David Hash

Carbon nanotube growth by PECVD: a review

Carbon nanotubes (CNTs), due to their unique electronic and extraordinary mechanical properties, have been receiving much attention for a wide variety of applications. Recently, plasma enhanced chemical vapour deposition (PECVD) has emerged as a key growth technique to produce vertically-aligned nanotubes. This paper reviews various plasma sources currently used in CNT growth, catalyst preparation and growth results. Since the technology is in its early stages, there is a general lack of understanding of growth mechanisms, the role of the plasma itself, and the identity of key species responsible for growth. This review is aimed at the low temperature plasma research community that has successfully addressed such issues, through plasma and surface diagnostics and modelling, in semiconductor processing and diamond thin film growth.

Growth of multiwall carbon nanotubes in an inductively coupled plasma reactor

A high density plasma from a methane–hydrogen mixture is generated in an inductively coupled plasma reactor, and multiwalled carbon nanotubes (MWNTs) are grown on silicon substrates with multilayered Al/Fe catalysts. The nanotubes are vertically aligned, and the alignment is better than the orientation commonly seen in thermally grown samples. A detailed parametric study varying inductive power, pressure, temperature, gas composition, catalyst thickness, and power to the substrate is undertaken. Transmission electron microscopy and Raman spectroscopy are used to characterize the nanotubes. Emission spectroscopy and a global model are used to characterize the plasma. The power in the lower electrode holding the substrate influences the morphology and results in a transition from MWNTs to nanofibers as the power is increased.

Model based comparison of thermal and plasma chemical vapor deposition of carbon nanotubes

A model-based comparison of thermal and plasma chemical vapor deposition (CVD) techniques for the growth of carbon nanotubes (CNTs) from methane feedstock is presented. In thermal CVD, the feedstock does not dissociate in the gas phase at temperatures commonly used for single- and multiwalled CNT growth (⩽900 °C) and the nanotube production is entirely due to surface reaction of CH4 on the catalyst surface. In contrast, plasma reactors produce, through electron impact as well as neutral reactions, significant amounts of acetylene, ethylene, a variety of CxHy radicals and ions from the methane/hydrogen feedstock, all of which contribute to the nanotube production. Such production of higher order stable hydrocarbons and radicals may make growth of single-walled CNTs difficult using low temperature plasma CVD techniques.

Carbon nanotubes by plasma-enhanced chemical vapor deposition

This paper presents the growth of vertically aligned carbon nanotubes by plasma-enhanced chemical vapor deposition (PECVD) using Ni catalyst and C2H2/NH3 feedstock. The role of plasma in aligning the carbon nanotubes during growth is investigated both experimentally and computationally, confirming that the field in the plasma sheath causes the nanotubes to be aligned. Experiments using a plasma analyzer show that C2H2 is the dominant precursor for carbon nanotube growth. The role of NH3 in the plasma chemistry is also investigated, and experimental results show how the interaction between NH3 and the C2H2 carbon feedstock in the gas phase explains the structural variation in deposited nanotubes for differing gas ratios. The effects of varying the plasma power during deposition on nanotube growth rate is also explored. Finally, the role of endothermic ion-molecule reactions in the plasma sheath is investigated by comparing measured data with simulation results.

Simulation of the dc Plasma in Carbon Nanotube Growth

A model for the dc plasma used in carbon nanotube growth is presented, and one-dimensional simulations of an acetylene/ammonia/argon system are performed. The effect of dc bias is illustrated by examining electron temperature, electron and ion densities, and neutral densities. Introducing a tungsten filament in the dc plasma, as in hot filament chemical vapor deposition with plasma assistance, shows negligible influence on the system characteristics.

An investigation of plasma chemistry for dc plasma enhanced chemical vapour deposition of carbon nanotubes and nanofibres

The role of plasma in plasma enhanced chemical vapour deposition of carbon nanotubes and nanofibres is investigated with both experimental and computational diagnostic techniques. A residual gas analysis (RGA) of a 12 mbar dc discharge with a C2H2/NH3 gas mixture is conducted near the Ni catalyst surface employed for carbon nanofibre growth. The results are corroborated with a 1D dc discharge model that solves for species densities, ion momentum, and ion, electron and neutral gas thermal energies. The effect of varying the plasma power from 0 to 200 W on the gas composition is studied. The dissociation efficiency of the plasma is demonstrated where over 50% of the feedstock is converted to a mixture of hydrogen, nitrogen and hydrogen cyanide at 200 W. Finally, the important role that endothermic ion–molecule reactions play in this conversion is, for the first time, established. Of these reactions, dissociative proton abstraction and collision-induced dissociation are of the greatest significance.

Impact of Gas Heating in Inductively Coupled Plasmas

Recently it has been recognized that the neutral gas in inductively coupled plasma reactors heats up significantly during processing. The resulting gas density variations across the reactor affect reaction rates, radical densities, plasma characteristics, and uniformity within the reactor. A self-consistent model that couples the plasma generation and transport to the gas flow and heating has been developed and used to study CF4 discharges. A Langmuir probe has been used to measure radial profiles of electron density and temperature. The model predictions agree well with the experimental results. As a result of these comparisons along with the poorer performance of the model without the gas–plasma coupling, the importance of gas heating in plasma processing has been verified.

Validation Process for Blowing and Transpiration-Cooling in DPLR

The process undertaken to verify the blowing boundary condition in the DPLR CFD code is presented. Blowing boundary condition in DPLR is used to compare simulated Blasius transformed blowing boundary layers to exact solutions. Results demonstrate that DPLR accurately predicts a blowing boundary layer for subsonic air-into-air flows. Flat plate heating results for subsonic blowing flows are analyzed, and the resulting heat transfer reduction compares well to an analytical function derived for a classical Couette flow. Finally, supersonic air-into-air blowing cases for a 5 degree half-angle cone are also shown to compare well to experimental data.

Monte Carlo sensitivity analysis of CF2 and CF radical densities in a c‐C4F8 plasma

A Monte Carlo sensitivity analysis is used to build a plasma chemistry model for octacyclofluorobutane (c‐C4F8) which is commonly used in dielectric etch. Experimental data are used both quantitatively and qualitatively to analyze the gas phase and gas surface reactions for neutral radical chemistry. The sensitivity data of the resulting model identifies a few critical gas phase and surface aided reactions that account for most of the uncertainty in the CF2 and CF radical densities. Electron impact dissociation of small radicals (CF2 and CF) and their surface recombination reactions are found to be the rate-limiting steps in the neutral radical chemistry. The relative rates for these electron impact dissociation and surface recombination reactions are also suggested. The resulting mechanism is able to explain the measurements of CF2 and CF densities available in the literature and also their hollow spatial density profiles.

Modelling of inductively coupled plasma processing reactors

A comprehensive model has been developed to study low-pressure, high-density plasma processing reactors. The model couples plasma generation and transport self-consistently to fluid flow and gas energy equations. The model and the simulation software have been used to analyse chlorine plasmas used in metal etching. The effect of the inductive coil frequency on the plasma characteristics has been examined and found to influence plasma uniformity only moderately. Model predictions for a CF4 plasma have been found to agree well with experimental results.