Department of Mechanical Engineering
Kenneth E. Goodson
Modern lithography equipment using extreme ultraviolet radiation demands precision optical components that can tolerate enormous radiation densities while resisting thermal damage. Many of these optical components involve novel nanostructures, and the absence of thermal conduction property data for these materials is a significant barrier to lifetime reliability engineering. One example is Mo/Si multilayers with ~ 7 nm period, which serve as mirrors for extreme ultraviolet (EUV) photolithography. Heat conduction in these mirrors strongly influences reliability due to thermally induced stress and atomic diffusion.
This project makes progress for EUV optical components at three levels. First, we use picosecond and nanosecond thermoreflectance, as well as electrical heating and thermometry in patterned nanobridges, to provide first-of-kind thermal conductivity data of the nanostructured optical components. Second, we investigate the impact of distributed optical absorption in these components through measurements and simulations based on the thermal property data obtained here. Third, at a more fundamental level, we use these unique nanostructured components to gain insight into the complex electron and phonon transport and energy covnersion processes. Progress includes measurements of the TaN barrier layers in mirror structures as a function of temperature history and, more recently, data for the in- and out-of-plane thermal conductivities of a Mo/Si multilayer sample with 6.8 nm period. Thermal conduction in the Mo/Si multilayers is influenced by phonon-phonon coupling across the interface, quasi-ballistic phonon transport across the metal layer, the electron-phonon nonequilibrium in the metal film, and the possibility of significant inelastic electron-interface scattering.
This research is having impact on our understanding of basic transport science at metal-dielectric interfaces and is yielding critical data for companies developing lithography equipment.