The most sophisticated integrated circuits, such as microprocessors and memory chips, are patterned by a process called projection lithography. In a typical lithographic process, a silicon wafer is coated with a radiation-sensitive polymer film ("resist") and exposed to a pattern of light. Radiation triggers a reaction that generates the latent chemical image, and patterns are developed by selectively washing away the exposed (or un-exposed) material. Industrial applications require high-throughput processes, so the resists must be highly sensitive to radiation. This is achieved with a process termed "chemical amplification". Chemically-amplified (CA) resists have two principal components: (i) a lipophilic polymer with acid-labile protecting groups; and (ii) a low concentration of photoacid generator (PAG). Exposing the resist to radiation generates a strong acid-counterion catalyst, and heating at moderate temperature promotes the acid-catalyzed decomposition of protecting groups along the polymer backbone. This deprotection reaction changes the polymer polarity for development in an aqueous base. CA systems are highly efficient because each photon absorbed by the resist generates approximately 0.3-2 acids (depending on the photon energy), and each catalyst cleaves hundreds of bonds so a low radiation dose is "amplified" through chemistry. However, the excellent sensitivity of CA resists comes at a price, because acid diffusion during reaction will limit the pattern resolution of CA resists. While CA resists have been studied for more than 40 years, there are no quantitative models that predict the spatial extent-of-reaction with nanoscale resolution. This poses an increasingly important roadblock for the semiconductor industry, as current research and development efforts are targeting sub-10 nm feature sizes.
Intellectual Merit:
The aim of this research program is to identify the fundamental physics and chemistry that produce anomalous catalyst transport and catalyst deactivation. The PI hypothesizes that (i) anomalous catalyst transport is induced by dynamic heterogeneities in the glassy polymer films; and (ii) catalyst loss is a signature of acid-catalyzed side reactions. A series of experiments are planned that will reveal these effects. The knowledge acquired through this research will be developed into a predictive lithography tool that can accelerate materials development and process optimization. The spatial extent-of-reaction in CA resists cannot be directly measured with experiments, so experimental investigation needs to be coupled to models that predict composition at nanometer length scales. The intellectual merit of this research is the development of an efficient, quantitative, and spatially-resolved reaction-diffusion model for CA resists that is applicable in a wide range of processing conditions with parameters determined by experimental investigation. To achieve this goal, the study will employ a minimal set of variables to be determined through optimization while incorporating the underlying physics and chemistry of the glassy polymer matrix and acid catalyst. The framework developed through this research will offer a means to investigate processing conditions and formulations that have yet to be realized in laboratory experimentation.
Broader Impacts :
The outcomes of this research program may immediately impact materials design and process development for next-generation lithography, as predictive models reduce or eliminate the need for iterative experimentation. After establishing the applicability of the anomalous transport model over a wide range of conditions, detailed descriptions of the method will be available to the general public via publications and presentations, furthering the development of lithography simulation that remains a bottleneck in industrial applications. The students engaged by this program will receive training in polymer physics and chemistry, computational methods, and designing experimental procedures. The PI has a record of engaging high school students and undergraduate students in laboratory research, and also participates in community outreach through NSF-funded K-12 camps at the University of Houston. Outcomes of this program will be incorporated into the PI's graduate course "Chemical Processing for Microelectronics."
