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United States Patent6453457
Pierrat , ; et al.September 17, 2002
Title
Selection of evaluation point locations based on proximity effects model amplitudes for correcting proximity effects in a fabrication layout
Abstract
Techniques for fabricating a device include forming a fabrication layout such as a mask layout, for a physical design layer, such as a design for an integrated circuit, and identifying evaluation points on an edge of a polygon corresponding to the design layer for correcting proximity effects. Included are techniques that correct for proximity effects associated with an edge in a layout corresponding to a design layer. An evaluation point is determined for the edge based on a profile of amplitudes output from a proximity effects model along a transect. The transect includes a target edge in the design layer corresponding to the edge. It is then determined how to correct at least a portion of the edge for proximity effects based on an analysis at the evaluation point. In other techniques, a dissection length parameter is derived based on a profile of amplitudes output by a proximity effects model along a transect. The transect includes a second edge in a second layout. An evaluation point is determined for a first edge based on the dissection length parameter. Then it is determined how to correct at least a portion of the first edge based on an analysis at the evaluation point.
Inventors:Pierrat; Christophe (Santa Clara, CA), Zhang; Youping  (Newark, CA)
Assignee:Numerical Technologies, Inc. (San Jose, CA)
Appl. No.:676356
Filed:September 29, 2000
Current U.S. Class:716/19 716/20 977/734 430/5 
Current International Class:G03F 1/14 (20060101)
Field of Search:716/19,20,21 430/5 257/618 438/129
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Primary Examiner: Smith; Matthew
Assistant Examiner: Pert; Evan
Attorney, Agent or Firm:Bever, Hoffman & Harms, LLP Harms; Jeanette S.
Parent Case Text


RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No. 09/675,582, entitled "Dissection Of Corners In A Fabrication Layout For Correcting Proximity Effects," filed on Sep. 29, 2000, invented by Christophe Pierrat and Youping Zhang.

This application is related to U.S. patent application Ser. No. 09/675,197, entitled "Dissection Of Edges With Projection Points in A Fabrication Layout For Correcting Proximity Effects," filed on Sep. 29, 2000, invented by Christopher Pierrat and Youping Zhang.

This application is related to U.S. patent application Ser. No. 09/676,375, entitled "Dissection Of Printed Edges From A Fabrication Layout For Correcting Proximity Effects," filed on Sep. 29, 2000, invented by Christophe Pierrat and Youping Zhang.

This application is related to U.S. patent application Ser. No. 09/728,885, entitled "Displacing Edge Segments On A Fabrication Layout Based on Proximity Effects Model Amplitudes For Correcting Proximity Effects," filed on Dec. 1, 2000, invented by Christophe Pierrat and Youping Zhang.

This application is related to U.S. patent application Ser. No. 09/130,996, entitled "Visual Inspection and Verification System," filed on Aug. 7, 1998.

This application is related to U.S. patent application Ser. No. 09/153,783, entitled "Design Rule Checking System and Method," filed on Sep. 16, 1998.

This application is related to U.S. patent application Ser. No. 09/544,798, entitled "Method and Apparatus for a Network Based Mask Defect Printability Analysis System," filed on Apr. 7, 2000.

This application is related to U.S. patent application Ser. No. 09/154,415, entitled "Data Hierarchy Layout Correction and Verification Method and Apparatus," filed on Sep. 16, 1998.

This application is related to U.S. patent application Ser. No. 09/154,397, entitled "Method and Apparatus for Data Hierarchy Maintenance in a System for Mask Description," filed on Sep. 16, 1998.

This application is related to U.S. patent application Ser. No. 09/632,080, entitled "General Purpose Shape-Based Layout Processing Scheme for IC Layout Modifications," filed on Aug. 2, 2000.
Claims

What is claimed is:
1. A method of correcting for proximity effects associated with an edge in a layout corresponding to a design layer, the method comprising: determining an evaluation point for the edge based on a profile of amplitudes output from a proximity effects model along a transect including a target edge in the design layer corresponding to the edge; and determining how to correct at least a portion of the edge for proximity effects based on an analysis at the evaluation point.

2. The method of claim 1, further comprising executing a routine implementing the proximity effects model for a plurality of points on the transect before said determining an evaluation point.

3. The method of claim 1, further comprising retrieving stored amplitudes for a plurality of points on the transect before said determining an evaluation point.

4. The method of claim 1, wherein the layout is the design layer for a printed features layer.

5. The method of claim 1, wherein the layout is an originally proposed fabrication layout for the design layer.

6. The method of claim 1, wherein: the layout is an adjusted fabrication layout for the design layer; and the adjusted fabrication layout includes at least a segment of the edge displaced from a corresponding target edge in the design layer.

7. The method of claim 1, before said determining an evaluation point, further comprising: determining whether the target edge is a critical edge; and if it is determined that the target edge is a critical edge, then executing a routine implementing the proximity effects model for a plurality of points on the transect.

8. The method of claim 7, wherein a target edge is a critical edge if it includes an edge of a true-gate.

9. The method of claim 7, wherein a target edge is a critical edge if it lies within a predetermined halo distance of another edge.

10. The method of claim 7, wherein a target edge is a critical edge if a corresponding edge in the layout has been adjusted for proximity effects in a previous round of corrections.

11. The method of claim 1, said determining an evaluation point further comprising determining a corresponding segment on the target edge, wherein said corresponding segment is associated with the portion of the edge corrected based on the analysis at the evaluation point.

12. The method of claim 11, said determining an evaluation point further comprising establishing an evaluation point where the profile substantially equals a predetermined percentage of a threshold value, wherein the threshold value defines an amplitude where the proximity effects model predicts an edge is printed in the printed features layer.

13. The method of claim 11, said determining the corresponding segment further comprising determining a corner segment extending from a vertex nearest the evaluation point to a corner segment dissection point where the profile exceeds a predetermined percentage of a threshold value, wherein the threshold value defines an amplitude where the proximity effects model predicts an edge is printed in the printed features layer.

14. The method of claim 11, said determining the corresponding segment further comprising determining a corner segment extending from a vertex nearest the evaluation point to a corner segment dissection point where the profile flattens.

15. The method of claim 11, said determining the evaluation point further comprising establishing a corner evaluation point where the profile substantially equals a predetermined fraction of an amplitude value where the profile flattens.

16. The method of claim 11, said determining the evaluation point further comprising establishing the evaluation point where the profile deviates more than a predetermined amount from an ideal profile having no proximity effects from another edge in the layout.

17. The method of claim 11, said determining the corresponding segment further comprising establishing the segment where the profile deviates more than a predetermined amount from an ideal profile having no proximity effects from another edge in the layout.

18. The method of claim 11, said determining the evaluation point further comprising establishing the evaluation point substantially between two local maxima in an absolute value of a first derivative of the profile.

19. The method of claim 11, said determining the corresponding segment further comprising establishing the segment to extend substantially from one local maximum in an absolute value of a first derivative of the profile to a next adjacent local maximum in the absolute value of the first derivative.

20. The method of claim 11, said determining the corresponding segment further comprising establishing the segment to extend substantially from a local maximum in an absolute value of a first derivative of the profile to a next adjacent local minimum in the absolute value of the first derivative.

21. The method of claim 11, said determining the evaluation point further comprising establishing the evaluation point substantially at a local maximum in an absolute value of a second derivative of the profile.

22. The method of claim 11, said determining the corresponding segment further comprising establishing the segment to extend substantially from a point where a second derivative of the profile crosses zero to a next adjacent point where the second derivative crosses zero.

23. A method of using a computer to correct for proximity effects associated with an edge in a layout corresponding to a design layer, the computer having a processor coupled to a computer-readable medium, the computer-readable medium storing at least a portion of the design layer, the design layer corresponding to a portion of an integrated circuit, the method comprising: determining an evaluation point for the edge based on a profile of amplitudes output from a proximity effects model along a transect including a target edge in the design layer corresponding to the edge; and determining how to correct at least a portion of the edge for proximity effects based on an analysis at the evaluation point.

24. A method of correcting for proximity effects associated with an edge in a layout corresponding to a design layer, the method comprising: determining dissection points for the edge based on a profile of amplitudes output from a proximity effects model along a transect including a target edge in the design layer corresponding to the edge; determining an evaluation point based on at least one of the profile and the dissection points; and determining how to correct a segment of the edge corresponding to two dissection points for proximity effects based on an analysis at the evaluation point.

25. A method of using a computer to correct for proximity effects associated with an edge in a layout corresponding to a design layer, the computer having a processor coupled to a computer-readable medium, the computer-readable medium storing at least a portion of the design layer, the design layer corresponding to a portion of an integrated circuit, the method comprising: determining dissection points for the edge based on a profile of amplitudes output from a proximity effects model along a transect including a target edge in the design layer corresponding to the edge, determining an evaluation point based on at least one of the profile and the dissection points; determining how to correct a segment of the edge corresponding to two dissection points for proximity effects based on an analysis at the evaluation point.

26. A method of correcting for proximity effects associated with a first edge in a first layout corresponding to a design layer for a printed features layer, the method comprising: deriving a dissection length parameter based on a profile of amplitudes output by a proximity effects model along a transect including a second edge in a second layout; and determining an evaluation point for the first edge in the first layout corresponding to the design layer based on the dissection length parameter; and determining how to correct at least a portion of the first edge for proximity effects based on an analysis at the evaluation point.

27. The method of claim 26, wherein the first layout is also the second layout.

28. The method of claim 26, wherein: the first layout is a proposed fabrication layout for the printed features layer; and the second layout is a test layout.

29. The method of claim 28, before said deriving a dissection length, the method further comprising retrieving amplitudes stored during building of the proximity effects model.

30. The method of claim 26, said deriving a dissection length parameter further comprising: determining a segment based on the profile; and setting the dissection length parameter based on a length of the segment.

31. The method of claim 30, wherein: a threshold value is an amplitude where the proximity effects model predicts an edge is printed in the printed features layer; said determining a segment further comprises establishing a corner segment extending from a vertex of the second edge to a corner segment dissection point where the profile exceeds a predetermined percentage of the threshold value; and the dissection length parameter is a corner dissection segment length.

32. The method of claim 30, wherein: said determining a segment further comprises establishing a corner segment extending from a vertex of the second edge to a corner segment dissection point where the profile flattens; and the dissection length parameter is a corner dissection segment length.

33. The method of claim 30, wherein: said determining a segment further comprises establishing a segment where the profile deviates more than a predetermined amount from an ideal profile with no proximity effects from another edge in the second layout; and the dissection length parameter is a detail dissection segment length.

34. The method of claim 30, wherein: said determining a segment further comprises establishing the segment to extend from one local maximum in an absolute value of a first derivative of the profile to a next adjacent local maximum in the absolute value of the first derivative; and the dissection length parameter is at least one of a detail dissection segment length and a maximum dissection segment length.

35. The method of claim 30, wherein: said determining a segment further comprises establishing the segment to extend from a local maximum in an absolute value of a first derivative of the profile to a next adjacent local minimum in the absolute value of the first derivative; and the dissection length parameter is at least one of a detail dissection segment length and a maximum dissection segment length.

36. The method of claim 30, wherein: said determining a segment further comprises establishing the segment to extend from a point where a second derivative of the profile crosses zero a next adjacent point where the second derivative crosses zero; and the dissection length parameter is at least one of a detail dissection segment length and a maximum dissection segment length.

37. The method of claim 30, wherein: the dissection length parameter is a halo distance; and said deriving the dissection length parameter comprises determining whether the profile deviates more than a predetermined amount from an ideal profile with no proximity effects from another edge in the first layout, if the profile deviates more than the predetermined amount, then determining a distance to a third edge in the second layout associated with each deviation, and including the distance in a set of distances; and setting the halo distance based on the set of distances.

38. The method of claim 30, wherein: said deriving a dissection length parameter further comprises, after said determining a segment based on the profile, including a length of the segment in a set of segment lengths, determining another segment based on another profile of amplitudes along another transect including another edge in the second layout, including another length of the segment in the set of segment lengths, and repeating said determining another segment for a plurality of edges in the second layout; and setting the dissection length parameter further comprises setting the dissection length parameter based on the set of segment lengths.

39. The method of claim 38, wherein: the dissection length parameter is a corner dissection segment length; a corner segment includes one vertex of an edge; and only a corner segment has a length included in the set of segment lengths.

40. The method of claim 38, wherein: the dissection length parameter is at least one of a detail dissection segment length and a maximum dissection segment length; a non-corner segment includes no vertex of an edge; and only a non-corner segment has a length included in the set of segment lengths.

41. The method of claim 38, wherein the dissection length parameter is based on a mode of the set of segment lengths.

42. A method of using a computer to correct for proximity effects associated with a first edge in a first layout corresponding to a design layer, the computer having a processor coupled to a computer-readable medium, the computer-readable medium storing at least a portion of the design layer, the design layer corresponding to a portion of an integrated circuit, the method comprising: deriving a dissection length parameter based on a profile of amplitudes output by a proximity effects model along a transect including a second edge in a second layout; and determining an evaluation point for the first edge in the first layout corresponding to the design layer based on the dissection length parameter; and determining how to correct at least a portion of the first edge for proximity effects based on an analysis at the evaluation point.

43. A method of correcting for proximity effects associated with a first edge in a first layout corresponding to a design layer for a printed features layer, the method comprising: deriving a dissection length parameter based on a profile of amplitudes output by a proximity effects model along a transect crossing a plurality of polygons in a second layout; and determining an evaluation point for the first edge in the first layout corresponding to the design layer based on the dissection length parameter; and determining how to correct at least a portion of the first edge for proximity effects based on an analysis at the evaluation point.

44. The method of claim 43, wherein the first layout is also the second layout.

45. The method of claim 43, wherein: the first layout is a proposed fabrication layout for the printed features layer; and the second layout is a test layout.

46. The method of claim 43, said deriving a dissection length parameter further comprising obtaining the profile by retrieving amplitudes stored during building of the proximity effects model.

47. The method of claim 43, wherein: the plurality of polygons comprises a plurality of polygon sets associated with corresponding pitch distances; a threshold value is a particular amplitude where the proximity effects model predicts an edge is printed in the printed features layer; and said deriving a dissection length parameter further comprises, determining whether a set of points along the profile associated with a polygon set includes a point having an amplitude that exceeds the threshold value, if it is determined that the set of points includes an amplitude that exceeds the threshold value, then including a pitch distance associated with the polygon set in a set of printable pitches, and deriving the dissection length parameter based on the set of printable pitches.

48. The method of claim 47, deriving the dissection length parameter comprises setting the dissection length parameter substantially equal to a smallest pitch distance in the set of printable pitches.

49. The method of claim 43, wherein: the plurality of polygons comprises a plurality of polygon sets associated with corresponding pitch distances; a threshold value is a particular amplitude where the proximity effects model predicts an edge is printed in the printed features layer; and the step of deriving a dissection length parameter based on the profile further comprises, finding a plurality of peak amplitudes corresponding to the plurality of pitch distances, each peak amplitude substantially equal to a maximum amplitude among all points on the profile associated with a polygon interpolating the plurality of peak amplitudes and the corresponding plurality of pitch distances to obtain a particular distance associated with the threshold value, and deriving the dissection length parameter based on the particular distance.

50. The method of claim 43, wherein the dissection length parameter is a corner dissection segment length used as a target length for a segment on the first edge that includes one vertex of the first edge.

51. The method of claim 43, wherein the dissection length parameter is a detail dissection segment length used as a target length for a segment on the first edge that includes neither vertex of the first edge.

52. A method of using a computer to correct for proximity effects associated with a first edge in a first layout corresponding to a design layer, the computer having a processor coupled to a computer-readable medium, the computer-readable medium storing at least a portion of the design layer, the design layer corresponding to a portion of an integrated circuit, the method comprising: deriving a dissection length parameter based on a profile of amplitudes output by a proximity effects model along a transect crossing a plurality of polygons in a second layout; and determining an evaluation point on the first edge in the first layout corresponding to the design layer based on the dissection length parameter; and determining how to correct at least a portion of the first edge for proximity effects based on an analysis at the evaluation point.

53. A computer readable medium for correcting proximity effects associated with an edge in a layout corresponding to a design layer, the computer readable medium carrying instructions to cause one or more processors to perform: determining an evaluation point for the edge based on a profile of amplitudes output from a proximity effects model along a transect including a target edge in the design layer corresponding to the edge; and determining how to correct at least a portion of the edge for proximity effects based on an analysis at the evaluation point.

54. A computer readable medium for correcting proximity effects associated with an edge in a layout corresponding to a design layer, the computer readable medium carrying instructions to cause one or more processors to perform: determining dissection points for the edge based on a profile of amplitudes output from a proximity effects model along a transect including a target edge in the design layer corresponding to the edge; determining an evaluation point based on at least one of the profile and the dissection points; and determining how to correct a segment of the edge corresponding to two dissection points for proximity effects based on an analysis at the evaluation point.

55. A computer readable medium for correcting proximity effects associated with a first edge in a first layout corresponding to a design layer for a printed features layer, the computer readable medium carrying instructions to cause one or more processors to perform: deriving a dissection length parameter based on a profile of amplitudes output by a proximity effects model along a transect including a second edge in a second layout; and determining an evaluation point for the first edge in the first layout corresponding to the design layer based on the dissection length parameter; and determining how to correct at least a portion of the first edge for proximity effects based on an analysis at the evaluation point.

56. A computer readable medium for correcting proximity effects associated with a first edge in a first layout corresponding to a design layer for a printed features layer, the computer readable medium carrying instructions to cause one or more processors to perform: deriving a dissection length parameter based on a profile of amplitudes output by a proximity effects model along a transect crossing a plurality of polygons in a second layout; and determining an evaluation point for the first edge in the first layout corresponding to the design layer based on the dissection length parameter; and determining how to correct at least a portion of the first edge for proximity effects based on an analysis at the evaluation point.

57. A carrier wave for correcting proximity effects associated with an edge in a layout corresponding to a design layer, the carrier wave carrying instructions to cause one or more processors to perform: determining an evaluation point for the edge based on a profile of amplitudes output from a proximity effects model along a transect including a target edge in the design layer corresponding to the edge; and determining how to correct at least a portion of the edge for proximity effects based on an analysis at the evaluation point.

58. A carrier wave for correcting proximity effects associated with an edge in a layout corresponding to a design layer, the carrier wave carrying instructions to cause one or more processors to perform: determining dissection points for the edge based on a profile of amplitudes output from a proximity effects model along a transect including a target edge in the design layer corresponding to the edge; determining an evaluation point based on at least one of the profile and the dissection points; and determining how to correct a segment of the edge corresponding to two dissection points for proximity effects based on an analysis at the evaluation point.

59. A carrier wave for correcting proximity effects associated with a first edge in a first layout corresponding to a design layer for a printed features layer, the carrier wave carrying instructions to cause one or more processors to perform: deriving a dissection length parameter based on a profile of amplitudes output by a proximity effects model along a transect including a second edge in a second layout; and determining an evaluation point for the first edge in the first layout corresponding to the design layer based on the dissection length parameter; and determining how to correct at least a portion of the first edge for proximity effects based on an analysis at the evaluation point.

60. A carrier wave for correcting proximity effects associated with a first edge in a first layout corresponding to a design layer for a printed features layer, the carrier wave carrying instructions to cause one or more processors to perform: deriving a dissection length parameter based on a profile of amplitudes output by a proximity effects model along a transect crossing a plurality of polygons in a second layout; and determining an evaluation point for the first edge in the first layout corresponding to the design layer based on the dissection length parameter; and determining how to correct at least a portion of the first edge for proximity effects based on an analysis at the evaluation point.

61. A computer system for correcting proximity effects associated with an edge in a layout corresponding to a design layout, the computer system comprising: a computer readable medium carrying data representing the edge and at least a portion of the design layout, the design layout corresponding to a portion of an integrated circuit; and one or more processors coupled to the computer readable medium, the one or more processors configured for determining an evaluation point for the edge based on a profile of amplitudes output from a proximity effects model along a transect including a target edge in the design layer corresponding to the edge, determining a correction based on an analysis of an amplitude from the profile at the evaluation point, and applying the correction to at least a portion of the edge.

62. A computer system for correcting proximity effects associated with an edge in a layout corresponding to a design layout, the computer system comprising: a computer readable medium carrying data representing the edge and at least a portion of the design layout, the design layout corresponding to a portion of an integrated circuit; and one or more processors coupled to the computer readable medium, the one or more processors configured for determining dissection points for the edge based on a profile of amplitudes output from a proximity effects model along a transect including a target edge in the design layer corresponding to the edge, determining an evaluation point based on at least one of the profile and the dissection points, determining a correction based on an analysis of an amplitude from the profile at the evaluation point, and applying the correction to at least a portion of the edge corresponding to two dissection points.

63. A computer system for correcting proximity effects associated with a first edge in a first layout corresponding to a design layout, the computer system comprising: a computer readable medium carrying data representing the edge and at least a portion of the design layout, the design layout corresponding to a portion of an integrated circuit; and one or more processors coupled to the computer readable medium, the one or more processors configured for deriving a dissection length parameter based on a profile of amplitudes output by a proximity effects model along a transect including a second edge in a second layout, determining an evaluation point for the first edge in the first layout corresponding to the design layer based on the dissection length parameter, executing a routine implementing the proximity effects model for the evaluation point to produce a model amplitude at the evaluation point, determining a correction based on an analysis of the model amplitude at the evaluation point, and applying the correction to at least a portion of the edge.

64. A computer system for correcting proximity effects associated with a first edge in a first layout corresponding to a design layout, the computer system comprising: a computer readable medium carrying data representing the edge and at least a portion of the design layout, the design layout corresponding to a portion of an integrated circuit; and one or more processors coupled to the computer readable medium, the one or more processors configured for deriving a dissection length parameter based on a profile of amplitudes output by a proximity effects model along a transect crossing a plurality of polygons in a second layout, determining an evaluation point for the first edge in the first layout corresponding to the design layer based on the dissection length parameter, executing a routine implementing the proximity effects model for the evaluation point to produce a model amplitude at the evaluation point determining a correction based on an analysis of the model amplitude at the evaluation point, and applying the correction to at least a portion of the edge.

65. A system for correcting proximity effects associated with an edge in a layout corresponding to a design layout, the system comprising: a means for determining an evaluation point for the edge based on a profile of amplitudes output from a proximity effects model along a transect including a target edge in the design layer corresponding to the edge, the proximity effects model for producing the profile; a means for analyzing an amplitude from the profile at the evaluation point to determine a correction; and a means for applying the correction to at least a portion of the edge.

66. A system for correcting proximity effects associated with an edge in a layout corresponding to a design layout, the system comprising: a means for determining dissection points for the edge based on a profile of amplitudes output from a proximity effects model along a transect including a target edge in the design layer corresponding to the edge and determining an evaluation point based on at least one of the profile and the dissection points, the proximity effects model for producing the profile; a means for analyzing an amplitude from the profile at the evaluation point to determine a correction; and a means for applying the correction to at least a portion of the edge corresponding to two dissection points.

67. A system for correcting proximity effects associated with a first edge in a first layout corresponding to a design layout, the system comprising: a means for deriving a dissection length parameter based on a profile of amplitudes output by a proximity effects model along a transect including a second edge in a second layout, a means for determining an evaluation point for the first edge in the first layout corresponding to the design layer based on the dissection length parameter, the proximity effects model for producing a model amplitude at the evaluation point; a means for analyzing the model amplitude at the evaluation point to determine a correction; and a means for applying the correction to at least a portion of the first edge.

68. A system for correcting proximity effects associated with a first edge in a first layout corresponding to a design layout, the system comprising: a means for deriving a dissection length parameter based on a profile of amplitudes output by a proximity effects model along a transect crossing a plurality of polygons in a second layout, a means for determining an evaluation point for the first edge in the first layout corresponding to the design layer based on the dissection length parameter, the proximity effects model for producing a model amplitude at the evaluation point; a means for analyzing the model amplitude at the evaluation point to determine a correction; and a means for applying the correction to at least a portion of the first edge.

69. A method for fabricating a printed features layer including features corrected for proximity effects, the method comprising: building an initial fabrication layout based on a design layer corresponding to the printed features layer, the fabrication layout including a edge of a polygon; determining an evaluation point for the edge based on a profile of amplitudes output from a proximity effects model along a transect including a target edge in the design layer corresponding to the edge, analyzing an amplitude from the profile at the evaluation point to determine a correction; applying the correction to at least a portion of the edge; producing a mask based on the portion of the edge corrected; and producing the printed features layer in a fabrication process using the mask.

70. A method for fabricating a printed features layer including features corrected for proximity effects, the method comprising: building an initial fabrication layout based on a design layer corresponding to the printed features layer, the fabrication layout including a edge of a polygon; determining dissection points for the edge based on a profile of amplitudes output from a proximity effects model along a transect including a target edge in the design layer corresponding to the edge; determining an evaluation point based on at least one of the profile and the dissection points; analyzing an amplitude from the profile at the evaluation point to determine a correction; applying the correction to at least a portion of the edge; producing a mask based on the portion of the edge corrected; and producing the printed features layer in a fabrication process using the mask.

71. A method for fabricating a printed features layer including features corrected for proximity effects, the method comprising: building an initial fabrication layout based on a design layer corresponding to the printed features layer, the fabrication layout including a first edge of a first polygon; deriving a dissection length parameter based on a profile of amplitudes output by a proximity effects model along a transect including a second edge in a second layout; determining an evaluation point for the first edge in the initial fabrication layout based on the dissection length parameter; executing a routine implementing a proximity effects model for producing a model amplitude at the evaluation point; analyzing the model amplitude to determine a correction; applying the correction to at least a portion of the edge; producing a mask based on the portion of the edge corrected; and producing the printed features layer in a fabrication process using the mask.

72. A method for fabricating a printed features layer including features corrected for proximity effects, the method comprising: building an initial fabrication layout based on a design layer corresponding to the printed features layer, the fabrication layout including a first edge of a first polygon; deriving a dissection length parameter based on a profile of amplitudes output by a proximity effects model along a transect crossing a plurality of polygons in a second layout; determining an evaluation point for the first edge in the initial fabrication layout based on the dissection length parameter; executing a routine implementing a proximity effects model for producing a model amplitude at the evaluation point; analyzing the model amplitude to determine a correction; applying the correction to at least a portion of the edge; producing a mask based on the portion of the edge corrected; and producing the printed features layer in a fabrication process using the mask.

73. A method of fabricating an integrated circuit using a mask, the mask including a segment corresponding to at least one portion of an edge in a design layer for the integrated circuit, the method comprising: determining an evaluation point for the edge based on a profile of amplitudes output from a proximity effects model along a target edge in the design layer collinear with the edge; and displacing at least a portion of the edge for proximity effects based on an analysis at the evaluation point.

74. The method of claim 73, before determining the evaluation point, further comprising: determining whether the target edge is a critical edge; and if it is determined that the target edge is a critical edge, then executing a routine implementing the proximity effects model for a plurality of points on the target edge.

75. The method of claim 74, wherein the target edge is a critical edge if it lies within a predetermined halo distance of another edge.

76. The method of claim 73, wherein determining an evaluation point includes establishing where the profile substantially equals a predetermined percentage of a threshold value, wherein the threshold value defines an amplitude where the proximity effects model predicts an edge is printed on the integrated circuit.
Description
BACKGROUND OF THE INVENTION

1. The Field of the Invention

This invention relates to the field of printed feature manufacturing, such as integrated circuit manufacturing. In particular, this invention relates to automatically identifying evaluation points where errors are computed and analyzed to achieve improved agreement between a design layout and an actual printed feature.

2. Description of Related Art

To fabricate an integrated circuit (IC), engineers first use a logical electronic design automation (EDA) tool, also called a functional EDA tool, to create a schematic design, such as a schematic circuit design consisting of symbols representing individual devices coupled together to perform a certain function or set of functions. Such tools are available from CADENCE DESIGN SYSTEMS and from SYNOPSYS. The schematic design must be translated into a representation of the actual physical arrangement of materials upon completion, called a design layout. The design layout uses a physical EDA tool, such as those available from CADENCE and AVANT!. If materials must be arranged in multiple layers, as is typical for an IC, the design layout includes several design layers.

After the arrangement of materials by layer is designed, a fabrication process is used to actually form material on each layer. That process includes a photo-lithographic process using a mask having opaque and transparent regions that causes light to fall on photosensitive material in a desired pattern. After light is shined through the mask onto the photosensitive material, the light-sensitive material is subjected to a developing process to remove those portions exposed to light (or, alternatively, remove those portions not exposed to light). Etching, deposition, diffusion, or some other material altering process is then performed on the patterned layer until a particular material is formed with the desired pattern in the particular layer. The result of the process is some arrangement of material in each of one or more layers, here called printed features layers.

Because of the characteristics of light in photolithographic equipment, and because of the properties of the material altering processes employed, the pattern of transparent and opaque areas on the mask is not the same as the pattern of materials on the printed layer. A mask design process is used, therefore, after the physical EDA process and before the fabrication process, to generate one or more mask layouts that differ from the design layers. When formed into one or more masks and used in a set of photolithographic processes and material altering processes, these mask layouts produce a printed features layer as close as possible to the design layer.

The particular size of a feature that a design calls for is the feature's critical dimension. The resolution for the fabrication process corresponds to the minimum sized feature that the photolithographic process and the material processes can repeatably form on a substrate, such as a silicon wafer. As the critical dimensions of the features on the design layers become smaller and approach the resolution of the fabrication process, the consistency between the mask and the printed features layer is significantly reduced. Specifically, it is observed that differences in the pattern of printed features from the mask depend upon the size and shape of the features on the mask and the proximity of the features to one another on the mask. Such differences are called proximity effects.

Some causes of proximity effects are optical proximity effects, such as diffraction of light through the apertures of the optical systems and the patterns of circuits that resemble optical gratings. Optical proximity effects also include underexposure of concave corners (inside corners with interior angles greater than 180 degrees) and overexposure of convex corners (outside corners with interior angles less than 180 degrees), where the polygon represents opaque regions, and different exposures of small features compared to large features projected from the same mask. Other causes of proximity effects are non-optical proximity effects, such as sensitivity of feature size and shape to angle of attack from etching plasmas or deposition by sputtering during the material altering processes, which cause features to have shapes and sizes that have decayed from or accumulated onto their designed shapes and sizes.

In attempts to compensate for proximity effects, the mask layouts can be modified. To illustrate, FIG. 1A shows mask items 170, such as windows or opaque areas on a mask, represented by edges 171 on one or more polygons, and some printed features 180 with exaggerated proximity effects. FIG. 1A does not represent an actual example, but is provided simply to illustrate the concepts of proximity effects and mask modifications to attempt to correct for proximity effects. To make apparent the illustrated proximity effects, the projection of the original polygons 171 is shown as a fine line around the printed features 180. Note that the printed features 180 includes a spurious connection feature 182, an edge 185 entirely displaced inside the corresponding edge of the original outline 171, and an end line 183 completely inside the outline of the original items 171.

FIG. 1B illustrates various ways mask items 190 are modified to correct for such effects. FIG. 1B is not a particular example of a particular set of corrections that actually mitigate the proximity effects illustrated in FIG. 1A. The corrections available include hammerheads 192 (i.e. hammerheads 192a and 192b) added to ends of items to compensate for overexposure of the entire end line of a feature. Also shown are biases 196 (i.e. biases 196a and 196b) applied along portions of a straight edge of a feature. A negative bias like 196 represents a portion of an opaque area made transparent (or a portion of a window made opaque). In this case the negative bias reduces the size of the item on a mask to avoid generating the spurious feature 182 of FIG. 1A. Also shown are assist features 194 (i.e. assist features 194a and 194b), which are separate items smaller than the resolution of the photolithographic process and thus too small to be formed in a photoresist layer, but which are sufficiently large to effect diffraction patterns that influence larger nearby features. The assist features 194 are intended to move the edge 185 of the printed features 180 in FIG. 1A toward the outline 171 of the original mask items 170. Also shown is a sub-resolution serif 193 of extra opaque material to compensate for overexposure at convex corners of opaque areas, and an anti-serif 197 indicating where opaque material, if any, is removed to compensate for underexposure at concave corners of opaque polygons. These corrections are listed to illustrate the concepts of correcting a mask to compensate for proximity effects. The illustrated corrections do not necessarily correct the depicted features.

In rule-based proximity corrections, corrections such as the serifs and biases of a predetermined size are automatically placed at corners and edges of fabrication layout shapes like the mask shapes 170. Other rules may include adding assist shapes to a mask near desired features from the design layout and adding hammerheads to short end lines of the desired features. Experience of engineers accumulated through trial and error can be expressed as rules, and applied during the fabrication design process. For example, a rule may be expressed as follows: if a feature is isolated, then widen the opaque region in the mask by a particular amount to compensate for expected overexposure, so that the feature prints properly.

The experience captured in rule-based corrections is garnered by going through the fabrication process repeatedly with different mask layouts, making adjustments and observing the results. However, this process consumes time and manufacturing capacity. Even if such experimental, rule-forming runs are made, the resulting rules often do not give satisfactory results as the features become more complicated, or become smaller, or interact in more confined areas, or involve any combination of these effects.

Because rule based corrections are often imprecise and time consuming to revise and test, a proximity effects model is often developed and used to predict the effects of a change in the mask layout with more precision and with fewer off line fabrication runs.

A proximity effects model is typically built for a particular suite of equipment and equipment settings assembled to perform the fabrication process. The model is often built by performing the fabrication process one or a few times with test patterns on one or more mask layouts, observing the actual features printed (for example with a scanning electron micrograph), and fitting a set of equations or matrix transformations that most nearly reproduce the locations of edges of features actually printed as output when the test pattern is provided as input. The output of the proximity effects model is often expressed as an optical intensity. Other proximity effects models provide a calibrated output that includes a variable threshold as well as an optical intensity. Some proximity effects models provide a calibrated output which indicates a printed edge location relative to a particular optical intensity as a spatial deviation. Thus some model outputs can take on values that vary practically continuously between some minimum value and some maximum value.

Once a proximity effects model is produced, it can be used in the fabrication design process. For example, the proximity effects model is run with a proposed mask layout to produce a predicted printed features layer. The predicted printed features layer is compared to the design layer and the differences are analyzed. Based on the differences, modifications to the proposed mask layout are made, such as by adding or removing serifs 193, or adding or removing anti-serifs 197. After making these corrections to the proposed mask layout, a final mask layout is generated that is used in the fabrication process.

A proximity effects model typically produces predictions that lead to corrections that are more accurate than rule based corrections. However, a proximity effects model consumes computational resources by an amount that is related to the number of points of interest where amplitudes are computed. For typical mask layouts, the number of points where the model could be run is large, and the computation time is prohibitive. Therefore it is typical to run the proximity effects model at selected evaluation points located on the edges of features, to determine the correction needed, if any, at each evaluation point, and then to apply that correction to all the points on a segment of the edge in the vicinity of the evaluation point.

If too many evaluation points are selected, the model runs too slowly and the correction procedure takes too long. If too few evaluation points are selected, the model may not detect critical locations where corrections are necessary. Another undesirable effect of too few evaluation points is that segments can become too large, so that even if a correction is accurate for the evaluation point, the correction is applied to too much of the edge. This causes undesirable changes on the edge away from the evaluation point.

What are needed are new ways to select evaluation points and define their vicinity for the mask layouts, so that a reasonable number of comparisons can be made and so that effective modifications to the mask layouts can be suggested.

SUMMARY OF THE INVENTION

According to techniques of the present invention, edges of polygons in a mask layout are dissected into segments defined by dissection points, with each segment having an evaluation point, according to modified dissection rules. These techniques allow the spacing of evaluation points and dissection points to be automatically adapted to portions of each edge where changes in the proposed mask layout are most likely needed. According to these techniques, dissection points are closer together where proximity effects are more significant and are farther apart where proximity effects are less significant. Thus unnecessary evaluation points are eliminated and the analysis process is speeded up, while still retaining needed evaluation points.

In one aspect of these techniques, all dissection points can be derived from just a few easily defined parameters. The first parameter is a corner dissection segment length, Lcor, used as a target length to compute dissection points for corner segments that include at least one vertex of the polygon when the length of the edge allows. The second parameter is a detail dissection segment length, Ldet, used as a target length to derive dissection points for segments along portions of an edge where changes in proximity effects are expected to be significant, such as adjacent to corner segments and around projection points when the length of the edge allows. A projection point is a location on an edge where a vertex of another edge comes close enough to introduce sharp changes in the proximity effects. A halo distance is a parameter that determines whether a vertex is close enough to introduce an important effect. A maximum dissection segment length, Lmax, by definition larger than either Lcor or Ldet, is used as a target length to further dissect long portions of the edge that would otherwise not be dissected, such as away from corners and projections points. Extra precision is achieved if a true-gate extension length parameter Lext is used on edges of shifters forming true-gates. The dissection parameters Lcor, Ldet, Lmax, halo and Lext are pre-set and accessed as needed.

In one aspect of the disclosed techniques, a profile of amplitudes from a proximity effects model along an edge of a feature in a design layout is used to dissect the edge. This amplitude profile reflects the level of proximity effects for the given layout and fabrication process. By using this information, a better dissection of the edge is achieved in which the segment length between successive dissection points depends on the magnitude of the proximity effects. For example, on a portion of an edge where the magnitude of the proximity effects is larger than another portion, the segment length is made longer. This kind of dissection avoids making the segment length too short. A segment that is too short, for example in a corner, may result in a very large correction, and may prevent the overall corrections for the layout from converging.

In another aspect, the dissection parameters are derived from the segment lengths based on profiles of amplitudes. According to the modified dissection rules, each evaluation point is placed along a segment at a predetermined fraction of the distance from one dissection point to the other defining each segment, where the predetermined fraction depends on the type of segment. For example, an evaluation point is placed closer to the non-vertex dissection point in a corner segment; and is placed midway between dissection points in other types of segments. Also according to the modified dissection rules, segments are located on polygon edges according to the type of edge, such as whether the edge is a line end, a turn end, or a longer edge. These edge types are deduced from the length of the edge compared to the dissection parameters and whether one or both vertices of the edge are concave or convex corners.

Specifically, according to some aspects of the invention, techniques correct for proximity effects associated with an edge in a layout corresponding to a design layer. An evaluation point is determined for the edge based on a profile of amplitudes output from a proximity effects model along a transect. The transect includes a target edge in the design layer corresponding to the edge. It is then determined how to correct at least a portion of the edge for proximity effects based on an analysis at the evaluation point.

According to a different embodiment, a computer system corrects proximity effects associated with an edge in a layout corresponding to a design layout. A computer readable medium carries data representing the edge and at least a portion of the design layout. The design layout corresponds to a portion of an integrated circuit. One or more processors are coupled to the computer readable medium. The processors are configured to determine an evaluation point for the edge. The evaluation point is based on a profile of amplitudes output from a proximity effects model along a transect. The transect includes a target edge in the design layer corresponding to the edge. A correction is determined based on an analysis of an amplitude from the profile at the evaluation point. The correction is applied to at least a portion of the edge.

In another embodiment, a mask for fabricating a printed features layer includes an opaque region having a segment corrected for proximity effects. The segment corresponds to at least one portion of a target edge in a design layer for the printed features layer. The segment is displaced from the corresponding portion in the design layer by a correction distance. The correction distance is based on analysis of an amplitude output by a proximity effects model at an evaluation point on the corresponding portion. The evaluation point is based on a profile of amplitudes output from a proximity effects model along a transect including the target edge.

According to other aspects, techniques correct for proximity effects associated with an edge in a layout corresponding to a design layer. Dissection points are determined for the edge based on a profile of amplitudes output from a proximity effects model along a transect. The transect includes a target edge in the design layer corresponding to the edge. An evaluation point is based on at least one of the profile and the dissection points. It is then determined how to correct a segment of the edge corresponding to two dissection points based on an analysis at the evaluation point.

According to other aspects, techniques correct for proximity effects associated with a first edge in a first layout corresponding to a design layer for a printed features layer. A dissection length parameter is derived based on a profile of amplitudes output by a proximity effects model along a transect. The transect includes a second edge in a second layout. An evaluation point is determined for the first edge based on the dissection length parameter. Then it is determined how to correct at least a portion of the first edge based on an analysis at the evaluation point.

According to a different embodiment, a computer system corrects proximity effects associated with a first edge in a first layout corresponding to a design layout. A computer readable medium carries data representing the edge and at least a portion of the design layout. The design layout corresponds to a portion of an integrated circuit. One or more processors are coupled to the computer readable medium. The processors are configured to perform the following steps. A dissection length parameter is derived based on a profile of amplitudes output by a proximity effects model along a transect. The transect includes a second edge in a second layout. An evaluation point is determined for the first edge based on the dissection length parameter. A proximity effects model is run for the evaluation point to produce a model amplitude at the evaluation point. A correction is determined based on an analysis of the model amplitude at the evaluation point. The correction is applied to at least a portion of the edge.

In another embodiment, a mask for fabricating a printed features layer includes an opaque region having a segment corrected for proximity effects. The segment corresponds to at least one portion of a target edge in a design layer for the printed features layer. The segment is displaced from the corresponding portion in the design layer by a correction distance. The correction distance is based on analysis of an amplitude output by a proximity effects model at an evaluation point on the corresponding portion. The evaluation point is based on a dissection length parameter based on a profile of amplitudes output by a proximity effects model along a transect including a second edge in a second layout.

According to other aspects of the invention, techniques correct for proximity effects associated with a first edge in a first layout corresponding to a design layer for a printed features layer. A dissection length parameter is derived based on a profile of amplitudes output by a proximity effects model along a transect. This transect crosses a plurality of polygons in a second layout. An evaluation point is determined for the first edge based on the dissection length parameter. Then it is determined how to correct at least a portion of the first edge based on an analysis at the evaluation point

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:

FIG. 1A is a plan view of two items on a mask and the resulting printed features showing proximity effects.

FIG. 1B is a plan view of elements on a modified mask after adjusting the mask of FIG. 1A in an attempt to correct for proximity effects.

FIG. 2 is a flow chart showing the sequence of processes and layouts utilized in the formation of devices having printed features layers according to one embodiment of the disclosed techniques.

FIG. 3 is a flow chart of a process for selecting dissection points according to one embodiment.

FIG. 4A is a plan view of elements on a mask used as input to an illustrative proximity effects model.

FIG. 4B is a plan view illustrating printed features with proximity effects that might result from the mask items of FIG. 4A superposed on an ideal projection of the mask items.

FIG. 4C is a graph of contours of constant model amplitude output on a two dimensional array resulting from the input of FIG. 4A for the exemplary proximity effects model.

FIG. 4D is a graph of a profile of model amplitude along a line shown in FIG. 4C for the exemplary proximity effects model.

FIG. 5A is a plan view of elements on a second mask layout used as input to a second proximity effects model.

FIG. 5B is a graph of a profile of model amplitude along an edge of a polygon from FIG. 5A.

FIG. 5C is a graph of a first derivative of the profile of FIG. 5B showing dissection points according to one embodiment.

FIG. 5D is a graph of a second derivative of the profile of FIG. 5B showing dissection points according to another embodiment.

FIG. 6 shows an exemplary test pattern for a fabrication process, a profile location, and the corresponding exemplary model output along the profile.

FIG. 7 is a flow diagram for setting dissection parameters according to one embodiment.

FIGS. 8A through 8E are partial plan views of polygons from mask layouts showing placement of dissection points and evaluation points according to one embodiment.

FIGS. 9A and 9B are flow diagrams for selecting dissection points using the dissection parameters according to one embodiments.

FIGS. 10A through 10C are partial plan views of overlapping polygons from two mask layouts, showing the placement of dissection points and evaluation points when not all of a polygon is printed, according to several embodiments.

FIG. 11 is a flow diagram of a process employed when changing edges for selecting dissection points and evaluation points according to one embodiment.

FIG. 12 is a block model of a computer system configured according to one embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Techniques are described for correcting fabrication layouts for proximity effects during the fabrication of printed features layers, such as in integrated circuits.

Functional Overview

FIG. 2 shows the processes and layouts produced according to the present techniques for designing and fabricating printed features layers 249 for devices 250. The conventional processes are modified to include new techniques for selecting evaluation and dissection points, as represented in FIG. 2 by process 260.

The conventional processes include the Functional EDA process 210 that produces the schematic diagram 215. The physical EDA process 220 converts the schematic diagram to a design layout made up of one or more design layers 225 (e.g. design layers 225a, 225b, and 225b). After mask layouts 235 (e.g. mask layouts 235a, 235b, and 235c) are produced, the conventional processes employ a fabrication process 240 to produce the printed features layer 249. The printed features layer 249 may be a layer in a printed circuit or the mask used to produce the layer in the printed circuit. In the former case, the fabrication process includes one process 243 for forming the mask and a second process 245 to produce the layer of the integrated circuit using the mask. If the printed features layer 249 is the mask, step 245 is skipped.

According to techniques of the present invention, the fabrication design process 230 includes a new process 260 for obtaining dissection and evaluation points. In one embodiment, these new dissection and evaluation points are determined for a proposed mask layout 231 and used with a proximity effects model in step 232 to produce a model output 233. The model output is related to the predicted location of edges in the printed features layer if a printed features layer were fabricated with a mask having the proposed mask layout. Hence, in FIG. 2, the model output is called the predicted printed features edge.

Each evaluation point is established for a segment of an edge from the proposed mask layout. The segment is defined by a pair of dissection points associated with the evaluation point. Once the dissection points and evaluation points are selected, the proximity effects model is run at the evaluation points. In step 234, the predicted printed features edge 233 for the evaluation point is compared to a target position for that edge in the design layer. Based on an analysis of the comparison, a correction distance is computed. The correction distance is then applied to the entire segment between dissection points associated with the evaluation point on the edge of the affected polygon. As each correction is applied, the mask layout is adjusted in step 236. As a result of the adjustments in step 236 the polygons on the adjusted mask layout may differ from the polygons in the proposed mask layout, as well as from the polygons in the design layers 225. The final mask layouts
235 are then based on the adjusted mask layouts produced by the adjustment process 236.

Obtaining Dissection and Evaluation Points

An overview of one embodiment for the new dissection/evaluation points selection process 260 is illustrated in FIG. 3. According to this embodiment, a few dissection parameters are established in response to predictions from a proximity effects model in step 310. In another embodiment, a human operator sets the dissection parameters. Those few dissection parameters represent the necessity of dissection along varying portions of various types of edges. These parameters are used to select all the dissection points and evaluation points on an edge of a polygon of a fabrication layout in step 330. In step 370, an evaluation point is placed on each segment defined by a pair of successive dissection points on the edge. The process then continues over all the edges needing dissection, on all the polygons in the layout.

For example, in step 390, it is determined whether the current edge is the last edge on the current polygon. If not, process flow passes to step 392 where the next edge requiring dissection on the polygon is selected and made the current edge. Flow then returns to step 330 in which the edge newly made current is dissected according to the dissection parameters. If the edge just processed by steps 330 and 370 is the last edge of the current polygon, process flow passes to step 394 in which it is determined whether the current polygon is the last one needing dissection in the fabrication layout. If not, the next polygon is made the current polygon in step 396, and control passes to step 392 in which the first edge needing dissection on the polygon is made the current edge. If the current polygon is the last on the layout then flow passes to step 232 of FIG. 2, where the proximity effects model is run at the selected evaluation points.

In an alternative embodiment, step 394 includes making another fabrication layout the current fabrication layout and then passing control to step 396. Such a subsequent layout may be necessary to dissect; for example, when the two fabrication layouts are used to doubly expose the same printed features layer, so that edges from the second fabrication layer become part of the printed features layer. This circumstance will be described in more detail later below.

Example of Selecting Dissection/Evaluation Points & Setting Dissection Parameters Using a Proximity Effects Model

According to one embodiment, the proximity effects model is used to set the dissection parameters, for example, in step 310. These parameters remain constant during the dissection of several polygons in step 330, on one or more proposed fabrication layouts during the fabrication layout design process, step 230.

Herein the term amplitude is used to describe the proximity effects model output associated with a location no matter what model is used. In general, the models are built to accept input representing polygons of opaque or transparent material. In general, an amplitude value is then produced for each location of interest. A threshold amplitude between the minimum amplitude value and the maximum amplitude value can be associated with the model. As used here, the threshold amplitude is associated with the location where the edge of a feature is actually printed. For example, where spatial deviation is output by the model, a threshold of zero marks the location of an edge. As another example, where the model output is simple optical intensity, a non-zero positive value associated with the printed edge serves as the threshold. In some models, the threshold value associated with an edge may itself be a function of several variables such as the optical intensity and intensity gradient at a location.

To understand how dissection parameters can be derived from proximity effects model output, consider FIGS. 4 and 5.

FIG. 4A shows two items 410 and 420 represented by polygons 411 and 421, respectively, in a mask layout provided as input to a proximity effects model that has been developed for a particular suite of equipment and settings. FIG. 4B shows the printed features 412 and 422, with shapes 413 and 423, respectively, that would be produced from a mask formed from the mask layout of FIG. 4A according to the proximity effects model. For reference, FIG. 4B also shows the polygons 411 and 421 if those were projected perfectly onto the printed features layer. The differences between the ideal and actually printed features are due to proximity effects. For example, arrow 415 shows the deviation of printed feature 413 from the design layout polygon 411
at one location on the polygon 411. Similarly, arrow 425 shows the deviation of printed feature 423 from the design layout polygon 421 at a location on that polygon 421. These deviations are output by one kind of proximity effects model.

FIG. 4C illustrates contours 450a-450g of constant amplitude on a two dimensional array of model output for one kind of proximity effects model, if it were to be run for every point on such an array. One particular contour representing the threshold value, contour 450c, is shown as a bold line. The shapes formed by the bold contour 450c represent this model's prediction of the shape of the printed features accounting for all proximity effects in the system. As can be seen by the bold contour 450c in FIG. 4C, this proximity effects model predicts printed features with shapes that agree with those that actually would be produced on the printed features layer, as shown in FIG. 4B.

For simplicity of explanation, the proximity effects model that outputs contours like those shown in FIG. 4C is used as an exemplary proximity effects model in the rest of this specification, unless otherwise stated. The techniques of the present invention, however, work with any proximity effects model, including those models which have been calibrated to produce amplitudes representing the predicted printed feature.

FIG. 4C also shows a double arrow 460 along which a profile of model amplitude is plotted in FIG. 4D. In FIG. 4D, model amplitude is expressed in arbitrary units on the vertical axis 484 from zero to a maximum represented by a value of 1.0. Distance along the double arrow of FIG. 4C from left to right is shown on the horizontal axis 482 of FIG. 4D, in arbitrary distance units. Along this profile the model amplitude begins at zero, rises steadily from about 3 distance units to about 5
distance units, flattens out to about 7 units, then dips at about 8 units, rises to a peak at about 9 distance units, and then falls off to zero again at about 13 distance units.

The threshold value 470 of about 0.3 model amplitude units corresponds to the value of contour 450c. Where the model output 465 is above the threshold value 470, a feature is predicted to be printed in the printed features layer; where the model output 465 is below the threshold value 470, no feature is predicted to be printed. Where the model output 465 equals the threshold value is where the edge of the printed feature is predicted to lie. Of course model output units are arbitrary and could be inversely related to what is printed, so that low amplitudes represent printed features and high amplitudes represent no printing. In such a case, the threshold would mark the value below which a feature is printed.

The selection of dissection points and the derivation of dissection parameters from a profile of model output is illustrated in FIGS. 5A, 5B, 5C and 5D.

FIG. 5A is a plan view of elements on a second mask layout used as input to another proximity effects model. The mask elements include two transparent items, 510 and 520, represented by a first polygon with edges 511 through 516 and a second polygon with edges 521 through 524, respectively. On the polygon of item 510, points 532 and 533 indicate where proximity effects are expected to vary sharply because of the corner point 538 between edges 514 and 515. Also shown are points 531, 534,
535 and 536 on the polygon of item 510 where proximity effects are expected due to the corner between edges 521 and 522 of the polygon of item 520. Points 531 through 536 are projection points, where proximity effects on one edge are expected to vary sharply because of the proximity of vertices of other edges.

A profile of model amplitudes along one of these edges reflects the size of proximity effects on that edge for this particular layout and model (where the model represents this particular suite of fabrication equipment and settings). By using information in this profile of amplitudes, a better dissection of this edge can be performed in which the segment length depends on the proximity effect magnitude.

FIG. 5B is a graph of a profile 540 of model amplitude along edge 511 of the polygon of item 510 shown in FIG. 5A, from its vertex in common with edge 516 to its vertex in common with edge 512. For purposes of illustration, the threshold for this model is taken as 0.23. This model then predicts that this edge 511 will print from about 100 nanometers (nm) to about 1100 nm. Arrow 541 indicates the location of projection point 531 and the value of amplitude at that projection point. Curve
543 indicates where the amplitude curve 540 would be expected to lie if there were no variation in proximity effects on edge 511 due to item 520. Arrow 542 indicates the location of projection point 532 and its amplitude. Curve 544 indicates where the amplitude profile 540 would be expected to lie if there were no variation in proximity effects on edge 511 due to corner point 538. As expected, the proximity effects represented by curve 540 vary in the vicinity of the projection points.

This profile 540 is used to illustrate how particular dissection points are selected on this edge, as well as how the dissection parameters used for dissecting other edges are derived from model output on this edge, according to this embodiment. In other embodiments, variations on the techniques described here are used.

This profile 540 illustrates a length scale of proximity effects at corners of a polygon. That corner length scale is found from a vertex of the edge to a position where the profile flattens near amplitude 0.3. That length scale is about 200
nm. In an alternative embodiment, the corner length scale is selected to be the distance to the threshold value of 0.23, which is about 150 nm. In still another embodiment, the corner length scale is taken to be a distance to an amplitude that is a given percentage of the threshold value. In this case, the given percentage is selected based on how aggressive a correction is sought in the vicinity of corners. Where more aggressive correction is desired, the percentage is chosen to be less than
100%. Where less aggressive correction is desired, the percentage is chosen to be greater than 100%. In still other embodiments, the corner length scale is associated with types of corners, e.g., concave or convex, or angle at the corner, or both. The corner length scale so derived provides a target length to use when dissecting corners where the edge allows.

This profile also suggests a length scale for details near projection points. Considering both sides of each projection point, that detail length scale should be about half the distance over which curve 540 deviates from curves 543 and 544. That detail length scale is about 100 nm. The detail length scale so derived provides a target length to use when dissecting details where the edge allows. This profile also suggests a maximum length scale where the curve 540 hardly varies at all, between the two projection points, of about 300 nm. The maximum length scale so derived provides a target length to use when dissecting edges away from corners and details, where the edge allows.

Along other edges, the distances for the amplitude curve to flatten, the distances over which projection points have noticeable effects, and the length of the undisturbed sections are expected to vary. Thus, dissection parameters based on these must be derived from the observed distributions of length scales. In a preferred embodiment, instead of using a single corner scale from one polygon of one test layout, a statistical corner scale, such as an average corner scale, or median corner scale, is used as a dissection parameter near corners. The statistical corner scale is derived from the corner segment lengths of several polygons on the same test layout or on several layouts. In other embodiments, different corner scales are derived for different corner types, e.g., concave or convex, or different ranges of angles or both. For example, corner segments associated with acute angles less than 45 degrees can be averaged separately from corner segments associated with intermediate angles from 45 through 80 degrees which themselves are averaged separately from corner segments associated with nearly right angles from 80 to 100 degrees.

In another model where proximity effects are different, the values of the length scales and their distributions are also expected to be different. For example, in an equipment suite with lower resolution optics and material processes, the length parameters are expected to be larger. That is, a distance from a vertex of an edge to a flattening point is expected to be greater for all edges tested, and, therefore, the derived dissection parameter is expected to be larger.

In one embodiment, an evaluation point for a corner segment is determined on this curve. According to this embodiment, the evaluation point is placed where the curve has a value equal to a given percentage of the amplitude value where the curve first flattens. For example, if the given percentage is set at 75%, then because curve 540 flattens at about 0.31 amplitude units, the evaluation point is selected where the curve has a value of 75% of 0.31, i.e., a value of 0.23. This occurs at a distance of about 100 nm. In another embodiment, the evaluation point is placed where the profile has a value equal to a given percentage of the threshold value, e.g., 90% of the threshold value of 0.23, which is 0.21. This occurs at about 90 nm on curve 543.

The following paragraphs define particular embodiments for determining dissection points and length scales for dissection parameters. Other embodiments rely on variations of these techniques known in the art.

FIG. 5C is a graph of a curve 550 representing a first derivative of the profile 540 of FIG. 5B. The first derivative is a measure of the slope of the profile 540 at each point along the profile. Where the profile 540 flattens the slope 550
approaches zero. A particular small value of slope, say 0.001, is chosen to indicate where a profile is flat. A profile is flat where the absolute value of its first derivative is less than this particular value. Starting from a distance of 0 nm, the curve 550 first becomes less than this particular value at about 200 nm. According to this embodiment, a corner segment is placed on this edge from 0 nm to 200 nm where the first derivative decreases to the particular value. The corner segment is marked by dissection points located at the vertex at 0 nm and at about 200 nm.

The 200 nm length of this corner segment is then submitted to a process to derive a corner dissection segment length parameter, Lcor. That process performs a weighted average of this corner length for this edge with other corner lengths from this and other edges to derive the corner dissection segment length, Lcor. In one embodiment, the weights are all the same.

Similarly, moving along edge 511 from its vertex in common with edge 512, at a distance of 1200 nm, the slope first increases above -0.001 at about 1000 nm. That is, the absolute value of the first derivative decreases below the particular value of 0.001 at about 1000 nm. This indicates a corner segment for this edge lies from distance 1000 nm to 1200 nm, and dissection points are placed at both those locations. As above, the 200 nm corner segment length is submitted to the process to derive the corner dissection segment length dissection parameter, Lcor.

FIG. 5C also shows arrows 551 and 552, which indicate the locations of projection points 531 and 532, respectively. Though the absolute value of slope at 551 may exceed the particular value of 0.001, this does not appear to be the case at 552. The slope near the projection point at 552 appears to be affected by the opposite slope associated with the corner segment. Along curve 550, the absolute value of the slope exceeds the particular value of 0.001 also near points indicated by arrows 553
and 554, at about 300 nm and 860 nm, respectively.

According to this embodiment, dissection points are added at each location where the absolute value of slope reaches a local maximum above the small particular value. One embodiment adds dissection points at 553, 551 and 554, which are located at about 300 nm, 480 nm and 860 nm, respectively. Thus, segments that extend from 200 to 300 nm, 300 to 480 nm, and 860 to 1000 nm are added to the two corner segments already defined for this particular edge 511. A residual edge that undergoes further dissection remains between points 551 and 554, from about 480 to 860 nm. Evaluation points are then placed on this edge, one evaluation point on each segment. In this embodiment, each evaluation point is placed midway between the dissection points on each non-corner segment.

The non-corner segment lengths obtained from these embodiments are sent to a process to derive other dissection parameters, such as a detail dissection segment length, Ldet, and a maximum dissection segment length, Lmax. In one embodiment of the process to derive dissection parameters, a histogram of segment lengths is formed and the detail dissection segment length, Ldet, is derived from a mode of the distribution associated with the smallest lengths. In this embodiment, a maximum dissection segment length is derived from a second mode associated with the next smallest lengths. In other embodiments, length scales may be derived from such a distribution of lengths using any methods known in the art. Then, dissection parameters are set based on the derived length scales.

In another embodiment of selecting dissection points from model output along a particular edge, dissection points are also added between these local slope maxima, either at a midpoint, or where the slopes are interpolated to closest approach zero. This embodiment will tend to produce twice the number of segments and cause the dissection parameters derived from the resulting segments to have about half the size. Evaluation points are then placed at the midpoints of the resulting segments.

In still another embodiment, dissection points are added at non-adjacent points where the slope curve 550 has a slope whose absolute value equals the particular minimum slope value, e.g., 0.001. This embodiment adds two dissection points for each local maximum such as at 553 and 554, one on either side of each local maximum.

In another embodiment, before adding dissection points, the second derivative of the profile 540 is computed and examined. FIG. 5D is a graph of a curve 560 representing a second derivative of the profile 540 of FIG. 5B. The second derivative measures the curvature of a profile. A positive value indicates that the profile is turning about a center above the profile, while a negative value indicates the profile is turning about a center below the profile. A zero indicates an inflection point where the profile is not curved. Arrows 561 and 562 indicate the locations of projection points 531 and 532, respectively.

Both projection points are located near points where curve 560 crosses zero. In this embodiment, dissection points are added wherever the second derivative is interpolated to cross zero. This method can be used even for corner segments if the crossing of zero closest to the corner is ignored. Using this method, one corner segment extends from 0 nm to the point indicated by arrow 565, at about 220 nm, and a second corner extends from the point at arrow 568, at about 990 nm to 1200 nm. These lengths of 200 nm and 210 nm, respectively, are sent to a process to derive a corner dissection segment length, Lcor.

This technique also produces dissection points as indicated by arrows 563 at about 320 nm, 561 (associated with projection point 531) at about 480 nm, 566 at about 760 nm, and 564 at about 880 nm. These points yield non-corner segments of lengths 100 nm, 160 nm, 280 nm, 120 nm, and 110 nm. These lengths are sent to the process for deriving non-corner dissection parameters, and support a detail dissection segment length Ldet of about 110 mn, and a maximum dissection segment length Lmax of about 300 nm.

According to one embodiment, an evaluation point is placed at the point on each segment where the absolute value of the second derivative is a maximum. For example, evaluation points are placed at 100 nm, 300 nm, 400 nm, 800 nm, 950 nm, and 900
nm, where the second derivative curve 560 reaches local extreme values between locations where the curve 560 crosses zero.

Using these example embodiments, a proximity effects model is run for a variety of locations along one or more edges of one or more polygons. The polygons can be selected from a proposed or final mask layout, or from a design layer of a design layout, or from a test mask used to build the proximity effects model. The profile of model output along the edge is used to select dissection points and evaluation points for that particular edge. The resulting segment lengths for the particular edge are used with segment lengths obtained for other edges to derive target lengths to use as dissection parameters.

This technique allows the proximity effects model to be run at a few locations, i.e., along certain edges, rather that at every point in a layout. Certain important edges are dissected directly, and the remaining edges are dissected using the dissection parameters. The proximity effects model is then run for all other edges only at the evaluation points. Thus a large number of model runs are avoided and the computation proceeds at a desirable rate. At the same time, the evaluation points are reasonably selected to give the necessary detail on all the edges.

Deriving Dissection Parameters From a Test Mask

The profiles such as 540 can be generated by any method known in the art. In one embodiment, the test patterns used to build the model are used. These test patterns already include a range of features at different scales and model output computed on a relatively dense output grid. Thus with substantially no additional computations than those already made to build the proximity effects model in the first place, model amplitudes along various polygon edges are already available for deriving dissection parameters.

In another embodiment, the corner length scale is derived from the model output at a series of features of varying pitch. Pitch refers to the smallest distance between repeated features in a layer. The use of features of varying pitch to derive a length scale appropriate for a given model is shown in FIG. 6.

FIG. 6 shows an exemplary test pattern 692 of elongated polygons for a suite of fabrication equipment. FIG. 6 also shows a profile location 695, and the corresponding exemplary model 690 amplitudes along the profile. The pitch associated with portion 692a of the test pattern 692 is illustrated by the separation 691a. Similarly, portions 692b, 692c and 692d have associated pitches 691b, 691c and 691d, respectively, of ever increasing size. The section 695 is perpendicular to the elongated polygons. The profile 690 on the section shows a portion 690a in which model output never exceeds the threshold amplitude 470 associated with a feature being printed. Thus this portion of the test pattern is too closely spaced to be printed with the equipment suite modeled by the proximity effects model. Portion 690b of profile 690 does cross the threshold amplitude, barely. Other portions 690c and 690d cross substantially above the threshold. In this example, the particular pitch 691b associated with profile portion 690b, represent a transition from unprintable to printable scales. Feature lengths smaller than this particular scale are not likely to have different printable effects. Consequently, dissecting features into segments smaller than this scale will likely generate too many segments and too many evaluation points. It is expected that corrections for such segments will not converge on a solution appropriate for the entire layout. If profile portion 690b had been entirely below the threshold, then a length scale representing the transition from unprintable to printable scales is obtained by interpolating to a pitch at the threshold amplitude on a plot of maximum amplitude versus pitch.

In one embodiment, the particular pitch length scale associated with maximum model output at the threshold value is used to set a corner dissection segment length. In another embodiment, this particular pitch length scale is used to set a detail dissection segment length.

Method for Deriving Dissection Parameters

FIG. 7 is a flow diagram for setting dissection parameters according to one embodiment of the present invention. This is one embodiment of step 310a for setting dissection parameters based on a proximity effects model output.

In step 710, a profile of proximity effects model amplitudes is generated along at least one edge of a polygon on a test layout. In the preferred embodiment, the test pattern is one used to generate the proximity effects model. In another embodiment, the polygon can be selected on-the fly from a proposed fabrication layout for an actual printed features layer.

In step 730, a corner dissection segment length parameter, Lcor, is set to a distance scale representing the distance from a corner, i.e., a vertex of the polygon defining the current edge, to a point on the profile where the profile flattens. The profile is flat, for example, where successive points undergo less than a certain change in