Optical Investigation of Deep Ultraviolet Degradation of Pellicles

Abstract

As optical lithography goes deep into the ultraviolet (UV) range for higher resolution, high-energy photons cause many problems that are associated with the photomask (PM). Upon repeated deep-UV exposure, unwanted crystals or layers can form on the PM surface following the photoreaction of surrounding contaminants. Moreover, the pellicle that is a part of the PM can react with deep UV, although it is designed to show high transparency and radiation durability. Our pellicle showed an initial reduction in thickness and then an increase in absorption after extended exposure to the 193 nm line of an ArF excimer laser. Both phenomena were responsible for the transmission loss of a pellicle. Particularly, the transmission loss caused by the thickness reduction was related to the interference effect. Lithography simulation showed that the reduced transmission by the pellicle led to the critical dimension variation. Moreover, a non-uniform reduction of thickness near the edge of the exposed area suggested that the apodization effect might occur near the edge of patterns.