Investigation on fabrication of nanoscale patterns using laser interference lithography

Nanoscale patterns are fabricated by laser interference lithography (LIL) using Lloyd's mirror interferometer. LIL provides a patterning technology with simple, quick process over a large area without the usage of a mask. Effects of various key parameters for LIL, with 257 nm wavelength laser, are investigated, such as the exposure dosage, the half angle of two incident beams at the intersection, and the power of the light source for generating one or two dimensional (line and dot) nanoscale structures. The uniform dot patterns over an area of 20 mm x 20 mm with the half pitch sizes of around 190, 250, and 370 nm are achieved and by increasing the beam power up to 0.600 mW/cm2, the exposure process time was reduced down to 12/12 sec for the positive photoresist DHK-BF424 (DongJin) over a bare silicon substrate. In addition, bottom anti-reflective coating (DUV-30J, Brewer Science) is applied to confirm improvements for line structures. The advantages and limitations of LIL are highlighted for generating nanoscale patterns.     

Maskless projection lithography for the fast and flexible generation of grayscale protein patterns

Protein patterns of different shapes and densities are useful tools for studies of cell behavior and to create biomaterials that induce specific cellular responses. Up to now the dominant techniques for creating protein patterns are mostly based on serial writing processes or require templates such as photomasks or elastomer stamps. Only a few of these techniques permit the creation of grayscale patterns. Herein, the development of a lithography system using a digital mirror device which allows fast patterning of proteins by immobilizing fluorescently labeled molecules via photobleaching is reported. Grayscale patterns of biotin with pixel sizes in the range of 2.5 μm are generated within 10 s of exposure on an area of about 5 mm(2) . This maskless projection lithography method permits the rapid and inexpensive generation of protein patterns definable by any user-defined grayscale digital image on substrate areas in the mm(2) to cm(2) range.     

Fabrication of three-dimensional photonic crystals by interference lithography with low light absorption

We report on the fabrication of polymer templates of photonic crystals by means of holographic (or interference) lithography. The grating is written in a SU-8 photoresist using a He-Cd laser of wavelength 442 nm. The use of the wavelength found within the photoresist low absorption band enables fabricating structures that are uniform in depth. Parameters of the photoresist exposure and development for obtaining a porous structure corresponding to an orthorhombic lattice are determined.     

Microstructure evolution of Fe-based nanostructured bainite coating by laser cladding

A Fe-based coating with nano-scale bainitic microstructure was fabricated using laser cladding and subsequent isothermal heat treatment. The microstructure of the coating was observed and analyzed using optical microscope (OM), field-emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD). The results showed that nanostructured bainitic ferrite and carbon-enriched retained austenite distributed uniformly in the coating. Blocky retained austenite was confined to the prior austenite grain boundaries resulting from the elements segregation. The bainitic microstructure obtained at 250 °C had a finer scale compared with that obtained at 300 °C. The volume fraction of austenite increased with increasing transformation temperature for the fully transformed bainitic coating. The bainitic transformation was accelerated as a result of the fine prior austenite generated during the laser cladding. The evolution of the carbon contents in bainitic ferrite and retained austenite revealed the diffusionless mechanism of the bainitic transformation.     

Epitaxial growth and oxidation behavior of an overlay coating on a Ni-base single-crystal superalloy by laser cladding

An overlay coating material was deposited on a single crystal superalloy SRR99 by laser cladding.The microstructure and oxidation behavior of this coating was investigated through scanning electron microscopy(SEM) and X-ray diffraction(XRD). The results indicated that although the composition of the coating was chosen based on the γ' composition in René N5 superalloy, the primary solidification phase of this coating during laser cladding was γ-Ni. Furthermore, under the laser cladding condition, fine parallel dendrites grew epitaxially in the coating from the substrate, indicating the single crystal structure of the substrate was reproduced. When the single crystal MCrAlY coating was oxidized at 1000?, both Al_2O_3 and Al_2O_3 formed during initial oxidation process. As the oxidation time proceeded, the presence of Al_2O_3 facilitated the formation of NiAl_2O_4 spinel oxide. Once the spinel was observed, it flourished and induced some porosity in the scale. When the scale thickness increased to 6–7 μm, large area spallation of the scale began.     

In situ monitoring of microfracture during plasma spray coating by laser AE technique

Atmosphere plasma spray coating materials include many pores and lamellar boundaries formed by flattened particles during spraying process although high reliability are required in ceramic coatings for turbines. These boundaries become an origin of the microcracks and following crack growth. As it is known that spraying parameters strongly affect the microstructure and strength of coating, it is expected to establish in situ monitoring technique for coating process. However, there is a limit to apply the existing non-destructive evaluation techniques to real-time monitoring at elevated temperature. We have investigated a non-contact measuring system to detect acoustic emission (AE) signals due to microfractures using a laser interferometer, and applied this technique for understanding microfracture process of ceramic coating at elevated temperature. In this paper, we evaluated the effect of several spraying parameters on the initiation and growth process of microcrack by detecting AE signals during coating process using a non-contact laser AE technique.     

Surface effects at the nanoscale significantly reduce the effects of stress concentrators

At nanoscales, the large surface over volume ratio is shown to be instrumental in eliminating or significantly reducing the adverse effects of nanoscale stress concentrators (NSCs) such as impurities, inclusions, pores, and cracks. Using molecular dynamics (MD) simulations, Cu crystals with and without NSCs are strained in tension and in shear, at two strain rates, one being an order of magnitude larger than the other. Cube-shape crystals with periodic boundary conditions show sensitivity to NSCs similar to macrosize samples where fracture mechanics works well. For such crystals, atomistic defects cluster near the loaded surfaces, the clustering being stimulated significantly by the NSCs. Crystals with non-periodic boundary conditions, however, show insensitivity to NSCs, for the sample sizes examined herein, i.e., cubes up to about 30 nm side length. Atomistic defects do not cluster near the loading surfaces but rather distribute over the entire sample. Even though the spatial distribution of atomistic defects depends on the presence of NSCs, the total number of such defects is found to be independent of the presence of NSCs for the cubic crystals. The reason for this is the presence of a “vast” amount of surfaces, for the non-periodic boundary conditions case, where numerous atomistic defects initiate, making the number of defects initiating from NSCs insignificant. Provided that the average energy in creating these defects is constant, a robust explanation of the insignificance of NSCs emerges.