Optical: systems and elements – Lens – With reflecting element
Reexamination Certificate
2000-12-22
2003-10-07
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
With reflecting element
C359S727000, C359S728000, C359S729000, C359S731000, C359S364000
Reexamination Certificate
active
06631036
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to catadioptric objective and the use thereof in a microscope or a microlithographic projection exposure apparatus. The catadioptric objective includes spherical and aspherical lens elements and two concave mirrors which face each other. All components of the catadioptric objective, including also the object field and the image field, are arranged centered to a linear optical axis. This class of catadioptric objectives has a central aperture obscuration.
BACKGROUND OF THE INVENTION
At wavelengths in the deep ultraviolet range, that is, wavelengths less than 250 nm, mirrors having a positive refractive power are used in combination with lenses of negative refractive power as suitable means for color correction.
A catadioptric microscope objective having two concave mirrors facing each other is disclosed in Russian patent publication 124,665. The 60× magnification of the catadioptric microscope objective is achieved without intermediate imaging. Because of the low field size, only a few spherical lenses are needed for correction. A composite lens is used in addition to the mirrors for color correction. This correction means is, however, no longer available in the deep ultraviolet wavelength range.
Catadioptric objectives for microlithography having only one concave mirror are known from U.S. Pat. No. 5,691,802 or European patent publication 0,475,020. In these systems, the optical axis must be bent at least once. If reticle and wafer are to be mounted parallel to each other, then a two-fold beam deflection is required. This leads to significant complexity with respect to construction. If, in addition, a purely reflective beam splitter is used, such as disclosed in U.S. Pat. No. 5,691,802, then only off-axis object fields can be imaged. The lenses of the objective near to the field are non-symmetrically illuminated whereby asymmetrical thermal deformations and therefore imaging errors which are difficult to correct occur because of the absorption of the lenses.
A centered arrangement of the optical components on a linear optical axis having two concave mirrors facing each other as shown in
FIGS. 1 and 2
does not have this disadvantage. In contrast, an aperture obscuration occurs because of the cutouts in the mirrors.
The effects of an aperture obscuration on the contrast transmission function is investigated in the article of S. T. Yang et al entitled “Effect of Central Obscuration on Image Formation in Projection Lithography” (SPIE Volume 1264, Optical/Laser Microlithography III (1990), pages 477 to 485. For incoherent illumination, the contrast is reduced for low spatial frequencies in comparison to an unvignetted system. The acceptance of obscured objectives can therefore be significantly increased when the aperture obscuration is further reduced. In addition, a reduction of the contrast transmission function must not necessarily lead to a reduction of the resolution capacity because of the nonlinear response function of the photoresist. By suitably selecting the photoresist, the break in the contrast transfer function continues to lie above the exposure threshold of the photoresist.
SUMMARY OF THE INVENTION
It is an object of the invention to further reduce the aperture obscuration and the lens diameters in objectives of the kind described above. It is a further object of the invention to provide excellent imaging and color correction for the field sizes typical for microlithography and an increase of the image end aperture compared to the state of the art with the least possible use of material.
The catadioptric objective of the invention transmits a light beam along a light path and defines an optical axis. The catadioptric objective includes in sequence of the travel of the light beam: a first lens group having a negative refractive power and arranged centered on the optical axis; a first concave mirror having a central cutout and being arranged centered on the optical axis downstream of the first lens group; a second concave mirror having a central cutout and being arranged centered on the optical axis downstream of the first concave mirror; the first and second concave mirrors being disposed so as to face each other; a second lens group having a negative refractive power and being arranged centered on the optical axis downstream of the second concave mirror; the first lens group having a first plurality of lenses arranged upstream of the first concave mirror; the second lens group having a second plurality of lenses arranged downstream of the second concave mirror; and, one of the first and second plurality of lenses having at least one aspheric lens surface.
The catadioptric objective of the invention is normally combined as a partial objective with at least one dioptric (purely refractive) partial objective to form a reduction objective. The combination of a catadioptric component objective with at least one dioptric partial objective and the use in a microscope or in a microlithographic projection exposure apparatus is also described.
In the catadioptric objective, the light rays starting from the object plane first pass through a first lens group having a negative refraction power and then impinge on a first concave mirror which has a hole at its center. This concave mirror is mounted concavely to the object plane. The light is reflected back and impinges on the second concave mirror which likewise has a central hole. This second concave mirror is mounted concavely to the image plane. In this way, the two concave mirrors face each other. The light rays are reflected back from this second concave mirror and pass through a second lens group having a negative refractive power before they impinge on the image plane of this catadioptric partial objective.
The cutouts in the mirrors make a continuous ray trace possible but lead to a central obscuration in the illumination of the diaphragm plane. All rays which would impinge in the region of the mirror cutouts when reflected at the concave mirrors do not contribute to imaging and have to be vignetted via suitable measures. An obscuration of the aperture rays occurs. The rays which proceed from an object point are characterized as aperture rays and these rays lie within a bundle of rays delimited by the system diaphragm.
The first lens group, the two concave mirrors and the second lens group are arranged centered on a common optical axis defining a straight line. The aperture obscuration and the use of material for the lenses is further reduced because of the targeted use of aspheric surfaces.
By using with one or several partial objectives, the intermediate image, which is generated by this catadioptric objective, shows intense aberrations which are then compensated with the additional objectives in the total image. The catadioptric objective is to exhibit a chromatic overcorrection and/or overcorrection of the Petzval sum as a compensation for combination with dioptric partial objectives.
It is especially advantageous when the lens elements directly forward of the first concave mirror and/or the lens elements directly after the second concave mirror have an intense negative refractive power. The lens elements can be individual negative lenses or can be several lens elements which, however, have to exhibit an overall negative refractive power. It is advantageous when these lenses having negative refractive power or adjacent lenses have aspheric lens surfaces. These lenses with negative refractive power generate a chromatic overcorrection. The amount of the chromatic axial aberration for lenses having a refractive power &PHgr; and a marginal ray height h
RD
is proportional to h
RD
2
·&PHgr; and the lenses having negative refractive power close to the mirror have a low marginal ray height because of the required low aperture obscuration. For this reason, the refractive power of the lenses has to be that much higher in order to achieve an adequate chromatic overcorrection.
It is advantageous when the lenses of the object end field lens group and/or the image end field lens
Carl-Zeiss-Stiftung
Ottesen Walter
Schwartz Jordan M.
Thompson Timothy J
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