SYSTEM AND METHOD FOR MEASURING AND ANALYZING LITHOGRAPHIC PARAMETERS AND DETERMINING OPTIMAL PROCESS CORRECTIONS

• US 20070035712A1
• Filed: 08/02/2006
• Published: 02/15/2007
• Est. Priority Date: 08/09/2005
• Status: Active Grant

• Current Assignee: ASML Netherlands BV (ASML Holding NV)
• Original Assignee: Brion Technologies Incorporated (ASML Holding NV)
• Inventors: Ye, Jun, Cao, Yu, Preil, Moshe, Hunsche, Stefan, Gassner, Michael
• Granted Patent: US 7,749,666 B2
• Time in Patent Office: Days
• US Class Current: 355/55
• CPC Class Codes: G03F 7/70641 Focus
• CPC Class Codes: G03F 7/70666 Aerial image, i.e. measuring the image of the patterned exposure light at the image plane of the projection system

Abstract

A method of using an in-situ aerial image sensor array is disclosed to separate and remove the focal plane variations caused by the image sensor array non-flatness and/or by the exposure tool by collecting sensor image data at various nominal focal planes and by determining best focus at each sampling location by analysis of the through-focus data. In various embodiments, the method provides accurate image data at best focus anywhere in the exposure field, image data covering an exposure-dose based process window area, and a map of effective focal plane distortions. The focus map can be separated into contributions from the exposure tool and contributions due to topography of the image sensor array by suitable calibration or self-calibration procedures. The basic method enables a wide range of applications, including for example qualification testing, process monitoring, and process control by deriving optimum process corrections from analysis of the image sensor data.

Claims

1. A method for identifying focal plane variations comprising:

measuring aerial images using an image sensor array at a plurality of focal planes in an exposure tool, the image sensor array including a plurality of image sensor elements;

determining a value for at least one image quality metric for the aerial image measured by each image sensor element at each of the plurality of focal planes to produce image quality metric data;

fitting a parametric function to the image quality metric data, the parametric function being a function of focus;

determining a best focus for each image sensor element using the fitted parametric function;

calculating a difference between the best focus for each image sensor element and a nominal best focus setting of the exposure tool; and

creating a focal plane map of the combination of the image sensor array and the exposure tool that indicates the differences between the best focuses of the image sensor elements and the nominal best focus setting.

2. The method of claim 1, further comprising:

producing aerial images using a mask including test patterns; and

calibrating the focal plane map to distinguish between contributions of the image sensor array and the exposure tool.

3. The method of claim 1, wherein the image sensor array is disposed in an image sensor unit that is capable of being loaded into the wafer stage of the exposure tool.

4. The method of claim 1, wherein the image sensor array is disposed in an image sensor unit that is integrated into the exposure tool.

5. The method of claim 1, wherein the image quality metric data are one or more values of image intensity, image log slope (ILS), image contrast, or critical dimension.

6. The method of claim 1, wherein the parametric function is one of a polynomial function, a Fourier series, a first-principles model, or a look-up table generated by simulation or calibration measurements.

7. The method of claim 2, further comprising mapping variations in optical aberrations that affect a best focus position of a projection lens of the exposure tool.

8. The method of claim 7, wherein the optical aberrations are astigmatism and spherical aberration.

9. The method of claim 7, further comprising using a set of optical aberration sensitive test patterns.

10. The method of claim 9, wherein the set of optical aberration sensitive test patterns includes a set of line-space patterns with varying pitches and varying orientations.

11. The method of claim 2, further comprising generating a plurality of focal plane maps from aerial images measured at different image sensor array orientations achieved by rotating the image sensor array by a multiple or multiples of 90 degrees to distinguish non-flatness contributions between the image sensor array and the exposure tool.

12. The method of claim 1, further comprising:

creating additional focal plane maps at different times to monitor changes in a focal plane of the exposure tool.

13. The method of claim 12, further comprising:

comparing the monitored changes in the focal plane to an averaged focal plane map over time to provide an early warning of a negative impact on exposure tool performance.

14. A method for characterizing pattern uniformity of a lithography process, comprising:

measuring aerial images using an image sensor array at a plurality of focal planes in an exposure tool, the image sensor array including a plurality of image sensor elements;

determining a value for at least one image quality metric for the aerial image measured by each image sensor element at each of the plurality of focal planes to produce image quality metric data;

fitting a parametric function to the image quality metric data, the parametric function being a function of focus;

determining a best focus for each image sensor element using the fitted parametric function;

generating a reconstructed image for each image sensor element at a best focus setting, wherein the reconstructed images have a best image quality across the entire field independent of focus variations due to topography of the image sensor array or to the exposure tool;

defining a pattern metric for a pattern uniformity characterization; and

using the reconstructed images at the best focus setting to analyze the pattern metric for the pattern uniformity characterization.

15. The method of claim 14, wherein, for each image sensor element, the reconstructed image at the best focus setting is generated by interpolating between the measured aerial images at the plurality of focal planes.

16. The method of claim 14, wherein the image sensor array is disposed in an image sensor unit that is capable of being loaded into the wafer stage of the exposure tool.

17. The method of claim 14, wherein the image sensor array is disposed in an image sensor unit that is integrated into the exposure tool.

18. The method of claim 14, wherein the image quality metric data are one or more values of image intensity, image log slope (ILS), image contrast, or critical dimension.

19. The method of claim 14, wherein the parametric function is one of a polynomial function, a Fourier series, a first-principles model, or a look-up table generated by simulation or calibration measurements.

20. The method of claim 14, wherein the pattern metric is one of a critical dimension metric, a line-end pullback metric, a pattern necking metric, or a pattern pinching metric.

21. The method of claim 14, further comprising:

generating a critical dimension uniformity map by calculating a critical dimension metric at every position within the field of the exposure tool.

22. The method of claim 14, further comprising:

calculating process corrections to minimize imaging errors across the field based on the reconstructed images generated for each image sensor element; and

applying the process corrections to the exposure tool.

23. A method for process control in an exposure tool, comprising:

defining a process window for a lithography process in a focus-exposure space;

selecting a set of focus settings in the process window;

selecting a set of exposure dose settings in the process window;

measuring a plurality of aerial images across a field of the exposure tool at a plurality of focal planes using an image sensor array, the image sensor array including a plurality of image sensor elements;

determining image quality metric data for each aerial image measured by each image sensor element at each of the plurality of focal planes;

fitting a parametric function to the image quality metric data for each image sensor element, the parametric function being a function of focus;

determining a best focus for each image sensor element of the image sensor array using the fitted parametric function for that image sensor element;

for each image sensor element, adjusting the selected set of focus settings in the process window to be relative to the determined best focus, wherein a zero focus setting is adjusted to the best focus and other focus settings in the process window are relative offsets from the best focus;

generating reconstructed images for each of the selected set of focus settings in the process window by interpolating the measured aerial images to the corresponding adjusted focus settings;

generating scaled aerial images for each selected exposure dose setting in the process window using the reconstructed images at each selected focus setting in the process window; and

analyzing the reconstructed and scaled aerial images for the plurality of image sensor elements at the plurality of focus settings and exposure dose settings in the process window to perform process control in the exposure tool.

24. The method of claim 23, further comprising:

generating pattern metric maps by calculating a pattern metric for each of a set of test patterns across the field of the exposure tool for each of the reconstructed images and scaled aerial images corresponding to the selected plurality of focus settings and exposure dose settings in the process window.

25. The method of claim 24, wherein the set of test patterns includes a set of line-space patterns with varying pitches and varying orientations.

26. The method of claim 24, wherein the pattern metric is one of a critical dimension metric, a line-end pullback metric, a pattern necking metric, or a pattern pinching metric.

27. The method of claim 24, further comprising:

generating process windows of the pattern metric vs. focus and exposure dose from the pattern metric maps for the set of test patterns.

28. The method of claim 27, wherein a process window is generated for each image sensor element by determining whether the pattern metrics for the reconstructed and scaled aerial images through varying focus and exposure dose settings fall within accepted tolerances for the lithography process.

29. The method of claim 27, further comprising:

generating a common process window by finding a smallest overlapped region of the process windows for the set of test patterns.

30. The method of claim 29, further comprising:

calculating optimal process conditions from the common process window; and

applying process corrections to the exposure tool so that the process conditions of the exposure tool match the optimal process conditions.

31. A method of generating reconstructed aerial images at best focus, comprising:

measuring aerial images using an image sensor array at a plurality of focal planes in an exposure tool, the image sensor array including a plurality of image sensor elements;

determining a value for at least one image quality metric for the aerial image measured by each image sensor element at each of the plurality of focal planes to produce image quality metric data;

fitting a parametric function to the image quality metric data for each image sensor element, the parametric function being a function of focus;

determining a best focus position for each image sensor element using the fitted parametric function; and

generating a reconstructed aerial image for each image sensor element at the best focus position, wherein the reconstructed image has best image quality across the entire field independent of focus variations due to topography of the image sensor array or to the exposure tool.

32. The method of claim 31, wherein the parametric function is one of a polynomial function, a Fourier series, a first-principles model, or a look-up table generated by simulation or calibration measurements.

33. The method of claim 31, wherein the best focus position for each image sensor element is determined by identifying a focus position at a maximum of the fitted parametric function.

34. The method of claim 31, wherein generating a reconstructed aerial image for each image sensor element at the best focus position is performed by interpolating the measured aerial images at the best focus position determined by the fitted parametric function.

35. The method of claim 31, wherein the image sensor array is disposed in an image sensor unit that is capable of being loaded into the wafer stage of the exposure tool.

36. The method of claim 31, wherein the image sensor array is disposed in an image sensor unit that it integrated into the exposure tool.

37. The method of claim 31, wherein the image quality metric data are one or more values of image intensity, image log slope (ILS), image contrast, or critical dimension.

38. A system for monitoring focal plane variations in an exposure tool, comprising:

an image sensor array capable of being loaded into the exposure tool, the image sensor array including a plurality of image sensor elements;

a computer in communication with the image sensor array, the computer including;

an image generator configured to generate measured aerial images using data from the image sensor array at a plurality of focal planes in an exposure tool;

an image quality metric calculator configured to calculate a value for at least one image quality metric for the aerial image measured by each image sensor element at each of the plurality of focal planes to produce image quality metric data;

a data fit module configured to fit a parametric function to the calculated image quality metric data, the parametric function being a function of focus, and to identify a best focus position for each of the plurality of image sensor elements using the fitted parametric function; and

a focus map module configured to generate a focus map that reflects differences between a best focus for each of the plurality of image sensor elements and a nominal best focus of the exposure tool.

39. The system of claim 38, wherein the image generator is further configured to generate reconstructed images at any focus position by interpolating the measured aerial images measured at a plurality of focal planes.

40. The system of claim 38, wherein the computer further includes a process control module configured to determine correctable errors in a lithography process performed by the exposure tool and track the correctable errors over time.

41. The system of claim 40, wherein the focus map module is further configured to create additional focus maps at different times to monitor changes in a focal plane of the exposure tool.

42. The system of claim 41, wherein the process control module is further configured to compare monitored changes in the focal plane of an exposure tool to an averaged focus map over time, to determine a focus correction, and to apply the focus correction to the exposure tool so that the process condition of the exposure tool matches a best focus condition.

43. The system of claim 38, wherein the parametric function is one of a polynomial function, a Fourier series, a first-principles model, or a look-up table generated by simulation or calibration measurements.

44. The system of claim 38, wherein the image quality metric data are one or more values of image intensity, image log slope (ILS), image contrast, or critical dimension.