A High-Resolution EUV Zinc-Oximate Resist: Why Post-exposure Bake Fails to Improve Lithography Performance

Abstract

Zinc oximatesorganometallic compounds combining zinc and oximate ligandswere previously used as precursors for zinc-oxide-based thin-film transistor applications. Recently, the potential of zinc oximates as high-resolution patterning materials was evaluated via electron beam lithography and extreme ultraviolet (EUV) lithography (EUVL). However, a detailed understanding of the lithographic reaction mechanisms is currently lacking due to limited exploration in material characterization and process tuning, particularly the crucial reaction mechanism that occurs during the post-exposure bake (PEB). Understanding how PEB affects zinc oximate resists is essential to further enhance the EUV sensitivity and line-edge roughness (LER) of this potentially new class of organometallic resists. In addition, the integration of zinc oximate resists into industrial EUV lithography processes has not yet been demonstrated. To address this knowledge gap, we first demonstrated the highest resolution possible (pitch-24 nm line-and-space) with a 0.33NA ASML EUV scanner tool and then performed a holistic investigation of the EUV exposure and thermally driven reaction mechanisms of a zinc oximate resist, zinc open-source nano-engineered (ZONE), which we link to on-wafer pitch-32 nm dense line-and-space EUV patterning performance. Specifically, we show that thermal treatment at 140 °C temperatures leads to the initiation of conversion into ZnO, thus preventing the optimization of patterning performance via the PEB. In contrast, no ZnO formation is observed under EUV exposure of 100 mJ/cm2, indicating a fundamentally different mechanism for the solubility switchone driven by preferential bond cleavage rather than bulk oxide formation. These competing chemical mechanisms manifest in a degradation of pattering performance (e.g., resolution, LER, etc.) under increasing PEB temperature (from 120 to 180 °C), which indicates thermally driven reactions indiscriminately cleave organic bonds and compromise the solubility contrast in ZONE. Nevertheless, the remarkably high resolution in EUV lithography suggests that metal oximate resists are a promising platform for future high-NA EUV lithography.