HyungJun Lim

Nanoimprint lithography with a focused laser beam for the fabrication of nanopatterned microchannel molds

We present a process based on nanoimprint lithography for the fabrication of a microchannel mold having nanopatterns formed at the bottoms of its microchannels. A focused laser beam selectively cures the resist in the micrometer scale during nanoimprint lithography. Nanopatterns within the microchannels may be used to control microfluidic behavior.

A modified squeeze equation for predicting the filling ratio of nanoimprint lithography

A numerical method using the modified squeeze model is proposed in this paper in order to overcome the limitation of the established squeeze equation and obtain filling ratios for nanoimprint lithography (NIL). Because the imprinting velocity is overestimated when the ratio of indenter width to polymer thickness is close to unity, the modified equation is critical. For verification, the numerical results are compared with the experimental data according to the various stamp geometries and pressure variation rates, for which a maximum difference of 10% is indicated. Based on these results, additional studies are conducted using the modified squeeze equation in order to obtain filling ratios according to the polymer thickness and temperature. The filling rates are enhanced through the increases in the temperature and the polymer thickness. The results demonstrate that the modified squeeze equation can be used to obtain and predict the filling ratio of sub-nanoscale NIL fabrication. It is expected that this study will assist in optimizing the experimental conditions and approaches for roll-to-roll NIL and step-and-flash NIL.

Laser-assisted selective lithography of reduced graphene oxide for fabrication of graphene-based out-of-plane tandem microsupercapacitors with large capacitance

We present a laser lithography technique that uses a focused laser beam to fabricate out-of-plane tandem microsupercapacitors (MSCs) from reduced graphene oxide (rGO) with large areal capacitance. By controlling the depth of focus in a laser beam focused by an objective lens during laser lithography on a graphene oxide (GO) film, a rGO/GO/rGO structure is formed in the GO film, and subsequently, two independent interdigitated electrodes (IDEs) were fabricated on the top and bottom surfaces of the GO film. The out-of-plane tandem MSC with a parallel assembly of two rGO-IDEs showed two times larger areal capacitance than an in-plane single MSC with one rGO-IDE in the same MSC device footprint. The laser-assisted selective lithography technique using a focused laser beam developed in this study can be further applied to improve the energy density of MSCs without increasing the electrode area by vertically stacking multiple out-of-plane tandem IDEs.

A new mold structure to replicate patterns over 1 microm in depth for substrate conformal imprint lithography.

This paper shows an improved mold replication process that uses polyurethane acrylate (PUA) and polyethylene terephthalate (PET) for the fabrication of an ultraviolet (UV) imprinting mold used in substrate conformal imprint lithography (SCIL). With the conventional replication process, which uses hard polydimethylsiloxane (h-PDMS) as a pattern layer, it is difficult to detach the mold from a silicon master for metal oxide semiconductor field effect transistor (MOSFET) that has patterns with over 1-micron depth. However, the method proposed in this paper allows us to easily replicate patterns that have more than 1-micron depth. The key idea of this method is to use PET film as a bonding layer to attach the PUA layer to the polydimethylsiloxane (PDMS) cushion layer to overcome the weak the adhesion force between the PUA and PDMS layer. We demonstrate how to make the modified replica mold and present imprinting results obtained using this replica mold in the SCIL process.