Abstract:
The invention discloses a method and apparatus for modifying, as appropriate, the geometries of a polygon. Based on various attributes associated with the polygon and its surroundings, modification of the location of the edge segments may conditionally occur. Additionally, if these modifications occur, a method to minimize the introduction of short edges during the modification is provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application No. 09/882,802, filed Jun. 14, 2001, now U.S. Pat. No. 6,574,784 priority from the filing date of which is hereby claimed under 35 U.S.C. §120. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of electronic design automation software. More specifically, the invention relates to the automatic adjustment of layout of integrated circuit designs. 
     BACKGROUND OF THE INVENTION 
     To be able to continually increase the gate count of semiconductor devices on fixed die size, integrated circuit (IC) designs have involved shrinking feature sizes. For the next decade, the outlook is strong for photolithography to continue to be the process by which IC are manufactured. When processing the features in today&#39;s deep sub-micron processes, the wavelength of light used in the photolithography process is less than that of the feature size. A result of the use of photolithography under these “tight” conditions is that the resulting design, notwithstanding the use of phase shift masking, does not precisely match the desired design. 
     A method of automatically correcting the resulting differences involves making subtle modifications to the mask or reticle used in the photolithography process (hereinafter collectively referred to as mask). These modifications are termed optical proximity corrections or optical and process corrections. Whether the term is referring to optical distortions alone or for process distortions in addition to optical distortions determines which term is the proper term to use. Regardless of the reason for these corrections, the discussions herein will generically refer to either or both of these types of corrections as OPC. 
     There are two basic types of OPC, rule-based and model-based. Rule based OPC applies corrections to the mask based on a predetermined set of rules. Thus, if an analysis of the mask determines that the mask meets a predetermined set of conditions, a process applies the appropriate correction to the mask for the conditions met. The corrections resulting from the rule-based approaches are typically less accurate, when compared to model based correction. However, rule-based corrections are more computationally efficient, and less costly. In contrast, a model-based OPC technique uses process simulation to determine corrections to the masks. The model-based OPC corrections, generated in accordance with the results of these simulations, generally provide for greater accuracy than the corrections provided by rule-based OPC. However, model-based OPC is computationally intensive and therefore time consuming as well as costly. 
     SUMMARY OF THE INVENTION 
     The invention discloses a method and apparatus for modifying, as appropriate, the geometries of a polygon. Based on various attributes associated with the polygon and its surroundings, modification of the location of the edge segments may conditionally occur. Additionally, if these modifications occur, a method to minimize the introduction of short edges during the modification is provided. 
     In one embodiment of the present invention, if the spacing between an edge segment and the nearest feature outside of a polygon comprising the edge segment is below a certain threshold, the edge segment will be negatively biased. 
     In one embodiment of the present invention, if the length of an edge segment, as a result of biasing, is too short as compared to a reference value, the edge will be lengthened by shortening adjacent edge segments and lengthening the short edge segment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 —A flowchart of process flow of one embodiment of the present invention. 
     FIG.  2 —A sample polygon layer to which the flowchart of the embodiment of FIG. 1 is applied. 
     FIG.  3 —A resulting polygon layer from the application of the process embodiment of the FIG. 1 to the sample polygon layer of FIG.  2 . 
     FIG.  4 —A more complex correction based on additional dimensions. 
     FIG.  5 —A table approach to correction of edge placement. 
     FIG.  6 —A polygon with no correction. 
     FIG.  7 —The polygon of FIG. 6 with 2-dimensional bias correction applied. 
     FIG.  8 —The, polygon of FIG. 7 with 3-dimensional bias correction applied. 
     FIG.  9 —The 2-dimentional table applied to the polygon of FIG. 6 establishing the bias shown in FIG.  7 . 
     FIG.  10 —A 3-dimentional table applied to the polygon of FIG. 6 establishing the bias shown in FIG.  8 . 
     FIG.  11 —illustrates an embodiment of the invention where length values of a polygon are taken into account in determining edge segment biases. 
     FIG.  12 —An example partial layer of polygon showing width and spacing to which an embodiment of this invention may be applied. 
     FIG.  13 —A 2-dimensional table of correction, in accordance with one embodiment, to be applied to the example partial layer polygon of FIG.  12 . 
     FIG.  14 —The resulting polygon from the application of the table of correction of FIG. 13 to the example partial layer polygon of FIG.  12 . 
     FIG.  15 —Resulting polygon from a space-priority based bias. 
     FIG.  16 —Example violation of minimum edge length during rule based OPC correction. 
     FIG.  17 —Results of applying short edge corrections to rule based OPC. 
     FIG.  18 —An example computer incorporated with an embodiment of the present invention. 
     FIG.  19 —An Electronic Design Automation (EDA) Tool Suite incorporated with the teachings of the present invention. 
     FIG.  20 —A networking environment suitable for practicing the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, various aspects of the present invention will be described. For purposes of explanation specific numbers, materials and configurations are set forth in order to provide a thorough understanding of the present invention. In some instances, well-known features are omitted or simplified in order not to obscure the present invention. 
     Various operations will be described as multiple discrete steps, in a manner that is most helpful in understanding the present invention, however, the order of description should not be construed as to imply that these operations are necessarily order dependent. In particular, these operations need not be performed in the order of presentation. Further, the description repeatedly uses the phrase “in one embodiment”, which ordinarily does not refer to the same embodiment, although it may. 
     Width and Space Based OPC 
     FIG. 1 shows a flowchart of a layout correction process in accordance with one embodiment of the present invention. This flowchart depicts a process for applying rules that advantageously determine whether to apply a correction to an edge segment corresponding to a portion of a line segment defining a side of a polygon. This polygon typically represents a feature on a layer of a design of an integrated circuit. The rules are based on the width of the polygon at the portion of the edge segment and the spacing between the portion of the edge segment of the polygon and the nearest structure. FIG. 3 shows the resulting polygons with at least one edge segment of Poly 1  modified when the process of FIG. 1 is applied to the polygon structure of FIG.  2 . 
     In this embodiment of the invention, each side of each polygon in a layer is processed. Each side of a polygon is processed as a line segment, and each line segment of a polygon is further divided into one or more edge segments  110 . Thus, an edge segment is at least a portion of a line segment (or side) of a polygon. Various embodiments of the invention determine which portion of the line segment defines an edge segment. 
     In one embodiment of the invention, the method ascertains which portion of a line segment defines an edge segment by determining at least two attribute sets along the line segment. The first attribute set is the spacing distances between the various portions of the line segment and their corresponding closest neighboring structures outside the polygon. The second attribute set is the widths of the polygon for the various portions of the, line segment. That is, the distance from the various portions of the line segment to corresponding portions of a line segment or segments on the opposite side of the polygon. An edge segment will be a contiguous portion of the line segment where each of these two attributes remains constant. In other words, when continuing on the line segment, if either the width of the polygon or the spacing between the polygon and another outside structure changes, then this signals the beginning of a new edge segment. 
     FIG. 2 shows an example of dividing a side, or line segment, of a polygon into edge segments. The “bottom” side of Poly 1  is divided by the present embodiment of the invention into the four edge segments as shown, edge segments  210 - 216  in FIG.  2 . In this example, the width of Poly 1  is constant (0.30) so any division of the bottom edge of Poly 1  is made based on spacing between the various portions of the line segment and the corresponding nearest neighboring outside structures. Beginning at the bottom right corner, the spacing between Poly 1  and the next structure, Poly 3 , is 0.7. In determining the edge segment definition, one continues along the bottom edge until either the space or the width attribute changes. In this example, this condition occurs at point  226 . The portion of the edge from the corner to this point  226  defines edge segment  216 . Continuation of this process for the remaining portions of the bottom edge of Poly 1  results in the definition of edge segments  210 - 216 . 
     In this embodiment of the present invention, during processing of an edge segment, the spacing between the edge segment and an outside nearest neighboring structure of the polygon, with respect to the edge segment currently being processed, is compared to a reference value  120 . In the example shown in FIG. 1, the reference value is 0.25 units. If the spacing is not below the reference value of this embodiment, then the process does not modify the edge segment, and the next edge segment is processed. If however, the spacing is less than the reference value, in this case 0.25, the process further checks the width of the polygon along the edge segment currently being processed. If the width of that polygon is above a certain reference value, in this case 0.27, then the process reduces the width of the polygon at the edge segment by 0.01 units. This reduction in the width is phrased as a negative bias, whereas an increase in the width of a polygon is phrased as a positive bias. 
     As previously mentioned, the application of the process in FIG. 1 to the polygon structures of FIG. 2 results in the modified polygon structures shown in FIG.  3 . In processing the polygon structure of FIG. 2, pursuant to the process of FIG. 1, only edge segments  218  and  212  result in affirmative responses to queries  120  and  130  of FIG.  1 . As a result, these are the only two edge segments which have biases correspondingly applied to them. In each case, the bias applied is a −0.01, and the resulting edge segments are shown in FIG.  3 . The width of Poly 1  at edge segment  312  is reduced to 0.29. Similarly, the width of edge segment  318  in Poly 2  is also reduced to 0.29. 
     In one embodiment, the processing and the bias applications are algorithmically effectuated, whereas in another embodiment, a table is preferentially employed. 
     FIG. 4 shows an example pseudocode for the handling of edge segments of a layer based on a slightly more complicated scheme than that shown above. The bias to an edge segment, as shown in FIG. 4, varies greatly in this embodiment of the present invention. In an alternate embodiment, a table that parallels the values of the algorithm is employed instead, as shown in FIG.  5 . In one embodiment of the present invention, the table is implemented via an array data structure. In another embodiment of the present invention, the table is implemented via a hash table and supporting functions. 
     By utilizing a table-based approach, each edge segment can be bucketized. That is, each edge segment can be placed in a bucket that corresponds to a unique cell entry in a table. Thus each bucket may have any number of edge segments that have attributes that match the requirements of each cell. For example, using the embodiment shown in FIG. 5, all edges with (0.4&lt;spacing&lt;=0.7) and (0.2&lt;=width&lt;0.3) will fall into a bucket corresponding to cell  510 . Thus, in one embodiment of the present invention, as the edges are processed by the present invention, when it is determined that an edge&#39;s bias should be changed, it is merely moved to a different bucket. In this manner, the edges are not immediately biased, the biases are performed at the end of the processing of the determination of each bias. This approach advantageously reduces the processing time vis-á-vis an embodiment where each edge is biased as the appropriate biased is determined. 
     Additional Dimension OPC Approach 
     Where more granularity of edge segment adjustment is needed vis-á-vis a two-dimensional approach, embodiments of the present invention may employ other dimensions, in addition to width and spacing, in determining the appropriate bias values. In one embodiment of the present invention, the length of the polygon at the edge segment undergoing processing is also used to determine the proper bias value. The length of the polygon in such an embodiment is the same as the length of the edge segment. For example, FIG. 6 shows a polygon with eight sides. Each side comprises a single edge segment except the bottom side whose length is 1.0 but which is comprised of 3 edge segments  670 ,  610 ,  680 , of 0.28, 0.40 and 0.32 length, respectively. Thus, in this embodiment, when processing the three edge segments, these individual lengths of the polygon at the edge segment only will be used in determining the OPC bias value vis-á-vis the length of the entire side. 
     FIG. 9 shows a table which applies a bias to edge segments based on two attributes of the edge segment, the width of the polygon at the edge segment and the distance of the edge segment from the nearest neighboring structure outside the polygon. Assuming the nearest neighboring structure to the edge segment of the polygon in FIG. 6 is greater than 0.7, FIG. 7 shows a resulting polygon with edge segments of FIG. 6 biased via the bias values as set out in the table of FIG.  9 . The two  710  edge segments are biased for a positive 0.02 units based on the proper determination from table of FIG.  9 . Note that other edge segments  720 - 750  on the polygon are biased by the same amount by applying the two-dimensional bias rules. The biasing that occurs is performed with respect to the width of the polygon and the spacing to the nearest neighboring structure outside the polygon. The biasing does not account for any extra dimensions such as length of the edge segment being processed by the present embodiment. 
     FIG. 8 shows the polygon from FIG. 6 biased via another embodiment of the present invention. This embodiment of the invention uses an additional dimension of length to ascertain the correct bias value. FIG. 10 shows a portion of a table used to determine the proper biases for this embodiment. Refer now to edge segment  820  in FIG.  8 . The width measurement for this edge segment, as shown in FIG. 8, is 1.0 and the length measurement for this segment is 0.2. Note in the table in FIG. 10 at row  1020 , that when the length value is less than 0.3, the bias value is to be 0.0. For edge segments  810 , the length of these edge segments is 0.4, with the width being 0.35. As a result, these edge segments will be biased by 0.2 as detailed in the table in FIG. 10 at row  1010 . The resultant polygon structures are shown in FIG.  8 . 
     Another embodiment of the present invention takes into account a second length value in determining bias for edge segments. FIG. 11 shows another set of polygons. A first polygon  1110  has an edge with a relatively long length L1. A second polygon  1120  has an edge with a relatively short length L2  1125  below some reference length Lref (not shown). In this embodiment of the invention a user specifics a second length such that when an edge segment of a layer under consideration for biasing  1115  is referencing another polygon with an reference edge  1125  whose length is below a minimum reference length Lref, then no biasing of the edge segment under consideration will occur. This may be desirable where users do not want narrow line-end edges to be used for determining the spacing measurements. In another embodiment of the invention, the edge segment under consideration for biasing  1115  is still biased, notwithstanding the proximity of the short edge, but the bias value is attenuated as a function of the length of the second edge segment  1125 . 
     Short Edge Interdiction 
     When performing biasing as discussed above, it is likely that different edge segments may be biased by different values. This can result in original (input) edge segments that are sub divided into smaller edge segments. This may result in (1) additional edge segments that (2) may be smaller than a threshold value. If each additional edge segment is sufficiently long so that there is not a problem with the manufacturability of those edges, then it may not be necessary to attempt to rid the design of those additional edges. 
     However there are times when making adjacent edge biases the same is desired. This would occur when the introduction of new edge segments results in edge segments below a threshold value. By having edge segments below a certain value there may be effects on design rules for the given manufacturing process. 
     An aspect of the invention is the ability to not allow short edges below a certain threshold. The present invention can accomplish this by resolving inconsistent biases under certain circumstances. In one embodiment of the present invention, the inconsistent biases are resolved when one edge segment is below a user specified value. In one embodiment, the inconsistent biases are resolved when one edge is below a process specific threshold value. In one embodiment all inconsistent biases are resolved. 
     Inconsistent Biases 
     Edge Merging 
     Another aspect of the invention is the ability to determine how to resolve adjacent edge segment biases, which may be inconsistent or even conflicting. For example, when two edge segments are to be biased, which bias measurement, if any, should an algorithm apply to prohibit the introduction of additional short edges? FIG. 12 shows a layer with polygons upon which one embodiment of the present invention operates. The figure also shows measurements of varying widths of polygon B (AA-AC). Additionally, FIG. 12 shows the spacing at different points between polygons A and B (A-D). 
     FIG. 13 shows a chart with the bias values that the present embodiment of the invention will apply to an edge segment. Space B and width AB define edge segment  1210 . Space C and width AB define edge segment  1220 . Edge 2  is therefore comprised of 2 edge segments  1210  and  1220 . Looking for the appropriate bias values for the B/AB space/width combination in FIG. 13, it is determined that the bias  1310  for edge segment  1210  is 0.2. In contrast, by looking up C/AB in FIG. 13 it is determined that the bias for edge segment  1220  is 0.1. This separate application of different biases to edge segments  1210  and  1220  results in an additional edge being created, one for each biased edge segment, as shown in FIG.  14 . The addition of too many extra edges during processing is undesired behavior. 
     Thus, when two biases for adjacent edge segments differ, various embodiments of the present invention apply a resolution function to determine the correct bias value to be employed, to avoid introduction of additional edges. This resolution may be performed based on any number of criteria. One embodiment of the present invention implements a priority scheme wherein a “maximizing spacing” scheme attempts to apply biases wherein a space-attribute bias determination assumes priority over a width-attribute bias determination. For example, refer again to FIG. 12, where original spacing at C is 1.35 and the spacing at B is 1.75. Applying each of the two possible biases from the table of FIG. 13 to both  1210  and  1220  results in spacing values of 1.65 and 1.35 for B and C, respectively, in the case of a 0.1 bias, and 1.55 and 1.25 for B and C in the case of the 0.2 bias. Thus, applying the 0.1 bias value results in the maximization of the spacing for both B and C. Accordingly, in the embodiment of the invention with a “maximizing spacing” resolution, the 0.1 bias value is applied, resulting in the corrected polygon of FIG.  15 . 
     In another embodiment of the present invention, the user may specify the method of determining the resolution. In yet another embodiment of the present invention, rules associated with the process used for the IC fabrication are used to determine what the resolution function will be. 
     A situation may arise where the weighing of both biases results in a “tie” as determined by the method of the embodiment. In this case, other application dependent heuristics may be employed for tie breaking. These may include choosing a weighted bias or user specified tie breaking rules. 
     Edge Lengthening 
     Another option for resolving the occurrence of short edges is to attempt to lengthen edge segment corrections. In one embodiment of the invention, the length of an edge segment is checked against a minimum edge segment length. If the embodiment determines that the edge segment does not meet the minimum segment length, the embodiment will check adjacent edge segments to determine their length. If the embodiment establishes that there is sufficient length in the short edge segment and the adjacent edge segment combined such that length can be removed from the adjacent edge segment and added to the short edge segment, resulting in two edge segments that meet the minimum length requirements, then the edge segments are so modified. 
     Refer now to FIG. 16 wherein an example of an edge-lengthening situation appears. In this embodiment, a polygon exists  1610  where an original edge  1620  has modifications based on requirements as previous discussed. Based on criteria, the two edge segments, edge seg 1   1630  and edge seg 2   1640  are to replace the original edge  1620 . This embodiment of the invention has a minimum segment length of 0.4. However, the edge seg 2   1640  has a length of 0.2 and is shorter than the minimum segment length of 0.4. In this embodiment of the invention, the adjacent segment edge seg 1   1630  will be checked. This segment has a length of 0.8. As a result, it is possible to modify the length of edge seg 2   1640  to meet the minimum requirement of 0.4 by taking length from edge seg 1   1630 . 
     The resulting corrections are shown in FIG.  17 . In this figure, edge seg 2   1740  has been extended to meet the minimum length requirement of 0.4. There is a shorter edge seg 1   1730  reflected in the modifications made to allow edge seg 2  to meet the minimum requirements. 
     In one embodiment of this invention, only part of the requirement addition to an edge segment is taken from a single edge. This results in a short edge that does not meet the minimum requirement, but is nevertheless closer than the original. In one embodiment of the present invention, a short edge is between to other edge segments. In this embodiment “length” is taken from two adjacent edge segments when an edge segment does not meet a minimum length requirement. 
     User Device Embodiment 
     Hardware 
     FIG. 18 illustrates one embodiment of a user apparatus suitable to be programmed with the utility application of the present invention. As shown, for the illustrated embodiment, user device  1800  includes processor  1802 , processor bus  1806 , high performance I/O bus  1810  and standard I/O bus  1820 . Processor bus  1806  and high performance I/O bus  1810  are bridged by host bridge  1808 , whereas I/O buses  1810  and  1820  are bridged by I/O bus bridge  1812 . Coupled to processor bus  1806  is cache  1804 . Coupled to high performance I/O bus  1810  are system memory  1814  and video memory  1816 , against which video display  1818  is coupled. Coupled to standard I/O bus  1820  are disk drive  1822 , keyboard  1824  and pointing device  1828 , and communication interface  1826 . 
     These elements perform their conventional functions known in the art. In particular, disk drive  1822  and system memory  1814  are used to store permanent and working copies of the electronic design system. The permanent copy may be pre-loaded into disk drive  1822  in factory, loaded from distribution medium  1832 , or down loaded from a remote distribution source (not shown). Distribution medium  1832  may be a tape, a CD, a DVD or other storage medium of the like. The constitutions of these elements are known. Any one of a number of implementations of these elements known in the art may be used to form computer system  1800 . 
     Certain embodiments may include additional components, may not require all of the above components, or may combine one or more components. Those skilled in the art will be familiar with a variety of alternative implementations. 
     EDA Tool Suite 
     Refer now to FIG. 19 wherein an EDA tool suite incorporated with a “Short edge management in rule based OPC” module of the present invention in accordance with one embodiment is shown. As illustrated, EDA tool suite  1900  includes OPC module  1902  incorporated with the teachings of the present invention as described earlier with respect to FIGS. 1-17. Additionally, EDA tool suite  1900  includes other tool modules  1904 . Examples of these other tool modules  1904  include but are not limited to synthesis module, DRC module and LVS module. 
     Remote Client 
     FIG. 20 shows an embodiment of the present invention  2000  with a remote client. In this embodiment, user controls, via a user client  2010 , execution of an EDA tool suite  2040  containing a “Short edge management in rule based OPC” module incorporated with the teachings of the present invention. The user interacts with a server  2030  executing the EDA tool suite  2040  through a network  2020 . The server  2030  executes the EDA tool suite  2040  which reads the user design data  2050 , performs operations on user data  2040  and provides feedback to user via user client  2010 . In various other embodiments of the present invention the EDA tool suite, user client and user design data can be distributed amount several network elements. 
     CONCLUSION 
     In the present description, an advantageous method of performing OPC to an IC mask layout as well as a method for managing short edge generation in the layout has been described.