Impact of plasma vacuum ultraviolet photons on ultrathin resists for EUV lithography

Ultrathin resists (thickness ≤50 nm) are now utilized in Extreme Ultraviolet (EUV) lithography where they act as masks for an etch based pattern transfer. However, typical cold plasmas used for dry etching are rich in Vacuum Ultraviolet (VUV) photons, which can interact with these resists, reducing the etch budget and complicating pattern transfer. In this study, we investigate the effects of VUV photons, argon ions, and argon plasma on three ultrathin negative tone chemically amplified resists, as well as polymethyl methacrylate, as a positive tone baseline resist. A protocol is employed to separate out the effects of VUV photons and Ar+ ions using characterization techniques such as ellipsometry, atomic force microscopy, and infrared spectroscopy. We examine the variations in etch rates, surface roughness, and bulk chemical changes of the resists. We also examine the validity of Ohnishi number and ring parameter models for the EUV resists and processing conditions used in the study.