Negative photoresist for 157-nm microlithography: a progress report

The design of 157 nm photoresists is a daunting task since air, water, and most organic compounds are opaque at this wavelength. Spectroscopic studies(1) led to the observation that fluorinated hydrocarbons offer the best hope for the transparency that is necessary for the design of an effective 157nm photoresist, and these classes of materials have quickly become the prominent platforms for a variety of research activities in this field. Our approach to the design of the resist polymer requires identification of a backbone that tethers the functional substituents and provides basic mechanical properties, an etch barrier that provides REE resistance, an acidic group that permits solubility in tetramethylammonium hydroxide (TMAH) developer. Fluorocarbon polymers have been identified as promising resist candidates for 157nm material design because of their relatively high transparency at this wavelength.(1) Numerous authors have discussed negative photoresists over the years. There are many uses for such materials at various levels in a semiconductor device.(2) One such use is with complementary phase shift mask thus eliminating the need for a second exposure step. This paper reports our recent progress toward developing a negative 157nm resist materials based on fluoropolymers with crosslinkers that are transparent at 157nm. The authors will report on the synthesis of the polymers used in this work along with the crosslinkers and other additives used in the formulation of the photoresist. Imaging experiments at practical film thicknesses at 157nm with binary and strong phase shifting masks will be shown demonstrating imaging capabilities. Spectroscopic data demonstrating chemical mechanisms and material absorbance will be shown along with other process related information.