Observation of High-Aspect-Ratio Nanostructures Using Capillary Lithography

,by K. Y. Suh, S.-J. Choi, S. J. Baek, T. W. Kim, and R. Langer, Adv. Mater. 2005, 17, No. 5, March 8

AgendaPeriodic nanostructureCapillary lithographyResults

Nanostructure for 300 nm High Mold for Different Film Thickness

Nanostructure for 500 nm High Mold for Different Film Thickness

Nanostructure for 450 nm High Mold for Different Polymer

Conclusion

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Periodic NanostructurePotential application

Photonic crystalsData storageNanometer-scale

biological sensor

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Photonic crystals, from S. M. Yang, G. A. Ozin, Chem. Commun. 2000, 2507

Data storage, from Y.-W. Chen, Y.-H. Tang, L.-Z Pei, and C. Guo,Adv. Mater 2005, 17, No. 5, March 8

Nanometer-scale biological sensor, from K.-B. Lee, S.-J. Park, C.-A. Mirkin, J. C. Smith, and M. Mrksich, Science 2002, 295, 1702.

Capillary Lithography (I)Temperature-Induced Capillarity

Solvent-Induced Capillarity

A small gap is present between the mold and the polymer.

No gap is present since the solvent wet the entire mold surface.

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Place the mold on the polymer surface

Heating

Cooling and mold remove

Slight pressing

Solvent evaporate

Remove the model

Solvent polymer

Mold

Capillary Lithography (II)Material for capillary

lithography Mold

Polyurethane acrylate Polymer

Poly(ethylene glycol)-based (PEG-based) random copolymer

Capillary rise is not expected but happens. It provides several unexpected nanostructure as a result of gas permeation and different wetting condition.

Permeability issue comes into play in the case of the polyurethane acrylate mold, especially when solvent-induced capillary was utilized.

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Capillary Lithography (III)Depending on the

etching conditions of the original silicon master, the height of the voids of the mode could be controlled.

The voids are of a truncated cone shape, with dimensions of 150-200 nm at the base and 70-100 nm at the top.

a) SEM images of the positive mold with step heights of 300 nm.

b) SEM images of the positive mold with step heights of 500 nm.

c) SEM image of the replicated negative mold.

d) An example of the sheet-type polyurethane acrylate mold.

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Nanostructure for 300 nm High Mold for Different Film ThicknessThe max height of the

structure is (2/3) of the mold height. Be achieved by

Maintain conformal contact over the entire surface without many trapped bubbles To avoid air trapped in the

voids to escape Air permeability of the

mode is very small No air escape through the

mode The solubility of air in the

polymer solution is smaller Valid for the water is the

solvent

Two different nanostructures Nanopillar has an average height of

300 nm (for thin films < 500 nm). Nanosphere has an average height of

170-200 nm (for thick films >800 nm).

• SEM images of the two types of nanostructures formed when the mold with a step height of 300 nm is used. a) Formation of nanopillars of reduced diameter. A dimple next to a nanopillar is indicated by an arrow in the inset. b) Formation of anao-hemispheres. Some hemisphere that are peeled off a shown as voids in the inset.

dimple

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Peel off

Nanostructure for 500 nm High Mold for Different Film Thickness (I) Thin film forms mushroom-like nanopillar

Mushroom-like nanopillar A structure in which the head is larger than the body. Caused by the polymer solution wets the adjacent dry region in the mold void

after reaching the ceiling. The orientation varies for each nanopillar because of different wetting path

Has a reduced diameter. Dimples are formed next the nanopillar

A schematic illustration for the formation of the two kinds of nanostructure, depending on wetting condition

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Nanostructure for 500 nm High Mold for Different Film Thickness (II) Thin film forms mushroom-like

nanopillar Mushroom-like nanopillar Has a reduced diameter. Dimples are formed next the

nanopillar. Thick film forms nanospheres

The diameter of the sphere is 150-170 nm.

The spaces between nanospheres is 500 nm.

Agrees with the mold.

SEM images of the two types of nanostructure formed using the mold with a step height of 500 nm. a, c) Formation of mushroom-like nanopillar with a reduced diameter; large-scale (a) and magnified (c) view are shown. A dimple next to a nanopillar is indicated by an arrow in (c). b, d) Formation of nanospheres; larger scale (b) and magnified (d) views are shown.

dimple

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Nanostructure for 450 nm High Mold for Different Polymer (II)Use 450 nm high mold with different polymer to form

nanopillar SPS has a reduced 90 nm diameter PEG has a reduced 110 nm diameter The formation of vertical sidewalls is quite reproducible. The black spots next to the pillars in the figures are dimples

formed by pressure buildup.

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SEM images of well-defined, high-aspect-ratio nanopillars using a) SPS and b) PEG copolymer. The diameters at the base are 90 nm and 110 nm, respectively.

ConclusionPresent the observation of several nanostructures, such

as mushroom-like nanopillars, vertical nanopillars, and nanospheres, using capillary lithography with UV-curable, polyurethane acrylate mold.

Air permeation during capillary plays an important role in pattern replication.

Depends on the thickness of the film, a nanopillar or nanosphere will be produced.

The step height of the mold could be adjusted to obtain well-defined vertical nanopillars less than the step height of mold.

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