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Vol. 34, No. 7
CARBON FILMS
1175
form of fine jets directed towards the center. The nickel electrodes, the gas delivery system,
and the substrate are all packed into the chamber. The gas delivery system is spot-welded to
the anode, which is insulated from the cathode with the help of a 3-mm thick polyethylene
ring. The cathode, in turn, is separated from the substrate by another 3-mm thick polyeth-
ylene ring. This assembly is suitably clamped, and the electrical connections are made as
shown in Figure 1.
A gas mixture containing 10% carbon dioxide and 90% nitrogen was used. The gas
metering and control were carried out using MKS mass flow controllers. The capacitor (3 F)
is charged to 335 Vdc with a neon lamp connected across it for monitoring its charge and
discharge cycles. The laser is operated at a repetition rate of 40 pulses per minute. It is
focused through the shroud of the gas onto the substrate to the extent that a dielectric
breakdown occurs in the gas mixture, thereby causing the discharge of the capacitor. The role
of the CO2 laser is twofold, namely, triggering of the plasma (capacitor) discharge and pulse
heating of the substrate, both synchronized to each other. The substrate used was a 1-mm
thick, 20-mm diameter disc of molybdenum. It was metallographically polished, ultrasoni-
cally cleaned in acetone, and vapor-degreased in isopropyl alcohol.
The capacitance of the capacitor and the composition of the gas mixture were optimized
based on the following observations: (1) larger capacitance produced a higher energy plasma
discharge, thereby causing significant sputtering of the electrodes/other construction mate-
rials, ablation of the substrate and leading to the formation of deep pits and spurious deposits
on it; and (2) 100% carbon dioxide gave rise to a very small amount of black-colored deposit
on the substrate. Thermodynamic calculations showed that the combined energy available
from the laser and the capacitor discharge was too little to decompose the amount of carbon
dioxide with which it was interacting; the excess carbon dioxide was, perhaps, quenching the
reactive intermediates. The hole size of 5 mm diameter in the electrodes was also optimized
by experiments.
About 13,000 pulses were used in the present experiments. The deposited film was stored
in an ambient environment before being characterized by scanning electron microscopy
(JEOL JSM 330A) for morphology and by X-ray photoelectron spectroscopy (XPS) and
Raman spectroscopy for composition. Before the XPS measurements, the sample was
sputter-cleaned. It was then excited by Mg K␣ radiation under 1.33 ϫ 10Ϫ7 Pa vacuum using
a Riber XPS system comprised of an X-ray source (model CX 700) and an electron energy
analyzer (model MAC2). The data were collected and analyzed using a PC-compatible
system. The binding energies were referenced using a Au-4f7/2 peak. Raman measurements
were carried out in backscattering geometry, using the 514.5 nm line of Arϩ laser at a power
of 200 mW.
RESULTS AND DISCUSSION
The deposit was grayish-white in appearance and covered an area of about 2.5 mm ϫ 2.5
mm, equal to the size of the laser spot falling on the substrate. Optical microscope exami-
nation showed the deposit to consist of an almost transparent and colorless particulate
material. Some particles dispersed the illuminating light into several colorful reflections.
SEM examination of the film revealed the particulate structure (Fig. 2), the particulate size
being roughly 0.5–2 m. The crystallites had a rounded polygonal shape, and the film was
semicontinuous and multilayered.
Saini
