Chemical Origins of Environmental Modifications to MOR Lithographic Chemistry

Key Takeaways

• O₂ drives carbonyl formation during post‑exposure bake
• CO₂ and humidity show minimal chemical impact
• O₂ enhances ligand loss only after EUV exposure
• Activation energy ~58 kJ mol⁻¹ for O₂‑mediated cleavage
• Up to 30% dose‑to‑gel reduction possible

Summary

Researchers at imec presented new findings on metal‑oxide resists (MORs) for EUV lithography, showing that atmospheric oxygen, not CO₂ or humidity, drives post‑exposure chemical changes. Using the BEFORCE platform, they demonstrated that O₂ induces carbonyl formation and accelerates ligand loss during post‑exposure bake, but only after EUV exposure creates reactive sites. Kinetic analysis revealed a first‑order O₂‑MOR reaction with an activation energy around 58 kJ mol⁻¹, leading to up to a 30% reduction in dose‑to‑gel. These insights suggest that controlled O₂ environments could improve MOR sensitivity without sacrificing resolution.

Pulse Analysis

The study highlights a pivotal shift in how the semiconductor industry approaches metal‑oxide resist (MOR) chemistry. While MORs have been praised for their high sensitivity, their interaction with ambient gases has long been a source of critical dimension drift. By isolating oxygen’s role using a custom‑controlled atmosphere, imec researchers proved that O₂—not CO₂ or water vapor—forms stable carbonyl groups during the post‑exposure bake, a reaction that only proceeds when EUV‑generated radicals are present. This discovery clarifies the long‑standing mystery of post‑exposure delay and offers a clear pathway to mitigate CD variation.

From a process engineering perspective, the kinetic data are especially valuable. The first‑order reaction kinetics and an activation energy of roughly 58 kJ mol⁻¹ indicate that modest temperature increases can dramatically accelerate ligand cleavage when O₂ is present. Moreover, the observed three‑fold enhancement in ligand loss at 50 % O₂ suggests that intentional oxygen dosing could lower the required EUV dose by up to 30 %, directly translating to higher throughput and reduced tool wear. Such dose‑to‑gel improvements are critical as the industry moves toward high‑NA EUV systems where photon budgets are tighter.

Strategically, these findings empower fabs to co‑optimize exposure, bake, and ambient control. By integrating O₂ management into the lithography workflow—potentially through inert‑gas purging or precise oxygen partial‑pressure regulation—manufacturers can achieve more stable patterning while exploiting the sensitivity gains. The work, funded by the EU’s Chips Joint Undertaking, positions MORs as a viable alternative to traditional chemically amplified resists, accelerating the roadmap toward sub‑3 nm nodes. Continued in‑situ studies will likely refine the balance between oxygen‑induced sensitivity and long‑term pattern fidelity.