Unraveling Dose Reduction in Metal Oxide Resists via Post-Exposure Bake Environment

In the realm of extreme ultraviolet (EUV) lithography, metal oxide resists (MORs) have emerged as promising candidates for advanced semiconductor patterning. However, their stability poses challenges, particularly interactions with clean-room environments like humidity and airborne molecular contaminants (AMCs) post-exposure. Researchers at imec, led by Ivan Pollentier, Fabian Holzmeier, Hyo Seon Suh, and Kevin Dorney, have developed a novel platform called BEFORCE to probe these effects. Presented at SPIE Advanced Lithography + Patterning in February 2026, their work unveils a dose reduction strategy by optimizing the atmospheric conditions during post-exposure delay (PED) and post-exposure bake (PEB).

BEFORCE integrates a bake and EUV system with Fourier-transform infrared (FTIR) spectroscopy and outgas measurements, enabling precise control over environmental variables. This setup allows evaluation of MOR in controlled atmospheres, addressing stability concerns that arise mostly after EUV exposure. The platform’s design facilitates experiments where gases like nitrogen (N2), carbon dioxide (CO2), and clean air (CA) are mixed with controlled relative humidity (RH%) and oxygen (O2) levels via mass flow controllers (MFCs). Initial findings from imec’s press release on February 25, 2026, highlight BEFORCE’s potential to enhance MOR performance.

A key focus is enhancing EUV dose response through PED/PEB environments. Dose-to-gel (D2G), a metric of photo-speed, serves as the primary indicator. Experiments show that oxygen concentration significantly influences condensation and dose requirements. In atmospheres with less oxygen than standard air (21% O2), condensation is minimal, but increasing O2 to 50% yields a 25-30% reduction in D2G. This suggests oxygen’s role is not saturated at nominal levels; higher concentrations accelerate the photochemical reactions leading to gelation, thus lowering the required EUV dose.

Humidity’s impact is nuanced and interdependent with oxygen. At low O2 levels, higher humidity improves photo-speed, mainly by aiding condensation in oxygen-scarce environments. Graphs from the study depict D2G decreasing sharply with rising humidity under low O2, but the effect plateaus or reverses in high-O2 settings. For instance, at 5% RH, increasing O2 from 0% to 50% reduces D2G by up to 30%. Conversely, in low-O2 conditions, humidity drives a steeper drop in D2G, indicating it compensates for oxygen’s absence in promoting resist cross-linking.

To disentangle PED and PEB contributions, the team conducted separate environment tests. Using a model MOR, they varied conditions: PED in air (21% O2, 45% RH) followed by PEB in vacuum, or vice versa. Results reveal that PEB atmosphere dominates condensation. PEB in air promotes significant film thickness changes indicative of condensation, while vacuum PEB suppresses it, regardless of PED conditions. Preliminary data with 120-second PED/PEB cycles underscore this: vacuum PEB yields higher D2G (slower photo-speed), but air PEB enhances sensitivity. This implies chemical transformations during baking are more sensitive to ambient gases than during delay.

Further inter-relations emerge with PEB temperature and time. At ~5% RH, oxygen trends hold across temperatures, but in zero-O2 environments, photo-speed remains stable, suggesting temperature independence without oxygen. With O2 present, longer PEB times significantly boost photo-speed, hinting at kinetic chemical effects. Kevin Dorney’s related talk (SPIE 13983-50) explores these origins, proposing mechanisms like ligand exchanges in MOR structures (e.g., OH to other groups).

The study opens avenues for co-optimization: tuning O2, humidity, temperature, and time could reduce doses by 25-30%, improving throughput in EUV lithography. For commercial MORs, humidity aids condensation without strong oxygen dependence, aligning with model resists but showing subtler effects.

Bottom Line: imec’s BEFORCE demonstrates that PEB environment is pivotal for MOR dose reduction. By elevating O2 and modulating humidity, manufacturers can enhance sensitivity without compromising stability. Acknowledgments go to Intel and resist suppliers for materials. Funded by the EU’s Chips Joint Undertaking and partners like Belgium and France, this research paves the way for efficient, environmentally tuned EUV processes, potentially revolutionizing high-volume semiconductor fabrication.

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