Abstract:
A plurality of cells include standard circuits having no power source line and no ground line and having signal terminals supplied with signals and power source terminals connected to a power source. Each of the cells is rotated in a desired direction and placed so as to reduce to a minimum the distance between the terminals of said cells and the corresponding terminals of another cell connected to those of the former cell and the cells are arranged in contact with each other. A plurality of wiring layers are arranged to connect the signal terminals of the cells together and the power source terminals thereof together. The wiring layers are insulated from the cells and placed above the cells.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a standard cell type semiconductor integrated circuit in which an LSI having desired functions is designed by combining standard cells such as inverter circuits, AND circuits and OR circuits, and particularly to the placement of a plurality of cells and the wiring between the cells. 
     2. Description of the Related Art 
     Recently, a poly-cell layout system in which a function block having a desired function is constructed by combining standard cells such as inverter circuits, AND circuits and OR circuits by use of the computer-aided design (CAD) has been developed. 
     FIG. 9 shows the poly-cell layout. In this layout, a plurality of standard cells (which are hereinafter simply referred to as cells) 12a, 12b, - - -, 12e are placed in an X direction to form a cell row 13a on a substrate 11. A power source line 12 1 , ground line 12 2  and signal terminals 12 3  are provided for each of the cells 12a, 12b, - - -, 12e, and the power source lines 12 1  and ground lines 12 2  of the cells are shown to be automatically connected in a continuous form when the cells 12a, 12b, - - -, 12e are placed. Likewise, on the substrate 11, cell rows 13b and 13c are paced in positions apart from the cell row 13a by preset distances in a y direction. The cell row 13b is constructed by cells 12f, 12g, - - -, 12j and the cell row 13c is constructed by cells 12k, 12l, - - -, 12o. Also, a power source line 12 1 , ground line 12 2  and signal terminals 12 3  are previously provided for each of the cells 12 f  to 12o. 
     An area between the cell rows 13a and 13b is defined as a first wiring area 14a and an area between the cell rows 13b and 13c is defined as a second wiring area 14b. The respective cells are interconnected by use of the first and second wiring areas 14a and 14b. 
     FIG. 10 shows interconnections between the cells 12a to 12o and between the cell rows 13a to 13c. That is, first layer metal wirings 15 indicated by broken lines are formed in the first and second wiring areas 14a and 14b. An insulative layer (not shown) is formed on the first metal wiring layers 15, and second layer metal wirings 17 indicated by solid lines are formed on the insulative layer. The first layer metal wirings 15 are selectively connected to the second layer metal wirings 17 through respective through holes 16 formed on the insulative layer. The second layer metal wirings 17 are connected to the signal terminals 12 3  formed on the respective cells 12. Further, the power source lines 12 1  of the cell rows 13a to 13c are connected together in the peripheral area of the substrate 11 and the ground lines 12 2  thereof are also connected together in the peripheral area. 
     FIG. 11 shows an example of the cell 12 used in the poly-cell layout, and portions which are the same as those of FIGS. 9 and 10 are denoted by the same reference numerals. 
     In the cell 12, the positions of the power source line 12 1  and the ground line 12 2  are predetermined and the signal terminals 12 3  are arranged on an area excluding the power source line 12 1  and ground line 12 2 . In FIG. 11, 91 denotes diffused layers, 92 polysilicon layers, 93 a buried contact, and 94 a position in which impurity ions for forming a depletion type transistor are implanted. 
     As shown in FIGS. 9 and 10, useless areas may occur in the wiring areas between the cell rows when higher cells and lower cells are placed. 
     Further, it is necessary to connect the power source lines which are previously provided for the respective cells together and the ground lines which are previously provided for the respective cells together, the rotation of the cells is limited by the power source lines and ground lines. Therefore, when the cells are connected to each other, some wiring lines must take a roundabout way and it is difficult to connect the cells to each other in the shortest way. 
     In principle, the widths of the power source line and ground line previously provided for each of the cells may be sufficient only if the power source line and ground line can pass a dissipating current of the cell. That is, in a case of the cell row 13a shown in FIGS. 9 and 10, the widths of the power source line and ground line of the cell 12e can be made smaller than those of the cell 12a when a current is supplied from the side of the cell 12a. However, the widths of the power source line and ground line of each cell are so determined that the power source line and ground line can pass a current for the whole portion of one cell row. Therefore, areas occupied by the power source line and ground line are large and it is difficult to reduce the size of the cell itself. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide a semiconductor integrated circuit in which useless areas in the cell and between the cells can be reduced and the length of wirings for connecting the cells to each other can be reduced. 
     The above object can be attained by a semiconductor integrated circuit including a plurality of cells having standard circuits constructed by semiconductor elements to constitute a desired circuit, comprising: 
     a plurality of cells including standard circuits having signal terminals supplied with signals and power source terminals connected to a power source, each of the cells being rotated in a desired direction and disposed so as to reduce the distance between the terminals of said cells and the corresponding terminals of another cell connected to those of the former cell to a minimum; and 
     a plurality of wiring layers for selectively connecting the signal terminals together and the power source terminals together, the wiring layers being insulated from the cells and formed above the cells. 
     According to preferred embodiments of this invention, the cell having the signal terminals and power source terminal is rotated in a desired direction and placed to reduce the distance between the signal terminals and power source terminal thereof and the signal terminals and power source terminal of another cell connected to those of the former cell to a minimum. Thus, unlike the conventional case, since it is not necessary to place their cells with the power source lines aligned, cells of different shapes can be placed in desired directions and useless areas can be reduced. 
     Further, since the lengths of the wirings for connecting the cells to each other can be reduced, the circuit operation speed can be enhanced. 
     In addition, the signal terminals are connected to one another by means of a plurality of wiring layers for connecting the cells with one another and the power source terminals are connected together by the wiring layers. Therefore, the conventional wiring areas are not necessary and useless areas can be reduced. 
     Each cell has signal terminals for receiving and outputting signals, and power source terminals connected to power sources but has no power source lines. Therefore, the size of the cell itself can be reduced. 
     Further, since the power source terminals are connected to each other by means of the wiring layer, the widths of the wiring layers connecting the power source terminals to each other can be determined according to the amount of current. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a construction view showing a first embodiment of this invention; 
     FIG. 2 is a construction view showing a structure obtained by forming wiring layers on the structure of FIG. 1; 
     FIG. 3 is a cross sectional view taken along the line 3--3 of FIG. 2; 
     FIG. 4 is a layout diagram showing an example of a standard cell applied to this invention; 
     FIG. 5 is a construction view showing a second embodiment of this invention; 
     FIG. 6 is a cross sectional view taken along the line 6--6 of FIG. 5; 
     FIG. 7 is a diagram showing the modifications of the cell, 
     FIG. 8 is a construction view showing a structure formed by arranging the cells having the same construction as that of FIG. 7; 
     FIG. 9 is a construction view showing the conventional cell layout system; 
     FIG. 10 is a construction view showing a structure obtained by forming wiring layers on the structure of FIG. 9; and 
     FIG. 11 is a layout diagram showing an example of the conventional standard cell. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     There will now be described an embodiment of this invention with reference to the accompanying drawings. 
     FIG. 1 shows a cell arrangement of this invention. The cell arrangement can be attained by arranging cells having the same shapes as those of FIGS. 9 and 10, and the cells of the same shapes are denoted by the same reference numerals. 
     In FIG. 1, the cell position and the direction of the cell arrangement of the cell row 13a are the same as those of the cell row 13a shown in FIGS. 9 and 10. However, in FIG. 1, the cells 12f to 12j constituting the cell row 13b are placed in contact with the cells 12a to 12e constituting the cell row 13a, respectively. In this case, the cell 12g is partly surrounded by the cells 12a, 12b and 12c, and the cell 12i is partly surrounded by the cells 12c, 12d and 12e. Further, the cells 12f and 12h are rotated by 90° in a clockwise direction with respect to those shown in FIGS. 9 and 10. 
     Likewise, the cell row 13c is placed in contact with the cell row 13b. The cell 12l is partly surrounded by the cells 12f, 12g and 12m and the cell 12m is rotated by 90° in a clockwise direction. Further, the cell 12o is rotated by 90° in a counter clockwise direction and placed in contact with the cells 12i and 12j. 
     The rotation direction of the cell is determined according to the length of a wiring for connecting the cells to each other and the cell is rotated in such a direction as to reduce the wiring length to a minimum. The algorithm for determining the direction of the cell rotation is well known in the CAD and therefore the explanation thereof is omitted. 
     FIGS. 2 and 3 show a structure obtained by forming wirings on the cell array of FIG. 1. A first insulative layer 24 is formed on the cells 12a to 12o and first layer metal wirings 21 indicated by broken lines are formed on the first insulative layer 24. The first layer metal wirings 21 are selectively connected to signal terminals 12 3  indicated by a □ mark, ground terminals 12 4  indicated by a X mark, or power source terminals 12 5  indicated by a symbol of a combination of the □ mark and X mark. 
     A second insulative layer 25 is formed on the first metal wiring layer 21 and the first insulative layer 24 and through holes 22 indicated by a o mark are formed in the second insulative layer 25. Second layer metal wirings 23 indicated by solid lines are formed on the second insulative layer 25 and the second layer metal wirings 25 are selectively connected to the first layer metal wirings 21 via the through holes 22. The second layer metal wirings 23 are selectively connected to the signal terminals 12 3  and the like formed on the respective cells 12a to 12 o . That is, the signal terminals 12 3 , ground terminals 12 4  and power source terminals 12 5  permit the first layer metal wirings 21 and the second layer metal wirings 23 to be connected to each other. 
     FIG. 4 shows an example of the cell used in the above embodiment. The cell can be obtained by omitting the power source terminal 12 1  and ground terminal 12 2  from the cell of FIG. 11 and partly modifying the cell of FIG. 11, and a power source line contact hole 31 and a ground line contact hole 32 are formed in the cell. The positions of the signal terminals 12 3 , power source line contact hole 31 and ground line contact hole 32 can be changed as required. 
     According to the above embodiment, the cell rows 13a, 13b and 13c are placed in contact with each other. Therefore, the wiring area can be reduced in comparison with that of the conventional case. Further, unlike the conventional case, the arrangement of the cells is not limited by the power source line and ground line and the cells can be rotated in a desired direction and placed in the rotated position. Therefore, the area of the function block constituted by a plurality of cells can be reduced. 
     The area may be dependent on the construction of the function block, but in the case of the construction of FIGS. 1 and 2, the area can be reduced by 30% in comparison with the construction of FIGS. 9 and 10. Therefore, function blocks of an amount increased by 30% can be taken out from the same wafer, the manufacturing yield can be enhanced and the manufacturing cost can be reduced by substantially 30% or more. 
     Further, the cell rows 13a, 13b and 13c can be arranged in contact with each other and the cells can be rotated and placed in the rotated position so as to reduce the wiring distance to a minimum. Since the operation speed of a transistor is limited by the capacitance of the wiring, the operation speed of the transistor can be enhanced by reducing the wiring length. 
     In the above embodiment, the wiring length can be reduced by 20% in comparison with that of the conventional case. Accordingly, the operation speed of the wiring can be enhanced by approx. 20%. 
     Since each cell has no power source line and no ground line, the area of the cell itself can be reduced. In FIG. 4, the structure which can be obtained by omitting the power source line and ground line from the structure of FIG. 11 is shown, but the area of each cell can be further reduced by designing the cell itself according to this invention. 
     Further, in this embodiment, the power source line and ground line are not previously provided for each cell and the power source line and ground line can be freely formed with the shortest wiring distance. Recently, the CAD performance is enhanced and the width of the wiring can be changed according to the amount of current. Therefore, in the function block, the area occupied by the power source line and ground line can be significantly reduced in comparison with that of the conventional case. 
     Next, a second embodiment of this invention is explained. FIGS. 5 and 6 show the second embodiment and portions which are the same as those of FIGS. 1 and 2 are denoted by the same reference numerals. 
     In the first embodiment, the first and second layer metal wirings 21 and 23 are used, but in the second embodiment, third layer metal wirings 41 indicated by one-dot-dash lines are additionally used. That is, as shown in FIGS. 5 and 6, a third insulative layer 26 is formed on the second metal layers 23 and second insulative layer 25 and the third layer metal wirings 41 are formed on the third insulative layer 26. The third layer metal wirings 41 are selectively connected to the first and second layer metal wirings 21 and 23 via through holes 22 formed in the third insulative layer 26 or through holes 22 formed in the second and third insulative layers 25 and 26. Thus, the entire wiring can be attained above the cell rows by using the third layer metal wirings 41. Therefore, the area can be further reduced. 
     In the above embodiments, the cell is formed in a rectangular form, but it is not limited to the rectangular form. For example, as shown in FIG. 7, cells 12p and 12q can be respectively formed in &#34;letter-L&#34; and &#34;letter-T&#34; shapes and placed as shown in FIG. 8 to attain the same effect as in the above embodiments. 
     Further, this invention can be variously modified without departing from the technical scope thereof.