Method of determining optimum exposure threshold for a given photolithographic model

• US 6,519,501 B2
• Filed: 01/23/2001
• Issued: 02/11/2003
• Est. Priority Date: 02/05/1998
• Status: Expired due to Fees

• Current Assignee: Micron Technology, Inc.
• Original Assignee: Micron Technology, Inc.
• Inventors: Burdorf, James, Pierrat, Christophe
• Primary Examiner(s): Picard, Leo
• Assistant Examiner(s): CABRERA, ZOILA E
• Application Number:
  US09/768,109
• Publication Number:
  US 20010029403A1
• Time in Patent Office: 749 Days
• Field of Search: 700/121, 700/30, 700/31, 700/28
• US Class Current: 700/121
• CPC Class Codes: C09K 8/42 Compositions for cementing, e.g. for cementing casings into boreholes; Compositions for plugging, e.g. for killing wells compositions for plastering C09K8/50
• CPC Class Codes: C09K 8/508 macromolecular compounds C09K8/512 takes precedence
• CPC Class Codes: C09K 8/80 Compositions for reinforcing fractures, e.g. compositions of proppants used to keep the fractures open
• CPC Class Codes: G03F 7/70558 Dose control, i.e. achievement of a desired dose
• CPC Class Codes: H01L 22/20 Sequence of activities consisting of a plurality of measurements, corrections, marking or sorting steps

Abstract

A system and method for enhancing process latitude (tolerances) in the fabrication of devices and integrated circuits. A measuring point is selected corresponding to a feature of critical dimension. Then the pattern is convolved with the model, and its value and rate of change are calculated over a range of corresponding values of a first process parameter. Next, an optimum threshold having the largest rate of change, or contrast, is selected. Finally, proximity correction is performed using relevant parameters.

Claims

1. A computer storage for storing instructions for determining an optimum process point for fabricating a device feature of a critical dimension, the instructions comprising:

selecting a measuring point on a computer representation of a wafer corresponding to the feature of the critical dimension;

calculating modeled behavior values and their rates of change over a range of corresponding values of a first process parameter representative of mask material edge position;

selecting an optimum threshold value having the largest rate of change around said measuring point; and

determining the first process parameter value corresponding to the optimum threshold value.

2. The computer storage of claim 1, wherein the instructions further include calculating the modeled behavior values and their rates of change over a range of corresponding values of the first process parameter representative of focus.

3. The computer storage of claim 1, wherein the instructions further include calculating the modeled behavior values and their rates of change over a range of corresponding values of the first process parameter representative of numerical aperture.

4. The computer storage of claim 1, wherein the instructions further include:

selecting a point on one side of the measuring point;

calculating a value of the modeled behavior at each of the points; and

calculating a slope through each of the points, wherein the slope is a function of the values of the modeled behavior at each point.

5. The computer storage of claim 4, wherein the instructions further include selecting a second point.

6. The computer storage of claim 1, wherein the instructions further include:

decrementing the value of the first process parameter;

calculating the value of the modeled behavior at the measuring point;

determining the value of the modeled behavior at a location offset from the measuring point in a first direction by a second value;

determining the value of the modeled behavior at a location offset from the measuring point in a second direction, opposite to the first direction, by the second value; and

calculating the rate of change of the modeled behavior corresponding to the first process parameter value.

7. The computer storage of claim 6, wherein the instructions further include:

calculating a difference of the modeled behavior values ascertained during determining the values of the modeled behavior; and

dividing the difference of the modeled behavior values by twice the second value.

8. A computer storage for storing instructions for determining an optimum process point for fabricating a device feature of a critical dimension, the instructions comprising:

selecting a measuring point on a computer representation of a wafer corresponding to the feature of the critical dimension;

calculating modeled behavior values and their rates of change over a range of corresponding values of a first process parameter representative of mask material edge position;

selecting an optimum threshold value having the largest rate of change around said measuring point;

determining the first process parameter value corresponding to the optimum threshold value; and

providing the optimum threshold value to a proximity effect correction process which modifies the mask pattern to compensate for proximity effects.

9. The computer storage of claim 8, wherein the instructions further include calculating the modeled behavior values and their rates of change over a range of corresponding values of the first process parameter of a computer representation of a mask.

10. The computer storage of claim 8, wherein the instructions further include calculating the modeled behavior values and their rates of change over a range of corresponding values of the first process parameter representative of focus.

11. The computer storage of claim 8, wherein the instructions further include calculating the modeled behavior values and their rates of change over a range of corresponding values of the first process parameter representative of numerical aperture.

12. A computer storage for storing instructions for determining an optimum process point for fabricating a device feature of a critical dimension, the instructions comprising:

selecting a measuring point on a computer representation of a wafer corresponding to the feature of the critical dimension;

calculating modeled behavior values and their rates of change over a range of corresponding mask material edge positions; and

selecting an optimum threshold value having the largest rate of change around said measuring point.

13. The computer storage of claim 12, wherein the instructions further comprise determining the mask material edge position corresponding to the optimum threshold value.

14. The computer storage of claim 12, wherein the instructions further comprise:

selecting a point on one side of the measuring point;

calculating a value of the modeled behavior at each of the points; and

calculating a slope through each of the points, wherein the slope is a function of the values of the modeled behavior at each point.

15. The computer storage of claim 12, wherein the instructions further comprise:

shifting the mask material edge position by a first value;

calculating the value of the modeled behavior at the measuring point;

determining the value of the modeled behavior at a location offset from the measuring point in a first direction by a second value;

determining the value of the modeled behavior at a location offset from the measuring point in a second direction, opposite the first direction, by the second value; and

calculating the rate of change of the modeled behavior value corresponding to the mask material edge position.

16. The computer storage of claim 15, wherein the instructions include:

calculating a difference of the modeled behavior values ascertained during determining the values of the modeled behavior; and

dividing the difference of the modeled behavior values by twice the second value.

17. The computer storage of claim 15, wherein the instructions further comprise providing the optimum threshold value to a proximity effect correction process which modifies the mask material edge position to compensate for proximity effects.

18. A computer storage for storing instructions for determining an optimum process point for fabricating a device feature of a critical dimension, the instructions comprising:

selecting a measuring point on a computer representation of a wafer corresponding to the feature of the critical dimension;

calculating modeled behavior values and their rates of change over a range of corresponding mask material edge positions, the calculating modeled behavior values and their rates of change including;

shifting the mask material edge position by a first value;

calculating the value of the modeled behavior at the measuring point;

determining the value of the modeled behavior at a location offset from the measuring point in a first direction by a second value;

determining the value of the modeled behavior at a location offset from the measuring point in a second direction, opposite the first direction, by the second value; and

calculating the rate of change of the threshold, the calculating the rate of change of the threshold including;

calculating a difference of the modeled behavior values ascertained during determining the values of the modeled behavior; and

dividing the difference of the modeled behavior values by twice the second value;

selecting an optimum threshold value having the largest rate of change; and

providing the optimum threshold value to a proximity effect correction process which modifies the mask material edge position to compensate for proximity effects.

19. A computer storage for storing instructions for determining an optimum process point for fabricating a device feature of critical dimension, the instructions comprising:

selecting a plurality of measuring points, wherein each measuring point corresponds to the feature of the critical dimension;

calculating values and rates of change of modeled behavior over a range of values of a first process parameter for each measuring point, the range of values representative of mask material edge position;

selecting an optimum threshold value having the largest rate of change for each measuring point;

selecting a threshold value from the plurality of optimum threshold values; and

providing the selected threshold value to a proximity effect correction process which modifies the mask material edge position to compensate for proximity effects.

20. The computer storage of claim 19, wherein the instructions further include determining the mean of the selected optimum threshold values for each measuring point.

21. The computer storage of claim 19, wherein the instructions further include determining the median of the selected optimum threshold values for each measuring point.

22. A computer for determining an optimum process point for fabricating a device feature of a critical dimension, comprising:

means for selecting a measuring point on a computer representation of a wafer corresponding to the feature of the critical dimension;

means for calculating modeled behavior values and their rates of change over a range of corresponding values of a first process parameter representative of mask material edge position;

means for selecting an optimum threshold value having the largest rate of change around said measuring point; and

means for determining the first process parameter value corresponding to the optimum threshold value.

23. The computer of claim 22, wherein the means for calculating includes means for calculating the modeled behavior values and their rates of change over a range of corresponding values of the first process parameter representative of focus.

24. The computer of claim 22, wherein the means for calculating modeled behavior values includes means for calculating the modeled behavior values and their rates of change over a range of corresponding values of the first process parameter representative of numerical aperture.

25. The computer of claim 22, wherein the means for calculating includes:

means for selecting a point on one side of the measuring point;

means for calculating a value of the modeled behavior at each of the points; and

means for calculating a slope through each of the points, wherein the slope is a function of the values of the modeled behavior at each point.

26. The computer of claim 25, wherein the means for calculating modeled behavior values includes selecting a second point.

27. The computer of claim 22, wherein the means for calculating threshold values and their rates of change includes:

means for decrementing the value of the first process parameter;

means for calculating the value of the modeled behavior at the measuring point;

means for determining the value of the modeled behavior at a location offset from the measuring point in a first direction by a second value;

means for determining the value of the modeled behavior at a location offset from the measuring point in a second direction, opposite to the first direction, by the second value; and

means for calculating the rate of change of the modeled behavior corresponding to the first process parameter value.

28. The computer of claim 27, wherein the means for calculating the rate of change includes:

means for calculating a difference of the modeled behavior values ascertained during determining the values of the modeled behavior; and

means for dividing the difference of the modeled behavior values by twice the second value.

29. A method, in a computer, for determining an optimum process point for fabricating a device feature of a critical dimension, comprising:

selecting a measuring point on a computer representation of a wafer corresponding to the feature of the critical dimension;

calculating modeled behavior values and their rates of change over a range of values of a first process parameter, including;

decrementing a value of the first process parameter by a first value;

calculating a value of the modeled behavior at the measuring point;

determining a value of the modeled behavior at a location offset from the measuring point by a second value in a first direction;

determining a value of the modeled behavior at a location offset from the measuring point by the second value in a second direction; and

determining a rate of change of the modeled behavior; and

determining whether a value and the rate of change of the modeled behavior has been calculated for a first process parameter equal to the first value, if so then end the method, and if not, then increment the first process parameter by a third value and repeat the above process in the order recited starting with calculating a value of the modeled behavior at the measuring period.

30. The method of claim 29, further comprising selecting an optimum threshold value having a largest rate of change, and providing the optimum threshold value to a proximity effect correction process which modifies a mask material edge position to compensate for proximity effects.

31. The method of claim 29, further comprising selecting the third value to be 2⁻

N times the first value.

32. The method of claim 29, further comprising selecting the third value is chosen such that first value can be divided by the third value without a remainder.

33. A method for determining an optimum process point for fabricating a device feature of a critical dimension, comprising:

selecting a measuring point on a computer representation of a wafer corresponding to the feature of the critical dimension;

calculating modeled behavior values and their rates of change over a range of corresponding values of an optical parameter of a stepper with respect to a constant mask material edge;

selecting an optimum threshold value having a largest rate of change around the measuring point; and

determining an optical parameter value corresponding to the optimum threshold value.

34. The method of claim 33, wherein calculating modeled behavior values comprises calculating the modeled behavior values and their rates of change over a range of corresponding values of the optical parameter representative of numerical aperture.

35. The method of claim 33, wherein calculating modeled behavior values comprises calculating the modeled behavior values and their rates of change over a range of corresponding values of the optical parameter representative of focus.

36. The method of claim 33, wherein calculating modeled behavior values comprises:

selecting a point on one side of the measuring point;

calculating a value of the modeled behavior at each of the points; and

calculating a slope through each of the points, wherein the slope is a function of the values of the modeled behavior at each point.

40. The method of claim 33, wherein selecting a measuring point, calculating modeled behavior values and their rates of change, selecting an optimum threshold value, and determining an optical parameter value are performed in a computer.

37. A method for determining an optimum process point for fabricating a device feature of a critical dimension, comprising:

selecting a measuring point on a computer representation of a wafer corresponding to the feature of the critical dimension;

calculating modeled behavior values and their rates of change over a range of corresponding values of an optical parameter of a stepper with respect to a constant mask material edge;

selecting an optimum threshold value having a largest rate of change around the measuring point; and

determining an optical parameter value corresponding to the optimum threshold value, wherein calculating modeled behavior values includes;

selecting a point on one side of the measuring point;

selecting a second point;

calculating a value of the modeled behavior at each of the points; and

calculating a slope through each of the points, wherein the slope is a function of the values of the modeled behavior at each point.

38. A method for determining an optimum process point for fabricating a device feature of a critical dimension, comprising:

selecting a measuring point on a computer representation of a wafer corresponding to the feature of the critical dimension;

calculating modeled behavior values and their rates of change over a range of corresponding values of an optical parameter of a stepper;

selecting an optimum threshold value having a largest rate of change around the measuring point; and

determining an optical parameter value corresponding to the optimum threshold value, wherein calculating modeled behavior values and their rates of change includes;

decrementing a value of the optical parameter;

calculating a value of the modeled behavior at the measuring point;

determining a value of the modeled behavior at a location offset from the measuring point in a first direction by a second value;

determining a value of the modeled behavior at a location offset from the measuring point in a second direction, opposite to the first direction, by the second value; and

calculating a rate of change of the modeled behavior corresponding to the optical parameter value.

39. The method of claim 38, wherein calculating the rate of change comprises:

calculating a difference of the modeled behavior values ascertained during determining the values of the modeled behavior; and

dividing the difference of the modeled behavior values by twice the second value.

41. A method for determining an optimum process point for fabricating a device feature of a critical dimension, comprising:

selecting a measuring point on a computer representation of a wafer corresponding to the feature of the critical dimension;

calculating modeled behavior values and their rates of change over a range of corresponding values of an optical parameter of a stepper with respect to a constant mask material edge;

selecting an optimum threshold value having a largest rate of change around the measuring point;

determining an optical parameter value corresponding to the optimum threshold value; and

providing the optimum threshold value to a proximity effect correction process which modifies a mask pattern to compensate for proximity effects.

42. The method of claim 41, wherein calculating modeled behavior values comprises calculating the modeled behavior values and their rates of change over a range of corresponding values of the optical parameter representative of one of focus, numerical aperture, exposure time, and on or off axis settings of a stepper.

43. The method of claim 41, wherein selecting a measuring point, calculating modeled behavior values and their rates of change, selecting an optimum threshold value, determining an optical parameter value corresponding to the optimum threshold value;

and providing the optimum threshold value to a proximity effect correction process are performed in a computer.

44. A method for determining an optimum process point for fabricating a device feature of a critical dimension, comprising:

selecting a measuring point on a computer representation of a wafer corresponding to the feature of the critical dimension;

calculating modeled behavior values and their rates of change over a range of corresponding values of exposure time of a stepper with respect to a constant mask material edge;

selecting an optimum threshold value having a largest rate of change around the measuring point; and

determining the exposure time corresponding to the optimum threshold value.

45. The method of claim 44, wherein calculating modeled behavior values comprises:

selecting a point on one side of the measuring point;

calculating a value of the modeled behavior at each of the points; and

calculating a slope through each of the points, wherein the slope is a function of the values of the modeled behavior at each point.

49. The method of claim 44, wherein selecting a measuring point, calculating modeled behavior values and their rates of change, selecting an optimum threshold value, and determining the exposure time are performed in a computer.

46. A method for determining an optimum process point for fabricating a device feature of a critical dimension, comprising:

selecting a measuring point on a computer representation of a wafer corresponding to the feature of the critical dimension;

calculating modeled behavior values and their rates of change over a range of corresponding values of exposure time of a stepper with respect to a constant mask material edge;

selecting an optimum threshold value having a largest rate of change around the measuring point; and

determining the exposure time corresponding to the optimum threshold value, wherein calculating modeled behavior values includes;

selecting a point on one side of the measuring point;

selecting a second point;

calculating a value of the modeled behavior at each of the points; and

calculating a slope through each of the points, wherein the slope is a function of the values of the modeled behavior at each point.

47. A method for determining an optimum process point for fabricating a device feature of a critical dimension, comprising:

selecting a measuring point on a computer representation of a wafer corresponding to the feature of the critical dimension;

calculating modeled behavior values and their rates of change over a range of corresponding values of exposure time of a stepper;

selecting an optimum threshold value having a largest rate of change around the measuring point; and

determining the exposure time corresponding to the optimum threshold value, wherein calculating threshold values and their rates of change includes;

decrementing a value of the exposure time;

calculating a value of the modeled behavior at the measuring point;

determining a value of the modeled behavior at a location offset from the measuring point in a first direction by a second value;

determining a value of the modeled behavior at a location offset from the measuring point in a second direction, opposite to the first direction, by the second value; and

calculating a rate of change of the modeled behavior corresponding to the exposure time.

48. The method of claim 47, wherein calculating the rate of change comprises:

calculating a difference of the modeled behavior values ascertained during determining the values of the modeled behavior; and

dividing the difference of the modeled behavior values by twice the second value.

50. A method for determining an optimum process point for fabricating a device feature of a critical dimension, comprising:

selecting a measuring point on a computer representation of a wafer corresponding to the feature of the critical dimension;

calculating modeled behavior values and their rates of change over a range of corresponding values of on/off axis settings of a stepper with respect to a constant mask material edge;

selecting an optimum threshold value having a largest rate of change around the measuring point; and

determining on/off axis settings corresponding to the optimum threshold value.

51. The method of claim 50, wherein calculating modeled behavior values comprises:

selecting a point on one side of the measuring point;

calculating a value of the modeled behavior at each of the points; and

calculating a slope through each of the points, wherein the slope is a function of the values of the modeled behavior at each point.

55. The method of claim 50, wherein selecting a measuring point, calculating modeled behavior values and their rates of change, selecting an optimum threshold value, and determining on/off axis settings are performed in a computer.

52. A method for determining an optimum process point for fabricating a device feature of a critical dimension, comprising:

selecting a measuring point on a computer representation of a wafer corresponding to the feature of the critical dimension;

calculating modeled behavior values and their rates of change over a range of corresponding values of on/off axis settings of a stepper with respect to a constant mask material edge;

selecting an optimum threshold value having a largest rate of change around the measuring point; and

determining on/off axis settings corresponding to the optimum threshold value, wherein calculating modeled behavior values includes;

selecting a point on one side of the measuring point;

selecting a second point;

calculating a value of the modeled behavior at each of the points; and

calculating a slope through each of the points, wherein the slope is a function of the values of the modeled behavior at each point.

53. A method for determining an optimum process point for fabricating a device feature of a critical dimension, comprising:

selecting a measuring point on a computer representation of a wafer corresponding to the feature of the critical dimension;

calculating modeled behavior values and their rates of change over a range of corresponding values of on/off axis settings of a stepper;

selecting an optimum threshold value having a largest rate of change around the measuring point; and

determining on/off axis settings corresponding to the optimum threshold value, wherein calculating threshold values and their rates of change includes;

decrementing a value of the on/off axis setting;

calculating a value of the modeled behavior at the measuring point;

determining a value of the modeled behavior at a location offset from the measuring point in a first direction by a second value;

determining a value of the modeled behavior at a location offset from the measuring point in a second direction, opposite to the first direction, by the second value; and

calculating a rate of change of the modeled behavior corresponding to the on/off axis setting.

54. The method of claim 53, wherein calculating the rate of change comprises:

calculating a difference of the modeled behavior values ascertained during determining the values of the modeled behavior; and

dividing the difference of the modeled behavior values by twice the second value.

56. A computer program product comprising a memory having computer program logic recorded thereon for enabling a processor in a computer system to determine an optimum process point for fabricating a device feature of a critical dimension, the computer program logic comprising:

a first calculating process enabling the processor to calculate a modeled behavior value associated with the device feature and a rate of change of the modeled behavior value over a range of corresponding values of an optical parameter of a stepper with respect to a constant mask material edge;

a second calculating process enabling the processor to select an optimum threshold value having a largest rate of change; and

a determining process enabling the processor to determine the value of the optical parameter corresponding to the optimum threshold value.

57. The computer program product of claim 56, further comprising:

a providing process enabling the processor to provide the optimum threshold value to a proximity effect correction process which modifies a mask pattern to compensate for proximity effects.

58. The computer program product of claim 56, wherein the optical parameter is one of focus, numerical aperture, exposure time and on or off axis settings of the stepper.

59. A computer system, comprising:

a processor;

a memory operatively coupled to the processor;

a first calculating process enabling the processor to calculate a modeled behavior value associated with a device feature and a rate of change of the modeled behavior value over a range of corresponding values of an optical parameter of a stepper with respect to a constant mask material edge;

a second calculating process enabling the processor to select an optimum threshold value having the largest rate of change; and

a determining process enabling the processor to determine a value of the optical parameter corresponding to the optimum threshold value.

60. The computer system of claim 59, further comprising a providing process enabling the processor to provide the optimum threshold value to a proximity effect correction process which modifies a mask pattern to compensate for proximity effects.

61. The computer system of claim 59, wherein the optical parameter is one of focus, numerical aperture, exposure time, and on or off axis setting.

62. A computer system, comprising:

a processor;

a memory operatively coupled to the processor;

a first calculating process enabling the processor to calculate a modeled behavior value associated with a device feature and a rate of change of the modeled behavior value over a range of corresponding values of an optical parameter of a stepper;

a second calculating process enabling the processor to select an optimum threshold value having the largest rate of change; and

a determining process enabling the processor to determine a value of the optical parameter corresponding to the optimum threshold value, wherein the rate of change of the modeled value indicates a process latitude of an edge of the device feature.

63. A computer system, comprising:

a processor;

a memory operatively coupled to the processor;

a first calculating process enabling the processor to calculate a modeled behavior value associated with a device feature and a rate of change of the modeled behavior value over a range of corresponding values of an exposure time of a stepper with respect to a constant mask material edge;

a second calculating process enabling the processor to select an optimum threshold value having a largest rate of change; and

a determining process enabling the processor to determine a value of an exposure time corresponding to the optimum threshold value.

64. The computer system of claim 63, further comprising a providing process enabling the processor to provide the optimum threshold value to a proximity effect correction process which modifies a mask pattern to compensate for proximity effects.

65. A computer system, comprising:

a processor;

a memory operatively coupled to the processor;

a first calculating process enabling the processor to calculate a modeled behavior value associated with a device feature and a rate of change of the modeled behavior value over a range of corresponding values of an exposure time of a stepper;

a second calculating process enabling the processor to select an optimum threshold value having a largest rate of change; and

a determining process enabling the processor to determine a value of an exposure time corresponding to the optimum threshold value, wherein the rate of change of the modeled behavior value indicates a process latitude of an edge of the device feature.

66. A computer system, comprising:

a processor;

a memory operatively coupled to the processor;

a first calculating process enabling the processor to calculate a modeled behavior value associated with a device feature and a rate of change of the modeled behavior value over a range of corresponding values of focus of a stepper with respect to a constant mask material edge;

a second calculating process enabling the processor to select an optimum threshold value having a largest rate of change; and

a determining process enabling the processor to determine a value of a focus corresponding to the optimum threshold value.

67. The computer system of claim 66, further comprising a providing process enabling the processor to provide the optimum threshold value to a proximity effect correction process which modifies a mask pattern to compensate for proximity effects.

68. A computer system, comprising:

a processor;

a memory operatively coupled to the processor;

a first calculating process enabling the processor to calculate a modeled behavior value associated with a device feature and a rate of change of the modeled behavior value over a range of corresponding values of focus of a stepper;

a second calculating process enabling the processor to select an optimum threshold value having a largest rate of change; and

a determining process enabling the processor to determine a value of a focus corresponding to the optimum threshold value, wherein the rate of change of the modeled value indicates a process latitude of an edge of the device feature.

69. A computer system, comprising:

a processor;

a memory operatively coupled to the processor;

a first calculating process enabling the processor to calculate a modeled behavior value associated with a device feature and a rate of change of the modeled behavior value over a range of corresponding values of a numerical aperture of a stepper with respect to a constant mask material edge;

a second calculating process enabling the processor to select an optimum threshold value having a largest rate of change; and

a determining process enabling the processor to determine a value of a numerical aperture corresponding to the optimum threshold value.

70. The computer system of claim 69, further comprising a providing process enabling the processor to provide the optimum threshold value to a proximity effect correction process which modifies a mask pattern to compensate for proximity effects.

71. A computer system, comprising:

a processor;

a memory operatively coupled to the processor;

a first calculating process enabling the processor to calculate a modeled behavior value associated with a device feature and a rate of change of the modeled behavior value over a range of corresponding values of a numerical aperture of a stepper;

a second calculating process enabling the processor to select an optimum threshold value having a largest rate of change; and

a determining process enabling the processor to determine a value of a numerical aperture corresponding to the optimum threshold value, wherein the rate of change of the modeled value indicates a process latitude of an edge of the device feature.

72. A computer system, comprising:

a processor;

a memory operatively coupled to the processor;

a first calculating process enabling the processor to calculate a modeled behavior value associated with a device feature and a rate of change of the modeled behavior value over a range of corresponding values of an on or off axis setting of a stepper with respect to a constant mask material edge;

a second calculating process enabling the processor to select an optimum threshold value having a largest rate of change; and

a determining process enabling the processor to determine a value of an on or off axis setting corresponding to the optimum threshold value.

73. The computer system of claim 72, further comprising a providing process enabling the processor to provide the optimum threshold value to a proximity effect correction process which modifies a mask pattern to compensate for proximity effects.

74. A computer system, comprising:

a processor;

a memory operatively coupled to the processor;

a first calculating process enabling the processor to calculate a modeled behavior value associated with a device feature and a rate of change of the modeled behavior value over a range of corresponding values of an on or off axis setting of a stepper;

a second calculating process enabling the processor to select an optimum threshold value having a largest rate of change; and

a determining process enabling the processor to determine a value of an on or off axis setting corresponding to the optimum threshold value, wherein the rate of change of the modeled value indicates a process latitude of an edge of the device feature.