Patent ID: 8009268

Claim:
An immersion lithography system, comprising: an optical source operable to produce light having a nominal wavelength; and an optical imaging system including an optical element in an optical path from said optical source to an article to be patterned by said optical imaging system, said optical element having a face adapted to contact a liquid including water occupying a space between said face and the article, said optical element including a material degradable by the liquid, said material including a fluoride of an alkaline earth metal; and a protective coating covering said degradable material at said face to protect said face from the liquid and being transparent to the light, stable when exposed to the light and stable when exposed to the liquid, said protective coating including an oxide of at least one element selected from the group consisting of hafnium (Hf), silicon (Si), zirconium (Zr), aluminum (Al) and scandium (Sc), said protective coating including a plurality of monolayers of said oxide, a first monolayer of said plurality of monolayers overlies and contacts said fluoride and each one of said plurality of monolayers other than said first monolayer overlies and contacts at least one other of said plurality of monolayers.