Contiguous Film Research Articles

Lubricated rolling over a pool

Experiments are conducted to explore the rolling of a cylinder over a pool of viscous fluid. The speed, width and loading of the cylinder are varied along with the initial depth and length of the viscous pool. Depending on the conditions, the cylinder will either ride on a lubrication film or remain in solid contact with the underlying substrate. For the former situation, a lubrication theory is presented that describes the pressure underneath the cylinder and the thickness of the film. The theory approximates the flow by the one-dimensional Reynolds equation with the addition of one term, with an adjustable parameter, to account for the flux of fluid to the cylinder sides. Once this parameter is calibrated against experiment, the theory predicts peak lubrication pressures, gap sizes and film thicknesses to within approximately ten per cent. For lubricated rolling, the film splits evenly between the cylinder and substrate downstream of the nip. The printer's instability arises during the splitting process, patterning the residual fluid films on the substrate and cylinder. If the pool length is less than the cylinder circumference, the fluid adhering to the cylinder is rotated back into contact with the substrate, and when there is sufficient adhered fluid a lubrication film forms that can again be modelled by the theory. Conversely, if there is insufficient adhered fluid, no contiguous lubrication film is formed; instead, the pattern from the printer's instability ‘prints’ from the cylinder to the substrate.

Electrodeposition of Cu(111) onto a Ru(0001) seed layer for epitaxial Cu interconnects

The electrodeposition of Cu onto epitaxial Ru(0001) seed layers was investigated from a sulfuric acid-based solution containing dilute copper(II) sulfate and chloride ions. Using galvanostatic deposition at −350 μA/cm2, Cu was deposited epitaxially onto a 30 nm-thick Ru(0001) seed layer, despite a compressive misfit strain between −6.9% and −8.3%, depending on the extent of strain relaxation of the Ru layer. However, rather than depositing as a single crystal, Cu grew as a bicrystal having a common out-of-plane orientation of Cu(111) and two equivalent in-plane orientations. The Cu grain size was large, on the order of micrometers, and the grain boundaries were identified as incoherent ∑3{211} twin boundaries. The Cu initially grew as isolated islands, coalescing into a contiguous film at thicknesses around 50 nm. The Cu film was rough, and thickness and coverage varied over the electrodeposited region. After the initial island growth, Cu void fraction and film roughness both decreased with thickness as the deposit transitioned into a planar film with nanometric islands growing on the film surface. However, at thicknesses exceeding 200 nm, anisotropic growth of large, faceted Cu islands on the planar Cu film again increased the surface roughness. The epitaxially deposited Cu bicrystal showed an improvement in resistivity when compared with polycrystalline Cu similarly electrodeposited onto a polycrystalline Ru seed.

Modeling and Simulation of Cu Diffusion in Porous Low-k Dielectrics

Integrating nanometer sized pores into low-k ILD films is one of the approaches to lower the RC signal delay and thus help sustain the continued scaling of micro-electronic devices. While increasing porosity of porous dielectrics achieves the desirable lowering of the dielectric constant (k), it also creates a myriad of reliability and implementation issues. One of the problems is the little understood metal diffusion in porous media. Here, we present a rigorous simulation method of Cu diffusion based on master equation with elementary jump probabilities within the contiguous dielectric film, along the pore boundary, from the dielectric matrix to the pore boundary, and from the pore boundary to the matrix material. In view of the diffusional jump distance being as large as 2 nm, the nano-pores being on a similar length scale, and the film thickness being only a few tens of nanometers, the conventional diffusion equation in differential equation form is grossly inadequate and elementary jump frequencies are required for a proper description of the Cu diffusion in porous dielectric. The present atomistic approach allows a consistent implementation of Cu ion drift in electric field by lowering and raising of the diffusion barriers along the field direction. Two numerical methods are being used to evaluate the atomistic Cu transport: 1) Diffusion tensor theory (4th rank tensor in 2D, 9th rank in 3D), 2) Sequential lattice point evaluation of elementary diffusion jumps. The 2nd method is very efficient in 3D, where the simulation time increases only linearly with the dimension of the matrix, while the 1st method allows an easy implementation of jump frequencies at the boundaries of the film and pore boundaries.

Photocatalytic reduction of nitrogen to ammonia on diamond thin films grown on metallic substrates

Recent studies have demonstrated that boron-doped diamond can act as a solid-state source of electrons in water when illuminated with above-bandgap light. Excitation of electrons to diamond's conduction band leads to facile emission of electrons into water; the resulting solvated electrons are potent reducing agents able to initiate many chemical reactions such as the reduction of N2 to NH3 and the reduction of CO2 to CO. Here, we report investigations of the photocatalytic activity of diamond thin films grown on Mo, Ni, and Ti substrates. Our results show that in each case, there is a maximum in the photocatalytic activity that occurs just when the diamond coalesces into a contiguous film, then decreasing as the film becomes thicker. These results suggest that electron emission arises in part from excitation of electrons in the metal substrate, followed by injection into the conduction band of the diamond film. More detailed studies on films grown on niobium show that at the open-circuit potential the films have a downward bend-bending of ~0.3V, which is favorable for barrier-free electron emission. Our result suggests that metal–diamond heterostructures may provide a scalable approach to achieving difficult photocatalytic reactions.

Diffusion-Controlled Growth of Molecular Heterostructures: Fabrication of Two-, One-, and Zero-Dimensional C60 Nanostructures on Pentacene Substrates

A variety of low dimensional C60 structures has been grown on supporting pentacene multilayers. By choice of substrate temperature during growth the effective diffusion length of evaporated fullerenes and their nucleation at terraces or step edges can be precisely controlled. AFM and SEM measurements show that this enables the fabrication of either 2D adlayers or solely 1D chains decorating substrate steps, while at elevated growth temperature continuous wetting of step edges is prohibited and instead the formation of separated C60 clusters pinned at the pentacene step edges occurs. Remarkably, all structures remain thermally stable at room temperature once they are formed. In addition the various fullerene structures have been overgrown by an additional pentacene capping layer. Utilizing the different probe depth of XRD and NEXAFS, we found that no contiguous pentacene film is formed on the 2D C60 structure, whereas an encapsulation of the 1D and 0D structures with uniformly upright oriented pentacene is achieved, hence allowing the fabrication of low dimensional buried organic heterostructures.

Model for the Mass Transport during Metal-Assisted Chemical Etching with Contiguous Metal Films As Catalysts

Metal-assisted chemical etching is a relatively new top-down approach allowing a highly controlled and precise fabrication of Si and Si/Ge superlattice nanowires. It is a simple method with the ability to tailor diverse nanowire parameters like diameter, length, density, orientation, doping level, doping type, and morphology. In a typical metal-assisted chemical etching procedure, a Si substrate is covered by a lithographic noble metal film and etched in a solution containing HF and an oxidant (typically H2O2). In general, the function of the metal is to catalyze the reduction of H2O2, which delivers electronic holes necessary for the oxidation and subsequent dissolution of the Si oxide by HF. However, the details of the etching process using contiguous metal thin films, especially the mass transport of reactants and byproducts are still not well understood. In this study, the etching mechanism was systematically investigated. Several models of metal-assisted chemical etching using a contiguous metal film...

ZnO coated nanospring-based chemiresistors

Chemiresistors were constructed using 3-D silica nanospring mats coated with a contiguous film of ZnO nanocrystals. Chemiresistors with an average ZnO nanocrystal radius <3 nm, or >20 nm, were found to exhibit a relative change in conductance of a factor of 50 upon exposure to a gas flow of 20% O2 and 80% N2 with ∼500 ppm of toluene and an operational temperature of 400 °C. Samples with an average ZnO nanocrystal radius of 15 nm were found to be the most responsive with a relative conductance change of a factor of 1000. The addition of metal nanoparticles (average radius equal to 2.4 nm) onto the surface of the ZnO nanocrystals (average radius equal to 15 nm) produced a relative change in conductance of a factor of 1500. For the optimum conditions (T = 400 °C, grain size ∼15 nm) well-defined spikes in conductance to explosive vapors (TNT, TATP) were obtained for 0.1 ms exposure time at ppb levels.

Removal of stacking faults in Ge grown on Si through nanoscale openings in chemical SiO 2

Nucleation and eventual coalescence of Ge islands, grown out of 5 to 7 nm diameter openings in chemical SiO 2 template and epitaxially registered to the underlying Si substrate, have been shown to generate a low density of threading dislocations (≪10 6 cm − 2 ). This result compares favorably to a threading dislocation density exceeding 10 8 cm − 2 in Ge films grown directly on Si. However, the coalesced Ge film contains a relatively high density of stacking faults (5 × 10 7 cm − 2 ), and subsequent growth of GaAs leads to an adverse root-mean-square roughness of 36 nm and a reduced photoluminescence intensity at 20% compared to GaAs grown on Ge or GaAs substrates. Herein, we find that annealing the Ge islands at 1073 K for 30 min before their coalescence into a contiguous film completely removes the stacking faults. However, the anneal step undesirably desorbs any SiO 2 not covered by existing Ge islands. Further Ge growth results in a threading dislocation density of 5 × 10 7 cm − 2 , but without any stacking faults. Threading dislocations are believed to result from the later Ge growth on the newly exposed Si where the SiO 2 has desorbed from areas uncovered by Ge islands. The morphology and photoluminescence intensity of GaAs grown on the annealed Ge is comparable to films grown on GaAs or Ge substrates. Despite this improvement, the GaAs films grown on the annealed Ge/Si exhibit a threading dislocation density of 2 × 10 7 cm − 2 and a minority carrier lifetime of 67 ps compared to 4 to 5 ns for GaAs on Ge or GaAs substrates. A second oxidation step after the high temperature anneal of the Ge islands is proposed to reconstitute the SiO 2 template and subsequently improve the quality of Ge film.

Comparative study of ZnO thin film and nanopillar growth on YSZ(111) and sapphire (0001) substrates by pulsed laser deposition

Abstract The growth of ZnO films by pulsed laser deposition was systematically studied on three different substrates, namely YSZ(1 1 1), Al 2 O 3 (0 0 0 1), and Si(1 0 0). The dependence of the film morphology on oxygen background pressure and substrate temperature was investigated. A strong dependence of the ZnO morphology on all three growth parameters was found allowing for tuning the microscopic structure of ZnO from contiguous film to a columnar growth, and to perfectly aligned nanopillars.

Air/water Interface Induced Folding And Self-assembly Of Amyloid-beta Peptide

The amphipacity of the natively unstructured amyloid-beta (Ab40) peptide may play an important role in its aggregation into beta-sheet rich fibrils that is linked to the pathogenesis of Alzheimer's disease. Using the air/water interface as an ideal hydrophobic interface, we characterized Ab's surface activity and the structure, assembly, and morphology of Ab adsorbed to the air/water interface. Ab dissolved in water readily adsorbed to the air/water interface to form a contiguous film with a surface pressure of approximately 14 mN/m and showed an apparent critical micelle concentration of about 100 nM. Adsorbed Ab was composed of a single molecular layer extending approximately 20 A into the aqueous subphase with in-plane ordering that gave rise to X-ray diffraction peaks. Analysis of the diffraction peaks showed that the air/water interface induced the otherwise unstructured Ab peptides to self-assemble into nano-size clusters with Ab peptides folded in a beta-sheet conformation. The presence of these clusters was further confirmed by imaging the morphology of the Ab film using atomic force microscopy. The formation of these ordered clusters was not affected by solution pH, ionic strength, or the presence of cosolutes sucrose and urea at concentrations that are known to stabilize and denature native proteins in solution, suggesting that the hydrophobic interface-driven Ab folding and assembly is robust and strongly favorable. Furthermore, Ab adsorbed at the air/water interface can seed fibril growth in solution when re-introduced into the bulk. Our results implicate that that interface-induced Ab folding and self-assembly may serve as a mechanism by which Ab aggregation occurs in vivo.

Forming carbon nanotube composites by directly coating forests with inorganic materials using low pressure chemical vapor deposition

Low pressure chemical vapor deposition has been used to fill carbon nanotube (CNT) forests with inorganic materials (polysilicon and silicon nitride). Forest filling proceeds by deposition around individual CNTs. As the coating thickness around each CNT increases, the free volume between adjacent nanotubes is filled and finally results in a contiguous composite film. The process maintains the forest height and alignment; however, the coating thickness around the CNTs is in general smaller at the base of the forest than it is at the top. This can cause a contiguous solid film to form at the top of the forest while the forest is only partially filled at the base. Once the top of the forest becomes filled, it prevents growth from occurring at the base. Consequently, the growth process can cap the top of the forest and leave voids between thinly coated CNTs at the base. Such composites have reduced hardness (4 GPa or less). Depositing at reduced temperatures and/or decreased precursor gas flow rates reduces the void fraction through improving the step coverage modulus. This allows one to produce thick (> 50 μm) polysilicon-CNT composite films having hardness approximately equal to that of polysilicon thin films (12.4 GPa).

Water-Induced Reconstruction that Affects Mobile Ions on the Surface of Calcite

Time-sequenced contact-force micrographs show that the (104) calcite cleavage surface reconstructs in humid air through pit formation and film growth. After 8 h at 80% relative humidity (RH), 50% to 80% of the surface is covered by islands that are flat-topped and 1-nm high. The lateral growth rates of individual islands are 4.2+/-0.4 nm min-1 in the 41 direction and 1.8+/-0.4 nm min-1 in the 48 direction, resulting in islands having distinct major and minor axes. On some samples, a contiguous, 1.5-nm-high film rapidly grows between the islands and the pits. The areal expansion rate of the film is 500 times faster than that of the islands. Gaps between the contiguous film and the islands expand and contract, which suggests that mass is exchanged between them and that both are loosely bound. Complementing the topographic images, polarization heights are simultaneously measured by polarization-force microscopy. The polarization heights of the islands and the contiguous film are -6 to -10 nm and -4 to -5 nm, respectively, compared to their respective topographic heights of +1.0 and +1.5 nm. Under our experimental conditions, the polarization heights are a surrogate for the local dielectric constant of the sample epsilon and arise from a convolution of the mobility and the density of surface ions. The polarization heights imply that epsilonsubstrate>epsilonfilm>epsilonisland. Changes in topographic and polarization heights at 20% and 50% RH suggest that the structures of the islands are in dynamic equilibrium with the adsorbed water. Our evidence suggests that the islands contain loosely bound water and may therefore be a hydrated calcium carbonate phase stabilized by the calcite surface.

A universal approach to electrically connecting nanowire arrays using nanoparticles—applicationto a novel gas sensor architecture

We report on a novel, in situ approach toward connecting and electricallycontacting vertically aligned nanowire arrays using conductive nanoparticles.The utility of the approach is demonstrated by development of a gas sensingdevice employing this nano-architecture. Well-aligned, single-crystalline zincoxide nanowires were grown through a direct thermal evaporation process at550 °C on gold catalyst layers. Electrical contact to the top of the nanowire array was establishedby creating a contiguous nanoparticle film through electrostatic attachment of conductivegold nanoparticles exclusively onto the tips of nanowires. A gas sensing device wasconstructed using such an arrangement and the nanowire assembly was found to besensitive to both reducing (methanol) and oxidizing (nitrous oxides) gases. This assemblyapproach is amenable to any nanowire array for which a top contact electrode is needed.

Design, Fabrication and Testing of a Novel Gas Sensor utilizing Vertically Aligned Zinc Oxide Nanowire Arrays

ABSTRACTWe report on a novel, non-destructive, in-situ approach toward connecting and electrically contacting vertically aligned zinc oxide nanowire arrays using conductive gold nanoparticles. A chemical gas-sensing device was constructed and tested using this nano-architecture. Well-aligned, single-crystalline zinc oxide nanowires were grown through a direct thermal evaporation process at 550 °C on gold catalyst layers. Electrical contact to the top of the NW array was established by creating a contiguous nanoparticle film through electrostatic attachment of conductive gold nanoparticles exclusively onto the tips of nanowires. The gas-sensing device fabricated through this approach was found to be sensitive to both reducing (methanol) and oxidizing (nitrous oxides) gases. This assembly approach is amenable to any array of one-dimensional nanostructures for which a top contact electrode is needed.

Stability of thin copper films on mesoporous dielectrics

We report observations of the morphological changes in sputter-deposited thin copper films (<100nm) deposited on solid SiO2 and varying porosity mesoporous silica substrates. Break-up of the copper film into islands (agglomeration) was observed using both an in situ hot-stage scanning electron microscope (SEM) system and an ex situ vacuum-annealing system followed by SEM characterization. The contributions of the underlying substrate to the film and substrate interfacial energy and the film’s elastic strain energy were evaluated. Detailed pore-size and roughness characterization of the mesoporous substrates were performed in order to evaluate the interfacial energy contribution. The intrinsic stress in the copper film was also determined as a function of underlying dielectric porosity. The stress is tensile in as-deposited films and does not dominate the interactions in our system, as the stress energy is two orders of magnitude lower than the surface/interfacial energies. We have observed an enhancement in the film stability up to a limiting porosity and pore-size due to an increase in the effective interfacial area and hence interfacial energy available for the contiguous copper film. The experimentally observed stability behavior is correlated to models that have appeared in literature, but which needed to be adapted for porous substrates.

Evaluation of plasma polymer-coated contact lenses by electrochemical impedance spectroscopy

The influence of ultra-thin (i.e., 5-50 nm) plasma polymer coatings on siloxane-based hydrogel contact lenses was investigated by electrochemical impedance spectroscopy (EIS). Impedance measurements as a function of frequency (Bode plots) were taken at regular intervals until steady impedance was obtained, indicating that the lenses were saturated. Appropriate equivalent circuit models were constructed to describe the salt intrusion characteristics of the plasma polymer-coated contact lenses. This provided information pertaining to the resistance and capacitance of interfacial and bulk layers in the plasma polymer-coated lenses. Resistance relates to ion permeability and capacitance for water uptake. This investigation showed that some of an ultra-thin layer of plasma polymer applied onto a dry hydrogel remains a contiguous film after the substrate hydrogel swells upon hydration. In some cases, however, the overall impedance of the coated lens is so low that the state of the plasma polymer layer after the hydration of the substrate hydrogel could not be judged by EIS.

Growth of nanometer thin ice films from water vapor studied using scanning polarization force microscopy

Atomic force microscopy (AFM) was used to study the growth and morphology of ice films on the cleavage surface of mica. Measurements performed in contact, as well as in noncontact operation modes of the microscope, allowed us to distinguish the solid and liquid parts of the film. At temperatures below −30 °C, supercooled water droplets formed on top of a thin (nanometer range) ice layer in contact with the substrate. After annealing, a contiguous flat film was formed. Between −20 and −10 °C and at a relative humidity of ∼83%, the film consisted of a solid ice layer ∼7 Å thick, covered by a liquid-like layer 50±5 Å thick. When the temperature was raised above 0 °C, droplets formed, which subsequently evaporated. Comparison of results obtained in the various AFM operation modes allowed us to conclude the existence of a liquid-like layer on the ice surface.

Structural studies of sputtered noble metal catalysts on oxide surfaces

Catalytically active noble metal additives are used in semiconductor gas sensors as supported catalysts in order to increase both the selectivity and the sensitivity of the sensors. This study considers the formation of nanoparticles from ultra-thin noble metal deposits on oxide surfaces during heating of the sputtered metal deposits. Ag, Au, Pd and Pt layers of various thicknesses were sputtered on SiO2 surfaces and on thin-film surfaces of semiconducting SnO2. The ultra-thin Ag, Au and Pt layers, contiguous after the sputtering, become discontinuous during the heating and form discrete metal nanoparticles. Because of the oxidation, the behaviour of Pd layers is very different from that of Ag, Au and Pt layers during heating. The structure of the metal layers was studied by atomic force microscopy and X-ray diffraction before and after the heat treatment. In addition, resistivity measurements were used to monitor the structural phenomena during the heat treatment. The agglomerated layer does not have the high conductivity of the contiguous film. A sudden conductivity decrease by many orders of magnitude was found at relatively low temperatures during the heat treatment. The conductivity decrease occurred within a narrow temperature range.

Molecular layer epitaxy by real-time optical process monitoring

In this paper we consider modern methods of optical process monitoring and control in the context of atomic layer epitaxy. One specific method, p-polarized reflectance spectroscopy (PRS), is chosen to assess details of layer-by-layer growth. We show that PRS monitoring under conditions of steady-state growth by pulsed chemical beam epitaxy (PCBE) can achieve the deposition of molecular layers of GaP on silicon (100) deposited with a precision of 5%, which can be improved by reducing the growth rate and increasing the period of time averaging of the reflectance data. Since in the nucleation period prior to formation of a contiguous heteroepitaxial film inhomogeneous surface chemistry and roughening complicates the modeling of the overgrowth process, advances in both experimental methods and theory are required for extending the control to non-steady-state growth conditions. Results of simultaneous single-wavelength PR monitoring and laser light scattering measurements in conjunction with atomic force microscopy studies of short period heteroepitaxial overgrowth processes are presented. The extension of PRS to the monitoring of organometallic chemical vapor deposition at higher pressures is also discussed.

Migration-enhanced pulsed chemical beam epitaxy of GaP on Si(001)

In this paper, we present results on the heteroepitaxial growth of GaP on Si by migration enhanced pulsed chemical beam epitaxy (PCBE) using additional hydrogen and filament assisted techniques. For the real-time monitoring we applied a new technique, p-polarized reflectance spectroscopy (PRS) which provides information on the bulk, interface and surface properties of the grown GaP/Si heterostructures. The monitoring is initiated close to the Brewster angle for the substrate material, achieving high sensitivity in the initial phase of heteroepitaxial overgrowth. The bulk properties and growth rate are monitored using the quarter wave periodic structure in the reflectance based on interference in the film. The surface vicinity properties are monitored by the periodic fine structures in the intensity of the reflected light, which are correlated with the switching cycles of the metalorganic precursor pulses and are amplitude-modulated in accord with the repeat cycle of the heteroepitaxial growth process. Additional information regarding the surface roughness was revealed by scattered light measurements during the growth of the film. The results of real-time monitoring of nucleation and growth are combined with results of the ex-situ characterization of the GaP/Si heterostructure by cross-sectional transmission electron microscopy (TEM) and atomic force microscopy (AFM) studies to gain an understanding of the critical phase, in which the silicon surface becomes sealed by a contiguous epitaxial film of GaP. The defects formed in the GaP film during this phase determine the quality of the epilayers obtained upon prolonged growth.