| 摘要 |
A method of determining at least one parameter that is characteristic of the angular distribution of light illuminating an object in a projection exposure apparatus is described. This parameter may be, for example, a pupil asymmetry. The method comprises the step of inserting a filter element in or in close proximity of a pupil plane of an illumination system that is arranged between a light source and the object. The filter element has a filter function that varies in an azimuthal direction with respect to the optical axis of the illumination system. Then the intensity of the light in a plane downstream of the pupil plane is measured. After rotating the filter element around the optical axis by an angle Phi, the measurement of the intensity is repeated. From the filter function, the angle Phi and the measured intensities the parameter is measured.
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