In order to clarify the role of hydrogen in diamond synthesis using a microwave plasma in a CO/H2 system, carbon films were grown by varying hydrogen mole fractions in a CO/H2/He microwave plasma. The correlation between film properties and plasma species was investigated through film characterization and plasma emission spectroscopy. C and C2 were formed in the gas phase of the CO/He system and only sootlike carbon was deposited. Hydrogen additions to the CO/He system were found to enhance diamond growth by suppressing the formation of C and C2, which inhibited diamond growth by blocking the nucleation sites. The complicated structure of amorphous hydrogenated carbon, diamond microcrystallites having a diameter of 100 Å, and graphitic carbon was formed in the CO(5%)/H2 (30%)/He system, while columnar polycrystallites were grown in the CO(5%)/H2 system. Almost the same amount of atomic hydrogen in the ground state was found to exist in both systems, whereas a larger amount of electronically excited atomic hydrogen existed in the CO/H2/He system than the CO/H2 system. The atomic hydrogen in the ground state enhanced diamond growth by removing the amorphous carbon deposits, while the electronically excited atomic hydrogen exhibited no contribution to diamond growth and reacted with the amorphous carbon deposits to form the amorphous hydrogenated carbon. Finally, oxygen-containing species and hydrocarbons were suggested to be important precursors for diamond crystallization.

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