Abstract:
A CMOS master slice having a plurality of regularly arranged basic cells improves an integration efficiency by optimizing size and arrangement of MOS transistors in the basic cells. Each of the basic cells comprises a first pair of transistors having gates thereof arranged to parallelly face each other, and a second pair of transistors having gate electrodes shorter in gate width than that of the first pair of transistors and parallel to the gate electrodes of the first pair of transistors. In adjacent basic cells, the gate electrodes of adjacent second transistors are substantially on a line so that a wasteful space is eliminated.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a master slice used in a gate array type LSI, and more particularly to a CMOS master slice in which a set of basic cells each comprising p-channel MOS transistors and n-channel MOS transistors are, arranged regularly. 
     2. Related Background Art 
     FIG. 1 shows a plan view of a basic cell used in a prior art gate spread type CMOS master slice. A basic cell 1 comprises a p-channel region 2 and an n-channel region 3. In the p-channel region 2, two p-channel large MOS transistors shown by gates 4 and 5, and two p-channel small MOS transistors shown by gates 6 and 7 are formed. In the n-channel region 3, two n-channel large MOS transistors shown by gates 8 and 9, and two n-channel small MOS transistors shown by gates 10 and 11 are formed. Numeral 12 denotes a p-well region formed on an n-type semiconductor substrate, and numerals 13-17 denote wiring tracks along the width of the gates. 
     At the beginning of the development of the gate array, it was a general practice that the gate widths of all MOS transistors in a basic cell are equal. However, since it is frequently advantageous in circuit performance the gate width is selectable depending on an application, MOS transistors having different gate widths are incorporated in one basic cell in a recent device. 
     In the basic cell, the basic MOS transistors 4, 5, 8 and 9 having the wider gate width are used as latch transistors, and n-channel additional MOS transistors 10 and 11 having the narrower gate width are used as path transistors to form one unit of a high performance SRAM memory circuit having a short access time. 
     However, where transistors having different gate widths are incorporated in one basic cell, wasteful spaces rae created on both sides of each of the pads of the narrow gate width transistors 6, 7, 10 and 11 and an integration efficiency is low. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a CMOS master slice having MOS transistors of different gate widths incorporated in a basic cell, which includes no wasteful space and offers a high integration efficiency. 
     It is a specific object of the present invention to provide a CMOS master slice having a basic cell comprising a group of p-channel MOS transistors and a group of n-channel MOS transistors, each of the transistor groups including a first pair transistors having gates thereof arranged to face parallelly to each other and second transistors disposed on both sides of the first transistor and having gate electrodes shorter in width than those of the first transistor and parallel to the gate electrodes of the first transistor, wherein the CMOS master slice include no wasteful space. 
     In order to achieve the above objects, in the CMOS master slice of the present invention, the transistors are arranged such that the gate electrodes of adjacent second transistors of adjacent basic cells are substantially on a line. 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a plan view of a basic cell used in a prior art gate spread type CMOS master slice, 
     FIG. 2 shows a plan view of a basic cell of a CMOS master slice in accordance with a first embodiment of the present invention, 
     FIG. 3 shows a circuit diagram of one cell of SRAM having wiring applied to the basic cell, 
     FIG. 4 shows a plan view of a basic cell of a CMOS master slice in accordance with a second embodiment of the present invention, 
     FIG. 5 shows a schematic plan view of a modification of the second embodiment in which the number of transistors is increased, and 
     FIG. 6 shows a plan view of a basic cell of CMOS master slice in accordance with a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown FIG. 2, a basic cell 21 comprises a p-channel region 22 and an adjacent n-channel region 23. In the present embodiment, a number of basic cells are arranged on an n-type semiconductor substrate, and a p-type well region 32 is formed in the n-channel region 23. In the p-channel region 22, two p-channel large MOS transistors shown by gates 24 and 25, and two p-channel small MOS transistors shown by gates 26 and 27 are formed. In the n-channel region 23, two n-channel large MOS transistors shown by gates 28 and 29, and two n-channel small MOS transistors shown by gates 30 and 31 are formed. 
     A gate width W 2  of each of the gates 26, 27, 30 and 31 of the small MOS transistors are selected to be no larger than one half of a gate width W 1 , of each of the gates 24, 25, 28 and 29 of the large MOS transistors. 
     The gates 26 and 27 of the small MOS transistors are staggered to each other in a direction of gate width (y direction) in the p-channel region 22 so that the gates do not overlap in the direction of gate width. Similarly, the gates 30 and 31 of the small MOS transistors are staggered to each other in the direction of gate width in the n-channel region 23 so that they do not overlap. 
     In the present arrangement, two right-hand small MOS transistors of a basic cell (not shown) which is left-adjacent to the basic cell 21 are arranged between the small MOS transistors which have the gates 26 and 30. In other words, those four small MOS transistors are arranged in line on a wiring track 33. On the other hand, two left-hand small MOS transistors of a basic cell which is right-adjacent to the basic cell 21 are arranged to hold the small MOS transistors having the gates 27 and 31 therebetween in line with them on a wiring track 37 to hold. Accordingly, where a number of basic cells are laterally (x direction) arranged, an average width per basic cell is equal to four wiring track width. The required area is 4/5 of that of the prior art basic cell which requires five wiring track width, and an integration factor is improved by a factor of 5/4. 
     FIG. 3 shows one unit of an SRAM memory circuit. It may be constructed by wiring the basic cell of FIG. 2. Wiring 40-53 shown an example thereof. Solid lines show a first layer wiring, broken lines show a second layer wiring and circles show contacts. The p-channel MOS transistors 61 and 62 of FIG. 3 correspond to the p-channel MOS transistors having the gates 24 and 25 shown in FIG. 2, and the n-channel MOS transistors 63, 64, 65 and 66 of FIG. 3 correspond to the n-channel MOS transistors having the gates 28, 29, 30 and 31 shown in FIG. 2. 
     FIG. 4 shows a plan view of a basic cell of a CMOS master slice in accordance with a second embodiment of the present invention. The present embodiment relates to a CMOS master slice suitable to construct a random logic. 
     In the present embodiment, like the first embodiment, the basic cells are arranged in an n-type semiconductor substrate, and a p-type well region is partially formed. The n-channel MOS transistors are formed on the p-type well region, and the p-channel MOS transistors are formed on other region of the substrate. Numeral 72 denotes one of such p-type well regions. 
     Noting to the basic cell 71, four n-channel MOS transistors shown by gate electrodes 77-80 are formed in a lower-half area of the drawing, and four p-channel MOS transistors shown by gate electrodes 73-76 are formed in an upper-half area. 
     The p-channel MOS transistor shown by the gate 73 and the n-channel MOS transistor shown by the gate 77 form a CMOS transistors pair. Similarly, the p-channel MOS transistors and the n-channel MOS transistors shown by the gates 74 and 78, 75 and 79, and 76 and 80, respectively, form three CMOS transistor pairs. In total, four CMOS transistor pairs are formed. 
     The CMOS transistor pair shown by the gates 73 and 77, and the CMOS transistor pair shown by the gates 74 and 78 are referred to as basic CMOS transistor pairs as they are located at the center of the basic cell 71 with respect to the gate length (x direction), and the two pairs of CMOS transistors having narrower gate width and located on both sides of the basic CMOS transistor pairs are referred to as additional CMOS transistor pairs. 
     In the basic CMOS transistors are interconnected by pads 81 and 82, respectively, because both gates are to be electrically connected in order to configure the random logic. On the other hand, In the additional CMOS transistor pairs on the both sides of the basic CMOS transistor pairs, the gates of the p-channel MOS transistors and the gates of the p-channel MOS transistors are isolated, because only the n-channel MOS transistors are used as path transistors separately from the p-channel MOS transistors when the SRAM memory circuit is configured. The isolation is only electrical and the transistors are closely arranged spatially. Accordingly, when both gates are to be connected for use as a random logic circuit, they may be easily interconnected by wiring. When those CMOS transistor pairs are viewed in the direction of gate length (x direction), the gates of the p-channel MOS transistors and the gates of the n-channel MOS transistors in each of the pairs are arranged in line extending along the gate length (x direction). Namely, the center positions of the respective pairs along the gate width are aligned in each basic cell, and the region in which the p-channel MOS transistors are formed and the region in which the n-channel MOS transistors are formed are symmetric around a line. 
     On the other hand, a gate width W 2  of the additional CMOS transistor pair including input terminals to the gates is selected to be no larger than one half of a gate width W 1  of the basic CMOS transistor pair including input terminals to the gates. Accordingly, the additional CMOS transistor pair on the right side of the basic cell 71, and the left-hand additional CMOS transistor pairs (not shown) in the basic cells 90 and 91 which are right-adjacent to the above additional CMOS transistor pair may be arranged on the same wiring track 96 so that a number of basic cells can be arranged on the substrate without gap. 
     In the present embodiment, the additional CMOS transistor pairs is arranged on each of the left and right sides of the basic CMOS transistor pairs, although the number of additional CMOS transistor pairs is not limited thereto. FIG. 5 shows an embodiment in which total of six CMOS transistor pairs 103-108, namely three pairs on each of the left and right sides of the basic CMOS transistor pairs 101 and 102 are arranged. As seen from FIGS. 4 and 5, in the additional CMOS transistor pair arranged on the left side of the basic CMOS transistor pair and the additional CMOS transistor pair arranged on the right side, a sum of the gate widths of the CMOS transistor pairs having the same sequence of arrangement in a predetermined direction along the gate length, including gate input terminals, is equal to a gate width W of the basic CMOS transistor pair including the gate input terminals. Referring to FIG. 5, a sum of the gate width of the additional CMOS transistor pair 103 and the gate width of the additional CMOS transistor pair 108, a sum of the gate width of the additional CMOS transistor pair 105 and the gate width of the additional CMOS transistor pair 106, and sum of the gate width of the additional CMOS transistor pair 107 and the gate width of the additional CMOS transistor pair 104 are equal to the gate widths of the basic CMOS transistor pairs 101 and 102, respectively. 
     An embodiment which improves an integration efficiency including the arrangement of body contact areas is described below referring to FIG. 6. The body contact areas are connecting areas to power supply wiring provided to set the substrate and the well region to a power supply voltage V dd  and a ground level GND in order to prevent a latch-up phenomenon. 
     A basic cell 110 comprises six n-channel MOS transistors shown by gate electrodes 112-117 and six p-channel MOS transistors shown by gate electrodes 118-123. Those n-channel MOS transistors and p-channel MOS transistors are arranged in mirror image along the gate width (vertical direction in the drawing). Body contact areas 124, 125 126 and 127 are formed between the gate electrodes 114 and 116, 115 and 117, 120 and 122 and 121 and 123, respectively. 
     A power supply wiring is connected to the body contact areas 124 and 125 to maintain the potential of the p-type well region 111 at a low potential power supply voltage GND, and another power supply wiring is connected to the body contact areas 126 and 127 to maintain the substrate potential at a high potential power supply voltage V dd . 
     Four MOS transistors shown by gate electrodes 112, 113, 118 and 119 are referred to as basic MOS transistors as they are located at the center of the basic cell 110 with respect to the gate length (lateral direction in the drawing), and eight MOS transistors having narrower gate electrodes 114-117, 120-123 located on both sides of the basic MOS transistors are referred to as additional MOS transistors. Namely, eight MOS transistors arranged on two outer wiring tracks 131 and 135 of the wiring tracks 131-135 are referred to as the additional MOS transistors, and four MOS transistors arranged on inner wiring tracks 132 and 134 are referred to as the basic MOS transistors. 
     A gate width W 2  of the additional MOS transistors is selected to meet 
     
         W.sub.2 &lt;(W.sub.1 -α)/2 
    
     Where W 1  is a gate width of the basic MOS transistor and α is a vertical length (in the drawing) of the body contact areas 124-127. Accordingly, by arranging the body contact areas 124-127 at the center of the gate width (vertical direction) on the left and right sides of the basic MOS transistor, the additional MOS transistors can be arranged on upper and lower sides of the basic MOS transistor within the range of the gate width W 1  of the basic MOS transistor. 
     The body contact areas 124-127 are integrated with the body contact areas of the left and right adjacent basic cells 136 and 137. In other words, the body contact areas 124 and 126 extend leftward beyond the area of the basic cell 110 so that they may be used as the body contact area of the basic cell 136 as well. Similarly, the body contact areas 125 and 127 extend rightward so that they may be used as the body contact area of the basic cell 137 as well. By sharing the body contact area by the adjacent basic cells, the area of the body contact can be widened. In order aspect, assuming that a minimum area for one body contact area is given and the area of the body contacts is governed by that condition, the area of the body contact for one basic cell can be reduced to one half by the integration. 
     In order to integrate the body contact areas of the adjacent basic cells, it is necessary that the body contact areas are arranged laterally symmetrically. In the present embodiment, it is met because all body contact areas are arranged at the centers of the gate widths of the basic MOS transistors. 
     In the present embodiment, the additional MOS transistors are arranged on the upper and lower sides of the body contact areas 124-127, although they may be provided on either one of the upper and lower sides. 
     From the invention thus described, it will be obvious that the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, an all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.