Unveiling chemical heterogeneity in underlayer materials via resonant soft X-ray reflectivity

BackgroundUnderlayers (ULs) play a critical role in the performance of photoresist systems used in extreme ultraviolet (EUV) lithography. Positioned between the resist and substrate, they influence adhesion, reflectivity, interfacial stability, and overall pattern fidelity. As EUV lithography progresses toward sub-10 nm patterning, the design and optimization of ULs have become increasingly important for mitigating stochastic effects. However, a comprehensive understanding of UL functionality is still lacking.AimOur goal is to develop a nondestructive methodology to characterize the chemical profile of underlayer materials, which is essential for understanding their functional role in EUV lithography.ApproachWe employ resonant soft X-ray reflectivity (RSoXR) near the carbon K-edge to quantitatively probe depth-dependent compositional variations in representative UL films. By tuning the X-ray energy across element-specific absorption edges, RSoXR with enhanced chemical contrast, enables direct observation of chemical heterogeneity within the underlayer.ResultsWe performed RSoXR measurements on spin-on glass (SOG) samples prepared under different curing conditions. A matrix-method algorithm was used to model the X-ray reflectivity and extract the corresponding optical constants and depth profiles. The simulated profiles indicate that the two curing methods result in different levels of curing uniformity, with light-cured SOGs exhibiting clear chemical heterogeneity.ConclusionsOur results highlight the capability of RSoXR to resolve subtle chemical variations in depth profile. This nondestructive metrology can be further extended to investigate EUV resists and UL-resist interfacial behavior, offering a comprehensive understanding of materials evolution throughout EUV lithography processes.