Deep-UV immersion interferometric lithography

Abstract

The limit of optical lithography is the minimum pitch between features. This pitch limit is given by Λ=λ/2nNA), where λ is the optical wavelength, n is the refractive index of the final medium of the optical system which is typically air (n = 1), and NA is the numerical aperture of the exposure device. A great deal of work has been done to decrease exposure wavelengths and increase the NA of exposure tools, however, until recently very little effort has been applied towards an immersion medium with n > 1. This paper examines extending minimum pitches through the use of such media. Exposures are at a wavelength of 213 nm, close to the current state-of-the-art 193-nm lithography node. The possible limits of lithography are examined using 193-nm resists exposed in air and comparing these limits to those possible when implementing liquid immersion lithography (LIL) exposures. Two immersion liquids were examined: deionized water, and Krytox a Perfluoropolyether (PFPE) oil. These liquids were compatible with 193-nm resist. A resolution enhancement factor of 28% for Krytox and of 41% for DI water was demonstrated. Images of good dense lines with a half pitch of 54 nm are presented.