US Patent 7224440 Lithographic apparatus and device manufacturing method

A lithographic apparatus includes an illumination system configured to provide a radiation beam; a patterning device configured to pattern the radiation beam to form a patterned radiation beam; and a projection system configured to project the patterned radiation beam onto a substrate. An optical assembly includes multiple optical elements two-dimensionally arranged between a radiation source and the patterning device to create a predetermined angular distribution of the radiation beam. In order to improve the uniformity of the radiation beam the optical elements are selected from a predetermined number of optical elements having different shapes and/or sizes.