Characterization of Surface Variation of Chemically Amplified Photoresist to Evaluate Extreme Ultraviolet Lithography Stochastics Effects

Extreme ultraviolet (EUV) lithography is required for advanced node semiconductor device fabrication. The stochastic effects in EUV lithography are problematic, especially with regards to pattern roughness and defect formation. In this study, we performed atomic force microscopy (AFM) on an EUV photoresist surface to determine the surface roughness, height histogram, line scan, area ratio, and power spectral density (PSD). Polymethyl methacrylate (PMMA) for nonchemically amplified resist (non-CAR), and poly(4-hydroxystyrene)(t-butyl acrylate) copolymer (PHS:tBA) and poly(4-hydroxystyrene)(polystyrene)(t-butyl acrylate) copolymer (PHSPS:tBA) with/ di-(t-butylphenyl)iodonium perfluorobutane sulfonate (TBPI-PFBS)/tetrabutylammonium lactate (TBAL) for chemically amplified resist (CAR) were examined. In this CAR system, the exposure and dark loss contributed to the surface variation of root mean square (RMS) of 1.5 nm and 0.95 nm under a nominal exposure dosage of 8 μC/cm2. The contribution of dark loss was further evaluated from the effects of backbone polymer composition and photoacid generator (PAG) loading. The dark loss induced surface roughness can be attributed to the competition of etch selectivity in the resist components. A skewness of the height histogram and change of correlation in PSD are related to the dark loss induced surface variation.