Broad spectrum ultraviolet catadioptric imaging system

• US 20020085271A1
• Filed: 11/06/2001
• Published: 07/04/2002
• Est. Priority Date: 07/22/1996
• Status: Active Grant

• Current Assignee: KLA-Tencor Corporation
• Original Assignee: KLA Instruments, Corporation
• Inventors: Chuang, Yung-Ho, Shafer, David R., Tsai, Bin-Ming B.
• Granted Patent: US 6,956,694 B2
• Time in Patent Office: Days
• US Class Current: 359/359
• CPC Class Codes: G02B 13/143 for use with ultraviolet radiation
• CPC Class Codes: G02B 17/0808 on-axis systems with at least one of the mirrors having a central aperture
• CPC Class Codes: G02B 17/0856 comprising a refractive element with a reflective surface, the reflection taking place inside the element, e.g. Mangin mirrors
• CPC Class Codes: G02B 17/0892 specially adapted for the UV
• CPC Class Codes: G02B 21/16 adapted for ultraviolet illumination ; Fluorescence microscopes G02B21/0076 takes precedence
• CPC Class Codes: G03F 1/84 Inspecting
• CPC Class Codes: G03F 7/70216 Mask projection systems
• CPC Class Codes: G03F 7/70225 Optical aspects of catadioptric systems, i.e. comprising reflective and refractive elements
• CPC Class Codes: G03F 7/70275 Multiple projection paths, e.g. array of projection systems, microlens projection systems or tandem projection systems
• CPC Class Codes: G03F 7/70616 Monitoring the printed patterns
• CPC Class Codes: G03F 7/7065 Defects, e.g. optical inspection of patterned layer for defects

Abstract

An ultraviolet (UV) catadioptric imaging system, with broad spectrum correction of primary and residual, longitudinal and lateral, chromatic aberrations for wavelengths extending into the deep UV (as short as about 0.16 μm), comprises a focusing lens group with multiple lens elements that provide high levels of correction of both image aberrations and chromatic variation of aberrations over a selected wavelength band, a field lens group formed from lens elements with at least two different refractive materials, such as silica and a fluoride glass, and a catadioptric group including a concave reflective surface providing most of the focusing power of the system and a thick lens providing primary color correction in combination with the focusing lens group. The field lens group is located near the intermediate image provided by the focusing lens group and functions to correct the residual chromatic aberrations. The system is characterized by a high numerical aperture (typ, greater than 0.7) and a large flat field (with a size on the order of 0.5 mm). The broad band color correction allows a wide range of possible UV imaging applications at multiple wavelengths.

Claims

1. A broad-band deep-ultraviolet achromatic catadioptric imaging system, comprising:

a focusing lens group including a plurality of lens elements, all formed from a single refractive material type, with refractive surfaces thereof disposed at first predetermined positions along an optical path of the system and having curvatures and said positions selected to focus ultraviolet light at an intermediate image within the system, and simultaneously to also provide in combination with the rest of the system, high levels of correction of both image aberrations and chromatic variation of aberrations over a wavelength band including at least 0.20-0.29 μ

m,

a field lens group with a net positive power disposed along said optical path proximate to said intermediate image, the field lens group including a plurality of lens elements formed from at least two different refractive materials with different dispersions, with refractive surfaces of the lens elements of the field lens group disposed at second predetermined positions and having curvatures selected to provide correction of chromatic aberrations including at least secondary lateral color of the system over said wavelength band,

a catadioptric group including a first optical element having at least a concave reflective surface with a central optical aperture therein disposed along said optical path proximate to said intermediate image so that ultraviolet light from the intermediate image can pass therethrough, the catadioptric group also including a second optical element which is a lens with a reflective mirror coating on a rear surface of said lens except for a central area on said rear surface where said mirror coating is absent, the optical elements of said catadioptric group being arranged such that ultraviolet light from the intermediate image transmitted through said central optical aperture of said first optical element of said catadioptric group passes through the lens portion of said second optical element of said catadioptric group, reflects from said reflective mirror coating on said lens rear surface, passes back through said lens portion towards said concave reflective surface of said first optical element, is reflected thereby and passes a third time through said lens portion of said second optical element and through said central area of said lens rear surface to form a final image beyond said catadioptric group.

2. The imaging system of claim 1 wherein said wavelength band includes 0.20-0.40 μ

m.

3. The imaging system of claim 1 wherein said wavelength band includes 0.193 μ

m.

4. The imaging system of claim 1 wherein said single refractive material type of said focusing lens group is fused silica.

5. The imaging system of claim 1 wherein said field lens group is formed from lens elements made of fused silica and a fluoride glass.

6. The imaging system of claim 1 wherein said first optical element of said catadioptric group comprises a concave mirror with a central hole therein forming said central optical aperture.

7. The imaging system of claim 1 wherein said first optical element of said catadioptric group comprises a meniscus lens with a concave reflective surface coating thereon.

8. The imaging system of claim 7 wherein said central optical aperture is formed by a central hole in said meniscus lens.

9. The imaging system of claim 7 wherein said central optical aperture is formed by a central area on said meniscus lens where said concave reflective surface coating is absent.

10. The imaging system of claim 1 wherein said catadioptric group is characterized by reflective surface curvatures selected to provide at least a 0.8 numerical aperture and a 0.5 mm field of view for said final image of the imaging system.

11. A catadioptric imaging system, comprising:

a first negative lens,

a second positive biconvex lens closely spaced to the first lens to form a substantially zero-power corrector group for chromatic variations of image aberrations,

a third negative meniscus lens spaced from the second lens,

a fourth negative meniscus lens with concave surfaces of the third and fourth lenses facing each other,

a fifth positive biconvex lens,

a sixth positive meniscus lens,

a seventh meniscus lens of near zero-power with concave surfaces of the sixth and seventh lenses facing each other, the third through seventh lenses being closely spaced together to form a focusing lens group with minimal image aberrations, said focusing lens group providing an intermediate image,

eighth, ninth and tenth lenses forming an achromatic field lens group positioned proximate to said intermediate image, said field lens group including at least one positive convex lens of a different refractive material type than all other lenses in the system and at least one negative meniscus lens, the field lens group having a net positive power,

an eleventh negative meniscus lens with a convex surface facing said first lens having a reflective coating thereon and with a first central optical aperture therein proximate to said intermediate image, and

a twelfth near zero-power, substantially flat lens with a reflective coating on a surface facing away from the first lens with a second central optical aperture therein, said twelfth lens spaced apart from the in eleventh lens, the eleventh and twelfth lenses with their respective reflective coatings forming a catadioptric group providing a light focusing relay for the intermediate image to provide a final image proximate to the second optical aperture.

12. The imaging system of claim 11 wherein the third lens is spaced at least 30 mm from the second lens, and said twelfth lens is spaced at least 30 mm from the eleventh lens.

13. The imaging system of claim 11 wherein the focusing lens group, field lens group and catadioptric group have refractive and reflective surfaces that are characterized by the following dimensional values:

14. The imaging system of claim 11 wherein the focusing lens group, field lens group and catadioptric group have refractive and reflective surfaces that are characterized by the following dimensional values:

17. The method of claim 16 wherein said object being inspected comprises a wafer surface including photoresist.

18. The method of claim 16 wherein said object being inspected comprises a phase-shift photomask.

19. The method of claim 16 further comprising analyzing said multi-wavelength ultraviolet image 50 as to identify and classify defects on said object.

15. A method of inspecting objects for defects comprising:

illuminating an object with ultraviolet radiation having a wavelength which is less than 400 nanometers;

forming a plurality of fluorescent ultraviolet images with a single broadband collector of ultraviolet radiation, each image detected at a wavelength band separated from other wavelength bands of images of the same object by at least 50 nanometers;

converting said plurality of images to visible light for visual inspection of defects.

16. A method of inspecting objects for defects comprising:

illuminating an object with ultraviolet radiation at multiple wavelengths,

forming a multi-wavelength ultraviolet image with a single broadband collector of ultraviolet radiation, and

detecting said multi-wavelength ultraviolet image.

20. A method of inspecting objects for defects comprising:

illuminating an object with ultraviolet radiation at multiple wavelengths separated by at least 10 nm,

for each ultraviolet illumination wavelength, forming a corresponding ultraviolet image slice of the object at a different focal plane from image slices corresponding to the other illumination wavelengths, and

forming a composite image by means of computer integration of the image slices, the composite image characterized by a greater depth of focus than any of its constituent image slices.