal positions of a column pass transistor in a semiconductor memory device of the present invention. Even if a third contact appears to be in contact with the active region through the gate polysilicon layer in FIG. 7, it should be understood by one skilled in the art that the third contact is actually kept in contact through a central insulating layer (not shown), for example, an oxide layer positioned at the upper portion of the active region far from the gate polysilicon layer.  
         [0036]    [0036]FIG. 3 includes explanatory diagrams for illustrating the layout structures applied to embodiments of the present invention that will be described below, that is, layout structures shown in FIGS. 4 through 7.  
         [0037]    In the first embodiment of the present invention, both bit lines  61  and section data lines  71  are named, first layers of metal lines (‘the first metal line’ or ‘metal line 1’). The first metal line is made, e.g., of aluminum Al. Also, the first and second contact lines relate to identical tungsten lines  80  (also referred to as ‘metal  100  line’) or they are differently named simply to distinguish contact lines having the bit lines  61  from those having the section data lines  71  for convenience. Accordingly, it should be noted that the first and second contacts are constructed as identical layers that are distinguished, for simplicity of description, in the embodiments of the present invention.  
         [0038]    On the other hand, even if both reference numerals  20 ,  30  indicate N type active regions in FIG. 3, they are distinguished for non-shared and shared active regions, respectively, in the embodiments of the present invention. In other words, reference numeral  30  indicates a drain.  
         [0039]    Hereinafter, a column path layout structure will be described in detail in accordance with the first embodiment of the present invention.  
         [0040]    [0040]FIG. 4 is a plan view illustrating a partial layout structure of column pass transistors of a semiconductor memory device in accordance with a first embodiment of the present invention. As shown in FIG. 4, a part ABCD surrounded with a dash-dot broken line shows a part of the layout structure. FIG. 5 is a top plan view illustrating an arrangement of a single transistor  100 - 1 - a  in the layout structure of column pass transistors in accordance with the first embodiment of the present invention. FIG. 6 is a top plan view illustrating the layout structure of a non-shared active region of transistors in accordance with the first embodiment of the present invention.  
         [0041]    In the column path layout structure of a semiconductor memory device of the present invention, first and second transistors [ 100 - i - a ] of the active regions  20  and  30  of the same conductivity type and arranged longitudinally are respectively connected with a plurality of memory cells (not shown in FIGS. 4 through 6). The transistors are further connected with the bit line pairs BL/BLB  61 . The longitudinal transistor region is arranged approximately in perpendicular to the longitudinal direction (Y-axis) of the bit line pairs  61 . In FIG. 4, all the bit line pairs respectively connected with the memory cells positioned at the upper part of the first and second transistors [ 100 - i - a ] are not shown to clearly illustrate the layout structure of the first and second transistors which are arranged in perpendicular to the bit line pairs  61 . However, FIG. 5 illustrates the layout structure constructed with first and second transistors [ 100 - i - a ] connected in perpendicular to the bit line pairs.  
         [0042]    The bit line pairs BLi/BLBi  61  are the first metal layer, and run in parallel with the Y-axis. Sources or drains  20  or  30  (one such source  20  being shown in FIG. 5) of the first and second transistors are arranged in perpendicular to the bit line pairs  61 . The drains  30  out of the active regions of the first transistors for bit lines BLi (not shown) are connected with the bit lines pair  61 , and the sources  20  are connected with the section data lines SDL_W  71 . Also, the drains  30  of the first transistors for bit line bar BLBi (not shown in FIG. 5) are connected with the bit line bars BLBi,  61 , and the sources  20  are connected with the section data line bars SDLB_W  71 . Furthermore, the first and second transistors connected to the bit line pairs of the identical memory cells and one side of sources  20  of the first and second transistors connected to the bit line pairs of adjacent memory cells are made in a shared arrangement. This is indicated in FIG. 5 by a single, contiguous shaded region  20 . The sources  20  are shared in their arrangement between neighboring memory cells to reduce the area occupied with transistors. The section data line pairs SDL_W/SDLB_W  71  (made of a first metal, e.g., aluminum (AL), identically with the bit line pairs  61 ) are positioned in parallel to the bit line pairs  61 . In order to use the layout space efficiently, the section data line pairs  71  are connected with the sources  20  of the entire or partial groups of transistors. In other words, to make the ratio of section data line pairs to transistors 1:n (where n is a positive integer), the sources of some or all transistors are connected to one of the section data line pairs  71 . As a result, there is a significant reduction in the layout space contrasted with the prior structure where the ratio between the section data lines and the number of transistors is 1:1. There will be described herein more embodiments of the layout structure in which the sources of the transistors are shared in arrangement with the section data line pairs  71 .  
         [0043]    [0043]FIGS. 5 and 6 illustrate the layout structures of the section data line pairs in accordance with the third and fourth embodiments of the present invention. Furthermore, drains  30  of the first and second transistors are respectively in contact with the first contact lines  80 - 1  of metal conductivity type layers, e.g. tungsten layers. First contacts  40 - 1  are formed where the first contact lines  80 - 1  cross one of the bit line pairs  61 , so that the drains  30  are connected to the bit line pairs  61 . The sources  20  of the first and second transistors are respectively in contact with the second contact lines  80 - 2  of the metal conductivity layers, e.g. tungsten layers. The second contacts  40 - 2  are formed where the second contact lines  80 - 2  cross one of the section line pairs  71  shown in FIG. 4, so that the sources  20  are connected to the section line pairs  71 . It is preferable that the third contacts  8  are formed at the sources and drains of the first and second transistors and at the first and second contact lines  80 - 1 ,  80 - 2  to reduce loading or contact resistance. The sources  20  contact the section data line pairs  71  through the second contact lines  40 - 2 . Also, the drains  30  contact the bit line pairs  61  through the first contact lines  40 - 1 .  
         [0044]    [0044]FIG. 5 shows the layout structure of a single column pass transistor [ 100 - 1 - a ] connected to the first bit line BLi arranged under the bit line pairs  61  connected to a plurality of memory cells. The bit lines BL 1 , the bit line bars BLBi to be connected to neighboring memory cells are arranged adjacently. For instance, in case of eight (8) memory cells, there is the same sequential arrangement of bit line pairs as in FIG. 5. In other words, the bit line pairs are arranged in parallel to the Y-axis, in a sequence of BL 1 , BLB 1 , BLB 2 , BL 2 , BL 3 , BLB 3 , . . . , BL 7 , BLB 7 , BLB 8 , BL 8 . Section data line pairs  71 , which are identical first metal lines neighboring the bit line BL 8  extending in parallel to the bit lines. FIG. 5 illustrates the first transistor connected to the first bit line BL 1 . There are sources  20 , gates  10  and drains  30  in the first transistor. Though not shown in FIG. 5, the gates  10  of the first transistor are connected with those of the second transistor to form gate electrodes. In drains  30 , non-shared (i.e. not connected in common) active regions of the first transistor are connected to the first contact line  80 - 1 , so as to contact the first bit lines BL 1  through the first contact  40 - 1 . In sources  20 , shared active regions of the first transistor are connected to the second contact line  80 - 2 , so as to contact the section data lines SDL_W through the second contact  40 - 2 . A number of the third contacts  8  are formed between the sources  20  and the second contact lines  80 - 2  to reduce loading.  
         [0045]    Similarly, a number of the third contacts  8  are formed between the drains  30  (not shown in FIG. 5 but shown in FIG. 6) and the first contact lines  80 - 1  to reduce loading. On the other hand, edges of the non-shared active region of the first transistors, drains  30 , may be formed in a proper shape, for instance, in a serrated shape as shown to reduce the junction area of bit lines. As shown in FIG. 6, edges of the drains, non-shared active regions of the next neighboring transistors, where they are in contact together, are formed in the serrated shape to be in a toothed (separated) mesh. A number of the first contact lines  80 - 1  and the third contacts  8  are formed at the teeth of the serrated active regions. The layout structure thus constructed is advantageous in reducing capacitance and minimizing the required layout area.  
         [0046]    Now, high integration of memory cells will be considered. Above all, it should be considered in what type of a layout structure is available for the first and second transistors in the size of the total memory cells that can reduce the layout area for the column path transistors. The first method is to make a multi-row layout structure of the first and second transistors arranged in perpendicular to the bit line pairs of a predetermined length within the size of memory cells. The second method is to make a matrix (multi-row and multi-column) layout structure of a predetermined number of the first and second transistors, arranged in perpendicular to the bit line pairs, of a predetermined length within the size of memory cells. (Refer to the layout structures shown in FIGS. 8 and 9.) For convenience, the first and second transistors, respectively related with bit lines BL and bit line bars BLB, can be arranged in the order of top/bottom or bottom/top.  
         [0047]    On the other hand, a predetermined size of spaces are formed toward both columns of a predetermined number of or the total number of the memory cells for gate signals Yi in the layout structure of the first and second transistors. (Refer to the layout structure shown in FIGS. 8 and 9.)  
         [0048]    Furthermore, the layout structure of the read path column pass transistors that have not been described in the first embodiment of the present invention can be arranged in the identical structure over or under the aforementioned layout structure of the write path column pass transistors. As shown in the prior art, the PMOD transistors for bit lines BL or bit line bars BLB may be also arranged in a layout structure in parallel to the bit line pairs.  
         [0049]    The layout structure of the read or write path column pass transistors (NMOS and PMOS transistors) applied to the present invention can be arranged as follows in Table 1.  
                                                         TABLE 1                                   Case 1   Case 2   Case 3   Case 4                                    NMOS   Top, vertical   Bottom,   Top, vertical   Bottom,           layout   vertical layout   layout   vertical layout       PMOS   Bottom,   Top, vertical   Bottom,   Top,           vertical layout   layout   horizontal   horizontal                   layout   layout                  
 
         [0050]    In case, the write path NMOS transistors and the read path PMOS transistors are respectively arranged at the top and bottom in the layout structure of the column path transistors of the memory cell interface, so that all the NMOS and PMOS transistors are arranged in perpendicular to the bit line pairs. In case 2, the write path NMOS transistors and the read path PMOS transistors are respectively arranged at the bottom and top parts in the layout structure of the column path transistors of the memory cell interface, so that all the NMOS and PMOS transistors are arranged in perpendicular to the bit line pairs. In case 3, the write path NMOS transistors and the read path PMOS transistors are respectively arranged at the top and bottom parts in the layout structure of the column path transistors of the memory cell interface, so that the NMOS transistors are arranged in perpendicular to the bit line pairs, but the PMOS transistors are arranged in parallel to the bit line pairs. In case 4, the write path NMOS transistors and the read path PMOS transistors are respectively arranged at the bottom and top parts in the layout structure of the column path transistors of the memory cell interface, so that the NMOS transistors are arranged in perpendicular to the bit line pairs, but the PMOS transistors are arranged in parallel to the bit line pairs. As shown in the first embodiment of the present invention, the layout structure of the column pass transistors connected with memory cells is formed to accomplish the aforementioned objects of the present invention. Another embodiment of the present invention will be described with reference to FIGS. 8 and 9.  
         [0051]    [0051]FIG. 8 is a top plan view illustrating the layout structure of column pass transistors applied to sixteen memory cells of a semiconductor memory device in accordance with the second embodiment of the present invention. FIG. 9 (specifically FIGS. 9 a  through  9   d ) is a top plan view illustrating the layout structure of column pass transistors applied to thirty-two memory cells of an SRAM in accordance with the third embodiment of the present invention. In the layout structure shown in FIGS. 8 and 9, only write path NMOS transistors connected with memory cells (as well as read path column pass transistors) are illustrated, but PMOS transistors can also be constructed in the same cases as shown in Table 1. FIG. 8 illustrates an embodiment of the layout structure of column pass transistors connected with sixteen column memory cells, wherein sixteen transistors for column memory cells are arranged in two columns with the bit line pairs connected with memory cells. Section data line pairs are arranged in parallel to the bit line pairs in the commonly arranged active regions of thirty-two transistors between the eighth and ninth memory cells MC 8 , MC 9 . All the transistors arranged in the same row are connected as one. Also, thirty-two non-shared active regions are respectively connected with bit lines BL or bit line bars BLB respectively belonging to their own areas. Spaces of a predetermined size are formed for gate signals Y 1 -Y 16  of transistors at both sides of the total sixteen column memory cells. The layout structure shown in FIG. 8 is similarly formed to that of the first embodiment of the present invention described above, so that repeated descriptions will be omitted. It is preferable that the contact lines should be arranged to make a short contact between the active regions and bit lines or section data lines. On the other hand, dummy cell arrays are positioned between the layout space of transistors and memory cells.  
         [0052]    [0052]FIG. 9 illustrates an embodiment of the layout structure of NMOS column pass transistors connected with thirty-two column memory cells. In FIG. 9, the section data line pairs are arranged in parallel to the bit lines in each space.  
         [0053]    Having described specific preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope of spirit of the invention as defined in the appended claims.  
         [0054]    As described above, the invention discloses a new layout structure and method for efficiently arranging column pass transistors within a space to be occupied with memory cells, the overall structure representing a far more efficient use of space than in the prior art.  
         [0055]    Also, the layout structure of the present invention is advantageous in reducing its own area of column pass transistors.  
         [0056]    In addition, there is an advantage in the present invention in that the section data lines commonly connecting bit lines and input/outputs may be made of the same material to reduce loading therein.  
         [0057]    The invention makes an essential contribution to minimizing the size of a chip in a semiconductor memory device.