Session W1B - Diamond and Diamond-like Carbon Deposition.
MIXED session, Wednesday morning, October 23
Lecture Hall,

Synthesis of diamond films using fullerene (C_60) precursors in an argon (Ar) microwave plasma without the addition of hydrogen has now been accomplished and strongly suggests that the diamond phase grows by a new and hitherto unexplored mechanism. Diamond films produced in this way are nanocrystalline, smooth (20-30 nm rms surface roughness) and highly reflective. They maintain their nanocrystallinity to thicknesses of more than 20 \mum. More recently is has been found by us that films with similar properties can also be grown from argon microwave plasmas containing 1% CH_4. Conditions using both C_60/Ar and CH_4/Ar mixtures are comparable to "standard" CH_4/H_2 plasma parameters: 1% carbon content, 100 sccm, 100 Torr, 1 KW microwave power, 700-800 #251#C substrate temperatures. Growth rates of 0.5-1 \mum/hr have been measured laser interferometrically on Si, Si_3N_4, SiC, W and WC substrates, which had been prepared by treatment with 0.1 \mum diamond powder or by using bias enhanced nucleation.

Emission spectra from CH4/H2 microwave plasmas show the presence of CH_3^. and CH molecules, as well as atomic H. By contrast, spectra from C_60/Ar and CH_4/Ar discharges are dominated by the well-known Swan bands of carbon dimer, C_2. Fragmentation of C_60 by intense laser irradiation as well as collisionally induced and surface-induced dissociation has for a long time been known to proceed by sequential loss of C_2 molecules at least until cage sizes of C_28 are reached. We now have found that fragmentation of C_60 in an argon microwave discharge also occurs via the C_2 pathway. Fragmentation in the plasma appears to be very efficient as a result of a number of different and competing processes, including collisionally induced dissociation by metastable argon atoms.

The experimental work has recently been suplemented by quantum chemical calculation, which strongly supports the idea that the chemically highly energetic C_2 molecule with an adsorption energy of 8 eV can grow diamond without the intervention of atomic hydrogen, while hydrocarbon precursors (CH_3^. or C_2H_2) with adsorption energies of 1 ev require the intervention of atomic hydrogen. Very high nucleation rates (10^10, cm^-2, sec^-1 are responsible for the microstructure of the films (average crystallite size of 15 \mum) grown from argon discharges.

Arguments will be advanced to show that diamond nucleation occurs in preference to graphite nucleation as a result of the thermodynamic stability of nanocrystalline diamond. Once established at the level of supercritical or

embryonic nuclear size, growth continues even in the absence or virtual absence of hydrogen because of the large energy barrier for the solid-state nucleation of the stable bulk graphite phase.

*Work supported by the U.S. Department of Energy, BES-Materials Sciences, under Contract W-31-109-ENG-38.

[W1B.02] Reaction Mechanisms for Growth of Diamond Thin-Films from Buckyball Precursors

Rapid growth of thin-films of diamond has been observed recently in experiments involving chemical vapor deposition following fragmentation of C_60 in a microwave discharge \footnote D. M. Gruen, S. Liu, A. R. Krauss, and X. Pan, J. Appl. Phys. 75, 1758 (1994). The C_2 molecule has been proposed as the principal growth species, with diamond growth occurring by insertion of C_2 into the C-H bonds of a hydrogen terminated diamond surface. Mechanisms for growth on the diamond surface, with dicarbon (C_2) as the growth species, have been examined using using ab initio and semi-empirical quantum mechanical methods. These calculations have provided information on the intermediates, reaction energies and activation energies of various possible reaction pathways for addition of C_2 to a diamond surface. Clusters of 18 and 48 carbon atoms terminated by hydrogen atoms are used to model the diamond surface in these calculations.

[W1B.03] Diamond Nucleation and Growth by Chemical Vapor Deposition

The growth of diamond thin films at low pressures, where diamond is metastable, is one of the most exciting developments in materials science of the last two decades. However, low growth rates and poor quality currently limit applications. Diamond growth is achieved by a variety of processes using very different means of gas activation and transport. Generalized models, coupled with experiments, show how process variables, especially pressure, gas activation temperature, characteristic diffusion length, and source gas composition, influence diamond growth rates and diamond quality. The modeling is sufficiently general to permit comparison between growth methods. The models indicate that typical processes, e.g., hot-filament, microwave and thermal plasma reactors, operate at pressures where concentrations of atomic hydrogen, [H], and methyl radicals, [CH3], reach maxima. The results strongly suggest that the growth rate maxima with pressure arise from changes in the gas phase concentrations rather than changes in substrate temperature. The results also suggest that, at one atmosphere pressure using only hydrocarbon chemistry, growth rates saturate at gas activation temperatures above 5000 K. Models of defect incorporation indicate that the amount of sp2, non-diamond material incorporated in the diamond is proportional to [CH3]/[H] and therefore can be correlated with the controllable process parameters. The unusual and interesting connection between diamond nucleation and growth with graphitic, sp2, precursors will be explored.

[W1B.04] Deposition of Diamond-Like Carbon Film Using RF Plasma Enhanced CVD foran Anti-Reflection Layer on Polarizers in TFT LCD Display

This paper describes the deposition of diamond-like carbon (DLC) film using RF plasma enhanced CVD for an anti-reflection layer on polarizers in TFT LCD display. The materials of polarizers in TFT LCD display are polymers, which are easily scratched. They need anti- reflection coating for using the LCD outside under sunlight. The DLC film has following properties : mechanically hard, chemically inert and optically transparent over visible light range. DLC film was deposited on some sublayers on polarizer by using RF plasma enhanced CVD method considering the trade-off between low optical absorption and hardness of DLC film with increasing the quantity of hydrogen. The RF plasma enhanced CVD method is suitable for larger substrates and economical to deposit DLC film. We got good results on them by measuring the optical properties from spectroscope and mechanical properties from scotch tape test : 0.4 absorption over visible light range, chemical inert and scratch resistant.

[W1B.05] Heavy-Ion Radiation-Induced Diamond Formation in Carbonaceous Materials

The feasibility of a radiation-induced diamond formation (RIDF) mechanism is demonstrated by the observation of nano-diamonds in carburanium, a U-rich fine-grained, coal-like assemblage containing amorphous carbonaceous material of Precambrian age from North Karelia, Russia. This mineral deposit represents an ideal natural environment for RIDF because the carbonaceous grains present have received a high fluence of energetic particles over a geological time scale. Fragments of carburanium were subjected to acid dissolution treatments to isolate any diamond present. Transmission electron microscopy on these acid residues identified 500 nm polycrystalline diamond aggregates. This observation and estimates of formation efficiencies supports the hypothesis that diamond can form in carbonaceous material irradiated by U decay fragments. Diamond concentration in bulk carburanium is #197# 30 ppm indicating that the RIDF efficiencies might be relatively low as compared to the competing formation of graphite; the acid treatment was an essential key in the recovery of diamond in carburanium. This fact could contribute to the lack of observation of diamond in well- studied ion-implanted carbons. Experiments to synthesize nano-diamonds by heavy-ion irradiation are scheduled for late 1996 at ANL's accelerator ATLAS.

Part W of program listing