Apodizing filter system useful for reducing spot size in optical measurements and other applications

• US 5,859,424 A
• Filed: 04/08/1997
• Issued: 01/12/1999
• Est. Priority Date: 04/08/1997
• Status: Expired due to Term

• Current Assignee: Tencor Instruments (KLA Corporation)
• Original Assignee: KLA-Tencor Corporation (KLA Corporation)
• Inventors: Carter, Joseph R., Norton, Adam E., Johnson, Kenneth C.
• Primary Examiner(s): LE, QUE TAN
• Application Number:
  US08/835,533
• Time in Patent Office: 644 Days
• Field of Search: 250/226, 250/237 G, 250/237 R, 250/559.04, 250/559.22, 250/201.2, 250/503.1, 250/492.2, 250/216, 356/225, 356/374, 356/381, 659/888, 659/738, 659/739
• US Class Current: 250/226
• CPC Class Codes: G02B 27/58 Optics for apodization or superresolution; Optical synthetic aperture systems
• CPC Class Codes: G03F 7/70616 Monitoring the printed patterns

Abstract

Because of diffraction effects caused by slits or apertures in optical measurement systems, the radiation energy which is directed towards a particular region on a sample will be spread over a larger area than desirable. By employing an apodizing filter in the radiation path in such system, diffraction tails of the system will be reduced. The apodizing filter preferably has a pattern of alternating high transmittance areas and substantially opaque areas where the locally averaged transmittance function is an apodizing function. In the preferred embodiment, the locally averaged transmittance function varies smoothly and monotonically from the periphery to the center of the filter.

Claims

1. A method for measuring a sample, including the steps of:

focusing radiation from a source into a sampling beam directed at the sample;

detecting radiation of the sampling beam that has been modified by a portion of the sample, said portion defining a measurement spot; and

apodizing radiation that is from the source, wherein said detecting step detects radiation that has been apodized in the apodizing step, to reduce size of the measurement spot.

2. The method of claim 1, further comprising polarizing radiation from the source, said focusing step focusing said polarized radiation to provide said sampling beam.

3. The method of claim 2, further comprising passing radiation from the source through an entrance slit or pinhole prior to focusing such radiation into said sampling beam.

4. The method of claim 2, said radiation from the source focused in the focusing step having a broad wavelength band, wherein said sample modifies said sampling beam, said detecting step including:

separating the modified sampling beam from the sample into wavelength components;

detecting the wavelength components to provide outputs; and

processing the outputs to determine changes of polarized state in amplitude and phase caused by the sample.

5. The method of claim 4, said focusing step focusing radiation to the sample at an oblique incidence angle by means of reflective optics.

6. The method of claim 4, said incidence angle being in a range from about 63.5 degrees to about 80.5 degrees.

7. The method of claim 4, said radiation having wavelengths in a range including infrared, visible and ultraviolet wavelengths.

8. The method of claim 4, said radiation having wavelengths in a range including microwave wavelengths.

9. The method of claim 1, said apodizing step apodizing radiation modified by the sample.

10. The method of claim 1, said apodizing step apodizing radiation of the sampling beam.

11. The method of claim 1, said focusing step focusing radiation that has been apodized in the apodizing step.

12. The method of claim 1, said measurement spot being less than about 50 by 30 microns.

13. The method of claim 1, wherein said focusing focuses radiation of a plurality of wavelengths into said sampling beam, and said apodizing apodizes radiation of said plurality of wavelengths.

14. The method of claim 1, wherein said focusing focuses broadband radiation into said sampling beam, and said apodizing apodizes broadband radiation.

15. The method of claim 1, said sampling beam having a diffraction point spread function, wherein said apodizing reduces tails of the point spread function relative to that of a clear, unapodized aperture.

16. The method of claim 1, wherein said apodizing causes amplitude of the radiation that is modified to vary smoothly between a maximum and a minimum.

17. The method of claim 1, wherein said detecting detects changes of polarized state of said radiation.

37. The apparatus of claim 8 said apodizing means including a plate having a two dimensional pattern of alternating high transmittance areas and substantially opaque areas, said pattern having a locally averaged transmittance function that is an apodizing function.

38. The apparatus of claim 37, said plate having a center portion and an edge portion surrounding said center portion, said filter having an effective area for filtering radiation, said effective area including the center portion and at least a part of the edge portion, said locally averaged transmittance function varying substantially monotonically by less than 50% of said high transmittance over any 10% of the length of any radial line.

39. The apparatus of claim 37, said locally averaged transmittance function being such that it has a substantially constant maximum value in a central part of the center portion of the plate.

40. The apparatus of claim 37, said pattern comprising a one or two dimensional diffraction grating with lines having a spatially varying structure, wherein the grating apodizes radiation passing therethrough in direction(s) transverse to the grating lines.

41. The apparatus of claim 37, wherein said pattern is a two dimensional grating comprising substantially opaque squares or rectangles on a clear background, or clear squares or rectangles on a substantially opaque background, with spatially varying square or rectangular dimensions.

18. An apparatus for measuring a sample, including:

a source which emits radiation;

means for focusing radiation from the source into a sampling beam to the sample;

means for detecting radiation of the sampling beam that has been modified by a portion of the sample, said portion defining a measurement spot; and

means for apodizing radiation that is from the source, wherein said detecting means detects radiation that has been apodized by the apodizing means, to reduce size of the measurement spot.

19. The apparatus of claim 18, wherein said focusing means focuses radiation of a plurality of wavelengths into said sampling beam, and said apodizing means apodizes radiation of said plurality of wavelengths.

20. The apparatus of claim 18, wherein said focusing means focuses broadband radiation into said sampling beam, and said apodizing means apodizes broadband radiation.

21. The apparatus of claim 18, said sampling beam having a diffraction point spread function, wherein said apodizing means reduces tails of the point spread function relative to that of a clear, unapodized aperture.

22. The apparatus of claim 18, wherein said apodizing means causes amplitude of said radiation that is modified to vary smoothly between a maximum and a minimum.

23. The apparatus of claim 18, wherein said detecting means detects changes of polarized state of said radiation.

24. The apparatus of claim 18, further comprising a polarizer that polarizes radiation from the source to produce a polarized beam, said focusing means focusing said polarized beam or a beam derived therefrom to provide said sampling beam.

25. The apparatus of claim 24, said apodizing means including at least a first apodizing filter, said filter filtering said polarized beam.

26. The apparatus of claim 25, said apodizing means including a second apodizing filter filtering unpolarized radiation from the source prior to polarization by the polarizer.

27. The apparatus of claim 25, further including an entrance slit or pinhole passing radiation originating from the source to the focusing means.

28. The apparatus of claim 27, further comprising:

at least one lens focusing radiation from the source through the polarizer to the entrance slit or pinhole, said at least one lens having conjugate planes;

two aperture stops at the two conjugate planes.

29. The apparatus of claim 28, said first apodizing filter being proximate to the aperture stop between the polarizer and the focusing means.

30. The apparatus of claim 25, said focusing means including reflective optics that reflectively focuses the sampling beam to the measurement spot on the sample, said detector means providing an output, said apparatus further comprising means for processing the detector means output to determine changes of polarization state in amplitude and phase caused by the sample.

31. The apparatus of claim 24, wherein the apodizing means is positioned to apodize radiation from the polarizer to the sample.

32. The apparatus of claim 24, wherein the apodizing means is positioned to apodize radiation from the source to the polarizer.

33. The apparatus of claim 24, wherein the apodizing means is positioned to apodize radiation modified by the sample.

34. The apparatus of claim 24, further including an entrance slit or pinhole passing radiation from the source to the focusing means.

35. The apparatus of claim 24, wherein the apodizing means is positioned to apodize radiation from the entrance slit or pinhole to the sample.

36. The apparatus of claim 34, wherein the apodizing means is positioned to apodize radiation from the source to the entrance slit or pinhole.

42. The apparatus of claim 18 said apodizing means including a spatially varying neutral density filter.

43. An apodizing filter comprising a plate having a two dimensional pattern of alternating high transmittance areas and substantially opaque areas, said pattern having a locally averaged transmittance function that is an apodizing function.

44. The filter of claim 43, said plate comprising:

a transparent substrate; and

a layer of substantially opaque material on or near the substrate, said layer defining said substantially opaque areas.

45. The filter of claim 44, wherein said substrate comprises quartz and said opaque material comprises chrome.

46. The filter of claim 43, said plate having a center portion and an edge portion surrounding said center portion, said filter having an effective area for filtering radiation, said effective area including the center portion and at least a part of the edge portion, said locally averaged transmittance function varying substantially monotonically by less than 50% of said high transmittance over any 10% of the length of any radial line.

47. The filter of claim 43, said locally averaged transmittance function being such that it has a substantially constant maximum value in a central part of the center portion of the plate.

48. The filter of claim 43, said pattern comprising a one or two dimensional diffraction grating with lines having a spatially varying structure, wherein the grating apodizes radiation passing therethrough in direction(s) transverse to the grating lines.

49. The filter of claim 48, wherein the grating comprises substantially opaque strips of varying widths, said strips having center lines, and wherein the spacings between the center lines of pairs of adjacent strips are substantially the same.

50. The filter of claim 43, wherein said pattern is a two dimensional grating comprising substantially opaque squares or rectangles on a clear background, or clear squares or rectangles on a substantially opaque background, with spatially varying square or rectangular dimensions.

51. The filter of claim 50, said grating comprising substantially opaque squares or rectangles on a clear background over a first portion of the plate and clear squares or rectangles on a substantially opaque background over a second portion of the plate.

52. The filter of claim 51, said first portion comprising a central portion of the plate and the second portion comprising a peripheral portion of the plate.

53. The filter of claim 52, said grating comprising substantially opaque squares on a clear background, or clear squares on a substantially opaque background, wherein at a boundary between the central and peripheral portions, the opaque and clear squares touch at their corners.

54. The filter of claim 50, said grating comprising substantially opaque squares on a clear background, or clear squares on a substantially opaque background, wherein the squares are centered at points of periodic square grids at substantially 45 degrees to the squares.

55. The filter of claim 54, wherein the clear squares are centered at points of a first periodic square grid and the substantially opaque squares are centered at points of a second periodic square grid, wherein the first and second grids have two dimensions and are displaced by half spacing along the two dimensions.

56. The filter of claim 43, wherein said pattern is a two dimensional grating comprising substantially opaque octagonal, circular, triangular or hexagonal regions on a clear background, or clear octagonal, circular, triangular or hexagonal regions on a substantially opaque background, with spatially varying dimensions of the regions.

57. The filter of claim 43, said sample modifying the sampling beam to derive a modified beam, wherein the sampling beam and the modified beam define an incidence plane, said locally averaged transmittance function varying in directions parallel to the incidence plane but having a substantially constant value in directions normal to the incidence plane.

58. The system of claim 43, said areas of high transmittance transmitting at least twice the amount of radiation transmitted by substantially opaque areas.

59. A radiation imaging system with improved resolution, comprising:

means for focusing radiation from an object surface to a focal surface;

an apodizer positioned in a radiation path between the object surface and the focal surface, said apodizer having a two dimensional pattern of alternating high transmittance areas and substantially opaque areas, said pattern having a locally averaged transmittance function that is an apodizing function; and

a filter in the radiation path following the apodizer, said filter blocking scattered radiation that is generated by, or passed by, the apodizer;

wherein radiation transmitted through the apodizer and the filter along said radiation path comprises image radiation, said image radiation having a diffraction point spread function;

wherein the apodizer reduces tails of the point spread function relative to that of a clear, unapodized aperture.

60. The system of claim 59, said filter blocking first and higher order diffracted radiation and passing zero order diffracted radiation passed by the apodizer.

61. The system of claim 60, said filter being a clear aperture, a baffle or a set of baffles.

62. A method for measuring ellipsometric parameters of a sample, including the steps of:

focusing polarized broadband radiation from a source into a sampling beam directed at the sample;

separating into wavelength components radiation of the sampling beam that has been modified by a portion of the sample, said portion defining a measurement spot;

detecting the wavelength components to provide outputs; and

determining from the outputs changes of polarized state caused by the sample; and

apodizing radiation that is from the source, wherein said detecting step detects radiation that has been apodized in the apodizing step, to reduce size of the measurement spot.

63. The method of claim 62, wherein said apodizing apodizes a plurality of wavelength components of said radiation.

64. The method of claim 62, said sampling beam having a diffraction point spread function, wherein said apodizing reduces tails of the point spread function relative to that of a clear, unapodized aperture.

65. The method of claim 62, wherein said apodizing causes amplitude of said radiation that is modified by the sample to vary smoothly between a maximum and a minimum.

66. An apparatus for measuring ellipsometric parameters of a sample, including the steps of:

means for focusing polarized broadband radiation from a source into a sampling beam directed at the sample;

means for separating into wavelength components radiation of the sampling beam that has been modified by a portion of the sample, said portion defining a measurement spot;

a detector detecting the wavelength components to determine changes of polarized state caused by the sample; and

an apodizer that apodizes radiation that is from the source and that is detected by the detector to reduce size of the measurement spot.

67. The method of claim 66, wherein said apodizer apodizes a plurality of wavelength components of said radiation.

68. The method of claim 66, said sampling beam having a diffraction point spread function, wherein said apodizer reduces tails of the point spread function relative to that of a clear, unapodized aperture.

69. The method of claim 66, wherein said apodizer causes amplitude of said radiation that is modified by the sample to vary smoothly between a maximum and a minimum.