Prospects for photolithography at 121 nm

Abstract

We have evaluated several subsystems that would be required for lithography at 121 nm: radiation sources, optics, ambient control, and resists. Dense plasma sources can generate significant power, >0.5 J of 121 nm radiation per pulse. An extensive survey of commercially available LiF, as well as specially grown LiF, shows that transmission losses are ∼20% per cm, a value which is at least 5× too high if LiF is to be used as lens material. At present it is not clear whether there is a fundamental limitation to LiF transparency at 121 nm, but the main source of absorption arises from lithium oxide or hydroxide in the crystal. Polymers, fluorinated or not, exhibit very high absorption at 121 nm, indicating that ultrathin resists or bilayer resists will have to be used at 121 nm. However, the photochemistry at 121 nm is probably not much different from that at longer wavelengths. The resists are sensitive, ∼1 mJ/cm2 required dose, and they have high contrast at 121 nm. Finally, we have also performed preliminary patterning experiments. Using contact printing with a phase shifting mask we have printed 45 nm features in an ultrathin photoresist.