Pillar Diameter Research Articles

Size-dependent deformation behavior of dual-phase, nanostructured CrCoNi medium-entropy alloy

The mechanical size effect of nanostructured, dual-phase CrCoNi medium-entropy alloy (MEA) was investigated by combining in-situ micro-compression testing with post-mortem electron microscopy analysis. The alloy possesses a superior yield strength up to ~4 GPa, primarily due to its hierarchical microstructure including column nanograins, preferred orientation, a high density of planar defects and the presence of the hexagonal close packed (HCP) phase. While the yield strength of the alloy has shown size-independency, the deformation behaviour was strongly dependent on the sample size. Specifically, with decreasing the pillar diameters, the dominant deformation mode changed from highly localized and catastrophic shear banding to apparently homogeneous deformation with appreciable plasticity. This transition is believed to be governed by the size-dependent critical stress required for a shear band traversing the pillar and mediated by the competition between shear-induced softening and subsequent hardening mechanisms. In addition, an unexpected phase transformation from HCP to face-centered cubic (FCC) was observed in the highly localized deformation zones, leading to strain softening that contributed to accommodating plasticity. These findings provide insights into the criticality of sample dimensions in influencing mechanical behaviors of nanostructured metallic materials used for nanoelectromechanical systems.

Enhanced Spray Cooling Using Micropillar Arrays: A Systematic Study

Abstract The role of contact-line evaporation on spray impingement heat transfer is systematically studied by spraying de-ionized water on silicon substrates with micropillar arrays. The height, the pillar diameter, and the spacing of the micropillar array were varied from 5 to 50 μm while keeping the porosity constant at 0.75. An air-assisted nozzle was used to create a liquid spray with a Sauter mean diameter (SMD) of ∼22 to 42 μm depending on flow conditions. Most test runs were conducted at a water flow rate of 30 ml/min and an air-liquid mass flow rate ratio of ∼0.57. The results show a continuous increase in the critical heat flux (CHF) as the pillar diameter is decreased. The effects of pillar height are nonmonotonic, with CHF and peak heat transfer coefficient attaining a maximum as the height-to-diameter ratio approaches unity. Values of CHF as high as 830 W/cm2 were achieved, along with cooling efficiencies of 49%. The effect of liquid flow rates and air-flow rates were also investigated independently using textured surfaces.

Size effects on strength and plasticity of ferrite and austenite pillars in a duplex stainless steel

The mechanical properties of the austenite and ferrite phases in a high Cr content 2205 duplex stainless steel (DSS) have been investigated using single crystal micropillars of size 200–2000 nm. Austenite pillars with axial orientation [123] show a strong size effect with strength increasing as pillar size decreases, following a power-law relation, with exponent -0.62. Pillars of diameter d = 300 nm show a strength >2.4 GPa. The strength of ferrite pillars with axial orientation [001] also shows a power-law with exponent -0.56 and strength > 1.5 GPa at d = 260 nm. However, [123]-orientated ferrite showed no size effect and a mean strength of 760 MPa. Both phases show localised multiple slip on non-parallel slip planes with high Schmid factors, with the ferrite showing a wider spacing of the slipped regions. On comparison with previous work, the strength of both phases in this alloy are shown to have greater strength than lower Cr content steels. Combining our data with results in the literature for larger diameter micropillar specimens supports the hypothesis that the high Cr content of the DSS leads to a larger bulk stress and a reduction in the magnitude of the apparent size effect exponent with larger pillar diameters.

Vertically-Aligned Multi-Walled Carbon Nano Tube Pillars with Various Diameters under Compression: Pristine and NbTiN Coated.

In this paper, the compressive stress of pristine and coated vertically-aligned (VA) multi-walled (MW) carbon nanotube (CNT) pillars were investigated using flat-punch nano-indentation. VA-MWCNT pillars of various diameters (30–150 µm) grown by low-pressure chemical vapor deposition on silicon wafer. A conformal brittle coating of niobium-titanium-nitride with high superconductivity temperature was deposited on the VA-MWCNT pillars using atomic layer deposition. The coating together with the pillars could form a superconductive vertical interconnect. The indentation tests showed foam-like behavior of pristine CNTs and ceramic-like fracture of conformal coated CNTs. The compressive strength and the elastic modulus for pristine CNTs could be divided into three regimes of linear elastic, oscillatory plateau, and exponential densification. The elastic modulus of pristine CNTs increased for a smaller pillar diameter. The response of the coated VA-MWCNTs depended on the diffusion depth of the coating in the pillar and their elastic modulus increased with pillar diameter due to the higher sidewall area. Tuning the material properties by conformal coating on various diameter pillars enhanced the mechanical performance and the vertical interconnect access (via) reliability. The results could be useful for quantum computing applications that require high-density superconducting vertical interconnects and reliable operation at reduced temperatures.

Surface elastic waves whispering gallery modes based subwavelength tunable waveguide and cavity modes of the phononic crystals

We studied the characteristics of a phononic crystal which consists of hollow pillars mounted on the surface of semi-infinite substrate. In this study, the existence of surface waves whispering gallery modes referred as localized cavity modes of the hollow pillars at the subwavelength frequency spectrum is reported. The interaction of the hollow pillars with the surface waves induce localized cavity modes and by tuning the inner radius of the hollow pillars these confined modes can be adjusted/tuned inside the bandgap region, thus introducing new applications for the surface waves based phononic crystals and acoustic metamaterial devices. These subwavelength confined cavity modes possess high-q narrow passband characteristics induced by the interaction of cavity modes with the surface waves propagating at the surface of semi-infinite substrate. We demonstrate the subwavelength waveguiding and other functionalities of these localized cavity modes by tuning the inner radius of the hollow pillars and allowing the specific wavelength frequencies to pass through the monochannel, multichannel, and linear cavity based compact waveguides. To make the study more general, we demonstrate the wave multiplexing phenomena for the identical frequencies (diameters of the hollow pillars are kept constant) for all types of waveguides. However, by varying the inner radii of the hollow pillars, the high-q localized frequencies can be tuned. All the physical phenomena presented shows excellent agreement and it indicates promising applications for the manipulation of surface waves in the surface acoustic waves devices, surface waves sensor and actuator technology, and other photonic/phononic and metamaterial applications.

Electrochemical Deposition of Sn-Cu Alloys for Applications in 3D Stacking in Microelectronics Industry

The use of eutectic Sn-Cu alloys in packaging applications in microelectronics industry is not something entirely new. However, this alloy was mostly used in large features and fabricated using metal powders and metallurgical deposition techniques. The diameter of an individual grain of the eutectic Sn-Cu powder is typically similar in size to the diameter of pillars or bumps currently used in 3D stacking, and if we were to check its usefulness as a solder on this scale, an alternative manufacturing technique had to be used.We have electrochemically deposited Sn-Cu alloys having up to 10 wt.% Cu and explored its possible benefits when used as a solder in combination with different Under bump metallization (UBM) materials. Sn-Cu alloys were deposited on blanket and patterned coupons, and promising experiments then transferred to a wafer-scale, i.e. Sn-Cu alloys were deposited on 300 mm wafers in an industrial-type plating tool. A typical sample/wafer had a Cu seed with a diffusion barrier (e.g. TiW) underneath and patterned features defined with a photoresist mask. Characteristic dimensions of the patterned features were on the order of mm and cm (lines), (diameter, d)50µm × (height, H)60µm (pillars), and (d)8µm × (H)15µm (bumps). Cu, Ni, or Co were used as UBM layers.Chemical composition of deposited alloys has been examined by using X-ray fluorescence (XRF), Micro X-ray fluorescence (Micro-XRF), Inductively coupled plasma mass spectrometry (ICP-MS), and Electron probe microanalysis (EPMA). Surface morphology and the shape of deposited features have been examined using Scanning electron microscopy (SEM) and Laser scanning microscopy (LSM), while thermodynamic properties have been studied using Differential scanning calorimetry (DSC). Within-wafer (WIW) and within-die (WID) height uniformity of the pillars has been analyzed using Falcon 630 Plus tool (Camtek Ltd), capable of capturing heights of all the structures on the wafer simultaneously. The kinetics of Intermetallic compound (IMC) formation have been studied in the number of selected samples by combining ex-situ Focused ion beam (FIB) and in-situ measurements of resistance change of the features during anneal/thermal ageing [1].We will discuss possible benefits and drawbacks for use of electrochemically deposited Sn-Cu alloy solders in combination with different UBM layers based on these results.

Polymer Micropillar Arrays for Colorimetric DNA Detection.

We describe the use of periodic micropillar arrays, produced from cyclic olefin copolymer using high-fidelity microfabrication, as templates for colorimetric DNA detection. The assay involves PCR-amplified gene markers for E. coli O157:H7 (rfbO157, eae, vt1, and vt2) incorporating a detectable digoxigenin label, which is revealed through an immunoenzymatic process following hybridization with target-specific oligonucleotide capture probes. The capacity of micropillar arrays to induce wicking is used to distribute and confine capture probes with spatial control, making it possible to achieve a uniform signal while allowing multiple, independent probes to be arranged in close proximity on the same substrate. The kinetic profile of color pigment formation on the surface was followed using absorbance measurements, showing maximum signal increase between 20 and 60 min of reaction time. The relationship between microstructure and colorimetric signal was investigated through variation of geometric parameters, such as pitch (10-50 μm), pillar diameter (5-40 μm), and height (16-48 μm). Our findings suggest that signal intensity is largely influenced by the edges of the pillars and less by their height such that it deviates from a linear relationship when both aspect ratio and pillar density become very high. A theoretical model used to simulate the changes in surface composition at the molecular level suggests that differences in the temporal and spatial accumulation of assay components account for this observation.

A closed-loop constant deposition current control method based on MCED circuit model

Meniscus-confined electrodeposition (MCED) is a simple and economical fabrication method for micro/nanoscale three-dimensional metal printing. In most applications of MCED of copper pillars using a deposition pipette probe, there exists a certain probe retraction speed range that can ensure successful pillar deposition. If the probe retraction speed exceeds this range, however, the morphological changes in the contact meniscus droplet bridge between the probe tip and substrate induce deposition current fluctuations. These fluctuations result in uneven pillar diameters (i.e., beaded wire deposition) and can even lead to nanowire breakage and growth termination. To analyze the MCED process, therefore, this paper first proposes a circuit model for the MCED process and then, based on this model, analyzes the mechanism underlying the deposition current fluctuations present as the probe retracts at higher speeds. To effectively suppress these deposition current fluctuations and improve the stability and quality of deposited copper pillars, a closed-loop constant-deposition-current control method is proposed. Compared with deposition using no control strategy, the constant-deposition-current control method is shown by simulations and experiments to significantly suppress the fluctuation of the deposition current and increase the stability of copper pillar growth. In addition, the constant-deposition-current control method is used for the rapid fabrication of copper pillars with high aspect ratios.

Spontaneous dewetting transition of nanodroplets on nanopillared surface

The spontaneous dewetting transition (SDT) of nanoscale droplets on the nanopillared surface is studied by molecular dynamics simulations. Three typical SDT modes, i.e. condensing, merging and coalescing with flying droplets are observed, and the underlying physical mechanism is clearly revealed by the potential energy analysis of droplets. We find that there exists a dimensionless parameter of the relative critical volume of droplet Ccri which completely controls the SDT of nanodroplets. Furthermore, the Ccri remains constant for geometrically similar surfaces, which indicates an intrinsic similarity of nanoscale SDT. The effect of pillar height, diameter and spacing on SDT has also been studied and it is likely to occur on the surface with longer, wider and thicker pillars, as well as pillars with cone-like shape and larger hydrophobicity. These results should be useful for a complete understanding of the nanoscale SDT and shed light on the design of smart superhydrophobic surfaces.

Effect of temperature on small-scale deformation of individual face-centered-cubic and body-centered-cubic phases of an Al0.7CoCrFeNi high-entropy alloy

High-entropy alloys (HEAs) represent an important class of structural materials because of their high strength, ductility, and thermal stability. Understanding the mechanical response of isolated phases of a FCC/BCC dual-phase HEA is integral to understanding the mechanical properties of these alloys in the bulk. We investigate the compressive response of single-crystalline cylinders with diameters between 400 nm and 2 μm excised from individual grains within FCC and BCC phases of the dual-phase Al0.7CoCrFeNi HEA at 295 K, 143 K, and 40 K. We observed a “smaller is stronger” size effect in the yield strength as a function of pillar diameter, D, of both alloy phases for all temperatures, with a power-law exponent, m, decreasing with temperature for the FCC phase, and remaining constant for all temperatures in the BCC phase. We found reduced work-hardening rates and more extensive strain bursts during deformation at lower temperatures in all samples. We performed molecular dynamics simulations of similar FCC and BCC HEA compression that displayed deformation dominated by dislocation slip at all temperatures. We discussed theories of low-temperature strengthening in HEAs, compared them to our experimental data and assessed how they manifest in the observed temperature-dependent size effect and work-hardening.

Fabrication of immobilized enzyme reactors with pillar arrays into polydimethylsiloxane microchip

This paper demonstrates the design, efficiency and applicability of a simple and inexpensive microfluidic immobilized enzymatic reactor (IMER) for rapid protein digestion. The high surface-to-volume ratio (S/V) of the reactor was achieved by forming pillars in the channel. It was found that pillar arrays including dimensions of 40 μm × 40 μm as pillar diameter and interpillar distance can provide both relatively high S/V and flow rate in the PDMS chip, the fabrication of which was performed by means of soft lithography using average research laboratory infrastructure. CZE peptide maps of IMER-based digestions were compared to peptide maps obtained from standard in-solution digestion of proteins. The peak patterns of the electropherograms and the identified proteins were similar, however, digestion with the IMER requires less than 10 min, while in-solution digestion takes 16 h.

Deep reactive ion etching of ‘grass-free’ widely-spaced periodic 2D arrays, using sacrificial structures

We fabricated two-dimensional arrays of periodic, widely-spaced pillars using deep reactive ion etching of silicon. To avoid the formation of micro-grass in the large open areas we used sacrificial structures surrounding the widely-spaced pillars. The use of sacrificial structures results in a denser pattern where the formation of grass is less likely to happen. We were able to remove the sacrificial structures without damaging the main array of pillars by using a modified Bosch process. The roughness remaining after removal of the sacrificial structures was evaluated using optical profilometry. Using this method, we were able to pattern grass-free arrays of widely-spaced 12 μm diameter pillars of 9:1 aspect ratio, with hexagonal and square distributions.

Novel CdZnTe micro pillar films deposited by CSS method

CdZnTe (CZT) micro pillar films with cubic zinc blend phase have been deposited on single layer graphene coated quartz substrate by close-spaced sublimation technique. Structural and phase purities of deposited micro pillar CZT films were investigated using X-ray diffraction and Raman spectroscopy. Scanning electron microscopy characterizations indicated that the diameters of CZT micro pillars were about 5 µm and their lengths were close to 57 µm. The band-gap of CZT micro pillar films was about 1.53 eV, which was confirmed by UV–Vis–NIR diffuse reflection spectra and PL spectra. The structure of Au/BGZO/CZT/graphene/quartz was fabricated to study its optoelectronic properties. Under the illumination of AM 1.5, CZT micro pillar films had good photo response, which indicated their great potential in the application of optoelectronic devices.

Ion bombardment induced formation of self-organized wafer-scale GaInP nanopillar assemblies

Ion sputtering assisted formation of nanopillars is demonstrated as a wafer-scale, lithography-free fabrication method to obtain high optical quality gallium indium phosphide (GaInP) nanopillars. Compared to binary materials, little has been reported on the formation of self-organized ternary nanostructures. Epitaxial (100) Ga0.51In0.49P layers lattice matched to GaAs were sputtered by nitrogen (N2) ions with relatively low ion beam energies (∼400 eV) to reduce ion bombardment induced damage. The influence of process parameters such as temperature, sputter duration, ion beam energy, and ion beam incidence angle on the pillar formation is investigated. The fabricated GaInP nanopillars have average diameters of ∼75–100 nm, height of ∼220 nm, and average density of ∼2–4 × 108 pillars/cm2. The authors show that the ion beam incidence angle plays an important role in pillar formation and can be used to tune the pillar shape, diameter, and spatial density. Specifically, tapered to near cylindrical pillar profiles together with a reduction in their average diameters are obtained by varying the ion beam incidence angle from 0° to 20°. A tentative model for the GaInP nanopillar formation is proposed based on transmission electron microscopy and chemical mapping analysis. μ-Photoluminescence and μ-Raman measurements indicate a high optical quality of the c-GaInP nanopillars.

Anomalous size-dependent strength in micropillar compression deformation of commercial-purity aluminum single-crystals

An anomalously reduced size-dependent strength of commercial-purity aluminum (Al) single-crystal micropillars with diameters ranging approximately from 1 to 10 μm is reported. High-purity Al (99.99%) single-crystal micropillars exhibited an obvious size dependence of the resolved shear stress for slip. The measured shear stress resolved onto a primary slip system (τi) scaled by the shear modulus (G) and the pillar diameter (d) scaled by the Burgers vector (b) showed the following correlation: τi/G = 0.33(d/b)−0.63, which agreed well with previous works. However, the commercial-purity Al samples exhibited a lower power-law exponent (0.19) for their size-dependent strength, resulting in (τi/G) = 0.006(d/b)−0.19. TEM characterization revealed the local presence of Al–Fe intermetallic precipitates surrounded by relatively high-density dislocations in annealed commercial-purity Al samples. These results indicate the relatively high-density dislocations could be responsible for the reduced size-dependent strength, which was confirmed by the remarkably reduced size dependence of their resolved shear stress by prior cold rolling. The reduced size-dependent strength can be rationalized using the stochastic model of the dislocation source length. Thus, this study provides a new insight to allow the application of the micropillar compression test to commercially produced Al alloys.

Review of Size Effects during Micropillar Compression Test: Experiments and Atomistic Simulations

The micropillar compression test is a novel experiment to study the mechanical properties of materials at small length scales of micro and nano. The results of the micropillar compression experiments show that the strength of the material depends on the pillar diameter, which is commonly termed as size effects. In the current work, first, the experimental observations and theoretical models of size effects during micropillar compression tests are reviewed in the case of crystalline metals. In the next step, the recent computer simulations using molecular dynamics are reviewed as a powerful tool to investigate the micropillar compression experiment and its governing mechanisms of size effects.

Optical‐Mode Structure of Micropillar Microcavities Containing a Fluorescent Conjugated Polymer

AbstractThe light emission from a series of micropillar microcavities containing a fluorescent, red‐emitting conjugated polymer, is explored. Cavities are fabricated by defining two dielectric mirrors either side of a polymer active region. Focused ion‐beam (FIB) lithography is then used to etch pillar structures into the planar cavity having diameters between 1 and 11 µm. The photoluminescence (PL) emission of the cavities is characterized using real‐space tomographic and Fourier‐space imaging techniques, with emission shown to be quantized into a mode‐structure resulting from both vertical and lateral optical confinement within the pillar. The optical‐confinement effects which result in a blue‐shift of the fundamental mode as the pillar diameter is reduced is demonstrated, with a model applied to describe the energy and distribution of the confined optical modes.

Micropillar compression investigation of size effect on microscale strength and failure mechanism of Calcium-Silicate-Hydrates (C-S-H) in cement paste

The compressive strength and failure of concrete and cement exhibit strong size effect over various length scales. To investigate possible size effect on compressive strength and failure mechanism of Calcium-Silicate-Hydrates (C-S-H) in cement paste, micropillar compression experiments were performed on micropillar geometries fabricated by focused ion beam milling on potential C-S-H locations identified through coupled backscatter electron imaging (BSE) and energy dispersive spectroscopy (EDS) analysis. The compressive strength of C-S-H (181–1145 MPa) measured from C-S-H micropillars of varying diameters indicated presence of a size effect with strong increase in strength with decreasing diameter. The deformation mode at failure also exhibited size effect: the dominant failure mode changed from axial splitting to plastic crushing as the pillar diameter was decreased. The observed relationship between strength and pillar diameter can be modeled by an inverse square root dependency which closely corresponds to Bazant's scaling law of quasi-brittle failure.

Efficient paradigm to enhance particle separation in deterministic lateral displacement arrays

Microfluidics based particle sorting and separation methods are gaining momentum to be applied for various applications. Deterministic lateral displacement (DLD) methods are prominent for high resolution in separation and there has been extensive studies to develop more efficient devices based on the DLD. However, it is still challenging to fully eliminate negative effects of the boundaries that degrade particle separation efficiency by perturbing the fluid flow in the channel. In this article, we present two equations to optimize channels’ geometry near the boundaries. Implementing the equations, the fluid behavior is improved around the pillars and thereby, separation efficiency is increased. The Boundary Correction Paradigm (BCP) enhances the microchannel’s functionality as much as 2–3 times and can be highly beneficial in microchannels. Also, an equation is proposed in order to recalibrate the BCP in microchannels with desired pillar diameters. The calibration equation assures high accuracy and resolution of the DLD devices corrected with the BCP.

Synergistic Effect of 3D Current Collectors and ALD Surface Modification for High Coulombic Efficiency Lithium Metal Anodes

Improving the performance of Li metal anodes is a critical bottleneck to enable next‐generation battery systems beyond Li‐ion. However, stability issues originating from undesirable electrode/electrolyte interactions and Li dendrite formation have impaired long‐term cycling of Li metal anodes. In this work, we have developed a bottom‐up fabrication process of vertically aligned and highly uniform Cu pillar arrays on a Cu foil via templated electrodeposition for use as a 3D current collector.[1] By rationally controlling geometric parameters of the 3D current collector architecture, including pillar diameter, spacing, and length, we have demonstrated the morphology of Li plating/stripping upon cycling can be controlled and optimal cycling performance can be achieved. In addition, we showed that deposition of an ultrathin layer of ZnO by atomic layer deposition on the current collector surface can facilitate the initial Li nucleation, which dictates the morphology and reversibility of subsequent cycling. This core–shell pillar architecture allows for the effects of geometry and surface chemistry to be decoupled and individually controlled to optimize the electrode performance in a synergistic manner. Using this model platform, we have demonstrated Li metal anodes with Coulombic efficiency up to 99.5% at 0.5 mA/cm2 and 99.4% at 1 mA/cm2, which are among the highest reported values to date. The results gained from this work can thus provide a pathway toward high‐efficiency and long‐cycle life Li metal batteries with reduced excess Li loading. [1] K.-H. Chen, A.J. Sanchez, E. Kazyak, A.L. Davis, N.P. Dasgupta, Adv. Energy Mater. 9, 1802534 (2019).