Plate correction of imaging systems

• US 5,392,119 A
• Filed: 07/13/1993
• Issued: 02/21/1995
• Est. Priority Date: 07/13/1993
• Status: Expired due to Term

• Current Assignee: Micron Technology, Inc.
• Original Assignee: Litel Instruments Incorporated
• Inventors: Smith, Adlai H., Hunter, Robert O. Jr., McArthur, Bruce B.
• Primary Examiner(s): Willis, Davis L.
• Assistant Examiner(s): WOLFE, RUSSELL
• Application Number:
  US08/091,669
• Time in Patent Office: 588 Days
• Field of Search: 356/355, 356/124, 359/859, 359/370, 359/558, 359/738
• US Class Current: 356/515
• CPC Class Codes: G02B 27/0025 for optical correction, e.g. distorsion, aberration
• CPC Class Codes: G03F 7/70308 Optical correction elements, filters or phase plates for manipulating imaging light, e.g. intensity, wavelength, polarisation, phase or image shift
• CPC Class Codes: G03F 7/706 Aberration measurement

Abstract

An object is projected through the lens system to be corrected to the image plane where the position of the diffraction limited ideal image is readily ascertainable. At least one primary image defect or Seidel Aberration is measured. These aberrations include distortion, curvature of field or Petzval curvature, spherical aberration, coma, and astigmatism. Interferometry is one technique typically used to measure the aberrations of the system. Based on the measurements, the location of an apparent object is computed. The apparent object is an imaginary location of the object which would cause the image of the object to register to the diffraction limited ideal image. Although only one corrector plate may be required to achieve the desired optical system performance improvements, in the preferred embodiment at least two corrector plate mounting planes are designed and mounts made for the insertion of first and second corrector plates to correct beam convergence and focus to the ideal image. In the analysis offered, provision is made to insert to a medial location a phase conjugate plate for the correction of systematic lens system errors. A matrix of test points is utilized to locally correct, on an individually measured basis, lens systems through customized corrector plates individually tailored to each lens system. There results a simplified lens system correctly customized to each particular lens system which assists in approaching the diffraction limit of the optical system. Both the process and the product of the process can be used to either improve existing lens trains or ease the fabrication of new lens trains.

Claims

1. A process of altering and improving the image quality of an imaging system comprising the steps of:

providing a radiation source;

illuminating an object with said radiation source;

projecting an image of said object along a projection axis through said imaging system to an ideal image plane where the position of a diffraction limited ideal image is readily ascertainable relative to an actual image of the object;

measuring at least curvature of field, distortion, and local astigmatism of said imaging system relative to said diffraction limited ideal image to determine an imaging system performance for a matrix of distributed field points through said imaging system;

calculating a set of apparent object points from a set of ideal image points and the measured imaging system performance such that each apparent object point corresponds to an ideal image point for said matrix of distributed field points through said imaging system;

providing at least two spaced apart corrector plate mounting planes substantially normal to the projection axis of the imaging system for the insertion of a first corrector plate and a second corrector plate within the object to image optical path;

designing a first corrector plate to produce areas of localized beam deflection for said matrix of field points such that a set of light rays emanating from a set of object points will intersect a set of imaginary rays emanating from a set of apparent object points at the second corrector plate;

designing at least a second corrector plate for said matrix of field points such that the direction of the set of light rays emanating from the object are altered so that the actual image and the diffraction limited ideal image coincide; and

,

inserting the first corrector plate and the second corrector plate into the corrector plate mounting planes.

2. The process of claim 1 and wherein:

said radiation source emits radiation in a visible wavelength regime.

3. The process of claim 1 and wherein:

said radiation source emits radiation in ultraviolet wavelength regime.

4. The process of claim 1 and wherein:

said radiation source emits radiation in soft x-ray wavelength regime.

5. The process of claim 2 and wherein:

said radiation source is monochromatic.

6. The process of claim 2 and wherein:

said radiation source is nearly monochromatic.

7. The process of claim 2 and wherein:

said radiation source is broad band.

8. The process of claim 1 and wherein:

said first corrector plate and said second corrector plate are equivalent to a first surface and a second surface of a single optic.

9. The process of claim 1 and including:

designing a third corrector plate to further coincide the actual image and the diffraction limited ideal image; and

,

inserting the third corrector plate into a corrector plate mounting plane.

10. The process of claim 1 and wherein:

one of said corrector plates includes provision for the correction of repeatable error within said imaging system.

11. The process of claim 1 and wherein:

at least one of said corrector plates include a grating.

12. The process of claim 1 and wherein:

one of said corrector plates is located within said imaging system; and

,

one of said corrector plates is located without said imaging system.

13. The process of claim 1 and wherein:

at least two corrector plates are located within said imaging system.

14. The process of claim 1 and wherein:

at least two corrector plates are located without said imaging system.

15. The process of claim 1 and including:

measuring for aberration including generating a plurality of interferograms of a set of discrete points on said ideal image;

looking for a common error on said corresponding points on said interferograms; and

,

placing a corrector plate within said imaging system having correction for said common error.

27. An imaging system having a first corrector plate at said first corrector plate plane and a second corrector plate at said second corrector plate plane, said corrector plates manufactured by the process of claim 1.

16. A process of altering and improving the image quality of an imaging system comprising the steps of:

providing a radiation source;

illuminating an object with said radiation source;

projecting an image of said object along a projection axis through said imaging system to an ideal image plane where the position of a diffraction limited ideal image is readily ascertainable relative to an actual image of the object;

measuring at least one type of aberration of said imaging system relative to said diffraction limited ideal image for a matrix of field points passing through said imaging system;

computing a required deflection of light rays relative to both a first corrector plate mounting plane and a second corrector plate mounting plane such that the image of the object produced by said deflected rays through said matrix of field points will converge with the diffraction limited ideal image; and

designing and inserting a first corrector plate into the first corrector plate mounting plane and a second corrector plate into the second corrector plate mounting plane to produce the required deflection of light rays for said matrix of field points such that rays corresponding to said measured field points converge toward said diffraction limited ideal image.

17. A process of altering and improving the image quality of an imaging system comprising the steps of:

providing a radiation source;

illuminating an object with said radiation source;

projecting an image of said object along a projection axis through said imaging system to an ideal image plane where the position of a diffraction limited ideal image is readily ascertainable relative to an actual image of the object;

measuring at least one type of aberration of said imaging system relative to said diffraction limited ideal image for a matrix of field points through said imaging system;

computing a required deflection of light rays relative to a corrector plate mounting plane such that the image of the object produced by said deflected rays will converge with the diffraction limited ideal image for said matrix of field points taken through said imaging system; and

designing and inserting a corrector plate into the corrector plate mounting plane to produce the required deflection of light rays towards said diffraction limited ideal image of said imaging system.

28. An imaging system having a corrector plate at said corrector plate plane, said corrector plate manufactured by the process of claim 17.

18. A process of altering and improving the image quality of an imaging system having an image and object plane comprising the steps of:

providing a radiation source;

illuminating an object with said radiation source;

measuring for aberration including generating a plurality of interferograms taken from a set of discrete field points on one of said planes;

determining a common error on said interferograms taken at said set of discrete field points;

subtracting said common error from each said interferogram to determine a local correction for each said discrete field point;

utilizing said local corrections to map aberration corrections for said set of discrete points;

providing at least two spaced apart corrector plate mounting planes substantially normal to the projection axis of the imaging system for the insertion of a first corrector plate and a second corrector plate within the object to image optical path;

designing a first corrector plate to produce areas of localized beam deflection such that said local correction for each discrete field point will cause a set of light rays emanating from said discrete point to intersect a set of imaginary rays at the second corrector plate having the desired corrected trajectory of light from said discrete field points;

designing at least a second corrector plate such that said local correction for each discrete point will alter the direction of the set of light rays emanating from the second corrector plate so that the rays converge to the desired trajectory of light from said discrete field points; and

,

inserting the first corrector plate and the second corrector plate into the corrector plate mounting planes.

19. The process of claim 18 and including:

designing a third corrector plate to compensate for said common error; and

inserting said third corrector plate near an aperture stop of the imaging system.

29. An imaging system having a first corrector plate at said first corrector plate plane and a second corrector plate at said second corrector plate plane, said corrector plates manufactured by the process of claim 18.

20. An imaging system product corrected towards a diffraction limited condition for projecting through the imaging system along a projection axis within a optical path an image to an image plane of an object from an object plane comprising:

providing said imaging system;

providing at least two spaced apart corrector plate mounting planes substantially normal to the projection axis of the imaging system for the insertion of first and second corrector plates within the object to image optical path with at least one of said corrector plate mounting planes being outside of said imaging system;

measuring at least curvature of field, distortion, and aberration of said imaging system for a matrix of field points through said imaging system;

designing and inserting to one of said corrector plate mounting planes a first corrector plate to produce beam deflection at said matrix of field points so that rays intersect a path at the second corrector plate to converge on the ideal image; and

,

designing and inserting a second corrector plate for said matrix of field points so that rays at the second corrector plate are given the required direction to converge on the ideal image.

21. The product of claim 20 and wherein:

said first corrector plate is an asphere producing beam deflection from a desired object position to intersect said path at said second corrector plate from said desired object position to said ideal image.

22. The product of claim 20 and wherein:

said second corrector plate is an asphere producing beam deflection to a desired trajectory to said ideal image.

23. The product of claim 20 and wherein:

one of said spaced apart corrector mounting planes is located within said imaging system; and

,

the other of said spaced apart corrector mounting planes is located without said imaging system.

24. The product of claim 23 and wherein:

one of said corrector plates is placed in the middle of said system and includes provision of for the correction of repeatable error within said imaging system.

25. The product of claim 20 and wherein:

both said plates are without said imaging system.

26. The product of claim 20 and including:

an asphere within said imaging system generated by;

measuring said imaging system for aberration including generating a plurality of interferograms of discrete field points on said ideal image;

looking for common error on said plurality of interferograms of said discrete field points; and

,

placing an asphere on said corrector plate within said imaging system having correction for said common error.

30. An imaging system having a first corrector plate at said first corrector plate plane and a second corrector plate at said second corrector plate plane, said corrector plates manufactured by the process of claim 20.