Ghost exposure of chemically amplified resist caused by acids generated in environmental air with KrF exposure

We discuss the ghost exposure of a chemically amplified resist in KrF excimer laser lithography, which is attributed to acids in the environmental air generated by laser irradiation. These acids diffused into the resist film and acted as catalytic acids. The ghost exposure expanded to more than 10 mm outside the exposure field and deteriorated the patterning characteristics. Controlling the environmental air so that it does not generate acids is important in order to improve CD control.