Assisted Nanoimprint Lithography Research Articles

Energetic carbon precursors for micro-supercapacitor printing

A highly energetic carbon precursor suitable for the solvent assisted nanoimprint lithography is presented. The interdigital electrodes in the fabricated micro-supercapacitors show excellent device capacitances and high cycling stability.

Characterisation and antibacterial investigation of a novel coating consisting of mushroom microstructures and HFCVD graphite

The resistance of bacteria toward antibacterial drugs is a rising problem. This threat is a major concern for space stations, where antibacterial surfaces would be ideal for materials which also need to be corrosion-resistant, hard and durable. Accordingly, the purpose of this work is to investigate novel coatings that have superhydrophobic mushroom microstructures. The microstructures are made of nickel deposited on a substrate which is composed of a gold layer on top of the silicon. The microstructures were fabricated with UV-light assisted nanoimprint lithography. These superhydrophobic microstructures have a well-defined alignment and shape which does not have any detrimental effect on the plastic deformation of the substrate. Similar structures were coated with carbon by hot-filament chemical vapour deposition (HFCVD) for a duration varying from 30 min to 120 min. Raman spectroscopy shows that the coating is composed of graphite, due to nickel-induced graphitisation during the deposition process. The antibacterial evaluation shows the bare nickel microstructures offer no antibacterial properties despite their superhydrophobic behaviour. On the other hand, the graphitic coated microstructures demonstrate significant antibacterial properties. Especially, 30 min HFCVD coated samples was antibacterial against E. coli and S. aureus with Gram-dependence and dependent on the coating deposition duration.

Fabrication and replication of re-entrant structures by nanoimprint lithography methods

In this work, the authors present and demonstrate a simple method to fabricate and mass replicate re-entrant structures. The method consists of the direct imprinting of polymer mushroomlike microstructures produced by a combination of photolithography and nickel up-plating process. In particular, they have studied the conditions to generate highly robust mushroomlike topographies and their topographical impact on the replication process. They discuss all the imprinting conditions suitable to replicate such topographies using both ultraviolet light assisted nanoimprint lithography (UV-NIL) and thermal NIL methods in two polymer films, poly(methyl methacrylate) and polypropylene, and a hybrid (organic–inorganic) UV light curable photoresist, namely, Ormocomp. Re-entrant topographies have been widely studied for liquid/oil repelling and dry adhesive properties, whereas in their experiments, they have proved evidence for their amphiphobic potential.

In-line metrology for roll-to-roll UV assisted nanoimprint lithography using diffractometry

We describe and discuss the optical design of a diffractometer to carry out in-line quality control during roll-to-roll nanoimprinting. The tool measures diffractograms in reflection geometry, through an aspheric lens to gain fast, non-invasive information of any changes to the critical dimensions of target grating structures. A stepwise tapered linear grating with constant period was fabricated in order to detect the variation in grating linewidth through diffractometry. The minimum feature change detected was ∼40 nm to a precision of 10 nm. The diffractometer was then integrated with a roll-to-roll UV assisted nanoimprint lithography machine to gain dynamic measurements in situ.

Nanopatterned Optical Fiber Tip for Guided Mode Resonance and Application to Gas Sensing

This paper reports on an efficient and convenient method of patterning nanostructures on the cleaved facet of an optical fiber to realize a high-performance fiber-optic gas sensor. The fabrication method utilizes an ultraviolet assisted nanoimprint lithography to transfer nanoscale patterns from a pre-formed stamp to the fiber tip. The novelty of this paper lies in utilizing simpler fabrication steps with better control over angle of contact at the fiber tip, which leads to rapid and precise formation of nanostructures with well-defined features. A periodic array of polymer nanoposts are formed at the fiber tip and coated with titanium dioxide to serve as a guided mode resonant (GMR) device. A gas sensor is realized by coating the GMR structure with a thin layer of graphene oxide (GO) nanosheets. We have utilized the resonance sensitivity of the nanopatterned fiber-tip gas sensor to surrounding refractive index. The abundant functional groups available at GO provides an effective adsorption surface for gas molecules. Microscopic imaging and spectroscopic studies are conducted to illustrate the structural and optical properties, and gas-sensing performance of the sensor. Volatile organic compounds, such as ethylene and methanol, associated with crop plant health, are detected by the sensor. The sensor provides sensitivities of 0.92 and 1.37 pm/ppm for ethylene and methanol vapors, respectively, with a three-fold enhancement in sensitivity and 50% reduction in response time compared with the non-GO coated counterpart. In addition, the sensor demonstrates good stability and reproducibility, thus having a great potential in fiber-optic remote sensing applications.

Highly sensitive detection of paclitaxel by surface-enhanced Raman scattering

The surface-enhanced Raman scattering (SERS) technique was shown to be an effective molecular analytical tool due to its high sensitivity. Here, we propose to exploit soft UV assisted nanoimprint lithography (UV-NIL) for the development of a reproducible and highly-sensitive SERS biosensor. Soft lithography is known to be advantageous for biological applications since it is compatible with insulating supports and large-area samples. In the present investigations, soft UV-NIL is used for the fabrication of large-sized arrays of gold nanocylinders on glass which were shown to be highly sensitive and highly specific sensing surfaces, with a limit of detection measured down to 1 nM. Employing the UV-NIL SERS substrate enable working ranges of nanomolar to micromolar concentrations in regards to our model paclitaxel analyte.

Order quantification of hexagonal periodic arrays fabricated by in situ solvent-assisted nanoimprint lithography of block copolymers

Directed self-assembly of block copolymer polystyrene-b-polyethylene oxide (PS-b-PEO) thin film was achieved by a one-pot methodology of solvent vapor assisted nanoimprint lithography (SAIL). Simultaneous solvent-anneal and imprinting of a PS-b-PEO thin film on silicon without surface pre-treatments yielded a 250 nm line grating decorated with 20 nm diameter nanodots array over a large surface area of up to 4′ wafer scale. The grazing-incidence small-angle x-ray scattering diffraction pattern showed the fidelity of the NIL stamp pattern replication and confirmed the periodicity of the BCP of 40 nm. The order of the hexagonally arranged nanodot lattice was quantified by SEM image analysis using the opposite partner method and compared to conventionally solvent-annealed block copolymer films. The imprint-based SAIL methodology thus demonstrated an improvement in ordering of the nanodot lattice of up to 50%, and allows significant time and cost reduction in the processing of these structures.

Obtaining nanoimprint template gratings with 10 nm half-pitch by atomic layer deposition enabled spacer double patterning

A strategy for fabricating nanoimprint templates with sub-10 nm line and 20 nm pitch gratings is demonstrated, by combining electron beam lithography and atomic layer deposition. This is achieved through pitch division using a spacer double-patterning technique. The nanostructures are then replicated using step-and-repeat ultra-violet assisted nanoimprint lithography.

Macroscopic high density nanodisc arrays of zinc oxide fabricated by block copolymer self-assembly assisted nanoimprint lithography

We report a facile means of creating nanodisc arrays of ZnO with high densities (∼22 Gbit in−2) and narrow distributions in size, shape and periodicities (<15%) using a combination of block copolymer self-assembly and nanoimprint lithography (NIL). ZnO nanodisc arrays with sub-100 nm spatial resolutions, using high-throughput and manufacturing compatible approaches are realized. The fabrication combines benefits from the use of NIL, which is a high-throughput and repeatable tool and from the use of block copolymer self-assembly which provides for low-cost production of high-resolution NIL molds. Preliminary results of the investigation of memory performance of these arrays within MOS capacitor devices show a flat-band voltage shift of 2.53 V at a relatively low operating voltage of 10 V. A high charge trap density of 2.3 × 1018 cm−3 combined with excellent retention of ∼80% after 1000 s of discharging is observed with low tunnelling oxide thickness of 3 nm, demonstrate significant promise of the ZnO nanodiscs to act as charge storage centers in non-volatile flash memory devices.

Large area nanopatterning by combined anodic aluminum oxide and soft UV–NIL technologies for applications in biology

Combining the nanoporous anodic aluminum oxide (AAO) technique with soft lithography constitutes a highly reproducible and quick method for large-area nanopatterning, with increasing potential for applications in biology. The AAO templates were used as master-molds to fabricate large areas (several square centimeters) of nanostructured and ordered tapered PDMS-bumps. The master-molds were filled using hexane-diluted PDMS of reduced viscosity compared to standard one. Viscosity evolution study for different current agent and hexane concentrations was achieved. The as-obtained stamps served subsequently to create large-area highly ordered nanostructured silicon substrates using UV assisted Nanoimprint Lithography (UV–NIL).

UV-nanoimprinting using non-transparent molds and non-transparent substrates

A new ultraviolet assisted nanoimprint lithography technique with an exposure through non-transparent molds and a nm-resolution capability is reported. The UV imprint material was not cured by direct irradiation, but substantially exposed to indirect and diffuse irradiation. The nanoimprint molds consisted of a transparent support and a non-transparent, patterned element. Successful imprints were conducted on transparent glass and polymer foils placed on non-transparent substrate holders as well as on SiO2 on Si. The reported technique enables the application of non-transparent mold materials like Si, new mold materials and alternative antisticking layers like metals in UV-NIL.

A versatile pattern inversion process based on thermal and soft UV nanoimprint lithography techniques

Si master molds are generally patterned by electron-beam lithography (EBL) that is known to be a time-consuming nanopatterning technique. Thus, developing mold duplication process based on high throughput technique such as nanoimprint lithography can be helpful in reducing its fabrication time and cost. Moreover, it could be of interest to get inverted patterns (holes instead of pillars) without changing the master EBL process. In this paper, we propose a two step process based on thermal nanoimprint lithography (T-NIL) (step 1) and soft UV assisted nanoimprint lithography (UV-NIL) (step 2) to invert a master EBL mold. After the two inversion steps, the grand-daughter Si mold exhibits the same pattern polarity as the EBL mold. For step 1, pattern transfer using ion beam etching (IBE) of a thin metallic underlayer is the critical step for dimension control due to the low NXR1020 resistance. For step 2, the optimized reactive ion etching (RIE) step allows transfer with good anisotropy even for nanostructures at the 50 nm-scale. For structures larger than 100 nm, this inversion process has been successfully applied to large field replication (up to 1.5 cm 2) on whole wafer.

Applications of excimer laser in nanofabrication

This paper addresses novel applications of an excimer laser (308 nm wavelength, 20 ns pulse duration) in nanofabrication. Specifically, laser assisted nanoimprint lithography (LAN), self-perfection by liquefaction (SPEL), fabrication of metal nanoparticle arrays, and the fabrication of sub-10-nm nanofluidic channels are covered. In LAN, a polymeric resist is melted by the laser pulse, and then imprinted with a fused silica mold within 200 ns. LAN has been demonstrated in patterning various polymer nanostructures on different substrates with high fidelity and uniformity, and negligible heat effect on both the mold and the substrate. SPEL is a novel technology that uses selective melting to remove fabrication defects in nanostructures post fabrication. Depending on the boundary conditions, SPEL is categorized into three basic types: Open-SPEL that takes place with surface open, Capped-SPEL where a cap plate holds the top surface of the nanostructures and Guided-SPEL where a plate held a distance above the structure guides the molten materials to rise and form a new structure with better profile. Using SPEL (in less than 200 ns), we have achieved a reduction of line edge roughness (LER) of Cr lines to 1.5 nm (3σ) (560% improvement from the original), which is well below what the previous technologies permit, and a dramatic increase of the aspect ratio of a nanostructure. We have used SPEL to make sub-25-nm smooth cylindrical NIL pillar molds and smoothing Si waveguides. Excimer laser is also used to make metal nanoparticles. Monolayers of particles are fabricated on various substrates (silicon, fused silica and plastics) by exposing thin metal films to a single laser pulse. Periodic nanoparticle arrays have been fabricated by fragmentation of metal grating lines. The periodicity of these nanoparticles can be regulated by surface topography such as shallow trenches. Finally, an excimer laser pulse has been used to melt the top portion of 1D and 2D Si gratings to seal off the top surface, forming enclosed nanofluidic channel arrays. The channel width has been further reduced to 9 nm using self-limited thermal oxidation. DNA stretching using 20 nm wide self-sealed channels is also demonstrated.

In situ real time monitoring of nanosecond imprint process

Using a real time imprint monitoring system (RIMS), the authors measured the process time for laser assisted nanoimprint lithography to be about 200ns. They found that during this short period of time, the mold had been fully pressed into the resist, resulting in a full pattern transfer with high fidelity. Their results also demonstrated the capability of RIMS for monitoring an ultrafast imprint process.

Compressed-carbon dioxide (CO 2) assisted nanoimprint lithography using polymeric mold

We have represented a compressed carbon dioxide (CO 2) assisted nanoimprint lithography (CO 2-NIL) technique using a fluoropolymer mold. Glass transition temperature ( T g) of polymethylmethacrylate (PMMA) under the compressed-CO 2 is apparently depressed by the absorption of CO 2, and the plasticized PMMS has enough mobility to rise into the groove of fluoropolymer mold. As a result, the 100 nm level of submicron features are uniformly fabricated on the polymer surface over a large area, and the use of fluoropolymer mold having a low interfacial energy assures the clean release from the patterned surface without a release agent. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) observation confirm these results. CO 2-NIL process could fabricate micro/nano-patterns without thermal cycle, high pressure and anti-adhesion layer compared to conventional NIL.

Laser pulse assisted nanoimprint lithography

A new method of nanofabrication—laser pulse assisted nanoimprint lithography—was investigated. The method is based on direct heating of a polymer film by laser pulse excitation, simultaneously with the imprint phase. Imprint was successfully performed into a S1805 photoresist film using fused silica stamp with additional pulses from a Nd:YAG laser excitation (power density 5, 3 GW/cm2, wavelengths 1064 and 355 nm). The imprint quality may be improved by the selection of an appropriate film material, which has high threshold of ablation and low softening point.

Ultrafast patterning of nanostructures in polymers using laser assisted nanoimprint lithography

We propose and demonstrate a nanopatterning technique, laser-assisted nanoimprint lithography (LAN), in which the polymer is melted by a single excimer laser pulse and then imprinted by a mold made of fused quartz. LAN has been used to pattern nanostructures in various polymer films on a Si or quartz substrate with high fidelity over the entire mold area. Here we show 200 nm pitch gratings with 100 nm linewidth and 90 nm height. The entire imprint from melting the polymer to completion of the imprint is less than 500 ns. The mold has been used multiple times without cleaning between each imprint. LAN not only greatly shortens the imprint processing time, but also significantly reduces the heating and expansion of the substrate and mold, leading to better overlay alignment between the two.