The invention relates to a catadioptric projection lens for imaging a pattern arranged in an object plane onto an image plane.
Projection lenses of that type are employed on projection exposure systems, in particular wafer scanners or wafer steppers, used for fabricating semiconductor devices and other types of microdevices and serve to project patterns on photomasks or reticles, hereinafter referred to generically as “masks” or “reticles,” onto an object having a photosensitive coating with ultrahigh resolution on a reduced scale.
In order create even finer structures, it will be necessary to both increase the image-end numerical aperture (NA) of the projection lens to be involved and employ shorter wavelengths, preferably ultraviolet light with wavelengths less than about 260 nm.
However, there are very few materials, in particular, synthetic quartz glass and crystalline fluorides, such as calcium fluoride, magnesium fluoride, barium fluoride, lithium fluoride, lithium-calcium-aluminum fluoride, lithium-strontium-aluminum fluoride, and similar, that are sufficiently transparent in that wavelength region available for fabricating the optical elements required. Since the Abbé numbers of those materials that are available lie rather close to one another, it is difficult to provide purely refractive systems that are sufficiently well color-corrected (corrected for chromatic aberrations). Although this problem might be solved by employing purely reflective systems, fabricating such mirror systems involves considerable expense and effort.
In view of the aforementioned problems, catadioptric systems that combine refracting and reflecting elements, i.e., in particular, lenses and mirrors, are primarily employed for configuring high-resolution projection lenses of the aforementioned type.
Whenever imaging reflective surfaces are involved, it will be beneficial to employ beam-deflection devices whenever images free of obscurations and vignetting are to be achieved. Both systems having geometric beamsplitters having one or more fully reflecting deflecting mirrors and systems having physical beamsplitters are known. Additional plane mirrors may also be employed for folding the optical path. However, folding mirrors are usually employed only in order to allow meeting space-occupation requirements, in particular, in order to allow orienting the object and image planes parallel to one another. However, folding mirrors are not absolutely necessary from the optical-design standpoint.
Employing systems having a physical beamsplitter in the form of, for example, a beamsplitter cube (BSC), has the advantage that it allows configuring on-axis systems. Polarization-selective reflective surfaces that either reflect or transmit incident radiation, depending upon the orientation of its polarization axis, are employed in such cases. The disadvantage of employing such systems is that hardly any suitable transparent materials are available in the desired, large volumes. Moreover, fabricating optically active beamsplitter coatings situated within beamsplitter cubes may prove extremely difficult, particularly in cases involving large angles of incidence on the reflective surfaces involved and/or radiation that is incident over a broad range of angles of incidence.
Some of the disadvantages of systems equipped with beamsplitter cubes may be avoided by employing systems having one or more deflecting mirrors in their beam-deflection devices. However, such systems have the inherent disadvantage that they are necessarily off-axis systems, i.e., systems with off-axis object fields.
A catadioptric reduction lens of that type is disclosed in European Patent EP 0 989 434, which corresponds to U.S. Ser. No. 09/364,382. Such lenses have a catadioptric first lens section having a concave mirror and a beam-deflection device arranged between their object plane and their image plane and a dioptric second lens section that is arranged following that first lens section. Their beam-deflection device, which is configured in the form of a reflecting prism, has a first reflective surface for deflecting radiation coming from their object plane to the concave mirror and a second reflective surface for deflecting radiation reflected by the concave mirror to their second lens section, which contains exclusively refractive elements. A positive lens whose refractive power is dimensioned such that the concave mirror lies in the vicinity of the pupil is arranged between the object plane and the first reflective surface. The catadioptric first lens section creates a real intermediate image that is situated a short distance behind the second reflective surface and far in front of the first lens of the second lens section. This intermediate image is thus readily accessible, which may be taken advantage of for the purpose of, e.g., installing an illuminated-field stop. Large maximum angles of incidence, particularly at the first reflective surface, impose stringent demands on the reflective coatings used on the mirrors in order that they will have largely uniform reflectances for all incident radiation.
Another reduction lens that has a beam-deflection device having a deflecting mirror is disclosed in U.S. Pat. No. 5,969,882, which corresponds to European Patent EP A 0 869 383. In the case of this particular system, the deflecting mirror is arranged such that light coming from the lens' object plane initially falls on the concave mirror of its first lens section, which reflects it to the beam-deflection device's deflecting mirror, which reflects it to another reflective surface that deflects it toward the lens of the entirely dioptric second lens section. The elements of the first lens section that are utilized for creating the intermediate image are configured such that the intermediate image lies close to the beam-deflection device's deflecting mirror. The second lens section refocuses the intermediate image onto the image plane, which may be oriented parallel to the object plane, thanks to the reflective surface that follows the intermediate image in the optical train.
U.S. Pat. No. 6,157,498 depicts a similar configuration whose intermediate image lies on, or near, the reflective surface of its beam-deflection device. Several lenses of its second lens section are arranged between the latter and a deflecting mirror located in its second lens section. An aspheric surface is also arranged in the immediate vicinity of, or near to, its intermediate image, and is included exclusively for the purpose of correcting for distortions, without affecting other imaging errors.
A projection lens having a reducing catadioptric lens section and an intermediate image situated in the vicinity of the deflecting mirror of a beam-deflection device is depicted in German Patent DE 197 26 058.
U.S. Pat. No. 5,999,333 depicts another catadioptric reduction lens having deflecting mirrors for which light coming from its object plane, after transiting a lens group having a positive refractive power, initially strikes a concave mirror that reflects it to the beam-deflection device's sole reflective surface. The intermediate image created by its catadioptric lens section lies close to this reflective surface, which reflects light incident thereon to a dioptric second lens section that images the intermediate image onto its image plane. Both its catadioptric lens section and its dioptric lens section create reduced images.
A similarly configured lens for which the intermediate image created by its catadioptric lens section also lies close to its beam-deflecting device's sole deflecting mirror is depicted in Japanese patent application JP-A-10010429. The surface of the lens of the dioptric lens section that follows that lies closest to the deflecting mirror is aspherical in order that it may make a particularly effective contribution to correcting for distortion.
Other lenses having off-axis object fields, a geometric beamsplitter, and a single concave mirror and single intermediate image followed by a dioptric lens section are known from U.S. Pat. No. 5,052,783 A, U.S. Pat. No. 5,691,802, and European patent application EP 1 079 253 A.
Systems whose intermediate image lies near, or on, a reflective surface may be compactly designed. They also allow keeping corrections for the field curvatures of these off-axis illuminated systems small, which simplifies correcting for distortion. However, intermediate images that fall on a reflective surface can be problematical, since, in that case, flaws on the reflective surface will be sharply imaged onto the image plane, and since extremely high irradiance levels may occur at the reflective surface.
Catadioptric systems that have beamsplitters generally have a group of lenses that is transited twice, i.e., are transited by light on its way from their object field to their concave mirror and its way from their concave mirror to their image field. In U.S. Pat. No. 5,691,802, it was proposed that this lens group have positive refractive power, which supposedly would allow employing a concave mirror with a smaller diameter. A system having a single positive lens that is transited twice situated near a deflecting mirror of its beamsplitter is disclosed in U.S. Pat. No. 6,157,498.
The high prices of the materials involved and limited availability of crystalline calcium fluoride in sizes large enough for fabricating large lenses represent problems, particularly in the field of microlithography at 157 nm for very large numerical apertures, NA, of, for example, NA=0.80 and larger. Measures that will allow reducing the number and sizes of lenses employed and simultaneously contribute to maintaining, or even improving, imaging fidelity are thus desired.