Poly(chloromethylstyrene): A high performance x-ray resist

Abstract

X-ray lithographic performance of poly(chloromethylstyrene) (PCMS), a high performance negative e-beam resist, was investigated. Superior resolution, i.e., resist profile and linewidth control, was obtained by x-ray exposure due to the essentially square x-ray exposure profile and the significantly reduced amount of backscattered electrons from the substrate. Sensitivity and resolution were evaluated as a function of polymer molecular parameters. Dry etching characteristics were evaluated under polysilicon, SiO2 and Al etching conditions. Excellent resolution, sensitivity and dry process compatibility were obtained and attributed to the high absorption of chlorine to Pd Lα radiation, the high chemical reactivity of the benzyl chloride moiety and the high plasma stability of the aromatic styrene moiety. Sensitivity of 300 000 molecular weight PCMS is 25–30 mJ/cm2 (D0.5g) to Pd Lα radiation; resolution is 1 μm in 1 μm final thickness resist with vertical walls at D0.9g (70 mJ/cm2), and submicron in thinner layers. Dry etch resistance is comparable to that of AZ-type photoresist. Postexposure deep UV hardening minimizes thermally induced flow and profile distortion even at temperatures greater than 300 °C. Lithographic performance of PCMS compares very favorably with other leading x-ray resists including the mixture of poly(2,3-dichloropropyl acrylate) and poly(glycidyl methacrylate-co-ethyl acrylate) (DCOPA), poly(allyl methacrylate-co-hydroxyethyl methacrylate) (EK88) and chloromethylated polystyrene (CMS).