Patent ID: 7477355

Claim:
A projection exposure apparatus for transferring an image of a patterned reticle onto a substrate, the projection exposure apparatus comprising: a) an illumination optical system for generating and directing an exposure light beam onto said reticle, and b) a projection optical system provided between said reticle and said substrate; said projection optical system comprising a plurality of imaging mirrors, each having a mirror support made of a support material; wherein each said support material is subject to thermal expansion during projection according to a respective coefficient of thermal expansion; said thermal expansion inducing imaging aberrations at substrate level, wherein said support materials are selected based on their respective coefficients of thermal expansion to minimize an aberration merit function by mutual compensation of contributions of said mirrors to at least one type of thermally induced aberration; and wherein said aberration merit function indicates the overall amount of said at least one type of said thermally induced aberrations; wherein said support materials are mutually different and have, when heated during exposure, mutually different coefficients of thermal expansion.