One of the obstacles hindering the transition from 193 nm to extreme ultraviolet (EUV) photolithography is photoresist performance. However, design of next generation chemicallyamplified EUV resists necessitates that we fully understand the mechanisms underlying photoacid generation. In particular, we would like to determine the effective distance the lowenergy electrons generated during EUV exposure travel within resists while continuing to induce photoacid generator (PAG) decomposition, since diffusion length carries important implications for resolution and line edge roughness. Here, we demonstrate two novel experimental approaches for obtaining electron diffusion length in resists using top-down electron beam exposure: thickness loss experiments and in situ mass spectrometry.
EUV photons expose photoresists by complex interactions starting with photoionization that create primary electrons (~80 eV), followed by ionization steps that create secondary electrons (10-60 eV).Ultimately, these lower energy electrons interact with specific molecules in the resist that cause the chemical reactions which are responsible for changes in solubility. The mechanisms by which these electrons interact with resist components are key to optimizing the performance of EUV resists. An electron exposure chamber was built to probe the behavior of electrons within photoresists. Upon exposure and development of a photoresist to an electron gun, ellipsometry was used to identify the dependence of electron penetration depth and number of reactions on dose and energy. Additionally, our group has updated a robust software that uses first-principles based Monte Carlo model called "LESiS", to track secondary electron production, penetration depth, and reaction mechanisms within materialsdefined environments. LESiS was used to model the thickness loss experiments to validate its performance with respect to simulated electron penetration depths to inform future modeling work.
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