Thermal Chemical Vapor Deposition Process Research Articles

Grafting of Poly(acrylic acid) onto an Aluminum Surface

Grafting of poly(acrylic acid) (PAA) onto an aluminum surface was successfully achieved by free radical polymerization of acrylic acid using typical radical initiators, benzoyl peroxide and 2,2'-azobisisobutyronitrile. Both spotlike and brush morphologies were achieved. A complete coverage of PAA on an aluminum surface was then achieved by using a thermal chemical vapor deposition process. The PAA thickness was determined by ellipsometry and the superficial chemical composition by X-ray photoelectron spectroscopy (XPS). Grazing angle Fourier transform infrared (FTIR) spectroscopy confirmed the presence of carboxylic acid groups on the surface, and the contact angle measurements revealed a decreasing free surface energy of aluminum due to the polymer surface covering.

Synthesis of Brushlike Carbon Nanotubes Using Wet-Processed Catalyst

For a large-scale process of synthesizing vertically aligned (brushlike) multiwalled carbon nanotubes using a thermal catalyst chemical vapor deposition process, a wet process for coating Fe catalyst onto a substrate has been investigated. Dimethylformamide was used for dissolving Fe(NO3)3 to form uniform films of Fe catalyst, and hence to synthesize uniform brushlike nanotubes on 6-in. Si substrates. The advantages of amine series solvent are discussed in terms of solubility in water and formation of Fe complexes.

Growth of carbon nanofiber coatings on nickel thin films on fused silica by catalytic thermal chemical vapor deposition: On the use of titanium, titanium–tungsten and tantalum as adhesion layers

Coatings of carbon nanofiber (CNF) layers were synthesized on fused silica substrates using a catalytic thermal chemical vapor deposition process (C-TCVD). The effects of various adhesion layers–titanium, titanium–tungsten and tantalum–under the nickel thin film on the attachment of carbon nanofibers and their morphological properties are presented. The diameter and the thicknesses of the CNF-coatings were analyzed by scanning electron microscopy, whereas the microstructure and crystallinity of the synthesized carbon nanofibers were investigated by transmission electron microscopy and Raman spectroscopy, respectively. Specific surface areas of CNF-coatings were determined with nitrogen adsorption–desorption isotherm measurements. Using C-TCVD of ethylene at 700 °C (1 h), well-attached, entangled, quasi-crystalline platelet carbon nanofibers were synthesized with tip-type growth mode on 25 nm thick nickel films with an adhesion layer of 10 nm Ta or Ti–W. The thickness of CNF-coating was ~ 3.5 μm, and the diameter of the fibers depended on the composition of the metallic thin film stack: 20–50 nm for Ni/Ta and 80–125 nm for Ni/Ti–W. The ultimate goal is the integration of these CNF-coatings as catalyst support in microfluidic devices, for which it is important to control CNF-coating characteristics such as fiber diameter, layer thickness, specific surface area and adhesion to the surface.

Investigation of growth properties of patterned and aligned carbon nanotubes for field emitter

We demonstrate the use of microstructures as an inducement for the growth of patterned and aligned carbon nanotubes (CNTs) during thermal chemical vapor deposition process. The growth of individual nanotube lines of vertical-standing 8–10 μm length can be controlled in almost any directions. In directions of approximate 90°, 60°, 45° or 0° relative to substrate surface, kinds of aligned CNTs patterns were synthesized. We also show that the flow of carrier gas and apical dominance like plant growth are the main factors of patterning CNTs, which indicates that the distribution of electric field can be controlled and the shielding effects can be weakened. The result establishes a method of patterned and aligned nanotubes, which is well suited for ordered arrays of CNTs field emitters. The field emission currents were observed to be fairly reproducible ranging from 375 to 1000 V and field emission measurements show a low turn-on field (1.25 V/μm).

Effect of Factors on Growth of Carbon Nanotubes by Thermal CVD

The characteristics of carbon nanotube obtained by thermal chemical vapor deposition (CVD) process were investigated as a function of ammonia (NH3) gas in hydrocarbon gas, growth time, growth temperature, and Fe catalyst thickness. Fe catalyst was prepared on Si by DC magnetron sputtering with thickness of 5–40 nm and pre-treated with NH3. CNTs were then grown with ammonia-acetylene gas mixture by thermal CVD. From our results, it was found that NH3 in C2H3 gas positively acted to improve CNT quality. The density and diameter as well as quality of CNTs were strongly affected by catalyst shape and thickness.

Hot-wire chemical vapor deposition of silicon nanoparticles on fused silica

Silicon nanoparticles on fused silica have potential as recombination centers in infrared detectors due quantum confinement effects that result in a size dependent band gap. Growth on fused silica was realized by etching in HF, annealing under vacuum at 700–750 °C, and cooling to ambient temperature before ramping to the growth temperature of 600 °C. Silicon particles could not be grown in a thermal chemical vapor deposition (CVD) process with adequate size uniformity and density. Seeding fused silica with Si adatoms in a hot-wire chemical vapor deposition (HWCVD) process at a disilane pressure of 1.1 × 10 − 5 Pa followed by thermal CVD at a disilane pressure of 1.3 × 10 − 2 Pa, or direct HWCVD at a disilane pressure of 2.1 × 10 − 5 Pa led to acceptable size uniformity and density. Dangling bonds at the surface of the as-grown nanoparticle were passivated using atomic H formed by cracking H 2 over the HWCVD filament.

Synthesis of Length-Controlled Aerosol Carbon Nanotubes and Their Dispersion Stability in Aqueous Solution

A one-step method combining spray pyrolysis and thermal chemical vapor deposition (CVD) processes was developed to grow hybrid carbon nanotube (CNT)-bimetallic composite particles. Nickel, aluminum, and acetylene were used as the catalytic site, noncatalytic matrix, and hydrocarbon source, respectively. The bimetallic particles (i.e., Al-Ni) were spray pyrolized and subsequently passed through thermal CVD. During the thermal CVD, the catalytic decomposition of acetylene occurred on the free-floating bimetallic particles so that sea urchin-like CNTs were radially grown. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses revealed the CNTs to have a uniform diameter of approximately 10 +/- 2 nm. The length of the CNTs was controlled by varying the residence time of the bimetallic nanoparticles with a length of 200-1000 nm. After nitric acid treatment, the CNTs were released by melting the bimetallic particles. The resulting CNTs were then dispersed in an aqueous solution to examine the effect of the length of CNTs on their dispersion stability, which is a critical issue for the stability and repeatability of the heat transfer performance in nanofluids. Ultraviolet-visible (UV-vis) spectrometer analysis showed that shorter CNTs were less stable than the longer CNTs due to the higher mobility-induced agglomeration of the shorter CNTs.

Synthesis of Vertically Aligned Multi-Walled Carbon Nanotubes on Copper Substrates for Applications as Thermal Interface Materials

AbstractVertically aligned carbon nanotubes (VACNTs) grown on bulk copper substrate are of great importance for CNT real-life applications as thermal interface materials in microelectronic packaging. However, their reproducible synthesis has been a great challenge so far. In this study, by introducing a well-controlled conformal Al2O3 support layer on the bulk copper substrate by atomic layer deposition (ALD) prior to the deposition of the iron catalyst layer, we reproducibly synthesize VACNTs of good alignment and high quality on the copper substrate, using a conventional thermal chemical vapor deposition process. The alignment and the quality are characterized by scanning electronic microscopy and Raman spectroscopy, respectively. The roles of the conformal Al2O3 support layer are discussed. A kinetics-controlled growth mechanism is shown. This progress provides a viable VACNT commercial application for thermal management, on the basis of which, we show a recent progress on a state-of-art Si/VACNT/Cu assembling process, named “chemical anchoring”. The high quality of the VACNTs on the copper growth substrate and the covalent bonding formed between the VACNTs and the silicon mating substrate greatly reduces the thermal resistance of the VACNT-mediated thermal interface.

Morphology study of carbon nanospecies grown on carbon fibers by thermal CVD technique

Multiwalled carbon nanotubes were uniformly grown directly on Ni catalyzed carbon fibers by catalytic decomposition of acetylene at 700 °C using thermal chemical vapour deposition process. Different process parameters such as growth temperature, time and gas flow rates were investigated in relation to morphology and yield of the products. The overall yield of the nanospecies demonstrated an initial nonlinear increase and eventually stabilized with increasing deposition time while it was found reaching to maxima at 700 °C and drops rapidly at temperatures on either side. The morphology of nanostructures was examined extensively using scanning and transmission electron microscopy which revealed formation of various carbon nanostructures such as nanofilaments, nanofibers, nanoladders, nanospirals, nanojunctions, nanocones, nanoonions etc. accompanying nanotubes. High resolution transmission electron microscopy images clearly showed single crystal structure of the catalyst particle and tip-growth mechanism of carbon nanotubes growth. Non-tip growth mechanism was observed in especial case of ferrocene derived floating Fe catalyst.

Synthesis and properties of triangular-shaped GaN nanorods via growth mode control

The synthesis of wurtzite gallium nitride (GaN) nanorods with triangular shape on c-Al 2O 3 substrates using a thermal chemical vapor deposition process was investigated. It was possible to control nanorod shape and growth mode of GaN nanorods by change of sample geometry in the chamber using a mixture of GaN powder and Ga metal with ammonia gas reaction. It was found that the GaN nanorods were grown via both vapor–liquid–solid and vapor–solid mode with change of sample placement in the chamber. The morphology of the GaN nanorods was observed by field-emission scanning electron microscopy in secondary electron and back-scattered electron mode. High-resolution transmission electron microscopy, and X-ray scattering measurements revealed the GaN nanorods to have a single-crystalline wurtzite structure.

Synthesis of GaN nanowires and nanorods via self-growth mode control

The synthesis of hexagonal wurzite one-dimensional (1D) GaN nanostructures on c-Al2O3 substrates was investigated using a thermal chemical vapor deposition (CVD) process. The diameter of the GaN nanostructures was controlled by varying the growth time using a mixture of GaN powder and Ga metal with the ammonia gas reaction. The morphologies of the GaN nanowires and nanorods were confirmed by field emission scanning electron microscopy. The micro-Raman spectroscopy and X-ray scattering measurements indicated that the GaN nanostructures had a hexagonal wurzite structure without any oxide phases. We investigated the difference in the structural properties between the GaN nanowires and nanorods. Deep-level emission bands were not observed in cathodoluminescence measurements from either the GaN nanowires or nanorods, indicating the incorporation of low-level impurities into our 1D GaN nanostructures.

Catalyst-Free Synthesis of ZnO Nanorods on Metal Substrates by Using Thermal Chemical Vapor Deposition

We report successful synthesis of ZnO nanorods on metal (Cr)-deposited Si substrates in a thermal chemical vapor deposition (CVD) process via a self-catalytic mechanism at relatively low growth temperatures of 400 and 500 C. The nanotips were densely grown onto the Cr layer while ZnO nanorods with low density were synthesized on the same substrates. Grazing-incident wide-angle X-ray scattering measurements showed a uniform distribution and a random directionality of the nanotips and the nanorods grown on Cr-deposited Si substrates. Free-exciton emission from the nanorods was clearly observed from the samples at a very low temperature of 50 K in the photoluminescence measurements. In addition, strong free exciton and relatively weak deep level emission bands were detected from both samples even at room temperature, indicating the ZnO nanotips and nanorods on Cr/Si substrates in this work had a low defect density.

Directional pyrolytic growth of microscale carbon fibers on electrochemically pretreated polyacrylonitrile-based carbon microfibers

Vapor-grown carbon microfibers were synthesized pyrolytically on electrochemically pretreated polyacrylonitrile-based carbon microfibers (PAN-CFs) without introducing any catalyst. Cyclohexane was used as the carbon source for the growth of fibers in a thermal chemical vapor deposition process (CVD). Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used for the characterization of the fibers. Microscale carbon fibers with about 1 to 2 µm in diameter were formed at 1123 K, while carbon microfibers of about 0.3 µm in diameter were synthesized at 1048 K. A pretreatment at negative potential or sequentially at both positive and negative potentials was found to be a prerequisite for the growth of carbon microfibers in the CVD process. Without electrochemical pretreatment or when only a positive potential was applied, the growth of microscale carbon fibers did not occur.

Electrodeposition of platinum-ruthenium nanoparticles on carbon nanotubes directly grown on carbon cloths for methanol oxidation

Dense carbon nanotubes (CNTs) were grown via a thermal chemical vapor deposition (CVD) process on titanium treated carbon cloths. Platinum-ruthenium (Pt-Ru) nanoparticles were then deposited on the CNT surfaces by potentiostatic electrodeposition in mixed sulfuric acid and ethylene glycol (EG) containing aqueous electrolytes. After the deposition process, the surface morphology of prepared specimens was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the electrochemical characteristics of the specimens were investigated by cyclic voltammetry (CV) in mixed sulfuric acid and methanol aqueous solutions saturated with nitrogen. Finely dispersed Pt-Ru nanoparticles were observed on the surfaces of the CNTs directly grown on carbon cloths. It was also observed that the performance of the Pt-Ru catalysts in terms of methanol oxidation efficiency and suppression of carbon monoxide adsorption would be influenced by the concentrations of sulfuric acid, EG, and Ru precursor salt during the electrodeposition processes.

Controlled Growth of Two-Dimensional ZnO Nanowalls by Thermal Chemical Vapor Deposition

Morphology and crystallographic properties of two-dimensional ZnO nanowalls epitaxially grown on catalytic Au-deposited GaN/c-plane sapphire substrates in a thermal chemical vapor deposition process were studied as a function of growth temperature. Vertically well-aligned ZnO nanowall networks with di erent wall size and morphology were formed at 880, 900 and 920 C, respectively. We observed that the ZnO nanowall networks were of a single-crystalline structure and showed a perfect epitaxial relation with the catalytic Au and the GaN substrate in high-resolution electron microscopy and synchrotron X-ray scattering experiments. It was found that the crystal quality of ZnO nanowall and catalytic Au was improved with increasing growth temperature in X-ray powder di raction measurements. The room-temperature photoluminescence spectrum showed a quite strong and sharp ultraviolet emission as well as relatively weak deep-level emission, indicating good optical properties of the ZnO nanowall samples in this work.

Synthesis of carbon nanotubes directly over TEM grids aiming the study of nucleation and growth mechanisms

A novel approach to produce electron-transparent multi-layer membranes over TEM grids for transmission electron microscopy analysis is presented. The membranes have been used to grow and analyze carbon nanostructured materials in a thermal–chemical vapor deposition process using Ni and Cu as catalysts and silicon oxide thin films as support layers, at temperatures as high as 900 °C. It is important that conditions of the synthesis using the electron-transparent multi-layer membrane system are similar to those for a conventional chemical vapor deposition process, where oxidized silicon wafers are employed. In particular, the thickness of the silicon oxide and the catalyst layers are the same, and similar carbon tubular structures were grown using both substrates. The use of membranes was crucial especially for the study of the nucleation mechanism for carbon nanotubes. These electron-transparent multi-layer membranes are relatively easy to obtain and they can be used for transmission electron microscopy studies of high-temperature synthesis of different nanostructured materials.

Low-temperature synthesis of one-dimensional ZnO nanostructures on screen-printed carbon nanotube films

One-dimensional (1D) ZnO nanostructures including nanowires, nanobelts, and nanorods were synthesized on screen-printed multi-walled carbon nanotube (MW-CNT) films in a thermal chemical vapor deposition process by gold (Au) nanoparticle-catalyst and self-catalyst driving at low temperatures of 400 and 500 °C. ZnO nanowires and nanobelts by introducing Au nanoparticles were grown via the vapor–liquid–solid (VLS) mechanism, while ZnO nanorods were realized via a self-catalytic VLS process. It was found that the diameter of ZnO nanowires strongly depends on that of Au nanoparticles, indicating the typical metal catalyst-driving VLS process. On the other hand, ZnO nanorods without Au-alloyed tips are comprised of hexagonal facets, suggesting self-catalytic VLS formation of the ZnO nanorods on the MW-CNT films. Room temperature photoluminescence spectra of the 1D ZnO nanostructures exhibit an ultraviolet emission band around 378 nm and deep-level emission band centered around 505 nm.

Multi-barrier layer-mediated growth of carbon nanotubes

Variation in the height of carbon nanotubes (CNTs) grown has been co-related to the type of multi-barrier-layer used. Initially, various types of barrier-layers such as Al, Al 2O 3, Al/SiO 2, Al 2O 3/SiO 2 were prepared onto a n-type Si (100) substrate. The thickness of SiO 2 was ∼ 550 nm, where as, Al 2O 3 and Al were ∼ 15 nm thick. These samples were covered with ∼ 1 nm thick Fe catalyst layer. The coated samples were subjected to the thermal chemical vapor deposition (T-CVD) process. SEM analysis showed that, for Al 2O 3/SiO 2 barrier layers, the average height of the CNTs was ∼ 10 μm, where as, for other types of samples it was less than ∼ 1 μm. To investigate this, multi-barrier layers were characterized by dynamic secondary ion mass spectrometry (D-SIMS). The observed variation in height of CNTs is attributed to the variation in diffusivity of Fe atoms into multi-barriers-layers. The results showed that, diffusion of Fe catalyst atoms could severally affect height of CNTs.

Simulation and optimization of silicon thermal CVD through CFD integrating Taguchi method

A steady laminar flow coupled with heat transfer, gas-phase chemistry, and surface chemistry model, was numerically solved for optimization of thermal chemical vapor deposition (CVD) from a gas mixture of silane and helium in the axis-symmetrical rotating/stagnating reactor with and without a rotational showerhead. At first, through computational fluid dynamics (CFD), the rate of deposited silicon on substrate was calculated and validated with the benchmark solutions from the literature. The computational model was then integrated with the dynamic model of Taguchi method to optimize the process parameters formulating a correlation to minimize thickness deviation of deposited silicon film on the substrate in the different sizes. In particular, the result shows thickness deviation of deposited silicon film can be reduced to 5.8% and 11% from 18% and 36% over a wafer in the diameters of 0.15 m and 0.3 m, respectively, in the thermal CVD process with the optimal conditions in this work.

Comparison of numerical methods for transient CVD simulations

The numerical modeling of laminar reacting gas flows in thermal Chemical Vapor Deposition (CVD) processes commonly involves the solution of convection–diffusion-reaction equations for a large number of reactants and intermediate species. These equations are stiffly coupled through the reaction terms, which typically include dozens of finite rate elementary reaction steps with largely varying rate constants. The solution of such stiff sets of equations is difficult, especially when time-accurate transient solutions are required. The latter is important for the study of start-up and shut-down cycli, but also for the study of inherently transient CVD processes, such as Rapid Thermal CVD (RTCVD) and Atomic Layer Deposition (ALD). In this study various numerical schemes for multidimensional transient simulations of laminar reacting gas flows with homogeneous and heterogeneous chemical reactions are compared in terms of efficiency, accuracy and robustness. As a test case, we study the CVD process of silicon from silane, modeled according to the classical 16 species, 27 reactions chemistry model for this process as published by Coltrin and coworkers [M.E. Coltrin, R.J. Kee, G.H. Evans, J. Electrochem. Soc. 136 (1989) 819]. We validate our results by comparison to steady state solutions in the benchmark paper of Kleijn [ C.R. Kleijn, Thin Solid Films, 365 (2000) 294]. It is concluded that, for time-accurate transient simulations of the stiff chemistry problems in CVD modeling the conservation of the non-negativity of the species concentrations is much more important, and much more restrictive towards the time step size, than stability. For the time integration methods studied in this paper, we give suggestions for the associated optimal nonlinear and linear solvers, such that the total computational costs are reduced as much as possible.