Extreme ultraviolet induced reactions of tin–oxo cage photoresists

Abstract

The performance of photoresists is a major challenge in extreme ultraviolet (EUV) lithography and needs to be improved for the future technology nodes that require higher resolution, patterning fidelity, and sensitivity. Hybrid inorganic/organic materials are considered for this crucial function, but the chemical mechanisms underlying their solubility switching are not well understood, which hampers the rational improvement of EUV photoresists. Here we study n-butyltin–oxo cages, a readily accessible “open source” representative negative tone resist. Upon exposure to EUV radiation (wavelength 13.5 nm), butane, butene and octane are the main volatile reaction products. Tin is fully retained in the films even after prolonged EUV exposure. It is found that the loss of only ∼18% of the butyl groups suffices to render the resist film insoluble. The initial quantum efficiency of Sn–C bond cleavage is Φ ≈ 5 per absorbed EUV photon, but this decreases rapidly with conversion of the material. After the primary Sn–C bond cleavage in a tin–oxo cage, induced by photoionization or capture of a (secondary) photoelectron, facile thermal reaction steps may occur that lead to additional Sn–C bond breaking. Although many questions remain, our work sheds new light on the reaction mechanisms at play and provides input for simulations of the lithographic process.