Chemical Vapor Deposited Silicon Carbide Research Articles

Improvement of uniformity in chemical vapor deposition of silicon carbide by using CFD

The chemical vapor deposition (CVD) of silicon carbide (SiC) on carbon has been widely used as a general method to suppress dust generation on carbon surfaces. For a CH3SiCl3 (MTS) and hydrogen system, computational fluid dynamic simulations to predict the growth rate of the silicon carbide are performed. The results of the simulations are consistent with the experimental results where the deposition rate depends highly on the H/Si composition and the specimen’s location. This simulation can provide guidance in optimizing the CVD process and improving the apparatus for CVD of SiC.

Chemical reactivity of CVC and CVD SiC with UO2 at high temperatures

Two types of silicon carbide (SiC) synthesized using two different vapor deposition processes were embedded in UO2 pellets and evaluated for their potential chemical reaction with UO2. While minor reactivity between chemical-vapor-composited (CVC) SiC and UO2 was observed at comparatively low temperatures of 1100 and 1300°C, chemical-vapor-deposited (CVD) SiC did not show any such reactivity. However, both CVD and CVC SiCs showed some reaction with UO2 at a higher temperature (1500°C). Elemental maps supported by phase maps obtained using electron backscatter diffraction indicated that CVC SiC was more reactive than CVD SiC at 1500°C. Furthermore, this investigation indicated the formation of uranium carbides and uranium silicide chemical phases such as UC, USi2, and U3Si2 as a result of SiC reaction with UO2.

Dynamic measurements of radiation-induced electrical-property modifications in CVD-SiC under fast-neutron irradiation

In situ measurements of the volume electrical conductivities of chemical-vapor-deposited silicon carbide (CVD-SiC) samples were carried out under irradiation by 2.5- and 14-MeV fast-neutron beams in air at room temperature. A slight radiation-induced conductivity (RIC) was detected under fast-neutron irradiation. A radiation-induced electrical degradation (RIED)-like behavior was observed in the form of degradation of the base conductivity in the absence of irradiation with increase in the fast-neutron fluence. The SEM micrograph images and XPS analysis of the surface of the fast-neutron-irradiated CVD-SiC samples revealed that the dissociation of carbon from existing SiOxC3−x compositions did not only occur via displacement damage, but also via ionizing effects (radiolysis), thereby leading to the observed RIED-like behavior as well as RIC in the electrical properties of the CVD-SiC samples.

Shape adaptive grinding of CVD silicon carbide

Because of the direct relationship between removal rate and surface roughness in conventional grinding, ultra-precision finishing of hard coatings produced by chemical vapour deposition (CVD) usually involves several process steps with fixed and loose abrasives. In this paper, an innovative shape adaptive grinding (SAG) tool is introduced that allows finishing of CVD silicon carbide with roughness below 0.4nm Ra and high removal rates up-to 100mm3/min. The SAG tool elastically complies with freeform surfaces, while rigidity at small scales allows grinding to occur. Since material removal is time dependent, this process can improve form error iteratively through feed moderation.

The effect of carbon on surface quality of solid-state-sintered silicon carbide as optical materials

The microstructure and the distribution of carbon (C) in silicon carbide (SiC) ceramics were investigated by scanning electron microscopy and transmission electron microscopy. The results show that C can restrain the growth of SiC grains and densify SiC ceramics with the increase of the C content, but residual C introduces a new phase-C to SiC ceramics. The hardness of C is less than that of SiC, so it's difficult to be polished as optical materials. The existence of C phase doesn't lead to the increase of surface roughness on SiC optical materials, but it leads to the decrease of the reflectance of SiC as the optical materials because the optical absorption of C in visible light is stronger than that of SiC. It indicates that C content is very important to the surface properties of SiC, which will affect the coating of chemical vapor deposition SiC or Si on the surface of SiC ceramics because of the different physical and chemical properties between C and SiC.

On the use of methane as a carbon precursor in Chemical Vapor Deposition of silicon carbide

It is generally considered that methane is not a suitable carbon precursor for growth of silicon carbide (SiC) epitaxial layers by Chemical Vapor Deposition (CVD) since its use renders epitaxial layers with very high surface roughness. In this work we demonstrate that in fact SiC epitaxial layers with high-quality morphology can be grown using methane. It is shown that a key factor in obtaining high-quality material is tuning the C/Si ratio of the process gas mixture to a region where the growth is limited neither by carbon nor by silicon supplies. From the growth characteristics presented here, we argue that the reactivity of methane with the SiC surface is much higher than generally assumed in SiC CVD modeling today.

대량 생산용 SiC CVD 리엑터에의 전산유체역학 시뮬레이션의 적용

Silicon carbide (SiC) materials are typical ceramic materials with a wide range of uses due to their high hardness and strength and oxidation resistance. In particular, due to the corrosion resistance of the material against acids and bases including the chemical resistance against ionic gases such as plasma, the application of SiC has been expanded to extreme environments. In the SiC deposition process, where chemical vapor deposition (CVD) technology is used, the reactions between the raw gases containing Si and C sources occur from gas phase to solid phases; thus, the merit of the CVD technology is that it can provide high purity SiC in relatively low temperatures in comparison with other fabrication methods. However, the product yield rarely reaches 50% due to the difficulty in performing uniform and dense deposition. In this study, using a computational fluid dynamics (CFD) simulation, the gas velocity inside the reactor and the concentration change in the gas phase during the SiC CVD manufacturing process are calculated with respect to the gas velocity and rotational speed of the stage where the deposition articles are located.

Cleaning Process Applicable to Silicon Carbide Chemical Vapor Deposition Reactor

The reactor cleaning process applicable to the silicon carbide chemical vapor deposition was designed and developed. The large silicon carbide particles formed on the carbon susceptor surface during the silicon carbide film deposition could be removed by etching the narrow contact regions between the particles and the susceptor surface using chlorine trifluoride gas at temperatures lower than 290°C. During this process, the carbon susceptor covered with the silicon carbide coating film did not suffer considerable damage. Thus, the reactor cleaning by detaching the silicon carbide particles while maintaining the silicon carbide coating film is possible when chlorine trifluoride gas is used at low temperatures.

Shortcomings of CVD modeling of SiC today

The active, epitaxial layers of silicon carbide (SiC) devices are grown by chemical vapor deposition (CVD), at temperatures above 1,600 °C, using silane and light hydrocarbons as precursors, diluted in hydrogen. A better understanding of the epitaxial growth process of SiC by CVD is crucial to improve CVD tools and optimize growth conditions. Through computational fluid dynamic (CFD) simulations, the process may be studied in great detail, giving insight to both flow characteristics, temperature gradients and distributions, and gas mixture composition and species concentrations throughout the whole CVD reactor. In this paper, some of the important parts where improvements are very much needed for accurate CFD simulations of the SiC CVD process to be accomplished are pointed out. First, the thermochemical properties of 30 species that are thought to be part of the gas-phase chemistry in the SiC CVD process are calculated by means of quantum-chemical computations based on ab initio theory and density functional theory. It is shown that completely different results are obtained in the CFD simulations, depending on which data are used for some molecules, and that this may lead to erroneous conclusions of the importance of certain species. Second, three different models for the gas-phase chemistry are compared, using three different hydrocarbon precursors. It is shown that the predicted gas-phase composition varies largely, depending on which model is used. Third, the surface reactions leading to the actual deposition are discussed. We suggest that hydrocarbon molecules in fact have a much higher surface reactivity with the SiC surface than previously accepted values.

Influence of SiC coating thickness on mechanical properties of SiCf/SiC composite

Silicon carbide (SiC) coatings with varying thickness (ranging from 0.14μm to 2.67μm) were deposited onto the surfaces of Type KD-I SiC fibres with native carbonaceous surface using chemical vapour deposition (CVD) process. Then, two dimensional SiC fibre reinforced SiC matrix (2D SiCf/SiC) composites were fabricated using polymer infiltration and pyrolysis (PIP) process. Influences of the fibre coating thickness on mechanical properties of SiC fibre and SiCf/SiC composite were investigated using single-filament test and three-point bending test. The results indicated that flexural strength of the composites initially increased with the increasing CVD SiC coating thickness and reached a peak value of 363MPa at the coating thickness of 0.34μm. Further increase in the coating thickness led to a rapid decrease in the flexural strength of the composites. The bending modulus of composites showed a monotonic increase with increasing coating thickness.

Silicon Carbide Oxidation in High‐Pressure Steam

Silicon carbide is a candidate cladding for fission power reactors that can potentially provide better accident tolerance than zirconium alloys. SiC has also been discussed as a host matrix for nuclear fuel. Chemical vapor–deposited silicon carbide specimens were exposed in 0.34–2.07 MPa steam at low gas velocity (~50 cm/min) and temperatures from 1000°C to 1300°C for 2–48 h. As previously observed at lower steam pressure of 0.15 MPa, a two‐layer SiO2 scale was formed during exposure to these conditions, composed of a porous cristobalite layer above a thin, dense amorphous SiO2 surface layer. Growth of both layers depends on temperature, time, and steam pressure. A quantitative kinetics model is presented to describe the SiO2 scale growth, whereby the amorphous layer is formed through a diffusion process and linearly consumed by an amorphous to crystalline phase transition process. Paralinear kinetics of SiC recession were observed after exposure in 0.34 MPa steam at 1200°C within 48 h. High‐pressure steam environments are seen to form very thick (10–100 μm) cristobalite SiO2 layers on CVD SiC even after relatively short‐term exposures (several hours). The crystalline SiO2 layer and SiC recession rate significantly depend on steam pressure. Another model is presented to describe the SiC recession rate in terms of steam pressure when a linear phase transition kl governing the recession kinetics, whereby the reciprocal of recession rate is found to follow a negative unity steam pressure power law.

Elimination of silicon gas phase nucleation using tetrafluorosilane (SiF4) precursor for high quality thick silicon carbide (SiC) homoepitaxy

AbstractTetrafluorosilane (SiF4) gas precursor is utilized to eliminate Si gas phase nucleation and Si parasitic deposition during silicon carbide (SiC) epitaxial growth, otherwise unachievable in similar growth conditions using conventional silane (SiH4) and dichlorosilane (SiCl2H2/DCS) precursors. Higher SiF bond strength (565 kJ mol−1) in SiF4 prevents early gas decomposition and Si cluster formation, essential for high temperature SiC chemical vapor deposition (CVD), and yet enables growth of high quality epitaxy in an improved particulate suppressed growth condition. High quality, thick 4H‐SiC epilayers >100 µm have been demonstrated using SiF4 with excellent surface morphology, polytype uniformity, crystallinity and low defect density. magnified imageSignificant (80%) suppression of parasitic deposition (left) using SiF4 reduces particulate generation during growth, and improves the epilayer quality (right).

Investigation of silicon/glass anodic bonding with PECVD silicon carbide as the intermediate layer

This paper describes the study of the conventional silicon/glass anodic bonding with plasma enhanced chemical vapor deposited silicon carbide (PE-SiC) as the intermediate layer, in order to evaluate the feasibilities of applying PE-SiC as the device's passivation layer prior to the packaging bonding and construction layer based on the bond-and-transfer technique. It is found that the mechanism of this bonding is similar to the traditional anodic bonding. As the PE-SiC thickened, the bond strength declined; meanwhile, the leak rate remained at the same level with the silicon/glass bonding. Further experiments revealed that the bonding increased the interlayer's tensile stress by 70.7 MPa and diminished the stress gradient by 24.6 MPa µm−1.

A Study of Predominated Pathway of Initial Processes in Chemical Vapor Deposition of Silicon-Carbide from Methyltrichlorosilane and Hydrogen System

The initial pathways in the CVD process of preparing silicon carbides with CH3SiCl3-H2 precursors was searched theoretically, which involves 21 well-defined transition states. The geometries of the species were optimized by employing the B3PW91/6-311G(d,p) method. The energy barriers and the reaction energies were evaluated with the accurate model chemistry method at G3(MP2) level after a non-dynamical electronic correlation detection. The heat capacities and entropies were obtained with statistical thermodynamics. The Gibbs free energies at 298.15 K and 1200 K for all the possible elementary reactions, including both direct decomposition and the radical attacking dissociations for MTS were reported. The energies at any temperature could be derived classically by using the analytical heat capacities. It was found that the free radical reactions have clearly kinetic ascendency to be the most efficient decomposition pathway.

A Study of Predominated Pathway of Initial Processes in Chemical Vapor Deposition of Silicon-Carbide from Methyltrichlorosilane and Hydrogen System

The initial pathways in the CVD process of preparing silicon carbides with CH3SiCl3-H2 precursors was searched theoretically, which involves 21 well-defined transition states. The geometries of the species were optimized by employing the B3PW91/6-311G(d,p) method. The energy barriers and the reaction energies were evaluated with the accurate model chemistry method at G3(MP2) level after a non-dynamical electronic correlation detection. The heat capacities and entropies were obtained with statistical thermodynamics. The Gibbs free energies at 298.15 K and 1200 K for all the possible elementary reactions, including both direct decomposition and the radical attacking dissociations for MTS were reported. The energies at any temperature could be derived classically by using the analytical heat capacities. It was found that the free radical reactions have clearly kinetic ascendency to be the most efficient decomposition pathway.

Low Temperature SiC Film Deposition Using Trichlorosilane Gas and Monomethylsilane Gas

In order to produce silicon carbide thin film through the process entirely at low temperatures, silicon carbide chemical vapor deposition on a silicon wafer surface was performed using trichlorosilane gas and monomethylsilane gas. First, the silicon thin film was formed using trichlorosilane gas at 800 degrees C and was cooled to room temperature in ambient hydrogen, in order to produce hydrogen-terminated silicon surface. Next, monomethylsilane gas was introduced at room temperature to produce silicon carbide thin film. The mirror-like appearance of the film obtained by this process was maintained after the exposure to hydrogen chloride gas at 800 degrees C. Furthermore, very thin Si-C layer was detected at the surface by means of the time-of-flight secondary ion mass spectrometry. Thus, the silicon carbide thin film was concluded to be formed through the process at temperatures below 800 degrees C.

Langasite Crystal Microbalance for Development of Reactive Surface Preparation of Silicon Carbide Film Deposition from Monomethylsilane Gas

A silicon carbide chemical vapor deposition (CVD) process at room temperature was developed using monomethylsilane (MMS) gas by an in situ measurement method using a langasite crystal microbalance (LCM). The phenomenon occurring on the LCM was evaluated by monitoring the frequency change during the processes and by comparing the frequencies in a steady state in ambient hydrogen without the use of any additional gases. The entire process was performed on the LCM, which could be used to evaluate the reactive surface preparation and the silicon carbide film deposition. To produce a reactive surface, a silicon-rich thin film was prepared by the CVD technique using MMS gas at 800 °C. Because the LCM frequency continuously decreased along with the MMS gas supply at room temperature, the silicon carbide film deposition was found to be continuous for 1 min.

Initial decomposition of methyltrichlorosilane in the chemical vapor deposition of silicon-carbide

All of the possible initial decomposition pathways in the CVD process of preparing silicon carbides with CH 3SiCl 3–H 2 precursors were searched theoretically. The geometries of the species were optimized with B3PW91/6-311G(d,p). The energy barriers and the reaction energies were evaluated with the accurate model chemistry at G3(MP2) level after a non-dynamical electronic correlation detection. The Gibbs free energies at 298.15 K and 1200 K for all the possible elementary reactions, including both direct decomposition and the radical attacking dissociations for MTS were reported. Nine transition states are firstly reported in initial decomposition of MTS. Comparisons with a previous theoretical study are excellent, but additional reactions studied here are found to be relevant as well.

Contact angle measurement of molten lead–lithium on silicon carbide surfaces

Measurements of the contact angles at the different temperatures of a molten lead–lithium eutectic alloy (PbLi) droplet on a silicon carbide (SiC) wall are needed for the research and development both of a magnetic confinement fusion (MCF) and an inertia confinement fusion (ICF) blankets. PbLi coolant/breeder flows in the coolant channel, which is made of the SiC walls, and will experience a flow slip at the wall, called as a magnetohydrodynamic (MHD) slip flow. The ICF blanket adopts a molten PbLi film flow along the first wall made of SiC. The PbLi contact angle database is necessary as the thermal property for numerically predicting the behavior of the flowing molten PbLi film. This study attempts the measurement of the contact angles between the molten PbLi and the various SiC surfaces. For example, in order to examine the initial PbLi wettability, we measured the contact angles of a chemical vapor deposition (CVD) SiC, a nano-infiltration and transient eutectic-phase (NITE) SiC/SiC composites, and a NITE SiC in an inert atmosphere. We obtained the contact angle database of the molten PbLi, varying the temperature of PbLi from 250 to 400 °C, on a surface-polished as well as an unpolished SiC.

Study on the Thermoelectric Properties of CVD SiC Deposited with Inert Gases

We deposited silicon carbide (SiC) by the chemical vapor deposition (CVD) method using the inert gases Ar and He. It was confirmed that SiC deposited with inert gases had a porous microstructure and high carbon content. We also studied the thermoelectric properties. SiC deposited with He gas had lower electrical and thermal conductivity compared with SiC deposited with Ar gas. Both samples using Ar and He exhibited a negative Seebeck coefficient, indicating n-type semiconductor behavior. The calculated figure of merit (Z) of SiC deposited with inert gases was improved compared with SiC deposited with H2 or N2 gas. The value for SiC deposited with He was higher than that for SiC deposited with Ar. The thermoelectric properties of porous silicon carbide deposited with inert gases were also compared with those of silicon carbide deposited with hydrogen or nitrogen gas.