In-Hyouk Song

Mold filling analysis of an alignment structure in micro hot embossing

Hot embossing is one of the most popular fabrication methods to replicate polymer microdevices in the field of micro-fluidics and micro-optics. Numerical models for hot embossing were constructed to analyze the advance of flow front of the molten polymer using commercial software, DEFORM-2D. A hemisphere tipped post, used as an alignment structure in the assembly of micro devices, was modeled to demonstrate the flow behavior of the molten polymer in mold filling. Hot embossing experiments were performed to validate the feasibility of the numerical models. Most of the simulations showed agreement with experiments. The mold filling was estimated with the heights of the embossed posts in the analysis. No significant mold filling with the molten polymer was observed below the glass transition temperature of 105 °C. The mold cavity was completely filled with the polymer at the molding temperature of 137.5 °C and 150 °C while the embossing forces were 300, 600, and 900 N.

Hydrothermal Growth of ZnO Nanowires on UV-Nanoimprinted Polymer Structures

Integration of zinc oxide (ZnO) nanowires on miniaturized polymer structures can broaden its application in multi-functional polymer devices by taking advantages of unique physical properties of ZnO nanowires and recent development of polymer microstructures in analytical systems. In this paper, we demonstrate the hydrothermal growth of ZnO nanowires on polymer microstructures fabricated by UV nanoimprinting lithography (NIL) using a polyurethane acrylate (PUA). Since PUA is a siloxane-urethane-acrylate compound containing the alpha-hydroxyl ketone, UV-cured PUA include carboxyl groups, which inhibit and suppress the nucleation and growth of ZnO nanowires on polymer structures. The presence of carboxyl groups in UV-cured PUA was substantiated by Fourier transform infrared spectroscopy (FTIR), and a Ag thin film was deposited on the nanoimprinted polymer structures to limit their inhibitive influence on the growth of ZnO nanowires. Furthermore, the naturally oxidized Ag layer (Ag2O) reduced crystalline lattice mismatches at the interface between ZnO-Ag during the seed annealing process. The ZnO nanowires grown on the Ag-deposited PUA microstructures were found to have comparable morphological characteristics with ZnO nanowires grown on a Si wafer.

Study on Stress and Thermal Contraction During Cooling and Demolding in Hot Embossing

Thermal stress and shrinkage were investigated to analyze pattern defects during the cooling and demolding steps in hot embossing. Hot embossing is one of the most popular fabrication methods that replicate polymer microdevices in fields such as micro-fluidics and micro-optics. Numerical models were developed to study the effect of thermal stress and adhesion force on a molded microstructure during the cooling and demolding steps of micro hot embossing. The alignment microstructure, a hemisphere-tipped post, was located at four radial locations of 5, 10, 15, and 20 mm from the center of the polymer substrate to determine the effect of thermal stress and adhesion forces, relative to pattern location, leading to pattern deformation. Results showed that thermal stress increased as the microstructure was moved further away from the center of the polymer substrate due to an increase in shrinkage velocity. Process parameters could be designed to improve replication fidelity by analyzing the stress distribution of a molded microstructure during cooling and demolding.Copyright

Finite Element Analysis for Reliable Replication of Alignment Structures in Micro Hot Embossing

Numerical analysis to predict the flow behavior of polymers was conducted using commercial finite element software, DEFORM-2D. An alignment structure, a hemisphere-tipped post with an annular ring, was modeled to demonstrate the flow front of the polymer during molding. The cavity filling process was evaluated by measuring the heights of the molded posts and confirmed and validated by experimental data. Estimated and measured heights showed better replication accuracy with the increase of molding temperature and embossing force. Results exposed complete cavity filling at embossing forces of 600 and 900 N at a molding temperature of 137.5°C. The FEM prediction tool can be used to select the adequate process parameters for a high fidelity replication of polymer microstructures.© 2013 ASME

MOLD ALIGNMENT SYSTEM FOR DOUBLE-SIDED MICRO HOT EMBOSSING USING KINEMATIC CONSTRAINTS

A cost effective alignment and positioning system was developed for double-sided hot embossing processes using kinematic principles. Alignment tooling located and transferred the top mold insert location to the proper location for the complementary mold insert and hot embossing was performed. The samples were checked for alignment accuracy and consistency. Adjustments were made and sampling repeated and measured. Results showed the alignment system can improve the locating and positioning of the complementary mold insert in relation to the top in a repeatable and cost effective manner.

Cavity Filling Analysis for the Fabrication of an Alignment Structure in Micro Hot Embossing

The flow behavior of polymers was analyzed using a commercial finite element software, DEFORM-2D, for the replication of microstructures. An alignment structure for microassembly, a hemisphere-tipped post, was modeled to demonstrate the polymer flow in hot embossing. The mold filling with polymer was evaluated by the heights of molded posts. The results of simulation showed better replication fidelity of the post with the increase of the molding temperature and displacement of the mold insert. The complete filling of a mold cavity was achieved at the molding temperatures of 112.5, 125, 137.5, and 150°C when the displacements were 1.5, 2.0 and 2.5 mm. The model can be used to design the process parameters for the reliable replication of microstructures.