Stretched Polymer Nanohairs by Tailored Capillarity and Adhesive Force
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0948-B05-09
Stretched Polymer Nanohairs by Tailored Capillarity and Adhesive Force Hoon Eui Jeong, Sung Hoon Lee, Pilnam Kim, and Kahp Y. Suh School of Mechanical and Aerospace Engineering, Seoul National University, Seoul, 151-742, Korea, Republic of
ABSTRACT We present a simple method for fabricating high aspect-ratio polymer nanostructures on a solid substrate by sequential application of molding and drawing of a thin polymer film. In this method, a thin polymer film is prepared by spin coating on a solid substrate and the temperature is raised above the polymer’s glass transition while in conformal contact with a poly(urethane acrylate) (PUA) mold having nano-cavities. Consequently, capillary force induces deformation of the polymer melt into the void spaces of the mold and the filled nanostructure was further elongated upon removal of the mold due to tailored adhesive force at the mold/polymer interface. The optimum value of the work of adhesion at the mold/polymer interface ranged from 0.9 to 1.1 times that at the substrate/polymer interface. Below or above this range, a simple molding or detachment occurred, corresponding to earlier findings. INTRODUCTION Fabrication of high aspect-ratio (AR) nanostructures has attracted much attention due to its wide range of applications such in sensor arrays [1], high capacitance in DRAM [2], field emitters [3], biological studies [4] and biomimetics [5-7]. In spite of increasing demand, high AR nanostructures are difficult to fabricate by conventional photolithography or electron-beam lithography because high AR structures are prone to collapse during developing. Typical microelectromechanical processes such as LIGA and deep reactive ion etching (DRIE) are also difficult to use due to their inherent limitations for the sub-100-nm patterning [8]. Unconventional contact-based methods (e.g., nanoimprint [9] or soft lithography [10]) have been suggested as an alternative for fabricating high AR structures. These techniques are relatively simple and economically viable, offering a low-expertise route to many polymeric micro/nanostructures. However, some limitations prevent the widespread use of these techniques. For example, nanoimprint lithography could not be directly applied to the fabrication of high AR structures due to the rigidity of the mold (Young’s modulus > ~100 GPa) and the limited film thickness. For soft lithography, polydimethylsiloxane (PDMS) mold is widely used for its favorable optical property and facile processibility. The potential limitation is that PDMS mold is difficult to be used for fabricating high aspect-ratio sub-100-nm structures due to its low Young’s modulus (~ 1.8 MPa) [11, 12]. Recently, a UV-curable polyurethane acrylate (PUA) mold was introduced for fabricating high AR nanostructures [13]. Since PUA mold is sufficiently rigid (Young’s modulus of ~ 40 MPa) for the sub-100-nm patterning, yet it is also flexible (~ 50 µm thickness), it can make an
intimate contact with the substrate over a large area, combining major advantages of imprint a
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