The highly oriented diamond films are deposited on the mirror-polished (100) silicon substrates in microwave plasma deposition system using a three-step process consisting of carburization, bias-enhanced nucleation(BEN) and growth.
In order to establish the selective growth condition of oriented nuclei formed during BEN step, texture-controlled diamond films are deposited on silicon wafers in a $CH_{4}-H_{2}$ gas mixture and the quality of the textured films is investigated using Raman spectroscopy. It is found that the quality of the (100) textured film is better than those of (110) or (111) textured films. The optimum growth rate maintaining the (100) texture is about 0.6 m/h, and its deposition conditions are as follows: substrate temperature, 800℃; methane concentration, 2%; microwave power, 1000W.
By adjusting the geometry of the substrate and substrate holder, very dense disc-shaped plasma is formed over the substrate when the bias voltage is about -250V. 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. From the results of the optical emission spectra of the dense disc-shaped plasma, it is found that the concentration of atomic hydrogen and hydrocarbon radicals are increased with negative bias voltage. It is also found that the highly oriented diamonds are deposited in the region, where the intensity ratios of carbonaceous species to atomic hydrogen are saturated.
The surface of highly oriented diamond film is composed by rectangular shaped grains which are formed parallel to each other. The {111} diamond pole figure of the film is clearly four-fold symmetry as expected for an epitaxial film. The full width at half maximum(FWHM) in the azimuthal angle and pole angle of the {111} diamond pole is about 9℃. The FWHM of the x-ray rocking curve of the diamond (004) reflex from the film is also about 9℃.
During the BEN process, a positive hydrogen ions and carbonaceous ions bombard to the substrate. Silicon atoms may be preferentially removed from the surface and near-surface layers of the silicon carbide deposited on silicon substrate. In order to sustain the lattice structure of silicon carbide, some of the carbonaceous ions fill in the vacant silicon sites immediately after the silicon atoms are etched. In this manner, the $sp^3$-bond carbon clusters can be formed with the orientation relationship between them and silicon carbide which was epitaxially grown on silicon substrate during the carburization and bias treatment. It is suggested that the heteroepitaxial diamond can be obtained by the combination of the formation of silicon carbide, selective etching of silicon in silicon carbide network, filling the vacant silicon sites by carbonaceous ions, adding hydrocarbon radicals to the oriented $sp^3$-bond carbon clusters and etching non-diamond component and off-oriented $sp^3$-bond carbon clusters during the BEN process. The deposition and etching rates of the oriented diamond, the off-oriented diamond and the non-diamond component is very fast with respect to the deposition without bias since there are large quantities of carbonaceous species, accelerated ions and atomic hydrogen in the dense plasma. The increased amount of the etching rate by applied bias is larger than that of the deposition rate of the diamond. The strongly-bonded embryos with substrates due to ordered chemical bonds are relatively difficult to etch away while the weakly-bonded embryos are easily etched by atomic hydrogen to become gasified or unstable embryos of diamond. Consequently, the fraction of oriented embryos in total embryos formed on the surface is increased by preferential etching of off-oriented embryos. As a result, the oriented embryos are grown by adding the diamond precursor to the stable nuclei. Finally, highly oriented diamond film is formed after a growth step, while a few off-oriented embryos, which are survived after the bias step, grow to form azimuthally misoriented grains.
