Phase Separation in Thin Polymer Films: From Self Stratification to Polymer Blend Lithography

This thesis describes the self-stratification as well as the purely lateral phase separation in a thin polymer blend film during spin coating. The solution system consists of polystyrene (PS) and poly(methyl methacrylate) (PMMA) in methylethylketon (MEK) as the solvent. It is shown that the formation of the morphology during spin-coating is influenced by a variety of parameters. These are the PS/PMMA mass ratio, the evaporation rate of MEK, the molecular weights (of PS and PMMA) and the humidity of the spin casting atmosphere. The dynamic of the layer formation was monitored by optical real-time in-situ reflectometry and the final morphologies were characterized by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). According to the model established in this study, the spin-casting humidity is the most important factor in determination of the phase separation ranging from layering to lateral phase separation between the two immiscible polymers. In (dry) nitrogen flow the PS/PMMA/MEK system forms a triple layer film following our „self-stratification model“. The sandwich-like triple layer (PMMA/PS/PMMA) was proven by different techniques including ellipsometry and Secondary Ion Mass Spectrometry (SIMS). With another model we describe the formation of a purely lateral phase morphology, where PS islands are separated in a PMMA matrix. Here, at a humidity of 40-50 %, this 3-dimensional phase separation occurs, whereby self-stratification was observed at lower humidity. Since the PS islands have contact both to the air and the substrate, this purely lateral morphology can be utilized in a novel lithographic technique, which we named „Polymer Blend Lithography (PBL)“ [1,2]. Using a selective solvent, one or the other polymer can be removed and the remaining one can then be used as a lithographic mask for a subsequent lift off process. If e.g. the PS Islands are removed a perforated PMMA film is left, which we call the “Swiss Cheese Structure”. In this study, two examples of the applications of PBL are demonstrated: 1. The fabrication of nano-patterned organic Self-Assembled Monolayer (SAM) templates, which we call Monolayer PBL and 2. Metal PBL which is specialized for the fabrication of micro/nanosized metal island arrays or perforated metal films with a typical hole density of 1 billion / inch2. These nanopatterned templates can be applied e.g. for surface-selective mineralization of ZnO films. Finally the wavelength-selective transmission of perforated aluminum films, which is based on the surface plasmonic effect, is demonstrated