Extreme Ultraviolet Lithographic Performance and Reaction Mechanism of Polymeric Resist─Utilizing Radical- and Acid-Amplified Cross-Linking

Extreme ultraviolet (EUV) lithography is a promising technology for making chips beyond 7 nm technology node; however, it comes with its own challenges. One of them is the need to find a suitable EUV resist. Photoresist performance is gauged by the resolution, LER, and sensitivity trade-off. However, the primary challenge for the photoresist in EUV lithography is the major shift in reaction chemistry relative to that in deep ultraviolet (DUV) lithography. The interaction of the EUV photon (92 eV) with the widely used conventional chemically amplified resist (CAR) is different than that of the DUV photon (5–6 eV). The DUV photon excites only the valence electrons of the photo-acid generator in CAR, which results into acid generation. However, a high-energy EUV photon can ionize the CAR polymer matrix. This leads to the generation of a cascade of secondary electrons. Thus, in addition to acid generation, other chemical reactions like bond scission and cross-linking occur by the interactions between resist and secondary electrons. Apart from chemically amplified reactions, CARs are not very efficient at utilizing the additional pathways offered by secondary electrons. Moreover, stochastics, arising from photon and chemical shot noise, are an additional challenge for an EUV resist. Therefore, there is a need for an alternative photoresist to CAR. Herein, we report a single polymer chain negative tone photoresist. The resist has an acid generator, an organotin moiety, and an acid reactive unit attached to a polymer chain. The resist is designed to use EUV-generated secondary electrons to generate acid and polymer-bound radicals. The polymer-bound radicals and acid amplification reaction can induce cross-linking among polymer chains to create a negative tone solubility switch. In this study, the EUV printability study and reaction mechanism of the resist are elucidated. Electron transfer reactions for radical generation were studied with the help of nanosecond pulse radiolysis and γ-radiolysis. Liquid chromatography and mass spectrometry were used to identify possible acid-amplified reactions.