Modified Chemical Vapor Deposition Process Research Articles

Progresses in Synthesis and Application of SiC Films: From CVD to ALD and from MEMS to NEMS.

A search of the recent literature reveals that there is a continuous growth of scientific publications on the development of chemical vapor deposition (CVD) processes for silicon carbide (SiC) films and their promising applications in micro- and nanoelectromechanical systems (MEMS/NEMS) devices. In recent years, considerable effort has been devoted to deposit high-quality SiC films on large areas enabling the low-cost fabrication methods of MEMS/NEMS sensors. The relatively high temperatures involved in CVD SiC growth are a drawback and studies have been made to develop low-temperature CVD processes. In this respect, atomic layer deposition (ALD), a modified CVD process promising for nanotechnology fabrication techniques, has attracted attention due to the deposition of thin films at low temperatures and additional benefits, such as excellent uniformity, conformability, good reproducibility, large area, and batch capability. This review article focuses on the recent advances in the strategies for the CVD of SiC films, with a special emphasis on low-temperature processes, as well as ALD. In addition, we summarize the applications of CVD SiC films in MEMS/NEMS devices and prospects for advancement of the CVD SiC technology.

Eye safe emission in Tm3+/Ho3+ and Yb3+/Tm3+ co-doped optical fibers fabricated using MCVD-CDS system

The RE-co-doped silica optical fibers emitting in the eye-safe spectral range are still attractive for new applications like LIDAR, pollution monitoring, navigation, and free-space optical communication. High gain and beam quality of fiber sources based on large mode area (LMA) active fibers can be fabricated by the well-known Modified Chemical Vapor Deposition (MCVD) technology improved by the chelate doping system. Considering silica matrix, the most interesting co-dopants in the region above 1400 nm are thulium and holmium. In the paper Tm3+/Ho3+ and Yb3+/Tm3+ co-doped silica optical fibers (refractive index and rare earth distribution profile, luminescence) are presented. Their respective emission spectra has been measured over the range 1550–2150 nm for Tm3+/Ho3+ (exc. at 796 nm) and 1600–2100 nm for Yb3+/Tm3+ (exc. at 976 nm). A wide luminescent spectrum has been recorded from RE co-doping of silica glass during the MCVD process and we were able to achieve the appropriate and uniform concentration for getting the conditions of energy transfer. The fabrication and characterization of Tm3+/Ho3+ and Yb3+/Tm3+ co-doped optical fibres are presented. The analysis of luminescence spectrum changes vs. fibre length has shown that there is Amplified Spontaneous Emission (ASE) at 1960 nm in Yb3+/Tm3+ co-doped optical fibre. The possibilities of obtaining novel constructions of RE co-doped optical fibers for eye-safe emission region will be also shown.

Low-Loss Waveguides by Planar Modified Chemical Vapor Deposition

We present a novel method to fabricate planar photonic devices for high power applications using modified chemical vapor deposition (MCVD) technology. The MCVD process, usually applied to optic fibers, is used here to deposit a layer of Ge doped silica on a fused silica slab. The doped layer was then heat-treated by a CO2 laser in order to achieve high optical quality and increase its damage threshold. Finally, direct laser lithography, followed by deep dry etching, was used to shape straight and curved large-core air-cladded waveguides with smooth sidewalls. The straight waveguides were shown to have an excellent propagation loss, 0.33 dB/cm, and the curved waveguide had a propagation loss of 1 dB/cm.

Strategic Architecture of Silicon Nanolayer-Embedded Graphite Hybrid for High Energy Lithium-Ion Battery Anodes

As the widespread emergence of modern technologies combined with human life, lithium ion battery (LIBs) has become one of the most important power supplier for mobile electronic devices, electric vehicles and stationary applications. However, the current LIBs provide a low energy density with approaching to the capacity limits, which emphasize the urgent need for high energy density battery systems. Herein, we have prepared the amorphous silicon nanolayer-embedded graphite/carbon (SGC) hybrids by chemical vapor deposition (CVD) method with developing the cost-effective and scalable pyrolysis system. With developing an industrial-relevant modified CVD process, sophisticated structure of optimum SGC hybrids achieved high reversible capacity (523 mAh g-1) and unprecedented coloumbic efficiency (92%) at a 1st cycle in the industrial standard electrode density (> 1.6 g cc-1) and areal capacity loading of > 3.3 mAh cm-2. Moreover, fabricated SGC electrode confirmed rapid increase of cycling efficiency upward of 99.5% over only 6th cycles and exactly allowed favorable cyclability of 96% capacity retention after 100 cycles and high rate capability comparable to the industrial graphite anode. In addition, the electrode composed of SGC hybrids entirely overcame the detrimental effects of the volume variation problems, exhibiting 23% of additional expansion excepting for graphite counterpart. This, in turn, completely preserved the electrical interconnectivity and mechanical integrity without any cracks and contact losses. Finally, a prototype full cell device is demonstrated with high voltage lithium cobalt oxide (LCO) cathode through the coin cell configuration, which achieved 92% of capacity retention after 100 cycles with considerable potential toward next generation target as practical devices. Consequently, this successful SGC hybrid anodes could be proposed for commercial extension to the next generation high-energy battery systems as a major breakthrough for electric vehicle or grid energy storage applications. Scheme 1. Schematic of fabrication process for SGC hybrids. Gas phase of Silane (SiH4) can easily adsorb and penetrate on a pristine graphite (PG), which leads to homogeneous and highly pure amorphous silicon nanolayer coating on both inner and outer surface of PG as silicon graphite (SG) hybrids. In the end, SGC hybrids are invented from SG through carbon coating with acetylene (C2H2) gas by CVD method. Figure 1

Confined-doped fiber for effective mode control fabricated by MCVD process

A confined-doped fiber was fabricated by a modified chemical vapor deposition (MCVD) process based on refractive index matching technology. With theory and experiments, we compared the confined-doped fiber and normal-doped fiber. We found that the confined-doped fiber with a core of 35μm and 0.07 numerical aperture could achieve single-mode output and improve the beam quality from 2.8 to 1.5 in the fiber laser. Meanwhile, it still possesses high laser efficiency and has good stability of beam quality with the increase in pump power. It suggests that the confined-doped fiber with a MCVD process may be the key material for a high-power fiber laser with excellent beam quality.

Novel synthesis route to graphene using iron nanoparticles

Abstract

A highly efficient Yb-doped silica laser fiber prepared by gas phase doping technology

In this paper we report on an alternative technique for the preparation of ytterbium (Yb)-doped silica fibers and their characteristics compared to the conventional modified chemical vapor deposition (MCVD) process in combination with solution doping and powder sinter technology (REPUSIL). In the case of the technique applied here, the active core diameter in the preform can be significantly increased via the deposition of Yb and the most important codopant, aluminum (Al), in the gas phase through the high-temperature evaporation of the Yb chelate compound and Al chloride in the MCVD process. The prepared preform shows a homogenous distribution of the refractive index and dopant concentration. The background loss of the drawn fiber was measured to be 25 dB km−1 at 1200 nm. Efficient lasing up to 200 W, showing a slope efficiency of about 80%, was demonstrated, which is comparable to fibers made via MCVD/solution doping and the REPUSIL technique.

CFD simulation of laser enhanced modified chemical vapor deposition process

The modified chemical vapor deposition (MCVD) process is one of the most widely used processes to manufacture the optical fiber preforms. There have been several experimental and numerical studies to establish that thermophoresis is the dominant mechanism for the transport of silica particles to the preform wall. There also exists several modification of this process to increase the deposition efficiency in the MCVD process. In this work we have carried out computational fluid dynamics simulation of the coupled equations of mass, momentum, energy and species transport in the laser enhanced thermophoretic deposition process using the conservative finite volume scheme. The effect of laser heating on different parameters such as thermophoresis, conversion rate and deposition efficiency is examined in this study. The presence of laser heating alters the velocity and temperature profile that leads to increase in the conversion and deposition efficiency due to high thermophoresis. The results of numerical simulations are in support of experimental and analytical studies. Our numerical simulations using the conservative finite volume scheme show that deposition efficiency increases with increasing power of the laser.

A computational fluid dynamics model for co-deposition of silica and germania in the MCVD process

The modified chemical vapor deposition (MCVD) process is used to create the doped silica preforms that are subsequently drawn to optical fiber. This paper reports on the extension of a computational fluid dynamics model of the MCVD process to include the simultaneous creation, transport and wall deposition of silica and germania particles. Simulations indicate the crucial role played by the interplay between chemical kinetics and equilibria along the substrate tube. Illustrative results show the likely impact of particle size on deposition patterns, along with a possible explanation for the observed high wastage of germanium in this process.

Impact of chlorine dissociation for modified chemical vapor deposition

Modified chemical vapor deposition (MCVD) is the platform technology used to create a wide range of silica-based optical fibers. This paper reports on the extension of the reaction scheme embedded within a computational fluid dynamics model of the MCVD process to include chlorine dissociation and recombination. Simulations employing this modified kinetic scheme indicate that chlorine dissociation acts primarily as a ‘heat sink’ in cases where the operating conditions promote a high peak temperature in the narrow reaction zone where most of the SiCl 4 oxidation occurs. The extended model allows a wider range of operating parameters to be examined in terms of the deposition profile of silica ‘soot’ particles on the substrate tube wall.

Fabrication of Yb-doped silica glass through the modification of MCVD process

A method to deliver Yb and Al compounds in vapor phase to the reaction/deposition zone has been devised for modified chemical vapor deposition (MCVD) process. By using this MCVD setup, we succeeded to prepare silica glass preforms presenting uniform dopants concentrations in the radial and longitudinal directions with good reproducibility. Preforms with a core diameter larger than 5 mm were easily prepared by depositing around 40 layers. By changing some parameters in the deposition step, such as carrier gases flow and temperatures of Yb(DPM) 3 and AlCl 3 furnaces, different concentrations of Yb and Al were incorporated into the core region of the silica glass preforms. By adjusting the Yb and Al concentrations, we succeeded to prepare preforms for large mode area (LMA) fiber with a change less than 10% of nominal refractive index.

Study on pure silica core optical fibers

An optimal refractive index profile of pure silica core optical fiber (PSCF) was designed, in combination with the characters of the modified chemical vapor deposition (MCVD) process. Techniques of preform fabrication by a new furnace round heating MCVD process and fiber drawing process were reviewed. Difficulties in doping fluorine in silica, widening the depressed-index cladding and maintaining the index of fiber core were discussed. Methods used to overcome these difficulties were given at the same time. Additionally, the optimal refractive index profiles of PSCF were presented.

Single-polarization polarization maintaining optical fiber with large stress birefringence and high homogeneity

A type of high-birefringence polarization maintaining (PM) fiber was designed and fabricated using a modified chemical vapor deposition (MCVD) process. Different from conventional stress-applied PM fibers, the shape of the stress jacket is like a “capsule.” By study of the cross sectional component distribution and a calculation of stress birefringence using the finite element method, the structure and composition of capsule PM fiber were optimized and the hybrid MCVD process was developed. With advantages in homogeneity and temperature performance, capsule fiber has great potential in applications such as fiber gyroscopes, fiber hydrophones, and other optical fiber sensors.

Immobilization of polysaccharides on a fluorinated silicon surface

A self-assembled monolayer (SAM) of fluoroalkyl silane (FAS) was deposited on a silicon surface by chemical vapor deposition (CVD) at room temperature under 1.01 × 10 5 Pa nitrogen. Using this new approach, the quality and reproducibility of the SAM are better than those prepared either in solution or by vapor phase deposition, and the deposition process is simpler. In this modified CVD process, the silane monomers, instead of the oligomeric species, are the primary reactants. Full coverage of the silicon surface by FAS molecules was achieved within 5 min. Heparin and hyaluronan, two naturally occurring biocompatible polysaccharides, were successfully covalently attached on the FAS SAM/Si surface by photo-immobilization. Atomic force microscopy (AFM) revealed the morphologic changes after the immobilization of heparin and hyaluronan, and X-ray photoelectron spectroscopy (XPS) confirmed the change in chemical compositions. Such combination of coatings is expected to enhance the stability and biocompatibility of the base material.

Microwave-assisted chemical vapor deposition process for synthesizing carbon nanotubes

A chemical vapor deposition (CVD) process is described that uses a susceptor to absorb the microwave and self-generate the heat for growing carbon nanotubes (CNTs) on a Si substrate. A high deposition rate of CNTs over a large area has been achieved, compared to the conventional CVD process, in which the reaction chamber was heated to the growing temperature (∼1100 °C) to trigger the reaction between the CH4 gas species and the catalyst. The modified CVD process possesses a pronounced advantage in the simplicity of the react on chamber design because no external heating is required. The size of the substrate is, in principle, unlimited as long as the susceptors can absorb the microwave uniformly. However, the design of the processing chamber and the choice of susceptors are very critical for successfully growing the CNTs using such a microwave-assisted CVD process.

A Three-Dimensional Analysis of the Flow and Heat Transfer for the Modified Chemical Vapor Deposition Process Including Buoyancy, Variable Properties, and Tube Rotation

A study has been made of the heat transfer, flow, and particle deposition relative to the modified chemical vapor deposition (MCVD) process. The effects of variable properties, buoyancy, and tube rotation have been included in the study. The resulting three-dimensional temperature and velocity fields have been obtained for a range of conditions. The effects of buoyancy result in asymmetric temperature and axial velocity profiles with respect to the tube axis. Variable properties cause significant variations in the axial velocity along the tube and in the secondary flow in the region near the torch. Particle trajectories are shown to be strongly dependent on the tube rotation and are helices for large rotational speeds. The component of secondary flow in the radial direction is compared to the thermophoretic velocity, which is the primary cause of particle deposition in the MCVD process. Over the central portion of the tube the radial component of the secondary flow is most important in determining the motion of the particles.

The Modified Chemical Vapor Deposition Process in a Concentric Annulus: An Extension for Focused High-Rate Deposition

A considerable percentage of the increasing production of glass fibers for telecommunication is based on the modified chemical vapor deposition (MCVD) process. The efficiency of this process is strongly hampered by the restricted and incomplete deposition of the expensive basic materials and the large taper effect. The introduction of an annular MCVD process yields not only complete and focused deposition but also an increased production rate. To investigate the conditions for optimal performance, the governing partial differential equations were solved. By identifying the characteristic parameters and their influence on the process, optimal design conditions were found for specific combinations of those parameters.

Germanium chemistry in the MCVD process for optical fiber fabrication

In the widely used modified chemical vapor deposition (MCVD) process for making optical fibers, germanium is the primary dopant for precisely increasing the refractive index of silica to form a guiding structure. The reactants in the process are usually GeCl 4 , SiCl 4 , and O 2 , often with POCl 3 , flowing in a silica support tube heated from the outside with an oxy-hydrogen flame. The oxide reaction products which are formed in the hot zone deposit as small particles downstream on the walls of the tube, and are consolidated into clear glass layers as the hot zone is traversed in the direction of flow. We have investigated the chemistry of germanium incorporation in the glass by infrared spectrophotometric analysis of the effluent gases, and by measurement of the properties of preforms and fibers produced by the process. It was found that SiCl 4 oxidizes completely to form the base glass, but GeCl 4 is incompletely reacted, with as much as 70 percent of the initial germanium present in the effluent as GeCl 4 . This is due to an unfavorable equilibrium in the reaction of GeCl 4 with O 2 to form GeO 2 , and we have developed a simple model for the process based on this equilibrium. At low temperatures the reactions in the hot zone are controlled by kinetics, but at the higher temperatures encountered in practice the gas concentrations and glass composition are controlled by thermodynamic equilibria. The composition of the glass produced can be quantitatively predicted by the model from such process parameters as the starting reactant concentrations, and the gas flow rates. It is necessary to include the dynamics of particle formation in the model to understand the inhomogeneity of multilayered deposits and the dependence of the final composition on the reaction zone maximum temperature. This results from the fact that the particles may grow large enough, especially with phosphorus present, to prevent diffusion of Ge from the particle interior to the equilibrating ambient at the surface. The model has been useful for predicting process parameters for the production of fibers with tightly controlled composition profiles.

Making Single-Mode Preforms by the MCVD Process

In the last few years, system technology has developed sufficiently to take advantage of the low-loss, high-transmission capability of single-mode fibers. First produced commercially in 1983, single-mode fiber now accounts for over 90 percent of all fiber made at AT&T Network Systems' Atlanta Works. Using the Modified Chemical Vapor Deposition (MCVD) process, glass rods, or preforms, of high-quality doped silica are created from which the fibers are then drawn. These fibers have losses below 0.40 dB/km and 0.22 dB/km at 1.31 μm and 1.55 μm, respectively, and transmission-rate capabilities in the gigabit-per-second range. Excellent concentricity of both the fiber core and outer diameter allows fibers to be joined into long lengths with a minimal additional loss. These characteristics allow single-mode fibers to be used in systems that operate at high bit rates with regenerator spacings of 30 km to 70 km apart.

Laser-Induced Thermophoresis and Particulate Deposition Efficiency

The interaction of laser radiation and an absorbing aerosol in a tube flow has been considered. The aerosol is produced by external heating of reactants as in the MCVD (Modified Chemical Vapor Deposition) process to produce submicron size particles in the manufacture of optical fiber preforms. These particles are deposited by thermophoretic forces on the inner wall of the tube as they are convected by a Poiseuille velocity profile. Axial laser radiation in the tube interacts with the absorbing particles, and the laser heating of the gas induces additional thermophoretic forces that markedly increase the efficiency of particulate deposition. A particle concentration dependent absorption coefficient that appears in the energy equation couples the energy equation to the equation of particle conservation, so that a nonlinear set of coupled partial integro-differential equations must be solved. Numerical solutions for aerosol particle trajectories, and thus deposition efficiencies, have been obtained. It is shown that laser enhanced thermophoresis markedly improves the deposition efficiency.