Increasing the Sensitivity of Nonchemically Amplified Resists by Oxime Sulfonate-Functionalized Polystyrene

Nonchemically amplified resists (n-CARs) based on oxime sulfonate-functionalized polystyrene (PSOS) were designed and prepared. The component ratios of the oxime sulfonate (OS) group in the polymer were well-controlled by changing the molar ratios of monomers, affording PSOSx (x = 100, 90, 60, and 40) polymers with oxime sulfonate ratios of 100, 89, 60, and 40%, respectively. The solubility, thermal stability, and film-forming ability for all of the polymers were evaluated, confirming the feasibility to use them as resist materials. The lithographic performances of PSOSx resists were studied by electron beam lithography (EBL) and extreme ultraviolet lithography (EUVL). PSOSx resists were demonstrated as dual-tone resists by using tetramethylammonium hydroxide (TMAH) as a positive tone developer and n-butyl acetate as a negative tone developer, respectively. Increasing the ratio of oxime sulfonate groups contributes to the negative tone development (NTD) patterns, while decreasing the ratio is conducive to the positive tone development (PTD) patterns. PSOS100 resists can resolve 20 nm half-pitch (HP) patterns at a dose of 920 μC/cm2 for NTD, and PSOS40 resists can resolve 40 nm HP patterns at a dose of 1214 μC/cm2 for PTD for EBL, respectively. The PSOS100 resist was further evaluated by EUVL, achieving patterning performance down to 18 nm HP at a dose of 87.1 mJ/cm2 with an LER of 2.3 nm for NTD. A comprehensive analysis of the patterning mechanism indicates that the decomposition of the oxime sulfonate group into sulfonic acid and other fragments upon exposure plays an important role in the solubility switch.