The Role of Interactions Between Photoresists and Underlayers in EUV Lithography

Extreme Ultraviolet (EUV) lithography is now thoroughly integrated into semiconductor manufacturing, and the industry is beginning to adopt high NA EUV as the next key step on the roadmap. EUV exposures have a significantly smaller depth of field relative to 193 nm, forcing the use of thinner resists, and high NA EUV will further reduce the resist thickness. As a result, interfacial effects are becoming increasingly dominant and are dictated by both the resist composition and the underlayer chemistry. The nature of the underlayer can have a significant effect on the resist performance by changing both the interaction of the resist with the EUV photons, but also though chemical interactions between components in the layers. The challenges presented by this complex materials stack are leading to innovations in both the processing approaches used to prepare the films, and also in development of new resist chemistries. Understanding these interactions requires spatially resolving chemical changes over the order of several nanometers. Resonant soft X-ray reflectivity (RSoXR) has been shown to be capable of providing both the chemical sensitivity and the spatial resolution needed to depth profile resist chemistries and how they changes at different processing stages. Resonant soft X-rays take advantage of the sensitivity to specific chemical functional groups near an atomic absorption edge, providing a route towards quantitatively characterizing the chemical depth profiles. This presentation will discuss several applications of this technique, including an investigation of conventional chemically amplified resists which still are the primary patterning material for the EUV process. The behavior of metal oxide resists will also be explored, as they are predicted to play a key role in high NA EUV patterning.