Electric Field Induced Self Assembly and Template Patterning of Polymer Microstructures
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ELECTRIC FIELD INDUCED SELF ASSEMBLY AND TEMPLATE PATTERNING OF POLYMER MICROSTRUCTURES CENGIZ S. OZKAN and *HUAJIAN GAO Mechanical Engineering Department, University of California, Riverside, CA *Max Planck Institute for Metals Research, Stuttgart, Germany
ABSTRACT We have developed a method for fabricating polymer microstructures based on electric field induced self assembly and pattern formation. A dielectric fluid placed in between to conductive plates experiences a force in an applied electric field gradient across the plates, which can induce a diffusive surface instability and self construction of the fluid surface. This process is exploited for the fabrication of self assembled polymer microstructures as well as replicated patterns through the use of pre-patterned plates or electrodes. We have used silicon wafers and transparent ITO (Indium-Tin Oxide) coated quartz substrates to fabricate the capacitor structures. The bottom silicon plate is spin coated with a 100-200 nm thick polystyrene film. The ITO substrate was placed over the polymer surface at a distance to leave a thin air gap using spacers. For directed pattern transfer, patterned ITO substrates were used. The capacitor setup was heated above the glass transition temperature of the polymer and a voltage was applied across the plates (50-150 Volts), which induces electric fields on the order of 107–10 8 V/m. The capacitor structure was quenched to observe the structures using optical microscopy and atomic force microscopy. The method described can be used to fabricate a variety of structures in the micron and nanometer scales including bio-fluidic MEMS, polymer optoelectronic devices and patterned templates for nanolithography.
INTRODUCTION Fabrication of structures in the micron and nanoscale via self assembly has attracted much interest in recent years. Self assembly methods range from fluidic self assembly of optoelectronic devices on silicon substrates (micron scale) to growing islands or quantum dots in heteroepitaxial film systems (nanoscale). These methods offer high throughput and low cost solutions compared to conventional patterning or lithography technologies, and they enable the fabrication of structures with dimensions beyond the wavelength of the light employed in most lithography processes [1-4]. Self assembly of islands in heteroepitaxial films is based on the formation of a surface instability via surface diffusion at elevated temperatures in a growth reactor [5]. The driving for this process is the biaxial film stress induced by the lattice mismatch between the heteroepitaxial film and the substrate. The energy release rate per unit amplitude A of the surface features (islands or undulations) for this process is given by [6],
C8.46.1
∂U = −πAY σ2 ∂A
(1)
where Y is called the surface admittance tensor and σ is the biaxial film stress. Surface roughening in heteroepitaxial films via surface diffusion has been studied extensively, and experimental results have shown that the surface evolution process takes place in the form of c
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