Thermal Nanoimprint Lithography Research Articles

Publisher's Note: “Improvements of Defects by Patterning Using Thermal Nanoimprint Lithography”

Publisher's Note: “Improvements of Defects by Patterning Using Thermal Nanoimprint Lithography”

Finite Element Method Simulation of the Molding Process for Thermal Nano-Imprint Lithography

We made a numerical study on the deformation of a viscoelastic polymethyl methacrylene (PMMA) resist when a rigid SiO2 stamp with a rectangular line pattern is imprinted into the PMMA resist for thermal nano-imprint lithography (NIL). The stress distribution in the polymer resist during the molding process is calculated by a finite element method (FEM). Our simulation results reveal that the asymmetric von Mises stress is distributed over the polymer around the external line, which seems to be due to the squeezing flow under the flat space. The stress seems to be concentrated at the sidewall close to the centerline of the whole structure. Our simulation also reveals that a micro gap is formed between the replicated structure and the outer wall of the mold.

Size-Dependent Shape Evolution of Patterned Polymer Films Studied in Situ by Phase-Retrieval-Based Small-Angle X-ray Scattering

Patterned polymer films are known to exhibit shape evolution when annealed at temperatures close to or above the glass transition temperature. Here, we employ small-angle X-ray scattering to address the size-dependent shape evolution of poly(methyl methacrylate) (PMMA) gratings obtained by thermal nanoimprint lithography. Using an iterative phase-retrieval scheme, we reconstruct the height profile of the grating in situ in a model-independent manner during a heating ramp through the glass transition. This allows us to directly observe the evolution from a trapezoidal shape, via a sinusoidal-like one, into a flat film, in agreement with ex-situ atomic force microscopy experiments. Moreover, we find the onset temperature of shape evolution to decrease monotonically with decreasing pattern size, the difference being ∼4 °C between gratings with periods of 240 and 80 nm. We primarily attribute the size-dependent shape evolution to a reduction in viscosity with decreasing pattern size, which is induced by eithe...

Novel Gold-Capped Nanopillars Imprinted on a Polymer Film for Highly Sensitive Plasmonic Biosensing

Herein, a nanoporous alumina was fabricated to use as a mold in transforming nanopillar structures onto a thin film polymer by thermal nanoimprint lithography (NIL). The size of the pores was successfully controlled by varying the applied voltages and etching time. These nanoporous structures were transferred to the Cyclo-olefin polymer (COP) film surface from the porous mold by a thermal nanoimprinting process. A plasmonic substrate was fabricated by sputtering a thin layer of gold onto this nanopillar polymer structure, and the refractive index response in a variety of media was evaluated. Finally, the biosensing capacity of this novel plasmonic substrate was verified by analysis of Human immunoglobulin and achieved a minimum detection limit of 1.0 ng/mL. With the advantages of mass production with consistent reproducibility stemming from the nanoimprint fabrication process, our gold-capped polymeric pillars are ready for the transition from academic interest into commercialization systems for practical use in diagnostic applications.

Publisher's Note: “Improvements of Defects by Patterning Using Thermal Nanoimprint Lithography”

Publisher's Note: “Improvements of Defects by Patterning Using Thermal Nanoimprint Lithography”

Microfabrication of Si-Based High-Index-Contrast-Grating Structure by Thermal Nanoimprint Lithography and Cl2/Xe-Inductively Coupled Plasma Etching

We demonstrated the fabrication of a Si-based high-index-contrast-grating (HCG) structure by thermal nanoimprint lithography and Cl2/Xe-inductively coupled plasma (ICP) etching. A vertical etching profile and a smooth etched surface, which satisfy the requirements for optical device application, were obtained. We believe that this proposed process is useful for the microfabrication of Si-based optical devices, such as the HCG structure, photonic crystals, narrow optical waveguides, and micro-electro-mechanical systems (MEMS).

Microfabrication of Si-Based High-Index-Contrast-Grating Structure by Thermal Nanoimprint Lithography and Cl2/Xe-Inductively Coupled Plasma Etching

Microfabrication of Si-Based High-Index-Contrast-Grating Structure by Thermal Nanoimprint Lithography and Cl₂/Xe-Inductively Coupled Plasma Etching

Experimental and Numerical Analysis of Polymer Deformation in Thermal Nanoimprint Lithography

Nanoimprint lithography has been widely used in various applications as a promising micro/nanofabrication tool. In the thermal nanoimprint process, optimization of important process parameters such as temperature, pressure, and pressure holding time is greatly needed to successfully imprint micro-and nanoscale patterns in a resist material. In this study, both experimentation and numerical simulation were performed to extend the understanding of the behaviour of polymer materials and to optimize process condition effectively during the thermal imprint process. The effects of the process conditions on the polymer deformation are discussed to achieve a rapid and high throughput thermal imprint process.

Enhanced uniformity and durability of antisticking layer in imprinting stamps

In the nanoimprint lithography (NIL) stamp, a fluorinated self-assembled monolayer (SAM) is used as an antisticking layer. The uniformity and durability of the antisticking layer should be improved for achieving an effective fabrication of imprinted devices. In order to achieve these goals, we manufactured a vaporized SAM coating system with tilt and rotation functions, and we uniformly formed the antisticking layer on the stamp surface. The antisticking layer was with the 0.5nm uniformity and the almost the same thickness of coating and re-coating. Moreover, to enhance durability, we conducted the pre-treatment before SAM coating and post-treatment after SAM coating – plasma cleaning, re-coating, deionized (DI) water deposition and annealing. The content of fluorine increased with re-coating, we could infer that the density of the antisticking layer increased via re-coating. To inspect the lifetime of the SAM-coated stamp, we measured the stamp surface contact angle in every five thermal NIL and identified that the durability of antisticking layer was dramatically improved.

A nanoporous silicon nitride membrane using a two-step lift-off pattern transfer with thermal nanoimprint lithography

Nanoimprint lithography is emerging as a viable contender for fabrication of large-scale arrays of 5–500 nm features. A fabrication process for the realization of thin nanoporous membranes using thermal nanoimprint lithography is presented. Suspended silicon nitride membranes were fabricated by low-pressure chemical vapor deposition (LPCVD) in conjunction with a potassium hydroxide-based bulk micromachining process. Nanoscale features were imprinted into a commercially available thermoplastic polymer resist using a prefabricated silicon mold. The pattern was reversed and transferred to a thin aluminum oxide layer by means of a novel two-stage lift-off technique. The patterned aluminum oxide was used as an etch mask in a CHF3/He-based reactive ion etch process to transfer the pattern to silicon nitride. Highly directional etch profiles with near vertical sidewalls and excellent Si3N4/Al2O3 etch selectivity were observed. One micrometer thick porous membranes with varying dimensions of 250 × 250 µm2 to 450 × 450 µm2 and a pore diameter of 400 nm have been engineered and evaluated. Results indicate that the membranes have consistent nanopore dimensions and precisely defined porosity, which makes them ideal as gas exchange interfaces in blood oxygenation systems as well as other applications such as dialysis.

Hydrogen silsesquioxane resist stamp for replication of nanophotonic components in polymers

We investigate an affordable, accurate and large-scale production method to fabricate subwavelength grating structures by hot embossing replication in polycarbonate substrates. We use inorganic hydrogen silsesquioxane (HSQ), a high resolution, binary, negative electron beam resist, on silicon substrate to make a stamp for replication. The stamp is fabricated without any etching processes and with simple process steps. The process starts by spin coating an HSQ-resist layer on a silicon substrate. The desired film thickness is achieved by adjusting the spinning speed and time. The resist material is then subjected to e-beam writing and development followed by a heat treatment to enhance the hardness and to obtain hot embossing stamp material properties comparable with solid SiO2. A comparison with and without the silicon etching is also performed. We demonstrate that a high quality stamp for thermal nano-imprint lithography for optical gratings can be fabricated using an inexpensive process without an etching step. The process results in a uniform imprinting density over the entire grating surface and high imprint fidelity. The reflectance spectra of replicated grating structures are also shown to be in agreement with theoretical calculations.

Enhanced Transmission through Gold Nanohole Arrays Fabricated by Thermal Nanoimprint Lithography for Surface Plasmon Based Biosensors

Abstract We present the fabrication of gold nanohole arrays using thermal nanoimprint lithography and integration into a microfluidic device for detection of biological analytes. The biosensor makes use of a surface-plasmon mediated effect known as enhanced optical transmission, in which the transmission of light is modulated. The sensitivity achieved with these gold nanohole arrays has been characterized and up-to 300 RIU/nm were obtained varying the array parameters. Detection of protein biomarkers via capture by specific antibodies has been achieved demonstrating the biosensing capabilities of the fabricated devices.

Reactive-monolayer-assisted Thermal Nanoimprint Lithography

Reactive-monolayer-assisted Thermal Nanoimprint Lithography

Robustly nano-tailored honeycomb structure for high-throughput antireflection polymer films

Although recently fabricated biomimetic nanostructures exhibit superior performance compared to corresponding nanostructures found in nature, their practical applications are limited due to the low mechanical stability of the nanostructures and the low productivity of the fabrication processes developed thus far. A nanoporous honeycomb structure that can alleviate any concentrated stress effectively was fabricated using roll-to-roll processable thermal nanoimprint lithography for high throughput and performance antireflection polymer films. A nanoscale positive (convex) antireflective structure of the moth eye, which shows broadband and omnidirectional antireflection properties, and a negative (concave) antireflective structure were produced using this process. The new nanoporous honeycomb structure exhibited high performance AR and enhanced mechanical stability. Moreover, a continuous process that can be used for industrial mass production was developed.

A “thiol-ene” photo-curable hybrid fluorinated resist for the high-performance replica mold of nanoimprint lithography (NIL)

Functional polyhedral oligomeric silsesquioxane (POSS)-based materials have been developed as an ideal material with high performance for nanoimprint lithography (NIL). A novel fluorinated hybrid resist as a soft mold for NIL, based on thiol-ene photopolymerization, was precisely designed and synthesized and is comprised of fluorinated mercaptopropyl polyhedral oligomeric silsesquioxane (POSS-F-SH) and a diluted crosslinker (2,2,3,3,4,4,5,5-octafluoro-1,6-hexyl diacrylate, DCFA4). The obtained fluorinated hybrid resists possess a variety of characteristics desirable for UV-NIL, including low viscosity (16–239 cP), a low bulk volumetric shrinkage (4.8–7.5%) and a good resistance to oxygen inhibition. The cross-linked hybrid resins exhibited high transparency to UV light and resistance to organic solvents. As a soft mold, the excellent mechanical property (Young's modulus: 0.31–1.56 GPa) and low surface energy (14–20.4 mJ m−2) of the fluorinated polymers provide a clean mold release without fracture or deformation of the embossed structures. The thermally stability (Td > 300 °C) render them capable of being used for both UV and thermal NIL duplication processes. The resultant soft mold exhibited a high resolution patterning capacity with feature sizes in the range of 200 nm to several microns. The economic efficiency of the mold fabrication, as well as the high durability and excellent releasing properties, could be quite valuable in NIL for the high-throughput fabrication of nano-devices.

Replication of large area nanoimprint stamp with small critical dimension loss

In this paper, the replication process of large area nanoimprint stamp with small critical dimension (CD) loss was investigated, using the thin residual layer nanoimprint lithography (NIL) technology. The residual layer thickness was optimized by changing the spin-coated resist thickness. The dependences of the residual layer etching rate on gas flow, chamber pressure, and RF power were investigated, and the optimized process conditions were established. By means of the thin residual layer NIL technique and optimized residual layer etching process, large area stamp with small CD loss and multi-orientation patterns was successfully replicated on 2-inch SiO2/Si wafer. The CD loss was controlled within 5 nm. The replicated stamp showed high performance in the patterning with thermal NIL. The replication process reported in this work could also be used to fabricate large area nanostructures with small CD loss.

High aspect ratio fine pattern transfer using a novel mold by nanoimprint lithography

To conduct our research, a Si mold with a high aspect nano trench pattern was fabricated using a new edge lithography process, and the pattern was replicated into PMMA films on Si wafer by thermal nanoimprint lithography. By using edge lithography, a SiO2 circular line pattern of 35 nm width and 3.5 μm height was obtained and the aspect ratio became 100. The Cr patterns were fabricated by a lift-off process by using the high aspect SiO2 nano patterns, and the Si substrate was etched by the advanced plasma etching of the gas switching process. A Si trench pattern of 25 nm width and 1.0 μm depth was obtained, and these Si trench patterns were used for nanoimprint molds. The aspect ratios of the nano trench patterns were about 10. The nano trench patterns were replicated into PMMA films with various molecular weights by thermal nanoimprint lithography. The pattern replication failed when the PMMA resins of 50 and 120 k molecular weights were used. It is found that the PMMA pattern was often broken during the demolding process and its strength was very important for the successful pattern replication of the high aspect pattern. A PMMA line pattern of 30 nm width and 230 nm height was successfully fabricated when the PMMA resin of 996 k molecular weight was used.

Experimental analysis for process control in hybrid lithography

The positive tone resist AZ 1505 is characterized with respect to hybrid lithography that uses thermal nanoimprint lithography to define the nanometric part of a lithography pattern in a first step, followed by optical lithography to define the micron-scaled patterns in the same resist layer in a second step. The parameters investigated are glass temperature and sensitivity (dose curves), both after thermal loading in a typical imprint sequence. The glass transition of the multicomponent resist formulation is derived from stress measurements and the dose curves are evaluated from development rate monitoring via laser interferometry at 532 nm. The results show that both parameters are affected by thermal loading, but adequate choice of the processing parameters makes it possible to compensate for the thermal degradation of the photoresist as long as the imprint temperature chosen does not exceed 130 °C. Based on the characterization results, successful hybrid lithography and lift-off has been demonstrated with AZ 1505.

Direct top-down ordering of diblock copolymers through nanoimprint lithography

In this work, thermal nanoimprint lithography (NIL) is used on full 8 in. silicon wafers to imprint a thin PS-b-PMMA block copolymer (BCP) layer. The authors show that the imprinted BCP layer can thermally self-organize after the NIL process or during the NIL process itself, depending on experimental conditions used. Self-organized imprinted features with good graphoepitaxy alignment are obtained with a cylindrical BCP. Nevertheless, a standard fluorinated silane mold treatment is shown not to be neutral to the BCP. Then, if the line features do not have a thickness exactly commensurable to the natural self-organizing period of the polymer l0, a surface wetting layer is observed.

Nanomanipulation of 2 inch wafer fabrication of vertically aligned carbon nanotube arrays by nanoimprint lithography

AbstractCarbon nanotube (CNT) arrays are typically defined by electron beam lithography (EBL), and hence limited to small areas due to the low throughput. To obtain wafer‐scale fabrication we propose large area thermal nanoimprint lithography (NIL). A 2‐inch stamp master is defined using EBL for subsequent pattern transfer to a hard metal mask before deep reactive ion etching (RIE) to form the stamp protrusions. This stamp master is then pressed against a wafer covered with nanoimprint resist, at a temperature above the glass transition temperature, transferring the pattern to polymer. Using this process, efficient production of wafer‐scale/larger arrays of CNTs has been achieved. The CNTs have been deposited by wafer‐scale plasma enhanced chemical vapour deposition (PECVD) of C2H2/NH3. Substrates containing such nanotubes have been used to automate nanorobotic manipulation sequences of individual CNTs and their assembly into prototypic CNT‐based devices.