Nanoscale Gratings Research Articles

Integrated plasmonic lens photodetector

We present a fabricated metal-semiconductor-metal (MSM) photodetector exhibiting an enhanced photocurrent by integrating a nanoscale metallic grating into its contacts. This serves to increase the incident photon flux about the aperture of the device by guiding incident photons as surface plasmon polaritons. High speed time response data shows that the device responsivity may be increased without sacrificing speed. We demonstrate both a photocurrent enhancement and responsivity increase of about 90% at the design wavelength in comparison to otherwise identical MSM photodetectors without integrated nanoscale gratings. The device retains the MSM advantages of simplicity, planarity, and monolithic integrability.

Correlation of anisotropic cell behaviors with topographic aspect ratio

In this study, we have used nanoimprinting to create a range of micro- and nanoscale gratings, or their combination, in bulk polystyrene plates to investigate anisotropic cell behaviors of human dermal fibroblasts with respect to the aspect ratio (depth/width) of gratings. The depth and width of the polystyrene gratings both show strong effects individually on cell alignment and elongation that are qualitatively similar to the results of other studies. However, consistent quantitative comparison of these individual parameters with different studies is complicated by the diversity of combinations of width and depth that have been tested. Instead, the aspect ratio of the gratings as a unified description of grating topography is a more consistent parameter to interpret topographic dependence of cell morphology. Both cell alignment and elongation increase with increasing aspect ratio, and even a shallow grating (aspect ratio of ∼0.05) is sufficient to induce 80% cell alignment. Re-plotting data recently published by other groups vs. aspect ratio shows a similar dependence, despite differences in cell types and surface structures. This consistency indicates that aspect ratio is a general factor to characterize cell behaviors. The relationship of cell elongation and alignment with topographic aspect ratio is interpreted in terms of the theory of contact guidance. This model provides simplicity and flexibility in geometry design for devices and materials that interface with cells.

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.

Nanoscale resist morphologies of dense gratings using electron-beam lithography

The authors report a systematic investigation, both by experiment and by numerical modeling, of resolution limits for dense nanoscale gratings fabricated in polymethylmethacrylate through low-energy electron-beam lithography (EBL) using 10keV electrons. In particular, they have studied the resist morphologies that develop in various exposure regimes for gratings with periods from 20to100nm and categorized the potential sources of resist damage in such gratings. Through comparison of their experimental and numerical results, they have elucidated the major mechanisms that limit the EBL process resolution at the stages of exposure and resist development. The authors have also suggested semiempirical criteria to predict the risk of resist damage when fabricating dense nanostructures.

Nanoimprint lithography technology with automatic alignment

Nanoimprint lithography (NIL) is recognized as one of the most promising candidates for the next generation lithography (NGL) to obtain sub-100 nm patterns because of its simplicity, high-throughput and low-cost. While substantial effort has been expending on NIL for producing smaller and smaller feature sizes, considerably less effort has been devoted to the equally important issue—alignment between template and substrate. A homemade prototype nanoimprint lithography tool with a high precision automatic alignment system based on Moire signals is presented. Coarse and fine pitch gratings are adopted to produce Moire signals to control macro and micro actuators and enable the substrate to move towards the desired position automatically. Linear motors with 300 mm travel range and 1 μm step resolution are used as macro actuators, and piezoelectric translators with 50 μm travel range and 1 nm step resolution are used as micro actuators. In addition, the prototype provides one translation (z displacement) and two tilting motion(α and β ) to automatically bring uniform intact contact between the template and substrate surfaces by using a flexure stage. As a result, 10 μm coarse alignment accuracy and 20 nm fine alignment accuracy can be achieved. Finally, some results of nanostructures and micro devices such as nanoscale trenches and holes, gratings and microlens array fabricated using the prototype tool are presented, and hot embossing lithography, one typical NIL technology, are depicted by taking nanoscale gratings fabrication as an example.

Fabrication of Nanoscale Gratings by Nanoimprint on Silicon Wafer

This paper presents an approach to fabricate nanoscale gratings by direct imprint on silicon substrate. Imprint conditions that affect the transcription accuracy, such as imprint temperature and pressure, are discussed, and the profile of the imprinted 80nm width gratings with a 250 nm pitch is checked by SEM. High fidelity and fine uniformity demonstrate that nanoimprint is an economical and efficient method to fabricate nanoscale gratings.

A spectral method for the nonconserved surface evolution of nanocrystalline gratings below the roughening transition

The evolution of nanoscale gratings on high-symmetry crystalline surfaces is studied below the roughening temperature in the nonconserved regime, where mass transport takes place via evaporation and condensation of atoms from the vapor to atomic-height surface steps. Since the equation for surface evolution is singular at a facet orientation, a modeling approach based on a weak formulation using a Fourier series expansion is employed. Our method allows for the consideration of a wide range of morphologies ranging from simple sinusoidal one-dimensional (1D) and two-dimensional (2D) gratings to more disordered sputter ripples, and gratings on vicinal surfaces where steps are introduced due to miscut from a facet orientation. Extensions of previous analyses of the decay of 1D sinusoids on vicinal surfaces are discussed; the miscut is seen to lead to the formation of flat regions of low step density at the profile extrema. We also suggest a tighter bound for the maximum miscut beyond which faceting does not occur. In distinct contrast to the case of conserved dynamics, 2D sinusoids do not show shape-invariant relaxation, but instead peaks and valleys shrink simultaneously leaving fourfold hypocycloidal regions around the saddle points. We, however, find that radially symmetric Gaussian bumps remain shape invariant during nonconserved relaxation. Our simulations on sputter ripples show lack of faceting during relaxation while the amplitude is found to decay, for a significant duration, in inverse proportion to the square root of the relaxation time, as observed in recent experiments on Ge(001) surfaces. We also consider relaxation experiments of sinusoids on Si(001) and find good agreement of the profiles observed at high temperature with our nonconserved calculations although the decay behavior at moderately high temperatures (900–1100 °C) is found to be consistent with the case where both conserved and nonconserved modes of mass transport are comparable.

Surface plasmons at nanoscale relief gratings between a metal and a dielectric medium with optical gain

Surface plasmons at the interface between metal and a dielectric with strong optical amplification are analyzed theoretically. It is shown that proper choice of optical indices of the dielectric medium results in an infinitely large effective refractive index of surface waves. Such resonant plasmons have extremely low group velocity and are localized within a vanishingly small distance near the interface. The plasmon-related anomalies in the UV reflection spectra are predicted for nanoscale gratings on a surface of a silver film covered by a concentrated dye solution with high optical amplification.

Decay Kinetics of Nanoscale Corrugation Gratings on Polymer Surface: Evidence for Polymer Flow below the Glass Temperature

The near-surface rheology of polystyrene films at temperatures around Tg was probed via the decay kinetics of shallow, nanoscale surface corrugation gratings. The gratings were created via hot embossing with a stepped silicon template. The wavelengths are distributed in a range between 25 and 60 nm and have a modulation depth of about 0.8 nm peak-to-peak. Upon heating, one observes a surface tension-driven decay of the surface corrugation at temperatures more than 30 deg below the bulk Tg. Such a fast decay is unexpected and indicates a strongly lowered glass temperature near the surface.

Fabrication of nanoscale gratings with reduced line edge roughness using nanoimprint lithography

Line edge roughness is an important factor contributing to the problem of performance degradation in various nanoscale devices. We have developed two smoothing techniques based on nanoimprint lithography for the fabrication of nanoscale gratings with significantly reduced line edge roughness. Compared with other smoothing techniques reported before, our methods are low-cost, effective, and easy to implement. These technologies have been used for the fabrication of smooth nanochannels for stretching and separating double-stranded DNA molecules. They are also compatible with a wide range of materials and applications, including subwavelength optics, bioanalysis and micro/nano-fluidic systems.

Nanoscale freestanding gratings for ultraviolet blocking filters

Ultraviolet (UV) blocking filters are needed for atomic flux imaging in environments where high levels of ultraviolet radiation are present. Freestanding gratings are a promising candidate for UV filtering. They have a high aspect ratio (∼13), narrow (∼40 nm) slots, and effectively block UV radiation. The grating fabrication process makes use of several etching, electroplating, and lithographic steps and includes an optional step to plug pinholes induced by particles during processing. Gratings were successfully manufactured and tested. Measured UV transmissions of ∼10−5 and particle transmissions of ∼10% are in agreement with theoretical predictions.

Faceting of stepped silicon (113) surfaces: Self assembly of nanoscale gratings

Synchrotron X-ray scattering studies of the phase behavior and phase transformations of stepped Si(113) surfaces tilted towards [001] are reviewed for temperatures between 300 and 1500 K. At the highest temperatures, these surfaces are uniformly stepped, and the intensity of near-specularly scattered X-rays increases with decreasing temperature. This is two-dimensional critical opalescence, which foreshadows a faceting transformation. At temperatures below the faceting transformation, (113) facets appear in coexistence with a stepped phase, leading to a mesoscopically grooved morphology. Both the misorientation angle at the phase boundary separating one- and two-phase regions and the intensity of the near-specular diffuse scattering may be described as power laws versus reduced temperature. This can be understood qualitatively on the basis of a mean-field theory, which incorporates an attractive interaction between steps. At lower temperatures, the surfaces are completely faceted, comprising (114) and (113) facets. The kinetics of faceting is also described. Following a quench from a one-phase region of the phase diagram into a two-phase region, the grooved superstructure forms and subsequently coarsens in time. For times between one and several hundred seconds, the surface morphology is self-similar at different times, with a characteristic groove size that evolves as a power law versus time. At later times, the groove size approaches a limiting value, as a result of elastic effects.