Fabrication of high-aspect-ratio microstructures using dielectrophoresis-electrocapillary force-driven UV-imprinting

Abstract

We propose a novel method for fabricating high-aspect-ratio micro-/nano-structures by dielectrophoresis-electrocapillary force (DEP-ECF)-driven UV-imprinting. The force of DEP-ECF, acting on an air–liquid interface and an air–liquid–solid three-phase contact line, is generated by applying voltage between an electrically conductive mold and a substrate, and tends to pull the dielectric liquid (a UV-curable pre-polymer) into the mold micro-cavities. The existence of DEP-ECF is explained theoretically and demonstrated experimentally by the electrically induced reduction of the contact angle. Furthermore, DEP-ECF is proven to play a critical role in forcing the polymer to fill into the mold cavities by the real-time observation of the dynamic filling process. Using the DEP-ECF-driven UV-imprinting process, high-aspect-ratio polymer micro-/nano-structures (more than 10:1) are fabricated with high consistency. This patterning method can overcome the drawbacks of the mechanically induced mold deformation and position shift in conventional imprinting lithography and maximize the pattern uniformity which is usually poor in capillary force lithography.

Fabrication of Microlens Arrays with Well‐controlled Curvature by Liquid Trapping and Electrohydrodynamic Deformation in Microholes

“…Importantly, the template was coated with a dielectric SiO 2 fi lm (about 1.2 μ m) grown by a thermal oxidization process. This is because a difference in dielectric properties between the SiO 2 and the liquid (i.e., a discontinuity of dielectrics at the solid-liquid interface) can induce a signifi cantly increased electrodynamic force on the liquid at the solid-liquid-air three-phase contact line, as was discovered experimentally and numerically; [ 17 ] therefore, the SiO 2 layer is desirable for generating a large curvature of the liquid surface. By physical vapor deposition (PVD), an ITO thin fi lm (which is transparent and conductive) was coated on the optical glass substrate and worked as another electrode.…”

“…A higher dielectric constant for the pre-polymer is desirable for enhancing electric fi eld strength at the solid-liquid-air contact line, as indicated by our numerical simulation. [ 17 ] For easy removal of the template from the cured polymer MLA, the template, after the SiO 2 growth by a thermal oxidation, was treated in a 1.0 wt% ethanol solution of heptadecafl uorodecyltrimethoxysilane (i.e., CF 3 (CF 2 ) 7 CH 2 CH 2 Si(OCH 3 ) 3 ) for 3 h, and subsequently baked at 150 ° C for 10 h. Figure 3 shows SEM images of concave MLAs generated by using a template with hexagonally arrayed cylindrical microholes. Figure 3 a indicates how the microlens curvature was infl uenced by the applied 5 Hz-square-wave voltage, U , ranging from 0 V to 200 V pp for the same pre-polymer thickness (roughly 12 μ m).…”

Electrowetting Assisted Air Detrapping in Transfer Micromolding for Difficult-to-Mold Microstructures

“…Our recent investigations suggested that the contact angle of a liquid UV-curable prepolymer dropped on a plane electrode coated with a dielectric layer (such as a SiO 2 /Si substrate) was decreased by applying a voltage between the plane electrode and a needle electrode inserted into the liquid drop. 35,36 Such an electrically induced decline of the contact angle of a liquid on dielectric/ conductor substrate is known as electrowetting on a dielectric (or EWOD). 37−42 The contact angle recovered to the initial value after the external voltage was switched off.…”

Step-Controllable Electric-Field-Assisted Nanoimprint Lithography for Uneven Large-Area Substrates

“…[26][27][28][29][30][31] For example, Chou et al successfully utilize electrostatic attraction force, equivalent to an external applied mechanical pressure as in the case of a conventional nanoimprint process, to pattern nanogratings with high fidelity and excellent uniformity in a photocurable resist on a 4′′ Si wafer. 28 Experimental results show no visible damage to the structures on the substrate.…”

“…In our previous work, fabrication of high-aspect-ratio (>10:1) microstructures using dielectrophoresiselectrocapillary force-driven UV-imprinting is demonstrated, which prevents drawbacks seen in conventional imprinting lithography such as mechanically induced mold deformation and position shift thereby maximizing pattern uniformity. 31 Moreover, a generalized formulation for numerical characterization of electrohydrodynamic patterning processes is presented by coupling liquid dielectrophoresis and the phase field of the air-liquid dual phase. 32 Nevertheless, in an uneven large-area substrate, for example, in the case of a curved gallium nitride (GaN) lightemitting diode (LED) epitaxial (epi-) wafer with high stiffness, fabrication of nanostructures with high fidelity still poses a real challenge.…”