Yuyang Liu

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.

Condensation-induced wetting state and contact angle hysteresis on superhydrophobic lotus leaves

In this paper, we demonstrate how condensed moisture droplets wet classical superhydrophobic lotus leaf surfaces and analyze the mechanism that causes the increase of contact angle hysteresis. Superhydrophobic lotus leaves in nature show amazing self-cleaning property with high water contact angle (>150°) and low contact angle hysteresis (usually <10°), causing droplets to roll off at low inclination angles, in accordance with classical Cassie–Baxter wetting state. However, when superhydrophobic lotus leaves are wetted with condensation, the condensed water droplets are sticky and exhibit higher contact angle hysteresis (40–50°). Compared with a fully wetted sessile droplet (classical Wenzel state) on the lotus leaves, the condensed water droplet still has relatively large contact angle (>145°), suggesting that the wetting state deviates from a fully wetted Wenzel state. When the condensed water droplets are subjected to evaporation at room conditions, a thin water film is observed bridging over the micropillar structures of the lotus leaves. This causes the dew to stick to the surface. This result suggests that the condensed moisture does not uniformly wet the superhydrophobic lotus leaf surfaces. Instead, there occurs a mixed wetting state, between classical Cassie–Baxter and Wenzel states that causes a distinct increase of contact angle hysteresis. It is also observed that the mixed Cassie–Baxter/Wenzel state can be restored to the original Cassie–Baxter state by applying ultrasonic vibration which supplies energy to overcome the energy barrier for the wetting transition. In contrast, when the surface is fully wetted (classical Wenzel state), such restoration is not observed with ultrasonic vibration. The results reveal that although the superhydrophobic lotus leaves are susceptible to being wetted by condensing moisture, the configured wetting state is intermediate between the classical Cassie–Baxter and Wenzel states.

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.

From nanocone to nanodisc: Structural transformation of gold nanoarrays via simple mechanical stresses

The authors demonstrate a simple fabrication process that enables to tune the shape, alignment, and dimension of gold nanocone arrays through mechanical stresses. The initial gold nanocone arrays were fabricated by lift-off process using the high-aspect-ratio nanopore arrays defined by interference lithography. Gold nanoarrays with continuous variation in their structural size, shape, orientation, and interspace were achieved by applying either single or combined mechanical stresses of normal pressure and tangential shear. With the tailored normal pressure level, the shapes of gold nanoarrays transformed from nanocones to nanopillars, and then nanodiscs. In the compression mode, the aspect ratio and interstructural gap spacing were controllable isotropically. When tangential shear stress was combined with the normal stress, the nanocones were bent anisotropically to the shearing direction. With an increase of the tangential stress level, the nanocone arrays were overturned and flattened, forming asymmetri...

3-D nanofabrication using nanostructured photoresist film as free-standing appliqué

This paper reports a three-dimensional (3-D) nanofabrication method using a nano-patterned photoresist (PR) film as a free-standing flexible template. In contrast to the conventional usage of PR as a two-dimensional (2-D) pattern transfer layer on a planar substrate in MEMS applications, the lithographically predefined PR layer is released from the supporting substrate in the form of a free-standing film and then applied as an appliqué for the 3-D (e.g., onto a curved surface) and hierarchical (e.g., onto a micro-textured surface) nano-patterning. The ultrathin free-standing PR films show excellent flexibility, integrality, and robustness under various mechanical manipulations and further pattern transfer processes such as deposition and etching after being appliquéd. Enabled by such versatility, periodic nanostructures are successfully demonstrated on various substrates via various fabrication techniques. The results suggest that the application of the free-standing PR films as appliqué will enables them to be used as flexible and re-useable templates or elements in MEMS fabrication and devices, especially for convenient and efficient 3-D nanofabrication over a large area.

Manipulation of the Superhydrophobicity of Plasma-Etched Polymer Nanostructures

The manipulation of droplet mobility on a nanotextured surface by oxygen plasma is demonstrated by modulating the modes of hydrophobic coatings and controlling the hierarchy of nanostructures. The spin-coating of polytetrafluoroethylene (PTFE) allows for heterogeneous hydrophobization of the high-aspect-ratio nanostructures and provides the nanostructured surface with “sticky hydrophobicity”, whereas the self-assembled monolayer coating of perfluorodecyltrichlorosilane (FDTS) results in homogeneous hydrophobization and “slippery superhydrophobicity”. While the high droplet adhesion (stickiness) on a nanostructured surface with the spin-coating of PTFE is maintained, the droplet contact angle is enhanced by creating hierarchical nanostructures via the combination of oxygen plasma etching with laser interference lithography to achieve “sticky superhydrophobicity”. Similarly, the droplet mobility on a slippery nanostructured surface with the self-assembled monolayer coating of FDTS is also enhanced by employing the hierarchical nanostructures to achieve “slippery superhydrophobicity” with modulated slipperiness.