Broad spectrum ultraviolet inspection methods employing...

Details Broad spectrum ultraviolet inspection methods employing... Broad spectrum ultraviolet inspection methods employing...

2000-06-16

2001-11-06

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

Ultraviolet light responsive means

C250S461100

Reexamination Certificate

active

06313467

ABSTRACT:

TECHNICAL FIELD
The present invention relates to optical systems adapted for imaging in the ultraviolet (UV) portion of the spectrum, and in particular to broadband UV catadioptric imaging optics, i.e. systems employing a combination of one or more lens elements and one or more reflecting (mirror) elements in series. The invention is addressed especially to systems that have been designed to correct for imaging and color aberrations.
BACKGROUND ART
Catadioptric imaging systems for the deep ultraviolet spectral region (about 0.15 to 0.30 &mgr;m wavelength) are known. U.S. Pat. No. 5,031,976 to Shafer and U.S. Pat. No. 5,488,229 to Elliott and Shafer disclose two such systems. These systems employ lens elements made from only a single refractive material, namely fused silica, since it is practically the only material that combines good transmission of deep UV light with desirable physical properties. For example, fluoride glasses (based on CaF
2
, LiF, etc.), while transmissive of deep UV light, are generally considered too soft, making lens formation difficult. Thus, fluoride glass materials are normally avoided whenever possible.
In the above-noted '976 Shafer patent, an optical system is disclosed, which is based on the Schupmann achromatic lens principle producing an achromatic virtual image, and which combines it with a reflective relay to produce a real image. The system, reproduced here as
FIG. 7
, includes an aberration corrector group of lenses
101
for providing correction of image aberrations and chromatic variation of image aberrations, a focusing lens
103
receiving light from the group
101
for producing an intermediate image
105
, a field lens
107
of the same material as the other lenses placed at the intermediate image
105
, a thick lens
109
with a plane mirror back coating
111
whose power and position is selected to correct the primary longitudinal color of the system in conjunction with the focusing lens
103
, and a spherical mirror
113
located between the intermediate image and the thick lens
109
for producing a final image
115
. Most of the focusing power of the system is due to the spherical mirror
113
. It has a small central hole near the intermediate image
105
to allow light from the intermediate image
105
to pass therethrough to the thick lens
109
. The mirror coating
111
on the back of the thick lens
109
also has a small central hole
119
to allow light focused by the spherical mirror
113
to pass through to the final image
115
. While primary longitudinal (axial) color is corrected by the thick lens
109
, the Offner-type field lens
107
placed at the intermediate image
105
has a positive power to correct secondary longitudinal color. Placing the field lens slightly to one side of the intermediate image
105
corrects tertiary longitudinal color. Thus, axial chromatic aberrations are completely corrected over a broad spectral range. The system incidently also corrects for narrow band lateral color, but fails to provide complete correction of residual (secondary and higher order) lateral color over a broad UV spectrum.
The above-noted '229 patent to Elliott and Shafer provides a modified version of the optical system of the '976 patent, which has been optimized for use in 0.193 &mgr;m wavelength high power excimer laser applications, such as ablation of a surface
121
′, as seen in FIG.
8
. This system has the aberration corrector group
101
′, focusing lens
103
′, intermediate focus
105
′, field lens
107
′, thick lens
109
′, mirror surfaces
111
′ and
113
′ with small central openings
117
′ and
119
′ therein and a final focus
115
′ of the prior '976 patent, but here the field lens
107
′ has been repositioned so that the intermediate image or focus
105
′ lies outside of the field lens
107
′ to avoid thermal damage from the high power densities produced by focusing the excimer laser light. Further, both mirror surfaces
111
′ and
113
′ are formed on lens elements
108
′ and
109
′. The combination of all light passes through both lens elements
108
′ and
109
′ provides the same primary longitudinal color correction of the single thick lens
109
in
FIG. 7
, but with a reduction in total glass thickness. Since even fused silica begins to have absorption problems at the very short 0.193 &mgr;m wavelength, the thickness reduction is advantageous at this wavelength for high power levels. Though the excimer laser source used for this optical system has a relatively narrow spectral line width, the dispersion of silica near the 0.193 &mgr;m wavelength is great enough that some color correction still needs to be provided. Both prior systems have a numerical aperture of about 0.6.
Longitudinal chromatic aberration (axial color) is an axial shift in the focus position with wavelength. The prior system seen in
FIG. 7
completely corrects for primary, secondary and tertiary axial color over a broad wavelength band in the near and deep ultraviolet (0.2 &mgr;m to 0.4 &mgr;m). Lateral color is a change in magnification or image size with wavelength, and is not related to axial color. The prior system of
FIG. 7
completely corrects for primary lateral color, but not for residual lateral color. This is the limiting aberration in the system when a broad spectral range is covered.
An object of the invention is to provide a catadioptric imaging system with correction of image aberrations, chromatic variation of image aberrations, longitudinal (axial) color and lateral color, including residual (secondary and higher order) lateral color correction over a broad spectral range in the near and deep ultraviolet spectral band (0.2 to 0.4 &mgr;m).
In addition to color correction, it is also desired to provide a UV imaging system useful as a microscope objective or as microlithography optics with a large numerical aperture for the final image and with a field of view of at least 0.5 mm. The system is preferably telecentric.
DISCLOSURE OF THE INVENTION
The object is met with a catadioptric imaging system in which an achromatic multi-element field lens is used, made from two or more different refractive materials, such as fused silica and fluoride glass. The field lens may be a doublet or preferably a triplet, which may be cemented together or alternatively spaced slightly apart. Because fused silica and fluoride glass do not differ substantially in dispersion in the deep ultraviolet, the individual powers of the several component elements of the field lens need to be of high magnitude. Use of such an achromatic field lens allows not only axial color, but also lateral color to be completely corrected over a broad spectral range. Only one field lens component need be of a different refractive material than the other lenses of the system.
An optical system according to the present invention includes a focusing lens group with plural lens elements, preferably all formed from a single type of material, with refractive surfaces having curvatures and positions selected to focus light to an intermediate image with high levels of correction of both image aberrations and chromatic variation of aberrations over a UV wavelength band of at least 0.20 to 0.29 &mgr;m, and preferably extending over 0.20 to 0.40 &mgr;m. Systems adapted for a UV band that includes the 0.193 &mgr;m wavelength are also possible. The system also includes the aforementioned field lens group positioned near the intermediate image to provide correction of chromatic aberrations including residual axial and lateral color. The intermediate image plane may be located either inside or outside the field lens group depending on the optimization. A catadioptric group includes a concave spherical reflector, which may either be a mirror or a reflectively coated lens element, and a planar or near planar reflector near the final image, which is a reflectively coated lens element. Both reflective elements have central optical apertures therein whe

Affiliated with

Also associated with

Rating

Broad spectrum ultraviolet inspection methods employing... does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Broad spectrum ultraviolet inspection methods employing..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Broad spectrum ultraviolet inspection methods employing... will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2599116