Vapor-Phase Polymer Infiltration with Al2O3 for Improved Nanoscale Patterning

Many scalable nanowire patterning approaches, such as selective coaxial lithography via etching of surfaces (SCALES), rely on polymer etching for patterning. Etching soft materials, such as poly(methyl methacrylate) (PMMA), faces issues such as uneven etching (leading to imprecise patterning) and a lack of compatibility with processes involving high temperatures or rapid etching. This research aims to resolve these issues by converting soft PMMA films to hard Al₂O₃ films via Al₂O₃ infiltration and subsequent pyrolysis of the films. Spuncoat PMMA films on planar Si wafers are first infiltrated with Al₂O₃ using high temperature vapor-phase infiltration (VPI) processes. Formation of dense alumina surface layers on top of the hybrid films indicated the VPI process reached saturation. These hybrid films then undergo a 600°C pyrolysis to remove the PMMA, leaving behind amorphous Al₂O₃ films. These films can be controllably etched using an alumina etchant composed of phosphoric, acetic, and nitric acid. When compared to standard alumina films formed through thermal Atomic Layer Deposition, the converted PMMA films etched comparably suggesting a dense alumina film that can also be used in further patterning steps. Throughout the process, films were characterized through x-ray photoelectron spectroscopy (XPS) depth profiling and ellipsometry. Achieving increased control over soft materials at the nanoscale promises to facilitate more precise polymer-based patterning.