In microlithography, a projection exposure apparatus serves to expose a photosensitive material on a wafer with an image of structures of a mask or reticle. To this end, the projection exposure apparatus usually contains an illumination system and a projection lens. The illumination system produces a desired radiation distribution for illuminating the structures of the mask while the projection lens images the illuminated structures on the photosensitive material of the wafer with a very high resolution.
In order to meet the requirements of the imaging properties, it is necessary to produce and position the optical elements used in the projection exposure apparatus with extremely high precision. To this end, the measurement both of a complete imaging optical system and of individual optical elements currently can be performed during a production or a readjustment of projection exposure apparatuses.
Phase shifting interferometry techniques in particular, such as e.g. shear or shearing interferometry or point diffraction interferometry are used for measuring an imaging optical system of the projection exposure apparatus. DE 103 16 123 A1 describes such an apparatus for measuring the wavefront of a microlithographic projection lens by way of shearing interferometry.
Diffractive optical arrangements as so-called zero optics often are used as an interferometric measurement apparatus for precisely measuring the shape of a surface of an optical element. Here, the wavefront of a test wave is matched to an intended shape of the surface by way of a diffractive element, for example a computer-generated hologram (CGH). Deviations from the intended shape can be determined by superposing a reference wave on the test wave that is reflected at the optical element. By way of example, such a measurement apparatus is described in DE 10 2012 217 800 A1.
The ever increasing demands on imaging optical systems in microlithography lead to an ever greater complexity of these systems. By way of example, use is made of an increasing number of mirrors in microlithographic projection lenses with extreme ultraviolet radiation (EUV). However, as already mentioned above, the above-described measurement apparatuses and methods only can measure an imaging optical system as a whole or each optical element individually. When measuring a complex imaging optical system as a whole, it is difficult to assign the measured aberrations to individual optical elements. For the purposes of measuring the optical element on its own, it may be necessary to take apart the complete imaging optical system, which is connected to an ever greater time outlay on account of the increasing complexity.