| 摘要 |
A projection optical system includes a first objective portion and an optical axis turn portion, each having an optical axis perpendicular to a reticle; an optical axis deflecting portion having an optical axis perpendicularly intersecting the above optical axis; and a second objective portion having an optical axis parallel to the above optical axis. An image-forming light beam from the reticle is returned toward the reticle by the first objective portion and a concave mirror provided in the optical axis turn portion to form a one-to-one image of the reticle. The returned image-forming light beam is led to the second objective portion through the optical axis deflecting portion to form a demagnified image on a wafer. During exposure, nitrogen gas is supplied through a gas changeover device to an illumination system unit, a reticle stage system unit, and a wafer stage system unit. During maintenance, air having ozone removed therefrom is supplied to these units through the gas changeover device. The projection optical system is constantly supplied with nitrogen gas.
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