Studies of Diamond Film Growth

Details of this work may by found in:
Journal of Vacuum Science and Technology A 14(3 pt. I)952

Background: Graphite vs. Diamond

The Observation:

Peter Bachmann observed that for all experiments resulting in the deposition of diamond films the ratios of carbon to hydrogen to oxygen fell into a narrow region (shown in red) of the ternary phase diagram for the three elements. He further demonstrated the concept by showing that diamond growth occurs over same range of elemental ratios, even when using two chemically different gas mixtures (ethylene + oxygen and acetone + oxygen) with different growth techniques (an atmospheric torch and a microwave plasma, respectively).

The Chemical Puzzles:

"Why does the chemical nature of the reactant gases have little or no influence on growth?"

"Why is it so easy to get a kinetic product: diamond and not the thermodyamically favored product: graphite?

Previous Work on the Chemistry of Diamond Growth

Our Approach:

Supersonic Pulse, Plasma Sampling Mass Spectrometry

Learn More About Supersonic Pulse, Plasma Sampling Mass Spectrometry

What Reactions Should be Observered

The plasma we use is an Electron Cyclotron Resonance (ECR) micowave plasma which operates at pressures below one-ten-millionth of an atmosphere. In this plasmas the power needed to maintain the discharge is coupled directly to the free electrons in the plasma. Since the diamond plasmas that we study are all >95% hydrogen, the most likey reaction will be the dissociation of hydrogen molecules to atoms. The majority of the hydrocarbon chemistry would then be expected to be driven from the reaction of the hydrogen with hydrocarbon species.

The Experiment:

Compare the Molecular Composition of
4% Carbon in Hydrogen Plasmas
Based on Four Chemically Different Hydrocarbons

The Results:

Composition of Each Plasma: Click on gas mixture to see actual mass spectral deconvolution

2% Ethane (C2H6) in Hydrogen (H2): 61% Ethane
14% Ethylene
16% Acetylene
3% Methane
6% Unassigned
2% Ethylene (C2H4) in Hydrogen (H2): 0% Ethane
46% Ethylene
49% Acetylene
2% Methane
3% Unassigned
2% Acetylene (C2H2) in Hydrogen (H2): 5% Ethane
12% Ethylene
82% Acetylene
1% Methane
0% Unassigned
4% Methane (CH4) in Hydrogen (H2): 0% Ethane
23% Ethylene
20% Acetylene
52% Methane
5% Unassigned

What May Be Learned:

Although the Ethane mixture reacts in the plasma to yield all other species, only the acetylene mixture shows any sign of producing ethane (and even that small amount is correlates to an ethane impurity in the acetylene feed stock). A large fraction of the ethane remains unreacted in the plasma.
In all cases, except the acetylene mixture, the amount of ethylene and acetylene are comparable.
Significant amounts of methane are observed only for the methane mixture, and a large fraction of the methane remains unreacted in the plasma.

Corresponding Conclusions:

The production and decomposition of ethane must be difficult and/or rare.
The conversion of acetylene to ethylene and ethylene to acetylene must be easy and common.
Carbon-carbon bonds are not easily broken or formed in these plasmas.

Final Note: The large fraction of acetylene that remains unreacted may imply that the normal molecule may be relatively unreactive, and the species in rapid interconversion with ethylene may be ethyne( a radical, also with mass 26).

Our Chemical Model of Diamond Plasmas

The arrows indicate the reaction of each species with H atoms in the plasma. The red region is the range of species that we currently believe to interconvert rapidly.