Changsheng Yuan

Hybrid Nanoimprint−Soft Lithography with Sub-15 nm Resolution

We developed a hybrid nanoimprint-soft lithography technique with sub-15 nm resolution. It is capable of patterning both flat and curved substrates. The key component of the technology is the mold, which consists of rigid features on an elastic poly(dimethylsiloxane) (PDMS) support. The mold was fabricated by imprinting a reverse image onto the PDMS substrate using a UV-curable low-viscosity prepolymer film. Patterns with sub-15-nm resolution were faithfully duplicated on a flat substrate without applying external pressure. Gratings at 200 nm pitch were also successfully imprinted onto the cylindrical surface of a single mode optical fiber with a 125 microm diameter.

Crack-free controlled wrinkling of a bilayer film with a gradient interface

We report a crack-free controlled wrinkling method based on a bilayer film system. A liquid UV-curable film is solidified on a uniaxially pre-stretched PDMS elastic sheet by UV-exposure. Subsequently, the sheet is released back to its initial non-stretched state, which results in spontaneous formation of grating wrinkles perpendicular to the stretching direction. An interface of gradient interpenetrating polymer networks (IPN) is considered to be formed between the stiff UV-cured film and the elastic support, which is practically beneficial for preventing crack formation and film delamination during the strain relaxation process. The periodicity of the gratings is tuned by controlling the thickness of the UV-cured polymer film and the amplitude of pre-strain of the elastic sheet. The imprinting results demonstrate that these self-formed wrinkles can serve as a mold to duplicate gratings by nanoimprint lithography. Furthermore, metal gratings are successfully fabricated from the wrinkling molds.

A New Strategy of Lithography Based on Phase Separation of Polymer Blends

Herein, we propose a new strategy of maskless lithographic approach to fabricate micro/nano-porous structures by phase separation of polystyrene (PS)/Polyethylene glycol (PEG) immiscible polymer blend. Its simple process only involves a spin coating of polymer blend followed by a development with deionized water rinse to remove PEG moiety, which provides an extremely facile, low-cost, easily accessible nanofabrication method to obtain the porous structures with wafer-scale. By controlling the weight ratio of PS/PEG polymer blend, its concentration and the spin-coating speed, the structural parameters of the porous nanostructure could be effectively tuned. These micro/nano porous structures could be converted into versatile functional nanostructures in combination with follow-up conventional chemical and physical nanofabrication techniques. As demonstrations of perceived potential applications using our developed phase separation lithography, we fabricate wafer-scale pure dielectric (silicon)-based two-dimensional nanostructures with high broadband absorption on silicon wafers due to their great light trapping ability, which could be expected for promising applications in the fields of photovoltaic devices and thermal emitters with very good performances, and Ag nanodot arrays which possess a surface enhanced Raman scattering (SERS) enhancement factor up to 1.64 × 108 with high uniformity across over an entire wafer.

Phase Separation of Silicon-Containing Polymer/Polystyrene Blends in Spin-Coated Films

In this Article, two readily available polymers that contain silicon and have different surface tensions, polydimethylsiloxane (PDMS) and polyphenylsilsequioxane (PPSQ), were used to produce polymer blends with polystyrene (PS). Spin-coated thin films of the polymer blends were treated by O2 reactive-ion etching (RIE). The PS constituent was selectively removed by O2 RIE, whereas the silicon-containing phase remained because of the high etching resistance of silicon. This selective removal of PS substantially enhanced the contrast of the phase separation morphologies for better scanning electron microscope (SEM) and atomic force microscope (AFM) measurements. We investigated the effects of the silicon-containing constituents, polymer blend composition, concentration of the polymer blend solution, surface tension of the substrate, and the spin-coating speed on the ultimate morphologies of phase separation. The average domain size, ranging from 100 nm to 10 μm, was tuned through an interplay of these factors. In addition, the polymer blend film was formed on a pure organic layer, through which the aspect ratio of the phase separation morphologies was further amplified by a selective etching process. The formed nanostructures are compatible with existing nanofabrication techniques for pattern transfer onto substrates.

Fabrication of wafer-scale nanopatterned sapphire substrate by hybrid nanoimprint lithography

A hybrid nanoimprint soft lithography (HNSL) technique was used to fabricate nanopatterned sapphire substrates (NPSSs) for light-emitting diodes (LEDs). HNSL combines the high resolution of nanoimprint lithography (NIL) and the conformal contact of soft lithography. The key component of HNSL is the hybrid mold, which consists of rigid nanopatterns with an anti-adhesion coating on an elastic poly(dimethylsiloxane) support. The mold was used to fabricate nanopatterns on a 2-in. sapphire substrate through a soft UV-NIL system with a double-layer resist, a top UV-curable layer, and an underlying PMMA layer. Nickel dot arrays were formed from the imprinted patterns through a lift-off process and used as the etching mask during the sapphire etching process due to nickels high etching resistance. A wafer-scale circular-truncated-cone shaped NPSS was achieved by chlorine-based inductively coupled plasma etching. Typical blue LEDs with emission wavelengths of 452 nm were grown by metal-organic chemical vapor depo...

High resolution soft mold for UV-curing nanoimprint lithography using an oxygen insensitive degradable material

Soft nanoimprint lithography has been developed to overcome the disadvantages of conventional nanoimprint lithography based on rigid molds. Hybrid nanoimprint-soft lithography mold is an efficient strategy to improve the resolution of soft nanoimprint because a rigid UV-curable material is used as the structural layer. In this paper, the authors design a novel UV-curable material for hybrid soft mold fabrication, which is degradable under mild acidic conditions and insensitive to oxygen during photopolymerization. The material comprises an acid-degradable cross-linker, 2,10-diacryloyloxymethyl-1,4,9,12-tetraoxaspiro[4.2.4.2] tetradecane, and an acyrlated polysiloxane, poly[(mercaptopropyl)methylsiloxane]. Oxygen sensitivity of acrylate groups during UV curing is avoided due to the cross-linking mechanism based on thiol-ene chemistry. The cured material can be decomposed into linear chains through the cleavage of acid-labile ketal links and dissolved in organic solvent when heated in an acidic solution. Th...

An oxygen-insensitive degradable resist for fabricating metallic patterns on highly curved surfaces by UV-nanoimprint lithography.

In this paper, an oxygen-insensitive degradable resist for UV-nanoimprint is designed, com-prising a polycyclic degradable acrylate monomer, 2,10-diacryloyloxymethyl-1,4,9,12-tetraoxa-spiro [4.2.4.2] tetradecane (DAMTT), and a multifunctional thiol monomer pentaerythritol tetra(3-mercaptopropionate) (PETMP). The resist can be quickly UV-cured in the air atmosphere and achieve a high monomer conversion of over 98%, which greatly reduce the adhesion force between the resist and the soft mold. High conversion, in company with an adequate Youngs modulus (about 1 GPa) and an extremely low shrinkage (1.34%), promises high nanoimprint resolution of sub-50 nm. The cross-linked resist is able to break into linear molecules in a hot acid solvent. As a result, metallic patterns are fabricated on highly curved surfaces via the lift off process without the assistance of a thermoplastic polymer layer.

A fast thermal-curing nanoimprint resist based on cationic polymerizable epoxysiloxane

We synthesized a series of epoxysiloxane oligomers with controllable viscosity and polarity and developed upon them a thermal-curable nanoimprint resist that was cross-linked in air at 110°C within 30 s if preexposed to UV light. The oligomers were designed and synthesized via hydrosilylation of 4-vinyl-cyclohexane-1,2-epoxide with poly(methylhydrosiloxane) with tunable viscosity, polarity, and cross-linking density. The resist exhibits excellent chemical and physical properties such as insensitivity toward oxygen, strong mechanical strength, and high etching resistance. Using this resist, nanoscale patterns of different geometries with feature sizes as small as 30 nm were fabricated via a nanoimprint process based on UV-assisted thermal curing. The curing time for the resist was on the order of 10 s at a moderate temperature with the help of UV light preexposure. This fast thermal curing speed was attributed to the large number of active cations generated upon UV exposure that facilitated the thermal polymerization process.

Wafer-scale fabrication of metal nanoring and nanocrescent arrays from nanoimprinted nanopillar arrays

Abstract. An approach was presented to fabricate wafer-scale nanoring structures based on nanopillar array templates fabricated by nanoimprint lithography. This fabrication method combined UV-curing nanolithography technology, metal deposition, and an etching process, which made it possible to tune the geometric properties of nanorings: height, diameter, and linewidth for various materials, such as Au and Ni. Nanoring arrays showed potential applications in many fields, including memory storage, biosensing, and optical devices. The optical measurement of Au nanorings (d=380  nm) showed its strong transmission resonances at the wavelength of 2.1  μm. A modified version of this fabrication method by depositing Ni in a controlled angle as a sacrificial layer was also utilized to create nanocrescent arrays. This modified method could easily tune the width of crescents through the nickel deposition angles and nanopillar template heights. The large-area gold nanocrescent arrays showed strong polarization-dependent transmission bands. Plasmonic crescent structures were expected to apply in infrared metamaterial and chemical sensing.

A high-performance and low cost SERS substrate of plasmonic nanopillars on plastic film fabricated by nanoimprint lithography with AAO template

As a powerful spectroscopy technique, surface-enhanced Raman scattering (SERS) can provide non-destructive and sensitive characterization down to a single molecular level. Aiming to the main challenges of high-performance SERS-active substrates for their real-world applications involving the ultra-sensitive and reproducible signals detection and signal uniformity with large-area, herein, a facile and reliable strategy based on combination of thermal imprinting polycarbonate (PC) film with porous anodic aluminum oxide (AAO) mold and E-beam evaporation of gold is provided to fabricate a high-quality SERS-active substrate consisting of ultra-dense hot-spots with large-area uniformity. Two kinds of sub-10 nm gaps were obtained, including the nanogaps between the neighboring gold coated PC-nanopillars and those between gold on the top of the nanopillars and that on the base, which actually build up a three-dimensional (3D) hot-spot network for high-performance SERS detection. The effect of structural parameter...