Electromigration in thin gold films on molybdenum surfaces

Electric current was passed through thin gold films deposited onto molybdenum surfaces. The edge of the gold film closer to the cathode was seen to move towards the anode, leaving a free molybdenum surface. Large hillocks and crystallites were seen to accumulate on the gold edge pointing toward the anode. The velocity of the film edge was assumed to represent the average drift velocity for electromigration.The velocity of the gold edge was found to be constant at any given temperature and current density. At any constant temperature, the velocity was proportional to the current density. Finally, the velocity depended exponentially on the temperature.Current densities of 10⁵–10⁶ Acm^-2 were used at temperatures of 260°–500°C. The “activation energy” for the drift velocity was found to be 0.6 eV in most samples, while some samples showed an energy of about 0.9eV. The drift velocities per unit current density ranged between 10^-13 and 10^-15 cm³ A^-1 s^-1 and the gold motion was always in the direction of electron flow.     

Optical and electrochemical properties of multilayer polyelectrolyte thin films incorporating spherical, gold colloid nanomaterials

Polyelectrolyte multilayer (PEM) films incorporating various types of spherical, gold nanomaterials (NMs) were investigated to assess the existence of electrochemical and/or optical signal enhancement effects directly attributable to embedded NMs and the relationship of these effects to film structure and composition. Specifically, electrostatically assembled films of cationic poly-l-lysine (PLL) and anionic poly(4-styrene sulfonate) (PSS) incorporating one of four types of spherical, gold colloid NMs were constructed on 3-(aminopropyl)trimethoxysilane (3-APTMS)-modified glass substrates for optical studies or 11-mercaptoundecanoic (MUA)-modified gold electrodes for electrochemical studies. The NMs inserted into the PEM films include citrate-stabilized gold nanoparticles, thioctic acid-stabilized gold nanoparticles (TAS-NPs), MUA-modified monolayer protected gold clusters, and hollow gold nanoshells (Au-NSs). Optical sensitivity of the NM-embedded films, in terms of absorbance, surface plasmon band shifts, and the dependence of these optical responses on film thickness, varied depending on the type of NM within the film (e.g., TAS-NPs versus Au-NSs) but exhibited no corresponding electrochemical effects in the diffusional voltammetry of a ferricyanide redox probe. While not correlated to optical responses, the increased Faradaic current achieved during voltammetry at NM-embedded PEM films suggested that electrochemical effects of NMs were less dependent on the type of NMs and were, instead, more related to their location within the film and the electrostatic interactions built into the interfacial chemistry of the films. These results should prove useful for developing strategies constructing thin films with NMs that are specifically designed for optical or electrochemical sensing, taking full advantage of the signal enhancements provided by individual types of NMs.     

Selective cell spreading, proliferation, and orientation on micropatterned gel surfaces

Hydrogel micropatterns of 10 approximately 200 microm in width were introduced during the polymerization of 2-acrylamido-2-methyl-propane sulfonic acid sodium salt (NaAMPS) on the surface of polyacrylamide (PAAm) gel. Behaviors of endothelial cells on the micropatterned PNaAMPS/PAAm gel surfaces were studied. Cells selectively proliferate on micropatterned PNaAMPS surface but not on PAAm surface, which requires no modification with any cell adhesive proteins or peptides. We found that decrease of the width of the micropatterns could increase the degree of anisotropic spreading of cells and the degree of cell orientation. These results demonstrated that the topographical micropatterns of hydrogel could control cell behaviors.     

Morphology and electrical resistance of thin gold films on air-cleaved potassium chloride surfaces

The structures of vacuum-evaporated thin gold films on air-cleaved KCl single-crystal surfaces were studied by transmission electron microscopy and electrical resistance measurements. Preferential growth of gold crystallites along the cleavage steps was observed and confirmed by the presence of one-dimensional conduction. Films of average thickness 1.5 nm or less showed an island structure and did not exhibit electrical conduction. Films with thickness of 2.0 nm and 2.5 nm had network structures and displayed one-dimensional conduction along the steps and two-dimensional conduction on the surface respectively. The electrical resistance was not stable for the 2.0 nm film, while the 2.5 nm film had stable values. The resistance of films of average thickness 2.0 and 2.5 nm decreased as the temperature decreased but the effect was small.     

Oxygen-ion-assisted deposition of thin gold films

Thin gold films have been deposited on glass and silicon substrates using ion-assisted deposition techniques. The adhesion of the films to the substrates is assessed by a scratch test. Deposition assisted by 100 eV-1 keV oxygen ions yields highly adhesive films that can only be removed by damaging the substrate. Argon and hydrogen ions produce films with relatively poor adhesion. The results show that the reflectance of oxygen-assisted films is reduced by trapping of the oxygen in the gold but no bulk chemical or structural changes are detected. It is proposed that a thin stable layer of gold oxide is formed during film growth and diffuses into the substrate, providing a strong bond for subsequent film deposition. Highly adhesive films with bulk optical properties are deposited on glass and silicon using oxygen-ion assistance only to the point of continuous film formation.     

Preparation of micropatterned poly(silsesquioxane) thin films using adamantylphenol molecules

Poly(methyl silsesquioxane) (PMSSQ) thin films with micropatterned surface structures have been prepared by use of adamantylphenol molecules as a photo-thermal sensitive moiety together with UV lithographic technique and mild heating process. Initially, PMSSQ films form positive patterns of micronscale due to the film densification triggered by photooxidation and photopolymerization of doped moieties within UV exposed region. With thermal treatment, negative patterns from these pre-patterned films are formed by the difference of polycondensation rates between non-exposed regions and irradiated areas without additional developing or etching steps. This structural transformation of PMSSQ thin films was investigated using Fourier-transformed infrared spectroscopy, ultraviolet visible spectroscopy, X-ray photoelectron spectroscopy, Auger electron spectroscopy, and field emission scanning electron microscopy.     

Fractal properties of gold, palladium and gold-palladium thin films on InP

Thermal interaction of indium phosphide (InP) bulk compound semiconductor with thin gold metal films was investigated in the course of the present work. The interaction of the InP/Au system resulted in a pattern showing fractal dimensions. The temperature dependence of the fractal parameters was investigated in a broad temperature range from 200 to 600 °C. No significant temperature dependence of the fractal dimension was observed.The same calculations will be presented for Au/InP and AuPd/InP systems. Our calculations show that the Pd-based contacts have a different behaviour than AuGe metallization where a strong temperature dependence of the fractal number was observed earlier.Another topology measure, the structural entropy is also calculated for the samples. The structural entropy is usually applied for determining the type of the localization of charge distributions, but it can also be used for generalized charges, such as the lightness of the pixels of an electron microscopy picture.     

Electro-reflectance and electro-transmittance in thin films of gold

The electro-reflectance and electro-transmittance of gold films have been measured using an electrolytic cell technique and the spectral variation of the effect has been established as a function of the film thickness. The electrolytic electric field causes a shift of the electro-reflectance peak to lower photon energies as the film becomes less than 20 nm thick. A relationship has been established between the normal reflectance and transmittance of the system and the changes in these with electric field. These effects have been predicted quantitatively with a theoretical model which takes into account only the modulation of the first surface of the charge carrier enhanced layer and uses the free electron component contribution to the optical constants together with interference within a thin absorbing film. This model has been used to predict the electro-reflectance effects previously observed with silver.

With very thin films (less than 10 nm) discrepancies arise due to the fact that the optical properties of the thin film material are no longer those of the bulk. In this case it has been shown that on the same model it is possible to use the derivative of the reflectance of the thin film with respect to frequency to obtain a prediction of the electro-reflectance exhibited.     

In-situ X-ray investigations of quench-condensed thin gold films

Thin gold films were deposited on float glass substrates held at cryogenic temperatures down to 77 K and investigated in-situ using X-ray reflectometry and surface sensitive reflection mode X-ray absorption spectroscopy (XAFS). The combination of these in-situ X-ray methods with simultaneous electrical resistivity measurements yields information about the surface and volume microstructure of the deposited films as a function of the deposition temperature and their changes induced by a subsequent annealing treatment. The surface sensitive XAFS experiments clearly proved that the films exhibit a polycrystalline structure throughout the temperature range studied here. The data were fitted using a correlated Debye-model. The results show that for film deposition at low substrate temperatures < 130 K, a significantly decreasing Debye-temperature was found, reaching values of about 100 K in comparison to 165 K for the polycrystalline bulk material. This decrease was interpreted to be predominantly related to defective film regions with an increased static disorder.     

Size effects on the mechanical behavior of gold thin films

The Membrane Deflection Experiment was used to test the mechanical response of freestanding thin film gold specimens. We present stress-strain curves obtained on films 0.3, 0.5, and 0.1 m thick. Elastic modulus was consistently measured in the range of 53–55 GPa. Several size effects on the mechanical properties were observed including yield stress variations with membrane width and film thickness. It was observed that a width of 2.5 m and a thickness of 0.5 m correspond to major transitions in the material deformation behavior.     

Microstructural characterization of thin gold films on a polyimide substrate

The study of thin metal films on flexible substrates is of interest for manufacture of many devices, such as implantable electrode arrays consisting of gold film on polyimide substrates. Adhesion of the film to the substrate is of utmost importance for device durability. Gold adhesion to a polyimide has previously been shown to increase when a variety of substrate treatments are performed prior to gold sputter deposition, but little microstructural analysis has been made to complete the process-structure-properties relationship. Here, the grain size is shown to increase slightly but statistically significantly if an oxygen plasma etch and adhesion layer treatment of the film is performed prior to deposition of the gold. A log-normal grain size distribution is found for gold on each sample, and the grains are shown to be columnar. Gold deposited on non-treated polyimide shows a strong {111} texture, but a random texture is seen in both pretreated systems, indicating that the pretreatment affects the surface energy of the polyimide and alters the gold film growth.     

Electrostatic immobilization of glucose oxidase in a weak acid, polyelectrolyte hyperbranched ultrathin film on gold: fabrication, characterization, and enzymatic activity.

In this paper we show that hyperbranched polymers can be used as a host matrix for electrostatic entrapment of enzymes. Specifically, amine-functionalized glucose oxidase (GOx+) and horseradish peroxidase, as well as poly(amidoamine) dendrimer-modified horseradish peroxidase, reversibly sorb into polyanionic, hyperbranched poly(sodium acrylate) (PAA-) films that are on the order of a few hundred angstroms thick. The quantity of GOx+ entrapped within the PAA- films depends on the nature of film preparation but is typically on the order of 0.06 unit/cm2. The extent to which entrapped GOx+ retains its activity depends on the film history, but for PAA-/GOx+ composites not exposed to glucose and stored at 4 degrees C, the original activity is retained for up to 68 days and perhaps longer.     

Ultrasmooth gold thin films by self-limiting galvanic displacement on silicon

Galvanic displacement (GD), a type of electroless deposition, has been used to obtain ultrasmooth gold thin films on silicon <111>. The novel aspect of the method presented herein is the absence of fluoride ions in the liquid phase, and its principal advantage when compared to previous efforts is that the process is inherently self-limiting. The self-limiting factor is due to the fact that in the absence of fluorinated species, no silicon oxide is removed during the process. Thus, the maximum gold film thickness is achieved when elemental silicon is no longer available once the surface is oxidized completely during the galvanic displacement process. X-ray photoelectron spectroscopy has been used as a tool for thickness measurement, using the gold to silicon ratio as an analytical signal. Three gold plating solutions with different concentrations of KAuCl? (2, 0.2, and 0.02 mM) have been used to obtain information about the formation rate of the gold film. This XPS analysis demonstrates the formation of gold films to a maximum thickness of ～3.5 ?. Atomic force microscopy is used to confirm surface smoothness, suggesting that the monolayer growth does not follow the Volmer-Weber growth mode, in contrast to the GD process from aqueous conditions with fluorinated species.     

Computational analysis of martensitic thin films using subdivision surfaces

This paper studies numerically the deformation of thin films made of materials undergoing martensitic phase transformations by using subdivision surfaces. These thin films have received interest as potential microactuators, and specifically a tent‐like configuration has recently been proposed. In order to model martensitic materials we use a multi‐well strain energy combined with an interfacial energy penalizing strain gradients. The study of such configurations requires adequate resolution of inhomogeneous in‐plane stretch, out‐of‐plane deformation and transition regions across which the deformation gradient changes sharply. This paper demonstrates that subdivision surfaces provide an attractive tool in the numerical study of such configurations, and also provides insights into the tent‐like deformations. Copyright © 2007 John Wiley & Sons, Ltd.     

A study of mixing in thermocapillary flows on micropatterned surfaces

The recent introduction of actuation mechanisms for microfluidic transport based on free surface flows raises a number of interesting questions involving efficient mixing configurations, especially in systems with small aspect ratios. This work investigates the characteristics of convective and diffusive mixing in continuous-mode streaming of thermocapillary microflows on chemically micropatterned surfaces. Mixing times and mixing lengths relevant to chemical microreactors or gas sensors are investigated for various geometries and parameter ranges. Scaling arguments and full numerical solutions are presented to extract optimal operating conditions. Confocal fluorescence microscopy measurements of the interfacial diffusive broadening in adjacent flowing streams confirm numerical predictions. Three important mixing regimes, based on analogues of purely diffusive dynamics, Rhines-Young shear-augmented diffusion and Taylor-Aris dispersion are identified and investigated for use in free surface flows with large surface-to-volume ratios.