Side Wall Degradation of Chemically Amplified Resists Based on Poly(4-hydroxystyrene) for Extreme Ultraviolet Lithography

The intensity of incident extreme ultraviolet (EUV) photons decreases with the distance from the resist surface according to Lambert's law. In chemically amplified resists, the initial gradient of acid concentration in the depth direction is moderated by diffusion during postexposure baking. However, acid diffusion is significantly restricted at the 22 nm node, at which EUV lithography is used with 40–75-nm-thick resists. In EUV resists, secondary-electron emission also affects the depth distribution of acids unlike that in photoresists. In this study, the effects of acid diffusion restriction and secondary-electron emission on the side wall profiles of poly(4-hydroxystyrene)-based resists were investigated using a simulation based on EUV sensitization mechanisms. Even at 75 nm thickness, the emission of secondary electrons affects the side wall profiles. However, in 22 nm line-and-space patterns, no side wall angle >88° can be obtained at a film thickness of 75 nm even if the transparency is 75% and the electron emission effect is taken into account.