High-RI resist polymers for 193 nm immersion lithography

A critical aim within the field of 193 nm immersion lithography is the development of high refractive index immersion fluids and resists. Increases in the refractive index (RI) of the immersion fluid will result in increases in the numerical aperture and depth of focus. Increasing the RI of resist polymers will improve exposure latitude for the process. A challenge for increasing the RI of resist polymers is to do so without detrimentally affecting other properties of the polymer such as transparency, line edge roughness, adhesion and plasma etch resistance. It is well known in the literature that introducing sulfur, bromine or aromatic groups into a polymer structure will increase its RI. However, due to the relatively strong absorption of phenyl groups at 193 nm these groups have to be avoided. Furthermore, the use of bromine poses problems associated with contamination of the silicon wafer. Hence, in this study, a systematic approach has been used to increase the sulfur content of 193 nm type resist polymers, by synthesis of sulfur-containing monomers and by performing bulk modifications of the polymer. The effect of sulfur content on the RI at 193 nm was then investigated. A broad study of the relationship between molecular structure and RI dispersion from 250-180 nm has also been undertaken, and conclusions drawn using QSPR methodologies. Finally, the effect of sulfur content on other lithography parameters, such as transparency, adhesion and plasma etch resistance, was also evaluated