Abstract: A method for qualifying a mask for use in lithography is proposed. The method includes the following steps: a provision of an apparatus for qualifying a mask, the apparatus comprising an optical system and an evaluation and control device; a detection of at least one first phase difference of light at the mask by use of the optical system and the evaluation and control device; loading the mask; detecting at least one second phase difference of light at the mask by use of the optical system and the evaluation and control device; and implementing a comparison of the first phase difference with the second phase difference by use of the evaluation and control device.

Abstract: A reflective optical element (17), in particular for an illumination optical unit of a projection exposure apparatus includes: a structured surface (25a) that preferably forms a grating structure (29), and a reflective coating (36) that is applied to the structured surface (25a). The reflective coating (36) covers the structured surface (25a) discontinuously, and the reflective optical element (17) has at least one protective layer (37) that covers the structured surface (25a) continuously. Also disclosed are an illumination optical unit (4) for a projection exposure apparatus (1) including at least one reflective optical element (17) of this type, to a projection exposure apparatus (1) including an illumination optical unit (4) of this type, and to a method for producing a protective layer (37) on a reflective optical element (17) of this type.

Abstract: In a method for producing an optical element for an EUV projection exposure apparatus, a shaping layer (221) is applied onto a substrate (20) so as to have a surface roughness of at most 0.5 nm rms directly after the application of the shaping layer onto the substrate.