Abstract:
In a semiconductor memory device, a plurality of straight word lines are arranged in parallel with each other, and a plurality of stepwise bit lines are arranged approximately perpendicular to the word lines. A plurality of memory cells of a one-transistor, one-capacitor type are connected between the word lines and the bit lines.

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 08/496,417, filed on Jun. 29, 1995, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor memory device, and more particularly, to a dynamic random access memory (DRAM) device having memory cells of a one-transistor, one-capacitor type. 
     2. Description of the Related Art 
     Generally, a DRAM device includes a plurality of word lines, a plurality of bit lines, and a plurality of memory cells each connected to one of the word lines and one of the bit lines. In this case, the memory cells are of a one-transistor, one-capacitor type. 
     In order to enhance the integration, in a prior art DRAM device (see: JP-A-HEI4-279055), the bit lines are sloped in relation to the word lines. This will be explained later in detail. 
     In the above-described prior art DRAM device, however, the integration is still low. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to further enhance the integration of a DRAM device. 
     According to the present invention, in a semiconductor memory device, a plurality of straight word lines are arranged in parallel with each other, and a plurality of stepwise bit lines are arranged approximately perpendicular to the word lines. A plurality of memory cells of a one-transistor, one-capacitor type are connected between the word lines and the bit lines. 
     The stepwise bit line configuration enhances the integration. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood from the description as set forth below, as compared with the prior art, with reference to the accompanying drawings, wherein: 
     FIG. 1 is a plan view illustrating a prior art DRAM device; 
     FIG. 2 is an equivalent circuit diagram of the device of FIG. 1; 
     FIGS. 3A and 3B are diagrams illustrating an area per one memory cell of the device of FIG. 1; 
     FIG. 4 is an enlarged partial plan view of the device of FIG. 1; 
     FIG. 5 is a plan view illustrating a first embodiment of the DRAM device according to the present invention; 
     FIG. 6 is an equivalent circuit diagram of the DRAM device of FIG. 5; 
     FIGS. 7A through 7E are cross-sectional views for explaining a method for manufacturing the device of FIG. 5; 
     FIG. 8 is a plan view illustrating a second embodiment of the DRAM device according to the present invention; 
     FIG. 9 is an equivalent circuit diagram of the DRAM device of FIG. 8; 
     FIG. 10A is a first layout diagram illustrating an entire DRAM including the memory cell array of FIG. 5 or 8; 
     FIG. 10B is a diagram illustrating the bit lines of FIG. 10A; 
     FIG. 11A is a second layout diagram illustrating an entire DRAM including the memory cell array of FIG. 5 or 8; 
     FIG. 11B is a diagram illustrating the bit lines of FIG. 11A; 
     FIG. 12A is a third layout diagram illustrating an entire DRAM including the memory cell array of FIG. 5 or 8; and 
     FIG. 12B is a diagram illustrating the bit lines of FIG. 11A. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before the description of the preferred embodiments, a prior art DRAM device will be explained with reference to FIGS. 1, 2, 3 and 4 (see: FIG. 5 of JP-A-HEI4-279055). 
     In FIG. 1, which is a plan view of the prior art DRAM device, a plurality of word lines WL 1 , WL 2 , . . . made of polycrystalline silicon are arranged in parallel with each other along a Y direction, while a plurality of bit lines BL 1 , BL 2 , . . . made of tungsten silicide are arranged in parallel with each other but are sloped in relation to an X direction. 
     Also, an element forming region R is sloped in the X direction and the Y direction. The element forming region R has two source regions connected to two capacitors (shown not in FIG. 1, but in FIG. 2), a drain region connected via a contact hole CONT to one of the bit lines such as BL 1 , and two channel regions over which two of the word lines such as WL 4  and WL 5  are located. That is, two memory cells are formed in one element forming region R. 
     As illustrated in FIG. 2, which illustrates an equivalent circuit diagram of the DRAM device of FIG. 1, a memory cell CL ij  (i, j=1, 2, . . . ) is provided at an intersection between one of the word lines WL 1 , WL 2 , . . . and one of the bit lines BL 1 , BL 2 , . . . . In this case, the memory cells CL 11 , CL 13 , . . . are provided for every two word lines and for every two bit lines. 
     Also, the bit lines BL 1 , BL 2 , . . . are connected to sense amplifiers SA 1 , SA 2 , . . . , respectively. Also, bit lines BL 1  &#39;, BL 2  &#39;, . . . are connected to the sense amplifiers SA 1 , SA 2 , . . . . Further, memory cells are CL 11  &#39;, CL 13  &#39;, . . . provided at intersections between the bit lines BL 1  &#39;, BL 2  &#39;, . . . and word lines BL 1  &#39;, BL 2  &#39;, . . . . 
     Thus, the DRAM device as illustrated in FIGS. 1 and 2 can be of an open bit line type. 
     A parallelogram area S per one memory cell is calculated as follows. As illustrated in FIG. 3A, a height H of the parallelogram is represented by 
     
         H=H1+H2+H3                                                 (1) 
    
     where 
     H1 is half of a width of one isolation region; 
     H2 is a width of one element forming region R; and 
     H3 is half of the width of one isolation region. 
     If the width of the isolation region and the element forming region R are both a minimum length f, then the formula (1) is replaced by ##EQU1## 
     Also, as illustrated in FIG. 3A, a width W of the parallelogram is represented by 
     
         W=W1+W2+W3+W4                                              (3) 
    
     where 
     W1 is half of the width of one word line; 
     W2 is the width of one capacitor contact hole forming region; 
     W3 is a width of one word line; and 
     W4 is a half of a width of a contact region. 
     If the width of one word line is the minimum length f, and a sum of W2 and W4 is the minimum length f, the formula (3) is replaced by ##EQU2## 
     Therefore, for the parallelogram illustrated in FIG. 3B, the area S thereof is 
     
         S=6f.sup.2 / sin θ                                   (5) 
    
     where θ is an acute angle of the parallelogram. 
     The minimum value of the area S will be explained next with reference to FIG. 4. That is, if a pitch of the word lines is 2f, 
     
         2f(cot θ+cot φ) sin θ=f                    (6) 
    
     
         2f(cot θ+cot φ) sin φ≧f               (7) 
    
     Therefore, from the formula (6), ##EQU3## 
     Also, from the formula (7), ##EQU4## 
     Thus, when cos θ=1/4, i.e., θ=75.5°, the area S is ##EQU5## In other words, when an angle between the bit lines and the element forming regions is about 29°, the integration is maximum. 
     In FIG. 5, which illustrates a first embodiment of the present invention, the bit lines BL 1 , BL 2 , . . . are arranged stepwise along the X direction. In more detail, each of the bit lines BL 1 , BL 2 , . . . includes first portions P1 including the contact hole CONT in parallel with the Y direction and second portions P2 in parallel with the X direction. Also, the element forming region R is in parallel with the X direction. 
     As illustrated in FIG. 6, which illustrates an equivalent circuit diagram of the DRAM device of FIG. 5, the bit lines BL 1 , BL 2 , . . . and the bit lines BL 1  &#39;, BL 2  &#39;, . . . are arranged stepwise. 
     In FIG. 5, an area S&#39; per one memory cell is rectangular. Therefore, the rectangular area S&#39; is calculated by putting a condition θ=90° into the formula (5): 
     
         S&#39;=6f.sup.2                                                (10) 
    
     Thus, the integration of the DRAM device of FIG. 5 can be enhanced as compared with that of the DRAM device of FIG. 1. 
     The method for manufacturing the DRAM device of FIG. 5 will be explained next with reference to FIGS. 7A through 7E. 
     First, referring to FIG. 7A, a local oxidation of silicon (LOCOS) is performed upon a P-type monocrystalline silicon substrate 1 with a mask of a silicon nitride layer (not shown), to create a thick silicon oxide layer 2. Then, a gate silicon oxide layer 3 is grown by thermally oxidizing the silicon substrate 1. 
     Next, referring to FIG. 7B, an N-type impurity doped polycrystalline silicon layer is deposited by a chemical vapor deposition (CVD) process and is patterned to form gate electrodes 4 (i.e., the word lines WL 1  and WL 2 ). Then, N-type impurities are doped into the silicon substrate 1 with a mask of the gate electrodes 4 and the thick silicon oxide layer 2, to create N-type impurity regions 5 (i.e., the drain region D and the source regions S) within the silicon substrate 1. Then, a silicon oxide layer 6 is deposited on the entire surface by a CVD process. 
     Next, referring to FIG. 7C, a contact hole 7 is perforated in the silicon oxide layer 6 by a photolithography and dry etching process. Then, tungsten silicide is deposited, and thereafter, the tungsten silicide is patterned and is left in the contact hole 7, thus forming a tungsten silicide layer 8 (i.e., the bit line BL 1 ). Then, a silicon oxide layer 9 is deposited on the entire surface by a CVD process. 
     In FIG. 7C, note that the tungsten silicide layer 8 (the bit line BL 1 ) is arranged in parallel with the gate electrodes 4 (the word lines WL 1  and WL 2 ), and therefore, the bit line BL 1  is not superposed onto the N-type impurity regions 5 (the source regions S). 
     Next, referring to FIG. 7D, a contact hole 10 is perforated in the silicon oxide layers 9 and 6 by a photohithography and dry etching process. Then, lower capacitor electrodes 11 made of N-type impurity doped polycrystalline silicon are formed by a CVD process and a photolithography and dry etching process. 
     Finally, referring to FIG. 7E, a capacitor insulating layer 12 made of silicon oxide is deposited by a CVD process. Then, a capacitor upper electrode 13 made of N-type impurity doped polycrystalline silicon is deposited by a CVD process. Thus, the capacitor comprising upper electrode 13, lower electrode 11, and insulation layer 12 is formed over the bit line 8 (BL1). Then, insulating layer and an aluminum connection layer (not shown) are formed to complete the device of FIG. 5. 
     In FIG. 8, which illustrates a second embodiment of the present invention, the bit lines BL 1 , BL 2 , . . . are also arranged stepwise along the X direction. In more detail, each of the bit lines BL 1 , BL 2 , . . . includes first portions P1&#39; including the contact hole CONT sloped in the Y direction and second portions P2&#39; in parallel with the X direction. Also, the element forming region R is in parallel with the X direction. 
     As illustrated in FIG. 9, which illustrates an equivalent circuit diagram of the DRAM device of FIG. 8, the bit lines BL 1 , BL 2 , . . . and the bit lines BL 1  &#39;, BL 2  &#39;, . . . are arranged stepwise with the sloped portions P1&#39;. 
     In FIG. 8, an area S&#34; per one memory cell is also rectangular. Therefore, the rectangular area S&#34; is also calculated by putting a condition θ=90° into the formula (5): 
     
         S&#34;=6f.sup.2                                                (11) 
    
     Thus, the integration of the DRAM device of FIG. 8 can be also enhanced as compared with that of the DRAM device of FIG. 1. 
     The manufacturing method of the device of FIG. 8 is similar to that of the device of FIG. 5. 
     In the second embodiment as illustrated in FIGS. 8 and 9, the bit lines can be reduced in length as compared with those of the first embodiment as illustrated in FIGS. 5 and 6. Therefore, the resistance of the bit lines is reduced. Also, since the space between the bit lines can be large, the capacitance therebetween can be reduced to enhance the resistance characteristics against noise. 
     In FIG. 10A, which is a layout diagram of an entire device including the memory cell array of FIG. 5 or 8, reference A1 designates an area of the memory cell array of FIG. 5 or 8, A2 designates an area of the sense amplifiers SA 1 , SA 2 , . . . of FIG. 6 or 9, and A3 designates a peripheral area. In this case, the bit lines BL 1 , BL 2 , . . . are arranged stepwise as illustrated in FIG. 10B. Therefore, since the area A1 is parallelogram, the area A3 is relatively large. Since this area A3 is used for peripheral circuits, the area A3 is preferably as small as possible. 
     In order to reduce the area A3, the bit lines BL 1 , BL 2 , . . . are made zigzag, i.e., staggered. For example, as illustrated in FIGS. 11A and 11B, each of the memory cell arrays A1 is divided into two blocks. Also, as illustrated in FIGS. 12A and 12B, each of the memory cell arrays A1 is divided into four blocks to further reduce the area A3. However, when the number of divided blocks is increased, space between the blocks is increased thus reducing the integration. Therefore, the number of divided blocks is selected at an optimum value in consideration of the area A3 and the space between the divided blocks. 
     As explained hereinbefore, according to the present invention, since an area per one memory cell is reduced, the integration of the device can be enhanced.