Optical: systems and elements – Lens – With support
Reexamination Certificate
1999-11-22
2001-10-23
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With support
Reexamination Certificate
active
06307688
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an optical system, in particular a projection-illumination unit used in microlithography, of the type defined in more detail in the preamble of claim
1
.
EP 0 678 768 A2 has described an optical system of the type described in the introduction, in which stepping and scanning processes are employed and only a narrow, slot-like strip is transmitted from a mask to a waver. To illuminate the entire field, a reticle and the waver are displaced laterally (scanning).
However, a drawback of the optical system according to this prior art is that the slot geometry results in a rotationally asymmetric illumination replication especially on lenses which are close to the waver. This means that the temperature distribution which results from the inevitable heating of the lens is likewise rotationally asymmetrical on the lens, and therefore, owing to the linear correlation between refractive index and temperature and owing to thermal expansion, image errors, such as for example astigmatism, arise on the optical axis.
A further problem with an optical system of this nature is the so-called compaction effect, i.e. aging caused by the irradiation, and this effect likewise leads to image errors.
In EP 0 678 768 A2, it is proposed to use a lens as a “final control element”, in order to correct the image error which is produced by uneven heating of the lens. For this purpose, forces which act in the radial direction are allowed to act on the lens. However, a drawback of this solution is that only compressive forces are generated, resulting in an asymmetric change in thickness.
EP 0 660 169 A1 has described a projection-illumination unit which is used in microlithography and in which objectives are provided with correction elements. These include a pair of lenses which can be rotated about the optical axis. In this case, the refractive power from the shape of the lens is altered by superimposing a cylindrical meniscus shape on a spherical lens.
SUMMARY OF THE INVENTION
The present invention is based on the object of providing an optical system of the type mentioned at the outset in which image errors, such as those which are formed, for example, by uneven temperature distribution and/or the compaction effect, can be corrected or at least significantly reduced by means of controlled astigmatism.
According to the invention, this object is achieved by means of the features listed in the defining part of claim
1
.
In contrast to the prior art, not only compressive forces, which result only in an asymmetric change in thickness, are produced, but also, by means of controlled tensile and/or compressive forces, the optical element, e.g. a lens, is deformed, this deformation being selected in such a way that image errors which arise can be compensated to a very substantial extent.
In one configuration according to the invention, it is possible, for the force-displacement transmission, to provide a lever transmission with a lever, at least one actuator acting on one end of the lever and the lever, at another end which is directed toward the inner ring, acting on the inner ring, in such a manner that a force which is parallel to the optical axis is generated on the inner ring.
By suitably arranging or distributing the tensile and compressive forces, which are accordingly distributed over the circumference, it is possible to produce, for example, a dual ripple for the inner ring which supports the lens, and therefore also for the lens. This is the case in particular if the tensile and compressive forces are in each case 180° opposite one another, with tensile and compressive forces in each case alternating at 90° intervals and the inner ring being fixedly connected to the mount by means of connections which are in each case located between these forces.
The astigmatism which is produced in this way makes it possible to substantially compensate for image errors. In practice, the optical element, such as for example a lens, assumes a saddle-like form with the deformation produced in this way.
If necessary, it is also possible to provide “over-compensation”, in order in this way to correct additional compensation for production defects or image errors from other optical elements in the system. In this way, it is possible, overall, to provide compensation for an entire objective or an illumination unit using simple means.
Various energy generators for generating the tensile and compressive forces are possible; generally, in order to increase the accuracy or to provide an appropriate precision adjustment, a lever transmission will be employed. As simple tensile and compressive force generators, it is possible to use actuators in the form of piezo-units or of hydraulic units or, advantageously, mechanical tensile and compressive units which can be arranged in the mount in a simple structure. This measure allows the mount at the same time to be used to seal the hydraulics or to accommodate the tensile and compressive units.
REFERENCES:
patent: 3957359 (1976-05-01), Meginnis
patent: 4017878 (1977-04-01), Hagiwara
patent: 4161120 (1979-07-01), Cloarec
patent: 5249082 (1993-09-01), Newman
patent: 5521764 (1996-05-01), Balogh et al.
patent: 5774274 (1998-06-01), Schachar
patent: 5966248 (1999-10-01), Kurokawa
patent: 0 660 169 A1 (1994-12-01), None
patent: 0 678 768 A2 (1995-04-01), None
Opto-mechanical systems design/Paul R. Yoder, Jr., 2nd Edition, rev. and expanded, vol. 35, 1992.
Becker Jochen
Merz Erich
Carl-Zeiss-Stiftung
Epps Georgia
O'Neill Gary
Welsh & Katz Ltd.
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