Design and realization of an interferometric microimaging system working in the EUV range

Abstract
A dedicated interferometric microimaging system has been designed and build to produce a magnified, diffraction limited image-interferogram of a coherently illuminated object, e.g. a laser plasma illuminated by a soft X-ray laser. The optical system consists of quasi-ellipsoïdal, multilayered mirror, illuminated at a 6 deg incidence from normal, associated with a grazing incidence Fresnel-bimirror interferometer. The working distance (optics to object) is [MATH]. The corrected aperture is 8x8 mm square. The magnification can be 10 or more, without loss of resolution. In a first realization, the ellipsoïdal mirror shape was approximated by a quasi-ideal torus, making the system not diffraction limited. The expected resolution (3-4 µm) was checked by imaging a metallic grid at 21.2 nm. In a second step, we have produced a better approximation of the ellipsoïd, which is diffraction limited at [MATH], under incidence 6.08 deg from normal. In this case, the actual corrected field is at least 0.9 mm, and the interferometric field is 0.6 mm perpendicular to fringes (these distances on object side). We discuss the design principles, leading to the choice of the various optical parameters, and the realization techniques.