1. Field of the Invention
The invention relates to a lens system. More specifically the invention relates to a projection lens system for semiconductor lithography with at least one exchangeable optical element, in particular the final optical element in the beam direction in the form of an end plate.
2. Description of the Related Art In the case of lens systems in semiconductor lithography, it is necessary, from time to time, to make subsequent corrections in order to eliminate aberrations or to increase the imaging accuracy. For this reason, it is possible for individual optical elements, e.g. the end plate, to be designed in an exchangeable manner, in order that, for example, lens-system aberrations may be optimized. The same also applies in the case of an optical element being damaged or subjected to wear. This applies, in particular, to the end plate in a projection lens system. In the case of such an exchange, however, the intention is to achieve reproducibility in order that new errors do not slip in.
For this purpose, U.S. Pat. No. 5,973,863 has already disclosed the practice of connecting the end plate of a lens system in semiconductor lithography in an exchangeable manner to the mount in order for it to be possible to correct spherical aberrations or other aberrations of the projection system. A screw-connection between the plate and the mount is proposed for this purpose.
In order to achieve exchangeability of an optical element in a lens system, it is already generally known, in principle, to provide a method of connection other than an adhesive connection between the optical element and its mount. For this purpose you are referred, for example, to the earlier application 199 29 403.8, in which an end element is connected to the adjacent optical element by wringing.