Patent Publication Number: US-6219271-B1

Title: Semiconductor memory device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor memory device in which a memory cell has a CMOS (Complementary Metal Oxide Semiconductor) configuration, such as a SRAM (Static Random Access Memory) cell with a six-transistor configuration. 
     2. Description of the Related Art 
     A SRAM cell generally has a latch and two transistors (word transistors). On-off operations of the transistors are controlled based on the voltage applied to a word line and thereby connection between each of two memory nodes of the latch and a bit line is made or broken. SRAM cells can be broadly divided into two types, namely, a MOS transistor load type and a high resistance load type; where the difference is based on a load element of the latch. SRAM cells of the MOS transistor load type, configured with six transistors, can be further broken down into two known types: a P-channel MOS transistor (called pMOS in the followings) load type and a TFT (Thin Film Transistor) load type, depending on the type of its load transistor. 
     FIG. 5 shows an example of a configuration pattern of a SRAM cell of the pMOS load type according to the related art. In FIG. 5, a SRAM cell is shown provided with a gate of the transistor. Wire connection inside the cell or upper wiring layers such as bit lines are omitted. Instead, FIG. 5 discloses the connection between portions effected by the upper wiring layers together with the pattern diagram. 
     The SRAM cell  100  of the pMOS load type has two p-type active regions  101   a  and  101   b,  and two n-type MOS active regions  102   a  and  102   b.  In the p-type active regions  101   a  and  101   b,  an n-channel MOS transistor (called nMOS in the followings) as a drive transistor is formed. In the n-type active regions  102   a  and  102   b,  a p-channel MOS transistor (called pMOS in the followings) as a load transistor is formed. The p-type active regions  101   a  and  101   b,  and the n-type active regions  102   a  and  102   b  are surrounded by an element separation insulating region of LOCOS (Local Oxidation of Silicon) or trench construction, for example. 
     In the SRAM cell  100  of the related art, the two p-type active regions  101   a  and  101   b  are turned outward approximately at a right angle in plan configuration. In the p-type active region  101   a,  a drive transistor Qn 1  and a word transistor Qn 3  are formed on both turned ends, sandwiching the bend. In the p-type active region  101   b,  a drive transistor Qn 2  and a word transistor Qn 4  are formed on both turned ends, sandwiching the bend. A word line (WL)  104 , which also works as gate electrodes of the word transistors Qn 3  and Qn 4 , is substantially orthogonal to both of the p-type active regions  101   a  and  101   b.  The word line  104  is provided through cells in the horizontal direction as shown in FIG.  5 . On the other hand, common gate lines  103   a  and  103   b,  which work as gate electrodes of the drive transistors Qn 1  and Qn 2 , are provided to each cell separately. The common gate line  103   a  is provided in the vertical direction as shown in FIG.  5  and orthogonal to the p-type active region  101   a.  The common gate line  103   b  is provided in the same direction and orthogonal to the p-type active region  101   b.    
     The common gate lines  103   a  and  103   b  are also orthogonal to the n-type active region  102   a  and  102   b,  respectively. Thereby, pMOS transistors (load transistors Qp 1  and Qp 2 ) are formed in the n-type active regions  102   a  and  102   b,  respectively. The load transistor Qp 1  and the drive transistor Qn 1  constitute a first inverter. The load transistor Qp 2  and the drive transistor Qn 2  constitute a second inverter. The first inverter and the second inverter constitute a latch. Each of the common gate lines  103   a  and  103   b  branches off at some mid point. As shown in the connection in FIG. 5, an input terminal of one inverter is connected to an output terminal of another inverter by a second wiring layer. Further, a Vcc (source voltage) supply line  105   a,  a Vss (common potential) supply line  105   b,  a bit line (BL 1 )  106   a  and a bit line (BL 2 )  106   b  are connected as shown. 
     Generally, in the SRAM cell with the above-described six-transistor configuration of the related art, DTw/WTw=1.0 and LTw/WTw=1.0, where DTw denotes a channel width of the drive transistors Qn 1  and Qn 2 , WTw denotes a channel width of the word transistors Qn 3  and Qn 4 , and LTw denotes a channel width of the load transistors Qp 1  and Qp 2 . In other words, the channel width of the drive transistors Qn 1  and Qn 2  and the channel width of the load transistors Qp 1  and Qp 2  are made to be equal to the channel width of the word transistors Qn 3  and Qn 4 . 
     However, in the case of a SRAM aimed for high-speed operations, a large cell current is required in order to minimize a delay in a bit line. As a result, an increase in the channel width WTw of the word transistors is needed. Thus, designing the SRAM cell while maintaining the above-described relationship, DTw/WTw=1.0 and LTw/WTw=1.0, causes an increase in, especially, the channel width LTw of the load transistors Qp 1  and Qp 2 , which are the only load transistors within the cell. This results in an increase in a cell size in a direction of the bit line, causing a problem in high integration. 
     The present invention is made in view of such a problem. An object of the invention is to provide a semiconductor memory device making it possible to reduce the size of memory cells and to achieve higher integration. 
     A semiconductor memory device according to the present invention comprises a plurality of memory cells, where each memory cell is provided with a pair of drive transistors of a first conductive type, a pair of load transistors of a second conductive type and a pair of word transistors of the first conductive type, wherein the relationship between a channel width DTw of the drive transistors and a channel width LTw of the load transistors is given by DTw/LTw&gt;1.0, more preferably, DTw/LTw&gt;1.4. 
     Particularly, the semiconductor memory device according to the present invention is suitable for a SRAM comprising a first active region in which a channel of the drive transistor and a channel of the word transistor are formed, a second active region in which a channel of the load transistor is formed, a word line which works as å gate electrode of the word transistor, and a common gate line which connects between a gate of the drive transistor and a gate of the load transistor, wherein a channel current direction of the drive transistor and a channel current direction of the word transistor are orthogonal to each other in the first active region, the word line is orthogonal to the first active region, and the common gate line is orthogonal both to the first active region and to the second active region. 
     In the semiconductor memory device according to the present invention, the channel width DTw of the drive transistor is made greater than the channel width LTw of the load transistor. Accordingly, the cell size can be reduced, as compared to the case with a semiconductor memory device of the related art with the relationship of DTw/LTw=1. 
     Other and Further objects, features and advantages of the invention will appear more fully from the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a pattern layout diagram of a SRAM cell of the pMOS load type according to a first embodiment of the invention. 
     FIG. 2 is a block diagram of a circuit of the SRAM cell shown in FIG.  1 . 
     FIG. 3 is a diagram for describing a relationship between DTw/LTw and a cell area of the SRAM cell shown in FIG. 1 
     FIG. 4 is a pattern layout diagram of an SRAM cell according to a second embodiment of the present invention. 
     FIG. 5 is a pattern layout diagram of a SRAM cell according to the related art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. 
     First Embodiment 
     First, a circuit configuration of a SRAM cell of the pMOS load type according to a first embodiment of the invention will be described with reference to FIG.  2 . 
     The SRAM cell of the pMOS load type, having a six-transistor configuration, includes n-channel MOS transistors (called nMOS in the followings) Qn 1  and Qn 2 , p-channel MOS transistors (called pMOS in the followings) Qp 1  and Qp 2 . The nMOS Qn 1  and Qn 2  work as drive transistors, while the pMOS Qp 1  and Qp 2  work as load transistors. The pMOS Qp 1  and Qp 2  load transistors, and the nMOS Qn 1  and Qn 2  drive transistors form two inverters (latch). The input terminals of the inverters are crossed over each other; an input terminal of one inverter is connected to an output terminal of another inverter, while an input terminal of the latter inverter is connected to an output terminal of the former inverter. 
     The nMOS transistors Qn 3  and Qn 4  are word transistors for controlling the connection of connecting points (memory nodes ND 1  and ND 2 ) of each inverter to bit lines BL 1  and BL 2  based on the voltage applied to word lines WL 1  and WL 2 . This cell configuration is common, and more detailed description of the connection is omitted. 
     In the SRAM cell of the pMOS load type, one bit line BL 1  is maintained at a high potential while a predetermined voltage is applied to the gate of the word transistors Qn 3  and Qn 4  through word lines WL 1  and WL 2 . Thereby, both transistors Qn 3  and Qn 4  are turned to ON to accumulate charge in the memory nodes ND 1  and ND 2 . The drive transistors Qn 1  and Qn 2 , and the load transistors Qp 1  and Qp 2  operate such that, when one memory node is “H (high),” another memory node is “L (low),” as is characteristic of a latch configuration. For example, when the memory node ND 1  is “H” and the memory node ND 2  is “L,” the drive transistor Qn 2  and the load transistor Qp 1  are turned to ON and the drive transistor Qn 1  and the load transistor Qp 2  are turned to OFF. As a result, the memory node ND 1  receives charge from a supply line of a source voltage Vcc, and the memory node ND 2  is continuously kept at the grounded potential. Conversely, if the memory node ND 1  is forced to “L” by turning the word transistor Qn 3  to ON when the bit line BL 1  potential is “L,” or if the memory node ND 2  is forced to “H” by turning the word transistor Qn 4  to ON when the bit line BL 2  potential is “H,” the drive transistors Qn 1  and Qn 2 , and the load transistors Qp 1  and Qp 2  are all inverted, and the memory node ND 2  receives charge from the supply line of the source voltage Vcc to keep the memory node ND 1  at the grounded potential. In this way, maintaining charge by the latch keeps charge in the memory nodes ND 1  and ND 2  statically. The potential of “L” or “H” is made to correspond to data of “0” or “1,” respectively, so that the data can be stored in six transistors within the cell. 
     Next, the pattern layout of a SRAM cell of a six-transistor type according to the first embodiment of the invention will be described with reference to FIG.  1 . The SRAM cell  10  includes p-type active regions  11   a  and  11   b  as a first conductive type region, and n-type active regions  12   a  and  12   b  as a second conductive type region. The p-type active regions  11   a  and  11   b,  and the n-type active regions  12   a  and  12   b  are surrounded by an element separation insulating region  16  of LOCOS or of trench construction, for example. 
     In the SRAM cell  10 , the p-type active regions  11   a  and  11   b  are turned inward in the form of an “L” in plan configuration. In this embodiment, the p-type active regions  11   a  and  11   b  are connected to each other. In the p-type active region  11   a,  a drive transistor Qn 1  and a word transistor Qn 3  are formed on both turned ends, sandwiching the bend. In the p-type active region  11   b,  a drive transistor Qn 2  and a word transistor Qn 4  are formed on both turned ends, sandwiching the bend. 
     A word line (WL)  14 , which works as a gate electrode of the word transistors Qn 3  and Qn 4 , is substantially orthogonal to both of the p-type active regions  11   a  and  11   b.  The word line  14  is provided through cells in the horizontal direction as shown in FIG.  1 . On the other hand, a common gate lines  13   a  and  13   b  (GL 1  and GL 2 ), which work as gate electrodes of the drive transistors Qn 1  and Qn 2 , are provided to each cell separately. The common gate line  13   a  is provided in the vertical direction as shown in FIG.  1  and orthogonal to the p-type active region  11   a.  The common gate line  13   b  is provided in the same direction and orthogonal to the p-type active region  11   b.    
     The common gate lines  13   a  and  13   b  are also orthogonal to the n-type active regions  12   a  and  12   b,  respectively. Thereby, a pMOS (load transistor Qp 1 ) is formed in the n-type active region  12   a  while a pMOS (load transistor Qp 2 ) is formed in the n-type active region  12   b.  The load transistor Qp 1  and the drive transistor Qn 1  constitute one inverter. The load transistor Qp 2  and the drive transistor Qn 2  constitute another inverter. Each of the p-type active regions  11   a  and  11   b  is electrically connected to a bit line (not shown) through first contacts  15   a  and  15   a,  respectively. Each of the p-type active regions  11   a  and  11   b  is also electrically connected to a supply line (not shown) of Vss (common potential) through a second contact  15   b.  The p-type active region  11   a  and the n-type active region  12   a  are electrically connected to each other through a third contact  15   c  and a fourth contact  15   d.  The p-type active region  11   b  and the n-type active region  12   b  are also electrically connected to each other through a third contact  15   c  and a fourth contact  15   d.  The n-type active regions  12   a  and  12   b  are commonly connected to a Vcc (source voltage) supply line (not shown) through a fifth contact  15   e.    
     The basic configuration of the SRAM cell  10  as described above is substantially the same as that of the SRAM cell  100  (FIG. 5) of the related art, except for the following relationships: 
     
       
           DTw/LTw&gt; 1.0  (1) 
       
     
     
       
           WTw/LTw&gt; 1.0  (2) 
       
     
     wherein DTw denotes a channel width of the drive transistors Qn 1  and Qn 2 , LTw denotes a channel width of the load transistors Qp 1  and Qp 2 , and WTw denotes a channel width of the word transistors Qn 3  and Qn 4 . 
     According to the embodiment, the channel width WTw of the word transistors Qn 3  and Qn 4  is, as described above, specified relatively larger than the channel width LTw of the load transistors Qp 1  and Qp 2  to minimize a delay in a bit line and to achieve speed-up. Further, the channel width DTw of the drive transistors Qn 1  and Qn 2  is specified relatively larger than the channel width LTw of the load transistors Qp 1  and Qp 2 . The reason why DTw is made greater than LTw will be described below. 
     Generally, when designing a SRAM cell with a six-transistor configuration, a cell current (I cell) and a static noise margin (called SNM in the followings) are items to be focused on as cell characteristics. A cell size of the SRAM is determined by the two items, the cell current and SNM. 
     Below is a list of combination examples, A-C, of the channel width (DTw, WTw, LTw) of each of the drive transistor, the word transistor and the load transistor, for obtaining a necessary and sufficient SNM (that is, SNM of 0.18 or more at a source voltage of 2.5V) at a certain cell current (I cell of 200 μA, for example). In other words, combinations of the channel width of the transistors for giving a solution of I cell=200 μA and SNM&gt;0.18V are, for example, as follows: 
     A. DTw=0.83 μm, WTw=0.83 μm, LTw=0.83 μm 
     B. DTw=0.96 μm, WTw=0.74 μm, LTw=0.48 μm 
     C. DTw=1.05 μm, WTw=0.70 μm, LTw=0.35 μm 
     Next, the relationship between the channel width (DTw, WTw, LTw) of the transistors and an SRAM cell area is as follows. In the SRAM cell shown in FIG. 1, denote a horizontal width of the element separation insulating region  16  between a horizontal edge portion of the cell and the p-type active region  11   a  by x 1 . Denote a horizontal width of the third contact  15   c  by x 2 . Denote a distance between the third contact  15   c  and the common gate line  13   a  by x 3 . Denote a horizontal width of the common gate line  13   a  by x 4 . Denote a distance between the second contact  15   b  and the common gate line  13   a  by x 5 . Further, denote a horizontal width of the second contact  15   b  by x 6 . Then, a width X of the SRAM cell  10  in the word line direction (the horizontal direction viewing the drawing) is given by the following equation: 
     
       
           X= ( x   1 ×½ +x   2   +x   3   +x   4   +x   5   +x   6 ×½)×2  (3) 
       
     
     To illustrate, assume that x 1 =0.20 μm, x 2 =0.20 μm, x 3 =0.11 μm, x 4 =0.18 μm, x 5 =0.11 μm, x 6 =0.20 μm, and that a transistor has the channel width as specified in A-C listed above. Then, X equals to 1.60 μm in each case. 
     On the other hand, denote a vertical width of the first contact  15   a  by y 1 . Denote a distance between the first contact  15   a  and a word line (WL)  14  by y 2 . Denote a width of the word line  14  by y 3 . Denote a distance between the word line  14  and the common gate line  13   a  (or  13   b ) by y 4 . Denote a length of a region protruded from the p-type active region  11   a  ( 11   b ) of the common gate line  13   a  (or  13   b ) by y 5 . Denote a vertical width of the drive transistor Qn 1  (or Qn 2 ) by y 6  (equals to DTw). Denote a distance between the drive transistor Qn 1  (or Qn 2 ) and the load transistor Qp 1  (or Qp 2 ), or a separation width between nMOS and pMOS, by y 7 . Denote a vertical width of the load transistor Qp 1  (or Qp 2 ) by y 8  (equals to LTw). Denote a length of a region protruded from the n-type active region  12   a  ( 12   b ) of the common gate line  13   a  (or  13   b ) by y 9 . Further, denote a vertical width of the element separation insulting region  16  between a bottom edge portion of the cell and the common gate line  13   a  (or  13   b ) by y 10 . Then, a cell length Y of the SRAM cell  10  in the bit line direction (in the vertical direction viewing the drawing) is given by a following equation: 
     
       
           Y=y   1 ×½ +y   2   +y   3   +y   4   +y   5   +y   6   +y   7   +y   8   +y   9   +y   10 ×½  (4) 
       
     
     To illustrate, assume that: y 1 =0.20 μm, y 2 =0.11 μm, y 3 =0.18 μm, y 4 =0.20 μm, y 5 =0.20 μm, y 6 =DTw, y 7 =0.50 μm, y 8 =LTw, y 9 =0.20 μm, y 10 =0.20 μm. Substituting these values into the equation (4), the following equation (5) is obtained: 
     
       
           Y =0.20×½+0.11+0.18+0.20+0.20 +DTw +0.50 +LTw +0.20+0.20×½(μm)= DTw+LTw +1.59(μm)  (5) 
       
     
     Consequently, a value of Y varies depending on values of DTw and LTw. 
     Accordingly, when using transistors of sizes as specified in A listed above, the cell width Y in the bit line direction is 3.24 μm. When using transistors of sizes as specified in B above, the cell width Y equals to 3.03 μm. When using transistors of sizes as specified in C above, the cell width Y equals to 2.99 μm. Therefore, the cell area (X·Y) is 5.18 μm 2  when using transistors of sizes as specified in A. The cell area (X·Y) is 4.85 μm 2  when using transistors of sizes as specified in B. The cell area (X·Y) is 4.78 μm 2  when using the transistors of sizes as specified in C. FIG. 3 shows the relationship between a ratio DTw/LTw of the channel width of the drive transistor to the channel width of the load transistor and the cell area in the cases with transistors of sizes as specified in either of A to C above. 
     As seen from FIG. 3, in the embodiment (cells of B and C above), the cell area can be reduced while achieving the cell current and the SNM equivalent to those of the SRAM cell of the related art by making the channel width DTw of the drive transistor greater than the channel width LTw of the load transistor. Further, the area can be relatively larger by increasing the channel width WTw of the word transistor. As a result, a delay in a bit line can be suppressed, and large cell current can be obtained. 
     Preferably, the ratio DTw/LTw of the channel width DTw of the drive transistor to the channel width LTw of the load transistor is preferably more than 1.4, more preferably more than 1.5, even more preferably more than 1.7 or more than 2.0. The rate of reduction in the cell area becomes bigger in this order named. 
     Second Embodiment 
     The present invention can be applied to an SRAM cell  20  with a pattern layout as shown in FIG.  4 . Since the basic pattern layout of the SRAM cell  20  is substantially the same as that of the SRAM cell  100  shown in FIG. 5, the same numerals are given to parts with the same functions as those of the first embodiment, and the specific description therefor is omitted. The SRAM cell  20  according to the embodiment has the relationship described in the equation (1) between the channel width DTw of drive transistors Qn 1  and Qn 2 , and the channel width LTw of load transistors Qp 1  and Qp 2 . The SRAM cell  20  also has the relationship described in the equation (2) between the channel width WTw of word transistors Qn 3  and Qn 4 , and the channel width LTw of the load transistors Qp 1  and Qp 2 . Similar to the SRAM cell  10  according to the first embodiment, the cell area can be reduced. In addition, a delay in a bit line can be suppressed to obtain a large cell current. 
     The present invention is not limited to the first and second embodiments described above but can be applied to an SRAM cell comprising an active region (a first active region) in which a channel of a drive transistor and a channel of a word transistor are formed, and another active region (a second active region) in which a channel of a load transistor is formed, a word line which works as a gate electrode of the word transistor, and a common gate line which connects between a gate of the drive transistor and a gate of the load transistor, wherein the word line is orthogonal to the first active region, and the common gate line is orthogonal both to the first active region and to the second active region. 
     As described above, according to the semiconductor memory device of the present invention, a cell size can be reduced and higher integration becomes possible since a channel width DTw of a drive transistor is made greater than a channel with LTw of a load transistor. 
     Further, especially, according to the semiconductor memory device of one aspect of the present invention, a delay in a bit line can be suppressed to obtain a large cell current since a channel width WTw of a word transistor is made greater than a channel width LTw of a load transistor. 
     Obviously many modification and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may bed practice otherwise than as specifically described.