Abstract:
A semiconductor device and a method of laying out the same includes routing primary power and ground distributions in the second metallization layer, rather than the first metallization as is conventionally done. This improves routability in the first metallization layer while providing sufficient current handling ability in the power and ground distributions.

Description:
This is a continuation of application Ser. No. 08/984,029 filed Dec. 2, 1997 now U.S. Pat. No. 5,981,987. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to semiconductor integrated circuits, and more particularly, to power and ground metallization routing in a multi-metal layer semiconductor device having a plurality of basic cell circuits such as standard cells and gate array cells. 
     2. Description of the Related Art 
     FIG. 1 illustrates a conventional integrated circuit having a number of rows  3  of cells  5 . The cells can have various widths W 1 , W 2 , W 3 , etc. and can be separated by small gaps (not shown). Power and ground are supplied to each cell from power and ground busses  7  and  9  via primary power and ground distributions  60  and  50 , respectively. The primary power and ground distributions are typically laid out in the first metallization layer (i.e., “metal  1 ”). Moreover, metals in adjacent layers are laid out perpendicular to each other. That is, for example in a four-metal layer integrated circuit, wirings in the first and third metallization layers are laid out in one direction, and wirings on the substrate surface (e.g. polygate) and the second and fourth metallization layers are laid out in a direction perpendicular to the wirings in the first and third metallization layers. 
     As integration increases, rows  3  begin to abut with each other, causing the distance D 1  to shrink to such a degree that the availability of the space between rows as channels for routing interconnections between cells in metal  1  is eliminated. Over-the-cell routers and other tools are thus required to route such interconnections in higher metal layers. 
     FIG. 2 illustrates the layout of a basic cell  5  that can be included in such a conventional integrated circuit as is illustrated in FIG.  1 . It includes a PFET device region  10 , a NFET device region  20 , polygate  30 , P-N device intraconnection  40 , primary ground distribution  50 , and primary power distribution  60 . Contacts  70  connect power from primary power distribution  60  to the PFET device region, and contacts  80  connect ground from primary ground distribution  50  to the NEFT device region. Input pints  85  are provided to connect devices in this cell with devices in other cells by contact to polygate  30  through contact  95 . 
     As can be seen, the primary power and ground distributions are laid out in metal  1  in an east-west direction. P-N intraconnections  40  and input pints  85  are also typically laid out in the first metallization layer. As should be apparent, to connect devices in cells in other rows to the input pins  85  and output pins (typically via connection to P-N intraconnection  40 ) of cell  5 , such connections must be routed up and over the primary power and ground distributions through higher metal layers and then back down to metal  1  through vias and contact holes and the like. 
     FIG. 3 is a side plan view of the basic cell in FIG. 2 taken along sectional line  3 — 3 . It shows primary power distribution  60  formed as the first metal layer over PFET device region  10 , with polygate  30  (i.e., a gate formed of a layer of doped polysilicon on the substrate) and first insulator layer  90  interposed therebetween. Device region  10  is formed in substrate  1  and is separated from other device regions by oxide  35 . Gate oxide layer  25  is interposed between polygate  30  and device region  10 . Input pin  85  is connected to polygate  30  by contact  95  through first insulator layer  90 . 
     The conventional technique of routing primary power and ground distributions in metal  1  is fraught with many problems. First, for example, due to the requirement of providing P-N intraconnections such as  40 , and the fact that cell integration restricts the availability of cell interconnections between rows, very few cell interconnections can be routed in metal  1 . Meanwhile, it is generally desirable to route as many interconnections as possible in lower metal layers so as to conserve routing resources in upper metal layers, and thus facilitate reduced average wire lengths. 
     Second, as cell integration increases, the number of devices per square area of the die increases, and hence the amount of current required to be carried on the primary power and ground distributions increases beyond the capabilities of the distribution lines. One solution to this problem involves making the primary power and ground distributions wider. However, certain minimum design distances such as D 2  and D 3  must be maintained so as to comply with the minimum feature requirements of the fabrication tools, for example. If the power and ground distributions are made wider, the device regions themselves must likewise be made wider, thus defeating higher cell integration. Moreover, an imbalance problem can arise even if the minimum feature requirements are maintained by increasing the size of a N device region, but without increasing the size of a P device region by a corresponding amount. This is because P devices are typically such weaker than N devices. 
     A second solution to the above-described current handling problem involves adding supplemental lines in metals  2  or  3 . 
     FIG. 4 illustrates the technique of laying out supplemental line  110  in an east-west direction in metal  3  in parallel with primary power distribution  60  in metal  1 . The primary and supplemental lines are connected through second insulator  100  and third insulator layer  105  by periodically provided stacked via and contacts  120 . This solution effectively increases the width of the primary power distribution line. However, this effective increase in width may not be sufficient in extreme circumstances where many cells in the same row require current at the same time. Moreover, cells may have different dimensions, causing the primary distribution line to snake north and south and making it difficult to align the primary and supplemental lines. 
     FIG. 5 illustrates the technique of providing supplemental lines in metal  2 . In this technique, supplemental power lines  115  are laid out in metal  2  in a north-south direction forming a matrix with the underlying primary power distributions. Inter-layer contacts are periodically provided to connect the supplemental power lines  115  and primary power distribution lines  60 . This technique permits the current in each of the primary power distributions  60  to be shared in parallel so that a “hot” row of devices can draw current from other primary power distributions  60  associated with other rows. It should be apparent from the foregoing that the same technique could be applied for ground as well as power. 
     Although providing supplemental lines in metal  2  improves the ability of the primary power and ground distributions to provided desired amounts of current, other problems are created. For example, the supplemental line  115  in metal  2  can interfere with metal  1  pin locations and thus can prevent picking up device input and output pins. This further problem is illustrated in FIG.  6 . As can be seen, when supplemental line  115  is laid out as shown in dashed lines, pin  85  is blocked, preventing any connection thereto unless a metal  1  interconnection can be made, which is unlikely. Accordingly, either the cell must be made wider or gaps must be provided between cells over which to lay out the supplemental line  115 , as shown in FIG.  6 . FIG. 7 is a side view of the cell in FIG. 6 taken along sectional line  7 — 7 . As should be clear, either widening the cell or providing larger gaps between cells defeats higher cell integration. 
     Accordingly, there remains a need in the art for effective primary power and ground distributions in a basic cell that provides sufficient current handling ability while not impeding metal  1  routability or increased integration. The present invention fulfills this need. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide effective primary power and ground distributions in an integrated circuit having a plurality of cells. 
     Another object of the invention is to provide sufficient current handling ability of power and ground distributions in an integrated circuit having a plurality of cells. 
     Another object of the invention is to improve device interconnection routability in an integrated circuit having a plurality of cells. 
     Another object of the invention is to improve cell integration. 
     Another object of the invention is to improve the ability to provide supplemental lines to primary power and ground distributions. 
     Another object of the invention is to improve P/N device balancing. 
     Another object of the invention is to reduce average wire lengths. 
     These and other objects of the invention are fulfilled by the present invention. In a preferred form, the invention includes primary power and ground distributions in the second metallization layer, rather than the first metallization as is conventionally done. This improves routability in the first metallization layer while providing sufficient current handling ability in the power and ground distributions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages the present invention, among others, will become apparent to those skilled in the art after considering the following detailed specification, together with the accompanying drawings wherein: 
     FIG. 1 illustrates the layout of a conventional integrated circuit having rows of cells; 
     FIG. 2 illustrates the layout of a basic cell in a conventional integrated circuit such as that illustrated in FIG. 1; 
     FIG. 3 is a side view of the conventional cell in FIG. 1 taken along sectional line  2 — 2 ; 
     FIG. 4 illustrates the conventional technique of providing supplemental power and ground distributions in metal  3  in the conventional integrated circuit; 
     FIG. 5 illustrates the conventional technique of providing supplemental power and ground distributions in metal  2  in the conventional integrated circuit; 
     FIG. 6 further illustrates the conventional technique of providing supplemental power and ground distributions in metal  2  in the conventional integrated circuit; 
     FIG. 7 is a side view of the conventional cell in FIG. 6 taken along sectional line  7 — 7 ; 
     FIG. 8 illustrates the layout of a basic cell with power and ground distribution routing in accordance with the present invention; 
     FIG. 9 is a side view of the basic cell illustrated in FIG. 8 taken along sectional line  9 — 9 ; 
     FIG. 10 illustrates providing supplemental lines in metal  3  and metal  4  in accordance with the principles of the present invention; 
     FIG. 11 illustrates inter-cell connections in metal  1  in accordance with the principles of the present invention; 
     FIG. 12 illustrates providing substrate and well ties in an integrated circuit in accordance with the invention. 
     FIG. 13 illustrates a multi-height basic cell in accordance with the principles of the present invention; and 
     FIG. 14 further illustrates providing multi-height basic cells in an integrated circuit in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 8 illustrates the layout of a basic cell layout using power and ground distribution routing in accordance with the present invention. It includes PFET device region  10 , NFET device region  20 , polygate  30 , P-N device intraconnection  240 , primary ground distribution  250 , primary power distribution  260 , cell output interconnection  242  and cell input interconnection  244 . Stacked via and contact hole  270  connects power from primary power distribution  260  to the PFET device region, as will be described in more detail below. 
     Contrary to the conventional techniques, the primary power and ground distributions are formed as the second metallization layer in the basic cell, and are routed in an east-west direction. P-N device intraconnection  240 , cell output interconnection  242 , and cell input interconnection  244  are formed in the first metallization layer, and can be routed in both north-south and east-west directions. Other elements can be the same as in the conventional cell and their repeated detailed explanation here is not necessary for an understanding of the invention. 
     As should be apparent, the routability of inter-cell connections in metal  1  is enhanced due to the lack of primary power and ground distributions in metal  1 . In the example illustrated in FIG. 8, P-N device intraconnection  240  can be connected to cell output interconnection  242  so as to provide the output of this cell to a cell in a northern row in metal  1 , while input pin can be connected to cell input interconnection  244  so as to supply the input of this cell from a cell in a southern row. Other examples and alternatives of connecting the inputs and outputs of cells in metal should be immediately apparent to those skilled in the art. 
     As should be further apparent, cell integration can be dramatically improved using the power and ground distribution routing in accordance with the invention. Not only does the improved routability of cell interconnections in metal  1  conserve routing resources in higher metal layers and reduce average wire lengths, but the device regions can be made smaller due to the ability, for example, to overlap portions of P-N device intraconnection  240  with primary power and ground distributions  250  and  260 . Further, the N device region can be made smaller relative to the P device region, thus allowing for better P/N balance. 
     FIG. 9 is a side plan view of the basic cell in FIG. 8 taken along sectional line  8 — 8 . It shows primary power distribution  260  formed as the second metallization layer over PFET device region  10 , with first insulator layer  90  and second insulator layer  100  interposed therebetween. It also further graphically illustrates how cell output interconnection  242  can be freely routed in metal  1  to connect devices in the basic cell in FIG. 8 via contact  210  with other cells north and south of the cell. 
     The primary power and ground distributions can be connected to the respective device regions using many known techniques. However, in a preferred embodiment of the invention illustrated in FIG. 8, primary power distribution  260  is connected to the PFET device region through stacked via and contact  270 . By using a stacked via and contact such as that illustrated, the use of metal  1  is minimized, thus further improving the routability of other interconnections in metal  1 . 
     Further advantages of routing the primary power and ground distributions in metal  2  rather than in metal  1  are as follows. First, the power and ground distributions in metal  2  can be made as wide as necessary to handle the current required to supply the integrated circuit devices. Moreover, metal  2  layers are trending toward being thicker than metal  1 , further enhancing the current capacity of the power and ground distributions in metal  2 . 
     As illustrated in FIG. 10, if supplemental power and ground distributions are still required, supplemental lines  215  can be provided in metal  3  in a matrix fashion with the primary distributions in metal  2 , with periodic connections therebetween. Furthermore, second supplemental lines  217  in metal  4  can be further provided in a matrix fashion with the supplemental lines in metal  3 , with periodic connections therebetween. It should be apparent that the pin blocking problem described with reference to FIG. 6 is alleviated in the present invention by the ability to access pins in metals  1  and  2 . 
     FIG. 11 illustrates how cells  5 -A and  5 -B in neighboring rows can be interconnected with each other and with other cells in metal  1  in accordance with the principles of the invention. This example shows the output of cell  5 -A connected with the input of cell  5 -B by cell interconnection  342 , while other inputs of both cells are connected with cells in the same and other rows by cell interconnections  344 ,  346  and  348 . 
     Although substrate and well ties can be provided in many known ways, FIG. 12 illustrates providing substrate and well ties in a manner preferred by the present invention. In the example shown in FIG. 12, substrate ties  303  and well ties  304  are provided at the corners of every cell, with adjacent cells in the same row sharing the same substrate and well ties, so as to connect ground and power respectively to substrate and N-wells in each cell. By providing the substrate and well ties in this manner, routability in metal  1  in both north-south and east-west directions is not significantly impeded. 
     Yet another advantage of the primary power and ground distribution routing of the present invention is illustrated in FIG.  13 . By virtue of the improved routability of cell interconnections in metal  1 , multi-height cells can be more easily provided that before. FIG. 13 shows an example of a double-height cell  305  linked together by device intraconnection  440  in metal  1 . Double-height cell  305  can be considered a stronger version of the basic cell illustrated in FIG. 8, with more input and output pin locations, thus further enhancing the routability of interconnections in metal  1 . FIG. 14 further illustrates how a multi-height cell such as double-height cell  305  can be provided in an integrated circuit having a plurality of single-height cells  5 . This advantage of the invention is particularly important for integrated circuit designs where complicated cell structures having many input and output pins are required. FIGS. 13 and 14 also illustrate another example of how substrate and well ties  303  and  304  are provided in accordance with the invention. 
     It should be noted that although the routing techniques of the present invention have been described hereinabove with particular reference to integrated circuits having standard cells, the principles of the invention can also be applied to gate arrays having predetermined basic gate array cells. 
     Accordingly, although this invention has been described in detail with reference to the preferred embodiments thereof, those skilled in the art will appreciate that various substitutions and modifications can be made to these examples without departing from the spirit of the invention as defined in the appended claims.