Highly oriented diamond particles are deposited on mirror-polished (100) silicon substrates in a bell-jar-type microwave plasma deposition system using a three-step process consisting of carburization, bias-enhanced nucleation and growth, By adjusting the geometry of the substrate and substrate holder, very dense disk-shaped plasma is formed over the substrate when the bias voltage is below -200 V. This dense plasma is a prerequisite for the high orientation and is not obtained simply by increasing microwave power, but is obtained by introducing a graphite block between the substrate and the substrate holder, The plasma is concentrated over the substrate with a thickness of several millimeters. From the results of the optical emission spectra of the disk-shaped dense plasma, it is found that the concentrations of atomic hydrogen and hydrocarbon radicals are increased with negative bias voltage. It is also found that the highly oriented diamonds ape deposited in the region, where the intensity ratios of carbonaceous species to atomic hydrogen are saturated. As a result, it is suggested that the highly oriented diamonds are obtained by a combination of the high dose of carbonaceous species and the increased hydrogen etching effects. (C) 1997 American Institute of Physics.