A Study of Photoacid Structure Dependence on Lithographic Performance in Chemically Amplified Resists.

The relationship between photoacid characteristics and lithographic performance in chemically amplified deep ultraviolet (DUV) resists was investigated. Two types of resist systems which consist of the same resin (tert-butoxycarbonyl (t-BOC) protected polyhydroxystyrene) and different photoacid generators (PAGs) were used. Resist 1 contains PAG 1 which generates aromatic sulfonic acid (acid 1; 2, 4- dimethylbenzensulfonic acid) and resist 2 contains PAG 2 which generates alkyl sulfonic acid (acid 2; cyclohexanesulfonic acid).The sensitivity of resist 1 was about four times higher than that of resist 2, and T-top profile became remarkable in resist 1, while the resolution capability was much the same. In order to clarify the reason of these differences, the photoacid characteristics such as the acid generation efficiency by exposure, the vapor pressure, the acid strength (pKa value), and the acid catalyzed deblocking reaction were investigated, together with the dissolution rate characteristics.Higher sensitivity of resist 1(aromatic sulfonic acid) was originated from larger amount of photogenerated acid (high absorbance), longer diffusion length and higher efficiency of deblocking reaction, as compared with resist 2 (alkyl sulfonic acid). Moreover, it was considered that T-topping profile was due to the evaporation of aromatic sulfonic acid 1.Based on these analysis, it was concluded that moderate absorbance, optimum diffusion length and lower evaporation property can realize ideal acid characteristics, leading to superior resist performance.