Conformable Solid‐Index Phase Masks Composed of High‐Aspect‐Ratio Micropillar Arrays and Their Application to 3D Nanopatterning

Abstract

Three-dimensional (3D) microstructures are essential elements in various technological applications, including optical coatings, [ 1 , 2 ] tissue engineering scaffolds, [ 3 ] microfl uidics, [ 4 ] energy storage devices, [ 5 , 6 ] plasmonics, [ 7 ] and photonic crystals. [ 8 , 9 ] While conventional methods, such as photo lithography and electron beam lithography, are costly and ineffective in the generation of complex 3D patterns over large areas, unconventional methods based on soft lithography, such as nanoimprint lithography (NIL), [ 10 ] nanotransfer printing (nTP), [ 11 ] microcontact printing ( μ CP), [ 12 ] and micromolding in capillaries (MIMIC), [ 13 ] provide simple and inexpensive routes to fabricate complex multidimensional nanostructures. Above all, proximity fi eld nanopatterning (PnP), [ 14 ] which uses elastomeric components similar to those used in soft lithography, can generate complex 3D nanostructures very rapidly over large areas. Optical interference from the conformable elastomers, working as phase shift elements, generates a complex 3D intensity distribution by the Talbot effect, [ 15 ] or self-imaging effect. The diffracted images of surface relief structures are periodically repeated at integer or fractional Talbot distances from the surface of the mask.Pattern resolution in soft lithography is mainly determined by the fi neness and the aspect ratio of elastomeric surface relief structures, which generally consist of poly(dimethylsiloxane) (PDMS), from a Si master that contains the desired relief patterns on its surface. The height of the relief structures is particularly important in PnP because it defi nes the magnitude of the phase shift as a function of the refractive index and the wavelength of the incident light. [ 16 ] Hence, relief structures with a high aspect ratio and narrow spacings ( < 1 μ m) between features must be well-controlled (i.e., straightness, duty cycle). Currently, two critical issues complicate control of these requirements. The fi rst issue is the limited thickness and physical damage of a patterned photoresist on a Si wafer that serves as a master; a thicker resist can lead to greater diffi culty and a higher potential for damage during the photopatterning and replication processes. The second issue is the low Young's modulus of soft elastomers ( ≈ 2-9 MPa), [ 17 ] which causes a collapse or a break in the neck of elastomeric relief structures, even after fi ne replication from the master. The low mechanical property also results in problems such as surface wrinkling, [ 18 ] sagging, buckling, and deforming [ 19 , 20 ] of the elastomers. Thus, reproducibility of the patterning cannot be guaranteed when using conformal contact-dependent methods. Several mold materials, such as polyolefi n, [ 21 ] modifi ed polyurethane, [ 22 ] PDMS, [ 23 , 24 ] and perfl uoropolyether (PFPE), [ 25 ] have been proposed to increase the Young's modulus and aspect ratio of the relief structures. Nevertheless, dense and fi ne relief structures with a high aspect ratio compose...

Shape‐Controllable Microlens Arrays via Direct Transfer of Photocurable Polymer Droplets

“…As shown, the ratio of the fi nal lens height to diameter (h F /d F = 0.271) was nearly the same with the equilibrium ratio (h E /d E = 0.28). Furthermore, the resulting polymer droplet showed a perfectly smooth plano-convex lens due to its high surface tension ( ∼ 25 dyne/ cm), [ 29 ] as determined by the three-dimensional Atomic Force Microscope (AFM) image of the transferred PUA lens array ( Figure S2). As can be seen in Figure S2 in the Supporting Information, the lens curvature was almost overlapped with the perfect sphere, suggesting that the surface be smooth and uniformly curved.…”

Three-dimensional nanonetworks for giant stretchability in dielectrics and conductors

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Flexible subwavelength gratings fabricated by reversal soft UV nanoimprint

“…Figure 1 shows the calculated results of the binary SWGs shown on the right of Fig. 1 transmission [11] . In the calculation, the grating period is 200 nm and the line width is 100 nm.…”

“…Using the lateral collapse theory established by Hui, Junyong Park et al discovered that cured PUA has a sufficiently high rigidity (≈1.7 GPa), greater than E * [11] c , which theoretically means no lateral collapse. However, during our experiments, collapse caused by the low rigidity still occurred when the curing dose was not sufficient to cure the PUA completely, as shown in Fig.…”