Quantitative line edge roughness characterization for sub-0.25 μm DUV lithography

As gate dimensions continue to shrink, improving CD control is a major challenge for sub-0.25 micron DW (248 nm) lithography. One concern is line edge roughness (LER) which takes the form of both high and low frequency effects. In this paper, high frequency line edge roughness (HFLER) refers to high frequency small amplitude CD variations (sidewall roughness) noted along the edge of a wet developed resist feature. Low frequency line edge roughness (LFLER) refers to the higher amplitude waviness (line width fluctuations) observed along the edge of developed features. Both these roughness parameters (LFLER in particular) could lead to significant variations in device characteristics. Several factors such as the resist formulation, quality of the aerial image and process conditions have in the past been attributed as possible sources of roughness. In this study, a quantitative characterization of wet developed feature roughness was conducted and attempts were made to determine the sources of its origin, along with the impact of plasma etch. High and low frequency LER was characterized using a Dektak SXM atomic force microscope (AFM) and a Hitachi 7800 scanning electron microscope (SEM). Nominal 0.20, 0.18 and 0.16 mu m isolated lines were studied following photolithography and the gate etch. Additional variables in this study included substrate type (organic and inorganic BARC), resist composition, develop time, focus and the impact of aerial image (attenuated phase shift mask in conjunction with quadropole illumination).