Method for determining an imaging aberration contribution of an imaging optical unit for measuring lithography masks

Abstract

Determining an imaging aberration contribution of an imaging optical unit for measuring lithography masks involves firstly focus-dependently measuring a 3D aerial image of the imaging optical unit as a sequence of 2D intensity distributions in different measurement planes in the region of and parallel to an image plane of an imaging of an object by use of the imaging optical unit. A spectrum of a speckle pattern of the 3D aerial image is then determined by Fourier transformation of the measured 2D intensity distributions having speckle patterns. For a plurality of spectral components in the frequency domain, a focus dependence of a real part RS(z) and an imaginary part IS(z) of said spectral component is then determined. From the determined values of the focus dependence of the real part RS(z) and the imaginary part IS(z), a contribution made to the speckle pattern spectrum by a mask structure, which contribution is to be eliminated, is then separated from an imaging aberration contribution made to the speckle pattern spectrum by the imaging optical unit. The imaging aberration contribution is then represented. This results in a method for determining the imaging aberration contribution of the imaging optical unit having little additional time expenditure in comparison with the measurement time on the respective lithography mask.