Multiscale Models of CVD Process: Review and Prospective.

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Carbon nanotubes (CNTs) are solid carbon allotropes that feature high strength and high transport capabilities. When large populations of CNTs are synthesized simultaneously, they self-assemble into vertically oriented CNT forests (also called CNT arrays or CNT turfs). In the current work, three-dimensional CNT forest microstructures are engineered by selectively activating CNT growth on specific regions of a patterned substrate. The process is realized using a combination of two distinct chemical vapor deposition (CVD) processes in concert with silicon substrates independently patterned with regions of alumina, iron, or alumina and iron. The CVD process of both techniques require a catalyst nanoparticle to facilitate CNT growth and a hydrocarbon gaseous precursor. Floating catalyst CVD, a process in which CNT catalyst is delivered to a substrate in the vapor state, is compatible with oxide-coated substrates and not compatible with bare silicon surfaces. Fixed catalyst CVD utilizes a static iron film catalyst layer to support CNT synthesis. Regions of alumina support selective CNT forest growth with floating catalyst CVD, while regions of layered Al2O3/Fe support both fixed and floating catalyst growth when the fixed catalyst is activated first. By selectively and sequentially activating specific regions of a lithographically-defined substrate using the various CVD processes, complex 3D microstructures are demonstrated. The resultant CNT forest microstructures are characterized using scanning electron microscopy (SEM) and nanoindentation. Atomic force microscopy is used to analyze the behavior of the fixed iron catalyst during the high temperatures required for CVD processing.

The deposition limiting step during the chemical vapor deposition (CVD) process of a film can be identified by the Arrhenius plot, which shows the effect of the wafer temperature on the deposition rate. The deposition limiting step, and as a consequence the Arrhenius plot, are affected by the operating conditions of the CVD reactor. By using a multiscale computational framework, it is shown that they are also affected by the existence of a micro-topography (e.g. micro-trenches) on the wafer. The origin of this effect is the loading phenomenon and the mutliscale computations are used to quantify it: The deposition rate decreases in the diffusion limited regime; the latter and the transition regime are shifted at lower temperatures compared to the flat wafer case. The evolution of the deposited film profile in the micro-trenches is calculated at all deposition regimes. The film conformity starts to decrease when the wafer temperature exceeds the maximum temperature of the reaction limited regime. Finally, it is shown that the effect of the micro-topography on the species concentrations in the reactor bulk is important only in the transition regime. The case studies are CVD of a) tungsten from tungsten hexafluoride and b) silicon from silane.

Epitaxial growth of SiC in a chimney CVD reactor

An approach to multiscale modelling with graph grammars.

Multi-scale Coupling Analysis of Partial Transverse Ventilation System in an Underground Road Tunnel

Multi-scale dimensionless prediction model of PEMFC sealing interface leakage rate based on fractal geometry

Size effect on the cyclic deformation behavior of superalloy ultrathin sheet: Characterization and multiscale modelling

The numerical computations for the species density and energy kinetics are used to elaborate the growth mechanism and differences between the graphene–carbon nanotube (CNT) hybrids grown by the chemical vapor deposition (CVD) and plasma‐enhanced CVD (PECVD) process. The modeling results show that the CNT, graphene, and graphene–CNT hybrids have enhanced growth rates in a plasma medium than in a neutral gas environment of a CVD method. Plasma enables low‐temperature growth of graphene–CNT hybrids and offers a broad temperature range for the growth than the CVD technique that operates only at elevated temperatures. The underlying reasons and mechanisms behind the better growth rates and temperature range offered by the PECVD process (~400–1,600 K) are explained through the results.

Chemical vapor deposition (CVD) process composes a complex system, where chemical reaction and heat and mass transfer interact with each other. And these macro-scale phenomena are deeply related to micro-scale mechanics. Hence multi-scale analysis is required to understand these complicated phenomena and to develop full-scale simulator of the CVD reactor. In this paper, we present the macro-scale simulation by the DSMC method. In those reactors, sometime the important species such as the reactive intermediates have extremely low density ratio. This causes the large statistical fluctuation in the DSMC method, where the number of particles and the calculation time are limited. We propose a new numerical method for this kind of problem and the whole process of silicon CVD is simulated by the new method. We simulated the following CVD process: the gas mixture of silane and hydrogen forms a free expansion jet through a nozzle orifice at the top of the reactor and interact with the heated substrate that is set ...

Growth mode during initial stage of chemical vapor deposition

In the 1980's appeared the first several scientific papers dealing with chemical vapor deposition (CVD) of nanostructured catalytic materials. Since then, the tremendous increase in the number of related publications indicates the significant importance of CVD for the preparation of catalysts. CVD has the capability to generate various types of catalytically attractive nano-scale structures by modifying the surface properties of massive or even nano-divided substrates. Relatively new CVD processes such as catalytic CVD, fluidized-bed CVD, rotary CVD, two-step CVD and large spot laser CVD allow the formation of nanoparticles, nanotubes, nanofibers, nanocomposites and porous or oriented films. Intensive research is being performed on the production of CNTs and the preparation of supported catalysts by CVD. This paper provides an overview of the relatively new CVD processes involved in the preparation of catalytic materials and some representative examples reported in the open literature.

Graphene, the atomic thin carbon film with honeycomb lattice, holds great promise in a wide range of applications, due to its unique band structure and excellent electronic, optical, mechanical, and thermal properties. Scientists are researching this star material because of the development of various emerging preparation techniques, among which chemical vapor deposition (CVD) has received the fastest advances in the past few years. For the CVD growth of graphene, the ultimate goal is to achieve the highest quality in the largest scale and lowest cost with a precise control of layer thickness, stacking order, and crystallinity. To meet this goal, researchers need a comprehensive understanding and effective controlling of the growth process, especially to its elementary steps. In this Account, we focus on our recent progresses toward the controlled surface growth of graphene and its two-dimensional (2D) hybrids via rational designs of CVD elementary processes, namely, process engineering. A typical CVD process consists of four main elementary steps: (A) adsorption and catalytic decomposition of precursor gas, (B) diffusion and dissolution of decomposed carbon species into bulk metal, (C) segregation of dissolved carbon atoms onto the metal surface, and finally, (D) surface nucleation and growth of graphene. Absence or enhancement of each elementary step would lead to significant changes in the whole growth process. Metals with certain carbon solubility, such as nickel and cobalt, involve all four elementary steps in a typical CVD process, thus providing us an ideal system for process engineering. The elementary segregation process can be completely blocked if molybdenum is introduced into the system as an alloy catalyst, yielding perfect monolayer graphene almost independent of growth parameters. On the other hand, the segregation-only process of predissolved solid carbons is also capable of high-quality graphene growth. By using a synergetic Cu-Ni alloy, we are able to further enhance the control to such a segregation technique, especially for the thickness of graphene. By designing a cosegregation process of carbon atoms with other elements, such as nitrogen, doped graphene could be synthesized directly with a tunable doping profile. Copper with negligible carbon solubility provides another platform for process engineering, where both carbon dissolution and segregation steps are negligible in the CVD process. Carbon atoms decomposed from precursors diffuse on the surface and build up the thermodynamically stable honeycomb lattice. As a result, graphene growth on copper is self-limited, and formation of multilayer graphene is generally prohibited. Being able to control this process better, as well as the high quality produced, makes copper-based growth the dominating synthesis procedure in the graphene community. We designed a two-temperature zone system to spatially separate the catalytic decomposition step of carbon precursors and the surface graphitization step for breaking this self-limited growth feature, giving high-quality Bernal stacked bilayer graphene via van der Waals epitaxy. We performed the so-called wrinkle engineering by growing graphene on nanostructured copper foil together with a structure-preserved surface transfer. In such a way, we controlled the wrinkling or folding on graphene and further fabricated graphene nanoribbon arrays by self-masked plasma etching. Moreover, by designing a two-step patching growth process on copper, we succeeded in synthesizing the mosaic graphene, a patchwork of intrinsic and nitrogen-doped graphene connected by single crystalline graphene p-n junctions. By following a general concept of process engineering, our work on the designed CVD growth of graphene and its 2D hybrids provides a unique insight of this research field. It enables the precise growth control of graphene together with the in-depth understanding of CVD growth process, which would further stimulate the pace of graphene applications.

Phase transformation and carbon precipitation of coal fly ash magnetospheres during a CVD process for microwave adsorption

Graphene has been considered as one of most attractive materials in nano-scaled electronic devices. Graphene film has been mainly grown on many metal (eg., Ni , Cu, Fe) by chemical vapor deposition (CVD) process and transferred on dielectric substrate. However, the complicated transfer techniques for metal-catalyzed graphene inevitably have resulted in wrinkles, holes, and metal etching residues. Thus, there is a substantial need to develop a scalable method for reliable production of large-area graphene directly on insulating substrates. We synthesized the nucleation of graphene on silicon nitride (Si3N4) substrate and report the influence of pre hydrogen flow during CVD process on silicon nitride which is the stable dielectric and suitable for carbon diffusion barrier. Test wafer was cleaned in 10:1 HF solution and nucleation of graphene was synthesized by CVD process using CH4, Ar, H2gas at 1000°C. We evaluated the graphene nucleation with and without pre hydrogen flow on silicon nitride film as shown in Fig.1. Although amorphous carbon cluster was formed on silicon nitride without pre hydrogen flow as shown in Fig. 2(a), Raman shift peak related graphene was obtained with pre hydrogen flow condition as shown in Fig. 2(b). It is assumed that hydrogen ions are bonded with silicon atom on silicon nitride surface during pre hydrogen process and then chemisorptions of carbon precursors on the H-terminated surface are easily made. Fig.3 shows the role of hydrogen [1]. To synthesize the graphene film on silicon nitride, it is essential to be terminated with hydrogen at silicon nitride surface. Reference [1] Jae-Hyun Lee et al, “Wafer-Scale Growth of Single-Crystal Monolayer Graphene on Reusable Hydrogen-Terminated Germanium”, Science, Vol. 344, 2014 Figure 1

ABSTRACTPolyxylylene thin films grown by the chemical vapor deposition (CVD) process have long been utilized to achieve uniform, pinhole-free conformal coatings. They have recently been cited as possible low dielectric constant films for intermetal layers in high-speed ICs. Homopolymer films are highly crystalline and have a glass transition temperature around room temperature. We have demonstrated that room temperature copolymerization with previously untested comonomers can be achieved during the CVD process. Copolymerizing chloro-p-xylylene with perfluorooctyl methacrylate results in the dielectric constant at optical frequencies being lowered from 2.68 to 2.19. Copolymerizing p-xylylene with vinylbiphenyl resulted in films which increase the temperature at which oxidative scission occurs from 320 to 450C. Copolymerizing p-xylylene with 9-vinylanthracene resulted in a brittle, yellow film.