Wenchuang Hu

Nano-confinement induced chain alignment in ordered P3HT nanostructures defined by nanoimprint lithography

Control of polymer morphology and chain orientation is of great importance in organic solar cells and field effect transistors (OFETs). Here we report the use of nanoimprint lithography to fabricate large-area, high-density, and ordered nanostructures in conjugated polymer poly(3-hexylthiophene) or P3HT, and also to simultaneously control 3D chain alignment within these P3HT nanostructures. Out-of-plane and in-plane grazing incident X-ray diffraction were used to determine the chain orientation in the imprinted P3HT nanostructures, which shows a strong dependence on their geometry (gratings or pillars). Vertical chain alignment was observed in both nanogratings and nanopillars, indicating strong potential to improve charge transport and optical properties for solar cells in comparison to bulk heterojunction structure. For P3HT nanogratings, pi-pi stacking along the grating direction with an angular distribution of +/-20 degrees was found, which is favorable for OFETs. We propose the chain alignment is induced by the nanoconfinement during nanoimprinting via pi-pi interaction and hydrophobic interaction between polymer chain and mold surfaces.

Three-dimensional SU-8 structures by reversal UV imprint

In this work, three-dimensional (3D) SU-8 micro- and nanostructures were fabricated using a reversal UV imprint process at low temperature and low pressure. The SU-8 polymer was coated on a patterned glass mold and then transferred onto various substrates by reversal UV imprint at a typical temperature of 50°C, pressure of 1MPa, and UV exposure of 1s. The lower temperature and pressure used compared to conventional thermal imprint shorten the imprint time and alleviate pattern distortion. A combination of silanes was used to generate a medium surface energy on the imprint molds to enable polymer spin coating and mold release after imprint. In addition, an O2 plasma was used for glass mold treatment to improve uniformity of silane coating and to increase substrate surface energy for better polymer adhesion. Using this technology, 100nm–1μm wide SU-8 gratings were fabricated on flat or patterned substrates with good fidelity. By repeating this process, multiple-level nanochannels, cavities, or air-bridging ...

Deposition and patterning of diamondlike carbon as antiwear nanoimprint templates

In this work, antiwear nanoimprint templates were made by depositing and patterning diamondlike carbon (DLC) films on Si and quartz. A capacitively coupled plasma enhanced chemical vapor deposition (PECVD) system was configured to deposit 100nm–1μm thick DLC films on Si and quartz substrates. These films were characterized with Raman spectroscopy, electron energy loss spectroscopy, atomic force microscopy, nanoindentation, contact angle measurements, and optical transmission measurements. The rf power and pressure of the PECVD process were varied to obtain uniform coating of DLC films with smooth surface (∼0.2nm rms), low surface energy (∼40mJ∕m2), and high hardness (∼22GPa). The resulting films’ wear resistance is more than three times better than quartz. The DLC films were patterned by nanoimprint lithography using polymethylmethacrylate (PMMA) followed by CF4 plasma etch. Thermal nanoimprint tests with DLC templates were performed in PMMA. Atomic force microscopy measurements indicated excellent patter...

Characterizing nanoimprint profile shape and polymer flow behavior using visible light angular scatterometry

The profile shape and the flow behavior of polymer nanoscale gratings made by a thermal nanoimprint process are precisely examined using visible light angular scatterometry. Nanoimprinted poly(methyl methacrylate) (PMMA) lines with 60–800nm width, 100–200nm height, and varied residual thicknesses of 70–400nm have been investigated using this optical approach, and insightful observations are made regarding residual stress buildup during thermal nanoimprint. In addition, a nonlinear profile model has been developed for scatterometry to monitor the “melting” behavior of PMMA gratings under annealing around its glass transition temperature. The polymer nanostructures were found to relax primarily at high stress regions.

Stability of HSQ nanolines defined by e-beam lithography for Si nanowire field effect transistors

Multiple instability states, e.g., grouped collapse, single collapse, wavy, and grouped wavy states, have been observed in hydrogen silses quioxane (HSQ) nanolines defined by electron beam lithography (EBL). Experimental data show that the critical aspect ratio of the HSQ lines dramatically increase when the line pitch reduced to sub-100-nm, which is opposite to theoretical models for capillary forces and swelling strain. Such contradiction can be well explained only if Young’s modulus is considered as a significantly varying factor. Further, experimental data show a dramatic decrease in swelling strain and increase in oxygen contents in HSQ with increasing EBL dose, indicating that it is the change in Young’s modulus rather than the capillary force or swelling strain that dominates the instability behaviors at the nanoscale. Stable high aspect ratio HSQ nanolines over metal pads were used to make working Si nanowire transistors on Si on insulator substrates. 12–14nm HSQ lines with aspect ratios of 11–14 ...

Surface energy induced patterning of polymer nanostructures for cancer diagnosis and therapy

We have developed a new simple method to pattern discrete polymer micro and nanostructures. A Si template is patterned by lithography and selective surface treatment to have spatially different surface energies that induce microfluidic self-patterning of a spincoated polymer layer. Biocompatible diblock co-polymer and SU8 are patterned using this method to form monodisperse and shape specific microstructures. After patterning, these particles are lifted off the template surface into aqueous solution. The template is then cleaned and re-used. These freestanding polymer particles with uniform and precise spherical morphology can be used as carriers for drug and imaging agents for biomedical applications.

Surface energy induced patterning of organic and inorganic materials on heterogeneous Si surfaces

A surface energy induced patterning (SEIP) method is developed to transfer resist patterns defined by lithography into various functional materials. A Si template is first chemically patterned using conventional lithography and selective attachment of trichlorosilane to achieve spatially different surface energies. Organic materials as well as inorganic films are deposited onto the chemically patterned template, followed by a thermal annealing process. The heterogeneous surface energies on the template induce material microfluidic reflow from the less to the more thermodynamically favorable areas. Using this method, patterned microstructures were achieved with SU-8, diblock copolymer, and aluminum film. In addition, the SEIP template was successfully used for atomic layer chemical vapor deposition to selectively pattern 200nm–2μm wide HfO2 structures.

Physical characterization of nanoimprinted polymer nanostructures using visible light angular scatterometry

Visible light angular scatterometry is applied to characterize the geometry and physical properties of sub-100-nm-wide polymer grat- ings fabricated using nanoimprint lithography and electron beam lithog- raphy. Measurement sensitivities to small variations in linewidth and slope angle were evaluated theoretically, which suggests that TM polar- ized incident light offers improved sensitivity for the measurements of sub-45-nm critical dimensions CDs. A variable angle scatterometer us- ing a red laser is built, and measurement results of various polymethyl- methacrylate PMMA gratings with sub-100-nm CDs reveals good ac- curacies and fit well to the scanning electron microscopy SEM measurements. In addition to geometry and dimension measurements, new functionality is implemented in the modeling to characterize polymer residue thickness, polymer flow dynamics, and evidence of stress in the nanoimprinted polymer gratings. Scatterometry is also applied to detect possible undercut line profiles resulting from electron beam lithography. The results promote using this low-cost and noninvasive technique to characterize polymer nanostructures as well as understand and control the underlying lithographic processes.

Visible light angular scatterometry for nanolithography

Visible light angular scatterometry is investigated for sub-100 nm line metrology. Measurement sensitivities to small variations in line dimensions are examined and some observations are made on the measurement conditions leading to improved sensitivities even for 20 nm CD. Experimental results with good accuracies are also shown for red light angular measurements of PMMA gratings made by e-beam lithography and nanoimprint lithography. The theoretical and experimental studies show high sensitivity and accuracy in characterization of the geometry and dimensions of nano-lines, particularly in measuring the polymer residue thickness of nanoimprinted polymer gratings and the undercut line profiles resulting from e-beam lithography. The results promote using this low cost and non-invasive technique to examine and control some underlying lithographic processes in nanofabrication.