Microlithographic system

Claim 1

In a microlithographic system for projecting a reduced image of a reticle onto the surface of a semiconductor wafer for creating circuit patterns thereon, an optical system comprising: a reticle holder; aligned with said reticle holder, a wafer stage; on the axis between said reticle holder and wafer stage, a lens for projecting an image of a reticle held in said holder onto a wafer held on said stage, said lens comprising a plurality of spaced, axially-aligned, optically-transmissive, refractor elements and tubular lens holder means for supporting and enclosing said elements, the holder means being essentially sealed to the two endmost of said elements, the interior spaces between successive elements being vented to each other; first port means communicating with the interior of said holder means between one endmost element and the respective adjacent element; second port means communicating with the interior of said holder means between the other endmost element and the respective adjacent element; means for applying a controlled flow of helium gas to said first port means; and means for venting helium from said holder through said second said port means, the space between the lens and the reticle holder and the space between the lens and wafer stage being open to the atmosphere and subject to barometric changes.