157-nm pellicles for photolithography: mechanistic investigation of the deep-UV photolysis of fluorocarbons

The advance of 157 nm as the next photolithographic wavelength has created a need to for transparent and radiation durable polymers for the use as pellicles. The most promising materials for the pellicles are fluorinated polymers, but the currently available fluorinated polymers undergo photodegradation and/or photodarkening upon exposure to 157 nm irradiation. To understand the mechanism of the photodegradation and photodarkening of fluorinated polymers, mechanistic studies on the photolysis of liquid model fluorocarbons such as, perfluoro butylethyl ether and perfluoro-2H-3-oxa-heptane, were performed employing UV, NMR, FTIR, GC, and GC/MS analysis. All hydrogen containing compounds showed decreased photostability compared to the fully perfluorinated compounds. Irradiation in the presence of atmospheric oxygen showed reduced photodarkening compared to deoxygenated samples. Irradiations were performed at 157 nm, 172 nm, 185 nm, and 254 nm and showed only minor wavelength dependence. Mechanisms for photodegradation of the fluorocarbons were proposed, where Rydberg excited states are involved.