ELECTRIC FIELD INDUCED SELF ASSEMBLY
Mo Yang, Cengiz S. Ozkan and Huajian Gao
Graduate Student, US
Keywords: Self Assembly, Template Formation, Nanolithography
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. Finite element modeling of the electrical potential distribution and the minimum wavelength of the surface features was conducted. As shown in Figure 1, the electrode chip are is 50x50 ìm2. The initial thickness of the polystyrene film is 200 nm and the spacer thickness (air gap) is 100 nm. A bias voltage of 120V was applied to the capacitor arrangement. The red color indicates is the maximum value of the electric potential. There is a discontinuity in the interface between the air gap and the polystyrene film for different dielectric constants. Figure 2 indicates the wavelength of the features formed upon applying the electric field. For experiments, 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 polystyrene solution was prepared by dissolving polystyrene pellets in toluene, at a weight ratio of 11%. Spin coating at 4000 rpm for 30 seconds and subsequent hot plate drying (120C, 5 min) results in a film thickness of 200 nm. The ITO substrate was placed over the polymer surface at a distance to leave a thin air gap using spacers. This gap provides the necessary free space for surface evolution. The applied voltage to induce surface evolution in the polymer strongly depends on the thickness of the air gap. 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 107–108 V/m. The capacitor structure was quenched to observe the structures using optical microscopy and atomic force microscopy. For the first experiment in Figures 3(a) and (b), the polystyrene film thickness was 200 nm and the spacer thickness was 600 nm. The capacitor arrangement was heated to 170 C, and a bias of 180 V was applied across the capacitor. The image shown in Figure 3(b) was obtained after 15 minutes of annealing. Average height of the features was 435 nm. In the second experiment (Figures 3(c) and 3(d)), the polystyrene film thickness was 200 nm and the spacer thickness was 500 nm. After 5 minutes of anneal time, the setup was quenched to observe the partially completed line segment in Figure 3(c), which reveals undulations similar to that in the self assembly process. Complete formation was observed after 8 minutes of total processing time, as shown in Figure 3(d). The method described can be used to fabricate a variety of structures in the micron and nanometer scales including NEMS, polymer optoelectronic devices and patterned templates for nanolithography.
NSTI Nanotech 2003 Conference Technical Program Abstract