Catadioptric reduction lens

Details Catadioptric reduction lens Catadioptric reduction lens

2002-01-23

2004-07-20

Epps, Georgia (Department: 2873)

Optical: systems and elements

Lens

With reflecting element

C359S633000

Reexamination Certificate

active

06765729

ABSTRACT:

The following disclosure is based on German Patent Application No. 101 04 177.2 filed on Jan. 24, 2001, which is incorporated into this application by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a catadioptric projection lens for projecting a pattern from an object plane onto an image plane.
2. Description of the Related Art
Such projection lenses are used in projection exposure systems for producing semiconductor devices and other microdevices, in particular in wafer scanners and wafer steppers. They are used to project patterns of photo masks or reticle plates (in the following simply called masks or reticles) onto an object coated with a photosensitive layer. The projection is performed with highest resolution and in reduced scale.
In order to create increasingly fine structures, it is necessary to increase the numerical aperture (NA) of the projection lens on one hand and to use increasingly shorter wavelengths on the other hand, preferably ultraviolet light with wavelengths of less than approx. 260 nm.
In this wavelength range there are only few sufficiently transparent materials for producing the optical components, in particular synthetic quartz glass and fluoride crystals, such as calcium fluoride, magnesium fluoride, lithium calcium aluminum fluoride, lithium strontium aluminum fluoride, barium fluoride, lithium fluoride, or the like. Since the Abbé constants of the available materials are relatively close together, it is difficult to provide pure refractive systems with sufficient correction of color aberrations (chromatic aberrations). In principle, this problem could be solved by using pure reflective systems. However, the fabrication of such mirror systems is costly.
Considering the problems mentioned above, catadioptric systems are preferable for projection lenses of very high resolution. In catadioptric systems refracting and reflecting components, therefore in particular lenses and mirrors, are combined.
When using mirror surfaces for projection, it is advantageous to use beam splitters to achieve obscuration-free and vignette-free images. There exist systems with geometrical beam splitters as well with physical beam splitters. A system with a geometrical beam splitter that uses two deviating mirrors is shown in EP 0 989 434 (corresponding to the U.S. Ser. No. 09/364382). Systems with a geometrical beam splitter have the disadvantage that they must necessarily be off-axis systems. By using a physical beam splitter, however, on-axis systems can be realized.
A system with a physical beam splitter and an intermediate image is known from EP-A-0 475 020 (corresponding to U.S. Pat. No. 5,052,763). This system has at least one catadioptric entry system and one dioptric exit system. The mask to be projected rests directly on a beam splitter, designed as a beam splitter cube (BSC). With the help of the beam splitter, part of the light reflected by the catadioptric system is diverted to the dioptric system. With the object to be projected resting directly on the beam splitter, the correction possibilities of the total system are restricted. Furthermore, this contact procedure has extremely high demands with respect to material quality and can cause mechanical problems due to the lack of working distance.
From EP-A-0 350 955 (corresponding to U.S. Pat. No. 4,953,960), a catadioptric projection lens without intermediate image is known. This projection lens system consists of a first lens group between the object plane and a physical beam splitter, a second lens group between the physical beam splitter and a concave mirror, and a third lens group between the physical beam splitter and the image plane. The lens group between the beam splitter and the concave mirror is supposed to correct comas of low degrees, spherical aberrations of the mirror, and the Gauss' error.
From DE-A-42 03 464 (corresponding to U.S. Pat. No. 5,402,267), a catadioptric projection lens with physical beam splitter and without intermediate image is known that permits high rear numerical aperture of at least 0.5 with a favorable construction and low adjustment sensitivity. The system distinguishes itself mainly by the fact that there is no lens group between the concave mirror and the beam splitter and that the concave mirror has a considerable reduction effect, i.e. a strongly reducing magnification. The correction of the chromatic longitudinal ray aberration (CHL) is mainly achieved with a strongly convergent ray trajectory in the beam splitter cube and may cause total achromatization of the chromatic longitudinal ray aberration. Typically the ray trajectory in front of the mirror, i.e. in the first passage through the beam splitter, is nearly collimated, while the ray trajectory behind the mirror, i.e. in the second passage through the beam splitter is normally strongly convergent. The system aperture is preferably located where the concave mirror is and is defined by the mirror rim. The aperture may also be defined on the mirror-side bounding surface of the beam splitter or between mirror and beam splitter. The strongly convergent ray trajectory after the concave mirror has the further advantage that only little positive focal power is needed after the beam splitter and that the beam heights are relatively small in this area so that negative effects on the chromatic aberration due to large beam heights in this area can be avoided. Projection lenses with these or comparable constructional and functional characteristics are called type I for the purpose of this application.
With these advantages, type I lenses have the disadvantage that the radiation reaches the beam splitter surface convergent, in particular in the second passage after being reflected by the concave mirror, causing a very large angle of incidence range. This has higher demands with respect to the quality of the beam splitter layer. In addition, the strong convergence of the ray trajectory after the concave mirror leaves very little room for lenses behind the beam splitter and thus little room for correctional measures. A further increase of the rear numerical aperture would require enlarging the beam splitter cube so that the image plane would have to be even closer to the beam splitter. For this reason, projection lenses of type I are also known as aperture limited.
Essentially similar problems also occur for other projection lenses that are constructed according to type I as far as the build-up and the ray trajectory is concerned. Among these are the projection lenses shown in the US patents U.S. Pat. Nos. 6,118,596, 6,108,140, 6,101,047. Large angles of incidences on the beam splitter surfaces may also occur in systems that create an intermediate image, shown for example in U.S. Pat. No. 5,808,805 or U.S. Pat. No. 5,999,333.
From U.S. Pat. No. 5,771,125, a catadioptric projection system with physical beam splitter and without an intermediate image is known where the rays are slightly divergent during their first passage through the beam splitter layer and are collimated during the second passage after being reflected by the concave mirror. This is to avoid deterioration of the image quality due to the dependence of the beam splitter layer's reflectivity on the angle of incidence. The collimation of the reflected light is achieved by keeping the focal power of the mirror group containing the concave mirror relatively low. In the system of EP-A-0 602 923 (corresponding to U.S. Pat. No. 5,715,084), however, a positive lens is provided in front of the physical beam splitter in order to collimate the rays that reach the beam splitter layer during the first passage. After being reflected by the concave mirror, it is convergent.
In order to minimize the angle of incidence on the beam splitter layer, DE-A-44 17 489 (corresponding to U.S. Pat. No. 5,742,436) suggests positioning at least one convergent lens on the object-side in front of the physical beam splitter in a catadioptric projection system without intermediate image in order to make the ray arriving at the beam splitter la

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