Surface Topology Measurements Research Articles

Selective Crystallization of Racemic Polymorph via Native Enantiomer Inhibition: dl-Methionine

Crystallization of chiral compounds is dictated by chiral recognition and molecular self-assembly in solution. However, their interplay remains elusive. The reason for the considerably reduced polymorphism in chiral molecules than that of nonchiral molecules remains unclear. Herein, we use a combination of experimental and computational techniques to show that excessive enantiomer functioning, as a native crystallization inhibitor, selectively suppresses the crystallization of racemic polymorphs, affording preferential crystallization of the metastable α polymorph of dl-methionine. Bulk crystallization assays show concomitant crystallization of the α and β polymorphs of racemic dl-methionine in the solution with no enantiomeric excess. However, when the solution contains excessive d-/l-methionine enantiomer, only the metastable α form can be crystallized. Crystal growth experiments, fluoresce confocal microscopy, and atomic force microscope surface topology measurements reveal the growth inhibition of both polymorphs with preferential suppression of the β form by excess native enantiomer. Binding energy calculations and molecular dynamic simulations further demonstrate the preferential adsorption of excessive enantiomers on the (0 0 2) facet of the β form over the α form. Overall, our results uncover a unique chiral self-positioning mechanism where the excess enantiomer solutes serve as a native growth inhibitor to disrupt the kinetics of racemic polymorphic crystallization, affording selective crystallization of only one polymorph of dl-methionine. Our results highlight the important effect of excess enantiomer in a solution on the polymorph occurrence of chiral molecules.

Single-shot wide-field topography measurement using spectrally multiplexed reflection intensity holography via space-domain Kramers-Kronig relations.

Surface topology measurements of micro- or nanostructures are essential for both scientific and industrial applications. However, high-throughput measurements remain challenging in surface metrology. We present single-shot full-field surface topography measurement using Kramers-Kronig holographic imaging and spectral multiplexing. Three different intensity images at different incident angles were simultaneously measured with three different colors, from which a quantitative phase image was retrieved using spatial Kramers-Kronig relations. A high-resolution topographic image of the sample was then reconstructed using synthetic aperture holography. Various patterned structures at the nanometer scale were measured and cross-validated using atomic force microscopy.

Active Balance Control of Humanoid Locomotion Based on Foot Position Compensation

A foot positioning compensator is developed in this paper for a full-body humanoid to retrieve its balance during continuous walking. An online Foot Position Compensator (FPC) is designed to improve the robustness of biped walking, which can modify predefined step position and step duration online with sensory feedback. Foot placement parameters are learned by the FPC based on the Policy Gradient Reinforcement Learning (PGRL) method. Moreover, the FPC assists the humanoid robot in rejecting external disturbances and recovering the walking position by re-planning the trajectories of walking pattern and the Center of Mass (CoM). An upper body pose control strategy is also presented to further enhance the performance of humanoid robots to overcome strong external disturbances. The advantages of this proposed method are that it neither requires prior information about the walking terrain conditions, nor relies on range sensor information for surface topology measurement. The effectiveness of the proposed method is verified via Webots simulation and real experiments on a full-body humanoid NAO robot.

Optimization-based extrinsic calibration of a three-dimensional sensor composed of an array projector and a single camera

We previously presented a technique calibrating a multiaperture array projector (MAAP) for performing high-speed three-dimensional (3-D) surface topology measurements with aperiodic sinusoidal fringe structured illumination. Although a stereo-camera setup used to be required for triangulation, the presented MAAP calibration technique re-enables photogrammetric 3-D measurements to be made using a single CCD or CMOS camera. For such a monocular-view photogrammetric system, intrinsic camera calibration, intrinsic projector calibration, and extrinsic system calibration are vital to obtain high-measurement performance. However, there is currently no such technique that can comprehensively perform the required extrinsic system calibration when using an MAAP and a single-camera setup. A comprehensive optimization-based method is proposed and the resulting measurement performance is evaluated. With the new calibration technique, we are able to achieve a surface standard deviation of ∼50 μm.

Optically transparent superhydrophobic polydimethylsiloxane by periodic surface microtexture

To achieve surface microtexture on poly(dimethysiloxane) (PDMS), laser processing of an aluminum sample and replication through hot embossing to a polycarbonate sheet, followed by PDMS thermal curing on both the etched aluminum and embossed polycarbonate is used. Surface topology measurements of the microtextured aluminum reveal microtexture height near 12μm, with slightly smaller microtexture height on the replicated surfaces. These PDMS samples exhibit optical transparency between 43% and 72% while retaining static contact angles above 150°, as the percent area without microtexture is increased from 53.8% to 81%. While samples replicated from the aluminum master exhibit roll-off angles of below 2°, samples produced using a PC master exhibit an almost complete loss of roll-off angle performance. This is a consequence of the microtexture profile being inverted at each embossing step. Optical analysis of evaporating water droplets revealed a strong pinning force at the smooth to microtexture interface, which also affects the observed roll-off angle. Samples with an untextured surface area above 81% exhibit a transition in wetting dynamics, resulting in a decrease in the static contact angle to near 130° and a change in the roll-off angle to values near 65° as exhibited on a smooth PDMS surface. The fabrication process is found to be repeatable over many months, without degradation in performance of the microtexture master or PDMS samples.

In-Line Dimensional Metrology in Nanomanufacturing Systems Enabled by a Passive Semiconductor Wafer Alignment Mechanism

One of the major challenges in nanoscale manufacturing is defect control because it is difficult to measure nanoscale features in-line with the manufacturing process. Optical inspection typically is not an option at the nanoscale level due to the diffraction limit of light, and without inspection high scrap rates can occur. Therefore, this paper presents an atomic force microscopy (AFM)-based inspection system that can be rapidly implemented in-line with other nanomanufacturing processes. Atomic force microscopy is capable of producing very high resolution (subnanometer-scale) surface topology measurements and is widely utilized in scientific and industrial applications, but has not been implemented in-line with manufacturing systems, primarily because of the large setup time typically required to take an AFM measurement. This paper introduces the design of a mechanical wafer-alignment device to enable in-line AFM metrology in nanoscale manufacturing by dramatically reducing AFM metrology setup time. The device consists of three pins that exactly constrain the wafer and a nesting force applied by a flexure to keep the wafer in contact with the pins. Kinematic couplings precisely mate the device below a flexure stage containing an array of AFM microchips which are used to make nanoscale measurements on the surface of the semiconductor wafer. This passive alignment system reduces the wafer setup time to less than 1 min and produces a lateral positioning accuracy that is on the order of ∼1 μm.

Threading dislocations in GaAs epitaxial layers on various thickness Ge buffers on 300 mm Si substrates

We have grown GaAs epitaxial layers on Ge buffers, themselves on Si (001) substrates, using an Applied Materials 300mm metal organic chemical vapor deposition tool. We varied the Ge buffer thickness between 0.36 and 1.38µm and studied the properties of a 0.27µm thick GaAs layer on top. We found that increasing the Ge buffer thickness yielded smoother GaAs films with an rms surface roughness as low as 0.5nm obtained on a 5×5µm2 area. The bow of the substrate increased following a linear law with the epitaxial stack thickness up to 240µm for a 1.65µm stack. We have also characterized the threading dislocations present in the GaAs layers using X-ray diffraction and cathodoluminescence. Increasing the Ge buffer thickness resulted in lower threading dislocation densities, enabling us to obtain anti-phase boundary – free GaAs films with a threading dislocation density as low as 3×107cm−2. In addition, atomic force microscopy surface topology measurements showed the presence of pits in the GaAs layers whose density agreed well with other threading dislocation density assessments. It thus seems that threading dislocations can in certain cases induce some growth rate variations, making them visible in as-grown GaAs films. Using thicker Ge buffers results in smoother films with less threading dislocations, with the side effect of increasing the bow on the wafer. If bow is not an issue, this is a practical approach to improve the GaAs (on Ge buffer) on silicon quality.

On-site holographic interference method for fast surface topology measurements and reconstruction

Based on holographic interferometry technique, we develop a quantitative method for on-site characterization of surface topology and 3D reconstruction. The optical set-up with 2D spatial light modulator for surface characterization of 3D micro-structures is presented. It is shown that this non-destructive and non-contact method made possible the rapid investigation of 3D polymer structures employing diffractive masks and phase modulation based on two beams interference. The experimental results obtained with this method demonstrated the quantitative high-resolution measurement of samples topology with faster processing details comparing to the actual screening methods which cannot be used on-site conditions.

Size measurement uncertainties of near-monodisperse, near-spherical nanoparticles using transmission electron microscopy and particle-tracking analysis

Particle-tracking analysis (PTA) in combination with systematic imaging, automatic image analysis, and automatic data processing is validated for size measurements. Transmission electron microscopy (TEM) in combination with a systematic selection procedure for unbiased random image collection, semiautomatic image analysis, and data processing is validated for size, shape, and surface topology measurements. PTA is investigated as an alternative for TEM for the determination of the particle size in the framework of the EC definition of nanomaterial. The intra-laboratory validation study assessing the precision and accuracy of the TEM and PTA methods consists of series of measurements on three gold reference materials with mean area-equivalent circular diameters of 8.9 nm (RM-8011), 27.6 nm (RM-8012), and 56.0 nm (RM-8013), and two polystyrene materials with modal hydrodynamic diameters of 102 nm (P1) and 202 nm (H1). By obtaining a high level of automation, PTA proves to give precise and non-biased results for the modal hydrodynamic diameter in size range between 30 and 200 nm, and TEM proves to give precise and non-biased results for the mean area-equivalent circular diameter in the size range between 8 and 200 nm of the investigated near-monomodal near-spherical materials. The expanded uncertainties of PTA are about 9 % and are determined mainly by the repeatability uncertainty. This uncertainty is two times higher than the expanded uncertainty of 4 % obtained by TEM for analyses on identical materials. For the investigated near-monomodal and near-spherical materials, PTA can be used as an alternative to TEM for measuring the particle size, with exception of 8.9 nm gold, because this material has a size below the detection limit of PTA.

Measurement uncertainties of size, shape, and surface measurements using transmission electron microscopy of near-monodisperse, near-spherical nanoparticles

Transmission electron microscopy (TEM) in combination with a systematic selection procedure for unbiased random image collection, semi-automatic image analysis, and data processing has been validated for size, shape, and surface topology measurements of silica nanoparticles. The validation study, assessing the precision and accuracy of the TEM method, consists of series of measurements on two colloidal silica-certified reference materials, with number-based modal area-equivalent circular diameters (ECD) of 19.4 nm (ERM-FD100) and 27.8 nm (ERM-FD304). The measurement uncertainties are estimated for the modal and median particle size, shape, and surface topology parameters of single primary particles. The single primary particles are distinguished from agglomerates using a linear discriminant analysis approach. After optimization of the binning process, the mode associated with the number-based particle size distribution is obtained by lognormal fitting. The methodology described in this paper relies on a high level of automation of calibration, image acquisition, image analysis, and data analysis and gives robust results for the modal ECD. The expanded uncertainty of the modal ECD is estimated to be about 3 %. The largest contribution to the expanded uncertainty stems from the uncertainty associated with the trueness of the TEM method.

Monte Carlo simulation of the field back-scattered from rough surfaces

A novel approach for the simulation of the field back-scattered from a rough surface is presented. It takes into account polarization and multiple scattering events on the surface, as well as diffraction effects. The validity and usefulness of this simulation is demonstrated in the case of surface topology measurement.

Effects of the surface Cu2−xSe phase on the growth and properties of CuInSe2 films

Drastic changes in average molecularities (m=Cu/In) from m≫1 to m=0.92–0.93 and in hole concentrations from p≫1019 cm−3 to as low as p=7.5×1016 cm−3 have been observed in molecular beam epitaxy grown CuInSe2 after selective etching of the Cu–Se phase by a KCN aqueous solution; high hole concentrations and Cu-excess compositions of the as-grown films were attributed to the Cu–Se phase. On the other hand, well-defined photoluminescence emissions were found characteristic of intrinsic CuInSe2. The presence of the Cu–Se phase made possible the growth of high-quality CuInSe2 epitaxial films at a temperature well below the melting point of any Cu–Se compound. Surface topology measurements showed that the surface of the as-grown films was not fully covered by Cu–Se grains, leaving holes with depths of 200–300 nm after KCN etching. The enhanced two-dimensional growth and the reduced defect concentration imply that a very thin Cu-excess surface layer controls the growth of CuInSe2 when grown under Cu-excess conditions.