Optical: systems and elements – Compound lens system – With curved reflective imaging element
Utility Patent
1999-03-05
2001-01-02
Nguyen, Thong (Department: 2872)
Optical: systems and elements
Compound lens system
With curved reflective imaging element
C359S357000, C359S727000, C359S731000
Utility Patent
active
06169627
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a catadioptric microlithographic reduction objective having two concave mirrors facing each other. Each of the concave mirrors has an axial-symmetric configuration and a central bore.
BACKGROUND OF THE INVENTION
Catadioptric reduction objectives for deep UV microlithography are known. A diagonally positioned polarizing beam splitter and a &lgr;/4 plate are required in the embodiment disclosed in U.S. patent application Ser. No. 08/845,384, filed Apr. 25, 1997, now U.S. Pat. No. 5,800,891 and the publications cited therein. The beam splitter and the &lgr;/4 plate present a manufacturing problem in the deep UV wavelength range. Furthermore, a deflection of the optical axis by approximately 90° is mandatory so that a second deflection is provided in order to maintain parallelism of reticle and wafer.
Other catadioptric systems are asymmetrically configured. These systems can, for example, be of the Dyson type or as shown in European patent publication 0,581,585.
U.S. Pat. No. 5,488,229 discloses a catadioptric microlithographic reduction objective which is configured to be axially symmetrical and has two concave mirrors facing toward each other. In principle, a central bore is given but this does not present a problem in view of the ever increasing significance of annular aperture illumination.
Both concave mirrors are configured as mangin mirrors and the second one of the mirrors operates in its center region as a lens. Thereafter, only the iris diaphragm and the wafer are arranged. At &bgr;=0.1, NA=0.6 and lambda—193 nm, five lenses and two mangin mirrors are sufficient. However, nothing is stated as to the image field size and as to the size of the center bore. With image field sizes, as they are obtained with the invention, mangin mirrors having a diameter up to one meter would be required. It is not seen that quartz glass or even another lens material could be made available in these dimensions in the mandatory quality for deep UV microlithography.
U.S. Pat. No. 5,031,976 discloses a similar configuration. However, the second mirror is planar and a separate thick lens is provided between the mirrors.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a reduction objective which provides an image field having a size corresponding to the production requirements of wafer-stepper machines and which has a suitable error correction. With the invention, a construction is provided which facilitates manufacture and avoids especially large and thick lenses.
The catadioptric microlithographic reduction objective of the invention defines an optical axis, provides a light path and forms an image of an object. The catadioptric microlithographic reduction objective includes: two concave mirrors mounted on the optical axis facing each other; each of the concave mirrors having a configuration symmetrical to the optical axis and having a central bore; a plurality of lenses being arranged along the optical axis toward the image downstream of the mirrors on the light path; and, the concave mirrors and the plurality of lenses being configured and arranged to provide a microlithographic reduction.
Within the plurality of lenses, the light beam is again reduced to a diameter which is considerably less than the mirror diameter so that correction lenses having reasonable diameters can be utilized.
According to another feature of the invention, the lenses at the object end are positioned forward in the intermediate space between the mirrors in the region of the central bore. Thus, the object end lens system projects into the mirror intermediate space in the region of the central bore thereby positively affecting chromatic aberrations.
According to still another feature of the invention, the light path between the concave mirrors is free of lenses, that is, in the region of the largest beam diameter. Accordingly, the objective of using small lenses is achieved.
An intermediate image is provided after the mirrors. In this way, surfaces conjugated to the mirrors result in the space between the intermediate image and the image. In the vicinity of these surfaces, elements for correcting the image errors, which are caused by the mirrors, can be optimally mounted. A meniscus pair is suitable for this purpose.
According to another feature of the invention, a corrective element is provided. More specifically, a convex air lens is included between two lenses and, in this way, good correction for astigmatism is achieved.
According to other features of the invention, the image field is greater than 20 mm in diameter and the numerical aperture at the image end is greater than 0.60. Also, the field curvature is less than 0.06 &mgr;m and the chromatic correction for a bandwidth of at least 6 pm (picometer) is achieved. With these features, the advantageous quality is provided by the invention, namely, with a large image field associated with a large numeric aperture, with low image field curvature and with adequate chromatic bandwidth.
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Carl-Zeiss-Stiftung
Nguyen Thong
Ottesen Walter
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