Abstract:
This invention discloses a semiconductor device including a substrate, at least first, second and third metal layers formed over the substrate, the second metal layer including a plurality of generally parallel bands extending parallel to a first axis, each band comprising a multiplicity of second metal layer strips extending perpendicular to said first axis, and at least one via connecting at least one second metal layer strip with the first metal layer underlying the second metal layer.

Description:
FIELD OF THE INVENTION 
     The present invention relates to gate arrays generally. 
     BACKGROUND OF THE INVENTION 
     Various types of gate arrays are well known in the art. Gate arrays comprise a multiplicity of transistors which are prefabricated. A specific application is achieved by customizing interconnections between the transistors. 
     Routing arrangements have been proposed for reducing the number of custom masks and the time needed to manufacture gate arrays by prefabricating some of the interconnection layers in two-metal layer gate array devices. Prior art devices of this type typically employ three custom masks, one each for the first metal layer, via layer and second metal layer. 
     U.S. Pat. No. 4,197,555 to Uehara describes a two-metal layer gate array device wherein the first and second metal layers are pre-fabricated and the via layer is customized. Uehara also shows use of pre-fabricated first metal and via layers and customization of the second metal layer. 
     U.S. Pat. Nos. 4,933,738; 5,260,597 and 5,049,969 describe a gate array which is customized by forming links in one or two prefabricated metal layers of a two-metal layer device. 
     U.S. Pat. No. 5,404,033 shows customization of a second metal layer of a two-metal layer device. 
     U.S. Pat. No. 5,581,098 describes a gate array routing structure for a two-metal layer device wherein only the via layer and the second metal layer is customized by the use of a mask. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide a multiple layer interconnection structure for a gate array device which has significant advantages over prior art structures. 
     The present invention employs at least three metal interconnection layers. Customization is preferably realized by customization of a via layer and a layer overlying that via layer. 
     There is thus provided in accordance with a preferred embodiment of the present invention a semiconductor device including a substrate, at least first, second and third metal layers formed over the substrate, the second metal layer including a plurality of generally parallel bands extending parallel to a first axis, each band including a multiplicity of second metal layer strips extending perpendicular to the first axis, and at least one via connecting at least one second metal layer strip with the first metal layer underlying the second metal layer. 
     Preferably the at least one via includes a repeating pattern of vias. 
     Further in accordance with a preferred embodiment of the present invention the third metal layer includes at least one third metal layer strip extending generally perpendicular to the second metal layer strips and being connected thereto by a via. 
     Still further in accordance with a preferred embodiment of the present invention the third metal layer includes at least one third metal layer strip extending generally parallel to the second metal layer strips and connecting two coaxial second metal layer strips by vias. 
     Additionally in accordance with a preferred embodiment of the present invention the first metal layer underlying the second metal layer includes a multiplicity of first metal layer strips extending generally parallel to the multiplicity of second metal layer strips. Furthermore, at least one of the first metal layer strips is electrically connected at ends thereof to different second metal layer strips for providing electrical connection therebetween. 
     Further in accordance with a preferred embodiment of the present invention the second metal layer strips include both relatively long strips and relatively short strips, at least one of the relatively short strips being connected to the first metal layer by a via. Preferably the relatively short second metal layer strips are arranged in side by side arrangement. Alternatively the relatively short second metal layer strips are arranged in spaced coaxial arrangement. 
     Additionally or alternatively the third metal layer includes a bridge connecting adjacent pairs of said relatively short second metal layer strips. 
     Still further in accordance with a preferred embodiment of the present invention the third metal layer includes at least one third metal layer strip extending perpendicular to the second metal layer strips and being connected thereto by a via. Furthermore, the third metal layer includes at least one third metal layer strip extending parallel to said second metal layer strips and connecting two coaxial second metal layer strips by vias. 
     Additionally in accordance with a preferred embodiment of the present invention the first metal layer comprises at least one first metal layer strip extending generally perpendicular to the second metal layer strips and being connected thereto by a via. Preferably the third metal layer includes at least one third metal layer strip extending perpendicular to the second metal layer strips and being connected thereto by a via. 
     Moreover in accordance with a preferred embodiment of the present invention the first metal layer includes first metal layer strips extending generally perpendicular to the second metal layer strips, the first metal layer strips being electrically connected at ends thereof by said vias to the second relatively short metal layer strips. 
     Still further in accordance with a preferred embodiment of the present invention the third metal layer comprises at least one third metal layer strip extending parallel to the second metal layer strips and connecting two coaxial second metal layer strips by vias. 
     Additionally in accordance with a preferred embodiment of the present invention also including at least one third metal layer strip extending parallel to the second metal layer strip and connecting two coaxial second metal layer strips. 
     There is also provided in accordance with a preferred embodiment of the present invention a semiconductor device including a substrate, at least first, second and third metal layers formed over the substrate, the second metal layer including a multiplicity of second metal layer strips extending perpendicular to the first axis, adjacent ones of the second metal layer strips having ends which do not lie in a single line. 
     Further in accordance with a preferred embodiment of the present invention the second metal layer strips are interlaced with one another. 
     Still further in accordance with a preferred embodiment of the present invention the third metal layer includes at least one third metal layer strip extending generally perpendicular to the second metal layer strip and being connected thereto by a via. 
     Additionally in accordance with a preferred embodiment of the present invention the third metal layer includes at least one third metal layer strip extending generally parallel to said second metal layer strips and connecting two coaxial second metal layer strips by vias. 
     There is provided in accordance with yet another preferred embodiment of the present invention a semiconductor device including a substrate, at least first, second and third metal layers formed over the substrate, the second metal layer including a plurality of generally parallel bands extending parallel to a first axis, each band comprising a multiplicity of second metal layer strips extending perpendicular to the first axis, and a plurality of mutually parallel relatively short second metal layer strips extending generally parallel to the first axis. 
     Further in accordance with a preferred embodiment of the present invention the third metal layer includes at least one third metal layer strip extending generally perpendicular to the second metal layer strips and being connected thereto by a via. Preferably at least one of the third metal strips connects two second metal layer strips by means of vias. 
     Still further in accordance with a preferred embodiment of the present invention the third metal layer includes at least one third metal layer strip extending generally parallel to the second metal layer strips and connecting two coaxial second metal layer strips by vias. Preferably at least one of the third metal strips connects two second metal layer strips by means of vias. 
     Additionally in accordance with a preferred embodiment of the present invention including at least one via connecting at least one second metal layer strip with the first metal layer underlying the second metal layer. 
     There is provided in accordance with yet another preferred embodiment of the present invention a semiconductor device including a substrate, at least first, second, third and fourth metal layers formed over the substrate, the second metal layer including a plurality of generally parallel bands extending parallel to a first axis, each band comprising a multiplicity of long strips extending parallel to the first axis, the long strips including at least one of straight strips and stepped strips, at least one electrical connection between at least one strip in the second metal layer to the third metal layer, which overlies the second metal layer. 
     Preferably the second metal layer comprises a repeating pattern. 
     Further in accordance with a preferred embodiment of the present invention the strips of the second metal layer are connected to one of the third metal layer and the fourth metal layer, both of which overlie the second metal layer, by least two electrical connections. 
     Alternatively most of the strips of the second metal layer are connected to one of the third metal layer and the fourth metal layer, both of which overlie the second metal layer, by least two electrical connections. 
     Further in accordance with a preferred embodiment of the present invention at least one of the strips of the second metal layer is electrically connected to another one of the strips of the second metal layer which is non-adjacent thereto. 
     Preferably the device forms part of a larger semiconductor device. 
     Still further in accordance with a preferred embodiment of the present invention the first metal layer includes a plurality of generally parallel bands extending parallel to a first axis, each band comprising a multiplicity of long strips extending parallel to the first axis, the long strips including at least one of straight strips and stepped strips, and at least one electrical connection between at least one strip in the first metal layer to the third metal layer, which overlies the first metal layer. 
     Additionally in accordance with a preferred embodiment of the present invention the first metal layer includes repeating pattern. 
     Further in accordance with a preferred embodiment of the present invention the strips of the first metal layer are connected to one of the third metal layer and the fourth metal layer, both of which overlie the first metal layer, by least two electrical connections. 
     Alternatively most of the strips of the first metal layer are connected to one of the third metal layer and the fourth metal layer, both of which overlie the first metal layer, by least two electrical connections. 
     Further in accordance with a preferred embodiment of the present invention at least one of the strips of the first metal layer is electrically connected to another one of the strips of the first metal layer which is non-adjacent thereto. 
     Additionally in accordance with a preferred embodiment of the present invention the semiconductor device forms part of a larger semiconductor device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: 
     FIG. 1 is a pictorial illustration of the lower two of the top three metal layers of a gate array device constructed and operative in accordance with a preferred embodiment of the present invention, prior to customization; 
     FIG. 2 is a pictorial illustration corresponding to FIG. 1 following customization thereof in accordance with a preferred embodiment of the present invention; 
     FIG. 3 is a schematic illustration corresponding to FIG. 1; 
     FIG. 4 is a schematic illustration corresponding to FIG. 2; 
     FIG. 5 is a schematic illustration corresponding to FIGS. 1 &amp; 3 but showing a variation in the arrangement of the lowest of the three metal layers; 
     FIG. 6 is a schematic illustration corresponding to FIG. 5 following customization thereof in accordance with a preferred embodiment of the present invention; 
     FIG. 7 is a schematic illustration corresponding to FIGS. 1 &amp; 3 but showing a variation in the arrangement of the middle of the three metal layers; 
     FIG. 8 is a schematic illustration corresponding to FIG. 7 following customization thereof in accordance with a preferred embodiment of the present invention; 
     FIGS. 9 and 9A schematically illustrate the lower four of the top five metal layers of a gate array device constructed and operative in accordance with another preferred embodiment of the present invention, prior to customization; 
     FIGS. 10 and 10A show a schematic illustration corresponding to FIGS. 9 and 9A following customization thereof in accordance with a preferred embodiment of the present invention; 
     FIGS. 11,  11 A, and  11 B schematically the lower four of the top five metal layers of a gate array device constructed and operative in accordance with yet another preferred embodiment of the present invention, prior to customization; 
     FIGS. 12,  12 A, and  12 B show a schematic illustration corresponding to FIGS. 11,  11 A, and  11 B following customization thereof in accordance with a preferred embodiment of the present invention; 
     FIG. 13 is a schematic illustration corresponding to FIG. 1 with additional bridges in the middle of the top three metal layers; 
     FIG. 14 is a schematic illustration corresponding to FIG.  13  and showing the top metal layer, prior to customization;  15  is a schematic illustration corresponding to FIG. 14 having via customization in accordance with a preferred embodiment of the present invention; 
     FIG. 16 illustrates a single cell routing unit, comprising layers M 4  to M 6  and I/O contacts in accordance with a preferred embodiment of the present invention; 
     FIG. 17 illustrates a single cell unit of similar construction to the single cell routing unit of FIG. 16, but without the I/O contacts; 
     FIG. 18 illustrates typical routing connections in the M 3  and M 4  layers, and the M 3 M 4  via and M 4 M 5  via layers, of the single cell routing unit, in accordance with a preferred embodiment of the present invention; 
     FIG. 19 illustrates an M 5  layer corresponding to the arrangement described hereinabove with respect to FIG. 5; 
     FIG. 20 illustrates an M 6  layer with vias M 5 M 6  corresponding to the M 6  layers of FIG. 9; 
     FIG. 21 illustrates a typical arrangement of 16 cells of M 3  and M 4  layers in a 4×4 matrix, in accordance with a preferred embodiment of the present invention; 
     FIG. 22 illustrates an M 5  layer comprising a 4×4 matrix of 16 cells, in accordance with a preferred embodiment of the present invention; 
     FIG. 23 illustrates an M 6  layer and M 5 M 6  via layer of a 4×4 cell matrix, in accordance with a preferred embodiment of the present invention; 
     FIG. 24 illustrates the layers M 3 , M 4 , M 5 , M 6  and M 7  in a 4×4 cell matrix, in accordance with a preferred embodiment of the invention; and 
     FIG. 25 illustrates seven metal layers, M 1 -M 7 , in accordance with a preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention is now described with reference to FIGS. 1-15, it being appreciated that the figures, illustrate parts of pre-customized circuits which parts are repeated multiple times in an actual circuit. The precustomized circuits may or may not form a part of a larger integrated circuit device. For reasons of practicality, an entire semiconductor device including such circuits cannot be illustrated to a resolution which enables the routing structure thereof to be discerned. 
     Reference is now made to FIG. 1, which is a pictorial illustration of the lower two of the top three metal layers of a gate array device constructed and operative in accordance with a preferred embodiment of the present invention, prior to customization and to FIG. 3 which is a schematic illustration corresponding thereto. 
     In accordance with a preferred embodiment of the invention, the gate array device of FIG. 1, when customized, includes a total of seven metal layers, identified as M 1 -M 7 . Metal layers M 1 -M 7  are depicted in FIG. 25, where M 1  is the bottom metal layer and M 7  is the top metal layer (vias between the layers are not shown). Metal layers M 1 -M 3  are employed for constructing logic units or cells. Layers M 4 -M 7  are employed for routing signals between cells. Generally, metal layers M 6  and M 7  are employed for relatively short or local routing paths, while metal layers M 4  and M 5  are employed for long or global routing. Typically, metal layers M 4  and M 6  provide routing generally in North-South directions, while metal layers M 5  and M 7  provide routing generally in East-West directions. 
     The present invention described herein with reference to FIGS. 1-15 shows various arrangements which provide such routing and in which metal layers M 1 -M 6  are fixed. Customization is carried out only on vias connecting metal layers M 6  and M 7 , here termed M 6 M 7  vias, or on both M 6 M 7  vias and on metal layer M 7 . 
     In FIG. 1, the top metal layer M 7  is not shown, inasmuch as this metal layer is added during customization, as will be described hereinbelow with reference to FIG.  2  and to FIG. 4, which is a schematic illustration corresponding thereto. 
     The basic structure shown in FIG. 1 comprises an M 6  metal layer which comprises multiple spaced bands  10  of parallel evenly spaced metal strips  12 , the center lines of which are preferably separated one from the other by a distance “a”. At a given periodicity, typically every twenty strips  12 , a plurality of pairs  14  of short strips  16  is provided. The number of pairs  14  of short strips  16  and their length is a matter of design choice. Strips  12  and  16  are shown running North-South. 
     Underlying the M 6  metal layer is an M 5  metal layer comprising parallel evenly spaced metal strips  22  extending East-West in the sense of FIG. 1 in bands  10 . In the illustrated embodiment of FIG. 1, strips  22  each underlie three pairs  14  of short strips  16  and are each connected at opposite ends thereof by means of an M 5 M 6  via  24  to a strip  16 . It is noted that adjacent ones of strips  22  begin and end at strips  16  of different pairs  14 , such that each pair  14  of strips  16  is connected to strips  22  extending along a different axis. It is appreciated that each strip  16  preferably is connected to only a single strip  22 . 
     It is appreciated that the embodiment of FIG. 1 is merely exemplary in that, for example, each strip  16  may overlie more than three strips  22  and thus each strip  22  may underlie more than three pairs  14  of short strips  16 . 
     Reference is now made to FIG. 2, which is a pictorial illustration corresponding to FIG. 1 following customization thereof. It is seen that in FIG. 2 an M 7  layer is added for customization of the gate array. The M 7  layer may include a bridge  30  connected by M 6 M 7  vias  32  to adjacent strips  16  of a pair  14 , thus effectively connecting two strips  22  lying along the same elongate axis. 
     The M 7  layer may also provide another type of connection, such as connections  36  between one of strips  16  and a strip  12 , by means of M 6 M 7  vias  38 . This type of connection provides a circuit connection between a strip  22  and a strip  12 . 
     The M 7  layer may additionally provide a further type of connection, such as connections  40  between strips  12  in two adjacent bands  10 , by means of M 6 M 7  vias  42 . This type of connection provides a North-South circuit connection by means of strips  12 . 
     It is appreciated that the customized structure of FIGS. 2 &amp; 4 enables a signal received along a strip  22  to be conveyed in an East-West direction via strips  22  and to be coupled to a strip  12  at an appropriate East-West location. In accordance with a preferred embodiment of the present invention, in the customization of the structure of FIGS. 1 &amp; 3, in each band  10 , a single elongate axis is employed for placement of bridges  30  for interconnecting underlying strips  22  to provide East-West routing and for placement of connections  36  between strip  12  and strip  22  for long routing of signals in East-West directions. The other parallel East-West elongate axes are employed for shorter East-West routing. 
     Reference is now made to FIG. 5, which is a schematic illustration corresponding to FIGS. 1 &amp; 3 but showing a variation in the arrangement of the lowest of the three metal layers. This variation is provided principally to help overcome problems of signal crosstalk between signals traveling alongside each other along strips  22  over a relatively long distance. In the arrangement of FIG. 5, each strip  44 , corresponding to strip  22  (FIGS. 1 &amp; 3) shifts its elongate axis at least one location therealong. As seen in FIG. 6, customization of the embodiment of FIG. 5 may include bridges  46  between adjacent strips  16  of a pair  14 , which provide a continuation of East-West routing and also produce a switch between the longitudinal axes of two adjacent strips  44 , thus decreasing crosstalk. This is accomplished by limiting the distance that signals travel alongside each other by means of switching and mixing the order of the long routing conductors. 
     It is appreciated that although the shift is shown embodied in the M 5  metal layer, it may be carried out using appropriate vias and an underlying metal layer. 
     Reference is now made to FIG. 7, which is a schematic illustration corresponding to FIGS. 1 &amp; 3 but showing a variation in the arrangement of the middle of the top three metal layers. This arrangement is provided in order to take into account often oversize strips in M 7  layers which, due to their size, could not be placed side by side to provide bridges for adjacent strips  12  without creating a short circuit therebetween. 
     The arrangement of FIG. 7 is distinguished from that of FIGS. 1 &amp; 3 in that whereas in FIG. 1 &amp; 3, strips  12  of each band  10  all terminate in a line, defining an elongate edge of band  10 , which is spaced from the corresponding elongate edge of an adjacent band  10 , in FIG. 7, the strips  52  of adjacent bands  54  do not terminate at the same North-South location. Thus, in the embodiment of FIG. 7, the strips of adjacent bands  54  are interlaced. As seen in FIG. 8, bridges  56  between strips  52  of adjacent bands  54  are thus offset from each other, providing ample spacing therebetween notwithstanding the relatively large width of the bridges. 
     Reference is now made to FIGS. 9 and 9A, which show a schematic illustration of the lower four of the top five metal layers of a gate array device constructed and operative in accordance with another preferred embodiment of the present invention, prior to customization. 
     In accordance with a preferred embodiment of the invention, the gate array device of FIGS. 9 and 9A, when customized, includes a total of seven metal layers, identified as M 1 -M 7 , the top metal layer being identified as M 7 . In FIGS. 9 and 9A, the top metal layer M 7  is not shown, inasmuch as this metal layer is added during customization, as will be described hereinbelow with reference to FIGS. 10 and 10A. 
     The basic structure shown in FIGS. 9 and 9A comprises a M 6  metal layer which comprises multiple spaced bands  110  of parallel evenly spaced metal strips  112 , the center lines of which are preferably separated one from the other by a distance “a”. Pair  114  provides connections to long routing conductors in North-South directions, which are implemented by M 4  strips  132  and  133  as described hereinbelow. 
     Underlying the M 6  metal layer typically is an M 5  metal layer comprising parallel evenly spaced metal strips  122  extending East-West in the sense of FIGS. 9 and 9A in bands  110 . In the illustrated embodiment of FIGS. 9 and 9A, strips  122  each extend across three pairs  115  of short strips  117  and are each connected at opposite ends thereof by means of an M 5 M 6  via  124  to a strip  117 . Pair  115  provides connections to long routing conductors in East-West directions. 
     It is noted that adjacent ones of strips  122  begin and end at strips  117  of different pairs  115 , such that each pair  115  of strips  117  is connected to strips  122  extending along a different axis. It is appreciated that each strip  117  preferably is connected to only a single strip  122 . The portion of the pattern which provides long routing conductors in East-West directions along M 5  strips  122  is described hereinabove with reference to FIGS. 1 &amp; 3. The M 5  layer also comprises a plurality of bridge elements  126  which extend parallel to strips  122 . 
     Underlying the M 5  metal layer there is provided an M 4  metal layer preferably comprising evenly spaced stepped strips  132  and straight strips  133 , extending generally North-South in the sense of FIGS. 9 and 9A across multiple bands  110 . At a given periodicity, typically every four to seven strips  112 , a plurality of pairs  114  of coaxial short strips  116  is provided. 
     FIGS. 9 and 9A show a single band  111  of parallel stepped strips  132  and straight strips  133 . Multiple similar bands  111  extending in the North-South directions are provided in a semi-conductor device. Strips  112  and  116  are shown running North-South. The Southmost end of each strip  132  is connected by an M 4 M 5  via  134  and an M 5 M 6  via  136  to a Northmost end of a strip  116  of a pair  114 . A facing end of a second strip  116  of pair  114  is connected by an M 5 M 6  via  136  to an Westmost end of a bridge element  126 , the Eastmost end of which is connected by an M 4 M 5  via  134  to a Northmost end of a strip  133 . 
     The Southmost end of a strip  133  is connected by an M 3 M 4  via  138  to the Northmost end of an L-shaped tunnel  140  embodied in an M 3  metal layer. The South-Westmost end of tunnel  140  is connected by an M 3 M 4  via  138  to the Northmost end of a strip  132 . 
     Reference is now made to FIGS. 10 and 10A, which is a schematic illustration corresponding to FIGS. 9 and 9A following customization thereof. It is seen that in FIGS. 10 and 10A an M 7  layer is added for customization of the gate array. The M 7  layer may include a bridge  141  connected by M 6 M 7  vias  142  to adjacent strips  116  of a pair  114 , thus effectively connecting two strips  132 . 
     The M 7  layer may also provide another type of connection, such as connections  153  between one of strips  116  and a strip  112 , by means of M 6 M 7  vias  142 . This type of connection provides a circuit connection between a strip  132  and a strip  112  employing short strip  116 , thereby to route signals over a relatively long distance in North-South directions. It is appreciated that the arrangement of FIGS. 10 and 10A enables all connections to North-South M 4  strips  132  and  133  to be made generally along one North-South axis  114 . 
     The M 7  layer may also provide a further type of connection, such as connections  150  between strips  112  in two adjacent bands  110 , by means of M 6 M 7  vias  142 . This type of connection provides a North-South circuit connection by means of strips  112 . Connections  152  between strips  112  in the same band and a connection  155  between strip  117  and strips  112  in the same band may also be provided. It is thus appreciated that the customized structure of FIGS. 10 and 10A enables a signal received along a strip  122  to be conveyed in an East-West direction via strips  122  and to be coupled to a strip  132  at an appropriate East-West location by properly employing the M 7  layer and the M 6 M 7  vias  142  using M 6  strips  112 ,  116  and  117 . FIGS. 10 and 10A show such a structure employing M 7  connections  150 ,  153  and  155 . 
     Reference is now made to FIGS. 11,  11 A, and  11 B which show a schematic illustration corresponding to FIGS. 9 and 9A but showing a variation in the arrangement of the M 3 , M 4  and M 5  metal layers. This variation is provided principally to help overcome problems of signal crosstalk between signals traveling alongside each other along strips  132  over a relatively long distance. In the arrangement of FIGS. 11,  11 A and  11 B, there is provided in the M 4  metal layer an arrangement which enables shifting of the elongate axis of North-South extending conductors in both East and West directions, thus enabling crosstalk to be decreased by appropriate switching of the order of strips  132 . This is accomplished by limiting the distance that signals travel alongside each other by means of switching and mixing the order of the long routing conductors. 
     FIGS. 12,  12 A, and  12 B show the configuration of FIGS. 11,  11 A, and  11 B following exemplary customization by the addition of a via M 6 M 7  and M 7  layers. 
     Reference is now made to FIG. 13, which is a schematic illustration corresponding to FIGS. 1 &amp; 3 with additional bridges  160  in the M 6  layer extending perpendicular to metal strips  161 , which correspond to strips  12  in the embodiment of FIGS. 1 &amp; 3. 
     FIG. 13 together with FIGS. 14 and 15, which is referred to hereinbelow, illustrate another preferred embodiment of the present invention wherein customization is effected only in M 6 M 7  vias. This embodiment provides savings in customization tooling by keeping the M 7  metal layer fixed. 
     FIG. 14 is a schematic illustration corresponding to FIG.  13  and showing the top metal layer M 7 , prior to customization. As seen in FIG. 14, the M 7  layer includes bridges  162  extending North-South and relatively long strips  164  extending East-West. Strips  164  partially overlie bridges  160  and bridges  162  partially overlie strips  161 . 
     FIG. 15 is a schematic illustration corresponding to FIG. 14 having via customization. It is seen that M 6 M 7  vias  166  interconnect strips  161  by employing bridges  162  in order to provide North-South routing. Other M 6 M 7  vias  168  interconnect strips  164  by employing bridges  160  in order to provide East-West routing. Additional M 6 M 7  vias  170  interconnect strips  161  with strips  164  in order to interconnect the East-West routing with the North-South routing. 
     The following drawings, FIGS. 16 to  22 , show typical designs of the various layers constructed and operative in accordance with a preferred embodiment of the present invention. 
     Reference is now made to FIG. 16, which illustrates a single cell unit  200 , comprising layers M 4  to M 6 , constructed and operative in accordance with a preferred embodiment of the present invention. The cell unit  200 , illustrated in FIG. 16, comprises 3 I/O contacts  202 ,  204  and  206  to a logic cell (not shown) at layer M 3 . The cell unit  200  in FIG. 16 also shows strips  44 / 22 , typically located in an E-W direction, and corresponding to the strips  44 / 22  shown in FIGS. 3 and 5. The cell unit  200  shows the strips  44 / 22  overlap the N-S strips  132  and  133 , as described hereinabove with respect to FIGS. 9 and 11. FIG. 17 shows a cell unit  208 , of similar construction to cell unit  200  of FIG. 16, but without the I/O contacts  202 ,  204  and  206 . 
     Reference is now made to FIG. 18, which illustrates typical routing connections in the M 3  and M 4  layers, and the M 3 M 4  via and M 4 M 5  via layers, of the cell unit  200 . The routing connections shown in FIG. 18, correspond to the straight strips  133  and the stepped strips  132  shown in FIGS. 9 and 11. FIG. 18 also shows the L-shaped tunnel  140 , embodied in the M 3  layer, connecting the Southmost end of strip  133  to the Northmost end of strip  132 . FIG. 18 further illustrates a series of S-shaped contacts  210 ,  212 ,  214  and  216 , in layer M 4 , for providing a shift between strips  44  of layer M 5  using the M 4 M 5  vias, as described hereinabove with respect to FIG.  5 . The contacts  210 ,  212 ,  214  and  216 , help to reduce the crosstalk between parallel strips, as discussed hereinabove with reference to FIG.  5 . FIG. 18 also shows multiple bands  217 ,  219 ,  221 ,  223  and  225 , which run in the North-South direction, corresponding to the band  111  of FIG.  9 . 
     Reference is now made to FIG. 19, which illustrates an M 5  layer corresponding to the arrangement described hereinabove with respect to FIG.  5 . The strips  44  in the E-W direction, shown in FIG. 19, correspond to the strips  44  of FIG.  5 . FIG. 19 also shows bridging elements  126  between strips  116  and  133  of FIGS. 9 and 11, and a series of M 4 M 5  vias  134 . 
     Reference is now made to FIG. 20, which shows the M 6  layer with vias M 5 M 6 , corresponding to the M 6  layers of FIG.  9 . Additionally, FIG. 20 shows the strips  16 / 117  and  12 / 112  corresponding to the strips in FIGS. 1 and 9, and strip  116  corresponding to the strips in FIG.  9 . FIG. 20 also shows typical I/O connections  230 ,  232 ,  234 ,  236  and  238 . 
     Reference is now made to FIG. 21, and shows a typical arrangement of  16  cells  200  of M 3  and M 4  layers, and the M 3 M 4  via and M 4 M 5  via layers, in a 4×4 matrix, in accordance with a preferred embodiment of the present invention. FIG. 21 also shows the strips  132  and  133 , corresponding to strips  132  and  133  of FIGS. 9 and 11. FIG. 21 further illustrates a series of S-shaped contacts  240 ,  242 ,  244 ,  246  in layer M 4 , as described hereinabove with respect to FIG. 18, for providing a shift between strip  44  of layer M 5  using the M 4 M 5  vias, as described hereinabove with respect to FIG.  5 . 
     Reference is now made to FIG. 22, which illustrates an M 5  layer comprising a 4×4 matrix of  16  cells  200 , in accordance with a preferred embodiment of the present invention. The M 5  layer comprises strips  44 / 22 , as shown in FIGS. 3 and 5, and also shows typical bridges  250  and  252 , corresponding to the bridge  126  of FIG.  11 . The bridge  250  is in the East direction and the bridge  252  is in the West direction. 
     Reference is now made to FIG. 23, which illustrates an M 6  layer and M 5 M 6  via layer of a 4×4 cell  200  matrix, in accordance with a preferred embodiment of the present invention. The M 6  layer typically comprises multiple spaced bands  260 ,  262 ,  264  and  266 , which run in the East-West direction. The multiple cells  260  to  266  correspond to the multiple spaced bands  10  of FIG. 1, and to the E-W bands  110  of FIG.  9 . 
     Reference is now made to FIG. 24, which illustrates the layers M 3 , M 4 , M 5 , M 6  and M 7  in a 4×4 cell matrix, in accordance with a preferred embodiment of the present invention. 
     It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove as well as modifications and variations thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not in the prior art.