In synthesis of diamond with a plasma jet under medium pressure, the m
orphology of the deposited particles depends strongly on the ratio of
the methane gas flow rate to the hydrogen gas flow rate and the distan
ce from the feed ring exit to the substrate. The relationship between
the morphology of the deposited particles and the chemical species in
the plasma jet is studied by means of emission spectroscopy, with a vi
ew toward achieving large-area deposition at a high rate. The intensit
y ratio of the CH to the H-alpha spectrum is strongly correlated with
the quality of the diamond synthesized and the deposition area.