Patent Publication Number: US-7913212-B2

Title: Method for determining a length of shielding of a semiconductor integrated circuit wiring

Description:
This application is a divisional of U.S. patent application Ser. No. 10/914,141, filed Aug. 10, 2004 now U.S. Pat. No. 7,361,975, which is a divisional of U.S. Application No. 10/694,997 filed Oct. 29, 2003 now U.S. Pat. No. 6,972,493, which is a Divisional of Application No. 09/963,469 filed Sep. 27, 2001 now U.S. Pat. No. 6,664,638, which claims priority to Application No. 2000-387890 filed Dec. 20, 2000 in Japan the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to semiconductor integrated circuits, and particularly relates to a semiconductor integrated circuit which has cross-talk noise thereof reduced by shielding wire lines. 
     2. Description of the Related Art 
     In electric circuits, inductance coupling and capacitance coupling between signal lines cause cross-talk noise. The greater the inductance coupling and capacitance coupling between lines, the greater the cross-talk noise is. In semiconductor devices such as large-scale integrated circuits, inductance coupling and capacitance coupling between lines increase as the circuit density increases thereby causing noise to appear more conspicuously. In order to reduce cross-talk noise caused by the inductance and capacitance coupling, shielding wire lines are used. 
     Use of shielding wire lines gives rise to problems in that they have relatively large wiring resistance per unit length. 
     First, large wiring resistance results in electric charging and discharging in the shielding wire lines being slowed, thereby reducing a shielding effect. 
     Second, when a shielding wire line is connected to the ground voltage or to the power supply voltage at several points along the line, an excessively large current runs through the shielding wire line, causing the problem of E-MIG (electro migration). The E-MIG occurs when the gradient of a power supply voltage in the LSI is ignored. For example, the power supply voltage differs between a point close to the power supply pin of the LSI and the center point of the LSI. If a shielding wire line is laid out between the points having a voltage difference, and is clipped to the power supply voltage at two end points thereof, an excessively large current ends up running through the shielding wire line. In order to circumvent this, analysis of power supply networks may be conducted to detect points where requirements of current density are not met, followed by making corrections one by one. Since there is no guarantee that this will converge, a time spent on the process of analyzing and making corrections would be regarded as a problem, especially when the speed of product development is given high priority. 
     Third, conventional shielding is made by providing a shield along all the extent of a shielded line. As a result, shielding wire lines consumes a wiring area to a large extent, giving rise to a problem in that an efficient layout becomes difficult to achieve. To obviate this, such measures as increasing the number of wiring layers or enlarging chip size may be conceivable. It is desirable, however, to provide efficient shielding wire lines under the given limitations of the number of wiring layers and chip-size. 
     Accordingly, there is a need for a method of laying out effective and efficient shielding wire lines and a semiconductor integrated circuit that has effective and efficient shielding wire lines. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide a semiconductor integrated circuit that substantially obviates one or more of the problems caused by the limitations and disadvantages of the related art. 
     Features and advantages of the present invention will be set forth in the description which follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by a semiconductor integrated circuit particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention. 
     To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a semiconductor integrated circuit, including a shielded wire line and a shielding wire line provided for the shielded wire line and divided into a plurality of segments in a longitudinal direction of the shielded wire line. 
     In the invention described above, the shielding wire line is divided into a plurality of segments, which makes smaller an effect of wiring resistance, thereby speeding up the electric charging and discharging of the shielding wire line. This improves the shielding effect. Further, division into the plurality of segments eliminates a need to connect a shielding wire line to a power supply potential or a ground potential at two positions far apart in the semiconductor integrated circuit. This obviates the problem of E-MIG. 
     According to another aspect of the present invention, the semiconductor integrated circuit as described above is such that at least one of the plurality of segments is connected to a first power supply potential, and remaining ones of the plurality of segments are connected to a second power supply potential different from the first power supply potential. 
     In the invention described above, the shielding wire line provided for the shielded wire line is connected to both the power supply potential and the ground potential. This configuration allows clippings to be made at a relatively large number of points, compared with a configuration in which shielding wire lines provided for a shielded wire line are connected to only either the ground potential line or the power supply potential line as in the case of conventional shielding wire lines. Provision of clippings at a larger number of points makes an effect of wire resistance lower, thereby increasing a shielding effect. 
     According to another aspect of the present invention, the semiconductor integrated circuit as described above is such that at least one of the plurality of segments is connected to a power supply potential only at a single connection point. 
     In the invention described above, the shielding wire line provided for the shielded wire line is divided into the plurality of shielding segments, each of which is fixed to a certain potential at a single clipping position. Since each shielding segment is provided with a potential through a single clipping position, the problem of E-MIG is eliminated. 
     Further, the present invention provides a semiconductor integrated circuit, including a shielded wire line and a shielding wire line provided for the shielded wire line and having a width broader than that of the shielded wire line. 
     Moreover, the present invention provides semiconductor integrated circuit, includes a shielded wire line and a plurality of shielding wire lines provided for the shielded wire line on one side of the shielded wire line. 
     Use of a broader shielding wire line or multiple shielding wire lines improves an shielding effect. 
     According to another aspect of the present invention, a semiconductor integrated circuit includes a shielded wire line and a shielding wire line provided along only a portion of an entire extent of the shielded wire line. 
     In the invention described above, a shielding wire line is provided only for a portion of an entire extent of the shielded wire line, thereby achieving shield wiring that makes an efficient use of wiring areas. 
     Further, the semiconductor integrated circuit as described above further includes a driver that transmits a signal to the shielded wire line, wherein the portion of the entire extent of the shielded wire line along which the shielding wire line is provided is a portion on a side of the driver. 
     In the invention described above, the shielding wire line is provided only for a portion of the entire extent of the shielded wire line on a side of the driver. A signal propagating through the shielded wire line has such a rising edge that the closer it is to the driver, the steeper the slope is. Accordingly, the effect of cross-talk noise that the shielded wire line on the noise source side has on the shielded wire line on the noise recipient side is the strongest near the driver, and becomes increasingly weak as it increases the distance from the driver and comes closer to the receiver. If a portion of the extent of the shielded wire line is shielded on the side of the driver, therefore, the effect of cross-talk noise can be sufficiently reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are drawings for explaining a semiconductor integrated circuit that has a shielding wire line provided according to a first embodiment of the present invention 
         FIG. 2  is a drawing showing a variation of shielding wire lines according to the present invention; 
         FIG. 3  is a drawing showing a variation of shielding wire lines according to the present invention; 
         FIG. 4  is a drawing showing a further variation of the shielding wire lines according to the present invention; 
         FIG. 5  is a drawing for explaining a semiconductor integrated circuit having shielding wire lines according to a second embodiment of the present invention; 
         FIGS. 6A through 6C  are drawings for explaining a shielding effect obtained when only a portion of the extent of the shielded wire line is shielded on the side nearer to the driver; 
         FIG. 7  is a flowchart of a process performed when the method of laying out shielding wire lines according to the present invention is applied to CAD; 
         FIG. 8  is a flowchart of a process of determining widths and numbers of shielding wire lines (ST 10 ) shown in  FIG. 7 ; 
         FIG. 9  is a flowchart of the step of determining lengths of shielding wire lines (ST 9 ) shown in  FIG. 7 ; and 
         FIG. 10  is an example of a table that shows the relationship between the length of parallel shielding wire lines and the length of necessary shielding wire lines. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, embodiments of the present invention will be described with reference to the accompanying drawings. 
       FIGS. 1A and 1B  are drawings for explaining a semiconductor integrated circuit that has a shielding wire line provided according to a first embodiment of the present invention.  FIG. 1A  shows a portion of shielding wire lines of a semiconductor integrated circuit laid out according to the present invention, and  FIG. 1B  shows a layout of conventional and typical shielding wire lines for the comparison purpose. 
     As shown in  FIG. 1A , in the present invention, shielding wire lines  1  are provided for a shielded wire line  2 . The shielding wire lines  1  are each divided into a plurality of shielding segments. Each shielding segment is connected to a power supply potential line  5  or to a ground potential line  7 . Here, connections between wire lines are made through contacts  6  or  8 . 
     In the present invention as described above, a shielding wire line  1  provided for the shielded wire line  2  is connected to both the power supply potential line  5  and the ground potential line  7 . The present invention allows clipping (connections) to be made at a relatively large number of points, compared with a configuration in which shielding wire lines  101  provided for a shielded wire line  102  are connected only to the ground potential line  7  and not to the power supply potential line  5  as in the case of conventional shielding wire lines shown in  FIG. 1B . Provision of clippings at a larger number of points makes an effect of wire resistance lower, thereby increasing a shielding effect. Namely, wire resistance is reduced, so that electric charging and discharging in the shielding wire lines become faster, resulting in a larger shielding effect. 
     In the present invention, further, a shielding wire line  1  provided for the shielded wire line  2  is divided into the plurality of shielding segments, each of which is fixed to a certain potential at a single clipping position. Since each shielding segment is provided with a potential through a single clipping position, the problem of E-MIG is eliminated. 
       FIG. 2  is a drawing showing a variation of shielding wire lines according to the present invention. In  FIG. 2 , the shielding wire lines  1  are designed to have a broader widths than the shielded wire line  2 . Enlargement of the width of the shielding wire lines  1  makes it possible to reduce wiring resistance, thereby enhancing the shielding effect. Namely, a reduction in the wiring resistance results in electric charging and discharging of the shielded wire lines being faster, further resulting in the increase of shielding effects. 
       FIG. 3  is a drawing showing a variation of shielding wire lines according to the present invention. In  FIG. 3 , a plurality of shielding wire lines  1  are provided for a single shielded wire line  2  on both sides thereof (or on one side thereof). Arranging the multiple shielding wire lines  1  adds to the shielding effect. 
       FIG. 4  is a drawing showing a further variation of the shielding wire lines according to the present invention. In  FIG. 4 , the shielding wire lines  1  provided for the shielded wire line  2  are divided into a plurality of shielding segments. Each shielding segment is connected to the ground potential line  7 , for example. Here, connections between the wire lines are made through contacts  8 . 
     In the configuration of  FIG. 4  described above, the shielding wire lines  1  provided for the shielded wire line  2  are each divided into a plurality of shielding segments, each of which is fixed to a certain potential at a single clipping point. Applying of potential to each shielding segment through a single clipping point eliminates the problem of E-MIG. In the example of  FIG. 4 , each shielding segment is connected to the ground potential line  7 , but may as well be connected to the power supply potential line  5 . In this manner, all the shielding segments may be connected to only either the power supply potential lines  5  or the ground potential lines  7  as shown in  FIG. 4 , rather than connected to both the power supply potential lines  5  and the ground potential lines  7  as shown in  FIG. 1 . This configuration as well obviates the problem of E-MIG. 
       FIG. 5  is a drawing for explaining a semiconductor integrated circuit having shielding wire lines according to a second embodiment of the present invention. 
     In  FIG. 5 , the shielded wire line  2  is driven by a driver  3 , and is provided with the shielding wire lines  1 . As shown in  FIG. 5 , the second embodiment of the present invention provides the shielding wire line  1  such that it shields only a portion of the entire extent L of the shielded wire line  2  on the side of the driver  3 . 
       FIGS. 6A through 6C  are drawings for explaining a shielding effect obtained when only a portion of the extent of the shielded wire line is shielded on the side nearer to the driver. 
       FIG. 6A  illustrates a signal that is supplied by a driver  3 A to a shielded wire line  2 A laid out parallel to the shielded wire line  2  of  FIG. 5 , and shows the signal on the noise source side.  FIG. 6B  illustrates a voltage waveform of a signal that is received by a receiver  4  as it is supplied from the driver  3  through the shielded wire line  2  of  FIG. 5 , and shows the signal on the noise recipient side. As shown in  FIG. 6A , a signal dropping to LOW is supplied from the driver  3 A to a receiver  4 A through the shielded wire line  2 A that extends parallel to the shielded wire line  2 . When a signal rising to HIGH is supplied from the driver  3  to the shielded wire line  2  of  FIG. 5 , as shown in  FIG. 6B , the voltage waveform of a signal received at the receiver  4  shows a delay ΔDelay due to cross-talk noise caused by the signal change of the shielded wire line  2 A. 
       FIG. 6C  shows an example of the relationship between the delay ΔDelay and the length of the shielding wire line  1  provided on the side of the driver  3  for the purpose of shielding a portion of the extent of the shielded wire line  2  when the length L of the shielded-wire line  2  of  FIG. 5  is 10 mm. The signal propagating through the shielded wire line has such a rising edge that the closer it is to the driver, the steeper the slope is. Accordingly, the effect of cross-talk noise that the shielded wire line on the noise source side has on the shielded wire line on the noise recipient side is the strongest near the driver, and becomes increasingly weak as it increases the distance from the driver and comes closer to the receiver. If a portion of the extent of the shielded wire line  2  is shielded on the side of the driver  3 , therefore, the effect of cross-talk noise can be sufficiently reduced. 
     In the example of  FIG. 6C , when approximately 70% (7 mm) of the total length of 10 mm of the shielded wire line  2  is shielded, the delay ΔDelay caused by cross-talk noise is substantially equal to that of the case in which the entirety of the shielded wire line  2  is shielded. Namely, in this example, a 70% shield length can achieve a delay reduction that is substantially the same as the 100% shield length. Further, if it is desired to set the delay ΔDelay to about 4, a shield that is 4 mm in length may be provided as can be seen from the figure. 
     At the stage of laying out shielding wire lines during the circuit design process, the relationship between the shield length and the delay time as shown in  FIG. 6C  may be provided as a table or a formula approximating the relationship where coupling capacitance and resistance may be provided as parameters. Such a table and an approximating formula may be provided as a library, and this library may be used to efficiently layout shielding wire lines. Details of shielding wire line layout will be described later. 
     In the second embodiment of the present invention as described above, a shielding wire line is provided only for a portion of the entire extent of the shielded wire line, thereby making it possible to arrange shielding wire lines that make efficient use of wiring areas compared with the related art. Further, the length of the shielding wire lines can be adjusted according to the extent of desired delay reduction, so that use of the wiring areas can be optimized, thereby significantly improving the layout efficiency. 
       FIG. 7  is a flowchart of a process performed when the method of laying out shielding wire lines according to the present invention is applied to CAD (computer aided design). 
     As major differences when compared with a typical process flow of a conventional CAD system, the method of laying out shielding wire lines according to the present invention is applied to step ST 5  of laying out shielding lines, step ST 6  of cutting shielding wire lines, step ST 9  of determining lengths of shielding wire lines, and step ST 10  of determining widths and numbers of shielding wire lines. 
     In  FIG. 7 , at step ST 1 , the arrangement of cells and macros is determined. 
     At step ST 2 , the positions of clock-signal lines and specified nets (specified wire line networks: wire lines necessary to be provided with shields) are determined. 
     At step ST 3 , spacing is performed with respect to the specified nets so as to secure wiring areas necessary for shielding wire lines. Further, step ST 10  is performed to determine widths and numbers of shielding wire lines. Details of the step ST 10  will be described later. 
     At step ST 4 , the positions of wire lines of other nets are determined. 
     At step ST 5 , the positions of shielding wire liens are determined. Further, step ST 9  is performed to determine lengths of the shielding wire lines. Details of the step ST 9  will be described later. 
     At step ST 6 , the shielding wire lines are cut. That is, the shielding wire lines are divided into a plurality of shielding segments at appropriate positions. This provides the advantages that were described in connection with  FIG. 1  and/or  FIG. 4 . Namely, provision of clippings at many positions reduces the effect of wiring resistance, and makes faster the electric charging and discharging of the shielding wire lines, thereby enhancing the shielding effect. Further, division into a plurality of shielding segments and connecting each shielding segment to a fixed potential at a single clipping point can eliminate the problem of E-MIG. 
     At step ST 7 , a circuit check is made by use of tools. If check results are not proper, errors are corrected at step ST 8 . After correction of errors, the procedure goes back to step ST 1  or step ST 2  to redesign the circuit, starting from a relevant designing stage. If the check results are proper, the procedure comes to an end. 
       FIG. 8  is a flowchart of a process of determining widths and numbers of shielding wire lines (ST 10 ) shown in  FIG. 7 . 
     At step ST 101 , a technology used for manufacturing the semiconductor integrated circuit is identified. 
     At step ST 102 , {T−(S+O)}/2 is calculated with respect to each wiring layer of the semiconductor integrated circuits, thereby obtaining a maximum shield width. Here, T is a total number of tracks, S the number of shielded nets, and O the number of other nets. The obtained maximum shield width indicates a maximum width that can be used for shielding wire lines. 
     At step ST 103 , a decision is made as to whether the width of shielding wire lines is set to a broader width or to a normal width. A table is provided that indicates whether broader width wire lines are usable with respect to each technology used for manufacturing of semiconductor integrated circuits, and a library including this table is referred to when deciding whether the broader width wire lines are available in the technology that is identified at step ST 101 . If the broader width is available, the procedure goes to step ST 104 . Otherwise, the procedure goes to step ST 105 . 
     At step ST 104 , the width of shielding wire lines is determined. This decision is made by referring to the maximum shield width obtained at step ST 102 . 
     At step ST 105 , the number of shielding wire lines is determined. This decision is made by referring to the maximum shield width obtained at step ST 102 . 
     In this manner, the step of determining widths and numbers of shielding wire lines (ST 10  of  FIG. 7 ) is carried out and completed. 
       FIG. 9  is a flowchart of the step of determining lengths of shielding wire lines (ST 9 ) shown in  FIG. 7 . 
     At step ST 201 , parallel wire lines are searched for based on net information. This is a step of detecting portions where the shielded wire lines and shielding wire lines run parallel to each other. 
     At step ST 202 , the lengths of shielding wire lines are determined. Here, a table is provided as a library such that the table indicates the relationships between the length of parallel wire lines and the length of necessary shielding wire lines, and this library is referred to when determining the length of shielding wire lines necessary for achieving a desired shielding effect. 
       FIG. 10  is an example of a table that shows the relationship between the length of parallel shielding wire lines and the length of necessary shielding wire lines. In the example of  FIG. 10 , when a 75% delay reduction is desired under the condition of parallel wire lines being 10 mm in length, for example, it can be learned from the table that a length of 6 mm is necessary as a shielding wire line. If a 65% reduction of the delay is sufficient, for example, it can be learned that a length of 5 mm suffices as a shielding wire line. In addition to the parallel wire line length of 10 mm, a parallel wire line length of 9 mm, 8 mm, and so on are listed, for which lengths of shielding wire lines necessary for desired delay reduction ratios are provided in a table format. Further, a plurality of tables using coupling capacitances and resistances as parameters may be provided as a library, so that a table matching relevant conditions is selected to obtain the length of shielding wire lines necessary for desired delay reduction ratios. 
     The relationship between the parallel wire line length and the necessary shielding wire line length may be provided as a library in the form of approximating formulas. 
     If the tolerable delay is known, a minimum wire line length for securing a desired delay is selected to attain an efficient arrangement of shielding wire lines that occupies no more than a necessary wiring area. Even if the tolerable delay is unknown, it is apparent that the shielding of the entire length of 10 mm of the shielded wire line is not different from the shielding of a partial extent of 7 mm in terms of an effect of cross-talk reductions in the example of  FIG. 10 . In such a case, a shield length of 7 mm may be selected so as to achieve shielding that makes efficient use of wiring areas. 
     Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention. 
     The present application is based on Japanese priority application No. 2000-387890 filed on Dec. 20, 2000, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.