Zhimou Xu

Accurate characterization of nanoimprinted resist patterns using Mueller matrix ellipsometry.

In order to control nanoimprint lithography processes to achieve good fidelity, accurate characterization of structural parameters of nanoimprinted resist patterns is highly desirable. Among the possible techniques, optical scatterometry is relatively ideal due to its high throughput, low cost, and minimal sample damage. Compared with conventional optical scatterometry, which is usually based on reflectometry and ellipsometry and obtains at most two ellipsometric angles, Mueller matrix ellipsometry (MME) based scatterometry can provide up to 16 quantities of a 4 × 4 Mueller matrix in each measurement and can thereby acquire much more useful information about the sample. In addition, MME has different measurement accuracy in different measurement configurations. It is expected that much more accurate characterization of nanoimprinted resist patterns can be achieved by choosing appropriate measurement configurations and fully using the rich information hidden in the measured Mueller matrices. Accordingly, nanoimprinted resist patterns were characterized using an in-house developed Mueller matrix ellipsometer in this work. We have experimentally demonstrated that not only more accurate quantification of line width, line height, sidewall angle, and residual layer thickness of nanoimprinted resist patterns can be achieved, but also the residual layer thickness variation over the illumination spot can be directly determined, when performing MME measurements in the optimal configuration and meanwhile incorporating depolarization effects into the optical model. The comparison of MME-extracted imprinted resist profiles has also indicated excellent imprint pattern fidelity.

Memristor‐Integrated Voltage‐Stabilizing Supercapacitor System

Voltage-stabilized supercapacitors: A single supercapacitor formed with PCBM/Pt/IPS nanorod-array electrodes is designed and delivers enhanced areal capacitance, capacitance retention, and excellent electrical stability under bending, while a significant voltage-decrease is observed during the discharging process. Once integrated with the memristor, the memristor-integrated supercapacitor systems deliver an extremely low voltage-drop, indicating greatly enhanced voltage-stabilizing features.

Porous Light-Emitting Diodes With Patterned Sapphire Substrates Realized by High-Voltage Self-Growth and Soft UV Nanoimprint Processes

Nanostructured GaN-based light-emitting diode (LED), with its high performance on light extraction efficiency, has attracted significant attention for the potential application in solid state lighting. However, patterning structures at nanoscale feature size with large area and low cost is of great importance and hardness. In this paper, a 2 inch anodic aluminum oxide (AAO) template was used as the initial mold to copy the photonic-crystal-like structures (PCLSs) on a blue-light LED by soft UV nanoimprint lithography. An additional solute, aluminum oxalate hydrate, is employed to overcome the burn-through issue in the high-voltage anodization which is critical for the fabrication of the large-pore (250-500 nm) AAO. The photoluminescence and electroluminescence enhancements of the patterned LED device with 150-nm-deep PCLSs are, respectively, 45% and 11.4% compared with the un-patterned LED device. A 3-D finite-difference time-domain simulation confirms that the light extraction efficiency is enhanced when PCLSs are formed. The proposed method is simple, cheap, repeatable in large area and compatible with the high volume production lines.

Mueller matrix ellipsometric detection of profile asymmetry in nanoimprinted grating structures

Mueller matrix ellipsometry (MME) is applied to detect foot-like asymmetry encountered in nanoimprint lithography (NIL) processes. We present both theoretical and experimental results which show that MME has good sensitivity to both the magnitude and direction of asymmetric profiles. The physics behind the use of MME for asymmetry detection is the breaking of electromagnetic reciprocity theorem for the zeroth-order diffraction of asymmetric gratings. We demonstrate that accurate characterization of asymmetric nanoimprinted gratings can be achieved by performing MME measurements in a conical mounting with the plane of incidence parallel to grating lines and meanwhile incorporating depolarization effects into the optical model. The comparison of MME-extracted asymmetric profile with the measurement by cross-sectional scanning electron microscopy also reveals the strong potential of this technique for in-line monitoring NIL processes, where symmetric structures are desired.

Resistive switching behavior of photochemical activation solution-processed thin films at low temperatures for flexible memristor applications

This study explores deep ultraviolet photochemically activated solution-processed metal-oxide thin films at room temperature for fabrication of flexible memristor active resistive layers. An annealing treatment was not required during the process. Solution processed undoped and Mn-doped ZnO thin films served as active layers in the resistive random access memory structure, prepared at 145 °C. The carrier transports in high and low electrical fields were dominated by Frenkel–Poole emission and thermionic emission, respectively. The trap energy level, which originated primarily from Vo or the singly charged oxygen vacancy, was calculated at 0.49 eV. A flexible structure consisting of Ag/DUV-ZnO/indium tin oxide/polyethylene terephthalate was fabricated successfully and its mechanical performance was investigated.

Polymer nanopillar array with Au nanoparticle inlays as a flexible and transparent SERS substrate

We report a facile and efficient way to fabricate a highly flexible, transparent and efficient surface-enhanced Raman scattering (SERS) substrate, in which Au nanoparticles (NPs) were embedded into the polymeric nanopillars via a nanoimprint lithography (NIL) method and using an anodic aluminum oxide (AAO) template. The obtained substrate exhibits prominent reproducibility and high sensitivity to Rhodamine 6G (R6G). Moreover, it possesses excellent transparency and flexibility. The SERS intensity acquired from these substrates almost remains constant after 200 bending cycles. Compared with traditional SERS substrates, this substrate represents a novel format with unique advantages, such as being highly flexible, transparent, lightweight, portable and easy to handle. More importantly, it can be scaled up for high-throughput production with low cost.

Integrated Tunable Optical Dispersion Compensator Consisting of Arrayed Waveguide Grating With Air Trench Output Waveguides

This paper proposes an integrated tunable optical dispersion compensator (TODC) which consists of an arrayed waveguide grating (AWG) with air trench output waveguides and waveguide-type phase modulators based on thermo-optic effect. The air trenches are formed on both sides of each output waveguide to minimize the size of AWG. One TODC design for compensating chromatic dispersion (CD) channel by channel with a tunable range from -402 ps/nm to 402 ps/nm and another TODC design for compensating chromatic dispersions of multiple channels simultaneously with a tunable range from -783 ps/nm to 783 ps/nm are introduced. Finally, a TODC consisting of two cascaded TODCs is analyzed, which peak-to-peak group delay ripple is as low as 0.9 ps, when its CD can be tuned from -776 ps/nm to 776 ps/nm.

Two-stage evaporated ordered nanoporous ultrathin metal films using reusable template

In this work, we present a versatile procedure for the formation of ordered nanoporous ultrathin metal film using reusable anodic aluminum oxide (AAO) template. Firstly, the Ag and Au films were evaporated on the bottom surface of AAO template sequentially, followed by selective removal of a sacrificial Ag film. This process results in an ultrathin nanoporous Au film was prepared without causing significant damage of the AAO replication template. The characterizations of SEM images indicate that obtained metal films have the hexagonal morphology similar to AAO template. The EDS analyses of the present Au film on the Si substrate indicate that Ag film was completely etched away. This novel fabrication method not only simplifies the preparation of ordered nanoporous ultrathin metal film, but also can be readily extended to other materials systems.

Fabrication of nanopore and nanoparticle arrays with high aspect ratio AAO masks

How to use high aspect ratio anodic aluminum oxide (AAO) membranes as an etching and evaporation mask is one of the unsolved problems in the application of nanostructured arrays. Here we describe the versatile utilizations of the highly ordered AAO membranes with a high aspect ratio of more than 20 used as universal masks for the formation of various nanostructure arrays on various substrates. The result shows that the fabricated nanopore and nanoparticle arrays of substrates inherit the regularity of the AAO membranes completely. The flat AAO substrates and uneven AAO frontages were attached to the Si substrates respectively as an etching mask, which demonstrates that the two kinds of replication, positive and negative, represent the replication of the mirroring of Si substrates relative to the flat AAO substrates and uneven AAO frontages. Our work is a breakthrough for the broad research field of surface nano-masking.

Dynamical thermal metamaterial response at terahertz frequencies

ABSTRACT Utilizing terahertz time domain spectroscopy, we have characterized the electromagnetic response of a planar array of complementary square metal frames fabricated upon a high resistivity Si substrate, and between the two layers existed an important ferroelectric layer strontium titanate(STO). The resonance frequency dependent magnetic and electric resonances are in excellent agreement with theory. We demonstrate, it has two distinctive reflection dips through reconfigures the metamaterial by etched a rectangular stripe, and the former metamaterial has only one resonance frequency. Cooling the metamaterial from 400 K to 200 K causes excellent blue shift. The resonance shift is owing to the variation of the dielectric constant of the low-temperature co-fired ceramic (LTCC) substrate. Because of the universality of metamaterial response over many decades of frequency, these results have implications for other regions of the electromagnetic spectrum.