Process margin in ArF lithography considering process window distortion

It is generally known that a highly transparent ArF resists is preferable for the bottom ARC process to maintain the vertical sidewall of resist patterns. Therefore we developed some transparent resists with a low concentration of photo acid generator. However, our careful analysis clarified that the completely transparent resist caused a serious deterioration of the exposure-defocus (E-D) process window. Therefore, there should be an optimum transparency in the ArF resists. Our experiment clarified that the medium transparency resists had the largest E-D process window. This resist produced 120 nm nested lines using an ArF stepper without any resolution enhancement technique.