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United States Patent5754299
Sugaya , ; et al.May 19, 1998
Title
Inspection apparatus and method for optical system, exposure apparatus provided with the inspection apparatus, and alignment apparatus and optical system thereof applicable to the exposure apparatus
Abstract
The present invention relates to inspection apparatus and method in which, based on images under a plurality of focus conditions formed by way of an optical system to be inspected, namely, using images under a plurality of defocal conditions, tendency in positional change or change of asymmetry between the images is calculated so as to specify at least one of aberration condition and optical adjustment condition of the optical system to be inspected as well as to exposure apparatuses and overlay accuracy measurement apparatuses provided with the inspection apparatus. In addition, the present invention relates to an image-forming optical system suitable to an alignment apparatus which is applicable to the exposure apparatuses. This image-forming optical system comprises a correction optical system for intentionally generating asymmetric aberration or symmetric aberration in the image-forming optical system and a decentering mechanism for decentering the correction optical system to cancel asymmetric aberration or symmetric aberration in the image-forming optical system.
Inventors:Sugaya; Ayako (Kawasaki, JP), Nakagawa; Masahiro  (Yokohama, JP), Nagayama; Tadashi  (Tokyo, JP)
Assignee:Nikon Corporation (, JP)
Appl. No.:584863
Filed:January 11, 1996
Foreign Application Priority Data
Jan 13, 1995 [JP] 7-003856
Aug 08, 1995 [JP] 7-222677
Oct 27, 1995 [JP] 7-303932
Current U.S. Class:356/401 356/399 
Field of Search:356/399-401 250/548,557,234,235,236 355/43,53,77
U.S. Patent Documents
4655598April 1987Murakami
4888614December 1989Suzuki et al.
5323207June 1994Ina
5448332September 1995Sakakibara
5461237October 1995Wakamoto
5473424December 1995Okumura
Foreign Patent Documents
4-273246Sep., 1992JP
4-65603Mar., 1992JP
5-118957May., 1993JP
6-69097Mar., 1994JP
Primary Examiner: Gonzalez; Frank
Assistant Examiner: Stafira; Michael P.
Attorney, Agent or Firm:Pennie & Edmonds
Claims

What is claimed is:
1. An inspection apparatus for specifying at least one of an aberration condition and an optical adjustment condition of an optical system to be inspected, said optical system being provided between a first surface and a second surface and used for projecting an image of a pattern on said first surface onto said second surface, said inspection apparatus comprising:
a defocus mechanism for defocusing the image of the pattern on said first surface, which is to be formed on said second surface through said optical system to be inspected, from a best focus condition of the image on said second surface by a predetermined defocus amount;
a detector for detecting the image formed on said second surface under a predetermined focus condition; and
an inspection unit for calculating a change in image position on said second surface within a predetermined defocus range, based on information concerning respective positions of a plurality of images detected by said detector under focus conditions different from each other, said plurality of images being formed on said second surface through said optical system to be inspected.

2. An inspection apparatus according to claim 1, wherein said inspection unit includes a calculator for calculating a measurement line which indicates a relationship between the amount of defocus and an image position corresponding thereto, based on information concerning the respective positions of said images on said second surface under at least three kinds of the focus conditions different from each other.

3. An inspection apparatus according to claim 1, wherein said inspection unit includes a calculator performing the steps of:
calculating, for each of a plurality of patterns on said first surface having forms different from each other, a change in image position on said second surface; and
specifying the aberration condition and optical adjustment condition of said optical system to be inspected, based on a difference in the change in image position on said second surface between the respective patterns.

4. An inspection apparatus according to claim 1, wherein said defocus mechanism includes a control system for moving at least one of the pattern on said first surface, said optical system to be inspected, at least one of optical elements of said optical system to be inspected, and said detector along an optical axis of said optical system to be inspected.

5. An inspection apparatus according to claim 1, further comprising a sensitivity control mechanism for controlling inspection sensitivity of said inspection apparatus.

6. A projection exposure apparatus comprising:
an illumination optical system for illuminating a mask on which a predetermined pattern is formed with exposure light having a predetermined wavelength;
a projection optical system provided between said mask and a photosensitive substrate, for transferring an image of the pattern on said mask onto said substrate; an inspection apparatus for specifying at least one of an aberration condition of said projection optical system and respective optical adjustment conditions of said projection optical system and said illumination optical system, said inspection apparatus comprising:
a defocus mechanism for defocusing the image of the pattern on said mask, which is to be formed on a major surface of said substrate through at least said projection optical system, from a best focus condition of the image on said major surface of said substrate by a predetermined defocus amount;
a detector for detecting the image formed on said major surface of said substrate under a predetermined focus condition; and
an inspection unit for calculating a chance in image position on said major surface of said substrate within a predetermined defocus range, based on information concerning respective positions of a plurality of images detected by said detector under focus conditions different from each other, said plurality of images being formed on said major surface of said substrate through at least said projection optical system;
an aberration correction mechanism for correcting the aberration of said projection optical system, based on the aberration condition of said projection optical system detected by said inspection apparatus; and
an optical adjustment mechanism for performing optical adjustments of said illumination optical system and said projection optical system, based on the respective optical adjustment conditions of said illumination optical system and said projection optical system specified by said inspection apparatus.

7. A projection exposure apparatus comprising:
an illumination optical system for illuminating a mask on which a predetermined pattern is formed with exposure light having a predetermined wavelength;
a projection optical system between said mask and a photosensitive substrate, for transferring an image of the pattern on said mask onto said substrate;
an alignment apparatus for positioning said substrate, said alignment apparatus comprising a mark illumination optical system for illuminating an alignment mark formed on said substrate and a mark detection optical system for detecting light from the alignment mark;
an inspection apparatus for specifying at least one of an aberration condition of said mark detection optical system and respective optical adjustment conditions of said mark illumination optical system and said mark detection optical system, said inspection apparatus comprising:
a detector for detecting the image of the alignment mark formed on a detection surface thereof under a predetermined focus condition;
a defocus mechanism for defocusing the image of the alignment mark, which is to be formed on the detection surface of said detector through at least said mark detection optical system, from a best focus condition of the image on said detection surface of said detector by a predetermined defocus amount; and
an inspection unit for calculating a change in image position on said detection surface of said detector within a predetermined defocus range, based on information concerning respective positions of a plurality of images detected by said detector under focus conditions different from each other, said plurality of images being formed on said detection surface of said detector through at least said mark detection optic system;
an aberration correction mechanism for correcting the aberration of said mark detection optical system, based on the aberration condition of said mark detection optical system detected by said inspection apparatus; and
an optical adjustment mechanism for performing optical adjustments of said mark illumination optical system and said mark detection optical system, based on the respective optical adjustment conditions of said mark illumination optical system and said mark detection optical system specified by said inspection apparatus.

8. An overlay accuracy measurement apparatus for measuring an overlay accuracy of a pattern formed on a photosensitive substrate to be exposed with exposure light having a predetermined wavelength, said measurement apparatus comprising:
a mark illumination optical system for illuminating the mark on said substrate;
a mark detection optical system for detecting light from the mark on said substrate;
an inspection apparatus for specifying at least one of an aberration condition of said mark detection optical system and respective optical adjustment conditions of said mark illumination optical system and said mark detection optical system, said inspection apparatus comprising:
a detector for detecting the image formed on a detection surface thereof under a predetermined focus condition;
a defocus mechanism for defocusing the image of the mark on said substrate, which is to be formed on said detection surface of said detector through at least said mark detection optical system, from a best focus condition of the image on said detection surface of said detector by a predetermined defocus amount; and
an inspection unit for calculating a change in image position on said detection surface of said detector within a predetermined defocus range, based on information concerning respective positions of a plurality of images detected by said detector under focus conditions different from each other, said plurality of images being formed on said detection surface of said detector through at least said mark detection optical system;
an aberration correction mechanism for correcting the aberration of said mark detection optical system, based on the aberration condition of said mark detection optical system detected by said inspection apparatus; and
an optical adjustment mechanism for performing optical adjustments of said mark illumination optical system and said mark detection optical system, based on the respective optical adjustment conditions of said mark illumination optical system and said mark detection optical system specified by said inspection apparatus.

9. A projection exposure apparatus comprising:
an illumination optical system for illuminating a mask on which a predetermined pattern is formed with exposure light having a predetermined wavelength;
a projection optical system for transferring an image of the pattern on said mask to a photosensitive substrate; and
an overlay accuracy measurement apparatus for measuring an overlay accuracy of a mark formed on said substrate to be exposed with exposure light having a predetermined wavelength, said measurement apparatus, comprising:
a mark illumination optical system for illuminating the mark on said substrate;
a mark detection optical system for detecting light from the mark on said substrate;
an inspection apparatus for specifying at least one of an aberration condition of said mark detection optical system and respective optical adjustment conditions of said mark illumination optical system and said mark detection optical system, said inspection apparatus comprising:
a detector for detecting the image formed on a detection surface thereof under a predetermined condition;
a defocus mechanism for defocusing the image of the mark on said substrate, which is to be formed on said detection surface of said detector through at least said mark detection optical system, from a best focus condition of the image an said detection surface of said detector by a predetermined defocus amount; and
an inspection unit for calculating a chance in image position on said detection surface of said detector within a predetermined defocus range, based on information concerning respective positions of a plurality of images detected by said detector under focus conditions different from each other, said plurality of images being formed on said detection surface of said detector through at least said mark detection optical system;
an aberration correction mechanism for correcting the aberration of said mark detection optical system, based on the aberration condition of said mark detection optical system detected by said inspection apparatus; and
an optical adjustment mechanism for performing optical adjustments of said mark illumination optical system and said mark detection optical system, based on the respective optical adjustment conditions of said mark illumination optical system and said mark detection optical system specified by said inspection apparatus.

10. An inspection method for specifying at least one of an aberration condition and an optical adjustment condition of an optical system to be inspected, said optical system including a system provided between a first surface and a second surface, for projecting an image of a pattern formed on said first surface onto said second surface, said method comprising the steps of:
defocusing the image of the pattern on said first surface, which is to be formed on said second surface through said optical system to be inspected, from a best focus condition on the image on said second surface by a predetermined defocus amount;
detecting the image formed on said second surface under a predetermined focus condition; and then
calculating a change in image position on said second surface within a predetermined defocus range, based on information concerning respective positions of a plurality of images detected on said second surface under focus conditions different from each other, said plurality of images being formed on said second surface through said optical system to be inspected.

11. An inspection method according to claim 10, wherein said method comprises the step of calculating a measurement line which indicates a relationship between the amount of defocus and an image position corresponding thereto, based on information concerning the respective positions of said images on said second surface under at least three kinds of focus conditions different from each other.

12. An inspection method according to claim 11, wherein said method comprises the steps of:
specifying a peak point on said measurement line within the defocus range where curvature of said measurement line is maximized; and
calculating the magnitude of adjustment of the optical adjustment condition in said optical system to be inspected, based on gradient of a tangential line passing through the peak point on said measurement line.

13. An inspection method according to claim 11, wherein said method comprises the steps of:
specifying a peak point on said measurement line within the defocus range where curvature of said measurement line is maximized;
calculating a maximum clearance between said measurement line and a tangential line passing through the peak point on said measurement line; and
calculating an amount of asymmetric aberration of said optical system to be inspected, based on the obtained maximum clearance.

14. An inspection method according to claim 11, wherein said method comprises the steps of:
calculating an approximation line approximating said measurement line; and
calculating the magnitude of adjustment of the optical adjustment condition in said optical system to be inspected, based on gradient of said approximation line of said measurement line.

15. An inspection method according to claim 11, wherein said method comprises the steps of:
specifying a peak point on said measurement line within the defocus range where curvature of said measurement line is maximized;
calculating an approximation line approximating said measurement line; and
calculating an amount of asymmetric aberration of said optical system to be inspected, based on a distance between said approximation line and the peak point on said measurement line.

16. An inspection method according to claim 11, wherein said method comprises the steps of:
specifying a peak point on said measurement line within the defocus range where curvature of said measurement line is maximized; and
calculating an amount of symmetric aberration of said optical system to be inspected, based on a defocus amount in the peak point on said measurement line.

17. An inspection method according to claim 10, wherein said method comprises the step of moving at least one of said first surface, said optical system to be inspected, at least one of optical elements of said optical system to be inspected, and said second surface along an optical axis of said optical system to be inspected, whereby defocusing the image to be formed on said second surface from the best focus condition by the predetermined defocus amount.

18. An inspection method according to claim 10, wherein said method comprises the steps of:
calculating, for each of a plurality of patterns on said first surface having forms different from each other, a change in image position on said second surface; and
specifying the aberration condition and optical adjustment condition of said optical system to be inspected, based on a difference in the change in image position on said second surface between the respective patterns.

19. An inspection apparatus for specifying at least one of an aberration condition and an optical adjustment condition of an optical system to be inspected, said optical system including at least a system for converging a supplied luminous flux on a predetermined surface, said inspection apparatus comprising:
a detector for detecting, on a predetermined detection surface thereof, a converged luminous flux of said luminous flux guided through said optical system to be inspected;
a defocus mechanism for defocusing a point of convergence of said converged luminous flux by a predetermined defocus amount with respect to the detection surface of said detector; and
an inspection unit for calculating a change in position of the converged luminous flux on said detection surface of said detector within a predetermined defocus range, based on information concerning respective positions of a plurality of converged luminous fluxes on said detection surface of said detector under focus conditions different from each other, said plurality of converged luminous fluxes being guided onto said detection surface of said detector through said optical system to be inspected.

20. An inspection apparatus according to claim 19, wherein said inspection unit includes a calculator for calculating a measurement line which indicates a relationship between the amount of defocus and a position of said converged luminous flux corresponding thereto, based on information concerning the respective positions of the converged luminous fluxes on said second surface under at least three kinds of focus conditions different from each other.

21. An inspection method according to claim 19, wherein said inspection unit includes a calculator performing the steps of:
calculating, for each of said plurality of luminous fluxes on said detection surface, a change in the position of the converged luminous flux on said detection surface, said plurality of luminous fluxes being supplied to said optical system to be inspected and having cross-sectional formes different from each other; and
specifying the aberration condition and optical adjustment condition of said optical system to be inspected, based on a difference in the change in the position of the converged luminous flux on said detection surface between the respective luminous fluxes.

22. An inspection apparatus according to claim 19, wherein said defocus mechanism includes a control system for moving at least one of a light source for supplying said luminous flux to said optical system to be inspected, said optical system to be inspected, at least one of optical elements of said optical system to be inspected, and said detector along an optical axis of said optical system to be inspected.

23. An inspection apparatus according to claim 19, further comprising a sensitivity control mechanism for controlling inspection sensitivity of said inspection apparatus.

24. A projection exposure apparatus comprising:
an illumination optical system for illuminating a mask on which a predetermined pattern is formed with exposure light having a predetermined wavelength;
a projection optical system provided between said mask and a photosensitive substrate, for transferring an image of the pattern on said mask onto said substrate;
an inspection apparatus for specifying at least one of an aberration condition of said projection optical system and said respective optical adjustment conditions of said projection optical system and illumination optical system, said optical system including at least a system for converging a supplied luminous flux on redetermined surface and comprising:
a detector for detecting, on a predetermined detection surface thereof, a converged luminous flux of said luminous flux guided through said projection optical system to be inspected;
a defocus mechanism for defocusing a point of convergence of said converged luminous flux by a predetermined defocus amount with respect to the detection surface of said detector; and
an inspection unit for calculating a change in position of the converged luminous flux on said detection surface of said detector within a predetermined defocus range, based on information concerning respective positions of a plurality of converged luminous fluxes on said detection surface of said detector under focus conditions different from each other, said plurality of converged luminous fluxes being guided onto said detection surface of said detector through said projection optical system to be inspected;
an aberration correction mechanism for correcting the aberration of said projection optical system, based on the aberration condition of said projection optical system detected by said inspection apparatus; and
an optical adjustment mechanism for performing optical adjustments of said illumination optical system and said projection optical system, based on the respective optical adjustment conditions of said illumination optical system and said projection optical system specified by said inspection apparatus.

25. A projection exposure apparatus comprising:
an illumination optical system for illuminating a mask on which a predetermined pattern is formed with exposure light having a predetermined wavelength;
a projection optical system provided between said mask and a photosensitive substrate, for transferring an image of the pattern on said mask to a photosensitive substrate;
an alignment apparatus for positioning said substrate, said alignment apparatus comprising a mark illumination optical system for illuminating an alignment mark formed on said photosensitive substrate and a mark detection optical system for detecting light from said alignment mark on said substrate;
an inspection apparatus for specifying at least one of an aberration condition of said mark detection optical system and respective optical adjustment conditions of said mark illumination optical system and said mark detection optical system, said optical system including at least a system for converging a supplied luminous flux on a redetermined surface and comprising:
a detector for detecting, on a predetermined detection surface thereof, a converged luminous flux of said luminous flux guided through at least said mark detection optical system;
a defocus mechanism for defocusing a point of convergence of said converged luminous flux by a predetermined defocus amount with respect to the detection surface of said detector; and
an inspection unit for calculating a change in position of the converged luminous flux on said detection surface of said detector within a predetermined defocus range, based on information concerning respective positions of a plurality of converged luminous fluxes on said detection surface of said detector under focus conditions different from each other, said plurality of converged luminous fluxes being guided onto said detection surface of said detector through at least said mark detection optical system to be inspected;
an aberration correction mechanism for correcting the aberration of said mark detection optical system, based on the aberration condition of said mark detection optical system detected by said inspection apparatus; and
an optical adjustment mechanism for performing optical adjustment of said mark illumination optical system and said mark detection optical system, based on the respective optical adjustment conditions of said mark illumination optical system and said mark detection optical system specified by said inspection apparatus.

26. An overlay accuracy measurement apparatus for measuring an overlay accuracy of a mark formed on a photosensitive substrate to be exposed with exposure light having a predetermined wavelength, said measurement apparatus comprising:
a mark illumination optical system for illuminating the mark on said substrate;
a mark detection optical system for detecting light from the mark on said substrate;
an inspection apparatus for specifying at least one of an aberration condition of said mark detection optical system and respective optical adjustment conditions of said mark illumination optical system and said mark detection optical system, said optical system including at least a system for converging a supplied luminous flux on a predetermined surface and comprising:
a detector for detecting, on a predetermined detection surface thereof, a converged luminous flux of said luminous flux guided through at least said mark detection optical system;
a defocus mechanism for defocusing a point of convergence of said converged luminous flux by a predetermined defocus amount with respect to the detection surface of said detector; and
an inspection unit for calculating a change in position of the converged luminous flux on said detection surface of said detector within a predetermined defocus range, based on information concerning respective positions of a plurality of converged luminous fluxes on said detection surface of said detector under focus conditions different from each other, said plurality of converged luminous fluxes being guided onto said detection surface of said detector through at least said mark detection optical system to be inspected;
an aberration correction mechanism for correcting the aberration of said mark detection optical system, based on the aberration condition of said mark detection optical system detected by said inspection apparatus; and
an optical adjustment mechanism for performing optical adjustments of said mark illumination optical system and said mark detection optical system, based on the respective optical adjustment conditions of said mark illumination optical system and said mark detection optical system specified by said inspection apparatus.

27. A projection exposure apparatus comprising:
an illumination optical system for illuminating a mask on which a predetermined pattern is formed with exposure light having a predetermined wavelength;
a projection optical system for transferring an image of the pattern on said mask to a photosensitive substrate; and
an overlay accuracy measurement apparatus for measuring an overlay accuracy of a mark formed on a photosensitive substrate to be exposed with exposure light having a predetermined wavelength, said measurement apparatus comprising:
a mark illumination optical system for illuminating the mark on said substrate;
a mark detection optical system for detecting light from the mark on said substrate;
an inspection apparatus for specifying at least one of an aberration condition of said mark detection optical system and respective optical adjustment conditions of said mark illumination optical system and said mark detection optical system, said optical system including at least a system for converging a supplied luminous flux on a predetermined surface and comprising:
a detector for detecting, on a predetermined detection surface thereof, a converged luminous flux of said luminous flux guided through at least said mark detection optical system;
a defocus mechanism for defocusing a point of convergence of said converged luminous flux by a predetermined defocus amount with respect to the detection surface of said detector; and
an inspection unit for calculating a change in position of the converged luminous flux on said detection surface of said detector within a predetermined defocus range, based on information concerning respective positions of a plurality o converged luminous fluxes on said detection surface of said detector under focus conditions different from each other, said plurality of converged luminous fluxes being guided onto said detection surface of said detector through at least said mark detection optical system to be inspected;
an aberration correction mechanism for correcting the aberration of said mark detection optical system, based on the aberration condition of said mark detection optical system detected by said inspection apparatus; and
an optical adjustment mechanism for performing optical adjustments of said mark illumination optical system and said mark detection optical system, based on the respective optical adjustment conditions of said mark illumination optical system detected by said inspection apparatus.

28. An inspection method for specifying at least one of an aberration condition and an optical adjustment condition of an optical system to be inspected, said optical system including at least a system for converging a supplied luminous flux on a predetermined surface, said method comprising the steps of:
defocusing, with respect to a predetermined detection surface for detecting a converged luminous flux of said luminous flux guided through said optical system to be inspected, a point of convergence of said converged luminous flux by a predetermined defocus amount;
detecting, on said detection surface, a converged luminous flux under a predetermined focus condition; and
calculating a change in the position of converged luminous flux on said detection surface within a predetermined defocus range, based on information concerning respective positions of a plurality of said converged luminous fluxes on said detection surface under focus conditions different from each other, said plurality of converged luminous fluxes being guided through said optical system to be inspected.

29. An inspection method according to claim 28, wherein said method comprises the step of calculating a measurement line which indicates a relationship between the amount of defocus and a position of the converged luminous flux corresponding thereto, based on information concerning the respective positions of said converged luminous fluxes on said detection surface under at least three kinds of focus conditions different from each other.

30. An inspection method according to claim 29, wherein said method comprises the steps of:
specifying a peak point on said measurement line within the defocus range where curvature of said measurement line is maximized and then
calculating a magnitude of adjustment of the optical adjustment condition in said optical system to be inspected, based on gradient of a tangential line passing through the peak point on said measurement line.

31. An inspection method according to claim 29, wherein said method comprises the steps of:
specifying a peak point on said measurement line within the defocus range where curvature of said measurement line is maximized;
calculating a maximum clearance between said measurement line and a tangential line passing through the peak point on said measurement line; and
calculating an amount of asymmetric aberration of said optical system to be inspected, based on the obtained maximum clearance.

32. An inspection method according to claim 29, wherein said method comprises the steps of:
calculating an approximation line approximating said measurement line; and
calculating the magnitude of adjustment of the optical adjustment condition in said optical system to be inspected, based on gradient of said approximation line of said measurement line.

33. An inspection method according to claim 29, wherein said method comprises the steps of:
specifying a peak point on said measurement line within the defocus range where curvature of said measurement line is maximized;
calculating an approximation line approximating said measurement line; and
calculating an amount of asymmetric aberration of said optical system to be inspected, based on a distance between said approximation line and the peak point on said measurement line.

34. An inspection method according to claim 29, wherein said method comprises the steps of:
specifying a peak point on said measurement line within the defocus range where curvature of said measurement line is maximized; and
calculating an amount of symmetric aberration of said optical system to be inspected, based on a defocus amount at the peak point on said measurement line.

35. An inspection method according to claim 28, wherein said method comprises the step of moving at least one of a light source for supplying the luminous flux to said optical system to be inspected, said optical system to be inspected, at least one of optical elements of said optical system to be inspected, and said detection surface along an optical axis of said optical system to be inspected, whereby defocusing the point of convergence of said converged luminous flux with respect to said detection surface from the best focus condition by the predetermined defocus amount.

36. An inspection method according to claim 28, wherein said method comprises the steps of:
supplying a plurality of luminous fluxes having cross-sectional forms different from each other to said optical system to be inspected;
calculating, for each of said plurality of luminous fluxes on said detection surface, a change in the position of the converged luminous flux on said detection surface; and
specifying the aberration condition and optical adjustment condition of said optical system to be inspected, based on a difference in the change in the position of the converged luminous flux on said detection surface between the respective luminous fluxes.

37. An inspection apparatus for specifying at least one of an aberration condition and an optical adjustment condition of an optical system to be inspected, said inspection apparatus comprising:
a detector for detecting, on a predetermined detection surface thereof, an image of a phase pattern formed through said optical system to be inspected;
a defocus mechanism for defocusing the image to be formed on said detection surface of said detector, from a best focus condition of the image on said detection surface of said detector by a predetermined defocus amount; and
an inspection unit for calculating a change in image asymmetry on said detection surface within a predetermined defocus range, based on information concerning respective asymmetry characteristics of a plurality of images detected by said detector under focus conditions different from each other, said plurality of images being formed on said detection surface of said detector through said optical system to be inspected.

38. An inspection apparatus according to claim 37, wherein said inspection unit includes a signal processor for detecting, for each of measurement points arranged in a predetermined measurement direction on said detection surface, a signal .SIGMA.V which is an integrated signal of individual signals V corresponding to a light intensity of the image of said phase pattern which are measured at respective measurement points arranged in a direction which is different from said measurement direction and a calculator for calculating, based on a relationship between said measurement direction and said integrated signal, asymmetry index .beta. of the image formed on said detection surface according to the following expression: ##EQU4## wherein: n: number of periodicity of said integrated signal with respect to said measurement direction;
V.sub.iL : first obtained minimum signal value with respect to said measurement direction, which is a minimum signal at i-th period of said integrated signal;
V.sub.iR : last obtained minimum signal value with respect to said measurement direction, which is a minimum signal at i-th period of said integrated signal;
V.sub.max : maximum value of said integrated signal, with respect to said measurement direction, in the whole measurement area; and
V.sub.min : minimum value of said integrated signal, with respect to the measurement direction, in the whole measurement area.

39. An inspection apparatus according to claim 37, wherein said inspection unit includes a calculation means for calculating, based on information concerning the respective asymmetry characteristics of the images on said detection surface under at least two kinds of focus conditions different from each other, a measurement line which indicates a relationship between the amount of defocus and the asymmetry index of the image corresponding thereto.

40. An inspection method according to claim 37, wherein said inspection unit includes a calculator performing the steps of:
calculating, for each of a plurality of phase patterns having forms different from each other, a change in image asymmetry on said detection surface on the predetermined defocus range; and
specifying the aberration condition and optical adjustment condition of said optical system to be inspected, based on a difference in change in image asymmetry on said detection surface between said phase patterns.

41. An inspection apparatus according to claim 37, wherein said defocus mechanism includes a control system for moving at least one of said phase pattern, said optical system to be inspected, at least one of optical elements of said optical system to be inspected, and said detector along an optical axis of said optical system to be inspected.

42. An inspection apparatus according to claim 37, further comprising a sensitivity control mechanism for controlling inspection sensitivity of said inspection apparatus.

43. A projection exposure apparatus comprising:
an illumination optical system for illuminating a mask on which a predetermined pattern is formed with exposure light having a predetermined wavelength;
a projection optical system for transferring an image of the pattern on said mask to a photosensitive substrate;
an inspection apparatus for specifying at least one of an aberration condition of said projection optical system and respective optical adjustment conditions of said projection optical system and said illumination optical system, said inspection apparatus comprising:
a detector for detecting, on a predetermined detection surface thereof, an image of a phase pattern formed through at least projection optical system,
a defocus mechanism for defocusing the image to be formed on said detection surface of said detector, from a best focus condition of the image on said detection surface of said detector by a redetermined defocus amount; and
an inspection unit for calculating a chance in image asymmetry on said detection surface within a predetermined defocus range, based on information concerning respective asymmetry characteristics of a plurality of images detected by said detector under focus conditions different from each other, said plurality of images being formed on said detection surface of said detector through at least said projection optical system,
an aberration correction mechanism for correcting the aberration of said projection optical system, based on the aberration condition of said projection optical system detected by said inspection apparatus; and
an optical adjustment mechanism for performing optical adjustments of said illumination optical system and said projection optical system, based on the respective optical adjustment conditions of said illumination optical system and said projection optical system specified by said inspection apparatus.

44. A projection exposure apparatus comprising:
an illumination optical system for illuminating a mask on which a predetermined pattern is formed with exposure light having a predetermined wavelength;
a projection optical system for transferring an image of the pattern on said mask to a photosensitive substrate;
an alignment apparatus for positioning said photosensitive substrate, said alignment apparatus comprising a mark illumination optical system for illuminating an alignment mark formed on said photosensitive substrate and a mark detection optical system for detecting light from said alignment mark;
an inspection apparatus for specifying at least one of an aberration condition of said mark detection optical system and respective optical adjustment conditions of said mark illumination optical system and mark detection optical system, said inspection apparatus comprising:
a detector for detecting, on a predetermined detection surface thereof, an image of a phase pattern formed through at least said mark detection optical system;
a defocus mechanism for defocusing the image to be formed on said detection surface of said detector, from a best focus condition of the image on said detection surface of said detector by a predetermined defocus amount; and
an inspection unit for calculating a change in image asymmetry on said detection surface within a predetermined defocus range, based on information concerning respective asymmetry characteristics of a plurality of images detected by said detector under focus conditions different from each other, said plurality of images being formed on said detection surface of said detector through at least said mark detection optical system;
an aberration correction mechanism for correcting the aberration of said mark detection optical system, based on the aberration condition of said mark detection optical system detected by said inspection apparatus; and
an optical adjustment mechanism for performing optical adjustments of said mark illumination optical system arid said mark detection optical system, based on the respective optical adjustment conditions of said mark illumination optical system and said mark detection optical system specified by said inspection apparatus.

45. An overlay accuracy measurement apparatus for measuring an overlay accuracy of a mark formed on a photosensitive substrate to be exposed with exposure light having a predetermined wavelength, said measurement apparatus comprising:
a mark illumination optical system for illuminating the mark on said substrate;
a mark detection optical system for detecting light from the mark on said substrate;
an inspection apparatus for specifying at least one of an aberration condition of said mark detection optical system and respective optical adjustment conditions of said mark illumination optical system and said mark detection optical system, said inspection apparatus comprising:
a detector for detecting, on a predetermined detection surface thereof, an image of a phase pattern formed through at least said mark detection optical system;
a defocus mechanism for defocusing the image to be formed on said detection surface of said detector, from a best focus condition of the image on said detection surface of said detector by a predetermined defocus amount; and
an inspection unit for calculating a chance in image asymmetry on said detection surface within a predetermined defocus range, based on information concerning respective asymmetry characteristics of a plurality of images detected by said detector under focus conditions different from each other, said plurality of images being formed on said detection surface of said detector through at least said mark detection optical system;
an aberration correction mechanism for correcting the aberration of said mark detection optical system, based on the aberration condition of said mark detection optical system detected by said inspection apparatus; and
an optical adjustment mechanism for performing optical adjustments of said mark illumination optical system and said mark detection optical system, based on the respective optical adjustment conditions of said mark illumination optical system and said mark detection optical system specified by said inspection apparatus.

46. A projection exposure apparatus comprising:
an illumination optical system for illuminating a mask on which a predetermined pattern is formed with exposure light having a predetermined wavelength;
a projection optical system provided between said mask and a photosensitive substrate, for transferring an image of the pattern on said mask onto said substrate; and
an overlay accuracy measurement apparatus for measuring an overlay accuracy of a mark formed on said substrate to be exposed with exposure light having a predetermined wavelength, said measurement apparatus comprising:
a mark illumination optical system for illuminating the mark on said substrate;
a mark detection optical system for detecting light from the mark on said substrate;
an inspection apparatus for specifying at least one of an aberration condition of said mark detection optical system and respective optical adjustment conditions of said mark illumination optical system and said mark detection system, said inspection apparatus comprising:
a detector for detecting, on a predetermined detection surface thereof, an image of a phase pattern formed through at least said mark detection optical system;
a defocus mechanism for defocusing the image to be formed on said detection surface of said detector, from a best focus condition of the image on said detection surface of said detector by a predetermined defocus amount; and
an inspection unit for calculating a change in image asymmetry on said detection surface within a predetermined defocus range, based on information concerning respective asymmetry characteristics of a plurality of images detected by said detector under focus conditions different from each other, said plurality of images being formed on said detection surface of said detector through at least said mark detection optical system;
an aberration correction mechanism for correcting the aberration of said mark detection optical system, based on the aberration condition of said mark detection optical system detected by said inspection apparatus; and
an optical adjustment mechanism for performing optical adjustments of said mark illumination optical system and said mark detection optical system, based on the respective optical adjustment conditions of said mark illumination optical system and said mark detection optical system specified by said inspection apparatus.

47. An inspection method for specifying at least one of an aberration condition and an optical adjustment condition of an optical system to be inspected, said method comprising the steps of:
defocusing an image of a phase pattern on a predetermined surface, which is to be formed on a predetermined detection surface through said optical system to be inspected, from a best focus condition of the image on said detection surface by a predetermined defocus amount;
detecting the image formed on said detection surface under a predetermined focus condition; and
calculating a change in image asymmetry on said detection surface within a predetermined defocus range, based on information concerning respective positions of a plurality of images detected on said detection surface under focus conditions different from each other, said plurality of images being formed on said detection surface through said optical system to be inspected.

48. An inspection method according to claim 47, wherein said method comprises the steps of:
detecting, for each of measurement points arranged in a predetermined measurement direction on said detection surface, a signal .SIGMA.V which is an integrated signal of individual signals V corresponding to a light intensity of the image of said phase pattern which are measured at respective measurement points arranged in a direction which is different from said measurement direction; and
calculating, based on a relationship between said measurement direction and said integrated signal, asymmetry index .beta. of the image formed on said detection surface according to the following expression: ##EQU5## wherein: n: number of periodicity of said integrated signal with respect to said measurement direction;
V.sub.iL : first obtained minimum signal value with respect to said measurement direction, which is a minimum signal at i-th period of said integrated signal;
V.sub.iR : last obtained minimum signal value with respect to said measurement direction, which is a minimum signal at i-th period of said integrated signal;
V.sub.max : maximum value of said integrated signal, with respect to said measurement direction, in the whole measurement area; and
V.sub.min : minimum value of said integrated signal, with respect to the measurement direction, in the whole measurement area.

49. An inspection method according to claim 47, wherein said inspection method comprises the steps of calculating a measurement line which indicates a relationship between the amount of defocus and the asymmetry index of the image corresponding thereto, based on information concerning the respective asymmetry characteristics of the images on said detection surface under at least three kinds of focus conditions different from each other.

50. An inspection method according to claim 49, wherein said method comprises the steps of:
specifying a peak point in a protruded portion of said measurement line within the defocus range; and
calculating an amount of asymmetric aberration of said optical system to be inspected, based on gradient of a tangential line passing through the peak point on said measurement line.

51. An inspection method according to claim 49, wherein said method comprises the steps of:
specifying a peak point in a protruded portion of said measurement line within the defocus range; and
calculating an amount of symmetric aberration of said optical system to be inspected, based on a difference between an asymmetry index at the peak point on said measurement line and an asymmetry index under the best focus condition.

52. An inspection method according to claim 49, wherein said method comprises the steps of:
calculating an amount of asymmetric aberration of said optical system to be inspected, based on gradient of said measurement line.

53. An inspection method according to claim 49, wherein said method comprises the steps of:
calculating an amount of symmetric aberration of said optical system to be inspected, based on an asymmetry index of said measurement line under the best focus condition.

54. An inspection method according to claim 49, wherein said method comprises the steps of:
specifying a peak point in a protruded portion of said measurement line within the defocus range; and
calculating a magnitude of the optical adjustment condition of said optical system to be inspected, based on an angle of a tangential line passing through the peak point on said measurement line with respect to said measurement line.

55. An inspection method according to claim 47, wherein said method comprises the step of moving at least one of said phase pattern, said optical system to be inspected, at least one of optical elements of said optical system to be inspected, and said detection surface along an optical axis of said optical system to be inspected, whereby defocusing the image to be inspected on said detection surface from the best focus condition by the predetermined defocus amount.

56. An inspection method according to claim 47, wherein said method comprises the steps of:
calculating, for each of a plurality of phase patterns having forms different from each other, a change in image asymmetry on said detection surface on the predetermined defocus range; and
specifying the aberration condition and optical adjustment condition of said optical system to be inspected, based on a difference in change in image asymmetry on said detection surface between said phase patterns.
Description
BACKGROUND OF THE INVENTION

1. Field of the Invention The present invention relates to inspection apparatus and method for optical systems as well as a projection exposure apparatus provided with the inspecting apparatus, and the like. In particular, it relates to inspection and adjustment of aberration conditions and optical adjustment conditions of projection optical systems applicable to projection exposure apparatuses used in lithography process for making semiconductor devices and liquid crystal display devices, for example, as well as optical systems (i.e., optical systems to be inspected) such as those of alignment apparatuses and overlay accuracy measurement apparatuses. Also, it relates to image-forming optical systems (including the above-mentioned optical systems to be inspected) which are suitable to alignment apparatuses by which a photosensitive substrate is positioned on the basis of an alignment mark (wafer mark) on the photosensitive substrate and the like in the projection exposure apparatuses, such as an optical system having a function to correct asymmetric aberration of the whole system, and alignment apparatuses provided therewith.

2. Related Background Art

Conventionally, in the projection exposure apparatus used in the lithography process for making semiconductor devices, liquid crystal display devices, and the like, a pattern formed in a mask is transferred to a wafer, which is a photosensitive substrate (e.g., silicon wafer or glass plate coated with a photosensitive material such as photoresist), by way of a projection optical system. Namely, with respect to a pattern which has already been transferred to the wafer, a projected image of the mask pattern formed by way of the projection optical system is positioned by an alignment apparatus (or alignment optical system disposed inside the projection exposure apparatus) so as to effect superposed exposure. Further, an overlay accuracy measurement apparatus disposed inside or outside the projection exposure apparatus judges whether the above-mentioned positioning is correctly effected or not.

When the optical adjustment of the projection optical system is insufficient or an aberration remains in the projection optical system, the projected image of the mask pattern cannot correctly formed on the surface of the photosensitive substrate, thereby forming a warped transfer pattern on the wafer. Also, when the optical adjustment of the alignment apparatus is insufficient or an aberration remains in the alignment apparatus, the mask and the wafer cannot be correctly positioned with respect to each other, thereby making it impossible to perform superposed exposure with a high accuracy. Further, in the overlay accuracy measurement apparatus, when the optical adjustment is insufficient or there remains an aberration, overlay accuracy measurement cannot be performed with a high accuracy.

Accordingly, there has been conventionally used a method for inspecting aberration of a projection optical system comprising the steps of transferring a plurality of light-shielding patterns formed at a light-permeable portion of a mask to a wafer by way of the projection optical system and then observing the amount of asymmetry of the resist image formed on the wafer through an electron microscope.

Also, Japanese Unexamined Patent Publication Hei No. 5-118957 discloses a method of inspecting asymmetrical aberration of a projection optical system (i.e., optical system to be inspected) comprising the step of detecting a spatial image of the mask light-shielding pattern formed by way of the projection optical system.

Further, with respect to the optical adjustment and the like of the optical system to be inspected, Japanese Unexamined Patent Publication Hei Nos. 6-69097 and 6-132197, for example, disclose a method in which deviation or inclination of an optical axis is corrected.

In addition, as the above-mentioned alignment apparatus, Japanese Unexamined Patent Publication Hei Nos. 4-65603 and 4-273246, for example, disclose an alignment apparatus which is of both off-axis type and image-pickup type. In this conventional alignment apparatus, light having a wide wavelength band emitted from a light source such as a halogen lamp illuminates an alignment mark (i.e., wafer mark). Then, an enlarged image of the wafer mark is formed on a image detection surface of an image pickup device. Thus obtained image pickup signal (i.e., electric signal) is subjected to an image processing so as to detect the position of the wafer mark on the wafer. The detection system of the image-pickup type alignment apparatus is also known as FIA (Field Image Alignment) system.

In the image-pickup type alignment apparatus, due to the wide band illumination, the influence of the thin-film interference on the photoresist layer on the wafer is decreased. Also, when the wafer mark to be inspected is an asymmetrical mark, a specific edge may be selected within the obtained enlarged image of the wafer mark, for example, so as to reduce the influence of the asymmetry.

Further, as the conventional alignment apparatus, there is an alignment apparatus which is of both TTL (Through The Lens) type and image-pickup type. In the TTL type alignment apparatus, the wavelength of exposure light and that of alignment detection light differ from each other, thereby generating an aberration with respect to the alignment detection light in the projection optical system used for exposure. Accordingly, in the TTL type alignment apparatus disclosed in Japanese Patent Publication Hei No. 2-35446, for example, asymmetric aberration generated in the projection optical system with respect to the alignment detection light is corrected by a sheet of a plane parallel plate disposed obliquely with respect to an optical axis, while an astigmatism generated by this plane parallel plate is corrected by additional two sheets of plane parallel plates.

SUMMARY OF THE INVENTION

The inventors have studied the foregoing prior art and found the following problems.

In the method in which a resist image is observed through an electron microscope, the resist image has to be actually formed on the wafer. Accordingly, prior to inspection, a complicated process takes a long time. Also, while a scanning type electron microscope (SEM) is usually used for inspecting the resist image, the resolution of SEM may vary depending on personal equation of operators and condition of the apparatus, thereby yielding a low reproducibility in inspection.

Also, in the method using the spatial image disclosed in Japanese Unexamined Patent Publication Hei No. 5-118957, sufficient effects cannot be obtained unless illumination a (ratio of illumination numerical aperture to image-forming numerical aperture) is made low. However, when the illumination a changes, the contribution of a luminous flux to wave aberration of the optical system changes. Accordingly, the aberration determined on the basis of the spatial image obtained under the condition where the illumination .sigma. is narrowed down may differ from the aberration under the practical condition.

Further, in the optical adjustment methods disclosed in Japanese Unexamined Patent Publication Hei Nos. 6-69097 and 6-132197, for example, only one of the condition where the optical axis and the principal ray are deviated from each other (referred to as "telecentricity" in the following) and the condition where a luminous flux is eclipsed (referred to as "eclipse" in the following) is adjusted. Accordingly, both of the telecentricity and the eclipse cannot be adjusted at the same time.

The object of the present invention is to provide inspection apparatus and method by which the aberration condition and optical adjustment condition of the optical system to be inspected can be inspected with a favorable reproducibility under a practical condition or a condition similar thereto.

Another object of the present invention is to provide a projection exposure apparatus, an alignment apparatus, and an overlay accuracy measurement apparatus which are provided with means for effecting correction of aberration or optical adjustment based on the aberration condition and optical adjustment condition of the optical system inspected by the above-mentioned inspection apparatus and method.

Still another object of the present invention is to provide an optical system which is suitably used in the above-mentioned alignment apparatus.

The first embodiment of the inspection apparatus in accordance with the present invention comprises, as shown in FIG. 13, a defocus mechanism for defocusing an image 15A of a pattern 15 on a first surface (i.e., surface of a reticle 13), which is to be formed on a second surface (i.e., surface PTa of a reference member PT), from a best focus condition by a predetermined defocus amount; a detector 19 for detecting the image 15A which is formed on the second surface PTa by the defocus mechanism under a predetermined focus condition; and an inspection unit 151 for calculating, based on the information concerning respective positions of a plurality of images which are formed on the second surface PTa under focus conditions different from each other and which are detected by the detector 19, a change in image position on the second surface within a predetermined defocus range. In particular, the inspection unit 151 comprises a signal processor 152 for capturing, from the detector 19, the information (i.e., electric signals) concerning the respective positions of the image 15A on the second surface PTa under at least three kinds of focus conditions different from each other and a calculator 153 for calculating, with reference to information in a table 154 prepared beforehand, a measurement line which indicates a relationship between the amount of defocus and the corresponding image position. Also, the above-mentioned defocus mechanism includes a control system for moving at least one of the pattern on the first surface (i.e., surface of the reticle 13), optical system to be inspected (e.g., projection optical system 16), and detector 19 along the optical axis of the optical system to be inspected. Accordingly, in the configuration shown in FIG. 13, this control system includes a controller 150 within a main control system 100 and at least one of a wafer stage control system 200, a projection optical system adjustment mechanism 300, and a reticle stage control system
400. Here, a configuration in which this defocus mechanism adjusts an illumination optical system, as shown in FIG. 19, is also preferable.

Also, the sensitivity in inspection of this inspection apparatus can be controlled when a sensitivity control mechanism which can change at least one of the illumination condition for the pattern, the form of the pattern, and the range of defocus. For example, in the configuration shown in FIG. 13, in order to adjust the illumination condition for the pattern, the sensitivity control mechanism includes a variable aperture stop 11 and an adjustor 20 therefor. Also, the above-mentioned defocus mechanism functions as the sensitivity control mechanism.

On the other hand, the light source for forming the image on the inspection surface as mentioned above may be a lamp light source or a laser source. A laser source is applicable to the second embodiment of the inspection apparatus in accordance with the present invention. Accordingly, in this embodiment, as shown in FIG. 18, with respect to a predetermined detection surface 25A, the defocus mechanism defocuses a point of convergence of a luminous flux, which is emitted from such a light source
22, formed by way of the optical system to be inspected, thereby enabling a detector 25 to detect the positions of the converged luminous flux under a plurality of focus conditions.

Further, in the third embodiment of the inspection apparatus in accordance with the present invention, a phase pattern is utilized for inspecting the aberration condition and optical adjustment condition of the optical system to be inspected. As shown in FIG. 19, the inspection apparatus in the third embodiment comprises a detector 19 for detecting an image of a phase pattern (i.e., wafer mark WM) on a predetermined surface (i.e., surface of a photosensitive substrate W); a defocus mechanism for defocusing the image to be formed on a detection surface of the detector 19 from a best focus condition by a predetermined defocus amount; and an inspection unit 151 for calculating, based on information concerning asymmetry of each of a plurality of images formed on the detection surface under a plurality of focus conditions different from each other, a change in asymmetry of the image on the detection surface within a predetermined defocus range. Here, the configuration of the defocus mechanism is similar to that of the first embodiment.

In particular, the inspection unit 151 in the third embodiment comprises a signal processor 152 for detecting, for each of measurement points arranged in a predetermined measurement direction S on the detection surface, a signal .SIGMA.V which is an integrated signal of individual signals V corresponding to the light intensity of the image of the phase pattern WM which are measured at respective measurement points arranged in a direction Sa, which is perpendicular to the measurement direction S, and a calculator 153 for calculating, based on a relationship between the measurement direction S and the integrated signal, asymmetry index .beta. of the image formed on the detection surface according to the following expression (cf. FIG. 8): ##EQU1## wherein: n: number of periodicity of the integrated signal with respect to the measurement direction S;

V.sub.iL : first obtained minimum signal value with respect to the measurement direction S, which is a minimum signal at the i-th period B.multidot.P of the integrated signal;

V.sub.iR : last obtained minimum signal value with respect to the measurement direction S, which is a minimum signal at the i-th period B.multidot.P of the integrated signal;

V.sub.max : maximum value of the integrated signal, with respect to the measurement direction S, in the whole measurement area excluding the both end portions of the image of the phase pattern WM; and

V.sub.min : minimum value of the integrated signal, with respect to the measurement direction S, in the whole measurement area excluding the both end portions of the image of the phase pattern WM.

Further, the calculator 153 calculates a

measurement line which indicates relationship between the amount of defocus and the asymmetry index of the corresponding image and, with reference to a table 154 prepared beforehand, the aberration condition and optical adjustment condition of the optical system to be inspected.

In addition, the first to third embodiments of the inspection apparatus in accordance with the present invention are applicable to an exposure apparatus, an alignment apparatus, and a superposing apparatus as well as an exposure apparatus provided with these alignment and overlay accuracy measurement apparatuses. As such an exposure apparatus, a one-shot exposure type exposure apparatus and a scanning type exposure apparatus have been known. The inspection apparatus in accordance with the present invention can be applied to any of these types of exposure apparatuses. Accordingly, this inspection apparatus can inspect the aberration conditions and optical adjustment conditions of the projection optical system, alignment optical system, and the like in each of these apparatuses.

Then, the inventors have studied the alignment apparatus to which the above-mentioned inspection apparatus is applicable and found the following problems.

In the image-forming optical system in the alignment apparatus of the conventional image pickup type, there remains a little aberration in manufacturing steps such as processing, assembling, and adjustment. When an aberration remains within the optical system, contrast of the wafer mark image on the image pickup surface may decrease or the wafer mark image may be warped, thereby generating an error in detection of the mark position. Recently, as the line width of circuit patterns becomes finer, alignment with a higher accuracy has become necessary. Accordingly, it has become difficult for the decrease in alignment accuracy caused by the little remaining aberration to be neglected.

In particular, among the aberrations remaining within the optical system, asymmetric aberration greatly influences the detection of the wafer mark image. Namely, when asymmetric aberration is generated within the optical system, the wafer mark image projected on the image-pickup surface is measured as being positionally deviated from the ideally formed image. Also, when there exists asymmetric aberration in the wafer mark image in cases where the form of the wafer mark, for example, its pitch, duty cycle, or difference in level, is changed or where the wafer is projected on the image-pickup surface under a defocused condition, the degree of the asymmetry of the wafer mark image may vary and the amount of deviation of the measurement position for the wafer mark may vary.

In the specification, the symmetric aberration means as an aberration that is symmetrically generated with respect to a principle ray or an optical axis of an optical system, and the asymmetric aberration means as an aberration that is asymmetrically generated with respect to a principle ray or an optical axis of an optical system. For example, at least spherical aberration is included in the above symmetric aberration, and at least coma is included in the above asymmetric aberration.

The wafer mark has different forms in respective steps for making a semiconductor. Accordingly, when the alignment (or positioning) of the wafer is effected by way of an optical system in which asymmetric aberration remains, accuracy in alignment may decrease due to process offset or deterioration of the reproducibility in superposing accuracy. Also, though the amount of asymmetric aberration tolerable in the optical system may somewhat vary depending on the optical characteristics (e.g., numeric aperture on the object side and magnification) of the image-forming optical system and the type of image processing, it is necessary for the amount of the remaining asymmetric aberration to be nearly nullified in order to realize highly accurate alignment.

In this regard, in the conventional alignment apparatus, the accuracy in manufacture of the optical system is made as high as possible so as to prevent various aberrations such as asymmetric aberration from generating. However, when the accuracy in manufacture of the optical system is improved alone, it is difficult for asymmetric aberration to be sufficiently eliminated so as to fulfill the demand for the above-mentioned accuracy in alignment. When the accuracy in manufacture of the optical system is to be increased so as to fulfill such a demand, the manufacturing cost may increase.

Also, as mentioned above, in the TTL type alignment apparatus disclosed in Japanese Patent Publication Hei No. 2-35446, a plane parallel plate is obliquely disposed in order to reduce asymmetric aberration. However, due to thus obliquely disposed plane parallel plate, aberrations such as astigmatism and dispersion may occur. Accordingly, in order to correct the aberration generated by the plane parallel plate, additional optical members may be necessary. Namely, when a plane parallel plate is asymmetrically (obliquely) disposed with respect to an optical axis to correct asymmetric aberration, various aberrations such as image-surface inclination, astigmatism, and dispersion may occur. As a result, the image quality of the mark image may deteriorate, thereby rather decreasing the accuracy in detection of the mark position. Also, since additional members for correcting these aberrations may be necessary, the optical system may have a larger size with a higher manufacturing cost.

With a simple configuration, the image-forming optical system in accordance with the present invention easily corrects asymmetric aberration in the whole system without influencing other aberrations. Also, the alignment apparatus to which this image-forming optical system is applied detects the position of the alignment mark with a high accuracy.

Specifically, the first embodiment of the image-forming optical system for the alignment optical system or the like in accordance with the present invention comprises, for example as shown in FIG. 24, an objective optical system 502 for converging light from a first surface (e.g., a photosensitive substrate W in which an alignment wafer mark WM is formed on a surface thereof); a condenser optical system 500B (504) which converges the light passing through the objective optical system
500A (502) so as to form, on a second surface (i.e., a detection surface 505a of an image pickup device 505), an image of the first surface and in which a predetermined amount of asymmetric aberration is generated; a correction optical system 550 which is disposed between the objective optical system 500A and the second surface and is movable in a vertical direction with respect to an optical axis of the condenser optical system 500B, the correction optical system 550 generating asymmetric aberration which offsets that generated by the condenser optical system 500B; and a decentering mechanism for making the correction optical system 550 a decentration condition with respect to the optical axis of the condenser optical system 500B. According to this configuration, in the first embodiment of the image-forming optical system, the correction optical system 550 decentered by the decentering mechanism so as to reduce asymmetric aberration in the whole image-forming optical system in accordance with the present invention, generates an eccentric asymmetrical aberration which offsets the asymmetric aberration generated by the condenser optical system 500B. In particular, the above-mentioned correction optical system 550 is substantially a same-magnification erect afocal optical system, while the space between this afocal optical system and the condenser optical system 500B is telecentric. Also, the focal length of the condenser optical system 500B is set longer than that of the objective optical system 500A, so that the image-forming optical system, as a whole, has an enlarging magnification.

In the specification, the eccentric asymmetrical aberration means as asymmetric aberration caused in the optical system by decentering the correction optical system with respect to an optical axis of the condenser optical system. The eccentric asymmetrical aberration functions so as to cancel asymmetric aberration generated by the objective optical system, and thereby asymmetric aberration of the whole optical system is effectively reduced.

Further, as shown in FIG. 24, in the first configuration of the above-mentioned decentering mechanism, as instructed by a main control system 100A, a driving system 250 makes the whole correction optical system 550 (i.e., a holder 553 holding the whole correction optical system 550) eccentric with respect to the whole optical system according to the present invention. Also, as mentioned above, when aberration occurs due to error in manufacture of the optical system, there is preferably used a configuration (i.e., second configuration) in which the optical positions of the condenser optical system 500B and the correction optical system 550 with respect to each other are fixed as shown in FIG. 25. In this second configuration, since the optical positions of the condenser optical system 500B and the correction optical system 550 with respect to each other are fixed beforehand, the aberration of the whole optical system is reduced, while decreasing the possibility of the aberration being newly generated in the optical system when in use.

Also, as shown in FIG. 31, the second embodiment of the image-forming optical system in accordance with the present invention has a configuration in which, when a predetermined amount of symmetric aberration occurs in a condenser optical system
500B, a decentering mechanism makes a correction optical system 550 eccentric with respect to an optical axis of the condenser optical system 500B as mentioned above. The correction optical system 550 generates symmetric aberration which offsets that generated by the condenser optical system 500B. Accordingly, the correction optical system 550, which has been made eccentric by the decentering mechanism so as to reduce asymmetric aberration in the whole image-forming system, generates symmetric aberration which offsets that generated by the condenser optical system 500B.

Here, the ratio of the amount of eccentricity of the above-mentioned correction optical system 550 to asymmetric aberration is proportional to the amount of symmetric aberration which is offset by the correction optical system 550 and the condenser optical system 500B to each other (cf. FIG. 34). In other words, when the amount of symmetric aberration which is offset by the correction optical system 550 and the condenser optical system 500B to each other is set appropriately and the correction optical system 550, which is adapted to have an eccentricity, is made eccentric by an appropriate amount, a desired amount of asymmetric aberration can be generated. Therefore, the asymmetric aberration occurring in the whole optical system including the correction optical system 550 can be reduced also when the correction optical system 550 is made eccentric so as to generate symmetric aberration.

Here, it is preferable that the correction optical system 550 in this embodiment is substantially a same-magnification erect afocal system while the space between the condenser optical system 504 and the correction optical system 550 is non-telecentric. Also, the focal length of the condenser optical system 500B is set longer than that of an objective optical system 500A, so that the image-forming optical system, as a whole, has an enlarging magnification.

Further, the third embodiment of the optical system in accordance with the present invention is an embodiment in which the above-mentioned first and second embodiments of the optical system are specifically applied to an alignment optical system (e.g., illumination optical system and image-forming optical system) utilizing a laser source (cf. FIGS. 35 and 36).

The above-mentioned first and second embodiments of the optical system are mainly applied to an alignment apparatus. In this case, the objective optical system 500A (502) in this image-forming optical system converges light from the alignment wafer mark WM on the photosensitive substrate W, while the condenser optical system 500B (504) converges the light passing through the objective optical system 500A so as to form an image of the wafer mark WM on a predetermined detection surface 505a. Also, this alignment apparatus includes at least an image-pickup device 505 for detecting the image of the wafer mark WM formed on the detection surface 505a by way of the image-forming optical system and an image processing system 350 for calculating, based on a signal output from the image-pickup device 505, the position of the image on the detection surface 505a. The third embodiment is configured in a similar manner.

As shown in FIG. 44, the fourth embodiment of the image-forming optical system in accordance with the present invention comprises a first image-forming optical system 700A (702) for converging light from a first surface so as to form an image of the first surface on a second surface; a second image-forming optical system 700B (704, 705) which converges light from the image of the first surface formed on the second surface so as to form an image of the first surface on a third surface and in which a predetermined amount of asymmetric aberration is generated; a correction optical system 750 which is formed in an optical path of the second image-forming optical system 700B and is movable in a vertical direction with respect to an optical axis of the second image-forming optical system 700B, the correction optical system 750 generating asymmetric aberration which offsets that generated by the second image-forming optical system 700B; and a decentering mechanism for decentering the correction optical system 750 with respect to the optical axis of the second image-forming optical system 700B. According to this configuration, the correction optical system which has been made eccentric by the decentering mechanism so as to reduce asymmetric aberration in the whole image-forming optical system generates asymmetric aberration which offsets that generated in the first image-forming optical system 700B. The above-mentioned decentering mechanism may have a configuration (i.e., first configuration) in which eccentricity is automatically made as shown in FIG. 44 as well as a configuration (i.e., second configuration) in which eccentricity has been fixed before use as shown in FIG. 25.

Also, the above-mentioned first image-forming optical system 700A has an enlarging magnification. The correction optical system 750 is substantially a same-magnification erect afocal optical system. The correction optical system 750 is disposed within a substantially telecentric space in the optical path within the second image-forming optical system 700A.

Further, as shown in FIG. 47, the fifth embodiment of the image-forming optical system in accordance with the present invention has a configuration in which, as mentioned above, when a predetermined amount of symmetric aberration occurs in a second image-forming optical system 700B, a decentering mechanism makes a correction optical system 750 eccentric with respect to an optical axis of the second image-forming optical system 700B. The correction optical system 750 generates symmetric aberration which offsets that generated by the second image-forming optical system 700B. Accordingly, the correction optical system 750 which has been made eccentric by the decentering mechanism so as to reduce asymmetric aberration in the whole image-forming optical system generates symmetric aberration which offsets that generated in the first image-forming optical system 700B.

Here, it is preferable that the correction optical system 750 in this embodiment is substantially a same-magnification erect afocal optical system, while the space between the second image-forming optical system 700B and the correction optical system 750 is non-telecentric. Also, the first image-forming optical system 700A has an enlarging magnification.

The above-mentioned fourth and fifth embodiments of the image-forming optical system are mainly applied to an alignment apparatus. In this case, the first image-forming optical system 700A (702) in this image-forming optical system converges light from the alignment wafer mark WM on the photosensitive substrate W, while the second image-forming optical system 700B (704, 705) converges the light passing through the first image-forming optical system 700A so as to form an image of the wafer mark WM on a predetermined detection surface 706a. Also, this alignment apparatus includes at least an image-pickup device 706 for detecting the image of the wafer mark WM formed on the detection surface 706a by way of the image-forming optical system and an image processing system 350 for calculating, based on a signal output from the image-pickup device 706, the position of the image on the detection surface 706a.

Also, in the fourth and fifth embodiments of the image-forming optical system in accordance with the present invention, not only the lamp light source but a laser source can be utilized.

The sixth embodiment of the image-forming optical system in accordance with the present invention comprises a first condenser optical system for converging a laser luminous flux from a first surface and a second condenser optical system for re-converging, on a second surface, the laser luminous flux from the first condenser optical system. In this embodiment, the correction optical system for generating asymmetric aberration which offsets that generated in the second condenser optical system is substantially a same-magnification erect afocal system and disposed within a substantially telecentric space in an optical path within the second condenser optical system. The first condenser optical system within the image-forming optical system has a reducing magnification. Further, this embodiment is also applicable to an alignment apparatus utilizing a laser source (cf. FIG. 52).

The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a first optical adjustment condition (telecentricity) of an optical system to be inspected, where a principal ray is deviated from an optical axis of the whole system;

FIG. 2 is a view showing a second optical adjustment condition (eclipse) of an optical system to be inspected, where an eclipse remains in the whole system;

FIGS. 3 to 5 are charts showing, in the first and second embodiments of the method of inspecting an optical system in accordance with the present invention, for individual optical adjustment conditions of the optical system to be inspected and aberrations generated, relationships between the amount of defocus in the optical system and the positional change of the pattern image;

FIG. 6 is a flow chart explaining the first and second embodiments of the method of inspecting an optical system in accordance with the present invention;

FIG. 7 is a view explaining a method of determining a cross-sectional configuration of a phase pattern and asymmetry of an image of the phase pattern;

FIG. 8 is a chart explaining asymmetry index .beta. of the phase pattern image, where horizontal axis indicates the measurement direction while vertical axis indicates an integrated value of a signal V corresponding to the light intensity at each measurement position;

FIGS. 9 to 11 are charts showing, in the third embodiment of the method of inspecting an optical system in accordance with the present invention, for individual optical adjustment conditions of the optical system to be inspected and aberrations generated, relationships between the amount of defocus in the optical system and the asymmetry index .beta. of the phase pattern image;

FIG. 12 is a flow chart explaining the third embodiment of the method of inspecting an optical system in accordance with the present invention;

FIG. 13 is a perspective view showing a schematic configuration of a projection exposure apparatus provided with the first embodiment of the inspection apparatus in accordance with the present invention;

FIGS. 14 to 17 are charts showing, in the first embodiment of the present invention shown in FIG. 13, for individual defocus conditions, relationships between the positional change of an image 15A of an inspection pattern 15 formed by way of a projection optical system 16 and the aberrations generated in the projection optical system 16;

FIG. 18 is a view showing a schematic configuration of a projection exposure apparatus provided with the second embodiment of the inspection apparatus in accordance with the present invention;

FIG. 19 is a view showing a schematic configuration of a projection exposure apparatus provided with the third embodiment of the inspection apparatus in accordance with the present invention;

FIG. 20 is a view showing a cross-sectional configuration of the photosensitive substrate shown in FIG. 19;

FIGS. 21 to 23 are charts showing, in the third embodiment of the present invention shown in FIG. 19, for individual defocus conditions, relationships between the positional change of an image of a wafer mark WM (i.e., phase pattern) formed by way of an image-forming optical system and the aberrations generated in the image-forming optical system;

FIG. 24 is a view showing a schematic configuration of a projection exposure apparatus provided with the first embodiment of the alignment apparatus in accordance with the present invention, including the first embodiment of the decentering mechanism;

FIGS. 25 to 27 are views showing configurations of alignment optical systems (i.e., image-forming optical systems) applicable to the alignment apparatus shown in FIG. 24, specifically, FIG. 25 showing an image-forming relationship of the alignment optical system (i.e., image-forming optical system) shown in FIG. 24, FIG. 26 showing a conjugate relationship of the pupils of the optical system shown in FIG. 25, and FIG. 27 showing an image-forming relationship of the optical system of a modified example for the optical system shown in FIG. 25;

FIGS. 28 and 29 are charts showing optical characteristics of the afocal optical system in the first embodiment (FIG. 24) and fourth embodiment (FIG. 44) of the alignment apparatus in accordance with the present invention, specifically, FIG. 28
showing a relationship between amount of shift .delta. and eccentric asymmetrical aberration .DELTA.HC in the afocal optical system and FIG. 29 showing a relationship between ratio .DELTA.HC/.delta. of eccentric asymmetric aberration AHC to amount of asymmetric aberration .DELTA.C in the afocal optical system;

FIG. 30 is a view showing the second embodiment of the decentering mechanism in the alignment apparatus in accordance with the present invention;

FIG. 31 is a view showing a schematic configuration of a projection exposure apparatus provided with the second embodiment of the alignment apparatus in accordance with the present invention;

FIG. 32 is a chart showing an image-forming relationship of the alignment optical system (i.e., image-forming optical system) shown in FIG. 31;

FIGS. 33 and 34 are charts showing optical characteristics of the afocal optical system in the second embodiment (FIG. 31) and fifth embodiment (FIG. 47) of the alignment apparatus in accordance with the present invention, specifically, FIG. 33
showing a relationship between amount of shift .delta. and eccentric asymmetrical aberration .DELTA.HC in the afocal optical system and FIG. 34 showing a relationship between ratio .DELTA.HC/.delta. of eccentric asymmetrical aberration .DELTA.HC to amount of symmetric aberration .DELTA.S in the afocal optical system;

FIG. 35 is a perspective view showing a configuration of a main portion of the projection exposure apparatus provided with the third embodiment of the alignment apparatus in accordance with the present invention;

FIG. 36 is a perspective view showing a configuration of the alignment optical system in the alignment apparatus shown in FIG. 35;

FIG. 37 is a view showing a relationship between a wafer mark and an irradiating sheet beam in the alignment optical system (i.e., enlarged plan view of the wafer surface) in the alignment optical system shown in FIG. 36;

FIG. 38 is a view showing an image-forming relationship of a light-emitting system within the alignment optical system shown in FIG. 36;

FIGS. 39 and 40 are charts showing optical characteristics of the afocal optical system in the third embodiment (FIG. 35) and sixth embodiment (FIG. 52) of the alignment apparatus in accordance with the present invention, specifically, FIG. 39
showing a relationship between amount of shift .delta. and eccentric asymmetrical aberration .DELTA.HC in the afocal optical system and FIG. 40 showing a relationship between ratio .DELTA.HC/.delta. of eccentric asymmetrical aberration .DELTA.HC to amount of asymmetric aberration .DELTA.C in the afocal optical system;

FIGS. 41 to 43 are views explaining influence of asymmetric aberration when the beam waist of a laser beam is relayed by a lens system;

FIG. 44 is a view showing a schematic configuration of a projection exposure apparatus provided with the fourth embodiment of the alignment apparatus in accordance with the present invention;

FIGS. 45 and 46 are views showing a configuration of an alignment optical system (i.e., image-forming optical system) applicable to the alignment apparatus shown in FIG. 44, specifically, FIG. 45 showing a conjugate relationship (i.e., image-forming relationship) between the object surface, intermediate image surface, and image surface of the alignment optical system (i.e., image-forming optical system) shown in FIG. 44 and FIG. 46 showing a conjugate relationship of the pupils in the optical system shown in FIG. 45;

FIG. 47 is a view showing a schematic configuration of a projection exposure apparatus provided with the fifth embodiment of the alignment apparatus in accordance with the present invention;

FIGS. 48 and 49 are views showing a configuration of an alignment optical system (i.e., image-forming optical system) applicable to the alignment apparatus shown in FIG. 47, specifically, FIG. 48 showing a conjugate relationship (i.e., image-forming relationship) between the object surface and image surface of the alignment optical system (i.e., image-forming optical system) shown in FIG. 47 and FIG. 49 showing a conjugate relationship of the pupils in the optical system shown in FIG.
48;

FIGS. 50 and 51 showing a configuration of a modified example of an alignment optical system applicable to the alignment apparatus shown in FIG. 47, specifically, FIG. 50 showing a conjugate relationship (i.e., image-forming relationship) between the object surface and image surface of the alignment optical system (i.e., image-forming optical system) and FIG. 51 showing a conjugate relationship of the pupils in the alignment optical system shown in FIG. 50;

FIG. 52 is a perspective view showing a configuration of a main portion of a projection exposure apparatus provided with the sixth embodiment of the alignment apparatus in accordance with the present invention;

FIG. 53 is a perspective view showing a configuration of the alignment optical system in the alignment apparatus shown in FIG. 52;

FIG. 54 is a view showing a relationship between a wafer mark and an irradiating sheet beam in the alignment optical system (i.e., enlarged plan view of the wafer surface) in the alignment optical system shown in FIG. 53; and

FIG. 55 is a view showing an image-forming relationship of a light-emitting system within the alignment optical system shown in FIG. 53.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, the embodiments of the present invention will be explained with reference to FIGS. 1 to 55. In the first place, the optical adjustment conditions of an optical system to be inspected by the inspection apparatus and method in accordance with the present invention will be explained with reference to FIGS. 1 and 2.

FIG. 1 shows a condition (i.e., telecentricity as a first optical adjustment condition) in which a principal ray is deviated from an optical axis of the optical system to be inspected. FIG. 2 shows a condition (i.e., eclipse as a second optical adjustment condition) in which an eclipse remains in the optical system to be inspected.

Depending on the adjustment condition of the optical system, when defocusing is effected at an object surface, an optical system, or a detector, positional deviation may occur at the detection surface of the detector.

FIG. 1 shows a typical example thereof. In this example, a light source and an aperture stop are symmetrically made eccentric with respect to the optical axis of the optical system. As a result, when the detection surface is defocused in the direction of the optical axis, positional deviation of the image occurs on the detection surface. Here, the quantitative magnitude (i.e., magnitude of deviation between the optical axis and the principal ray) of such an optical adjustment condition is defined as telecentric index.

Also, depending on the adjustment condition of the optical system, there is a case where the distribution of diffracted light components are asymmetrically eclipsed (i.e., shielded) with respect to an aperture stop (i.e., image-forming aperture stop) within an image-forming optical system. FIG. 2 shows a typical example thereof. In this case, only the image-forming aperture stop is made eccentric with respect to the optical axis of the optical system. As a result, the image on the detection surface usually collapses asymmetrically. Here, the optical adjustment condition where such a phenomenon occurs is referred to as a condition in which "eclipse" remains. Also, the quantitative magnitude of such an optical adjustment condition is defined as amount of eclipse.

In practice, the optical adjustment condition of the optical system is a mixed state of the conditions shown in FIGS. 1 and 2 (i.e., first and second optical adjustment conditions).

Next, the embodiments of the method of inspecting an optical system in accordance with the present invention will be explained.

FIGS. 3 and 4 are charts showing, within a predetermined defocus range, for individual defocus condition, relationships between the positional change of a pattern image and various aberrations in the first embodiment.

In the first embodiment of the inspection method, for example, a position x of a spatial image of a pattern formed by way of an optical system to be inspected is measured under a plurality of defocus conditions (with an amount of defocus Z) holding the best focus condition (Z=0) therebetween. Here, the best focus position (Z=0) of the optical system to be inspected can be determined, for example, as a position where the light intensity of a bright and dark pattern image formed by way of the optical system to be inspected is maximized.

When there is no remaining aberration in the optical system to be inspected and the optical adjustment of the optical system to be inspected is ideal, the image position x is constant, as indicated by straight line L1 in FIG. 3, regardless of the amount of defocus Z.

On the other hand, when the optical adjustment condition of the optical system to be inspected includes a condition such as that shown in FIG. 1, the image position x depends on the amount of defocus Z such that the relationship between the image position x and the amount of defocus Z is substantially linear as indicated by straight line L2 in FIG. 3. The gradient of line L2 (x/Z) is substantially proportional to the telecentric index.

Also, when only symmetric aberration remains in the optical system to be inspected, a measurement line which indicates the relationship between the image position and the amount of defocus becomes line L1 shown in FIG. 3. Namely, the image position x is constant regardless of the amount of defocus Z.

However, when only asymmetric aberration remains in the optical system to be inspected, the measurement line becomes curve L3 shown in FIG. 4. Namely, the image position x changes like a high-order curve depending on the amount of defocus Z. Amount of deviation .alpha. of curve L3 in the x direction within a predetermined defocus range is substantially proportional to the amount of asymmetric aberration.

In the specification, the measurement line includes a straight line, curve, or the like as shown in FIGS. 3-5 and 9-11.

Here, the amount of deviation a is determined as follows. Namely, a peak point P of a protruded portion of the measurement line is specified. This point P is a point where the curvature of the measurement line is maximized (i.e., point where the radius of curvature is minimized). The amount of deviation .alpha. is defined as the maximum clearance between the tangential line passing through the peak point P and the measurement line within the predetermined defocus range.

When symmetric aberration remains in addition to asymmetric aberration in the optical system to be inspected, the resulting measurement line is curve L4 which is formed when curve L3 is moved in parallel to the direction of the vertical axis (i.e., Z-direction) as shown in FIG. 4. Amount of defocus Zp corresponding to the peak point P in curve L4 is substantially proportional to the amount of symmetric aberration in the optical system to be inspected.

Here, the best focus position (Z=0) of the optical system to be inspected can be determined, for example, as a position where the light intensity of a bright and dark pattern image formed by way of the optical system to be inspected is maximized.

In the following, the first embodiment of the inspection method in accordance with the present invention will be explained with reference to the flow chart of FIG. 6. Here, in order to facilitate the explanation, reference will also be made to FIG. 13 which shows a configuration of an inspection apparatus realizing this embodiment. Also, the following explanation of operations will be made on the assumption that adjustment conditions and aberration conditions of a projection optical system in a projection exposure apparatus are inspected.

First, as an initial condition, by using autofocus systems 21A and 21B, a main control system 100 controls control systems 200, 300, and 400 for a reticle stage 14, wafer stages 12 and 17, and the like, such that an image is formed on a detection surface PTa (i.e., reference surface) for the image position under the best focus condition for the image (step ST1). Though the image position under the best focus condition is used as a reference in this embodiment, other image positions under a defocus condition may be used therefor.

This image position under the best focus condition is captured into a signal processor 152 in the main control system 100 by way of a detector 19 (step ST2). This step is repeated at least three times (including the detection of the image under the best focus condition) while, according to an instruction from the main control system 100 (i.e., controller 150), defocusing the image on the detection surface PTa by a predetermined amount of defocus (steps ST3 and ST4). When the image position x is detected at least three times, it can be judged whether the measurement line becomes linear as shown in FIG. 3 or forms a curve as shown in FIG. 4.

After the foregoing detection of the image position x is performed a predetermined number n (.gtoreq.3) of times, a calculator 153 in the main control system 100 calculates, with reference to data (i.e., data of reference measurement lines which have already been obtained) within a table 154 prepared beforehand, a measurement line Lx (measurement curve) which is adaptive to the resulting relationship between the amount of defocus Z and the image position x (step ST5). Here, as the number of detections of the image position increases, the measurement line more favorably reflecting the actual optical adjustment conditions and aberration conditions of the optical system to be inspected can be obtained.

In the actual optical system to be inspected, the above-mentioned various aberrations and the optical adjustment condition shown in FIG. 1 exist in a mixed state. Accordingly, the resulting measurement line Lx is tilted by a predetermined angle as shown in FIG. 5. This is because the measurement line Lx includes the component of the measurement line L2 concerning the optical adjustment condition shown in FIG. 1 and the component of the measurement line L4 concerning the amounts of asymmetric and symmetric aberrations (or measurement line L3 concerning the amount of asymmetric aberration).

The calculator 153 initially specifies a peak point P of thus obtained measurement line Lx. As mentioned above, this peak point P is a peak of a protruded portion of the measurement line Lx within a predetermined defocus range, namely, a point in which the curvature of the measurement line Lx is maximized (i.e., radius of curvature is minimized). Then, the calculator 153 calculates a tangential line of the measurement line Lx passing through thus obtained peak point P, thereby calculating, within the defocus range, the maximum clearance between the resulting tangential line and the measurement line Lx as an amount of deviation a (step ST6). Since thus obtained amount of deviation .alpha. is substantially proportional to the amount of asymmetric aberration of the optical system to be inspected, the calculator 153 calculates the amount of asymmetric aberration with reference to the prepared table 154 or on the basis of a constant of proportionality (i.e., known constant) specific to the optical system to be inspected.

Also, the amount of symmetric aberration can be easily obtained when the above-mentioned peak point P is specified on the resulting measurement line Lx. This is because the amount of movement L (i.e., amount of defocus Zp) of the peak point P with respect to the best focus condition is substantially proportional to the amount of symmetric aberration of the optical system to be inspected (step ST7). Accordingly, the calculator 153 calculates the amount of symmetric aberration with reference to the prepared table 154 or on the basis of a constant of proportionality (i.e., known constant) specific to the optical system to be inspected.

Further, the magnitude (i.e., telecentric index) of the optical adjustment condition such as that shown in FIG. 1 can be obtained from the gradient (x/Z) of the tangential line of the measurement line Lx passing through the above-mentioned peak point P (step ST8). Then, the calculator 153 calculates the telecentric index with reference to the prepared table 154 or on the basis of a constant of proportionality (i.e., known constant) specific to the optical system to be inspected. Here, an inspection unit 151 includes the signal processor 152 and the calculator 153.

As explained in the foregoing, in cases where asymmetric aberration, symmetric aberration, and the optical adjustment conditions shown in FIG. 1 exist in a mixed state in the optical system to be inspected, the amount of asymmetric aberration and the amount of symmetric aberration can be detected respectively on the basis of the amount of deviation .alpha. of thus obtained curve Lx and the amount of defocus Zp corresponding to the peak point P in the curve Lx. Also, based on the tangential line at the peak point P of the curve Lx, the telecentric index can be obtained. Here, in place of the tangential line, a line approximating the curve Lx, as a whole, may be utilized as well.

The approximation line is easily calculated with method of least square, or the like. The telecentric index can be calculated on the basis of the gradient of the approximation line, and the amount of asymmetric aberration can be calculated on the basis of the distance between the peak point P and the approximation line.

In this manner, based on thus obtained curve Lx, the amount of asymmetric aberration, amount of symmetric aberration, and telecentric index can be detected simultaneously with a favorable reproducibility.

In the first embodiment of the inspection method in accordance with the present invention, the image position of a pattern formed by way of an optical system to be inspected (e.g., projection optical system in FIG. 13) is measured. By contrast, in the second embodiment of the inspection method in accordance with the present invention, the image position of a converged luminous flux formed by way of an optical system to be inspected is measured. In the second embodiment, based on a principle similar to that of the first embodiment, the amount of asymmetric aberration, amount of symmetric aberration, and telecentric index can be detected simultaneously with a favorable reproducibility. The operations of the second embodiment are similar to those of the flow chart shown in FIG. 6.

On the other hand, in the third embodiment of the inspection method in accordance with the present invention, based on a change in asymmetry of a phase pattern image under individual defocus conditions, the amount of asymmetric aberration, amount of symmetric aberration, and amount of eclipse of an optical system to be inspected can be simultaneously detected. The phase pattern image can be formed on the detection surface, for example as shown in FIG. 7, when protrusions (i.e., phase pattern) is disposed with a predetermined pitch on a surface Ms of a mask which is permeable to light.

FIG. 8 is a chart in which integrated signal .SIGMA.V, which is obtained as signals V corresponding to the light intensity of the phase pattern image are integrated in a non-measurement direction Sa, is plotted with respect to a measurement direction S in order to explain asymmetry index .beta. of the phase pattern image.

As shown in FIG. 8, the integrated signal .SIGMA.V changes with each period BP (B: magnification from the pattern to the image-pickup surface; P: pitch of the pattern) along the measurement direction S. In order to quantify the asymmetry of the phase pattern image, the minimal signal values at the left and right in the drawing at the i-th period in the distribution of the integrated signal .SIGMA.V are defined as V.sub.iL and V.sub.iR, respectively. Also, in the whole area of the integrated signal .SIGMA.V excluding the both ends thereof extending over all the periods, the maximum value and minimum value are defined as V.sub.max and V.sub.min, respectively.

Then, the asymmetry index .beta. of the phase pattern image is determined by the following expression: ##EQU2## wherein: n: number of periodicity and

.SIGMA.: sum mark of i=1 to n.

FIGS. 9 and 10 are charts showing, in this third embodiment, under individual defocus conditions, relationships between the change in the asymmetry index .beta. of the phase pattern image and various aberrations.

Under the ideal optical adjustment condition where there is neither remaining aberration nor eclipse in the optical system to be inspected, as indicated by line L5 in FIG. 9, the index .beta. is zero regardless of the amount of defocus Z. Also, when there is only symmetric aberration in the optical system to be inspected, as indicated by line L5 in FIG. 9, the index .beta. is constant regardless of the amount of defocus Z.

However, when only asymmetric aberration exists in the optical system to be inspected, as indicated by line L6 in FIG. 9, the index .beta. depends on the amount of defocus Z such that the relationship between the index .beta. and the amount of defocus Z becomes linear. Then, the gradient (.beta./Z) of this line L6 is substantially proportional to the amount of asymmetric aberration.

When symmetric aberration remains in addition to asymmetric aberration in the optical system to be inspected, the resulting measurement line is curve L7 which is formed when curve L6 is moved in parallel to the direction of the horizontal axis (i.e., .beta.-direction) as shown in FIG. 9. Amount of parallel movement L at the best focus position (Z=0) is substantially proportional to the amount of symmetric aberration. Here, the best focus position (Z=0) of the optical system to be inspected can be determined, for example, as a position where the light intensity of a bright and dark pattern image formed by way of the optical system to be inspected is maximized.

Also, when eclipse exists in the optical system to be inspected, as indicated by polygonal line L8 in FIG. 10, the index .beta. becomes substantially linear depending on the defocus direction and the amount of defocus Z. Bending amount A (.beta./Z) of polygonal line L8 with respect to line L5 (or tangential line at an apex P' of the measurement line) is substantially proportional to the amount of eclipse.

In this manner, also in the third embodiment, based on the change in asymmetry of the phase pattern image under individual defocus conditions, the amount of symmetric aberration, amount of asymmetric aberration, and amount of eclipse can be detected simultaneously with a favorable reproducibility.

In the following, the third embodiment of the inspection method in accordance with the present invention will be explained with reference to the flow chart of FIG. 12. Here, in order to facilitate the explanation, reference will also be made to FIG. 19 which shows a configuration of an inspection apparatus realizing this embodiment. Also, the following explanation of operations will be made on the assumption that optical adjustment conditions and aberration conditions of an alignment optical system in an alig