Elucidation of Steplike Output Energy Decrease Observed in ArF Excimer Laser for Microlithography

Abstract

A steplike decrease in the output energy of an ArF excimer laser for microlithography is caused by the laser light absorption in the laser gas. To identify the absorbing gas component, the light absorption characteristics of the laser gas inside the discharge chamber were investigated under various gas conditions using a continuous xenon light source in the ultraviolet region and an ArF excimer laser at 193 nm. As a result, the oxygen-fluorine compound F x O y generated in the laser discharge was identified as the absorbing molecule, because a strong steplike absorption of the 193 nm laser light was only observed when oxygen and fluorine were added together to the discharge gas. The lifetime of this absorbing molecule was examined against gas temperature during the operation of the laser discharge. The data obtained were useful in setting the best operational conditions for the ArF excimer laser for microlithography.