(365f) High Aspect Ratio Pillar Arrays Formed Via Electrohydrodynamic Instabilities
Self-assembly based patterning techniques are appealing because of their ability to harness natural phenomena to form useful structures. Recently, a technique has emerged that is capable of forming ordered polymeric pillar arrays. Pillars are formed by the amplification of thin film surface instabilities through the application of an electric field normal to the film surface. Experimentally this is achieved by placing a thin film coated substrate within planar proximity of another surface, forming a simple capacitor. Applying an electric field across the gap results in the formation of an array of uniformly sized pillars. Pillars form due to the force imbalance at the film interface where the electric field amplifies film undulations against the restoring forces of gravity and surface tension.
Pillar formation is modeled using linear stability analysis, which describes the initial amplification of film undulations. The intermediate growth of pillars is usually explained as slowly growing undulations that eventually span the capacitor gap. Our experimental observations show that this proposed mechanism is incomplete. We have observed the formation of small fibrils that emanate from the undulations and project across the gap to initiate the pillar growth process. These polymeric fibrils have very high aspect ratios and can be used to form novel 3-D cage type structures through the use of a bi-layer film stack (e.g. PS on PMMA).
In a parallel effort to create high aspect ratio pillar arrays, we have developed a sophisticated tool capable of stretching pillars and controlling the key physical parameters in pillar formation (i.e. gap size and e-field strength). In the absence of stretching, the pillar structures reported to date have had low aspect ratios (less than unity). Here we will show that the aspect ratio of these structures can be uniformly increased by stretching. The tool utilizes three servo motors to manipulate the positioning of the upper electrode of the capacitor device and the resulting gap is measured using light interferometry. The systematic control provided by this tool allows the study of the effects of geometry on pillar formation dynamics.
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