Biomedical Engineering Reference
In-Depth Information
growth process used, and the specific process conditions applied. Crystal sizes can be micro- or nano-
crystalline depending on the temperature, pressure, and the feed gases used. The success of the CVD
methods expands the potential application areas of diamond compared to natural or HPHT-synthetic
diamond into automotive, consumer, defense, and space applications
[22,28,31]
.
CVD from energetically activated hydrocarbon/hydrogen gas mixtures is the most successful pro-
cess for depositing diamond. The process is defined as the formation of a thin solid from the gaseous
phase via a chemical reaction onto a substrate at an elevated temperature
[22,29-31]
.
A CVD reactor provides several functions including:
Transport the reactant and diluents gases to the substrate
Provide sufficient activation energy to reactants
Maintain specific system pressure and temperature
Allow film deposition to proceed
Remove the gaseous by-products.
There are several different approaches for activating precursor gases for diamond
[29-34]
, which
include the following.
15.3.1
Plasma-Enhanced CVD
Plasmas generated by various forms of electrical discharges or inductions heating have been
employed for many years to deposit diamond films. The plasma contains atomic hydrogen and other
species containing carbon necessary for diamond growth. The efficiencies of the different plasma
processes vary from method to method. Three plasma frequency regimes are commonly employed.
These are
microwave
plasma-enhanced CVD, which typically uses excitation frequencies of
2.45 GHz,
radio-frequency
(RF) plasma excitation, which employs frequencies of usually 13.56 MHz,
and
direct-current
(DC) plasmas, which can be run at low electric powers (a “cold” plasma) or at high
electric powers (which create an
arc
or a
thermal
plasma).
15.3.1.1 Microwave Plasma-Enhanced CVD
Microwave plasma-enhanced CVD has been used more extensively and successfully than any other
plasma-based method for the growth of diamond films. Microwave plasmas are different from other
plasmas in that the microwave frequency can oscillate electrons. Collision of electrons with gase-
ous atoms and molecules generates a high degree of ionization. This method of diamond film growth
has a number of advantages over other plasma methods of diamond film growth. Microwave depo-
sition, being an electrode-less process, avoids contamination of the films due to electrode erosion.
Furthermore, the microwave discharge at 2.45 GHz, being a higher-frequency process than the RF,
produces a higher plasma density with higher-energy electrons resulting in higher concentrations of
atomic hydrogen and other hydrocarbon radicals producing efficient high-quality diamond films. In
addition, the plasma is confined to the center of the deposition chamber as a ball, preventing carbon
deposition onto the walls of the chamber.
15.3.1.2 RF Plasma-Enhanced CVD
In this method, as the name implies, RF power is applied to two electrodes to creating a plasma in
an inductively coupled or a capacitively coupled parallel plate arrangement. The growth of diamond
