Ishan Wathuthanthri

Highly Ordered Hollow Oxide Nanostructures: The Kirkendall Effect at the Nanoscale

Highly ordered ultra-long oxide nanotubes are fabricated by a simple two-step strategy involving the growth of copper nanowires on nanopatterned template substrates by magnetron sputtering, followed by thermal annealing in air. The formation of such tubular nanostructures is explained according to the nanoscale Kirkendall effect. The concept of this new fabrication route is also extendable to create periodic zero-dimensional hollow nanostructures.

Large-area pattern transfer of metallic nanostructures on glass substrates via interference lithography

In this paper, we report a simple and effective nanofabrication method for the pattern transfer of metallic nanostructures over a large surface area on a glass substrate. Photoresist (PR) nano-patterns, defined by laser interference lithography, are used as template structures where a metal film of controlled thickness is directly deposited and then transferred onto a glass substrate by the sacrificial etching of the PR inter-layer. The laser interference lithography, capable of creating periodic nano-patterns with good control of their dimensions and shapes over a relatively large area, allows the wafer-scale pattern transfer of metallic nanostructures in a very convenient way. By using the approach, we have successfully fabricated on a glass substrate uniform arrays of hole, grating, and pillar patterns of Ti, Al, and Au in varying pattern periodicities (200 nm-1 µm) over a surface area of up to several cm(2) with little mechanical crack and delamination. Such robust metallic nanostructures defined well on a transparent glass substrate with large pattern coverage will lead to advanced scientific and engineering applications such as microfluidics and nanophotonics.

Two degrees-of-freedom Lloyd–mirror interferometer for superior pattern coverage area

In this study, we have developed a tunable Lloyd-mirror interferometer with two degrees of freedom, in contrast to a traditional system with one degree of freedom. This new Lloyd-mirror interferometer allows an angular rotation of the mirror independently from that of a sample stage, resulting in an increased pattern coverage area with tunable pattern periodicity. Both theoretical and experimental results verify that the tunable characteristic of the modified configuration enhances the nanopatterning capabilities of the Lloyd-mirror interference lithography system especially in achieving greater pattern coverage area for larger pattern periodicities.

Tunable two-mirror interference lithography system for wafer-scale nanopatterning

We have designed and analyzed a novel (to the best of our knowledge) two-beam interference lithography system for large-area (wafer-level) nanopatterning with enhanced tunability of pattern periodicities. The tunable feature has been achieved by placing two rotational mirrors in the expanded beam paths at regulated angles for a desired period. Theoretical analyses show that the effective pattern coverage area greater than a 4 in. (10 cm) wafer scale is attainable with a 325 nm (30 cm coherence length) HeCd laser and 4 in. (10 cm) mirrors, while the pattern coverage area is restrained by the overruling effects between the optical coherence and mirror size. The experimental results also demonstrate uniform nanopatterns at varying periods (250-750 nm) on 4 in. (10 cm) substrates, validating the theoretical analyses. The tunable two-mirror interferometer will offer a convenient and robust way to prepare large-area nanostructures on a wafer scale with superior tunability in their pattern periodicities.

Wafer-scale pattern transfer of metal nanostructures on polydimethylsiloxane (PDMS) substrates via holographic nanopatterns.

In this paper, we report on a cost-effective and simple, nondestructive pattern transfer method that allows the fabrication of metallic nanostructures on a polydimethylsiloxane (PDMS) substrate on a wafer scale. The key idea is to use holographic nanopatterns of a photoresist (PR) layer as template structures, where a metal film is directly deposited in order to replicate the nanopatterns of the PR template layer. Then, the PDMS elastomer is molded onto the metal film and the metal/PDMS composite layer is directly peeled off from the PR surface. Many metallic materials including Ti, Al, and Ag were successfully nanopatterned on PDMS substrates by the pattern transfer process with no use of any adhesion promoter layer or coating. In case of Au that has poor adhesion to PDMS material, a salinization of the metal surface with 3-(aminopropyl)-triethoxysilane (APTES) monolayer promoted the adhesion and led to successful pattern transfer. A series of adhesion tests confirmed the good adhesion of the transferred metal films onto the molded PDMS substrates, including scotch-tape and wet immersion tests. The inexpensive and robust pattern transfer approach of metallic nanostructures onto transparent and flexible PDMS substrates will open the new door for many scientific and engineering applications such as micro-/nanofluidics, optofluidics, nanophotonics, and nanoelectronics.

Transfer patterning of large-area graphene nanomesh via holographic lithography and plasma etching

The authors present a high-throughput fabrication technique to create a large-area graphene nanomesh (GNM). A patterned negative photoresist layer was used as an etch mask atop chemical vapor deposition grown graphene on Cu foil. Shielded by the periodic nanopatterned photoresist mask, the graphene layer was selectively etched using O2 plasma, forming a GNM layer. A poly(methyl methacrylate) layer was spun on the GNM atop copper foil, and the GNM was subsequently transferred onto a SiO2/Si substrate by etching away the copper foil. Large-area (5 × 5 cm), periodic (500 and 935 nm in pitch), uniform, and flexible GNMs were successfully fabricated with precisely controlled pore sizes (200–900 nm) and neck widths (down to ∼20 nm) by adjusting the pattern generation of holographic lithography and the O2 plasma etching process parameters. This holographic lithography-based transfer method provides a low-cost manufacturing alternative for large-area, nanoscale-patterned GNMs on an arbitrary substrate.

Dual applications of free-standing holographic nanopatterns for lift-off and stencil lithography

In this article, the authors report a new lift-off process to obtain nanoporous free-standing trilayer film of metal/photoresist/antireflective coating (ARC) stack and to reuse the thin and flexible membrane as a versatile stencil lithography mask for the dual purposes. For the initial lift-off process of metal nanostructures, nanoperiodic pore patterns of the photoresist/ARC bilayer were first defined by holographic lithography and reactive ion etching on a silicon substrate. Then a thin metal layer was deposited through the nanopores, forming the uniform array of metal nanostructures on the silicon substrate. Different from a traditional lift-off process, the by-product of the metal/photoresist/ARC trilayer was not dissolved away but released intact from the substrate as a free-standing membrane by using a specially designed solution (NH3/H2O2/H2O). It uniquely allows the use of the free-standing membrane as a novel stencil for direct bonding and continuous release onto/from various new substrates, incl...

Fabrication of hierarchical nanostructures using free-standing trilayer membrane

Hierarchical nanostructures are typically fabricated with multiple lithography steps on prepatterned micro- or nanostructures. However, such conventional multiple lithography steps result in poor coverage and uniformity, especially when patterning on high-aspect-ratio micro- or nanostructures. In this work, the authors present a new fabrication method which can make hierarchical nanostructures even on high-aspect-ratio prepatterns, using a free-standing trilayer membrane. The nanoporous trilayer composite films, consisting of metal/photoresist/antireflective coating, is created on a planar silicon substrate by using laser interference lithography, reactive ion etching, and e-beam deposition. Then, a customized solution of ammonia (NH3), hydrogen peroxide (H2O2), and water (H2O) is used to release the composite film from the supporting substrate. The free-standing composite membrane is then transferred on a prepatterned silicon substrate. Showing no major defects on the membrane surface, the flexible and s...

Stencil Lithography for Scalable Micro- and Nanomanufacturing

In this paper, we review the current development of stencil lithography for scalable micro- and nanomanufacturing as a resistless and reusable patterning technique. We first introduce the motivation and advantages of stencil lithography for large-area micro- and nanopatterning. Then we review the progress of using rigid membranes such as SiNx and Si as stencil masks as well as stacking layers. We also review the current use of flexible membranes including a compliant SiNx membrane with springs, polyimide film, polydimethylsiloxane (PDMS) layer, and photoresist-based membranes as stencil lithography masks to address problems such as blurring and non-planar surface patterning. Moreover, we discuss the dynamic stencil lithography technique, which significantly improves the patterning throughput and speed by moving the stencil over the target substrate during deposition. Lastly, we discuss the future advancement of stencil lithography for a resistless, reusable, scalable, and programmable nanolithography method.

Large-Scale Fabrication of Porous Gold Nanowires via Laser Interference Lithography and Dealloying of Gold–Silver Nano-Alloys

In this work, we report on an efficient approach to fabricating large-area and uniform planar arrays of highly ordered nanoporous gold nanowires. The approach consists in dealloying Au–Ag alloy nanowires in concentrated nitric acid. The Au–Ag alloy nanowires were obtained by thermal annealing at 800 °C for 2 h of Au/Ag stacked nanoribbons prepared by subsequent evaporation of silver and gold through a nanograted photoresist layer serving as a mask for a lift-off process. Laser interference lithography was employed for the nanopatterning of the photoresist layer to create the large-area nanostructured mask. The result shows that for a low Au-to-Ag ratio of 1, the nanowires tend to cracks during the dealloying due to the internal residual stress generated during the dealloying process, whereas the increase of the Au-to-Ag ratio to 3 can overcome the drawback and successfully leads to the obtainment of an array of highly ordered nanoporous gold nanowires. Nanoporous gold nanowires with such well-regulated organization on a wafer-scale planar substrate are of great significance in many applications including sensors and actuators.