Step Pinning Research Articles

Resin bleed improvement on surface mount semiconductor device

Resin bleed is a transparent layer of epoxy compound which occurs during molding process but is difficult to be detected after the molding process. Resin bleed on the lead on the unit from the focused package, SOD123, can cause solderability failure at end customer. This failed unit from the customer will be considered as a customer complaint. Generally, the semiconductor company has to perform visual inspection after the plating process to detect resin bleed. Mold chase with excess hole, split cavity & stepped design ejector pin hole have been found to be the major root cause of resin bleed in this company. The modifications of the mold chase, changing of split cavity to solid cavity and re-design of the ejector pin proposed were derived after a detailed study & analysis conducted to arrive at these solutions. The solutions proposed have yield good results during the pilot run with zero (0) occurrence of resin bleed for 3 consecutive months.

Heterogeneous nucleation of pits via step pinning during Si(100) homoepitaxy

Using scanning tunneling microscopy (STM), we investigate oxide-induced growth pits in Si thin films deposited by molecular beam epitaxy. In the transition temperature range from 2D adatom islanding to step-flow growth, systematic controlled air leaks into the growth chamber induce pits in the growth surface. We show that pits are also correlated with oxygen-contaminated flux from Si sublimation sources. From a thermodynamic standpoint, multilayer growth pits are unexpected in relaxed homoepitaxial growth, whereas oxidation is a known cause for step pinning, roughening, and faceting on elemental surfaces, both with and without growth flux. Not surprisingly, pits are thermodynamically metastable and heal by annealing to recover a smooth periodic step arrangement. STM reveals new details about the pits’ atomistic origins and growth dynamics. We give a model for heterogeneous nucleation of pits by preferential adsorption of Å-sized oxide nuclei at intrinsic growth antiphase boundaries, and subsequent step pinning and bunching around the nuclei.

A Mesocrystal‐Like Morphology Formed by Classical Polymer‐Mediated Crystal Growth

AbstractGrowth by oriented assembly of nanoparticles is a widely reported phenomenon for many crystal systems. While often deduced through morphological analyses, direct evidence for this assembly behavior is limited and, in the calcium carbonate (CaCO3) system, has recently been disputed. However, in the absence of a particle‐based pathway, the mechanism responsible for the creation of the striking morphologies that appear to consist of subparticles is unclear. Therefore, in situ atomic force microscopy is used to investigate the growth of calcite crystals in solutions containing a polymer additive known for its ability to generate crystal morphologies associated with mesocrystal formation. It is shown that classical growth processes that begin with impurity pinning of atomic steps, leading to stabilization of new step directions, creation of pseudo‐facets, and extreme surface roughening, can produce a microscale morphology previously attributed to nonclassical processes of crystal growth by particle assembly.

Antimalarials inhibit hematin crystallization by unique drug–surface site interactions

In malaria pathophysiology, divergent hypotheses on the inhibition of hematin crystallization posit that drugs act either by the sequestration of soluble hematin or their interaction with crystal surfaces. We use physiologically relevant, time-resolved in situ surface observations and show that quinoline antimalarials inhibit β-hematin crystal surfaces by three distinct modes of action: step pinning, kink blocking, and step bunch induction. Detailed experimental evidence of kink blocking validates classical theory and demonstrates that this mechanism is not the most effective inhibition pathway. Quinolines also form various complexes with soluble hematin, but complexation is insufficient to suppress heme detoxification and is a poor indicator of drug specificity. Collectively, our findings reveal the significance of drug-crystal interactions and open avenues for rationally designing antimalarial compounds.

Role of Molecular Recognition in l-Cystine Crystal Growth Inhibition.

l-Cystine kidney stones—aggregates of single crystals of the hexagonal form of l-cystine—afflict more than 20 000 individuals in the United States alone. Current therapies are often ineffective and produce adverse side effects. Recognizing that the growth of l-cystine crystals is a critical step in stone pathogenesis, real-time in situ atomic force microscopy of growth on the (0001) face of l-cystine crystals and measurements of crystal growth anisotropy were performed in the presence of prospective inhibitors drawn from a 31-member library. The most effective molecular imposters for crystal growth inhibition were l-cystine mimics (aka molecular imposters), particularly l-cystine diesters and diamides, for which a kinetic analysis revealed a common inhibition mechanism consistent with Cabrera–Vermilyea step pinning. The amount of inhibitor incorporated by l-cystine crystals, estimated from kinetic data, suggests that imposter binding to the {0001} face is less probable than binding of l-cystine solute molecules, whereas imposter binding to {101̅0} faces is comparable to that of l-cystine molecules. These estimates were corroborated by computational binding energies. Collectively, these findings identify the key structural factors responsible for molecular recognition between molecular imposters and l-cystine crystal kink sites, and the inhibition of crystal growth. The observations are consistent with the reduction of l-cystine stone burden in mouse models by the more effective inhibitors, thereby articulating a strategy for stone prevention based on molecular design.

Termination of hollow core nanopipes in GaN by an AlN interlayer

Nanopipes associated to screw dislocations are studied by transmission electron microscopy in Si-doped GaN films grown on silicon substrates. The observations revealed that dislocations had an empty core and that an AlN interlayer is suited to block their propagation. The termination mechanism is discussed in terms of strain and kinetic growth factors, which may affect the creation and propagation of nanopipes. According to the observations, it is proposed that either step pinning or lateral overgrowth occurring at the proximity of the defect assists in capping the nanopipe.

Kinetics of LPP crystal nucleation and interface morphology studies

The nucleation parameters such as interfacial energy (γ), critical free energy (ΔGcr), critical radius (rcr) and critical nucleus number (ncr) of LPP have been evaluated. Two types of growth patterns (spiral growth and nucleation at facet edges or corners) have been observed. Berg effect and impurity adsorption are responsible for the hopper or skeletal morphology. Step pinning can give rise to diffusion-field overlap in the face's center which can eventually lead to loss of stability of the step trains. The formation of line defect is linked to the embedding of microcrystals of the same phase as growth takes place. Ultimately, it can repair itself after multiple layers have grown.

Phase-Field Modeling of Step Dynamics on Growing Crystal Surface: Step Pinning Induced by Impurities

The step pinning hypothesis is widely accepted as an explanation for the inhabitation of crystal growth by impurity molecules. This hypothesis enables us to evaluate the average step velocity and the critical impurity density above which no step can move on the crystal surface polluted by impurities. The average step velocity and the critical impurity density were obtained analytically in some literature; however, they have not been verified with other approaches. Here, we propose a new method for the numerical simulation of the step pinning by impurities in the framework of a phase-field technique and report a quantitative study on the average step velocity and critical impurity density for the first time. Our results clarify the limitations of the physical assumptions of the analytical formulas and their applications. Our phase-field method provides a good numerical tool for the quantitative investigation of the effect of impurities on step dynamics if step pinning works as described in the Cabrera–Verm...

An Investigation of Shorting Pin Effects on Circular Disc Microstrip Antennas

In this study, shorting pin addition, one of the common bandwidth enhancement techniques, is performed on traditional circular disc microstrip antennas that have three different dielectric substrate thicknesses and designed in three different resonant frequencies. Numbers of added shorting pins and positions of these pins are changed in every step, systematically and different pin combinations are tried for three antennas that design parameters are present in literature. Later, optimal pin combinations that give the intended resonant frequencies and the largest bandwidths are determined. For these optimal combinations, by keeping the resonant frequencies constant, the bandwidths of the antennas are increased to %1.440, %2.780 and % 6.024, respectively. Compared to measurement results in the literature, for bandwidths, significant improvements are obtained. At the same time, it is observed that the technique of shorting pin addition increases the bandwidths in higher percentages on thicker microstrip antennas. For three antennas, it is seen that all pin combinations give similar resonant frequency variations depended on pin positions.

Friction stir lap welding of 5456 aluminum alloy with different sheet thickness: process optimization and microstructure evolution

The influences of friction stir welding process parameters on microstructure evolution and mechanical properties of lap-welded 5456 aluminum alloy plates with different thickness and temper conditions were investigated. The upper plate was 5 mm thick, cold rolled aluminum alloy 5456-T321 and the lower plate was 2.5-mm annealed sheet (5456-O). Four different pin geometry types (conical thread pin, cylindrical–conical thread pin, stepped conical thread pin, and flared-triflute pin tool) and two rotational speeds (600 and 800 rpm) were used to produce the joints. Microstructures and microhardness values in the weld nugget (WN), thermomechanically affected zone (TMAZ), and the heat-affected zone (HAZ) were examined and correlated with selected processing conditions. Specifically, the influence of tool geometry on the flow of the plasticized material in the nugget zone, extent of hooking defect, and mechanical properties (microhardness) of the FSW joints were documented and quantified. It was found that weld joints made by using the stepped conical thread pin tool produced a homogeneous microstructure with finer grain size (5.4 μm) and higher microhardness levels than the other tools. The optimum processing conditions resulting in sound and defect-free joints with highest mechanical properties were obtained with the stepped conical thread pin and 600-rpm rotational speed. The evolution of the microhardness in each region is characterized and related to processing conditions.

A Preliminary Study of Deformation Behavior of Friction Stir Welded Ti-6Al-4V

A preliminary study of deformation behavior of friction stir welded (FSW) Ti-6Al-4V was performed using two different tools with cylindrical and stepped spiral pin design for the welding process. The nugget regions experienced temperature above β transus and the matrix transformed to fine acicular α during cooling of the nugget. By using stepped spiral pin design, a local region with much refined grain structure and significant tool debris particles were observed. Room temperature tensile test showed increased strength and decreased ductility in the material from this region. Fractographic analysis revealed that tool debris particles served as void nucleation sites. Tensile tests of FSW material were carried out at 625 °C in the strain rates of 3 × 10−4 and 1 × 10−3 s−1. The strength was higher as compared to the as-received material. Microstructural evolution during tensile test was also investigated. Results showed that dynamic globularization occurred during the high temperature tensile test.

Influence of tool geometry and rotational speed on mechanical properties and defect formation in friction stir lap welded 5456 aluminum alloy sheets

Friction stir welding of AA5456 aluminum alloy in lap joint configuration is with two different tempers, T321 and O, and different thicknesses, 5mm and 2.5mm was investigated. The influences of tool geometry and various rotational speeds on macrostructure, microstructure and joint strength are presented. Specifically, four different tool pin profiles (a conical thread pin, a cylindrical–conical thread pin, a stepped conical thread pin and Flared Triflute pin tool) and two rotational speeds, 600 and 800rpm, were used. The results indicated that, tool geometry influences significantly material flow in the nugget zone and accordingly control the weld mechanical properties. Of particular interest is the stepped conical threaded pin, which is introduced for the first time in the present investigation. Scanning electron microscopy investigation of the fracture location of samples was carried out and the findings correlated with tool geometry features and their influences on material flow and tension test results. The optimum microstructure and mechanical properties were obtained for the joints produced with the stepped conical thread pin profile and rotational speed of 600rpm. The characteristics of the nugget zone microstructure, hooking height, and fracture location of the weld joints were used as criteria to quantify the influence of processing conditions on joint performance and integrity. The results are interpreted in the framework of physical metallurgy properties and compared with published literature.

Influences of Tool Geometry on Metallurgical and Mechanical Properties of Friction Stir Welded Dissimilar AA 2024 and AA 5052

The present work investigates on the characterization of friction stir welded dissimilar aluminium alloys AA2024 and AA5052. Performance of commonly used tool profiles were compared against the newly developed stepped pin profile. H13 tool steel was used as friction stir tool with various pin profiles which includes cylindrical, cylindrical-threaded, squared, tapered and stepped types. Tensile and hardness studies were done to analyze mechanical behaviour. Macrographs of cylindrical and tapered pins showed minute discontinuities whereas other profiles did not. Microstructure and SEM/EDAX analysis were done to examine the metallurgical changes for every trial. The optimized process parameters of speed and feed were identified to be lOOOrpm and 40mm/min respectively. Extensive studies revealed that sound welds were produced in the plates that were welded with new stepped pin tool.

High Surface Capacity Li-Ion All Solid State 3D Microbattery Based on Anatase TiO2 Deposited by ALD on Silicon Microstructures

The proposed study deals with the design, the fabrication and the characterization of 3D silicon microstructures decorated with step conformal and pin hole free anatase TiO2 negative electrode deposited by atomic layer deposition. A high aspect ratio and dense array of 3D silicon micropillars or microtubes are fabricated on silicon wafer. The obtained surface capacity of the TiO2 layer (150nm) deposited on 3D microtubes (area enlargement factor =25) reaches 0.15 mAh/cm2 at C/4. The reported 3D topologies enhance the capacity of standard TiO2 2D thin film electrode.

Theory of Impurity Induced Step Pinning and Recovery in Crystal Growth from Solutions

We extend the terrace-step-kink model of crystal growth to impure solutions where the impurities act as barriers to step motion. The effects of supersaturation, step curvature, step repulsions, and impurities on step motion are treated in a unified free energy framework. The model reproduces several features seen in experiments on growth of potassium dihydrogen phosphate crystals, wherein a dead zone at low supersaturations and a recovery of crystal growth by motion of large coherent step bunches at larger supersaturations are observed. We identify a key feature of solution growth that leads to these effects.

Growth inhibition of calcium oxalate monohydrate crystal by linear aspartic acid enantiomers investigated by in situatomic force microscopy

The inhibitory effect of linear enantiomers of L- and D-Asp6 on the growth of calcium oxalate monohydrate crystal has been investigated using in situ atomic force microscopy. The inhibitory magnitude of D-Asp6 on the growth of the [00] step on the (010) face is about 10% larger than that of L-Asp6. While no chiral effect is observed or expected on the growth of the [0] step on the (01) face by both enantiomers, their inhibitory effect on this step is much stronger than that on the [00] step on the (010) face. In both cases, the step morphology indicates that these enantiomers create the impurity pinning along the steps, while the dependence of step speed on supersaturation shows that they also produce a reduction of the step kinetic coefficients. Analysis of the step speed data within the context of an existing model for step pinning and kink blocking shows that the major impact of Asp6 is to block active kink sites. The larger inhibition of the [00] step growth by D-Asp6 over L-Asp6 and the substantially larger inhibition of the [0] step over the [00] step by both enantiomers both result from larger affinity for adsorption to the (010) face and the (01) face, respectively. This is because the larger adsorption leads to a higher density of blocking kink sites along the steps. The estimated difference in binding energy of L- and D-Asp6 to the respective faces from the kinetics model is consistent with the trend predicted by our molecular modeling of the enantiomer binding to the faces.

Sticky steps inhibit step motions near equilibrium

Using a Monte Carlo method on a lattice model of a vicinal surface with a point-contact-type step-step attraction, we show that, at low temperature and near equilibrium, there is an inhibition of the motion of macrosteps. This inhibition leads to a pinning of steps without defects, adsorbates, or impurities (self-pinning of steps). We show that this inhibition of the macrostep motion is caused by faceted steps, which are macrosteps that have a smooth side surface. The faceted steps result from discontinuities in the anisotropic surface tension (the surface free energy per area). The discontinuities are brought into the surface tension by the point-contact-type step-step attraction. The point-contact-type step-step attraction also originates "step droplets," which are locally merged steps, at higher temperatures. We derive an analytic equation of the surface stiffness tensor for the vicinal surface around the (001) surface. Using the surface stiffness tensor, we show that step droplets roughen the vicinal surface. Contrary to what we expected, the step droplets slow down the step velocity due to the diminishment of kinks in the merged steps (smoothing of the merged steps).

Crystalline Quality and Surface Morphology of 3C-SiC Films on Si Evaluated by Electron Channeling Contrast Imaging

Electron channeling contrast imaging (ECCI) has been utilized to evaluate the surface morphology and crystalline quality of 3C-SiC films grown by chemical vapor deposition (CVD) on (100) and (111) Si substrates. ECCI in this study was performed using an electron backscatter diffraction (EBSD) system equipped with forescatter diode detectors and mounted inside a commercial scanning electron microscope (SEM). This nondestructive method permits direct dislocation imaging through local fluctuations in forescattered electron yield attributable to lattice strain. Coordinated ECCI, SEM, and EBSD analysis of film surfaces allowed correlations between film orientation, surface morphology, and dislocation behavior. Evidence of lateral dislocations parallel to <110> directions and atomic step pinning by dislocations was observed.

Kinetic Monte Carlo simulation of an atomistic model for oxide island formation and step pinning during etching by oxygen of vicinal Si(100)

A lattice-gas model is developed to describe the simultaneous oxidation and etching of Si(100) surfaces exposed oxygen. The model incorporates nucleation of oxide islands via conversion of on-surface to back-bonded oxygen, together with an observed transformation in the shapes of just-formed islands from linear to two-dimensional. Model analysis via Kinetic Monte Carlo simulation quantifies oxygen uptake and oxide island nucleation kinetics, including possible enhanced nucleation at step edges. Simulated etching of vicinal Si(100) surfaces reveals that receding steps are pinned by oxide islands and transform into finger-like structures even at higher temperatures where oxide island growth is inhibited.

Interaction between Epsomite Crystals and Organic Additives

A number of phosphonates and carboxylates were tested as potential crystallization inhibitors for epsomite (MgSO4·7H2O). Epsomite nucleation is strongly inhibited in the presence of amino tri(methylene phosphonic acid) (ATMP), diethylenetriaminepentakis (methylphosphonic acid) (DTPMP), and poly(acrylic acid) sodium salt (PA). These additives also act as habit modifiers promoting the growth of acicular crystals elongated along the [001] direction. Environmental scanning electron microscopy (ESEM), Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), and molecular modeling of additive adsorption on specific epsomite (hkl) faces are used to identify how these additives inhibit epsomite crystallization. Additives attach preferentially on epsomite {110} faces, at edges of monolayer steps parallel to [001]. Step pinning and the eventual arrest of step propagation along ⟨110⟩ directions account for the observed habit change. Hydrogen bonding between the functional groups of additive mol...