Simulation of mold deformation and pattern interaction in nanoimprint lithography
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Simulation of mold deformation and pattern interaction in nanoimprint lithography Nicolas Cleveland, Hongwei Sun Francis School of Engineering, University of Massachusetts Lowell, United States
Abstract As an emerging manufacturing technique, nanoimprint lithography (NIL) can fabricate micro and nanoscale features of microfluidic devices at very high accuracy and reliability. In high-temperature TNIL process, a polymer melt such as polymethyl-methacrylate (PMMA) is heated beyond the melting temperature so that it behaves predominantly as a fluid during the imprint process. The process parameters such as pressure, temperature, and material properties play critical roles in the NIL process. In this work, the process of thermal nanoimprint lithography (TNIL) is studied computationally with emphasis on the effect of soft-mold deformation on polymer melt flow and finished result by-way-of fluid-structure interaction (FSI) technology. Process is assumed isothermal at 180 °C. Applications of this modeling technique range from micro- and nano-patterns used in micro-channels for biomedical devices to other applications such as biological/particle sensors or super-hydrophobic surfaces. The simulation result is compared to experimental results, and traits observed in TNIL done with soft mold are supported and explained through numerical results.
I.
Introduction
Nanoimprint lithography (NIL), an emerging technology, is a process used to produce micro- and nano-scale structure on moldable polymeric surfaces. Originally patented by Stephen Chou [1], contributions to NIL have been made through experiment and numerical analysis by others since [2-12]. Thermal nanoimprint lithography (TNIL) is a NIL variant which uses heat to melt the moldable polymer surface to achieve imprinting. Microscale soft lithography, in which the stamp is often made of a very flexible polydimethylsiloxane (PDMS) due to its accessibility and that it does not damage the nano-pattern during removal. However, the deflection of the PDMS stamp during TNIL imprinting can be significant, and can cause distortions in the finished imprinted pattern. Foresight of the final resist quality of a given lithography prototyping process is valuable information to those developing TNIL inventions with PDMS molds. A closed-form solution of simultaneous stamp and resist deformation has been proposed, where resist is viscous and stamp and substrate are elastic for a variety of geometries [12]. However, this open form numerical simulation allows for general TNIL geometries to be analyzed.
II.
Methodology a.
Fluid Dynamics
For this case the general Navier-Stokes flow equation, (1) is applied in order to solve flow-field variables. Equation (1) is governs the conservation of mass and is simplified to (2)
where density is assumed constant and
is equal to unity. To solve for the velocity field, equation (1) becomes (3)
Viscosity varies with phase and shear rate throughout the flow-field, so spatial variation in is considered in equation (3). The advancing free surface is
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