Microlithography is used for producing microstructured components, such as integrated circuits or LCDs, for example. The microlithography process is carried out in a so-called projection exposure apparatus having an illumination device and a projection lens. The image of a mask (reticle) illuminated via the illumination device is in this case projected via the projection lens onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In projection lenses designed for the EUV range, i.e. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable light-transmissive refractive materials, mirrors are used as optical components for the imaging process.
As EUV radiation source the use of a free electron laser is known besides plasma sources and synchrotrons. The lasers have the advantage, inter alia, that the generated radiation is restricted to the desired EUV radiation, i.e. the desired wavelength range, and the contaminations that arise in the case of plasma sources owing to the target materials involved there are also avoided.
During the operation of a projection exposure apparatus it is desirable to set specific polarization distributions in the pupil plane and/or in the reticle in a targeted manner in the illumination device for the purpose of optimizing the imaging contrast and also to be able to carry out a change in the polarization distribution during the operation of the projection exposure apparatus.
In principle, in a free electron laser, polarized radiation is generated by the use of an undulator arrangement including a plurality of magnets for generating EUV light by deflecting the electron beam. FIG. 7A and 7B in each case show a possible construction of a free electron laser including an electron source 710 for generating an electron beam 705, an accelerator unit 720 for accelerating the electron beam 705, and an undulator arrangement 700 including a plurality of magnets for generating EUV light by deflecting the electron beam 705, wherein the undulator arrangement 700 here includes two undulators 701, 702. Since the polarization of the generated radiation is predefined by the concrete arrangement of the magnets of the undulator arrangement 700, in principle in accordance with FIGS. 7A,7B with the use of an undulator arrangement 700 including two undulators 701, 702 it is possible to generate light beams S1, S2 having mutually different polarization directions (e.g. horizontally and vertically polarized light), wherein as indicated in FIG. 7B it is also possible to realize a spatial separation of the respective beam paths e.g. by tilting the undulators 701, 702 in relation to one another (relative to the respective direction of propagation of the electron beam within the relevant undulator).
Even if the principle described above with reference to FIGS. 7A and 7B fundamentally allows the setting of different polarized illumination settings (including the generation of effectively polarized radiation upon the superimposition of horizontally and vertically polarized light), here in practice the problem occurs that, depending on the desired one of the polarized illumination settings the light of the undulator having the polarization state respectively not desired is not used or is lost, as a result of which the performance of the projection exposure apparatus is impaired.
With regard to publications regarding changing the polarization distribution in projection exposure apparatuses designed for the EUV range, merely by way of example reference is made to DE 10 2008 002 749 A1, US 2008/0192225 A1, WO 2006/111319 A2 and U.S. Pat. No. 6,999,172 B2.