The Interdependence of Exposure and Development Conditions when Optimizing Low-Energy EBL for Nano-Scale Resolution

Abstract

Electron beam lithography (EBL) is the major direct-write technique to controllably fabricate nanoscale features. A focused beam of electrons induces a chemical change in a layer of radiation sensitive material (resist), such as chain scissioning in positive tone polymethylmethacrylate (PMMA) polymer photoresist. The localized fragmented region is rendered more soluble in a suitable developer solution and removed. In negative tone resists, such as hydrogen silsesquioxane (HSQ) or calixarene, the radiation damage eventually results in bond cross-linking, generating structures locally more resistant to dissolution. Limitations of the technology are related largely with unwanted exposure of the resist away from the impact of the focused electron beam due to scattering of the primary electrons in the resist (often described as the forward scattering), generation of secondary electrons, and backscattering from the substrate (the proximity effect). The exposure and development processes have been optimized and routinely used for fabrication of submicron features. However, as requirements for lithography have progressed toward the sub-20 nm regimes, major challenges have emerged of introducing controllable radiationinduced changes at molecular-size scales, within a reasonable tradeoff with the applicability of the standard materials, as well as cost and simplicity of the processes. Due to the proximity effect, this becomes particularly demanding when dense patterns with closely positioned features must be fabricated. Achieving deep nanoscale resolutions in high density patterns at industrially-relevant throughputs requires new approaches to EBL. Novel EBL processes that would extend capabilities of the technology significantly into the deep nanoscale regime entail new approaches to resist design, exposure strategies, and development techniques (Haffner et al., 2007; Liddle et al., 2003; Ocola & Stein, 2007; Word et al., 2003). To achieve this will require a much more detailed understanding of the molecular mechanisms involved in both the electron-resist interaction and in the polymer dissolution (development) stages of the nanolithography process. Despite a significant research effort and vast literature on electron beam lithography, the detailed molecular mechanisms are still inadequately understood. Published modeling studies address