Patent ID: 8797507

Claim:
An illumination system having a pupil surface, the illumination system comprising: a mirror array comprising a plurality of mirror elements, each mirror element comprising a mirror support and a reflective coating, and each mirror element being configured to direct light produced by a primary light source toward the pupil surface; and a temperature control device, wherein: each mirror element comprises structures that have different coefficients of thermal expansion; for each mirror element, the structures are fixedly attached to each other; the temperature control device comprises a radiation system configured to selectively direct radiation to target areas on the reflective coating and is configured to variably modify a temperature distribution within the structures of the mirror elements to individually change a shape of each mirror element; the mirror elements have concave mirror surfaces, and the temperature control device is configured to modify a temperature distribution within the structures so that the mirror elements have different focal lengths in two orthogonal planes; and the illumination system is configured to be used in a microlithographic projection exposure apparatus.