1. Field of the Invention
The invention concerns an illumination system for wavelengths ≦100 nm, wherein the illumination system has an object plane and a field plane. In illumination systems ≦100 nm, the problem exists that light sources of such illumination systems emit radiation that can lead to an undesired exposure of the light-sensitive object in the wafer plane. Furthermore optical components of the exposure system, such as, for example, the multilayer mirror, can be heated up in this way.
In order to filter out the undesired radiation, transmission filters are used in illumination systems for wavelengths ≦100 nm. Such filters have the disadvantage of high light losses. In addition, they can be disrupted very easily by heat stress.
The object of the invention is to provide an illumination system for wavelengths ≦100 nm, particularly in the EUV range, in which the above-named disadvantages can be avoided.
According to the invention, this object is solved by an illumination system that has at least one grating element and at least one physical diaphragm in a diaphragm plane. The physical diaphragm is situated in the beam path from the object plane to the field plane after the grating element.
2. Description of the Prior Art
Grating elements, for example, reflection gratings, particularly echelette gratings, which are also known as blazed gratings, have been known for a long time from monochromator construction for synchrotron radiation sources. For these elements good experiences, particularly with very high fluxes, were made.
With respect to the use of diffraction gratings in monochromators, reference is made to the following publications, whose disclosure content is incorporated to the full extent in the present Application:                H. Petersen, C. Jung, C. Hellwig, W. B. Peatman, W. Gudat: “Review of plane grating focusing for soft x-ray monochromators”, Rev. Sci. Instrum. 66(1), January 1996.        M. V. R. K. Murty: “Use of convergent and divergent illumination with plane gratings”, Journal of the Optical Society of America, Vol. 52, No. 7, July 1962, pp. 768–773.        T. Oshio, E. Ishiguro, R. Iwanaga: “A theory of new astigmatism and coma-free spectrometer”, Nuclear Instruments and Methods 208 (1993) 297–301.        